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An Experimental System for Imaging the Reversible Adsorption of Amphiphiles at Aqueous-Liquid Crystal Interfaces Jeffrey M. Brake and Nicholas L. Abbott* Department of Chemical Engineering, University of WisconsinsMadison, 1415 Engineering Drive, Madison, Wisconsin 53706 Received December 3, 2001. In Final Form: April 26, 2002 We report an experimental system that permits optical imaging of the reversible adsorption of amphiphiles at stable, planar interfaces formed between aqueous phases and immiscible thermotropic liquid crystals. Copper grids (hole sizes of 19-292 µm and thicknesses of 18-20 µm) supported on glass surfaces treated using octadecyltrichlorosilane were impregnated with nematic 4-cyano-4′-pentylbiphenyl (5CB). Films of 5CB confined within the grids were stable (did not dewet the grid) to immersion into either water, aqueous solutions of sodium dodecyl sulfate (SDS), or aqueous solutions of SDS containing NaCl. Whereas the anchoring of 5CB on the copper grid dominated the appearance of the 5CB when using grids with hole sizes of 19 µm, reversible changes in the orientation of the liquid crystal (observed using polarized light) caused by adsorption of SDS at the liquid crystal-aqueous interface were readily observed when using grids with hole sizes larger than 19 µm. With increasing concentrations of SDS in the aqueous phase, a series of reproducible and distinct surface-driven distortions were observed within the 5CB confined to the grid. We also observed the effects of added NaCl on the distortions induced within the liquid crystal to be consistent with increased adsorption of SDS caused by screening of the electrostatic interactions between the SDS adsorbed at the interface. This result suggests that the orientation of the 5CB is influenced by the areal density of SDS molecules at the liquid crystal-aqueous interface (probably through steric interactions). Because the aqueous phase contacting the liquid crystal can be exchanged (thus permitting the addition and removal of reactants), this experimental system is a simple and broadly useful one for investigations in which liquid crystals are used to amplify interfacial phenomena at fluid interfaces into optical images.
Introduction This paper reports a simple and broadly useful experimental system for the study of interfacial phenomena (e.g., adsorption and association) at fluid interfaces that is based on the orientational behavior of liquid crystals (LCs) (Figure 1).1,2 The approach permits the formation of stable and approximately planar interfaces between a waterimmiscible LC and aqueous phases that can be readily exchanged. The planar interface permits straightforward interpretation of the anchoring of the LC at the interface that results from reversible adsorption phenomena. Whereas past studies have demonstrated that LCs can be used to amplify biomolecular interactions occurring on the surfaces of solids that present covalently immobilized receptors,3-6 the lateral mobility of species within biological membranes is essential for many types of biological function (e.g., association of proteins involved in signal transduction across membranes; complexation of enzymes and substrates within biological membranes).7 The work reported here, therefore, aims to establish an experimental system that permits the use of LCs to amplify binding events occurring at fluid interfaces. Here we report the development of an experimental system that permits the * To whom correspondence should be addressed. E-mail: abbott@ engr.wisc.edu. Fax: 608-262-5434. (1) Cognard, J. Mol. Cryst. Liq. Cryst. 1982, 1 (Suppl.), 1. (2) Je´roˆme, B. Rep. Prog. Phys. 1991, 54, 391. (3) Gupta, V. K.; Skaife, J. J.; Dubrovsky, T. B.; Abbott, N. L. Science 1998, 279, 2077. (4) Skaife, J. J.; Abbott, N. L. Langmuir 2000, 16, 3529. (5) Skaife, J. J.; Brake, J. M.; Abbott, N. L. Langmuir 2001, 17, 5448. (6) Skaife, J. J.; Abbott, N. L. Langmuir 2001, 17, 5595. (7) Stryer, L. Biochemistry, 4th ed.; W. H. Freeman and Co.: New York, 1995.
Figure 1. Schematic illustration of the alignment of 5CB induced by adsorption of SDS at the 5CB-aqueous interface: (A) SDS-free interface; (B) SDS-laden interface.
use of LCs to report reversible adsorption at fluid interfaces by using sodium dodecyl sulfate as a model adsorbate.8-10 The approach reported in this paper builds from several past studies.8-18 First, a number of past investigations (8) Poulin, P.; Weitz, D. A. Phys. Rev. E 1998, 57, 626. (9) Poulin, P.; Stark, H.; Lubensky, T. C.; Weitz, D. A. Science 1997, 275, 1770. (10) Drzaic, P. S. Liquid Crystal Dispersions, Series on Liquid Crystals; World Scientific: Singapore, 1995; Vol. 1. (11) Sonin, A. A. The Surface Physics of Liquid Crystals; Gordon and Breach: New York, 1995. (12) Perez, E.; Proust, J. E.; Ter-Minassian-Saraga, L. Colloid Polymer Sci. 1978, 256, 784. (13) de Mul, M. N. G.; Mann, J. A., Jr. Langmuir 1994, 10, 2311.
