From Powders to Dispersions in Water: Effect of Adsorbed Molecules

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From Powders to Dispersions in Water: Effect of Adsorbed Molecules on the Redispersion of Alumina Particles S. Desset,† O. Spalla,*,† P. Lixon,† and B. Cabane‡ CEA Saclay, Service de Chimie Mole´ culaire, 91191 Gif sur Yvette, France, and Laboratoire PMMH, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France Received January 31, 2001. In Final Form: June 22, 2001 Aqueous dispersions of colloidal alumina particles have been submitted to processes where the dispersion is aggregated, dried, and then redispersed in water. This cycle was performed with alumina particles that had different surface states: bare surfaces with monovalent counterions; surfaces that have been covered with small molecule ligands and have their surface charge reversed. In all cases, redispersion was produced by an increase in the ionic pressure due to the counterions, after immersion of the powder in water at the appropriate pH. In all cases, the pH that produced the appropriate ionic pressure was 5-6 pH units away from the isoelectric point of the surfaces. This was related to the fact that the surfaces were, in all cases, separated by a thin water film, which had a constant thickness (8 Å) set by the strength of binding of water to the alumina surfaces. However, for surfaces that had been covered with high amounts of macromolecular ligands, the minimum separation of surfaces was increased, and redispersion could be obtained at a pH which was closer to the isoelectric point. These phenomena can be accounted for, quantitatively, through a calculation of the balance of surface forces, including van der Waals attractions, hydration forces, and ionic pressures.

I. Introduction Most solids can be dispersed in water as colloidal particles.1 The dispersed state, where the particle surfaces are separated by water, is metastable2 with respect to any of the aggregated states, where they have been brought into contact. This is because the energies of the aggregated states are lowered by interactions between surfaces, such as chemical bonds, H bonds, and dispersion forces. Consequently, all lyophobic aqueous dispersions aggregate irreversibly upon drying.3 Once in the dry state, the addition of water will not produce spontaneous redispersion, and the input of mechanical energy is required to break interparticle bonds; even so, redispersion to the level of the primary particles is generally not achieved, unless brute force methods are used. The passage through a dry state is, however, unavoidable for many applications of colloidal dispersions. In some cases, water must be removed because the particles will be used in a nonaqueous environment. In many other cases, it is simply more convenient to store the dispersion as a dry powder and add water only at the application stage. Also, in ceramic manufacturing, there is a need for aqueous dispersions that can be deposited as thin films, dried, and then removed selectively by redispersion in water. At present, the efficiency of these processes is limited by the aggregation that occurs upon drying. In a previous paper,4 we reported that alumina particles could be dispersed in water, aggregated, dried, and then redispersed in water through the application of very weak forces or even with no forces at all. Similar observations † ‡

CEA Saclay. Laboratoire PMMH.

(1) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: New York, 1989. (2) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the stability of lyophobic colloids The interaction of sol particles having an electric double layer; Elsevier: Amsterdam, 1948. (3) Frens, G.; Overbeek, J. T. G. J. Colloid Interface Sci. 1972, 38, 376. (4) Desset, S.; Spalla, O.; Cabane, B. Langmuir 2000, 16, 10495.

were made earlier by Rohrsetzer et al.5,6 At first sight, this would seem to conflict with the statements made above, regarding the irreversible character of aggregation caused by drying. However, close examination of the alumina powder obtained after drying revealed that the particles had retained at least one layer of water molecules on their surfaces. Moreover, redispersion in water took place beyond a threshold in pH; therefore, electrostatic forces between the hydrated surfaces must have been the driving forces. It was concluded that redispersion took place when the balance of surface forces (van der Waals attractions and electrostatic repulsions) became even or repulsive at the interparticle distance set by the remaining water layer.7-9 Therefore, these redispersion phenomena do not conflict with the general statements regarding surface energy.10 To use these phenomena in practical applications, it is necessary to have some control of the conditions where they take place. For the alumina particles used in the previous study, full redispersion took place only at very low (12) pH. This restricted range severely limits the usefulness of spontaneous redispersion: for most applications, it would be preferable to obtain redispersion through addition of water at neutral pH. Since redispersion is caused by surface forces, this problem is equivalent to obtaining the right surface forces between surfaces that are immersed in a given aqueous phase. Throughout the history of colloid science, much work has been done on the control of surface forces for the purpose of enhancing colloidal stability or delaying the onset of colloidal aggregation. There is good reason to believe that (5) Rohrsetzer, S.; Paszli, I.; Csempesz, F.; Ban, S. Colloid Polym. Sci. 1992, 270, 1243. (6) Rohrsetzer, S.; Ban, S.; Kovacs, P.; Paszli, I. Colloid Polym. Sci. 1995, 273, 189. (7) Parsegian, V. A.; Evans, E. A. Curr. Opin. Colloid Interface Sci. 1996, 1, 53. (8) Israelachvili, J. Intermolecular and surface forces; Academic Press: Ltd.: New York, 1992. (9) Leneu, D.; Rand, R. P.; Parsegian, V. A.; Gingell, D. Biophys. J. 1976, 18, 209. (10) Wennestro¨m, H. Colloids and Surf., A 2000, 167, 209.

10.1021/la010166t CCC: $20.00 © 2001 American Chemical Society Published on Web 09/18/2001

