Langmuir 1994,10, 3529-3535
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Colloidal Single Crystals of Silica Spheres in Alcoholic Organic Solvents and Their Aqueous Mixtures T.Okubo Department of Polymer Chemistry, Kyoto University, Kyoto 606-01, J a p a n Received April 1, 1994. In Final Form: July 5, 1994@ Single crystals of colloidal silica spheres, 110 f4.5nm in diameter, are visually observed in suspensions of purely alcoholic organic solvents, Le., methyl alcohol, ethyl alcohol, and ethylene glycol and in their aqueous mixtures in the exhaustively deionized conditions. Single crystals appear also for aqueous mixtures of propyl alcohol and n-butyl alcohol. Close-up color photographs of the single crystals are taken. Two kinds of colloidal singlecrystals, i.e.,blocklike crystal grow up from the homogeneousnucleationmechanisms in the bulk phase for from the cell wall and the pillar-likeones from the heterogeneous nucleation mechanism along the cell wall, are observable in these solvent systems clearly. The size ofthe single crystals increases significantlyas the sphere concentration decreases,and the largest crystals appear at sphere concentrations slightly higher than the critical concentration of melting (q5c, in volume fraction). @c values are around 0.0002in pure water and increase sharply as the fraction of organic solvent increases. The 4cvalues in 100% of methyl alcohol, ethyl alcohol, and ethylene glycol range from 0.01 to 0.02,which are substantially low compared with the reference values reported hitherto. The change in 4cis explained well with the change in the dielectric constants of the solvent mixtures. The important role of the expanded Debyescreening length around spheres and the intersphere repulsion is supported strongly. Colloidal suspensions show the extraordinary distributions, i.e., gaslike, liquid-like, and crystal-like structures, especially in deionized states.'-15 Colloidal crystals, in which colloidal particles distribute in the ordered array, and quite similar to metals and protein crystals in morphology and phase diagram. They are also very convenient systems to understand the fundamental properties of states of substances and electrostatic interactions of macroionic systems. Recently, we have observed very large single crystals, 2-8 mm in size for the very diluted aqueous suspension (ca. 0.0002in volume fraction) and also exhaustively deionized suspension with ion-exchange resins more than 3 weeks in a test tube of 13 mm in outside diameter.16-18 Colloidal crystals are formed mainly with a n electrostatic interparticle repulsion and an expanded electrical double layer around the spheres in the deionized state.12J5 Therefore, the role of the solvent is very important for these interparticle interactions. Colloidal crystals have been formed for the concentrated suspension of colloidal spheres in the nonaqueous solvents such as 1,4-dioxane, Abstract published in Advance ACS Abstracts, September 1, 1994. @
(1)Luck, W.; Klier, M.; Wesslau, H. Ber. Bunsen-ges. Phys. Chem. 1963,67,75,84. (2) Vanderhoff, W.; van de Hul, H. J.;Tausk, R. J. M.; Overbeek,J. Th. G. Clean Surfaces: Their Preparation and Characterization for Interfacial Studies; Goldfinger, G., Ed.; Dekker: New York, 1970. (3) Hiltner, P. A.; Papir, Y. S.; Krieger, I. M. J. Phys. Chem. 1971, 75,1881. (4) Kose, A.; Ozaki, M.; Takano, K.; Kobayashi, Y.; Hachisu, S. J . Colloid Interface Sci. 1973,44,330. (5) Crandall, R. S.; Williams, R. Science 1977,198,293. (6) Mitaku, S.; Otsuki, T.; Okano, K. Jpn. J.Appl. Phys. 1978,17, 305. (7) Clark, N. A.; Hurd, A. J.; Ackerson, B. J. Nature 1979,281,57. (8) Lindsay, H. M.; Chaikin, P. M. J . Chem. Phys. 1982,76,3774. (9) Tomita, M.; Takano, K.; van de Ven, T. G. M. J. Colloid Interface Sci. 1983,92,367. (10)Pieranski, P. Contemp. Phys. 1983,24,25. (11)Pusey, P. N.; van Megen, W. Nature 1986,320,340. (12) Okubo, T. Ace. Chem. Res. 1988,21,281. (13)Ottewill, R. H. Langmuir 1989,5,4. (14)Russel, W. B., Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, 1989. (15)Okubo, T. Prog. Polym. Sei. 1993,18,481. (16) Okubo, T. Naturwissenschaften 1992,79,317. (17) Okubo, T. Colloid Polym. Sci. 1993,96,61. (18) Okubo, T. Langmuir 1994,10,1695-1702.
benzene, and bromoform by several r e s e a r c h e r ~ . In ~J~~~~ this report, formation of the colloidal crystals has been investigated in alcoholic nonaqueous solvents, which are dehydrated completely and further deionized exhaustively with the ion-exchange resins more than 3 or 4 weeks. It will be described in this report that the critical concentrations of melting (&) for the suspensions containing these organic solvents are substantially low (0.0002-0.025in volume fraction) compared with the reference values reported hitherto, and the influence of the deionization is substantial.
