Langmuir 1987, 3, 18-25
18
Articles Structural Component of the Swelling Pressure of Clays Philip F. Low+ Contribution No. 10582, Department of Agronomy, Purdue University Agricultural Experiment Station, West Lafayette, Indiana 47907 Received January 29, 1986. In Final Form: September 23, 1986 Experimental data were preaented to show that the value of q6,the electric potential at the outer Helmholtz plane, is - 4 0 mV for Na-mectites. This value of +$ was used with electric double-layer theory to predict the relation that would exist between II,the swelling pressure, and A, the distance between adjacent smectite layers, if swelling were due to excess osmotic pressure in the interlayer solution. Data showing the observed relation between n and X were also presented. These data were derived from a study in which X was determined by X-ray diffraction at succesaively higher values of II. Comparison of the predicted relation with the observed relation demonstratedthat the former was relatively insignificant. Hence, it was concluded that the osmotic contribution to II is negligible. This conclusion was discussed with reference to the work of other investigators. The results of previous studies were used to establish the fact that both (II + 1 ) and Ji/JF, the ratio of the value of any property, i, of the water in the smectite-water system to that of pure water, are exponential functions of l / t , the reciprocal of the average thickness of the water films on the surfaces of the smectite layers. Elimiition of l / t betwean the two functions yielded an equation relating (n + 1 ) to Ji/J:. This equation indicated that the development of XI and the deviation of Jifrom J: were caused by the same factor, i.e., the structural perturbation of the water under the influence of the layer surfaces. Hence, it was concluded that the structural component of the swelling pressure is primarily responsible for the swelling of smectites. Aluminosilicate minerals with an expanding layer lattice, commonly called smectites, are abundantly and widely distributed in the earth's crust where they are responsible for many reactions and processes. Not only are they of great practical importance in geology, hydrology, civil and petroleum engineering, and soil science, but they are also of great theoretical importance because of their separable layers with controllable spacing, high specific surface area, and inherent charge. Indeed, they are ideally suited for the study of interparticle forces and surface-water interaction. The structure of a typical smectite like montmorillonite is illustrated in Figure 1. It is composed of two structural units, the silica tetrahedron and the alumina octahedron. The former consists of a silicon ion surrounded tetrahedrally by four oxygen ions; the latter consists of an aluminum ion surrounded octahedrally by six oxygen (or hydroxyl) ions. These structural units are interconnected by the sharing of oxygen ions at their common corners and/or edges so that a layer of tilted alumina octhedra is interposed between two layers of silica tetrahedra. The result is a flat, composite aluminosilicate layer of appreciable areal extent but only -10 A thick. Several such layers are stacked one above the other, in parallel arrangement, to form the crystal. During or after the formation of the smectite crystal, there is a partial replacement of A13+in octahedral sites by Mg2+and Fe3+ or Fe2+ and, to a lesser degree, a replacement of Si4+in tetrahedral sites by AP+. Thus, the layers of the crystal have a net negative charge. This charge is neutralized by the adsorption of interlayer, exchangeable cations. When water at Pa,the pressure of the atmosphere, comes in contact with a smectite crystal, it penetrates between Professor of Soil Chemistry.
0743-7463/87/2403-0018$01.50/0
the layers of the crystal and forces them apart. Thus, the c-axis spacing of the crystal (i.e., the dimension of its unit cell Ito the plane of cleavage) increases. In other words, the crystal swells. In order to prevent swelling, P, an absolute pressure in excess of Pa,must be applied to the crystal. At equilibrium, P - Pa equals II, the swelling or disjoining pressure of the crystal. Figure 2 shows an aqueous suspension of clay (a generic term for colloidal silicates, including smectites) separated from pure water at Paby a semipermeable membrane. In order to keep water from being transferred into the suspension and increasing X, the distance between the superimposed layers, an absolute pressure, P, must be applied to the suspension by means of the piston. This pressure is transmitted uniformly to all the clay particles through the surrounding water. Hence, as before, P - Pa = II for the given value of X. Now, by means of thermodynamics, it can be shown' that, for the system being considered, (G, - GWo)p,= -o,(P - Pa)= -own where G, and GWoare the partial molar free energies of the water in the suspension and in the pure state, respectively, and Ow is the partial molar volume of the water in the suspension. If the suspension is equilibrated with an aqueous electrolyte solution instead of pure water, GwO is replaced by the partial molar free energy of the water in the solution. Otherwise, the relation expressed by e_q 1 remains unchanged. Hence, any factor that affects G, also affects II. In the past, most investigators have assumed that 0, is affected only by the ions dissolved in the interlayer solution and, therefore, that the repulsive pressure, p, tending to force the layers apart, is the difference between (1) Low, P. F.;Anderson, D.M. Soil Sci. 1958, 86,251.
