Nikolay O. Mchedlov-Petrossyan

This article presents the results of a study of the colloidal properties of multiwalled carbon nanotubes (MWCNTs) in water and two organic solvents. This work continues the systematic study of the aggregation stability and coagulation of carbon nanoparticles in various solvents with the aim of identifying the contribution of solvation to the colloidal properties of these widely used systems. The samples were characterized by EDS and XPS methods. The suspension was prepared using ultrasonic treatment in N-methyl-pyrrolidin-2-one and diluted 100-fold with water, acetonitrile, or dimethyl sulfoxide. The working concentration of MWCNTs carbon nanotubes was 5×10–4 g L–1. The size of negatively charged particles in water, 95 vol% aqueous CH3CN, and 95 vol% DMSO was determined by dynamic light scattering: the Zaver values are 249±15, 265±42 and 146±8 nm, respectively, which corresponds to the diameter of the equivalent sphere. TEM images show that the CNT diameter in aqueous suspension is approximately 9–11 nm and the length is approximately 40–170 nm. Critical coagulation concentrations (CCC) of NaCl were determined using the Fuchs function. These values differ significantly in water and 95% acetonitrile: CCC = 230 and 1.0 mM, respectively. In 95% DMSO, coagulation is less pronounced. The effects were discussed in terms of Gutmann’s Donor Numbers for organic solvents and the specificity of hydration of non-polar materials in water. The CCC of HCl in water is 1.5 mM, which suggests the role of acid-base properties in the formation of the negative charge of the MWCNT colloidal particles.

Colloidal solutions of ionic liquids extend the concepts of surface science. Cationic oligomeric silsesquioxanes formed the silsesquioxane core and cationic organic substituents, namely OSS(C₃N⁺Br⁻)_n and OSS(C₁₀N⁺Br⁻)_n, are amorphous with a glass transition temperature below 0°C and possess prerequisites for the formation of supramolecular aggregates in aqueous solutions. The hydrodynamic size and zeta potential values of colloidal species were determined by dynamic and electrophoretic light scattering. The local surface potential and acidity were estimated by the spectrophotometric study of acid‐base equilibria of bound bromophenol blue. The kinetic effect of these colloidal species in the idea of micellar rate effect emphasizes the importance of the “diverting” effect of cationic head groups introduced recently. The nanospecies morphology, size, and charge densities vary with the length of the alkyl substituent of OSS. This modification opens up potentially broader applications of cationic oligomeric silsesquioxanes, which are synthesized as ionic liquids. The behavior in an aqueous solution determines the future environmental fate of the ionic liquids, which are defined as environment‐friendly compounds, basically as non‐volatile and non‐flammable.

This article reports equilibrium and kinetic data for three fluorescein dyes containing the nitro group in the residue of the phthalic acid, 3'-, 4'-, and 5'-nitrofluorescein, in water and 50 mass % aqueous ethanol. These compounds, particularly the recently synthesized 3'-nitrofluorescein, were less considered polyprotic acids than many other fluorescein dyes. The dyes behave as diprotic acids, which can also be protonated: H3R+⇌ H2R ⇌ HR–⇌ R2–. The dissociation constants in water and 50 mass % aqueous ethanol were obtained in buffer solutions of diluted HCl using the spectrophotometric method accompanied by a potentiometric determination of pH at an ionic strength of 0.05 M and 25.0 ± 1 °C. The dye concentrations in the working solutions were, as a rule, within the range of (5–10)×10–6 M. The thermodynamic values of the dissociation constants were calculated using the Debye–Hückel equation, the second approach, for ionic activity coefficients. The absorption spectra in the visible region of the individual molecular and ionic forms were determined for the above-mentioned compounds and the ester of 3'-nitrofluorescein. For the last, the dissociation constants were determined in aqueous ethanol. Whereas in water the fraction of the zwitterionic tautomer of nitro­fluoresceins increases as compared with the parent compound, in aqueous ethanol, the colorless lactone predominates. The microscopic dissociation constants were determined and their values are in line with the influence of the electrophilic nitro group. The alkaline hydro­lysis of the nitrofluorescein diacetates was investigated in the two aforementioned solvents. The obtained results allowed us to single out the rate constants of the two-step kinetics of the hydrolysis of the diacetyl derivatives of nitrofluoresceins.

