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A QUANTITATIVE STUDY OF THE ADSORPTION IN SO- LUTION AND AT INTERFACES OF SUGARS, DEXTRIN,
STARCH, GUM ARABIC, AND EGG ALBUMIN, AND THE MECHANISM OF THEIR ACTION AS
EMULSIFYING AGENTS.
BY GEORGE L. CLARK” AND WILLIAM A. MANN.
(From the Furman Chemical Laboratories, Vanderbilt University, Nashsills.)
(Received for publication, January 21, 1922.)
There is a wide divergence in opinion at the present time as to what the real mechanism of the action of colloid protectors and emulsifying agents may be. While it is generally known that the same substances may be used as protectors of very finely divided solid precipitates by preventing agglomeration, and as emulsifying agents by stabilizing liquid droplets dispersed in a liquid medium, and that the most efficient of these substances are themselves pep- tized colloids, it is by no means generally agreed that the mecha- nism of the two processes is the same. On the other hand there have been numerous attempts to embody in a single generaliza- tion the properties and conditions necessary for colloid peptiza- tion and protection, and for emulsification. Thus Plateau’ and Quincke2 without understanding the true nature of adsorption con- sidered that the most important factor required in an emulsifying agent was that it should lower the surface tension of one of the liquids though they also realized the large influence of viscosity. PickeringS presented the theory that the emulsifying agent must form small insoluble particles to cover the droplets. Fischer’ be- lieved that ability to combine with water is the most essential
* National Research Fellow in Physical Chemistry, Harvard University. 1 Plateau, J., Ann. Physik., 1870, cxli, 44. * Quiucke, G., Ann. Physik., 1888, cclxxi, 580. * Pickering, S. U., J. Chem. Sot., 1907, xci, 2001. 4 Fischer, M. H., and Hooker, M. O., Science, 1916, xliii, 468.
157
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Emulsifying Agents
attribute. Bancroft? states that an emulsifying agent is adsorbed into the interface between dispersed phase and dispersing medium and produces a coherent film whioh by its viscous nature prevents coalescence of the particles. He points out that while most emul- sions are made with gelatinous colloids as emulsifying agents, the same purpose would be served by any substance which will be adsorbed to the interface and make it viscous. Similarly the mechanism of stabilizing or protecting colloids is to be explained by adsorption of the agent which is either soluble in water or is solvated by water.’ Holmes,’ in working with gelatin as an emul- sifying agent, could get no evidence of any concentration of gelatin by adsorption around oil droplets or that as oil content is increased the gelatin content must be increased as would be the case if adhe- sion layers were formed. He considers that the leading factor is simply viscosity, not the maximum but the “most favorable.” These results are in direct contradiction to the work of Winkel- blech* who shook a dilute solution of gelatin with benzene and found a film of gelatin at the dineric interface, a phenomenon which he proposed as a method of analysis for gelatin. He declared that the adsorption of the emulsifying agent at the interface was inversely proportional to the ease of peptization of the substance by one of the liquids in the emulsion. In a paper before the Ameri- can Chemical Society at Rochester, N. Y., April 1921, Holmes showed that the formation of an adsorbed film was the principal factor in the use of cellulose nitrate.
It is at once apparent that quantitative data on the action of peptizing colloids and related substances is almost completely lacking, and that satisfactory conclusions cannot be drawn with- out such data. The object of the present research was, therefore, to study by the most accurate known methods the behavior of five substances which, to a greater or less extent, act as colloid protectors or emulsifying agents, viz. sugars, dextrin, starch, gum arabic, and egg albumin, throughout the greatest possible range of concentration, each alone and, for the purpose of studying ion
s Bancroft, W. D., J. Phys. Chem., 1913, xvii, 501; Applied colloid chem- istry; general theory, New York, 1921,225.
* Bancroft,& p. 225. 7 Holmes, H. N., and Child, W. C. J., Am. Chem. Sot., 1920, xlii, 2049. * Winkelblech, K., 2. angezo. Chem., 1907, xix, 1953.
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G. L. Clark and W. A. Mann 159
adsorption, in the presence of the following electrolytes: hydro- chloric acid, sodium hydroxide, iodide, sulfate, and bicarbonate, as respectively, acid, base, liquefying or peptizing salt, precipita- ting or pectizing salt, and a biologically significant substance. It is, of course, unnecessary to point out the manifold application of such data to the fields of adsorption and digestion in physiology, many industrial processes including the manufacture of soaps, paints, medical preparations, oils, inks, and numerous other prod- ucts, flotations, sewage disposal, electroplating-in short, in every case where minute “drops, bubbles, grains, filaments or films” require stabilization.
EXPERIMENTAL METHODS.
Each dilution of each one of the five substances under investiga- tion, alone and in the presence of each of the five electrolytes, was studied under the following heads: (a) Surface tension at 25”. (5) Interfacial tension between solution and benzene, and in some cases caprylic alcohol for purposes of comparison between aromatic hydrocarbon and long chain alcohol. (c) Viscosity. (d) Density. (e) Actual observations upon emulsions made up uniformly with benzene and kerosene and studied for heaviness of creaming and stability as measured by time of existence.
The surface tension of the solutions was determined by the drop, weight method, using a Morgan drop-weight apparatus@ with cali- brated1° tip with radius of 0.2673 cm., and the values calculated by
the use of the Harkins and Brown11 formula: y = E ?r *(-I
;+ ,
where 7 is the surface tension in dynes per cm., m the weight of a drop, g the acceleration due to gravity, r the radius of the tip, V the molecular volume determined from the density, and $ the function corresponding to the fraction of an ideal drop which actually falls. There can be no criticism of the accuracy of this method as shown by the recent confirmatory results of Richards and Carver’* in their critical study of the capillary height method.
*Morgan, J. L. R., J. Am. Chem. Sot., 1911, xxxiii, 849. *a Harkiis, W. D., and Humphrey, E. C., J. Am. Chem. Sot., 1910, xxxviii,
228. 11 Harkine W. D., and Brown, F. E., J. Am. Chem. Sot., 1919, xii, 615. 11 Richards, T. W., and Carver, E. K,, J. Am. Chem. Sot., 1921, xliii, 827.
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TABL
E I.
Effe
ct
of E
mul
aif&
g Ag
ents
on
Be
nzen
e-W
ater
an
d Ke
rose
ne-W
ater
Em&&
ma.
Cond
ition
of
em
ulsi
on.
4 3 6 3
2 9 6 9 -
fj -
5 4
6 6
9 8
6 9 8 8 9 9
2
0 c -
Oil
pham
e.
Ebtto
lpte
.
-
.d B
f
1 4
-
10
10
0 0 0 0 0 0 0 0 0 0 0 0 0 0 H
CI
pcr=
-l
Benz
ene
......
