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Adsorption sequence of the alkali cations at the tungsten trioxide–water interface
Colloids and Surfaces A: Physicochemical and Engineering Aspects 154 (1999) 149 – 156
Adsorption sequence of the alkali cations at the tungsten trioxide–water interface Freddy Dumont *, Paul Verbeiren, Claudine Buess-Herman Faculte´ des Sciences CP 255, Uni6ersite´ Libre de Bruxelles, 2 Boule6ard du Triomphe, 1050 Brussels, Belgium
Abstract The stability of pyrogenic tungsten trioxide hydrosols is studied in the presence of various monovalent cations between pH 2 and 12. The hydrosol critical coagulation concentrations (c*) are determined by extrapolation of the experimental log W –log c curves up to log W =0. The c* of K + , Rb + and Cs + slightly increase with the pH of the solution while the c* of Li + presents a pronounced maximum at pH 4.5. A less marked maximum is also observed at the same pH in the presence of Na + . The corresponding ionic adsorption sequence at the water – WO3 interface deduced from these experimental data is Cs + \ Rb + \ K + \ Na + \ Li + in the pH domain under investigation. This overall behavior is successfully explained by the model proposed by Gierst who has generalized the Gurney concept describing the ion–ion interactions in solution to the ion – surface interactions. The results reported in this work are also compared with those obtained previously for other oxides; they confirm the validity and the generality of the conclusions already drawn concerning the ionic adsorption sequences at the oxide – water interface. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Adsorption sequences; Gierst model; Tungsten trioxide hydrosol; Coagulation; Point of zero charge
1. Introduction Many properties of colloidal dispersions in aqueous solution depend directly on the adsorption of ions on the particle surface. Among others, the stability of an hydrosol at a given pH is closely related to the nature of the ions present in the solution: the stronger the ion adsorption, the lower the concentration required to coagulate the
Dedicated to the memory of Julie and Me´lissa, Ann and Eefje, and Loubna * Corresponding author. Tel.: + 32-2-6502989; fax: + 32-26502934. E-mail address: [email protected] (F. Dumont)
hydrosol. A good knowledge of the mechanisms that govern the ion adsorption is of primary interest since it could allow prediction or at least understanding of the behavior of colloidal dispersions in solution. In the case of oxides, the electrical surface charge originates from the acido–basic reactions of the surface hydroxyl groups according to M OH − U M OH2+ M OH + OH − U M O − + H2O The net electrical surface charge is equal to the difference between the number of the negatively and the positively charged sites, its value depends
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directly on the pH of the solution and on the chemical nature of the surface through the equilibrium constants of the acido – basic reactions. Therefore, for a certain pH value the net electrical surface charge is equal to zero. This particular pH, known as the point of zero charge (pzc), is a fundamental property of the oxide [1,2]. Previous works were devoted to the study of the ionic adsorption sequences at several oxide– water interfaces with pzc values ranging from 2.8 to 8.5. They have clearly shown that the adsorption sequences of cations and anions depend on the pzc value of the oxide [3 – 6]. The present work was undertaken to verify if the results obtained in the study of tungsten trioxide, an oxide with a very low pzc value (around pH 1–2), confirm or invalidate the conclusions drawn previously.
2. Materials The tungsten trioxide hydrosols were prepared by precipitation of tungstic acid from potassium tungstate solutions at a pH close to 1. The precipitate was then boiled for 1 week under reflux conditions. After this maturation-dehydration step, the solid was thoroughly washed by successive centrifugation cycles, the sediment being redispersed in bi-distilled water. This process was repeated until the conductivity of the supernatant remained constant (5 10 − 5 S cm − 1). The exact composition of the so-obtained tungsten oxide is generally poorly defined. This problem can nevertheless be solved by a high temperature treatment of the solid that leads to the stable WO3. The sol resulting from the first step was gently heated to dryness and the obtained powder was then maintained at 650°C for 12 h. Finally, the powder was ultrasonically redispersed in bi-distilled water. Finally, the monodispersity of the resulting hydrosol was improved by successive centrifugation cycles at low and high fields to eliminate the largest and the smallest particles. X-ray analysis indicated that the particles were essentially amorphous. The transmission electron microscopy pictures showed that the particles were quasi-spherical,
their mean diameter and the corresponding standard deviation were found to be equal to 45 and 10 nm, respectively. Photon correlation spectroscopy measurements led to slightly different values, 39 and 15 nm, respectively, as deduced from the auto-correlation function analysis by the cumulants method. The agreement between both sets of results may nevertheless be considered as satisfactory.
