Forces between Lipid-Coated Interfaces

Forces between Lipid-Coated Interfaces By Per M. Claesson LABORATORY FOR CHEMICAL SURFACE SCIENCE, DEPARTMENT OF CHEMISTRY, PHYSICAL CHEMISTRY, ROYAL INSTITUTE OF TECHNOLOGY, S-100 44 STOCKHOLM AND INSTITUTE FOR SURFACE CHEMISTRY, P.O. BOX 5607, S-114 86, STOCKHOLM, SWEDEN

1 Introduction The surfaces of food emulsion droplets are often coated with a complex mixture of lipids, proteins and carbohydrates.' The presence of these compounds generates repulsive forces that are essential for stabilizing the emulsion. There are several types of forces that may give rise to increased emulsion or dispersion stability.* Repulsive electrostatic double-layer forces, which essentially are entropic in origin, are present when the adsorbed species are net charged.* This is the case for some lipids, for most proteins, and for some polysaccharides. Large and flexible polymers anchored to the interface also give rise to long-range repulsive forces; these are known as steric forces. The molecular mechanism behind this repulsion is primarily an unfavourable reduction in entropy of the system due to an enhanced segment density and a reduced number of available polymer conformation^.^ A stabilizing repulsive force like this is observed when the surfaces are coated with polysa~charides~ or flexible proteins.5 The importance of interparticle forces for the properties of colloidal dispersions, biomembranes and biological cells has been realized for a long time. This has inspired the development of a range of techniques for studying how these interactions depend on surface separation and on the molecular nature of the interface. A recent review6 focuses on some of the force-measuring techniques available and their advantages and drawbacks. One of these techniques, the interferometric surface force technique,' has been used for studying surfaces coated with a range of different proteins. Much of the work done in this area was recently r e ~ i e w e dAt . ~ the present stage it appears that it is comparatively easy to understand (i) the long-range forces, and also many aspects of the short range forces, induced by the presence of small, structurally stable globular proteins (like insulin and lysozyme), and (ii) the forces induced by very flexible proteins (like B-casein and proteoheparan sulphate). The most difficult pro201

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teins to understand seem to be the ones that have a globular structure but are not very stable, e.g. albumin.8 Forces between surfaces coated with polysaccharides have been studied much less. However, from the studies presented so far, it is clear that the forces generated by this class of compounds are similar to those observed in other polymer and polyelectrolyte systems. For instance, the importance of the solvency for adsorption of, and the interaction between surfaces coated by, a chemically modified polysaccharide has been investigated.’-“ Further, the forces acting between negatively charged surfaces coated with chitosan, a weak cationic polysaccharide, have been measured at various pH valuesI2 and these have been found to correlate with the stability of chitosan-coated emulsion droplets.13 Interactions between lipid layers have been very intensely investigated during the last 20 years using different techniques, mainly osmotic stress and surface force measurement^.'^^'^ This topic is also the focus of this review article. First, the principles of some of the techniques used for studying interactions between lipid-coated surfaces are presented. Then, we present some data that illustrate the interactions between uncharged lipids and how the forces generated by the lipids depend on the hydrocarbon chain fluidity and the nature of the polar group. Finally, the molecular origin of the short-range repulsion between lipid layers is discussed, a topic which is currently hotly debated in the scientific literature. 19-24

2 Methods Osmotic Stress Measurements The osmotic stress method has been employed to determine interactions between lipid and surfactant bilayers in lamellar and gel phases. This information is obtained by measuring the water activity, a , from which the osmotic pressure II in the system is calculated fromI4

where V , is the partial molar volume of water and a. is the activity of pure water. The osmotic pressure thus equals the swelling pressure for the system relative to contact with pure water. The water activity in the sample was determined by measuring the relative water vapour pressure using a headspace chr~matograph.~’ This method works well at low water activities, but the scatter in the data at high water activities, i.e. at low pressures, is too large to allow accurate measurements. In order to determine the swelling pressure curve one also has to have the means to measure the separation between the bilayers. This was done by using an X-ray diffraction technique allowing the

