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Langmuir 2006, 22, 4552-4557
Synthesis and Characterization of Charged Polystyrene-Acrylic Acid Latex Particles P. S. Mohanty,† R. Kesavamoorthy,‡ Kozo Matsumoto,† Hideki Matsuoka,*,† and K. A. Venkatesan§ Department of Polymer Chemistry, Kyoto UniVersity, Kyoto 615-8510, Japan, Materials Science DiVision, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India, and Fuel Chemistry DiVision, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India ReceiVed NoVember 7, 2005. In Final Form: February 18, 2006 Novel, monodisperse charged colloidal particles of polystyrene cross linked with divinylbenzene and surfacegrafted with acrylic acid were synthesized by emulsion polymerization and were characterized by estimating the dissociable surface charge by conductivity titration, the particle effective charge by conductivity verses particle concentration, and the particle size by dynamic light scattering and atomic force microscopy. The structural ordering and dynamics were investigated as a function of the volume fraction of the particles using static and dynamic light scattering, respectively. Furthermore, from the electrophoresis measurements, these particles are found to have a high salt tolerance due to increases in charge as a function of salt concentration.
Introduction The most fascinating aspect of charged colloidal suspensions is the appearance of long-range order.1-3 These structures, commonly known as colloidal crystals, exhibit iridescence arising from the Bragg diffraction of visible light.1 These colloidal crystals are very important because of their use as Bragg-diffraction devices3, nanoswitches,4,5 chemical sensors,6 and templates for preparing photonic band gap materials.7,8 Recently, these colloidal crystalline arrays (CCAs) have been immobilized into an expandable transparent hydrogel to obtain a gel-sensing device.7-11 Apart from crystalline order,1,12 these colloidal suspensions also exhibit gas,12-14 liquid,12,14,15 and even glasslike12,16,17 order. The structural ordering and phase transitions in charged colloidal suspensions can be easily tailored by tuning the range and strength of the interparticle interaction energy U.12 This interaction energy U can be varied over a wide range by changing the Debye * Corresponding author. E-mail:
[email protected]. † Kyoto University. ‡ Materials Science Division, Indira Gandhi Centre for Atomic Research. § Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research. (1) Mohanty, P. S.; Tata, B. V. R.; Yamanaka. J.; Sawada, T. Langmuir 2005, 21, 11678. (2) Matsuoka, H.; Yamamoto, T.; Harada, T.; Ikeda, T. Langmuir 2005, 21, 7105. (3) Guisheng, P.; Kesavamoorthy, R.; Asher, S. A. Phys. ReV. Lett. 1999, 178, 3860. (4) Asher, S. A.; Pan, G.; Kesavamoorthy, R. Nonlinear Opt. 1999, 21, 343. (5) Kamenjicki, M.; Lednev, I. K.; Mikhonin, A.; Kesavamoorthy, R.; Asher, S. A. AdV. Mater. 2003, 13, 774. (6) Asher, S. A.; Peteu, S. F.; Reese, C. E.; Lin, M. X.; Finegold, D. Anal. Bioanal. Chem. 2002, 373, 632. (7) Wijnhoven, J. E. G. J; Vos, W. L. Science 1998, 281, 802. (8) van Blaadern, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (9) Sharma, A. C.; Jana, T.; Kesavamoorthy, R.; Shi, L.; Virji, M. A.; Feingold, D. N.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 2971. (10) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. (11) Asher, S. A.; Sharma, A. C.; Goponenko, A. V.; Ward, M. M. Anal. Chem. 2003, 75, 1676. (12) Mohanty, P. S. Ph.D. Thesis, Madras University, 2005 (unpublished). (13) Brown, J. C.; Pusey, P. N.; Goodwin, J. W.; Ottewill, R. H. J. Phys. A: Math. Gen. 1975, 8, 664. (14) Tata, B. V. R.; Mohanty, P. S.; Yamanaka, J.; Kawakami, T. Mol. Simul. 2004, 30, 153. (15) Kesavamoorthy, R.; Tata, B. V. R.; Arora, A. K.; Sood, A. K. Phys. Lett. 1989, 138A, 208. (16) Pusey, P. N.; van Megen, W. Phys. ReV. Lett. 1987, 59, 2083. (17) Ha¨rtl, W.; Versmold, H.; Zhang-Heider, X. J. Chem. Phys. 1995, 102, 6613.
