Effect of Alkane Chain Length on Crystallization in Emulsions During Supercooling in Quiescent Systems and Under Mechanical Stress

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Article Effect of Alkane Chain Length on Crystallization in Emulsions during Supercooling in Quiescent Systems and under Mechanical Stress

Serghei Abramov * ID , Kinza Shah, Lydia Weißenstein and Heike Petra Karbstein

Karlsruhe Institute of Technology (KIT), Institute of Process Engineering in Life Sciences (BLT), Section I: Food Process Engineering (LVT), Kaiserstr. 12, 76131 Karlsruhe, Germany; [email protected] (K.S.); [email protected] (L.W.); [email protected] (H.P.K.) * Correspondence: [email protected]; Tel.: +49-721-608-42497

Received: 15 November 2017; Accepted: 12 January 2018; Published: 15 January 2018

Abstract: Crystallization behavior of hexadecane (C16H34), octadecane (C18H38), eicosane (C20H42), and docosane (C22H46) dispersions of similar mean droplet diameter (x50.2 ≈ 15 µm) was investigated in quiescent systems and compared to crystallization under mechanical stress. In quiescent systems, the required supercooling decreased with increasing chain length of the alkanes to initiate crystallization. Crystallization of alkane dispersions under mechanical stress resulted in similar onset crystallization supercooling, as during quiescent crystallization. Increase of mechanical stress did not affect the onset crystallization supercooling within alkane dispersions.

Keywords: emulsion; dispersion; crystallization; melt emulsiﬁcation; crystallization index; aggregation; nucleation

1. Introduction Crystalline dispersions are widely used in the chemical and life science industry [1–3]. They act as high performance lubricants or release agents, are applied as delivery systems with site-specific and controlled release kinetics or are added to foods and feeding stuffs to increase the bioavailability and biocompatibility of encapsulated active ingredients [4–10]. Due to the distinctive viscoelastic behavior of the encapsulating material, these dispersions are usually produced in a two-step melt emulsification process: in the first step, the dispersed phase is emulsified above its melting temperature and transferred under supercooling into a fine crystalline dispersion in the second step [11–13]. During emulsification, molten bulk (dispersed phase) is homogenized to countless small volume units—the droplets. Different to bulk crystallization in a macroscopic volume element, crystallization in dispersed droplets occurs individually and separately in each droplet, resulting in polydisperse crystallization behavior during supercooling [14,15]. The individual crystallization behavior in dispersions is strongly dependent on the formulation and process parameters, such as droplet size, additives, dispersed phase content, cooling rate, external forces and supercooling, as the driving force for crystallization in molten droplets [3,9,16–18]. Ideally, those parameters are adjusted to transform all droplets into spherical crystalline particles of the same size. Eventually, crystallization to different structures, incomplete crystallization and/or colloidal processes often occur during supercooling and destabilize the dispersions [19–24]. The final size, shape and physical state and thus the application properties of the dispersions are strongly connected to the nucleation within single droplets [25–27]. Usually, the formation of nuclei proceeds by heterogeneous nucleation mechanism due to catalytic impurities, which lower the nucleation barrier and allow crystallization at low supercooling [12]. In dispersions, the number of catalytic impurities is much lower than the number of droplets and require higher supercooling

Processes 2018, 6, 6; doi:10.3390/pr6010006 www.mdpi.com/journal/processes Processes 2018, 6, 6 2 of 16 for crystallization compared to the bulk of the same material [3,12]. The droplet size becomes a critical parameter for nucleation in dispersions, since with increasing droplet size the probability of heterogeneous nucleation increases and crystallization becomes thermodynamically favorable [26,28]. In most crystalline dispersions droplets are comparatively small (diameter <1 µm up to some µm) and the size distribution is polydisperse. This causes incomplete crystallization depending on the supercooling [14,15]. In past decades, many researchers investigated crystallization in dispersed systems with a focus on the initiation of the nucleation. Different approaches, such as surface-initiated, collision-mediated and energy-induced nucleation were discussed [28–36]. One of the most promising approaches, combining surface-initiated, collision-mediated and energy-induced nucleation, is stress-controlled nucleation [15,37–42]. Mechanical stress-induced crystallization is already industrially applied in the confectionery (chocolate) or dairy (whipped cream) industry to control and adjust morphology, texture and mouthfeel [28,42–46]. However, the impact of mechanical stresses on crystallization in dispersed systems is still not fully understood and is a subject of many investigations. In our previous investigations we discussed the individual and polydisperse crystallization in hexadecane (C16H34) dispersions [14] and the influence of rotation shear on the crystallization behavior in hexadecane dispersions of different mean droplet size [15]. This current investigation expands the homologous series by octadecane (C18H38), eicosane (C20H42), and docosane (C22H46) dispersions in order to compare the crystallization behavior of substances with similar chemical properties but different chain lengths, and focuses on the influence of oscillation mechanical stress, and discusses its impact on crystallization intensification in dispersed systems.

2. Materials and Methods

2.1. Materials All substances were commercially available and used as purchased without further puriﬁcation ◦ or processing. Hexadecane (HD, C16H34, purity 99%, melting point at 18 C) was purchased from ® ◦ Sigma-Aldrich (St. Louis, MO, USA). Octadecane (OD, C18H38, purity 99%, melting point at 28 C) ® was purchased from Sigma-Aldrich (St. Louis, MO, USA). Eicosane (EC, C20H42, purity 99%, melting point at 37 ◦C) was purchased from Sigma-Aldrich® (St. Louis, MO, USA). Docosane (DC, ◦ C22H46, purity 95%, melting point at 44 C) was purchased from Sasol (Johannesburg, South Africa). Polyoxyethylen-20-sorbitanmonolaurat (Tween® 20) was purchased from Carl-Roth® (Karlsruhe, Germany). Water was puriﬁed in a Milli-Q® instrument Q-POD® at 18.2 MΩ (Darmstadt, Germany).

2.2. Emulsion Preparation Every emulsion consisted of 1 wt % or 10 wt % alkane (dispersed phase: hexadecane (HD), octadecane (OD), eicosane (EC) or docosane (DC)), 1 wt % Tween® 20 (surfactant) and 98 wt % or 89 wt % purified water (continuous phase). First, Tween® 20 was dissolved under continuous stirring in tempered purified water 10 K above the melting temperature of the alkane (ϑm,alkane) in a glass vessel. Afterwards, the dispersed phase was added to the surfactant solution and tempered with the gear-rim dispersing element (rotating part) for another 15 min 10 K above ϑm,alkane without stirring. Then, the dispersed phase was homogenized with a gear-rim dispersing machine (IKA® T25 digital, ULTRA-TURRAX®, Staufen im Breisgau, Germany) at 2.2 m/s tangential speed (3200 rpm, 13 mm outer diameter of the rotating part) 10 K above ϑm,alkane for 10 min in a glass vessel of 25 mm inner diameter. After emulsification, samples were taken for the quiescent crystallization (cryo-polarizing microscopy of 1 wt % emulsions) and crystallization under mechanical stress (oscillation mechanical stress of 10 wt % emulsions) analysis. Between emulsification and analysis, the emulsions were continuously stirred above ϑm,alkane to avoid creaming, crystallization and inhomogeneous sampling. During the sample preparation for the analytical part, emulsions did not show any sign of instability. Processes 2018, 6, 6 3 of 16

