Differential Scanning Calorimetry
Figure 5 a lecithin showed a low-intensity endothermic peak at 47.2°C, which was the chain melting transition point where it transformed into a liquid crystalline form from a gel form with enhanced ability to act as a drug carrier (36). Two endotherms were observed for chitosan, where 83.8°C was attributed to water loss and polymeric chain decomposition at 303.7°C (37). PEG 2000 endothermic peaks at 53.5°C, indicating melting process of the sample (36).
Fig. 5 Characterisation of PALN nanoparticles. a DSC thermogram of PALNs, physical mixture, ACV, lecithin, PEG 2000, chitosan; b XRD Diffractogram for PALNs
ACV had four thermal effects shown in Fig. 5 a. The analysis for ACV used was ACV polymorph form VI, with 1:2 ratio of ACV to water. The first endothermic peak at 116.5°C was assigned to the dehydration process. It leads to recrystallisation of ACV form VI to ACV form V, which undergoes a phase transition to IV at 174.6°C. Further heating leads to endotherm at 255.7°C, indicating ACV melting point (MP). Last exothermic peak at 259.1°C was assigned to the exothermic decomposition of the ACV (38).
The physical mixture for the first endotherm was assigned to the lecithin and PEG2000. ACV endothermic peak diminished but a new peak emerged at 151°C. It might be due to molecular interaction between drug, lecithin and chitosan. The interaction causes ACV to have decreased crystallinity and increased amorphous properties of acyclovir, or turned into semi-solid form, or dissolution in the matrix melt (39). For PALN1, peak diminishments of lecithin, chitosan, and PEG 2000 were observed. An endotherm observed at 242.3°C was assigned to the left shift of the ACV MP. Although the left shift was evidence for crystallinity changes, the amplitude of shifting was abnormal. Table III showed that heating rate for acyclovir and PM was higher than PALN1. Higher heating rate results in MP shifted to a higher temperature which is in agreement from the previous report of acyclovir MP (40).