Method for the Determination of the Surface Tension of Solids, from Their Melting Points in Thin Wedges

A method is proposed which allows one to measure the surface tension of solids whose liquids wet glass and steel and whose solid/liquid contact angle against those materials is zero. A thin wedge, formed of optically flat steel and glass slabs, is filled with liquid substance and then allowed to solidify. It is then immersed in a large heat bath whose temperature is raised at a rate of about 1.5°C per 24 hours, and is known at any time within ±0.002°C. The solid substance starts melting from the thinnest part of the wedge, well below its ``normal'' melting point T_m. From the lowering of the melting point ΔT corresponding to a measured thickness h of the wedge in the solid/liquid interface, the surface tension of the solid σ_S is then arrived at from the equation: $${\sigma }_S=\frac{Q_{\mathrm{f\rho S}}}{{\text{2T}}_m}{\mathrm{h\Delta T+\sigma }}_L,$$ where Q_f is the heat of fusion in erg/g, ρ_S the density of the solid, and σ_L the surface tension of the substance in liquid state. The method was applied to stearic and myristic acids. For myristic acid four independent measurements gave an average value of σ_S=116±10 erg/cm², all four samples showing similar orientation of the crystal lamellae (gross crystal orientation) with respect to the interface solid/liquid. For stearic acid three similar gross orientations gave an average σ_S=179±8 erg/cm². One gross orientation, markedly different from the others, yielded the value of σ_S=135±6 erg/cm². Thickness h was determined from the interference lines (sodium light). The gross orientation was observed in polarized sodium light.