Interaction of Ionic Species and Fine Solids with a Low Energy Hydrophobic Surface from Contact Angle Measurement

JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.

204, 342–349 (1998)

CS985585

Interaction of Ionic Species and Fine Solids with a Low Energy Hydrophobic Surface from Contact Angle Measurement Z. A. Zhou, Hish Hussein, Zhenghe Xu, Jan Czarnecki,* and Jacob H. Masliyah1 Department of Chemical and Materials Engineering, University of Alberta, Canada; and *Syncrude Canada, Ltd., Edmonton Research Center, Canada Received November 17, 1997; accepted April 14, 1998

Interactions of ionic species, (organic and inorganic) precipitates, and fine solids with a low energy hydrophobic surface were examined using a model system of paraffin wax in aqueous solutions. Contact angle measurement was used to evaluate the interactions between paraffin wax and testing variables. No changes in contact angle were observed with various types of metal and metal hydroxyl ions, metal hydroxyl precipitates, fine silica, and alumina powders, suggesting weak or absence of interactions between these species and paraffin wax. At pH <9, the presence of amine reduced the contact angle, but no pH dependence on contact angle was observed for a given amine concentration. A sharp decrease in contact angle was observed at higher pHs, where precipitates of amine molecules formed probably on wax surfaces. In the presence of lauric acid, on the other hand, contact angles reduced at a pH below 8, due to the formation of precipitates, but the reduction was less significant, compared with the reduction by amine precipitates. At high pHs, adding lauric acid did not show any effect on the measured contact angles. The significant effect of fine solids on contact angle was observed only when the solids were made hydrophobic by adsorbed surfactants. The present study further demonstrated that both the thermodynamic criteria and the interactions among substrate/solids/surfactants/metal ions must be considered in identifying the effect of different factors on the wettability of low energy hydrophobic surfaces. © 1998 Academic Press Key Words: hydrophobic surfaces; paraffin wax; ionic species; surfactant; precipitates; fine solids; contact angle measurement.

INTRODUCTION

Interactions arising from the hydrophobic/hydrophilic nature of molecules or surfaces have been an active area of research, especially during the last decades, due to their important role in a wide range of industrial applications (e.g., coagulation, emulsification, flotation, solid/liquid separation). In processes such as minerals separation, de-inking of recycled paper pulp, and bitumen recovery from oil sands using flotation, both hydrophobic and hydrophilic particles are present in a slurry containing different types and amount of surfactants and metal ions (either added on purpose or present naturally by dissolu1

To whom correspondence should be addressed. E-mail: jacob.masliyah@ ualberta.ca. 0021-9797/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.

tion, chemical reaction, and recycle of processing water). Since the separation by flotation is based on differences in surface properties, such as hydrophobicity of solid particles, the presence of such chemical species and fine mineral (clay) particles in the system may drastically change the hydrophobicity of other solids, thus complicating the separation process. The success of separation using flotation techniques, therefore, relies largely on the understanding of how different species interact with each other and affect the wettabilities/hydrophobicities of individual particles. A commonly encountered problem in flotation is the socalled ‘‘slime coating’’ (1), i.e., hydrophobic (hydrophilic) particles become coated by a layer of hydrophilic (hydrophobic) fines. Slime coating interferes with the flotation process through the build-up of steric barriers by coated hydrophilic fines which prevent the direct contact of air bubbles with the hydrophobic particles, thus reducing the recovery (2, 3). Slime coating also deteriorates the froth quality by carrying hydrophilic fines coated on hydrophobic particles into the froth product (4). In bitumen recovery from Canadian oil sands, it was found (5– 8) that the presence of a certain amount of fines (minerals and clays) in the feed severely depressed bitumen flotation. A close examination by Dai and Chung (9) revealed the coating of naturally hydrophilic silica fines on the bitumen droplets, even though bitumen is a low energy and naturally hydrophobic surface (10). The interference of fine solids on minerals and bitumen flotation has been explained based on the classical electrical double layer theory (1, 8, 9, 11). However, recent development in both interfacial sciences and flotation theory (12, 13) indicates that bubble/particle attachment and, in some cases, slime coating in flotation are mainly driven by hydrophobic force. Therefore, structural forces must be included in the classical DLVO theory to account satisfactorily for the observed flotation phenomena (14, 15). The study on bitumen (or hydrocarbon oil)/silica coagulation (16) also showed that naturally hydrophilic silica did not coagulate with pure hydrocarbon oil/bitumen, unless the silica was rendered hydrophobic by adsorbed surfactants. This finding suggests that when dealing with hydrophilic particles interacting with oil droplets, thermodynamic criterion must be considered. To extend our study