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determination of the repeat distance d. Hence, the swelling pressure versus separation curve can be measured by preparing a set of samples with different water activities and determining the repeat distance in the samples. For lipids in the lamellar or gel phase, the lipid bilayer thickness dl and the water layer thickness d, can to a first approximation be calculated from the measured repeat distance26 dl =

WlVl

((1 - W J V ,

+

d = d@, WlVl

where wIis the weight fraction of lipid, v, and v, are the partial specific volumes of lipid and water, and @ is the volume fraction of lipid. The area per lipid molecule A is26

where

v

“

Mv =I N

is the molecular volume of the lipid, M is the molecular weight, and N is Avogadro’s number. In the calculations it is assumed that the specific molar volume of lipid is independent of the pressure, that there is no water in the lipid layer, and that no lipid is dissolved in the interlamellar water (indicating a sharp boundary between the lipid phase and the aqueous phase). In reality the lipidwater interface is less sharp, and other methods to determine the location of the interface have been proposed. For instance, the electron density profile can be determined from low-resolution X-ray diffraction analysis and a maximum in electron density can be obtained corresponding to the mean location of the centre of the polar For a more detailed discussion of the osmotic stress technique, the reader is referred to the review by Rand and Parsegian.”

Interferometric Surface Force Apparatus A schematic illustration of the interferometric surface force apparatus’ (SFA) is provided in Figure 1. It consists of a stainless steel measuring chamber that encloses the two interacting surfaces. The substrate surfaces are silvered on their backside and glued onto cylindrical silica discs. These surfaces are mounted in a crossed cylinder configuration. Collimated white light is directed perpendicularly to the surfaces. The light passes through the lower surface and becomes multiply reflected between the silver layers. Due to constructive

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j m l 560 nm

(a) Tospectrometer

t

550 nm

F.E.C.O.

t

White light

Mica sheets crossed cylinders configuration

Figure 1

A schematic illustration of the main components of the interferometric

surface force apparatus (part a). The stainless steel measuring chamber contains the two interacting surfaces. One mica surface is glued to a silica disc that is attached to a piezo-electric crystal. The other surface, also glued to a silica disc, is mounted on a double cantilever force measuring spring. The surfaces are oriented in a crossed cylinder configuration (part b). White light enters through the window in the bottom of the chamber. It is multiply reflected between the silver layers, and a standing wave pattern, called fringes of equal chromatic order (F.E.C.O.), is generated (part c). The standing waves exit through the top window, and the wavelength and fringe shape are analysed in a spectrometer

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interference for certain wavelengths, multiple-beam interference fringes (or fringes of equal chromatic order, F.E.C.O.) are transmitted through the upper surface. The F.E.C.O. wavelengths depend on the thickness T of the mica surfaces and the separation between them, D. The surface separation can be determined by comparing the wavelengths when the surfaces are in contact and apart. Under good conditions, i.e. with bright fringes produced by multiple reflections between highly reflecting layers on thin mica sheets, we have found that the distance can be measured with an accuracy of ca. 0.1 nm. We note that this distance is an absolute value measured relative to the pre-determined zero separation. When the experimental geometry is that of crossed cylinders, the fringe pattern has a parabolic shape, and the local radius of the interacting surfaces can be determined from the fringe shape. The distance between the two interacting surfaces is normally changed stepwise by either a synchronous motor or, more accurately, by a stepwise change in the voltage applied to a piezo-electric crystal. The movement of the piezo-electric crystal is calibrated at large surface separations using the F.E.C.O. The expansion of the piezo-electric crystal, AD, is linear in the applied voltage, AV, as long as no force acts between the surfaces; ix.,we have

AD = cAV.

(6)

The force F, between the two surfaces in the crossed cylinder configuration is measured by expanding or contracting the piezo-electric crystal by a known amount AD and then measuring interferometrically the actual distance that the two surfaces have moved relative to one another ADO.Any difference in the two distances when multiplied by the spring constant k gives, using Hooke’s law, the difference between the forces at the initial and the final separations

AF, = k(AD

-

ADO) = ~ ( c A V- ADO).

