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

Abstract

Abstract:

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.

Key words: NMR spectroscopy, transverse relaxation dispersion, surfactant, CTAB, critical micelle concentration

CLC Number:

O482.53

ZHANG Hong-Mei, YANG Xiao-Yan, LEI Hao. Millisecond-Scale Molecular Dynamics in Aqueous Solution of Surfactant CTAB Studied by NMR Transverse Relaxation Dispersion[J]. Chinese Journal of Magnetic Resonance, 2005, 22(3): 235-243.

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