International Journal of Pharmaceutics 232 (2002) 11 – 22 Moist heat treatment on physicochemical change of chitosan salt films Garnpimol C. Ritthidej a, *, Thawatchai Phaechamud b , Tamotsu Koizumi c a Faculty of Pharmaceutical Sciences, Department of Industrial Pharmacy, Chulalongkorn Uniersity, Bangkok 10330, Thailand b Faculty of Pharmacy, Department of Pharmaceutical Technology, Silpakorn Uniersity, Nakorn Pathom 73000, Thailand c Faculty of Pharmaceutical Sciences, Department of Pharmaceutics, Toyama Medical and Pharmaceutical Uniersity, Toyama 930 -01, Japan Received 20 April 2001; accepted 17 September 2001 Abstract Chitosan salt films were prepared by casting method using acetic, citric, formic, glycolic, lactic, malic and propionic acids as solubilizers. The films were then exposed to moist heat at 60 °C and 75% relative humidity for several time intervals. The influence of moist heat treatment on their physicochemical characteristics was investigated. All freshly prepared films were soluble in deionized water and HCl buffer solution. Chitosan citrate film also dissolved in phosphate buffer solution. After treatment, the percentage of water sorption and dissolution of chitosan films in three media were gradually decreased. Longer alkyl group and less carboxyl and hydroxyl groups in the molecule of organic acid resulted in lower percentage of water sorption and dissolution of treated films. The FT-IR spectra revealed that there was amide formation between chitosan and organic acids after treatment especially in chitosan acetate and propionate films. However, the absorption peaks of ammonium [’NH 3 + ] and free carboxylate groups were still remained in treated chitosan citrate and malate films. Change in the degree of crystallinity from powder X-ray diffractogram and thermal characteristic from DSC thermogram were also related to the water sorption and dissolution of films. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Chitosan salt film; Moist heat treatment; Physicochemical change www.elsevier.com/locate/ijpharm 1. Introduction Chitosan could be used as film former for coated pharmaceuticals (Hall and Wallace, 1998). Since it is soluble in dilute acidic medium, attempt to prepare in insoluble form is to expand its use in controlled release systems. Dry heat treatment to chitosan acetate film was reported to increase its water resistance which was attributed to the crosslinking of chitosan molecules and/or the for- mation of anhydrous crystalline in the structure (Lim and Wan, 1995). The coloration of chitosan salt film was intensified with the increasing tem- perature and duration of treatment. Moreover, conversion of chitosan acetate film to chitin film was reported by heating at 80–140 °C undergo- * Corresponding author. Tel.: +66-2-218-8272; fax: +66- 2-218-8279. E-mail address: rgarnpim@chula.ac.th (G.C. Ritthidej). 0378-5173/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0378-5173(01)00894-8