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Surfactant Systems
Journal of Colloid and Interface Science 212, 384 –389 (1999) Article ID jcis.1998.6044, available online at http://www.idealibrary.com on
Interactions in Calcium Oxalate Hydrate/Surfactant Systems M. Sikiric´,* N. Filipovic´-Vincekovic´,† ,1 V. Babic´-Ivancˇic´,† N. Vdovic´,† and H. Fu¨redi-Milhofer‡ *Faculty of Agriculture, †Department of Chemistry, Ruder Bosˇkovic´ Institute, 10001 Zagreb, Croatia; and ‡Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel Received July 22, 1998, accepted December 17, 1998
i.e., monomer or supramolecular organization in solution. In addition, crystal aggregation may be induced (11) or inhibited (7), and the type of the predominant crystallizing phase may be changed. Thus, it has been shown (6, 8 –11) that micellar solutions of anionic surfactants promote crystallization of the metastable calcium oxalate dihydrate (COD, CaC 2O 4(2 1 x)H 2O, where x # 0.5), evident because of the thermodynamically stable monohydrate (COM, CaC 2O 4 H 2O), which is the dominant crystal form in control systems and in the presence of low (ionic) concentrations of surfactants. In order to better understand the above phenomena we performed systematic studies of the interaction of surfactants with prepared, well defined crystals of COM and COD under conditions resembling those in urine. In a recent publication (13) the adsorption of anionic sodium diisooctyl sulfosuccinate (AOT) from 0.3 M NaCl solutions onto COM and COD crystals was discussed. Given all other parameters identical, adsorption was strongly dependent on the ionic structures of the respective substrate surfaces, with COD crystals having significantly lower affinity for surfactant adsorption than COM. In the present paper we discuss recent findings about the adsorption of an anionic (sodium dodecyl sulfate, SDS) and a cationic (dodecylammonium chloride, DDACl) surfactant onto the same substrates. The influence of the adsorbed surfactants on the phase transformation of COD to COM and on adsorption behavior will be explained on the basis of the ionic structure of the different crystal faces of COM and COD, respectively.
Phase transformation of calcium oxalate dihydrate (COD) into the thermodynamically stable monohydrate (COM) in anionic (sodium dodecyl sulfate (SDS)) and cationic (dodecylammonium chloride) surfactant solutions has been studied. Both surfactants inhibit, but do not stop transformation from COD to COM due to their preferential adsorption at different crystal faces. SDS acts as a stronger transformation inhibitor. The general shape of adsorption isotherms of both surfactants at the solid/liquid interface is of two-plateau-type, but differences in the adsorption behavior exist. They originate from different ionic and molecular structures of crystal surfaces and interactions between surfactant headgroups and solid surface. © 1999 Academic Press Key Words: calcium oxalate hydrate; surfactant adsorption; transformation.
INTRODUCTION
A number of studies have been carried out to investigate the effects of various additives on nucleation, crystal growth, aggregation, and phase transformation of calcium oxalate hydrates which are the most common constituents of urinary calculi (1–12). In our group particular attention has been paid to the role of surfactants (6 –12) for the following reasons: (i) Many organic constituents in human urine, including proteins, have surfactant properties. Among other organic molecules, human urine contains biological surfactants, bile salts, which are responsible for lowering urinary surface tension and have a potential inhibitory role in urinary stone development (9). (ii) Because of their unique properties (self-assembly, adsorption at interfaces) and ready availability in many different designs, surfactants lend themselves as ideal model systems for systematic studies of the effect of organic molecules on the formation and the transformation of ionic crystals.
EXPERIMENTAL
Materials
In previous work (6 –11) we investigated the effect of order various model surfactants on in situ formation of calcium oxalate from high ionic strength solutions. The obtained results revealed (7, 11) that the crystal growth kinetics and morphology depend on the type and the concentration of the surfactant,
Solutions were prepared from analytical grade chemicals (Merck, Germany) and triple distilled water and standardized by using conventional analytical methods. SDS, a BDH (England) product, was purified by repeated recrystalization from ethanol–water mixtures. DDACl was prepared by neutralization of dodecylamine with HCl and purified by repeated recrystallization from benzene– ethylether mixtures. The critical micelle concentration, (CMC) under actual experimental conditions (i.e., solutions containing 0.30 and 0.01 mol dm 23
1
To whom correspondence should be addressed. E-mail: filipovic@ rudjer.irb.hr. 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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calcium chloride) was found to be 5 3 10 24 mol dm 23 for SDS and 1 3 10 23 for DDACl as determined by surface tension measurements using an interfacial tensiometer (model K8, Kru¨ss, Hamburg). COM crystals were prepared by simultaneously dropping 250 ml of 0.04 mol dm 23 solutions of calcium chloride and sodium oxalate into 1500 ml of triply distilled water at 75°C under stirbar stirring. After synthesis the suspension was kept at the same conditions for 2 h and afterward slowly cooled to room temperature. After 24 h the crystals were filtered through a 0.45-mm Milipore filter, washed with deionized water, and dried in a vacuum dessicator. The crystals were six-sided plates with an average surface area of 6.4 m 2 g 21. COD crystals were precipitated by the method of Brown et al. (14) by consecutive addition of calcium chloride (0.37 ml of 4.00 mol dm 23) and sodium oxalate (1.53 ml of 0.25 mol dm 23) solutions to 200 ml of solution containing 0.01155 mol dm 23 magnesium sulfate, 0.00963 mol dm 23 sodium citrate, and 0.0637 mol dm 23 potassium chloride. Solutions were gently swirled and left at room temperature for 10 min. Crystals were filtered through a 0.45-mm Milipore filter, washed with deionized water and ethanol, and dried in a vacuum dessicator. The crystals had the shape of tetragonal bipyramids with an average surface area of 5.0 m 2 g 21. Experimental Systems Both phase transformation and adsorption studies were carried under the same conditions that were employed in previous in situ kinetic and morphological studies on the crystallization of calcium oxalates in the presence of additives (1, 2, 6 –12). Thus we have maintained high ionic strength (0.3 mol dm 23 NaCl) and an initial pH 6.5. The pH was chosen within the range of urinary pH (5–7). Although the particular value is somewhat high (the average urinary pH is 5.8), it was convenient for interpreting kinetic studies because the predominant oxalate species at this pH is C 2O 422 ion. In keeping with the above principles 0.5 g dm 23 prepared COM and COD crystals were conditioned at 37°C in solutions containing 0.3 mol dm 23 NaCl and 0.01 mol dm 23 CaCl 2. The pH of the solutions was adjusted to 6.5 with NaOH solution. Known concentrations of SDS or DDACl were added to the solutions prior to pH adjustment. Transformation of COD into COM was studied in a blank system without surfactant and in the systems with micellar concentrations of cationic and anionic surfactants. Crystals were suspended in electrolyte solutions by magnetic stirring (G 5 5 6 1 s 21) and periodically sampled onto glass slides and observed by bright field and polarized light optical microscopy (Leitz Ortophan photographic microscope). At given time intervals, crystals were collected by filtration, and their composition was determined qualitatively by X-ray diffraction powder patterns and quantitatively by thermogravimetric analysis. Quantitative analysis by thermogravimetry is based on the
