Alignment of Discotic Lyotropic Liquid Crystals at Hydrophobic and

May 19, 2012 - Anna Akinshina , Martin Walker , Mark R. Wilson , Gordon J. T. Tiddy , Andrew J. Masters , Paola Carbone. Soft Matter 2015 11 (4), 680-...
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Alignment of Discotic Lyotropic Liquid Crystals at Hydrophobic and Hydrophilic Self-Assembled Monolayers Zuhoor H. Al-Lawati,† Ban Alkhairalla,† Jonathan P. Bramble,† Jim R. Henderson,† Richard J. Bushby,‡ and Stephen D. Evans*,† †

Molecular and Nanoscale Physics, School of Physics and Astronomy and ‡Centre for Molecular Nanoscience, University of Leeds, Leeds, LS2 9JT, United Kingdom ABSTRACT: The alignment of the lyotropic discotic liquid crystal 2,3,6,7,10,11-hexa-(1,4,7-trioxaoctyl)-triphenylene (TP6EO2M) on −CO2H and −CH3 self-assembled monolayers (SAMs) has been investigated in order to determine whether the surface free energy could be used to control anchoring in such a system. Irrespective of the SAMs used we found that the alignment tends to be with the director in the plane of the surface. To understand why the different SAM types produce the same bulk behavior we considered the adsorption of TP6EO2M from dilute solutions. In the case of −CH3 surfaces we found the first adsorbed DLC layer lies with its triphenylene core close to the hydrophobic SAM, leaving the hydrophilic chains extending from the surface. For the −CO2H surfaces the first DLC layer is believed to be edge-on again, leading to hydrophilic chains extending away from the surface. These adsorbed DLC layers give rise to planar alignment of the bulk nematic and columnar phases.

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most types of discotic LC.25 Confinement is generally a more effective way to control DLCs rather than relying on the nature of the surface (the surface energy or roughness).26 In general, homeotropic anchoring is commonest at the substrate−LC interfaces (confinement between two identical substrates), whereas planar anchoring is normal at the LC−air interface (single substrate).26,27 However, controlling the alignment of DLCs is an experimental challenge.28 Lyotropic LCs are common in biological systems, and control over their alignment could prove useful for development of new sensors, for example, lyotropic chromonic LCs have been used as a real-time microbial sensor to detect immune complexes.29 It is somewhat surprisingly therefore that while there is substantial literature on the alignment of thermotropic liquid crystals (TLCs), there are few reports on the alignment of lyotropic liquid crystals (LLCs). One main reason for this is that the main lyotropic phases (columnar and lamellar) are difficult to align either on surfaces or by using external fields. In principle, this problem could be overcome by exploiting lyotropic nematic phases, but most of these only occur over very small ranges of temperature and composition. However, there are a few exceptional systems that have a wide nematic range and in which the columnar or lamellar phases can be aligned through the nematic phase.30−32 One of these, the discotic lyotropic liquid crystal (DLLC) TP6EO2M (2,3,6,7,10,11-hexa-(1,4,7-trioxaoctyl)-triphenylene, Figure 1), is the subject of the present study.

INTRODUCTION Our ability to control the anchoring of liquid crystal (LC) materials at surfaces is important for many applications, particularly in the display industry but also potentially for development of novel chemical and biological sensors. Traditionally this has been achieved by controlling the topography of the surface through the use of rubbed polyimide surfaces,1 ZBD surfaces,2 or obliquely evaporated surfaces3 or via the chemistry of the surface such as lecithin layers,4 polymer films,5 or SAM layers.4,6,7 The alignment of thermotropic calamitic (rod-like) LCs on SAM surfaces has been the subject of many studies.3,7−19 Many factors are involved. For example, in the nematic phase 5CB and MBBA align homeotropically on a mixed SAM of nalkanethiols containing long (CH3(CH2)15SH) and short (CH3(CH2)4SH) aliphatic chains. However, both align nonuniformly in a planar or tilted manner on single-component alkanethiol SAMs (CH3(CH2)nSH, 2 < n > 15).7 Varying the ω-functional group of alkanethiol SAMs offers another way of controlling anchoring. The alkylcyanobiphenyl family (nCB, n = 5−9) has been studied on SAMs with different ω-functional groups.9 Low-energy surfaces with a CH3-terminated group and nCBs, where n is large, result in homeotropic anchoring, while decreasing n or/and increasing the surface free energy leads to planar anchoring and a transition from complete to nonwetting.8 Patterning the SAMs has also been shown to provide additional control over the azimuthal alignment of LCs in the nematic phase and controlled formation of defects in the smectic phase.20−24 Treated surfaces can be used to control the alignment of calamitic LCs. However, surface treatment is ineffective for © 2012 American Chemical Society

