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Langmuir 2000, 16, 2248-2253
Interactions between Mica Surfaces across Triglyceride Solution Containing Phospholipid and Polyglycerol Polyricinoleate Andra Dedinaite* and Bruce Campbell Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden, and Kraft Food Technology Center, 801 Waukegan Road, Glenview, Illinois 60025 Received July 27, 1999. In Final Form: October 28, 1999 Results obtained by direct measurements of the forces acting between polar mica surfaces interacting across solutions of triolein containing phosphatidylethanolamine (PE), polyglycerol polyricinoleate (PGPR), and a PE/PGPR mixture are presented. It was shown that PE adsorbed on mica from anhydrous triolein and thus rendered the surface nonpolar. The change in ordering of the liquid triolein molecules induced by bringing two such surfaces together gives rise to a structural force with two force barriers. In contrast, the adsorption of PGPR from anhydrous triolein resulted in a steric force barrier with a range of 120 Å. It was also found that from the mixture of PE and PGPR in triolein both additives adsorbed as a complex on mica surfaces. The presence of these aggregates on the surfaces gave rise to a very long-range strong repulsive force. We discuss the implication of the measured forces to colloidal stability of particle dispersion in nonpolar media and compare the efficiency of additives as dispersion stabilizers. We also show that the presence of water has an effect on the adsorbed layer structures. When PE is used as a dispersing agent, water induces formation of aggregates, which would provide strong repulsive barriers between the particles. In contrast, when PGPR is used as additive, water preferentially adsorbs on the polar mica surface and at water saturation gives rise to a capillary formation around the surface contact position. Finally, when a mixture of PE and PGPR is used, water is found to result in formation of a viscous, sticky, adsorbed layer that would flocculate the particles.
Introduction Forces between surfaces interacting across media of low polarity, containing various additives, are of great interest when aimed at understanding the behavior of food colloids. Many additives adsorb onto particle surfaces, thereby modifying the surface properties and changing the interactions between the dispersed particles. A better knowledge about the forces generated by the adsorbed additives would be beneficial for understanding and controlling the behavior of particle dispersions in media of low polarity. Commonly used additives in the food industry are phospholipids and polymers. Phospholipids (e.g. soybean lecithin) have been widely accepted due to their natural origin. A polymer representative, polyglycerol polyricinoleate (PGPR) (see Figure 1) as an additive in food has been accepted after many years of toxicity tests. It has been proved that PGPR is of no hazard to human health, and currently in Europe PGPR is used alone as a lecithin substitute or mixed with lecithin.1 Very little is known about the forces generated by adsorption of PGPR and PGPR-phospholipid mixtures on particle surfaces. The only study up to now is by Wells who, using a surface force apparatus, investigated interactions between polar mica surfaces coated with PGPR and immersed in the nonpolar solvents toluene and cyclohexane.2 He found that PGPR generates a steric force barrier that would prevent particle flocculation. Toluene was shown to be a better solvent for PGPR, and in this liquid the steric barrier generated by PGPR was extending further into the bulk compared to in cyclohexane. These (1) Wilson, R.; Van Schie, B. J.; Howes, D. Food Chem. Toxicol. 1998, 36, 711. (2) Wells, M. A. Emulsifiers in Chocolate; Wells, M. A., Ed.; SCI: London, 1998; p 73.