difference in the content of crystalline water in COM and COD (15). X-rays (Philips counter diffractometer) and thermogravimetric analysis (Mettler TA4000 System) was performed immediately after filtration. X-ray diffractograms were recorded from moist samples, while TGA samples were air-dried to constant weight before the onset of heating. In this way further hydrate transformation during sample preparation was avoided. For adsorption experiments 0.5 g dm 23 of the respective solid phase (COM or COD) was equilibrated for 24 h at pH 6.5 and 37°C in electrolyte solution with different concentrations of DDACl or SDS. After completion of the experiment the solutions were centrifuged and aliquot solutions were analyzed for DDACl or SDS by an Orion surfactant electrode (model 93-42) in connection with an Ag–AgCl double junction reference electrode (model 90-02, Orion Research, USA). The specific surface area of the crystals was determined by the Brunauer–Emmett–Teller (BET) method (16) (Quantasorb, Quantachrome Corp.) using nitrogen. From the amount of surfactant adsorbed in the plateau regions and the specific surface area of respective crystals, the plateau adsorption densities were calculated. Electrophoretic measurements were made with an automated microelectrophoretic instrument (Pen Kem, type S3000 Bedford Hills, NY). Aliquots of 15 ml of equilibrated suspensions were left to settle and the supernatant suspensions were separated and injected into the detector cell. The measurement was performed at low voltage (50 V) because of the high ionic strength of suspension. RESULTS
When suspended in electrolyte solutions, metastable COD transforms into the thermodynamically stable COM. In Fig. 1 time dependent changes of solid phase composition in electrolyte solutions without surfactant and in the presence of micellar concentrations of SDS or DDACl are represented. As revealed by TG analysis, without surfactant up to 40% of COD transformed into COM within 24 h, and within 7 days transformation was complete (curve 1 in Fig. 1). The corresponding X-ray diffractograms show only the characteristic peaks of COD and COM, the peaks intensity of COD decreasing and those of COM increasing with time. Increasing content of COM in COD–COM mixtures was also observed by optical microscopy. In the beginning, the characteristic tetragonal bipyramids typical of COD crystals were observed, but aging in electrolyte solution induced pronounced rounding and a decrease in crystal size typical of dissolution. The dissolution of COD crystals was followed by the formation of plate-like COM crystals with a wide size distribution. In the presence of micellar concentrations of the surfactants significant inhibition of phase transformation occurred (Fig. 1, curves 2, 3,) with SDS being a somewhat stronger inhibitor. The results of thermogravimetric analyses were confirmed by X-ray diffractograms exhibiting that samples aged for 7 days in
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FIG. 1. The increase of the mass fraction of COM present in the solid phase with aging time, t A. Transformation kinetics of COD as observed in control system (F) and in micellar surfactant solutions (Œ, c(DDACl) 5 0.004 mol dm 23; E, c(SDS) 5 0.0008 mol dm 23). The amount of COD was 0.5 g dm 23, concentration of CaCl 2 5 0.01 mol dm 23, NaCl 5 0.3 mol dm 23, pH 6.5, and temperature, T 5 37°C.
surfactant solutions consisted of mixtures of COD and COM with a very small portion of COM, in contrast to the control system where in aged samples peaks characteristic of COM prevailed. No significant morphological differences between COD crystals aged in micellar solutions of SDS and DDACl, respectively, were observed. Electrophoretic mobility spectra of COM and COD particles, suspended under the working experimental conditions (i.e., in solutions containing 0.3 M NaCl and 0.01 M CaCl 2) are shown in Fig. 2. The spectra clearly show surface heterogeneity; i.e., the solid particle surfaces consist of a distribution of patches of different surface charge densities. Although both negative and positive electrokinetic charges are present, it is seen that positive electrokinetic charges prevail especially at the COD/solution interface. This is possibly due to the adsorption of excess calcium ions from the electrolyte solution. The net surface charge density displayed by z-potential measurements is an average of the charges on the local patches. In Figs. 3 and 4 the adsorption isotherms of SDS and DDACl on COM and COD are compared with the respective z-potential vs equilibrium concentration curves. All adsorption isotherms (Figs. 3a, 4a) are of the two-plateau-type (LS) (17), exhibiting a region of low adsorption (appearing as a pseudoplateau) and a region of high adsorption with an inflection between them. At concentrations approaching the CMC, the adsorption isotherms reached a short second plateau. The usual leveling off of the adsorption isotherms was observed only in a narrow concentration range in the vicinity of the CMC because with further increase in concentration, surfactant precipitation occurred and was confirmed by X-ray analysis (see
also Ref. 6). Because the surfaces of both hydrates adsorb more SDS than DDACl, the surface area of both crystals occupied by one dodecylammonium ion is somewhat higher than that occupied by one dodecyl sulfate ion. Higher concentrations of both surfactants were adsorbed on COM, so that the density of adsorbant was greater on those crystals. The calculated adsorption densities in the regions of low and high adsorption are listed in Table I. Changes of the z-potential of both COM and COD particles with increasing SDS concentration are shown in Fig. 3b. Initially, under the given experimental conditions (0.3 M NaCl and 0.01 M CaCl 2) calcium oxalate particles bear an overall small positive electrokinetic charge, with an average z-potential of 0.1 mV for COM and 7.5 mV for COD, respectively. In contrast to the respective adsorption isotherms (Fig. 3a) the z-potential vs concentration curves show two inflections at low and high surfactant concentrations, respectively. As expected the z-potential becomes more negative with the adsorption of more anionic SDS. The changes in the z-potential of COM and COD suspensions with DDACl revealed a slight increase of z-potential with increasing surfactant concentration (Fig. 4b). DISCUSSION
Phase Transformation of COD into COM in the Presence of Surfactants Metastable calcium oxalate hydrates (COD) and calcium oxalate trihydrate, (COT) (CaC 2O 4 (3 2 x H 2O), where x # 0.5) transform into the thermodynamically stable COM by a
CALCIUM OXALATE HYDRATE/SURFACTANT SYSTEMS
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FIG. 2. Change of relative signal (arbitrary units) intensity, I, with electrophoretic mobility for control COM (top) and COD (bottom) dispersions. The amount of solid phase was 0.5 g dm 23, concentration of CaCl 2 5 0.01 mol dm 23, NaCl 5 0.3 mol dm 23, pH 6.5, and temperature, T 5 37°C.
