A Study on Two-Dimensional Phase Transitions in Langmuir

5992

J. Phys. Chem. B 1999, 103, 5992-6000

A Study on Two-Dimensional Phase Transitions in Langmuir Monolayers of a Carboxylic Acid with a Symmetrical Triphenylbenzene Ring System Patrycja Dynarowicz-Ła¸tka, Anantharaman Dhanabalan, 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 ReceiVed: January 19, 1999

A detailed investigation on phase transitions in Langmuir monolayers of 5′-phenyl-1,1′:3′1′′-terphenyl-4carboxylic acid (PTCA) at the air-water interface is presented. Surface pressure and surface potential isotherms were obtained using varying experimental parameters such as spreading volume, subphase pH, ionic strength, subphase temperature, and compression speed. The isotherms exhibit two liquid-condensed regions separated by a broad plateau with a 2-fold decrease in area/molecule. PTCA monolayers were visualized by Brewster angle microscopy, transferred as multilayer LB films and characterized by Fourier transform infrared spectroscopy, X-ray diffraction (XRD), and surface potential measurements. On the basis of the analysis of surface pressure and surface potential isotherms, it was not possible to distinguish between bilayer formation and conformational changes of the triphenylbenzene ring system upon compression as the origin of the plateau. The LB transfer from monolayers in the plateau was unsuccessful and thus characterization of LB films does not help in elucidating the possible cause for the transition.

Introduction

SCHEME 1: Chemical Structure of PTCA Molecule

Phase transitions in surface pressure-area (π/A) isotherms of Langmuir monolayers of various film materials have been the subject of numerous investigations. A characteristic plateau in which there is no or little change in surface pressure upon monolayer compression has been observed with Langmuir monolayers of model amphiphilic compounds such as long chain fatty acids,1,2 alcohols,3,4 alkylammonium salts,5 or phospholipids.6-8 Such a plateau region, which usually disappears at higher subphase temperatures, is attributed to the coexistence of either vapor/liquid phases at low surface pressures or liquid expanded/liquid condensed phases at high-pressure regions. The “plateaulike” shape in the pressure-area curves of bipolar molecules 9-11 has been modeled as a first-order phase transition associated with the lifting off of one polar group from the subphase during compression.12 The existence of strongly temperature-dependent plateau regions in monolayers of biphenyl13-17 and terphenyl derivatives,13,18,19 as well as the plateau region of liquid-crystalline compounds,20 have been interpreted as the result of the reversible collapse of a monolayer into a multilayer state. The noticeable difference between these compounds and the simple amphiphilic molecules is the inverse temperature dependence of the plateau surface pressure of the latter compounds in relation to the former ones. Nontemperature sensitive plateau regions for some polymers21 and physiologically active compounds22-25 were related to molecular orientational changes upon compression. For calcitonin monolayers, the existence of the plateau was attributed to dissolution of the film-forming material,26 since the lateral compression forced the molecules to adopt a close-packed looplike conformation, causing them to desorb from the air-water interface into the subphase. For mixed monolayers with a typical amphiphilic compound and a semiamphiphilic polymer, the plateau was ‡ Present address: Jagiellonian University, Faculty of Chemistry, Krakow, Poland.

related to the squeezing out of the polymer from the monolayer of the amphiphilic compound.27 In a preliminary report, it has been shown that the pressure/ area isotherms for polyphenyl carboxylic acids with a symmetrical triphenylbenzene ring system, namely, 5′-phenyl-1,1′: 3′,1′′-terphenyl-4-carboxylic acid (in short PTCA) (Scheme 1) and its 4′-methyl derivative, exhibit a characteristic plateau28 which spans over the region corresponding to a 2-fold decrease in area/molecule. Interestingly, Langmuir monolayers formed by a series of 4′-alkyl[1,1′-biphenyl]-4-carbonitriles yield isotherms with even broader plateaus, for which the ratio of the area/molecule values at the beginning of the plateau to those found at its termination amounts to 3.13-15,17 Similar behavior was reported for p-terphenyl derivatives.13,18,19 The PTCA molecule (Scheme 1) can actually be treated as a derivative of m-terphenyl. Thus it appears that amphiphiles with a biphenyl, p- or m-terphenyl core exhibit a common ability to form monolayers with peculiar plateau regions in a broad range of areas. For monolayers of biphenyl and p-terphenyl derivatives it has been suggested that such a broad plateau is due to the formation of a triple layer, consisting of a bilayer on the top of a monolayer, via a “roll-over” mechanism.14,17,19 By analogy to this model, the plateau with a 2-fold decrease in area observed for PTCA could be attributed to a transition of a monolayer into a bilayer. However, for such molecules as PTCA, possessing an ionizable carboxylic group, the plateau could arise from partial material dissolution into the water subphase. Also, the

