molecules Article Monitoring Silane Sol-Gel Kinetics with In-Situ Optical Turbidity Scanning and Dynamic Light Scattering Abul Bashar Mohammad Giasuddin and David W. Britt * Department of Biological Engineering, Utah State University, Logan, UT 84322-4105, USA * Correspondence: [email protected]; Tel.: +1-435-787-2158 Received: 13 June 2019; Accepted: 2 August 2019; Published: 6 August 2019 Abstract: Organosilanes (e.g., R’-SiOR3) provide hydrophobic functionality in thin-film coatings, porous gels, and particles. Compared with tetraalkoxysilanes (SiOR4), organosilanes exhibit distinct reaction kinetics and assembly mechanisms arising from steric and electronic properties of the R’ group on the silicon atom. Here, the hydrolysis and condensation pathways of n-propyltrimethoxy silane (nPM) and a tri-fluorinated analog of nPM, 3,3,3-trifluoropropyl trimethoxy silane (3F), were investigated under aqueous conditions at pH 1.7, 2.0, 3.0, and 4.0. Prior to hydrolysis, 3F and nPM are insoluble in water and form a lens at the bottom (3F) or top (nPM) of the solutions. This phase separation was employed to follow reaction kinetics using a Turbiscan instrument to monitor hydrolysis through solubilization of the neat silane lens while simultaneously tracking condensation-induced turbidity throughout the bulk solution. Dynamic light scattering confirmed the silane condensation and particle aggregation processes reported by the turbidity scanning. Employing macroscopic phase separation of the starting reactants from the solvent further allows for control over the reaction kinetics, as the interfacial area can be readily controlled by reaction vessel geometry, namely by controlling the surface area to volume. In-situ turbidity scanning and dynamic light scattering revealed distinct reaction kinetics for nPM and 3F, attributable to the electron withdrawing and donating nature of the fluoro- and organo-side chains of 3F and nPM, respectively. Keywords: organosilane; fluorosilane; ORMOSIL; hydrophobic nanoparticle; aqueous sol-gel synthesis; Turbiscan 1. Introduction Tri-alkoxysilanes, R’Si(OR)3, are widely employed to introduce functionality via the R’ chemistry into sol-gel processes, with specific applications as adhesion promoters and cross-linkers, as resin precursors, and to capture and encapsulate biological molecules [1,2]. For alkyl or aryl R’-groups, the resulting particles or gel are referred to as organically modified silicas (ORMOSILs). When fluorine is present on the R’ chain, they may be referred to as F-ORMOSILs. The hydrophobic alkyl- and fluoro R’ groups lower the surface energy of the resulting sol-gel based thin films, gels, or particles, and have been widely explored to create non-reflective, non-fouling, and superhydrophobic materials [3–11]. The hydrolysis and condensation chemical reactions generally occur through nucleophilic substitution reactions, with water attacking the Si center to liberate alcohol. Methoxy and ethoxy moieties are the two most common alkoxy leaving groups, and their hydrophobicity renders alkyl- and fluoro-silane precursors insoluble in aqueous solution. An R’ alkyl- or fluoro-group on the silane further reduces solubility, and an organic cosolvent, such as ethyl alcohol, is frequently employed to improve solubility and control reaction pathways. The sol-gel reactions for a trialkoxy ORMOSIL are depicted as: Molecules 2019, 24, 2931; doi:10.3390/molecules24162931 www.mdpi.com/journal/molecules MoleculesMolecules 201 2019, 92, 42,4 ,x x FOR FOR PEERPEER REVIEW REVIEW 2 of 15 2 of 15 Molecules 2019, 24, 2931 2 of 14 employed to improve solubility and control reaction pathways. The sol-gel reactions for a trialkoxy employed to improve solubility and control reaction pathways. The sol-gel reactions for a trialkoxy ORMOSIL are depicted as: ORMOSIL are depicted as: R’Si(OR)R’Si(OR)33 ++ nHnH22OO R’Si(OR)R’Si(OR)3 n3(OH)-n(OH)n +n nROH+ nROH Hydrolysis Hydrolysis (1) (1) − R’Si(OR)R’Si-OR3 ++ HOnH-2SiR’O R’SiR’Si(OR)-O-SiR’ + 3ROH-n(OH) n + nROH A lcohol condensation Hydrolysis (2) (1) R’SiR’Si-OR-OH + HO+ HO-SiR’-SiR’ R’SiR’Si-O-SiR’-O- SiR’+ H2O + ROH Water c ondensation Alcohol condensation (3) (2) R’Si-OR + HO-SiR’ R’Si-O-SiR’ + ROH Alcohol condensation (2) R’Si-OHAt + aHO minimum,-SiR’ for complete R’Sihydrolysis-O-SiR’, three + H 2molesO of water are Waterrequired c ondensationper mole of (3) trialkoxysilane. Excess water, along with acid catalysis, favors full hydrolysis (030) prior to initiationAt a minimum,of condensation. for completeControlling hydrolysis the reaction, threerates molesof tri-alkoxysilane of water arehydrolysis required and per mole of R’Si-OH + HO-SiR’ R’Si-O-SiR’ + H2O Water condensation (3) trialkoxysilane.condensation affords Excess control water, over along the