6044
Langmuir 1996, 12, 6044-6052
Measurements of the Krafft Point of Surfactant Molecular Complexes: Insights into the Intricacies of “Solubilization” Hirotaka Hirata,* Akiko Ohira, and Nahoko Iimura Niigata College of Pharmacy, 2-13-5 Kamishin’eicho Niigata 950-21, Japan Received April 12, 1996. In Final Form: September 3, 1996X Of the surfactant molecular complexes widely obtained by us in the system of quaternary ammonium cationic surfactants such as CTAB (cetyltrimethylammonium bromide) and various aromatic substances as additives, we have already reported several solution characteristics which reveal them to be novel surfactant species. In the report it was disclosed that they had their own critical micelle concentrations (cmc’s) different from those of the mother surfactants and that the solubilization of such surfactantssolubilizates (additives) was just the dissolution of the surfactant complexes generated in the solubilization processes. In this successive study through measurements of the Krafft point of the surfactant molecular complexes, it will again be clarified that the “solubilization” is not a special phenomenon but merely one which is recognized to be the dissolution of those surfactant complexes like that of any other crystalline material. The facts that the solubilization is obviously operative even at such a low surfactant concentration as below its cmc and that the presence of surfactants far above their cmc suppresses the solubilization to such an extent that it is much lower than that of the solubility of the solubilizates in pure water might be explained only by the novel concept we already presented, by which the obsolete theory for the solubilization so far should be substituted.
Introduction As one of the most conspicuous properties of surfactant solutions, the phenomenon “solubilization” deserves to be pointed out first of all. Solubilization has long been studied on the basis of the micellar aggregation theory, which had played an important role ridding researchers of an anomalous aspect of their colloidal solutions in the early days.1 Solubilization has, of course, been explained as one of the most remarkable effects of those micelles in the medium, the formation of which is accepted to be caused above some critical concentration called the critical micelle concentration cmc.2-7 Today, the phenomenon “solubilization”, which stands for making insoluble or sparingly soluble materials (solubilizates) soluble and the resulting solutions clear and homogeneous in water, is generally believed to be due to the existence of surfactant micelles. The formation of micelles, above all, precedes then the process of making non-water-soluble materials homogeneous follows it, in accordance with the observation that below the cmc and before micelle formation no solubilization occurs.2-7 In these explanations the specially dispersed phase containing the oleophilic solubilizates in the micelle has been argued to be thermodynamically stable.8 In regard to the solubilization, apart from these explanations and knowledge piled up so far, there are yet many vague points and abstruse areas left to be solved. Under these circumstances we have recently discovered the wide formation of the surfactant molecular complexes X Abstract published in Advance ACS Abstracts, November 15, 1996.
(1) McBain, J. W.; Hutchinson, E. Solubilization and Related Phenomena; Academic Press: New York, 1955. (2) Preston, W. C. J. Phys. Colloid Chem. 1948, 52, 84. (3) Klevens, H. B. Chem. Rev. 1950, 47, 1. (4) Shinoda, K.; Nakagawa, T.; Tamamushi, B.; Isemura, T. Colloid Surfactants; Some Physico-Chemical Properties; Academic Press: New York, 1963. (5) Shinoda, K. Solvent Properties of Surfactants Solutions; Marcel Dekker, Inc.: New York, 1967. (6) Elwothy, P. H.; Florence, A. T.; MacFarlane, C. B. Solubilization by Surface-Active Agents and its Application in Chemistry and Biological Sciences; Chapman and Hall: London, 1968. (7) Mittal, K. L., Ed. Micellization, Solubilization, and Microemulsions; Plenum: New York, 1977. (8) MacBain, J. W. Advances in Colloid Science; Interscience Publishers, Inc.: New York, 1942; Vol. 1.