10.1021/jp9902102 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/07/1999

Phase Transitions in Langmuir Monolayers

J. Phys. Chem. B, Vol. 103, No. 29, 1999 5993

plateau’s rising might be due to conformational changes of the triphenylbenzene ring system upon compression. On the basis of the analysis of π/A isotherms, one cannot distinguish between these possibilities. Thus, in the present work, we undertook further studies, supplemented by electric surface potential measurements and Brewster angle microscopy (BAM) images, to gain further insight into the origin of the plateau region in π/A isotherms of the PTCA monolayers. The monolayers were also transferred onto substrates as multilayer LB films and characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and surface potential measurements, in the hope of using the data to help identify the plateau’s origin. Experimental Details PTCA was synthesized according to the method already reported28 and characterized by nuclear magnetic resonance and FTIR techniques. The spreading solution for the LB experiments was prepared by dissolving PTCA 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 subphase. Subphase pH and ionic strength were adjusted by addition of HCl or NaCl. PTCA was spread on a subphase containing 4 × 10-4 M cadmium chloride and 5 × 10-5 M sodium bicarbonate whose pH was ca. 6.0 to obtain the monolayer of cadmium salt of PTCA. Monolayer studies and LB deposition were carried out with a KSV-5000 LB system (total area ) 730.5 cm2) placed on an antivibration table in a class 10000 clean room. Monolayers were usually compressed with a barrier speed of 25 mm/ min unless otherwise specified. The monolayer stability in preand postplateau regions was verified by monitoring the change in area per molecule while holding the surface pressure constant. However, due to experimental difficulties, the stability at the plateau was examined by recording the change in surface pressure by keeping the area constant. The monolayer morphology at the air/water interface was investigated using a Brewster angle microscope (mini-BAM, Nanofilm Technologie GmbH, Go¨ttingen, Germany). Monolayers were transferred at different target surface pressures with a dipping speed of 3 mm/min. At the end of the lifting off the substrate from the subphase, the deposited monolayer was dried in air for 10 min. BK7 glass, gold-coated glass slides, and calcium fluoride (CaF2) plates were used as substrates that were cleaned thoroughly prior to use. FTIR measurements were carried out with a BOMEM-MB102 Michelson series instrument (128 scans, resolution 4 cm-1) in the transmission mode. XRD measurements were made with a Rigaku Rotaflex (Model RU200B) X-ray diffractometer in the 2θ range of 3-20° using a Cu target. Surface potential measurements of the LB film deposited on gold/glass substrates were performed with a Trek 320B electrostatic voltmeter. Results A. Langmuir Monolayers. Surface pressure-area (π/A) isotherms of PTCA recorded upon spreading the same number of molecules on different subphases at 20 °C are shown in Figure 1. The isotherms have the same shape with a characteristic broad plateau that separates two regions of low compressibility. The main differences are the shift to larger areas for the acidic subphase and the higher plateau pressure for the subphase containing cadmium ions. The latter observation can be attributed to the enhanced stability of the Cd-PTCA monolayer in comparison with the corresponding acid mono-

Figure 1. π/A isotherms of the PTCA monolayer on different subphases at 20 °C. Spreading volume ) 150 µL, and compression speed ) 25 mm/min.

Figure 2. π/A isotherms of PTCA monolayer on the acidic subphase (10-3 M HCl) with the spreading of different numbers of molecules at 20 °C. Compression speed ) 25 mm/min.

layer, similarly to the behavior of cadmium salts of fatty acids.32 When the volume spread was varied (Figure 2), the plateau was shifted slightly to lower areas for increasing volumes spread on acidic subphases, as expected, because spreading may be not as efficient with larger volumes. For pure water, on the other hand, the surface pressure curves were shifted toward higher areas for larger volumes spread (Figure 3). This occurred because there is some material loss for monolayers spread on pure water, as confirmed by the large shifts toward lower areas shown in Figure 4 when the compression speed was decreased. Indeed, the longer the compression time, the higher was the amount of material lost to the subphase. Furthermore, material loss was apparent when a given monolayer was compressed-

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Dynarowicz-Ła¸tka et al.

