Monoglyceride surface films: Stability and interlayer interactions

Monoglyceride Surface Films: Stability and Interlayer Interactions ISABELLE PEZRON, 1'2'* ERWOAN PEZRON, 3'* PER M. CLAESSON,*"~ AND BJORN A. BERGENSTAHL* *The Institute for Surface Chemistry, Box 5607, S-11486 Stockholm, Sweden; and "~The Department of Physical Chemistry, The Royal Institute of Technology, S-10044 Stockholm, Sweden Received September l i, 1990; accepted November 15, 1990 Interactions between surfaces coated with monolayers of monoglycerides immersed in water have been studied using the surface force apparatus. The monolayers were deposited onto hydrophobized mica surfaces by means of the Langmuir-Blodgett technique. The measured forces between monopalmitin monolayers in the gel state show the existence of an adhesive minimum at a separation of about 10 A; this attraction decreases when the temperature increases. At even shorter separations a strong repulsion dominates the interaction. The forces between monoolein monolayers deposited in the liquid expanded state are repulsive at all separations and the range of the hydration/steric force is 10-15 A. Deposited monolayers of monopalmitin in the gel state exhibit a good stability and remarkable rehealing properties. In contrast, deposited monolayers ofmonoolein are unstable and readily transform into different structures. © 1991AcademicPress,Inc. INTRODUCTION

Because of their importance in biology, interactions between layers of polar lipids such as phospholipids are the focus of much interest and recent studies have been devoted to the measurement of forces between such lipid model membranes using the osmotic stress technique of Rand, Parsegian, and co-workers ( 1, 2) or the surface force technique developed by Israelachvili and co-workers (3). In particular, for uncharged bilayers of lipids, e.g., zwitterionic phospholipids or neutral galactolipids (4), both types of investigations have pointed out the predominant role of strongly repulsive hydration forces when these hydrophilic surfaces approach each other to distances less than 30 A. The molecular origin of

1 To whom correspondence should be addressed. 2 Present address: Department G6nie Biologique, URA CNRS No. 858, Universit6 de Technologie de Compi~gne, BP 649, F-60200 Compi~gne, France. 3 Present address: Research Center TOTAL-CHIM1E, BP 22, F-60550 Verneuil-en-Halatte, France.

this short-range repulsive force is still debated: a new interpretation in terms ofa "steric protrusion force" has been proposed recently by Israelachvili and Wennerstrom (5). Monoglycerides are a different kind of polar lipid of biological relevance as intermediates in lipid digestion and metabolism. They also rank among the most commonly used emulsifiers in the food industry (6). The interactions between monoglyceride-coated emulsion droplets affect the stability of the emulsion toward coalescence and flocculation (7). Recent studies have been devoted to the measurement of short-range repulsive pressures in monoglyceride lamellar and gel phases (8, 9 ). However, in the context of food emulsions, direct studies of interactions between adsorbed layers of monoglycerides as a function of their separation should provide new and complementary insights concerning the influence of these neutral polar lipids on droplet interactions. Another motivation for this study was to learn more about interactions between polar but uncharged groups. Previously, the interactions between ethylene oxide (10), amine oxide

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0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. All rightsof reproductionin any formreserved

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groups ( 11 ), and phosphine oxide (12) have been investigated by adsorbing appropriate surfactants on hydrophobic surfaces and measuring the forces by the surface force technique. An overview of the different behavior of these nonionic surfactants will be published elsewhere (13). In this work, monolayers of monopalmitin or monoolein were deposited onto hydrophobic surfaces using the Langmuir-Blodgett procedure. The forces acting between deposited monoglyceride layers, in fluid state or in gel state, were measured at different temperatures using the surface force apparatus. This paper focuses on two points: (i) the difference in stability between monopalmitin and monoolein monolayers and (ii) the temperaturextependence of the force curves. EXPERIMENTAL

