J. Phys. Chem. B 2000, 104, 1701-1707
1701
Langmuir Monolayers of Polyphenyl Carboxylic Acids Patrycja Dynarowicz-Ła¸ tka,†,‡ Anantharaman Dhanabalan,† Ailton Cavalli,†,§ and Osvaldo N. Oliveira, Jr.*,† Instituto de Fı´sica de Sa˜ o Carlos, UniVersidade de Sa˜ o Paulo, CP 369, CEP 13560-970, Sa˜ o Carlos, SP, Brazil, and Instituto de Biologia Letras e Cieˆ ncia Exatas, UniVersidade do Estado de Sa˜ o Paulo Sa˜ o Jose´ do Rio Preto, SP, Brazil ReceiVed: July 14, 1999; In Final Form: September 24, 1999
A series of 4′-substituted (methyl, phenyl, biphenyl, toluilyl, nitro, cyano, and p-nitrophenyl) 5′-phenyl-mterphenyl-4-carboxylic acids (PTCAs) have been synthesized, and their Langmuir monolayers were characterized using surface pressure and surface potential isotherms under a variety of experimental conditions. A long plateau appears at the pressure-area isotherms for most compounds, which is attributed to the tilting of molecules upon compression. A model is presented for the molecular arrangements which explains the decrease in dipole moment in the plateau region. As expected, the monolayer stability was higher for derivatives with more hydrophobic substituents, whereas the introduction of an additional hydrophilic group, viz. -CN or -NO2, in the parent PTCA molecule yielded negative surface potentials since the latter groups contributed negatively, in opposition to the positive contribution from the COOH group.
Introduction Traditional amphiphiles employed in the fabrication of Langmuir monolayers1 and Langmuir-Blodgett2 films include mainly long-chain aliphatic compounds with a highly hydrophilic headgroup, of which n-octadecanoic acid (stearic acid) is perhaps the most common one for monolayer studies. In contrast to the vast literature on aliphatic amphiphiles, very little work has been done on the monolayer behavior of purely aromatic analogues of aliphatic amphiphiles, either with uncondensed or fused benzene rings. The exceptions are represented by works of Demchak and collaborators3,4 who studied p-terphenyl derivatives, and by a few reports on polyaromatic compounds attached to long alkyl chains (-R), for example, 4′-cyano-4′′-alkylbiphenyls or terphenyls,5-10 4′-alkylaniline,11 long-chain pyridine derivatives,12 and amphiphilic derivatives of dyes.13 Molecules with solely an arenic hydrophobic moiety consisting of nonrigid polyphenyl systems appear, therefore, to represent an almost unexplored class of compounds as far as Langmuir monolayer studies are concerned. We have instituted a project in which a variety of polyphenyl carboxylic acids have been synthesized and are being employed for Langmuir monolayer and LB film characterization. In addition to investigating interesting optical properties of these materials in the long run, a more immediate aim is to understand their basic monolayer properties. In previous studies, we have shown that compounds containing a triphenylbenzene ring system (see Scheme 1, where R is H, CH3, and C6H5, compounds 1, 2, and 3, respectively)14-16 may form stable monolayers at the air-water interface. This means that the right balance between two opposing effects, i.e., the attraction of a polar group to the water and the presence of a hydrophobic * Corresponding author. Fax: +55 16 271 3616. Phone: +55 16 273 9825. E-mail:
[email protected]. † Universidade de Sa ˜ o Paulo. ‡ Present address: Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Krakow, Poland § Universidade do Estado de Sa ˜ o Paulo Sa˜o Jose´ do Rio Preto.
