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Effect of Surfactant Structure on the Orientation of Liquid Crystals at Aqueous-Liquid Crystal Interfaces† Jeffrey M. Brake, Andrew D. Mezera, and Nicholas L. Abbott* Department of Chemical Engineering, University of WisconsinsMadison, 1415 Engineering Drive, Madison, Wisconsin 53706 Received January 25, 2003. In Final Form: March 26, 2003 It is known that the orientations assumed by thermotropic liquid crystals (LCs) in contact with water are sensitive to the types and concentrations of surfactants and/or polymers present in the aqueous phase. This work expands upon these past observations by developing criteria for surfactants that give rise to a particular orientation of a contacting nematic LC formed formed from 4′-pentyl-4-cyanobiphenyl (5CB). We observe surfactants that have a bolaform structure ((11-hydroxyundecyl)trimethylammonium bromide (HTAB), dodecyl-1,12-bis(trimethylammonium bromide) (DBTAB), 11-(ferrocenylundecyl)trimethylammonium bromide (FTMA)) and which adopt looped configurations at air-water/oil-water interfaces cause planar anchoring of 5CB. In contrast, classical surfactants (alkyltrimethylammonium halides (CnTABs, n > 8), sodium dodecyl sulfate (SDS), and N,N-dimethylferrocenylalkylammonium bromides (FCnABs, n > 12)) that assume tilted orientations at air-water/oil-water interfaces can give rise to a homeotropic orientation of 5CB. By comparing SDS, dodecyl trimethylammonium halide (DTAB), and tetra(ethylene glycol) monododecyl ether (C12E4), we conclude that the nature of these headgroups does not measurably influence the orientation of the LC. However, the orientation of the LC is found to depend on the aliphatic chain length and the areal density of the adsorbed surfactant. When using surfactants with short alkyl chain lengths (n ) 8 for CnTAB and n ) 7 and 12 for FCnAB), we observe the orientation of 5CB to remain parallel to the interface up to concentrations at which the 5CB begins to be solubilized by the surfactant. These results, when combined, lead us to conclude that interactions between the aliphatic chains of the surfactant and 5CB, which are influenced by the conformation of the surfactant, largely dictate the orientation of the 5CB.
Introduction The work described in this paper revolves around an experimental investigation of the relationship between the molecular structure of surfactants adsorbed at interfaces between aqueous phases and thermotropic liquid crystals and the resulting orientations of the liquid crystals. Whereas a number of past studies have exploited the influence of surfactants adsorbed at interfaces between liquid crystals and aqueous phases on the orientations of liquid crystals,1-3 the relationship between the structures of the surfactants and the resulting orientations of the liquid crystals has not been elucidated. By comparing the influence of a series of surfactants (Figure 1) on the orientations of liquid crystals, we sought to establish criteria for surfactants that give rise to particular orientations of liquid crystals. Our experimental approach builds from the results of a recent study in which we reported an experimental system that provides approximately planar and stable interfaces between aqueous phases and liquid crystals.1 Because the interface between the liquid crystal and aqueous phase is not highly curved (in contrast to past studies of nematic emulsions),2,3 the orientations of the liquid crystals can be determined unambiguously using standard methods based on the transmission of polarized light through the liquid crystals. Our past study based on this experimental system focused on the anchoring of 4′* To whom correspondence should be addressed. E-mail:
[email protected]. Fax: 608-262-5434. † Part of the Langmuir special issue dedicated to David O’Brien. (1) Brake, J. M.; Abbott, N. L. Langmuir 2002, 18, 6101. (2) Poulin, P.; Stark, H.; Lubensky, T. C.; Weitz, D. A. Science 1997, 275, 1770. (3) Drzaic, P. S. Liquid Crystal Dispersions. Series on Liquid Crystals; World Scientific: Singapore, 1995; Vol. 1.
Figure 1. (A) Chemical structure of 5CB. (B) Chemical structures of surfactants that adopt looped configurations at air-water/oil-water interfaces. (C) Chemical structures of surfactants that adopt tilted configurations at air-water/oilwater interfaces.
