Solvation and hydrophobic forces

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Molecular ordering at surfaces

Israelachvili, Fig. 13.1

The liquid density at:

  • (a) The vapor/liquid interface.
  • (b) The solid/liquid interface.
  • (c) The solid/liquid/solid interface.

This effect seems to be on a molecular level but does it have any truth on a macroscopic level? Or microfluidics? What length scales are important?

Look who worked on this topic! Strange coincidence or the one and the same? Below is a paper abstract.

Ordering in a Fluid Inert Gas Confined by Flat Surfaces
Stephen E. Donnelly,1* Robert C. Birtcher,2 Charles W. Allen,2 Ian Morrison,1 Kazuo Furuya,3 Minghui Song,3 Kazutaka Mitsuishi,3 Ulrich Dahmen4
High-resolution transmission electron microscopy images of room-temperature fluid xenon in small faceted cavities in aluminum reveal the presence of
three well-defined layers within the fluid at each facet. Such interfacial layering of simple liquids has been theoretically predicted, but 
observational evidence has been ambiguous. Molecular dynamics simulations indicate that the density variation induced by the layering will cause xenon, 
confined to an approximately cubic cavity of volume approx  8 cubic nanometers, to condense into the body-centered cubic phase, differing from the 
face-centered cubic phase of both bulk solid xenon and solid xenon confined in somewhat larger (>=20 cubic nanometer) tetradecahedral cavities in 
face-centered cubic metals. Layering at the liquid-solid interface plays an important role in determining physical properties as diverse as the 
rheological behavior of two-dimensionally confined liquids and the dynamics of crystal growth.
1 Joule Physics Laboratory, Institute for Materials Research, University of Salford, Manchester M5 4WT, UK.
2 Materials Science Division, Argonne National Laboratory, Argonne IL 60439, USA.
3 National Institute for Materials Science, 3-13 Sakura, Tsukuba 305, Japan.
4 National Center for Electron Microscopy, LBNL, Berkeley, CA 94720, USA.

Solvation force - oscillatory

Israelachvili, Fig. 13.2

Note that the number of spheres in contact with the surface also varies with the maxima and minima.

The corresponding oscillatory solvation forces are shown in the lower graph.

Measured oscillatory forces

Israelachvili, Fig. 13.5

Forces between mica sheets separated by a liquid. The doted line is the theoretical calculation.

Not so good!!

Molecular “ordering” at a surface is a significant factor.

The hydrophobic efffect

Hydrophobicity refers to the physical property of a molecule that is repelled from a mass of water. Hydrophobic molecules tend to be non-polar and thus prefer other neutral molecules and nonpolar solvents. Hydrophobic molecules in water often cluster together forming micelles. Water on hydrophobic surfaces will exhibit a high contact angle. Examples of hydrophobic molecules include the alkanes, oils, fats, and greasy substances in general. Hydrophobic materials are used for oil removal from water, the management of oil spills, and chemical separation processes to remove non-polar from polar compounds.

According to thermodynamics, matter seeks to be in a low-energy state, and bonding reduces chemical energy. Water is electrically polarized, and is able to form hydrogen bonds internally, which gives it many of its unique physical properties. But, since hydrophobes are not electrically polarized, and because they are unable to form hydrogen bonds, water repels hydrophobes, in favour of bonding with itself. It is this effect that causes the hydrophobic interaction — which in itself is incorrectly named as the energetic force comes from the hydrophilic molecules. Thus the two immiscible phases (hydrophilic vs. hydrophobic) will change so that their corresponding interfacial area will be minimal. This effect can be visualized in the phenomenon called phase separation.

Israelchvili, Fig. 13.12

Water near a “hydrophobic” surface re-arranges to maintain hydrogen bonding. This decreases the entropy and hence raises the free energy.

Most significant in molecular interactions:

  • (a) Solubility
  • (b) Micellization
  • (c) Association
  • (d) Protein folding

Less so in

  • (e) Adhesion
  • (f) Wetting
  • (g) Flocculation
  • (h) Flotation.

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