solution mediated process (18 –21). In general the extent of phase transformation depends on the rate of dissolution of the metastable, and the rates of nucleation and growth of the stable phase, all of which are sensitive to solution conditions (pH, ionic strength, temperature, and the presence of impurities). It has been shown (19) that certain amino acids when present in solution change the rate controlling mechanism of COD dissolution from diffusion to a surface controlled mechanism; i.e., the kinetics of COD dissolution are controlled by processes at the solid-solution interface. In accordance with the above findings and taking into account the results of the above-described adsorption studies (Figs. 3 and 4 and Table I), it seems reasonable to assume that the strong inhibition of COD to COM phase transformation is a consequence of adsorption of the surfactants at both the COM and the COD solid–liquid interfaces. When COD crystals are brought into contact with a solution containing micellar concentrations of SDS or DDACl the rate of dissolution of COD crystals is greatly reduced because of adsorption and so is the rate of building up the supersaturation necessary for crystallization of COM. At the same time, growth of any nuclei of COM crystals is also inhibited by strong adsorption of surfactant at the COM solid–liquid interface. The observed difference between the rate of phase transformation in the presence of SDS and DDACl, respectively (Fig. 1), is in accordance with the difference in adsorption densities from micellar solutions of the two surfactants (see values for the second plateau in Table I).
Adsorption of Surfactants at the Surfaces of COM and COD Crystals Adsorption is a consequence of interactions at the solid– liquid interface and thus depends on the crystal structure and electrokinetic charges at the interface. A number of models (22–29) have been proposed, most of which are based on experimental and theoretical studies dealing with the adsorption of ionic surfactants at the oxide–liquid interface. All models agree that at very low surfactant concentrations the primary force involved in adsorption is of electrostatic origin; i.e. the headgroups of the adsorbed surfactant are oriented toward the solid surface and the hydrocarbon tails toward the solution. The subsequent rapid increase of the adsorption at higher surfactant concentrations is generally explained by surfactant aggregation at the solid–liquid interface involving mechanisms similar to those causing the formation of micelles in aqueous solutions, i.e., hydrophobic tail–tail interactions. Finally the leveling off of adsorption starting at the CMC is ascribed to a chemical potential sink caused by the presence of micelles in the bulk solution (24, 26). The general shape of the isotherms characterizing the adsorption of SDS on calcium oxalate particles (Fig. 3a) is in accordance with the above-described models. It should be noted that the final adsorption densities are unusually high (20.5 and 31.9 molecules nm 22 for COD and COM, respectively, Table I) as compared to previously reported adsorption densities of anionic surfactants on oxide surfaces (23, 24, 27,
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A number of studies most relevant to the above results have been reviewed by Cases and Villieras (29). The authors discuss the importance of surface heterogeneity in the adsorption process. Adsorption on heterogeneous surfaces occurs first on the most energetic homogeneous domains followed by successive phase transitions on domains with lesser energies. As the solution concentration increases the adsorption isotherms develop step-like discontinuities related to the infilling of surface domains. The discrete steps may or may not be observable depending on the size and distribution of the homogeneous areas and the energies of patches. When sparingly soluble salts are used as substrates, adsorption of surfactants may go far beyond monolayer capacity because in addition to true adsorption, retention at the surface may be caused by surface condensation. Although in the control systems both COM and COD particles exhibited an overall positive charge, adsorption of the cationic surfactant did occur, although to a lesser extent than adsorption of SDS (Figs. 4a and Table I). The general shapes of the respective isotherms (Fig. 4a) are similar to those for
FIG. 3. (a) Adsorption isotherms of SDS on COM and COD and (b) z-potential values vs the equilibrium surfactant concentration (c eq). G is the adsorbed amount of surfactant at given concentration. The amount of solid phase in COM and COD dispersions was 0.5 g dm 23, concentration of CaCl 2 5 0.01 mol dm 23, NaCl 5 0.3 mol dm 23, pH 6.5, and temperature, T 5 37°C.