S0743-7463(96)00353-8 CCC: $12.00
irrespective of their surfactant chemical nature.9-11 Especially quaternary ammonium cationic surfactant species such as CTAB were quite pertinent to provide the complexes in the crystalline state.10 By X-ray analyses the well-grown crystals not only afforded us the detailed structural knowledge of the surfactant molecular complexes but substantially proved the stable existence of the complexes, not with thoughts of interaction or complexation so far between surfactants and various materials but with firm bases as their stable isolation of the complexes.12 These crystalline surfactant molecular complexes always arose from the homogeneously solubilized solution systems that were allowed to stand under cool conditions for a period of time after attainment of solubilization equilibrium of any solubilizates in an appropriately adjusted surfactant solution.10,11 When the solutions were warmed up, the precipitated complex crystals were easily dissolved again and all solution characteristics were recovered and the original solubilized solutions instantaneously realized.10,11 Thus the crystallization and the dissolution were perfectly reversible in the solubilized solution systems like those of any common crystalline material. The observations strongly suggested that the phenomenon so far referred to as “solubilization” was not special but simple and quite the same to what was conventionally experienced as the dissolution of any substance.13 Successive studies for the solution behaviors of the surfactant molecular complex species through electric conductivity measurements revealed that the species were all novel surfactant species which had their own characteristic cmc’s different from those of the mother surfactants and they, moreover, showed different slopes in the well-known linear relation for each complex homologous series in the diagram of log(cmc) against carbon numbers in the surfactant alkyl chain.13 (9) Hirata, H.; Kanda, Y.; Sakaiguchi, Y. Bull. Chem. Soc. Jpn. 1989, 62, 2461. (10) Hirata, H.; Kanda, Y.; Ohashi, S. Colloid Polym. Sci. 1992, 270, 781. (11) Hirata, H.; Iimura, N. J. Colloid Interface Sci. 1993, 157, 297. (12) Kitamura, T.; Ohashi, Y.; Iimura, N.; Hirata, H. In preparation. (13) Hirata, H.; Yagi, Y.; Iimura, N. J. Colloid Interface Sci. 1995, 173, 151.
© 1996 American Chemical Society
Krafft Point of Surfactant Molecular Complexes
On the basis of these results we proceeded further to understand how the situation was correlated between the solubilization by several surfactants with various solubilizates and the dissolution of the complex species yielded from the same component systems. Through this survey of the Krafft point detection of the surfactant complexes we disclosed several new facts that each surfactant complex species has its own different characteristic Krafft point from that of the mother surfactant species and that the temperature region of beginning of the dissolution of the surfactant complexes (Krafft point) corresponded very well to the set up point of the solubilization by any surfactant concerned. Experimental Section All the surfactant molecular complexes were, on one hand, produced from surfactants and solubilizates through a conventional treatment of solubilization in aqueous media. The preparation method is described in detail in the former reports.10,11 The molar composition ratios of the complexes were easily known from the elementary analysis, UV spectrometry, electric conductivity measurement, and most accurately by the X-ray crystallography.13 Of course, the results sufficiently corresponded to each other. On the other hand, the specimen solutions of solubilization which were to be compared with those from complexes were supplied through the conventional solubilization treatment after equilibrium achievement in a long period of time at a definite temperature of the measurement. UV spectra at a definite temperature were obtained by using a UV-visible recording spectrophotometer (UV-160A Shimadzu Corp., Kyoto). Measurements of the electric conductivity (κ) were carried out by using a conductivity meter (CM-30ET TOA Electronics Ltd., Tokyo). Before measurements all the specimen solutions were carefully adopted to a definite temperature of measurement to avoid a perturbation coming from nonequilibrium states which might arise from some temperature shifts.
