Law for heat exchange and evaporation..

I wonder if there is any law for heat exchange. I have learnt that heat exchange between systems occurs when the temperature of respective systems differ. But in case of evaporation, the process creates a cooling effect even in an isothermal condition ( I mean by isothermal condition to be a isothermal situation between the evaporating liquid and the surrounding).

I would like to discuss the cooling effects produced by evaporation under isothermal condition.