Whether decorative, protectiveor functional, coatings mustcontend with water in theenvironment. A problem dis-tinct from the issue of wet

adhesion and hydrolytic stability of coatings is con-trolling the interaction of water with a coated surface.Very often the descriptors hydrophobic or hydrophilicare applied to coated surfaces. Although the termshydrophobic and hydrophilic are casually used, theyare usually not defined. A growing number of applica-tions ranging from architectural coatings to aorticstents require a precise control and, therefore, precisedefinition of substrate interaction with water. Silanes

are playing an increasing role in controlling the inter-action of water with a surface.

Silanes are silicon chemicals that possess a hydrolyti-cally sensitive center that can react with inorganic sub-strates such as glass to form stable covalent bonds andorganic substitution that alters the physical interac-tions of treated substrates (Figure 1). Different thanmost additives, which have a limited performancerange, they can achieve surface properties ranging fromhydrophobic to hydrophilic. They may be a sole activeingredient or a component in a coatings formulation,controlling the interaction of water over a broad spec-trum of requirements. In order to understand howsilanes can affect hydrophobicity and hydrophilicity, it isimportant to understand some of the fundamentals ofthe interaction of water with surfaces.

Water, Hydrophobicity and HydrophilicityHydrophobic and hydrophilic are frequently useddescriptors of surfaces. A surface is hydrophobic if ittends not to adsorb water or be wetted by water. A sur-face is hydrophilic if it tends to adsorb water or be wet-ted by water. More particularly, the terms describe theinteraction of the boundary layer of a solid phase withliquid or vapor water. Silanes can be used to modifythe interaction of boundary layers of solids with waterwith a high degree of control, effecting variabledegrees of hydrophobicity or hydrophilicity.

Since the interaction of water with surfaces is fre-quently used to define surface properties, a brief reviewof its structure and properties can be helpful. Although

By Barry Arkles | Gelest Inc., Morrisville, PA

Hydrophobicity,Hydrophilicity

and SilanesWater, water everywhere is the refrainfrom the rhyme of the ancient marinerand a concern of every modern coatingstechnologist.

organic substitution allows permanent property modification

hydrolyzable alkoxy (alcohol) groups

Property modifications include:HydrophobicityReleaseDielectricAbsorptionOrientationHydrophilicityCharge Conduction

Applications include:Architectural CoatingsWater-RepellentsMineral Surface TreatmentsFillers for CompositesPigment DispersantsDielectric CoatingsAnti-fog CoatingsRelease CoatingsOptical (LCD) Coatings

CH 3CH 2CH 2CH 2CH 2CH 2CH 2CH 2— Si — OCH 2CH 3

OCH 2CH 3

OCH 2CH 3

Figure 1 | Silanes and surface modification.

Reprinted with permission from the October 2006 issue of Paint & Coatings Industry magazine

Hydrophobicity, Hydrophilicity and Silanes

the structure of water is a subject of early discussion inthe study of physical sciences, it is interesting to notethat the structure of liquid water is still not solved and,even so, most technologists lose appreciation of what isknown about its structure and properties.

The quantum calculation of the structure of an iso-lated H2O molecule has evolved to the currentlyaccepted model which demonstrates a strong dipole,

but no lone electron pairs associated with sp3

hybridized orbitals of oxygen. This model of isolatedH2O conforms most closely to the vapor state, andextrapolation often leads to the conclusion that wateris a collection of individual molecules that associatewith each other primarily through dipole interactions.The polar nature of water, with its partial positive andpartial negative dipole, explains why bulk water read-ily dissolves many ionic species and interacts withionic surfaces. The difference between isolated vaporphase water and bulk liquid water is much moreextreme than can be accounted for by a model relyingonly on dipole interaction.

The properties of bulk liquid water are stronglyinfluenced by hydrogen bond interactions. In the liq-uid state, in spite of 80% of the electrons being con-cerned with bonding, the three atoms of a water mol-ecule do not stay together as discrete molecules. Thehydrogen atoms are constantly exchanging betweenwater molecules in a protonation-deprotonationprocess. Both acids and bases catalyze hydrogenexchange and, even when at its slowest rate ofexchange (at pH 7), the average residence time of ahydrogen atom is only about a millisecond.

