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The Effect of the Nature of the Organic Cosolvent on Surface Charge Density of Silica in Mixed Solvents
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
179, 128–135 (1996)
0194
The Effect of the Nature of the Organic Cosolvent on Surface Charge Density of Silica in Mixed Solvents MAREK KOSMULSKI 1 Polish Academy of Sciences, Institute of Catalysis and Physical Chemistry of Interfaces, Laboratory of Adsorption and Physical Chemistry of Interfaces, Pl. M.C.Sklodowskiej 3, 20031 Lublin 41, Poland Received June 5, 1995; accepted October 9, 1995
Potentiometric titrations of silica in mixtures of water with alcohols, diols, polyalcohols, ketones, ethers, DMSO, DMF, and acetonitrile have been carried out. Usually, the negative surface charge s0 of silica at a given pH and ionic strength decreases when the concentration of the organic component in the mixed solvent increases. Polyalcohols are an exception: their presence does not affect s0 , and erythritol makes it even more negative. Generally, the absolute value of s0 of silica at a given concentration of an organic cosolvent is lower for the cosolvents with a longer hydrocarbon chain, e.g., water ú methanol ú ethanol, and higher for cosolvents with a higher number of polar groups, e.g., propanol õ propanediol õ propanetriol. The magnitude of s0 at a given pH, ionic strength, and organic cosolvent concentration is correlated with Reichardt’s ET (30) and Kosower’s Z and Z * polarity parameters; namely, the absolute value of s0 increases when these parameters increase. Even better correlation can be achieved when the solvent is characterized by a linear combination of one of three above-mentioned solvent parameters or a hydrogen bond donation ability with b hydrogen bond acceptance ability or with DN Gutman’s donor number. In solvents with high values of the later two parameters, the negative surface charge of silica is low. q 1996 Academic Press, Inc.
Key Words: silica; surface charge; mixed solvent; solvent polarity.
INTRODUCTION
Studies of colloids in nonaqeuous and mixed solvents have been recently reviewed (1–3). The dielectric constant e is the most common parameter used in colloid chemistry to characterize solvents (1–3). As a mater of fact, many properties related to colloid stability, z potentials etc., are entirely different in nonpolar (low e ) and polar (high e ) media even if the same dispersed solid is involved. On the other hand, it is well known that the dielectric constant often fails in explaining the solvent dependent physico-chemical properties and phenomena (4, 5). Therefore, several experimental scales have been introduced, based on heats of solvation, NMR shifts and solvatochromic shifts of specially selected 1
To whom correspondence should be addressed.
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a hydrogen bond donation ability b hydrogen bond acceptance ability p* polarity/polarizability parameter Reichardt’s ET (30) parameter DN, Gutman’s donor number AN, Gutman’s acceptor number Acity Basity Kosower’s Z and Z * polarity parameters.
ET (30), DN, Z, and Z * are expressed in kcal/mol, AN is a dimensionless parameter and ranges from 0 to 100 for most solvents, and the other dimensionless parameters range from 0 to 1. The available values of these parameters for different solvents have been recently summarized (4, 5). Unfortunately, a complete set of the 10 parameters is available only for a few solvents. Basically, the above solvent parameters can be measured not only for one-component solvents but also for solvent mixtures. The values of ET (30) for mixtures of many organic solvents with water are available. However, it is generally agreed that nonlinear dependence between ET (30) and mole fraction of organic component observed in many mixed solvent systems is chiefly due to preferential solvation of the solvatochromic dye. Thus, the values of ET (30) measured in solvent mixtures characterize the solvent composition near the chromophore group in the probe molecule rather than the bulk composition so application of such values to characterize processes in solvent mixtures is limited (4, 6). Therefore, in the present study, values of the parameters for pure organic solvents are used to characterize solvent mixtures with water even if corresponding data for the solvent mixture are also available. Many solvent dependent quantities are correlated with one of the mentioned above experimental solvent parameters. For many other quantities, the one-parameter approach fails,
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0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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probe molecule(s) in particular solvents. For many organic solvents, the values of the following parameters can be found in the literature:
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but a good correlation is observed with a properly selected linear combination of solvent scales (4, 5), (XYZ ) j Å (XYZ )0 / aAj / bBj / cCj / . . .
[1]
where XYZ is a solvent-dependent property, the subscripts 0 and j denote a medium in which A0 Å B0 Å C0rrr Å 0 (for most solvent scales this corresponds to gas phase or completely inert solvent) and the given solvent, respectively, uppercase letters denote parameters characterizing the solvent j, and lowercase letters denote parameters dependent only on the property XYZ. For certain combinations of solvent scales in Eq. [1], a considerably better correlation is observed than with single parameter. However, it often happens that introduction of additional parameters does not lead to a substantially better fit. For example, with phenomena insensitive to basic properties of the solvent, solvent scales expressing solvent basicity are usually not involved in Eq. [1]. Moreover, the solvent scales are interrelated. Mutual relationship between solvent scales as well as the most ‘‘efficient’’ combinations of parameters in Eq. [1] are discussed in detail in (5). Some sets of solvent scales, i.e., a, b and p*; Acity and Basity were so designed that the solvent effect of any solvent dependent process might be expressed as a linear combination of the respective three or two solvents scales. In view of empirical character of these solvent scales, it is difficult to predict a priori which combination of solvent scales will be the most successful or to anticipate the values of the coefficients in Eq. [1] for a given solvent dependent quantity. Applications of the above solvent scales in surface chemistry are rare; some of them are summarized in (4). In the present study, different solvent scales are used to interpret the effect of the nature of organic cosolvent on the surface charge density of silica in mixed aqueous–organic solvents. It is well known, that an admixture of an organic solvent (alcohols, dioxane) leads to a decrease of the absolute value of surface charge density s0 of silica and rutile in 1:1 electrolytes at a given pH and ionic strength (1) but so far little attention has been paid to the role of the nature of the organic cosolvent in this phenomenon. The relative decrease of the surface charge is little sensitive to the pH, ionic strength and the nature of the supporting electrolyte. To the contrary, the surface charge densities of other oxides like anatase, hematite and alumina are not affected by addition of alcohols or dioxane. It is interesting, that a substantial effect of admixtures of organic cosolvents on the z potentials is observed (at a sufficiently high ionic strength) for all oxides, even those whose s0 is not affected (1). The nature of these effects and their mutual relationship are still unknown. Alcohols have been found to adsorb from aqueous solutions on silica (strong solvent effect on s0 ) but not on alumina (no effect). Thus the decrease of s0 may be
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related to adsorption of alcohols which leads to blockade of some surface hydroxyl groups, which are responsible for formation of the surface charge of oxides. To the contrary, the solvent may affect the surface charge density through different solvent properties. For example the acid–base properties of the solvent are probably involved in the proton adsorption–description equilibrium. Formation of the surface charge of silica is a complex physicochemical process. Therefore, it is not obvious which solvent scale(s) can be used to predict the solvent effect on s0 . The solvent composition may also affect the chemical potential of the protons in solution, the potential of the liquid junction, and the behavior of the glass electrode. Discussion of the pH measurements in water-rich mixed solvents and the corresponding literature references can be found elsewhere (9). The display of a pHmeter in equimolar solutions of strong acids and bases is little sensitive to replacement of water with small (up to 30%) admixtures of organic cosolvents, so the above-mentioned effects are negligible or they cancel out (9). In several studies mediocre correlations have been found between the values of the z potential of silica and other oxides in ‘‘pure’’ organic solvents and selected solvent scales (e.g., 7, 8). In spite of apparent similarities, the relevance of the studies carried out in nonaqueous media to the present study is limited. The quoted papers treat on nonaqueous or nearly nonaqueous, often apolar solvents while in the present paper water is the main component. There is no simple relationship between z potential and surface charge density, and the parameters used to characterize the experimental system in the present study (pH, nature and concentration of the electrolyte) are usually disregarded in nonaqueous solvents. EXPERIMENTAL
Chromatographic silica from Merck (388 m2 /g) was washed with nitric acid and then with water in order to remove water soluble impurities, dried at 1207C and then stored for 1 year before the experiments. Analytical or HPLC grade organic solvents from Aldrich, Fluka, Merck, and Ubichem were used without purification. In some instances, the experiments were repeated with reagents from different sources. Mixed solvents (up to 20% of the organic cosolvent if solubility permits) were prepared gravimetrically. Potentiometric titrations of silica in 0.1 mol dm03 KCl solution were carried out for aqueous solutions of the following compounds: acetone, acetonitrile, 1,4-butanediol, 2,3butanediol, dioxane, DMF, DMSO, erythritol, ethanediol, ethanol, ethoxyethanol, glycerol, D-mannitol, methanol, 1,5pentanediol, 1,2-propanediol, 1,3-propanediol, 1-propanol, 2-propanol, and THF and several higher diols and ketones. Titrations in 0.01–1 mol dm03 KCl and 0.1 mol dm03 LiCl, NaCl, and CsCl solutions were carried out in water, 20% DMSO, and 20% glycerol. The salt solution in a given
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FIG. 1. Negative surface charge of silica in 0.1 mol dm03 KCl solutions in DMSO–water mixtures.
