Theoretical And Experimental Study On The Salt Tolerance Capability Of Anionic Surfactant

Anionic Surfactant can greatly enhance the oil recovery, but at the high salt reservoirstrata it is invalidated for encountering with the large amounts of inorganic ions. Thedevelopment of salt-type surfactant agent is urgent, but now it is great difficult to design anddevelop new salt-type surfactant because the salt tolerance mechanism of surfactant is not yetclear. In view of this, the present master thesis chooses two famous anionic surfactants:Sodium lauryl sulfate (SDS) and Sodium lauryl polyoxyethylene ether sulfate (AES) as studyobject. Adopting the spinning drop interfacial tension measurement, the effect of inorganicions on the oil/surfactant/water interfacial tension is investigated; Using quantum chemicalcalculation investigated the binding model and strength of inorganic ions and surfactant, andanalysis the charge change of the surfactant headgroup effect of the inorganic ions; Finally,molecular dynamics simulation was been used to investigate the inorganic ions type andconcentration influence on the nature of surfactants self-assembled structure at oil-waterinterface, and the mechanism of the impact on oil-water interfacial tension, in depthunderstanding the interaction mechanism of inorganic ions and surfactant. The innovation liesof this master thesis is using the experiments and molecular modeling combination method, toreveal the surfactant salt tolerance ability and microscopic mechanism from the perspective ofinterfacial tension.The experimental results show that, with the increase in the concentration of inorganicsalts, the ability of surfactants in oil-water interfacial tension reduction first increases and thendecreases which means that the interfacial tension of systems decreases firstly and thenincreases; AES has a higher optimal salinity and a higher ions concentraions when the newphase appearing, so the salt tolerance ability of AES is better than SDS; the effect strengthfollows the order: Ca salt <Mg salt <Na salt. Simulation results have shown that the surfactant ability of salt tolerance is related withthe binding strength of headgroup and inorganic ions, the most mode of the inorganic ionsbinding with surfactant is2:1type and binding strength follows the order: Ca>Mg>Na, so theeffect strength follows the order: Ca salt <Mg salt <Na salt; the binding strength of AES andions is weak than SDS, furthermore the effect strength of ions on the headgroup charge ofAES is weaker than SDS, so the salt tolerance ability of AES is better than SDS whichconsisted with experimental results; with the increase of the concentration of MgCl2, moreMg2+aggregating at interface, the effect on screen electrostatic repulsions between headgroupstrengthen, so the surfactant self-assembled monolayer structure is changed from disorder toorder then disorde, which lead to the interaction with surfactant and the oil/water increasingfirstly and then decreasing, that means the excess energy of interfacial molecules decreasesfirstly and then increases with the increasing of the concentration of MgCl2. So, the interfacialtension of the system decreases firstly and then increases with the increasing of theconcentration of MgCl2, consisting with experimental results.