Langmuir 1990,6, 478-481
478
external surface, oxy en can still effectively quench 'pyrene*. However, T1 does not quench 'pyrene* under these experimental conditions.
also wants to thank Dr. M. Wolszczak and Dr. K. Koike for a helpful discussion on the electron-tunneling mechanism.
Acknowledgment. We thank the National Science Foundation for the support of this work. Kai-Kong Iu
Registry No. 0,, 7782-44-7; T1, 7440-28-0; H,O, 7732-18-5; pyrene, 129-00-0;perylene, 198-55-0.
Q
Partitioning and Stability of Aqueous Dispersions. Particle Size of Dye Dispersionst Erik Kissa Research and Development Division,$ Jackson Laboratory, Chemicals and Pigments Department, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19898 Received April 25, 1989. I n Final Form: August 17, 1989 The particle-water interface in aqueous dye dispersions stabilized with a polymeric dispersant has been probed with a water-immiscible solvent in which the dye but not the dispersant is soluble. Unprotected dye particles dissolve in the solvent on contact and are extracted into the solvent phase. Dye particles protected by sodium lignosulfonate, a polymeric dispersant, are hydrophilic and favor the aqueous phase when extracted with the solvent. The extraction rate of the dye from the aqueous phase into the solvent phase depends on the resistance of the protective barrier to penetration of the solvent and indicates the effectiveness of particle shielding. When the composition of the dispersion and the extraction conditions are constant, the extraction rate decreases exponentially with the increasing fineness of the dispersion and the stability to sedimentation and coagulation. The extraction technique promises to be a useful tool for studying dispersion stability and probing the particle-water interface.
Introduction Water-insoluble dyes, pigments, and pharmaceutical and agricultural chemicals are dispersed in water by reducing their particles to a size which can be stabilized by a di~persant.'-~The dispersant aids wetting of dye particles with water, facilitates breaking of agglomerate^,^,^'^ and stabilizes the dispersion by forming a steric or an electrostatic barrier by adsorption on the surface of the Ionic polymers stabilize aqueous disper-
'
Presented at the 62nd Colloid and Surface Science Symposium, J u n e 22, 1988, University Park, PA. Publication No. 648. (1) Dispersion of Powders in Liquids, 2nd ed.; Parfitt, G. C., Ed.; Wiley: New York, 1973. (2) Callahan, W. B.; Manz, W. J.Oil Colour Chem. Assoc. 1964,47, 1964. (3) Parfitt, G. D. J. Oil Colour Chem. Assoc. 1967, 50, 822. (4) Carr, W. J. Oil Colour Chem. Assoc. 1971,54, 155. (5) Carr, W. Powder Technology 1977,17, 183. (6) Rehbinder, P. A. Colloid J. USSR 1968,20,493. (7) Kissa, E. Text. Res. J. 1989,59, 66. (8) Napper, D. H. J. Colloid Interface Sci. 1977,58, 390. (9) Ottewill, R. H. J . Colloid Interface Sci. 1977,58, 357. (10) Parfitt, G. D.; Peacock, J. In Surface and Colloid Science; Matievic, E., Ed.; Plenum Press: New York, 1978; Vol. 10, p 163. (11) Ottewill, R. H. Colloid Polym. Sci. 1980, 67, 71. (12) Tadros, Th. F. Adv. Colloid Interface Sci. 1980,12, 141. (13) Laible, R.; Hamann, K. Adv. Colloid Interface Sci. 1980, 13, 65. (14) Sato, T.; Ruch, R. Stabilization of Colloidal Dispersions by Polymeric Adsorption; Marcel Dekker: New York, 1980. (15) Vincent, B.; Whittington, S. In Surface and Colloid Science; Plenum Press: New York, 1981; Vol. 12, p 1. (16) Tadros, Th. F. In The Effects of Polymers on Dispersion Properties; Tadros, Th. F., Ed.; Academic Press: London, 1982; p 1. (17) Napper, D. H. Polymeric Stabilization of Colloidal Dzspersions; Academic Press: London, 1983.
