Colloidal properties of aqueous fullerenes: isoelectric points and aggregation kinetics of C60 and C60 derivatives.

Aqueous colloidal suspensions of C60 (aqu/C60) and the C60 derivatives PCBM ([6,6]-phenyl C61-butyric acid methyl ester) and the corresponding butyl and octyl esters, PCBB and PCBO (aqu/PCB-R, where R is an alkyl group), were produced by stirring in double deionized water for 5 months. Kinetically stable fullerene aggregates were formed using this procedure that ranged in intensity-averaged hydrodynamic diameter (Dh) from 193 +/- 2 nm (95% C.L.) for aqu/C60 to 259 +/- 6 nm for aqu/PCBO. Measured zeta potentials (zeta) were < -50 mV, and the isoelectric points (p) were < 1.0 for all of the aqu/fullerenes. Time-resolved dynamic light scattering (TRDLS) was used to measure aqu/fullerene Dh change with time and as a function of background solution ionic strength. The critical coagulation concentration (CCC) values for the aqu/PCB-R fullerenes were significantly higher than that of aqu/C60, indicating that the phenyl alkyl ester moieties of the equ/PCB-R fullerenes were impeding the aggregation process.