Article pubs.acs.org/Biomac
Electrooptical Behavior of Aqueous (Hydroxypropyl)cellulose Liquid Crystals Containing Imidazolium Salts Mitsuhiro Ito, Yoshikuni Teramoto, and Yoshiyuki Nishio* Division of Forest and Biomaterials Science, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan ABSTRACT: A dynamic control of the cholesteric coloration and optical clarity of aqueous (hydroxypropyl)cellulose (HPC) lyotropics is attainable under a weak electric field by employing a fluctuating ionic additive as P and Tc shifter (P, cholesteric pitch; Tc, cloud point). The present Article demonstrates some examples of time-evolving gradation in reflection color and transparency for HPC liquid crystals containing various N-alkyl-substituted methylimidazolium salts ([CnMim][X]); this was perceivable when each anisotropic solution was sealed in a layer form between parallel slide glasses spaced by a pair of carbon electrodes and then electrified with a direct circuit. The electrooptical phenomenon was interpreted as being primarily due to generation of a disproportional dislocation of cation (CnMim+)/anion (X−) constituents. Even after the electric supply was ceased, an appreciable potential difference remained in the color-gradated samples. It is suggested that the salt-containing liquid-crystalline system behaves like a quasi-capacitor as a viscous electrolytic medium of high resistance.
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temperature.7,8 In brief, at HPC concentrations of more than 45 wt %, the polymer solution is optically anisotropic, and, particularly at 50−70 wt %, it imparts a color because of the selective light reflection originating from a well-developed cholesteric arrangement; the supramolecular helical pitch decreases with increasing polymer concentration and with decreasing temperature. The binary system also shows a unique phase-separation behavior with the lower critical solution temperature (LCST); a so-called cloud point is situated around 40 °C with regard to isotropic HPC solutions (≤40 wt %), but it is rather lowered at higher HPC concentrations, leading to the mesophase formation. As described in detail in previous studies,9−12 the cholesteric periodicity and LCST-type phase boundary, and ensuing optical properties of the lyotropic system of HPC/water, are significantly affected by the addition of a small amount of inorganic salts9 or organo-ionic liquids such as imidazolium salts12 as the third component. As a universal rule, it was shown that both the cholesteric pitch (P) and the phase-separation temperature, and accordingly the wavelength of selective light reflection (λM) and the cloud point (Tc), varied systematically with a change in strength of a so-called “chaotropic” effect of the ions constituting the additive salts. In general, an increase in the ionic chaotropicity weakens the hydrophobic interaction of (the side chains of) water-soluble polymers. In the addition of N-alkyl-substituted methylimidazolium salts ([CnMim][X]; n, carbon number of 1-alkyl substituent), however, particular attention was directed to a habitual surfactant-like action of the organocation moiety;12 ultimately, this salted HPC lyotropic system was considered to be phase-equilibrated under the multi-influences comprising mutually competitive electrostatic and hydrophobic factors derived from the amphiphilic nature of
INTRODUCTION It is well-known that cellulose derivatives as polymer molecule and even its microfibrils (nanocrystallites) have a mesoscopic character of self-assembling to make a liquid crystalline order.1−5 When these cellulosics are condensed to form an anisotropic phase in a suitable solvent, the packing arrangement of the solute is usually of a cholesteric (or chiral nematic) type. The cellulose derivative whose cholesteric liquid-crystallinity was first clearly reported is (hydroxypropyl)cellulose (HPC; see Figure 1a).6,7 A certain significance of the finding is that the
Figure 1. Structural formulas of (a) (hydroxypropyl)cellulose (HPC) and (b) representative two series of N-alkyl-substituted methylimidazolium salts.
solvent can solely be water, which is the most common liquid substance and easily provides a diversity of ionic media by dissociating various electrolytes: acid, base, and salt compounds. A phase diagram of the HPC/water system has been established satisfactorily as a function of composition and © 2012 American Chemical Society
Received: December 9, 2011 Revised: January 4, 2012 Published: January 27, 2012 565
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CnMim+ and an additional chaotropic effect of the counterion X−. A successful example of functional development was the ionmediated dynamic control of cholesteric coloration and optical turbidity by use of weak electrical stimulation for the HPC lyotropics including solutions13 and swollen networks.14 In those studies, conventional metallic salts were employed for the additive ions as P or Tc shifter. The dynamic concept was based on a possible electrophoretic migration of the dissociated ionic particles. A similar electrooptical phenomenon should be expected to take place in the addition of a series of imidazolium salts, too, and it is an actual fact, although we solely referred to a preliminary observation in the preceding article.12 The present Article shows further specific examples of the electrical manipulation of visual appearance for the [CnMim][X]containing aqueous HPC cholesteric system. Insight is also provided into the mechanism of the observed electrooptical function, through examination of a potential difference for the electrified liquid-crystalline samples.