screening parameter κ, which can be achieved by changing the particle concentration np, the charge on the particle Z, or the salt concentration Cs. Most of the past studies on structural ordering, dynamics, and phase transitions have been investigated on charged polystyrene particles having a strong acid group such as sulfonate. Because of the complete dissociation of the sulfonic groups, these particles in general carry more charges than particles having weak acid group. Hence, they can be ordered at lower volume fraction1 compared to the latter one. However, in the case of a strong acid group, the particles are not sensitive to pH, and pH control is very important for a drug delivery system. In the case of a weak acid group, the particles will have high charge at high pH and a low charge at low pH. Therefore, these particles can be effectively used as drug carriers in a drug delivery systems. One can design the drug release mechanism at both low and high pH, but this is not possible in the case of a polystyrene particle having a strong sulfonate group. Apart from this, these particles can also be self-assembled at higher volume fraction and at higher pH to form colloidal crystals, which is also very useful for preparing photonic band gap materials. So far, there have been no reports on the synthesis, characterization, and study of structural ordering and dynamics using static and dynamic light scattering. This motivates us to synthesize polystyrene latex particles having COO- ions on their surfaces and to study their basic properties. In this article, we will report the synthesis and characterization of polystyrene-divinyl benzene-acrylic acid copolymer colloidal particles. The synthesis is carried out according to the general procedures used by Asher and co-workers.18-20 They have used a sulfonated co-monomer such as sodium 1-allyloxy-2-hydroxypropane sulfonate (COPS1) to charge the particles. Here we have used acrylic acid as the co-monomer. We carried out conductivity titration to determine the dissociable surface charge number (Zt) and conductivity verses particle concentration to estimate the effective particle charge (Z). The size distribution was estimated using dynamic light scattering and atomic force (18) Reese, C. E.; Asher, S. A. J. Colloid Interface Sci. 2002, 248, 41. (19) Reese, C. E.; Guerrero, C. D.; Weissman, J. M.; Lee, K.; Asher, S. A. J. Colloid Interface Sci. 2000, 232, 76. (20) Xu, X.; Friedman, G.; Humfeld, K. D.; Majetich, S. A.; Asher, S. A. Chem. Mater. 2002, 14, 1249.
10.1021/la052995a CCC: $33.50 © 2006 American Chemical Society Published on Web 04/14/2006
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microscopy. The structural ordering and dynamics are studied as a function of the volume fraction using static and dynamic light scattering. The salt effect on very dilute suspensions was studied using dynamic light scattering and electrophoresis. Experimental Section Synthesis. Monodisperse, charged polystyrene colloidal spheres with divinylbenzene cross linking and acrylic acid surface charging were synthesized by emulsion polymerization using a three-necked round-bottomed reaction vessel that contained a reflux condenser and a glass stirrer powered by a high-torque mechanical stirrer. The vessel also contained a thermometer and an argon/reagent inlet. The temperature was maintained by the silicone oil bath surrounding the vessel. Styrene (Aldrich) and divinylbenzene (DVB, Aldrich) were used after the inhibitor was removed by passing through a column of aluminum oxide. Sodium dodecyl sulfate (SDS) surfactant (Ranboxy, India), acrylic acid (AA) co-monomer (Ranboxy, India), sodium bicarbonate (NaHCO3) buffer, and ammonium persulfate radical initiator (Sigma) were used as received. The reaction vessel was charged with 220 mL of Millipore water with Ar gas bubbling. The stirrer was kept at about 100 rpm. After 5 min, 200 mg of NaHCO3 dissolved in 3 mL of water was added to the contents of the reactor vessel as a buffer. After another 30 min, 150 mg of SDS dissolved in 3 mL of water was added to the reactor, and the stirring speed was increased to 200 rpm. SDS is a surfactant, and it formed spherical micelles in buffered water. Froth appeared slowly. After 15 min, the Ar gas inlet needle was raised from the water and given a gas blanket in the vessel over the reactants’ surface. The temperature of the vessel was raised to 42 °C, and the froth disappeared. After 20 min, 26.4 g of styrene, 3 g of DVB, and 2 g of AA were placed in a beaker, purged with Ar gas, and pored into the reagents’ dropping funnel fitted onto the vessel. Temperature was raised to 71 °C and maintained