Coupling of the Orientations of Liquid Crystals to Electrical Double

May 4, 2001 - We report the orientations of thermotropic liquid crystals to be coupled through dipolar interactions to the electric fields of electric...
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J. Phys. Chem. B 2001, 105, 4936-4950

Coupling of the Orientations of Liquid Crystals to Electrical Double Layers Formed by the Dissociation of Surface-Immobilized Salts Rahul R. Shah† and Nicholas L. Abbott* Department of Chemical Engineering, 1415 Engineering DriVe, UniVersity of Wisconsin, Madison, Wisconsin 53706 ReceiVed: NoVember 7, 2000; In Final Form: March 7, 2001

We report the orientations of thermotropic liquid crystals to be coupled through dipolar interactions to the electric fields of electrical double layers that form upon contact of the liquid crystals with model surfaces presenting sodium carboxylate salts. The surfaces were prepared by the self-assembly of HOOC(CH2)10SH on semi-transparent films of gold. The density of sodium carboxylate groups was controlled by pretreatment of the surfaces with aqueous solutions buffered between pH 3.2 to 10.6 and quantified by using X-ray photoelectron spectroscopy. We used this well-defined experimental system to test predictions of a model that describes the dipolar coupling of the orientations of liquid crystals to an electric field formed at a surface through dissociation of sodium ions into the liquid crystal. Four predictions of the model, namely (I) the influence of the sign of the dielectric anisotropy of the liquid crystal, (II) the effect of the thickness of the film of liquid crystal relative to the Debye length within the liquid crystal, (III) the influence of the density of sodium carboxylate groups on the surface, and (IV) the influence of the concentration of electrolyte (NaI) dissolved within the liquid crystal, were found to be consistent with our experimental observations and thus support the proposed orientational coupling of the liquid crystal to the electric field formed by dissociation of the surface immobilized sodium carboxylate salt. A comparison of the orientational behavior of nematic phases of 4-cyano-4′-pentylbiphenyl (5CB) to the predictions of the model also revealed the sodium carboxylate5CB interface to be held close to a constant potential (rather than constant density of charge) upon addition of electrolye to the liquid crystal.

1.0. Introduction Numerous past studies have reported investigations of electrical double layers formed by the ionization of acids and/or the dissociation of salts that are immobilized on surfaces of solids.1 These studies have largely focused on electrical double layers formed by the contact of the surfaces with isotropic aqueous and nonaqueous liquids.2 Although these past investigations have permitted the testing of theoretical descriptions of electrical double layers and have provided valuable insights into the origin of physical phenomena such as wetting,3 lubrication,4 and colloidal stability,1c,5 few past studies have focused on the formation of electrical double layers within anisotropic, nonaqueous liquids such as nematic liquid crystals and the influence of the electrical field within the double layer on the orientations assumed by liquid crystals near these surfaces. The work described in this paper addresses this latter topic. This paper describes an experimental investigation of the coupling of the orientations of thermotropic liquid crystals to electrical double layers formed near model surfaces presenting controlled areal densities of sodium carboxylate salts. The orientations assumed by thermotropic liquid crystals near surfaces are known to depend on orientation-dependent interactions between the molecules within the liquid crystal (mesogens) that are near (within nanometers) to the surface and the material that comprises the surface.6 Because the mesogens within the liquid crystal that are near the surface can communicate their * To whom correspondence should be addressed. Email: [email protected] † Current address: 3M Center, 3M Corporation, 201-1W-28, St. Paul, MN 55101.

orientations to mesogens that are a distance of 10-100 µm from the surface, even short-range interactions between surfaces and mesogens can direct the orientations of bulk liquid crystalline phases. This phenomena, the so-called anchoring of bulk liquid crystals by surfaces, is easily studied because most liquid crystalline phases are optically anisotropic (birefringent), and thus, their orientations can be measured by using polarized light.6 Past studies have investigated the anchoring of liquid crystals on monomolecular films (e.g., deposited by the method of Langmuir and Blodgett), mechanically sheared polymeric films, and inorganic films such as obliquely deposited SiOx.7 These studies and others8 have provided a long list of possible interactions (van der Waals forces, electrostatic interactions, steric interactions, etc) between surfaces and liquid crystals that likely lead to the anchoring of liquid crystals. The goal of the research reported in this paper is to define a model experimental system that makes possible the systematic manipulation of a single, well-defined interaction between a surface and liquid crystal and to use this capability to test hypotheses regarding the influence of this interaction on the orientation of a liquid crystal. In short, we have prepared surfaces that present increasing areal densities of sodium carboxylate salts and have measured the orientations of liquid crystals on them. We hypothesized that, on surfaces with sufficiently high densities of sodium carboxylate salts, partial dissociation of the sodium ions from the surface-immobilized salts would lead to formation of an electrical double layer within the liquid crystal, and that the electric field within the electrical double layer would promote the liquid crystal to orient in a direction parallel to the field lines. At the same time, because

10.1021/jp004073g CCC: $20.00 © 2001 American Chemical Society Published on Web 05/04/2001

Orientations of Liquid Crystals

Figure 1. Schematic illustration of the bulk orientation of a liquid crystal anchored by a surface. See text for details.

the dielectric properties of a liquid crystal depend on its orientation, we hypothesized that the structure of the electrical double layer would depend on the orientation of the liquid crystal. In this paper, we focus on electrostatic interactions that drive the average orientation of a liquid crystal out-of-the-plane of the supporting surface. Hereafter, we use the term “the director” to describe the average orientation of the long axis of the molecules that form the liquid crystal. We characterize the orientation of the director by the polar angle, θ, which is measured with respect to the normal of the surface (see Figure 1). Finally, we note that we use the term “anchoring energy” to refer to the angle-dependence of the free energy of the interface between the liquid crystal and supporting surface. Specifically, we define the anchoring energy, W, as W ) W(θ ) 90°) W(θ ) 0°). From this definition, it follows that the director of the liquid crystal will assume an orientation that is perpendicular to the surface (θ ) 0°) if W > 0 and parallel to the surface (θ ) 90°) if W < 0. It is well know that application of an external electrical potential difference between two parallel conducting plates that confine a liquid crystal can lead to a change in the orientation of the liquid crystal. A local electric field (Eˆ ) so-formed within the liquid crystal will act on the mesogens by aligning the dipoles, pˆ , of the mesogens within the liquid crystal in a direction that is parallel to Eˆ .9 The torque, τˆ , that orients the dipoles of the mesogens can be evaluated as τˆ ) pˆ × Eˆ . Equivalently, the influence of the electric field on the orientation of the liquid crystal can be described at the continuum level, which leads to the conclusion that the liquid crystal orients such that its largest dielectric constant is parallel to the lines of the applied electric field. For reasons similar to those stated above, it has also been theorized that a liquid crystal can be oriented by the internal electric field of an electrical double layer formed in a liquid crystal near a surface supporting a density of charge.10 Here, we note three essential ideas contained in past discussions of electrical double layers formed at charged surfaces in liquid crystals. First, the proposed mechanisms leading to the possible accumulation of charge at a surface immersed in a liquid crystal include (I) the specific adsorption of ionic species from the liquid crystalline phase onto the surface and (II) the ionization of surface functional groups.11 Second, it has also been proposed that the structure of the electrical double layer within the liquid crystal is influenced by ionic species that are either present at the conclusion of its synthesis, formed by the chemical degradation of mesogens, or formed by the ionization of entrained water (H+ and OH-).12 Third, similar to the case of isotropic liquids, the structure of the electrical double layer formed at a surface has been proposed to depend on the dielectric properties of the contacting liquid. In isotropic liquids, this effect is generally described in terms of the influence of the dielectric properties of the liquid on the Debye length (κ-1).1