3. Measurement of the ionic adsorption sequences The critical coagulation concentration of an hydrosol, c*, is defined as the transition concentration between the fast and the slow coagulation regimes. In the fast coagulation or Schmoluchovski domain, the half coagulation time, Ts, is independent of the electrolyte concentration. On the contrary, in the slow coagulation conditions the half coagulation time, T, increases when the electrolyte concentration decreases. T is related to the Smoluchovski half coagulation time by the equation T= W TS
(1)
where W is the stability factor of the hydrosol. In the slow coagulation domain, the log W– log c relationship is quasi-linear. The c* values can thus be determined by extrapolation of the experimental log W–log c curves up to log W=0. The theory of colloidal suspension stability (DLVO theory) indicates how c* is related to the Hamaker constant, A, and to the Stern potential, cd, by the following equation [7] c*= B
c 4d A2 z2
(2)
where B is a constant and z is the ion valency. This equation was deduced from an oversimplification of the DLVO theory and is, consequently, of very little use. Nevertheless, its great merit is to stress clearly that c* is extremely sensitive to the cd potential. As a matter of fact, the electrical surface potential, c0, is imposed by the solution pH—it is nearly independent of the nature of the counter-ion—at least, in the absence of strong specific adsorption. On the contrary, the
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Fig. 1. Examples of the variation of the absorbance of the coagulating hydrosol with time. (1) Smoluchovski fast coagulation; (2) slow coagulation. The slopes at the origin allowing the calculation of the stability factor W are broken lines.
Stern potential, cd, depends strongly on the ion adsorption, the stronger the ionic adsorption, the lower the cd value. The ionic adsorption sequence can therefore be deduced from the experimental c* values. Toelstra and Kruyt [8] have demonstrated that, during the coagulation process, the absorbance of an hydrosol increases linearly with time, t, according to the equation
As =As0 1+
2t W Ts
curves in the Smoluchovski region to the measured one at the actual electrolyte concentration. The measurements were consequently performed at 630 nm using a Bausch and Lomb Spectronic 700 spectrophotometer. The details of the experimental setup are given elsewhere [3]. The chloride salts (analytical grade) of the cations were used. The pH of the solution was adjusted with HCl in acid solution and with the cation hydroxide in alkaline solution.
(3)
where As0 is the absorbance of the stable hydrosol. This equation is valid when the particle size is significantly smaller than the incident radiation wavelength (Rayleigh law). Therefore, this linear relationship is not expected to hold very long since the mean particle size increases when the coagulation progresses, so that the aforementioned conditions cease to be fulfilled. Nevertheless, if the stable hydrosol meets the Rayleigh conditions, the stability factor W is merely equal to the ratio of the slope at the origin of the As –t
4. Results Examples of As –t curves measured at two electrolyte concentrations are presented in Fig. 1. Curve 1 corresponds to the Schmoluchovski coagulation while curve 2 is related to a slow coagulation process. The negative curvature is more pronounced in the Schmoluchovski regime, a consequence of the aforementioned phenomenon. Fig. 2 summarizes the most important features of the measured log W–log c curves.
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Fig. 2. Log W –log c curves measured in the presence of various cations. (1) Cs + , pH 4.1; (2) K + , pH 11.2; (3) K + , pH 3.4; (4) Na + , pH 6.0; (5) Li + , pH 10.0; (6) Li + , pH 4.3.
As expected, log W decreases linearly with log c in dilute electrolyte solution and becomes constant when the concentration is further increased. The slope of the reported curves depends neither on the pH nor on the nature of the studied cation; this clearly suggests that the coagulation mechanism remains the same in all the experimental conditions of this work. The variations of the critical coagulation concentrations with the pH for the five studied cations Li + , Na + , K + , Rb + and Cs + are given in Fig. 3. The c*–pH curve measured in the presence of Li + shows a decided maximum at pH 4.5, a less important maximum is also observed at the same pH for Na + whereas the c* values of K + , Rb + and Cs + increase slightly with the pH of the solution until they reach a constant value above pH 7. The extrapolation of the c* curves to zero leads to the determination of the isoelectric point (iep) of the hydrosol; its value amounts to approximatively 1.8. In this case, as a strong specific adsorption of the studied ions is never observed, the iep and the pzc values coincide.
The alkali adsorption sequence deduced from the c* values may be written as Cs + \ Rb + \ K + \ Na + \ Li + It remains unchanged in the whole pH range under investigation. Finally, it was obviously not possible to determine the adsorption sequence of the anions because the electrolyte concentration required to fix the pH below the pzc is too high and causes an immediate coagulation of the hydrosol.
5. Discussion The first detailed studies of ionic adsorption have been carried out at the mercury–water interface. The adsorption of the alkali cations at the water–mercury [9] interface follows the sequence Cs + \ K + \ Na + \ Li + A first explanation of this sequence, known as the Hofmeister lyotropic sequence, is based on the
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Fig. 3. Variation of the critical coagulation concentrations (c*) with the pH of the solution. (1) Li + ; (2) Na + ; (3) K + ; (4) Rb + ; (5) Cs + .
