Molecular segregation observed in a concentrated alcohol–water solution

Abstract

When a simple alcohol such as methanol or ethanol is mixed with water, the entropy of the system increases far less than expected for an ideal solution of randomly mixed molecules. This well-known effect has been attributed to hydrophobic headgroups creating ice-like or clathrate-like structures in the surrounding water, although experimental support for this hypothesis is scarce. In fact, an increasing amount of experimental and theoretical work suggests that the hydrophobic headgroups of alcohol molecules in aqueous solution cluster together. However, a consistent description of the details of this self-association is lacking. Here we use neutron diffraction with isotope substitution to probe the molecular-scale structure of a concentrated alcohol water mixture (7:3 molar ratio). Our data indicate that most of the water molecules exist as small hydrogen-bonded strings and clusters in a 'fluid' of close-packed methyl groups, with water clusters bridging neighbouring methanol hydroxyl groups through hydrogen bonding. This behaviour suggests that the anomalous thermodynamics of water alcohol systems arises from incomplete mixing at the molecular level and from retention of remnants of the three-dimensional hydrogen-bonded network structure of bulk water.

Long-range specific ion-ion interactions in hydrogen-bonded liquid films

“…The failure of AN to propagate LR-SIE confirms our conjecture, but the experiments in IPA appear to contradict it. However, we consider that the latter result represents a false negative because IPA, in contrast with H 2 O and MeOH, [49][50][51][52][53][54][55][56] lacks an extensive H-bond network. Alkyl groups restrict the number of configurations amenable to hydrogen bonding, and this effect increases with the number and size of the alkyl groups surrounding O-H moieties.…”

Organic molecules on the surface of water droplets – an energetic perspective

“…Even though molecules like methanol and water can be mixed over the whole concentration range, the methyl groups still tend to cluster to some degree in methanol solutions 26 . Dielectric relaxation measurements of a number of alcohol/water mixtures have revealed mixing to be a complex function of alcohol concentration [27][28][29][30] .…”

Microstructure of neat alcohols

“…Associated liquids, such as water and alcohols, for example, belong to a special class because of the particularity of the hydrogen bonding (HB) that is highly directional, and tend to enhance the structure the liquid locally. One particularly interesting example of this phenomena is the microheterogeneous nature of aqueous mixtures, which has attracted a recent upsurge of interest [1,2,3,4,5,6,7]. Perhaps the most remarkable reported fact is that watermethanol mixtures show local immiscibility at microscopic level, while being miscible at macroscopic level [1,2].…”

“…One particularly interesting example of this phenomena is the microheterogeneous nature of aqueous mixtures, which has attracted a recent upsurge of interest [1,2,3,4,5,6,7]. Perhaps the most remarkable reported fact is that watermethanol mixtures show local immiscibility at microscopic level, while being miscible at macroscopic level [1,2]. In order to appreciate this result it is interesting to compare it to microemulsions where bicontinuous phases are usually observed, and micro-immiscibility operates with domain sizes ranging from 100 nanometers to few micrometers, while those mentioned here are around few nanometers-that is about few molecular diameters.…”

“…From a statistical mechanical point of view, homogeneity is expressed by the fact that the order parameter, in this case the one body density, which formally depends on both the position and orientation of a single particle 1 (as in a crystal or a liquid crystal, for example), is a constant throughout the sample: ρ (1) (1) = ρ = N/V , where N is the number of particles per volume V. As a consequence, the microscopic description of the structure of a neat liquid starts from the two-body density function ρ (2) (1, 2) = ρ (1) (1)ρ (1) (2)g(1, 2) that expresses the density correlations between particles 1 and 2, and reduces in this case to ρ 2 g(1, 2) , where g(1, 2) is the pair distribution function. Associated liquids, such as water and alcohols, for example, belong to a special class because of the particularity of the hydrogen bonding (HB) that is highly directional, and tend to enhance the structure the liquid locally.…”