Langmuir 1999, 15, 2477-2483
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Molecular Orientation in Langmuir Monolayers Containing the [M(dmit)2]n- Anion Determined from Surface Pressure and Surface Potential Isotherms A. V. Hughes, D. M. Taylor,* and A. E. Underhill Institute of Molecular and Biomolecular Electronics, University of Wales, Dean Street, Bangor, Gwynedd, LL57 1UT, U.K. Received June 17, 1998. In Final Form: December 17, 1998 The behavior of three dmit containing semiamphiphilic salts ([Ni(dmit)2][DDI]2, [Ni(dmit)2][DDI], [Pd(dmit)2][DDI]2) has been investigated in detail at the air-water interface. An osmotic equation of state was fitted to the liquid expanded regions of the π-A isotherms, while the surface potential isotherms were analyzed using the Helmholtz equation (HE). A molecular orientation was proposed for each material based on the cross-sectional areas calculated from the equation of state, and these were consistent with the apparent dipole moments calculated from the HE. The behavior of the palladium salt was found to differ from that of the two nickel salts, with plots of isotherm gradient against water content being coincident for the nickel salts but not so for the palladium containing material. This was explained as being due to the almost identical lipid-lipid Lucassen-Reynders (LR) interaction coefficients calculated for the two nickel salts, which differed significantly from the coefficient calculated for the palladium derivative. The magnitudes of the coefficients were qualitatively in agreement with the molecular orientations proposed.
1. Introduction There has been considerable interest in recent years in conductive LB films, both in their own right and as potential sensor materials.1 Much of the work has focused on the investigation of organic or organometallic molecules containing large delocalized π systems.2 These have the potential for a high degree of intermolecular orbital overlap and hence for the formation of a band structure conducive to electronic conduction. Of the many classes of material which have been investigated, the bis(dithiolene) transition metal complexes, to which the [M(dmit)2]n- (where M ) Ni, Pd, Pt; dmit ) dimercaptoisotrithione) anion shown in Figure 1 belongs, are one of the most promising.3 This particular molecule is attractive for two reasons. First, the extensive π-orbitals of the sulfur atoms potentially allow intermolecular orbital overlap in more than one direction, thus suppressing the Peierls distortion inherent in low dimensional materials.4 Second, the presence of the transition metal atom leads to several stable oxidation states and hence potentially to a degree of control over band occupancy. Materials containing dmit complexes have been shown to exhibit interesting behavior as single crystals,3 but for application purposes, it is necessary to process them into thin, uniform films. Although the dmit anion itself has no amphiphilic character, it has been shown that LB film formation is possible using a suitable amphiphilic countercation, and films containing alkylpyridinium,5 alkylammonium,6 and, more recently, alkylimidazolium7 counterions have been studied. (1) Roberts, G. G. In Langmuir-Blodgett Films; Roberts, G. G., Ed.; Plenum Press: New York, 1990. (2) Acker, D. S. J. Am. Chem. Soc. 1962, 84, 3770. (3) Cassoux, P.; Valade, L.; Kobayashi, H.; Kobayashi, A.; Clark, R. A.; Underhill, A. E. Coord. Chem. Rev. 1991, 110, 115. (4) Peierls, R. E. Quantum Theory of Solids, Oxford University Press: Oxford, England, 1955. (5) Dhindsa, A. S.; Badyal, J. P.; Pearson, C.; Bryce, M. R.; Petty, M. C. J. Chem. Soc. Chem. Commun. 1991, 5, 322. (6) Nakamura, T.; Tanaka, H.; Matsumoto, M.; Tachibana, H.; Manada, E.; Kawabata, T. Chem. Lett. 1988, 10, 1667.
Figure 1. The three complexes investigated in this work.
