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Langmuir 1997, 13, 6644-6649
Influence of the Dielectric Constant of the Media on Oxide Stability in Surfactant Solutions M. Colic* and D. W. Fuerstenau Department of Materials Science and Mineral Engineering, University of California, Berkeley, California 94720 Received March 12, 1997. In Final Form: July 25, 1997X
The influence of the dielectric constant of the suspension on the colloidal stability of alumina in the presence of surfactant was investigated. Colloid stability measurements were supplemented with electrokinetic, conductivity, and surfactant adsorption studies. Studies with physadsorbed sodium dodecyl sulfate were performed in the presence of different volume percents of short chain alcohols or dioxane in order to vary the dielectric constant of the suspension. It was observed that the surfactant adsorption decreased with the dielectric constant of the media. At 50 vol % of different alcohols, the addition of surfactant could not reverse the alumina charge and restabilize coagulated particles. To understand the influence of lower dielectric constant on surfactant adsorption at the surface or in the second layer, (hydrophilic head pointing toward the solution), alumina and silica particles were chemically precoated with dodecyl chains. The addition of anionic surfactant suspended such hydrophobic particles and increased the negative surface charge. Cationic surfactant also successfully dispersed dodecyl-coated alumina and increased the positive charge on the surface. When sufficient alcohol was added to the solution, no change in ζ potential was evident as the surfactant concentration was increased. This indicated the absence of surfactant adsorption in the second layer in low-dielectric-constant media. Conductivity measurements indicated that approximately 40% of the surfactant molecules were dissociated, even at 50 vol % of ethanol. Adsorption measurements indicated no surfactant adsorption. These results strongly suggested that, in suspensions with low dielectric constant, no surfactant molecules adsorb in the second layer with hydrophilic heads oriented toward the solution.
Introduction Surfactant adsorption at solid/liquid interfaces has been a rather popular subject of research.1-6 Fuerstenau and co-workers1,2 have developed the so-called “hemimicelle” model to describe adsorption, elecktrokinetic, and contact angle studies on the mechanisms of the surfactant adsorption at the solid/liquid interface. According to that model, most cationic and anionic surfactants physadsorb only at the oppositely charged surfaces. Surfactant adsorption occurs as individual ions at low adsorption densities, but they begin to associate into patches called hemimicelles due to hydrocarbon chain association on the surface. Somasundaran and co-workers5 demonstrated with certain spectroscopic and classical techniques that some surfactant molecules must be adsorbed in the reverse orientation with their heads pointing toward the bulk aqueous phase even at low surface coverage before a full monolayer is formed. Somasundaran proposed a modified version of the hemimicelle model to account for these findings and named it the “reverse hemimicelle model”. Cases and co-workers4 developed a mathematical model which takes into account reverse hemimicelle formation. Scamerhorn and co-workers3 proposed what they termed the “admicelle” model to describe anionic surfactant adsorption on oxide surfaces. According to their model, * To whom correspondence should be addressed. Current address: Materials Department, College of Engineering, University of California, Santa Barbara, California 93106 X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Gaudin, A. M.; Fuerstenau, D. W. Min. Eng. (Littleton, Colo) 1955, 7, 1. (2) Fuerstenau, D. W. Pure Appl. Chem. 1970, 24, 135. (3) Scamerhorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 463. (4) Cases, J. M.; Vilieras, P. Langmuir 1992, 8, 1251. (5) Somasundaran, P.; Kunjapu, J. T. Miner. Metall. Process. 1988, 6, 68. (6) Kellar, J. J.; Cross, W. M.; Miller, J. D. Sep. Sci. Technol. 1990, 25, 2133.
