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Solution Behavior of Anionic Surfactant Molecular Complexes
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
191, 510–513 (1997)
CS974981
NOTE Solution Behavior of Anionic Surfactant Molecular Complexes
We have already reported the solution behavior as examined by electrical conductivity for cationic surfactant molecular complexes composed of quaternary ammonium salts such as CTAB and various aromatic additive species such as phenols and amines. By the same electrical conductivity method, we have obtained many valuable results which establish the newly obtained anionic surfactant molecular complexes to be novel surfactant species displaying their own characteristic CMCs and Krafft points different from those of their mother surfactants. Through the solution behavior, furthermore, we could deduce that a solubilized solution system that was of molar composition equal to that of the complex in the surfactant and the solubilizate (additive) was perfectly identical to the dissolution system of the complex. The facts satisfactorily confirmed that a solubilized solution system was only the dissolution system of the complex which spontaneously resulted in the process of solubilization. q 1997 Academic Press Key Words: anionic surfactant; molecular complex; novel surfactant; cmc; Krafft point; solubilization theory.
INTRODUCTION Succeeding a series of discoveries of the existence of crystalline surfactant molecular complexes of quaternary ammonium cationic surfactants with various aromatic additive substances (1–3), we have recognized analogous formations of surfactant complexes between anionic surfactants, sodium alkylsulfates such as SLS (sodium laurylsulfate), and various phenolic compounds and others as additives. Especially well grown single crystals yielded from the sodium octyl sulfate/2-naphthol system unambiguously afforded firm evidence of the occurrence of these complex species, revealing structural details through X-ray analysis (4). The crystalline anionic surfactant molecular complexes were generally obtained from homogeneously solubilized solution systems of the homologous sodium alkyl-sulfates with those additives as solubilizates. After conventional treatment of the solubilization by, for instance, SLS the surfactant molecular complexes always precipitated in powdery form when kept standing at a cool temperature. The powdery complexes were promptly dissolved when warmed in the solutions, recovering all the solution characteristics and thus resulting in the original solubilized solution systems. The crystallization and the dissolution of the complex species were always perfectly reversible with temperature change, like a phenomenon commonly observable in any crystalline substance (2, 3). The generation of the complexes and the solution behavior with temperature shift were entirely the same as those of the cationic species (2, 3). The observation suggested reconsideration of the fundamentals of the theory of solubilization developed on the basis of the arguments persisting in ab initio existence of the micelle (5–8) and reformation of the old belief for new thoughts that solubilization is a simple phenomenon easily recognized as just being the dissolution of the complex species naturally yielded in the process of solubilization. Concerning the analogous facts already observed in cationic
JCIS 4981
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6g2e$$$281
EXPERIMENTAL Materials. Sodium alkyl sulfates donated by Kao Soap Co. Ltd. (Wakayama Lab.) were repeatedly recrystallized, initially from MeOH solutions and then from aqueous solutions. All of the additives purchased from Wako Pure Chemical Industries Ltd. (Osaka) and from Tokyo Kasei Kogyo Co. Ltd. (Tokyo) were recrystallized once by the conventional method. Complex preparation. From aqueously solubilized solution systems, not by special but by common procedures, the surfactant complexes were obtained. The details were described in the preceding papers (1–3). Electrical conductivity measurements. Electrical conductivity was measured using a CM-30ET meter (TOA Electronics Ltd., Tokyo).
