The Meaning of the Words

Hydrophobicity comes from the Greek words for water (hydor) and fear of something (phobos), while hydrophilicity comes from the Greek words for water (hydor) and to love (phileo).  Hydrophobic means water-hating, while hydrophilic means water-loving.

The Effects

Hydrophobic materials do not like water and they will do anything to get away from the water.  Oil and gasoline, for instance, float on water.  But they are also hydrophobic, so in the process of going to the surface, they are getting away from the water and minimizing their contact with the water.  When oil and gasoline are floating on water, the only interface between them and the water is the lower oil or gasoline surface which is against the upper water surface.  Were the oil or gasoline dispersed in the water with a high intensity disperser, small droplets of oil and gasoline would form within the water, but they would once again move to the surface where they would coalesce to form thin layers atop the water surface. 

Most organic materials, and especially those used as additives in ceramic slips and bodies, are also hydrophobic (or at least partially hydrophobic.)  As a result, they too will be positioned at interfaces to minimize their contact with the water.  When such additives are mixed into a mixture of oil and water, for example, the hydrophobic part of the additives will associate with the oil and the hydrophilic part with the water.  As a result, one would expect to find such additives at the oil/water interface.

To add some complexity to this topic, consider that many organic additives to ceramic slips, slurries, and bodies, are partially hydrophilic and partially hydrophobic.  This isn't just a rare phenomenon.  It is more the rule than the exception.  Part of each additive likes water;  part of it doesn't.  Just as in the example above, when this is the case, the hydrophilic part of the additive aligns itself to interact with the water, while the hydrophobic part aligns itself away from the water.

What is the Cause and What is the Effect?

The effect, when a hydrophobic material is mixed into water, is that it tries to get out of the water.  But having said that, what is the real driving force behind the retreat from contact with water by hydrophobic additives'?  Do the additives try to get out of the water, or does the water push the additives out?

You can think of the hydrophobic effect as if the additives hate water and do all that they can to minimize their contact with water.  In fact, the name of the phenomenon is consistent with this point of view.

But the real driving force is the water.  The easy way to explain this is to say that the water likes to be in contact with water more than it likes to be in contact with hydrophobic materials.  As a result, the water pushes hydrophobic materials out to the surfaces.  Water pushes all hydrophobic materials to any available surface with the water.  This can take the form of pushing the hydrophobic materials to the water/air interface at the upper surface of the water, or toward beaker/water or tank/water interfaces at the side walls or bottom, or toward particle/water interfaces which are spread throughout the slip, slurry, or body.  This last position is the desired position for partially hydrophobic additives in ceramic systems.

C. Tanford (The Hydrophobic Effect:  Formation of Micelles and Biological Membranes, 2nd ed., John Wiley and Sons, NY, 1980) said, "The free energy of attraction between water and hexane or octane at 25^oC is about
-40kJm^-2 of contact area, but the free energy of attraction of water for itself is -144kJm^-2.  It is clearly the latter alone that leads to a thermodynamic preference for elimination of hydrocarbon-water contacts;  the attraction of hydrocarbon for itself is essentially the same as its attraction for water."  This statement clearly labels the water as the cause, and the expulsion of the hydrophobic materials as the effect.

According to Tanford, the free energy of attraction between water and these two hydrophobic materials is about
-40kJm^-2.  The free energy of attraction between these hydrophobic materials and themselves is approximately the same value.  But the free energy of attraction of water for itself is more than three times as strong.  So in the process of each water molecule trying to associate itself with other water molecules, the hydrophobic materials are pushed out of the water.  That is, hydrophobic materials are pushed to any available interfaces between the water and something else -- air, glass, tank, or particle.

How Strong Is the Hydrophobic Effect?

So far in this series, we have discussed van der Waals attractive forces.  Hydrophobic forces are stronger than van der Waals attractive forces.  Another type of force we will consider in the next article is electrostatic.  Hydrophobic forces are also stronger than electrostatic forces.

What is the significance of this force?  When hydrophobic additives are dispersed in ceramic slips or bodies, hydrophobic forces will push additives onto powder surfaces.  Even when the hydrophobic additives have net negative ionization sites and the particles have net negative surface charges, the hydrophobic effect will overpower the electrostatic repulsion! 

Let me say this again.  Even when particle surface charges are negative, anionic hydrophobic additives can be disperse into suspensions or bodies and they will enhance the negative surface charges of the particles.  Hydrophobic additives will coat particles -- even when the additives are electrostatically negative and the particles are also electrostatically negative.  Normally, one would expect like-charges ( negative and negative) to repel, but the hydrophobic effect is stronger -- and the additives will be forced onto the particle surfaces anyway.  When this happens, negative surface charge densities are usually enhanced.

Additive Orientation Effects

In the same book, Tanford also made this comment:  "Molecules of a dual nature, with one part soluble in water, and one part that is expelled from it, are forced by their duality to adopt unique orientations with respect to the aqueous medium, and to form organized structures."

Let's consider a typical organic dispersant (which is an anionic polyelectrolyte) for ceramic systems .  Several common dispersants fall into this category.  These are polymers, which contain side groups (or legs) along the length of the organic backbone.  I like to picture them as millipedes with negative charges at the end of each leg.  The backbone of the polymeric additive (which corresponds to the body of the millipede) is hydrophobic.  The negative charge-entities at the end of each leg are hydrophilic.  Frequently, these additives are sodium or ammonium salts.  When mixed into solution, the sodium and ammonium ions move off into solution, leaving behind their negatively charged sites. 

The picture, then, is this:  the millipedes' backbones (the polymeric additive backbones) lay down onto particle surfaces with their negatively-charged hydrophilic legs (the negatively-charged side groups) dangling up into the water.  If particle surfaces were originally negatively charged, each millipede's negatively charged legs will dangle out into the water near the surface and the particle's net negative surface charge will be enhanced.  If particle surfaces were originally positively charged, some of the millipede's negatively charged legs will cancel some positive surface sites, and other negatively charged legs will dangle out into the water to change the particle's surface charge from net positive to net negative.

This phenomenon is advantageous in some particle systems which contain both net negative and net positively charged surfaces simultaneously.  Consider kaolinite crystals, for example.  Surface charges of the broad planes can be negative while edges are positive.  Consider other mixtures, for example silica plus alumina.  Alumina is net positively charged up to about pH 10, while silica is net negatively charged at all pHs higher than about 2.  In most mixtures of alumina and silica, alumina will be positively charged while the silica will be negatively charged.  Experience shows that a relatively small addition (a fraction of a percent) of one of these polymeric anionic polyelectrolytes can convert all particle surfaces to net negative.   

Summary

The hydrophobic effect causes organic and polymeric additives with hydrophobic character, when dispersed throughout ceramic slips and bodies, to coat particles as their contact with water is minimized. 

The hydrophobic effect is stronger than both van der Waals and electrostatic forces.  Regardless of the existing surface charge, the hydrophobic effect will push hydrophobic additives onto particle surfaces.  With the help of the hydrophobic effect, negative surface charge densities can be enhanced using commonly available anionic polyelectrolyte dispersants.

Miscellany

Suggested topics for future issues of this E-zine .... Please continue to send your ideas or questions for future topics.  Thanks.  Until next time ...

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