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cases, the electric supply was ceased after the passage of 60−90 min and, successively, the time course of a possible electric discharge was examined by following the change of a potential difference remaining in the electrified sample. A digital multimeter TR6846 (Advantest) was used for this purpose. All of these experiments were carried out at ambient temperature (ca. 22 °C).
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RESULTS AND DISCUSSION Time-Course Observations of Visual Appearance. Figure 3a−c shows a result of electrooptical observations for
EXPERIMENTAL SECTION
Materials and Sample Preparation. The HPC used was a commercially available powder sample (Scientific Polymer Products), the same as that described in the preceding paper:12 weight-average and number-average molecular masses, Mw = 11.9 × 104 and Mn = 4.4 × 104, respectively (from GPC measurements); degree of side-group substitution, DS = 2.06 and MS = 4.04 (from 1H and 13C NMR measurements15), where DS and MS denote an average number of substituted hydroxyls and that of introduced hydroxypropyl groups, respectively, per anhydroglucose residue. All of the N-substituted imidazolium salts used were the same as those previously synthesized via N-alkylation/quarternization of 1methylimidazole with different alkyl halides.12 Two major salt series of 1-normal-alkyl-3-methylimidazolium halides, [CnMim][Br] (n = 2, 4, 6) and [C4Mim][X] (X = Cl, Br, I), were assorted. (See Figure 1b.) Additional imidazolium salts with the N-alkyl part modified by terminal hydroxylation or by chain-branching, such as 1-(6hydroxyhexyl)-3-methylimidazolium bromide ([C6OHMim][Br]) and 1-sec-butyl-3-methylimidazolium bromide ([s-C4Mim][Br]), were also used. Cholesteric liquid crystals of 62.5 wt % HPC were prepared by mixing weighed HPC dry powder and distilled water containing a prescribed quantity of imidazolium salt in a glass vial over a period of ∼4 weeks, with the aid of repeated centrifugation to accelerate the polymer dissolution. Throughout this study, the salt concentration was 2.5 × 10−4 mol/g-HPCaq, denoted as a molar amount of the salt per gram of HPC aqueous solution. Measurements. Selected liquid-crystalline samples were sealed into a cell, which was made up of two slide glasses spaced parallel, two carbon plates (500 μm thick) as a pair of inert electrodes and spacers, and another sealing Teflon spacer, as illustrated in Figure 2. The
Figure 3. Time course of the visual appearances of [CnMim][X]added HPC aqueous liquid crystals at 23 °C, each solution was prepared at polymer and salt concentrations of 62.5 wt % and 2.5 × 10−4 mol/g-HPCaq, respectively, and subjected to the action of an electric field of E = 4.5 V/12 mm. Added imidazolium salt: (a) [C4Mim][Cl]; (b) [C4Mim][Br]; and (c) [C4Mim][I].