Stern picture of the inner part of the double layer: in the absence of any specific chemical effect, the adsorbability of an ion at an interface is determined by its electrical charge (the higher the valency of the counter-ion, the higher its adsorption) and its size (the larger the size of the adsorbed species, the lower its adsorption). The size of the adsorbate imposes its distance of closest approach to the surface. Accordingly, the reported experimental Hofmeister sequence leads to conclude that the alkali cations remain hydrated when they adsorb at the mercury – water interface. However, in the presence of pre-adsorbed pyridine [10] at the same interface, the reversed adsorption sequence is obtained Li + \Na + \K + \Cs + Therefore, from this anti-Hofmeister sequence, it must be concluded that the cations undergo a dehydration during the adsorption. Such a different behavior in very similar electrical field conditions is difficult to understand; moreover, as the cations have no particular affinity for the pyridine molecule, it is hard to explain this inversion by
some specific chemical effect. However, there is actually no physical reason to reject the charge and size effects of the Stern model and it must therefore be concluded that the size of the ion is not the only parameter that plays a role in fixing the ion–surface distance of closest approach. Gierst et al. [11] solved this problem by proposing a generalization of the Gurney model of ion– ion interactions in solution to ion–surface interactions. According to Gurney [12], the local structure of water near an ion is different from that in the bulk phase as a result of the action of the local electric field of the ion on the solvent molecules. The interaction forces between an ion and a water molecule may be stronger or weaker than the interaction forces between two water molecules. In the first case, the solvent is more structured in the vicinity of the ion than in the bulk phase. This situation is encountered with polyvalent and small monovalent ions such as Li + , Na + and F − which are known as structure-maker or promoter. In spite of their relatively large size, IO3− and BrO3− are also structure-makers; this unexpected
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property is well explained by Nightingale [13]. In the second case, the water molecules are less structured near the ion than in the bulk phase. The larger alkali cations K + , Rb + and Cs + , halide anions like I − , Br − and Cl − and several polyatomic anions like ClO4− , SCN − , ClO3− , NO3− belong to this structure-breaker family. In ionic solutions, the most important ion–ion interactions are electrical, but according to Gurney, short range forces originating from the respective action of each partner on the water structure are superimposed to the purely electrical force. These forces are attractive when both partners exert the same structural action on the solvent and repulsive in the opposite situation. The Gurney description leads for instance to a coherent interpretation of the activity coefficients of the alkali halides. Gierst et al. [11] extended this concept to the description of the ion – surface interactions; the surface is then assimilated to a macro ion which is able to promote or destroy the water structure in its vicinity. Accordingly, at a given surface potential, a structure-maker ion will be more adsorbed at a structure-maker surface than a structure-breaker one and inversely. This model explains successfully not only the inversion of the alkali ion adsorption sequence at the water–mercury interface in the presence of pyridine but also the effect of the nature of the so-called indifferent ions on the kinetics of many reduction reactions at the mercury electrode [14]. This model was used later by Berube and Debruyn [15] to explain the Li + \Cs + adsorption sequence deduced from potentiometric titrations of titanium dioxide suspensions. Dumont et al. [3 – 6] studied a series of oxides. In these works, the experimental ionic adsorption sequences were used as a tool to analyse the surface properties. This approach led to the conclusion that the low pzc oxide surfaces behave as structure-breakers wheareas the high pzc ones are structure-makers. The transition pzc value was around pH 4. The pzc of an oxide is related to its heat of immersion (− DHi) in water according to the semi-empirical equation of Healy and Fuerstenau [16]
(−DHi)= 4.606 RT pHpzc + (−DHc)
(4)
where (− DHc) is an integration constant. This relationship indicates clearly that the structuring power of an interface increases with its heat of immersion. It is worth noting that the same correlation exists between the heat of hydration of an ion and its structuring power [4]. We have also found that the structuring power of a surface increases with its electrical charge. In particular, it was ascertained that a surface with a slightly pristine structure-breaker character becomes a structure-maker when its surface charge is increased. This phenomenon is well illustrated by previously reported results on a TiO2 hydrosol (pzc 4.6) [5] and a SnO2 hydrosol (pzc 3) [6] where the alkali cations adsorption sequence goes from Cs + \ Li + to Li + \ Cs + as the surface charge is increased by moving the solution pH away from the pzc. This may be explained merely by assuming that the actual structuring power of the surface results at least from the sum of two effects: the pristine action of the dispersed phase and the orientation of the water dipoles by the electrical forces originating in the superficial charges. The experimental ionic adsorption sequence Cs + \ Li + observed in this work indicates that the WO3 surface is structure-breaker in the whole explored pH range 2–12. Moreover, the relatively small values of the critical coagulation concentrations c* of K + , Rb + and Cs + indicate a relative strong adsorption of these structure-breaker cations. This conclusion agrees completely with the predictions of the model since the WO3 pzc is significantly smaller than 4. The fact that the sequence remains unchanged constitutes an indication of the high structurebreaker character of WO3 because the surface electrical charge is unable, by its sole action, to change the intrinsic properties of the surface. Nevertheless, the c* values of Li + and Na + pass through a maximum around pH 4.5 and go down slowly until pH 7. This decrease of c* corresponds to a slight increase of the adsorption of these ions, an effect that can only be attributed to a slight attenuation of the structure-breaker power of the surface, in close agreement with the previous description. Finally, above pH 7, the c*