SCHEME 1: Chemical Formulas of the Compounds Investigated
moiety that prevents the molecule from dissolving in the water, has been achieved. These monolayer experiments were carried out with 4′-methyl (compound 2, in short MPTCA) and 4′-phenyl (compound 3, in short DTCA) derivatives of the basic compound 1, in short PTCA. In this paper, we extend the research to various polyphenyl carboxylic acids with the following substituents at the 4′ position: R ) nitro, cyano, p-biphenyl, p-nitrophenyl, and p-toluilyl (Scheme 1, compounds 4-8, respectively). The monolayers are characterized by surface pressure (π) and electric surface potential (∆V) measurements carried out under various experimental conditions. This study is expected to provide valuable information on both the steric and electronic influence of different electron acceptor/releasing substituents on the monolayer characteristics of the parent compound, PTCA. Experimental Section The compounds were synthesized according to the procedure reported earlier.14,16 The spreading solution for the Langmuir experiments was prepared by dissolving any given compound
10.1021/jp992396a CCC: $19.00 © 2000 American Chemical Society Published on Web 02/05/2000
1702 J. Phys. Chem. B, Vol. 104, No. 8, 2000
Figure 1. Surface pressure-area (π/A) isotherms of PTCA and its derivatives spread on 10-3 M HCl aqueous subphase under standard experimental conditions (20 °C, compression speed ) 7.5 × 1017 Å2/ min).
in spectroscopic grade chloroform with a typical concentration of ca. 0.4 mg/mL. Ultrapure water produced by a Nanopure (Infinity) water purification system coupled to a Milli-Q water purification system (resistivity ) 18.2 MΩ cm) was used as a subphase. Subphase pH and ionic strength were adjusted by addition of HCl or NaCl. For obtaining monolayers of cadmium salt of the compounds, solutions were spread on a subphase containing 4 × 10-4 M cadmium chloride and 5 × 10-5 M sodium bicarbonate whose pH was ca. 6.0. Monolayer studies were carried out with a KSV-5000 LB trough (total area ) 730.5 cm2) placed on an antivibration table in a class 10 000 clean room. The surface pressure of the floating monolayer was measured to an accuracy of 0.1 mN m-1 using a Wilhelmy plate (made of chromatography paper, ashless Whatman Chr 1) connected to an electrobalance. Simultaneously, surface potential was recorded using a vibrating plate located ca. 2 mm above the water surface. The reference electrode, made from platinum foil, was placed in the water subphase. The surface potential measurements were reproducible to (10 mV. Monolayers were usually compressed with a barrier speed of 25 mm/min (equivalent to a compression rate of 7.5 × 1017 Å2/min) unless otherwise specified. Results and Discussion Pressure-Area Isotherms. Surface pressure-area isotherms on 10-3 M HCl aqueous subphases at 20 °C of various PTCA derivatives are shown in Figure 1. The striking feature is that the area per molecule for the onset of surface pressure cannot simply be related to the molecular size. While the introduction of biphenyl and p-toluilyl groups at the 4′-position of PTCA increased the molecular area in comparison to the parent PTCA, analogously to what occurred for MPTCA and DTCA, the molecular area decreased for the 4′-nitro, cyano, p-nitrophenyl derivatized PTCAs. In analogy with aliphatic counterparts, in which 10-12 carbons are required in the alkyl chain to form a stable monolayer, parent PTCA lies just on the borderline, for it possesses four phenyl groups, and the hydrophobicity of a phenylene group is comparable to that of four methylene groups.17 Therefore, the methyl, phenyl, biphenyl, and p-toluilyl derivatives of PTCA are expected to form more stable monolayers, due to enhanced hydrophobicity when compared to the PTCA molecule. The introduction of another polar group such as nitro, cyano, and p-nitrophenyl, on the other hand, is expected to disrupt the hydrophilic-hydrophobic balance that exists in PTCA, which may in turn affect monolayer formation.