Received: March 19, 2012 Revised: May 14, 2012 Published: May 19, 2012 12627

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EXPERIMENTAL SECTION

Materials. Dichloromethane 99.9% (DCM), hydrogen peroxide (27.5 wt %), methanol (MeOH) 99.9%, 16mercaptohexa-decanoic acid (C16H32O2S), and 1-hexadecanthiol (C16H34S) were used as received from Sigma-Aldrich. Sulfuric acid (98%) was supplied by Fisher Scientific. Glass microscope slides (thickness 0.8 mm) were purchased from Agarand and cut to approximately one-half of the original length. Millipore Milli-Q water with a resistivity better than 18.2 MΩ·cm was used throughout. High-purity (99.99%) temper-annealed gold wire (0.75 mm diameter) was supplied by Goodfellow. 2,3,6,7,10,11-Hexa-(1,4,7-trioxaoctyl)triphenylene (TP6EO2M) DLLC was synthesized as previously described and had physical and spectroscopic properties identical to those previously reported.30 Glass equilateral prisms with a high refractive index of 1.85 (Tih53) were purchased from Galvoptics, U.K. Substrate Preparation. Glass microscope slides and high refractive index prisms (Tih53) were first cleaned by ultrasonication for 15 min in a 10% solution of Decon 90 in Milli-Q water. Each slide and prism was rinsed 10 times in Milli-Q water and then dried under a stream of oxygen-free nitrogen. The slides were ultrasonicated in dichloromethane, and the prisms were ultrasonicated in methanol for 15 min; both were removed and dried in a stream of nitrogen, rinsed under MilliQ water, and dried under nitrogen. The slides were immersed in piranha solution (70:30, v/v, H2SO4/H2O2) at ∼85 °C for 10 min. The substrates were then rinsed in Milli-Q water and dried under nitrogen. Slides and prisms were placed separately in an Edwards Auto 306 thermal evaporator. A 30 nm gold layer was deposited onto a chromium adhesion layer (3 nm thick) on slides, and in the case of the prisms a 20 nm gold layer was evaporated directly onto the base of the prisms. The deposition rate was maintained at 0.08 nms−1 and at a base pressure of 6 × 10−7 mbar. Samples were rotated during the evaporation process of the gold in order to produce a uniformly deposited gold film which will reduce the likelihood of uniform azimuthal alignment.3 The gold-coated slides were cleaned immediately prior to use by placing them in freshly prepared piranha solution for 2 min, followed by a rinse with Milli-Q water, while the gold-coated prisms were rinsed with methanol, dried with nitrogen, rinsed with Milli-Q water, and dried with nitrogen before use. SAM Adsorption. Clean gold-coated slides and prisms were immersed in 1 mM alkanethiol solution in methanol for 18 h at 23 °C. Samples were then removed from solution, rinsed with

Figure 1. Chemical structure of TP6EO2M (2,3,6,7,10,11-hexa-(1,4,7trioxaoctyl)triphenylene) discotic lyotropic liquid crystal (DLLC) .

