CSIR NET - Biophysical Chemistry - Protective Colloids - Biology Life sciences

Protective Colloids 

There are many insoluble constituents in the biological fluids like urine and bile. But, surprisingly, they do not precipitate out normally. Only under abnormal conditions they precipitate out in the form of gallstones and kidney stones. 

How is a normal person safeguarded 

from these pathologies? 

The answer lies with protective colloids. Protective colloids or peptizing colloids, as they are often called, do not form a separate type or class of colloidal systems. They are nothing but lyophilic colloidal systems which when added in sufficient quantities to a lyophobic sol hinder its precipitation by electrolytes (see Figure 3-5). 

In other words, the suspensoid is protected by the emulsoid. The emulsoid therefore, acts in this case as a protective colloid. 

An opposite phenomenon occurs if the reCipe is altered, the ingredients remaining the same. It is called sensitization. 

In this. if a small amount of the lyophilic system is added to the lyophobic system. precipitation is facilitated. The suspensoid gets sensitized and is precipitated out by a very small amount of electrolytes. This is because the lyophilic colloid is now enveloped 

by the lyophobic system facilitating flocculation (see Figure 3.6).

The protective action of various lyophilic substances was studied by Zsigmondy. He advised a quantitative measurement called the Gold Number in order to measure the relative protective powers of hydrophilic collOids. The Gold Number is the weight in mgs of protective colloids that when added to 10 ml of a Zsigmondy gold sol (contains negatively charged hydrophobic collOidal particles). just fails to prevent the colour change from red to blue on the addition of 1 ml of a 10% solution of sodium chloride. 

The smaller the gold number. the greater is the protective action of a particular lyophilic substance. With the unprotected gold sol. the addition cf sodium chloride brings about charge neutralization. aggregation of the sol particles, and finally, precipitation. 

The gold number of various fluids of our body presents a picture of our health profile and thus can be used for diagnostic purposes by a clinical chemist. 

Examples of protective colloids are gelatin. gum arabic. albumin etc.

source: Biophysical Chemistry Principles and Techniques, Upadhyay Upadhyay Nath, 2001