Oct., 1958
IONOTROPY-A SPECIAL CASEOF GELATION
STUDIES OF IONOTROPY-A
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SPECIAL CASE OF GELATION
BY W. T. HIGDON Publication No. 166, Shell Development Company, Exploration and Production Research Diuision, Houston, Texas Received March 3,1968
Clay particles in aqueous suspension are oriented by the addition of crystals of electrolyte. The region of oriented particles in the suspension exhibits a figure similar to the cross of isocline when viewed with crossed polarizers. A mechanism for the orientation of clay particles in suspension by an electrolyte has been proposed and discussed in the light of several experiments. The roles of the electrolyte are (a) the surface of the electrolyte crystal forms the initiating surface for the oFientation, and (b) the succession of increasing concentrations of electrolyte from the dissolving crystal depresses the re ulsion forces between clay platelets and allows the van der Waals-London forces of attraction to orient the cla plateets. Subsequent flocculation and aggregation values of electrolyte concentration in the gel fix the particles in orientedlcondition.
P
Introduction H. Thielel coined the. term “ionotropy” to designate the formation of a gel with ordered particles by the addition of an electrolyte to a colloidal suspension. The particles are oriented with respect to each other and to the source of the electrolyte. This phenomenon gives a new approach to the observation of the fundamental principles of the in6eraction of clay particles in suspension. In the discussions to follow, the term “suspension” means the salt-free sol of charged colloidal particles which are acted upon by the electrolyte. The electrolyte which effects the ionotropy will be termed the “orienting electrolyte” to differentiate it from other electrolytes involved in the experiments. The term “ordered gel” will be used to label the jelly or paste of oriented colloidal particles obtained by placing the orienting electrolyte into the suspension. Preparation and Description The phenomenon of ionotropy can be simply demonstrated by placing a thin layer of a 2% suspension of saltfree sodium Wyoming bentonite on a glass slide and then placing a crystal of sodium chloride on the layer of suspension. When the ordered gels are observed microscopically with unpolarized white light they a pear darker than the surrounding suspension, which an increased concentration of clay particles in the ordered gel region. Surrounding the darker region is a halo of a light regionlighter than the unaffected suspension-indicating this to be a zone in which the concentration of clay is lowered. When the polarizer and analyzer are inserted into the microscope in crossed position, the circular region of increased clay concentration appears as a bright field with a dark cross. The arms of the cross are parallel with the planes of polarization. A diamond-shaped dark region is eft in the center of the bright field; this is the trace of the salt crystal. The resemblance of the cross obtained in the orientation of clay platelets by electrolytes to the cross of isocline-observed when dilute clay suspensions are in circular flow between crossed polarizers-led to the comparison of the two. The qualitative explanation of the figure obtained in the cross of isocline is as follows. The suspension is subjected to circular flow. The particles of the suspension are oriented according to the resultant of the shearing forces set up by the circular flow tending to orient the clay particles and the kinetic forces of the Brownian movement tending to destroy the orientation of the clay particles. Along some line a t an angle with the planes of polarization, depending on the shearing forces and the temperature, many of the particles will be oriented parallel with the planes of polarization. Conse uently, the particles in this position will be at extinction, wiich gives rise to the dark arms of the cross. In all other sectors of the circle the particles are at some uniform angle with the extinction position, and consequently the areas are light.
ideates
(1) H. Thiele, Nalurwise., 34, 123 (1947).
