Abstract

Behaviors of nonionic-anionic surfactant sodium fatty Alcohol polyoxyethylene Ether Carboxylate (AEC) at dodecane/water interface influenced by inorganic salts NaCl, CaCl2 and MgCl2 were investigated by interfacial tension methods and molecular dynamics simulation. Contrasted distributions of various salts at interface and in aqueous solutions and resulting lowering the interfacial tension have been observed. Composition of surfactants-salts complex at interface surrounding different cations within 2.5 Å were found to be in two categories: (i) the octahedral complexation of divalent cation was generated by the participation of surfactant head group and water molecule, (ii) only water molecules were involved in the complexation. According to the simulation results, all the $Na^+$ involved in type ii, without any direct interaction with surfactant head group, while $Ca^{2+}$ and $Mg^{2+}$ involved in type I, following the formula: $C_a^{2+}􏰲􏰅􏱮+4(􏰆surfactant)􏰊+2(􏰆H􏰅_2O)􏰊 ⇄ [Ca 􏰆(surfactant)_4(􏰊􏱣􏰆H_2􏰅O)_2􏰊􏰅]􏰅􏱮^{2+},$ $M􏴓􏰅􏱮_g^{2+}+(􏰆surfactant)􏰊+5􏰆(H􏰅_2O)􏰊 ⇄ [Mg(􏰆surfactant􏰊) 􏰆(H􏰅_2O)_5􏰊􏰟]^{2+}$. This strongly chelate interaction between $Ca^{2+}$ and surfactants makes surfactants more stretched at interface, thus more effective in oil/water interface.

Keywords:

Dodecane/water interface, interfacial tension, molecular dynamics simulation, surfactant AEC,

References

Competing interests

The authors have no competing interests.

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