.. 0
Kero
sene
g
Bens
ene
......
. 0
......
.. 0
.Ol
Kero
eene
...
....
0 .O
l Be
nzen
e ...
.....
0 .l
Kero
sene
.. ...
...
0.1
Benz
ene.
...
....
0.91
8 Ke
roee
ne
......
. 0.
918
Bens
ene
......
.. 1
.O
Kero
sene
...
. . .
. 1
.O
Benz
ene.
...
....
10 .O
Ke
rose
ne
......
. 10
.O
Benw
ne.
......
. 30
.O
Kero
sene
...
....
30 .O
Be
nsen
e ...
.....
0.00
6
10
10
In
the
tabl
e 0
indi
cate
a Di
sapp
eare
d;
1, T
race
a;
2, E
xtre
mel
y sli
ght;
3, V
ery
sligh
t; 4,
Slig
ht;
5, F
air;
6, G
ood;
7,
Ver
y go
od;
8, H
eavy
; 9,
Ver
y he
avy;
10
, Exc
elle
nt.
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Kero
sene
...
....
Benz
ene
...
. ....
Ke
rose
ne
......
. Be
nzen
e.
......
. Ke
rose
ne
......
. Be
nzen
e.
......
. Ke
rose
ne
......
. Be
nzen
e.
......
. Ke
rose
ne
......
. Be
nzen
e.
......
. Ke
rose
ne
......
. Be
nzen
e.
......
. Ke
rose
ne
......
Be
nzen
e.
......
. g
Kero
sene
...
....
- Be
nzen
e ...
.....
Kero
sene
...
....
Benz
ene.
...
....
Kero
sene
...
....
Benz
ene.
...
....
Kero
sene
...
....
Benz
ene.
...
...
Kero
sene
...
....
Benz
ene.
...
...
Kero
sene
...
....
Benz
ene.
...
...
Kero
sene
...
....
Benz
ene.
...
....
Kero
sene
...
....
0.00
5 0.
05
0.05
0.
5 0.
5 0.
439
0.43
8 5.
0 5.
0 0.
005
0.00
5 0.
05
0.05
0.
5 0.
5 0.
459
0.45
9 5.
0 5.
0 0.
005
0.00
5 0.
05
0.05
0.
5 0.
5 0.
459
0.45
9 5.
0 5.
0
HC
I H
Cl
HC
l H
Cl
HC
l H
Cl
HC
l H
Cl
HC
l N
aI
NaI
N
aI
NaI
N
aI
NaI
N
aI
NaI
N
aI
NaI
Na
OH
NaO
H Na
OH
NaO
H Na
OH
NaO
H Na
OH
NaO
H Na
OH
NaO
H
5 6 0 2
5 6
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TABL
E &f
Jonc
ltukd
. - I
- i -
Cond
ition
of e
mul
eiin
.
I- Be
Einn
ing.
- 2 5 4 5
- d B m
-
5 5 5 5 -
6 8
4 5 6 6 -
6 9
-
Afte
r 7
week
a - z z 0 2 3 2 5 3 0 2 4 2 5 5 -
- z!
m’
- 5 2 5 2 -
- 3 d - - 2 2 2 5 2 2 2 2 5 5 -
5 8 4 9 -
- 2 2 - 4 2 5 2 -
- 2 f a 8 - 5 8 4 9 -
- .d
B
3 b - 10
10
LO
LO
-
CoIlCe
ll-
;&io
n of
ag
ent.
- a L 2 - 4 5 2 5 2 5 2 5 2 5 2 5 2 5 2 6 -
oil
phee
e.
ElW
trolyt
e.
/ .- -
per
cent
0.00
5 0.
005
0.05
0.
65
0.5
0.5
0.45
9 0.
459
5.0
50
0.00
5 0.
005
0.05
0.
05
0.5
0.5
0.45
9 0,
459
5.0
5.0
Benz
ene.
...
....
Kero
sene
...
....
Benz
ene.
...
....
Kero
sene
...
....
s Be
nzen
e ...
.....
h3 K
eros
ene
......
. Be
nzen
e.
......
. Ke
rose
ne
......
. Be
nzen
e.
......
. Ke
rose
ne
......
. Be
nzen
e.
......
. Ke
rose
ne
......
. Be
nzen
e.
......
. Ke
rose
ne..
.....
Benz
ene
......
.. Ke
roze
ne
......
. Be
nzen
e.
......
. Ke
rose
ne
. _ . _
. Be
men
e.
....
.; .
Kero
sene
.. ...
. .
NaFf
COs
NaHC
O,
NaHC
Oa
NaHC
O,
NaHC
Os
NaHC
OI
NaHC
O,
NaHC
OI
NaHC
O,
NaHC
Os
Na&h
N&
304
N&30
4 Na
sh
N&30
4 N
&30.
N
&304
Na
tSO
N&30
4
NM04
10
10
10
10
10
10
10
10
10
10
10
10
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G. L. Clark and W. A. Mann 163
The interfacial tensions were measured by the corresponding drop-volume method using a modified form of the apparatus described by Harkins and Humphreyn with the correction curve for calculation. Two or more runs were made in each case to insure accurate checking.
The relative viscosities were measured by means of a special Ostwald type viscosimeter and with the following precautions: constant temperature throughout; same treatment of the solution as regards shaking and previous treating; recognition of the large effect of viscosity changes in emulsoids with time so that runs were made only after the emulsoids were allowed to stand for some time, and at the ssme time that surface tension measurements and actual emulsions were made. In most cases, of course, the emul- soids were so dilute that a practically constant viscosity was ob- served, agreeing with the observations of von Schroeder.”
Density measurements were particularly important for the exact calculation of surface tension. They were made at 25’ with pycnometer or carefully calibrated Westphal balance.
The emulsions were all made up in the same manner, with the same relative volumes, 75 per cent oil to 25 per cent solution or emulsoid. The benzene was dehydrated and distilled over metallic sodium at 30-32”. Kerosene was purified and dehydrated over metallic sodium. The emulsion was made by adding 30 cc. of oil in 10 cc. portions to 10 cc. of the solution. After each addition of oil the bottles (glass-stoppered specimen bottles of uniform sise) were shaken a full 10 minutes or for all three additions 4,800 strokes. These were then placed in a quiet place, or in some cases centri- fuged, and observed after various lengths of time. The results are tabulated together in Table I in which the emulsions are &&led on a scale of ten from the standpoint of efficiency of emulsification.
EXPERIMENTAL RESULTS.
Sugars.
