T H E COKDUCTANCE O F SOME SODICJI OLEATE SOLUTIONS I N RELATION TO INTERFACIAL ADSORPTION. * BY R A L P H F. S I C K E R S O S AXD P A U L SEREX
Introduction In a recent investigation Johlin’ has found that the relative values of surface tension of sodium oleate solutions between concentrations .0019 N and . o j 9 N are much greater than those obtained by other investigators. Leeten* and Dennhardt3 studied the conductivity of these solutions and reached the conclusion that the hydrolysis constant of sodium oleate is comparatively small. Hahne4 studied the effect of benzene on the properties of the soap in aqueous solution, but the magnitude of the interface benzene/ sodium oleate was unknown. The present investigation was undertaken for the purpose of redetermining the conductances of sodium oleate solutions in the range of concentrations stated, to compare the results with surface tension data, and to measure the relative interfacial adsorption between these solutions and various oils, in order that the function of the interface in emulsification of oil-in-aqueous sodium oleate solution might be more clearly defined. Apparatus and Materials The conductivity bridge was essentially the same as that described by Hall and Adamsj except that a two-stage audio frequency amplifier was interposed between the bridge output and the head phones. The alternating current used had a frequency of 1000cycles/sec. The bridge circuit was calibrated according to the method suggested by Wark.6 The procedure described by PopofP was followed in the preparation of the platinum black surfaces on the electrodes of the conductivity cell. The cell constant was about 0 . 3 5 9 5 , (Subject to the calibration curve of Wark’s method.) “Equilibrium” water with a specific conductance of about .9 X IO-^ mhos was used throughout the investigation. Kahlbaum’s “Purest” sodium oleate was recrystallized twice from absolute alcohol, and dried in a vacuum oven. The purified product was used to make a stock solution from which various concentrations were obtained by dilution. * Condensed from the thesis submitted by Ralph F. Kickerson at Massachusetts State College, 1932, in partial fulfillment of the requirements for the degree of Master of Science. Johlin: J. Biol. Chem., 84, 534 (1929). Leeten: Z. deut. 01-Fett-Ind., 43, 50, 65, 81 (1923). , Dennhardt: “Handbuch der Chernie u. Tech. der le du. Fette.” Goldschmidt, 3,417 (1910). Hahne: Z. deut. 01-Fett-Ind., 45, 245-8, 263-64, 274-76, 289-90, 308-10 (192j). j Hall and Adams: J. Am. Chem. SOC., 41, 1515-25 (1919). Wark: J. Phys. Chem., 34, 885-6 (19 0) Popoff: “Quantitative Analysis,” zd id.; 4jO (1927).
’
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RALPH F. NICKERSON AND PAUL SEREX
Benzene, toluene and carbon tetrachloride were high grade, obtained from the Eastman Kodak Company. Meta-xylene, hexane and heptane were technical grade from the same company.
Experimental Conductivities of the several concentrations of sodium oleate were obtained by placing the solutions in the cell and allowing them to stand in the thermostat at 2 5 O C for half an hour before measurements were taken. Aliquots were transferred carefully to prevent frothing. Contamination by COz was minimized as much as possible. The data are presented in Table I.
FIG.I The variation of equivalent conductance (a) and relative conductance (b) with logarithm of concentration of sodium oleate. The dotted line is Johlin's surface tension data.
TABLE I Equivalent Conductance of Sodium Oleate a t z g0C No ,.A Equivalent Conductance Sodiuln Oleate (Mhos) at 25OC HzO .83 X IO-^
,1216 ,0608 ,0304 ,0152
,0076 ,0038
24.75 24,47 26.87 31 . o s 37.88 47 . g o
Normality Equivalent Conductance Sodium Oleate (Mhos) at 25°C H20 .83 X IO-^
,0019 ,0009j ,00048
,00024 ,00024(13hrs.