Evanescent wave ellipsometry (EWE) is a surface-sensitive technique that can be used to monitor the anchoring and orientational wetting behavior of the LCs on SAMs by tracking the change in the Brewster angle (θB) as a function of temperature. The Brewster angle, θB, occurs above the critical angle, making it a sensitive probe of the alignment close to the substrate−liquid interface.9,10,13,33 In this paper we present a study of the anchoring of TP6EO2M on single-component organothiol SAMs displaying different surface functional groups using evanescent wave ellipsometry. TP6EO2M was first synthesized by Boden et al. in 1985.34 The molecules are disk-like in shape made up of a rigid hydrophobic triphenylene core with six flexible nonionic hydrophilic ethylenoxy side chains surrounding the core.34 When TP6EO2M is dissolved in water, the molecules stack to form columnar aggregates to minimize the interface between the hydrophobic core and the water. At higher concentration these rod-like aggregates self-organize to give a nematic phase of columns, NC mesophase. Increasing the concentration still further leads to formation of a columnar hexagonal, Colh, mesophase.30,35,36 The average length of the columns in the isotropic solution and in the nematic phase increases with increasing concentration. The experimental phase diagram of TP6EO2M/D 2O was established by a combination of deuterium NMR spectroscopy and X-ray diffraction by Boden et al. (Figure 5).8,37,38 The two-phase regions in the figure are marked by tie lines. Everywhere else is a homogeneous singlephase region. The nematic phase is intermediate between the isotropic phase at higher temperature/lower concentration and a columnar hexagonal phase at lower temperature/higher concentrations.30 The transitions from isotropic to nematic and nematic to columnar are first order.35 In our study H2O rather than D2O has been used as the solvent, which is expected to reduce the I/Nc and I/Colh phase boundary by ∼4 K.

Figure 2. Evanescent wave ellipsometry (EWE) experimental setup. Architecture of the interface (not to scale) and exponential decay of the electric field into LC phase are shown in the inset. 12628

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different concentrations of TP6EO2M in isotropic phase at 298 K using the critical angle to determine the refractive indices.42

methanol, dried in a stream of nitrogen, rinsed with Milli-Q water, and dried once more in the nitrogen stream. The alkanethiol materials used were 16-mercaptohexadecanoic acid (HS−(CH2)15−CO2H) to give a high-energy surface (hydrophilic SAM) and 1-hexadecanethiol (HS−(CH2)15−CH3 for a low-energy surface (hydrophobic SAM). The SAM-functionalized surfaces were characterized immediately prior to use using water contact angles measurements (average of 3 measurements on 3 samples). The values of the static (θS), advancing (θa), and receding (θr) contact angles and the hysteresis of the SAMs formed from HS−(CH2)15−CO2H were 34 ± 1°, 39 ± 1°, 30 ± 1°, and 9°, respectively while those for SAM formed from HS−(CH2)15−CH3 were 108 ± 1°, 112.5 ± 1°, 104.5 ± 2°, and 8°, respectively. The value of the advancing contact angle is higher than previously reported39 for the acid-terminated SAM which probably reflects a degree of contamination of this high-energy surface. However, the value is in the acceptable range and close to that frequently found in the laboratory. The magnitude of the values in each case and the small values of hysteresis (less than 10°) show that both SAM types were consistently produced and of good quality. Wetting Measurements. A First−Ten-Angstrom 4000 Goniometer was used to measure the contact angles of water on the SAMs under ambient conditions. Droplets of Milli-Q water were advanced and receded on at least three places on the surface of the SAMs using a microsyringe needle. Cell Arrangement. Cells were comprised of two SAMcoated surfaces. The SAM-coated prism formed the top surface, while a glass slide, coated with a SAM of the same type, formed the lower surface (Figure 2). TP6EO2M LC, at the required concentration, was placed on the SAM glass slide, and 23 μm thick PET spacers were placed around the LC on all sides, leaving a small gap for air bubbles and excess material to escape from the cell. The SAM-coated prism was placed on the LC, forcing it to fill the cell. The alignment of TP6EO2M on the SAM was then studied using EWE. Ellipsometry. A Jobin-Yvon UVISEL spectroscopic ellipsometer was used to determine the Brewster angle, θB, at which rp is minimum and the phase shift of reflected light Δ = 270° (or the ellipticity, ρ = 0). The temperature of the cell was maintained with an accuracy of 0.05 °C using a PID-controlled Peltier heater. The temperature was allowed to reach equilibrium for 6 min before each reading was taken. The angular resolution was 0.01°, and the wavelength was set to 633 nm. The phase-modulated ellipsometer was operated in mode III (with the analyzer and modulator set to A = +45° and M = −45°, respectively).40,41 The change in the Brewster angle as a function of temperature as the sample was cooled through the isotropic−nematic or columnar transition of the LCs was traced. Data was collected on both cooling and heating runs to check for hysteresis. The beam diameter was ∼1 mm2 and was much smaller than the surface area of the cell and therefore is unaffected by possible edge effects, and thus, the sample could be regarded as an infinite surface. In addition, the LC film is much thicker (23 μm) than the depth of the evanescent field, meaning that the data represents changes in alignment taking place close to the prism SAM interface. However, the directorfluctuation-mediated interactions between the two opposing boundaries of the cell cannot be ignored. Consequently, identical opposing boundaries of known properties were used, so the alignment is enforced at the surface. Abbe Refractometry. A Bellingham and Stanley Abbe refractometer was used to measure the refractive indices at

