AMINOFUNCTIONAL SORBENTS FOR THIN LAYER CHROMATOGRAPHY

ABSTRACT

The obtaining of sorbents was developed one-stage technological approach of preparation of dispersed silica-polysaccharide materials using sol-gel process in situ. Based on hydrolytic polycondensation of tetraethoxysilane at presence an ionogen polysaccharide chitosan resulting to formation of the microspherical porous material with selective properties caused by specificity of structure as silica and chitosan and allow to using it as an adsorbent for thin-layer chromatography.

The physico-chemical and chromatographic properties of the synthesized sorbents were studied.

АННОТАЦИЯ

Для получения сорбентов был разработан одностадийный технологический подход приготовления дисперсных кремнеземно-полисахаридных материалов с использованием золь-гель процесса in situ. На основе гидролитической поликонденсации тетраэтоксисилана в присутствии ионогенного полисахарида хитозана образуется микросферический пористый материал с селективными свойствами, обусловленными специфичностью строения, как кремнезема, так и хитозана, что позволяет использовать его в качестве адсорбента для тонкослойной хроматографии. Исследованы физико-химические и хроматографические свойства синтезированных сорбентов.

Keywords: polysaccharide chitosan, sol-gel process, silica gel, sorbent, chromatography.

Ключевые слова: полисахарид хитозан, золь-гель процесс, силикагель, сорбент, хроматография.

Introduction

The accession of natural polysaccharides with silica is an effective strategy that can be applied to new hybrid structures in the various fields such as material sciences and environmental sciences. Particularly, adsorbents were obtained by the sol – gel method exhibit synergistic properties of the starting components and have a good thermochemical, rheological and optical properties, which is important for the formation of various products from them in chemical, optical (films, membranes, powders), petrochemical, and pharmacology, perfume industries [1-3].

Sorbents based on tetraalkoxysilanes TROS ((R = ethoxy, n-propoxy, or n-butoxy) and biopolymers (chitosan, fibroin, pectin, etc.) are of interest as biohybrid materials as a stationary phase for chromatography. Known methods for producing sorption materials based on chitosan [4]. Studies in the synthesis of selective chitosan-silica sorbents for high-performance liquid (HPLC), high-performance thin-layer chromatography (HPTLC) and thin-layer chromatography (TLC) are very relevant and promising, attracting the attention of specialists.

TLC has been known for over 100 years and is one of the most inexpensive chromatographic procedures used to detect and quantify various classes of substances. It provides simultaneous and parallel separation of several samples. Our group in [5] prepared a chitosilicagel, i.e. silica gel modified with chitosan by sol-gel technology, and studied the interaction of chitosan with polyethoxysilane PEOS. This sorbent modified by chitosan physical adsorption, was tested by TLC of alkaloids, using chloroform : methanol mixture as a mobile phase.

This investigation describes the synthesis of the chitosan-silica for their use as sorbent for thin-layer chromatography. The chitosan-silica microspherical sorbents using TEOS obtained as a partially hydrolyzed TEOS as a silica precursor. The chromatographic properties of the obtained sorbent samples were evaluated by the thin layer chromatography method in the analysis of a test mixture of nitroaniline isomers.

Experimental procedure

A chitosan sample was obtained by the Muzarelli method [6] from the domestic pupa of the silkworm Bombyx mori. The degree of deacetylation of the sample 82% was determined by the analytical method and the average molecular weight of 148000 by the viscometry method using 2% acetic acid (CH₃COOH) as a solvent.

Microspherical silica gel (I) was obtained from polyethoxysiloxane (PEOS) oligomer by the sol-gel method described in [7]. The chitosan–silica sorbent (II) synthesized from the TEOS and chitosan obtained from Bombyx mori chitin.

Conditions of chromatography. One-dimensional ascending chromatography in a saturated chamber was performed; saturation time was 1 h at 25°C. The stationary phases were microspherical silica gel (MSS) and the chitosan–silica sorbent deposited from suspensions in isopropanol onto 10 ×10cm glass supports, which were dried first in air and then in an oven 70 °C for 3 hours, then the sorbents were stored in a desiccator at room temperature. The mobile phase was hexane-chloroform-isopropanol (7:2:1 (v/v) ratio). The eluent zone was 78 mm; separation time was 19 min on MSS and 12 min on chitosan–silica at 25°C. For the visualization of the spots of the substances on the chromatograms, the latter were dried at ~25°C for 30 min and spots were developed in iodine vapour.

