The hydrophobic chain length of a surfactant has a significant effect on its critical micelle concentration CMC in water. As the length of the hydrophobic chain increases, the CMC of the surfactant decreases.The reason behind this is that surfactants with longer hydrophobic chains have stronger hydrophobic interactions, which promote the formation of micelles. These stronger interactions make it energetically more favorable for the surfactant molecules to aggregate into micelles, thus requiring a lower concentration of surfactant to reach the CMC.In summary, increasing the length of the hydrophobic chain of a surfactant will decrease its CMC in water, making it more effective at forming micelles at lower concentrations.