Micelle kinetics of photoswitchable surfactants: Self-assembly pathways and relaxation mechanisms

Victoria Ariel Bjørnestad; Xinmeng Li; Christophe Tribet; Reidar Lund; Michele Cascella

doi:10.1016/j.jcis.2023.05.057

Micelle kinetics of photoswitchable surfactants: Self-assembly pathways and relaxation mechanisms

J Colloid Interface Sci. 2023 Sep 15:646:883-899. doi: 10.1016/j.jcis.2023.05.057. Epub 2023 May 15.

Authors

Victoria Ariel Bjørnestad¹, Xinmeng Li¹, Christophe Tribet², Reidar Lund³, Michele Cascella⁴

Affiliations

¹ Hylleraas Centre for Quantum Molecular Sciences, Department of Chemistry, University of Oslo, Sem Sælands vei 26, Oslo, 0371, Norway.
² PASTEUR, Département de Chimie, École Normale Supérieure, PSL University, Sorbonne Université, CNRS, Paris, 75005, France.
³ Hylleraas Centre for Quantum Molecular Sciences, Department of Chemistry, University of Oslo, Sem Sælands vei 26, Oslo, 0371, Norway. Electronic address: reidar.lund@kjemi.uio.no.
⁴ Hylleraas Centre for Quantum Molecular Sciences, Department of Chemistry, University of Oslo, Sem Sælands vei 26, Oslo, 0371, Norway. Electronic address: michele.cascella@kjemi.uio.no.

PMID: 37235934
DOI: 10.1016/j.jcis.2023.05.057

Abstract

Hypothesis: A key question in the kinetics of surfactant self-assembly is whether exchange of unimers or fusion/fission of entire micelles is the dominant pathway. In this study, an isomerizable surfactant is used to explore fundamental out-of-equilibrium kinetics and mechanisms for growth and dissolution of micelles.

Experiments: The kinetics of cationic surfactant 4-butyl-4'-(3-trimethylammoniumpropoxy)-phenylazobenzene was studied using molecular dynamics simulations. The fusion and exchange processes were investigated using umbrella sampling. Equilibrium states were validated by comparison with small-angle X-ray scattering data. The photo-isomerization event was simulated by modifying the torsion potential of the photo-responsive group to emulate the trans-to-cis transition.

Findings: Micelle growth is dominated by unimer exchange processes, whereas, depending on the conditions, dissolution can occur both through fission and unimer expulsion. Fusion barriers increase steeply with the aggregation number making this an unlikely pathway to equilibrium for micelles of sizes that fit with the experimental data. The barriers for unimer expulsion remain constant and are much lower for unimer insertion, making exchange more likely at high aggregation. When simulating photo-conversion events, both fission and a large degree of unimer expulsion can occur depending on the extent of the out-of-equilibrium stress that is put on the system.

Keywords: Azobenzene; Cationic surfactant; Micelle kinetics; Molecular dynamics; Photoswitch; SAXS.