Redispersion of Alumina Particles

the same procedures could be used to control the reverse process, i.e., redispersion. It has been known for a long time that small adsorbing molecules can modify the surfaces of inorganic particles and thus alter (or improve) the stability of dispersed mineral suspensions.11-17 In the ceramic community,18-21 such adsorbed molecules have been called “stabilizers”, “peptizers”, or “dispersants”. In a chemical sense, all of them are ligands of the surface sites. For instance, the most common dispersants used at low pH for oxide dispersions are polycarboxylic acids such as citric acid or polyacrylic acid, which can form inner- or outer-sphere complexes with the surface hydroxyls. Nevertheless, from a fundamental point of view, the exact role of these ligands is not completely understood. Indeed, the addition of these molecules may change three types of colloidal properties. First, the addition of such molecules to a stable dispersion may prevent the flocculation that would be caused by a shift in pH: this is the stabilizing effect of “dispersants”. Second, the addition of such molecules to an aggregated aqueous suspension may enhance the repulsion between accessible surfaces and cause the particles to redisperse more easily: this is the peptizing effect of “dispersants”. Third, the addition of such molecules before the drying of the suspension may facilitate the redispersion of the dry powder because they keep the surfaces apart during drying: this is the protecting effect of “dispersants”. The stabilizing effect has been demonstrated many times.22,23 On the other hand, the peptizing and/or protecting effects are not clearly separated in the experimental reports and even the differences between these two notions have not been clearly recognized. The aims of this work were to measure, quantitatively, the stabilizing, peptizing, and protecting effects of ligands for a chosen colloidal dispersion and to determine which one of these effects is the cause of the redispersion of the dried powder into water. As a colloidal system, we used the same R-Al2O3 particles that were already studied in our previous work.4 The ligands were selected among the dispersants that are most frequently used for ceramic manufacturing: citric acid; other R-hydroxy carboxylic acids; polyacrylic acid. In a first step, we characterized the interaction of these molecules with the alumina surfaces, mainly through adsorption isotherms. Then we measured the “stabilizing” effect of these ligands on the R-Al2O3 dispersions in water, characterized as a shift in the range of pH where colloidal stability was achieved. Subsequently we measured their (11) Biggs, S.; Scales, P.; Leong, Y.-K.; Healy, T. J. Chem. Soc., Faraday Trans. 1995, 91, 2921. (12) Fauconnier, N. Ph.D. Thesis, Universite´ de Paris VI, 1996. (13) Chevalier, Y.; Brunel, S.; Le Perchec, P.; Mosquet, M.; Guicquero, J.-P. Prog. Colloid and Polym. Sci. 1997, 1997, 6. (14) Mosquet, M.; Chevalier, Y.; Brunel, S.; Guicquero, J.-P.; Le Perchec, P. J. Appl. Polym. Sci. 1996, 65, 2545. (15) Peyre, V.; Spalla, O.; Belloni, L.; Nabavi, M. J. Colloid Interface Sci. 1997, 187, 184. (16) Peyre, V.; Spalla, O.; Belloni, L. J. Am. Ceram. Soc. 1999, 82, 1121. (17) Guldberg-Pedersen, H.; Bergstrom, L. Acta Mater. 2000, 48, 4563. (18) Cesarano, J.; Aksay, I. A.; Bleier, A. J. Am. Ceram. Soc. 1988, 71, 1062. (19) Graule, T.; Hidber, P.; Hofmann, H.; Gauckler, L. In EuroCeramics, Basic Science and Processing of Ceramics; Ziegler, Ed.; 1993; p 299. (20) Hidber, P.; Graule, T.; Gauckler, L. J. Am. Ceram. Soc. 1996, 79, 1857. (21) Laucournet, R.; Pagnoux, C. J. Am. Ceram. Soc. 2000, 83, 2661. (22) Biggs, S.; Healy, T. J. Chem. Soc., Faraday Trans. 1994, 90, 3415. (23) Hidber, P.; Graule, T.; Gauckler, L. J. Eur. Ceram. Soc. 1997, 17, 239.

Langmuir, Vol. 17, No. 21, 2001 6409 Scheme 1. Cryo-TEM Image of an Aqueous Suspension of AKP50 Dispersed at pH 5 with Ultrasounds

“peptizing” effect, by making a powder from the dispersion of bare R-Al2O3 particles and then adding either pure water or a solution of the ligands and measuring the extent redispersion according to the protocols defined in our earlier work. Finally we measured their “protecting” effect by adding the ligands to the aqueous dispersion before turning it into a powder and then measuring the extent of redispersion in water according to the same protocols. The analysis of these results was done in two ways. First, the effect of the ligands was modeled as an adsorbed layer characterized by a thickness and a density of surface charges. Then the balance of surface forces and applied forces that operate during the redispersion protocol was determined. This gave a prediction for the extent of redispersion according to the thickness of the gap between two particles and the pH of the aqueous phase. Finally, these predictions were compared with the measured redispersion ratios. This gave a clear view of the mechanisms by which different ligands promote redispersion. According to these mechanisms, it is possible to picture the “ideal complexing agent” that would be best suited for promoting the redispersion of dry powders. II. Materials and Methods A. Materials. 1. Alumina. The source of alumina was the R-Al2O3 powder manufactured by Sumitomo (type AKP50). This type of alumina is synthesized through the hydrolysis of very pure organometallic precursors (AlR3 or Al(OR)3) followed by calcination at high temperature (T > 1100 °C) and then fragmentation through ball milling.24 Examination of the powder through scanning electron microscopy shows large aggregates with polydisperse sizes ranging from a few micrometers to a few hundreds of micrometers. These aggregates are held together by very strong forces and do not break up when they are dispersed in water; consequently, this powder cannot be used as it is for aggregation-redispersion cycles. Aqueous suspensions were made by mixing this powder with water and using high-power ultrasound to break all aggregates. With this procedure, suspensions containing 10% alumina by weight were prepared in this way either at pH 5 or at pH 11. The particles had an average diameter of 0.2 µm, with a narrow size distribution (see photograph in Scheme 1). Zetametry and acid base titration experiments indicate that the isoelectric point (IEP) of the particles was at pH ) 9.3 ( 0.15.25 At pH values close to the IEP (8.5-10), the suspensions were flocculated. At all other pH values, they had colloidal stability. These stable colloidal suspensions were used for all the aggregation-redispersion studies reported here. 2. Ligands. The ligands were chosen among molecules that are known to form surface complexes with alumina.19,23,26-28 Such (24) Horikiri, S. In FC Annual report for overseas readers; 1986; p 23. (25) Desset, S. Ph.D. Thesis, CEA Saclay DRECAM/SCM, Universite´ Paris VI, 1999. (26) Graule, T. J.; Gauckler, L. J. In Third Euro-Ceramics; 1993; p 491.