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4937 Because liquid crystals possess anisotropic dielectric properties, the orientation of the liquid crystal (and thus the dielectric property experienced by the electric field) will determine the extent of penetration of the electric field into the liquid crystalline phase (as captured by the Debye length). Whereas theory describing the possible action of an electric field within an electrical double layer on the orientation of a liquid crystal is well established, experimental studies that are based on systematic manipulation of electrical double layers formed through the mechanisms described above have not been reported. Numerous past studies have measured properties of the electrical double layer imposed by an external electric field applied using conductive electrodes that confine a liquid crystal. These studies have reported measurements of capacitance13 and conductivity.14 These studies, however, have not reported changes in the orientations of liquid crystals due to the imposed electrical double layers. Barbero and co-workers have provided two pieces of experimental evidence for a long-range interaction between a surface and a liquid crystal that is plausibly due to the influence of an electrical double layer on the orientation of a liquid crystal.15,16 First, they measured the anchoring energies, W, of nematic 4-cyano-4′-pentylbiphenyl (5CB) and ZLI-1800100 between surfaces of either rubbed films of Formvar, poly(acrylic acid) or polyxyethylene alkyl ester. Although the orientation of the liquid crystal was always parallel to the surfaces (i.e., planar anchoring thus corresponding to a negative value of W), the anchoring energies were measured to increase (became less negative) as the distance between the two confining surfaces was increased from 2.5 to 115 µm.15 Because the anchoring energy varied with distance between the two surfaces, this result was interpreted to indicate that a long-ranged electrostatic interaction dominated the anchoring energies of the liquid crystals in these systems. Second, liquid crystals of p-methoxybenzylidene-p-n-butylaniline (MBBA) or mixtures of MBBA and p-ethoxybenzylidene-p-n-butylaniline (EBBA) when confined between optically flat substrates coated with 30 nm thick films of either chromium or indium/tin oxide, were observed to change orientation from homeotropic to planar with increasing distance (∼30 µm to ∼60 µm) between the confining surfaces. Because the nematic phases of MBBA and EBBA possess a negative dielectric anisotropy (∆ 0 or ∆ < 0) of liquid crystal, and (IV) thickness of liquid crystalline layer, on the orientation of the liquid crystal.

4938 J. Phys. Chem. B, Vol. 105, No. 21, 2001

Shah and Abbott

TABLE 1: Dissociation Constants for HOOCCH3 (pKa) and NaOOCCH3 (pKd) in Aqueous and Non-Aqueous Solvents Measured by using Potentiometric, Spectroscopic, and Conductance Techniques solvent water 95% HOCH3 5% water HOCH3 70% dioxane 30% water HOOCCH3



HOOCCH3 (pKa)

78 40

4.7521 7.625

30 18

9.625 8.222c,25

6.5

∼1324

NaOOCCH3 (pKd) ∼122a 2.525 2.825 2.522c 6.623a,23c

In this paper, we report a study of the anchoring of liquid crystals using an experimental system that permits a high level of control over the structure of surfaces, including the areal density of charge supported on the surface. The choice of our experimental system is guided by two observations. First, contact angle titrations,17 force-distance measurements,18 measurements using a quartz crystal microbalance,19 and adhesion measurements,20 all indicate that carboxylic acids tethered at surfaces and immersed into aqueous solutions ionize as a function of increasing pH. It has also been reported that, for the same bulk solution pH, the extent of ionization of the carboxylic acid on the surface is less than that of a carboxylic acid in the bulk aqueous solution (pKa ) 4.75).21 These results led us to propose that it should be possible to prepare surfaces that present controlled densities of COOH and COO-Na+ groups for studies of the orientations of liquid crystals. Second, past measurements (Table 1) of bulk dissociation constants for HOOCCH3 (Ka, pKa) and NaOOCCH3 (Kd, pKd) in various solvents (aqueous and nonaqueous)22-25 reveal that the bulk dissociation constants for sodium carboxylate salts (Kd) are substantially larger (>4 pK units) than the corresponding dissociation constants for carboxylic acid (Ka). Although the dielectric environment of a SAM will change both the pKa and pKd from their values measured in bulk solution, these results lead us to propose that it should be possible to control the density of charge presented at a surface when contacted with a liquid crystal by manipulation of the density of carboxylic acid and sodium carboxlate salts at a surface. This proposition is based on the observation that the areal charge density established by the process of dissociation of the salt will be much higher than the areal charge density developed through the ionization of the acid. In this study, we have tested these propositions by using self-assembled monolayers (SAMs) formed from ω-HOOC(CH2)10SH chemisorbed onto semi-transparent gold. By immersion of the SAMs into aqueous solutions possessing increasing pHs, we controlled the extent of conversion of the terminal COOH moiety to its sodium salt as is schematically illustrated in Figure 2A.26 We hypothesized that the extent of ionization of the carboxylic acid would be small compared to the dissociation of the sodium salt when exposed to a liquid crystal (Figure 2B and 2C).27 To guide our experiments, we have also calculated the influence of an electrical double layer on the anchoring energy of a liquid crystal using a simple model based on solution of the Poisson-Boltzmann equation in an anisotropic dielectric medium. The results of this calculation are significant because the parameters that enter the model, namely the surface charge density (σ) or surface potential (φ), Debye length (κ-1), dielectric anisotropy (∆), and cell thickness (d) can be systematically manipulated in our experimental system. We test the predicted coupling of the electrical double layer and orientation of liquid crystal by (I) comparing and contrasting the out-of-plane orientation of liquid crystals that possess dielectric anisotropies

Figure 2. (A) Schematic illustrations of SAMs formed from HOOC(CH2)10SH that have been pretreated by immersion and withdrawl from aqueous solutions at three pHs. (B) Schematic illustration of the interface between a surface presenting carboxylic acid groups and a liquid crystal. (C) Schematic illustration of an electrostatic double layer formed upon contact of a liquid crystal with a surface presenting sodium carboxylate groups.