solvent was put in a Teflon vessel thermostated at 257C and stirred under nitrogen for 20 min. Then 1 g of silica was added and the suspension was titrated with 200 ml portions of 0.1 mol dm03 KOH (LiOH when LiCl was the supporting electrolyte, etc.). A new portion of titrant was added every 15 min and the pH-meter display was recorded. It has been found in preliminary experiments that a slower titration yields practically the same results at pH õ 8. Above this pH value, slow titration produces substantially higher apparent s0 values due to slow dissolution of silica. A f-71 Beckman pH-meter with glass and calomel reference electrode was used. The pH electrode was calibrated by means of aqueous buffers. This simplified procedure was applied because pH buffers in most solvent studied in this paper are not available. However, it was shown in a previous paper (9) that for short-chain alcohols and dioxane mixtures with water, a calibration procedure involving buffer solutions in a given solvent gives pH-values only slightly different (up to 0.1 pH unit) from those obtained using the ‘‘simplified’’ calibration, when the concentration of the organic cosolvent is 20% or lower. Apparent s0 was calculated by comparison of titration curves with and without silica. Most of blank curves in mixed solvents were very close to those of water. Data for solvents which contained measurable amount of titrable acid are not reported in this paper. The raw s0 data were corrected for basic impurities of silica (20 mmol g 01 ). The s0 of silica is negative over the studied pH range and ‘‘decrease of s0 ’’ always means decrease of És0É in the further text.
density at a given pH clearly decreases when the DMSO concentration increases. In 20% DMSO, s0 of silica is equal to half of that in water, so the effect is quite spectacular. To the contrary, s0 (pH) curves of silica in glycerol–water mixtures are independent of the solvent composition. The dielectric constant of DMSO–water and glycerol–water mixtures over the studied range of organic cosolvent concentration is only slightly lower than that of pure water (10). Thus, the values of s0 of silica in mixtures of DMSO and glycerol with water are not correlated with the dielectric constant of the medium. Comparison of the effects of different solvents shows some general trends. An increase of the length of the hydrocarbon length leads to a more pronounced solvent effect, as shown in Fig. 2 for water and 10% methanol, ethanol, and propanol. Only 2-propanol data are shown in the figure, but the s0 (pH) curve for 10% 1-propanol is practically identical. There is no substantial difference between the effect of ethanol and propanol per kilogram of organic cosolvent, but the effect per mole is more pronounced for propanol. Similar trends: lower s0 for a longer hydrocarbon chain and no substantial difference between isomers is observed for the other concentrations of monoalcohols (5, 15, 20%) and also for diols. To the contrary, an increasing number of hydroxyl groups at a constant length of hydrocarbon chain gives a higher s0 of silica; data for C3 (propanol, propanediol, propanetriol) are presented in Fig. 3. Similar trend is observed with C2 and C4 . In 20% aqueous meso-erythritol(1,2,3,4butanetetrol), s0 of silica is higher by about 10% than that in water. In this respect, erythritol is unique among the organic compounds studied in this paper, only with glycerol is some increase in s0 as compared with pure water also observed, but it is much less pronounced than that with erythritol. For aqueous D-mannitol, s0 (pH) curves are practically the same as in pure water. Generally, the effect of organic compounds with one hydroxyl group per carbon atom on the s0 (pH)
RESULTS AND DISCUSSION
The effect of the length of the hydrocarbon chain and of the number and nature of polar groups in the molecule of organic cosolvent. A typical dependence of s0 of silica on the pH and organic cosolvent concentration for DMSO– water mixed solvent is shown in Fig.1. The surface charge
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FIG. 2. Effect of hydrocarbon chain length on s0 of silica in 0.1 mol dm03 KCl solutions in mixtures of water with alcohols.
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the effect of DMSO on s0 of silica may be characterized by a single parameter Ds Å 0 (( s0, water 0 s0, solvent )/( s0, water cDMSO ))pH
[2]
Å 0.17 kg mol 01 ,
FIG. 3. Effect of the number of hydroxyl groups on s0 of silica in 0.1 mol dm03 KCl solutions in mixtures of water with C3 compounds.
curves for silica is small, while the compounds with extra carbon atoms cause a decrease of surface charge density of silica. However, application of this rule is limited to alcohols. Comparison of alcohols with the compounds of the same length of the hydrocarbon chain and the same number but different kind of polar groups, e.g., propanols with acetone or butanediols with ethoxyethanol, shows that for the same number of carbon and oxygen atoms in the molecule of the organic cosolvent, the decrease of s0 of silica in mixtures of ketones and ethers with water is more pronounced than for corresponding alcohols for most combinations of pH and concentration of the organic cosolvent. The effect of the nature and concentration of the supporting electrolyte. In all systems studied in this paper ( 0.1 mol dm03 alkali chlorides, 0.01 – 1 mol dm03 KCl ) one observes s0, 20% glycerol É s0, water and s0, 20% DMSO É 0.5 s0, water . However, s0, solvent / s0, water for both 20% glycerol and 20% DMSO decreases with the ionic strength and it also decreases in the series KCl ú NaCl ú LiCl ú CsCl. For example, s0, 20% glycerol ú s0, water in 0.01 mol dm03 KCl, s0, 20% glycerol Å s0, water in 0.1 mol dm03 KCl and s0, 20% glycerol õ s0, water in 1 mol dm03 KCl and in 0.1 mol dm03 solutions of the other alkali chlorides. Also the decrease of s0 of silica in 20% DMSO in CsCl solution is more pronounced than in KCl. The origin of this effect is unknown; further studies are intended. Correlation with solvent scales. A promising method to find an universal rule to predict the solvent effect on the surface charge density, is to check the correlation between the solvent scale(s) based on the value of s0 of silica in mixed solvents ( s.s.s.) and the solvent scales called in the Introduction. In DMSO-water mixtures (Fig. 1) the relative decrease of s0 is approximately proportional to the concentration of the organic cosolvent and pH independent. Thus
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where cDMSO is expressed in mol/kg. Only a few solvents exerting a measurable effect on s0 of silica show pH and solvent-composition-independent Ds; e.g., for methanol it equals 0.05 kg mol 01 . For most solvents, Ds decreases when the concentration of the organic cosolvent increases. Usually, a concentration range (e.g. up to 5%) can be found for a given organic cosolvent, where the relative decrease of s0 at a given pH is proportional to the solvent concentration so Ds is solvent concentration independent. The course of (1 0 s0, solvent / s0, water ) as a function of concentration of the organic cosolvent resembles the Langmuir adsorption isotherm and the range of proportionality is analogous to the Henry range. For some solvents exerting a substantial effect on s0 of silica, e.g., ethoxyethanol, a plateau is also observed; i.e., d s0 / d cethoxyethanol É 0 at a sufficiently high concentration of the organic cosolvent. The extent of Henry range is reduced and the trend to reach a plateau is more pronounced when the length of the hydrocarbon chain in alcohols and diols increases. Nearly linear dependence between s0 of silica and concentration of organic cosolvent over a wide concentration range is observed only for some cosolvents showing e ( pure solvent ) ú 30. In many solvent systems ( acetone, dioxane, THF ) , the low-concentration Ds values sharply decrease when the pH increases. In other systems ( acetonitrile ) , the low-concentration Ds value is insensitive to the pH and in most studied solvent systems ( diols, ethanol, propanols ) , Ds at pH 8 is lower than at pH 7 by about 30%. High values of low-concentration Ds may be explained in terms of preferential adsorption of organic components on silica. For certain cosolvents (acetone, dioxane, THF) this preferential adsorption is probably more pronounced at lower pH values. To the contrary, with organic cosolvents for which Ds is independent of the pH and solvent composition (DMSO, methanol), the composition of the solvent in the interfacial region probably does not differ very much from the bulk composition. In view of the discussed above variability, the values of Ds at different organic cosolvent concentrations and pH give many different s.s.scales. This is a general problem with solvent mixtures which is not encountered with onecomponent solvents. It is illustrated in Fig. 4, which shows plots of an imaginary solvent-dependent property x as a function of the cosolvent concentration for three different cosolvents A, B, and C. At the concentration C1 , the order of A, B, and C on the ‘‘x-solvent scale’’ is exactly opposite to that at C3 .