results provided some understanding about how PGPR governs forces between polar surfaces immersed in nonpolar medium. As shown in Wells’ work, the nature of the solvent does play a role, and therefore it remains interesting to investigate surface interactions in a more complex nonpolar liquid, such as food oil. For our study we chose triolein as a suitable example of a triglyceride. Triolein also offers the additional convenience of being liquid at room temperature. In this article we investigate forces generated by PGPR adsorbed on polar mica surfaces. We show how these forces are influenced by water activity in mixtures with L-R-phosphatidylethanolamine (PE). Experimental Section Surface Force Measurements. The forces acting between msucovite mica surfaces (Reliance, New York) in triolein containing 200 ppm PGPR, 200 ppm PE, and PGPR and PE mixtures in anhydrous oil at different water activities were measured employing the interferometric surface force apparatus (SFA)3 Mark IV.4 The details concerning our experimental setup can be found in previous publications.5,6 Results from the measurements are plotted as force, F, normalized by the undeformed geometric mean radius of the surfaces, R, as a function of surface separation. This allows direct comparison between results obtained using surfaces with different curvature. The experiments were carried out using a drop of 60 µL inserted between the mica surfaces. Various water activities (aw) at room temperature (20 °C) were generated in the sample by equilibration with the vapor of (3) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (4) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135. (5) Claesson, P. M.; Dedinaite, A.; Bergenståhl, B.; Campbell, B.; Christenson, H. Langmuir 1997, 13, 1682. (6) Dedinaite, A.; Claesson, P. M.; Campbell, B.; Mays, H. Langmuir 1998, 14, 5546.
10.1021/la991018u CCC: $19.00 © 2000 American Chemical Society Published on Web 01/28/2000
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Figure 1. The principal structure of PGPR. PR is polyricinoleate. In PGPR, at least one of the groups marked R is PR while the rest are either hydrogens, fatty acid residues, or PR.1,14 saturated salt solution. KCNS (Fluka, puriss p.a.) was used to obtain aw ) 0.47, ZnSO4‚7H2O (Merck, p.a. grade) was used to obtain aw ) 0.9, and pure water was employed to obtain aw ) 1. To obtain close to zero water activity, the atmosphere in the measuring chamber was dried with P2O5 powder (Merck, p.a. grade). Triolein was purchased from Nu-Chek-Prep, Inc. with purity >99%. Prior to use, the triolein was centrifuged for approximately 6 h at 300000g (Optima L-90K Ultracentrifuge, Beckman) in order to concentrate the particules present in the commercial triolein in the bottom part of the centrifuge tube. The upper portion of the centrifuged sample was used in further experiments. The polyglycerol polyricinoleate (PGPR) (trade name ADMUL WOL) obtained from Quest International, Naarden, The Netherlands, was dissolved to 10 000 ppm in triolein and purified by centrifugation, as described above, to remove dust particles. The principal formula of PGPR is shown in Figure 1. The PGPR preparation which we used was polydisperse. From size exclusion chromatographic data, it contained molecular species ranging in molecular weight from several hundred to over four thousand g/mol. The L-R-phosphatidylethanolamine (PE) powder from soybean, with purity greater than 99%, was purchased from Avanti Polar Lipids, Inc., and was used as received. The plant-derived product contains a mixture of saturated and unsaturated fatty acids with main components being (18:2) 47.3%, (16:0) 21.5%, (18:0) 8.8%, (18:3) 7.2%, and (18:1) 5.7%.7 This PE mixture was in our previous article6 erroneously referred to as L-R-oleoylpalmitoylphosphatidylethanolamine.
Figure 2. Force normalized by radius as a function of surface separation between mica surfaces interacting across anhydrous triolein containing 200 ppm PE. The dashed line marks the thickness of a PE bilayer (one layer on each surface). Two clear force barriers are observed. The surfaces jump into the region of the inner force barrier from a distance of ≈100 Å (upper arrow). The lower arrows show outward jumps occurring during separation from the positions of the inner and the outer force barriers. Solid squares represent the force curves measured upon approach; open squares show the force curves measured upon separation.
