ARTICLE pubs.acs.org/Langmuir
Influence of the Degree of Ionization and Molecular Mass of Weak Polyelectrolytes on Charging and Stability Behavior of Oppositely Charged Colloidal Particles Amin Sadeghpour, Emek Seyrek, Istvan Szilagyi, Jose Hierrezuelo, and Michal Borkovec* Department of Inorganic, Analytical, and Applied Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1205 Geneva, Switzerland ABSTRACT: Positively charged amidine latex particles are studied in the presence of poly(acrylic acid) (PAA) with different molecular masses under neutral and acidic conditions by electrophoresis and time-resolved dynamic light scattering. Under neutral conditions, where PAA is highly charged, the system is governed by the charge reversal induced by the quantitatively adsorbing polyelectrolyte and attractive patchcharge interactions. Under acidic conditions, where PAA is more weakly charged, the following two effects come into play. First, the lateral structure of the adsorbed layers becomes more homogeneous, which weakens the attractive patchcharge interactions. Second, polyelectrolyte adsorption is no longer quantitative and partitioning into the solution phase is observed, especially for PAA of low molecular mass.
’ INTRODUCTION Polyelectrolytes are frequently used as additives to control the stability and rheological properties of colloidal suspensions. Examples of important industrial applications include papermaking, water treatment, and the formulation of foods, cosmetics, paints, and drugs.15 More recently, polyelectrolytes have been shown to represent flexible building blocks in the fabrication of multilayered surface coatings or capsules of tunable permeability.68 In all of these cases, the adsorption of polyelectrolytes on oppositely charged colloidal particles and subsequent charge reversal are essential in determining the interaction forces between particles and, as a consequence, the suspension stability.914 When a small dose of a polyelectrolyte is added to a suspension of oppositely charged particles, the polyelectrolyte adsorbs on the particle surface and reduces their surface charge and thereby the suspension stability. At a sufficiently high polyelectrolyte dose, the suspension becomes unstable. Further addition of polyelectrolyte leads to a charge reversal at the isoelectric point (IEP) and suspension restabilization. Rapid aggregation near the IEP originates from attractive van der Waals forces, which dominate the interactions between the particles in this regime. Away from IEP, the suspension is stabilized by repulsive forces between the electrical double layers. This characteristic behavior can be rationalized within the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO).1517 Certain phenomena in oppositely charged polyelectrolyte particle systems cannot be explained within the classical DLVO picture, and they indicate the presence of additional attractive non-DLVO forces. In particular, aggregation rates near the IEP may exceed aggregation rates in excess salt, especially at low salt levels. This effect was explained by the laterally uneven structure of adsorbed polyelectrolyte layers and the resulting interactions between patchcharge heterogeneities.3,911 The existence of such patchy surfaces and the resulting attractive forces are now r 2011 American Chemical Society
well evidenced by differential electrophoresis and atomic force microscopy techniques.1821 Moreover, these attractive patch charge forces also accelerate the particle aggregation away from the IEP. This characteristic behavior is mainly documented in systems containing strong polyelectrolytes or highly charged weak polyelectrolytes.913,22,23 Relatively few studies are available that focus on polyelectrolytes with a low line charge density or weak polyelectrolytes at a low degree of ionization.24,25 Such systems are relevant in natural waters where positively charged iron oxides are stabilized by negatively charged humic acids.26,27 The architecture of polyelectrolyte multilayer assemblies containing weak polyelectrolytes can be influenced by their degree of ionization.2831 The permeability of capsules made out of such multilayers can be tuned via pH when suitable weak polyelectrolytes have been incorporated.32 For these reasons, studies of colloidal dispersions in the presence of weak polyelectrolytes are well justified. The most important weak anionic polyelectrolyte in industry is poly(acrylic acid) (PAA). Few reports exist on the adsorption of PAA on positively charged latex particles33 and in polyelectrolyte mutilayers.34,35 However, little is known on how PAA influences the stability of colloidal suspensions. The present study fills this gap and clarifies how the degree of ionization and the molecular mass of PAA influence the stability of positively charged colloidal particles.