Figure 5. Hysteresis isotherms of PTCA monolayer on the water subphase at 20 °C. The monolayer was compressed to (A) postplateau, (B) plateau, and (C) preplateau region.

Figure 3. π/A isotherms of PTCA monolayer on the water subphase with the spreading of different numbers of molecules at 20 °C. Compression speed ) 25 mm/min.

Figure 6. π/A isotherms of PTCA monolayer on 10-3 M HCl at different subphase temperatures. Compression speed ) 25 mm/min.

Figure 4. π/A isotherms of PTCA monolayer on the water subphase with various compression speeds at 20 °C.

decompressed several times, since the pressure area curves were shifted to lower areas in subsequent compressions. As can be seen in Figure 5, strong hysteresis on water was observed for monolayers compressed up to the midplateau or postplateau regions. Because such a material loss was not observed for monolayers spread on acidic subphases,28 most of the subsequent studies to be reported here employed such subphases. For PTCA monolayers spread on HCl subphases, Figure 6 shows that the length of the plateau increases and its pressure decreases as the subphase temperature is increased. The compressibility modulus isotherm of the PTCA monolayer on an acidic subphase is shown in Figure 7, which also includes the pressure isotherm for comparison. As one should expect, the compressibility

modulus increases with the surface pressure upon compression in the pre- and postplateau regions, and remains low and constant in the plateau region. The stability of compressed PTCA monolayers on acidic subphases was studied at different surface pressures. At a target surface pressure of 7 mN/m, the monolayer was reasonably stable with a decrease in surface pressure by only about 5%. However, the monolayer was highly unstable in the plateau and postplateau regions, with the decrease in area by ca. 30% for monolayers compressed to 15 mN/m (at the plateau) and a decrease in pressure by ca. 60% for those compressed to 25 mN/m. In all cases, more than 60% of the total decrease in pressure or area occurred during the initial 5 min. The plateau region may denote substantial changes in molecular packing or even a nonmonomolecular arrangement for the film, which can cause significant changes in the normal component of the molecular dipole moments. Surface potential isotherms were therefore obtained and analyzed for the PTCA monolayers. A typical potential-area isotherm is shown in Figure 8, together with the surface pressure and effective dipole moment isotherms for a monolayer on an acidic subphase at


Figure 7. Surface pressure and compressibility modulus - area isotherms of PTCA monolayer on 10-3 M HCl at 20 °C.

Figure 8. Surface potential-area (solid) and effective dipole momentarea (dotted) isotherms of PTCA monolayer on 10-3 M HCl at 20 °C. The surface pressure-area isotherm of the PTCA monolayer (dasheddotted) from Figure 1 is reproduced here for comparison.

20 °C. The dipole moment-area isotherm will be discussed later. For acidic subphases, the initial surface potential, at large areas per molecule, was close to zero provided that the amount of material spread was low, typically 100 µL for a 0.4 mg/mL solution. When larger quantities were spread, say, 150-200 µL, so that full surface pressure and surface potential isotherms could be obtained simultaneously, a nonzero initial surface potential was observed which varied between 150 and 200 mV. Upon compression, the surface potential onset occurred at an area, usually referred to as the critical area (ca. 68 Å2), where the surface pressure was still close to zero (gaseous state of the


Figure 9. Surface potential-area isotherms of PTCA monolayer on the surface of different subphases at 20 °C.