1987 American Chemical Society
Langmuir, Vol. 3, No. I , 1987 19
Structural Component of Swelling Pressure
leagues (e.g., ref 9-11) and Peschel et a l . I 2 have advanced the concept that surface-water interaction also contributes to p . Additional evidence for this concept will he given in the present paper. In using electrical double-layer theory to assess $, and, thereby, the double-layer or osmotic contribution t o p , it is essential that ILS, the electrical potential in the outer Helmholtz plane (OHP), or us,the surface charge density in the same plane, he known. In this regard, recall that electrical double-layer theory applies only to the diffuse layer and that this layer begins at the OHP. Values of $6 and u6 have recently been determined in the author's laboratory by four different methods for three smectites having Na+ as the exchangeable ionic ~pecies.'~ In method I, the 2 potential of the clay particles was measured by electrophoresis at several values of K, which is defined by
2 = 8mne2u2/tkT
(3)
where 6 is the dielectric constant. Then, In tanh (uef/4kT) was plotted against K and the intercept and slope of the resulting straight line were used with the equation of Eversole and Boardman," viz.,
Figure 1. Structural model of a smectite
In tanh (ue{/4kTJ = In tanh (ve+*/4kT) - KT (4) to calculate the respective values of $6 and T, the distance from the OHP to the plane of shear. In method 11, measured values of Vex, the anion exclusion volume, were plotted against the correspondingvalues of Z / K . According to an equation derived by Chan et al.,'5
Vex= ( 2 S / ~ ) [ l - eXP(ve$~/2kT)I
LSemipermeable Membrane Figure 2. Illustration of a two-phase system, clay and water, which requires the application of a pressure, P,to the clay phase to maintain equilibrium. the osmotic pressures of the interlayer and external solutions. Provided this assumption is correct. it can be shown* that
where S is the specific surface area of the clay. Since the value of S was known for each clay and since the plot of Vexagainst Z/K was always linear, the slope of this plot yielded the value of in keeping with the equation. In method 111, E,, the potential drop between a clay suspension and the solution in equilibrium with it, was measwed at each of several values of I, the ionic strength. The results were then used to calculate the corresponding values of npQ, where n p is the number of particles per centimeter cubed of suspension and Q is the charge per particle, by means of the following equation:"
E,,, = 5 W n p Q R T / P I
u6 =
(3) Verwey, E. J. W.: Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids; Ehvier: New York, 1948. (4) Low,P. F.; Deming, J. M. Soil Sei. 1963, 75,178. (5) Ravina. I.; Low,P. F. Cloys Clay Miner. 1972,20,109. (6)Viani, B. E.: Low,P. F.; Roth, C. B. J. Colloid Znterfoce Sci. 1981, 96,229.
npQ/WS
(7)
where W is the mass of clay per centimeter cubed. Finally, ugwas plotted against K and fi6was determined from the (7) Derjspuin, B. V.; Titijevaskaia, A. 5.; Abrieassova, I. S.;Malinka, A. D. Faraday SOC.Diacuss. 1964,18.24. (8) Derjndn, B. V.; Chursev, N. V. J. Colloid Znterfeee Sei. 1974.49. 249. (9) Israelachvili. J. N.; Adams, G. E. J . Chem. Soc.. Faraday Trow. I 1978, 74,975.