In the ordered gels of ionotropy, the influence of Brownian movement is absent, so the angle between the arms of the cross and the planes of olarization should be zero, Le., the arms of the cross shouldie parallel with the planes of polarization. Since this is observed in every case, the conclusion is prompted that the particles are oriented symmetrically about the source of the electrolyte and are fixed against Brownian movement. The specific orientation-particles radially or tangentially oriented-may be determined by the observation of the ordered gel with the Red I plate in the polarizing microscope. This causes the general dark field to become a fistorder red. The dark arms of the cross also become firstorder red. The northwest-southeast quadrants of the bright birefringent area become a second-order blue, and the northeast-southwest quadrants become a 6rst-order yellow. Since the Red I plate was inserted into the microscope in.a northwest-southeast position and the color observed 111 these quadrants is one of increasing order (blue), the clay platelets in this region must be oriented with their “fast” and “slow” directions congruent with the fast and slow directions of the Red I plate. The data in API Report 49* show all montmorillonites to be negatively birefringent and the Clayspur Wyoming bentonite t o have the specific indices of refraction of CY = 1.502, p = 1.515 and y = 1.515. I n negatively birefringent crystals the index of refraction corres onding to the highest velocity light ray is identified with t i e c crystallo raphic axis of the mmeral. The c crystallographic axis o f the bentonites is perpendicular to the face of the clay platelets. Consequently, the clay platelets in the circular ordered gel region must be oriented tangentially t o the circular diffusion front. Figure 1 shows a schematic drawing of the various regions of an ordered gel. Zone A is an o en region left by the deposition of the crystal of orienting eyectrolyte. The outline of the crystal is maintained even after the crystal itself has dissolved. Zone B is the zone of oriented aggregation of the clay platelets. This zone is the region containing the dark or red cross parallel to the planes of polarization when it is viewed in polarized light. Zone C is an isotropic region of random coagulation or flocculation of the clay particles. The colloid concentration in this zone is decreased. The outer boundary of zone C is faint and difficult to fix. The equations governing diffusion for the boundary conditions of this system, salt diffusing outward from a crystal source, are given by Gemant.’ From the relations given by Gemant, the diffusion coefficient of sodium chloride in water and geometric values taken from photographs of ionotropic gels, diffusion times of the order of 6 minutes for gels of 0.8 mm. in diameter and 12 minutes for gels of 1.2 mm. in diameter are calculated. This time of formation is the same order of magnitude that is found in the experimental formation of ionotropic gels of similar size.
Explanations of the Mechanism of Ionotropy Thiele4-* has developed an explanation of the mechanism of ionotropy in the course of his investi(2) M. S. Main, P. F. Kerr and P. K . Hamilton, Arnar. Petrol. Xnst. Res. Proj., 49, 50 (1949). (3) A. Gemant, J . A p p l . P h y s . , 17, 1076 (1946). (4) H. Thiele and H. Micke, Kolloid-2.. 116, 1 (1950). (5) H.Thiele and G. Kienast, ibid., 137, 134 (1952). (6) H. Thiele, {bid., 136, 80 (1954). (7) H. Thiele, Disc. Faraday Soc., 18, 294 (1954). (8) H.Thiele and G. Andorson, KolEoid-Z., 140, 76 (1955).
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gation of the subject. He suggests that the first stage of the process is the adsorption and exchange of gegenions at the sol-electrolyte solution boundary. The second stage of the process is the discharging and dehydration of the colloidal particles by the electrolyte which causes the “lattice forming forces” to become preponderant. These forces cause the linking of the colloidal particles in more or less parallel orientation. By this means a rigid structure is built. Alternative mechanisms based on density gradients, concentration effects or effects on the solvent activities cannot be considered since simple sugar molecules do not cause ionotropy. This phenomenon is not associated with the sign of the diffusion front since sodium chloride (anion more mobile than cation, resulting in a negative front) gave the same qualitative phenomenon as did potassium chloride (anion and cation with almost equal mobilities) and sodium acetate (cation more mobile than the anion, resulting in a positive front). An alternative hypothesis was examined, namely, that there exists between clay platelets forces which will Fig. 1.-Schematic drawing of ionotropic region. tend to orient the platelets with reference to one another if the appropriate chemical environment is available. The diffusing electrolyte provides such an environment. The obvious choice for the orienting forces is the van der Waals-London attractive forces. Verwey and Overbeekg describe Random orien- Edge-to-face ori- Parallel orientathese forces for two flat plates. One may examine A. tation .in dilute entatioq in dilute tion in ionotropic the phenomenon of ionotropy in view of Verwey- suspension. suspension. gel. Overbeek t,heory by considering a plane cut through a volume element of a suspension which contains Fig. 2.