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Langmuir 1997, 13, 5920-5924
The Surface Potential of Langmuir Monolayers Revisited Osvaldo N. Oliveira, Jr.,* and Cla´udia Bonardi Instituto de Fı´sica de Sa˜ o Carlos, USP, CP 369, 13560-970 Sa˜ o Carlos, SP, Brazil Received March 10, 1997. In Final Form: June 12, 1997X The surface potential characterization of Langmuir monolayers of simple, aliphatic compounds is revisited. Analogously to what has already been observed for fatty acids, the monolayer surface potential for esters and alcohols is close to zero at very large areas per molecule and only starts to increase at a given critical area. No problems of reproducibility are noted, which means that domains known to exist even in earlier stages of monolayer compression do not make the monolayer macroscopically heterogeneous. The appearance of a critical area is consistent with findings from a number of other experimental techniques employed by research groups around the world, including lateral conductance, optical reflectance, Maxwell displacement current, and ellipsometry.
I. Introduction In recent years considerable progress has been made on the interpretation of surface potentials (∆V) of Langmuir monolayers. Using the Demchak-Fort (DF)1 approach, in which a monolayer is considered as a threelayer capacitor with distinct dielectric constants, Taylor and collaborators2,3 have shown that measured ∆V can be related to group dipole moments for a number of aliphatic compounds. The DF model has also been applied to model potential vs area per molecule (∆V/A) isotherms of stearic acid monolayers4 and mixed monolayers of palmitic and trifluoropalmitic acid.5 This was made possible after Taylor et al.6 discovered that reproducible ∆V-A curves could be obtained if the water employed in the subphase was adequately purified. At these large areas ∆V is close to zero and remains constant until a critical area is reached on compression, after which a steep increase is observed. In the past, surface potentials for large areas per molecule were usually not reproducible owing to the formation of islands and aggregates in the monolayer, with ∆V being nonzero and varying for different monolayers of a given compound and even for different compression-expansion cycles of the same monolayer. Interestingly, such a formation of islands was first suggested by Harkins and Fischer in 1933.7 In this paper we show that reproducible curves, similar to that of stearic acid, can also be obtained for other compounds such as alcohols, amines, and esters possessing long aliphatic tails. Another important feature of the surface potential technique is that it allows the Langmuir monolayer to be probed at much earlier stages of monolayer compression, in comparison with surface pressure isotherms. The appearance of a critical area at which the surface potential rises sharply is also worth mentioning as it indicates the structuring of the monolayer that is also detected by a number of other experimental techniques. Though the * To whom correspondence may be addressed: tel, +55 16 2715365; fax, +55 16 2713616; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Demchak, R. J.; Fort,T. J., Jr. J. Colloid Interface Sci. 1974, 46, 191. (2) Oliveira, O. N., Jr.; Taylor, D. M.; Lewis, T. J.; Salvagno, S.; Stirling, C. J. M. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1009. (3) Taylor, D. M.; Oliveira, O. N., Jr.; Morgan, H. J. Colloid Interface Sci. 1990, 139, 508. (4) Oliveira, O. N., Jr.; Taylor, D. M.; Morgan, H. Thin Solid Films 1992, 210, 76. (5) Oliveira, O. N., Jr.; Riul Jr., A.; Leal Ferreira, G. F. Thin Solid Films 1994, 242, 239. (6) Taylor, D. M.; Oliveira, O. N., Jr.; Morgan, H. Thin Solid Films 1989, 173, L141. (7) Harkins, W. D.; Fischer, E. K. J. Chem. Phys. 1933, 1, 852.
