Langmuir 2005, 21, 6805-6814
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Influence of Surfactant Tail Branching and Organization on the Orientation of Liquid Crystals at Aqueous-Liquid Crystal Interfaces Nathan A. Lockwood, Juan J. de Pablo, and Nicholas L. Abbott* Department of Chemical & Biological Engineering, University of WisconsinsMadison, 1415 Engineering Drive, Madison, Wisconsin 53706 Received January 26, 2005. In Final Form: April 22, 2005 We have examined the influence of two aspects of surfactant structurestail branching and tail organizationson the orientational ordering (so-called anchoring) of water-immiscible, thermotropic liquid crystals in contact with aqueous surfactant solutions. First, we evaluated the influence of branches in surfactant tails on the anchoring of nematic liquid crystals at water-liquid crystal interfaces. We compared interfaces that were laden with one of three linear surfactants (sodium dodecyl sulfate, sodium dodecanesulfonate, and isomerically pure linear sodium dodecylbenzenesulfonate) to interfaces laden with branched sodium dodecylbenzenesulfonate. We carried out these experiments at 60 °C, above the Krafft temperatures of all the surfactants studied, and used the liquid crystal TL205 (a mixture of cyclohexanefluorinated biphenyls and fluorinated terphenyls), which forms a nematic phase at 60 °C. Linear surfactants caused TL205 to assume a perpendicular orientation (homeotropic anchoring) above a threshold concentration of surfactant and parallel orientation (planar anchoring) at lower concentrations. In contrast, branched sodium dodecylbenzenesulfonate caused planar anchoring of TL205 at all concentrations up to the critical micelle concentration of the surfactant. Second, we used sodium dodecanesulfonate and a commercial linear sodium dodecylbenzenesulfonate to probe the influence of surfactant tail organization on the orientations of liquid crystals at water-liquid crystal interfaces. Commercial linear sodium dodecylbenzenesulfonate, which comprises a mixture of ortho and para isomers, has been previously characterized to form less ordered monolayers than sodium dodecanesulfonate at oil-water interfaces at room temperature. We found sodium dodecanesulfonate to cause homeotropic anchoring of both TL205 and 4′-pentyl-4cyanobiphenyl (5CB, nematic at room temperature), whereas commercial linear sodium dodecylbenzenesulfonate caused predominantly planar and tilted orientations of both TL205 and 5CB. These results, when combined, lead us to conclude that (1) interactions between the aliphatic tails of surfactants and liquid crystals largely dictate the orientations of liquid crystals at aqueous-liquid crystal interfaces, (2) the interactions that orient the liquid crystals at these interfaces are sensitive to the branching and degree of disorder in the surfactant tails, and (3) differences in the chemical composition of TL205 and 5CB, most notably fluorination of TL205, lead to subtle differences in the orientations of these two nematic liquid crystals.
Introduction The orientations assumed by thermotropic liquid crystals near interfaces are known to be sensitive to the nature of the interactions between the mesogens forming the liquid crystal and the confining medium.1,2 This phenomenon, which is typically controlled by energetics on the order of 10-2-10-3 J/m2, is referred to as the anchoring of liquid crystals by interfaces.2 Depending on the structure of the interface, the liquid crystal may align normal to the interface (homeotropic anchoring), parallel to the interface (planar anchoring), or at an angle relative to the interface (tilted anchoring). The anchoring of liquid crystals in contact with a wide variety of solids and chemically modified solids has been studied.2 Factors that influence the anchoring of liquid crystals at solid interfaces include the nanometer- to micrometer-scale topography of the interface,3 intermixing of mesogens and monolayerdecorated interfaces,4 and interactions such as hydrogen bonding between mesogens and the interface.1,2 * To whom correspondence should be addressed: e-mail
[email protected]; fax (608) 262-5434. (1) Cognard, J. Mol. Cryst. Liq. Cryst. 1982, 1, 1-74. (2) Je´roˆme, B. Rep. Prog. Phys. 1991, 54, 391-451. (3) Janning, J. L. Appl. Phys. Lett. 1972, 21, 173-174. (4) Hiltrop, K.; Stegemeyer, H. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 582-588.
