J. Phys. Chem. 1996, 100, 17249-17254
17249
Solid and Solution Properties of Alkylammonium Perfluorocarboxylates Hiromi Furuya, Yoshikiyo Moroi,* and Kozue Kaibara Department of Chemistry, Faculty of Science, Kyushu UniVersity 33, Hakozaki, Fukuoka 812-81, Japan ReceiVed: May 6, 1996; In Final Form: July 1, 1996X
Seven alkylammonium perfluorocarboxylates (CnH2n+1NH3+ CmF2m+1COO-; n + m ) 15, n ) 2, 4, 6, 8, 10, 12, 14) were synthesized, and their physicochemical properties were investigated. Melting points (mp) and heats of fusion of the solids were determined by differential scanning calorimetry, and the crystal structures were examined by X-ray diffraction analysis. The critical micelle concentration and aqueous solubility for C14H29NH3+CF3COO- and C12H25NH3+C3F7COO- were obtained from the electric conductivity method, while reproducible data were not obtainable for the other five amphiphiles. The size of the molecular aggregates was observed by a phase contrast microscope, and very large aggregates of micrometers in size were found to be formed. These results indicate that the properties of solids and solutions depend not only on hydrocarbon but also on fluorocarbon chains. The time dependence of the electric conductance was traced immediately after dilution of concentrated C4H9NH3+C11F23COO- solution, and the dissociation rate of the aggregates to monomers was obtained over the temperature range from 20 to 45 °C. The activation energy of the dissociation process was found to be 76 kJ mol-1.
Introduction Characteristics of an ionic amphiphile depend on the properties of both cationic and anionic groups in the molecule. Especially when a counterion of amphiphile has a hydrophobic chain, the physicochemical properties are much different from those of amphiphiles with a metallic or a halogen counterion. The amphiphiles that have both a hydrocarbon group ion and a fluorocarbon group ion have recently been subjects of interest and have been investigated from the colloid and surface chemical point of view.1-3 As is well-known, the miscibility of fluorocarbon and hydrocarbon chains is nonideal, and the interaction between them is very weak.4,5 For example, as for the micellization in solutions of fluorocarbon and hydrocarbon mixed surfactants, two kinds of micelles, fluorocarbon-dominant micelles and hydrocarbon-rich micelles, are formed.6-10 In addition, fluorocarbons tend to form larger aggregates than hydrocarbons11 because of an increased hydrophobicity of fluorocarbon compared with hydrocarbon. These properties of the fluorocarbon group lead to the formation of vesicles for single-chain amphiphiles,12,13 although multichain amphiphiles are usually necessary to form vesicles for hydrocarbon amphiphiles. In our preceding study,14 micellizations of three dodecylammonium perfluorocarboxylates of different fluorocarbon chain lengths as counterion were investigated, from which the aggregates formed were found to be much larger in size than conventional micelles whose aggregation number is several hundreds at most. In this study, seven alkylammonium perfluorocarboxylates of the same number of total carbon atoms as dodecylammonium perfluorobutylate, 16, but of different numbers of hydrocarbon and fluorocarbon atoms were synthesized, and physicochemical properties in their solid and solution states were examined. As for the solids, melting points (mp’s) and heats of fusion were measured by differential scanning calorimetry, and the structures of crystals were made by X-ray diffraction analysis. The aqueous solubility and critical micelle concentration (cmc) were determined from the electric conductivity method, and the sizes of the aggregates were pursued by static and dynamic light scatterings and by phase contrast microscopy. * E-mail address:
[email protected]. X Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01280-4 CCC: $12.00
As mentioned above, amphiphiles made of hydrocarbon group ion and fluorocarbon group counterion show physicochemical properties much different from those of two hydrocarbon group ions.15 This strongly suggests that the former are very promising compounds to develop functional materials. In this sense, the effect of fluorocarbon group ion on the properties has to be positively explored. One possible way to this is to change the chain length of fluorocarbon counterion, keeping the chain length of hydrocarbon ion constant. The other is to change the both ions, keeping the total number of their carbon atoms constant. In the first step, the latter method was adopted to find out the direction for further investigation of the present amphiphiles. Experimental Section Materials. Amphiphiles