Diffusion of Water and Electrolytes in Mesoporous Silica with a Wide Range of Pore Sizes

MARTÍNEZ CASILLAS, DIANA C.; LONGINOTTI, M. PAULA; BRUNO, MARIANO M.; VACA CHÁVEZ, FABIÁN; ACOSTA, RODOLFO H.; CORTI, HORACIO R.

JOURNAL OF PHYSICAL CHEMISTRY C

AMER CHEMICAL SOC

Lugar: Washington; Año: 2018 vol. 122 p. 3638 - 3647

1932-7447

The diffusion of alkaline chlorides (LiCl, KCl, and CsCl)and water in mesoporous silica samples with pore sizes covering the rangefrom micropores (2 nm) up to mesopores larger than 30 nm have beenmeasured by resorting to a simple diffusional technique in the case ofelectrolytes and 1H NMR in the case of water. The morphology of thesilica samples varies from a microporous structure, an interconnectednetwork of pores, and typical mesoporous materials with ink-bottle pores,with increasing pore size. The release of electrolytes from the silica as afunction of time exhibits two differentiated regimes, at short and longtimes, which correlates quite well with the size of the pores and that ofnecks of the pores, respectively. The diffusion of water inside the poresfollows the same trend with pore size that the diffusion of electrolytes,indicating a coupling between the ions and water diffusional mobilities.The tortuosity effect on the diffusion of all studied electrolytes and water shows a monotonic slight increase with decreasingdiameter for pores larger than 5 nm, while the tortuosity factor increases markedly for smaller pores. In microporous andmesoporous silica with pore sizes below 10 nm, the tortuosity factor of Li+ ion is much larger than those for K+ and Cs+ ions,since its diffusion is hindered by a stronger electrostatic interaction with the ionizable silanol groups on the pore wall; and alsolarger than that for water diffusion which it is retarded by a weaker hydrogen bond interaction with the silanol groups. Thedifferences in tortuosity factors among alkaline chlorides and water become negligible for pore sizes larger than 10 nm. Thespin−lattice relaxation time measurements of 1H-water and Li+ ions confirm this behavior.