YOTES
288
octadecyl phosphate from the air-water interface14 as is seen in Table I. In conclusion, for uncharged monolayers satisfying the assumptions of the theory of Ter Minassian-Saraga, the activity coefficients may be obtained from desorption kinetics by means of the relationship y* = F;,/k,". Furthermore, if k , # ksoy*,monolayer desorption is not diffusion limited but is more likely controlled by an energy barrier at the surface.
Colloidal Suspensions of Chrysotile Asbestos : Specific Anion Effects
by A. W. Naumann and W. H. Dresher Union Carbide Corpwation Research Center, Tuxedo, New Y O T ~ (Received June 18,1966)
Unlike other silicate minerals, the zero point of charge (z.P.c.) of chrysotile asbestos is above pH 7. Values for the Z.P.C. of pH 10.1 and 11.8 have been reported.102 In acidic solutions, therefore, chrysotile fibrils bear ;t positive charge that tends to increase as pH decreases. It has been found that, with the proper choice of conditions, the surface charge can be increased to the point that metastable dispersions of chrysotile in water can be prepared. Such suspensions are susceptible to flocculation, and the conditions for preparing them are quite critical. I n general terms, dispersions are prepared when: (a) sufEcient energy is imparted to an asbestos slurry to defibrillate the asbestos-fiber aggregates, thus liberating the individual crystalline fibrils; (b) the pH is appropriate for a high surface charge; and (e) the kind and concentrations of anions in the suspending media are controlled to preserve the surface charge. The latter is of particular importance, since pronounced specific anion effects are involved. This communication deals with the magnitude and nature of these anion effects.
Experimental Section The chrysotile used in this investigation was obtained from the New Idria serpentinite formation of central California. This deposit and the type of material found in it have been described elsewhere.a Chrysotile concentrations of 0.5% were used for all experiments. Dispersed suspensions of chrysotile were prepared by thrashing a slurry of asbestos and deionized water in a Waring blender for 3 min. in the The JoUTnd of Physical Chemistry
presence of acetic acid. Acid additions were chosen to 0.1. I n the absence of addigive a final pH of 4.2 tional electrolytes, suspensions prepared in this way are of low viscosity (1 to 5 cp.) and exhibit the pearly shimmer characteristic of colloidal suspensions of anisotropic particles. After standing for several hours, the suspensions revert to gels. This transition occurs rapidly once initiated. Viscosity often increases from a few centipoises to more than 100 cp. in less than 1min. The addition of electrolytes shortens the period of stability. With increasing concentration, the period of stability is progressively shortened until at a critical concentration, low-viscosity dispersions are prepared that revert almost immediately to gels. Above this concentration the low-viscosity regimes are not obtained. I n the present investigation, the critical coagulation concentration (c.c.c.) was arbitrarily taken to be the amount of salt (when added prior to blending) that gave dispersions with a viscosity of 1 to 3 cp. 1 min. after blending, and more than 20 cp. after standing 5 min. A Brookfield Model LVF viscometer was used for the viscosity measurements. The concentrations established in this way were reproducible to *ti%.
*
Results Critical coagulation concentrations for salts of nine monovalent anions were determined and are listed in Table I. Table I
Anion
Added as
KMn04 NaCNS
KI NaN03
KBr KC104 NaNOa NaCl KC1 NH4Cl CaClz MgClz
Critical coagulation concn., m m o1es/I.
0.4 0.6 0.9 1.2 2.0 2.0 2.1 4.0 4.0 4.3 4.7 4.7 4.0
(1) F. L.Pundsack, J. Phya. Chem., 59, 892 (1955). (2)E.Martinez and G. L. Zucker, ibid., 64, 924 (1960). (3) R. C. Munro and K. M. Reim, Can. Mining J., 83, No. 8, 45 (1962).
