Amide i IR probing of core and shell hydrogen-bond structures in reverse micelles

Abstract

The properties of N-methylacetamide (NMA) molecules encapsulated in the reverse micelles (RMs) formed by anionic surfactant aerosol OT (AOT), are studied with vibrational spectroscopy and computation. Vibrational spectra of the amide I"" mode of the fully deuterated NMA-d7 show gradual increase of peak frequencies and line broadening as the size of RMs decreases. Analyses of the spectral features reveal the presence of three states of NMA-d7 that correspond to NMA located in the core of water phase (absorption frequency of 1606 cm-1) and two types of interfacial NMA near the surfactant layer (1620 and 1644 cm"1). In larger RMs with water content w0= [D2O]/[AOT] ≥ 10, only the first two states are observed, whereas in smaller RMs, the population of the third state grows up to 25 % at w0 = 2. These results indicate the general validity of the two-state core/shell model for the confined aqueous solution of NMA, with small modifications due to the system-dependent solute-interface interaction. However, simulations of small RM systems with w0 ≤ 15 show continuous variations of the population, frequency shifts, and the solute-solvent interaction strengths at solute-interface distance less than 4 Å. Thus, the distinction of solute core/shell states tends to be blurred in small RMs but is still effective in interpreting the average spectroscopic observables.