Interaction between Gelatin and Sodium Dodecyl Sulfate at the Air

Physical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, U.K.. A. M. Howe and E. A. Simister. Kodak European Research, Headstone Drive, Harro...
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Langmuir 2000, 16, 6546-6554

Interaction between Gelatin and Sodium Dodecyl Sulfate at the Air/Water Interface: A Neutron Reflection Study D. J. Cooke, C. C. Dong, and R. K. Thomas* Physical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, U.K.

A. M. Howe and E. A. Simister Kodak European Research, Headstone Drive, Harrow, Middlesex, HA1 4TY

J. Penfold ISIS, CLRC, Chilton, Didcot, Oxon, OX11 0QX Received January 31, 2000. In Final Form: May 3, 2000 Neutron reflection has been used to study the composition and structure of layers adsorbed at the air/water surface of solutions of gelatin and sodium dodecyl sulfate (SDS) and these results have been compared with the surface tension of the same solutions. Above a concentration where free micelles of SDS can be expected to form in the bulk solution the layer is exactly as would be expected for solutions of SDS on its own. However, at low SDS concentrations the presence of gelatin greatly enhances the adsorption of SDS in comparison with solutions just containing surfactant, and in the intermediate range of SDS concentration, where the surface tension is relatively constant, the surface excess of SDS is also constant at gelatin concentrations of 0.1 wt %. The thickness of the surfactant layer in the two lower ranges of SDS concentration is much larger than a simple surfactant layer, ranging from 35 down to 22 Å (in comparison with 19 Å for the pure surfactant layer), suggesting that the layer is not only roughened by binding of gelatin at the surface but that a proportion of the bound SDS molecules are completely immersed just below the surface. This is confirmed by measurements of the layer structure at different isotopic compositions. The presence of gelatin at the surface and the enhancement of the adsorption of SDS indicate that complexes of gelatin and SDS are strongly surface active. Furthermore, the measured thickness of the SDS layer at the surface shows that these complexes probably do not contain surfactant in the form of micelles. This further suggests that it may not be reliable to interpret the first discontinuity in the surface tensionlog(concentration) plot in such strongly interacting systems as the point at which there is an onset in aggregation of the surfactant on the polyelectrolyte (critical aggregation concentration, or CAC).

Introduction Gelatin has a wide range of commercial applications because of its ability to act as an emulsifier, stabilizer, and film forming binder. In several of these applications surfactant is added in order to provide emulsification capacity or to control interfacial tension or because the interaction between surfactant and gelatin is important in determining the properties of the system. Gelatin interacts strongly with anionic surfactants, although not as strongly as many proteins. The surface tension plots for gelatin/anionic surfactant mixtures1-3 show features similar to those found for more weakly interacting systems such as poly(ethylene oxide) (PEO)/sodium dodecyl sulfate (SDS) but, because the interactions are so much stronger there are reasons for believing that the underlying behavior for gelatin and for other polyelectrolyte/surfactant systems may be different from that of uncharged polymers and surfactants.4 In particular, the adsorption behavior of simple polyelectrolyte/surfactant mixtures at the air/liquid interface indicates the formation of highly * Please address all communications to Dr. R. K. Thomas, Physical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, U.K. (1) Knox, W. J.; O’Parshall, T. J. Colloid Interface Sci. 1970, 33, 16. (2) Knox, W. J.; O’Parshall, T. J. Colloid Interface Sci. 1972, 40, 290. (3) Muller, D.; Malmsten, M.; Bergenstahl, B.; Hessing, J.; Olijve, J.; Mori, F. Langmuir 1998, 14, 3107. (4) Goddard, E. D. Colloid Surf. 1986, 19, 301.

surface active macromolecular/surfactant complexes.5-11 Until recently most experimental investigations into the adsorption and aggregation behavior of this type of system have focused on the aggregation in the bulk solution (see, for example, the extensive work by Dubin et al.12-14 or on adsorption at the solid/liquid interface.3,15-17 The reason is that there have been few techniques capable of determining the surface composition of mixed layers at the air/water interface and it is difficult to interpret the relatively few surface tension data unambiguously. Neu(5) Bergeron, V.; Langevin, D.; Asnacios, A. Langmuir 1996, 12, 1550. (6) Regismond, S. T. A.; Winnik, F. M.; Goddard, E. D. Colloid. Surf. 1996, 119, 221. (7) Merta, J.; Stenius, P. Colloid. Surf. 1997, 122, 243. (8) Zhang, J. Y.; Zhang, L. P.; Tang, J. A.; Jiang, L. Colloid. Surf. 1994, 88, 33. (9) Merta, J.; Stenius, P. Colloid Polym. Sci. 1995, 273, 974. (10) Buckingham, J. H.; Lucassen, J.; Hollway, F. J. Colloid Interface Sci. 1978, 67, 423. (11) Asnacios, A.; Langevin, D.; Argillier, J. F. Macromolecules 1996, 29, 7412. (12) Li, Y. J.; Dubin, P. L.; Havel, H. A.; Edwards, S. L.; Dautzenberg, H. Langmuir 1995, 11, 2486. (13) Li, Y. J.; Dubin, P. L.; Dautzenberg, H.; Luck, U.; Hartmann, J.; Tuzar, Z. Macromolecules 1995, 28, 6795. (14) Li, Y. J.; Dubin, P. L.; Havel, H. A.; Edwards, S. L.; Dautzenberg, H. Macromolecules 1995, 28, 3098. (15) Sidora, M.; Golub, T.; Musabekov, K. Adv. Colloid Interface Sci. 1993, 43, 1. (16) van de Steeg, H. G. M.; Cohen Stuart, M. A.; de Keizer, A.; Bijterbosch, B. H. Langmuir 1992, 8, 2538. (17) Meadows, J.; Williams, A.; Garvey, M. J.; Harrop, R.; Phillips, G. O. J. Colloid Interface Sci. 1989, 132, 319.