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RESULTS AND DISCUSSION The refractive index at each concentration (w%) of TP6EO2M was determined in the isotropic phase at 298 K from refractometry measurements (Figure 3). At the highest

Figure 3. Refractive index as a function of the TP6EO2M concentration. Solid squares indicate the refractive indices determined by refractometry of bulk solutions in the isotropic phase (niso) at 298 K. Values for n⊥ are based on the work of Alkhairalla8, and those for n∥ are calculated from niso and n⊥. Error in the concentration is 0.03%, and error in the refractive index values is 0.0001.

TP6EO2M concentration (61%) at 298 K the sample was just in the isotropic phase, close to the boundary of the twophase region (see EWE data for H2O in Figure 5). The refractive indices in the isotropic phase (niso) at concentrations greater than 61% were determined by extrapolation from the measured data. The refractive index perpendicular to the director (n⊥) for the NC phase was previously determined by Alkhairalla,8 and the refractive index parallel to the director (n∥) was obtained using the relation 1 n iso 2 = (2n⊥2 + n 2) (1) 3 It is evident that the values for n∥ are lower than those for the corresponding isotropic phase, while the n⊥ values are higher for the same concentration. The refractive index in the isotropic phase is linearly dependent on the concentration, and we can therefore use the Brewster angle, in this phase, to determine/check the TP6EO2M concentration. For the isotropic solution we found that θB = (50.2 + 0.2[CTP6EO2M])°

(2)

The variation of θB expected during growth of an adsorbed TP6EO2M film (53%), beneath an isotropic phase, was modeled (Figure 4) using the scattering matrix method.41 The scattering matrix calculations were written with Maple 13 assuming a four-layer system, prism/gold/TP6EO2M-film/ TP6EO2M bulk. The prism, gold, and LC bulk phase (T > TIN) were assumed to be isotropic, while the LC film was assumed to be modeled as a uniaxial thin film. From knowledge of the refractive indices, for each layer as well as the thickness of the gold and LC films it was possible to calculate the ellipticity and reflected intensity for the systems under study, assuming all interfaces were both abrupt and flat. The wavelength of the 12629

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Figure 5. Comparison between the phase diagram for TP6EO2M/ D2O determined by 2H NMR spectroscopy and the experimental transition temperatures for TP6EO2M/H2O measured by EWE. Phase diagram of the TP6EO2M/D2O system is taken from Boden et al.8,37,38 EWE transition temperatures were taken from the temperatures at which abrupt changes were noted in the normalized Brewster angle. Solid diamonds indicate isotropic to the coexistence of isotropic and LC transitions on cooling, and open diamonds indicate LC to the coexistence of isotropic and LC transitions on heating. Error in the concentration is 3 × 10−4%.

