3447
SURFACE CONDUCTIVITY OF ALUMINOSILICATES
Electrochemistry of the Interface between Some Aluminosilicate Crystals and Salt Solutions. I.
Surface Conductivity
by S. D. James' Department of Physical Chemistry, University of Melbourne, Victoria, Australia
(Received April 6 , 1966)
Interfacial conductivities have been measured for crystal plugs of chabazite, beryl, and microcline (with particular emphasis on chabazite) bathed by salt solutions. These conductivities are much too high to arise through excess ions in the diffuse layer. It is suggested that they originate in a skin of silica gel surrounding crystals, formed by the leaching of A1 from crystals by H+ and subsequent recondensation of polysilicic acid with the leached surface. In the case of chabazite-sodium chloride system, it is shown that the conductivity is due solely to acidic dissociation of SiOH groups. In other solutions, adsorption of salt into the gel layer may contribute significantly to the conductivity. It is suggested that the surface conductivity of borosilicate glasses in aqueous solution may arise in an analogous manner by the hydrolytic extraction of B.
Although the electrokinetic properties of alumina, silica, and the clay aluminosilicates have been extensively investigated, similar data on other crystalline aluminosilicates appear to be very sparse. In particular, the crystalline zeolites seem to have been totally neglected in this respect. In view of the well-defined structures of these cation-exchanging crystals and the large variation in framework charge and openness exhibited by different zeolitic species, a systematic investigation of their surface electrochemistry should be of interest. Apart from interest attaching to the effect of crystal structure on surface properties, the zeolites provide structurally well-defined analogs of the silicaalumina gels of such importance as cracking catalyts in the petroleum industry. The zeolite chosen for the present study was chabazite (0.5Ca, Na(A1Si20a).3H20). As with all the 3d framework zeolites, the structure3is composed of tetrahedra of SiO4*- and A10d5- in which each 0 is shared between two adjacent tetrahedra, the unit negative charge on tetrahedral aluminum being compensated by an adjacent cation. The stacking of tetrahedra is such that a pattern of intersecting channels is repeated regularly throughout the structure and the cations and water molecules located in these channels can migrate throughout the crystal.4a Both uni- and bivalent cations may exchange between chabazite and external
salt solutions, but anions are excluded from the crystal by the Donnan effect. This Donnan exclusion is cerowing to the tainly complete below 2 rn external large negative charge on the framework (3.9 mequivl hydrated g) and the relative density of the structure (free diameter of the eight-membered silicate rings, limiting migration, is roughly 5.9 A). Chabazite is particularly suitable for electrokinetic studies as the naturally occurring crystals are widely distributed and sufficiently large for streaming potential work on porous plugs. Although highly conducting compared with the average ionic crystal, chabazite is of sufficiently low conductivity relative to the aqueous solutions used in the present work to make electrokinetic measurements of well-defined significance. For purposes of comparison, electrokinetic measurements were made on two other crystalline aluminosilicates, beryl and a feldspar. The feldspar used (microcline) is a 3d framework bearing a negative charge similar in magnitude t o that of chabazite and compensated mainly by the univalent cations E(+ and ~
(1) Nuclear Engineering Department, Brookhaven National Laboratory, Upton, N. Y. 11973. (2) R. M. Barrer, Proc. Chem. SOC.,99 (1958). (3) L. S. Dent and J. V. Smith, Nature, 181, 1794 (1958). (4) (a) R.M. Barrer and D. C. Sammon, J. Chem. SOC.,2838 (1955); (b) R. M. Barrer and W. M. Meier, ibid., 299 (1958).
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Na+. An important difference from chabazite is that the feldspars are the most dense of the crystalline aluminosilicates and the cations are inaccessible to ion exchange, being trapped within the ~ t r u c t u r e . ~ Beryls is built from the packing of discrete silicate rings, (-O-Si02-)612-.2A13+, 3Be2+. The A1 and Be are located between adjacent 12-membered rings which are stacked, one above the other, giving a system of parallel, nonintersecting pores of free diameter 2.52.6 A. The framework charge is effectively aero, since the small polyvalent cations are virtually undissociated from the silicate rings.
