Low-angle X-ray diffractions observed in highly viscoelastic solutions of cationic surfactant-aromatic additive systems

LETTER TO THE EDITOR Low-Angle X-Ray Diffractions Observed in Highly Viscoelastic Solutions of Cationic Surfactant-Aromatic Additive Systems In some systems composed of CTAB, a cationic surfactant, and aromatic additives such as salicylic acid or its alkaline metal salts in which a remarkable viscoelasticity has been reported to be induced, the authors have found very strong X-ray diffractions at low-angle regions ranging over 20 = 1°-5°. It seems that this fact could provide a key to solving the viscoelastic solution structure. © 1988AcademicPress,Inc. Several cationic surfactants, cetyltrimethylammonium bromide (CTAB), cetylpyridinium bromide (CPB), etc., induce a remarkable viscoelasticity in solution even in very diluted states when added with some aromatic substances such as salicylic acid or its sodium salt and some other phenolic derivatives (1). The authors have already reported the occurrence of enormously elongated gigantic micelles in the system, entanglement of which might induce the viscoelasticity in solution through image detection under an electron microscope (2). We believe that the findings have provided firm evidence of the so-called gel structure where fibrous materials entangle each other so as to frame networks (3). Extensive data about these systems through measurements of light scattering (4), neutron scattering (5), etc. have been accumulated in order to obtain knowledge of micelle size and shape, but it appears that there are yet many ambiguities: we cannot recognize the solution behavior inclusively. In these circumstances we have recently found that very strong X-ray diffraction can be observed at the low-angle region of 20 = 1°-5°. In Fig. 1 sharp diffraction peaks are shown as an example observed in a solution of CTAB-

cesium salicylate (CS). Several s h a ~ diffraction peaks are observed only at the low-angle region; in the wider-angle region only a broad and diffuse peak is observed. Judging from the 20 values obtained, it is clearly shown that no structures exist in a shorter range on the order of several angstroms but do exist only in very long spacings on the order of 30-40 A or more. The specimen solution of CTAB-CS accompanies a remarkable viscoelasticity and a soft gel appearance and except for emitting a faint opalescence, it is still a clear fluid with high viscosity. It seems that this fact clearly represents the occurrence of considerably large bodies that scatter the natural sunlight, but none of the crystalline suspension, of course, was found in specimens. The general features in the wider-angle region as seen in Fig. 1 also support this observation. The fact that any diffraction is not found there reflects that there is no small spacing structure based on a crystalline material in the solution. Any surfactant is known to provide characteristic liquid crystalline phases under a considerably high concentrated condition but the solution in question is very dilute containing materials less than 3-4% at most. By using a polarizing microscope we proved the state of the solution to be isotropic.

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55

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FIG. 1. X-ray diffraction pattern in the system of CTAB-CS at the molar ratio CS/CTAB = 1.0, in CTAB concentration 1 X 10 -1 mole/liter. 3O0 0021-9797/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, VoL 121, No. 1, January 1988

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LETTER TO THE EDITOR Other examples of the systems examined that exhibited sharp diffractions are summarized in Table I. All of these systems providing strong diffractions were, of course, highly viscoelastic. Those that induced viscosity only in solution, such as a system of CTAB-cyelohexane carboxylic acid, did not show any diffraction nor, of course, did those in simple fluid systems without any behavior. It is interesting to note the time dependence in diffraction patterns. Such an example is shown in Fig. 2, where the two sharp diffraction peaks, those which assumed the initial profile as seen in Fig. 1, represent a clear trend in exchanging their intensities for about 1 h or so. The time-dependent nature which appears in every measurement is characteristic of these highly viscoelastic and gel-forming systems (6). These effects of aging might be due to a change of solution structure, such as destruction and reformation, which may be induced by a process of sample charging into a cell, or for other reasons. In addition to aging, thermal effects, that is, annealing, are also evident in every specimen solution examined. Those will be gathered and reported in detail in a future publication. According to these facts the sharp diffractions observed at the low-angle region appear to be entirely systematic, being involved only in the viscoelastic systems examined. The sharp and extremely intense diffraction peaks imply the occurrence of a firmly constructed ordered structure even in diluted and apparently isotropic solutions. Studies about this structure, based on a diffraction measurement using much higher-energy beams and SAX in connection with the previously obtained electron microscopic images, are under way. What ordered structures exist and how they are constructed in the solution will be explained according to the data provided and will soon be reported elsewhere. X-ray diffraction measurement was carried out by using a Rigaku Denki Co. D/MAX-RVA rotating anode highpower X-ray refractometer (vertical type), which was furnished with a vertically arranged goniometer and a conventional counter to be streamed by a reflected X-ray beam at the specimen surface of a flatly held sample. The X-ray TABLE I Other Diffraction Data in CTAB-Additive Systems Additive

20 (°)

d (~)

Remarks

Naphthylamine

2.78 5.40

31.5 16.4

Very strong Strong

Potassium biphthalate

2.45

36.04

Weak and broad

Salicylic acid

1.51

58.48

Very strong

o-Iodophenol

3.40 4.35

25.96 20.30

Very strong Strong

Note. CTAB concentration was 1 × 10-~ mole/liter and the mixing molar ratio, additive/CTAB, was always unity.

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FIG. 2. X-ray diffraction patterns showing aging effects of a specimen that is the same as that in Fig. 1. Time lapse is counted in minutes starting at (a), which assumed as an origin, but it should be noted that those are retrospective to the profile shown in Fig. 1 (see text); (b) after 12 rain; (c) after 22 min; and (d) after 65 min, respectively.

beams used were CuKc~ radiation selected by a curved graphite monochrometer. ACKNOWLEDGMENTS We thank Dr. Y. Kitayama of Niigata University for her helpfttl suggestions and ardent encouragement and Mr. H. Minakawa for his support in operating the X-ray apparatus. REFERENCES 1. Nash, T., ,L Col[oidSci. 13, 134 (1958); Hirata, H., Thesis, Osaka University, 1959. 2. Sakaiguchi, Y., Shikata, T., Uraami, H., Tamura, A., and Hirata, H., J. Colloid Interface Sci. 119, 291 (1987). 3. Ferry, J. D., Adv. Protein Chem. 4, 1 (1948); Katchalsky, A., Prog. Biophys. Biophys. Chem. 4, 1 (1954). 4. Imae, T., and Ikeda, S., J. Colloid Interface Sci. 98, 363 (1984). 5. Hoffmann, H., Kalus, J., and Thrun, H., Ber. Bunsenges. Phys. Chem. 87, 1120 (1983). 6. Hirata, H., private communications. HIROTAKA HIRATA YUKO SAKAIGUCHI

Department of Physical Chemistry Niigata College of Pharmacy Niigata, Japan 950-21 Received June 8, 1987; accepted September 30, 1987 Journal of Colloid and Interface Science, Vol. 121, No. 1, January 1988