THE AIR WATER INTERFACE OF N-DODECANEPHOSPHONIC ACID-SOLUTIONS/

The air/water interface of the system n-dodecanephosphonic acid (H2DP) -water was studied by surface tension, ion-selective electrodes and ev aporation in an electrobalance. The combination of surface tension and ion-selective electrodes measurements enables to study the adsorption of soluble surfactants above the cmc using the Gibbs' equation. H2DP formed a nonideal monolayer at the air/water interface with A(molec) = 0.995 nm(2) below the cmc. Above the cmc there was a reduction in ads orption giving A(molec) = 6.32 nm(2), which remained almost constant i n the explored concentration range. This adsorption reduction may be d ue to a change composition on micellization, or to a thermodynamic adv antage of micellization on adsorption. The reduction in the evaporatio n rate of water was mainly due to the reduction of the water activity, caused by the presence of solutes in bulk. This-is because the strong changes in the surface coverage did not have significant influence on the evaporation rate.