Polarization Modulation Infrared Reflection Absorption Spectroscopy of Gibbs Monolayer at the Air/Water Interface

Abstract

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was applied to investigate Gibbs monolayers of ethylene glycol mono-n-dodecyl ether (EGDE), spontaneously adsorbed at the interface of air andaqueous EGDE subphase. Brewster angle microscopic images reveal that the EGDE monolayer forms circular domains in a condensed state. The PM-IRRAS measurements reveal a ν a s (CH 2 ) band at 2918.2 cm - 1 and a δ(CH 2 ) band at 1469.2 cm - 1 , indicating respectively the planar trans zigzag conformation and crystalline packing of the -(CH 2 ) 1 1 - chain. The ν a s (C-O-C) mode gives a negative band, implying that the ether linkage preferentially stands up in the Gibbs monolayer. The Gibbs monolayer was successfully transferred onto solid substrates such as Au and CaF 2 , using the conventional Langmuir-Blodgett technique. Infrared transmission and reflection measurements of the transferred monolayer were applied to obtain the anisotropic complex refractive indexes. Using these indexes, the PM-IRRAS spectrum of the EGDE monolayer at the air/subphase interface was simulated and compared to the experimental one. It is concluded that the -(CH 2 ) 1 1 - chain is tilted at 21° with respect to the surface normal and twisted at 56° on the aqueous subphase. The physical act of LB transfer onto solid substrates has caused reorientation of the -(CH 2 ) 1 1 - chain and the ether linkage as well as deterioration of the chain conformational order.