The Tensile Strengths of Liquids. I. Thermodynamic Considerations

An investigation is made of the thermodynamic stability of liquids under tension. Comparisons of the Helmholtz free energies show that small samples of liquid under tension are unstable with respect to two-phase liquid-vapor equilibrium at the same volume and temperature. Defining the tensile strength as the negative pressure at which (∂P/∂V)T=0 for the liquid, it is possible to calculate ``maximum'' possible tensile strengths from equations of state. The van der Waals and Berthelot equations in reduced form can be adapted to such explicit calculation and give values much lower, and therefore much closer to experimental values, than previous calculations. For non-associated liquids at 27°C, the van der Waals maximum tensile strengths are about 150 atmos. whereas the Berthelot values are about 750 atmos. The temperature dependence for the two equations is about 1 and 6 atmos. °C, respectively. Comparisons with observation of the calculated reduced liquid volume, energy of vaporization, coefficient of thermal expansion, and bulk compressibility support the use of the van der Waals and Berthelot equations in computing liquid properties.