Trapped non-equilibrium states in aqueous solutions of oppositely charged polyelectrolytes and surfactants: effects of mixing protocol and salt concentration

Abstract

The very slow equilibration time in oppositely charged systems makes it necessary to control not only the concentration of the species but also the details of the mixing process. This has been demonstrated for processes occurring at interfaces where order of addition effects can be of great importance. In this investigation we set out to study the bulk properties of aqueous mixtures of a highly charged cationic polyelectrolyte mixed with an anionic surfactant with the aim to learn if long-lived non-equilibrium states were formed also in this case, and thus if the details of the mixing procedure would affect the structure of the aggregates formed. For simplicity we chose two mixing protocols, denoted "PTS" and "STP". In the PTS-method the polyelectrolyte is added to the surfactant solution whereas in the STP-method the surfactant is added into the polyelectrolyte solution. The properties of the mixtures in aqueous solutions, with different NaCl concentrations and as a function of time, were followed by conducting turbidity, electrophoretic mobility and dynamic light scattering measurements. The results demonstrate that the mixing protocol indeed has a great impact on the size of the aggregates initially formed and that this size difference persists for long times. Hence, trapped non-equilibrium states do play an important role also in the bulk solution. We found that in excess surfactant solutions the smaller aggregates formed by the STP-method are more resistant than the larger ones formed by the PTS-method to colloidal instability induced by electrolytes (NaCl). Based on our results we suggest that for producing small and stable polyelectrolyte-surfactant aggregates in systems with excess surfactant, the surfactant should be added last, while the opposite should be applied for systems with excess polyelectrolyte.

Dissolved and Colloidal Substances (Dcs) and the Charge Demand of Papermaking Process Waters and Suspensions: A Review

“…In cases where the amphiphylic species are negatively charged, e.g. with carboxylate or sulfonate groups, relatively strong interactions with cationic polyelectrolytes have been documented (Dubin and Davis 1985;Leung and Goddard 1985;Naderi et al 2005). Without calling into question the foregoing evidence of cationic demand of various extractive materials, it also has been reported that non-charged, unsaponifiable compounds may make up the majority of extractives from some species, including eucalyptus (Prinsen et al 2012).…”

“…As was noted in Table 6, some researchers have reported that the presence of surfactants can interfere with the endpoints of polyelectrolyte titrations (Dubin and Davis 1985;Leung and Goddard 1985;Naderi et al 2005). Figure 15 represents a situation in which an anionic surfactant molecule, e.g.…”

“…Effects of time on polyelectrolyte complexation have also been reported (Dautzenberg and Karibyants 1999;Phipps 1999;Naderi et al 2005;Hubbe 2006;Vanerek and van de Ven 2006). In practical terms this means that if one wants to achieve a certain state of cationic demand at the forming zone of a paper machine, then the amount of highly charged additive will have to be increased, in accordance with how far back in the system it has been added to the furnish (Strazdins 1972;Goossens and Luner 1976).…”

“…Kabanov & Zezin 1984;Ström and Stenius 1987 The structure of DCS leads to slow kinetics, so that the system does not approach equilibrium within the time allowed for titration. Ström and Stenius 1987;Phipps 1992;Dautzenberg & K. 1999;Naderi et al 2005;Vanerek & van de Ven 2006 Weak PEs (especially with carboxyl groups and nonquaternary amine groups) may be uncharged, dependent on the pH. Hattori et al 1994 Weak PEs (especially with carboxyl groups and nonquaternary amine groups) may form ion pairs during complexation.…”

“…Michaels et al 1965;Kekkonen et al 2001 Participation of surface-active species in the complexation complicates endpoints. Dubin & Davis 1985;Leung & Goddard 1985;Naderi et al 2005 The inflexibility of polyelectrolyte chains is predicted to prevent 1:1 pairing in some cases, according to molecular dynamics simulations.…”