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molecules usually contain carboxyl and/or hydroxyl groups, which enable the molecule to coordinate to the alumina surface. They are citric acid (from Prolabo, Rectapur), polyacrylic acid (abbreviated PAAH, Mw ) 2000 g/mol from Aldrich), 1,2-dihydroxy3,5-dibenzenedisulfonic acid, (Tiron, Aldrich), 1,2,3,4-butanetetracarboxylic acid (Aldrich), and glycolic acid (Aldrich). Their concentration in solution was determined using a total organic carbon analyzer (TOC) (Dohrman DC-180). B. Methods. 1. Adsorption Isotherms. For the interpretation of redispersion experiments, it was important to know the extent of coverage of the alumina surfaces by adsorbed ligands. These adsorbed amounts were measured through the depletion of the ligand concentration in the supernatant. Samples were prepared as indicated above, by mixing alumina powder into an aqueous solution of the ligands and using high-power ultrasound to obtain an aqueous suspension of the 0.2 µm particles. In the case of ligands that were polymers, the pH of the ligand solution was always set initially to 9 to avoid the irreversible adsorption that would occur at lower pH values. These samples were then left to equilibrate for 2 days, while the pH was adjusted at regular intervals through additions of dilute solutions of NaOH and/or HNO3. The samples were then centrifuged for 5 min at 25 000 rpm, and the concentration of ligands remaining in the supernatant was determined by TOC. With strong ligands, some aluminum ions became complexed by the excess ligands and dissolved in the aqueous phase. The concentration of these dissolved aluminum ions was measured through atomic absorption spectroscopy. 2. Adsorption-Desorption Kinetics. It was important to know how the adsorbed ligands respond to a change in solution conditions, such as a pH jump. Indeed, redispersion was achieved by a pH jump from conditions where the particles are aggregated to conditions where they repel each other. Accordingly, a suspension containing 10 wt% of alumina and 0.085 wt% of citric acid was prepared as described above and then divided into four samples which were submitted to different equilibration conditions and pH jumps: (i) A sample initially at pH 5 was brought to pH 9. Another one took the opposite path (pH 9 to 5). (ii) A sample initially at pH 5 was centrifuged for 10 min at 9000 rpm. The supernatant was separated, and the remaining sediment was dried under P2O5 for 1 week. Afterward, the stored supernatant was added to the dry powder and pH brought to 9. Another sample took the opposite path (pH 9 to 5). These 4 cycles were also repeated with a suspension containing 10 wt% alumina and 0.09 wt% polyacrylic acid. In each case the amounts of citric acid or polyacrylic acid in the supernatant were monitored after the pH jump, until equilibrium was reached. 3. Colloidal Stability. The adsorption of ligands changed the surface charge of the particles. This produced a shift in their IEP and also in the pH ranges where the suspensions were either stable or flocculated. For most samples, these ranges could be determined visually, through assessment of the sedimentation rate of the suspensions: suspensions that settled in less than 5 min were classified as flocculated. These results were used to construct a stability diagram for the suspensions, according to surface coverage and pH. For samples with a high surface coverage of ligands, the flocculation threshold was determined more accurately through a turbidity measurement. An example of the procedure is as follows. Two suspensions containing 10 wt% alumina and either 0.2% citric acid or 0.4% polyacrylic acid were dispersed at pH 9 with ultrasound. These suspensions were maintained for 2 days at pH 9 to let them reach equilibrium and then diluted to 6 g/L in citric acid (2 × 10-4 mol/l) or polyacrylic acid (2 × 10-3 mol/l of monomer units) at different pH values. Then a turbidity measurement (see below) was used to measure the proportion of particles that had remained dispersed as a function of the final pH. 4. Drying. Once the particle surfaces were covered with the chosen amount of ligands, the particles were recovered through centrifugation, and the sediment was dried with P2O5 (relative

humidity 0-3%). In this way we obtained the powders that were used in all subsequent redispersion experiments. 5. Redispersion. The dry powders with ligand-covered surfaces were compared according to their ability to redisperse under the effect of the hydrodynamic force caused by sedimentation. For each sample, a small amount of powder was poured into a 50 mL test tube (to give a weight fraction of 0.1%) filled with water and the pH was adjusted with HNO3 or NaOH. These tubes were slowly rotated (60 rpm) about an axis normal to their length. The typical duration was 2 h. Then the quality of the redispersion was measured through turbidimetry 6. Turbidimetry. The fraction of redispersed particles was assessed through turbidimetry. The dilute suspension was set to rest in a vertical position for 18 h. Sedimentation took place during this time. It can be calculated from the Stokes law that, in this time, all aggregates larger than 530 nm have traveled down more than 3 cm. At the end of the sedimentation time, 2 mL of the suspension, located at 3 cm below the surface, were extracted. This volume corresponds to a total height of 3 mm in the sample tube. According to its location in the sedimentation gradient, this volume must contain unit particles or small aggregates that have diameter smaller than 500 nm. Consequently, this procedure selects all nonaggregated or weakly aggregated particles. This is a very good approximation to the fraction of nonaggregated particles. Indeed, the complete size distributions measured at various stages of redispersion are bimodal,4,29 containing mostly large aggregates and elementary particles and very few aggregates of intermediate sizes.4 The total mass of these particles was then determined through a measurement of the turbidity of the collected volume. Indeed, the turbidity values can be used to determine particle concentrations, provided that all particles have a known standard size. For this purpose, the samples collected from the sedimentation experiment were submitted to strong ultrasound irradiation for 30 s. The turbidity values of these homogenized suspensions are proportional to the alumina concentration, which can be determined through a calibration procedure.4 7. Electrophoretic Mobility. The electrophoretic mobilities of particles were measured under different ionic conditions with a Coulter Delsa 440 SX. The concentration of particles was very low (0.5 g/L), and a background of 1 mM KNO3 was added to the suspensions. 8. Complete Separation and Redispersion Cycles. In summary, we started from a powder in which the primary particles were aggregated through very strong bonds. We dispersed this powder in water through brute force methods (ultrasound), and then we modified the particle surfaces through ligand adsorption. These dispersions were then dried to make powders in which the interparticle contacts were controlled, first because ligand molecules remained bound to the surfaces and also because the extent of drying was controlled (the surfaces retained one monolayer of water molecules). These powders with controlled contacts were used for redispersion experiments, according to a protocol that included agitation through tumbling, sedimentation, sampling, homogenizing through ultrasound irradiation, turbidimetry, and also pH measurements at various stages. This protocol applies a minimal amount of mechanical energy to the aggregates before sampling, so that the effects of changes in the state of surfaces or in solution conditions show up clearly. The results are redispersion thresholds expressed in terms of surface coverage and pH.

(27) Gourmand, M. Ph.D. Thesis, ESPCI, Universite´ Paris VI, 1998. (28) Laucournet, R.; Pagnoux, C. J. Eur. Ceram 2001, in press.

(29) Laarz, E.; Zhmud, B. V.; Bergstro¨m, L. J. Am. Ceram. Soc. 2000, 83, 2394.

III. Results A. Interaction between Complexants and Alumina. 1. Citric Acid. (a) Adsorption and Desorption Kinetics. The effects of pH jumps on the amount of citric acid adsorbed on alumina are presented in Figure 1. In every case, the adsorption and desorption kinetics were fast: after 5 min, the adsorbed amount was close to its final value, which was reached in 2 h. Moreover, the kinetics measured for the dried powder were just as fast as those measured for the undried samples. This revers-


Figure 1. Adsorbed amount of citric acid versus time elapsed after changing the pH of the suspensions: black circle, from 5 to 9; white circle, from 5 to 9 after drying; gray square, from 9 to 5; white square, from 9 to 5 after drying.