Experimental Section Materials. A sample of natural chabazite from Collingwood, Victoria, was kindly provided by Dr. Beasley of the National Museum, Melbourne. The crystals were separated from inclusions of stillbite and ground to a suitable size (60-120 mesh) for streaming measurements. The crystals were characterized by refractive index and X-ray powder diffraction patterns. The atomic ratio of Si to A1 was determined by chemical analysis as 2.09, as compared with the “ideal” formula ratio of 2.00. To avoid surface contamination, no chemical cleaning was performed on the crystals after grinding. For some of the measurements, the natural, calcium-rich crystals were converted to the Na form by exhaustive ion exchange from near-boiling, 20% NaCl solutions. Samples consisting of 14 g of 60-120 mesh crystals were given overnight treatments with three successive 800-ml portions of well-stirred solution on a steam bath. Samples of beryl and microcline from Londonderry, W. A., were obtained from Mr. W. Trahar, C.S.I.R.O., Melbourne. The crystals had been cleaned by cycling with HC1 and NaOH while coarse, followed by grinding to about 7 5 mesh size and storing under conductivity water. Salts used were of reagent grade or recrystallized materials. Only doubly distilled water was allowed to contact the crystals in all measurements. The second distillation was from alkaline permanganate in an allglass Pyrex still. The conductivity of the water varied from 1 to 1.6 X lov6mho/cm. Acetone was redistilled in an all-glass Pyrex still. Its conductivity never mho/cm. exceeded 0.08 X Measurement of Surface Conductivity. RIeasurement of the conductivity of crystal plugs was normally made in flowing salt solutions. The streaming cell used was that of Buchanan and Heymann6 except that the perforated Pt electrodes enclosing the crystals were given a light platinization instead of an Ag-AgC1 coating. T h e Journal of Physical Chemistry
The cylindrical crystal plugs were 1.1 cm in diameter 3 cm long, and usually contained about 8 g of crystals. Solutions were forced through the plug under controlled NZ pressures. Plug resistances (R, ohms) were measured with a Philoscope universal measuring bridge at 2000 cps. The cell constant (C, cm-l) of the plug was determined by measuring the ratio of plug resistance to the bulk specific resistance of solution flowing through the plug, as a function of increasing salt concentration, This ratio increases with concentration and finally becomes constant at high concentrations. The constant value of the ratio is the cell constant. At high concentrations, surface conductivity becomes a negligible fraction of the measured plug conductivity which is thus determined wholly by the conductivity of solution in intercrystalline pores. Then, at lower concentrations, where a significant fraction of (l/R) is due to surface conductivity
M, = (1/R - H / C ) mho
(1)
where M , is the contribution of surface to measured plug conductivity and H is the bulk specific conductivity of solution flowing through the plug. I n general, A[, is the total crystalline contribution to (l/R) and includes both bulk and surface crystalline conductivity. The attribution of Ad, purely to the crystal surfaces will be justified in the discussion. H was measured as the specific conductivity of solution remaining in the streaming reservoir immediately after each run.
Results Natural Chabazite. The results of Figure 1 were obtained with a plug of natural chabazite (plug I), initially 3.6 ern long, composed mainly of 60-120 mesh crystals, sandwiched between 10-22 mesh crystals adjacent to the electrodes. Expressed as per cent of total plug conductivity (l/R), M , varied from about 90% in distilled water to about 20% in N solutions. Sodium Chabazite. The results of Figure 2 refer to a plug of Na chabazite (plug 11),initially 3.2 cm long consisting wholly of 22-60 mesh crystals. These results are plotted against specific conductivity of streaming solution instead of equivalent concentration since the comparison in Figure 2c of results in acetone and water is facilitated by this type of plot. As per cent of (l/R), M , varied from about 96% in distilled water to about N solutions. 30% in ~~
~~
( 5 ) C. E. Marshall, “Colloid Chemistry of the Silicate Minerals,”
Academic Press Inc., New York, N. Y., 1949. (6) A. S. Buchanan and E. Heymann, Proc. Roy. SOC.(London), A195, 150 (1948).
SURFACE CONDUCTIVITY OF ALUMINOSILICATES
I
0
c
I
3449
I
-
LiOH 0 LlCl Q LIZ so4 KOH *KCI A KzS04
Q
6-
E
ID
0
-
x
i
4-
-
2-
I
0.
I
I
0 .D
A
NaOH NaCl Naz SO.
*A
L
IO-^ N
lo-*
( = X I OF STREAMING SOLN. (EQUIV./L)
IO-^
Figure lb.
