J. Phys. Chem. 1992, 96, 8180-8183
8180
References and Notes
central part of the chain and reduce ordering at the termini. However, as simple inspection of Figure 1 shows, the surviving dipoledipole ordering bias changes sign upon reversing the polarity of either the solvent or the solute dipoles (in the absence of substantial dipoltdipole correlations). It is thus expected that if, for example, a solute with the dipoles pointing away from the molecular center were to interact with a solvent in which the dipole moment points toward the molecular center, these trends would be reversed. Accordingly, a judicious mixture of solvents each with different dipole arrangements could produce a null effect for the dipolar bias experienced by a particular solute with a specific dipole moment composition (i.e., the null effect will be solute dependent). In summary, we feel that the significance of dipole generated ordering in the nematic phase has been overlooked in NMR studies of molecular ordering in nematics (perhaps due to some vague notion about the apdarity of the nematic phase and a simdtanmus neglect of excluded-volume/short-distancecontributions to the averaging of directional interactions), Clearly, the implications of dipoltdipole (and d i p o l e i n d u d dipole) interactions should be studied more extensively before positing a role for higher rank terms of the multipole expansion (e&, quadrupoles9J6) in explanations of nematic solvent effects.
(1) de Jeu, W. H. Phil. Trans. R. Soc. London 1983, A309, 217. (2) Cladis, P. E. Phys. R N . Lett. 1975, 35, 48. (3) McMillan, W. L. Phys. Reu. 1973, A8, 1921. (4) Gelbart, W. M. J. Phys. Chem. 1982, 86. 4298. (5) Cotter, M. A. Phil. Trans. R. Soc. London 1983, A309, 127. (6) Romano, S. Liq. Cryst. 1988, 3, 323. (7) Perera, A.; Patey, G. N. J . Chem. Phys. 1989, 93, 3045. (8) Zarragoicoechea, G. J.; Levcsque, D.; Weis, J. J. Mol. Phys. 1992, 75, 989. (9) Emsley, J. W.; Palke, W. E.; Shilstone, G. N. Liq. Crysr. 1991,9,643, 649. (10) Strictly speaking, the pair distribution function is not invariant with
respect to independent inversions of the directions of the two molecules even if their shape is symmetric. When the dipole moment composition of the molecules is not symmetric, the dipole interaction will break the full inversion symmetry of the pair distribution wen in a nonpolar medium. (The single molecule distribution is of course inversion symmetric.) Accordingly, there will be an additional source of incomplete averaging of the dipole interaction at short distances as a result of dipoltdipole correlations. This additional source produces a dipole contribution to the effective orientational potential for molecules carrying a single dipole moment wen when the dipole is positioned at the center of the molecule (Le., 8,s‘ = 0). See: Dunmur, D. A.; Palffy-Muhoray, P. Mol. Phys. 1992, 76, 1015 and references therein. (1 1) Photinos, D. J.; Samulski, E. T.; Toriumi, H. J. Chem. Soc., Furaday Trans. 1992, 88, 1875. (12) (a) Photinos, D. J.; Samulski, E. T.; Toriumi, H. J. Phys. Chem. 1990, 94,4688. (b) Photinos, D. J.; Samulski, E. T.; Toriumi, H. J . Phys. Chem. 1990, 94,4694. (13) Photinos, D. J.; Samulski, E. T.; Toriumi, H. Mol. Cryst. Liq. Crysr. 1991, 204, 161. (14) Manuscript in preparation. (15) Dunmur, D. A,; Toriyama, K.Liq. Crysr. 1986, 1, 169. (16) Est, A. J. van der; Kok, M. Y.; Burnell, E. E. Mol. Phys. 1987,60, 397.
Acknowledgment. This work was supported by a subcontract from the University of Pennsylvania (DARPA/ONR Grant NOOO14-90-J- 1559). Registry No. DBr-C9, 4549-33-1; DBr-C10, 4101-68-2; 5CB, 40817-08-1; SOCB, 52364-71-3; C10, 16416-29-8; Phase V, 37268-47-6.