Results Regarding any ionic surfactant, a somewhat anomalous dissolution behavior involved by a sharp rise of the solubility with temperature was historically found by Krafft.14 Today the behavior is recognized as follows: At low temperature the solubility of the ionic surfactants is quite limited until the temperature reaches some critical point. At the critical temperature an abrupt increase of the solubility occurs accompanied by a fundamental change in the dissolution state of the surfactant; above that point dissolved surfactant species commence aggregation, also known as micelle formation, which causes a sudden increase of the solubility, i.e., at low temperature monomer solubility, which is very low, determines the total solubility in the system, while at higher temperature when the monomer solubility has reached the cmc, the micelle solubility, which is far much higher than that of monomer, determines the system solubility.4,15,16 Consequently the Krafft phenomenon is often characterized by a critical micelle temperature.17 On the basis of these understandings, it is possible to deduce the solution characteristics of the surfactant species. We conducted thereof the measurements of the Krafft point of several crystalline surfactant molecular complexes in comparison with the solubilized solution systems composed of the same components in order to get to a broader perspective view and to see the connection between the solubilization and the surfactant molecular complex formation. As described below the Krafft points in each system corresponded well (14) Krafft, F.; Wiglow, H. Chem. Ber. 1895, 28, 2566. Krafft, F. Chem. Ber. 1899, 32, 1596. (15) Murray, R. C.; Hartley, G. S. Trans. Faraday Soc. 1935, 31, 183. (16) Tartar, H. V.; Wright, K. A. J. Am. Chem. Soc. 1939, 61, 539. (17) Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1976, 80, 1075.
Langmuir, Vol. 12, No. 25, 1996 6045 Table 1. Surfactant Molecular Complex Molar Composition Ratio and their cmc surfactant complex (surfactant/additive)
molar composition cmc of the ratio (surfactant/ surfactant complex additive) species (mol/dm3)
CTAB/p-phenylphenol CTAB/o-iodophenol
2/1 1/1
CTAB/2-naphthol CTAB/indole CTAB/R-naphthylamine
1/1 3/2 2/1
CTAB/diphenylamine a
c
2/1
0.40 × 10-3 a 0.70 × 10-3 b 0.88 × 10-3 c 0.60 × 10-3 b 0.62 × 10-3 b 0.42 × 10-3 b 0.44 × 10-3 c 0.57 × 10-3 a
b
At 45 °C, by UV spectroscopy. At 30 °C, by electric conductivity. At 35 °C, by UV spectroscopy.
between each system of complexes and of the correlated solubilization systems. In this study we dealt with systems of surfactant molecular complexes which were yielded from quaternary ammonium cationic surfactants such as CTAB and several phenolic or aromatic amines. Almost all of the complexes derived from those cationic surfactants were of crystalline nature.10 Of course, these crystalline substances are not simple cationic surfactant precipitates, which might be caused by the deprotonation of added phenols and by a subsequent shifting of medium pH, but crystals of genuine complexes which have crystallographically been proved by the X-ray structural analyses. The crystalline surfactant molecular complexes dealt with in this report are summarized in Table 1, where the composition, the molar composition ratios, and the cmc values of the complex species are gathered. It is interesting to note that all the values of the cmc of the complex species are considerably depressed in comparison with that of the mother species (CTAB). The results correspond well to accepted common knowledge that most surfactants always tend to be affected in the presence of a third material such as solubilizate or some other coexisting material. Furthermore, a good coincidence in the behavior will be described below, regarding cmc’s and Krafft points in comparison of the surfactant complex systems with those of the corresponding solubilized solutions composed of equimolar composition to the complexes. However, such cmc depressions and the Krafft point modifications in the solubilized solution systems should not be superficially recognized by the effect of the coexistence of a third material with surfactant but should properly be recognized by the inherent characteristics exhibited by those surfactant complexes. According to our opinion based on our findings the recognition for these effects to be caused by a coexisting third material is not substantial, at least in so far as the systems of the surfactant complex formation are concerned. Regarding the formation and the stable isolation of those surfactant molecular complexes in detail, a series of our recent papers has demonstrated the whole picture of the unambiguous existence of the complexes and moreover deeply suggestive and new aspects of the solution characteristics.10 The object of this article is to elucidate that (1) the cationic surfactant species are novel surfactant species which have their inherent cmc’s and Krafft points different from their mother surfactant (CTAB), based on the knowledge already presented,9-11 (2) the composite solution systems (solubilization systems) composed of CTAB and phenols etc., as solubilizates, in equimolar composition to the surfactant complexes are quite identical to each other, and (3) depending on the above (1) and (2), the interpretation upon the phenomenon “solubilization” based on the micelle theory which generally prevails to date is not substantial but apparent, in so far as our
6046 Langmuir, Vol. 12, No. 25, 1996
Figure 1. Temperature dependence of κ in the systems of surfactant complex (CTAB/p-phenylphenol) solution (4) and the same molarly composed solubilized solution (O) as the complex. Both systems which contain CTAB of 5 × 10-3 mol/ dm3 corresponded well to each other, clarifying the identity of the systems. A sharp rise of κ with temperature is due to the Krafft phenomenon. An arrow in the diagram shows the Krafft point of the complex surfactant species.
