Langmuir 1992,8, 2491-2500
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Surface Tension Anisotropy and Relaxation in Uniaxially Compressed Langmuir Monolayers T. M. Bohanon,*t+A. M. Lee, J. B. Ketterson, and P. Dutta Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208 Received May 4,1992. In Final Form: June 29,1992
We have performed capillary wave measurements, both parallel and perpendicular to the direction of compression, on uniaxially compressed Langmuir films. We observed surface tension anisotropy only in the A and B (CS and Lz”) phases of heneicosanoic acid. These are also the phases whose reported X-ray diffraction peak widths are resolution limited. When surface tension anisotropy developed, it decayed only slightly over 30 min. Surface pressure measurements are ambiguous for phases in which the surface tension is anisotropic. At room temperature, where uniaxially compressed acid monolayers are isotropic, even though the films are known to calcium ions caused anisotropy only at the highest pH values (=ll), be stiff at moderate pH values (-6). We observed a large change in surface tension anisotropy with a small change in pH, signaling a phase transition.
Introduction Langmuir monolayers are often observed to be rigid or highly viscous. In fact, one of the first studies of monolayers’ determined how the viscosity and rigidity of a monolayer behaved as a function of pressure. However, pressure measurements may be meaningless if a monolayer is viscoelastic or solid. The surface pressure of a monolayer, the change in surface tension between clean water and film-covered water, is a scalar quantity.2 This quantity becomes undefined when the surface tension is anisotropic or inhomogeneous. Compressing a monolayer uniaxially results in a shear stress. Whether or not uniaxial compression will cause surface tension anisotropy depends on the mechanical properties of the monolayer. If a monolayer is a fluid with low viscosity, relaxation times will be short and the surface tension will be isotropic. If the monolayer is highly viscous, viscoelastic, or solid, it may be impractical to wait for the surface tension to become isotropic. In the latter case, compression should be performed isotropically or in a shape-preserving manner; uniaxial compression clearly does not fit this requirement. Several studies of the effect of uniaxial compression on monolayers have been performed,3 but only three have quantitatively studied surface tension anisotropy. In the first study, Miyano4 used orthogonal Langmuir balances to show that the surface tension of stearic acid with aluminum ions in the subphase is anisotropic. However, a Langmuir balance measures agross average of the surface tension and can only measure the surface tension a t the boundary of the monolayer (a severe disadvantage if the film is nonuniform).5 In the second study, Halperin et ale6used Wilhelmy plates to measure the difference in surface tension parallel and perpendicular to compression. They reported that monolayers of stearic acid and lignoceric acid are solids and a monolayer of pentadecanoic acid is not. However, viscous or rigid films force the t Present address: Johannes Gutenberg Universitiit, Institut fiir Organische Chemie, J. J. Becher-Weg 18, 6500 Mainz, Germany. (1) Langmuir, I.; Schaeffer, V. J. J. Am. Chem. SOC. 1937,59, 2400. (2) Pressure may ale0 be defined as the trace of the stress tensor;this makes it possible to always define a pressure. However, we will only discuss the more familiar definition. (3) See e.g. (a) Malcolm, B. R. Thin Solid Films 1989, 178, 17; (b) Peng, J. B.; Barnes, G. T. Langmuir 1990,6,578. (4) Miyano, K.; Maeda, T. Rev. Sci. Instrum. 1987,58, 428. (5) Miyano, K. Langmuir 1990, 6, 1254. (6) Halperin, K.; Ketterson, J. B.; Dutta, P. Langmuir 1989,5, 161.
