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The effect of increasing hydrogen ion concentrationupon the emulsifying power of sodium oleate
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The heights are plotted against the pH values in
Figure 4. It is apparent that the curve obt 'ained closely
parallels that for emulsion stability as opposed to those
for viscosity and surface tension. The indication is
clearly that a determinative relationship may well exist
between emulsifying power and the tendency to fora stable
interfacial films as postulated by the modern protective
film theory.
The Application of the Mudd Interfacial
Technique to Single Emulsion
Droplets at Different
pH Values*
The interfacial technique developed by Mudd and
Mudd (29) and used by them in the study of bacteria, red
blood cells etc., has recently been applied by Nugent (so)
to the examination of single emulsion droplets. The
reader is referred to his paper for the details of this
application which cannot be discussed in full here.
Briefly, in employing this technique, a drop of the
emulsion to be studied and a drop of a suitable oil are
placed close together upon a microscope slide. A coverglass
is then placed over both drops which spread under it with
the formation of an oil -water interface. The oil then tends
to displace the water from contact ith the glass, and,.
as a result, the interface moves slowly in the direction of
the water side.
The moving interface is followed with a microscope,
usually employing dark field illumination, and observation
is made of the behavior of the single emulsion droplets
when the interface makes contact with them. Nugentts
paper describes various types of behavior which have been
observed under different conditions, and suggests an
interpretation for each.
In the present work, ten cc. portions of M/15
phosphate buffer solutions (31) were placed in test tubes,
and five drops of an olive oil or benzene emulsion made with
standard one per cent sodium oleate at pH 9.37 were added.
It was assumed that under these conditions the droplets in
the diluted emulsions would still bear protective films, if
such had originally been present, and that this film would
exhibit properties characteristic of the pH of the particular
buffer solution. The general behavior observed was entirely
similar to that previously reported by ¿ugent in the study
of the fat droplets from cream. In the preparations which
exhibited the least protective power, the droplets
coalesced with the advancing oil phase the moment that
contact was made. In those which exhibited the greatest
protective power, the droplets were picked up by the advanc-
ing interface and carried by it more or less permanently..
Under these conditions, the droplets appeared to be
entirely on the aqueous side, being apparently just
tangent to the interface* As in the case of the cream
fat studies, intermediate protective powers were clearly
distinguishable between these extremes, characterized
by temporary carry followed by coalescence with the oil
phase. In such cases the droplets are picked up by the
interface and carried for varying short periods of time,
when they suddenly disappear due to coalescence with the
oil* The times of carry were observed for a number of
droplets and the average time of carry was taken as a
measure of the resistance to coalescence*
The mechanism whereby temporary carry is possible
is somewhat obscure, but the reproducible nature of the
phenomenon and the parallelism which has been shown to
exist between this behavior and the relative stability
of emulsions, makes it highly probable that its
interpretation as an indication of intermediate
protective action is correct*
TABLE V indicates the results obtained with the
standard olive oil and benzene emulsions diluted with
M/15 phosphate buffer solutions as described, and each
studied with two interfacial oils, Kahlbaums, triolein
and commercial sNujolv,
Table V.
THE BEHAVIOR ON TRE APPLICAT.IßN OF THE MUDD INTERFACIAL
TECHNIQUE TO SINGLE EMULSION DROPLETS AT BIFFERENT
pH VALUES.ESi
Olive oil emulsions.,
p Average time of carry in seconds'
Triolein as interfacial Eujol as interfacialoil. oil.
8.0 . Perm.snent 26.9 , 10 1.5.9 Less than one Less than one-5 . 3 0 o
Benzene emulsions.
8.0 Permanent6.9 105.9 25.3 Less than one ,
10
1Less than one
It is believed that in every case, when coalescence
fails to occur at the moment when contact is made, proof
i$ obtained of the existence of some sort of protective film,
either on the droplets, on the advancing interfacial front,
or on both. If this is true, then the experimental results
obtained in Table V prove the existence of more or less
protective films at all pH values investigated above pH 6.
If time of carry is taken as a measure of the relative
protective power of such films, then a clearcut gradual
decrease in protective power is indicated as the pH value
decreases from pH 8 to pH 5 in all cases. These results
then are in accord with the variation of the emulsifying
power with pH over the range, and constitute perhaps the
most direct proof of the protective action of films of
emulsifying agent yet obtained.
