Surface tension and rheology of aqueous dispersed systems containing a new hydrophobically modified polymer and surfactants.

Abstract This article reports data supporting that the hydroxypropyl cellulose-methyl methacrylate (HCMMA) hydrophobically modified polymer studied is surface-active at the air–water interface due to its amphiphilic nature. Surface tension measurements of diluted solutions point to the formation of a complex between this copolymer and a polyoxyethylene nonylphenyl ether non-ionic surfactant of high HLB. Conversely, no indications of specific interactions were found either with a polyoxyethylene nonylphenyl ether non-ionic surfactant of intermediate HLB or with an anionic surfactant such as sodium dodecyl sulfate (SDS). The physical stability of such dispersions depended on the surfactant used. The HCMMA/SDS systems studied showed phase separation shortly after preparation, while the dispersions with the non-ionic surfactant of higher HLB exhibited at least short-term stability and Newtonian behaviour. Foam-like dispersions of HCMMA–surfactant systems with intermediate HLB presented long-term stability, underlying the important role of hydrophobic interactions in these systems. One of the latter dispersions and the corresponding continuous phase were rheologically characterised by small amplitude oscillatory shear and flow curve experiments and exhibited a high Newtonian viscosity up to a critical shear stress followed by a shear thinning as well as weak-gel viscoelastic properties. The results obtained support that (a) the continuous phase presents a complex microstructure, which required the use of a serrated sensor system to avoid the occurrence of wall depletion phenomena, (b) it controls the rheology of the whole dispersion and (c) the latter showed both physical stability and rheological properties suitable for applications as controlled release systems in pharmacy or cosmetics.