Long chain ionic surfactants: the understanding of adsorption mechanisms from the resolution of adsorption isotherms

The authors present the different steps required for understanding the adsorption of long chain ionic surfactants at the solid-aqueous solution interface. First, the physical chemistry of the surfactant in solution has to be well documented. This requires to know the phase diagram of the surfactant that yields the Krafft point and the value of the critical micellar concentration (CMC) of the spherical micellar phase (T>TKrafft) or the saturation concentration of the hydrated-crystal phase (T<TKrafft). Second, a thermodynamical approach, 2-D condensation on heterogeneous surfaces, should be applied in the domain where true adsorption takes place (Δμ<0). The physical significance of the plots θ versus ln Ce and θ versus Δμ are discussed in details: they allow to determine surface heterogeneity (energetic distribution function) and to characterize the state of the condensed adsorbed layer (liquid-crystal state or hydrated-crystal state) from either the cross sectional area of the hydrocarbon chain in the adsorbed layer or the value of the free energy change of a CH2 group, Δg. Two different systems (aggregative process) are discussed; (i) systems with strong normal and lateral bonds; (ii) systems with weak normal bonds and strong lateral bonds. The conditions for possible surface precipitation are also presented. In all cases, the choice of an appropriate reference phase is necessary and in those conditions, the use of sophisticated models to estimate the conformational entropy of the hydrocarbon chain is not needed.