Charge modification as a mechanism for tunable properties in polymer–surfactant complexes

Abstract

Oppositely charged polymer–surfactant complexes are frequently explored as a function of phase space defined by the charge ratio Z, (where Z = [+polymer ]/[−sur f actant ]), commonly accessed through the surfactant concentration. Tuning the phase behaviour and related properties of these complexes is an important tool for optimising commercial formulations; hence, understanding the relationship between Z and bulk properties is pertinent. Here, within a homologous series of cationic hydroxyethyl cellulose (cat-HEC) polymers with minor perturbations in the degree of side chain charge modification, phase space is instead explored through [+polymer ] at fixed Cpolymer . The nanostructures were characterised by small-angle neutron scattering (SANS) in D2 O solutions and in combination with the oppositely charged surfactant sodium dodecylsulfate (h-or d-SDS). Scattering consistent with thin rods with an average radius of ∼7.7 Å and length of ∼85 Å was observed for all cat-HEC polymers and no significant interactions were shown between the neutral HEC polymer and SDS (CSDS < CMC). For the charge-modified polymers, interactions with SDS were evident and the radius of the formed complexes grew up to ∼15 Å with increasing Z. This study demonstrates a novel approach in which the Z phase space of oppositely charged polymer–surfactant complexes can be controlled at fixed concentrations.