RESULTS AND DISCUSSION A detailed description of the crystalline anionic surfactant complexes utilized in this study will appear in an appropriate paper in the near future. Therein it will be argued that we have succeeded in proving the stably isolable existence of those species, of course, with a definite molar ratio as well as the cationic ones. Some limited data on the complex molar composition ratios of those species which concern this investigation of the solution behaviors are collected in Table 1. An interesting discrimination of the anionic surfactant complexes from the cationic ones is that most of the former complexes contain some water molecules in their crystalline structure. The water content shown in Table 1, assayed by the Karl–Fischer method, was quite sufficiently consistent, to the minute order, with the complex composition molar ratio, which was most reliably obtained from the elemental analysis and rather handily from UV spectroscopy. The values of the molar ratios, of course, corresponded well to each other. In the conductometric performances in aqueous media, therefore, the complexes which were composed of minimum integral numbers of surfactants and additives play the role of charge carriers, associated with conditions of variously aggregated or nonaggregated form. In the anionic surfactant molecular complex formation seen in Table 1, phenols also appeared to be preferable to any other materials and more easily available for measurement, as in the case of cationic species (2, 3). The fact requires that complex formation be independent of charge because
510
0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
AID
surfactant complex systems we have criticized the obsolete theory and proposed a new concept of solubilization by which we are able to recognize it as one type of abstrusely complicated phenomenon (9). Recently we have studied the solution behavior of these anionic surfactant molecular complexes through electrical conductivity ( k ) measurement. Similar, to the former findings in the case of the cationic surfactant complexes we have been able to establish that (i) these anionic surfactant complexes also are a novel surfactant species; i.e., they have their own CMCs different from those of their mother surfactant species; (ii) they show the Krafft phenomenon as an inherent surfactant species; (iii) they homologously satisfy the well-known linear relation of log(cmc) vs surfactant carbon number, and (iv) any solution system of the surfactant molecular complexes is quite identical to that which is provided by solubilization in the same molar composition ratio (surfactant to additive) as that of the surfactant molecular complex species.
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TABLE 1 The Molar Composition Ratios of the Anionic Surfactant Molecular Complex Species Treated within This Study Complex (surfactant/additive)
Molar composition ratio (surf./add.)
SLS /2-naphthol /p-iodophenol /p-bromophenol SMS /2-naphthol /p-iodophenol SCS/2-naphthol SSS /2-naphthol /p-iodophenol
TABLE 2 The CMC Values of the Anionic Surfactant Molecular Complexes and Those of the Mother Surfactants at 457C
Water content (complex/H2O)
1.5 1.5 2.0
1/4 1/3 1/3
1.0 1.0 1.0
1/1 0 1/1
1.0 2.0
/ /
Note. SLS, sodium laurylsulfate; SMS; sodium myristylsulfate; SCS, sodium cetylsulfate; and SSS, sodium stearylsulfate. 0, Unknown yet; /, anhydrous.
Surfactant species
cmc(1103 mol/dm3)
SLS SLS /2-naphthol /p-iodophenol /p-bromophenol /p-ethoxyphenol SMS /2-naphthol /p-iodophenol SCS /2-naphthol SSS /2-naphthol /p-iodophenol
9.10, 9.10,a 8.40,b 8.44a,b 4.00 4.00 4.10b 4.40b 2.09, 2.31a 1.30 1.31 0.61, 0.58d 0.48 0.20, 0.28e 0.18 0.10
a
Ref. 16. Data at 307C. c Ref. 17. d Ref. 18. e Ref. 19. b
the sulfate anion of the surfactant, for example, apparently refuses the approach of rather acidic phenols. However, such trends of almost no effect of the electrostatic factors in the complexation are always very common in every surfactant complex formation of both ionic species. Typical solution behavior of the electrical conductance ( k ) vs the concentration of the complex component species is represented in Fig. 1, in comparison with the behavior of the solubilized solution systems of various molar ratios of surfactants to solubilizates (complex additives). In the diagram the profile of the surfactant complex species consists of two lines with different slopes. The profile is quite similar to that of single ionic surfactant species, for instance to that of the mother surfactant (SLS), as depicted comparatively in the same diagram. According to common knowledge the kink point at which two different slope lines intersect with each other designates the CMC of the surfactant species. Hence, directly from the diagram, it is quite feasible to estimate the CMC values characteristic of the newly obtained anionic surfactant complex species as listed in
FIG. 1. Comparison of the k vs concentration behaviors of the mother surfactant, SLS (see footnote to Table 1) ( l ), and its complex (SLS/ pbromophenol) solutions ( s ) at 307C with those of the various molarly solubilized solution systems in the ratio of SLS/ p-iodophenol; ª, n, and h correspond to 1/1, 3/2, and 2/1, respectively. Note the perfect agreement of the behavior between systems of the complex solution and of 2/1 molarly solubilized solution, and this agreement deduces the composition molar ratio of the complex species.