The parallelism demonstrated between these results and
the emulsifying páwers is further in entire accord with the
previous results and predictions of Nugent,, and therefore
serves to confirm the value, and to extend the application
of this new method for emulsion research.
The Variation of the Cataphoretic
Mobility of Single Emulsion
Droplets at Different
pH Values..
It is generally assumed that the direction and speed
of migration of colloidal or microscopic objects under
standard conditions of cataphoresis serve respectively as
an indication of the sign of any surface charge borne by
the particles, and as a measure of the relative extents
of this charge under varying conditions.
As has been pointed out,_ one possibility is that
surface films of an emulsifying agent may bear sufficiently
high charges to prevent actual contact of emulsion
droplets when mechanical forces tend to bring such contact
about. Under such conditions the charge due to the films
might well be the factor of outstanding importance in
determining the protective action of the emulsifying agent.
It was believed, that a conclusion as to this matter could
be reached in the present case by a simple comparison of the
cataphoretic mobilities of the emulsion droplets at different
p$ values, and a comparison of these relative mobilities with
the corresponding emulsifying powers*
Accordingly five drops of either a standard olive oil .
or a standard benzene emulsion were added to 10 cc. portions
of various M /15 phosphate buffer solutions, the same
assumptions being made as in the case of the diluted
emulsion's used in the interfacial technique werk. The
buffers ranged from straight dibasic sodium phosphate) whose
pH value was determined to be 3.20 to the mixture of pH 5.2.
The mobility observations were made using a Northrop- Kunits
microcataphoresis cell. (32) with accessory apparatus as
suggested by Mudd. (33) All observations were nade at 0.21
of the depth of the cell from the bottom. At this level the
electrical endosmotic movement of the liquid in the closed
cell is theoretically zero, (34) and therefore the observed
mobilities of the particles approximate the true mobilities.
Dark - field illumination was obtained with a Bausch and
Lomb cardioid condenser. A const ant potential was applied
from three 45 volt radio IRBY batteries.
In the case of each preparation the times were noted,
by means of a stopwatch, which were required for the droplets
to traverse five divisions of-an arbitrary ocular micrometer
scale. This was taken in all cases as the mean of a number
of observations on different droplets with the direction of
the applied potential reversed after each reading by means
of a commutating switch. From these values the mobilities
of the droplets were calculated im scale divisions per
second. The results are shown in TABLE VI.
PH
THE RELATIVE MaBILITIES OF OIL EMULSION
DROPLETS WITH SODIUM OLEATE AS THE
EMULSIFYING AGENT AT DIFFEREN T pH VALUES.
Time required to traversefive scale divisions inseconds.
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An indirect method was employed in the salting out
experiments. Several benzene emulsions were made with the
sodium oleate solutions at each of the decreasing pH values*
Gradually increasing volumes of saturated sodium chloride
solution were added to the series of emulsions at each pH
value, until that volume was found which resulted in a
definite cracking of the emulsion within ten minutes time.
The results are shown in TABLE VIII.
Table VIII.
THE VARIATION Ili THE VOLUME OF SATURATED SODIU2
CHLORIDE SOLUTION JUST LUFFICIENT TO CAUE THE
CRACKING OF A SERIE OF EMULSIONS AT DIFFERENT
pa VALUES.
PII Volume of saturated sodium chloridesolution in cc.
9.97- 0.69.32 0.47.55 0.57.01 0.14.86 0.0
It is generally believed that the salting out of
hydrophilic colloids is due to a dehydrating action exerted
by the added salt. (36) On this basis, it seems probable
that the greater the degree of hydration of the material
-33-
the greater the concentration of a given salt necessary to
complete the process. The data in Table VIII clearly
demonstrate a decreasing concentration of salt .:ith decreas-
ing pH, which may then be assumed to indicate a decreasing
hydration of the dispersed material with pßì and as in the
case of the turbidity results, a decrease in the extent of
the hydration of the films of emulsifying agent with
decrease in emulsifying power.
The general conclusion from the two indirect methods
employed to test the variation of the states of hydration
of the protective films of sodium oleate or modified sodium
oleate with decreasing pH values, is thus, that there is a
continuous decrease in the cxtent of the hydration of the
films under these conditions. This decrease parallels the
continuous decrease in emulsifying power under the same
conditions. &ince it has been pointed out on theoretical
grounds that the protective action of films of emulsifying
agents might sell depend largely upon their hydrous nature,
these experiments may be taken as confirming this theory.