For all these so-called semiamphiphilic salts it has been shown that although monolayers can be formed at the air-water interface, considerable care and attention to detail is necessary to ensure uniformity. When spread from solution, the materials initially form aggregates which dissipate only slowly into monolayers, and long spreading times of several hours are usually necessary to ensure complete dispersal.7,8 The relative orientations of the component molecules is a crucial factor in governing the electrical characteristics of molecular materials, since this will influence the nature of the molecular orbital overlap and hence the band structure of the solid.9 It is desirable, therefore, to determine and ultimately to control the molecular ori(7) Hughes, A. V.; Rees, J. A.; Taylor, D. M.; Underhill, A. E. Supramol. Sci. 1997, 4, 309. (8) Taylor, D. M.; Gupta, S. K.; Underhill, A. E.; Wainwright, C. E. A. Thin Solid Films 1992, 210/211, 287. (9) Cox, A. The electronic Structure and Chemistry of Solids, Oxford Science Publications: Oxford, England, 1987.
10.1021/la9807154 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/11/1999
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entation in such materials. For an LB film the first stage is to establish the structure of the precursor floating monolayer at the molecular level. Direct spectroscopic investigations of the air-water interface are possible but difficult. An ability to determine the orientation of the molecules from more readily measurable properties of the interface (e.g. the surfacepressure and surface-potential isotherms) is thus highly desirable and the purpose of the investigations reported in this paper is to evaluate whether this may be achieved for relatively complex systems such as the semiamphiphilic dmit salts. For simple amphiphiles, a powerful method for investigating molecular orientation is to fit a suitable equation of state to the π-A isotherm, and then to interpret the “best fit” values of the disposable parameters in terms of molecular factors.10 A recent review by Smamby and Brockman11 concluded that the equation which most closely represented the behavior of a number of amphiphiles is the osmotic-type equation derived by Wolfe et al.,12 and it is this equation which will be applied here to the isotherms of the three materials shown in Figure 1. Surface potential isotherms are generally interpreted according to the Helmholtz equation,13 which is derived by assuming that the monolayer and the interface may be treated as a parallel plate capacitor. Although improvements to this basic theory have been proposed,14 in all cases, extending the theory has invariably led to an increase in the number of disposable parameters required to fit the experimental data. For the case of simple amphiphiles, detailed investigations have revealed the range of values which would be expected for the parameters in these extended theories.15 For more complex systems such as those described here, however, it is by no means certain that the parameters will take similar values to those found to be appropriate for simple fatty acids. For simplicity, therefore, the surface potential isotherms of the three materials studied here will be interpreted in terms of Helmholtz’ original equation. 2. Experimental Section The synthesis of the materials shown in Figure 1 was carried out according to the method of Steimecke et al.,16 and has been reported elsewhere.7 The dual barrier LB trough was constructed from PTFE and mounted on an antivibration table located in a class 10 000 semiconductor clean room. Ultrapure water (UPW) for the subphase and for cleaning was obtained from a Millipore SuperQ system, comprising reverse osmosis, activated carbon, nuclear grade deionizer cartridges and a 0.25 µm point of use filter. Spreading solutions were prepared in chloroform (Aldrich, HPLC grade) at concentrations between 0.5 and 1 mg mL-1. The monolayers were spread for sufficient time to allow the complete dispersion of material into a monomolecular film7 (5 h for complexes a and b; 16 h for complex c). The equation of state was fitted to the π-A isotherms using a commercial package (Jandel Sigmaplot) which utilizes a (10) Gaines, G. L. J. Chem. Phys. 1978, 69, 924. (11) Smamby, J. M.; Brockman, H. L. Langmuir 1991, 7, 1031. (12) Wolfe, J.; Brockman, H. L. Proc. Natl. Acad. Sci. U.S.A. 1988, 85, 4285. (13) Helmholtz, H. Abhandlungen zur Thermodynamik Chemister Vorga¨ nge; Herausgegeben von Dr. Max Planck: Leipzig, Germany, 1902; pp 51-83. (14) Demchak, R. J.; Fort, T. J., Jr. J. Colloid. Interface Sci. 1974, 46, 191. (15) Oliveira, O. N., Jr.; Taylor, D. M.; Lewis, T. J.; Salvagno, S.; Stirling, C. J. M. J. Chem. Soc., Faraday Trans. 1989, 85, 1009. (16) Steimecke, G.; Sieler, H. J.; Kirmse, R.; Hoyer, E. Phosphorus and Sulphur 1979, 7, 49.