Dynarowicz-Ła¸ tka et al. Another intriguing feature is that no clear plateau could be observed for the 4′-cyano, p-biphenyl, and p-nitrophenyl derivatives, in contrast to the other derivatives of PTCA (4′-methyl, phenyl, nitro, p-toluilyl) where the plateau spans over the region corresponding to a decrease in area/molecule of about 2-fold. For cyano derivative only a small, low-pressure (ca. 0.5 mN/ m) transition occurs, while for p-biphenyl PTCA a “kink” at a surface pressure of 10 mN/m appears. No plateau, however, can be seen in the course of the π/A isotherm for the p-nitrophenyl derivative. This cannot be explained by either steric or electronic effects. p-Nitrophenyl PTCA monolayer exhibits a liquid condensed type of isotherm and collapses at a surface pressure of ca. 40 mN/m. It should be mentioned that electronic effects from the substituent make it unlikely for aromatic rings in PTCA and its derivatives to be coplanar, particularly in the case of the central benzene ring and the one attached to the -COOH group. For example, for the PTCA molecule, semiemprical MOPAC computation18 in AM1 parametrization19 revealed an optimum conformation in which each phenyl ring is twisted by 40° with respect to the central one.15 The polyphenyl ring system is flexible, and the aromatic rings can rotate with respect to each other, thus adopting different conformations, since the bond connecting the phenyl groups is essentially a single bond with some double bond character. The variation of single double bond character depends on the presence of electron-withdrawing/electron-releasing substituents on the phenyl ring and may restrict rotation of the phenyls. While the bulky hydrophobic biphenyl and hydrophilic cyano/ nitrophenyl substitutions led to isotherms with no clear plateau, the hydrophilic nitro and hydrophobic phenyl/p-toluilyl substituents resulted in the appearance of a broad plateau. To understand such a behavior, we performed monolayer experiments under different subphase conditions, as described below. In a set of control experiments, we observed that changing the spreading volume did not influence significantly the isotherms for any compound. This is in contrast to the shift toward higher areas measured for the parent PTCA as the spreading volume increased.15 For the PTCA derivatives investigated here, the compression speed is not important for the mean area per molecule, but the plateau surface pressure slightly increases with increasing compression speed. As an example, for the p-toluilyl derivative, the plateau surface pressure changes from 23.5 mN/m to 25.5 mN/m with increasing the compression speed from 1.5 × 1017 Å2/min to 30 × 1017 Å2/min, respectively. Particularly for the methyl derivative of PTCA, the compression speed has an important effect, since the monolayer collapsed at a much lower surface pressure (∼10 mN/m) when compressed slowly, say at 0.3 × 1017 Å2/min. Further effects from compression speeds are apparent for monolayers of p-biphenyl PTCA, with the small “kink” observed under standard experimental conditions (pure water subphase, 20 °C, compression speed 7.5 × 1017 Å2/min) becoming a clear plateau when the compression speed is lowered to 0.3 × 1017 Å2/min. For the compounds whose isotherms have no plateau (cyano and p-nitrophenyl derivatives), the compression speed did not have any significant effect. When comparing surface pressure-area isotherms of different PTCA-derivatives on an acidic solution (see Figure 1) to those recorded on pure water (results not shown here), we note that the pressure lift-off is shifted toward larger areas on a 10-3 M HCl aqueous subphase for the cyano (from ca. 30 to 40 Å2) and nitro (from 35 to 47 Å2) derivatives of PTCA, analogously to observed with the parent PTCA monolayer (from 41 to 47 Å2). However, no such shift was noticed for the other PTCA
Langmuir Monolayers of Polyphenyl Carboxylic Acids
Figure 2. π/A isotherms of PTCA on different subphases under standard experimental conditions.
Figure 3. Influence of subphase temperature on the π/A isotherms of 4′-p-toluilyl PTCA spread on water; compression speed ) 7.5 × 1017 Å2/min.
derivatives, which are more hydrophobic. One of the possible reasons for the lower mean area per molecule on pure water is partial material dissolution into the subphase since the -COOH headgroup is partially ionized at this subphase pH (∼6.0). Changing the subphase composition may affect the monolayer properties, with a particularly strong effect from introducing cadmium ions in the subphase, as illustrated in Figure 2 for PTCA. The increase in the ionic strength, with the addition of NaCl in the subphase increases both the area per molecule for the onset of surface pressure and the plateau surface pressure. The plateau pressure is also affected by the subphase temperature. Figure 3 shows that this pressure increases with decreasing temperature, as it will be discussed later. For the least stable monolayers, viz. cyano and nitro derivatives of PTCA, material dissolution is significant at higher temperatures, which reflected in a significant shift (ca. 20 Å2) toward lower areas per molecule with the increase in subphase temperature from 20 to 30 °C. The onset for surface pressure is shifted slightly by 7-8 Å2 for the methyl, p-nitrophenyl, and biphenyl derivatives upon increasing the subphase temperature, but no change was noted for the other compounds. It has been already mentioned that the presence of cadmium ions in the subphase strongly affect the π/A characteristics. In a previous work, we noticed that complexation of cadmium ions