S0743-7463(97)00279-5 CCC: $14.00
surfactants adsorb as patches of bilayers even at low adsorption densities. Spectroscopic results from Miller’s6 and Somasundaran’s5 groups strongly suggest that the reverse hemimicelle model best describes the actual situation. Colloid stability of particles suspended in surfactant solutions has received considerably less attention.7-9 Surfactants may either stabilize or flocculate particles suspended in liquid media. Stabilization is explained by the formation of a protective layer of adsorbed surfactant chains while flocculation is commonly considered in terms of interaction of hydrophobic tails from hemimicelles adsorbed on different particles (the so-called bridging mechanism8). In addition, ionic surfactants induce particle aggregation by charge neutralization7 through the adsorption of surfactant ions charged oppositely to the surface, followed by restabilization as the ζ potential increases in magnitude again but of opposite sign. An additional way of stabilizing a suspension of oxide particles would be through the formation of reverse hemimicelles or bilayers. Reverse hemimicelles or bilayers carry significant charge, with the charged groups oriented toward the solution, thereby causing strong interparticle repulsion.8,9 The influence of the dielectric constant of the media on surfactant adsorption mechanisms has received almost no interest among surface scientists, with only four papers (to our knowledge) dealing with problems in that interesting area.10-13 Current EPA regulations restrict the (7) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Colloid Interface Sci. 1966, 22, 599. (8) Colic, M.; Kallay, N. J. Surf. Sci. Technol. 1988, 4, 53. (9) Liang, L.; Morgan, J. J. Aquat. Sci. 1990, 52, 32. (10) Lyklema, J.; deWit, J. N. Colloid Polym. Sci. 1978, 256, 1110. (11) Kandori, K.; Konno, K.; Kitahara, A. Bull. Chem. Soc. Jpn. 1984, 57, 3419. (12) Esumi, K.; Ikemoto, M.; Meguro, K. Colloids Surf. 1990, 46, 231. (13) Esumi, K.; Kobayashi, T.; Meguro, K. Colloids Surf. 1991, 54, 189.
© 1997 American Chemical Society
Oxide Stability in Surfactant Solutions
use of solvents in paint technology and many companies have developed mixed water-based solvent systems.14,15 Very little literature exists on the mechanisms of particle dispersion in such systems. Alumina powder is often used, either on its own or as a surface coating of rutile. Meguro and co-workers studied the influence of the solvent dielectric constant on adsorption of lithium perfluorocarbon sulfonates.12,13 They concluded that surfactant adsorption with the hydrophilic head directly on the alumina surface is decreased in low-dielectric-constant solvent due to weaker dissociation of surface groups as well as that of surfactant counterions. Using fluorescence and ESR probe spectroscopy, they also indicated the strong possibility that at low dielectric constant no surfactant ions adsorb in the second layer with their heads pointing toward the solution. Since probe molecules are also influenced by the change in dielectric constant of the media, no conclusive data could be obtained. We have attempted to understand the dispersion of alumina in mixed aqueous solvents containing different amounts of alcohols or dioxane. To avoid any inconclusive results about the adsorption of surfactant ions in the second layer, we compared the results on physadsorption of surfactant to that of surfactant adsorption on the alumina and silica particles which were chemically precoated with dodecyl chains. The results of our investigation are presented in this paper. Experimental Section Experimental Materials. Salts, acids, bases, alcohols, and surfactants were purchased from Aldrich Chemicals, Milwaukee, WI. All salts used in this work were of the highest purity available. Nitric acid and sodium hydroxide were used to adjust pH. The alumina powder used in this study, AKP-50, 99.9% pure (R-Al2O3), was used as received from Sumitomo Chemical Company , New York, NY. The specific surface area of this sample was 10 m2 g-1, and its isoelectric point (iep) was found to be at pH 9. The mean particle size was 300 nm, as determined by photon correlation spectroscopy (PCS). High-purity silica powder with a mean particle size of 2 µm and surface area of 8 m2 g-1 was used as received from Johnson Matheson. Electrokinetics Measurements. In the electrophoretic mobility studies, 0.01 g of oxide was added to 100 mL of triply distilled water. After a short conditioning period in an ultrasonic bath, the suspensions were equilibrated for an additional 15 h at pH 4 under argon atmosphere. Before the initial pH adjustment, the appropriate amount of salt was added and the suspensions were titrated with NaOH or HNO3 and agitated for 20 min on a gyratory mixer. Then, the pH was measured again and, if necessary, adjusted to the desired value. After pH equilibration, the appropriate amount of surfactant was added. The systems were agitated for 20 min before final pH adjustment. Electrophoretic mobility measurements were conducted with a Zeta Meter 3.0 system manufactured by Zeta Meter Inc., New York, and with a PEN KEM 501 manufactured by PEN KEM Inc. In addition, some electrophoretic mobilities were also measured with a Malvern Zeta Sizer 3 electrophoretic lightscattering system. Excellent correlation was observed with measurements performed with the different instruments. Electrophoretic mobilities measured with three different techniques were reproducible within (2 mV. Colloid Stability Measurements. A series of systems with the same oxide concentration, pH, and ionic strength but increasing surfactant concentration were prepared in test tubes. After they were equilibrated at 20 °C overnight, turbidity was measured with a Brinkman nephelometer. Alternatively, the mean particle size of the alumina in suspensions was determined with a Malvern Zeta Sizer 3 photon correlation spectrometer (14) Morgan, R. E. Mater. Perform. 1996, 35, 31. (15) Weismantel, G. E.; Paul, B. O.; Hrickiewicz, M. Chem. Process. 1996, 59, 31.
