## 3 Results and Discussion  
Unloaded SLNs, prepared by the PIT method, showed a small mean particle size, which was not affected by LP2 loading (see Table 1). PDI values were lower than 0.300, suggesting the presence of a homogeneous population of nanoparticles in the investigated samples. Analogously to mean particle size and PDI, \(\zeta\)-potential values were similar for unloaded and LP2-loaded SLNs. As reported in the literature [27], \(\zeta\)-potential values greater (as an absolute value) than 30 mV are needed to provide stable colloidal dispersions. However, unloaded and LP2-loaded SLNs showed good stability during storage for two months at room temperature, as no significant change of particles size, PDI, and \(\zeta\)-potential values was observed (data not shown). All samples remained clear during the storage period with no sign of drug precipitation. SLNs with similar technological properties (small mean size, low PDI and \(\zeta\)-potential values, and good stability during storage at room temperature) were obtained in previous works using the PIT method [23]. The presence of long polyoxyethylene chains of the surfactant oleth-20 on the nanoparticle surface, which could provide a steric stabilization, was considered responsible for the good stability of the resulting nanocarriers [23]. As illustrated in Table 1, LP2-loaded SLNs showed a PIT value greater than that of unloaded SLN, thus suggesting different interactions among the lipid core components in the presence of LP2. Therefore, DSC studies were carried out to assess the thermal behavior of the investigated SLNs. The LP2 calorimetric peak was centered at 57.78 \({}^{\circ}\)C. The calorimetric curve of unloaded SLN was characterized by the main peak at 41.90 \({}^{\circ}\)C, and a shoulder at a lower temperature was present (Figure 2). Such shoulder could indicate that the surfactant was not homogeneously distributed in the SLNs' structure. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & \begin{tabular}{c} **Size \(\pm\) S.D.** \\ **(nm)** \\ \end{tabular} & **PDI \(\pm\) S.D.** & \
\begin{tabular}{c} **PIT** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} &
\begin{tabular}{c} **Zeta** \\ **(mV)** \\ \end{tabular} \\ \hline SLN A & 27.22 \(\pm\) 2.06 & 0.128 \(\pm\) 0.030 & 65 & \(-\)10.5 \\ SLN LP2 & 29.79 \(\pm\) 1.50 & 0.126 \(\pm\) 0.029 & 71 & \(-\)10.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mean size (size), polydispersity index (PDI), phase inversion temperature (PIT), and \(\zeta\)-potential (Zeta) of unloaded (SLN A) and LP2-loaded SLNs. The calorimetric curve of LP2-loaded SLNs was quite different, as there was no evidence of the previously mentioned shoulder, and the main peak was sharper and centered at 46.89 \({}^{\circ}\)C. These results suggest that the incorporation of LP2 affected the calorimetric behavior of SLNs; in particular, the absence of the shoulder could indicate a homogeneous distribution of the components of the SLN. Many parameters can be taken into account to study the calorimetric behavior of SLNs. The most important and used are the transition temperature (T), which marks the transition from an ordered state to a disordered state; the enthalpy variation (\(\Delta\)H); and the \(\Delta\)T1/2, which can be taken as a measure of the cooperativity of the transition (the number of lipid molecules undergoing simultaneous transition); in particular, \(\Delta\)T1/2 is inversely proportional to cooperativity. The comparison of these parameters for unloaded and LP2-loaded SLNs could provide information on the effect of LP2 incorporation into SLNs. LP2-loaded SLNs showed a transition temperature increase of about 5 \({}^{\circ}\)C; therefore, the presence of LP2 in the SLNs structure favored the ordered state. \(\Delta\)H was \(-\)11.90 J/g in unloaded SLNs and \(-\)18.02 J/g in LP2-loaded SLNs and a decrease in \(\Delta\)T1/2 (calculated by the DSC software as the temperature width at half peak height) from 5.99 \({}^{\circ}\)C in unloaded SLN to 4.40 \({}^{\circ}\)C in LP2-loaded SLN was observed.