Effect of Fatty Acids on Interfacial Tensions of Novel Sulfobetaines Solutions

The dynamic interfacial tensions (IFTs) of two zwitterionic surfactants with different hydrophobic groups, alkyl sulfobetaine (ASB) and benzyl substituted alkyl sulfobetaine (BSB), against hydrocarbons, acidic model oils containing fatty acids, and three crude oils have been investigated by a spinning drop interfacial tensiometer. The influences of concentration and alkyl chain length of fatty acids on the IFTs of two betaine solutions were expounded. The effect of the alkyl chain carbon number (ACN) of the oil phase on the IFTs has also been researched. The experimental results show that the whole hydrophilic part of the betaine molecule (anionic-cationic part and the hydroxyl) is almost flat at the interface, which results in the larger occupied space of the hydrophilic part at the interface. Therefore, the branched and benzyl-substituted betaine, BSB, has a larger sized hydrophobic part and can form a more compact adsorption film than linear ASB molecules. The IFT values decrease obviously when fatty acids are added into the oil phase due to the formation of mixed adsorption films. This synergism in reducing IFT is controlled by the chain length compatibility and the optimum acid concentration. The ultralow IFT values can be reached when the carbon number of the alkyl chain of the fatty acid is more than 12. The IFTs of both BSB and ASB solutions against different crude oils are lower than those against pure hydrocarbons due to the formation of mixed adsorption films of betaine and petroleum acid molecules. These experimental results also confirm that the acidic model oils can represent crude oil for IFT studies to some extent.