A study on optimum surfactant to multiwalled carbon nanotube ratio in alcoholic stable suspensions via UV-Vis absorption spectroscopy and zeta potential analysis

Abstract

Common ultrasonication and centrifugation techniques could be utilized for dispersion of carbon nanotubes in solvents. However, there are few studies investigating the effect of concentration of species on stability of surfactant modified suspensions focusing on specific dispersants. Therefore, in this study, UV-Vis absorption spectroscopy, sedimentations tests, zeta (zeta) potential and dynamic light scattering analyses (DLS) were used as tools to estimate the optimal surfactant to multi walled carbon nanotubes (MWCNTs) weight ratio in stable centrifuged suspensions. MWCNTs colloids were prepared via addition of sodium dodecylbenzene sulfonate (SDBS) with ultrasonication and centrifugation in isopropyl alcohol (IPA). Coupling information gathered from overlapped absorption spectra and weighed dried sediments enabled estimation of MWCNTs concentrations in centrifuged suspensions (supernatant). By utilizing sediments weight and zeta potential observations, optimum SDBS/MWCNTs weight ratios were calculated. The results have shown that the SDBS/MWCNTs ratio should be 9 or higher in supernatants in order to obtain stable suspensions. It was also observed that increasing SDBS/MWCNTs concentrations over 9 in supernatants gradually decreased the zeta potentials of studied suspensions. Higher SDBS/MWCNTs ratios, also decreased settling velocities that were attributed to increased viscosity of supernatants. In addition to overlapping spectra technique, which relies on utilization of spectra related to centrifuged and uncentrifuged suspensions only for quantitative analysis, differential UV-Vis spectra were also utilized for the assessment of SDBS/MWCNTs ratios in supernatants. Presence of SDBS in supernatants (prepared by removing contribution of MWCNTs by using COOH modified MWCNTs' UV-Vis absorption spectrum) exhibited an unexpected hyperchromic shift in maximum wavelength in spectra leading to misleading estimations of surfactant concentrations.