Variegated Micelle Surfaces - American Chemical Society

Langmuir 2004, 20, 7313-7322

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Variegated Micelle Surfaces: Correlating the Microstructure of Mixed Surfactant Micelles with Bulk Solution Properties P. C. Griffiths,*,† A. Y. F. Cheung,† C. Farley,† I. A. Fallis,† A. M. Howe,‡ A. R. Pitt,‡ R. K. Heenan,§ S. M. King,§ and I. Grillo| School of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF10 3TB, United Kingdom, Kodak, Limited, Research & Development, Headstone Drive, Harrow, Middlesex HA1 4TY, United Kingdom, ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom, and Institut Laue Langevin, 6 rue Jules Horowitz, Grenoble, Cedex 9, France

Downloaded by CENTRAL MICHIGAN UNIV on September 10, 2015 | http://pubs.acs.org Publication Date (Web): July 17, 2004 | doi: 10.1021/la0493013

Received March 17, 2004. In Final Form: June 10, 2004 Electron paramagnetic resonance, viscosity, and small-angle neutron scattering (SANS) measurements have been used to study the interaction of mixed anionic/nonionic surfactant micelles with the polyampholytic protein gelatin. Sodium dodecyl sulfate (SDS) and the nonionic surfactant dodecylmalono-bis-Nmethylglucamide (C12BNMG) were chosen as “interacting” and “noninteracting” surfactants, respectively; SDS micelles bind strongly to gelatin but C12BNMG micelles do not. Further, the two surfactants interact synergistically in the absence of the gelatin. The effects of total surfactant concentration and surfactant mole fraction have been investigated. Previous work (Griffiths et al. Langmuir 2000, 16 (26), 9983-9990) has shown that above a critical solution mole fraction, mixed micelles bind to gelatin. This critical mole fraction corresponds to a micelle surface that has no displaceable water (Griffiths et al. J. Phys. Chem. B 2001, 105 (31), 7465). On binding of the mixed micelle, the bulk solution viscosity increases, with the viscosity-surfactant concentration behavior being strongly dependent on the solution surfactant mole fraction. The viscosity at a stoichiometry of approximately one micelle per gelatin molecule observed in SDS-rich mixtures scales with the surface area of the micelle occupied by the interacting surfactant, SDS. Below the critical solution mole fraction, there is no significant increase in viscosity with increasing surfactant concentration. Further, the SANS behavior of the gelatin/mixed surfactant systems below the critical micelle mole fraction can be described as a simple summation of those arising from the separate gelatin and binary mixed surfactant micelles. By contrast, for systems above the critical micelle mole fraction, the SANS data cannot be described by such a simple approach. No signature from any unperturbed gelatin could be detected in the gelatin/mixed surfactant system. The gelatin scattering is very similar in form to the surfactant scattering, confirming the widely accepted picture that the polymer “wraps” around the micelle surface. The gelatin scattering in the presence of deuterated surfactants is insensitive to the micelle composition provided the composition is above the critical value, suggesting that the viscosity enhancement observed arises from the number and strength of the micelle-polymer contact points rather than the gelatin conformation per se.

Introduction The literature on polymer-(single) surfactant systems is extensive and excellent reviews exist,1-3 but currently there are only a handful of papers describing one-polymertwo-surfactant “ternary” systems, even though these bear a much closer resemblance to real formulations involving polymer-surfactant systems. The aqueous ternary system of one polymer and two surfactants comprising gelatin, sodium dodecyl sulfate (SDS), and the sugar-based surfactant dodecylmalono-bis-N-methylglucamide (C12BNMG, Chart 1) can be described as a “selective” mixture because the anionic surfactant interacts with the polymer and the nonionic surfactant in the respective binary combinations, but there is no interaction between the nonionic surfactant †

Cardiff University. Kodak, Limited, Research & Development. § Rutherford Appleton Laboratory. | Institut Laue Langevin. ‡

(1) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Polymer-Surfactant Systems; Surfactant Science Series; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 2000. (3) Currie, E. P. K.; van der Gucht, J.; Borisov, O. V.; Cohen Stuart, M. A. Pure Appl. Chem. 1999, 71, 1227.

and gelatin,4-10 except perhaps an osmotic interaction at high surfactant concentrations. The interaction between the mixed surfactant micelle and polymer and the properties of the mixture can be “tuned” by the nonionic (noninteracting) surfactant mole fraction. These observations are not unique to the current system, with similar conclusions having been reached for the system comprising poly(vinylpyrollidone) PVP/SDS/hexaethylene glycol monon-dodecyl ether C12E6.11 The concept of the “selectivity” of the surfactant is a convenient distinction in these systems. When the polymer (4) Griffiths, P. C.; Bales, B. L.; Howe, A. M.; Pitt, A. R.; Roe, J. A. J. Phys. Chem. B 2000, 104, 264-270. (5) Griffiths, P. C.; Roe, J. A.; Howe, A. M.; Pitt, A. R.; Bales, B. L. Langmuir 2000, 16, 8248-8254. (6) Griffiths, P. C.; Whatton, M. L.; Kwan, W.; Abbott, R. J.; Pitt, A. R.; Howe, A. M.; King, S. M.; Heenan, R. K. J. Colloid Interface Sci. 1999, 215, 114. (7) Griffiths, P. C.; Stilbs, K.; Paulsen, K.; Howe, A. M.; Pitt, A. R. J. Phys. Chem. B 1997, 101, 915. (8) Griffiths, P. C.; Roe, J. A.; Jenkins, R. L.; Reeve, J.; Cheung, A. Y. F.; Hall, D. G.; Pitt, A. R.; Howe, A. M. Langmuir 2000, 16, 9983. (9) Griffiths, P. C.; Pettersson, E.; Stilbs, P.; Cheung, A. Y. F.; Howe, A. M.; Pitt, A. R. Langmuir 2001, 17, 7178. (10) Griffiths, P. C.; Roe, J. A.; Howe, A. M.; Pitt, A. R. Colloid Polym. Sci., in Press. (11) Li, Y.; Xu, R.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 8677.

