What Is the Value of Water Contact Angle on Silicon?

materials

Article What Is the Value of Water Contact Angle on Silicon?

Paweł Bryk 1, Emil Korczeniewski 2, Grzegorz S. Szyma ´nski 2, Piotr Kowalczyk 3, Konrad Terpiłowski 4 and Artur P. Terzyk 2,*

1 Department of Chemistry, Chair of Theoretical Chemistry, Maria Curie-Skłodowska University, 20-031 Lublin, Poland; [email protected] 2 Faculty of Chemistry, Physicochemistry of Carbon Materials Research Group, Nicolaus Copernicus University in Toru´n,Gagarin Street 7, 87-100 Toru´n,Poland; [email protected] (E.K.); [email protected] (G.S.S.) 3 College of Science, Health, Engineering and Education, Murdoch University, Murdoch WA 6150, Australia; [email protected] 4 Department of Chemistry, Chair of Physical Chemistry of Interfacial Phenomena, Maria Curie-Skłodowska University, 20-031 Lublin, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-56-61-14-371

Received: 4 March 2020; Accepted: 26 March 2020; Published: 27 March 2020

Abstract: Silicon is a widely applied material and the wetting of silicon surface is an important phenomenon. However, contradictions in the literature appear considering the value of the water contact angle (WCA). The purpose of this study is to present a holistic experimental and theoretical approach to the WCA determination. To do this, we checked the chemical composition of the silicon (1,0,0) surface by using the X-ray photoelectron spectroscopy (XPS) method, and next this surface was purified using different cleaning methods. As it was proved that airborne hydrocarbons change a solid wetting properties the WCA values were measured in hydrocarbons atmosphere. Next, molecular dynamics (MD) simulations were performed to determine the mechanism of wetting in this atmosphere and to propose the force field parameters for silica wetting simulation. It is concluded that the best method of surface cleaning is the solvent-reinforced de Gennes method, and the WCA value of silicon covered by SiO2 layer is equal to 20.7◦ (at room temperature). MD simulation results show that the mechanism of pure silicon wetting is similar to that reported for graphene, and the mechanism of silicon covered by SiO2 layer wetting is similar to this observed recently for a MOF.

Keywords: wetting; silicon; contact angle; molecular dynamics simulation; hydrocarbons hypothesis

1. Introduction The process of cleaning is a crucial stage in the study of surface properties, and more or less advanced cleaning procedures have been proposed prior to the contact angle (CA) measurements to remove the adsorbed oil, hydrocarbons, and other impurities [1]. It has been well documented experimentally that physical adsorption of hydrocarbons by changing a surface hydrophobicity can simultaneously affect the values of CA especially if water droplet is sitting on a surface. As it was shown by us recently (see, e.g., in [2] and references therein) the influence of airborne adsorbed hydrocarbons on water contact angle (WCA) depends on the coverage and the nature of a surface. Generally, it was shown that the increase or a decrease in the WCA value after airborne hydrocarbons adsorption can be observed. Recently, we also proved that if the affinity of water to a substrate is high water nanodroplet can completely remove light hydrocarbons from a surface and robust self-cleaning properties of a surface are observed [3]. There are few major topics that should be considered during silicon surface wetting. As it was pointed out by de Gennes et al. [1], it is well known that a silicon surface is covered by a thin layer of SiO2. This thin oxide layer can be removed [4]

Materials 2020, 13, 1554; doi:10.3390/ma13071554 www.mdpi.com/journal/materials Materials 2020, 13, 1554 2 of 17

by a treatment of Si with HNO3, HF, and CH3COOH mixture; however, after this process, a sample should be stored in an inert atmosphere. Moreover, the silicon surface is additionally covered by this layer of adsorbed hydrocarbons. Shinozaki et al. [5] studied the methods of SiO2 surface cleaning from adsorbed hydrocarbons, using three different methods of surface cleaning. The thickness of adsorbed hydrocarbons layer is in the range of ~0.2 nm. Choi et al. [6] showed that the UV/O3 plasma treatment removes adsorbed hydrocarbons completely. However, during a plasma treatment, surface roughness appears and it is more visible for longer plasma treatment time. This roughness influences the CA values. Due to the above-mentioned reasons, WCA reported experimentally and obtained by molecular simulations for silicon differ remarkably. For example, the WCA equal to 86–88◦ was reported from experimental studies for etched silicon [7]. Computer simulation results of a wetting process of a (0,0,1) SiO surface [8] led to WCA equal to 24 1.28 . Han et al. determined the WCA on 2 ± ◦ a smooth silicon surface as equal to 54◦ [9]. Isaev et al. [10] assumed that WCA is in the range of 87◦. Similar WCA value on silicon was obtained from simulation by Barisik and Beskok [11]. These authors assumed a Stillinger–Weber potential for silicon and the SPC/E model of water. They quote [7] a range between 86 and 88◦ as the true experimental WCA on etched silicon. However, as we mentioned above, a fresh silicon surface quickly develops a ~1.4–1.5 nm thick layer of silicon dioxide, leading to significant lowering of WCA. One of the conclusions of the Barisik and Beskok study is that it is important to include the Stillinger–Weber potential between silicon atoms in order to get the correct wetting behavior of this system. The key argument (cf. Figure 4 in [11]) is that WCA as a function of the substrate–water interaction strength (ε) develops two wetting regimes characterized by different slopes of the plot. Unfortunately, we find this argument to be flawed. As it is well known [12,13], there is a linear dependence of cosθ (where θ = CA) vs. ε (and not θ vs. ε) close to the first order wetting transition, i.e., for small values of 1-cosθ [2]. Although this relationship in practice persists up to fairly large WCA values, for systems sufficiently well removed from the wetting transition, the plot of cosθ vs. ε will develop a curvature (see [14]). The inclusion of silicon–silicon interactions will lead to vibrations of the silicon lattice substrate structure and will alter water particles in the nearest vicinity of the surface but this cannot qualitatively alter the wetting properties of the silicon. Adding insult to injury, Barisik and Beskok [11] used spherical droplets; therefore, their WCA values can be affected by the line tension contribution [15]. It is well known that this contribution can have different signs depending on the value of the CA, with a positive diverging value close to the wetting transition [16]. Summing up simulation approaches does not consider the onset of the silicon dioxide thin layer, nor do they consider the presence of the airborne surface hydrocarbons and their influence on CA value. This presence must be taken into account because the CA is usually measured in the air atmosphere. The aim of our study is to present a holistic experimental and theoretical approach to evaluate the effect of alkane adsorption on water–silicon CA. First of all, we present the experimental results. Different silicon surface cleaning methods are applied and, after cleaning, the WCA is measured in an inert as well as in hydrocarbons atmosphere. To confirm the influence of hydrocarbons some chromatographic measurements are additionally performed. Finally, the results of Molecular Dynamics (MD) simulations are reported to determine the wetting mechanism in light hydrocarbons atmosphere and to obtain the correct force field parameters leading to agreement between experiment and simulation.