three liquid-crystalline solutions of HPC containing [C4Mim][X] of X = Cl, Br, and I, respectively, all being prepared at a polymer concentration of 62.5 wt % and at a salt concentration of 2.5 × 10−4 mol/g-HPCaq. The strength of applied electric field was, in common, E = ∼3.8 V/cm (4.5 V/12 mm). In the initial stage free of the electric field (see the top of the respective Figures), the [C4Mim][Cl]-containing sample was bluish and the [C4Mim][Br]-containing one gave an orange color mingled with yellowish green, whereas the other sample containing [C4Mim][I] was substantially colorless, even though a faint reddish hue was perceived for the solution stocked in a glass vial. Therefore, the incorporation of imidazolium salts into the aqueous HPC liquid crystal, which assumed a greenish appearance in the salt-free state at 62.5 wt % and at ∼23 °C, causes a serious change of the reflection color. The above example also well-characterizes the anion-type dependence of the wavelength of maximum reflectance, λM, and therefore the cholesteric pitch, P; this structural parameter increases according to the chaotropic strength of X−, that is, in a manner satisfying the order of Cl− < Br− < I−. The voltage imposition onto the three samples gave rise to a distinct variation in the reflection color with a regional dependence. After a lapse of 60 min, the [C4Mim][Br]added system gained a reddish hue in the vicinity of the positive electrode and also exhibited a sequence of yellow, green, and blue striations toward the side of the negative electrode (Figure 3b, middle). With the passage of another 60 min, as shown in
Figure 2. Sample cell devised for electrooptical examination. sectional planes of the twinning carbon plates were apart face to face at a distance of 10−12 mm on both sides of the layered viscous liquid crystal. An electric generator, Multifunctional Synthesizer 1946 (NF Electric Instruments), was used to electrify the sample-charged cell. An electromotive force of 3.5 to 4.5 V was loaded onto the cell. In some 566
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the bottom of Figure 3b, the reflective colors of the sample ranged from profound red (positive side) through orange, yellow, and green (central part) to blue and pale violet (negative side), and a colorless ultraviolet zone appeared in close proximity to the negative carbon plate. In the [C4Mim][I]-added system, the change in visual appearance was relatively slow; a red-colored band was produced in the negative-side area just after electrification for 120 min (Figure 3c, middle). Another 120 min supply of electricity resulted in formation of almost full spectral colors ranging from red to violet, whereas half of the sample was left uncolored (i.e., λM > ∼800 nm) but with a dull brown tincture at the positive side-edge, indicating condensation of iodine ions (Figure 3c, bottom). The other system containing [C4Mim][Cl] imparted no clear array of various color bands in the course of the electrification process, as illustrated in Figure 3a. Instead, a colorless zone of λM < 400 nm emerged on the negative side of the sample before 20 °C, however, the [C2Mim][Br]-added system (Tc ≈ 20 °C) was gelatinous and opaque (but tinged with pale violet). Under a special condition where the sample cell was cooled on a wet towel ( 800 nm) in the initial nonelectrified state at 23 °C, and, when 3 h passed after electrification, the negative side of the sample was covered with an entire spectrum of reflection colors, as demonstrated in Figure 4a. The transformation in visual appearance with time is just like the situation of the electrified [C4Mim][I]-containing sample (Figure 3c) but with a little difference in the coloring speed. When an imidazolium salt [C6OHMim][Br] with the N-hexyl tail modified by terminal hydroxylation was employed, the salted HPC liquid crystal was uniformly colored reddish orange in the initial sandwiched state (Figure 4b, top); that is, the attachment of a hydrophilic OH group at the alkyl end seriously reduced P, relative to that observed for the [C6Mim][Br] addition. In the course of electrification, the [C6OHMim][Br]added sample showed a definite variation in the reflection color, as illustrated in Figure 4b (middle and bottom); a dark red spread over a large part of the positive half area and a sequence of narrow bands of yellow, green, blue, and violet colors
Figure 4. Time-evolving gradations for (a) [C6Mim][Br]- and (b) [C6OHMim][Br]-added HPC aqueous liquid crystals (HPC conc., 62.5 wt %; salt conc., 2.5 × 10−4 mol/g-HPCaq) observed at 23 °C under application of E = 4.5 V/12 mm.
