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was low. The FSC/SSC of cellulose nanofibrils, which are known to be very swollen in water, was 6.6. The obtained results indicated that the determined FSC/ SSC values could in fact be connected to particle density or swelling. By mixing together different types of polyelectrolytes into PECs, FSC/SSC values of 1.1–8.7 have so far been obtained, indicating that the choice of polyelectrolytes and the ratio between them will results in very different populations of PEC particles. The hydrophobicity of PEC particles was deter- mined by FCM analyses with the added hydrophobic staining agent Nile red. A small volume of Nile red dissolved in methanol was added to the different water samples. It was expected that the added Nile red mainly adsorbed onto the surface of the different PEC particles, since the solubility of Nile red in water is negligible and the migration of Nile red into the PECs is unlikely (Greenspan and Fowler 1985; Jose and Burgess 2006). Once activated, Nile red emitted fluorescent light that was detected in the red spectra of the FCM apparatus. The PEC particles interacted and activated the added Nile red, but the detected red intensities were quite low, which indicated that the PECs were ultimately hydrophilic in nature (Fig. 7). The average hydrophobicity of the PEC particles were of the same size range for all of the measured polyelectrolyte combinations at different cation/anion ratios. The average hydrophobicity of the PECs was lowest around the theoretical point of neutralization for all of the PEC combinations, and increased once this cation/anion ratio was exceeded. The results showed that the added Nile red was activated to a lesser extent close to the neutralization points, which
indicated that the particles were slightly less hydrophobic. It has been reported that the hydrophobic domains exist within polyelectrolytes, and that these tend to vary with the conformational changes (Abe et al. 1977). It was reported that a polyelectrolyte with many hydrophilic ionic sites will show a hydrophobic atmosphere, or a low-polar solvent-like environment, within its domains. The hydrophobicity of a dense polyelectrolyte conformation is higher than for a polyelectrolyte with extended conformation. The presented FCM results would indicate that the poly- electrolyte chains on the surfaces of the PEC particles were less dense around their neutralization points, due to the slight decrease in hydrophobicity. More extended conformations of the polyelectrolytes may have been favored around the neutralization point. Additionally, the determined hydrophobicities of the PECs followed their FSC/SSC values to some extent; increases in structural density of the PECs seemed to be connected to increases in hydrophobicity.
Time dependency: coagulation
To study the time dependency of PECs over a short time scale, a coagulating PEC mixture was chosen for FCM analysis. From the preliminary turbidity mea- surements, it was known that a mixture of Raifix 01015 SW and 250 kDa CMC, DS 0.9, at a ratio of 5:1, was unstable over time. The two polyelectrolytes were quickly mixed together and diluted. The first sample was collected after 6 s of polyelectrolyte contact time, and it was quickly fed into the FCM apparatus. The initial number of particles in suspension was about 41000/ l L, and the measured FSC/SSC value of the PEC population was 3.4 (Fig. 8). The number of particles per l L decreased quite rapidly within the first 5 min of contact time, from 41000 to about 10000/ l L, most likely due to secondary aggregation of the initial particles. Within the first 5 min, the FSC/SSC value of the particle population also increased from 3.4 to 8.7, indicating the formation of large, swollen PEC particles with low structural density. The particle count continued to decrease over time, and only about 500/ l L remained at 53 min. From the FCM density plots, it was visible that the largest, and most unstable aggregates disappeared over time. The largest aggregates formed macroscopic flocs, that were either left in suspension or attached to the glass beaker walls.
Fig. 7 Determined average hydrophobicity of particle popula- tions from different cation/anion ratios of polycations with 700 kDa CMC
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