Introduction Heteroaggregation of colloidal materials having different composition, charge, shape or size is critical to many technological processes, such as mineral flotation [1], core–shell nanocomposites [2] and cell recovery [3], among others. The simpler case of homoaggregation, where only one type of particle is present, is well studied and documented in the literature [4]. In this case, two limiting regimes have been observed: (1) diffusion-limited cluster aggregation (DLCA), in which every collision between particles results in the formation of a bond [5], and (2) reaction-limited cluster aggregation, in which a large number of particle collisions is necessary before a stable bond is established [6]. However, when systems are composed of several species, the reaction kinetics is much more complex and no general kinetic theory is so far available [4, 7]. In this work, heteroaggregation processes arising in aqueous suspensions of colloidal particles with opposite charge were monitored experimentally by means of single-cluster light scattering (SCLS). So far, mainly static and dynamic light scattering experiments have been carried out [8]. Both techniques assess only some average cluster size. SCLS, however, allows us to follow the time evolution of the detailed cluster size distribu- tions up to heptamers. The aim of this work is to study the influence of the electrostatic interactions on the kinetics of charge-heteroaggregation processes arising in 1:1 mixtures of oppositely charged model colloids. For this purpose, the range of the electrostatic interactions was modified by changing the electrolyte concentration of the dispersion medium. At high ion concentration, the particle charge is completely screened and the electrical double layer around the particle is confined to the particle surface. In this case, interactions of electrostatic Progr Colloid Polym Sci (2004) 126: 151–154 DOI 10.1007/b94020 Ó Springer-Verlag 2004 J. M. Lo´pez-Lo´pez A. Schmitt J. Callejas-Ferna´ ndez R. Hidalgo-A ´ lvarez Time-resolved measurements of the cluster-size distributions arising in charge-heteroaggregation processes J.M. Lo´pez-Lo´pez (&) Æ A. Schmitt J. Callejas-Ferna´ndez Æ R. Hidalgo-A ´ lvarez Biocolloid and Fluid Physics Group, Department of Applied Physics, University of Granada, Campus de Fuentenueva, 18071 Granada, Spain e-mail: jmlopezl@ugr.es Tel.: +34-958-246175 Fax: +34-958-243214 Abstract Charge-heteroaggregation processes arising in 1:1 mixtures of positively and negatively charged particles were studied at low and high electrolyte concentration. Single-cluster light scattering was employed to monitor and resolve the detailed cluster size distribution up to hexamers. The results obtained show that long-range electrostatic interactions between oppositely charged particles favour aggregation at low electrolyte concentration. Specially, dimers are formed from oppositely charged monomers at an increased rate which is almost twice the value found for pure diffusion- limited aggregation. Keywords Single-cluster light scattering Æ Colloidal heteroaggregation Æ Diffusion- limited cluster aggregation