28). A possible explanation for such high adsorption densities may be found by considering details of the isotherms together with the corresponding z-potential vs equilibrium concentration curves (Figs. 3a and 3b). It is seen that adsorption starts at very low surfactant concentrations while the slightly positive surface potential hardly changes. This is in accordance with an electrostatically driven, head-to-surface adsorption mechanism. With increasing surfactant concentration the adsorption isotherms reach a pseudoplateau, while the corresponding z-potential becomes negative, indicating that aggregation has taken place and that SDS molecules are partly arranged in the form of negatively charged bilayers. The adsorption densities in this region (3 and 4 molecules nm 22 for COD and COM, respectively, Table I) are comparable with the maximum adsorption density of SDS on Al 2O 3 (4.2 molecules nm 22, Ref. 28). Subsequently, while adsorption further increases, the z-potential vs concentration curves pass through a near plateau followed by a second relatively steep inflection. This could indicate filling in of lower energy surface sites by head-on adsorption of surfactant molecules and their subsequent aggregation. Considering the high adsorption densities obtained, the formation of multilayer arrangements and/or surface condensation (29) cannot be excluded.
FIG. 4. (a) Adsorption isotherms of DDACl on COM and COD and (b) z-potential values vs the equilibrium surfactant concentration (c eq). G is the adsorbed amount of surfactant at given concentration. The amount of solid phase in COM and COD dispersions was 0.5 g dm 23, concentration of CaCl 2 5 0.01 mol dm 23, NaCl 5 0.3 mol dm 23, pH 6.5, and temperature, T 5 37°C.
CALCIUM OXALATE HYDRATE/SURFACTANT SYSTEMS
TABLE I Adsorption Densities of DDACI and SDS on COM and COD at 37°C for the First Plateau and for the Second Plateau, Respectively
System
COM 1 DDACI
COM 1 SDS
COD 1 DDACI
COD 1 SDS
molecules/nm 2 molecules/nm 2
3.4 16.3
4.3 31.9
2.4 13.3
3.0 20.5
SDS while the corresponding positive z-potential slightly increases with increasing concentration (Fig. 4b). An explanation for the adsorption of the cationic surfactant at prevailing positively charged COM and COD crystal faces is given by the presence of both positively and negatively charged patches as demonstrated by the electrophoretic mobility spectra (Fig. 2). As previously determined by morphological studies (7, 12), COM crystals present quite different sites for preferential adsorption of anionic and cationic surfactants. When crystals where grown in the presence of an anionic surfactant (AOT) (12), preferential adsorption occurred at the {101} and {010} crystal faces resulting in thin, elongated crystals. The cationic surfactant, DDACl, in contrast promoted the formation of thin, rhomb-like crystals, indicating preferential adsorption on the {101} crystal faces. Another important result of our experiments is the fact that adsorption of both surfactants is at all concentrations lower than onto COM crystals (Figs. 3a and 4a and Table I). This is true particularly for the anionic surfactant where the ratio of the respective adsorption densities onto COM vs COD (in molecules/nm 2) is 1.5 (second plateau in Table I). This is in accordance with the respective ratio 2 of the adsorption densities of sodium diisooctyl sulfosuccinate (AOT) onto COM and COD crystal surfaces, respectively (Table I in Ref. 13). The lower adsorption capacity of COD as compared to that of COM may in part be due to poisoning of the crystal surface by adsorbed Mg 21 and/or citrate ions which have been used in the process of preparation of the seed crystals (in accordance to Ref. 10). However, at least part of the difference can be ascribed to the difference in the ionic structures of the adsorbing crystal faces of the two substrates, which has been discussed in detail in Ref. 13. In this treatise it has been shown that COM crystal faces have high negative charge densities and that adsorption of the anionic surfactant to these surfaces occurs via solution calcium ions. On the other hand, crystal planes of COD expose rows of oxalate ions and water molecules, which increase their hydrophilic character and shield the surfaces from additional adsorption (13). In conclusion, the adsorption results indicate that the adsorption of surfactants at the calcium oxalate/solution interfaces strongly depends on the molecular and/or ionic structure of the substrate surface and specific head group/surface interactions. The overall adsorption capacity of both substrates for DDACl
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is significantly lower and so is the difference between them (the ratio of adsorption densities on COM and COD in micellar solutions is 1.2). ACKNOWLEDGMENTS The financial support granted by the Ministry of Science, Technology, and Informatics of the Republic Croatia (Grant 0098602) is gratefully acknowledged.
