Molecular emulsions: from charge order to domain order

TL;DR

This paper uses large-scale simulations to explain why scattering experiments do not detect domain pre-peaks in aqueous mixtures, revealing that charge order cancels out domain correlations, challenging previous interpretations of scattering data.

Contribution

It demonstrates that domain pre-peaks in atom-atom structure factors cancel out in total scattering intensity, providing a new interpretation of micro-segregation in mixtures.

Findings

Domain pre-peaks cancel in total scattering intensity.

Charge order analogy explains suppression of pre-peaks.

Scattering experiments cannot detect homogeneous domain segregation.

Abstract

Aqueous mixtures of small molecules, such as lower n-alkanols for example, are known to be micro-segregated, with domains in the nano-meter range. One consequence of micro-segregated domains would be the existence of long range domain-domain oscillatory correlations in the various atom- atom pair correlation functions, and subsequent pre-peaks in the corresponding atom atom structure factors, in the q-vector range corresponding to nano-sized domains. However, no such pre-peak have ever been observed in the large corpus of radiation scattering data published so far. Here, through large scale simulations of aqueous-1propanol mixtures, I report that the domain pre-peak contributions in the atom-atom structure factors exactly cancel each other in the total scattering intensity, thus suppressing the pre-peak in agreement with the experimental findings. This cancellation is explained by drawing an analogy between the charge order found in ionic fluids and the segregated domain order. This finding opens new interpretation of the well known scattering pre-peak observed in micro-emulsions. In particular, it implies that scattering experiment cannot detect homogeneous domain segregation, hence cannot lead to a proper microscopic description of atom-atom correlations in domain ordered mixtures.