Zhisheng Gao

NMR studies in surfactant and polymer-surfactant systems: Micelle formation of sodium ω-phenyldecanoate and interaction with poly(ethylene oxide)

Abstract The micellization process of sodium ω-phenyldecanoate and the interaction between this surfactant and poly(ethylene oxide) (PEO) in aqueous solution have been studied, using 1 H NMR spectroscopy. The critical micelle concentration (CMC) and the micellar aggregation number of sodium ω-phenyldecanoate are found to be 0.0121 ± 0.0001 m and 26 ± 1, respectively. Upon micellization, large upfield (low frequency) chemical shifts are observed for the aromatic protons and the methylene protons near the terminal phenyl group, while the shifts of protons attached to methylene carbons near the headgroup are very small. These observations imply that the terminal phenyl group is on average located in the micellar interior. In the presence of PEO, two critical concentrations are observed, corresponding to, respectively, the formation of a polymer-surfactant aggregate and the coexistence of the polymer-surfactant aggregate and surfactant micelle. The first critical concentration, X 1 , is at 0.00885 ± 0.00004 m and is independent of the PEO molecular weight and concentration. The second critical concentration, X 2 , is dependent on PEO concentration. The binding ratio, ( X 2 − CMC)/ C PEO , is 0.44 ± 0.01 ω-phenyldecanoate ion per monomer unit of PEO. In the PEO/ω-phenyldecanoate system large 1 H aromatic ring current-induced shifts are observed for the surfactant protons near or on the terminal phenyl group and for the PEO protons. Very small shifts are observed for the surfactant protons near the head group. These observations suggest that the terminal phenyl group and most of the PEO are in close proximity, and on average in the interior of the polymer-surfactant aggregate.

Solubilization equilibria of alcohols and polymers in micellar solutions: NMR paramagnetic relaxation studies

Abstract The solubilization equilibria of some typical alcohols in cationic, anionic, and nonionic surfactant micellar solutions were investigated using the NMR paramagnetic relaxation method [J. Phys. Chem., 93 (1989) 2190]. The degree of solubilization of solubilizate in the micellar phase, p , was determined by monitoring the change in the 1 H spin-lattice relaxation rate of the solubilizate upon the addition of paramagnetic ions to the aqueous phase. Hydrophilic paramagnetic ions, which should have the same sign of charge as the micellar surface for the case of ionic surfactant systems, were used to enhance the spin-lattice relaxation rate of solubulizate in the aqueous phase. The degree of solubilization obtained is, within experimental error, independent of the concentration of the paramagnetic ions. The error in the degree of solubilization, p , decreases with increasing paramagnetic ion concentration. The results obtained using different paramagnetic ions (i.e., 3-carboxylate-PROXYL (charge: − 1) and Mn (EDTA) 2− in anionic micellar systems, and Mn (D 2 O) 2+ 6 and Mn (EDTA) 2− in nonionic micellar systems) are in good agreement. The paramagnetic relaxation method was also used to determine the degree of solubilization of poly (ethylene oxide) in two surfactant solutions, sodium ω-phenyldecanoate and sodium dodecyl sulfate.

Nuclear magnetic resonance paramagnetic relaxation method for the determination of micellar solubilization equilibria applied to anionic micelles

The NMR paramagnetic relaxation method has been developed (Z. Gao, R. E. Wasylishen and J. C. T. Kwak, J. Phys. Chem., 1989, 93, 2190) as an alternative procedure for the determination of micellar distribution equilibria. The 3-carboxyproxyl anion has been suggested as a suitable relaxation agent for the case of anionic micelles. In a recent paper (P. J. Bratt, H. Choudhury, P. B. Chowdhury, D. G. Gillies, A. M. L. Krebber and L. H. Sutcliffe, J. Chem. Soc. Faraday Trans., 1990, 86, 3313) it was noted that the acid form 3-carboxyproxyl is partially located in the micellar phase, and therefore cannot be used as a relaxation agent in this method. In this paper, the difference between 3-carboxyproxyl (acid form) and sodium-3-carboxyproxyl (salt form) is demonstrated by measuring 1H T1 values of sodiumdodecylsulphate alkyl protons and of completely solubilized tert-butylcyclohexane in the presence of these two relaxation agents. The paramagnetic 3-carboxyproxyl anion does not influence the T1 values of surfactant or solubilizate protons in the micellar phase. It is concluded that the anionic form of 3-carboxyproxyl can be used to determine distribution coefficients of neutral solubilizates in anionic micelles.

Spectroscopy of rhodamine 6G solubilized in complexes of anionic polymers with cationic surfactant

Abstract Absorption and emission spectra of rhodamine 6G (R6G) are reported in mixed solutions of dodecyltrimethylammonium bromide (DTAB) with various polyelectrolytes, including the sodium salts of poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), poly(styrenesulfonic acid) (PSS), polygalacturonic acid (pectate), and the alternating copolymers of maleic acid with ethylene (PMA-E) and styrene (PMA-St). In the systems of PAA, PMA, PMA-E and PMA-St, in the absence of surfactant, the R6G absorption spectrum has a strong dimer band and exhibits remarkable quenching of the emission spectrum indicating self-association of R6G in the polyion domain. Upon addition of DTAB, the dimer band disappears, the monomer band increases and the emission intensity recovers at a longer wavelength than that for the monomer band in aqueous solution. These observations are explained by the self-association of R6G in the polyion solutions, followed by the solubilization of R6G in the monomeric form in polyion-DTA complexes. In the case of PSS a red-shift of the R6G monomer band is observed in the absorption spectrum of R6G in solution of PSS alone, and the dimer band is absent. In this case DTAB addition does not affect the monomer band position and induces only a small dimer band maximum at high DTAB concentration, indicating territorial site-binding of R6G and DTA. In the pectate system, only minor changes in absorption and emission spectra are observed in the solution of pectate alone, and addition of DTAB leads to full recovery of the free solution spectra, indicating a weak interaction of R6G with pectate. These results are discussed in terms of different competitive, cooperative binding processes of R6G and DTAB by the various polyions.

NMR studies of the interactions of neutral polymers with unilamellar vesicles

Abstract The NMR paramagnetic relaxation method is used to investigate the interactions between poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) and small unilamellar vesicles (SUVs) of phospholipids. The results indicate the PEO does not interact with phospholipid vesicles; it is located exclusively in the bulk water phase. Low molecular weight PPO polymers distribute between the aqueous and lipid phases. The vesicles remain impermeable to Pr3+ ions when PEO or PPO is present.