S. Ezrahi

Dielectric and Calorimetric Characteristics of Bound and Free Water in Surfactant-Based Systems

ABSTRACT The hydration behavior of the system polyoxyethylene (10) oleylalcohol [C18:1 (EO)10 or Brij 97]/water/dodecane/butanol (model system B) was investigated along a dilution line for which the respective weight ratio of dodecane:butanol:Brij 97 is 3:3:4. Two experimental methods were applied: time domain dielectric spectroscopy (TDDS) and sub-zero temperature differential scanning calorimetry (SZT-DSC). Two types of bound water (with melting peaks at -25 and -11°C) were detected by SZT-DSC (using the endothermic mode), whereas TDDS revealed only one such type. Nevertheless, roughly the same total amount of bound water was estimated from these two techniques. The average number of bound water molecules per ethylene oxide (EO) group, NW/EO is 2.3 (TDDS data) or 2.5 (SZT-DSC data) in good agreement with the observation that 1–2 water layers are formed in the hydration of ethoxylated surfactants. We have also shown that butanol is involved in the formation of the bound water that melts at -25°C. We suggest that butanol molecules occupy binding sites within the second hydration shell, thereby reducing the expected total bound water content.

Sub-zero temperature behavior of water in non-ionic microemulsions

Sub-zero temperature DSC measurements were conducted to evaluate the behavior of water in non-ionic microemulsions. Two surfactant systems were studied. The first, based on ethoxylated fatty alcohol, octaethylene glycol monon-dodecylether [hereafter referred to as C12(EO)8] and also containing water, pentanol and dodecane at a fixed weight ratio of 1:1. The second system, based on oligomeric ethoxylated siloxanes, water and dodecanol as oil phase. In both systems it was found that in up to 30 wt.% of the total water content, all water molecules solubilize in the amphiphilic phase and are bound to the ethylene oxide (hereafter referred to as EO) head-groups. No free water exists in the surfactant aggregates’ core. Up to three molecules of water are bound to each EO group.In the first system, the behavior changes significantly upon adding more water. The added pentanol allows further swelling and the water penetrates into the amphiphile structures and forms a reservoir of free water. Structures are deformed and grow from elongated channels (up to 15–20 wt.% water), via illdefined (one-dimensional growth) local lamellar structures (up to ca. 60 wt.% water) to spherical normal, O/W micelles (at ≥85 wt.% water).In contrast, the oligomeric systems, due to geometrical restrictions of the amphiphiles and the nature of their curvature that prevents inversion, cannot further solubilize water in the surfactant aggregates’ core, causing phase separation to occur.

Time domain dielectric study of non-ionic microemulsions☆

Abstract The dielectric behavior of a microemulsion prepared from dodecane, butanol, water and polyoxyethylene (10) oleylalcohol (Brij 97) has been studied by time domain dielectric spectroscopy over the frequency range of 10 5 –10 10 Hz for temperatures between 10 and 40°C with varying water content. Several dielectric dispersion regions with the typical characteristic relaxation times of the order of nanoseconds and tens of picoseconds are observed. They are analyzed in time domain in terms of macroscopic dipole correlation functions and are attributed to the relaxation mechanisms of the different chemical compounds within the microemulsion. The appearance and intensity of the peaks for the relaxation processes depend on the water content. The types of motion of different kinetic units that can be manifested as local relaxation processes are proposed. The dielectric properties of microemulsions depend on the underlying phase behavior and on the consequent morphology in these systems. The results suggest that changes in the microstructure of polarization and relaxation mechanisms occur in the water fraction content intervals of 5–10, 10–35 and 35%.