2. Materials and Methods Three types of silicon samples were used in our experiment: A (1,0,0) plate (ON Semiconductor Co, Raznov, Czech Republic, obtained by using Czochralski method) cut on squares (10 10 mm) × using a diamond edge; a (1,1,1) silicon wafer (ITME, Warszawa, Poland) obtained by using a Float Zone method and laser cutting (7 7 mm); and a silicon powder (purity 5N-99.999%), 100–200 µm, × produced by Sabon Plant, Silicio FerroSolar SLU, Madrid, Spain. The (1,0,0) and (1,1,1) samples were applied in the X-ray photoelectron spectroscopy (XPS) and wetting studies, and the powder in the chromatographic study. XPS spectra were measured using the UHV multi-chamber analytical system from Prevac Ltd. (Rogów, Poland). The system has been equipped with sources (MX-650 from Gamma Materials 2020, 13, 1554 3 of 17 data Scienta, anode Kα-Al) or achromatic X-rays (anode: Mg/Al or Ag, resolution <1 meV). Before WCA measurements, three separate purification processes were performed: the procedure taken form original de Gennes book [1] (labeled as DG), the DG procedure followed by a pre-treatment with solvents (called SDG), and the procedure of purification in acetone (samples were labeled as A and LA, respectively). In the DG method, the plates (stored in polypropylene boxes) were removed from a polyvinylchloride tape and placed immediately in an acidic bath containing H2SO4/H2O2 (30 min, 70:30, H2SO4—95% pure p.a., Chempur, Piekary Sl´ ˛askie,Poland, H2O2—30%, POCH, Gliwice, Poland) following the procedure proposed in [1]. During the SDG method, a silicon sample was removed from a polyvinylchloride tape and placed in ultrasonic bath in acetone (pure p.a., Chempur, Piekary Sl´ ˛askie,Poland) for 45 min (after 30 min. the solvent was removed and replaced by a fresh one). Next, each silicon plate was washed out using the surfactant Rosulfan L (PCC Exol SA, Brzeg Dolny, Poland). This surfactant is composed of sulfuric acid, C12-14-alkyl(even numbered) esters, and sodium salts. Next, the samples were washed out with deionized water (containing 3–6 ppm of ions) and placed immediately in an ultrasonic bath in ethanol (pure p.a., Chempur, Piekary Sl´ ˛askie,Poland) for 30 min (after 15 min. the solvent was removed and replaced by a fresh one). Next, ethanol was replaced by isopropanol (pure p.a., Chempur, Piekary Sl´ ˛askie,Poland, 30 min; after 15 min. the solvent was removed and replaced by a fresh one). Finally, the same was done with acetone (pure p.a., Chempur, Piekary Sl´ ˛askie,Poland, 20 min; after 10 min. the solvent was removed and replaced by a fresh one). Next, the DG procedure standard described above was performed. Acetone purified materials were obtained by placing a silicon sample in acetone (pure p.a., Chempur, Piekary Sl´ ˛askie,Poland) for 3 days (sample labeled as A) and for one month (LA). Finally, the samples from all studied series (excepting A and LA) were washed out at least 5 times in deionized water and placed in small glass containers (each sample separately) in an oven (373 K, 30 min). Each sample was next cooled for 4 min in a closed cell. Next, the initial WCA was measured, and the samples were subjected to the C10H22 atmosphere following a similar procedure to that described in detail previously [2]. WCA was measured at 298.15 K (at relative humidity equal to 40 %). A goniometer was equipped with a camera Grasshopher3 GS3–U3–32S4C–C, 3.2 Mpx connected to the specially designed optical system consisting of perfectly located in the optical axis two elements: Edmund Optics 1.0x Telecentric Lens 55350 and Edmund Optics Telecentric Illuminator Lens 62760 with a bright diode placed in it (MicroBrite ™ Spot/Coaxial Light model: SL223 470 IC, Light wavelength: 470 nm) leading to polarized parallel light. Between these two optical elements, a precise measuring cuvette made of optical glass (Hellma Analytics, Optical Glass, Light Path 50 mm, model: 704-003-50-10) was placed in the optical axis, having a glass lid with a precise hole with a diameter corresponding to the outer diameter of the dispensing needle. Drops were dispensed using a syringe dispenser manufactured by CERKO, Poland, Gdynia, equipped with a Hamilton syringe, enabling precise dosing of the set volume of the measuring liquid. The needles used were disposable needles manufactured by John P. Kummer GmBH (Needle Tip all in PTFE, size G30, in length 2 inches, model: PDS-Z592130). The probe liquid was deionized water with an ion content of 3–6 ppm, thermostated at the measurement temperature for 30 min. Ten microliter drops were used for all measurements. Liquid hydrocarbon was added to the system in a volume of 1 mL into a small weighing vessel placed inside the cuvette described, with a constant evaporation surface of 260 mm2 each time. Between each measuring series, the entire measuring cuvette was thoroughly cleaned using detergents and acetone, and next dried in an ambient atmosphere. Static contact angles were measured based on the obtained images using the ImageJ software with plugin Drop Analysis -LB-ADSA method [17] with optimization of all available parameters through gradient energy approach. Each measurement was repeated at least 3 times using fresh silicon samples and reagents. For the samples from the A and LA series, the WCA was measured immediately after acetone evaporation. Finally, we also checked the method of purification in HF however, the AFM analysis of studied samples revealed strong roughness and this is why the samples purified using this method were Materials 2020, 13, 1554 4 of 17 excluded from our study. No roughness creation was observed for the samples purified using other methods. The chromatographic investigations were carried out using a Chrom 4 gas chromatograph with a flame ionization detector (FID) using helium as a carrier gas. A computer was connected to the gas chromatograph to control, acquire, and process the chromatographic data. The adsorbent was placed in a glass column (50 cm 2 mm I.D.) with an absorbent bed length of 40 cm, which corresponded × to 3.15 g of silicon used (particle diameter of 0.1–0.2 mm). Before the adsorption experiments, the column with the adsorbent was conditioned at 423K for 1 h under a flow of helium. N-decane and n-pentadecane (pure p.a., Chempur, Piekary Sl´ ˛askie,Poland) were used as adsorbates. The adsorbates were injected into the column by means of a Hamilton microsyringe. The size of the injected samples was 0.2 µL. The temperature of the injection device was set at 423 K and 453 K for n-decane and n-pentadecane, respectively. Additionally, in some experiments, to investigate the effect of water on hydrocarbons adsorption, different amounts of water (1.2 µL) were dosed before a hydrocarbon injection. The reagent injections were performed in the range of 333 to 353 K (n-decane) or 373 to 423 K 3 1 (n-pentadecane) at carrier gas flow rate 12 cm min− (measured by means of a bubble gauge). To perform theoretical studies, we model the silicon substrate as layers of frozen particles interacting via the Lennard–Jones (12–6) potential with the oxygen from the SPC/E model of water. The silicon atoms of diameter σSi = 2.095 Å are arranged on a diamond cubic lattice with 5.431 Å spacing. All dispersion interactions have a cut-off radius rcut = 15 Å. We use the SPC/E[18] model of water with σ = 3.166 Å, ε = 0.6497752 kJ/mole, q = 0.8476, q = 0.4238, HOH angle 109.47 , and OH bond OO OO O − H ◦ length 1 Å. We assume the additivity of diameters σij = (σii + σjj)/2. However, the dispersion interactions 1/2 between the silicon and oxygen are weaken, εSiO = kd(εSiSiεOO) with εSiSi = 209.199544kJ/mole. The parameter kd will be estimated numerically in order to recover the experimental values of WCA. The system of SPC/E water molecules is first arranged on a parallelepiped lattice and placed on the silicon substrate. A minimization of the potential energy is performed followed by molecular dynamics simulation in the NVT ensemble (with time step 0.002 ps) using Nose–Hoover thermostat (τ = 0.1) and SHAKE algorithm for keeping the water molecules rigid. Equilibration was performed for 2 ns while the time averages were accumulated by at least 40ns (up to 60ns for very small WCAs). Long-range Coulomb interactions were evaluated using the Particle Mesh Ewald (PME) method with 15 Å taken as a value demarcating the real-space and k-space PME calculations. All simulations were carried out using OpenMM molecular simulation package [19–23] at constant temperature T = 298.15 K. The SPC/E water on pure silicon calculations were divided into three phases: First we evaluated the kd parameter using cylindrical droplets, so that the experimental value [7] of the WCA on silicon are recovered in simulation. To this end, we prepared a parallelepiped silicon surface of dimensions 543 Å 32.58 Å x 16.29 Å (x,y,z dimensions, respectively) containing 15,600 Si atoms with × (1,0,0) plane being the top surface facing in the z-direction. On the top of this surface, 3971 SPC/E water molecules were placed. The simulations were carried out at constant temperature T = 298.15 K, and the simulation box was enclosed by a cylindrical repulsive wall of radius 150 Å. From the positions of the water molecules the two dimensional density profiles (Cartesian grid) of the cylindrical droplets were calculated. The WCA of cylindrical droplets was determined by finding the density contour satisfying 0.5 0.03 g/cm3. ± 3. Results

3.1. Silicon Characterization and WCA Measurements The results of the XPS measurements for a typical silicon sample are shown in Figure1. High-resolution XPS spectra recorded the following elements; carbon, oxygen, sodium, chlorine, and silicon. Elements such as carbon, sodium, and chlorine were found in trace amounts. In the case of silicon, on the surface there is a 78.4 mass concentration of silicon and 16.7 of oxygen. These two components are crucial for experiment [24]. The Si 2p B peak appeared at 103.2 eV and it is associated Materials 2020, 13, 1554 5 of 17

with the Si (+IV) state corresponding to SiO2 structure on the surface (Figure1, Table1). Pure Si bonds were detected at 99.22 and 99.82 eV. 24.7% of silica occurs on the surface in combination with oxygen [25,26]. Materials 2020, 13, x FOR PEER REVIEW 5 of 17

2p 1s Figure 1. 1. DeconvolutionDeconvolution of of th thee Si Si 2p(a) (anda) and O Osignals1s signals (b) of (b the) of Si the (1,0,0) Si (1,0,0) wafer wafer..