Figure 2. Equilibrium adsorption isotherms of citric acid onto alumina for different pH values.

ibility of the adsorption of small molecules onto mineral surfaces was, to our knowledge, not clearly established before. It has important consequences for the redispersion processes, since it limits the amount of electrical charges that can be brought by small molecule ligands. (b) Adsorption Isotherms. The equilibrium amounts of adsorbed citrate were measured in samples that had been kept for 1 week, with regular pH adjustments to pH values chosen between 4.2 and 10.5. The resulting adsorption isotherms are shown in Figure 2. They show a first stage of adsorption with a very strong affinity of citrate for R-alumina and then a slower rise to the plateau of adsorption. At every pH, this plateau is reached when the free citrate concentration is less than 2 mM. Moreover, the amount adsorbed at the plateau is lower at higher pH, as observed earlier by Hidber.20 Finally, it is worth noting that, at pH 10.5, there remains only 0.42 µmol/m2 adsorbed and none at pH 12. Free citrate ions can also dissolve aluminum ions from the alumina particles. The measured concentrations of soluble aluminum are presented in Figure 3. This shows that the solubilization process becomes significant only when the plateau level of adsorption has been reached. Beyond this point, the concentration of solubilized aluminum rises with the concentration of free citrate; for instance, at pH 6.5 and with a free citrate concentration of 3 mM, the concentration of aluminum in the solution reaches 2 mM.

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Figure 3. Soluble aluminum versus concentration of free citric acid in the alumina suspensions.

Figure 4. Adsorbed amount of PAAH versus time elapsed after a change in the pH for suspensions: black circle, from 5 to 9; white circle, from 5 to 9 after drying; gray square, from 9 to 5; white square, from 9 to 5 after drying.

2. Polyacrylic acid. The adsorption of polymers30 on colloidal particles is quite different from that of small molecules. The macromolecules adsorb through a large number of monomers (therefore the affinity is higher), but an even larger number of monomers are not located at the surface (therefore the adsorbed layer is thicker). A complication occurs when the macromolecules are long, because they can bridge the surfaces together, causing flocculation of the suspension. this would make redispersion difficult. To avoid this problem, we have used short polymers, with an average molar mass Mw ) 2000 g/mol. (a) Adsorption and Desorption Kinetics. The effects of pH jumps on the amount of polyacrylic acid adsorbed on alumina are presented in Figure 4. The adsorption kinetics, measured in a jump from pH 9 to pH 5, was fast in every case: in the first 5 min, the adsorbed amount increased to a value that was close to its final value, measured after 10 h. On the other hand, the desorption kinetics, measured in a jump from pH 5 to pH 9, was too slow to be measured: after 10 h, the adsorbed amount remained much higher than the equilibrium value of the adsorption at pH 9. This hysteresis is in sharp contrast to the reversibility observed with molecular complexants. It is due to the polymeric nature of PAAH2000, because the desorption of a macromolecule requires the simultaneous desorption of many monomers. (30) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T., Vincent, B. Polymers at Interface; Chapman & Hall: New York, 1993.

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Figure 5. Adsorption isotherms for PAAH2000 onto alumina at different pH values: 0, pH 5; O, pH 7; 4, pH 9; 9, pH 10.5. The gray squares presents the remaining adsorbed amounts 2 h after bringing the pH from 5 to the final value noted near the point. The gray cercles are the remaining adsorbed amounts 2 h after bringing the final pH from 9 to the final value noted near the point.

The redispersion test used in this study was to immerse a small amount of dry powder in a large amount of water at a fixed pH with 2 mM of free PAA and to tumble the test tube during 2 h. As the pH of redispersion can be different from the pH at which the powder was dried, the adsorbed amount of PAA 2 h after an initial pH jump were systematically measured. To do so, two dilute suspensions were first equilibrated at pH 5 and pH 9 with 2 mM of free PAA in the supernatant. Both suspensions were then divided into 6 parts and their pH shifted by an addition of acid or base to final values of 5.5, 6.5, 7.5, 8.5, 9.5, and 10.5. After 2 h, the solid was separated from the supernatant through centrifugation and the free amount measured by COT. The deduced adsorbed amounts are collected in Figure 5. Two main conclusions can be drawn from these experiments: (i) When the final pH is lower than the initial pH, the adsorbed amount increases to the equilibrium amount which again means that the adsorption is rapid (less than 2 h). (ii) When the final pH is higher than the initial one, a measurable desorption is observed: jumping from pH 5 to pH 10.5 it decreases from 9.5 to 6.4 µmol/m2 meanwhile jumping from pH 9 to pH 10.5 it decreases from 3.5 to 2.4 µmol/m2. This shows that at pH 10.5 both coverage are not at the equilibrium since the direct adsorption at pH 10.5 yields to a value of 1.64 µmol/m2. This hysteresis also has consequences for the coverage of alumina surfaces during redispersion experiments: contrary to the case of small molecules, the amount of PAAH adsorbed at a given pH depends not only on the total amount of PAAH in solution but also on the sample history and, particularly, on the pH at which the initial adsorption had been performed. (b) Adsorption Isotherms. The isotherms obtained at pH 5, 7, and 9 are presented in Figure 5. As with citric acid, the adsorption at the plateau decreases when the pH increases. At the same pH, the adsorbed amount is twice as high as for citric acid. The shape of the isotherms is less abrupt, but this is not significant, because this shape is controlled by the polydispersity of the polymer, as previously demonstrated by Cohen-Stuart.31 Finally, it is worth noting that, at pH 10.5, there remains only 1.64 µmol/m2 adsorbed and none at pH 12. (31) Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Fleer, G. J. J. Polym. Sci. Polym. Phys. Ed. 1980, 18, 559.

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Figure 6. Stability diagram versus pH and adsorbed amount of citric acid: O, stable dispersion; b, flocculated suspension.

B. Redispersion of Complexed Particles. The effects of the ligands on the properties of the suspensions are presented in this section. There are three possible effects: (i) Ligands can be added to an aqueous suspension, and they change its colloidal stability (stabilizing effect). (ii) A powder of bare particles can be redispersed in water that contains some ligands (peptizing effect). (iii) Ligands can be added to an aqueous suspension, which is dried to yield a powder with protected surfaces, and then the redispersion of this powder in water is measured (protecting effect). 1. Stabilizing Effect. When citric acid is added to alumina suspensions, it causes a change in the surface charge of the particles and a corresponding shift of the pH range in which the suspension flocculates, toward lower pH values. A stability diagram was established by using suspensions that were kept at a set pH and adding increasing amounts of citric acid. Flocculation was observed when the amount of citric acid matched the end of the first stage of adsorption (high-affinity binding). At this point, the ζ potential of the particles had vanished. Further addition of citric acid gave suspensions that regained colloidal stability; in these suspensions the particles had a negative surface potential according to their electrophoretic mobility (-4.9 µm cm/Vs). This behavior shows that the addition of citric acid first cancels and then reverses the surface charge of the alumina particles. As a result, the flocculation region appears in the stability diagram as a band that crosses the diagram at the location of charge cancellation (Figure 6). The mobility of bare particles has been measured versus pH and is reported in Figure 7. The addition of PAA2000 also caused a similar shift of the pH range in which the suspension flocculates. Therefore, the added macromolecules also progressively cancel and then reverse the surface charge of the alumina particles Further experiments were done in which the stability diagram was crossed in the other direction. Alumina suspensions were made with a fixed concentration of citric acid or PAAH (corresponding to saturating amounts at pH 4). Then the pH was varied and the flocculation range was determined by measuring the fraction of dispersed particles through turbidimetry. The results (Figure 8) show the shift in the flocculation region, in comparison with the suspensions or bare particles. The bare particles were flocculated in a narrow pH range around the IEP at pH 9. The addition of a saturating amount of citric acid shifted this range to values below pH 4. With polyacrylic acid, it was pushed below pH 3. This location is in