10.1021/la0493013 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/17/2004


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Chart 1. C12BNMG

increases and the solubilization level decreases. The importance of steric interactions was also highlighted by Bakski and Kaur16 who reported that the pyridinium headgroups of hexadecylpyridinium bromide interacted more strongly with dextran sulfate and poly(acrylic acid) than with the benzylic headgroup of benzylhexadecyldimethylammonium chloride. An analogy may be drawn with polymers adsorbing to particle surfaces of opposite charge17-19 or heterogeneous surfaces with “patches” on the particle surface that have a higher affinity for the polymer.20 Both approaches lend themselves to computer simulation, and parameters describing the polymer conformation are accessible. For the oppositely charged polymer-particle case, the strongest complexes are observed when the polymer (“polyion”) is flexible and the polymer folds or wraps around the charged surface, the “macroion”. With increasing polyion stiffness, a range of structures are observed for the complex as the polyion lifts off the surface of the macroion, ultimately reaching the point where there is a single contact between the polyion and the macroion surface. Our goal here is to explore the relationship between the macroscopic property of the system (e.g., viscosity) and the microscopic structure of the bound micelle, in particular the structure of the headgroup region. To this end, we have employed electron paramagnetic resonance (EPR) to probe the headgroup region of the micelle into which the polymer binds. The EPR method is based on the linear dependence of the hyperfine coupling constant (the separation of the resonance lines) of nitroxide spin probes for a given temperature on a nonempirical polarity index, defined to be the molar concentration ratio of -OH dipoles in a solvent relative to that in water.21 The nitroxide moiety of doxyl stearic acid methyl ester randomly and rapidly samples all portions of the polar layer surrounding the hydrocarbon core of a micelle and reports an average value of the polarity index. Thus, the polarity index is simply the volume fraction of OH groups within the polar shell of the micelle. Changes in the polarity index due to the binding of gelatin provide an estimate of the amount of gelatin bound to the micelle surface. These approaches rely heavily on the characterization of the mixed SDS/C12BNMG surfactant micelle in the absence and presence of the polymer. Previously, we have employed a range of techniques to this end: SANS6 (size, shape, and composition), electrophoretic NMR (ENMR)9 and pulsed-gradient spin-echo NMR7 (PGSE-NMR; charge, hydrodynamic size, and composition), and dodecyl anion selective electrodes (unimer concentration and, by inference, bound micelle composition).10

binds to both surfactants in their respective binary mixtures, the polymer is nonselective, whereas if the polymer interacts with only one of the two surfactants, the polymer is selective. For example, Zanette and Frescura12 has studied the critical micelle concentrations (cmc’s) in the ternary mixture poly(ethylene oxide) (PEO)/ SDS/sodium dodecanoate, showing that the surfactants exhibited “ideal” mixing in both the absence and presence of the polymer. In all cases, the cmc in the presence of the polymer, cmc(1), are reduced compared with the equivalent cmc that was recorded in the absence of the polymer. For this system, the polymer interacts with both surfactants and, thus, the polymer is “nonselective”. By contrast, WynJones et al.,11 our group,4-8 and Lijekvist and Kronberg13 have focused on selective systems in which the polymer interacts with only one of the two surfactants; Lijekvist and Kronberg considered predominantly cmc data for PVP/ decyl-β-maltoside/dodecylbenzenesulfonate, while WynJones et al.11 undertook a somewhat more detailed study of PVP/SDS/C12E6 using small-angle neutron scattering (SANS), dodecyl anion selective electrodes, and isothermal titration. Their results showed clearly that SDS is systematically removed from the polymer by the addition of the nonionic surfactant. During this process, mixed micelles are observed both in solution and bound to the polymer, a conclusion in good agreement with an earlier NMR study on the SDS/C12BNMG system.14 On the basis of the absolute scattering intensity of their SANS data, Wyn-Jones et al. suggest that around 150 monomer units of PVP are associated with the micelle surface, and this decreases with increasing nonionic surfactant mole fraction. Moore et al.15 have studied the binding of the protein zein to mixed SDS/nonionic C12En surfactant micelles. They show that the solubilization of the zein starts near a surfactant concentration corresponding to the onset of cooperative binding. The nonionic surfactant decreases the ability of the mixed micelle to solubilize the zein, with the effect being more pronounced with an increase in ethylene oxide content, n. They rationalized these observations in terms of steric repulsion between the zein and the surfactant headgroups: as n increases, the repulsion (12) Zanette, D.; Frescura, V. L. A. J. Colloid Interface Sci. 1999, 213, 379. (13) Liljekvist, P.; Kronberg, B. J. Colloid Interface Sci. 2000, 222, 165. (14) Griffiths, P. C.; Abbott, R. J.; Stilbs, P.; Howe, A. M. J. Chem. Soc., Chem. Commun. 1998, 1, 53. (15) Moore, P. N.; Puvvada, S.; Blankschtein, D. Langmuir 2003, 19, 1009.

Experimental Section Various experimental methods have been employed in this work; here we present techniques that probe both microscopic and macroscopic behavior, namely, EPR to detect the micelle surface polarity and SANS to probe the microstructure and viscosity as a measure of the macroscopic behavior. Viscosity Measurements. Viscosity measurements were performed using a range of calibrated Ostwald capillary viscometers suspended in a temperature-regulated water bath, at 45 °C. (16) Bakski, M. S.; Kaur, I. Colloids Surf. 2003, 224, 185. (17) Jonsson, M.; Linse, P. J. Chem. Phys. 2001, 115, 3406. (18) Jonsson, M.; Linse, P. J. Chem. Phys. 2001, 115, 10975. (19) Linse, P.; Akinchina, A. J. Phys. Chem. 2003, 107, 8011. (20) van der Linden, C. C.; van Lent, B.; Leermakers, F. A. M.; Fleer, G. J. Macromolecules 1994, 27, 1915. (21) Mukerjee, P.; Ramachandran, C.; Pryter, R. A. J. Phys. Chem. 1982, 86, 3189.


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SANS Studies. SANS measurements were performed on the fixed-geometry, time-of-flight LOQ diffractometer at the ISIS Spallation Neutron Source (Oxfordshire) and on the steady-state reactor source D11 diffractometer at the ILL (Grenoble). Neutron wavelengths of 2-10 Å (LOQ) and 8 Å (D11) were employed to span Q ranges of approximately 0.001-0.25 Å -1. On D11, this required three separate instrument configurations (sampledetector distances and collimation). On both instruments, the samples were contained in 2-mmpath-length, UV-spectrophotometer grade quartz cuvettes (Hellma) and mounted in aluminum holders on top of an enclosed, computer-controlled sample changer. Sample volumes were approximately 0.4 cm3. Temperature control was achieved through the use of a thermostated circulating bath pumping fluid through the base of the sample changer. Under these conditions, a temperature stability of (0.5 °C can be achieved. All measurements were carried out at 40 °C, although temperature has a very small effect on scattering from these samples. Experimental measuring times were between 40 and 80 min. Depending on which instrument was used, all scattering data were (a) normalized for the sample transmission and incident wavelength distribution, (b) background corrected using an empty quartz cell (this also removes the inherent instrumental background arising from vacuum windows, etc.), and (c) corrected for the linearity and efficiency of the detector response using the instrument-specific software package. The data were put onto an absolute scale by reference to the scattering from a wellcharacterized partially deuterated polystyrene-blend standard sample (LOQ) or flat scatterer such as water (D11). EPR. Experimental details for the EPR measurements are also identical to those described previously, and, therefore, only essential details are repeated here. Stock solutions of the two surfactant solutions with spin-probe/surfactant molar ratios of 1:400 were mixed to yield solutions with SDS micelle mole fractions ranging from xSDS ) 0 to xSDS ) 1. Parallel series were prepared with 5 wt % gelatin. These nondegassed samples were sealed with a gas-oxygen torch into melting point capillaries, which were housed within a quartz EPR tube for the measurements. Five spectra were measured at X-band frequency with a Bruker ESP-300 spectrometer. EPR Line Shape Fitting and Analysis. The line shapes were fitted to the Voigt approximation to separate the Gaussian and Lorentzian components of the spectral lines and to locate the resonance fields of the three EPR lines to a precision of a few milligauss. The separation A+ of the low and center lines (MI ) +1 and MI ) 0) is directly related to the polarity index H(25 °C), defined as the molar ratio of OH groups in a given volume relative to water:

