J. Phys. Chem. C 2007, 111, 14365-14370
14365
Electric Birefringence of Electrolytes near Charged Surfaces, II: Effect of Polymeric Additives Sourav Saha and Lynden A. Archer* School of Chemical and Biomolecular Engineering, Cornell UniVersity, Ithaca, New York 14853 ReceiVed: January 2, 2007; In Final Form: April 22, 2007
Electric birefringence of electrolytes containing linear polyacrylamide and polyethylene oxide near a negatively charged silica glass surface is studied using evanescent wave laser polarimetry. We find that addition of polymers to electrolytes near charged surfaces leads to substantially larger levels of electric birefringence than the already anomalously large electric birefringence observed in pure electrolytes near charged surfaces. The polymer contribution to the birefringence increases with surface charge, ionic strength of the electrolyte, and electric field, but is only a weak function of polymer molecular weight and polymer concentration. Unusually high levels of polymer orientation are observed for moderate electric fields (10-30 V/cm). These observations are discussed in terms of large local shear gradients experienced by polymer chains subject to electro-osmotic flow in the electric double layer near charged surfaces.
I. Introduction Neutral polymers are widely used as separation media for size-based separation of various charged molecules/objects using electrophoresis.1 Capillary electrophoresis (CE), a commonly used analytical tool for nucleic acid sizing, sequencing, and genotyping, uses a capillary tube filled with a solution of uncross-linked neutral polymer to separate polyelectrolytes. In a recent study, we compared electrophoretic mobility and dynamics of fluorescently labeled linear and star-branched DNA in semidilute LPA solutions.2 Surprisingly, we found that although electrophoretic migration dynamics of the fluorescently labeled branched and linear polyelectrolytes are rather different, their electrophoretic mobilities are nearly identical for comparable molecular weight analytes. This observation suggests that polymer resistance to DNA migration is localized in a region near the DNA segment invisible to our fluorescence visualization measurements. From the field and the electrolyte concentration dependence of the mobility, we tentatively concluded that nonNewtonian flow of neutral polymer induced by electro-osmotic flow (EOF) in the electric double layer (EDL) sets the polymer resistance. If correct, this explanation implies that electrophoretic separation of all charged objects in CE in semidilute polymer solutions is likely governed by local, complex, likely nonNewtonian flows of neutral polymer. Given that the typical Debye lengths, κ-1, in CE are 1 or more orders of magnitude smaller than the random coil size, Rg, of neutral polymers used in these measurements, the mechanism by which polymer chains are oriented and stretched in the double layer remains an open question. In this article, we consider the related but, in principle, simpler problem of electric-field-induced changes in conformation of neutral polymers near a negatively charged planar surface. To quantify orientation, we employ a recently developed optical tool, evanescent wave laser polarimetry (EWLP),3 which relies on total internal reflection of laser light at an interface to report field-induced orientation anisotropy of molecular segments near * To whom correspondence should be addressed. E-mail: laa25@ cornell.edu.
a glass substrate. In our previous EWLP study,4 we investigated the electric birefringence of adsorbed ions on the glass surface. A model based on the idea that oppositely charged ions in solution can form ionic clusters at the glass surface was used to explain the anomalously large birefringence produced by simple buffers near silica glass surfaces. This model contends that field-induced polarization and orientation of the clusters lead to anomalously high apparent Kerr law constants near charged surfaces. A significant finding from the previous study is that the surface birefringence produced by a polymer-free electrolyte solution is independent of the electrolyte pH. This result is essentially opposite of what one would expect for birefringence produced by strong EOF-induced shear flow near a charged surface. The solution pH therefore provides a convenient variable for isolating surface birefringence contributions from the sought-after EOF induced shear flow in the EDL from the already quite large effects produced by ionic clusters. II. Experiment Figure 1 is a schematic diagram of the EWLP experimental setup. The flow cell consists of a 30 × 10 × 1 mm open channel cut into polyoxymethylene base with platinum-black-coated electrodes, separated by 30 mm, located at each end of the channel. Polymer solutions are sealed in the channel using a silica glass hemisphere (borosilicate, radius ) 20 mm, refractive index n1 ) 1.52), and the charge at the glass surface is varied by changing the pH of the solutions. A Biorad, PowerPac 1000 power supply is used to apply moderate dc electric fields (1030 V/cm) across the channel. These electric fields are deliberately kept low to circumvent problems such as ohmic heating and electrolysis related to the application of high voltages across conducting media. A helium-neon laser with wavelength λ ) 633.2 nm incident at an angle, φ, of 83° to the direction y (see Figure 1) is employed to generate an evanescent field in the electrolyte solution near the 1-2 (glass-polymer solution) interface. The refractive index of the polymer solution is n2 = 1.33; the critical angle, φc ) asin(n2/n1), is then close to 61°. Thus, for φ > φc the laser light is totally reflected at the 1-2
10.1021/jp0700231 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/08/2007
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Figure 1. Schematic diagram of EWLP experimental arrangement used in the study. The silica glass hemisphere is unrestrained in these experiments; it is held in place by its weight and the surface tension of the liquid sample.
