On the stiffness of chitosan hydrochloride in acid-free aqueous solutions R. Signini a , J. Desbrie `res b , S.P. Campana Filho a, * a Instituto de Quı ´mica de Sa ˜o Carlos, Universidade de Sa ˜o Paulo, Caixa Postal 780, CEP: 13560-970, Sa ˜o Carlos/SP, Brazil b Centre des Recherches sur les Macromole ´cules Ve ´ge ´tales, CERMAV/CNRS, affiliated with University Joseph Fourier, BP:53-38041 Grenoble Cedex 9, France Received 24 June 1999; accepted 17 December 1999 Abstract Chitosan hydrochlorides F A  0:226; i.e. DA  22:6% were randomly degraded by ultrasonication and characterized by viscosity measurements in aqueous acid-free solutions. It is shown that acid-free aqueous solutions of chitosan hydrochloride of variable ionic strengths 0:06 M  m  0:3M are free of aggregation as evaluated by the values of the Huggins constants 0:31  k  0:63: These solutions were employed to study the solution properties of chitosan hydrochloride at different ionic strengths, which allowed the determina- tion of its salt tolerance as well as its characteristic stiffness parameter. Following Smidsrod’s approach chitosan hydrochloride gave a stiffness parameter B  0:06; in agreement with the value reported by Rinaudo et al. It is suggested that the higher values of B reported for chitosan in the literature may be attributed to aggregation.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Chitosan hydrochloride; Acid-free aqueous solutions; Stiffness parameter; Huggins constant 1. Introduction Chitosan is a cationic polyelectrolyte when dissolved in dilute acid solutions. It is usually prepared by the deacetyla- tion of chitin, an abundant naturally occurring polysacchar- ide (Muzarelli, 1978). Ideally, the primary structures of chitin and chitosan correspond to those of poly[b(1 ! 4)- 2-acetamido-2-deoxy-d-glucopyranose] and poly[b(1 ! 4)- 2-amino-2-deoxy-d-glucopyranose], respectively. Chitin and chitosan usually exist in nature as components of complex composites. In these composites, such as the cell walls of fungi and the exoskeleta of sea animals and insects, chitin and chitosan are combined with other substances such as proteins, polysaccharides, lipids, pigments and inorganic material (Roberts, 1992a). Chitin usually contains some 2-amino-2-deoxy-d-gluco- pyranose units. It is not known if these are present in the native chitin or if they occur as a consequence of the hydro- lysis of acetamido groups, since isolation from these composites generally requires strong alkaline media, the exact conditions depending on the source and purity of the final product (Roberts, 1992a). Chitosan generally also contains 2-acetamido-2-deoxy-d- glucopyranose in addition to 2-amino-2-deoxy-d-gluco- pyranose units since fully deacetylated chitosan is rarely prepared due to the occurrence of severe depolymerization (Domard & Rinaudo, 1983). Nevertheless, the use of mild reaction conditions and the presence of oxygen scavengers in the reaction medium may prevent chain depolymeriza- tion. Recently, a new method for the deacetylation of chit- osan using aqueous sodium hydroxide (5–10%) was patented, which allows the preparation of extensively deace- tylated chitosan with little polymer degradation (Rinaudo, Le Dung & Milas, 1993a). Although another nomenclature system has been proposed (Roberts, 1992b; Roberts, 1997), chitin and chit- osan are terms of long standing use and the most practical way to distinguish between them is based on the proportion of 2-acetamido-2-deoxy-d-glucopyranose and 2-amino-2- deoxy-d-glucopyranose units present in their chains, expressed as the average degree of acetylation (DA) or as the mole fraction of acetylated glycosamine units (F A ), and also on their solubilities. Chitin has mostly acetylated units and the literature reports that it can be dissolved in a limited number of solvents such as aqueous solutions of neutral salts, concen- trated solutions of strong acids and in some organic solvents (Roberts, 1992c). The best solvents for highly acetylated a-chitin are DMAc/LiCl and N-methylpyrrolidone/LiCl since they are non-degrading solvents and also they allow the dissolution of a substantial amount of chitin. Other solvents are not so good since they cause degradation and Carbohydrate Polymers 43 (2000) 351–357 0144-8617/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0144-8617(00)00182-X www.elsevier.com/locate/carbpol * Corresponding author.