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The other problem with comparison of the effects of different solvents is related to different concentration scales. For cosolvents of a high molecular weight, even relatively insignificant decrease of s0 of silica at a given weight concentration gives a high Ds value which is related to concentrations expressed in mol/kg. Thus, the position of particular organic cosolvents on the solvent scale based on a property of mixed solvents may differ when this property is related to mass on one hand and to number of moles on the other hand. In order to avoid arbitrary choices, two different parameters are used to express the solvent effect on s0 of silica: ( s0, solvent / swater )pH and Ds (Eq. [2]). The values of these two parameters combined with three different organic cosolvent concentrations (5, 10, and 20%) and three pH values (7, 7.5 and 8) give 18 s.s.scales. At lower pH values, s0 of silica is too low to get a quantitative comparison between different solvents. Each of the 18 s.s.s. is analyzed separately. A linear correlation between these s.s.s. and the solvent scales named in the Introduction, a, b, p*, ET (30), DN, AN, Acity, Basity, Z, Z *, and also with the Hildebrand solubility parameter d (11) (all parameters for pure organic solvents) and e value for a solvent mixture was checked first. Unfortunately, the values of all 12 parameters are available only for a few organic solvents; thus, a preliminary analysis using a singleparameter version of Eq. [1] with XYZ Å s.s.s. (one of 18) and A Å one of a, b, p*, ET (30), DN, AN, Acity, Basity, Z, Z *, d (pure solvent), and e (mixture) was carried out for a set of 10 solvents: water, methanol, ethanol, 2-propanol, acetone, acetonitrile, DMF, DMSO, dioxane, and THF. Preliminary analysis: Correlations between s.s.s. and 12 solvent scales for 10 solvent systems. Although the ( s.s.s.)0 and a values in one-parameter Eq. [1] obtained for a given solvent scale A in terms of least square error vary from one s.s.s. to another, a general rule was found that the correlation coefficient r for a given A is little affected by the choice of
FIG. 4. Typical problem with mixed solvents: at different concentrations the order of cosolvents A, B, and C on the ‘‘ x - solvent scale’’ may be different.
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s.s.s. The b, DN and Basity solvent scales always give a very poor correlation with s.s.s. (r ! 0.5). The ET (30), Z, and Z * are much better correlated with s.s.s. (r ú 0.8 for most s.s.s.). With highly polar organic cosolvents (high ET (30), Z, and Z * values), high s0, solvent / s0, water and low Ds values in mixed solvents are observed. This suggests that the magnitude of surface charge depends on solvent polarity rather than on its donor/acceptor properties. The correlation between ET (30) and s.s.s. considerably improves when the concentration of the organic cosolvent increases, e.g., for 20% solutions r ú 0.85. This effect confirms the suggestion that the Ds values at low concentrations of the organic cosolvent are affected by preferential adsorption. Due to adsorption, the concentration of the organic cosolvent in the interfacial region for a given bulk concentration is different for each organic cosolvent. The a and d solvent scales give typically a 0.7 õ r õ 0.8 correlation coefficient with s.s.s.; with certain s.s.s., d gives even r ú 0.9. The surface charge density of silica increases with hydrogen bond donation ability a and Hildebrand solubility parameter d of the organic cosolvent. Certainly, higher r values may be achieved with a twoparameter version of Eq. [1]. The best linear correlations with s.s.s are obtained using ET (30) or Z, or Z * as A and b or DN as B in this equation. The former three solvent scales express chiefly the solvent polarity, while the latter two scales express solvent basicity. The signs of a and b in Eq. [1] with such combinations are always opposite, i.e., a combination of high polarity (high ET (30) or Z, or Z * ) and low basicity (low b or DN) of the organic cosolvent give high s0, solvent / s0, water and low Ds in mixed solvents. Also with a two - parameter approach, the r-values increase with the concentration of the organic cosolvent to exceed 0.95 for a correlation between s0, 20% mixed solvent / s0, water and the best linear combinations of ET (30) with DN or with b. However, it is difficult to point a combination of parameters that would be considerably better than the other five combinations in terms of high r-values; for example, with mixed solvents containing 5% of organic cosolvent, the combinations involving Z give a higher r than those involving ET (30) while with 20% solvents the latter are considerably better. A correlation between s.s.s. and linear combination of a with b or DN solvent scales is only slightly worse than with the above six combinations. Also here, low basicity of the organic cosolvent gives high s0, solvent / s0, water and low Ds in mixed solvents while the effect of solvent acidity ( a ) is opposite. This preliminary analysis shows that only some solvent scales give promising correlations in the one- and two-parameter Eq. [1] with XYZ Å s.s.s., namely, ET (30), Z, and Z * in the one-parameter approach and combinations of a or ET (30) or Z or Z * as A and b or DN as B in a two-parameter approach. Protic and aprotic solvents considered separately. The solvents studied in this paper may be divided into two
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TABLE 1 The Parameters of Eq. [3] Calculated for Seven Protic Solvents pH 7
7.5
8
% Organic cosolvent
(s0,solvent /s0,water)0
a
r
5 10 20 5 10 20 5 10 20
1.27 1.39 1.45 1.25 1.37 1.42 1.2 1.28 1.26
00.55 00.82 00.96 00.50 00.77 00.90 00.40 00.58 00.60
0.98 0.95 0.96 0.99 0.97 0.97 0.99 0.96 0.94
groups. Water and alcohols have a-values about unity and they are strong proton donors and acceptors, as well. To the contrary, acetone, acetonitrile, dioxane, DMF, DMSO, and THF have a-values equal or close to zero and their strong interaction with water is chiefly due to their proton acceptor properties. Different mechanism of interaction with water and probably also with hydroxylated silica surface justifies an idea to consider protic and aprotic solvents as two separate groups. The following data, a, b, p*, ET (30), AN, Z, e, and d are available for the following seven protic solvents: water, methanol, ethanol, 1- and 2-propanol, ethanediol, and glycerol. The solvent scale Z * is linearly correlated with Z, while the other solvent scales which are omitted here were not successful in the preliminary analysis. For the seven protic solvents, all 18 s.s.s. considered in this study are very well correlated with the b solvent scale. The best-fit parameters of the equation s0, solvent / s0, water Å ( s0, solvent / s0, water )0 / a b
[3]
are summarized in Table 1 along with the correlation coefficients.
FIG. 5. Correlation between ET (30) of organic solvents and s0 of silica at pH 7 in 0.1 mol dm03 KCl in 20% aqueous solutions of these solvents. Thirteen solvents used to calculate the solid line: circles (open—aprotic; full—protic). The dashed line was calculated for six alcohols (full symbols except for water). Data represented by squares were not used in the calculations.
Excellent correlation (r ú 0.95) is also observed when A (Eq. [1]) Å e(mixed solvent). Thus, application of e to characterize solvents is very reasonable for protic solvents in view of the present results (but it clearly fails, e.g., with DMSO, Fig. 1). Also ET (30) is correlated with s.s.s. (r É 0.9). On the other hand, application of Eq. [3] with parameters from Table 1 to aprotic solvents gives completely unrealistic results, e.g., s0, dioxane / s0, water ú 1. Since the one-parameter Eq. [1] gives satisfactory correlations for protic solvents, there is no need to introduce a second parameter. Attempts to find a linear correlation between s.s.s. and one of the solvent scales called in the Introduction completely failed for aprotic solvents. For most s.s.s., a and Z yield better than with the other solvent scales, but still poor correlation.