In the liquid state, water molecules are bound toeach other by an average of three hydrogen bonds.Hydrogen bonds arise when a hydrogen that is cova-lently bound to an oxygen in one molecule of waternears another oxygen from another water molecule.The electrophilic oxygen atom “pulls” the hydrogencloser to itself. The end result is that the hydrogen isnow shared (unequally) between the oxygen to whichit is covalently bound and the electrophilic oxygen towhich it is attracted (O-H...O). Each hydrogen bondhas an average energy of 20 kJ/mol. This is much lessthan an O-H covalent bond, which is 460 kJ/mol. Eventhough an individual hydrogen bond is relativelyweak, the large number of hydrogen bonds that existin water that pull the molecules together have a signif-icant role in giving water its special bulk properties. Inice, water molecules are highly organized with fourhydrogen bonds. Liquid water is thought to be a com-bination of domains of molecules with 3-4 hydrogenbonds separated by domains with 2-3 hydrogenbonds, subject to constant turnover - the flickering clus-ter model (Figure 2).

This brief description of water is provided in orderto give the insight that whenever a solid surface inter-acts with bulk water it is interacting with a soft matterstructure, not simply a collection of individual mole-cules. Surface interactions with water must competewith a variety of internal interactions of liquid phasewater: van der Waals forces, dipole interactions,hydrogen bonding and proton exchange.

molecule of water showing dipole

2 molecules showing hydrogen bond

ice - molecules of water with 4 hydrogen bonds

HydrogenOxygen

water

liquid water - flickering cluster modelregions of molecules with 3-4 hydrogen bondsseparated by regions with 2-3 hydrogen bonds

(not shown: out of plane hydrogen bonds)

Figure 2 | The water molecule.

Hydrophobic-“poor wetting”

Ordinary Surface-“typical wetting”

Hydrophilic“good wetting”

Figure 3 | Suface wetting types.

Wettability and Contact AngleA surface is said to be wetted if a liquid spreads overthe surface evenly without the formation of droplets.When the liquid is water and it spreads over the sur-face without the formation of droplets, the surface issaid to be hydrophilic. In terms of energetics, thisimplies that the forces associated with the interactionof water with the surface are greater than the cohe-sive forces associated with bulk liquid water. Waterdroplets form on hydrophobic surfaces, implying thatcohesive forces associated with bulk water are greaterthan the forces associated with the interaction ofwater with the surface (Figure 3).

Practically, hydrophobicity and hydrophilicity arerelative terms. A simple, quantitative method fordefining the relative degree of interaction of a liquidwith a solid surface is the contact angle of a liquiddroplet on a solid substrate (Figure 4). If the contactangle of water is less than 30°, the surface is desig-nated hydrophilic since the forces of interactionbetween water and the surface nearly equal thecohesive forces of bulk water, and water does notcleanly drain from the surface. If water spreads overa surface, and the contact angle at the spreadingfront edge of the water is less than 10°, the surface isoften designated as superhydrophilic provided thatthe surface is not absorbing the water, dissolving inthe water or reacting with the water. On a hydropho-bic surface, water forms distinct droplets. As thehydrophobicity increases, the contact angle of thedroplets with the surface increases. Surfaces withcontact angles greater than 90° are designated ashydrophobic. The theoretical maximum contactangle for water on a smooth surface is 120°. Micro-textured or micro-patterned surfaces with hydropho-bic asperities can exhibit apparent contact anglesexceeding 150° and are associated with superhy-drophobicity and the “lotus effect.”

Critical Surface Tension and AdhesionWhile the contact angle of water on a substrate is agood indicator of the relative hydrophobicity orhydrophilicity of a substrate, it is not a good indica-tor for the wettability of the substrate by other liq-uids. Critical surface tension is associated with thewettability or release properties of a solid. It servesas a better predictor of the behavior of a solid witha range of liquids.

Liquids with a surface tension below the critical sur-face tension (γc) of a substrate will wet the surface, i.e.,show a contact angle of 0 (cosθe = 1). The critical sur-face tension is unique for any solid and is determinedby plotting the cosine of the contact angles of liquidsof different surface tensions and extrapolating to 1.

Figure 4 | Water contact angles on various surfaces.

Figure 5 | Contact angle and wettability.

θheptadecafluorodecyltrimethoxysilane* 115°poly(tetrafluoroethylene) 108-112°poly(propylene) 108°octadecyldimethylchlorosilane* 110°octadecyltrichlorosilane* 102-109°tris(trimethylsiloxy)silylethyl-

dimethylchlorosilane 104°octyldimethylchlorosilane* 104°dimethyldichlorosilane* 95-105°butyldimethylchlorosilane* 100°trimethylchlorosilane* 90-100°poly(ethylene) 88-103°poly(styrene) 94°poly(chlorotrifluoroethylene) 90°human skin 75-90°

diamond 87°graphite 86°silicon (etched) 86-88°talc 50-55°chitosan 80-81°steel 70-75°gold, typical (see gold, clean) 66°intestinal mucosa 50-60°kaolin 42-46°platinum 40°silicon nitride 28-30°silver iodide 17°soda-lime glass <15°gold, clean <10°

*Note: Contact angles for silanes refer to smooth treated surfaces.