Langmuir, Vol. 13, No. 25, 1997 6645 and used as a measure of colloid stability. Systems with larger mean particle size were considered to be less stable. Surfactant Adsorption Measurements. A known weight of alumina (1-5 g) was equilibrated at the desired pH and ionic strength with 50 mL of surfactant solution. The pH of the suspension was adjusted with 0.1 M HNO3 at the start of the experiment and again 1 h before the end of the equilibration period. The suspension was aggitated in an Environ shaker at 200 rpm and the temperature was maintained at 20 °C. Thirty milliliters of the final suspension was centrifuged in a refrigerated centrifuge at 20 °C, and 1-10 mL of the resulting supernatant was used for analysis. Dodecyl sulfate was analyzed colorimetrically16 after two-phase extraction (water-toluene). The following standard procedure was followed: An aliquot of the supernatant solution was diluted to a final volume of 100 mL and a concentration between 1 × 10-7 and 5 × 10-7 M in a 125 mL Erlenmeyer flask with a ground stopper. Five milliliters of acetate buffer (pH 5) and five milliliters of 0.001 M ethyl violet (cationic dye) solution were added to the resulting dodecyl sulfate solution. After the Erlenmeyer flask was shaken for 30 s, 5 mL of toluene was added and the flask was shaken for 10 more minutes in an Environ shaker at 200 strokes per minute. Afterward, the flask was left standing for 60 min in order to achieve phase separation. Finally, the toluene phase was carefully removed and the absorbance was measured at 615 nm. The calibration curve was linear in the range from 0 to 5 × 10-7 M of dodecyl sulfate in the aqueous solution.
Results The influence of the dielectric constant of the media on the colloidal stability of metal oxides in surfactant solutions was investigated. Dioxane, methanol, ethanol, propanol, and 2-propanol were used in mixture with water to lower the dielectric constants of the suspensions studied. For the sake of brevity only the results for ethanol-water mixtures are shown. A similar trend was observed with all other solvents studied. One of the aims of this study was to show that surfactant adsorption in the second layer (in the absence of specific, superequivalent adsorption) is neccessary to reverse the charge and restabilize suspended metal oxide particles. Most of the studies described in this paper were performed with sodium dodecyl sulfate. To show that sulfate ions cannot reverse the charge and restabilize oxide particles on their own, electrophoretic mobility and colloid stability studies with the alumina-sulfate-water systems were performed. Figure 1 shows the effect of sodium sulfate addition on (a) the colloid stability and (b) the electrophoretic mobility of suspended alumina particles. It is clearly shown that no charge reversal and restabilization of coagulated particles occurred even at 0.5 M sodium sulfate. The results of colloidal stability and electrophoretic mobility studies in mixed solvents are presented in Figure 2. Figure 2 shows the influence of the dielectric constant of the medium (ethanol-water mixtures of different compositions) on the stability (a) and the electrophoretic mobility (b) of the alumina at pH 3 in 0.01 M NaNO3 as a function of the sodium dodecyl sulfate concentration. At 0, 10, and 30 vol % of ethanol, the addition of surfactant caused coagulation and subsequent oxide restabilization. At 50 vol % of ethanol, restabilization of the oxide suspension did not occur. The electrophoretic mobility of alumina particles (Figure 2b) was lower in media with more ethanol (lower dielectric constant). At 50 vol % of ethanol, charge reversal of the oxide did not occur. Similar experiments were performed in water-dioxane, watermethanol, water-propanol, water-2-propanol, and waterbutanol media. At 50 vol % of alcohol, the restabilization (16) Motomizu, S.; Fujiwara, S.; Fujiwara, A.; Toel, K. Anal. Chem. 1982, 54, 392.