Continuous Deionization of Latex Suspensions T. Palberg,*>tW. Hart1,t U. Wittig,g H. Versmold,* M. W M 4 t and E. Simnacbert Department of Physics, University of Konstanz. Konstanz, FRG. Institute of Physical Chemistry II, RWTH Aachen, Aachen, FRG, and Rcithel GmbH & Co., Bochum. FRG (Received: March 13, 1992; In Final Form: June 15, 1992)
A new, fast, and reproducible method of preparing ordered colloidal suspensions at very low salt concentrations is reported. Preparation times are reduced by 2 orders of magnitude compared to those for the usual methods. Particle concentration and salt contents can be monitored and controlled by optical and conductivity measurements. Transient effects appearing during continuous deionization of the sample are reported, and qualitative explanations of these effects are suggested.
Introduction Monodisperse, charged submicron spheres interacting via screened Coulomb potentials may form systems of fluidlike, crystalline, or amorphous interparticle structure, if the salt concentration in the suspension is low enough. For these systems, the terms “colloidal fluid”, “colloidalcrystal”, or “colloidal glass” are used, indicating that ordered colloids provide a model system for atomic substances. In contrast to the latter, typical length scales are in the order of a few hundred nanometers; thus, the structure and dynamics can be investigated by light-scattering methods. Also time scales are altered significantly. In recent publications, a number of effects have been reported and measured quantitatively, which are not always easily investigated in atomic substances, either because of unaccessible time scales or because experimental procedures demand very high expenditure. Examples include nucleation and crystal growth processes from an undercooled melt,’J nonequilibrium phase t r a n s i t i ~ ntransversal6 ,~~ and longitudinal’ lattice vibrations, structural relaxation processes, ‘University of Konstanz. *RWTH Aachen. IR6thel GmbH & Co.
0022-365419212096-8180$03.00/0
and two-dimensional phenomena.* Experiments are supported by extensive theoretical work.+’* Quantitative experiments are to be performed under well-defined and reproducible conditions concerning particle and salt concentrations. A number of sample preparation methods are reported, which usually include the more or less complete removal of salt ions and the subsequent addition of a known amount of salt. Alternatively, Myers and Saville” presented a sedimentation-decantation method, where the desired salt concentration is reached directly. We will, however, deal mainly with the former technique and focus on the deionization problem. Diutzis in 1952 passed protein solutions through a column of both anionic and cationic ion-exchange resinsI4 (IEX). This procedure can also be applied to suspensions, but care has to be taken at high volume fractions, because colloidal particles may coagulate on the IEX;’* suspended material is lost, and aggregates may contaminate the suspension. The amount of coagulation may be reduced significantly, if particla are predialyzed and the IEX is conditioned by special methods.16J7 Further methods of latex purification are dialy~is~*.~~ or agitation with mixed-bed IEX.M The choice of the preparation method has no influence on the chemical properties of the latices, as determined 0 1992 American Chemical Society
Continuous Deionization of Latex Suspensions by conductometric and potentiometric titration.21 AmctaN and Fujita pointed out that impurity levels of lr5and be reached with the suspension in contact with air.22 lower IXMO~ The authors " m e n d e d the use of a modified columnar method and subsequent dialysis under N2. By far the most common technique is simply the introduction of IEX into a (precleaned) sample contained in a (sealed) cell. The exchange process is believed to be completed when particle ordering appears after a period of several weeks. The process may be made faster by repeatingshaking of the sample. As the progress of the exchange process is not monitored, there may result severe inaccuracies in the experimental conditions. In nonagitated samples, a transient vertical-phase separation may occura with pronounced differences in particle concentrations of a crystalline, a fluidlike ordered, and a depleted phase. However, this separation seems to be restricted to the presence of IEX in the cell. If the resin is removed from the suspension, different phase separation phenomena do occur.24 These show much less pronounced density changes. The nature of the phenomenon is discussed to be highly controversal and may indeed be due to a combination of different causes.7,11,16,23,2s-27 To summarize the characteristics of the above-mentioned purification methods, we can state that low impurity levels can well be reached. However, the methods are time-consuming, they neither include an in situ control of the exchange process nor include subsequent addition of salt. Moreover, the presence of IEX in the measuring cell may lead to severe effects on the concentration and spatial distribution of both salt ions and particles. The above-mentioned methods also do not allow for different experiments performed simultaneously on one sample. To circumvent these disadvantages, we developed a technique combining the advantages of the former methods with a newly designed preparation circuit. We first separated the IEX from the measuring cell using two different cells connected via a peristaltically driven pump c i r ~ u i t . By ~ ~this, , ~ ~very fast deionization was achieved and the light-scattering properties were measured with high reproducibility at conditions of no added salt or with known amounts of salt added into the (disconnected) measuring Further use of a N2 atmosphere permitted precise conductometric titration^.^^,^^ The inclusion of additional measuring cells, a reservoir, and a bypass of the IEX then allowed for simultaneous measurements at arbitrary but equal particle and salt concentrati~ns.~~ Finally, with the setup presented here, these suspension parameters are precisely monitored and rigorously controlled by in situ determinati~n.'~
