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

Authors

• Mesfin Tsige, The University of AkronFollow
• Tianbo Liu, The University of Akron
• Javad Tamnanloo, The University of Akron
• Abdol Hadi Mokarizadeh, The University of Akron

Author 1 OrcID

https://orcid.org/0000-0002-7540-2050

Author 2 OrcID

https://orcid.org/0000-0002-8181-1790

Author 3 OrcID

https://orcid.org/0000-0001-8552-1523

Author 4 OrcID

https://orcid.org/0000-0001-5922-7654

Academic department

School of Polymer Science and Polymer Engineering

Description

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.

Publisher name

ACS Publications

Grant Information

N/A

Data Management

N/A

Document Type

Article

Digital Object Identifier (DOI) Link

https://doi.org/10.1021/acs.langmuir.5c05734

Publication Date

1-24-2026

Publication Title

Langmuir

Volume

42

Issue

4

First Page

3468

Last Page

3478

Recommended Citation

Tsige, Mesfin; Liu, Tianbo; Tamnanloo, Javad; and Mokarizadeh, Abdol Hadi, "The Role of Side-Chain Length and Counterion Mediation on Dimerization of Rigid Sphere-Rod Amphiphiles: A Molecular Dynamics Investigation" (2026). University Research. 38.
https://ideaexchange.uakron.edu/university_research/38

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.