The main reason behind the poor order in neutral surfactants is the weak (S0H+)(X−I+) interaction which becomes even worse in the absence of mixing. This weak attraction of silica-surfactant click here seeds plus the slow structuring step associated with quiescent growth are unfavorable for pore ordering. Enhancement of structural order in the (S0H+)(X−I+) route of MSU-type silica

was achieved in earlier studies by operating at a surfactant concentration higher than 16 wt% in acidic conditions (pH <2) [54] or by addition of a fluoride mineralizing agent (e.g., NaF) at neutral [50] or pH >2 conditions [55]. Our system achieved the mesostructure at 0.7 wt% surfactant concentration, so we believe that ordering can be improved in quiescent interfacial growth by the addition of a structure-enhancing agent. Mechanism of quiescent interfacial growth The above studies indicate that the quiescent interfacial approach for acidic synthesis of mesoporous silica is sensitive to growth parameters. TBOS or TEOS placed as a top layer diffuses

through the stagnant interface, hydrolyzes with water, and then condenses with surfactant seeds in the water. Similar to the colloidal phase separation mechanism in mixed systems [31], silica-surfactant composites in quiescent growth phase-separate and undergo further condensation, pore restructuring, and aggregation steps. Baf-A1 chemical structure Interrelation among these simultaneous steps, driven by the growth conditions, is not clear in quiescent approach, but they clearly dictate the final shape and structure. The product develops slowly into rich textural morphologies composing mainly of fibers attached to the interface and/or particulate shapes in the water bulk. These shapes possess wormlike mesochannels of uniform size and pore arrangement ranging from poorly ordered (particulates) to well-ordered p6mm-type hexagonal structures (fibers). The external morphology and internal structure vary with the type and content of the silica precursor, acid source (counterion), and surfactant type. The slow growth nature of the quiescent approach (order of days)

is attributed to the absence of mixing plus the slow interdiffusion among the hydrophobic (TEOS/TBOS)-hydrophilic (water) constituents. Silica source diffuses slowly from the top layer into the water causing a distribution Progesterone of silica concentration in the stagnant water bulk. This distribution can drive the condensation faster or slower. Moreover, the distribution is highly influenced by selleck inhibitor solvent concentration (water + alcohol) in the water phase driven by their tendency to evaporate at the interface [56]. By removing the solvent from the interface upon hydrolysis, surfactant seeds become more closely packed which enhances the structural order. Similarly, evaporation brings uncondensed silica species in contact which drives the system into faster condensation. Thus, the rate of silica diffusion and solvent evaporation are key determinants of shape and structure in the quiescent approach.