interface, and the evanescent wave carries information about the optical properties of the material in medium 2 located within a characteristic penetration depth dP ) λ/(2π xn12sin2φ-n22) , which is ∼140 nm for our experiment configuration.3,5 This value is comparable to but larger than the radius of gyration for the highest molecular weight polyacrylamide solutions (LPA5 (Rg ≈ 110 nm), LPA2 (Rg ≈ 70 nm)) used in the study. The optical arrangement shown in Figure 1 therefore allows the time-dependent birefringence and orientation of optically anisotropic molecules “near” (i.e., within a few Rg) the interface to be simultaneously determined. To carry out these measurements, the laser light is first polarized parallel to the direction of the electric field, x, using a linear polarizer (P), and subsequently modulated using a photoelastic modulator (PEM), and a quarter wave plate (Q) oriented in the configurations shown. Following total internal reflection at the 1-2 interface, the light beam is analyzed using a circular polarizer (CP) and photodetector (D) and demodulated using lock-in amplifiers. The time-dependent intensity I(t) measured by the detector is related to the intensity of the incident light, I0; the polarization modulation frequency, ω; the birefringence retardation, δ′ ) 2πdeff∆n′/λ, of the electrolyte medium; and the apparent orientation angle, χ′, by the following expression,3
I(t) ≈
I0 [1 - 2J1](A) sin(δ′) cos(2χ′) sin(ωt) + 2 2J2(A) sin(δ′) sin(2χ′) cos(2ωt)] (1)
where, deff ≈ 2dP/(cos φ) is the effective optical path length, which is ∼2.3 µm for the optical configuration used in Figure 1. ∆n′ is the induced birefringence of the medium near the glass surface. J1(A) and J2(A) are Bessel function coefficients that arise from the modulation.6 Both coefficients can be determined from separate calibration experiments using materials of known birefringence retardation with the PEM tuned to achieve J0(A) ) 0.3,6 An alternative form of eq 1, I(t) = Rdc - Rω sin(ωt) + R2ω cos(2ωt), indicates that two lock-in amplifiers tuned to ω and 2ω with appropriate phases and a low pass (dc) filter are sufficient to measure all Fourier amplitudes, Rk, of the optical signal needed to determine δ′ (or ∆n′) and χ′ simultaneously. Linear polyacrylamide (LPA) was supplied by Ciba Specialty Chemicals (UK) and, in some cases, purchased from Scientific Polymer Products, as indicated in Table 1. Polyethylene oxide (PEO) used in the study was purchased from Polymer Source (Dorval, Quebec, Canada). LPA and PEO solutions suitable for electrophoresis studies were made by dissolving the appropriate amounts of polymer in 4 mM TAE (Tris-acetate-EDTA, pH
TABLE 1: Characterization Information for Polymer Solutions polymer solutions LPA0.6-2a LPA1.2-2a LPA2.0-0.75a LPA2.0-2a LPA2.0-3a LPA2.0-6a LPA5-3b PEO1.2-1.2c PEO0.5-7.4c
Mw (MDa)
PI
C (%w/v)
0.59 1.2 1.7 2.05 2.05 2.05 5-6 1.2 0.53
1.7 2.0 1.9 1.9 1.9 1.9
2 2 0.75 2 3 6 3 1.2 7.4
1.7 1.07
τc (s) 0.0067 0.0036 0.077 0.11 0.54
a Ciba Specialty Chemicals. b Scientific Polymers Products. c Polymer Source.