Experimental Section Materials. ColloidaI silica spheres of CS-91 in aqueous suspension were a gift from Catalyst & Chemicals Ind. Co. (Tokyo). Diameter (d), standard deviation ( 6 ) from the mean diameter, and polydispersity index (6/d)were 110 nm, 4.5 nm, and 0.041, respectively. The values of d and 6 were deteranined from an electron microscope. The charge density of the spheres was determined by conductometrictitration with a Wayne-Kerr autobalance precision bridge,Model B331,mark I1(BognerRegis, Sussex). Charge density of strongly acidic groups was 0.48 pCl cm2, The sphere sample was carefully purified several times using an ultrafiltration cell (Model 202, membrane: DiafloXM300, Amicon Co.). Then the sample was treated on a mixed bed of cation- and anion-exchange resins (Bio-Rad,AG501-X8(D), 20-50 mesh) and further on the molecular sieves (type3A1/6,Wako Pure Chemicaks, Osaka)for at least 1month. Water used for the purification and for suspension preparation was deionized by using cation- and anion-exchange resins (Puric-R, type G10,Organo Co., Tokyo),purified by a Milli-Q reagent grade system (Millipore Co., Bedford, MA), and further treated with the ion-exchangeresins t o Bio-Rad. Organic solvents used were water-soluble,i.e., methyl alcohol,ethyl alcohol,n-propylalcohol, n-butyl alcohol, and ethylene glycol, which were most purified grade samples commercially available and used after treatment of the dehydration and also deionization with the coexistence with molecular sieves and Bio-Rad resins more than a month. CS-91 spheres in methyl alcohol, ethyl alcohol, and ethylene glycol were also gifts from Catalyst & ChemicalsInd. Co. These suspensions were obtained from aqueous suspension by the ultrafiltration technique against the organic solvents using a hollow-fiber-typeultrafiltration module, type SIP-1013,Asahi (19)Kose, A.; Hachisu, S. J. Colloid Interface Sci. 1974,46,460. (20) Rodriguez, B. E.; Wolfe, M. S.; Kaler, E. W. Langmuir 1993,9, 12.
0743-7463/94/2410-3529$04.50/00 1994 American Chemical Society
Okubo
3530 Langmuir, Vol. 10, No. 10, 1994 Chemical Ind. Co., Tokyo. For the preparation of 100%methyl alcohol, ethyl alcohol, and ethylene glycol suspensions, the molecular sieve were kept in the suspension in addition to the Bio-Rad resins in order to eliminate water contamination as completely as possible. Colloidal suspensions were prepared in the test tubes with stoppers, 13 and 15 mm inside and outside diameters. The sample suspensions in test tubes were further treated with a small amount of Bio-Rad resins in the test tubes more than 3 or 4 weeks with inverted mixing several times a day. Close-upPhotographing and Video Films. Phase equilibria between the crystal-like and liquid-like structures were obtained clearly with the naked eye. Photographingof colloidal crystals in a test tube was made with a Canon EOSlO camera,
macrolens (EF50mm,f= 2.51,and life-sizeconverter EF. Velvia film (Fujichrome,RVP35, IS0=50) was used. Light source was a pocket-type flash light (Xenontype, BF-775,National). Closeup video films were taken with a video-camera recorder (Hi-8, CCD-VSOO, Sony). Color photographs from the video films were made with a color printer (NV-MP1,Panasonic).