Hughes et al. Levenberg-Marquart least-squares algorithm.17 For each material, it was found that the best fit (i.e. lowest residuals) was obtained if the curve fit was weighted in proportion to the reciprocal of the experimental surface pressures. The surface potential data was collected using the vibrating plate capacitor method. The apparatus was built in house and has been described in detail elsewhere.18
3. Theory 3.1. Osmotic Equation of State. The derivation of Wolfe et al.’s osmotic equation of state has been given elsewhere.12 Briefly, it is derived from Gibbs’ formulation of surface thermodynamics by assuming that the monolayer is in equilibrium, and equating the chemical potentials of the components (i.e. the monolayer molecules and those of the subphase water) in the surface phase to those in the bulk aqueous phase. Two forms of the expressions for the chemical potentials are written with the deviations from ideal behavior accounted for with an activity coefficient on the one hand, and an osmotic coefficient on the other. Combining the two expressions leads to Wolfe et al.’s equation of state, viz
π)
[(
)]
ω1 qkT 1 ln b 1 + ω1 A - ω2 f1
(1)
where π is the experimental surface pressure, A the area per molecule, k Boltzmann’s constant, T the temperature, ω1 the cross-sectional area of the water molecules, ω2 the cross-sectional area of the lipid molecules, and f1b a constant related (but not equal) to the activity coefficient of the water in the surface phase, and q is a parameter related to the osmotic coefficient. Equation 1 becomes negative as the area per molecule becomes infinitely large since the value of f1b is invariably greater than unity. This contrasts with the behavior of real systems where the surface pressure is always zero or positive, and the gradient of the isotherm is discontinuous at the lift-off area. The equation of state therefore has no thermodynamic significance for areas per molecule larger than the lift-off area. Smamby and Brockman11 limited the application of the equation to the range from 1 mN m-1 up to the collapse pressure of their films, and it is this convention which was used in the present work. The nature and magnitude of the interaction between the molecules within a liquid expanded film may be investigated by calculating the Lucassen-Reynders (LR) interaction coefficients from the best-fit parameters. According to Lucassen-Reynders analysis,19 the logarithm of the water activity coefficient in the surface phase (f1s) can be described by a linear function of the product of the mole fractions of each pair of interacting components, viz
ln f1s )
1 kT
∑ Hijxisxjs
(2)
where Hij are coefficients whose values reflect the strength, and whose signs represent the nature (+ve ) attractive, -ve ) repulsive) of the interactions between the ith and jth components. The logarithm of the activity coefficient (17) Brown, K. M.; Dennis, J. E. Numer. Math. 1972, 18, 289. (18) Taylor, D. M.; Oliveira, O. N., Jr.; Morgan, H. J. Colloid Interface Sci. 1990, 139, 508. (19) Lucassen-Reynders, E. H. J. Colloid Interface Sci. 1973, 42, 554.
