TRANSITIORS IS SOAP-OIL SYSTEMS
53
REFERESCES (1) BUERGER, SMITH,DE BRETTEYILLE, A N D RYER:Proc. Natl. Acad. sci. C . S. 28, 526 (1942). (2) BUERGER, SMITH,RYER,. ~ N DSPIKE:Proc. Natl. Acad. Sci. U. S. 31,226 (1945). (3) BUSHEY:Ph.D. Thesis, The Rice Institute, 1948. M.A. Thesis, The Rice Institute, 1949. (4) DRAPER: (5) DRAPER A N D JfILLIGas: Texas J. sci. 2, 209 (19s). (6) FERGUSOX: Oil & Soap 21, 6 (1944). (7) FERGUSON, ROSEVEAR, A N D SORDSIECK: J. Am. Chem. SOC.89, 141 (1947). (8) FERGUSON, ROSEVEAR, A N D STILLMAN: Ind. Eng. Chem. 36, 1005 (1943). BUERGER, A N D SMITH: J. Phys. Chem. 49,417 (1945). (9) GARDINER, (IO) MCBAIN,VOLD,A N D JOHNSTON: J. Am. Chem. SOC.63, 1000 (1941). (11) MILLIGAN,SxmsoN, BUSHEY,RACHFORD, ASD DRAPER: I n process of publication. (12) MILLIGAN A N D WATT:Unpublished results reported a t the 110th Meeting of the American Chemical Society, which was held in Chicago, Illinois, September 9-13, 1946. M.A. Thesis, The Rice Institute, 1943. (13) SIMPSON: (14) THIESSEN:Angew. Chem. 61, 318 (1938). AND STAUFF:Z. physik. Chem. 176A, 397 (1936). (15) THIESSEN A N D STAUFF:%. physik. Chem. 177A, 398 (1936). (16) THIESSEN (17) VOLD:J. Phys. Chem. 49, 315 (1945). (18)WATT:M.A. Thesis, The Rice Institute, 1946. (19) WEISERAXD MILLIGAN:J. Phys. Chem. S8, 1175 (1934).
TRANSITIOSS I N SOAP-OIL SYSTEMS BY DIELECTRIC ABSORPTION' TODD M. DOSCHER AND SANFORD DAVIS Department of Chemistry, University of Southern California, Los Angeles 7, California
Received August 10, 1960 I. INTRODUCTION
Very small quantities of water and other polar molecules have a very pronounced effect on the physical behavior of soap-oil systems and their industrial utilization. Attempts to understand the role played by water have led to studies of the phase relations in these systems (5, 6, 17, 18) and studies of their stability (7). However, in previous investigations it was not always certain that the last traces of water had been removed from the soap. I t was one of the purposes of this investigation to determine the minimum quantities of water which are required to produce significant changes in the temperatures a t which phase transitions have been recorded. Further, measurements of the capacitance and dielectric absorption of these systems are capable of revealing the existence of any changes in the orderliness of packing and the freedom from intermolecular restraint of the soap aggregates a t or between transition temperatures. 1 Presented a t the Twenty-Fourth National Colloid Symposium, which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society a t St. Louis, Missouri, June 15-17, 1950.