TableTable 1. 1. TheThe results results of ofX- X-rayray photoelectron photoelectron spectroscopy spectroscopy (XPS (XPS)) analysis analysis..

PeakPeak Name Name PositionPosition (eV) (eV) %At %At conc. conc. Species Species Si 2pSi 32p/2 A3/2 A 99.2299.22 68.5 68.5 main silicon main peak silicon (Si 2p peak doublet) (Si 2p doublet) Si 2pSi 12p/2 B1/2 B 99.8299.82 - - Si (0) Si (0)second silicon second peak silicon (Si 2p peak doublet) (Si 2p doublet) Si 2pSi B2p B 103.2103.2 24.7 24.7 Si (+IV) Si (+IV)SiO2 SiO2 Si 2pSi C2p C 101.63101.63 0.8 0.8 Si (+III) Si (+III) Si 2pSi D2p D 100.95100.95 1.3 1.3 Si (+II) Si (+II) Si 2pSi E2p E 100.41100.41 1.3 1.3 Si (+I) Si (+I) Si 2pSi F2p F 98.7998.79 3.3 3.3 Si (0) Si (0)Deffective Destructuresﬀective structures O 1s A 532.6 94.7 O (-IV) SiO2 O 1s A 532.6 94.7 O (-IV) SiO2 O 1s B 531.4 5.3 O (-II) hydroxides O 1s B 531.4 5.3 O (-II) hydroxides

Summing up, up, it it is is seen seen that that on on the the surface surface of ofour our samples samples the the SiO SiO2 layer2 layer is created is created.. The results of of measurements measurements in in C C1010H22H atmosphere22 atmosphere show show no remarkable no remarkable differences differences between between the the (1,0,0)(1,0,0) and (1,1,1) samples. samples. Moreover Moreover,, we we do do not not observe observe the the dependence dependence of WCA of WCA on hydrocarbon on hydrocarbon expositionexposition time time,, and and only only oscillati oscillationsons of of WCA WCA values values around around an anaverage average value value (this (this behavior behavior will be will be explainedexplained by the the results results of of MD MD simulations simulations,, see see below). below). Figure Figure 2 presents2 presents the the comparative comparative analysis analysis ofof thethe average WCA WCA values values measured measured for for (1,0,0) (1,0,0) samples samples purified purified using using different different methods methods.. One Onecan can observeobserve that the didifferencesfferences in WCA depend on the method of of purification purification,, and and the the smallest smallest value value,, i.e., 20.7i.e., 20.72.7 ± 2.7is observed° is observed for thefor the samples samples cleaned cleaned using using the the SDG SDG procedure. procedure. This This value value is assumedis assumed for the ± ◦ scalingfor the scaling of the forceof the field force during field during MD simulations MD simulations (see below).(see below).

Materials 2020, 13, x FOR PEER REVIEW 6 of 17 Materials 2020, 13, 1554 6 of 17 Materials 2020, 13, x FOR PEER REVIEW 6 of 17

Figure 2. The average water contact angle (WCA) values for silicon (1,0,0) samples subjected10 to C FigureFigure 2. The 2. Theaverage average water water contact contact angle angle ((WCAWCA) values for for silicon silicon (1,0,0) (1,0,0) samples samples subjected subjected to C to C1010 H expositionH22 exposition at 298.15at 298.15 K. K. H22 exposition at 298.15 K.

3.2. Chromatographic3.2. Chromatographic Measurements Measurements 3.2. Chromatographic Measurements TheThe aim aim of theof the chromatographic chromatographicresults results was to to confirm conﬁrm that that water water repels repels light light hydrocarbons hydrocarbons fromThefrom a silicon aima silicon of surface. the surface. chromatographic In In fact, fact, the the results results results collectedcollected was in into Figure Figureconfirm 33 confirm conﬁrm that water this this hypothesis, hypothesis,repels light as we ashydrocarbons observe we observe fromthe progressive thea silicon progressive surface. decrease decrease In fact, in in both theboth hydrocarbonsresults hydrocarbons collected retention in Figure times times 3 confirmafter after inject injection thision hypothesis,of water of water to the as to systemwe the observe system.. theThis progressiveThis phenomenon phenomenon decrease isa is function a infunction both of hydrocarbons of temperature temperature retention and hydrocarbon hydrocarbon times after type, type, inject and and asion it as wasof it water was experimentally experimentally to the system . Thisevaluated phenomenonevaluated by us by thatus is that fora function f theor the system system of temperature n-pentadecane n-pentadecane and water hydrocarbon it it vanishes vanishes type, at at~423 ~423and K. as K. it was experimentally evaluated by us that for the system n-pentadecane water it vanishes at ~423 K.

Figure 3. Chromatograms of dosed 0.2µl n-decane ((a) 333 K; (b) 353 K) and 0.2µl pentadecane ((c) 373 K, (d) 398 K), without (green lines) and after the injection of 1 µl (red line) and 2 µl (blue line) of water.

µ µ Figure 3. ChromatogramsChromatograms of of dosed dosed 0.2µl 0.2 nL-decane n-decane ((a (() a333) 333 K; K;(b) ( b353) 353 K) K)and and 0.2µl 0.2 pentadecaneL pentadecane ((c) µ µ 373((c) 373K, (d K,) 398 (d) 398K), K),without without (green (green lines) lines) and and after after the the injection injection of of1 µl 1 (redL (red line) line) and and 2 µl 2 (blueL (blue line) line) of water.of water.

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Materials 2020, 13, x FOR PEER REVIEW 7 of 17 3.3. Molecular Simulations of SPC/E Water on Silicon 3.3. Molecular Simulations of SPC/E Water on Silicon Figure S1 shows a water contour calculated for kd = 0.147. The bottom of the droplet is defined 3 as aFigure part of S1 the shows density a water profile contour near thecalculated surface for with kd = the 0.147. density The ofbottom at least of the 0.5 droplet g/cm and is defined from this as afigure part of we the identify density this profile at z =near17.6 the Å. surface Next, the with least-square the density method of at least of fitting0.5 g/cm the3 and contour from to this the figure circle wewas identify applied, this taking at z the= 17.6 points Å. Next, that arethe atleast-square a distance atmethod least 10of Åfitting from the the contour surface. to Figure the circle S2 shows was applied,the evolution taking of the the points WCA that with are time at a for distance kd = 0.147. at least In 10 general, Å from the the variation surface. ofFigure WCA S2 is shows within the the evolutionbracket of of experimentally the WCA with reported time for valueskd = 0.147. [7]. In The general, calculations the variation were carried of WCA out is for within a set the of dibracketfferent ofwater–silicon experimentally interaction reported strengths: values [7]. kd The= 0.1, calculations 0.12, 0.142, were 0.145, carried 0.146, out 0.147, for 0.15, a set 0.18, of different 0.19, 0.196, water– and silicon0.22. We interaction note that strengths: the dependence kd = 0.1, of 0.12, WCA 0.142, vs. k0.145,d varies 0.146, from 0.147, 117.7 0.15, for k0.18,d = 0.10.19, (corresponding 0.196, and 0.22. to WeεSiO note= 1.1659 that the kJ/mole) = to 16.8 for kd = 0.22 (corresponding to εSiO = 2.5650 kJ/mole) and it is nonlinear. Instead we find nice linear relationship for the dependence of the cosine of WCA vs. kd, see Figure4.

FigureFigure 4. 4. DependenceDependence of of the the simulated simulated cosine cosine of of WCA WCA vs. vs. k kdd atat 298.15 298.15 K. K.