F. Dumont et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 154 (1999) 149–156
values become constant and independent of the solution pH for all the studied cations. Such an effect indicates that the Stern potential and consequently the surface charge does not increase anymore with the solution pH. This kind of situation is only encountered when the fraction of the dissociated surface groups is extremely close to 1. This suggests that the intrinsic dissociation constant of the hydroxyl groups is high and also that their number density is relatively small, this conclusion needs to be confirmed by potentiometric titrations of WO3 suspensions. A similar situation prevails at the SiO2 – water interface (pzc 2–3). The ionic adsorption sequence at this interface was deduced from potentiometric titration data [17]. The observed sequence remains of the structure-breaker type between pH 2 and 13, but, at the same time, the relative difference between the amounts of the adsorbed cations Li + , Na + , K + and Cs + is found to lessen when the solution pH goes up. In a recent paper, Colic et al. [18] studied the viscosity of high volume fraction alumina suspensions in the presence of alkali cations. An electrophoretic study was first performed, it had shown that the adsorption at the alumina–water interface decreases from Li + to Cs + . This structure-maker sequence is in full agreement with the high pzc value (9.0) of alumina. The viscosity measurements have clearly demonstrated that the behavior of the suspension is determined by the particle interaction forces and more precisely by the depth of the interparticle interaction potential well. This one results from the balance between the van der Waals attraction forces (assumed independent of the nature of the cation) and the repulsive forces originating in the presence of the adsorbed cations. The corresponding equilibrium separation distance between two particles, although difficult to be assessed, increases significantly from Li + to Cs + . As a matter of fact, the heavily hydrated structure-maker Li + is well embedded inside the structured water layer at the particle surface whereas the poorly hydrated structure-breaker Cs + is maintained far away as a consequence of the incompatibility between the solvent structure at the surface and around the cation. All these facts constitute also a substantial
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support to the Gierst model. However, in the same paper, the authors have summarized several preliminary results of experiments performed on a series of materials including silica. They have reported that the cations sequence was found to be the same for all the dispersions investigated and identical to that observed on alumina. In the particular case of silica, this result appears to be at variance not only with the predictions of the Gierst model but also with the experimental adsorption sequence deduced from potentiometric titration experiments [17]. In our opinion, this contradiction is just apparent and may be removed easily by remembering that the behavior of silica suspensions is actually far from comparable with any other colloidal system. The most important reason for this particular behavior lies in the very low value of the Hamaker constant of the SiO2 –water system, which results in very weak van der Waals attraction forces between two silica particles dispersed in water [19]. This explains why the small sized silica hydrosols are stable at their point of zero charge [20], a quite unusual behavior in colloidal science. However, the same hydrosols, paradoxically, coagulate in alkaline solutions where their surface charge should act as a supplementary stabilizing factor [21,22]. Such a general behavior is far from tractable by the DLVO theory [23]. Nevertheless, this problem was solved by Depasse and Watillon [22,24] who demonstrated that the coagulation in alkaline solutions is due to the formation of some kind of chemical bond between the particles and, moreover, that the strength of this bond decreases in the order of the alkali cations acidity character: Li + , Na + , K + , Cs + . Consequently, the interparticle attraction forces which are responsible for the suspension viscosity increase in the order Li + to Cs + , identical—but for quite different reasons—to the one observed in the DLVO suspensions like zirconia and alumina.
6. Conclusions The experimental results observed at the WO3 – aqueous electrolyte solution interface confirm that the general description of the ionic adsorption at
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interface follows the Gierst model. Presently, to the best of our knowledge, no available experimental data invalidate this model, we may conclude its general validity. However, we are quite aware that the weak point of this description rests in the fact that it is only qualitative and, therefore, unable to make any quantitative prediction. This indicates that firstly, a complete theory of the ionic adsorption phenomenon is needed and, secondly, that theoretical works and molecular dynamic approaches that would ignore the effect of the surface on the solvent structure are doomed to be incomplete, except in very particular conditions. Accordingly, the theory of the ion size effects on the adsorption in molecular solvent (Torrie et al [25]) should only be applied to describe the adsorption at an oxide surface with a pzc close to 4 since this particular surface is known to have very little influence on the water structure.
Acknowledgements One of the authors (Paul Verbeiren) acknowledges gratefully the David et Alice Van Buuren Foundation for financial support.
References [1] G.A. Parks, Chem. Rev. 65 (1965) 177. [2] R.H. Yoon, T. Salman, G. Donnay, J. Colloid Interface Sci. 70 (1979) 483. [3] F. Dumont, A. Watillon, Discuss. Faraday Soc. 52 (1971) 324.
.
