Effect of amphiphilic drugs on the cloud point of hydroxypropylmethyl cellulose: Modulation with salt excipients

In this paper we have reported the effect of two amphiphilic drugs, i.e., amitriptyline hydrochloride (AMT) and promethazine hydrochloride (PMT), on the cloud point (TCP) of hydroxypropylmethyl cellulose (HPMC) in the absence and presence of electrolytes. TCP of HPMC increased on increasing the [drug] and both drugs behaved almost in a same manner. In the presence of a small amount of NaCl (0.01 mol kg− 1) TCP of the drug-polymer system showed a slight increase with a similar pattern as observed in its absence. The pattern of TCP change was different in the presence of high concentrations of NaCl (0.1 and 0.2 mol kg−1) which displayed a minimum in [drug] vs. TCP plots while the steepness and appearance of minimum depend upon the electrolyte concentration. The effect of other monovalent salts (NaBr, KCl, KBr) was also seen and it was noted that bromide and sodium ions were found to be more effective as compared to chloride and potassium ions, respectively, for affecting the TCP of HPMC. The energetics of the cloud point have been calculated and discussed.