FromFrom this figurefigure wewe estimateestimate the the value value of of the the substrate–water substrate–water interaction interaction strength strength at which at which the firstthe firstorder order wetting wetting transition transition takes takes place place kw = 0.2237kw = 0.2237 or εSiO or,w ε=SiO2.6081,w = 2.6081 kJ/mole. kJ/mol Fore the. For largest the largest CA values CA valueswe find we that find the that deviations the deviations from linearity from linearity set in, set as expected.in, as expected. This figure This f confirmsigure confirms the quality the quality of the ofobtained the obtained results. results. The calculations The calculations presented presented above indicateabove indicate a good silicon–watera good silicon interaction–water interaction strength strengthkd = 0.147, kd or= ε 0.147,SiO = 1.71387548725 or εSiO = 1.71387548725 kJ/mole. This kJ/mol valuee. has This been value used has in subsequent been used calculations. in subsequent calculations.In the second stage of calculations, we estimate the influence of the finite system size (i.e., the effect of theIn linethe tensionsecond onstage the of WCA). calculations To do this, we weestimate created the a surfaceinfluence of Siof ofthe dimensions finite system 21.74 size nm (i.e.21.74, the × effectnm of1.629 the nmline (x,y,ztension dimensions, on the WCA respectively)). To do this containing we created 41,600 a surface Si atoms of Si of with dimensions 100 top plane 21.74facing nm × × 21.74the z-direction. nm × 1.629 On nm the (x,y,z top ofdimensions, this surface respectively) we placed a containing certain number 41,600 SPC Si/ Eatoms water with molecules. 100 top Similar plane facingto the simulationsthe z-direction. for cylindricalOn the top droplets of this surfac the simulationse we placed of a the certain spherically number symmetric SPC/E water drops molecules. were also Similarcarried to out the at simulations constant temperature for cylindrical T = 298.15 droplets K with the thesimulations same thermostat of the spherically parameters. symmetric The simulation drops werebox wasalso toppedcarried out by aat spherical constant temperature repulsive wall T = of 298.15 the radius K with 10 the nm. same Once thermostat the initial parameters. minimization The sofimulation the positions box was ofwater topped molecules by a spherical was accomplished, repulsive wall the of spherically the radius symmetric 10 nm. Once drop the is readily initial minimizationformed allowing of the the positions calculation of water of the radiallymolecules symmetric was accomplished density profile., the spherically From the radially symmetric averaged drop istwo-dimensional readily formed allowing density profile the calculation%(r, z), we of evaluated the radially the symmet densityric contours density satisfying profile. From0.5 the0.03 radially g/cm3 . ± averagedFigure5 shows two-dimensional the water drop density contour profile calculated ρ(r, z), we for evaluated k d = 0.147 the and density for three contours different satisfying numbers 0.5 of± 0.03water g/cm molecules,3. Figure N 5SPC shows/E = 2000,the water 4000, drop and 6000, contour respectively. calculated for kd = 0.147 and for three different numbers of water molecules, NSPC/E = 2000, 4000, and 6000, respectively. Similar to the cylindrical droplet, the bottom of the spherical droplet is defined as the part of the density profile near the surface with the density of at least 0.5 g/ cm3, and again from Figure 5, we infer it to be z = 17.6˚A. Materials 2020, 13, x FOR PEER REVIEW 8 of 17

Likewise, the least-square method of fitting the contour to the circle was applied, taking the points that are at a distance at least 10 Å from the surface. We find the WCA values of 86.84°, 86.83° and 86.41° for the spherical droplets comprising of 2000, 4000, and 6000 water molecules, respectively. These results show that there is very little dependence of the WCA on the droplet size. Consequently,

Materialsthe line2020 tension, 13, 1554 finite size effects are negligible when considering droplets consisting of 6000 8water of 17 molecules.

Figure 5. Figure 5.Map Map of of the the mass mass density density profiles profiles (left (left column) column) and and the the spherical spherical droplets droplets contours contours (right (right column) of water droplet on bare silicon surface, for the droplets consisting of 2000 (a), 4000 (b), column) of water droplet on bare silicon surface, for the droplets consisting of 2000 (a), 4000 (b), and and 6000 (c) water molecules. The circles denote the position of water density profile with value 6000 (c) water molecules. The circles denote the position of water density profile with value 0.5 ± 0.5 0.03 g/cm3. 0.03g/cm± 3. Similar to the cylindrical droplet, the bottom of the spherical droplet is defined as the part of the density profile near the surface with the density of at least 0.5 g/ cm3, and again from Figure5, we infer it to be z = 17.6 A. Likewise, the least-square method of fitting the contour to the circle was applied, taking the points that are at a distance at least 10 Å from the surface. We find the WCA values of 86.84◦, 86.83◦ and 86.41◦ for the spherical droplets comprising of 2000, 4000, and 6000 water molecules, respectively. These Materials 2020, 13, x FOR PEER REVIEW 9 of 17

In the third phase of calculations, we simulated the water drops on alkane preadsorbed silicon surfaces. The simulation system was prepared by creating a surface of Si of dimensions 21.74 nm × 21.74Materials nm ×2020 1.629, 13, nm 1554 (x,y,z dimensions, respectively) containing 41,600 Si atoms with 100 top plane9 of 17 facing the z-direction, i.e., the same number of silicon atoms as for the spherical water drop-bare silicon surface systems. Next, on the top of silicon surface we placed a number of n-decane molecules results show that there is very little dependence of the WCA on the droplet size. Consequently, the line in order to achieve the required surface density. We use the OPLS all-atom model of n-decane [27]. tensionMaterials 20 finite20, 13 size, x FOR eff ectsPEER are REVIEW negligible when considering droplets consisting of 6000 water molecules.9 of 17 The set of interaction parameters is collected in Table S1. In the third phase of calculations, we simulated the water drops on alkane preadsorbed silicon We assume the additivity of diameters σij = (σii+σjj)/2 and the Lorentz–Berthelot mixing rule. The surfaces.In the The third simulation phase of systemcalculations was prepared, we simulated by creating the water a surface drops of on Si alkane of dimensions preadsorbed 21.74 silicon nm Coulomb and LJ interactions between atoms separated by three bonds within the same molecule were × 21.74surfaces. nm The1.629 simulation nm (x,y,z system dimensions, was prepared respectively) by creating containing a surface 41,600 of Si atomsof dimensions with 100 21.74 topplane nm × scaled down× by multiplying them by 0.833333 and 0.5, respectively. All dispersion interactions have facing21.74 nm the × z-direction, 1.629 nm (x,y,z i.e., the dimensions, same number respectively) of silicon atoms containing as for the41, spherical600 Si atoms water with drop-bare 100 top silicon plane a cut-off radius rcut = 1.5 nm, and the same distance was used to switch from a real-space to Fourier surfacefacing the systems. z-direction, Next, oni.e. the, the top same of silicon number surface of silicon we placed atoms a number as for the of n-decane spherical molecules water drop in order-bare space calculations of the electrostatic interactions. The alkane–silicon system was allowed to tosili achievecon surface the requiredsystems. surfaceNext, on density. the top We of silicon use the surface OPLS all-atomwe placed model a number of n-decane of n-decane [27]. molecules The set of equilibrate for 1.0 ns. Subsequently, 6000 SPC/E water molecules were placed above the alkane- interactionin order to parametersachieve the isrequired collected surface in Table density. S1. We use the OPLS all-atom model of n-decane [27]. covered silicon surface. The resulting water–alkane–silicon system was equilibrated for 10 ns The setWe of assume interaction the additivity parameters of diameters is collectedσ in= (Tableσ +σ S1)/2. and the Lorentz–Berthelot mixing rule. The followed by another 10 ns of gathering the timeij averages,ii jj and the same procedure of evaluating the CoulombWe assu andme LJ interactionsthe additivity between of diameters atoms σ separatedij = (σii+σjj)/2 by threeand the bonds Lorentz within–Berthelot the same mixing molecule rule. were The WCA was adopted. Figures 6–8 show simulation snapshots of an alkane covered silicon surface. scaledCoulomb down and by LJ multiplying interactions them between by 0.833333 atoms separated and 0.5, respectively.by three bonds All within dispersion the same interactions molecule have were a cut-oscaledff radiusdown by rcut multiplying= 1.5 nm, and them the sameby 0.833333 distance and was 0.5, used respec to switchtively. from All dispersion a real-space interaction to Fouriers space have calculationsa cut-off radius of the rcut electrostatic= 1.5 nm, and interactions. the same distance The alkane–silicon was used to switch system from was alloweda real-space to equilibrate to Fourier forspace 1.0 ns.calculations Subsequently, of the 6000 electrostatic SPC/E water interactions. molecules were The placedalkane– abovesilicon the system alkane-covered was allowed silicon to surface.equilibrate The for resulting 1.0 ns. water–alkane–siliconSubsequently, 6000 SPC/E system water was equilibratedmolecules were for 10placed ns followed above the by another alkane- 10covered ns of gathering silicon surface. the time The averages, resulting and water the same–alkane procedure–silicon of system evaluating was the equilibrated WCA was for adopted. 10 ns Figuresfollowed6– 8by show another simulation 10 ns of snapshots gathering ofthe an time alkane averages, covered and silicon the same surface. procedure of evaluating the WCA was adopted. Figures 6–8 show simulation snapshots of an alkane covered silicon surface.

Figure 6. Simulation snapshots (top view) of water droplet on n-decane-covered silicon at CHYDR = 0.73 molecules/nm2. The snapshot of the alkane on silicon before the water drop was introduced (a), the configurations of alkane molecules after the water drop was introduced ((b) water molecules are not shown), and the snapshots of water drop with the alkane molecules (c). Silicon surface is not shown for clarity.

