College Park, Maryland      June 6 - 10 , 2004

M4-C2 (4:00 PM): Liquid-Gas Critical Phenomena Under Confinement: SANS Studies of CO2 in Aerogel

Y. B. Melnichenko, G. D. Wignall (Condensed Matter Sciences Division, Oak Ridge National Laboratory), D. R. Cole (Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA), H. Frielinghaus (Forschunszentrum Jülich GmbH, Institut für Festkörperforschung, D-52425 Jülich, Germany)

The effect of spatially fixed (quenched) impurities on phase transitions and critical phenomena remains a challenging area for both theory and experiment. Small angle neutron scattering (SANS) is a well-established technique for investigating the behavior of confined binary liquid solutions, as it can probe the correlation length (ξ) and susceptibility (χ) in pores on length scales ~ 1 - 100 nm. We report results of the first SANS measurements of the critical behavior of a single-component fluid (CO2) in a highly porous aerogel and compare it with the critical behavior of the bulk fluid. We demonstrate that, although the scattering from blank aerogel exceeds that from CO2 by two orders of magnitude, the temperature and pressure variation of thermodynamic properties of confined fluids can be extracted and explored. Pressure scans at temperatures, both far away and close to the liquid-gas critical temperature, TC reveal the importance of the molecule-surface interactions, which become increasingly important in the critical region. ξ and χ for the bulk fluid diverge as T~TC and their variation is described by the Ising model exponents (ν = 0.63 and γ = 1.24, respectively). Furthermore, the scaling exponents of confined fluid are close to mean field predictions (ν = 0.5 and γ = 1.0) and the absolute value of ξ does not exceed the pore size of aerogel (~ 70 Å) even at T ~ TC. The results demonstrate that quenched disorder induced by aerogel works to depress density fluctuations in the critical region. Below TC, the long-lived methastable domains are formed. Despite the negligible volume occupied by aerogel (< 4 %), the macroscopic phase separation of confined CO2 into coexisting liquid and gaseous phases characteristic of the bulk fluid, is completely suppressed. Experimental data show that critical absorbtion is as important as the effect of confinement, even in highly porous systems.

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