Inorganic Ion Specific Effect Of The Micellization Of Surfactants Mixed Systems

The micellization of trimethylene-1,3-bis(dodecyl dimethyl quaternary ammonium)(12-3-122+,2X-)with different counterions/dodecyl sulfonate(Y+AS-)mixed systems has been systematically investigated in this paper.The following investigations have been conducted:1.The first critical micelle concentration(CMC1)of ionic surfactants 12-3-122+,2X-(X-= F-,Cl-,Br,NO3-,Ac-,1/2SO42-)?Y+AS-(Y+=Na+,K+,Rb+)and mixed systems 12-3-122+,2X-/Y+AS-/H2O have been measured by surface tension method at 318.15 K.Experimental results show that the CMC1 of single ionic surfactants with different counterions 12-3-122+,2X-or Y+AS-shows specific ion effect,the CMC1 values increase according to the following sequence:SO42-<NO3-<Br<Cl-<AC-<F-;Rb+<K+<Na+,which are in well accordance with the Hofmeister series.For mixed systems 12-3-122+,2X-.Na+AS-/H2O with Y+AS-in excess,their aggregates are negatively charged,counterions are all Na+,X-as co-ions have little effect on CMC1,the sequence of CMC1 is as follow:Br-? AC-? F-? Cl-? SO42-<NO3-.The mixed systems with 12-3-122+,2X-in excess,the aggregates are positively charged,X-ions as counterions,show specific ion effect on CMC1 of mixed systems,the sequence of CMC1 is as follow:Br-<Cl-? AC-<F-<SO42-<NO3-.According to Collins' concept of matching water affinities,the ability of forming ion pairs is weak for NO3-and Na+,and the influences between NO3-and Na+ on the ability of their counterion binding with surfactant ions are small;Though SO42-ions can form ion pairs with Na=ions,while SO42-ion as divalent counterion,its electrostatic screening effect is stronger,therefore these counterions can effectively weaken the electrostatic attraction between the oppositely charged headgroups,leading to larger CMC1.For the 12-3-122+,2Br-/Y+AS-/H2O mixed systems,when Y+AS-is in excess,the CMCi increases following the order:Rb+<K+<Na+;when 12-3-122+,2Br-is in excess,there are no significant differences between the CMCi of different mixed systems.At the same time,the second critical micelle concentration CMC2 of 12-3-122+,2Br-/Na+AS-/H20 and 12-3-122+,2Br-/K+AS-/H20 mixed systems have been determined by electrical conductivity method:When surfactant Y+AS-is in excess,K+ ion shows stronger effect on reducing CMC2.2.The influence of changing cationic surfactant molecular structure on the micellization of the mixed systems,as well as the influences of temperature change,the addition of alcohol and inorganic salts on the micellization of mixed 12-3-122+,2Br-/Na+AS-/H2O systems have been studied by surface tension method.The cationic surfactant with different molecular structure dodecyl trimethyl ammonium bromide(DTAB)with one-head and one-tail,gemini surfactant(12-3-122+,2Br-)with two-head and two-tail,didodecyl dimethyl ammonium bromide(DDAB)with one-head and two-tail,bolaform surfactant dodecane-1,12-bis(trimethyl Ammonium bromide)with two-head and one-tail,the CMCi of the mixed systems and Na+AS-increases accoding to the order:DDAB/Na+AS-/H2O<12-3-122+,2Br-/Na+AS-/H20<DTAB/Na+AS-/H2O<Bola/Na+AS-/H2O,which is in accordance with the order of the single ionic surfactant.The CMCi of the single ionic surfactant and the mixed systems with one of the surfactant in large excess changed insignificantly with the increase of temperature.However,the CMC1 of mixed 12-3-122+,2Br-/Na+AS-/H2O systems near to the isoelectric point decreases first and then increases with the increase of temperature.With the addition of propanol,the CMC1 of single ionic surfactant decreases first and then increases,while the CMCi of the mixed systems increases monotonously.With the addition of inorganic salt,the CMCi of single ionic surfactants decreases significantly,the changes of CMCi for the mixed systems are related to the salt concentration and the molar ratio of the mixed systems.Meanwhile,the influence of the addition of the salt YX formed by the constituent counterions Y+ and X-of surfactant on CMCi of 12-3-122+,2X-/Y+AS-/H2O has been investigated.The addition of inorganic salt can compress the micellar double layer and reinforce the electrostatic screening the repulsion between the same charged headgroups of surfactant,thus leading to the decrease of CMCi.3.The mixed micelle composition Xim is approachable to equivalent molar ratio 0.50 when the bulk composition between 0.20?0.80,in the other composition,large differences between mixed micelle composition and bulk composition,which was calculated using the regular solution theory.But the mixed micelle composition Xlm is close to the isoelectric point 0.333 in the all range composition according to the dissociated Margules model.Interaction parameters ?m of the regular solution theory calculation is negative values,and it's absolute value is larger,the results mean high synergistic effect between the cationic surfactant and the anionic surfactant in the mixed micelles.Dissociated Margules model proposed by((r1A12+ r2A21)/(ri + r2)parameters instead of ?m can be used a measure of synergies and the strength of interaction between mixed system.The calculated surface excess adsorption ?max mixed system larger than ion surfactant from Gibbs adsorption formula,reached largest at the isoelectric point,along of the excess a particular ion surfactant,?max decrease,the minimum surface area per surfactant molecule Amin of the mixed system is smaller than ion surfactant accordingly,and change little near the isoelectric point,surfactant molecules are closely packed,and the mixed system the lowest surface tension ?CM all smaller than ion surfactant.It is that mixed systems have high surface activity.