Preparation of vesicles composed of 2-(hexadecyloxy) cinnamic acid and N-[3-(dimethylamino) propyl]-octadecanamide and their photo- and pH-responsive release property

Abstract

Vesicles composed of N-[3-(dimethylamino) propyl]-octadecanamide (DMAPODA) and 2-(hexadecyloxy) cinnamic acid (HOCA) in an equimolar ratio were prepared by taking advantage of salt bridge formed between the amino group and the carboxylic group. The structure of vesicle was observed on a Transmission electron microscopy (TEM), and the size was determined on a dynamic light scattering equipment. The phase transition of the vesicular membrane was found to be around 35 °C on a differential scanning calorimeter. HOCA of the vesicular membrane was readily dimerized under the irradiation of a UV light (λ = 254 nm, 6 W). The release degree of rhodamine B from vesicle suspended in distilled water (pH 6.8) was about 70 % in 1 h at 25 °C, and the UV irradiation during the release experiment had little effect on the release degree. However, it had a significant effect when the temperature of release medium was 40 °C. Upon the photodimerization, HOCA would readily change its orientation in the vesicular membrane vesicle at 40 °C, possibly because the vesicular membrane is in a liquid crystalline state at the temperature, which is higher than the phase transition temperature (around 35 °C). In addition, the vesicle released rhodamine B in a pH-dependent manner. The release degrees were the highest at pH 3.0 and the lowest at pH 9.0 among the pH values tested. The salt bridge formed between DMAPODA and HOCA is labile at a strong acidic condition so it would be responsible for the extensive release at pH 3.0.