Viscosity

An important piece of equipment in Joe's lab is the viscometer, used to check the viscosity of the samples. Viscosity measurements can be used to determine the product's quality and efficiency, and this method is also very quick and reliable for analyzing product performance. By controlling the viscosity of products, and ensuring that they are in proper ranges, Joe enhances the quality and performance of the product as well as reduce production cost. There are many types of viscometers that Joe uses, such as a rotational viscometer, a capillary viscometer, and a vibro viscometer. A rotational viscometer works by inserting a cylindrical rotor into the sample and the rotor starts to rotate at constant speed. By using the principle that viscosity is directionally proportional to running torque, the twist angle caused by the torque is then proprtional to viscosity. This value is shown on the a meter on the viscometer.

Above is an illustration of how a rotational viscometer works.

Viscosity is a liquid's resistance to flow, but it is also dependent on the strength of intermolecular forces as well as the shape of the molecules. Usually, liquids that have higher intermolecular forces, which are polar molecules and/or can form hydrogen bonds, are more viscous than non-polar liquids. Sulfuric acid is a good example of this. It's intermolecular forces due to hydrogen bonding causes it to viscous, whereas an non-polar liquid with weak intermolecular forces like liquid nitrogen have low viscosity. A larger molecule may also be much more viscous, because of stronger dispersion forces as well as how relatively easy it is for the molecular chains to be "tangled" together. This is why long chained alkanes like fuel oil, and saturated fats are extremely viscous. Joe uses various substances to adjust the viscosity of products to a desired value. For instance, propylene glycol (1,2-propanediol) is used to increase the viscosity, due to its two hydroxyl groups which enable it to hydrogen bond, therefore increasing its intermolecular forces. Sodium xylene sulfonate is also used to lower the viscosity of cleaning products, because it is a hydrotrope. The concept of intermolecular forces that I learned in organic chemistry can be applied to viscosity which is vital to the chemistry of quality control in the cleaning industry.Viscosity is important to these products because of the importance of the texture and thickness of the liquid (like in shampoo and detergents) that feels nice to the hands, as well as produce a good amount of lather.

Above is the structure of propylene glycol. 

Above is the structure of sodium xylene sulfonate.