March 18, 2007
What do hair mousse and malaria mosquitoes have in common? Aerosol!
The first use of aerosol technology was way back during World War II, when aerosol cans were introduced as a way to apply an insecticide intended to combat the malaria mosquito. Now, we use aerosol cans for many applications, including as dispensers of foamy mousse to combat flat hair.
Although dispensing foamy mousse from a can seems magical, the mechanism can be explained using physical chemistry. The principle at work is pressure. Warning labels on aerosol containers say that contents inside the can are under pressure mean...and they don't joke! Product in an aerosol can is pushed out by a propellant. Inside a can, the propellant exists either as a gas above or as a liquefied gas distributed throughout the product.
In both cases, when the can's nozzle is pressed, the propellant forces the product out of the can to release pressure.
In the first case, the propellant is compressed into the small space above the product. The key to the propellant's function is compression. Gases expand to fill whatever container they're in; the larger the container, the lower the force exerted by colliding gas particles, and the lower the pressure. Compression occurs when many gas particles are packed into a small area, resulting in a greater force and pressure, due to increased collisions between particles. Gases that take liquid form when highly compressed are called liquefied gases, which are used in the second case.
Did you know? The liquefied gas type of aerosol is commonly used for foams such as hair mousse.
How exactly is that foamy consistency achieved? Most aqueous (i.e. water-based) foams are composed of 95% gas and 5% liquid. As the product/propellant mixture leaves the can, this foam ratio is achieved because the liquefied gas is released from the high pressure environment and returns to its gaseous state. The gas particles fragment the liquid into a network of tiny bubbles; depending on how tightly the bubbles are packed, the foam can have part-gas, part-liquid and sometimes even part-solid properties.
The size and shape of the bubbles also impacts the foam's properties and resulting application. Strangely though, the behavior of foam is not predictable and much of what is known comes from trial and error.
Did You Know? No theories exist to design foam with a specific degree of stiffness.
However, surfactant additives are known to keep foams more rigid. Surfactants, which are commonly added to hair mousse, are complex molecules that act as wetting agents at the foam surface. They contain both hydrophobic (water repelling) and hydrophilic (water-loving) groups. Surfactants protect hydrophobic foam from water in the air and prevent it from collapsing. Chemical additives are used as surfactants in products such as hair mousse to provide stiffness..
Eventually, stiff foam releases its gas component to the air and the product returns to its original liquid state. This explains why hair mousse foam doesn't stay foamy once it's deposited and rubbed around on hair. When the mousse is applied to hair, the propellant gas evaporates and leaves the styling product, surfactant, and an improved hairdo, behind.
Hair mousse is fascinatingly foamy and seemingly magical, but in its essence it is the fashionable embodiment of physical chemistry!
Alison Palmer has a Bachelor of Science degree in chemistry from the University of British Columbia and recently completed a Master’s degree in chemical biology at McGill University. Her research involved modifying DNA to make it do tricks. She loves to talk and write about science and plans to pursue a career in science journalism.