J. Phys. Chem. B, Vol. 104, No. 8, 2000 1703 with 4′-phenyl derivative of PTCA (DTCA) brought substantial changes in the nature of the isotherm, in comparison to the parent PTCA monolayer, since an additional plateau appeared for DTCA.16 For the methyl and p-toluilyl PTCAs, there was an increase in the plateau surface pressure, analogously to the parent PTCA monolayer (see Figure 2). For p-nitrophenyl PTCA, however, a clear plateau appeared owing to the introduction of cadmium ions in the subphase (the isotherm on water shows no plateau region). Similar to the effect observed for DTCA, the onset of surface pressure occurs at larger areas per molecule for the cadmium salt. For example, the surface pressure liftoff for DTCA and Cd-DTCA occurs at 60 and 75 Å2, respectively, for p-nitrophenyl derivative of PTCA and its cadmium salt it occurs at 35 and 60 Å2, respectively. On the other hand, the plateau region disappeared upon complexation with cadmium ions for monolayers produced from biphenyl PTCA. The influence of subphase temperature on isotherms of monolayers of cadmium salts of PTCA derivatives has also been studied. For the p-nitrophenyl derivative, the plateau became prominent upon increasing the subphase temperature, and only a little shift was noticed toward lower areas per molecule. No change in the nature of the isotherm could be seen for the biphenyl and other derivatives, though for the biphenyl derivative the isotherm was shifted toward higher areas with increasing temperatures. However, it is evident that except for the phenyl derivative, no other derivative of PTCA displays an isotherm with two plateau regions on an aqueous Cd2+ subphase. From these studies, it is clear that changing subphase and compression conditions influences the monolayer characteristics of PTCA derivatives differently. This can be understood in terms of a dependence of molecular structuring in the monolayer on the substituents attached to PTCA. In general, the introduction of polar groups such as -NO2 and -CN substituents seems to affect the monolayer stability; no clear plateau could be observed for cyano, biphenyl and p-nitrophenyl derivatives of PTCA on the pure water surface. However, for PTCA and its methyl, phenyl, p-toluilyl, and nitro derivatives, a similar course of the π/A isotherms with a broad plateau region was observed. The stability of compressed films, studied at different target surface pressures, indicates that the monolayers are very stable in the preplateau region, with a decrease in area per molecule by only 5%. However, in all cases, the monolayers were unstable in the plateau and postplateau regions, with a decrease in area of 30-60%. As has already been mentioned, the plateau in the π/A isotherm is temperature dependent and the surface pressure at which the plateau occurs decreases with increasing temperatures. Such a behavior is inverse to that of plateaus originating from phase coexistence, observed with Langmuir monolayers formed by model amphiphiles (e.g., n-pentadecanoic acid).20,21 However, plateau regions attributed either to the collapse of a monolayer into a multilayer state22 or due to orientational changes upon compression23-25 exhibit analogous trend with temperature as observed for PTCA and its derivatives. On the basis of the analysis of surface pressure/area isotherms only, and considering that the plateau region corresponds to a ca. 2-fold decrease in area, the following reasons may be put forward to explain the plateau. (i) Transition of a monolayer into a bilayer structure; this can explain the 2-fold decrease in area, by the formation of a second layer on top of a monolayer (Figure 4, Ia) as well as the immersion of the first layer into the water subphase (Figure 4, Ib). (ii) Formation of an interdigitated molecular arrangement; this kind of ordering can
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Dynarowicz-Ła¸ tka et al.
Figure 5. Surface potential (∆V) vs area (A) isotherms of 4′-p-toluilyl PTCA on different subphases under standard experimental conditions. Surface pressure-area isotherm on water (solid line) is also reproduced for comparison. Figure 4. Possible orientations for the molecules at the plateau region, explanation in the text.