10.1021/la011746t CCC: $22.00 © 2002 American Chemical Society Published on Web 06/27/2002
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have reported on the influence of surfactants (including SDS) on the orientations of LCs that possess interfaces with aqueous systems by using emulsions of aqueous solutions and nematic LCs.8-11 While the optical appearance of the nematic droplets does reveal the orientation of the LC at the interface with the aqueous phase, the complex geometry makes interpretation of the orientation less obvious than when using a planar interface.10 The stability of the emulsions also depends strongly on the anchoring of the LC as well as the presence and type of surfactants present.8-10 The experimental system we report here permits observation of the orientation of the LC at a planar interface with an aqueous phase. Second, several groups have described the orientations of thin films12-14 and droplets15 of LC deposited onto the surface of water by using optical microscopy12,15 and nonlinear optical methods.13,14 These films, however, are not stable upon introduction of surfactants into the aqueous subphase; the LC dewets the aqueous subphase (see below). To address this issue in studies of the structure of films of smectic LCs, Kraus et al. reported a simple and elegant experimental system that permits observation of the orientation of a 6 mm diameter, freely suspended film (thickness of hundreds of nanometers or less) of smectic LC that is lowered into contact with a droplet of an immiscible liquid (e.g., water, glycerol).16 Whereas smectic LCs readily form freely suspended films, nematics do not form films with comparable stability.17 Thus here we report an experimental geometry in which nematic films are stabilized by confinement within copper grids and contact with a supporting solid surface. Our use of copper grids builds on the report of Nazarenko and Nych who reported observations of nematic phases of 4-cyano-4′-pentylbiphenyl (5CB) confined to copper grids and in contact with air (not aqueous solutions).18 Experimental Section Materials. Sodium dodecyl sulfate (SDS) at 99+% purity, hexadecanethiol, and aluminum oxide (activated, 50-200 µm, neutral) were obtained from Sigma (St. Louis, MO). The SDS was initially purified by recrystallization from ethanol (Aaper Alcohol and Chemical Co., Shelbyville, KY).19 However, the purification had no measurable effect on the results reported in this paper. 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), sodium chloride, and heptane were all obtained from Fisher Scientific (Pittsburgh, PA). The 5CB was purchased from EM Sciences (New York, NY). The glass microscope slides were Fisher’s Finest, Premium Grade obtained from Fisher. Titanium (99.99+%) and gold (99.999%) were obtained from International Advanced Materials (New York, NY). Copper specimen grids with thicknesses of 18-20 µm, hole sizes of 19, 55, 115, and 292 µm, and bar widths of 6, 7, 10, and 48 µm, respectively, were obtained from Electron Microscopy Sciences (Fort Washington, PA). Treatment of Glass Microscope Slides with OTS. Glass microscope slides were cleaned according to a procedure detailed in an earlier publication.20 Briefly, the slides were immersed in a piranha solution (70% (v/v) sulfuric acid and 30% (v/v) hydrogen peroxide) for 1 h at approximately 80 °C (Warning: piranha (14) La¨uger, J.; Robertson, C. R.; Frank, C. W.; Fuller, G. G. Langmuir 1996, 12, 5630. (15) Press, M. J.; Arrott, A. S. Phys. Rev. Lett. 1974, 33, 403. (16) Kraus, I.; Pieranski, P.; Demikhov, E.; Stegemeyer, H.; Goodby, J. Phys. Rev. E 1993, 48, 1916. (17) Sonin, A. A. Freely Suspended Liquid Crystalline Films; John Wiley & Sons: New York, 1998. (18) Nazarenko, V.; Nych, A. Phys. Rev. E 1999, 60, R3495. (19) Shin, J. Y.; Abbott, N. L. Langmuir 1999, 15, 4404. (20) Skaife, J. J.; Abbott, N. L. Chem. Mater. 1999, 11, 612.
