Rheological properties of creams with four different surfactant combinations - effect of storage time and conditions.

Changes in the rheological properties of four o/w cream formulations differing in the combination of surfactants were studied. The non-ionic surfactants used were soybean derivatives, polyethylene glycol 10 and 25 soya sterol, and sorbitol derivatives, sorbitan monooleate and trioleate. Combinations of the soybean and sorbitol derivatives were used. The rheological properties were tested during a 28-day storage period at three different storage conditions (cold, room temperature and accelerated conditions). In addition to dynamic and static rheological tests, droplet size distributions and conductivities of the creams were also determined. The consistency of the creams containing polyethylene glycol 10 soya sterol decreased during storage. Despite the greatest decreases in consistency, the creams containing polyethylene glycol 10 soya sterol exhibited the most viscoelastic structures with linear viscoelastic behaviour. Storing the creams for 28 days in the three different storage conditions made the differences in the consistency of the formulations smaller. All three storage conditions were involved when the conditions of the most viscoelastic cream of each formulation was specified. In the case of linearly viscoelastically behaving creams containing polyethylene glycol 10 soya sterol, all the rheological tests correlated with the droplet size distributions and the conductivity tests.