THE EFFECT OF SMALL QUANTITIES OF CALCIUM ON THE ADSORPTION OF SODIUM DODECYL-SULFATE AND CALCIUM AT THE GAS-LIQUID INTERFACE

The surface excess concentrations of an anionic surfactant (sodium dodecyl sulfate, SDS; the hydrophobic nonionic counterpart, monododecyl tetraethylene glycol, C12E4; and calcium ions) have been measured directly at the gas-liquid interface, using radiochemical techniques. A bulk SDS concentration hiatus was found between the filled gas-liquid interface, in the end of which the surface tension continued to fall despite a filled gas-liquid interface, which also failed to coincide with the critical micelle concentration (CMC). An alignment of SDS molecules below the interface is proposed to explain the results. Only in the presence of NaCl concentration ≥ 1 × 10-1 mol dm-3 did this concentration hiatus disappear, mainly as a result of a lowered CMC. Calcium ions migrated only to the anionic surfactant interface, and left before the formation of micelles. Neither calcium nor sodium chloride salts appear to have any effect on the surface excess concentration of C12E4. The results show that the Gibbs adsorption equation cannot be used in a blanket manner to obtain surface excess concentrations, but has to be adjusted to the particular surfactant, counterion, and conditions prevailing. © 1994 by Academic Press, Inc.