Langmuir 1998, 14, 2050-2054
Interactions between Sodium Dodecyl Sulfate and Hydrophobically Modified Poly(acrylamide)s Studied by Electron Spin Resonance and Transmission Electron Microscopy Yilin Wang, Daohui Lu, Chengfen Long, Buxing Han, and Haike Yan* Institute of Chemistry, Chinese Academy of Science, Beijing 100080, People’s Republic of China
Jan C. T. Kwak Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3 Received August 7, 1997. In Final Form: January 30, 1998 The interactions of hydrophobically modified poly(acrylamide)s (HMPAM) and unmodified poly(acrylamide) (PAM) with sodium dodecyl sulfate (SDS) have been studied by electron spin resonance using 2,2,6,6-tetramethylpiperidine-1-oxyl and 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl as spin probes. The morphologies of dried solutions of the polymers with and without SDS have also been observed by transmission electron microscopy. The rotational correlation time τc of the spin probes reflects the microenvironment of the polymer-micelle aggregates, which indicates that the higher hydrophobicity of HMPAMs leads to a much more compact packing in the polymer-micelle aggregates. The interactions between surfactants and polymers cause morphological transitions of polymers, which results in the polymer chains stretching out.
Introduction Hydrophobically modified (HM) polymers have recently become an important and unique class of water-soluble polymers. They have attracted increasing attention because of their unusual solution properties and numerous industrial applications.1 HM polymers can be modified by molecular weight,2 charge density,3 extent of hydrophobic substitution,4 and nature of the hydrophobic group.5 But usually, HM polymers consist of a water-soluble backbone onto which a low number of hydrophobic groups have been chemically attached1 and the hydrophobic side groups often consist of long alkyl chains. Although the substitution degree of hydrophobic groups is very low, HM polymers often have outstanding behaviors as compared to their unmodified parent polymers. For instance, the viscosity of a solution containing a HM polymer and a small amount of surfactant can be several orders of magnitude higher than that in the corresponding system with the unmodified parent polymer.6-9 Studying the specific interaction pattern of HM polymers with surfactants will provide a possibility of controlling viscosity and the other properties and (1) Glass, J. E., Ed. In Polymers in Aqueous Media: Performance through Association; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (2) Wormuth, K. R. Langmuir 1991, 7, 1622. (3) Shimizu, T.; Seki, M.; Kwak, J. C. T. Colloids Surf., A 1986, 20, 89. (4) Biggs, S.; Selb, J.; Candau, F. Langmuir 1992, 8, 838. (5) Walderhaug, H.; Hansen, F. K.; Abrahmsen, S.; Persson, K.; Stilbs, P. J. Phys. Chem. 1993, 97, 8336. (6) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B. Prog. Colloid Polym. Sci. 1992, 89, 118. (7) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994. (8) Ka¨stner, U.; Hoffmann, H.; Do¨nges, R.; Ehrler, R. Colloids Surf. 1994, 82, 279. (9) Hulde´n, M. Colloids Surf. 1994, 82, 263.
designing various systems for various applications; therefore many works about this type of systems have been reported.10-16 To explore the nature of the interactions between HM polymers and surfactants, a series of studies17-19 have been reported on the aqueous solutions of hydrophobically modified poly(acrylamide)s (HMPAM) and ionic surfactants. Kwak et al.17 reported experimental evidence for an associative phase separation in the mixtures of HMPAM and sodium dodecyl sulfate (SDS) using NMR spectra, fluorescence spectra, transition temperature measurements, and viscosity. The binding of alkylbenzenesulfonates to various HMPAMs which differ in the degree of hydrophobic modification was also studied by 1 H NMR.18 The results show that the interaction between surfactants and polymers was influenced by the structure of the surfactant and the degree of hydrophobic substitution of the polymer. Recently, the interactions of HMPAMs and their unmodified parent polymer PAM with SDS and tetradecyltrimethylammonium bromide have been studied by microcalorimetry.19 The critical aggregation concen(10) Thuresson, K.; Nystro¨m, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730. (11) Thuresson, K.; Nilsson, S.; Lindman, B. Langmuir 1996, 12, 530. (12) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909. (13) Kevelam, J.; van Breemen, J. F. L.; Blokzijl, W.; Engberts, J. B. F. N. Langmuir 1996, 12, 4709. (14) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967. (15) Anthony, O.; Zana, R. Langmuir 1996, 12, 3590. (16) Winnik, F. M.; Regismond, S. T. A.; Goddard, E. D. Langmuir 1997, 13, 111. (17) Effing, J. J.; McLennan, I. J.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 2499. (18) Effing, J. J.; McLennan, I. J.; Van Os, N. M.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 12397. (19) Wang, Y. L.; Han, B. X.; Yan, H. K.; Kwak, J. C. T. Langmuir 1997, 13, 3119.
