Camel back shaped Kirkwood-Buff Integrals

TL;DR

This paper explains the camel back shape of Kirkwood-Buff Integrals in certain binary mixtures by showing that aggregate formation leads to dual extrema, reflecting complex concentration fluctuations.

Contribution

It introduces the concept that aggregate formation in binary mixtures causes dual extrema in KBI, expanding understanding of micro-segregation and concentration fluctuations.

Findings

Dual extrema occur when one species forms aggregates.

Meta-extremum appears at low concentration of aggregates.

The duality between fluctuations and micro-segregation is illustrated.

Abstract

Some binary mixtures, such as specific alcohol-alkane mixtures, or even water-tbutanol, exhibit two humps camel back shaped KBI. This is in sharp contrast with usual KBI of binary mixtures having a single extremum. This extremum is interpreted as the region of maximum concentration fluctuations, and usually occurs in binary mixtures presenting appreciable micro-segregation, and corresponds to where the mixture exhibit a percolation of the two species domains. In this paper, it is shown that two extrema occur in binary mixtures when one species forms "meta-particle" aggregates, the latter which act as a meta-species, and have their own concentration fluctuations, hence their own KBI extremum. This "meta-extremum" occurs at low concentration of the aggregate-forming species (such as alcohol in alkane), and is independant of the other usual extremum observed at mid volume fraction occupancy. These systems are a good illustration of the concept of the duality between concentration fluctuations and micro-segregation.