*
0743-7463/90 f 2406-0478$02.50/0
sions by the electrosteric mechanism, which combines electrostatic and steric mechanism^.'^^'^*^*^ One of the mechanisms may be dominant under certain conditions. Dilute dispersions are stabilized mainly by electrostatic repulsion, but in concentrated dispersions the steric stabilization mechanism is dominant. The objective of this paper is to show that the stability of dispersions can be examined and the particle-water interphase probed by extraction with a waterimmiscible solvent in which the dispersed solid but not the dispersant is soluble. We studied aqueous dye dispersions stabilized with sodium lignosulfonate, an ionic amphiphilic polymer which is derived from lignin and features sulfonic acid and phenol The sta(18) Vincent, B. In Polymer Adsorption and Dispersion Stability; Symposium Series 240; Goddard, E. D., Incent, B., Eds.; ACS American Chemical Society: Washington, DC, 1984; p 1. (19) Hirtzel. C. S: Raiaeoualan. R. Colloidal Phenomena: Noves: Park Ridge, NJ,.1985.' ' (20) Lyklema, J. In Flocculation, Sedimentation, and Consolidation; Moudgil, B. M., Somasundaran, P., Eds.; United Engineering Trustees. 1985. (21) Silberberg, A. J. Colloid Interface Sci. 1986, 111,486. (22) Fleer, G. J.; Scheutjens, J. M. H. M.; Cohen Stuart, M. A. Colloids Surf. 1988, 31, 1. (23) Herb, C. A.; Ross,S. Colloids Surf. 1980, 1, 57. (24) Buscall, R. J. Chem. SOC.,Faraday Trans. 1 1981, 77,909. (25) Glennie, D. W. In Lignins; Sarkanen, K. V., Ludwig, C. H., Eds.; Wiley: New York, 1971. (26) Le Bell, J. C.; Hurskainen, V. T.; Stenius, P. J. J.Colloid Interface Sci. 1976, 55, 60. (27) Tadros, Th. F. Colloid Polym. Sci. 1980,258 439. (28) Heimann, S. Melliand Textilber 1982,53,885. (29) Heath, D.; Tadros, Th. F. Colloid Polym. Sei. 1983,261,49. (30) Hatfield, G. R.; Maciel, G. E.; Erbatur, 0.;Erbatur, G. Anal. Chem. 1987,59, 172.
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0 1990 American Chemical Society
I
_
Partitioning and Stability of Aqueous Dispersions
bility of an aqueous dispersion can be visualized to depend on the coverage of the particles by an adsorbed or attached polymer and the nature of the interface formed. We observed that milling of the dye in the presence of the dispersant reverses the partitioning of the dye in a twophase system consisting of water and the solvent. This observation suggested that the rate at which the solvent extracts the particles from the aqueous dispersion indicates the effectiveness of particle protection and the stability of the dispersion.
Experimental Section Materials. The yellow dye31 (I) and the navy blue dye32 (11) have the following structures:
No.
c.0
(LI
The dispersant (Lignosol SFX 65), a sodium lignosulfonate, was obtained from Reed Lignin, Greenwich, CT. The solvents were reagent grade chemicals. Preparation of Dispersions. The sand-milling procedure for the preparation of dye dispersions has been described in one of our previous articles.' The dye (15.0 g or proportionally more, if a wet filter cake was milled), 30 mL of water (or proportionally less, if a wet filter cake was milled), and 9.0 g of the dispersant (Lignosol SFX 65) were milled with 220 g of sand agitated by a stainless steel disk rotating at 1700 rpm in a stainless steel beaker. The pH of the slurry was adjusted to 6.4 with aqueous ammonia. The temperature was maintained at 40-50 O C . Five minutes before milling was terminated, 0.15 g of sodium silicofluoride, 0.15 g of paraformaldehyde, and 12.0 g of sorbitol (70% in water) were added. The slurry was filtered through a milk filter, and the sand was washed with 15 mL of water, unless a higher concentration of the dispersion was needed. Partitioning between Water and a Solvent. The dye dispersions contained as an average 20% w/w dye and 12% WJW of dispersant. To facilitate phase separation, the dispersions were diluted with water, usually to a concentration of 0.2 gJL of dye, immediately before the extraction with the solvent. The dilution of the dispersion assures that (a) the solubility of the dye in the solvent is not exceeded; (b) the dispersant does not interfere with the separation of the phases; and (c) the concentration of the dispersion can be adjusted to a constant value for a particle size comparison. A 50-mL aliquot of the diluted dispersion was transferred to an 8-ozjar, and 50 mL of a solvent (tetrachloroethylene or chloroform) was added. The jar was closed and rotated end over end on a rotating mixer (Fisher Kendall) for the specified time. The jar was allowed to stand undisturbed for 10 min. Absorbances of both phases were determined spectrophotometrically. Five milliliters of the water layer was diluted to 100 mL with dimethylformamide (DMF) and 15 mL of water. An aliquot portion of the solvent layer was diluted with the same (31) Cole, J. E.; Dunbar, R. A.; Gumprecht, W. H. Ger. Offen.2,125,058, 1972. (32) Blackwell, J.; Tropea, A. US.Patent 3,678,028, 1972.
