Organization of Four Thermotropic Liquid Crystals of Different

Langmuir 2004, 20, 7991-7997

7991

Organization of Four Thermotropic Liquid Crystals of Different Polarities on Model Liquid and Solid Surfaces Mounia Badis,† M. Hassan Guermouche,‡ Jean-Pierre Bayle,§ Marek Rogalski,| and Ewa Rogalska*,† Equipe de Physico-chimie des Colloı¨des, UMR 7565 CNRS/Universite´ Henri Poincare´ Nancy 1, Faculte´ des Sciences, BP 239, 54506 Vandoeuvre-le` s-Nancy Cedex, France, Laboratoire de Chromatographie, Institut de Chimie, USTHB, B.P. 32, El-Alia, Alger, Alge´ rie, Laboratoire de Chimie Structurale Organique, ICMO, Bt. 410, 91405 Orsay Cedex, France, and Laboratoire de Thermodynamique du Milieu Polyphase´ , Universite´ de Metz, 57045 Metz Cedex, France Received April 9, 2004. In Final Form: June 30, 2004 The thermodynamic and surface properties of four structurally related thermotropic liquid crystals (LC) were investigated to understand their organization at gas-liquid and gas-solid interfaces. In this study, LC with a benzoyloxy azobenzene mesogenic core substituted with heptyloxy and/or dioxyethylene ether groups were used. The propensity of the LC to form self-assembled multilayers was demonstrated in the films spread at the air/aqueous interface using the Langmuir technique and Brewster angle microscopy and on the solid surfaces of Chromosorb WHP and silica, using differential scanning calorimetry. On the basis of the results obtained, a molecular recognition mechanism underlying separation processes using LC as selectors in gas chromatography is proposed.

1. Introduction The organization of molecules adsorbed at a solid surface is imposed by the established interactions. The organization induced by the solid surface may reach several layers of the adsorbed molecules1 and change the thermodynamic properties of the film formed, compared to the bulk phase.2,3 This phenomenon is well illustrated by a significant increase of the glass transition temperature in poly-(2′)-vinylpyridine interacting with a polar substrate4 and a decrease of the glass transition temperature in a polystyrene film adsorbed on silicon.5 Overall, the physical properties of a liquid in contact with a solid surface are dominated by the bulk properties except in the case of some particular liquids, such as thermotropic liquid crystals (LC).3,6,7 LC self-organize and exhibit different types of long-range orientational order, depending on their anchoring at the interface. The term “surface anchoring” refers to the phenomenon of a preferential molecular orientation in a liquid crystal at a phase boundary.8,9 This orientation is a complex function of the LC-interface interactions involving physical forces, hydrogen bonding, * To whom correspondence should be addressed. E-mail: [email protected]. † Universite ´ Nancy 1. ‡ USTHB, Alger. § ICMO, Orsay. | Universite ´ de Metz. (1) Cristofolini, L.; Arisi, S.; Fontana, M. P. Synth. Met. 2001, 124, 151. (2) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsvier Pub. Co.: New York, 1948. (3) Benichou, O.; Cachile, M.; Cazabat, A. M.; Poulard, C.; Valignat, M. P.; Vandenbrouck, F.; Van Effenterre, D. Adv. Colloid Interface Sci. 2003, 100-102, 381. (4) van Zanten, J. H.; Wallace, W. E.; Wu, W. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1996, 53, R2053. (5) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett. 1994, 27, 59. (6) Sheng, P. Phys. Rev. Lett. 1976, 37, 1059. (7) van Effenterre, D.; Ober, R.; Valignat, M. P.; Cazabat, A. M. Phys. Rev. Lett. 2001, 87, 125701/1. (8) Sonin, A. A. The Surface Physics of Liquid Crystals; Gordon & Breach: Langhorne, PA, 1995.

