Protonation, Hydrolysis, and Condensation of Mono- and Trifunctional Silanes at the Air/Water Interface

Abstract

The protonation, hydrolysis, and condensation kinetics of octadecyldimethylmethoxysilane (OMMS) and octadecyltrimethoxysilane (OTMS) at the air/water interface were investigated using a monolayer trough. OTMS chemical condensation within physically condensed phases was observed in transferred monolayers using fluorescence microscopy. Molecular area increases and decreases attributed to protonation and hydrolysis, respectively, of silane methoxy groups were measured by a surface balance. These area changes at constant surface pressure suggested a stepwise protonation and hydrolysis of the three OTMS methoxy groups. In contrast, only a single protonation and hydrolysis event was observed for monofunctional OMMS. The influences of monolayer spreading time, silane packing density, and subphase pH on the reaction kinetics are presented.

Separating Octadecyltrimethoxysilane Hydrolysis and Condensation at the Air/Water Interface through Addition of Methyl Stearate

“…12 Due to the reactive nature of OTMS, the π-A isotherms are highly dependent on incubation times and compression rates. 5 This reactivity is evident in the A(t) isobars shown in Figures 3 and 4. Figure 3 shows the influence of SME on the OTMS A(t) isobars (π = 10 mN/m).…”

“…For the 98:2 OTMS:SME ratio, electrostatic repulsion is prevalent and two molecular area increases are observed, possibly corresponding to a stepwise protonation of methoxy groups on OTMS as previously reported. 5 Nonetheless, even 2% SME is sufficient to accelerate the reaction rate 2-fold (reaction complete in 15 h) as compared to the pure OTMS monolayer in Figure 3 which does not reach a limiting molecular area until 30 h.…”

“…5 The interfacial reactivity of the OTMS sol-gel process have been monitored either as molecular area (A) or surface pressure (π) changes over time. The resultant OTMS isobars and iso-area curves display features which have been attributed to the formation of protonated reaction intermediates (A or π increases) 5 followed by hydrolysis and condensation of these intermediates (A or π decreases). [4][5][6][7] These reactions are given below: (1) (2) (3) (4) where R stands for the methoxy group, -OCH 3 , and R for the octadecyl chain, -(CH 2 ) 17 -CH 3 .…”

Preparation of silane-coated TiO2 nanoparticles in supercritical CO2

“…Characteristic bands corresponding to the organic part of the different used silane molecules were those appearing at around 2955 and 2870 cm À1 (CH 3 antisymmetric and symmetric stretching, respectively), 2918 and 2851 cm À1 (CH 2 antisymmetric and symmetric stretching, respectively) and 1467 cm À1 (CH 2 symmetric bending) [12,40,45,46]. The pattern of alkyl groups in the silane monolayer was assessed by means of the position of the CH 2 stretching bands [12], which has a value of $2924 cm À1 for disordered structures like liquid alkane and is shifted to a lower wavenumber ($2915-2918 cm À1 ) in crystalline alkanes [47]. Hence, according to the literature CH 2 stretching frequencies obtained for supercritically silanized samples corresponded to well-ordered structures.…”

Interfacial Behavior of a Reacting Alkoxysilane Monolayer System

“…The initial protonation (Reaction 1a) of the alkoxide group to form the oxonium ion intermediate brings about electrostatic repulsion between the head groups resulting in the initial expansion as noted elsewhere. [23] The initial expansion eventually results in a net area reduction as the alkoxide groups are converted to silanols or Si-O-Si covalent bonds.…”