Langmuir 1990,6, 883-885 root-mean-squared end-to-end distance RF = 68 A. Coulombic repulsions between micelles at both chain ends would tend to elongate the chain. The fully extended (helical) length of this polymer, 504 A, corresponds to a sequence of (TTGTTG) states. We can estimate the conformational entropy loss on the cyclization step I11 IV in the following way. Assume the micelle on the chain end represents a sphere of radius ca.16.7 A, Le., a normal SDS micelle radius." If we assume that the other chain end is located on average within a sphere of radius R,, the fraction of this volume occupied by the micelle is ca. 0.015. If any conformation of the chain placing both pyrenes within the same micelle is deemed to be cyclized, the entropy loss corresponding to cyclization, AS = -R In (0.015), is ca. 8.3 cal K-' mol-'. At SDS concentrations below the peak in Figure 2, the entropy loss for the reaction I11 IV is offset by the gain in free energy through the hydrophobic binding of the second pyrene to the micelle. At higher SDS concen-
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(15) Gruen, D.W. R. Prog. Colloid Polym. Sci. 1985, 70, 6 .
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trations, the equilibrium is among 111, IV, and V. Here the entropy loss on cyclization should be much less severe. The polymer containing micelles a t both ends should have a significantly lower entropy than the random chains I because of the tendency in V toward chain elongation due to Coulombic effects.
Summary We have examined by fluorescence spectroscopy aqueous solutions of the polymer'Py-PEO-Py (1) in the presence and absence of SDS. The most striking result is that the hydrophobic end groups interact cooperatively with the polymer and the surfactant to promote surfactant aggregation with the polymer at much lower concentrations than the surfactant cmc even in the presence of the unsubstituted polymer. At low SDS concentration, the polymers tend to cyclize so that the two end groups share a single mixed micelle. Acknowledgment. We thank NSERC Canada for its support of this research.
Effect of the Spreading Solvent on Monolayers of Valinomycin Herman E. Ries, Jr. Department of Molecular Genetics and Cell Biology, The Uniuersity of Chicago, Chicago, Illinois 60637 Received October 24, 1989 Pressure-area isotherms for valinomycin, a cyclic dodecadepsipeptide,are significantly different when the film is spread from four different solvents: benzene, chloroform, n-hexane, and cyclohexane. This effect of the solvent is in sharp contrast to that found with films of long-chain lipid-type compounds, which show no significant solvent effect. The highly polar internal ring structure of valinomycin in contrast to the relatively small terminal polar group of a typical lipid may account for the difference.
Introduction Because few film-forming compounds spread spontaneously, the use of volatile solvents has become an integral part of monolayer spreading.'.* For many years, possible effects of the spreading solvent on monolayer properties have been of some concern. Early studies of the solvents were performed on long straight-chain compounds such as fatty acids and a l ~ o h o l s . ~With - ~ conventional solvents, benzene, chloroform, and n-hexane, no significant differences were observed in the important parts of the pressure-area isotherms, that is, in the long central linear portions from which the extrapolated areas are obtained. However, in current studies on monolayers of valinomycin, marked differences are observed (1) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; Wiley: New York, 1982; p 114. (2) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Intarscience: New York, 1966;pp 30-33. ( 3 ) Cook, H. D.; Ries, H. E., Jr. J. Phys. Chem. 1956,60, 1533. (4) Cook, H. D.; Ries, H. E., Jr. J . Am. Chem. SOC.1969,81, 501. (5) Walker, D. C.; Ries, H. E., Jr. Nature 1964,203,292.
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in the isotherms obtained with four spreading solvents: benzene, chloroform, n-hexane, and cyclohexane. Valinomycin is a cyclic dodecadepsipeptide, an antibiotic, and an ionophore. It carries ions through the cell membrane because of its cyclic polar center and its nonpolar exterior. A schematic drawing of the valinomycin molecule is shown in Figure 1. Its structure is clearly different from those of the long-chain lipid-like molecules.
