Relationship between the effects on bacterial activity of selected disinfectants and the hydrophobic characters of dibasic acid diesters

We prepared test solutions which contained 80% (v/v) ethanol and 0.2% (w/v) chlorhexidine (CH) or benzalkonium chloride (BC) with or without a dibasic acid diester. After complete evaporation of the ethanol from the solution on filter paper, an overnight broth culture (Staphylococcus aureus) was repeatedly inoculated onto the filter paper, and viable Bacterial counts were measured at 5 min after the last inoculation. By comparison with viable counts for CH or BC alone, we estimated the potentiating effects of dibasic acid diester on the bactericidal activity of CH or BC, and confirmed that this activity of the two disinfectants was potentiated in the presence of certain compounds in the homologs of di-n-butyl esters of aliphatic dibasic acid, and di-alkyl esters of adipic and phthalic acid. Diisobutyl adipate, one of the most effective diesters, substantially enhanced the bactericidal activities of benzethonium chloride, cetyl pyridinium chloride and didecyl dimethyl ammonium chloride, as well as CH and BC, but not those of polyhexamethylene biguanide or alkyldiaminoethyl glycinate. The potentiating effects of dibasic acid diesters observed for both CH and BC seemed to be affected by the hydrophobic character of these diesters themselves and are also expressed well by a particular quadratic equation as a function of these characters: namely, capacity factors, as determined by high-performance liquid chromatography.