10.1021/la0001171 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/07/2000

Gelatin-SDS Interactions at AW Interface

tron reflection is capable of giving structural information about a mixed layer simultaneously with information about its composition18 and we have already applied the technique to the PEO/SDS, PEO/MDS, and poly(vinylpyrrolidone) (PVP)/SDS systems.19-23 Penfold et al. have applied the technique to mixtures of more than one surfactant with polyelectrolytes24-26 and Lee et al.27,28 have applied it to poly(isopropylacrylamide)/SDS mixtures at the air/water interface. Rennie et al. have used neutron reflection to examine SDS/gelatin mixtures at the polystyrene/water interface,29,30 Lu et al. have used it to examine the interaction of SDS and the globular protein bovine serum albumin at the silica/water interface,31,32 and Lee et al. have used it to explore lipase-surfactant interactions at the air/water interface.33 Since gelatin lacks the complications of secondary and tertiary structure, existing as a random coil above 37 °C, it is also a useful model for surfactant/protein interactions. Experimental Details Protonated sodium dodecyl sulfate (hSDS) (Polysciences Inc.) was purified by recrystallization from ethanol. Deuterated SDS (dSDS) was prepared from deuterated dodecanol and purified as described elsewhere.34 The purity of the SDS was assessed as satisfactory by the absence of a minimum in the plot of surface tension against the log of concentration. This is generally accepted as a criterion of purity, although it may not eliminate some contamination by divalent ions, which can cause discrepancies between the neutron reflectivity results and those from the Gibbs equation.35-37 Three samples of gelatin were used with the following specifications: (i) Drygel 867 (abbreviated to Drygel): isoelectric point ) 5.0, Ca2+ < 50 ppm, Na+ < 3000 ppm. (ii) Kodak Pathe DI (abbreviated to KP): 11.2% water, isoelectric point ) 5.0, Ca2+ + Mg2+ ) 40 ppm, Na+ < 250 ppm. (iii) Fractionated gelatin (alpha enriched, abbreviated to Alpha): isoelectric point ) 5.0, exact ion concentrations not

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Figure 1. Surface tension of aqueous solutions of SDS on its own (O) and with 0.1 wt % (×) and 1 wt % (b) Drygel gelatin at 35 °C. The points marked with arrows are the values of T1 and the critical micelle concentrations. available but similar to KP, Mn )76 100, Mw/Mn ) 1.33. The Alpha gelatin was given to us by T. H. Whitesides. It was prepared by fractional precipitation from water with methanol and NaNO3 and its distribution of chain lengths is distinctly monomodal, consisting mainly of alpha chain.38 All solutions were prepared from a stock solution of gelatin, prepared by gentle stirring and heating to 44 °C (hydrolysis occurs above 50 °C). The solutions were equilibrated for 2 h before use at their natural pH. The neutron reflectivity measurements were made on the reflectometer SURF39 at ISIS, England. The instrument was calibrated using the reflectivity profile of pure D2O, and a flat background determined at high momentum transfer was subtracted before processing the data. The solutions were contained in Teflon troughs mounted in a sealed thermostated container. Surface tension measurements were done on a Kruss K10 maximum pull tensiometer using a platinum-iridium ring as described previously.34 All the measurements were made at a temperature of 35 °C.

