Modified Trouton’s Rule for the Estimation, Correlation, and Evaluation of Pure-Component Vapor Pressure Paul M. Mathias,* Garry Jacobs, and Jesus Cabrera Fluor Corporation, 3 Polaris Way, Aliso Viejo, California 92698, United States ABSTRACT: Insights from the venerable Trouton’s Rule have been used to guide the development of an applied-thermodynamic method for the estimation, correlation, and evaluation of pure-component vapor pressure. Trouton’s Rule very simply and succinctly states that the entropy of vaporization of fluids at their normal boiling point is a constant (≈10.5 times the gas constant). Detailed evaluation of the data for many families of chemical compounds reveals the subtle patterns of departures from the rule, and facilitates the devel- opment of a useful new correlation. Several examples are presented to demonstrate the value of the new correlation to estimate, corre- late, extrapolate, and evaluate vapor-pressure data, and to under- stand the patterns of vapor-pressure behavior. The methodology provides a guide for the development of thermodynamic correlations, and the resulting correlations are expected to be useful for the practice of applied thermodynamics. ■ INTRODUCTION The vapor pressure of pure fluids is an extremely important physical property, and extensive efforts in applied thermo- dynamics have been devoted to developing methods for the analysis, correlation and estimation of vapor pressure. Many of these approaches have used the Clausius-Clapeyron equation, which provides an approximate relationship between the tem- perature derivative of the vapor pressure and the enthalpy of vaporization. 1 The Gibbs-Helmholtz equation provides a similar relationship between the temperature derivative of the saturated fugacity and the liquid enthalpy departure. 2 Trouton’s Rule 3,4 is an empirical and classical observation that relates the enthalpy of vaporization of fluids to the temperature of vapor-liquid phase change. In this paper, we combine the insights from the Clausius-Clapeyron equation, the Gibbs-Helmholtz equation, and Trouton’s Rule to develop a practical vapor-pressure corre- lation procedure that has a small number of parameters, and demonstrate its utility. Today, chemical technologists have access to a large number of electronic databases and correlations, 5 and these include: NIST, 6 NIST WebBook, 7 DIPPR, 8 PPDS, 9 DETHERM ··· on the WEB, 10 and the Korean Thermophysical Properties Data Bank. 11 The databases provide a powerful way to evaluate and develop property models, as is demonstrated in this paper through the use of the DIPPR database. 8 But databases must also be tested and evaluated, and an effective way to do this is visually and through intuitive relationships. 12 Also, engineering property correlations are improved when based upon intuitive understanding in addition to rigorous theoretical and scientific relationships. Here we demonstrate the development and application of a robust and intuitive correlation based upon Trouton’s Rule. While today we have access to extensive databases, many new compounds (e.g., pharmaceutical active ingredients, new chemicals, degradation compounds, etc.) have only minimal data, often just a single vapor pressure and a liquid density. Even if additional data are available, they must be evaluated and tested. Correlations with just a few adjustable parameters enable a useful technique to evaluate and extrapolate data, and we also demonstrate this capability of the modified Trouton’s Rule correlation presented in this paper. The quantitative analyses in this paper have been executed using Solver in Excel. ■ THERMODYNAMICS The goal of this work is to develop a useful correlation by exploiting the relationship between vapor pressure and various enthalpies-which must be unambiguously defined and objec- tively chosen. We begin by discussing exact and approximate relationships from thermodynamics and applying them to data for representative substances. The Clausius equation is an exact thermodynamic equation that relates the temperature derivative of the vapor pressure to the enthalpy of vaporization and the difference between the saturated vapor and liquid molar volumes. = Δ Δ P T H T V d d S VL VL (1) Special Issue: In Honor of Cor Peters Received: August 29, 2017 Accepted: December 5, 2017 Article pubs.acs.org/jced Cite This: J. Chem. Eng. Data XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.jced.7b00767 J. Chem. Eng. Data XXXX, XXX, XXX-XXX