Adsorption of ionic species to the surface of polystyrene latexes

Salt concentration and particle density dependence of electrophoretic mobilities of spherical colloids in aqueous suspension

Using laser Doppler velocimetry in the superheterodyne mode, we conducted a systematic study of the electrophoretic mobility of dispersions of small silica spheres (a=18 nm) suspended in water at different salinities and particle concentrations. The concentration of NaCl was varied from 40 microM up to 16 mM, while the particle concentrations were varied between 4.2x10(18) and 2.1x10(20) m-3. We find a decrease of mobility with increasing salt concentrations and an increase with increased particle number densities. The latter observation is not backed by the standard cell model of electrophoresis with Shilov-Zharkikh boundary conditions. Rather, if the experimental data are interpreted within that model, an unexpected change of the zeta potential at constant added salt concentration results. Interestingly, all experimental data collapse onto a single master curve, if plotted versus the ratio C* of particle counterions to added salt ions. We obtain a logarithmic increase of mobility for C*<1 and a plateau for C*>1. This may indicate a change of the Stern layer structure not yet included in the theoretical model.

Drude-type conductivity of charged sphere colloidal crystals: Density and temperature dependence

We report on extensive measurements in the low-frequency limit of the ac conductivity of colloidal fluids and crystals formed from charged colloidal spheres suspended in de-ionized water. Temperature was varied in a range of 5 degrees C < Theta < 35 degrees C and the particle number density n between 0.2 and 25 microm(-3) for the larger, respectively, 2.75 and 210 microm(-3) for the smaller of two investigated species. At fixed Theta the conductivity increased linearly with increasing n without any significant change at the fluid-solid phase boundary. At fixed n it increased with increasing Theta and the increase was more pronounced for larger n. Lacking a rigorous electrohydrodynamic treatment for counterion-dominated systems we describe our data with a simple model relating to Drude's theory of metal conductivity. The key parameter is an effectively transported particle charge or valence Z(*). All temperature dependencies other than that of Z(*) were taken from literature. Within experimental resolution Z(*) was found to be independent of n irrespective of the suspension structure. Interestingly, Z(*) decreases with temperature in near quantitative agreement with numerical calculations.

An experimental test of Booth's primary electroviscous effect theory

The primary electroviscous effect has been investigated in dilute suspensions of titanium oxide (anatase), the viscosities of which were measured by means of a capillary viscometer with automatic timing. The linear relation between viscosity and solids volume fraction was first determined at the isoelectric point of the particles when the particles are uncharged, and the electroviscous contribution to the intrinsic viscosity was then determined at other values of pH. Booth's theory (Proc. R. Soc. London Ser. A203, 533 (1950)) agrees well with the experimental results when the particle zeta potential is small and the double layer is thin (kappa alpha approximately 7.3), but agreement is poor when the double layer is thick (kappa alpha approximately 0.6).

Interaction potentials, structural ordering and effective charges in dispersions of charged colloidal particles

As colloidal dispersions of charged particles exhibit a wide variety of commercial, technological and scientific applications, a considerable theoretical effort has been devoted to finding an effective interaction potential from primitive models. The forces derived from this potential should justify the spatial ordering experimentally observed under certain conditions. This paper reviews the advances in these theoretical studies as well as some experiments (based on the mentioned order) that try to corroborate them. Special attention has been paid to the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential. Nowadays, many of these theoretical investigations suggest that it could be applied if some of its parameters are renormalized. Nevertheless, to achieve a renormalization procedure in a strict way (from a primitive model) is a difficult task as a result of the size and charge asymmetries between small ions and macroions. Thus, several procedures for computing renormalized charges in a simple way have been developed. However, the notion of effective charge has also been widely used (as a adjustable parameter) in order to justify results found for several kinds of colloids (like solid particle dispersions or micellar systems) by means of quite different experimental techniques. Renormalization (as well as ion condensation) approaches, experiments and the controversial relationship between theoretical and phenomenological effective charges are also reviewed in this work.

Capillary zone electrophoresis of sub-microm-sized particles in electrolyte solutions of various ionic strengths: size-dependent electrophoretic migration and separation efficiency

To gain insight into the mechanisms of size-dependent separation of microparticles in capillary zone electrophoresis (CZE), sulfated polystyrene latex microspheres of 139, 189, 268, and 381 nm radius were subjected to CZE in Tris-borate buffers of various ionic strengths ranging from 0.0003 to 0.005, at electric field strengths of 100-500 V cm(-1). Size-dependent electrophoretic migration of polystyrene particles in CZE was shown to be an explicit function of kappaR, where kappa(-1) and rare the thickness of electric double layer (which can be derived from the ionic strength of the buffer) and particle radius, respectively. Particle mobility depends on kappaR in a manner consistent with that expected from the Overbeek-Booth electrokinetic theory, though a charged hairy layer on the surface of polystyrene latex particles complicates the quantitative prediction and optimization of size-dependent separation of such particles in CZE. However, the Overbeek-Booth theory remains a useful general guide for size-dependent separation of microparticles in CZE. In accordance with it, it could be shown that, for a given pair of polystyrene particles of different sizes, there exists an ionic strength which provides the optimal separation selectivity. Peak spreading was promoted by both an increasing electric field strength and a decreasing ionic strength. When the capillary is efficiently thermostated, the electrophoretic heterogeneity of polystyrene microspheres appears to be the major contributor to peak spreading. Yet, at both elevated electric field strengths (500 V/cm) and the highest ionic strength used (0.005), thermal effects in a capillary appear to contribute significantly to peak spreading or can even dominate it.