In contrast to the widely studied behavior of liquid crystals on solid surfaces, the deformable and fluid nature of interfaces between thermotropic liquid crystals and immiscible aqueous phases leads to new types of interfacial phenomena and possible mechanisms of anchoring of liquid crystals.5-9 Several past studies have reported on the influence of surfactants on the orientations of liquid crystals when the surfactants are adsorbed at aqueousliquid crystal interfaces.5-9 For example, water-liquid crystal-surfactant emulsions and inverted emulsions have been used to characterize the orientations of liquid crystals at aqueous-liquid crystal interfaces.7-9 Studies of emulsions revealed that the type and strength of anchoring at the aqueous-liquid crystal interface determines the director profile and the types of topological defects formed in the liquid crystals.9 The anchoring of liquid crystals at the interface of aqueous droplets dispersed in liquid crystals also leads to the ordering of (5) Brake, J. M.; Abbott, N. L. Langmuir 2002, 16, 6101-6109. (6) Brake, J. M.; Mezera, A. D.; Abbott, N. L. Langmuir 2003, 16, 6436-6442. (7) Drzaic, P. S. Liquid Crystal Dispersions; Series on Liquid Crystals; World Scientific: Singapore, 1995. (8) Poulin, P.; Stark, H.; Lubensky, T. C.; Weitz, D. A. Science 1997, 275, 1770-1773. (9) Mondain-Monval, O.; Dedieu, J. C.; Gulik-Krzywicki, T.; Poulin, P. Eur. Phys. J. B 1999, 12, 167-170.
10.1021/la050231p CCC: $30.25 © 2005 American Chemical Society Published on Web 06/16/2005
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the water droplets due to interactions mediated by topological defects in the director profile of the liquid crystals.8 A second approach that permits investigation of the anchoring of liquid crystals at aqueous-liquid crystal interfaces relies on the creation of approximately planar interfaces between aqueous phases and liquid crystals.5 This geometry enables study of the reversible adsorption of surfactants and other amphiphiles at aqueous-liquid crystal interfaces and permits quantification of the orientations of the liquid crystals with standard polarized light microscopy. Past studies of planar aqueous-liquid crystal interfaces focused on the anchoring of 4′-pentyl4-cyanobiphenyl (5CB) in the presence of linear anionic, cationic, and nonionic surfactants.5,6,10 The linear surfactants used in these past studies possessed one hydrophilic headgroup and one hydrophobic tail and are known to adopt tilted configurations at air-water and oil-water interfaces.11-15 Sum frequency generation15 and optical second harmonic generation12 studies have shown that the tilt angles of the surfactant tails vary strongly with the surface excess concentration of the surfactant15 as well as the type of interface (e.g., oil-water or air-water).12 In general, as the density of surfactant adsorbed at the oil-water interface increases, the average tilt of the surfactant tails approaches the interface normal.15 At the water-5CB interface, the anchoring of 5CB changes from planar to homeotropic with increasing interfacial density of linear surfactant adsorbed at the interface.5,6,8 The dependence of anchoring on interfacial density of adsorbed surfactant concentration has recently been described within the context of a thermodynamic framework.16 Past studies have also explored the anchoring of 5CB at planar aqueous-5CB interfaces in the presence of bolaform surfactants, which have two hydrophilic headgroups linked by a hydrophobic tail.6 At air-water and oil-water interfaces, bolaform surfactants adopt looped conformations independent of the interfacial density of the surfactant at the interface.17,18 In contrast to linear surfactants with a single headgroup, bolaform surfactants cause planar anchoring of 5CB at all areal densities of adsorbed surfactant.6 These past studies5,6 and others19 suggest that interactions between the tails of adsorbed surfactants and mesogens largely dictate the orientations of liquid crystals. For example, orientational transitions have been observed in mixed Langmuir monolayers of 4′-octyl-4-cyanobiphenyl (8CB) and pentadecanoic acid on water.19 Second-harmonic generation and surface pressure measurements revealed that compression of the mixed monolayers results in a decrease of the tilt angle (measured from the interface normal) of 8CB and the formation of multilayers of 8CB. The change in tilt angle of the 8CB was attributed to interactions between 8CB and the tails of the pentadecanoic acid.19 (10) Brake, J. M.; Mezera, A. D.; Abbott, N. L. Langmuir 2003, 21, 8629-8637. (11) Fina, L. J.; Valentini, J. E.; Tung, Y. S. ACS Symp. Ser. 1994, 581, 44-54. (12) Grubb, S. G.; Kim, M. W.; Rasing, T.; Shen, Y. R. Langmuir 1988, 4, 452-454. (13) Lu, J. R.; Thomas, R. K. ACS Symp. Ser. 1995, 615, 342-354. (14) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143-304. (15) Messmer, M. C.; Conboy, J. C.; Richmond, G. L. J. Am. Chem. Soc. 1995, 117, 8039. (16) Rey, A. D. Langmuir 2004, 20, 11473-11479. (17) Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 4116-4124. (18) Menger, W. R.; Wrenn, S. J. Phys. Chem. 1974, 78, 1387. (19) Barmentlo, M.; Vrehen, Q. H. F. Chem. Phys. Lett. 1993, 209, 347-351.
