Abstract

The diffusivity coefficients of lithium sulfate solutions in water and water and silica gels over the concentration range 0.2 to 2.5 molal have been measured for the first time with micro-Raman spectroscopy for a horizontal-tube capillary, conventional 90° Raman scattering for a narrow-bore vertical-tube system, and FT-Raman spectroscopy for the gels. Data were obtained for axial concentration profiles as function of the distance and for time elapse at fixed positions on the tube axis with the use of integrated intensity measurements of the <i>v</i>₁ (SO₄) symmetric stretching band at 980 cm^-1. The use of computer model calculations to fit the experimental data yielded values of <i>D_A</i> = 0.4 × 10^-5 cm² s^-1 for the vertical system and 0.3 × 10^-5 cm^-2 s^-1 for the gel system. Discrepancies observed for the horizontal system have been ascribed to the presence of convective flow. A convective flow rate of 5.10^-2 cm h^-1 was calculated in this case with the use of computer simulation and used in the fitting of the experimental data through a general form of the second Fick's law. Under these conditions, a <i>D_A</i> value of 0.2 × 10^-5 cm² s^-1 was obtained.

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