J. Phys. Chem. 1986, 90, 6051-6053
6051
Adsorption and Diffusion at Low Electrolyte Concentrations Alexandre Goman,+ Simo Liukkonen,* Helsinki University of Technology, Department of Chemistry, SF-021 50 Espoo. Finland
and Pentti Passiniemi Neste Oy, Technology Group, SF-06850 Kulloo, Finland (Received: December 17, 1985; In Final Form: June 6, 1986)
Adsorption and surface diffusion were measured inside the Pyrex glass capillary by using labeled 22NaC1in about M aqueous sodium chloride solution. The closed capillary method was applied. It was found that the amount of adsorption in a 2-mm-diameter capillary represents typically 5%, corresponding to a surface layer of 30 pm thickness. In about 2-mm and 1-mm (diameter) capillaries, the diffusion coefficient measured was 3.8% and 7%, respectively, higher than the bulk M solution. This corresponds to a surface diffusion coefficient of 2.33 X m2s-' compared to 1.332 coefficient in a X lo4 m2 s-l in bulk solution.
Introduction Adsorption is frequently encountered when solid-liquid interfaces are presnt in experiments. There are some statements about adsorption connected with diffusion measurements,'-' too. However, quantitative data are often lacking for adsorption effects and also their interpretation is not clear. Only some remarks on simultaneous adsorption and diffusion are d i s c ~ s s e d .Also ~ when measuring diffusion coefficients with the diaphragm cell method the peculiar results below about 0.05 M electrolyte solutions have been attributed to adsorption on the glass capillaries of the diaphragm. The aim of this work was to measure both phenomena under suitable conditions and to interpret the results quantitatively. We chose the closed capillary method, which can be applied to very low concentrations where adsorption can be ~ i g n i f i c a n t . ~The .~ diffusion under study was the tracer diffusion of sodium (22Na+) M aqueous sodium chloride solution. The ions in a 1.1 X capillary material on which the adsorption of 22Na+took place was commercial Pyrex glass. Experimental Section Materials. Sodium chloride solutions (1.1 X M) were prepared by diluting the standard NaCl (Normanal N / 10, No. 7653) without further purification. The water used for dilutions was triple distilled. The radioactive tracer solution was made by labeling the above solution with carrier-free 22NaC1supplied by Radiochemical Centre, Amersham, England. The specific activity of the stock solution was about 200 &i/cm3. The pH of the active solution was 6.5. The capillaries and their closing system are described in an earlier paper.s Adsorption Measurements. Capillaries of different diameters and of nearly the same length (3 cm) were filled with an active solution containing the tracer sodium chloride. Before the first filling the capillaries were not treated in order to avoid any changes on the inner surface of the capillary. In all cases the capillaries were inserted into polyethylene bottles which were then closed. The activity in the capillaries was measured in a well-type scintillator connected to a high-voltage supply and a single-channel analyzer (Wallac Co, Turku, Finland). After a chosen time the active solution was carefully removed from the capillary with a glass syringe. The activity remaining in the capillary was measured. The capillary was then refilled with the same active solution that was first removed and the activity was then again measured. This procedure was repeated several times. The total contact time of the active solution on the surface varied from about 0.5 to 2 h (cf. Figure 1). The height of the plateau b-c represents 2 4 % Present address: Technical University of Prague, Faculty of Nuclear Science and Physical Engineering, Biehovi 7, 11519 Prague, Czechoslovakia.
