Self-assembly of the cationic surfactant n-hexadecyl-trimethylammonium chloride in methyltrimethoxysilane aqueous solution: classical and reactive molecular dynamics simulations

High-Performance Methylsilsesquioxane Aerogels: Hydrolysis Mechanisms and Maximizing Compression Properties

Synthesis of methylsilsesquioxane aerogels by ambient pressure drying instead of supercritical drying has recently emerged as a major trend, but the issues of low mechanical strength and unstable performance still need to be resolved. This work reveals the microscopic formation mechanisms of gel skeleton based on the kinetic characteristics of methyltrimethoxysilane (MTMS) precursor hydrolysis and the associated sol-gel reactions. The effects of oxalic acid concentration (cOA) and hydrolysis time of MTMS solution (th) on the gelation time, morphologies, microstructures, chemical structure, and compression properties of the as-synthesized methylsilsesquioxane aerogels are investigated. The optimal cOA and th are 38.4 mmol/L and 120 min, respectively, endowing the methylsilsesquioxane aerogels with a compression strength of 0.170 MPa and a maximum compression strain of 61.2%. Precise control of the hydrolysis conditions ensures the formation of branched particle-to-particle networks, which is crucial for maximizing the compression properties of methylsilsesquioxane aerogels synthesized under industry-relevant conditions.

The correlation between the micelle morphology of surface-active ionic liquids with self-assembly and thermodynamic characteristics: coarse-grained MD simulation and experiment

Surface-active ionic liquids (SAILs) show great promise as novel green solvents due to their low vapor pressure, high thermal stability, high electrical conductivity, and bio-friendly nature to replace traditional volatile organic solvents in industrial processes. In the present work, the combination of coarse-grained (CG) molecular dynamics (MD) simulations with conductivity measurements was employed to explain the correlation between the micelle morphology and physicochemical and thermodynamic properties of self-assembly. A homologous series of SAIL molecules, 1-n-alkyl-3-methylimidazolium bromide [Cnmim][Br] (n = 4, 6, 8, 10, and 12), were chosen at various concentrations to shed light on this issue. Simultaneously two factors of concentration and alkyl chain length affected the morphology to control the physical and thermodynamic features. Moreover, the nature of the headgroup for two SAILs with the longest alkyl chain was assessed by shifting from imidazolium into ammonium. First, the critical micelle concentration (CMC), the degree of counterion dissociation of micelles, and the standard Gibbs energy of micellization of SAILs were determined using conductivity data. The micelle morphology such as the aggregation number, micelle radius, and moment of inertia was computed before, around, and after the CMC by MD simulation. Simulated results in accordance with the experimental measurements provide a quantitative understanding of the micellar properties. Increasing the alkyl chain length was associated with a non-spherical bigger micelle while the ammonium-based surfactant with a lower repulsion between neighboring monomers in micelles induced bigger and more spherical aggregates. Raising the SAIL concentration did not considerably influence the sphericity of the micelle except for the SAIL with the longest tail. The umbrella sampling method calculated the potential of mean force (PMF) for pulling a monomer of SAIL from a pre-assembled micelle into the solution. The dissociation energy of a SAIL monomer from a micelle increased with the tail length or with shifting into the ammonium head group and was substantially influenced by micelle morphology. Comparison between a sphere micelle with an oval one demonstrated that the dissociation of a SAIL monomer from a non-spherical shape needed a higher amount of energy. An improved understanding of how the shape of the SAIL micelles controls the physicochemical properties and stability helps to extend their application to different chemical processes.