(7)

Spring deflections of the order of 1nm can be determined in this way. Hence, using a spring with a spring constant of about 100N/m and surfaces with a radius of about 2 cm gives a detection limit in force of N, corresponding to a force normalized by radius of ca. 5pNlm. The mechanical spring system is unstable in the regions of the force-distance curve where the gradient of the force (dFJdD) exceeds the spring constant; under such circumstances the surface separation changes suddenly, and a jump occurs.28 The force measured in the crossed cylinder geometry is related to the force between two spheres, F,, and the free energy of interaction per unit area between flat surfaces, Gf, according to the Derjaguin approximation: 2,29

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Forces between Lipid-Coated Interfaces

It follows that the pressure in the flat geometry, Pf, is related to the force gradient in the curved geometry by

where R is the geometric mean radius.

Thin Film Balance A thin film balance (TFB) was to measure the forces acting between the two air-liquid interfaces in a single foam lamella. The main parts of the thin film balance are shown in Figure 2. Single thin-liquid foam films are formed in the hole drilled through a fritted glass disc, onto which a glass capillary is fused. The solution-permeable film holder is placed in a gas-tight measuring cell with the free end of the capillary tube exposed to atmospheric pressure. The gas pressure in the cell is regulated by a syringe pump. The force per unit area, known as the disjoining pressure, II, in a plane-parallel film is given by3'

I

cell

interference

..- . . -

Figure 2

. ~.~. .. I L .....~

-

A schematic illustration of the main components of the thin film balance. A macroscopic f o a m film is formed in a hole drilled in a porous glass frit. The surfactant solution is contained in the frit, in the glass capillary, and at the bottom of the closed cell. The film thickness is determined using interferometry. The reflected light is viewed by a video camera and the intensity of a selected wavelength is measured with a photomultiplier tube ( P M T ) . The pressure in the measuring cell is varied by means of a syringe p u m p and measured by a pressure transducer

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where PG and PRare the gas and reference (atmospheric) pressures respectively, y is the surface tension of the solution, r is the radius of the capillary tube, Ap is the density difference between the aqueous surfactant solution and the gas, h, is the height of solution in the capillary tube above the film, and g is the gravitational constant. Each term on the right hand side of equation (10) is measured independently, providing a direct measurement of n. Film thicknesses are measured interferometrically. White light from a 100W halogen lamp is passed through a heat filter and focused at normal incidence onto the individual foam film formed in the porous-plate holder. Reflected light from the film is then split and sent to a video camera and a fibre optic probe placed in the microscope ocular. The light from the fibre optic is filtered ( A = 546 nm) and analysed with a sensitive photomultiplier tube. The so-called equivalent film thickness is then calculated from the standard Scheludko interferometric equation which assumes a constant refractive index across the film.32 This equivalent thickness is slightly thicker than the true film thickness h, because the surfactant adsorption layers at each film interface have a higher refractive index than the aqueous core. To correct for this difference the multilayer correction factor system derived by D ~ y v iiss adopted. ~~ Finally, the thickness of the film's aqueous core is determined by substracting the thickness of the adsorbed layers from the total film thickness. The force curve, the disjoining pressure isotherm, is generated by measuring the equilibrium film thickness as a function of the applied capillary pressure to the film. The capillary pressure is changed by altering the gas cell pressure. This makes it possible to determine the repulsive branch of the disjoining pressure isotherm. Additional experimental details can be found elsewhere.34

3 Results and Discussion Monoglycerides and phospholipids are common stabilizers for emulsions in food and pharmaceutical applications. These compounds (see Figure 3) are nonionic and zwitterionic, respectively. This means that these emulsifiers do not generate any long-range repulsive electrostatic double-layer forces. Hence, according to DLVO theory, emulsion droplets coated with monoglycerides or phospholipids ought to flocculate due to the action of atttractive van der Waals forces. However, in practice this does not occur due to the presence of rather short-range repulsive forces. These forces have been thoroughly investigated using osmotic stress and surface force techniques. I6I8 The short-range forces acting between some monoglycerides in the lamellar and gel phase are illustrated in Figure 4.This set of data was obtained in our laboratory employing the osmotic stress technique.25 The molecules are packed into bilayer structures which in the lamellar state have fluid hydrocarbon chains. The area per molecule of monopalmitin in the lamellar phase (at 60 "C)is ca. 32 A*. In this case a repulsive force acts between the bilayers out to a separation of ca. 12 A. The force decays exponentially with increasing water layer thickness and the decay constant is ca. 2.4 A. The monopalmitin sample changes into a metastable gel phase when the temperature is decreased. The