REFERENCES 1. Sˇkrtic´, D., Fu¨redi-Milhofer, H., and Markovic´, M., J. Crystal Growth 80, 113 (1987). 2. Markovic´, M., Komunjer, Lj., Fu¨redi-Milhofer, H., Sˇkrtic´, D., and Sarig, S., J. Crystal Growth 88, 118 (1988). 3. Antinozzi, P. A., and Brown, C. M., J. Crystal Growth 125, 215 (1992). 4. Cody, A. M., and Cody, R. D., J Crystal Growth 135, 235 (1994). 5. Tomazˇic´, B., and Nancollas, G. H., J. Colloid Interface Sci. 75, 149 (1980). 6. Sˇkrtic´, D., and Filipovic´-Vincekovic´, N., J. Crystal Growth 88, 313 (1988). 7. Sˇkrtic´, D., Filipovic´-Vincekovic´, N., and Fu¨redi-Milhofer, H., J. Crystal Growth 144, 118 (1991). 8. Sˇkrtic´, D., Filipovic´-Vincekovic´, N., and Bisˇc´an, J., Ber. Bunsen-Ges. Phys. Chem. 97, 114 (1993). 9. Sˇkrtic´, D., Filipovic´-Vincekovic´, N., Babic´-Ivancˇic´, V., and Tusˇek-Bozˇic´, Lj., J. Crystal Growth 133, 189 (1993). 10. Fu¨redi-Milhofer, H., Tunik, L., Bloch, R., and Garti, N., Mol. Cryst. Liq. Cryst. 248, 199 (1994). 11. Fu¨redi-Milhofer, H., Tunik, L., Filipovic´-Vincekovic´, N., Sˇkrtic´, D., Babic´-Ivancˇic´, V., and Garti, N., Scanning Microsc. 9, 1061 (1995). 12. Tunik, L., Addadi, L., Garti, N., and Fu¨redi-Milhofer, H., J. Crystal Growth 167, 748 (1996). 13. Tunik, L., Fu¨redi-Milhofer, H., and Garti, N., Lanmguir 14, 3351 (1998). 14. Brown, P., Ackerman, D., and Finlayson, B., in “Urolithiasis” (R. A. L. Sutton, E. C. Cameron, V. Walker, and B. Robertson, Eds.), p. 75. Plenum, New York, 1989. 15. Babic´-Ivancˇic´, V., Fu¨redi-Milhofer, H., Purgaric´, B., Brnicˇevic´, N., and Despotovic´, Z., J. Crystal Growth 71, 655 (1985). 16. Brunauer, S., Emmettt, Ph., and Teller, E., J. Am. Chem. Soc. 60, 309 (1938). 17. Zhu, B. Y., and Gu, T., Adv. Colloid Interface Sci. 37, 1 (1991). 18. Cardew, D. T., and Davey, R. J., Proc. R. Soc. London A 389, 415 (1985). 19. Brecˇevic´, Lj., and Kralj, D., J. Crystal Growth 79, 178 (1986). 20. Sˇkrtic´, D., and Fu¨redi-Milhofer, H., J. Crystal Growth 129, 449 (1993). 21. Fu¨redi-Milhofer, H., Babic´-Ivancˇic´, V., Brecˇevic´, Lj., Filipovic´-Vincekovic´, N., Kralj, D., Komunjer, Lj., Markovic´, M., and Sˇkrtic´, D., Colloids Surf. 48, 219 (1990). 22. Gaudin, A. M., and Furstenau, D. W., Trans. AIMIE 202, 958 (1955). 23. Scamehorn, J. F., Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 85, 463 (1982). 24. Hankins, N. P., O’Haver, J. H., and Harwell, J., Ind. Eng. Chem. 35, 2844 (1996). 25. Somasundaran, P., and Kunjappu, J. T., Colloids Surf. 37, 245 (1989). 26. Bohmer, M. R., and Koopal, L. K., Langmuir 8, 1594 (1992). 27. Boemher, M. R., and Koopal., L. K., Langmuir 8, 2660 (1992). 28. Somasundaran, P., Krishnakumar, S., and Kunjappu, J. T. in “Surfactant Adsorption and Surface Solubilization” (R. Sharma, Ed.), ACS Symposium Series 615, p. 104. American Chemical Society, Washington, DC, 1995. 29. Cases, J. M., and Villieras, F., Langmuir 8, 1251 (1992).
Interactions in Calcium Oxalate Hydrate/Surfactant Systems M. Sikiric´,* N. Filipovic´-Vincekovic´,† ,1 V. Babic´-Ivancˇic´,† N. Vdovic´,† and H. Fu¨redi-Milhofer‡ *Faculty of Agriculture, †Department of Chemistry, Ruder Bosˇkovic´ Institute, 10001 Zagreb, Croatia; and ‡Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel Received July 22, 1998, accepted December 17, 1998
i.e., monomer or supramolecular organization in solution. In addition, crystal aggregation may be induced (11) or inhibited (7), and the type of the predominant crystallizing phase may be changed. Thus, it has been shown (6, 8 –11) that micellar solutions of anionic surfactants promote crystallization of the metastable calcium oxalate dihydrate (COD, CaC 2O 4(2 1 x)H 2O, where x # 0.5), evident because of the thermodynamically stable monohydrate (COM, CaC 2O 4 H 2O), which is the dominant crystal form in control systems and in the presence of low (ionic) concentrations of surfactants. In order to better understand the above phenomena we performed systematic studies of the interaction of surfactants with prepared, well defined crystals of COM and COD under conditions resembling those in urine. In a recent publication (13) the adsorption of anionic sodium diisooctyl sulfosuccinate (AOT) from 0.3 M NaCl solutions onto COM and COD crystals was discussed. Given all other parameters identical, adsorption was strongly dependent on the ionic structures of the respective substrate surfaces, with COD crystals having significantly lower affinity for surfactant adsorption than COM. In the present paper we discuss recent findings about the adsorption of an anionic (sodium dodecyl sulfate, SDS) and a cationic (dodecylammonium chloride, DDACl) surfactant onto the same substrates. The influence of the adsorbed surfactants on the phase transformation of COD to COM and on adsorption behavior will be explained on the basis of the ionic structure of the different crystal faces of COM and COD, respectively.
Phase transformation of calcium oxalate dihydrate (COD) into the thermodynamically stable monohydrate (COM) in anionic (sodium dodecyl sulfate (SDS)) and cationic (dodecylammonium chloride) surfactant solutions has been studied. Both surfactants inhibit, but do not stop transformation from COD to COM due to their preferential adsorption at different crystal faces. SDS acts as a stronger transformation inhibitor. The general shape of adsorption isotherms of both surfactants at the solid/liquid interface is of two-plateau-type, but differences in the adsorption behavior exist. They originate from different ionic and molecular structures of crystal surfaces and interactions between surfactant headgroups and solid surface. © 1999 Academic Press Key Words: calcium oxalate hydrate; surfactant adsorption; transformation.
INTRODUCTION
A number of studies have been carried out to investigate the effects of various additives on nucleation, crystal growth, aggregation, and phase transformation of calcium oxalate hydrates which are the most common constituents of urinary calculi (1–12). In our group particular attention has been paid to the role of surfactants (6 –12) for the following reasons: (i) Many organic constituents in human urine, including proteins, have surfactant properties. Among other organic molecules, human urine contains biological surfactants, bile salts, which are responsible for lowering urinary surface tension and have a potential inhibitory role in urinary stone development (9). (ii) Because of their unique properties (self-assembly, adsorption at interfaces) and ready availability in many different designs, surfactants lend themselves as ideal model systems for systematic studies of the effect of organic molecules on the formation and the transformation of ionic crystals.