and the physical properties of the surface, such as planarity, texture, and lattice structure. The preferred anchoring angle corresponds to the minimum of the Gibbs energy of the system, determined by the entropy/energy balance. The flat arrangement of the LC molecules on the solid surface is entropically favored compared to the vertical arrangement, as the latter allows less freedom of molecule motion.10,11 However, the flat arrangement of LC can take place only if the LCsurface bonds substitute the bonds existing between the LC mesogen moieties. For such a situation to occur, a good steric accommodation of LC and strong interactions with the interface are required. When the LC-surface interactions are weaker9 than the mesogen-mesogen bonds, the LC molecules are oriented vertically or tilted to the interface. In the case of nematic ordering of the LC, the molecules in the bulk are organized along an axis. Thus, the overall properties of the LC may depend on the orientation of the axis imposed by the solid-LC interactions. This phenomenon is postulated to occur with certain LC used as stationary phases in gas chromatography (GC).12-14 For this application, LC are adsorbed on different solid substrates such as modified silica. While the mechanism of molecular recognition using LC has not yet been elucidated, it is supposed that the ordering of LC adsorbed on the solid substrates may play a decisive role in chromatographic separations.15-17 Little is known, how(9) Jian, K.; Shim, H.-S.; Tuhus-Dubrow, D.; Bernstein, S.; Woodward, C.; Pfeffer, M.; Steingart, D.; Gournay, T.; Sachsmann, S.; Crawford, G. P.; Hurt, R. H. Carbon 2003, 41, 2073. (10) Uchida, T.; Seki, H. In Liquid Crystals. Applications and Uses; Bahadur, B., Ed.; World Scientific: Singapore, 1990; Vol. 3. (11) Hooks, D. E.; Fritz, T.; Ward, M. D. Adv. Mater. 2001, 13, 227. (12) Kelker, H. Ber. Bunsen-Ges. 1963, 67, 698. (13) Kelker, H. Z. Anal. Chem. 1963, 198, 254. (14) Dewar, M. J. S.; Schroeder, J. P. J. Am. Chem. Soc. 1964, 86, 5235. (15) Witkiewicz, Z. Nowe Kierunki w Chromatografii; WNT: Warszaw, 1988. (16) Vetrova, Z. P.; Karabanov, N. T.; Jashin, J. A. Chromatographia 1977, 10, 341.

10.1021/la049093e CCC: $27.50 © 2004 American Chemical Society Published on Web 08/04/2004

7992

Langmuir, Vol. 20, No. 19, 2004

Badis et al.

Figure 1. Chemical structures of the LC used.

ever, about the ordering of stationary phases in GC, and the existing information is contradictory. While some authors proposed that the silica surface influences ordering of the adsorbed LC in the range of 100 nm18 or even 100 µm,19 the others concluded that the support has only a marginal influence on the long-range organization of the stationary phase.20 A model presenting GC stationary phases as ordered monomolecular films in equilibrium with bulk capillary liquids21 has been used previously for interpreting chromatographic results obtained with n-docosane and noctadecanol adsorbed on hydroxylated siliceous supports.22,23 The hypothesis of the formation of a monolayer of n-octadecanol molecules oriented nearly perpendicularly relative to the solid surface was supported by the convergence of molecular area values obtained from GC and Langmuir monolayer experiments.24,25 However, the conclusions on stationary phase organization reached for n-docosane and n-octadecanol should not be hastily extrapolated to more complex molecules, whose organization depends on structural features such as the geometry around the sNdNs moiety.26 Indeed, we showed recently that some LC have a high propensity for multilayer rather than monolayer formation.27 The latter result was confirmed by other studies.28-30 On the other hand, it has (17) Vetrova, Z. P.; Karabanov, N. T.; Yashin, Y. I. Dokl. Akad. Nauk SSSR 1980, 250, 1165. (18) Chow, L. C.; Martire, D. E. J. Phys. Chem. 1969, 73, 1127. (19) Grushka, E.; Solsky, J. F. J. Chromatogr. 1975, 112, 145. (20) Sokolova, E.; Vlasov, A. Fluid Phase Equilib. 1998, 150-151, 403. (21) Giddings, J. C. Anal. Chem. 1966, 34, 458. (22) Serpinet, J. J. Chromatogr. 1972, 68, 9. (23) Serpinet, J. J. Chromatogr. 1973, 77, 289. (24) Serpinet, J. J. Chromatogr. 1976, 119, 483. (25) Serpinet, J. Anal. Chem. 1976, 48, 2264. (26) Zawisza, I.; Bilewicz, R.; Luboch, E.; Biernat, J. F. Thin Solid Films 1999, 348, 173. (27) Rogalska, E.; Rogalski, M.; Judeinstein, P.; Bayle, J. P.; Guermouche, M. H. J. Mol. Liq. 2001, 94, 221. (28) Wang, L.; Tian, Y.; Xi, S.; Ren, Y. J. Phys. Chem. B. 1998, 102, 8353.

