Langmuir 1991, 7, 2694-2699
2694
Measurements of the Elastic Properties of Surfactant and Lipid Monolayers Y. L. Chen,? C. A. Helm,$ and J. N. Israelachvili**+ Department of Chemical & Nuclear Engineering and Materials Department, University of California a t Santa Barbara, Santa Barbara, California 93106, and Znstitut fiir Physikalische Chemie, Johannes-Gutenberg- Universittit, Jakob- Welder- Weg 11, 0-6500 Mainz, Germany Received May 8, 1991. I n Final Form: July 29, 1991 The surface forces apparatus technique was used to study the elastic properties of amphiphilic monolayers and bilayers on solid surfaces of mica. Both single-chained and double-chained surfactants (or lipids) were used, and two techniques were used in their preparation: controlled Langmuir-Blodgett deposition and spontaneousself-assembly (adsorptionfrom solution). Our aim was to measure the thickness and compressibility properties of these films and to study how different methods of preparation (e.g., surfactant density) affect these properties. The compressibilities of adsorbed monolayers are in the range (0.5-2) X 108 N/mZ, which is about an order of magnitude higher than those of the bulk materials, but are similar to the values of free lipid bilayer membranes in aqueous solutions.
Introduction Self-assembling and Langmuir-Blodgett (LB) films deposited on surfaces are used in optical and electronic devices, as chemical sensors, protective coatings, surface modifiers, information storage devices, and for producing low adhesion and low friction surfaces. Some of the properties of these films such as their density and molecular orientation, thickness and thermal stability, adhesion and friction, have been studied by a number of technique5.l-3 However, it has not been easy to measure the elastic properties of monolayers on solid surfaces. A number of techniques, such as Brillouin ~cattering,~ electron energy loss spectroscopy (EELS): and helium atom scattering (HAS)? have been used. Unfortunately, none of these techniques are currently sensitive enough to measure the properties of single monolayers. The most detailed results have come from a Brillouin scattering study of the Rayleigh, Love, and Sezawa modes off cadmium arachidate multilayer films on Mo containing 11 to 401 monolayers. The elastic moduli both perpendicular and parallel to the multilayers were found to be similar to that of bulk organic materials," viz., 109-1010 N/m2. However, because of the strong electrostatic interaction between the first monolayer and the solid substrate (which is often very different from the interaction between the monolayers themselves), the elastic modulus of a single adsorbed monolayer is expected to be different, and it cannot be inferred from measurements on a multilayer film. In contrast, the elastic properties of free surfactant and lipid bilayers in water have been measured using electrocompression2 and pipet aspiration methods.3 Typical values for the lateral area compressibility modulus of a t University of California at Santa Barbara. t Johannes-Gutenberg-Universit&,
(1) Swalen,J. D.; Allara, D. L.; Andrade, J. D.;Chandroes, A. E.; Garoff, s.;Israelachvili, J. N.;McCarthy, T. J:; Murray, R.; Pease! R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,9,932. Mhbiue, D.: Mhhwald, H.Adu. Muter. 1991,9, 19. (2) Alvarez, 0.;Latorre, R. Biophys. J. 1978, 21, 1. (3) Kwok, R.; Evans, E.A. Biophye. J. 1981,35,637. Evans, E. A,; Kwok, R. Biochemistry 1982,21,4874. Evans, E.; Needham,D. J.Phys. Chem. 1987,91,4219. Cevc, G.;Mareh, D. PhospholipidBilayers; Wiley: New York. 1987. (4) Zanoni, R.; NaaeUi, J.; Bell,J.; Stageman,G. I.; Seaton,C. T. Phys. Reo. Lett. 1986, 57 (22), 2838. (5) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic Press: New York, 1982. (6)Toennis, J. P. J. Vac. Sci. Technol., A 1984, 2, 1055.