AID
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Table 2 compared with those of the mother surfactants. The CMCs derived from the kink point of each surfactant complex species are clearly different from those of the mother. The fact decisively demands that each complex species should be another novel surfactant species different from the mother surfactants. In scrutinizing the conductance profiles, it is found that the shift of the kink points appears to depend on the composition of each system. Surfactant (SLS) surplus to the corresponding complex molar ratio enhances conductivity under a situation of mixed micelle formation, while surfactant deficiency causes conductivity decrease through several effects which would presumably be involved in solubilization of the remainder additive (pbromophenol) by the complex surfactant (SLS/p-bromophenol) and resulted in an increase in micelle volume and in adsorbed counterions and an according decrease in the numbers of charge-carrying ions. It should also be emphasized that the behavior of the surfactant complex solution system is always perfectly identical to that of the solubilized solution system of equal molar composition. On this basis we again claim that a solubilized solution system is just a dissolution system of the surfactant/additive complexes spontaneously yielded in the solubilization processes when both systems are well matched in their molar composition ratios (9). In addition, the variously prepared complex surfactant species in this series of sodium alkyl sulfates revealed a characteristic phenomenon of dissolution with temperature called the ‘‘Krafft point’’ which was widely known to be realized in ionic surfactants (11, 12). A typical profile with the abrupt enhancement of k which is characteristically caused by a sudden solubility increase of the complex surfactant species over a very narrow temperature range is presented in Fig. 2 (13). The profile, easily predictable for any ionic surfactant species, was already utilized to prove the complex surfactants of cationics to be novel surfactant species (9). Through observation it was disclosed that the Krafft points of the complex species resulted so far in these sodium alkylsulfate series were all appreciably decreased compared with their mother species, while in a series of CTAB the points were almost unaffected by complexation with various materials (9). This finding is very suggestive in interpreting the crystal structure, the disclosure of which is gradually being shown to be connected to the clear differences found between cationic and anionic complexes (4). The Krafft points detected are gathered in Table 3. The facts of inherent CMCs and Krafft points would also unambiguously indicate that the anionic surfactant molecular
coidal
512
NOTE
FIG. 2. Krafft phenomenon detected in the k measurements as to SSS/ p-iodophenol complex solution system of 5.0 1 10 04 mol/dm3 surfactant concentration. The temperature causing an abrupt increase of k corresponds to the Krafft point of the system.
complexes widely obtained in sodium alkylsulfates with various phenolic substances are novel kinds of surfactants, to be dealt with separately from their mother surfactants. Moreover, as one of the stronger bases for establishing that the surfactant complexes obtained are novel kinds of surfactants, the well-known plots regarding the relationship of log (CMC) vs the carbon numbers of the complex surfactant alkyl chain are illustrated in Fig. 3. In the homologous series of sodium alkylsulfates containing p-bromophenol and 2-naphthol the distinct linearity which has obviously different slopes from the mother homolog was quite satisfactorily clarified (14, 15). From the diagram two constants, A and B, in the linear relationship of log (CMC) Å A{BN are presented in Table 4 along with the mother’s homolog.
CONCLUSION As for the stably isolated anionic surfactant molecular complexes electric conductivity ( k ) measurements were carried out. Through these measure-
TABLE 3 The Krafft Points of the Mother Surfactants and Their Complexes
AID
Surfactant species
Krafft point (7C)
SLS /2-naphthol /p-iodophenol SMS /2-naphthol /p-iodophenol SCS /2-naphthol SSS /2-naphthol /p-iodophenol
12.0 5.3 0.4 29.0 4.0 1.0 46.0 21.2 56.0 31.0 30.4
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FIG. 3. The relationship of cmc vs carbon number of the mother surfactant homologs and their two complex homologs at 457C. n, s, and h correspond to each homolog of the mother surfactants, /2-naphthol complex, and /p-iodophenol complex homologs, respectively.