Lockwood et al.
In this paper, we report the results of an investigation that sought to further our understanding of the relationship between surfactant molecular structure and the orientations of liquid crystals at aqueous-liquid crystal interfaces.6,10 Our investigation focused on the effects of branching of surfactant tails as well as surfactant tail organization at aqueous-liquid crystal interfaces. We note here that similar investigations of the branching of polymer side chains in polymer-dispersed liquid crystal systems have been reported.20 Polymers of 1-methylheptyl acrylate caused planar anchoring of the liquid crystals 5CB and TL205 (a mixture of cyclohexane fluorinated biand terphenyls) at all temperatures studied. In contrast, polymers with branches at any other position on the side chain (e.g., 3-methylheptyl acrylate) caused homeotropic anchoring of 5CB and TL205 below a defined homeotropicplanar transition temperature. The authors suggest that the conformation of 1-methylheptyl acrylate limits the extent of side-chain packing relative to the other branched isomers and that the differences in packing are reflected in the anchoring of 5CB and TL205 by the polymers. The studies of the influence of the organization of surfactant tails on the anchoring of liquid crystals that we report in this paper are founded in part on the results of past work by Richmond and co-workers, who used infrared-visible sum frequency vibrational spectroscopy to characterize the organization of surfactants at wateroil interfaces.21,22 A study by Watry and Richmond22 reported on the order of the aliphatic tails of linear sodium dodecanesulfonate (LDS) and commercial linear dodecylbenzenesulfonate (L-DBS) at planar water-CCl4 interfaces. The aliphatic tails of LDS at the water-CCl4 interface displayed increasing ordersthat is, a tendency toward an extended, all-trans conformationswith increasing bulk LDS concentration. In contrast, the aliphatic tail of commercial L-DBS at the water-CCl4 interface assumed a disordered state at all bulk commercial L-DBS concentrations. The authors attribute the disorder in commercial L-DBS tails to a staggering of the surfactant at the water-CCl4 interface caused by the phenyl ring in the surfactant headgroup. The past studies described above, when combined, led us to design experiments to test two hypotheses regarding the effect of surfactant structure and organization on the anchoring of liquid crystals at aqueous-liquid crystal interfaces. First, we hypothesized that surfactant molecules with branched aliphatic tails would cause planar rather than homeotropic anchoring of the liquid crystal because branched surfactants cannot pack as efficiently as linear surfactants at the aqueous-liquid crystal interface. This difference in packing is analogous to decreased melting temperatures in branched versus linear alkanes.23 Second, we hypothesized that surfactants which have been reported to assume disordered states at aqueous-oil interfaces (such as commercial L-DBS studied by Watry and Richmond22) would likewise cause planar anchoring of a liquid crystal. We tested our first hypothesis by comparing the anchoring of liquid crystals caused by linear and branched surfactants at the water-liquid crystal interface. In these experiments, we used the linear surfactants sodium dodecyl sulfate, sodium dodecanesulfonate, and isomerically pure (para) linear sodium (20) Zhou, J.; Collard, D. M.; Park, J. O.; Srinivasarao, M. J. Am. Chem. Soc. 2002, 34, 9980-9981. (21) Conboy, J. C.; Messmer, M. C.; Richmond, G. L. Langmuir 1998, 23, 6722-6727. (22) Watry, M., R.; Richmond, G. L. J. Am. Chem. Soc. 2000, 122, 875-883. (23) Burch, K. J.; Whitehead, E. G., Jr. J. Chem. Eng. Data 2004, 49, 858-863.
