In this report, we studied the effect of halide salt concentration on Arachidic Acid (AA) Langmuir monolayers formed on salt solutions, such as NaCl, NaI, NaF by using a SumFrequency Generation Vibrational Spectroscopy. In the SFG spectra, we observed the peak intensities of CH3FR and CH3SS vibrational modes decreased when increasing the salt concentration. This observation indicates that the structure of the AA monolayer was disordered by the interaction of the dissolved ions with the carboxyl head groups of the monolayer. | VNU Journal of Science: Mathematics – Physics, Vol. 33, No. 2 (2017) 42-47 Study of Langmuir Monolayers of Arachidic Acid on Saline Solutions Using Sum-frequency Generation Vibrational Spectroscopy Nguyen Thi Hue1,2, Vu Thi Thanh Tam1, Nguyen Anh Tuan1,* 1 Faculty of Physics, VNU University of Science, 334 Nguyen Trai, Hanoi, Vietnam 2 Hung Vuong University, Phu Tho, Vietnam Received 13 March 2017 Revised 28 April 2017; Accepted 25 May 2017 Abstract: In this report, we studied the effect of halide salt concentration on Arachidic Acid (AA) Langmuir monolayers formed on salt solutions, such as NaCl, NaI, NaF by using a SumFrequency Generation Vibrational Spectroscopy. In the SFG spectra, we observed the peak intensities of CH3FR and CH3SS vibrational modes decreased when increasing the salt concentration. This observation indicates that the structure of the AA monolayer was disordered by the interaction of the dissolved ions with the carboxyl head groups of the monolayer. Keywords: Langmuir monolayer, interfacial structure, Sum-frequency vibrational spectroscopy. 1. Introduction Langmuir monolayer is a single monomolecular layer formed on the air/liquid interface. Structures and properties of Langmuir monolayers are often affected by external conditions such as temperature, dissolved ion concentration, and pH. The fatty acid molecules have both hydrophilic and hydrophobic parts. The head group which contains hydrophilic compounds is attracted to water. The tail group of the hydrocarbon chain is a hydrophobic part, which is out of the water and forms a single layer of molecules on the water surface [1]. Langmuir monolayers have attracted studies because their structural resembles biological membranes. The interaction in the interface comes from hydrogenbonding between the head groups and the water molecules. In addition, the presence of dissolved ions affects molecular orientation and structure of the interfacial layers. Sum-frequency generation vibrational .
