Material Sciences and Engineering, Miskolc, Volume 36/2. (2011) pp. 11–20
ELECTROKINETIC POTENTIAL OF POLYSTYRENE LATEX PARTICLES IN POLYELECTROLYTE MIXTURES SÁNDOR BÁRÁNY University of Miskolc, 3515 Miskolc-Egyetemváros, Hungary
[email protected]
*
The effect of binary mixtures of cationic and anionic polyelectrolytes (PE) on the electrokinetic potential of monodisperse carboxylated polystyrene (PS) particles as a function of the mixture dose, its composition and way of adding the polymers to the suspension has been studied In mixtures of cationic and anionic PE, the ζ-potential of particles is determined by the adsorbed amount of the anionic polymer independently of the charge density of polyelectrolytes, the ratio between positively and negatively charged segments and the sequence of addition of the mixture components. It has been shown that the isoelectric point of the surface is reached at the adsorbed amount of positive charges that is only a small fraction of the “total” surface charge density. The laws observed were explained by features of macromolecules conformation in adsorbed mixed polyelectrolyte layers. Keywords: polystyrene, particles, suspension, anionic and cationic polyelectrolytes, mixture, electrokinetic potential, isoelectric point, polyelectrolyte layers, conformation. Introduction Polymers are widely used as flocculants for suspensions in many industrial applications, such as mineral processing, papermaking, water treatment and biotechnology. In recent years there has been considerable interest in the use of multi-component flocculants, especially dual-polymer systems. In the latter case, there can be significant advantages over the use of single polymers. The effect of non-ionic polymer mixtures on flocculation of hydrophobic sols has been studied in detail by Csempesz et al. [1, 2]. A short review on the issue is given in our recent papers [3-5]. The effect of polymers on colloidal stability is mainly due to adsorption of macromolecules onto the particles surface and the structure of adsorbed layers formed [6-8]. Electrokinetic measurements are a powerful tool to investigate the structure of interfacial polymer layers [9-11]. At low ionic strength, formation of fairly thick hydrodynamically impermeable adsorbed layers of non-ionic polymers shifts the slip plane further out into the bulk liquid phase, which causes a decrease in zeta-potential and hence a reduction in the electrical repulsion forces between particles with adsorbed polymer. However, such adsorbed layers can also 11
Material Sciences and Engineering, Miskolc, Volume 36/2. (2011) pp. 11–20
give appreciable steric repulsion [6, 7, 12], which would often outweigh the electrokinetic effect. From the difference between electrokinetic potentials of bare particles and those with adsorbed nonionic polymer, the ‘electrophoretic’ thickness of the adsorbed layer can be evaluated. It is also well known that addition of oppositely charged polyelectrolytes to a colloidal dispersion usually leads to a significant decrease in the ζ-potential and to a change in the sign of the particle charge, which is accompanied by destabilization of the system. Adsorption of polyelectrolytes bearing charges of the same sign as the particles is responsible for the increase in the electrokinetic potential and stability of dispersions[11, 12]. So, electrokinetic data provide valuable information about the effect of adsorbed polymers on the parameters of the electrical double layer and mechanisms of destabilization of colloids by polymers. Suspension of polystyrene latex particles is an excellent model system to study the effect of polymer mixtures adsorption on the electrokinetic potential and mechanism of flocculation of dispersions. In this paper, the results on the electrokinetic measurements of monodisperse carboxylated polystyrene particles in solution of cationic and anionic polyelectolytes of different charge density and their binary mixtures of different composition are presented. 1. Materials and methods Monodisperse carboxylated polystyrene (PS) particles with 4,2 µm in diameter were synthesized as described in paper [13]. The sample was kindly provided by Prof. A. Menshikova, Institute of Macromolecular Compounds, SaintPetersburg, Russia. The particles surface contains polyvinyl pyrrolidone (PVP) grafted by PS chains, which anchored in PS matrix. Commonly, surface concentration of carboxylic groups originated from the initiator (4,4'-azobis, 4cyanoisovaleric acid) for such samples reaches about 2*10-6 mol/m2. The surface charge density determined by conductometric titration for our sample was 0.9*10-6 mol/m2 (repeated twice). This smaller value in comparison with 2*10-6 mol/m2 might be a result of storage of the microspheres in pure water, when some carboxylic groups could migrate into particle volume. The calculated specific surface area of particles was S=65 m2.g-1. As high-molecular substances SNF FO type cationic polyelectrolytes, and SNF AN grade anionic polyelectrolytes of different charge density (CD) produced by SNF A.S., France, have been used. Stock solutions of polymers with concentration 0.5 g/l have been prepared, then working solutions of designated concentration (typically 0.1-0.05 g/l) were prepared by dilution. The latter were used within 5 days after dilution of stock solutions. Preliminary tests showed that within this period of time the viscosity of polymer solutions remained practically constant. 12
Material Sciences and Engineering, Miskolc, Volume 36/2. (2011) pp. 11–20
Characteristics of the polyelectrolytes applied are presented in Table 1. Table 1 Polyelectrolytes used Name
Type
MW/106
SNF FO 4125 SH SNF FO 4400 SH SNF AN 905 SH SNF AN 956 SH
Cationic Cationic Anionic Anionic
6–7 6–7 8–9 8–9
Charge density (mol%) 6 30 5 50
The electrokinetic potential of particles was measured using ZetaPlus, Brookhaven Inc. instrument. Determination of the electrophoretic mobility in this instrument is based on the Doppler-effect. The program used in this instrument converts the electrophoretic mobility of particles into electrokinetic potential using the classical Smoluchowsky formula. This procedure is acceptable for our systems with relatively big particles as the criterion Rel (or Dukhin number)= exp[(ψ1/2-1)/ κa] in our experiments was much less than unity (where ψ1 is the Stern potential, κ is the reciprocal Debye length and a is the particle radius, the magnitude of the initial ζ-potential was taken for ψ1 ) As shown by Dukhin [14], the magnitude of Rel (or Du) characterizes the contribution of the electric double layer polarization to the electrosurface phenomena and this contribution can be neglected for Rel
