The monotonicity behavior of density profiles at vapor-liquid interfaces of mixtures

In their seminal monograph 'Molecular Theory of Capillarity', Rowlinson and Widom describe different possible shapes of density profiles at the vapor-liquid interface of mixtures. They postulated that in some instances, density profiles could possibly be non-monotonic, exhibiting either a maximum and/or a minimum. This contribution revisits this statement in the light of four decades of posterior research. We summarize the distinct morphologies at the vapor-liquid interface suggested in the literature recognizing that the condition of a single minimum in the profile has not yet been reported. Interfacial density profiles with a single maximum as well as fully monotonic density profiles have been observed and reported extensively. The case of a simultaneous maximum and minimum is more controversial, as it has only been predicted using theoretical approaches such as density gradient theory (DGT). This ambiguity is further investigated in this work using the example of the vapor -liquid interface of cyclohexane + butanol. Both DGT in combination with several distinct equations of state and molecular dynamics simulations are used. The results from the two methods are found to be contradictory: while the DGT results predict a maximum/minimum structure, the computer experiment results indicate only a single maximum in the density profiles. This work thereby emphasizes that results from DGT for highly non-ideal mixtures should not be taken for granted.

Automated summary

This article revisits the description of density profiles at the vapor-liquid interface of mixtures in the seminal monograph 'Molecular Theory of Capillarity' by Rowlinson and Widom. It summarizes the different suggested morphologies at the interface and acknowledges the absence of reports on profiles with a single minimum. Both profiles with a single maximum and fully monotonic profiles have been extensively observed and reported. The presence of both a maximum and a minimum in the density profiles is controversial and has only been predicted using theoretical approaches like density gradient theory (DGT). This study investigates this ambiguity using the example of the vapor-liquid interface of cyclohexane + butanol, employing DGT and molecular dynamics simulations. The results from the two methods contradict each other, with DGT predicting a maximum/minimum structure while the computer experiment results show only a single maximum. Thus, this work highlights the need for caution when interpreting DGT results for highly non-ideal mixtures.