Collective and self-diffusion coefficients in an ionic critical mixture: 3-methylpyridine+water+NaBr

María-Paula Hernández; Francisco Monroy; Francisco Ortega; Ramón G Rubio

doi:10.1063/1.1856455

Collective and self-diffusion coefficients in an ionic critical mixture: 3-methylpyridine+water+NaBr

J Chem Phys. 2005 Mar 8;122(10):104501. doi: 10.1063/1.1856455.

Authors

María-Paula Hernández¹, Francisco Monroy, Francisco Ortega, Ramón G Rubio

Affiliation

¹ Departamento de Química Física I, Facultad de Química, Universidad Complutense, 28040-Madrid, Spain.

PMID: 15836326
DOI: 10.1063/1.1856455

Abstract

The dynamics of concentration fluctuations of three critical samples of the 3-methylpyridine (3MP)+water+NaBr system have been measured by photon correlation spectroscopy. The collective-diffusion coefficient D shows the usual Ising behavior near the critical temperature T(c). However, as |T-T(c)| increases, the dynamic correlation length calculated from D, xi, takes values higher than the correlation length of the critical fluctuations calculated from static light scattering, xi(s). At the largest |T-T(c)| measured, xi approaches the value, xi(0,d) approximately equal to 1.13 nm, while the amplitude of xi(s) is xi(0,s)=0.38 nm. Pulsed-gradient NMR spectroscopy points out the existence of two dynamic contributions. One of them is consistent with the existence of molecular entities of hydrodynamic radius 0.31 nm, while the other one indicates the existence of aggregates rich in 3MP of radius 1.16 nm. The existence of the aggregates may explain the apparent anomalous behavior of the dynamic light scattering experiments for this system far from the critical point.