104
Langmuir 1987, 3. 104-106
surface, it would be expected that the molecules with higher electron density in the oxygen atom interact more strongly with surface electron-accepting sites (in the case of hydroxylated surface, the hydrogen atom of surface hydroxyls).6 Therefore, the molecules having longer carbon chains form stronger hydrogen bonding with surface hydroxyls, which results in the increased amount of irreversible adsorption on the hydroxylated surface. On the other hand, on the surface with a lesser hydroxyl content, the value of Vir, is almost the same for three kinds of alcohols, i.e., about 3 molecules per nm2. Jones and Hockey have assumed that most of the external plane of rutile is composed of three planes, the (110) plane in 60% and (101) and (100) plane each in 20%.18 From their model, the number of fivefold Ti4+ ions interacting with alcohol molecules can be estimated to be 5.1,7.9, and 7.4 ions per nm2for the (110), (IOl), and (100) plane, respectively.18,22Taking into account the steric hindrance by the larger size of alcohol molecules, only half of these ions should interact with alcohols. From the ratio of each cleavage plane and the number of Ti4+ions accessible for alcohol molecules in the plane, the value of 2.9 Ti4+ions per nm2 can be evaluated as the number of effective sites interacting with alcohols as a whole, which is
in good agreement with the value of about 3 molecules per nm2 for Vbr on the dehydroxylated surface. The possibility of substitutional reaction by water for adsorbed alcohol was examined for the present rutile sample, as in the case of the ZnO-alcohol-water system.6 The adsorption isotherm of alcohol was first determined on the dehydroxylated surface and then the sample was evacuated, followed by exposure to saturated water vapor at 25 "C, during which there is a possibility of substitutional reaction of water with adsorbed alcohol. Next the sample was evacuated again and the second adsorption isotherm of alcohol was measured. This isotherm was found to be quite the same as that of the second adsorption on the sample with preadsorbed alcohol and without exposure to water vapor. This implies that the substitutional reaction of water for adsorbed alcohol does not occur on the rutile surface, which was also confirmed by infrared spectroscopic observation. Thus, it can be concluded that the interaction of the rutile surface with alcohol molecules is stronger than that with water, in contrast to the case of ZnO. Registry No. TiOz,13463-67-7; MeOH, 67-56-1; EtOH, 64-17-5; PrOH, 71-23-8.
Langmuir-Blodgett Deposition of a Ring-Shaped Molecule (Valinomycin) J. B. Peng,+ B. M. Abraham, P. Dutta,* and J. B. Ketterson Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60201
H. Frank Gibbard Power Conversion Incorporated, Elmwood Park, New Jersey 07407 Received July 21, 1986. I n Final Form: October 3, 1986 Valinomycin, unlike most surfactants, is a ring-shaped molecule; its hydrophilic groups are evenly distributed around the ring. By the Langmuir-Blodgett (LB) technique, a single monolayer of valinomycin can be deposited on withdrawing a (hydrophilic)glass or mica substrate from the subphase, but the contact angle is not appreciably changed by the deposition of this flat molecule. The monolayer then peels off as the substrate is reintroduced into the water. However, if a thin film of silver is first evapoated on the substrate or three monolayers of lead stearate deposited on it, multiple layers of valinomycin can be built up. LB multilayers can also be deposited on poly(methy1methacrylate) (PMMA). Monitoring the actual (dynamic)contact angle during the process of deposition suggests that, for successful LB deposition, the contact angle must be greater than 90° on immersion and less than 90" on withdrawal. For valinomycin this sequence occurs only with substrates whose contact angles are hysteretic even in clean water.
Introduction Valinomycin is a cyclic dodecadepsipetide. Three units, each consisting of D-valin, L-lactic acid, L-valine, and Dhydroxyisovaleric acid, are joined sequentially to form a ring. The study of Langmuir monolayers of this material,1-3 and of its Langmuir-Blodgett (LB) deposition characteristics, is particularly interesting because its molecular structure is quite different from that of typical film-forming compounds. The compound has also received attention because it selectively transports K+ across both natural and synthetic membra ne^.^^ Smith et al.' determined the structure of crystalline valinomycin; Neu'Permanent address: University of Science and Technology of China, Hefei, Anhui, People's Republic of China.
0743-7463/87/2403-OlO4$01.50/0
pert-Laves et aL8 determined the structure of crystalline K+ complexed valinomycin. The results of the two determinations established a clear difference in the conformation, unit cell size, and space group between the complexed and uncomplexed crystalline compound. Some years ago, Gibbardg prepared thick films of valinomycin in an organic binder which was deposited on the surface (1) Ries, H.E.,Jr.; Swift, H. J. Colloid Interface Sci. 1978, 64, 111. (2) Abraham, B. M.; Ketterson, J. B. Langmuir 1985, I, 461. (3) Kemp, G.; Wenner, C. E.Biochim Biophys. Acta 1972, 282, 1. (4) Crisp, D. J. J. Colloid Sci. 1946, 1, 49. (5) Ries, H.E.,Jr.; Walker, D. C. J. Colloid Sci. 1961, 16, 361. (6) Blank, M.J. Phys. Chem. 1962, 66, 1911. (7) Smith, G.D.; Daux, W. L.; Langs,D. A.; DeTitta, G . T.;Edmonds, J. W.; Rohrer, D. C.; Weeks, C. M. J. Am. Chem. SOC.1975,97, 7242. (8) Neupert-Laves, K.;Dolber, M. Helu. Chim. Acta 1975, 58, 432. (9) Gibbard, H.F., unpublished results.
