hydrophobic interactions are usually considered the two Competition between mono-and divalent ions in the association most important. Electrostatic binding is a process that has of counterions to the headgroups of amphiphiles was studied in its most famous description in the DLVO theory for the one surfactant system with organic counterions (piperidine / / diffuse electrical double layer (6). The association is favored piperazine 2/ octanesulfonate) and one with inorganic counterions by the energetic attractive contribution coming from the di-(Na / /Ca 2/ octyl sulfate). By conductivity and 13 C NMR chemical rect electrostatic interactions and is disfavored by the deshift measurements the critical micelle concentration (CMC) was crease in entropy when counterions are ordered near the found to decrease drastically when small amounts of divalent surface aggregate. The second mechanism occurs due to counterions were present in the system. Self-diffusion coefficients hydrophobic interactions between the counterion and the agof surfactant ions and organic counterions were measured in the micellar phase by the Fourier transform pulsed-gradient spin-echo gregate. These interactions tend to be more difficult to gener-(FT-PGSE) NMR method. The degree of counterion binding in the alize into a concise theory than the electrostatic interactions. micellar system with piperidine / /piperazine 2/ counterions was A strong dependence on the hydrophobicity is noted in the obtained from FT-PGSE NMR measurements. It was observed literature though, and for instance, the relation between the that the divalent counterions were more strongly bound than the number of CH 2 groups in the counterion and counterion monovalent counterions. The experimental results were compared binding has been given theoretical descriptions (7).
with theoretical Poisson-Boltzmann calculations. The cell model
The valency of the counterion plays, for electrostatic reawas used to study the electrostatic effects. Good agreement besons, an important role in the association process. In surfactween electrostatic theory and experiment was observed; however, tant systems with multivalent ions usually the Krafft point an attractive force exists between the monovalent piperidine counis increased so that the surfactant will precipitate. The critical terions and the micelle, probably because of hydrophobic interacmicelle concentration (CMC) is often lower than with monotions.