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Wilhelmy plate away from the vertical, which produces erroneous measurements, and the plate can obstruct film flow7or deformation. Both of these problems depend on the orientation of the Wilhelmy plate, making it difficult to know if the surface tension would be anisotropicwithout the plate in the film. In the third study, Miyano5 used externally generated capillary waves to study surface tension anisotropy. He found the surface tension of a monolayer of stearic acid to be isotropic (in contrast to the work of Halperin et al.) and the surface tension of a monolayer stearic acid with aluminum ions in the subphase to be extremely nonuniform and anisotropic. Like Miyano, we used externally generated capillary waves to study surface tension anisotropy. The capillary wave probe, unlike the other two methods, is a nonintrusive method of studying surface tension. Also, only the surface tension in the direction of the propagating plane wave is measured. This makes direction-dependent measurementa less ambiguous than the other two methods. By using an optical fiber to detect the capillary waves and two orthogonal blades to generate the waves (at separate times), we have eliminated the need to rotate the system, in contrast to Miyano’s apparatusesBecause of monolayer relaxation, it is advantageous to take orthogonal measurements as quickly as possible; our setup allows these measurements to be taken in less than 10 s. Bibo, Knobler, and Peterson8 have shown that many different materials exhibit the same monolayer phases. The fatty acids, ethyl esters, other esters, and alcohols can all be put on a generalized T-T diagram (the temperature and pressure scales vary with chain length and the nature of the head group). We chose to study heneicosanoic acid because it exhibits most of the phases common to the above mentioned materials in a temperature range from 1 to 35 “C. We studied surface tension anisotropy in these phases and compared our resulta with reported X-ray diffraction peak widths. Finally, we added calcium ions to the subphase and studied the surface tension anisotropy as a function of pH and pressure. It is well-known that adding ions to the subphase can cause monolayers to become stifCgJOthere(7) Malcolm, B. R.
J. Colloid Interface Sci. 1986, 104, 521. (8)Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591. (9) Spink, J. A.;
Sanders, J. V. Tram. Faraday SOC.1955, 61, 1154. (IO)Deamer, D. W.; Cornwell, D. G. Biochim. Biophys. Acta 1966,
116, 555.
0 1992 American Chemical Society
Bohanon et al.
2498 Langmuir, Vol. 8, No. 10, 1992
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fore, we expected to observe a high degree of surface tension anisotropy. We chose to do this study at 20 "C to facilitate comparison with other studies, which are typically performed at room temperature.
Experimental Procedure The surface tension anisotropy was measured using a capillary wave probe with a fiber-optic detection system. Details of this technique have been presented elsewhere." Briefly, a capillary wave is generated by applying a sinusoidal voltage, -250 V, to a platinum blade held approximately 0.5 mm above the water. The wave is detected interferometrically by an optical fiber at a distance 1 cm away from the blade. The detected signal is differentiated and full wave rectified; the envelope of the processed signal is a sine wave that is 4 times the frequency of the applied signal. A change in surface tension causes a relative phase change between the applied and detected signal. The surface tension was calculated using the Kelvin equation, u = p&/k3-where u is the surface tension, p is the density of water, and w and k are the angular frequency and wave vector of the capillary wave-with a correction factor applied to account for the nonzero elasticity of the monolayer. For a discussion of how the surface pressure obtained from the Kelvin equation is related to the true surface pressure, obtained from the dispersion relation for capillary waves, see ref 5. For all measurements the capillary wave frequency was 625.0 Hz. The resolution of the surface tension measurement is i0.3 mN/m. All measurements (except the time dependence study) were taken at constant "pressure", as measured using a Wilhelmy plate. Of course, constant "pressure" in an anisotropic film only refers to the change in surface tension measured at a particular position and plate orientation; this topic will be discussed below. The subphase water was deionized and filtered through a Barnstead Nanopure I1 system until it had a resistivity of 118.0 Mil cm. The pH of the subphase was lowered by adding HCl and raised by adding NaOH. For the calcium ion study the concentration of Ca2+ions was adjusted to 0.5 mM using CaC12. All films were compressed uniaxially in a temperaturecontrolled trough. The compressionspeed was 0.5 (&/molecule)/ min. The temperature of the apparatus can be controlled in the range from 0 to 60 OC with a reproducibility of 0.1 "C. Details have been presented elsewhere.12 Figure 1 shows the relative positions of the capillary-wave probe blades, optical fiber, Wilhelmy plate, compressional barriers, and elastic bands.