SECTION

Materials. Monopalmitin (1-monohexadecanoylglycerol CH2OHCHOHCHzOOC(CHz)I4CH3) and monoolein ( 1-mono(cis-9octadecenoyl) glycerol CHzOHCHOHCH20O C ( C H 2 ) v C H z C H ( C H 2 ) 7 C H 3 ) , both of purity larger than 99%, were obtained from NU CHEK, Elysian, Minnesota, and used as supplied. The spreading solvent used for deposition and surface balance experiments was a mixture of redistilled chloroform (from Merck) and ethanol (2%). Hydrophobic surfaces were prepared by deposition of a 1:1 mixture of eicosylamine and eicosanol (EA/ EO). Green muscovite mica was received from Mica Supplies Ltd, London. The water used for all experiments was purified by a modified MilliQ unit described previously ( 11 ). In addition, the water used in the surface force apparatus was deaerated for several hours. Surface balance experiments. All experiments were performed with a computerized Langmuir trough system developed by KSV Chemicals, Helsinki, Finland. The force measuring device is a Wilhelmy plate. The troughs are made of Teflon, the barrier of Delrin. A thermostated trough was used for surface Journal of Colloid and Interface Science, Vol. 144, No. 2, July 1991

ET AL.

pressure-area isotherms, area relaxation at constant surface pressure, and equilibrium spreading pressure determinations. The temperature was controlled to within _+0.2°C at 20°C and to within +_0.4°C at 40°C. Depositions were carried out at room temperature ( 21-22 °C ) using a second trough. Surface pressure-area isotherms were recorded at a barrier speed of 10 m m / m i n which corresponded to a compression rate of 2 ~2 per molecule per minute. The purity of the subphase was tested prior to each experiment by compressing at 10 m m / m i n the surface without any lipid. The surface was regarded as clean when the surface pressure rise upon compression was less than 0.15 m N / m . About 5 min was allowed for the spreading solvent to evaporate. For relaxation measurements, the monolayer was compressed at a rate of 2 A 2 / m o l e / m i n and then automatically mainmined at the desired surface pressure. The time dependence of the film area was then recorded. For equilibrium spreading pressure (ESP) determinations, monopalmitin crystals were sprinkled on a clean subsolution and left until a stable pressure value was attained. If no further increase in surface pressure was observed on addition of new crystals then the final surface pressure value (Zre) was taken as the ESP. Surface preparation. Molecularly smooth mica sheets were coated with pure silver on one side and glued (silvered side down) onto cylindrical silica discs (radius of curvature about 2 cm). The surfaces were prepared in two steps: (i) the mica surfaces were made hydrophobic by deposition of a monolayer of E A / E O and (ii) a monoglyceride layer was deposited on these bydrophobic surfaces, so that the polar monoglyceride head group is oriented toward the solution. Indeed, contrary to the case of phospholipids (4), it was not possible to form directly a stable monoglyceride bilayer on mica. For the first deposition the surface pressure was automatically kept constant at 30 mN / m while the surfaces were raised through the water/air interface. Under these conditions the transfer ratio of EA / EO from the air/water interface to the mica/air

FORCES

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MONOGLYCERIDE

interface was 1.0. The advancing contact angle of a water droplet on such hydrophobic surfaces is 109 ° . A layer of monoglyceride (monoolein or monopalmitin) molecules was deposited from pure water by lowering the hydrophobized mica surfaces into the Langmuir trough until complete immersion. Again the surface pressure was kept constant during the deposition and the area per lipid head group (oriented toward the solution) was calculated from the transfer ratio. The transfer ratio of monopalmitin onto E A / E O coated mica was found to be 1.0 at surface pressures of 20 and 30 r a N / m. The transfer ratio o f m o n o o l e i n was 0.9 at rr = 20 m N / m and 1.0 at rr = 30 m N / m . The monopalmitin monolayers deposited at rr = 20 m N / m were in a liquid condensed state (A = 22-23 A2) with a packing similar to that in the gel phase according to X-ray data (9). On the other hand, monoolein monolayers deposited at rr = 30 or 35 m N / m were in a fluid state (A = 32-35 A2) with a packing resembling that in the lameUar phase (9). These conditions were chosen because the lipid lamellar and gel states are the most relevant for the formation and stability of emulsions. The silica discs with their bilayer-coated surfaces were kept under water during the transfer to and the mounting in the surface force apparatus. Surface force measurements. Interactions between monoglyceride-coated surfaces immersed in water were determined using a surface force apparatus developed by Israelachvili and co-workers (14). This apparatus allows measurement of the force acting between two surfaces, m o u n t e d in a crossed cylinder configuration, as a function of their separation. The distance between the surfaces is controlled and measured (with an accuracy of 2 A) by an optical interference technique (15). The force (detection limit 10-7 N) is determined from the small deflection of a spring supporting one of the surfaces. According to the Derjaguin approximation ( 16, 17), the force Fc(d) is related to the free energy of interaction per unit area between flat surfaces G f ( d ) via