342

343

INTERACTION OF IONIC SPECIES

FIG. 1. Schematic of experimental setup used for contact angle measurement.

from solid/oil droplet systems to hydrophobic/hydrophilic particle systems, and to simulate the wetting behavior of bitumen as in practice, the interactions of ionic species, (inorganic and organic) precipitates, and fine solids with a low energy hydrophobic paraffin wax were studied, and the results are presented in this paper. EXPERIMENTAL

Chemicals and Testing Materials ACS certified grade inorganic salts (NaCl, CaCl2, AlCl3) were purchased from Fisher Scientific. Surfactants tested included dodecyl amine hydrochloride, DAH (Eastman, Kodak), and lauric acid (Aldrich). Paraffin wax flakes were purchased from Aldrich. In some experiments, fine silica (25 mm from US Silica Company) and alumina (210 mm from Aldrich) powders were added into the solution. The chemicals were used as received. The solids were roasted in an oven at 500°C for overnight to remove any organic contaminants. All the measurements were conducted at room temperature in deionized water, and the solution pH was adjusted by reagent grade HCl and NaOH.

captive bubble method was used to measure water contact angles on a paraffin wax layer with a NRL Rame-Hart goniometer. To minimize the changes of contact angles with different bubble sizes as reported by Drelich et al. (17), Good and Koo (18), and Leja and Poling (19), a bubble of relatively large volume (2–5 mL in volume) was used in the present work. A bubble was generated on the tip of a microsyringe (25 mL, without needle) and kept in the solution for a given period of time before being brought into contact with the wax surface. To avoid the bubble moving around during changing the bubble volume via pushing or pulling the plunger (see Fig. 1), as encountered in the work by Drelich et al. (17), the formed bubble was constrained between the bottom surface of the syringe and the substrate. Both the advancing and the receding contact angles were measured from the left and right sides and then averaged. A set of baseline experiments was conducted to examine the effect of bubble aging time on contact angle. Virtually no change was observed for the aging time from 10 to 30 min. Therefore, a 10-min equilibrium period was used in all contact angle measurements. For each solution condition, 5 to 10 measurements were conducted by varying bubble volumes and changing the testing spots on the substrate. Determination of true equilibrium contact angles is often complicated, if not impossible, due to the surface roughness, chemical heterogeneous, and mechanical instabilities (20, 21). In addition, chemical reaction, static and dynamic conditions, line tension due to the drop size, penetration of the liquid into solids, and swelling of the solid by liquid can all contribute to variations of the measured contact angle values (22, 23). Therefore, the current work was focused on the changes in contact angle with solution conditions. The measurement errors were within 5°. Since the receding contact angles are more relevant to the flotation process (24), all reported values in this paper are the receding contact angles, unless otherwise stated. RESULTS AND DISCUSSION

Substrate Preparation

General Observation

Before conducting a measurement, a small piece of glass slide with a dimension of ca. 2 3 2 cm was placed on a hot plate, and then a few flakes of paraffin wax were added on the slide. The wax melted at an elevated temperature and spread on the slide, forming a thin layer. The heating element of the hot plate was then switched off, and the wax cooled down. The prepared paraffin wax surface was placed in a glass vessel containing the testing liquid (see Fig. 1). To avoid any contamination, a new substrate was prepared for each experiment having different solution conditions.