’ EXPERIMENTAL METHODS Materials. Amidine-terminated, surfactant-free polystyrene latex particles were purchased from Interfacial Dynamic Corporation (IDC, Received: May 26, 2011 Revised: June 23, 2011 Published: June 27, 2011 9270
dx.doi.org/10.1021/la201968b | Langmuir 2011, 27, 9270–9276
Langmuir
ARTICLE
Table 1. Degree of Ionization of PAA at Different pH Values and Ionic Strength Estimated with the Cylinder Stern Model PAA degree of ionization ionic strength (M)
pH 5.8
pH 4.0
pH 3.0
0.001
0.24
0.028
0.0036
0.01
0.37
0.053
0.0072
0.05
0.51
0.093
0.014
Portland). The supplier reported a mean particle radius of 110 nm with a coefficient of variation of 4.3% determined by transmission electron microscopy (TEM) and a surface charge density of 0.13 C/m2 obtained by conductometry. Prior to use, particles were dialyzed with a poly(vinylidine fluoride) membrane with a molecular mass cutoff of 250 kg/mol (Spectrum, Breda, Netherlands) against Milli-Q water until the conductivity of the surrounding water was 6, resembling the behavior presented in an earlier report.38 Experiments were performed only for pH 36 because the present study does not focus on effects caused by variations in the charge of the bare particles. Samples of poly(acrylic acid) (PAA) with number-average molecular masses of 5.7, 20, and 88 kg/mol were purchased from Polymer Source Inc. (Montreal, Canada). According to the manufacturer, the polydispersity indices, defined as the weight-average molecular masses divided by the number-average molecular masses were 1.09, 1.09, and 1.12, respectively. The ionization of the PAA sample of 88 kg/mol was studied by potentiometric titrations and quantitatively interpreted with the cylinder Stern model.39 The degree of ionization was estimated with that model at the respective ionic strength and pH relevant to this study (Table 1). The degree of ionization is assumed to be independent of the molecular mass.40 Hydrochloric acid was used to adjust the pH to 3.0, 4.0, and 5.8. Analytical-grade solid potassium chloride (KCl) was used as a background electrolyte to set the ionic strength. The experiments were carried out at a temperature of 25.0 ( 0.2 °C. Electrophoretic Mobility. Laser Doppler velocimetry was used to carry out the electrokinetic measurements with a Zetasizer Nano ZS (Malvern Instruments, Worcestershire, U.K.). Samples were prepared by mixing water with the appropriate volume of electrolyte solution to reach the desired ionic strength. Subsequently, particles were added from a concentrated stock solution to give a final particle concentration of about 4.5 mg/L, which corresponds to a number density of 7.5 1014 m3. Finally, the polyelectrolyte was added from pre-equilibrated polyelectrolyte solutions to obtain final polyelectrolyte concentrations in the range from 5 104 to 5 mg/L. The final volume of each sample was 4 to 5 mL. The electrophoretic mobilities were recorded after 2 to 3 h of equilibration time after the addition of the polyelectrolyte to the particle suspension. Particle Aggregation. Aggregation rate constants were measured with time-resolved dynamic light scattering. A compact goniometer
system (ALV/CGS-3, Langen, Germany) with a He/Ne laser with a wavelength of 633 nm and an avalanche photodiode detector was used. Borosilicate glass cuvettes were cleaned using piranha solution, which is a boiling mixture of concentrated H2SO4 and a 30% solution of H2O2 in a volume ratio of 3:1. Subsequently, the cuvettes were extensively rinsed with Milli-Q water and dried in a dust-free oven. Samples were prepared directly in cuvettes with a particle concentration of 4.5 mg/L and appropriate polyelectrolyte concentrations to achieve the desired polyelectrolyte dose. Aggregation measurements were started immediately after the addition of polyelectrolyte to the particle suspension. The correlation function was accumulated for 20 s at a scattering angle of 90°. A second-order cumulant fit was used to analyze the data. The apparent hydrodynamic radius Rh(t) was obtained as a function of time t from the first cumulant. The second cumulant also increases with time but scatters substantially. The experiment was typically run for 30 min, whereby the hydrodynamic radius did increase by
