Micelle and Inverse Micelle Structure Driven Viscoelasticity and Phase Separation in 2-Isobutoxyethanol–Water Mixtures: Insights from All-Atom Simulations

The molecular association between 2-isobutoxyethanol (IBE) and water in mixed polar solvents was investigated by using all-atom molecular dynamics simulations over a wide composition range. Structural analyses, including radial distribution functions (RDFs) and potentials of mean force (PMFs), reveal that hydrogen bonds between IBE-water are significantly stronger than those in IBE-IBE and water-water pairs, as evidenced by higher RDF peak intensities, deeper contact minima, and slower hydrogen-bond autocorrelation decay. These robust interactions promote the formation of large, stable heterogeneous clusters connected via hydrogen-bond networks. Dynamical properties show a pronounced slowdown of IBE self-diffusion with decreasing the mole fraction of 2-isobutoxyethanol (<i>x</i> _IBE). We observed the presence of the inverse micelle structure in <i>x</i> _IBE = 0.5 and micelle structures in <i>x</i> _IBE = 0.1. Stress-stress autocorrelation functions indicate enhanced viscoelasticity at <i>x</i> _IBE ≤ 0.5 due to the presence of inverse micelle and micelle structures, and predominantly liquid-like behavior at <i>x</i> _IBE > 0.5. At <i>x</i> _IBE = 0.1, phase separation is observed, consistent with previous experimental research (J. Chem. Thermodynamics, 2000, vol. 32, 729-741). This combined structural-dynamical picture highlights the critical role of hydrogen bonding in controlling microstructure, viscoelastic response, and phase stability in amphiphile-water mixtures, offering insights for designing functional aqueous solutions in industrial and biochemical applications.