In the absence of solids and precipitates, virtually no changes in contact angle were observed experimentally when the bubble volume changed from 2 to 5 mL. Although the paraffin wax surface was not macroscopically flat, due to the contraction during solidification, the measured contact angle hysteresis was small (about 5°), which was in general agreement with the work by Extrand and Kumagai (25) using polymer surfaces as substrates. This is in contrast to Teflon and gold-coated surfaces which had a contact angle hysteresis of about 20 to 60°, respectively (18, 26). To calibrate the system and establish a baseline, water contact angles on the paraffin wax layer were measured as a function of pH (Fig. 2). A contact angle of 105° 6 5° was obtained. This value compares well with the values reported by

Contact Angle Measurement Interactions of different species with paraffin wax were evaluated from changes in contact angle on the wax surface. A

344

ZHOU ET AL.

FIG. 2.

Effect of pH and bubble volume on contact angles.

Arbiter et al. (108 –111°) (27) and Zisman (105–110°) (28) and from theoretical calculations (111°) (29). Effect of Metal Ions and pH The effect of salt concentration and pH on water contact angles was reported in the literature for various hydrophobic substrates. For example, water contact angles on mercury decreased with halide concentration and increased with pH (21); while on paraffin wax, no changes in contact angles were observed with pH (27). From the thermodynamic point of

view, whether an event can occur spontaneously or not depends on the changes in the free energy of a system. Only an event accompanied by a decrease in the system energy can occur thermodynamically. Since mercury has a surface energy (ca. 480 mJ/m2 (30)) much higher than the cohesion energy of water (144 mJ/m2), when adding halides to a water/mercury system, these halides may specifically adsorb on the mercury/ water interface to reduce the total system energy. The consequence is that water contact angles on mercury are reduced, due to the hydration of these adsorbed halides, as demonstrated by Xu et al. (21). Unlike mercury, paraffin wax is a low energy hydrophobic substance with a surface energy of 25.5 mJ/m2 (29), which is significantly lower than the cohesion energy of water. Therefore, the adsorption of metal ions on the paraffin wax would increase the total system energy, which is energetically unfavorable. Hence, addition of metal ions and the change in solution pH should not affect water contact angles on paraffin wax. To verify the above argument, the effect of metal ions such as Na1, Ca21, Al31 and their corresponding derivative metal hydroxyl ions on contact angle was investigated. The results given in Fig. 3a showed little changes in contact angle, regardless of solution pH and type of metal ions. This finding suggests the absence of specific adsorption of OH2, metal, and metal hydroxyl ions on paraffin wax, as anticipated. To further confirm this, the effect of metal ion concentration on contact angle was studied and the results are shown in Fig. 3b. Again, no changes in contact angle were observed, even at a metal ion concentration as high as 1 M where hydration effect of metal ions is anticipated to play a role (31). This observation suggests

FIG. 3. (a) Effect of metal ions on contact angles (concentration: 1 mM; bubble volume: 5 mL). (b) Effect of metal ion concentration on contact angles (bubble volume: 5 mL).

INTERACTION OF IONIC SPECIES

345

sured contact angles, suggesting that hydrophilic solids could not adhere to hydrophobic paraffin wax, regardless of whether the electrostatic force is attractive (for alumina/wax) or repulsive (for silica/wax). An important factor which contributed to the above observation may be due to the presence of a thin layer of water film on the hydrophilic particles, resulting from the strong interaction between the polar groups on the hydrophilic surfaces and water dipoles. Therefore, the interaction between hydrophilic particles (or metal hydroxyl precipitates) and paraffin wax is not through direct contact between the two entities, but via a layer of water film sandwiched in between. Since water could not adhere to, or spread over, paraffin wax, i.e., water is mobile or slips on the hydrophobic surfaces (35, 36), the interaction between paraffin wax and metal hydroxyl precipitates or hydrophilic particles may not be sufficient to affect the contact angle measured using the present technique. FIG. 4. Effect of presence of fine particles (25 mm silica or 210 mm alumina) on contact angles of water on paraffin wax.

