Final answer:
When two liquids that are completely mixed, they form a single phase and so there is no interfacial tension between them. However, the mixture will have its own surface tension at the boundary with air, which is based on the net intermolecular forces of the combined liquids.
Step-by-step explanation:
If two liquids are completely mixed, the concept of interfacial tension typically refers to the tension at the boundary between two different phases, such as liquid and gas or between two immiscible liquids. When two liquids are completely mixed, meaning that they are miscible and form a single homogenous phase, there is no longer an interface to exhibit surface tension. However, the mixed liquid itself will display surface tension at its boundary with air or another non-mixing substance, which is determined by the combined intermolecular forces within the new mixture.
Surface tension is the elastic force within the liquid's surface that acts to minimize the surface area of the liquid. It is caused by the unbalanced forces acting on molecules at the surface of a liquid compared to those in the bulk of the liquid. Molecules at the surface are attracted to other molecules in the liquid, causing the surface to contract, leading to the formation of droplets or allowing denser objects to float on the surface if the surface tension is strong enough compared to the object's weight and shape.
Essentially, when two liquids mix, the intermolecular forces of the constituent liquids will determine the new surface tension of the mixture. For example, water possesses strong hydrogen bonds and therefore has a high surface tension, whereas alcohol has weaker intermolecular forces and a lower surface tension. A mixture would have a surface tension reflective of the intermolecular strengths within the new combined liquid.