systems of the molecular complex formation are concerned. The argument originates from the observation that those surfactant complexes are simply precipitated in the crystalline form at a cool condition from the solubilized solution systems and on warming they are again dissolved instantaneously causing homogeneous solubilized solution systems in which the solution characteristics are wholly recovered. The change of the precipitation (crystallization) and the dissolution is always occurred, as ordinarily as observed in any crystalline material, in a perfectly reversible manner toward both directions with the temperature shifts. Thus, the cmc depression and the Krafft point modification observed everywhere in the following diagrams of the CTAB systems should not simply and superficially be understood to be caused by the addition of phenols etc., but these effects should be recognized to be a result of an inherent nature of the surfactant complexes yielded in the systems as clarified in comparison of the solubilization systems with the corresponding complex systems. Determination of the Krafft point relied on the following two methods: one was the measurement of electric conductivity (κ) and the other was spectroscopy.18 Both methods gave results which corresponded well to each other. In each specimen solution over its cmc, the temperature dependences of κ are described by sigmoidalshaped curves (Figures 1-5). In the low-temperature region rather low values of κ can be ascribed to a suppressed condition of occurrence of the charge carriers which come from the dissolution of the surfactant complex species. A sudden rise of the curves with temperature increase followed by the suppressed κ region corresponds to an abrupt increase in the amount of the dissolved (18) Ino, T. Nippon Kagaku Zasshi 1959, 80, 456.
Hirata et al.
Figure 2. Temperature dependence of κ in the systems of surfactant complex CTAB/o-iodophenol solution (4) and the same molarly composed solubilized solution (O) as the complex. Both systems which contain CTAB of 5 × 10-3 mol/dm3 corresponded well to each other, clarifying the identity of the systems. A clear change of dissolution of complex species is noted to be caused by the Krafft phenomenon. An arrow in the diagram shows the Krafft point of the complex surfactant species.
species. An appropriately appointed temperature within the sharp rise region should designate the Krafft point of the surfactant complex species; i.e., above this temperature surfactant species are abruptly dissolved and commence aggregation all at once to form micelles.18 In this study we generally determined the Krafft points of the complex surfactant species as the onset of the sharp rise in the κ-temperature behavior as illustrated by an arrow in the diagrams. The following slower increase or reversed slope region appears to be due to a transition of the aggregation states. In fact, in the system of CTAB/o-iodophenol (Figure 2) in which a remarkable heavy viscoelasticity is caused, suggesting the existence of enormously gigantic micelles in solution, the conductivity rise continues after a rather low κ region at a low temperature until the temperature reaches a certain point at which the heavy viscoelasticity in solution involved in the giant micelles disappears,19-21 although the point is not seen on the diagram yet. It is very important to note that in all those diagrams two profiles of the surfactant complexes and of the solubilization systems have a good correspondence to each other. The fact clearly proves the identity of both systems; i.e., the solubilized solution systems are quite identical to the dissolution systems of the complex species in the same molar composition. The absorbance-temperature diagrams over cmc concentration of each species also represent the same sigmoidal curve behavior as those of the κ-temperature. (19) Shikata, T.; Sakaiguchi, Y.; Urakami, H.; Tamura, A.; Hirata, H. J. Colloid Interface Sci. 1987, 119, 291. (20) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081. (21) Ulmius, J.; Wennerstrom, H.; Johansson, L. B. A.; Lindblom, G.; Gravsholt, S. J. Phys. Chem. 1979, 83, 2232.
