Behavior of Hydroxypropylmethylcelluloses of Different Molecular Structure and Water Affinity at the Air–Water Interface

Introduction ........................................................................................................ 455

Materials and Methods ..................................................................................... 456

Equilibrium Surface Pressure .................................................................. 456

Surface Film Balance ................................................................................. 457

Automatic Drop Tensiometer................................................................... 457

Results.................................................................................................................. 457

Conclusions......................................................................................................... 460

Acknowledgments ............................................................................................. 460

References ........................................................................................................... 461

Hydroxypropylmethycellulose (HPMC) is a water-soluble nonionic macro-

molecule derived from cellulose (Yoguchi et al., 1995) that is able to gel upon

heating. It has interesting applications in food, pharmaceutical, and cosmetic

areas, mainly for controlled release (Fyfe and Blazek, 1997; Avranas and

Tasapoulus, 2000). The surface properties of HPMC solutions depend on the

length and distribution of trains, loops, and tails. This can be explained in

terms of the structure of the polymer, which is a consequence of the

manufacturing process that involves heterogeneous reactions. The presence

of crystalline and amorphous regions in the solid-state form of cellulose

results in a nonuniform distribution of substituents along the backbone of

the cellulose molecule (Nahringbauer, 1995). As a result there may exist

regions of the cellulose backbone that are hydrophobic in nature, regions

rich in methoxyl groups, and others which are hydrophilic, being rich in

hydroxypropyl groups. Thus, once adsorbed, the hydrophobic regions that

would correspond to the so-called trains would be located at the air-water

interface and the hydrophilic groups, the loops and tails, would be

immersed in the subphase. HPMCs exhibit different surface activity

depending on their varying methoxyl/hydroxypropyl ratio.