Pry.

Chrm. Fats orher Lipids. Vol. 16. pp. 163-169. Pergumon Press. 1918. Printed in Grcal Britain

.

STABILITY

OF EMULSIONS

FORMED

BY POLAR

LIPIDS

KARE LARSON Division

of Food

Science,

Chemical

Center,

University

of Lund,

S-220

07 Lund,

Sweden

CONTENTS I.

II. III.

INTRODUCTION

163

EXPERIMENTAL

163

RESULTS AND DISCUWON A. Surface film behavior B. Emulsion stability and surface film properties C. Interracial structure and emulsion stability

164 164 166 169

of some

mixed

systems

ACKNOWLEDGEMENT

169

REFERENCES

169 I. INTRODUCTION

Polar lipids, e.g. phospholipids, monoglycerides and long-chain esters of fruit acids, are important food additives. In these applications, they are usually classified as food emulsifiers although other effects than emulsification often are utilized, such as complex formation with proteins and starch. Emulsions based on polar lipids are also used in the topological administration of drugs. Knowledge of the structure of the lipid film at the oil/water interface is necessary in order to understand the physical properties of these emulsions, such as stability, rheology and diffusion of substances through the interfacial layer. The molecular arrangement of lipids at interfaces will be considered here and a procedure for identification of the structure corresponding to the highest emulsion stability will be demonstrated. Polar lipids used for formation of emulsions give aqueous phases with water which can be characterized by X-ray diffraction, and they form condensed monolayers at the air/water interface which can be studied by surface balance technique. Boyd and coworkers’ have studied the rheological properties of surface films, and on this basis, have been able to correlate increasing emulsion stability with increasing surface viscosity and elasticity. The occurrence of multilamellar liquid-crystalline particles above the limit of swelling of monoglycerides and related emulsifiers have been demonstrated,6 and the importance of such films with regard to emulsion stability has been pointed out by Friberg and Rydhag.3 In systems corresponding to food emulsions, however, there is often an interfacial film with crystalline hydrocarbon chains.” Such a film can be formed by lipid crystals, exposing a hydrophobic surface towards the oil phase and hydrophilic surface groups towards water. This possibility of formation of two alternative surface structures of lipid crystals (with dominating surfaces parallel to the bimolecular unit layers) means that the crystalline state exhibits emulsification properties, a feature which seems to have been neglected previously. The interfacial film can also be formed by lipid bilayers with crystalline hydrocarbon chains alternating with water layers, a structure characterizing the gel state.6 The hydrocarbon chains in the gel phase possess some degree of disorder, and such a multilamellar inter-facial film is therefore more flexible than that formed by true crystals. This type of structure seems to correspond to the highest emulsion stability as will be demonstrated below. II.

EXPERIMENTAL

A continuously recording surface balance of the vertical Wilhelmy type was used for the recordings of surface pressure us molecular area’ (IIA). The lipids used were 163

Kare Larsson

164

dissolved in hexane+thanol(9: 1) and spread by an “Aglo” microsyringe. The nomenclature of surface film phases according to Ha&ins4 was used. X-ray diffraction data were obtained as previously described.6 The food emulsifiers used were kindly supplied by Dr. Krog, AS Grindstedvaerket, Denmark, and the lecithin samples were prepared according to standard methods and checked by thin layer chromatography (TLC). The emulsions were prepared by mixing in a standard shaking equipment (gyrotoric type, New Brunswick) for 20 min. The emulsion stability was expressed as the time required for visible phase separation. The following emulsifiers were studied: L-a-dimyristoyland dipalmitoyl lecithin, distilled monoglycerides of fully hydrogenated lard (termed monoglycerides in the following text), tetraglycerol monostearate, sodium stearc161$8 oyl-Zlactylate, I-monolinolein, sorbitan monolaurate (SPAN 20), and polyoxyethylene (n = 20) sorbitan monooleate (TWEEN 80).

