# Hydrophilic vs water soluble?

Is the term 'hydrophilic' a synonym of 'water soluble' or can you have a molecule that is water insoluble but hydrophilic (or water soluble and not hydrophilic)?

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• Interesting question, but I think this is more on-topic in chemistry. There are substances as cellulose which are hydrophilic but not water-soluble. – Chris Feb 3 '16 at 9:16
• Possible duplicate of biology.stackexchange.com/q/23763/3340 – WYSIWYG Feb 3 '16 at 9:55
• Consider this example: glass is hydrophilic but not soluble. – Zhe May 22 '17 at 22:22

Hydrophilic substances are not necessarily water soluble, and the two terms are not synonymous.

On a molecular scale, "hydrophilic" is defined by the IUPAC Gold Book$^1$ as:

'Water loving'. The capacity of a molecular entity or of a substituent to interact with polar solvents, in particular with water, or with other polar groups.

Note that the term hydrophilic does not necessarily refer to an entire molecule, and that interacting strongly with a molecule or a part of a molecule does not imply that it must be soluble in water.

Also, entire macroscopic surfaces can be said to be hydrophilic, meaning that they interact strongly with water but certainly are not soluble in water. In this context, the degree of hydrophilicity is measured by the contact angle, which can be simply stated as the angle formed when a droplet of water is placed on a surface, where a small angle (as seen from inside the droplet) means that the droplet has spread out over the surface, and that the surface is therefore hydrophilic.

So, although small, hydrophilic molecules are frequently water soluble, solubility plays no part in the definition of "hydrophilic" on any scale or context.

1) IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8

Yes, they mean the same thing actually. Going through any one of the several online sources available, you can clearly understand this: A Hydrophilic molecule is one in which interaction with water is thermodynamically more favorable than interaction with oil or other hydrophobic solvents. Such substances are generally capable of forming hydrogen bonding with water, which makes them soluble in water. One can in fact say that these types of molecules are soluble in all polar solvents, like alcohol and ammonia.

• Solubility depends on polarity but that doesn't mean that all polar substances will dissolve in water or any other polar solvent. – WYSIWYG Feb 3 '16 at 18:54
• @WYSIWYG That is the general rule: like dissolves like. Sure, there might be some exceptions. – ShankRam Feb 4 '16 at 1:55

Seems to me that a drop of oil hitting water beads up, showing no tendency to increase its area of contact with the water. On the other hand, a drop of gasoline, e.g. Heptane or Octane, will immediately spread over the water surface to such a degree that the actual thickness of the film will be one molecule. Such a tendency to maximize the area of contact sounds to me like hydrophilic while not soluble.