Examining the Role of Surfactant Packing in Phase Transformations of Periodic Templated Silica/Surfactant Composites

Abstract

In this work, we examine the role of curvature and surfactant packing in controlling the structure of periodic silica/surfactant composites by driving such materials through a transformation from a hexagonal to a lamellar phase. We focus on how the interplay of desired packing and volume constraints dictates the resulting structures. In general, surfactants expand in a complex way upon heating, and this can cause a change in the optimal packing geometry. However, the presence of a rigid silica framework may prevent surfactants from reaching this preferred volume and/or curvature. Real-time in situ X-ray diffraction is used to monitor the structural evolution of these materials heated under hydrothermal treatments. Because the thermal-driven disorder of the surfactant tails drives the phase transition, we examine four types of composites with varying tail density. Ordinarily, composites consist of surfactants with one 20-carbon tail and one positively charged ammonium headgroup. Tail density is varied by replacing a small amount (0-16%) of these single-tail, single-head surfactants with single-tail, double-head 'gemini' surfactants. A greater head--tail ratio indeed produces different results, causing the phase transition to occur at higher temperatures. Using simple geometric models to gain better understanding of our experimental results, we find that, while both unfavorable curvature and limited volume may exist for the surfactants in these composites, the constrained curvature appears to be the dominant effect in driving structural rearrangement.

Minute-made and low carbon fingerprint microwave synthesis of high quality templated mesoporous silica

“…S3 ‡). 38 Concomitantly, the C BET parameter of the BET equation exhibited a considerable increase from about 105 up to 211. These two increases are assigned to a profound surface restructuration, whereas there was no significant change of the silanol concentration according to 29 Si NMR analyses.…”

Effect of Benzyltrimethylammonium Ion as a Co‐directing Agent on Phase Transitions in a Nanostructure Silica/Surfactant Composite

“…The percentage of BTMA + is increased from hexagonal to lamellar mesophase (about 1% for each transition phase). The phase transition can be understood by applying the simple model of Israelachvili on the conical shape and the space occupied by the hydrophilic and hydrophobic groups of the surfactant 16,22 . The local effective parameter is defined as g=V/a 0 l c , where V and l c are the total volume and the length of the hydrophobic portion of the surfactant and a 0 is the polar head area.…”

“…In this pathway, the associated silica oligomers with surfactant head groups can polymerize and a highly ordered rigid framework forms. The overall topology of this nanostructure is determined by surfactant curvature which can be described by the dimensionless effective surfactant packing parameter (g) 16 . This model explains favored phase transition from lamellar to cubic and hexagonal phases.…”

Mechanisms and Intermediates in the True Liquid Crystal Templating Synthesis of Mesoporous Silica Materials

“…Since their discovery in 1992, , many different synthesis approaches have been developed which we roughly categorize into three classes: the first and most commonly used approach is the so-called “liquid crystal templating” in which diluted surfactant solutions are used, from which the MSM precipitates upon the addition of a polymerizable silica precursor . This process has been studied extensively; − however, the numerous chemical components and physical parameters involved as well as the simultaneous formation of an ordered structure from the formerly isotropic solution and the incipient polymerization of the silica precursor render it difficult to predict the structure and properties of new MSMs. Even though, this method is called “liquid crystal templating,” no actual liquid crystal phase is involved in the process.…”