NOTES
-
289
-
-
The observed order, C1NOz- > ClodBr- > NOa- > I- > CNS- > Mn04-, is similar to that found for the selectivity anion-exchange resins,4 C1- < NOz- < Br- < NO3- < I-. The order differs, however, from that found for the coagulation concentrations for other positively charged colloids, namely: CXS- > NO,- > C1- for positive A1&, and I- > Br- > NO3- > C1- for positive FezO8,S and also differs from the specific anion effects that have been observed with positive silver iodide,6 barium sulsulfate,’ and mercury surfaces.* There were no pronounced specific cation effects observed when the cation was varied in a series of chloride salts. 103-
Discussion Chrysotile Suspensions-General Considerations. Three factors combine to make suspensions of chrysotile well suited for the study of variables that influence the stability of lyophobic colloids: the size, the shape, and the surface composition of the particles. The basic repeat unit of the chrysotile structure is a composite sheet consisting of a tetrehedral silica layer and an octahedral magnesium hydroxide layer joined together so that the unshared oxygens of the silica tetrahedra and one of the surfaces of the octahedral layer lie in a common plane.9 A mismatch in the natural repeat distances of the two layers causes the composite sheet to curve. The sheets curve and close to form tubes of extreme length-to-diameter ratio.1° The magnesium hydroxide layer has the longer repeat distance and is exposed at the cylinder surfaces. The lengths of chrysotile fibrils range from a few to many tens of microns; diameters are subject to much less variation and usually range from 250 to 350 8. This is another result of the lattice mismatch that causes curvature and tube formation, for there is a strain-free configuration at only one unique radius. Strain increasts with increasing departure from this ideal radius, limiting the number of layers that can be built up into a stable cylindrical wall and both average fibril diameter and the distribution of sizes around the average. Thus, suspensions of chrysotile consist of particles of simple and known geometry, Le., long rods of essentially constant diameter, a shape for which mathematical forms are available to describe particle motion, electrical double-layer structure, etc. Equally significant, particle surfaces are of known and uniform composition. There is no possibility for a multiplicity of crystal faces except at the ends of the fibrils, which make up only a small portion of the total surface. The net result of these factors is a high susceptibility of chrysotile suspensions to flocculation. The time
required for the number of particles of an initially monodispersesuspension to be reduced by one-half is
k tl/, = RDva where k is a constant, R is the radius of influence of a given particle, D is the particle diffusion constant, v is the initial particle concentration, and CY is the fraction of particle-particle encounters that lead to a lasting contact.s Collision diameter (R) is determined by the longest dimension of the particles, while, to a first approximation, the diffusion constant (0)varies as the mean particle dimension. The coefficient a is the probability that particles undergoing collision have sufficient kinetic energy to overcome any potential barrier to particle-particle contact. The sensitivity of chrysotile suspensions to flocculation is due primarily to unusually large values for R and a. As mentioned previously, the lengths of chrysotile fibrils, hence the collision diameters (R),are of the order of several tens of microns. The sticking coefficient (CY)is large because the probability of overcoming a potential barrier of given height increases as the area involved in particle-particle contact decreases.*‘ For random (unaligned)encounters between asbestos fibrils, area of overlap is of the order of the square of the diameter of the fibrils; in general, less than lo-“ cm.2. The areas involved are small enough to require large barriers (of the order of several K T ) to prevent flocculation. #pee@ Anion Ejects. Within the framework of the Derjaguin-Landau-Verwey-Overbeek theory, l1 the over-all interaction between two colloidal particles is the sum of an attractive potential resulting from van der Waals-London interactions and a repulsive potential resulting from the interaction of the electrical double layers associated with the particles. Suspensions are stable when the sum of the potentials gives an energy barrier around the particles high enough to reduce collisions caused by thermal movements in the (4) R. M.Wheaton and W. C. Bauman, I d . Eng. Chem., 43, 1088 (1951). (5) H.R. Kruyt, “Colloid Science,” Vol. I, Elsevier Publishing Co., Amsterdam, 1952. (6) J. S. BeeMey and H. 5. Taylor, J. Phys. Chem., 29, 942 (1925). (7) I. M. Kolthoff and W. M. MacNevin, J . Am. Chem. SOC.,58, 1543 (1936). (8) D.C.Grahame, M. A. Poth, and J. I. Cummings, ibid., 74,4422 (1952). (9) E. J. W.Whittaker, Acta Cryst., 9, 855 (1956). (10) J. Turkevich and J. Hillier, Anal. C h m . , 21, 475 (1949). (11)E. J. W.Verwey and J. T. G. Overbeek, “Theory of the Stability of Lyophobic Colloids,” Elsevier Publishing Co., Amsterdam and New York, 1948.