been demonstrated that the organized adsorbed films achieve the thermodynamic equilibrium by adjusting their thickness to a well-defined value.7 In light of these results, it can be expected that the LC deposed on GC stationary phases may form ordered thin films coexisting in equilibrium with the bulk LC. While the existing information gives some insight into the LC-solid surface interactions, it is not sufficient for a rational engineering of the GC/LC chromatographic process at a molecular level. Our present work aims at bringing more understanding of the relation between the molecule organization and the molecular recognition leading to a chromatographic separation. Four different LC used in this study have the same mesogenic core (benzoyloxy azobenzene) while the differences in polarity and ordering capacity are conferred by the terminal heptyloxy (C7) or dioxyethylene ether (DOE) chains (Figure 1). Recently, we demonstrated that these LC can be successfully used to separate compounds such as the isomeric linalool and citronellal and xylene, tetraethylbenzene, and cresol isomers.31 In the absence of direct experimental methods allowing determination of the organization of LC molecules spread on the chromatographic supports, we have decided to study the organization and conformation of the four LC using the Langmuir technique,32,33 Brewster angle microscopy (BAM), and differential scanning calorimetry (DSC). While the Langmuir technique and BAM yielded information (29) Mu, J.; Okamoto, H.; Takenaka, S.; Feng, X. Colloids Surf., A 2000, 172, 87. (30) Ibn-Elhaj, M.; Riegler, H.; Mohwald, H.; Schwendler, M.; Helm, C. A. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1997, 56, 1844. (31) Ammar-Khodja, F.; Guermouche, S.; Guermouche, M. H.; Berdague, P.; Bayle, J. P. Chromatographia 1999, 50, 338. (32) Birdi, K. S. Self-Assembly Monolayer Structures of Lipids and Macromolecules at Interfaces; Kluwer Academic/Plenum: New York, 1999. (33) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966.

