Prof. Dr. Peter Fischer

Viviane Lutz-Bueno, PSI, Swiss Light Source, 5232 Villigen, Switzerland Marianne Liebi, PSI, Swiss Light Source, 5232 Villigen, Switzerland Rosanna Pasquino, University of Naples Federico II, 80125 Naples, Italy Joachim Kohlbrecher, PSI, Laboratory for Neutron Scattering, PSI, 5232 Villigen, Switzerland Peter Fischer, Institute of Food, Nutrition and Health, ETH Zurich, 8092 Zurich, Switzerland

The salt curve revisited: Electrostatic charges govern the viscoelastic properties of micellar solutions

Abstract

 

In ionic surfactant micelles, interactions among surfactant monomers, their counterion, and additives are fundamental to tune molecular self-assembly and thus the rheological properties. Here we propose a combination of 1H-NMR, integrated small-angle neutron scattering (SANS) and small-angle X-ray scattering, and rheology to probe the molecular arrangements of the individual molecules within the micelle and the resulting flow properties [1-3]. Shifts in the 1H-NMR signal show the penetration of counterions and additives into the micellar surfactant structure while SANS and SAXS determine specific intramicellar length scales and intermicellar interactions. SANS signals are sensitive to the contrast between the solvent (deuterium) and the hydrocarbonic tails in the micellar core (hydrogen) and SAXS access the inner structure of the polar shell because the headgroups, counterions, and penetrated salt have higher electron densities compared to the solvent and to the micellar core.

The number density, intermicellar distances, aggregation number, and inter/intramicellar repulsions are discussed on the basis of the dependence of the structure factor and form factor on the micellar aggregate morphology. The results allow us to propose a modified packing parameter model taking electrostatic charges into account. As a consequence, micellar growth and structure can be discussed by variations in the flexibility and size of the headgroup as well as the ionic dissociation rate of its counterion. Additionally, we show that the counterion binding is even more significant to the development of viscoelasticity than the headgroup structure of a surfactant molecule [2, 4, 5]. This somewhat surprising finding shows the importance of electrostatic charges in the self-assembly process and its consequences on the rheological fingerprint of viscoelastic surfactant solutions.

 

[1] V. Lutz-Bueno, R. Pasquino, M. Liebi, J. Kohlbrecher, P. Fischer: Langmuir 32 (2016) 4239.

[2] V. Lutz-Bueno, M. Liebi, J. Kohlbrecher, P. Fischer: Langmuir 33 (2017) 2617.

[3] V. Lutz-Bueno, S. Isabettini, F. Walker, S. Kuster, M. Liebi, P. Fischer: PCCP 19 (2017) 21869

[4] V. Lutz-Bueno, J. Zhao, R. Mezzenga, T. Pfohl, P. Fischer, M. Liebi: Lab Chip 16 (2016) 4028.

[5] V. Lutz-Bueno, R. Pasquino, S, Haward, A. Shen, P. Fischer: J. Rheol. 61 (2017) 769.



Date: 9/2/2018
Time:12:00 (coffee & cookies will be served at 11:45)
Place:FORTH Seminar Room 2