Langmuir 2002, 18, 785-787
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Confirmation of the Differentiating Effect of Small Cations in the Shift of the Isoelectric Point of Oxides at High Ionic Strengths Marek Kosmulski* Department of Electrochemistry, Technical University of Lublin, 20618 Lublin, Nadbystrzycka 38A, Poland, and Department of Physical Chemistry, A° bo Akademi University, Porthansgatan 3-5, 20500 Turku, Finland Received September 6, 2001 High concentrations of certain 1-1 electrolytes in water induce a shift in the isoelectric point (IEP) of metal oxides to pH values substantially higher than the pristine IEP. With large cations, the nature of the anion does not affect the magnitude of these shifts, but with Li and Na the shift depends on the nature of the anion. This effect is termed the differentiating effect of small cations, and it has been studied in detail for anatase. The present results confirm the existence of the differentiating effect of Na for alumina. The anion sequence I > Br > Cl > ClO4 > NO3 observed for alumina is identical with that reported for anatase.
Introduction At low electrolyte concentrations and near the point of zero charge (PZC), the adsorption of ions from 1-1 electrolytes is chiefly electrostatic, and the position of the PZC/IEP (isoelectric point) of metal oxides and related materials, their surface charge density σ0, and the ζ potential (at constant pH and ionic strength) are independent of the nature of the 1-1 electrolyte. This is because the ions are fully hydrated and the surface does not “see” individual ions but only their charges. Farther from the PZC, the nature of the counterions affects the σ0, that is, the nature of the cation affects only the negative branch, and the nature of the anion affects only the positive branch of the charging curve.1 On the other hand, the effect of the nature of the 1-1 electrolyte on the electrokinetic curves at low ionic strengths is rather insignificant. In contrast, at concentrations of >0.1 mol dm-3 the nature of the 1-1 electrolyte affects not only the value but even the sign of the ζ potential. Typically, the isoelectric point shifts to pH values higher than the pristine IEP, and at sufficiently high concentrations of certain 1:1 electrolytes, there is no IEP at all and the ζ potential is positive over the entire pH range.2 Classical electrokinetic methods are not suitable for measurements at high ionic strengths. Fortunately, the newly introduced electroacoustic method made such studies possible. The shifts in the IEP induced by different salts comply to the following rules: Salts with large cations (Cs) have a rather insignificant and anion-independent effect. Salts with small cations (Li, Na) have a significant and aniondependent effect. Salts with small anions (Cl) have a rather insignificant and cation-independent effect. Salts with large anions (I) have a significant and cation-dependent effect. The differentiating effect of small cations and large anions was thoroughly examined only for one adsorbent, namely, anatase. Studies of zirconia and alumina at high * Fax: +48 81 5254601. E-mail:
[email protected]. (1) Kosmulski, M. Chemical Properties of Material Surfaces; Marcel Dekker: New York, 2001. (2) Kosmulski, M.; Rosenholm, J. B. J. Phys. Chem. 1996, 100, 11681.
ionic strengths confirm the above rules,3 but the electrokinetic behavior of these oxides was only studied in the presence of a few salts, so the hypothesis that the differentiating effect is common for all metal oxides has a rather speculative character. Moreover, in the original publication reporting the high ionic strength electrokinetic data for alumina4 the authors interpret their results as existence of the cation effect (different nitrates were studied and only in LiNO3 was the IEP substantially shifted to high pH) and nonexistence of the anion effect (different potassium salts were studied and the IEP was not shifted). We show here that this interpretation is only valid for potassium which is a nondifferentiating cation, but it must not be generalized for other cations. Namely, for different sodium salts the nature of the anion substantially affects the observed shifts in the IEP of alumina. Experimental Section The electrokinetic potential and particle size in aqueous dispersions of alumina were determined using Acustosizer (Colloidal Dynamics) with the 1.13 software version. All measurements at ionic strengths higher than 0.02 mol dm-3 were corrected for the electrolyte background. According to the manufacturer, the instrument operation range is up to 4 S/m. This corresponds to about 0.5 mol dm-3 of common salts. The titrations were carried out without any protection of the samples from the atmospheric CO2. A detailed description of the titration procedure and discussion of the significance of the results and difficulties can be found elsewhere.2 Reagent grade alumina from Fluka (R form, 3 µm average diameter) was purified following the procedure described elsewhere.5 The electroacoustic measurements were carried out at the volume fraction of alumina of 6%. The dispersions were unstable even at low ionic strengths and far from the IEP. The water was MilliQ, and the experiments were carried out at 25 °C. The other chemicals were reagent grade. It was demonstrated previously2 that the purity of the reagents (e.g., 99.995% versus regular AR) does not affect the shifts in the IEP of anatase at high ionic strengths. Thus, it is very unlikely that impurities (3) Kosmulski, M.; Rosenholm, J. B. Langmuir 1999, 15, 8934. (4) Johnson, S. B.; Scales, P. J.; Healy, T. W. Langmuir 1999, 15, 2836. (5) Kosmulski, M.; Matijevic, E. Colloids Surf. 1992, 64, 57.
10.1021/la0155653 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/21/2001
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Figure 1. The electrokinetic potential of alumina as a function of pH and different concentrations of NaNO3.
Kosmulski
Figure 2. The electrokinetic potential of alumina as a function of pH and different concentrations of NaBr.
present in AR reagents might play a crucial role in analogous shifts for alumina.
