Desorption of Fatty Acids from Monolayers at the Air/Water Interface to β-Cyclodextrin in the Subphase

• Published in: Langmuir 1996-11, Vol.12 (23), p.5664-5668
• Main Authors: Slotte, J. Peter, Illman, Sara
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Gas-liquid interface and liquid-liquid interface; General and physical chemistry; Surface physical chemistry
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la960401l

In this study the cyclodextrin (CD)-induced desorption of fatty acids from monolayers at the air/water interface was examined. Desorption rates were determined while maintaining the monolayer at constant surface pressure. Of the three fatty acids examined (tetra-, penta-, and hexadecanoic acids), tetra- and pentadecanoic acids displayed a liquid-expanded (LE) to liquid-condensed (LC) phase transition at ambient temperature. We examined the effect of the phase transition on desorption rates with pentadecanoic acid monolayers. The cyclodextrin-induced desorption of pentadecanoic acid was a linear function of time (zero-order kinetics) when the monolayer surface pressure was kept constant during the course of the experiments. With different fatty acid monolayers, the desorption rates (with the monolayers in the LC state) were observed to decrease as the fatty acid chain length increased (from 14 to 16 carbons). The desorption rates with pentadecanoic acid monolayers increased curvilinearly with increasing temperature (the monolayer was in the LC state at all temperatures examined; between 9 and 29 °C). To study the effect of the LE → LC phase transition on desorption kinetics, pentadecanoic acid monolayers were prepared at 30 °C. At this temperature, the LE → LC transition region (plateau) was between 13 and 17 mN/m. The desorption rate versus surface pressure function increased linearly with increasing surface pressure throughout the LE → LC transition region. At pressures above 20 mN/m (when the pentadecanoic acid monolayer was altogether in the LC state), however, the rate of desorption appeared to increase faster as the lateral surface pressure increased.