The weight and number average masses of aggregates are represented as a function of aggregation time in Fig. 4; the vertical dashed line indicates the transition between two rates of aggregation. In the first period, the aggregate masses grow like S(t) « N(t) « t0 66 whereas in the second period S(t) « N(t) « t0'33 is valid. The variations of S(t) and N(t) are concomitant and the values of z = w being smaller than 1 determine the diffusion-limited process to be reversible [39,40]. Collisions between latex particles systematically lead to aggregate formation, but some links are unstable and break up, reverting to the situation that existed prior to collision. This mechanism that preserves the aggregate mass polydisper-sity, S(t)/N(t) = 2, is different from the reaction-limited process for which some collisions fail to induce aggregation. The bell-shaped mass frequency curves (not represented) corresponding to the establishment of the asymptotic domain (z = w = 0.33) indicates that the distribution of 3770 IU IgM molecules on the
IgG-coated latex allows each collision between coated latex particles to be efficient toward aggregation (t being close to 1 as shown in Fig. 1). However, the rate of aggregate growth indicates that some links are not definitely established and break up . The retardation effect due to the partial reversibility in aggregation on S(t) and N(t) may be expressed as follows:
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