In Situ Time-Resolved SAXS Study of the Formation of Mesostructured Organically Modified Silica through Modeling of Micelles Evolution during Surfactant-Templated Self-Assembly

Abstract

The mechanisms of formation of organically modified (phenyl, vinyl, and methyl) silica materials with cubic Pm3̅n and hexagonal p6m periodic mesostructures obtained in one step in the presence of the cetyltrimethylammonium bromide (CTA+B) surfactant are reported in this study. Understanding the way these complex materials form is difficult but undoubtedly necessary for controlling the material structure and its properties because of the combined presence of surface organic groups and large surface areas. Here, the mechanism of formation is clarified on the basis of the modeling of time-resolved in situ small angle X-ray scattering (SAXS) experiments, with a specific focus on the micelle evolution during material formation. Their fast self-assembly is followed for the first time with a quick temporal resolution of a few seconds using a third-generation synchrotron radiation source. To better understand the behavior of the complex organic-containing mesostructure, we perform a comparative study with the corresponding organo-free, isostructural materials obtained from three different surfactants (CTA+, CTEA+, and CTPA+) having a constant chain length (C16) and an increasing polar head volume (met-, et-, and prop-). Numerical modeling of SAXS data was crucial to highlighting a systematic sphere-to-rod micellar transition, otherwise undetected, before the formation of the 2D hexagonal phase in both organo-free and organo-containing systems. Then, two different pathways were found in the formation of the cubic Pm3̅n mesostructure: either an ordering transition from concentrated flocs of spherical micelles (from CTEA+ or CTPA+) for pure TEOS systems or a structural transformation from an intermediate 2D hexagonal mesophase in organosilane systems (from CTA+). Combining the comparison between organo-free and organo-containing systems with numerical modeling, we find that the hexagonal-to-cubic phase transition in the organically modified materials seems to be strongly influenced not only by the obvious presence of the organic group but also by the quicker and more massive condensation kinetics of silicate oligomers on the CTA+ micellar surface. Finally, quite unexpectedly, we find a wormlike-to-sphere micellar transition in the CTPA+ system.

Probing chemistry and kinetics of reactions in heterogeneous catalysts

“…For deuteriated benzene, DSLD ¼ 1.93 Â 10 À6ÅÀ2 compared with DSLD ¼ 3.47 Â 10 À6ÅÀ2 for the empty pore thus reducing the contribution of pore-pore features in the F(Q). The increased signal complexity above 1Å À1 is consistent with additional atomic-scale correlations corresponding to the absorbed benzene-d 6 . In real space, the decrease in intensity of the rst diffraction peak is matched by a 'washing out' of the periodic pore correlation (Fig.…”

Nanoscale Platelet Formation by Monounsaturated and Saturated Sophorolipids under Basic pH Conditions

“…In other words, by an unfortunatec oincidence, the diffraction peak could be hidden by the ellipsoidf orm factor, ac lassical phenomenon that is known to affect the relative intensities among Bragg peaks in ordered micellar systems. [75] Figure 7. Scheme summarizing the self-assembly behavioro fC18:1 and C18:0 SLs with pH.Atbasic pH, the presence of the COO À groupd rivest he formation of micelles andnanoscaleplatelet aggregates for bothcompounds.…”

An optimized protocol for the analysis of time-resolved elastic scattering experiments

“…This elastic scattering is collected at a detector that records the spatial position of the particle and the time of acquisition. Scattering from stationary or steady-state materials provides microstructural information about the quiescent or locally time-independent state; thus the material properties can be represented by the time-average over the experimental period, T. However, there has been a recent increase of interest surrounding time-resolved scattering experiments, where materials such as biological macromolecules, 1,2 gels, [3][4][5] vesicles and membranes, [6][7][8] polymers and polymer crystal phases, 9,10 and micelle solutions [11][12][13][14] have been investigated. The structure of materials responding to an applied excitation should not be analyzed by this simple time-average of the scattering, as the time-dependent nature of the response cannot be discerned.…”