Figure 6 | Critical surface tensions.

γc

heptadecafluorodecyltrichlorosilane 12.0polytetrafluoroethylene 18.5octadecyltrichlorosilane 20-24methyltrimethoxysilane 22.5nonafluorohexyltrimethoxysilane 23.0vinyltriethoxysilane 25paraffin wax 25.5ethyltrimethoxysilane 27.0propyltrimethoxysilane 28.5glass, soda-lime (wet) 30.0poly(chlorotrifluoroethylene) 31.0poly(propylene) 31.0poly(propylene oxide) 32polyethylene 33.0trifluoropropyltrimethoxysilane 33.53-(2-aminoethyl)-aminopropyl-

trimethoxysilane 33.5poly(styrene) 34p-tolyltrimethoxysilane 34

cyanoethyltrimethoxysilane 34aminopropyltriethoxysilane 35polymethylmethacrylate 39polyvinylchloride 39phenyltrimethoxysilane 40.0chloropropyltrimethoxysilane 40.5mercaptopropyltrimethoxysilane 41glycidoxypropyltrimethoxysilane 42.5polyethyleneterephthalate 43poly(ethylene oxide) 43-45copper (dry) 44aluminum (dry) 45iron (dry) 46nylon 6/6 45-6glass, soda-lime (dry) 47silica, fused 78titanium dioxide (anatase) 91ferric oxide 107tin oxide 111

Note: Critical surface tensions for silanes refer to smooth treated surfaces.

The contact angle is given by Young’s equation:γsv – γsl = γlv • cosθe

where γsl = interfacial surface tension, γlv = surfacetension of liquid.

Hydrophilic behavior is generally observed by sur-faces with critical surface tensions greater than 45dynes/cm. As the critical surface tension increases,the expected decrease in contact angle is accompaniedwith stronger adsorptive behavior and with increasedexotherms associated with the adsorption.

Hydrophobic behavior is generally observed by sur-faces with critical surface tensions less than 35dynes/cm. At first, the decrease in critical surface ten-sion is associated with oleophilic behavior, i.e., thewetting of the surfaces by hydrocarbon oils. As thecritical surface tensions decrease below 20 dynes/cm,the surfaces resist wetting by hydrocarbon oils and areconsidered oleophobic as well as hydrophobic.

Silane treatment has allowed control of thixotropicactivity of silica and clays in paint and coating applica-tions. In the reinforcement of thermosets and thermo-plastics with glass fibers, one approach for optimizingreinforcement is to match the critical surface tension ofthe silylated glass surface to the surface tension of thepolymer in its melt or uncured condition. This has beenmost helpful in resins with no obvious functionalitysuch as polyethylene and polystyrene. Growth of manymicrobial organisms is reduced on surfaces treated withalkylsilanes of C8 or less substitution due not only towettability factors but because dry surfaces depriveorganisms of metabolic water requirements.

How Does a Silane Modify a Surface?Most of the widely used organosilanes have oneorganic substituent and three hydrolyzable sub-stituents. In the vast majority of surface treatmentapplications, the alkoxy groups of the trialkoxysi-lanes are hydrolyzed to form silanol-containingspecies. Reaction of these silanes involves four steps.Initially, hydrolysis of the three labile groups occurs.Condensation to oligomers follows. The oligomersthen hydrogen bond with OH groups of the substrate.Finally, during drying or curing, a covalent linkage isformed with the substrate with concomitant loss ofwater (Figure 7). Although described sequentially,these reactions can occur simultaneously after theinitial hydrolysis step.

At the interface, there is usually only one bondfrom each silicon of the organosilane to the sub-strate surface. The two remaining silanol groups arepresent either in condensed or free form. The Rgroup remains available for covalent reaction orphysical interaction with other phases.

Hydrophobicity, Hydrophilicity and Silanes

B. Arkles, CHEMTECH, 7, 766, 1977

Figure 7 | Hydrolytic deposition of silanes.

Δ - CH3OH

SiH3C

OCH3

CH3

R

+

O

Si CH3H3C

R

OH

Figure 8 | Anhydrous deposition of silanes.

Silanes can modify surfaces under anhydrous condi-tions consistent with monolayer and vapor phase depo-sition requirements (Figure 8). Extended reaction times(4-12 hours) at elevated temperatures (50-120 °C) aretypical. Of the alkoxysilanes, only methoxysilanes areeffective without catalysis. The most effective silanes forvapor phase deposition are cyclic azasilanes.

