# Encapsulation of \(\alpha\)-Lipoic Acid in Halloysite Nanotubes
## 2 Materials and Methods
### Methodology for Determining the Concentration of \(\alpha\)-Lipoic Acid  
For the analytical control of \(\alpha\)-lipoic acid content, a chromatographic method of analysis was used, which has several advantages. Namely, the substance practically does not change during separation, which is very important for further chemical research. This method is quite suitable for the separation of liquid and gaseous compounds and can be used for the fractionation of mixtures of substances that are similar in chemical composition, properties, and structure. In addition, chromatography has high accuracy in the separation of substances and is relatively easy to use.  
Reverse-phase liquid chromatography was chosen for the study. During reverse phase chromatography (RPC), hydrophobic substances dissolved in a polar solvent selectively interact with the stationary phase. The liquid phase consists of an aqueous buffer containing an organic modifier dissolved in water, and this modifier forms a liquid interface between the two phases, the mobile being polar and the immobile being hydrophobic.  
The sorbent for this method of identifying \(\alpha\)-lipoic acid in chromatography consists of a basic matrix to which organic ligands, which are alkylated hydrocarbon chains, are \"sewn\".  
The lipophilic medium of the stationary part of the chromatographic system was selected in such a way as to attract molecules of the organic modifier to form a liquid adsorption phase. Octadecyl groups--C18--were used as matrix coatings.  
Since there is no method of identification of \(\alpha\)-lipoic acid in the literature, this study developed a method of identification and determination of the quantitative content of this active pharmaceutical ingredient by liquid chromatography.  
A mixture of distilled water and methanol in a 1:1 ratio was used as a solvent. To prepare the comparison solution, 15 mg of a-lipoic acid was dissolved in 5 mL of methanol and brought to a volume of 50.0 mL with the solvent. For the investigated solutions, an aliquot of 5.0 mL of the sample was taken and adjusted to a volume of 20.0 mL with the solvent, then 2.0 mL of the obtained solution was adjusted to a volume of 20.0 mL with z the solvent.  
Chromotagraphy was performed on a liquid chromatograph with a spectrophotometric detector. The \(\alpha\)-lipoic acid peak was identified using a Zorbax C-18 chromatographic column (Agilent, Kiev, Ukraine). Acetonitrile and 0.2 M potassium dihydrogen phosphate solution were used as mobile phases. The flow rate of the mobile phase was 1.7 mL/min. The temperature of the chromatographic column is 30 \({}^{\circ}\)C. The optical absorption density is 220 nm. The volume of the injected sample was 10 \(\upmu\)L. The concentration of lipoic acid was determined by the response (in mAU--milli Absorbance Unit) of the chromatograph detector.  
In this study, tests were performed on the separation of \(\alpha\)-lipoic acid from halloysite using different types of Agilent membrane filters. Two-layer membrane filters are used to purify the sample solution. Such a filter consists of two parts: a preliminary coarse glass fiber filter and, in fact, a membrane filter. The prefilter consists of borosilicate glass fibers. It is chemically inert and resistant to most solvents. The presence of a prefilter with a large surface area in the design ensures its large capacity and the possibility of repeated use.  
This study is necessary to establish which of the filter types is best suited for the separation of \(\alpha\)-lipoic acid from halloysite. In the course of laboratory studies, it was determined that the membrane filter Agilent PES 0.1 mkm is best suited for the performance of the tasks. The accuracy of the results is greater than 99.74%.