Langmuir, Vol. 6, No. 2, 1990 479 solvent, if necessary. The partition ratio and the percent dye extracted were calculated from the dye concentrations. The initial concentration of the dispersion was determined by diluting a weighed sample (about 150 mg) of the dispersion to 250 mL with DMF.33 A 10-mL aliquot volume of the solution and 20 mL of water were transferred to a 100" volumetric flask and diluted with DMF to the mark. The dye content of the dispersion was calculated from the absorbance of the solution and the absorptivity of the pure dye. Particle Size. The particle size of the dye dispersions was determined by a Joice-Loebl disk ~ e n t r i f u g e . ~ ~ - ~ ~ Sedimentation Rate. A sample of the dispersion was diluted with water for the extraction rate determination, according to the procedure described above. About 40 mL of the undiluted dispersion (about 20% WJW dye) was transferred into a 50-mL centrifuge tube and the weight of the dispersion determined. The sample was centrifuged at the maximum speed for 2 h in the International Clinical centrifuge, Model CL. The tube was inverted and held in the vertical position to allow the liquid to drain until dripping stopped. The percent sedimentation was calculated from the weight of the residue in the centrifuge tube. The value obtained was corrected for incomplete drainage of the dispersion, determined without centrifugation by the same draining procedure. Storage Stability. Dispersions having a different particle size were prepared by varying the milling time of the dye (11) with Lignosol SFX 65. The dispersions were filtered, and their dye content and extraction rates were determined. After 6 weeks of undisturbed storage at ambient temperature, most of the dispersions had formed agglomerates. The sediment was separated and the dye content of the dispersions redetermined.
Discussion The stability of a dispersion depends on the coverage of the particles by an adsorbed or attached polymer and on the nature of the interfacial region formed. When the particle-water interface of an aqueous dispersion is probed with a solvent which can dissolve the unprotected solid, but not the polymeric dispersant sheath around the particle, the extraction of the disperse phase from the aqueous dispersion into the solvent depends on effective shielding of the particles by the polymeric barrier and consequently on the stability of the dispersion. Unprotected dye particles in a two-phase system, consisting of water and the solvent, dissolve rapidly in the solvent. The partitioning of the dye between water and the solvent reverses itself when the dye particles are protected by sodium lignosulfonate, an amphiphilic polymeric dispersant. An aqueous dispersion of the dye, prepared by milling the dye in the presence of sodium lignosulfonate, is miscible with water. In the two-phase system, the dispersed dye stays in the aqueous phase, indicating that the dye particles surrounded by the ionic amphiphilic polymer are hydrophilic. When the two-phase system containing the dye dispersion is agitated, the dye is transferred slowly from the aqueous phase into the solvent phase. The rate at which the dye is extracted from the aqueous phase into the solvent phase depends on the extraction conditions and the stability of the dispersion. In order t o extract the dye from the aqueous dispersion, the solvent has to penetrate the protective shell and displace the dispersant on the particle. The solvent may diffuse through the protective sheath around the particle in the aqueous phase or, more likely, displace the dispersant when the particle collides with the solvent phase. (33) Kissa, E. In The Analytical Chemistry of Synthetic Dyes; Venkataraman, K., Ed.; Wiley: New York, 1977. (34) Allen, T. Particle Size Measurement, 3rd ed.; Chapman and Hall:New York,1981. (35) Atherton, E.; Cooper, A. C.; Fox, M. R. J. SOC.Dyers Colour. 1964,80, 521. (36) Beresford, J. J . Oil Colour Chem. Assoc. 1967,50, 594.