TABLE 2 The Parameters of Eqs. [4] and [5] Calculated for 13 Solvents Eq. [4] pH 7
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Eq. [5]
% Organic cosolvent
(s0,solvent /s0,water)0
a
r
(s0,solvent /s0,water)0
a
r
5 10 20 5 10 20 5 10 20
0.35 0.10 00.29 0.43 0.17 00.66 0.55 0.34 00.05
0.010 0.014 0.020 0.009 0.013 0.027 0.007 0.010 0.017
0.78 0.82 0.90 0.79 0.82 0.88 0.71 0.80 0.91
0.20 00.05 00.51 0.31 0.03 00.81 0.43 0.21 00.23
0.009 0.011 0.016 0.007 0.010 0.020 0.006 0.008 0.013
0.82 0.80 0.88 0.82 0.80 0.79 0.76 0.81 0.90
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TABLE 3 The Parameters of Eq. [6] Calculated for 13 Solvents pH 7
7.5
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TABLE 5 The Parameters of Eq. [8] Calculated for 13 Solvents
% Organic cosolvent
(s0,solvent /s0,water)0
a
b
r
5 10 20 5 10 20 5 10 20
0.45 0.26 00.13 0.54 0.33 00.62 0.64 0.47 0.06
0.010 0.015 0.021 0.009 0.014 0.028 0.008 0.011 0.017
00.2 00.3 00.4 00.2 00.3 00.1 00.2 00.3 00.3
0.85 0.92 0.97 0.90 0.94 0.89 0.84 0.93 0.96
It was also impossible to find a combination of two solvent scales which would give a satisfactory correlation with all s.s.s. for aprotic solvents. Although some combinations gave even r ú 0.99 for certain s.s.s., the same combinations fail (r õ 0.8) in explaining the other s.s.s. This negative result is not surprising in view of different nature of the functional groups in the studied aprotic solvents. To the contrary, it is interesting that in aprotic solvents, s0 of silica is better correlated with a hydrogen bond donation ability, while in protic solvents, rather with b hydrogen bond acceptance ability. Correlations between s.s.s. and selected solvent scales for 13 solvent systems. Thirteen solvents considered in the previous section (protic vs. aprotic) will be analyzed in this section as one group. The results are similar to those of the preliminary analysis what is not surprising because only three solvents (1-propanol, ethanediol, and glycerol) were added to the sample. Again, ET (30) and Z solvent scales give higher correlation coefficients than the other solvent scales with most s.s.s. The best-fit parameters of the equations
pH 7
7.5
8
% Organic cosolvent
(Ds)0
a
b
r
5 10 20 5 10 20 5 10 20
0.84 0.62 0.45 0.73 0.55 0.42 0.49 0.42 0.37
00.017 00.011 00.008 00.015 00.011 00.008 00.011 00.009 00.007
0.3 0.2 0.1 0.3 0.2 0.1 0.3 0.2 0.1
0.90 0.95 0.96 0.92 0.95 0.95 0.87 0.93 0.92
s0, solvent / s0, water Å ( s0, solvent / s0, water )0 / aZ
[5]
and the corresponding correlation coefficients are summarized in Table 2. As an example, Fig. 5 shows the correlation between ET (30) of organic solvents and s0 of silica in 0.1 mol dm03 KCl in 20% aqueous solutions of these solvents at pH 7. The 13 solvents used to calculate the slope are denoted by circles; other solvents are denoted by squares. The agreement between the calculated (line) and experimental (symbols) s0 of silica in solvents which were not involved in the calculation is satisfactory. Full and open symbols are used to distinguish between protic and aprotic solvents, although they were treated as one group in the calculation. The equation represented by the solid line considerably underestimates s0 of silica in glycerol and ethylene glycol and overestimates s0 of silica in ethanol. On the other hand, the broken line (best fit for aliphatic monoalcohols, ethanediol and glycerol) gives a satisfactory prediction for diols (squares) but it completely fails in predicting s0 of silica in water and aprotic solvents. The following two-parameter equations have been found to give the best correlations with s.s.s.:
s0, solvent / s0, water Å ( s0, solvent / s0, water )0 / aET (30) [4] s0, solvent / s0, water Å ( s0, solvent / s0, water )0
and TABLE 4 The parameters of Eq. [7] Calculated for 13 Solvents pH 7
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TABLE 6 The Parameters of Eq. [9] Calculated for 13 Solvents
% Organic cosolvent
(s0,solvent /s0,water)0
a
b
r
5 10 20 5 10 20 5 10 20
0.29 0.11 00.36 0.41 0.19 00.79 0.53 0.34 00.14
0.009 0.011 0.017 0.008 0.011 0.020 0.006 0.009 0.014
00.2 00.3 00.3 00.2 00.3 00.05 00.2 00.3 00.2
0.88 0.89 0.94 0.92 0.92 0.79 0.88 0.93 0.94
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pH 7
7.5
8
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% Organic cosolvent
(Ds)0
a
b
r
5 10 20 5 10 20 5 10 20
1.06 0.73 0.53 0.91 0.66 0.50 0.63 0.52 0.44
00.013 00.009 00.006 00.012 00.008 00.006 00.009 00.007 00.005
0.2 0.2 0.1 0.2 0.2 0.1 0.3 0.2 0.1
0.90 0.90 0.91 0.93 0.91 0.90 0.89 0.91 0.88
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/ aET (30) / b b
[6]
s0, solvent / s0, water Å ( s0, solvent / s0, water )0 / aZ / b b [7] Ds Å ( Ds )0 / aET (30) / b b
[8]
Ds Å ( Ds )0 / aZ / b b.
[9]
The best-fit parameters of these equations and the corresponding correlation coefficients are summarized in Tables 3–6. Only slightly worse correlation was observed between s.s.s. and linear combination of a and b solvent scales. In all combinations shown in Tables 3–6, a is positive in Eqs. [6] and [7] and negative in Eqs. [8] and [9], while b is negative in Eqs. [6] and [7] and positive in Eqs. [8] and [9]. This means that s0 of silica is high and Ds is low in solvents of high polarity (high ET (30) and Z ) and low hydrogen bond acceptance ability b. Moreover, for most s.s.s., the best correlation is observed with an approximately 015:1 blend of b with ET (30) or Z; however, the proportions vary from one s.s.s. to another. The absolute values of a and b coefficients in Eqs. [6] and [7] increase and in Eqs. [8] and [9] decrease with the organic cosolvent concentration. An analogous trend is observed for most combinations of solvents scales and with single-parameter approach, e.g., Eqs. [3] – [5] (Table 1 and 2). This corresponds to the discussed earlier observation that Ds in most solvent mixtures decreases when the concentration of the organic cosolvent increases, probably due to Langmuirtype adsorption isotherm of organic cosolvents in the studied concentration region. Taking into account only the solvents studied in this paper, ET (30) ranges from 36 (dioxane) to 63.1 (water), Z ranges from 58.8 (THF) to 94.6 (water) and b ranges from 0.37 (dioxane) to 0.9 (1-propanol). Considering typical values of these solvent parameters and range of their variability, and data from Tables 3–6 one can notice that in Eqs. [6] – [9] the contribution of polarity dominates over that of hydrogen bond acceptance ability b. The present study shows that the surface charge density of silica in mixed solvents may be rationalized in terms of linear combinations of well-known experimental solvent scales. Combinations of solvent polarity with hydrogen bond acceptance ability b give the highest correlation coefficients. The observed trend, i.e., a high És0É when ET (30) is high and b is low is not surprizing. The negative surface charge of silica is formed as a result of dissociation of the surface
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hydroxyl groups, which is promoted in polar solvents like any other process in which electric charges are separated. It follows from the present results that the empirical solvent polarity scales ET (30) and Z are more appropriate than e to characterize the solvent effect on the surface dissociation reaction. Indeed, the dielectric constant characterizes the properties of bulk liquid rather than the ability of solvent molecules to solvate particular species. The charge of the surface |Si{O 0 groups in aqueous solutions and in mixed solvents rich in water is almost entirely neutralized by the adsorption of the cations of the supporting electrolyte (1); thus, surface dissociation is accompanied by transfer of these cations from bulk solution to the surface layer. Such a transfer is promoted by low ability of the solvent to solvate cations (low b ). The major difficulty in prediction of s0 of silica in given solvent is the preferential adsorption of organic cosolvents at low concentrations which interferes with the influence of solvent polarity and hydrogen-bond acceptance ability. The other problem is related to the selection of a representative sample of solvents. Availability of data is a limiting factor here. Overrepresentation of certain types of solvents, e.g., monoalcohols, in the sample may bias the results. ACKNOWLEDGMENTS This study was inspired by criticism of author’s earlier work raised by Professors E. Dutkiewicz and Z. Galus. The experiments have been carried out by M. Geca and T. Urban.
REFERENCES 1. Kosmulski, M., Colloids Surf. 95, 81 (1995). 2. Morrison, I. D., Colloids Surf. 71, 1 (1993). 3. Van der Hoeven, Ph.C., and Lyklema, J., Adv. Colloid Interface Sci. 42, 205 (1992). 4. Reichardt, Ch., Chem. Rev. 94, 2319 (1994). 5. Marcus, Y., Chem. Soc. Rev. 22, 409 (1993). 6. Skwierczynski, R. D., and Connors, K. A., J. Chem. Soc. Perkin Trans. 2, 467 (1994). 7. Labib, M. E., and Williams, R., J. Colloid Interface Sci. 97, 356 (1984). 8. Spange, S., Simon, F., Heublein, G., Jacobasch, H.-J., and Bo¨rner, M., Colloid Polym. Sci. 269, 173 (1991). 9. Kosmulski, M., and Matijevic, E., Langmuir 8, 1060 (1992). 10. Janz, G. N., and Tomkins, R. P., ‘‘Nonaqueous Electrolytes Handbook,’’ pp. 86 and 100. Academic Press, New York, 1972. 11. Barton, A. F., ‘‘CRC Handbook of Solubility Parameters and Other Cohesion Parameters,’’ p. 142. CRC Press, Boca Raton, FL, 1983.