be attained either by overlapped triphenylbenzene rings (Figure 4, IIa) or by partial immersion of some phenyl rings attached to the carboxylic group into the subphase (Figure 4, IIb), and a 2-fold decrease of area per molecule can be expected in the closely packed arrangement. (iii) Conformational changes of the triphenylbenzene ring system; as already analyzed for the PTCA monolayer,15 changes in conformation of peripheral phenyl rings under compression, from one with twisted side phenyls with respect to the central benzene ring to one with in-plane structure, influence significantly the areas of projection of the vertically oriented molecules. The ratio of the projection areas of the extreme conformations is about 2. (iv) Inclination of molecules (Figure 4, III). The extrapolation of the pre-plateau region of the isotherm to π ) 0 coincides with the crosssectional area of the molecule. However, gradual inclination of molecules with partial interdigitation of phenyl rings, as illustrated in Figure 4, III, may lead to molecular arrangements of smaller areas, thus accounting for the plateau region. To distinguish between these possibilities, a systematic study of surface potential-area isotherms was performed, as described below. Surface Potential Isotherms. It is known that a quantitative relationship between the molecular dipole moments and the measured surface potentials of Langmuir monolayers can be made for model amphiphilic aliphatic compounds such as alkanoic fatty acids.26,27 Interestingly, the modeling is based on the Demchak-Fort model,3 which was originally developed for aromatic compounds. However, Oliveira et al.28 suggested that the parameters in the DF model, i.e., the relative dielectric constants where the molecular dipole moments are embedded, should be different depending on whether aliphatic or aromatic compounds were under consideration. Owing to the wide availability of surface potential data, the model could be tested for the aliphatic compounds, but the same could not be done for the aromatic analogues. This work is therefore aimed at obtaining data for a number of compounds, to pave the way for theoretical treatments to be carried out soon. Furthermore, a qualitative analysis suffices to identify the most probable reason for the plateau’s appearance, as explained later. Typical surface potential-area isotherms for the p-toluilyl substituted PTCA obtained with various subphases (water, 10-3 M HCl, 10-3 M NaCl, and 4 × 10-4 M CdCl2 + 5 × 10-5 M
NaHCO3) are shown in Figure 5. For the purpose of comparison, the pressure/area isotherm on water is also shown in the same graph. The initial surface potential, at large areas per molecule, was always close to zero. Upon compression, the surface potential onset occurred at the so-called critical area, where the surface pressure was still zero, and increased gradually, reaching a maximum value at areas per molecule corresponding to the beginning of the plateau region in the π/A isotherm. As for other substituted PTCAs, the critical area increases according to the order 4 × 10-4 M CdCl2 + 5 × 10-5 M NaHCO3 < 10-3 M NaCl = 10-3 M HCl < water. Since the critical area denotes the coming together of molecules and hence the beginning of structuring of the molecules at the air/water interface,26 one would expect this area to provide information on the molecular size and the impact from dipoles on the intermolecular interactions at early stages of compression. The maximum surface potential, on the other hand, obeys the order 4 × 10-4 M CdCl2 + 5 × 10-5 M NaHCO3 < water = 10-3 M NaCl < 10-3 M HCl subphases. The lower surface potential value for the monolayer spread on the subphase containing cadmium ions is attributed to the large negative contribution from the doublelayer formed by carboxylate ions and cadmium counterions, since the monolayer is fully ionized at the subphase pH (∼6.2) employed. By the same token, the highest surface potentials are observed for nonionized PTCA’s on acidic subphases (10-3 M HCl), while the partially ionized monolayers spread on pure water and salt solutions (10-3 M NaCl) display intermediate potentials. As for the latter subphases which have essentially the same pH (ca. 6.0), the surface potential for the salt solutions is slightly higher due to the effect of the ionic strength on the double-layer potential, though it is small even for an increase of NaCl concentration from millimolar to 0.1 M (about 40 mV). However, changing the subphase pH while keeping the ionic strength has a much more pronounced effect on surface potential, since the degree of ionization of acid headgroups is affected. Maximum surface potentials (∆Vmax) occur in the beginning of the plateau regions when the films are quite stable, and may therefore be used to compare the PTCA’s monolayers on the various subphases. Table 1 shows that the surface potentials for the neutral methyl and p-toluilyl substituted PTCA’s (on acidic subphases) are higher than for the parent PTCA. This is consistent with earlier reports on the small increase in surface potentials with the length of the alkyl chain in fatty acid monolayers,29 with a 15 mV increment being observed for each additional pair of carbon atoms in the chain.30,31 On the other
Langmuir Monolayers of Polyphenyl Carboxylic Acids
J. Phys. Chem. B, Vol. 104, No. 8, 2000 1705
TABLE 1: Parameters from the Surface Pressure and Surface Potential Isotherms, for the PTCA’s Derivatives Investigateda area/molecule [Å2] compound
Ao
Ae
PTCA 4′-methyl 4′-phenyl 4′-toluilyl 4′-biphenyl 4′-nitro 4′-cyano 4′-nitrophenyl
47 46 60 52 55 47 41 41
21 23 31 31 49 27 26 37
∆Vmax [mV] πp 10-3 [mN/m} water NaCl 15 19 21 25 14
10-3 HCl
Cd2+
450 440 550 490 465 479 562 320 405 400 494 280 492 488 594 380 326 325 445 300 -639 -563 -481 -611 -135 -110 -30 -380 -500 -500 -300 -700
R Ao and Ae are extrapolated areas to π ) 0 for the preplateau and postplateau regions, respectively. πp is the average surface pressure at the plateau. ∆Vmax corresponds to the maximum surface potential of any given isotherm.
