Adsorption of the organic dibasic acids at the liquid interfaces. I. Neutral monolayers of the unionized acids

Abstract

The interfacial pressures (π) of oil-water and air-water systems are measured extensively in the presence of increasing aqueous concentrations ( C) of unionized pimelic ( C 7), suberic ( C 8), azelaic ( C 9), and sebacic ( C 10) acids near pH 2. The pressure-area curves for these acid monolayers at an oil-water interface are found to follow the Langmuir equation of state. The limiting co-area A o per molecule for the adsorbed monolayers of odd and even chain dibasic acids are found to be 51 and 34Å 2, respectively. The results also indicate that the dibasic acid molecules do not possess any cohesional attraction for each other within the film. Values of the area ( A) per adsorbed dibasic acid molecules at an oil-water interface for various values of C are found to fit the modified Langmuir adsorption isotherm. The free energy (−ΔG°) of adsorption for the four dibasic acids does not increase in a regular manner with increase of chain length. However,− ΔG CH 2 o for the odd and even chain series of dibasic acids can be calculated separately and in each case its value is observed to be 714 cal/mole. From the comparison of the π − A curves at the oil-water and air-water interfaces, the cohesive pressure ( π s ) of the film at air-water interface has been calculated using the Davies equation. The two-dimensional van der Waals constants for the films of dibasic and monobasic acids are also compared critically.