Tuning the polydispersity of alcohol ethoxylates for enhanced oily soil removal

Commercially available alkyl alcohol ethoxylates have a broad distribution of ethylene oxide (EO) units and also a somewhat narrower distribution of alkyl chain length. Generally, the purer the surfactant sample (narrower distribution), the better is its detergency performance, and detergency peaks at the phase inversion temperature (PIT) for a given oil. However, in real detergency processes this may not hold true since soils are typically mixtures of several oily components, and temperature variations are significant. Therefore, if a polydispersity index (PDI) of ethoxylates is defined as the ratio of weight average EO moles to number average EO moles in the sample, then it is conceivable that an optimal PDI might be obtained. We compared the detergency of hexadecane for pentaethylene glycol monododecyl alcohol (C₁₂EO₅) samples in a broad PDI range, using an oil-soluble dye. While detergency at 55°C (PIT of hexadecane with C₁₂EO₅) decreases monotonically with increasing creasing PDI, average detergency over a 20°C temperature range around the PIT tends to show a maximum at PDI of ca. 1.1 (narrow-range ethoxylate). Similarly, for a mixture of undecane/hexadecane/tetracosane (30∶50∶20 w/w/w) for which the average PIT is approximately the same as that of hexadecane detergency at 55°C shows a maximum as a function of PDI at a value of ∼1.37 (broad-range ethoxylate). All detergency results are in general agreement with the reverse trends in oil/water interfacial tension and suggest that, having decided the optimal EO moles for a given application based on PIT, one can further improve the performance of alcohol ethoxylates in real detergency processes by tuning their polydispersity.