8.3) and 89 mM TBE (Tris-borate-EDTA, pH 8.3) buffer, respectively, with continuous stirring for several days. For pHdependent study, LPA solutions were prepared in various pH buffers with 5 mM NaCl. The pH of the buffers made from maleate (pKa ) 2.0), formate (pKa ) 3.75), acetate (pKa ) 4.76), pyridine (pKa ) 5.23), phosphate (pKa ) 7.2), and Tris (pH ) 8.06) were adjusted to approximately 2, 3, 4, 5, 7, and 8 respectively, by titrating with strong base (NaOH) or strong acid (HCl) as required. LPA concentrations ranging from 0.75% w/v to 6.0% w/v, and PEO concentrations of 1.2% w/v and 7.2% w/v were used for this study. The characteristic bulk relaxation time, τc, the reciprocal of the crossover shear rate corresponding to the onset of shear thinning, of the LPA solutions was characterized using independent mechanical rheometry experiments (see Table 1). Organic and other adsorbed impurities on the glass surface were cleaned by treating the glass hemisphere and cover slips in a 50/50 mixture of sulfuric acid and 30% v/v hydrogen peroxide at 100 °C for 1 h before carrying out the birefringence experiments. The surfaces were subsequently thoroughly rinsed in DI water. To evaluate the effect of surface chemistry on the measurements, borosilicate hemispheres were functionalized with a short polyethylene glycol (oligo-PEG) self-assembled monolayer using CH3(CH2CH2O)6-9(CH2)3Si(OCH3)3 (OEGOEt) (Gelest, Inc.). As discussed later, the zeta potential of the oligo-PEG-coated glass is close to that of the bare substrate, which allows the effect of polymer surface adsorption to be isolated without interfering with the underlying EOF. Dynamic contact angle (DCA) (Cahn Instrument, DCA-315) and ellipsometry (Rudolph Research, Auto EL, model no. 2C, 4A) measurements on SAM-coated substrates revealed that nearly complete coverage by a well ordered monolayer coating is achieved with the grafting procedure used in the current work.
Electric Birefringence of Electrolytes, II
Figure 2. Time-dependent evolution of ∆n′(t) at E ) 30 V/cm after field imposition and removal. Results are for LPA5-3 in buffer (dark lines) and for the neat buffer solutions without polymer (gray lines) at (a) pH 2 and (b) pH 8. (c) Plot of polymer contribution to the measured birefringence, ∆n′p, versus pH for LPA5-3.
