Five essential oils and nine of their components were compared to diethyl toluamide (DEET) for their repellent activity against the human body louse, Pediculus humanus humanus. The absolute or intrinsic repellency of the compounds was tested by applying the repellent to corduroy patches and comparing them with untreated patches. It was found that the most effective repellents were DEET and citronella, whose activity lasted at least 29 days. The activity of rosemary lasted at least 18 days and that of eucalyptus more than 8 days. The repellent activity of the oil components such as citronellal and geraniol lasted more than 15 and 8 days, respectively. DEET remained effective at a dilution of 1:32, geraniol at 1:8, citronella at 1:4 and rosemary and citronellal at 1:1. The comparative or standard repellency of the candidate repellents was examined with the aid of a new screening technique using hairs treated with ammonium bicarbonate which is attractive to lice. Using this technique it could be shown that the repellent activity of citronella and geraniol lasted 2 days and that of rosemary and citronellal for only one day. DEET was active for less than one day. Serial dilutions of these substances also revealed that citronella was the most potent repellent for lice, followed by citronellal, rosemary, geraniol and DEET. The differences however, were not significant.
The formation of microcapsules which contain rosemary oil, is herewith described. The process is based on two steps: (a) formation of oil-in-water emulsions, by using lecithin as emulsifier, thus imparting negative charges on the oil droplets; (b) addition of a cationic biopolymer, chitosan, in conditions that favor the formation of an insoluble chitosan-lecithin complex. Zeta potential measurements revealed that addition of very low concentrations of chitosan to lecithin stabilized emulsions, led to reversal of charge. At a suitable pH range the chitosan precipitated around the oil droplets, forming positively charged microcapsules. The chitosan-lecithin insoluble complex is composed of a 1:1 molar ratio of the chitosan monomeric unit and lecithin, as evaluated by elementary analysis and turbidity measurements.
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