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Colic and Fuerstenau
a
b
Figure 1. Stability (a) and electrophoretic mobility (b) of monodisperse alumina particles at pH 4 as a function of sodium sulfate concentration.
and charge reversal of alumina particles in suspension did not occur (data shown only for water-ethanol suspensions). Figure 3 presents the adsorption density of sodium dodecyl sulfate on alumina as a function of the sodium dodecyl sulfate concentration at pH 3 with 0.01 M NaNO3 in water and in a water-ethanol mixture (50 vol % of ethanol). It can be seen from the plots given in Figure 3 that the affinity of dodecyl sulfate ions for the oxide surface is significantly decreased in the ethanol-water mixture, as indicated by an order of magnitude decrease in maximum surface coverage (around 10-5 mol m-2 for pure water and around 10-6 mol m-2 for the 50-50 waterethanol mixture). It is not possible to predict from these results whether the decrease in adsorption is due to a decrease in the surface charge of alumina and the dissociation constant of sodium dodecyl sulfate or due to a lack of dodecyl sulfate adsorption in a second layer. Experiments with the chemically coated silica and alumina powders were performed in order to understand whether media with low-dielectric-constants prevent surfactant adsorption in the second layer. Silica and alumina were partially covalently coated with the dodecyl chains by heating powders with an equivalent amount of dodecanol under reflux17 (Al-OH + ROH ) AlOR + H2O). Powders were washed 10 times with 2-propanol to remove excess dodecanol. The presence of dodecyl chains on the surface was identified with FTIR and TGA/GC/MS analysis. Residual charge on the surface was measured with ζ potential measurements. The covalent carbon-oxygen bond is very stable, and no appreciable hydrolysis occurs at pH’s between 3 and 12. (17) Iler, R. K. The Chemistry of Silica, stability, polymerization, colloids and surface chemistry and biochemistry; Wiley: New York, 1979.
Figure 2. (a) Stability and (b) electrophoretic mobility of monodisperse alumina particles as a function of the concentration of sodium dodecyl sulfate at pH 3 in 0.01 M NaNO3 in water-ethanol mixtures of various composition. Stability results are expressed as the mean particle size.
Figure 3. Adsorption density of sodium dodecyl sulfate on alumina as a function of the equilibrium concentration of sodium dodecyl sulfate at pH 3 in 0.01 M NaNO3 in water-ethanol mixtures of various composition.
Figure 4 shows the influence of the addition of dodecyl sulfate on the electrophoretic mobility of dodecyl-coated silica. At pH 2.2 silica is close to its isoelectric point (pH 1.8) and is only weakly charged (-1.17 µm/(s V cm) electrophoretic mobility). On noncoated silica, no adsorption of sodium dodecyl sulfate (SDS) or change in electrophoretic mobility with SDS addition was observed. On the other hand, as presented in Figure 4, the addition of SDS decreased the electrophoretic mobility of dodecylcoated silica from -1.17 µm/(s V cm) without SDS to -4.68 µm/(s V cm) at 0.008 M SDS. Adsorption measurements (data not shown) also indicated the adsorption of SDS on dodecyl-coated silica. Figure 5 compares the effect of SDS addition on the electrophoretic mobility of dodecyl-coated silica in the presence of 25 and 50 vol % of ethanol. At 25 vol % of
Oxide Stability in Surfactant Solutions
Figure 4. Influence of sodium dodecyl sulfate on the electrophoretic mobility of silica and dodecyl-coated silica particles at pH 2.2.
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Figure 7. Influence of dodecyltrimethylammonium bromide on the electrophoretic mobility of dodecyl-coated alumina in water-ethanol mixtures of various composition at pH 8.5. Table 1 conductivity/µΩ cm-1
Figure 5. Influence of sodium dodecyl sulfate on the electrophoretic mobility of dodecyl-coated silica in water-ethanol mixtures of various composition at pH 2.2.
Figure 6. Influence of sodium dodecyl sulfate on the electrophoretic mobility of dodecyl-coated alumina in water-ethanol mixtures of various composition at pH 11.