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Forces between Lipid-Coated Interfaces (a)

(b) H

/

0

1

H /

0

0

1

CH2 - CH - CH2

I

/I

H3C - P - CH3

0

c=o

Figure 3

Chemical structures of three of the studied surfactants: (a) monopalmitin, (h) dimethylphosphine oxide, and (c) octyl-&$ucoside

gel phase is also a bilayer structure, but in this phase the hydrocarbon chains are frozen. The area per molecule in the gel phase is 23 A2, about 72% of that found in the lamellar phase. The repulsive short-range force in the gel state is at a given water layer thickness about a factor of 4-5 weaker than that in the lamellar state. It has a measurable range of about 8 A. Monoolein, which has the same polar group as monopalmitin, contains one cis double bond and therefore it forms a lamellar phase (area per molecule ca. 36-32 A2) at a lower temperature than monopalmitin. The forces measured between monoolein lamellar bilayers at 23°C are very similar to those found between monopalmitin lamellar layers at 60"C, and thus much stronger than those in the monopalmitin gel state at 23 "C. From these observations it is clear that the short-range force in the lamellar state is much stronger than in the gel state. Further, it is the state of the hydrocarbon chains (fluid or frozen) which is of prime importance for the strength of the force, whereas the temperature and structure of the hydrocarbon chain ( i . e . , saturated or unsaturated) is of importance primarily because these parameters determine the state of the hydrocarbon chains. It is

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5

10

15

Water Layer Thickness Figure 4

Swelling pressure as a function of water layer thickness in the lamellarphase of monopalmitin at 60 "C (O), in the gel state of monopalmitin at 23 "C (O), and in the lamellar state of monoolein at 23 "C (m). The upper solid line is an exponential force law (C ' ex ( DIA)), where the pre-expon$ntial factor has a value of 1 . 1 x l@Nlm , and the decay constant is 2 . 4 A . The lower solid line represents a force law with the same decay constant and a preexponential factor of 0.26 x 108Nlm2

4 -

worth noting that the area per molecule, and thus the area of the unfavourable hydrocarbon-water contact, is larger in the lamellar phase than in the gel phase. Nevertheless, the repulsive force is larger in the lamellar phase. What is the reason for this? At present there are two main ideas about the molecular origin of shortrange forces between uncharged lipids: hydration repulsion and steric repulsion. Let us consider how these contributions are thought to orginate and how they are expected to change when going from the gel to the lamellar state. The O H groups in the polar part of the monoglyceride (Figure 3) are hydrophilic and they interact favourably with water. Water molecules close to the interface respond to the presence of these hydrophilic groups and so they adopt a nonrandom orientation. The influence of the interface extends a few molecular layers out into the solution. As the two bilayers of monoglycerides are approaching each other, their respective solvation layers start to overlap and as a consequence the free energy of the system changes. This generates a repulsive force that is known as a 'hydration force'. When the gel phase melts, the hydrocarbon-water contact area increases and this reduces the hydrophilicity of the interface. This is expected to result in a less long-range hydration force, the opposite to what is observed. However, another effect may come into play, namely, as the gel phase melts, intra-layer hydrogen bonds between

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Forces between Lipid-Coated Interfuces