LAYERS

Fc(d)/R = 2 r r G f ( d ) ,

451 [1]

where R is the geometric mean radius of the surfaces and R > d. Force measurements were performed at different temperatures (from 18 to 40°C). A heating fan directed toward the surface force apparatus was used in order to control the temperature. The temperature in the measuring chamber was determined by a thermistor and found to change less than 1°C during determination of a force-distance curve. RESULTS

SURFACE FILMS OF MONOGLYCERIDES AT THE AIR/WATER INTERFACE For our interlayer force measurements, the quantitative control and preparation of surfaces by means of the Langmuir-Blodgett technique obviously requires a thorough knowledge of the monolayer film behavior of monopalmitin and monoolein at the air/water interface.

Surface Pressure-Area Isotherms of Monopalmitin and Monoolein Surface pressure (rr)-area per molecule (A) isotherms of monopalmitin (Cj6 saturated 1monoglyceride) and monoolein (Cls cis unsaturated 1-monoglyceride) have been recorded in the temperature interval 6 to 40°C (Figs. 1 and 2). Monopalmitin forms a fully condensed film at 8°C. At temperatures higher than 16°C, a liquid expanded (LE) state appears and extends with increasing temperature to larger areas per molecule. The 20°C isotherm shows that at low surface pressures monopalmitin is an expanded liquid; the limiting area per molecule is 70-75 ~2. A transition from the LE to the LC (liquid condensed) state occurs at a surface pressure of 4 m N / m . The extrapolated limiting area in the LC state is around 24.5 ~2. At ~- ~ 31 m N / m , a change in the slope of the rr-A curve indicates the transition from the LC to the SC (solid condensed) state. The film fractures in the SC state at ~rf ~ 53 Journal of Colloid and Interface Science, Vol. 144, No. 2, July 1991

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6O

50 E

40

ao ~o fo

"E lo o lo

20

30

40

50

mean molecu]ar area

60

B'0

70

[~moZec]

FIG. 1. Surface pressure-area isotherms of 1-monopalmitin on water at 8°C (@), 20°C (A), 25°C (m), 31°C ( 0 ) , and 40°C (O).

m N / m for a molecular area slightly lower than 20 A 2. The same basic features are present in all isotherms between 20 and 40°C. Monoolein monolayers are in the liquid expanded state under all conditions explored. A noticeable increase in surface pressure occurs at an area of 70-72 ~k 2 (at 20°C). The collapse of the film is observed at 7r ~ 40-42 m N / m for a molecular area about 30 A 2. Increasing the temperature causes a slight expansion of the monolayer.

Stability of Monopalmitin and Monoolein Films Experiments giving the time dependence of the monolayer area at a constant surface pres-

9°I

sure provide a simple way to study these instability phenomena (18). The equilibrium spreading pressure for monopalmitin increases with temperature, and this value is actually expected to rise until the melting temperature of the hydrated lipid bulk phase is reached. The equilibrium spreading pressure is 23 m N / m at a temperature of 20°C. Hence, monopalmitin spreads spontaneously from crystals to form a liquid condensed film with a mean area per lipid of about 22 A 2. Liquid condensed or expanded films obtained at surface pressures below 7re are thus thermodynamically stable. For 7r > ~re, the film is not in thermodynamic equilibrium and indeed the surface pressure of a monopalmitin monolayer overcompressed to the fracture quickly falls with time. However, a monopalmitin film kept at a constant ~r over the equilibrium spreading pressure (but below 7rr) remains in a metastable state for hours. Figure 3 shows that the area of a monopalmitin film kept at a constant surface pressure of 30 m N / m decreases very slowly with time. No collapse occurs and the shape of the area relaxation curve shows that the slow relaxation is due to a slight solubility of monopalmitin in water. Monoolein monolayers display clearly different behavior. The equilibrium spreading pressure (determined at 20 and 30°C) is higher: ~re ~ 44 m N / m . Accordingly, the compressed monoolein monolayer collapses in the liquid-expanded state at a surface pressure value (40-42 m N / m ) close to but slightly 1,00 "

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45

50

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65

70

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FIG. 2. Surface pressure-area isotherms of 1-monoolein on water at 6°C (A), 20°C (IlL and 39°C ( 0 ) . Journal of Colloid and Interface Science, Vol. 144, No. 2, July 1991

2'0 time

4'0

6'0

(ran)

FiG. 3. Time dependence, at T = 20°C, of the area of monolayers of monoolein (E]) and monopalmitin ( 0 ) kept at constant surface pressure ( r = 30 m N / m ) .

