ARTICLE pubs.acs.org/JPCC
Electrostatics and Metal Oxide Wettability Gary Hanly, Daniel Fornasiero, John Ralston,* and Rossen Sedev Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, Adelaide, SA 5095 ABSTRACT: The wettability of a titania surface, whose surface was partially covered with a strongly based octadecyltrihydrosilane, was studied above and below the isoelectric point (or pHiep). The advancing water contact angle is at a maximum at the pHiep, decreasing symmetrically on either side in a Lippmannlike manner. The change in wettability, for a given pH, became more pronounced with increasing salt concentration. Using a non-Nernstian model of the electrical double layer, the experimental dependence of contact angle on both pH and salt concentration was satisfactorily predicted.
’ INTRODUCTION Wetting is strongly influenced by electrostatics, irrespective of whether an external voltage is applied across a solidliquid interface, as in the case of electrowetting,1,2 or the spontaneous formation of an electrical double layer, due to the adsorption of potential determining ions, which causes a decrease in contact angle.3,4 Moller was the first to investigate how electrical polarization of a metal/solution interface influenced its contact angle.5 Contact anglepotential capillary curves were obtained for a range of noble metals, with a maximum occurring at the same potential for all metals examined. Frumkin et al.6 confirmed the parabolic dependence of contact angle on potential, but found that the maxima occurred at different potentials for different metals.7 For the mercuryelectrolyte system, there are a number of studies existing where the influence of electrostatics has been critically examined.8,9 In these systems, the celebrated Lippmann equation illustrates the dependence of the interfacial tension of the mercurysolution interfaces on potential and charge:10 ! ∂γHg-sol ¼ σo ∂E T, μS
where E is the applied potential, σ0 is the charge per unit area at the mercurysolution interface, T is the absolute temperature, and μs is the chemical potential of the supporting electrolyte. When the YoungDupre equation is invoked, the dependence of contact angle on E and σ0 is obvious.8,9 For solidliquid interfaces, the role of the electrical double layer in influencing wettability has received scant attention. The dependence of the solidliquid interfacial tension for solid Fe2O3 in contact with an aqueous solution has been examined, with the interfacial tension reaching a maximum at the point of r 2011 American Chemical Society
zero charge.11 Hough and Ottewill3 and Laskowski and Kitchener12 showed that the contact angle, θ, of silver iodide and methylated silica, measured through the aqueous phase, reached a maximum at the point of zero charge of the solid and was strongly dependent on the concentration of potential determining ions. Gribanova et al. conducted similar experiments on a variety of quartz and glass surfaces, observing a similar trend.13 Fokkink and Ralston4 used a simple electrical double layer model to describe the pAg dependence of θ while Chatelier et al.14 extended this treatment to plasma-deposited films. Vittoz et al.15 adopted an approach similar to that of Fokkink and Ralston when examining the wettability of silica and alumina. In the various studies reported to date, the key element that is lacking is the ability to examine a specific solidliquid interface where the number of hydrophobic and hydrophilic (charged and uncharged) groups is precisely controlled. pH and ionic strength may then be varied in a wide range around the point of zero charge, enabling the influence of the electrical double layer to be explored and related to the surface population of particular groups. TiO2 is an excellent candidate for this investigation, with a surface chemistry that is well-defined,16,17 a negligible solubility over a wide pH range, an easily accessible point of zero charge, and where surface modification is feasible using a silane that is chemically bound to the solid surface.18 In this study, we have studied the θ (pH, ionic strength) dependence for hydrophobized TiO2 and developed a quantitative model linking the population of different surface groups to the wettability. The outcomes have wide ramifications for both static and dynamic contact angle behavior. Received: April 20, 2011 Revised: June 7, 2011 Published: June 28, 2011 14914