Without the water drop, the alkane molecules exhibit locally preferential orientation at 45 or 135 degrees wrt. the x-axis of the simulation box. This is particularly well visible at the CHYDR = 1.29 Figure 6. Simulation snapshots (top view) of water droplet on n-decane-covered silicon at C = Figure 26. Simulation snapshots (top view) of water droplet on n-decane-covered silicon at CHYDRHYDR = 0.73 molecules/nm (Figure 72). Once the water droplet is introduced it prefers to be in direct contact with 0.73molecules/nm molecules/2nm. The. Thesnapshot snapshot of the of thealkane alkane on onsilicon silicon before before the the water water drop drop was was introduced introduced (a), thethe the siliconconﬁgurations surface, creating of alkane a hole molecules in the alkane after the coverage. water drop Th wase so introduced-called ”dimple” ((b) water state molecules [2] is preserved are not even at configurations very high hydrocar of alkanebon molecules surface after coverage the water (cf. drop Fig urewas 8introduced). Figure 9(( bshows) water themolecules water are droplet not shown),shown), andand thethe snapshotssnapshots ofof waterwater dropdrop withwith thethe alkanealkane moleculesmolecules ((cc).). SiliconSilicon surfacesurface isis notnot shownshown contours calculated for three hydrocarbon surface concentrations, CHYDR = 0.73, 1.29, and 2.92 forfor clarity.clarity. molec/nm2. Without the water drop, the alkane molecules exhibit locally preferential orientation at 45 or 135 degrees wrt. the x-axis of the simulation box. This is particularly well visible at the CHYDR = 1.29 molecules/nm2 (Figure 7). Once the water droplet is introduced it prefers to be in direct contact with the silicon surface, creating a hole in the alkane coverage. The so-called ”dimple” state [2] is preserved even at very high hydrocarbon surface coverage (cf. Figure 8). Figure 9 shows the water droplet contours calculated for three hydrocarbon surface concentrations, CHYDR = 0.73, 1.29, and 2.92 molec/nm2.

FigureFigure 7. Simulation 7. Simulation snapshots snapshots (side (side view view)) of ofwater water droplet droplet on on n- n-decane-covereddecane-covered silicon silicon at atCHYDR CHYDR = = 2 1.291.29 molec/nm molec/nm2. The. Thesnapshot snapshot of the of thealkane alkane on silicon on silicon before before the thewater water drop drop was was introduced introduced (a), (thea), the configurationsconﬁgurations of alkane of alkane molecules molecules after after the thewater water drop drop was was introduced introduced ((b) (( waterb) water molecules molecules are arenot not shown),shown), and and the the snapshots snapshots of ofwater water drop drop with with the the alkane alkane molecules molecules ( (cc).). Silicon Silicon surface surface is is not shown for for betterbetter clarity.clarity.

Figure 7. Simulation snapshots (side view) of water droplet on n-decane-covered silicon at CHYDR = 1.29 molec/nm2. The snapshot of the alkane on silicon before the water drop was introduced (a), the configurations of alkane molecules after the water drop was introduced ((b) water molecules are not shown), and the snapshots of water drop with the alkane molecules (c). Silicon surface is not shown for better clarity.

Materials 2020, 13, x FOR PEER REVIEW 10 of 17

All three profiles correspond to the ”dimple” state of the water nanodroplet. We observe that the WCA gradually increases with increasing CHYDR with WCA = 88.7°, 103.6°, and 111.0°, respectively. The summary of simulations of water nanodrops on alkane covered bare silicon surface is presented in Figure 9, showing WCA dependence on the hydrocarbon surface coverage. We note that at low to moderate surface coverages WCA is practically independent of CHYDR. The WCA Materials 2020, 13, x FOR PEER REVIEW 10 of 17 inMaterials 2020, 13, 1554 10 of 17 All three profiles correspond to the ”dimple” state of the water nanodroplet. We observe that the WCA gradually increases with increasing CHYDR with WCA = 88.7°, 103.6°, and 111.0°, respectively. The summary of simulations of water nanodrops on alkane covered bare silicon surface is presented in Figure 9, showing WCA dependence on the hydrocarbon surface coverage. We note that at low to moderate surface coverages WCA is practically independent of CHYDR. The WCA increase is observed at coverages close to a full monolayer (which is estimated to be 1.46 molec/nm2). Once the hydrocarbon surface density reaches 145% of monolayer coverage, i.e., 2.12 molec/nm2, WCA becomes independent of CHYDR again with the elevated level of WCA around 111°.

Figure 8. Simulation snapshots (side view) of water droplet on n-decane-covered silicon at CHYDR = Figure 8. Simulation snapshots (side view) of water droplet on n-decane-covered silicon at CHYDR = 2.92 molec/nm2. The snapshot of the alkane on silicon before the water drop was introduced (a), the 2.92 molec/nm2. The snapshot of the alkane on silicon before the water drop was introduced (a), the conﬁgurations of alkane molecules after the water drop was introduced ((b) water molecules are not configurations of alkane molecules after the water drop was introduced ((b) water molecules are not shown), and the snapshots of water drop with the alkane molecules (c). Silicon surface is not shown for shown), and the snapshots of water drop with the alkane molecules (c). Silicon surface is not shown better clarity. for better clarity. Without the water drop, the alkane molecules exhibit locally preferential orientation at 45 or 135 degrees wrt. the x-axis of the simulation box. This is particularly well visible at the CHYDR = Figure 8. Simulation snapshots (side view) of water droplet on n-decane-covered silicon at CHYDR = 1.29 molecules/nm2 (Figure7). Once the water droplet is introduced it prefers to be in direct contact 2.92 molec/nm2. The snapshot of the alkane on silicon before the water drop was introduced (a), the with the silicon surface, creating a hole in the alkane coverage. The so-called ”dimple” state [2] is configurations of alkane molecules after the water drop was introduced ((b) water molecules are not preserved even at very high hydrocarbon surface coverage (cf. Figure8). Figure9 shows the water shown), and the snapshots of water drop with the alkane molecules (c). Silicon surface is not shown droplet contours calculated for three hydrocarbon surface concentrations, C = 0.73, 1.29, and for better clarity. HYDR 2.92 molec/nm2.

Figure 9. The spherical water droplet contour for the water-alkane-silicon system with 6000 molecules of water calculated at three hydrocarbon surface concentrations, CHYDR = 0.73 (a), 1.29 (b), and 2.92 (c) molec/nm2. Circles denote the position of the water density profile with value 0.5 ± 0.03 g/cm3.

Figure 9. The spherical water droplet contour for the water-alkane-silicon system with 6000 molecules Figure 9. The spherical water droplet contour for the water-alkane-silicon system with 6000 molecules of water calculated at three hydrocarbon surface concentrations, CHYDR = 0.73 (a), 1.29 (b), and 2.92 (c) of water calculated at three hydrocarbon surface concentrations, CHYDR = 0.73 (a), 1.29 (b), and 2.92 (c) molec/nm2. Circles denote the position of the water density proﬁle with value 0.5 0.03 g/cm3. molec/nm2. Circles denote the position of the water density profile with value 0.5 ± 0.03 g/cm3. All three proﬁles correspond to the ”dimple” state of the water nanodroplet. We observe that the WCA gradually increases with increasing CHYDR with WCA = 88.7◦, 103.6◦, and 111.0◦, respectively. The summary of simulations of water nanodrops on alkane covered bare silicon surface is presented in Figure9, showing WCA dependence on the hydrocarbon surface coverage. We note that at low to moderate surface coverages WCA is practically independent of CHYDR. The WCA increase is observed at coverages close to a full monolayer (which is estimated to be 1.46 molec/nm2). Once the hydrocarbon surface density reaches 145% of monolayer coverage, i.e., 2.12 molec/nm2, WCA becomes independent of CHYDR again with the elevated level of WCA around 111◦.

