Location of anti-TB drugs and microstructural changes in organized surfactant media using optical properties

Microstructure elucidation is a critical step in the understanding of organized surfactant media such as microemulsions. In this work, a systematic approach for revelation of microstructure changes of Tween 80 microemulsion has been carried out using photophysical properties. Microemulsion structural changes have been monitored using pyrene and different solvatochromic probes for the presence of oil-rich, water-rich, and bicontinuous microstructures. The hydrophobic optical probe, Nile red (NR), has been used to report from the side of the oil-surfactant interface and to complement this, a hydrophilic optical probe tris(2,2'-bipyridine)ruthenium(II) dichloride (RC) from the side of the water-surfactant interface. The results show the partitioning of probes into different microenvironments within the system. NR occupies the interface toward the apolar side and RC toward the polar. The analysis also indicates conspicuous dependence of the emission/absorbance behavior of optical probes on the water:surfactant molar ratio (ω). Quantitative estimates have been obtained for the polarities (ET(30)) of solubilization sites of the probes in different regions of microemulsions and are well correlated with a Stokes shift. Lastly, these probes of different solubilities have been used to locate the position of anti-tuberculosis drugs, i.e., rifampicin (hydrophobic), pyrazinamide, and isoniazid (hydrophilic) of variable solubilities loaded in Tween 80 systems.