Место издания:University of Trondheim Trondheim, Norway
Первая страница:488
Последняя страница:488
Аннотация:In present investigation we fulfilled CARS approach to registration of different states of carbon dioxide confined in nanopores. The nanoporous glass sample (NGS) with internal surface area ~250 m2/g and void-solid ratio ~30% was filled with gaseous carbon dioxide in high-pressure cell. CARS-spectra of high-frequency (“blue”) Fermi-dyad component 1 Q-branch were collected at temperature T=21 oC (this value is far enough from critical Tcr=31 oC) at pressure range ~30÷57 atm (Psat=57 atm is saturation value) that corresponds to density range ~35÷100 amagat.
Spectra structure was defined by contributions of molecules from three different states, collected spectra resulted from interference of these contributions. “Gaseous” contribution was caused by gaseous carbon dioxide from core bulk inside pores as well as from gaps between NGS and cell windows. “Adsorptive” contribution was caused by layers of molecules adsorbed on internal surface of NGS. Manifestation of “adsorptive” leaded to visually well-defined asymmetric spectra shape (Fig.1) at pressures below 52 atm (~0.9 Psat). At pressure values 54 atm and higher condensation inside pores leaded to appearance of “liquid” contribution, which manifestation is visually well-defined in appearance of low-frequency peak (Fig.1). Non-resonant background was negligible in our experiments.
Procedure of spectra fitting confirmed the possibility of identification of each state existence inside nanopores. At pressures below ~0.9 Psat spectra shapes calculated using “liquid” linewidth value essentially differed from collected spectra shapes in the low-frequency wing region (Fig. 1, dashed line). Good accordance with collected spectra shapes was achieved by using “adsorptive” linewidth value which is ~2 times larger than “liquid” one (Fig. 1, solid line). At pressure 54 atm all three contributions were significant and influenced on spectrum shape. At pressures higher than 54 atm double-peak spectra shape was in a good accordance with calculations with usage of “liquid” linewidth value that confirmed condensation inside nanopores.
Magnitude of each state contribution depends on a fraction of molecules in this state. The ratios between three fractions are defined by pressure and topological characteristics of NGS. Namely, the value of internal surface area of NSG defines “adsorptive” fraction of molecules. Pore diameter defines pressure value at which condensation inside nanopores takes place. Total number of molecules inside porous sample is defined by its void ratio, so one can compare magnitude of signal from nanopores with magnitude of signal from gaps between NSG and cell windows or with non-resonant background if it exists. Thus, presented approach allows one to measure the porosity parameters of NG sample by measurements of CARS-spectra broadening with following analysis of relations between spectral contributions.
The work is supported by Russian Foundation for Basic Research, grant #07-02-01331-а