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Results and Discussion Figure 2 shows isotherms of heneicosanoic acid at various (11)Bohanon, T.M.; Mikrut, J. M.; Abraham, B. M.; Ketterson, J. B.; Dutta, P. Reu. Sci. Inatrum. 1991,62, 2959. (12)Bohanon, T.M.; Mikrut, J. M.; Abraham, B. M.; Ketterson, J. B.;
Dutta, P.; Jacobson, S.; Floeenzier, L. S.;Torkeleon, J. M. Rev. Sci. Instrum. 1992,63, 1822.
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Figure 2. Surface "pressurenas measured by the Wilhelmy plate (circles), and the difference between surface tensions parallel and perpendicular to compression as measured by the capillary wave technique (squares),versus molecular area of heneicosanoic acid, at various temperatures. All the phases of heneicosanoic acid appear at one or more of the temperatures shown.
temperatures on a pH = 2 subphase. The circles are the surface "pressures" obtained using a Wilhelmy plate. The squares are the difference in surface tensions obtained using a fiber-optic capillary wave probe. All the phases of heneicosanoic acid are observed in one or more of the five temperatures shown in the figure. A t the temperatures of 25,20, and 10 "C, the D, C, C', A', rotator I, and rotator I1 phases occur. None of these phases displayed surface tension anisotropy within the resolution and time scale of our apparatus. Next we turn to 5.0 and 2.0 "C. Surface tension anisotropy was not observed in the D or the C phases at either temperature. However, anisotropy was observed immediately after the phase transitions, to the A phase at 5.OoCand to the B phase at 2.0 "C. Further, the anisotropy increased at the B to A transition. Figure 3 is the phase diagram of heneicosanoic acid monolayers. The only phases in which surface tension anisotropy occurs have been shaded. Surface pressure measurements are meaningful in all but the A and B phases. Fatty acid monolayers in these phases have been studied using X-ray diffraction.13-15 The phases which occur at 25, 20, and 10 "C have X-ray diffraction peaks that are broader than the instrumental res01ution.l~ That is, they show some degree of order, but they do not have longrange order. Along our 5.0 and 2.0 "C isotherms, only the A and B phases have resolution-limited peaks. Thus, the (13)Lin, B.; Shih, M. C.; Bohanon, T. M.; Ice, G. E.; Dutta, P. Phys. Reu. Lett. 1990, 65, 191.
(14)Kenn,R.M.;Bohm,C.;Bibo,A.M.;Peterson,I.R.;Mohwald,H.;
Kajaer, K.; Als-Nielsen, J. J. Phys. Chem. 1991, 95,2092. (15)Shih, M. C.;Bohanon, T. M.; Mikrut, J. M.; Zachack, P.; Dutta, P.Phys. Reu. A 1992,45, 5734.
Langmuir, Vol. 8, No. 10, 1992 2499
Surface Tension Anistropy in Langmuir Monolayers 30
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TEMPERATURE Figure3. Phase diagram of heneicosanoic acid. Shaded regions are the phases which show surface tension anisotropy. These phases are common to many fatty acids, ethyl esters, and alcohols. All phases show some degree of order, but only the shaded phases are solids. 23.0 i n
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heneicosanoic acid spread on a solution of 0.5 mM calcium chloride. The surface tension anisotropy is within the resolution of the apparatus.
