Colloids mutual flocculation

When the sensitisation occurs in a system where hydrophobic and hydrophilic colloids arc oppositely charged, sensitization is easily explained as a form of mutual flocculation. An example is the flocculation of an (acid) gold sol which is negatively charged with an acidified (positively charged) gelatin solution In their dependence on the electrolyte concentration the flocculations show strong resemblance to complex coacervations (see volume II, chapter X). 

A colloidal solution therefore consists of small solid particles, maintained dispersed in a liquid by a set of forces that prevent their aggregation and flocculation. It includes two phases (liquid and solid), with a mutual boundary that constitutes an interface. Exchanges between the two phases take... 

In applying these potential curves to the problem of the stability of hydrophobic colloids, we observe first of all that the particles in a sol or suspension are subject to Brownian movement. This implies that encounters take, place continually between two (or more) particles. If the potential curve between two particles is of the type (a), the potential barrier will prevent lasting contact between the particles and after the encounter the two particles retain their independnece. If, on the contrary, the potential curve is of type ( ), the particles attract each other and if they are brought together by Brow nian motion, they will yield to their mutual attraction and form a lasting combination — difficult to separate — so that after the encounter one double particle results instead of two single ones, which means that flocculation has set in. 

They will attempt to reduce the still very large boundary surface between colloid-poor and colloid-rich phase by enlarging the mutual contact spots of the primary particles. Floccules have therefore in general the tendency to contract. Secondary processes can now also occur. When the colloid-rich phase has a coacervate nature, that is to say, is liquid — although possibly very viscous — then the contact spots between adjoining very small coacervate drops can transform into coalescence spots... 

Electric surface charge tends to keep the particles from flocculating and precipitating, since bodies of like charge are mutually repellant. The importance of the surface charge is shown % the fact that colloids are least stable when at the isoelectric point. 

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