A+ ) 14.309 + 1.419H(25 °C)

(1)

For simple SDS solutions, H(25 °C) corresponds to the volume fraction of water in the polar shell. However, some of the 10 sugar -OH moieties per nonionic surfactant can also contribute to H(25 °C). For the SDS/C12BNMG pair, we found that 7.5 -OH groups contributed to H(25 °C), independent of the SDS mole fraction xSDS. Materials. The gelatin used here (Kodak European Research, molecular weight 150 000 g/mol) is a polydisperse lime-processed ossein photographic grade gelatin with an isoelectric point of approximately 4.9. This sample has been decalcified and had its pH adjusted to 5.7 with sodium hydroxide, NaOH. SDS (98% purity, Aldrich Chemical Co., Ltd.), was purified by repeated recrystallization from absolute alcohol until no dip could be seen in the surface tension behavior around the cmc (surface tension/ log concentration data). The deuterated SDS (d-SDS; Aldrich) was used as received. Previous work has shown that the C12BNMG contains some impurities, namely, small amounts of alcohol (unreacted starting material) but about 5 wt % of the single-headed analogue derived from decarboxylated starting material. It was not viable to purify (by HPLC) the large quantity of surfactant required for the viscosity experiments, so these studies used the “as-received” surfactant. A purified sample was used for the SANS, EPR, and surface tension work. However, subsequent surface tension

measurements with the pure sample showed that surfactant purification has very little effect on the qualitative behavior. There are some differences in cmc values and the composition under which the interaction with gelatin first starts, but, nonetheless, the overall behavior is close to that observed with the nonpurified C12BNMG material. A deuterated version of the sugar surfactant, d23-C12BNMG, was prepared in four steps from d23-lauric acid (Aldrich). In a small-sized Schlenk tube, d23-lauric acid (996 mg, 4.4 mmol) was dissolved in dry, freshly distilled tetrahydrofuran (THF; 20 mL) under a nitrogen atmosphere. The solution was cooled to 0 °C with an ice-salt bath. A total of 1.5 equiv (∼7 mL) of a 1.0 M borane solution in THF was added over 5 min, and the resulting solution was allowed to come to room temperature and was stirred overnight. The reaction was quenched by the cautious addition of water followed by 10 mL of a 1 M NaOH solution. The THF was removed on a rotary evaporator, 50 mL of water was added, and resulting mixture extracted with dichloromethane (3 × 20 mL). The combined organic layers were dried with MgSO4 and filtered, and the solvent was removed on a rotary evaporator to afford the crude product. Bulb-to-bulb (Ku¨gelrohr) distillation afforded d23-dodecanol [HOCH2(CD2)10CD3] as a colorless oil. The yield was 878 mg (94%). d23-Dodecanol (812 mg, 3.9 mmol) was dissolved in 5 mL of hydrobromic acid (30 wt %) in glacial acetic acid and was stirred overnight. The reaction mixture was diluted with 50 mL of chloroform, and the resulting solution was washed with distilled water (5 × 20 mL). The organic layer was dried with MgSO4 and filtered through a pad of silica, and the solvent was removed using a rotary evaporator to afford the crude product. Bulb-to-bulb (Ku¨gelrohr) distillation afforded d23-dodecyl bromide [BrCH2(CD2)10CD3] as a colorless oil. The yield was 887 mg (81%). To an excess of dimethyl malonate (1.32 g, 10 mmol) was added d23-dodecyl bromide (605 mg, 2.2 mmol) and potassium carbonate (0.69 g, 5 mmol) dissolved in freshly distilled acetonitrile (10 mL). The reaction mixture was heated (80 °C) and stirred under a nitrogen atmosphere for 12 h. The reaction mixture was cooled, diluted with dichloromethane (30 mL), filtered through a fluted filter paper, and washed with 0.5 M HCl (20 mL), saturated NaHCO3 (20 mL), and water (20 mL). Removal of the solvent on a rotary evaporator affords the crude product as a yellow oil. The excess dimethyl malonate was removed by bulb-to-bulb distillation. The residual oil was diluted with ether and filtered through a 2 cm × 2 cm silica column using ether (50 mL) as an eluent. Removal of the solvent afforded dimethyl-(d23-dodecyl)-malonate [MeO2C)2CH(CH2(CD2)10CD3] as a very pale yellow oil. The yield was 556 mg (77%). Dimethyl(d23-dodecyl)-malonate (446 mg, 1.38 mmol) and N-methyl glucamine (1.95 g, 10 mmol) were dissolved in 4 mL of dry, freshly distilled dimethylformamide and heated at 120 °C under a nitrogen atmosphere for 24 h. The solvent was removed by bulbto-bulb distillation, and the residue was heated under a vacuum at 100 °C for a further 4 h. The crude surfactant was prepurified on silica with MeOH as an eluent and subsequently by reverse phase HPLC using an Agilent 1100 series chromatography platform on a 25 × 2.1 cm LC-C18 Supercosil column with MeOH/ H2O (4:1 at 8 mL/min) as an eluent with peak detection by fixedwavelength UV at 224 nm. Removal of solvent from four chromatography runs afforded the pure surfactant as a white amorphous powder. The yield was 356 mg (61%).