appeared on the negative side, at nearly the same speed of time elapsing as that in the case using [C4Mim][Br]. Interpretation of Time-Evolving Gradations under Electrification. The imidazolium salt-added HPC lyotropic system exercises a dynamic electrooptical function when coupled to an electric circuit of low voltage, as has been demonstrated above. The observations of the time-evolving gradation in visual appearance may be interpreted as coming from some perturbation in ionic environment by the electrification, probably from the generation of an uneven distribution of the additive cation/anion constituents to shift P and Tc of the lyotropics. In this interpretation, close care should be taken for the specific system using organo-ionic liquids, lest the gradation mechanism is unreasonably simplified. As we saw in the preceding subsection, the addition of the comparatively bulky CnMim+s of n = 4 and 6 raised P relative to the nonionic reference of the HPC mesophase, even though the annular imidazolio moiety itself can act as an essentially antichaotropic ion to this aqueous system.12 The P elevation is primarily due to the surface-active agency derived from the amphiphilic structure of CnMim+; viz., the voluminous N-alkyl tail would merge into the hydrophobic side-chain domain surrounding each oriented HPC trunk so that the cholesteric twist angle (an azimuth difference between adjacent nematic layers) of the mesophase is changed, commonly reduced to raise P, which was actually demonstrated with the aid of X-ray diffractometry.12 In solubility, thereby, the polymer molecule becomes more stable electrostatically because of the newly evolved ionic surface. This surfactant-like activity of the organocation should be more pronounced with increasing carbon number n and suppressed with attachment of a hydrophilic OH group at the N-alkyl-chain end. In the above context, we run back over the electrooptical behavior in question. The behavior is phenomenologically parallel to that found in the previous experiment13 using conventional alkali-metallic salts for similar additives. Those salts are completely ionized and quite susceptible to electrophoresis in aqueous media; therefore, the electrical stimulation commensurately allows the ionic particles to migrate directly 567
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lyte, a high-resistant electrolyte, and a dielectric substance, each medium being electrified between a pair of inert electrodes. In Model A applicable to common electrolyte media such as ionized salt solutions in mere water, there is only a steep gradient in contact with the respective electrode faces, arising due to formation of a so-called electric double layer. In Model C, the potential gradient supposed is due to the polarization accompanied by orientation of dipoles, as in insulators providing a large electric capacitance. Model B is set up for the imidazolium salt-added aqueous liquid crystals of HPC; the system would behave as an extremely viscous electrolytic medium of relatively high resistance. On the basis of the abovestated interpretation of the time-evolving gradation in color, an electric potential gradient should spread over the medium in the direction of the negative side from the positive one of the electrodes during electrification; however, an interfacial sharp gradient contiguous to the respective electrode faces may be admitted in common with ordinary electrolytes. It is assumed that the inner prevailing gradient is due to the electrophoretic migrations of dissociated salt ions, driven by an electrostatic force of attraction between the cation/anion and the corresponding electrodes. If Model B is much more relevant to the present system concerned, then we will see an aftereffect on ceasing the supply of electricity to the sample; that is, the inner potential gradient generated until then is retained for some time before the ionic distribution becomes uniform in the viscous medium. All the while the sample cell should behave like a capacitor, but the mechanism of potential storage is different from that in the case using conventional insulators (Model C). Acting on the anticipation of finding such a storage effect, we carried out a potentiometry for electrified samples of concentrated HPC solutions, each containing an imidazolium salt. The result is exemplified in Figure 6 for [s-C4Mim][Br]-added and [C6Mim][Br]-added liquid crystals of 62.5 wt % HPC. Each
and localize in the HPC lyotropic system. In the present case using [CnMim][X], the process generating the imbalance in distribution of the cation/anion constituents may be more complicated. The applied electric field would first induce a change in orientational allocation of the organocations encapsulating HPC solute molecules in the initial quiescent state, so as to yield a preferred orientation parallel to the nematic layers. This spatial perturbation should reduce the surfactant-like stabilization effect and be followed smoothly by the migration of the voluminous cations toward the negative pole; thereby the negative side of the sample tends to assume a cholesteric arrangement of shorter P, coupled together with a rise of the inherent antichaotropic effect of the condensed imidazolio moieties. Then, the orientational reallocation and electrophoretic migration of C6Mim+ can be expected to be slower than the sequence for C4Mim+ of shorter alkyl tail, and this view is actually supported by comparing the two sets of data given in Figures 3b and 4a. In the employment of C6OHMim+ (Figure 4b), the gradation of the liquid crystal with electrifying time was made at somewhat higher speed, compared with the case using C6Mim+; this is attributable, most probably, to shallow and crooked intrusion of the hexyl chain of C6OHMim+ into the hydrophobic side-chain region of HPC, even in the initial stage without electrification. It should be noted further that the dissociation of imidazolium ionic liquids in aqueous solutions is generally imperfect and, accordingly, the higher the dissociation degree, the stronger the surface-active agency.16 As exemplified in Figure 3, the [C4Mim][I]-containing HPC liquid crystal showed an exceptionally slower change in the time-evolving gradation process, compared with the other two samples containing [C4Mim][Cl] or [C4Mim][Br]. A higher dissociation character of the iodide may be responsible for the observation; that is, the dissociability results in a greater extent of the action of C4Mim+ to stabilize the HPC assembly in the initial nonelectrified state as well as in a slower mobility of the better-hydrated chaotropic anion I− under electrification. Needless to say, the electronegativity of iodine is relatively lower, and thus the heteropolar bond energy of the imidazolium iodide should be weaker than those of the corresponding bromide and chloride. In any of the salted samples, however, the applied electric field should promote dissociation of the imidazolium halide left ion-paired. Observation of Residual Electric Potential. Figure 5 illustrates three models (A−C) of electric potential distribution idealized for, respectively, an electrically low-resistant electro-
Figure 6. Electric potential change with time for imidazolium saltadded HPC aqueous liquid crystals (HPC conc., 62.5 wt %; salt conc., 2.5 × 10−4 mol/g-HPCaq), followed after cessation of the electrification of E = 4.5 V/12 mm for 80 min at 21 °C. Added salt: (△) [s-C4Mim][Br]; (●) [C6Mim][Br].