at T ) 71 ( 1 °C. Then the reagents were dropped into the vessel at a rate of ∼100 drops/min. In 30 min, all of the reagents went into the vessel. Then, 100 mg of sodium persulfate dissolved in 10 mL of water was added to the reagents through the drop funnel at a rate of ∼100 drops/min. The polymerization reaction started at this moment, and the reaction was left to reflux for 5 h. Then the heater was turned off to stop the reaction. Upon completion of the reaction, the product appeared to be milky white. The coagulation was found to be less than 2%. After the solution cooled overnight, the colloid was filtered through previously boiled glass wool and was dialyzed (Pierce snakeskin pleated dialysis tubing with a 10 000 MWCO) against water, which was replaced at least daily for 25 days. Mixed-bed ion-exchange resins (BioRad) were added to the suspension for storage in a PET bottle. Conductometric Titration. Titration was carried out at a temperature of 25.0 ( 0.1 °C using a conductivity meter (model DS-8M, Horiba, Japan). Light Scattering. Structural ordering and dynamics were investigated using a Photal SLS-6000HL light-scattering apparatus (Otsuka Electronics, Osaka, Japan) equipped with a digital correlator (Photal GC-1000). A He-Ne laser (wavelength 632.8 nm) was used for the measurement. Atomic Force Microscopy (AFM). AFM studies were carried using a Seiko SPI3800 probe station and an SPA300 unit system of the SPI3900 series scanning probe microscopy system in dynamic force mode (noncontact mode). The microcantilever (SI-DF-A, Seiko) was made of silicon, and its spring constant was 2 N/m. For sample preparation, an aqueous suspension of colloidal particles was dropped onto a microscope slide glass (IWAKI, Japan) and air dried. Electrophoresis. The mobility as a function of salt concentration is investigated using an electrophoresis instrument (Photal ELS800, Otsuka, Japan). The corresponding zeta potential was estimated using the Smoluchowski and Huckel approximation.
Results and Discussion Particle Size Estimation. The particle diameter was estimated from the dynamic light scattering (DLS) and atomic force
Figure 1. (A) g(1)(Q, t) vs t. The solid line is the line fit to the experimental data using eq 1. The size distribution obtained from CONTIN analysis is shown as an inset. (B) AFM image of dried colloidal particles on microslide glass.
microscopy (AFM) measurements. It is obtained by fitting the field correlation function g(1)(Q, t) to a single exponential12 using eq 1 (Figure 1A). The diameter is found to be 97 ( 7 nm. The corresponding size distribution obtained from CONTIN analysis is shown as an inset in Figure 1A. The polydispersity (pd) is found to be less than 10%. The particle size estimated from AFM (∼100 nm) is shown in Figure 1B. Hence, the size estimated from DLS is found to be in close agreement with that from AFM. DLS gives the hydrodynamic diameter of the colloidal particle in water, whereas AFM gives the dry particle diameter. It is found that the two estimates are the same value within the standard deviation of 10%. Because acrylic acid is hydrophilic, one may expect the acrylic acid-grafted particle to swell in an aqueous medium. However, the swelling here is negligible because of the high degree of DVB cross linking of polystyrene in the particle. Because AA is water-soluble, most of the acrylic acid would have stayed at the particle surface instead of going into the core of the particle. Hence, water cannot be forced into the particle core. Dissociable Surface Charge (Zt) Estimation: Conductivity titration was carried out with aqueous NaOH to estimate Zt. Figure 2A shows the typical conductivity titration curve for a weak acid with a strong base. The weak acid (COOH) is weakly ionized in aqueous solution. The conductivity initially increases
4554 Langmuir, Vol. 22, No. 10, 2006
Figure 2. (A) Conductivity titration curve for charged latex particles. Conductivity variation is plotted as a function of the titrated amount of 0.008 M NaOH against the colloidal suspension. The arrow indicates the amount of NaOH(aq) needed to fully neutralize the COO- acid present in the suspension. The two solid lines are the linear fit to the conductivity data, and the intersection of the two lines is the equivalence point (denoted by the vertical arrow). From this equivalence point, the total number of H+ ions per particle was estimated using the number density of the polystyrene particle. (B) Conductivity is measured as a function of the particle volume fraction. The straight line drawn is the linear fit to the data points. From the slope of the curve, the effective charge density is estimated.