ments it was established that those complex species behaved themselves quite differently from their mother surfactants, showing their own characteristic CMC’s and Krafft points and subsequently that they were novel surfactant species different from their mothers. The finding that the anionic surfactant molecular complexes are another novel kind of surfactant species was still more evidently confirmed through the fact that the homologous series of the complex surfactant species satisfied the well-known linear relationship log (CMC) Å A{BN with different slopes from each other and from their mother species. As recognized through the method which allows us to determine complex molar composition by electric conductivity measurements based on comparing behaviors of solubilized solution systems of various molarity with that of the system of the complex solution and on selecting the system of identical behavior to each other, it was also deduced that a solubilized solution system composed of a surfactant and an additive in equal molarity to that of a complex composition was perfectly identical to the complex solution system and, hence, that a solubilized solution system was only a dissolution system of spontaneously yielded complex species in the course of solubilization.
TABLE 4 Constants in the Relationship of log(cmc) Å A–BN, Determined in the Anionic Surfactant Homologous Series and Its Complex Species at 457C Homolog
A
B
Surfactanta Surfactant 2-naphtholb /p-iodophenolb
1.083
0.265
0.397 0.811
0.232 0.265
a
As for the following homologous series of SLS, SMS, SCS, and SSS. Complex homologous series composed of the surfactants cited above and each additive. b
coidal
513
NOTE
ACKNOWLEDGMENT The authors express their gratitude to Wakayama Lab. of Kao Soap Co. Ltd. for its kind gift of anionic surfactants of a series of the sodium alkylsulfate homologs.
REFERENCES 1. Hirata, H., Kanda, Y., and Sakaiguchi, Y., Bull. Chem. Soc. Jpn. 62, 2461 (1989). 2. Hirata, H., Kanda, Y., and Ohashi, S., Colloid Polym. Sci. 270, 781 (1992). 3. Hirata, H., and Iimura, N., J. Colloid Interface Sci. 157, 297 (1993). 4. Sawada, K., Ohashi, Y., Iimura, N., and Hirata, H., Presented at 1996 Annual Meeting of the Crystallographic Society of Japan, Himeji, 1996. 5. Klevens, H. B., Chem. Rev. 47, 1 (1950). 6. McBain, J. W., and Hutchinson, E., ‘‘Solubilization and Related Phenomena.’’ Academic Press, New York, 1955. 7. Elworthy, P. H., Florence, A. T., and Mac Farlane, C. B., ‘‘Solubilization by Surface Active Agents and its Application in Chemistry and Biological Sciences.’’ Chapman and Hall, London, 1968. 8. Mittal, K. L. (Ed.), ‘‘Micellization, Solubilization, and Microemulsion.’’ Plenum, New York, 1977.
AID
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9. Hirata, H., Ohira, A., and Iimura, N., Langmuir 12, 6044 (1996). 10. Hirata, H., Yagi, Y., and Iimura, N., J. Colloid Interface Sci. 173, 151 (1995). 11. Krafft, F., and Wiglow, H., Chem. Ber. 28, 2566 (1895); Krafft, F., Chem. Ber. 32, 1596 (1899). 12. Mazer, N. A., Benedek, G. B., and Carey, M. C., J. Phys. Chem. 80, 1075 (1976). 13. Ino, T., Nippon Kagaku Zasshi 80, 465 (1959). 14. Klevens, H. B., J. Am. Oil Chem. Soc. 30, 74 (1953). 15. Herrman, K. W., J. Phys. Chem. 66, 295 (1962). 16. Goddard, E. D., and Benson, G. C., Can. J. Chem. 35, 986 (l957). 17. Flockhart, B. D., J. Colloid Sci. 16, 484 (1961). 18. Evans, H. C., J. Chem. Soc. 579 (1956). 19. Go¨tte, E., Int. Congr. Surf. Act. 3rd. 1, 45 (l960). Hirotaka Hirata 1 Nahoko Iimura Niigata College of Pharmacy 2-13-5 Kamishin’eicho, Niigata, Japan Received December 10, 1996; accepted May 9, 1997
1
To whom correspondence should be addressed.
