1,2-Hexanediol is a polyol that is colorless and transparent with a mild sweet smell. When used as a cosmetic ingredient, it has good antibacterial and moisturizing effects (antibacterial mechanism: it can enter the cell membrane of microorganisms, then destroy the integrity of the cell membrane, allowing cell fluid to seep out, and finally causing the microorganisms to die). At the same time, it has no sensitive side effects on human skin.

1,2-Hexanediol has a structural formula. It exhibits good miscibility with other substances (easily soluble in water, ethanol, etc.) and can be well combined with other antimicrobial agents to achieve antibacterial and anti-allergic effects. Furthermore, it has been reported that 1,2-hexanediol is used in deodorants, showing significant deodorizing and antiperspirant effects on human skin, and is gentle on the skin.

Because of its antibacterial effects and its absence from the preservative list, 1,2-hexanediol is often used as an alternative preservative in preservative-free systems.

1.Moisturizing: Kong Qiuchan et al. studied the moisturizing efficacy of 1,2-hexanediol. 

The results of transdermal water loss and moisturizing capacity tests are shown in Figures 1 and 2. 0.5% 1,2-hexanediol showed slightly better moisturizing properties than 3.0% glycerin, exhibiting a very good moisturizing effect. Moreover, 1,2-hexanediol has a lower viscosity than glycerin, resulting in a more suitable skin feel in products.

2. Alternative Preservatives - Individual Antibacterial Effects

Kong Qiuchan et al. studied the individual preservative efficacy of 1,2-hexanediol. The results of the preservative challenge test with 1,2-hexanediol added alone in aqueous solutions are shown in Table 1. 1,2-hexanediol showed good inhibitory effects against Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. At a 1,2-hexanediol mass fraction of 2.0%, all three bacteria were completely killed by day 2. However, 1,2-hexanediol showed significantly weaker killing ability against Candida albicans and Aspergillus niger; at a 1,2-hexanediol mass fraction of 2.0%, they were only completely killed by day 7 and day 21, respectively.

The results of the preservative challenge test of 1,2-hexanediol in aqueous solutions against mixed bacteria and mixed fungi are listed in Table 2. 1,2-Hexanediol showed excellent preservative ability against mixed bacteria at a mass fraction of 1.5%, with the inoculated bacteria being completely killed by day 2. Similar to the test results of Aspergillus niger alone, 1,2-hexanediol showed weaker preservative ability against mixed fungi; at a mass fraction of 2.0%, it took 21 days to completely kill the mixed fungi.

Mixed bacteria: Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa were co-inoculated, with the inoculation numbers of the three bacteria maintained at the same order of magnitude.

Mixed fungi: Aspergillus niger and Candida albicans were co-inoculated, with the inoculation numbers of the two bacteria maintained at the same order of magnitude.

Table 2. Preservative Challenge Test of 1,2-Hexanediol against Mixed Bacteria and Fungi

3. Alternative Preservatives - Compound Antibacterial Effects

Since 1,2-Hexanediol exhibits good inhibitory effects on bacteria but slightly weaker inhibitory effects on fungi, it needs to be compounded with other alternative preservatives to achieve better antibacterial effects. These include p-hydroxyacetophenone, 1,2-pentanediol, octyl glycol, and ethylhexylglycerin.

As shown in Table 3, numbers 3 and 5 refer to the mass ratios of 1,2-hexanediol and p-hydroxyacetophenone of 0.5:0.5 and 0.8:0.4, respectively. The results in Table 3 show that in emulsions, gels, and creams, except for emulsions using compound formulation 3, which still had a small amount of bacterial residue on the second day, all other bacterial species were completely killed, demonstrating a very significant preservative effect. Furthermore, as the water content and water activity in the formulation decrease, the bacterial growth rate decreases, and the preservative effect is enhanced.

In summary, 1,2-hexanediol is a polyol with moisturizing and antibacterial properties. Regarding antibacterial activity, 1,2-hexanediol exhibits significant inhibitory effects against bacteria and yeast, but its inhibitory effect on mold is slightly weaker. Therefore, when used as an alternative preservative, 1,2-hexanediol needs to be combined with other ingredients. For example, when the ratio of 1,2-hexanediol to p-hydroxyacetophenone is 0.5:0.5 or 0.8:0.4 (by mass), or when the ratio of 1,2-hexanediol to propylene glycol is 0.8:0.2 or 0.8:0.3 (by mass), it can pass the preservative challenge test.