that the examined metal ions have little affinity to paraffin wax and bubbles, as anticipated for low energy, nonpolar surfaces. Effect of Hydrophilic Contaminants The effect of hydrophilic impurities contained in a polymer on contact angles was reported by Fowkes et al. (32). In their systems, the hydrophilic impurities were part of the polyethylene surface structure, which could not be removed, and caused a reduction in contact angles. If the hydrophilic components were added externally or formed extrinsically, as encountered in flotation, they may behave differently. To examine the interaction of metal hydroxyl precipitates with wax, contact angles at high metal ion concentrations (e.g., Al31 at 1 M) above the corresponding hydroxide precipitation pHs (e.g., pH 8 and 11.5) were measured. Visually a layer of precipitates was observed to deposit on the substrate, probably due to the gravity. However, on approaching an air bubble to the substrate, this layer of precipitates was pushed away readily, and no change in contact angle was measured, as shown in Fig. 3b. Aluminum hydroxide precipitates have a PZC around pH 9.5 (33), and paraffin wax/water interface, on the other hand, can be assumed to be similar to air/water interface, which has a PZC around pH 2– 4 (27, 34). Based on electrical double layer theory, an electrostatic attraction between precipitate and paraffin wax is anticipated. To illustrate whether electrostatic force plays a dominant role in the interaction between paraffin wax and hydrophilic particulates, the effects of fine silica (negatively charged) and alumina (positively charged over a wide pH range) on contact angles were examined. The results given in Fig. 4 showed again negligible changes in the mea-

Effect of Surfactant Addition In surfactant solutions, the changes in contact angle depend on the orientation of surfactant molecules on a surface. This orientation is dictated by the competition of hydrophobic interaction with electrostatic interaction between the surfactant and the substrate, and is influenced by the structure of both the surfactant and the substrate, as well as the degree of hydrophobicity of the substrate (37, 38). For a strongly hydrophobic surface, it is usually accepted that the hydrocarbon chain of surfactant faces the surface, leaving the polar group exposed to water, and thus causing a reduction in contact angle. This pattern has been recently detected and verified by an infraredvisible sum-frequency spectroscopy (39). On a moderate hydrophobic surface, Bision et al. (40) proposed that there were three different surfactant molecular orientation patterns, depending on the electrostatic interaction between the surfactants and the surface. Lessa and Carmona-Ribeiro (41) found that the surface charge of polystyrene had a significant effect on modifying the surface wettability by surfactant adsorption. To simulate the role of surfactant in a bitumen extraction process, the effects of cationic amine and anionic carboxylates on contact angle of water on paraffin wax were studied, and the results are shown in Figs. 5 and 6. Amine. In Fig. 5, the effect of DAH concentration on contact angles is presented, and three important points can be noted: (i) contact angles decreased with increasing DAH concentration; (ii) contact angles remained constant below pH 8 for a given amine concentration; and (iii) contact angles dropped sharply when the pH was higher than 8, especially at high surfactant concentration, where the critical pH of bulk precipitation was reached. A contact angle as low as 30° (i.e., a reduction of 60°) was observed at pH .10 with amine concentration above 0.5 mM. Thermodynamic solution speciation diagram for amine systems (42) showed a constant concentration of protonated (cat-

346

ZHOU ET AL.

effect on the contact angles. At low pHs where lauric acid was in the form of precipitates, a continuous reduction in contact angle with increasing pH was observed, reaching a minimum value of ca. 85° at a pH corresponding to the PZC of lauric acid precipitates. Further increase in pH above 7 resulted in a less reduction in contact angle, and eventually contact angle returned to the value as in the absence of lauric acid at a pH higher than 8 or 10. Over these pH ranges, lauric acid is in anionic form. A long range electrostatic interaction between carboxylate anions and paraffin wax may prevent the close proximity of carboxylates to the surface to such a degree that no hydrophobic interaction could be recognized. Therefore, there is no substantial adsorption of carboxylates on the paraffin wax, which accounts for little changes in contact angles over the surfactant concentration range studied. The above explanation appears to agree with Keurentjes et al. (37) who reported that there was a narrow region of hydrophobicity in which virtually no adsorption of surfactants occurred. FIG. 5. Effect of DAH addition on contact angles (bubble volume: 5 mL). (The data for critical pH of bulk precipitation are from Laskowski et al. (46).)

