Surface phase transition of C₁₂E₁ at the air/water interface: A study by dynamic surface tension, external RA FT-IR, and 2D IR correlation methods

The surface conformational states of the Gibbs monolayer of ethylene glycol mono-n-dodecyl ether (C₁₂E₁) at the air/water interface was studied using dynamic surface tension, external reflection-absorption FT-IR spectroscopy (ERA FT-IR), and two-dimensional infrared (2D IR) correlation methods at constant temperature. The dynamic surface tensions were measured at different bulk concentrations of C₁₂E₁, and it was observed that a constant surface tension region appears at ∼38.5 mN m ^-1 in a dynamic surface tension profile at concentrations higher than 11 μmol kg^-1. This constant surface tension region corresponds to the surface phase transition from liquid expanded (LE) to liquid condensed (LC). Two sets of ERA FT-IR spectra were collected, one at different bulk concentrations but after equilibrium time (equilibrium measurements) and another at constant bulk concentration (m = 16 μmol kg^-1) but at different times (dynamic measurements). The first set of these measurements show that the peak area increases in the range of 11 < m ≤ 16 μmol kg ^-1, which means the increase in the number of surfactant molecules at the air/water interface. Also, the wavenumber of antisymmetric CH₂ stretching decreases gradually from ∼2923 cm^-1 (for 10 and 11 μmol kg^-1) to ∼2918 cm^-1 (for m ≥ 16 μmol kg^-1) with increasing concentration. The wavenumbers of 2923 and 2918 cm^-1 were assigned to LE and LC phases, respectively, and the decrease of wavenumber in the concentration range of 11 < m ≤ 16 μmol kg^-1 were correlated to the surface phase transition (LE → LC), or in other words, in the mentioned concentration range, two phases coexist. The dynamic ERA FT-IR measurements at 16 μmol kg^-1 also confirm the surface phase transition from LE to LC. The 2D IR correlation method was applied to the both equilibrium and dynamic IR spectra of the C₁₂E ₁ monolayer. The synchronous correlation maps show two strong autopeaks at ∼2922 and ∼2851 cm^-1 and also show a strong correlation (cross-peaks) between antisymmetric CH₂ stretching (v_a) and symmetric CEk stretching (vs). The asynchronous correlation maps show that both observed bands of v_a and v_s in one-dimensional IR split into two components with the characteristic of overlapped bands, which reveals the coexistence of two phases (LE and LC) at the interface at 11 < m ≤ 16 μmol kg^-1. The synchronous and asynchronous maps that were obtained from dynamic IR spectra closely resembled the equilibrium map.