Organization of Liquid Crystals on Model Surfaces

about the behavior of LC spread on aqueous NaCl solution used as a model polar surface,34 DSC made it possible to determine characteristic parameters of phase transitions and multilayer formation in LC adsorbed on silica and on diatomit (Chromosorb WHP). Analogies between water and silica surfaces in the interactions with organic probe molecules were showed recently.35 Indeed, while the anchoring of LC molecules on the water and silica is different due to a different structure of the two surfaces, it can be proposed that the effect of the polar surface on the molecule orientation and ordering is very similar. On the basis of the calorimetry and self-assembly results, we propose that in the LC layers either the nonpolar C7 hydrocarbon chain or the polar central ester group is preferentially exposed into the gas phase, depending on the molecule structure and orientation induced by the support. The efficiency of GC separations using these LC36 would depend directly on selective, orientation-controlled interactions of LC ester or hydrocarbon moieties with the solutes. 2. Materials and Methods 2.1. Chemicals. Chromosorb WHP with a grain diameter of 0.125-0.140 mm and specific area of 0.5 m2/g was supplied by Supelco. Silica (Silice 60 ACC) with a grain diameter of 0.1800.500 mm and specific area of 350 m2/g was from SDS. Four LC compounds used in this study have the same mesogenic core (benzoyloxy azobenzene) and C7 or DOE chains as presented in Figure 1. The LC synthesis has been described previously.31 2.2. DSC. Chromatographic silica WHP and silica were modified by soaking in CH2Cl2 solutions of LC. After evaporation of the solvent under a vacuum, the samples containing 10 or 50 wt % LC were equilibrated at 423 K for 4 h. DSC measurements were performed using a TA 2920 device (TA Instruments, New Castle, U.S.A.). All scans were carried out at a heating rate of 8 deg/min. 2.3. Compression Isotherms and BAM. Monolayer experiments were carried out with a KSV 5000 barostat (KSV, Helsinki). A Teflon trough (15 cm × 58 cm × 1 cm) with two hydrophilic Delrin barriers (symmetric compression) was used in all experiments. The system was equipped with an electrobalance and a platinum Wilhelmy plate (perimeter 39.24 mm) as a surface pressure sensor. The apparatus was closed in a Plexiglas box, and the temperature was kept constant at 20 °C. Before each utilization, the trough and the barriers were cleaned using cotton soaked in chloroform, gently brushed with ethanol and then with tap water, and finally rinsed with water purified by reverse osmosis (Millipore, France). All solvents used for cleaning the trough and the barriers were of analytical grade. To overcome the slight solubility of the LC in pure water, an aqueous 5 M NaCl solution, with a well-known salting-out effect on DOE polymers,27 was used as a subphase in all experiments. Water used to prepare the NaCl solutions was purified by reverse osmosis and had a surface tension of 72.75 mN/m at 20 °C. Any residual surface-active impurities were removed before each experiment by sweeping and suction of the surface. Monolayers were spread, using calibrated solutions of LC, prepared with spectrophotometric grade chloroform (Aldrich, A.C.S.), at a concentration ∼0.5 mg/mL. After the equilibration time of 20 min, the films were compressed at the rate of 1 cm/min. A personal computer and KSV software were used to control the surface pressure-molecular area (Π-A) compression isotherm experiments. The morphology of the films was imaged with a computerinterfaced KSV 2000 barostat combined with a Brewster angle microscope (KSV Optrel BAM 300, Helsinki). The Teflon trough (34) Suresh, K. A.; Blumstein, A.; Rondelez, F. J. Phys. 1985, 46, 453. (35) Huruguen, J. P.; Amara, M.; Mear, A. M.; Hamraoui, H.; Olier, R.; Privat, M. J. Phys.: Condens. Matter 2001, 13, 4939. (36) Ammar-Khodja, F.; Guermouche, S.; Guermouche, M. H.; Rogalska, E.; Rogalski, M.; Judeinstein, P.; Bayle, J. P. Chromatographia 2003, 57, 249.

Langmuir, Vol. 20, No. 19, 2004 7993 dimensions were 6.5 cm × 58 cm × 1 cm; other experimental conditions were as described above.

3. Results and Discussion 3.1. LC Films Spread at the Air/Aqueous NaCl Interface. The aqueous surface on which LC are spread can be considered as a model of a polar chromatographic support surface. The organization of LC molecules at the air/aqueous interface can be evaluated using Π-A compression isotherms. The isotherms are obtained by a lateral compression of the LC molecules spread on the aqueous surface, which results in an increase of the surface pressure and a decrease of the surface area available per molecule. The Π-A compression isotherms of the four LC studied and the morphology of the films visualized with BAM are presented in Figure 2a-d. The characteristic parameters of the isotherms are collected in Table 1. The analysis of the structure of the films using BAM allowed detecting the gas-liquid phase transition at surface pressure values close to 0 with all four LC (Figure 2, images 1). At higher surface pressure values, corresponding to the first conspicuous transition points of the isotherms, images characteristic for the collapse of the monolayers were registered (Figure 2, images 2). Interestingly, similar images of collapsed monolayers were observed recently with another family of amphiphilic macromolecules, namely, calixarene derivatives.37 The morphology of the multilayer films at the end of the compression is shown in Figure 2, images 3. The C7-C7 isotherm shows the first transition point at Π ) 11 mN/m and A ) 22 Å2 corresponding obviously to the monolayer collapse. It can be noted that the molecular area of C7-C7 at the collapse of the monolayer is roughly equal to the cross section of the long-chain fatty acids containing in their structure the azobenzene moiety,38 which can be assimilated to the LC used here. This implies that the C7-C7 molecules are oriented perpendicularly to the water surface. Such orientation may be due to the strong intermolecular interactions between the mesogenic moieties and van der Waals interactions between hydrocarbon chains, acting as the only force stabilizing the monolayer in the absence of a polar headgroup in the C7-C7 structure.39 The molecular areas of C7-DOE and DOE-C7 at the collapse point of the monolayers are 51 and 37 Å2, respectively. This indicates that the tilt of the hydrophobic chain relative to the water plane is more pronounced in C7-DOE than in DOE-C7. This may be due to the fact that the polar headgroup in C7-DOE comprises the ester group and, thus, is bigger than the DOE moiety headgroup present in DOE-C7 (Figure 3). Consequently, the hydrophobic chain stabilizing the film via π-π and van der Waals interactions is shorter in C7-DOE than that in DOE-C7. In the case of DOE-DOE, the molecular area at the collapse of the monolayer is 220 Å2. This value, corresponding roughly to the area of the LC mesogenic core,38 indicates that it has a tendency to lie flat on the water surface, while the terminal DOE moieties are immerged in the aqueous subphase. One consequence of the proposed LC organization would be different screening from the gas phase of the ester moiety in the four molecules. In the case of the tightly packed C7-C7 film, the ester moiety would be inaccessible for the gas phase while its accessibility would (37) Van der Heyden, A.; Regnouf-de-Vains, J.-B.; Warszynski, P.; Dalbavie, J.-O.; Zywocinski, A.; Rogalska, E. Langmuir 2002, 18, 8854. (38) Marciniak, W.; Witkiewicz, Z. J. Chromatogr. 1985, 324, 299. (39) Huang, Z.; Acero, A. A.; Lei, N.; Rice, S. A.; Zhang, Z.; Schlossman, M. L. J. Chem. Soc., Faraday Trans. 1996, 92, 545.