Experimental Section The particles used in this work are carboxylated polystyrene latex spheres with nominal diameters of 109 and 91 nm and a titrated charge of 2.2 X 104 e- per particle for the larger particles (DOW Chemical, purchased at Serva, Heidelberg, FRG). The hydrodynamic diameters determined by dynamic light scattering are = 102 and 81 nm; for the larger particles, the effective surface charge determined by torsional resonance detection (calculated after ref 6,assuming ideal polycrystallinity) is Z* = 490 f 20 at a volume fraction @ = 0.0033; the effectively transported charge from low-frequency conductivity and electrophoresis with no added salt is 2,= 370 f 20 (calculated after ref 30 with the electrophoretic mobility pEL= 10 p n s-'/(V m-I). A stock suspension of volume fraction @ = 1% was prepared by dilution with doubly distilled water (uW= 65 nS) and stored over mixed bed ion-exchange resin (Amberlite MB1, Serva, Heidelberg, FRG, processed by the abovementioned methodd6J7). At this stage of first contact of particles with IEX, there seems to be a considerable amount of aggregation (ca. 2-10%) always present in the suspension. The reason for this is not yet understood, as lowering the salt concentration actually should result in an enhanced colloid stability. From the stock suspension, samples with a particle concentration slightly above the desired value were prepared by further dilution and then passed through 0.2-pmmembrane filters (Schleicher & SchOll, FRG)to remove aggregates and dust.
I'
- - - R m -
c
--
c :
C
'
v?
Palberg et al.
8182 The Journal of Physical Chemistry, Vol. 96, No, 20, 1992 11
Ii,
I
d 0
450
950
1450
time /sec
Elgun 2. Conductivity vs time for the deionization of a 8 X 10-4 mol L-l solution of NaCl at different pump velocities v and masses m of IEX: (1) u = 1 mL s-l, m = 14 g; (2) u = 1 mL s-I, m = 7 g; (3) v = 0.5 mL s-I, m=lg.
200
1h
sw
603
100
t lsec
Figure 4. Conductivity (-) and the intensity of transmitted light ( 0 ) vs time for the deionization of a dilute, nonordering suspension of cp = 10" m-3 and a starting concentration cso = 2 X 10-4 M NaCI. Note the pronounced extrema and their difference in position and width. Deionization is considered complete when both plateau values are reached. 30
0 0
- -0.0
125
250
375
NaCl conc. r10'4/mol
500
t i m e /3ec
Figure 3. Conductivity vs time for the addition of a small amount of salt
..................
to deionized water.
rate dbcreases over several days to reach rates below 1 nS h-l but drastically increases again when the tubing is replaced by a new one. We thus attribute the contamination mainly to ions desorbing from the tube surface. Leakage of C 0 2through tube connections, seals, or the tubing itself is believed to play a minor role, as long as no silicone rubber parts are used. The very small contamination rate allows measurements at extremely low salt concentrations, which can be considered constant in time for at least a few hours. Contamination rates in the measuring cell can be reduced even further by use of tight valves at the connections to the tubing system, thus reducing the surface of the actual sample. However then, in situ control of impurity levels is no longer possible. The addition of a small amount of NaCl to completely deionized water is shown in Figure 3. Small oscillations stem from the pumping geometry and flatten out within a few cycles, indicating a homogeneous salt concentration. Upon successive addition of salt, the conductivity at the plateau values rises in quantitative agreement with literature data.3* Next we repeated the measurement with several latex suspensions of known diameter, surface charge, and particle concentration. The data of all these show qualitatively the same behavior, so we only present data on the 109-nm samples. A typical example of the conductivity and light transmission vs time is shown in Figure 4 for a dilute, nonordering suspension of particle concentration cp= 10'5 m-3. The starting concentration of NaCl was cp = 1 x 1V mol L-I, The decay of the conductivity resemblar that of pure NaC1, except that a minimum is observed and a higher plateau value is reached after 15-30 min due to the conductivity contribution of the p a r t i ~ l e s . ~The ' ~ ~transmitted ~ intensity displays a maximum and returns to the original value after 10-15 min. Visual inspection of the exchange cell shows a temporarily elevated particle concentration in the vicinity of the ion exchanger, preferentially around anion-exchanger beads. We consider dilute suspensions to be salt free, when the plateau value of the conductivity is reached. If now salt is added under bypass conditions, a behavior similar to the pure salt case is observed: a rise to a plateau value and flattening oscillations, indicating homogeneous salt distribution after two to three cycling times. However, successive addition
I-'
Figure 5. Conductivity vs concentration of NaCI. Note the absence of a minimum in conductivity in contrast to the deionization.
