Effects of organic acids on sol?gel transition of silicic acid?Arheological study

Abstract

The sol–gel transition in the silica system is dependent upon the degree of hydrolysis and polymerization reaction in the system [1]. The polymerization reaction for the formation of the gel network occurs in different stages, i.e. polymerization of monomer to form particles, growth of particles and finally linking of particles into chains and the formation of the network which extends throughout the liquid medium. The structural evolution through the growth of silicate polymers from the genesis to the gel point has been well established by various in-situ methods such as NMR, vibrational spectroscopy and small-angle scattering. However, a rheological study can be an important tool to characterize sols and sol–gel transition and can give a qualitative information on the structure at the time of gelation. Many processing parameters, such as the presence of a catalyst, H2O:Si ratio, type of solvent, pH, temperature, steric and inductive factors, etc. effect the mechanisms of hydrolysis and condensation of silicon alkoxide [1, 2]. For the hydrolysis and condensation rate of silicon alkoxide, the effect of a catalyst was investigated by Pope and Mackenzie [3], and the dependence of gel time on H2O:Si ratio was observed by Klein [4]. Artaki et al. [5] showed the effect of different solvents on the condensation rate of silicon alkoxide. Colby et al. [6] reported the effects of temperatures on the gelation of silicon alkoxide and the pH dependence on the hydrolysis and condensation of the same system is also well known [1, 2]. Voronkov et al. [7] pointed out that steric and inductive factors influenced the condensation rate of silicon alkoxides. In the present work, a rheological study, i.e. the determination of gelling time tg, the dynamic flow behaviour, the viscoelastic property of the sols and gels and its relation to relaxation exponent n and fractal dimension df in a three-dimensional space during hydrolysis and condensation of tetraethyl orthosilicate (TEOS), in the presence of different organic acids, i.e. oxalic acid, citric acid and succinic acid was carried out at the same pH, temperature and H2O:Si ratio in the system. Silicic acid sols were prepared by using (initially) alcohol-free TEOS (Fluka Chemica, Purum grade), deionized water and different organic acids, i.e. oxalic acid, citric acid and succinic acid (AR grade, S. D. Fine Chem. Pvt. Ltd.) in the molar ratio of 14:1:0.2 respectively at 32 ± 1 ◦C. The system was stirred magnetically and the temperature was increased to 70–80 ◦C. After stirring for about 15 min, a clear sol was obtained [8]. The pH of each sol was adjusted to 3.5 by addition of dilute NH3 (25 wt%, G. R. grade, E. Merck, India). The rheological measurements of the sols were carried out by using a Haake rheometer (Rotovisco, model: RT20) at 30 ◦C. The instrument was connected to a computer, loaded with Haake software (Version V3). A cone and plate sensor (c60/1◦) was used for all measurements. Steady shear flow measurements of the samples in controlled rate (CR) mode were carried out by increasing the shear rate from 0 to 200 s−1 over 30 s followed by an immediate decrease in the shear rate from 200 s−1 back to zero in another 30 s. The dynamic flow behavior, i.e. loss modulus (G ′′) and storage modulus (G ′) of the sols and gels was determined using oscillatory flow with varying radial frequency (ω) i.e., 0.924, 1.985, 4.279 and 19.855 rad s−1. Loss tangent (tan δ) can be obtained as,