monolayer). A small “kink” was noticed in the surface potential isotherm at ca. 49 Å2 that corresponds to the onset area for surface pressure. This “kink” was observed only while spreading smaller amounts of material. Note that the area for maximum surface potential (∆Vmax ) 550 mV) coincides with the beginning of the plateau region in the surface pressure isotherm. When pure water was used as the subphase, the initial surface potential for the PTCA monolayer was highly irreproducible, varying between 50 and 200 mV. Only for very low volumes spread was an almost zero surface potential observed, with a maximum value of ca. 450 mV. Isotherms experiments were also conducted for PTCA monolayers spread on subphases containing either NaCl or cadmium ions, as shown in Figure 9. In all cases, the onset for surface potential occurs at a critical area that is larger than the onset area for surface pressure. For NaCl subphases, various ionic strengths were employed for estimation of the double-layer potential. ∆Vmax decreased with the ionic strength, being 470, 460, 440, and 420 mV for 10-1, 10-2, 10-3, and 10-4 M NaCl, respectively. These potentials are lower than the potential for an unionized monolayer spread on an acidic subphase. The double-layer potential (ψo) calculated by subtracting the surface potential of an unionized monolayer (subphase ) 10-3 M HCl, ∆V ) 550 ( 10 mV) from the value measured with the same salt concentration (subphase ) 10-3 M NaCl, ∆V ) 440 ( 10 mV) is ca. -110 mV, which is close to that for a fatty acid monolayer (ψo ∼ -100 mV).33 Similarly to that observed for NaCl subphases, ∆Vmax was lower for the monolayer spread on the subphase containing cadmium ions (ca. 490 mV) than for the monolayer on the acidic subphase. This difference could again be attributed to the negative doublelayer potential which now consists of the negative layer of ionized carboxylate anions (refer to FTIR results of LB films described in the following section) and the positive layer of counter metal ions. A similar behavior has been reported for a model fatty acid monolayer.34 Brewster angle microscopy was applied to visualize the PTCA monolayer. Figure 10a-d shows BAM images at different states of compression. Figure 10a shows the gas-liquid coexistence (region I). In region II (not shown here), the black (gas) holes

5996 J. Phys. Chem. B, Vol. 103, No. 29, 1999


Figure 10. Brewster angle microscopy images at different regions of compression, explanation in the text.

disappear and the monolayer becomes homogeneous until the middle part of the plateau is reached. Upon further compression, characteristic stripes appear (Figure 10b), which grow in size (Figure 10c) and eventually the film collapses (Figure 10d). The plateau can thus reflect the coexistence of regions of different molecular rearrangements. However, the existence of the stripes can be attributed either to molecules with a more planar conformation whose number increases upon compression or to patches of multilayer structures. B. LB Film Transfer and Characterization. Monolayer transfer has been attempted at various target surface pressures in different regions of the surface pressure isotherm. In the preplateau region, the monolayer was transferred at a constant surface pressure of 5 mN/m both in the case of PTCA and that of Cd-PTCA. For Cd-PTCA, the monolayer was, in general, transferred as Y-type LB films, i.e., transfer during both lifting and dipping with no significant difference for different substrates. On the other hand, for the PTCA monolayer, the deposition was initially Z-type, i.e., transfer only during lifting, which switched over to Y-type after a few deposition cycles. The number of layers for the switch to occur was irreproducible. The poor monolayer stability at the initial stages of holding the monolayer at constant surface pressure affected the transfer ratio (TR) estimation. In such cases, TR was higher than unity for the first few depositions and close to unity after that. However, TR was close to unity for all layers if the deposition started after a waiting period of an hour or so. The attempts to transfer PTCA and Cd-PTCA monolayers in the plateau and postplateau condensed regions were not successful, because of the poor monolayer stability. Actually, for aromatic amphiphiles the presence of at least four aromatic groups is necessary for a good

spreading and the PTCA molecule just lies on the border. However, the nontransferability of the PTCA monolayer in the plateau region could also suggest a bilayer formation (which usually can be transferred only using the horizontal lifting method) or the existence of a particular orientation of molecules which is not favorable to transfer. The transferred LB films from monolayers in the preplateau region were visually uniform and characterized by FTIR, XRD, and surface potential measurements. The FTIR spectra of PTCA LB films produced from monolayers spread on an acidic subphase and water-containing cadmium ions are shown in Figure 11. The film transferred from the acidic subphase (curve a) exhibited a broad O-H stretching vibrational absorption band in the region of 3200-2500 cm-1 with embedded aromatic C-H stretching vibrational absorption peaks at ca. 3000 cm-1. The fine structures observed on the lower wavenumber side of the O-H band can be assigned to overtones and combination tones of the lower wavenumber fundamental vibration bands. Also observed were a strong absorption peak at 1688 cm-1 corresponding to the CdO stretching vibration, multiple absorption peaks centered at 1600 cm-1 due to ring CdC stretching vibrations and at 1410 cm-1 corresponding to C-O-H bending, and a strong band at 1300 cm-1 for C-O stretching. The appearance of intense vibrational absorption peaks for both C-O-H bending and C-O stretching may indicate dimer formation. These FTIR features are similar to those of a PTCA cast film, indicating no significant ionization of COOH groups for an acidic subphase. In contrast, the FTIR spectrum of the LB film transferred from a subphase containing cadmium ions (curve b) was markedly different and exhibited the following vibrational absorption peaks: weak absorption



Figure 12. Powder XRD pattern of LB film of Cd-PTCA (21 layers).

