Thermodynamics of Transfer of Amphiphiles between the Liquid− Air

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Langmuir 1997, 13, 1746-1757

Thermodynamics of Transfer of Amphiphiles between the Liquid-Air Interface and a Solid SurfacesWetting Tension Study of Langmuir-Blodgett Films Vasili Yaminsky,* Tommy Nylander,† and Barry Ninham† Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Institute of Advanced Studies, The Australian National University, Canberra, A.C.T. 0200, Australia Received July 19, 1996. In Final Form: January 6, 1997X Wetting tension was determined by measuring the meniscus height on a mica surface during spreading, compression, and subsequent Langmuir-Blodgett deposition of an insoluble monolayer of DSPE (distearoylphosphatidylethanolamine) spread at the water-air interface, and on reimmersion of the surface in pure water. While such systems are essentially nonequilibrium and irreversible in various aspects of their behavior, we show that fundamental principles behind Langmuir-Blodgett phenomena can be understood by consideration of the thermodynamic equilibrium at the three-phase line. At large areas per molecule, long before the monolayer at the liquid-vapor interface is condensed, it undergoes spontaneous condensation and compression at the three-phase line. By this thermodynamically driven mechanism the contact angle increases and the layer is transferred onto the surface of the solid. This is a general effect which occurs also for soluble surfactants. Free energies of transfer of a lipid molecule between the interfaces that coexist at the three-phase line are the basic parameters. This is revealed by a unified thermodynamic and kinetic analysis which explains mechanisms involved in deposition, and the nature of instability of deposited monolayers. The results explain contact angle hysteresis and hydrophobic interactions in such systems. The assumption that monolayers of insoluble surfactants are stable in aqueous environments so long as the contact angle is large is shown to be erroneous. A consequence is that long range attractions seen between such surfaces in water are related to capillary condensation.

1. Introduction In many biological systems surface active molecules, like lipids, have a low solubility in water. These molecules can be transported not through the bulk but from one of the coexisting surfaces onto the other. Mono- and multimolecular layers can be deposited by repeatedly passing a solid substrate through the surface of water on which a monolayer is spread. This was demonstrated by Langmuir and Blodgett about 60 years ago,1,2 and the interest in the problem continues to grow.3-7 Such a technique for assembling “nanostructures” has appeal for proposed manufacture of electronic and electro-optic devices as well as in biomedical applications.8 Hydrophobic and insulating coatings of a single molecular thickness can be so formed on various (hydrophilic) substrates. It is often taken for granted that these monolayers are stable in aqueous environments as long as they appear to be hydrophobic. However, the actual behavior of deposited films in their interaction with water is masked and obscured by a large contact angle hysteresis. This has never been explained. It occurs even on smooth and homogeneous substrates like mica and silica.9 * Author to whom all correspondence should be addressed: Fax: (06) 249 0732. E-mail: [email protected]. † Present address: Physical Chemistry 1, Lund University, P.O. Box 124, S-221 00 Lund, Sweden. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Langmuir, I. Trans. Faraday Soc. 1920, 15, 62-74. (2) Blodgett, K. B. J. Am. Chem. Soc. 1935, 57, 1007-1022. (3) Birch, W. R.; Knewtson, M. A.; Garoff, S.; Suter, R. M.; Satija, S. Langmuir 1995, 11, 48-56. (4) Riegler, J. E.; LeGrange, J. D. Phys. Rev. Lett. 1988, 61, 24922495. (5) Honig, E. P. Langmuir 1989, 5, 882-883. (6) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Prog. Colloid Polym. Sci. 1991, 84, 184-188. Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Ye, X. Colloids Surf. A 1995, 94, 279-289. (7) Spratte, K.; Chi, L. F.; Riegler, H. Europhys. Lett. 1994, 25, 211217. (8) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991.

S0743-7463(96)00716-0 CCC: $14.00

We will show that hydrophobicity is not a sufficient criterion for stability of Langmuir-Blodgett films. There is a distribution of the lipid between the three interfaces, and spontaneous compression occurs across the threephase line. The “piston action” of the wetting perimeter effectively takes the function of the barrier on a trough, and the balance of the surface pressures at the three interfaces at the line of their coexistence is responsible for regulation of the contact angle. The diffusioncontrolled adsorption transfer from the surface of water onto the solid is fast. The conditions of a slower reverse transfer by desorption are quite different. This results in contact angle hysteresis, which is particularly large for insoluble monolayers. We first briefly summarize the theme and content of this paper. For the usual LB technique a spread monolayer is deposited at a high constant pressure. This has often been presumed to guarantee an ideal ordered structure for the condensed layer attained at the water-air interface to translate onto the solid. High deposition pressure is commonly believed to be an important factor for stability of the layer. We will show that this is not necessarily so. This structure is “frozen” only so long as the substrate stays out of liquid. Compressed to a high pressure on the trough and retaining its structure while in air, the stored energy is released and the monolayer “springs back” in the presence of water. The head group-substrate interaction which consolidates the monolayer and restricts its lateral mobility is weakened by water. The molecules can diffuse and rearrange themselves under water. When partly immersed, the confinement condition is no longer applicable. The lipid deposit in whatever structural form is now present (the original monolayer, reoriented bilayer, patches of a segregated bulk phase, etc.) slides off the plate and spreads back onto the surface of water. (9) Yaminsky, V. V.; Claesson, P. M.; Eriksson, J. C. J. Colloid Interface Sci. 1993, 161, 91.