were synthesized by neutralization of alkylamines with perfluorocarboxlic acids. C2H5NH2 was obtained from Nacalai Tesque, and C8H17NH2, C12H25NH2, and C3F7COOH were obtained from Aldrich Chemical Co., Inc. C4H9NH2, C6H13NH2, C10H21NH2, C14H29NH2, and C7F15COOH were from Tokyo Chemical Industry Co., Ltd. CF3COOH was from Kanto Chemical Co., Inc., and C5F11COOH and C9F19COOH were from Daikin Chemical Co., Ltd. C11F23COOH was from Central Medical Co., Ltd., and C13F27COOH was from Exfluor Research Corporation. Tetradecylammonium perfluoroacetate was synthesized by the following process: CF3COOH was dissolved in water, and the concentration of the acid was determined by titration with NaOH solution. An equivalent amount of C14H29NH2 by weight dissolved in acetone was added stepwise to the acid solution. C14H29NH3+CF3COO- (Ch14Cf1) formed was purified by recrystallization from water. Ethanol was used as an antiforming agent. C12H25NH3+C3F7COO- (Ch12Cf3), C10H21NH3+C5F11COO- (Ch10Cf5), C8H17NH3+C7F15COO- (Ch8Cf7), C6H13NH3+C9F19COO- (Ch6Cf9), C4H9NH3+C11F23COO(Ch4Cf11), and C2H5NH3+C13F27COO- (Ch2Cf13) were also similarly synthesized, where acetone-water mixed solvent was employed for the reactants whose aqueous solubility is small. Concerning longer homologues as to perfluorocarboxylates, aqueous solution of amine was neutralized by an equivalent © 1996 American Chemical Society
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TABLE 1: Conditions of DSC Measurement amphiphiles
initial temp/°C
final temp/°C
scanning rate/°C min-1
Ch14Cf1 Ch12Cf3 C10Cf5 Ch8Cf7 Ch6Cf9 Ch4Cf11 Ch2Cf13
25 0.0 -30 0 25 70 100
100 50 30 60 100 130 160
1.0 0.2 1.0 1.0 1.0 1.0 1.0
amount of acid in a suspension state, where the neutralization took place without any difficulty. The purities were checked by the elemental analysis, and the found values agreed with the calculated values within experimental error ((0.3% for C and N and (0.1% for H). The water used was distilled twice from alkaline permanganate. Differential Scanning Calorimetry (DSC). Melting points (mp’s) and heats of fusion were measured by DSC apparatus (Shimadzu; DSC-50). As for the samples, the crystals were once melted and then used for DSC measurement in order to eliminate the solvent-induced effect on the crystal structures. About 1 mg of the solid crimped in an aluminum cell was used as a sample, while an empty cell was used as a reference. The conditions are shown in Table 1. The final data were determined as the average over three experiments. X-ray Diffraction. The crystal structures were analyzed by an X-ray diffractometer (Shimadzu XD-D1) with the Geiger counter at room temperature. The scanning speed was 1 deg/ min., and the range of measurement angle was 3.0-45.0°. The crystals were ground to a powder after their melting, too. Ch10Cf5 was liquid at room temperature, and the data was not obtained. Aqueous Solubility and cmc. The solubility was determined by two methods. One was from the change of specific conductance with temperature at a given concentration for temperatures above the micellization temperature16 (or conventional Krafft point17) and the other from the change of the conductance with surfactant concentration at a given temperature for below the micellization temperature.2 The cmc was also determined from electric conductance measurements as their inflection point. Phase Contrast Microscope. Phase contrast microscope observation was performed as follows: the amphiphile solution was placed in-between two pieces of glass plates with a hollow space in the lower plate. The plates were set on a temperatureregulated (0-80 °C) stage of the phase contrast microscope (Olympus IMT-2). The picture was input in the image processor (Hamamatsu photonics Argus-10) through a CCD camera (Hamamatsu photonics C3077-C3754). The output picture from the image processor was recorded on a videotape. Finally, the picture from videotape was processed by a computer and by the image analysis program (NIH Image, public domain software by W. Rasband). Dissociation of Aggregates. Concentrated solution of Ch4Cf11 above the cmc (about 0.8 mmol kg-1) was diluted below the cmc at constant temperature. Time dependence of the conductances was traced by a conductometer (TOA Electronics Ltd. CM-60S). Results and Discussion Solid Phase. The DSC curves are illustrated in Figure 1, where they show two or three endothermic peaks for the samples except for Ch4Cf11 and Ch2Cf13. Especially, those of Ch8Cf7 and Ch6Cf9 are clearly divided. The melting process was also observed by the phase contrast microscope, and it was confirmed
Figure 1. DSC curves of alkylammonium perfluorocarboxylates.
