Langmuir 2003, 19, 8629-8637
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Articles Active Control of the Anchoring of 4′-Pentyl-4-cyanobiphenyl (5CB) at an Aqueous-Liquid Crystal Interface By Using a Redox-Active Ferrocenyl Surfactant Jeffrey M. Brake, Andrew D. Mezera, and Nicholas L. Abbott* Department of Chemical and Biological Engineering, University of WisconsinsMadison, 1415 Engineering Drive, Madison, Wisconsin 53706 Received March 18, 2003. In Final Form: June 25, 2003 We report an experimental study that demonstrates control of the anchoring of a nematic liquid crystal (LC), 4′-pentyl-4-cyanobiphenyl (5CB), at an aqueous-LC interface by the reversible adsorption of a redoxactive surfactant, 11-(ferrocenylundecyl) trimethylammonium bromide (FTMA). Whereas the anchoring of 5CB was found to be insensitive to the oxidation state of FTMA when using aqueous solutions of FTMA, oxidation of FTMA in aqueous mixtures of cetyltrimethylammonium bromide (CTAB) and FTMA led to a near-planar-to-homeotropic transition in the orientation of 5CB. The change in orientation of 5CB was attributed to the competitive adsorption of CTAB (which causes homeotropic anchoring) and either FTMA or oxidized FTMA (both of which cause near-planar anchoring). Because oxidized FTMA is less surface active than FTMA, oxidation of FTMA in a mixture containing CTAB and FTMA leads to an increase in the surface concentration of CTAB. Reversible control of the anchoring of 5CB was achieved by sequential contact of the liquid crystal with aqueous solutions containing CTAB and FTMA and then CTAB and oxidized FTMA. The addition of chemical oxidizing or reducing agents was also observed to change the orientation of the liquid crystal. Time-dependent changes in the optical texture of 5CB were quantitatively interpreted in terms of the tilt angle of the 5CB at the aqueous-LC interface. These results suggest new principles for control of the anchoring of LCs at interfaces as well as new experimental approaches for the study of surfactant adsorption at liquid-liquid interfaces.
Introduction The work described in this paper was motivated by the goal of coupling the orientation of liquid crystals (LCs) to the oxidation states of redox-active surfactants that are dissolved in aqueous solutions contacting water-immiscible LCs. Past studies of the adsorption of the redoxactive surfactant 11-(undecylferrocenyl) trimethylammonium bromide (FTMA) at aqueous-air interfaces have revealed large changes in the surface tension and surface excess concentration of the surfactant as a function of the oxidation state of the ferrocene group of FTMA.1,2 Recently, however, studies of the anchoring of 4′-pentyl-4-cyanobiphenyl (5CB) in contact with solutions of either FTMA or oxidized FTMA revealed the orientation of the LC to be insensitive to the oxidation state of FTMA.3 The orientation of 5CB was found to be parallel to the interface for both oxidation states of FTMA. In contrast, aqueous solutions of sodium dodecyl sulfate are known to cause homeotropic (perpendicular) anchoring of 5CB at high concentrations and planar anchoring at low concentrations.4 The difference in the anchoring of 5CB caused by * To whom correspondence should be addressed. E-mail: abbott@ engr.wisc.edu. Fax: 608-262-5434. (1) Gallardo, B. S.; Hwa, M. J.; Abbott, N. L. Langmuir 1995, 11, 4209. (2) Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 4116. (3) Brake, J. M.; Mezera, A. D.; Abbott, N. L. Langmuir 2003, 19, 6436. (4) Brake, J. M.; Abbott, N. L. Langmuir 2002, 18, 6101.