ionic) amines below pH at which the precipitation starts to occur. This corresponded to an observed constant contact angle below pH 9 at a given amine concentration. The pH independence of contact angles suggests that hydrophobic forces may dominate the interaction between amine and paraffin wax even at pHs where the amine is in the cationic form. As a result, the amine molecules are oriented on the paraffin wax surfaces in such a way that the hydrocarbon chains face the paraffin wax, with the polar groups facing water, thus reducing the contact angles with increasing amine concentrations. This is opposite to the talc/amine system (43), where adding amine increased the water contact angle on talc, probably due to a lower hydrophobicity of talc (with contact angle u 5 62°). A sharp decrease in contact angle to 30° was observed in the presence of amine precipitates at high pHs. In this case, the precipitates are neutral amine molecules and, therefore, should be originally hydrophobic due to the lack of hydrolyzed polar groups (44). The direct force measurement by Ravishankar and Yoon (45) showed that no extra hydrophobic forces were detected in amine systems over the pH range where the amine molecules form precipitates. They proposed that the precipitates became hydrophilic, probably resulting from the reverse orientation of cationic amine on the precipitates. Such orientation of amine ions increased the number of hydrophilic sites on the precipitate surface. This hypothesis was supported by the zeta potential measurement. It was found (46) that zeta potentials of amine precipitates reduced sharply (from positive to negative) with pH, suggesting the adsorption of the excess OH2, on the precipitates. Carboxylates. The effect of lauric acid on contact angle is shown in Fig. 6. Compared with DAH, lauric acid has less

Effect of Surfactant and Fine Solids The results in Fig. 4 demonstrated that naturally hydrophilic particles will not adhere to naturally hydrophobic surfaces. However, it has been recognized in flotation practice that fine hydrophilic particles do coat on hydrophobic particles (slime coating) and, therefore, interfere with the flotation process. To explain this apparent discrepancy, the effect of surfactant-solid interaction was examined. As shown in Fig. 7, the contact angles dropped significantly (from ca.100 to 40°) at pH below 6 when silica was present, in contrast to the DAH system in the absence of silica where contact angle was found unaffected. Above pH 9, contact angles returned to values similar to those

FIG. 6. Effect of lauric acid addition on contact angles (bubble volume: 5 mL). (The data for critical pH of bulk precipitation and PZC of precipitates are from Laskowski et al. (46).)

INTERACTION OF IONIC SPECIES

FIG. 7. DAH.

Effect of fine silica addition (0.4 g/L) on contact angles in 0.5 mM

in the absence of silica. Because no change in contact angle was observed with silica/paraffin wax systems, as shown in Fig. 4, the results in Fig. 7 indicate that the surfactant induced the interaction between silica and paraffin wax below pH 9. Similar changes in contact angle with pH were observed in the silica/DAH system (47). The results here may suggest that the measured contact angle corresponded to that of silica deposited on paraffin wax. To verify whether this is the case, contact angle measurement on a clean glass slide (without paraffin wax coating) in a solution with the same DAH concentration (0.5 mM) was conducted. It was found that the data correlated well with the silica powder/DAH/paraffin wax system, confirming that silica was coated on the paraffin wax. The results also suggest that the solid/liquid ratio (0.04%, w/w) in this experiment is sufficiently low so that the surfactant concentration in the solution did not change substantially despite the adsorption on fine particles. It should be noted that similar results were obtained at a lower solid/liquid ratio, further confirming that the effect of solid surface area on the change of bulk surfactant concentration could not be detected at the solid/liquid ratio used in our experiments. Since silica is negatively charged in the pH range studied, the adsorption of cationic amine on a negatively charged silica surface made silica hydrophobic (47). The hydrophobilized silica then coated on the paraffin wax through hydrophobic interaction with DAH as a bridge. It is interesting to note that over a pH range from 8 to 10, the measured contact angle for silica powder is substantially higher than that for glass slide. The observed difference could be related to a ‘‘rougher’’ surface of deposited silica powder layer on the paraffin wax. However, it is more likely that the paraffin wax surface is not fully covered by the hydrophobilized silica particles so that the