-Schematic diagram of orientational possibilities of two clay platelets with respect t o one another. two particles as schematically represented in Fig. 2. In the figure the dashed line envelope around the of the van der Waals-London theory, including the solid line traces of the particles represents an energy assumption of closest approach, in ionotropy there barrier which defines a point of closest approach should be a concentration as well as parallel alignof the particles. Such an energy barrier might ment of the particles. be either the Born repulsion present whenever two Two dFfferent experimental observations were atoms approach each other closely or the presence made of the concentration changes occurring in of a few layers of water adsorbed on the clay sur- ionotropic gels. The first was an attempt to faces with energies of hydration in excess of the measure the concentration change of the gel directly energies of attraction. It is possible that the point by weighing. Table I contains typical concentraof closest approach may be the secondary minimum tion data from ionotropic gels formed in suspensions . described by Verwey and Overbeekg (p. 184). of sodium Wyoming bentonite, obtained by direct This is indicated by the relatively low concentra- weighing. tions of the ordered gels which indicate a rather TABLE I wide spacing between the platelets even a t the point of closest approach. CHANGES IN CONCENTRATION OF CLAYIN ICNOTROPIC GEL If rotation only is allowed then b gives the orienFORMATIONS Trial no. % Clay (by weight) tation of greatest energy of attraction due to the van der Waals-London forces. If rotation of the par1 6.99 ticle about its mass center plus translation of the 2 6.85 particle to a position of closest approach is allowed, 3 6.87 then c represents the position of the greatest energy 4 7.82 5 7.28 Av. 7.16 of attraction between the two platelets. Vold’O Control 4.91 has calculated the energy of attraction between two finite plates of various dimensions and orientaControl 4.81 tions. She showed in general that when a point Control 5.02 Av. 4.91 of closest approach is assumed, the orientation of These data show about a 46% increase in the the broadest face of one particle opposite the broadest face of another particle is the position of clay concentration in the ionotropic gel as commaximum attraction energy. As a consequence pared with the clay suspension from which the gel was formed. (9) E. J. W. Verwey and J. Th. G. Overbeek, “Theory of the StaThe other method employed to demonstrate conbility of Lyophobic Colloids,” Elsevier Publishing Co., New York, centration changes accompanying ionotropic gel N. Y., 1948, p. 184. formation was the following. The suspension (10) Marjorie J. Vold, J . CoZEoid Sci.,9, 451 (1954).
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IONOTROPY-ASPECIALCASEOF GELATION
Oct., 1958 VI
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e ; z 0
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z w
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RANDOM
FLOCCULATION
o c :
i
0 2
0 W
2
Z O N E OF INCREASED CONCENTRATION
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Fig. 3.-Densitometer
Z O N E OF DECREASED CONCENTRATION
ou wmt -4 z u w e t -
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trace of ionotropic gel showing the concentration changes accompanying gel formation.
layer containing the gel region was photographed in ordinary light. The negative was then scanned with tlle densitometer. The assumption was made that the light falling on the film was an inverse function of the concentration of the clay particles in the suspension layer, and that the light coming through the negative and falling on the photocell of the densitometer was also a function of the clay concentration. Figure 3 is a schematic representation of the recorded galvanometer deflection of such an operation. This figure shows the concentration change which accompanies ionotropic gel formation. This observation-the concentrating of clay particles in ionotropy-predictable from considerations of the theory that van der WaalsLondon forces are predominantly operative in clay particle orientation by electrolytes, supports this viewpoint of the mechanism of ionotropy. The theory proposed assigns to the diffusing orienting electrolyte the role of creating the chemical conditions under which the platelets can orient. The important feature of the diffusing orienting electrolyte in this theory is the succession of concentration levels in a given microscopic region. Four concentration ranges are important in this regard: (1) the first concentration range of interest is the first few milliequivalents of electrolyte per liter which reduce the viscosity of a concentrated suspension. This is discussed by van Olphen.ll This range, symbolized as C1, is around 3 to 4 meq./liter of sodium chloride in a suspension of sodium Wyoming bentonite. (2) The second concentration range is the concentration of electrolyte sufficient to cause flocculation of the suspension. This range, symbolized as Cf, occurs at 20 to 40 meq./liter for various salts according to Darley. l 2 (3) I n the third salt concentration range are those concentrations a t which aggregation takes place. Aggregation is defined as the stacking of the individual clay unit layers face to face to make one kinetic unit. This concept is discussed by Garrison and Ten Brink.13 This range, symbolized as C,, is above 400 meq./liter of sodium chloride for sodium (11) H. van Olphen, Disc. Faraday Soc., 11, 82 (1951). (12) H.C. H.Darley, J . Petrol. Technol., 210, 93 (1957). (13) A. D. Garrison and K. C. Ten Brink, Petrol. Technol., 2, I (1939).
