J. Phys. Chem. 1995, 99, 4672-4678
4672
Interactions of Ionic Surfactants with a Nonionic Cellulose Ether in Solution and in the Gel State Studied by Pulsed Field Gradient NMR H. Walderhaug, B. NystriSm,” and F. K. Hansen Department of Chemistry, The University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo 3, Norway
B. Lindman Physical Chemistry 1, Chemical Center, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received: October 11, 1994@ Surfactant and polymer NMR self-diffusion measurements were carried out at different temperatures on gelling and nongelling aqueous systems of ethyl(hydroxyethy1) cellulose (EHEC) in the presence of sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB). The surfactant self-diffusion experiments revealed strong surfactant-EHEC interactions. By using a two-site model it was demonstrated that the degree of surfactant binding to EHEC, at a given total surfactant concentration, is higher in the presence of CTAB than with SDS. In both cases, the amount of surfactant bound to the polymer is roughly independent of temperature, even when the gel transition region is approached. The results for a more polar EHEC sample show that in the presence of SDS (a nongelling system in the temperature range considered) a significant increase of the binding parameter is observed at elevated temperatures, whereas no change of the parameter is found in solutions containing CTAB. In the analysis of the interaction situation, the interplay between surfactantinduced associations and enhanced polymer-polymer interactions is considered. The polymer self-diffusion results suggest that the decay of the spin-echo attenuation can initially be described by a stretched exponential (“fast” diffusion) followed by a single exponential (“slow” diffusion). The observations indicate that the interactions are stronger in the EHEUSDS system as compared with the corresponding EHEC/CTAB system.
Introduction The interaction between ionic surfactants and water-soluble nonionic polymers has gained a growing in recent years, mainly due to the importance of this type of systems in various industrial applications (paints, pharmaceuticals, oil recovery, etc.). Aqueous solutions of ethyl(hydroxyethy1) cellulose (EHEC) in the presence of a surfactant constitute a system of this Various experimental approaches, such as cloud point (cP),~N M R s e l f - d i f f u s i ~ nelectrical , ~ ~ ~ ~ conductivity,”,l2 time-resolved fluorescence quenching,l1%l2 rheological,13J4 static and dynamic light ~ c a t t e r i n g ~measurements, ~J~ as well as measurements using a surfactant-selectiveelectrode,16 have been employed to study different physicochemical properties of EHEC in aqueous solution in the presence of a surfactant. The picture that emerges from these investigations is that both anionic and cationic surfactants interact strongly with the EHEC polymer. It is well established that aqueous solutions of EHEC in the presence of an ionic surfactant exhibit a thermoreversible sol-gel transition in the semidilute regime, where the polymer molecules overlap each other. Upon heating, these systems undergo a transformation from a moderate viscous solution to a clear and stiff gel. The strength of the gel and the exact position of the gel point depend on factors such as type of substituent, degree and heterogeneity of substitution, and type of surfactant. Quite recently,14the sol-gel transition of aqueous solutions of EHEC in the presence of an anionic (sodium dodecyl sulfate, SDS) and a cationic (cetyltrimethylammonium bromide, CTAB) surfactant was studied with the aid of dynamic light scattering and shear stress relaxation measurements. These experiments revealed that the presence of the anionic surfactant gave rise to much stronger network formation than with the cationic surfactant. In order to gain further insight into the nature of the complex interaction phenomena between the @
Abstract published in Advance ACS Absfracts, March 1, 1995.
0022-365419512099-4672$09.00/0
polymer and the surfactant, it is necessary to understand the behavior and function of the surfactant in the system. In the present study, we have carried out N M R self-diffusion measurements of the surfactant in the systems EHEC/SDS/D20 and EHEC/CTAB/D;?O over an extended temperature range. hevious 1 ~ 1 2 ~ 1utilizing 6 different experimental methods, on aqueous EHEC/surfactant systems indicate that binding of surfactant to the polymer increases with temperature. However, most of the N M R experiments in this study were conducted at a surfactant concentration (4 mm) where the present EHEC/surfactant systems give gels at elevated temperature. Under these conditions we have found that the amount of surfactant bound to the polymer is practically independent of temperature. This indicates that a further increase in the degree of surfactant binding with increasing temperature is not a necessary condition for these systems to form gels. In addition, some NMR measurements on a more polar EHEC sample in the presence of SDS or CTAB have been performed. We will also report some preliminary results from polymer self-diffusion N M R measurements on dilute and semidilute EHEC/SDS/H2O and EHEC/CTAB/HzO systems over a broad temperature region. These results reveal a complex pattern of behavior when the gel zone is approached.