monoglyceride groups are broken, making it possible to form more hydrogen bonds with ~ a t e r .This ~ ~effect , ~ ~is likely to make the interface more hydrophilic and increase the range of the hydration force. Clearly, since .both the hydrocarbon-water interfacial area and the number of hydrogen bonds with water increase as the monoglyceride gel phase melts into the lamellar phase, it is not easy to predict if the overall hydration force contribution should increase or decrease as a result of this phase transition. The bilayer arrangement in the lamellar or gel state is not static due to the molecular and collective motions of the lipids.23This means that the interface between the lipid bilayer and the aqueous layer is not well defined. The motions of the bilayer membranes give rise to a steric force with contributions from collective bending and squeezing motions as well as movement of individual molecules perpendicular to the interface.22 All these motions are restricted when two bilayers come close to each other and this lowers the entropy of the system, thus generating a repulsive force. It has been argued22 that it is this movement of the individual molecules, causing them to protrude from the average location of the interface, that often generates the most important force contribution, known as the protrusion force.22 Since the motion of the molecules increases as the gel phase melts into the lamellar phase, it is clear that this force contribution is also expected to increase, in line with the experimental findings for mono glyceride^.^^^^^ A comparison of the difference in swelling pressure in the gel and in the lamellar phase of a typical monoglyceride and a typical phospholipid is provided in Figure 5 . First, it is clear that phospholipids generate a stronger short-range force than do monoglycerides. Secondly, for phospholipids, just as for monoglycerides, a stronger repulsive force is observed in the lamellar phase than in the gel phase.15 It is hard to rationalize this in terms of a more hydrophilic interface in the lamellar phase, where the area per molecule is larger than in the gel phase. Instead, it appears that the steric protrusion force contribution is most important in this case. This conclusion explains why changes that increase the fluidity or the size of the polar group increase the range of the short-range repulsion. For instance, consider the following three observations. (i) The range of the short-range force increases when phosphatidylethanolamine is methylated. (ii) The range of the hydration force is often larger for lipids with unsaturated chains than for lipids with saturated chains. (iii) An increase in chain heterogeneity increases the range of the short-range force. From these findings one may draw the conclusion that an efficient phospholipid stabilizer for food emulsions should be used above its chain melting temperature. Furthermore, it ought to contain a mixture of hydrocarbon chains and a high fraction of unsaturated chains. Finally, phosphatidylcholine, due to its

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P. M . Claesson

0

5

10

15

20

25

30

35

Water Layer Thickness (A> Figure 5

Typical swelling pressure against water layer thickness curves for the exponentially decaying part of the short-range interaction for monoglycerides and phospholipids. Upper thick line represents DPPC with melted chains (50°C);lower thick line represents DPPC with frozen chains (25 "C). Upper thin line represents monopalmitin with melted chains (60"C);lower thin line represents monopalmitin with frozen chains (23 "C)

larger headgroup size, ought to be a better stabilizer than phosphatidylethanolamine. The short-range interaction is expected to increase with increasing temperature as long as molecular protrusion is the predominant force contribution. This is what is observed for phospholipids and monoglycerides. However, there are also some nonionic surfactants which generate forces that have an opposite temperature d e p e n d e n ~ e . ~ ' -Most ~ ~ well known of these are the ethylene oxide based surfactants, but the same phenomenon is also observed for dimethylphosphine oxide (Figure 3), as illustrated in Figure 6. The reason for this inverse temperature dependence of the interaction is that the polar groups become more hydrophobic as the temperature increases. For ethylene oxide based surfactants the molecular mechanism behind this has been suggesteda to be a temperature induced change in conformation from gauche to trans of the 0442-0 segments in the ethylene oxide chain. The molecular mechanism behind the increased hydrophobicity of the dimethylphosphine oxide group at higher temperature^^^ cannot be explained in the same way. We note that the hydration model of Kjellander41742 may rationalize the behaviour of both the dimethylphosphine oxide and the ethylene oxide surfactants. In general, it is clear that both hydration and protrusion forces contribute to the short-range interactions between nonionic and zwitterionic lipids. Experi-

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Forces between Lipid-Coated interfaces

h

Ed

!3

E 2m

2

PI

0 Figure 6

5

10

15

Water Layer Thickness (A>

Swelling pressure in the lamellar phase of dimethyldodecylphosphine oxide as a function of water layer thickness. Measurements were carried out at 18°C (U) and 48 "C (@)

