The effect of temperature on the critical micelle concentration CMC of a given surfactant can be complex and depends on the specific surfactant being studied. Generally, an increase in temperature can lead to a decrease in the CMC of ionic surfactants, while the effect on nonionic surfactants can be more variable. Experimental evidence:A study by Rosen and Kunjappu 2012 investigated the effect of temperature on the CMC of various surfactants. They found that for ionic surfactants, such as sodium dodecyl sulfate SDS , the CMC decreased with increasing temperature. This is because the increased temperature weakens the electrostatic interactions between the head groups of the surfactant molecules, making it easier for them to aggregate and form micelles.For nonionic surfactants, such as polyoxyethylene 20 sorbitan monolaurate Tween 20 , the effect of temperature on CMC can be more complex. In some cases, the CMC may decrease with increasing temperature, while in others, it may increase or show no significant change. This is because nonionic surfactants rely on hydrophobic interactions between their tail groups, which can be influenced by temperature in different ways depending on the specific surfactant structure.Explanation of observed changes in CMC with respect to temperature:For ionic surfactants, the decrease in CMC with increasing temperature can be attributed to the weakening of electrostatic interactions between the head groups. As temperature increases, the thermal energy of the system increases, which can overcome the electrostatic repulsion between the charged head groups. This allows the surfactant molecules to come closer together and form micelles more easily, resulting in a lower CMC.For nonionic surfactants, the effect of temperature on CMC depends on the balance between the hydrophobic effect and the solvation of the surfactant molecules. As temperature increases, the hydrophobic effect becomes stronger, favoring micelle formation and leading to a decrease in CMC. However, increased temperature can also enhance the solvation of the surfactant molecules, making it more difficult for them to aggregate and form micelles, which would result in an increase in CMC. The overall effect of temperature on the CMC of nonionic surfactants depends on the specific balance between these two factors, which can vary depending on the surfactant structure and the solvent system.Reference:Rosen, M. J., & Kunjappu, J. T. 2012 . Surfactants and Interfacial Phenomena 4th ed. . John Wiley & Sons.