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in terms of the best fit parameters from eq 1 is given by Feng et al.20 as
ln f1s ) kT[q ln f1b + (q - 1) ln x1s]
(3)
Combining these two equations and writing for a two component system leads to
kT[q ln f1b + (q - 1) ln x1s] ) H11x1s2 + H12x1sx2s + H22x2s2 (4) where component 1 is the subphase water, while component 2 is the spread monolayer. Following Feng et al., writing this equation for three values of the mole fraction (at the lift off area, at the mean area and at collapse) gives three equations in three unknowns from which the values of the interaction coefficients may be calculated. 3.2. Surface Potential. In Helmholtz’ description of the surface potential of a Langmuir monolayer, the individual charges constituting the dipoles are assumed to be smeared out so as to approximate uniformly charged sheets. That is, the monolayer is assumed to be analogous to a parallel plate capacitor, whose plates carry the positive (+Q) and negative (-Q) charges which form the dipoles. The charge density on each “plate” will be given by σ ) (nQ, where n is the number of molecules per unit area. If the plates are separated by a distance l, the potential difference between them is given by σ/C, where the capacitance per unit area, C, equals 0/l. 0 is the permittivity of free space, and is the permittivity of the material between the plates, which is assumed to be unity. From these relations, the Helmholtz equation can be established,13 i.e
∆V )
NQl Nµ j µ j σ )( ) ) C 0 0 A0
(5)
where A is the area per molecule, and µ j is the effective molecular dipole moment at the surface. 4. Results and Discussion (i) π-A Isotherms. A typical π-A isotherm for the materials studied here is sketched in Figure 2. As can be seen, there are three distinct regions. From the lift-off area (i.e. the area at which the surface pressure begins to rise) to the sudden decrease in gradient which occurs at between 30 and 35 mN m-1 the isotherms are liquid expanded in character (i.e., they are continuous and show no evidence of any phase transitions). Above about 35 mN m-1 a plateau is observed which extends to areas far smaller than would be expected for a monomolecular film. The plateau is almost completely reversible with only slight hysteresis being observed between the compression and decompression isotherms. A similar reversible plateau has been observed previously in a protein-containing film21 and was attributed from X-ray diffraction and ellipsometry investigations to a folding of the monolayer into a bilayered structure. SEM investigations of LB films of these materials showed that films deposited within the liquid expanded region were far more uniform than those deposited above the transition to the plateau,22 suggesting (20) Feng, S. S.; Brockman, H. L.; Macdonald, R. C. Langmuir 1994, 10, 3188. (21) Riegler, H. Mol. Membr. Biol. 1995, 12, 93. (22) Gupta, S. K.; Taylor, D. M. Unpublished results.
Figure 2. Sketch of a typical isotherm of these materials shown in compression and expansion. The isotherm is typically liquid expanded until this gives way to a reversible plateau. A further transition to a steep vertical region is also sometimes observed (dashed line).
the plateau seen here also represents a reorganization of the film into a multilayered structure. Since the monomolecular nature of the films is assumed in the derivation of eq 1, it evidently cannot be applied to the plateau region and above. We have recently identified the conditions necessary for the complete spreading of the three materials investigated here into uniform monolayers at the airwater interface,7 and the isotherms investigated here are taken under conditions where a uniform, monomolecular film would be expected. Evidently, if this is not the case then the best-fit values of ω2 obtained from curve fitting will not represent the true molecular cross sectional areas at the interface. Equation 1 is derived for a monolayer in thermodynamic equilibrium. Strictly speaking, all Langmuir monolayers are metastable and are thus never truly in an equilibrium state. However, unless the monolayers in question are extraordinarily unstable, then it seems to be generally accepted10-12 that most systems are close enough to equilibrium to justify the use of equilibrium models. Monolayers which are considered to be of a reasonable stability are usually those whose isotherms are invariant with the rate of compression and show little or no hysteresis between the compression and expansion of the film. For the three materials investigated here, no compression rate effects were noted, and as was stated earlier, the degree of hysteresis was small. The equation of state is intended to be applied to isotherms which are “liquid expanded”. According to Smamby and Brockman,11 the surface potential of an isotherm in the liquid expanded region should show a good linearity when plotted against the reciprocal of the area per molecule, and this is true of the regions of interest of all these materials. Thus, it would seem that Langmuir monolayers of the materials shown in Figure 1 do conform to the necessary criteria of liquid expanded, monomolecular films in a pseudoequilibrium state necessary for justifying the application of eq 1. The equation was therefore fitted to the regions between 1 mN m-1 and the transition to the plateau of the isotherms of the three materials, and the resulting experimental and “best fit” isotherms are shown in Figure 3. The
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Hughes et al. Table 1. Best-Fit Parameters from Applying Eq 1 to the Isotherms of the Three Materials
Figure 3. Experimental (points) and theoretical (lines) isotherms for the three materials.
