AbstractThe increase in the viscosity of an aqueous solution or gel of hydroxypropylmethylcellulose (HPMC) in the presence of an anionic surfactant is attributed to a two‐step process: (a) the surfactant undergoes ion–dipole interaction with the ethereal oxygen in HPMC, in the absence or presence of water, and (b) hydrocarbon moieties in the now pendant surfactant molecules undergo hydrophobic bonding, in the presence of water, resulting in an apparent increase in the molecular weight of the HPMC. DSC of dry blends and dried gels or films confirmed HPMC–sodium lauryl sulfate interaction. Work‐to‐break measurements on films cast from aqueous solutions of HPMC, in the absence or presence of anionic surfactants, indicated an apparent increase in the molecular weight of HPMC in the presence of su
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