J. Phys. Chem. 1989,93,1051-1059
7057
Effects of Humidity on the Structure and Adhesion of Amphiphilic Monolayers on Mica Y. L. E. Chen, M. L. Gee, C. A. Helm, J. N. Israelachvili,* and P. M. McGuiggan Department of Chemical Engineering and Materials Department, University of California. Santa Barbara, California 93106 (Received: July 10, 1989)
We report the results of experimental measurements on various physical properties of single-chained and double-chained surfactant monolayers adsorbed on mica surfaces exposed to both water and organic vapors. We find that varying the vapor pressure can have very dramatic effects on the adhesion (surface energy), film thickness, mobility, and other molecular relaxation mechanisms of monolayers. The results of these studies point to a very intimate involvement of vapors on the static and dynamic properties of both isolated monolayers and two adhering monolayers.
Introduction Surfactant and lipid monolayers and multilayers adsorbed onto solid surfaces have long been the focus of both fundamental and applied research. Such films can be deposited onto surfaces by use of the Langmuir-Blodgett technique or by adsorption from solution (self-assembling films). The properties of well-ordered, close-packed monolayer films have been characterized by use of a large variety of techniques.' However, there is less concrete data when the chains are in the amorphous state and it is still not possible to unambiguously distinguish between chains in the solid amorphous or liquid states. In addition, surprisingly few studies have been made on the effects of humidity or other vapors on monolayer-coated surfaces exposed to ambient conditions. It has long been known that the thickness, phase state (solid or liquid), and swelling of surfactant and lipid bilayers in water can be very dependent on the water content2 and the type of organic solvent resent.^ These effects are due to the penetration of these molecules into the hydrophilic head-group region or hydrophobic chain region of the bilayers. However, little work has been done on the behavior of monolayers adsorbed onto solid substrates in air where the conditions are equivalent to an undersaturation of both water and organics. What effects does varying the ambient conditions produce on the structure and properties of the monolayer? This is an interesting and important question, since such monolayers are increasingly being developed and used as chemical sensors, in optical and electronic devices, protective coatings, information storage devices, and for producing low-adhesion and low-friction surfaces.' We have carried out a series of experiments on the effects of vapors on monolayers with head groups which hydrate differently and report here the most novel of our findings to date. These indicate that the physical properties of monolayers can be very sensitive to atmospheric conditions in ways that do not appear to have been previously reported or appreciated. Experimental Section Surfactants and Lipids. The single-chained CTAB (hexadecyltrimethylammonium bromide, CH3(CH2)15N+(CH3)3Br-, Sigma, recrystallized from acetone/ethanol), double-chained DHDAA (dihexadecyldimethylammonium acetate, [CH3(CH2)15] 2N+(CH3)20Ac-,Sogo, ion exchanged from the bromide form4) and double-chained D M P E (L-a-dimyristoylphosphatidylethanolamine, Sigma, used as received) were used for these studies. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, A. E.; Garoff, S.; Israelachvili, J. N.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Williams, R. M.; Chapman, D. Prog. Chem. Fats Lipids 1970,11, 3 . Sirota, E. B.;Smith, G. S.; Safinya, C. R.; Plano, R. J.; Clark, N. A. Science 1988, 242, 1406. (3) Gruen, D. W. R.; Haydon, D. A. Biophys. J. 1981,33, 167. Pope, J. M.; Littlemore, L. A.; Westerman, P. W. Biochim. Biophys. Acta 1989, 980, 69
(4) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W. J. Phys. Chem. 1986, 90, 5841.
Preparation of Surfactant-Coated Mica Surfaces. CTAB and DHDAA monolayers were formed by immersing the mica in 5 X lo4 M and I X lo-' M aqueous solutions of the surfactant respectively for 30 min. The surfaces were then retracted, dipped in triply distilled water, and then dried with pure, dry N2. The surfactant cations readily displace the K+ ions on mica and adsorb from the solution forming an approximately 16-A-thick surfactant m~nolayer.~ In contrast, DMPE is highly insoluble in water. Therefore a monolayer was deposited onto mica by the LB t e c h n i q ~ eunder ~.~ a pressure of 38 mN/m yielding a compact monolayer of thickness 23-24 A and molecular area 43 A2 as determined from transfer ratios. Experimental Procedure. The surfactant-coated surfaces were installed into a surface forces apparatus,' and the relative humidity of the chamber was controlled by using various saturated salt solutions, allowing at least 12 h of equilibration time. The optical interference technique7 using fringes of equal chromatic order (FECO) enables measurements of film thicknesses to within about 1 A. In addition, from the shapes of the interference fringes one can also ascertain whether a capillary condensed bulk-liquid meniscus (bridge) had formed between the two cylindrically curved surfaces on coming into contact.I0 From the measured adhesion or "pull off" force, F, needed to separate the two surfaces from contact, the solid/vapor adhesion energy, or surface energy, ysv, was determined according to the well-known equation'' ysv = F / h R
(1)
where R is the radius of the curved surfaces. Note that in the presence of a capillary condensed liquid annulus bridging the two surfaces, the measured adhesion force is now mainly due to the Laplace pressure arising from the liquid/vapor interface. Hence the surface energy yLv is now given by" yLv
F / ~ T R ( c o s8)
(2)
where 8 is the contact angle of the liquid on the solid. Results Effect of Humidity on Monolayer Thickness, Swelling, and Structure. At zero or low humidity the structure of the singlechained CTAB monolayer is likely to be as shown in Figure 1A. The chains of double-chained surfactants are expected to be more tightly packed, straight, and aligned vertically to the surface. The (5) Israelachvili, J. N.; Perez, E.; Tandon, R. K. J. Colloid Interface Sci. 1980, 78, 260. Pashley, R. M.; Israelachvili, J. N. Colloids Surf.1981, 2, 169. (6) Marra, J. J. Colloid Interface Sci. 1985, 107,446; Ibid. 1986,109, 11. (7) Israelachvili, J. N.; Adams, G. E. J. Chem. SOC.,Faraday Trans. 1 1978, 74, 975. (8) McGuiggan, P. M.; Pashley, R. M. Colloids Surf. 1987, 27, 277. (9) Marra, J.; Israelachvili, J. N. Biochemistry 1985, 24, 4608. (10) Fisher, L. R.; Israelachvili, J. N. Colloids Surf.1981, 3, 303. (1 1) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, New York, 1985.