mentally, only the total force is measured. Conclusions about which force contribution is more important are based on data interpretation. As indicated above, the protrusion force concept is able to rationalize several aspects of the short-range forces observed in phospholipid and monoglyceride systems in a consistent way. However, other researchers have preferred to interpret the short-range force observed in these systems as a hydration force, and this point is still being debated in the scientific literature. 19-24 Lipid layers adsorbed to solid surfaces also generate short-range repulsive forces. In this case we d o not have any forces due to collective motions of the lipid molecules, but both protrusion forces and hydration forces are expected to be present. The forces acting between hydrophobized mica surfaces coated with lipid and surfactant monolayers have been investigated in several studies'7v18employing the interferometric surface force apparatus. Some data obtained in our laboratory43are shown in Figure 7, including the forces acting between deposited layers of monopalmitin. The zero distance point in this graph is defined as the contact between the hydrophobic surfaces prior to deposition of monopalmitin. The long-range repulsive force observed in this case is due to residual charges on the hydrophobic surface. At a separation of cu. 70 A, a van der Waals attraction dominates the interaction and the surfaces jump inwards to a separation of cu. 50A. The short-range force, observed in the distance range 50-40 A, is due to protrusion and hydration forces. Observe that the effective range of this force, ca. 10 A,is the same as the range of the short-range interaction observed in the monopalmitin lamellar and gel phases using the osmotic stress technique. It is not trivial to compare quantitatively the interactions between solid surfaces coated with a particular lipid, obtained from surface forcc mcasure-

213

P . M . Claesson 8 6

I

4

eE4 - 2 -4

0

50

100

150

200

Distance
Force F normalized by radius R as a function of separation between monolayers of monopalmitin (m) and between monolayers of octyl-Bglucoside (0)adsorbed on hydrophobized mica surfaces. The zero distance is defined as the point of contact between the hydrophobized mica surfaces. The arrows indicate inward or outward jumps

ments with the corresponding interactions in the liquid crystalline state obtained from osmotic stress measurements. One reason for this is that one has to take into account the difference in geometry, crossed cylinders in the SFA and flat surfaces in the osmotic stress measurements. This is normally done using the Derjaguin approximation; see equations (8) and (9). Another reason is that the lipid-water interface is not very well defined and it is difficult to establish how the zero separation used in the two techniques correspond to each other. A third difference is that surface force measurements are carried out using a constant chemical potential of water whereas the chemical potential of water is varied during osmotic stress measurements. Despite these difficulties and differences, the results obtained with the two techniques agree qualitatively. 25,39,44 The force between hydrophobic surfaces coated with an adsorbed monolayer of the sugar-based surfactant octyl-/3-glucoside (Figure 3) across a 25 mM surfactant solution close to the critical micelle concentration (cmc) is shown in Figure 7. The driving force for adsorption is a hydrophobic interaction between the surface and the surfactant tails. Hence, the surfactant is oriented with the sugar group directed towards the aqueous phase. Just as for monoglycerides, the short-range interaction is dominated by a van der Waals force, and at smaller distances by a protrusionhydration force.45 The short-range force wall is located at smaller distances for the sugar-based surfactant than for monopalmitin. This is due to the smaller molecular size of octyl-/3-glucoside which makes the adsorbed layer thinner. It is also worth noting that the depth

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Forces between Lipid-Coated Interfaces 10000

0

100

200

300

400

500

600

Distance (A) Figure 8

Disjoining pressure across a single foam film stabilized by octyl-/3-glucoside as a function of water layer thickness. The bulk surfactant concentration is 25mM (close to the cmc). The arrow indicates the transition from a common black film to a Newton black film

of the attractive minimum observed for the sugar surfactant is smaller than that observed for monopalmitin. This may be due to a larger mobility and size, and thus a larger protrusion force component, for octyl-/?-glucoside. The forces acting between two air-water interfaces stabilized by octyl-Pglucoside across a 25 mM surfactant solution have been determined34 employing a thin-film balance. The results are shown in Figure 8. A small charge at the air-water interface (450 nm*/charge) gives rise to a weak repulsive doublelayer force that dominates the long-range interaction. At a separation of ca. 120& an attractive van der Waals force induces a transition from a thicker common black film to a very thin (1-2 nm) Newton black film. The thickness of this film is determined by similar protrusion and hydration forces to those found to act at short distances between surfactant-coated solid surfaces and between bilayers in lamellar and gel phases.