REGION
>
u
Wyoming bentonite. (4) Saturated solutions of the orienting electrolyte are the fourth concentration studied. These are symbolized by C,. The fact that the concentration is the important parameter of the electrolyte in ionotropy 4s supported by some experiments in which amounts of electrolyte in the concentration ranges of C1, Cr and C, were added to a suspension of polydispersed sodium Wyoming bentonite. The specific concentrations added to the suspension of sodium Wyoming bentonite were 8, 40 and 600 meq./liter, respectively. After thorough mixing of the electrolyte with the clay suspension, crystals of sodium chloride were deposited in the thin layers of suspension which contained electrolyte. Ionotropy OCcurred in the suspension containing Cl amounts of electrolyte (0.008 meq./liter) which was qualitatively the same as in the absence of the electrolyte. Ionotropy occurred in the suspension containing Cf amounts of electrolyte (0.04 meq./ liter) with the addition of a sodium chloride crystal but at a greatly reduced distance from the crystal and at a reduced degree of ordering of the clay platelets. No evidence of ionotropy was observed around the crystal of sodium chloride placed in a thin layer of Wyoming bentonite suspension which contained C, values of electrolyte (0.6 meq./liter). The role of the electrolyte in the orientation of colloidal particles (ionotropy) is explained in the following manner. Consider the particle adjacent to one already fixed in an ionotropic gel. This particle is fixed in a thixotropic gel or concentrated suspension so that Brownian motion is negligible. The first salt concentration level to reach the particle from the origin of the diffusion electrolyte is Cl. This amount of electrolyte liberates the clay particle and makes it possible to move both rotationally and by translation. Successively increasing electrolyte concentrations depress the double layer of the particle and decrease thereby the repulsive forces between the particles, which allows the attractive forces of van der WaalsLondon to pull the platelet toward the one previously fixed. As the concentration of the salt about the particle reaches Cf then the particle will become fixed a t first contact with another particle. If C,
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W. T. HIGDON
Vol. 62
(B) cp
*P
I
1
1
t
LEGEND
-BLUE
Fig. 4.-Schematic
-YELLOW
1 7 -
RED
representation of the microscope observations of the orientation of clay platelets with polarized light and Red I plate.
