On the mesoscopic origins of high viscosities in some polyelectrolyte-surfactant mixtures

Abstract

Oppositely charged polyelectrolyte (PE) surfactant mixtures allow the control of rheological parameters of a solution even at fairly low concentrations. For example, addition of 0.3 wt. % of anionic surfactant to a 1 wt. % solution of the polycation JR 400 increases the viscosity by 4 orders of magnitude. Recently, we could show that this increase is related to the formation of mixed, rod-like PE/surfactant aggregates which interconnect several polyelectrolyte chains [Hoffmann et al., Europhys. Lett. 104, 28001 (2013)]. In this paper, we refine our structural model of the aggregates to obtain a more consistent picture of their internal structure for different anionic surfactants. Combining small angle neutron scattering (SANS) and neutron spin-echo (NSE) allows us to determine the size of the aggregates. By comparing different contrasts, the internal structure of the aggregates can be elucidated and it is seen that the PE in the aggregates retains a relatively high freedom of movement. We proceeded to investigate the influence of the surfactant concentration and the surfactant type on structure and dynamics of the mixed aggregates. It is seen that the structural parameters of the aggregates depend very little on the surfactant concentration and headgroup. However, it is crucial to incorporate a sufficient amount of PE in the aggregates to increase the viscosity of the aggregates. By comparing viscous samples at 1 wt. % PE concentration with samples at a PE concentration of 0.3 wt. %, where no significant increase in viscosity is observed, we find that similar aggregates are formed already at this lower PE concentrations. However, the amount of PE incorporated in them is insufficient to interconnect several PE chains and therefore, they do not increase viscosity. So, our detailed investigation combining contrast variation SANS and NSE does not only allow to explain the viscosity behavior but also to deduced detailed information regarding the structures and the dynamics especially of the polyelectrolyte within the complexes.

Structure and dynamics of polyelectrolyte surfactant mixtures under conditions of surfactant excess

“…aggregates on the PE rich side of the phase diagram, these aggregates have a relatively well defined PE shell 27,43 (see Fig. S3).…”

“…Therefore, attempts to extract the fraction of bound PE by fitting two decays instead of one turned out to be unsuccessful. Attempting to do so is further complicated by the fact that the PE retains some of its freedom of movement in the aggregates as has been shown before 27,43 which is why taking D app from the measurements with Atto647 at the same SDS concentration is not feasible and we decided to fit data with Eq. (13).…”

“…The peculiar self-aggregation behaviour of oppositely charged polyelectrolyte (PE) surfactant mixtures has lead to quite a number of publications over the past decades. Many different structures can be observed, 6,17,[25][26][27] which depend on various parameters of the PE, such as charge density, backbone rigidity, molecular weight or the exact location of the charge with respect to the backbone, as well as on the properties of the surfactant, such as its chain length, the type of headgroup or the number of chains in a surfactant molecule.…”

Intermolecular Interactions in Polyelectrolyte and Surfactant Complexes in Solution

“…The formation of mixed rod-like polyelectrolyte-surfactant aggregates increases the viscosity for the cationically modified hydroxyethyl cellulose polyelectrolyte JR 400/surfactant system according to Hoffmann et al [128,133,134]. The viscosity is polyelectrolyte concentration dependent.…”

“…The viscosity is polyelectrolyte concentration dependent. At high polyelectrolyte concentrations, the viscosity is high due to the interconnectivity between chains from physical crosslinks between PESC aggregates (Figure 14) [128,132,134,135]. The solutions go from a viscoelastic fluid to more elastic as the phase becomes more gel-like [132,135].…”