© 1997 American Chemical Society

Wetting Tension Study of LB Films

There is another side of the problem. The deposition of a condensed monolayer onto a solid does not necessarily require the layer to be compressed at the aqueous-air interface. There is a distribution of the lipid between the two interfaces, essentially as for the distribution of a solute between two immiscible solvents in extraction. It is through this mechanism that spontaneous compression of the monolayer occurs at the three-phase line. And it is this process which gives rise to an increase of the contact angle in the presence of a surfactant. It is already well-known that mica and silica can be rendered hydrophobic by soluble cationic surfactants.10-17 Cetyltrimethylammonium bromide (CTAB) is a typical example. Maximum hydrophobicity is reached already at such low concentrations that the surface pressure at the solution-air interface is vanishingly small, orders of magnitude lower than typical values used in LangmuirBlodgett deposition.16,17 The adsorbed layer is in a gaslike state, and the area per molecule is very large. Adsorption on the surface of mica which is under the solution is also vanishingly small. Nevertheless, a condensed monolayer is deposited when a mica sheet is retracted from such a solution. But although this is understood, the corresponding phenomena that occur with insoluble surfactants have at best been imperfectly explored. In the present work we have used the almost completely insoluble zwitterionic phospholipid, distearoylphosphatidylethanolamine, which provides an interesting contrast to the studies with CTAB. No adsorption through the bulk is anticipated for such lipids, and the state of the film should be less dependent on charge effects. The chemical potential with insoluble amphiphiles cannot be changed by varying bulk concentration. But it can readily be controlled by varying the molecular area and by measuring the surface pressure on a Langmuir trough. Langmuir-Blodgett-coated mica surfaces have been used extensively to study forces between hydrophobic surfaces.18-20 A long range attractive force is usually observed in such systems.21 All of the as yet unsatisfactory attempts to interpret these experiments have rested on the assumption that the layers are stable. Long range “hydrophobic attraction” seen with water soluble surfactants can be attributed to changes of adsorption with separation.22 But the question of insoluble LB film interaction which shows similar attraction at a further greater range of distances has remained open. We show that precisely the same mechanism is involved. It relates to lateral mobility of hydrophobic monolayers that allows the system to minimize its free energy with separation by formation of bridging condensates. (10) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989. (11) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (12) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088. (13) Ke´kicheff, P.; Christenson, H. K.; Ninham, B. W. Colloid Surf. 1989, 40, 31. (14) Zorin, Z. M.; Churaev, N. V.; Esipova, N. E.; Srgeeva, I. P.; Sobolev, V. D.; Gasanov, E. K. J. Colloid Interface Sci. 1992, 152, 170. (15) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706. (16) Yaminsky, V. V.; Yaminskaya, K. B. Langmuir 1995, 11, 936. (17) Eriksson, L. G. T.; Claesson, P. M.; Eriksson, J. C.; Yaminsky, V. V. J. Colloid Interface Sci., in press. (18) Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986, 114, 234-242. (19) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390. (20) Wood, J.; Sharma, R. J. Adhesion Sci. Technol. 1995, 9, 1075. (21) Christenson, H. K. The Long-Range Attraction between Macroscopic Surfaces. In Modern Approaches to Wettability: Theory and Applications; Schrader, M. E., Loeb, G., Eds.; Plenum Press: New York, 1992; p 29. (22) Yaminsky, V. V.; Ninham, B. V.; Christenson, H. K.; Pashley, R. M. Lamgmuir 1996, 12, 1936.