(∆) that are opposite in sign (5CB and MBBA), (II) manipulating the ionic strength and thus the Debye length of the liquid crystal, (III) manipulating the thickness of the film of liquid crystal in the cell, and (IV) manipulating the density of charge on the surfaces of the cell by controlling the degree of conversion of the surface-confined acid to sodium carboxylate salt. We also compare the predictions of the model and our experimental measurements to determine if the density of charge on the surface remains constant (constant charge) or is regulated (constant potential) as electrolyte is added to the liquid crystal. 2.0. Materials and Methods 2.1. Materials. We synthesized HOOC(CH2)10SH by using previously published methods.28 The liquid crystal 5CB (Tni )

Orientations of Liquid Crystals 34.5 °C), manufactured by BDH, was purchased from EM Industries (Hawthorne, NY). H3C(CH2)2SH, H3C(CH2)15SH, NaCl (99.9%), NaBr (99.99%), and NaI (99.99%) were purchased from Aldrich (Milwaukee, WI). MBBA (Tni ) 43 °C) was purchased from TCI (Portland, OR) and purified by chromatography by using a column of Alumina (Brockman I). The clearing temperatures (Tni) of the liquid crystals were measured on a Leica (Leitz, Germany) heated microscope stage. The salts, NaI, NaCl, and NaBr were heated in a vacuum oven for 24 h at 250 °C before use. Glass microscope slides (Fisher’s Finest) were purchased from Fisher (Los Angeles, CA). Titanium (99.999%) and gold (99.999%) were purchased from Advanced Materials (Spring Valley, NY). 2.2. Methods. Cleaning of Substrates. Microscope slides were cleaned sequentially in piranha (70% H2SO4, 30% H2O2) and base solutions (70% KOH, 30% H2O2) using nitrogen to provide agitation (1 h at ∼80 °C). Warning: Piranha solution should be handled with extreme caution; in some circumstances, most probably when it has been mixed with significant quantities of an oxidizable organic material, it has detonated unexpectedly. The slides were then rinsed thoroughly in deionized water (18.2 MΩ-cm), ethanol, and methanol and dried under a stream of nitrogen. The clean slides were stored in a vacuum oven at 110 °C. All other glassware was cleaned in piranha solution prior to use.29 Uniform Deposition of Gold Films. Semi-transparent films of gold with thicknesses of ∼100 Å were deposited onto glass slides mounted on rotating planetaries (no preferred direction or angle of incidence) by using an electron beam evaporator (VES-3000-C manufactured by Tek-Vac Industries, Brentwood, NY). The rotation of the substrates on the planetaries ensured that the gold was deposited without a preferred direction of incidence. We refer to these gold films as “uniformly deposited gold films”. A layer of titanium (thickness ∼50 Å) was used to promote adhesion between the glass microscope slide and the film of gold. The rates of deposition of gold and titanium were ∼0.2 Å/sec. The pressure in the evaporator was less than 1 × 10-6 Torr before and during each deposition. The gold source was periodically cleaned by cycling sequentially immersing it in aqua regia (70% HNO3, 30% HCl) and piranha solutions at 50 °C (30 min in each solution). The cycle was repeated 3-4 times, rinsing between cycles in deionized water. Oblique Deposition of Gold Films. Semi-transparent films of gold with thicknesses of ∼130 Å were deposited onto glass microscope slides mounted on stationary holders by using the electron beam evaporator described above. The gold was deposited from a fixed direction of incidence and a fixed angle of incidence of either ∼40° or ∼50° (measured from the normal of the surface). We refer to these gold films as “obliquely deposited gold films”. A layer of titanium (thickness of ∼50 Å) was used to promote adhesion between the glass and the film of gold. Formation and Pretreatment of SAMs. Self-assembled monolayers (SAMs) were formed on the surfaces of uniformly and obliquely deposited films of gold by immersing the films in ethanolic solutions containing 1 mM HOOC(CH2)10SH for 1 h. After rinsing in ethanol and drying under nitrogen, the SAMs were immersed for approximately thirty seconds in aqueous solutions buffered between pH 2 and pH 12. Upon removal from an aqueous solution, the surface of a slide was placed under a stream of nitrogen gas to displace excess solution from the surface. Unless stated otherwise, aqueous solutions were buffered by using the following acids and sodium salts: pH 1-2.5 (0.01 M HCl), pH 2.5-4 (1 mM NaH2PO4/H3PO4), pH

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4939 4-5 (1 mM NaO2CCH3/HO2CCH3), pH 6-7 (1 mM Na2HPO4/ NaH2PO4), pH 8-9 (1 mM Na2CO3/NaHCO3), pH 9-11 (1 mM Na2HPO4/Na3PO4), pH 11.5-12 (10 mM NaOH), pH 1213 (0.1 M NaOH). All buffered solutions were used within 12 h of preparation. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (Surface Science, Mountain View, CA) was used to determine the extent of conversion of immobilized carboxylic acid groups to their sodium carboxylate salts. Details of the procedure have been reported in a past publication.26 Orientations of Liquid Crystals. We measured the orientations of nematic phases of 5CB and MBBA on SAMs by fabricating optical cells from two films of gold, each of which supported SAMs. First, the two gold films were aligned (facing each other). For obliquely deposited films of gold, the direction of deposition of the gold in each film was parallel. Second, the gold films were clipped together (binder or bulldog clips) using a thin film of Mylar or Saran Wrap (nominal thickness ∼2, 12, 25, 50, 100 µm) to keep the two surfaces apart. A drop of 5CB or MBBA, heated into its isotropic phase (T < 45 °C), was then drawn by capillarity into the cavity between the two surfaces of the optical cell. The cell was subsequently cooled to room temperature and the optical texture analyzed with an Olympus BX-60 polarizing light microscope (Tokyo, Japan) in transmission mode. Doping Liquid Crystals with Electrolytes. We doped 5CB using salts (NaI, NaBr, NaCl) that were heated in a vacuum oven at 250 °C for 24 h prior to use. First, the salt was dissolved into methanol to a concentration of 2 mM. Second, microliter volumes of the salt in the methanolic solution were transferred into vials and desiccated under vacuum for at least 12 h. After the methanol had evaporated, 100 µL of 5CB was added to each sample vial and heated into the isotropic state. The 5CB was mixed with the salt (in the isotropic state) using a vortex mixer and allowed to equilibrate over 24 h (heating and mixing the solution each couple of hours). Measurement of ConductiVity of Doped Liquid Crystals. Conductivity (k) measurements were performed to estimate ion densities (F) and Debye lengths (κ-1) within liquid crystals with and without added electrolyte. Measurements were made using a liquid crystal cell of known dimensions (thickness ∼14 µm, area 150-250 mm2) and a Pine Bipotentiostat AFCBP1 (Grove City, PA). The liquid crystals were aligned parallel to the surfaces of the cell. Planar alignment was induced by using SAMs formed from H3C(CH2)2SH supported on uniformly deposited gold films. The SAMs prevented the irreversible adsorption of ions onto the gold30 and preserved the alignment of the liquid crystal throughout the experiment. We applied 500 mV DC across the cell (which is below the threshold voltage required to drive a change in orientation of 5CB) and measured the plateau in current reported by the bipotentiostat ∼1 ms after application of the potential. The conductivity of the 5CB was estimated from knowledge of the applied voltage, measured current and cell dimensions. Estimates of the conductivity based on this procedure were found to be in good agreement with values reported by the supplier of 5CB.31 The ion density within the liquid crystal was estimated from the ratio of the ionic conductivity, k, and the equivalent conductance (Λo) of salts within 5CB (for NaI, Λo ≈ 10 cm2mol-1Ω1-). We estimated Λo for 5CB from the equivalent conductance for benzonitrile (for NaI, Λo ) 47.6 cm2mol-1Ω1-).32 Because Λo is defined as the ratio of the ionic conductivity and the ion density in the solvent and because the ionic conductivity is inversely proportional to the viscosity of the solvent, we estimated Λo for 5CB

