Thermodynamics of micellization and adsorption of zwitterionic surfactants in aqueous media

Titration microcalorimetry has been applied to study the micellization and adsorption of the following zwitterionic surfactants: (1) the C-12 and C-14 homologues of (alkyldimethylammonio)ethanoate and (alkyldimethylammonio)-1-propanesulfonate (referred to as C12N1C, C14N1C, C12N3S, and C14N3S, respectively); (2) (dodecyldimethylammonio)butanoate (C12N3C); (3) three surfactants of the [(dodecyldimethylammonio)alkyl]phenylphosphinate type with the intercharge alkanediyl group being C3H6, C6H12, and C10H20 (C12N3PPh, C12N6PPh, and C12N10PPh, respectively). In regard to the basic principles of both the micellization and the adsorption on a hydrophilic silica surface, the zwitterionic surfactants studied fall into line with polyoxyethylenated nonionics. Enthalpy of dilution measurements allowed determining the standard molar enthalpies of micellization (Delta(mic)h degrees) and thus, basing on the appropriate critical micelle concentration values, estimates could be made of the standard molar energies of micellization (Delta(mic)g degrees) to furnish the corresponding entropy changes, Delta(mic)s degrees. At room temperatures, the negative free energy term is the result of an unfavorable positive enthalpy change and a favorable increase in entropy, the latter contribution dominating. The positive values of Delta(mic)h degrees decrease with rising temperature, addition of NaCl, decreasing hydrophilic character of the zwitterionic headgroup, and increasing length of the alkyl chain. Adsorption isotherms for the same surfactants on two hydrophilic silica surfaces, Spherosil XOB015 and precipitated silica RP 63-876. were measured using the solution depletion method. The related changes in the differential molar enthalpy of displacement (Delta(dpl)h) were detected calorimetrically as a function of surface coverage, Theta. Adsorption of surfactant betaines is the interplay of two mechanisms: (1) ion-dipole bonding between the ionic surface sites and the zwitterionic headgroups oriented with the quaternary nitrogen group close to the surface and the anionic substituent group away from it; (2) formation of surface aggregates, induced by the hydrophobic effect. At higher coverages, Delta(dpl)h is a constant function of Theta, the trends with changing temperature, salt content, and molecular structure of the surfactant essentially paralleling the corresponding Delta(mic)h degrees changes. Strong cooperativity in the adsorption phenomenon results in extended surface aggregates or even bilayers in the adsorption plateau region; otherwise the adsorbed phase is fairly fragmented.