Results When two surfactants are present in solution, convention and convenience dictate that the system is described in terms of the ratio of the two surfactants on a molar basis and the total surfactant concentration. The concentration ratio of the two surfactants may be expressed in terms of the total surfactant present in solution, the solution mole fraction, RSDS, or in terms of the micellized surfactant only, the micelle mole fraction, xSDS. When the non-micellized concentration of each surfactant is low or mutually comparable, then RSDS ≈ xSDS. For surfactants with two quite different cmc’s (and, therefore, unimer concentrations) and at low total surfactant concentrations, RSDS and xSDS can be quite different, as is the case here.


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Figure 1. Surface tension derived cmc’s and EPR based polarity (A+) measurements for binary mixtures of SDS and C12BNMG, in the presence (trianglessopen, cmc; filled, EPR) and absence (circlessopen, cmc; filled, EPR) of 0.25 wt % gelatin (cmc) or 5 wt % gelatin (EPR).

This must be borne in mind wherever comparisons are made between data recorded from various techniques at different concentrations. At higher surfactant concentrations, when the nonaggregated component constitutes a small amount of the total surfactant present, RSDS ≈ xSDS. For the system comprising SDS and C12BNMG, which have a constant aggregation number for SDS micelle mole fractions xSDS > 0.3, we have previously reported4 that replacing each SDS molecule with a molecule of the more bulky-headed nonionic C12BNMG molecule expels about 18-20 molecules of water, equivalent to that amount associated with .≈2 SDS molecules. Therefore, each of the nonionic molecules renders a volume of polar shell inaccessible to water. Accordingly, the surface of these mixed micelles reaches the lower number of associated water molecules somewhere around a micelle mole fraction xSDS .≈ 0.3. Over the region xSDS < 0.3, there is a slight decrease in aggregation number Figure 1 shows the correlation between two quite different experimental studies: the changes in the cmc (we use cmc to denote the concentration of surfactant at the onset of micellization when gelatin is absent; cmc(1) is used when the micelles are bound to gelatin) determined by surface tension and the polarity experienced by the spin probe located within the headgroup region of the micelle surface, for the mixed micelles alone and the mixed micelles in the presence of gelatin. The EPR study was performed with a 5 wt % gelatin concentration and at 25 mM total surfactant concentration, whereas the surface tension study was undertaken with a gelatin concentration of 0.25 wt % and an inherently varying surfactant concentration. Nonetheless, all the data have been recast in terms of the micelle mole fraction, xSDS, on the basis of the previous SANS,6 PGSE-NMR,7 and electrode analyses.8 The gelatin concentration has no effect on the cmc(1), as expected as the gelatin merely “nucleates” the formation of micelles. Both experimental approaches show that below the critical micelle composition,8 xSDScrit ) 0.3, neither the cmc nor the polarity change in the presence

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of gelatin. This composition we have previously interpreted as an indication of the onset of interaction.8 For xSDS > 0.3, there are decreases in both the cmc and polarity when gelatin is present; the differences between the respective pair of cmc’s (with, without polymer) and polarities increase with increasing xSDS. The peak in the cmc behavior around xSDS ) 0.975 and the discontinuity in the polarity over the region are due to the presence of two micellar environments.10 The polarity increase over the region 0 < xSDS < 0.3 is due to a slight decrease in aggregation number that increases the micelle curvature. Clearly, there is a simple correlation between the surface character of the micelle and the binding of gelatin to it. This may be expressed as an amount of water within the micellar shell populated by the surfactant headgroups region that is displaced by the polymer. Alternatively, it may also be thought of in terms of a decreasing sodium counterion binding concomitant with the expulsion of the water, at least as xSDS decreases to xSDS ) 0.3. Below this value, our ENMR studies have shown that the counterion binding increases dramatically as the displaceable water associated with the headgroups decreases to zero.9 The micelle surface may be thought of as “variegated” because the fractional area of the micelle surface occupied by the sulfate headgroups will bind gelatin whereas that fraction occupied by sugar headgroups will not. It must be remembered that that the gelatin is binding to the area of micellar core that is exposed to the continuous phase rather than directly to the sulfate groups. This area, which may be equated as the difference between the crosssectional area of the headgroup and the area of the micelle surface it occupies, is significant for the sulfate group because of the electrostatic repulsion between them, but minimal for the nonionic headgroup. It is also important to remember that the dynamics of the micelle means that any specific arrangement of polymer and surfactant headgroups will not persist for any appreciable time and that an average degree of “variegation” will result that can be quantified in terms of the micelle composition. This is, in essence, an extension of ideas discussed by (a) Nagarajan22 and Ruckenstein et al.23 who theorized polymer-surfactant complexation in terms of an area (apol) of the micelle surface that becomes “shielded” by the polymer upon complexation and (b) the similarity between the polymer binding to a (dynamic) micelle surface and polymers adsorbing to “patchy”17-20 (static) particle surfaces. The “variegation” analogy illustrates how the binding of the polymer will decrease the free energy of formation of the micellar core-water interface, but it also increases the free energy of the system because of the steric repulsions between the polymer segments and the surfactant headgroups at the micelle surface. Compared to anionic surfactants, cationic and nonionic surfactants tend to have larger headgroups, which allows less penetration of water into the region next to the hydrophobic tails, and the potential benefit of shielding by the polymer is much smaller. Furthermore, binding of the polymer would induce much more steric repulsion at the micelle surface. This approach led to the simple prediction, verified by limited data on SDS/Triton X-100/PEO mixtures that bound mixed micelles will form at high SDS mole fractions (shielding), but no binding will occur at low SDS mole fractions (steric repulsion).22,23 The EPR technique reports the amount of water associated with the surfactant headgroups in the polar (22) Nagarajan, R. Colloids Surf. 1985, 13, 1. (23) Ruckenstein, E.; Huber, G.; Hoffmann, J. Langmuir 1987, 3, 382.



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Figure 2. EPR derived adsorption isotherm for the adsorption of gelatin to the variegated SDS/C12BNMG micelle surface.