pentyl-4-cyanobiphenyl (5CB) at the aqueous-5CB interface in the presence of sodium dodecyl sulfate (SDS).1
10.1021/la034132s CCC: $25.00 © 2003 American Chemical Society Published on Web 05/21/2003
Surfactant Effects on Liquid Crystal Orientations
SDS is a classical anionic surfactant having one hydrophilic headgroup (sulfate) and one hydrophobic tail (dodecyl chain). Surfactants having this type of molecular architecture adopt tilted configurations at air-water and oil-water interfaces.4-7 The tilt angles of the surfactant tails vary strongly with the surface excess concentration of the surfactant4,5 as well as the type of interface6 (e.g., oil-water, air-water). In general, as the density of surfactant adsorbed at these interfaces increases, the average tilt angle of the tails of the surfactant approaches the normal to the interface.4,5 At the 5CB-water interface, the anchoring of 5CB changes from planar (parallel to the interface) to homeotropic (perpendicular to the interface) with increasing areal density of SDS adsorbed at the interface.1,2 The results of our past study1 and the past studies of others8 suggest that interactions of the tails of the adsorbed surfactants and the liquid crystal largely dictate the orientations of the liquid crystals. For example, one past study has demonstrated anchoring transitions in mixed Langmuir monolayers of 4′-octyl-4-cyanobiphenyl (8CB) and pentadecanoic acid (PDA) on water.8 Compression of these monolayers resulted in a decrease of the tilt angle of 8CB (measured from the normal) and the formation of multilayers of 8CB, as probed by second-harmonic generation and surface pressure measurements. The change in tilt angle of the 8CB was attributed to interactions between 8CB and the tails of the PDA.8 In the study reported this paper, we sought to further test this hypothesis regarding the role of the surfactant tails in dictating the orientation of the liquid crystal by systematically varying the nature of the tails of the surfactants. As shown in Figure 1, we compare and contrast the influence of surfactants that possess different tail structures (e.g., bolaform and classical structures) as well as different headgroups. We also report on the influence of redox-active surfactants on the orientation of the liquid crystals. The results of this study provide insight into the mechanisms by which surfactants preferentially orient LCs at interfaces with aqueous phases. Materials and Methods Materials. Cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB), octyltrimethylammonium bromide (OTAB), sodium dodecyl sulfate (SDS), tetra(ethylene glycol) monododecyl ether (C12E4), lithium sulfate, and aluminum oxide (activated, 50-200 µm, neutral) were obtained from Sigma-Aldrich (St. Louis, MO). Deionization of a distilled water source was performed using a Milli-Q system (Millipore, Bedford, MA) to give water with a resistivity of 18.2 MΩ cm. Octadecyltrichlorosilane (OTS), methanol, methylene chloride, sulfuric acid, hydrogen peroxide (30% w/v), and heptane were all obtained from Fisher Scientific (Pittsburgh, PA). Ethanol was obtained from Aaper Alcohol and Chemical Co. (Shelbyville, KY). 4′-Pentyl-4-cyanobiphenyl (5CB) was obtained from EM Sciences (New York, NY). 11-(ferrocenylundecyl)trimethylammonium bromide (FTMA)9-12 was obtained from Dojindo (Japan). All chemicals were used as obtained without further purification. The glass microscope slides were Fisher’s Finest Premium Grade obtained from Fisher Scientific. Copper specimen grids (18 µm (4) Messmer, M. C.; Conboy, J. C.; Richmond, G. L. J. Am. Chem. Soc. 1995, 117, 8039. (5) Tung, Y. S.; Fina, L. J.; Gao, T.; Rosen, M. J.; Valentini, J. E. Polym. Mater. Sci. Eng. 1991, 65, 308. (6) Grubb, S. G.; Kim, M. W.; Rasing, Th.; Shen, Y. R. Langmuir 1988, 4, 452. (7) Lu, J. R.; Thomas, R. K. The Structure of Surfactant Monolayers at the Air-Water Interface Studied by Neutron Reflection. In Applications of Neutron Scattering to Soft Condensed Matter; Gabrys, B. J., Ed.; Gordon and Breach Science Publishers: New York, 2000. (8) Barmentlo, M.; Vrehen, Q. H. F. Chem. Phys. Lett. 1993, 209, 347.