S0743-7463(97)00272-2 CCC: $14.00
idea of a critical packing density was suggested some years ago,6,8 it has been largely overlooked in the study of Langmuir monolayers. Iwamoto and co-workers9-11 do mention a critical area in recent papers, albeit with a different definition from that employed here. They include a theoretical approach for the phase transition of molecular orientation,9 but the calculated value for the critical area for long-chain aliphatic compounds is not consistent with surface potential experiments. This is discussed in detail here. II. Materials and Methods Langmuir monolayers were spread from chloroform solutions of stearyl alcohol, methyl stearyl ester, and stearylamine, onto aqueous subphases using a KSV5000 system mounted on an antivibration table and housed in a temperature-controlled clean room. All chemical compounds were purchased from Sigma Co. and used as received. Spreading conditions such as barrier compression speeds, concentration of the solutions, and volume dispensed onto the aqueous subphase were varied in order to identify any possible effect on monolayer characteristics. Monolayer behavior was monitored by surface pressure and surface potential-area isotherms. Surface pressure was measured using the Wilhelmy method to an accuracy of 0.1 mN/m, while the surface potential was measured with a Kelvin probe to an accuracy of 10 mV. The subphase water was supplied by a Millipore purification system consisting of a Milli-RO system coupled to Milli-Q polishing cartridges.
III. Results and Discussion 3.1. Updating Surface Potential-Area Data. Figures 1-3 show typical surface pressure and surface potential isotherms for monolayers spread onto ultrapure water subphases from stearyl alcohol, stearyl methyl ester, and stearylamine, respectively. The surface pressure curves are essentially the same as those presented in the literature. The surface potential isotherms, however, differ from old published data (see for instance, Chapter 5 of ref 12) in that ∆V remains constant (close to zero) at large areas per molecule and only increases at a given critical area, Ac. For the amine, the surface potential is nonzero even at large areas per molecule, probably owing to a large, positive contribution from the double-layer, as discussed later. In all results presented here, the surface (8) Oliveira, O. N., Jr. Ph.D. Thesis, University of Wales, Bangor, 1990. (9) Sugimura, A.; Iwamoto, M.; Zhong-can, O. Y. Phys. Rev. E 1995, 50, 614. (10) Iwamoto, M.; Kubota, T.; Muhamad, M. R. J. Chem. Phys. 1995, 102, 9368. (11) Iwamoto, M.; Mizutani, Y. Phys. Rev. B 1996, 54, 8186. (12) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966.
© 1997 American Chemical Society
Surface Potential of Langmuir Monolayers
Figure 1. Surface pressure and surface potential isotherms of a stearyl alcohol monolayer, obtained by spreading an aliquot of 200 µL of a chloroform solution (0.25 mg/mL) on ultrapure water (pH ≈ 5.8). The critical area, Ac, for the onset of the surface potential is marked with an arrow. The compression speed was 15 mm/min and the experiments were carried out at room temperature (20 ( 1 °C).
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Figure 3. Surface pressure and surface potential isotherms for a stearylamine monolayer. Experimental conditions were identical to those of Figure 1.
potential values were reproducible within (20 mV while the critical areas were reproducible within (5 Å2. The increase in surface potential at the critical area for the compounds shown in the figures is not particularly sharp. In fact the initial slope of increase varies from one compound to another and is very sharp for phospholipids, for instance.13 Subsidiary experiments were carried out for palmitic and stearic acid, but the data are not presented here as they are practically the same as those in refs 6 and 8. The shape of the surface potential curves was not altered to any significant extent when subphases of different pH values (from 1 to 10) were employed (data not shown here). Neither was the shape affected by adding NaCl to the subphase; that is to say, upon compression the surface potential still increased at practically the same critical area. The only change was obviously in the surface potential amplitude for amine and acid monolayers whose degree of ionization depends on the subphase pH.