later)
58.65 66.00 69.03 69.90 72.20
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The curye (Fig. 1-8) represents graphically the conductivity-log. concentration relationship. A decided break in the trend of the curve may be seen at about ,002 N. Leeten fails to point out this “break” although his data. show it. To eliminate the possibility that the change of trend of the conductance curye was due to the less highly conducting salt, NasCOs, which could result from the interaction of NaOH, a consequence of the hydroiysis of sodium oleate, and the COSof the “equilibrium” water, oonductances of sodium oleate and NaOH of the various concentrations were determined simult,aneously on the same water. Suhtracbion of the specific conductance of the NaOH solu-
The effect
Re. z
the appeehranm of sodium oleate solu!~lons. Photngrsph t a k a nine days sfter the dilutions were made. of d i l d o n on
tion from the corresponding figure for the sodium oleat,e solution gave a curve which showed the same effect (Fig. r-h), The curve (Fig. r-h) was taken, therefore, as the graphical representation of pure increases of mobility and hydrolysis with dilution. An intense hrhidity developed almost immediately in solutions of concentrations less than ,002 N,hut. very gradually and much fainter in concentrations greater than ,002 N. Tho phot,ograph (Fig. 2 ) clearly illustri&testhis phenomenon. The conductances of the highly turbid solutions fall in the “break” of the conductance-log. concentration curve. A small quantity of dilute alkali solution quiokIy removed this turbidity. Solutions of concentration .ooz N o r grcater produced stable foams when shaken, whpreas concentrations lesv than ,002 N did not,. A number of investigators have shown that the minimum surface tension of aqueous sodium oleate solutions corresponds to ahout .ooa N.
Discussion From the photograph (Fig. 9) and the conductance-log. concentrat.ian wlationship (Fig. r ) together wit,h Johlin’s surface tension data (Fig. I) which have been plotted for comparison i t is evident, first, that surface tension de-
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RALPH F. NICKERSON AND PAUL SEREX
creases as conductance increases from a .os9 N solution down to a concentration of ,002 N ; second, that both surface tension and conductance areconstant above .os9 N at least as high as .120 N ; and third, that the appearance of an intense turbidity is coincident with abrupt changes in the trends of the conductance and surface tension curves. In solutions of concentration .02 N or greater the colloidal material flocculates and settles on standing. McBain’ and his students find that “acid soap’’ results from hydrolysis. If increasing conductance be taken as a criterion of increasing hydrolysis it is apparent that surface tension decreases as hydrolysis proceeds between the limits .os9N and .002 N. The lowering of the surface tension over this interval must be attributed to the hydrolytically formed “acid sodium oleate” and increases of mobility of the conducting particles.
Adsorption at the Oil/Sodium Oleate Interface In order to inquire into the mechanism of oil-in-water stabilization by sodium oleate, the following studies were carried out on sodium oleate solutions with varying interfacial conditions. Experimental The first series of experiments consisted of varying the concentration of the sodium oleate solution under constant interfacial conditions. For each of the various concentrations of sodium oleate the procedure was as follows: IO cc. of the solution were placed in the conductivity cell and permitted to reach surface and thermal equilibrium a t 2 5 O C . The conductance of the solution was then determined. A 2 cc. portion of benzene was then carefully layered on the surface of the solution in the cell and after 23 hours the conductance redeterTABLE I1 The Effect of a Surface Layer of Benzene on the Conductance of Sodium Oleate in 23 Hours a t 2 5 O C Equivalent Conductance 26.89
Concentration
.0608 N .0304 “ .OI52 ,0076 ‘‘ ,0038 ‘ I . 00I 9 I‘
28.85 32.8s 38.81 49.16 59.16 73.2’ 73.48
‘(
,0010 .0005
Equivalent Conductance after 23 hours
27.29 30.07 37.22 53.53 73. I4 82.99 86.95 87.37
A Equivalent
Conductance .40 1.22
‘4.37 14.72 23.98 23.83 13.74 13.89
Controls .oos N N a O H
.00I
235
229.5
-2%
216
210.
-3 -3.5
(L
.ooos (‘ 205 197.5 McBain: J. SOC.Chem. Ind., Jubilee Number, 1931, 238.