2.3. Droplet Size Distribution Analysis The droplet size distributions (DSD) of emulsions were investigated using a customized polarizing light microscope (Eclipse Ci-L, Nikon, Shinagawa, Tokyo, Japan) equipped with an optically accessible temperature controlled stage (LTS 420, Linkam Scientific, Tadworth, UK). The optical components were customized by Nikon for long distance investigations and consisted of CFI ocular (magnification: 10×), CFI p-Fluor objective (magnification: 10×; optical aperture: 0.3; working distance: 15.2 mm) and LWD achromatic condenser (working distance 10 mm). The combined magnification during optical observations was 100×. To differentiate crystalline structures from liquid droplets the optical set up was expanded with polarization filter LV-PO, polarization filter C-SP and analyzer LV-FLAN FL (more details at Nikon homepage). After the emulsification, 25 µL of 1 wt % alkane emulsions were pipetted between two microscope cover slips glued on a tempered microscope object slide using tempered pipette, then covered with a third cover slip and sealed with silicone 10 K above ϑm,alkane. The distance between the object slide and the cover slip was approximately 130 µm (sample thickness). Then, the sealed samples were cooled to ϑm,alkane and equilibrated for 5 min. Afterwards, images were taken of the emulsions in liquid state. The obtained micrographs were processed with ImageJ 1.46r (National Institute of Health, Bathesda, MD, USA) software to determine the characteristic values such as number, size and size distribution by area of the emulsions. Details are given in [14].

2.4. Quiescent Crystallization Analysis In analogy to DSD analysis, same thermo-optical equipment and sample preparation procedure were applied to investigate quiescent crystallization behavior in alkane dispersions, as described in Section 2.3. First, onset crystallization temperature and melting temperature of the dispersions were determined, since these values are subject of Kelvin effect and might vary in dependency of mean droplet size [47]. The samples were cooled to 18 K (entire crystallization of all droplets within the dispersion) supercooling with a cooling rate of 1 K/min. During supercooling, picture sequences of the crystallizing dispersions were taken every 0.2 K. The onset crystallization temperature of the dispersions was determined optically by the appearance of the ﬁrst crystallized droplet (gray transparent droplet → green opaque spheroid) in the volume. Then, the samples were re-heated to 10 K above ϑm,alkane with a heating rate of 1 K/min. During heating, picture sequences of the melting dispersions were taken every 0.2 K. The melting temperature of the dispersions was determined optically by change of color and shape of the droplets (green opaque spheroids → gray transparent droplets). Inﬂuence of the cooling rate on crystallization in alkane dispersions was investigated by varying the cooling rate. The samples were cooled to supercooling of 18 K with a cooling rate ∆T/∆t of 0.1 K/min, 1 K/min and 10.0 K/min. During supercooling, picture sequences of the droplets were taken every 0.2 K. Due to polarization the crystallized droplets change their color from gray transparent to green opaque and can be easily differentiated from liquid droplets. The number of crystallized droplets was optically followed in dependency of cooling rate. To exclude the effect of time dependency during crystallization in alkane dispersions isothermal observations were performed. Since the dependency of cooling rate on crystallization in alkane dispersions decreased with increasing chain length (see AppendixC, Figure A3), only HD dispersions were investigated. HD samples were cooled to supercooling of 2.5, 4.5 and 8.5 K and held at each supercooling for one hour. During the isothermal observations, picture sequences of the droplets were taken every 12 s. Due to polarization the crystallized droplets change their color from gray transparent to green opaque and can be easily differentiated from liquid droplets. The number of crystallized droplets was optically followed in dependency of time. The main objective of thermo-optical investigations was to describe and quantify the crystallization behavior in alkane dispersions by determination of crystallization index by number Processes 2018, 6, 6 4 of 16

Processes 2018, 6, 6 4 of 15 CIN in quiescent systems. Similar to previous investigations, the alkane dispersion samples were cooled to 18 K supercooling with a cooling rate of 1 K/min. During supercooling, picture sequences of the crystallizing dispersions were taken every 0.2 K. The obtained micrographs were processed of the crystallizing dispersions were taken every 0.2 K. The obtained micrographs were processed with ImageJ 1.46r software (National Institute of Health, Bathesda, MD, USA) to differentiate with ImageJ 1.46r software (National Institute of Health, Bathesda, MD, USA) to differentiate between between liquid and crystalline structures, to determine the characteristic values such as number, size liquidand size and distribution crystalline structures,of crystallized to determine and supercooled the characteristic droplets valuesand to such calculate as number, the number-based size and size distribution of crystallized and supercooled droplets and to calculate the number-based crystallization crystallization index within the dispersions: index CIN within the dispersions: Ncr,,p CIN == (1)(1) Nd+ +Np wherewhere Ncr,,p is is the the number number of of crystalline crystalline droplets, droplets,N dthe the number number of of droplets droplets and andN pthe the number number of allof solidall solid structures structures (amorphous (amorphous or crystalline or crystalline droplets).Details droplets).Details of the thermo-optical of the thermo-optical method aremethod shown are in ourshown earlier in our paper earlier [14]. paper [14].

2.5. Crystallization under Mechanical Stress Crystallization behavior of bulk alkane and alkanealkane inin waterwater dispersionsdispersions underunder oscillationoscillation mechanical stress stress was was investigated investigated using using a rotation a rotationalal rheometer rheometer (Physica (Physica MCR MCR 301, Anton 301, Anton Paar, Graz, Paar, Graz,Austria) Austria) equipped equipped with witha double a double gap geometry gap geometry DG 26.7 DG 26.7(26.7 (26.7 mm mm outer outer diameter, diameter, 464 464 µmµ widthm width of ofthe the outer outer gap gap and and 419 419 µmµ mof ofthe the inner inner gap) gap) in inoscillation oscillation mode. mode. At thisthis point,point, nono rheologicalrheological evaluationevaluation hashas beenbeen donedone yet.yet. The rotationalrotational rheometerrheometer has beenbeen merely usedused asas aa preciseprecise temperedtempered shearshear cell.cell. OnlyOnly mechanicalmechanical values,values, suchsuch asas torquetorque ((M) as aa signalsignal forfor crystallization,crystallization, and deformation ( )γ) oror frequencyfrequency ((f )) asas settingsetting parametersparameters toto adjustadjust oscillationoscillation mechanical stress, have been usedused toto describedescribe thethe crystallizationcrystallization behaviorbehavior underunder mechanicalmechanical stressstress during supercoolingsupercooling inin alkanealkane dispersionsdispersions (see(see FigureFigure1 1).).

Figure 1. Torque M as as aa functionfunction ofof supercooling at constant deformation deformation γ of 1% and frequency f of 11 HzHz ofof 1010 wtwt %% eicosaneeicosane dispersion.dispersion. MicrographsMicrographs on the right areare depicteddepicted resultsresults ofof quiescentquiescent crystallizationcrystallization (without(without mechanicalmechanical treatment).treatment).