shell of the micelle. On addition of gelatin to two otherwise identical mixed micelles, any reduction in polarity, expressed in terms of the water volume fraction φH2O, corresponds to the amount of water that is displaced by gelatin. For simple SDS, φH2O ) 0.72 whereas on addition of gelatin φH2O ) 0.54. Therefore, the difference in the degree of hydration is the amount of gelatin bound to the micelle surface, expressed in terms of the micelle polar shell volume (Figure 2). For the simple SDS case just polarshell presented, this corresponds to φgelatin ) 0.18. The form of Figure 2 is analogous to a conventional Langmuir adsorption isotherm. Similarly, for xSDS) 0.3, φH2O) 0.35 and we see again the clear effect of a critical threshold value below which no binding occurs. The amount of gelatin within the polar shell of the micelle is quite significant over the SDS-rich part of the composition range; for xSDS ) 0.7, φH2O ) 0.42 and φgelatin ) 0.1. These estimates are comparable to the ∼150 monomer units of PVP per SDS micelle proposed by Wyn-Jones et al.30 The question then arises as to how this binding effects the bulk properties of the solution such as the viscosity of a gelatin solution? Adding gelatin to solutions of anionic surfactants gives rise to large increases in viscosity,24,25 which depends on gelatin molecular weight, ionic strength, and surfactant character. The onset of thickening occurs at cmc(1), a concentration which decreases with increasing alkyl chain length. For alkyl chain lengths greater than 10 carbons, there is a maximum in the viscosity at a surfactant concentration corresponding to a stoichiometry of one micelle per gelatin chain. This bound micelle acts as a transient cross-link between adjacent gelatin strands. (24) Greener, J.; Contestable, B. A.; Bale, M. D. Macromolecules 1987, 20, 2490. (25) Griffiths, P. C.; Roe, J. A.; Abbott, R. J.; Howe, A. M. Imaging Sci. J. 1997, 45, 224. (26) Griffiths, P. C.; Stilbs, P.; Howe, A. M.; Whitesides, T. H. Langmuir 1996, 12, 5302. (27) Cosgrove, T.; White, S. J.; Zarbakhsh, A.; Heenan, R. K.; Howe, A. M. Langmuir 1995, 11, 744. (28) Cosgrove, T.; White, S. J.; Zarbakhsh, A.; Heenan, R. K.; Howe, A. M. J. Chem. Soc., Faraday Trans. 1996, 92, 595. (29) Griffiths, P. C.; Teerapornchaisit, P.; Fallis, I. A.; Grillo, I. Langmuir 2001, 17, 2594 (30) Li, Y; Xu, R.; Couderc, S.; Bloor, D. M.; Warr, J.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2001, 17, 5657.

Figure 3. (a) Relative viscosity of 5 wt% aqueous standard gelatin solutions as a function of total surfactant concentration critical at low SDS mole fractions; RSDS/RSDS ) 0 (open squares), critical critical ) 0.7 (filled squares), RSDS/RSDS RSDS ) 1.2 (open circles), critical and RSDS/RSDS ) 2.0 (filled circles). (b) Relative viscosity of 5 wt % aqueous standard gelatin solutions as a function of total surfactant concentration at high SDS mole fractions; RSDS/ critical critical RSDS ) 4 (open circles), RSDS/RSDS ) 3.6 (open triangles), critical critical RSDS/RSDS ) 3.3 (filled squares), RSDS/RSDS ) 2.7 (open critical squares), and RSDS/RSDS ) 2.0 (filled circles).

Similar viscosity maxima are also observed for PVP or PEO with SDS, but have a different origin.5 The magnitude of the viscosity increase on addition of surfactant to a gelatin solution is reduced when the noninteracting C12BNMG surfactant is also present, Figure 3a,b. These data are presented in terms of the SDS solution mole fraction, RSDS, (as opposed to the SDS micelle mole fraction xSDS) normalized to the critical critical for the impure surfactant solution mole fraction RSDS critical such that for RSDS/RSDS < 1 no change in the cmc is observed when gelatin is added; that is, no interaction critical for the between the micelle and the gelatin occurs (RSDS pure surfactant is 0.80 while the equivalent quantity for the impure surfactant is ∼0.6). This ratio removes the


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relative effects of the sugar surfactant impurity on the cmc and cmc(1) and facilitates the comparison with the purified sample used for the EPR and SANS data. The viscosity data are rather complex, showing several distinct features dependent on the total surfactant concentration and the surfactant mole fraction, (Figure critical ) 0 and 0.7 (Figure 3a), there is a 3a,b). For RSDS/RSDS negligible increase in viscosity and, by inference, no change in polymer conformation for surfactant concentrations below RSDS concentration in a C12BNMG/SDS/gelatin RSDS system would show some difference from the simple C12BNMG/SDS systems around the binary SDS/gelatin cmc(1), that is, 0.6-1.0 mM; no such difference was observed in the SDS unimer concentration extracted from our electrode study.8 In our opinion, it is more probable that small changes in ionic strength and pH give rise to subtle changes in the gelatin conformation, and it is the consequence of these changes that we detect in the viscosity. It is clear, however, that nonadsorbed micelles do not significantly alter the viscosity of the solution and, hence, the polymer conformation. Concomitantly, it also shows that the nonionic micelle, regardless of surfactant purity, does not bind to the gelatin. critical ) 1.2 data set is fundamentally difThe RSDS/RSDS ferent, showing a weak increase in viscosity around cmc(1) and a subsequent increase at 5 mM total surfactant, the same surfactant concentration as observed in the RSDS/ critical RSDS ) 0 and 0.7 data sets. However, the viscosity critical ) 2 system is very different. behavior for the RSDS/RSDS Here, the viscosity increases markedly around cmc(1), rising to a plateau around 10 mM surfactant before increasing once more to a plateau around 50 mM. At higher total surfactant concentrations (∼50 mM), the magnitudes of the viscosities are greater and correlate more clearly with the surfactant composition; that is, for total surfactant concentrations comparable to >50 cmc(1), the viscosities increase with SDS mole fraction. Clear maxima are observed for the four highest SDS mole fractions around the stiochiometry of one micelle per gelatin strand (Figure 3b, 60 mM total surfactant). This is in agreement with the previous work of Greener et al. 24 on a range of gelatin/anionic surfactant systems who showed that the adsorbed micelle acts as a transient crosslink between the gelatin chains and that this cross-linking is most efficient at a stiochiometry of one micelle per gelatin chain. The viscosity at this same stoichiometry for the gelatin-mixed micelle system shows a simple dependence on the amount of micelle surface area occupied by the sulfate groups, whether the micelle size is varied via alkyl chain length26 or composition in the case of the

Griffiths et al.