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0194
The Effect of the Nature of the Organic Cosolvent on Surface Charge Density of Silica in Mixed Solvents MAREK KOSMULSKI 1 Polish Academy of Sciences, Institute of Catalysis and Physical Chemistry of Interfaces, Laboratory of Adsorption and Physical Chemistry of Interfaces, Pl. M.C.Sklodowskiej 3, 20031 Lublin 41, Poland Received June 5, 1995; accepted October 9, 1995
Potentiometric titrations of silica in mixtures of water with alcohols, diols, polyalcohols, ketones, ethers, DMSO, DMF, and acetonitrile have been carried out. Usually, the negative surface charge s0 of silica at a given pH and ionic strength decreases when the concentration of the organic component in the mixed solvent increases. Polyalcohols are an exception: their presence does not affect s0 , and erythritol makes it even more negative. Generally, the absolute value of s0 of silica at a given concentration of an organic cosolvent is lower for the cosolvents with a longer hydrocarbon chain, e.g., water ú methanol ú ethanol, and higher for cosolvents with a higher number of polar groups, e.g., propanol õ propanediol õ propanetriol. The magnitude of s0 at a given pH, ionic strength, and organic cosolvent concentration is correlated with Reichardt’s ET (30) and Kosower’s Z and Z * polarity parameters; namely, the absolute value of s0 increases when these parameters increase. Even better correlation can be achieved when the solvent is characterized by a linear combination of one of three above-mentioned solvent parameters or a hydrogen bond donation ability with b hydrogen bond acceptance ability or with DN Gutman’s donor number. In solvents with high values of the later two parameters, the negative surface charge of silica is low. q 1996 Academic Press, Inc.
Key Words: silica; surface charge; mixed solvent; solvent polarity.
INTRODUCTION
Studies of colloids in nonaqeuous and mixed solvents have been recently reviewed (1–3). The dielectric constant e is the most common parameter used in colloid chemistry to characterize solvents (1–3). As a mater of fact, many properties related to colloid stability, z potentials etc., are entirely different in nonpolar (low e ) and polar (high e ) media even if the same dispersed solid is involved. On the other hand, it is well known that the dielectric constant often fails in explaining the solvent dependent physico-chemical properties and phenomena (4, 5). Therefore, several experimental scales have been introduced, based on heats of solvation, NMR shifts and solvatochromic shifts of specially selected 1
To whom correspondence should be addressed.
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a hydrogen bond donation ability b hydrogen bond acceptance ability p* polarity/polarizability parameter Reichardt’s ET (30) parameter DN, Gutman’s donor number AN, Gutman’s acceptor number Acity Basity Kosower’s Z and Z * polarity parameters.
ET (30), DN, Z, and Z * are expressed in kcal/mol, AN is a dimensionless parameter and ranges from 0 to 100 for most solvents, and the other dimensionless parameters range from 0 to 1. The available values of these parameters for different solvents have been recently summarized (4, 5). Unfortunately, a complete set of the 10 parameters is available only for a few solvents. Basically, the above solvent parameters can be measured not only for one-component solvents but also for solvent mixtures. The values of ET (30) for mixtures of many organic solvents with water are available. However, it is generally agreed that nonlinear dependence between ET (30) and mole fraction of organic component observed in many mixed solvent systems is chiefly due to preferential solvation of the solvatochromic dye. Thus, the values of ET (30) measured in solvent mixtures characterize the solvent composition near the chromophore group in the probe molecule rather than the bulk composition so application of such values to characterize processes in solvent mixtures is limited (4, 6). Therefore, in the present study, values of the parameters for pure organic solvents are used to characterize solvent mixtures with water even if corresponding data for the solvent mixture are also available. Many solvent dependent quantities are correlated with one of the mentioned above experimental solvent parameters. For many other quantities, the one-parameter approach fails,
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0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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probe molecule(s) in particular solvents. For many organic solvents, the values of the following parameters can be found in the literature:
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but a good correlation is observed with a properly selected linear combination of solvent scales (4, 5), (XYZ ) j Å (XYZ )0 / aAj / bBj / cCj / . . .
[1]
where XYZ is a solvent-dependent property, the subscripts 0 and j denote a medium in which A0 Å B0 Å C0rrr Å 0 (for most solvent scales this corresponds to gas phase or completely inert solvent) and the given solvent, respectively, uppercase letters denote parameters characterizing the solvent j, and lowercase letters denote parameters dependent only on the property XYZ. For certain combinations of solvent scales in Eq. [1], a considerably better correlation is observed than with single parameter. However, it often happens that introduction of additional parameters does not lead to a substantially better fit. For example, with phenomena insensitive to basic properties of the solvent, solvent scales expressing solvent basicity are usually not involved in Eq. [1]. Moreover, the solvent scales are interrelated. Mutual relationship between solvent scales as well as the most ‘‘efficient’’ combinations of parameters in Eq. [1] are discussed in detail in (5). Some sets of solvent scales, i.e., a, b and p*; Acity and Basity were so designed that the solvent effect of any solvent dependent process might be expressed as a linear combination of the respective three or two solvents scales. In view of empirical character of these solvent scales, it is difficult to predict a priori which combination of solvent scales will be the most successful or to anticipate the values of the coefficients in Eq. [1] for a given solvent dependent quantity. Applications of the above solvent scales in surface chemistry are rare; some of them are summarized in (4). In the present study, different solvent scales are used to interpret the effect of the nature of organic cosolvent on the surface charge density of silica in mixed aqueous–organic solvents. It is well known, that an admixture of an organic solvent (alcohols, dioxane) leads to a decrease of the absolute value of surface charge density s0 of silica and rutile in 1:1 electrolytes at a given pH and ionic strength (1) but so far little attention has been paid to the role of the nature of the organic cosolvent in this phenomenon. The relative decrease of the surface charge is little sensitive to the pH, ionic strength and the nature of the supporting electrolyte. To the contrary, the surface charge densities of other oxides like anatase, hematite and alumina are not affected by addition of alcohols or dioxane. It is interesting, that a substantial effect of admixtures of organic cosolvents on the z potentials is observed (at a sufficiently high ionic strength) for all oxides, even those whose s0 is not affected (1). The nature of these effects and their mutual relationship are still unknown. Alcohols have been found to adsorb from aqueous solutions on silica (strong solvent effect on s0 ) but not on alumina (no effect). Thus the decrease of s0 may be
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related to adsorption of alcohols which leads to blockade of some surface hydroxyl groups, which are responsible for formation of the surface charge of oxides. To the contrary, the solvent may affect the surface charge density through different solvent properties. For example the acid–base properties of the solvent are probably involved in the proton adsorption–description equilibrium. Formation of the surface charge of silica is a complex physicochemical process. Therefore, it is not obvious which solvent scale(s) can be used to predict the solvent effect on s0 . The solvent composition may also affect the chemical potential of the protons in solution, the potential of the liquid junction, and the behavior of the glass electrode. Discussion of the pH measurements in water-rich mixed solvents and the corresponding literature references can be found elsewhere (9). The display of a pHmeter in equimolar solutions of strong acids and bases is little sensitive to replacement of water with small (up to 30%) admixtures of organic cosolvents, so the above-mentioned effects are negligible or they cancel out (9). In several studies mediocre correlations have been found between the values of the z potential of silica and other oxides in ‘‘pure’’ organic solvents and selected solvent scales (e.g., 7, 8). In spite of apparent similarities, the relevance of the studies carried out in nonaqueous media to the present study is limited. The quoted papers treat on nonaqueous or nearly nonaqueous, often apolar solvents while in the present paper water is the main component. There is no simple relationship between z potential and surface charge density, and the parameters used to characterize the experimental system in the present study (pH, nature and concentration of the electrolyte) are usually disregarded in nonaqueous solvents. EXPERIMENTAL
Chromatographic silica from Merck (388 m2 /g) was washed with nitric acid and then with water in order to remove water soluble impurities, dried at 1207C and then stored for 1 year before the experiments. Analytical or HPLC grade organic solvents from Aldrich, Fluka, Merck, and Ubichem were used without purification. In some instances, the experiments were repeated with reagents from different sources. Mixed solvents (up to 20% of the organic cosolvent if solubility permits) were prepared gravimetrically. Potentiometric titrations of silica in 0.1 mol dm03 KCl solution were carried out for aqueous solutions of the following compounds: acetone, acetonitrile, 1,4-butanediol, 2,3butanediol, dioxane, DMF, DMSO, erythritol, ethanediol, ethanol, ethoxyethanol, glycerol, D-mannitol, methanol, 1,5pentanediol, 1,2-propanediol, 1,3-propanediol, 1-propanol, 2-propanol, and THF and several higher diols and ketones. Titrations in 0.01–1 mol dm03 KCl and 0.1 mol dm03 LiCl, NaCl, and CsCl solutions were carried out in water, 20% DMSO, and 20% glycerol. The salt solution in a given
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FIG. 1. Negative surface charge of silica in 0.1 mol dm03 KCl solutions in DMSO–water mixtures.