FORCES

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MONOGLYCERIDE

lower than 7re. The surface pressure remains constant after the monolayer collapse. At a constant surface pressure below ~-e,the monolayer is thermodynamically stable and the time dependence of the area does not exhibit the sigmoid shape characteristic of a nucleation and growth collapse process (cf. Fig. 3). Instead, dissolution of monoolein in water induces significant area losses.

mitin monolayers (one on each hydrophobic surface) can be compared with the bilayer thickness observed in the crystal (46 A) and in the gel phase (48 A at a water content of 5%) (9). The good correspondence between these values shows that the deposited monopalmitin monolayer remains stable in water and indeed has a structure close to a half-bilayer in the gel state, where the molecules are tilted. Upon separation of the monopalmitin layers from contact, an adhesive minimum is observed when the monoglyceride surfaces are 10 A apart (D = 55 A). The attractive force is consistent with a van der Waals attraction. Monopalmitin layers in the gel state have good mechanical strength: a high pressure is needed to induce fusion. Nevertheless, when the surfaces are pressed together strongly, it is possible to squeeze out the monopalmitin layer and reach contact between the hydrophobic monolayers. This contact is characterized by a much larger adhesion (>30 m N / m). After separation from hydrophobic contact the interactions between the surfaces were measured again to see the effect of this local disruption of the deposited monoglyceride layers. The resulting force-distance curve was very similar to that obtained before the layer was disrupted locally as shown in Fig. 5. Hence, monopalmitin monolayers in the gel state exhibit remarkable healing properties. Moreover, the monopalmitin monolayer was

INTERACTIONS BETWEENMONOGLYCERIDE LAYERS

Monopalrnitin Monolayers in the Liquid Condensed State Forces between monopalmitin monolayers. The forces between monopalmitin layers in water were measured as a function of the surface separation defined as the distance between hydrophobic monolayers. Figure 4a illustrates the forces at room temperature ( T = 20°C). At large surface separations (d > 100 A) a double-layer repulsion is observed, characterized by an exponential distance decay. When the surfaces are approaching each other, an attractive force caused the surfaces to jump inward from a separation of about 60 A to about 50 A. At shorter separations a strong repulsion dominates the interaction. The contact between monopalmitin layers is located at a separation of 45 >, from the hydrophobic contact. This thickness of the two monopal-

15

453

LAYERS

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FIG. 4. M e a s u r e d forces, in water, b e t w e e n surfaces o f m i c a c o a t e d with a h y d r o p h o b i c m o n o l a y e r a n d a m o n o l a y e r o f m o n o p a l m i t i n in the gel state; the solid circles c o r r e s p o n d to the i n w a r d r u n a n d the o p e n circles to the o u t w a r d run. F o r this figure, as for the following, the line serves as a guide for the eye; the d o t t e d part illustrates a region w h i c h is not accessible to force m e a s u r e m e n t s . ( a ) T = 2 0 ° C ; ( b ) T = 30°C. Journal of Colloid and Interface Science. Vol. 144, No. 2, July 1991

