Influence of the Partial Hydrophobization of Fumed Silica by Hexamethyldisilazane on Interactions with Water

Abstract

Fumed silicas unmodified (S BET = 378 m2/g) and modified (379−285 m2/g) by hexamethyldisilazane reacting with silanols (concentration of grafted trimethylsilyl groups C TMS = 0.09−0.79 mmol/g) were studied by means of the NMR, IR, differential thermogravimetry, atomic force microscopy, microcalorimetry, adsorption, and theoretical methods. Variation in the surface composition and changes in the structural characteristics of primary and secondary particles lead to the nonlinear dependence of the Gibbs free energy of interfacial water, the heat of immersion of silicas in water, and the chemical shift δH(T) of water adsorbed from air on C TMS, despite a nearly linear decrease in water adsorption (at p/p 0 ≈ 0.8 for 24−72 h) with C TMS. An increase in C TMS causes alterations in the structure of the hydrogen bond network in the interfacial water and other properties of water strongly (700−300 mg/g silica) and weakly (1400−500 mg/g) bound to the silica surfaces.

Structured water in partially dehydrated yeast cells and at partially hydrophobized fumed silica surface

“…Similar modified silicas were described in details elsewhere [35,55,56]. The amounts of grafted TMS groups for chosen series of modified silicas (Table 1, C TMS ) correspond to 30-50% of initial content of surface SiOH groups; i.e., significant amounts of silanols remain.…”

“…The preparation of modified silicas and their structural and adsorptive properties were described in details elsewhere [35][36][37][38][39]. Choice of modified fumed silica is caused by a weaker effect of confined space between primary particles in their aggregates and between these aggregates in agglomerates in comparison with porous adsorbents [30,31,[35][36][37][38][39]. There are several effects controlling interaction of water with nonpolar solvents or function- Table 1 Structural characteristics of silica samples modified by HMDS Note.…”

“…A technique of determination of the pore size distributions (as gaps between spherical primary particles of fumed silica) used for calculations of certain parameters shown in Table 1 was described in detail elsewhere [11,32,[35][36][37][38][39]. Calculation of the fractal dimension (Table 1, D AJ ) was performed using the FrenkelHalsey-Hill equation [42] applied to the nitrogen adsorption data at p/p 0 0.85.…”

Nanocomposites with fumed silica/poly(vinyl pyrrolidone) prepared at a low content of solvents

“…The use of the pore model with a mixture of cylindrical pores (since the structure of treated silica samples becomes close to that of silica gel characterised by cylindrical pores) and gaps between spherical particles leads to significantly smaller values of the deviation (Dw mix is between 0.1 and 0.25, but Dw sph is between 0.6 and 1.2). Typically modification of silica surface by different functionalities leads to diminution of the adsorption potential and the fractal dimension (D AJ ) [14,16,23,[38][39][40]. The PVP immobilization onto the silica surface leads to a decrease in the D AJ value in comparison with that of the initial silica.…”

Interaction of unmodified and partially silylated nanosilica with red blood cells

“…This modification reduces the specific surface area by approximately 15% but the hydrophilic properties are virtually untouched since the heat of immersion in water and Gibbs free energy of the interfacial water layer are close to the corresponding values for the initial silica A-300 [15][16][17][18]. Aqueous suspensions of unmodified and modified nanosilicas were prepared by mixing of the dry powder with 3.8 wt.% solution of trisodium citrate (TSC).…”