Radioactive elements (Ra and RaD) were used by V. G. Khlopin and B. A. Nikitin as indicators in determining the nature of a new kind of mixed Gramm crystals. These studies showed a fundamental difference between true mixed crystals in the spirit of Eilhard Mitscherlich, when the substitution of one component for another is expressed in the form of ion for ion, or atom for atom, molecule for molecule, and mixed crystals of a new kind, in which such a simple substitution is impossible, and proceeds by means of very small sizes of the ready sections of the crystal lattice of each component. Scientists have shown that mixed crystals of a new kind fundamentally differ from true mixed crystals by the presence of a low miscibility limit - they are not formed at low concentration of one of the components at all. In this case, they are similar to anomalous mixed crystals (as shown experimentally by V. G. Khlopin and M. A. Tolstaya), and relate to the latter approximately as a colloidal solution with suspension. These works (on the structure and properties of mixed crystals of a new kind and anomalous mixed crystals) led V. G. Khlopin to the idea of the need to classify isomorphic bodies not by considering the structure of isomorphic mixtures in static equilibrium (as it was done, for example, by V. G. Goldschmidt and his school), but according to the methods of substitution of components - taking into account the dynamics of the formation of an isomorphic mixture. In this case, all isomorphic bodies are strictly divided into two groups according to the method of substitution:
(a) Isomorphic compounds in the spirit of E. Mitscherlich, truly isomorphic. Substitution in the formation of mixed crystals by such compounds occurs according to the first principle: ion on ion, etc. The above distribution laws apply to such crystals. Such compounds have similar chemical composition and molecular structure.
(b) All other isomorphic compounds, when the formation of mixed crystals is conditioned by the second principle: substitution of sites from the unit cell or close to them (mixed crystals of a new kind or isomorphic of the 2nd kind according to W.G. Goldschmidt), up to microscopic - anomalous mixed crystals such as FeCl — NHCl, Ba(NO), Pb(NO), methylene blue KSO - Ponsorot, etc., showing heterogeneity).
3.Thanks to the works discussed in the previous two paragraphs, V. G. Khlopin was able to present in a new form the law of E. Mitscherlich, which makes it possible to judge the composition and molecular structure of unknown compounds on the basis of their formation of isomorphous mixtures with compounds whose composition and molecular structure are known. V.G. Khlopin proposed the method of isomorphous co-crystallization from solutions for fixation of weightless and unstable chemical compounds and determination of their composition. The method made it possible to discover and determine the composition of individual compounds of divalent and hexavalent polonium (V. G. Khlopin and A. G. Samartseva).
4. Studying adsorption of isomorphous ions on the surface of crystalline precipitates, V. G. Khlopin showed that adsorption equilibrium is established in 20–30 minutes; adsorption of isomorphous ions does not depend on the charge of the adsorber surface when its solubility does not change. Correctly reproducible results of adsorption study and full reversibility of this process are achieved only if the adsorber surface remains unchanged throughout the experiment - if the adsorber solubility remains unchanged; in case of changes in the liquid phase composition or under other additional conditions, when the adsorber solubility changes, adsorption acquires a more complex character, which is accompanied by co-crystallization distorting the results. Studying the adsorption kinetics, a similar phenomenon was encountered by L. Imre. V. G. Khlopin gave a formula for determining the surface of crystalline precipitates by adsorption of an isomorphic ion on them and experimentally confirmed its applicability (V. G. Khlopin, M. S. Merkulova).