Hydrogels in aqueous phases of polyvinylalcohol (PVA), surfactants and clay minerals
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Aqueous solutions of synthetic clay minerals have been studied in the presence of surfactants and water-soluble polyvinylalcohol (PVA). The PVAs (PVA 1, PVA 2) had a molecular weight of about 105 Dalton and a degree of hydrolysis of 82%. The PVA-samples were surface active and lowered the surface tension to 43 mN/m. As a consequence of their amphiphilic nature the PVA molecules bind strongly to clay mineral particles. On saturation the clay mineral particles adsorb the fivefold weight of PVA of their own weight. It is concluded that the thickness of the adsorbed layers on both sides of the clay mineral is in the range of the hydrodynamic diameter of the PVA-coils in the bulk phase.
When the clay mineral particles are not saturated with PVA, they act as cross-linking agents for the PVA. The whole systems are physically cross-linked and assume gel-like properties. Rheological measurements show that samples behave like soft matter with a yield stress value. All of them have a frequency independent storage modulus which is an order of magnitude larger than the loss modulus. The hydrogels become stronger as PVA concentration increases.
Small amounts of cationic surfactants bind on the clay mineral. The interface of the clay mineral becomes more hydrophobic and the binding of the PVA on the clay mineral is strengthened. With rising concentration of the surfactant the surfactant molecules bind on PVA and the PVA becomes hydrophilic. As a consequence the PVA can no longer bind on the clay mineral and the gels transform to viscous and turbid solutions. Small amounts of cationic surfactants therefore stiffen the hydrogels while larger amounts cause phase separation and a solution with low viscosity. Anionic surfactants like SDS do not bind on the clay mineral, but strongly on the PVA. With increasing SDS concentration, the hydrogels become stiffer at first but thereafter they break and transform to viscous fluids.
In PVA-solutions without the clay minerals both cationic and anionic surfactants bind to the PVAs in the aqueous solution. With increasing concentration of surfactant, the viscosities of the solutions pass over a maximum. In this respect the PVAs behave like hydrophobically modified water soluble polymers. The surfactants bind to the hydrophobic microdomain and thereby crosslink the polymer molecules. On saturation the polyvinyl alcohol with anionic surfactant become hydrophilic and the network character disappears to a certain extent.
KeywordsHydrogels Clay minerals Polyviny lalcohol Polymer surfactant interaction
The authors would like to thank Wacker Polymer Systems GmbH & Co. KG in Burghausen (Germany) for supplying the compounds for this investigation and, in particular for the financial support for Dr. Jing Liu.
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