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m after 20 h (Fig. 6). This shows that monoolein molecules do desorb and that fusion between monoolein layers occurs more easily as the area per molecule, and consequently the F/R exposure of the hydrocarbon chains to water, (mN/m) increases. In case (ii) it was possible to reproducibly measure the interactions between apparently 0 well-defined monolayers ofmonoolein during ' 1 '5 J 0 5 10 20 25 the initial state of the experiment. Figure 7 D (nm) FIG. 5. Measured force for monopalmitin at 20°C; the shows the force-distance curve determined at solid circles correspondto a run previous to hydrophobic 20°C. At large distances there is again a recontact and the crosses to just after hydrophobiccontact pulsive double-layer interaction. The shortfor the same positionofthe cylinders. Onlyforcesmeasured range part of the force-distance curve contains on approach are included here. a weak m i n i m u m located in the repulsive regime at a separation of 50 #,, corresponding to a water layer thickness of 15-17 A. Hence, stable over at least 3 days even after different there is, due to the presence o f charges on the cycles of heating and cooling (the force curve surfaces, no net attraction between monoolein is reproducible). monolayers in the fluid state, which is in conThe temperature effect. The force-distance trast to the significant adhesion observed becurves mesured at 30 and 40°C show the same tween monopalmitin monolayers in the gel features as at 20°C. No significant change in state. At shorter separations the interaction the double-layer force was observed. The only becomes strongly repulsive due to hydration significant temperature effect appears to be and steric interactions. It is worth noting that that the adhesion force decreases and that the the thickness of two deposited monoolein position of the attractive m i n i m u m moves to monolayers in the fluid state determined from slightly larger separations with increasing surface force measurements ( 33 A) is in good temperature. Figure 4b shows a force-distance agreement with values obtained for bilayers in curve obtained at 30°C. At 40°C, the adhesion lamellar liquid crystals (9). The monolayer force was about 20% lower than at 30°C. When cooling back to 20°C, the adhesion 7 force recovers its former value. 6 "t - - q

52

Monoolein Layers in the Fluid State Experiments were performed under two different conditions: (i) two mica surfaces carrying deposited monoolein layers (deposition pressure 20 or 30 r a N / m ) were mounted in the surface force apparatus filled with pure water and (ii) monoolein-coated surfaces were mounted in the apparatus filled with water containing some monoolein crystals. In case (i) the deposited monoolein layer slowly desorbed with time, which resulted in a change in surface interaction. For instance, the force needed in order to induce fusion decreased from 6 m N / m after 2-3 h to 2 r a N / Journal of Colloid and Interface Science, Vol. 144, No. 2, July 1991

F/R (mN/m)

,

4J 32 1 0

!

J

10

20

3J0

40

50

D (nm)

FIG. 6. Measured forces, at T = 20°C, between surfaces of mica coated with a hydrophobic monolayer and a monolayer of monoolein in the fluid state deposited at ~r = 20 mN/m. The circles represent one of the first runs performed (solid circles, inward run; empty circles, outward run), the crosses a run performed 1 day later. The arrows show the jump to hydrophobiccontact.

FORCES BETWEEN MONOGLYCERIDE LAYERS 10 86F/R

(raN/m)

4

-

10

2f0 3~0 D (nm)

40

50

FIG. 7. Measured forces,at T - 20°C, in water presaturated with monoolein, between surfaces of mica coated with a hydrophobic monolayer and a monolayer of monoolein in the fluid state deposited at ~r = 35 mN/m. The solid circles correspond to the inward run and the open circles to the outward run. The crosses correspond to a run performed on a different position of the crossed cylinders.

structure was not rebuilt after the layers had been disturbed locally by forcing them into hydrophobic contact (Fig. 8). This is in sharp contrast with the rehealing properties of monopalmitin monolayers in the gel state. It appears that after disruption of the monolayer the monoolein molecules are aggregated in some other structure, presumably similar to the structures found in the cubic phase in equilibrium with excess water• The remaining monoolein molecules can also be squeezed out under a stronger pressure. It is noted that, when not perturbed by hydrophobic contact, the force curves obtained are reasonably reproducible (Fig. 7).

455

the first E A / E O layer or from ionic impurities in the second layer (most likely fatty acids). The water from the purification unit contains, according to conductivity measurements, about 5 X 10 -6 M monovalent electrolyte• Such a low electrolyte concentration gives rise to a very slow decaying double-layer force (the expected Debye length is 1300 A). The measured Debye length is considerably shorter (450-600 A), corresponding to a background monovalent electrolyte of about 4 X 10 -5 M. This is at least partly due to experimental difficulties in measuring weakly decaying longrange forces using as thin mica sheets as in these experiments. The surface charge density can be estimated from the magnitude of the interaction and the measured Debye length. Such calculations show that there is less than one charge per 7000 A 2 for monopalmitin and less than one charge for 9000 A2 for monoolein. THE ADHESIVE FORCE: TEMPERATURE DEPENDENCE The van der Waals attraction increases slightly with increasing temperature (due to the zero-frequency term which is proportional to kT). Thus the decrease in adhesion when the temperature increases in the monopalmitin system originates from a significant increase of the short-range steric and hydration repulsion between deposited monopalmitin 7

DISCUSSION The surface force experiments allowed us to determine all force-distance curves for hydrophobic surfaces coated with monopalmitin and monoolein.

