Effect of surfactants on the thermoresponse of PNIPAM investigated in the brush geometry

Hypothesis: Anionic surfactants have been reported to interact with poly(N-isopropyl acrylamide) (PNIPAM), suppressing its thermoresponse. Scattering and NMR studies of the anionic sodium dodecyl-sulfate (SDS) system propose that the PNIPAM-surfactant interaction is purely hydrophobic. However, prior phenomenological investigations of a range of surfactant identities (anionic, cationic, nonionic) show that only anionic surfactants affect the thermoresponse and conformation of PNIPAM, implying that the hydrophilic head-group also contributes. Crucially, the phenomenological experiments do not mea-sure the affinity of the tested surfactants to the polymer, only their effect on its behaviour.Experiments: We study the adsorption of six surfactants within a planar PNIPAM brush system, elucidat-ing the polymer conformation, thermoresponse, and surfactant adsorption kinetics using ellipsometry, neutron reflectometry (NR), optical reflectometry and the quartz crystal microbalance technique. NR is used to measure the distribution of surfactants within the brush.Findings: We find that only anionic surfactants modify the structure and thermoresponse of PNIPAM, with the greater affinity of anionic surfactants for PNIPAM (relative to cationic and nonionic surfactants) being the primary reason for this behaviour. These results show that the surfactant head-group has amore critical role in mediating PNIPAM-surfactant interaction than previously reported. Taking inspira-tion from prior molecular dynamics work on the PEO-surfactant system, we propose an interaction mech-anism for PNIPAM and SDS that reconciles evidence for hydrophobic interaction with the observed head- group-dependent affinity.CO 2022 Elsevier Inc. All rights reserved.

Automated summary

The interaction between anionic surfactants and PNIPAM is primarily hydrophobic, but only anionic surfactants affect the thermoresponse and conformation of PNIPAM, suggesting that the hydrophilic head-group also plays a role. The greater affinity of anionic surfactants for PNIPAM compared to cationic and nonionic surfactants is the main reason for this behavior. These findings highlight the critical role of the surfactant head-group in mediating PNIPAM-surfactant interaction.