The Chemistry of Liquids, Part 2 - Hydrophilic Interactions & The "Special" Properties of Water

Hydrophilic Solvents

The word, "Hydrophilic", literally means, "Water Loving".  Hydrophilic liquids are more like water than other less polar solvents.  Examples of hydrophilic solvents are acetonitrile and acetone, of which both exhibit dipole moments causing both to attract water molecules.  This attraction with water molecules causes hydrophilic solvents to be highly miscible with water.

The Special Properties of Water

Water does not behave like the low molar mass compound that it is.  Other compounds, much heavier than water, exist in the gas state  at ambient conditions.  Carbon Dioxide, CO2, and Chlorine, Cl2, are two examples.  Lighter compounds existing as liquids at ambient conditions do so because of electrostatic intermolecular attractive forces resulting from dipole moments of individual molecules.  Water is just such a compound, existing as a liquid between zero and one hundred degrees Celsius.  The attractive forces between water molecules are a special case of dipole-dipole interactions called hydrogen bonding.  The overall strength of water hydrogen bonds are greater than the average kinetic energy of individual molecules below one hundred degrees Celsius, thus causing water to exist as clusters of molecules.  These liquid water molecule clusters are in a disordered and rapidly changing state of flux.  Carbon Dioxide molecules, on the other hand, lack net dipole moments and so do not attract each other.  Both spaces between and  kinetic energies of carbon dioxide molecules are much greater when compared to the corresponding properties of water.  The following figure provides a visual depiction of the disparities between water and carbon dioxide.

 

 

Notice the cluster-like formation of liquid water molecules vs. the complete separations and greater translational velocities of gaseous carbon dioxide molecules.

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