Abstract
Series of molecular dynamics simulations for 2-propanol-water mixtures, as a function of temperature (between freezing and room temperature) and composition (xip = 0, 0.5, 0.1, and 0.2), have been performed for temperatures reported in the only available experimental structure study. It is shown that when the all-atom optimized potentials for liquid simulations interatomic potentials for the alcohol are combined with the TIP4P/2005 water model, then the near-quantitative agreement with measured X-ray data, in the reciprocal space, can be achieved. Such an agreement justifies detailed investigations of structural, energetic, and dynamic properties on the basis of simulation trajectories. Here, we focus on characteristics related to hydrogen bonds (HB): cluster-, and in particular, ring formation, energy distributions, and lifetimes of HB-s have been scrutinized for the entire system, as well as for the water and isopropanol subsystems. It is demonstrated that similar to ethanol-water mixtures, the occurrence of 5-membered-hydrogen-bonded rings are significant, particularly at higher alcohol concentrations. Concerning HB energetics, an intriguing double maximum appears on the alcohol-alcohol HB energy distribution function. HB lifetimes have been found significantly longer in the mixtures than they are in pure liquids.
Highlights
Aqueous solutions of alcohols, the latter showing both hydrophobic and hydrophilic characters, have provided an excellent testing ground, from many aspects, for scientific research for many decades.[1−24] these solutions are of basic importance in numerous fields, ranging from fundamental science to industrial applications.Recently, the temperature-dependent structure of methanol/ water[25] and ethanol/water[26] liquid mixtures, as well as the microscopic dynamics[27] in ethanol/water solutions, has been investigated extensively in water-rich mixtures, using molecular dynamics (MD) simulations
The energetic definition[53] of the H-bond was applied as follows: two molecules were considered hydrogen bonded to each other if they were found at a distance r(O···H) < 2.5 Å, and the interaction energy is smaller than −3 kcal/mol
This definition has less arbitrariness than the pure geometrical definition, as we showed in an earlier publication,[53] where all analysis using both the energetic and the geometric definition (r(O···H) < 2.5 Å, and H−O···O angle between 30°) were carried out
Summary
The latter showing both hydrophobic and hydrophilic characters, have provided an excellent testing ground, from many aspects, for scientific research for many decades.[1−24] these solutions are of basic importance in numerous fields, ranging from fundamental science to industrial applications.Recently, the temperature-dependent structure of methanol/ water[25] and ethanol/water[26] liquid mixtures, as well as the microscopic dynamics[27] in ethanol/water solutions, has been investigated extensively in water-rich mixtures, using molecular dynamics (MD) simulations. The temperature-dependent experimental work of Takamuku[15] extends to aqueous solutions of yet another alcohol, isopropanol (a.k.a. 2-propanol or propan-2-ol), again, at low alcohol concentrations 0.2). The first explanation to the perturbation of a hydrophobic probe (originally alcohols) induced to the water structure was proposed by Frank and Evans in 1945,1 and was based mainly on the thermodynamic properties of these solutions. The proposed “iceberg” formation of water molecules around hydrophobic solutes has provoked a significant debate in the scientific community. There is still no consensus concerning this hypothesis and about the details of the atomistic picture that cause nonideal behavior in terms of several of their macroscopic properties at low alcohol concentrations. There is still no consensus concerning this hypothesis and about the details of the atomistic picture that cause nonideal behavior in terms of several of their macroscopic properties at low alcohol concentrations. 2-Propanol is the simplest example of a secondary alcohol, where the alcohol carbon atom is attached to two other carbon atoms and in this sense, the OH-group of this molecule has a compact hydrophobic environment
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