Electrical conductivity and self diffusion-NMR studies of the system: Water/sucrose laurate/ethoxylated mono-di-glyceride/isopropylmyristate

Abstract

We studied the electrical conductivity and the self diffusion behaviors of cosmetic and pharmaceutical grade mixed nonionic surfactant microemulsion systems in order to determine the microstructural association changes. The systems studied were: water (W)/sucrose laurate (L1695)/ethoxylated mono-di-glyceride (EMDG)/isopropylmyristate (IPM) for the self diffusion investigations. For the electrical conductivity determination the water was replaced by an aqueous solution of 0.05 M NaCl. The system studied by electrical conductivity has a mixed surfactant content γ equals 31 wt% and a weight ratio of EMDG in the surfactants mixture δ equals 50 wt%. The mass fraction of NaCl in the aqueous phase, ɛ, equals 0.29 wt%. Electrical conductivity was determined as function of temperature and the mass fraction of oil in the mixture of water and oil (i.e. α (wt%)). The electrical conductivity ( κ) of the system varies from 4.3 × 10 −3 at α = 10 wt% to 1.3 × 10 −4 at α = 90 wt% and varies linearly with temperature. The system: W/L1695/EMDG/IPM was investigated using pulsed gradient spin echo-nuclear magnetic resonance (PGSE-NMR) in the following two mixed surfactant contents and distributions: [A] γ = 31 wt%, δ = 50 wt% and [B] γ = 26 wt%, δ = 25 wt%. It was found that the relative self-diffusion coefficients ( D/ D 0) of water and oil vary as α varies. In the case where γ = 31 wt%, δ = 50 wt%, the relative diffusion coefficients ( D/ D 0) decrease from 0.83 to 0.11 in the case of water, and increase from 0.001 to 0.36 in the case of oil as α varies from 5 to 85 wt%. The hydrodynamic radius ( R H) and the area ( a) per polar head group were estimated as 7.3 nm and 75 Å 2 for α = 10 wt%, γ = 31 wt%, and δ = 50 wt%.