Langmuir, Vol. 13, No. 6, 1997 1747

From this point of view the idea of hydrophobicity has to be treated with caution. Here a high contact angle effectively means that the lipid, rather than water, preferentially wets mica. The basic principle of two-phase wetting holds even though the amount of the lipid originally confined to a monolayer is limited. It is this mobile lipid deposit which pushes water off the solid substrate and raises the contact angle, instead of being transferred under water. The contact angle is high even though there are no hydrophobic moieties being exposed to water. An associated effect by which lipid escapes water when two surfaces interact is capillary condensation. The relation of this hydrophobic attraction to monolayer stability has been pointed out earlier,9,22 but relevant monolayer properties are here explored in detail. Adhesion, dewetting, contact angle hysteresis, and long range forces are then all linked thermodynamically to surfactant adsorption at the three-phase line16,17 and in the gap between interacting surfaces.22,23 In the following we focus attention on the importance of thermodynamic arguments for proper assessment of the events that take place along the three-phase line. That consideration is profoundly important, in spite of the fact that Langmuir-Blodgett phenomena are essentially irreversible in most respects. The irreversibility derives from kinetic reasons, but as we shall show, thermodynamic arguments do reveal the directions of evolution. The condition of thermodynamic equilibrium is easily violated globally at areas of the solid surface distant from the threephase line and in the bulk phases in which the lipid is absent. But this condition is readily satisfied locally on the coexistence line. This is the essential boundary at which things happen. It is related to contact angle phenomena and intrainterfacial mass transfer. The adsorption equilibration from receding conditions does not need activation and proceeds rapidly. This is precisely the condition of monolayer deposition. 2. Basic Theory Before proceeding to experimental results, we rehearse some basic thermodynamic aspects of wetting, that will provide the grounds for subsequent analysis. We do not discuss details of capillarity for heterogeneous and nonequilibrium surfaces which will be seen to be irrelevant for our argument. Clean mica is a uniform surface with no hysteresis on contact with pure water. We are mainly preconcerned at this stage with a question of what will happen after adsorption equilibrium is attained. Under this ultimate condition, which probably can never be reached experimentally in some cases, and recalling that mica does not dissolve in water or change irreversibly in some other way, the Young and the Gibbs equations are applicable without limitation. This is so in spite of the fact that a normal stress at the threephase line is present in any situation other than that zero contact angle.4 In the lateral plane the surface tension is counterbalanced by the wetting tension

τ ) γLV cos θ The mechanical equilibrium is achieved by positioning the surface of the liquid at an angle θ to the solid. The Young equation defines thermodynamically the equilibrium contact angle between the liquid-vapor interface and the solid-liquid interface:

γSV - γSL ) γLV cos θ

(1)

It defines the thermodynamical equilibrium value of the wetting tension also. In our convention the index S designates solid, L is liquid, and V is vapor. A change in interfacial tension (γ) is related via the chemical potential (µ) to the surface excess (Γ) of a component by the Gibbs equation (23) Yaminsky, V. V. Langmuir 1994, 10, 2710.

1748 Langmuir, Vol. 13, No. 6, 1997 -dγ ) Γ dµ

Yaminsky et al. (2)

For a dilute solution of a soluble surfactant

dµ ) (kT/c) dc where c is the bulk concentration. With an insoluble lipid this last approximation cannot be made. For a monolayer formed out of such a surfactant it is rather the change in chemical potential that is defined by the Gibbs equation (eq 2). Indeed, it can be related to the surface pressure (Π), which is measured, and the area per molecule (A), which is controlled, on a Langmuir trough. Since dγ ) -dΠ and A ) 1/Γ, we have dµ )A dΠ. It follows from the Gibbs equation that positive adsorption is accompanied by an increase of surface pressure (reduction of surface tension). By applying the Gibbs equation, with the definition of the equilibrium wetting tension, we obtain

-dτ ) (ΓSV - ΓSL) dµ

(3)

This immediate result of the two equations taken together, known as the Lucassen-Reynders24 equation, has already been applied to explain hydrophobic effects of cationics.16,17 The equilibrium wetting tension decreases in magnitude in the presence of an adsorbing component, provided that adsorption at the solidvapor interface is larger than adsorption at the solid-liquid interface. Indeed, the difference in the equilibrium value of the wetting tension for the pure solvent (τ°) and in the presence of an adsorbing species (τ) is equal to the difference of the surface pressures:

τ° - τ ) ΠSV - ΠSL

(4)

Surface pressure is defined by

Π ) γ° - γ )

∫

n

Γ dµ

(5)

-∞

where γ° and γ are the surface tensions before and after spreading the monolayer. Equation 4 is the integral form of eq 3 and is written with the use of definition 5. This is an exact result as long as the change of the chemical potential of water by the solute is negligible. This is precisely so for an insoluble lipid which is present only on the surface. For any interface the absolute value of the surface pressure is related to the corresponding adsorption isotherm by the Gibbs equation (eq 2). Although only the difference of the absolute values of γSV and γSL which cannot be determined independently is involved, the absolute values of ΠSV and ΠSL are physical quantities. Their full values are defined by eq 5 in terms of adsorption excesses. From eq 1 it now follows that the contact angle must increase if γSV is reduced by surfactant adsorption more than the sum γSL + γLV of the other two tensions together. In other words, if the surface pressure at the solid-vapor interface is larger than the sum of the corresponding values for the solid-liquid and liquidvapor interfaces, contact angle increases. These classical definitions have been used to explain dewetting effects of solutions of cationic surfactants on silica16 and mica.17 They provide a framework for subsequent considerations of the more complicated nonequilibrium phenomena involved in deposition of insoluble monolayers on solid substrates and of their effects on wetting.