Figure 4. Fitting residuals for the three materials.
residuals (i.e. the difference between the theoretical and experimental pressures over the course of the isotherms) for each of the three materials are shown in Figure 4. For each material, the theoretical surface pressure is slightly lower than the experimental pressure at large areas, while the opposite is true at lower areas. The best fit was obtained for the dianionic nickel salt, with a largest absolute residual of ca. 0.35 mN m-1. This compares favorably with the residuals obtained by Smamby and Brockman11 for the lipids investigated in their survey. For the monoanionic nickel and dianionic palladium salts, the equation did not fit the data quite so well, and largest residuals (ignoring the two highest values at around 2530 mN m-1 which represent the onset of the plateau) are approximately twice that of the dianionic nickel salt. However, these are still not unreasonable compared to the values reported for some other materials.11 The values of the three best fit parameters for each material are given in Table 1. It has been suggested by other workers23 that in Langmuir monolayers of the semiamphiphilic dmit salts, the anion lies flat at the air-water interface so as to
material
ω2 (Å2)
q
f1b
[Ni(dmit)2][DDI]2 [Pd(dmit)2][DDI]2 [Ni(dmit)2][DDI]
93.33 68.57 57.70
7.432 4.028 7.258
1.0826 1.1087 1.065
account for the large areas per molecule observed. However, in a liquid expanded monolayer, the molecules are not close packed, and the area per molecule of the isotherm reflects the area occupied by the film molecules plus that of the intervening water molecules, rather than just of the film molecules themselves. The true area occupied by the film molecules is given by the area approached asymptotically as the isotherm is extrapolated to infinite pressure i.e., the value of ω2 obtained from curve fitting. To determine the orientation of the molecules within the film, therefore, it is necessary to consider which orientations will give rise to the values of ω2 given in Table 1. The magnitude of the [M(dmit)2]n- anion was considered by Gupta et al.,24 and following their calculations it can be approximated as a rectangular box, of dimensions 16.1 Å × 6.3 Å × 3.2 Å. An approximate value for the area occupied by the 1,3-didodecylimidazolium (DDI) counterion can be obtained by assuming that each alkyl chain is vertically oriented, therefore occupying an area of 18.6 Å2. The minimum possible area occupied by a monoanionic complex will therefore be approximately 57 Å2, while that of a dianionic complex will be 94 Å2. The cross-sectional area of the dianionic nickel complex (93.3 Å2) is close to the expected value and is thus consistent with all three moieties (i.e. the [Ni(dmit)2]2- anion and the two DDI counterions) being almost vertically oriented and in contact with the water surface. Similarly, the value of ω2 obtained for the monoanionic nickel salt (57.7 Å2) is exactly as expected if both moieties are in contact with the water surface and untilted. However, determining the orientation of the molecules of the palladium salt from the cross sectional area is problematic. ω2 is far lower than expected, which implies that none of the moieties are tilted and that all three cannot be in contact with the water surface. That is, the headgroups of the counterion and the dmit anion cannot lie in the same plane, and one or more of the molecules must be shifted normal to the air-water interface relative to the others such that the film becomes multimolecular. It was argued by Gupta et al.24 that in their alkylammonium salts, the dmit anion rotated over the course of the isotherm from a flat lying configuration at the lift off area, to the vertical at the point of the transition to the plateau. However, this cannot be the case since during compression of a liquid expanded monolayer, the orientation of the film molecules remains effectively constant and the decrease in area is accommodated by the expulsion of water molecules from the surface phase into the bulk.19 For a film with a high water content, therefore, any applied tensile stress can easily be compensated for by this removal of water, resulting in a highly flexible film and a low isotherm gradient. It is reasonable to expect that the compressibility (i.e. the gradient) of an isotherm at any given point will be substantially a function of the number of water molecules available for expulsion in this way. (23) Taylor, D. M.; Underhill, A. E.; Gupta, S. K.; Wainwright, C. E. A. Makromol. Chem. Makromol. Symp. 1991, 46, 199. (24) Gupta, S. K.; Taylor, D. M.; Underhill, A. E.; Wainwright, C. E. A. Synth. Met. 1993, 58, 373.
