THE PRINCIPLE OF CONGRUENCE AND ITS APPLICATION TO COMPRESSIBLE STATES

The principle of congruence claims that certain thermodynamic and tran sport properties of a mixture of n-alkanes are the same as those of th e pure n-alkane of the mole-fraction-averaged carbon number. We discus s the origin of the principle, and attempts at theoretical justificati on and validation with respect to experimental data. We demonstrate th at the principle applies even in cases where no fundamental justificat ion exists. Whenever the principle applies, prediction of thermodynami c behavior of a mixture of n-alkanes is inherently simpler and usually more accurate than that based on empirical equations of state, molecu lar interaction parameters, and mixing and combining rules. Our experi mental verification serves to pinpoint those properties that can be sa fely and accurately predicted on the basis of the principle of congrue nce. The solubility of hydrogen in mixtures of long-chain n-alkanes is used as a prime example.