Chemically-Tailored Surfaces of Silica Gel and Alumina Examined Using Color

Freshman students engaged in a three-session laboratory project attached functional group moieties bearing established chemical scavengers onto the surfaces of porous silica gels and alumina and assayed the tailored materials for their ability to remove specific agents out of protic solutions. The students deposited organosilanols featuring amine-type scavenger groups onto the starting materials, affording, in particular, surfaces tailored with (3-aminopropyl)silyl and {3-[2-(2-aminoethylamino)ethylamino]propyl}silyl moieties. The modified surfaces were succinylated thereafter, converting amino groups into succinamic acid derivatives and affording a second generation of scavengers. Chromogenic ninhydrin analyses confirmed the surface-pendant amino groups by a positive color-yield and the surface-pendant succinylated amino groups by a greatly reduced or negative color-yield. The students easily studied the interaction between engineered surfaces and solutions containing model acids, aldehydes, bases, and transition metals by virtue that each compound examined was inherently colored or color-yielding. The binding of glutaraldehyde was recognized by the development of a fixed orange color, while cupric ion was noted by various shades of blue. Surface-immobilized morpholine was noted by the development of a pink solution. Bromocresol green produced a dark blue surface, and potassium permanganate gave a rich violet color. In all cases, native silica gel remained colorless after similar treatments, implying that the surface properties of modified silica gel were markedly altered through rational design. The students concluded their project with a report, having experienced the merits of surface engineering and color-facilitated analyses.