The interaction of hydrophobic groups with water is crucial in many self-organizing processes like the folding of proteins and the formation of lipid membranes. In the recent past we have studied the dynamics of water surrounding hydrophobic methyl groups with femtosecond mid-infrared spectroscopy and dielectric relaxation spectroscopy. We observed that the molecular reorientation of hydrating water is slowed down strongly: at room temperature the hydrating water shows a reorientation time constant >10 ps, over 4 times slower than the molecules in bulk liquid water. We also observe that the slowing down effect strongly decreases with temperature, in line with the thermodynamic observations. The strong effect of hydrophobic methyl groups on the reorientation of water shows that hydrophobic methyl groups frustrate the collective hydrogen-bond network dynamics of water that are required to locally reorient a water molecule.
In the future we will study the interaction of water with hydrophobic membrane systems that are specifically targeted at the desalination and purification of water. Examples are polydimethylsiloxane and poly(ether-block-amide) membranes. The membrane material is hydrophobic, but still allows the fast passage of water vapour through pores as wide as possible, but not so wide that water can enter as a liquid. Hence, the purification of water relies on the fact that water can only pass as single molecules or small clusters and not as a liquid solution. The interaction of a hydrophobic membrane with water, and the related structure of water at the membrane surface and inside the pores are poorly understood and form an experimental challenge. Femtosecond mid-infrared spectroscopic techniques in combination with macroscopic measurements will help in obtaining a molecular-level understanding of these membrane systems and to develop new concepts and membrane materials to make clean water using solar powered membrane distillation technology. This project is part of an industrial partnership program (IPP) with Wetsus.
Lukasz Piatkowski , Adam D. Wexler , Elmar C. Fuchs , Hinco Schoenmaker and Huib J. Bakker, Ultrafast vibrational energy relaxation of the water bridge Phys. Chem. Chem. Phys., (2011)