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 similar to 38.5 mN m(-1) in a dynamic surface tension profile at concentrations higher than 11 mu mol 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 mu mol 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 mu mol 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 similar to 2923 cm(-1) (for 10 and 11 mu mol kg(-1)) to similar to 2918 cm(-1) (for m <= 16 mu mol 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 mu mol 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 mu mol 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 C12E1 monolayer. The synchronous correlation maps show two strong autopeaks at similar to 2922 and similar to 2851 cm(-1) and also show a strong correlation (cross-peaks) between antisymmetric CH2 stretching (v(a)) and symmetric CH2 stretching (v(s)). 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 mu mol kg(-1). The synchronous and asynchronous maps that were obtained from dynamic IR spectra closely resembled the equilibrium map.