A fast method for the analysis of the phase diagrams of lyotropic compounds was employed to study the formation of liquid crystals from two surfactants, cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPBr) in water. Studies were also carried out in the protic solvents, glycerol (G), formamide (FA), ethylene glycol (EG), and N-methylformamide (NMF), and the aprotic solvents, dimethylformamide (DMF) and N-methylsydnone (NMS). While the normal succession of ordered phases appeared to be governed by geometric constraints of interface curvature, the differences in behavior were accounted for by the differences in cohesion energy of the solvents and the different natures of the polar heads of the two surfactants. In DMF, a solvent with low cohesion energy, both surfactants showed only lamellar phases, whereas CPBr with a highly delocalized charge on the polar head displayed a succession of conventional phases in all the other solvents. CTAB with a localized charge formed only lamellar phases in NMF and NMS. This behavior was interpreted as resulting from headgroup solvation due to dipoledipole interactions or hydrogen bonding. The particular case of NMS was accounted for by better stacking between the planar molecules of this solvent and the pyridinium rings of CPBr.

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We report here a small angle neutron scattering (SANS) study of the micellization of SDS in formamide. We show that a compound with a hydrophobic chain containing 12 carbon atoms forms micelles in formamide. The mechanism of self-association is in agreement with a multiple equilibrium model rather than a pseudophase model, as the aggregates were shown to increase in size over a large concentration domain. This was attributed to smaller solvophobic effects and lower interfacial energy in formamide than in water. IntroductionThe formation of micelles and ordered lyotropic phases of cationic and nonionic surfactants in formamide (FA) and ethylene glycol (EG) has been demonstrated for compounds whose hydrophobic chains contain 14-16 carbon atoms. 1,19 Micellization is possible as the cohesion energy density of these two solvents is higher than 1.1 J‚m -3 . However the solvophobic effect is lower in formamide than in water and a progressive micellization is observed for chain lengths of 16 carbons and above. The rather high cmc value depends on experimental methods; the ionic surfactant aggregates do not show up well in conductivity measurements; 7 the curves show no clear

The Krafft temperatures and critical micellar concentrations (cmc) of N-alkylpyridinium halides (C m PyX, m = 12, 16, or 20 and X = Cl, Br, or I) in formamide were determined. The size, structure, and charge of the micelles as a function of chain length and nature of counterion were calculated from the small angle neutron scattered intensity. At a chain length of 12 carbons, only small, relatively unstructured aggregates were formed. At a chain length of 16 carbons, some micelles were also observed at a concentration of twice the cmc and above. At a chain length of 20 carbons, micelles were the sole species. The radii of the cores of the spherical micelles in formamide were always lower than the radii of the corresponding micelles in water. The micelles bear a higher charge in formamide than in water, but similar solvation effects were noted in the two solvents on changing the counterion (I-, Br-, or Cl-).

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We report here a study of the micellization of different surfactants to observe the effects of the chain length, counterions and headgroups in a non-aqueous polar solventformamide. Small-angle neutron scattering measurements were performed in a PACE spectrometer in L.L.B. The solvent used was a 2/3 deuterated formamide. The aggregation number and the spontaneity of the association depend on the chain length ; but, in formamide, micelles always are smaller and more ionized than in water. Their growth is much more low in formamide than in water where "rod-like micelles" form especially with Br". CI" in formamide leads preferentially to the formation of small micelles as it does in water. The micellar ordered phase is observed in the binary Cjg Py Cl/formamide system at high concentration. With regard to the headgroup N + (CH 3) 3 , the pyridinium ring encouraged the micellization in formamide close to 20°C.