3. Materials and Methods 3.1. Materials. We have used a zwitterionic phospholipid, distearoylphosphatidylethanolamine (DSPE), as a typical example of water insoluble surfactants used for Langmuir-Blodgett deposition. The DSPE sample (lot B9705) was kindly provided by Genzyme Pharmaceutical and Fine Chemicals, Haverhill, U.K. Water used was desalted in a reverse osmosis unit, distilled in a glass still, and passed through a Milli-Q water purification (24) Lucassen-Reynders, E. H. J. Phys. Chem. 1963, 67, 969. (25) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590.

system (Millipore Corporation, Bedford, MA). All other chemicals used were of analytical grade. The substrate was green muscovite mica. 3.2. Monolayer Techniques. A surface film balance (JoyceLoebl, Gateshead, England) with a custom-built Teflon trough (maximum surface area of 13 × 25 cm2, volume of about 2 L) and a single movable Teflon barrier was used for surface pressure versus area (Π-A) measurements and for deposition of DSPE. The surface tension was measured (0.1 mN/m accuracy) with the Wilhelmy plate method, using a filter paper of 1 cm width. Prior to the measurements the paper was rinsed and prewetted with water for at least 30 min. After the trough was filled with water, the surface was further cleaned by sweeping it with the barrier and with suction from the interface. To check cleanliness, a Π-A isotherm of the bare interface was recorded at the same speed as that used for recording isotherms with DSPE. The process was repeated to assure that the surface pressure was below 0.2 mN/m upon compression to 5% of the initial area. Just before starting the experiment, the Teflon barrier was moved to the maximum surface area. In deposition and wettability experiments the solid sample was then partly immersed in the water in the trough. The wettability test for a clean surface was done as described below. Two hundred microliters of a 0.23 or 0.047 mg/mL solution of DSPE in a spreading solvent (chloroform/methanol as 2:1 by volume) was carefully applied at the interface by a microsyringe. Its tip was placed at a low angle to the interface in such a way that a large meniscus was formed under the needle. During spreading and compression of the phospholipid monolayer, the geometry of the meniscus on the immersed surface was recorded. A time between spreading and compression of at least 10 min was allowed, until no changes in the meniscus height were observed. This allowed for evaporation of the solvent and ensured complete spreading. The compression speed for the isotherm records was 1.6 or 1.0 cm/min when spreading was done from the 0.23 and 0.047 mg/mL solutions, respectively. This corresponds to about 6 or 16 Å2/(molecule s). The deposition was made at a constant pressure of 26 or 31 mN/m, and the speed of the dipper was 0.2 cm/min. The mica used for deposition had a width of 2 cm and was cleaved at most 20 min before immersion in water. The substrate was partly immersed in the water, and the heights of both receding and advancing menisci were measured before spreading the phospholipid at the interface. This assured that the contact angle is effectively zero and that no hysteresis occurs. In a separate deposition experiment a mica sheet was completely immersed before spreading the lipid. The transfer ratio of DSPE onto mica was measured by depositing onto a larger area of mica (two sheets with dimensions 5 cm in width by 10 mm in height). The transfer ratio is defined as the ratio of the decrease of the area of the monolayer maintained upon retraction at a constant pressure and the retracted area of the mica sheet. To measure reverse transfer, the subphase was removed from the trough, new water was added, and the sheets were then reimmersed in clean water. The film of DSPE, transferred from the surface of mica onto the surface of water, was compressed, and the surface pressure (Π) versus area (A) was recorded. From this Π-A isotherm, and the calibration isotherm for known amounts spread, the amount transferred was calculated. 3.3. Wetting Techniques. The wetting tension, τ ) γLV cos θ, and the contact angle on a partly immersed plate were measured by two capillarographic techniques: with the Wilhelmy plate and from the meniscus height. By suspending the mica plate from the microbalance (of the Langmuir trough) and recording the force (F) versus time (t) and/or depth (z) of immersion, the wetting tension was obtained from the equation

F ) Fcapillary + Fbuoyancy ≈ LγLV cos θ + S∆Fgz

(6)

Here the sign approximately equal is to remind us that in the absence of adsorption equilibrium the wetting perimeter can be nonuniform and the contact angle can vary locally; θ is then an averaged quantity. Here L is twice the width of the plate (the wetting perimeter), S is the horizontal cross-sectional area of the substrate, ∆F is the difference in density between liquid and

Wetting Tension Study of LB Films

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Figure 1. Schematic representation of the wetting of a hydrophilic surface. The upper half shows complete wetting by water. Here h0 represents the meniscus height and θ is the contact angle. The lower half shows the effect of spreading an insoluble amphiphile at the liquid-vapor interface at low surface area per molecule (very low surface pressure at the liquid-vapor interface). The amphiphile forms a condensed layer behind the three-phase line, which causes the contact angle to increase. The meniscus moves downward (the meniscus height h1 is lower), leaving a condensed layer at the solid surface. air, and g is acceleration due to gravity. Since the mica plates used were quite thin, the buoyancy force is almost negligible in this case. For control of depth z and of speed dz/dt, the balance was mounted on a microtranslation stage driven by a dc motor. The position was measured by measuring the rotation of the shaft with a potentiometer. In these experiments the substrate with a deposited layer was immersed in water in a 50 mL beaker. When the balance was used for simultaneous measurement of the surface tension, the substrate was suspended directly from the translation stage. The meniscus formed by the liquid on the substrate immersed in the trough (shown schematically in Figure 1) was monitored with a videocamera (Sony DXC 151 AP) attached to a microscope. The images were recorded with a videotape recorder (Sony U-matic). The height (h) of the meniscus was measured on the recordings by using a videomicrometer (model 305, Colorado Video Inc., Boulder, CO). For a meniscus on a flat wall, the contact angle, θ, can be calculated from the meniscus height, h, as

sin θ ) 1 - ∆Fgh2/(2γLV)