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mined. Controls consisting of benzene layered on dilute solutions of sodium hydroxide were determined in the same way. The data obtained are presented in Table 11. These data have been plotted (Fig. 3) together with the surface tension measurements of Johlin for reference. It was observed, also, that the layer of benzene caused the colloidal turbidity to dissolve in a few hours with the result that the aqueous layer was clear and transparent. The second series of experiments consisted of varying the oil while the other conditions were kept constant. The procedure was the same as that just outlined except that I O cc. portions of a .o118 N sodium oleate solution
FIQ.3 A diagram showing the incremes of equivalent conductance of various concentrations of sodium oleate in contact with benzene for 23 hours. The surface tension curve after Johlin.
were used in each case, and z cc. portions of different oils were layered on the surface. The rate of change of conductance was obtained by noting the time. Oil-NaOH controls were determined also. The experimental error, as manifested by the controls, is taken into consideration in the graphical representation of this data (Fig. 4). The third series of experiments was concerned with a varying interfacial area, while the other conditions were held constant. Pyrex vessels, which gave different areas of the benzene/solution interface with the same volumes of sodium oleate solution and benzene, were set up in the thermostat. A 2 5 cc. aliquot of the same sodium oleate solution was placed in each container. A
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RALPH F. NICKERSON AND PAUL SEREX
2 5 cc. portion of benzene was then layered on the interface in each vessel. After 30 hours an aliquot from the aqueous layer of each vessel was transferred to the conductivity cell and the measurement taken. Toluene, carbon tetrachloride, and m-xylene were determined in the same way. The data are plotted (Fig. 5 ) . Discussion A layer of benzene on sodium oleate solutions whose concentrations varied from .os9 N to ,001N brought about increases in the conductances of these solutions. I n the case of concentrations .oozN or greater the more turbid the solution, the smaller was the increase of conductance. Under the conditions
FIG.4 The rate of percentage increase of conductance of .OI18 N eodium oleate solution due to surface layers of various oils. The controls consist of the same oils on &lute sodium hydroxide.
of the experiments the increments of conductance were negligible in concentrations greater than ,059 N. The experimental errors due to the diffusion of COz through the benzene layer and its subsequent interaction with the NaOH resulting from the hydrolysis of sodium oleate, and to the distribution of impurities in the benzene layer between the two liquid phases, were negative with respect to the increments of conductance. The results with sodium oleate solutions are, therefore, probably slightly low. The colloidal “acid sodium oleate” which was present as a finely divided white solid when the interface of the aqueous sodium oleate solution was in contact with its saturated vapor, went into solution when benzene was layered on the surface. This transition from a turbid to a clear solution was not observed when the concentration was less than .ooo5 N. The addition of a small quantity of dilute alkali had the same action on any of these solutions, namely, a clarification.
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1591
The areas of the vessels used in the surface variation experiments could not be measured with precision and the errors involved in transferring aliquots to the conductivity cell are certainly not negligible, but the data taken under the same conditions show that the conductance increases with larger and larger areas of contact oil/sodium oleate solution.
Conclusions The presence of benzene on the surface of sodium oleate solutions under the conditions already mentioned brings about increases of conductance of these solutions. The shifts in conductance increase with time. The magnitude of
TOWCNCON .OWN
~OLUTTL
I)CN?XYc(*1 .OImN
0
IO
A~LA?C?)
NAOLtrrr
-
u)
FIG.5 The changes of equivalent conductance of sodium oleate solutions caused by varying the interfrtcial area between the oil and the solution.
the increment in any specific case is limited by the amount of “acid sodium oleate” which is present as a colloidal suspension in the solution when benzene is absent from the interface. As the conductance increases, the solution clears. From the assumption that .ooz Nisthe minimum concentration of sodium oleate for which the interface against air is saturated’ and that all solutions of this soap of greater concentration than .ooz N have saturated interfaces, it is evident that the surface activity of these sodium oleate solutions is determined to a great extent by the “acid sodium oleate” which results from hydrolysis of the neutral salt. The surface tension is a minimum at .ooz N and Harkins, Daviea and Clark: J. Am. Chem. SOC., 39, 54 (1917).