0743-7463/91/2407-2694$02.50/0
variety of double-chained bilayers in the fluid state range from K A = 100to 230 mN/m, corresponding to thickness compressibility moduli of K = K A /=~0.5 X 108 N/m2 (5 X 108dyn/cm2),where 6 is the bilayer thickness. However, it is not obvious that these values also apply to adsorbed bilayers or monolayers. Other types of compressibility measurements include routine pressure-area (II-A) measurements of insoluble monolayers a t the air/water interface. These give a wide range of values for the lateral compressibility depending on the phase state and type of headgroup. Concerning thickness measurements, a number of techniques, such as X-ray specular diffraction, small angle neutron scattering (SANS), capacitance, ellipsometry, and other optical techniques are available to determine the thickness of both adsorbed monolayers or free bilayers in solution. Our first aim was to measure the thickness of adsorbed monolayers trapped between two mica surfaces and compare these with previouslymeasured values using other techniques. Our second aim was to measure the elastic properties of these films and to study how these depend on the method of preparation (e.g., surfactant density).
Materials and Methods Materials. The single-chained CTAB (hexadecyltrimethylammonium bromide) with molecular structure CH&H&N+(CH~)SB~was purchased from Sigma Co. and recrystallized from acetone/ethanol mixture. The double-chained DHDAA (dihesadecyldimethylammoniumacetate) with strucwas purchased from Sago Pharture [CH~(CH~)~&N+(CHS)~ACmaceutical Co., Japan,and ion exchanged from the bromide form (made by Dr. P. McGuiggan). The phospholipids DMPE ( ~ - a dimyristoylphoephatidylethanolamine) was purchased from Sigma Co. and used as received (quoted purity, 99.9%). Surface Force Apparatus (SFA). We used a standard SFA (Mk11) to measure the externally applied force between two monolayer-coatedmica surfaces in contact and, simultaneouely, the contract area and thickness of the trapped films between the two surfaces. Details of this apparatus are described in the references.' In this apparatus two thin sheets of atomically smooth mica are glued to cylindrical lenses of glass with the cylindrical axes at right angles. This geometry is locally equivalent to a sphere near a flat surface. The shapes of the surfaces and the separation between them are measured by (7) Ieraelachvili,J. N.; Adam, G. E. J. Chem. SOC., Faraday Tram. I 1978, 74, 975.
0 1991 American Chemical Society
Langmuir, Vol. 7,No. 11, 1991 2695
Elastic Properties of Monolayers employing an optical interferencetechnique using fringesof equal chromatic order (FECO). These fringes, which can be viewed through the eyepiece of a spectrometer or recorded on a video camera, enable surface deformations and other shape changes to be continually monitored in real time and at the angstrom resolution level. Langmuir-Blodgett Deposition Technique. The Langmuir-Blodgett deposition was carried out in a Teflon-coated Joyce-Loebl trough with a surface area of 575 cm2, filled with deionized Millpore purified water as a subphase. The surface pressure was measured with a Wilhelmy-type surface balance. Lipids were dissolved in a mixture of chloroform and methanol at concentrationsof 1mg/mL. Before spreadingthe lipid solution on water, bare mica mounted disks were immersed into the subphase quickly. After the lipid solution was spread, 15-20 min was allowed for solvent evaporation. The monolayer was then compressed to the desired pressure, no,and the bare mica surfaces were slowly withdrawn from the solution at a speed of 0.4 cm/ min at constant surfacepressure. The transfer ratio was measured separately to determine the deposited lipid density (area per molecule). Immediately after deposition,the mica mounted disks were placed into the SFA chamber, purged with pure dry nitrogen gas, and equilibrated at a controlled temperature for 12 h. Surfaces Prepared by Adsorption from Solution. The surfactant solutions we& prepared one day before the experiments. The concentrations of the solutions were close to the critical micelle concentration (cmc), viz., 5 X lo-' M for CTAB and 7 X IOd M for DHDAA, and the pH was typically 5.7 f 0.2. Two mica sheets, previously mounted on glass disks, were immersed in the appropriate surfactant solutionfor 30min, during which time a monolayer would adsorb at a density of about 60 A2 per CTAB molecule and 75 A2 per DHDAA molecule, as ascertained from subsequent ESCA and optical thickness measurements (see below). The mica was slowly withdrawn from the surfactant solution, emerging dry, and briefly rinsed (washed) with Millipore filtered water in order to remove any possible second layer. Immediately after this, the surfactant-coated mica sheets were placed in the SFA. The chamber was then purged with dry filtered nitrogen and, as in the case of LB-deposited layers, equilibrated over P205 at a controlled temperature for 12 h. Unless otherwise stated, all the results presented here are for fully dried monolayers. Monolayer Thickness Measurements. UV radiation at wavelengths below 400 nm causes rapid degradation of organic materials.