III. Results and Discussion A. Electric Birefringence of Polymer Solutions near Glass Surface. Effect of pH. Figure 2a-b compares time-dependent EWLP raw data for LPA5-3 in buffer and for the buffer without polymer. The results presented in the figure are for pH 2 and 8, respectively. A fixed electric field E ) 30 V/cm was applied by manually turning on the power supply at a preset time. The field is removed by manually turning the power supply off at a chosen time. The birefringence signal is observed to rise in response to the imposition of the field and to approach a steadystate value after a period of time. Upon removal of the field, the birefringence decays to zero after a period of time. Significantly, for pH 2, which is close to the isoelectric point (IEP) of the glass surface, the presence of LPA in the buffer solution does not significantly influence the birefringence amplitude or its time-dependence (see Figure 2a). On the other hand, for pH 8, at which the glass surface is negatively charged, the polymer solution is seen to manifest much higher birefringence levels than what is observed for the buffer solution alone (see Figure 2b). In the previous paper in this series, Kerr-like surface birefringence of the pure buffer solutions was explained in terms
J. Phys. Chem. C, Vol. 111, No. 39, 2007 14367 of electric-field-induced polarization and alignment of ionic clusters at the glass surface.4 The birefringence increase observed in the presence of polymer can be attributed to at least two sources: (i) The solvated polymer chains are incorporated in individual clusters or bridge multiple clusters. In either case, polarization of the clusters in an external field should lead to strong polymer segment orientation and an increased birefringence. (ii) The polymer contribution originates from electricfield-induced alignment and stretching of neutral polymers by the electroosmotic shear flow (EOF) in the Debye layer. This latter contribution is expected to be a strong function of the zeta potential of the glass surface and is thereby an increasing function of pH relative to the isoelectric point for silica glas.7 The polymer contribution to the measured electric birefringence, ∆n′p can be estimated by subtracting the steady-state birefringence signal for the pure buffer solution, ∆n′0, from the steady-state signal measured in the same buffer containing polymer. Figure 2c shows the effect of buffer pH on ∆n′p. It is evident from the figure that unlike ∆n′0, which is independent of pH,4 ∆n′p is a strong increasing function of the solution pH. In particular, ∆n′p approaches zero near the isoelectric point of silica glass (pH ≈ 2) and increases as the solution pH is increased above this value. This result indicates that the zeta potential of the surface has a significant effect on ∆n′p, but no measurable effect on ∆n′0. It also shows that the polymer contribution to the surface birefringence is unlikely to originate from the same processes as for the pure electrolyte; the effect of surface charge on ∆n′p is, in fact, essentially what would be expected for a polymer birefringence contribution by mechanism ii. Effect of Electric Field. If EOF in the Debye layer is responsible for the polymer contribution to the birefringence, we would also expect the effect of electric field on ∆n′p to be a strong function of surface charge and salt concentration in the buffer. The effect of electric field E on ∆n′p is summarized in Figure 3a for LPA5-3 at pH 2 and 7. At pH 2, that is, close to the IEP of the silica glass surface, ∆n′p is close to zero and independent of E; whereas at pH 7, ∆n′p is seen to increase with increasing E. As a first approximation, the shear rate in the EDL produced by EOF can be estimated as, γ˘ ≈ νeof/κ-1, where νeof ) Eζ/ηs is the EOF velocity,7 ζ is the zeta potential of the glass surface, and and ηs are the permittivity of water and buffer viscosity, respectively (neutral polymers at moderate concentrations do not alter appreciably8). κ-1 is the Debye layer thickness and inversely proportional to the square root of electrolyte concentration.7 Hence, γ˘ increases with E except around the IEP (ζ ∼ 0), where γ˘ should be close to zero for all E, which is consistent with our observation. If the ζ potential for silica glass does not decrease too strongly with increasing ionic strength, γ˘ should also increase with increasing ionic strength of the buffer. From the reported zeta potential values of silica glass in monovalent salt solutions at pH ) 8.3,9 ζ is expected to change from 75 mV in a 4 mM TAE buffer to ∼55 mV in 20 mM TAE (see inset of Figure 3b). Figure 3b illustrates the effect of salt concentration in the buffer solution on ∆np for LPA2.0-2 in 0.1 X TAE (4 mM Trisacetate, pH ∼ 8.3) and 0.5 X TAE (20 mM Tris-acetate, pH ∼8.3) buffer solutions at various E. An increase in ∆n′p with increasing electrolyte concentration is observed for all electric fields studied. The effect is evidently small at low fields, but very large at high fields (in reality, somewhat larger than the field-dependence might have suggested). This discrepancy can be rationalized in terms of the more complex shape of the
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Figure 4. (a) First and second lock-in amplifier output waveforms for the LPA5-3 in buffer solution (dark line) and the buffer solution without LPA (gray line) at pH 8; inset shows the same data at pH 2. In both cases, measurements are for a single electric field, E ) 30 V/cm, and the larger signal amplitude corresponds to Rω. (b) Plot of χ′ versus E for various LPA solutions in TAE buffer. Inset is a schematic of the orientation angle.