(7)

or expressed in terms of the wetting tension, τ:

τ ) γLV cos θ ) h[∆Fg(γLV - ∆Fgh2/4)]1/2

(8)

4. Results 4.1. Spreading. We investigated in situ effects of DSPE spread on the surface of water on the wetting behavior of mica. The plate was partly immersed in pure water in the trough. The solutions of DSPE (concentrations 0.047 (I) or 0.23 (II) mg/mL) were spread on the surface of water to give initial areas per molecule (A) of about 400 Å2 (I) or 90 Å2 (II). There is no measurable surface pressure at such low coverages (see Figure 3). Nevertheless, after spreading begins, in ΓLV or, more generally, ΓSV > ΓLV + ΓSL if ΓSL is not negligible. Provided the plate is retracted slowly enough so that the attainment of equilibrium could follow a displacement of the plate, the monolayer will cover the retracted area of the plate. The deposited layer at the surface of the solid is at a higher pressure than the layer at the water-air interface, and the area per molecule is generally smaller. In all such cases, soluble surfactant or not, whether amphiphiles are transferred directly from one interface onto another or adsorbed from bulk solution, there are similar adsorption events that take place at the threephase line. These determine the conditions of the wetting and deposition. The same principle applies for layers deposited from water and nonpolar solvents.29 However, large contact angles occur only for water and not for hydrocarbons. The highest angles are typically observed for surfactants for which the charge of the head group is opposite to that of the surface. With these preliminary remarks in mind, in discussing our results, we compare and contrast with wetting studies of similar surfaces hydrophobed by a soluble cationic. We first recapitulate the driving forces for adsorption of surfactants at the solid-liquid, solid-vapor, and liquidvapor interfaces. We take the bulk of the solvent as the reference state simply because this is the usual convention in dealing with solutions. However, simply by adding a constant which would be the free energy of the corresponding transfer, the argument could be repeated with reference to any bulk phase or an interface. 5.1. Solid-Liquid Interface. The free energy of transfer of a cationic surfactant from bulk solution to a silica-water or mica-water interface comprises two major components. One contribution corresponds to an ion exchange energy between the positively charged head group of the surfactant ion and a negatively charged site on the surface. For mica this is a lattice aluminate, and for silica it is a dissociated silanol. There can be zero energy balance for this contribution if nonspecific electrostatic interaction for both exchanging ions occurs. The contribution can be negative (attractive, favorable) or positive, which would depend on the position in the Hofmeister series. The other contribution results from the hydrophobic free energy gain on transfer of the hydrocarbon tail of the amphiphile from the aqueous medium surrounding it in (29) Tsao, Y.-H.; Yang, S. X.; Evans, D. F.; Wennerstro¨m, H. Langmuir 1991, 7, 3154.

Wetting Tension Study of LB Films

the bulk water to the solid-liquid interface, where a tail is surrounded by other tails of the neighboring molecules in the adsorbed layer. This contribution is essentially negative for a condensed monolayer and increases with chain length. It does not occur when the layer is in a gaslike state, in which case tails are isolated from each other. In this limit the adsorption of a surfactant is not greater than that for inorganic ions. It then increases cooperatively with concentration on approaching the point of zero charge (pzc). Here condensation occurs given that the charge site density is high enough to allow condensation to proceed. For mica with its high surface charge this is particularly the case. 5.2. Solid-Vapor Interface. Basically the same mechanisms as for the solid-liquid interface apply. Saturated water vapor adsorbs in the form of mono- or multilayer films on hydrophilic substrates. The equilibrium thickness is infinite if the contact angle equals zero. Electrostatic binding within such a film would be similar to that at the solid-liquid interface. An amphiphile can undergo ion exchange interaction or, if a double layer is not developed within a thinner film when the contact angle is higher, replace a surface proton and form a salt “bond”. The hydrophobic contribution, however, is much larger for the solid-vapor interface than for the solid-liquid interface. The hydrocarbon tails in the latter case are at least partly exposed to water. The terminal methyl groups of the acyl chains are still exposed to water, even if the monolayer is close packed. The latter contribution alone is up to 30 mJ/m2 of unfavorable energy per square meter, as follows from consideration of the surface tension of a hydrocarbon-air (about 20 mJ/m2) and hydrocarbonwater (50 mJ/m2) interface, or a similar value of the wetting tension for pure water on a solid paraffin. 5.3. Liquid-Vapor Interface. Adsorption at the water-air interface is favored by a release of the hydrophobic energy on transfer of the tail from bulk water out into air. This hydrophobic contribution to the free energy of adsorption is large and almost the same for solid-vapor and liquid-vapor interfaces. However, the electrostatic contribution to the free energy of adsorption at the waterair interface is essentially positive (repulsive). It thus acts in the opposite direction as compared to that for adsorption at a negatively charged solid surface under water. It results from repulsion between the charged polar head groups, exposed in this case to water, which tends to diminish the adsorption. The electroneutrality here is due to counterions in the diffuse double layer. The repulsive energy is particularly large at low ionic strengths when the Debye length is large so that the screening of the electrostatic repulsion by counterions is weak. With these mechanisms in mind it follows that the adsorption densities can indeed be much higher at the solid-vapor interface than at the other two interfaces. Both the lower free energy of adsorption at the solidliquid interface and the larger free energy of adsorption at the solid-vapor interface are the factors which contribute to dewetting. It can easily be observed that the hydrophobic monolayer need not be present at the mica-water interface at all for a large contact angle to be formed. The opposite statement would rather be true. The smaller the equilibrium adsorption at the solid-liquid interface and the larger it is at the solid-vapor interface, the larger the equilibrium contact angle formed. Indeed, the bigger the free energy of adsorption, the smaller the concentrations or the chemical potentials at which a surfactant monolayer will begin to adsorb. Condensed monolayers of CTAB on mica-air and silica-

