silane modification of glass and silica surfaces

Silane modification of glass and silica surfaces to obtain equallyoil-wet surfaces in glass-covered silicon micromodel applications

Jay W. Grate,1 Marvin G. Warner,1 Jonathan W. Pittman,1 Karl J. Dehoff,1 Thomas W. Wietsma,1

Changyong Zhang,1 and Mart Oostrom1

Received 16 November 2012; revised 16 May 2013; accepted 16 June 2013; published 5 August 2013.

[1] Wettability is a key parameter inuencing capillary pressures, permeabilities, ngeringmechanisms, and saturations in multiphase ow processes within porous media. Glass-covered silicon micromodels provide precise structures in which pore-scale displacementprocesses can be visualized. The wettability of silicon and glass surfaces can be modied bysilanization. However, similar treatments of glass and silica surfaces using the same silanedo not necessarily yield the same wettability as determined by the oil-water contact angle.In this study, surface cleaning pretreatments were investigated to determine conditions thatyield oil-wet surfaces on glass with similar wettability to silica surfaces treated with thesame silane, and both air-water and oil-water contact angles were determined. Borosilicateglass surfaces cleaned with standard cleaning solution 1 (SC1) yield intermediate-wetsurfaces when silanized with hexamethyldisilazane (HMDS), while the same cleaning andsilanization yields oil-wet surfaces on silica. However, cleaning glass in boilingconcentrated nitric acid creates a surface that can be silanized to obtain oil-wet surfacesusing HMDS. Moreover, this method is effective on glass with prior thermal treatment at anelevated temperature of 400C. In this way, silica and glass can be silanized to obtainequally oil-wet surfaces using HMDS. It is demonstrated that pretreatment and silanizationis feasible in silicon-silica/glass micromodels previously assembled by anodic bonding, andthat the change in wettability has a signicant observable effect on immiscible uiddisplacements in the pore network.

Citation: Grate, J. W., M. G. Warner, J. W. Pittman, K. J. Dehoff, T. W. Wietsma, C. Zhang, and M. Oostrom (2013), Silanemodification of glass and silica surfaces to obtain equally oil-wet surfaces in glass-covered silicon micromodel applications, WaterResour. Res., 49, 47244729, doi:10.1002/wrcr.20367.

1. Introduction

[2] Micromodels are planar microuidic devices with atleast one transparent face that enables visualization of u-ids within spatially structured pore networks created byetching or other microfabrication techniques [Berejnovet al., 2008; Buckley, 1991; Gunda et al., 2011; Javadpourand Fisher, 2008; Karadimitriou and Hassanizadeh, 2012;Lenormand, 1999]. Processes that are studied include, butare not limited to, displacement of a wetting uid by an im-miscible nonwetting uid (drainage), or displacement of anonwetting uid by an immiscible wetting uid (imbibi-tion). For visualization in micromodels, the two uids aredistinguished from one another by their native differencesin color (e.g., when one is a dark-colored oil), by differen-ces in refractive index (when both are clear and colorless),

or using colored or uorescent dyes to facilitate distinctionof two otherwise clear colorless uids [Chomsurin andWerth, 2003; Grate et al., 2011; Lenormand, 1999]. Dis-placements in porous media are relevant to petroleum re-covery [Abdallah et al., 2007; Buckley, 1991, 2001;Dandekar, 2006; Zhao et al., 2010], nonaqueous phase liq-uid contaminant transport [Bradford et al., 1999; Demondand Roberts, 1991; Dwarakanath et al., 2002; Hofsteeet al., 1998; OCarroll et al., 2004; Powers et al., 1996],and geological carbon sequestration [Benson and Cole,2008; Chalbaud et al., 2010; Chiquet et al., 2007; Espi-noza and Santamarina, 2010; Krevor et al., 2011; Zhanget al., 2011b].[3] Wettability is a key parameter inuencing capillary

pressures, permeabilities, ngering mechanisms, and nalsaturations in displacement processes within porous media[Abdallah et al., 2007; Anderson, 1986; Lenormand,1999]. The wettabilities of surfaces in micromodels forstudying multiphase ow phenomena are typically depend-ent on the materials of micromodel fabrication. Micromo-dels have been developed in materials such as glass, epoxyresins, silicon, and most recently, polydimethylsiloxane(PDMS). Epoxy resin [Bergslien et al., 2004; Buckley,1991; Lenormand, 1999] and PDMS models are oil-wet,although PDMS can be treated with plasma to create a

Additional supporting information may be found in the online versionof this article.

1Pacic Northwest National Laboratory, Richland, Washington, USA.

