Direct observation of a hydrophobic bond in loop-closure of a capped (-OCH2CH2-)n oligomer in water

TL;DR

This study demonstrates the direct observation of a hydrophobic bond in water by analyzing the end-to-end distance distribution of a capped oligomer, revealing insights into hydrophobic interactions beyond methane in aqueous solutions.

Contribution

It provides the first experimental-like evidence of a hydrophobic bond in water using oligomer end-to-end distance distributions, bypassing methane's solubility issues.

Findings

The oligomer's end-to-end distance probability density resembles the methane potential of mean force.

Hydration causes the chain globule to swell, affecting hydrophobic interactions.

The volume of the globule influences local concentration and interaction probabilities.

Abstract

The small r variation of the probability density P(r) for end-to-end separations of a -CH2CH3 capped (-OCH2CH2-)n oligomer in water is computed to be closely similar to the CH4 ... CH4 potential of mean force under the same circumstances. Since the aqueous solution CH4 ... CH4 potential of mean force is the natural physical definition of a primitive hydrophobic bond, the present result identifies an experimentally accessible circumstance for direct observation of a hydrophobic bond which has not been observed previously because of the low solubility of CH4 in water. The physical picture is that the soluble chain molecule carries the capping groups into aqueous solution, and permits them to find one another with reasonable frequency. Comparison with the corresponding results without the solvent shows that hydration of the solute oxygen atoms swells the chain molecule globule. This supports the view that the chain molecule globule might have a secondary effect on the hydrophobic interaction which is of first interest here. The volume of the chain molecule globule is important for comparing the probabilities with and without solvent because it characterizes the local concentration of capping groups. Study of other capping groups to enable X-ray and neutron diffraction measurements of P(r) is discussed.