EXPERIMENTAL
Materials
In previous work (6 –11) we investigated the effect of order various model surfactants on in situ formation of calcium oxalate from high ionic strength solutions. The obtained results revealed (7, 11) that the crystal growth kinetics and morphology depend on the type and the concentration of the surfactant,
Solutions were prepared from analytical grade chemicals (Merck, Germany) and triple distilled water and standardized by using conventional analytical methods. SDS, a BDH (England) product, was purified by repeated recrystalization from ethanol–water mixtures. DDACl was prepared by neutralization of dodecylamine with HCl and purified by repeated recrystallization from benzene– ethylether mixtures. The critical micelle concentration, (CMC) under actual experimental conditions (i.e., solutions containing 0.30 and 0.01 mol dm 23
1
To whom correspondence should be addressed. E-mail: filipovic@ rudjer.irb.hr. 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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calcium chloride) was found to be 5 3 10 24 mol dm 23 for SDS and 1 3 10 23 for DDACl as determined by surface tension measurements using an interfacial tensiometer (model K8, Kru¨ss, Hamburg). COM crystals were prepared by simultaneously dropping 250 ml of 0.04 mol dm 23 solutions of calcium chloride and sodium oxalate into 1500 ml of triply distilled water at 75°C under stirbar stirring. After synthesis the suspension was kept at the same conditions for 2 h and afterward slowly cooled to room temperature. After 24 h the crystals were filtered through a 0.45-mm Milipore filter, washed with deionized water, and dried in a vacuum dessicator. The crystals were six-sided plates with an average surface area of 6.4 m 2 g 21. COD crystals were precipitated by the method of Brown et al. (14) by consecutive addition of calcium chloride (0.37 ml of 4.00 mol dm 23) and sodium oxalate (1.53 ml of 0.25 mol dm 23) solutions to 200 ml of solution containing 0.01155 mol dm 23 magnesium sulfate, 0.00963 mol dm 23 sodium citrate, and 0.0637 mol dm 23 potassium chloride. Solutions were gently swirled and left at room temperature for 10 min. Crystals were filtered through a 0.45-mm Milipore filter, washed with deionized water and ethanol, and dried in a vacuum dessicator. The crystals had the shape of tetragonal bipyramids with an average surface area of 5.0 m 2 g 21. Experimental Systems Both phase transformation and adsorption studies were carried under the same conditions that were employed in previous in situ kinetic and morphological studies on the crystallization of calcium oxalates in the presence of additives (1, 2, 6 –12). Thus we have maintained high ionic strength (0.3 mol dm 23 NaCl) and an initial pH 6.5. The pH was chosen within the range of urinary pH (5–7). Although the particular value is somewhat high (the average urinary pH is 5.8), it was convenient for interpreting kinetic studies because the predominant oxalate species at this pH is C 2O 422 ion. In keeping with the above principles 0.5 g dm 23 prepared COM and COD crystals were conditioned at 37°C in solutions containing 0.3 mol dm 23 NaCl and 0.01 mol dm 23 CaCl 2. The pH of the solutions was adjusted to 6.5 with NaOH solution. Known concentrations of SDS or DDACl were added to the solutions prior to pH adjustment. Transformation of COD into COM was studied in a blank system without surfactant and in the systems with micellar concentrations of cationic and anionic surfactants. Crystals were suspended in electrolyte solutions by magnetic stirring (G 5 5 6 1 s 21) and periodically sampled onto glass slides and observed by bright field and polarized light optical microscopy (Leitz Ortophan photographic microscope). At given time intervals, crystals were collected by filtration, and their composition was determined qualitatively by X-ray diffraction powder patterns and quantitatively by thermogravimetric analysis. Quantitative analysis by thermogravimetry is based on the
difference in the content of crystalline water in COM and COD (15). X-rays (Philips counter diffractometer) and thermogravimetric analysis (Mettler TA4000 System) was performed immediately after filtration. X-ray diffractograms were recorded from moist samples, while TGA samples were air-dried to constant weight before the onset of heating. In this way further hydrate transformation during sample preparation was avoided. For adsorption experiments 0.5 g dm 23 of the respective solid phase (COM or COD) was equilibrated for 24 h at pH 6.5 and 37°C in electrolyte solution with different concentrations of DDACl or SDS. After completion of the experiment the solutions were centrifuged and aliquot solutions were analyzed for DDACl or SDS by an Orion surfactant electrode (model 93-42) in connection with an Ag–AgCl double junction reference electrode (model 90-02, Orion Research, USA). The specific surface area of the crystals was determined by the Brunauer–Emmett–Teller (BET) method (16) (Quantasorb, Quantachrome Corp.) using nitrogen. From the amount of surfactant adsorbed in the plateau regions and the specific surface area of respective crystals, the plateau adsorption densities were calculated. Electrophoretic measurements were made with an automated microelectrophoretic instrument (Pen Kem, type S3000 Bedford Hills, NY). Aliquots of 15 ml of equilibrated suspensions were left to settle and the supernatant suspensions were separated and injected into the detector cell. The measurement was performed at low voltage (50 V) because of the high ionic strength of suspension. RESULTS
When suspended in electrolyte solutions, metastable COD transforms into the thermodynamically stable COM. In Fig. 1 time dependent changes of solid phase composition in electrolyte solutions without surfactant and in the presence of micellar concentrations of SDS or DDACl are represented. As revealed by TG analysis, without surfactant up to 40% of COD transformed into COM within 24 h, and within 7 days transformation was complete (curve 1 in Fig. 1). The corresponding X-ray diffractograms show only the characteristic peaks of COD and COM, the peaks intensity of COD decreasing and those of COM increasing with time. Increasing content of COM in COD–COM mixtures was also observed by optical microscopy. In the beginning, the characteristic tetragonal bipyramids typical of COD crystals were observed, but aging in electrolyte solution induced pronounced rounding and a decrease in crystal size typical of dissolution. The dissolution of COD crystals was followed by the formation of plate-like COM crystals with a wide size distribution. In the presence of micellar concentrations of the surfactants significant inhibition of phase transformation occurred (Fig. 1, curves 2, 3,) with SDS being a somewhat stronger inhibitor. The results of thermogravimetric analyses were confirmed by X-ray diffractograms exhibiting that samples aged for 7 days in
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FIG. 1. The increase of the mass fraction of COM present in the solid phase with aging time, t A. Transformation kinetics of COD as observed in control system (F) and in micellar surfactant solutions (Œ, c(DDACl) 5 0.004 mol dm 23; E, c(SDS) 5 0.0008 mol dm 23). The amount of COD was 0.5 g dm 23, concentration of CaCl 2 5 0.01 mol dm 23, NaCl 5 0.3 mol dm 23, pH 6.5, and temperature, T 5 37°C.