solvent was put in a Teflon vessel thermostated at 257C and stirred under nitrogen for 20 min. Then 1 g of silica was added and the suspension was titrated with 200 ml portions of 0.1 mol dm03 KOH (LiOH when LiCl was the supporting electrolyte, etc.). A new portion of titrant was added every 15 min and the pH-meter display was recorded. It has been found in preliminary experiments that a slower titration yields practically the same results at pH õ 8. Above this pH value, slow titration produces substantially higher apparent s0 values due to slow dissolution of silica. A f-71 Beckman pH-meter with glass and calomel reference electrode was used. The pH electrode was calibrated by means of aqueous buffers. This simplified procedure was applied because pH buffers in most solvent studied in this paper are not available. However, it was shown in a previous paper (9) that for short-chain alcohols and dioxane mixtures with water, a calibration procedure involving buffer solutions in a given solvent gives pH-values only slightly different (up to 0.1 pH unit) from those obtained using the ‘‘simplified’’ calibration, when the concentration of the organic cosolvent is 20% or lower. Apparent s0 was calculated by comparison of titration curves with and without silica. Most of blank curves in mixed solvents were very close to those of water. Data for solvents which contained measurable amount of titrable acid are not reported in this paper. The raw s0 data were corrected for basic impurities of silica (20 mmol g 01 ). The s0 of silica is negative over the studied pH range and ‘‘decrease of s0 ’’ always means decrease of És0É in the further text.
density at a given pH clearly decreases when the DMSO concentration increases. In 20% DMSO, s0 of silica is equal to half of that in water, so the effect is quite spectacular. To the contrary, s0 (pH) curves of silica in glycerol–water mixtures are independent of the solvent composition. The dielectric constant of DMSO–water and glycerol–water mixtures over the studied range of organic cosolvent concentration is only slightly lower than that of pure water (10). Thus, the values of s0 of silica in mixtures of DMSO and glycerol with water are not correlated with the dielectric constant of the medium. Comparison of the effects of different solvents shows some general trends. An increase of the length of the hydrocarbon length leads to a more pronounced solvent effect, as shown in Fig. 2 for water and 10% methanol, ethanol, and propanol. Only 2-propanol data are shown in the figure, but the s0 (pH) curve for 10% 1-propanol is practically identical. There is no substantial difference between the effect of ethanol and propanol per kilogram of organic cosolvent, but the effect per mole is more pronounced for propanol. Similar trends: lower s0 for a longer hydrocarbon chain and no substantial difference between isomers is observed for the other concentrations of monoalcohols (5, 15, 20%) and also for diols. To the contrary, an increasing number of hydroxyl groups at a constant length of hydrocarbon chain gives a higher s0 of silica; data for C3 (propanol, propanediol, propanetriol) are presented in Fig. 3. Similar trend is observed with C2 and C4 . In 20% aqueous meso-erythritol(1,2,3,4butanetetrol), s0 of silica is higher by about 10% than that in water. In this respect, erythritol is unique among the organic compounds studied in this paper, only with glycerol is some increase in s0 as compared with pure water also observed, but it is much less pronounced than that with erythritol. For aqueous D-mannitol, s0 (pH) curves are practically the same as in pure water. Generally, the effect of organic compounds with one hydroxyl group per carbon atom on the s0 (pH)
RESULTS AND DISCUSSION
The effect of the length of the hydrocarbon chain and of the number and nature of polar groups in the molecule of organic cosolvent. A typical dependence of s0 of silica on the pH and organic cosolvent concentration for DMSO– water mixed solvent is shown in Fig.1. The surface charge
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FIG. 2. Effect of hydrocarbon chain length on s0 of silica in 0.1 mol dm03 KCl solutions in mixtures of water with alcohols.
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the effect of DMSO on s0 of silica may be characterized by a single parameter Ds Å 0 (( s0, water 0 s0, solvent )/( s0, water cDMSO ))pH
[2]
Å 0.17 kg mol 01 ,
FIG. 3. Effect of the number of hydroxyl groups on s0 of silica in 0.1 mol dm03 KCl solutions in mixtures of water with C3 compounds.
curves for silica is small, while the compounds with extra carbon atoms cause a decrease of surface charge density of silica. However, application of this rule is limited to alcohols. Comparison of alcohols with the compounds of the same length of the hydrocarbon chain and the same number but different kind of polar groups, e.g., propanols with acetone or butanediols with ethoxyethanol, shows that for the same number of carbon and oxygen atoms in the molecule of the organic cosolvent, the decrease of s0 of silica in mixtures of ketones and ethers with water is more pronounced than for corresponding alcohols for most combinations of pH and concentration of the organic cosolvent. The effect of the nature and concentration of the supporting electrolyte. In all systems studied in this paper ( 0.1 mol dm03 alkali chlorides, 0.01 – 1 mol dm03 KCl ) one observes s0, 20% glycerol É s0, water and s0, 20% DMSO É 0.5 s0, water . However, s0, solvent / s0, water for both 20% glycerol and 20% DMSO decreases with the ionic strength and it also decreases in the series KCl ú NaCl ú LiCl ú CsCl. For example, s0, 20% glycerol ú s0, water in 0.01 mol dm03 KCl, s0, 20% glycerol Å s0, water in 0.1 mol dm03 KCl and s0, 20% glycerol õ s0, water in 1 mol dm03 KCl and in 0.1 mol dm03 solutions of the other alkali chlorides. Also the decrease of s0 of silica in 20% DMSO in CsCl solution is more pronounced than in KCl. The origin of this effect is unknown; further studies are intended. Correlation with solvent scales. A promising method to find an universal rule to predict the solvent effect on the surface charge density, is to check the correlation between the solvent scale(s) based on the value of s0 of silica in mixed solvents ( s.s.s.) and the solvent scales called in the Introduction. In DMSO-water mixtures (Fig. 1) the relative decrease of s0 is approximately proportional to the concentration of the organic cosolvent and pH independent. Thus
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where cDMSO is expressed in mol/kg. Only a few solvents exerting a measurable effect on s0 of silica show pH and solvent-composition-independent Ds; e.g., for methanol it equals 0.05 kg mol 01 . For most solvents, Ds decreases when the concentration of the organic cosolvent increases. Usually, a concentration range (e.g. up to 5%) can be found for a given organic cosolvent, where the relative decrease of s0 at a given pH is proportional to the solvent concentration so Ds is solvent concentration independent. The course of (1 0 s0, solvent / s0, water ) as a function of concentration of the organic cosolvent resembles the Langmuir adsorption isotherm and the range of proportionality is analogous to the Henry range. For some solvents exerting a substantial effect on s0 of silica, e.g., ethoxyethanol, a plateau is also observed; i.e., d s0 / d cethoxyethanol É 0 at a sufficiently high concentration of the organic cosolvent. The extent of Henry range is reduced and the trend to reach a plateau is more pronounced when the length of the hydrocarbon chain in alcohols and diols increases. Nearly linear dependence between s0 of silica and concentration of organic cosolvent over a wide concentration range is observed only for some cosolvents showing e ( pure solvent ) ú 30. In many solvent systems ( acetone, dioxane, THF ) , the low-concentration Ds values sharply decrease when the pH increases. In other systems ( acetonitrile ) , the low-concentration Ds value is insensitive to the pH and in most studied solvent systems ( diols, ethanol, propanols ) , Ds at pH 8 is lower than at pH 7 by about 30%. High values of low-concentration Ds may be explained in terms of preferential adsorption of organic components on silica. For certain cosolvents (acetone, dioxane, THF) this preferential adsorption is probably more pronounced at lower pH values. To the contrary, with organic cosolvents for which Ds is independent of the pH and solvent composition (DMSO, methanol), the composition of the solvent in the interfacial region probably does not differ very much from the bulk composition. In view of the discussed above variability, the values of Ds at different organic cosolvent concentrations and pH give many different s.s.scales. This is a general problem with solvent mixtures which is not encountered with onecomponent solvents. It is illustrated in Fig. 4, which shows plots of an imaginary solvent-dependent property x as a function of the cosolvent concentration for three different cosolvents A, B, and C. At the concentration C1 , the order of A, B, and C on the ‘‘x-solvent scale’’ is exactly opposite to that at C3 .
