Single particle optical sizing : aggregation of polystyrene latices by salt and polymer

The subject of this thesis is the development of a Single Particle Optical Sizer (SPOS) which is capable of measuring in detail discrete particle size distributions in the colloidal size range. With this instrument we studied the aggregation of latices induced by polymer and salt, and found evidence for non-equilibrium flocculation.

Chapter 2 is an inventory of the existing methods of measuring aggregation. A comparison is made with our SPOS instrument. The techniques are classified into three groups: classical, multi particle detection and single particle detection methods. Only very global information is obtained about the aggregation process with the classical methods. In the case of turbidity, only the initial rate of the total aggregation process can be obtained. Multiparticle detection methods are able to determine accurately a particle size in monodisperse samples (laser beat spectroscopy). For large spherical particles (d>1 μm) a particle size distribution can be measured (laser diffraction spectroscopy). With small angle light scattering an initial rate of the aggregation can be determined. Single particle detection methods are able to measure discrete particle size distributions. With electron microscopy very small particles can be individually sized. However this technique is rather tedious and unsuitable for the study of aggregation kinetics. With SPOS, also a single particle detection method, fast and reliable particle size and aggregate distribution can be measured as a function of time.

The SPOS instrument operates on the principle of low angle light scattering. In order to determine the measurable size range of the SPOS, we present in chapter 3 numerical results of the light scattering intensity as a function of size, type, solvent and detection angle, as obtained with the Mie theory.

In chapter 4, the design of the SPOS is described and several test experiments on the operation of the instrument are presented. In the instrument the particles are hydrodynamically focused into a very narrow stream, and they pass one-by-one through an elliptical laser focus. Upon passage, each of them emitts a flash of light which is detected by a photomultiplier and converted into an electronic pulse which is stored according to its intensity in a multichannel analyzer. The number of signals of each size can be displayed and renders a complete particle size distribution.

Much attention is paid to the possible influence of the hydrodynamic forces in the instrument on the disruption of aggregates. We conclude that only for very weakly bond aggregates de-aggregation may occur before monitoring.

In chapter 5 the instrument is used to study the coagulation process (aggregation induced by salt) of latex dispersions. We describe the preparation of the latices and determine the rate constants of three initial aggregation steps (singlet+singlet, singlet+doublet and singlet+triplet). This enables us to check the primary assumption of the Von Smulochowski theory which states that all these rate constants are the same. We measured a difference between the value of the determined rate constants. Furthermore we used three different mixing cells to study the effectivity of mixing and the influence on the aggregation rate.

In chapter 6 we use the SPOS method to study the aggregation induced by polymer. The experimental results are on first sight rather surprising. In many cases the flocculation does not obey second order kinetics. Nevertheless, the data can be well understood if the dynamical aspects of the polymer adsorption are taken into account. From our experiments a clear distinction in behaviour was found between polymers in a relaxed or a non-relaxed state on the latex surface, leading to equilibrium or non-equilibrium flocculation, respectively. In the latter case the rates of polymer attachment and particle collision are faster than the rate of reconformation of adsorbed polymer. We propose a new model for polymer induced bridging flocculation which incorporates these two mechanisms and predicts the occurrence of these mechanisms as a function of particle concentration, molecular weight of the polymer, shear forces and double layer repulsion.