Corresponding author: J. W. Grate, Pacic Northwest National Labora-tory, PO Box 999, Richland, WA 99352, USA. ([email protected])

2013. American Geophysical Union. All Rights Reserved.0043-1397/13/10.1002/wrcr.20367

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WATER RESOURCES RESEARCH, VOL. 49, 47244729, doi:10.1002/wrcr.20367, 2013

water-wet surface temporarily [Berejnov et al., 2008; Bhat-tacharya et al., 2005; Javadpour and Fisher, 2008]. Glassmicromodels, typically prepared by wet etching, are water-wet [Buckley, 1991; Conrad et al., 1992; Javadpour andFisher, 2008; Lenormand, 1999]. The etched pore networkin the surface is sealed with either a at glass plate or asymmetrically etched pore network in glass, creatingmicromodels where all surfaces are made of the same mate-rial, and hence have the same wettability. Silicon micromo-dels are prepared by dry etching techniques in siliconwafers [Chomsurin and Werth, 2003; Gunda et al., 2011;Lenormand, 1999; Willingham et al., 2008; Zhang et al.,2011a, 2011b], yielding structures with vertical walls andmuch higher precision than wet-etched glass. The siliconsurfaces are oxidized to create a water-wet silica interface,and the models are sealed with a borosilicate glass plate byanodic bonding at (typically) 400C. We will call these sili-con-silica/glass micromodels. While this technique pro-vides strongly bonded devices with near-perfect structures,it does create models with different materials for the porenetwork and the cover plate.[4] The combination of silicon-silica and glass is nor-

mally of little consequence for obtaining micromodels ofuniform wettability so long as both surfaces are thoroughlycleaned to provide water-wet surfaces. In our treatment, weuse the denition by Anderson [1986], where water-wetcorresponds to oil-water contact angles from 0 to 75,intermediate-wet from 75 to 115, and oil-wet from 115 to180. Glass and silica surfaces can be modied with silanesto alter the wettability, a process that is called silaniza-tion [Arkles et al., 2009; Fadeev, 2006; Grate andMcGill, 1995; Jain et al., 2002; McGill et al., 1994; Mena-wat et al., 1984; Shahidzadeh-Bonn et al., 2004; Wei et al.,1993]. Alkylsilane or arylsilane reagents in the liquid form,or adsorbed from the vapor phase, react with surface sila-nols to create new siloxane bonds that covalently bond thealkyl or aryl groups to the surface, altering surface wett-ability. We have previously reported the air-water and oil-water contact angles obtained from diverse silanes on silicasurfaces and found a linear correlation relating these con-tact angles (and their cosines) [Grate et al., 2012]. Thisstudy identied silanes that produce intermediate-wet andoil-wet surfaces on silica, and demonstrated the surfacemodication of an assembled silicon-silica/glass micromo-del to obtain an intermediate-wet micromodel.[5] Here, we turn our attention especially to the prepara-

tion of oil-wet surfaces on glass in comparison with silica.

When using silanes intended to yield hydrophobic oil-wetsurfaces, we have found that the resulting silica and glasssurfaces do not necessarily have the same wettabilitieseven when treated under the same conditions. These differ-ences between silica and glass represent a potential issuefor multiphase ow studies in surface-modied silicon-silica/glass micromodels, where all surfaces should ideallyhave the same wettability. Glass and silica are differentmaterials and their surfaces change with thermal treatmentsthat affect surface silanol coverage. The availability of sur-face silanols then inuences the silanization process, thesurface density of attached organic groups, and the wett-ability outcome. For example, in the eld of capillary gaschromatography, capillaries of glass, fused silica, andquartz have all been examined and noted to have differentcharacteristics depending on the material, its thermal his-tory, and the chemical pretreatments such as basic etchingor acid leaching, and these treatments inuence subsequentsilanization processes [Bartle et al., 1981; Grob, 1986].The advantageous features of silicon-silica/glass micromo-dels, and the importance of wettabilities in displacementprocesses, have prompted us to study the silanization ofglass surfaces in comparison with silica surfaces. Our goalis to develop surface modication procedures that can beapplied in an anodically bonded silicon-silica/glass micro-model, where both the glass and silica surfaces of a micro-model will be similarly oil-wet.