Figure 4. Simple relationship between the composition and size of a bound micelle expressed in terms of the micelle surface area and the bulk viscosity of the solution; open symbols correspond to the alkyl sulfates (ref 26), and the closed symbols correspond to the mixed micelles data presented here.

mixed dodecyl tailed surfactants (Figure 4). At surfactant concentations greater than 150 mM, the viscosity of gelatin/SDS solutions increases once more, associated with the sphere-to-rod transition of the (nonbound) SDS micelles present in solution. There is no reason to assume that the mixed systems here would not show a similar behavior, but due to the limited availability of the sugar surfactant, this region has not been explored. Accordingly, it is not possible to state whether the limiting viscosity critical in the RSDS/RSDS ) 2 case is a “plateau” or a “peak”. SANS is a more direct method for probing the conformation of the gelatin or the size of the micellar species. In a SANS experiment, it is possible to highlight the scattering from a particular component using selective deuteration because deuterated materials show little scattering in a deuterated solvent, as illustrated in Table 1. In this work, because only protonated gelatin is available, the number of contrasts possible is fewer than ideal. We have, therefore, focused on three: (1) h-gelatin/ h-SDS/h-C12BNMG/pure D2O; (2) h-gelatin/d-SDS/d23-C12BNMG/D2O, and (3) h-gelatin/h-SDS/h-C12BNMG/(50% H2O/50% D2O), that is, conditions of contrast match for the gelatin. Therefore, case 1 is scattering from all the components, while case 2 arises predominantly because of gelatin only and case 3 because of the surfactants only. Previous studies27,28 of gelatin/surfactant solutions using deuterated SDS have shown that when a SDS micelle binds to gelatin, the gelatin collapses onto the micelle surface. Accordingly, a significant amount of the scattering from the gelatin resembles that of the micellar component, and there is little scattering consistent with a structurally unperturbed gelatin. Similarly, in our previous SANS studies of hydrophobically modified gelatin with SDS,29 we employed two contrasts, h-gelatin/h-SDS/D2O and h-gelatin/d-SDS/D2O. The gelatin scattering could be described by a two-length scale model in the absence of the surfactant, but on addition of the surfactant, this approach fails because of the strong peak in the data due to the surfactant-induced change in the gelatin conforma-


Langmuir, Vol. 20, No. 17, 2004 7319


Table 1. Approximate Relative Scattering Contributions for the Three Contrast Variations Studied Here

contrast term

study 1 (h-gelatin/h-SDS/ h-C12BNMG/pure D2O)

study 2 (h-gelatin/d-SDS/ d23-C12BNMG/D2O)

study 3 [h-gelatin/h-SDS/ h-C12BNMG/(50% H2O/50% D2O)]

(Fsurfactants,head - Fsolvent)2 (Fsurfactants,core - Fsolvent)2 (Fpolymer - Fsolvent)2

∼1-2.5; depends on RSDS ∼46 ∼12

∼1-2.5; depends on RSDS ∼0.3 ∼12

∼0 ∼11 ∼0

tion described by the structure factor S(Q). However, we were able to extract the surfactant micelle morphology by fitting the h-gelatin/h-SDS/D2O data set using the corresponding h-gelatin/d-SDS/D2O data set as a “background”. It was found that the bound micelles were of comparable size to nonbound micelles, in agreement with our fluorescence studies.25 More recently, we have fitted the neutron scattering data from gelatin/fluorosurfactant mixtures to a form factor comprising a solid ellipse (surfactant) plus a Lorentzian correlation length (gelatin). Again, there was no difference in size between the bound and nonbound micelle. We adopt an analogous approach here, similar to that used by Wyn-Jones et al.,11,30 in which the total scattering is written as the sum of that arising from the polymer and the surfactant, with no interference term:

I(Q) ) Ipolymer(Q) + Isurfactant(Q) + Binc

(2)

where the separate I(Q) terms are each the product of two terms, one describing the size and shape of the scattering species called the form factor, P(Q), and one accounting for spatial arrangement of the scatterers in solution (a consequence of the intermicellar interactions) called the structure factor, S(Q). Binc is the incoherent scattering. The structure factor S(Q) is calculated from the rescaled mean spherical approximation approach based on a repulsive Yukawa tail (an exponentially damped electrostatic term) and the hard-core potential.31,32 The model is described via four parameters: a hard sphere volume fraction φhard sphere and particle radius Rhard sphere, the micellar charge, and the inverse Debye screening length. In all cases, when polymer is complexed with the surfactant micelle even at contrast match for the polymer, it is not straightforward to calculate or even estimate what these quantities should be. Indeed, such parameters are not meaningful and accordingly have been treated as fit parameters, but care was taken to ensure that the resultant fits did not involve nonphysical parameters. Above the gel temperature, the gelatin adopts a random coil conformation, although previous work has shown that temperature has a negligible effect on gelatin scattering over the Q range probed here.27,28 For solutions above the critical overlap concentration, we invoke a simple Lorentzian function to describe the scattering from the localized structure of the gelatin network:

Iξ(Q ) 0) Ipolymer(Q) ) (Fpolymer - Fsolvent)2 [1 + Q2ξ2]

(3)

where ξ is the network correlation length. Previously, we and others have used a two-correlation length approach to fit gelatin scattering data but this approach requires data at substantially lower Q values than we accessed in this study. Therefore, the approach embodied in eq 3 is perfectly adequate. Pezron et al.33 found that the persistence length was around 20 Å for dilute gelatin solutions, (31) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 651. (32) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (33) Pezron, I.; Djabourov, M.; Leblond, J. Polymer 1991, 32, 3200.

an estimate in good agreement with the later work of Cosgrove et al.27,28 (ξ ) 50 Å) and our studies on modified gelatin (ξ ) 36 Å), at higher gelatin concentrations. The surfactant scattering is modeled using the widely used charged core-shell model:

I(Q)surfactant ) nm[S(Q)〈F(Q)〉2 + 〈|F(Q)|〉2 - 〈F(Q)〉2] + Binc (4) The bracketed term, [S(Q)〈F(Q)〉2 + 〈|F(Q)|2〉 - 〈F(Q)〉2], calculated from the decoupling approximation,34 describes the morphology of the scattering species, where F(Q) ) V1(F1 - F2)F0(QR1) + V2(F2 - Fsolvent)F0(QR2). For an elliptical micelle, both F(Q) and F(Q)2 require numerical integration over an angle γ between Q and the axis of the ellipsoid to account for the random distribution of orientations of the ellipse. This has been omitted for clarity. The first term represents the scattering from the core (subscript 1) of radius Ri and axial ratio X, and the second, the polar shell (subscript 2). Vi ) 4/3πXRi3, and F0(QRi) ) 3ji(QRi)/(QRi) (ji is the first-order spherical Bessel function of the first kind). The results of two separate experiments are presented. One set of data was recorded at ISIS using only protonated surfactants and polymers but two solvent compositions. The second set was recorded at the ILL where deuterated surfactants were used. In each experiment, two total surfactant concentrations have been studied for a series of mole fractions. The scattering behavior for a series of different SDS mole fractions for 5 wt % gelatin at the total surfactant concentration of 10 mM is shown in Figure 5. The 10 mM total surfactant corresponds to a concentration well above the onset of the interaction but below that required to produce on average one micelle per strand, which would correspond to ∼50 mM surfactant. In all these data, there is significantly more scattering than the gelatin-only case, indicating that the scattering is dominated by the surfactant term.11 The RSDS ) 1.0 and RSDS ) 0.1 systems have very different shapes; there is a peak around Q ) 0.04 Å in the RSDS ) 1.0 data because of the surfactantinduced S(Q). The RSDS ) 0.1 data set merely exhibits a plateau in scattering over this Q range. The intermediate mole fractions exhibit a peak with increasing intensity RSDS associated with the increasing ionic character of the bound micelle. For those systems where there is no binding of gelatin to the micelle, the scattering should simply correspond to be the summation of two terms, one arising from the gelatin, the other from the surfactants. To fit these data, we use the measured scattering from a 5 wt % gelatin solution as a “background” to account for the unperturbed gelatin. The scalar p describes “how much” unperturbed gelatin is required:

I(Q) ) pIpolymer(Q) + Isurfactant(Q) + Binc

(5)

A core-shell morphology for the micelle has been adopted which has successfully treated the binary surfactant (34) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022.


