Determination of monolayer density by potentiometric titration

Potentiometric titration of acidic groups originating from the functionalization of nanosized maghemite particles by a diacidic bolaamphiphile (DBSC-10) is described. The titrations are achieved in the solvent dimethyl-formamide (DMF), using 0.01 N potassium hydroxide (KOH) in water as titrant. A reproducible packing density of 3.4 mu mol of DBSC-10/m(2) of maghemite is found. This value falls between the approximated values estimated from molecular dimensions for vertically and horizontally oriented surfactant molecules, indicating that DBSC-10 adopts a tilted-vertical orientation with the maghemite surface. Titration does not affect the. stability of the monolayer at higher pHs, as confirmed by infrared measurements on titrated particles. New strong peaks do appear at 1603 cm(-1) (v(COO- asymmetrical stretch)) and 1348 cm(-1) (v(COO- symmetrical stretch)), indicating the formation of ionic complexes between the free carboxylic moieties of the monolayer and K+ ions from titrant.