Three major impacts on the torque during oscillation mechanical treatment are connected to the Three major impacts on the torque during oscillation mechanical treatment are connected to the supercooling of organic dispersions: (1) constant increase in torque due to supercooling (increase in supercooling of organic dispersions: (1) constant increase in torque due to supercooling (increase in viscosity); (2) additional increase in torque due to crystallization/solidification of organic substances viscosity); (2) additional increase in torque due to crystallization/solidification of organic substances (increase of inner friction due to solidification) and (3) crystallization/solidification driven (increase of inner friction due to solidification) and (3) crystallization/solidification driven aggregation aggregation leading to fluctuation and further increase in torque (aggregate formation and breakage processes, dependent on the mechanical stress). Sample preparation: 3.8 mL of 10 wt % alkane emulsion were pipetted into the tempered

measuring cup using a tempered pipette 10 K above ,. The measuring bob was placed into the gap and tempered with alkane emulsion for additional 5 min 10 K above ,. To reduce

Processes 2018, 6, 6 5 of 16 leading to ﬂuctuation and further increase in torque (aggregate formation and breakage processes, dependent on the mechanical stress). Sample preparation: 3.8 mL of 10 wt % alkane emulsion were pipetted into the tempered measuring cup using a tempered pipette 10 K above ϑm,alkane. The measuring bob was placed into the gap and tempered with alkane emulsion for additional 5 min 10 K above ϑm,alkane. To reduce impurities/alien nuclei, the measuring geometry was washed with water, then isopropanol, and afterwards dried for 5 min in dust and lint free environment. Impact of mechanical stress on crystallization: Samples of 10 wt % alkane dispersion were cooled to 18 K supercooling with a cooling rate of 1 K/min. During cooling three different deformations γ, 1, 100 and 10,000%, and three different frequencies f , 1, 10 and 100 Hz, were applied. Deformation and frequency are proportional in square to the dissipated energy during oscillation and were used to vary the mechanical stress intensity during crystallization. To compare the torque within the experimental series, normalized torque was calculated:

M Mnorm = (2) Mmax

Mechanical treatment is reported to accelerate nucleation and thus the crystallization progress in dispersed systems [3,48]. Acceleration of the crystallization process can be identified by: (1) decrease of required supercooling for crystallization and (2) increase of crystalline content. Therefore, we determined the onset crystallization supercooling (supercooling, at which first crystallization effects on torque were detected) and phase transition torque factor (quotient of torque after and before crystallization) to compare the impact of mechanical stress intensity on crystallization behavior in emulsions. Onset crystallization supercooling: To determine the onset crystallization supercooling onset ∆Tcr we evaluated the torque as function of supercooling and stress intensity. For this, we determined the first significant increase in torque (over 10%) as a result of supercooling and mechanical stress (see Figure1, magnified diagram): Mi+1 onset ∆Tcr(M) = ∆T ≥ 1.1 (3) Mi

Phase transition torque factor: To estimate the crystallization progress in dispersions we determined the phase transition torque Mpht in dependence of mechanical stress intensity. Therefore, we calculated the mean torque Mcr between 16 K and 17 K supercooling (every droplet was crystalline at this supercooling during quiescent crystallization in alkane dispersions) and related it to the mean torque Mliq between 0 and 1 K supercooling (every droplet was liquid at this supercooling during quiescent crystallization in alkane dispersions):

Mcr Mpht = Mliq

2.6. Statistical Analysis Every emulsion and every experiment was prepared/performed three times for triple determination. Uncertainty was calculated using commercial software MATLAB R2017a (the MathWorks, Natick, MA, USA), Microsoft Excel 2016 or Origin 2017 and expressed as standard deviation.

3. Results and Discussion

3.1. Quiescent Crystallization Three parameters—droplet size (AppendixA), melting point (AppendixB) and time dependency (AppendixC)—of crystallization behavior in alkane dispersions were analyzed prior the determination Processes 2018, 6, 6 6 of 16 of crystallization index. Since these parameters were needed for the quiescent crystallization analysis, but are not the main topic of our discussion, details are given in the AppendicesA–C. During the thermo-optical observations of quiescent crystallization behavior in 1 wt % alkane dispersion three different physical states occurred: liquid droplets above melting temperature (grey, Processes 2018, 6, 6 6 of 15 transparent spheres), supercooled liquid droplets below melting temperature (grey, transparent spheres) and polyspheres) crystalline and poly structures crystalline below structures melting below temperature melting temperature (green, opaque (green, spheroids). opaque spheroids). We did notWe observe any amorphousdid not observe structures any amorphous (grey, opaque structures spheroids) (grey, opaque or recrystallization spheroids) or torecrystallization single crystalline to single structures (colored,crystalline transparent structures polygons) (colored, due transparent to low supercooling polygons) ratedue andto low short supercooling experiment rate duration and short of 20 min. experiment duration of 20 min. Alkane droplets crystallized to poly crystalline spheroids and Alkane dropletsretained nearly crystallized spherical to (see poly Figure crystalline 2). spheroids and retained nearly spherical (see Figure2).

Figure 2. Micrographs of 1 wt % alkane dispersions at different supercooling with a cooling rate of 1 Figure 2. Micrographs of 1 wt % alkane dispersions at different supercooling with a cooling rate of K/min. Length of the scale bar is 200 µm. 1 K/min. Length of the scale bar is 200 µm. At low supercooling, droplets crystallized individually and stochastically distributed in the Atvolume low supercooling, independent of dropletstheir size. This crystallized crystallization individually behavior is andconnected stochastically to the impurity distributed initiated in the crystallization, where impurities reduce the nucleation barrier in droplets and cause crystallization volumeat independent relatively low ofsupercooling their size. [3,48]. This In crystallization emulsions, the behaviornumber of impurities is connected is usually to the much impurity smaller initiated crystallization,than the number where impuritiesof droplets. Therefore, reduce the crystallizat nucleationion in barrier emulsion in droplets droplets containing and cause no crystallizationimpurities at relativelyrequires low supercoolinghigher supercooling [3,48 and]. In is emulsions,strongly dependent the number on the droplet of impurities size. The is general usually experience much smaller than theshows, number that of with droplets. increasing Therefore, purity and crystallization decreasing droplet in emulsion size high droplets supercooling containing is required no impuritiesfor requiresnucleation higher supercooling in emulsions [3,15]. and is strongly dependent on the droplet size. The general experience Within the homologues series HD (C16H34), OD (C18H38), EC (C20H42) and DC (C22H46), we shows, that with increasing purity and decreasing droplet size high supercooling is required for observed different crystallization behavior of droplets in dependence of the alkane chain length. nucleationSimilar in emulsionsto onset crystallization [3,15]. temperature (Appendix B), the onset crystallization supercooling Withindecreased the with homologues increasing serieschain length HD (Cof 16theH alka34),nes. OD To (C quantify18H38), the EC crystallization (C20H42) and progress DC in (C 22H46), we observedalkane dispersions, different crystallization we calculated th behaviore crystallization of droplets index by in number dependence CIN (see ofFigure the alkane3). chain length. Similar to onset crystallization temperature (AppendixB), the onset crystallization supercooling decreased with increasing chain length of the alkanes. To quantify the crystallization progress in alkane dispersions, we calculated the crystallization index by number CIN (see Figure3). In HD dispersions, crystallization index remained nearly zero at low supercooling and increased continuously and linear during further supercooling. At a critical supercooling (∆T = 12 K) all observed droplets crystallized. Similar to HD dispersions, crystallization index in OD dispersions increased continuously and linear, until complete crystallization of the droplets (∆T = 10.5 K). The crystallization index of EC dispersion showed nonlinear increase with a stagnation region (stagnation between 6 K and 8 K supercooling). In case of DC dispersions, the nonlinear behavior of the crystallization increased with a more pronounced stagnation region (stagnation between 5 K and 10 K supercooling). The stagnation during EC and DC crystallization indicates two different nucleation types, low nucleation barrier (crystallization at low supercooling, type I) before the stagnation and high nucleation barrier (crystallization at high supercooling, type II) [28,48,49]. Those stagnation Figure 3. Number-based crystallization index CIN of 1 wt % alkane dispersions as a function of regions mightsupercooling be the cause ∆ with of a impurities, cooling rate of different 1 K/min. Length nucleation of the scale mechanisms bar is 200 µm. or most likely formation of intermediate structures (polymorphism).