Results and Discussion Methyl Alcohol and Aqueous Methyl Alcohol Suspensions. In order to obtain the exhaustively deionized and dehydrated methyl alcohol suspension, both the Bio-Rad resins and the molecular sieves were in the suspension more than 4 weeks. Figure 1shows a closeup color picture of the CS-91 suspensions thus obtained. Small (0.2- 1mm) but strongly brilliant single crystals appeared! It should be noted that the sphere concentration for this figure is not very high, 0.0177 in volume fraction. The critical concentration of melting (4,)is 0.01 and around 30-fold higher than that of aqueous suspension (4, = 0.0004). It is highly plausible that in methyl alcohol suspension, the number of the effectively dissociated ions of colloidal spheres is small compared with that of the aqueous suspension.21 The dielectric constant of methyl alcohol is lower than that of water. These differences may introduce the higher 4, value for methanol. Size of the single crystals first increased slightly and then turned to decrease as the sphere concentration increased, which differs slightly from the sharp change in size for the aqueous system.16-18 Note here that the crystal growing rate evaluated from the reflection spectroscopy was very slow compared with that of the aqueous suspension. Kinetic analysis on the crystal growth will be discussed later in a separate paper. Figure 2 shows a typical example of the close-up color pictures of the single crystals formed for the aqueous methyl alcohol mixture a t x (fraction of methyl alcohol in aqueous organic solvent mixture) = 0.6. Large or very large single crystals are seen. The change in colors from bluish to reddish is due to the change in the incident angle of light through the curved wall of the test tube cell. The shape of the crystals in the bulk phase far from the cell wall is blocklike and varied in shape. For several crystals, the crystal planes are recognized. These crystals are grown up from the homogeneous nucleation mechanism. Furthermore, lined and pillar-like crystals are also observable along the cell wall, though not so clear in this figure. These are crystals from the heterogeneous nucleation from the cell wall. These morphological features of the colloidal single crystals are quite similar to those of purely aqueous suspension.18 Figure 3 shows the phase diagram ofthe aqueous methyl alcohol mixtures. Clearly, 4, values increased gently as the fraction of methyl alcohol ( x ) increased from 0 to 0.6. However, 4, increased sharply for the higher x-values. Five different sizes of open circles in the figure indicate the round size of the single crystals which appeared, i.e., smaller than 0.1 mm (shown by the smallest open circles (21) Okubo, T. Ber. Bunsen-ges. Phys. Chem. 1987,91, 1064.
in the figure), 0.1 to 0.5 mm (secondary small open circles), 0.5 to 1.5 mm (medium size of open circles), 1.5 to 2.5 mm (secondary large open circles), and larger than 2.5 mm (largest open circles), respectively. For the aqueous suspension,18the largest single crystals were observed for the suspension a bit higher than the critical concentration of melting, and the size decreased sharply with increasing sphere concentration. However, in the methyl alcohol system, the largest crystals appeared a t a much higher concentration than the 4, and the size decreased gently with sphere concentration. Furthermore, the range of sphere concentrations, where the size increases, was recognized clearly and widely for methyl alcohol containing systems. For example, see the change in size shown by open circles at x = 0.6 in the figure. According to the effective hard-sphere mode1,22-28 colloidal crystal is formed when the effective diameter (d,R) of the colloidal spheres, which includes the Debye screening length (Dl), is close to or larger than the observed intersphere distance ( D ) ,i.e., def=2 x D1+ diameter(d)l 1. D. In crystal-like structures, the spheres fluctuate around their equilibrium points. When d,Eis comparable to or a bit shorter than the D value, the distribution of the spheres is usually liquid-like, and the spheres move without keeping their positions. When d,fis much shorter than D , a gas-like distribution is observed. Note here that the observed intersphere spacing ( D )is always close to the calculated mean intersphere distance (D,,). The Debye-screening length, D1 is given by eq 1
D,= ( 4 7 ~ e ~ n / c k , 1 1 - ~ ’ ~
(1)
where e is the electronic charge, E is the dielecric constant of the solvent, k g is the Boltzmann constant, and n is the concentration of free-state cations and anions in suspension and given by n = n, n, no, where n, is the concentration (number of ions per cm3) of diffusible counterions, n, is the concentration of foreign salt, sodium chloride, for example. no is the concentration of both H+ and OH- from the dissociation of water. In order to estimate n,, the fraction of free-state counterions (/?I must be known. Note that the maximum value of D1 observed hitherto was ca. 1pm in water. The similar value of D1, Le., 1.2 pm, is also estimated from eq 1by taking no = 2 x (mol/dm3)x NAx ~ m -whereNAis ~, Avogadro’s number. It should be mentioned here that /?-values are very small for typical colloidal particle^,^^^^,^^^^^ and the n-values calculated using the stoichiometric charge number (2) on a colloidal surface (instead of /?Z)is always overestimated. Table 1compares the D1 values observed and calculated in methanol-water mixtures. The observed values of the Debye screening length, DI,,,~, are obtained from eq 2
+
+
where D,is the intersphere distance a t the critical sphere concentration of melting, &. Here, /? was assumed to be 0.1 irrespective of the methanol content, though the /? value is expected t o decrease slightly with increasing (22) Baker, J. A.; Henderson, D. J. Chem. Phys. 1967,47,2856. (23) Wadachi, M.; Toda, M. J. Phys. SOC.Jpn. 1972,32,1147. (24) Hachisu, S.; Kobayashi, Y.; Kose, A. J. Colloid Interface Sci. 1973.42.342. (25) Bkenner, S. L. J. Phys. Chem. 1976,80, 1473. (26) Takano, K.; Hachisu, S. J . Chem. Phys. 1977,67,2604. (27) Barnes, C. J.;Chan, D. Y.; Everett, D. H.; Yates, D. E. J . Chem. SOC.,Faraday Trans. 2 1978,74,136. (28) Voeglli, L. P.; Zukoski, C. F., IV J . Colloid Interface Sci. 1991,
-141 - - , 79 .- .