Effect of Acid on Chabazite. When or N HC1 was passed through a plug of natural chabazite (plug 111), the resistances of both plug and effluent solution were strikingly dependent on flow rate; e.g., for N HC1 at 1 ml/min, R = 0.622 X lo6 ohm, effluent resistance = 0.86 X lo4 ohm-cm, and p H is 6.2. The same quantities at 300 ml/min were 0.237 X lo5 ohm, 0.27 X lo4 ohm-cm, and 3.0, respectively. Even at 300 ml/min, the resistance of the effluent solution was some 12% higher than that of the ingoing solu-
tion. It is evident that an extremely rapid reaction was removing H + from intercrystalline solution. No values of M , could be calculated for the hydrochloric acid-chabazite system, but it was established that M , in NaCl solutions had fallen by about 20% after prolonged N HC1. exposure of the plug to lod4and The Murata test' was applied to a sample of natural chabazite. A sample of 1.2 g of chabazite was shaken with 1 ml of HzO and 2 ml of concentrated AR HC1. The mixture became gradually more viscous and gentle heating immediately produced a gel containing clumps of feathery material. Effect of Water on Chabazite. The resistance of chabazite plugs in HzO was independent of the flow rate above about 250 ml/min. At lower flow rates, R decreased; e.g., it was about 10% smaller a t 40 ml/ min. A cessation of water flow caused R to fall very
with all three plugs studied; it was not merely an initial effect exhibited by a newly formed plug, but persisted throughout the life of a plug. The value of R invariably fell upon stagnation, independently of the previous history of the plug. I n a subsidiary experiment, a glass electrode in equilibrium with H,O was surrounded with 22-60 mesh crystals of natural chabazite. The initial pH of about 5 increased in 24 hr to a constant value of about 10. Similar decreases in R with stagnation were observed in neutral salt solutions although the magnitude of these changes was not so striking owing perhaps to the relatively high initial conductivities. With solutions of K I and LiCl in acetone (containing less than 1% HzO), changes in R upon stopping flow were much smaller and gmerally in the opposite direction. Effect of Aluminate on Chabazite. Surface conductivity was drastically reduced after the prolonged exposure of chabazite plugs to a solution containing the aluminate ion ( 1 c, LiOH; 0.05 c, AlC13); e.g., exposure of plug I1 (Na chabazite) for 24 hr to flowing alumiN LiOH and NaCl to nate solution reduced M , in mho) before this 25% of their values (-20 X treatment. Surface Conductiuity of Beryl and Microcline. The plugs of beryl and microcline studied were quite similar in cell constant to those already described for chabazite. (The cell constants of all plugs mentioned in this paper lay between 8 and 10 cm-I.) Surface conductivity was studied in HzO and in solutions of NaCl, (7) K. J. Murata, Am. Mineralogist, 28, 548 (1943).
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LizSOd, HCI, and NaOH up to N . For the beryl plug, in all cases, M , was zero within the experimental error and no change in R was detected upon stopping the flow of any of these solutions through the plug. The feldspar plug possessed a finite surface conductivity which was, however, much lower than that of any of the chabazite plugs studied. The feldspar results are not illustrated. Ma lay between 0.1 and 0.5 X lom6mho for the solutions mentioned above. No effect of stagnation on the feldspar plug resistance was noted except in N NaOH where R fell from 0.316 X lo6 to 0.224 X lo6 ohm after 25 min of stagnation.