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’ FREE ENERGY OF FORMATION OF AN ELECTRICAL DOUBLE LAYER AND ITS IMPACT UPON WETTABILITY a. General Approach. The approach here is based on the change in electrochemical potential, μ̅ , when a potential determining ion is transferred from the bulk solution to an initially uncharged surface. If dΓ moles per unit area are transferred, then the change in free energy, dF can be obtained. Recall that μ̅ consists of both chemical and electrochemical parts. The process occurs of course at constant temperature, volume, surface area, and overall composition. We proceed by calculating the work required to form the electrical double layer from a system which is in an uncharged state initially. [The information contained in the numerous references cited in this paper is widely dispersed, frequently uses different symbols and generally covers only one or two of the theory aspects. We have developed one coherent set of equations and arguments here to help both current and future readers.] The change in surface force energy per unit area, dFdl, when dΓ moles per unit area are transferred from the bulk solution to the surface is given by (following Verwey and Overbeek,19 Grahame,20 Chan and Mitchell,21 Hunter,22 Trauble et al.23 and Payens,24 with the more recent insights of Biesheuvel)25
dF dl ¼ ðμ̅ μ̅ Þ dΓ s
ð1Þ
B
where μ̅ and μ̅ are, respectively, the electrochemical potential of the potential-determining ions at the surface (s) and in the bulk solution (B). Furthermore, s
To evaluate the first integral, the functional form of ψo (σ) is required. For a simple GouyChapman model22 applied to a flat interface, 2kT eσ D 1 sinh ψo ðσÞ ¼ ψo ðσÞ ¼ ð7Þ e 2kεεo kT where ψD o (σ) is a particular potentialcharge relationship that reflects the ion distribution in the diffuse electrical double layer, k is the Boltzmann constant, T is the absolute temperature, ε is the dielectric constant of water, εo is the permittivity of free space, and k is the Debye reciprocal length. In the case of the second integral, the bulk electrochemical potential of the adsorbing ions, μ̅ B is taken to be constant. The amount of adsorbed charge is σ ¼ qΓ thus we can define a function ψso(σ) by qψso ðσÞ ¼ ½μs ðΓÞ μ̅ B ¼ ½μs ðσ=qÞ μ̅ B
ð2Þ
where ψo is the surface electrostatic potential and Ze is the charge of the potential determining ion, with Z as the ion valency and e as the electronic charge. Thus the specific surface free energy ΔFdl is Z σo Z Γo ΔF dl ¼ ψo ðσÞ dσ þ ðμs μ̅ B Þ dΓ ð3Þ o
o
where the surface charge density σo is given by σo ¼ ZeΓo ¼ qΓo
ð4Þ
and Γo is the equilibrium surface concentration of potential determining ions. The first integral on the right-hand side (rhs) of eq 3 is the electrical work done in creating the electrical double layer, while the second integral is called the chemical component of the free energy. Assuming that μs ¼ μs ðΓÞ
ð5Þ
i.e., the chemical component of μ̅ s depends only on the quantity of ions adsorbed and not on the surface potential ψo, this means that all electrostatic contributions to μ̅ s in eq 2 are described by qψo while μs contains nonelectrostatic contributions, such as dispersion interactions, ionsolvent interactions, and the like, as noted elsewhere.25,26 ΔFdl now becomes Z σo Z Γo ΔF dl ¼ ψo ðσÞ dσ þ ðμs ðΓÞ μ̅ B Þ dΓ ð6Þ o
ð9aÞ ð9bÞ
If the amount of adsorbed charge is σ, then the surface potential is ψso(σ) and at equilibrium ψDo ðσo Þ ¼ ψso ðσ o Þ
B
μ̅ s ¼ μs þ Zeψo
ð8Þ
ð10Þ
thus
2kT εσo 1 s σo 1 B sinh ¼ μ μ̅ e q 2kεεo kT e
for a planar electrical double layer. Using eqs 8 and 9, eq 6 now appears as Z σo fψDo ðσÞ ψso ðσÞg dσ ΔF dl ¼
ð11Þ
ð12Þ
o
b. Nernstian Surfaces, Constant Potential. Recalling the approach of Verwey and Overbeek,19 μs(Γ), the chemical component of the surface electrochemical potential, is assumed to be independent of Γ, the moles per unit area of potentialdetermining ions adsorbed. This means in eq 9 that ψso = ψo, the constant potential case. Thus eq 12 becomes Z σo ΔF dl ¼ ψo ðσÞ dσ σo ψo ð13Þ o
