Research Summary
The main purpose of the present study was to gain understanding of the rheological (elastic, viscoelastic, and viscous) properties of pharmaceutical creams containing sorbitan fatty acid ester surfactants. These surfactants include sorbitan monolaurate, sorbitan monopalmitate, sorbitan monostearate, sorbitan monooleate, and sorbitan trioleate. The study investigated the effects of an increase in the hydrocarbon chain length, the presence of double bonds in the hydrocarbon chains, and the concentration of the surfactant on the rheological properties of the sorbitan fatty acid ester surfactant-containing creams. Additionally, the study examined the effects of the volume of the inner phase and short-term storage on the rheological properties of these creams.
The creams analyzed were either simple, three-component creams or complex, multi-component creams. The rheological properties were determined through dynamic oscillation stress sweep, oscillation frequency sweep measurements, static creep recovery measurements, and time-dependent viscosity measurements. An increase in the length of the saturated hydrocarbon chain of the surfactant enhanced the elastic nature of the creams. The presence of a double bond in the hydrocarbon chain of the surfactant decreased elasticity, whereas an increase in the number of double bonds within the hydrocarbon chain increased the elastic nature of the creams. Furthermore, an increase in the concentration of the surfactant and the volume of the inner phase contributed to the elastic nature of the creams.
The study found that storage time decreased elasticity, and storing the creams under three different conditions leveled off the differences in elastic properties. In the three-component cream formulations, the rheological properties were mainly attributed to the interfacial properties of the surfactants. In contrast, in the multi-component cream formulations, the rheological properties were also influenced by the multiphase structures of the creams in addition to the surfactants used.
Determining the rheological properties, including unrecoverable viscous, partly recoverable viscoelastic, and recoverable elastic properties, provided valuable insights into the structural and intermolecular properties of the creams. Rheological measurements were shown to be sensitive tools for detecting structural changes in pharmaceutical creams and should be regarded as an integral part of the quality evaluation process for such products.
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