Mixed aggregate formation and synergistic interactions of binary surfactant mixtures of di-n-decyldimethylammonium chloride, [DiC(10)][Cl], with polyoxyethylene alkyl ethers, CiEj (i = 10, 12, j = 4, 6, 8), have been investigated for various [DiC(10)][Cl]/CiEj ratios. The critical aggregation concentration of the binary mixtures has been determined by tensiometry, and the aggregate characteristics (i.e., size and composition, free ammonium concentration) have been estimated using the pulsed field gradient NMR spectroscopy and a [DiC(10)]-selective electrode. Diffusion coefficient measurements of micelles confirmed the synergistic interaction between the surfactants. It is thus shown that the formation of surface monolayers and mixed aggregates from [DiC(10)][Cl]/C10Ej mixtures is driven by both tail/tail and head/head interactions, whereas [DiC(10)][Cl]/C12Ej co-aggregation is mainly driven by tail/tail interactions. As a consequence, the co-aggregation phenomenon notably influences the biocidal activity of [DiC(10)][Cl] on the Candida albicans fungi. In the presence of C12Ej, the biocidal activity of the ammonium salt is inhibited due to the trapping of the cationic surfactants in the mixed aggregates, whereas in the presence of C10Ej, the biocidal activity of the surfactant mixture is maintained. The mode of action is also confirmed by a faster increase in the zeta potential of a C. albicans suspension in the presence of [DiC(10)][Cl]/C10E8 than in the presence of [DiC(10)][Cl]/C12E8. Therefore, a judicious adjustment of the alkyl (i) and polyoxyethylene (j) chain lengths of CiEj avoids its antagonistic effect on the biocidal activity of [DiC(10)][Cl]. (C) 2012 Elsevier Inc. All rights reserved.