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Partitioning and Stability of Aqueous Dispersions. Effect of Electrolytes on the Stability of Aqueous Dye Dispersionst Erik Kissa Jackson Laboratory, Chemicals and Pigments Department, E. I . du Pont de Nemours and Co., Wilmington, Delaware 19898 Received October 12,1989. In Final Form: January 16,1990 The effect of electrolytes on the stability of an aqueous dye dispersion has been studied by extracting the dispersion with a solvent in which the dye but not the dispersant is soluble. The amount of dye extracted from ita dilute aqueous dispersion stabilized with a polymeric dispersant (sodium lignosulfonate) was found to increase with the 213 power of extraction time. Electrolytes increase the extraction rate in qualitative agreement with the Schulze-Hardyrule. The extraction of the dye from its dispersion containing electrolytes involves one or both of the following mechanisms: (a) the electrolyte causes flocculation, and the solvent extracts the aggregates;(b) the electrolytereduces the stability of the dispersion,and the solvent penetrates the perturbed polymer barrier around the particle. The latter mechanism dominates when the extraction rate is more rapid than the focculation rate. The extraction rate is limited by the transfer rate of the dye from ita dispersed state to the solvent phase. The amount of dye extracted within a time interval increases with increasing dye concentration in the dispersion to a transfer-rate-dependent maximum concentration of the dye in the solvent phase. The transfer rate increases with decreasing stability of the dye dispersion. This extraction technique can be used to evaluate dispersion stability and detect destabilization by electrolytes, heat, or agitation.
Introduction We have studied the stability of aqueous dye dispersions and probed the particle-water interface with a waterimmiscible solvent in which the unprotected dye but not the polymeric dispersant is soluble.lV2 The extraction rate of the dye from the aqueous dispersion into the solvent phase is correlated with the particle when the composition of the dispersion and the extraction conditions are constant. The extraction rate decreases with the increasing stability of the dispersion.2 T h e ratedetermining step is the penetration of the dispersant sheath around the particle by the solvent, with the consequent displacement of the dispersant and the dissolution of the dye particle in the solvent. The dispersions were stabilized with sodium lignosulfonate, a polymeric dispersant featuring phenolic and sulfonic acid hydrophiles. Around a dye particle, sodium lignosulfonate forms a sheath which is oriented with its ionic and hydrophilic sulfonate groups toward water. According to current theories, ionic polymers stabilize lyophobic dispersions by an electrosteric mechanism which combines electrostatic and steric stabilization mechanism^.^^^ It is believed that the concentrated dispersions are stabilized mainly by the steric mechanism, but in dilute dispersions electrostatic stabilization dominates.'* The surface charge of the dispersed particles depends on t h e concentration of t h e dispersant and other electrolytes present in the bulk solution. In dilute dispersions, the countercharge of the adsorbed dispert Research and development division publication no. 657. Presented at the American Chemical Society 197th National Meeting, Dallas, TX, April 10, 1989. (1)Kissa, E., Text. Res. J. 1989,59,66. (2) K i m , E. Langmuir 1990,6,478. (3)Pditt, C. D., Ed. Dispersion of Powders in Liquids, 2nd ed.;Wiley
New York, 1973. (4) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Colloid Science; Academic Press: London, 1983. (5) Herb, C. A.; Rosa, S.Colloids Surf. 1980,1,57. (6)Buscall, R. J. Chem. SOC.,Faraday Trans. 1 1981, 77, 909.
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sant on the dye particle is diffuse and the counterions are far from the particle. Electrostatic repulsion prevents particles from approaching each other and aggregating. However, a high ionic strength of electrolytes can cause flocculation by compressing the electrostatic barrier around the particle. The reduced electrostatic repulsion allows the colliding particles to approach to a distance where attraction exceeds repulsion. Agglomeration or flocculation results if ionic repulsion is the only stabilization mechanism. Since the dye is extracted with a nonionic solvent, it was intriguing to find out how the electrostatic effects of electrolytes on dispersion stability affect extraction with a nonionic solvent. It can be visualized that the solvent extracts the dye flocculated by the electrolyte or penetrates the protective polymer barrier perturbed by the electrolyte, displaces the dispersant, and dissolves the dye.
Experimental Section The navy blue dye7 used in this study has the following structure:
\-I
I
Procedures. The preparation of aqueous dye dispersions and the methods for the determination of the particle size and the partition ratio of the dye dispersion with a water-immiscible solvent have been described in our previous The dispersions A, B, and C were made by milling the dye I and sodium lignosulfonate (60% of the weight of dye) for 6.7, 0.5, and 0.5 h, respectively. The dye used for preparing the dispersion C was dried before milling; the other dispersions were made by milling the filter cake of the dye without drying. The dispersions contained as an average 20% w/w of dye and 12% (7) Blackwell, J.; Tropea, A. US.Patent 3,678,028, 1972.
0 1990 American Chemical Societv
Kissa
1218 Langmuir, Vol. 6, No. 7, 1990 % Dye Extracted
E,(%)
Dye Extracted
c, (W) 70.010
:T
-I
0.008
tCd'(hC6,)
Figure 1. Amount of dye I extracted with tetrachloroethylene as a function of extraction time (dispersionsA, B, and C).
I 0.5
w/w of dispersant. The average particle size of the dispersions increased in the order A < B < C, indicated by the cumulative weight percent of particles under 0.3 pm: A, 83%;B, 42%; C
