Growth of AOT Reversed Micelles and the Solvent Effect Investigated by Dielectric and Light-Scattering Measurements

Abstract

The relative permittivity (εr) and the conductivity (κ) at a frequency (f) of 50 kHz, and the dynamic light-scattering were measured for ternary systems of (AOT + r·H2O) solved in cyclohexane, heptane, octane, decane, and dodecane at a temperature of 298.15 K as a function of the amount ratio (r) of water to AOT (r = [H2O]/[AOT]) up to the onset of percolation. The molality (m) of AOT for those solutions was 0.15 mol kg−1, and increased to 0.30 mol kg−1 for solutions with dodecane. By increasing r, spherical reversed micelles were formed at r ≈ 6 in straight-chain hydrocarbons. The light-scattering measurements indicated that the mean hydrodynamic diameter of the micelles increased with increasing r, showing a sigmoid curve. The growth of micelles was promoted in a longer chain hydrocarbon. The percolation transition occurred as the particles grew to a critical diameter of about 50 nm. On the other hand, the growth of particles in c-hexane was moderate, and percolation did not occur up to r = 90, where the solution turned turbid. The growth of particles found from light-scattering measurements was well correlated to dielectric observations.

Studies on the hydration of Aerosol OT and the formation of reversed micelles in decane by a calorimetry and ab initio calculations

“…The purification of AOT (Aldrich Chemical Co., mole fraction purity >0.99) to remove salts was similar to that in the previous process [2,21]. According to the Karl FischerÕs method, the mole fraction of water in AOT was 0.02.…”

“…As those assemblies grow they form rod-like micelles, and then bi-continuous microemulsion, from the measurements of heat capacity [20] and dielectric properties [2,21]. It is expected that the water in w/o type particles becomes bulky with increasing r due to the network formation of hydrogen-bonds.…”

“…Previously, we reported the results of ab initio calculations for AOT in different conformations [2]. In the present calculations, the hydrocarbonchains of AOT were exchanged with methyl groups to reduce the number of atoms.…”

“…Aerosol OT {AOT; sodium bis (2-ethylhexyl) sulfosuccinate} is one of the most studied surfactants since it forms a stable w/o microemulsion without adding a co-surfactant, and its organized solutions reveal an excellent ability to solve a substantial amount of water in oil. In order to study the structure and the physicochemical properties of w/o microemulsion various techniques, such as light scattering [1,2], X-ray scattering [3,4], spectroscopy by IR and Raman [5][6][7][8], dielectric spectroscopy [9][10][11][12], and NMR [13,14] were employed. In earlier studies, it was commonly considered that AOT forms reversed-micelles in hydrocarbons without water [15][16][17][18][19].…”

“…We found that water is necessary for the formation of reversed-micelles by means of heat capacity measurements [20]. We also suggested that spherical reversed-micelles are created from anti-symmetric premicelles [2,21]. Thus, the molecular interactions that act between the surfactant and water molecules are the essential subject to understand the formation of assemblies.…”