4940 J. Phys. Chem. B, Vol. 105, No. 21, 2001 by scaling the equivalent conductance of benzonitrile by the ratio of the viscosity of 5CB and benzonitrile. The viscosity of benzonitrile at 25 °C is 1.5 cP21 and the average Miesowicz viscosity for 5CB is ∼8 cP at 25 °C.33 We also point out that the ionic conductivity depends on the solvated radius, and thus, it may also be different when NaI is dissolved in 5CB as compared to benzonitrile. Because we were not able to obtain an estimate of the solvated radius of the ions, we have not corrected our estimate of the equivalent conductance for this factor. Measurement of Out-of-Plane Orientations of Liquid Crystals (Crystal Rotation Technique). A home-built optical apparatus was used to measure the out-of-plane orientation (tilt) of 5CB within the optical cells. The optical cells were placed between cross-polars, illuminated at normal incidence using a polarized He-Ne laser, and then rotated from -20° to +20° with respect to the normal. A plot of the intensity of light transmitted through the cell against the angle of incidence was used to estimate the tilt of the optical axis of the liquid crystal from the surface of the cell.34 Measurement of In-Plane Orientations of Liquid Crystals. We measured the azimuthal orientations of liquid crystals supported on SAMs (on obliquely deposited gold films) by using white light, crossed polarizers, a quarter wave plate (QWP), and ∼2 µm-thick optical cells. The orientation of the optical axis of the liquid crystal (the director) was determined by rotating the cell between cross polarizers and observing the shift in interference colors upon insertion of the QWP into the optical path. The in-plane orientation of the liquid crystal was defined as the one for which the insertion of the QWP resulted in the maximal positive shift in interference colors. The interference colors were interpreted by using a Michel-Levy color chart, as described in a previous study.35 Measurement of Transmission of Light through Optical Cells of Liquid Crystals. We measured the transmission of unpolarized light (at 632.8 nm) through an optical cell filled with liquid crystal using a Cary 1E UV-vis spectrophotometer (Varian Instruments, Sugar Land, TX). The raw transmittance data was corrected for cell thickness using Beer’s Law (where the path length is taken as the thickness of the liquid crystal cell). 3.0. Model for Electrostatic Contribution to Anchoring Energy of Liquid Crystal 3.1. General Considerations. To guide our experiments, we have evaluated a simple model that describes the action of an electric field generated within an electrical double layer on the alignment of a liquid crystal. This model is based on a description of the liquid crystal as an anisotropic dielectric continuum and a solution of the Poisson-Boltzmann equation within an anisotropic dielectric continuum. Because the liquid crystal possesses anisotropic dielectric properties, the electric field within the electrical double layer induces a torque on the mesogens thereby causing the liquid crystal to orient with its largest dielectric constant oriented parallel to the lines of the electric field.9 Similarly, properties of the electrical double layer, namely the Debye length, also depend on the orientation of the liquid crystal and thus the dielectric constant experienced by the electric field. This model captures the coupling of the orientation of the liquid crystal and the structure of the electrical double layer through calculation of the electrostatic contribution to the anchoring energy, W, of the liquid crystal. The experiments we report in this paper are based on liquid crystal cells that are fabricated using two identical surfaces, each of which possess a density of surface charge and an electrical

Shah and Abbott

Figure 3. Schematic illustrations of the electrical potential profile near charged surfaces. (A) Electrical potential (ψ) near an isolated plate. (B-D) Electrical potential near an isolated plate (ψ) and between two charged plates (Ψ) separated by a distance, d.

double layer. The extent of overlap of the electrical double layers from the opposing surfaces depend on the relative thickness of the liquid crystal cell and the Debye length. As illustrated in Figure 3A, the potential profile of an isolated surface depends only on κ-1 (and not the thickness of the cell). In contrast, and as shown schematically in Figure 3B-D for the case of a constant surface potential, the electric field at the surface of a liquid crystal cell increases with increasing cell spacing, d (at constant κ-1). We anticipate, therefore, that the greatest electrostatic contribution to the anchoring energy will occur when κd.1 and the electric fields from the two surfaces of the cell do not overlap (Figure 3D). 3.2. Model for the Electrostatic Anchoring Energy. We evaluate the electrostatic anchoring energy, W, as the work performed to reorient an anisotropic dielectric medium from a homeotropic (θ ) 0°) alignment to a planar alignment (θ ) 90°) as

W)

0 ∫-d/2 ∫0π/2τˆ dθdz

(1)

where τˆ ) pˆ × Eˆ (z) and pˆ is the induced polarization parallel and perpendicular to the director (taken along the coordinate axes), and d is the thickness of the cell. We consider the case of the liquid crystal 5CB. The magnitude of Eˆ (z) is dependent upon the orientation of 5CB (perp ) 6.7 (planar) and para )

Orientations of Liquid Crystals

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4941

19.7 (homeotropic))9 and is calculated from the PoissonBoltzmann equation that was linearized based on the DebyeHu¨ckel approximation. This approximation is valid because of the extremely low charge density at the surface (as discussed below).36 As illustrated in Figure 3B-D, we have calculated the profile of the electric field assuming symmetrically charged surfaces. This gives rise to a null electric field at the midplane of the cell. We have utilized both constant potential and constant charge boundary conditions when calculating the magnitude of the electric field at the surface. The constant charge case has been related to the constant potential case through the standard equation1a,1b

qo ) -oE(z)|d/2

(2)

where  is the dielectric constant of the nematic liquid crystal in the isotropic state, and E(z) is the electric field at the surface calculated from constant potential boundary conditions. In the calculations reported in this paper, we have assumed a surface potential of -7 mV. The actual value of surface potential assumed in our calculations does not affect the conclusions of this paper. This value is estimated from force-distance measurements reported by Bard and co-workers in aqueous solution (at pH 10) using SAMs formed from 3-mercaptopropionic acid on gold. They reported a surface potential of -62 mV.18a Whereas water has a relative dielectric constant of  ) 78, the relative dielectric constants of 5CB lie between 6.7 and 19.7. We expect, therefore, that the potential of a surface in 5CB will be substantially less than in water. To account for this expectation, we base our calculations on a surface potential of -7 mV. We note that a surface charge density of 1 electronic charge per 400 000 nm2 was calculated from Gouy-Chapman theory assuming κ-1)2.5 µm and a surface potential of -7mV. For the cases of constant surface potential and constant surface charge, eq 1 can be simplified to

