Mechanism responsible for the reduction of interfacial tension by extended surfactants

The dynamic interfacial tensions (IFTs) of four extended surfactants (S-C12PO13S, L-C12PO4S, L-C16PO4S and L-C12PO11S) with different alkyl chain length and polypropylene oxide (PO) group against hydrocarbons were investigated. The mechanism responsible for the IFT reduction for extended surfactant has been detected. According to experimental results, we found that only when the size of hydrophobic part is enlarged to match the size of anionic head, the ultralow IFT will appear at optimized hydrophilic-lipophilic balance. For extended surfactants, with an increase of PO number, the curled PO chains occupy larger spaces at interface and match the size of anionic head. As a result, the ultralow IFT appears against alkanes when PO number is 13. The increase of alkyl chain length shows little effect on the size of hydrophobic part and ability of reducing IFT. Moreover, with an increase of n-alkane carbon number (ACN), the size of hydrophobic part increases as a function of ACN. Therefore, the increase of size of hydrophobic part compensates the decrease of adsorbed amount when ACN is higher than n(min), value (the n-alkanes carbon number when the IFTs reach the minimum), which results in the "L" shape of IFT curve with a function of ACN.