Volume 70,Number 1
Januarg 1966
NOTES
290
suspension. For particles of a given geometry, the height of tJhe energy barrier is determined by the surface potential ($0) of the colloidal particles and the charge type (monovalent, divalent, etc.) of the counterions of the suspending medium. In terms of the simple theory, the effect of ions of the same charge type should be identical, and the charge type of the ions bearing the same charge as the particles should have no influence on stability. The latter condition is met by chrysotile suspensions, but the former is not. Several extensions of the original theory have been made to account for specific ion effect^.^^-'^ The usual approach is to have the height of the potential barrier depend upon a “Stern layer” potential ($d), which is a function of the composition of the system. One of the many obstacles to making such extensions is an inexact knowledge of the chemical forces involved in lowering the effective surface potential from $0 to $d. The results of the chrysotile coagulation experiments suggest that these forces are similar to those responsible for ion-exchange selectivities, for which a number of explanations have been proposed.16-1s One of the more widely accepted views is that selectivity is determined by the polarization of ions in the fields of the resin exchange sites. The equivalent phenomena for interactions of anions and chrysotile would be short-range interactions between the ions and localized charged sites on the fibril surfaces. Under the conditions of the flocculation experiments, coagulation concentrations were chosen to give essentially constant flocculation rates for the various anions. Following eq. 1, R, D,Y, and k were constants for the particular chrysotile system employed, so that at the C.C.C. the flocculation coefficient, a, was also the same for all anions. It has been shown that in the presence of a potential barrier, coagulation rate is reduced by a factor, W I T
V max
W=e-
kT
where Vmaxis the maximum of the potential barrier.l‘ Hence, the essentially constant flocculation rates at the critical coagulation concentrations reflect essentially constant values for Vmaxand, to a first approximation, essentially constant values for #d. The constancy of $d’s, in turn, reflects essentially equivalent concentrations of adsorbed counterions. The c.c.c., therefore, correspond to the solution concentrations of anions needed to bring about the necessary degree of anion adsorption to cause flocculation, Le., equilibrium of the sort
A- (solution) The Journal of Physical Chemistry
A- (surface)
(3)
0.81
I
I
I
I
i
I
j~ g 0.2
g
0.0
0 0
a 0 (3
Q
J
-0.2
-0.4
I
I
‘ION VOLUME,
A’
Figure 1. Correlation between critical coagulation concentration and ion volume. Volumes for C1-, Br-, and I- are based on Pauling radii. Volumes for oxyanions are based on the volumes of two, three, or four oxygen atoms and that of CNS- on the sum of the volumes for N and S.
For such reactions AFO = RT In
aA-(solution)
a A -(surface)
=
RT In
CA
- -/-
(4)
p
FA
where F A is the surface concentration, CA is the solution concentration, and p is the activity coefficient term. Little is known about the activity coefficients of adsorbed ions, but we may suppose that at the C.C.C. F A and p are nearly the same for all anions. If so, log C(solution) is a measure of the energetics of adsorp11s. polarizability, or tion, and a semilog plot of C.C.C. (12) A. Packter, 2. physik. Chem. (Leipaig), 214, 63 (1960). (13) M. J. Sparnaay, Rec. trav. chim., 81, 395 (1962). (14) V. M. Barboi, Y. M. Glazman, and I. >I. Dykman, Colloid J. (USSR), 24, 382 (1962). (15) S. Levine and G. M. Bell, J. Phys. Chem., 67, 1408 (1963). (16) H. P. Gregor, J. Belle, and R. A. Marcus, J. Am. Chem. SOC., 77, 2713 (1955). (17) S. A. Rice and F. E. Harris, 2. physik. Chem., 8 , 207 (1956). (18) B. Chu, D. C. Whitney, and R. M. Diamond, J. Imrg. Nucl. Chem., 24, 1405 (1962).