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spheres) and poly crystalline structures below melting temperature (green, opaque spheroids). We did not observe any amorphous structures (grey, opaque spheroids) or recrystallization to single crystalline structures (colored, transparent polygons) due to low supercooling rate and short experiment duration of 20 min. Alkane droplets crystallized to poly crystalline spheroids and retained nearly spherical (see Figure 2).

Figure 2. Micrographs of 1 wt % alkane dispersions at different supercooling with a cooling rate of 1 K/min. Length of the scale bar is 200 µm.

At low supercooling, droplets crystallized individually and stochastically distributed in the volume independent of their size. This crystallization behavior is connected to the impurity initiated crystallization, where impurities reduce the nucleation barrier in droplets and cause crystallization at relatively low supercooling [3,48]. In emulsions, the number of impurities is usually much smaller than the number of droplets. Therefore, crystallization in emulsion droplets containing no impurities requires higher supercooling and is strongly dependent on the droplet size. The general experience shows, that with increasing purity and decreasing droplet size high supercooling is required for nucleation in emulsions [3,15]. Within the homologues series HD (C16H34), OD (C18H38), EC (C20H42) and DC (C22H46), we observed different crystallization behavior of droplets in dependence of the alkane chain length. Similar to onset crystallization temperature (Appendix B), the onset crystallization supercooling Processesdecreased2018, 6, 6with increasing chain length of the alkanes. To quantify the crystallization progress in 7 of 16 alkane dispersions, we calculated the crystallization index by number CIN (see Figure 3).

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In HD dispersions, crystallization index remained nearly zero at low supercooling and increased continuously and linear during further supercooling. At a critical supercooling (ΔT = 12 K) all observed droplets crystallized. Similar to HD dispersions, crystallization index in OD dispersions increased continuously and linear, until complete crystallization of the droplets (ΔT = 10.5 K). The Figure 3. Number-based crystallization index CIN of 1 wt % alkane dispersions as a function of Figurecrystallization 3. Number-based index of EC crystallization dispersion showed index nonlinear CI of increase 1 wt % with alkane a stagnation dispersions region as (stagnation a function of supercooling ∆ with a cooling rate of 1 K/min. LengthN of the scale bar is 200 µm. supercoolingbetween 6∆ TK withand a8 coolingK supercooling). rate of 1 K/min.In case Lengthof DC dispersions, of the scale the bar nonlinear is 200 µm. behavior of the crystallization increased with a more pronounced stagnation region (stagnation between 5 K and 10 K supercooling). The stagnation during EC and DC crystallization indicates two different nucleation Thetypes, chain low length nucleation of investigated barrier (crystallization alkanes at had low asupercooling, great inﬂuence type I) on before the the crystallization stagnation and progress in alkanehigh dispersions, nucleation barrier as shown (crystallization in Figure at 3high in supercooling, micrographs type on II) the [28,48,49]. right. Those At 5 stagnation K supercooling alkane dispersionsregions might of be similarthe cause mean of impurities, droplet different size showednucleation differentmechanisms crystallization or most likely formation progress at the of intermediate structures (polymorphism). same formulation and process conditions: no crystallization in HD dispersions (CIN,HD = 0.00); The chain length of investigated alkanes had a great influence on the crystallization progress in OD dispersionsalkane dispersions, crystallized as shown to 1% in (CI FigureN,OD 3= in 0.01); micrographs 24% of on all the droplets right. At crystallized5 K supercooling inEC alkane dispersions (CIN,EC =dispersions 0.24); crystallization of similar mean of 40% droplet by numbersize showed in DCdifferent dispersions crystallization (CIN,DC progress= 0.40). at the same formulation and process conditions: no crystallization in HD dispersions (CIN,HD = 0.00); OD 3.2. Crystallizationdispersions undercrystallized Mechanical to 1% (CI StressN,OD = 0.01); 24% of all droplets crystallized in EC dispersions (CIN,EC = 0.24); crystallization of 40% by number in DC dispersions (CIN,DC = 0.40). Different to direct number-based observation of crystallization in previous chapter, torque was used as indirect3.2. Crystallization integral under signal Mechanical to follow Stress crystallization in alkane dispersions under mechanical stress. Liquid-solidDifferent phase transition, to direct number-based such as crystallization, observation of crystallization is connected in toprevious an increase chapter, intorque inner was friction or ﬂow resistanceused as duringindirect integral mechanical signal treatment.to follow crystall Therefore,ization incrystallization alkane dispersions under under mechanicalmechanical stress. stress results in increasedLiquid-solid torque phase signal, transition, which such can beas crystallization, used as an indicator is connected for to crystallizationan increase in inner events friction in or dispersed systems [flow40–42 resistance]. during mechanical treatment. Therefore, crystallization under mechanical stress results in increased torque signal, which can be used as an indicator for crystallization events in Compareddispersed to systems the quiescent [40–42]. crystallization in Section 3.1, alkane dispersions under low mechanical stress ( f = 1HzComparedand γ =to 1%the) showquiescent similar crystallization trend in in the Section start of3.1, crystallization: alkane dispersions With under increasing low chain length themechanical onset crystallization stress (=1 supercooling and =1%) decreasesshow similar in trend alkane in the dispersions start of crystallization: (see Figure 4With). However, the onsetincreasing crystallization chain length supercooling the onset crystallization was equivalent supercooling within decreases the standard in alkane deviationdispersions to(see quiescent Figure 4). However, the onset crystallization supercooling was equivalent within the standard crystallizationdeviation behavior to quiescent (see crystalliz Figure4ationb). behavior (see Figure 4b).

(a) (b)

Figure 4. (a) Normalized torque of 10 wt % alkane dispersions under low oscillation Figure 4. (a) Normalized torque Mnorm of 10 wt % alkane dispersions under low oscillation mechanical mechanical stress as a function of supercooling ∆ ; (b) Comparison of onset crystallization stress as a function of supercooling ∆T;(b) Comparison of onset crystallization supercooling supercooling during quiescent crystallization ∆ and crystallization under low during quiescentmechanical crystallizationstress ∆,%onset.∆ Tquiscent and crystallization under low mechanical stress onset∆Tf =1Hz,γ=1%. Two general effects of non-destructive mechanical treatment might influence the crystallization in dispersed systems: (1) Energy input over the mechanical work connected to immobilization of molecules for nucleation [50] and (2) collision of moving droplets/particles in the flow field leading

Processes 2018, 6, 6 8 of 16

Two general effects of non-destructive mechanical treatment might influence the crystallization in dispersed systems: (1) Energy input over the mechanical work connected to immobilization of Processes 2018, 6, 6 8 of 15 molecules for nucleation [50] and (2) collision of moving droplets/particles in the flow field leading to secondary nucleation [[48].48]. Dissipated Dissipated energy energy during during oscillation mechanical stress treatment is proportional in square to the frequency f and deformation γ andand cancan bebe adjusted by these two parameters. Theoretically, increasingincreasing the the frequency frequency at at low low deformation deformation (such (such as as 1%) 1%) mostly mostly results results in inenergy energy input input (effect (effect 1) and 1) increasingand increasing the deformation the deformation at low at frequency low frequency (such as (such 1 Hz) as mostly 1 Hz) results mostly in resultsflow field in mediatedflow field collisions mediated (effect collisions 2). Therefore, (effect 2). we Therefore, varied the we frequency varied ftheat lowfrequency deformation at low and deformation γandat low deformation frequency to observeat low frequency the influence to observe of mechanical the influence stress on of the mechanical onset crystallization stress on thesupercooling onset crystallization in alkane dispersions supercooling (see in Figure alkane5 ).dispersions (see Figure 5).