SDS and FTMA was traced to the different configurations assumed by these two surfactants at the aqueous-LC interface. Surfactants such as FTMA and oxidized FTMA assume looped configurations that cause planar anchoring of 5CB, whereas classical surfactants have tails which extend into the 5CB (e.g., sodium dodecyl sulfate) and cause homeotropic anchoring.3,4 In this paper, we report a study that couples the orientation of 5CB to the oxidation state of FTMA by exploiting the competitive adsorption of FTMA from a mixed surfactant system containing cetyltrimethylammonium bromide (CTAB, which causes homeotropic anchoring) and FTMA (which causes nearplanar anchoring). Materials and Methods Materials. CTAB and lithium sulfate were obtained from Sigma-Aldrich (St. Louis, MO). FTMA was obtained from Dojindo (Japan). 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), hydrogen peroxide (30% w/v), and L-ascorbic acid were all obtained from Fisher Scientific (Pittsburgh, PA). 5CB was obtained from EM Sciences (New York, NY). All chemicals were used as obtained without further purification. The glass microscope slides were Fisher’s Finest Premium Grade obtained from Fisher. Copper specimen grids (18 µm thickness, 292 µm grid spacing, and 55 µm bar width) were obtained from Electron Microscopy Sciences (Fort Washington, PA). Preparation of Optical Cells. The details regarding the preparation, examination, and properties of the optical cells used in this study can be found in a past publication.4 Briefly, glass
10.1021/la034469u CCC: $25.00 © 2003 American Chemical Society Published on Web 09/19/2003
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microscope slides were cleaned according to published procedures5 and coated with OTS.4 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.6,7 Clean copper specimen grids were then placed onto the surface of OTS-treated glass. 5CB (1 µL) 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 20 ( 1 °C. Optical Examination of LC Textures. The orientation of 5CB was examined by using plane-polarized 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% in order to collimate the incident light. Homeotropic alignments were determined by first observing no transmission of 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.8 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.9,10 All images were captured using a digital camera (Olympus C-2040 Zoom) mounted on the microscope and 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. Each alignment was observed to be stable for at least 4 h, typically >24 h. Preparation of FTMA Solutions. Aqueous solutions of FTMA were prepared in their reduced state in deaerated 0.1 M Li2SO4 at pH 2. Experiments using the reduced ferrocenyl surfactant were conducted immediately following the preparation of the solutions so as to minimize oxidation of the ferrocene. Oxidized solutions of the ferrocenyl surfactants were prepared by electrochemical oxidization using a bi-potentiostat (Princeton Applied Research) set at +0.350 V (versus a saturated calomel electrode) for g3 h. The working electrode and counter electrode were platinum gauze, and the reference electrode was saturated calomel. The extent of oxidation of the ferrocene was monitored by visible spectrophotometry. FTMA absorbs strongly at 630 nm, whereas the oxidized FTMA absorbs strongly at 440 nm. Competitive Adsorption of CTAB and FTMA. The optical cells containing 5CB were exposed to CTAB and FTMA using three different methods. (I) The optical cells were immersed in aqueous mixtures of CTAB and either FTMA or oxidized FTMA for at least 90 min. (II) The optical cells were first immersed in aqueous solutions containing CTAB for at least 90 min to allow the 5CB to obtain its preferred alignment. Then, either FTMA or oxidized FTMA was added to the solution from a concentrated aqueous solution of either FTMA or oxidized FTMA. (III) The optical cells were first immersed in aqueous solutions of either FTMA or oxidized FTMA for at least 90 min to allow the 5CB to obtain its preferred alignment. Then, CTAB was added to the solution from a concentrated aqueous solution of CTAB. In all cases, the aqueous solution contained 0.1 M Li2SO4 adjusted to pH 2 using sulfuric acid. Subsequent exchange of the aqueous mixtures containing CTAB and either FTMA or oxidized FTMA which contacted the 5CB was performed by two methods. (I) A peristaltic pump was used to displace at least 6 volumes of the original aqueous solution with a new aqueous mixture containing CTAB and either FTMA or oxidized FTMA. (II) The optical cells were physically withdrawn from the original aqueous solution (5) Skaife, J. J.; Abbott, N. L. Chem. Mater. 1999, 11, 612. (6) Cognard, J. Mol. Cryst. Liq. Cryst. 1982, 1 (Suppl.), 1. (7) Yang, J. Y.; Mathauer, K.; Frank, C. W. Microchemistry 1994, 441. (8) Bloss, F. D. An Introduction to the Methods of Optical Crystallograpy; Holt, Rinehart and Winston: New York, 1961. (9) Drzaic, P. S. Liquid Crystal Dispersions. Series on Liquid Crystals; World Scientific: Singapore, 1995; Vol. 1. (10) Sonin, A. A. Freely Suspended Liquid Crystalline Films; John Wiley & Sons: New York, 1998.