plot renders the time course of a potential change followed after cessation of the electrification of the sample for 80 min under a field of E = 4.5 V/12 mm. This time-coursing change virtually reflects a process of electric discharge. In the two examples, data of 2.1 ([s-C4Mim][Br] addition) and 1.6 V ([C6Mim][Br] addition) were obtained as an initial potential difference, and the values diminished sharply to ∼1.1 and ∼1.2 V, respectively, in several minutes from the beginning
Figure 5. Schematic representations of electric potential distribution for three different media, each electrified between a pair of inert electrodes. Medium: (A) low-resistant electrolyte (dilute solution); (B) high-resistant electrolyte (viscous solution); and (C) dielectric (insulator). 568
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precede: sufficient dissociation of the ion pair and orientational reallocation of the organo-cation initially stabilizing solute HPC in a surfactant-like way, both promoted by the external electric force. The electrooptical function found for such an ionic system of HPC liquid crystals is potentially applicable to some displays that do not require a short response time. The color patterning would also be diversified by alteration of the sort and shape of the electrodes contacting the viscous sample.
of the respective measurements. This kind of rapid decay may be associated with a relaxation process mainly to quench a steep potential gradient vicinal to each electrode, which had developed in the ascendant of the electric double layer. Actually, however, there is a possibility of missing a much faster decay because it took a few tens of seconds to set up the circuit in which a potentiometer was inserted after the original circuit was broken by separating from the electric source. After 10 min passed, the rate of the potential decay became gradual in any of the samples, and, even after a 120 min lapse, the [C6Mim][Br]added sample retained 1.02 V and the other one containing [sC4Mim][Br] did 0.93 V. These retentions of an appreciable level of potential imply a persistence of imbalance in the positive/negative charge distribution, reflecting slower delocalizing diffusion of the salt ions in the highly viscous medium. Correspondingly, over the time period of ∼2 h, an array of gradated colors induced in advance for the liquid crystals was kept with a slight relaxation. In continuation of the exploration, eventually, it took at least 2 days for the gradated samples to restore a uniformly colored state. The potentiometric observation was duplicated by measurements for HPC liquid crystals electrified with [C4Mim][Br] or [C6OHMim][Br] addition under the same condition as that in the above. The [C4Mim][Br]-added sample gave a decay curve quite similar to that observed for the [s-C4Mim][Br]-added sample. As for the [C6OHMim][Br]-added sample, the decaying potential was plotted at a level intermediate between the corresponding two sets of data for the [C6Mim][Br]-added and [s-C4Mim][Br]-added samples. Remarks on Electrochemical Reaction. It was not noticeable under the present conditions of electrification, whereas, in the use of Pt electrodes giving a lower hydrogen overvoltage,17 a generation of H2 gas was evident from the negative side when a relatively high strength of electric field (e.g., ∼5 V/10 mm) was applied to the aqueous liquidcrystalline series of HPC/imidazolium salt. This is due to electrolysis of water containing ionized salts following the formation of an interfacial electric double layer. (See models A and B in Figure 5.) Concomitantly, the sample loaded into the electrifying cell should, more or less, undergo a pH change. However, it is quite rash to take that the possible timedependent shift in pH is the main cause yielding the gradation of reflection colors. In fact, when 62.5 wt % HPC liquid crystals were prepared at various pHs of 1−12.5 with HNO3 and LiOH, all of the samples took on a greenish hue (λM = 510−550 nm) at 18 °C.13 The adjusters NO3− and Li+ are comparatively weak chaotropic and antichaotropic ions, respectively.9
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AUTHOR INFORMATION
Corresponding Author
*Phone: +81 75 753 6250. Fax: +81 75 753 6300. E-mail:
[email protected].