Figure 4. Numerical modeling of the change of the normalized Brewster angle during growth of a columnar hexagonal phase of TP6EO2M (53%) beneath an isotropic solution. Solid squares indicate homeotropic growth, while open oriented up and down triangles represent planar anchoring parallel and perpendicular to the plane of incidence, respectively. Solid diamonds indicate the random planar orientation of the LC director. Refractive indices used for prism and gold were nprism = 1.85 and nAu= 0.25−3.2i, respectively. TP6EO2M refractive indices were taken to be niso = 1.445 for the isotropic phase and n⊥ = 1.46316 and n∥ = 1.40976 for refractive indices perpendicular and parallel to the director in the columnar phase. Thickness of the gold film was 20 nm, and wavelength of the incident light λ = 633 nm. Brewster angle in the isotropic phase θB* = 57.3°.

CsPFO/H2O system is lower by 3.9 and 4.3 °C at 0.15 (volume fraction) concentrations than the ones for CsPFO/D2O. The graphs (A1−A3) in Figure 6a show the variation of the Brewster angle data (θB), normalized to the value at T ≫ TIN/IC (θB*), referred to in the text as the normalized Brewster angle as a function of temperature. Data are shown for three examples (28%, 47.2%, and 60.8%) on cooling and heating through the isotropic−nematic and isotropic−columnar phase transitions at CH3-terminated surfaces. From A1 to A2 the concentration of TP6EO2M in water was increased from 28% to 47.2%. In both cases, the increase in the normalized Brewster angle reveals planar anchoring in the nematic phase. As the concentration was increased further the change in the normalized Brewster angle became slightly negative. Hysteresis was observed at all concentrations as a result of entering the region of coexistence from different directions. Figure 6b shows the change in normalized Brewster angle as a function of concentration of TP6EO2M (by weight) at CH3-functionalized SAM surfaces. The shift was taken for each concentration from T ≫ TIN/IC at T = 303.7 K to T ≪ TIN/IC at T = 291.5 K. The results from modeling the expected against the weight % of a semi-infinite thick film of TP6EO2M of random planar, isotropic, and homeotropic alignment (dashed, dashed dotted, and dotted lines, respectively) are superimposed on the EWE experimental results. The normalized Brewster angle (solid diamonds), in the nematic phase, increased with the TP6EO2M concentration up to 54%, after which it decreased. The positive shift at low concentration (below 54%) is identical to the theoretical prediction for random planar alignment (dashed line). For concentrations between 54% and 60% the normalized Brewster angle (solid diamonds) decreases, indicating that as one enters the columnar phase there is either a decrease in the order within the film or that an increasing proportion of a homeotropic layer is being grown at the interface. After ∼60% of TP6EO2M the shift in the Brewster angle changes to a negative value, indicating a degree of homeotropic alignment.

light used was 633 nm for both experiment and modeling. Four different systems were modeled representing the possible orientations of the director with respect to the surface and the plane of incidence. Growth of a homeotropically aligned columnar film (with the columns perpendicular to the surface) would lead to a reduction in θB (solid squares), whereas for planar alignment (columns parallel to the surface) θB increases. Three possible planar configurations are shown with the LC director parallel, perpendicular, or randomly distributed relative to the plane of the incidence. Practically, the angular resolution of the evanescent wave ellipsometer is 0.01°, which equates to a sensitivity in thickness of a few Angstroms of LC film. Figure 4 shows that the EWE is approximately linearly proportional to changes in thickness up to ∼50 nm, becoming relatively insensitive to variation in film thickness for thicknesses greater than 200 nm. From Figure 4 it is evident that if the system is cooled from the isotropic into the columnar (or nematic) phase, the Brewster angle should show a positive change for planar anchoring or a reduction for homeotropic alignment. Figure 5 shows the EWE data for the transition temperatures for TP6EO2M in H2O superimposed on the phase diagram for TP6EO2M in D2O determined by 2H NMR spectroscopy. Solid diamonds indicate the transition from isotropic to the coexistence of isotropic and LC region. Open diamonds represent the transition from the LC phase to the coexistence area during heating. EWE data (H2O) shows systematic deviation from the NMR data (D2O). The largest difference in TICO is a reduction of ∼3.8 K for TP6EO2M/H2O than TP6EO2M/D2O at 65%. Previous studies40,43 have also shown that the phase transitions for lyotropic liquid crystal in H2O are generally a few degrees lower than in D2O. For instance, Boden et al.40 found that the CsPFO/H2O phase diagram is similar qualitatively to CsPFO/D2O. However, TIN and TNL for 12630