347

measured contact angle represents a composite value between paraffin wax and hydrophobic silica particles. Above pH 10, data points fell on the same line for all the cases studied, indicating that silica and glass slide were coated by amine precipitates under these conditions, as in the case of DAH/ paraffin wax system, and confirmed from zeta potential measurements (46). To further justify the bridging role played by a surfactant, the effect of anionic carboxylate ions in combination with alumina powder (positively charged over a wide pH range) on water contact angle was investigated. The results given in Fig. 8 showed that compared with no alumina addition, contact angles further decreased with increasing pH. This is in contrast to the case where alumina powder was used alone (see Fig. 4). The presence of alumina has no effect on contact angles of water on paraffin wax in lauric acid solutions when pHs were higher than the PZC of alumina, i.e., when alumina became negatively charged. At pH ,9, alumina is positively charged, and negatively charged carboxylate ions were physically adsorbed on the alumina. The observed reduction in contact angle with the addition of alumina indicated that a layer of alumina was coated on paraffin wax, a situation similar to the silica/ DAH/paraffin wax system. What we measured was the contact angle of a layer of alumina powder, with adsorbed carboxylate ions, deposited on the paraffin wax. At pH . PZC, negatively charged alumina was unable to adsorb carboxylate ions. As a result, alumina remained hydrophilic and did not coat on paraffin wax, as indicated in Fig. 4. The above observations suggest that the role of surfactant in bridging hydrophilic particles with hydrophobic surfaces can only occur when the surfactant interacts with the solid particles. Our study on the interactions between fine solids and par-

FIG. 8. Effect of fine alumina addition (0.4 g/L) on contact angles in 0.5 mM lauric. (The data for PZC of alumina are from Modi and Fuerstenau (48).)

348

ZHOU ET AL.

affin wax in the presence and absence of surfactants further demonstrated that solids will not adhere to paraffin wax (and therefore, there was no change in contact angles) unless they are made hydrophobic by adsorbed surfactants, as shown in Figs. 7 and 8. This has a practical implication in flotation. Since the work presented here showed that pure naturally hydrophilic particles cannot attach to naturally hydrophobic surfaces, the problems of slime coating may partly be due to the fact that these originally hydrophilic slimes have been contaminated by surfactants present in the system, thus sticking to naturally hydrophobic particles. In oil sands processing, it was found that (8, 49) carboxylates adsorbed on fine minerals and clays affected bitumen flotation and froth quality. This study provided fundamental evidence for the practical issues. Our study with model systems also suggests that efforts should be directed to minimizing surfactant contamination when searching for solutions to fine particle interference in bitumen flotation. CONCLUSIONS

1. A relatively constant water contact angle on paraffin wax was observed at different pHs. This value was not affected by the presence of different types and concentrations of metal and corresponding metal hydroxyl ions, metal hydroxyl precipitates, fine silica, and alumina powders. This is attributed to the low surface energy of paraffin wax, which prevented adsorption and adhesion of species associated with an increase in system energies. 2. Increasing the concentration of amine reduced the contact angle, due to the adsorption of amine on paraffin wax by hydrophobic interaction. A sharp decrease in contact angle was observed at higher pHs, where amine molecules precipitate preferably on paraffin wax surfaces. The change of amine precipitates from hydrophobic to hydrophilic was attributed to the reversal orientation of surfactant on the precipitate surface. 3. The presence of lauric acid reduced the contact angle over a pH range from 2 to 7, due to the formation of precipitates, but the reduction was much smaller, compared with the amine precipitates. At higher pHs, virtually no differences in contact angle were observed with or without lauric acid, due to the electrostatic repulsion between the carboxylate ions and the wax-water interface. 4. Fine silica or alumina, although not interacting with paraffin wax by themselves, can coat on paraffin wax surfaces in the presence of surfactants. The induced hydrophobicity by surfactant adsorption on the solids and the hydrophobic interaction between the surfactants adsorbed on the solids and paraffin wax surfaces are responsible. ACKNOWLEDGMENTS Financial support for this work provided by NSERC-Industrial Research Chair Program in Oil Sands (held by J.H.M.) and by Alberta Department of Energy is gratefully acknowledged. The support and assistance of the person-

