Langmuir 1996,11, 2853-2855
2853
Restricted Motion of a Particle Trapped inside a Void in a Colloidal Dispersion Hiroshi Yoshida,*rt Norio Ise,* and Takeji Hashimotot Hashimoto Polymer Phasing Project, ERATO, Research Development Corporation of Japan, Keihanna Plaza, Hikaridai, Seika, Kyoto 619-02, Japan, and Central Laboratory, Rengo Co., Ltd., 186-1-4, Ohhikari, Fukushima, Osaka 553, Japan Received April 24, 1995@ The dynamics of a particle trapped inside a void in colloidal dispersion was studied by confocal laser scanning microscopy. "he motion of the trapped particle was found to be highly restricted compared to that of the particles in the surrounding high-density region. The restricted motion must be the result of a very long range interaction with the surrounding particles, which has been overlooked in current colloidal interaction theory.
Introduction Colloidal dispersions have long been believed to be more or less homogeneous in particle distribution. However, recent studies have clearly revealed that these macroscopically homogeneous dispersions are microscopically inhomogeneous in both static and dynamic features.l+ One example is the non-space-filling, localized ordered structure of colloidal particles, which has been observed by micros~opy,*,~ by traverse photography,6 and by Another outstanding case of the structural inhomogenities is void formation2J0-16 observed in deionized dilute dispersions of highly charged colloidal spheres. The void region of low particle density was observed to coexist with a surrounding liquid-like region of high particle density. Ise et al. observed with a confocal laser scanning microscope (CLSM) that the voids were stable (for more than 60 min) and huge (for example, 50 x 50 x 150 pm) and existed in the internal material of the dispersion, ruling out an artifactual nature due to the glass-dispersion i n t e r f a ~ e . ~ JThe ~ J ~coexistence of liquid and gas phases in dilute colloidal dispersions has also been demonstrated experimentally and discussed in terms of Monte Carlo
* To whom correspondence should be addressed. t Hashimoto Polymer Phasing Project. f Central Laboratory. Abstract published inAduance ACSAbstracts, August 1,1995. (1)Ise, N.; et al. Langmuir 1990,6, 296. (2)Dosho, S.;et al. Langmuir 1993,9,394. (3)Schmitz, K. S.Macroions in Solution and Colloidal Suspension; VCH Publishers: New York, 1993. (4)Ito, K.; Nakamura, H.; Ise, N. J. Chem. Phys. 1988,85, 6136. (5)Ito, K.; Nakamura, H.; Yoshida, H.; Ise, N. J. Am. Chem. SOC. 1988,110,6955. (6)Yoshiyama, T.; Sogami, I. S. Langmuir 1987,3,851. (7)Ise, N. et al. J. Am. Chem. SOC.1980,102,7901. (8)Matsuoka, H.; Ise, N. Adv. Polym. Sci. 1994,114,187. (9)Konishi, T.; Ise, N.; Matsuoka, H.; Yamaoka, H.; Sogami, I.; Yoshiyama, T. Phys. Rev. E 1995,51,3914. (10)Kose, A,; et al. J. Colloid Interface Sci. 1973,44,330. (11)Ise, N. In Orderingand Organization in Ionic Solutions; Ise, N., Sogami, I. S., Eds.; World Scientific: Singapore, 1988;p 397. (12)Ise. N.: Matsuoka.. H.:. Ito.. K.:.Yoshida. H. Discuss. Faradar SOC. 1990,go, i53: (13)Ito, K.; Yoshida, H.; Ise, N. Chem. Lett. 1992,2081. (14)Ito, R;Yoshida, H.; Ise, N. Science 1994,263,66. (15)Kesavamoorthv. R.: Raialakshimi., M.:, Rao.. C. B. J. Phvs.: Condens. Matter 1989,'1,7149: (16)Structural inhomogenities were also reported for other ionic systems. Uyeda et al. photographed the coexistence of solid, liquid, and gas structures in the LB films of fatty acid5.l' Ringsdorf et al. provided atomic force micrographsshowingdomain formationin "liquid@
like" repions.18 ~~~~. ~.
(17)Ezsumoto, M.;,etal. Nature 1987,327,319; Thin Solid Films 1993,223,358. (18)Ringsdorf, H.; et al. Science 1993,259,213.
0743-746319512411-2853$09.00/0
simulations using the Sogami potential,lgwhich has, in addition to the Yukawa type Coulombic repulsive term, an attractive tail at large interparticle separations.20The void structure and the gas-liquid transition positively suggest the unignorable role of electrostatic attraction which has been overlooked in the widely accepted DLVO theory.21 In this paper, we report another extraordinary new observation, that of the restricted motion of a particle trapped inside avoid, which appearsto be highly important to the correct understanding of the interparticle interaction.