that these substituents are oriented in the opposite direction to the dipole of the carboxylic acid headgroup. This leads to a most probable molecular orientation at the air-water interface in which the strongly polar -COOH headgroups are anchored at the air-water interface and the hydrophobic polyaromatic rings are pointed toward the air. Another possible molecular orientation in which the strong polar -COOH group is removed from the water can be considered as thermodynamically unfavorable. The effects from changing the subphase, in terms of the double-layer contribution, are nevertheless similar to the parent PTCA and the other derivatives. That is to say, a similar surface potential is measured on pure water surface and on a 10-3 M NaCl aqueous subphase, a less negative surface potential is measured for the acidic subphase, while the surface potential is more negative for the subphase containing cadmium ions. A simple comparison of the surface potentials of the parent PTCA (550 mV) and that of 4′-nitro PTCA (-480 mV) revealed a lowering of maximum surface potential of approximately 1000 mV. But a comparative small decrease (about 580 mV) was observed for cyano PTCA, though the group dipole moment of cyano and nitro groups are comparable (about 4 D).32 This probably has to do with distinct packing arrangements assumed by the derivatives and will be the subject of further investigation with a quantitative analysis of surface potentials in terms of the effective dipole moments. One may nevertheless inspect qualitatively the effective dipole moment-area isotherm in order to investigate the nature of the plateau transition region. The effective dipole moment is calculated using the Helmholtz equation,33
∆V ) µn/Ar0 Figure 6. Surface potential (∆V) vs area (A) isotherms of 4′-pnitrophenyl PTCA on different subphases under standard experimental conditions.
hand, a lower maximum surface potential was observed for aryl (phenyl and biphenyl) substituted PTCA’s. A comparison of the surface potentials of nonionized, on acidic subphases, and fully ionized monolayers, on cadmium-containing subphases, yields, in principle, the double-layer contribution for any given compound. One should therefore expect the same contribution since all compounds possess a carboxylic group. However, this is not the case, and the difference is much higher for substituted PTCA’s (145 mV for biphenyl PTCA, 214 mV for toluilyl and phenyl PTCA’s, 242 mV for methyl PTCA) than for the parent PTCA (60 mV). This points to the existence of distinct packing arrangements, which affect the vertical component of the dipole moment differently for distinct PTCA’s in the compressed state, as dictated by the substituents at the 4′-position. The difference in packing arrangement could simply be due to different degrees of tilting of side phenyl groups with respect to the central one, caused by the substituents at the 4′-position. The possibility of material solubility into the subphase, i.e., poor monolayer stability, may be ruled out, since solubility is expected to be minimum both on the acidic subphase as well as on a subphase containing cadmium ions (due to complexation of PTCA’s with cadmium ions). As for PTCA’s with polar substituents, a typical surface potential-area isotherm is shown in Figure 6 for 4′-nitrophenyl PTCA monolayer on various subphases at 20 °C. The surface potential decreases upon monolayer compression from the initial zero, owing to the large negative contribution to the vertical dipole moment introduced by the polar substituent, which means