Hydrolysis ConsiderationsWater for hydrolysis may come from several sources. Itmay be added, it may be present on the substrate sur-face, or it may come from the atmosphere. The degreeof polymerization of the silanes is determined by theamount of water available and the organic sub-stituent. If the silane is added to water and has low sol-ubility, a high degree of polymerization is favored.Multiple organic substitution, particularly if phenyl ortertiary butyl groups are involved, favors formation ofstable monomeric silanols.

The thickness of a polysiloxane layer is also deter-mined by the concentration of the siloxane solution.Although a monolayer is generally desired, multilayeradsorption results from solutions customarily used. Ithas been calculated that deposition from a 0.25%silane solution onto glass could result in three to eightmolecular layers. These multilayers could be eitherinter-connected through a loose network structure, orintermixed, or both, and are, in fact, formed by mostdeposition techniques. The orientation of functionalgroups is generally horizontal, but not necessarily pla-nar, on the surface of the substrate.

The formation of covalent bonds to the surface pro-ceeds with a certain amount of reversibility. As wateris removed, generally by heating to 120 °C for 30 to 90minutes or evacuation for 2 to 6 hours, bonds mayform, break, and reform to relieve internal stress.

Selecting a Silane for Surface ModificationInorganic Substrate Perspective

Factors influencing silane surface modification selec-tion include:• Concentration of surface hydroxyl groups;• Type of surface hydroxyl groups;• Hydrolytic stability of the bond formed; and• Physical dimensions of the substrate or substrate

features (Figure 9).Surface modification is maximized when silanes

react with the substrate surface and present the max-imum number of accessible sites with appropriate sur-face energies. An additional consideration is the phys-ical and chemical properties of the interphase region.The interphase can promote or detract from total sys-tem properties depending on its physical properties

such as modulus or chemical properties such aswater/hydroxyl content.

Hydroxyl-containing substrates vary widely in con-centration and type of hydroxyl groups present. Freshlyfused substrates stored under neutral conditions have aminimum number of hydroxyls. Hydrolytically derivedoxides aged in moist air have significant amounts ofphysically adsorbed water which can interfere withcoupling. Hydrogen bonded vicinal silanols react morereadily with silane coupling agents, while isolated orfree hydroxyls react reluctantly.

Silanes with three alkoxy groups are the usual start-ing point for substrate modification. These materialstend to deposit as polymeric films, effecting total cov-erage and maximizing the introduction of organicfunctionality. They are the primary materials utilizedin composites, adhesives, sealants and coatings. Limi-tations intrinsic in the utilization of a polylayer depo-sition are significant for nano-particles or nano-com-posites where the interphase dimensions generated bypolylayer deposition may approach those of the sub-strate. Residual (non-condensed) hydroxyl groupsfrom alkoxysilanes can also interfere in activity.Monoalkoxy-silanes provide a frequently used alterna-tive for nano-featured substrates since deposition islimited to a monolayer.

If the hydrolytic stability of the oxane bond betweenthe silane and the substrate is poor or the applicationis in an aggressive aqueous environment, dipodalsilanes often exhibit substantial performance improve-

OHH

OHO

O

OH

O

H

H

H

O

H

EXCELLENT

GOOD

SLIGHT

POOR

SUBSTRATES

SilicaQuartzGlassAluminum (AlO(OH))Alumino-silicates (e.g. clays)SiliconCopperTin (SnO)TalcInorganic Oxides (e.g. Fe2O3, TiO2,Cr2O3)Steel, IronAsbestosNickelZincLeadMarble, Chalk (CaCO3)Gypsum (CaSO4 )Barytes (BaSO4)GraphiteCarbon Black

Estimates for Silane Loading on Siliceous Fillers

Average Particle Size Amount of Silane(minimum of monolayer coverage)

1.5%<1 micron1.0%1-10 microns

10-20 microns 0.75%0.1% or less>100 microns

Water droplets on a (heptadecafluoro-1,1,2,2-tetrahy-drodecyl)trimethoxysilane-treated silicon wafer exhibit high contact angles, indicative of the low surface energy. Surfaces are both hydrophobic andresist wetting by hydrocarbon oils. (Water droplets contain dye for photographic purposes).

Silane Effectiveness on Inorganics

Figure 9 | Silane selection.

ments. These materials form tighter networks andmay offer up to 105 x greater hydrolysis resistancemaking them particularly appropriate for primerapplications.

Hydrophobic Silane Surface TreatmentsFactors that contribute to the ability of an organosi-lane to generate a hydrophobic surface are itsorganic substitution, the extent of surface coverage,

residual unreacted groups (both from the silane andthe surface) and the distribution of the silane on thesurface (Figure 10).

