Wettability of psoralen powders: influence of bile salts on their contact angles and surface free energy components

Abstract

The purpose of this work was to assess the surface free energy (SFE) of bare and bile-salt-precontacted pellet surfaces of psoralens (lipophilic molecules). This study was carried out according to the approach of van Oss et al. to the solid-liquid interfacial free energy using the Lifshitz-van der Waals component, electron acceptor and electron donor parameters of the SFE. With the help of a hydraulic press, powders of 8-methoxypsoralen (8-MOP), 5-methoxypsoralen (5-MOP) and 4,5',8-trimethylpsoralen (TMP) were compacted sufficiently to obtain smooth and glossy disc surfaces. One set of pressed discs was used directly after preparation and another set was treated with a bile salt (sodium cholate/sodium deoxycholate mixture) aqueous solution of 0.1 mol l −1 for 2 h at 37°C before droplet deposition. The surface free energies of bare and bile-salt-treated discs were determined by advancing contact angles using polar (water and formamide) and apolar (diiodomethane) standard liquids. The measurements were carried out at 21.5 ± 1.5°C using a surgical microscope fitted with a slanted mirror and equipped with a video camera linked to a computer. In addition, the surface and interfacial tensions of a biphasic system of octanol and water in which psoralens were solubilized were measured using the De Noüy ring method. The results were correlated with the lipophilicity and solubility of these drugs. The data of contact angle values of psoralen bare surfaces indicated the hydrophobicity order 8-MOP < 5-MOP < TMP. The percentages of Lifshitz-van der Waals ( γ s LW) components corroborated this. 8-MOP showed the same acidic and basic values ( γ s AB = 6.5 mJm −2, while γ s AB was equal to 1.8 and 0.4 mJm −2 for 5-MOP and TMP respectively. However, when the psoralens were treated with sodium cholate sodium deoxycholate ( NaC NaDC ) mixture, the presence of micelles on the surface modified the previous data by decreasing the contact angles with polar solvents and γ s LW. In contrast, the total surface free energy of all the psoralens increased, especially the electron onor component ( γ −) and γ s AB. When comparing the three psoralens, 5-MOP behaved peculiarly as it had the lowest γ s − (29.5 mJm −2) and γ s AB (21.4 mJm −2). Theoretical explanations concerning the reactivity of the psoralens vis à vis standard liquids were assumed. The results of this work enabled us to understand qualitatively and quantitatively the types of interaction between the molecules of each studied psoralen and to predict their behaviour when they permeate through a given biological membrane (e.g. human skin). From this work the adsorption of bile salts on to the hydrophobic surface was confirmed for the psoralen case. This result could constitute the beginning of a physicochemical explanation of the digestion step for orally taken psoralens.