(29) Alexander, S.; Chaikin, P. M.; Grant, P.; Morales, G. J.; Pincus, P.; Hone, D. J. Chem. Phys. 1984,80,5776. (30) Okubo, T. J. Colloid Interface Sci. 1988,125, 380.
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Langmuir, Vol. 10, No. 10, 1994 3531
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Figure 1. A close-up color picture of CS-91 spheres in 100 vol % methyl alcohol 4
min after inverted mixing the suspension which has been treated with Bio-Radresins and the molecular sieves for 3 weeks. C$ = 0.0177.
Figure 2. A close-up color picture of CS-91 spheres in 60 vol % methyl alcohol aqueous mixture 30 min after inverted mixing
the suspension which has been treated with Bio-Rad resins for 4 weeks. q5 = 0.00227. Exposure time = 1.5 s, iris = 22.
Table 1. Parameters in the Effective Hard-Sphere Model for the Aqueous Methyl Alcohol Mixtures 0 0.2 0.4 0.6 0.8 1
0.00040 0.00044
1350 1310 1240
620 600 565
0.00076
1090
0.0023
490
750 450
320 170
0.00052 0.011
770 710 640 540
340 150
fraction of methanol.21 As is clear from the table, observed values of the Debye length agreed with the calculation excellently, which again supports the validity of the effective hard sphere model for the organic solvent systems.
Ethyl Alcohol and Aqueous Ethyl Alcohol Suspensions. Figures 4 and 5 show the close-up color pictures of the colloidal single crystals of CS-91 spheres in 100% ethanol and 60% ethanol aqueous suspensions. Clearly, the single crystals were observed for both the suspensions. Size of the single crystals was a bit smaller than that of the corresponding aqueous methanol mixtures. For the purely ethanol suspension, which has been treated with the Bio-Rad resins and the molecular sieves for a long time more than 4 weeks, single crystals from the heterogeneous nucleation were not recognized with the naked eye (see Figure 4). Figure 6 shows the phase diagram for the aqueous ethanol mixtures. Profiles of the critical sphere concentrations as a function of ethanol content are quite similar to those of aqueous methanol mixtures shown in Figure 3. However, the former are a bit larger than the latter. It should be mentioned here that the blinking of the colloidal single crystals, which has been observed by this
8
X
Figure 3. Phase diagram of CS-91spheres in aqueous methyl
alcohol mixtures. x denotes volume fraction of methyl alcohol. Open and solid circles show the “crystal” and “liquid states, respectively. author recently,31appeared very often for the aqueous alcohol mixtures in the sphere concentrations close to the values.
Aqueous Propyl Alcohol and Aqueous n-Butyl Alcohol Suspensions. Figure 7 and 8 show the close(31) Okubo, T.J . Colloid Interface Sei. 1992,153, 587.
3532 Langmuir, Vol. 10, No. 10, 1994
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Figure 6. Phase diagram of CS-91 spheres in aqueous ethyl
alcohol mixtures. x denotes volume fraction of ethyl alcohol. Open and solid circles show the “crystal”and “liquid” states, respectively.
Figure 4. A close-up color picture of CS-91 spheres in 100vol % ethyl alcohol 50 min after inverted mixing the suspension,
which has been treated with Bio-Radresins and the molecular sieves for 3 weeks. = 0.00757. Exposure time = 2 s, iris = I$
22.