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The other problem with comparison of the effects of different solvents is related to different concentration scales. For cosolvents of a high molecular weight, even relatively insignificant decrease of s0 of silica at a given weight concentration gives a high Ds value which is related to concentrations expressed in mol/kg. Thus, the position of particular organic cosolvents on the solvent scale based on a property of mixed solvents may differ when this property is related to mass on one hand and to number of moles on the other hand. In order to avoid arbitrary choices, two different parameters are used to express the solvent effect on s0 of silica: ( s0, solvent / swater )pH and Ds (Eq. [2]). The values of these two parameters combined with three different organic cosolvent concentrations (5, 10, and 20%) and three pH values (7, 7.5 and 8) give 18 s.s.scales. At lower pH values, s0 of silica is too low to get a quantitative comparison between different solvents. Each of the 18 s.s.s. is analyzed separately. A linear correlation between these s.s.s. and the solvent scales named in the Introduction, a, b, p*, ET (30), DN, AN, Acity, Basity, Z, Z *, and also with the Hildebrand solubility parameter d (11) (all parameters for pure organic solvents) and e value for a solvent mixture was checked first. Unfortunately, the values of all 12 parameters are available only for a few organic solvents; thus, a preliminary analysis using a singleparameter version of Eq. [1] with XYZ Å s.s.s. (one of 18) and A Å one of a, b, p*, ET (30), DN, AN, Acity, Basity, Z, Z *, d (pure solvent), and e (mixture) was carried out for a set of 10 solvents: water, methanol, ethanol, 2-propanol, acetone, acetonitrile, DMF, DMSO, dioxane, and THF. Preliminary analysis: Correlations between s.s.s. and 12 solvent scales for 10 solvent systems. Although the ( s.s.s.)0 and a values in one-parameter Eq. [1] obtained for a given solvent scale A in terms of least square error vary from one s.s.s. to another, a general rule was found that the correlation coefficient r for a given A is little affected by the choice of
FIG. 4. Typical problem with mixed solvents: at different concentrations the order of cosolvents A, B, and C on the ‘‘ x - solvent scale’’ may be different.
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s.s.s. The b, DN and Basity solvent scales always give a very poor correlation with s.s.s. (r ! 0.5). The ET (30), Z, and Z * are much better correlated with s.s.s. (r ú 0.8 for most s.s.s.). With highly polar organic cosolvents (high ET (30), Z, and Z * values), high s0, solvent / s0, water and low Ds values in mixed solvents are observed. This suggests that the magnitude of surface charge depends on solvent polarity rather than on its donor/acceptor properties. The correlation between ET (30) and s.s.s. considerably improves when the concentration of the organic cosolvent increases, e.g., for 20% solutions r ú 0.85. This effect confirms the suggestion that the Ds values at low concentrations of the organic cosolvent are affected by preferential adsorption. Due to adsorption, the concentration of the organic cosolvent in the interfacial region for a given bulk concentration is different for each organic cosolvent. The a and d solvent scales give typically a 0.7 õ r õ 0.8 correlation coefficient with s.s.s.; with certain s.s.s., d gives even r ú 0.9. The surface charge density of silica increases with hydrogen bond donation ability a and Hildebrand solubility parameter d of the organic cosolvent. Certainly, higher r values may be achieved with a twoparameter version of Eq. [1]. The best linear correlations with s.s.s are obtained using ET (30) or Z, or Z * as A and b or DN as B in this equation. The former three solvent scales express chiefly the solvent polarity, while the latter two scales express solvent basicity. The signs of a and b in Eq. [1] with such combinations are always opposite, i.e., a combination of high polarity (high ET (30) or Z, or Z * ) and low basicity (low b or DN) of the organic cosolvent give high s0, solvent / s0, water and low Ds in mixed solvents. Also with a two - parameter approach, the r-values increase with the concentration of the organic cosolvent to exceed 0.95 for a correlation between s0, 20% mixed solvent / s0, water and the best linear combinations of ET (30) with DN or with b. However, it is difficult to point a combination of parameters that would be considerably better than the other five combinations in terms of high r-values; for example, with mixed solvents containing 5% of organic cosolvent, the combinations involving Z give a higher r than those involving ET (30) while with 20% solvents the latter are considerably better. A correlation between s.s.s. and linear combination of a with b or DN solvent scales is only slightly worse than with the above six combinations. Also here, low basicity of the organic cosolvent gives high s0, solvent / s0, water and low Ds in mixed solvents while the effect of solvent acidity ( a ) is opposite. This preliminary analysis shows that only some solvent scales give promising correlations in the one- and two-parameter Eq. [1] with XYZ Å s.s.s., namely, ET (30), Z, and Z * in the one-parameter approach and combinations of a or ET (30) or Z or Z * as A and b or DN as B in a two-parameter approach. Protic and aprotic solvents considered separately. The solvents studied in this paper may be divided into two
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TABLE 1 The Parameters of Eq. [3] Calculated for Seven Protic Solvents pH 7
7.5
8
% Organic cosolvent
(s0,solvent /s0,water)0
a
r
5 10 20 5 10 20 5 10 20
1.27 1.39 1.45 1.25 1.37 1.42 1.2 1.28 1.26
00.55 00.82 00.96 00.50 00.77 00.90 00.40 00.58 00.60
0.98 0.95 0.96 0.99 0.97 0.97 0.99 0.96 0.94
groups. Water and alcohols have a-values about unity and they are strong proton donors and acceptors, as well. To the contrary, acetone, acetonitrile, dioxane, DMF, DMSO, and THF have a-values equal or close to zero and their strong interaction with water is chiefly due to their proton acceptor properties. Different mechanism of interaction with water and probably also with hydroxylated silica surface justifies an idea to consider protic and aprotic solvents as two separate groups. The following data, a, b, p*, ET (30), AN, Z, e, and d are available for the following seven protic solvents: water, methanol, ethanol, 1- and 2-propanol, ethanediol, and glycerol. The solvent scale Z * is linearly correlated with Z, while the other solvent scales which are omitted here were not successful in the preliminary analysis. For the seven protic solvents, all 18 s.s.s. considered in this study are very well correlated with the b solvent scale. The best-fit parameters of the equation s0, solvent / s0, water Å ( s0, solvent / s0, water )0 / a b
[3]
are summarized in Table 1 along with the correlation coefficients.
FIG. 5. Correlation between ET (30) of organic solvents and s0 of silica at pH 7 in 0.1 mol dm03 KCl in 20% aqueous solutions of these solvents. Thirteen solvents used to calculate the solid line: circles (open—aprotic; full—protic). The dashed line was calculated for six alcohols (full symbols except for water). Data represented by squares were not used in the calculations.
Excellent correlation (r ú 0.95) is also observed when A (Eq. [1]) Å e(mixed solvent). Thus, application of e to characterize solvents is very reasonable for protic solvents in view of the present results (but it clearly fails, e.g., with DMSO, Fig. 1). Also ET (30) is correlated with s.s.s. (r É 0.9). On the other hand, application of Eq. [3] with parameters from Table 1 to aprotic solvents gives completely unrealistic results, e.g., s0, dioxane / s0, water ú 1. Since the one-parameter Eq. [1] gives satisfactory correlations for protic solvents, there is no need to introduce a second parameter. Attempts to find a linear correlation between s.s.s. and one of the solvent scales called in the Introduction completely failed for aprotic solvents. For most s.s.s., a and Z yield better than with the other solvent scales, but still poor correlation.