2. Experimental Methods

[6] In our experiments, the borosilicate glass was Pyrex7740 obtained from Sensor Prep Services (Elburn, Ill.) andthe silica surfaces were thermal oxide grown on silicon wa-fer material (Virginia Semiconductor Inc., Fredericksburg,Va.). Clean surfaces for silanization reactions on 3 cm 1cm rectangles of silicon were obtained with a series ofwater and organic solvent rinses prior to nal cleaning bystandard cleaning solution 1 for 30 min (SC1, a 7080Csolution of ve parts deionized (DI) water, 1 part 27% am-monium hydroxide, and 1 part 30% hydrogen peroxide)[Kern, 2008], UV-ozone [Kern, 2008] for 30 min using aJelight model 342 UV-ozone cleaner, concentrated nitricacid (68%, boiling point 120C, cleaning conditions speci-ed in Tables 1 and 2), or aqua regia for 10 min. Surfacescleaned with liquid reagents were thoroughly rinsed withdeionized water. For silanization reactions, cleaned drysamples were put in an aluminum foil lined glass jar and

Table 1. Contact Angle Measurements on Silanized Silica and Glass Surfaces Using Silanes Intended to Yield Oil-Wet Surfacesa

Contact Angles Air-Water Oil-Waterb

Surface Modification Surface Mean Std Dev n Mean Std Dev n Cleaning Method

Hexamethyldisilazane Silica 97 2.2 9 140 4.9 9 UV-ozonea

Glass 97 1.1 9 91 3.0 9 Aqua regiaGlass 96 2.3 9 114 2.7 9 SC1Glass 100 1.8 9 150 3.8 9 Boiling nitric acidc (30 min)

Dodecyltriethoxysilane Silica 102 1.9 9 148 4.7 9 UV-ozonea

Glass 102 1.5 9 110 2.6 9 UV-ozone

aHexadecane is the oil.bResults on silica from our previously published work [Grate et al., 2012].cConcentrated nitric acid.

GRATE ET AL.: SILANE MODIFICATION OF GLASS AND SILICA SURFACES

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the silane was drop cast over the surfaces. The jar wasclosed and placed in a 90C oven for 15 h. Samples weresolvent-rinsed with water, methanol, and hexanes after sila-nization and allowed to dry thoroughly before contact anglemeasurements. All contact angles were determined on aRame-Hart Model 500 Advanced Goniometer (Netcong, N.J.) as described in detail previously [Grate et al., 2012].Static contact angles were stable within minutes for allsurfaces except glass surfaces modied with silanes yield-ing hydrophobic interfaces. The latter sometimes requiredseveral minutes to reach stable contact angles. Measure-ments were typically made on three locations on each ofthree samples for nine total measurements per silane/mate-rial combination.

3. Results and Discussion

3.1. Contact Angles of Silanized Glass and SilicaSurfaces

[7] Using silanes previously identied [Grate et al.,2012] to yield oil-wet surfaces on silica, we found that sila-nization reactions producing hydrophobic oil-wet surfaceson silica do not necessarily yield oil-wet surfaces on glass.Moreover, this may not be apparent from air-water contactangles, which may be similar on the two materials ; insteadthese differences are revealed by the oil-water contactangles. Examples collected from several experiments areshown in Table 1. On silica, silanization with hexamethyl-disilazane (HMDS) gave air-water and oil-water contactangles of 97 and 140, respectively, the latter of which isclearly in the oil-wet range. However, on glass cleanedwith aqua regia prior to silanization we observed air-waterand oil-water contact angles of 97 and 91, respectively.The glass surface is not oil-wet by the oil-water contactangle measurement. In a repeat experiment on glass,cleaned with SC1, contact angles were 96 and 114, respec-tively. Again, the silanized glass is not oil-wet, and again,the oil-water contact angle on the glass is quite differentfrom that on silica (140). On the other hand, glass cleanedin boiling concentrated nitric acid prior to silanization withHMDS had air-water and oil-water contact angles of 100and 150, respectively, which is clearly in the oil-wetrange.[8] Our rationale for using boiling concentrated nitric

acid (boiling point 120C) was that it would act as an oxi-

dizer to thoroughly clean the surfaces, while simultane-ously hydrating and cleaving surface siloxane linkages tocreate a surface with abundant surface silanols for reactionwith silanes. In addition, alkali metal ions migrate to thesurface of heated glass, and these ions can be exchangedout of the glass by acid treatment, leading to a surfaceenriched in silica [Elmer, 1980; Jang et al., 2001; Wrightet al., 1980]. Wright et al. [1980] noted that leaching glasswith boiling 20% hydrochloric acid (110C) resulted insurfaces that were nearly pure silica, while Jang et al.[2001] noted that boiling glass surfaces in concentrated ni-tric acid reduced surface concentrations of sodium, cal-cium, and aluminum atoms.[9] With dodecyltriethoxysilane (see Table 1), another