S0743-7463(97)00890-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/28/1998
Interactions between Polymers and Surfactants
Figure 1. Structures and nomenclature of the nitroxide radicals and the polymers used.
tration and the thermodynamics parameters of the surfactants in the presence of HMPAMs and PAM have been obtained from the mixing enthalpy curves. This paper is an important complement to the previous works.17-19 We present a comparative study of the interactions between SDS and HMPAMs or PAM, respectively. Electron spin resonance (ESR) is used to study the microenvironments experienced by nitroxide radicals 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-oxo2,2,6,6-tetramethylpiperidine-1-oxyl (OTEMPO) in SDS solutions with PAM and HMPAMs. Transmission electron microscopy (TEM) is concentrated on the aggregated morphologies of PAM and HMPAMs with and without SDS. The aim of these investigations is to gain a deeper understanding of the morphologies and the other factors which affect the viscosity properties of such systems. Experimental Section Materials. Two kinds of hydrophobically modified poly(acrylamide)s and their unmodified analogue were prepared by radical copolymerization of acrylamide and N-alkylacrylamide in tert-butyl alcohol with azobis(isobutyronitrile) as initiator at 60 °C. The synthetic method was the same as described previously.17,18 Structure and nomenclature of the polymers are shown in Figure 1. The average molecular weight as determined by viscometry was approximately 200 000 for all three polymers.19 SDS (BRL, 99.5%) was used as received. The spin probes TEMPO and OTEMPO were purchased from Sigma Chemical Co. and were used without further purification. Figure 1 shows the structures of the nitroxide radicals investigated. ESR Measurements. All solutions were prepared by weight in twice-distilled water. The polymer concentrations were kept constant at 0.1 wt %. The concentrations of TEMPO and OTEMPO were 1.5 × 10-4 mol kg-1, which were small enough to be considered as a negligible perturbation. The ESR samples were equilibrated for 2 days at 10 °C and then stabilized for 3 h at 25 °C before ESR spectroscopy. The electron spin resonance spectra were recorded at 25 °C on a Brucker ESP300 spectrometer operating at X-band with 100-kHz magnetic field modulation at 1.28-mW microwave power to avoid power saturation. All spectra were run using a flat cell. The average relative error for the rotational correlation time was better than 10%. TEM Measurements. TEM samples were prepared from the dilute solutions of PAM, PAM-C10-2% and PAM-C12-2% with and without SDS (just above critical micelle concentration), respectively. The polymer concentrations were 0.1 wt %. Thin films of samples were formed by placing a drop of solution on a Formvar polymer support film which had been mounted on the surface of a standard 300-mesh TEM copper grid. After drying, the sample films were shadowed with platinum-carbon at an
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Figure 2. Representative ESR spectra of TEMPO and OTEMPO in SDS-PAM-C10-2% systems containing 0.1 wt % PAM-C10-2% at 25 °C: (a) below the cac of SDS; (b) above the cac of SDS. angle of about 15°. The samples were imaged in a Hitachi H-800 transmission electron microscope at an operating voltage of 100 kV.