0022~3654/89/2093-1051$01.50/0 0 1989 American Chemical Society
7058 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
Letters
Figure 3. Capillary condensed water at bifurcation of two CTAB monolayer-coated mica surfaces exposed to humid air near saturation. Aqueous regions are shown by the shaded patches.
Figure 1. (A) Schematic impression of a single-chained surfactant monolayer (Le., CTAB, for which the head-group area is larger than the hydrocarbon chain area) on mica under dry conditions. (B) Hydrated CTAB monolayer, Le., monolayer exposed to humid atmosphere, showing that water molecules (black dots) have penetrated into the headgroup/interfacial region. The surfactant molecules are now highly mobile and can diffuse laterally as well as flip-flop across the monolayer. (C) Penetration of organic molecules (e.g., alkanes) into monolayer chain region. 2.0
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of the relative hydration of the head groups. As can be seen from Figure 2, the swelling was most pronounced for CTAB (the most hydratable head group) and least pronounced for DMPE (the least hydratable head group). Eflect of Humidity on Fluidity of Monolayers. With increasing humidity there are also clear indications (see below) that the monolayers become less tightly bonded to the mica surfaces and that the surfactant molecules become increasingly more mobile. The effect was most pronounced for the single-chained CTAB monolayer where at humidities above 55% there was a transition from an immobile solidlike monolayer to one that was clearly in the fluid state where the molecules could now diffuse laterally and transversely, i.e., overturn or “flip-flop”, across the monolayers. These effects were ascertained by carefully monitoring the time dependence of the changing shapes of the FECO fringes when the two surfaces were in contact and separated, for different humidities. Flip-flop of the CTAB chains was inferred from the increase in hydrophilicity of the surfactant-coated surfaces at relative humidities above 55-75%, since capillary condensed water could be easily “seen” at the periphery of the contact zone from the FECO fringe pattern.1° This indicates that the contact region had become more hydrophilic due to overturning of CTAB molecules, as shown in Figure 3. More evidence of this phenomenon was obtained by the observed thinning of the CTAB monolayer at the three-phase contact line. Furthermore, the condensing liquid meniscus grew slowly with time in contact. If the chamber was rapidly dried by a sudden purge of dry N2while the surfaces were still in contact, the water evaporated immediately leaving behind its residual surfactant at the periphery. This precipitated surfactant (seen as small bumps in the FECO fringes) remained immobile under dry conditians but reincorporated back into the monolayer once water vapor was reintroduced. By contrast, if the surfaces were separated under humid conditions, the water meniscus evaporated leaving behind some surfactant molecules. Within seconds, these burrow their way back into the monolayer returning it to its original pristine state (Figure 1B) as ascertained from the flatness of the FECO fringes on bringing the surfaces back into contact. All these effects were reversible and reproducible and clearly indicate that the molecules in the hydrated monolayers are highly mobile. By carrying out a variety of time-dependent measurements on surfaces in contact, and then separating them for different time intervals and bringing them back into contact again, it was possible to ascertain that the flip-flop rates were of the order of 1-10 ,s for fully hydrated CTAB monolayers, whereas no flip-flop occurred at all for monolayers in the dry state (at least over a period of many hours). That the CTAB monolayers were in the fluid state when exposed to vapor above a certain humidity was also
0 20
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Figure 2. Change in monolayer thickness and surface energy as a function of relative humidity of CTAB, DHDAA, and DMPE monolayers on mica in a nitrogen atmosphereat 25 OC. Surface energies were deduced frorh the measured adhesion forces by use of eq 1. Both the swelling thickness and surface energies are normalized to the values at zero humidity (see text). The relatively large range of adhesion and swelling values obtained with CTAB is partly dependent on the monolayer density (coverage).