This review article is devoted to the colloidal properties of fullerene solutions. According to generally accepted understandings, all solvents in relations to fullerenes are divided into “good”, “poor”, and “reactive”. We have consistently considered the state of fullerenes in these systems. In “good”, predominantly non-polar aromatic solvents and CS₂, non-equilibrium dissolution methods lead to the formation of colloidal aggregates, whereas the utilization of equilibrium methods results in the formation of molecular solutions. The latter, however, have some unusual properties; new results considered in this review confirm previously expressed ideas about colloidal properties of these solutions. In “poor” (polar) solvents, lyophobic colloidal systems appear. Both “bottom-up” and “top-down” methods of preparation are well documented in the literature. However, N-methylpyrrolidine-2-one, DMSO, and DMF dissolve fullerenes quite easily and with less energy consumption. These solvents can be considered a subset of “poor” solvents that have some features of being “reactive” at the expense of basic properties. New data confirm that hydrosols of fullerenes are typical hydrophobic colloids that obey the Schulze–Hardy rule and other regularities in the presence of electrolytes. Organosols in acetonitrile and methanol are much less stable with respect to the effects of electrolytes. This allows us to assume a non-DLVO stabilizing factor in the hydrosols. Accordingly, a new estimate of the Hamaker constant of fullerene–fullerene interaction is proposed. In DMSO and DMF, the coagulation of fullerene sols is hindered due to strong solvation with these basic solvents.

This review article is devoted to the colloidal properties of fullerene solutions. According to generally accepted understanding, all solvents in relations to fullerenes are divided into “good” (or “strong” or “high-solubility” ones), “poor” (usually polar) and “reactive”. Water occupies a special place, close to “poor” solvents. We have consistently considered and discussed the state of fullerenes in these systems. In "good", predominantly non-polar aromatic solvents and CS2, nonequilibrium dissolution methods lead to the formation of colloidal aggregates, whereas utilization of equilibrium methods results in the formation of molecular solutions. The latter, however, have some unusual properties; new results confirm previously expressed ideas about colloidal properties of these solutions. Both experimental and theoretical methods devoted to such systems are considered. In “poor” (polar) solvents, typical lyophobic colloidal systems appear. Both “bottom-up” and “top-down” methods of preparation are now well documented in the literature. However, some of these polar solvents dissolve fullerenes rather easily and with less energy consumption. In addition to N-methylpyrrolidin-2-one, which has been well studied for fullerenes, DMSO and DMF have now been described. These solvents can be considered a subset of poor solvents that have some features of being “reactive” at the expense of basic (“cationophilic”) properties. Room temperature ionic liquids are also used to dissolve fullerenes, but there is little experimental research. Hydrosols and aqueous suspensions of fullerenes are typical hydrophobic colloids that obey the Schulze–Hardy rule and other regularities in the presence of electrolytes. New experimental data confirm the previously obtained results. Organosols in acetonitrile and methanol are much less stable with respect to electrolytes; the difference in critical coagulation concentrations of electrolytes reaches two to three orders of magnitude. This allows us to conclude that there is an additional, so-called non-DLVO, stabilizing factor in the hydrosols. Accordingly, the value of the Hamaker constant of fullerene-fullerene interaction may be higher than that derived from coagulation data in water. Contrary to the mentioned “poor” polar solvents, the stability of fullerene sols in DMSO is even higher than in water. These results, as well as data obtained with DMF demonstrate strong solvation of fullerenes with basic solvents.