Composition and Structural Transitions of Polyelectrolyte鈥揝urfactant Complexes in the Presence of Fatty Acid Studied by NMR and Cryo-SEM

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• 作者：Christian Totland ; Jose Martinez-Santiago ; Kavssery P. Ananthapadmanabhan ; Ponisseril Somasundaran
• 刊名：Langmuir
• 出版年：2015
• 出版时间：February 10, 2015
• 年：2015
• 卷：31
• 期：5
• 页码：1623-1631
• 全文大小：436K
• ISSN：1520-5827

文摘

Insoluble complexes formed when a cationic polyelectrolyte is neutralized by the oppositely charged surfactant sodium dodecylethersulfate (SDES) in the presence and absence of lauric acid (LA) have been examined directly using NMR spectroscopy and cryo-SEM. Below the SDES critical micelle concentration (CMC鈥? the insoluble complex contains about 10 times more water than just above CMC鈥? This is related to a structural transition of the complex, where water is contained mainly in larger compartments below CMC鈥?and then mainly in narrower compartments above CMC鈥? The structure of the complex鈥檚 solid matrix was monitored by recording two-dimensional T₂-diffusion correlation spectra of the water proton resonance, which reveal the presence of several different water environments which correspond to different complex structures. Structural features in the micrometer range were confirmed using cryo-SEM. When LA is present, the larger water compartments seen below CMC鈥?are to some extent present in the entire SDES concentration range, which is not the case in the absence of LA. Furthermore, the inclusion of LA into the SDES aggregates above CMC鈥?leads to a lamellar sheetlike organization of the polyelectrolyte-stabilized surfactant phase. In the absence of LA, a stringy network of fibers is seen in cryo-SEM images, indicating a spherical or rodlike SDES phase. Consequently, the complex without LA holds about 1.7鈥?.9 times more water than the complex with LA above the SDES CMC鈥? T₁ relaxation, ¹³C chemical shifts, and ¹H resonance line widths of SDES in the system support the above observations. The combination of MAS NMR, T₂-diffusion correlation, and cryo-SEM proved to be an effective method for studying structural transitions in the surfactant鈥損olyelectrolyte(鈭扡A) insoluble complexes.