FIGURE 1.
VARI4TIO.V I\'
\VETTING
ACIDRATIOOF ACID MIXTURES
P R O P E R T I E S WITH ALKALI-FATTY
AQUEOUS
SODIUM
HYDROXIDE-OLEIC
Shaded areas indicate positive spreading wefficients: reference liquid, mineral oil; temperature, 26' C. (77' F.)
Wetting and Spreading Properties Oleic Acid-Sodium of Aqueous Solutions
Hydroxide Mixtures
H. L. CUPPLES Bureau of Entomology and Plant Quarantine, Department of Agriculture, Washington, D. C.
NSECTICIDAL dipping and spraying solutions are often applied to greasy or waxy surfaces. Therefore, the efficiency of these fluids may depend upon their wetting power for such surfaces. Of the various materials that have been found to improve the wetting properties of aqueous solutions, soaps have been extensively used, and in numerous cases the addition of soap has been found greatly to increase the efficiency of insecticidal dipping and spraying solutions. The use of soaps in nicotine sulfate spray solutions is now quite common. Cooper and Nuttall (1) state that, whereas a plain aqueous solution of sodium arsenite containing 1 part of arsenious oxide in 600 parts of water is ineffective as a sheep and cattle clip, a solution of the same concentration containing a small percentage of soap and oil to increase the wetting power proves to be satisfactory. O'Kane and eo-workers (b), studying the penetration of the tracheae of insects by solutions containing various concentrations of soap and other wetting agents, found that a 1per cent solution of saponin, a 15 per cent solution of sodium caproate, and dilute sodium oleate solutions 7Tei-e ineffective in promoting penetration of the tracheae, but that more concentrated solutions of sodium oleate (0.5 per cent and higher) were effective. The objects of the present investigation were to develop a relatively simple physical-chemical test that would quantitatively nienwre the wetting and spreading properties of
aqueous solutions of soaps and other wetting ageiits, and to use the test for the evaluation of such materials.
Previous Methods of Measuring Wetting and Spreading Properties Various methods have been proposed for evaluating the wetting and spreading properties of aqueous solutions. For instance, i t has been suggested that the wetting power is inversely proportional to the surface tension of the solution, but attempts to correlate the wetting powers of solutions with their surface tensions have been unsuccessful. Lovett (4) has studied the rise of spray solutions in glass tubes coated with wax from the surface of apples and has The wetting and spreading properties of aqueous solutions may be evaluated by determining their spreading coefficients with reference to a neutral mineral oil, from measurements of the surface and interfacial tensions made with a du Noiiy tensiometer. The significance of the spreading coefficients as a measure of these properties is demonstrated by a simple visual test. The wetting and spreading properties of aqueous sodium hydroxideoleic acid mixtures are very sensitive to variations i n the alkali-fatty acid ratio. A slight excess of alkali may profoundly change these properties. 1219
1220
INDUSTRIAL AND ENGINEERING CHEsfISTRY
used this as a measure of wetting power. It seems proLaLle that such a measurement may indicate the wetting poner for immersional wetting, but not for spreading wetting (6). In any event, the method is apparently not well suited for general application, or for obtaining fundamental and accurate measurements of the physical properties involved. O’Kane and eo-workers (6) have attempted to evaluate the wetting and spreading properties by measurements of the angles of contact between small drops of the solutions and the integument of insects This method appears to be based on sound fundamental theory, but, as the authors have stated, the measurement of angles of contact on the irregularly curved surfaces of the insects is a matter of considerable practical difficulty. Moreover, since all solutions having positive spreading coefficients (9)will theoretically have zero angles of contact, the method is restricted to solutions having relatively poor wetting powers for the surface involved. Cooper and S u t t a l l (1) have measured the wetting powers of insecticidal dips and sprays by determining a quantity corresponding to the spreading coefficient. (The spreading coefficient, S, of liquid A over liquid B is defined by the equation, S = T B - T A - T A B where , T A and T B are the surface tensions of the respective liquids, and T A Bis the interfacial tension.) They reasoned that in actual practice the surface to be wetted is nearly always coated with a greasy or waxy secretion, and that in the comparison of wetting powers a thick oil might be taken to represent the solid surface. Accordingly, they determined the spreading coefficients (wetting power) on “liquid vaseline” for a number of spraying solutions and for a wide variety of soaps, using the Donnan pipet for determining the interfacial tensions. Their results indicated that various soaps did not differ markedly from one another, and their values for the spreading coeficient were uniformly negative a t all concentrations tested (up t o 2 per cent fatty acid content). Furthermore, they have emphasized that the method was successfully applied only t o solutions containing soap as a basis, and that with such materials as saponin i t was impossible to obtain satisfactory values for the interfacial tensions.
