The structural-functional relationship of polymer-surfactant complexes relevant to personal care product

Abstract

Polymer-surfactant (PS) mixtures are widely used to control both solution and surface properties. The link between the molecular structure of polymers and surfactants and their associative behaviours is of great interest and it is not very well understood. The examination using several different methods in the colloidal systems is to link the function of PS complexes to their microstructure from different aspects.

My thesis aims to investigate how oppositely charged PS complexes can affect the interaction and the adhesive force of drops to surfaces and link these attributes to the target functions of a formulation, including shelf life stability and drop deposition or adhesion of an emulsion formulated chemical products, for example, personal care products. This was achieved by using both novel microscopic methods to quantify adhesion interactions and probe the adsorption and microstructure of PS complexes coated on drops and model surfaces as well as correlating these data to macroscopic methods for bulk solution properties.

In this work, cellulose based cationic polymer and anionic surfactants, sodium lauryl (or dodecyl) sulphate surfactants were used based on current key ingredients in personal care product formulation. We have studied when drops will stick to surfaces in the presence of PS complexes by systematically varying the components of PS complexes (e.g. polymer type, surfactant concentration and type, and electrolyte concentration) and correlating the observed drop adhesion to hydrophobic surfaces with the phase diagrams of PS complexes. This observed that polydispersity in anionic surfactant headgroup can drive different drop adhesion, which motivated studies on surfactants in the absence of polymer to see how polydispersity of head group affects the micellization of the surfactant by measuring their critical micelle concentration (cmc) as a function of polydispersity degree and added electrolyte as well as the shape and dimension of the micelle using small angle neutron scattering (SANS). These measurements demonstrated that by controlling the degree of polydispersity in surfactant headgroup, the micelle character and their interaction with polymer can be possibility predicted.

The measurements of drop adhesion were then compared to the adsorption of the PS complexes in order to explain how the structure of PS complexes on different surfaces can affect drop adhesion. The adsorption of PS complexes onto model surfaces that have more complexity, relevant to skin, hair or textiles were studied by measuring the adsorbed PS layer thickness using AFM imaging as well as force measurements in combination with measures of the adsorbed amount using QCM-D. By combing the observation of the layer thickness and adsorbed mass of PS complexes upon surfactant and electrolyte dilutions, and the effect from surface character, more insights of the mechanism of the structure change of PS complexes is understood.