Interfacial complexation of a neutral amphiphilic ‘tardigrade’ co-polymer with a cationic surfactant: Transition from synergy to competition

Anna Slastanova, Richard A. Campbell, Luisa Islas, Rebecca J.L. Welbourn, John R. P. Webster, Mauro Vaccaro, Meng Chen, Eric Robles, Wuge H. Briscoe

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

Abstract

Hypothesis: Neutral amphiphilic PEG-g-PVAc co-polymer (a “tardigrade” polymer consisting of a hydrophilic polyethylene glycol, PEG, backbone with hydrophobic polyvinyl acetate, PVAc, grafts) can form complexes at the air–water interface with cationic dodecyltrimethylammonium bromide (DTAB) via self-assembly. Compared to anionic SDS, cationic DTAB headgroups are expected to interact strongly with the negatively charged OH groups from the partial dissociation of the PVAc grafts. We anticipate a transition from synergistic to competitive behaviour, which is expected to be dependent on the surfactant structural characteristics and concentration. Experiments: DTAB/PEG-g-PVAc mixtures were investigated using a combination of dynamic and equilibrium surface tension measurements, neutron reflectivity (NR) at the air–water interface, and foaming tests. We varied the concentrations of both the DTAB (0.05 to 5 critical micelle concentration, cmc) and that of PEG-g-PVAc (0.2 and 2 critical aggregation concentration, cac). Findings: Our results show that the interfacial interactions between DTAB and PEG-g-PVAc were both synergistic and antagonistic, depending sensitively on the surfactant concentration. At DTAB concentrations below its cmc, a pronounced cooperative adsorption behaviour was likely driven by the hydrophobic interactions between the DTAB tail and the PVAc grafts and the attraction between the DTAB headgroups and the partially dissociated –O- groups in the partially hydrolysed PVAc grafts, forming a mixed layer. This synergistic adsorption behaviour transitioned to a competitive adsorption behaviour at DTAB concentrations above its cmc, leading to polymer-surfactant partition, forming a “hanging” polymer layer underlying a surfactant monolayer at the interface. We postulate that DTAB/PEG-g-PVAc complexation in the bulk contributed to partial depletion of the mixture from the interface. We therefore consider this polymer/surfactant system to be a moderately interacting system at the air–water interface. No discernible differences in the foaming behaviour were observed between the DTAB/PEG-g-PVAc systems and the pure surfactant. Our results suggest that surfactant headgroup characteristics (particularly charges) were crucial in determining the structure and composition of polymer-surfactant complexes at the air–water interface, as well as the foamability and foam stability, whilst the coexistence of the synergistic and competitive adsorption behaviour is attributed to the unique architecture of the tardigrade polymer with amphiphilicity and partial charge, facilitating different surfactant-polymer interactions at different DTAB concentrations.

Original languageEnglish
Pages (from-to)1064-1076
Number of pages13
JournalJournal of Colloid and Interface Science
Volume606
DOIs
StatePublished - Jan 15 2022
Externally publishedYes

Funding

We acknowledge funding from the Engineering and Physical Science Research Council (EPSRC Case Awards) and Procter & Gamble. Deuterated DTAB was kindly supplied by the ISIS Deuteration Facility (Dr Peixun Li). We thank Institut Laue-Langevin and ISIS Neutron and Muon Source for allocations of neutron beamtime: 9-12-488, (DOI: 10.5291/ILL-DATA.9-12-488, at FIGARO, ILL), and (DOI: 10.5286/ISIS.E.84939893 and 10.5286/ISIS.E.RB2000016 at INTER, ISIS). Dr Magdalena Wlodek is thanked for her help during the ILL experiment. Prof Julian Eastoe is thanked for the suggestion of measuring surface tension during addition of adsorbing species to an equilibrated interfacial layer. Mr Gerard Langley is thanked for the tardigrade illustration. We acknowledge funding from the Engineering and Physical Science Research Council (EPSRC Case Awards) and Procter & Gamble. Deuterated DTAB was kindly supplied by the ISIS Deuteration Facility (Dr Peixun Li). We thank Institut Laue-Langevin and ISIS Neutron and Muon Source for allocations of neutron beamtime: 9-12-488, (DOI: 10.5291/ILL-DATA.9-12-488, at FIGARO, ILL), and (DOI: 10.5286/ISIS.E.84939893 and 10.5286/ISIS.E.RB2000016 at INTER, ISIS). Dr Magdalena Wlodek is thanked for her help during the ILL experiment. Prof Julian Eastoe is thanked for the suggestion of measuring surface tension during addition of adsorbing species to an equilibrated interfacial layer. Mr Gerard Langley is thanked for the tardigrade illustration.

FundersFunder number
INTER
Institut Laue-Langevin and ISIS Neutron and Muon Source9-12-488
Procter and Gamble
Engineering and Physical Sciences Research Council

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