Studies on adsorption characteristics and mechanism of adsorption of chlorhexidine mainly by carbon black

The extent of adsorption of chlorhexidine to carbon black and sanitary cott on was determined by measuring the amounts of chlorhexidine adsorbed to car bon black or sanitary cotton from the chlorhexidine solution containing spe cific amount of carbon black or sanitary cotton. As another comparative ant iseptic example of adsorption phenomena, adsorption of acrinol to sanitary cotton was also studied. The specific surface area of carbon black was meas ured by the BET method of adsorption isotherm. The pattern of adsorption of chlorhexidine to carbon black was temperature-dependent Langmuir isotherms , and the amounts adsorbed increased as the temperature was raised. Since c hlorhexidine, whose pKa's are 2.2 and 10.3, is considered to exist in aqueo us solution as the di-cation, an ion-ion interaction should be formed betwe en protonated biguanide and anionic portions of carbon black or sanitary co tton. The chlorophenyl and hexane moieties interact with hydrophobic portio ns of carbon black or sanitary cotton. The perturbation experiment conducte d on this interaction system showed that the nature of interaction was irre versible. The enthalpy change calculated from Langmuir constants was small, indicating the existence of ion-ion interaction. The entropy values, 27.4 to 28.2 e.u. obtained in this system, suggested that the hydration shells o f the ions were rather tightly bound. The area occupied by a chlorhexidine molecule, 548 (A), was twice greater than the projection area, 276 (Angstro m)(2), suggesting that chlorhexidine was adsorbed in such a way that each m olecule is sufficiently well spaced. (C) 2001 Wiley-Liss, Inc. and the Amer ican Pharmaceutical Association.