Results (18) Lu, J. R.; Penfold, J.; Thomas, R. K. Adv. Colloid Interface Sci. (in press). (19) Lu, J. R.; Blondel, J. A. K.; Cooke, D. J.; Thomas, R. K.; Penfold, J. Prog. Colloid Polym. Sci. 1996, 100, 311. (20) Cooke, D. J.; Dong, C. C.; Lu, J. R.; Thomas, R. K.; Simister, E. A.; Penfold, J. J. Phys. Chem. B 1998, 102, 4912. (21) Cooke, D. J.; Lu, J. R.; Thomas, R. K.; Wang, J. B.; Han, B. X.; Yan, H. K.; Penfold, J. Langmuir 1998, 14, 1990. (22) Purcell, I. P.; Thomas, R. K.; Howe, A. M.; Blake, T. D.; Penfold, J. Colloid Surf. A 1995, 94, 125. (23) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637. (24) Penfold, J.; Staples, E. J.; Tucker, I.; Creeth, A.; Hines, J. D.; Thompson, L.; Cummins, P. G.; Thomas, R. K.; Warren, N. Colloid Surf. A 1997, 128, 107. (25) Creeth, A.; Cummins, P. G.; Staples, E. J.; Thompson, L.; Tucker, I.; Penfold, J.; Thomas, R. K.; Warren, N. Faraday Discuss. 1996, 104, 245. (26) Staples, E. J.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. J. Phys. Chem. B 1999, 103, 5204. (27) Jean, B.; Lee, L. T.; Cabane, B. Langmuir 1999, 15, 7585. (28) Lee, L. T. Curr. Opin. Colloid Interface Sci. 1999, 4, 205. (29) Turner, F.; Rennie, A. R.; Thomas, R. K.; Thirtle, P. N. Imaging Sci. J. 1997, 45, 270. (30) Turner, F.; Clarke, S. M.; Rennie, A. R.; Thirtle, P. N.; Li, Z. X.; Thomas, R. K.; Langridge, S.; Penfold, J. Prog. Colloid Polym. Sci. 1999, 112, 206. (31) Lu, J. R.; Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir 1999, 14, 6261. (32) Lu, J. R.; Su, T. J.; Thomas, R. K. J. Phys. Chem. B 1998, 102, 10 307. (33) Lee, L. T.; Jha, B. K.; Malmsten, M.; Holmberg, K. J. Phys. Chem. B 1999, 103, 7489. (34) Lu, J. R.; Marrocco, A.; Su, T. J.; Thomas, R. K.; Penfold, J. J. Colloid Interface Sci. 1993, 158, 303. (35) An, S. W.; Lu, J. R.; Thomas, R. K.; Penfold, J. Langmuir 1996, 12, 446. (36) Cross, A. W.; Jayson, J. J. J. Colloid Interface Sci. 1994, 162, 45. (37) Hines, J. D. J. Colloid Interface Sci. 1996, 180, 488.

Surface Tension. Surface tension measurements were made for SDS mixtures with all three gelatin samples at a concentration of 0.1 wt % and at 35 °C. Measurements were also made at a gelatin concentration of 1 wt % for the Drygel gelatin. It is always difficult to be certain that a true equilibrium surface tension has been measured in polymer/surfactant systems because the measurement itself is a dynamic one and it is made at a finite time after the creation of the surface. We adopted the standard of waiting for 20 min before making the measurements, which was sufficient to establish some kind of steady state, if not equilibrium. Figure 1 shows the results for Drygel at two different polymer concentrations and Figure 2 compares the results for the three gelatin samples at a fixed polymer concentration of 0.1 wt %. In earlier measurements the low concentration onset of the plateau was found to be at 1 mM SDS for 0.5 wt % gelatin at a pH of 4.1.1,2 Our measurements were done at the natural pH (see the next paragraph) and we also obtain the value of 1 mM at 0.1 wt % Drygel gelatin, but the value is much lower when the Drygel gelatin concentration is 1 wt % and it is somewhat lower for the two other gelatin samples. The curves for Drygel have the general features of two discontinuties separated by a plateau region, as exhibited by most surfactant/polymer mixtures, such as the SDS/ (38) Whitesides, T. H.; Miller, D. D. Langmuir 1994, 10, 2899. (39) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899.

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Figure 3. Variation of solution pH of 0.1 wt % Alpha gelatin as a function of SDS concentration at 35 °C. Figure 2. Surface tension of aqueous solutions of SDS on its own (O) and with 0.1 wt % Drygel (×), Kodak Pathe DI (b), and Alpha (+) gelatins at 35 °C.

PEO system.40 The discontinuities are commonly attributed to a critical aggregation concentration (CAC) and a critical micelle concentration (cmc), the former being the surfactant concentration at which aggregates of surfactant start to form on the polymer and the latter being the concentration at which free surfactant micelles form. The neutron reflection results presented below will show that the identification of the first break point as a CAC is not necessarily justified in strongly interacting polymer/surfactant systems and we will therefore designate this point as T1.40 The curves for the other two gelatin samples are different from that for Drygel in that there is no plateau; instead, the surface tension increases steadily from about 43 mM m-1 at the first break point up to a maximum of about 47 mM m-1 before decreasing again to the second break point. In the higher range of SDS concentration the curves for the three gelatin samples are identical within experimental error. We defer the discussion of the γ - ln[S] plots until after the presentation of the neutron reflection data. However, here we note that some of the observed differences between the three gelatins are expected. The natural pH of the KP and Alpha gelatins is closer to their isoelectric point (5.0) than for Drygel. In the preparation of Drygel sodium hydroxide is added to the gelatin after removal of the Ca2+ ions which leads to a natural pH of the gelatin solution of about 5.7 whereas the natural pH of the other two gelatin samples is 5.0, just above the isoelectric point. Binding of surfactant to the lower charged species should initially enhance the surface activity more than for the higher charged species. Thus the binding of a small amount of SDS should make the KP and R samples much more surface active than the Drygel sample, as observed (Figure 2). As further SDS is complexed the increase in hydrophobicity of the polyelectrolyte-surfactant complex is offset by its increasing negative charge. This arises both from neutralization of positive charge by the negative surfactant ion and because the adsorption of the negative surfactant ion may shift some of the dissociation equilibria of acid residues on the polymer