[4] F. Dumont, D. Van Tan, A. Watillon, J. Colloid Interface Sci. 55 (1976) 678. [5] F. Dumont, J. Warlus, A. Watillon, J. Colloid Interface Sci. 138 (1990) 543. [6] F. Dumont, S. Contreras, M. Diaz y Alonso, An. Quim. 91 (1995) 635. [7] E.J.W. Verwey, J.Th.G. Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. [8] S.A. Troelstra, H.R. Krayt, Kolloid Beih. 54 (1943) 225. [9] D.C. Grahame, Chem. Rev. 41 (1947) 441. [10] L. Gierst, P. Herman, Z. Anal. Chem. 216 (1966) 238. [11] L. Gierst, L. Vandenberghen, E. Nicolas, A. Fraboni, J. Electrochem. Soc. 113 (1966) 1025. [12] R.W. Gurney, Ionic Processes in Solutions, Dover, New York, 1953. [13] E.R. Nightingale, in: B.E. Conway, R.G. Barradas (Eds.), Chemical Physics of Ionic Solutions, Wiley, New York, 1967, p. 53. [14] L. Gierst, E. Nicolas, L. Tytgat-Vandenberghen, Croat. Chem. Acta 42 (1970) 117. [15] Y. Be´rube´, P.L. de Bruyn, J. Colloid Interface Sci. 28 (1968) 92. [16] T.W. Healy, D.W. Fuerstenau, J. Colloid Sci. 20 (1971) 376. [17] Th.F. Tadros, J. Lyklema, J. Electroanal. Chem. Interfacial Electrochem. 22 (1969) 9. [18] M. Colic, G.V. Franks, M.L. Fisher, F.F. Lange, Langmuir 13 (1997) 3129. [19] F. Dumont, The Colloid Chemistry of Silica, in: H.E. Bergna (Ed.), ACS Advances in Chemistry Series No. 234, Washington DC, 1994, pp. 143. [20] R.K. Iler, The Chemistry of Silica, Wiley Interscience, New York, 1979. [21] L.H. Allen, E.G. Matijevic, J. Colloid Interface Sci. 31 (1969) 287. [22] J. Depasse, A. Watillon, J. Colloid Interface Sci. 33 (1970) 430. [23] T.W. Healy, The Colloid Chemistry of Silica, in: H.E. Bergna, (Ed.), ACS Advances in Chemistry Series No. 234, Washington DC, 1994, pp. 147. [24] J. Depasse, J. Colloid Interface Sci. 194 (1997) 260. [25] G.M. Torrie, P.G. Kusalik, G.N. Patey, J. Chem. Phys. 91 (1989) 6367.
Adsorption sequence of the alkali cations at the tungsten trioxide–water interface Freddy Dumont *, Paul Verbeiren, Claudine Buess-Herman Faculte´ des Sciences CP 255, Uni6ersite´ Libre de Bruxelles, 2 Boule6ard du Triomphe, 1050 Brussels, Belgium
Abstract The stability of pyrogenic tungsten trioxide hydrosols is studied in the presence of various monovalent cations between pH 2 and 12. The hydrosol critical coagulation concentrations (c*) are determined by extrapolation of the experimental log W –log c curves up to log W =0. The c* of K + , Rb + and Cs + slightly increase with the pH of the solution while the c* of Li + presents a pronounced maximum at pH 4.5. A less marked maximum is also observed at the same pH in the presence of Na + . The corresponding ionic adsorption sequence at the water – WO3 interface deduced from these experimental data is Cs + \ Rb + \ K + \ Na + \ Li + in the pH domain under investigation. This overall behavior is successfully explained by the model proposed by Gierst who has generalized the Gurney concept describing the ion–ion interactions in solution to the ion – surface interactions. The results reported in this work are also compared with those obtained previously for other oxides; they confirm the validity and the generality of the conclusions already drawn concerning the ionic adsorption sequences at the oxide – water interface. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Adsorption sequences; Gierst model; Tungsten trioxide hydrosol; Coagulation; Point of zero charge
1. Introduction Many properties of colloidal dispersions in aqueous solution depend directly on the adsorption of ions on the particle surface. Among others, the stability of an hydrosol at a given pH is closely related to the nature of the ions present in the solution: the stronger the ion adsorption, the lower the concentration required to coagulate the
Dedicated to the memory of Julie and Me´lissa, Ann and Eefje, and Loubna * Corresponding author. Tel.: + 32-2-6502989; fax: + 32-26502934. E-mail address: [email protected] (F. Dumont)
hydrosol. A good knowledge of the mechanisms that govern the ion adsorption is of primary interest since it could allow prediction or at least understanding of the behavior of colloidal dispersions in solution. In the case of oxides, the electrical surface charge originates from the acido–basic reactions of the surface hydroxyl groups according to M OH − U M OH2+ M OH + OH − U M O − + H2O The net electrical surface charge is equal to the difference between the number of the negatively and the positively charged sites, its value depends
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directly on the pH of the solution and on the chemical nature of the surface through the equilibrium constants of the acido – basic reactions. Therefore, for a certain pH value the net electrical surface charge is equal to zero. This particular pH, known as the point of zero charge (pzc), is a fundamental property of the oxide [1,2]. Previous works were devoted to the study of the ionic adsorption sequences at several oxide– water interfaces with pzc values ranging from 2.8 to 8.5. They have clearly shown that the adsorption sequences of cations and anions depend on the pzc value of the oxide [3 – 6]. The present work was undertaken to verify if the results obtained in the study of tungsten trioxide, an oxide with a very low pzc value (around pH 1–2), confirm or invalidate the conclusions drawn previously.