as a function of NaOH concentration because of the ionization of the COOH group, and Na+ is released as a counterion. After the complete ionization of the COOH group, the conductivity increases further, which is due to added Na+ and OH- ions. This clearly shows that the charge of the particle increases as a function pH (i.e., as a function of added NaOH), but in the case of the particle having a sulfonate group, the degree of dissociation is 1. The charge of the particle do not increase further, so the conductivity initially decreases as a function of added NaOH because of the exchange of H+ by Na+ until neutralization. The surface charge estimated from the neutralization point (shown by the arrow in Figure 2A) is found to be 2400 charges per particle. Furthermore, to determine the effective charge per particle, conductivity is recorded as a function of particle concentration. Figure 2B shows the volume fraction φ as a function of conductivity. It can be seen that conductivity versus φ shows good linearity. The solid line is the linear fit to the conductivity data points. The intercept is identical to the value of the conductivity of the deionized water ( Qmax; and 2, Q < Qmax.
This further confirms that the particles are noninteracting at φ ≈ 0.00001. The solid line is the fitted line for free diffusion coefficient Do with 4.8 × 10-8 cm2/s. Samples prepared with a volume fraction of up to 0.002, which are in contact with mixed-bed ion-exchange resins, were not iridescent in visible light, implying that the samples are not crystalline. To characterize the structural ordering in these suspensions, the time-averaged scattering intensity Is(Q) is measured as a function of the scattering wave vector Q. The structure factor S(Q) obtained by correcting to P(Q) for the sample with φ ) 0.00045 is shown in Figure 4A. S(Q) shows the first peak occurring at Qmax ) 0.69 × 105 cm-1 and a smooth second peak suggesting that structural ordering is liquidlike.12,14,22 The volume fraction is estimated from the first peak position Qmax using the following relations:
np )
( ) ( )
1 Qmax x2 2π
φ)
πd3 n 6 p
3
(2)
(3)
φ is found to be same as that prepared by diluting the original suspension with deionized water. The same is observed for other samples, which did not show any iridescence to the naked eye. These results imply that the samples are homogeneous and possess liquidlike order. The dynamics of the liquidlike ordered suspension is studied by measuring g(2)(Q, t) at three different Q values: Q ) Qmax, Q () 0.49 × 105 cm-1) < Qmax, and Q () 2.48 × 105 cm-1) > Qmax. Figure 4B shows Q2t as a function of ln[g(1)(Q, t)]. It can be seen from the Figure that in the case of an
(5)
where
Ds )
Do S(Q)
(6)
Figure 4A shows that Do/Ds is obtained by performing a cumulant analysis on the measured g(1)(Q, t). Hence the variation of Do/Ds obtained for different values of Q is compared with S(Q) measured from the angle-resolved static light scattering experiment. We observed good matching of the reciprocal short-time diffusion coefficient (Do/Ds) with S(Q) for all values of Q. Pusey et al.21 and Gru¨ner and Lehmann22 have also shown good agreement between (Do/Ds) and S(Q) in a dilute aqueous suspension of polystyrene particles as well as in silica colloidal particles in an ethylene glycol/water mixture.12 Hence, our results are in good agreement with studies on dilute aqueous suspensions of polystyrene particles and silica particles. The long-time diffusion coefficient DL has been obtained from the long-time slopes of g(1)(Q, t). The reciprocal diffusion coefficient Do/DL is plotted against Q in Figure 4A. Though the shape is same as S(Q), the value is higher and is also found to be in agreement with those reported in previous studies of polystyrene latex particles. Samples with a volume fraction of φ g 0.002 showed iridescence due to the Bragg diffraction of visible light. This suggests that the structural ordering of the suspension is crystalline. This sample also showed a sharp Bragg spot under the illumination of a focused laser beam of λ ) 632.8 nm. The sample was placed in our light-scattering system, and we oriented those crystals whose Bragg spots lie in the scattering plane. The measured S(Q) as a function of Q is shown in Figure 5A. S(Q) shows four sharp, intense peaks. The first peak corresponds to Bragg reflection from the (110) plane. The calculated peak positions show that the crystal structure is body-centered cubic (BCC). The volume fraction estimated using the Q110 peak position is found to be same as that determined by dilution of the original suspension. Furthermore, the AFM study is also carried out on (23) Dynamic Light Scattering: Applications of Photon Correlation Spectroscopy; Pecora, R., Ed.; Plenum: New York, 1985; p 108.
