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Chemical Reaction: The Salt of the Earth

By: Steve Herman
Posted: August 28, 2008, from the March 2007 issue of GCI Magazine.

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Salt is usually a cubic crystal with alternating sodium and chlorine ions. The presence of even a slight amount of moisture will cause the salt to dissolve and the crystals to stick together. Caking affects flow and makes some processes less efficient, and additives have been used to alleviate the problem. But even the shape of a salt crystal is subject to the march of technology. A new shape has recently been developed by researchers in India2 to reduce these caking effects. The addition of a small amount of glycine changes the shape from cubic to a decahedron, which is nearly spherical. This shape has much better flow. In addition, glycine is more resistant to water, so humid conditions have less effect on the crystals.

Glycine is costly, but a significant portion can be recycled. The final product has 0.5–1.0% glycine, which is an essential amino acid and imparts a slight refreshing and sweet flavor. As a bonus, the aqueous solutions are clear—whereas traditional anti-caking agents create cloudy solutions.

Salt’s Added Value

The most extensive use of sodium chloride is viscosity control. Surfactant micelles assume a larger effective volume by associating with added salt. This effective swelling of micelles increases viscosity. This goes only so far before additional salt causes the micelles to split. The process is routinely tracked by a graph called the salt curve. Wise formulators keep the salt adjustment to the left side of the peak so that a precipitous drop in viscosity will not occur if a mistake is made.

Salt is not often found in emulsions, but it can have value in w/o emulsions. The salt will increase in polarity in the internal phase, pulling more of the hydrophilic end of the emulsifier into the water. This process helps reduce the interfacial tension. Salt typically destabilizes conventional o/w emulsions.

Salt comes into its own as a primary ingredient in scrubs and bath soaks. In scrubs, it is an abrasive, but the soaks lead to an unexpected area of science—diffusion and osmotic pressure.