Aliphatic hydrocarbon substituents or fluorinatedhydrocarbon substituents are the hydrophobic entitiesthat enable silanes to induce surface hydrophobicity.Beyond the simple attribute that in order to generate ahydrophobic surface the organic substitution of thesilane must be non-polar, more subtle distinctions canbe made. The hydrophobic effect of the organic substi-tution can be related to the free energy of transfer ofhydrocarbon molecules from an aqueous phase to ahomogeneous hydrocarbon phase. For non-polar enti-ties, van der Waals interactions are predominant fac-tors in interactions with water and such interactionscompete with hydrogen bonding in ordering of watermolecules. Van der Waals interactions for solid surfacesare primarily related to the instantaneous polarizeabil-ity of the solid, which is proportional to the dielectricconstant or permitivity at the primary UV absorptionfrequency and the refractive index of the solid. Entitiesthat present sterically closed structures that minimizevan der Waals contact are more hydrophobic thanopen structures that allow van der Waals contact.Thus, in comparison to polyethylene, polypropyleneand polytetrafluoroethylene are more hydrophobic.Similarly, methyl-substituted alkylsilanes and fluori-nated alkylsilanes provide better hydrophobic surfacetreatments than linear alkyl silanes.

Surfaces to be rendered hydrophobic usually arepolar with a distribution of hydrogen bonding sites. Asuccessful hydrophobic coating must eliminate or mit-igate hydrogen bonding and shield polar surfaces frominteraction with water by creating a non-polar inter-phase. Hydroxyl groups are the most common sites forhydrogen bonding. The hydrogens of hydroxyl groupscan be eliminated by oxane bond formation with anorganosilane. The effectiveness of a silane in reactingwith hydroxyls impacts hydrophobic behavior notonly by eliminating the hydroxyls as water adsorbingsites, but also by providing anchor points for the non-polar organic substitution of the silane, which shieldsthe polar substrates from interaction with water.

Strategies for silane surface treatment depend on thepopulation of hydroxyl groups and their accessibilityfor bonding. A simple conceptual case is the reaction oforganosilanes to form a monolayer. If all hydroxylgroups are capped by the silanes and the surface iseffectively shielded, a hydrophobic surface is achieved.Practically, not all of the hydroxyl groups may react,leaving residual sites for hydrogen bonding. Further,there may not be enough anchor points on the surfaceto allow the organic substituents to effectively shieldthe substrate. Thus the substrate reactive groups of the

Hydrophobicity, Hydrophilicity and Silanes

Figure 11 | Hydrophobicity vs water permeability.

Although silane- and silicone-derived coatings are ingeneral the most hydrophobic, they maintain a highdegree of permeability to water vapor. This allowscoatings to breathe and reduce deterioration at thecoating interface associated with entrapped water.Since ions are not transported through non-polarsilane and silicone coatings, they offer protection tocomposite structures ranging from pigmentedcoatings to rebar-reinforced concrete.

complete coverage

incomplete hydroxyl reaction

= (CH 3)3Si = trimethylsilyl

Pyrogenic silica has 4.4-4.6 OH/nm2. Typically less than 50% are reacted. Other substrates have feweropportunities for reaction.

Figure 10 | Hypothetical trimethylsilylated surfaces.

silane, the conditions of deposi-tion, the ability of the silane toform monomeric or polymeric lay-ers and the nature of the organicsubstitution all play a role in ren-dering a surface hydrophobic. Theminimum requirements ofhydrophobicity and economicrestrictions for different applica-tions further complicate selection.

Hydrophobicity is frequently associated witholeophilicity, the affinity of a substance for oils, sincenon-polar organic substitution is often hydrocarbon innature and shares structural similarities with manyoils. The hydrophobic and oleophilic effect can be dif-ferentiated and controlled. At critical surface tensionsof 20-30, surfaces are wetted by hydrocarbon oils andare water repellent. At critical surface tensions below20, hydrocarbon oils no longer spread and the sur-faces are both hydrophobic and oleophobic. The mostoleophobic silane surface treatments have fluorinatedlong-chain alkyl silanes and methylated medium-chain alkyl silanes.

Superhydrophobic surfaces are those surfaces thatpresent apparent contact angles that exceed the theoret-ical limit for smooth surfaces, i.e., >120°. The most com-mon examples of superhydrophobicity are associatedwith surfaces that are rough on a sub-micron scale andcontact angle measurements are composites of solid sur-face asperities and air, denoted the Cassie state (Fig-ure12). Perfectly hydrophobic surfaces (contact anglesof 180°) have been prepared by hydrolytic deposition ofmethylchlorosilanes as microfibrillar structures.

