Coexistence Phenomena in the Critical Region: V. The Gravity Effect in the Binary Liquid System Aniline-Cyclohexane From Light Scattering

Abstract

Transll~ittance measurements were used to investigate vertical concentration gradients, a t temperatures slightly above the critical, in a binary liquid system. The gradients, which are predicted theoretically, did not begin t o form during time intervals of several hours. I t was found that a theoretical relation between transmittance and concentration gradient did not apply, even qualitatively, t o the results obtained. The significance of these observations is discussed. INTRODUCTIONIn previous publications (3, 4), the authors reported large vertical gradients in density near the liquid-vapor critical point in one-component systems. These gradients, due t o gravitational compression, were anticipated owing to the large compressibility of a fluid in the critical region.The case of vertical concentration gradients in a binary liquid system, a t temperatures slightly above the critical, forms an analogue which is not so obvious. I t was, however, shown by Yvon (9) that such concentration gradients are predicted theoretically. Since Rice and Rowden (6) reported a flat apex on the coexistence curve for aniline-cyclohexane, it was of interest to see if this flat top could be ascribed t o concentration gradients by analogy to the gravity effect in one-component systems (3,4,7,8).The critical solution point for a binary liquid system is defined by the conditions where p1 ancl cl are respectively the chemical potential and mole fraction of component 1 in the solution.For the variation of concentration with height in a binary liquid mixture, Yvon (9) obtained an expressioil which may be simplified to the form where x is distance measured positive downward and G(x) is a finite function of x. From Equation [2], Yvon concluded that when the solution approaches the critical state, i.e. when (dpl/d~I)T,p + 0, (dcl/dx), becomes very large.ldl~anzl.script rrceived Nouenrber 1 , 1957.

Solutions of Nonelectrolytes

“…Murray & Mason (33) have continued work on coexistence phenomena in the critical region, unsuccessfully attempting to detect by light scattering a gravity-produced concentration gradient in the aniline-cyclohexane binary liquid system at temperatures just above the critical mixing temperature [an effect predicted by Yvon (34)]. They infer that the flat apex of the co existence curve for this system [see Rowden & Rice (35)] is not gravity pro duced (as has been suggested for certain one-component, liquid-gas systems).…”