# Immobilization of Levocyetririzine on Mesoporous Silica for Antiallergenic Gel Formulation
## 3 Results and Discussions
### Immobilization of LevdC on Mesoporous Material  
The absorption-desorption isotherms of LevdC-immobilized carrier and Syloid are shown in Figure 2. The decrease occurred in the specific surface area values of the mesoporous substrate after the immobilization of the drug allowed for calculating the coverage of the substrate surface with the drug [22]. It was assumed that the decrease in the SSA of the Syloid was due to the adsorption of drug molecules on the surface. Accordingly, the changes in SSA suggest that the active substance covered only 4 of the surface of the Syloid carrier in the case of samples _SyLev1_ and _SyLev4_, while in the other samples, this was true to a lesser extent (Table 2). The maximum coverage of 24.5\(\%\) was achieved after 2 h of mixing in aqueous medium. In a methanolic medium, the same coverage was reached only after 24 h of mixing, although the coverage values did not show considerable differences with respect to mixing time. It follows that adsorption was completed within the shortest time tested (2 h) irrespective of the medium used, and the 24.5\(\%\) coverage seems to be an equilibrium in the given conditions. This result may be of a surprise considering that the binding efficiency of the active ingredient to the surface is better when using a less polar solvent [24; 33] characterized by lower permittivity [34; 35]. The permittivity of methanol (\(\varepsilon_{\rm r}\) = 32.7) is approximately a third of that of water (\(\varepsilon_{\rm r}\) = 80.1) [36], so, theoretically, better surface bonding was expected when using a methanol solution. In our case, the concentration of the solution (which was identical for tests) seemed to have a decisive influence on the adsorption efficiency. The 24.5% coverage obtained with LevdC in a methanol medium is much higher than the 9% observed in a metronidazolezole fixation study using a Syloid carrier and made using similar experimental conditions and procedures [24]. The difference can be explained by the chemical structure and the molecular size. To achieve the highest possible drug coverage, the ratio of the pore diameter to the size of the drug molecule should theoretically be larger than 1. The pore accessibility for the drug molecule is adequate if the ratio of the pore to the drug molecule is above 3 [37]. The average pore size of Syloid is 11.3 nm in diameter, and the molecular size of LevdC is approximately 2 \(\times\) 2 nm, while metronidazoleazole is approx. 1 \(\times\) 1.5 nm, which suggests that both can fit comfortably into the mesopores of Syloid. The ratio in the case of LevdC is approx. 5, while for metronidazoleazole it is approx. 10. From the same number of molecules, LevdC can cover a larger surface area due to its larger size, which can also contribute to the greater affinity resulting from its chemical structure. The XPS tests showed a small increase in the surface carbon content in all samples (samples _SyLev1-SyLev5_) made with LevdC in aqueous and methanol media. However, no such increase in the nitrogen and chlorine content was detected up to a depth of 10 nm. Considering that the results obtained from XPS are typically characteristic only of the outermost surface of the analyzed sample, the results suggest that the active ingredient is not located on the outer surface but within the pores. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Samples** & **Specific Surface Area (m\({}^{2}\) g\({}^{-1}\))** & **Covered Area (\%)** \\ \hline Syloid & 368 & – \\ \hline _SyLev1_ & 278 & 24.5 \\ _SyLev2_ & 357 & 3.0 \\ _SyLev3_ & 288 & 21.8 \\ _SyLev4_ & 278 & 24.5 \\ _SyLev5_ & 301 & 19.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specific surface area and percent surface coverage of samples made in aqueous and methanol solutions containing levocetririarine dihydrochloride on the surface of Syloid carrier according to nitrogen adsorption measurements. Figure 2: Adsorption isotherms of levocetriazine dihydrochloride–MSM and Syloid materials.