where, as noted by Verwey and Overbeek19 and Hunter,22 σoψo is the chemical free energyR decrease per unit area of surface, Δμchem, whereas the term σo oψo(σ) dσ represents the electrical work and is equivalent to charging a capacitor. Thus chem ΔF dl ¼ ΔF elec dl þ ΔF dl
ð14Þ
for constant potential. Note that ΔFchem is independent of the dl state of charge of the surface, which is true only at low charge density (see below for the more general case). By partial integration, eq 13 becomes Z ψo σ dψ ð15Þ ΔF dl ¼ -
o
o
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We observe that the free energy is always negative, as expected. If the potential is low, where the DebyeHuckel approximation holds, then, using σo = εkψo 1 1 ΔF dl ¼ εεo kψ2o ¼ σ o ψo 2 2
ð16Þ
For large potentials, σo increases with exp(Zeψo/2kT) and thus ΔFdl falls between the limits of 1/2σoψo and σoψo. In eq 15, inserting σo = (2kTεεok/e) sinh(Zeψo/2kT) yields 4k2 T 2 εεo k Zeψo ΔF dl ¼ cosh 1 ð17Þ Ze2 2kT as the free energy of formation of a single, isolated diffuse electrical double layer for the case of constant potential or Nernstian surfaces, where the chemical component of μ̅ i is constant and independent of Γ. c. Non-Nernstian Surfaces. This treatment stems from the work of Chan and Mitchell,21 with the original platform provided by Verwey and Overbeek19 and Payens,24 particularly. Chan and Mitchell21 demonstrated how to remove the restriction of low charge density surfaces. We consider a monoacidic surface with the following reaction: MOH S MO þ Hþ
The change in chemical potential for reaction18 is then Δμ ¼ Δμo þ kTðln½Hþ þ ln½MO ln½MOHÞ
with Δμo ¼ μo ðMO Þ þ μo ðHþ Þ μo ðMOHÞ ¼ kT ln K a2 Concentrations can be related to surface charge, σ, through the expression σ ¼ R 3 e 3 Ns and R¼
ð18Þ
Incorparation of eq 24 into eq 21 gives Z ( kT σo ½Hþ chem ΔF dl ¼ ln þ lnðeN s σÞ þ ln σg dσ e o K a2
Now ΔF elec dl ¼
After integration and rearrangement, eq 25 becomes14,21,27 (
ΔFchem dl
Z
σo o
þ
At equilibrium, Δμ = 0, and eq 24 becomes σo ½Hþ ln ¼0 þ ln K a2 eN s σo
ΔF dlchem Z
ψo dσ ¼ σo ψo
ψo
σ o dψ
o
and, using eq 7, ΔF elec dl ¼ σ o ψo εεo k
2kT e
2
cosh
eψo 1 2kT
ð20Þ
, Chan and Mitchell21 and Chatelier et al.14 show For Δchem dl that Z 1 σo ¼ Δμ dσ ð21Þ ΔF chem dl e o where, for any reaction, Δμ = Σμproducts Σμreactants. The chemical potentials of products and reactants in reaction 18 are then expressed as a function of their respective concentration, such as, for example, μðMO Þ ¼ μo ðMO Þ þ kT ln½MO
¼ kT 3 N s
½Hþ R ln þ ð1 RÞ lnð1 RÞ þ R ln R K a2
)
ð26Þ
where H+ refers to the potential-determining ions. Clearly, in eq 19, ΔF dl ¼
ð25Þ
ΓHþ
ΔF dlelec
½MO ½MOHT
where Ns is the number of surface sites, MOH and MO, and [MOH]T = [MOH] + [MO]. As R is always a positive quantity, we use the absolute value of σ. Then eq 23 becomes ( ) ½Hþ lnðeN s σÞ þ ln σ ð24Þ Δμ ¼ kT ln K a2
which has a dissociation constant Ka2. The surface charge density σo = e(ΓH+ ΓS), where ΓH+ is the amount of H+ adsorbed per unit area, and ΓS (= Ns) is the total number of ionizable groups per unit area. From eq 9a we can see that eψos = μ̅ B μs(ΓH+). Recalling eq 6, i.e. Z σo Z Γs ΔF dl ¼ ψo ðσÞ dσ þ ðμs μ̅ B Þ dΓHþ ð19Þ o
ð23Þ
ð22Þ
ð27Þ
The surface potential, Ψo can be expressed as a function of surface potential using the Boltzmann equation, [H+] = [H+]b 3 exp(eΨo/kT), and eq 27 ([H+]b is the proton concentration in the bulk solution). " # kT K a2 eN s ln ψo ¼ 1 ð28Þ e ½Hþ b σ o Combining eqs 7 and 28 gives " # eσo K a2 eN s 1 2 sinh 1 ¼0 þ ln 2kTεεo k ½Hþ b σo
ð29Þ
which can be solved to give σo for given values of Ns, Ka2, and [H+]b. A similar approach can be used for reaction at a monobasic surface with a dissociation constant Ka1: MOH þ Hþ S MOH2 þ
ð30Þ
with Δμ ¼ Δμo þ kTðln½MOH2 þ ln½MOH ln½Hþ Þ
where μ°(MO) is the standard chemical potential of MO. 14916
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Δμ ¼ kT ln
σ K a1 þ ln þ eN s σ ½H
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ð31Þ
K a1 ΔFdlchem ¼ kT 3 Ns R ln R þ ð1 RÞ lnð1 RÞ þ R ln þ ½H
ð32Þ At equilibrium, Δμ = 0, and eq 31 becomes σo K a1 ln þ ln þ ¼ 0 eN s σ o ½H
ð33Þ