W)

∫-d/2 0

[

]

paraE2(para,z) perpE2(perp, z) dz 2 2

(3)

We have evaluated eq 3 using various cell thicknesses and Debye lengths for the case of constant charge (Figure 4A) and the constant potential (Figure 4B). 3.3. Predictions of Model. The model described above for the coupling of an electrical double layer to the orientation of a liquid crystal leads to a number of predictions. These predictions are outlined below. In Section 4.0, we systematically test each prediction and report our experimental findings. 3.3.1. Effect of Charge Density at Surface. The first, and perhaps most obvious, prediction of the above-described model is that surfaces possessing a high density of charge should cause nematic phases of 5CB to orient perpendicular to the surface (because the dielectric constant of nematic 5CB is maximal in a direction that is approximately parallel to the director). Because the results of past studies lead us to predict that the extent of ionization of carboxylic acid groups at the surface will be negligible when in contact with 5CB as compared to the extent of dissociation of sodium carboxylate groups (see Introduction and Table 1) when in contact with 5CB, we predict that maximal surface charge densities will be realized when SAMs formed from HOOC(CH2)10SH are pretreated with aqueous solution with high pHs. That is, we predict homeotropic anchoring of nematic 5CB when supported on SAMs presenting sodium carboxylate groups to the liquid crystal. We present experimental tests of these predictions in Sections 4.2 and 4.3.

Figure 4. Electrostatic contribution to the anchoring energy of liquid crystal calculated using boundary conditions of (A) constant charge and (B) constant potential within cells of thickness ∼2 to 100 µm. The Debye length is indicated next to each curve.

3.3.2. Effect of the Thickness of the Optical Cell. The model predicts that the electrostatic contribution to the anchoring energy of the liquid crystal, W, will increase with the separation of the two SAMs forming the liquid crystal cell when the separation is small compared to the Debye length within the liquid crystal. This prediction is qualitatively similar when assuming either constant charge or constant potential boundary conditions in the model. In short, the model predicts homeotropic anchoring of 5CB in thick (compared to Debye length) liquid crystal cells, whereas in thin liquid crystal cells the orientation of the liquid crystal will be dominated by nonelectrostatic (e.g., van der Waals) contributions to the anchoring energy. The mathematical basis for this prediction is shown in eq 3, where W is shown to depend on the area under a plot of the square of the electric field profile for an electrical double layer originating from a surface. We report experiments that test these predictions in Section 4.4. 3.3.3. Effect of the Dielectric Anisotropy of Liquid Crystals. The predicted influence of the electrical double layer on the orientation of the liquid crystal is dependent on the sign of the dielectric anisotropy of the liquid crystal. Whereas nematic phases of 5CB have a positive dielectric anisotropy (∆ > 0), and thus, the predicted effect of the electrical double layer is to promote homeotropic orientation of nematic 5CB, the dielectric anisotropy of the nematic phase of MBBA is negative (∆ < 0). We predict, therefore, that the influence of the electrical double layer on the orientation of nematic MBBA will be to promote orientations of MBBA that are parallel to the surface of the SAM. That is, in contrast to 5CB, the model predicts that nematic phases of MBBA should not adopt homeotropic orientations on SAMs presenting presenting sodium carboxylate groups to the liquid crystal. We report experiments that test this prediction in Section 4.5. 3.3.4. Predicted Influence of Debye Length of Liquid Crystal. Perhaps the least obvious prediction of the model is that of the influence of the Debye length of the liquid crystal on the electrostatic contribution to the anchoring energy of the liquid

4942 J. Phys. Chem. B, Vol. 105, No. 21, 2001

Figure 5. Fractional conversion, Χ, of immobilized carboxylic acid groups to sodium carboxylate as a function of pH of pretreatment.

crystal. The model predicts the influence of electrolyte added to the liquid crystal to depend on the electrical nature of the SAM-liquid crystal interface. Past studies using surface forces techniques have defined two limits for the electrical nature of charged surfaces interacting with solvents, namely constant charge or constant potential behavior.1a Depending on whether the SAM-liquid crystal interface follows constant charge or constant potential behavior, we predict the effect of the added electrolyte on the electrostatic contributions to the anchoring energy to be qualitatively different. First, if the charge density on the surface is not regulated upon the addition of electrolyte (constant charge), the electrostatic contribution to the anchoring energy of the liquid crystal W decreases with decreasing Debye length. In this case, the electric field is screened near the surface with increasing ionic strength. Second, and in contrast to the case of constant surface charge, if the density of charge is regulated (by the exchange of ions between the bulk of the liquid crystal and the SAM-liquid crystal interface) so as to lead to constant potential conditions at the SAM-liquid crystal interface, the model predicts that the electrostatic contribution to the anchoring energy will increase with decreasing Debye length. We report experimental measurements of the effects of added electrolyte in Section 4.6. 4.0. Results 4.1. Characterization of Surfaces Presenting Carboxylic Acid and Sodium Carboxylate Groups. The first goal of our experiments was to prepare surfaces that presented controlled densities of carboxylic acid groups and sodium carboxylate salts. We prepared these surfaces by immersion of the SAMs into aqueous solutions buffered between pH 1.7 and ∼13 (see Methods section for details). In a past study, we characterized surfaces prepared by this procedure using contact angles, ellipsometry, reflectance-absorbance FTIR, and XPS.26 These measurements led us to conclude that (I) our experimental procedures permit the preparation of SAMs presenting COOH and COO-Na+ groups, (II) these SAMs do not support excess salt in the pH range 3.2 to 10.6, and (III) the conversion of the acid to salt does not measurably (by RA-FTIR) perturb the hydrocarbon regions of the SAMs.26 In our past paper, we also demonstrated that the extent of conversion of carboxylic acid groups to sodium carboxylate salts can be determined by using XPS (Figure 5). The results shown in Figure 5 indicate that the conversion of COOH to COONa can be systematically controlled between 0 and 100% using the methods described above. We make four