* The method works well in air partly because the UV light breaks covalent bonds and partly because the ozone generated by the UV light in the present of oxygen reacts with the resulting organic radicals (known as photooxidation) liberating volatile oxides such as COS. The effectiveness of UV degradation is very dependent on the intensity of the radiation, the irradiation time, and the distance between the lamp and the surface (see below). We and otherseJOhave found that surfactant monolayerson mica can be totally removed from the surfaces by this method. We also found that the heat generated during this process can result in a significant, approximately 35 "C, increase in temperature inside the SFA chamber-an effect that appears to enhance the UV degradation process. In the present experiments, UV degradation was carried out at the end of each experimental run with a pair of surfactantcoated mica surfaces. The two surfaces were separated, while still mounted inside the SFA chamber, until they were about 1 cm apart. A thin UV pen-lamp with wavelength 254 nm and intensity 4.5 mW/cmz at 2.5 cm, supplied by UV Products, Inc., was carefully inserted between the surfaces. During irradiation the chamber was connected to the outaide air via a filter. This was to allow entry of oxygen gas into the chamber, needed to form the ozone for oxidizing the surfactant layers. The monolayers were completely removed after about 2 h. (8) Graesie, N.; Scott, G. Polymer Degradation & Stabilization;
Cambridge University Press: London, 1985. Ranby, B.; Rabek, J. F. Photodegradation, Photo-oxidation and Photostabiliza tion of Polymer; John Wiley & Sons: London and New York, 1975. (9) Helm, C. A.; Tippmann-Krayer,P.; Mohwald, H.; Ale-Nielsen,J.; Kjaer, K. Biophys. J., in press. (,lo) Hirz, S. J.; Homola, A. M.; Hadzioannou, G.;Frank, C. W h n g mutr, in press.
Figure 1. Elastic sphere pressed against a flat surface, where each surface has an adsorbed monolayer of thickness l/p.Dr. The adhesion energy y is defined at the plane where the two surfaces meet and separate, as shown. After the monolayers had been removed, purified water was injected between the surfaces. The SFA was then purged with dry nitrogen and allowed more than 2 h to cool back to the original tempqrature (controlled to within 0.1 OC). This is important since for the optical thickness measurements to be accurate the mica sheets have to be at the same temperature, within 0.5 "C, before and after the monolayers are removed (mica has a thermal expansion coefficient of 0.26 f 0.03 (A/pm)/"C, so that a 0.5 "C change in temperature causes the thickness of two contacting 1.5-pm sheets to change by 0.4 A). After the temperature had equilibrated, the surfaces were brought into contact again under a range of external loads and the following were measured: the contact area as a function of load and the changing thickness of the mica sheets as a function of load. Prolonged UV irradiation for periods from 2 h to more than 4h no longer results in any measurable thickness change,although the adhesion between the mica surfaces increases slightly. This may due to the final removal of submonolayeramounts of residual or airborne contaminants.lO
Theoretical Background Compressibility Measurements via the JKR Theory. The Johnson-Kendall-Roberts (JKR) theoryl1-l4 is the basic theory that describes the "contact mechanics" of two adhering elastic spheres in contact under various externally applied loads. We have used the JKR theory to calculate the surface energy and stress distribution within the contact zone under negative, zero, or positive loads. In the JKR theory two spheres of radii R1 and R2, bulk elastic moduli KB,and surface energy W = 2y per unit area will flatten when pressed together under an external load or force, F, such that their contact area will have a radius a given by R a3 = - [F+ 6urR + (127ryRF + ( 6 ~ r R ) ~ ) " (1) ~l
KB
+
where R = R1R2/(R1 R2). For a sphere of radius R on a flat surface (Figure 1)or for two cross cylinders of radius R (the geometry we have here) we may put R2 = 03, R1 = R in the above equation. Under zero load (F = 0) the contact radius is finite and given by a. = (l27ryR2/KB)l/'
(2)
Equation 1further shows that under small negative loads (F< 0) the solids still adhere until a t some critical negative
force the surfaces suddenly jump apart. The adhesion or (11) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. SOC.London, A 1971,324, 301. (12) Israelachvili, J. N.; Perez, E.; Tandon, R. K. J. Colloid Interface Sci. 1980, 78, 260. (13) Horn, R. G.;Israelachvilli,J. N.; Fribac, F. J. Colloid Interface Sci. 1987, 115, 480-491. (14) Tabor, D. J . Colloid Interface Sci. 1977, 58, 2.