(a) (b)

Figure 5. (a) Onset crystallization supercooling of 10 wt % alkane dispersions ∆,.as Figure 5. (a) Onset crystallization supercooling of 10 wt % alkane dispersions onset∆Tcr,dispersion aas function a function of offrequency frequency ; f(;(b)b )Onset Onset crystallization crystallization supercooling supercooling of of 10 10 wt wt % % alkane dispersionsdispersions ∆, as a function of deformation . onset∆Tcr,dispersion as a function of deformation γ.

In general, we observed no or only little influence of frequency or deformation during oscillation In general, we observed no or only little inﬂuence of frequency or deformation during oscillation mechanical stress on onset crystallization supercooling in alkane dispersions. Onset crystallization mechanical stress on onset crystallization supercooling in alkane dispersions. Onset crystallization supercooling tended to increase with increasing frequency and deformation and thus with increasing supercooling tended to increase with increasing frequency and deformation and thus with increasing mechanical treatment. According to the comparatively small increase, standard deviations and the mechanical treatment. According to the comparatively small increase, standard deviations and the used analytical method, this might be also connected to the sensitivity of the method (no detection of used analytical method, this might be also connected to the sensitivity of the method (no detection of crystallization events on single droplet level, several crystallization events are required for detection). crystallization events on single droplet level, several crystallization events are required for detection). The onset supercooling in dispersions represents only a small number of crystalline droplets. This The onset supercooling in dispersions represents only a small number of crystalline droplets. This small small number of droplets, which crystallized at low supercooling, might also crystallize according to number of droplets, which crystallized at low supercooling, might also crystallize according to impurity-initiated mechanism and independent of mechanical treatment. However, the major impurity-initiated mechanism and independent of mechanical treatment. However, the major fraction of droplets in dispersions crystallizes at higher supercooling. Consequently, to describe the fraction of droplets in dispersions crystallizes at higher supercooling. Consequently, to describe crystallization progress in dispersions requires quantitative determination of crystallization index CIi the crystallization progress in dispersions requires quantitative determination of crystallization index as a function of mechanical stress over relevant supercooling range. Therefore, we evaluated the CI as a function of mechanical stress over relevant supercooling range. Therefore, we evaluated crystallizationi progress in alkane dispersions by calculation of the phase transition torque factor (see the crystallization progress in alkane dispersions by calculation of the phase transition torque factor Figure 6), which takes the mean torque at high supercooling (between 16 K and 17 K, every droplet (see Figure6), which takes the mean torque at high supercooling (between 16 K and 17 K, every was crystalline during quiescent crystallization analysis) into ratio to mean torque at low droplet was crystalline during quiescent crystallization analysis) into ratio to mean torque at low supercooling (between 0 and 1 K, every droplet was liquid during quiescent crystallization analysis). supercooling (between 0 and 1 K, every droplet was liquid during quiescent crystallization analysis). Since liquid-solid phase transition causes higher flow resistance, dispersions with high crystallinity Since liquid-solid phase transition causes higher ﬂow resistance, dispersions with high crystallinity result in increase of torque during mechanical treatment. Additionally, crystallizing droplets do not result in increase of torque during mechanical treatment. Additionally, crystallizing droplets do not only change their physical state but also form crystalline structures, which differ from the shape of only change their physical state but also form crystalline structures, which differ from the shape of spherical droplets. Those crystalline structures provide more interactive surface, than point contact spherical droplets. Those crystalline structures provide more interactive surface, than point contact of of spheres and result in stronger aggregation due to van der Waals forces. During oscillation spheres and result in stronger aggregation due to van der Waals forces. During oscillation mechanical mechanical treatment, aggregation results in supplementary increase in torque, in addition to the treatment, aggregation results in supplementary increase in torque, in addition to the increase due to increase due to supercooling and phase transition. Consequently, high torque transition factor indicates high state of crystallinity and/or crystallization initiated aggregation in dispersed systems.

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supercooling and phase transition. Consequently, high torque transition factor indicates high state of

Processescrystallinity 2018, 6, 6 and/or crystallization initiated aggregation in dispersed systems. 9 of 15

(a) (b)

Figure 6. (a) Phase transition torque factor of 10 wt % alkane dispersions .as a function of Figure 6. (a) Phase transition torque factor of 10 wt % alkane dispersions Mpht as a function of frequency frequency ; (b) Phase transition torque factor of 10 wt % alkane dispersions as a function of f ;(b) Phase transition torque factor of 10 wt % alkane dispersions Mpht as a function of deformation γ. deformation . At low frequency and low deformation, the phase transition torque factor was comparatively high At low frequency and low deformation, the phase transition torque factor was comparatively in range of several thousand indicating advanced crystallization and/or crystallization-dependent high in range of several thousand indicating advanced crystallization and/or crystallization- agglomeration in dispersions. Different to low mechanical stress during supercooling, we measured dependent agglomeration in dispersions. Different to low mechanical stress during supercooling, we a decrease in phase transition torque factor by applying both high frequencies and high deformations, measured a decrease in phase transition torque factor by applying both high frequencies and high indicating less ﬂow resistance and consequently less crystallization and/or crystallization initiated deformations, indicating less flow resistance and consequently less crystallization and/or aggregation. This was in contrast to the expectations, since mechanical treatment, especially at high crystallization initiated aggregation. This was in contrast to the expectations, since mechanical deformation, increases the collision rates and therefore the secondary or collision-mediated nucleation treatment, especially at high deformation, increases the collision rates and therefore the secondary or in dispersions [3,48]. collision-mediated nucleation in dispersions [3,48]. At this point it should be noted, that two aspects are important for the efﬁciency of collision At this point it should be noted, that two aspects are important for the efficiency of collision mediated nucleation: (1) collision frequency/probability, which increases with increasing mechanical mediated nucleation: (1) collision frequency/probability, which increases with increasing mechanical treatment; and (2) collision time, which decreases with increasing mechanical stress. Therefore, treatment; and (2) collision time, which decreases with increasing mechanical stress. Therefore, there there is a limit in mechanical stress, which is able to promote secondary nucleation in dispersions. is a limit in mechanical stress, which is able to promote secondary nucleation in dispersions. This This limit is the collision time. The collision time must be larger than the required time for colliding limit is the collision time. The collision time must be larger than the required time for colliding supercooled droplet to wet the surface of crystallized droplet in contact, and initiate secondary supercooled droplet to wet the surface of crystallized droplet in contact, and initiate secondary nucleation. Otherwise, mechanical treatment will result in frequent collisions without nucleation nucleation. Otherwise, mechanical treatment will result in frequent collisions without nucleation leading to low crystallinity in dispersions. In addition, from thermodynamic point of view, every leading to low crystallinity in dispersions. In addition, from thermodynamic point of view, every energy input, such as work or heat during mechanical treatment, increases the free energy of the energy input, such as work or heat during mechanical treatment, increases the free energy of the system and might inhibit the nucleation. Of course, this strongly depends on ability of the system to system and might inhibit the nucleation. Of course, this strongly depends on ability of the system to convert and save the energy in order to increase its overall energy. convert and save the energy in order to increase its overall energy. Still, the increase in torque during supercooling cannot be clearly connected to crystallization or Still, the increase in torque during supercooling cannot be clearly connected to crystallization or aggregation/aggregation-breakage effects separately using torque as an indicator for crystallization aggregation/aggregation-breakage effects separately using torque as an indicator for crystallization in the tempered shear cell. To identify the mechanisms of crystallization under mechanical stress in the tempered shear cell. To identify the mechanisms of crystallization under mechanical stress requires a combination of optical, mechanical and thermal measuring method, and is a subject of our requires a combination of optical, mechanical and thermal measuring method, and is a subject of our ongoing work. ongoing work. 4. Conclusions 4. Conclusions Crystallization behavior of alkane dispersions with different chain lengths, hexadecane Crystallization behavior of alkane dispersions with different chain lengths, hexadecane (C16H34), (C16H34), octadecane (C18H38), eicosane (C20H42), and docosane (C22H46), showed strongly chain octadecane (C18H38), eicosane (C20H42), and docosane (C22H46), showed strongly chain length- length-dependent crystallization behavior. With increasing chain length, the required supercooling dependent crystallization behavior. With increasing chain length, the required supercooling and thus and thus the driving force for the initiation of crystallization decreased within the investigated alkanes. the driving force for the initiation of crystallization decreased within the investigated alkanes. Unlike Unlike the entire and instantaneous crystallization after nucleation in pure alkanes, crystallization the entire and instantaneous crystallization after nucleation in pure alkanes, crystallization in investigated alkane droplets occurred individually and were stochastically distributed among the observed supercooling range. Different to hexadecane and octadecane dispersions, eicosane and docosane dispersions had a stagnation region within the crystallization process indicating two