surfactant solutions consisted of mixtures of COD and COM with a very small portion of COM, in contrast to the control system where in aged samples peaks characteristic of COM prevailed. No significant morphological differences between COD crystals aged in micellar solutions of SDS and DDACl, respectively, were observed. Electrophoretic mobility spectra of COM and COD particles, suspended under the working experimental conditions (i.e., in solutions containing 0.3 M NaCl and 0.01 M CaCl 2) are shown in Fig. 2. The spectra clearly show surface heterogeneity; i.e., the solid particle surfaces consist of a distribution of patches of different surface charge densities. Although both negative and positive electrokinetic charges are present, it is seen that positive electrokinetic charges prevail especially at the COD/solution interface. This is possibly due to the adsorption of excess calcium ions from the electrolyte solution. The net surface charge density displayed by z-potential measurements is an average of the charges on the local patches. In Figs. 3 and 4 the adsorption isotherms of SDS and DDACl on COM and COD are compared with the respective z-potential vs equilibrium concentration curves. All adsorption isotherms (Figs. 3a, 4a) are of the two-plateau-type (LS) (17), exhibiting a region of low adsorption (appearing as a pseudoplateau) and a region of high adsorption with an inflection between them. At concentrations approaching the CMC, the adsorption isotherms reached a short second plateau. The usual leveling off of the adsorption isotherms was observed only in a narrow concentration range in the vicinity of the CMC because with further increase in concentration, surfactant precipitation occurred and was confirmed by X-ray analysis (see
also Ref. 6). Because the surfaces of both hydrates adsorb more SDS than DDACl, the surface area of both crystals occupied by one dodecylammonium ion is somewhat higher than that occupied by one dodecyl sulfate ion. Higher concentrations of both surfactants were adsorbed on COM, so that the density of adsorbant was greater on those crystals. The calculated adsorption densities in the regions of low and high adsorption are listed in Table I. Changes of the z-potential of both COM and COD particles with increasing SDS concentration are shown in Fig. 3b. Initially, under the given experimental conditions (0.3 M NaCl and 0.01 M CaCl 2) calcium oxalate particles bear an overall small positive electrokinetic charge, with an average z-potential of 0.1 mV for COM and 7.5 mV for COD, respectively. In contrast to the respective adsorption isotherms (Fig. 3a) the z-potential vs concentration curves show two inflections at low and high surfactant concentrations, respectively. As expected the z-potential becomes more negative with the adsorption of more anionic SDS. The changes in the z-potential of COM and COD suspensions with DDACl revealed a slight increase of z-potential with increasing surfactant concentration (Fig. 4b). DISCUSSION
Phase Transformation of COD into COM in the Presence of Surfactants Metastable calcium oxalate hydrates (COD) and calcium oxalate trihydrate, (COT) (CaC 2O 4 (3 2 x H 2O), where x # 0.5) transform into the thermodynamically stable COM by a
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FIG. 2. Change of relative signal (arbitrary units) intensity, I, with electrophoretic mobility for control COM (top) and COD (bottom) dispersions. The amount of solid phase was 0.5 g dm 23, concentration of CaCl 2 5 0.01 mol dm 23, NaCl 5 0.3 mol dm 23, pH 6.5, and temperature, T 5 37°C.
solution mediated process (18 –21). In general the extent of phase transformation depends on the rate of dissolution of the metastable, and the rates of nucleation and growth of the stable phase, all of which are sensitive to solution conditions (pH, ionic strength, temperature, and the presence of impurities). It has been shown (19) that certain amino acids when present in solution change the rate controlling mechanism of COD dissolution from diffusion to a surface controlled mechanism; i.e., the kinetics of COD dissolution are controlled by processes at the solid-solution interface. In accordance with the above findings and taking into account the results of the above-described adsorption studies (Figs. 3 and 4 and Table I), it seems reasonable to assume that the strong inhibition of COD to COM phase transformation is a consequence of adsorption of the surfactants at both the COM and the COD solid–liquid interfaces. When COD crystals are brought into contact with a solution containing micellar concentrations of SDS or DDACl the rate of dissolution of COD crystals is greatly reduced because of adsorption and so is the rate of building up the supersaturation necessary for crystallization of COM. At the same time, growth of any nuclei of COM crystals is also inhibited by strong adsorption of surfactant at the COM solid–liquid interface. The observed difference between the rate of phase transformation in the presence of SDS and DDACl, respectively (Fig. 1), is in accordance with the difference in adsorption densities from micellar solutions of the two surfactants (see values for the second plateau in Table I).
