Decanol Effect on Micellar Structure and Phase Transitions Cila ˆ ine Vero ˆnica Teixeira, Rosangela Itri,* and Lia Queiroz do Amaral Instituto de Fı ´sica da USP, C.P. 66318, Sa ˜ o Paulo, 05315-970, Brazil Received May 22, 1998. In Final Form: October 5, 1998 Concentrated micellar solutions of the binary system sodium lauryl sulfate-water and the ternary system sodium lauryl sulfate-water-decanol have been studied by X-ray diffraction in the vicinity of the isotropic (I)-type I hexagonal (HR) liquid-crystal phase transitions. Results show how decanol affects the I f HR phase transition. In the binary system there is a close packing of micelles in a local hexagonal order in the I phase, and the HR phase has a behavior typical of finite/hard objects. In contrast, in the ternary system the close packing is relaxed through a micellar growth, and the HR phase has a behavior typical of infinite/flexible objects. An intriguing phase sequence HR-nematic cylindrical (Nc) with an increase in the decanol content is also investigated, and it is attributed to a combined effect of micellar growth and change in rigidity with decanol addition. I. Introduction Concentrated solutions of amphiphilic materials in water present a rich polymorphism 1,2 depending on concentration and temperature. The typical sequence of positional growing order with increasing amphiphile concentration is isotropic (I)-type I hexagonal (H R )- lamellar (L R ), where the R symbol refers to disordered paraffin chains. Addition of a long-chain alcohol and/or salt to concentrated amphiphilic/water systems can yield lyotropic nematic N phases with finite anisotropic ag- gregates having long-range orientational order. 3,4 The role of alcohol in changing the aggregation process, leading to islands of nematic phases in ternary phase diagrams, is not yet understood. The amphiphile sodium dodecyl (lauryl) sulfate (SLS) has been widely studied in isotropic micellar solutions. The binary SLS/water system shows H R and L R phases 2 with some intermediate phases with long-range positional order, 5 while a nematic domain 6-8 is obtained in the ternary system by decanol addition. The SLS/water system has been studied by small-angle X-ray scattering (SAXS) in our group from lower concen- trations 9 up to the vicinity of the I-H R phase transition, 10,11 which occurs at 40 wt % of SLS (mole ratio water/SLS, M w ) [water]/[SLS], of 24.0), as well as its H R domain. 12 In addition, I samples with decanol addition to the binary system with 26 wt % of SLS (M w ) 45.2) up to mole ratio decanol/SLS, M d ) [decanol]/[SLS], of 0.139 were also previously investigated. 11 Further, an H R phase and a nematic cylindrical (N c ) were encountered at M d ) 0.195 and 0.286, respectively. 11 The sequence of phases I-H R - N c -N d (nematic discotic)-L R was observed by Quist et al. 8 in the region of higher amphiphilic concentration. The phase diagram of Quist et al. 8 reproduced the nematic domain previously found by Amaral et al. 6,7 at lower SLS concentration and the limits of the H R phase obtained by Ekwall 2 when the nematic domain was not known yet. In a previous paper, 11 analysis of the isotropic curves in terms of particle form factor and interference function 13 allowed the determination of the paraffinic micellar anisometry, which corresponds to the ratio between the longest and the shortest axes of a paraffinic prolate ellipsoid. Results were compared with theories of self- assembling, 14 which predicted I-H and I-N-H phase transitions. However, some questions remain open such as how decanol affects the micellar structure and induces phase transitions. Decanol allows the appearance of an inter- mediate long-range orientational order phase (nematic) between two long-range positional order phases, H R and L R , in contrast with the system without decanol. It also reverts the phase sequence, since the transitions observed with decanol addition are I-H R -N c , while theories predict I-N-H. In a recent paper 15 the N c -N d transition in three different amphiphilic systems was explained in terms of changes in the micellar shape (from spherocylinder to planar) induced by requirements of the elastic bending energy of the polar/apolar interface. This happens because it is energetically favorable for the decanol to stay at flatter interfaces. To investigate the micellar structures at I-H R -N c phase transitions, samples at fixed M w ) 45.2 with M d ranging from 0.14 to 0.38 are now studied by means of X-ray diffraction, complementing the previous work. 11 The results show a narrow H R domain, confirming the phase sequence I-H R -N c -N d . Accordingly, both binary and (1) Luzzati, V. In Biological Membranes; Academic: London, 1968; Chapter 3, p 71. Tiddy, G. J. T. Phys. Rep. 1980, 57, 2. (2) Ekwall, P. In Advances in Liquid Crystals; Academic: London, 1975; Vol. 1, p 1. (3) Amaral, L. Q.; Pimentel, C. A.; Tavares, M. R.; Vanin, J. A. J. Chem. Phys. 1979, 71, 2940. (4) Charvolin, J.; Levelut, A. M.; Samulski, E. T. J. Phys. Lett.s Paris 1979, 40, L587. Hendrikx, Y.; Charvolin, J. J. Phys.sParis 1981, 42, 1427. (5) Kekicheff, P.; Cabane, B. J. Phys.sParis 1987, 48, 1571. (6) Amaral, L. Q.; Helene, M. E. M.; Bittencourt, D. R.; Itri, R. J. Phys. Chem. 1987, 91, 5949. (7) Amaral, L. Q.; Helene, M. E. M. J. Phys. Chem. 1988, 92, 6094. (8) Quist, P. O.; Halle, B.; Furo ´, I. J. Chem. Phys. 1991, 95, 6945. (9) Itri, R.; Amaral, L. Q. J. Phys. Chem. 1991, 95, 423; J. Appl. Crystallogr. 1994, 27, 20. (10) Itri, R.; Amaral, L. Q. J. Phys. Chem. 1990, 94, 2198. (11) (a) Itri, R.; Amaral, L. Q. Phys. Rev. E 1993, 47, 2551. (b) Erratum in Phys. Rev. E 1988, 58, 1173, regarding the H R phase in the ternary system in the phase sequence I-HR-Nc with increasing decanol amount. (12) Amaral, L. Q.; Gulik, A.; Itri, R.; Mariani, P. Phys. Rev. A 1992, 46, 3548. Itri, R.; Amaral, L. Q.; Mariani, P. Phys. Rev. E 1996, 54, 5211. (13) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 6, 651. (14) Taylor, M. P.; Herzfeld, J. Phys. Rev. A 1991, 43, 1892. (15) Amaral, L. Q.; Santin Filho, O.; Taddei, G.; Vila-Romeu, N. Langmuir 1997, 13, 5016. 936 Langmuir 1999, 15, 936-939 10.1021/la980606p CCC: $18.00 © 1999 American Chemical Society Published on Web 01/22/1999