Hydrophilic Silane Surface TreatmentsThe vast majority of surfaces are hydrophilic, andwater is omnipresent in the environment, yet the pre-cise nature of interaction of water with specific sur-faces is largely unknown. Water adsorption may beuniform or in isolated patches. It may be driven by anumber of different physical and chemical processes.The adsorption of water by a surface may be assistedor retarded by other adsorbents present in the envi-ronment. The purpose of applying a hydrophilic sur-face treatment is to control the nature and extent ofinteraction of water with a surface.

The controlled interaction of water with substratescan offer various degrees of hydrophilicity, rangingfrom physi-sorption to chemi-sorption and centers forion interaction. The utility of hydrophilic surfacesvaries widely. Anti-fog coatings exploit high surfaceenergies to flatten water droplets rather than allowingthem to form light-scattering droplets. In biologicalsystems, hydrophilic surfaces can reduce nonspecific

bonding of proteins. Hydrophiliccoatings with hydrogen bondingsites allow formation of tightlyadherent layers of water withhigh lubricity in biological sys-tems and the ability to resist oiladsorption in anti-graffiti coat-ings. They can also be used to dis-perse particles in aqueous coat-ings and oil-in-water emulsions.

Hydrophilic coatings with ionic sites form antistaticcoatings, dye receptive surfaces and can generate con-ductive or electrophoretic pathways. Thick films canbehave as polymeric electrolytes in battery and ionconduction applications.

In general, surfaces become more hydrophilic in theseries: non-polar < polar, no hydrogen-bonding <polar, hydrogen-bonding < hydroxylic, < ionic. Thenumber of sites and the structure and density of theinterphase area also have significant influence onhydrophilicity.

Much of the discussion of hydrophobicity centersaround high contact angles and their measurement.As a corollary, low or 0° contact angles of water areassociated with hydrophilicity, but practically thecollection of consistent data is more difficult. Dis-criminating between surfaces with a 0° contactangle is impossible. The use of heat of immersion is amethod that generates more consistent data for solidsurfaces, provided they do not react with, dissolve orabsorb the tested liquid (Figure 13). Another impor-tant consideration is whether water adsorbed is“free” or “bound.” Free water is water that is readilydesorbed under conditions of less than 100% relativehumidity. If water remains bound to a substrateunder conditions of less than 100% relative humid-ity, the surface is considered hygroscopic. Anotherdescription of hygroscopic water is a boundary layer

Figure 13 | Heats of immersion.

Heats of Immersion in Water mJ/m2

Titanium dioxide 225-250Talc 220-260Aminopropyltriethoxysilane* 230-270Silicon dioxide 210-225Glass 200-205Vinyltris(methoxyethoxy)silane* 110-190Mercaptopropyltrimethoxysilane* 80-170Graphite 32-35Polytetrafluoroethylene 24-25

*Data for silane treated surfaces in this table is primarily from B.

Marciniec et al, Colloid & Polymer Science, 261, 1435, 1983 recal-

culated for surface area.

Superhydrophobic Surface

Figure 12 | Cassie state.

of water adsorbed on a surface less than 200 nmthick that cannot be removed without heating. Ameasure of the relative hygroscopic nature of sur-faces is given by the water activity, the ratio of thefugacity, or escaping tendency, of water from a sur-face compared to the fugacity of pure water.

The hydrophilicity of a surface as measured ordetermined by contact angle is subject to interferenceby loosely bound oils and other contaminants. Heatsof immersion and water activity measurements areless subject to this interference. Measurements ofsilane-modified surfaces demonstrate true modifica-tion of the intrinsic surface properties of substrates. Ifthe immobilized hydrophilic layer is in fact a thinhydrogel film, then swelling ratios at equilibriumwater absorption can provide useful comparative data.

Controlling hydrophilic interaction with silanesurface treatments is accomplished by the selectionof a silane with the appropriate hydrophilic substitu-tion. The classes of substitution are: polar, non-hydrogen bonding; polar, hydrogen bonding; hydrox-ylic; and ionic-charged.