(10)Iaraelachvili, J. N.; Paahley, R. M. Nature (London) 1983, 306, ""0
(2) Langmuir, I. J. Chem. P h p . 19sS,6,873.
(6)
in which R is the molar gas constant and F is the Faraday. Once the values of npQ were known, the respective values of u6 were calculated by using the relation
(2)
where n is the concentration of the symmetrical electrolyte in the external solution, k is the Boltzmann constant, T is the absolute temperature, Y is the ionic valence, e is the electronic charge, and Gh is the electric potential midway between the parallel smectite layers. Therefore, if $h can be determined by means of electrical double-layer theory: p can be calculated. However, the author and his colleagues (e.g., ref 4+3, Derjaguin and his colleagues (e.g., ref 7, 8), and, in recent years, Israelachvili and his col-
(5)
20 Langmuir, Vol. 3, No. 1, 1987
Low
Table I. Values of $6 for Three Na-Smectites As Determined by Different Methods mV method I1 method I11
(esu cm-*)
$6,
smectite Na-Otay Na-Texas Na-Upton
method I -60 -50 -59
-6 1 -46 -51
-59
method IV
-56
slope of the resulting straight line in accordance with the equation fl6
= ( ~ k T ~ / 2 7 ~sinh u e )(ue+,/2kT)
c
(8)
Equation 8 is a familiar equation of double-layer theory for the case of nonoverlapping double layer^.^ Method IV relied on the ion product principle from the Donnan theory of membrane equilibrium, namely,
nrl X nr2= nrrlX nrr2
A
Mexican
x
Otoy
'i t
Observed
/ I
1
I
-
1
I
(9)
in which the subscripts 1 and 2 represent the cation and anion, respectively, the single prime denotes one phase (the clay suspension), and the double prime denotes the other phase (the solution). In this method, n$, nrrl,and n'? were measured and the results were substituted into eq 9 to determine nrl. Thereafter, ug was calculated from the relation u6 = ue(n', - n r 2 ) / W S
(10)
This process was repeated for different electrolyte concentrations in the solution and then uswas plotted against K . As before, was determined from the slope of the resulting line. Values of +a obtained by the different methods are reported in Table I. Note that the four methods gave consistent results for each smectite and that the results for the different smectites were similar. Also recall that the reported values are independent of electrolyte concentration because their determination involved different values of K . If these values of are used in eq 8, it becomes evident that u6 is small relative to ao, the surface charge density at the solid-solution interface, at all values of K . For example, by using the values of +a obtained by method I, we find that the values of a6for the Na-Otay, Na-Texas, and Na-Upton smectite in a M solution of NaCl are 516,400, and 504 esu/cm2, respectively. The correspondingvalues of a,, as determined from the cation-exchange capacity and S, are 5.11 X lo4, 3.90 X lo4,and 3.25 X lo4 esu/cm2. Hence, in every case, u6/ao < 0.02. The necessary conclusion is that the fraction of counterions in the diffuse layer is very small and that this layer is poorly developed. In other words, almost all of the counterions are in the Stern layer. The same conclusion holds true for other smectites as we1l.l' The relation between +6, +h, and h, the distance from the OHP to the midplane between the parallel clay layers, is given3 by L u ( 2 cosh y - 2 cosh u)-lj2dy = -Kh
+
(17) Low, P. F. Soil Sci. SOC.A m . J . 1981, 45, 1074.
d = drain tube
b = porous ceramic filter
e, e' = beryllium windows
c = gas port f = support Figure 4. Apparatus for measuring the c-axis spacing of the clay crystals in a clay film as a function of the difference in pressure, P - Pa,across the film.
