Hydroxypropylmethylcellulose at the oil–water interface. Part I. Bulk behaviour and dynamic adsorption as affected by pH

The bulk and oil–water (O/W) surface behaviour of four different commercial types of hydroxypropylmethylcelluloses (HPMCs) was studied at pH3 and 6. Dynamic light scattering (DLS) analysis of particle size distribution in HPMC water solutions indicated cluster formation at pH6 for most types. The dynamics of adsorption showed that the surface pressure (π) values and the rate of adsorption/penetration were lower at pH3. The surface dilatational modulus of films at pH6 showed a continuous increase indicating a slow formation of a viscoelastic interfacial film. Nevertheless, at pH3 in most cases a viscoelastic film was not formed. The results obtained at the oil–water interface well correlated with DLS. Among all the HPMC studied, E5LV showed over all the best performance at this oil–water interface due to its low molecular weight and high decrease of interfacial tension. Its high hydrophobicity allows a strong tendency to interact at the interface to form elastic films.