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trends in X-ray peak widths are correlated with surface tension anisotropy. We next address the question of whether the anisotropy persists. Figure 4 shows the time dependence of the orthogonal surface “pressures” in the B phase at T = 1.5 “C. In the case surface “pressure” refers to the change in surface tension relative to pure water in a particular direction as measured by the capillary wave technique. The area was held constant and the surface “pressures” parallel and perpendicular to compression were measured at regular intervals. The circles are the surface “pressure” parallel to compression and the squares are the surface “pressure” perpendicular to compression. The surface “pressure”parallel to compression relaxes faster and longer than the surface “pressure”perpendicular to compression, but after 25 min, the two surface “pressures” are still significantly different. What would happen if we waited longer? On typical monolayer compression time scales (a complete isotherm in approximately one hour) equilibrium will not be achieved. If one does wait until the film comes to equilibrium, further compression will cause more anisotropy and require more waiting. Moreover, if anisotropy does develop, it means that a shear stress is being applied to a solid film. This shear stress may exceed the yield point of the material. In phases where anisotropy occurs, it is best to compress in a shape-preserving manner, thus preventing the anisotropy that would be developed upon compressing uniaxially.12 Figure 5 shows 20.0 “Cisotherms of heneicosanoic acid with calcium ions in the subphase at various pH values. The lines are the surface pressure obtained using a
19
20
21
22
23
24
Area (A2/molecule) Figure 6. Isotherms of heneicosanoic acid spread on a solution of 0.5 mM calcium chloride at pH = 11. The line is the Wilhelmy plate measurement. The circles and squares are the surface “pressure”parallel and perpendicular to compression, as measured by the capillary wave technique.
Wilhelmy plate and the squares are the surface tension difference obtained using the capillary wave probe. Surface tension anisotropy is not observed up to pH = 10.5. This result is unexpected because-as previously mentioned-these films are stiff even at medium pH. At pH = 11.0 surface tension anisotropy is observed (Figure 6). Because the anisotropy curve would overlap the Wilhelmy plate isotherm, the two capillary wave measurements are plotted independently. Also, notice the Wilhelmy plate measurement is much lower than either of the capillarywave probe measurements. We only point this out qualitatively, because in this phase the Wilhelmy plate measurement depends on its position and orientation; i.e. surface pressure measurements are ambiguous. We can draw several conclusions from this pH study. Even though a film may be highly viscous, compressing it uniaxially may not cause surface tension anisotropy. The large change in surface tension anisotropy for a small change in pH can only be the result of a nonsolid to solid phase transition. A solid phase appears at pH = 11.0.
Bohunon et al.
2500 Langmuir, Vol. 8, No. 10, 1992 These results agree with recent X-ray diffraction results on heneicosanoic acid with calcium ions in the subphase at 5 OC.l6 A new, solid phase was observed at pH = 11. Below this pH, the phase transitions continually moved to lower pressures as the pH increased; just below pH = 11,where the isotherms were featureless,a highest pressure phase (A’) was observed at all pressures. At 20 O C , the highest pressure phase is a rotator phase; therefore, we suggest that we are in a rotator phase at all pressures when the isotherms are featureless at pH = 10.5. This would explain why the surface tension is isotropic up to this pH.
Conclusion Uniaxial compression will cause surface tension anisotropy-and meaningless pressure measurements-in only two of the eight heneicosanoic acid monolayer phases. The anisotropy that is developed does not disappear on typical monolayer compression time scales. The two (16) Shih, M. C.; Bohanon,T. M.; Mikrut, J. M.; Zschack, P.; Dutta,
P.J. Chem. Phys. 1992,96, 1556.
phases that develop anisotropy are the only phases that have resolution-limited X-ray diffraction peaks. Even though the other six phases show some degree of order (i.e. they are not liquids), they can be compressed uniaxially. Many different materials show these phases; we expect monolayers in the correspondingphases of other materials to behave similarly. Although heneicosanoic acid monolayers with calcium ions in the subphase are highly viscous at pH = 6 to 10.5, they can be compressed uniaxially. A slight change in pH, at pH = 11,causes a large change in surface tension anisotropy signaling a mesophase to solid phaee transition. At and above pH = 11,compressing uniaxially will cause surface tension anisotropy.
Acknowledgment. This work was supported by the
U.S.Department of Energy under Grant No. DEFGO2MER45125. Registry No. Heneicosanoic acid, 2363-71-5; calcium, 744070-2.