Langmuir, Vol. 19, No. 16, 2003 6437 thickness, 292 µm grid spacing, and 55 µm bar width) were obtained from Electon Microscopy Sciences (Fort Washington, PA). (11-Hydroxyundecyl)trimethylammonium bromide (HTAB) and N,N-dimethylferrocenylalkylammonium bromides (FCnAB, n ) 7, 12, 18) were available in our laboratory from previous studies.9,10 The structures of these surfactants and 5CB are shown in Figure 1. Dodecyl-1,12-bis(trimethylammonium bromide) (DBTAB) was prepared according to the methods of Menger and Wrenn.13 The DBTAB was then purified by extraction (3×) of the reactants into a hexane-acetone mixture (50/50 v/v) at 40-50°C. The purity and structure of the DBTAB were then verified by thin layer chromotography and NMR, respectively. NMR (methanol) δ 3.3 (4 H, CH2N+(CH3)3), 3.1 (18 H, N+(CH3)3), 1.8 (4 H, CH2CH2 N+(CH3)3), 1.3 (16 H, CH2CH2CH2). Preparation of Optical Cells. A detailed description of the methods used to prepare and examine the optical cells can be found in an earlier publication.1 Briefly, glass microscope slides were cleaned according to published procedures14 and coated with octadecyltrichlorosilane (OTS).1 The quality of the OTS layer was assessed by checking the alignment of the 5CB confined between two OTS-treated glass slides. Any surface not causing homeotropic anchoring of 5CB was discarded.15,16 Copper specimen grids that were cleaned sequentially in methylene chloride, ethanol, and methanol were then placed onto the surface of OTStreated glass slides. One microliter of 5CB was dispensed onto each grid, and the excess LC was removed by contacting a 25 µL capillary tube (Fisher) with the 5CB droplet on the grid. The optical cell was heated to ∼50 °C and then immediately immersed in the aqueous solution of interest held at room temperature (20 °C). Optical Examination of LC Textures. The orientation of 5CB within each optical cell was examined by using planepolarized light in transmission mode on an Olympus BX60 microscope with crossed polarizers. The cells were placed on a rotating stage located between the polarizers. Orthoscopic examinations were performed with the source light intensity set to 50% of full illumination and the aperture set to 10% so as to collimate the incident light. Homeotropic alignments of the LC were determined by first observing the absence of transmitted light during a 360° rotation of the sample. Insertion of a condenser below the stage and a Bertrand lens above the stage allowed conoscopic examination of the cell. An interference pattern consisting of two crossed isogyres indicated homeotropic alignment.17 In-plane birefringence was indicated by the presence of brush textures, typically four-brush textures emanating from a line defect, when the sample was viewed between crossed polarizers during orthoscopic examination.18 All images were captured using a digital camera (Olympus C-2040 Zoom) mounted on the microscope that was set to an f-stop of 2.6 and a shutter speed of 1/320 s. All images shown in this paper correspond to the equilibrium alignment of the LC (typically measured 5-15 min after exposure of the LC to the aqueous surfactant solution). The alignment was observed to be stable for at least 4 h, typically >24 h. Preparation of Ferrocenyl Surfactant Solutions. The ferrocenyl surfactants (FTMA and FCnAB) were prepared in their reduced state in deaerated 0.1 M Li2SO4 adjusted to pH 2 by the addition of sulfuric acid. Experiments using the reduced ferrocenyl surfactant were conducted immediately after dissolution (9) Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 4116. (10) Aydogan, N.; Rosslee, C. A.; Abbott, N. L. Colloids Surf., A 2002, 201, 101. (11) Bennett, D. E.; Gallardo, B. S.; Abbott, N. L. J. Am. Chem. Soc. 1996, 118, 6499. (12) Aydogan, N.; Gallardo, B. S.; Abbott, N. L. Langmuir 1999, 15, 722. (13) Menger, W. R.; Wrenn, S. J. Phys. Chem. 1974, 78, 1387. (14) Skaife, J. J.; Abbott, N. L. Chem. Mater. 1999, 11, 612. (15) Cognard, J. Mol. Cryst. Liq. Cryst. 1982, 1 (Suppl.), 1. (16) Yang, J. Y.; Mathauer, K.; Frank, C. W. Microchemistry 1994, 441. (17) Bloss, F. D. An Introduction to the Methods of Optical Crystallograpy; Holt, Rinehart and Winston: New York, 1961. (18) Sonin, A. A. Freely Suspended Liquid Crystalline Films; John Wiley & Sons: New York, 1998.
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Figure 3. Optical images of 5CB confined to a copper grid that was supported on an OTS-coated glass slide and contacted with solutions of (A) DBTAB (10 mM) and (B) HTAB (10 mM) in 0.1 M Li2SO4 at pH 2. Optical examination was performed orthoscopically using crossed nicols and a 4× objective. All scale bars represent 300 µm.