3.4. Molecular Simulations of SPC/E Water on Silicon Covered with SiO2 The results shown in the previous section indicate an increase in the WCA with increasing hydrocarbon surface density (Figure 10). In order to recover the experimentally observed insensitivity of WCA to alkane surface coverage (Figure2) a change in the surface–adsorbate interaction characteristics

Materials 2020, 13, 1554 11 of 17

has to be introduced. We recall that the freshly etched silicon surface quickly develops a 1.4–1.5 nm thick film of silicon dioxide. This layer significantly alters the nature of the surface–adsorbate interactions by adding the electrostatic component to the dispersion interactions exerted by the bare silicon surface. In our modeling it is sufficient to consider only the SiO2 layer since we assume that the dispersion interactions have a cut-off at 1.5 nm and the underlying Si surface is located beyond the cut-off range. Our calculations are again divided into three stages. In the first stage, we determine the surface–adsorbate force field by considering cylindrical water nanodroplets and adjusting the interaction parameters, so that they match the experimental WCA values. In the second stage, we estimate how big the spherical nanodrop has to be in order to have negligible WCA finite size effects. Finally, in the third stage we determine the influence of the hydrocarbon surface density on water nanodroplets. In our calculations, we adopt the same computational procedures as those applied in Materialsthe previous 2020, 13, x section. FOR PEER REVIEW 11 of 17

FigureFigure 10. 10.WCA WCAdependence dependence on hydrocarbon on hydrocarbon surface coverage, surface coverage,for water nanodroplets for water nanodroplets on n-decane- on coveredn-decane-covered bare silicon surface. bare silicon surface.

3.4. MolecularLet us Simulations begin with cylindricalof SPC/E Water nanodrops on Silicon on Covered SiO2 surface. with SiO We2 assume that the surface atoms are completely immobile, whereas the interactions with water and hydrocarbon molecules have both the electrostaticThe results and shown dispersion in the (LJ) previous components. section Inindicate the literature, an increase there in are the a number WCA with of di increasingﬀerent force hydrocarbonﬁelds for silicon surface dioxide density [8,28 (Figure–31]. For 10 example,). In order in [ 8 to] the recover charge the associated experimentally with the silicon observed atom insensitivity of WCA to alkane surface coverage (Figure 2) a change in the surface–adsorbate qSi = 0.6e, whereas in [31], qSi = 2.4e. In the present work, we started with the former value of the interaction characteristics has to be introduced. We recall that the freshly etched silicon surface silicon charge and gradually increased it with accompanying decrease of the LJ energy parameter εSi quickly develops a 1.4–1.5 nm thick film of silicon dioxide. This layer significantly alters the nature so, that the experimental WCA was recovered. We have found that the set of the surface parameters qSi of = the1.1e surfaceε = 0.0502416–adsorbate kJ / interactionmole, σ =s4.0534 by adding Å, q the = electrostatic0.55e ε component= 0.286377 to kJ / themole, dispersion and σ Si Si O,SiO2 − O,SiO2 O,SiO2 interactions= 2.8598 Å exerted accurately by the reproduces bare silicon the surface. experimentally In our modeling observed it behavior is sufficient of WCA, to consider as will only be shown the SiO2 layer since we assume that the dispersion interactions have a cut-off at 1.5 nm and the underlying below. The SiO2 substrate was composed by repeating the 12-atom α-crystoballite unit cell along x, y, Si surfaceand z-directions is located 108, beyond 7, and the 3 times,cut-off respectively.range. Our calculations This process are created again adivided surface into of the three size stages. 53.7624 In nm the first3.4846 stage nm, we2.0844 determine nm. On the the surface top of that–adsorbate surface 3000 force SPC field/E water by considering molecules werecylindrical placed water and the × × nanodropsimulationlets wasand carriedadjusting out the using interaction the protocol parameters described, so earlier.that they Figure match 11 the shows experimental the contour WCA of the values. In the second stage, we estimate how big the spherical nanodrop has to be in order to have cylindrical water nanodroplet on the SiO2 surface. negligible WCA finite size effects. Finally, in the third stage we determine the influence of the hydrocarbon surface density on water nanodroplets. In our calculations, we adopt the same computational procedures as those applied in the previous section. Let us begin with cylindrical nanodrops on SiO2 surface. We assume that the surface atoms are completely immobile, whereas the interactions with water and hydrocarbon molecules have both the electrostatic and dispersion (LJ) components. In the literature, there are a number of different force fields for silicon dioxide [8,28–31]. For example, in [8] the charge associated with the silicon atom qSi = 0.6e, whereas in [31], qSi = 2.4e. In the present work, we started with the former value of the silicon charge and gradually increased it with accompanying decrease of the LJ energy parameter εSi so, that the experimental WCA was recovered. We have found that the set of the surface parameters qSi = 1.1e εSi = 0.0502416 kJ/mole, σSi = 4.0534 Å, qO,SiO2 = −0.55e εO,SiO2 = 0.286377 kJ/mole, and σO,SiO2 = 2.8598 Å accurately reproduces the experimentally observed behavior of WCA, as will be shown below. The SiO2 substrate was composed by repeating the 12-atom α-crystoballite unit cell along x, y, and z- directions 108, 7, and 3 times, respectively. This process created a surface of the size 53.7624 nm × 3.4846 nm × 2.0844 nm. On the top of that surface 3000 SPC/E water molecules were placed and the simulation was carried out using the protocol described earlier. Figure 11 shows the contour of the cylindrical water nanodroplet on the SiO2 surface.

Materials 2020, 13, x FOR PEER REVIEW 12 of 17 Materials 2020, 13, 1554 12 of 17 Materials 2020, 13, x FOR PEER REVIEW 12 of 17

Figure 11. The cylindrical droplet contour calculated for the water–silicon dioxide surface system. The circles denote the position of the water density profile with value 0.5 ± 0.03 g/cm3. The thick red FigureFigure 11.11. TheThe cylindrica cylindricall droplet droplet contour contour calculated calculated for forthe thewater water–silicon–silicon dioxide dioxide surface surface system. system. line denotes the droplet profile obtained by applying the fitting procedure. The dashed line denotes TheThe circles circles denote denote the the position position of the of the water water density density profile profile with with value value 0.5 0.5 ± 0.030.03 g/cm g/cm3. The3. The thick thick red red the line tangent to the drop that meets the bottom of the nanodroplet at the± big red point. WCA lineline denotes denotes the the droplet droplet profile profile obtained obtained by by applying applying the the fitting fitting procedure. procedure. The dashed lineline denotesdenotes the dependence on hydrocarbon surface coverage for water nanodroplets on n-decane covered bare theline line tangent tangent to to the the drop drop that that meets meets the bottom the bottom of the of nanodroplet the nanodrop at thelet big at red the point. big red WCA point. dependence WCA silicon surface. dependenceon hydrocarbon on hydrocarbon surface coverage surface for coverage water nanodroplets for water nanodroplets on n-decane covered on n-decane bare silicon covered surface. bare silicon surface. WeWe note note that that there there is isa very a very thin thin (6Å (6Å thick) thick) layer layer composed composed of ofwater water molecules molecules very very strongly strongly adsorbed on the silicon dioxide surface. This is due to the strong electrostatic surface–adsorbate adsorbedWe note on that the there silicon is a dioxide very thin surface. (6Å thick) This layer is due composed to the strong of water electrostatic molecules surface–adsorbate very strongly interactions. Nonetheless, a good profile of the nanodroplet can be obtained, which fits very nicely adsorbedinteractions. on the Nonetheless, silicon dioxide a good surface. profile This of the is nanodroplet due to the strong can be obtained,electrostatic which surface fits– veryadsorbate nicely to to the theoretical circular drop profile (cf. Figure 12, thick red line). From simulations we estimate interactions.the theoretical Nonetheless, circular drop a good profile profile (cf. Figureof the nanodrop12, thick redlet can line). be From obtained, simulations which wefits estimatevery nicely WCA WCA to be 21.6°. Figure 13 shows the contour of the spherical nanodroplet on the SiO2 surface to tothe be theoretical 21.6◦. Figure circular 13 shows drop the profile contour (cf. ofFig theure spherical 12, thick nanodroplet red line). From on the simulations SiO2 surface we evaluated estimate for evaluated for the surface comprising of 66,564 atoms obtained by replicating the α-crystoballite unit WCAthe surface to be 21.6 comprising°. Figure of 13 66,564 shows atoms the contour obtained of by the replicating spherical thenanodropletα-crystoballite on the unit SiO cell2 surface along x,y, cell along x,y, and z directions 43, 43, and 3 times, and for 8000 SPCE water molecules. We note that, evaluatedand z directions for the surface 43, 43, comprising and 3 times, of and 66,564 for 8000atoms SPCE obtained water by molecules. replicating We the note α-crystoballite that, similar unit to the similar to the cylindrical nanodroplet, a thin layer of strongly adsorbed water molecules is easily cellcylindrical along x,y, nanodroplet,and z directions a thin 43, layer 43, and of strongly3 times, and adsorbed for 8000 water SPCE molecules water molecules. is easily detectable We note tha ont, the detectable on the nanodroplet contour. Again, a well-defined nanodroplet fits very well to the similarnanodroplet to the cylindrical contour. Again, nanodroplet, a well-defined a thin layer nanodroplet of strongly fits veryadsorbed well towater the theoreticallymolecules is expectedeasily theoretically expected circular drop profile (cf. Figure 12, red line). The WCA in this case equals 21.75° detectablecircular drop on the profile nanodroplet (cf. Figure contour. 12, red line). Again, The a WCAwell-defined in this case nanodrop equalslet 21.75 fits◦ which very well is close to theto the which is close to the WCA obtained for the cylindrical nanodroplet. theoreticallyWCA obtained expected for the circular cylindrical drop profile nanodroplet. (cf. Figure 12, red line). The WCA in this case equals 21.75° which is close to the WCA obtained for the cylindrical nanodroplet.