III. RESULTS

AND

DISCUSSION

A. Surface Film Behavior Food emulsifiers form three-dimensional aqueous phases with excess water of either the liquid-crystalline or the gel type.6 The relations between the molecular arrangements in surface films and in three-dimensional phases of such amphiphiles have not been considered earlier, taking our present knowledge on liquid-crystalline structures into account. These relations in the case of two synthetic lecithins, L-a-dimyristoyl lecithin (DML) and L-a-dipalmitoyl lecithin (DPL), will first be discussed. It should be mentioned that numerous surface film studies of lecithins have been published, but the structural relation between three-dimensional phases and monolayer phases have not been analyzed. An isotherm of DML recorded at 14.8”C is shown in Fig. 1. A liquid condensed phase of the Ll-type is formed at low pressure with a molecular area of about 58 A2 at highest condensation. At further compression, a condensed phase of the LZ-type is formed by a first-order phase transition and it has a collapse point corresponding to an area of about 46 A2 per molecule. The structural relations between surface film phases and three-dimensional phases were discussed in an earlier paper.’ The lamellar liquid-crystalline phase and the liquid-condensed phase show the same molecular dimensions and similar mobilities, whereas the molecular mobility and the cross-section of the phases L2 and LS indicate that they correspond to the gel-phases in three-dimensional systems. In the case of vertical molecules, the surface film is of the LS type existing at rather high pressures, whereas tilted molecules in same chain packing give surface film phases of the LZ-type. The high molecular mobility as well as the high compressibility of the tilted L2 phase can be explained by the hexagonal chain packing allowing rotational movement of the molecules. A confirmation of these proposed structural relations between surface film phases and three-dimensional aqueous phases is

60 50E ,” r =. 0

c

4030zoIO-r 66.5 a’/ M

T; 6lmM

Fig. 1. Pressure-area isotherm of dimyristoyl lecithin at 14.8”C.

Stability of emulsions formed by polar lipids

Multilayer

165

region

lr L2 0rLS

LI

7+ -es-

______

-------

_-_----

FIG. 2. Schematic illustration of the pressure-temperature phase diagram of a typical food emulsifier. The broken line indicates the lower pressure limit for formation of condensed monolayers.

given by the temperature ranges in which these phases exist. The DML sample used showed a transition gel to liquid-crystalline phase (crystalline z$ melted chains) at 22°C. The surface film isotherms recorded at intervals of 3°C showed that the highest temperature at which the L2 phase exist is in the range 20-23°C. The collapse behavior, with the film going over the edges of the surface balance trough, makes an exact determination impossible. The monolayer phase behavior of DPL has recently been described.2 Below the chain melting temperature at about 42°C there are two gel phases in aqueous systems, one with vertical chains (42-34”(Z), and the other with tilted chains (below 34”C).12 The same phases seem to occur in the monolayers-an L2 phase which can be observed up to about 34°C and an LS phase up to about 42°C. A pressure-temperature phase diagram characteristic for most food emulsifiers is shown schematically in Fig. 2. These surface film phase relations were observed in the case of the lecithins described above and in systems of other emulsifiers which will be considered further below. The equilibrium spreading pressures were also determined and were always found to be equal to the pressure at which the Ll phase and L2 or LS phase coexist, and above the critical temperature for existence of the L2 or LS phases the equilibrium spreading pressure is equal to the collapse pressure. One finding of the present work is that emulsions involving the food emulsifiers examined have an oil/water interface of liquid-crystalline type or of gel type depending upon whether the temperature is above or below the highest temperature at which the L2 or LS phase can exist.

FIG. 3. Pressure-area isotherm of C,&,s

monoglycerides at 18°C.

166

KAre Larsson 60

t

IO t

r 34.&!M

?24.0 it'/ M

FIG. 4. Pressure-area isotherm recorded at 11.8”C of monolinolein to the left and to the right of the same amount monolinolein mixed with hexadecane in molecular proportions I : 10.

The pressure-area isotherm of a Ci& s monoglyceride sample is shown in Fig. 3. As in ‘the lecithin samples, an Ll-phase is first formed, which at about 30 AZ per molecule is transformed into a phase which, according to molecular area (about 27 A2 per molecule at collapse), must have tilted molecules. The high compressibility shows that this is not an L2-phase but a true solid condensed phase. This can be correlated with the three-dimensional state, where the gel-phase formed at cooling is very unstable and is transformed into the P-crystalline form. Pressure-area isotherms of monolinolein alone and mixed with hexadecane are shown in Fig. 4. It can be seen that the paraffin molecules are solubilized in the Ll phase of monolinolein, and at higher pressure, they are successively squeezed out from the film so that the film consists of monolinolein only at the collapse point. Hexadecane and triolein were examined also in mixtures with the other food emulsifiers and they were always squeezed out from the L2, LS or solid-condensed phases showing that the phases which are more condensed than the Ll phase cannot accommodate the oil molecules in its close-packing. It should also be pointed out that monolinolein forms a cubic liquid-crystalline phase in excess of water7-a structure with no counterpart in a monomolecular surface film. Another difference between surface films and threedimensional phases is observed when the hydrocarbon chains are so short so that no condensed surface films are formed. 1-Monolaurin, for example, forms a lamellar liquid crystal with water above 43°C but no condensed monomolecular films can be obtained. By the addition of sodium chloride, however, the stability of the monolayer is increased, so that area-pressure isotherms can be recorded. ” It has also been observed that the transition temperature between the lamellar liquid crystal and crystals dispersed in water is increased by the addition of sodium chloride to the binary system l-monolaurinwater.’ B. Emulsion Stability and Surface Film Properties of some Mixed Systems