2. Materials The tungsten trioxide hydrosols were prepared by precipitation of tungstic acid from potassium tungstate solutions at a pH close to 1. The precipitate was then boiled for 1 week under reflux conditions. After this maturation-dehydration step, the solid was thoroughly washed by successive centrifugation cycles, the sediment being redispersed in bi-distilled water. This process was repeated until the conductivity of the supernatant remained constant (5 10 − 5 S cm − 1). The exact composition of the so-obtained tungsten oxide is generally poorly defined. This problem can nevertheless be solved by a high temperature treatment of the solid that leads to the stable WO3. The sol resulting from the first step was gently heated to dryness and the obtained powder was then maintained at 650°C for 12 h. Finally, the powder was ultrasonically redispersed in bi-distilled water. Finally, the monodispersity of the resulting hydrosol was improved by successive centrifugation cycles at low and high fields to eliminate the largest and the smallest particles. X-ray analysis indicated that the particles were essentially amorphous. The transmission electron microscopy pictures showed that the particles were quasi-spherical,
their mean diameter and the corresponding standard deviation were found to be equal to 45 and 10 nm, respectively. Photon correlation spectroscopy measurements led to slightly different values, 39 and 15 nm, respectively, as deduced from the auto-correlation function analysis by the cumulants method. The agreement between both sets of results may nevertheless be considered as satisfactory.
3. Measurement of the ionic adsorption sequences The critical coagulation concentration of an hydrosol, c*, is defined as the transition concentration between the fast and the slow coagulation regimes. In the fast coagulation or Schmoluchovski domain, the half coagulation time, Ts, is independent of the electrolyte concentration. On the contrary, in the slow coagulation conditions the half coagulation time, T, increases when the electrolyte concentration decreases. T is related to the Smoluchovski half coagulation time by the equation T= W TS
(1)
where W is the stability factor of the hydrosol. In the slow coagulation domain, the log W– log c relationship is quasi-linear. The c* values can thus be determined by extrapolation of the experimental log W–log c curves up to log W=0. The theory of colloidal suspension stability (DLVO theory) indicates how c* is related to the Hamaker constant, A, and to the Stern potential, cd, by the following equation [7] c*= B
c 4d A2 z2
(2)
where B is a constant and z is the ion valency. This equation was deduced from an oversimplification of the DLVO theory and is, consequently, of very little use. Nevertheless, its great merit is to stress clearly that c* is extremely sensitive to the cd potential. As a matter of fact, the electrical surface potential, c0, is imposed by the solution pH—it is nearly independent of the nature of the counter-ion—at least, in the absence of strong specific adsorption. On the contrary, the
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Fig. 1. Examples of the variation of the absorbance of the coagulating hydrosol with time. (1) Smoluchovski fast coagulation; (2) slow coagulation. The slopes at the origin allowing the calculation of the stability factor W are broken lines.
Stern potential, cd, depends strongly on the ion adsorption, the stronger the ionic adsorption, the lower the cd value. The ionic adsorption sequence can therefore be deduced from the experimental c* values. Toelstra and Kruyt [8] have demonstrated that, during the coagulation process, the absorbance of an hydrosol increases linearly with time, t, according to the equation
As =As0 1+
2t W Ts
curves in the Smoluchovski region to the measured one at the actual electrolyte concentration. The measurements were consequently performed at 630 nm using a Bausch and Lomb Spectronic 700 spectrophotometer. The details of the experimental setup are given elsewhere [3]. The chloride salts (analytical grade) of the cations were used. The pH of the solution was adjusted with HCl in acid solution and with the cation hydroxide in alkaline solution.
(3)
where As0 is the absorbance of the stable hydrosol. This equation is valid when the particle size is significantly smaller than the incident radiation wavelength (Rayleigh law). Therefore, this linear relationship is not expected to hold very long since the mean particle size increases when the coagulation progresses, so that the aforementioned conditions cease to be fulfilled. Nevertheless, if the stable hydrosol meets the Rayleigh conditions, the stability factor W is merely equal to the ratio of the slope at the origin of the As –t
4. Results Examples of As –t curves measured at two electrolyte concentrations are presented in Fig. 1. Curve 1 corresponds to the Schmoluchovski coagulation while curve 2 is related to a slow coagulation process. The negative curvature is more pronounced in the Schmoluchovski regime, a consequence of the aforementioned phenomenon. Fig. 2 summarizes the most important features of the measured log W–log c curves.
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Fig. 2. Log W –log c curves measured in the presence of various cations. (1) Cs + , pH 4.1; (2) K + , pH 11.2; (3) K + , pH 3.4; (4) Na + , pH 6.0; (5) Li + , pH 10.0; (6) Li + , pH 4.3.
As expected, log W decreases linearly with log c in dilute electrolyte solution and becomes constant when the concentration is further increased. The slope of the reported curves depends neither on the pH nor on the nature of the studied cation; this clearly suggests that the coagulation mechanism remains the same in all the experimental conditions of this work. The variations of the critical coagulation concentrations with the pH for the five studied cations Li + , Na + , K + , Rb + and Cs + are given in Fig. 3. The c*–pH curve measured in the presence of Li + shows a decided maximum at pH 4.5, a less important maximum is also observed at the same pH for Na + whereas the c* values of K + , Rb + and Cs + increase slightly with the pH of the solution until they reach a constant value above pH 7. The extrapolation of the c* curves to zero leads to the determination of the isoelectric point (iep) of the hydrosol; its value amounts to approximatively 1.8. In this case, as a strong specific adsorption of the studied ions is never observed, the iep and the pzc values coincide.
