Lifetime distribution of clusters in binary mixtures involving hydrogen bonding liquids

<jats:title>Abstract</jats:title><jats:p>Hydrogen bonded liquids are associated liquids and tend to exhibit local inhomogeneity in the form of clusters and segregated sub-nano domains. It is an open question as to whether Hbonded clusters in pure water have common features with the water segregated pockets observed in various aqueous binary mixtures, such as water–alcohol mixtures, for example. In the present study, we demonstrate through classical molecular dynamics studies of the lifetime distributions of the hydrogen bonds in different types of binary mixtures, that these lifetimes exhibit the same universal features in the case of the pure liquids, independently of the species concentrations. The same types of three distinct lifetimes are observed, all of them in the sub picosecond regime. The primary lifetime concerns that of Hbonded dimers, and strongly depends on Hbonding criteria such as the bonding distance. The two others are independent of bonding criteria and appear as universal accross many liquids and mixtures. The secondary lifetime (<jats:inline-formula><jats:alternatives><jats:tex-math>$$\tau _1 \approx 20$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>τ</mml:mi> <mml:mn>1</mml:mn> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>20</mml:mn> </mml:mrow> </mml:math></jats:alternatives></jats:inline-formula> fs) concerns Hbonded cluster lifetimes, while the tertiary lifetime (<jats:inline-formula><jats:alternatives><jats:tex-math>$$\tau _2 \approx 50$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>τ</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>50</mml:mn> </mml:mrow> </mml:math></jats:alternatives></jats:inline-formula> fs) concerns the topology of these clusters, such as chains or globules, for example. This surprizing separation in three distinct lifetimes suggests the existence of associated three distinct kinetic mechanisms in the very short sub-picosecond time scales, with, in addition, an appealing connection to the concepts of local energy and entropy.</jats:p>