5-

4F/R (mN/m)

3 -

21

THE LONG-RANGE INTERACTION Measurements performed in pure water reveal a long-range electrical double-layer interaction in both cases. The surface charge m a y originate from a mismatch between the mica lattice charge and charged amine groups in

0 10

20 30 O (rim)

40

50

FIG. 8. Measured force for monoolein at 20°C; the solid circlescorrespondto a run previousto hydrophobiccontact and the crosses to just after hydrophobic contact for the same position of the cylinders. Journal of Colloid and Interface Science, Vol. 144, No. 2. July 1991

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layers in the gel (LC) state. This is consistent with X-ray experiments performed with monopalmitin in the gel phase which show that the maximum swelling of the gel phase increases with temperature (9). T H E S H O R T - R A N G E INTERACTION

Different types of interactions may contribute to the short-range interactions observed in many amphiphilic systems: steric, hydration, steric-protrusion, and undulation forces (2, 5, 8, 19). The main feature observed for monoglyceride systems is the increase in the repulsion when the system goes from the gel state to the lamellar state, in agreement with osmotic pressure measurements published in a previous article (9). It can be explained qualitatively in different ways. According to the Israelachvili and Wennerstrom model (5), the repulsive force arises from thermal protrusion of individual surfactant molecules into the solution: in the gel state, where the hydrocarbon tails are in a crystalline state, the motion of the surfactant molecules is much more reduced than in the lamellar state and so will be the repulsive interaction. The increase in repulsion with increasing temperature for monopalmitin in the gel phase can also be rationalized by a change in the number of hydrogen bonds formed with water. In the crystal each hydroxyl oxygen participates in two hydrogen bonds (one interlayer bond and one intralayer bond (20)). In the structurally similar gel phase the interlayer hydrogen bonds have been disrupted, whereas most intralayer hydrogen bonds should remain intact. As the temperature increases, thermal motion will disrupt more and more intralayer hydrogen bonds, which results in an increased number of hydrogen bonds with water and consequently a more hydrophilic surface and a stronger hydration force. When the short-range interactions between different hydrophilic but neutral groups are compared, some differences and similarities emerge. At room temperature, the interactions Journal of Colloid and Interface Science, Vol. 144,No. 2, July 1991

between amine oxide layers, phosphine oxide, ethylene oxide layers, and monoglyceride layers are all characterized by an attractive minimum followed at shorter separations by a hydration and steric repulsion. However, the temperature dependence of the interaction is very different for these three polar nonionic groups. No temperature dependence is observed in the case of amine oxide (11). In contrast, a decrease in repulsion at elevated temperatures is observed in the cases of ethylene oxide and phosphine oxide (10, 12). This difference, which is consistent with the phase behavior, can be explained in terms of the interaction with water: the amine oxide group is a structure breaker whereas the ethylene oxide group is, according to the model of Kjellander (21), essentially a structure maker. We note that Karlstrrm (20) has proposed an alternative explanation for the temperature-dependent interaction between ethylene oxide chains based on conformational changes. STABILITY OF THE DEPOSITED LAYERS

Deposited monopalmitin layers in the gel state and deposited monoolein layers in the fluid state display very different behavior at interfaces. Monopalmitin monomolecular films are very stable and exhibit remarkable rehealing properties. Monoolein monolayers are unstable and in saturated solutions larger aggregates are formed on the surface. This can be explained by the fact that the gel state of monopalmitin can exist in equilibrium with excess water whereas the lamellar state of monoolein is unstable and converted to a cubic phase with increasing amounts of water. Force curves between surfaces coated by monolayers of monopalmitin in the liquid condensed state do present an adhesive minimum at about 10 A from contact, which indicates that emulsions stabilized by monoglycerides in the gel state may flocculate in a weak hydration minimum with an approximately 10-A thin (four to five water mole-

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cules) water film between the flocculated emulsion droplets. From a practical point of view, it is worth noting that an increased flocculation will destabilize dilute dispersions but will stabilize concentrated dispersions against settling. It is also clear that a few adsorbed polymers at the interface--polysaccharides or proteins--would prevent flocculafion in this weak minimum. The good mechanical properties of the film of monoglyceride in the gel state, as demonstrated by surface force experiments, are expected to provide an efficient barrier against coalescence. This statement is supported by the finding that emulsions coated with a monolayer in the gel state at the oil/ water interface can be much more stable than when a lamellar liquid crystalline monolayer is present at the interface (6, 22). CONCLUSION