nel at Syncrude Canada Ltd., Edmonton Research Center, are also acknowledged. One of the authors (Z.A.Z.) thanks the Quebec Government for providing financial assistance in the form of FCAR Postdoctoral Research Scholarship.

REFERENCES 1. Fuerstenau, M. C., Miller, J. D., and Kuhn, M. C., ‘‘Chemistry of Flotation.’’ SME and AIME, New York, 1985. 2. Fuerstenau, D. W., Gaudin, A. M., and Miaw, H. L., Trans. AIME 211, 792 (1958). 3. Gaudin, A. M., Fuerstenau, D. W., and Miaw, H. L., Trans. Can. Inst. Miner. Met. 63, 668 (1960). 4. Iwasaki, I., Cooke, S., Harraway, D. H., and Choi, H. S., Trans. AIME 223, 97 (1962). 5. Grant, G., Sarker, S., and Phillips, C. R., Ind. Eng. Chem. Process Des. Dev. 19, 185 (1980). 6. Sanford, E. C. Can. J. Chem. Eng. 61, 554 (1983). 7. Schramm, L. L., Smith, R. G., and Stone, J. A., AOSTRA J. Res. 1, 147 (1985). 8. Hepler, L. G., and Smith, R. G., ‘‘The Alberta Oil Sands: Industrial Procedures for Extraction and Some Recent Fundamental Research,’’ pp. 1–247. AOSTRA Technical Publication Series No. 14, Alberta Oil Sands Technology and Research Authority, Edmonton, Canada, 1994. 9. Dai, Q., and Chung, K., Fuel 74, 1858 (1995). 10. Bowman, C. W., in ‘‘Proc. 7th World Petrol. Congr.,’’ Vol. 3, p. 583, 1967. 11. Takamura, K., and Wallace, D., J. Can. Pet. Tech. 27 (Nov-Dec.), 98 (1988). 12. Churaev, N. V., in ‘‘17th International Mineral Processing Congress,’’ Vol. 3, p. 1. Dresden, Germany, 1991. 13. Yoon, R. H., in ‘‘17th International Mineral Processing Congress,’’ Vol. 3, pp. 16 –31. Dresden, Germany, 1991. 14. LaFrance, P., and Grasso, D., Environ. Sci. Tech. 29, 1346 (1995). 15. Yoon, R. H., and Mao, L., J. Colloid Interface. Sci. 181, 613 (1996). 16. Zhou, Z. A., Zhenghe Xu, Masliyah, J. H., and Czarnecki, J., submitted for publication. 17. Drelich, J., Miller, J., and Hupka, J., J. Colloid Interface Sci. 155, 379 (1993). 18. Good, R. J., and Koo, M. N., J. Colloid Interface Sci. 71(2), 283 (1979). 19. Leja, J., and Poling, G. W., in ‘‘V International Mineral Processing Congress,’’ Paper No. 17, pp. 325–341. Group III, London, 1960. 20. Bourges-Monnier, C., and Shanahan, M. E. R., Langmuir 11, 2820 (1995). 21. Xu, Z., Liu, Q., Ling, J., and Summers, A., Langmuir 12, 547 (1996). 22. Miyama, M., Yang, Y., Yasuda, T., Okuno, T., and Yasuda, H. K., Langmuir 13, 5494 (1997). 23. Kwok, D. Y., Gietzelt, T., Grundke, K. Jacobasch, H. J., and Neumann, A. W., Langmuir 13, 2880 (1997). 24. Drelich, J., and Miller, J. D., ‘‘SME Annual Meeting, Denver, Colorado, March 6-9, 1995,’’ preprint No. 95-11. 25. Extrand, C. W., and Kumagai, Y., J. Colloid Interface Sci. 191, 378 (1997). 26. Drelich, J., Wilbur, J. L., Miller, J. D., and Whitesides, G. M., Langmuir 12, 1913 (1996). 27. Arbiter, N., Fujii, Y., Hansen, B. an Raja, A., in ‘‘Advances in Interfacial Phenomena of Particulate/Solution/Gas Systems; Applications to Flotation Research’’ (P. Somasundaran and R. B. Grieves, Eds.), Vol. 71, p. 176. AIChE Symposium Series 150, New York, 1975. 28. Zisman, W. A., Ind. Eng. Chem. 55(10), 19 (1963). 29. Fowkes, F. M., Ind. Eng. Chem. 56(12), 40 (1964). 30. Lide, D. R. (Ed.) ‘‘Handbook of Chemistry and Physics,’’ 71st ed. CRC Press, Boston, 1991. 31. Craig, V. S. J., Ninham, B. W., and Pashley, R. M., J. Phys. Chem. 97, 10192 (1993).

