Liquid crystalline phases in emulsions

Liquid Crystalline Phases in Emulsions STIG F R I B E R G The Swedish Institute for Surface Chemistry, Drottning Kristinas vgg 45, 8-114 28 Stockholm, Sweden

Received December 21, 1971; accepted May 18, 1971 The earlier hypothesis of the presence of a liquid crystalline layer at the interface of a stable emulsion has been experimentally verified by means of visual observation in a microscope. It is shown that the emulsions reach the high level of stability, which characterizes emulsions containing liquid crystals, when the liquid crystals begin to be dispersed as separate aggregates besides their location at the interface. Calculations of the influence on van der Waal's interaction between the emulsified droplets show that the relative size of the Hamaker constant of the liquid crystalline layer is a more important factor for the stability than the thickness of the adsorbed layer.

INTRODUCTION

give a rational explanation to the problems The relations between the behavior of regarding changes of stability (7), viscosity emulsions and the properties of the interface (9) with emulsifier concentration, and also oil/water have been investigated by a great to the problem concerning the difference of emulsional behavior when different hydronumber of chemists (1, 2). In spite of this several questions have re- carbons are present (8). The present article will show that the dismained without an answer. Davies in 1964 (3) showed the difference in behavior of two persion of the liquid crystalline phase takes emulsions between which the only difference place around the droplets and also g simple was the hydrocarbon phase being benzene in calculation of the reduction of the dispersion one case and petrol ether in the other. The forces between the emulsion droplets due to emulsion containing benzene gave rise to a multilayer adsorption of the emulsifier. flocculation but no coalescence even after EXPERIMENTAL prolonged storage, contrary to the behavior Materials. The trioctanoin was >99 % of the emulsion with petrol ether showing extensive coalescence with almost no floc- pure from Eastman Kodak, and the egg culated drops at all. In the same manner lecithin was prepared according to a simpliSherman 1965 in a remark (4) brought the fled method (11). Distilled water was used. Emulsification. Mixtures of water, egg attention to the sudden changes in stability, viscosity, and even inversion taking place lecithin, and trioctanoin were dispersed in when the concentration of the emulsifier is each other and the emulsions centrifuged in a Martin Christ preparative ultracentrifuge altered. The knowledge of the existence of liquid at 20,000 rev/min for 5 hr. The three phases crystalline phases in connection with emul- were dispersed in each other anew and the sions is quite old (4-6) but the investigations emulsions stored at 20°C for several months at our institute (7-10) were obviously the to observe their stability. They were photofirst ones to give the relation between the graphed at intermediate times to record the emulsion properties and equilibrium condi- separation process. After a few months they tion in phase diagrams. The results could were once again centrifuged to show the Journal of Colloid and Interface ,Science, Vol. 37, No. 2, October 1971

291

292

STIG FRIBERG

FIG. 1. Emulsions after different times of storage. Treatment 2 X 1 min in an ultrasonic Son Blaster at 20°C. presence of the liquid crystalline phase. Part of the emulsion was observed in a microscope equipped with polarizers to visually observe the liquid crystalline phases. RESULTS

Emulsion Stability. Figure 1 shows the photographs of the emulsions, when stored at 20°C without any agitation after the

emulsification process. The emulsion without emulsifier has separated completely after a few months. In the emulsions with 1 and 2 % emulsifier, a certain separation can be observed while the emulsions with higher contents of the emulsifier have about the same appearance as initially. Ultracentrifugal emulsification gave two liquid phases and one liquid crystalline phase of a lamellar structure in the emulsions, where lecithin had been added. Microphotographs. Figure 2 presents microphotographs of the emulsions immediately after preparation. In the emulsions prepared with 1 and 2 % of emulsifier a wide distribution of drop sizes is observed. No signs of optically anisotr0pie regions can be found. When the emulsifier concentration was increased to 5 % some of the droplets show a wrinkled surface when viewed in the normal light. These parts give rise to illuminated regions in the photograph, when polarized light is used in the microscope (Fig. 2C). The liquid crystalline phase is distributed at the interface between trioctanoin and water since all optically anisotropic parts are situated at the surface of the droplets. Some of the aggregates could consist entirely of liquid crystalline substance, but some of them show the optical anisotropy only in a layer at the interface and must be interpreted as an emulsified drop covered with a

FIG. 2. Microphotographs of emulsion containing lecithin. A, 2% emulsifier normal light; B, 5% emulsifier normal light; C, 5% emulsifier polarized light. Journal of Colloid and Interface Science, Vol. 37, No. 2, October 1971

