Real-time and quantitative investigation of zwitterionic surfactant interaction at oil-water interface: Interferometry experimental and MD simulation insights

In this work, the interaction performances at octadecane oil–water interface were investigated for zwitterionic surfactants of dodecyl dimethyl hydroxypropyl sulfobetaine (HSB-12) and hexadecyl dimethyl hydroxypropyl sulfobetaine (HSB-16) from both experimental and simulation insights. For macroscopic experiment, optical interferometry methodology was developed to real-time measure the interfacial interaction process and obtain the quantitative interfacial thickness and mass results. The four-step dynamic process (diffusion, adsorption, arrangement and aggregation) was characterized by kinetic analysis, indicating arrangement process as slow-limiting step. The rate of the four step process (diffusion rate k 1 ( d i f ) $k_{1(\mathrm{d}\mathrm{i}\mathrm{f})}$ , adsorption rate k 2 ( a d s ) $k_{2(\mathrm{a}\mathrm{d}\mathrm{s})}$ , arrangement rate k 3 ( a r r ) $k_{3(\mathrm{a}\mathrm{r}\mathrm{r})}$ , and aggregation rate k 4 ( a g g ) $k_{4(\mathrm{a}\mathrm{g}\mathrm{g})}$ ) is represented as k 3 ( a r r ) < k 1 ( d i f ) , k 2 ( a d s ) , k 4 ( a g g ) $k_{3(\mathrm{a}\mathrm{r}\mathrm{r})}<k_{1(\mathrm{d}\mathrm{i}\mathrm{f})}{,k}_{2(\mathrm{a}\mathrm{d}\mathrm{s})},k_{4(\mathrm{a}\mathrm{g}\mathrm{g})}$ . The Langmuir model was characterized by thermodynamic analysis, deducing the aggregation spontaneity of HSB-16 > HSB-12. The free energy of the aggregation stages of HSB-16 and HSB-12 was expressed as Δ G a g g ( H S B 16 ) = - 3.95 ${\mathrm{\Delta }G}_{\mathrm{a}\mathrm{g}\mathrm{g}(\mathrm{H}\mathrm{S}\mathrm{B}16)}=-3.95$ kJ/mol <  Δ G a g g ( H S B 12 ) = - 3.13 ${\mathrm{\Delta }G}_{\mathrm{a}\mathrm{g}\mathrm{g}(\mathrm{H}\mathrm{S}\mathrm{B}12)}=-3.13$ kJ/mol. For microscopic simulation, the interaction configurations and interaction energies were obtained through molecular dynamics simulation (MDS) calculation. The interaction stability and interaction strength were indicated to be HSB-16 > HSB-12. The interaction mechanism was explained by proposing molecular-orientation model below critical micelle concentration (CMC) and layer-interaction model above CMC. The different mechanisms were attributed to electronic attraction between surfactant molecules and hydrophobic interaction between surfactant and oil molecules. This work successfully constructs the interfacial platform for oil–water interface analysis, which not only elucidate the surfactant-oil intermolecular interactions but also provide mechanism understanding from macroscopic and microscopic scales.