Results

We studied the possibility of synthesizing microspherical polymer-silica sorbents using the sol-gel process, using natural chitosan as an organic template, studied the properties of silica gels by physicochemical methods. Chitosan from silkworm pupae with a degree of deacetylation of 82% and a molecular weight of 148 kDa.

The obtained chitosan-silica microspherical sorbents synthesized using TEOS, which obtained as a partially hydrolyzed TEOS as silica precursor. The use of TEOS leads to the formation of a powdery material consisting of particles of about 5 μm in size of spherical, oval and others shapes (Fig.1).

Figure 1. SEM images of chitosan-silica sorbents

The elemental analysis for nitrogen was 0.61–0.93%, which indicates the presence of chitosan in the silica gel network in sorbents obtained by the sol – gel method. The IR spectra of the obtained chitosan-silica sorbents have bands at 1090–1020 cm^-1, which are related to stretching vibrations of Si – O – Si and Si – O – C. Changes in the absorption bands of the amide group of chitosan in the region of 1650 cm^-1 and the silanol group in the region of 960 cm^-1 suggest the formation of hydrogen bonds between the silanol, amide and oxy groups of chitosan (Fig. 2).

Figure 2. IR spectra of the samples. 1 – silica, 2 – chitosan, 3 - chitosan-silica sorbent

The shift of the 1340 cm^-1 band, assigned to the deformation vibrations of the primary OH group, may indicate the formation of an ether bond between the primary alcohol group of chitosan and the silanol group formed from TEOS of the silica network. This binding of chitosan is of significant importance, since unbound amino groups allow various chemical transformations to be carried out in order to obtain products with valuable properties (sorbents for chromatography, biocatalysts, etc.).

The chromatographic properties of the sorbent samples were evaluated by the TLC (thin layer chromatograpy) method in the analysis of a test mixture of nitroaniline isomers. The chromatographic properties of chitosan-silica sorbents different from the initial MCC (Fig. 3). The results shown in Fig. 3 shows an increase in the R_f value (where R_f is the retention rate of the components to be separated) of these compounds compared to the R_f values obtained on the starting silica gel, which indicates a decrease in their adsorption activity.

Figure 3. Data on the TLC of nitroaniline isomers, eluent: hexane-chloroform-isopropanol (7: 2: 1)

The efficiency of chromatographic separation largely depends on the active centers, which include the functional groups of chitosan on the surface of sorbents responsible for intermolecular interactions in the separation processes. The obtained hybrid chitosan-silica sorbent has more functional chitosan groups on the surface of the sorbent and inside the matrix, as a result of which it has a large number of active centers, which leads to an increase in the Rf value of nitroaniline isomers.

Conclusion

Finally, the obtained data indicate that the sol-gel method is the most effective for the synthesis of chitosan-silica sorbent with spherical particles in one stage. Moreover, the functional groups characteristic of the polymer are on the surface of the sorbent and in the entire volume of the silica gel matrix, which ultimately leads to an increase in the number of functional groups responsible for the selectivity and effectiveness of the sorbent.

References:

1. Hua Zou , Shishan Wu, Jian Shen. Polymer/Silica Nanocomposites: Preparation, Characterization, Properties, and Applications. Chem. Rev. 2008, 108, 9, pp.3893–3957.
2. Помогайло А.Д. Гибридные полимер - неорганические нанокомпозиты // Успехи химии. - 2000. - №1 (69). - С.60-89.
3. Mohammad Abdul Sattar, Interface Structure and Dynamics in Polymer‐Nanoparticle Hybrids: A Review on Molecular Mechanisms Underlying the Improved Interfaces, ChemistrySelect, 2021, 6, 20, (5068-5096).
4. Riham El Kurdi,Mazhar Chebl, MikaSillan. Chitosan oligosaccharide/silica nanoparticles hybrid porous gel for mercury adsorption and detection. Materials today communication, 2021, v.8. pp.102770.
5. B. D. Kabulov, D. Sh. Shakarova, O. A. Shpigun , S. S. Negmatov. Chitosan-silica nanocomposite sorbent for thin-layer chromatography of alkaloids. Russian Journal of Physical Chemistry A 2008, Focus on Chemistry volume 82, pp.924–927.
6. R. A. A. Muzzarelli Chitosan Chemistry: Relevance to the Biomedical Sciences, Adv Polym Sci (2005) 186: 151–209.
7. B. D. Kabulov, Zalyalieva S.V., Graboskya T.A. Microspherical silica gel for HPLC. // Russian Journal of Physical Chemistry - 1993. - № 10 (67) - pp. 2070-2072.