Comparison of traditional and fullerene-based adsorbents for extraction of 1,4-dioxane and 2-methyl-1,3-dioxolane from milk адсорбентов и адсорбентов для детектирования 1,4-диоксана в молоке

Using density functional methods, the results of the analysis of traditional adsorbents and adsorbents based on nanosized particles capable of trapping 1,4-dioxane and 2-methyl-1,3-dioxolane molecules in milk are presented. We considered the following interacting compounds: 1,4-dioxane — primary amine, 1,4-dioxane — secondary amine, 1,4-dioxane — fullerene C 20 , 1,4-dioxane — a fragment of the structure of activated carbon, 2-methyl-1,3-dioxolane — primary amine, 2-methyl-1,3-dioxolane — secondary amine, 2-methyl-1,3-dioxolane — fullerene C 20 , 2-methyl-1,3-dioxolane — a fragment of the structure of activated carbon. We determined the optimal configurations of the corresponding interacting structures, estimated their binding energies and chemical potentials. The highest binding energy was obtained for 1,4-dioxane adsorbed on C 20 fullerene. At the same time, the energy gaps between the occupied HOMO and unoccupied LUMO molecular states were calculated, which makes it possible to characterize the reactivity and stability of molecules. Compounds of 1,4-dioxane and 2-methyl-1,3-dioxolane with amines have rather large gaps HOMO-LUMO. Using the concept of the electronic localization function, we found that a covalent bond is formed between 1,4-dioxane and C 20 fullerene with a sufficiently high degree of electron localization in the bond region. In other cases, the value of the localization function indicates the absence of a chemical bond between the compounds. The proposed study gives recommendations on the adsorption of 1,4-dioxane and 2-methyl-1,3-dioxolane for further solid-phase microextraction, which will allow them to be found in milk by gas chromatography using a flame ionization detector.


Introduction
The planetary deterioration of the ecological situation is inextricably linked with the deterioration of the quality of food, the safety of which is the key to preserving human health. It is especially important to preserve the naturalness and purity of baby food as a guarantee of the preservation and transmission of the "correct" genetic information. One of the environmental pollutants is 1,4-dioxane, the chemical formula of which is С 4 Н 8 О 2 . It is widely used in industry as a strong solvent in the production of adhesives, textiles, papers, lacquers, pharmaceuticals and other chemicals, but at the same time, it is a toxic substance, a potential carcinogen that can damage the central nervous system, liver, and kidneys [1]. 1,4-dioxane is found in drinking water and the ambient air. In addition, it is known that in the context of preserving children's health, 1,4-dioxane can be transmitted to infants during lactation. According to pharmacokinetic models [2], it was shown that 1,4-dioxane, along with carbon tetrachloride, methylchloroform, perchloroethylene, has the property of easily binds to milk, which is inhaled from the ambient air to nursing mothers. In this regard, the problem of determining foreign chemical inclusions and maintaining the chemical purity of breast milk is relevant and promising. The detection of 1,4-dioxane is an independent task and can be considered in conjunction with the detection of 2-methyl-1,3-dioxolane since the latter is the most frequent derivative of 1,4-dioxane, often obtained as a result of the decomposition of 1,4-dioxane and indicates its presence. Finding two substances at once in breast milk and milk products for children will more accurately characterize its purity. It is important to be able to detect these two substances simultaneously.
Due to the high solubility of 1,4-dioxane in water, its detection seems to be a serious problem [3]. There are various physical and chemical methods of analysis. Standard analytical methods based on closed-loop stripping analysis [4] are not suitable to determine 1,4-dioxane owing to taste and odor are almost absent [3]. The physicochemical method for separating substances, based on the distribution of the components of the analysed mixture between two immiscible phases moving relative to each other (the gas chromatography method), can be very reliable in this case. 1,4-dioxane, which has the properties of volatility and thermal stability, meets the requirements of this method. The following detectors can be used to continuously measure the concentration of substances at the outlet of the chromatographic column: thermal conductivity detector, electron capture detector, mass spectrometry, and others. A detector for the determination of 1,4-dioxane must simultaneously combine the properties of versatility, non-destructive measurements, and high sensitivity to organic compounds. The Flame ionization detector reacts to all compounds that can be ionized by a flame and are a dedicated organic compound detection tool. To apply this method, a preliminary concentration of dioxane and 2-methyl-1,3-dioxolane is required.
To pre-concentrate the dioxane and dioxolane, one of three methods must be used: liquid-liquid extraction (LLE), purge and trap (PT), and solid-phase extraction [5]. The PT pre-concentration method requires extended purge time or additional heat. The reported limit for the PT method, observed in [3], was 150 ng / L. The detection limit in the LLE method was 200 ng / L [6]. The separation of solid-phase mixtures using solid sorbents in the SPE method allows one to increase the dioxane concentration to 1 µg / L [7 -9]. A variation of the SPE method is the solid-phase microextraction (SPME) with involves the use of a fibre coated with an extracting phase. The SPME approach can be effectively used in combination with gas chromatography due to improvement in time-saving, economic factors, and detection limits [10].
The stationary phase in the gas chromatography method is solid particles, namely adsorbents with a high specific surface area (10 -1000 m 2 / g). The search for such adsorbents by the methods of quantum chemistry is the subject of this article, i. e., we hope to obtain a comparative picture of the degree of adsorption of dioxane and methyl-1,3-dioxolane on known materials and, based on the obtained data, to give appropriate recommendations for the selection of adsorbents based on quantum chemistry. We will take activated carbon, amines, fullerenes, and other carbon nanoparticles as the analysed adsorbents. Activated carbon as a well-known sorbent is used in the field of pharmacology, in the food industry, in the military, in devices for purifying drinking water [11,12], and in many other areas of life. The effectiveness of primary and secondary amines as adsorbents was discussed in [13], where, in particular, it was shown that the processes of adsorption and desorption for the primary, secondary, and tertiary amines proceed in opposite directions. The study of hybrid materials based on mixed amines and oxides for the capture and separation of CO 2 from bio-hydrogen was carried out in [14]. Further, fullerenes are stable, biocompatible, and highly reproducible nanoparticles. It was shown in [15,16] that fullerenes, fullerene-, graphene-like materials [17], due to their large specific surface area, can chemically bind a large number of particles. Although C 60 and nanotubes are more suitable due to their larger area, we consider C 20 as the minimum fullerene, it is the most curved and therefore the most active. The efficiency of adsorption using functionalized nanoparticles by different groups is also an interesting subject for research.