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Figure 6. (a) Variation of Brewster angle (θB) normalized to the value at T ≫ TIN/IC (θB*) as a function of temperature on both cooling and heating runs (solid and open diamonds, respectively) through the I−N and I−Colh phase transitions at 28%, 47.2%, and 60.8% concentrations of TP6EO2M at CH3-terminated organothiol surfaces (A1−A3 graphs). (b) Change in the normalized Brewster angle as a function of TP6EO2M concentration at CH3-functionalized SAM (solid diamonds). Data for the Brewster angles were taken from the EWE results. Shifts were taken as the difference between the values at T = 303.7 (T ≫ TIN/IC) and 291.5 K (T ≪ TIN/IC). Dotted and dashed lines show calculated behavior for a 3000 Å thick layer of homeotropic and planar anchoring, respectively. Error in the concentration is ∼0.03%.

experimentally, to increase with the concentration of TP6EO2M up to 50% and after which it decreased to 0.01 but remained positive. The behavior below 50% is that expected for a random planar alignment. From 50% to 66.5% the normalized Brewster angle (solid diamonds) decreases to 0.01, which indicates a decrease in the order within the film, although planar alignment remains dominant. Polarizing Optical Microscopy Measurements. Figure 8 shows the texture obtained for a 53.8% TP6EO2M sample sandwiched between glass slides coated with CO2H- (Figure 8a−c) and CH3-functionalized (Figure 8d−f) SAMs and cooled at 0.1 °C/min from the isotropic into the Colh phase. The bright mosaic texture, which is also observed upon rotation of the sample relative to the polarizer, is consistent with domains in which the director is oriented in a random/random planar manner. This agrees with the EWE results on both SAMs. Monolayer Formation for TP6EO2M at CH3 and CO2H Interfaces in Very Dilute Solutions. In dilute solutions of surfactants there is usually equilibrium between the monomer in solution and an absorbed monolayer at the water/air interface, at the interface of the solution hydrophobic surfaces, and sometimes at the interface between the aqueous solution

However, the change is much smaller than that predicted for complete homeotropic alignment. Figure 7 shows the variation in the normalized Brewster angle as a function of temperature (Figure 7a) on both cooling and heating runs (solid and open diamonds, respectively) through the isotropic−nematic and isotropic−columnar phase transitions at 26.3%, 39.8%, and 66.1% concentrations of TP6EO2M on −CO2H-terminated organothiol SAMs (A1− A3). In all cases a positive shift in the normalized Brewster angle was observed indicative of planar anchoring. Random planar anchoring on CO2H-terminated SAMs is seen both when the LC is in the nematic phase (A1 and A2) and when it is in the columnar phase (A3). For all concentrations, hysteresis was observed as a result of entering the region of coexistence from different directions.The predicted shift in the normalized Brewster angle (Δ(θB − θB*)/θB*) versus the weight % of TP6EO2M together with the results from experimental EWE (solid diamonds) are presented in Figure 7 b. The modeling shows the shifts in the changes expected for a semi-infinite film of TP6EO2M of random planar, isotropic, and homeotropic alignment (dashed, dashed dotted, and dotted lines). The normalized Brewster angle (solid diamonds) was observed, 12631

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Figure 7. (a) Variation of Brewster angle data (θB) normalized to the value at T ≫ TIN/IC (θB*) as a function of temperature on both cooling and heating runs (solid and open diamonds, respectively) through the I−N and I−Colh phase transitions at 26.3%, 39.8%, and 66.1% concentrations of TP6EO2M at CO2H-terminated organothiol surfaces (A1−A3). (b) Change in the normalized Brewster angle as a function of TP6EO2M concentration at a CO2H-functionalized SAM (solid diamonds). Dotted and dashed lines show calculated behavior for a 3000 Å thick layer of homeotropic and planar anchoring, respectively. Error in the concentration is 0.03%.

