Electrokinetic behavior and colloidal stability of polystyrene latex coated with ionic surfactants
Graphical abstract
Introduction
It is well known that the adsorption of surfactants changes the hydrophobic/hydrophilic character of the surface on which they are adsorbed. In the case of ionic surfactants, they also alter the surface charge. As a result, ionic surfactants are broadly used as stabilizers/destabilizers of colloidal dispersions in several fields (i.e., industry, medicine, biology, mineral processing, and treatment of wastewater) [1], [2].
In principle, a colloidal dispersion with low affinity for the medium (as polystyrene latex particles in water) is thermodynamically unstable and tends to aggregate spontaneously. However, if repulsive interactions take place among the particles, this aggregation is slow enough to consider the system as kinetically stable. This is the mean of the term “colloidal stability.” In the case of polystyrene latex, the synthesis procedure leads to some polar and/or charged groups on the surface of the particles. This causes the colloidal stability of the system by means of the repulsion originated by the overlapping of the electric double layers of the particles. Van der Waals attractive forces are also present. When ionic surfactant molecules are adsorbed on latex, they modify the balance of these forces, above all, those related to electrostatic contributions. The adsorption of chains on particles with the same sign of charge should increase the electrostatic repulsion. However, the adsorption on opposite charged surfaces would give rise to the charge cancellation and, as a consequence, to the destabilization of the system. Overcoming the adsorbed surfactant amount necessary to destabilize the system, stable complexes could be found again. Other contribution to the repulsive interaction among complexes is the steric interaction (osmosis and mixing [3], [4]). This contribution could be strong enough to maintain the colloidal stability of the complexes in absence of electrostatic repulsion (at high electrolyte concentration). Analysing how the ionic surfactants modify the properties of a model system (with known characteristics) will help to understand their role on different surfaces. Polystyrene latices covered by nonionic surfactants (Triton X-100 and Triton X-405) have been already extensively studied [5]. The case of ionic surfactant-polystyrene latex complexes is analyzed in this work. Latices with different surface charge sign and value were examined. In addition, the effect of the type of surfactant (positive and negative) was considered. A study of the electrokinetic behavior of such ionic surfactant–latex complexes was performed from two points of view. On the one hand, the electrophoretic mobility as a function of the adsorbed surfactant amount was analyzed, which gave information about different trends observed in the adsorption isotherm published in a previous paper [6]. On the other hand, the mobility of particles with different surfactant coverage was studied by increasing the electrolyte (NaCl) concentration. Higher salt concentrations than those usually reported were reached in this work. This makes it interesting for understanding the charging of aqueous-hydrophobic interfaces, which is especially important in many industrial and biological systems where moderate or high ionic strengths are required [7]. These measurements were also very useful to understand the colloidal stability results. The study of the aggregation of the complexes by adding NaCl was done by means of nephelometry. The critical coagulation concentration (ccc) was obtained for complexes with different surfactant coverage. Finally, a theoretical treatment of the electrophoretic mobility data allowed us to calculate different magnitudes related to the interface of the covered latex systems, that is, the zeta potential and the surface density charge of the complexes.
Section snippets
Reagents
All chemicals used were of analytical grade. NaCl salt was purchased from Merck. The water was purified by a Milli-Q Academic Millipore system. Buffered solutions (borate for pH 9, phosphate for pH 7, and acetate for pH 5) present a constant ionic strength of 2 mM. Nonbuffered solutions were obtained by adding to water the amount of HCl or NaOH necessary to get the required pH. Ionic strength was kept constant when mixing of these solutions was necessary.
Latex particles
Two different polystyrene latices,
Results and discussion
Different adsorbent surfaces were used in order to analyse the influence of the latex surface charge (sign and value) on the electrophoretic mobility of the latex–surfactant complexes: Lx(COOH) at pH 9 and 7, and Lx(anfo) at pH 9 as negative surfaces, and Lx(anfo) at pH 5 as a positive surface. The adsorbed surfactant amount of the complexes at such conditions is known through the adsorption isotherms presented in a previous work [6].
The results for each ionic surfactant will be presented
Summary
Looking through the mobility and colloidal stability results corresponding to the different complexes analyzed, we can conclude that the electrostatic repulsion is the main responsible for the colloidal stability of the latex-ionic surfactant complexes. Above certain adsorbed amount, the complexes are always colloidally stable at low electrolyte concentrations. That means that even though the surfactant is adsorbed on a surface with opposite sign of charge, overcoming the amount needed to
Acknowledgements
Financial support from ‘Comisión Interministerial de Ciencia y Tecnología,’ Projects MAT2003-01257 and AGL2004-01531/ALI (European FEDER support included) is gratefully acknowledged.
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