Re: Why are some plant pigments are more soluble than others in a solvent?

Aaron,
    The solubility of compounds, including plant pigments, depends on their 
molecular structure.  The general rule for solubility is, "like dissolves 
like."  That is, polar solvents such as water will dissolve other substances 
that are polar.  A polar bond is one in which the sharing of electrons is 
unequal between the two atoms involved, with the result that the bond has an 
unequal distribution of charge – partial positive and partial negative 
charges at opposite ends of the bond.  These partial charges allow for 
interactions with water molecules, which are themselves polar, allowing the 
water to effectively solvate the polar molecule.  Ionic compounds like salt 
are very soluble in water, because the separated positive and negative ions 
can both be very well solvated by the polar water molecules.  Plant pigments 
are covalently bonded molecules, and cannot dissociate into ions, but can 
still be well solvated if polar groups are present.
      Thus, plant pigments which have polar groups on the molecule (such as 
alcohol, amine, or carboxylic acid groups) will be much more soluble in 
water that molecules that have few or no polar substituents.  Such nonpolar 
pigments would be more soluble in a nonpolar solvent, such as benzene.  

A web site that contains information about the polarity of water, and how it 
interacts with charges to assist solvation is the following:
 http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/page3.html