The Effects of Dissolved Hydrophobic and Hydrophilic Groups on Water Structure
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The Effects of Dissolved Hydrophobic and Hydrophilic Groups on Water Structure Qiang Sun1 Received: 16 November 2019 / Accepted: 31 March 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract In this work, Raman spectroscopy is employed to investigate the effects of hydrophobic and hydrophilic groups on water structure. In aqueous methanol, ethanol, 1-propanol solutions, the addition of methylic groups (-CH3) raises the intensity ratio of the 3430 cm−1 sub-band to the 3220 cm−1 peak, and also increases the intensity of free OH vibrations. From our recent studies, this means that the increase of hydrophobic groups raises the ratio of DA (single donor-single acceptor) to DDAA (double donor-double acceptor) hydrogen bonding. Additionally, increasing the hydrophilic groups (–OH) as 1-propanol, 1,2-propanediol and 1,2,3-propanetriol solutions, also increases the ratio of DA to DDAA structural motifs, but decreases the free OH vibrations. The results of this study do not support the “iceberg” model of hydrophobic effects. Additionally, the dissolved hydrophobic and hydrophilic groups may have different effects on free OH vibrations. Keyword Water · Hydrogen bonding · Hydrophobic effects · Raman spectroscopy
1 Introduction The hydrophobic effect, the tendency of non-polar molecules or molecular surfaces to aggregate in an aqueous solution (or to be expelled from water into an aggregate), is regarded to be the major driving force in biological processes ranging from membrane formation to protein folding and ligand binding, and is thus of fundamental importance in physical chemistry and biology. Hydrogen bonding is stronger than van der Waals interactions, so that water should play an important role in the process of hydrophobic effects. Therefore, it is necessary to study the effects of dissolved solutes on the structure of water. Historically, the classical mechanism for hydrophobicity was proposed by Frank and Evans [1], and advanced by Kauzmann [2], Tanford [3], and many others. To explain the negative entropy of solvation of non-polar molecules, it predicts that water immediately surrounding the hydrophobic solutes is more “ordered” than bulk water [4–6]. This is terrmed “iceberg” formation (or clathrate structure) around the hydrophobic molecule [1]. * Qiang Sun [email protected] 1
Key Laboratory of Orogenic Belts and Crustal Evolution, The School of Earth and Space Sciences, Ministry of Education, Peking University, Beijing 100871, China
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Vol.:(0123456789)
Journal of Solution Chemistry
To unravel the physical origin of hydrophobicity, many works have been conducted on the effects of a hydrophobic group on water structure. In agreement with “iceberg” model, some studies [7–11] support the increased structure for water in contact with hydrophobic groups. In a study by Ben-Amotz et al. [7], isotopic and temperature-dependent Raman scattering measurements indicated that the hydrophobic hydration shells of linear alcohols ranging from methanol to heptanol have a hydrophobi
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