TABLE 1: Surface Potentials of PTCA and Cd-PTCA LB Films

Figure 11. Transmission FTIR spectra of LB films of (a) PTCA (35 layers) and (b) Cd-PTCA (25 layers).

peaks at ca. 3000 cm-1 for aromatic C-H stretching vibration, strong absorption peak at ca. 1600 cm-1 for ring CdC stretching vibration, and strong absorption peaks at 1520 and 1390 cm-1 for the asymmetric and symmetric CdO stretching vibrations of the carboxylate anion, respectively. The absence of a vibration absorption peak at 1688 cm-1 for the free acid group indicates complete ionization of the acid headgroups in the presence of cadmium ions in the pH ) 6.0 subphase. The broad absorption in the 3500-3000 cm-1 region disappears upon drying, and its appearance may be attributed to entrained water within the LB film. The XRD pattern observed for the LB film of cadmium salt of PTCA (11 layers) transferred at a constant target surface pressure of 7 mN/m is shown in Figure 12. In contrast to the XRD pattern of LB films from cadmium salts of long chain fatty acids that usually exhibits a set of intense diffraction peaks,35 only two peaks are observed for the Cd-PTCA LB film. The bilayer distance calculated by assigning the diffraction peaks as 001 and 002 peaks was 25 ( 0.3 Å. This value is close to that expected from the estimated length of a PTCA molecule, i.e., ca. 11.5 Å (the length of the molecular long axis varies by 0.5 Å for the different conformations of the PTCA molecule), as determined by the HyperChem computer modeling program. It must be stressed that, with the routine XRD pattern, it is not possible to draw any conclusion about the layer structure in the Cd-PTCA LB film. The result may simply indicate the presence of Cd-PTCA microcrystallites in the LB film, and further X-ray reflectivity studies are required for probing the layered structure within the LB film. Surface potential measurements have been carried out with the Cd-PTCA LB films with different numbers of layers

subphase

no. of layers

VLB (mV)

water with Cd2+ ions water with Cd2+ ions water with Cd2+ ions water with Cd2+ ions 10-3 M HCl

1 3 5 15 15

200 240 340 440 290

transferred at 5 mN/m onto gold/glass substrates. While scanning the probe over the film surface, the surface potential varied by ( 10 mV, which is the accuracy limit of the set up used in the present study, indicating the film uniformity at least at the macroscopic level. The surface potentials for Cd-PTCA LB films with different numbers of layers are listed in Table 1. The surface potential was approximately 200 mV for a onelayer LB film, then increasing with the number of layers, reaching 440 mV for a 15-layer LB film. Surface potentials in Y-type LB films usually increase with the number of layers before saturation owing to the influence from the substrate.36 The surface potential for the transferred LB film is VLB ) Vo + Vi, where Vo and Vi are contributions from the dipoles within the film and from the film-substrate interface, respectively. It is not certain whether the double-layer potential, existing in the monolayer at the air/water interface, should also contribute to the surface potential of an LB film. The surface potentials for the LB films with one and three monolayers are lower than ∆Vmax for the Langmuir monolayer of Cd-PTCA at the air/ water interface. Further, surface potential measurements were carried out with the PTCA LB film (15 layers) transferred from the compressed monolayers (5 mN/m) on an acidic subphase. Note that, with the acidic subphase, the transfer was Y-type with relatively poor deposition during the dipping cycle (TR ∼ 0.5), and hence a direct comparison of the LB films of PTCA and Cd-PTCA with the same number of layers is not straightforward. The surface potential of the 15-layer PTCA LB film (290 mV) is lower than ∆Vmax of the corresponding monolayer (ca. 550 mV) at the air/water interface. This is probably due to the poor transfer of the PTCA monolayer and the negative contribution from the substrate/film interface.