Direct measurements of interactions between monoglyceride monolayers in water show the existence of strong short-range repulsive hydration and steric forces, for both monopalmitin in the gel state (range ~ 8-10 A) and monoolein in the fluid state (range

10-15 ]~). The attraction measured between monopalmitin monolayers in the gel state decreases when the temperature increases; for monoolein monolayers in the fluid state no net attraction was detected, and the slight adhesive minimum observed was in the repulsive regime. Hence, it appears that the interactions between monoglyceride films change with the state of the hydrocarbon chain. When going from the more condensed to the fluid state the attraction between monolayers decreases whereas the repulsion increases. Deposited monolayers of monopalmitin in the condensed state are stable and exhibit remarkable healing properties after a local disruption. In contrast, deposited monolayers of monoolein are readily transformed into less defined structures. This difference between monopalmitin and monoolein can be ratio-

nalized by considering which phases exist in equilibrium with excess water. ACKNOWLEDGMENTS E.P. and I.P. thank the CNRS, the "Minist~re des Affaires Etrang~res," the Swedish Institute, and the Association for Surface Chemistry Research for financial support. REFERENCES 1. Leneveu, D. M., Rand, R. P., and Parsegian, V. A., Nature (London) 259, 601 (1976). 2. Rand, R. P., and Parsegian, V. A., Biochim. Biophys. Acta 988, 351 (1989). 3. Marra, J., and Israelaehvili, J. N., Biochemistry 24, 4608 (1985). 4. Marra, J., J. Colloid Interface Sci. 107, 446 ( 1985 ). 5. Israelachvili, J. N., and Wennerstrom, H., Langmuir 6, 873 (1990). 6. Krog, N. J., Riisom, T. H., and Larsson, K., in "Encyclopedia of Emulsion Technology" (P. Becher Ed.), Vol. 2, Dekker, New York, 1985. 7. Bergensffthl, B. A., and Claesson, P. M., in "Food Emulsions," (K. Larsson and S. Friberg, Eds.), Dekker, New York, 1990. 8. Mclntosh, T. J., Magid, A. D., and Simon, S., Biophys. J. 55, 897 (1989). 9. Pezron, I., Pezron, E., Bergenst~l, B. A., and Claesson, P. M., J. Phys. Chem. 94, 8255 (1990). 10. Claesson, P. M., Kjellander, R., Stenius, P., and Christenson, H. K., 3. Chem. Soc. Faraday Trans. 1 82, 2735 (1986). 11. Herder, C. E., Claesson, P. M., and Herder, P. C., J. Chem. Soc. Faraday Trans. 1 85, 1933 (1989). 12. Herder, C. E., submitted for publication. 13. Claesson, P. M., Eriksson, J. C., Herder, C. E., Bergenst~,hl, B. A., Pezron, E., Pezron, I., and Stenius, P., Faraday Discuss., in press. 14. Israelachvili, J. N., and Adams, G. E., J. Chem. Soc. Faraday Trans. 1 74, 975 (1978). 15. Israelachvili, J. N., J. Colloid Interface Sci. 44, 259 (1973). 16. Derjaguin, B. V., Kolloid-Z. 69, 155 (1934). 17. lsraelachvili, J. N., "Intermolecular and Surface Forces." Academic Press, London, 1985. 18. Smith, R. D., and Berg, J. C., Z Colloidlnterface Sci. 74, 273 (1980). 19. Lyle, I. G., and Tiddy, G. J. T., Chem. Phys. Lett. 124, 432 (1986); Carvell, M., Hall, D. G., Lyle, I. G., and Tiddy, G. J. T., Faraday Discuss. Chem. Soc. 81, 223 (1986). 20. Karlstr6m, G., Z Phys. Chem. 89, 4962 (1985). 21. Kjellander, R., J. Chem. Soc. Faraday Trans. 1 78, 2025 (1982), 22. Larsson, K., Prog. Chem. Fats Lipids 16, 163 ( 1978 ).

Journal of Colloid and Interface

Science.VoL 144,No. 2, July 1991