Processes 2018, 6, 6 10 of 16 in investigated alkane droplets occurred individually and were stochastically distributed among the observed supercooling range. Different to hexadecane and octadecane dispersions, eicosane and docosaneProcesses dispersions 2018, 6, 6 had a stagnation region within the crystallization process indicating two10 differentof 15 nucleation types. At high critical supercooling (13 K) and independent of the alkane chain length, everydifferent droplet nucleation crystallized types. within At high the investigatedcritical supercooling sample. (13 K) and independent of the alkane chain Applyinglength, every mechanical droplet crystallized stress duringwithin the supercooling investigated sample. in alkane dispersions and following the Applying mechanical stress during supercooling in alkane dispersions and following the crystallization process by measuring the torque led to qualitatively similar crystallization behavior, crystallization process by measuring the torque led to qualitatively similar crystallization behavior, as inas quiescent in quiescent systems. systems. Mechanical Mechanical stress stress diddid notnot affect the the required required supercooling supercooling for initiation for initiation of of the crystallizationthe crystallization process. process. Consequently, Consequently, increaseincrease of frequency frequency or or deformation deformation during during oscillation oscillation mechanicalmechanical treatment treatment led led to similarto similar onset onset crystallization crystallization supercooling. supercooling. Increase Increase of mechanical of mechanical stress stress causedcaused an unexpectedan unexpected decrease decrease in torque torque indicating indicating incomplete incomplete crystallization. crystallization. This is contrary This is to contrary the to theexpected expected increase increase in secondary in secondary nucleation. nucleation. However, However, the evaluation the of evaluation torque allows of torqueonly qualitative allows only qualitativestatements statements of the ofcrystallization the crystallization process processin dispersed in dispersed systems systems under mechanical under mechanical stresses. stresses.A A combinationcombination of of optical, optical, mechanicalmechanical and and thermal thermal meth methodsods to quantify to quantify the crystallization the crystallization behavior behavior of dispersed systems is a subject of our ongoing investigations and will be presented in our following of dispersed systems is a subject of our ongoing investigations and will be presented in our papers. following papers. Acknowledgments: The authors thank AiF for funding our IGF-project 18462 N and the research association Acknowledgments: GVT. The authors thank AiF for funding our IGF-project 18462 N and the research association GVT. Author Contributions: Serghei Abramov and Heike Petra Karbstein provided the idea, conceived and Author Contributions: Serghei Abramov and Heike Petra Karbstein provided the idea, conceived and designed designed the experiments; Serghei Abramov, Kinza Shah and Lydia Weißenstein performed the experiments; the experiments; Serghei Abramov, Kinza Shah and Lydia Weißenstein performed the experiments; Serghei Serghei Abramov, Kinza Shah and Lydia Weißenstein analyzed the data; Serghei Abramov wrote the paper; HeikeAbramov, Petra Karbstein Kinza Shah corrected and Lydia the Weißenstein paper. analyzed the data; Serghei Abramov wrote the paper; Heike Petra Karbstein corrected the paper Conﬂicts of Interest: The authors declare no conﬂict of interest. Conflicts of Interest: The authors declare no conflict of interest. Appendix A. Droplet Size Distribution Appendix A. Droplet Size Distribution DropletDroplet size size distributions distributions (DSD) (DSD) of of 1 1 wt wt %% alkanealkane dispersions, dispersions, prepared prepared with with the thesame same energy energy inputinput Ev but Ev atbut different at different temperatures temperatures (10 (10K K aboveabove the melting melting temperature temperature of respectively of respectively alkanes), alkanes), are shownare shown in Figure in Figure A1 .A1.

Figure A1. Cumulative sum distribution by area Q2 of 1 wt % alkane in water emulsions stabilized Figure A1. Cumulative sum® distribution by area Q2 of 1 wt % alkane in water emulsions stabilized with with 1 wt % Tween20 and measured at , with PLM/IP. Triangles: DSD of hexadecane 1 wt % Tween20® and measured at ϑ with PLM/IP. Triangles: DSD of hexadecane dispersions. dispersions. Pentagons: DSD of octadecanem,alkane dispersions. Diamonds: DSD of eicosane dispersions. Pentagons:Hexagons: DSD DSD of octadecaneof docosane dispersions. Length Diamonds: of the scale DSD bar of eicosaneis 200 µm. dispersions. Hexagons: DSD of docosane dispersions. Length of the scale bar is 200 µm. We chose polarizing light microscopy and image processing instead of laser diffraction and Wedynamic chose light polarizing scattering light for the microscopy characterization and of image the DSD processing to ensure the instead measurement of laser of diffraction only liquid and dynamicdroplets. light For scattering this we worked for the at characterization exact alkane melting of the temperature DSD to ensure during the the measurement measurement ofand only were liquid able to detect possible crystalline structures due to polarization. This technique also allowed us to

Processes 2018, 6, 6 11 of 16 droplets. For this we worked at exact alkane melting temperature during the measurement and were able to detect possible crystalline structures due to polarization. This technique also allowed us to avoid unwanted measurements of agglomerates/aggregates, which might be formed due to colloidal Processes 2018, 6, 6 11 of 15 processes in dispersed systems. As we showed in our previous investigations [14,15] the mean droplet diameteravoid measured unwanted withmeasurements thermo-optical of agglomerates/aggre method is almostgates, identical which might within be formed the standard due to colloidal deviation to the moreprocesses statistic in dispersed laser diffraction systems. measurements As we showed and in our will previous be used investigations as the characteristic [14,15] the size mean value for furtherdroplet discussions. diameter The measured mean dropletwith thermo-optical diameter of themethod alkane is almost dispersions identical is givenwithin in the Table standard A1. deviation to the more statistic laser diffraction measurements and will be used as the characteristic

Tablesize value A1. forMean further droplet discussions. diameter The x50.2 meanand drople numbert diameter of characterized of the alkane droplets dispersions N with is standardgiven in deviations.Table A1. Measured with polarizing light microscopy/image processing.