Anchoring of 5CB by FTMA. Past studies have demonstrated control of the surface excess concentration of FTMA at an air-water interface by manipulation of the oxidation state of the ferrocene moiety.19 At bulk concentrations between 0.001 and 10 mM, the equilibrium surface tensions and surface excess concentrations of solutions of FTMA and oxidized FTMA were measured to vary significantly. Here we report measurements of the anchoring of 5CB in contact with aqueous solutions of FTMA and oxidized FTMA as a function of their bulk concentrations. The optical textures of 5CB following the adsorption of FTMA (0.01 mM) and oxidized FTMA (0.01 mM) to the aqueous-5CB interface are shown in panels A and B of Figure 2, respectively. Brushlike textures, particularly four-brush textures, emanating from a single line defect are observed within most of the grid squares. These textures indicate an alignment of 5CB that is parallel to the interface between 5CB and the aqueous solution of FTMA and oxidized FTMA (planar anchoring). Initially, we found this result to be surprising because (I) past studies by us1 and others2 have revealed the orientation of 5CB to be influenced by the adsorption of SDS at the aqueous-LC interface and (II) our past studies demonstrate the surface tension/surface excess concentration of FTMA to undergo large changes upon oxidation of the ferrocene group of FTMA.9,19 Under the conditions of the experiments shown in Figure 2A (0.01 mM FTMA) and Figure 2B (0.01 mM oxidized FTMA), the oxidationinduced difference in surface tension of an air-water
interface is ∼11 mN/m while the difference in surface excess concentration is ∼2 µmol/m2.9 The observation of planar anchoring of 5CB upon exposure to a 0.01 mM solution of oxidized FTMA is consistent with a low level of adsorption of oxidized FTMA such that 5CB is contacting a largely surfactant-free interface. However, when the ferrocene is in a reduced state, the 5CB-aqueous interface is expected to be nearly saturated with FTMA. Under these conditions, we also observed the anchoring of the 5CB to be planar. In contrast, at bulk concentrations of SDS that lead to high surface coverages of SDS at airwater and oil-water interfaces, the orientation of 5CB is observed to be homeotropic.1 Measurements of the orientation of 5CB performed at other bulk concentrations of FTMA (0.001-10 mM) and oxidized FTMA (0.001-10 mM) failed to reveal the presence of homeotropic anchoring of 5CB. Anchoring of 5CB by Bolaform Surfactants. The results above led us to hypothesize that the apparent insensitivity of the anchoring of 5CB to the surface coverage of FTMA (in both oxidation states) was a manifestation of the conformations assumed by FTMA and oxidized FTMA at the 5CB-water interface. Past studies have shown that both FTMA and oxidized FTMA adopt a looped configuration at the air-water interface (Figure 2C).9,12 In contrast, classical surfactants such as SDS orient at air-water and oil-water interfaces with their tails in extended conformations (tilted relative to the interface).4-7 To test the role of the looped conformation of a bolaform surfactant in determining the orientation of 5CB at an aqueous interface, we measured the orientation of 5CB at interfaces with aqueous solutions containing one of two bolaform surfactants, dodecyl-1,12-bis(trimethylammonium bromide) (DBTAB) or (11-hydroxyundecyl)trimethylammonium bromide (HTAB) (Figure 1A). These surfactants adopt looped configurations at airwater interfaces.9,13 First, we examined DBTAB, which has been found to have a limiting surface area at an air-water interface of ∼107 Å2.13 This value is much greater than the limiting surface area of analogous classical surfactants (e.g., ∼63 Å2 for DTAB) which adopt tilted configurations at airwater interfaces.19 Because the free energy of hydration of the trimethylammonium moieties (ca. -100 kJ/mol)20 is large, we expect the conformation of DBTAB at the 5CB-water interface to be similar to that at the airwater interface. We measured the alignment of 5CB in contact with aqueous solutions of DBTAB to be planar at all concentrations studied (0.01-100 mM, Figure 3A). This concentration range spans the critical micelle concentration (cmc) of DBTAB (20-50 mM) in addition to being
(19) Gallardo, B. S.; Hwa, M. J.; Abbott, N. L. Langmuir 1995, 11, 4209.
(20) Ben-Naim, A. Solvation Thermodynamics; Plenum Press: New York, 1987.
Figure 2. (A and B) Optical images of 5CB confined to a copper grid that was supported on an OTS-coated glass slide and contacted with aqueous solutions of FTMA and oxidized FTMA at various bulk concentrations: (A) 0.01 mM FTMA in 0.1 M Li2SO4 at pH 2; (B) 0.01 mM oxidized FTMA in 0.1 M Li2SO4 at pH 2. Optical examination was performed orthoscopically using crossed nicols and a 4× objective. All scale bars represent 300 µm. (C) Proposed representative configuration of FTMA and oxidized FTMA at the air-water interface. of the FTMA so as to minimize oxidation of the ferrocene. Oxidized solutions of the ferrocenyl surfactants were prepared by electrochemical oxidation using a bipotentiostat (Princeton Applied Research) set at +0.350 V for g3 h. The working electrode and counter electrode were platinum gauze, and the reference electrode was saturated calomel. The progress and extent of oxidation of the ferrocene were monitored by visible spectrophotometry. The reduced ferrocene absorbs strongly at 630 nm while the oxidized ferrocene absorbs strongly at 440 nm. During the time course of the experiments reported in this paper, the oxidation state of the ferrocene changed by less than 2%.
Results and Discussion
Surfactant Effects on Liquid Crystal Orientations
well above the observed onset of surface activity for DBTAB at an air-water interface (10 116 20-50 107
FTMA Oxidized FTMA FC12AB
0.1 M Li2SO4, pH 2 (25 °C) 0.1 M Li2SO4, pH 2 (25 °C) 0.1 M Li2SO4, pH 2 (25 °C)
a
Ferrocenyl Surfactants 0.1 85 >30 75 0.2 69
Conset mM
γlim (mN/m)
ref
0.1 10 0.05 0.0005