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RALPH F. NICKERSON AND PAUL SEREX
the solution is clear (Fig. 2) which indicates a maximum concentration of the “acid sodium oleate” in the interface. The surface tension increases as hydrolysis decreases because the ratio of the more water soluble component sodium oleate to the less soluble “acid sodium oleate” in the surface becomes larger and larger until the surface becomes saturated with sodium oleate and the surface tension as well as the conductivity becomes constant. The result of the presence of benzene in the interface is the displacement of sodium oleate from the interfacial film and the substitution‘ of “acid sodium oleate” which exists largely in colloidal suspension in the case of the solution/vapor interface. The exchange takes place until the colloidal excess becomes exhausted. The removal of the excess “acid sodium oleate” from the solution is followed by further hydrolysis of the sodium oleate which is recorded as increments of equivalent conductance (Fig. 3). A study of the changes in the equivalent conductance that take place as the interfacial area is increased (Fig. 5 ) leads to the conclusion that hydrolysis of the sodium oleate induced by the presence of the oil ceases when the concentration of alkali reaches a “suppressing value.” Further adsorption must remove from the solution both sodium oleate and “acid sodium oleate.” The various oils used in this investigation show marked differences in the induction of hydrolysis (Fig. 4). Such differences may be interpreted as meaning that the interfacial energy between the oil and the sodium oleate solution determines the quantity of “acid sodium oleate” adsorbable and hence the concentration of alkali which is capable of suppressing further hydrolysis. Increments of conductance of these sodium oleate solutions due to the influence of an oil layer on the interface indicate that the surface active material is in equilibrium with that dissolved in the main bulk of the solution. The “acid sodium oleate” is more highly adsorbed a t the oil/solution interface than it is a t the vapor/solution interface for the same solution of sodium oleate because its fugacity from the aqueous phase is determined by its solubility in the interface. The solubility in the interface, in turn, is conditioned by the interfacial energy. I n discussing the results of Briggsl, Clayton3says : “This assumption may not be strictly true, as possibly the difficultly-soluble acid sodium oleate produced by hydrolysis may be a factor in emulsification.” I t is hoped that this investigation clarifies the action of “acid sodium oleate” in emulsoid systems. Frothing is observed only in those solutions which have a saturated surface and an excess in colloidal suspension or as a crystalloidal precipitate. Stable foams may be obtained] therefore, on solutions in which there is an excess in equilibrium with the surface and the hydrolytic system. Further investigations along this line are being carried on a t this laboratory. ‘Nonaka: J. SOC.Chem. Ind. Japan, 32, 115-20 (1929)reaches essentially this conclusion from cataphoretic studies. 1 Briggs: J. Phys. Chem., 19, 210-31(1915). 2 Clayton: “Theory of Emulsions,” ad Ed., 84 (1928).
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I593
s-ary At concentration .ooz N sodium oleate shows an abrupt change in the trend of the concentration-conductivity relationship. This change of trend is coincident with minimum surface tension and loss of frothing power. 2. Surface tension of sodium oleate solutions varies inversely with hydrolysis between the limiting concentrations .os9 N and .ooz N. 3 . More “acid sodium oleate” is adsorbed a t the oil/solution interface than at the vapor/solution interface of the same sodium oleate solution. 4. Various oils have differing capacities to adsorb “acid sodium oleate.” 5. The mechanism of the buffer action of sodium oleate in solution has been demonstrated. 6 . The rale of “acid sodium oleate” in emulsification has been much underated. 7 . Frothing is attributed to a hydrolytic system in equilibrium with a saturated surface and an excess, colloidal, or colloidal and crystalloidal, depending on the concentration. I.
Goessmann Chemistry Laboratory, Mmsachusett-3 State College, Amherst, Mass. Februaty,1938.