Langmuir, Vol. 13, No. 6, 1997 1755

air interfaces form at concentrations less than 10-7 M. At the solid-liquid interface significant adsorption arises only on approaching the point of zero charge, 5 × 10-6 M for mica and 5 × 10-5 M for silica. It is restricted to the value of the original charge of the surface in pure water. The adsorption difference remains positive and large over this huge concentration interval. The wetting tension decreases with concentration. Even when the adsorption difference is constant, the surface pressure difference increases with increasing concentration because the chemical potential increases. The differences in how the dewetting occurs for mica and silica can be correlated with the fact that the surface charge on silica is an order of magnitude lower at normal pH.14-17 Note that the cmc (critical micelle concentration) for CTAB is 10-3 M.30 Compared to the case for cationic surfactants for DSPE, the free energy of electrostatic binding of the zwitterionic head group to the negatively charged substrate is smaller. However, our experimental results indicate that the deposited layer of DSPE also undergoes a spontaneous compression on going from the water-air to the mica-air interface in a similar fashion if not to the same extent as does CTAB. The reasons may be the presence of the cationic amine group which interacts with the negatively charged surface groups. Because of this effect the head group interacts more strongly with mica than with water. On going from the solid-vapor to the solid-water interface, adsorption is again smaller. For the insoluble DSPE it can be expected to be even lower than that for cationic surfactants. The electrostatic contribution is less favorable for a zwitterion than for a cation, and a larger chain represents a more unfavorable hydrophobic contribution. As the strength of the head group-substrate interaction diminishes, the difference between a larger surface pressure at the solid-vapor interface and a smaller surface pressure at the water-air interface becomes smaller. In the absence of specific binding of the head group of the surfactant to the surface of the solid, dewetting effects are no longer pronounced. Contact angle does not increase or change significantly when the change of the wetting tension equals the change of the surface tension. In the limit of zero contact angle, adsorption and surface pressures at water-air and solid-air interfaces are equal. This situation occurs for anionic surfactants such as soaps (salts of fatty acids) or sodium dodecyl sulfate (SDS),3 for which an ion exchange mechanism of binding to silica or mica can be ruled out, as the surface sites also are anionicacidic. Adsorption and surface pressure at the solidwater interface are small. For such surfactants adsorption at both the liquidvapor and solid-vapor interfaces is driven by hydrophobic effects alone. The monolayer is transferred onto the plate at the same value of density at which it is present at the water-air interface. It is in this case of zero contact angle that the touching technique is applicable. Such a monolayer is deposited spontaneously but also is more easily washed away by pure water. Unlike cationics, nonionics and anionics if not mediated by complexation with polyvalent cations do not show dramatic effects on wetting of negatively charged hydrophilic substrates. It is clear that the strength of the specific binding of the head group to the substrate is an important factor responsible for the extent of hydrophobicity achievable for each given pair of surfactant and solid. When the liquid is not subject to any forced movement, amphiphiles with a stronger affinity to the solid-air (30) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; NSRDS-NBS 36; U.S. Government Printing Office: Washington, DC, 1971.