Most of the work on sugars was conducted with purest sucrose. While solutions of sugar are not colloidal or gelatinous in the sense
I* Harlsms and Humphrey,10 p. 235. I4 von Schroeder, P., 2. physik. Chem., 1903, xlv, 75
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164 Emulsifying Agents
that those of dextrin, starch, gum arabic, and egg albumin are this non-electrolyte is of great theoretical importance, although it may not be considered nearly so effective a colloid protector or emulsifying agent as the emulsoids. Sugar is usually considered to be molecularly dispersed in aqueous solutions but Lobry de Bruyn and Wolff’6 have shown that concentrated solutions show the Tyndall phenomenon characteristic of a colloid. It is inter- esting, therefore, to discover if this tendency may account for such powers as an emulsifying agent as sugar solutions may pos- sess. Surprisingly little work has been done with sugar solu- tions and none quantitatively. There are, of course, numerous -es where strong sugar solutions prevent the formation of visible solid precipitates, such aa calcium silicate,1b silver chromate, silver chloride,l7 lime,‘* and hydrous oxides of copper, uranium, and iron.lg Bancroft20 states that no work has been done on the direct peptization of precipitates by such a dissolved non-electrolyte. Grimauxgl found that invert sugar was seven times aa effective as cane-sugar in holding up ferric oxide. Bancroft considers that sols are stabilized by non-electrolytes such as sugar by very strong adsorption of the sugar so that the colloid will behave aa though water-soluble. There is no data cited on this point however. It would necessarily follow from this idea that sugar could act as an emulsifying agent if it will go into the interface of an oil-water mixture and make it suf%iently viscous. On the other hand if molecular dispersion in water solution is considered aa the maxi- mum limiting case of peptization no concentration of sugar would be found in the interface in keeping with the general rule that the amount adsorbed at the interface is inversely proportional to the ease of peptization by one of the liquids. The above references constitute the present knowledge of the action of sugar. Aside from limited data such as may be found in Landolt-Biirnstein tables on the surface tension of sugar solutions, there is nothing quantitative.
16 Lobry de Bruyn, C. A., and Wolff, L. K., Rec. trau. chim. Pays-Bue, 1904, xxiii, 155.
I6 W&berg, J., Bull. Sot. Chim., 1890, xv, series 3,1097. 1’ Lobry de Bruyn, C. A., Bet-. them. Gee., 1902, xxxv, 3079. 18 Graham, T., J. Chem. Sot., 1862, xv, 253. ** Riffard, E., Compt. rend. Acud., 1873, Ixxvii, 1103. *O Bancroft, p. 163. *I Grimaux, E., J. Chem. Sot., 1334, xlvi, 966.
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G. L. Clark and W. A. Mann
The experimental values of cane-sugar solutions on specific gravity, relative viscosity, surface tension, and interfacial tension against benzene are tabulated in Table II. While in all cases the values of 7 are above those of pure water there is a very interesting variation discovered in the dilute solutions. There is a gradual slight rise above 72.05 dynes for pure water to 0.01 per cent solu- tions, followed by a decrease to 0.1 per cent and another increase at somewhat less than 1 per cent which continues up to the most concentrated solution, 35 per cent. It will be shown later that the same rise and depression in y for very dilute solutions of the four emulsoids, dextrin, starch, gum arabic, and egg albumin, is observed, although the second ascent is not observed for them. The peculiar behavior for sucrose, which in solution seems to be uniformly negatively adsorbed, is difficult to explain, though it may be related to the hydration’ of the sugar molecules, agglom- eration, or both.
The values in the table show at a glance that, except in the cases of dilute solutions and of HCl, the presence of the electrolytes enhances the value of r. In 0.005 per cent solutions made 0.5 M with the electrolytes, hydrochloric acid and sodium iodide have no effect, but sodium hydroxide lowers ‘y, to a considerable extent. Thus, though 0.005 per cent sucrose has a value of 72.13 and the hydroxide alone of 72.60, the solution of both is 71.77, even below that of pure water, This can be explained only upon the basis of chemical reaction in very dilute solutions or a very specific adsorp tion of hydroxyl ions. It will be shown that dextrin and starch act in exactly the same way. In the more concentrated solutions .the hydroxide acts similarly to the sulfate and bicarbonate. Ex- cept in the case of the 0.1 per cent solution which marks the maxi- mum point of deflection, hydrochloric acid, which alone has prac- tically no effect upon the surface tension of water, lowers the value of the sugar solution. The specific effect of the iodide ion which is so markedly adsorbed and peptizing upon emulsoids is even here quite apparent. This entire study is one of great inter- est inasmuch as two factors enter in: the effect of electrolytes on sugar solutions and of sugar solutions on the electrolytes. The variations are to be accounted for by differences in adsorption of ions upon sugar molecules and of water upon both sugar and the ions. It should be noted that 0.5 per cent sugar solutions, though
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TABLE 11.
Sugar (&&TOSS).
c!.mcen tmtion <
su-b might.
w f-'
None. “
“
“
“
“
0.005 0.005 0.005 0.005 0.005 0.005 0.01 0.05 0.05 0.1 0.5 0.5 0.5 0.5 0.5 0.5 1.0 5.0 5.0
10.0 10 .o 15.0 15 .o 15 .o 15 .o 15.0 15 .o 20 .o 30.0 35.0
-
Electldyte. Specific Relative gravity. viscosity.
Hz0 0.9970 1.000 HCl 1.0068 1.041 NaI 1 SO525 1.020 NaOH 1.0210 1 .I17 NaHCO‘ 1.0267 1.123 N&30, 1 .cGOl 1.245
None. 1.003 HCI 1.0072 1.045 NaI 1.0532 1.025 NaOH 1.0211 1.118 NaHCO, 1.0268 1.122 NalSOc 1.0661 1.223 None. 0.9972 1.005
‘4 1 .Oll HCl 1.0079 1.051 None. 0.9998 1.017
“ 1.028 HCI 1.0087 1.049 NaI 1.0544 1.026 NaOH 1.0228 1.129 NaHCO, 1.0294 1 .I40 NazS04 1.0737 1 ,235 None. 1.0024 1.03b
“ 1.174 HCI 1.0270 1.185 None. 1.0377 1.328 HCl 1.0489 1.389 None. 1.482 HCl 1.0727 1677 NaI 1 .I170 1.669 NaOH 1.0833 1.939 NaHCO, l.OBO5 1.836 NatSO 1.1237 2 .lc@ None. 1.@305 1.636
“ 1.1257 3.030 “ 1.1510 4.098
SlUfSCe tension.
Surface t-maim at interface.
dunes per cn
72.05 72.09 72 .&3 72.60 73.04 73.36
72.16 72.15 72.14 71.77 73.49 73.52 72.27 72.20 72.27 72.14 72.31 72.18 72.85 72 97 72.80 73.34 72.48 72.57 72.50 72.69 72.59 72.90 73 .lO 73.14 73.41 73 A5 74.32 73 .ll 73 43 73.58
dp3s per Eta.