Experimental Section Materials and Solution Preparation. The EHEC samples were manufactured by Berol Nobel (Stenungsund, Sweden). Most of the Fourier transform pulse-gradient spin-echo (FTPGSE) measurements were carried out on a sample designated Bermocoll DVT 89017. The number average molecular weight M,, for this sample was about 100 000, and cp = 34 O C in pure water. The degree of substitution of ethyl groups DSethyl= 1.9 per anhydroglucose unit and the molar substitution of ethylene oxide groups MSEO = 1.3 per anhydroglucose unit. Some measurements were also performed on a more hydrophilic 0 1995 American Chemical Society
. I . Phys. Chem., Vol. 99, No. 13, 1995 4673
Interactions of Ionic Surfactants EHEC sample (Bermocoll E351 FQ; henceforth referred to as EHEC,) with M,,= 150 000, cp = 68 "C, DSethyl= 1.1, and MSEO= 2.1. The cationic CTAB and the anionic SDS were both obtained from Fluka and were used without further purification. Desalted and purified EHEC solutions were prepared by dialysis against distilled water for one week. Hollow fibers made of regenerated cellulose were used as dialyzing membrane (molecular cut-off at 8000; Spectrum Medical Industries). After freeze-drying, the polymer was redissolved in water. Samples were prepared by weighing the components, and the solutions were homogenized by stirring at room temperature for several days. NMR Experiments. All NMR experiments were carried out in 5 mm NMR tubes on a Bruker CPX 200 spectrometer, equipped with a proton field gradient probe (from Cryomagnetics Inc., Indianapolis) and a home-built field gradient driver unit. In the present configuration this system is capable of delivering gradients from ca. 10 to 75 mT/m. By using a variable temperature control unit, the temperature constancy was controlled to within & O S "C. The temperature settings were calibrated by using an ethylene glycol sample giving easily measurable shift differences between the two proton NMR signals that depend strongly on temperature. All the surfactant NMR self-diffusion measurements were performed on samples dissolved in D20 (Fluka, 99.8 atom % D). At the lowest surfactant concentrations with polymer present, a very large number of scans was accumulated in order to obtain an acceptable signal-to-noise ratio. In this study the FT-PGSE technique17 was utilized. The selfdiffusion coefficients (Os) of the surfactants were evaluated by following the attenuation of the intensities of the terminal methyl signals with aid of the Stejskal-Tanner equationla
R
= Z(S)/Z(O)
= exp(-2A/T2) exp[-k2tefPs]
(1)
where I(6) denotes the attenuated signal intensity at the gradient duration 6 and I(0) is the intensity in the absence of a gradient, k yg6 and teff = A - 6/3. Here A (typically 0.07 s) is the time interval between the pulses (close to the effective experimental observation time for the process), T2 is the proton spinspin relaxation time, y is the proton gyromagnetic ratio, and g is the magnetic field gradient amplitude. The measurements were performed by varying 6 and by keeping g and A fixed during an experiment. The magnetic field gradient amplitudes were calibrated using a sample of 5% light water in heavy water together with known diffusion coefficient^'^ for the HDO species. The value of T2 was found to vary somewhat depending on temperature and composition of the system. In some cases the low values of T2 enforced a reduction of A in order to obtain measurable signals (see eq 1). The nonlinear least squares program UNIFITZ0was used to extract surfactant self-diffusion coefficients from spin-echo intensity raw data. For the EHEC polymers the NMR signal intensities from terminal methyl groups on the side chains (1-2 ppm region) were used to construct the echo attenuation plots in Figures 5 and 6 . A possible overlap between this signal and the signals of the surfactant methylene protons was avoided by extending A to 0.14 s (vide infra). At this length of A and at a magnetic field gradient amplitude of 75 mT/m all the NMR signals from the surfactant had disappeared from the spectra. The N M R results for the motion of the polymer chains exhibit a more intricate behavior than that of the surfactant diffusion. A tentative model for the analysis of the polymer NMR data will be suggested in connection with the discussion of the results.
0.70
t
(1.I 0.40
I
I
0.00 20.0
25.0
30.0
35.0
40.0
45.0
Temperature PC) Figure 1. Temperature dependence of the observed surfactant selfdiffusion coefficient at a fixed SDS concentration of 4.0 mm for SDSI D2O (+), for EHECISDSDzO at EHEC concentrations of 0.5 wt % (0) and 1.0 wt % (O), and for EHECJSDSD20 at an EHEC, concentration of 1.0 wt % (*). The solid lines and the dashed curve are drawn to show the trends.
Results and Discussion Surfactant Self-Diffusion. The interaction between EHEC and an ionic surfactant as a function of temperature has been the subject of research in a number of recent contribut i o n ~ . It~was , ~observed ~ ~ ~for ~ certain ~ ~ ~surfactants and certain conditions that more and more surfactant binds to the polymer with increasing temperature. With the aid of the FT-PGSE technique the surfactant self-diffusion coefficient can be monitored and the amount of surfactant, kinetically bound to the polymer, can be q~antified.43~As a first approximation, the diffusion data can be analyzed according to a two-site model, where it is assumed that the surfactant is either bound to the polymer (as either monomers or clusters) with a diffusion coefficient equal to the polymer diffusion coefficient Db or is free in the solution and diffuses as single monomers with a diffusion coefficient Df. Due to the rapid exchange, which is much faster than the time scale of the NMR experiment, between the free and the polymer-bound states, the observed selfdiffusion coefficient Dabs is the population-averaged mean of the self-diffusion coefficients for the two states, Le.,
Where p denotes the fraction of the total amount of surfactant that is bound to the polymer. Since in this study Db
