Chemistry in your cupboard

Making the active ingredient soluble (1 of 3)

PCMX is, as expected, not very soluble in water; only 330 mg dm^-3. This is because only the OH group can form hydrogen bonds with water. Because they are acidic, phenols and substituted phenols are more soluble in alkaline solutions. However, strongly alkaline solutions are caustic and damage the skin so a slightly alkaline solution, pH 10, is used. In fact, PCMX shows slightly less activity at pH 10 than in a neutral solution.

Dettol actually contains 4.8% w/v PCMX.

w/v stands for weight per unit volume, so this is 4.8 g in 1000 cm³ or 48 g dm^-3 (48 000 mg dm^-3), nearly 150 times as much as its solubility in pure water. How is this achieved?

PCMX is soluble in relatively non-polar solvents - one such is pine oil which is used in the formulation of Dettol. This oil is derived, not surprisingly, from needles, twigs and cones of pine trees. Pine oil is a mixture composed largely of alpha-terpineol, which is moderately polar because of its single -OH group, Figure 9.

Figure 9: Structure of alpha-terpineol

The rest of the pine oil consists largely of hydrocarbons. It contains terpenes, a group of hydrocarbons which are essentially polymers of isoprene, Figure 10.

Figure 10: Structure of isoprene