Interplay of formation kinetics for highly oriented and mesostructured silicate-surfactant films at the air-water interface

Kinetic details of mesostructured silicate-surfactant films formed at the air-water interface of acidic solutions of cetyltrimethylammoium bromide and tetraethyl orthosilicate were systematically studied. Time-resolved grazing-incident small-angle X-ray scattering was adopted to capture the formation kinetics of the free-standing films comprising mesostructured silicate channel domains highly oriented to the air-water interface. Evolutions of the ordered domain size and phase volume are interpreted on the basis of the Avrami analysis, from which intermediate phases and corresponding phase transitions (as modulated by temperature, pH level, and/or composition) during film formation were quantitatively identified. Extracted kinetic parameters, complemented with the rate constant of silicate hydrolysis obtained via Raman spectroscopy, revealed details of the dynamic interplay between silicate polymerization and inorganic-organic self-assembling. Effects of the air-water interface on the formation of the silicate-surfactant films were illustrated in terms of the highly oriented mesostructure, the greatly enhanced kinetics characterized by sporadic nucleation and diffusion-controlled growth, and the reduced activation energy for silicate polymerization, in sharp contrast to solutions without the air-water interface. A schematic diagram of the free energy vs. surfactant headgroup area is constructed to correlate the observed kinetics pathways for mesostructure formation during film fabrication at the air-water interface. Advantages of the interface coupling with either air-water or solution-substrate on film formation are discussed.