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.
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-).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.