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Surfactants decrease surface tension between two fluids. What do you call a substance that increases the surface tension between two fluids? How is this effect acheived?

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Briefly, the cause for surface tension is the difference in the energetic state between a molecule at the surface compared to a molecule inside the liquid phase: the molecule inside has lower energetic state as it has more inermolecular interactions (or, to get to the surface, a molecule needs to break intermolecular bonds, which needs energy). This is more pronounced with higher polarity or when ions (charges) are around as the interactions are then stronger.

You can increase the polarity of a (polar) solvent to get higher surface tension, by dissolving salts.

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The traditional specific term for a substance that, when added to a fluid, increases its surface tension is tensoionic. It is derived from the fact that it was first observed in solutions of ionic salts on water.

However, the recent trend is to consider all solutes which have an effect on surface tension as surfactants, i.e. surface agents, whether their contribution to surface tension is positive or negative. This usage can however be ambiguous.

The effects driving this phenomenon are complex and thermodynamic in nature: at low concentrations it seems to be driven mostly by the Gibbs isotherm behaviour of mixtures, but at higher concentration, as solvation takes over an important fraction of the solvent, an entropic effect dominates, and all strong electrolytes tends to the same, entropic-driven asymptotic behaviour.

For a discussion on the thermodynamics involved see Levin, Y. Interfacial tension of electrolyte solutions. J. Chem. Phys. 2000, 113, 9722-9726

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