Results and Discussion Unpaired electrons are not inherently present in the surfactant-polymer systems, so a spin probe must be introduced in order to enable ESR studies. By careful selection of the structure of the probe, it may be constrained in a location of interest. The spin probes can report information on the microenvironments in which they are located. The spin probes used in this study, shown in Figure 1, are TEMPO and OTEMPO. The studied systems are SDS, SDS-PAM, SDS-PAM-C10-2% and SDS-PAMC12-2%. Representative ESR spectra of TEMPO and OTEMPO in polymer systems with different concentrations of SDS are presented in Figure 2. The ESR line shapes are related to the surfactant concentrations. Both the spin probes in the SDS solutions with and without polymers exhibit the usual three-line pattern below critical micelle concentrations (cmc’s) or critical aggregate concentrations (cac’s), which has been determined in ref 19. Above the cmc or cac values of SDS, the high-field lines are broadened as a result of solubilization of the probes in the SDS micelles or in the polymer-micelle aggregates. By analysis of the ESR spectra, the nitrogen hyperfine coupling constant and the rotational correlation time can be obtained.20 The hyperfine coupling constant AN can give information about the micropolarity of the microenvironment sensed by the nitroxide probe.21 A more polar environment produces a larger value of AN due to a greater electron density at nitrogen. Below the cmc and cac of SDS, the AN values of TEMPO are all 17.2 G and essentially invariant with and without polymers, which means that the micropolarities of the probe microenvironments do not change in this concentration range. Above the cmc and cac of SDS, the AN also shows a constant 16.9 G. For OTEMPO, the AN values are 16.1 and 15.8 G corresponding to the values below and above the cmc or cac of SDS, respectively. Obviously, for all the systems, AN values are lower for the spin probes in the micelle and polymermicelle aggregate than in bulk water, which reflects the reduced micropolarities at the binding sites of the probes in the micelle and the polymer-micelle aggregate. How(20) Jolicoeur, C.; Friedman, H. L. J. Solution Chem. 1978, 7, 813. (21) Knauer, B. R.; Napier, J. J. J. Am. Chem. Soc. 1976, 98, 4395.
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Figure 4. Sketch of the structures of TEMPO and OTEMPO in micellar aggregate.
Figure 3. Variation of rotational correlation time, τc, of TEMPO in SDS-polymer aqueous solutions.
ever, there is no detectable influence of different polymers on AN values. The rotational correlation time τc measured from the ESR spectrum is reflective of the probe mobility and can be used to monitor changes in the microviscosity experienced by the probe. τc may be regarded as the time needed for a molecular to rotate for an angle of π.22 As an approximation, from the ESR spectra, the rotational correlation time can be calculated from the following equation23, 24
[( ) ( ) ]
τc ) (6.6 x 10-10)Wo
where Wo represents the peak-to-peak line width of the ESR mid field line (in gauss) and h-1, h0, and h+1 are the peak-to-peak heights of the low-, mid-, and high-field lines, respectively. The constant 6.6 × 10-10 has been calculated for di-tert-butyl nitroxide,25,26 but to a good approximation, it can be used for other nitroxide radicals as well. τc can be correlated with the microviscosity of the probe by the following relation
τc ) 4πηa3/3kT where a is the hydrodynamic radius of the probe, η is the viscosity, and k and T represent the Boltzmann constant and the temperature in K, respectively. Figure 3 shows the variations of rotational correlation time of TEMPO as a function of the SDS concentration in SDS, SDS-PAM, SDS-PAM-C10-2%, and SDS-PAMC12-2% systems, respectively. In each system, there is a pronounced increase in τc at the onset of micelle or aggregate formation, which corresponds to the cmc or cac value of SDS, respectively. For TEMPO probe, when the SDS concentrations are below cmc and cac, τc shows no significant variation and the spin probe experiences low rotational friction just like (22) Krishnakumar, S.; Somasundran, P. J. Colloid Interface Sci. 1994, 162, 425. (23) Kivelson, D. J. Chem. Phys. 1960, 33, 1094. (24) Schreier, S.; Ernandes, J. R.; Cuccovia, I.; Chaimovich, H. J. Magn. Reson. 1978, 30, 283. (25) Martinie, J.; Michon, J.; Rassat, A. J. Am. Chem. Soc. 1975, 97, 1818. (26) Yoshioka, N. J. J. Colloid Interface Sci. 1978, 63, 378.