The selection of the class of hydrophilic substitutionis dependent on the application. If it is sufficient forwater to spread evenly over a surface to form a thinfilm that washes away and dries off quickly withoutleaving ‘drying spots’, then a polar aprotic silane ispreferred. If a coating is desired that reduces non-spe-cific binding of proteins or other biofoulants, then apolar hydrogen-bonding material such as a polyetherfunctional silane is preferred. A very different applica-tion for a polar non-hydroxylic material is thin filmproton conduction electrolytes. Lubricious coatingsare usually hydroxylic since they require a restrainedadsorbed phase of water. Antistatic coatings are usu-ally charged or charge-inducible as are ion-conductivecoatings used in the construction of thin-film batter-ies. A combination of hydrophilicity and hydrophobic-ity may be a requirement in coatings that are used asprimers or in selective adsorption applications such aschromatography. Formulation limitations mayrequire that hydrophilicity is latent and becomesunmasked after application.

Factors affecting the intrinsic hydrolytic stabilityof silane-treated surfaces are magnified when thewater is drawn directly into the interface. Even puresilicon dioxide is ultimately soluble in water (at alevel of 2-6 ppm), but the kinetics, low concentrationfor saturation and phase separation, make this anegligible consideration in most applications. Theequilibrium constant for the rupture of a Si-O-Sibond by water to two Si-OH bonds is estimated at10-3. Since at a minimum three Si-O-Si bonds mustbe simultaneously broken under equilibrium condi-

Hydrophobicity, Hydrophilicity and Silanes

Figure 14 | Range of water interaction with surfaces.

Interaction Description Surface Measurement -Example Parameter

Low Superhydrophobic Contact angleOleophobic FluorocarbonLipophobicOleophilic Water-sliding angleLipophilic Hydrocarbon Critical surface tensionHydrophobic

Moderate Polar Polymer Heat of immersionHydrophilic Oxide surfaceHygroscopic Polyhydroxylic Water activity

Strong Hydrogel film Equilibrium water Absorption swell

Figure 17 | Dipodal silane structure.

Figure 16 |

Common leaving groups

.

Bond Dissociation Energy(kcal/mole)

Me3Si-NMe2 98Me3Si-N(SiMe3)2 109Me3Si-Cl 117Me3Si-OMe 123Me3Si-OEt 122Me3Si-OSiMe3 136

Figure 15 |

Bond dissociation energies

.

Type Advantage Disadvantage

Dimethylamine Reactive, ToxicVolatile byproduct

Hydrogen chloride Reactive, CorrosiveVolatile byproduct

Silazane (NH3) Volatile Limited Availability

Methoxy Moderate reactivity, Moderate Neutral byproduct Toxicity

Ethoxy Low toxicity Lower reactivity

tions to dissociate an organosilane from a surface, inhydrophobic environments the long-term stability isa minor consideration. Depending on the conditionsof exposure to water of a hydrophilic coating, thelong-term stability can be an important considera-tion. Selection of a dipodal, polypodal or other net-work-forming silane as the basis for inducinghydrophilicity or as a component in the hydrophilicsurface treatment is often obligatory (Figure 14).

Reacting with the SubstrateLeaving Groups

The reaction of an organofunctional silane with a sur-face-bearing hydroxyl group results in a substitutionreaction at silicon and the formation of the silylatedsurface where the silicon is covalently attached to thesurface via an oxygen linkage. This connection may beformed directly or in the presence of water through areactive silanol intermediate. In general, the reactivityof hydroxylated surfaces with organo-functionalsilanes decreases in the order: Si-NR2 > Si-Cl > Si-NH-Si > Si-O2CCH3 > Si-OCH3 > Si-OCH2CH3. An analysisof the relevant bond energies indicates that the for-mation of the Si-O-surface bond is the driving force forthe reaction under dry and aprotic conditions. Sec-ondary factors contributing to the reactivity oforganofunctional silanes with a surface are: thevolatility of the byproducts; the ability of the byprod-uct to hydrogen bond with the hydroxyls on the sur-face; the ability of the byproduct to catalyze furtherreactions, e.g., HCl or acetic acid; and the steric bulk ofthe groups on the silicon atom (Figures 15 and 16).

Although they are not the most reactive organosi-lanes, the methoxy and ethoxysilanes are the mostwidely used organofunctional silanes for surface mod-ification. The reasons for this include the fact that theyare easily handled and the alcohol byproducts arenon-corrosive and volatile. The methoxysilanes arecapable of reacting with substrates under dry, aproticconditions, while the less reactive ethoxysilanesrequire catalysis for suitable reactivity. The low toxic-ity of ethanol as a byproduct of the reaction favors theethoxysilanes in many commercial applications. Thevast majority of organofunctional silane surface treat-ments are performed under conditions in which wateris a part of the reaction medium, either directly addedor contributed by adsorbed water on the substrate oratmospheric moisture.