7994

Langmuir, Vol. 20, No. 19, 2004

Badis et al.

Figure 2. Surface pressure-molecular area isotherms of the four LC spread at the air/aqueous NaCl interface: (a) C7-C7, (b) C7-DOE, (c) DOE-C7, and (d) DOE-DOE. BAM images show (1) gas-liquid-phase transition, (2) collapse of the monolayer, and (3) structure of the multilayer at the end of the compression. Scale: the length of snapshots 1 and 3 is 560 µm (1 cm ) 109.4 µm); the length of snapshots 2 is 300 µm (1 cm ) 58.6 µm). Table 1. Characteristic Parameters of the Compression Isotherms collapse value

condensed phase

LC

Acol (Å2)

Πcol (mN/m)

A0a (Å2)

Cs-1 (mN/m)

C7-C7 C7-DOE DOE-C7 DOE-DOE

22 51 37 220

11 9 30 2

27 70 50 270

69 32 124 7

a

A0 values were measured at the most condensed phase of the monolayer.

be higher in the less tightly packed C7-DOE. The ester moiety would be still more easily accessible for the gas phase in the DOE-C7 films because it is closer to the top of the film than in the case of C7-DOE or C7-C7. Finally, in the DOE-DOE films, the ester moiety is situated at the air/water interface and readily accessible for both the aqueous and the gas phases. To evaluate film properties, its stability and compressibility can be considered.32,33 The surface pressure value corresponding to the collapse point reflects film stability (S). For the LC studied here, it decreases in the following

order: SDOE-C7 (30 mN/m) > SC7-C7 (11 mN/m) > SC7-DOE (9 mN/m) > SDOE-DOE (2 mN/m). The film compressibility is defined as follows:

Cs ) -

1 ∂A A ∂Π

( )

(7)

In general, films of high fluidity are characterized by high compressibility values. The compressibility modulus (Cs-1), that is, the rigidity of the films of the four LC studied, decreases in the same order as their stability: Cs,DOE-C7-1 (124 mN/m) > Cs,C7-C7-1 (69 mN/m) > Cs,C7-DOE-1 (32 mN/ m) > Cs,DOE-DOE-1 (7 mN/m). The fact that the DOE-C7 film has the highest stability and rigidity among the four LC studied is due both to its anchoring to the aqueous subphase via the DOE moiety and to a relatively long hydrophobic moiety composed of three aromatic rings oriented into the air. The C7-C7 film cannot be stabilized by the interactions with the aqueous subphase because C7-C7 molecules do not have a clearly delimitated polar headgroup and, thus, is less stable and more fluid than the DOE-C7 film. The lower stability and higher fluidity

Organization of Liquid Crystals on Model Surfaces

Figure 3. Proposed organization of LC molecules at the air/ aqueous surface. Color code: carbon, cyan; nitrogen, blue; oxygen, red; and hydrogen, white.