almost identical when fully complexed with surfactant, exactly as observed. That there is such an effect of pH is shown by the effect of added SDS on the pH of the gelatin solution shown in Figure 3 and it agrees with the effects of surfactant on pH obtained in earlier work by Klotz41 and by Knox and Parshall.2 Similar surface tension deviations have been observed by Merta et al. for cationic starch/anionic surfactant mixtures9 and by Muller et al. for sodium dodecyl benzene sulfonate/gelatin mixtures.3 Note that the two minima for KP and Alpha gelatin cannot result from impurities in the SDS because no minimum is observed either for SDS on its own or for the SDS/ Drygel system. Neutron Reflection and Surface Coverage. For the determination of the adsorbed amount of any species at the air/aqueous solution interface the ideal experiment is to measure the reflectivity from the surface of mixtures with each species in turn in its deuterated form in null reflecting water (NRW). NRW consists of a mixture of D2O in H2O in the molar ratio 0.088:1 and has a neutron refractive index identical to that of air. The neutrons sense no refractive index boundary at the surface of pure NRW and are therefore not reflected. The presence of a layer at the surface whose refractive index is different from that of NRW will give rise to a reflected signal. In general, the protonated version of a surfactant or polymer also has a scattering length close to zero and therefore the neutron reflection experiment gives the coverage only of the deuterated species in the mixture. Gelatin, however, presents some difficulties for such experiments because it cannot be fully deuterated, it will exchange protons with a partially deuterated solvent, and its composition in NRW is such that it does not have a zero scattering length. Thus, it may not be possible to eliminate its contribution entirely. Initially, we measured the reflectivity from solutions of protonated SDS (hSDS) and gelatin in NRW to determine just how much signal resulted from adsorbed gelatin. The reflectivities of such solutions were found to be negligible at high SDS concentrations and relatively small at low SDS concentrations and the approximate values of the thickness, τ, and scattering length density, F, of the gelatin layer are given in Table 1. The scattering length density is defined by

+ RCO2H h RCO2 + H

F ) Σ bini

to the left. This should cause an increase in the pH and a decrease in the activity of the polymer/surfactant complex. As the system moves further away from the isoelectric point the electrostatic charge is better able to balance the hydrophobicity and the surface activities of the three gelatin samples should then converge, becoming

where ni is the number density of nucleus i and bi is its scattering length. The sum is over all the constituent nuclei. The combination of the scattering length density and thickness of the gelatin layer proved to be sufficiently small that the SDS coverage could be determined reasonably accurately from the reflectivities of dSDS solutions

(40) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36.

(41) Klotz, I. M. Proteins 1954, B2, 740.

(1)

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Figure 4. Neutron reflectivity profiles of dSDS in a solution of 0.1 wt % Alpha gelatin in null reflecting water. The SDS concentrations are 30.0 (O), 9.0 (+), 6.4 (b), 0.8 (×), and 0.1 (4) mM. The continuous lines are the best fits for a single uniform layer model using the parameters in Table 2.

Figure 6. Neutron reflectivity profiles of dSDS in a solution of 0.1 wt % Drygel gelatin in null reflecting water. The SDS concentrations are 12.0 (O), 6.4 (+), 0.8 (b), 0.3 (×) and 0.1 (4) mM. The continuous lines are the best fits for a single uniform layer model using the parameters in Table 4. Table 2. SDS Adsorption Isotherms with 0.1 wt % Alpha Gelatin Obtained from Neutron Reflection Data at 308 K cSDS/mM 106 F/Å-2 τ ( 2/Å A(mol.) ( 10%/Å2 1010 Γ/mol cm-2 0.1 0.3 0.8 1.6 3.2 4.8 6.4 9.0 12.0 30.0

1.4 1.9 2.1 2.1 2.2 2.4 2.8 3.4 3.6 3.7

32 30 28 28 27 24 21 19 19 19

61 48 47 47 47 48 48 43 40 40

2.7 3.4 3.5 3.5 3.5 3.5 3.5 3.9 4.2 4.3

Table 3. SDS Adsorption Isotherms with 1 wt % Alpha Gelatin Obtained from Neutron Reflection Data at 308 K

Figure 5. Neutron reflectivity profiles of dSDS in a solution of 1.0 wt % Alpha gelatin in null reflecting water. The SDS concentrations are 32.0 (O), 15.0 (+), 9.0 (b), 3.2 (×), and 0.1 (4) mM. The continuous lines are the best fits for a single uniform layer model using the parameters in Table 3. Table 1. Single Layer Fits to Neutron Reflectivity from hSDS/Gelatin in NRW at 308 K gelatin

