The vacuum distillation of heat-sensitive substances such as organic oils, dyes, and drugs requires special equipment from that available for atmospheric distillation. Study in the resulting field, high vacuum distillation, has subsequently led to the development of such equipment. Three types of high vacuum distillation and distillation equipment are recognized: conventional distillation, unobstructed-path distillation, and molecular distillation. Conventional vacuum distillation employs standard equipment under relatively high vacuum conditions. For unobstructed-path distillation, the equipment is modified so that the vapor path between the evaporating and condensing surfaces is clear.

When the vapor path is unobstructed and the condenser is separated from the evaporator by a distance less than the mean free path of the evaporating molecules, the phenomenon is called molecular distillation. Molecular distillation is the limiting type of high vacuum distillation with the absolute pressure range approximately 1 to 7 microns of mercury. The use of the centrifugal molecular still permits the distillation of organic substances which might suffer thermal decomposition even under the conditions of the conventional high vacuum still. Against the advantages of prevention of thermal decomposition and shorter time requirements must be placed the poor separatory power of distillation and the necessity of numerous redistillations. The molecular still is valuable not because it gives good separation but because distillation is accomplished 50 to 250 degrees centigrade below temperatures of conventional atmospheric distillation.

In ideal molecular distillation, equilibrium does not exist between the vapor and the liquid, no molecules re-enter the distilland after once being vaporized. Under these ideal conditions, Langmuir's equation predicts that the quantity of a given material distilling at a given temperature is proportional to P/√M where P is the vapor pressure and M is the molecular weight. Previous investigations at the Virginia Polytechnic Institute and other laboratories substantiate the importance of molecular weight and structure. Since most of these investigations have been only exploratory, insufficient work has been done to permit any conclusions of major importance.

The purpose of this investigation was to determine the effects of various molecular structures of fatty acids on their distillation characteristics when distilled in a laboratory-size centrifugal molecular still. Saturated (C₁₅, C₁₆, C₁₇, C₁₈, C₁₉, and C₂₀) and unsaturated (C₁₈ with one, two, and three double bonds) fatty acids were used in the study. Such effects as the chain length, presence of double bonds and their position in the molecule, and cis-trans and positional isomerism were studied.