Adsorption of surfactin at water with ethanol mixture-air interface

Abstract

The addition of alcohol to surfactant changes, among others, its adsorption properties, therefore the surface tension of water/ethanol solution of surfactin was measured at 293 K using the ring detachment method. The measurements were made in the range of surfactin concentration from 0 to 40 mg/dm3 and in the whole range of ethanol concentration so their molecules were present in solution in the monomeric and aggregated forms. The values of solution surface tension were also predicted using the Miller et al., Connors and independent adsorption methods. Based on these measurement results, the surfactin Gibbs surface excess concentration at the solution-air interface was determined. The ethanol Gibbs surface excess concentration was established using the Gibbs and Guggenheim-Adam concepts as well as that of the total concentration one. Taking into account the Gibbs surface excess concentration of surfactin and total ethanol concentration, the fraction of the area occupied by surfactin and ethanol at the solution-air interface was deduced. Additionally, composition of mixed monolayer at the solution-air interface was calculated and compared to the theoretical composition resulting from the independent adsorption of surfactin and ethanol. Based on the Gibbs surface excess concentration of ethanol and surfactin, the Gibbs standard free energy of adsorption was calculated using different methods. The obtained results and their consideration indicate that there is a mutual influence of surfactin and ethanol on their adsorption at the solution-air interface which depends on the concentration of both surfactin and alcohol.