0 1987 American Chemical Society
Langmuir, Vol. 3, No. 1, 1987 105
LB Deposition of Valinomycin of a silver wire. When the silver had been partially converted electrochemically to silver chloride, the device functioned as a sensor for potassium ions in aqueous solutions. Several other groups1*l2 have also explored the use of this material in sensor applications. If thin, regular films of valinomycin can be produced on an appropriate substrate, a variety of sensitive sensing techniques using small sensors may become feasible. Surface plasmon resonance (SPR) is a promising probe of K+ concentration in (say) a physiological fluid in contact with a valinomycin LB film. This technique has been used by Pockrand et al.13 on calcium arachidate LB films deposited on silver; the use of SPR in other sensor systems has been demonstrated by Nylander et al.14 and by Liedberg et al.15 With this as added motivation, we have attempted to prepare LB films of valinomycin and to study the conditions under which successful deposition occurs.
Materials and Method Valinomycin was obtained from the Aldrich Chemical Co. A stock solution was prepared by dissolving a weighed quantity in Burdick and Jackson ethanol-free chloform. The solutions were prepared so that the initial area per molecule was approximately 460 A2 for a 1OO-gL aliquot. The water for the subphase was deionized by passing through a Barnstead NANOpure I1 ion-exchanger system followed by a micropore filter. The resistivity of this water, which was used unbuffered, was >18 MQ cm. The surface pressure/molecular area ( F A ) diagrams were determined in the usual manner by spreading an aliquot of valinomycin solution on the purified water substrate and measuring the change in surface tension with a modified Langmuir balance. A ribbon barrier driven by a dc motor could be set either to go to a given position (for 7r-A diagrams) or to maintain a constant pressure (for LB deposition). The trough was enclosed within a temperature-controlled hermetically sealed box. A complete description has appeared elsewhere.16 Attempts to deposit LB films were made on the familiar substrates glass and mica. In addition, we used PMMA (“plastic” cover slips purchased from VWR Scientific Co.), mica substrates on which silver layers (approximately 540 A) had been evaporated, and mica covered with three LB monolayers of lead stearate. The glass and mica substrates were degreased with chloroform and then rinsed with acetone. They were then cleaned in Pirhana solution (concentrated nitric acid and hydrogen peroxide). Finally, after the substrates were rinsed with purified water, they were placed in a plasma cleaner for 3 h. The PMMA was washed with dilute detergent solution and then repeatedly rinsed with purified water. The Ag and PbSt overlayers were used as prepared. The various substrates were suspended from a Cahn electrobalance, and the deposition procedure was carried out by moving the balance up and down to move the substrate into or out of the subphase. In this way a continuous measurement of the vertical force could be recorded, which enabled us to determine the contact angle (10)Wakida, S.; Tanaka, T.;Kawahara, A.; Hiiro, K. Bunseki Kagaku 1984, 33, 556. (11)Thompson, M.;Krull, U. J.; Worsfold, P. J. Talanta 1976, 26, 1015. (12)Oehme, M.;Simon, W. Anal. Chim. Acta 1976,86,21. (13)Pockrand, I.; Swalen, J. D.: Gordon, J. G.: Philpott, M. R. Surf. Sci. 1977,74,237. (14)Nylander, C.;Liedberg, B.; Lind, T. Sens. Actuators 1982,3,79. (15)Liedbera, B.:Nvlander, C.: Lundstrom, I. Sens. Actuators 1983. 4,299. (16)Dutta, P.; Halperin, K.; Ketterson, J. B.; Peng, J. B.; Schaps, G.; Baker, J. Thin Solid Films 1985,134,5.
40
3 VALINOMYCIN o t
t”
+
temp. 2 C temp. 25 c temo. 51 c
-
. E 0 VI
21
0 W
U
3
m m W
a U
El
m AREA
[i /mol)
Figure 1. Monolayer pressure-area isotherms (linear scales) for valinomycin at three different temperatures.