0
2
1
3
/ ( I O 5 cm
Figure 6. Static structure factor of a suspension of cp = 3.4 X loi8m-3 completely deionized 90 min after taking the sample off the stock suspension.29
of salt does not lead to a minimum in conductivity, as is clearly seen in Figure 5. Here the particle concentration remains unchanged. At higher particle concentration, where ordering is achieved during deionization, the conductivity behaves the same as with dilute suspensions, whereas the transmitted intensity is a complicated function of particle and salt concentrations, phase behavior, and wavelength. During the continuous deionization process, the transmission usually displays a series of oscillations, goes through a deep minimum, and f d y reaches a plateau value after 40-80 min, the earlier, the faster the pump velocity. Also the magnitude of transmitted intensity is pump velocity depmdeat. We consider the exchange process to be completed when the final plateau value of transmitted intensity is reached. Having reached both plateau values, the suspension readily crystallizes, when the pump is stopped. Figure 6 shows a static structure factor of a completely deionized sample of 91-nm particles measured only 90 min after taking the sample off the peak, stock suspension.29 From the existence of the seventh hg# the sample is identified to be a polycrystalline boc solid and from the position of Bragg peaks, the concentration of particle8 is m-3. determined as cp = 3.4 X Discussion Compared to the usual ways of preparing samples of colloidal crystals or fluids, the method reported herc has several advantages.
Continuous Deionization of Latex Suspensions First of all, it is much faster. Typical times for deionization in the order of 1 h are reached compared to weeks for standing preparation. Second of all, we can easily detect the completion of the exchange proctss when a constant conductivity value is reached and, for dilute samples, the starting value of transmitted intensity is reached, whereas the deionization is wnsidmd complete for denser when the Of transmitted intensity is reached* Cyclic preparation of the sample allows us to confirm space and time invariant conditions concerning particle and salt concentrations. Thus, as a third point, we are able to perform a number Of mtasurements On One in diffmntlY designed measuring cells under precise in situ control of sample conditions. Equally well, different experiments at different setups may be performed at identical preparation conditions. The experiments presented here clearly show that a temporal increase in particle concentration occurs in the vicinity of the ion-exchange beads. No fluctuations in particle concentration are observed under bypass conditions. This supports arguments for an active role of the ion exchanger in some of the observed phase transition^.'*^^^*^ A transient decrease in particle concentrationdoes not suffice to explain the minimum in conductivity observed during deionization. In fact, the minimum is not present in the salt concentration dependence of the conductivity upon addition of salt. F'reliminary experiments on the exchange of NaOH and HCl point toward different exchange rates for Na+ and C1-, leading to a transient exctss concentration of NaOH upon continuous removal of neutral NaCl from the suspension. That in turn may lead to a decrease in conductivity in analogy to a conductometric titration. However, this still remains speculative, and more experiments are now being conducted to clarify the issue.
conclusions We developed a fast and reproducible method of preparing colloidal suspensions at arbitrary but constant low salt concentrations controlled by in situ measurements. Transient effects on conductivity and intensity of transmitted light are reported and are qualitatively explained by temporal appearance of an excess concentration of NaOH and the accumulation of particles next to IEX beads, respectively. Acknowledgment. We love to thank the group of R. Weber for many inspiring discussions. Financial support of Deutsche Forschungsgemeinschaft is gratefully acknowledged.
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8183
References a d Notes (1) Aastuen, D. J. W.; Clark, N. A.; Cotter, L. K.; Ackerson. B. J. Phys. Rev. Lett. 1986, 57, 1733. (2)Gast, A. P.; Monovoukas, Y. Nature 1991, 351, 553. (3) Dozier, W. D.;Chaikin, P. M. J . Phys. 1982,43,843. (4)Ackerson, B. J.; Chowdbury, A. H. J . Chem. Soc., Furuday Discuss. 1987,83, 1. (5)Xue,W. G.; Grest, G. S.Phys. Rev. Lett. 1990, 64,419.