Figure 2. Change of melting point with the number of carbon atoms of the fluorocarbon counterion.
TABLE 2: Results of Melting Point, Heat of Fusion, and the Entropy Change of Fusion Ch14Cf1 Ch12Cf3 Ch10Cf5 Ch8Cf7 Ch6Cf9 Ch4Cf11 Ch2Cf13
mp/°C
∆H/kJ mol-1
∆S/kJ K-1 mol-1
80.0-80.5 38-39 11.8 33.2, 37.0 48, 57-61 101-103 143.4-144.5
31.8 ( 0.87 29.5 ( 2.96 25.6 ( 1.51 28.9 ( 2.01 20.0 ( 1.68 26.03 ( 1.16 25.40 ( 1.51
0.0900 ( 0.0025 0.0947 ( 0.0095 0.0898 ( 0.0053 0.0939 ( 0.0065 0.0612 ( 0.0051 0.0694 ( 0.0031 0.0609 ( 0.0036
that the endothermic peaks correspond to melting of the solid. According to the above observations, the first peak of Ch8Cf7 and Ch6Cf9 comes from a transition to a thermotropic liquid crystal and the second one does from a complete melting of the liquid crystal. As for Ch10Cf5, the heat was exothermic at -7 to 5 °C, which suggested that the solid molecules were coagulated in a nonstabilized state and that when temperature was increased, the molecules were rearranged into more stable state, accompanying an exothermic heat. The results of mp and heats of fusion are given in Table 2. The changes of mp show a V-shaped curve. The left line of V-shape is the changes of mp with the length of hydrocarbon chain (Ch), while the right one is those of fluorocarbon chain (Cf), where shorter chain ion acts as the counterion. Since the attractive interaction between alkyl chains of the same kind becomes weaker with decreasing the chain length, melting occurs at lower temperature as both chain lengths become shorter. As for Ch8Cf7 and Ch6Cf9, there are two mp’s due to the respective chain. Two kinds of chains should influence the melting, but the effect of Ch on mp still appears for Ch6Cf9
Properties of Alkylammonium Perfluorocarboxylates
Figure 3. Model of the crystal structure of alkylammonium perfluorocarboxylates.
that has longer Cf and Ch. This is because the attractive interaction between hydrocarbon chains is stronger than that between fluorocarbon chains. As to a hydrocarbon-hydrocarbon amphiphile such as the long chain ester made from n-fatty acid and n-alcohol, the mp changes give a U-shaped curve.18 In addition, at the same chain length for both fatty acid and alcohol, the curve has a small maximum, at which a slight stability is induced in the crystal structure. For hydrocarbonfluorocarbon amphiphiles, however, such a maximum is not observed in the mp curve.
J. Phys. Chem., Vol. 100, No. 43, 1996 17251 The relationship between the heats of fusion and the lengths of both chains may not be quite clear, but it can be said that the solids with longer Ch need more heat to melt compared with those with longer Cf. By dividing the heat values by mp, the entropy changes of fusion can be obtained (Table 2). Now, they are found to be divided into two groups; one is ∆S ) 0.09 kJ K-1 mol-1 for the entropy change of Ch, and the other is ∆S ) 0.06 kJ K-1 mol-1 for Cf. This difference is caused by the difference in interaction and flexibility or stiffness between two kinds of chains. The fact that the ∆S value of Ch8Cf7 which has the same carbon number in both hydrocarbon and fluorocarbon chains still belongs to the Ch group is also caused by stronger interaction and increased flexibility of Ch. As far as melting is concerned, the hydrocarbon chain is more effective than the fluorocarbon chain. The crystal structure is thought to be as shown in Figure 3. Two molecular layers of alkylammonium cations and perfluorocarboxylate anions are alternately aligned. The distances between the planes of ammonium head groups and between those of carboxylate groups are evaluated by X-ray diffraction. Two examples of diffraction patterns are given in Figure 4a,b. For Ch14Cf1 (Figure 4a) which has the longest Ch, the peaks up to the fifth order of Ch are observed, whereas any peak due to short Cf is not found. For Ch8Cf7 (Figure 4b) which has the same carbon number for both hydrocarbon and fluorocarbon
Figure 4. (a) Spectrum of X-ray diffraction of Ch14Cf1. (b) Spectrum of X-ray diffraction of Ch8Cf7.