FigureFigure 12. 12.TheThe spherical spherical droplet droplet contour contour calculated calculated for the for water the– water–siliconsilicon dioxide dioxide surface surface system. system. The 3 circlesThe circlesdenote denotethe position the position of the water of the density water density profile profile with value with value0.5 ± 0.03g/cm 0.5 0.033. gThe/cm thick. The red thick line red Figure 12. The spherical droplet contour calculated for the water–silicon dioxide± surface system. The denotesline denotes the droplet the droplet profile profile obtained obtained by applying by applying the fitting the fitting procedure. procedure. The Thedashed dashed line line denotes denotes the the circles denote the position of the water density profile with value 0.5 ± 0.03g/cm3. The thick red line lineline tangent tangent to the to the drop drop that that meets meets the the bottom bottom of ofthe the nanodrop nanodropletlet at atthe the big big red red point. point. denotes the droplet profile obtained by applying the fitting procedure. The dashed line denotes the line tangent to the drop that meets the bottom of the nanodroplet at the big red point.

Materials 2020, 13, x FOR PEER REVIEW 13 of 17

Finally, let us turn to water nanodroplets on alkane-preadsorbed SiO2 surfaces. In our calculations, we used 21.4054 nm × 21.4054 nm × 2.0844 nm surface comprising of 66,564 atoms obtained by repeating the α-crystoballite unit cell along x, y, and z directions 43, 43, and 3 times, and for 8000 SPCE water molecules. The simulation protocol was identical to that described in the previous section. Figure 13 shows the snapshots of water–alkane–silicon dioxide systems taken at three different hydrocarbon surface concentrations, CHYDR = 0.59 (Figure 13a), 1.47 (Figure 13b), and 2.78 molec/nm2 (Figure 13c). At a first glance, one important difference to snapshots with the bare silicon surface (Figures 6–8) is clearly noticeable. The n-decane molecules, instead of covering the whole surface, tend to group themselves into “islands” composed of stacked hydrocarbon chains (cf. Figure 13, top left-hand corner panels). The reason for such behavior is the strong electrostatic interaction between water molecules and the SiO2 surface. This leads to the formation of a thin layer of adsorbed water molecules covering the majority of the surface (cf. Figure 13, top right-hand corner panels). The alkane stacking minimizes the number of hydrocarbon–water contacts as they are Materialsunfavorable 2020, 13, x, FORand PEER is furthe REVIEWrmore promoted due to the weaker dispersion interactions between13 of 17 the Materials 2020, 13, 1554 13 of 17 n−decane and the surface. Water nanodroplets are located between the alkane “islands” (Figure 13, bottomFinally, left let-hand us corner turn topanels). water nanodroplets on alkane-preadsorbed SiO2 surfaces. In our calculations, we used 21.4054 nm × 21.4054 nm × 2.0844 nm surface comprising of 66,564 atoms obtained by repeating the α-crystoballite unit cell along x, y, and z directions 43, 43, and 3 times, and for 8000 SPCE water molecules. The simulation protocol was identical to that described in the previous section. Figure 13 shows the snapshots of water–alkane–silicon dioxide systems taken at three different hydrocarbon surface concentrations, CHYDR = 0.59 (Figure 13a), 1.47 (Figure 13b), and 2.78 molec/nm2 (Figure 13c). At a first glance, one important difference to snapshots with the bare silicon surface (Figures 6–8) is clearly noticeable. The n-decane molecules, instead of covering the whole surface, tend to group themselves into “islands” composed of stacked hydrocarbon chains (cf. Figure 13, top left-hand corner panels). The reason for such behavior is the strong electrostatic interaction between water molecules and the SiO2 surface. This leads to the formation of a thin layer of adsorbed water molecules covering the majority of the surface (cf. Figure 13, top right-hand corner panels). The alkane stacking minimizes the number of hydrocarbon–water contacts as they are unfavorableFigure, and 13. Simulation is further snapshotsmore promoted of water dropletdue to on the n-decane-covered weaker dispersion silicon interactions dioxide at surface between density the Figure 13. Simulation snapshots of water2 droplet on n-decane-covered silicon dioxide at surface n−decan0.59e and (a), the 1.47 surface. (b), and 2.78Water (c) molecnanodroplets/nm . The are top located left-hand between corner panel the alkane shows the“islands” snapshot (Fig ofure only 13, thedensity alkane 0.59 molecules. (a), 1.47 ( Theb), and top 2. right-hand78 (c) molec/nm corner2. panel The top shows left- onlyhand the corner water panel molecules. shows the The snapshot bottom bottom left-hand corner panels). left-handof only the corner alkane panel molecules. shows the The water top molecules right-hand but corner only thosepanel with shows z-coordinate only the water greater molecules. than 2.55 nm.The Thebottom bottom left right-hand-hand corner corner panel panel shows shows the water the snapshot molecules of water but only nanodroplet those with with z-coordinate alkane molecules. greater Thethan silicon 2.55 nm. dioxide The surfacebottom is right not- shownhand corner for better panel clarity. shows the snapshot of water nanodroplet with alkane molecules. The silicon dioxide surface is not shown for better clarity. Finally, let us turn to water nanodroplets on alkane-preadsorbed SiO2 surfaces. In our calculations, we usedFigure 21.4054 14 nmshows21.4054 the contours nm 2.0844 for nm the surface nanodroplets comprising for of systems 66,564 atoms presented obtained in Fig byure repeating 13a–c. × × theDespiteα-crystoballite large differences unit cell in alongthe hydrocarbon x, y, and z directionssurface concentration 43, 43, and, 3 the times, nanodrop and forlets 8000 are SPCEquite similar water molecules.with WCAs The equal simulation to 18.8°, protocol21.8°, and was 21.3 identical° for CHYDR to that= 0.59, described 1.47, and in 2.78 the molecules/nm previous section.2, respectively. Figure 13 shows the snapshots of water–alkane–silicon dioxide systems taken at three different hydrocarbon 2 surface concentrations, CHYDR = 0.59 (Figure 13a), 1.47 (Figure 13b), and 2.78 molec/nm (Figure 13c). At a first glance, one important difference to snapshots with the bare silicon surface (Figures6–8) is clearly noticeable. The n-decane molecules, instead of covering the whole surface, tend to group themselves into “islands” composed of stacked hydrocarbon chains (cf. Figure 13, top left-hand corner panels).Figure The 13. Simulation reason for snapshotssuch behavior of water is the droplet strong on electrostatic n-decane-covered interaction silicon between dioxide waterat surface molecules anddensity the SiO 0.592 surface. (a), 1.47 ( Thisb), and leads 2.78 to (c the) molec/nm formation2. The of top a thin left layer-hand ofcorner adsorbed panel watershows moleculesthe snapshot covering theof majority only the ofalkane the surface molecules. (cf. The Figure top 13right, top-hand right-hand corner panel corner shows panels). only Thethe water alkane molecules. stacking The minimizes thebottom number left of-hand hydrocarbon–water corner panel shows contacts the water as theymolecules are unfavorable, but only those and with is furthermore z-coordinate promotedgreater due to the weaker dispersion interactions between the n decane and the surface. Water nanodroplets are than 2.55 nm. The bottom right-hand corner panel shows− the snapshot of water nanodroplet with locatedalkane between molecules. the The alkane silicon “islands” dioxide surface (Figure is 13not, bottomshown for left-hand better clarity. corner panels). FigureFigure 1414. shows The spherical the contours water d forroplet the contour nanodroplets for the water for systems–alkane– presentedSiO2 system in with Figure 8000 13 moleculesa–c. Despite of water calculated at three hydrocarbon surface concentrations, CHY DR = 0.59 (a), 1.47 (b), and 2.78 largeFigure diff erences 14 shows in the the hydrocarbon contours for surface the nanodroplets concentration, for the systems nanodroplets presented are in quite Figure similar 13a–c. with 2 3 (c) molec/nm . The circles denote the position of the water density profile with value 0.52 ± 0.03 g/cm . Dely.WCAs equal to 18.8◦, 21.8◦, and 21.3◦ for CHYDR = 0.59, 1.47, and 2.78 molecules/nm , respectively.