Chen et al.
2696 Langmuir, Vol. 7, No. 11,1991 "pull-off" force needed to separate the surfaces is given by
Fs = - 3 ~ R 7
Stress. a Stress, o
(3)
It is noteworthy that according to the JKR theory a finite elastic modulus, K B ,while having an effect on the contact area, has no effect on the adhesion force, Fs, an interesting and intuitively unexpected result that has nevertheless been verified experimentally."J2 The pressure or stress distribution within the contact circle is given byl1J2
where x = r/a (see Figure 1). When y = 0 we have the much simpler case of two nonadhering spheres, and the equations of the JKR theory reduce to those of the Hertzls theory: adhesion force, FS = 0; contact radius, a3 = RF/KB; pressure or stress distribution, u ( x ) = 3F(1 - x2)/27ra2. The last equation shows that at the center ( x = 0) the pressure is 3F/2?ra2, which is simply 1.50 times the mean pressure across the contact area. Most of the equations of the JKR theory and all the equations of the Hertz theory have been experimentally tested for molecularly smooth surfaces and found to apply extremely Thus, it has been verified that the contact area of two surfaces as a function of applied load is excellently described by eq 1for both nonadhering and adhering surfaces.13 In addition, eq 3 relating the adhesion force to the surface or interfacial energy has been found to be correct to within 25% for a variety of surfaces in vapors or liquids.12J6J7 The main drawback of the JKR theory is that it predicts an infinite stress at the contact boundary where the two surfaces bifurcate. This unphysical prediction comes from assuming that the adhesive forces between the two contacting surfaces have an infinitesimally short range (essentially a 6 function at contact with no interaction outside the contact zone). These deficiencies in the JKR theory can be neglected when calculating stress distributions well inside the contact zone. Accordingly, the stress or pressure was always measured at the center of the contact circle (at x = 0) where it is given by
do) = 1.5F + 67ryR + (121ryRF+ ( 6 ~ y R ) ~ ) ' l( ~5 ) maz
Results Thickness and Compressibility Measurements. The following parameters were measured during experiments (cf. Figures 1 and 2): (i) the undeformed radius, R, of the separated surfaces (to i3%); (ii) the contact radius a, versus applied compressive load F,from which the adhesion energy y was determined (to *lo% ) using eq 1;(iii) the mica thickness Dm and the film thickness Df (to fO.l nm), both at x = 0. The mica and film compressibilities were determined graphically from plots of u against AD. First, we note that the thickness compress(15) Hertz, H. J . Reine Angeur. Math. 1881, 92, 156. Also in Miscellaneous Papers; Macmillan: London, 1896; p 146. (16) Shchukin, E. D.; Amelina, E. A.; Yaminsky, V. V. Colloid Surf. 1981,2,221. Shchukin, E. D. In Microscopic Aspects of Adhesion and Lubrication;Georges, J . M., Ed.;Elsevier: Amsterdam, 1982; pp 389402. (17) Simmons, G.; Wang,H. Single Crystal E b t i c Constants and Calculated Aggregate Properties, 2nd ed.; MI" Press: Cambridge, MA, 1971; p 321.