M ( K = 3.25 X lo5 cm-l) by numerical integration of the equation. These values of u and h were used subsequently with eq 2 to obtain p as a function of h. Then I1 was obtained as a function of X by making use of the following relations
(11)
in which y = ve+/kT, z = ue+,/kT, u = ue+,JkT, and is the electric potential at any distance from the OHP. Since +g is independent of electrolyte concentration, we assumed that it is also independent of interlayer distance. Therefore, we let z in eq 11 have a constant value corresponding to = -60 mV, which is representative of the data reported in Table I, and determined the respective values of u and h for an electrolyte concentration of
a = oriented clay film
n=p-f
(12)
f = A/67rX3
(13)
X = 2(h + 6 )
(14)
and in which f is the van der Waal's attractive force, A is the Hamaker constant, and 6 is the distance between the solid-solution interface and the OHP. The value assigned J (ref 9) and the value assigned to 6 to A was 2.2 X was 5.5 A. Thus, the theoretical curve in Figure 3 was obtained. To test the adequacy of the osmotic concept of swelling as described by the foregoing equations, Viani et a1.6J8
Langmuir, Vol. 3, No. 1, 1987 21
Structural Component of Swelling Pressure determined the actual relation between II and A for eight different Na-saturated smectites and Li-vermiculite. A simplified diagram of their apparatus is shown in Figure 4. An oriented film of clay, a, was formed on the porous ceramic filter, b, by depositing a suspension of the clay on the filter and admitting nitrogen gas at an elevated pressure, P, to the chamber, A, through the port, c. The underside of the filter was connected via the drain tube, d, to the outside atmosphere at the pressure, Pa. Therefore, water was expressed from the suspension until P - Pa = II. Since the chamber was provided with beryllium windows, e and e', and was held to the axis of an X-ray goniometer by the adjustable support, f, it was possible to determine the c-axis spacing of the clay crystals at the given value of II by X-ray analysis. The corresponding value of X was obtained from this spacing by subtracting 9.3 A-the thickness of the individual smectite layers. Thus, values of X were determined a t several values of II for each of the eight smectites and vermiculite. The common curve of II vs. X for three of these smectites is shown in Figure 3. It is described by the empirical equation
II + 1 = exp[ k.(
-
;)]
= b exp(k,/X)
I
I
I
I
x
I
I
I
I
MONTE AMIATA CALIFORNIA RED TOP
A
20
I
HIGH GRADE WESTERN
Q
I
I
I
I
40
I
I
60
80
I
I
t .(X) Figure 5. Relation between n and t for Na-smectites having single values of B and k'= in eq 18.
(15) 8.0I
in which Xo is the interlayer spacing when II = 0, k, = 75.73
I
...........
A, and b = exp(-k,/XO) = 0.642. The curves for the other
I
I
I
$=43.44 a, B =0.8
I
I
=45.751, B=O.78 whenlT 2 / K . In addition, the slopes and intercepts
of these relations provide values of K and #&,respectively, that are consistent with the values determined by other means. Therefore, it appears that, with layer lattice silicates like mica and vermiculite, double-layer repulsion predominates at large values of A. This is not true, however, at smaller values of A. Then another force becomes dominant?^^^^^^ This force has been ascribed to surface hydration. Lubetkin et a1.28also made graphs of log II vs. X for a montmorillonite from Clay Spur, WY, that was saturated with different monovalent cations and equilibrated with solutions containing different concentrations of the respective chloride salts. These graphs were curvilinear but became increasingly linear as X increased. Whether or not they eventually became linear is uncertain, especially at the lower salt concentrations. Nevertheless, they were regarded as being linear when X > 2 / and ~ eq 22 was applied to two of them, namely, those for the Li-montmorillonite in lo4 and M LiCl solutions, to obtain the corresponding values of #6.The values so obtained were 250 and 150 mV, respectively. However, because eq 21 was used to calculate the values of X used in the graphs, these graphs were probably displaced upward from their true locations. Such a displacement would cause the resulting values of to be too high. Note that they were far higher than the values reported for similar silicates in Table I and elsewhere.9,12,14.15,17,37-39