coidal
191, 510–513 (1997)
CS974981
NOTE Solution Behavior of Anionic Surfactant Molecular Complexes
We have already reported the solution behavior as examined by electrical conductivity for cationic surfactant molecular complexes composed of quaternary ammonium salts such as CTAB and various aromatic additive species such as phenols and amines. By the same electrical conductivity method, we have obtained many valuable results which establish the newly obtained anionic surfactant molecular complexes to be novel surfactant species displaying their own characteristic CMCs and Krafft points different from those of their mother surfactants. Through the solution behavior, furthermore, we could deduce that a solubilized solution system that was of molar composition equal to that of the complex in the surfactant and the solubilizate (additive) was perfectly identical to the dissolution system of the complex. The facts satisfactorily confirmed that a solubilized solution system was only the dissolution system of the complex which spontaneously resulted in the process of solubilization. q 1997 Academic Press Key Words: anionic surfactant; molecular complex; novel surfactant; cmc; Krafft point; solubilization theory.
INTRODUCTION Succeeding a series of discoveries of the existence of crystalline surfactant molecular complexes of quaternary ammonium cationic surfactants with various aromatic additive substances (1–3), we have recognized analogous formations of surfactant complexes between anionic surfactants, sodium alkylsulfates such as SLS (sodium laurylsulfate), and various phenolic compounds and others as additives. Especially well grown single crystals yielded from the sodium octyl sulfate/2-naphthol system unambiguously afforded firm evidence of the occurrence of these complex species, revealing structural details through X-ray analysis (4). The crystalline anionic surfactant molecular complexes were generally obtained from homogeneously solubilized solution systems of the homologous sodium alkyl-sulfates with those additives as solubilizates. After conventional treatment of the solubilization by, for instance, SLS the surfactant molecular complexes always precipitated in powdery form when kept standing at a cool temperature. The powdery complexes were promptly dissolved when warmed in the solutions, recovering all the solution characteristics and thus resulting in the original solubilized solution systems. The crystallization and the dissolution of the complex species were always perfectly reversible with temperature change, like a phenomenon commonly observable in any crystalline substance (2, 3). The generation of the complexes and the solution behavior with temperature shift were entirely the same as those of the cationic species (2, 3). The observation suggested reconsideration of the fundamentals of the theory of solubilization developed on the basis of the arguments persisting in ab initio existence of the micelle (5–8) and reformation of the old belief for new thoughts that solubilization is a simple phenomenon easily recognized as just being the dissolution of the complex species naturally yielded in the process of solubilization. Concerning the analogous facts already observed in cationic
JCIS 4981
/
6g2e$$$281
EXPERIMENTAL Materials. Sodium alkyl sulfates donated by Kao Soap Co. Ltd. (Wakayama Lab.) were repeatedly recrystallized, initially from MeOH solutions and then from aqueous solutions. All of the additives purchased from Wako Pure Chemical Industries Ltd. (Osaka) and from Tokyo Kasei Kogyo Co. Ltd. (Tokyo) were recrystallized once by the conventional method. Complex preparation. From aqueously solubilized solution systems, not by special but by common procedures, the surfactant complexes were obtained. The details were described in the preceding papers (1–3). Electrical conductivity measurements. Electrical conductivity was measured using a CM-30ET meter (TOA Electronics Ltd., Tokyo).
