Micelles of different dimeric amphiphiles Br -, n-C 16 H 33 NMe 2 + -(CH 2 ) m -N + Me 2 -n-C 16 H 33 , Br -(where m ) 3, 4, 5, 6, 8, 10, and 12) adopt different morphologies and internal packing arrangements in aqueous media depending on their spacer chain length (m). Detailed measurements of small angle neutron scattering (SANS) cross sections from different bis-cationic, dimeric surfactant micelles in aqueous media (D 2 O) are reported. The data have been analyzed using the Hayter and Penfold model for macro ion solution to compute the interparticle structure factor S(Q) taking into account the screened Coulomb interactions between the dimeric micelles. The SANS analysis clearly indicated that the extent of aggregate growth and the variations of shapes of the dimeric micelles depend primarily on the spacer chain length. With spacer chain length, m e 4, the propensity of micellar growth was particularly pronounced. The effects of the variation of the concentration of dimeric surfactants with m ) 5 and 10 on the SANS spectra and the effects of the temperature variation for the micellar system with m ) 10 were also examined. The critical micelle concentrations (cmc) and their microenvironmental feature, namely, the microviscosities that the dimeric micellar aggregates offer to a solubilized, extrinsic fluorescence probe, 1,6-diphenyl-1,3,5-hexatriene, were also determined. The changes of cmcs and microviscosities as a function of spacer chain length have been explained in terms of conformational variations and progressive looping of the spacer in micellar core upon increasing m values.

Micellar solutions of a highly hydrophilic ethylene oxide−propylene oxide triblock copolymer, pluronic F88 (EO10 3PO39EO10 3), in aqueous solution are examined by small-angle neutron scattering (SANS) at different concentrations and temperatures and in the presence of different salts. At temperature less than 30 °C, F88 solution in water (5 wt %) showed unimers which are fully dissolved Gaussian chains. The unimer-to-micelle transition takes place when the temperature or concentration is increased. Added neutral salt favors micellization of the copolymer at lower concentration/temperature. At temperature close to ambient, block copolymer forms micelles that consist of a central core, presumably dominated by the propylene oxide blocks, surrounded by a corona of highly hydrated ethylene oxide subchains. SANS analysis shows that the size of hydrophobic core increases as a part of PEO adjacent to the PPO core loses water with increasing temperature or salt (KCl) concentration. The micellar volume fraction increases with increasing concentration of block copolymer. The salting out effect of different electrolytes on structure of micelles is in the order KCI > KBr > KI. The effect of temperature on micellar solutions in the presence of KCl is also examined. It has been observed that the effect of added salt on the structural phase behavior of block copolymer is analogous to that of temperature. In all the above study, micelles are found to be spherical.

Micellar solutions of cationic surfactant cetyltrimethylammonium bromide (CTAB) in the presence of sodium salicylate (NaSal) show a viscoelastic behavior. Small-angle neutron-scattering (SANS) and viscosity studies from CTAB/NaSal micellar solutions are reported. Zero-shear viscosity of these solutions as a function of NaSal concentration (C s ), at all the four measured surfactant concentrations (C d ) of 12.5. 25, 50 and 100 mM, show a double-peak behavior. The effect of C s /C d on two viscosity maxima and a minimum has been examined, and the scaling relations are obtained. SANS experiments have been carried out at different NaSal concentrations, beyond the first viscosity maximum for two surfactant concentrations of 25 and 100 mM. It is found that micelles are rigid rods and their exponential length distribution shows that they behave as living polymers beyond the first viscosity maximum. The micellar structure does not change in the living polymer regime, whereas viscosity varies with increase in NaSal concentration. The variation in viscosity seems to be connected with the change in the intermicelle interactions.