Isothermal compressions were measured for thirteen high-purity liquid hydrocarbons and two binary mixtures of liquid hydrocarbons. These hydrocarbons have a molecular weight range of 170 to 351 and included normal paraffins, cycloparaffins, aromatics, and fused ring compounds. The pressure range for these measurements was from atmospheric to as high as 10 000 bars, being limited to lower values for some compounds to avoid possible solidification of the liquid. The volume changes due to pressure were measured at six temperatures spaced about equally in the range 37.8°C to 135.0°C. The volume changes and pressures were measured by methods similar to those of P. W. Bridgman. Pressure-volume isotherms can be described adequately by the Tait equation, v0—v=C log(1+P/B), or for pressures above a certain minimum, whose value depends on the compound, by the Hudleston equation log[v2/3P/(v01/3−v1/3)]=A+B(v01/3−v1/3).For the Tait equation the parameter C can be predicted for hydrocarbon liquids from the relation C=0.2058 v0. Compressibility for a given hydrocarbon decreases with increasing pressure at constant temperature and increases with increasing temperature at constant pressure. The compression, and the compressibility, of liquid hydrocarbons are strongly dependent on molecular structure. Cyclization introduces a rigidity of molecular shape which decreases the compressibility markedly. Furthermore, fused ring cyclization as exemplified by naphthyl and decalyl structures has a considerably greater effect in decreasing compressibility than cyclization to nonfused rings such as cyclopentyl, cyclohexyl, or phenyl, even at equivalent carbon atom in ring percentages. Isobars and isochores were drawn and studied over the full range of temperature and pressure. The coefficient of thermal expansion, (1/v0) (δv/δT) P, for a given hydrocarbon, decreases with increasing pressure at constant temperature. (δ2v/δT2) P undergoes a sign change at a certain pressure, whose value depends on the compound; (δv/δT) P increases with increasing temperature below this pressure and decreases with increasing temperature above this pressure. The pressure coefficient, (δP/δT) v, is not a function of volume alone but is also dependent on the temperature and pressure. (δE/δv) T and (δE/δP) T go to zero and then reverse sign for compounds that can be studied to sufficiently high pressures.

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Using the rolling-ball method the viscosity of seven pure hydrocarbons, having 25 or 26 carbon atoms, and three binary mixtures of them has been measured to 3450 bars at 37.8°, 60.0°, 98.8°, and 135°C. The compounds included isoparaffinic, cycloparaffinic, and aromatic types. The increase in viscosity with pressure was found to be strongly dependent on molecular structure. The viscosity-temperature coefficient 1/η(∂η/∂T)p increased with increased pressure while the viscosity-pressure coefficient 1/η(∂η/∂P)T decreased with increased temperature. The behavior of the binary mixtures corresponded within 5% over a range of 50–100 fold change in viscosity to that of the pure compounds equivalent to them in molecular weight and average molecular structure. This remarkable agreement is interpreted to mean that the viscosity of these compounds is some additive function of their constituent groups whether these groups are combined in the same or different molecules as long as the basic molecular symmetry is unchanged. The values of the Eyring theory ΔF±, ΔH±i, ΔS±, and ΔV± for these data are discussed. For the saturated compounds at constant temperature, an approximately linear relation was found between log η and [(v/v0)4—(v/v0)2] where v is the specific volume and η the absolute viscosity.

INTERPRETATIONof the effect of molecular constitution on the physical properties of pure substances has contributed greatly to man's understanding of the physical world, and has often permitted the segregation and evaluation of intraand intermolecular effects. Because of the variety of molecular structures possible, compounds of carbon have been especially useful in such investigations, and the compounds of carbon and hydrogen are usually taken as reference structures to which other carbon compounds are related. For these and other cogent reasons,

The viscosity of fourteen very pure high molecular weight hydrocarbon liquids has been determined at six temperatures, ranging from 37.78°C to 135°C, and at pressures ranging from atmospheric to 3400 bar, by use of a rolling-ball viscometer. For nine of the liquids whose PVT data were not available, a flexible bellows piezometer was used to determine the density between the same limits of temperature and pressure as employed for the viscosity measurements. The hydrocarbons selected vary in molecular weight from 268.5 to 535.0 and comprise several structural symmetry families involving progressive cyclization to cyclohexyl, phenyl, and decalyl groups. Also included are an alkylnaphthalene, an alkyldecahydronaphthalene, two hydrochrysenes, an homologous series of branched paraffins, and a series of 1,1-diphenylalkanes. Correlations are made of structure with viscosity, rate of change of viscosity with pressure, and enthalpy of activation for viscous flow. The effects of molecular weight and of molecular structure on viscosity are discussed.