Independent ion migration in suspensions of strongly interacting charged colloidal spheres

We report on systematic measurements of the low-frequency conductivity sigma in aqueous suspensions of highly charged colloidal spheres. Sample preparation in a closed tubing system results in precisely controlled number densities of 10(16) m-3 < or = n < or = 10(19) m-3 (packing fractions of 10(-7) < or = phi < or = 10(-2)) and electrolyte concentrations of 10 < or = c < or = 10(-3) mol l-1. Due to long-range Coulomb repulsion, some of the systems show a pronounced fluid or crystalline order. Under deionized conditions we find sigma to depend linearly on the packing fraction with no detectable influence of the phase transitions. Further, at constant packing fraction sigma increases sublinearly with the increasing number of dissociable surface groups N. As a function of c the conductivity shows pronounced differences depending on the kind of electrolyte used. We propose a simple yet powerful model based on the independent migration of all species present and the additivity of the respective conductivity contributions. It takes account of small ion macro-ion interactions in terms of an effectively transported charge. The model successfully describes our qualitatively complex experimental observations. It further facilitates quantitative estimates of sigma over a wide range of particle and experimental parameters.

Surface Electrical Properties of Polystyrene Latex

A polystyrene latex suspended in HCl has been studied by dielectric response and dynamic mobility measurements. The aim was to see if these data, along with previous results from other techniques, could be interpreted in a consistent way within the standard electrokinetic model to yield information about the surface electrical properties of the latex. It was found that the zeta-potentials deduced from dynamic mobility and conductivity measurements were reasonably consistent, but did not agree well with those deduced from the electrophoretic mobility. The use of a dynamic Stern layer did not improve the agreement to any great extent. It is conjectured that differences in the methods of preparation, in particular the use of centrifugation, may be responsible for some of the disagreement. The pH-dependence of the dynamic mobility was also investigated and found to be weak. The dielectric response measurements showed significant discrepancies from the predictions of the standard electrokinetic model in the kHz regime, and the dynamic Stern layer model with realistic parameters could not improve the agreement very much. However the experimental relaxation frequencies were in good agreement with the predictions of the theory. Copyright 1999 Academic Press.

Surface Electrical Properties of Polystyrene Latex

The surface electrical properties of a polystyrene latex have been studied with several experimental methods. Previous studies by other authors have shown discrepancies between the surface potential and the surface charge density determined by different methods. This work is an attempt to produce more experimental data for the discussion concerning these discrepancies. Both electrophoretic mobility and electrical conductivity methods have been used. Also, gel permeation chromatography has been used to estimate the number of endgroups on the latex particle that carry charged surface groups. Synthesis and cleaning methods of polystyrene latex are also discussed. Calculations on a Poisson-Boltzmann level of approximation have been performed in an attempt to connect the quantities determined with dynamic (electrophoresis and static conductivity) methods and static (titration) methods. The discrepancies can be explained by introducing a pH- and electrolyte-dependent surface structure such as the hairy layer model. A Stern layer conductance model can also to some extent explain these discrepancies, but this model contains several parameters not directly attainable by experiments. This means that a unique set of parameters cannot be determined without further studies on several latex systems. Copyright 1999 Academic Press.

Adsorption of Anions on the Surface of Charge-Free Latex Particles as Revealed by Turbidimetric Measurements

The preferential adsorption of anions on the surface of latex particles without permanent charges is investigated. The latex consists of particles of 83 nm diameter stabilized by a thin layer (1.3 nm) of polyethylene oxide chains. The serum of the latex is adjusted to an ionic strength of 0.001 by adding different salts (KCl, KI, LiF, LiI, NaI). The structure of the latex particles has been studied by small-angle X-ray scattering and by dynamic light scattering whereas the interaction of the particles was investigated by turbidimetry. The latter method gives clear indication that the particles repel each other due to the electrostatic repulsion. The effect is much more pronounced in the presence of I- anions than in the presence of Cl- or F-. Since all other mechanisms of long-range repulsive interactions can be ruled out, the results presented herein demonstrate the preferential adsorption of anions, in particular of I- onto the latex surface. Copyright 1998 Academic Press.

A Study of Colloidal Particle Brownian Aggregation by Light Scattering Techniques

Aggregation kinetics and aggregate structure are studied for monodisperse polystyrene latex particles of diameter 60 and 140 nm. The experimental part consists of measurements over a rather broad range of electrolyte concentrations (0.1 to 0.8 M NaCl) and for particle volume fractions 10(-6) to 10(-5), using dynamic and static light scattering techniques. The aggregation kinetics data obtained can be fitted into a single curve with a proper scaling of time and size for both particle sizes. For relatively long times, we observe power-law type kinetics, characteristic of diffusion limited aggregation (DLA). Fractal dimensions in all cases are close to 1.7 (as expected for DLA). The light scattering behavior of aggregates of arbitrary size is also studied numerically by employing computer simulations, and existing models (Lin et al., 1990. J. Colloid Interface Sci. 137, 263) are tested and improved for certain conditions (large angles and/or large particles). The shape of the experimental kinetics curves is successfully described with numerical simulations, further suggesting that (under the conditions tested) DLA aggregation prevails for short times as well. Copyright 1997 Academic Press. Copyright 1997Academic Press