J. Phys. Chem. 1996, 100, 4909-4918
4909
Binding of SDS to Ethyl(hydroxyethyl)cellulose. Effect of Hydrophobic Modification of the Polymer Krister Thuresson,*,† Olle So1 derman,† Per Hansson,‡ and Geng Wang§ Physical Chemistry 1 and DiVision of Thermochemistry, Chemical Center, UniVersity of Lund, P.O. Box 124, S-221 00 Lund, Sweden, and Department of Physical Chemistry, Uppsala UniVersity, P.O. Box 532, S-751 21 Uppsala, Sweden ReceiVed: July 20, 1995; In Final Form: NoVember 22, 1995X
The binding of SDS to cellulose polymers in the semidilute concentration regime has been studied by means of NMR, ion-selective electrode, and a time-resolved fluorescence technique. Two polymers have been used, differing only in a low degree of hydrophobic modification of one of them. NMR self-diffusion and activity measurements show that the binding of SDS to the nonmodified polymer has a fairly pronounced critical aggregation concentration (cac), while binding to the hydrophobically modified polymer is less cooperative up to a concentration of about the cac in the nonmodified polymer/SDS system. NMR T2 relaxation and fluorescence studies indicate that surfactants bound to the hydrophobically modified polymers in the noncooperative regime have slow dynamics compared to micellized surfactants, to surfactants bound to the unmodified polymer, and to surfactants bound to the hydrophobically modified polymer in the cooperative regime. Furthermore, in the non-cooperative regime the fluorescence studies imply that the SDS aggregation number of the mixed micelles is low and that the number of hydrophobic zones is invariant with respect to the surfactant concentration.
Introduction Mixtures of polymers and surfactants in aqueous solutions have through the years been used in many applications. The number of scientific investigations in the field of polymer surfactant interactions is therefore vast, and the techniques used include phase studies, surface tension, conductivity, surfactant activity, rheology, time-resolved fluorescence, and NMR.1-21 Some years ago a new type of polymers, often referred to as hydrophobically modified (HM) polymers, consisting of a water soluble backbone to which a low number of hydrophobic tails have been chemically attached, was introduced.22 As such polymers comprise both hydrophilic and hydrophobic parts, they have an amphiphilic behavior in aqueous solutions. Despite the fact that the substitution degree is low (typically e5 mol %), they often have superior performance as compared to their unmodified relatives. For instance, the viscosity of a solution containing an HM polymer and a proper amount of surfactant can be several orders of magnitude higher than in the corresponding system with the unmodified polymer.23-25 These effects are usually discussed in terms of the surfactant’s ability to assemble at the hydrophobic moieties of the polymers. To gain a deeper understanding of the interactions underlying the differences in solution behavior on hydrophobic modification of a polymer (e.g., increased viscosity on addition of surfactants23-25 and changes in phase behavior26) more information on the polymer/surfactant complex is desired. In this paper we therefore present a comparative study of the interaction between an anionic surfactant and an HM polymer or the parent polymer, respectively. Time-resolved fluorescence quenching was used to gain knowledge about the size and the concentration of those complexes. The dynamics of the surfactant molecules bound to the polymer were probed with a fluorescence technique as well as with NMR relaxation methods. In order to interpret * To whom correspondence should be addressed. † Physical Chemistry 1, University of Lund. ‡ Uppsala University. § Division of Thermochemistry, University of Lund. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4909$12.00/0