RESULTS AND DISCUSSION A detailed description of the crystalline anionic surfactant complexes utilized in this study will appear in an appropriate paper in the near future. Therein it will be argued that we have succeeded in proving the stably isolable existence of those species, of course, with a definite molar ratio as well as the cationic ones. Some limited data on the complex molar composition ratios of those species which concern this investigation of the solution behaviors are collected in Table 1. An interesting discrimination of the anionic surfactant complexes from the cationic ones is that most of the former complexes contain some water molecules in their crystalline structure. The water content shown in Table 1, assayed by the Karl–Fischer method, was quite sufficiently consistent, to the minute order, with the complex composition molar ratio, which was most reliably obtained from the elemental analysis and rather handily from UV spectroscopy. The values of the molar ratios, of course, corresponded well to each other. In the conductometric performances in aqueous media, therefore, the complexes which were composed of minimum integral numbers of surfactants and additives play the role of charge carriers, associated with conditions of variously aggregated or nonaggregated form. In the anionic surfactant molecular complex formation seen in Table 1, phenols also appeared to be preferable to any other materials and more easily available for measurement, as in the case of cationic species (2, 3). The fact requires that complex formation be independent of charge because
510
0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
AID
surfactant complex systems we have criticized the obsolete theory and proposed a new concept of solubilization by which we are able to recognize it as one type of abstrusely complicated phenomenon (9). Recently we have studied the solution behavior of these anionic surfactant molecular complexes through electrical conductivity ( k ) measurement. Similar, to the former findings in the case of the cationic surfactant complexes we have been able to establish that (i) these anionic surfactant complexes also are a novel surfactant species; i.e., they have their own CMCs different from those of their mother surfactant species; (ii) they show the Krafft phenomenon as an inherent surfactant species; (iii) they homologously satisfy the well-known linear relation of log(cmc) vs surfactant carbon number, and (iv) any solution system of the surfactant molecular complexes is quite identical to that which is provided by solubilization in the same molar composition ratio (surfactant to additive) as that of the surfactant molecular complex species.
07-11-97 07:48:03
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511
NOTE
TABLE 1 The Molar Composition Ratios of the Anionic Surfactant Molecular Complex Species Treated within This Study Complex (surfactant/additive)
Molar composition ratio (surf./add.)
SLS /2-naphthol /p-iodophenol /p-bromophenol SMS /2-naphthol /p-iodophenol SCS/2-naphthol SSS /2-naphthol /p-iodophenol
TABLE 2 The CMC Values of the Anionic Surfactant Molecular Complexes and Those of the Mother Surfactants at 457C
Water content (complex/H2O)
1.5 1.5 2.0
1/4 1/3 1/3
1.0 1.0 1.0
1/1 0 1/1
1.0 2.0
/ /
Note. SLS, sodium laurylsulfate; SMS; sodium myristylsulfate; SCS, sodium cetylsulfate; and SSS, sodium stearylsulfate. 0, Unknown yet; /, anhydrous.
Surfactant species
cmc(1103 mol/dm3)
SLS SLS /2-naphthol /p-iodophenol /p-bromophenol /p-ethoxyphenol SMS /2-naphthol /p-iodophenol SCS /2-naphthol SSS /2-naphthol /p-iodophenol
9.10, 9.10,a 8.40,b 8.44a,b 4.00 4.00 4.10b 4.40b 2.09, 2.31a 1.30 1.31 0.61, 0.58d 0.48 0.20, 0.28e 0.18 0.10
a
Ref. 16. Data at 307C. c Ref. 17. d Ref. 18. e Ref. 19. b
the sulfate anion of the surfactant, for example, apparently refuses the approach of rather acidic phenols. However, such trends of almost no effect of the electrostatic factors in the complexation are always very common in every surfactant complex formation of both ionic species. Typical solution behavior of the electrical conductance ( k ) vs the concentration of the complex component species is represented in Fig. 1, in comparison with the behavior of the solubilized solution systems of various molar ratios of surfactants to solubilizates (complex additives). In the diagram the profile of the surfactant complex species consists of two lines with different slopes. The profile is quite similar to that of single ionic surfactant species, for instance to that of the mother surfactant (SLS), as depicted comparatively in the same diagram. According to common knowledge the kink point at which two different slope lines intersect with each other designates the CMC of the surfactant species. Hence, directly from the diagram, it is quite feasible to estimate the CMC values characteristic of the newly obtained anionic surfactant complex species as listed in
FIG. 1. Comparison of the k vs concentration behaviors of the mother surfactant, SLS (see footnote to Table 1) ( l ), and its complex (SLS/ pbromophenol) solutions ( s ) at 307C with those of the various molarly solubilized solution systems in the ratio of SLS/ p-iodophenol; ª, n, and h correspond to 1/1, 3/2, and 2/1, respectively. Note the perfect agreement of the behavior between systems of the complex solution and of 2/1 molarly solubilized solution, and this agreement deduces the composition molar ratio of the complex species.
