Mixing of Partially Fluorinated Carboxylic Acids with Their Hydrocarbon Analogs at the Air–Water Interface

Abstract

The mixing behavior of 1-(perfluorobutyl)undecanoic acid–pentadecanoic acid (C15), 1-(perfluorohexyl)undecanoic acid–heptadecanoic acid (C17), and 1-(perfluorooctyl) undecanoic acid–nonadecanoic acid (C19) mixtures was investigated at the air–water interface. The compression isotherms of the fluorocarbon acid–hydrocarbon acid mixtures were recorded at various compositions on hydrochloric acid (pH 1.9, 37±2°C) as a subphase. The phase transition, limiting molecular area, area at collapse pressure, and collapse pressure were determined for all π–A isotherms. The mixing behavior was assessed by analyzing the concentration dependence of the average molecular area at constant film pressure (area/mole fraction or A–X diagram) and the concentration dependence of the phase transition, where possible. All three acid mixtures show a negative deviation from ideal behavior at surface pressures between 5 and 20 mN/m, which is indicative of an attractive interaction of both compounds in the mixed monolayer at the air–water interface. The miscibility apparently decreases with increasing chain length of the carboxylic acids (C15>C17>C19).