Krafft Point of Surfactant Molecular Complexes
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Figure 3. Temperature dependence of κ in the systems of surfactant complex CTAB/2-naphthol) solution (4) and the same molarly composed solubilized solution (O) as the complex. Both systems containing CTAB of 5 × 10-3 mol/dm3 corresponded well to each other, proving them to be identical systems. The κ behavior with temperature is in the same manner as the previous figure. An arrow shows the Krafft point of the complex surfactant species.
Figure 4. Temperature dependence of κ in the systems of surfactant complex (CTAB/indole) solution (4) and same molarly composed solubilized solution (O) as the complex. Both systems containing 5 × 10-3 mol/dm3 CTAB corresponded well to each other, proving them to be identical systems. The κ behavior with temperature is seen in the same manner as the above. An arrow shows the Krafft point of the complex surfactant species.
Much clearer detection of the Krafft points is achieved through the diagrams than through the κ-temperatures as shown by an arrow in each diagram. Absorbances were measured at the characteristic wavelength of each additive (solubilizate) species. At the low-temperature region before the Krafft point of the species, the limited very low solubility of the complexes led to very minute values of absorbance. Contrary, above the critical temperature the abruptly enhanced solubilities brought a steeply increasing absorbance as illustrated in Figures 6-10. In each system of the complex species of a definite concentration the absorbance would reach some limited and expected value which is specified by the sigmoidal curve as well as seen in κ vs temperature profiles. Also in these diagrams the correspondence between both systems of complex species and of the solubilized solutions of the same molar composition is quite satisfactory. In the absorbance-concentration diagrams at a given temperature the difference of the dissolution behavior is very remarkable in accordance with whether it is above or below a critical temperature as depicted in Figures 11-13. All profiles in each diagram clearly consist of two lines with different slopes, and they intersect at a certain concentration point. The concentration obviously corresponds to the cmc of the complex surfactant species. The prominent absorbance at a higher temperature system is, of course, due to the higher level of dissolution of the species. In contrast, lower temperature profiles indicate a limited dissolution of very minute levels. The large difference of the profiles depends simply on the temperature at which the measurements were carried out and distinctly reflect the clear existence of the Krafft phenomenon. On this basis it can be explained that a solubilization which appears not to proceed at some temperature just implies that at the temperature the
dissolution of the generated complexes composed of the surfactants and the solubilizates is not possible because the temperature is too low. Thus for the lower temperature measurements in these diagrams, the concentration of the surfactant species plotted on the abscissa is just apparent, since in the systems the surfactant species can only be dissolved at limited values at saturation. In these cases the specimen solutions for the absorbance measurement has been filtered to take off the nondissolved residuals. In these diagrams we should take notice of significant lower temperature behaviors in which we can directly see an effect of suppressed solubilization, i.e., the effect of limited solubility of solubilizates (additives) in the presence of the surfactant compared with their rather high values by themselves in pure water at the same temperature. The effect is very distinctive in CTAB/o-iodophenol (Figure 12) and /R-naphthylamine (Figure 13) systems. Thus we have here discovered a yet unknown phenomenon of “reverse solubilization”. Of course, however, it is quite ready to recognize this apparently curious phenomenon due to the simple dissolution of the surfactant complex species associated with their Krafft phenomenon. It is also important to note that the situation is quite similar to the above; the profiles of the same molarly composed solubilization systems which are always accompanied and compared with those of complex species in each diagram are perfectly superposed onto those of dissolution systems of the surfactant complexes. The fact obviously demonstrates that as for some materials as solubilizates the system of the solubilization is quite identical to that which is brought about by dissolution of the preformed complex species between the same surfactants and the solubilizates. In regard to Figures 11-13, especially to Figure 11 of
6048 Langmuir, Vol. 12, No. 25, 1996
Figure 5. Temperature dependence of κ in the systems of surfactant complex (CTAB/R-naphthylamine) solution (4) and the same molarly composed solubilized solution (O) as the complex. Both systems containing 5 × 10-3 mol/dm3 CTAB corresponded well to each other, proving them to be identical systems. The κ behavior is the same as the above. An arrow shows the Krafft point of the complex surfactant species.