In order to ensure complete reproducibility of the data, isotherms were obtained under various experimental conditions so that any effect from experimental parameters could be detected.14 For example, for these simple compounds no effect was to be expected when the concentration of the spreading solution was varied. However, while the surface pressure is unaltered when the concentration is changed from 1 to 0.25 mg/mL, fluctuations in ∆V are significantly decreased for the 0.25 mg/mL concentration. As will be discussed later, it is clear that formation of aggregates (domains) is favored for the higher concentration. One could then suppose that using very dilute solutions would be ideal. This is not the case, though, as we observed in experiments with concentrations lower than 0.25 mg/mL. Because now the solution needed to be dispensed in several steps, thus progressively perturbing the monolayer, fluctuations started to increase upon decreasing the concentration below 0.25 mg/mL.14 With regard to the barrier compression speed, hysteresis experiments showed that at low speeds (5-15 mm/min), there is very little change in the overall behavior of the isotherms. No pronounced hysteresis is observed (that is to say, the surface potential curves on decompression are essentially the same as during monolayer compression). For higher speeds, however, not only the hysteresis but also the fluctuations are increased. Another important parameter relates to the time elapsed between spreading and the first monolayer compression-expansion cycle. This period of time should be sufficient not only for the solvent (e.g., chloroform) to evaporate but also for the molecules to be uniformly spread over the aqueous surface. Experiments in which pure chloroform is spread show that after 5 min there is no detectable trace (in surface pressure and potential measurements), but usually 1015 min was allowed before compression started. In spite of the differences observed here for the region of large areas per molecule, the surface potential values of closely packed monolayers were essentially within the published values. Table 1 summarizes the results obtained with the Langmuir monolayers and also includes the expected surface dipole moment when the modified Demchak-Fort model2 is employed. According to the Demchak-Fort model, the monolayer surface potential
(13) Morgan, H.; Taylor, D. M.; Oliveira, O. N., Jr. Thin Solid Films 1989, 178, 73.
(14) Bonardi, C. MSc. Dissertation Thesis, Instituto de Fı´sica de Sa˜o Carlos, Universidade de Sa˜o Paulo, at Sa˜o Carlos, Brazil, 1995.
Figure 2. Surface pressure and surface potential isotherms for a methyl stearyl ester monolayer. Experimental conditions were identical to those of Figure 1.
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Oliveira and Bonardi
Table 1. Dipole Moment Values for Condensed Monolayers of Various Aliphatic Compounds, Including Stearic Acid Data Not Shown in the Papera compound
∆Vmax (mV)
µ⊥ exp (mD)
µ⊥ DF (mD)
acidb
390
206
210
400 500 835
212 265 442
210 230 320
stearic (pH ) 2, un-ionised) alcoholc methyl esterc aminec
a The measured surface potentials are displayed in the second column, which were used for obtaining the experimental normal component of the dipole moment (µ⊥exp ) ∆Vmax A 2.65 × 10-2). Here A is the area per molecule in Å2 and the constant 2.65 × 10-2 was employed for converting the dipole moment into debye units (1 D ) 3.33564 × 10-30 C m). The third column brings the calculated dipole moments using the modified Demchak-Fort model with the parameters and group dipole moments suggested in ref 2: 3 ) 2.8, 2 ) 6.4, µ1/1 ) -65 mD; µ3 ) 330 mD, and µ2 ) 990, 1000, 1110, and 150 mD for the acid, alcohol, ester, and amine headgroups, respectively. A double-layer contribution was considered only for the amine which was supposed to be fully ionized at pH 5.8; the Gouy-Chapman theory, eq 2, was employed in the calculation. b Subphase containing HCl. c Ultrapure water subphase.
results from three main dipolar contributions
∆V )
[
]
1 µ1 µ2 µ3 + + A0 1 2 3
(1)
where µ1 is the normal component of the dipole moment due to the reorientation of the water molecules owing to the presence of the monolayer, µ2 is the normal component of the dipole moment from the hydrophilic headgroups at the monolayer-water interface, and µ3 is the normal component of the dipole moment from the hydrophobic tail groups at the monolayer/air interface. i are the effective dielectric constants of the respective regions in which the dipoles are embedded. When the monolayer is at least partially ionized, a double-layer is formed which contributes, according to the Gouy-Chapman theory, with
ψ0 )
[
]
2kT eR sinh-1 e A((5.88 × 10-7)cT)1/2
(2)
where ψ0 is the double-layer potential, R the degree of dissociation of the monolayer headgroups, e the protonic charge, k the Boltzmann constant, T the absolute temperature, c the ionic concentration in moles/liter, A the area per molecule, and the dielectric constant of the subphase. As expected, the measured surface dipole moment (i.e., surface potential) agrees to a reasonable extent with the values predicted by the DF model using the parameters determined by Oliveira et al.2 for aliphatic compounds (3 ) 2.8, 2 ) 6.4, µ1/1 ) -65 mD). The group moments used were also extracted from ref 2: µ3 ) 330 mD for the terminal CH3, and µ2 ) 990, 1000, 1110, and 150 mD for the normal component of dipole moment of acid, alcohol, ester, and amine headgroups, respectively. With regard to the parameters used, it is worth mentioning that the value of 2.8 for 3 has been corroborated in a theoretical model by Taylor and Bayes15 for calculating the effective dielectric constant at the film/air interface. The only exception in Table 1 is amine for which a discrepancy appears between experiment and the predicted value, in spite of the addition of a large GouyChapman double layer contribution which was calculated from eq 2 assuming the monolayer to be fully ionized. Incidentally, such a large contribution from the double layer is the reason for a positive (around 200 mV) (15) Taylor, D. M.; Bayes, G. F. Phys. Rev. E 1994, 49, 1439.
