Studies on Rolling-up Mechanism on the Removal of Oily Soil Mixture of Liquid Paraffin and Oleic Acid

Abstract

The rolling-up mechanism on the removal of oily soil mixture of liquid paraffin (Nujol) and oleic acid from polyester (PET) substrate in aqueous surfactant solutions was studied by measuring the contact angle and the interfacial tension. Surfactants used were nonylphenyl poly (oxyethylene) ether (EO : 11) (NP-11) and sodium dodecylsulfate (SDS).In the NP-11 solution, Nujol alone placed on PET as a droplet, the contact angle of which was about 110°, was not rolled up, however, Nujol mixed with oleic acid was removed easily from PET by rolling-up. The addition of 5% oleic acid into Nujol resulted in the most efficient removal.As for the effect of surfactants on the interfacial tension at Nujol containing 5% oleic acid/water interface, the NP-11 lowered the interfacial tension markedly almost to 0dyn/cm, while the SDS lowered only to about 3dyn/cm. Furthermore, the equilibrium contact angle of Nujol with 5% oleic acid on PET increased in NP-11 solution and decreased in SDS solution as compared with those of Nujol alone.The calculated residual works of detergency (WR, v) in the system of Nujol with 5% oleic acid on PET were 0.09dyn/cm in 5×10-3% NP-11 solution and 3.3dyn/cm in 5×10-1% SDS solution, respectively. Therefore, in both NP-11 and SDS systems, WR, v decreased in the presence of 5% oleic acid in Nujol.From these results, it is concluded that the addition of small amount of oleic acid into Nujol is effective for increasing the equilibrium contact angle and especially for lowering the oil/water inter-facial tension to facilitate the removal of Nujol by rolling-up.