Small-angle neutron scattering from microemulsions near the disorder line in water/formamide–octane-CiEj systems

Abstract

A small-angle neutron scattering study of nonionic surfactant molecules at the water-oil interface: Area per molecule, microemulsion domain size, and rigidity Small angle neutron scattering near Lifshitz lines: Transition from weakly structured mixtures to microemulsions Small-angle neutron scattering measurements were performed on symmetric microemulsions containing equal volume fractions of water/formamide, n-octane, and a sufficient amount of nalkyl polyglycol ether (CiE j ). By changing the surfactant chain length in the order C 8 E 3 , . C 6 E 2 , and C 4 E 1 in pure water, as well as for given CgE3 and C 6 E2 by increasing the relative amount of formamide in the water/formamide mixture, the amphiphilic strength (the amphiphilicity) is reduced. We observe that the characteristic scattering peak becomes weaker and its position moves into q = 0 showing that the microstructure becomes disordered.However, a q -2 and a q -4 behavior for large q in film and bulk contrast, respectively, is still observed indicating the persistence of internal interfaces. As the microstructure is further weakened, a disorder line is passed at which the behavior of the real space correlation function changes from a damped oscillatory behavior to a monotonically decreasing one. We determine that this line has been passed from fits to the bulk scattering intensity. Simultaneously, the film scattering intensities show a transition from correlated to uncorrelated films as the disorder line is passed. Closely related to the weakening of the microstructure is the occurence of a n~nwetting ~ wetting transition in the vicinity of a Lifshitz line. Roughly speaking, mlcroemulslOns, that do show a scattering peak do not wet the water-oil interface. As predicted by Landau-Ginzburg theories, the wetting transition occurs on the microemulsion side of the disorder line. As the amphiphilicity is even further reduced, a tricritical point is reached with the interesting observation that substantial local structure still persists. 8532

Structural Polymorphism in a Four-Component Nonionic Microemulsion

“…Two-shell micelle model of radius R. R c is the radius of the hydrocarbon core and t m is the thickness of the polar mantle. The electron density of the core, r c , is taken as 00.051 e/Å 3 relative to water (0.33 e/Å 3 ) and the A third parameter, which can also be defined, is f a , the electron density of the mantle, r m , is calculated from molecular composition and amphiphilicity factor (16)(17)(18), which relates to the behavior volumes to be 0.026 e/Å 3 relative to water, as described under Experimental.…”

“…The swelling with water can be described by (55) oil and water domains. In our case, we know, based on the size of the C 12 (EO) 8 molecule ( 23) , that the surfactant must play a dominant role in determining the thickness of the [16] nonwater domains. Consequently, although from a geometrical point of view, neither the oil nor the water are completely circumscribed by surfactant, still the obstruction to molecular diffusion provided by the surfactant molecules prevents where d is the measured periodicity, d 0 is the thickness of the (surfactant / oil / alcohol) layer.…”

Introductory Chapter: Microemulsions

“…However, due to the confluence of particle size variation and shape anisotropy, analyzing, and deconvoluting, these data may not always be straightforward. SANS is particularly versatile and can characterize the structure, and interactions of colloidal systems with a wide range of interactions as well as differences in orientation and droplets with non-spherical shapes [17].…”

The Effect of Anomeric Head Groups, Surfactant Hydrophilicity, and Electrolytes onn-Alkyl Monoglucoside Microemulsions

“…The procedure used to determine the phase boundaries for sections through the phase prism for the water-C k OC 2 OC k -C m G 1 mixtures follows closely that of Schubert and Strey (25). Phase boundaries were determined with a precision of Ϯ0.05°C.…”