cSDS/mM

106 F/Å-2

τ ( 2/Å

alpha alpha alpha alpha drygel 867

0.17 1.8 5.6 9.0 3.2

0.6 0.6 0.5 background 0.6

50 50 35 s 50

in gelatin in NRW by ignoring the gelatin contribution to the reflectivity altogether (the reflectivity varies as the square of the product of thickness and scattering length density). The reflectivities obtained for different dSDS concentrations in 0.1 and 1.0 wt % Alpha gelatin and in 0.1 wt % Drygel in NRW at 35 °C are shown in Figures 4, 5, and 6. Measurements were also made at temperatures of 37 and 42 °C to check for any temperature dependence of the adsorption around 37 °C where the gelatin chains are reported to form random coils,42-44 but no changes were observed. This agrees with results by Cosgrove et al. who used small angle neutron scattering to examine possible changes in the bulk solution.43,44 The reflectivities were (42) Djabourov, M. Contemp. Phys. 1988, 29, 273. (43) Cosgrove, T.; White, S. J.; Zarbakhsh, A.; Heenan, R. K.; Howe, A. M. J. Chem. Soc., Faraday Trans. 1996, 92, 595. (44) Cosgrove, T.; White, S. J.; Zarbakhsh, A.; Heenan, R. K.; Howe, A. M. Langmuir 1995, 11, 744.

cSDS/mM 106 F/Å-2 τ ( 2/Å A(mol.) ( 10%/Å2 1010 Γ/mol cm-2 0.05 0.1 0.18 0.3 0.8 1.6 3.2 4.8 6.8 9.0 15.0 32.4 40 45

1.4 1.4 1.7 1.7 1.8 1.9 1.9 2.1 2.1 2.3 2.7 3.4 3.7 3.7

34 34 34 34 34 33 33 31 31 28 25 20 19 19

59 59 47 46 45 44 44 42 42 43 41 40 40 40

2.8 2.8 3.5 3.6 3.7 3.8 3.8 3.9 3.9 3.9 4.0 4.1 4.2 4.2

fitted using a single uniform layer model and the optical matrix method to calculate the reflectivity, as described in previous papers.20,23 It has been shown elsewhere that this procedure gives a coverage that is independent of the model,45 although the thicknesses obtained are not independent of the model. Nevertheless, the relative values of the thickness are determined accurately. The results, in terms of the thicknesses and coverages, are given in Tables 2, 3, and 4. The variations of the surface excess of SDS with bulk concentration of SDS at the two concentrations of R gelatin are compared in Figure 7. The first important observation is that the presence of gelatin greatly enhances the amount of adsorbed SDS at the lower end of the concentration range. Thus, at 0.1 mM there is extensive adsorption of SDS when gelatin is present whereas there is an almost undetectable level of adsorption for SDS on its own.23 For (45) Lu, J. R.; Lee, E. M.; Thomas, R. K. Acta Crystallogr. 1996, A52, 11.

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Figure 7. The surface excess of SDS as determined from neutron reflection data in the absence of gelatin (×) and in the presence of 0.1 wt % (b) and 1 wt % (O) Alpha gelatin. The continuous lines are to guide the eye. The surface excesses of SDS at log10 [SDS] ) -3 and -4 are respectively 7 × 10-11 and less than 1 × 10-11 (ref 23).

Figure 8. The surface excesses of SDS (b) and surface tensions (O) for solutions of SDS in (a) 0.1 wt % Alpha and (b) 0.1 wt % Drygel gelatin. Table 4. SDS Adsorption Isotherms with 0.1 wt % Drygel 867 Obtained from Neutron Reflection Data at 308 K cSDS/mM 106 F/Å-2 τ ( 2/Å A(mol.) ( 10%/Å2 1010 Γ/mol cm-2 0.1 0.3 0.8 1.6 6.4 12.0

1.2 1.4 1.9 1.9 2.5 3.6

39 36 28 28 26 19

57 54 51 51 54 57

2.9 3.0 3.2 3.2 4.0 4.1

the 0.1 wt % sample there are two discontinuities in the surface excess which correspond with the values of T1 and cmc in the surface tension curve (Figure 8) and the first of these discontinuities is better defined from the neutron data. The surface coverage is also constant over the region from T1 to [SDS] = 12 mM. At the higher concentration of Alpha gelatin there is a break in the surface excess corresponding to T1 in the surface tension curve but at higher SDS concentrations the surface excess just increases monotonically. For the Drygel the SDS surface excess was not measured over such a fine grid of concentrations but the pattern of discontinuities is again consistent with the breaks in the surface tension curve.