The alkali adsorption sequence deduced from the c* values may be written as Cs + \ Rb + \ K + \ Na + \ Li + It remains unchanged in the whole pH range under investigation. Finally, it was obviously not possible to determine the adsorption sequence of the anions because the electrolyte concentration required to fix the pH below the pzc is too high and causes an immediate coagulation of the hydrosol.
5. Discussion The first detailed studies of ionic adsorption have been carried out at the mercury–water interface. The adsorption of the alkali cations at the water–mercury [9] interface follows the sequence Cs + \ K + \ Na + \ Li + A first explanation of this sequence, known as the Hofmeister lyotropic sequence, is based on the
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Fig. 3. Variation of the critical coagulation concentrations (c*) with the pH of the solution. (1) Li + ; (2) Na + ; (3) K + ; (4) Rb + ; (5) Cs + .
Stern picture of the inner part of the double layer: in the absence of any specific chemical effect, the adsorbability of an ion at an interface is determined by its electrical charge (the higher the valency of the counter-ion, the higher its adsorption) and its size (the larger the size of the adsorbed species, the lower its adsorption). The size of the adsorbate imposes its distance of closest approach to the surface. Accordingly, the reported experimental Hofmeister sequence leads to conclude that the alkali cations remain hydrated when they adsorb at the mercury – water interface. However, in the presence of pre-adsorbed pyridine [10] at the same interface, the reversed adsorption sequence is obtained Li + \Na + \K + \Cs + Therefore, from this anti-Hofmeister sequence, it must be concluded that the cations undergo a dehydration during the adsorption. Such a different behavior in very similar electrical field conditions is difficult to understand; moreover, as the cations have no particular affinity for the pyridine molecule, it is hard to explain this inversion by
some specific chemical effect. However, there is actually no physical reason to reject the charge and size effects of the Stern model and it must therefore be concluded that the size of the ion is not the only parameter that plays a role in fixing the ion–surface distance of closest approach. Gierst et al. [11] solved this problem by proposing a generalization of the Gurney model of ion– ion interactions in solution to ion–surface interactions. According to Gurney [12], the local structure of water near an ion is different from that in the bulk phase as a result of the action of the local electric field of the ion on the solvent molecules. The interaction forces between an ion and a water molecule may be stronger or weaker than the interaction forces between two water molecules. In the first case, the solvent is more structured in the vicinity of the ion than in the bulk phase. This situation is encountered with polyvalent and small monovalent ions such as Li + , Na + and F − which are known as structure-maker or promoter. In spite of their relatively large size, IO3− and BrO3− are also structure-makers; this unexpected
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property is well explained by Nightingale [13]. In the second case, the water molecules are less structured near the ion than in the bulk phase. The larger alkali cations K + , Rb + and Cs + , halide anions like I − , Br − and Cl − and several polyatomic anions like ClO4− , SCN − , ClO3− , NO3− belong to this structure-breaker family. In ionic solutions, the most important ion–ion interactions are electrical, but according to Gurney, short range forces originating from the respective action of each partner on the water structure are superimposed to the purely electrical force. These forces are attractive when both partners exert the same structural action on the solvent and repulsive in the opposite situation. The Gurney description leads for instance to a coherent interpretation of the activity coefficients of the alkali halides. Gierst et al. [11] extended this concept to the description of the ion – surface interactions; the surface is then assimilated to a macro ion which is able to promote or destroy the water structure in its vicinity. Accordingly, at a given surface potential, a structure-maker ion will be more adsorbed at a structure-maker surface than a structure-breaker one and inversely. This model explains successfully not only the inversion of the alkali ion adsorption sequence at the water–mercury interface in the presence of pyridine but also the effect of the nature of the so-called indifferent ions on the kinetics of many reduction reactions at the mercury electrode [14]. This model was used later by Berube and Debruyn [15] to explain the Li + \Cs + adsorption sequence deduced from potentiometric titrations of titanium dioxide suspensions. Dumont et al. [3 – 6] studied a series of oxides. In these works, the experimental ionic adsorption sequences were used as a tool to analyse the surface properties. This approach led to the conclusion that the low pzc oxide surfaces behave as structure-breakers wheareas the high pzc ones are structure-makers. The transition pzc value was around pH 4. The pzc of an oxide is related to its heat of immersion (− DHi) in water according to the semi-empirical equation of Healy and Fuerstenau [16]
(−DHi)= 4.606 RT pHpzc + (−DHc)
(4)
where (− DHc) is an integration constant. This relationship indicates clearly that the structuring power of an interface increases with its heat of immersion. It is worth noting that the same correlation exists between the heat of hydration of an ion and its structuring power [4]. We have also found that the structuring power of a surface increases with its electrical charge. In particular, it was ascertained that a surface with a slightly pristine structure-breaker character becomes a structure-maker when its surface charge is increased. This phenomenon is well illustrated by previously reported results on a TiO2 hydrosol (pzc 4.6) [5] and a SnO2 hydrosol (pzc 3) [6] where the alkali cations adsorption sequence goes from Cs + \ Li + to Li + \ Cs + as the surface charge is increased by moving the solution pH away from the pzc. This may be explained merely by assuming that the actual structuring power of the surface results at least from the sum of two effects: the pristine action of the dispersed phase and the orientation of the water dipoles by the electrical forces originating in the superficial charges. The experimental ionic adsorption sequence Cs + \ Li + observed in this work indicates that the WO3 surface is structure-breaker in the whole explored pH range 2–12. Moreover, the relatively small values of the critical coagulation concentrations c* of K + , Rb + and Cs + indicate a relative strong adsorption of these structure-breaker cations. This conclusion agrees completely with the predictions of the model since the WO3 pzc is significantly smaller than 4. The fact that the sequence remains unchanged constitutes an indication of the high structurebreaker character of WO3 because the surface electrical charge is unable, by its sole action, to change the intrinsic properties of the surface. Nevertheless, the c* values of Li + and Na + pass through a maximum around pH 4.5 and go down slowly until pH 7. This decrease of c* corresponds to a slight increase of the adsorption of these ions, an effect that can only be attributed to a slight attenuation of the structure-breaker power of the surface, in close agreement with the previous description. Finally, above pH 7, the c*