B
A
Figure 2. (A) Two contacting mica sheets of total thickness D, = 2-8 ccm. (B)"Sandwich"consisting of two mica sheets with two adsorbed monolayersof total thickness Df= 3-5 nm trapped between them.
ibility K of any material is defined by18 K = D(au/aD) where D is the thickness of the film. Thus, we may obtain K from measurements of u and AD using the following equation
where AD is the change in thickness of the material. Equation 6 applies to any material or film (mica sheets, trapped monolayers, or a composite sandwich of both as in Figure 2B). Strictly because the FECO method measures the distance between the two outer silvered layers on the mica sheets, any change in the thickness measured, AD, actually includes the sum of the changes due to the mica sheets, AD,, and the trapped surfactant monolayers or film, ADf. Thus, what is actually measured is AD, = ADm + ADf (7) The question is, how can one extract the monolayers compressibility when this can only be measured with a thick sheet of mica sandwichingit on either side. Luckily, while the mica sheets are about lo00 times thicker than the trapped monolayers, they are also about lo00 times less compressible, and this fact has allowed us to measure the compressibilities of the much thinner, but also much more compressible,monolayers. This was done as follows. A precalibration of the compressibility of the mica sheets, with no surfactant layers between them (Figure 2A), was carried out using eq 6 and the results are shown in Figure 3. The slope of the line passing through the origin gives a compressibility modulus of Km = Kmia = D(au/dD,) = (0.91 f O.l)lO1l N/m2, which may be compared with the literature values for micas 08' (0.51.0) X 10" N/m2. We note that the above analysis ignores any possible variation of the refractive index of mica with applied pressure. This effect will modify the value of K,,, by a small (but unknown) amount. However, this effect may be ignored when analyzing of the compressibility of trapped monolayers between mica sheets, since it is already included in Km by definition. The above value obtained for K mwas used as a baseline for subsequent measurements with monolayer films between the mica surfaces (Figure 2B). In these experiments the total compressibility,KTOT,of the composite sandwich was measured, viz. _ _ ~
And since the normal stress u must be uniform throughout (18) Forapureieotropicmaterialthethicknesacompreesibilitymoddun
-
K is related to the bulk elastic compressibility KB that appears in tho JKRtheory by& = 2K/3(1- uz), where Y is Poisson's ratio. For typical values of u = 0.26-0.35, we have K KB. Note, however, that for the monolayer-mica-glue-glass system we have here the two values are quite different.
Elastic Properties of Monolayers
Langmuir, Vol. 7, No. 11, 1991 2691
60 50
-
MICA
DMPE 40
-
43 a 2
E 1
-5 z E
*
OO
2
4
6
30-
&! n 20E
2
0
10-
(lo-*) Figure3. Measured isothermal compressibilityof three different muscovite mica samples at 25 "C using eq 6 for mica: K , = oD,/AD,. Typical experimental values were D, = 2-8 pm, R = 1cm, y = 40-50 mN/m, F = 0-5 g, and a = 10-50 Mm. The line coresponds to an elastic modulus of K , = 0.91 X 1011N/m2. Strain, -AD,/D,
0-
I
I
I
I
Area Per Molecule on Water Surface (A2)
48
MICA with DMPE (67 i z )
Film Thickness, Df (A) 40 30 /
I
20 /
E.
