Reload Index (ZRiChK UMCS)
THE INFLUENCE OF ASCORBIC ACID ON RHEOLOGICAL PROPERTIES OF MICROEMULSION REGION
OF THE SDS, PENTANOL AND WATER SYSTEM
Rheology occurs to be an excellent tool for information about changes in the colloidal structures and their interactions. It is well known that the molecules of associated colloids generally form spherical aggregates in the aqueous solution above the CMC concentration value. Under appropriate conditions of concentration, salinity, temperature, presence of counter ions these spherical micelles grow in size or change their shape resulting in the formation of rodlike structures or even long flexible micelles. For the dilute ionic surfactant solution, a simple way to transform spherical to cylindrical micelles is to change the packing parameter i.e. to decrease the effective headgroup area by adding salt, which diminishes the electrostatic repulsion between the headgroups and allows them to be located closer. With the increasing salt concentration the cylindrical aggregates grow to flexible wormlike micelles and behave like polymers. The formation of such structures modifies strongly rheological properties of surfactant solutions. This effect is more pronounced in the systems with cationic surfactants than in the systems containing non-ionic and anionic surfactants.
Our objective is to investigate the effect of the water-soluble antioxidant, i.e. ascorbic acid (vitamin C, AA) on the viscosity of microemulsion of SDS, pentanol and water.
This work is part of our wider studies of influence of the microenvironment (i.e. type of micellar system) on activity of antioxidants typified of various hydrophobicity [1-5]. The additional advantage of investigations of antioxidant behaviour in the molecular aggregates is that they can help to understand naturally occurring processes. It is because micelles can be considered as the simplest model of the cell membrane present in the biological system. The knowledge about rheology of emulsion seems to be useful not only in studying the biological processes but first of all in various pharmaceutical and cosmetic formulations and processing variables that result in a change of dispersed phase volume, size distribution and particle-particle interaction as well as particle deformability.
As follows from our own experiments ascorbic acid is dissolved/ solubilized up to 60% in the microemulsion region of SDS/ pentanol/ water system. The addition of AA to the system caused reduction of the microemulsion region in the phase diagram. Ascorbic acid serves as a competitive organic anion to the surfactant changing W/O microemulsion system towards O/W one.
Knowing the microstructure helps understand the rheology of emulsion systems. The relationship between the structure and rheology is nicely illustrated in Fig.1, which shows how the viscosity of a mesophase structure such as a lamellar phase can be many orders of magnitude higher than that of an isotropic swollen micellar phase that may exist with a composition only slightly different from that of the mesophase.
Experimental viscosity values presented in Fig. 2 change in the same manner as is typical for emulsions presented in Fig. 1.
We can sum up obtained results in a form of the following general conclusions:
1. The W/O and O/W microemulsion region of the SDS/ pentanol/ water system behaves as typical Newtonian fluid – viscosity remains constant independent of the shear rate. For the bicontinuous system the shear thinning behaviour can be observed.
2. The ascorbic acid addition to the system caused the microemulsion viscosity increase, the highest in the bicontinuous region and in the lowest temperature.
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