Figure 3. (a) ∆n′p versus E for LPA5-3 in buffer solution at various pHs. (b) LPA2.0-2 in TAE buffer at varying buffer concentrations. The plots of ζ against ion (monovalent) concentration for silica glass (from ref 9). (c, d) LPA solutions in 0.1 × TAE buffer; inset in (c) shows corresponding results for PEO.
velocity profile in the EDL than the linear profile assumed in our estimate of γ˘ . Figure 3c-d shows that for all LPA and PEO solutions investigated, ∆n′p increases with increasing E. The inset in Figure 3c summarizes the results for PEO solutions. These results demonstrate that the electric-field-induced birefringence of polymers observed near charged surfaces is quite general, that is, not limited to LPA. Figure 3d shows that for a fixed LPA Mw, ∆n′p is a weakly increasing function of electric field and a very weak function of polymer concentration over the
range studied. Figure 3c shows that ∆n′p is also a rather weak function of polymer molecular weight. In the next section, we will show that both observations are, in fact, consistent with the high shear rates produced by EOF in the EDL. B. Orientation Angle. The steady-state apparent orientation angle χ′ ) 1/2 tan-1(R2ω/Rω) can be obtained from the relative magnitude of the first (Rω) and second (R2ω) lock-in signals (see eq 1). Figure 4a plots Rω (t) and R2ω (t) for LPA5-3 in buffer solutions with pH 2 and 8. For comparison, data is also provided for the respective buffer solutions without LPA. It can be clearly seen from this figure that the steady-state Rω is about an order of magnitude higher than R2ω for both the neat buffer and for the LPA solutions, irrespective of pH. This means that χ′ is close to zero, whether polymer is present or absent. Virtually identical results are observed for all polymer solutions (PEO and LPA) studied over the entire range of electric fields considered. Figure 4b depicts steady-state χ′ values for various LPA solutions to illustrate this point. The steady-state χ′ values reported were obtained by fitting the measured Rω(t) and R2ω(t) data to a single-exponential growth function and using eq 1 to compute χ′. It is again evident from Figure 4b that for all polymer solutions studied and over the full range of E examined, the steady-state values of χ′ are close to zero, signifying very high levels of orientation is achieved; χ′ ≈ χ (see Figure 4b (inset)) for φ () 83°) close to 90°. In our previous study,4 we also found χ ≈ 0° for the polymer-free buffer solutions and suggested that this observation follows from surface-attached ionic clusters (i.e., ionic clusters at the surface rotate in the x-z plane because these clusters always remain bound to the glass surface). The fact that χ ≈ 0° for the polymer solutions could originate from at least two sources: (i) The high level of orientation of ionic clusters obfuscates any possible misalignment of polymer segments with the imposed field. (ii) The polymer segments
Electric Birefringence of Electrolytes, II are themselves highly oriented either by coupling to the clusters or by the EOF shear flow near the glass surface. On the basis of the effect of pH on the polymer contribution to the steadystate birefringence of the solutions, we are encouraged to pursue the last possibility. To do this, we first need an estimate for the effective Weissenberg number, Wi ) γ˘ τc, where γ˘ is the apparent shear rate in the EDL and τc is a characteristic polymer relaxation time. For 4 mM TAE, κ-1 is ∼5 nm and ζ is ∼75 mV (for pH ∼ 8.3).9 This means that at a field of just 30 V/cm, γ˘ = 33 000 s-1, an extremely large value. This result implies that for LPA2.0-0.75 (the solution with the lowest bulk relaxation time; see Table 1), Wi ≈ 120 at E ) 30 V/cm. Even larger Wi values characterize the near surface flow for the other polymer solutions (e.g., Wi ≈ 2540 for LPA2.0-2, and Wi ≈ 17 800 for LPA2.0-6). These are exceedingly high Weissenberg numbers by any measure. Specifically, at such high Wi values, polymer chains are anticipated to be quite strongly aligned with the shear direction, and χ is expected to be close to zero for the polymer independent of any possible contribution from oriented clusters. The usually strong effects of polymer concentration and molecular weight on birefringence are also expected to significantly weaken, which is also observed from the birefringence experiments (see Figure 