The Micellization Of Cationic/anionic Surfactant-Mixed Systems With Various Counterion Effects

The micellization of cetyl trimethyl quaternary ammonium(CTA+X-)and dodecanoate(Y+L-)mixed systems with different counterions have been systematically investigated in this paper,the main works are as follows:1.The critical micelle concentration(CMC)of ionic surfactants CTA+X-(X-＝F-,Cl-,Br,N03-,Ac-,1/2SO42-).Y+L-(Y+=Na+,K+,Rb+)and CTA+X-AY+L-/H2O mixed systems have been measured by surface tension method at 298.15 K.Experimental results show that,there is specific ion effect on the CMC of ionic surfactants CTA+X~and Y+L-with different counterions,the CMC values increase in the following sequence:SO42-<NO3-<Br<Cl-<Ac-<F-;Na+<K+<Rb+.For CTA+X-/SLi/H20 mixed systems,the CMC values increase according to the sequence:F-<Cl-<Br:<Ac-<SO42-<NO3-.For the CTA+Br/Y+L-/H2O mixed systems,the CMC values increase according to the sequence:Rb+<K+<Na+.The results illustrate that,the CMC values of CTA+F-/Na+L-/H2O and CTA+Br-/Rb+L-/H2O mixed systems with high degree of matching water affinities between counterions F-and Na+ or Br’ and Rb+ are close to each other,and less than the other mixed systems,while CTA+NO3-/Na+L-/H2O mixed system with high degree of mismatching water affinities between counterions NO3-and Na+ gives the biggest CMC values.Based on this result,we have chosen another CTA+X-/Y+L-/H2O mixed system with high degree of matching water affinities between oppositely charged counterions(i.e.,Cl-and K+)to investigate the influence of matching water affinities.According to the concept of matching water affinities,oppositely charged inorganic ions with higher degree of matching water affinities have stronger formation ability of ion pairs,leading to weaker electrostatic screening effect.The ability weaken the electrostatic repulsion between the same charged headgroups and electrostatic attraction between the-oppositely charged headgroups becomes weaker,the overall effect strengthens the attraction,therefore leads to the effective decrease of CMC.Meanwhile,the CMC values of CTA+Br-and Y+L-have been determined by electrical conductivity method at different temperatures and the corresponding thermodynamic functions of micellization have been calculated.The CMC values of CTA+Br-and Y+L-increase when the temperature increases from 298.15 K to 318.15 K.2.The chain length effect of cationic surfactant on the micellization of the CnTAB/SL/HO mixed systems has been investigated by using surface tension method,and the effects of temperature,the concentration and molecular structure of organic salts on the micellization of CTA+Br-/Na+L-/H2O mixed systems have also been investigated.The cationic/anionic surfactant mixed systems with more symmetrical chain length show stronger synergistic effect.The CMC values of the single ionic surfactant and the cationic/anionic surfactant mixed systems with one of the surfactant in large excess increase with increasing temperature,while the CMC of the CTA+Br-/Na+L-/H2O mixed systems near to the equimolar charge point only has little change.The CMC values of single surfactant and the mixed systems decrease significantly with the increase of the concentration of sodium salicylate(NaSAL).The molecular structures of the additive organic salts have a significant influence on the micellization of CTA+Br-/Na+L-/H2O mixed system.For example,for the organic salts(i.e.,NaSAL and its isomer sodium p-hydroxybenzoate NaPHB)which can penetrate into the micelle core by the way of embedding,hydrophobic interaction and electrostatic interaction between the organic ions and cationic/anionic surfactants show influence on the micellization.The bile salt sodium deoxycholate(NaDC)that is composed of a rigid steroid skeleton with a polar face and a nonpolar face is another organic salt used as additive in this investigation.The bile salt molecules are difficult to penetrate into the micelle core,while they lie flat at the micelle surface between headgroups due to steric effect,thus separating these headgroups,meanwhile NaDC have two hydroxyl groups,can easily form stable intramolecular hydrogen bond.It usually shows steric effect,hydrogen bond and electrostatic interaction effect on the micellization of cationic/anionic surfactant mixed systems.3.The mixed micellar composition Xlm has been calculated by the dissociated Margules model.The calculated X1m values are close to 0.50 when the bulk compositions are in the range between 0.30 and 0.70.While in the other bulk compositions,there are large differences between X1m values and bulk compositions.Large negative values of micellar interaction parameters βm calculated by regular solution theory mean strong synergistic effect between the cationic surfactant and the anionic surfactant in mixed micelles.At equimolar composition,the value of(r1A12 + r2A21)/(r1 + r2)in the dissociated Margules model is equivalent with the value of βm,illustrating that this value can be used as a measure of synergism and interaction strength between the two different surfactants in mixed systems.The calculated maximum surface excess concentrations Γmax in the mixed systems are larger than those of single ionic surfactants by Gibbs adsorption formula,and the minimum areas per surfactant molecule Amin in the mixed systems are smaller than those of single ionic surfactants.Γ mar shows maximum and Amin shows minimum at equimolar composition,Γmax decreases and Amin increases with the increase of excess degree of one ionic surfactant.The surface tension at CMC(γCMC)in mixed systems is lower than that of single ionic surfactant,illustrating that the ability of the mixed systems to reduce the surface tension of water is stronger than that of single ionic surfactant.The Pc20 of single ionic surfactant is small,it increases with the addition of oppositely charged surfactant,and reaches maximum at equimolar composition,illustrating that the efficiency of the mixed systems to reduce the surface tension or interfacial tension is the highest at equimolar composition.