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opens up a vast array of new possibilities (Decher 1997; de Vasconcelos et al. 2006). The use of polyelectrolyte complexes (PECs), or polyelectrolyte multilayers (PEMs) have received quite a lot of attention in the field of research (Philipp et al. 1989; Dautzenberg 1997; Decher 1997; Dautzenberg and Karibyants 1999). Multi-layered structures on surfaces can be formed from a wide variety of components, either from solutions or dispersions. The most common approach for PEC or PEM formation is the electrostatic attraction between oppositely charged groups. The formation of PECs is driven by the increase in entropy when counter ions are liberated from the oppositely charged polyelec- trolytes during associative phase separation into colloidal particles (Piculell and Lindman 1992; Kekkonen et al. 2001). Strong ionic binding between the opposite charges of the polyelectrolytes results in kinetically frozen structures (Dautzenberg and Kar- ibyants 1999). It has been reported that the core of PEC particles consists of a neutralized polyelectrolyte core, while the outer shell consists mainly of excess polyelectrolyte, which stabilizes the colloids against aggregation (Dautzenberg and Karibyants 1999). The use of PECs and PEMs have also found their way into the field of pulping and papermaking (Petzold and Lunkwitz 1995; Petzold et al. 1996; Gernandt et al. 2003; Hubbe et al. 2003, 2005; Ga¨rdlund et al. 2005; Lofton et al. 2005; Torgnysdot- ter and Wa˚gberg 2006; Myllytie et al. 2009; Ren- neckar and Zhou 2009; Lin and Renneckar 2011; Granberg et al. 2012; Ankerfors and Wa˚gberg 2013; Marais et al. 2014). The use of PECs and PEMs in papermaking is somewhat expected, since the proper- ties of paper and retention of fillers commonly have been tailored by additions of polyelectrolytes, in single or dual systems, for a long time (Hubbe 2006). Additionally, surfaces of pulp fibers carry anionic charges that are convenient anchor points for poly- electrolytes and PECs. In papermaking applications, the size of the PECs can be tailored to exceed the average pore radius of pulp fibers, to ensure that the PECs are retained mainly on fiber surfaces. In papermaking, surface-specific strengthening agents are especially interesting, since the additive should be adsorbed within the cross-over region of two fibers in order to enhance paper strength (Stratton 1989). The properties and behavior of PECs in suspension has previously been studied by measurements of
turbidity, static light scattering, dynamic light scatter- ing, polyelectrolyte titration, viscosity, surface charge analysis, FTIR spectroscopy, and electrophoretic mobility (Buchhammer et al. 1995; Dautzenberg 1997; Dautzenberg and Karibyants 1999; Dragan and Cristea 2001; Kekkonen et al. 2001; Hubbe et al. 2005; de Vasconcelos et al. 2006). It has been reported that the formation of PECs is sensitive towards the nature of the cationic polyelectrolyte (Hubbe et al. 2005). It is known that the formation of PECs is path dependent, which means that all aspects of the PEC formation will influence the result (Ko¨tz 1993; Feng et al. 2008). Due to the wide variety of available polyelectrolytes, additional information about their interactions and the complexes they form is still needed. Adsorption experiments and modeling have been used to gain information about formed layers of polyelectrolytes (Van de Steeg et al. 1992; Fleer et al. 1993). Adsorption studies using QCM-D and SPR techniques have revealed that polyelectrolytes adsorb differently depending on their chemical properties, such as molar mass and charge density, onto model cellulose or silica surfaces (Tammelin et al. 2004; Kontturi et al. 2008; Strand et al. 2017). A polyelec- trolyte with a high charge density will adsorb as a stiffer and more rigid layer, compared to a polyelec- trolyte with a low charge density (Kontturi et al. 2008). The charge density of the surface itself in adsorption studies is also of great importance, which was shown by adsorption experiments onto cellulose and silica surfaces. While the QCM-D and SPR methods provide information about adsorption of polyelectrolytes onto surfaces, the behavior of unretained polycations and polyanions is an open question. An analysis method that could bridge the gap between the previously used analyses for PECs and the techniques used in adsorp- tion studies would be helpful for gaining additional insight into the behavior of polyelectrolyte mixtures. Flow cytometry (FCM) is a relatively new tech- nique in the field of pulping and papermaking. The technique was adapted from medical science, where it is used mainly for the counting of cells (Shapiro 2003). FCM also measures the light scattering properties of particles in suspension in forward and side direction. Further, FCM can be used to measure the fluorescence of particles at different wavelengths, which is very useful when combined with addition of selective dyes to samples. In the field of pulping and papermaking,
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