54
TODD M. DOSCHER AND SANFORD DAVIS 11. EXPERIMENTAL TECHNIQUES
r h e capacitance and copductance of systems containing 2-13 per cent of sodium stearate in cetane and up to 0.5 per cent added water were measured with a General Radio Type 916A capacitance bridge between 0.1 and 200 kc. A Hewlett Packard Type 200C oscillator was used as the signal source, and a General Radio amplifier and null detector, Type 1231A, was used as the detector up to frequencies of 10 kc. For the higher range of frequencies a U. S. Navy Model RBL3 radio receiver (Wells Gardner Company) was used as the detector. The systems were contained in a Pyrex-glass cell (see figure 1) which was suspended by the lead wires in an air oven. The oven was provided with Pyrex-glass
FIG.1. Capacitance cell. A, 25-mm. i.d. Pyrex cylinder, 60 mm. high; B, platinum electrodes; C, glass saddle for positioning electrodes; D , tungsten seals; E, 1.5-mm. i.d. capillary. ports t o permit visual observation to be made during a run. The temperature of the air oven could be varied between 50°C. and 250°C. and controlled to within 0.5"C. The temperature was recorded and controlled with the use of a Brown Instrument Company strip-chart potentiometer and a calibrated iron-constantan thermocouple. Since it was desired to view the contents of the cell, no guard electrodes were used; instead, the oven was lined with sheet metal and connected to the ground lead. The ground lead to the cell was a +-in. copper rod, which was led through the side of the oven and which could be cranked through an arc of about 270°, permitting the cell to be oscillated and the contents agitated between measurements. The high lead was made of braided wire and was led into the oven through three concentrically arranged glass tubes which were spaced with glass washers. The capacitance and loss factor of the leads were checked several times during the course of the investigation over the entire range of temperatures and
TRANSITIONS I N SOAP-OIL SYSTEMS
55
frequencies which were used. The cells were initially evacuated and sealed in order to be calibrated. The capacitance of the three cells used ranged between 4.0 and 4.2 N p f . The cells were designed so that a minimum of free space was available for the vapors of cetane and water. The sodium stearate was prepared as previously described (5), and after grinding was filled into a dried and weighed cell. The cell was heated to 175°C. under a pressure of 1 mm. of mercury for 72 hr. I t was sealed while still connected to the vacuum line by fusing the capillary. The cell was reweighed to obtain the weight of sodium stearate; it was then transferred to a dry-box and the seal was broken. Freshly distilled cetane was admitted through the capillary with a 6-in. hypodermic needle and syringe. The cell was then capped with a rubber gland which was cemented to the capillary. The amount of cetane added to the cell was determined by weighing the syringe before and after delivery of the cetane. The cell was removed from the dry-box and sealed. Water was admitted in much the same manner as the cetane, except that a microsyringe was used. The weight of these systems varied between 10 and 15 g., so that 10 mg. of water corresponded to the addition of about 0.1 per cent of water. The measurements were made by heating the systems to 250°C. and keeping them a t that temperature for 48 hr. before cooling them at an average rate of 1°C. per 15 min. Faster cooling rates (see results) did not give reproducible results. The reproducibility of the results a t the slow cooling rate is adequately shown by the conformity of the experimental observations (each curve of figures 2, 3, and 4 represents the values obtained in three or more cooling cycles). 111. EXPERIMENTAL RESULTS AXD DISCUSSIOS
E$ecl of frequency In all the systems which were investigated, both the conductancc and the capacitance fell off with increasing frequency between 0.4 and 200 kc. The effect was very small in the anhydrous systems but increased as the water content of the systems was increased. Unfortunately, reliable measurements could not be obtained below 0.4 kc. to detect the existence of any maxima of lower frequency. As the frequency mas increased (see figure 2) the capacitance and conductance curves were smoothed out, and a t 200 kc. the maxima in the capacitance-temperature curves were scarcely discernible. This dependence on frequency is strongly indicative that the dielec-tric absorption in these systems is due to Na~rvcllWagner polarization (6, 10, 11): the coexistence of two phases, one (soap-rich) more conductive than the other (cetane-rich). One entire series of runs was duplicated with gray platinized electrodes. The electrodes, before being sealed into the cell, were platinized in a platinum chloride-lead acetate solution, then heated in a flame for 10 min., replatinized, and reheated. The gray electrodes were then extracted with water for 48 hr. in a Soxhlet apparatus to remove traces of electrolytes. The results obtained with these electrodes were exactly similar to those obtained with bright platinum
56
TODD M. DOSCHER AND SANFORD DAVIS
~yatems.
CH 2a 175
I50 125
-25
-8.5
- 9.5
-10.5
FIG.3. Log specific conductance of 2.0 per cent sodium stearate-cetane system, l-kc. Solid lines are cooling curves at 1°C.per 15 min. Dashed lines are for quenched systems (see text).
electrodes, indicating that the frequency variation was not due to galvanic polarization. Plots of d’/(d - e-) us. frequency (12) did not give straight lines, an
57
TRANSITIONS IN B O A W I L SYBTEX8
indication that there is more than one relaxation time or mechanism in these systems.