3c-d), because greater levels of shear thinning manifested by more concentrated solutions compensate for their initially higher zero-shear viscosity and slow-flow primary normal stress coefficient values. C. Relaxation Time. Finally, we consider the relaxation time of the polymer contribution, ∆n′p. The characteristic relaxation time of the birefringence decay can be estimated by plotting the product of the normalized birefringence decay signal and time (∆n′/∆n′max)t against the time t, where ∆n′max is the maximum value of the birefringence signal. It is straightforward to show that for a single Debye-like (exponential) relaxation function, the time, τB, corresponding to the maximum in (∆n′/ ∆n′max)t is the characteristic relaxation time. If the relaxation function is a sum of discrete Debye functions with wellseparated time constants, a plot of (∆n′/∆n′max)t will manifest multiple maxima, each locating a characteristic time scale for the relaxation process. It was shown in the previous paper in this series4 that the characteristic birefringence relaxation time obtained using this approach is identical to the value determined by fitting birefringence relaxation data to a single-exponential function. Figure 5 shows the birefringence relaxation time obtained from the birefringence decay signals for LPA5-3 at various pHs (5a), as well as for varying polymer molecular weight and repeat unit chemistries (5b). The inset compares (∆n′/∆n′max)t versus t for the buffer at a fixed pH of 8.0, with and without polymer. The plots in the inset show a single strong maximum irrespective of the conditions, indicating that a single dominant process with characteristic time τB ≈ 20 s governs birefringence relaxation under all of the conditions explored. Significantly, the position of the maximum is seen to be the same whether polymer is present or absent from the buffer (see inset). In addition, the characteristic birefringence decay time, τB, is unaffected by solution pH, polymer molecular weight, or polymer chemistry LPA/PEO. The insensitivity of the birefringence relaxation time confirms that this time scale does not reflect relaxation of the polymer component in solution. This observation is surprising because one would expect to see a fast relaxation due to reorientation of polymer chains after cessation of shear by EOF, followed by a slow relaxation due to reorientation of the ionic clusters at the glass surface.4 The fact that this is not observed and, furthermore, that the characteristic decay time is unaffected
J. Phys. Chem. C, Vol. 111, No. 39, 2007 14369
Figure 5. (a) Plot of τB versus pH for LPA5-3. Left inset shows the plot of τB versus E for LPA solutions in 0.1 × TAE buffer. These measurements were performed using OEG-OEt-SAM-coated glass. Right inset plots (∆n′/∆n′pmax)t versus time t for LPA5-3 at pH 8. (b) tB versus electric field for various polymer molecular weights and solution concentrations.
by variables such as the solution pH, already shown to strongly influence the magnitude of ∆n′p, proves that this time scale is unrelated to the polymer birefringence relaxation. As discussed in the previous article,4 slow relaxation of surface birefringence from the buffer can be rationalized in terms of slow diffusion of ionic clusters at the surface by local adsorption/desorption processes. It is possible to investigate this effect indirectly by modifying the surface chemistry of the glass. To this end, we grafted a dense self-assembled monolayer of PEO oligomers (OEG-OEt) to the silica glass substrate. This oligomer was chosen for a variety of reasons, including its hydrophilicity and the fact that the oligo-PEG SAMs are known to have high surface charge due to preferential hydroxide ion adsorption. ζ for the modified glass surface at 4 mM buffer concentration is ∼70 mV;10 hence, the presence of SAM should reduce the physical adsorption of polymer chains to the glass surface without significantly changing the electro-osmotic flow near the surface. The thickness of the SAM is found by ellipsometry to be 2.2 ( 0.06 nm, which is close to its fully extended length, confirming a densely packed SAM on the silica surface. The inset in Figure 5a summarizes τB data measured in polymer solutions near the SAM-coated substrates. It is apparent from this figure that τB is minimally, if at all, affected by the surface treatment. This result appears