Langmuirian surfactant adsorption

Surfactant adsorption close to the cmc may appear Langmuirian, but this does not automatically imply a simple orientation. For example, rearrangement from a horizontal to a vertical orientation or electrostatic interaction and counterion binding may be masked by simple adsorption isotherms. It is essential, therefore, to combine the adsorption isotherms with other techniques such as microcalorimetry and various spectroscopic methods in order to obtain a full picture of surfactant adsorption. 

In the case of ionic surfactants the adsorption of surfactant ions is accompanied by binding of counterions. In addition, the concentrations of the ionic species vary across the EDL (even at equilibrium). These effects are taken into account in Equation 5.57, which can be used as an expression for Q in the case of Langmuirian barrier adsorption of an ionic surfactant. 

The adsorption isotherm of nonionic surfactants are in many cases Langmuirian, much like those of most other highly surface active solutes adsorbing from dilute solutions, and the adsorption is generally reversible. However, several other adsorption types are produced [29], and these are illustrated in Figure 5.7. The steps... 

This is attributed to the failure of the large molecules to enter the pores of the solid. More complex isotherm shapes are encountered as in the case of the adsorption of alkyl surfactants on silica and alumina. For example, the adsorption isotherm of sodium dode-cylsulfonate on alumina consists of four regions depending on the dominant adsorption mechanism. Adsorption of polymeric reagents on minerals typically results in a pseudo-Langmuirian type isotherm as shown in Fig. 4.7 for the adsorption of polyacrylamide on Na-kaolinite (Hollander et al., 1981). 

The comparison of the empirical Szyszkowski equation (II. 18) with the Gibbs equation (II.5) indicates that Langmuir adsorption isotherm (11.22) is well suited also for the description of adsorption at the air - surfactant solution interface. It is interesting to point out that at the gas - solid interface, for which eq. (11.22) was originally derived various deviations from Langmuirian behavior are often observed. 

One sees that for Langmuirian adsorption the Gibbs elasticity grows linearly with the surfactant concentration cj. Since the concentration of the monomeric surfactant cannot exceed the critical micellization concentration, Cj[Pg.624]

Fig. 1.9 shows typical Langmuirian curves for the adsorption of a series of polyoxyethylated non-ionic surfactants on to graphitized carbon black... 

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