A 1D‐ and 2D‐NMR Study of an Anionic Surfactant/Neutral Polymer Complex

Abstract

NMR chemical shifts and linewidth measurements were examined for mixtures of sodium 10-phenyldecanoate (Na x-PhDec) in deuterated aqueous solutions in the presence of varying compositions of poly(ethylene oxide) (PEO) polymers of 2000 and 4000 molecular weight. In addition, variable temperature NMR spectra and NMR spin lattice relaxation times (T 1 ) were obtained for the PEO-4000/Na x-PhDec system as a function of varying polymer concentrations. As expected, the polymer/surfactant systems exhibit the behaviour typical of that of an anionic surfactant/ neutral polymer system with well defined critical aggregation concentrations (CMC) corresponding to the formation of polymer/surfactant complexes below the CMC of the free surfactant. The 1 H-NMR linewidths acquired for the Na xPhDec/PEO-4000 system before and after the CMC region of the surfactant indicate that the maximum in the linewidth of the PEO proton peak is reached at approximately twice the CMC of the free surfactant. 2D-NMR NOESY measurements on this system exhibit cross peaks between the PEO protons and the protons on the surfactant backbone, consistent with the location of the phenyl group in the micellar interior. All these NMR experiments are interpreted in terms of the structure of the polymer/surfactant complexes as a function of the system composition.

NMR investigations of self-aggregation characteristics of SDS in a model assembled tri-block copolymer solution

“…The value of C 2 obtained from relaxation measurements agreed well with that of proton chemical shifts and self-diffusion investigations. Such proton relaxation measurements for monitoring cac and C 2 of SDS-PEO systems were reported earlier [33,34,48]. The PEO methylene proton R 2 data of mixture-II against SDS concentration is also portrayed in Fig.…”

“…It is well-known that R 1 data is sensitive to fast motions, while R 2 can afford valuable information on slow and fast motions [18,23,48]. Hence, DR (=R 2 À R 1 ) is a measure of slow mo- Table 2 Proton spin-lattice relaxation rates (R 1 ) (in s À1 ) of SDS in the absence and presence of the polymer-I of mixture-I (at 25°C).…”

“…Therefore, the present proton spin relaxation data can be envisaged as the contribution from two different molecular motions like local (fast) motions of surfactant/polymer chains and global (slow) motions arising from surfactant-polymer aggregate/micelle or the lateral diffusion of surfactant over the surfactant-polymer mixed aggregate/micelle. In essence, the present relaxation data can be viewed in the light of model free approaches i.e., two-step model [18,23,48], rather than the evaluation of spin-relaxation contribution from intra as well as inter molecular processes. Hence, R 2 measurements were made over a similar range of concentrations to that of R 1 .…”

An NMR study of macromolecular aggregation in a model polymer-surfactant solution

“…NMR spectroscopy has been used as a powerful technique to study macromolecular dynamics 31 in polymersurfactant systems ͑and the PEO-SDS system in particular͒ since it can report on molecular motion inside the aqueous solutions via the longitudinal relaxation time T 1 and the transverse relaxation time T 2 . 11,32 In addition, pulsed-fieldgradient diffusion NMR spectroscopy is used to measure the molecular self-diffusion coefficient. 33,34 All of these physical quantities will change due to the interaction between molecules, molecular aggregation, and micellization.…”

“…According to the two-step model for relaxation in surfactant systems, 32,[39][40][41] both the proton longitudinal relaxation rate R 1 and the transverse relaxation rate R 2 at different SDS concentrations are sensitive to fast local motions ͑"free state"͒ as well as slower aggregate motions ͑"aggregate state"͒. The difference ⌬R=R 2 −R 1 reports 41 on the slower ͑aggregate͒ motions.…”