By letting #a = 150 mV for the Li-montmorillonite, Lubetkin et al.28were able to use eq 2 and 11to obtain a theoretical curve of II vs. X which extended to low values of A. Since this curve compared favorably with the corresponding experimental curve, it was inferred that double-layer theory describes clay swelling even when X < 2 / ~ . However, this inference is untenable. The theoretical curve was invalid because both its position and shape depended on the value of #aand the value assigned to this quantity was too high. Further, when Low and Margheim40used the data of Callaghan and O t t e ~ i 1 to 1 ~plot ~ In (II + 1)vs. l / X for the Na-saturated variety of the same montmorillonite in a M NaCl solution, they obtained a linear relation in keeping with eq 15. As noted earlier, doublelayer theory is not consistent with such a relation. The author and his colleagues414 have collected abundant data on the relation between II (or a related quantity, the tension of the water) and m, f m, for mixtures of NaUpton montmorillonite and water at very high values of m,/m, and, hence, of A. If these data are combined to construct a common plot of In II vs. m,/m,, a curvilinear relation results. On the other hand, if the same data are used to make a common plot of In (II + 1)vs. m,/m,, a linear relation is found down to values of m,/m, as low as -0.1, correspondingto a value of II equal to -0.05 atm. It is reasonble to conclude, therefore, that double-layer repulsion does not predominate in colloidal clay systems when X is large and that the primary forces in these systems remain the same over a wide range of A. The effect of the sol-gel transition on II also supports the preceding conclusion. When a clay-water system is (36) Pashley, R. M. J. Colloid Interface Sci. 1981,80, 153. (37) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (38) Friend. J. P.: Hunter. R. J . Claw Clay Miner. 1970. 18. 275. (39) Hunter, R. J.; Alexander, A. E.-J.Coiloid Sci. 1963; 18; 833. (40) Low, P. F.; Margheim, J . F. Soil Sci. SOC.Am. J. 1979, 43, 473. (41) Davey, B. G.;Low, P. F. Trans. Int. Congr. Soil Sci., 9th 1968, 1, 607. (42) Banin, A.; Davey, B. G.;Low, P. F. Soil Sci. SOC.Am. Roc. 1968, 32, 306. (43) Kay, B. D.; Low, P. F. Soil Sci. SOC.Am. R o c . 1970, 34, 373. (44) Low, P.F. Trans. Int. Cong. Soil Sci., 10th 1974,1, 42.
Langmuir, Vol. 3, No. 1, 1987 23 in the sol state, II has an infinitesimally small value, whereas as soon as the system gels, II assumes a finite v a l ~ e . ~ The l * ~sol-gel ~ transition may be produced by allowing the system to stand after it has been converted to a sol by a mechanical disturbance, or it may be produced by increasing m,/m, beyond the critical value at which instant gelation occurs. It is noteworthy that this critical value corresponds to (m,/m,)O in eq 16. For example, a suspension of Na-Upton montmorillonite undergoes instant gelation when m J m , reaches a value of -0.0441149 and (m,/m,)O for this montmorillonite is within experimental error of the same value.21 Now, the sol-gel transition should have no appreciable effect on double-layer overlap at a given value of m,/m, and, consequently, should have no appreciable effect on double-layer repulsion. However, since the Brownian motion of the clay particles ceases as gelation occurs, the sol-gel transition would affect the stability of any arrangement of water molecules induced by the particle surfaces and, thereby, G, in eq 1. For this reason, it was concluded that II develops with gelation because of the concomitant development of hydration shells (i.e., zones of perturbed water) around the particles. This conclusion is supported by two additional observations, namely, that a clay sol expands when it gelsmand that a clay gel can be converted to a clay sol by the application of pressure.51 Layers of mica and vermiculite have a higher density of charge, a greater proportion of the charge in tetrahedral sites, and larger lateral