Special Topic - Dipodal SilanesFunctional dipodal silanes and combinations of non-functional dipodal silanes with functional silanes havesignificant impact on substrate bonding, hydrolyticstability and mechanical strength of many composites

systems. They possess enabling activity in many coat-ings, particularly primer systems and aqueous immer-sion applications. The effect is thought to be a result ofboth the increased crosslink density of the interphaseand a consequence of the fact that the resistance to

(C2H5O)3 Si CH2CH2 Si(OC 2H 5)3

(C2H5O)3Si CH2 CH2CH2CH2CH2CH2CH2CH2 Si(OC2H5)3

SIB1817.0

SIB1824.0

SIB1831.0

Si(OCH3 )3

Si(OCH3)3

SIB1829.0

CH2CH2 Si(OCH3)3

CH2CH2(CH3 O) 3Si

Figure 18 | Hydrophobic dipodal silanes.

SIB1834.0SIB1833.0

N

H

(CH 3O) 3Si

CH2

CH2

CH2

CH2CH2 N

H

Si(OCH3)3

CH2

CH2

CH2

N

H

CH2

CH2

CH2

Si(OCH3)3(CH 3O) 3Si

CH2

CH2

CH2

NCH2

CH2

CH2

Si(OC2 H 5 )3

H

O

C(CH 2CH 2O)OH

O

CN

CH2

CH2

CH2

(C2 H 5 O) 3Si

n

SIB1824.82

(CH 3O) 3SiCH 2CH2CH 2

NCH 2CH 2OH

CH2

CH2

NCH 2CH2OH

(CH 3O) 3SiCH 2CH2CH2

SIB1142.0

Figure 19 | Hydrophilic dipodal silanes.

Wet adhesion to metals (N/cm)

Primer on metal10% in i leets dellor-dloCmuinatiTHOrP-

liNliNenalis oNMethacryloxypropylsilane 0.25 7.0Methacryloxypropylsilane + 10% dipodal 10.75 28.0

(cohesive failure)

90º peel strength after 2 h in 80ºC water.

P. Pape et al, in Silanes and Other Coupling Agents, ed. K. Mittal, 1992, VSP, p105

Figure 20 | Effect of dipodal –SiCH2CH2Si- on the bond strength of acrosslinkable ethylene-vinyl acetate primer formulation.

Note: Designations are Gelest product codes.

Note: Designations are Gelest product codes.

hydrolysis of dipodal materials (with the ability toform six bonds to a substrate) is estimated at close to100,000 times greater than conventional couplingagents (with the ability to form only three bonds to asubstrate) (Figures 17-20).

Both because dipodal silanes may not have functionalgroups identical to conventional coupling agents orbecause of economic considerations, conventional cou-pling agents are frequently used in combination with anon-functional dipodal silanes. In a typical application adipodal material such as bis(triethoxysilyl)ethane is com-bined at a 1:5 to 1:10 ratio with a traditional couplingagent. It is then processed in the same way as the tradi-tional silane coupling agent. �

ReferencesSilane Coupling Agents – General References and

Proceedings

1. Arkles, B. Tailoring Surfaces with Silanes, CHE MTECH

1977, 7, 766-778.

2. Plueddemann, E. Silane Coupling Agents, Plenum, 1982.

3. Mittal, K. Silanes and Other Coupling Agents, VSP, 1992.

4. Leyden, D.; Collins, W. Silylated Surfaces, Gordon & Breach, 1980.

5. Leyden, D. Silanes, Surfaces and Interfaces, Gordon &

Breach, 1985.

6. Steinmetz, J.; Mottola, H. Chemically Modified Surfaces, Else-

vier, 1992.

7. Blitz, J.; Little, C. Fundamental & Applied Aspects of Chemi-

cally Modified Surfaces, Royal Society of Chemistry, 1999.

Substrate Chemistry – General Reference and Proceedings

8. Iler, R. The Chemistry of Silanes, Wiley, 1979.

9. Pantelides, S.; Lucovsky, G. SiO2 and Its Interfaces, MRS

Proc. 105, 1988.

Hydrophobicity and Hydrophilicity

10. Tanford, C. The Hydrophobic Effect, Wiley, 1973.

11. Butt, H.; Graf, K.: Kappl, M. Physics and Chemistry of Inter-

faces, Wiley, 2003.

12. Adamson, A. Physical Chemistry of Surfaces, Wiley, 1976.

13. Fowkes, F. Contact Angle, Wettability and Adhesion, Amer-

ican Chemical Society, 1964.

14. Quere, D. Non-sticking Drops Rep. Prog. Phys. 2005, 68,

2495.

15. McCarthy, T. A Perfectly Hydrophobic Surface, J. Am. Chem.

Soc., 2006, 128, 9052.

Hydrophobicity, Hydrophilicity and Silanes