34.28 34.47 34.82 34.70 34.65 36.16
-- 34.49 34.46 35.14 35.00 35.24 36.72 34 68 34.74 34.80 34.79 34.03 34 33 34.60 35.08 35.35 36.39 35.57 33.80 34.57 33 97 34 67 33 94 34.07 34.27 34.87 34.22 36.51 33.90 33.54 32.85
* Determinations made with 90 per cent bensene, sp. gr. 0.8715. t Values for sucrose solutions without electrolytes, percentage con-
centrations 0.005,0.05, K5,5.0. and 15, taken from curves. All other values in the table are actual determinations.
166
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G. L. Clark and W. A. Mann 167
possessing a higher surface tension than 0.01 per cent, produce a marked lowering of the surface tension of sodium bicarbonate, al- though the 0.005 per cent raises it, and that sodium sulfate in 0.5 per cent sugar solution has the same value as if it were present in pure water, The bicarbonate has the same surface tension both in 0.005 and 15 per cent sugar solutions.
The effect of the presence of hydrochloric acid in solutions of sucrose is particularly interesting because of inversion with time. Inversion has not before been followed by means of possible varia- tions in surface tension with time. In Table III are some repres- entative data. It is at once apparent that the changes are exceedingly small though generally there is a slight increase which has been confirmed by repeated observations. The small structural effect is shown by the fact that a 1 per cent sugar solution made 0.5 M with hydrochloric acid has a surface tension of 72.18 upon mix- ing, and 72.26 after inversion, while a 1 per cent levulose solution with acid is 72.23, a 1 per cent dextrose solution with acid is 72.25, and a 1 per cent equal mixture of levulose and dextrose with acid is 72.25. Absolutely no evidence has been obtained to confirm the statement of Grimaux21 that invert sugar is seven times as e5cient in holding up suspensoids or emulsoids as sucrose. In equal concentrations no difference has been detected in any case.
Considering the surface tension values at the interface between sugar solutions and benzene, very interesting results are obtained. Starting with the value of 34.28 for the interfacial tension of water- benzene and adding sugar, there is at first an increase to 0.1 per cent solutions, then a sharp decrease, another increase between 1 and 10 per cent solutions, an almost constant value between 10 and 26 per cent, and a final continual decrease with increase in concentration up to 35 per cent. Even with these variations, all sugar solutions above 1 per cent in concentration, lower the inter- facial tension between water and benzene, a result which is ap- parently favorable for the action of sugar as an emulsifying agent. Since sugar does not lower the surface tension of water alone and is seemingly negatively adsorbed in the system solution-vapor, it at once follows that the positive effect at the benzene interface must be specific for the benzene at the higher concentrations of sugar. That this is true is quite clearly shown by the fact that at the interface between water and caprylic alcohol, an increase
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168 Emulsifying Agents
in sugar increases the tension, following closely the action upon water alone. This is shown in Table IV. It must be concluded,
TABLE III.
Sugar-Eflect of Inveraaon of Sucrose on Surface Tension, Interfacial Surface Tension, and Viscosity.
sucrose. pstfsnt
0.005 0.005 0.06 0.05 0.5 0.6 5.0 5.0
10 .o 10.0 15 .o 15 .o
None. HCl None. HCl None. HCl None. HCl None. HCl None. HCl
1.0972
1 .Ou79
l.OcE57
1 .O276 1.0377 1 AI489
1.0727
1.903 1 .@I5 1 .Oll 1.051 1.028 1.049 1.174 1.186 1.328 1.389 1.432 1.077
dynes per cm.
72 .I0 72.15 72.20 72.27 72.31 72.18 72.57 72 50 72.69 72 59 72 90 73 .lO
Levulose.
dyne8 per em.
72 29
72 22
72.26
72.33
72.76
73 27
34.49 34.46 34.74 34.80 34.03 34.33 33.80 34 57 33.97 34.67 33.94 34.07
34.33
34.73
34.40
34.82
34 89
34.08
Dextrose.
ii:: 1 ZY- / ::E3I ::iz 1 Ez 1 1 ii:: 1
Levulose and dextrose.
::6” 1 tie* j ::zz / ::ti.i 1 E:ii / / ii:: j
* 0.5 M HCl. t With aolutione fresh. $ Determinations made with 99 per cent benzene, ep. gr. 0.8715.
therefore, that the chemical nature of the oil used in making an emulsion will determine, from the standpoint at least of surface energy relationships, the efficiency of sugar as an emulsifying
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G. L. Clark and W. A. Mann 169
agent. It will be shown later that kerosene forms much better emulsions in the presence of sugar than benzene following a greater decrease of the interfacial tension kerosene-water.
All of the electrolytes increase the interfacial tension values of sugar solution-benzene, following in a somewhat parallel way the variations of sugar concentration. Here again the expected does not occur, e.g. that HCI should lower the values as in the case of solution-vapor. Obviously, interpretations are not justified until experiments varying the concentration of electrolyte with given sugar content are made.
TABLE IV.
Capq&c Alcohol. Surface tension = 26.35. Relative viscosity = 3.126. Density -
0.82026.
Water.. . . . . . . . . . . . . . . . . . . . . . . . . . . . psrd dunes par cm. dVMnrm.
None. 9.80 34 28’ 33.72t
Sucrose.. . . . . . . , . . . . . . . . . . . . . . . . . . . 30 .O 10.88 33 54 Dextrin........................... 10.0 3.85 36.41 Starch.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 .O 10.17 35 .lO Gum arabic.. . . . . . . . . . . . . . . . . , . . . . 10.0 9.24 22.87
* Determination made with 90 per cent benzene, sp. gr. 0.8715, used with sucrose.
t Determination made with 90 per cent benzene, sp. gr. 0.8745, ueed with dextrin, starch, and gum arabic.
Parallel in importance with the effect of sugar upon interfacial tension is the viscosity effect. It has already been pointed out that the view of Bancroft is that any substance which is adsorbed lo an interface and is su&&ntly viscous will act as an emulsifying agent while Holmes found that favorable viscosily alone may ac- count for such an action. Of course, the viscosity of sugar, a molecular dispersoid, increases in a regular way with concentra- tion. Electrolytes increase viscosity slightly in dilute solutions but produce a very great lowering in the more concentrated solu- tions. It is noteworthy that this effect is not only observed with sugar, a molecular dispersoid, but also with the emulsoids.
