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Abstract
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The limited understanding of how fatty acid alkyl chain length influences the interfacial behavior and stability of
menthol-based hydrophobic deep eutectic solvents (HDESs) in aqueous environments hinders their rational
design for biphasic applications. This study integrates molecular dynamics (MD) simulations and COSMO-RS
modeling to investigate the effect of fatty acid alkyl chain length (valeric acid, C5; enanthic acid, C7; pelar-
gonic acid, C9) on the structural, interfacial, and dynamic properties of menthol-based hydrophobic deep
eutectic solvents (HDESs) in aqueous environments. Our integrated approach reveals that increased hydropho-
bicity and steric hindrance of longer-chain fatty acids are the key mechanisms driving enhanced phase stability.
COSMO-RS predicts increasing log Kow and decreasing water selectivity with chain length. MD validates these
predictions through minimal water penetration, shorter FA–water hydrogen bond lifetimes (VAL: 1.9ps; PEL: 0.8
ps), and more negative excess volume for MPEL. The stability factors confirm partial miscibility with stability
order PEL > ENA > VAL. FA–water interaction energies become less negative with chain length, while men-
thol–PEL interactions strengthen upon hydration. Diffusion coefficients decrease with chain length, and distri-
bution coefficients (DFAs/Dwater) decline markedly for MPEL, confirming reduced mobility. The strong
concordance between MD and COSMO-RS establishes a validated mechanistic framework linking alkyl chain
length to HDES stability, enabling rational design of task-specific hydrophobic DESs for biphasic extraction,
contaminant removal, and pharmaceutical processing
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