Behavior above 11roc. The conductance of the systems containing 2.0 per cent and 12.8 per cent sodium stearate is shown in figures 3 and 4, and the capacitance of the 2.0 per cent system in figure 2. These curves are typical of those obtained for intermediate and higher concentrations, respectively. The capacitance-temperature relations for the anhydrous systems indicate that the mobility of any ionic species as well as rotation of polar aggregates is appreciably lost when the systems have
-710
-a0
-
9.0
-rao
FIG.4. Log specific conductance of 12.8 per cent sodium stearatecetane system, 1 kc. Cooling curves at 1°C. per 15 min.
been cooled t o 205"C., and their immobilization is practically complete a t 168OC. The systems are still isotropic a t 205"C., although an abrupt rise in viscosity occurs at 205°C.; the system is transformed from a mobile liquid to an isotropic gel. At 168°C. an anisotropic phase is precipitated. Arresting the temperature between 205°C. and 168°C. did not result in the appearance of any turbidity. When the tubes were oscillated in this temperature range however, birefringemt st& were developed which disappeared on standing. These reaults indicate that the soap molecules are aggregated in the isotropic gel between 205OC. and 168OC. (1). The addition of small amounts of water incresses the mobility above 168OC., 80 that the systems are mobile isotropic fluids down to this temperature. Although the conductance and capacitance of the aqueous syatema above 168OC. are significantly higher than those of the anhydrous systems, a change in the rate at which the conductance is falling with temperature is also observed in the
58
TODD M. DOSCHER AND SANFORD DAVIS
former a t 205°C. This suggests that the intrinsic nature of the aggregation of the soap molecules between 205°C. and 168°C. is not altered by the presence of water, but that the water acts &s a peptizing agent in permitting the formation of smaller and free-draining micelles (1). At 168°C. the anhydrous systems become turbid, although there is no marked change in the capacitance or conductance a t this temperature. Below this temperature, the conductance and capacitance drop regularly with decreasing temperature but a t a very much lower rate than a t higher temperatures. In the aqueous systems a very marked drop in conductance occurs a t 168"C., and below this temperature both the conductance and the capacitance begin to rise to a maximum value. At first, no turbidity was reported in the aqueous systems coincident with the changes observed by the bridge measurements. However, when the cells were not oscillated, it was found that a t temperatures slightly under 168°C. a very small amount of a fine precipitate had collected a t the bottom of the cells. The decreased viscosity of the system evidently permits the material to settle rather rapidly. Shaking the cell distributed the precipitate so finely that it could not be readily observed through the oven windows. I t was not until the system had been cooled to several degrees below 168°C. that the turbidity had increased to a point where it could be readily observed. These results again indicate that the water does not interfere with the transitions Tvhich occur in its absence, but merely acts as a peptizing agent in reducing the size and ability of the aggregates to interact with each other. The mobility of the aggregates in the aqueous system is further borne out by the observed increase in capacitance and conductance below 168°C. The liquid crystalline aggregates which begin to form in these systems at 168°C. and continue to increase below this temperature are either ionized in the presence of water or have a finite, net electric moment and therefore can respond to the electrical field, resulting in the increase in conductance below 168°C. (8, 14). This increase is halted when the growth of these aggregates causes the internal viscosity of the system to increase to a point where the mobility of the aggregates is impaired (14). The greater the ratio of water to soap in the system (peptization being greater, viscosity less), the lower is the temperature a t which the maximum occurs. In the anhydrous systems the aggregates are neither ionized nor do they have a net electric moment; or the aggregates are so much larger and interact with each other to such a degree that they cannot respond to the electrical field. The higher viscosity of these systems indicates that the latter effect, a t least, is operative. It should be pointed out that the temperatures 168°C. and 205OC. are temperatures a t which mesomorphic transitions have been reported in solvent-free sodium stearate: waxy to superwaxy and superwaxy to subneat (19). Further, all three mesomorphic phases have been reported to be liquid crystalline on the basis of x-ray diffraction investigations (13), although the waxy phase has a more restricted liquid crystalline arrangement of molecules than the less-ordered neat phases. The microscopic appearance (5) of the sodium stearate-cetane systems below 170°C., it may be concluded, should not be attributed to the formation of
TRANSITIONS I N SOAP-OIL SYSTEMS
59
a new and unique phase but rather t o the greater fluidity and consequent increase in the development of liquid crystalline structures which occur in the solvent-free soap.