Computational details
The considered atomic model was driven to energy-optimal configurations using the GAMESS software package [18]. Based on the density functional theory using the set of basis functions B3LYP / 6-311G, the diameter of the molecules, their total energies corresponding to a given local energy minimum, and the bond lengths were calculated. All structures are optimized while a Hellman-Feynman force convergence exceeds 10 −2 eV / Å. Iteration tolerance is 10 −5 . The dispersion interaction was taken into account by the DFT-D2 method of Grimme [19]. The density function theory method is also used to calculate the HOMO-LUMO sites and Electron Localization Function (ELF). The full visualization process is carried in the ChemCraft package. Fig. 1 shows the models for the considered 1,4-dioxane and 2-methyl-1,3-dioxolane, primary amines, secondary amines, fullerene C 20 , and activated carbon. All of them considered molecules were optimized as described above. The dark grey balls denote the carbon atoms, the light grey balls denote the hydrogen atoms, the red balls denote the oxygen atoms, and the blue balls denote the nitrogen atoms.

Results and Discussions
The binding (adsorption) energy of 1,4-dioxane and 2-methyl-1,3-dioxolane molecules on primary and secondary amines, fullerene C 20 , and a fragment of the structure of activated carbon was given by [20] where E tot is the total optimized energy of the complex interacting structures, E mol is the energy of the free, relaxed 1,4-dioxane molecule (or 2-methyl-1,3-dioxolane molecule), and E ads is the same energy one of four adsorbents.
As shown in [21], the density functional theory allows getting a set of physical quantities. One of them is the HOMO-LUMO gap. It is defined as the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital [21]. So, the optimized parameters for the adsorption of the 1,4-dioxane molecule and 2-methyl-1,3-dioxolane molecule on the four adsorption species were tabulated in Table 1.
As shown in Table 1, the largest bonding was found for 1,4-dioxane -fullerene C 20 because the E b = −0.435 eV. At the same time, 2-methyl-1,3-dioxolane and secondary amine are weakly bonded. 1,4-dioxane and 2-methyl-1,3-dioxolane have a general tendency when adsorbed on the considered materials, namely, the binding energies of these molecules with "pure" carbon materials (fullerene C 20 and activated carbon) are higher than with amines. In general, obtained values are close to the adsorption energy of oxygen molecules physisorbed on two-dimensional tungsten disulfide [22]. As to the HOMO-LUMO gap, the compounds of 1,4-dioxane and 2-methyl-1,3-dioxolane molecules with amines have a large HOMO-LUMO gap. The HOMO-LUMO gap is reduced as one moves to other adsorbents.
To understand the bonding character of the considered interacting structures, we calculated the ELF as [23] ELF where D is the kinetic energy density of the considered system and D 0 is the kinetic energy density of a homogeneous electron gas. If ELF takes values close to 0, then such space regions contain not localized electrons and, conversely, if the value is 1, high localization of electrons can be observed. [23]. Fig. 2 shows the results of isosurfaces of ELF for all interacting structures.
As shown in Fig. 2 c, the high electron localization is reached between 1,4-dioxane and fullerene C 20 . In remain cases, ELF is in the range from 0.07 to 0.15, indicating the absence of covalent bonding between them. These captures are obtained for the same levels of ELF. On the contrary, Fig. 3 shows the results of maps of ELF for all interacting structures when one sees different values of ELFs. These images are designed to show the shortest distance between two molecules. The figure clearly shows the areas of covalent bonding inside the molecules depicted in red colour. At the same time, intermolecular regions considered structures (except the 1,4-dioxane -fullerene C 20 ) are marked in blue and green.
2-methyl-1,3-dioxolane. This information will help to carry out solid-phase microextraction and find them in milk using gas chromatography with a flame ionization detector method.