5998 J. Phys. Chem. B, Vol. 103, No. 29, 1999 In summary, transfer experiments under different experimental conditions indicated a better Y-type transfer of monolayer with the subphase containing Cd2+ ions. FTIR revealed the ionization state of PTCA molecules with different subphases: complete ionization and complexation with metal ions at the neutral subphase pH and no ionization with an acidic pH. The macroscopic uniformity of the LB films was inferred by no or little variation in surface potential while scanning the film surface with the potential probe. The difference between the surface potential values observed with LB film and corresponding monolayer at the air/water interface is explained by taking into account the negative contribution from the substrate/film interface. Possible Origins for the Plateau. As indicated in the Introduction, the plateau transition in surface pressure isotherms for most amphiphilic molecules is usually attributed to multilayer formation, conformational changes, or partial material dissolution into the subphase. In all these cases, the plateau pressure decreases with increasing temperatures, as observed for the PTCA monolayers (see Figure 6). To distinguish between these possibilities, isotherms obtained under various experimental conditions were analyzed. First of all, we may rule out possible experimental artifacts, as the plateau transition region was observed reproducibly under a variety of experimental conditions. While some material loss was identified for monolayers spread on pure water, as inferred from the isotherms recorded with various spreading volumes and compression speeds and also from hysteresis experiments, this did not apply to acidic subphases for which the plateau was also present. Therefore, material loss can also be discarded as the cause for the plateau appearance. This difference between pure water and acidic subphase may be related to the degree of ionization of the PTCA monolayer, as an almost complete dissolution was noticed with basic subphases (pH g 10). However, the double-layer potential for PTCA monolayers lies in the range from -80 to -110 mV for varying ionic concentrations, i.e., close to that of a stearic acid monolayer.33 Though the pKa of PTCA is not known, one may assume it to be close to that of stearic acid, and therefore the degree of dissociation at pH 5-6 is bound to be only a few percent, in analogy to that observed for stearic acid.33 It is likely then that other mechanisms, such as a less effective spreadability, may be behind the material loss for PTCA monolayers on pure water. For acidic subphases, the two remaining possibilities for the plateau appearance are the molecular rearrangement and the multilayer formation. The effective dipole moment (µ), which is the vertical component of the dipole moment of the free molecule, was calculated at different stages of compression using the Helmholtz equation37 i.e., ∆V ) µ/Α0, wherein 0 is the dielectric permittivity of free space and corresponds to a relative permittivity assumed to be 1 for the sake of calculation of µ, as it is usually done38 (for the actual values of in a monolayer, see ref 34). Figure 8 shows that µ j increases in the gaseous state of the monolayer (region I) where the surface pressure is still close to zero. The maximum value of 0.57 D [1 D ) 3336 × 10-30 C m] was reached at a molecular area of ca. 42 Å2, which coincides with the beginning of the plateau region in the surface pressure isotherm. Upon further compression, a dramatic decrease in µ j could be seen in the plateau (region III). In principle, the decrease in dipole moment fits well with the hypothesis of a bilayer formation during the plateau, because the molecules would be likely to be arranged in a centrosymmetric way, thus causing the effective dipole moment to drop substantially. The bilayer formed would not be amenable to


Figure 13. Model optimum conformation of the PTCA molecule in the gas phase. Projection of the molecule along (a) x-axis, (b) y-axis, and (c) z-axis.

transfer either, which then explains why LB films could only be transferred for monolayers in the preplateau region. However, changes in molecular packing may also lead to decreases in the effective dipole moment, as follows. At large molecular areas (region I), PTCA molecules lie almost horizontally at the air/water interface and upon compression the hydrophobic triphenyl group lifts off from the water surface gradually to a vertical position while the hydrophilic acid group is anchored at the interface. The increase in dipole moment in this region supports such orientational changes39 at large molecular areas. The projection area of a vertically oriented PTCA molecule estimated with the HyperChem computer program is 44.8 Å2 which is close to the lifting-off area in the π/A isotherms. During the plateau in the surface pressure isotherm, the effective dipole moment dropped almost to the initial dipole moment value, which could be due to orientational changes associated with different conformations of the peripheral phenyl groups with respect to the central benzene ring. It is known that the equilibrium dihedral angle between the planes of the phenyl rings in a biphenyl molecule changes from 45° in the gas phase to 22° in the melt and 0° in the crystal at room temperature.40 By the same token, one can anticipate significant conformational changes in the triphenyl ring system upon film compression. Computer modeling indicated two cases of orientation for the side phenyl groups of PTCA, one with both rings parallel and another with both rings perpendicular to each other. Semiempirical MOPAC41 computations in AM142 parametrization revealed an optimum conformation with the dihedral angle close to 40° in the gas phase of the molecule, as shown in Figure 13. The peripheral phenyl rings for both parallel and antiparallel conformations have been then rotated from 0 to 100° with respect to the central phenyl group. As can be seen in Figure 14, the calculated dipole moment varies within the range of ca. 2.87-2.93 D; for antiparallel conformation having the maximum value at about 50° which drops down to ca. 2.87 D for 0°. The decrease in effective dipole moment in the plateau region may therefore be related to the change in conformation of the side phenyl groups from one with twisted side phenyl groups (ca. 20°), characteristic of the liquid state, to one with in-plane phenyl groups (0°), characteristic of the solid state. To calculate the energy input necessary to force a twisted conformation of the side rings to a planar one in the triphenylbenzene ring system, two extreme conformations of the PTCA