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ACKNOWLEDGMENTS This work was partially financed by a Grant-in-Aid for Scientific Research (A) (no. 23248026 to YN) from the Japan Society for the Promotion of Science.
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REFERENCES
(1) Guo, J.-X.; Gray, D. G. Chapter 2. In Cellulosic Polymers, Blends and Composites; Gilbert, R. D., Ed.; Hanser: New York, 1994. (2) Gray, D. G.; Harkness, B. R. Chiral Nematic Mesophases of Lyotropic and Thermotropic Cellulose Derivatives. In Liquid Crystalline and Mesomorphic Polymers; Shibaev, V. P., Lam, L., Eds.; Springer: New York, 1994; pp 298−323. (3) Gray, D. G. Carbohydr. Polym. 1994, 25, 277. (4) Fukuda, T.; Takada, A.; Miyamoto, T. Chapter 3. In Cellulosic Polymers, Blends and Composites; Gilbert, R. D., Ed.; Hanser: New York, 1994. (5) Zugenmaier, P. Chapter 9. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley: Weinheim, Germany, 1998; Vol. 3. (6) Werbowyj, R. S.; Gray, D. G. Mol. Cryst. Liq. Cryst., Lett. Sect. 1976, 34, 97. (7) Werbowyj, R. S.; Gray, D. G. Macromolecules 1980, 13, 69. (8) Fortin, S.; Charlet, G. Macromolecules 1989, 22, 2286. (9) Nishio, Y.; Chiba, R.; Miyashita, Y.; Oshima, K.; Miyajima, T.; Kimura, N.; Suzuki, H. Polym. J. 2002, 34, 149. (10) Nishio, Y.; Chiba, R. Ekisho 2003, 7, 218. (11) Nishio, Y. Adv. Polym. Sci. 2006, 205, 97. (12) Chiba, R.; Ito, M.; Nishio, Y. Polym. J. 2010, 42, 232. (13) Chiba, R.; Nishio, Y.; Miyashita, Y. Macromolecules 2003, 36, 1706. (14) Chiba, R.; Nishio, Y.; Sato, Y.; Ohtaki, M.; Miyashita, Y. Biomacromolecules 2006, 7, 3076. (15) Ho, F. F. -L.; Kohler, R. R.; Ward, G. A. Anal. Chem. 1972, 44, 178. (16) Modaressi, A.; Sifaoui, H.; Mielcarz, M.; Domańska, U.; Rogalski, M. Colloids Surf., A 2007, 302, 181. (17) See, for example: Koryta, J.; Dvořaḱ , J.; Kavan, L. Principles of Electrochemistry, 2nd ed.; John Wiley & Sons: Chichester, England, 1987; pp 245−409 (Chapter 5).
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CONCLUSIONS Visual appearance of aqueous HPC liquid crystals containing an additive of N-alkyl-substituted methylimidazolium salts ([CnMim][X]) can be temporally varied by electrical stimulation under ambient conditions. Namely, a time-evolving gradation in cholesteric reflection color or optical clarity of the lyotropics is attainable by imposing a relatively weak electromotive force (e.g., ∼4.5 V) onto appropriate electrodes (e.g., twins of carbon plate) in contact with the sample sealed in a cell. This may be interpreted as due to the electrophoretic migrations and localizations of CnMim+ and X− as P and/or Tc shifters, the ionic fluctuation giving rise to an inner gradient of electric potential in the viscous medium of high resistance. Before distinct formation of the imbalance in distribution of the cation and anion, however, the following processes would 569
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