表面活性剂全氟庚酸和全氟辛酸的^(19)F NMR研究

^(19)F NMR STUDY ON PERFLUOROHEPTANOIC ACID AND PERFLUOROOCTANOIC ACID

测定了全氟庚酸 (PFHA)和全氟辛酸 (PFOA)在三氟三氯乙烷或正戊醇溶液中的不同浓度的1 9FNMR化学位移并归属了谱图 .研究了活性剂浓度对化学位移影响的机理和对不同核的不同影响 .由化学位移δ值对浓度倒数作图 ,可求得活性剂的临界胶束浓度 (cmc) :全氟庚酸 :0 .0 1 95mol/L (戊醇溶剂 )和 0 .0 4 0 6mol/L(三氟三氯乙烷溶剂 ) ;全氟辛酸 :0 .0 5 4 7mol/L(戊醇溶剂 ) .用计算模拟法使δ对浓度倒数作图同实验数据作图相比较 ,可推测得全氟辛酸戊醇溶液的单体分子与胶束大分子的平衡常数为K =2 1 (mol/L) - (n - 1 ) 和胶束聚集数n =3.9.

Surfactants perfluoroheptanoic acid (PFHA) and perfluorooctanoic acid (PFOA) were studied in solvents 1,1,2 trifluorotrichloroethane (TFTCE) and n pentanol (C\-5 OH) by 19 F NMR. The spectra of both compounds in both solvents displayed only one set of time averaged spectral peaks, indicating that molecular exchange rate between micelle and monomer is faster than NMR timescale. All 19 F resonance observed were assigned. Within the concentration range studied, all 19 F nuclei in PFHA had lower chemical shifts (upfield shift) in TFTCE than in C 5 OH, indicating the existence of larger deshielding effects in C 5 OH. When PFHA dissolved in C 5 OH, the 19 F nuclei in the α CF\-2 group had the largest chemical shift ( δ ), those in the ε CF\-2 and ω CF\-3 groups had the smallest δ ; and the 19 F nuclei in the β and δ CF 2 groups had chemical shifts in between. When dissolved in different solvents, the chemical shift differences (in TFTCE vs. in C 5 OH) of the 19 F nuclei in the α CF 2 and ω CF 3 groups of PFHA, which locate at the ends of the fluorocarbon chain, were larger than those of the other 19 F nuclei. By plotting chemical shift vs. the inverse of its concentration (1/ C ), the critical micellar concentrations (cmc) of PFHA was obtained, and is the concentration where the δ 1/C relationship starts to deviate from a linear relationship. cmc of PFHA in C 5 OH and in TFTCE calculated in this study were 0.0195 mol/L and 0.0406 mol/L, respectively. The cmc of PFOA in C 5 OH measured was 0.0547 mol/L. Using the chemical shift vs. 1/C relationship of different 19 F nuclei to calculate cmc resulted in different values, and this might be an advantage for determining cmc using this method. By comparing the δ vs. 1/C relationship obtained experimentally with that calculated by stimulation, equilibrium constant ( K ) and aggregation number ( n ) for PFOA in C 5 OH were obtained, and were 21(mol/L) -( n -1) and 3.9, respectively.