Shah and Abbott observations using the data shown in Figure 5. First, the extent of conversion of acid to salt within SAMs pretreated with aqueous solutions buffered at pH ∼4 is ∼8%. The onset of conversion of acid to sodium carboxylate is lower than estimates based on other surface analytical techniques (pH 5-6) such as contact angle titrations,17 force-distance curves,18 and quartz crystal microbalance.19 This difference may reflect the use of XPS to directly measure the cation (Na) associated with the carboxylate on the surface or it may be caused by the procedure we used to pretreat our surfaces (immersion and withdrawl).37 Second, our XPS measurements indicate that ∼50 ( 8% of the carboxylic acid groups are converted to their sodium salts following pretreatment in aqueous solutions buffered at pH 8.5. Third, the extent of conversion of acid to sodium salt on surfaces pretreated at pH 10.6 is ∼90%. Fourth, whereas our XPS measurements indicate that the pretreatment of the SAMs with buffered aqueous solutions at pH 10.6 does not lead to the deposition of excess salt on the surface, when pretreated at pH 11.7, we observed a 10% decrease in the area of the Au (4f) peaks in the XPS spectra. We also measured an increase in the ellipsometric thickness of the SAM of ∼2 Å, a result that is consistent with the deposition of excess salts on these SAMs when pretreated at pH 11.7.26 4.2. Dependence of the Out-of-Plane Orientation of 5CB on the Extent of Conversion of Carboxylic Acid to Sodium Carboxylate Salt. The essential proposition underlying the experiments reported in this paper is that control of the extent of conversion of surface-immobilized carboxylic acid groups to the sodium carboxylate moieties will permit manipulation of the density of surface charge that forms when the surface is contacted with liquid crystal. As discussed above, this proposition is based on the results of past studies that have revealed the extent of dissociation of sodium carboxylate salts to be substantially higher than the extent of ionization of carboxylic acids when dissolved in nonaqueous solvents (see Table 1). This proposition leads us to predict that 5CB will tend to assume a homeotropic orientation presenting sodium carboxylate salts because the dissociation of sodium ions from the surface will lead to the formation of an electrical double layer. We tested the above prediction by measuring the orientation of 5CB within optical cells that where fabricated from two SAMs, each formed from HOOC(CH2)10SH and pretreated by an aqueous solution buffered between pH 1.7 and 12 (Figure 6). The two SAMs were spaced by a distance of 45 ( 5 µm in these experiments. We extract three observations from images shown in Figure 6. First, the orthoscopic and conoscopic images in Figure 6 reveal that a transition from planar (at pH 3.1) to homeotropic (pH > 9.6) orientation does occur with increasing conversion of carboxylic acid to sodium carboxylate on the surface. This change in orientation is consistent with the predicted influence on the orientation of 5CB of an electrical double layer formed at the SAM-liquid crystal interface. Second, at pHs between 3.1 and 9.6, it is also evident that there is an increase in the domain size of the 5CB as the carboxylic acid is increasingly converted to sodium carboxylate. Past investigations by Kle`man and co-workers have established that a weakening of the anchoring of 5CB on a surface can lead to an increase in the domain size of the optical texture of the liquid crystal.38 The observed increase in the domain size with increase in extent of conversion of acid to salt is, therefore, consistent with the predicted electrostatic contribution to the anchoring energy (weakening the tendency of the liquid crystal to assume a planar orientation). We quantitatively followed the change in domain size of the liquid crystal by measuring the transmission

Orientations of Liquid Crystals

Figure 6. Optical textures (cross polars) formed by nematic 5CB within optical cells prepared from uniformly evaporated films of gold that support SAMs formed from HOOC(CH2)10SH. The SAMs were pretreated by immersion into aqueous solutions with pH 1.7-12. The thickness of each optical cell was 45 ( 5 µm. The inset, shown in the images corresponding to 9.6 < pH < 12 is a conoscopic image indicating homeotropic anchoring. The bottom figure shows the transmission of unpolarized light at 632.8 nm as a function of pH of pretreatment.

of unpolarized light (without polarizers) through the liquid crystal. A plot of transmission (%) versus pH in Figure 6 shows a continuous increase in the domain size of 5CB between pH 3.1 to 8.5. As the domain size increases, less light is scattered by the liquid crystal, thus resulting in an increase in the intensity of light transmitted through the cells. At pHs greater than 9.4, the intensity of light transmitted through the cell remained constant because the liquid crystal assumed a homeotropic orientation. The third observation extracted from Figure 6 is that there is a change in the qualitative nature of the optical texture of the liquid crystal with increasing pH of pretreatment. At pH 3.1, the texture of the liquid crystal is grainy in nature. This texture converts to a marbled texture at pH 8.0. At pH 8.5, we observed schlieren textures for 5CB. The schlieren

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4943 texture (s ) (1/4) observed at pH 8.5 indicates a tilted (oblique) orientation of 5CB.6 A hometropic alignment was observed for pretreatments at pH>9.4. 4.3. Quantitative Dependence of Out-of-Plane Orientation of 5CB on Extent of Conversion of Acid to Salt. Whereas the results in Sections 4.2 reveal that the orientation of 5CB changes from planar to homeotropic with increasing conversion of carboxylic acid to sodium carboxylate, the observations are qualitative. In this section, we report quantitative measurements of the out-of-plane orientation of 5CB by using the crystal rotation technique. Because this method requires a uniform azimuthal orientation of a liquid crystal, we performed these measurements using liquid crystals supported on obliquely deposited film of gold.39 The thickness of the liquid crystal cell was 45 ( 5 µm. The results shown in Figure 7 reveal the orientation of 5CB to be planar on SAMs pretreated at pH < 8.0 and homeotropic on SAMs pretreated at pH > 9.4. When pretreated at pHs between 8.0 and 9.4, we measured the orientation of 5CB to be tilted between planar and homeotropic. We measured the tilt of 5CB from the normal to increase from 5° to 20° as the pH of pretreatment decreased from pH 9.4 to pH 8.6. Over the same range of pretreatment pHs, we observed schlerien textures for 5CB supported on SAMs on uniformly deposited gold (Figure 6). As noted above, the presence of a schlieren texture with s ) (1/4 defects is consistent with the presence of a tilted (oblique) orientation of 5CB.6 In summary, the results presented in Section 4.2 and 4.3 are qualitatively consistent with the predicted influence of an electrical double layer on the anchoring of nematic phases of 5CB, namely the tendency of 5CB to orient homeotropically on SAMs presenting sodium carboxylate to the liquid crystal. 4.4. Effect of Thickness of Liquid Crystal (Distance between Confining Surfaces). The model described in Section 3 leads to the prediction that the electrostatic contribution to the anchoring energy should be an increasing function of the thickness of the film of liquid crystal. This prediction holds true for cases where the thickness of the film of liquid crystal, and thus, the distance between the confining surfaces is small compared to the Debye length of the liquid crystal. As shown in Figure 4, it also holds true for both constant surface charge and constant surface potential boundary conditions. Here, we report a test of this prediction by measurement of the orientation of 5CB between two identical surfaces spaced by distances that are greater than ∼2 µm and less than 100 µm. The surfaces of the cells were pretreated at either pH 3.2 (100% COOH) or at a pH less than 10.6 (∼90% COONa). The gold films supporting the SAMs were uniformly deposited. Figure 8A-E shows the optical appearances of nematic 5CB supported on SAMs pretreated at pH 3.2. The distances between the two surfaces of the cells were ∼2, 12, 25, 50, and 100 µm, respectively, for Figure 8A-E. Inspection of these images reveals that the orientations of 5CB do not change measurably with increasing thickness of the liquid crystal layer. The textures possess domain sizes that are 0), the predicted influence of an electrical double layer on nematic MBBA would be to promote planar anchoring of MBBA. The optical measurements of the orientation of MBBA shown in Figure 9 reveal the orientation of MBBA to be planar for all cell thicknesses and pHs of pretreatment (pH 3.2 and 10.6). Schlieren textures were observed when using surfaces pretreated at pH 3.2 and 10.6, independent of the cell thicknesses. Defects