Table A1. Mean droplet diameter x50.2 and number of characterized droplets N with standard Alkane (Dispersed Phase) x50.2 (µm) N (−) deviations. Measured with polarizing light microscopy/image processing. C16: hexadecane (HD) 17.2 ± 1.2 1297 ± 306 C18:Alkane octadecane (Dispersed (OD) Phase) 16.1 x50.2 ±(µm)0.4 N 1653 (−) ± 148 C20:C16: eicosane hexadecane (EC) (HD) 14.6 17.2 ±± 1.20.2 1297 1774 ± 306± 65 C22:C18: docosane octadecane (DC) (OD) 12.8 16.1 ±± 0.41.0 1653 2506 ± 148± 86 C20: eicosane (EC) 14.6 ± 0.2 1774 ± 65 C22: docosane (DC) 12.8 ± 1.0 2506 ± 86 Applying the same Ev in the emulsification process resulted in different DSDs and x50.2 values. The differences are caused by different emulsification temperatures and material properties resulting Applying the same Ev in the emulsification process resulted in different DSDs and x50.2 values. in differentThe differences viscosity are ratios caused and by interfacial different emulsification tensions between temperatures the dispersed and material and continuous properties resulting phase. Both, the viscosityin different ratio viscosity and interfacial ratios and tension, interfacial influence tensions the between emulsification the dispersed efficiency and andcontinuous thus the phase. final DSD. Consequently,Both, the viscosity x50.2 decreased ratio and within interfacial the homologoustension, influence series the from emulsification 17.2 µm (HD, efficiency lowest and emulsification thus the temperature)final DSD. to Consequently, 12.8 µm (DC, x highest50.2 decreased emulsification within the temperature).homologous series from 17.2 µm (HD, lowest emulsification temperature) to 12.8 µm (DC, highest emulsification temperature). Appendix B. Onset Crystallization Temperature and Melting Point Appendix B. Onset Crystallization Temperature and Melting Point The onset crystallization and melting point in 1 wt % dispersions were determined optically using polarizedThe onset cryo-microscopy crystallization and and melting are shown point in 1 Figure wt % A2dispersions, micrographs were determined on the right. optically Since the using polarized cryo-microscopy and are shown in Figure A2, micrographs on the right. Since the crystallization in dispersions is an individual stochastic process, the onset crystallization temperature crystallization in dispersions is an individual stochastic process, the onset crystallization temperature was determined by the first crystallized droplet. The melting temperature was determined by the was determined by the first crystallized droplet. The melting temperature was determined by the meltingmelting of all of crystalline all crystalline structures structures within within the the dispersion.dispersion.

Figure A2. Onset crystallization temperature , and melting point/ temperature Figure A2. Onset crystallization temperature onset ϑcr,dispersion and melting point/ temperature , of the alkane dispersions. Micrographs on the right show one of three exemplary ϑm,dispersion of the alkane dispersions. Micrographs on the right show one of three exemplary determination of onset crystallization temperature and melting point in hexadecane dispersions. determination of onset crystallization temperature and melting point in hexadecane dispersions. Length of the scale bar is 200 µm. Length of the scale bar is 200 µm.

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The melting temperature was always higher, then the onset crystallization. With increasing Processes 2018, 6, 6 12 of 16 chain length of the alkanes, the difference between melting temperature and onset crystallization decreased. Consequently, with increasing chain length of the alkanes lower supercooling was requiredThe for melting crystallization temperature in alkane was always dispersions. higher, Melting then the temperature onset crystallization. and onset crystallization With increasing are chainsummarized length in of Table the alkanes, A2. the difference between melting temperature and onset crystallization decreased. Consequently, with increasing chain length of the alkanes lower supercooling was required for crystallizationTable A2. Onset in alkane crystallization dispersions. temperature Melting temperature and onset, melting crystallization point/ temperature are summarized in Table, A2. of the alkane dispersions and reference melting temperature of the bulk ,, according to [51]. Table A2. Onset crystallization temperature onset ϑcr,dispersion, melting point/ temperature ϑm,dispersion Alkane (Dispersed Phase) Onset θcr,dispersion/°C θm,dispersion/°C θm,ref,bulk/°C [51] of the alkane dispersions and reference melting temperature of the bulk ϑm,re f ,bulk according to [51]. C16: hexadecane (HD) 12.9 ± 0.5 18.5 ± 0.1 18.2 C18: octadecane (OD) 25.3 ± ◦0.8 28.8 ± 0.2◦ 28.2 ◦ Alkane (Dispersed Phase) Onset θcr,dispersion/ C θm,dispersion/ C θm,ref,bulk/ C[51] C20: eicosane (EC) 35.3 ± 0.2 37.0 ± 0.1 36.6 C16: hexadecane (HD) 12.9 ± 0.5 18.5 ± 0.1 18.2 C18: octadecaneC22: docosane (OD) (DC) 25.3 ± 43.50.8 ± 0.1 28.8 44.5± ± 0.20.0 44.1 28.2 C20: eicosane (EC) 35.3 ± 0.2 37.0 ± 0.1 36.6 We comparedC22: docosane the (DC) melting temperatures 43.5 ± 0.1 of the dispersions 44.5 ± 0.0 with the reference 44.1 melting temperature of the bulk (data from [51]) and detected differences in the range of 0.5 K. Those differencesWe compared result from the melting the resolution temperatures of the ofmethod the dispersions (signal per with time the unit), reference the dispersed melting temperature state (bulk ofvs. the dispersion) bulk (data and from the [51 purity]) and detectedof the substances. differences In in this the rangepaper, of we 0.5 therefore K. Those refer differences the melting result fromtemperature the resolution , of or the supercooling method (signal ∆ per to the time determined unit), the dispersed, state using (bulk our vs.thermo-optical dispersion) andmethod. the purity of the substances. In this paper, we therefore refer the melting temperature ϑm,alkane or supercooling ∆T to the determined ϑm,dispersion using our thermo-optical method. Appendix C. Time Dependency of Crystallization Process Appendix C. Time Dependency of Crystallization Process Nucleation is a time-dependent process during the crystallization procedure. Exceeding the drivingNucleation forces, such is a time-dependentas supersaturation process and/or during supercooling, the crystallization is usually procedure.required to Exceedingovercome the drivingnucleation forces, barrier such and as still supersaturation does not necessarily and/or supercooling,result in immediate is usually crystallization. required to The overcome induction the nucleationtime, which barrier is required and still doesfrom not certain necessarily supercooling/supersaturation result in immediate crystallization. to nucleation, The induction differs time, in whichdependence is required of formulation from certain and supercooling/supersaturation process. To describe the time to dependency nucleation, differson nucleation in dependence in alkane of formulationdispersions we and varied process. the Tocooling describe rate theand time followed dependency optically on the nucleation crystallization in alkane process dispersions (see Figure we variedA3). the cooling rate and followed optically the crystallization process (see Figure A3).