during the entire deposition process. The method has been described fully by Neumann:’ Neumann and and by Peng, Abraham, Dutta, and Ketter~0n.l~
Results and Discussion In Figure 1, we display a-A diagrams for valinomycin spread on unbuffered water at three different temperatures 2,25, and 51 OC. It should be noted that the TA - diagram is not observably affected by temperature, an unexpected result. Room temperature 7r-A diagrams have been reported by Ries and Swift’ and by Abraham and Ketterson2 on unbuffered water and by Kemp and Wenner3 on 0.2 M KC1. Incidentally, our curves agree better with those of Kemp and Wenner than with those of the other two groups, although we used unbuffered water. All a-A diagrams of valinomycin begin to show concavity toward the pressure axis well before the film is compressed to an area corresponding to the hard-core diameter of the molecule. This may simply be a consequence of a continuous change in conformation as the film is compressed or an indication that molecules continuously leave the monolayer for the subphase. With many hydrocarbon chain compounds, deposition is macle in a pressure range where the slope of the T-A diagram is very large, and there is a well-defined maximum (“solid”) surface density of about 20 A2 per chain. No such region exists on the valinomycin a-A diagram. This creates a problem in the selection of a deposition pressure and in the interpretation of the transfer ratio, defined as the decrement in trough area divided by the area of the substrate. No special significancecan be attached to a transfer ratio of unity; when transfer ratios at different pressures are compared, the possibility that deposited monolayers (17)Neumann, A. W. 2. Phys. Chem. 1964,41, 339; 1964,43, 71. (la)Neumann, A. W.; Good, R. J. Surf.Colloid Sci. 1979,11. (19)Peng, J. B.; Abraham, B. M.: Dutta, P.; Ketterson, J . B. Thin Solid Films 1985,134,187.
106 Langmuir, Vol. 3, No. 1, 1987
Peng et al.
Table I. Contact Angles of.Water on Various Substrates dynamic contact angles, deg substrate glass mica Ag PMMA
static contact angles, deg
8,,
aA
a,,
aA
0 0
0 0
0 0
0
54 50
101 96
67 66
96 85
0
may not have either the same density as the monolayer on water or a constant density independent of deposition pressure must be considered. For example, the values of transfer ratio greater than 1.0 shown later in this paper may be due to an increase in the density of the monolayer upon deposition, an option not available to hydrocarbon chain compounds. Except for PMMA (where we attempted to deposit a t three different pressures), film depositions were made a t -19 dyn/cm, near the inflection point of the a-A diagram. Before attempting deposition, the contact angle of clean water on all the substrates used (except the lead stearate LB films, whose stability in clean water is uncertain) was determined by the vertical force method described above. These values are tabulated in Table I. Dynamic values were determined while lowering or raising the substrate at the rate of about 1.2 cm/min, the same rate used for deposition; static values were determined by stopping during the raising or lowering process and waiting until the reading was constant for several minutes. It is clear that for Ag and PMMA (and presumably also for PbSt, which has hydrocarbon chains on the surface) the static and dynamic values are different from each other and both are extremely hysteretic (i.e., depend on the direction of current or most recent motion). For the substrates on which valinomycin deposition was attempted, the contact angles and transfer ratios during deposition are summarized in Table 11. When deposition occurred, the range listed includes only values observed after the first two monolayers had been deposited. On both glass and mica only one layer was deposited, on withdrawal from the subphase; on reintroducing the substrate the layer peeled off. The contact angle was always -0'; it did not change with dipping direction. Frequently, the deposition of a monolayer on withdrawal changes a hydrophilic surface to hydrophobic; the absence of this effect with valinomycin is probably due to the fact that
Table 11. Contact Angles and Transfer Ratios during Deposition of Valinomycin Monolayers contact angles, deg substrate glass mica PbSt Ag PMMA
T,
dyn/cm 19 19 19 19 14.5 16.5 19.0
8,,
0 0 25-29 13-19 34-36 32-33 28-29
8~
0 0 95-98 67-83 87-89 91-93 97-98
transfer ratios TA
T,, 1.1 0.9 0.9-1.1 1.0-1.1 0.5-0.8 0.8-1.1 1.1-1.3
-1.1 -0.8
0.7 0.0 to 0.0 to 0.3 to 0.8 to
-0.2 -0.1 0.5 1.1
it is a relatively thin disk. Multilayers were produced, however, on PbSt and Ag, where there was a large difference between the contact angle going down (8,) compared to the contact angle coming up (eu). The variations in contact angle and transfer ratio are probably due to low resolution, backlash, etc; however, some patterns are visible. The contact angle during withdrawal is always much less than 90"; deposition always occurs on withdrawal. On immersion, deposition occurs if dd > 90'; there is no deposition if e d S 90'; and if ed ,'O as with glass or mica, any monolayer already deposited peels off. This last effect may be due to increased penetration into the region between the deposited monolayer and the substrate at low contact angles.
-
Conclusion We have successfully deposited Langmuir-Blodgett films of valinomycin on silver, LB-deposited lead stearate, and PMMA. In the successful substrates, the contact angles show a large hysteresis even with clean water. The sequence Bu < go", 8, > 90°, is well-known to anyone familiar with LB deposition; valinomycin (unlike, say, fatty acid salts) does not generate this sequence during attempted deposition on glass or mica. However, multilayers are readily produced by using substrates on which the appropriate contact angle sequence occurs. We believe this may be a general rule applicable to other intractable materials. Acknowledgment. This work was supported by the
U.S. Department of Energy under Grant DE-FG0284ER-45125. Registry No. Ag, 7440-22-4; valinomycin, 2001-95-8; lead stearate, 7428-48-0.