Results Forces across PE Solutions in Triolein. Interactions between polar mica surfaces across a 200 ppm dispersion of PE in anhydrous triolein and in triolein saturated with water have been investigated. The forces are significantly different from those measured between mica across pure triolein.5 The resulting force curve across the anhydrous PE dispersion displayed two clear force barriers (Figure 2). An outer rather weak force barrier was located at a separation of 100-120 Å with a magnitude of 0.8 mN/m. The inner force barrier was located at a separation of 7075 Å. This force barrier could not be overcome even by applying high compressive forces. The pull-off force measured from the position of 100-120 Å was about -0.5 mN/m and from the region of 70-75 Å about -2 mN/m. The force curve obtained when the PE dispersion in triolein was saturated with water was significantly different. When the surfaces were brought together for the first time a long-range repulsion, extending to about 300 Å, was observed. The repulsive force increased slowly with decreasing separation, reaching 0.8 mN/m (Figure 3) at a separation of 53 Å. At this point, the repulsive force changed character and became very steep, nearly like a hard wall. The weak long-range repulsion was significantly reduced during subsequent measurements. Irrespective of the number of approaches, the hard-wall-type repulsion remained the same. In particular, it appeared at the same separation each time. The pull-off force measured when (7) Shaw, W. A. Private communication.
Figure 3. Force normalized by radius as a function of surface separation between mica surfaces interacting across triolein containing 200 ppm PE at a water activity of 1. The first measurement (squares) and the second measurement (circles) are shown. Forces measured upon approach and separation are presented by solid and open symbols, respectively. The arrow shows an outward jump occurring upon separation. The dashed line marks the thickness of an PE bilayer between the surfaces.
withdrawing the surfaces from a separation of 53 Å was -0.8 mN/m. Forces across PGPR Solutions in Triolein. The interactions between two mica surfaces across triolein containing 200 ppm PGPR have been measured in a range of water activities. The results are shown in Figure 4. As seen, PGPR adsorbed from triolein solution onto the mica surfaces and generated repulsive forces, which ranged to approximately 150 Å when the water activity was close to zero. This corresponds to 75 Å long loops and tails of the polymer extending into the bulk from each mica surface. The magnitude of the force increased rather slowly
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Figure 4. Force normalized by radius as a function of surface separation between mica surfaces interacting across triolein containing 200 ppm of PGPR at various water activities: aw ≈ (open circles), aw ) 0.47 (open triangles), aw ) 0.9 (solid diamonds), and aw ≈ 1 (solid circles). The arrow shows an inward jump occurring upon approach at aw ≈ 1.
Figure 5. Force normalized by radius as a function of surface separation between mica surfaces interacting across triolein containing 200 ppm PGPR at water activities of close to 0 (open circles) and 0.47 (open triangles). Two distinct force regimes are observed: the magnitude of a force increases slowly with decreasing surface separation until a separation of 60 Å is reached. From 60 Å force increases steeply.
with decreasing surface separation until the surfaces were pushed to a separation of 60 Å (see Figure 5). At 60 Å, the steric force increased very strongly. Such a force profile indicates that the adsorbed layer consists of an outer region that is dilute with respect to polymer segments and an inner dense region next to the surface. At a compression of 2 mN/m the adsorbed PGPR layers had become so compact that the measured force had a hard-wall character. The compressed PGPR double-layer thickness was approximately 45 Å, corresponding to an approximately 22 Å thick polymer layer on each mica surface. The polymer layers were very strongly adsorbed onto the surfaces and it was not possible to squeeze them out even by applying compressive forces reaching over 20 mN/m. When the PGPR-coated surfaces were separated, it was found that an adhesion had developed. The magnitude of the pull-off force was about 1 mN/m. An increase in water activity to 0.47 and 0.9 hardly influenced the surface interaction. It is interesting to note, however, that when the water activity was 0.9 with the surfaces in contact for a prolonged period of time under a constant compression of 6.5 mN/m (see Figure 6), the distance changed with time. The change in separation was very limited for short times. However, after about 1000 s the surfaces rather quickly slid from a separation of 45 Å to a separation of 10 Å. We interpret this as being due to a slow condensation of water around the contact position that displaced most of the PGPR from the surfaces.
