Vibrational Sum Frequency Spectroscopy Study of the Liquid/Vapor Interface of Formic Acid/Water Solutions

The liquid/vapor interface of aqueous formic acid solutions has been studied by vibrational sum frequency spectroscopy (VSFS) in the concentration range extending from water to pure formic acid. Throughout the whole concentration range, the acid molecules at the surface were found to be protonated, and no evidence of the dissociated form was found. A comprehensive picture of the adsorption and structural behavior was obtained by targeting the CH/OH and the CO stretching regions. The main spectral features were the CH stretch at ∼2926 cm⁻¹ and the CO stretching vibration at ∼1720 cm⁻¹, in addition to the OH stretching bands of water. Information regarding the orientation of the interfacial formic acid molecules was also obtained. The tilt angles (θ) of the CH and CO bonds from the surface normal were determined to be basically concentration independent and attained values in the ranges of 27° < θ_CH < 43° and 42° < θ_CO < 57°. Various models for the surface structure are discussed based on the spectral evolution in the whole concentration range, and a comparison is made to the adsorption behavior of acetic acid from an aqueous solution, which has been previously studied under similar conditions.