J . Phys. Chem. 1991,95, 5385-5386
5385
Cubic Phase Formation In Polar Nonaqueous Solvents T. A. Bleasdale, C.J. T. Tiddy,* and E. Wyn-Jones Department of Chemistry and Applied Chemistry, University of Salford, The Crescent, Savord, England M5 4 WT (Received: April 17, 1991)
We report the first observation of surfactants that form a cubic (I,) mesophase in the nonaqueous polar solvents glycerol and formamide where they do not m u r with water. Two series of cationic surfactants (N-alkylpyridinium chlorides, C,,PyCI, n = 12-16, and N-alkylpyridinium bromides, C,,PyBr,n = 12-20) were examined by polarizing microscapy in water, ethylene glycol, glycerol, formamide, and ethylammonium nitrate. Cubic (I,) mesophases were observed with water (CI2PyCI,C,QYcI), glycerol (c16w1), and formamide (C,&Cl, C18PyBr,C a B r ) , but none occumd with ethylene glycol and ethylammonium nitrate. The results indicate that ionic strength and the extent of specific counterion binding are key factors for cubic phase formation.
Introduction The formation and structure of lyotropic liquid crystals (or mesophases) with water as the solvent have been much studied over the years'-' These have shown that the most common mesophases are hexagonal (H1/H2),cubic (11/12and Vl/V2), and lamellar (La). The 11/12phases are made up of small globular micelles packed in a cubic array and are found between the micellar solutions and HI/H2phases. The Vl/V2 phases are a bicontinuous network of both surfactant and solvent with cubic symmetry. These are found between HI/H2and La.Recently, however, attention has begun to focus on the use of highly polar nonaqueous solvents as substitutes for water." Solvents such as ethylene glycol, formamide, and glycerol have all been shown to form mesophases with ionic and/or nonionic surfactants. For example, cetyltrimethylammonium bromide (CTAB)5-7 forms hexagonal (HI),bicontinuous cubic (VI),and lamellar (La) phases with formamide, glycerol, and ethylene glycol in an analogous way to the aqueous system. Longer chain poly(oxyethy1eneglycol) alkyl ether surfactants (C16E04+6E08)8 also exhibit HI phases (E06/E08) and a Laphase (EO,) with formamide. In addition, both the CI&08/formamide and aqueous systems form an Il cubic phase between the micellar solution and H1 phase. To date, however, there appears to be no account of an 1, cubic phase with ionic surfactants in any solvent other than water. In this Letter we are reporting not only the first II phases with cationic surfactants in formamide and glycerol but also that these occur with a bromide salt derivative using formamide. (In aqueous/cationic surfactant systems I, phases are usually restricted to chlorides only.) The surfactants studied are a series of Nalkylpyridinium chlorides (C,PyCI, n = 12-16) and N-alkylpyridinium bromides (C,PyBr, n = 12-20).
Experimental Section Materials. Ethylene glycol (99+%), formamide (99+%), and glycerol (99.5%) were used as received from Aldrich Chemical Co. with a water content of