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TABLE 3: Results of Distance between Planes (d)
Ch14Cf1 Ch12Cf3 Ch10Cf5 Ch8Cf7 Ch6Cf9 Ch4Cf11 Ch2Cf13 a
dCb/Å
dFf/Å
25.31 (42.27)a 20.51 (37.12) (32.32) 23.43 (27.20) 16.10 (22.37) (17.23) (12.34)
(11.36) (16.32) (21.28) 22.01 (26.24) 23.87 (31.20) 23.06 (36.16) 23.78 (35.33)
The values in parentheses are obtained from the CPK model.
Figure 5. Results of inclination of the chains (θ).
Figure 7. Phase contrast micrograph of Ch14Cf1 (a, 10 mmol kg-1), Ch12Cf3 (b, 2.9 mmol kg-1), and Ch6Cf9 (c, 1.6 mmol kg-1).
Figure 6. Changes of solubility and cmc with temperature for Ch14Cf1 and Ch12Cf3.
chains, the two series of peaks are shown: the series of peaks of Ch and Cf are observed. The results of their long spacing, a distance between the planes, are summarized in Table 3. For the distances between planes, those for Ch become longer with increasing chain length, while those of Cf remain almost constant regardless of the chain length. In Figure 5 are shown the inclination angles of the chains, θ, which are calculated from the ratio of the distance obtained above and the molecular length evaluated from the CPK atomic model. In the cases of Ch8Cf7 and Ch6Cf9 which have comparable length for Ch and Cf each, they have similar θ values. The longer the Cf chain is, the smaller the θ value becomes. That is, when the chain length becomes longer, the molecules align less perpendicularly to the plane of the head groups (Figure 3), although this is not quite clear for Ch. Solution. Aqueous solubility and cmc were measured for Ch14Cf1 and Ch12Cf3 (Figure 6). As for the rest amphiphiles, they could not be determined by the electric conductivity method, because the monomeric concentration was very low and because the conductance drift with time was also observed.
The cmc of Ch12Cf3 is higher than that of Ch14Cf1, although both surfactants have the same total carbon number. This means that micellization is controlled by the stronger effect of longer Ch length regardless of shorter Cf length and that micelles are formed in which hydrocarbon ions are acting as a surfactant ion. As for the solubility (S), the enthalpy changes of the dissolution are determined from a slope of the plots of ln S vs T-1. The values are 40.1 and 26.8 kJ mol-1 for Ch14Cf1 and Ch12Cf3, respectively. When aqueous solubility was investigated by decreasing the carbon number of the Cf counterion of Ch12Cf3 in the preceding study,14 the difference in the enthalpy change of the dissolution remained quite small, within about 1-2 kJ mol-1 per one carbon atom. Therefore, such a difference of about 13 kJ mol-1 is due to the difference in hydrocarbon chain length between the two. Similar energy differences due to the chain length have also been observed for sodium alkylsulfonates.19 The authors tried to determine the size of the aggregates by static and dynamic light-scattering methods. However, the determination turned out to be impossible, because the aggregates were too big to measure and the change in size with time was also observed. Hence, the sizes of the aggregates were determined by phase contrast microscopy at 40 °C (Figures 7 and 8). The sizes of Ch14Cf1 (10 mmol kg-1), Ch12Cf3 (2.9 mmol kg-1), and Ch6Cf9 (1.6 mmol kg-1) aggregates were about 2-3 µm and 5-6 µm for small and large spherical aggregates, respectively (Figure 7). That of Ch4Cf11 (1.1 and 3.4 mmol kg-1) was around 10 µm in length, and the shape was rodlike (Figure 8a,b). The photograph of b was the one observed immediately after preparing the solution. A few hours later, the aggregates became very big, and the size became about
Properties of Alkylammonium Perfluorocarboxylates
J. Phys. Chem., Vol. 100, No. 43, 1996 17253
Figure 9. Changes of specific conductance with time at three different temperatures.