3
.2
0
i> 0
Co(OH)2 CaCI2 COSOI
Discussion In Figures 1 and 2, it can be seen that M , is increased almost by a factor of 10 in or low2N salt solutions, as compared with its value in water (ionic conN). Also, equilibrium centration roughly 5 X values of M , were obtained, normally, in less than 2 min after changing the concentration of solution flowing through the plug of crystals. Both these facts show that M , is a property of the surface rather than of the interior of the crystals since (a) there is no salt absorption into the intracrystalline pores of chabazite in this concentration range and (b) the relative density of the chabazite structure precludes equilibration of the interior of crystals by ionic diffusion in such short times. Thus we are justified in describing Maas a surface conductivity. Origin of Surface Conductivity. The experimental values of M , are much greater than could be attributed to excess ions in the diffuse or Gouy part of the electrical double layer surrounding crystals. For instance, the experimental Ma in N KC1 was about 5 X mho. In the same experiment, the { potential of the surface was obtained as - 18 mv (see part 11). Substitution of this value of { in the appropriate equation of the diffuse layer,* ignoring electroosmosis, gives the surface conductivity of the Gouy layer as roughly lo-” mho-cm-2. Even allowing for the surface area of the plug (400 cmZ),the experimental greatly exceeds the calculated value. Furthermore, crystals of natural chabazite, after ion exchange to the Na form, showed a greatly increased surface conductivity together with a greatly reduced { potential ( M , was raised by a factor of 3, while { was reduced by an order of magnitude). Since the surface conductivity of a diffuse layer increases with l, it is obvious that the experimental M , has no connection with the diffuse layer. Ma must therefore originate in an electrokineticallv inactive region within the crystal surfaces. It is suggested that M. arises in a skin of silica gel ~~
~
I
I
I
IO-^
10:~
IO-’
N ( = z c ) OF STREAMING SOLN. (EOUIV./L)
Figure IC. I
I
I ’
I
I
t 0
IO+ IO-^ 10-3 CONDUCTIVITY OF STREAMING SOLN. (mhokm)
Figure 2a. Effect of salt type and concentration on the surface conductivity of a plug of sodium chabazite.
formed by the abstraction of aluminum from the surface of the aluminosilicate crystals, as shown in Figures 3a and b. Silanol groups formed in this way give rise to M, by adsorbing salt from external solution and/or by acidic dissociation and ion exchange. Ionic mobilities are assumed to be higher in this leached surface phase owing to its greater openness relative to the original crystal. The reactions shown in Figures 3a and b are merely schematic. The state of A1 leached from the crystals will depend on pH as shown by Ottewill;B e.g., between
u
The Journal of Physical Chemistry
(8) H. R. Kruyt, Ed., “Colloid Science,” Vol. I, Elsevier Press, London, 1952,p 237.
SURFACE CONDUCTIVITY OF ALUMINOSILICATES
3451
(b)
(c)
I
-0
IO-^
IO-^
I
1
Si0 No’ Si01A1--OHt4H*OHsi0
’
1
-
AI3: No’, 4 0 H SiOH
-1
SiO,
SioH t A t ( 0 H ) i No* SiOH
,A.t(OH)i
No’ 2 H 2 0
si0
Figure 3. Probable reactions at the surface of chabazite: (a) reaction with acid; (b) reaction with water; (c) reaction with aluminate.
IO-^
CONDUCTIVITY OF STREAMING SOLN. (mho/cm)
The result of a Murata test on a sample of the chabazite used in this work shows it to belong to the latter class, in accordance with the Si:A1 ratio of 2.09 obtained analytically. It was also found that 0.1 g of the chabazite dissolved completely in 15 ml of 0.2 N HC1 giving a glass-clear solution. Thus it would appear that the leaching of A1 from chabazite could not by itself produce the high experimental values of M,, since discrete chains of silicic acid would leave the zeolite surface and residual SiOH groups could extend for only a few molecular layers into the crystal. Relatively thick, coherent surface layers of silica gel could be formed by the recondensation of partially or completely detached chains of PSA with the remaining surface SiOH groups as
Figure 2b.
surfaceSi0H
--f
surfac-Si-0-Si=PSA
0 lo-s
+ HOSiPSA
lo-’ CONDUCTIVITY OF STREAMING SOLN. (mho/crn)
Figure 2c.
p H 4 and 10, A1 exists as polynuclear hydrolyzed cab ions. The leaching of A1 from zeolites is well established and explains the effects of water and acid on the conductivity of chabazite plugs. Thus it has been observedlo that moist, powdered chabazite reacts alkaline to litmus. It is also known’ that zeolites are readily de composed by strong acids, leaving a residue of silica. With excess acid, the more siliceous zeolites (Si :AI > 3) are converted to a coherent pseudomorph of hydrated silica retaining the external habit and optical properties of the original zeolite crystal. The less siliceous zeolites (Si:Al < about 2.0) normally decompose to a silica sol or gel, since here the total removal of A1 produces isolated chains of polysilicic acid (PSA).