Experimental Section High-purity valinomycin preparations from the Calbiochem Corp. and from the Aldrich Chemical Co. gave monolayer properties that are essentially identical. The four spreading solvents were high-purity materials from American Burdick and Jackson; extremely dilute solutions were prepared gravimetrically. The monolayer experimentswere performed on a horizontalfloat film balance of the Langmuir-Adam-Harkins type? Sur(6) Ries, H. E., Jr.; Swift, H. J. Colloid Interface Sci. 1982,89, 245.
0 1990 American Chemical Society
884 Langmuir, Vol. 6, No. 4, 1990
Letters
0 FIRST COMPRESSION
FIRST EXPANSION 0 SECOND COMPRESSION
(i
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1
I
1
~~~
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F i g u r e 1. Schematic drawing of the valinomycin molecule.
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100 150 200 250 300 350 400 AREA, SOUARE ANGSTROMS PER MOLECULE
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Figure 3. Compression-expansion isotherms for valinomycin spread from benzene.
a
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z W
a
i
AREA, SOUARE ANGSTROMS PER MOLECULE
Figure 2. Pressurearea isotherms for valinomycin spread from four different solvents. Table I. Effect of the Spreading Solvent on Monolayers of Valinomycin collapse area,' pressure, compressibility,* solvent A*/molecule dyn/cm cm/dyn benzene 370 23 0.0150 300 chloroform 23 0.0145 n-hexane 280 24 0.0135 cyclohexane 260 23 0.0141 a Extrapolated area at zero pressure. * Compressibility is (a, - al!/ad,, where a, is the extrapolated area at zero pressure and a1 1s a smaller area a t pressure fl. faces in contact with the film were solid Teflon (float), F E P Teflon (foils), and Teflon coatings (trough and barriers). Thus no wax coatings could complicate the solvent effect. Temperatures of the distilled water substrate rarely varied more than 0.1 "C during a single experiment. No temperature effect was observed in the room temperature range, 20-25 "C.
Results and Discussion Pressure-area isotherms for valinomycin obtained with the four different spreading solvents are plotted in Figure 2 and the data therefrom presented in Table I. The isotherm contours are quite similar, as expected. Unexpected are the marked displacements along the area axis. Extrapolations of the central linear portions to zero pressure give molecular areas of 370, 300, 280, and 260 A2, respectively, for the benzene, chloroform, n-hexane, and cyclohexane solvents. Collapse pressures are essentially identical. Benzene yields a compressibility of 0.0150cm/ dyn, which is slightly greater than those for the others.
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Figure 4. Compression-expansion isotherms for valinomycin spread from cyclohexane. Table 11. Physical Properties of Spreading Solvents boiling solubility point at in H,O," surface molecular density 760 mm, g/IOOO g tension,b solvent weight (20/4) "C ofH,O dyn/cm benzene 78.12 0.8787 80.1 1.8 28.9 chloroform 119.38 1.4832 61.7 8 27.1 n-hexane 86.18 0.6603 68.9 0.01 18.4 cyclohexane 84.16 0.7786 80.7 0.07 25.5 'At 25 O C . Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Cas Interfaces; Interscience; New York, 1966; p 32. b A t 25 OC.
A tentative interpretation may be helpful to those working with similar and related systems. Possibly the benzene simply associates more strongly with the valinomycin than do the other solvents. Because of the polarizability of benzene, the inner multipolar portion of the valinomycin molecule may induce dipole moments that effect adhesion of the two molecules. The small single polar extremities of the fatty acid and alcohol molecules should be considerably less effective in this respect. Furthermore, the cyclic structure of benzene and the inner cyclic geometry of valinomycin provide additional compatibility. Although chloroform has a moderate dipole moment, it lacks a compatible molecular geometry. The nhexane and cyclohexane (predominantly, the chair form) lack polarizability as well as compatible geometry. The physical properties of the four solvents listed in Table
Langmuir 1990, 6, 885-888
I1 indicate minor similarities and differences. The representative compression-expansion-recompression isotherms plotted in Figures 3 and 4 show effectively no hysteresis. Such data establish that there is a negligible loss of the film-forming material during the experiment. Thus losses due to leakage, evaporation, or dissolution in the substrate are negligible. Moreover, the benzene that presumably becomes associated with the valinomycin does not dissociate during the experiment. In conclusion, it should be emphasized that although the spreading solvents may not have significant effects
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on the isotherms of vertically oriented long chain molecules, they may have pronounced effects on the film properties of other compounds with larger polar structures such as those with cyclic or related configurations. Acknowledgment. I am indebted to Professor Hewson Swift for helpful discussions and to Dr. Christopher Chou for valuable assistance in some of the experimental work. These studies were supported in part by a grant from the United States Public Health Services (CA14599).