CTAB/p-phenylphenol, it should intensively be noted to a gradual increase in absorbances with increase of the surfactant concentration. The gradual increase just begins at the origin of the diagrams, even in such dilute condition of the surfactant as below cmc. The fact that the absorbance comes from the solubilizates proves that even below cmc the surfactants like CTAB, etc., can solubilize the solubilizates in severe contrast to the generally accepted views so far. The apparent inconsistency, however, can quite readily be understood as the dissolution of once yielded surfactant molecular complexes. The yielded surfactant molecular complex species are all new surfactant species different from their mother surfactants as demonstrated in the previous report.13 They behave like any common surfactant; they have their own characteristic cmc and they assume their own aggregation states in the medium according to the ambient conditions.13 Other cases agreeable to such aspects can be referred to in the data already obtained as shown in Figure 14.13 Throughout the diagrams verifying the identity of both systems of the surfactant complex solutions and the solubilized solutions of the same molarity, the profiles simultaneously confirm that the clear κ increase which exactly starts from the origin of the diagrams should be interpreted to be caused by the dissolution of the new complex surfactant species, generated in the course of the solubilization which has never been accepted so far in such a low surfactant concentration range below the cmc. The aggregation states of those complex surfactant species seem to be directly reflected in the slope change of κ-concentration diagrams. In the low surfactant concentration ranges below their cmc’s, the surfactant species are in the premicellar form; i.e., the aggregation states consisted of small numbers of the complex surfactant species. Above their cmc’s the aggregation is modified to the ordinal micelles in a comparatively sudden manner
Hirata et al.
Figure 6. Temperature dependence of absorbance in the systems of surfactant complex (CTAB/p-phenylphenol) solution and the same molarly composed solubilized solution as the complex. Symbols respectively show that complex (4) and solubilization (O) at 5.0 × 10-3 mol/dm3 CTAB. The absorbance was measured at a wavelength of 325 nm in accordance with the characteristic absorption of p-phenylphenol. A very sharp rise of absorbance is due to the sudden enhancement of dissolution of species and corresponds to the Krafft phenomenon. An arrow shows the Krafft temperature of the complex surfactant species.
at a definite concentration. The behavior is rationally accepted by the reason that the complex surfactant species are, in themselves, new different species as distinctly illustrated, in the diagrams of Figure 14, by the total profile of κ-concentration which is quite similar to their mother surfactant, CTAB. With respect to each complex surfactant species the estimated Krafft points through these techniques are given in Table 2 in which the value of the mother surfactant, CTAB, is put at the margin of the table as a reference.22 In the table it is easy to see that all the values of the Krafft temperature are variously modified in comparison with that of the mother CTAB. It is worth while recognizing, however, that in our opinion the Krafft temperature modification is not due to the addition of a third material such as phenol but due to one of inherent characteristics of the newly found complex surfactants, though the modification seemingly appears due to a simple effect by the material addition. The findings that the newly yielded surfactant complexes are another different novel surfactant species from their mother, CTAB, have already been documented in a previous report.13 It should again be emphasized that each surfactant complex species has its own characteristic Krafft point showing that it is a different surfactant species from the mother species. Through these results the points are much more decisively determined as a temperature of the sharp rise of the curves of the absorbance-temperature diagrams preferably to those of κ-temperature. Both values, however, have acceptable agreement with each other. (22) Adam, H. K.; Pankhurst, K. G. A. Trans. Faraday Soc. 1946, 42, 523.
Krafft Point of Surfactant Molecular Complexes
Figure 7. Temperature dependence of absorbance of CTAB/ o-iodophenol systems in two different concentrations of CTAB contained in each system. Symbols show that complex (3) and solubilization (y) at 1.5 × 10-2 mol/dm3 CTAB and complex (4) and solubilization (O) at 5.0 × 10-3 mol/dm3 CTAB, respectively. The absorbance wavelength at 279 nm was used as the characteristic absorption of o-iodophenol. In the diagram an arrow corresponding to the very sharp rise of absorbance shows the Krafft point of the complex surfactant species.
Figure 8. Temperature dependence of absorbance of CTAB/ 2-naphthol systems. The absorbance wavelength is 327 nm for 2-naphthol. Symbols and the arrow are the same as those in Figure 7.