concentrated solution was diluted below the cmc, although the cmc value was not definite but rough due to the poor reproducibility, and the conductance change was followed up for about 1 h. The conductance changes are shown in Figure 9. The higher the measurement temperature was made, the shorter became the time for the conductance to reach a constant value. The dissociation of the aggregates to monomers is expressed by k
A 98 M
(1)
where A, M, and k are the aggregate, the monomer, and the rate constant, respectively. The rate equation is given by Figure 8. Phase contrast micrograph of Ch4Cf11; the concentrations are 1.1 mmol kg-1 (a) and 3.4 mmol kg-1 (b, c).
5 µm in width and 20 µm in length (Figure 8c). Since Ch4Cf11 of 3.4 mmol kg-1 was not completely soluble in water at 40 °C, these aggregates can be regarded as the ones that were growing toward the crystal. The sizes of Ch2Cf13 (0.1 and 1.0 mmol kg-1) were similar to that of Ch14Cf1. Change of the size with the solution concentration was not observed. The whole photographs in Figures 7 and 8 are the aggregates of larger size which are detectable by phase contrast microscope and, therefore, do not contradict the presence of smaller aggregates like spherical micelles. The reasons why such large aggregates are formed are thought to be an increase in both hydrophobicity and bulkiness of the Cf chain. F. Giulievi and M. P. Krafft13 have predicted the aggregation behavior of singlechain perfluoroalkylated amphiphiles, using the amphiphiles’ self-organization model proposed by J. Israelachvili et al.20 Although the model is made for hydrocarbon amphiphiles, it seems applicable to fluorinated amphiphiles because of the agreement of Giulievi’s prediction with their experimental results. Unfortunately, the size and shape of the aggregates could not be predicted in this study, since the surface-activity data was not available. However, it is reasonably expected that the size of the aggregates in these systems depends on such parameters as surface area per molecule, length of amphiphile, and volume of hydrophobic chain, which are necessary to calculate the packing parameter for speculation of shape of aggregates. Kinetics for Dissociation of Ch4Cf11 Aggregates. It was mentioned above that the conductance change with time was observed on determining cmc. The change was pursued, and the rate constants for the dissociation of the aggregates to monomers were obtained over the temperature range from 20 to 45 °C. The measurement was carried out as follows: the
d[AM] ) -k[AM] dt
(2)
and [AM] is the equivalent concentration in the aggregate form to be converted to monomers. When [AM0] is the concentration of [AM] at t ) 0, eq 2 is give as
[AM] ) [AM0]e-kt
(3)
If total concentration of the amphiphile is [St], the concentration of the monomer, [M], is expressed by using eq 3
[M] ) [St] - [AM0]e-kt
(4)
On the other hand, the conductance κ is given by the following
κ ) λA[A] + λM[M]
(5)
where λA and λM are the molar conductivity of aggregate and monomer, respectively. Since the size of the aggregates is much larger than that of the monomer, λA ≈ 0 or λM . λA. Then eq 5 can be set as
κ ≈ λM[M]
(6)
By substituting eq 4 into eq 6, κ is expressed as
κ ) λM[St]{1 - ([AM0]/[St])e-kt}
(7)
When the time is taken to be sufficiently long and all aggregates are dissociated to monomers, the conductance value κ () λM[St]) is given as κ∞. Then, eq 7 leads to the following equation
κ ) κ∞{1 - ([AM0]/[St])e-kt}
(8)
17254 J. Phys. Chem., Vol. 100, No. 43, 1996
Furuya et al. chains is stronger. In the solution, on the other hand, very large aggregates are formed, whereas the molecular aggregates of n-alkylammonium and of n-perfluorocarboxylate themselves are rather small. The larger aggregates are caused by increased hydrophobicity and bulkiness of fluorocarbon counterions. That is, the hydrocarbons influence the state of solid more effectively, while the fluorocarbons influence the state of solution more effectively. As for the rate constant for dissociation of the Ch4Cf11 aggregates to monomers, the reasonable values are obtained by electric conductance measurements. The rate value was referred to some activation energies.