thickness of CTAB and DHDAA monolayers increased with increasing humidity, as shown in Figure 2. This indicates that water diffuses into the head-group region (shown schematically in Figure 1B for CTAB) and that the hydrated head groups become slightly separated from the mica surface by a hydration layer. No measurable swelling was observed for close-packed DMPE monolayers. These observations can be explained in terms
Letters established by the much increased compressibility of the monolayers (to be described in a later publication). The effects seen with the two double-chained surfactants were much less pronounced than with CTAB. The swelling was less (Figure 2); there was no observable capillary condensation, and there were no indications that the monolayers ever became mobile/fluid above a certain humidity. Moreover, the monolayers could be easily damaged and, in contrast to CTAB, never self-annealed. Effect of Humidity on Adhesion Forces and Energy. At zero relative humidity the adhesion energies of all three monolayers, as deduced from eq 1, were all in the range ysV= 23-40 mJ/m2, which covers the range expected for hydrocarbon surfaces composed of CH3 and/or CH2 groups. With increasing humidity the adhesion energies generally increased, especially above 55%, as shown in Figure 2. In the case of CTAB, we have already noted that water condenses at the periphery of the contact zone which becomes more hydrophilic due to surfactant flip-flop. Some flip-flop also occurs, but to a lesser extent, within the contact zone itself. This is illustrated in Figure 3 and has been inferred previously from contact angle measurements of aqueous surfactant solutions on CTAB-coated and DHDAA-coated mica.8 The existence of this water meniscus leads to an additional Laplace pressure, which is responsible for the increased adhesion since eq 2 now replaces eq 1. Since the capillary condensed water contains some surfactant at the water-air meniscus interface (Figure 3), its surface tension yLvis reduced below the value for bulk water but remains above that for two purely hydrocarbon surfaces in dry conditions. Effect of Organic Vapors on Monolayer Properties. We have studied the effects of saturated organic vapours of cyclohexane and various alkanes from c6 to c16 and have found that these molecules can also cause the monolayers to increase their thickness (i.e., swell) by a few angstroms, presumably by penetration of these hydrophobic molecules into the chain regions (Figure 1C). We have observed (i) that lower molecular weight alkanes penetrate more readily than the higher alkanes, (ii) that penetration is progressively reduced to zero as the chain packing density (monolayer coverage) increases, and (iii) that the compressibility of alkane-swollen monolayers is higher than for alkane-free monolayers. Quantitative data on these systems will be discussed later.
Discussion and Conclusions The thickness changes of CTAB, DHDAA, and DMPE monolayers on mica when exposed to water vapor correlates with the hydrophilicity (maximum hydration) of their head groups. The results point to a mechanism that involves water penetration into the head groups and the formation of a thin interfacial hydration
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7059 layer and reduced bonding between the head groups and the mica surfaces. In this regard, the swelling mechanism appears to be similar to that occurring in lyotropic liquid crystals* where the interbilayer separation increases as a function of water content and head-group hydrophilicity. With increasing hydration the CTAB molecules become significantly more mobile once the humidity exceeds 55%. These findings also correlate with the well-known reduction in the melting/transition temperatures and phase changes of multilamellar bilayers induced by increasing water content? The observed penetration of alkane molecules into monolayers from vapor also correlate with the penetration of alkanes into lipid bilayers from the bulk liquid^.^ From our adhesion studies we also conclude that the structure of isolated and contacting monolayers are different. When two monolayers come into contact in the presence of water vapor, some of the surfactant molecules overturn and even exchange between the two monolayers. The whole physical state of contacting monolayers appears to change in the contact region resulting in, among other things, an increased adhesion (surface energy) between them. The Gibbs adsorption isotherm, which would predict a reduced surface energy, does not appear to be valid under such conditions. The effect of humidity on the thickness, phase state, and adhesion of monolayers appears to depend on both the nature of the head group and the number of chains. For fully condensed monolayers the swelling is largely determined by the nature of the head groups while the adhesion depends also on the number of chains (see Figure 2). We have found that these properties also depend on the surface density or coverage of the monolayers and can require long times to reach equilibrium, again depending very much on the type of monolayer and the relative humidity (longer times are generally needed for drier monolayers). A full account of our findings, including the time-dependent effects, will be reported a t a later date. In conclusion, our results imply that we must be more aware of the crucial role of ambient conditions on the structure, phase state, bonding, adhesion, hydrophobicity, lubricity, molecular relaxation mechanisms, and “annealing” times in monolayers adsorbed onto solid surfaces. Acknowledgment. Y.L.E.C. thanks IBM for a graduate fellowship sponsored by SUR Research Award No. 800612; we also thank the N S F for financially supporting this work under grant No. CBT87-21741 and C.A.H. thanks the Deutsche Forschungsgemeinschaft for a postdoctoral research scholarship. We gratefully acknowledge inspirational and encouraging discussions with S. Bubbles and R. Duckie.
