Surface activity of sodium salts of alpha‐sulfo fatty esters: The air‐water interface

Adsorption at the air‐water interface of sodium hexyl α‐sulfopelargonate, sodium heptyl α‐sulfopelargonate, sodium methyl α‐sulfomyristate, and sodium methyl α‐sulfopalmitate has been investigated. The amount of surfactant adsorbed at the interface has been obtained by the application of a simple form of the Gibbs equation to surface tensions of aqueous solutions of varying surfactant ion concentration but at constant counterion (Na+) concentration. Surface tensions were determined by using the drop‐volume method. Counterion concentrations of 0.01 and 0.04N were used. Cross‐sectional areas for the surfactant anions are in the range 44 to 59Å2 (independent of concentration) and are consistent with close packing in which the carbon chains are perpendicular to the interface. For each surfactant, increase in counterion concentration from 0.01 to 0.04N results in a surface‐tension decrease of about 7.5 dyne/cm. This effect can be predicted by using bulk activity coefficients from the Debye‐Hückel theory. Dependence of surface tension on carbon chain length indicates that the carbon chains of the four compounds behave similarly at the air‐water interface.