The surface potential, Ψo, can be expressed as a function of surface potential using the Boltzmann equation for the proton concentration and eq 33. " # kT ½Hþ b eN s ln 1 ð34Þ ψo ¼ e K a1 σo Combining eq 7 and 34 gives " # eσo ½Hþ b eN s 1 1 ¼0 2 sinh ln 2kTεεo k K a1 σo
ð35Þ
which can be solved to give σo for given values of Ns, Ka1, and [H+]. From these values of σo, eq 7, 20, and 26 or 32 were then used to calculate the surface potential and the electrical and chemical components of the free energy of formation of the double layer, respectively. d. Applicability to Contact Angle Changes on Charged Surfaces. As shown by Fokkink and Ralston,4 in the presence of surface charge, the specific free energy of the solidliquid interface is γSL ¼ γ°SL þ ΔF dl
ð36Þ
where ΔFdl represents the free energy of formation of the ionizable surface relative to the point of zero charge (pzc) for an ionizable surface with acidic or basic groups, where H+ is the potential determining ion. Using the YoungDupre equation, cos θðpHÞ ¼ cos θðpzcÞ
ΔF dl γLV
ð37Þ
chem as noted above and elsewhere.3,7 where ΔFdl = ΔFelec dl + ΔFdl elec chem ΔFdl and ΔFdl are calculated through eqs 20 and 26 or 32, as outlined above.
’ MATERIALS AND METHODS Silicon wafers covered with a thin layer of TiO2 were produced by sputtering titanium in an argon/oxygen environment for 41 min using a radio-frequency (RF) magnetron source operating at 2 kW. Prior to sputtering, silicon wafers were dipped in hydrofluoric acid to remove any native oxide layer. X-ray photoelectron spectroscopy (XPS) detected only the presence of Ti and O in a 1:2 ratio (detection limit 0.1 atomic %). This indicated that the surface layer was pure titanium dioxide. X-ray diffraction showed that the deposited titanium dioxide was amorphous.17 Imaging by AFM showed a root-mean-square (rms) roughness of 0.3 nm with a peakto-valley height of 1.5 nm over 1 μm.28 The isoelectric point (iep), determined by streaming potential measurements, falls at 4.4 ( 0.1,
as we have described elsewhere, along with other characteristics.17,29 The TiO2-coated wafers were cut to a 10 mm 10 mm size with a diamond tip pen, cleaned in ethanol, submerged in a 10% w/w KOH solution for 1 min, then rinsed with copious amounts of highpurity water and finally dried under a stream of high purity nitrogen and plasma cleaned with a Harris Plasma Cleaner. A test performed on a set of wafers showed that water completely wets the surface. The high-purity water used in this study had a specific conductivity of less than 0.4 μS/cm, a surface tension of 72.8 mN/m at 20 °C, and a bubble residence time of less than 1 s. Octadecyltrihydrosilane (OTHS, CH3(CH2)17SiH3) was obtained from Sigma-Aldrich (97.5% purity) and was used as supplied. AR cyclohexane (Chem-Supply) was dried with molecular sieves (Sigma-Aldrich) for a minimum of 24 h before use. All salts, acids, and bases (HNO3 and KOH) were of analytical grade or better, and solutions were prepared daily. All glassware was cleaned in a warm solution of 30% potassium hydroxide for an hour, then washed thoroughly with high-purity water and placed in an oven at 110 °C for 2 h. Measurements were conducted in a Class 100 clean room at an ambient humidity of 45% and temperature of 22 °C. Surface Modification Using OTHS. Titania surfaces were hydrophobized using OTHS, using a method adapted from Fadeev and McCarthy.18 A stock solution of OTHS was prepared and diluted to the required concentration with cyclohexane. Freshly cleaned TiO2 wafers were immersed in an OTHS solution for 15 h (preliminary experiments showed that equilibrium was achieved within this period), washed several times with cyclohexane in an ultrasonic bath, followed by ethanol and then high-purity water. The TiO2 wafers were stored under high-purity water before use to minimize any adventitious contamination. Contact Angle Determinations. The contact angle was measured using the captive bubble technique. The sample was submerged in an aqueous solution of predetermined ionic strength that had been purged with high purity nitrogen gas for at least an hour. The cell was placed on an adjustable xyz stage, and a charge-coupled device (CCD) camera (Jai CV M10BX) was used to take tagged image file format (TIFF) images of the