The better definition of T1 in the neutron reflection isotherms for Alpha gelatin shows clearly that it occurs at a lower SDS concentration than the corresponding break in the Drygel isotherm. There is also a generally higher level of adsorbed SDS for the Alpha relative to the Drygel system which is consistent with the generally lower surface tension in the plateau region. The variation in the thickness of the layer follows a different pattern to the surface excess (Tables 2, 3, and 4). At large concentrations of SDS the thickness (and composition) of the layer of 19 ( 2 Å is exactly the same as would be found for the surfactant on its own.34 At concentrations of SDS below T1 the surfactant forms a much thicker layer indicating that it is not only roughened by the presence of the strongly interacting gelatin but that some surfactant molecules may actually be fully immersed in the aqueous subphase. We emphasize here that this is a measurement of the SDS distribution, not the gelatin, so that it is a thick SDS layer that is being observed, much thicker than the fully extended length of an SDS molecule. The only interaction that could lead to the magnitude of the thickness observed is some sort of binding of SDS on the gelatin strands. The magnitude of this effect decreases gradually as the SDS concentration is increased and continues to do so across the plateau even though the overall composition of the layer is constant. The value of the thickness is not independent of the model used to fit the data, but the relative values for a given choice of model are accurate to about (10%, which is smaller than the variations described above. The reflection measurements described so far only give information about the thickness of the SDS layer, although at low SDS concentrations the effect of the gelatin at the surface can clearly be seen. The power of the neutron reflection method is that further information, particularly as regards the location of the solvent in the surface layer, can be obtained by making measurements at different isotopic compositions. For this type of system, the effect of the adsorbed layer on the water distribution normal to the interface can be ascertained by measuring the reflectivity from hSDS and protonated polymer in D2O, when the two solutes will be approximately invisible. The relative position of SDS and water would normally be obtained by combining the measurements using dSDS in NRW and hSDS in D2O with that from dSDS in D2O. The situation with gelatin is complicated by its H/D exchange with the solvent and it therefore has a variable scattering length so, for example, it will make more of a contribution to the hSDS/D2O profile than to the dSDS/NRW profile. Also, the ideal measurement of the gelatin structure at the air/water interface would require fully deuterated gelatin, which is not available. To try to remove any ambiguity from the structure determination we measured more than the minimum number of reflectivities outlined above, using the set hSDS/D2O, hSDS/water3.5, dSDS/D2O, dSDS, NRW, and dSDS/water3.5, where water3.5 refers to an isotopic composition of H2O and D2O such that the scattering length density is 3.5 × 10-6 Å-2, which is about halfway between those of H2O and D2O. Measurements were made using 0.1 wt % R gelatin at two concentrations of SDS, 15 and 1.18 mM, chosen to be just above the cmc and on the plateau region, and at 1.0 wt % Alpha gelatin and 1.18 mM SDS. Rather than estimate the amount of H/D exchange in the gelatin using the known formula in combination with assumptions about the exchange, the actual scattering length density of the layer was determined by measuring the scattering length density of an adsorbed layer of the gelatin at the surface of water of different isotopic composition. Although this cannot be

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Table 5. Structural Parameters for the Adsorbed Layer at the Surface of 1.18 mM SDS in Alpha Gelatin at 35 °C 0.1 wt % gelatin

1.0 wt % gelatin

parameter

layer 1

layer 2

layer 1

layer 2

thickness/Å 106 FdSDS/D2O/Å-2 106 FdSDS/NRW/Å-2 106 FdSDS/3.5/Å-2 106 FhSDS/D2O/Å-2 106 FhSDS/3.5/Å-2 volume fraction of gelatin volume fraction of water proportion of SDS molecule in layer

9.5 4.1 4.1 4.1 0.0 0.0 0 0 0.66

20 6.1 1.0 3.8 5.1 2.8 0.06 0.80 0.34

11 3.2 3.2 3.2 0.0 0.0 0 0 0.58

27 6.0 1.0 3.7 5.0 2.8 0.09 0.78 0.42

Table 6. Structural Parameters for the Adsorbed Layer at the Surface of 15 mM SDS in 0.1 wt % Alpha Gelatin at 35 °C parameter

layer 1

layer 2

thickness/Å 106 FdSDS/D2O/Å-2 106 FdSDS/NRW/Å-2 106 FdSDS/3.5/Å-2 106 FhSDS/D2O/Å-2 106 FhSDS/3.5/Å-2 volume fraction of gelatin volume fraction of water proportion of SDS molecule in layer

10 5.5 5.5 5.5 0.0 0.0 0 0 0.8

10 6.5 1.4 4.2 5.1 2.8 ∼0 0.8 0.2

used to give an accurate estimate of the extent of H/D exchange, it is sufficiently accurate for the present purposes where the gelatin is not the dominant contributor to the reflectivity. We chose a relatively simple structural model to fit the data because the resolution is not high enough to justify a more elaborate model. The model consists of two layers, the upper one containing SDS, air, and gelatin, but no water, and the lower one containing gelatin, a fraction of the SDS, and water. This was fitted simultaneously to the reflectivity profiles from all five isotopic compositions and an excellent fit could be obtained with the parameters given in Tables 5 and 6 and an example of the quality of the fits to one set of data is shown in Figure 9. At the higher SDS concentration the structure of the adsorbed layer is within error the same as for solutions of SDS on its own at or just above the cmc. This in itself indicates that no gelatin is present in the layer since gelatin would be expected to perturb the structure. However, there is a lower limit below which neutron reflection could not detect gelatin. For this layer structure and the particular isotopic compositions used the presence of gelatin in the lower of the two layers could not be identified with certainty below a volume fraction of about 5%. Since this second layer is thin (10 Å) this means that there is negligible gelatin in the layer, which is in agreement with the observation that the surface tension of solutions of this concentration is not affected by the presence of gelatin. About one fifth of the SDS is immersed in the water layer and this is as expected from the degree of immersion of other surfactants for this surface concentration. At the lower SDS concentration the SDS is clearly dragged down into the thick lower layer by gelatin (Figure 10). If each molecule were merely becoming more extensively immersed the second layer would form an intermediate layer between the top layer and bottom layer, and this intermediate layer would contain only gelatin and water. This would not fit the data as is shown simply by considering the measured thicknesses of the layers from the reflectivity of dSDS in NRW (+gelatin). The mean contribution of the lower 20% of those molecules that form the surface to the fraction of SDS in the lower layer is

