What other, rather easy to obtain, salts are more hydrophilic than $\ce{NaCl}$? Is there a hydrophilic scale for substances like the Mohs scale of mineral hardness?


As per DavePhD's answer, the contact angle of NaCl with water is zero, so we need another measure.

I think the word you are looking for is hygroscopic. http://en.wikipedia.org/wiki/Hygroscopy Substances which are so hygroscopic that they dissolve in the water they absorb from the atmosphere are said to be deliquescent, although the property of deliquesence in itself does not directly indicate the strength of a substance as a drying agent.

The strength would best be measured as the equilibrium vapour pressure at a certain temperature, or alternatively, the temperature required to dehydrate: i.e. the temperature at which the vapour pressure of water vapour above the hydrated form of the substance becomes equal to atmospheric.

Cheap substances which are hygroscopic find applications as desiccants

Here is a list of some common ones http://en.wikipedia.org/wiki/List_of_desiccants. It includes the workhorse laboratory desiccants: silica gel for use in desiccators where products are dried under vacuum; molecular sieves, for drying solvents; CaCl2 for drying gases; and MgSO4 and CaSO4 in their anhydrous form for a quick drying of organic solutions. I would assume (though I'm not sure) that all of these would be stronger than NaCl.

Unfortunately the really strong ones are missing, so here's a list of mine. The weaker ones above tend to act by physical absorption, or by changing from anhydrous crystals to hydrated crystals. Stronger ones tend to react chemically.

Concentrated sulfuric acid Strong enough to dehydrate sugar to "charcoal." One of the reasons for the affinity is the energy released when H2SO4 proponates H2O, forming HSO4- + H3O+. More of a curiousity than a desiccant. Not widely used as a desiccant because of its acidity.

Phosphorous pentoxide Reacts with water to form various different polymeric phosphoric acids, ultimately ending up with 2P2O5 + 6H2O= 4H3PO4 though strength is gradually reduced as water is absorbed. Commonly used in the lab when a strong desiccant is required.

Calcium Oxide The reaction CaO + H2O = Ca(OH)2 has been proposed for large scale energy storage (the storage occurs in the reverse direction), but so far has been used only for smaller applications. As the calcium species are basic, exposure to air causes absorption of CO2, which then irreversibly degrades the material to bicarbonate and carbonate (unless heated to a much higher temperature.)

Sodium Metallic sodium, extruded into wire using a press to increase its surface area, is the best way to ensure absolutely dry solvents.

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The scale for degree of being hydrophilic is contact angle.

Contact angle ranges from 0 to 180 degrees, 0 being the most hydrophilic and 180 being the most hydrophobic.

180 degrees means that a drop of water on the surface forms a sphere, which only contacts the solid surface at one tangential point.

0 degrees means that the water spreads in a thin, flat layer on the solid surface.

NaCl and NaF have contact angles of 0 degrees according to The Significance of Interfacial Water Structure in Soluble Salt Flotation Systems Journal of Colloid Interface Science vol. 235, page 150, so no salts are more hydrophlic.

KCl has a contact angle of 8 degrees, and KI 25 degrees.

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  • $\begingroup$ Curious. Is the measurement of contact angle still well-defined when the liquid being measured is able to dissolve the surface it's on? $\endgroup$ – Nicolau Saker Neto Feb 19 '15 at 16:48
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    $\begingroup$ actually, those values are for salt-saturated solution contacting the solid salt, not pure water $\endgroup$ – DavePhD Feb 19 '15 at 17:02
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    $\begingroup$ There are salts, like calcium chloride (CaCl2), that are sufficiently hydrophilic to dissolve in water vapor from the surrounding air if not kept in an airtight container. NaCl doesn't do this, so i would have assumed CaCl2 is more hydrophilic than NaCl - but clearly, a contact angle smaller than 0 is impossible. Is there a different word than "hydrophilic" for this, so CaCl2 is more "whatever" than NaCl? $\endgroup$ – Guntram Blohm supports Monica Feb 19 '15 at 18:44
  • $\begingroup$ @GuntramBlohm CaCl2 is much more "hygroscopic" than NaCl. $\endgroup$ – DavePhD Feb 19 '15 at 18:54
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    $\begingroup$ I wonder if the OP is referring to something besides hydrophilicity - maybe solubility, hygroscopy, or something like enthalpy or entropy in solution? For hydrophilicity though, this is a great answer. $\endgroup$ – thomij Feb 19 '15 at 20:42

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