Comments The Equation of State Approach for Interfacial Tensions: Comments to Johnson and Dettre In a recent publication,' Johnson and Dettre attempted to criticize the equation of state approach for interfacial tensions. Their main point, in our understanding, is of a dual nature, as follows: the number of degrees of freedom for a two-component three-phase system is supposed to be one, contrary to our contention that it is two for a two-component solid-liquid-vapor system; there is, within this concept, no difference between a solid-liquidvapor system and a liquid-liquid-vapor system. Both assertions are false, as we shall show below. We have shown? by considering the Gibbs-Duhem relations for a two-component three-phase solid-liquid-vapor system, that Tal = f(TW71") (1) where yetis the solid-liquid interfacial tension, ysv the solid-vapor interfacial tension, and ylv the liquid-vapor interfacial tension. Clearly, eq 1implies that there should be two degrees of freedom in contrast to the above contention that there should be only one. The phase rule from which Johnson and Dettre infer that there should be only one degree of freedom is the appropriate relation only for what Callen3 calls a "simple composite system". This definition excludes explicitly systems for which surface effects are considered. It is well-known that the phase rule for systems containing surfaces, e.g., as formulated by Defay and Prigogine; can differ drastically from that for a simple composite system. Recently, we have reexamined the phase rule for systems containing moderately curved interfaces.' It may be interesting to note that in the refereeing process it was pointed out to us repeatedly that our paper was, in a way, only an extension of the arguments of Defay and Prigogine.' In brief, our paper' has shown the following: (1) Johnson, R. E., Jr.; Dettre, R. H. Langmuir 1989,5, 293.
(2) Ward, C. A.; Neumann, A. W. J. Colloid Interface Sci. 1974, 49, 286. (3) Callen, H. B. Thermodynamics and an Introduction to Thermostatistics, 2nd ed.; Wiley: New York, 1985. (4) Defay, R.; Prigogine, I. Surface Tension and Adsorption (A. Bellemane, collab. with D. H. Everett, trans.); Longmans, Green & Co.: London, 1966. (5) Li, D.; Gaydoa, J.; Neumann, A. W. Langmuir 1989,5, 1133-1140.
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Figure 1. Curvature effect of a moderately curved interface on capillary rise.
The phase rule for a heterogeneous system containing moderately curved surfaces is given by f=r+l-N (2) where f is the number of degrees of freedom of this system, r is the number of independent chemical components, and N is the number of P* = P@ type pressure equality relations among the mechanical equilibrium conditions. By applying this phase rule (2) to two-component solid-liquid-vapor systems containing moderately curved liquid surfaces, such as a sessile drop resting on an ideal solid surface in equilibrium with the liquids vapor, it has been shown' that the number of degrees of freedom is two. This result implies that any two of the intensive variables used to describe this system are sufficient to characterize the system completely; in other words, any two intensive variables may be chosen as the independent variables, and the remaining intensive variables can be expressed as functions thereof. Thus, if one chooses ysv and ylv as the two independent variables, then one may express yal,the remaining surface tension, as a function of ysv and ylv;i.e. one recovers eq 1. We conclude that Johnson and Dettre's assertion that a two-component system consisting of three bulk phases and their interfaces should have only one degree of freedom is false. Johnson and Dettre argued that the equation of state (l),if applicable to solid-liquid systems, should be applicable to liquid-liquid systems too. They presented in their Figure 1 the contact angle results of alkanes on two liquid substrates, pentafluoropropanol (PPA)and water. Based on these data, their Table I showed the differences between the measured liquid substrate surface tensions and the liquid substrate surface tensions calculated by using the combination of an explicit form of the equation of state, eq 3 as shown below, with the Young equation; hence, they used these results as one of the two tests to show that equation of state (1) does not hold. 0 1990 American Chemical Society
