Adsorption of alkyldimethylamine and alkyldimethylphosphine oxides at curved aqueous solution/silica interfaces, studied using microcalorimetry

We report on microcalorimetric studies of the adsorption of octyldimethylamine oxide (C8DAO), decyldimethylamine oxide (C10DAO), dodecyldimethylamine oxide (C12DAO), and decyldimethylphosphine oxide (C10DPO) on mesoporous, low surface charge density silica gel from aqueous solutions at 298.15 K. Displacement enthalpies combined with adsorption isotherms were measured at natural pH, at which the dilute solutions of the alkyldimethylamine oxides were mixed cationic-nonionic. All surfactants studied adsorb exothermally from dilute solution by forming hydrogen bonding between nonionic headgroup and surface silanol groups, and by electrostatic interactions between ionic adsorbates and oppositely charged surface sites. All the cationic surfactant species and dissociated silanol groups do not lose all their counterions and hydrated water when interacting electrostatically with each other. The surfaces with the exothermally adsorbed constrained species induce endothermic, hydrophobic interactions that increase the adsorption and result in surface self-assembled structures at surfactant contents in solution well below the bulk critical micelle concentration (cmc). The endothermic differential molar enthalpies of displacement of the decyl group substituted surfactants studied show a maximum when the adsorption saturates near the cmc. They form globular surface micelles at the solution/SiO2 interface. Due to interaggregate interactions, the interfaces between neighboring globular surface assemblies are not fully hydrated. C12DAO suddenly forms hemimicelles in the low-affinity adsorption region involving rearrangement of adsorbate species, and probably ellipsoidal micellar aggregates in the high-affinity region at the hydrophilic silica interface. The aggregation number is four C12DAO species per hemimicelle on the average. The adsorption mechanism of C12DAO seems to be different from those of C10DAO and C10DPO.