Behavior below 117°C. The large increase in capacitance and conductance which begins a t 117°C. and reaches a maximum between 89°C. and 93°C. cannot be attributed t o water per se. Calculations made on the assumption that the water is a continuous phase, or that it is uniformly dispersed (20) in a soap-cetane gel which has the electrical characteristics observed for the anhydrous system, yield results which are much lower than those actually observed. The increase in capacitance and conductance can be due either t,o a significant increase in the orderliness and continuit'y of packing of the polar heads of the soap molecules engendered by the presence of water, or to hydration of the sodium ions in a structure which would have become ordered without the presence of water. In either case, the end result would be the same and the conductance would be attributed to the oscillation and possibly even migration of the hydrated sodium ions betwecn neighboring polar groupsa mechanism similar to those which have been advanced for proton transfer in accounting for the large, low-frequency conductance in well-ordered polyamides (2). Arresting the temperature a t a point a feiv degrees below 117°C. did not result in any increase in the conductance. I t was necessary to lower the temperature to 90°C. to reach a masimum value. The activation energy for conductance between 90°C. and 50°C. (after the 117"C.-O0°C. transition is completed), calculated from the values obtained at 1 kc., varies between 5 and 7 kcal. per mole, a fact which further substantiates the hypothesis that the conductance is due to solvated ions in a well-ordered structure. An important question t o be decided in view of the influence of water on the stability is whether the loiv values of conductance in the anhydrous systems are due to the absence of ordered arrays of soap molecules or merely to the absence of water of hydration. The temperature of 117°C. is that a t which profound changes in structure occur in solvent-free sodium stearate. Calorimetric investigations have shown that the heat absorbed on heating the solvent-free soaps a t this temperature is proportional to the length of the soap molecules (IO), and x-ray investigations have shown that nt this temperature pure sodium stearate is transformed from a crystalline t o a liquid crystalline phase (13). Also, above 117°C. the inclination of the hydrocarbon chains to the planes of the polar heads is significantly greater than at lower temperatures, and this increased tilt permits closer interaction and decreased mobility of the polar heads (10). I t may be concluded from this evidence that the transition which begins (on cooling the soapcetane systems) at 1li"C. involves a change in the nature of the principal forces holding the soap molecules together in aggregates-from dipole-dipole interaction above 117°C. to the dispersion forces around the hydrocarbon chains a t lower tempei,atures. I t would be expected that water would promote this morphotropic transition (3) from liquid crystalline to crystalline mesomorph by solvating the polar heads of the soap molecules and thereby weakening their interaction.
60
TODD M. DOSCHER AND SANFORD DAVIS
Crystallization would therefore be permitted to occur at the highest possible temperature, with a minimum of undercooling, facilitating the growth of large intermeshed crystalline aggregates. The resulting stability of these systems would be directly related to the presence of these intermeshed crystalline fibers or sheets (4, 9), in which the oil is immobilized. On the other hand, the conductance, viscosity, and visual appearance of the anhydrous sodium stearatecetane systems point to the existence of large intermeshed liquid crystalline aggregates above 117°C. In the absehce of water, crystallization may be expected to be retarded as the systems are cooled below this temperature, but as long as this meshwork persists a t lower temperatures, the systems should be stable. Anhydrous sodium stearate-cetane gels are in fact momentarily stable and transparent immediately after quenching (7), but since the liquid crystalline aggregates are metastable a t ordinary temperatures, they will slowly and randomly crystallize. This will result in a breakdown of the continuity of liquid crystalline structure before it can be replaced by a