in water. When the SDS concentrations reach the cmc and cac, τc begins to increase, which is attributed to the motional restriction of the probe within the polymermicelle aggregates. At about 21, 22, 26, and 26 mmol kg-1 concentrations of SDS in SDS, SDS-PAM, SDSPAM-C10-2%, and SDS-PAM-C12-2% systems, respectively, the τc values do not increase any more, where the order of τc values in various systems is as follows: SDS < SDS-PAM < SDS-PAM-C10-2% < SDS-PAM-C122%. Ramachandran et al.27 reported that most of TEMPO solubilized in the micelles is located at the micelle-water interface. The nitroxide group of TEMPO is in contact with water and forms the strong hydrogen bonding with water. The hydrophobic cycle is oriented toward the hydrocarbon core of the micelle. The actual situation of TEMPO in micellar aggregate could be represented as sketched in Figure 4. Therefore the τc order indicates that the microviscosity at the micelle-water interface and micelle-polymer interface increases in the same order. This conclusion depends on the interaction between polymers and surfactants. For unmodified polymer PAM, the micellar aggregate formation is enhanced by an interaction between the surfactant and the polymer backbone. The “necklace model” of polymer-surfactant binding describes the polymer-surfactant aggregate as a series of spherical micelles whose surfaces are covered by polymer segments and connected by polymer strands. On addition of surfactants to the HMPAMs, the side chains of hydrophobic polymer act as nucleation sites onto which the surfactants preferentially adsorb and are formed to be the micelle-like aggregate with the side chains of the hydrophobic polymer. The interactions between polymers and surfactants cause τc in micellar aggregates to be greater than that in unperturbed micelles. The packing degree of the micellar aggregates on the polymer is decided by the strength of the interaction between polymer and surfactant, so hydrophobic side chains of the HM polymers yield more tightly compact structures in the micellar aggregates than in unmodified polymer, then cause τc to increase with the polymer hydrophobicity. That is to say, the polymer hydrophobic chains result that the rotation of the probe becomes slow and the binding interface of HMPAM with SDS has a higher microviscosity. The longer alkyl chain leads to the stronger hydrophobic interaction between polymer side chain and surfactant and also leads to more compact polymer-micelle aggregates. This observation helps to understand our (27) Ramachandran, C.; Pyter, R. A.; Mukerjee, P. J. Phys. Chem. 1982, 86, 3198.
Interactions between Polymers and Surfactants
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Figure 5. TEM images of (a) PAM, (b) PAM-C10-2%, (c) PAM-C12-2%, (d) PAM-C10-2% with SDS, and (e) PAM-C12-2% with SDS.
