Interfacial behavior and micellization of binary surfactant mixtures in the concentrated sodium aluminate solution

Abstract Methods to eliminate the negative effects of organics on alumina production have been required for many decades. Similar to sodium laurate (SL) and sodium stearate (SS), compounds such as sodium oxalate (SO), sodium benzoate (SB), and sodium citrate (SC) also exist as surfactants in concentrated sodium aluminate solutions. Herein, elevating the temperature reduced the maximum surface excess (Γmax) and saturated adsorption layer thickness (δ) but increased the cross-sectional headgroup area (Amin) of the surfactant molecules. The order of critical micellar concentration (CMC) was: SO > SC > SB > SL > SS, while the micellar size followed the sequence: SS > SL > SC > SB > SO. Addition of SO (or SC) increased the CMC of SB, SL, SS, and SC (or SO). However, SO and SC were observed to play different roles in the interfacial properties of the organics. Addition of SO increased the Γmax values of SC, SL, and SS, and decreased the Amin values. In contrast, adding SC reduced Γmax and δ values of SB, SO, and SS, and raised Amin values. SO could be readily incorporated into micelles relative to SC, possibly leading to formation of the small micelles in SB. Additionally, increasing the temperature reduced the molar fraction of SO (or SC) in the mixed micelles of the binary surfactant mixtures, and SC could assemble at the air/solution interface and be incorporated into mixed micelles at increasing temperatures. Finally, the CMC values calculated using a non-ideal model agreed well with those determined from experiments.