Surfactant Effects on Liquid Crystal Orientation
Figure 1. Chemical structures of surfactants and liquid crystals used in this work. Surfactants include sodium dodecyl sulfate (SDS), linear dodecanesulfonate (LDS), linear dodecylbenzenesulfonate (L-DBS), and branched dodecylbenzenesulfonate (BR-DBS). Liquid crystalline materials are TL205 and 4′-pentyl-4-cyanobiphenyl (5CB). The liquid crystal TL205 is a proprietary mixture of cyclohexane-fluorinated biphenyls and fluorinated terphenyls; a representative structure is shown.
dodecylbenzenesulfonate (SDS, LDS, and pure L-DBS, respectively) and the branched surfactant sodium dodecylbenzenesulfonate (BR-DBS; Figure 1). We evaluated our second hypothesis by using LDS and a commercial version of L-DBS (obtained from the same source used by Watry and Richmond21,22 and which we report in this paper to comprise a mixture of ortho and para isomers) to probe the influence of surfactant tail organization on the orientations assumed by liquid crystals at the waterliquid crystal interface. Finally, we report a comparison of the anchoring of the liquid crystals 5CB and TL205 at surfactant-laden interfaces to assess the dependence of our conclusions on the chemical composition of these two nematic liquid crystals. Materials and Methods Materials. Sodium dodecyl sulfate (SDS) was obtained from Sigma-Aldrich (St. Louis, MO). Linear dodecanesulfonate (LDS) and commercial linear dodecylbenzenesulfonate (L-DBS) were obtained from TCI America (Portland, OR). SDS was recrystallized from ethanol (Aaper Alcohol and Chemical Co., Shelbyville, KY),24 and LDS was recrystallized from hot water. Branched dodecylbenzenesulfonate (BR-DBS) was obtained as an acidified paste (Sulfonic 100) from Stepan (Northfield, IL) and was neutralized with NaOH prior to use. Thin-layer chromatography showed a single component after neutralization and separation. Octadecyltrichlorosilane (OTS), methanol, methylene chloride, sulfuric acid, hydrogen peroxide (30% w/v), and heptane were obtained from Fisher Scientific (Pittsburgh, PA). The liquid crystals TL205, a proprietary mixture of cyclohexane-fluorinated biphenyls and fluorinated terphenyls, and 4-pentyl-4′-cyanobiphenyl (5CB) were obtained from EMSciences (New York). (24) Shin, J. Y.; Abbott, N. L. Langmuir 1999, 15, 4404-4410.
Langmuir, Vol. 21, No. 15, 2005 6807 Deionization of a distilled water source was performed with a Milli-Q system (Millipore, Bedford, MA) to give water with a resistivity of 18.2 MΩ‚cm. Glass microscope slides were Fisher’s Finest Premium Grade obtained from Fisher Scientific. Gold specimen grids (bars 20 µm thick and 55 µm wide, spaced 283 µm apart) were obtained from Electron Microscopy Sciences (Fort Washington, PA). An isomerically pure (para) form of linear dodecylbenzenesulfonate (pure L-DBS) was prepared by the direct sulfonation of phenyldodecane with chlorosulfonic acid (both from SigmaAldrich, Milwaukee, WI).25 Phenyldodecane was dissolved in a small quantity of chloroform and cooled in an ice bath under a nitrogen atmosphere. A stoichiometric quantity of chlorosulfonic acid was added dropwise and the solution was stirred for 1 h. The chloroform was removed under reduced pressure. The resulting alkylbenzenesulfonic acid was dissolved in water and neutralized with NaOH to produce the sodium salt. The product was purified by recrystallization from hot water. NMR and mass spectrometry confirmed the product as linear dodecylbenzenesulfonate. 1H NMR (DMSO, 300 MHz, ppm): δ 0.8 (t, CH3-, 3H), 1.2 [br s, CH3-(CH2)10-CH2-, 20H], 1.5 [t, CH3-(CH2)10-CH2-, 2H], 7.1 (d, phenyl ring, ortho to sulfonate, 2H), 7.5 (d, phenyl ring, meta to sulfonate, 2H). Electrospray TOF mass spectrometry: [M]- 325.3, expected 325.5. Preparation of Optical Cells. A detailed description of the methods used to prepare and examine the liquid crystal hosted within optical cells can be found in a previous publication.5 Briefly, glass microscope slides were cleaned according to published procedures26 and coated with octadecyltrichlorosilane (OTS).5 The quality of the OTS layer was assessed by checking the alignment of 5CB confined between two OTS-treated glass slides. Any surface not causing homeotropic anchoring of 5CB was discarded.1,27 Gold specimen grids that were cleaned sequentially in methylene chloride, ethanol, and methanol were placed onto the surface of OTS-treated glass slides. Approximately 1 µL of TL205 or 5CB was dispensed onto each grid and the excess liquid crystal was removed with a syringe. The liquid crystalimpregnated grid supported on an OTS-treated glass slide was quickly (