monolayer surface potential at large areas per molecule. That an almost flat curve is observed in this area range is explained by a weak area dependence of the doublelayer contribution (cf. eq 2). It could be argued that the GC theory leads to poor results for fully ionized monolayers but if anything this theory is bound to overestimate the double-layer contribution.8 Therefore, any correction to the GC theory would widen the gap between calculated and experimental values for the monolayer surface potential. This discrepancy for amine is a long-standing problem that has already been mentioned by Demchak and Fort1 and Oliveira et al.2 and is probably associated with the low value suggested, 150 mD, in ref 1 for the normal component of the NH2 in the headgroup. Because the dipole moment of a N-H bond is 1.39 D,16 it is likely that any other orientation for NH2 could lead to an increase in the calculated dipole moment. 3.2. The Existence of a Critical Packing Density of Langmuir Monolayers. The most widely employed measurements for characterizing Langmuir monolayers, i.e., pressure-area isotherms, generally bring information only for the closing stages of monolayer compression, when molecules are very close to each other and short-range interactions are manifest with the surface pressure becoming nonzero. There are obviously exceptions for monolayers of some types of compounds such as those which possess polar groups in the tail17-19 or highly charged polar headgroups. Other techniques provide information for earlier compression stages as the presence of the monolayer can be detected at much larger areas per molecule. This is the case of ellipsometry, Brewster angle microscopy (BAM), surface potential, and lateral conductance measurements. The surface potential technique was certainly the first to be used among the mentioned techniques, but interpretation of the surface potential results was fraught with difficulties, in particular because of non-reproducibility of the data. Such a state of affairs remained until the late 1980s when Taylor and collaborators6 discovered that, at least for fatty acids, non-reproducible surface potential results at large areas per molecule were caused by tiny amounts of impurity in the water subphase. It was then shown6 that if the subphase water was adequately purified, a stearic acid monolayer should possess zero surface potential at large areas per molecule, the potential increasing reasonably sharply at a critical area before attaining a maximum value in the condensed state. If, on the other hand, aged water was employed in the subphase, ∆V would be nonzero from the start and irreproducible data would be obtained even for several compression-expansion cycles of the same film. The existence of a critical packing density (or critical area per molecule) above which monolayer behavior changed drastically was suggested in refs 13 and 20, in which surface potential and lateral conductance measurements were compared for fatty acids and phospholipids. Taylor’s group, however, was not the first to observe sharp changes in isotherms; Vogel and Mo¨bius21 and Ducharme et al.22 published data where such changes were apparent. Since then a large number of papers have appeared with indications of a critical area in surface (16) CRC Handbook of Chemistry and Physics, 73rd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1992. (17) Oliveira, O. N., Jr.; Taylor, D. M.; Stirling, C. J. M.; Tripathi, S.; Guo, B. Z. Langmuir 1992, 8, 1619. (18) Vogel, V.; Mo¨bius, D. Thin Solid Films 1985, 132, 205. (19) Kellner, B. M. J.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 452. (20) Morgan, H.; Taylor, D. M.; Oliveira, O. N., Jr. Biochim. Biophys. Acta 1991, 1062, 149. (21) Vogel, V.; Mo¨bius, D. Thin Solid Films 1988, 159, 73. (22) Ducharme, D.; Salesse, C.; Leblanc, R. M. Thin Solid Films 1985, 132, 83.
