Langmuir films and mechanical properties of polyethyleneglycol fatty acid esters at the air-water interface

• This study is a comprehensive theoretical and experimental study of the Langmuir films of Polyethyleneglycol Fatty Acid Esters.
• It is possible to associate a correlation between the tail group and hydrophilic chain and the ensuing physicochemical properties.
• Shorter polyoxyethylene polar chains lead to more ordered films, but longer chains may be more effective in stabilizing the monolayer film.

Polyethylene glycol (PEG) fatty acid esters are used in various industries for their interfacial properties, especially because their emulsifying characteristics can be varied considerably by altering the size of the PEG or fatty acid chains. In this study, we combine experimental surface-specific methods and molecular dynamics (MD) simulations to probe the effects from changing both the hydrophilic and hydrophobic chain sizes. PEG fatty acids containing either stearic or palmitic acid ethoxylated PEG chains of average molecular weight 200 or 400 g mol−1 (referred to as PEG200 and PEG400, respectively) had critical aggregation concentrations between 30 and 50 μM, and formed Langmuir films at the air/water interface with compressibility modulus typical of liquid-expanded phases. The area per molecule was consistently smaller for the PEG200 series owing to an increased ordering of the hydrophilic chains inferred from the polarization-modulated infrared reflection absorption (PM-IRRAS) spectra, which was corroborated by MD simulations. With PM-IRRAS and MD simulations, we also noted that the hydrophobic chain had increased order when its size increased from palmitic to stearic acid, which was attributed to a higher melting temperature for longer saturated hydrocarbon chains.

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