Mosquito repellent thermal stability, permeability and air volatility

The effectiveness of mosquito repellents, whether applied topically on the skin or released from a wearable device, is determined by the evaporation rate. This is because a repellent has to be present in the form of a vapour in the vicinity of the exposed skin that needs protection. Therefore, gravimetric techniques were used to investigate the direct evaporation of selected liquid repellents, their permeation through polymer films, and their release from a microporous polyethylene matrix.

Evaporation of a repellent into quiescent air is determined by its air permeability. This is a product of the vapour pressure and the diffusion coefficient, i.e. $$ . It was found that repellents could be ranked in terms of decreasing volatility as: ethyl anthranilate > citriodiol > dimethyl phthalate > N,N-diethyl-meta-toluamide (DEET) > decanoic acid > ethyl butylacetylaminopropionate > Icaridin. Experimental S_A values, at 50 °C, ranged from 0.015 ± 0.008 mPa m²  s^-1 for the least volatile repellent (Icaridin) to 0.838 ± 0.077 mPa m²  s^-1 for the most volatile (ethyl anthranilate). The release rate from microporous polyethylene strands, produced by extrusion-compounding into ice water baths followed a similar ranking. These strands featured an integral skin-like membrane that covered the extruded strands and controlled the release of the repellent at a low effective rate.

The high thermal and thermo-oxidative stability together with the low volatility of the mosquito repellents ethyl butylacetylaminopropionate and Icaridin make them attractive candidates for long-lasting wearable mosquito-repellent devices. Such anklets/bracelets may have utility for outdoor protection against infective mosquito bites in malaria-endemic regions. © 2019 Society of Chemical Industry.