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170 Emulsifying Agents
Freundlich22 has shown that increase in viscosity in any case goes hand in hand with increase in agglomeration which in turn involves decrease in the amount of available free water. Another type of agglomeration, however, may be accompanied by a de- crease in viscosity. This point will be fully discussed under dex- trin and starch. It would seem, therefore, that the electrolytes prevent the agglomeration of sugar in higher concentrations. On the other hand the real effect may be of the sugar acting on the electrolytes. The well grounded fact that ions are more strongly hydrated than electrically neutral undissociated molecules ex- plains why increase in dissociation and increase in viscosity go hand in hand. As a result of the magnetic field about the charged particles the ions may very strongly adsorb the solvent-a phenom- enon which would be greatly hindered in the presence of adsorbed sugar probably. In justification of this it is, of course, well known that viscous sugar or glycerol solutions have a marked effect on ionic conductivity, though equally viscous solutions of such sub- stances as gelatin, agar, and proteins have practically no hindrance whatever to the motion of ions and crystalloid molecules.5a Here again interpretations must be incomplete until observations have been made varying the concentration of electrolyte. Suffice it to say that below 20 per cent solutions the electrolytes increase vis- cosity while above 20 per cent they lower it. If viscosity were a predominating factor, therefore, it would be expected that the best emulsions would be obtained in the presence of electrolytes, particularly sodium sulfate, below 20 per cent, and with pure sugar solutions above 20 per cent, and that if the maximum vis- cosity is the “most favorable” the very best emulsions would be obtained with 35 per cent sugar solution without electrolytes. Such is found to be the case if the rating of the emulsions in Table I is considered. But it should also be remembered that 35 per cent sugar solution without electrolytes produces the maximum lowering of interfacial tension between water and either benzene or kerosene, and, therefore, there are two superimposed favorable effects for the emulsifying action of sugar. There is no question, however, that viscosity is the more important of the two factors,
*a Freundlich, H., 1’~. Faraday SOL, 1913, ix, 66. ** Cj. Walden, P., 2. physik. Chem., 1906, Iv, 209.
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for the stability of the emulsions follows more nearly the increase in viscosity aided both by increase in concentration and by pres- ence of electrolytes in the more dilute solutions, than it does the changes in adsorption as measured by interfacial tension. Thus emulsions of caprylic alcohol in water are increasingly better with increase in viscosity of the sugar solution added, although the interfacial tension is also gradually increased. While for benzene the maximum viscosity is also the most favorable, this is not true with kerosene emulsions. A 10 per cent sugar solution produces much better emulsions than 30 per cent solutions, paralleling the discovery of Holmes that too viscous gelatin solutions were not desirable for emulsions of the same oil.
By a consideration of sugar alone, t.herefore, it is apparent that no general rule for the mechanism of the action of emulsifying agents can be laid down, nor can any one general rule be enunci- ated for the action of one a.gent under all conditions. Much de- pends upon the chemical nature of the liquids being emulsified. With sugar the primary attribute is viscosity-usually maximum -even though there is evidence also of adsorption at higher con- centrations upon the emulsoid particles so that the colloid tends to act like a water-soluble one. On the other hand viscosity cannot be considered the only prime requisite of an emulsifying agent; for though sugar furnishes a large and favorable viscosity, it does not compare as an emulsifying agent with even minute concentra- tions of other substances in which the viscosity factor is largely negligible.
Dextrin a.nd Starch.
These two substances will be considered together because of their close relation. Added interest is placed in these substances as a result of the recent work of Herzfeld and Klinger24 who declare that dextrin is not a split-product of starch at all but simply a more highly dispersed starch. Thus the blue color of iodine ad- sorbed upon starch and the brown color upon dextrin is merely a result of the greater dispersion ifi the latter case. Diastatic action, is, like dextrinization of starch by simple adsorption of the surface by solutions, merely a process of alteration of the degree of disper-
I4 Herzfeld, E., and Klinger, R., Biochem. Z., 1920, cvii, 268.
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172 Emulsifying Agents
sion to form a large increase in surface which is an essential prep- aration for later hydrolysis. If then this view be true, the tran- sition from starch through soluble starch (more highly dispersed) to dextrin (most highly dispersed colloidally) should be a contin- uous one, and the differences in the emulsifying action of dextrin, soluble starch, or ordinary starch should be related merely to the degree of dispersion of the emulsoid. The relations are further complicated, however, by the graduations between molecular solubility in extremely dilute solutions, the suspensoid state in dilute solutions, and emulsoid state at higher concentrations. This will be considered from the standpoint of viscosity. The whole question is one of great importance biologically, inasmuch as animal glycogen is really dextrinized or highly dispersed starch and quantitative measurements should throw considerable light upon its action in living organisms.
Dextrin and soluble starch are water-peptizable colloids and consequently serve to act as colloid protectors26 and emulsifying agents. Zsigmondy2” has determined the gold number (number of mg. required to prevent the color change in a 10 cc. red gold solution containing 0.0053 to 0.0058 per cent gold, when 1 cc. of a 10 per cent solution of sodium chloride is added) of dextrin as 10 to 20 and of potato starch as 25. Winkelblech* pointed out that soluble starch formed a film at dineric interfaces because of the difficulty of peptization by water. Friedentha12’ found that soluble starch exhibited a slight depression of the freezing point and a molecular weight of 9,450 while ordinary starch has a molec- ular weight of at least 32,400.28
Little work has been done on surface tension. Ostwald2e states that gelatinizing colloidal mixtures have a lower surface tension than their pure dispersion medium. In Table V are shown the surface tension relationships of dextrin and in Table VI, those of soluble starch together with values for ordinary starch. The close
16 Lachaud, M., Bull. Sot. Chim. de Paris., 1896, xv, series 3,1105. *6 Zsigmondy. R., 2. anal. Chem., 1901, xl, 697. ‘7 Friedenthsl, H., Cl&. Physiol., 1898-99. xii, 849. ** Brown, H. T., and Morris. G. H., J. Chem. Sot,, 1888, liii, 610. 19 Ostwald, W., A handbook of colloid chemistry; The recognition of
colloids, the theory of colloids, and their general physico-chemical prop- erties, t.ranslated by Fischer, M. H., Philadelphia, 2nd edition, 1919,51.
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G. L. Clark and W. A. Mann 173
similarity is very marked in the more dilute solutions. In both cases there is a gradual rise in surface tension above that of pure water until a 0.01 per cent solution is reached, followed by a regular decrease to exactly the same degree. Thus 1 per cent dextrin has a value for y of 71.98 and 1 per cent starch of 71.96. At higher
TABLE V.
Dextrin.
ChICen- “;Brbl;,f
by weight
fw;; Relative Surface Surfare
Electrolyte. viscosity. tension. tension at interface.’
per rent
None. “ I‘ “ ‘I “
-
--
_ -
-- dunes per em. &mes per cm.
