加盐导致的全氟辛酸盐表面活性的反常行为

Abnormal Surface-Active Behavior of Perfluorooctanoates Induced by Salt

通过测定全氟辛酸铵(APFO)、全氟辛酸三甲铵(TMHPFO)、全氟辛酸三乙铵(TEHPFO)在不同浓度NaCl(0.1,0.3,0.5 mol?L-1)存在时水溶液的表面张力曲线,考察不同反离子的氟表面活性剂其表面活性随无机盐浓度的变化.结果表明,NaCl对APFO的胶束化有明显的促进作用;对于TMHPFO和TEHPFO则在NaCl浓度较低时有很小的促进作用,NaCl浓度较高时由于Na+和N(CH3)+3(或N(CH2CH3)+3)之间的离子交换作用反而临界胶束浓度(cmc)增大.APFO,TMHPFO的最低表面张力(γcmc)随着NaCl浓度的增大而增大.而对于TEHPFO,少量NaCl的加入有利于降低γcmc、然后随着NaCl浓度的增大TEHPFO的γcmc增大.这说明,加盐溶液中始终存在着屏蔽效应和离子交换作用的竞争,随着NaCl浓度增大离子交换趋势增大;对于疏水性较高的三乙铵离子在NaCl存在的情况下离子交换作用导致的γcmc升高需要更高的NaCl浓度才能显现.通常认为外加无机盐是增强表面活性剂的表面活性的方法之一,本工作表明,对于有些反离子为有机阳离子的氟表面活性剂,外加无机盐,如NaCl,不仅不能起到显著的增效作用,浓度大时甚至会降低表面活性剂的效能.所以这类表面活性剂在实际使用时应尽量避免高盐环境.

The surface tension of the aqueous solutions of ammonium perfluorooctanoate （APFO）, trimethylammonium perfluorooctanoate （TMHPFO） and triethylammonium perfluorooctanoate （TEHPFO） with the addition of different concentration of NaC1 （0.1, 0.3, 0.5 moloL-1） was measured to investigate the surface activity change of fluorinated surfactants with different counterions upon the addition of inorganic salt. The results showed that, NaC1 significantly facilitated the micellization of APFO. While for TMHPFO and TEHPFO, slight promotion on their micellization was found when the concentration of NaC1 was low, and when the concentration of NaC1 was large, the critical micelle concentration （cmc） increased due to the ion exchange between Na＋and N（CH3） ＋ （or N（CH2CH3）3 ＋ ）. The minimum surface tensions （Tome） of APFO and TMHPFO were increased with the increase of NaC1 concentration. For TEHPFO, small amount of NaC1 favored to lower 7cmc, then ？cnac of TEHPFO was increased with the increase of NaC1 concentration. It showed that, there was always competition between the shielding effect and ion exchange effect in the solution containing salt, and the tendency of ion exchange became large with the increase of NaC1 concentration. For the relatively hvdroohobic triethvlammonium ions in the oresence of NaC1. the aooearance of the increase of ycmc induced by ion exchange needed higher concentration of NaC1. Since it is well known that the addition of inorganic salt is one of the methods to enhance the surface activity of surfactants, this work showed that for certain anionic fluorinated surfactants with organic counterions, the addition of inorganic salt such as NaC1 could not obviously enhance and even lower the efficiency of surfactants in the case of large concentration of salt. Therefore, this kind of surfactants should avoid the environment of high salinity in application.