# Role of Side-Chain Length and Counterion Mediation on Dimerization of Rigid Sphere-Rod Amphiphiles: A Molecular Dynamics Investigation

## TL;DR

This paper uses molecular dynamics to study how side-chain length and counterions affect the dimerization of rigid sphere-rod amphiphiles in mixed solvents.

## Contribution

The study reveals how hydrophobic interactions and counterion mediation stabilize dimerization despite electrostatic repulsion.

## Key findings

- Longer side chains enhance hydrophobic rod/side-chain interactions, strengthening dimer stability.
- TBA+ counterions screen electrostatic repulsion between Keggins, enabling stable dimer formation.
- Solvent reorganizes cooperatively, with water depletion in rod gaps and THF accumulation on hydrophobic surfaces.

## Abstract

Rigid sphere-rod amphiphiles (RSRAs) comprising a Keggin
polyoxometalate
(POM) headgroup and an oligofluorene rod with different side chain
lengths (C2, C6, C10, C16, are the number of carbon atoms per side
chain) were probed by all-atom molecular dynamics in THF/water mixtures
(15 and 33 vol % THF) with tetrabutylammonium (TBA+) counterions
to elucidate the molecular origins of dimerization. Despite strong
Keggin-Keggin electrostatic repulsion, stable dimers form through
a synergy of (i) hydrophobic rod/side-chain interaction that strengthens
with side-chain length and (ii) counterion-mediated attraction by
TBA+, which localizes near the Keggins to screen electrostatic
repulsion. Packing evolves from near-parallel rods for short chains
to interdigitated, tilted arrangements for long chains, while solvent
reorganizes cooperatively. Water is depleted from the inter-rod gap,
while tetrahydrofuran (THF) accumulates on exterior hydrophobic surfaces
as a loose solvation shell. Around Keggins, terminal and bridging
oxygens sustain a structured hydration layer with long water residence
time. Dynamically, single-molecule root-mean-square deviation (RMSD)
increases with side-chain length while dimer self-diffusion decreases
modestly. Trends are consistent across solvent fractions, with expected
shifts in magnitudes. These results provide an atomistic framework
linking side-chain architecture, counterion screening, and solvent
organization to the thermodynamic stabilization and dynamic behaviors
of RSRA dimers, clarifying early events that preceded higher-order
self-assembled structures.