Brake and Abbott solution reacts strongly with organic compounds and should be handled with extreme caution; do not store the solution in closed containers.) The slides were then rinsed with water, ethanol, and methanol and dried under a stream of gaseous nitrogen followed by heating to ∼110 °C for >2 h prior to OTS deposition. A 0.5 mM OTS solution was prepared by adding OTS to heptane that was dried by passage through an aluminum oxide column. The column was approximately 10 cm high and 5 cm in diameter and was equipped with a glass wool frit to prevent leakage of alumina into the solvent. Hydrostatic pressure was used to flow the heptane through the column. The slides were immersed in the 0.5 mM OTS in heptane solution for 30 min at room temperature. They were then rinsed with methylene chloride and dried under nitrogen. The quality of the OTS layer was tested by forming a sandwich of treated OTS slides spaced by ∼15 µm using Saran wrap spacers. 5CB was introduced between the slides and the resulting optical texture was examined using polarized light to confirm homeotropic anchoring.1,21 Any sample not exhibiting homeotropic anchoring of 5CB was rejected. Formation of Monolayers of Alkanethiols on Films of Gold. Glass slides were cleaned as described above. Gold was deposited onto these slides by physical vapor deposition using an electron beam evaporator (model VES-3000-C, TekVak Industries Inc., Brentwood, NY). An 8 nm layer of titanium was first deposited to promote adhesion of a 20 nm thick film of gold to the glass slide. The deposition was performed obliquely at an angle of 45° with respect to the normal to promote uniform azimuthal anchoring of 5CB.22 The gold-coated slides were then immersed in an ethanolic solution containing 1 mM hexadecanethiol for 1 h. After removing the slides from the solution, they were rinsed in ethanol and dried under nitrogen. Each monolayer formed from an alkanethiol was tested by forming a sandwich from two alkanethiol-treated slides spaced by ∼15 µm using Saran wrap spacers. 5CB was injected between the slides, and the anchoring of the 5CB was examined by polarized light microscopy. Each sample that did not demonstrate uniform planar anchoring of the LC with an azimuthal orientation parallel to the direction of deposition of the gold was rejected.22,23 Formation of a monolayer was confirmed by ellipsometry (Rudolph Auto EL, Flanders, NJ). The details of the ellipsometric measurements can be found elsewhere.3 Unconfined LC in Contact with an Aqueous Phase. Two systems were explored in which the LC was unconfined. First, 5 µL droplets of 5CB were placed onto the surface of an aqueous phase in a glass dish with a diameter of 60 mm. The thickness of the fully spread film was calculated to be ∼2 µm. Second, 5 µL droplets of 5CB were placed on glass slides treated with OTS or gold films treated with hexadecanethiol, as described above. The slides hosting 5CB were subsequently submerged into aqueous solutions of SDS. In both cases, the SDS concentration of the aqueous phase ranged from 0 to 10 mM. Preparation of Optical Cells. The copper specimen grids were cleaned sequentially in methylene chloride, ethanol, and methanol, dried under nitrogen, and then heated at 110 °C for 24 h. The grids were then placed onto the surface of either OTStreated glass or hexadecanethiol-treated gold. 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. This procedure led to the formation of a stable film of 5CB within the grid. The surface of the 5CB in contact with the aqueous phase was approximately flat as determined by concurrent focus of the grid and 5CB under an optical microscope at objective powers ranging from 4× to 50×. Optical Cells Formed with Two Solid Surfaces. Optical cells formed with two solid surfaces were prepared by placing a second solid surface (OTS-treated glass or alkanethiol-treated gold) on top of the 5CB confined to the grids. The two surfaces were then secured using binder clips. The optical cell was heated to ∼50 °C (above the nematic-isotropic transition temperature of 5CB of ∼35 °C)4 using a heat gun to remove the effects of flow of 5CB during formation of the optical cell. (21) Yang, J. Y.; Mathauer, K.; Frank, C. W. Microchemistry 1994, 441. (22) Gupta, V. K.; Abbott, N. L. Langmuir 1996, 12, 2587. (23) Gupta, V. K.; Abbott, N. L. Phys. Rev. E 1996, 54, R4540.
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Figure 3. Optical images of 5CB deposited onto the surface of (A) water and (B) water containing 10 mM SDS. The images were obtained by orthoscopic examination between crossed nicols using a 4× objective.
Figure 2. Schematic representation of the procedure used to prepare the experimental system reported in this paper. Optical Cells Exposed to an Aqueous Phase. The 5CB impregnated grid supported on a solid surface was quickly (