Discussion The crystalline surfactant molecular complexes in this subject were always provided in homogeneously solubilized solution systems composed of surfactants and solubilizates in aqueous media, after an attainment of the solubilization equilibrium.10,11 To date many of their structures have been identified thus establishing their stable existence of the complex species.12 The precipitated complex crystals were easily dissolved again instantaneously by warming
Langmuir, Vol. 12, No. 25, 1996 6049
Figure 9. Temperature dependence of absorbance of CTAB/ R-naphthylamine systems. The absorbance wavelength is 320 nm for R-naphthylamine. Symbols and the arrow are the same as those in Figure 7.
Figure 10. Temperature dependence of absorbance of CTAB/ diphenylamine systems at CTAB concentration of 5.0 × 10-3 mol/dm3. The absorbance wavelength is 279 nm for diphenylamine. Symbols and the arrow are the same as those in Figure 6.
the systems after recovering all their solution characteristics.10,11 The crystallization and the dissolution were perfectly reversible like any common crystalline material.10,11 The observation gives a definitive clue to elucidate the “solubilization”, that it is not a special phenomenon but just a conventionally observable and daily experienced one which can readily be grasped as the dissolution of any crystalline substance.10,11,13 One of other remarkable features of the surfactant complexes was that each of them was a novel species of
6050 Langmuir, Vol. 12, No. 25, 1996
Figure 11. The difference of absorbances of CTAB/p-phenylphenol systems at different temperatures. Symbols show the complex (3) and the solubilization (y) at 30 °C and the complex (4) and the solubilization (O) at 45 °C, respectively. Increasing absorbance is in good accordance with increase of chromatic species in the systems, while one kept at low levels means that the species dissolution is limited at low concentration of the saturation because of an extremely low solubility of the species below the Krafft temperature. In the latter case the concentration scaled on the abscissa is only apparent. Two broken lines (b and 0) just starting from the origin illustrate the absorbance of a single additive (p-phenylphenol) in pure water at 45 and 30 °C, respectively. At both temperatures solubility saturation is observed. In the profiles of each temperature in the low concentration region it should be noted that “reverse solubilization” occurs in the whole concentration range at 30 °C and in a limited range at 45 °C, while at 45 °C solubilization is normally promoted (see text).
the surfactant; they showed distinctively different cmc’s of their own from the mother surfactants and satisfied the well-known linear relation between log(cmc) and the carbon numbers of the surfactant alkyl chains in their homologous series.13,23,24 The knowledge offered through the results of κ measurements tempted us to develop further the ideas for overthrowing and replacing the obsolete thoughts of “solubilization” which insist on an ab initio existence of micelles in the medium. The results obtained through measurements of Krafft points by two different methods regarding the new surfactant species of the surfactant molecular complexes between those cationic surfactants and several aromatic additives provided a clear conclusion that they behaved like any surfactant, showing a sharp increase of solubility at a certain temperature point, which should be designated as their own Krafft point. At the same time it was established that all of the behavior by two different methods of Krafft point measurements corresponded very well between each system of surfactant complexes and same molarly composed solubilization system. Repeatedly the results decisively prove that the phenomenon “solubilization” is just the dissolution of the precomposed surfactant complexes which contain, of course, the sur(23) Klevens, H. B. J. Am. Oil Chem. Soc. 1953, 30, 74. (24) Herman, K. W. J. Phys. Chem. 1962, 66, 295.
Hirata et al.
Figure 12. The difference of absorbance of CTAB/o-iodophenol systems at different temperatures. Symbols show the complex (3) and the solubilization (y) at 16 °C and the complex (4) and the solubilization (O) at 35 °C, respectively. In the diagram the same feature as Figure 11 is seen, reflecting the existence of the Krafft phenomenon. A broken line denotes the single additive (o-iodophenol) absorbance, which is due to its solubility in pure water at 16 °C. Note the clear occurrence of “reverse solubilization” associated with that shown in lower level absorbance at the same temperature.