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Figure 6. Distance as a function of time. The two PGPR-coated surfaces are pressed together with a constant force of 6.5 mN/m at aw ) 0.9. Initially, the change in separation is very limited. After about 1000 s the surfaces rather quickly slides from a separation of 45 Å to a separation of 10 Å.
Figure 7. Force normalized by radius as a function of surface separation between mica surfaces interacting across triolein containing 200 ppm of PGPR and 200 ppm of PE at water activity close to 0. Solid squares represent the forces measured upon approach; open squares represent the forces measured on separation. The arrow shows an outward jump.
No similar time effects were observed at lower or higher water activities. At water saturation (Figure 4), there was no strong interaction between the surfaces until a separation of about 130 Å. From this distance the force was attractive, and the surfaces jumped into mica-mica contact. This force is, like the one previously observed between mica in water-saturated triolein without PGPR, 5 due to the formation of a water capillary. The pull-off force was 170 ( 20 mN/m. Forces across Mixtures of PE and PGPR in Triolein. A very long-range repulsion dominated the interaction between mica surfaces immersed in anhydrous triolein containing a mixture of 200 ppm PE and 200 ppm PGPR. The PE/PGPR layers on the mica surfaces were not homogeneous, as shown by the fact that the range of the repulsive force varied between different spots on the surfaces. One example of a force curve showing a repulsion with a range of ≈1500 Å is shown in Figure 7. However, in most cases the repulsion had a considerably longer range. We attribute this force to adsorption of large aggregates. The force observed upon separation was lower and a small adhesion of -1 mN/m was found. Raising the water activity in the sample in a stepwise manner permitted the investigation of the effect of water on the interfacial properties of the mixtures. The repulsive
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Figure 8. Force normalized by radius as a function of surface separation between mica surfaces interacting across triolein containing 200 ppm PGPR and 200 ppm PE at saturation with water. Five consecutive force runs of increasing depth of approach are plotted: run to a separation of 400 Å and back (stars), to a separation of 217 Å and back (open circles), to a separation of 112 Å and back (solid diamonds), to a separation of 71 Å and back (open triangles), and to a separation of 24 Å and back (solid squares). The arrows pointing to the right show outward jumps. The dashed arrow indicates the attractive force that was needed to be reached for the surfaces to separate from a distance of 24 Å. Table 1 separation, Å
pull-off force, mN/m
separation, Å
pull-off force, mN/m
400 217 112
0.5 0.5 0.9
71 24
4.2 50
force remained very long-ranged at water activity of 0.47. However, at aw ) 0.9, the range of repulsion was significantly reduced compared to that at lower water activities. A further increase in water activity to aw ) 1 had a dramatic effect on the surface interaction. The forces measured across the PE/PGPR mixture in triolein at water saturation are shown in Figure 8. On approach, the surface interactions were dominated by repulsive forces from a separation of about 700 Å. However, as soon as the adsorbed layers had come into contact they stuck together. This is seen as an attractive force on separation. The absolute magnitude of the attraction was increasing with decreasing separations, as seen from Table 1. We note that the adsorbed layer was very viscous and equilibrium forces could not be measured. However, the general trends observed in Figure 8 and Table 1 were reproducible. Adsorbed layers with similar sticky and viscous properties were formed also when an initially hydrated PE/PGPR mixture was allowed to adsorb onto the mica surfaces (Figure 9). However, the adsorbed layers were significantly thinner. We note that the water activity effect was reversible, i.e., if the sample was redried the large aggregates that gave rise to very long-range repulsive forces were again adsorbed. Discussion When a polar mica surface is immersed into anhydrous triolein containing a dispersed phospholipid, PE, a spontaneous adsorption of phospholipid on the surface takes places. The phospholipid molecules assemble with their polar zwitterionic groups on the mica surface and with their hydrocarbon tails directed into the bulk liquid of low polarity. Hence, the mica surface becomes coated with a phospholipid monolayer and acquires a nonpolar character. We have previously argued6 that such surfaces interact in nonpolar media in the same way as two
Figure 9. Force normalized by radius as a function of surface separation. An initially hydrated PE/PGPR mixture was allowed to adsorb onto the mica surfaces. Three consecutive force runs of increasing depth of approach are plotted: run to a separation of 198 Å and back (solid triangles), to a separation of 116 Å and back (open circles), and to a separation of 45 Å and back (solid squares).
