Interfacial Surfactant Mixing

Abstract

This thesis is concerned with the interfacial mixing behaviour of surfactant and biosurfactant systems at the air/liquid and solid/liquid interface. The systems were primarily studied using neutron reflectometry, which was used to investigate the structure and composition of adsorbed surfactant layers at the interface. The motivation for the project is related to the growing desire for more environmentally friendly commercial surfactant formulations. The mixed CMCs and adsorbed surface compositions, at the air/water interface, of SLES EO3/C12E6 and SLES EO3/C12E8 mixtures were investigated at 10 oC, 25 oC, and 40 oC. The data was fitted with the pseudophase approximation (PPA) model using a single interaction parameter. The increase in temperature significantly decreased the CMC values for each mixture, but the surface compositions were largely unaffected by the change in temperature. The PPA model predicted mild synergism, with a change in the free energy upon mixing between -0.2 and -0.6 RT. The mixed CMCs and adsorbed compositions, at the air/water interface, of R2/C12E8 and R2/SLES EO3 mixtures in the ratio 50:50 were investigated as a function of solution pH. An increase in pH resulted in a significant decrease in the adsorbed amount of R2: between pH 7 and 8, the adsorbed amount of R2 mixed with C12E8 decreased from 1.3 x10^(-10) mol cm^(-2) to 0.5 x10^(-10) mol cm^(-2). The adsorption behaviour of R1:LAS4:CaCl2 and R2:LAS4:CaCl2 mixtures at the solid/liquid interface were investigated as a function of concentration, composition, and temperature. The electrolyte induced multilayer formation of LAS4 at the interface, and small quantities of R1 and R2 enhanced LAS4 adsorption. Between 20 oC and 40 oC, the adsorption of pure LAS4 increased whereas the adsorption of a R2:LAS4 mixture decreased. The mixed CMCs and adsorbed compositions of 5 binary surfactant mixtures have been fitted using a Markov chain model. The CMC and surface compositions for each system can be modelled using the same two reactivity ratios, g1 and g2, which indicates the interaction strength of the surfactants in both phases are the same, within model limitations. The fittings were used to predict the mixing behaviour of C12E8/R1 and SLES EO3/LAS4 mixtures, and the predictions are supported by literature, showing great potential for further developing this approach to treat surfactant mixing.

Date of Award	31 Dec 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jian Lu (Supervisor) & Henggui Zhang (Supervisor)