The geometry, arrangement, and orientation of a quaternary ammonium surfactant flanked by two methyl groups, a benzyl head, and an octyl tail were assessed at the air–water and air-deuterium oxide (D2O) interfaces using sum frequency generation vibrational spectroscopy. Remarkably, symmetric and asymmetric N–CH3 stretches (at ∼2979 and ∼3045 cm–1, respectively, in the SSP polarization combination) were visible in water but negligible in deuterium oxide. We concluded that D2O addition triggers the average reorientation of the dimethyl amino units parallel to the interface and possibly changes the overall conformation of the surfactant. A reduced number of gauche defects in the surfactant octyl chain is also observed in D2O. Tilt angles for the octyl chain (1.0–10.8°) are consistent with an ordered monolayer at the air–liquid interface.

Polymers containing poly­(ethylene glycol) (PEG) units can exhibit excellent antifouling properties, which have been proposed/used for coating of biomedical implants, separation membranes, and structures in marine environments, as well as active ingredients in detergent formulations to avoid soil redepositioning in textile laundry. This study aimed to elucidate the molecular behavior of a copolymer poly­(MMA-co-MPEGMA) containing antiadhesive PEG side chains and a backbone of poly­(methyl methacrylate), at a buried polymer/solution interface. Polyethylene terephthalate (PET) was used as a substrate to model polyester textile surfaces. Sum frequency generation (SFG) vibrational spectroscopy was applied to examine the interfacial behavior of the copolymer at PET/solution interfaces in situ and in real time. Complementarily, copolymer adsorption on PET and subsequent antiadhesion against protein foulants were probed by quartz-crystal microbalance experiments with dissipation monitoring (QCM-D). Both applied techniques show that poly­(MMA-co-MPEGMA) adsorbs significantly to the PET/solution interface at bulk polymer solution concentrations as low as 2 ppm, while saturation of the surface was reached at 20 ppm. The hydrophobic MMA segments provide an anchor for the copolymer to bind onto PET in an ordered way, while the pendant PEG segments are more disordered but contain ordered interfacial water. In the presence of considerable amounts of dissolved surfactants, poly­(MMA-co-MPEGMA) could still effectively adsorb on the PET surface and remained stable at the surface upon washing with hot and cold water or surfactant solution. In addition, it was found that adsorbed poly­(MMA-co-MPEGMA) provided the PET surface with antiadhesive properties and could prevent protein deposition, highlighting the superior surface affinity and antifouling performance of the copolymer. The results obtained in this work demonstrate that amphiphilic copolymers containing PMMA anchors and PEG side chains can be used in detergent formulations to modify polyester surfaces during laundry and reduce deposition of proteins (and likely also other soils) on the textile.

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Long-distance transmission of oil is usually carried out in large-diameter steel pipelines; water present therein may cause severe internal corrosion. An effective method of mitigating such corrosion is to inject organic corrosion inhibitors (CIs). Their surface adsorption, via heteroatom functionalities, can markedly enhance the corrosion resistance of metals. In this study, three CI model compounds with different head groups but the same alkyl tail length (-C14H29), specifically tetradecyltetrahydropyrimidinium (THP-C14), tetradecylphosphate ester (PE-C14) and tetradecylimidazolinium (IMID-C14) were synthesized, their purities being determined using nuclear magnetic resonance spectroscopy (NMR). The critical micelle concentrations (CMCs) of each compound were measured using surface tensiometry (Du Noüy ring) and fluorescence spectroscopy techniques, differences being found between these indirect and direct methods. In addition, linear polarization resistance was used to determine inhibition efficiencies (IEs) for carbon steel immersed in a 5 wt.% NaCl electrolyte saturated with CO2. CI surface saturation concentrations, with maximum IEs, were compared with the determined CMCs. Excellent IEs were observed at concentrations of THP-C14, PE-C14 and IMID-C14 which do not correspond with their CMCs, the differences involved being significantly greater than what has previously been reported for tetradecylbenzyldimethylammonium (BDA-C14). These results demonstrate that there is no direct link between CMC and metal surface saturation/corrosion inhibition efficiency that can be made on a generalized basis for the different head groups but with the same tail length corrosion inhibitors, indicating that the selection of the appropriate CI concentration for an industrial application should not be based on CMC alone.

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