Hz0 0.9970 1 .cKlo 72.05 33.72 HCl 1.0668 1.041 72.09 34.38 NaI 1.0525 1.020 72.86 34.55 NaOH 1.0210 1.117 72.60 34.97 NaHCOa 1.0267 1.123 73.04 34.37 Na&JO, 1.0601 1.245 73.36 34.50
-- None. 1.0145 72.23 34.16 HCI 1.0076 1.085 ,71.96 34.45 NaI 1.0512 1.025 72.31 34.45 NaOH 1.0202 1.120 72.67 35.13 NaHCOt 1.0265 1.134 72.60 35 .oo NazSOc 1.0592 1.237 73 33 36.20 None. 0.9992 1.029 72.41 34 6.1
‘I 1.6020 1.066 71.96 33.87 ‘I 1.230 69.76 35 .Ol
HCI 1.0249 1.473 68.38 33 54 NaI 1.0699 1.415 M.82 33.69 NaOH 1.0373 1.631 64.56 25 35 NaHCO, 1.0442 1.638 72 .Ol 32 .I55 Na2S04 1.0774 1.809 72.03 33 94 None. 1.0353 2 647 68.06 36.41
- * Determinations made with 90 per cent benzene, sp. gr. 0.8745. t Values for dextrin, percentage concentrations 0.005 and 5.0, taken
from curves. All others actual determinations.
0.005t 0.005 0.005 0.005 0.005 0.005 0 .Ol 1 .o 5.0 5.0 5.0 5.0 5.0 5.0
10.0
-
_ -
- -
concentrations the rate of decrease for dextrin is slightly greater than for starch. In the presence of electrolytes, however, a con- siderable difference is found. Most noteworthy is the very large effect of sodium hydroxide in lowering the surface tension of dex- trin by almost 6 dynes in 5 per cent solution. It also has a large effect on the more concentrated starch solutions. Sodium bicar-
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174 Emulsifying Agents
bonate and sulfate, while both increase the surface of dextrin, have descending curves, while for starch they ascend with increase in concentration. This would not be expected, however, in more concentrated starch emulsoids.
At the interface water-benzene, dextrin acts exactly opposite to sugar, i.e. there is a depression in 7 to 1 per cent and then a steady
TABLE VI.
Starch.
co+?tm- %t%hof by weight.
SWkW2. Ele&.rolyta. tension at
interface.* Specific gravity.
RddiV0 vmmsity.
SurfSee tension.
fvnea per cm. dytwa pa‘ cm.
None. 1.005 72.20 34.03
HCI 1 JO79 1.044 72 29 34.24
NsI 1 .uO525 1.016 72.73 34.86
NaOH 1.0210 1.149 72.74 35 .Ol NaHCO, 1.0270 1.129 72.67 34.95 NaBO, 1.0594 1.236 73 22 35.62 None. 0 9979 1.099 72.36 34.34
“ 0.9985 1 .Oll 72.26 34.23 “ 1.012 72.13 34.52
HCl 1.0089 1.088 72.23 34.28 NaI 1.0537 1.072 72.72 33.80 NaOH 1.0214 1.189 72.29 32.93 NaHCO, 1.0277 1 .I80 72.74 32 88 Na,SO, 1.0606 1.318 73 4.3 34.30 None. 1.0099 1.105 71.98 35 .lO
None. 0 9987 1.097 71.40 33.28
l Determinations made with 90 per cent benzene, ep. gr. 0.8745. t Values for starch, percentage concentrationa 0.005 and 0.5, taken from
curves. All others actual determinations. $ Corn-&arch uzed. All other determinations made with soluble starch.
pstm
0.005t
0.005 0.005 0.005 0.005 0.005 0.01 0.1 0.5 0.5 0.5 0 5 0.5 0.5 1 .o
0.009~
increase until with 10 per cent dextrin the interfacial tension is more than 2 dynes higher than in its absence. The change in state of dextrin at concentrations of 1. per cent or more un- doubtedly accounts for this speci.6~ action towards benzene--a phenomenon which is not apparent at the interface solution-vapor. Exactly the same thing is true with starch except that the increase begins at only 0.1 per cent. This seems to point very clearly to a
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similarity between dextrm and starch except with respect to de- gree of dispersion. All electrolytes in both cases produce a con- tinuous lowering with increase in concentration with the case of sodium hydroxide again notable. This shows a marked pep- tization by all ions, even the SO4 ion which under ordinary circum- stances has the greatest precipitating power for an emulsoid. Much of the effect upon surface tension, however, must be linked with the fact that both of these emulsifying agents are hydrous.
Dextrin and starch are not greatly different in viscosity. In both cases all electrolytes increase viscosity at small concentrations but the effect is linear while at higher concentrations solutions of the emulsoids alone increase rapidly and, as in the case of sugar, exhibit higher values than in the presence of the electrolytes. Two kinds of agglomeration must be distinguished. Agglomera- tion with increase in concentration into homogeneous larger parti- cles cannot account for an increase in viscosity for, as a matter of fact, such a process should result in a decrease of viscosity because of the decrease in surface and the amount of bound water. Agglom- eration may, however, be accomplished by increase in viscosity if there is enclosure of water between the masses, and not increase in the size of the particles by coalescence of smaller ones. This is what must happen, therefore, in the case of dextrin and starch, considered separately, and is an explanation of the results obtained by MeyelsO on the viscosity of starch solutions.
The results on both dextrin and starch, taken separately, con- firm the work of Harrison31 on the viscosity formula of Hatscheksa as applied to starch: zl = q ( 1 + Kf> where z1 is the viscosity of the emulsoid, T.I that of the pure water, f the ratio of the volume of suspended matter to the total volume, and K = 4.5. Following are results for dextrin at two concentrations:
Per cent.
1 10
f (observed).
0.012 0.24
/ (calculatsd)
0.015 0.25
so Meyer, A., Kolloidchem. Beihefte, 1913-14, v, 1. 81 Harribon, W., J. Sot. Dyer8 and Colorists, 1911, xxvii, 84. ** Hatschek, E., 2. Chem. Ind. Kolloid, 1910, vii, 301.