0022-3654/86/2090-6051$01.50/0
of the initial activity depending on the capillary - diameter. The final activity (i.e., &) is ;oughl;lO% of the value for plateau b-c. Figure 1 gives the idea that there exist several adsorption layers, one of which consists mainly of a thin liquid layer. This layer is easily removed. There is also a stronger bounded thin layer which is more difficult to remove. This layer indicates heterogeneous isotope exchange between the active 22Na+ions of the solution and the inactive sodium atoms of the glass surface. To support the above idea of the probable existence of an adsorbed thin liquid layer inside the liquid content of the capillary we also performed one careful experiment of the following type: A capillary of 2.23-mm i.d. was first filled with the active solution and its activity was measured. The lower part of the capillary was then immersed into benzene, and its upper part was tightly connected to a syringe having a middle bottle between the capillary and the syringe. The temporary bottom of the capillary was removed under the level of the benzene liquid. The active solution in the capillary was thus replaced by sucking the active solution out into the middle bottle. After rejoining the bottom to the open lower end of the capillary, the activities of the capillary and the bottle were separately measured by the method described above. The sum of these two activities corresponded to that of the capillary initially. The activity remaining in the capillary was of the same magnitude as in Figure 1 (at the level of plateau b-c). In a third type of experiment glass rods of 4.5-5-mm diameter and of 2-3-cm length were immersed into water for about 5 min. The rods were drawn out of water, put into polyethylene cylinders which were immediately closed and weighed. A possible drop at the lower end of a rod was removed by a gently shaking. In one version the rods were closed inside a polyethylene syringe and water was pushed away by pressing the piston quite suddenly. The water layer remaining on the rods was weighed and its thickness was calculated by assuming a homogeneous spreading of water (connected with the wetting power of the glass) on the surface. These results (28 experiments) are in accordance with the former experiments. Diffusion Measurements. The arrangement for the measurements was the same as given in ref 8 and 9. The capillary length (1) Crank, J. Mathematics of Diffusion; Clarendon: Oxford, 1975; 2nd ed, p 350. (2) Tyrrel, H. J. V.; Harris, K. R. Diffusion in Liquids; Butterworths:
London. 1984: D 109. (3) Standing H. A,; Warwicker, J. 0.; Willis, H. F. J . Text. Inst., Trans. 1947, 1335-1349. (4) Podhajecky, P.; Benes, P.; Gosman, A. Radioanal. Chem. 1971, 51, 315-324. ( 5 ) Mills, R. Rev. Pure Appl. Chem. 1961, 11, 78-91. (6) Robinson, R. A,; Stokes,R. H. Electrolyte Solutions; Butterworths: London, 1959; 2nd ed, pp 256 and 316. (7) Mysels, K. J.; McBain, J. W. Colloid. Sci. 1948, 3, 45-60. (8) Passiniemi, P. J. Solution Chem. 1983, 12, 801-813. (9) Passiniemi, P.; Liukkonen, S.;Noszticzius, Z. J . Chem. SOC.,Faraday Trans. 1 1977, 73, 1834-1839. ~
0 1986 American Chemical Society
6052
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 ACT I V I TY
t
Gosman et al. Cs = KCb (3) in which K is the adsorption equilibrium coefficient. The ratio of the surface volume to the total geometric volume of the capillary can be expressed as
I
- d r - 6Yh
p = - vs= V
TIME
---*I P-
rr2h
2 - (ti) 2 r
(4)
in which r is the radius and h the height of the capillary. In eq 4 B is apparently a poorly geometric factor having the limiting values
Figure 1. Schematic view of an adsorption experiment. The scales on
both axes are given in arbitrary units: (a) Activity (counts/min) of the capillary first filled; (b-c) activity of the capillary emptied; (c-d) first rinsing with water; (d-e) repeated rinsings. The average time scale was for a-b and c-d was several minutes, for b-c some 10 min, and for d-e of the order of hours.
=
p
-
-
0, when r
1, when r
-
(constant c)
(5a)
6 (constant c )
(5b)
03
After an analysis of the physical meaning of eq 3, it results that c, increases with increasing cb but only to a saturation value &at). The total adsorption layer 6, in reasonable experimental time intervals (excluding diffusion in deep glass layers), has a limiting capacity. Thus the limiting value of K, representing the slope in c, vs. cb coordinates, equals lim K =
when cb
t,
-
c(max)
(6)
where c(max) represents the maximum solubility of the solute and e can be very small, i.e. O