Figure 14. A plot of dipole moment versus the angle between central and peripheral phenyl rings.

molecule were assumed as shown in Figure 15. The first had a coplanar arrangement in which the peripheral rings and central benzene ring are in the same plane (Figure 15 a), while in the second an out-of-plane arrangement was assumed in which the peripheral rings are twisted out perpendicular to the plane of the central benzene ring (Figure 15 b). The ratio of projection areas of these extreme conformations is about 2, which coincides with the reduction in area within the plateau region, i.e., ratio of the areas at which the plateau starts and ends. The energy input during monolayer compression of PTCA within the plateau region was estimated using: ω ) πS, where ω is the work done during compression, π is the mean value of surface pressure in the plateau region (14.5 mN/m), and S is the difference in area within the plateau (∆A ) 43-18.5 Å2 per molecule). This gives 2.14 kJ/mol, i.e., 0.5 kcal/mol, which is sufficient for the anticipated conformational changes as the energy input necessary for the cis-trans photoisomerization has been calculated as 2-3 kcal/mol.43 The energy requirements for such a transformation should increase with a decrease in temperature, and indeed, we observed in Figure 6 that the plateau surface pressure increases with decreasing subphase temperatures. The BAM images confirmed that at the plateau region the monolayer had not collapsed, for pictures at this stage are considerably different from those of a collapsed monolayer. The change in behavior, with formation of stripes, in the middle of the plateau would appear to indicate formation of nonmonomolecular structures, but the possibility of reflecting domains with molecules of distinct orientation cannot be ruled out. Having failed to distinguish in an unequivocal way between bilayer formation and molecular repacking as the cause of the plateau’s existence on the basis of monolayer characterization, we had hoped to get some insight from transfer characteristics. That transfer cannot take place when monolayers had been compressed up to the plateau region may be indicative of bilayer formation, which would certainly make it difficult for monolayer transfer. However, the existence of unfavorable molecular orientations within the monolayer can also prevent the monolayer from being transferred. Unfortunately, the characterization of LB films does not help in elucidating the possible cause for the plateau, for the mere reason that transfer was only successful

Figure 15. Different projections of model coplanar (a) and out-ofplane (b) conformations of PTCA molecule, explanation in the text.

for monolayers in the preplateau region. By analogy with similar compounds under investigation, we may say that the hypothesis of formation of nonmonomolecular structures is more likely. Conclusions A systematic monolayer study indicates that triphenyl carboxylic acid (PTCA) is capable of forming Langmuir monolayers at the air/water interface. A clear and reproducible phase transition appeared as a broad plateau in the π/A isotherms which spans over a region corresponding to the decrease of molecular area by a factor of ca. 2. π/A isotherms are influenced by the spreading conditions employed, and the sizes of the lift-off areas are comparable with those of the projection areas of the vertically oriented molecule. The electric surface potential as well as the effective dipole moment increases upon compression of the monolayer in the 2D gaseous state. Both preplateau surface pressure and surface potential attain their maximum at the same surface coverage. On the basis of surface pressure, surface potential measurements, BAM images, and LB transfer experiments, we could not identify a unique explanation for the plateau transition, which can be either due to a bilayer formation or to molecular reorientational changes. The hypothesis of bilayer formation appears to be more likely. The PTCA monolayer may be transferred onto solid supports as multilayer

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A Study on Two-Dimensional Phase Transitions in Langmuir

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