Present Method of Measuring Wetting and Spreading Properties I n the present work the various aqueous solutions have been evaluated by a determination of their spreading coefficients with reference to a refined mineral oil, a du S o d y interfacial tensiometer being used for measuring the surface and interfacial tensions. Since in certain cases the interfacial tension has been found to vary with the age of the interface, all the interfacial tensions were measured 10 minutes after formation of the interface. In making these measurements the instrument was first calibrated against analytical weights, and the calibration was then checked by determining the surface tensions of water and benzene, using the appropriate ring correction factors as determined by Harkins and Jordan (3). Measurements h a r e been made with various soap solutions, saponin, and several of the more recently developed sulfonated wetting agents, and, so far as can be judged from the results now arailable, the method is generally applicable to all these solutions, The experimental results presented in this paper, however, are restricted to the field of oleic acid-sodium hydroxide mixtures containing various concentrations of oleic acid and with varying alkali-fatty acid ratios. A positive spreading coefficient indicates that the solution qhould spontaneously spread to a thin film if a drop of it is placed on a surface of the reference liquid (in this case, refined mineral oil). A negative spreading coefficient indicates that the solution should fail to spread over such a surface. I n other words, the greater the algebraic value of the spreading coefficient the better are its wetting and spreading properties.
\;OL. 27, h 0 . 10
It is not feasible to test the spreading properties of these aqueous solutions simply by placing a drop on the surface of a I olurne of the oil in some form of container, since the aqueous solution, because of its higher specific gravity, will sink through the oil. The writer has therefore developed a simple visual test to determine whether the aqueous solution has a positive or negative spreading coefficient with reference to the oil. If a drop or two of the oil is placed on a level sheet of celluloid and spread with the finger to a thin film, and a drop of the aqueous solution is placed a t the center of this film, the drop of aqueous solution will spread only if it has a positive spreading coefficient with reference to the oil. This test has been found to substantiate conclusions drawn from the values of the spreading coefficient., and by this means a convincing, xisual demonstration may be given which shows the practical importance of the differences between the various wlutions. Sodium Oleate as a Wetting and Emulsifying Agent Sodium oleate is used extensively as a wetting and spreading agent. As a common soap it is used in spraying solutions containing nicotine and in inpecticidal dipping solutions. It is used as both emulsifier and spreader in oil-emulsion sprays. R h e n used for oil-emulsion sprays, the sodium oleate is sometimes formed during the preparation of the spray by the interaction of oleic acid with sodium hydroxide. This procedure is convenient in practical spraying operations, but the possibility of thus obtaining solutions with inferior or variable wetting properties has not previously been raised. I t is evident that in preparing a sodium oleate solution “on the job” by causing oleic acid to react with sodium hydroxide, it will be relatively difficult to obtain an exactly “neutral” qolution, or, rather, a solution containing equivalent amounts of alkali and fatty acid. I n practical operations it may also he necessary to make allowance for the acidity or alkalinity of the available water supply. I t has apparently been assumed that small departures from neutrality would not substantially alter the wetting properties, and the impression seems to be prevalent that a slight exce- of alkali would probably improve the wetting a i d spreading propertiei. The work here reported, however, di--
d
4 SURFACE TENSION yi
aes
0 50 GRAMS OLElC ACID
I 0 75 PER 1 0 0 CC
L
100
FIGURE 2. VARIATION IN WETTINGPROPERTIES WITH COYCEiYTRATION FOR .kQUEOUS SODIUM
HY-
DROXIDE-OLEICACID MIXTURESOF CONSTAVT ALKALI-FATTY ACID RATIO Shaded areas indicate positive spreading coefficients; reference liquid, mineral oil: temperature. 25’ C . (77 F.)
OCTOBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLEI. VARIATION IS WETTIKGPROPERTIES WITH ALKALI-FATTYACID RATIOFOR SODIUM HYDROXIDEOLEICACID MIXTURES Ratio, Moles Na#H/Moles Oleic .%rid
0 50 0.61 0.71 0.81 0.91 0.95 0.96 0.97 0.98
0.99 1.00 1.01 1.02 1.03
1 04 1 05 1.14 1.21 1.31 1.41 1.51 2.02 3.02 4 03
0 50 0.70 0.90 1.00 1.10 1.25 1.40 1.60 1.80 2.00 3 00 4.00 0.50 0.70 0.90 1.00 1.10 1.25 1.40 1.60 1.80
2.00 3.00 4.00
Interfacial Surface Tension Tension against Oil Dynes/cm Dynes/cm. 1.00 Grain Oleic l c i d per 100 Cc. 25.3 3.4 25.4 2 4 25.5 1.3 25.7 0.8 25.4 0.9 25 0 0.8 25.0 0.8 25.3 1.4 25.3 1.4 25.3 1.4 25.4 1.7 26.4 2.1 26.8 2.4 26.8 2.4 27.9 3.5 28.6 3.6 30.6 4.5 30.8 4.6 30.5 4.5 31.4 4.3 31.5 4.4 30.1 3.4 30.3 3.0 29.7 2.7 0.30 Gram Oleic Acid per 100 Cc. 24.8 4.5 25.1 2.4 25.2 2.0 25 0 1.7 25.' 2.0 26.4 2.9 29 1 3.7 30.3 4.1 30.7 4.4 31 2 4.7 31 I 4 5 31.6 4.5 0.10 Gram Ol& Acid per 100 cc. 25.6 13.7 8.3 24.7 7.8 24.8 4.8 24.9 3.9 25,l 3.3 25.1 3.6 24.8 25." 3.5 4.0 27.2 4.1 28.4 4.4 30.9 4.7 31.6
Spreading Coefficient on Oil D y n e s / c m,
+1.8 - t 2 .I -t3. i -+4 .t4 0 2 +4.i +4.7 +3.8
+3.8
-t3.s +3 4 t2 0 t1.3 fl 3 -0 g -l., -4.6 -4.9 -5 2 -4.5 -5.4
-3 0 -2.8
-1.9
-f1 2 +3.0 +3.3 +3.8 .t3.3
+l.2
-2.3 -3.9 -4.6 -5.5 -5.7 -5.6
to adjust the alkali-fatty acid ratio to a value which depends on the concentration of the solution. At the concentration of 1.00 gram of oleic acid per 100 cc. of solution (Figure 1, left) optimum spreading properties are obtained a t an alkali-fatty acid ratio of approximately 0.95. At a ratio of 1.00 the spreading coefficient is slightly below its maximum, and with further additions of alkali it decreases rapidly, becoming negative in sign a t a ratio of 1.04, and continuing t o decrease to a ratio of about 1.50, and then slowly increasing. -4t t h e concentration of 0.30 gram of oleic acid per 100 cc. (Figure 1, center) optimum spreading properties are obtained a t a ratio of about 1.00-that is, with equivalent amounts of oleic acid and sodium hydroxide. The maximum value of the spreading coefficient is somewhat lower than a t a concentration of 1.00 gram of oleic acid per 100 cc., and i t does not decrease so rapidly with increased amounts of alkali. The general form of the curve, however, is similar to that found a t the higher concentration. At the concentration of 0.10 gram of oleic acid per 100 cc. (Figure 1, right) optimum spreading properties are obtained a t an alkali-fat'ty acid ratio of about 1.35-that is, with a TABLE 11. V.iRIATIoS IN WETTING PROPERTIES WITH COXCESTRATION FOR SODIVM HYDROXIDE-OLEIC ACID MIXTURES OF COSSTAXT ALKALI-FATTY APID RATIO Ratio Moles NaOH/Mples Oleic .%ad
-2.1
closes that solutions of sodium oleate t h a t have excellent wetting and spreading properties may be adversely affected b y a slight excess of alkali.
Wetting and Spreading Properties of Oleic Acid-Sodium Hydroxide Mixtures The same general procedure has EXPERIMENTAL PROCEDURE. been followed in preparing each of the soap solut'ions for which data are given in this report. To about 100 cc. of water in a 500-cc. flask, the desired volume of standard sodium hydroxide solution was added from a buret. The desired amount' of oleic mid (neutralizing value equal to 98.7 per cent of the theoretical) n-as then measured into the flask from a graduated pipet. The mixture was diluted t o the desired volume, heated to boiling, and shaken. All the mixtures thus prepared were allowed to stand for 1 hour or longer before being tested. The recorded alkali-fatty acid ratios are based on the neutralizing capacity of the oleic acid. Table I and Figure 1 shorn how the wetting and spreading properties vary with the alkali-fatty acid ratio for solutions containing 1.OO, 0.30, and 0.10 gram, respectively, of oleic acid per 100 cc. Table I1 and Figure 2 illustrate the variation in wetting and spreading properties with the concentration of oleic acid, a t the two constant alkali-fatty acid ratios of 0.90 and 1.25. It is evident from Figure 1 that the wetting properties of oleic acid-sodium hydroxide mixtures containing the same amount of oleic acid depend greatly upon the alkali-fatty acid ratio. A slight variation in bhis ratio may cause a profound change in the wett'ing properties of the solution, and in order to obtain optimum wetting properties it is necessary
0 05 0 10 0 25 0 50 1 00
-8.8 -2.5
+2.1 +l.8 -0.7 -2.0 .-4.8
Oleic Acid Grams/lOO Cc.