network of intermeshed crystalline aggregates. Further evidence that there is an energy barrier in the transition from the liquid crystalline structure to the crystalline mesomorph is obtained from the results observed on quenching the aqueous systems from above 168°C. to temperatures below 117°C. The initial values of the conductance (see figure 4) are less than the equilibrium values obtained on slow cooling, indicating that the transition from isotropic solution to the liquid crystalline aggregates, stable between 168°C. and 117"C., has been impaired. On standing, however, the conductance and capacitance rise rapidly and in some cases surpass the values obtained on slow cooling. This indicates that crystallization from the supercooled solution proceeds more rapidly than from the equilibrium systems in which the liquid crystalline phase has been allowed to develop. SUMMARY
It may be concluded from the results and discussion presented above that the role played by water in stabilizing sodium stearatecetane systems is related to the ability of water both to peptize the liquid Crystalline aggregates above 117°C. and to promote the transition from the liquid crystalline phase to an intermeshed network of crystalline soap on cooling the systems below this temperature. Neither the presence of cetane nor small quantities of water appear to change the characteristic transitions of the solvent-free sodium stearate. REFERENCES (1) ARKIN,L., AND SINGLETERRY, C . : J. Colloid Sci. 4, 537 (1949). (2) BAKER,W., AND YAGER, W.: J. Am. Chem. SOC.64,2164,2171 (1942). (3) BERNAL, J., AND CROWFOOT, D.: Trans. Faraday Soc. 29, 1042 (1933). (4) BONDI, A , , CRAVATH, A., MOORE, R . , AND PETERSON, W.: Inst. Spokesman 13, No. 12, 12 (1950). (5) (6) (7) (8) (9)
DOSCHER, T . , AND VOLD,R . : J. Colloid Sci. 1, 299 (1946). DOSCHER, T.,AND VOLD,R . : J. Phys. & Colloid Chem. 62.97 (1948). DOSCEER, T., AND VOLD,R.: J. Am. Oil Chemists' SOC.26, 515 (1949). ERRERA, J.: Trans. Faraday SOC. 90, 775 (1934). FARRINQTON, B., AND BIRDSALL, D . : J. Phys. & Colloid Chem. 62, 1415 (1948).
ELECTROCHEMICAL PROPERTIES OF MINERAL MEMBRANES. IX
61
(IO) GARNER, W., AND RYDER,E.:J. Chem. SOC.l27,I, 720 (1925). GARNER, W., MADDEN, F . , AND RUSHBROOKE, J.: J. Chem. SOC.129,II. 2491 (1926). (11) HOFFMAN, J., AND SMYTHE, C.: J. Am. Chem. Soc. 71,431 (1949). (12) MURPHY, E.:Trans. Am. Electrochem. SOC.46, 133 (1934). H.,ROSEVEAR, F., AND FERGUSON, R.: J. Chem. Phys. 16, 175 (1948). (13) NORDSIECIK, L., AND KAST,W.:Trans. Faraday SOC.29, 931, 1064 (1933). (14) ORNSTEIN, (15) RACE,H.:Trans. Am. Inst. Elec. Engrs. 62, 682 (1933). (16) RACE,H.,AND KIENLE,R . : Trans. Am. Electrochem. SOC.46, 87 (1934). G . : J. Am. Oil Chemists’ SOC.24, 353 (1947). (17) SMITH, (18) SYITH,G., AND MCBAIN,J.: J. Phys. & Colloid Chem. 61, 1189 (1947). (19) VOLD,R.: J. Am. Chem. Soo. 63, 2915 (1941). Physics 7,434 (1933). (20) YAGER,W.:
T H E ELECTROCHEMICAL PROPERTIES OF MINERAL MEMBRANES. I X MEMBRANE CHARACTERISTICS OF CLAYPASTES~,~
S. K . MUKHERJEE’
AND
C. E . MARSHALL
Department of So&, University of Missouri, Cohmbia, Missouri Received August 10, 1960 INTRODUCTION
In a previous paper (2) a new approach has been made in interpreting the electrochemical behavior of mineral membranes. According to the theory therein proposed, the membrane, in addition to possessing an exchange property, is supposed to act as a reversible electrode system with respect to the cationic solution in contact with it. The exchange of a surface cation for another in the solution is attended with heat changes (heat of adsorption, cationic bonding energy), the amount being determined by the nature of the cation and the mineral characteristics of the membrane. Equating the heat, osmotic, and electrical effects which occur during the passage of 1faraday of electricity through the cell
we have the following expression for the total potential (2): Presented before the Twenty-Fourth National Colloid Symposium, which waa held under the auspices of the Division of Colloid Chemistry of the American Chemicd Society a t St. Louis, Missouri, June 15-17, 1950. * Contribution from the Department of Soils, Missouri Agricultural Experiment Station, Columbia, Missouri. Journal Series No. 1214. * Lecturer in Chemistry and Ghose Travelling Fellow, University of Calcutta, Calcutta, India.