Figure 9. Fitted reflectivity profiles for 1.18 mM SDS in 0.1 wt % Alpha gelatin, (a) dSDS in NRW, (b) hSDS in D2O, (c) dSDS in D2O, and (d) hSDS (+) and dSDS (b) in in water3.5. The continuous lines are calculated using the structural data in Table 5.

approximately 0.1 at a gelatin concentration of 0.1 wt % and therefore most of the observed fraction of 0.34 SDS is caused by molecules that are more completely or totally immersed in the water. At the higher gelatin concentration of 1.0 wt % the gelatin layer is thicker and a correspondingly greater fraction of SDS is present in the lower layer. Note the reason that neutron reflection is more sensitive to the presence of gelatin at these lower SDS concentrations is because of the greater thickness of the layer;

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Figure 10. Schematic diagram of the structure of the layer adsorbed at the air/water interface of SDS/gelatin solutions (a) above the cmc of SDS (15 mM SDS) and (b) in the plateau region (1.18 mM SDS). There is no gelatin at the surface in (a) and in (b) the gelatin chain is drawn with side branches to represent the fact that many of the important amino acid residues in the interaction have a significant length (e.g., lysine). No effort has been made to represent the interaction of the charges in the drawing because this would not be warranted by the resolution of the experiment.

reflection responds to the total amount of gelatin in the surface region. Qualitatively, the picture is broadly consistent with those proposed by Goddard for mixtures of polycation with anionic surfactants4 and also by Kamiyama et al. for the adsorption of gelatin on mica.46 Measurements of the thickness of mixed gelatin/sodium alkyl benzene sulfonate on a hydrophobic surface using ellipsometry indicate a much thicker gelatin layer3 than apparently observed here. Although the systems are different, one would not expect that the differences would be quite so large. However, the neutron reflection signal is so dominated by the more concentrated region of the layer that, in cases where the layer becomes more diffuse toward the bulk solution, it is often insensitive to the presence of a diffuse tail. Discussion The following conclusions can be drawn from the neutron reflection results: (i) At low [SDS], adsorption of surfactant only occurs because of a strong gelatin/surfactant interaction at the interface; (ii) At low [SDS], the gelatin/surfactant interaction at the surface has a strong influence on the structure of the layer; causing the surfactant distribution normal to the surface to be much broader than for a simple surfactant solution; (iii) There is significant adsorption of gelatin right up to the cmc; (iv) At 0.1 wt % gelatin, the structure and composition of the mixed surface layer is constant along the plateau of the surface tension; (v) The thickness and coverage of the surfactant layer above the cmc is as expected for the pure surfactant layer. (46) Kamiyama, Y.; Israelichvili, J. Macromolecules 1992, 25, 5081.

Cooke et al.

The result (i) is not surprising. There is evidence that SDS binds to gelatin in the bulk solution in the region below T1 with an approximate stoichiometry of five SDS molecules per strand.38 Provided that the points of surfactant attachment to the gelatin and to the surface do not cause too much loss of configurational entropy this complex should be surface active because of the added hydrophobicity introduced by the presence of the surfactant chains. The adsorption of a surface active complex is also consistent with the large depression of the surface tension observed in this concentration range. Others (e.g., ref 9) have drawn similar conclusions from the surface tension data. However, the further observation (ii) from neutron reflection suggests that there is some competition between the requirements of the large hydrophobicity of the single surfactant/monomer unit and the configurational entropy of the chain. Thus, the relatively large thickness and the position of the layer relative to the aqueous subphase shows that a significant fraction of SDS molecules in the layer are totally immersed in the aqueous subphase below T1. Since this is below the point where it is presumed that SDS forms aggregates on the gelatin, there can be no possibility that this added thickness is caused by the presence of micelles; it is, in any case, significantly less than expected for a micelle. The extra thickness in comparison with a pure surfactant layer therefore results from a roughening of the layer. The obvious explanation is that this results from a balance between configurational entropy and interaction of the individual hydrophobic units with the interface. It has been suggested for the system tetradecyl trimethylammonium bromide/carboxymethylchitin47 that surfactant micelles may form part of the polymer/surfactant complex bound at the interface. This seems unlikely in the present case because above T1 the composite layer is both too thin to be formed from micelles (even when allowance is made for the smaller micelles generally found to be bound to polymers48 and is thinner than the layer below T1, where there can be no micelles. Observation (iv) would also not fit well with an interpretation based on adsorption of micelles because if the T1 is a true CAC, the density of bound micelles should increase with surfactant concentration and this would lead to an increasing surface coverage of SDS, which is not observed. In earlier measurements on PEO/SDS and PVP/SDS the surfactant concentration increased continuously through T1 (the CAC for these systems) with no evidence of any kind of discontinuity. In the plateau region, in terms of the Gibbs isotherm,