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interface than for the other two interfaces will slide off the surface of the liquid, migrate across the three-phase line, and accumulate on the solid surface at the vapor side. This process, during which the originally high hydrophilic meniscus is pushed downward along the solid surface, is initially controlled by diffusion of the lipid molecules from the water-air interface to the three-phase line. The process does not need activation and so occurs spontaneously and quite rapidly. However, as adsorption increases locally and the layer condenses, subsequent equilibration slows down. A rearrangement of the condensed layer at the solid-vapor interface involves mechanisms of solid state (self) diffusion rather than liquid state interfacial diffusion. The large molecular size of the bulky lipid and its almost total lack of solubility in water are what make the activation barriers particularly high for DSPE and similar compounds. Even CTAB, which has a smaller molecule and is water soluble, shows a greater tendency to reach the final state of equilibrium, but DSPE does not. For the latter the advancing wetting tension does not change even after several days. While spontaneous rearrangement is now almost prevented when molecular frictional effects come into play while the monolayer condenses, the layer can undergo further forced compression by the advancing wetting front. However, wherever the forced movement of the wetting line is stopped, it is there pinned and no spontaneous changes to the meniscus further occur. Only after the maximum compression which the layer can withstand without shearing is reached does the liquid begin to advance. Here we have a high limiting value for the obtuse advancing angle. When the surface pressure at the solidvapor interface reaches the collapse value, and this is almost 50 mN/m, the theoretical wetting tension would be 20 mN/m or the contact angle would be 70-80°. This then could be the equilibrium value. The maximum experimental advancing values of the contact angle are higher still. They indicate that a supercompression is possible. With cationic surfactants, consider what happens if the liquid is forced to advance after the hydrophobic band forms by diffusion. This occurs when a clean plate is continuously moved slowly down into the solution. The meniscus jumps over this band and spreads over the still clean polar surface above it. Then the equilibration process is repeated. The contact angle again rises while a new band is formed. The previously formed band which is now under water gradually dissolves. The mechanism gives rise to the characteristic stick-slip movement of the three-phase line often observed on the first immersion of a hydrophilic substrate in water containing a surfactant. This effect occurs for cationic surfactants on silica and mica with a strong head group-substrate interaction.16,17 For DSPE with the weaker bonds between the head groups and the substrate, the deposit is more easily pushed back up along the solid-vapor interface by the advancing wetting line rather than being transferred under water. The nonequilibrium, thermodynamically unfavorable transfer occurs for the more strongly adhering and frictionally resistant monolayers of the less hydrophobic cationic surfactant, and not for the lipid. The stick-slip mode is characteristic of CTAB but not of DSPE. A spontaneous spreading of surfactant molecules to distant areas of the surface far away from the meniscus, while thermodynamically favorable, does not occur for kinetic reasons. Even for CTAB, which is more mobile than DSPE, as shown by shorter equilibration times and the smoothness of the equilibrium meniscus, diffusion onto more distant areas of the solid-vapor interface goes

Yaminsky et al.

prohibitively slowly. Only if the liquid is forced to recede, that is when the plate is retracted from the solution, is a uniform hydrophobic layer left behind the retreating line. It then covers the entire macroscopic area of the solid evenly as it emerges out of the liquid. The receding line moves smoothly also for CTAB because it does not now meet hydrophobic barriers in its progression. If the speed of the retraction is slow enough, the deposited densities are as high as those for equilibrium adsorption at the solid-vapor interface. Of course, the most perfect layers can be deposited with surfactants that give zero zipper angle because there is no hysteresis and the monolayer slides smoothly onto the solid. But this layer will be the least stable in water. For more stable, zippering layers microjumps of the irregular meniscus can spoil ideal deposition. For the line to be smoother, conditions should be closer to equilibrium. But this implies longer retraction times. Our test for the deposition from underwater then might be worth further examination. Otherwise at lower ΠLV the ideal meniscus forms for CTAB, not for DSPE. The meaning of the zipper angle will now be clarified. The basic considerations follow thermodynamically. When the angle is large, deposition of an essentially condensed monolayer at the solid-vapor interface occurs even when the adsorbed layer at the liquid-air interface is in a gaslike state. The surface pressures are orders of magnitude less than those typically used for Langmuir-Blodgett depositions. Condensed layers of CTAB are formed at interfaces of mica or silica in air at very small sub-pzc concentrations. Here there is no measurable surface pressure at the solution-air interface.16,17 For CTAB, as the chemical potential (concentration) further increases, adsorption at the water-air interface also becomes significant. The surface pressure, ΠLV, now increases in a range of concentrations, from almost zero at concentrations at the pzc up to about 35 mN/m at the cmc. For CTAB on silica and mica the wetting tension goes through a minimum in this range.16,17 This is because adsorption at the solid-liquid interface, which remains small up to the pzc when the maximum of hydrophobicity is reached, also increases dramatically on approaching the cmc as a bilayer adsorbs at this interface. As opposed to the case for soluble CTAB, the wetting tension for the water insoluble lipid DSPE decreases monotonically with increasing chemical potential over the whole range of surface pressures at the water-air interface. These range from zero to values of about 30 mN/m, as high as those for micellar solutions of soluble surfactants. This shows that the lipid does not adsorb at the water-mica interface either as a monolayer or in a bilayer form, even when prescribed values of the chemical potential are high. Further, when a mica or silica surface is retracted from CTAB solution and then placed in pure water, the surface remains hydrophobic over a long period of time. Large contact angles are still observed after many hours or even days of immersion. While desorption into the bulk of water even for soluble surfactants like CTAB is slow, the layer, which adheres more strongly to the surface than does DSPE, is still laterally mobile. Molecules travel from the bulk and along the surface and stick to the boundary of the meniscus. Trace amounts of CTAB present by desorption on reimmersion in a limited volume of pure water would be enough to support finite contact angles. The equilibrium contact angles are already high at concentrations as low as 10-8 to 10-7 M. The rapid decrease in the wetting tension of the mica sheet when DSPE is spread at the water-air interface is also observed with water soluble surfactants. When a