dimensions than layers of montmorillonite. Also, as regards the swollen crystals, a greater number of layers are superimposed with greater threedimensional order in vermiculite than in montmorillonite. None of these distinctions seems to account for the fact that the equation which describes the swelling of mica and vermiculite is different from that which describes the swelling of montmorillonite at high values of X. However, this difference disappears as X becomes smaller. Viani et ala6and Pashley and have observed that the experimental curve of II vs. X for mica is similar to that for montmorillonite over a wide range of X and have discussed minor corrections that may be made to bring the two curves together. Also, Viani et al.18 have observed that the curves of II vs. X for vermiculite and montmorillonite are coincident from X > 100 A to X 38 A. At 38 A, X ceases to change with increasing II in the vermiculite. Several experiments have shown that X decreases with increasing salt concentration (e.g., ref 25, 29, 28) and the decrease has been interpreted to mean that double-layer forces are involved. Such an interpretation is not necessarily correct. The ions of a salt can affect the structural arrangement of the water near a silicate surface and, thereby, the magnitude of any hydration force. This fact is shown by the work of P a s h l e ~ . ~l?n~addition, ' different methods involving different silicates (including mica, vermiculite, montmorillonite, and fused silica) have shown that f, +6, and II do not always change uniformly as the salt concentration changes but often exhibit a maximum M.3739539" In discussing this at a concentration of N (45) Kolaian, J. H.; Low, P. F. Clays Clay Miner. 1962, 9,71. (46) Leonard, R. A.; Low, P. F. Clays Clay Miner. 1964,12, 311. (47) Day, P. R.; Ripple, C. D. Soil Sci. SOC.Am. Proc. 1966, 30,675. (48) Ripple, C. D.; Day, P. R. Clays Clay Miner. 1966, 14, 307. (49) Baker, D. E.;Low, P. F. Soil Sci. SOC.Am. Proc. 1970, 34, 49. (50) Anderson, D. M.; Leaming, G. F.; Sposito, G. Science (Washington, DE.)1963,141, 1040. (51) Hiller, K.H. Nature (London) 1964,201, 1118. (52) Pashley, R. M.; Quirk, J . P. Colloids Surf. 1984, 9,1. (53) Low, P.F. Special Report 40, 1985; Highway Research Board, Washington, DC; p 55.
24 Langmuir, Vol. 3, No. 1, 1987
Low
1.6 I I phenomenon, Friend and Hunter%suggested that it is due I I to structural changes in the electrolyte near the solid surface. In view of the evidence presented in Figure 3 and the I. foregoing analysis, we believe that the double-layer or 14 osmotic component of II is negligible in clays and that the I. structural component, i.e., the component due to structural changes in the water under the influence of the particle surfaces, is paramount. From a thermodynamic viewpoint, 1.2 it may be said that clay-water interaction lowers G, relative to GWoat atmospheric pressure and that, to establish equilibrium between the clay-containing phase and the JJJ~ pure water, the pressure on the former must be increased until G, becomes equal to GWo.Reference to eq 1 shows that the required increase in pressure is P - Pa.It may be argued that there is no theoretical basis for this belief, but recent computer simulations have indicated that the Q0 interaction of water with solid surfaces can influence the structure of the water to an appreciable depthmg and can produce a repulsive force.B0 It may also be argued that any force arising in this manner is unimportant and that 3. the discrepancy between the two curves in Figure 3 is 0.6attributable to another force of unknown origin or to errors in the theory on which the theoretical curve is based. In order to discount such arguments and to verify the primary role that the structural component plays in disjoining clay I I I I surfaces, the following discussion is included. 