*3.5. Structural Analysis*

FTIR spectrum depicting characteristic vibrational frequencies of isoniazid in its unformulated state and encapsulated within the RDS formulation were compared (Figure 4). The vibrational band assignments for formulated and unformulated isoniazid were executed based on their positions, nature, magnitudes, and intensities relative to each characteristic functional group. The key bands plus wavenumbers identified and compared for both the RDS formulation and isoniazid in its pure state are presented in Table 1. The vibrational frequencies of key functional groups as it relates to the chemical backbone structure of isoniazid compares well for both pure drug and RDS formulation, with characteristic peaks presenting with varying intensities. The isoniazid loaded RDS formulation appeared overly dominant in most regions with some of its peaks suppressing those of pure isoniazid. This suggests physical solid–liquid/solid–solid dispersion and significant drug encapsulation. The O–H stretch recorded at 3676 cm<sup>−</sup><sup>1</sup> for the RDS formulation further supports the physically dispersed state in the water/oil medium, followed by snap-freezing and lyophilization.

**Figure 4.** The Fourier transform infrared (FTIR) spectrum of the unformulated isoniazid and RDS formulation, showing different characteristic vibrational bands.

**Table 1.** FTIR vibrational frequencies representing formulated and unformulated isoniazid key functional groups.