(6)Chaikin, P. M.; DiMeglio, J. M.; Dozier, W. D.; Lindsay, H. M.; Weitz, D. A. In Physics of Complex und Supermolecular Fluids; Safran, S. A., Clark, N. A., us.; Wiley: New Yo&, 1987. (7)Cotter, L. K.; Clark, N. A. J . Chem. Phys. 1987. 86, 6616. (8) Murray, C. A,; Van Winkle, D. H. Phys. Rcu. Lcrr. 1987,58, 1200. (9)Alexander, S.;Chaikin, P. M.; Grant, P.; Morales, G. J.; Pincus, P.; Hone, D. J . Chem. phys. 1984, 80, 5776. (10) Krause, R.;Nigele, G.; Karrer, D.; Schneider, J.; Klein, R.;Weber. R. P h p . A 1988, 153, 400. (1 1) Voegtli, L.P.; Zukaski, C. F., IV J . Colloid Interface Sci. 1991,141, 79. (12) Lawen, H.; Hansen, J. P.; Roux, J.-N. Phys. Reu. A , in press. (13) Myers, D. F.; Saville, D. A. J. Colloid Interface Sci. 1989, 113,448. (14) Diutzis, H. M. Ph.D. Thesis, Harvard, 1952. (15) Schenkel, J. H.; Kitchener, J. A. Nuture 1959, 182, 131. (16) Vanderhoff, J. W.; Van den Hul, H. J.; Tausk, R.J. M.; Overbeek, J, T.G, suflaces; New YorL, 1970. (17)Helffrich, F. Ion Exchange; McGraw-Hill: New York, 1962;p 230. (18) Ottewill, R. H.; Shaw, J. N. Kolloid 2.Z . Polym. 1967, 215, 261. (19) Ottewill, R. H.; Shaw, J. N. Kolloid Z . Z . Polym. 1967, 218, 34. (20)Van den Hul, H. J.; Vanderhoff, J. W. Proc. Inr. Congr. SurJ Acr. Subst., Vth, 1968, 2, 319, (21)Stone Masui, J.; Wattilon, A. J. Colloid Inrerfuce Sci. 1975,52,479. (22)Ametani, K.; Fujita, H. Jpn. J . Appl. Phys. 1978, 17, 17. (23)Arora, A. K.; Tata, B. V. R.;Sood, A. K.; Kcesavamoorthy, R.Phys. Reu. 1988* 60*2438. (24) Hachisu, S.;Kobayashi, Y.; Kosc, A. J . Colloid Inreflice Sci. 1973, 42,342. (25)Sogami, I.; Ise, N. J . Chem. Phys. 1984, 81, 6320. (26) Luck, W. A. P. Phys. Blrirter 1967. 7 , 304. (27)Hiltner, P. A.; Krieger, I. M. J. Phys. Chem. 1969, 73,2386. (28)Hirtl, W. Ph.D. Thesis, Dortmund. 1985. (29)Wittig, U. Ph.D. Thesis, Dortmund, 1989. (30)Hirtl, W.; Versmold, H.; Wittig, U.; Marohn, V. Mol. Phys. 1983, 50, 815. (31) Hirtl, W.; Versmold, H. J . Chem. Phys. 1984,80, 1387. (32)Hirtl, W.; Versmold, H. J. Chcm. Phys. 1984, 81, 2507. (33)Hirtl, W.; Versmold, H.; Wittig, U.Ber. Bunsenges. Phys. Chem. 1984*88* lM3(34) Hirtl, W.; Versmold, H. J . Chem. Phys. 1988, 88, 7157. (35) HUser, M. Private communication. (36)Palberg, T. Ph.D. Thesis, Dortmund, 1988. (37) Deggelmann, M.; Palberg, T.; Hagenblichle, M.; Maier, E. E.; Krause, R.;Graf, C.; Weber, R.J. Colloid Interfuce Sci. 1991, 143, 341. (38) Ebert,H., Ed.Physikulisches Tuschenbuch;Vieweg: Braumhweig, 1978, (39) Russel, W. B.; Saville, D. A.; Schowalter, T. Colloidal Dispersions; Academic Press: Cambridge, MA, 1990; p 316.
