Silica computer simulations

Garofalini, S.H. (1990) Molecular dynamics computer simulations of silica surface structure and adsorption of water molecules, J. Non-Cryst. Solids, 120, 1. 

CT, 1999, pp. 441—451. The Dynamics of Non-Crystalline Silica Insight from Molecular Dynamics Computer Simulations. 

Magnetic nanoparticles in the sol-gel silica glass were obtained by heat treatment at 1000°C in air during 6 h and identified by X-ray and Mossbauer spectroscopy as y-Fe203 (maghemite) [11], The magnetic parameters of maghemite used in computer simulations of the SPR spectra, are Ms = 370 kA m 1 and = —4.64 kJ m"3 (cubic symmetry) [35],... 

The extensive studies on the structure [72, 89] and Raman and Brillouin spectra [68-70, 73], as well as computer simulation results [77-82] have revealed that in the 8-50 GPa pressure range and at room temperatures, the silica glass is subject to a broad transformation accompanied by a change in the short-range order structure and an increase in the coordination number from 4 to 6. It should be noted that during coordination transformation at intermediate pressures, many silicon atoms have a fivefold coordination. The main part of the transformation takes place in a narrower pressure range of 10-40 GPa. 

Rabouille C. and Gaillard J. F. (1990) The validity of steady-state flux calculations in early diagenesis a computer simulation of deep-sea silica diagenesis. Deep-Sea Res. 37, 625-646. 

The potential of Eq. (1) with parameters determined in Refs. [10, 11] was thoroughly tested in computer simulations of silica polymorphs. In Ref. [10], the structural parameters and bulk modulus of cc-quartz, a-cristobalite, coesite, and stishovite obtained from molecular dynamics computer simulations were found to be in good agreement with the experimental data. The a to / structural phase transition of quartz at 850 K ha.s also been successfully reproduced [12]. The vibrational properties computed with the same potential for these four polymorphs of crystalline silica only approximately reproduce the experimental data [9]. Even better results were reported in Ref. [5] where parameters of the two-body potential Eq. (1) were taken from Ref. [11]. It was found that the calculated static structures of silica polymorphs are in excellent agreement with experiments. In particular, with the pressure - volume equation of state for a -quartz, cristobalite, and stishovite, the pressure-induced amorphization transformation in a -quartz and the thermally induced a — j3 transformation in cristobalite are well reproduced by the model. However, the calculated vibrational spectra were only in fair agreement with experiments. 

Now we consider simulations of the atomic structure of amorphous oxides especially at their surfaces. We begin with the best studied structure of amorphous silica. The structure of the bulk amorphous silica is generally obtained in computer simulation by first simulating liquid silica at a high temperature of about 4000 K [6] or even 6000 K [7]. This stage is followed by the simulation of quenching when the temperature steadily decreases at the rate of 10 lO " K/s. This rate of cooling is the lowest practically achievable in... 

It is well known that the energy of interaction of an atom with the continuous solid is 2-3 times less than with the discrete (atomic) model (cf., e.g., Ref. [38], Figs. 2.2-2.4). Thus, to obtain the same Henry s Law constants with the two models, one has to increase e for the continuous model. This, however, does not discredit the continuous model which is frequently used in adsorption calculations. In particular, we can use the above mentioned results of Ref. [37] to predict the value of e for Ar which would have been obtained if one had carried out Henry s Law constant calculations for Ar in the AO model of Ref. [17] and compared them with experiment. One can multiply the value of e for CH4 obtained from AO model by the ratio of e values for Ar and CH4 in the CM model [36] to obtain tjk = 165A for Ar in the AO model. This is very close to the value of 160 K obtained in Ref. [21, 28] by an independent method in which the value of the LJ parameter e for the Ar - oxide ion interaction was chosen to match the results of computer simulation of the adsorption isotherm on the nonporous heterogeneons surface of Ti02. Considering the independence of the calculations and the different character of the adsorbents (porous and nonporous), the closeness of the values of is remarkable (if it is not accidental). The result seems even more remarkable in the light of discussion presented in Ref. [28]. Another line of research has dealt with the influence of porous structure of the silica gel upon the temperature dependence of the Henry constants [36]. 

Figure 8. Transmission electron micrographs of typical clusters of gold, silica, and polystyrene colloids, prepared by both diffusion-limited and reaction-limited cluster aggregation and by computer simulation. There is a striking similarity in the structure of the clusters of different colloids in each regime.

Figure 4. 187-MHz CRAMPS spectra ofMCB silica gel (A) evacuated at 100 °C and (B) evacuated at 500 °C. Plot C is a deconvolution of spectrum A, and plot D is a computer simulation based on C.

Figure 2 shows H CRAMPS spectra (39) of silica gels. Parts A, A, and A" of Figure 2 show that the spectrum obtained on the untreated sample can be computer-simulated as the sum of contributions from relatively sharp, symmetrical peaks centered at 1.7 and 3.5 ppm and an asymmetrical peak or band extending from about 8 to about 1 ppm. The spectrum of the silica gel after a 25 °C vacuum dehydration (Figure 2B) allows the peak centered at 3.5 ppm in Figure 2A to be easily identified as physisorbed water. 

Figure 16. 29Si NMR spectra of spinnable silica sol prepared from TEOS (HsO/Si =1.5). Spectrum a, experimental, 1.0-Hz exponential line broadening spectrum b, experimental, 30-Hz exponential line broadening spectrum c, computer simulation and spectrum d, resonance components of computer simulation. (Reproduced with permission from reference 112. Copyright...

The chemistry and surface properties of porous and non-porous silica were extensively reviewed [8,9]. We have organized this review in several sections starting with a very brief account of mesoporous solids synthesis and their main structural characteristics (see the section on Mesoporous SUica ). This section also comments on the preparation of chemically modified solids and their properties as well as how the mesoporous structure can be controlled. We also present some theoretical results obtained through computer simulations. Those smdies model the sohd structure of sdica that will be employed in other simulations. Section 111 is devoted to the... 

Computer simulation proved to be an excellent tool to study a large variety of problems. Stmctore of solids and interfacial processes were largely studied by numerical simulations of the corresponding real systems [49-52]. The specific cases of bulk silica and silicate glasses were reviewed quite recently [53]. Amorphous silica was the material chosen to study by computer simulation in several cases [41,54-56]. 

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