where A is the area available for a molecule in the monolayer, µn (so-called effective dipole moment) is the normal component of the dipole moment of a film molecule, and r and 0 are the dielectric constant of the monolayer and the permittivity of free space, respectively. In calculating µn, the dielectric constant was taken as one, since no quantitative treatment is aimed at (see, however, the importance of dielectric constants for the analysis of dipole moments in ref 27). Figure 7 presents the effective dipole moment-area dependence for PTCA, which is representative for all the monolayers formed by 4′-PTCA derivatives with hydrophobic substituents. The surface pressure and electric surface potential-area isotherms are reproduced in the same graph for comparison. At large areas, the film molecules are believed to be well separated and noninteracting with each other (Figure 7, region a). At this stage both surface pressure and surface potential are zero, i.e., the pressure and potential probes depict the values for pure water. Under film compression, the film molecules start to interact (region b), and at a critical area the surface potential starts to increase owing to the decrease in dielectric constant in the vicinity of hydrophilic groups.34,35 van der Waals attraction between aromatic moieties causes the surface pressure to increase until the plateau pressure is reached. At the area per molecule corresponding to the beginning of the plateau, both surface potential and effective dipole moment reach their maximum value. On further compression, the surface potential decreases since the effective contribution of the dipole moment, µn, diminishes. Such a decrease allows us to rule out the following possibilities for the plateau appearance. (i) Interdigitated arrangements (Figure 4, IIa and IIb); in this case, the vertical component of the dipole moment (as well as the surface potential) should increase due to an increased dipole density. (ii) Conformational changes of side phenyl groups, since molecular modeling18,19
Dynarowicz-Ła¸ tka et al.
1706 J. Phys. Chem. B, Vol. 104, No. 8, 2000
Figure 7. Surface pressure (dash-dotted line), surface potential (solid line), and effective dipole moment (dashed line) vs area isotherms of PTCA spread on a 10-3 M HCl subphase under standard experimental conditions, explanation in the text.
revealed that the dipole moment component of a free molecule along the long molecular axis increases on going from a twisted conformation (expected for an expanded state) to a planar structure (characteristic of a condensed state). The two remaining possibilities for the origin of the plateau are the hypotheses of (i) bilayer formation, which could indeed result in a decrease in dipole moment, and (iv) inclination of film molecules with partial interdigitation of phenyl rings, which can also lead to lower vertical components of the dipole moment. The 2-fold decrease in area per molecule in the plateau and its temperature dependence, by analogy to the model from refs 6, 10, and 22, may be attributed to a transition of a monolayer into a bilayer. It is, however, difficult to suggest an adequate, energetically favorable model since the molecular arrangements presented in Figure 4, Ia and Ib, which involve either the formation of a second layer of molecules with polar groups exposed to the air (Figure 4, Ia) or immersion of one layer of molecules into the water subphase (Figure 4, Ib), are both unlikely from the thermodynamic point of view. In fact, a 2-fold decrease may also result from the formation of multilayer (e.g., trilayer) patches coexisting with a monolayer. However, it is unlikely that multilayer patches form just in the beginning of the plateau. It is well-known that upon multilayer formation, the isotherms are highly irreproducible, even for a model filmforming molecule like stearic acid. Notwithstanding, isotherms for polyphenyl carboxylic acids, especially the pressure at which the plateau occurs, are very reproducible. Moreover, Brewster angle microscopy (BAM) images for PTCA,15 which are representative for all 4′-derivatives investigated here, indicate monolayer homogeneity up to the mid plateau region. Therefore, multilayer formation is only likely to occur at the end of the plateau region, and the decrease in dipole moment in the beginning of the plateau should be attributed to the tilting of molecules (hypothesis iv). Conclusions The influence of attaching polar and nonpolar substituents at the 4′ position of 5′-phenyl-m-terphenyl-4-carboxylic acid (PTCA) on the monolayer characteristics has been investigated.