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Griffiths et al.

Figure 5. SANS data for 5 wt % gelatin in the presence of 10 mM h-SDS and h-C12BNMG in D2O. Data were recorded on the LOQ diffractometer. RSDS ) 1 (filled circles), 0.8 (open circles), 0.5 (filled squares), 0.2 (open squares), and 0.1 (open triangles). The open diamonds correspond to the scattering from 5 wt % gelatin in the absence of any surfactant, ξ ) 26 Å.

Figure 6. SANS data for 5 wt % gelatin in the presence of 60 mM h-SDS and h-C12BNMG in D2O. Data recorded on the LOQ diffractometer. RSDS ) 1 (filled circles), 0.8 (open circles), 0.5 (filled squares), 0.2 (open squares), and 0.1 (open triangles). The open diamonds correspond to the scattering from 5 wt % gelatin in the absence of any surfactant, ξ ) 26 Å.

scattering; the core radius of the mixed micelle is set to be equivalent to the all-trans length of a dodecyl chain (16.7 Å), and the shell thickness is fixed at 5 Å. The structure factor has been included as per the HayterPenfold model for charged spheres, discussed earlier. The critical solid lines through the data points for RSDS < RSDS systems correspond to this simple model with p ≈ 1.0 ((0.1), that is, scattering is simply the sum of the independent micelle and polymer scattering. Thus, we can indeed conclude that there is no binding of these micelles to the gelatin, in good agreement with both the viscosity and cmc analyses. The slight increase in peak intensity in the RSDS ) 0.2 system compared with the RSDS ) 0.1 case arises as a result of the presence of a weak peak from the micelle structure factor because the micelles contain a greater component of the anionic surfactant. critical (∼0.6) systems The scattering from the RSDS > RSDS critical systems, are quite different from that of the RSDS < RSDS showing much more pronounced maxima. Attempts to fit critical data to the additive polymer plus the RSDS > RSDS surfactant model just described (eq 5) were not successful. Given that there appears to be little unperturbed gelatin scattering present in these data, the scattering has been treated in terms of the simple core-shell morphology, allowing the ellipticity of the scatterer and its size to vary, that is, much larger and less spherical scatterers are permitted. The Hayter-Penfold model is used to treat the structure factor, although as stated previously the parameters must be treated as largely empirical. A scaled 5 wt % gelatin solution “background” was initially included, but it was soon apparent that there was probably very little of such residual scattering. Similarly, introducing a simple Lorentzian or a two-correlation length term did not improve the “fits”. The lines through the data points are the fits, and because may be seen, they are very good. The parameters describing these fits were very similar to those describing the mixed micelle in terms of the micelle size (Rcore ) 16.7 Å; shell thickness 5 Å), but the ellipticity

X was much greater; X ≈ 4.5 for RSDS ) 1.0, X ≈ 7.5 for RSDS ) 0.8, and X ≈ 7.6 for RSDS ) 0.5. The shell scattering length density has been estimated given the composition of the bound micelle and the amount of water present, as quantified from the EPR experiment. Similar results were obtained for fluorosurfactant/gelatin mixtures.35 It is unlikely that the micelles are this elliptical because this would require that the aggregation number increased dramatically. Rather, we conclude that part of the gelatin molecule collapses onto that proportion of the micelle surface occupied by SDS but is repelled by that part occupied by the C12BNMG, and it is this shape that the SANS experiment perceives. Indeed, the ellipsoid model used here is only an approximation to the gelatin-decorated micelle; gelatin/micelle cross terms, implicitly ignored in our fitting, will add longer distances to the pair correlation function at small Q and gelatin self-terms at high Q, and both will make the particle look more extended. Figure 6 shows analogous data to that presented in Figure 5, but at a higher surfactant concentration, 60 critical and RSDS mM. The distinction between the RSDS > RSDS critical < RSDS is far clearer - the pronounced peaks in the interacting system, that are absent in the noninteracting system. Again the scattering is dominated by surfactanttype scattering, suggesting significant perturbation to the polymer conformation. The solid lines again refer to fits critical ) as to a particle scattering -only model (for RSDS > RSDS detailed previously but with smaller ellipticities and a simple sum of polymer plus surfactant scattering for RSDS critical < RSDS . While this approach offers a valuable insight into these systems, it is more useful to use neutron contrast variation to separate the scattering from the surfactant and the polymer. Such an approach is detailed in Figures 7a,b and 8; SANS has been recorded from gelatin solutions in (35) Griffiths, P. C.; Cheung, A. F. Y.; Jenkins, R.; Pitt, A. R.; Howe, A. M.; Heenan, R. K.; King, S. M. Langmuir 2004, 20, 1161.



Langmuir, Vol. 20, No. 17, 2004 7321

Figure 8. SANS data for 5 wt % gelatin in the presence of 60 mM h-SDS and h-C12BNMG under conditions of contrast match for the gelatin, 50% H2O. Data recorded on the LOQ diffractometer. RSDS ) 1 (filled circles), 0.8 (open circles), 0.5 (filled squares), 0.2 (open squares), and 0 (open triangles).

Figure 7. (a) SANS data for 2 wt % gelatin in the presence of 1.5 mM d-SDS and d-C12BNMG in D2O. Data recorded on the ILL diffractometer. RSDS ) 1 (open squares), 0.95 (open circles), 0.90 (hexagons), 0.80 (open triangles, up), and 0.50 (open triangles, down). The filled symbols correspond to a 5 wt % gelatin solution corrected for the difference in concentration. The solid line is a fit to a hollow core-shell ellipse particle form factor, with Rcore ∼ 17((1) Å and X ∼ 5, with a weak Lorentzian term. The inset shows a “zoom” of the data around the peak to emphasize the very subtle changes in the various data sets. (b) SANS data for 2 wt % gelatin in the presence of 15 mM d-SDS and d-C12BNMG in D2O. Data recorded on the ILL diffractometer. RSDS ) 0.95 (open circles), 0.75 (open squares), 0.45 (open triangles), and 0.25 (open diamonds). The RSDS ) 1 has been omitted for clarity. The insert shows a “zoom” of the data around the peak to emphasize the very subtle changes in the various data sets.

