Unexpected Colloidal Stability of Fullerenes in Dimethyl Sulfoxide and Related Systems

Abstract

It is known that fullerenes are poorly soluble in polar solvents, but readily form colloidal solutions in such media. These solutions are typically solvophobic (hydrophobic when prepared in water), that is, thermodynamically unstable colloidal systems with negatively charged particles. To understand the stability factors of a colloidal system, the thresholds for coagulation of a sol or suspension by electrolytes are of key importance. While hydrosols and aqueous suspensions coagulate at concentrations of 1:1 inorganic electrolytes about 0.1− 0.2 M, in acetonitrile and methanol, the corresponding critical concentrations of coagulation are ca. 3 orders of magnitude lower. Given the wide variety of properties of organic solvents, it seemed important to complete the picture to study solvents with more basic properties. This is all the more reasonable since electrophilic fullerenes are in fact Lewis acids. Our choice was dimethyl sulfoxide, DMSO, and related solvent systems. The colloidal solutions of fullerenes C 60 and C 70 in DMSO and N,N-dimethyl formamide, DMF, are unexpectedly easy to prepare by mechanical methods, and addition of water leads to formation of relatively stable organo-hydrosols. UV−visible spectra and dynamic light scattering were used to characterize the solutions of C 60 and C 70 in DMSO, benzene−DMSO, acetonitrile−DMSO, and benzene−acetonitrile−DMSO systems, as well as in DMF. Our present study demonstrated that, in contrast to organosols in methanol and acetonitrile, colloids of C 70 and C 60 fullerenes in DMSO and DMF are surprisingly as stable with respect to electrolytes as the corresponding hydrosols are. Such high stability is caused by the non-DLVO interactions, or, in terms proposed by Churaev and Derjaguin, by the so-called structural effect. These results shed light on the nature of the solvation of colloidal fullerene particles in solvents of various chemical natures.

Stability of Aqueous Suspensions of COOH-Decorated Carbon Nanotubes to Organic Solvents, Esterification, and Decarboxylation

“…Comparison with the CCC values of C 60 and C 70 fullerenes in water, methanol, acetonitrile, DMF, and DMSO , reveals a striking similarity with the trends described in the present report. However, the current case is more complicated because of the presence of COOH groups on SWCNT surface.…”

“…For example, acidic admixtures appear in DMSO during sonication. 14 Therefore, only previously prepared initial aqueous suspensions are diluted with purified organic solvents. The above-mentioned ζ values obtained in our study may reflect various processes, including the dissociation of the carboxylic group.…”

“…The results obtained now (Table 1) are in line with the solvent influence on the CCCs for electrolytes in C 60 and C 70 colloids in acetonitrile and methanol, on the one hand, and in DMSO and DMF, on the other. 13,14 Fullerenes are Lewis acids, and their interaction with proto-and cationophilic solvents can be expected. The CCCs in these solvents are even higher than in water.…”

“…Our recent studies on C 60 and C 70 fullerenes revealed that comparison of the CCCs determined in water and in solvents of different nature allows one to shed light on the intrinsic properties of colloids. 13,14 Considering similar features of CNTs and fullerenes, we expect that corresponding study of SWCNTs can provide important insights.…”

“…Sonication was performed in both cases, and positive ζ values were observed only in the case of Cl-containing solvents. , We decided not to use the sonication in entire organic solvents because of possible unexpected side effects. For example, acidic admixtures appear in DMSO during sonication . Therefore, only previously prepared initial aqueous suspensions are diluted with purified organic solvents.…”