Figure 7. Mobility of alumina particles versus pH: (() bare particles; (O) in the presence of 0.2 mM free citrates; (b) in the presence of 2 mM of PAA 2 h after bringing the pH from 5 where the coverage was first equilibrated to the final value; (]) in the presence of 2 mM of PAA 2 h after bringing the pH from 9 where the coverage was first equilibrated to the final value.

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Figure 9. Stability diagram for alumina/PAA mixtures: O, stable dispersion; b, flocculated suspension.

Figure 10. Redispersion of alumina powders dried at pH 9 under P2O5 in the absence of complexing molecules in ([) pure water, (0) a citric acid solution (0.2 mM), and (b) a polyacrylic acid solution (2 mM monomer concentration). Figure 8. Flocculation curves, where the shaded areas indicate the pH ranges where the suspensions are flocculated: [, bare alumina; 0, alumina particles in equilibrium with 0.2 mM citric acid; b, alumina particles in equilibrium with 2 mM polyacrylic acid (monomer concentration).

agreement with that shown in the stability diagram through ligand addition at fixed pH. (Figure 9). Thus, the adsorption of the ligands provides a wide pH region (all pH values > 5) where the particles have taken a negative surface charge and the suspensions have colloidal stability. 2. Peptizing Effect. When an aqueous solution of ligands is added to a dry powder, the alumina surfaces equilibrate with all species contained in the solution, including the ligands. Consequently the electrical charges of all free surfaces are modified, and the forces between these free surfaces must change accordingly. This may shift the redispersion thresholds. For these experiments, dry powders of bare alumina particles (30-50 mg) were immersed in solutions (55 g) of the ligands, at pH values set through addition of HNO3 and NaOH. The concentration of free ligands was chosen to be large enough to provide saturation coverage of the surfaces (beginning of the plateau) at all pH values, large enough to ensure that the adsorption or desorption of these molecules with pH does not change significantly their bulk concentration, but not too large so that the ionic strength of the solution is not significantly increased. We chose 2 × 10-4 mol/L for molecular complexants such as citric

acid and 2 × 10-3 mol/L of monomer units for polyacrylic acid. Finally, the standard redispersion protocol was applied. Figure 10 presents the fraction of redispersed particles as a function of solution pH, for redispersion in water, in solutions of citric acid and in solutions of PAAH. At basic pH, all redispersion ratios are the same regardless of the presence or absence of ligands in the solution. For every solution condition, the redispersed fraction goes from nearly zero to 100% in less than a unit pH either on the acid or basic side. Thus, a pH threshold of redispersion can be ascribed to the pH value where 50% of the powder is redispersed. At basic pH, the threshold is located, in every case, near pH 12, i.e., 3 pH units above the IEP. Therefore the addition of ligands, at lower pH values (between the IEP and pH 12), has not changed the interparticles forces sufficiently to promote redispersion. At acid pH, only the bare particle redisperse, with a threshold near pH 4.3, i.e., 5 pH units below the IEP. In this case the addition of ligands prevents redispersion, which is easily understood since the ligands bring negative changes that compensate the positive charges of the bare surfaces. Thus, contrary to expectations, the redispersion of alumina particles is not helped by the adsorption of ligands during the redispersion. Consequently the “peptizing” effect, as defined above, does not exist for alumina particles with citric acid or polyacrylic acid.

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Figure 11. Redispersion in a citric acid solution (0.2 mM) of the alumina powder dried in the presence of ligands with P2O5. The corresponding surface coverage during drying are ([) no citric acid, (9) citric acid dried at pH 9 (0.6 µmol/m2), and (0) citric acid dried at pH 5 (1.5 µmol/m2).

Figure 12. Redispersion in a polyacrylic acid solution (at 2 mM) of an alumina powder dried with P2O5. Influence of the polyacrylic acid coverage during drying: [, no polyacrylic acid; b, polyacrylic acid dried at pH 5 (9.5 µmol/m2); O, polyacrylic acid dried at pH 9 (3.5 µmol/m2).

3. Protecting Effect. When ligands are added to an alumina suspension, and subsequently water is removed through drying, the ligands may remain on the alumina surfaces. This would cause an increase in the minimal separation of alumina surfaces and, consequently, make redispersion easier. However, the ligands may also be expelled from the interparticle contact areas by the capillary pressures exerted during drying, or they may be too small to keep the surfaces sufficiently far apart. For these experiments, the alumina suspensions were prepared with different amounts of ligands: either the saturating amount at pH 9 (0.2% citric or 0.4% polyacrylic acid) or the saturating amount at pH 5 (0.55% citric or 0.85% polyacrylic acid). The suspensions were then centrifuged for 5 min at 9000 rpm. The sediments were recovered, spread on glass plates, and dried with P2O5, yielding powders with protected surfaces. The supernatants were analyzed by measuring their TOC content; from this analysis we calculated the coverage of the particles by citric acid (or polyacrylic acid) for each suspension. Finally the dry powders (30-40 mg) were immersed in solutions (55 g) of the ligands (citric acid at 2 × 10-4 mol/L or polyacrylic acid at 2 × 10-3 mol/L of monomer units) containing the appropriate amounts of HNO3 and NaOH. The redispersion protocol was then applied. (a) Protection by Citric Acid. The redispersion ratios for powders with surfaces protected by citric acid (either 0.6 and 1.5 µmol/m2) are shown in Figure 11. Both powders have a marked redispersion threshold at pH 10.7, even if the better protected particles present a rate of redispersion around 30% in the range pH 5-10. The figure also shows the redispersion ratios for the powder with bare surfaces, redispersed in the same citric acid solution (already shown in Figure 10): this powder has a redispersion threshold at pH 12.7. Since all samples were redispersed in identical citric acid solutions, the lower threshold at pH 10.5 must originate from the presence of citric acid on the surfaces before the aggregation of particles, which amounts to 0.42 µmol/m2 as shown before. At pH 10.5, the electrophoretic mobility is equal to -5.3 µm cm/(V s) in the presence of citric acid whereas it is only of -2.8 µm cm/(V s) in pure pH solution. According to these results, the preadsorbed citric acid molecules provide a definite (but limited) protection against irreversible aggregation during drying. This protection is not improved at the higher amount of