intrinsically hydrophobic particles. The triolein molecules are forced to order in structured layers in the confined space between the two hydrophobic surfaces, and due to a change in this order, when two surfaces come together, two clearly expressed force barriers are seen in the force curve (Figure 2). There are no polar interactions between triolein and the PE monolayer coated surface. Hence, no preferential triolein molecular conformation with the polar glyceryl part directed toward the surface and the three oleic acid residues directed to the bulk solution as observed at polar surfaces5 is expected. Instead, the periodicity of the force barriers, 20 Å, suggests a random orientation of the triolein molecules between the surfaces. The change in packing density with separation of triolein in random conformations gives rise to the oscillatory force. We note that the molecular solubility of PE in anhydrous triolein is very low, and the 200 ppm dispersion of this phospholipid is turbid. Nevertheless, no aggregates are attached to the PE-coated mica surface. This conclusion can be drawn from the fact that no long-range repulsive forces can be observed in the surface interaction profile. Phospholipids are hygroscopic and when phospholipid crystals are exposed to the open air, they absorb moisture. Similarly, lecithin preserves its hygroscopic properties even when dissolved in nonpolar media; such lecithin dispersions are hygroscopic.8 The same is true for dispersions of PE. The turbid anhydrous PE dispersion becomes clear after equilibration with water vapor. This change in solution appearance indicates that the presence of some water results in formation of smaller or, alternatively, different types of aggregates with a refractive index that closely matches that of triolein. In a recent work Mays et al.9 showed that a 200 ppm PE dispersion in triolein saturated with water contained isotropic aggregates with a radius in the range of several thousand angstroms. These aggregates were suggested to be reversed vesicles or possibly a dispersed reversed bicontinuous cubic phase. In Figure 3, which shows the forces between mica surfaces across the PE dispersion in triolein at saturation with water, a rather long-range repulsive force is seen on the first approach. However, once the surfaces have been in PE-PE contact, the subsequent approaches resulted in the near disappearance of repulsive forces. A possible explanation of the observation of the long-range repulsion (8) Elworthy, P. H. J. Chem. Soc. 1961, 5385. (9) Mays, H.; Almgren, M.; Dedinaite, A.; Claesson, P. M. Langmuir Submitted.
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Figure 10. Force normalized by radius as a function of surface separation. The forces generated by adsorption of PE (solid squares) and PGPR (open circles) from anhydrous triolein are compared.
is adsorption of phospholipid aggregates or their fragments. These aggregates, being weakly attached, can be removed from the surface contact spot by mechanically pushing them into the bulk or along the surface when the surfaces are approached. The range of the repulsive force, 300 Å, is much less than the size of aggregates which were observed by the dynamic light scattering technique,9 about 6000 Å. This can be interpreted as being due to strong deformation or break down of the aggregates when they contact the surface. A break down of aggregates upon adsorption has been observed before, when aqueous phospholipid liposomes solutions were studied using the SFA. In that study it was concluded that the liposomes break down at the surface and the surfaces become covered by a disordered layer of phospholipids.10 Similarly, a study employing atomic force microscopy (AFM) showed11 that phosphatidylcholine (PC) liposomes in aqueous solutions containing MgCl2 adhere to mica surface to form bilayers. Liposomes of PE, on the other hand, were found to first form a complete bilayer, and subsequently, deformed PE liposomes adhered to the PE bilayer. We recognize that the aqueous liposome solutions in many respects are different from our phospholipid dispersion in nonpolar oil. Nevertheless, in both studies an inner compact layer is formed next to the surface. In the nonpolar oil we do not observe any formation of ordered phospholipid layers when the aggregates contact the already formed PE monolayer. The forces induced by adsorption of the polymeric additive, PGPR, on mica surfaces from a 200 ppm solution in triolein are shown in Figure 4. A purported structure of this additive is shown on a Figure 1. Clearly, the actual composition of this polymer can vary to quite a large extent. As seen from Figure 4, the steric repulsion starts at a surface separation of approximately 150 Å, and this corresponds to 75 Å long loops and tails of the polymer. Such loops and tails would provide a good steric barrier, preventing hydrophilic particles from flocculating in nonpolar media. Our measurements indicate that additives such as PGPR are more efficient in providing a strong repulsive force barrier between the particles than PE is. A comparison of the surface interaction profiles in anhydrous triolein containing PE and containing PGPR is made in Figure 10. As seen from the figure, the repulsive (10) Blomberg, E.; Claesson, P. M.; Wa¨rnheim, T. Colloids Surf. In press. (11) Egawa, H.; Furusawa, K. Langmuir 1999, 15, 1660.
