A STUDY OF T H E SORPTION OF ACID SODIUM OLEATE’ R. F. NICKERSON3 Department 01 Chemistry, Massachusetts State College, Amherst, Massachusetts Received August 1, 1086
The colloid theory of liquid-liquid system has been envisaged in a broad, general formulation by Cofman (2), who redefined the classical thermodynamic variables and then applied them in the classical manner to colloid systems. One of the consequences of this approach is the statement that colloids are characterized by energy changes of a potential rather than a thermal nature. In other words, the energy exchanges of molecularly dispersed systems involve a heat effect which originates in the haphazard kinetic motion. On the contrary, the energy transfers of colloidally dispersed matter which, by definition, possesses a restricted degree of motion, must be associated with chemical or electrochemical processes. The same opinion had been expressed earlier by Einstein (2). It is true that classical thermodynamics in the form of Gibbs’ adsorption equation has proved to be both “infertile and incomplete” (16) for the liquid-liquid interface. Several reasons have been given for this failure, among which are the following: entropy changes in the adsorbing region contingent upon adsorption (lo), inadequacy of the analytical methods employed to check the theory (10, 4), and the use of the equation in a questionably valid form (11). The investigation reported here is a further study of the liquid-liquid interface by a method already outlined (15). The influence of some hydrocarbon oils on interfacial free energy is the subject of inquiry. The data are considered in the light of Cofman’s theory. METHOD
The method employed consisted of a study of the kinetics of sorptiona a t the interface between sodium oleate solutions and hydrocarbon oils. 1 Condensed from the dissertation presented to the Faculty of the Massachusetts State College in partial fulfillment of the requirements for the degree of Doctor of Philosophy, June, 1934. * Present address: Worcester State Hospital, Worcester, Massachusetts. a McBain (11) applied the term “sorption” to surface effects that are not distinguishably adsorption or absorption.
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R. F. NICKERSON
Briefly the technique depends on the hydrolytic equilibrium (NaOl), Colloid micelles
e NaOl + HzO $ NaOH + Acid sodium oleate
which has been worked out by McBain and his students (12, 14). It is evident that acid sodium oleate has the greatest escaping tendency of all the components of the equilibrium and, therefore, should be sorbed most readily. In the event of such sorption the equilibrium is displaced in the direction of formation of additional free sodium hydroxide. The measurements of increments of sodium hydroxide were made at 25 &O.Ol"C. with a conductivity apparatus. A 10-cc. volume of aqueous sodium oleate solution was introduced into a Freas type conductivity cell with attendant care to avoid frothing. Half an hour later the resistance of this solution was determined with a Wheatstone-Kohlrausch bridge sensitized with a tmo-stage audio frequency amplifier. Then, a 5-cc. aliquot of the hydrocarbon oil under investigation was allowed to run down the inside wall of the cell and to become an unbroken layer on the surface of the sodium oleate solution. The ground glass stopper of the conductivity cell was sealed with molten paraffin. Determinations of the resistance of the sodium oleate solution were recorded a t specified intervals with time reckoned from the formation of the liquid-liquid interface. MATERIALS
Sodium oleate was prepared from oleic acid of the best quality. Approximately molar equivalents of oleic acid and freshly made sodium alcoholate, both in absolute alcohol, were poured together. The precipitated sodium oleate was washed on a suction filter with cold absolute alcohol. The partially purified product was precipitated again from absolute alcohol, washed, and finally dried a t reduced pressure and low temperature to avoid decomposition. Solutions of sodium oleate were prepared by a special procedure in order to avoid foams, exposure in transference, and contamination from soft glass. The calculated quantity of dry sodium oleate was weighed and introduced into a dry glass-stoppered Pyrex bottle and the necessary amount of conductivity water added. Like the sodium oleate solutions of Du Nouy (3), solutions treated as described remained clear almost indefinitely, provided they were sealed with paraffin and stored in the dark. Sodium hydroxide solutions were obtained from the dilution of 20 M sodium hydroxide with the proper amount of conductivity water (1). The hydrocarbon oils-o-, m-, and p-xylenes, toluene, benzene, nhexane, n-heptane, n-octane, 95 per cent n-nonane, n-decane, and kerosene -were purified by double distillation from metallic sodium through an
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all-glass Pyrex distilling apparatus. The fraction of kerosene which distilled between 180 and 24OOC. was taken as representative. The 95 per cent n-nonane was made available through the courtesy of the Bureau of Standards, American Petroleum Institute Project No. 4. EXPERIMENTAL
New data have demonstrated that benzene, used in the previous study (15), is not a good standard of reference for the hydrocarbons. For this reason the sorption phenomenon was examined over a range of concentra-
- -- HYDROLYSIS BY INDICATORS
FIG. 1. Variations of the conductance induced by m-xylene with concentration of sodium oleate. Dotted line shows hydrolysis found by indicators
tioris of sodium oleate with m-xylene superimposed. The layer of hydrocarbon oil on the surface of the dilute sodium oleate solution displaces the hydrolytic equilibrium in the direction of greater hydrolysis. The niagnitude of the displacements can be estimated accurately as increments of the equivalent conductance of the aqueous phase. The initial and final conductances of sodium oleate solutions over a range of concentrations are given in graphical form (figure I), together with the percentages of normal hydrolysis obtained through indicators by McBain and Hay (13) in the same concentration interval. The final conductance was read after twenty-two hours.
