1) Presence of impurities either on the surface or dissolved in it.
The presence of impurities on the surface or dissolved in the liquid greatly affects the surface tension of the liquid. Highly soluble substances can increase the surface tension of a liquid while others with low solubility substances decrease the surface tension of a liquid. A great example of this water. Seawater has more surface tension than distilled water while distilled water has more surface tension compared to the oil-water mixture.
Another factor is temperature.
Temperature is an important factor for surface tension. it is because of viscosity. The hotter the liquid is, the lesser is the viscosity of a liquid which is a more fluid liquid has lesser surface tension. When the temperature reaches boiling point, the viscosity of the liquid becomes almost like that of gas, and this effect causes surface tension becomes zero. Whereas, liquids with a lower temperature has more viscosity, and more viscosity means more surface tension. When the liquid is near and or on its freezing point, that is when viscosity of the liquid is almost like of a solid, where the surface tension of the liquid is at maximum. A great example of it is crude oil. Hot crude oil has lesser surface tension, which causes the liquid move almost like water in pipes and pumps and cold crude oil, which has more surface tension, clogs up in pipes and pumps.