Results and Discussion The IEP of alumina in 10-3 mol dm-3 NaNO3 at pH 9.3 (Figure 1) is independent of the direction of titration (no hysteresis in acid and base titrations). This IEP is slightly higher than most pristine values reported in the literature.1 On the other hand, the IEP in Figure 1 matches most literature values obtained by means of the electroacoustic method.6 Thus, the present result confirms that the electroacoustic method produces a systematically higher IEP of alumina than classical electrokinetic methods. The absolute values of the ζ potential of alumina far from the IEP in 10-3 mol dm-3 NaNO3 in Figure 1 are considerably lower than typical values reported in the literature for the same ionic strength.1 This is probably because the instrument software is unable to properly interpret the signal from large particles. Most likely, the actual ζ potential can be obtained by multiplying the results presented in Figure 1 by a factor of 3-4. Therefore, also the absolute values obtained at higher ionic strengths in this study are not reliable. It is believed, however, that the presented results properly reflect the sign of the ζ potential and the position of the IEP. The particle size results calculated by the instrument software are scattered, and they do not show any clear pH or ionic strength dependence. Moreover, many runs failed to produce any particle size (only ζ potential data were obtained). In view of the unusually low ζ potential produced by the instrument software, the physical sense of the calculated particle size is questionable and these results are not reported here. Figure 2 shows the same type of electrokinetic behavior at high ionic strengths as was reported for anatase. The IEP shifts to higher pH as the salt (in this case NaBr) concentration increases. The results obtained in 10-3 mol dm-3 NaNO3 are used as the low ionic strength reference in Figure 2 and in the other figures. It is well established in the literature (e.g., ref 7) that the IEP of alumina and other metal oxides and even the value of the ζ potential (6) Kosmulski, M. Pr. Nauk. Inst. Gorn. Politech. Wroclaw. 2001, 95, 5.
Figure 3. The electrokinetic potential of alumina as a function of pH and different concentrations of NaI.
at a certain pH at low ionic strength are rather insensitive to the nature of the 1-1 salt (NaCl versus NaNO3, etc.). Even more substantial shifts than those shown in Figure 2 were observed with NaI (Figure 3), and at concentrations of >0.3 mol dm-3 the ζ potential was positive over the entire studied pH range (up to 11.5). Very likely, such sign reversal over the entire pH range can be also induced at sufficiently high NaBr concentrations (Figure 2). With 0.1 mol dm-3 NaClO4 (Figure 4), the IEP of alumina is shifted to pH 9.7, but higher concentrations of this salt did not induce a further shift. Finally, with NaNO3 (Figure 1) up to 0.2 mol dm-3 there was no shift in the IEP at all, and the results obtained at 0.3-0.5 mol dm-3 NaNO3 suggest even a small shift in the IEP to lower pH (although negative values at pH < 9.5 do not exceed -1 mV). Such shifts in the IEP to low pH at high ionic strengths were (7) Franks, G. V.; Johnson, S. B.; Scales, P. J.; Boger, D. V.; Healy, T. W Langmuir 1999, 15, 4411.
Effect of Small Cations on IEP Shift of Oxides
Figure 4. The electrokinetic potential of alumina as a function of pH and different concentrations of NaClO4.
not reported for anatase,2 but the situations when the IEP shifts to high pH, reaches the maximum, and on further increase in the ionic strength shifts back in the direction of the pristine IEP are quite common. The results reported in ref 4 suggest the absence of any shift in the IEP of alumina at NaNO3 concentrations up to 1 mol dm-3, but with data points every 1 pH unit, the IEP can be only roughly estimated from these results. On the other hand, the same publication shows the shift in the IEP of alumina in 1 mol dm-3 CsNO3 to a pH below the pristine IEP. This may suggest that such downshifts are commonplace for alumina. Rowlands et al.8 report a shift in the IEP of alumina from the low ionic strength value of 9.1 to about (8) Rowlands, W. N.; O’Brien, R. W.; Hunter, R. J.; Patrick, V. J. Colloid Interface Sci. 1997, 188, 325.
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10 in 0.5 mol dm-3 NaCl and sign reversal to positive at pH up to 13 in 3 mol dm-3 NaCl. Results obtained with different samples of alumina are not necessarily compatible, but combination of the results presented in this study and the literature data gives a rather consistent picture. First, the existence of the differentiating effect of Na for alumina is confirmed. The anion sequence I (the strongest shift in the IEP to high pH at relatively low salt concentrations) > Br > Cl > ClO4 > NO3 (the absence of any shift or perhaps even a slight shift in the IEP to low pH) for alumina qualitatively confirms that reported for anatase (for the electrolyte concentration range of interest). It should be emphasized that the relative position of salts in this series depends on the concentration range of interest. For some salts (e.g., NaBr, Figure 2), the increase in salt concentration results in a continuous shift in the IEP to high pH, while for other salts the plateau or even reversal in the trend (on further increase in the ionic strength, the IEP shifts back in the direction of the pristine IEP) occurs at relatively low concentrations. The present study supports the hypothesis2,9 that the shifts in the IEP at high ionic strengths are mainly due to ion-ion and ion-solvent interactions in solution and the specific ion-surface interactions play the secondary role. This explains the identical sequence in the ability of particular salts to induce a shift in the IEP of anatase on one hand and of alumina on the other and the correlation of this sequence with Marcus’ structure breaking and making scale of anions and cations.10 On the other hand, the similar behavior of lithium and sodium salts observed for anatase2 and the large gap between NaNO3 and LiNO3 reported for alumina4 indicate a substantial contribution of specific interaction between Li and the alumina surface. This result is in line with the very high negative σ0 of alumina11 in the presence of Li compared with other alkali metal anions. LA0155653 (9) Yaroshchuk, A. E. J. Colloid Interface Sci. 2001, 238, 381. (10) Marcus Y. J. Solution Chem. 1994, 23, 831. (11) Kosmulski, M.; Plak, A. Colloids Surf., A 1999, 149, 409.