Figure 10. Optical textures (cross polars) of MBBA confined within optical cells prepared with SAMs formed from HOOC(CH2)10SH and pretreated (A-E) at pH 3.2 and(F-J) at pH 10.6 with thickness (A,F) ∼4 µm, (B,G) 12 µm, (C,H) 25 µm, (D,I) 50 µm, (E,J) 100 µm. Defects with strength s ) (1/4 are indicated by arrows labeled with a 1 and s ) (1/2) defects are indicated by arrows labeled with 2. The gold films were uniformly deposited (see text for details).

with s ) (1/4 are indicated in Figure 10 by arrows labeled with a 1 and s ) (1/2 defects are indicated by arrows labeled with 2; the s ) (1/2 defects observed in all cells indicate the planar orientation of MBBA. We confirmed these conclusions by quantitative measurement of the orientation of MBBA using the crystal rotation method and obliquely deposited films of gold. For pretreatment pHs of 3.0, 7.5, and 10.6, the tilt angle of MBBA was measured to be NaBr > NaCl.43 When using a concentration of electrolyte of 15mM, we measured the conductivities of 5CB to increase with the size of the halide anion: NaI (1.4 ( 0.3 × 10-6 S/cm), NaBr (4.5 ( 0.5 × 10-9

Orientations of Liquid Crystals

Figure 13. Optical textures (cross polars) of 5CB doped with 15 mM NaCl (A,B,G), NaBr (C,D,G), and NaI (E,F,G) confined within optical cells fabricated using uniformly evaporated gold supporting SAMs formed from HOOC(CH2)10SH and pretreated at pH 10.6. (A,C,E) corresponds to cells with thickness of ∼4 µm and (B,D,F) corresponds to cells with thickness of 12 µm. (G) is the texture and alignment observed for cells doped with NaCl or NaBr or NaI with thickness greater than ∼12 µm. The inset in (B,D-G) is a conoscopic image indicating homeotropic alignment.

S/cm), and NaCl (2.1 ( 0.4 × 10-9 S/cm). When doped with 15mM electrolyte, and when confined to cells with a thickness of ∼4µm, we measured 5CB containing NaCl and NaBr to be aligned in a planar orientation on SAMs pretreated at pH 10.6 (Figure 13). In contrast, 5CB containing 15mM NaI was oriented hometropically. When confined to cells with thicknesses of 12µm, we also measured a progression toward homeotropic anchoring with the sequence NaCl, NaBr, and NaI. When confined to cells with thicknesses greater than 25µm, all electrolytes promote homeotropic alignment. Consistent with the predicted influence of an electrostatic contribution to the anchoring energy of nematic 5CB, we observed the degree of dissociation of the electrolytes within 5CB (as assessed by measurements of conductivity) to affect the out-of-plane orientation of 5CB (cell thicknesses of ∼2 µm to 50 µm) supported on surfaces formed from HOOC(CH2)10SH pretreated at pH 10.6. These results, when combined, provide further support that (I) electrostatic effects contribute to the anchoring of 5CB on SAMs, and (II) the electrical behavior of the SAM-liquid crystal interface is closer to constant-potential than constant-charge. 5.0. Discussion The results reported in this paper support our proposition that an electrical double layer formed at the interface between a SAM presenting sodium carboxylate groups and a liquid crystal can dictate the orientation assumed by the liquid crystal near the interface. Our results are also consistent with the proposition that contact of the liquid crystal with the SAM results in an extent of ionization of the carboxylic acid that is small compared to the dissociation of a sodium carboxylate salt. That is, our results suggest that the density of charge at the liquid crystalSAM interface can be controlled by the extent of conversion of the acid to the salt. These interfaces are stable and can be prepared reproducibly, and thus provide an useful experimental

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4947 system for exploring the effects of electrical double layers on the orientations of liquid crystals. Whereas a number of laboratories have directly measured, by using an AFM cantilever modified with gold-coated silica sphere, electrostatic forces between two surfaces that present SAMs formed from HOOC(CH2)10SH in aqueous solutions, few past studies have reported direct force measurements in nonaqueous systems (isotropic or anisotropic).18,44 In a preliminary study using benzonitrile, an isotropic solvent with similar dielectric properties ( ) 25) to 5CB (para ) 19), our forcedistance measurements indicate that SAMs presenting the COOH moiety are not measurably charged when placed into contact with desiccated benzonitrile. However, upon conversion of ∼90% of the COOH groups to COONa by pretreatment of the SAMs in aqueous solution at pH 10.6, we measured a repulsive, electrostatic interaction within desiccated benzonitrile containing 0.1 M tetrabutylammonium bromide. These results, which will be described in detail elsewhere, support our conclusions based on the orientational behavior of 5CB that the density of charge on SAMs contacted with a nonaqueous solvents can be controlled by the extent of conversion of carboxylic acid to its sodium carboxylate salt.45 We also point out that the orientational behavior of the liquid crystal shown in Figure 8 can be used to infer the range of the interaction associated with the electrical double layer in the liquid crystal (without added electrolyte). Inspection of Figure 8 reveals that the transition from planar to homeotropic orientation occurred within cells with thicknesses of ∼12 µm. By comparing this result to the predictions of the model in Figure 4, we estimate the magnitude of the Debye length to be 2-5 µm (without added electrolyte). Although this estimate is slightly greater than the estimate based on conductivity (κ-1 ≈ 1 µm; Figure 12), both techniques lead to estimates of the Debye length that are of the same order of magnitude. Although these values for the Debye length appear small as compared to the cell thickness, it is important to note that both surfaces of the cell support the SAMs, and thus, the behavior of the liquid crystal in the cells is influenced by the overlap of the diffuse double layers from each surface. The experimental study of electrostatics in thermotropic liquid crystals reported in this paper required that we address a number of issues that are less important in studies of electrostatics in aqueous systems. First, in solvents with low dielectric constants ( < 30), the extent of dissociation of electrolytes is typically low,25 and thus, conductance or spectroscopic techniques must be used to measure the extent of dissociation of ions in order to estimate the ion density. Because liquid crystals are anisotropic, it is also important to align the liquid crystals in a known orientation during conductivity measurements.47 In our experiments, we exploited the self-assembly of alkanethiols on gold to align liquid crystals on the gold electrodes. Second, the ionic content of liquid crystals have been reported to vary widely because of variations in water content and other ionic impurities. Thus, it is important to measure the ionic density of the liquid crystal in each experiment. For instance, Thurston et al48 reported the Debye length to be ∼0.6 µm by using DC switching studies within a nematic mixture, 〈E7〉. The electrolyte concentration due to impurities was calculated to be 4 µM. Using conductivity measurements, Cognard estimated the Debye length to be ∼1 µm when using 50 nM of tetrabutylammonium perchlorate dissolved in nematic 5CB.49 Lu et al.13b directly estimated the Debye length to be ∼25 µm within MBBA by measuring the electric field distribution within a cell (thickness ∼1 cm) with a 5 kV applied potential. Colpaert measured the