D2O in the presence of 1.5 mM deuterated SDS and deuterated C12BNMG and from gelatin solutions in the

presence of 60 mM surfactants under conditions of contrast match for the gelatin (50% D2O/50% H2O). The gelatin-only is scattering illustrated in Figure 7a,b for two total surfactant concentrations, 1.5 and 15 mM, for a series of mole fractions. Unfortunately, the paucity of the deuterated sugar necessitated the lower gelatin concentration. Using deuterated surfactants and D2O in this manner means that the recorded scattering arises predominantly from the gelatin, both unperturbed and bound to the micelle, plus a very small contribution from the corona of surfactant headgroups. In effect, the polymer-micelle complex will appear as a (hollow) shell of scattering plus some unperturbed gelatin. Also shown are the scattering from a 5 wt % gelatin solution scaled linearly to account for the difference in concentration. Clearly, the gelatin/d-surfactant scattering has a character much more reminiscent of surfactant-like scattering (only critical < RSDS have been recorded here). The systems with RSDS fit to the RSDS ) 1 data invokes a model based on a hollow core-shell (core contrast matched) with Rcore ≈ 17 Å ((1) and X ≈ 5 using a shell scattering length density estimated from the EPR-derived composition, plus a very small contribution from a Lorentzian term (eq 3). This fit provides a useful insight, is in excellent agreement with the data presented in Figures 5 and 6, and is commensurate with the schematic picture of the gelatin/mixed micelle complex: a surfactant core with a block of gelatin adsorbed to the micelle surface. Of course, the distribution of the gelatin segments normal to the micelle surface is likely to be exponential or Gaussian in form, rather than a crude block. The deuterated surfactant/gelatin SANS data are replotted (on a linear scale) as insets to Figure 7a,b to emphasize the changes in the peak intensity in the data. The peak intensities fall into two bands; the first encompasses the RSDS ) 1.0, 0.95, and 0.90 series, with the RSDS ) 0.90 being the most intense, while the second contains the RSDS ) 0.80 and 0.50 series. This ostensibly peculiar order arises as a result of a complex interplay


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between the amount of bound surfactant [these systems are very close to cmc(1) so any subtle changes in cmc(1) are magnified for total surfactant concentrations this close to cmc(1)] and the degree of counterion binding. The degrees of counterion binding, which are comparable in the first series, are quite different between the two series and indeed quite different within the second series, assuming that the mixed micelle/gelatin system has a behavior similar to the mixed micelle behavior.9 The RSDS ) 0.95 data set seems most “out of order”, but there is evidence of two micelle types at this stoichiometry.10 Analogous behavior is observed much more clearly for the equivalent data sets recorded at the higher surfactant concentration (Figure 7b), that is, 15 mM total surfactant concentration, where subtle changes in the cmc(1) are now irrelevant. The peak intensity increases with decreasing SDS micelle mole fraction, that is, less ionic surfactant in the micelle, as a result of an increasing degree of counterion dissociation. While fitting the gelatin plus surfactant scattering can yield information regarding the micelle morphology, a more appropriate approach is to consider the case where the gelatin is contrast matched, tha tis, 50%:50% H2O/ D2O (Figure 8). Under these conditions, the scattering arises solely from the protonated surfactants. The solid lines through the data sets are fits to the core-shell model, with Rcore ∼ 16.7 Å, a shell thickness of ∼5 Å, and an ellipticity much smaller than that suggested earlier, X ∼ 1.2 ( 0.2, when the gelatin also contributes to the scattering. Indeed, this micelle morphology is comparable to the nonbound micelle morphology. The only difference between the data sets is the effective ionic character of the micelle shown predominantly by the peak generated by the structure factor S(Q), which increases with increasing SDS mole fraction. These results support the generally accepted notion that the bound micelles have effectively the same size and shape as nonbound micelles, at an equivalent composition. There is, however, a second peak in the data around Q ∼ 0.1 Å arising because of the structure peak. This indicates that the local concentration of the micelles, their effective charge, or the Debye length is far greater than that calculated from the known composition of the system. This further illustrates the need to treat these parameters as semiempirical. That said, the effective volume fraction of the micelles used to fit the data in Figure 8 is roughly φhard sphere ≈ 0.15, with particle radius Rhard sphere ≈ 30 Å.

Griffiths et al.

Conclusions SANS, EPR, and viscosity have been used to probe the solution structure of the polymer-surfactant complex comprising gelatin/SDS/C12BNMG. A direct measure of the polymer conformation and size/shape of the surfactant aggregate is accessible through SANS, and this has been correlated to the local structure and composition of the polar shell of the micelle. It is shown that by varying the nature of the headgroups in the polar region of a surfactant micelle the interaction with a polymer such as gelatin may be altered in a controlled manner. Two extremes are shown: one where there is a noninteracting surfactant micelle and, therefore, no interaction with gelatin and the other extreme, where the interaction is present. The two extremes are delineated by a critical micellar mole fraction for the SDS. Above this critical micelle composition, mixed micelles bind to gelatin causing a change in polymer conformation and an increase in viscosity. The magnitude of the increase in viscosity is both surfactant-concentration- and composition-dependent. At a stoichiometry of one micelle per gelatin strand, the viscosity increase scales linearly with the amount of micelle surface area available onto which the polymer may bind. The EPR results show that the amount of gelatin associated with the micelle also depends linearly on the micelle composition. Interestingly, the gelatin conformation is not that sensitive to the micelle character; two quite different but mutually comparable behaviors are observed delineated by the critical mole fraction. Below this value, there is no change in gelatin conformation while above this mole fraction, the gelatin “wraps” around the surfactant micelle. For the latter, the gelatin conformation does not vary significantly with composition. The amount of gelatin at the micelle surface is dependent on the micelle composition. When the gelatin binds to the micelle surface, the polymer conformation is determined by localized lateral interactions between the cationic gelatin residues and the anionic headgroups plus some hydrophobic residue/ micellar core contribution. At low surfactant concentrations, the cationic-anionic interaction dominates, with the interaction attaining a more hydrophobic character with increasing surfactant concentration. It would appear that the electrostatic interactions dominate the thermodynamics that govern the polymer conformation. LA0493013

Variegated Micelle Surfaces - American Chemical Society

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