ethanol, the electrophoretic mobility of dodecyl-coated silica was less influenced with the addition of SDS than at 0 vol% of ethanol and changed from -0.23 µm/(s V cm) with no SDS present to -2.03 µm/(s V cm) with 0.008 M SDS. At 50 vol % of ethanol, no change in electrophoretic mobility was observed with the addition of SDS to dodecylcoated silica. Adsorption measurements showed no surfactant adsorption at 50 vol % of ethanol in suspension (data not shown). Figure 6 presents the effect of SDS addition on the electrophoretic mobility of dodecyl-coated alumina at pH 11. It was shown earlier that SDS does not adsorb on
SDS conc/M
no ethanol
25 vol % ethanol
50 vol % ethanol
5 × 10-4 1 × 10-3 5 × 10-3
36.5 62.2 289.3
23.2 45.1 207.2
14.2 26.2 128.5
pure alumina above the isoelectric point (pH 9). However, the addition of SDS to dodecyl-coated alumina caused a change of electrophoretic mobility from -2.03 µm/(s V cm) (without SDS) to -3.125 µm/(s V cm) (0.008 M of SDS). At 50 vol % of ethanol, no change in electrophoretic mobility occurred with the addition of SDS to suspension (-0.78 µm/(s V cm) with and without SDS). In the absence of dodecyl coating, addition of SDS does not change the electrophoretic mobility of alumina with or without ethanol added. Figure 7 shows similar results with dodecyl-coated alumina and the cationic surfactant dodecyltrimethylammonium bromide (DTAB). A positive alumina surface (pH 8.5) was studied in that case to avoid any electrostatic adsorption of DTAB. As in the case of negative surfaces and SDS, the addition of DTAB changed the electrophoretic mobility of dodecyl-coated alumina from 0.85 µm/ (s V cm) without surfactant to 3.23 µm/(s V cm) at 0.008 M DTAB but did not influence that of pure alumina. At 50 vol % of ethanol, no change in electrophoretic mobility was observed. Adsorption measurements at 50 vol % of ethanol indicated the absence of surfactant adsorption (data not shown). The conductivity of SDS solutions of different concentrations was measured in water and in water-ethanol mixtures of different compositions. The results are shown in Table 1. The conductivity of SDS solutions decreased approximately 30% at 25 vol % of ethanol and ca. 60% at 50 vol % of ethanol. This indicated a significant decrease in the dissociation of surfactant molecules but also the presence of at least 40% of dissociated surfactant molecules at 50 vol % of ethanol. Discussion The behavior of metal oxides in pure aqueous or pure organic media has been a rather popular subject of research,1-6 whereas only a few publications report on the results of studies of the surface properties of oxides in mixed water-organic media.10-13 It is quite reasonable to assume that only “classical” hemimicelles with hydrophilic surfactant heads adsorbed at the oxide surface exist in the more hydrophobic media
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because the reverse surfactant orientation is thermodynamically not favorable. Our experiments were performed in water-methanol, water-ethanol, water-propanol, water-butanol, and water-dioxane media of various compositions. In all the cases studied, the oxide particles could not be restabilized in a low-dielectric-constant medium. The electrophoretic mobility studies showed that oxide particles, which could not be restabilized, remained positively charged or neutral at all surfactant concentrations. Adsorption density measurements showed that a significant decrease in the maximum surface coverage occurred in media with low dielectric constants. ESR spectroscopy measurements of solubilized probes13 indicated that adsorbed surfactant aggregates are more densely packed in a medium of high dielectric constant. ESR and fluorescence probe measurements of Esumi and co-workers13 also showed that the microviscosity of the adsorbed surfactant aggregates did continously change in water suspensions but did not significantly change in the media with low dielectric constants. This was interpreted by Esumi and co-workers as the absence of surfactant adsorption in the second layer in low-dielectricconstant media. Numerous solvents were investigated in order to avoid any specific effects in the ethanol-water-sodium dodecyl sulfate system. The fact that restabilization and charge reversal did not occur in any of the systems investigated suggests that decreased hydrophobic chain-chain interactions between adsorbed surfactant molecules are responsible for the absence of second-layer adsorption. Even if molecules adsorbed in the second layer were fully nonionized due to the low dielectric constants of the media, their adsorption would restabilize oxide particles due to steric stabilization. Fully quantitative analysis of the adsorption isotherms with the low-dielectric-constant media was unfortunately not possible because some surfactant-dye complex species used to extract surfactant molecules into organic solvents for colorimetric analysis could have remained in the aqueous mixed solvent system. This would result in somewhat higher adsorption densities. Yet combination of the electrophoretic mobility, colloid stability, and adsorption studies described in this paper along with the ESR and fluorescence probe studies performed by Esumi and co-workers13 strongly suggests the absence of second-layer adsorption in low-dielectricconstant media. Another interesting behavior observed in this study was a jump in particle size (decrease in colloid stability of alumina) which occurred before charge neutralization of oxide particles was achieved. The total interparticle interaction force is a sum of electrostatic double-layer forces, van der Waals forces, short range repulsive forces, steric forces, and hydrophobic forces. As surface coverage of oxide is increasing with increase in surfactant concentration, more and more surface charge is neutralized. This effect in itself decreases the electrostatic doublelayer repulsions. In addition to that, the increase in surface coverage with the hydrophobic surfactant tails pointing toward solution is followed by the increase in the hydrophobic attractions. When hydrophobic attractions start to dominate electrostatic double-layer repulsions, a jump in the decrease in the colloid stability of the oxide particles studied occurs. Yoon and co-workers18 recently observed the increase in hydrophobic constant (measured with the atomic force microscope surface force measurements) with the increase in surface coverage with the hydrophobic chains pointing toward solutions. Yaminski (18) Yoon, R. H.; Flinn, D. H.; Rabinovich, Y. I. J. Colloid Interface Sci. 1997, 185, 363.