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force starts at roughly the same distance with both additives. However, already at a distance of 120 Å the surfaces coated with PE experience a weak attractive minimum of 0.5 mN/m when separated. Further in, at a distance of 75 Å, one stronger repulsive force barrier exists. At this separation a strong adhesive minimum of 2 mN/m is measured. With PGPR, on the other hand, the repulsive force increases until a separation of about 50 Å is reached. At this position, an adhesive minimum (not shown on the graph), with a magnitude less than 1 mN/m, was sometimes observed, i.e., half or less of that measured for the PE-coated surfaces at the larger separation of 75 Å. It is worth noticing that the repulsive forces measured with PE are due to changes in the triolein structuring in confinement between the surfaces. Such forces are significant just outside atomically smooth surfaces, but they become smeared out in the presence of random surface irregularities. The steric repulsive force found between PGPR-coated surfaces would not be equally sensitive to surface roughness and thus PGPR would be a more efficient stabilizer for particles with rough surfaces. It is well-known that PGPR, due to the presence of the hydrophilic polyglycerol chains, has a large water binding capacity and PGPR is a powerful water-in-oil emulsifier.1 In this context, it is interesting to notice that introduction of water to the triolein PGPR mixture up to aw ) 0.9 has no measurable effect on surface interactions between mica surfaces coated with PGPR. On the other hand, at aw ) 0.9, when surfaces are left in PGPR-PGPR contact and pressed together with a constant force of 6.5 mN/m (Figure 6), they slowly drift into a separation of 10 Å. This demonstrates that due to water condensation around the contact position a partial expulsion of adsorbed PGPR from the surfaces takes place. At saturation with water, however, a water capillary forms around the surface position very readily and the long-range surface interaction is dominated by strong attractive force acting from a distance of ≈130 Å. Thus the stabilizing steric repulsive force is totally removed. In contrast, a stabilizing repulsive force exists between PE-coated surfaces also at saturation with water (compare Figures 4 and 3, respectively), and thus at high water activities PE ought to be a better dispersion stabilizer than PGPR. The force due to capillary formation around the contact position is to a first approximation given by12
F ) 4πγow cos θ R
(1)
where γow is the oil-water interfacial tension and θ is the contact angle for water on mica in triolein, which is close to zero.5 The pull-off force measured due to water capillary formation in PGPR containing triolein is -170 ( 20 mN/ m, corresponding to an interfacial tension of 13 mN/m. This value is much lower than that between pure triolein and water, 33 mN/m.13 In comparison, the pull-off force measured in pure triolein due to water capillary formation is much higher, -350 ( 50 mN/m,5 which is a result of the higher oil-water interfacial tension. The surface force data show that an anhydrous mixture of 200 ppm PGPR and 200 ppm PE in triolein generates very long-range and strong repulsive forces (Figure 7). No similar forces were observed with only PE or with only (12) Christenson, H. K. J. Colloid Interface Sci. 1988, 121, 170. (13) Johnson, M. C., R.; Saunders: L. Chem. Phys. Lipids 1972, 8, 112. (14) Adams, W. F.; Feuerstein, A.; Go¨litz, H.; Kielmeier, F.; Kroll, J.; Schraml, D.; Schuster, G.; Stenf, G.; Wagner, R. M. Emulgatoren fu¨ r Lebensmittel; Springer-Verlag: Berlin, 1985.
