ADSORPTION AND THERMODYNAMIC PROPERTIES OF CHITOSAN–SILICA NANOCOMPOSITE MATERIALS PREPARED BY THE SOL–GEL METHOD DURING BENZENE VAPOR ADSORPTION

ABSTRACT

Chitosan–silica nanocomposite materials synthesized via the sol–gel method were investigated in terms of their adsorption and thermodynamic properties toward benzene vapor. Adsorption isotherms were obtained and analyzed using the Brunauer–Emmett–Teller (BET) method to evaluate specific surface area and pore structure parameters. The results revealed that the incorporation of silica into the chitosan matrix significantly enhances surface area and porosity, while additional modification with glycerin leads to a substantial increase in sorption capacity and pore volume.

Thermodynamic analysis showed that the Gibbs free energy values become more negative in modified systems, indicating stronger polymer–solvent interactions and improved affinity toward benzene molecules. The obtained findings demonstrate that the structural and surface properties of chitosan–silica nanocomposites can be effectively tuned through compositional variation.

These results highlight the potential of such hybrid materials for application in adsorption-based separation processes, environmental purification, and advanced sorption technologies.

АННОТАЦИЯ

Нанокомпозитные материалы на основе хитозана и диоксида кремния, синтезированные золь–гель методом, были исследованы с точки зрения их адсорбционных и термодинамических свойств по отношению к парам бензола. Изотермы адсорбции были получены и проанализированы с использованием метода Брунауэра–Эмметта–Теллера (BET) для определения удельной поверхности и параметров пористой структуры.

Результаты показали, что введение диоксида кремния в матрицу хитозана приводит к значительному увеличению удельной поверхности и пористости, тогда как дополнительная модификация глицерином способствует существенному повышению сорбционной ёмкости и объёма пор.

Термодинамический анализ показал, что значения энергии Гиббса становятся более отрицательными в модифицированных системах, что свидетельствует об усилении взаимодействий полимер–растворитель и повышении сродства к молекулам бензола. Полученные результаты демонстрируют возможность эффективного регулирования структурных и поверхностных свойств нанокомпозитов хитозан–диоксид кремния путём изменения их состава.

Данные материалы представляют значительный интерес для применения в адсорбционных технологиях, процессах очистки окружающей среды и создании функциональных сорбентов.

Keywords: chitosan, silicon dioxide, sol–gel method, nanocomposite, benzene vapor, adsorption isotherm, porous structure, polymer–solvent interaction, Gibbs energy

Ключевые слова: хитозан, диоксид кремния, золь–гель метод, нанокомпозит, пары бензола, изотерма адсорбции, пористая структура, взаимодействие полимер–растворитель, энергия Гиббса.

Introduction. Nowadays, nanocomposite materials based on natural polymers are attracting great interest from both scientific and practical standpoints. Chitosan is a biodegradable, environmentally safe polymer rich in functional groups, capable of adsorbing various solvents and gases. Silica, on the other hand, is characterized by its high specific surface area, mechanical stability, and chemical inertness.

Chitosan–silica nanocomposites prepared by the sol–gel method allow for the control of pore structure and surface properties. Studying polymer–solvent interactions in such materials is important for assessing their sorption capabilities. In particular, the adsorption of benzene vapors is an effective method for determining hydrophobic properties.

Chitosan is a natural polysaccharide, and its high biocompatibility, along with the presence of functional amine and hydroxyl groups, makes it widely used as an adsorption material. In the literature, it is noted that chitosan primarily exhibits hydrophilic properties and adsorbs water vapor to a high degree [1,2]. At the same time, its interaction with aromatic hydrocarbons, including benzene, is relatively low [3]. Silica-based materials, however, are considered effective in adsorbing organic vapors due to their high specific surface area and developed pore structure. In silica materials obtained via the sol-gel method, it is possible to control pore size and volume, which directly influences the mechanism of the adsorption process [4,5]. It has been shown that silanol groups on the surface of silica can interact with organic molecules through physical adsorption [6].

In recent years, organic–inorganic hybrid materials, especially chitosan–silica nanocomposites, have attracted significant scientific interest. Scientific sources have noted that the interaction between the polymer and inorganic phases significantly alters the mechanical, sorption, and thermodynamic properties of nanocomposites [7,8]. In such systems, the silica–chitosan chains reorient, leading to the formation of a porous structure.

In assessing polymer–solvent interactions, thermodynamic functions calculated from adsorption isotherms, including the mean free energy and Gibbs energy, are important indicators. According to recent thermodynamic studies, polymer–solvent interactions in hybrid nanocomposites significantly affect Gibbs free energy and adsorption capacity [14,15]. According to Flory–Huggins theory, the free energy of mixing allows for an assessment of the intensity of polymer–polymer and polymer–solvent interactions [9]. In the literature, benzene vapor adsorption has been shown to be an effective method for determining the relative hydrophobicity of materials and their true specific surface area [10].

Recent studies (2021–2025) have demonstrated that chitosan–silica nanocomposites exhibit enhanced adsorption performance toward volatile organic compounds due to tunable pore structures and surface functionalities [9–11].