The emulsion stability of a triolein-water 1: 10 (w/w) emulsion with 1% (w/w) of an emulsifier mixture consisting of Ci &, s monoglycerides-TWEEN 80 is illustrated in Fig. 5. The surface film behavior of this monoglyceride mixture is shown in Fig. 4. TWEEN 80 does not form a condensed monolayer. It can be seen that the emulsion stability is much higher at room temperature than at 60°C and furthermore, the most stable emulsions are obtained at a weight ratio TWEEN/monoglycerides of about 1: 1. Microscopic examinations indicated the existence of an ordered phase at the oil/water interface, detectable by the occurrence of birefringence. The emulsions formed by the 1: 1 emulsifier mixture and those formed by TWEEN and the monoglycerides separately were centrifuged (2000g for half an hour), and the phase separating between the oil and water layers was examined by X-ray diffraction. It was found that the 1 :l mixture gives a gel-phase with a dominating short-spacing line at 4.2 A, whereas the emulsion with monoglycerides alone gives a P-crystal form (dominating short-spacing at 4.6 A)

Stability of emulsions formed by polar lipids

I IO

I

I

20

30

6VC I 50 60 70 60 90

1-1

40

%TWEENBO(w/w)in

167

C.&C,,

monoglycerides

FIG. 5. Emulsion stability according to the time for separation in a system consisting of 10% (w/w) triolein, 89% water and I % of an emulsifier C, .&I, s monoglycerides-TWEEN 80 in various proportions. The stability us the weight ratios of the two emulsifiers are shown at 60°C (upper curve) and at 25°C (lower curve).

and that with TWEEN alone gives a liquid-crystalline phase (liquid chains as evident from the 4.5 8, halo). The long-spacing of the gel-phase could not be seen on the X-ray photograph, indicating that it was larger than about 300& which was the resolution of the low-angle Luzzati camera used. The first order of such lamellar phases are usually very strong whereas higher orders are quite weak and sometimes missing. This indicates that the presence of the TWEEN molecules, although nonionic, results in swelling of the monoglycerides above the usual limit of about 20a in water layer thickness (c.f.

\ 30-

\

\

\

\

\\

‘L-

---------

20-

x’

x’

X-X-

f 7’ IO-

I I I I IO 20 30 40

I 50

I I 60 70

I 60

I 90

% (w/w) C,,/C,,monoglycerrdes In TWEENBO FIG. 6. Surface film behavior as evident from the average molecular areas at monolayer phase transition or collapse us composition for mixtures of C,,/C,s monoglycerides-TWEEN 80. The broken line shows the molecular areas of the liquid-condensed phase Ll at highest condensation, and the solid line curve shows the collapse of the solid-condensed phase. The shaded region corresponds to the existence of an L2 phase.

168

KAre Larsson

The surface film behavior of this binary emulsifier system is shown in Fig. 6. The characteristic molecular areas at phase transitions within the monolayer are given as a function of the composition. The isotherms correspond to the air/water interface, and it was checked that the presence of hexadecane in excess did not change the transition points. In the composition range corresponding to the highest emulsion stability, an L2 phase is formed according to compressibility and the high molecular mobility. It was not possible to obtain good dimensions for this phase at high amounts of TWEEN due to limitations in the compression range of our surface balance. There is obviously complete mutual solubility in the Ll-phase, whereas the solid-condensed monoglyceride phase can dissolve very small amounts of TWEEN. The surface film data and the X-ray diffraction suggest independently that the high emulsion stability in the case of the 1: 1 mixture is due to the occurrence of a gel-phase forming the oil/water interface.

% (w/w) monolinolein in sodturn steoroyl-24octylate FIG. 7. Emulsion stability and surface film behavior of binary emulsion mixtures between sodium-stearoyl-24actylate and 1-monolinolein examined as described in Figs. 5 and 6. The broken lines illustrate the emulsion stability at 25°C. whereas the upper solid line corresponds to the LI-phase and the lower one to the LZ-phase.