Figure 10. Arrhenius plot for dissociation rate constant of Ch4Cf11 (b). Plots of 4 and 0 are for a formation-dissolution of micelles of sodium tetradecylsulfate21 and nickel dodecylsulfate,22 respectively.
Further, it is rewritten as
ln(1 - κ/κ∞) ) -kt + ln([AM0]/[St])
(9)
The slope for the plots of ln(1 - κ/κ∞) vs t then becomes the rate constant. Time dependence of the electric conductance was analyzed according to eq 9, and the rate constants were evaluated from good linear relationships. The activation energy for the present dissociation process was found to be 76 kJ mol-1 from the Arrhenius plots (Figure 10). The activation energies for formation-dissolution of micelles of sodium tetradecylsulfate21 and nickel dodecylsulfate22 are evaluated to be 144 and 67 kJ mol-1, respectively, from the relaxation times. On the other hand, the energy for dissolution of aluminum fluoride trihydrate crystal23 whose process is more similar to the present process is 72 kJ mol-1. Judging from these reference values, the activation energy for the present process can be considered to be reasonable and acceptable. Conclusion The physicochemical properties of ionic amphiphiles which consisted of hydrocarbon and fluorocarbon group ions were investigated. The solid properties are effected more by the length of the hydrocarbon chain than by the length of the fluorocarbon chain. This is because the hydrocarbon chain is more mobile and more flexible than the fluorocarbon chain and because the molecular interaction between the hydrocarbon
Acknowledgment. The authors are deeply indebted to Dr. Y. Murata of Fukuoka University for making available dynamic light scattering. This work was partly supported by Shorai Foundation for Science and Technology and partly by Sumitomo Foundation, which are greatly acknowledged. References and Notes (1) Hoffman, H.; Platz, G.; Ulbricht, W. J. Phys. Chem. 1981, 85, 1418. (2) Sugihara, G.; Nagadome, S.; Yamashita, T.; Kawachi, N.; Takagi, H.; Moroi, Y. Colloids Surf. 1991, 61, 111. (3) Morisue, T.; Moroi, Y.; Shibata, O. J. Phys. Chem. 1994, 98, 12995. (4) Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. Regular and Related Solutions, The Solubility of Gases, Liquids, and Solids; Van Nostrand Reinhold: New York, 1970; Chapter 10. (5) Kissa, E. Fluorinated Surfactants; Marcel Dekker: New York, 1993; Chapter 7. (6) Mukerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, 80, 1388. (7) Mysels, K. J. J. Colloids Interface Sci. 1978, 66, 331. (8) Funasaki, N.; Hada, S. J. Phys. Chem. 1980, 84, 736. (9) Burkitt, J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 628. (10) Moroi, Y.; Furuya, H. J. Colloid Interface Sci. 1996, 180, 296. (11) Guo, W.; Brown, T. A.; Fung, B. M. J. Phys. Chem. 1991, 95, 1829. (12) Krafft, M. P.; Giulieri, F.; Riess, J. G. Colloids Surf. A 1994, 84, 113. (13) Giulier, F.; Krafft, M.-P. Colloids Surf. A 1994, 84, 121. (14) Furuya, H.; Moroi, Y.; Sugihara, G. Langmuir 1995, 11, 774. (15) Anacker, E. W.; Underwood, A. L. J. Phys. Chem. 1981, 85, 2463. (16) Moroi, Y.; Matuura, R. Bull. Chem. Soc. Jpn. 1988, 61, 333. (17) Moroi, Y. Micelles; Theoretical and Applied Aspects; Plenum Press: New York, 1992; Chapter 6. (18) Kono, H.; Onoue, S.; Motomura, K.; Matuura, R. Mem. Fac. Sci., Kyushu UniV. 1973, C8, 219. (19) Moroi, Y.; Sugii, R.; Akine, C.; Matuura, R. J. Colloid Interface Sci. 1985, 108, 180. (20) Israelachvili, J.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (21) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905. (22) Baumu¨ller, W.; Hoffmann, H.; Ulbricht, W.; Tondre, C.; Zana, R. J. Colloid Interface Sci. 1978, 64, 418. (23) Nielsen, A. E.; Altintas, N. D. J. Cryst. Growth 1984, 69, 213.
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