Adsorption of Surfactants at the Surfaces of COM and COD Crystals Adsorption is a consequence of interactions at the solid– liquid interface and thus depends on the crystal structure and electrokinetic charges at the interface. A number of models (22–29) have been proposed, most of which are based on experimental and theoretical studies dealing with the adsorption of ionic surfactants at the oxide–liquid interface. All models agree that at very low surfactant concentrations the primary force involved in adsorption is of electrostatic origin; i.e. the headgroups of the adsorbed surfactant are oriented toward the solid surface and the hydrocarbon tails toward the solution. The subsequent rapid increase of the adsorption at higher surfactant concentrations is generally explained by surfactant aggregation at the solid–liquid interface involving mechanisms similar to those causing the formation of micelles in aqueous solutions, i.e., hydrophobic tail–tail interactions. Finally the leveling off of adsorption starting at the CMC is ascribed to a chemical potential sink caused by the presence of micelles in the bulk solution (24, 26). The general shape of the isotherms characterizing the adsorption of SDS on calcium oxalate particles (Fig. 3a) is in accordance with the above-described models. It should be noted that the final adsorption densities are unusually high (20.5 and 31.9 molecules nm 22 for COD and COM, respectively, Table I) as compared to previously reported adsorption densities of anionic surfactants on oxide surfaces (23, 24, 27,
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A number of studies most relevant to the above results have been reviewed by Cases and Villieras (29). The authors discuss the importance of surface heterogeneity in the adsorption process. Adsorption on heterogeneous surfaces occurs first on the most energetic homogeneous domains followed by successive phase transitions on domains with lesser energies. As the solution concentration increases the adsorption isotherms develop step-like discontinuities related to the infilling of surface domains. The discrete steps may or may not be observable depending on the size and distribution of the homogeneous areas and the energies of patches. When sparingly soluble salts are used as substrates, adsorption of surfactants may go far beyond monolayer capacity because in addition to true adsorption, retention at the surface may be caused by surface condensation. Although in the control systems both COM and COD particles exhibited an overall positive charge, adsorption of the cationic surfactant did occur, although to a lesser extent than adsorption of SDS (Figs. 4a and Table I). The general shapes of the respective isotherms (Fig. 4a) are similar to those for
FIG. 3. (a) Adsorption isotherms of SDS on COM and COD and (b) z-potential values vs the equilibrium surfactant concentration (c eq). G is the adsorbed amount of surfactant at given concentration. The amount of solid phase in COM and COD dispersions was 0.5 g dm 23, concentration of CaCl 2 5 0.01 mol dm 23, NaCl 5 0.3 mol dm 23, pH 6.5, and temperature, T 5 37°C.
28). A possible explanation for such high adsorption densities may be found by considering details of the isotherms together with the corresponding z-potential vs equilibrium concentration curves (Figs. 3a and 3b). It is seen that adsorption starts at very low surfactant concentrations while the slightly positive surface potential hardly changes. This is in accordance with an electrostatically driven, head-to-surface adsorption mechanism. With increasing surfactant concentration the adsorption isotherms reach a pseudoplateau, while the corresponding z-potential becomes negative, indicating that aggregation has taken place and that SDS molecules are partly arranged in the form of negatively charged bilayers. The adsorption densities in this region (3 and 4 molecules nm 22 for COD and COM, respectively, Table I) are comparable with the maximum adsorption density of SDS on Al 2O 3 (4.2 molecules nm 22, Ref. 28). Subsequently, while adsorption further increases, the z-potential vs concentration curves pass through a near plateau followed by a second relatively steep inflection. This could indicate filling in of lower energy surface sites by head-on adsorption of surfactant molecules and their subsequent aggregation. Considering the high adsorption densities obtained, the formation of multilayer arrangements and/or surface condensation (29) cannot be excluded.
FIG. 4. (a) Adsorption isotherms of DDACl on COM and COD and (b) z-potential values vs the equilibrium surfactant concentration (c eq). G is the adsorbed amount of surfactant at given concentration. The amount of solid phase in COM and COD dispersions was 0.5 g dm 23, concentration of CaCl 2 5 0.01 mol dm 23, NaCl 5 0.3 mol dm 23, pH 6.5, and temperature, T 5 37°C.
CALCIUM OXALATE HYDRATE/SURFACTANT SYSTEMS
TABLE I Adsorption Densities of DDACI and SDS on COM and COD at 37°C for the First Plateau and for the Second Plateau, Respectively
System
COM 1 DDACI
COM 1 SDS
COD 1 DDACI
COD 1 SDS
molecules/nm 2 molecules/nm 2
3.4 16.3
4.3 31.9
2.4 13.3
3.0 20.5
SDS while the corresponding positive z-potential slightly increases with increasing concentration (Fig. 4b). An explanation for the adsorption of the cationic surfactant at prevailing positively charged COM and COD crystal faces is given by the presence of both positively and negatively charged patches as demonstrated by the electrophoretic mobility spectra (Fig. 2). As previously determined by morphological studies (7, 12), COM crystals present quite different sites for preferential adsorption of anionic and cationic surfactants. When crystals where grown in the presence of an anionic surfactant (AOT) (12), preferential adsorption occurred at the {101} and {010} crystal faces resulting in thin, elongated crystals. The cationic surfactant, DDACl, in contrast promoted the formation of thin, rhomb-like crystals, indicating preferential adsorption on the {101} crystal faces. Another important result of our experiments is the fact that adsorption of both surfactants is at all concentrations lower than onto COM crystals (Figs. 3a and 4a and Table I). This is true particularly for the anionic surfactant where the ratio of the respective adsorption densities onto COM vs COD (in molecules/nm 2) is 1.5 (second plateau in Table I). This is in accordance with the respective ratio 2 of the adsorption densities of sodium diisooctyl sulfosuccinate (AOT) onto COM and COD crystal surfaces, respectively (Table I in Ref. 13). The lower adsorption capacity of COD as compared to that of COM may in part be due to poisoning of the crystal surface by adsorbed Mg 21 and/or citrate ions which have been used in the process of preparation of the seed crystals (in accordance to Ref. 10). However, at least part of the difference can be ascribed to the difference in the ionic structures of the adsorbing crystal faces of the two substrates, which has been discussed in detail in Ref. 13. In this treatise it has been shown that COM crystal faces have high negative charge densities and that adsorption of the anionic surfactant to these surfaces occurs via solution calcium ions. On the other hand, crystal planes of COD expose rows of oxalate ions and water molecules, which increase their hydrophilic character and shield the surfaces from additional adsorption (13). In conclusion, the adsorption results indicate that the adsorption of surfactants at the calcium oxalate/solution interfaces strongly depends on the molecular and/or ionic structure of the substrate surface and specific head group/surface interactions. The overall adsorption capacity of both substrates for DDACl
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is significantly lower and so is the difference between them (the ratio of adsorption densities on COM and COD in micellar solutions is 1.2). ACKNOWLEDGMENTS The financial support granted by the Ministry of Science, Technology, and Informatics of the Republic Croatia (Grant 0098602) is gratefully acknowledged.
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