INTERACTION OF IONIC SPECIES 32. Fowkes, F. M., McCarthy, D. C., and Mostafa, M. A., J. Colloid Interface Sci. 78(1), 200 (1980). 33. Kusaka, E., Nakahiro, Y., and Wakamatsu, T., Miner. Eng. 8, 818 (1995). 34. Yoon, R. H., and Yordan, J. L., J. Colloid Interface Sci. 113(2), 430 (1986). 35. Vinogradova, O. I., J. Colloid Interface. Sci. 169, 306 (1995). 36. Vinogradova, O. I., Langmuir 11, 2213 (1995). 37. Keurentjes, J. T. F., Cohen Stuart, M. A., Btinkman, D., Schroen, C. G., and van’t Riet, K., Colloids Surf. 51, 189 (1990). 38. Wang, Y., Han, B., Yan, H., and Kwak, J. C., Langmuir 13, 3119 (1997). 39. Ward, R. N., Davies, P. B., and Bain, C. D., J. Phys. Chem. 97, 7141 (1993). 40. Bision, P. D., Vartledge, J. G., Keesom, W. H., and Radke, C. J., J. Colloid Interface Sci. 78, 225 (1980). 41. Lessa, M. M., and Carmona-Ribeiro, A. M., J. Colloid Interface. Sci. 182, 166 (1996).

349

42. Fuerstenau, M. C., in ‘‘Principles of Flotation’’ (P. King, Ed.), p. 1. South African Institute of Mining and Metallurgy, Johannesburg, 1982. 43. Ouyang, J., Lu, S., and Wu, L., in ‘‘Processing of Hydrophobic Minerals and Fine Coal’’ (L. S. Laskowski and G. W. Poling, Eds.), p. 355–361. CIM, Canada, 1995. 44. Laskowski, J. S., Yordan, J. L., and Yoon, R. H., Langmuir 5, 373 (1989). 45. Ravishankar, S. A., and Yoon, R. H., Miner. Metall. Process. 10 (May, 1997). 46. Laskowski, J. S., Vurdela, R. M., and Liu, Q., ‘‘XVI International Mineral Processing Congress,’’ pp. 703–715. Elsevier Science, Amsterdam, 1988. 47. Fuerstenau, D. W., Trans. AIME. 208, 1365 (1957). 48. Modi, H. J., and Fuerstenau, D. W., J. Phys. Chem. 61, 640 (1957). 49. McIntyre, D. D., Montgomery, D. S., and Strausz, O. P., AOSTRA J. Res. 2, 223 (1986).