NOTES
a property related to polarizability, provides a test for the hypothesis that the specific anion effects observed with chrysotile are due to interactions between anions and localized charged sites on the particle surfaces or, more specifically, are controlled by the polarization of anions in the field of the charged sites. This correlation is shown in Figure 1. Procedures are available for inferring polarizabilities from crystal properties,lg but, unfortunately, self-consistent sets of data are not available to provide values for many of the anions studied. Lacking polarizabilities, the C.C.C. were plotted against ion size, which is a related property. A definite correlation between critical coagulation concentration and ion size was obtained. Of the nine anions studied, only one, perchlorate, was seriously out of line. One may speculate why the specific effects reported here have not been found for other materials with hydroxyl surfaces as, for example, oxidm and other silicate minerals. Possibly the effects are operative but not readily observed except for systems sensitive to flocculation (such as chrysotile). It is interesting to note that nitrate, frequently used as an “indifferent” electrolyte for colloid studies, is strongly adsorbed by chrysotile surfaces. Acknowledgment. The authors are grateful to Prof. R. S. Hansen of Iowa State University for helpful discussion of the subject of this communication. (19) J. R. Tessman, A. H. Kahn, and W. Shockley, Phys. Rev., 92, 890 (1953).
Empirical Relation between Electrical Conductivity and Pressure for Organic Solids1
by Y. Okamoto Research Division, School of Engineering and Science, New York University, University Heights, New Y O T ~ (Received July 6 , 1966)
Recently, the effects of pressure on the electrical conductivity of various organic solids have been inve~tigated.~-~ The conductivity and compressibility of organic crystals are generally highly anisotropic.6 The conductivities of pentacene under pressure are found to display large quantitative differences in single crystal and powder samples.’ Therefore, conductivity measurements are preferably performed on single crystals. However, large single crystals are presently available only for a limited number of or-
291
ganic compounds. I n addition, the conductivity measurements of single crystals in one specific direction under high pressure are extremely involved. Therefore, in order to obtain the preliminary data of electrical conductivity of organic solids under pressure, powder specimens are generally employed. In general, the observed conductivity increases with increasing pressure, and this phenomenon is usually reversible. The mechanism of electron conduction in most organic compounds is not yet clear. This is especially true of measurements made on other than single crystals. However, whether conducting via the band model or hopping-model mechanism, the increase in the conductivity with pressure may be accounted for by the increase in the amount of aorbital overlap between adjacent molecules.* The effect of pressure on the conductivity varies among the compounds. This phenomenon is generally less pronounced for compounds having low resistance at atmospheric pressure than with those of high resistance. The small change in resistance with pressure may be explained by the fact that low-’resistance compounds have achieved near maximum orbital overlap at atmospheric pressure. Therefore, only small variations of resistance with pressure are generally observed. This is clearly shown in Table I in which the effect of pressure on the electrical resistance of 15 various types of charge-transfer complexes are summarized. Figure 1 shows the plot of log R/R1 bar vs. logarithmic resistivity ( p ) of the compounds at room temperature under 1 atm. of pressure. R and R1 bar are the resistance at 12.5 kbars and 1 bar, respectively. By the analysis of variance, the plot shows a fairly good linear relationship between log R/R1 bar and log p1 bar (probability P < 0.001). The slope was calculated by the least-squares method and the following equation was obtained R P1 bar log - = mlogR1 bar C
+b
(1) Part 111: Effect of Pressure on Organic Compounds; part 11: Y. Okamoto, S. Shah, and Y . Matsunaga, J . Chem. Phys., 43, 1904 (1965). (2) G.A. Samara and H. G. Drickamer, ibid., 37, 474 (1962). (3) P. Harada, Y.Maruyama, I. Shirotani, and H. Inokuchi, Bull. Chem. SOC.Japan, 37, 1378 (1964). (4) M. Schwarz, H. W. Davies, and B. J. Dobriansky, J. Chem. Phys., 40, 3257 (1964). (5) Y.Okamoto, S. Shah, and Y. Matsunaga, part I1 of this series. (6) 9. 9. Kabalkina, Fiz. Tverd. Tela, 4, 3124 (1962). (7)R. B. Aust, W. H. Bentley, and H. G. Drickamer, J . Chem. Phys., 41, 1856 (1964). (8) Y. Okamoto and W. Brenner, “Organic Semiconductors,’’ Reinhold Publishing Corp., New York, N. Y.,1964,Chapters 2, 3.
Volume 70, Number 1 January 1966