bubble placed on the silanated titania surface, using a frame-grabbing software package. Analysis was performed with an in-house software program. This instrumental arrangement minimized contamination of the sample and allowed easy manipulation of both the pH and the ionic strength. A clean stainless steel needle was used to produce a bubble of approximately 2 mm in diameter on the surface, and the advancing and receding contact angles, measured through the aqueous phase, were recorded. All measurements were performed at least in quadruplicate. The standard deviation in contact angle for all measurements was less than 3°. The pH of the electrolyte solution was adjusted to specified values by addition of small amounts of acid or base. Streaming Potential Measurements. Streaming potential measurements were performed using an apparatus based on the design of Scales et al.29 Electrolyte solution (104 and 103 M analytical grade KCl with a pH adjusted with analytical grade KOH or HCl) was circulated through the cell under constant hydrostatic pressure, P, and the streaming potential, E, was measured. The zeta potential, ζ, was calculated using the Smoluchowski equation: ζ¼ 14917
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Figure 1. Advancing contact angle of water on a titania surface as a function of pH at 103 M KNO3 and OTHS surface coverage; from top: 100%, 87%, 68%, 54%. The lines are shown to guide the eye.
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Figure 2. Advancing contact angle of water on a titania surface partially covered with OTHS (54% surface coverage) as a function of pH and KNO3 concentration. The lines are shown to guide the eye.
where λ is the conductivity of the capillary, ε0 is the permittivity of free space, η is the viscosity of the liquid, and ε is its dielectric constant. Two identical, clean (see above) plates were always used. All procedures have been described in detail elsewhere.30,31
’ RESULTS AND DISCUSSION a. Contact Angles for Titania Surfaces. The maximum water advancing and receding contact angles obtained were 118° ( 2° and 101° ( 1°, respectively, representing the contact angles of a titania wafer at maximum OTHS surface coverage. These results compare favorably with the respective values of 117° and 100° reported by Fadeev and McCarthy18 at maximum surface coverage of OTHS on titania, but are slightly higher than the advancing contact angle reported for a surface fully covered with methylterminated groups, ∼110°.32 The water advancing and receding contact angles decrease with decreasing OTHS concentration to 43° and 20°, respectively, at the smallest OTHS concentration currently used in these experiments. In the absence of OTHS, the titania advancing and receding contact angles were zero, i.e., the surface completely wets. The surface coverage of OTHS was determined using atomic force microscopy.28,31,33 The stability of the OTHS layer on titania surfaces of intermediate surface coverage was investigated as a function of pH.31 A change in pH could remove the OTHS layer and therefore affect the resulting contact angle.34 For titania surfaces with a partial coverage of OTHS (e.g., θa of 82°), there was no detectable change in wettability for pH values between 2 and 12 for immersion times up to 24 h. Thereafter the contact angle decreased by up to 11° over the next 1000 h. Ex situ XPS surface analysis and in situ tapping-mode atomic force microscopy investigations showed that there was no change in either surface composition or topography over a 24 h period. Water contact angles for titania surfaces as a function of OTHS surface coverage and pH are shown in Figure 1, and as a function of KNO3 concentration and pH at a fixed surface coverage of OTHS in Figure 2. At surface coverages less than 100%, both pH and salt concentration influence the wettability. In Figure 2, there is a maximum in the contact angle versus pH curve at pH values between 4.0 and 4.3. The contact angle
Figure 3. Surface potential as a function of pH and KNO3 concentration obtained from eq 17 and 37 with θ(pzc) = 61.9° (T = 22 °C, γLV = 0.0725 J m2; 2kT/e = 50.89 mV; k = 3.3 109 [KNO3]0.5 m1; z = 1 for H+). The lines are shown to guide the eye.