Experimental Section An inverted type CLSM (LSM 410,Carl Zeiss, Oberkochen, Germany) was used with a 5-mW HeNe laser and x40 oil immersion type objective (N.A. 1.30).23 By use of the CLSM, direct observation of particles in the internal regions of milky colloidal dispersions was feasible even at a distance of 100 pm from the coverslip. The thickness of the focal plane was about 0.5 pm. Polystyrene-based latex particles (N700, 0.7 pm diameter, 3.9pClcm2analytical charge density, Sekisui Chemical Osaka, Japan) were used. Sample dispersions were extensively purified as follows:24First, the latex dispersion was washed with fresh Milli-Q grade water using an ultrafiltration cell (Amicon Model 8050, Amicon Inc., MA,USA) and a 0.1-pm pore size membrane (VCWP, Millipore Corp., MA, USA), and then ionexchanged using analytical grade ion exchange resins (Bio-Rad Laboratories, AG 501-X8(d), CA, USA) before use. The density of the dispersion medium was adjusted to that of the particles (1.05)by using a H20/D20 = 1/1 mixture, to avoid gravitational effectes. A quartz cell (10 mm in diameter, 15 mm in height, bottom made of a coverslip) was used for the observation. A 1.0% dispersion (0.7 mL) was introduced into the cell with 30 mg of the ion-exchange resins, which were kept 5 mm away from the cell bottom using a nylon mesh screen. Half of the protons of the cation exchange resins were exchanged to deuterium ions for the density matching. The cell was then shaken by a Bio-Shaker (Sibata Hario Glass, Tokyo, Japan) for 6 h and kept inside a constant temperature chamber at 25 "C for 2 days. (19)Tata, B. V. R.; Arora, A. K.; Valsakumar, M. C. Phys. Rev. E 1993,47,3404. (20)Sogami, I. S.; Ise, N. J. Chem. Phys. 1984,81,6320. (21)Gas-liquid phase separation in colloidal dispersions has been described in terms of a depletion effect for semidilute (around 10%) dispersionswith addition of free polymer or for dispersionsof particles with polymer hairs attached.22This is not the case for the systems we report here, which are dilute (lessthan 1%) dispersionsoflatex particles without any additional polymer. (22)Russel, W. B. Colloidal Dispersions; Cambridge University Press: Cambridge, 1989;Chapter 10. (23)Confocal Microscopy Wilson, T., Ed.; Academic Press: London, 1990.
(24)Ito, K.; Nakamura, H.; Yoshida, H.; Ise, N. J.Am. Chem. Soc. 1988,110,6955.
0 1995 American Chemical Society
2854 Langmuir, Vol. 11, No. 8, 1995
Letters
Figure 1. Void structure in a polymer latex dispersion observed by CLSM at a vertical distance of 28pm from the coverslip (cell bottom): scan time, 0.7 s/frame; scale bar a t lower left corner, 50 pm; latex, N700; latex concentration, 1.0%. The thickness of the focal plane was 0.5 pm. The CLSM observation was made at room temperature, controlled around 25 "C.The scanning speed was set to 0.2-0.7 s/frame by limiting the scanning region. Scanned images were stored in a videotape and analyzed by a Macintosh Quadra 650 personal computer with public domain image analyzing software (NIH Image, version 1.55, National Institutes of Health, USA). No convection was detected in the dispersion during the observation.
Results and Discussion Figure 1 shows a CLSM image of void structures in the internal region at a vertical distance of 28 pm from the coverslip (cellbottom). The distance was calculated from the measured mechanical displacement of the objective, taking the influence of the refractive index difference between the coverslip and the dispersion medium into account. This structure was maintained for more than 3 h although fluctuation of the interfaces of the voids was observed. The void structures could be easily destroyed by shaking the cell, which led to a homogeneous distribution. After this randomized dispersion in the sample chamber was left for 3 days, voids were again formed. In most cases, no particles were found inside the voids. However, as shown in Figure 2, some.voids did contain particles. Figure 2a is a horizontal cut (parallel to the coverslip of the cell bottom) of such a void at a distance of 15 pm from the coverslip. A single particle (a white dot) is seen inside the void. Figure 2b is a cross section of the void and particle in a plane perpendicular to the coverslip. It is obvious that the trapped particle really exists in isolation from other particles. The nearest neighbor particlesfrom the trapped one were roughly 8- 10 pm away in all directions. In Figure 3a, the distribution of the particles around and in the void shown in Figure 2a is reproduced. Figure 3b is a cumulative plot of the centers of gravity of the particles in 50 successive scans (25 s). The motion of the particles in the surrrounding region is so hectic that the available space appeared to be almost uniformly covered in the 25-s time span, whereas the motion of the trapped particle is very restricted. The latter stayed in the vicinity
Figure 2. CLSM images of a void containing a particle: scan time, 0.7 s/frame; scale bars at lower left corners, 20 pm; latex, N700; concentration, 1.0%. The trapped particle is marked by arrows. (a)Cross section of a void at a distance of 15pm parallel to the coverslip. (b) Perpendicular cross section of the same void, obtained by computer-slicing of a 3D CLSM image of the void shown in (a), constructed from 24 horizontal cross sections (each 1.6 pm apart). The distances from the coverslip to the upper and lower edges of (b) are 39 and 2 pm, respectively.
