Polymer—surfactant interactions: (Hydroxypropyl)cellulose with ionic and ion-ionic surfactants

Abstract

The influence of ionic and non-ionic surfactants on the cloud point and viscosity under shear behaviour of an aqueous 1 wt.% (hydroxypropyl)cellulose (HPC) solution has been investigated. The surfactants used were hexadecyltrimethylammonium chloride, hexadecyltrimethylammonium bromide, dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, sodium dodecylsulphate, octaethyleneglycol mono n-dodecyl ether and dodecyl-β- d-maltoside (designated CTAC, CTAB, DTAC, DTAB, SDS, C 12E 8 and DM, respectively). Both the cloud point and rheological behaviour in HPC/charged surfactant systems have been interpreted in terms of the surfactant first adsorbing on the HPC molecules in a cooperative manner and then forming bulk micelles in solution. The cloud points in the HPC/DTAB, HPC/DTAC and HPC/SDS systems initially decrease with increasing surfactant concentration. Conversely, the cloud points in the HPC/CTAB and HPC/CTAC systems initially increase. This behaviour has been explained in terms of the relative contributions of the hydrophobic and electrostatic components to the total interaction force. It has been found that modifying HPC through surfactant adsorption, by either increasing the degree of hydrophobic chain substitution or the charge density, can result in an increase in both the relative viscosity and the shear-thinning. The formation of bulk micelles in the HPC/charged surfactant systems negates the initial surfactant adsorption effects. At relatively high surfactant concentrations ( ≫ CMC), the HPC/DTAC and HPC/DTAB systems have been found to exhibit different cloud point and rheological behaviour to the HPC/SDS, HPC/CTAB and HPC/CTAC systems. This has been attributed tentatively to aggregate size effects. Upon normalisation of the results to the CMC, SDS has been found to display a much stronger affinity for HPC than the cationic surfactants. These affinity differences can account for the differences in the values of the rheological parameters obtained. At the surfactant concentrations studied (i.e. up to at least 10 −2 mol dm −3), both the C 12E 8 and DM molecules and micelles exhibit no significant interaction.