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Figure 6. g(1)(Q, t) as a function of t for 0.1 M salt concentration. The inset is the corresponding size distribution.
Figure 5. (A) S(Q) vs Q for a crystalline suspension with a volume fraction of φ ) 0.003. (B) g(1)(Q, t) as a function of t for crystalline order. The inset is the mean square displacement as a function of time.
the crystalline sample at higher φ () 0.1). One drop of the crystalline suspension is placed on a clean glass plate and allowed to dry for 1 day. The AFM image of the dried crystalline suspension is shown in Figure 1B. From the image, it can be clearly seen that the particles are ordered and monodisperse. The dynamics of the crystalline suspension is investigated by measuring the intensity-intensity correlation function at the first peak position. The g(1)(Q, t) extracted from g(2)(Q, t) shows a nondecay in g(1)(Q, t), which is also shown in Figure 5B. Because in a crystal the long-time diffusion is arrested (i.e., DL ) 0), the mean square displacement 〈r2(t)〉 is expected to show saturation and is calculated from g(1)(Q, t) using the following equation: 12,14
g(1)(Q, t) ) exp
(
)
-Q2〈r2(t)〉 6
(7)
Indeed, we observe saturation in 〈r2(t)〉 versus t, which is plotted as the inset of Figure 5B. Salt Effect. The salt effect is investigated on a noninteracting suspension at a very dilute volume fraction (φ ) 0.00007) using dynamic light scattering and electrophoresis. The diameter estimated from g(1)(Q, t) does not change significantly up to a salt concentration of 0.1 M (Figure 6). Furthermore, the size distribution shown in the inset does not change significantly at a salt concentration of 0.1 M. Above this salt concentration (>0.1 M), aggregation is found to occur. Details of the aggregation and the stability ratio will be reported separately. The electrophoretic mobilities µ of the diluted sample suspensions (φ ≈ 10-5) were measured as a function of added salt using light-scattering electrophoresis, and the corresponding particle zeta potentials ζ were calculated using the following
Figure 7. (A) Electrophoretic mobility and (B) zeta potential of colloidal particles as a function of salt concentration.
equation24
ζ)f
( ) ηµ o
(8)
with f ) 1 for κa . 1 and exp(ezζ/2kBT)/κa , 1 (Smoluchowski approximation) and f ) 3/2 for κa , 1 (Hu¨ckel approximation). In Figure 7A, mobility is plotted as a function of salt concentration. It can be seen from the Figure that initially the mobility of the latex particle increases with increasing salt concentration up to 0.005 M and then decreases continuously with further increasing salt concentration. The corresponding zeta potential is calculated using eq 8 and is shown in Figure 7B. It has also been seen in the case of polystyrene latex colloids that the electrokinetic potential curves pass through a maximum as a function of increasing salt concentration.25 This contradicts the double-layer models, which predict a continuous decrease in potential. Various explanations have been proposed for this maximal behavior,26,27 but in the case of our acrylic latex particles, we understand the initial increase in the zeta potential as a function of salt concentration and then the decrease in the zeta potential (24) Hunter, R. J. In Zeta Potential in ColloidScience: Principles and Applications; Ottewill, R. H., Rowell, R. L., Eds.; Academic Press: New York, 1981. (25) Elimelech, M.; O’Mella, C. R. Colloids Surf. 1990, 44, 165. (26) Zukoski, C. F.; Saville, D. A. J. Colloid Interface Sci. 1986, 32, 114. (27) van den, Th. J. J.; Bijsterbosh, B. H. Colloids Surf. 1987, 22, 187.