the fluorescence and NMR relaxation data, the surfactant binding needed to be quantified. Unfortunately, binding isotherms of surfactants to HM polymers are up to today rather scarce.27,28 Therefore parts of this work concern the determination of the binding of SDS to the polymers. Traditionally two techniques, equilibrium dialysis1,29-32 and surfactant-specific electrodes,1,33-35 have been used to obtain such isotherms. In the first method, a polymer solution is placed in equilibrium with a surfactant solution through a semipermeable membrane. In the second method, the activity of the SDS molecules is probed directly. Both of these methods have drawbacks. An extended time (several days) is required to reach equilibrium in the dialysis experiment. The concentration of polymer-bound SDS can then be calculated from a determination of the total SDS concentration on both sides of the membrane. The second method also suffers drawbacks. Those will be discussed later as the ion-selective electrode is one of the methods utilized in this investigation. A third method for determination of binding of surfactants to polymers is NMR self-diffusion,5,36 and recently this method was used to obtain the binding of SDS to an associative polymer.27 The measurement, which is nonperturbing, offers a convenient and quick way to determine the SDS binding to the polymers, and despite a rather low sensitivity, which is the main drawback of this method, we succeeded in measuring the binding of SDS to a concentration below 1 mmolal. Experimental Procedures Materials. Two ethyl(hydroxyethyl)cellulose (EHEC) polymers, Figure 1, were supplied by Akzo Nobel Surface Chemistry AB, Stenungsund, Sweden. They are identical except for a small hydrophobic modification of one of them, consisting of 1.7 mol % nonylphenol (corresponding to 0.64 mmolal nonylphenol in a 1 w/w % polymer solution). Both have substitution degrees of hydroxyethyl and ethyl groups of MSEO ) l.8 and DSethyl ) 0.6-0.7, respectively, referring to the average number of bonded groups per repeating unit of the polymer. Their molecular weight is approximately 100 000. © 1996 American Chemical Society
4910 J. Phys. Chem., Vol. 100, No. 12, 1996
Thuresson et al.
Figure 1. Structure of EHEC and HM-EHEC.
Figure 2. Stimulated spin echo sequence used in the diffusion experiments.
MSEO, DSethyl, and the molecular weight were given by the manufacturer, while the nonylphenol substitution of HM-EHEC was determined by measuring the absorbance of the aromatic ring at 275 nm with a Shimadzu UV-160 spectrophotometer using phenol as a reference. Before use the polymers were purified as described elsewhere.26 SDS of specially pure grade was obtained from BDH Laboratory Supplies, Poole, Dorset, England, and R-deuterated SDS was synthesised by Syntestja¨nst AB, Lund, Sweden. Both SDS samples were used as received. Pyrene (analytical grade) from Serva FeinBiochemica GmbH & Co., Heidelberg, Germany, was used as supplied. NDodecylpyridinium chloride (DoPC) from Aldrich-Chemie, Steinheim, Germany, was recrystallized several times from acetone. D2O (99.8% isotopic pure) from Dr. Glaser AG, Basel, Switzerland, was used as solvent in the NMR self-diffusion measurements, and deuterium-depleted H2O from Sigma Chemical Co., St. Louis, Mo, in the T2 relaxation measurements. In all other experiments water of Millipore quality was utilized. Methods. The samples, prepared by weighing, were carefully mixed by tilting end over end at 25 °C for 2-3 days before any measurements were performed. The concentration of polymer was kept constant at 1 g of polymer per 100 g of Millipore water (1 w/w %) throughout the investigation. Polymer and surfactant concentrations were compensated for changed molecular weights originating from enrichment of a specific hydrogen isotope (i.e., NMR measurements). Selfdiffusion measurements were performed on ordinary SDS with D2O as solvent, while T2 relaxation was measured on R-deuterated SDS containing D2O-depleted water as solvent. Self-diffusion measurements were performed on a Bruker MSL 100 NMR spectrometer equipped with a thermostated
probe ((0.1 °C) suited for determination of proton diffusion at a resonance frequency of 100 MHz. A stimulated spin echo pulse sequence, Figure 2, was used due to rapid transverse relaxation of the protons of the polymers. The magnitude of the field gradients generated by the diffusion probe were calibrated by measuring the diffusion of water at 25 °C.37 Both the SDS, with as high accuracy as possible, and the polymer diffusion, with a somewhat lower demand on the accuracy, were obtained simultaneously. The experiments were performed by varying the length, δ, of the field gradient pulses and keeping everything else constant. Each diffusion experiment consisted typically of 15-25 different values in which the spectrum was accumulated several times to obtain a satisfactory signal to noise ratio. T2 relaxation experiments were performed on a Varian Unity 300 NMR spectrometer. The line width of the deuterium nucleus in R-deuterated SDS was measured at half-height of the signal at a resonance