ИСТИНА

Characteristics of detonation nanodiamonds strongly depend on the surface state of the particles, which is determined by the method of synthesis and the subsequent treatment. When the reaction chamber is filled with inert gas (the so-called dry synthesis), the surface relaxation of particles formed is chiefly driven by C–C interactions. When the explosion is carried out in the presence of either water or ice (wet synthesis, which is more efficient judging from the diamond fraction in the primary particles), water molecules can be involved in the surface formation. An important aspect related to the applications of nanodiamonds is the formation of their stable aqueous suspensions, i.e., disaggregation of particles the sizes of which may be hundreds or even thousands nanometers. There are diverse methods of disaggregation, some of which (bead-assisted sonication or laser heating) just destroy non-diamond interlayers and covering shells and, in this way, initiate the reorganization of fresh surfaces rich in unsaturated carbon atoms that have lost their close neighbors. Since the process usually takes place in water, the solvent molecules should inevitably be involved in the surface reconstruction. Thus, both at the stage of primary formation in the course of wet synthesis and at the stage of subsequent disintegration of nanodiamond aggregates, water molecules are expected to play an important role and determine to a certain degree the state of nanodiamond particles, namely their surface modification with O- and/or H-containing functional groups. To clarify the situation, nonempirical calculations of Cn(H2O)m systems (n = 20–110 and m=4–35) were carried out. Two possible variants of the water involvement in the structure relaxation process were considered. One variant implies that water molecules can directly participate in the process at the stage of the primary formation of carbon particle surfaces. Another variant is the interaction of fully relaxed carbon subsystem, the surface of which is rich in diverse unsaturated or variously coordinated carbon atoms, with water molecules, whose initial arrangement is arbitrary, which means that it is not necessarily H-bonded water clusters, but rather independent water molecules. Geometry optimization was carried out at the density functional level with a hybrid exchange-correlation functional of B3LYP type and 6-31G Gaussian basis set augmented with polarization and in some cases diffuse functions on the nuclei, which is known to provide accurate description of the electron density redistribution at the breakage and formation of intramolecular OH bonds and intermolecular hydrogen bonds. More accurate energy estimates were obtained in the second order of the Moeller-Plesset perturbation theory. Calculations were carried out with the use of PC GAMESS and Firefly 8.0 program packages on Chebyshev supercomputer of the Supercomputing Center of Lomonosov Moscow State University. It was found that chemical interaction between active surface sites of carbon particles and water molecules becomes possible only when a certain threshold amount of molecules is present close to the surface and covers a surface part that involves at least two active sites. The probability of the dissociation of a water molecule that leads to the formation of either neighboring or non-neighboring C–OH and C–H surface groups depends on the type of surface sites and their arrangement. Under certain conditions, carboxyl groups can appear on the surface, and surrounding water cluster may become protonated. The discovered general convexo-concave character of the water hydration shells over surface segments of carbon nanoparticles is found to be determined by the hydrophobic-hydrophilic balance of the surface sites and related to the properties of hydrogen-bonded water molecules, particularly the strength of hydrogen bonds between them. Calculated vibrational spectra of functionalized carbon particles enabled us to supplement the conventional interpretation of the corresponding experimental spectra.