3786
Cellulose (2018) 25:3781–3795
of one of either anionic or cation polyelectrolyte results in a stabilizing shell around a neutralized polyelectrolyte core (Dautzenberg and Karibyants 1999). The PEC particles carry either negative or positive net charge due to the cation/anion ratio of the mixture. Measurements have shown that the sign of the PEC particles charge changes sharply within a quite narrow cation/anion ratio (Kekkonen et al. 2001). The maximum turbidity for these mixtures appear close to the theoretical neutralization point, but not necessarily precisely on the neutralization point. Similar observations have been reported earlier in several studies (Chen et al. 2003). It was reported that a slight excess of polyDADMAC was needed to completely neutralize the polyanion used (Kekkonen et al. 2001). Similarly, it was reported that the near- zero electrophoretic mobility of PECs from poly- DADMAC and CMC was achieved at a ratio of 8:10 of cationic to anionic groups (Hubbe et al. 2005). Deviations from a 1:1 when using irregularly branched polyelectrolytes are quite common (Ko¨tz 1993). This was noticeable in the determined turbidity values when cationic starches Raifix 01015 SW and Rais- abond 15, which both contain a high amount of branched amylopectin, were mixed together with the different CMC grades (Fig. 1). Steric factors, differ- ences in chain lengths, and differences charge densi- ties have previously been used to explain deviations from 1:1 stoichiometric neutralization points between polyelectrolytes of opposite charge (Philipp et al. 1989; Buchhammer et al. 1995; Kekkonen et al. 2001). It has previously been reported that the maximum turbidity decreases with increasing molar mass of the polyelectrolyte (Hubbe et al. 2005). This correlation was not apparent in these experiments, but instead it was noted that the maximum turbidity depended on the charge density of the cationic polyelectrolyte, when the amount of anionic charges were kept constant in all trial points.
PEC mixtures that contained an excess of either polycation or polyanion, due to electrostatic stabiliza- tion of the particles. It is known that the PEC particles consist of a core of neutralized polyelectrolytes, and an outer shell is formed from the polyelectrolyte in excess (Dautzenberg and Karibyants 1999). It was clear that some changes occurred in the PEC particles over time, and that these changes were most likely an effect of secondary aggregation of particles. Decreases and increases in turbidity over time have previously been reported for other PEC mixtures (Hubbe et al. 2005). Others have reported colloidally stable PECs over time periods of 48 h, and in some cases even 2 months (Gernandt et al. 2003). However, some very noticeable decreases in tur- bidity took place at certain cation/anion ratios. Com- binations of Raifix 01015 SW with the different CMCs resulted in unstable PEC mixtures at cation/anion ratios close to the theoretical point of neutralization. In these experiments, Raifix 01015 SW was the only tested cationic starch produced from waxy maize, i.e. it consisted approximately of 98% branched amy- lopectin, unlike the other cationic starches from potato (Fredriksson et al. 1998). This low-molar mass and branched structure seemed to form unstable aggregates effectively, given enough time. Also a 5.5:1 ratio of Raisabond 15 and DS 1.2 CMC also resulted in unstable aggregates over time, i.e. once again close to the theoretical point of neutralization. An incomplete, and less noticeable turbidity decrease was seen for Raisabond 15 with DS 0.7 CMC at the ratio of 4:1, which also was close to the theoretical neutralization point at 3.8.
Flow cytometry
Polyanion was mixed together with polycations under agitation. The PECs were diluted, a small amount of staining agent for hydrophobic components (Nile red) was added, and the mixture was analyzed by flow cytometry. The Argon laser in the FCM instrument scanned the PECs. The light scattered by each colloidal particle was recorded in the forward direc- tion (FSC), side direction (SSC), and in the red spectra. The different light scattering properties of the PEC particles were plotted against each other, in order to create particle populations in FCM density plots. The PEC particle populations were quite visible when FSC was plotted versus SSC, as illustrated in Fig. 3a of a
Long-term stability of PECs
The long-term stability of PEC mixtures was evalu- ated by turbidity measurements 24 h after PEC formation. The measured turbidity values after 24 h were lower than the previously measured values for almost all of the tested PEC combinations (Fig. 2). Only small changes in turbidity were expected for
123
Made with FlippingBook Digital Publishing Software