+ HzO
A sprinkling of such siloxane linkages would result in a stable, highly porous layer containing large numbers of accessible SiOH groups. The existence of a recondensation process is indicated by the following evidence. The highest M , values were exhibited by natural crystals which had been ionexchanged by a batch method over a period of days from hot 4 m NaCl solutions at pH 6.7. According to Iler,” the condensation of SiOH groups is most favored at pH 5-8 and especially if excess salt is present. Hence the pH, ionic strength, and batch nature of the ion-exchange procedure were all conducive to SiOH (9) E. Matijevic, K. G . Mathai, R. G . Ottewill, and M. Kerker, J . Phya. Chem., 65, 826 (1961). (10) J. W. Mellor, “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. VI, Longmans, Green and Co.,London, 1925,p 733. (11) R. K. Iler, “Colloid Chemistry of Silica and Silicates,” Cornel1 University Press, Ithace, N . Y.,1955.
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3452
condensation and thus to the formation of a surface layer of silica gel. Also, the prolonged action of flowing HC1 a t pH 3-4 caused a 20% reduction of M , in NaCl solutions. I n this case, the pH and low ionic strength were least favorable to SiOH condensation and the extremely rapid abstraction of AI from the crystalgel interface could cause fragments of the gel layer to be detached and swept away, thus reducing M,. Two further facts point to the presence of crystals of a superficial layer of hydrated silica forming the source of interfacial conductivity. (a) If the surface conductivity is due to SiOH groups, then any treatment chemically modifying these groups should drastically affect the conductivity. Thus, plugs were subjected to prolonged exposure to a flowing solution containing the aluminate ion. The striking reductions in M , produced by this treatment are strong evidence for associating M , with superficial layers of silica. The probable reaction is shown in Figure 3c. In sterically favorable cases, this reaction could go to completion eliminating four SiOH groups. This formation of aluminosilicate links will directly eliminate some SiOH groups and probably render others inaccessible. The feasibility of the reaction is shown by the fact that crystals and rigid gels of aluminosilicate are synthesized by reaction of PSA with aluminate under quite similar conditions.12 (b) Rabinowitch13 noted that the ion exchange of natural chabazite to the Na form reduced its sorption capacity for Nz. Sorption studies are preceded by exhaustive outgassing a t 300" to remove intracrystalline water. Barrer', found sorption capacities to vary considerably with the ion-exchange procedure. He considered that prolonged ion exchange hydrolyzes the crystal surface to a gel-like skin which, in the subsequent outgassing, seals off crystal surfaces. These observ% tions correlate well with the assumption that M , values originate in a siliceous surface layer. The known properties of a silica surface" show that exhaustive outgassing a t 300" will remove large amounts of constitutive water by condensation of adjacent SiOH groups to give siloxane links, -Si-0-Si. This process could well result in a much denser, quartz-like skin, impermeable to Nz. Nature of Conducting Species. It was stated earlier that M , could arise either by adsorption of salt from external solution onto the SiOH groups or by their acidic dissociation. An indication that ionization rather than adsorption is mainly responsible for the surface conductivity of chabazite plugs is provided by the work of Barrer and James15 showing that a membrane molded from sodium chabazite (200 mesh) and polystyrene powders behaved as a sodium elecThe Journal of Physical Chemistry
S. D. JAMES
trode in NaC1 solutions below 0.1 m, if care was taken to limit penetration of ambient solution into the membrane. Since the sodium chabazite was prepared by exhaustive ion exchange from hot 4 m KaC1 solutions, the crystalline contribution to membrane conductivity must have been almost wholly interfacial. Furthermore, imbibed salt solution could play no part in membrane conduction since this was purely cationic. Hence, in NaCl solution only cations are mobile in the surface film responsible for M,. This indicates that ionization of cations from fixed surface anions determines M , since if physically adsorbed ion pairs were significantly contributing, some anionic conductance would exist. Although the electrode behavior of chabazite was studied only in NaC1 solutions, the above is strong evidence that ionization rather than adsorption is mainly responsible for M , in aqueous solution. Variation of M , with Salt Type. On the foregoing assumption that the acidic dissociation of SiOH groups largely determines M,, a substantial variation of M , with pH would be expected. However, a notable feature of the results of Figures 1 and 2 is the lack of a large or consistent variation of M , with pH. M , values are not greatly different between the hydroxide and chloride or sulfate of a given metal