changes were rapid (reaching equilibrium as quickly as the pH could be altered) and reversible (shown by repeated cycling of the pH). The decrease in contact angle with change in pH is more pronounced at the higher salt concentration. Moreover, the contact angle appears to remain constant at pH values larger than 8 and 9 at 101 M and 103 M KNO3, respectively. b. Calculation of Contact Angles: Nernstian Surfaces at Constant Potential. Equations 17 and 37 were used to obtain Ψo (the only unknown value) from the experimental contact angles in Figure 2 and θ(pzc) = 61.9° at 103 and 101 M KNO3, using an iterative numerical method at each pH value. The value of θ(pzc) equal to 61.9° was assumed to be independent of KNO3 concentration and was estimated by taking an average of the highest contact angle values at pH around 3.94.6. Values of the surface potential, Ψo, as a function of pH are shown in Figure 3. The surface potential is zero at pH values between 3.9 and 4.6 where the curves for 101 M and 103 M KNO3 concentrations intercept (in agreement with the measured 14918
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Figure 4. (circles) Surface charge as a function of pH at KNO3 concentrations of (filled symbols) 0.001 M and (empty symbols) 0.1 M calculated from surface potential values in Figure 3 and eq 38. (triangles) Experimental surface charges of Yates et al.16 for TiO2 particles. The surface charges versus pH curves of Yates et al. were shifted to lower pH values so that their point of zero charge (pH 5.6) coincides with that in the present study (pH 4.4). The lines are shown to guide the eye.
Figure 5. Relationship between the experimental zeta potential and contact angle of water on OTHStitania particles at pH 10 (103 M KNO3). The line is shown to guide the eye.
iep of 4.5). The magnitudes of surface potential increase sharply as the pH is move away from this interception point, but remain more or less constant at pH values larger than 6 or less than 3. Figure 4 shows the values of surface charge, σo, calculated using eq 38 and the values of Ψo in Figure 3. 2kTεεo k eψo sinh σo ¼ ð38Þ e 2kT As expected, the surface charge increases in magnitude with increasing salt concentration, while the opposite is true for the surface potential (Figure 3). These values are compared with the experimental surface charge data of TiO2 particles of Yates and Healy16 following the scaling of pHpzc approach of Fokkink et al.35 The calculated surface charge values are more than 4 times larger than the experimental data. We do not expect a change in the electrical properties of TiO2 particles for OTHS coverage of less than 60% for θ less than 46° (see Figure 5), as predicted elsewhere.36 We also note that the surface charges remain constant for pH g 8, which is not expected for surface chargepH curves,
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Figure 6. (symbols) Experimental and (lines) calculated contact angle of water on titania wafer partially covered with OTHS (54%) in 101 M KNO3 as a function of pH and values of Ka1 and Ka2 (see text for explanation).
Figure 7. (symbols) Experimental and (lines) calculated contact angle of water on titania wafer partially covered with OTHS (54%) in 103 M KNO3 as a function of pH and values of Ka1 and Ka2 (Ns = 3 1018 sites/m2; see text for explanation).