4 Conclusions We have shown how the use of various force measuring techniques can provide a detailed knowledge about interactions between solid surfaces, between two air-water interfaces in a foam lamella, and between bilayers in liquid crystalline states. It is found that, to a very large degree, the same forces are generated by a given surfactant or lipid irrespectively of whether it is present in a liquid crystalline phase or is adsorbed at the surface of an emulsion droplet or a hydrophobic particle, or at t h e air-water interface. The main distinction

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between the interactions in the various systems can be related to a difference in adsorbed amount of the surfactant and to a difference in interface flexibility. The short-range forces generated by the lipids are due to a combination of steric/protrusion forces and hydration forces. The main body of the results obtained can most easily be rationalized by considering that the protrusion force is the most important cause of the repulsion. When this is the case, it is clear that stronger repulsive forces are expected to be generated by fluid lipid layers than by frozen ones. Also, it is expected that a large polar group will generate more long-range repulsion than a small polar group.

References 1 . B. Bergenstihl and P. M. Claesson, in ‘Food Emulsions’, ed. K. Larsson and S. E. Friberg, Marcel Dekker, New York, 1990, p. 41. 2. J. N. Israelachvili, ‘Intermolecular and Surface Forces’, 2nd edn, Academic Press, 1991. 3. G. J . Fleer, M. A . Cohen Stuart, J . M. H. M. Scheutjens, T. Cosgrove, and B. Vincent, ‘Polymers at Interfaces’, Chapman & Hall, London, 1993. 4. P. M. Claesson, H. K . Christenson, J . M. Berg, and R . D . Neuman, J . Colloid Interface Sci., 1995, 172,415. 5. P. M . Claesson, E. Blomberg, J . C. Froberg, T. Nylander, and T. Arnebrant, A d v . Colloid Interface Sci.,1995, 57, 161. 6. P. M. Claesson, T. Ederth, V. Bergeron, and M. W. Rutland, A d v . Colloid Interface Sci., in press. 7. J. N. Israelachvili and G. E. Adams,J. Chem. SOC.Faraduy Trans. I , 1978,74,975. 8. E. Blomberg, P. M. Claesson, and R. D. Tilton,./. ColloidInterfaceSci., 1994,166, 427. 9. M. Malmsten, P. M. Claesson, E. Pezron, and I. Pezron, Langmuir, 1990,6, 1572. 10. M. Malmsten and P. M. Claesson, Langmuir, 1991,7,988. 11. I . Pezron, E . Pezron, P. M. Claesson, and M. Malmsten, Langmuir, 1991,7,2248. 12. P. M. Claesson and B. W. Ninham, Langmuir, 1992,8, 1406. 13. P. Falt, B. Bergenstihl, and P. M. Claesson, Colloids Surf. A , 1993,71, 187. 14. V. A . Parsegian, N. Fuller, and R. P. Rand, Proc. Natl. Acad. Sci. USA, 1979,76, 2750. 15. R. P. Rand and V. A. Parsegian, Biochim. Biophys. Acta, 1989,988,351. 16. V. A . Parsegian, R. P. Rand, and N. L. Fuller, J . Phys. Chem. 1991,95,4777. 17. J . Marra and J. N. Israelachvili, Biochemistry, 1985, 24, 4608. 18. J. L. Parker, J . Colloid Inferface Sci., 1990, 137, 571. 19. J. N. Israelachvili and H. Wennerstrom, Langmuir, 1990,6, 873. 20. V. A . Parsegian and R. P. Rand, Langmuir, 1991,7, 1299. 21. J . N. Israelachvili, Langmuir, 1992, 8 , 1501. 22. J. N. Israelachvili and H . Wennerstriim, J . Phys. Chem., 1992, 96, 520. 23. S.-J. Marrink, M. Berkowitz, and H. J . C . Berendsen, Lnngmuir, 1993,9,3122. 24. J . N. Israelachvili and H. Wennerstrom, Science, 1996,379,219. 25. I . Pezron, E. Pezron, B. Bergenstihl, and P. M. Claesson, J . Phys. Chem., 1990, 94, 8255. 26. K. Fontell, L. Mandell, H . Leitinen, and P. Ekvall, Acta Polytech. Scand., Chem. Ser., 1986, 74, 111. 27. T. J . McIntosh and S. A. Simon, Biochemistry, 1986,25,4058.

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Forces between Lipid-Coated Interfaces

28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

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38. 39. 40. 41. 42. 43. 44. 45.