previous work,19 where the thermodynamic parameters indicate that the process of surfactant aggregation in the presence of polymer is strongly entropy-driven. In SDS, SDS-PAM, SDS-PAM-C10-2%, and SDSPAM-C12-2% systems, at about 21, 22, 26, and 26 mmol kg-1 of SDS, respectively, the τc values pass maximums and begin to decrease above these concentrations. This observation means that the surfactant binding to the polymer must be close to the saturation for the concen-
trations of polymers at the surfactant concentrations mentioned above. Then these surfactant concentrations are called saturation concentrations, where the microviscosity reaches the maximum. The curve of apparent viscosity vs SDS concentration for the SDS-PAM-C102% system also has a peak between the SDS concentrations of 10 and 20 mM.17 So the microviscosity and apparent viscosity are relevant. Above the saturation concentrations, the regular free micelle begins to form. Thus the
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influence of the polymer hydrophobic chains on the τc values gradually decreases, which makes the τc values for different polymer systems tend to be close at higher SDS concentrations. This tendency for the same systems has been observed in calorimetric curves.19 The above results indicate that the rotation of TEMPO is closely related to the hydrophobicity of the polymers; however, the result of OTEMPO shows that the τc values in the presence of polymers are identical with that in the absence of polymers. OTEMPO has two hydrophilic moieties. Molecular models indicate that the OTEMPO can exist on the micellar surface in a sidewise orientation in which both polar groups are exposed to water and interact with micellar headgroups and to the some extent without a serious loss of hydrophobic interactions in micelles.27,28 The sketch for OTEMPO interacting with the micellar aggregate is shown in Figure 4. OTEMPO is on the micellar aggregate surface, so the polymersurfactant interaction cannot influence the rotation of OTEMPO obviously, although the interaction has a strong effect on the packing situation of micellar aggregate. Additionally, the strong hydrophilicity of OTEMPO results are that it likes to reside in the water phase; thus above the cmc or cac of SDS, the increase magnitude of τc for OTEMPO is smaller than that of TEMPO during the same concentration period of SDS. However, the amount of OTEMPO adsorbed on the micellar surface increases with the micelle concentration, which makes the τc increase gradually with the micellar concentration of SDS. Owing to the two sites of OTEMPO interacting with the micelles or micellar aggregates, the rotation of OTEMPO is much slower at the higher SDS concentration than TEMPO. These results of OTEMPO could help us to understand the behavior of TEMPO. TEMPO and OTEMPO are both located in the micelle-water and micelle-polymer interface,27,28 but their behaviors in τc are very different; therefore the hydrophilicities of the nitroxide group and the CdO group are of importance in interpreting the microenvironmental effects experienced by TEMPO and OTEMPO in micellar systems. The ESR results are also supported by the study of transmission electron spectroscopy, which is concentrated on the aggregated morphologies of polymers with and without SDS after drying. The aggregation process occurs along with the evaporation of water. Figure 5 shows representative images of the polymers. It could be noted that the unmodified PAM, PAM-C10-2%, and PAM-C12(28) Pyter, R. A.; Ramachandran, C.; Mukerjee, P. J. Phys. chem. 1982, 86, 3206.
Wang et al.
2% without SDS are all the spheres in the micrographs, and the size increases with the increase of the polymer hydrophobicity. The hydrophobic moieties that are along the polymer backbone and the hydrophobic side chains tend to associate with one another. These spherical aggregates are formed by intramolecular associations of the hydrophobic moieties to minimize their contact with water. The stronger the hydrophobicity, the larger the spherical aggregates. Upon addition of SDS, the morphologies of the polymers are all changed. For unmodified PAM, no clear morphology could be seen, because the aggregate of PAM is dispersed by SDS micelles. For PAM-C10-2% and PAMC12-2%, fine structures are observed. The two polymers present branched wormlike morphologies. It has been known from ESR results that surfactant micelles are tightly wrapped by the polymer chains, which leads to the conclusion that the polymer sites along their chains are completely occupied by the SDS micelles and the micellar surfaces have the same charges; hence, the chains of the polymers stretch out to be less tightly aggregate, even to be single strands. Addition of SDS or not is obviously different in the aggregated morphologies of polymers during drying. Conclusions The interactions of SDS with HMPAMs and their parent polymer PAM have been investigated. The results indicate that the difference in the strength of interaction in surfactant-polymer systems is often more obviously seen in the microenvironmental properties of micellar binding to the polymers and is easily seen in the morphological transitions of the polymers. On addition of SDS to the polymers, the interaction between SDS and the polymers results in micellar aggregates that are more tightly compacted than the unperturbed micelle, where the chains of the polymers spread out. The hydrophobic side chains of the HM polymers act as nucleation sites onto which SDS molecules aggregate, so the packing situation of micellar aggregate is strongly affected by the hydrophobicity of the polymer. Higher hydrophobicity causes the more compact packing of the micellar aggregate. Acknowledgment. This work is supported by State Science and Technology Commission of China and the National Natural Science Foundation of China under Grant 29573144 and by the National Sciences and Engineering Research Council of Canada. LA9708905