where N solutions, for instance, differs by 6 the pH in units. Neither is M , in hydroxide always greater than in neutral solutions of the same concentration. Although it has been suggested16 that the acidity of a surface SiOH group is much greater than that of Si(OH), (pK, = 9.8 a t 20"),17 the measurements of Bolt18 show that the ionization of silica has a much stronger pH dependence than that demonstrated in the present work for the surface of chabazite. Thus, the leached chabazitic surface can by no means be identified with a pure silanol surface. Commercial silica sols are sometimes stabilized by A1 salts19 and Iler is quoted as saying that even 1% A1 on a silica surface increases its acidity.20 Hence, the leached chabazitic (12) E.g., R. M. Barrer and E. A. D. White, J . Chem. SOC.,1267 (1951); 1561 (1952). (13) E. Rabinowitch and W. D. Wood, Trans. Faraday SOC.,32, 947 (1936). (14) R. M. Barrer, private communication. (15) R. M. Barrer and S. D. James, J . Phys. Chm., 64,417 (1960). (16) L.B. Ryland, M. W. Tamele, and J. N. Wilson in "Catalysis," Vol. 111, P. H. Emmett, Ed., Reinhold Publishing Corp., New York, N. Y.,1960, Chapter 1. (17) 5. A. Greenberg, J . Chem. Educ.,36, 218 (1959). (18) G. H.Bolt, J . Phya. Chem., 61, 1166 (1957). (19) J. W. Ryznar in "Colloid Chemistry," Vol. VI, J. Alexander, Ed., Reinhold Publishing Corp., New York, N. Y.,1946,Chapter 64. (20) 5. A. Greenberg, T. N. Chang, and R. Jarnutowski, J . Polyner Sci.. 58, 147 (1962).
SURFACE CONDUCTIVITY OF ALUMINOSILICATES
surface which is likely to contain some residual A1 might behave as a fairly strong acid. This would explain the fact that surface conductivity is determined by the ionic concentration rather than by the pH of ambient solutions. A complete explanation of the specific effects evident in Figures 1 and 2 is difficult to obtain. In general, the cation is of more importance than the anion in determining ,!I,. The anionic effect is, however, greater than would be expected from the previous conclusion that M , is generally due to cation exchange. Possibly, the specific effects of anions occur as a result of a partial contribution of s& adsorption to M , in some salt solutions. The experiments illustrated in Figure 2c using acetone as solvent were performed in the expectation of a uniform lowering of M , compared with corresponding results in aqueous solution. Although limiting free mobilities of ions in acetone are greater, owing primarily to this solvent’s greater fluidity, ion-pair formation at finite ionic strengths lowers mobilities in acetone relative to those in water. Figure 2c shows that the addition of acetone to the extent of 1% by volume causes a slight increase of A4, in NaCl solutions. This increase is paralleled by the attendant increase in solvent fluidity, and therefore the interface is not significantly accumulating acetone in preference to water, since an interfacial film rich in acetone would favor ion pairing and a lowering of M,. LiCl is particularly prone to ion pairing because of its small cationic size, and this explains the very low values of M , for the lithium chloride-acetone system. The maximum and subsequent leveling-off would not be expected if M , were due solely to the presence of cation-exchange groups, -SiO-Li+. If salt adsorption were determining M,, the shape of the curve might be caused by an eventual precipitation of LiCl in the interfacial gel layer. No explanation is offered for the very high values of M , in the potassium iodide-acetone system. In N solution, M , in acetone was more than twice that in water. A better understanding of specific differences in M , exhibited by salts of the same valence type must await a systematic study of interfacial transport numbers derived from potential measurement^.'^ As discussed above, these measurements can produce an estimate of the relative contribution of surface ionization and surface adsorption of salt to M , in any given environment. Surface Conductivity of Beryl and Microcline. The low and indetectable M , values for the feldspar and beryl plugs, respectively, are consonant with the higher chemical resistivity of these materials relative
3453
to that of chabazite. Marshal121 mentions the low solubility of feldspars and the fact that only in HC1 is an analyzable quantity of A1 released into solution. The feldspar used in the present work, microcline, is a much more closely packed structure than chabazite. According to i\/aarshall15the potassium ion in orthoclase, a structure almost identical with microcline, is surrounded by oxygens and has only about 0.1 A free space. Thus the hydrolytic extraction of A1 from microcline can occur only at its external surface. This process will necessarily be much faster in chabazite, where H30+ from external solution can penetrate the crystal quite rapidly by ion-exchange diffusion through relatively wide intracrystalline channels. The beryl structure evidently is virtually completely protected from hydrolytic breakdown by its covalent character. Surface Conductivity of