0.90
-to.! +1.o +2.1
1221
1.25
0 05
0 08 0 10 0 15 0 20 0 25 0 30 0 40 0 50 1 00
Surface Tension
Interfacial Tension against Oil
Spreading Coefficient on Oil
Dunes/cm. 24.8 24 8 25.1 25.1 25.1
Dunss/cm. 14.4 7 8 2.0 1.2 0.8
D$nes/cm.
24.8 24.9 25 1 24.9 25.1 26.3 28.7 29 2 30 7 30 8
-8.7 -2 1 +3.4 +4.? t4.6
8.8 6 %
-0.6
3 3
+2.1 +2.4
3.2 2.9 3.4 3.1
3 8 4 5 4 5
-3.1
+2.5 +0.8 -1.9
-2 5 -4 I -4.8
considerable excess of alkali. With still greater amounts of alkali the spreading coefficient gradually decreases, becoming negative in value a t a ratio of about 1.75. The results as a whole indicate that the surface properties of aqueous sodium hydroxide-oleic acid mixtures are closely related to the degree of hydrolysis of the sodium oleate, or to the amount of "acid soap" present in the mixture. Thus, a t lower concentrations a higher ratio of alkali to fatty acid would be required to maintain a given degree of hydrolysis which might be associated n-ith maximum wetting and spreading properties. It is logical t o expect that changes in the degree of hydrolysis might substantially alter the colloidal characteristics of the mixture and thus affect its surface properties. Figure 2 illustrates the variation in wetting and spreading properties with change in concentration for two different values of the alkali-fatty acid ratio-namely, 0.90 and 1.25, With a ratio of 0.90 the spreading coefficient increases rapidly with increase in concentration u p to about 0.25 per cent and beyond this the increase is slight, whereas with a n alkali-fatty acid ratio of about 1.25 the spreading coefficient increases with concentration only u p t o about 0.17 per cent and then decreases. At a Concentration of 0.20 per cent the spreading coefficient of the mixture with a ratio of 1.25 is +2.5, but a t a concentration of 0.50 per cent it is -4.7. That is, dilution from a concentration of 0.50 per cent to one of 0.20 per cent greatly improves the wetting and spreading properties.
1222
INDUSTRIAL AND EKGIXEERING CHEMISTRY
Literature Cited (1) Cooper, W. F., and Nuttall, W. H., J . Agr. Sci , 7, 219-39 (1915). (2) Harkins, W. D., and Feldman, Aaron, J . Am. Chem. SOC..44. 2665-85 (1922).
(3) Harkins, W. D., and Jordan, H. F., I b X , 52,1751-72 (1930). (4) Lovett, A. L., Oreg. Agr. Expt. Sta., Bull. 169 (1920).
VOL. 27, NO. 10
(5) O'Kane, W. C., et al., N. H. Agr. Expt. Sta., Tech. BuU. 39 (1930); 46 (1931); 48 (1932); 51 (1932). (6) Osterhof, H. J., and Bartell, F. E., J . Phys. C h m . , 34,1399-1411 (1930).
RECEIVED April 3, 1935. Presented (by title) before the Division of Agricultural and Food Chemistry a t the 89th Meeting of the American Chemical Society, New York, N. Y.,April 22 t o 2 6 , 1935.
Lecitho-Protein The Emulsifying Ingredient in Egg Yolk H. RI. SELL, ,4. G. OLSEN, AND R. E. KRWIERS General Foods Corporation, Battle Creek, 3Iich.