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Figure 5. Variation in isotherm gradient with film water content for each material. Table 2. LR Interaction Coefficients for the Monolayers of Each Material material
H11(kT)
H12(kT)
H22(kT)
[Ni(dmit)2][DDI]2 [Pd(dmit)2][DDI]2 [Ni(dmit)2][DDI]
0.586 0.411 0.472
-2.577 -1.057 -3.622
-9.802 -4.636 -9.573
According to Wolfe and Brockman,12 the mole fraction of water in a liquid expanded film (x1s) will be given by
x1s )
(A - ω2)/ω1
(6)
1 + (A - ω2)/ω
and it is then a simple matter to convert this mole fraction into an absolute value for the number of water molecules per complex. The gradient of the theoretical isotherms will be given by
dπ ) dA
[
-qkT
(
)]
ω1 (A - ω2) 1 + A - ω2 2
(7)
and if the degree of water content is the most critical factor in determining the shape of the three isotherms, plots of dπ/dA as a function of the number of water molecules per film molecule should coincide. Such a plot is shown in Figure 5, and as can be seen, the curves for the two nickel complexes coincide almost exactly. The curve for the palladium salt is similar, but is shifted along the abscissa relative to the other two. This is consistent with the anomalous value of the lipid cross-sectional area calculated for the palladium salt. That is, shifting one of the ions of this salt out of the plane of the air-water interface would substantially alter the environment of the hydrophilic region compared to the two nickel salts. It is to be expected that the isotherm, which according to Figure 5 is substantially a function of the interaction between the film molecules and the water, would reflect this difference. The LR interaction coefficients for the three monlayers were calculated and are given in Table 2. The interactions between the water molecules (H11) for all three films are attractive, and comparable to the values obtained by Feng et al. for the lipids which they studied. The [M(dmit)2] anion has a very low affinity for water, and its presence may explain why these materials all display slightly
Figure 6. Surface potential-area isotherms for the three materials.
repulsive interactions with the surface water molecules (i.e. negative H12). The value of H12 is lowest for the palladium salt. This would be consistent with the [Pd(dmit)2]2- moiety not being in contact with the water surface for this material. The slightly higher value observed for the monoanionic nickel than for the dianionic complex is also as expected since the imidazolium counterions would probably shield the dmit anion from the surface water to a greater extent in the former than in the latter. The lipid-lipid interaction coefficients (H22) are all highly repulsive, with approximately equal values being observed for the two nickel complexes, while that of the palladium derivative is significantly smaller. In previous investigations of these materials, the absolute rates of spreading of the nickel salts were found to be almost identical and more rapid than that of the palladium derivative.7 This is then consistent with the differences calculated between the interactions of the molecules within the films. That is, the almost identical interaction which occurs between the molecules of the two nickel salts gives a plausible explanation for the similarity in the behavior of these two materials. Also, the anomalous interaction coefficient calculated for the palladium salt is consistent with a molecular arrangement in the floating monolayer significantly different to those of the nickel analogues. (ii) ∆V-A Isotherms. The surface potential isotherms of the three materials are shown in Figure 6. As can seen, in each case the rise in potential upon film compression commences at areas per molecule which are significantly larger than the lift-off areas for the π-A isotherms. For example, ∆V for the dianionic nickel salt begins to increase when the area per molecule has decreased to ∼550 Å2: approximately double the area at which any significant increase in surface pressure is observed. For the two nickel salts, the ∆V isotherm rises monotonically until the surface pressure begins to rise, at which area ∆V rises much more slowly. Similar behavior is observed for the dianionic palladium salt but with an added discontinuity in the isotherm between 350 and 250 Å2 per molecule when little change in potential occurs. Assuming ) 1, applying eq 5 to the curves in Figure 6 yields the apparent molecular dipole moment, µ j , for each material (Figure 7). For the two nickel compounds, µ j rises monotonically to a maximum value at areas per
2482 Langmuir, Vol. 15, No. 7, 1999
Figure 7. Dipole moment-area iostherms for the three materials.