LIQUID CRYSTALLINE PHASES IN EMULSIONS substance having a higher degree of order than a liquid. DISCUSSION The results are related to a phase diagram water/lecithin/trioetanoin which has been published previously (10). The lecithin is sparingly soluble in the two phases and consequently any addition of lecithin in excess of a fraction of a percent will give rise to the formation of a liquid crystalline phase. The liquid erystM is also observable as a separate layer in the ultraeentrifugation cell even after prolonged eentrifugation. Dispersion of a liquid clystalline phase in one solution by means of an ultrasonic device will give rise to occlusions of small droplets of the solution separated from the rest of the solution by multilayers (12). The solution where one liquid phase is occluded as small droplets in the same phase was named a dispersion by the original author. The present resuRs have to be interpreted as a similar phenomenon with two liquid phases present. The treatment with the ultrasonic device disperses the liquid crystalline phase in such thin layers around the droplets that they cannot be observed as anisotropic layers under polarized light in the microscope. The drop-size distribution is wide, the number of small drops is high and the presence of submieroseopieM droplets can not be excluded. However, counting and determining the size of the drops gives an area of 2.101° m2/gm dispersed substance when the density is taken as one. Two percent of the emulsifier will give a layer thickness of about 0.3 ~m if it is assumed that the liquid crystalline phase is evenly distributed. One should notice that the liquid erystMline phase can be removed from the droplets by means of ultraeentrifugation. The results also give an indication that the long-term stability is dependent on the dispersion method of the emulsifier. When aggregates of the liquid crystalline phases actually me observable the emulsion is con-

293

siderably more stable than is the ease with emulsions of lower concentrations of the emulsifier where no optically anisotropie regions are observable. This could be explained by the assumption of an extensive formation of too small droplets of the emulsifier to be observable in the microscope, which would reduce the thickness of the layer as calculated above considerably. The explanation could, however, also be that the thick layers would have an influence on the forces which govern the reduced coalescence rates of the emulsion. Regarding this we considered an estimation of the change of the van der Waal's attraction forces due to the adsorbed multilayers to be of value. Vold (13) treated the influence of adsorbed layers on the van der Waal's interaction between colloid particles. By summation of the interaction energies of the particles, the adsorbed layers and the "phantom" particles of the medium she gave the following expression for the total interaction energy: 1 2 V = ( A ~ I/2 -- As1/~)2Hs q- ( A s 1 / 2 - Ap~n)2He + 2 ( A ~ 1~ - A~ ~ ) (A~ ~/~ - A g ) H ~ ,

--

[1]

where A~ = Hamaker interaction constant; M denotes the medium; S denotes the sorbate; and P denotes the particle. The function H is determined by the geometry of the system according to H ( x , y) = Y x ~ ÷ xy ÷ x q_

Y x2-q-xyq-x x2 +

q-y

[2]

my q- x

q- 2lnx2.q_ x y q- x q- y ' where x = a/2R; y = R2/R1;

A = distance between particle surfaces; R1 < R2 = radii of the paltieles.

Journal of Colloid and Interface Science, Vol. 37, No. 2, October 1971

294

STIG FRIBERG

For particles at close distances lim H ( x , y) Y x-+0 x(1 4- y"

[3]

Using this, an estimation can be made of the reduction of the interaction energy of two emulsified drops when they are covered with a layer of a liquid crystal. The liquid crystal in the present system has a lamellar structure and the calculations should contain the summation of the interaction from subsequent layers consisting of double the hydrocarbon chains and double the polar parts plus the water which is associated with the polar groups. Such a detailed calculation will later be published in connection with a study over the specific stabilizing influence of different liquid structures. In the present calculation the liquid crystalline layer will be considered as a continuous medium with a ttamaker constant between those for the continuous medium and the droplets. The size of the two drops is assumed to be a, the distance between the surfaces is A, and the thickness of the adsorbed layer is ~. Applying Eqs. [1] and [3] gives

>1~

0.5

"5 "E

0

ul

~.

•

0 4

"

"

'

-05

-1D FIG. 3. l~elative change ((V1 -- V2)/V2) of v a n der Waal's i n t e r a c t i o n when emulsified droplets of radius 1 ~m are covered by a layer (~ A): V1 = i n t e r a c t i o n energy of covered drops; Ve = i n t e r action energy of i n i t i a l drops; A M = I t a m a k e r i n t e r a c t i o n c o n s t a n t for t h e s o l v e n t ; A s = Ham a k e r i n t e r a c t i o n c o n s t a n t for t h e liquid crystal; Ap = H a m a k e r i n t e r a c t i o n c o n s t a n t for t h e emulsified droplet; A s = K I . A M ; A F = K s . A M .