adsorbed citric acid. A qualitative explanation is that the adsorbed citric acid molecules create a steric hindrance when two particles approach. This steric hindrance is obtained at partial surface coverage already; it is not improved at higher coverage, because the thickness of the adsorbed layer does not depend on coverage. (b) Protection by Polyacrylic acid. The redispersion ratios are shown in Figure 12, together with the results obtained for a bare alumina powder redispersed in a polyacrylic solution (Figure 12). The results show a large shift in the redispersion thresholds for the powders made from suspensions with different amounts of PAAH: the higher the amount of adsorbed PAAH, the lower the redispersion pH. Since all samples were redispersed in identical PAAH acid solutions, the lower thresholds must originate from the presence of PAAH acid on the surfaces before the aggregation of particles. The comparison of results shown in Figure 11 and in (Figure 12) indicates that the protection offered by PAAH molecules at a coverage of 3.5 µmol/m2 (dried at pH 9) is comparable to that offered by citric acid molecules. However, it is possible to reach a much higher coverage with PAAH (9.5 µmol/m2 dried at pH 5), and this results in a more efficient protection (threshold at pH 8.5). To quantify the electrostatic contribution to the redisperison improvement, the mobilities of the particles at the redispersion pH were measured. To do so, two dilute suspensions were first equilibrated at pH 5 and pH 9 with 2 mM free PAA in the supernatant. Both suspensions were then divided into 6 parts and their pH shifted by an addition of acid or base to final values of 5.5, 6.5, 7.5, 8.5, 9.5, and 10.5. After 2 h, the electrophoretic mobility of each suspension was measured. The results are reported in Figure 7. They show that the mobility is negative for every pH. Its value is saturated showing the ζ potential is high (unfortunately, the Smoluchovsky and Henry equations cannot be applied for these sorts of particles (Κr ) 10) and high mobilities), except for the lowest pH (5.5) which shows that the IEP is not far (4). In particular, there is nothing marked at pH 8.5 (for the particles jumping from pH 5) nor pH 10.5 (for the particles jumping from pH 9) corresponding to the threshold of redispersion. (c) Other Molecular Ligands. Similar redispersion experiments were performed using powders made from alumina suspensions with surfaces protected by tiron, tetracarboxylic acid, and glycolic acid. All redispersion results have a common feature: the weight percentage of


Figure 13. Redispersion threshold, defined as the pH where 50% of the powder is redispersed, as a function of the adsorbed amount of protecting ligands.

redispersed particles rises rapidly in a narrow pH range. For particles complexed by anionic ligands, this range is located at high pH. We define a threshold pH as the pH where 50% of the particles are redispersed. The location of this threshold on the pH scale measures the efficiency of the ligand: efficient ligands cause redispersion to occur at pH values that are less extreme than in the case of bare particles. Thus, a synthetic view of the protection efficiency of the various additives can be gained by plotting the threshold pH as a function of the amount of adsorbed molecules. This plot shows two distinct groups of ligands (Figure 13). The first group contains molecules that do not change the threshold; these molecules are weakly adsorbed and they may not remain bound to the surfaces during drying (e.g. glycolic acid at pH 5 and pH 9), or else they were adsorbed in very low amounts (tetracarboxylic acid at pH 9). Molecules of the second group shift the threshold from pH 12.2 down to pH 10.2; these molecules (tetracarboxylic acid at pH 5, citric acid, and tiron) have a definite protection efficiency. This efficiency is obtained with adsorbed amounts that are at least 2-3 µmol/m2, which is not a complete coverage. Higher adsorbed amounts of these molecules do not provide a better protection. IV. Discussion The data presented above show that the adsorption of ligands has a profound effect on the aggregationredispersion behavior of alumina suspensions. The thresholds for aggregation and for redispersion are shifted across the pH scale, and the amplitude of the aggregationredispersion cycle may also be changed. The purpose of this section is to rationalize these effects according to the nature of the ligands and their surface densities. A. Summary of Results. The results can be sorted out according to the effects of ligands on the different stages of the aggregation-redispersion cycle. (i) The addition of ligands to a nonaggregated suspension did provide colloidal stability in a pH region (neutral to basic pH) where particles with bare surfaces would aggregate. This made it possible to use alumina suspensions over a more practical range of pH values. However, this was nothing more than a shift in the location of the IEP. (ii) Conversely, the addition of ligands to an aggregated suspension suppressed redispersion at acid pH but did not promote redispersion at basic pH. In that sense, there was no peptization of the aggregated suspension. In the literature, previous reports of a peptizing effect relate to

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experiments where a strong mechanical shear was applied. In this case, the function of the ligands was to prevent reaggregation after fragmentation, as in the stabilizing effect described above. (iii) The addition of ligands to a nonaggregated suspension, before the aggregation and drying stage, made it possible to redisperse the powder at pH values where particles with bare surfaces remain aggregated. The efficiency of this protection is measured by the amplitude of the hysteresis cycle that combines flocculation at acid pH and redispersion at basic pH. With small molecule ligands, this cycle had the same amplitude as for particles with bare surfaces (but, due to the shift in the IEP, it took place on the basic side of the IEP instead of the acidic side). With high amounts of macromolecular ligands, this cycle had a reduced amplitude, indicating that the surfaces were truly protected by the ligands. The first two points in this list are easily taken care of. The stabilizing effect (i) is well-known. It is well explained by the variations of surface charge caused by the ligands and the resulting shift in the IEP. The absence of a true peptizing effect, as noted in point ii, may originate from a heterogeneous adsorption of ligands in the aggregate: indeed, the ligands added at the redispersion stage may not have reached the regions of interparticle contact. On the other hand, the use of ligands at the beginning of the adsorption-redispersion cycle, before aggregation and drying, has the desired effects at the redispersion stage. A quantitative discussion of these effects is necessary to allow further progress in the control of redispersion mechanisms. B. General Redispersion Mechanism. In our previous work,4 we demonstrated that the aggregated particles are redispersed through a change in the balance of interparticle forces. In the dry powder, alumina particles are separated by thin films of adsorbed water and held together by van der Waals forces. After immersion in water, repulsive surface forces (ionic pressure and hydration) cause redispersion if the net adhesion force becomes nearly zero (redispersion through thermal agitation) or less than the applied force (redispersion through sedimentation). All redispersion experiments show a clear threshold for redispersion. For a given state of hydration in the dry powder, immersion in water at a set pH produces redispersion if this pH is sufficiently remote from the IEP of the surfaces. This corresponds to redispersion when the effective surface charge density exceeds a certain threshold. For bare particles, we found that the threshold is crossed when the pH is brought down from 9 (IEP) to 4.5. According to measurements of the number of ionized surface sites and to triple layer model (TLM)32,33 calculations, the corresponding effective surface charge density is 0.045 e/nm2. These phenomena may be analyzed quantitatively through a calculation of the balance of forces.4 The van der Waals attractions have been calculated for spherical particles separated by a hydration film of thickness d0. The ionic repulsions have been calculated according to the TLM model, in the planar geometry with the same hydration film, and then transformed to the spherical geometry through the Derjaguin approximation. The results of this calculation give a threshold according to the minimum separation, d0, and to the effective charge density of the alumina surfaces, σ. The location of this (32) Davis, J. A.; James, R. O.; Leckie, J. O. J. Colloid Interface Sci. 1977, 63, 480. (33) Westall, J.; Hohl, H. Adv. Colloid Interface Sci. 1980, 12, 265.