Surface Potential of Langmuir Monolayers
potential,23-27 lateral conductance,13,20 optical reflectance,28 Maxwell displacement current,29 ellipsometry,30 etc. Most importantly of all is the fact that critical areas, obtained with different techniques for a given compound, usually agree within experimental errors. As for the measured values for the critical area, they are usually close to twice the area for a condensed monolayer. A model is lacking which could explain such an observation. In the discussion of surface potential curves, it is interesting to analyze data obtained with the Maxwell displacement current technique,31 for the measured current can be related to the normal component of the molecules dipole moment in the same way as the surface potential. As expected, for fatty acids the current became nonzero only at a critical area of ca. 35 Å2,32 which agrees very well with the critical area determined by surface potential measurements. Good examples of more recent studies supporting the existence of a critical area will be commented upon. Ahuja et al.32 showed that large increases in surface potential occur at given areas per molecule for monolayers of dioctadecyldimethylammonium bromide (DOMA) spread on halogen-containing subphases. The area at which the potential rises, called here critical area, decreased by adding Br-, Cl-, and especially I-. There was no critical area for the monolayer on the NaF subphase. The behavior on NaF was explained by the authors as being caused by a more homogeneous monolayer. F- is the smallest ion investigated and the most strongly hydrated. In our view, this strong hydration may be the cause for the absence of a critical area, as small domains would be highly connected to each other even at very large areas per molecule. The authors32 attribute the increase in potential to the fact that the monolayer front comes under the surface potential detector which is located closer to the fixed barrier. However, by obtaining surface potential isotherms with the Kelvin probe placed at various locations above the trough, Oliveira and Cavalli23 showed that, within experimental errors, the surface potential increases always at the same critical area. The measurement of the UV reflection spectrum has allowed Ahuja et al.28 to probe interactions between dimyristoylphosphatidic acid (DMPA) and tetracation cyclobis(paraquat-p-phenylene) (BBP4+) molecules at the air/water interface. In this method, only those molecules that resonantly contribute to the enhanced surface reflection are detected. In general, the reflectance displayed a sharp increase at 100 Å2, similar to ∆V-A isotherms for these compounds.28 In yet another observation of a critical area, Fichet et al.30 presented surface potential and ellipsometric results from film compression of dithioliumanion salts. Surface potentials increase very sharply at a given critical area which depends slightly on the subphase temperature but very strongly on the type of (23) Oliveira, O. N., Jr.; Cavalli, A. J. Phys.: Condens. Matter 1993, 5, A307. (24) Luckham, P.; Wood, J.; Froggatt, S.; Swart, R. J. Colloid Interface Sci. 1993, 156, 164. (25) Dupart, E.; Agricole, B.; Ravaine, S.; Mingotaud, C.; Fichet, O.; Delhaes, P.; Ohnuki, H.; Munger, G.; Leblanc, R. M. Thin Solid Films 1994, 243, 575. (26) Befort, O.; Mo¨bius, D. Thin Solid Films 1994, 243, 553. (27) Kondo, T.; Ahuja, R. C.; Mo¨bius, D.; Fujihira, M. Bull. Chem. Soc. Jpn. 1994, 67, 315. (28) Ahuja, R. C.; Caruso, P.; Mo¨bius, D.; Wildburg, G.; Ringsdorf, H.; Philp, D.; Preece J. A.; Stoddart, J. F. Langmuir 1993, 9, 1534. (29) Ohara, K.; Nakajima, M. Thin Solid Films 1993, 226, 164. (30) Fichet, O.; Ducharme, D.; Gionis, V.; Delhae`s, P.; Leblanc, R. M. Langmuir 1993, 9, 491. (31) Iwamoto, M.; Majima, Y. Jpn. J. Appl. Phys. 1988, 27, 721; 1989, 29, 564. (32) Ahuja, R. C.; Caruso, P.; Mo¨bius, D. Thin Solid Films 1994, 242, 195.