.144.
Summary and Conclusions.
Emulsions of one volume of olive oil or benzene
in two volumes of one per cent sodium oleate solution have
been shown to decrease continuously in stability with
increase in the hydrogen ion concentration of the aqueous
phase from the pH value of about ten, shown by dispersions
of the soap in distilled water,,,, to about pH 4.5 where -no
stability is shown. These changés are due to a decrease
in the emulsifying power of the sodium oleate or modified
sodium oleate under conditions of increasing acidity,, this
emulsifying power becoming zero at the lower pH value
mentioned. Marked emulsifying power is exhibited by sodium
oleate only at pH values greater than seven.
In . contrast with the decrease in emulsifying power,
the curve obtained on plotting the relative viscosity and
surface tension values of sodium oleate solutions against
decreasing pH values show respectively a well defined
maximum and a well defined minimum in the region pH 8.0 --
pH 9.0. The shapes of these curves indicate clearly that
neither of these properties is of primary importance in
determining the emulsifying powers at the different pH values.
On the other hand the foaming tendency of sodium oleate
solutions decreases continuously with pH thus paralleling the
behavior of the emulsifying power. The foaming tendency
la taken as a measure of the tendency of the solutions to
form stable interfacial films. The parallelism between
foaming tendency and emulsifying po er is in accord with
the best accepted theory of the mechanism of the action of
emulsifying agent namely that the fundamental action is
the formation of stable viscous or plastic films in the
interface surrounding each emulsion droplet.
Further evidence to the same effect is afforded by
the application of the Mudd interfacial technique to the
study of the single emulsion droplets at decreasing pH
values. It is believed that the experiments performed
under this heading afford remarkably direct evidence for
the existence and importance of surface films in emulsification.
Having obtained the foregoing evidence for the
existence of protective surface films., it was next of
interest to determine to what extent surface charge due to
the presence of such films might be the factor of prime
importance in promoting stability. Theoretically it might
well play this role. The relative surface charges were
determined at decreasing p$ values by means of microcat-
aphoresis experiments. The curve obtained on plotting
relative mobilities against pH values shows a clearcut
maximum in the region pH 8.0 - pH 8.5, apparently correspond-
ing to the maximum in the viscosity curve and the minimum
An the surface tension curve. The shape of the curve
definitely rules out surface charge as the factor of
primary importance in determining the emulsifying power
of the sodium oleate solutions.
In contrast with the cataphoretic mobility curve,
further experiments demonstrated that sodium oleate
solutions show a continuous increase in turbidity over the
pH range exhibiting the continuous decrease in emulsify.
Ing power. Over the same range, it was also shown that
continuously decreasing quantities of sodium chloride are
required to salt out the soap. Both of these results admit
of but one reasonable interpretation, namely, that there
is a corresponding continuous decrease in the extent of
the hydration of the dispersed material in the sodium
oleate solutions with pH, and a corresponding decrease in
the extent of the hydration of the protective films formed.
The general conclusions are reached that, first, the
emulsifying power of the sodium oleate or modified sodium
oleate under the conditions of the experiments performed
is dependent upon the formation of stable -Viscous or plastic
films about the dispersed emulsion droplets in accord with
the general criterion of Bancroft, and secondly that the
state of hydration of the films so formed is a factor of
greatest importance in promoting emulsifying power. While
this hydrous nature may be simply a contributory factor to
the formation of stable interfacial films, the theory is
-57-
advanced here that it is in all probability a factor of
primary importance acting .to minimize the aggregation of
emulsion droplets this latter being necessarily the first
step in any coalescing process, and further to minimize
coalescence if aggregation does occur.
Microcataphoresis experiments demonstrate that
surface charge is not a factor of primary importance in
the emulsions studied.
The application of the Mudd interfacial technique
referred to above has confirmed the importance and extended
the application of this new method for emulsion research.
In final conclusion a tentative explanation is offered
for the occurrence of maxima in the pH - viscosity and the
pa cataphoretic mobility curves and a minimum in the
pH - surface tension curves, all in the range pH 8.0 - 9.0.