to rule out slow surface rearrangement of ionic clusters by adsorption/desorption processes as the source of the long relaxation times. The current experimental setup does not allow us to consider the effect of temperature on the relaxation times; however, it allows us to pursue other processes. For example, independent measurements of the much faster current rise and decay characteristics rule out slow ohmic heating/cooling as the source for the long decay times. This conclusion is also supported by the lack of dependence of τB on ionic strength (conductivity) of the solutions, magnitude of the electric field, and length of the field and magnitude of the field imposition time. It is, of
14370 J. Phys. Chem. C, Vol. 111, No. 39, 2007 course, possible that an “unknown” conduction pathway near the charged surface (perhaps in the EDL) provides a local source of heat that leads to uneven stresses (and hence, birefringence) in the glass hemisphere. Arguments against this mechanism include the fact that a single Fourier amplitude, Rω, of the optical signal is disproportionately affected. This would be unexpected if the birefringence originates from a temperature gradient between the submerged surface of the hemisphere and its exposed parts (Figure 1). In addition, the absence of fieldinduced birefringence in bulk (transmission) polarimetry measurements performed at electric fields as high as 100 V/cm is inconsistent with this mechanism. We have also performed real-time visualization of the solution glass interface in an optical microscope (100×) to determine whether interference produced by some unusual field-induced wetting phenomena or field-induced formation/alignment of trapped gas bubbles (e.g., produced by electrolysis of H2O) could be the source of the long birefringence relaxation times. Even at fields as high as 100 V/cm (i.e., about 3 times higher than the electric fields used for the birefringence runs), the solution glass interface was found to be optically “silent”. IV. Conclusions In this article, the second in a series of two, direct measurements of electric-field-induced orientation of neutral polymers is performed near charged planar substrates. We observe large enhancements in birefringence due to the polymers near the glass surface. The polymer contribution to the measured birefringence is a strong function of surface charge and concentration of ions in the buffer. These observations are discussed in terms of the large velocity gradients produced by EOF near the glass surface. Specifically, even at modest electric fields, we show that EOF
Saha and Archer induces strong shearing of neutral polymer chains in the EDL near charged surfaces, which can induce high levels of molecular alignment. We also observe that the characteristic time for relaxation of the field-induced birefringence in polymer solutions is similar to the relaxation time of adsorbed ions at a silica glass surface soaked in buffer. Silica glass surfaces modified by chemical attachment of oligo-PEO self-assembled monolayers (SAMs) are also found to have little, if any, effect on the relaxation time, indicating that the ionic cluster relaxation process is unlikely to proceed by a surface adsorption/desorption mechanism. Acknowledgment. We are grateful to Abraham Stroock for helpful discussions. The study was supported by the National Science Foundation (Grants DMR0551185 and DMR 0404278). References and Notes (1) Albarghouthi, M. N.; Barron, A. E. Electrophoresis 2000, 21, 4096. (2) Saha, S.; Heuer, D. M.; Archer, L. A. Electrophoresis 2006, 25, 3181. (3) Dao, T. T.; Archer, L. A. Langmuir 2001, 17, 4042. (4) Saha, S.; Archer, L. A. J. Phys. Chem. C 2007, 111, 14358. (5) Harrick, N. J. Internal Reflection Spectroscopy; Harrick Scientific Corp.: Ossining, NY, 1987; Chapter 2. (6) Fuller, G. G. Optical Rheometry of Complex Fluids; Oxford University Press: New York, 1995; Chapter 3. (7) Hunter, R. J. Foundation of Colloid Science; Clarendon Press: Oxford, 1993; Vol. 1, Chapter 6, p 332; Chapter 9, p 554. (8) Brooks, D. E.; Seaman, G. V. F. J. Colloid Interface Sci. 1973, 43, 670. (9) Wiese, G. R.; Sanders, R. S.; Chow, R. S.; Masliyah, J. H. J. Colloid Interface Sci. 1995, 174, 230. Wiese, G. R.; James, R. O.; Healy, T. W. Discuss. Faraday Soc. 1971, 52, 302. (10) Chan, Y. M.; Schweiss, R.; Warner, C.; Grunze, M. J. Colloid Interface Sci. 2003, 19, 7380.