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176 Emulsifying Agents
Viscosity does not therefore seem to depend upon the degree of dispersion as affected by change in concentration since the same formula holds true for starch; depending only on the volume of the dispersed phase. It should be noted, however, that the viscosity curve for dextrin really lies slightly above that of starch and its deviation from a linear ratio is also greater. This rather beauti- fully parallels the work of Ode+ on sulfur sols where the viscosity of a dispersion of particles of 0.01 p diameter lay above that of sols of the same substance consisting of particles of 0.1~. This then is the genuine case of agglomeration into larger homogeneous particles by coalescence setting free because of the smaller total surface some of the previously bound water and seems to point very clearly to the fact that dextrin is of the same chemical nature as starch but smaller homogeneous particles, thus confirming the view of Herzfeld and KlingeP for the first time experimentally. If, therefore, viscosity is the predominating effect upon the action of dextrin and starch as emulsifying agents, it would be expected that dextrin would be better than starch, that in dilute solutions the presence of electrolytes should be more favorable than dextrin alone, and in concentrated solutions less favorable. Such is found to be the case when Table I is examined. With viscosity, however, must be compared the interfacial tension relationships. From this point of view dextrin in the presence of sodium hydroxide which produces by adsorption of the hydroxyl ions the greatest diminution in interfacial tension should give the best emulsions, even though the viscosity is not so favorable as with dextrin alone. It is a singular fact entirely in keeping with the view here pre- sented that 10 per cent dextrin alone with maximum viscosity produces the heaviest cream in every emulsion, but the most stubk emulsion measured by time of existence is produced by 5 per cent dextrin with 0.5 M sodium hydroxide in which there is the maximum effect in reducing interfacial tension. In the case, therefore, of these emulsoids viscosity is not so important a factor as in the case of sugar. It would be interesting to discover what effect the two types of viscosity exhibited by a molecular dispersoid such as sugar and an emulsoid respectively, will have on emulsifying properties. Sugar has a true internal friction which resists some molecular motion while dextrin, starch, gelatin, albumin, etc., in solutions
** Oden, 2. physik. Chem., 1912, lxxx, 799.
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G. L. Clark and W. A. Mann 177
of equal viscosity offer no hindrance. This is related, of course, to the structure of the gel-forming substances in solution whether fibrile, net-like, or granular through the interstices of which ions or crystalloid molecules may pass unhindered, and which exhibit viscosity by virtue of deformation by the force required to drag through the capillary of a viscosimeter. It is reasonable to expect, however, that toward colloid particles the heterogeneous structure of such substances even in solution may offer resistance which is not possible towards ions or crystalloid molecules. In this case it is clear that a more effective prevention of coalescence is at- attained by actual film formation around each droplet than by simple hindrance of motion due to viscosity of the medium.
Gum Arabic.
Gum arabic in many ways is very similar to dextrin or starch in its relative di%lculty of peptization by water, and gelatinous nature, but in general its effect at corresponding dilutions upon surface energy relationships is larger. The surface tension values are shown in Table VII. After a slight increase as observed pre- viously there is a continuous decrease in surface tension with in- crease in concentration. These results are quite contrary to the findings of Quinckes4 and ZlobickP who state that gum arabic increases the surface tension of water. Unlike dextrin, however, all of the electrolytes give lower values, with hydrochloric acid most effective in this respect as contrasted with the alkali for dextrin and starch. As a matter of fact adsorption of the hydroxyl ion causes precipitation of the colloid in 10 per cent solution. At the interface solution-benzene exactly the same effects are observed although sodium hydroxide is found to have far the greatest lower- ing effect in solutions up to 0.5 per cent after which the acid is most effective. In all cases the lowering is as much as 10 dynes or more from the value water-benzene in solutions from 5 to 10 per cent, a considerably greater effect than for dextrin or starch.
The viscosity shows a relation reverse to that of dextrin in that all electrolytes even in dilute solutions lower it, very markedly in .
84 Quincke, Wied. Ann., 1888, xxxv, 582. Drude’s Ann. Phys., 1901, vii, series 4, 631; 1902, ix, 969; 1903, x, 507; 1904, xv, 1.
u Zlobicki, L., Bull. Acad. SC. Cracovie, 1906,488.
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178 Emulsifying Agents
the more concentrated solutions. Here again in the emulsions both viscosity and interfacial tension lowering produce favorable emulsifying conditions. Thus while a 10 per cent solution with the highest viscosity produces emulsions equally good for benzene and kerosene, equally good emulsions are obtained in 0.5 per cent
TABLE VII.
Gum Arabic.
C0nWl- tmtion of urn ambic
t y weight.
O.Oli 0.1 0.5 0.5 0.5 0.5 0.5 0.5 1 .o 5.0 5.0 5.0 5 .OJ 5.0 5.0
10.0
Surface Electrolyte. tension at
interface:
Relative viwxeity.
Surface tension.
None. “
“
HCl NaI NaOH NaHCOs NagSO None.
“
HCI NaI NaOH NaHCO, NaBOt None.
dpra per em. dynes per Evn.
0.9985 1.021 72.26 34.47 0.9996 1.115 72.30 34.27
1.380 72.18 33 35 1 JO87 1.114 68.75 31.40 1.0534 1.107 72.24 31.82 1.0220 1 .lQO 70 38 27.10 1 xl276 1.208 72.03 31.42 1.0604 1.343 73.18 31.47 1.0017 1.703 72.04 32.68
1.370 69.69 28.28 1.0251 2.101 61.98 20.21 1.0705 2.479 63.28 23.52 1.0434 2.624 65.41 24.50 1.0438 2.604 65.56 23.51 1.0778 2.887 66.29 25.05 1.0362 8.781 61.49 22.87
* Determinations made with 90 per cent benzene, sp. gr. 0.8745. t Values for gum arabic, percentage concentrations 0.5 and 5.0, taken
from curves. All others actual determinations. $ Heavy precipitate formed. Determinations made with solution
taken from top.
solution in the presence of sodium hydroxide or iodide in the case of benzene, and sodium sulfate and bicarbonate in the case of kerosene. This is another clear indication of the fact that where viscosity is chiefly operative as a favorable factor in emulsification it is largely independent of the nature and the quantity of the oil being emulszed; while on the other hand adsorption to inter- faces is determined by the nature of the oil, the emulsifying agent, and the relative quantities particularly when electrolytes are pres- ent. Thus sodium hydroxide and iodide enable largest adsorp-
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G. L. Clark and W. A. Mann 179
tion of the gum arabic at benzene-water interfaces, and sodium sulfate and bicarbonate at kerosene-water interfaces.
Egg Albumin.
This substance was chosen for purposes of comparison inasmuch as viscosity effects are ahnost negligible. Furthermore, its ten-
TARLR VIII.
Relative vidty.
concen- tmtion of surfaoe SurfaCe
% albumi Electrolyte. tension. tension at
y weight. intefme.*
dune8 per cm. dvnsr per em.