Colic and Fuerstenau
and Ninham19 discussed such behavior in a recent manuscript. To further understand the surfactant adsorption in the second layer and the influence of dielectric constant on the process, we partially coated alumina and silica particles (to avoid investigation of effects specific only for alumina) with covalently attached dodecyl chains. Such particles were hydrophilic enough to be dispersable in water but had dodecyl chains pointing toward the solution at all dielectric constants. The experiments presented in Figures 4-7 indicated that additions of surfactants can significantly change the electrophoretic mobility of these particles. Adsorption measurements also confirmed surfactant adsorption onto dodecyl-coated particles with similar sign of charge. Unfortunately, it was not possible to quantitavely compare the maximum adsorption densities in the second layer with noncoated and coated alumina particles. Esumi and co-workers20 recently showed that more surfactant is adsorbed in the second layer with precoated particles (the higher the surface density of precoated alkyl chains, the higher the maximum adsorption). However, this does not influence our conclusions. At 50 vol % of ethanol, no change of electrophoretic mobility was observed and also no surfactant was adsorbed at the particle surface. Since we used particles with the same sign of charge as that of surfactant, no electrostatic physadsorption of surfactant ions with the head down was possible, as indeed indicated with the adsorption measurements on the uncoated particles at the same pH. Consequently, the results presented here strongly suggested that, at 50 vol % of ethanol, the adsorption of surfactant in the second layer was completely prevented. At low dielectric constant of the media, surfactant molecules are thermodynamically more stable in solution. Recent studies of Somasundaran and co-workers21 also indicated that the addition of short chain alcohols to aqueous suspensions of oxides decreased surfactant adsorption. Conductivity measurements indicated a 60% decrease of surfactant counterion dissociation in low-dielectricconstant media. However, the concentration of dissociated ions was significant enough to expect adsorption to change the electrophoretic mobility of dodecyl-coated alumina or silica particles. Direct adsorption measurements indicated the absence of any surfactant adsorption in lowdielectric-constant suspensions. Consequently, it is believed that the above presented results strongly suggest the absence of statistically significant surfactant adsorption in the second layer in low-dielectric-constant media. More direct spectroscopic studies should be performed in order to further examine the proposed adsorption mechanism. Conclusions The influence of the dielectric constant of the media on the colloid stability of alumina and silica particles in surfactant solutions was investigated. Results obtained with physadsorbed sodium dodecyl sulfate were compared to results obtained with alumina and silica particles coated with covalently attached dodecyl chains. At high volume percentages of alcohols such as methanol, ethanol, or propanol, oxide charge reversal and restabilization never occurred in the mixed solvent suspensions. Similar results (19) Yaminsky, V.; Jones, C.; Yaminsky, F.; Ninham, B. Langmuir 1996, 12, 3531. (20) Esumi, K.; Uda, S.; Gaino, M.; Ishiduki, K. Langmuir 1997, 13, 2803. (21) Fu, E.; Somasundaran, P.; Maltesh, C. Colloids Surf. 1996, 112, 55.
Oxide Stability in Surfactant Solutions
were obtained with the alumina and silica coated with the dodecyl chains. These results strongly suggested that surfactant adsorption in the second layer with the hydrophilic head oriented toward the solution does not happen in mixed solvent suspensions with low dielectric constants. It also seems that surfactant adsorption in the second layer is needed for the charge reversal and restabilization of oxides coagulated with the physadsorbed surfactant.
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Acknowledgment. Partial support of this research by the Bureau of Mines through a grant (Grant No. G1174106) to the California MMRRI is acknowledged. Also, we wish to acknowledge Matthew Fisher for help in correcting the manuscript and performing some experiments described in this paper, particularly coating and characterization of powders. LA9702796