Langmuir, Vol. 20, No. 19, 2004 7995

Figure 5. DSC thermograms obtained with LC adsorbed on WHP: (a) C7-C7, (b) C7-DOE, (c) DOE-C7, and (d) DOEDOE. The weight percent of LC in the LC/silica phase was 10%. The second-order transition is well seen with the adsorbed C7C7 and DOE-C7 while in DOE-DOE and C7-DOE it is superposed on the nematic-liquid transition occurring at the same temperature.

Figure 4. DSC thermograms obtained with pure LC: (a) C7C7, (b) C7-DOE, (c) DOE-C7, and (d) DOE-DOE.

Figure 6. DSC thermograms obtained with LC adsorbed on silica: (a) C7-C7, (b) C7-DOE, (c) DOE-C7, and (d) DOEDOE. The weight percent of LC in the LC/silica phase was 10 (solid line) or 50% (b).

of the C7-DOE and DOE-DOE compared to the other two films result obviously from their strong interactions with the aqueous subphase, as discussed previously. The film compressibility and stability can be tentatively correlated with the tendency of the four LC to form multilayers on aqueous subphases. It can be observed that the most fluid DOE-DOE builds up a multilayer structure continuously while the other three, more rigid LC, do so in a step-by-step way. The comparison of the extrapolated “zero pressure” areas, A0,33 of the four LC at the most condensed phase of the monolayer (27, 270, 70, and 50 Å2 for C7-C7, DOE-DOE, C7-DOE, and DOE-C7, respectively) with the A0 values measured at the points corresponding to the highest surface pressure of the multilayered films (14, 50, 35, and 25 Å2 for C7-C7, DOEDOE, C7-DOE, and DOE-C7, respectively) indicates that all four LC form multilayers. While C7-C7, C7-DOE, and DOE-C7 oriented more or less perpendicularly to the aqueous surface would form bilayers, the horizontally oriented DOE-DOE would build up films five layers thick. It can be expected that the ordering imposed on the LC

by their intrinsic structure and by the aqueous subphase is preserved on long distances in the multilayers and that the same ordering of the LC films, which is observed at the water surface, would be preserved at the chromatographic support surface. As a consequence of the screening of the ester groups from the gas phase, the chromatographic layers formed with C7-DOE would establish apolar interactions with the solutes present in the gas phase and in this respect would have properties close to those of the C7-C7 layers. The DOE-C7 layers would have properties closer to those of DOE-DOE, as in both the polar ester groups are available for interactions with the solutes present in the gas phase. 3.2. Calorimetric Studies of Phase Transitions in LC. DSC experiments were carried out using both pure LC and LC adsorbed on silica or on Chromosorb WHP. The corresponding thermograms are presented in Figures 4-6. The characteristic parameters of the phase transitions are collected in Table 2. In the case of C7-C7 and C7-DOE, a solid-solid transition denoted K1 f K is observed below the melting temperature. Two solid-solid transitions, denoted K2 f K1 and K1 f K, are observed

7996

Langmuir, Vol. 20, No. 19, 2004

Badis et al.

Table 2. Temperatures and Enthalpies of the Phase Transitions in the Bulk and Adsorbed LC phase transition K2 f K1 LC C7-C7

sample

t (°C)

∆Hc (J/g)

bulk LC + WHPa LC + silicaa

75.2

4.4

K1 f K

KfN

NfI

t (°C)

∆Hc (J/g)

t (°C)

∆Hc (J/g)

t (°C)

∆Hc (J/g)

81.8

37.5

102.6 101.7

47.1 29.3

215.3 214.4

3.6 1.7

C7-DOE

bulk LC + WHPa LC + silicaa

96.2 93.2

58.2 43

176.4 179.4

2.3 1.0

DOE-C7

bulk LC + WHPa LC + Silicaa LC + Silicab

83.9 80.3

74.7 33.7

189.2

1.7

93.6

4.8

169.4

0.5

DOE-DOE

bulk LC + WHPa LC + silicaa

101.1 98.2

36.5 34.6

183.2 182.0

1.4 1.0

a

73.4

1.1

85.1

0.9

10 wt % LC. b 50 wt % LC. c ∆H is reported for 1 g of LC.