silane that is expected to produce hydrophobic surfaces,results similar to those with HMDS were observed. Air-water contact angles on silica and glass were both 102,while the oil-water contact angles were 148 and 110 onsilica and glass, respectively (glass not cleaned in boilingnitric acid). Again, the glass surface was not oil-wet; it wasquite different from the silica surface treated with the samesilane, and the difference was only apparent in the oil-water contact angle. The oil-water contact angle appears toprobe the surface differently than the air-water contactangle, at least for the glass surfaces.[10] The results in Table 1 were collected from experi-

ments on glass that has been prepared for silanization bycleaning and treatment methods, as we sought conditions toachieve oil-wet glass surfaces. Hot concentrated nitric acidwas the most promising pretreatment method for glass toenable silanizations leading to oil-wet surfaces. Therefore,we set up a series of more directly comparable experimentsusing the same pretreatment methods on silica and glasssurfaces followed by identical simultaneous silanizationreactions with HMDS. In these experiments, we also investi-gated the effect of prior thermal treatments of 400C. Micro-models are assembled by anodic bonding at 400C, andtherefore we needed to investigate silanizations on surfaceswith similar prior thermal histories. Results are shown in Ta-ble 2. HMDS is a monofunctional silane with regard to reac-tions with surface silanols. It can only bond to the surfaceand not to other adjacent silanes, and thus creates a mono-layer on the surface. Trichloro and trialkoxysilanes, by con-trast, can bond to the surface and polymerize with oneanother, potentially leading to disordered silane multilayers.

Table 2. Contact Angle Measurements on Simultaneously HMDS-Silanized Silica and Glass Surfaces After Various PretreatmentMethods

Contact Angles Air-Water Oil-Watera

Thermal History Surface Pretreatment Surface Mean Std Dev n Mean Std Dev n

RT Nitric acid, RT, 1 weekb,c,d Silica 101 1.6 9 154 2.7 9Glass 102 1.3 9 148 1.0 3

400C Nitric acid, RT, 1 week Silica 100 0.8 9 152 3.0 9Glass 95 1.5 9 102 5.0 9

400C Nitric acid, boiling, 30 min Silica 95 2.1 9 140 7.5 9Glass 98 2.0 9 141 2.2 9

aHexadecane is the oil.bRT is room temperature. The rst entry in the rst column indicates the prior thermal history of the glass.cThose labeled 400C were baked at 400C for 25 min and then cooled prior solution cleaning.dConcentrated nitric acid in each of these experiments.


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HMDS therefore represents a controlled test for silane with-out major risk of siloxane polymer formation on the surface.[11] In the rst experiments listed in Table 2, we exam-

ined whether boiling the concentrated nitric acid was neces-sary. Glass and silica surfaces, with no prior 400Ctreatment, were soaked in concentrated nitric acid at roomtemperature for 1 week. After silanization with HMDS, air-water and oil-water contact angles on silica were 101 and154, respectively, while those on glass were 102 and 148,respectively. This treatment gave oil-wet surfaces with simi-lar contact angles on silica and glass, and the results wereconsistent with the prior boiling nitric acid treatment onglass with no prior 400C treatment as seen in Table 1 (air-water and oil-water contact angles of 100 and 150 on glass).However, in a similar experiment where the glass was rstbaked at 400C for 25 min before room temperature nitricacid treatment for 1 week, silanization with HMDS producedglass surfaces with air-water and oil-water contact angles of95 and 102, respectively. This glass surface is not oil-wet,and baking the glass at high temperature clearly did have aneffect on the subsequent silanization and surface wettabilityresults. Heating glass at high temperature dehydrates theglass with adjacent surface silanols condensing to form si-loxane linkages. Hence, there are fewer surface silanols forsubsequent modication by the hydrophobic silane.[12] However, when glass with a history of baking at

400C is subsequently treated with boiling concentrated ni-tric acid, and then silanized, HMDS yields oil-wet surfaceson both silica and glass. The air-water and oil-water contactangles on silica were 95 and 140, respectively, while thoseon glass were 98 and 141, respectively. These results withHMDS provide a method for obtaining similar oil-wetsurfaces on silica and glass that have prior exposure to hightemperatures such as those associated with anodic bondingof silicon-silica/glass micromodels.