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Figure 14. Redispersion limits of alumina powders in the axis effective charge distance of contact separation d0. The diameter of the particle is 150 nm, and the Hamaker constant is equal to 9 kT. The two lines report for two different ionic forces the distances d0 for which, for the given effective charge, the contact force is equal to -0.041 nN, which was shown to be the force threshold for redispersion in the preceding paper: (continuous line) I ) 1 mM; (dashed line) I ) 0.05 mM. The insert shows the variation of the structural and effective surface charge for bare alumina particles (extracted from ref 4).

boundary in the d0, σ plane is shown in Figure 14: at surface charges exceeding the boundary traced in the figure, it is predicted that sedimentation (applied force 0.041 nN) will cause redispersion. In practice, redispersion is produced by pH changes. Therefore, it is necessary to relate the scale of surface charge densities to those pH changes. For particles with “bare” surfaces (small counterions only, no specific ligands), we found that the effective surface charge density, σ, is a quasi-linear function of the change of pH from the IEP, ∆pH (insert of Figure 14). This approximation is expected to be accurate for surfaces that meet the following conditions: (i) The pK’s of the surface sites are located symmetrically on either side of the IEP. (ii) There is a high density of surface sites, and relatively few of them have been ionized. (iii) There are no specifically bound ions or ligands. According to this relation between σ and ∆pH, the redispersion boundary traced in Figure 14 can be read in d0, σ or in d0, ∆pH coordinates. In our experiments, particles with bare surfaces were redispersed at ∆pH ) 5, which corresponds to σ ) 0.045 e/nm2. According to the calculations, particles with this surface charge will redisperse if their minimum separation d0 is at least 8 Å (Figure 14). This thickness is consistent with the measured residual hydration of the particles. If the powder had a lower residual hydration (d0 < 5 Å), an enormous increase in surface charge or ∆pH would be required to produce redispersion. Conversely, if the surfaces were kept sufficiently far apart (d0 > 20 Å), redispersion would occur with very weak forces or none at all. Now we consider surfaces that have been protected by ligands and examine whether a similar analysis can be made. C. Redispersion of Particles Covered with Citrate Ions. The adsorption of citrate ions produces a major change in the surface charge of the alumina particles, pushing the IEP from pH 9 to pH 4. As a result, the redispersion of alumina particles is now obtained on the basic side of the IEP (near pH 10.5), instead of the acidic side in the case of bare surfaces. In this range of pH, the “new” surface has a surface charge opposite to the original surface (Figure 6); it may be described as follows. The

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ionized surface sites have bound citrate ions, and their original nitrate counterions have been released. The surface charge is reversed because each citrate carries 3 carboxylate groups, some of which are not compensated by the charge of the surface sites. This excess negative charge is compensated by sodium counterions, which may be condensed on the new surface or dispersed in the diffuse ionic layer. According to this simple view, the new surface is made of “free” carboxylate groups and sodium counterions. This new surface meets the criteria listed above for a quasilinear variation of the effective surface charge with pH changes. Therefore, at the same value of ∆pH, it should carry the same effective surface charge as the original surface. Remarkably, the redispersion of alumina particles covered with citrate ions takes place at the same ∆pH as with bare surfaces. Accordingly, the surface charge density must be the same, at this threshold, as for bare surfaces at their redispersion threshold. Therefore the repulsive ionic force must have the same strength in both cases. According to the balance of surface forces (Figure 14), redispersion with this ionic repulsion is possible if the minimum interparticle separation is d0 ) 8 Å, as in the case of bare surfaces. According to this argument, the alumina particles with adsorbed citrate ions must have been aggregated at the same minimum separation d0 as the particles with bare surfaces, and they redisperse at the same threshold in surface charge density. Thus, the hydration forces that maintain a water film between surfaces must have remained the same, and the repulsive ionic forces at the redispersion threshold are also the same; the only thing that has changed in practice is the pH range where redispersion takes place. This implies that the citrate ions fit into the water layers that are adsorbed on the surfaces and do not disturb the hydration forces within these layers. We now examine whether this substitution is indeed possible. D. Molecular Scale Model of the Gap. The gap that separates surfaces contains bound water, free water, and ions. When particles are pulled toward each other by van der Waals attractions or by capillary forces, free water is expelled, and only bound water remains. Measurements of the minimum distance of particles with bare surfaces (d0 ) 8 Å) indicate that the layer of bound water on each surface is 4 Å thick. This amount of bound water agrees with direct measurements of the residual hydration for alumina surfaces that have been dehydrated through exchange with P2O5. Therefore, this water is indeed strongly bound water. In the classical picture of ionic distributions near charged surfaces, a fraction of the counterions are condensed on the charged surfaces, within this layer. This layer is then called the Stern layer. A refinement of this picture is the triple layer model, where some counterions lose water to form ion pairs with the surface sites, while the others remain fully hydrated and are condensed only in the electrostatic sense (Figure 15). The detailed picture of the surface layers would then comprise a first layer, with dehydrated ions and some strongly bound water, and a second layer, with hydrated ions and hydration water. If the particles were not pulled together, there would also be a third layer containing free counterions and free water. If the particles are pulled together by van der Waals forces, the free water is expelled, but the first and second layers of bound water are retained. Now we may examine how citrate ions fit in these layers. Since they are ligands of the alumina surfaces, we assume