It has been reported that adsorption of benzene vapors on hybrid nanomaterials is strongly influenced by pore size distribution and surface energy parameters [12,13].

Therefore, studying the sorption of benzene vapor in chitosan–silica nanocomposite materials allows for a deep analysis of their pore structure, surface properties, and polymer–solvent interaction mechanism.

Materials and methods

To study the sorption properties of a hybrid chitosan-silica nanocomposite material, a universal high vacuum adsorption apparatus and a Tiana-Calve type differential microcalorimeter (DAC-1-1A) was used. Measurements were carried out at a residual pressure of 10⁻³–10⁻⁴ Pa using a KM-8 cathetometer. When the required high vacuum for the sorption experiments was reached, the change in the quartz balance was observed, and the weight of the dry sample was determined from the equation based on the difference in the manometer reading before and after the high vacuum was established:

g_C=g_ВЛ-(r /1000)        (1)

w, g·VL and gC – polymer weight before and after vacuum formation;  mm/mg;  – difference between the catheterometer's readings before and after vacuum formation, mm.A constant is calculated for each sample weight to determine the amount of sorbate adsorbed on the polymer during each measurement:

K = (r·100) / (1000·gC)           (2)

The lengthening of the spring by 1 mm corresponds to the percentage of sorbed vapor, K. The percentage of sorption is calculated for each relative pressure using the following equation:

%S = K                            (3)

Here, Δ is the difference between the catenometer reading after high vacuum has been established and the subsequent equilibrium state of the sorbate at the characteristic relative pressure Pi/Pi0, where Pi is the sorbate vapor pressure measured during adsorption; Pi0 is the pressure of the sorbate's saturated vapor).  The powder X-ray structural parameters of the samples were calculated from the BET (Brunauer–Emmett–Teller) method. The monolayer capacity is determined from the adsorption isotherms according to the BET equation:

X/Xm = C(P/P0)(1 – P/P0)(1 + (C – 1)P/P0)    (4)

Results and discussion

The study of benzene vapor sorption can provide interesting information about the structure of polymer materials. As can be seen from the figure, the curves of the isotherm have a convex S-like structure characteristic of polymers at low relative pressures. Based on the shape of the isotherm curves, the influence of different types of macromolecules can be inferred.

Porous Structure and Interaction Energies of Chitosan-Silica Nanocomposite Samples with Benzene Vapors Table 1

Table 1.

Porous Structure and Interaction Energies of Chitosan-Silica Nanocomposite Samples with Benzene Vapors

The values of the thermodynamic functions of the porous structure and solvent interactions of the chitosan–silica nanocomposite materials prepared by the sol–gel method are presented in the table.

Figure 1. Plot of the average free energy of polymer–solvent interaction of the studied samples as a function of concentration: 1. chitosan/TEOS.

Based on the obtained isotherms, the initial samples were prepared with chitosan and silica, as well as with chitosan in various ratios -silica nanocomposites, the solvent's thermodynamic stability was qualitatively and quantitatively assessed by calculating the solvent chemical potential ∆μ1 and the polymer chemical potential ∆μ2, yielding the average free energy of polymer–solvent mixing ΔG₍mix₎. was calculated. For all studied samples shown in Figure 1, their Gibbs energies were determined from the concentration dependence of ∆g₍m₎.

In Figure 1, the polymer–solvent interaction—i.e., the average free energy of mixing Δg_m—determined using the chemical potentials of the solvent and polymer, is more negative for chitosan–silica–GL (5:1:1).  Compared to the other studied samples, the intensity of polymer–solvent interactions in these systems increases relative to polymer–polymer interactions. This indicates a higher hydrophobicity for these systems compared to the others. The improved sorption characteristics observed for chitosan–silica–glycerin systems are consistent with recent findings on VOC adsorption using biopolymer–silica composites [16]. For the chitosan–silica and chitosan–silica nanocomposites at a 5:1 ratio, the polymer–solvent nanocomposite exhibited a relatively lower average free energy and Gibbs energy. The polymer–solvent interaction in these systems is explained by the strengthening of intermolecular interactions between molecules of different chemical natures.

Table 2.

Sorption characteristics of samples of chitosan–silica nanomaterials synthesized by the sol–gel method, showing their pore structure and thermodynamic function values.

The porous structure of the sol–gel–derived samples differs from that of the encapsulated samples, as shown in the table: the encapsulated samples have a larger specific surface area and smaller mean pore radii.

Thermodynamic calculations show that the interaction between the nanomaterial and benzene in these samples differs markedly.

Conclusions

The obtained results confirm that structural modification of chitosan with silica and glycerin significantly enhances adsorption efficiency and thermodynamic stability. These materials demonstrate high potential as effective adsorbents for volatile organic compounds, particularly benzene vapor.The obtained results confirm that structural modification of chitosan with silica and glycerin significantly enhances adsorption efficiency and thermodynamic stability. These materials demonstrate high potential as effective adsorbents for volatile organic compounds, particularly benzene vapor.

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