Figure 13. The difference of absorbance of CTAB/diphenylamine systems at different temperatures. All illustrations and explanations are the very same as in Figure 12. In this system also the “reverse solubilization” is significantly observed at lower temperature than the Krafft point of the surfactant complex species.
factants and the solubilizates at a definite molar composition. Figures 11-13 support the thoughts from another view point; the apparent effect that at a low temperature a surfactant has no ability to solubilize any solubilizate is just due to the temperature being too low to dissolve the once yielded surfactant complex species. Other important points to be emphasized are that the surfactant complex species dealt with here are quite stable in aqueous media. In figures 11-13, broken lines just starting from the origin represent the absorbance values due to singly dissolved solubilizates in pure water. In the presence of surfactant, the absorbance values are all
Krafft Point of Surfactant Molecular Complexes
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Figure 14. Diagrams showing dependence of κ on the surfactant concentration at 30 °C. It should be noted that even below cmc the solubilization is operative, deduced from behaviors of both the complex solutions (4) and the solubilized solutions (O). Each profile corresponds to the systems of (a) CTAB/o-iodophenol, (b) CTAB/2-naphthol, (c) CTAB/o-cresol, and (d) CTAB/indole, respectively. A dotted line in each diagram shows the behavior of the single CTAB of which cmc is denoted by the kink point.
suppressed in the low-temperature systems. This remarkable effect means that apparently the solubilizates are subjected to a condition of difficult solubilization, in other words, in the presence of surfactants “reverse solubilization” by surfactants occurs. The curious effect, of course, deeply correlates the decreased solubility of the complex surfactant species associated with that of the temperature being below their Krafft points as stated above. The effect, on one hand, suggests the high stability of the surfactant complex species in the medium. In the surfactant complexes the mother surfactants tightly hold the component solubilizates like a mother holding her daughter in her arms. Provided the stability of the
Table 2. Krafft Point of the Surfactant Molecular Complexesa Krafft point (°C) surfactant complex (surfactant/additive)
UV spectroscopy
electric conductivity (κ)
CTAB/p-phenylphenol CTAB/o-iodophenol CTAB/2-naphthol CTAB/indole CTAB/R-naphthylamine CTAB/diphenylamine
36.0 22.6 22.7 20.0 24.0 35.3
34.7 18.6 19.7 19.2 23.5
a
Reported Krafft point of CTAB is 24 °C (see ref 22).
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complex species is very poor, i.e., the equilibrium point significantly deviates to the dissociation side for the complex species, the liberated species (solubilizates) should increase the absorbance and ultimately regain the broken line levels. However, the fact appears not to be that which is assumed, reflecting none of recognition of liberated materials in the system. Such thermally variable effects also suggest promising applications for various fields such as recovery or removal of organic pollutants in water, carrying out capture and release at the same time just by changing temperature.25,26 The conceivable stability of the surfactant complex species in the aqueous medium, on the other hand, infers how the dissolved state of those species is. The stability of the surfactant complex is quite sufficient so that they exist as a unit and conduct themselves in a firmly (25) Lee, B.-H.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1990, 6, 230. (26) Scamehorn, J. F.; Harwell, J. H. Surfactant-Based Separation Process; Marcel Dekker, Inc.: New York, 1989.
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connected form in the media not only statically but kinetically or dynamically. In this context we need to touch on interesting but somewhat complicated problems to be solved in this stage, e.g., the problem of the position of the solubilizates in micelles.27,28 On the basis of our information obtained here, solubilizates always appear to be associated with the surfactants in the solubilization system, being not separately operative but cooperative in their any physical performance. The arguments developed here, moreover, seem to be deeply related to very important problems of material transport through living cell membranes and, of course, directly to the applicational areas in many industrial fields. It looks like we are now passing the turning point regarding the basic comprehension of the phenomenon “solubilization” not by commonplace thoughts but by a novel concept. LA9603535 (27) Eriksson, J. C. Acta Chem. Scand. 1963, 17, 1478. Eriksson, J. C.; Gillberg, G. Acta Chem. Scand. 1996, 20, 2019. (28) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978, 82, 1620.