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subphase. For dithiolium-TCNQ the onset of surface potential (critical area) and the change in the ellipsometric angle occur at the same area (110 Å2). The reason why a critical packing density exists for monolayer compression seems to be related to the “coming together” of the microdomains known to exist in Langmuir monolayers. Indeed, Pen˜acorada et al.33 have shown by Brewster angle microscopy that the large increase in surface potential corresponds to the most significant change in the relative surface coverage of condensed islands. This aggregation of domains appears to result in a structuring of the monolayer which has several effects. First of all, because water is probably removed from the headgroup/subphase interface the dielectric constant of such an interface is decreased sharply. This causes the increase in surface potential. The optical density of the monolayer is also affected, and therefore ellipsometric signal as well as BAM and optical reflectance are modified. Monolayer structuring causes facile proton conduction (enhancing the lateral conductance), probably by establishing proton pathways. That the monolayer surface potential is directly related to the formation of domains and islands was further demonstrated by Gupta et al.34 working with divalent salts (2C12)2 [M(dmit)2] (dmit ) 4,5-dimercapto-1,3-dithiole-2-thionato), in which M is a metal cation, possessing poor spreading characteristics. They observed that within 5 min of spreading ∆V increased to 290-350 mV, before returning to zero. But if monolayers were compressed before the surface potential had decreased to zero, the ∆V-A was shifted to smaller areas and the salt could not be considered to be fully spread. In their own words: “until then, presumably, the complex exists as islands of highly condensed material on the water surface.” The idea of a critical area has also been proposed by Iwamoto and co-workers recently. In ref 9 a thermodynamical model was presented based on the interaction of dipolar molecules with a liquid surface. The model predicted a first-order transition that corresponded to a sharp pulse in the Maxwell displacement current for a short-chain liquid crystal. For 4-cyano-4′,5-alkyl-biphenyl (5CB), the measured peak occurred at 90 Å2, which agreed well with the predicted critical area (πL2 where L is the chain length). In this context, the critical area values defined by Iwamoto9 should be the same as those observed in surface potential measurements. The critical area obtained by the Maxwell displacement current technique was seen to increase with chain length for this class of liquid crystal, from 5CB to 10CB.10 Such an increase, however, did not obey the L2 dependence, but rather tended to saturate as the chain length increased, as shown in Figure 3 of ref 10. Indeed, the predicted L2 behavior breaks completely for long chain molecules. For instance, using the model mentioned, Iwamoto and Mizutani11 suggested that a sharp change in the monolayer dielectric constant should be observed at a critical area of 1385 Å2 for a long-chain, aliphatic compound. Obviously, we can hardly expect any significant change in monolayer behavior at such large areas. The reason why the model fails for long-chain compounds is probably associated with the importance given to the chain orientation, as the chemical structure and even monolayer behavior are somewhat different from the cyanobiphenyl compounds. The measured critical area for a stearic acid monolayersfrom surface potential as well as the displacement (33) Pen˜acorada, F.; Reiche, J.; Katholy, S.; Brehmer, L.; RodriguezMe´ndez, M. L. Langmuir 1995, 11, 4025. (34) Gupta, S. K.; Taylor, D. M.; Dynarowicz, P.; Barlow, E.; Wainwright, C. E. A.; Underhill, A. E. Langmuir 1992, 8, 3057.
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current resultssis around 40 Å2, an area at which the chains must assume a direction (almost normal) very close to that of condensed monolayers. Any substantial change must therefore be attributed to the headgroup/ water interface, as emphasized in this paper. It might be possible that a change in surface pressure may occur at the critical area, but the accuracy of the surface pressure measurements reported here is not enough to detect such a change. IV. Conclusions Surface potential isotherms have been presented for Langmuir monolayers of some long-chain aliphatic compounds. While the surface potential values for condensed
Oliveira and Bonardi
monolayers were essentially the same as those published in the literature a long time ago, the area dependence was markedly different from that of earlier works. Of particular importance was the appearance of a critical area per molecule below which the surface potential was seen to increase. Such an increase has been attributed to a sharp lowering of the local dielectric constant at the monolayer/water interface when the monolayer becomes structured, which is also manifested in monolayer data obtained with other techniques such as ellipsometry, Maxwell displacement current, and lateral conductance measurements. LA970272O
