Difference in the hydration state of water at the hydrophobic interface of structural isomers of propanol investigated by U.V visible absorption and Raman spectroscopic study

Abstract

In this work, the nature of hydration shell water around hydrophobic groups of alcohol has been investigated by studying the U.V visible absorption energy shift of p-nitro-aniline upon interaction with water-alcohol mixture in different proportion. Analysis shows that, at low mole fraction of alcohol the absorption energy shift with composition of alcohol-water mixture originates from the interaction of hydrophobically modified water with PNA. It is further observed that, the absorption energy changes nonlinearly with the chain length of alkyl group of alcohol. From Raman studies it is apparent that, the absorption energy shift originates from the changing population of dangling OH interacting with PNA. The central part of the study has been to investigate the difference in the hydrophobic environment due to variation in the arrangement of hydrophobic groups in propanol. The U.V visible absorption studies of PNA in 1-propanol and 2-propanol water mixture show that 1-propanol bears higher population of dangling OH compared to 2-propanol. In addition, we have also performed Raman spectroscopic studies in the region of OH stretch. The spectra of hydration shell water are extracted by RD-SCF method. The characteristic peak due to dangling OH was analyzed and it also shows 1-propanol to possess relatively higher amount of dangling OH compared to 2-propanol. This joint study confirms that arrangement of hydrophobic moiety in small molecules can affect the water hydrogen bonding of water at the hydrophobic interface.