A small molecular size system giving unexpected surface effects: α-Cyclodextrin + sodium dodecyl sulfate in water

Maximum drop volumes (MDV) and the resultant surface tension values (σ) of α-cyclodextrin (α-CD) + sodium dodecyl sulfate (SDS) aqueous mixtures have been determined over a broad concentration range of both solutes at 283.15, 293.15, 303.15, 313.15, and 323.15 K. Drops significantly larger than those of pure water (up to ∼25% larger) were observed at low temperatures for solutions with [α-CD]/[SDS] concentration ratios, ≳2, producing unexpectedly high surface tension values. Our results indicate that at certain solute concentration ratios and temperatures, the drop volume method provides wrong values for equilibrium surface tensions. This is due to the high viscoelasticity of the surface film whose effect is important even though the injection rate of the drops was slow and the solutes molecular sizes are small.

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