Phase Behavior of Propylene Glycol Sticks

Deodorant sticks are a combination of ingredients suspended in a propylene glycol gel.

Propylene glycol is a double alcohol compound. Sodium stearate is a major component of some types of soap. Sodium stearate causes propylene glycol to gel into a solid, forming the structural composition of many commercial cosmetic and deodorant sticks.

Ternary Phase System

Cosmetic and deodorant sticks also contain water- and nonwater-soluble components, such as mineral oil, fragrance and antibacterial chemicals mixed in an oil-and-water emulsion. When a surfactant, such as sodium stearate, is added to an oil-and-water emulsion, a ternary phase system is formed. Commercial propylene glycol sticks exhibit ternary phase system behavior.

Ternary Phase Diagrams

The ternary phase system includes an oil phase, an aqueous phase and the surfactant. The relative amounts of each of these components may be expressed in a ternary-phase diagram: drawings of triangles that are used to represent the equilibrium formed between the three components as a function of temperature.

Phase Behavior Application

When deodorants or cosmetics are applied to the skin, temperature change and water evaporation cause the relationship between the ternary phases to shift. Using a phase diagram to record the physical properties of a compound through all phase shifts is crucial for establishing the usefulness of a particular formulation of propylene glycol-based sticks.

References

• "Encyclopedia of Emulsion Technology"; Paul Becher, editor; 1988
• "The Physical Chemistry of Cosmetic Formulations"; Toufique Choudhury

Resources

• "Palm-Based Sodium Dihydroxystearate Deodorant Stick"; Zahariah Ismail, et al.
• "Thermodynamics of Solutions"; Alcohol Water Solutions; Isidoro Martinez
• "Ternary Phase Diagrams", Guna Selvaduray, Oct. 2004