表面活性剂CTAB水溶液中的T2弛豫分散研究

用CPMG脉冲序列测定了表面活性剂十六烷基三甲基溴化铵（CTAB）分子中的氮甲基(N-CH₃)质子的横向弛豫时间（T₂表观），并发现测得的T₂表观\}与序列中的重聚脉冲间隔时间的一半τ _cp有关，说明存在横向弛豫分散现象. 当在τ_cp≤1 ms时，T₂表观与τ²_cp}呈线性关系；而当τ_cp≥4.6 ms时，T₂表观变得与τ_cp无关. 利用Luz-Meiboom两体化学交换模型计算了不同浓度的CTAB溶液中的N-CH₃质子的本征横向弛豫时间（T₂本征）和化学交换速率k_ex，发现k_ex与T₂本征和自扩散系数D一样，在临界胶束浓度（CMC）附近发生突变. 这个突变反映了CTAB分子在从单体到胶束的转变过程中其动力学特性发生了改变. 

Millisecond-Scale Molecular Dynamics in Aqueous Solution of Surfactant CTAB Studied by NMR Transverse Relaxation Dispersion

Apparent proton transverse relaxation times of the hydrophilic group (N-CH₃) were measured in aqueous solution of surfactant CTAB with different concentrations using the CPMG pulse sequence, and found to be dependent on the interpulse delay between the refocusing pulses (τcp). This indicates that diffusion and/or chemical exchange contribute to the transverse relaxation process of the N-CH₃ protons. It was observed that the apparent transverse relaxation time has a linear relationship with τ²_cp when τ_cp≤1 ms, but becomes independent on τ_cp when τ_cp≥4.6 ms. Using the Luz-Meiboom two-site chemical exchange model, the chemical exchange rates of the N-CH₃ protons at different concentrations were calculated and found to change from 1.57 ms^-1 to 1.15 ms^-1 at the critical micelle concentration (CMC), behaving similarly as the intrinsic transverse relaxation time and self diffusion coefficient D. Change of the chemical exchange rate at CMC is probably a reflection of alteration of the exchange dynamics of adjacent CTAB molecules when micelles are formed.