Mechanisms of Silicon Alkoxide Hydrolysis–Oligomerization Reactions: A DFT Investigation

Abstract

Silica aerogels possess a variety of unique and remarkable properties, but the mechanisms of silicon alkoxide, Si(OR)(4), hydrolyses and oligomerization in the initial stage of sol-gel processes are still not well understood. On the basis of density functional theory calculations at the B3LYP/6-31G(d,p)//B3LYP/6-311++G(d,p) basis set level, the hydrolysis and oligomerization reactions of Si(OR)(4) in neutral, acidic, and alkaline solutions were systematically investigated and we found that in acidic solutions the precursor Si(OCH(3))(4) was inclined to hydrolyze rather than to condense and the hydrolysis processes were energetically more favorable than the neutral ones. In alkaline solutions, the hydrolysis products oligomerize through an S(N)1 dimerization mechanism and the condensation rates are fast to form denser colloidal aerogels. Our calculations also testify that the subsequent cyclization reactions are energetically unfavorable.

AFM-compatible microfluidic platform for affinity-based capture and nanomechanical characterization of circulating tumor cells

“…2a). In this way, the reversible physical bond between the PDMS and glass surfaces was feasible via NH-O hydrogen bonds formed between the silicon alkoxides (Si-O − ) 18 and protonated nitrogens ( Fig. 2a).…”

Zeolite Beta Formation from Clear Sols: Silicate Speciation, Particle Formation and Crystallization Monitored by Complementary Analysis Methods

“…The main difference between syntheses from heterogeneous hydrogels and from clear sols is the presence of the amorphous solid phase (gel) during the initial stage of crystallization in hydrogels, which has a direct impact on the crystallization pathways. Clear sols are prepared from silicon sources such as tetraethylorthosilicate (TEOS), producing monomeric silicic acid, which subsequently condenses into a score of oligomeric silicates . If the Si/OH ratio exceeds 1, a critical aggregation concentration is reached and oligomers aggregate into micellar hydrated ion pairs, designated as nanoaggregates (NAs).…”

The Practicality of Mesoporous Silica Nanoparticles as Drug Delivery Devices and Progress Toward This Goal

“…The pH of aqueous solutions used in MSN synthesis enables control of hydrolysis and condensation reaction rates needed to form an ordered silicon dioxide matrix (Scheme 1). The extent and rate of alkoxysilane hydrolysis plays a major role in oligomerization of the silica (8,9) which determines the stability and morphology of the MSN. The silica matrix is non-crystalline and contains abundant surface silanol groups, which allow further post-synthetic transformation of the silica surface.…”