E
GG YOLK is the stabilizing ingredient in
mayonnaise because of its powerful emulsifying action, and it has been assumed generally that lecithin is the constituent of egg yolk which is effective in producing the emulsions (IO). It was therefore natural to expect t h a t the addition of more lecithin would increase the stability of mayonnaise. The addition of lecithin was tried, with the uniform result that the mayonnaises so produced had poor consistency. This observation led t o a study of the effect of each of the known major constituents of egg yolk on mayonnaise. When i t was found that none of these substances is capable of producing the consistency derived from the whole yolk, the composition of the latter was reinvestigated. The out'come was the demonstration that egg yolk owes its emulsifying action to a n unstable complex containing both lecithin and protein, which has been called for the purposes of this paper a "lecitho-protein."
Emulsifying Properties of Yolk Constituents The better known organic constituents of egg yolk are the fatty oil, a solid fat, protein, lecithin, cephalin, cholesterol, and pigments. Their emulsifying properties with respect to mayonnaise were found to be as follows: Reasonably pure fatty oils have very little emulsifying action and egg oil is no exception. By analogy to previous work, the residue remaining after the removal of oil from egg yolk and the complete extraction of lecithin therefrom by alcohol was considered to be protein. It, too, was devoid of emulsifying action. Corran and Lewis (3) found that lecithin favored an oil-inwater type emulsion, of which mayonnaise is an example. To determine its effect on mayonnaise, lecithin was added with the egg or with part of the oil, or was dispersed in part of the vinegar, but in every case it lowered the consistency and decreased the stability of the mayonnaise. Consequently its effect is harmful rather than beneficial. It was assumed t'hat cephalin would act like lecithin. Corran and Lewis also noted that cholesterol favored the water-in-oil emulsion. With mixtures of lecithin and cholesterol they observed antagonistic effects. Inversion of the emulsion occurred when the ratio of the lecithin to cholesterol was about 8 t o 1, provided both were added with the water phase, but it occurred between 2 to 1 and 1 t o 1 when the cholesterol was dissolved in the oil phase and the lecithin in the water phase prior to emulsification. These results would indicate that no beneficial effects are to be expected from the addition of cholesterol to mayonnaise. The addition of cholesterol in small amounts to either the egg yolk or the oil resulted in mayonnaise emulsions that were identical with those made with egg yolk alone. A weakening of the mayonnaise emulsion occurred when the cholesterol content was increased to about four times that normally present in the egg yolk. The pigments were not tested in these experiments, but were subsequently shown to be without noticeable effect.
These negative results suggested that some unstable constituent was destroyed during the ceparation of the constituents. A consideration of Barbieri's procedure ( 2 ) indicated that for physico-chemical purposes the dehydration of the yolk with alcohol was a mistake. T o avoid the latter, use was made of hexane as the first solvent. However, hexane would not make contact with egg yolk because of its high percentage of moisture. This difficulty was obviated by the use of acetone, which is both miscible with water and a better f a t solvent than alcohol. Tests showed that a mixture of 4 parts of hexane with 1 part of acetone did not denature the protein. Acetone alone completely denatured it.
Fractionation Experiments I n order to make the test material comparable to that used commercially, fresh egg yolks were salted and used as the starting material in the fractionation experiments. The eytraction n-as begun with 1 to 4 acetone-hexane mixture and was completed with hexane alone. The solvent was removed from each extract by evaporation from tared beakers set in a water bath at 50" to 60" C. Residual traces of solvent were removed in a vacuum desiccator. Details covering the extraction of 1804 grams of salted egg yolk (1604 grams yolk, 200 grams sodium chloride) are as follows: Extraction
Solvent Volume
1
2000
2
800
3
700
4
500
5
400
6 7 8
400 400 300
cc.
Solrent Composition Parts 4 hexane 1 acetone 4 hexane
{
1 acetone 11.5 hexane 1 acetone Hexanea
Hexanea Hexane0 HexaneQ
Egg 011 Extracted Grama 222 139 72 62
4 3 2 1
Total 505 a When no separation of liquid occurred, more hexane was added t o break the emulsion.
These nianipulations separated the salted yolk into a soluble lipoid fraction and an insoluble residue. The latter consisted of sodium chloride and the protein-like material of yolk. It was a light yellow powder which, when mixed with twice its weight of water, swelled to a sticky mass t h a t in many ways resembled the original yolk. It was completely denatured by alcohol or acetone. T h a t it contained a relatively large amount of lecithin was shown by an analysis according t o the A. 0. A. C. method ( 1 ) . A 1.1965-gram and a 1.2310-gram portion gave 0.0220gram and 0.0241-gram portions of magnesium pyrophosphate