which normally increase exponentially as the pH moves away from the point of zero charge.36,37 c. Calculation of Contact Angles: Non-Nernstian Surfaces at Constant Potential. Equations 20, 26 or 32, and 37 were used to calculate the contact angle as a function of pH using a nonlinear least-squares routine to fit the experimental contact angle versus pH data with Ka1 and Ka2 as fitting parameters. The calculated contact angles are compared with the experimental data for 101 M and 103 M KNO3 in Figures 6 and 7, respectively, and various values of Ka1 and Ka2. A site density Ns = 8 1018 sites/m238 obtained for rutile was used. It was found that the calculated contact angle was relatively insensitive to variations in site density; therefore, a Ns value of 3 1018 sites/m2, which takes account of the loss of (10054)% ionizable groups because of their reaction with OTHS (54% surface coverage by OTHS for the data in Figures 6 and 7) was used in the final calculations. For the 101 M KNO3 data in Figure 6, the fit of the experimental contact angles is good up to pH 7 for Ka1 and Ka2 values of 1.5 104 and 105 M1, respectively. At pH values 14919
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Figure 8. Calculated fraction of positively and negatively charged sites, R+ and R respectively, at the titania surface (Ka1 = 1.5 104 and Ka2 = 105 for 101 M KNO3; Ka1 = 2 105 and Ka2 = 104 for 103 M KNO3; Ns = 3 1018 sites/m2).
Figure 10. Calculated surface charge as a function of pH and KNO3 concentrations (Ka1 = 1.5 104 and Ka2 = 105 for 101 M KNO3; Ka1 = 2 105 and Ka2 = 104 for 103 M KNO3; Ns = 3 1018 sites/m2).
Figure 9. Calculated surface potential as a function of pH and KNO3 concentrations (Ka1 = 1.5 104 and Ka2 = 105 for 101 M KNO3; Ka1 = 2 105 and Ka2 = 104 for 103 M KNO3; Ns = 3 1018 sites/m2).
Figure 11. Calculated electrical and chemical components of the free energy of formation of the ionizable surface as a function of pH and KNO3 concentrations (Ka1 = 1.5 104 and Ka2 = 105 for 101 M KNO3; Ka1 = 2 105 and Ka2 = 104 for 103 M KNO3; Ns = 3 1018 sites/m2).
higher than 8, the experimental contact angles remain constant while the calculated ones decrease. A similar trend is observed at 103 M KNO3 in Figure 7 with Ka1 value of 2 105 M1 and Ka2 values between 105 and 103 M1, although the fit is rather poor. The contact angle data at pH values larger than 8 cannot be explained by this model. At pH 8, the surface charge is approximately 0.4 C/m2 at 101 M KNO3 (see Figure 10), which corresponds to an R value (proportion of charged surface sites) of 0.7 (see Figure 8). Figures 9 and 10 show surface potential and surface charge versus pH curves, respectively, calculated with Ka1 = 1.5 104 and Ka2 = 105 M1 for 101 M KNO3, and Ka1 = 2 105 M1 and Ka2 = 104 for 103 M KNO3. The magnitude of surface potential and surface charge, and trend with pH or electrolyte concentration are the same as those in Figures 3 and 4 calculated with the Nernstian approximation. The Ka1 and Ka2 values obtained in this study compare favorably to the values of Ka1 = 105.2 and Ka2 = 106.6 calculated
by Westall and Hohl39 for the experimental data of Yates et al.16 for rutile particles. The calculated electrical and chemical components of the free energy of formation of the double layer are shown in Figure 11 as a function of pH and KNO3 concentration. They both increase in magnitude as the pH moves away from the point of zero charge. ΔFelec is positive while ΔFchem is negative. ΔFchem is larger than ΔFelec. ΔFelec is relatively independent of salt concentration, while ΔFchem is slightly larger at 101 M KNO3.
’ SUMMARY This study is a combined experimental and theoretical approach aimed at explaining how electrostatics influence the wettability of metal oxides. The wettability of a titania surface, whose surface was partially covered with a strongly based OTHS, was studied above and 14920
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The Journal of Physical Chemistry C below the isoelectric point (or pHiep). The advancing water contact angle is a maximum at the pHiep, decreasing symmetrically on either side in a Lippmann-like manner. The change in wettability, for a given pH, became more pronounced with increasing salt concentration. Using a non-Nernstian model of the electrical double layer, the experimental dependence of contact angle on both pH and salt concentration was satisfactorily predicted. The outcomes of this investigation are important in many fields. We have shown that it is possible to alter the wettability of a coated metal oxide surface by simply changing the pH and/or salt concentration. This has wide ramifications for mineral flotation, pigment dispersion and formulation, as well as microfluidics.
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