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Although the curve of percentages of sodium oleate hydrolyzed shows a rapid and continuous increase with dilution, the curve of final conductances under m-xylene is not symbatic. If the conductance increments were a measure simply of the phase distribution of oleic acid, the equivalent conductance of sodium hydroxide (240 mhos) should be approached. This is, however, not the case. After passing through a maximum, the final conductance begins to decrease in spite of the fact that normal hydrolysis increases. These data indicate that the effect measured is not simply the extraction of free oleic acid by the oil phase but, rather, is a true adsorption a t the interface. TABLE 1 T h e sorption potentials of some hwdrocarbons at 85°C. IN IT I A L 2ONDUCTANCE
FINAL 2ONDUCTANCE
Hexane, . . . . . . . . . . . . . . . . . . . . . .
36.72
49.33
Heptane. ..................... Octane, ....................... Nonane ....................... Decane. . . . . . . . . . . . . . . . . . . . . . . .
38.30 38.20 38.20 38.20
50.77 50.56 49.97 48.69
Benzene ......................
36.51
55.15
Toluene. ...................... o-Xylene ......................
36.10 36.55
58.00 60.96
m-Xylene .....................
36.35
61.12
p-Xylene . . . . . . . . . . . . . . . . . . . . . . Kerosene, .....................
36.59 36.84
60.16 48.36
OIL
INCREMENT
8 0R P T IO N POTENTIAL
0.509 12.47 12.36 11.77 10.49
{;:E 21.90 24.41
{z 23.57 11.52
0.503 0.498 0.476 0.424 0.7531 0.801? 0.885
0.987 1,000 0,952 0.466
Sorption potentials of hydrocarbon oils The conductance increments of a series of hydrocarbon oils have been determined. The hydrocarbons were investigated two a t a time in closely similar cells. Simultaneously, with each pair a control determination was made on m-xylene. Aliquot parts (10 cc.) of the same iM/100 sodium oleate solution were used for all of the hydrocarbons given in table 1. The ratio of the total conductance increment for a given hydrocarbon to that for m-xylene is tabulated as the relative sorption potential. Rates of sorption of the hydrocarbom The rate of sorption during the first few minutes of the process is an important datum for any hydrocarbon oil. This initial velocity is an
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approximate measure of the potential gradient for acid sodium oleate between the aqueous solution and the liquid-liquid interface. I n each portion of the same sodium oleate solution the thermodynamic potential of acid sodium oleate is the same; hence, the relative potential gradient is determined solely by the free interfacial energy. Therefore, the rate of sorption at the outset of the process is proportional to the free interfacial energy. The free sodium hydroxide which accumulates as a result of the displacement of the hydrolytic equilibrium comRlicates the subsequent rates and alters the thermodynamic potential of the acid soap. The initial rate of sorption for each hydrocarbon oil has been calculated. The conductance increment for the first quarter of an hour of the velocity process was multiplied by four in order to convert it to mhos per hour. TABLE 2 Relative initial free interfacial energies and total conductance increments in contact with M/100 aqueous sodium oleate solution RELATIVE INTERFACIAL ENERQY AB MAOS PER A O U R
OIL
_____ ......................
....................
Benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p-Xylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T o l u e n e ,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . m-Xylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o-Xylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.68 2.56 1.32 4.40, 4.60 6.72 11.52
TOTAL CONDUCTANCE INCREMENT
12.6 11.5 18.6 23.6 21.9 24.8 24.4
The first part of the conductance-timc curve may be assumed to be linear without much error. These initial slopes or relative interfacial free energy values are given in table 2 for a few hydrocarbons. The complete conductance-time data are given in figure 2 for all of the hydrocarbons except the alkanes heptane, octane, nonane, and decane. These alkanes differ only slightly from hexane; therefore they ate omitted. The curves demonstrate clearly that a disturbing factor exists in the case of benzene. The data in both tables 1 and 2 indicate a peculiarity also. Closer inspection of figure 2 has led to the suspicion that the same disturbance influences the toluene and p-xylene curves, although to a lesser degree. The interpretation of these results is greatly simplified when the structure of surfaces given by Harkins, Clark, and Roberts (5) and Harkins, Davies, and Clark (6) is employed. These authors and Langmuir (9) have shown that the benzene ring, uniformly polarized by its double bonds, lies flat in the surface of pure water. Harkins and his collaborators have stated also that toluene molecules are slightly tilted as a result of the polarizing influence of the methyl group; m- and o-xylene make larger and larger
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R. F. KICKERSON
angles respectively with the plane of the int'erface. The intensit'y of the stray fields of thew molpcules varies with the angle of tilt. The order of intensity of thew stray electric fields adjudged from structure only should hr o-xylene > m-xylene > toluene > p-xylene > henzrne. The widencc given above (table 2) i s in complete agreemcnt with these conclusions of Harkins and Langmuir, and seems to indicate that the initial slope of the lime-sorption curve is a measure of the free interfacial energy present subject to the (Bonditions imposed. The work of McBain (14) and Walker (17) has elucidated t,he nature of the aqueous sodium oleate solutions. At the concent'rat'ion (M/100) used in the prcsrnt expcrinicnts sodium oleat,e is largrly in the form of miccllcs.
,
%2s
FIG. 2. The time increments of equivalent conduc.taiirc f o r a few p u w hydrocarbons. T h e solid lines indicate the readings taken
These micclles are rather highly polar as a consequence of their structure. Surface tension measurements (7) show that, colloid micelles of sodium oleate exert a: much smaller surface pressure than molecular soap does. As concentration increases, the surface pressure of sodium oleate decreaw after a maximum has been passed. It may be concluded, therefore, that the surface of a M/100 sodium oleate solution is somewhat polar in nature because the surface solute is principally in t,he form of polar micelles. Under these conditions the stray fields of the impinging hydrocarbons probably exist in almost the same relationship to the sodium oleate interfarc as they do to pure water. 1he conductancc mcaswcwc~ut*rcportcd herc are evidently duc to thc r ,
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283
selective sorption of acid sodium oleate. The selective sorption and orientation of molecular soap must of necessity destroy the polar nature of the aqueous surface, and create in its place a non-polar surface of oleate chains on which the hydrocarbon oil impinges. The end result, then, must be a re-orientation of the benzene rings and a straightening of the tilted rings to the perpendicular. The previously mentioned flattening of the time-conductance curves seems to be evidence of this phenomenon. The fact that benzene and all of its derivatives tend to nearly the same total increment regardless of initial slope, bears out this statement also. In accordance with Gibbs’ minimum free energy principle any changes in surface structure must be in the direction of increases of potential energy. These re-orientation effects seem, therefore, to indicate “changes of entropy” in the adsorbing layers of the hydrocarbon oil. Storage of potential energy It has been suggested (2) that energy transfers in colloid systems involve chemical or electrochemical processes. Some support for this statement is contained intrinsically in the data above, but an experiment was designed to yield more direct proof. To the same M/100 sodium oleate solution known amounts of sodium hydroxide were added stepwise and aliquots of the sodium oleatesodium hydroxide mixture were removed between additions. An accurately measured amount of each aliquot was investigated with m-xylene at 2 5 T . The data are plotted in figure 3. The additional alkali suppresses normal hydrolysis. Some of the added sodium hydroxide disappears by combination with the acid sodium oleate. The straight line in figure 3 is the conductance-concentration curve expected if none of the sodium hydroxide combined. The shaded area shows the amount and .limit of normal hydrolysis. m-Xylene induces hydrolysis even in the presence of added alkali which has reduced normal hydrolysis to a negligible degree. It is probable, then, that Cofman’s idea is wholly tenable. Free energy in the system is converted to potential energy. I n this induced hydrolysis water is broken up into its constituent ions; free energy is thereby stored as electrochemical potential.