4948 J. Phys. Chem. B, Vol. 105, No. 21, 2001

Shah and Abbott

TABLE 2: Effect of Electrolyte (1:1) Concentration on the Surface Charge Density Estimated from Surface Forces Measurements with Surfaces Separated by 30 nma

solvent

Debye length (nm)

surface charge (C/m2) 0.001 0.0015 0.006



electrolyte

acetone2f

20

TBABb

ethylene glycol2g

37

LiCl

28 14 40

propylene carbonate2e

65

TEABc

8 27

0.015 0.0045

water46

78

KBr

9 145 14

0.007 0.002 0.01

a The two sets of data for each type of solvent correspond to two concentrations of electrolyte. b TBAB - Tetrabutylammonium Bromide (H3C(CH2)3)4NBr. c TEAB - Tetraethylammonium Bromide (H3C(CH2))4NBr.

ion concentration due to impurities in purified ZLI-4757 (Merck, New Jersey) to be ∼5 nM which is similar to measurements by He´rino for liquid crystal methoxybutylazoxybenzene (MBAB)50 and Sprokel for recrystallized MBBA.13a,c Our measurements of conductivity of 5CB lead to estimates of the Debye length of ∼1 µm (measured in a direction that is perpendicular to the director). These measurements are in reasonable agreement with the observed orientational behavior of 5CB, which leads to estimates of the Debye length of 2-5 µm. Our experimental measurements of orientations of liquid crystals reported in this paper also provide insights into the electrical nature of the SAM-liquid crystal interface. By comparing the experimentally measured influence of added electrolyte (Figures 12 and 13) to the predictions of the model (Figure 11), we conclude that the charge at the interface is regulated by the changes in ionic strength so as to possess an approximately constant-potential nature. These results were obtained from experiments performed with a fixed separation between the surfaces. For comparison, we note that surfaces forces measurements performed with mica (and various 1:1 electrolytes and solvents) have revealed both constant potential and constant charge behavior as a function of separation of the two mica surfaces.2,46 Whereas the electrical behavior of the surfaces were determined in these past studies by analyzing the force as a function of separation, the experiments reported herein were performed at constant separation as a function of the concentration of electrolyte. To permit comparison of surface force measurements to our experiments, we have compiled past estimates of the surface charge density obtained from surface forces measurements for a fixed separation of 30 nm. Table 2 presents these results for a variety of polar and hydrogenbonding solvents with added 1:1 salts. Inspection of the results in Table 2 reveals that the charge of the mica surface is regulated upon addition of salt. The results reported in this paper are consistent with the proposition that the homeotropic orientation of 5CB observed on surfaces presenting sodium carboxylate is driven by the formation an electrical double layer at the interface. Changes in short-range interactions between the liquid crystals and the substrates cannot account for the observed dependence of the orientations of the liquid crystals on the thickness of the optical cells. In the absence of the double layer, that is, when using a surface pretreated at low pH, we observe the anchoring of 5CB to be planar. Because we also observe planar anchoring of nematic phases of 5CB on SAMs formed from alkanethiols (methyl group presented at the surface), we conclude that the

planar anchoring is the result of van der Waals forces operating at this interface.51 Thus, the out-of-plane anchoring transition that we observe on SAMs formed from HOOC(CH2)10SH with increasing pH of pretreatment likely reflects the dominant influence of van der Waals forces at low pH and the dominant influence of electrostatic interactions resulting from the formation of an electrical double layer at high pH. The results presented in this paper demonstrate that the orientational behavior of liquid crystals can be used to estimate the Debye length within a liquid crystal as well as the dominant electrical nature of the liquid crystal-SAM interface. Here, we illustrate how these observations can also be used to estimate the surface potential at the SAM-liquid crystal interface. Past studies by Miller and Abbott have measured the anchoring energy W for 5CB on SAMs formed from H3C(CH2)15SH to be approximately ∼0.1 mJ/m2.51 This anchoring energy, which causes a planar orientation of 5CB, results from van der Waals interactions between the liquid crystal and the SAM-gold interface. Because the van der Waals forces act between the underlying gold substrate and the supported liquid crystal across the SAM, these interactions are present when using SAMs terminated with the carboxylic acid and sodium carboxylate functional groups. Thus, the transition we observe in the outof-plane orientation of liquid crystals reflects a change in the balance of van der Waals and electrostatic forces. Because we observe a transition from planar to homeotropic orientation within cells with thickness between ∼12-25 µm (Figure 8), we can estimate the surface potential from Figure 4B (for the constant potential case) by matching the calculated electrostatic contribution to the anchoring energy to the van der Waals contribution (∼0.1 mJ/m2). We estimate the surface potential of the SAM-5CB interface to be 2, 3, and 8 mV for Debye lengths of 1, 2, and 5 µm, respectively. We have interpreted the experiments reported in this paper in terms of an interaction between an electrical double layer at a surface and a medium that possesses anisotropic dielectric properties. Barbero and co-workers have noted that other types of electrostatic coupling can exist between an electric field associated with an electrical double layer and a liquid crystal.10 For example, flexoelectricity has been theorized as the basis of an additional mechanism that couples the gradient in an electric field to the quadrupolar properties of the mesogens. Although these types of interactions may well exist in the experimental system that we have investigated, our experimental results conform to the predictions of a model based on a dipolar coupling of the liquid crystal and the electric field with an electrical double layer. In a recent study, we report on the azimuthal orientations assumed by nematic phases on 5CB when supported on SAMs formed from HOOC(CH2)10SH that had been pretreated with aqueous solutions with increasing pH.26 These studies were performed on obliquely deposited gold films. We observed a change in the azimuthal orientation of the liquid crystal upon conversion of the acid to the sodium carboxylate salt. Here, we simply point out that this past study was performed using “thin” (