Wetting Tension Study of LB Films

small amount of CTAB is added to pure water, an irregular nonwetting meniscus immediately forms. In this case, however, relaxation leads to formation of a smooth wetting line. This is what is expected for adsorption equilibrium at a uniform substrate. For the insoluble lipid the receding wetting tension responds to an increase of the surface pressure at the water-air interface in a thermodynamically determined way. The meniscus however stays more or less irregular over much longer periods of time even when the relaxation occurred from receding conditions. This shows that the final approach to equilibrium, under which condition the rates of adsorption and desorption are balanced and no structural rearrangements of the layer adjoining the three-phase line occur, is more retarded for the DSPE layer. The reduction of the meniscus height is irreversible. The three-phase line stays pinned if the DSPE layer at the water-air interface is subsequently decompressed. This is the effect which gives rise to the wetting hysteresis observed as the DSPE layer on mica is immersed in initially pure water. 6. Conclusions The mere fact that a monomolecular layer can change wetting dramatically is well-known since the famous works of Langmuir.31 If only hydrophobic chains are exposed, the surface is as hydrophobic as paraffin, irrespective of the nature of the substrate that underlies the monolayer. These are straightforward considerations as long as one assumes that the layer does not undergo changes on contact with pure water. This does not necessarily hold for LB monolayers. Lateral mobility and detachment, molecular reorientation and segregation, and desorption transfer back to the liquid-air interface take place. A poor solubility of the amphiphile used for deposition, or a perfect structure of the layer formed in the course of deposition when taken as given, provides little guarantee that the surface will be stable in changing environments. We have shown that during deposition at high pressure the layer can be maintained at equilibrium with the monolayer at the liquid-air interface. Initially there is no hysteresis when a clean plate is first immersed and then retracted. While the substrate is kept in air after deposition, the surface pressure, ΠSV, of the deposited layer is high. We have seen that it is even larger than the pressure controlled on the trough. When the surface is subsequently immersed in pure water, it is therefore in a highly nonequilibrium state. Even if desorption into the bulk of water is hindered by low solubility, the layer can easily be transferred back to the water-air interface. Adsorption re-equilibration at the three-phase line occurs. It can be prevented only if the head groups of an (31) Langmuir, I. Chem. Rev. 1929, 6, 451.

Langmuir, Vol. 13, No. 6, 1997 1757

irreversibly chemisorbed surfactant are immobilized on the solid substrate by e.g. nonhydrolyzable covalent bonds.20 When pushed away from the solid-liquid interface, ordinary monolayers condense at the threephase line. Such a condensation equally occurs on a clean plate partly immersed in water even at the low surface pressures at the liquid-air interface. A high contact angle can be maintained when hardly any of the initially deposited monolayer is left on the substrate under water. Hence, it can be concluded that hydrophobicity and monolayer stability are not one and the same issue. When two such surfaces are brought together in a surface force experiment, the lateral mobility of the lipid deposit under water, which we find can be even greater than that for a soluble surfactant such as CTAB, allows the deposited lipid material to condense in the narrow contact slit. A similar effect arises also under reversible conditions with soluble cationic surfactants. For these it has already been shown to be responsible for the long range attraction between silica and mica surfaces in CTAB.22 This relation between monolayer instability and the even longer range hydrophobic attraction between LB monolayers has been suggested earlier.9 The similarity between this attraction and an extremely long range capillary force between hydrophilic surfaces in water vapor due to ionic condensates (“polywater”) will be considered separately.32,33 Dewetting is a general consequence of physical surfactant adsorption at the three interfaces which meet together at the three-phase line. In the case of insoluble amphiphiles that show specific head group-substrate interaction, dewetting is achieved by spontaneous migration of molecules from the liquid-air to the solid-vapor interface. It always arises when the free energy of adsorption at the solid-air interface exceeds the free energy of adsorption at the solid-liquid interface. The effect gives rise to a surface pressure discontinuity at the three-phase line, and its origin can be revealed by the same thermodynamic argument which ordinarily applies to surfaces of surfactant solutions. Acknowledgment. We appreciate help and discussions with H. K. Christenson, who is greatly thanked for his generosity. We thank Professor Ka˚re Larsson for discussions. T. N. acknowledges financial support from the Swedish Council for Forestry and Agricultural Research. LA9607161 (32) Yaminsky, V. V. Langmuir 1997, 13, 2. (33) Yaminsky, V. V. Colloids Surf., in press.