Interaction of aqueous suspensions of single-walled oxidized carbon nanotubes with inorganic and organic electrolytes

Abstract

This article reports the colloidal behavior of the single-walled carbon nanotubes, SWCNT, decorated by COOH groups, in water. Energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) methods were used to characterize the solid samples. The SWCNT suspension was characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM) methods, as well as taking into account the dependence of the electrokinetic potential ς on pH. The negatively charged particles with a diameter of an equivalent sphere about ≈200 nm possess a ς value of –48 mV at pH 5.7 ± 0.1. Deceleration of the reaction of cationic and anionic triphenylmethine dyes (methyl violet and nitrophenol violet, respectively) with hydroxide ion in the presence of the SWCNT was observed. This effect can be associated with a lower local concentration of hydroxide ion compared to the bulk phase owing to a substantially interfacial concentration of the COO– groups.Critical coagulation concentrations, CCC, in electrolytic solutions were determined using the rate of increase in size. Coagulation with different inorganic electrolytes demonstrated that the system under study obey the Schulze–Hardy rule. Comparison with the literature data allowed confirming that the CCC for NaCl can be an effective criterion of oxidation degree of the carbon materials of this type. The coagulating influence of surface-active organic cations, including dyes, is much expressed. Besides the overcharging of the negatively charged colloidal species by cetyltrimethylammonium bromide, the same effect was revealed in the case of the cationic dyes neutral red and methyl violet, and even for the Ca2+ and La3+ cations.Methylation of the carboxylic groups, COOH → COOCH3, of the SWCNT during 380 h, as well as heating up to 1000 °C results in a substantial and sometimes opposite changes in the CCC values for different electrolytes. Along with decreasing in the CCC for NaCl, an increase in the corresponding values for multi-charged cations was observed. This reveals the important role of the simultaneous interactions of multi-charged cations with several COOH (or COO–) groups in the case of the initial highly oxidized SWCNT, because this effect became less possible after methylation or removal of the carboxylic groups via heating. A substantial negative ς-potential in aqueous media of the colloidal particles of preheated, i.e, devoid of carboxylic groups SWCNT, allows expecting an alternative charging mechanism, characteristic of oil drops and gas bubbles.