with DOE-DOE. K f N and N f I are solid-nematic and nematic-liquid transitions. The analysis of the data reveals significant differences in phase transition parameters of the pure and adsorbed LC. These differences can be correlated with the organization of LC at the solid surfaces, as discussed in the following. 3.2.1. Results Obtained with Silica. The phase behavior of LC adsorbed on silica is very different compared to that of pure bulk LC. Indeed, with the silica samples containing 10 wt % LC, a high and continuous increase of the system heat capacity was observed in a large temperature range (Figure 6a-d) while no first-order phase transition was detected. However, in the silica samples containing 50 wt % of DOE-C7, both solid-nematic and nematic-liquid transitions characteristic of pure bulk LC were observed (Figure 6c). The ratio of the LC molecules in the bulk and in the adsorbed film was estimated from the calorimetric results presented in Figure 6c, and the heat effects of the transitions were measured in the latter samples (Table 2). This estimation indicates that about 15 wt % of LC is in a bulk solid state. Taking into account the high specific surface of silica, the remaining 85% may be organized as a mono- or a bilayered film. 3.2.2. Results Obtained with Chromosorb WHP. The temperatures of solid-nematic and nematic-liquid transitions (clearing point) in LC are not significantly altered upon contact with the solid surface of Chromosorb WHP (the observed differences are of the order of 1-3 °C), indicating that the general character of the LC phase behavior is the same in both systems; the exception is the solid-nematic transition in DOE-C7 which occurs at a lower temperature than in the bulk. The heat effects of the solid-nematic transition decrease with all four LC in the presence of the solid support compared to pure LC. This can be easily understood if we assume that at the clearing point the heat effect of the solid-nematic transition in the adsorbed LC comes only from these LC molecules which do not interact directly with the solid support and which consequently have bulk properties. These molecules which do interact with the solid support would be at the origin of an additional transition observed below the LC clearing temperature (Figure 5a,b). We propose that this additional effect corresponds to the smectic-nematic transition occurring in the adsorbed LC film, the smectic multilayer ordering of the LC being induced by the solid support. The absence of the smectic order in the pure bulk LC used in this work supports this proposal. On the basis of the results obtained, we further

propose that the adsorbed LC films are composed of two phases in equilibrium: ordered smectic multilayers and a bulk nematic phase filling the pores of the solid support. This model may be theoretically approached using a quasithermodynamic treatment of inhomogeneous systems based on the density gradient theory combined with an equation of state.40 With the temperature increase, at the LC nematicliquid phase transitions the corresponding enthalpies are either slightly lower in the presence of the solid surfaces or not altered at all compared to pure LC. This result indicates that the whole amount of the LC adsorbed on the solid support undergoes the clearing transition and that the ordering effect of the solid support disappears at this point. It should be noticed that the efficiency of chromatographic separations deteriorates above the clearing point.27,36 Thus, it can be postulated that chromatographic separations depend on LC-solid support ordering interactions. The ratio of the nematic and smectic LC molecules in the adsorbed film was obtained from the quantitative analysis of calorimetric peaks corresponding to the LC melting in the reference bulk phase and the bulk phase immobilized on the solid surface (Figures 5 a-d and from Table 2). Because the same phase transition occurs in both cases, the differences in the measured heat effects reflect the ratio of the LC in the two phases. In the case of C7-C7, this ratio was found to be 6:4. Calculations using the latter result, as well as the specific area of the Chromosorb WHP (0.5 m2/g) and the molecular area of 38 Å2 obtained at the lift-off point of the compression isotherm of C7-C7, indicate that approximately 40 ordered layers are formed. In the case of the flat-lying DOE-DOE (molecular area 220 Å2), the ratio of LC molecules in the film and in the bulk is 1:10 and the resulting number of layers is around 48. With the three LC, the calculated number of layers, NL, in the film decreases in the same order as the film compressibility and stability, that is, NL DOE-C7 (90) > NL C7-C7 (40) > NL C7-DOE.25 The only exception from this rule is NL of DOE-DOE. The latter is certainly due to the flat arrangement of DOE-DOE molecules on the surface. Consequently, the thickness of one layer of DOE-DOE molecules is significantly lower than the thickness of the films formed with the other three LC. This finding indicates that the thickness of the LC film formed on a solid support can be correlated with the (40) Panayiotou, C. Langmuir 2002, 18, 8841.