0.01t None. 0.9980 1.005 72.28 33.90 0.05 u 1.014 70.30 28.72 0.05$ HCl 1.0078 1.060 65.03 23.58 0.05 NaI 1.0532 1.022 66.88 23.02 ‘0.05 NaOH 1.0206 1.118 57.61 15.96 0.05 NaHCO, 1.0262 1.124 66.43 26.11 0.05 NaaSO. 1.0595 1.248 61.35 24.81 0.1 None. 0.9996 1.024 68.99 26.38 0.409 I‘ 1.05$ 66’. 27 20.39 0.409 HCl 1.0076 1.373 62.82 20.24 0.409 NaI 1.0537 1.057 69.64 19.66 0.409 NaOH 1.0214 1.169 52.62 12.03 0.409 NaHCOa 1.0276 1.147 58.20 20.58 0.409 N&30, 1.0610 1.274 60.76 21.20 0.415 None. 0.9994 1.056 66.30 20.15 0.4150 “ 1.0000 1.056 65.09 25.49 0.918 “ 1.0012 1.057 61.67 21.62
* Determinations made with 90 per cent benzene, zp. gr. 0.8745. t Values for egg albumin, percentage concentrations 0.05 and 0.409,
taken from curves. All others actual determinations. $ Heavy precipitate formed in both cases with HCI. Determinatione
made with solution taken from top: 0 Determinations made with fresh solution. All other determinations
with egg albumin were made with solutions 2 weeks old. In the latter caze albumin settled out of solution which was filtered off without redeter- mining the percentage strength of the solution.
!bztmin.
dency to form films by adsorption at liquid interfaces may be easily seen with the eye if water is dropped into benzene and not shaken. The surface tension of egg albumin solutions has been the subject of considerable controversy. Quinckes” and ZlobickP and others state that it diminishes the tension at water-vapor surfaces,
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180 Emulsifying Agents
while Iscovesco36 found that it increases the surface tension. As a matter of fact it does both-increasing in the most dilute solu- tions and decreasing in the more concentrated. The surface tension values shown in Table VIII are reminiscent of gum arabic in that all electrolytes lower, with sodium hydroxide most effective. Hydrochloric acid, of course, is adsorbed and causes precipitation. The same relations hold true at the solution-benzene interface. Thus 0.5 M sodium hydroxide in 0.918 per cent egg albumin lowers the interfacial tension by more than 22 dynes, considerably the greatest effect observed in any of the emulsifying agents. Chemi- cal action such as the formation of sodium albuminate and hydro- lytic cleavage account for this.37
The viscosity of albumin emulsoids has been the subject of numerous investigations.38 Hnrdy3g and Bottazzi40 have shown that viscosity is dependent upon protein ions, is at a minimum when these are absent, and that addition of acids and bases should increase viscosity. It is clearly evident from Table VIII that in these experiments variations in viscosity are not important. Ef- ficiency as an emulsifying agent must,, therefore, be related here to adsorption and film formation and it would be expected that the best results would be obtained in the presence of sodium hydrox- ide which lowers the interfacial tension to the greatest extent. This is found to be true. Excellent emulsions were obtained with every solution of egg albumin, all by far better than any with the preceding four emulsifying agents, of these the most stable had present sodium hydroxide. This is entirely in keeping with the finding of Zsigmondvz” who gives the following gold numbers for proteins contained in egg albumin:
Globulin ................................................ Ovomucoid .............................................. Crystallized egg albumin ................................ Other con-albumin ...................................... Alkali albuminate .......................................
Gold number.
0.02 -0.05
0.04 -0.08 2.0 -8.0 0.03 -0.05 0.006-0.04
36 Iscovesco, H., Compt. Tel&d. Sot. biol., 1910, lsix, G22; 1911, lxx, 11, 66. 37 Handovsky, I~., Biochent. %., 1910, 510. xxv, 38 Cf. Ostwald,26 156. p. 39 Hardy, W. B., J. Physiol., 19OSO6, xxxiii, 251. 4o Bottazzi, F., AIli. R. Accad. Lincei, 1913, sxii, 141, 263.
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G. L. Clark and W. A. Mann 181
SUMMARY.
1. The purpose of the researches described in this paper has been to determine by quantitative measurement what the attributes essential for an emulsifying agent or colloid protector are, and what kind of subst.ance is best in this respect among non-elec- trolytes.
2. Quantitative measurements are made by the most accurate methods known of surface tension, interfacial tension, and vis- cosity of solutions of sugars, dextrin, starch, gum arabic, and egg albumin at all concentrations, both alone and in the presence of each of five electrolytes: hydrochloric acid, sodium hydroxide, iodide sulfate, and bicarbonate. To enable a practical test of theoretical principles a study of actual emulsions is also made with each of the above five substances alone and in the presence of each of the five electrolytes.
3. There are two factors of predominating importance in con- sidering sugar as an emulsifying agent, i.e. viscosity and ability to lower the interfacial tension, with viscosity of primary impor- tance. The chemical nature of the oil used in making emulsions will determine to some extent the efficiency of sugar as an emulsi- fying agent while the chemical nature of the liquids being emulsi- fied is also important.
4. The inversion of sucrose, measured for the first time by sur- face tension methods, is accompanied by a slight increase in surface tension. Experimental data shown herewith indicate that invert sugar is no more efficient than is sucrose in holding up sus- pensoids or emulsoids.
5. For the first time experimental confirmation is given of the fact that dextrin is of the same chemical nature as starch but smaller homogeneous particles.
6. Dextrin is found to be a better emulsifying agent than starch. Viscosity and lowering of interfacial tension is here again of pre- dominating importance in producing the best emulsions, with, however, viscosity of only secondary importance.
7. Two types of viscosity from the standpoint of emulsification are discussed: that of a molecular dispersoid which has a true in- ternal friction resisting some molecular motion and that of an cmulsoid which offers no hindrance.
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182 Emulsifying Agents
8. Lowering of interfacial tension and viscosity produce favor- able emulsifying conditions for solutions of gum arabic.
9. Efficiency as an emulsifying agent for egg albumin is related to adsorption of film formation. Better emulsions were obtained with egg albumin than with any one of the four other substances under consideration.
10. No one general rule can be made as to the effect which pro- duces the best emulsions for any one substance nor can any one generalization be made for the effect which produces best emul- sions for all substances under all conditions. Those which seem to be of predominating importance are viscosity and film forma- tion, the latter effect being of primary importance since it was with egg albumin, where a film formation is quite marked, that the best emulsions were obtained among all five of the substances under consideration in this study.
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George L. Clark and William A. MannAGENTS
THEIR ACTION AS EMULSIFYING ALBUMIN, AND THE MECHANISM OF
STARCH, GUM ARABIC, AND EGG INTERFACES OF SUGARS, DEXTRIN,ADSORPTION IN SOLUTION AND AT
A QUANTITATIVE STUDY OF THE
1922, 52:157-182.J. Biol. Chem.
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