Figure 8. Optical textures observed under crossed polarizing conditions for a sample of 53.8% TP6EO2M on CO2H- (a−c) and CH3-functionalized (d−f) SAMs. Images were taken with a magnification of 20×, and the stage relative to the polarizer is at (a and d) 0°, (b and e) 45°, and (c and f) 90°, respectively.

functionalized surfaces. Previous studies of the effect of concentration of TP6EO2M on the surface tension at the air/water interface have already established that it behaves as a

and hydrophilic surfaces. If this is also true for TP6EO2M in water it will undoubtedly have a profound effect on the alignment of the NC and Colh phases on the SAM12632

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increasing concentration. Numerical modeling of the changes expected for adsorption of a ‘neat’ TP6EO2M film from the dilute solution is shown in Figure 10b. Although the changes are very small they are within the sensitivity of the instrument and significantly larger than the experimental error. For both surfaces the adsorption profiles display S-shaped curves as a function of concentration, showing formation of films of finite thickness in each case. The shift in the Brewster angle for dilute 10−3 M solutions on the CH3terminated SAM is about 8 × 10−4. The modeling suggests that this corresponds to a 3.5 Å thick planar oriented monolayer (not physically realistic unless the coverage is low) or a 5.4 Å thick homeotropic oriented layer. The shift in the normalized Brewster angle for CO2H SAM was about 2 × 10−3. The modeling suggests this would represent a 9 Å thick layer with a planar alignment or a 13 Å homeotropic layer (unrealistic for a monomolecular layer). Further evidence for adsorption taking place at the SAM/ water interface in dilute solution is provided by measurements of the contact angles for dilute TP6EO2M solutions on CH3and CO2H-functionalized surfaces. On the CH3-functionalized surface the static contact angle for 1.6% (1.7 × 10−2 M) and 19.5% (2.6 × 10−1 M) TP6EO2M solutions was ∼70°, a marked decrease from the value of 103° found for pure water. On the CO2H surface a smaller change was observed from 23° in H2O to 16° and 20° for 1.6% and 19.5% solutions, respectively.

surfactant and suggested that at this interface the molecules form a monolayer in which the aromatic cores lie parallel to the surface and exposed to the air while the hydrophilic side chains point down into solution (Figure 9a).44 Hence, in order to

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DISCUSSION AND CONCLUSIONS The alignment of discotic liquid crystals at surfaces is not well understood. On the basis of results obtained for phthalocyanine-based DLCs26,45,46 and for hexabenzacoronene-based DLCs27 it has been suggested that one way to control their alignment is by introducing additional ether oxygens into the side chains and that this will promote homeotropic anchoring on high-energy surfaces. From the results of the present study it is clear that this is not true for this particular lyotropic triphenylene-based DLC. Whereas almost all triphenylenebased DLCs with simple alkoxy side chains align in a homeotropic manner on both high- and low-energy surfaces it is evident that TP6EO2M, which has additional ether

Figure 9. Schematic representation of the alignment of the proposed structures of TP6EO2M monolayers. (a) ‘Homeotropic’ layer at the air−water interface. (b) ‘Homeotropic’ layer at the water−hydrophobic SAM interface. (c) ‘Planar’ layer at the water−hydrophilic SAM interface.

understand the alignment of TP6EO2M at SAM surfaces more fully, we undertook adsorption studies of dilute solutions of TP6EO2M in the concentration range from 1 × 10−10 to 1 × 10−3 M, well below the concentration required to form the NC or Colh phase. Adsorption of TP6EO2M at CO2H and CH3 interfaces was followed experimentally using EWE. Figure 10a shows the variation in the normalized Brewster angle with