AID
JCIS 4981
/
6g2e$$$281
07-11-97 07:48:03
Table 2 compared with those of the mother surfactants. The CMCs derived from the kink point of each surfactant complex species are clearly different from those of the mother. The fact decisively demands that each complex species should be another novel surfactant species different from the mother surfactants. In scrutinizing the conductance profiles, it is found that the shift of the kink points appears to depend on the composition of each system. Surfactant (SLS) surplus to the corresponding complex molar ratio enhances conductivity under a situation of mixed micelle formation, while surfactant deficiency causes conductivity decrease through several effects which would presumably be involved in solubilization of the remainder additive (pbromophenol) by the complex surfactant (SLS/p-bromophenol) and resulted in an increase in micelle volume and in adsorbed counterions and an according decrease in the numbers of charge-carrying ions. It should also be emphasized that the behavior of the surfactant complex solution system is always perfectly identical to that of the solubilized solution system of equal molar composition. On this basis we again claim that a solubilized solution system is just a dissolution system of the surfactant/additive complexes spontaneously yielded in the solubilization processes when both systems are well matched in their molar composition ratios (9). In addition, the variously prepared complex surfactant species in this series of sodium alkyl sulfates revealed a characteristic phenomenon of dissolution with temperature called the ‘‘Krafft point’’ which was widely known to be realized in ionic surfactants (11, 12). A typical profile with the abrupt enhancement of k which is characteristically caused by a sudden solubility increase of the complex surfactant species over a very narrow temperature range is presented in Fig. 2 (13). The profile, easily predictable for any ionic surfactant species, was already utilized to prove the complex surfactants of cationics to be novel surfactant species (9). Through observation it was disclosed that the Krafft points of the complex species resulted so far in these sodium alkylsulfate series were all appreciably decreased compared with their mother species, while in a series of CTAB the points were almost unaffected by complexation with various materials (9). This finding is very suggestive in interpreting the crystal structure, the disclosure of which is gradually being shown to be connected to the clear differences found between cationic and anionic complexes (4). The Krafft points detected are gathered in Table 3. The facts of inherent CMCs and Krafft points would also unambiguously indicate that the anionic surfactant molecular
coidal
512
NOTE
FIG. 2. Krafft phenomenon detected in the k measurements as to SSS/ p-iodophenol complex solution system of 5.0 1 10 04 mol/dm3 surfactant concentration. The temperature causing an abrupt increase of k corresponds to the Krafft point of the system.
complexes widely obtained in sodium alkylsulfates with various phenolic substances are novel kinds of surfactants, to be dealt with separately from their mother surfactants. Moreover, as one of the stronger bases for establishing that the surfactant complexes obtained are novel kinds of surfactants, the well-known plots regarding the relationship of log (CMC) vs the carbon numbers of the complex surfactant alkyl chain are illustrated in Fig. 3. In the homologous series of sodium alkylsulfates containing p-bromophenol and 2-naphthol the distinct linearity which has obviously different slopes from the mother homolog was quite satisfactorily clarified (14, 15). From the diagram two constants, A and B, in the linear relationship of log (CMC) Å A{BN are presented in Table 4 along with the mother’s homolog.
CONCLUSION As for the stably isolated anionic surfactant molecular complexes electric conductivity ( k ) measurements were carried out. Through these measure-
TABLE 3 The Krafft Points of the Mother Surfactants and Their Complexes
AID
Surfactant species
Krafft point (7C)
SLS /2-naphthol /p-iodophenol SMS /2-naphthol /p-iodophenol SCS /2-naphthol SSS /2-naphthol /p-iodophenol
12.0 5.3 0.4 29.0 4.0 1.0 46.0 21.2 56.0 31.0 30.4
JCIS 4981
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6g2e$$$281
07-11-97 07:48:03
FIG. 3. The relationship of cmc vs carbon number of the mother surfactant homologs and their two complex homologs at 457C. n, s, and h correspond to each homolog of the mother surfactants, /2-naphthol complex, and /p-iodophenol complex homologs, respectively.