Preliminary experiments involving the titration of
known quantities of sodium oleate in solution with standard
hydrochloric acid and the plotting of the number of cc. acid
added against the corresponding pH values obtained, indicate
that the region pH 8.0 - 9.0 corresponds to the addition
of an amount of hydrochloric acid equivalent to one third
of the sodium oleate originally present and to the possible
formation of an acid soap of the constitution (sodium oleate)2.
(oleic acid) . A further investigation of titration curves
of this type might well lead to valuable information with
-33-
regard to the general question of the formation of acid
soaps. As pointed out earlier in the paper, the
conclusions reached herein are believed to be independent
of the extent of the formation of compounds of this type.
BIBLIOGRAPHY
1. Bancroft: *Applied Colloid Chemistry,* 3rd ed., McGraw-Rill Book Co*, Inc., New York and London, 1932, ChapterXII.
2. Bancroft: ibid., p. 364.
3. McBain and McClatchiet J. Am. Chem. Soc., 3266 (1932);McBain and Stewart; J. Chem* Soc., 1300 1392 (1927);.Holmes: *Introductory Colloid ChemistrYow John Wiley andBons, Inc., New York, 1934, p. 98.
4. Holmes and Child: J. Am, Chem. Soc., 42, 2049 (1920);Nugent: J. Phys. Chem., Ag., 449 (193217
5. Krantz and Gordon; Colloid Symposium Monograph, 9., 173(1928).
6. Nugent: loc. cit.
7. Mudd, Nugent, and Bullock: J. Phys Chem., §g40 237 (1932).
8. Kruyt (translated by van Klooster); *Colloids,* End, ed.,John Wiley and Eons, Inc., New York, 1930, p. 266.
9. Kruyt (translated by van Klooster): loc* cit., p. 265.
10. Bancroft: loc cit.* p. 367; Gortner: *Outlines of Bio-chemistry,* John Wiley and Vons Inc., New York, 1929, p.34.
11. Bancroft; J. Phys.. Chem., 17, 415 (1913); 19, 275 (1915);Clayton: The Theory of Emulsions and Their TechnicalTreatment** P. Blakistonts Son and Co., Philadelphia, 1928,p. 120.
12. Bancroft: *Applied Colloid Chemistry,* 3rd. ed., McGrawHill Book Co., Inc., New York and London, 1932, p. Z.
13. Bancroft: ibid,, Chapter XII.
14. Mudd, Nugent, and Bullocks loc. cit.
15. Kruyt (translated by an Klooster): loc. cit., p. 192.
16. Holmes and Child: loc. cit.; Clayton: *The Theory ofEmulsions and Their Technical Treatment,* P. Blakiston'sBon and Co*, Philadelphia, 1928.
_40_
17. Briggs: J. Phys. Chem., .2.1,4 120 (1920) .
18. Popoff, Kunz, and Snow: J. Phys.. Chem., 32 1056 (1928) .
19. Clark: "The Determination. of Hydrogen Ions," 2nd. ed.,Williams and Wilkins Co., Baltimore,, 1925, p. 114.
20. Clayton: loc. cit., Chapters IV, VI, and VIII; Holmesand Child: loe. cit; Krantz and Gordon: loe. cit.
21. Holmes and Child: loc. cit.; Clayton: 'The Theory ofEmulsions and Their Technical Treatments P. BlakistontsSon and Co., Philadelphia' 1920.
22. Findlay: "Practical. Physical Chemistry,* 5th ed.,Lontmans, Green and Co., London, New York and Toronto,1931, p. 74.
23. Findlay: ibid., p. 75.
24. Clayton: loe. cit.
25. Findlay: loc. cit., p. 85.
26. Findlay: ibid., p. 85.
27. Findlay: ibid., footnote p. 85.
28. Bancroft: loc. cit., p. 374.
29. Mudd and Mudd: J. Exptl. Med., 1106 633, 647 (1924);43, 127 (1926); 46, 167, 173 (1927); J. Gen. Physiol,14, 733 1930 31T
33. Nugent: loc. cit.
31. Clark: loc. cit.
32. Northrop and Kunitzt J. Gen. Physiol., 7,, 723 (1924 -25) .
33. The cell with accessory apparatus may be purchased fromthe krthur H. Thomas Co., Philadelphia.
34. McCutcheon, Mudd, Strumia, and Lucke: J. Gen. Physiol.,13, 663 (1930) .
35. Bancroft: loc. cit., p. 223.
36. Kruyt (translated by van Klooster): loc. cit., p. 194.