3.2. Oil-Wet Micromodel

[13] Based on our studies with HMDS, we modied theinterior surface of a silicon-silica/glass micromodel that hadbeen assembled by anodic bonding at 400C in our laborato-ries, using methods described in the supporting information.These micromodels have been described previously [Grateet al., 2012; Zhang et al., 2011a, 2011b]. Figure 1 comparesthe displacement of water by hexadecane in micromodelswith water-wet and oil-wet surfaces. Displacements inwater-wet micromodels were described in detail previously[Zhang et al., 2011a]. At low ow rates, displacement in thewater-wet model is unstable and is characterized by capillaryngering with multipore blobs left undisplaced. By contrast,hexadecane displaced water from the oil-wet micromodelwith ease, displacing the vast majority of the water withoccasional (mostly) single-pore residual water left behind.The entry pressure for the hexadecane into the oil-wet porenetwork was only approximately 50 Pa, compared to 3500Pa in water-wet pore networks with the same pore geometry.Figure 2 shows hexadecane and water in contact with eachother and a uid channel boundary in the entry section of theoil-wet micromodel. The wetting of the surface by hexade-cane in preference to water is apparent, and stands in con-trast with similar images previously published by Grateet al. [2012] for water-wet micromodel surfaces. The oil wetsurface was stable for at least several days as repeated dis-

placement experiment were performed, with alcohol rinsesto wash the model in between experiments.

4. Discussion

[14] Our observations can be interpreted in terms of sur-face coverage by the silane, which varies depending on theinitial surface chemical composition of the glass. In thisinterpretation, thermal histories and pretreatments leadingto lower densities of surface silanols, and hence lower den-sities of silane residues after reaction with HMDS, wouldlead to intermediate-wet surfaces, while surfaces that yieldhigher densities of silane residues after silanization wouldlead to oil-wet surfaces. The oil-water contact angle ismore sensitive to these surface coverage differences than

Figure 1. Comparison of the displacement of water withhexadecane in (a) water-wet and (b) oil-wet micromodels.Micromodels and methodology were described in detail inreference [Zhang et al., 2011a]. Hexadecane contains NileRed and the images were obtained by epiuorescence mi-croscopy. Flow is from left to right at 5 L/h, correspond-ing average linear velocity is 2.1 cm/h.

Figure 2. Annotated uorescence microscopy image ofNile Red dyed hexadecane (oil) in the entry region of themicromodel, shown where the two liquid phases contactthe solid channel boundary.


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the air-water contact angle. Some precedence for this inter-pretation can be found in the work by Menawat et al.[1984], who treated soda-lime glass samples, previouslycleaned in warm nitric acid for 20 min, with various mono-functional organosilanes. Silane concentrations in liquid-phase silanization reactions were varied, leading to differ-ences in contact angles that were interpreted as differencesin surface silane coverage. The oil-water advancing contactangles (oil used was xylene) showed large differences incontact angle with concentration, from 70 to 100 at 0.05mg/L silane concentration in n-hexane, up to valuesapproaching 160 at silane concentrations from 5 to 40 mg/mL. Variations in air-water contact angles were not soextreme. In one case, on increasing the t-butyldiphenyl-chlorosilane concentration from 0.05 to 40 mg/mL, the air-water contact angle varied from about 68 to 72, while theoil-water contact angle varied from 100 to 155.[15] In summary, in this technical note, we have demon-

strated that borosilicate glass and silica surfaces do not nec-essarily have the same wettabilities when silanized underthe same conditions with the same silane. By nding pre-treatment procedures that yield similar oil-wet surfaces onglass and silica after silanization, even after prior exposureto elevated temperatures (400C), we have addressed apractical problem that is relevant to the surface modica-tion of anodically bonded silicon-silica/glass micromodels.It is, in general, more difcult to obtain oil-wet surfaces onglass after silanization, especially glass with a history ofprior high temperature treatment. In particular, we nd thatSC1 does not provide glass surfaces that yield oil-wetsurfaces after silanization. However, treatment with boilingconcentrated nitric acid does restore 400C treated glass toa state where it can be silanized with HMDS to yield oil-wet surfaces similar to that of HMDS-silanized silica surfa-ces. Therefore, we generally recommend boiling with con-centrated nitric acid as a treatment to prepare the interiorsurfaces of silicon-silica/glass micromodels prior to silani-zation reactions. To prepare oil-wet micromodels withHMDS, we specically recommend this pretreatment, andwe have demonstrated that it is feasible to do so.

[16] Acknowledgments. The Carbon Sequestration Initiative of theLaboratory Directed Research and Development Program at the PacicNorthwest National Laboratory (PNNL) supported this research. A portionof this research was carried out in the William R. Wiley EnvironmentalMolecular Sciences Laboratory (EMSL), a national scientic user facilitysponsored by the Department of Energys Ofce of Biological and Envi-ronmental Research and located at PNNL. PNNL is a multiprogramnational laboratory operated for the DOE by Battelle Memorial Institute.

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