Synergy, competition, and the “hanging” polymer layer: Interactions between a neutral amphiphilic ‘tardigrade’ comb co-polymer with an anionic surfactant at the air-water interface

Anna Slastanova,Richard A Campbell,Tim Snow,Elizabeth Mould,Peixun Li,Rebecca J.L Welbourn,Meng Chen,Eric Robles,Wuge H Briscoe

doi:10.1016/j.jcis.2019.11.017

Abstract

Understanding the structure of polymer/surfactant mixtures at the air-water interface is of fundamental importance and also of relevance to a variety of practical applications. Here, the complexation between a neutral ’tardigrade’ comb co-polymer (consisting of a hydrophilic polyethylene glycol backbone with hydrophobic polyvinyl acetate grafts, PEG-g-PVAc) with an anionic surfactant (sodium dodecyl sulfate, SDS) at the air-water interface has been studied. Contrast-matched neutron reflectivity (NR) complemented by surface tension measurements allowed elucidation of the interfacial composition and structure of these mixed systems, as well as providing physical insights into the polymer/surfactant interactions at the air-water interface. For both polymer concentrations studied, below and above its critical aggregation concentration, cac, (0.2 cac and 2 cac, corresponding to 0.0002 wt% or 0.013 mM and 0.002 wt% or 0.13 mM respectively), we observed a synergistic cooperative behaviour at low surfactant concentrations with a 1–2 nm mixed interfacial layer; a competitive adsorption behaviour at higher surfactant concentrations was observed where the polymer was depleted from the air-water interface, with an overall interfacial layer thickness ~1.6 nm independent of the polymer concentration. The weakly associated polymer layer “hanging” proximally to the interface, however, played a role in enhancing foam stability, thus was relevant to the detergency efficacy in such polymer/surfactant mixtures in industrial formulations.

Highlights

Polymer/surfactant interfacial organisation is important to many processes such as foaming [13], detergency [4], solubilisation [5], flotation [6], encapsulation [7], and lubrication [8], as well as applications such as personal care products [9], pharmaceuticals [10, 11], and oil industries [12]
0.5 cmc Sodium dodecyl sulfate (SDS) concentration, which had been attributed to polymer partial desorption from the interface [65]; this led to a slight increase in the surface tension compared to that of pure SDS
In the case of 0.5 cmc-SDS mixtures with higher polymer concentration (2 cac polyethylene glycol (PEG)-g-PVAc), the surface tension values from 2 measurements showed a synergistic effect, whilst 3 other measurements pointed towards a competitive behaviour with less polymer adsorbing to the interface

Summary

Introduction

Polymer/surfactant interfacial organisation is important to many processes such as foaming [13], detergency [4], solubilisation [5], flotation [6], encapsulation [7], and lubrication [8], as well as applications such as personal care products [9], pharmaceuticals [10, 11], and oil industries [12]. In the case of 0.5 cmc-SDS mixtures with higher polymer concentration (2 cac PEG-g-PVAc), the surface tension values from 2 measurements showed a synergistic effect, whilst 3 other measurements pointed towards a competitive behaviour with less polymer adsorbing to the interface.

Results

Conclusion