1Opm
Figure 3. Locations of particles (a)in a single scan and (b) in 50 successive scans: scan time, 0.5 s/frame; distance from the coverslip, 15,um;latex, N700; concentration, 1.0%. The centers of gravity of the particles in (b) are shown as dots.
of its initial position for more than 10 min. In Figure 4, trajectories of (a)the trapped particle, (b) a particle at the void interface, and ( c ) a free particle (at a concentration of 2.0 x are compared. Clearly, the diffusion motion of the trapped particle is highly damped in comparisonwith the others. Consideringthat the particle concentration is practically nil in the void over the experimental time scale, the diffusion of the trapped particle was expected to be the same as that of free particles. 'This was not the case, however. It has to be emphasized that phenomenon discussed above was not
Letters
Figure 4. Trajectories of three particles, (a)trapped inside a void, (b) at the interface of the void, and (c) a free particle. Trajectories(a)and (b)were determinedby tracing the particle centers (aand /3 in Figure 3a). Trajectory c was obtained from an independent CLSM observation of a particle in a dilute where interparticleinteractionscan dispersionof 2.0 x be ignored. The trajectorieswere for 5.8 s. Scan time was 0.2 dframe. only observed for one particular particle. Particles which were trapped inside voids commonly showed restricted motion. According to the Einstein-Stokes equation, the d i f i sion of a noninteracting particle is determined by the temperature, viscosity of the medium, and its size. It is unrealistic to assume that the local temperature in the void is different from that of the surrounding region, since a small temperature difference would cause convection in the dispersion medium and destroy the structure. This was not observed. It is also difficult to admit that the local viscosity is higher in the void than in the bulk, though direct proof is not available. Particle aggregation, which would cause damped diffusion, can be ruled out, since monomeric and dimeric forms can be easily distinguished by the CLSM. It has been suggested that the voids should contain a higher electrolytec o n c e n t r a t i ~ n but , ~ ~in, ~terms ~ of direct interactions, such an effect should lead to enhanced diffusionbecause the particle trapped inside the void would be more strongly screened. Although a detailed analysis is lacking, it would therefore seem that the observed phenomenon is due to many-body contributions rather than direct local interactions. The particles in the surrounding region experience both (1)the interaction with neighboring particles and (2)that with distant particles. On the other hand, the trapped particle feels only the second interaction. Two features of these interactions must be noted. First, because of the large number of distant particles, the influence of their Brownian motion on the trapped particle would be smeared. Therefore the trapped particle is affected by the very long range interaction, weak or strong, homogeneously from all directions. This may be the cause of its restricted motion. Secondly, the influence of distant particles on the particles in the surrounding region would be less significant than that of their neighboringparticles (25) Delville, A. Langmuir 1994, 10, 395. (26) Smalley, M. V. Personal communication.
Langmuir, VoZ. 11, No. 8, 1995 2855 due to the differencein the working distances. This would cause random motion of the particles in the surrounding region. There remains a possibility that the trapped particle might be an “impure”particle that has a different number of surface charges from the others. However, free noninteracting particles obey Einstein’s theory of Brownian motion for neutral particles to the first approximation because electrostatic charges on the latex surface are practically neutralized by counterion c~ndensation.~’ Therefore, the restricted motion of the trapped particle cannot be explained without introducing a very long range interaction even if the charged state ofthe trapped particle is different from the others. The nature of the very long range interaction causing the restricted motion could be repulsive or attractive. However, the formation of the void structure can be explained only by a long-range attraction. Therefore we believe that it is more natural to account for the restricted motion in terms of the attractive interaction. It is worth mentioning the recent unanticipated findings of positive adsorption of charged particles or micelles in the vicinity of a like-charged i n t e r f a ~ e .(Compare ~ ~ ~ ~ ~ the particle numbers at three distances from the coverslip shown in Figure 14 ofref2.) This positive adsorption was observed in the distance range of 5 to 50 ym for a latex dispersion. It is difficult to escape the conclusionthat the adsorption is caused by a very long-range attraction between the particles (or micelles) and the interface. We believe that a similar kind of attraction is operative between the trapped particle and distant particles in the surrounding region and that this gives rise to the restricted motion observed. Recently, atomic force microscopy and surface force apparatuses have been applied to colloidal systems to determine the interparticle interaction. However, because those techniques are only applicable to single particleflat plate or single particle-single particle geometries at relatively short separations, only relatively strong shortrange interactions could be detected. Although more detailed analyses are required at large interparticle separations, the present results show the existence of a very long range interaction between the particles which has been overlooked in current colloidal science.
Acknowledgment. We thank Dr. Martin V. Smalley for helpful discussions and for his kind help in preparing the manuscript. LA950322U (27) Ito. K.:Ise. N.: Okubo. T . J . Chem. Phvs. 1985. 82. 5732. (28) Lu;J. R.;Simister, E.’A.;Thomas, R. K.; Penfold,’J.J . Phys. Chem. 1993,97,13907. (29) Ito, K.;Muramoto, T.; Kitano, H. J . Am. Chem. SOC. 1995,117, 5005.
