title: Diffusion of dissolved ions from wet silica sol-gel monoliths: implications for biological encapsulation.
abstract: Divalent nickel (Ni(2+)), Cu(II)EDTA, methyl orange, and dichromate were used to investigate diffusion from hydrated silica sol-gel monoliths. The objective was to examine diffusion of compounds on a size regime relevant to supporting biological components encapsulated within silica gel prepared in a biologically compatible process space with no post-gelation treatments. With an initial sample set, gels prepared from tetraethoxysilane were explored in a factorial design with Ni(2+) as the tracer, varying water content during hydrolysis, acid catalyst present during hydrolysis, and the final concentration of silica. A second sample set explored diffusion of all four tracers in gels prepared with aqueous silica precursors and a variety of organically modified siloxanes. Excluding six outliers which displayed significant syneresis, the mean diffusion constant (D(gel)) across the entire process space of sample set 1 was 2.42×10(-10) m(2) s(-1); approximately 24% of the diffusion coefficient of Ni(2+) in unconfined aqueous solution. In sample set 2, the tracer size and not gel hydrophobicity was the primary determinant of changes in diffusion rates. A strong linear inverse correlation was found between tracer size and the magnitude of D(gel). Based on correlation with the tracers used in this investigation, the characteristic 1-h diffusion distance for carbonate species relevant to supporting active phototrophic organisms was approximately 1.5mm. These results support the notion that silica sol-gel formulations may be optimized for a given biological entity of interest with manageable impact to the diffusion of small ions and molecules.
authors.affiliation: College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Building, Corvallis, OR 97331, USA.
authors.forename: David J
authors.initials: DJ
authors.lastname: Dickson
authors.forename: Bethany
authors.initials: B
authors.lastname: Lassetter
authors.forename: Benjamin
authors.initials: B
authors.lastname: Glassy
authors.forename: Catherine J
authors.initials: CJ
authors.lastname: Page
authors.forename: Alexandre F T
authors.initials: AF
authors.lastname: Yokochi
authors.forename: Roger L
authors.initials: RL
authors.lastname: Ely
mesh_terms: D004058:Diffusion; D005782:Gels / Q000737:chemistry*; D012821:Silanes / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry*; D012833:Siloxanes / Q000737:chemistry
chemical_list: D005782:Gels; D012821:Silanes; D012833:Siloxanes; C040733:tetraethoxysilane; D012822:Silicon Dioxide