Figure A3. Micrographs of 1 wt % alkane dispersions at ΔT = 7.5 K during supercooling with different Figure A3. Micrographs of 1 wt % alkane dispersions at ∆T = 7.5 K during supercooling with different cooling rates. Length of the scale bar is 200 µm. cooling rates. Length of the scale bar is 200 µm.

We observed decreasing time dependency of the crystallization in dispersions with increasing We observed decreasing time dependency of the crystallization in dispersions with increasing chain length from HD (C16) over OD (C18) and EC (C20) to DC (C22). In DC dispersions, the amount of chain length from HD (C ) over OD (C ) and EC (C ) to DC (C ). In DC dispersions, the amount of crystallized droplets within16 triple determination18 was20 comparatively22 equal independent of the cooling crystallized droplets within triple determination was comparatively equal independent of the cooling rate. EC and OD dispersions showed similar crystallization behavior at low (0.1 K/min) and medium rate. EC and OD dispersions showed similar crystallization behavior at low (0.1 K/min) and medium (1.0 K/min) cooling rates as DC dispersions. At high (10 K/min) cooling rate, both EC and OD Processes 2018,, 6,, 66 13 of 15 16

(1.0 K/min) cooling rates as DC dispersions. At high (10 K/min) cooling rate, both EC and OD dispersions, showedshowed a a lower lower number number of crystallizedof crystalliz droplets.ed droplets. The timeThe dependencytime dependency at the highat the cooling high coolingrate was rate higher was in higher OD dispersions in OD dispersions than in ECthan dispersions. in EC dispersions. In case ofIn HDcase dispersions, of HD dispersions, the number the numberof crystallized of crystallized droplets increaseddroplets increased with decreasing with decreasing cooling rate cooling and showedrate and more showed time-dependent more time- dependentcrystallization crystallization behavior as behavior OD, EC andas OD, DC EC dispersions. and DC dispersion In order tos. evaluateIn order theto evaluate time dependency the time dependencycompared to compared the driving to force the driving of supercooling force of wesupercooling performed we isothermal performed observations isothermal of observations crystallization of crystallizationin HD dispersions in HD (see dispersions Figure A4 ).(see Figure A4).

Figure A4.A4.Micrographs Micrographs of of 1 wt 1 %wt HD % dispersionsHD dispersions at different at different supercooling supercooling during one-hour during isothermalone-hour isothermalobservations. observations. Length of the Length scale of bar the is scale 200 µ barm. is 200 µm.

During the isothermal observation at low supercooling (ΔT = 2.5 K) we did not detect any During the isothermal observation at low supercooling (∆T = 2.5 K) we did not detect any crystallization in HD dispersions within one-hour observation, which corresponds to the onset crystallization in HD dispersions within one-hour observation, which corresponds to the onset crystallization investigations in Appendix B. Increasing the supercooling (to ΔT = 4.5 K) led to crystallization investigations in AppendixB. Increasing the supercooling (to T = 4.5 K) led to crystallization of few droplets within one minute. The number of crystallized ∆droplets remained comparativelycrystallization equal of few within droplets one-hour within observation. one minute. At The high number supercooling of crystallized (ΔT = 8.5 droplets K) more remained droplets T crystallizedcomparatively in HD equal dispersions within one-hour within one observation. minute. Al Atso high here, supercooling the number of (∆ crystallized= 8.5 K) more droplets droplets did notcrystallized change within in HD one dispersions hour. within one minute. Also here, the number of crystallized droplets did not changeThe influence within one of hour.supercooling was higher than the time dependency during isothermal observationsThe influence in HD of supercoolingdispersions with washigher no significant than the timechange dependency in number during of crystallized isothermal observationsdroplets after in oneHD dispersionsminute. According with no significant to no change change in innumber number of of crystallized droplets droplets after after one one minute. minute According in HD dispersionsto no change and in number nearly no of time crystallized dependency droplets in investigated after one minute time in scale HD in dispersions OD, EC and and DC nearly dispersions no time wedependency chose 1 K/min in investigated as a cooling time rate scale for in further OD, EC investigations. and DC dispersions we chose 1 K/min as a cooling rate for further investigations. References References 1. Müller, R.H.; Petersen, R.D.; Hommoss, A.; Pardeike, J. Nanostructured lipid carriers (NLC) in cosmetic 1. dermalMüller, products. R.H.; Petersen, Adv. Drug R.D.; Deliv. Hommoss, Rev. 2007 A.;, Pardeike,59, 522–530, J. Nanostructureddoi:10.1016/j.addr.2007.04.012. lipid carriers(NLC) in cosmetic 2. Pardeike,dermal products. J.; Hommoss,Adv. Drug A.; Müller, Deliv. Rev. R.H.2007 Lipid, 59 nanoparticles, 522–530. [CrossRef (SLN, ][NLC)PubMed in cosmetic] and pharmaceutical 2. dermalPardeike, products. J.; Hommoss, Int. J. Pharm. A.; Müller, 2009, R.H. 366, Lipid170–184, nanoparticles doi:10.1016/ (SLN,j.ijpharm.2008.10.003. NLC) in cosmetic and pharmaceutical 3. McClements,dermal products. D.J. Int.Crystals J. Pharm. and 2009crystallization, 366, 170–184. in oil-in [CrossRef-water][ emulsions:PubMed] Implications for emulsion-based 3. deliveryMcClements, systems. D.J. Adv. Crystals Colloid and Interface crystallization Sci. 2012 in, 174 oil-in-water, 1–30, doi:10.1016/j.cis.2012.03.002. emulsions: Implicationsfor emulsion-based Adv. Colloid Interface Sci. 2012 174 4. Rostami,delivery systems.E.; Kashanian, S.; Azandaryani, A.H.;, Fara, 1–30.marzi, [CrossRef H.; Dolatabadi,][PubMed ]J.E.N.; Omidfar, K. Drug 4. targetingRostami, E.; Kashanian,using solid S.;Azandaryani, lipid nanoparticles. A.H.; Faramarzi, Chem. H.; Dolatabadi, Phys. J.E.N.;Lipids Omidfar, 2014, K. Drug181, targeting56–61, using solid lipid nanoparticles. Chem. Phys. Lipids 2014, 181, 56–61. [CrossRef][PubMed] doi:10.1016/j.chemphyslip.2014.03.006. 5. Wissing, S. Cosmetic applications for solid lipid nanoparticles (SLN). Int. J. Pharm. 2003, 254, 65–68. [CrossRef] 5. Wissing, S. Cosmetic applications for solid lipid nanoparticles (SLN). Int. J. Pharm. 2003, 254, 65–68, 6. Müller, R. Solid lipid nanoparticles (SLN) for controlled drug delivery: A review of the state of the art. Eur. J. doi:10.1016/S0378-5173(02)00684-1. Pharm. Biopharm. 2000, 50, 161–177. [CrossRef] 6. Müller, R. Solid lipid nanoparticles (SLN) for controlled drug delivery: A review of the state of the art. Eur. J. Pharm. Biopharm. 2000, 50, 161–177, doi:10.1016/S0939-6411(00)00087-4.

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