Brake et al. and placed into a new aqueous mixture containing CTAB and either FTMA or oxidized FTMA. During the transfer process, care was taken to ensure that a thin film of water covered the 5CB at all times. Chemical Reduction and Oxidation of FTMA/CTAB Solutions. The LC cells were first immersed for at least 90 min in a prescribed solution of CTAB and either FTMA or oxidized FTMA (volume ∼ 20 mL) to allow the 5CB to obtain its preferred alignment. The FTMA in the mixture was then chemically oxidized by the addition of an equi-equivalent concentration of hydrogen peroxide. Likewise, the oxidized FTMA in the mixture was then chemically reduced by the addition of an equi-equivalent concentration of L-ascorbic acid. The dynamics of the response of 5CB to the change in FTMA oxidation state was monitored by polarized light microscopy (see above). The change in the oxidation state of the ferrocene group following the addition of L-ascorbic acid or hydrogen peroxide was verified by visible spectroscopy. Determination of the Tilt Angle of 5CB at the Aqueous5CB Interface. Tilt angles at the aqueous-5CB interface were determined by the numerical solution of eq 3 (see Results and Discussion). The experimental parameters in eq 3, namely, no, ne, and ∆neff, were determined as follows. The indices of refraction of 5CB were taken as constant using the values reported for λ ) 632 nm at 25 °C, no ) 1.711 and ne ) 1.5296.11 The colors from at least five random grid squares in each optical image were matched by eye against a Michel-Levy chart to determine ∆neff at a thickness of 18 µm. The thickness of the 5CB film was determined by observing the grid surface and the LC film surface to lie in the same focal plane when imaged by optical microscopy. For each value of ∆neff, the tilt angle of the 5CB at the aqueousLC interface was determined by numerical solution of eq 3. The tilt angles reported are the average of the tilt angles obtained using the different grid squares with the error in the tilt angles being typically 5 µM) at a fixed concentration of oxidized FTMA (50 µM). Likewise, mixtures containing lower concentrations of oxidized FTMA (5 min after exchange of the solution). The 5CB film is confined to a copper grid (292 µm hole size) that was supported on an OTS-coated glass slide. The images were obtained by orthoscopic illumination of the sample between crossed nicols at 4× magnification. All scale bars represent 300 µm.
and 50 µM oxidized FTMA by physically moving the optical cell between solutions or by exchange of the solution using a peristaltic pump. Both procedures gave indistinguishable results. After the optical texture of the 5CB reached a new steady state (typically 30 min). The results above are significant for two reasons. First, they demonstrate it is possible to quantify the dynamic process of reorientation of the LC. Second, they suggest the basis for a novel approach to measure the timedependent adsorption of amphiphiles at surfaces. To develop such an approach, however, knowledge of the dependence of the orientation of the LC on the surface concentration of the surfactant is required. Conclusions We have demonstrated that it is possible to couple the orientation of LCs to the oxidation state of a redox-active surfactant, FTMA. Because aqueous solutions containing FTMA cause planar anchoring of the LC independent of the oxidation state of the FTMA, we investigated mixed surfactant systems comprised of FTMA and CTAB. The coadsorption of mixtures of FTMA and CTAB to the aqueous-LC interface led to orientations of the LC that depend on the oxidation state of FTMA. The dependence of the orientation of the LC on the oxidation state of FTMA was found to reflect the competitive adsorption of CTAB and either FTMA (resulting in near-planar anchoring) or oxidized FTMA (resulting in homeotropic anchoring) for the interface. This mechanism permits reversible coupling of the anchoring of 5CB to in situ changes in the oxidation state of FTMA. Finally, we quantified the time-dependent orientation of the LC during transitions between homeotropic and near-planar anchoring that was driven by the competitive adsorption of FTMA and CTAB at the aqueous-LC interface. Acknowledgment. This research was supported by the Office of Naval Research (N000149910250), Center for Nanostructured Interfaces (NSF DMR 0079983), the National Institutes of Health (NIH 5 T32 GM08349), the Camille and Henry Dreyfus Foundation (Teacher-Scholar Award), the donors of the Petroleum Research Fund (ACSPRF35409-AC7), and the National Science Foundation (CTS-9911863). We also thank Craig Rosslee for his assistance with the electrochemistry of the ferrocenyl surfactants. Appendix: Director Profile with Splay and Bend Distortion of 5CB Due to Asymmetric Anchoring Boundary Conditions at the Limiting Surfaces (OTS and Water)
Figure 9. (A) Cartoon representation of the asymmetric boundary conditions. (B) Definition of the tilt and azimuthal angles that specify the orientation of 5CB.
