| Mixed cationic and anionic surfactant systems can form rich microstructures spontaneously,and exhibit very complicated phase behaviors and unique rheological properties,indicating a broad application prospect in many areas.In this paper,the phase behaviors and rheological properties and aggregate microstructures of the mixed systems have been investigated by changing the molecular structure of organic salts,surfactant concentration,the molecular structure of surfactants and other factors.1.For 0.10 mol·kg-1 CTAB/AS mixed system,the effect of organic salts(such as NaSAL,NaPHB,TMAB,TBAB,SDC and so on)with different molecular structures on the phase behaviors and rheological properties has been investigated.Because of the complicated molecular structures of organic salts which are different from those of inorganic salts,they show different effects on the phase behaviors and rheological properties of mixed surfactant systems.For example,the bile salts SDC with rigid facial structure,due to the steric effect,bile salt molecules can not penetrate into the micelle core,but lie flat at the aggregate surface between headgroups of the conventional surfactant,which is not conducive to the growth of the micelles.Thus,with the concentration increase of SDC,the space effect and electrostatic interaction strengthen,which affect the microstructures of micelles or aggregates,and lead to the gradual disappearance of the original heterogeneous region,liquid crystalline phase regions and aqueous two-phase regions of the mixed systems,and finally only the isotropic single-phase region exists;while the shear viscosity of the isotropic single-phase region decreases with the concentration increase of SDC.Other organic salts such as NaSAL?NaPHB and TMAB?TBAB can penetrate into the micelle core by the way of embedding,similar to inorganic ions,the organic counterion effects on the phase behaviors and rheological properties are significant;However,inorganic coion effects are usually weak to be ignored,while the organic coion effects are also relatively significant.The effect of SAL-ions and the effect of PHB-ions on phase behaviors of mixed systems with excess AS are significantly different,SAL-ions have strong hydrophobic interaction and can strongly weaken the effect of Na+ ions,thus significantly changing the phase behaviors of the mixed systems,leading to the disappearance of aqueous two-phase region at higher salt concentration.Therefore,the addition of organic salts with different molecular structures can induce mixed surfactant systems to show different phase behaviors and rheological properties.2.The phase behaviors of CTAB/AS and 12-3-12/AS mixed systems with different total surfactant concentrations in the absence and in the presence of different concentrations of organic salts were studied.The effect of the same organic salt on the phase behaviors of mixed surfactant systems with different total concentrations:When the total surfactant concentration is 0.01 mol.kg-1,only isotropic single-phase region and heterogeneous phase region were observed for the mixed CTAB/AS system and the mixed 12-3-12/AS system,and with the increase of the total surfactant concentration,the liquid crystalline phase region and aqueous two-phase region occur gradually;In the mixed surfactant systems with low total concentration in the presence of NaSAL,the concentration of organic counterions increases with the concentration increase of the NaSAL,and then the molar ratio between counterions and surfactant ions increases,thus reinforcing the hydrophobic and electrostatic interactions.Therefore,the influences of organic salts on the phase behaviors of the mixed surfactant systems with lower total concentration are greater,and the addition of organic salt induces the phase boundaries of aqueous two-phase region far away from equimolar ratio.However,for the mixed CTAB/AS system with lower total surfactant concentration,the required concentration of SDC to induce all the mixed systems in the whole molar ratio range to be isotropic single-phase systems is lower.While,for the mixed 12-3-12/AS systems regardless of the total surfactant concentration,the addition of SDC can induce the formation of liquid crystalline systems and aqueous two-phase systems,and the phase boundaries of aqueous two-phase regions move far away from equimolar ratio with the concentration increase of SDC.3.The effect of molecular structure of surfactants on the phase behaviors and rheological properties of the mixed surfactant systems has been investigated.(1)The phase behaviors of the mixed CTAB/AS system and the mixed 12-3-12/AS system with different molecular structure of cationic surfactant have been studied.When the conventional single head-single tail CTAB is replaced by the two head-two tail gemini surfactant 12-3-12,for the additive organic salts such as NaSAL or NaPHB which can penetrate into the micelle core,aqueous two-phase systems and liquid crystalline systems with excess AS were not observed;the phase boundaries of aqueous two-phase region for the mixed 12-3-12/AS system with excess cationic surfactant are further away from equimolar ratio.In addition,aqueous two-phase separation was induced for the single 12-3-12 system with the concentration increase of organic salt.For the additive organic salt SDC with rigid facial structure,however,the heterogeneous phase region,liquid crystalline phase region and aqueous two-phase region have not disappeared with the concentration increase of SDC,while the phase boundaries of aqueous two-phase region move far away from equimolar ratio and the phase region becomes wider.(2)The effect of chain length of cationic surfactants on phase behavior of the mixed surfactant systems.At 303.15K,the effects of two different types of organic salts(NaSAL and SDC)on the phase behaviors of the mixed DTAB/SL system and the mixed CTAB/SL system have been investigated.The results indicate that:the required lowest concentration of SDC to induce all the mixed surfactant systems in the whole molar ratio range to be isotropic colorless transparent solutions increases with the increase of chain length of the cationic surfactant.At 303.15 K,the phase behaviors of the mixed CTAB/SL systems with pure water or NaSAL aqueous solutions of low concentration as solvent are similar to those of the mixed CTAB/AS system when the composition is far away from equimolar ratio.However,when the composition is close to equimolar ratio,with the composition increase of CTAB,the original heterogeneous region located between the two liquid crystalline phase regions is replaced by the sequentially occurred aqueous two-phase region(i.e.,phase region in which the aqueous two-phase systems with emulsion polarization top phase and bluish translucent bottom phase),heterogeneous region,aqueous two-phase region,heterogeneous region and aqueous two-phase region.This aqueous two-phase regions will disappear to be replaced by heterogeneous region with the concentration increase of NaSAL,and then the phase behaviors of the mixed CTAB/SL system are analogous to those of the mixed CTAB/AS system.However,for the mixed DTAB/SL system,the heterogeneous phase region will disappear to be replaced by the liquid crystalline phase region with the concentration increase of NaSAL. |
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