molecule corresponding to the onset of surface pressure increase. For the palladium salt, the plateau in the ∆V isotherm results in a discontinuous increase in µ j , which nevertheless passes through a maximum at the area corresponding to the increase in surface pressure. The [M(dmit)2]n- anion would not be expected to contribute directly to surface potential since this moiety has a high degree of symmetry and, hence, no net dipole moment. A simple molecular mechanics computer model suggests that an imidazolium group should have a moment of 1.2 D parallel to the C2 rotational axis and in the same direction as the alkyl chains. It has also been suggested18 that a linear, uncoiled alkyl chain has a moment of 330 mD parallel to the chain and in the direction of the terminal methyl group. Assuming that the chains of the counterions are all straight and vertically orientated, then the molecular moment of the four chain dianionic compounds is expected to be 3.7 D while that of the two chain monoanionic salt will about half this value. Clearly, the values in Figure 7 are much smaller. It is well-known that molecular moments calculated by applying the Helmholtz equation to surface potential data are smaller than expected. One explanation is that the value of unity assumed for the relative permittivity is incorrect, although the actual value is still a matter of debate and may differ between different classes of lipid.25 A second explanation is that monolayer molecules are tilted from the vertical. However, given the small area available to each alkyl chain in the present materials, this is unlikely to be the case here. More likely is that the lower than expected values of the dipole moments are due to the subphase water. It has been shown for phospholipids that the changes which occur in the orientations of the surface water molecules due to the presence of the monolayer make a large contribution to the measured value of the surface potential.26 That is, the presence of the monolayer imposes an orientation on the water molecules in the surface phase such that their net dipole moment opposes that of the film, thus decreasing the measured surface potential. Implicit in the discussion to date is that the negative charges on the anions and the positive charges on the (25) Adam, N. K.; Danielli, J. F.; Hardley, J. B. Proc. R. Soc. London, Ser. A 1934, 147, 491. (26) Wilson, M. A.; Pohorille, A. J. Am. Chem. Soc. 1994, 116, 1490.
Hughes et al.