Thickness Change, -AD (1) Figure 4. Measured thickness changes AD with stress u for a DMPE monolayer film (area per molecule 67 Az)between two mica sheets, from which Kf is determined using eqs 8-10. the composite, we also have K,,, = uDm/ADm and Kf = uDf/ADf (9) We may therefore express Kf in terms of the five measurable quantities ADTOT,Dm, Df, u, and Km as
Thickness Change, -AD, (A) Figure 5. Compressibilities of Langmuir-Blodgett deposited DMPE monolayers at different surfacecoveragesor phase states, as indicated by the E A curves for DMPE on water (top). Since the transfer ratios were not unity, the area marked along the horizontal axis (on water) is not the same as the molecular area deposited on the solid surfaces (shown by arrows). The monolayer compressibilities go from 0.6 X 108to 1.8 X 108N/m2as the phase state goes from liquid to solid. are resented for different surface coverages ranging from 43 per DMPE molecule (corresponding to crystalline close packed monolayers) to 67 A* per molecule (corresponding to more loosely packed monolayers in the liquidcrystalline state). The results are plotted in such a way that ADf = 0 refers to the full thickness (Df = 48 A) of two crystalline monolayers under zero stress (corresponding to 24 A per monolayer). Thus the intercepts at 0.2 and 1.2 nm for the 43- and 53-A2monolayers imply that these monolayers are thinner by 0.1 and 0.6 nm, respectively, than the crystalline one (in addition to being more compressible). The general trends shown in Figure 5 is of a compressibility modulus that decreases with decreasing surface coverage (density). The values ranged from 0.5 X 108 to 2 X lo8 N/m2 and are thus about an order of magnitude less than the bulk compressibility of -109 N/m2. We checked that no extrusion of the monolayers occurred during their compression; this was established by noting that the thickness returned to the original starting value (within f0.5 A) on decompression. Similar measurements were carried out using "selfassembling" monolayers of single-chained CTAB and double-chained DHDAA adsorbed from solution. The results are shown in Figure 6. These yielded similar results to those with DMPE in the more fluidlike states but are
l2
Since ADm, u, and Km are measured or previously calibrated, and since Dm and Df are determined from the positions of the FECO fringes before and after UV degradation, one may plot uDf againt (ADTOT- uDm/Km). The line drawn through the experimental points should pass through the origin and its slope gives the film compressibility, Kf. This procedure for determining both KTOTand Kf is shown in Figure 4 for a trapped DMPE film. In all the measurements, after R and a had been measured and y ascertained from a plot of a against F,l9 the stress wascalculated a t the contact center ( x = 0) using eq 5. Note that under zero external load (F= 0) there is already a finite compressive stress a t x: = 0. It is for this reason that the experimental points of Figures 3 and 4 generally start at some finite value of u rather than a t u = 0. Thickness and Compressibility of Surfactant Monolayers, Figure 5 shows the stress-strain curvesfor various Langmuir-Blodgett deposited DMPE monolayers. Data (19) Chen, Y. L.; Helm, C. A.; Ieraelachvili, J. N. J. Phys. Chem., in
prese.
2698 Langmuir, Vol. 7, No.11, 1991
Chen et al.
Table I. Thickness Compressibilities (in Order of Decreasing Compressibility) surfactant coverage (area per molecule),Az phase state compressibility, 108 N/m2 method SFA (this expt, fully dried monolayers) no deposition (mica) solid crystal 910 LB DMPE 43 cryst condensed 1.78 53 cryst expanded 0.91 LB DMPE 15 solid amorphous 0.78 adsorbed DHDAA' LB DMPE 61 cryst expanded 0.56 adsorbed CTABO 60 amorphous 0.51 other methods (on fully hydrated bilayers) electrocompression PE (bacterial) fluid 1.1 pipet aspiration PC (egg) fluid 0.5 a
Chen, Y. L.;Chen, S.; Frank, C. W.;Israelachvili, J. N. Paper in preparation. Film Thickness, D,
6)
4Y--71 I
I
I
I
I O
; 50
40
OO
5
10
Thickness Change, -ADf
15
Figure 6. Compressibilities of self-assembled monolayers of double-chained DHDAA and single-chained CTAB surfactants are found to be 0.8 X 108 and 0.5 X 108 N/m2, respectively.