40 a0 120 160 t (5, In earlier papers from the author's l a b o r a t ~ r y , ~ ~ f ' ~ + ~ evidence was presented to show that the thermodynamic, Figure 7. Relation between Ji/J: and t for three different properties of the water in Na-smectites: namely, the apparent hydrodynamic, and spectroscopic properties of water in specific expansibility, (d~$~/dT')p,the apparent specific comclay-water systems differ from those of pure bulk water pressibility, ( ~ 3 4 ~ / d Pand ) ~ , the molar absorptivity at the frequency and are described by the following empirical equation of 0-D stretching, c. (Note: each symbol represents a different smectite having its own surface characteristics.) Ji/ JiO = exp[Oi(m,/mw) 1 (23) in which Jiand J: are the values of any property, i, of the water in the clay-water system and in pure bulk water, respectively, and p is a constant that is characteristic of both the property and the clay. Moreover, Mulla and Low62and Sun et a1.20have shown that pi = kiS (24) where hi is a constant that is characteristic of the property alone. By combining eq 17, 23, and 24, we find Ji/J: = exp(ki/p,t) (25) The validity of eq 25 is demonstrated in Figure 7 for three different properties of the water (H,O or HDO) in claywater systems: namely, the apparent specific expansibility, (d$JdT)B the apparent specific compressibility, (d$u/dP)T, and the molar absorptivity, E , at the frequency of 0-D stretching. The data for these properties were taken from the work of Sun et and Mulla and Low.62 Each curve in this figure is described by eq 25 with the appropriate value of ki/p,. Appropriate values of this constant for curves 1,2, and 3 are 7.22 X lo4, -0.807 X lo4, and -13.68 X g cm-2, respectively, when t is expressed in centiI
(54) Peschel, G.; Adlfinger, K.H.; Schwartz, G. Naturwissemchaften 1974, 61,215. (55) Gruen, D. W. R.;Marcelja, S.; Pailthorpe, B. A. Chem. Phys. Lett. 1981, 82, 315. (56) Christou, N. I.; Whitehouse, J. S.; Nicholson, D.; Parsonage, N. G. Symp. Faraday Sac. 1981,16, 139. (57) Jonsson, B. Chem. Phys. Lett. 1981,82, 520. (58) Lee, C. Y.; McCammon, J. A.; Rossky, P. J. J. Chem. Phys. 1984, 80, 4448. (59) Mulla, D. J.; Low, P. F.; Cushman, J. H.; Diestler, D. J. J. Colloid Interface Sci. 1984, 100, 576. (60) Magda, J. J.; Tirrell, M.; Davis, H. T. J. Chem. Phys. 1985, 83, 1888. (61) Low, P. F. Soil Sci. SOC.Am. J . 1979, 43, 651. (62) Mulla, D. J.; Low, P. F. J. Colloid Interface Sci. 1983, 95, 51. (63) Oliphant, J. L.; Low, P. F. J. Colloid Interface Sci. 1983,95,45.
I
Langmuir 1987,3,25-31 clays represented in figure 5 and the values ki that are characteristic of the water properties represented in Figure 7, we obtain the relations between II and Ji/J? shown in Figure 8. Since Sun et a1.20have reported the values of kifor several additional water properties, II can also be related to them. Thus, we have found a general relation between the swelling pressure of a clay-water system and any property of the water within it. Such a relation would be expected only if both depend on the same factor.
25
Consequently, we conclude that interaction between the water and the adjacent surfaces of the clay layers affects the structure of the water and, thereby, ita structuresensitive properties. Since one of these properties is G,, II is also affected in accordance with eq 1. I t may be said, therefore, that the structural component of II is primarily responsible for the swelling of clays. Or,alternatively, it may be said that the swelling of clays is due to hydration of their surfaces.
Aqueous Biphase Formation in Polyethylene Oxide-Inorganic Salt Systems K. P. Ananthapadmanabhan and E. D. Goddard* Research and Development Department, Specialty Chemicals Division, Union Carbide Corporation, Tarrytown, New York 10591 Received April 22,1986. In Final Form: September 29,1986 Inorganic salts such as Na2S04,MgS04, and Na3P04have been reported to form aqueous two-phase systems with polyethylene glycols (PEGS). Recent studies show that the above phenomenon is very general in the sense that a number of inorganic salts, even certain uni-univalent salts, form aqueous two-phase systems with PEG. The relative concentration of various salts to form two-phase systems was found to depend upon the valency and hydration (size) of the ions as well as "specific" interactions of the ions with the polymer. Possible mechanisms leading to the formation of PEGinorganic salt-water aqueous two-phase systems are discussed.