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values become constant and independent of the solution pH for all the studied cations. Such an effect indicates that the Stern potential and consequently the surface charge does not increase anymore with the solution pH. This kind of situation is only encountered when the fraction of the dissociated surface groups is extremely close to 1. This suggests that the intrinsic dissociation constant of the hydroxyl groups is high and also that their number density is relatively small, this conclusion needs to be confirmed by potentiometric titrations of WO3 suspensions. A similar situation prevails at the SiO2 – water interface (pzc 2–3). The ionic adsorption sequence at this interface was deduced from potentiometric titration data [17]. The observed sequence remains of the structure-breaker type between pH 2 and 13, but, at the same time, the relative difference between the amounts of the adsorbed cations Li + , Na + , K + and Cs + is found to lessen when the solution pH goes up. In a recent paper, Colic et al. [18] studied the viscosity of high volume fraction alumina suspensions in the presence of alkali cations. An electrophoretic study was first performed, it had shown that the adsorption at the alumina–water interface decreases from Li + to Cs + . This structure-maker sequence is in full agreement with the high pzc value (9.0) of alumina. The viscosity measurements have clearly demonstrated that the behavior of the suspension is determined by the particle interaction forces and more precisely by the depth of the interparticle interaction potential well. This one results from the balance between the van der Waals attraction forces (assumed independent of the nature of the cation) and the repulsive forces originating in the presence of the adsorbed cations. The corresponding equilibrium separation distance between two particles, although difficult to be assessed, increases significantly from Li + to Cs + . As a matter of fact, the heavily hydrated structure-maker Li + is well embedded inside the structured water layer at the particle surface whereas the poorly hydrated structure-breaker Cs + is maintained far away as a consequence of the incompatibility between the solvent structure at the surface and around the cation. All these facts constitute also a substantial
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support to the Gierst model. However, in the same paper, the authors have summarized several preliminary results of experiments performed on a series of materials including silica. They have reported that the cations sequence was found to be the same for all the dispersions investigated and identical to that observed on alumina. In the particular case of silica, this result appears to be at variance not only with the predictions of the Gierst model but also with the experimental adsorption sequence deduced from potentiometric titration experiments [17]. In our opinion, this contradiction is just apparent and may be removed easily by remembering that the behavior of silica suspensions is actually far from comparable with any other colloidal system. The most important reason for this particular behavior lies in the very low value of the Hamaker constant of the SiO2 –water system, which results in very weak van der Waals attraction forces between two silica particles dispersed in water [19]. This explains why the small sized silica hydrosols are stable at their point of zero charge [20], a quite unusual behavior in colloidal science. However, the same hydrosols, paradoxically, coagulate in alkaline solutions where their surface charge should act as a supplementary stabilizing factor [21,22]. Such a general behavior is far from tractable by the DLVO theory [23]. Nevertheless, this problem was solved by Depasse and Watillon [22,24] who demonstrated that the coagulation in alkaline solutions is due to the formation of some kind of chemical bond between the particles and, moreover, that the strength of this bond decreases in the order of the alkali cations acidity character: Li + , Na + , K + , Cs + . Consequently, the interparticle attraction forces which are responsible for the suspension viscosity increase in the order Li + to Cs + , identical—but for quite different reasons—to the one observed in the DLVO suspensions like zirconia and alumina.
6. Conclusions The experimental results observed at the WO3 – aqueous electrolyte solution interface confirm that the general description of the ionic adsorption at
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interface follows the Gierst model. Presently, to the best of our knowledge, no available experimental data invalidate this model, we may conclude its general validity. However, we are quite aware that the weak point of this description rests in the fact that it is only qualitative and, therefore, unable to make any quantitative prediction. This indicates that firstly, a complete theory of the ionic adsorption phenomenon is needed and, secondly, that theoretical works and molecular dynamic approaches that would ignore the effect of the surface on the solvent structure are doomed to be incomplete, except in very particular conditions. Accordingly, the theory of the ion size effects on the adsorption in molecular solvent (Torrie et al [25]) should only be applied to describe the adsorption at an oxide surface with a pzc close to 4 since this particular surface is known to have very little influence on the water structure.
Acknowledgements One of the authors (Paul Verbeiren) acknowledges gratefully the David et Alice Van Buuren Foundation for financial support.
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