Interfacial Tensions near Critical Endpoints: Experimental Checks of EdGF Theory

TL;DR

This paper compares experimental measurements of interfacial tensions near critical endpoints in a binary mixture with predictions from the EdGF theory, confirming some theoretical aspects and highlighting the need for more precise data.

Contribution

It provides experimental validation of the EdGF theory's predictions for universal scaling functions and background contributions near critical endpoints.

Findings

Antonow's rule is confirmed.

Residual singular behavior matches scaling predictions.

Identifies a three-fold discrepancy in surface tension magnitude.

Abstract

Predictions of the extended de Gennes-Fisher local-functional theory for the universal scaling functions of interfacial tensions near critical endpoints are compared with experimental data. Various observations of the binary mixture isobutyric acid $+$ water are correlated to facilitate an analysis of the experiments of Nagarajan, Webb and Widom who observed the vapor-liquid interfacial tension as a function of {\it both} temperature and density. Antonow's rule is confirmed and, with the aid of previously studied {\it universal amplitude ratios}, the crucial analytic ``background'' contribution to the surface tension near the endpoint is estimated. The residual singular behavior thus uncovered is consistent with the theoretical scaling predictions and confirms the expected lack of symmetry in $(T-T_c)$. A searching test of theory, however, demands more precise and extensive experiments; furthermore, the analysis highlights, a previously noted but surprising, three-fold discrepancy in the magnitude of the surface tension of isobutyric acid $+$ water relative to other systems.