In this report, on investigation of swelling of the didodecyldimethylammonium bromide (DDAB) interfacial layer in w/o microemulsions is discussed. An increased incorporation of the oil yields a swelling of the layer and subsequently smaller water droplets due to a higher layer curvature. We have examined the effect of the oil chain length by using hexane, octane, and dodecane as the continuous phase. The calculated water droplet radii and oil incorporations show that the shortest alkane yields the highest swelling. Further, mixture of hexane and dodecane were used. It was concluded that up to 40 mol % of the shortest alkane, it completely determines the interaction between the oil phase and interfacial layer._x000B_ Finally, the influence of added NaCl on the DDAB/hexane/water system was investigated, and it was found that added salt decreases the water droplet radius.