(169bi) Hydration Free Energies of Linear Alkanes (C1-C20) and Surfactants in Different Water Models

We report differences in the hydration free energies of alkanes ranging from methane to eicosane (C20) between different water models (SPC/E, TIP4P2005, OPC) calculated using the free energy perturbation method in molecular simulations. We find that the tail correction for the potential function and the potential cut-off distance associated with the alkane-water interactions have significant impact on the calculated hydration free energies. We also calculate the hydration free energy of cationic and charge-neutral surfactants with differently sized polar head groups. We find that cationic surfactants have a much higher affinity to get solvated in water as compared to charge-neural surfactants. The cationic surfactants are associated with formation of a water finger at the water-air interface, which changes the shape of the free energy profile near the interface. Lastly, we show how the solubility of surfactants can be significantly enhanced by changing the single substitutions on the alkyl tail. These results reveal the contributions of the polar head and the alkyl tail on the overall hydration free energy, and thus are useful for designing surfactants with the desired aqueous solubility needed for various applications.