Stationary production began of 2.6nm mono-disperse diamond particles

We reported on the determination of the average diameters of detonation nanodiamond as 2.6°ř0.5nm in the News column of the January 2018 issue in our Home Page. However, soon after that we had difficulties reproducing this size for considerable length of time. More particularly, we often ended up with particles having average diameters of 4~5nm, about twice as large compared to the true size. Therefore, we suspected that the 2.6nm particles might have been formed once when milling is completed, but soon aggregate to form dimers in the course of post-processing for some unknown reason. We thereupon checked all the possible causes of dimerization one by one, and finally found °∆dilution with water°« responsible.
We used to wash inside of circulation path after each milling operation and noted the milling space containing agitator and bead retained considerable amounts of ND particles after collecting the product solution to give black color to the washings. Hence, we used to combine all the washing water with the collected colloidal solution of DND, and if necessary removed excess water by using rotary evaporator. Furthermore, after many steps of further purification, concentration of the product solution was again adjusted to 2.5% by diluting with water to meet the commercial norm. Thus, we have twice repeated the hazardous dilution with water during the final stage of dispersion.
We had already found that dispersed nanodiamond particles aggregated to small extents upon diluting with water, not only in our solutions but also in those from other vendors. We recognized the °∆dilution aggregation°« a general phenomenon among nanoparticles, but never paid much attention due to its minor effect. However, it is now clear that once the particle sizes are reduced to below 3nm, the dilution aggregation appears more significantly, perhaps because of the increased surface.
Upon rigorous exclusion of dilution with water after milling, undesirable aggregation disappeared and 2.6nm particles became consistently obtained. As the concentration of agglutinate in the initial slurry before disintegration is kept at 4.0% at the moment,°°the concentration of dispersed nanodiamond is also 4.0%. For the moment we will sell only the 4% solution of 2.6nm diamond particles. Then a new problem arises on how to prepare solutions with concentrations less than 4%. We will shortly handle this problem and report as soon as it is possible to sell other concentrations.
The discovery of dilution aggregation poses more challenging target to explain. It is likely that this phenomenon represents reverse of the well-known phenomenon in stable colloidal solutions, °∆concentration dispersion.°« Namely, the higher the concentration, the more stable will be the dispersion. This phenomenon can be understood readily by invoking colloidal crystal. These interesting phenomena seem more important in nanotechnology than in chemistry, where the opposite rules prevail.