Figure 10. (a) Shift in the Brewster angle due to a thin TP6EO2M film adsorbed from a dilute TP6EO2M solution on CH3 and CO2H SAMs, respectively. θB* is the Brewster angle at the concentration of the 10−10 M solution. Error in the concentration is 3 × 10−4 M. (b) Numerical modeling showing the variation in the normalized Brewster angle during growth of a thin adsorbed layer beneath a dilute bulk phase (10−4 M). Random planar, isotropic, and homeotropic films are indicated by dotted, solid, and dashed curves. Refractive indices used for prism and gold were nprism = 1.85 and nAu= 0.25−3.2i, respectively. 12633

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oxygens in the side chains, aligns in a planar manner in the NC phase on both high- and low-energy surfaces, and although the anchoring of the Colh phase is weak, homeotropic anchoring is only (slightly) favored on the ‘low-energy’ CH3-terminated SAM surface! The fact that the NC phase of TP6EO2M aligns in the same way on both CH3- and CO2H-terminated SAM surfaces studied can be understood in light of the EWE studies of dilute solutions (Figure 10). From these it is clear that above a concentration of ∼10−4 M (∼9.4 × 10−3%) the surfactant coats the SAM, presumably rendering both the ‘high-energy surface’ and the ‘low-energy surface’ hydrophilic and presenting very similar interfaces to the bulk solution. The form of the reduced Brewster angle versus log concentration plots in this region show that the surface films formed are finite, and this means that they are probably monomolecular. In the case of the CH3terminated SAM on the basis of EWE results and by analogy with the surface tension studies44 we can feel reasonably confident that this unimolecular layer is of the type shown in Figure 9b in which the aromatic cores lie parallel to the surface and the ethylenoxy chains extend out into the water. Previous studies have shown that TP6EO2M is strongly surfactant and below a concentration of 10−5 M the surface tension (at the air−water interface) versus log (concentration) plot is linear. From the slope of this line the derived area per molecule at the interface is 115 Å2 corresponding to a disk of radius 5.76 Å, very close to the radius of the rigid core (including the ether oxygens) of 5.6 Å.34 Hence, it was suggested that at the water− air interface the molecules are arranged such that the hydrophobic cores are parallel to the surface and exposed to air with the hydrophilic ethylenoxy chains pointing down into the water as shown in Figure 9a. This is also consistent with other studies of triphenylenes with hydrophilic side chains at the air−water interface.47,48 The behavior at the hydrophobic SAM interface (Figure 9b) seems to be analogous to that at the air interface and also to be governed by a requirement for the aromatic core to lie parallel to the hydrophobic surface. The nature of the layer formed on the CO2-terminated SAM in dilute solution (Figure 10b) is less clear. The change in the reduced Brewster angle is too large to be explained in terms of a ‘homeotropic’ layer (this would need to be ∼13 Å thick, which is too thick even for a bilayer), but if the layer is ‘planar’(Figure 9c), the thickness would be ∼9 Å, a little less than the diameter of the aromatic core, so too thin for a ‘planar’ monolayer. The most likely explanation is that the molecules are indeed ‘edgeon’ but that they are tilted. Above a concentration of ∼20% cooling of the isotropic solution lead to formation of an NC phase, and on both CH3and CO2H-terminated SAMs it is clear that this is well aligned in a planar manner. Above a concentration of ∼50% cooling of the isotropic solution leads to formation of a Colh phase. As is usual in DLC systems this is much more difficult to align than the nematic phase and even at the very slow cooling rates used in the EWE experiments alignment is imperfect. In the case of the CO2Hterminated SAM the predominant alignment of the liquid crystal remains planar up to ∼67%, but in the case of the CH3terminated SAM homeotropic alignment is slightly favored above a concentration of 60%.

Article

AUTHOR INFORMATION

Corresponding Author

*Phone: 0113 343 3852. Fax: 0113 34 33900. E-mail: s.d. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Education in Sultanate of Oman for the provision of a Ph.D. scholarship.

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