frequency of 46 MHz. All NMR measurements were carried out at 25 °C. SDS activity measurements, at 25 °C, were performed with a home-built electrode. The carrier complex was prepared from equivalent amounts of cetyltrimethylammonium bromide and SDS.38 The purified complex was dissolved in o-nitrotoluene to a concentration of 1 mM, and the functional liquid membrane was formed by immersing a 0.022 µm Teflon filtration membrane in the organic solvent. The membrane was mounted over the open end of a glass tube with a mechanical support, and the organic phase was covered with an aqueous solution containing 1 mM NaCl and 1 mM SDS. The reference electrode consisted of a 10 mM NaCl solution in a glass tube with a ceramic liquid junction plug in the bottom. Ag/AgCl electrodes were dipped into the solutions of the working and the reference electrodes. The electrode potential was measured with a PHM64 Research pH-meter (Radiometer), and generally the equilibrium value was reached within 30 s. The reproducibility was about (1 mV. NaCl (10 mM) was used as the support electrolyte in all measurements. Before each run the membrane of the electrode was replaced. The surfactant-sensitive membrane sets an upper concentration limit for the range in which the electrode can be used. Once the surfactant activity has reached the point at which aggregates can form (micelles), the surfactants incorporated in the membrane may dissolve in the micelles, and, as a consequence, the electrode’s response is changed. Hence, this technique gives best results well below the critical micelle
Binding of SDS to Ethyl(hydroxyethyl)cellulose
J. Phys. Chem., Vol. 100, No. 12, 1996 4911
concentration (cmc) of the surfactant. A simple way to test the response of the electrode is to perform repeated measurements in a SDS solution of known concentration. The electrode should give the same response after measuring the SDS activity of the unknown sample as it did before the measurement. The fluorescence decay data were collected with the single photon counting technique. A detailed description of the timeresolved fluorescence quenching (TRFQ) experimental technique and equipment used is given elsewhere.39 All measurements were performed at 25 °C in equilibrium with air. The fluorescence from the probe (pyrene) was monitored at 395 nm following excitation at 320 nm. Results Evaluation of Self-Diffusion Data. The evaluation of the NMR self-diffusion experiments was complicated by the fact that the peaks from the polymer and the surfactant partly overlap. The raw spectra were integrated over the two interesting peaks, and the peak areas, rather then the peak heights, were used in the data evaluation. One of the peaks originated only from the polymer, while the other peak contained contributions both from the polymer and the surfactant. As the polymers have the possibility to form clusters which have an inhomogenous size distribution, the attenuation of the first peak could be fitted by assuming a Kohlrausch-Williams-Watts distribution of diffusion coefficients.40 Therefore, the Stejskal-Tanner expression is given by40-44
[(
(
I(τ1 + τ2) ) I(0) exp - (γg)2D2,appδ2 ∆ -
δ 3
β
)) ]
(1)
I(τ1 + τ2) is the measured peak area at time τ1 + τ2, where τ1 and τ2 specify the time for the two radio frequency pulses (see Figure 2), I(0) is the peak area when δ ) 0, γ denotes the magnetogyric ratio of protons, g is the strength of the field gradient, δ represents the duration of the magnetic field gradient pulses, ∆ is the time from leading edge to leading edge between the pulsed gradients, D2,app represents the apparent diffusion coefficient of the polymer molecules, and, finally, β is a measure of the width of the distribution of polymer diffusion coefficients. When β approaches unity, the polymer diffusion coefficients become more homogenous while a lower value represents a broader distribution. I(τ1 + τ2) was measured, and δ and ∆ were known parameters from the setup of the experiment. (γg)2, which is a constant associated with the spectrometer, was obtained from the calibration on water. I(0), D2,app, and β were obtained from a nonlinear least-squares fit (using a Levenberg-Marquardt method) of eq 1 to the I(τ1 + τ2) values. The attenuation of the peak used for the SDS diffusion determination was fitted with a biexponential expression (see eq 2), with one term for the SDS diffusion and one for the polymer diffusion. The latter consists of a spectrum of diffusion coefficients (vide supra).