FigureFigure 14.14. TheThe spherical spherical water water droplet droplet contour contour for forthe the water water–alkane–SiO–alkane–SiO2 system2 system with with 8000 8000 molecules molecules of waterof water calculated calculated at three at three hydrocarbon hydrocarbon surface surface concentrations, concentrations, CHY CHY DR DR = 0.59= 0.59 (a), ( a1.47), 1.47 (b) (,b and), and 2.78 2.78 (c)(molec/nmc) molec/nm2. The2. The circles circles denote denote the the position position of the of the water water density density profile proﬁle with with value value 0.5 0.5 ± 0.030.03 g/cm g/cm3. 3. ± Materials 2020, 13, 1554 14 of 17

4. Discussion The major purpose of our study was to perform a holistic experimental and theoretical research on silicon wetting. Our XPS results (Figure1) show that the SiO 2 layer is present on our samples and the contents of other impurities can be neglected. This means that we expect hydrophilic nature of silicon covered by SiO2. The question arises if light airborne hydrocarbons adsorption occurs in remarkable amount. Hydrophilic nature of silicon surface is confirmed by the chromatographic experiment results (see Figure3). Moreover, this experiment confirms that we expect remarkable influence of water on the repellence of light hydrocarbons from the droplet bottom. As it is proved, the method of sample purification has a strong influence on the WCA values (Figure2). The most optimal method is the SDG procedure based on a series of treatments before a routine DG procedure. Obtained in this method WCA is equal to 20.7◦ and is the smallest value of our study. This WCA was next used as a reference for the calibration of the force field for the MD simulations. The differences in WCA values for the samples treated by using different methods are caused not by adsorption of light, but long hydrocarbons present in air. This is the reason why we observe only oscillations of WCA after exposure of silicon samples to C10H22 atmosphere. The simulation results show that the influence of hydrocarbons on the WCA of bare silicon should be remarkable, and the WCA increases by ~20◦ after exposure. In this mechanism, the repellence of hydrocarbons by water molecules from the bottom of the droplet at small coverages occurs. However, at larger surface hydrocarbons concentrations, water cannot repel hydrocarbons from the substrate and WCA increases. In this case the wetting mechanism is analogous to reported by us for a graphene [2]. In contrast for SiO2 covered surface, as light hydrocarbons are removed from the bottom of a droplet, the WCA is only slightly modified by hydrocarbons located at the droplet edge. In this case, hydrocarbons alter the shape of the contact line. Thus, the mechanism of wetting is similar, but not the same, to this observed recently for a strongly hydrophilic MOF [3]. Therefore, it can be stated that SiO2 covered silicon shows robust self-cleaning properties with respect to light hydrocarbons. All our computational results are summarized in Figure 15, where we compare the WCA dependence on CHYDR for the both, the bare silicon surface and the silicon dioxide surface. We note, that for the bare silicon surface (cf. Figure 15, circles), the WCA increases ~20◦ with the increase of the hydrocarbon surface density, whereas for the SiO2 surface (cf. Figure 15, squares), the WCA remains approximately independent of the alkane surface coverage, according to experimental data. The small (of about 3◦) WCA variations in the latter case are due to the fact, that water nanodroplet is located between the alkane-rich domains. These alkane-rich domains may slightly alter the shape of the droplet leading to a small variation in the WCA. Thus, simulations provide molecular-level insight into the interaction between the airborne-adsorbed light hydrocarbons and water droplets. On freshly etched silicon surfaces, the strong dispersion interactions lead to strong adsorption of light hydrocarbons and ultimately to an increase of WCA. On the other hand, the thin layer of SiO2 present on all silicon surfaces when exposed to oxygen leads to significant screening of the dispersion component of the interactions. This promotes the electrostatic interactions between the thin layer of SiO2 and water molecules. Simultaneously this leads to a decrease in WCA and renders it almost insensitive to the airborne-adsorbed light hydrocarbons. MaterialsMaterials 20202020, 13,,13 x ,FOR 1554 PEER REVIEW 15 of15 17 of 17

FigureFigure 15. 15. WCAWCA dependence dependence on onhydrocarbon hydrocarbon surface surface coverage, coverage, for for water water nanodroplets nanodroplets on onn-decane n-decane coveredcovered bare bare silicon silicon (circles) (circles) and and silicon silicon dioxide dioxide (squares) (squares) surfaces. surfaces.

5. Conclusions5. Conclusions Our XPS study proves that the studied silicon surface is covered by a SiO layer. Among the Our XPS study proves that the studied silicon surface is covered by a SiO2 layer.2 Among the studiedstudied methods methods of ofsilicon silicon surface surface purification purification,, the the SDG SDG method method is the is the most most efficient. efficient. The The WCA WCA for for (1,0,0) and (1,1,1) silicon surfaces are equal to 20.7 2.7 . Light hydrocarbons strongly influence the (1,0,0) and (1,1,1) silicon surfaces are equal to 20.7 ± 2.7± °. Light◦ hydrocarbons strongly influence the wetting properties of bare, freshly etched silicon, and have small influence on the wetting of SiO wetting properties of bare, freshly etched silicon, and have small influence on the wetting of SiO2 2 covered surface. Molecular dynamics simulations reveal that the thin layer of SiO on the top of the covered surface. Molecular dynamics simulations reveal that the thin layer of SiO2 on2 the top of the siliconsilicon surface surface scr screenseens the the dispersion dispersion interactions. interactions. This This gives gives a rise a rise to tothe the weak weak adsorption adsorption of ofairborne airborne lightlight hydrocarbons hydrocarbons and and the the insensitivity insensitivity of ofWCA WCA to tothe the light light hydrocarbon hydrocarbon surface surface coverage. coverage. The SiO covered silicon surface shows robust self-cleaning properties against light hydrocarbons. The SiO2 2 covered silicon surface shows robust self-cleaning properties against light hydrocarbons.Therefore the Therefore differences the reported differences in Figure reported2 are in caused Figure by2 are longer caused hydrocarbons by longer remaininghydrocarbons on a remainingsurface due on a to surface application due to of application insufficient of cleaning insufficient method. cleaning Based method. on our Based results on those our results hydrocarbons those hydrocarbonsare removed are by removed modified by de modified Gennes approach, de Gennes according approach to, theaccording procedure to the described procedure in thisdescribed study. in this study. Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/13/7/1554/s1, Figure S1: The cylindrical droplet contour calculated for the water–bare silicon surface system with kd = 0.147. SupplementaryThe xc and zc. Materials:Cartesian coordinates The following denote are the available position of online the water at www.mdpi.com/xxx/s1 density profile with value, Figure 0.5 0.03 S1: The g/cm 3, cylindricalFigure S2: droplet Time contour dependence calculated of the for simulated the water water–bare contactsilicon surface angle for system the system with kd with = 0.147. kd = The0.147. ±xc and Table zc. S1: CartesianForcefield coordinates parameters denote applied the position during simulations of the water of density n-decane. profile with value 0.5 ± 0.03g/cm3, Figure S2: Time dependenceAuthor Contributions: of the simulatedConceptualization water contact angle A.P.T., for the P.B., system G.S.S., with and kd = P.K.,E.K.; 0.147. Table methodology S1: Forcefield P.B., parameters E.K., K.T., appliedA.P.T.,G.S.S. during simulations and P.K.; investigation of n-decane. P.B., E.K., A.P.T., G.S.S. and K.T.; writing—original draft preparation, A.P.T.; P.B.; G.S.S. writing—review and editing, all authors. All authors have read and agreed to the published Authorversion Contributions: of the manuscript. Conceptualization A.P.T., P.B., G.S., and P.K.,E.K.; methodology P.B., E.K. K.T., A.P.T.,G.S. and P.K.; investigation P.B., E.K., A.P.T.,G.S and K.T.; writing—original draft preparation, A.P.T.; Funding: This research was funded by Polish NCN, grant number OPUS 13 UMO—2017/25/B/ST5/00975. P.B.; G.S. writing—review and editing, all authors. All authors have read and agreed to the published version of theConflicts manuscript. of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or in the decision to Funding:publish This the results.research was funded by Polish NCN, grant number OPUS 13 UMO—2017/25/B/ST5/00975. 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