In general, PTCA’s with nonpolar substituents formed more stable monolayers than the parent compound, due to possible enhancement in the hydrophilic-hydrophobic balance. Because molecular packing was also affected by the substituents, parameters such as onset areas for surface pressure and surface potential and the maximum surface potential also depended on the substituent. Such dependence could not be attributed to steric effects only. For the hydrophilic substituents, the changes were even more drastic, with an inversion in sign of the surface potential and the disappearance of the plateau in the pressure isotherm for some compounds. Upon analyzing the surface pressure and surface potential isotherms, we could ascribe the beginning of the plateau region to a gradual inclination of molecules, which may lead to the formation of 3-D multilayer structures at the termination of the plateau. Acknowledgment. The authors acknowledge financial support of FAPESP and CNPq (Brazil) for this work. References and Notes (1) Gainess, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (2) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (3) Demchak, R. J.; Fort, T., Jr. J. Colloid Interface Sci. 1974, 46, 191. (4) Cadenhead, D. A.; Demchak, R. J. J. Chem. Phys. 1968, 49, 1376. (5) Daniel, M. F.; Lettington, O. C.; Small, S. M. Thin Solid Films 1983, 99, 61. (6) Xue, J.; Jung, C. S.; Kim, M. W. Phys. ReV. Lett. 1992, 69, 474. (7) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (8) De Mul, M. M. G.; Mann, J. A. Langmuir 1994, 10, 2311. (9) Dent, D.; Grundy, M. J.; Richardson, R. M.; Roser, S. J.; McKeown, N. B.; Cook, M. J. J. Chim. Phys. 1988, 85, 1003. (10) Schro¨ter, J. A.; Plehnert, R.; Tschierske, C.; Katholy, S.; Janietz, D.; Penacorada, F.; Brehmer, L. Langmuir 1997, 13, 796. (11) Jones, C. A. Ph.D. Thesis, University of Durham, United Kingdom, 1987. (12) Mo¨bius, D. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 848. (13) Kuhn, H.; Mo¨bius, D.; Bu¨cher, H. In Techniques of Chemistry; Weissberger, A., Rossiter, B. W., Eds.; Wiley: New York, 1972; Vol. 1, Part 3B, pp 577-702.
Langmuir Monolayers of Polyphenyl Carboxylic Acids (14) Czapkiewicz, J.; Dynarowicz, P.; Milart, P. Langmuir 1996, 12, 4966. (15) Dynarowicz-Ła¸ tka, P.; Dhanabalan, A.; Oliveira, O. N., Jr. J. Phys. Chem. B 1999, 103, 5992. (16) Dynarowicz-Ła¸ tka, P.; Dhanabalan, A.; Oliveira, O. N., Jr. Langmuir, submitted (17) Van Oss, N. M.; Kok, P.; Bolsman, T. A. M. B. Tenside Surf. Det. 1992, 3, 175. (18) Stewart, J. J. P. Quantum Chemistry Programme Exchange #455. (19) Dewar, M. J. S.; Zoebish, E. G.; Healy, E. G.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902. (20) Pallas, N. R.; Pethica, B. A. Langmuir 1995, 1, 509. (21) Marcelja, S. Biochem. Biophys. Acta 1974, 367, 165. (22) Seitz, M.; Struth, B.; Preece, J. A.; Plesnivy, T.; Brezesinski, G.; Ringsdorf, H. Thin Solid Films 1996, 284/285, 304. (23) Kellner, B. M. J.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 452. (24) Vogel, V.; Mo¨bius, D. Thin Solid Films 1985, 132, 205.
J. Phys. Chem. B, Vol. 104, No. 8, 2000 1707 (25) Vila Romeu, N.; Min˜ones Trillo, J.; Conde, O.; Casas, M.; Iribarnegaray, E. Langmuir 1997, 13, 71. (26) Oliveira, O. N., Jr.; Bonardi, C. Langmuir 1997, 13, 5920. (27) Taylor, D. M.; Bayes, G. F. Phys. ReV. E 1994, 49, 1439. (28) 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. (29) Kuchhal, Y. K.; Katti, S. S.; Biswas, A. B. J. Colloid Interface Sci. 1974, 49, 48. (30) Harkins, W. D.; Fisher, E. K. J. Chem. Phys. 1933, 1, 852. (31) Adam, N. K.; Harding, J. B. Proc. R. Soc. (London) A 1932, 138, 411. (32) Smith, J. W. Electric Dipole Moments; Butterworth: London, 1955. (33) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1963. (34) Oliveira, O. N., Jr.; Taylor, D. M.; Morgan, H. Thin Solid Films 1992, 210/211, 76. (35) Leite, V. B. P.; Cavalli, A.; Oliveira, O. N., Jr. Phys. ReV. E 1998, 57, 6835.
