Colloid and Interface Science in a Historical Perspective

The scientific approach of interfacial phenomena started in the second half of the eighteenth century with B. Franklin s reports in 1765 on the amount of oil needed to cover the surface of Clapham Pond in England. Later, in the nineteenth century. Lord Rayleigh pursued these experiments and A. Pockels and I. Langmuir did the first quantitative studies on the properties of monolayers of surface-active substances at liquid/air interfaces. 

After discovering and explaining some typical phenomena of colloidal dispersions during the last half of the nineteenth centnry, the distinction between hydrophobic and hydrophilic (if water is not the continnous medium lyophobic and lyophilic) colloids was clearly formnlated at the beginning of the twentieth century (E. Perrin, 1905 W. Ostwald, 1909). 

In hydrophobic colloidal systems, the water molecules have a higher affinity for one another than for the colloidal particles. Consequently, the particles would stick together on each encounter, nnless they repel each other. It was discovered by H. Schulze in 1883 that addition of electrolyte destabilizes hydrophobic colloidal dispersions, and W.D. Hardy in 1900 showed that the destabilization is accompanied by a reduction in the electrophoretic mobility of the particles. From this, it was inferred that colloidal stability is maintained by electrostatic repulsion between charged particles. 

The behavior of hydrophilic colloids is quite different. Hydrophilic colloids often, but not always, are electrically charged as well. Addition of electrolyte lowers the electrophoretic mobility, but it usually does not lead to destabilization of the colloidal dispersion. The obvious explanation is that the particles interact favorably with water molecules. The particles are surrounded with a layer of hydration water that keeps the particles apart. 

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