cations are coplanar. If, during compression, the anions were displaced vertically upward in the monolayer, then a negative contribution to the surface potential and, hence, the apparent molecular moment would ensue. Therefore, the lower than expected values for the molecular moment in these materials would be consistent with a vertically orientated dmit anion with its charge center, normally located around the central metal atom, positioned above the imidazolium moiety. Furthermore, it has already been inferred from the π-A isotherm that the [Pd(dmit)2]2was not in contact with the water subphase and was likely to be shifted upward into the hydrocarbon tail region of the monolayer. The appearance of the plateau in the surface potential isotherm during compression for this salt and the resulting reduction in ∆V at smaller areas is consistent with such a process occurring. Whether a small upward shift of the anion is responsible for the smaller than expected molecular moment in the dianionic nickel salt is not clear. Certainly, at a given area per molecule, such an effect would be smaller for the monoanionic salt because of the smaller charge on the anion. This is consistent with observation. There is substantial evidence to suggest that, at large areas per molecule, the alkyl chains of most materials lie flat on the water surface27 so that the apparent molecular moment and hence the surface potential is expected to decrease to zero at large areas. The rise in potential under compression is then seen as a gradual rotation of molecules from the horizontal to the vertical, a process which increases the vertical component of the molecular moment. For the nickel salts, this process is smooth and continuous whereas for the palladium salt the anion is forced out of the water into the hydrophobic region of the monolayer, giving rise to a plateau in the surface potential isotherm. The apparent molecular moment of each material passes through a maximum at an area per molecule corresponding to the rise in surface pressure. We have argued that upon further compression, the shape of the π-A isotherm is dictated by expulsion of water molecules from the monolayer. If this is the case, and if the water molecules make a such large contribution to the dipole moment as has been suggested, then the reduction in apparent molecular moment may reflect the change in the water content of the interfacial phase as the monolayer is compressed. 5. Conclusions In our previous investigations of the spreading behavior of the three complexes described here, we have shown that the absolute rates of spreading of the two nickel complexes are almost identical, while the palladium salt shows some differences compared to the other two. A similar pattern has emerged from the application of the osmotic equation of state to the liquid expanded regions of the isotherms of the three materials. Although the isotherms of the nickel complexes appear to be different, it has been shown that the isotherms differ only in the degree of water content and the cross sectional areas of the component molecules. When the gradient of the isotherms are plotted as a function of the number of water molecules per film molecule, the curves for the nickel complexes are almost coincident. In investigations of materials such as these, authors have previously inferred a molecular orientation based on the large lift off areas of the isotherms, as one might do with a simpler, close-packed film such as stearic acid. However, the fact that these isotherms can be represented by an equation describing a two component system derived (27) Pethica, B. L. Thin Solid Films 1987, 3, 152.
Molecular Orientation in Langmuir Monolayers
largely from the point of view of the solvent (i.e. the subphase water) suggests that the water content of the material is critical to their behavior, and represents a significant portion of the film during compression of the isotherm. The actual cross-sectional areas suggest that in the dianionic nickel complex, all three molecules are vertically oriented and in contact with the water surface. For the monoanionic complex, the two moieties are again in contact with the water and essentially vertical. The cross sectional area of the palladium complex is far smaller than expected, necessitating a multilayered arrangement of the molecules where the three ions cannot be in contact with the water surface. Thus, given that there are significant differences between the arrangement of the molecules in the palladium film as compared to the other two, its “anomalous” behavior is entirely to be expected. The differences in the behavior of the palladium film is more striking in the surface potential studies. These confirm the orientations inferred from the analysis of the π-A isotherms, and for the palladium salt, the surface potential isotherm shows some evidence that the [Pd(dmit)2]2- moiety is pushed up out of the plane of the headgroups during compression. Effective dipole moments calculated from the Helmholtz relationship are lower than expected. It is suggested that this is probably due to the interaction of the monolayer with the subphase water molecules, as has been found to be the case for other materials.26
Langmuir, Vol. 15, No. 7, 1999 2483
The Lucassen-Reynders (LR) interaction coefficients have been calculated from the best fit parameters of the equation of state, according to the method of Feng et al. The degree of interaction between the nickel complexes is almost identical, and thus gives a plausible reason for the similarity in the behavior of these two materials. Differences in the strength of the interactions between the subphase water and the molecules of the different complexes are consistent with the orientations described above. That is, the negative values of H12 may reflect the low affinity of the dmit anion for water. The largest interaction is seen for the monoanionic nickel salt in which the dmit would be expected to be more exposed, while the lowest interaction is seen for the dianionic palladium salt, where the anion is furthest from the subphase molecules. It is also suggested that the lower lipid-lipid interaction calculated for the palladium complex is consistent with the different molecular orientation inferred for this material. Acknowledgment. A.V.H. is grateful to EPSRC for the award of a studentship We are also indebted to Prof. C. J. M. Stirling (Department of Chemistry, University of Sheffield) for the gift of 1,3-didodecylimidazolium bromide. LA9807154