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PGPR present in triolein (Figures 2 and 4, respectively). One can interpret these observations as follows. The PGPR, being able to strongly adsorb on mica surfaces with the polar polyglycerol part of the molecule and on PE aggregates (the existence of solidlike PE aggregates is inferred from the fact that an anhydrous PE and PGPR mixture in triolein is turbid and the measured force curves are of different nature compared to when soft aggregates are attached to the surfaces at higher water activities) with the nonpolar polyricinoleate part, has the capacity of anchoring the PE crystals on mica surface. The strong and long-range repulsive forces are thus due to the fact that crystals attached on the two surfaces come into contact with each other. It is interesting to notice that after the surfaces are pushed together and separated, an adhesion of -1 mN/m is measured. The existence of this adhesion once again indicates interaction between PGPR and PE. Due to PGPR forming bridges between crystals attached to the two opposing surfaces, an adhesion is developed. Introduction of water to the system containing the mixture of PE and PGPR adsorbed on mica surface in triolein leads to a significant change in the properties of the adsorbed layer. At saturation with water (Figure 8), the crystalline PE aggregates have melted and the measured interactions were dominated by a steric repulsion ranging approximately 700 Å. The adsorbed layer was homogeneous and of roughly equal thickness all over the surface. The adsorbed layer was also highly viscous and sticky: as soon as the two layers came into contact, an adhesion was developed, as observed upon separation. The internal structure of these layers is not known, but it is clear that it contains PGPR, PE, and water, since no similar “gel-like” layers are observed with only two of the components present in the triolein. Conclusions Phosphatidylethanolamine (PE) adsorbs with the zwitterionic headgroup on the mica surface from anhydrous
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dispersion in triolein to form a monolayer, and this renders the surfaces nonpolar. Dispersed crystals of PE do not adsorb on such surfaces. Between nonpolar surfaces the ordering of the triolein molecules changes as the surfaces come together, and this gives rise to a structural force with two oscillations. Introduction of water into the mixture induces structural changes in the molecular arrangement: reversed phospholipid aggregates are formed. In contact with the surface these aggregates break and their fragments weakly adsorb on hydrocarbon surfaces, thus giving rise to a long-range repulsive force. Polyglycerol polyricinoleate (PGPR) adsorbs on mica surface from anhydrous solution in triolein and this gives rise to a steric repulsive force. The structure of the adsorbed layers remains unchanged until the water activity in the solution is raised to 0.9. At this water activity, slow water condensation and slow expulsion of the adsorbed polymer from the surface contact position takes place. At saturation with water, fast water condensation resulting in formation of a capillary condensate occurs, and as a result, PGPR is completely expelled from the contact position of the surfaces. This leads to a highly increased surface adhesion. In the anhydrous mixture of PE and PGPR in triolein, PGPR facilitates the adsorption of PE crystals on mica surfaces. When the surfaces are brought together, the crystals give rise to a strong very long-range repulsive force. Hydration of the mixture up to aw ) 0.9 does not lead to qualitative changes in the adsorbed layer, though the aggregate size is reduced. Saturation with water leads to formation of a viscous and sticky adsorbed layer containing PGPR, PE, and water. Acknowledgment. We gratefully acknowledge Per Claesson and Paul Smith for valuable discussions. Andra Dedinaite acknowledges financial support from Kraft Foods. LA991018U
