Estimating surface tension of moth sex pheromones

The surface tension of chemical compounds is a crucial parameter in many interfacial processes of importance in sexual communication through odorants. To date, it has not been measured for any insect sex pheromone, including those from worldwide pests or well-studied organisms. Instead, it can be retrieved from dedicated black-box software, which delivers poorly described theoretical calculations and lacks clarity regarding error values. Using the low variability in the chemical structures of the sex pheromones from several moth species as an asset to avoid compounding effects, we estimated their surface tension using the pendant drop method, a well-known method in soft matter chemistry and physics yet seldom used by biologists despite being well suited for small samples. We examined 12 compounds: Z8-12:OH, E7Z9-12:Ac, E8E10-12:Ac, Z8-12:Ac, E8-12:Ac, E10Z12-16:Ald, E10Z12-16:OH, Z11-16:Ald, E3Z8Z11-14:Ac, Z9-14:Ac, Y11Z13-16:Ac, Z11-16:Ac. The species associated with these pheromones include the pests Lobesia botrana, Cydia splendana, Grapholita molesta, Bombyx mori, Cydolima perspectalis, Tuta absoluta, Spodoptera frugiperda, Thaumetopoea pityocampa and Mamestra brassicae. The measured values, ranging from 26 to 31 mN/m, are in accordance with the stated values in the available ACD/ChemSketch software and with our own theoretical predictions based on their densities and Parachor calculations, known as Macleod’s formula.