Glasses. Many experimental determinations of the specific surface conductivity of borosilicate glasses8 have been made using slits or capillaries of Pyrex or Jena glass. The results of this work are very disparate. For instance, the values of different workers for N KC1 vary from to mho. To explain the lowest of these values in terms only of the diffuse layer would require a p potential of about 110 mv and a value of mho would require an impossible high {. Urban, White, and Strassner22reconciled their experimental and calculated conductivities by assuming that ionic adsorption into the Stern layer mainly d e termined surface conductivity and they calculated ionic adsorption potentials necessary to augment diff use-layer conductivities to their experimental values. It seems inherently unlikely that these adsorption energies would vary sufficiently among glasses of very similar composition to account for the 1000-fold variation of surface conductivity observed experimentally. It is more likely that the conductivity arises in an electrokinetically inactive swollen gel layer, as suggested by Overbeek.* This gel layer is probably a skin of silica gel formed analogously to that of chabazite by the hydrolytic extraction of boron from the borosilicate surface. Work summarized by Eite123shows clearly that leached surface layers of hydrated silica exist on any glass which has been exposed to aqueous solution. The thickness and hence conductivity of these swollen films will vary considerably with the
(21) V. E. Nash and C. E. Marshall, University of Missouri, Agricultural Experiment Station, Research Bulletin 613,Sept 1956. (22) F. Urban, H. L. White, and E. A. Strassner, J. Phys. Chem., 39, 311 (1935). (23)W. Eitel, “Physical Chemistry of Silicates,” University of Chicago Press, Chicago, Ill., 1954,pp 978,1348, 1358.
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chemical resistivity of the glass specimen and with the duration and conditions of its exposure to aqueous solutions.
Acknowledgment. The author wishes to thank Professor A. S. Buchanan for many helpful discussions during the course of this work.
Electrochemistry of the Interface between Some Aluminosilicate Crystals and Salt Solutions. 11. Electrokinetic Charge
by A. S. Buchanan and S. D. James1 Department of Physical Chemistry, University of Melbourne, Victoria, Australia
(Received April 6, 1966)
Electrokinetic charge densities have been calculated from streaming potentials measured for crystal plugs of chabazite, beryl, and microcline (with particular emphasis on chabazite) bathed by salt solutions. The results are interpreted in terms of cation-exchange and ionadsorption processes between added salt and the crystal surfaces.
This work is directly complementary to the data on surface conductivity presented in the previous paper.2
Experimental Section The materials and streaming cell and the method of measuring plug resistances (R) and cell constant (C) were identical with those of part I of this work.2 Streaming Potential. Streaming potentials ( V ) varying from 20 to 1400 mv were measured with a Cambridge valve potentiometer to i=1 mv. The nitrogen pressure (5-40 cm) forcing solutions through the plug was controlled within i=0.05cm by a mercury “blowoffJJdevice and measured to the same accuracy with a mercury manometer. A detailed study of streaming potential as a function of driving pressure ( P ) over the range 0-40 cm confirmed that V was an accurately linear function of P , so the system was following the Helmholtz-Smoluchowski equation. I n subsequent runs, V was measured at three pressures only as a quick check on linearity. Each run was begun with 5 1. of solution in the streaming reservoir and streaming was commenced at the highest P to be used in order to clear the plug of solution from the previous run and establish a steady state The Journal of Physical Chemistry
as soon as possible. The value of R was measured as a function of P only after the V-P measurements were finished, since R measurements were found to polarize the plug. The value of R was normally independent of P, except in acidic or alkaline solutions. I n solutions which reacted with the crystals composing the plug, thus changing the composition of intercrystalline solution and causing the plug resistance R to vary with flow rate, the driving pressure P was increased until R became constant and V-P measurements were made in this region. Under this condition, the conductivities of ingoing and effluent solutions were equal and the compositions of intercrystalline and ingoing solutions were assumed to be identical. The temperature of solution remaining in the streaming reservoir was measured immediately after each run. The { potentials were calculated from the modified Helmholtz-Smoluchowski equation, l’ = -4a77VC/ DPR, where D and 7 are the dielectric constant and viscosity in the diffuse layer (both assumed equal to those (1) Nuclear Engineering Department, Brookhaven National Laboratory, Upton, N. Y. 11973. (2) 5. D. James, J. Phys. C h a . , 70, 3447 (1966).