The critical amount of the 1: 1 emulsifier required to give a stable emulsion was also tested. A mixture of 20% (w/w) triolein in emulsifier/water was kept homogeneous during cooling from 65°C to 20°C. Only about 0.6% (w/w) emulsifier is needed in order to get a stable emulsion, and at 0.3% the emulsion is stable for a few hours. Much larger amounts of the emulsifier are needed in order to get a stable emulsion above the transition temperature liquid-crystal 2 gel. This is probably due to the lateral van der Waals interaction in the emulsion bilayers, which must be much higher due to the closer packing in the gel-phase compared to the liquid-crystalline phase. Another binary system of food emulsifiers is shown in Fig. 7. The surface film data indicate that there is almost no molecular solubility of monolinolein in the gel-phase of sodium stearoyl-24actylate, whereas there is complete mutual solubility in the Ll-phase. The emulsion stability could also be correlated with the existence of the gel-phase as in the system described above. An emulsion system consisting of tetraglycerol monostearate as one emulsifier and SPAN 20 as the other was also examined in the same way. SPAN 20 gives a lamellar liquid-crystalline phase in water and liposomes in excess of water just like the monoglycerides. The only surface film phase it forms is the Ll-phase. Emulsions with limited

Stability of emulsions formed by polar lipids

169

stability could only be obtained at small amounts of SPAN (up to about 2:8 in weight ratio). An L2 surface film phase was found also in this system in the case of the highest emulsion stability. Although there are indications of an interfacial film with the structure of the gel-phase in these emulsions, the emulsion stability is much smaller than observed in the system TWEEN 8O-C,6/Cls monoglycerides. This is believed to be related to the viscoelastic properties of the interfacial film. The gel-phase formed in the tetraglycerol monostearate-SPAN system was not found to give swelling above the water layer thickness of about 20 A, which is the usual limit in nonionic systems. The existence of a gel-phase with very thick water layers between the bilayer units should be expected to give more flexible interfacial films than those with low water content (cf. Ref. 9).

C. Interfacial

Structure and Emulsion Stability

As suggested in a previo& work” and confirmed by the results presented here, the state of the hydrocarbon chains is an important factor in emulsions. In the case of ordered chains, the hexagonal chain packing seems to give the highest emulsion stability according to the model systems examined here. One reason could be that it is disordered enough to allow deviations from a planar layer arrangement. If an interfacial film with the gel type of structure is compared with a film of the lamellar liquid-crystalline type, it is obvious that the molecular association in the plane of the film is stronger due to the crystalline close-packing in the bilayers, and the higher emulsion stability obtained by such films is certainly related to this. The liquid structure of the chains in the multilamellar liquid crystalline particles (or films) on the other hand can often solubilize a large proportion of nonpolar lipid molecules, like fats and oils, and this structure can therefore exhibit a higher emulsion capacity than those with crystalline chains. It is thus useful to know the phase transitions of the interfacial film, particularly the liquid crystal+ gel transition. A simple technique to obtain this information is to use the surface balance. Acknowledgment-This work was made possible by the inspiration Stina and Einar Stenhagen.

and support from the late Professors

REFERENCES 1. ANDERSSON,H. J. I., ST~LBERG~TENHAGEN, S. and STENHAGEN,E. The Suedberg, Almqvist-Wiksell, Stockholm (1944). 2. BOYD, J., PARKINSON,C. and SHERMAN,P. J. Coil. Interface Sci. 41, 359 (1972). 3. FRIBERG,S. and RYDHAG, L. Kolloid 2. Z. Polym. 244, 233 (1971). 4. HARKINS, W. D. The Physical Chemistry of Films, 2nd edn., Reinhold Publishing Corp., New York (1954). 5. KRCIG,N. and LARSSON,K. Chem Phys. Lipids 2, 129 (1968). 6. LARSX)N, K. Z. Phys. Gem. (Neue Folge) 56, 173 (1967). 7. LARSSON,K. Chem. Phys. Lipids 9, 181 (1972). 8. LARSSON,K. Surface and Colloid Science, Vol. 6, p. 261, Ed. E. MATUEMC, Wiley, New York (1973). 9. LARSSON,K. and KROG, N. Chem Phys. Lipids 10, 177 (1973). 10. LARSSON,K. Chem. Phys. Lipids 14, 233 (1975). 11. MERKER, D. R. and DALIBERT,B. F. J. Am. Chem. Sot. 80, 516 (1958). 12. RAND, R. P., CHAPMAN, D. and LARSSON,K. Biophys. J. 15, 1117 (1975).

Stability of emulsions formed by polar lipids.

Pry. Chrm. Fats orher Lipids. Vol. 16. pp. 163-169. Pergumon Press. 1918. Printed in Grcal Britain . STABILITY OF EMULSIONS FORMED BY POLAR LIP...
494KB Sizes 0 Downloads 0 Views