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The Real Reason Why Oil and Water Don't Mix
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Abstract
The majority of introductory chemistry and organic chemistry textbooks state that oil and water don't mix because of enthalpic effects. These texts generally make the argument that the mixing process is endothermic, reasoning that the water-water hydrogen bonds that must be broken in order to accommodate the solute are much stronger than the subsequent solvent-solute dipole-induced dipole intermolecular forces that are formed. In fact, in most cases the mixing process is exothermic, so the immiscibility of the two liquids must be explained by a loss of entropy in the system. The widely accepted model explaining the hydrophobic effect invokes the formation of icelike clathrate hydrate "cages" around nonpolar solute molecules. Water molecules at the surface of these relatively rigid clathrate structures are strongly hydrogen-bonded to one another. The formation of these solvent "cages" explains why both Delta H and Delta S are negative for the solution process, and the endergonicity of solvation is thus due to entropy and not enthalpy. Authors should remove from their textbooks the incorrect enthalpic/hydrogen-bond explanation for the hydrophobic effect. Because aspects of the correct entropic/clathrate "cage" explanation lie beyond the scope of introductory or organic chemistry courses, it may be wisest to omit any detailed physical explanation of the "like dissolves like" phenomenon. If the overall format of the text permits, a brief discussion of solvation entropy effects might be included in the section dealing with the immiscibility of oil and water
Keywords (Audience):
High School / Introductory ChemistryKeywords (Domain):
Physical ChemistryKeywords (Pedagogy):
Textbooks / Reference BooksKeywords (Subject):
Theoretical ChemistryCiting Articles
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This article has been cited by 9 ACS Journal articles (5 most recent appear below).
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Carmen V. Gauthier , Gabriel PintoJournal of Chemical Education2009 86 (11), 1281Any student that has spent time in the kitchen knows that hot vegetable oil will pop and spatter violently after coming into contact with water such as that on the surface of foods (meat, fish, potatoes, etc.). This well-known effect can be used as an ...
Hydrophobic Solvation NOT via Clathrate Water Cages
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Todd P. SilversteinJournal of Chemical Education2008 85 (7), 917I read with great interest Konrad and Lankau’s recent article in this Journal on the hydrophobic effect. These authors discuss the thermodynamics underlying Henry’s law to shed light on the process of hydrophobic solvation. The approach strikes me as ...
Contrasting Nonaqueous against Aqueous Solvation on the Basis of Scaled-Particle Theory
Henry S. Ashbaugh, Lawrence R. PrattThe Journal of Physical Chemistry B2007 111 (31), 9330-9336Contrasting Nonaqueous against Aqueous Solvation on the Basis of Scaled-Particle Theory
Henry S. Ashbaugh, Lawrence R. PrattThe Journal of Physical Chemistry B2007 111 (31), 9330-9336Normal hexane is adopted as a typical organic solvent for comparison with liquid water in modern theories of hydrophobic hydration, and detailed results are worked-out here for the C-atom density in contact with a hard-sphere solute, ρCG(R), for the full ...
Control of Chemical Equilibrium by Solvent: A Basis for Teaching Physical Chemistry of Solutions
Oleg V. Prezhdo and Colleen F. Craig , Yuriy Fialkov , Victor V. PrezhdoJournal of Chemical Education2007 84 (8), 1348Control of Chemical Equilibrium by Solvent: A Basis for Teaching Physical Chemistry of Solutions
Oleg V. Prezhdo and Colleen F. Craig , Yuriy Fialkov , Victor V. PrezhdoJournal of Chemical Education2007 84 (8), 1348The mechanisms by which solvent can be used to control chemical equilibria are analyzed from a thermodynamic perspective and illustrated with several examples. The solvent effect is described in terms of universal- and specific-solvation components, and ...
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- Received: August 03, 2009
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