(
[
(
I(τ1 + τ2) ) I(0) p exp -(γg)2D1δ2 ∆ -
[(
δ + 3
)]
(
(1 - p) exp - (γg)2D2,appδ2 ∆ -
δ 3
β
)) ]) (2)
D1 represents the surfactant self-diffusion coefficient, and p is the fraction of the surfactant contribution to the peak area. All other variables are as in eq 1. p, D1, and I(0) were fitted to the δ and I(τ1 + τ2) data pairs, with the polymer diffusion (D2,app and β) held constant at values obtained from the fitting of the attenuation of the first peak to eq 1. In Figure 3a,b the best
Figure 3. (a) NMR attenuations of the peak containing signal from EHEC, open symbols, and from the signal containing contributions from both EHEC and SDS, filled symbols. The lines represent the best fits of eqs 1 and 2, respectively, to the data points: 1 w/w % EHEC and 3.5 mmolal SDS (O, b), 1 w/w % EHEC and 8 mmolal SDS (0, 9), and 1 w/w % EHEC and 20 mmolal SDS (], [). (b) Same as a above but for 1 w/w % HM-EHEC and 0.65 mmolal SDS (O, b), 1 w/w % HM-EHEC and 4 mmolal SDS (0, 9), and, finally, for 1 w/w % HMEHEC and 12 mmolal SDS (], [).
fits of eqs 1 and 2 to the NMR attenuations of the peak containing the signal only from the polymer and of the peak containing the signal from both polymer and surfactant are shown for EHEC and HM-EHEC samples, respectively, at some different surfactant concentrations. The parameter values obtained from the best fits are given in Table 1. Due to rapid exchange of the surfactants between their different locations on the time scale set by the experiment (a few hundred milliseconds), the measured attenuation of the SDS peak is a population-weighted average of the different contributing diffusion processes. There will be contributions from free surfactants, from surfactants bound in micelles, and from surfactants bound to the polymers. However, the number of different contributing diffusion processes can be reduced. The free micelle concentration is set to zero in the investigated SDS concentration regime, and the surfactants bound to the polymers
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Thuresson et al.
TABLE 1: Parameter Values Giving the Best Fits of Equations 1 and 2 to the Experimental NMR Attenuationsa D2,app ×1010 (m2 s-1)
β
D1 ×1010 (m2 s-1)
p
D2,mean ×1010 (m2 s-1)
0.023 0.014 0.068 0.0097 0.0074 0.19
0.48 0.65 0.62 0.38 0.34 0.40
3.77 2.26 0.92 2.72 3.10 1.53
0.64 0.79 0.85 0.16 0.63 0.75
0.011 0.010 0.047 0.0024 0.0013 0.059
EHEC + 3.5 mmolal SDS EHEC + 8.0 mmolal SDS EHEC + 20.0 mmolal SDS HM-EHEC + 0.65 mmolal SDS HM-EHEC + 4.0 mmolal SDS HM-EHEC + 12.0 mmolal SDS
a The results are displayed in Figure 3a,b. In the fit of data to eq 2, D2,app and β are held constant at values given from the fit of polymer diffusion data to eq 1 (see text). D2,mean is given by eq 4.
are approximated to have the distribution of diffusion coefficients of the polymer (which varies with the surfactant concentration). Given those assumptions the diffusion-controlled NMR signal attenuation is due to the diffusion of free surfactants and to the diffusion of surfactants bound to the polymers. With the aid of eq 3 the fraction of polymer-bound surfactant, pbound, could be obtained. The results are presented in Figure 4a,b.
Dobs ) pboundDbound + (1 - pbound)Dfree
a
(3)
Dbound in eq 3 is set to be the mean value of the polymers’ selfdiffusion coefficients, D2,mean, at the corresponding surfactant concentration. D2,mean is obtained through a transformation of D2,app using eq 4 where Γ(1/β) is the Γ-function of (1/β).40,43,44
D2,mean )
D2,app 1 1 Γ β β
()
(4)
Dfree is the free monomer surfactant diffusion, which was determined in a separate experiment to (4.99 ( 0.06) 10-10 m2 s-1. In this context it should be mentioned that the polymerbound surfactant gave a negligible (
