Adsorption at Liquid-Vapor Boundary

Surface tension curves, such as the one shown in Fig. 4.2, yield useful information, in addition to defining the minimum surface tension at cmc, ycmc- The surfactant concentration needed to attain the surface excess concentration, c, corresponds to surface saturation, F, . The slope dy/d log c of the linear portion of the curve is related to the surface excess concentration, F ,. 

The excess surface concentration F, can be determined from the linear portion of the surface tension-log surfactant concentration curve. Although direct determination of r is experimentally more difficult, several satisfactoiy methods have been developed [12-30]. McBain and co-workers [13,14] analyzed a slice of the surface removed with a microtome. Radioisotope tracer methods [15-21] are simpler and the agreement with calculated values has been satisfactory. Recently, the structure and composition of the adsorbed surface layer has been determined by neutron reflection [22-30]. 

Downer et al. [29,30] used neutron reflection and surface tension measurements to investigate the adsorption of four fluorinated surfactants at the air-water interface. The surfactants used were two single-chain carboxylates, sodium per-fluorononanoate (NaPFN) and sodium 9H-perfluorononanoate, and two doublechain sulfosuccinates, sodium bis(l/ir, lH-perfluoropentyl)-2-sulfosuccinate (DCF4) and sodium bis(lH, IH, 5H, 5/f-octafluoropentyl)-2-sulfosuccinate (DHCF4). The replacement of a terminal fluorine by hydrogen creates a permanent dipole in the hydrophobic chain and consequently, increases the cmc and the limiting surface tension as well. 

Rosen [11 ] has suggested that fluorination of the hydrophobe has apparently only a small effect on r in contrast to the large effeet fluorination has on surface tension. If fluorination does not affect the surface excess concentration F, significantly the large effeet of fluorination on surface tension must be related to the chemical potential of the fluorinated surfactant [Eq. (3)]. The chemical potential depends on the nature of the hydrophobe, as well as the hydrophile. The main factors in reducing surface tension are the decrease in the work needed to transport a surfactant molecule to the surface and the low surface energy of the monomolec-ular film formed by adsorbed surfactant molecules. 

Under conditions of surface saturation, the Gibbs equation can be used to determine the maximum extent of surfactant adsorption on the surface. The area occupied by a molecule, (expressed in nm ) can be calculated from the surface excess concentration F , of the saturated surface (mol/cm )  

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