Figure 15. Schematic representation of the contact region when citrates molecules and their counterions are adsorbed. Some (2/3) of the carboxylate functions of the adsorbed citrate molecule are in direct contact with the surface and are dehydrated. Each of the hydrated carboxylate functions is compensated by a counterion which can either be condensed (but still hydrated) or in the diffuse layer.

that they form inner-sphere complexes with the surface sites. A plausible model of an inner-sphere complex has a bidentate geometry where 2 carboxylates and the hydroxyl group of the citrate are directly coordinated to the surface site, while the third carboxylate group points away from the surface and remains hydrated. In the TLM picture, the first layer would contain the carboxylates that are directly coordinated to surface sites, plus some chemisorbed water. The second layer would then contain the carboxylate groups that point away from the surface, with the water molecules that form their hydration (2 molecules/ion), and the sodium counterions that condense on this negatively charged surface, also with the water molecules that hydrate them (6 molecules/ion). Since the number of adsorbed citrate ions is known, we may calculate how many water molecules are needed to hydrate all the ionized groups. For alumina surfaces at pH 10.5, the number of adsorbed citrate ions is 0.42 µmol/ m2. According to the binding model presented above, there is one free carboxylate group per citrate and one sodium counterion that compensates the charge of this carboxylate. The hydration of these ions amounts to 8 water molecules/citrate, i.e., 3.36 µmol/m2 of hydration water. However, according to measurements of the amount of adsorbed water, the amount of surface water actually amounts to 22 µmol/m2. Therefore, the citrate ions and their sodium counterions easily fit into the first 2 layers of the TLM, and they use up only 15% of the adsorbed water for their hydration. In summary, adsorbed citrate ions bind to the surface sites, displace the original counterions (nitrate) of the surface, reverse the surface charge, and bring some of their own counterions (sodium). However, they fit in the Stern layer and do not change the range of hydration forces nor the minimum separation of alumina surfaces. Redispersion occurs through a rise in the surface charge density, caused by a pH change equivalent to that required for bare surfaces. E. Redispersion of Particles Protected by Polyacrylate. The adsorption of polyacrylate ions produces the same changes in the surface charge. However, the effects on redispersion depend on the adsorbed amount. At low adsorbed amounts, the redispersion thresholds are exactly the same as with citrate ions. This is easily

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understood through the previous molecular scale model of the gap. For surfaces which adsorbed polyacrylate at pH 9 and were then brought to pH 10.5, the surface density of adsorbed monomers is 2.4 µmol/m2. We assume, as above, that one-third of these carboxylate groups remains free, fully hydrated, and compensated by sodium counterions. The hydration of these ions amounts to 8 water molecules/free carboxylate, i.e., 6.4 µmol/m2 of hydration water. This is still a small fraction of the amount of surface water, which amounts to 22 µmol/m2. Therefore, the polyacrylate ions at this surface density easily fit into the first 2 layers of the TLM and use up only 30% of the adsorbed water for their hydration. For surfaces which adsorbed polyacrylate at pH 5 and were then brought to pH 8.5, the surface density of adsorbed monomers is 6.4 µmol/m2. With the same assumptions, the hydration of all the ions carried by the polyacrylate uses 18 µmol/m2 of hydration water. This is comparable to the total amount of water that hydrates the alumina surfaces (22 µmol/m2). Therefore, the polyacrylate ions at this surface density may add a substantial amount of hydration water. It is easy to see that the minimum separation of surfaces must then be increased. In the most restrictive model, a gap containing only the polyacrylates, the sodium counterions, and their hydration water would have a thickness d0 )12 Å. In the other extreme, where the gap would also contain all the water molecules that were originally bound to the surfaces, it would be d0 ) 18 Å. In summary, high amounts of adsorbed polyacrylates can retain a substantial amount of hydration water and push the range of hydration forces beyond their original location. As a result, redispersion takes place at a lower surface charge density or a smaller pH change compared to that required for bare surfaces. V. Conclusion Powders made of aggregated colloidal particles can be redispersed in water if the particle surfaces have remained hydrated during the aggregation and drying stages. This residual hydration prevents direct contact between the particle surfaces: in the case of alumina, which is a strongly hydrophilic surface, the minimal separation due to residual hydration is d0 ) 8 Å. In these conditions, spontaneous redispersion can be obtained through a shift in the balance of surface forces. This is achieved through pH change such that the surfaces acquire a high surface charge (typically 1 elementary charge/nm2), and the resulting ionic pressure overtakes the van der Waals attractions. For most mineral surfaces, these conditions require a high concentration of H+ or OH- ions and, therefore, rather extreme pH values (typically 5 pH units away from the IEP). Thus, there are two practical constraints for spontaneous redispersion: the surfaces must have remained hydrated, and the pH must be changed to conditions where they become strongly charged. In this work we have found that there are two ways around these problems: (a) It is possible to obtain a high enough surface charge at less extreme pH values if the mineral surfaces are modified through the adsorption of ligands that carry many elementary charges per molecule. Many of these molecules are polycarboxylic acids, but molecules that carry sulfate groups are also capable of producing this change in surface charge. (b) It is possible to set the minimal separation of surfaces through the adsorption of ligands. In the case of alumina, which naturally retains a substantial amount of surface

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water, small molecule ligands do not increase the minimal separation d0. If it is desirable to reduce the strength of attractions further, to the point where a small surface charge is enough to cause redispersion, then macromolecular ligands must be used. In the case of other, less hydrated surfaces, spontaneous redispersion may be impossible unless ligands have been adsorbed to set a minimal separation between the surfaces. At this stage, it is possible to define the nature of ligands that would be most efficient to perform these tasks. According to condition (a) above, they must carry a strong electrical charge: at least 3 charged groups/molecule or, in the case of macromolecules, at least one charged group for every 2 monomers. According to condition (b) above, they must be quite bulky; small molecules, with molar masses below 1000, do not produce a sufficient increase in the minimal separation of surfaces. There are two type of macromolecules that may fit these requirements.

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Statistical copolymers may form thick adsorbed layers, particularly if they are of the associative type; indeed, experiments by Gourmand27 indicate that associative copolymers may form strong protecting layers. Diblock copolymers may be even more efficient, since the nonadsorbing tails of these copolymers may produce a strong swelling pressure up to separations where the van der Waals attractions become quite small. Indeed, experiments (not reported here) using diphosponate poly(oxyethylenes)14 produced spontaneous redispersion at all pH values; however, full redispersion was not achieved, presumably because the surface coverage was incomplete. Thus, there is still plenty of room for improving the redispersion properties of powders through the adsorption of specially designed ligands. LA010166T

From Powders to Dispersions in Water: Effect of Adsorbed Molecules

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