Figure 7. A close-up color picture of CS-91 spheres in 60 vol % propyl alcoholaqueous mixture 183 min after inverted mixing
the suspension which has been treated with Bio-Rad resins for
2 weeks. 4 = 0.00757. Exposure time = 0.7 s, iris = 22.
Figure 5. A close-up color picture of CS-91 spheres in 60 vol % methyl alcohol‘aqueousmixture 18min after inverted mixing the suspension, which has been treated with Bio-Rad resins for 3 weeks. 4 = 0.00530. Exposure time = 1.5 s, iris = 22.
up color pictures of single crystals in the aqueous suspensions containing 60 vol % and 20 vol % of propyl alcohol. Colloidal single crystals in the 5%n-butyl alcohol aqueous mixtures are shown in Figure 9. Suspensions containing propyl alcohol or n-butyl alcohol more than 60 vol % or 5 vol %, respectively, were not available
Single Crystals of Silica Spheres
Langmuir, Vol. 10, No. 10,1994 3533
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0.2
0.8
0.4
i
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Figure 10. Phase diagram of CS-91spheres in aqueous propyl
alcohol mixtures. x denotes volume fraction of propyl alcohol. Open and solid circles show the “crystal” and “liquid states. respectively.
Figure 8. A close-up color picture of CS-91 spheres in 20 vol % propyl alcoholaqueous mixture 60 min after inverted mixing
the suspension which has been treated with Bio-Rad resins for 2.5 weeks. 4 = 0.000227. Exposure time = 2 s, iris = 22.
0
0.2
0.4
0.6
0.8
1
X
Figure 11. Phase diagram of CS-91spheres in aqueousn-butyl alcohol mixtures. x denotes volume fraction of n-butyl alcohol.
Open and solid circles show the “crystal” and “liquid states, respectively. homogeneously, and not studied. Suspensions of sphere concentrations a bit higher than the critical concentration of melting showed large single crystals. Shapes of the single crystals were also very similar to those of aqueous methyl alcohol mixtures. The phase diagrams of aqueous propyl alcohol and aqueous n-butyl alcohol mixtures are shown in Figures 10 and 11,respectively. It is rather surprising that the critical concentrations and sizes of single crystals in these solvents are quite similar to those in aqueous methyl alcohol mixtures. Figure 9. A close-up color picture of CS-91 spheres in 5 vol % n-butyl alcoholaqueousmixture 60 min after inverted mixing
the suspension which has been treated with Bio-Rad resins for 4 weeks. 4 = 0.000757. Exposure time = 1.5 s, iris = 22.
Ethylene Glycol and Aqueous Ethylene Glycol Mixtures. Formation of colloidal crystals was also observed for 100% ethylene glycol. Strongly brilliant beautiful blue colors were observed for the comparatively concentrated suspensions. Figure 12 shows a close-up
Okubo
3534 Langmuir, Vol. 10,No. 10,1994
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0.6
0.8
1
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Figure 14. Critical concentrations of melting for CS-91
suspensions in aqueous-methyl alcohol(O), -ethyl alcohol( x >, -propyl alcohol (A), -n-butyl alcohol(O), and -ethylene glycol ( 0 )mixtures as a function of mixing ratio (x).
Figure 12. A close-up color picture of CS-91 spheres in 80 vol % ethylene glycolaqueous mixture 26 min after inverted mixing
the suspension which has been treated with Bio-Radresins for 4 weeks. 4 = 0.00606. Exposure time = 2 s, iris = 22.
I
E112 $c-1/3 vs plots in water (0)and aqueous-methyl alcohol (01, -ethyl alcohol (x), and -ethylene glycol ( 0 ) mixtures.
Figure 15.
0
0.2
0.6
0.4
0.8
1
X
Figure 13. Phase diagram of CS-91 spheres in aqueous
ethylene glycolmixtures. x denotes volume fraction of ethylene glycol. Open and solid circles show the “crystal” and “liquid states, respectively. color photograph in 80 vol % ethylene glycol aqueous mixture. The small size of single crystals is recognized in the figure. Figure 13 shows the phase diagram, which is quite similar t o that of aqueous methanol systems.
Critical Concentrations of Melting in Aqueous Alcohol Mixtures. Critical concentrations of melting (&) for all the suspensions examined in this work are shown in Figure 14. The 4cvalues increased sharply as
the fraction of organic solvent increased. Furthermore, the order (3) seems to hold in the & values.
water