Organization of Liquid Crystals on Model Surfaces

stability of the corresponding films spread at the air/ aqueous NaCl interface. 4. Conclusions Chromatographic results published recently27,36 showed that probe molecules of different polarities could be separated using the four LC described in the present study. As expected intuitively, the nonpolar m- and p-xylene separated well on the C7-C7 phase but they did not separate at all on the hydrophilic DOE-DOE phase. On the contrary, the polar citronellal and linalool did not separate with the hydrophobic C7-C7 phase while they separated with the other three LC phases. However, the polar and nonpolar interactions between the probe molecules and the LC could not account in a straightforward manner for the other chromatographic results obtained. Indeed, tetraethylbenzenes separated well on the C7-C7 and C7-DOE but not on the DOE-DOE and DOE-C7 phases, while the m- and p-cresols separated on the DOE-C7 and DOE-DOE but not on the C7-C7 and C7-DOE phases. While the latter results show that the C7-DOE’s properties are closer to those of C7-C7 and those of DOE-C7’s are closer to DOE-DOE’s, the structural differences between C7-DOE and DOE-C7 alone do not indicate convincingly a higher polarity of the latter. We propose that the organization of the two closely related LC on the surface of the chromatographic support is responsible for their different properties. Indeed, as shown using monolayer experiments, the ester moiety in C7-DOE interacts with the water phase while in DOEC7 it is exposed to the gas phase. If the same ordering of LC molecules was preserved at the solid surface of the hydrophilic chromatographic support, the solutes would have an easier access to the polar ester moiety in the case of DOE-C7 than in the case of C7-DOE. By the same token, in the least polar C7-C7 the ester group would be inaccessible to the solutes as a result of a tight packing of the LC molecules oriented vertically relative to the surface of the chromatographic support. Finally, the ester moiety in DOE-DOE may be freely accessible to the solutes, because this LC is expected to lay flat on the

Langmuir, Vol. 20, No. 19, 2004 7997

chromatographic support’s surface, by analogy to the situation in monolayers. Thus, the ordering-dependent accessibility of the ester moiety for the solutes would be the main factor responsible for molecular recognition and separation using the four LC. The other structural motifs present in the four LC would be mainly responsible for the organization of the molecules and their orientation relative to the surface. The LC systems are characterized by large fluctuations and by a high density of energetically close states. Therefore, the ordering induced by a solid surface does not necessarily influence the LC bulk properties such as melting and clearing temperatures and enthalpies. The dramatic changes observed in these properties using DSC indicate that the LC adsorbed on the solid surfaces exist in at least two energetically different states. Indeed, phase transitions characteristic for the bulk LC were observed together with new transitions. Accordingly, we propose that at the solid surface the bulk LC and the multilayer film exist in equilibrium. This equilibrium, as well as the thickness of the film, is determined by the properties of the solid surface such as polarity and specific area and by the polarity, wetting, and propensity to ordering of the LC. The influence of the solid surface on LC phase transitions discussed in this work are in accord with other authors’ results described recently in the literature.41-44 Acknowledgment. The authors thank Jeff Rice and Dr. C. Kowal for revising the English in the manuscript. Fre´de´ric Brusseaux, Alexis Martin, and Jean-Louis Vaucher from Service Technique de l’UHP are acknowledged for the technical assistance. LA049093E (41) Bahr, C.; Booth, C. J.; Fliegner, D.; Goodby, J. W. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1995, 52, R4612. (42) Lucht, R.; Bahr, C.; Heppke, G. J. Phys. Chem. B. 1998, 102, 6861. (43) Schlauf, D.; Bahr, C.; Glogarova, M.; Kaspar, M.; Hamplova, V. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1999, 59, 6188. (44) Lucht, R.; Bahr, C. Phys. Rev. Lett. 2000, 85, 4080.