Introduction tems for various bioseparations. Recently, a number of An aqueous solution of polyethylene glycol 3350 (4%)other investigators have used such systems for bioseparations6-14and to study the fundamentals of such partiwhen mixed with an aqueous solution of dextran (-5%, t i ~ n i n g . ~ ~ ~The J ~ Jmost ~ J ~widely used phase system in MW 5 X los) or an inorganic salt, such as potassium such studies has been the PEGdextran system. As phosphate or magnesium sulfate, separates into two mentioned earlier, PEG also forms two-phase systems with phases. The unique feature of this two-phase system is inorganic salts such as potassium phosphate. The use of that both phases have almost 90-95% water. Also, each PEGpotassium phosphate systems for the purification of of the phases is relatively rich in one of the solute comindustrial enzymes has been pioneered by Kula and coponents. Reported results' show that the density differworker~.~~*'~ ence between the two phases is small and the interfacial Fundamentals involved in the formation of polymertension between the two phases is small and in the range polymer-solvent two-phase systems have been examined of lo4 to lo-' mN/m. Aqueous two-phase systems of this to some e~tent.'~-~O Scott,17for example, has conducted type have been reported by Beijernick2 as early as 1896. a thermodynamic analysis of two-phase formation and has Almost 50 years later, Dobry and Boyer-Kawenoki3 condeveloped equations to predict critical conditions for phase ducted a systematic study of the miscibility behavior of separation in certain model systems. Phase-forming pairs of different polymers in aqueous and nonaqueous media. The results of their study clearly showed that incompatibility between polymers, leading to two-phase (6) Walter, H.; Brooke, D. E.; Fisher, D. Partitioning in Aqueous Two formation with the same solvent in both the phases, is a Phase Systems; Academic: New York, 1985. very general phenomenon. (7) Walter, H. In Methods of Cell Separation; Catslmpoolaa,N., Ed.; The biaqueous nature of the two-phase system and Plenum: New York, 1977; Vol. 1, p 307. (8)Johansson, G.Biochim. Biophys. Acta 1970,221, 387. difference in properties of the phases makes it possible to Taylor, P.; Barondes, 5.H. Nature (London) 1975, (9) Flanagau, S.D.; use them for the partitioning and separation of biological 254. _. 441. materials such as cells, organelles, enzymes, proteins, etc. (lO)Fisher, D. Biochem. J. 1981,196,l. Note that the use of oil-water-type two-phase systems for (11) Harris, J. M.;Case, M. G.; Hovanes, B. A. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 86. such biological separations may not, in general, be feasible (12) Matiasson, B.; Ling, T. G. I. J. Immunol. Methoda 1980,38,217. because of possible denaturation of biologically active (13) Kula,M.R.;Kroner, K. H.; Hustedt, H. Adu. Biochem. Eng. 1982, components in the nonaqueous medium. Albertsson4i6has 24, 73. (14) Kroner, K. H.;Hustedt, H.; Kula, M. R. Process Biochem. 1984, done pioneering work on the use of aqueous biphase sys170. (15) Brooke, D. E.;Seaman, G. V. F.; Tamblyn, C. H.; Walter, H. ~
(1) Albertsson, P. A. Partition of Cell Particles and Macromolecules; Wiley-Interscience: New York, 1971. (2) Beijernick, M. W. Zbl. Bakt. 1896,2,627,698; Kolloid-2. 1910, 7, 16. (3) Dobry, A.; Boyer-Kawenoki, F. J. Polym. Sci. 1947,2 (I), 90. (4) Albertsson, P.A. Biochim. Biophys. Acta 1968,27,378. (5) Albertason, P. A. Adu. Protein Chem. 1970, 24, 309.
Biophys. J. 1976,15, 142a. (16) Bamberger, S.; Seaman, G. V. F.; Brown, J. A.; Brooks, D. E. J. Colloid Interface Sci. 1984, 99 (l),187. (17) Scott, R.C. J. Chem. Phys. 1949, 17 (3), 268. (18) Tompa, H.Trans. Faraday SOC.1949,45, 1142; 1960, 46, 970. (19) Flory, P.J. Principles of Polymer Chemistry; Comell Univ.: 1953. (20) Ogston, A. G.Biochem. J. 1970,116, 171.
0743-7463/87/2403-0025$01.50/00 1987 American Chemical Society