size with and acidmorphology catalysis, of polycondensation favors full hydrolysis products that (030) prior to initiationassembleAt aof minimum,into condensation. nanoparticles for complete (sol) Controlling th hydrolysis,at may undergo the three reaction growth moles/ ofaggregation rates water areof into requiredtri -largeralkoxysilane perparticles mole and ofhydrolysis and condensationtrialkoxysilane.form gels depending affords Excess water,oncontrol reaction along over withconditions, the acid catalysis,size such and as favors acidmorphology or full base hydrolysis catalysis of (030)polycondensation, as priorillustrated to initiation in the products that reaction diagram in Figure 1. In the reaction scheme, (x,y,z) report the status of hydrolysis and assembleof condensation. into nanoparticles Controlling the reaction(sol) th ratesat may of tri-alkoxysilane undergo growth hydrolysis/aggregation and condensation into alargerffords particles and controlcondensation over the size and morphology of polycondensation products that assemble into nanoparticles form gels depending on reaction conditions, such as acid or base catalysis, as illustrated in the (sol) that may undergo growth/aggregation into larger particles and form gels depending on reaction reactionconditions, diagram such as in acid Figure or base 1. catalysis, In the asreaction illustrated scheme in the reaction, (x,y,z) diagram report inthe Figure status1. In of the hydrolysis and condensationreaction scheme, (x,y,z) report the status of hydrolysis and condensation X,Y,Z = (OR,OH,OSi) Figure 1. Schematic of the reaction scheme of hydrolysis and condensation kinetics of tri- alkoxysilanes under acidic and basic conditions in aqueous media, along with a numerical reaction scheme [11]. Here, (300), (030), and (003) represent the -OR, -OH, and -OSi functionalities, respectively, around a single Si atom. The neat silane consists of three alkoxy groups (300), which under acidic conditions are sequentially hydrolyzed to three hydroxylX,Y,Z = (OR,OH,OSi) groups (030), which may subsequently condense to produce three siloxane bonds around the Si center (003). FigureFigure 1.1.Schematic Schematic of the of reaction the schemereaction of hydrolysisscheme andof condensationhydrolysis kinetics and ofcondensation tri-alkoxysilanes kinetics of tri- alkoxysilanesunderOn a acidicgiven and underSi basicatom, acidic conditions x indic andates in basic aqueous the conditionsnumber media, of along inalkoxy aqueous with (- aOR) numerical media, leaving reactionalong groups with scheme present, a numerical [11]. y the reaction numberHere, of (300), (-OH) (030), groups, and (003) and represent z the number the -OR, of -OH, (-O- andSi) groups -OSi functionalities,, as depicted respectively, in the schematics. around a Thus, scheme [11]. Here, (300), (030), and (003) represent the -OR, -OH, and -OSi functionalities, for asingle trialkoxysilane Si atom. The under neat silane acid consists catalysis of three, the alkoxyprecur groupssor (300) (300), follows which a under stepwise acidic hydrolysis conditions are to form (210)respectively,sequentially to (120), and hydrolyzed around (030) aproducts. to single three hydroxyl Si Under atom. groups basic The (030), conditions,neat which silane may condensation consists subsequently of three pathways condense alkoxy toare produce groups promoted (300),, which yieldingunderthree siloxanehigheracidic bondsdimensionalconditions around are theproducts Sisequentially center ear (003).lier inhydrolyzed the reaction to. Thethree reaction hydroxyl schematics groups shown (030), forwhic h may trialkoxysilanesubsequently precursorscondense tothat produce yield threeORMOSIL siloxane and bonds F-ORMOSIL around the are Si centeralso descriptive(003). of tetraalkoxysilanes,On a given Si atom, e.g., x(400) indicates. While the the number same rules of alkoxy for acid (-OR) and leaving base catalysis groups present, apply, there y the are number some ofkey (-OH) distinct groups,ions. and z the number of (-O-Si) groups, as depicted in the schematics. Thus, for a trialkoxysilaneOn a given under Si atom, acid catalysis, x indic theates precursor the number (300) follows of alkoxy a stepwise (-OR) hydrolysis leaving to formgroups (210) present, y the numberThe of sol(-OH)-gel reactionsgroups, andand final z the products number formed of (- Ofrom-Si) alkylgroups- and, asfluoro depicted-trialkoxy insilanes the schematics. are Thus, todistinct (120), andfrom (030) tetraalkoxysilanes products. Under. First, basic and conditions, most notably, condensation the chemical pathways cross-linking are promoted, capacity yielding to form forhigher siloxanea trialkoxysilane dimensional (Si-O-Si) bonds products under is reduced. earlier acid incatalysis Thus, the reaction.