(i.e., θ(z)) while the azimuthal angle, ψ, remains constant at a fixed position in the xy plane (see Figure 9B). The director can then be expressed as19
n ) {sin θ(z) cos ψ, sin θ(z) sin ψ, cos θ(z)} (A.2) Substitution of eq A.2 into eq A.1 gives
Fd ) (1/2)K1 sin2 θ(z)
( ) dθ(z) dz
2
+
(1/2)K3 cos2 θ(z)
( ) dθ(z) dz
2
(A.3)
By making the one-constant approximation (i.e., K1 ) K3 ) K), eq A.3 can be rewritten as
Fd ) (1/2)K
( )
2
dθ(z) dz
(A.4)
The director profile, n, within a film of a nematic LC in contact with interfaces having asymmetric anchoring boundary conditions (i.e., θ1 * θ2, see Figure 9A) can be solved using continuum theory. According to the continuum theory of nematic LCs, the local distortion energy of a nematic, Fd, under the combined effects of splay, twist, and bend deformations can be expressed as
For 5CB, this approximation is justified by a K1/K3 ratio of ∼0.92.20 Integration of eq A.4 over the thickness of the nematic film gives the total elastic energy per unit area, Fd,
Fd ) (1/2)K1 (div n)2 + (1/2)K2 (n ‚ curl n)2 +
The equilibrium director profile within the nematic film satisfies the minimization of eq A.5 with respect to the profile of the distortion such that
(1/2)K3(n x curl n)2 (A.1) where K1, K2, and K3 are the splay, twist, and bend elastic constants, respectively.18 For bend and splay deformations propagating perpendicular to the limiting interfaces (in the z direction), the tilt angle, θ, measured relative to the z-axis varies with position relative to the limiting surfaces
∫0d(
0)
∫0d(
d dz
)
dθ(z) dz
Fd ) (1/2)K
)
dθ(z) dz
2
dz
(A.5)
2
dz
(A.6)
(20) Madhusudana, N. V.; Pratibha, R. Mol. Cryst. Liq. Cryst. 1982, 89, 249.
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The derivative with respect to z can be distributed within the integral to yield
0)
∫0
d
(
2
)
dθ(z) d2θ(z) dz + dz dz2
( )| dθ(z) dz
2
0)
∫0d
(
2
dz2
(A.9)
Solving the differential equation in eq A.9 subject to the boundary conditions θ(0) ) θ1 and θ(d) ) θ2 gives
( )|
d(d) dθ(z) dz d dz
2
d(0) (A.7) 0 dz
z θ(z) ) (θ2 - θ1) + θ1 d
)
dθ(z) d2θ(z) dz dz dz2
(A.8)
In the presence of bend and splay distortions, eq A.8 can only be satisfied by
(A.10)
where d is the thickness of the nematic film. Substitution of this director profile into eq A.5 yields
(θ2 - θ1)2 d
Under the assumption of strong anchoring at the two fixed limiting surfaces, the second and third terms of eq A.7 vanish yielding
0)
d2θ(z)
Fd ) (1/2)K
(A.11)
By taking K ∼ 5 × 10-12 N, θ1 ) 0, θ2 ) π/2, and d ) 2 × 10-5 m, the elastic energy stored within the nematic film is ∼6 × 10-4 mN/m, significantly less than the change in surface energies (order 101 mN/m) responsible for the anchoring transition observed within the nematic.20 LA034469U
