Abstract:The rapid coagulation of polystyrene latex was investigated. The number of singlets, doublets and triplets as function of time was measured with a single particle scattering photometer. The experimental results are in contradiction to Smoluchowski theory.Key words: rapid coagulation, rate constants, light scattering.The rate of rapid coagulation of dispersed colloidal particles is determined by the velocity of Brownian diffusion of the particles one towards another. According to von Smoluchowski [1] the rate of decrease of the particle concentration n with time t is given byni is the number of aggregates with i particles, k is rate constant of coagulation, k was assumed by von Smoluchowski as independent of the aggregate size. From this follows(2) 3// k~ = Boltzmann constant, t/= viscosity, T = temperature, the value for k at 298 K = 6,1.10 -18 m 3 s-'.It follows from literature [2-6] the k values obtained experimentally are lower than the von Smoluchowski value and about 2-3" 10 _'8 m 3 s-'. The discrepancy was explained by the so called hydrodynamic interaction.In our experiments coagulation kinetics was investigated by low angle laser light scattering of single particles [7,8]. We have investigated homodisperse polystyrene latexes with different particle radius, different surface charge and different polar groups. They were prepared by emulsion polymerisation [9,10]. The scattered light was analyzed using the RayleighDebye approximation. From the scattering data the number of singlets, doublets and so on were calculated at different times. In figure 1 the particle distribution was shown of the original dispersion (curve 1) and after coagulation has started with the formation of doublets and triplets (curve 2). Both curves were directly obtained from the display of the multichannel analyzer (NTA 1024 EMG, Budapest). On the ordinate is written the number of impulses (I) and on the abscissa the amplitude of the impulses (U) corresponding with the size of the aggregates,In figure 2 is shown the alteration of the number of singlets, doublets and triplets for a polystyrene latex with a = 0,25 #m in 0,3 mol/1 KNO3. From many of such curves mean values for the coagulation constants were calculated. The most important result was that at the beginning of coagulation the number of singlets decreased linerarily with time. This is in contradiction to Smoluchowski theory.At the beginning of coagulation this means t--* 0 only the formation of doublets has be considered: nl t=0 nt = number of singlets, ni ~ number of aggregates of kind i. The constant for doublet formation can be calculated from the slope according to equation (3). We obtained for the doublet formation kll = (2,38 + 0,24) W528 79
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