Investigating the properties of ionic water at the surfactant-water interface using time resolved and 2D-Sum Frequency Generation (SFG) spectroscopy

A surfactant is a chemical substance which has both water-attractant (hydrophilic) and water-repellant (hydrophobic) parts in its structure. The most common property of a surfactant is its ability to become adsorbed at the water (as well as oil or any hydrophobic substance) surface. The hydrophilic (water-attractant) parts remain in contact with water and the hydrophobic (water repellant) parts remain away from water. Since the water-attractant parts of the surfactant molecules have their own chemical nature, they influence the “fundamental” chemical nature of the water molecules at their vicinity. This effect becomes very prominent when these hydrophilic parts of the surfactant are ionic (charged). Depending on the charge (positive/ negative) vicinal water molecules behave completely differently.
Using time resolved sum frequency generation methods we have studied water in contact with the positively charged lipid DPTAP (1,2-dipalmitoyl-3-trimethylammonium-propane) and the negatively charged surfactant SDS (sodium dodecyl sulfate). For positively charged lipids, the water displayed heterogeneity and dynamical behavior that is indistinguishable from bulk water. A simple model developed for bulk water, taking into account ultrafast energy transfer between water molecules and vibrational relaxation via the bend overtone, fully accounts for the observations. This contrasts the behavior of water in contact with negative lipids, for which at least two distinct sub-ensembles of water can be directly identified. In between these two sub-ensembles, ultrafast energy transfer is occurring. The difference between the positive and negative surfactant can be understood by noting the alignment of the O-H groups. For positively charged lipids, the charge-induced alignment of water molecules results in interfacial water molecules with O-H groups pointing toward the bulk, so that the interfacial water interacts directly with bulk water. For negative surfactants the O-H group is pointing toward the surfactant.