The addition of foreign particles reduces the surface tension between particles of the substance under observation. This is something we accept and something that makes common sense. But how do we explain the common observation that soap water has higher surface tension than pure water , considering it's ability to form soap bubbles? Or is it just because for the soap bubble we have 2 surfaces and hence 2 times the force in spite of lower surface tension?
As far as I know, soap reduces surface tension in water.
This happens because water molecules in the liquid attract each other thanks to their polarity, more precisely, the polarity causes an electric attraction between them. In the surface, water molecules don't have any other water molecules above them, so they attract with a stronger force their neighbouring particles, which creates surface tension as we know. Now, if you add soap, soap molecules make their way between the water molecules, so the distance between them increases and the electric force decreases, which in turn lowers surface tension.
This is what I have always understood when I think about the subject, but maybe there's a more precise answer to your question that proves an increase in surface tension.