Charged Polystyrene-Acrylic Acid Latex Particles
at higher salt concentration as follows. It is known that in the case of acrylic acid-based latex particles most of the charge sites do not dissociate because it is a weak acid. As the salt concentration increases, the dissociation of COOH groups increases, leading to more Na+ ions from the charge sites. This results in an increase in the number of Na+ ions near the double layer, hence the zeta potential increases up to 0.005 M salt concentration. As the salt concentration is increased beyond 0.005 M, the salt ions enter the diffusive double layer. Some of the counterions get neutralized with the salt ions. This results in a compression of the diffusive double layer, hence the mobility decreases as the salt concentration is further increased and the corresponding zeta potential is also less negative. The increase in the value of Z with salt concentration up to 0.005 M NaCl is a desirable feature. The colloidal crystal melts as the impurity is added to the suspension because the interaction strength and the range decrease. However, if Z increases the interaction increases and the colloidal crystal can survive with a nominal number of impurity ions. Furthermore, the increase in Z with the added salt concentration has also been noticed in the case of block copolymers.28 Finally, it is important to discuss the summary of our synthesized colloidal particles and compare with that of the conventional charged colloidal systems having strong sulfonic groups. First, we observe that our colloidal particles have low charge at low pH, compared to particles with strong acid groups, because of the partial dissociation of the COOH group. However, the particle charge can increase as a function of pH, as is evident from conductometry titration. This is a very important property of these latex particles, which the strong acid latex particles do not have. These particles can also self-assemble to form crystalline order at comparatively higher volume fraction than for the charged colloidal systems having strong acid groups. Apart from this, these particles also exhibit gas and liquidlike order at lower volume fraction, which is very similar to the other one. Furthermore, from electrophoresis, it is seen that the particle charge increases at intermediate salt concentration. This is also an important and useful property of these latex particles because the crystalline order can be maintained even at higher impurity concentrations than in conventional colloidal systems with strong acid groups. It is very well known that the most dominant interparticle interaction among the like-charged colloidal particle in a chargestabilized colloidal suspension is the screened Coulomb repulsion. The complete phase diagram of sulfonic group latex particles has been investigated with respect to the particle concentration, charge, and salt concentration.29 However, there has been no detailed investigation of such a phase diagram for latex particles
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having COO- groups. There have also been experimental observations of a gas-liquid transition30 and a gas-solid transition1 in bulk suspension in the case of sulfonic group particles. These experimental observations have served as evidence for the existence of a long-range attractive component in like-charged colloids in the effective interparticle interactions of like-charged colloids.12,31 So far, no systematic investigations have been carried out on polystyrene particles having a weak acid group to observe gas-liquid and gas-solid transitions. Our synthesized colloidal particles may be also an ideal system for the investigation of such transitions by controlling parameters such as pH, salt concentration, and volume fraction. We will be working toward these investigations in the near future.
Conclusions Novel, monodisperse, charged colloidal particles of DVB crosslinked polystyrene with acrylic acid surface grafting with 97 (7 nm diameter were synthesized and characterized by investigating the surface properties by conductivity titration and the size distribution by dynamic light scattering and atomic force microscopy. The structural ordering and dynamics are studied as a function of volume fraction using static and dynamic light scattering. As a function of volume fraction, the suspension shows gas, liquid, and crystalline order. The crystalline-like phase is found have BCC ordering. Furthermore, the salt effect shows that particles are highly stable without any aggregation up to a salt concentration of 0.1 M. The mobility and zeta potential are found to increase initially as a function of salt concentration and then decrease at higher salt concentration. The initial increase in the zeta potential may be due to the dissociation of more counterions, which increases the width of the diffusive doublelayer potential. The decrease in the zeta potential at higher salt concentration is due to the charge neutralization of salt ions with counterions in the vicinity of the double layer. Acknowledgment. This work was financially supported by a Grant-in-Aid for Scientific Research (A15205017) from the Ministry of Education, Science, Sports and Technology of Japan, to whom our sincere gratitude is due. This work was also supported by 21st century COE program for a United Approach to New Materials Science. LA052995A (28) Mouri, E.; Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H.; Torikai, N. Langmuir 2004, 20, 10604. (29) Sirota, E. B.; Ou-Yang, H. D.; Sinha, S. K.; Chaikin, P. M.; Axe, J. D.; Fujji, Y. Phys. ReV. Lett. 1989, 62, 293. (30) Tata, B. V. R.; Arora, A. K. In Ordering and Phase Transitions in Charged Colloids; Arora, A. K., Tata, B. V. R., Eds.; VCH: New York, 1996; p 149. (31) Tata, B. V. R. Curr. Sci. 2001, 80, 948.
