Dimensional and concentration dependences of diffusion of aqueous alkaline-earth metal nitrate solutions in porous-glass membranes

Abstract

Diffusion coeffi cients characterizing penetration of aqueous solutions of magnesium, calcium, strontium, and barium nitrates across porous-glass membranes with predominant pore radii of 4.5-70 nm were determined. A decrease in the pore radii of the membranes is accompanied in all cases by an exponential fall of the diffusion mobility. A reversal of the diffusion mobility series was observed for membranes with pore radii of 4.5 and 70 nm on the background of the general tendency toward a concentration-related rise in the diffusion coeffi cients.The decrease in the mobility of solution components in membranes, observed as the radius of transport channels becomes lower, seems to be apparent. However, the nature and, moreover, the analytical form of this dependence can only be found by performing experiments on a number of membranes of the same chemical nature with variable and rather precisely settable parameters. The fact that this opportunity is provided in the case of a porous glass (PG) creates exclusive conditions for studies and practical application of the dimensional specifi c features of membrane separation and concentration processes. The development of studies in this area is mostly restricted by the unavailability of a set of PGs as a commercial product. At the same time, both general principles and particular specifi c features of the related preparative procedures can be considered suffi ciently developed [1][2][3][4][5], which served as a basis for fabrication of mechanically strong GS membranes with reliably controlled structure of their inner space. The use of these membranes in a series of studies [6-9] made it possible to reliably confi rm the known concept [10,11] that the transport of electrolyte solutions across thinporous hydrophilic membranes is largely controlled by specifi c properties of structured near-wall layers. For example, it has been shown [6-9] that there is a stable "dimensional" dependence of the diffusion coeffi cients of aqueous solutions of a number of inorganic salts on the pore radius of PG membranes D = D 0 exp(-K p /r p ), where D 0 corresponds to a free solution (r p → ∞), and parameter K p characterizes the effective thickness of the near-wall structured layer with a hindred diffusion.The substantial decrease in the numerical values of K p and the corresponding rise in the diffusion mobility may be due to the specifi c destructive effect of electrolytes, which causes a decrease in the thickness of near-wall layers with increasing concentration of aqueous salt solutions [6-8, 10, 11].In this communication, we report generalized results of a study of the diffusion transport of alkaline-earth metal nitrates in PG membranes. The porous structure parameters of the membranes and the experimental conditions of mass-transfer measurements have been described in detail previously [6]. The dependences shown in Fig.1a demonstrate specifi c features of diffusion for the example of calcium nitrate solutions, typical of all the systems studied. The diffusion

Diffusion transport of Ca(NO3)2 aqueous solution in porous glass membranes

“…[3,4] The quantitative dimensional peculiarities of aqueous salts solutions diffusion transport through the set of PG membranes with varied pore radius (r p ) have been studied only in our works. [3,5,6] The base of the results interpretation and numerical simulation was proposed in the form of the equation:…”