J. V. Joshi

Combined SANS and SAXS studies on alkali metal dodecyl sulphate micelles

Combined small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS) studies have been performed on charged micelles of anionic surfactants of alkali metal dodecyl sulphate (MDS, M = Li, Na, Rb and Cs) in aqueous solution. The dimensions of the micelles have been obtained using SANS as the neutrons are mostly scattered by the hydrogenous core of the micelles. The contrast for the head groups and the counterions is very small, so they are not seen with the neutrons. On the other hand, x-rays are scattered by the electron density fluctuation, which is proportional to the atomic number, and thus x-rays have been used to see the high-Z components of the micelles directly. SAXS from LiDS gives the headgroup thickness, and the counterion condensation is measured directly by the scattering from the high-Z counterions of MDS (M = Na, Rb and Cs) micelles. Based on the results of these two techniques, the role of the different counterions in deciding the structure of micelles has been explained in terms of counterion condensation and the thickness over which they are condensed around the charged micelle. It is found that as the hydrated size of the counterion decreases, the counterion condensation increases and counterions are condensed over a smaller thickness, which results in larger charge neutralization and hence a larger size of the micelles.

Viscoelastic behaviour of cetyl trimethyl ammonium bromide/sodium salicylate/water system: Effect of solubilisation of different polarity oils

Rheological behaviour of cetyl trimethyl ammonium bromide (CTAB)/sodium salicylate/oil/water system (being termed as oil-in-water microemulsions) was studied. Three monomers with different polarities, viz. acrylic acid, methylmethacrylate and styrene, were used to formulate isotropic oil-in-water microemulsions. Compositions of all the systems studied were on the boundary of the semidilute and concentrated regime (<5 wt.%). CTAB/NaSal mole ratio was kept constant ([CTAB]/[NaSal]=1) and effect of the solubilisation of oils on the viscoelasticity was studied. All measurements were made in linear viscoelastic region. Microemulsions formulated were found to obey Maxwells model. Various parameters such as G′, G′′, η*, η0 and τm obtained by rheological studies were correlated with polarity and partitioning behaviour of monomers in between continuous phase and micelles. Characteristic relaxation time τm for all the systems studied was in the range 0.02–0.56 s. The mechanism of the relaxation behaviour of the systems was discussed on the basis of the ‘existing models’ proposed for the thread-like micellar systems. It is seen that the nature of oil is quite important in deciding the viscoelastic properties of above microemulsions. The behaviour of the oils was compared with that of hydrocarbons and alcohols.

Small-angle neutron scattering studies of sodium butyl benzene sulfonate aggregates in aqueous solution

The aggregation behaviour of a hydrotrope, sodiumn-butyl benzene sulfonate (Na-NBBS), in aqueous solutions is investigated by small-angle neutron scattering (SANS). Nearly ellipsoidal aggregates of Na-NBBS at concentrations well above its minimum hydrotrope concentration were detected by SANS. The hydrotrope seems to form self-assemblies with aggregation number of 36–40 with a substantial charge on the aggregate. This aggregation number is weakly affected by the hydrotrope concentration.

Small-angle neutron scattering studies of nonionic surfactant: Effect of sugars

Micellar solution of nonionic surfactantn-dodecyloligo ethyleneoxide surfactant, decaoxyethylene monododecyl ether [CH3(CH2)11(OCH2CH2)10OH], C12E10 in D2O solution have been analysed by small-angle neutron scattering (SANS) at different temperatures (30, 45 and 60° C) both in the presence and absence of sugars. The structural parameters like micelle shape and size, aggregation number and micellar density have been determined. It is found that the micellar structure significantly depends on the temperature and concentration of sugars. The micelles are found to be prolate ellipsoids at 30° C and the axial ratio of the micelle increases with the increase in temperature. The presence of lower concentration of sugar reduces the size of micelles and it grows at higher concentration of sugar. The structure of micelles is almost independent of the different types of sugars used.

Small angle neutron scattering and viscosity studies of micellar solutions of bis-cationic surfactants containing hydroxyethyl methyl quaternary ammonium head groups

The effect of head group polarity and spacer chain length of bis-cationic surfactants on shape, size and rheology of surfactant aggregates in aqueous solution has been investigated through small angle neutron scattering (SANS) and viscosity measurements. A series of bis-cationic surfactants C12H25N+(CH3)(C2H4OH)–(CH2)s–N+(CH3)(C2H4OH)C12H25 2Br−, referred to as 12-s-12 MEA, where two highly polar quaternary amine centers are covalently connected through polymethylene spacers at head group level, has been synthesized and characterized. Critical micellar concentration of these surfactants in aqueous solution determined by tensiometry and conductometry was observed to be 10–100 times lower than conventional bis-cationic surfactants [C12H25N+(CH3)2–(CH2)s–N+(CH3)2C12H25 2Br−, referred to as 12-s-12 DMA]. The extent of aggregate growths and variation in shape and size of micelles was observed to be strongly dependent upon head group polarity, spacer chain length and experimental conditions such as concentration and temperature.

Structural Changes in Micelles of Different Sizes on Hydrocarbon Solubilization as Studied by SANS

Micellar structures of cationic alkyltrimethylammonium bromide (CnTAB) surfactants in aqueous solution on the addition of hydrocarbon have been studied by small‐angle neutron scattering (SANS). Measurements have been performed for different chain lengths of the surfactants (n=12, 14, and 16) at a fixed concentration (0.1 M) and varying concentration of two hydrocarbons, benzene and hexane, in the range 0 to 0.2 M. Data have been analyzed for interparticle structure factors considering the screened Coulomb interactions between the charged micelles and single particle form factors calculated by treating these micelles as prolate ellipsoid. It is found that the effect of addition of hydrocarbon is more pronounced for larger size of the micelles having longer chain lengths and also for the hydrocarbon benzene than hexane. The axial ratio of the micelles shows a large increase and the effective charge on the micelles decreases on addition of hydrocarbon. There is only a small increase in semiminor axis on addition of hydrocarbon, which suggests that the distribution of hydrocarbon is uniform inside the micellar core.

SANS Study of Micellar Structures on Oil Solubilization

Micellar structures of cationic alkyltrimethylammonium bromide (CnTAB) surfactants for different chain lengths (n = 12, 14 and 16) in aqueous solution on the addition of two oils benzene and hexane have been studied by small‐angle neutron scattering (SANS). Measurements have been performed for fixed 0.1 M concentration of surfactant and oil. It is found that the size and the aggregation number of the micelles increase on oil solubilization with increase in oil concentration. The effect is more pronounced for larger size of the micelles having longer chain length and also for the oil benzene than hexane. The axial ratio of the micelles shows a large increase and the effective charge on the micelles decreases on addition of oil. There is only a small increase in semiminor axis on addition of oil, which suggests that distribution of oil is uniform inside the micellar core.