Ultrasonic speeds and thermodynamic properties for tetramethylsilane, tetraethylsilane and tetraethoxysilane under high pressures

Ultrasonic speeds u in tetramethylsilane TMS, tetraethylsilane TES and tetraethoxysilane TOS were measured by a "sing-around" technique operated at a frequency of 2 MHz. The present results cover temperatures from 283.15 to 333.15 K and pressures up to 200 MPa for TMS and 100 MPa for the other two liquids, with an observed error of less than ± 0.3%. The u values for these compounds, affected more largely by temperature and pressure changes than those for toluene and poly(dimethylsiloxane), show that the order of magnitude is TMS < TOS < TES, which differs from that for the densities reported elsewhere. Among the compounds studied, the pressure effects on the u values in TMS and TES are notably larger than those in TOS, in spite of the fact that the densities of TOS displayed a larger pressure effect than did the densities of TES. It was found that the order of magnitude of numerical u value and the pressure effect on the ultrasonic speeds correspond excellently to those on the intermolecular free lengths derived by Eyring's liquid model using pVT.