RESUMEN
The multispectral modality and technique for optically dense samples of optoacoustic spectroscopy were applied to measure spectra and high absorbances of concentrated aqueous dispersions of undoped nanodiamonds. The data from optoacoustic and optical transmission measurements and DSC data of the mean particle size by the Gibbs-Kelvin equation are compared to estimate the difference in composition of various nanodiamond trademarks. Optoacoustic spectra confirm the contribution of surface dimer chains into the absorption of nanodiamonds in the long wavelength range. Optoacoustic and conventional absorption spectra of aqueous solutions of nanodiamond fractions after centrifugation (15300g) and ultracentrifugation (130000g) revealed a separation of a highly absorbing non-diamond sp2 phase. The two-step separation by ultracentrifugation followed by extra centrifugation made it possible to isolate a highly absorbing and soluble nanodiamond phase with the particle size of 3.6 nm, showing a change in spectra compared to the starting nanodiamond material.
RESUMEN
The optoacoustic (OA) technique capable for light-absorption measurements in highly concentrated solutions is applied for direct solubility assessment in highly absorbing saturated solutions without any dilution or sample preparation. Maximum solubility of Phenol Red in aqueous solutions at pH>11 is found to be 1.3±0.1 g/L. The experimental dependence of solubility of Phenol Red on solution acidity was obtained with optoacoustic and conventional spectrophotometric measurements. The data from both methods show complementary results. The model of the solubility of Phenol Red on solution acidity describing the experimental dependence is proposed, which cannot be attained at pHs above 7 without the OA-technique.