480 Langmuir, Vol. 6, No. 2, 1990
Kissa M Dye
Particle Size (pm)
l.o
Extracted
r
r
100
I
5 t
\
0.2 i-
2 -
c 1 0.1 I 10
-
1-
, 20
I 30
I
1
I
40
50
100
-
0.5
Milling Time (min)
Figure 1. Particle size of dye dispersions as a function of milling time: yellow (I), navy blue (II), cumulative 70% under size. In either case, the extraction rate depends on effective shielding of the particle. When the particle is protected, only a fraction of solvent-particle collisions leads to dissolution of the particle in the solvent phase analogous to slow flocculation induced by particle-particle collisions. Since the reversal of partitioning is not abrupt but continuous, partitioning of the dispersion gives information on dispersion stability and the particlewater interphase. Milling of a dye in the presence of water and a dispersant decreases the size of dye particles, as shown in Figure 1 for dyes I and 11. The relative amount of dye extracted, E,, decreases with the increasing milling time (Figure 2). On logarithmic scales, a plot of E, against the milling time yields straight lines. Consequently, a plot of the particle size versus the relative amount of dye extracted, E,, at time t is linear on logarithmic scales (Figure 3). The slope of the straight lines suggests that the relative amount of dye extracted from the dispersion at time t is exponentially related to the average particle size
E , = Adn (1) where A is a coefficient, d is the average particle size (measured in rm), and n is an exponent. The value of the coefficient A depends on the composition of the dispersion, the solvent, and the extraction conditions. The value of the exponent n was found to be 2.5 for the yellow (I) dye and 2.2 for the navy blue (11). An assumption that the extraction rate is proportional to the surface area of the particle requires n to have a value of 2. A deviation from the expected value can be explained by polydispersity and irregular shape of particles. The particle size shown in Figures 2 and 3 is not the physical size of ideal spherical particles but the equivalent size of spherical particles having the same weight as the actual particles. The remarkably good correlation of dye partitioning with the particle size of the dispersion warrants some thoughts on the partitioning mechanism. Milling reduces the particle size of the dispersion. The number of particles and the specific surface area, the surface area per unit weight of the particles, increase. The resulting increase in the total surface area should facilitate contact of the particles with the solvent and increase the extraction rate. In contrast, the extraction rate decreased
O2 0.1
t
10
20 50 100 Milling Time (min)
200
Figure 2. Extraction of a dye from ita dispersion in water with tetrachloroethylene vs milling time: yellow (I), navy blue (11). Extraction time 60 min. Particle Size (pm)
r
l.o
70% US
0.3 0.2
0.1
,
0.5
10
1.0
50
E I (Yo)
Figure 3. Extraction of yellow dye (I) from ita dispersion in water with tetrachloroethyleneas a function of the particle size (50% and 70% under size). Extraction time 60 min. when the particle size was reduced. The decrease in the extraction rate suggests therefore that smaller particles are more effectively stabilized. This conclusion is consistent with the observation by Matthews and Rhodes" and Watillon and JosephPetit? that larger particles of polystyrene latices coagulated more rapidly than smaller particles. The theory derived by Bagchi3' predicts that, for slow flocculation of dispersions with the same adsorption layer thickness, the dispersion becomes progressively unstable with the increasing particle size. Although the stability of a dispersion usually increases with decreasing particle size, the reduction of particle size increases the interfacial energy and may lead ultimately to instability.' The complexity of the particle size effect on dispersion stability has been discussed in several papers.4M2 The critical flocculation concentration of polystyrene latices has been found to vary with the particle size, reaching a maximum and then decreasing gradually with the diameter of the (37) Matthews, B. A.; Rhodes, C. T. J . Colloid Interface Sci. 1968, 28, 71. (38) Watillon, A.; Joseph-Petit, A. M. Discuss. Faraday SOC.1966, 42. - - , 143. - .-. (39) Bagchi, P. Colloid Polym. Sci. 1976,254, 890.
