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Fig. 8 Determined number of particles per l L (Open circle, green) and FSC/SSC values (Diamond, blue) of a coagulating PEC mixture as a function of contact time. FCM density plots of
the PEC population after different contact times, plotted as SSC versus FSC, are also presented. (Color figure online)
The FSC/SSC value of the particle population seemed to stabilize at about 5.5 after 53 min, but the number of particles remaining in suspension was only about 1% of the initial amount. The large change in the observed PEC population should be of special interest in wet-end chemistry, where the contact times between polycations and polyanions can be extremely short. These studied additives required a short contact time before large, swollen, and easily retainable PEC particles were formed. In adsorption studies at high flux, an initial over-adsorption of polymer has been noted, and the over-adsorption was corrected by partial desorption over time (Fleer et al. 1993). In a previous study, it was noted that the response to additions of salt took up to 60 min for some PEC systems (Dautzenberg and Karibyants 1999). In another study, the PEC mixtures were stirred for 10 min, and allowed to stabilize for 2 h prior to characterization (Mende et al. 2002). Therefore, the time dependency of PECs should be studied in more detail, and also be taken into account
when studying mixtures of this nature. The situation at initial polyelectrolyte contact, where a papermaker usually operates, may be very different from the equilibrium states commonly studied at laboratory scale. The salt concentration should also be taken into account, since this will influence the results due to the introduction of screening effects during PEC formation. It should be mentioned again, that every step of PEC formation will influence the result (Ko¨tz 1993; Feng et al. 2008). Slowly introducing the polycation to its counterpart, which is a common approach in literature, in order to study the cationically stabilized PECs at cation/anion ratios exceeding the neutraliza- tion point will cause the interaction path to pass through the unstable region. By suddenly introducing the polycation, it is possible to circumvent this phenomenon and obtain cationically stabilized parti- cles that are not products of secondary aggregation.
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