(A): (X) 10; (O) 40; (Y) 1000; (a) 5OO0; (A) 10,000.

Vt

F(A~2

al/2~2 a 4-

=-5i2L +

]

V2)/V~ is calculated as function of K1/K2. The constants are defined according to

a

_

+ 4- 2(A~ 2

As~/2) (A 1/2 4- A~/2)

--

a4-~ +

A s = KI"AM; Ap = K2"AM,

[4]

+

] '

for the van der Waals interaction energy for particles with adsorbed layers in the flocculated state. For uncovered particles at the same distance one has V2 = 1 / ~ 2 [ ( A ~ 2 - - / J - ~ / 2 ) 2 - ~ I

[5]

The calculation covers the case of droplets with a radius 1 t~m and with a distance between the surface of 5A. This distance is between the outer surfaces of the adsorbed layers for covered droplets. The ratio (V1 --

and K2 is put equal to 4, which is of the right order. Figure 3 shows the relative change of the energy of the emulsified droplets in the flocculated state when they became covered. For low values of K1/K2 the relative change of energy is always positive which means a reduced attraction between the particles. When K1 = }~.K2, A s = AM, and consequently the adsorbed layer gives exactly the same result as an increase of the distance between the surfaces which means that the energy will be reversibly proportional to the distance between the surfaces. Since this distance is small initially (5A) for the floe-

Journal of Colloid and Interface Science, Vol. 37, No. 2, October 1971

LIQUID CRYSTALLINE PHASES IN EMULSIONS eulated drops even a layer of only 10 A reduces the interaction energy to one fom'th of the initial value. Equation [3] is no longer valid for the two highest values of ~. A more refined calculation according to Eq. [2] gives, however, a small difference which is not noticable in the diagram. When K 1 = K2, A s = Ap, and the adsorbed layer causes the same change as an increase of the size of the droplets. The interaction energy is increased linearly to the radius according to [5]. The increase udll be small as long as the droplet radius is large compared to the thickness of the adsorbed layer. The overall effect of an adsorbed layer is dominated by the ratio K 1 / K > For small ratio a reduction of the interaction energy will always take place and the reduction will be more pronounced with the thickness of the adsorbed layer. Increasing K I / K 2 will lead to a reversa] of this; observable in the figure already when A s > A>~~/~. In general the van der Waal's interaction between droplets is high. Putting A = 5A; a = 104A, and (A~, I/2 - A~,I/;) ~ = 0.5 X 10-la erg gives V ~- 150 KT. In order to prevent flocculation the van der Waal's interaction should be reduced at, least 90 %. The figure gives evidence that this is impossible when A e > A ~ / 2 . A p ~/2. In general the figure gives evidence that the ratio of the interaction constants is more important than the thickness of the covering layer.

295

Regarding this it appears probable that other factors also are responsible for the protective action of liquid crystalline phases on emulsion. ACKNOWLEDGMENTS The author is grateful to Prof. J. Lyklema for suggesting the calculation of van der Waal's interaction. Mrs. L. Rydhag is gratefully acknowledged for excellent experimental assistance. REFERENCES 1. BEeriER, P., "Emulsions, Theory and Practice," Reinhold, New York (1966). 2. SHERMAN, P., Ed. "Emulsion Science," Academic Press, New York (1968). 3. DAVIES,J. T., Recent Progr. Surface Sci. 2,129 (1964). 4. BURT, g. W., or. Soe. Cosmet. Chem. 16, 465

(1965). 5. JAMES, R. T., AND GOLDENBERG, R. L., 11, 461 (1960). 6. SALISBURY, R., LEUALLEN, E. E., &ND CHA_VKIN, L. W., Or. Amer. Pharm. Ass. Sci. Ed. 43,

117 (1954). 7. FRIBERG, S., MANDELL,L., AND LARSSON,M., Or. Colloid Interface Sci. 29, 155 (1969). 8. FRIBERG,S., ANDWILTON, I., Amer. Soap Perf. 85, 27 (1970). 9. FRIBERG,S., AND SOLYOM, P., Kolloid-Z. Z. Polym. 236, 173 (1970). 10. FRIBEnG,S., AND MANDELL, L., J. Pharm. Sci. 59, 1001 (1970). 11. SINGLETON,W. S., G~2~Y,M. S., BROWN,M. L., AND WHITE, J. L., or. A m e r . Oil Chem. Soc.

42, 53 (1965). 12. LARSSON,K., Z. Phys. Chem. (Frankfurt am Main) 56, 173 (1967). 13. VoLI), M. J., J. Colloid Interface Sci. 16, 1 (1961).

Journal of Colloid and Interface Science, Vol. 37, No. 2, October 197i