Evidence for confinement induced phase separation in ethanol–water mixture: a positron annihilation study

Abstract

We report an experimental evidence for the phase separation of ethanol-water mixture confined in mesoporous silica with different pore size using positron annihilation lifetime spectroscopy (PALS). A bulk-like liquid in the core of the pore and a distinct interfacial region near the pore surface have been identified based on ortho-positronium lifetime components. The lifetime corresponding to the core liquid shows similar behavior to the bulk liquid mixture while the interfacial lifetime shows an abrupt rise within a particular range of ethanol concentration depending on the pore size. This abrupt increase is attributed to the appearance of excess free-volume near the interfacial region. The excess free-volume is originated due to microphase separation of confined ethanol-water primarily at the vicinity of the pore wall. We envisage that probing free-volume changes at the interface using PALS is a sensitive way to investigate microphase separation under nanoconfinement.

Microphase separation of a miscible binary liquid mixture under confinement at the nanoscale

“…They showed that a small fraction of the ethanol molecules seemed to be anchored at the silica surface. 34 A similar result was also established by Guo et al 14 who have studied confinement of the ethanol-water mixture between two planar silica walls. A partial and local separation between water and ethanol close to the silica surface was thus evidenced.…”

“…Whereas these effects have been largely investigated in the case of confined single components [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and immiscible binary mixtures, [28][29][30] less works have been devoted to the confinement of miscible liquid mixtures. 15,[31][32][33] Recently, Muthulakshmi et al 34 reported an experimental evidence of a partial phase separation of an ethanol-water mixture confined in mesoporous silica using positron annihilation lifetime spectroscopy. They showed that a small fraction of the ethanol molecules seemed to be anchored at the silica surface.…”

Phase stability condition and liquid–liquid phase separation under mesoscale confinement

“…universally, both for alcohol-water mixtures [7][8][9][10][11] and macromolecules [4,5] To achieve these aims, the stability theory is indispensable, because the study of preferential adsorption on its own cannot give a complete insight into phase separation for the following reasons. Here, adsorption refers to 〈𝑁 𝑚 𝑁 1 〉 and 〈𝑁 𝑚 𝑁 2 〉, whereas, from Eq.…”

“…Aqueous biomolecular solutions stable in macroscopic bulk, such as the ones containing DNA, PEG and NaCl [4] or DNA and alginate [5], undergo phase transition when confined in a droplet, and increasingly so as the droplet size decreases [4,5]. Simple solution mixtures [6], even ethanol-water [7][8][9][10][11] mixtures, go through phase separation under confinement. Thus, confinement may affect phase stability.…”

Confinement Effects on Glass-Forming Aqueous Dimethyl Sulfoxide Solutions

“…For example, the phase behavior can be affected by such multitude of interactions [ 41 ]. In particular, several studies found an increased tendency for microphase separation of confined binary mixtures as a consequence of preferential interactions with the pore walls [ 42 , 43 , 44 , 45 ]. Furthermore, it was reported that the dependence of the glass transition temperature on the composition differs for confined and bulk hydrogen-bonded mixtures [ 38 ].…”