follows Cf closely with little time for random sticking of the particles the probability of the particles taking the preferred parallel arrangement close to one anbther is great. If there is a large time interval between Cfand C, in passing the particle, the degree of random, long-range flocculation compared to oriented, short-range linking, as in aggregation, will be greater. If only Cf is attained in the region of the particle only random flocculation will occur. The role, therefore, of the salt in ionotropy is to create the environment in which the attractive forces can orient the clay particles. The velocity of movement of the electroIyte front through the suspension also can be estimated. A time interval for the diffusion of the electrolyte across the annular space between the two cylinders can be found by calculating the times for the electrolyte to diffuse to two concentric cylinders. Assuming a constant velocity across this space, one can estimate a value for the mean velocity of the front. Such a calculation obtained a value of 25 p per minute as the velocity of the diffusing electrolyte front through the suspension. With data from Kahn and Lewisl4 the rotational diffusion coefficient of the clay particle can be estimated. From these coefficients ample time was found to allow for both rotation and translation of the clay particle while the diffusing electrolyte front was passing the particle. The foregoing only explains the parallel orientation of the clay platelets with reference to one another in ionotropy. The cylindrical symmetry of the gel must be accounted for. The initial orientation of the particle can be understood from the observation that any surface imposed on a suspension causes orientation. Air bubbles, droplets of water or solutions of any material, or pieces of solid materials extruded into or placed into a layer of suspension cause the clay particles to orient parallel to the interface. When the suspension is dilute so that there is considerable Brownian movement of the particles the orientation is transient; in concentrated suspensions the orientation of the clay particles about an imposed interface is long lasting. (14)A. Kahn and D. R. Lewis, Tu18 JOURNAL, 68, 801 (1054).
Consequently, the deposition of the salt grystal would cause at least transient orientation parallel to the crystal faces. Alternatively, the orienting crystal face would have the van der Waals-London forces between it and the adjoining clay platelet, thereby causing the clay platelet to orient itself parallel to the crystal face by means of the mechanism proposed above. The following experiment was conducted to distinguish between the two roles played by the salt in ionotropy and to provide information concerning the role of the geometry of the diffusion front in the specific orientation of the particles in ionotropy. The observations from this experiment are schematically represented in Fig. 4. Brass vanes were fixed perpendicular to the glass surface of a microscope slide. The vanes, 90° with respect to one another, extended from the sides of the slide in to the middle leaving a small area about the dimension of a salt crystal at the point where they almost touched. A suspension was placed in the cell that would be classed as a dilute suspension by the criterion that Brownian movement was possible. Yellow and blue regions were observed along the brass vanes with crossed polarizers and the Red I plate' in the optical system. The color fringes, represented in Fig. 4a, showed that a thin layer of suspension next to the vanes had p&rticles oriented parallel to the vanes. Salt crystals were introduced at the junction of the vanes and were allowed to dissolve in the suspension and create a birefringent region. The colored sectors when viewed through the Red I plate (see Fig. 4b) showed that in the great portion of the sector between the vanes the orientation of the clay platelets was the same as in normal ionotropy, &e., tangential to the diffusion front. When this region was rotated by 45' SO that the vanes were at 45O with the planes of polarization it was apparent from the colors that the orientation of the clay particles persisted parallel to the vanes rather than perpendicular to the vanes which would have been the orientation of the particles in normal ionotropy (see Fig. 4c). This experiment leads to the conclusion that the geometry of the diffusion front, or the stream of diffusing
Oct., 1958
IONOTROPY-ASPECIALCASEOF GELATION
ions, is not the determinative factor in the orientation of the clay particles. Orientation of the particles tangential to the diffusion front in normal ionotropy occurs only because the interface initiating the orientation is coincident with the source of the diffusion front. There is another consequence of the van der Waals-London attraction of particles in their orientation by electrolytes. There should be a point where the sol is so dilute, ie., the particles are so far apart, that the energy of attraction between particles is too small to effect orientation. Preliminary investigations place this point a t about 0.5y0 clay by weight for a polydispersed sodium Wyoming bentonite oriented by sodium chloride. Another experiment on the nature of the ionotropic region involved the effect of an electric field.
1281
If an electric field of a sufficient strength to cause rapid movement of the clay particles in the suspension is imposed on the system after the formation of the gel region, the gel region is unaffected. If the electric field is imposed simultaneously with the initiation of orientation then only one-half of the field is formed, that half toward which the clay particles are moving. The other half does not form because the clay particles are moving away from the source of the electrolyte. The nature of the clay mineral affects the quantitative aspects of ionotropy but not the qualitative ones. Nontronite, att,apulgite aqd illite were all found to exhibit ionotropy to a greater or lesser degree, with the clay platelets oriented tangentially with the diffusion front. Kaolinite did not form oriented gels.