Acid sodium oleate and surface tension The increments of equivalent conductance a t different concentrations of sodium oleate are plotted in figure 4 for comparison with the surface pressure measurements of Johlin (8). Surface pressure is the force exerted by the surface solute against the surface tension of the pure solvent; numerically, it is the difference between the surface tension of the pure solvent and that of the solution. The similarity of the two curves suggests that a part of the surface pres-
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R . F. NICKERSON
sure exerted by sodium oleate is a function of hydrolysis as measured by conductance. In concentrations of 0.02 il.I and greater, sodium oleate exists largely in the form of micelles. It appears, therefore, that the micelles alone exert a surface pressure of about 46 dynes and that greater
%do
39a
jfioo
4& /
%a
99da
MOLES NAOH PER LITLR OF ~ ; W N R O L L A T C FIG.3. The effect of sodium hydroxide on the increment of conductance due t o m-xylene
surface pressures are exerted by acid sodium oleate. It follows then, that the surface pressure of a sodium oleate solution which impinges on its vapor has a very limited value when that datum is applied to the liquid-liquid interface between that same sodium oleate solution and an oil phav.
SORPTION O F ACID SODIUM OLEATE
285
Owing to the fact that acid sodium oleate exerts the greater surface pressure, together with the fact that the hydrocarbon phase increases the quantity of it by selective adsorption and the resultant hydrolysis, the surface pressure of solute for a given sodium oleate solution in contact with an oil phase can exceed the surface pressure which obtains when the same solution impinges on its vapor.
FIG.4. .4 comparison of surface pressures with m-xylene conductance increments a t different concentrations CONCLUSIONS
The failure of Gibbs’ equation must be attributed a t least in part to a lack of knowledge of the processes involved. The inflexion points and negative slopes in the surface pressure-concentration relationship for sodium oleate are probably due to changes in the solute. If the concentration function were differentiated and each of the various forms of aggregates considered as a separate solute with individualistic pronerties, nega tive values of r would probably not arise. Moreover, it seems that changes in the structure of surfaces not covered by the theory of orientation take place during and as a result of adsorption. There is probably a molecular re-orientation in the surface configuration of some oils at the plane of contact. As a consequence free and potential surface energies of the interface are altered. These changes must be included in Gibbs’ equation. The selective adsorption of acid sodium oleate by hydrocarbon oils alters
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R. I?.
PTICKERSON
the surface pressure by a n amount not predict'able from surface tension measurements. The characteristics of the oil phase must be considered. Relative sorption potentials, which should be useful data in the quantizat,ion of emulsions, are tabulated. Some evidence in support of t)he idea that colloids involve potential energy changes has been given. In the case of sodium oleate and hydrocarbon oils free interfacial energy is stored in t'he cleavage of water. The author is indebted to Dr. Paul Serex, whose sustained interest was invaluable to the progress of the work. REFERENCES (1) Assoc. Official Agr. Chem.: Xethods of rlnalysis, 3rd edition, p. 28. Washington, D. C. (1930). (2) C o ~ u a x V.: , Chem. Rev. 4, 1-49 (1927). (3) Du NoUY, P. L.: Surface Equilibria, American Chemical Society Monograph. The Chemical Catalog Co., Inc., New York (1926). (4) HARKINS,W. D.: in Alexander's Colloid Chemistry, Vol. 1, p. 192. T h e Chemical Catalog Co., Inc., New York (1926). (5) HARKINS,W, D., CLARK,G. L ~ . ,~ N DROBERTS,L. E.: J. Am. Chem. SOC.42, 700 (1920). (6) HARKINS, W.D., DAVIES,E. C. II.,AND CLARK,G. L.: J. Am. Chem. Sot. 39, 584 (1917). 17) ~, JOHLIN. J. bf.: J . Biol. Chem. 84, 543 (1929). (8) JOHLIN, J. 31.: Private communication. (9) LANGMUIR, I.: J. Am. Chem. Soc. 39, 1848 (1917). (10) LEWIS,W.C . McC.: Phil. Mag. 16, 499 (1908). (11) MCBAIX,J. W.:Sorption of Gases a n d Vapours, p. 8. Routledge & Sons, London (1932). (12) MCBAIN,J. W,:J. Soc. Chem. Ind. 37, 249T (1918). (13) ~ ~ C B A IJ. N ,W.,AXD H A Y : International Critical Tables, Vol. V, p. 459. McGraw-Hill Book Co., New York (1929). (14) MCBAIN,J. W.,AND JWNKISS, W.J.: J. Chem. SOC. 121, 2325 (1922). (15) NICKERSON,R . F., AND SEREX~ P.: 9. Phys. Chem. 36, 1585 (1932). (16) PENNYCUICK, S. W.: J. Am. Chem. Soc. 62, 4621 (1930). (17) n7.4LKER, E. E.: J. Chem. Soc. 119, 1521 (1921).