again 10-50 times smaller than the Brillouin-scattering values for an 11-layer cadmium arachidate film on Mo, which showed that the elastic moduli parallel and perpendicular to the surfaces of these multilayers are again similar to that of bulk organic materials. Our results suggest that the elastic properties of both adsorbed and deposited single anhydrous monolayers on a solid substrate are different from those of deposited multilayers. On the other hand, our measured values are very similar to those of bilayer membranes in aqueous solution^.^-^ Our results for the compressibilities are summarized in Table I, where they are also compared to previously reported values obtained by using different techniques. Monolayer Thicknesses. It is well-known that both the area and thickness of monolayers on water change in a complex way with lateral pressure.g*21Accordingly, we determined to study the variation of monolayer thickness with molecular area of LB-deposited DMPE monolayers, exposed to both dry and humid conditions. Previous results have shown that adsorbed surfactant monolayers can swell when exposed to humid air22 due to wa@r penetration into the headgroup region, but no systematic study was made as a function of coverage. Our results, shown in Figure 7, show that the less compact monolayers are more susceptible to picking up water and swelling. Figure 7 also shows a comparison of the thicknesses measured with the SFA at 100%humidity and those previously measured using an X-ray specular (20)Cevc, G.; Marsh, D. Phospholipid Bilayers; John Wiley & Sons: New York, 1987. Marsh, D. CDC Handbook of Lipids; CRC Prees: Boca Raton, FL, 1990. (21) Geines, L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Intencience: New York, 1966. (22) Chen,Y. L.; Gee, M. L.; Helm, C. A.; Israelachvili,J. N.; McGuiggan, P. M. J. Phys. Chem. 1989, 93, 7057.
70
Area Per Molecule (A2)
20
(i)
60
Figure 7. Comparison of monolayer thicknesses as measured by the SFA technique using UV desorption to desorb the monolayers (measured at 0% and 100% humidity) and X-rays specular diffraction.9
Table 11. Monolayer Thicknesses As Determined by Different Methods coverage, monolayer methods surfactant A2 thickneea, A SFA CTAB 60 17 i 1.5 75 18.5 1.5 SFA DHDAA SFA CaABS 66 17 1.5 19* 1 neutron scatterin@ CaABS
**
Reference 23.
diffraction technique on fully hydrated monolayers at the air/water s u r f a ~ e .The ~ results are very similar. Finally, the thicknesses of some self-assembled (rather than LB deposited) monolayers adsorbed on mica are tabulated in Table I1 and, where possible, compared with the results of neutron scattering measurements.23 Again, there is good agreement.
Summary and Conclusions The SFA technique has been used to measure thickness and elastic properties of single monolayers either LB deposited or self-assembledfrom solutionon mica surfaces. The monolayers are about an order of magnitude more compressible than the bulk solid materials or multilayer films. They exhibit compressibilities close to 108 N/m2, which are similar to those of lipid bilayer membranes in aqueous solutions. Decreasing the monolayer coverage on the surfaces increasestheir compressibility (decreasing compressibility modulus), with monolayers in the liquid crystalline state being 4 times more compressible than those in the solid crystalline state. The combination of SFA, FECO, and UV desorption techniques allows one to accurately measure monolayer (23)Markovic, I.; Ottewill, R. H.; Cebula, D. J.; Field, I.; Marsh, J. F. Colloid Polym. Scr. 1984,262, 648.
Elastic Properties of Monolayers thicknesses under both dry and wet conditions in air. The thickness of a DMPE monolayer on mica a t 100%humidity is about 2 A more than in the dry state (this includes the water of hydration) and is very similar to previous measurements on DMPE monolayers a t the air/water interface. Results on the elastic properties of hydrated monolayers (asa function of adsorbed water content) will be published later. The results suggest that even when monolayers are adsorbed on a solid surface, they may not be as rigid or as strong as when they interact with other, similar monolayers within a solid-crystalline multilamellar phase. Clearly, the existence of a hard, rigid, and smooth surface is not sufficient to fully freeze the monolayers, probably because the substrate lattice is not "matched" to that of the monolayer (in the language of materials science: there
Langmuir, Vol. 7, No. 11, 1991 2699 is an epitaxial mismatch between the adsorbed layer and the substrate surface). We conclude, therefore, that the better molecular "packing" afforded by the other surfactant monolayersin a multilayer structure is more effective for providing material strength than a hard supporting substrate. Whether this applies to other surfactantsubstrate systems remains to be seen.
Acknowledgment. Y. L. Chen thanks Exxon Chemical Company and IBM for a graduate fellowship (IBM SUR Research Award No. 800612) and C. A. Helm thanks the Deutsche Forschungsgemeinschaft for a postdoctoral research scholarship. We also thank Drs. Helmuth MOhwald and Sid Simon for helpful discussions. Registry No. CTAB, 57-09-0; DHDAA, 71326-37-9;DMPE,
998-07-2.