TABLE 2 The Parameters of Eqs. [4] and [5] Calculated for 13 Solvents Eq. [4] pH 7
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Eq. [5]
% Organic cosolvent
(s0,solvent /s0,water)0
a
r
(s0,solvent /s0,water)0
a
r
5 10 20 5 10 20 5 10 20
0.35 0.10 00.29 0.43 0.17 00.66 0.55 0.34 00.05
0.010 0.014 0.020 0.009 0.013 0.027 0.007 0.010 0.017
0.78 0.82 0.90 0.79 0.82 0.88 0.71 0.80 0.91
0.20 00.05 00.51 0.31 0.03 00.81 0.43 0.21 00.23
0.009 0.011 0.016 0.007 0.010 0.020 0.006 0.008 0.013
0.82 0.80 0.88 0.82 0.80 0.79 0.76 0.81 0.90
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MAREK KOSMULSKI
TABLE 3 The Parameters of Eq. [6] Calculated for 13 Solvents pH 7
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TABLE 5 The Parameters of Eq. [8] Calculated for 13 Solvents
% Organic cosolvent
(s0,solvent /s0,water)0
a
b
r
5 10 20 5 10 20 5 10 20
0.45 0.26 00.13 0.54 0.33 00.62 0.64 0.47 0.06
0.010 0.015 0.021 0.009 0.014 0.028 0.008 0.011 0.017
00.2 00.3 00.4 00.2 00.3 00.1 00.2 00.3 00.3
0.85 0.92 0.97 0.90 0.94 0.89 0.84 0.93 0.96
It was also impossible to find a combination of two solvent scales which would give a satisfactory correlation with all s.s.s. for aprotic solvents. Although some combinations gave even r ú 0.99 for certain s.s.s., the same combinations fail (r õ 0.8) in explaining the other s.s.s. This negative result is not surprising in view of different nature of the functional groups in the studied aprotic solvents. To the contrary, it is interesting that in aprotic solvents, s0 of silica is better correlated with a hydrogen bond donation ability, while in protic solvents, rather with b hydrogen bond acceptance ability. Correlations between s.s.s. and selected solvent scales for 13 solvent systems. Thirteen solvents considered in the previous section (protic vs. aprotic) will be analyzed in this section as one group. The results are similar to those of the preliminary analysis what is not surprising because only three solvents (1-propanol, ethanediol, and glycerol) were added to the sample. Again, ET (30) and Z solvent scales give higher correlation coefficients than the other solvent scales with most s.s.s. The best-fit parameters of the equations
pH 7
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% Organic cosolvent
(Ds)0
a
b
r
5 10 20 5 10 20 5 10 20
0.84 0.62 0.45 0.73 0.55 0.42 0.49 0.42 0.37
00.017 00.011 00.008 00.015 00.011 00.008 00.011 00.009 00.007
0.3 0.2 0.1 0.3 0.2 0.1 0.3 0.2 0.1
0.90 0.95 0.96 0.92 0.95 0.95 0.87 0.93 0.92
s0, solvent / s0, water Å ( s0, solvent / s0, water )0 / aZ
[5]
and the corresponding correlation coefficients are summarized in Table 2. As an example, Fig. 5 shows the correlation between ET (30) of organic solvents and s0 of silica in 0.1 mol dm03 KCl in 20% aqueous solutions of these solvents at pH 7. The 13 solvents used to calculate the slope are denoted by circles; other solvents are denoted by squares. The agreement between the calculated (line) and experimental (symbols) s0 of silica in solvents which were not involved in the calculation is satisfactory. Full and open symbols are used to distinguish between protic and aprotic solvents, although they were treated as one group in the calculation. The equation represented by the solid line considerably underestimates s0 of silica in glycerol and ethylene glycol and overestimates s0 of silica in ethanol. On the other hand, the broken line (best fit for aliphatic monoalcohols, ethanediol and glycerol) gives a satisfactory prediction for diols (squares) but it completely fails in predicting s0 of silica in water and aprotic solvents. The following two-parameter equations have been found to give the best correlations with s.s.s.:
s0, solvent / s0, water Å ( s0, solvent / s0, water )0 / aET (30) [4] s0, solvent / s0, water Å ( s0, solvent / s0, water )0
and TABLE 4 The parameters of Eq. [7] Calculated for 13 Solvents pH 7
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TABLE 6 The Parameters of Eq. [9] Calculated for 13 Solvents
% Organic cosolvent
(s0,solvent /s0,water)0
a
b
r
5 10 20 5 10 20 5 10 20
0.29 0.11 00.36 0.41 0.19 00.79 0.53 0.34 00.14
0.009 0.011 0.017 0.008 0.011 0.020 0.006 0.009 0.014
00.2 00.3 00.3 00.2 00.3 00.05 00.2 00.3 00.2
0.88 0.89 0.94 0.92 0.92 0.79 0.88 0.93 0.94
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pH 7
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% Organic cosolvent
(Ds)0
a
b
r
5 10 20 5 10 20 5 10 20
1.06 0.73 0.53 0.91 0.66 0.50 0.63 0.52 0.44
00.013 00.009 00.006 00.012 00.008 00.006 00.009 00.007 00.005
0.2 0.2 0.1 0.2 0.2 0.1 0.3 0.2 0.1
0.90 0.90 0.91 0.93 0.91 0.90 0.89 0.91 0.88
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SURFACE CHARGE IN MIXED SOLVENTS
/ aET (30) / b b
[6]
s0, solvent / s0, water Å ( s0, solvent / s0, water )0 / aZ / b b [7] Ds Å ( Ds )0 / aET (30) / b b
[8]
Ds Å ( Ds )0 / aZ / b b.
[9]
The best-fit parameters of these equations and the corresponding correlation coefficients are summarized in Tables 3–6. Only slightly worse correlation was observed between s.s.s. and linear combination of a and b solvent scales. In all combinations shown in Tables 3–6, a is positive in Eqs. [6] and [7] and negative in Eqs. [8] and [9], while b is negative in Eqs. [6] and [7] and positive in Eqs. [8] and [9]. This means that s0 of silica is high and Ds is low in solvents of high polarity (high ET (30) and Z ) and low hydrogen bond acceptance ability b. Moreover, for most s.s.s., the best correlation is observed with an approximately 015:1 blend of b with ET (30) or Z; however, the proportions vary from one s.s.s. to another. The absolute values of a and b coefficients in Eqs. [6] and [7] increase and in Eqs. [8] and [9] decrease with the organic cosolvent concentration. An analogous trend is observed for most combinations of solvents scales and with single-parameter approach, e.g., Eqs. [3] – [5] (Table 1 and 2). This corresponds to the discussed earlier observation that Ds in most solvent mixtures decreases when the concentration of the organic cosolvent increases, probably due to Langmuirtype adsorption isotherm of organic cosolvents in the studied concentration region. Taking into account only the solvents studied in this paper, ET (30) ranges from 36 (dioxane) to 63.1 (water), Z ranges from 58.8 (THF) to 94.6 (water) and b ranges from 0.37 (dioxane) to 0.9 (1-propanol). Considering typical values of these solvent parameters and range of their variability, and data from Tables 3–6 one can notice that in Eqs. [6] – [9] the contribution of polarity dominates over that of hydrogen bond acceptance ability b. The present study shows that the surface charge density of silica in mixed solvents may be rationalized in terms of linear combinations of well-known experimental solvent scales. Combinations of solvent polarity with hydrogen bond acceptance ability b give the highest correlation coefficients. The observed trend, i.e., a high És0É when ET (30) is high and b is low is not surprizing. The negative surface charge of silica is formed as a result of dissociation of the surface
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hydroxyl groups, which is promoted in polar solvents like any other process in which electric charges are separated. It follows from the present results that the empirical solvent polarity scales ET (30) and Z are more appropriate than e to characterize the solvent effect on the surface dissociation reaction. Indeed, the dielectric constant characterizes the properties of bulk liquid rather than the ability of solvent molecules to solvate particular species. The charge of the surface |Si{O 0 groups in aqueous solutions and in mixed solvents rich in water is almost entirely neutralized by the adsorption of the cations of the supporting electrolyte (1); thus, surface dissociation is accompanied by transfer of these cations from bulk solution to the surface layer. Such a transfer is promoted by low ability of the solvent to solvate cations (low b ). The major difficulty in prediction of s0 of silica in given solvent is the preferential adsorption of organic cosolvents at low concentrations which interferes with the influence of solvent polarity and hydrogen-bond acceptance ability. The other problem is related to the selection of a representative sample of solvents. Availability of data is a limiting factor here. Overrepresentation of certain types of solvents, e.g., monoalcohols, in the sample may bias the results. ACKNOWLEDGMENTS This study was inspired by criticism of author’s earlier work raised by Professors E. Dutkiewicz and Z. Galus. The experiments have been carried out by M. Geca and T. Urban.
REFERENCES 1. Kosmulski, M., Colloids Surf. 95, 81 (1995). 2. Morrison, I. D., Colloids Surf. 71, 1 (1993). 3. Van der Hoeven, Ph.C., and Lyklema, J., Adv. Colloid Interface Sci. 42, 205 (1992). 4. Reichardt, Ch., Chem. Rev. 94, 2319 (1994). 5. Marcus, Y., Chem. Soc. Rev. 22, 409 (1993). 6. Skwierczynski, R. D., and Connors, K. A., J. Chem. Soc. Perkin Trans. 2, 467 (1994). 7. Labib, M. E., and Williams, R., J. Colloid Interface Sci. 97, 356 (1984). 8. Spange, S., Simon, F., Heublein, G., Jacobasch, H.-J., and Bo¨rner, M., Colloid Polym. Sci. 269, 173 (1991). 9. Kosmulski, M., and Matijevic, E., Langmuir 8, 1060 (1992). 10. Janz, G. N., and Tomkins, R. P., ‘‘Nonaqueous Electrolytes Handbook,’’ pp. 86 and 100. Academic Press, New York, 1972. 11. Barton, A. F., ‘‘CRC Handbook of Solubility Parameters and Other Cohesion Parameters,’’ p. 142. CRC Press, Boca Raton, FL, 1983.
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