Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 5, pp. 658−661.
Pleiades Publishing, Ltd., 2013.
Original Russian Text © G.S. Akuzhaeva, S.V. Chaika, Yu.Yu. Gavronskaya, V.N. Pak, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 5,
Comparative Characterization of the Diffusion Mobility
of Aqueous Calcium Salt Solutions in Porous-Glass Membranes
G. S. Akuzhaeva, S. V. Chaika, Yu. Yu. Gavronskaya, and V. N. Pak
Herzen State Pedagogical University of Russia, St. Petersburg, Russia
Received February 12, 2013
Abstract—Diffusion coefﬁ cients of calcium acetate, nitrate, and chloride, which characterize the transport of their
aqueous solutions across porous-glass membranes with predominant pore radii of 4.5–70 nm, were determined. A
decrease in the membrane pore radii is accompanied in all cases by an exponential fall of the diffusion mobility.
The noticeable difference between the anion mobilities, which favors salt separation, grows with increasing radii
of transport channels. In a membrane with a pore radius of 4.5 nm, the salt diffusion mobility series is changed.
The possibility of successfully solving a number of
problems associated with concentration and separation
of solution components is governed by the development
and application of membranes with nanometer transport
channels. The important properties of membranes
include, together with their spatial structure, the
composition and structure of their surface, mechanical
strength, chemical stability, and radiation hardness.
These requirements are fully satisﬁ ed by porous-glass
(PG) membranes obtained as plates pierced by through
channels by successive acid and alkaline treatments
of liquated glasses of the sodium borosilicate system
[1–4]. Depending on the synthesis conditions, PG
membranes may have channels with radii in the range
from several to hundreds of nanometers [1–4]. The
structure and properties of the hydroxylated surface of
PG membranes are similar to those of the well studied
varieties of amorphous silica: silica gels and aerosils.
To design and perform membrane ﬁ ltration
processes, it is necessary to determine how the diffusion
coefﬁ cients depend on the radius of transport channels
and chemical nature and concentrations of solutes. On
the whole, it has been reliably established [2, 3, 5–8]
that the diffusion rate of electrolytes in PG membranes
is noticeably lower than that in free solutions. It has
been shown [5–8] that the limitation of the mass transfer
of aqueous salt solutions is reliably represented by an
exponential dependence of diffusion coefﬁ cients D on
the radius r
of transport channels in PG:
D = D
→ ∞) approaches the value for a bulk
solution, and the dimensional parameter K
the size of the water layer structured by the silica surface.
The exclusion (or weak involvement) of the near-wall
water in the transport of electrolytes, reﬂ ected by the
numerical value of K
, is clearly manifested in studies
of the concentration and temperature dependences of the
mass transfer [5–8]. It has also been found [5–8] that the
salt diffusion rate in PG membranes strongly depends
on the ability of cations to disintegrate the boundary
layer of water, determined by their charge and tendency
to undergo hydration. In this context, it seems necessary
to determine the nature and extent of the inﬂ uence
exerted by the type of anions on the rate of the diffusion
transport of salts. In the present study, this problem
is considered for the example of diffusion of aqueous
solutions of calcium chloride, nitrate, and acetate in a
set of PG membranes with pore radii of 4.5 to 70 nm.