ments it was established that those complex species behaved themselves quite differently from their mother surfactants, showing their own characteristic CMC’s and Krafft points and subsequently that they were novel surfactant species different from their mothers. The finding that the anionic surfactant molecular complexes are another novel kind of surfactant species was still more evidently confirmed through the fact that the homologous series of the complex surfactant species satisfied the well-known linear relationship log (CMC) Å A{BN with different slopes from each other and from their mother species. As recognized through the method which allows us to determine complex molar composition by electric conductivity measurements based on comparing behaviors of solubilized solution systems of various molarity with that of the system of the complex solution and on selecting the system of identical behavior to each other, it was also deduced that a solubilized solution system composed of a surfactant and an additive in equal molarity to that of a complex composition was perfectly identical to the complex solution system and, hence, that a solubilized solution system was only a dissolution system of spontaneously yielded complex species in the course of solubilization.
TABLE 4 Constants in the Relationship of log(cmc) Å A–BN, Determined in the Anionic Surfactant Homologous Series and Its Complex Species at 457C Homolog
A
B
Surfactanta Surfactant 2-naphtholb /p-iodophenolb
1.083
0.265
0.397 0.811
0.232 0.265
a
As for the following homologous series of SLS, SMS, SCS, and SSS. Complex homologous series composed of the surfactants cited above and each additive. b
coidal
513
NOTE
ACKNOWLEDGMENT The authors express their gratitude to Wakayama Lab. of Kao Soap Co. Ltd. for its kind gift of anionic surfactants of a series of the sodium alkylsulfate homologs.
REFERENCES 1. Hirata, H., Kanda, Y., and Sakaiguchi, Y., Bull. Chem. Soc. Jpn. 62, 2461 (1989). 2. Hirata, H., Kanda, Y., and Ohashi, S., Colloid Polym. Sci. 270, 781 (1992). 3. Hirata, H., and Iimura, N., J. Colloid Interface Sci. 157, 297 (1993). 4. Sawada, K., Ohashi, Y., Iimura, N., and Hirata, H., Presented at 1996 Annual Meeting of the Crystallographic Society of Japan, Himeji, 1996. 5. Klevens, H. B., Chem. Rev. 47, 1 (1950). 6. McBain, J. W., and Hutchinson, E., ‘‘Solubilization and Related Phenomena.’’ Academic Press, New York, 1955. 7. Elworthy, P. H., Florence, A. T., and Mac Farlane, C. B., ‘‘Solubilization by Surface Active Agents and its Application in Chemistry and Biological Sciences.’’ Chapman and Hall, London, 1968. 8. Mittal, K. L. (Ed.), ‘‘Micellization, Solubilization, and Microemulsion.’’ Plenum, New York, 1977.
AID
JCIS 4981
/
6g2e$$$281
07-11-97 07:48:03
9. Hirata, H., Ohira, A., and Iimura, N., Langmuir 12, 6044 (1996). 10. Hirata, H., Yagi, Y., and Iimura, N., J. Colloid Interface Sci. 173, 151 (1995). 11. Krafft, F., and Wiglow, H., Chem. Ber. 28, 2566 (1895); Krafft, F., Chem. Ber. 32, 1596 (1899). 12. Mazer, N. A., Benedek, G. B., and Carey, M. C., J. Phys. Chem. 80, 1075 (1976). 13. Ino, T., Nippon Kagaku Zasshi 80, 465 (1959). 14. Klevens, H. B., J. Am. Oil Chem. Soc. 30, 74 (1953). 15. Herrman, K. W., J. Phys. Chem. 66, 295 (1962). 16. Goddard, E. D., and Benson, G. C., Can. J. Chem. 35, 986 (l957). 17. Flockhart, B. D., J. Colloid Sci. 16, 484 (1961). 18. Evans, H. C., J. Chem. Soc. 579 (1956). 19. Go¨tte, E., Int. Congr. Surf. Act. 3rd. 1, 45 (l960). Hirotaka Hirata 1 Nahoko Iimura Niigata College of Pharmacy 2-13-5 Kamishin’eicho, Niigata, Japan Received December 10, 1996; accepted May 9, 1997
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