(40) Ottewill, R. H.; Shaw, J. N. Discuss. Faraday SOC.1966, 42,
154.
(41) Chung, H. S. Colloids Surf. 1985, 15, 119.
(42) Penners, N. H.
G.;Koopal, L. K.Colloids Surf. 1987,28, 67.
Langmuir, Vol. 6, No. 2, 1990 481
Partitioning and Stability of Aqueous Dispersions Dye in Dispersion
% Dye Extracted
(%)
l”R 1.0
-
0
0 2
O b
06
10
0.8
Dispersant Added (giL)
Figure 4. Effect of sodium lignosulfonate on partitioning of the dye (11). Initial dispersion concentration: dye, 0.2 g/L; dispersant, 0.12 g/L. Extracted for 15 min with chloroform. Sedimentation (%) 100,
I
7
0
0
p a r t i ~ l e . ~The ~ ? stability ~~ of our aqueous dye dispersions did not decrease, however, with decreasing particle size, at least within the particle size range studied. While the extraction mechanisms remain to be elucidated in detail, it is evident that the extraction of the dye from the aqueous dispersion involves displacement of the dispersant on the dye particle by the solvent. Hence, the extraction rate should depend on the adsorption of the dispersant on the particle surface and consequently on the concentration of the dispersant in the bulk aqueous phase. The adsorbed dispersant can be visualized to exist in an equilibrium with the dispersant in the bulk aqueous phase. Dilution of the dispersion water lowers the dispersant concentration in the bulk aqueous phase and shifts the equilibrium toward desorption. Consequently, increasing the dispersant concentration in the dispersion should favor adsorption and increase the stability of the dispersion. We found indeed that adding more dispersant to a dispersion decreases the extraction rate (Figure 4). It may be argued that the extraction rate indicates only the stability of a dispersion to the displacement of the protective dispersant shell by the solvent. To show that (43) Kotera, A.; Furusawa, K.; Kudo, K. Kolloid 2.2.Polym. 1970, 240, 837. (44) Wiese, G.R.;Healy, T.W.Trans. Faraday SOC.1970,66,490.
,” 1
2
3
5
10
20
30
50
E,
Figure 6. Storage stability of navy blue dye (11) dispersions containing initially 20% dye, expressed as the percent dye remaining in dispersion after 6 weeks of storage and plotted against percent dye extracted from diluted dispersions before storage. Solvent, tetrachloroethylene; extraction time, 5 min.
extraction data are also valid for a general assessment of dispersion stability, we correlated the extraction rate, determined for diluted dispersions containing less than 1g/L of dye, with the sedimentation and coagulation stability of concentrated dispersions containing 14-20 ?4 dye solids. The relationship between extraction and sedimentation was examined for dye I and I1 dispersions differing in particle size. Sedimentation in dilute dispersions obeys the Stokes law for spherical uncharged particles in the absence of particle-particle interactions. However, the sedimentation velocity of concentrated dispersion is not necessarily a simple function of particle size because of particle-particle and particle-liquid interaction^.^^ We found for dispersions containing 20% dye that sedimentation increases with increasing extraction rate. On logarithmic scales, the relationship is linear (Figure 5), indicating that the sedimentation rate increases also with increasing particle size. The extraction rate of diluted dispersions correlated also with the stability of concentrated dispersions to agglomeration. Dispersions containing 20% of dye I1 were filtered to remove any agglomerates present and were allowed to stand undisturbed for 6 weeks. The agglomerates which formed during storage were separated, and the amount of dye remaining in the dispersion was redetermined. The stability of the dispersions, expressed as the percent of dye remaining in the dispersion, increased with the decreasing extraction rate of the dispersions (Figure 6) and consequently with the decreasing particle size. These experiments provide evidence that extraction of a dispersion with a solvent, in which the dispersed solid but not the dispersant is soluble, is a useful tool for studying dispersion stability. This novel technique for probing the interface of dispersed solids is applicable also to emulsions and merits further attention. Registry No. I, 34791-88-3; 11, 34446-26-9. (45) Schmitz, R. Colloid Polym. Sci. 1978,256, 578.