dγ ) - ΓSdln[S] - ΓPdln[P] ) 0

(2)

where S and P refer to surfactant monomer and uncomplexed polymer respectively, which requires that both dln[S] ) 0 and ΓPdln[P] = 0. The latter condition is easily fulfilled because ΓP refers to the molar surface excess of polymer, which will always be small, although dln[P] may not be zero across the plateau because of the formation of polymer/surfactant complex. For the PEO/SDS system neutron reflection measurements confirmed that ΓP ) 0 above the CAC. Thus, in the standard situation where any adsorbed polymer is removed from the surface by complexation with surfactant micelles the surface tension measurement effectively monitors the bulk surfactant concentration and hence the phase behavior in the bulk (47) Babak, V. G.; Vikhoreva, G. A.; Lukina, I. G. Colloids Surf. A 1997, 128, 75. (48) Cabane, B. J. Phys. Chem. 1977, 81, 1639.

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solution. Any discontinuities in the γ-lnc curve must then be caused by discontinuities in the concentration of surfactant in the bulk solution. The pattern of two discontinuities in the γ-lnc plot, one at the CAC and one at the cmc, is common and even in systems where surface interactions between polymer and surfactant might be expected, which would make the surface behavior more dominant, such behavior is widely interpreted as indicating the presence of both a CAC and a cmc (e.g., ref 11). In contrast, neutron reflection shows that in the gelatin/ SDS system the surface consists of both polymer and surfactant right up to the cmc. It is interesting to examine the consequences of this observation for the behavior of the surface tension. The strongest commonly encountered interactions between polymers and surfactants are electrostatic interactions between a charged group on the polymer and an oppositely charged surfactant. This arises in polyelectrolyte/surfactant systems where either the polyelectrolyte carries a charge opposite to the surfactant or where the polymer is of mixed charge such as gelatin. In such a system, it is probable that a surface active complex is formed between one polymer molecule and a small number n of surfactant molecules (smaller than necessary to form a micelle) just as has been suggested for gelatin/SDS.38 The factors affecting the formation of such surface active complexes will be (i) the surfactant ion will bind to a polyelectrolyte segment to form a unit much more hydrophobic than the two isolated fragments, (ii) the loss of entropy of binding of the surfactant ion will be compensated by a corresponding gain for the counterion liberated from the polyelectrolyte, and (iii) the loss of conformational freedom of the polymer on binding to the surface will not be large if only a few segments of the polymer are anchored to the surface. The formation of a surface active complex does not exclude the possibility that micelles also form on the polymer giving a complex PSm where m is assumed to be much larger than n. However, because n , m the surface active complex will form below the CAC. Assuming for the moment that only these two complexes exist and that n is well defined then the formation of the surface active complex will be governed by the equilibrium constant

Kn )

[PSn] ([P]0 - [PSn])[S]n

(3)

where [P]0 is the overall polymer concentration. Since n , m and we are presuming a strong interaction to form PSn, PSn will initially be formed under circumstances where there is negligible PSm. Equation 3 can be rearranged to give

Kn[S]n[P]0 [PSn] = 1 + Kn[S]n

(4)

Because [S]n varies very rapidly with [S] the polymer will change more or less completely into the complex over a very short range of addition of surfactant. Denoting this concentration [S]R, [PSn] will be approximately constant between [S]R and the concentration where PSm starts to form (the CAC). If the complex is the dominant adsorbing species at the interface its adsorption will be determined by the Gibbs isotherm

dγ) -ΓPSnRTdln[PSn]

(5)

Since [PSn] is approximately constant between [S]R and

the CAC, dln[PSn] must be vanishingly small in this concentration range. Thus dγ must also be correspondingly small, i.e., there will be an approximate plateau on the γ-ln[S] plot below the CAC, provided that there are no dramatic changes in ΓPSn. In fact, since PSn is presumed to be very surface active, the surface can be expected to be saturated with the complex in this range and ΓPSn will therefore be constant and nonzero. Still sticking to the assumption that the only two types of complex are PSn and PSm, the surfactant added above T1 stays in the form of monomer until it reaches a concentration where it can micellize on the complex PSn to form PSm, i.e., the true CAC. At this point PSm will start to be formed and the concentration of PSn will start to be reduced. Depending on the strength of the surface activity of PSn and assuming that PSm is surface inactive this will eventually cause PSn to start desorbing from the surface. The surface may then respond to further addition of surfactant in three ways depending on the surface activity of the monomer surfactant at the CAC. The surface tension may increase because the surfactant monomer is not sufficiently surface active to maintain the low value of γ, it may remain approximately constant by replacement of PSn by surfactant, or it may decrease because the surfactant is sufficiently surface active to assist in pushing PSn off the surface. The first type of surface behavior has been observed in cationic starch/SDS mixtures by Merta and Stenius9 and appears to be the behavior observed in the KP and Alpha gelatin/SDS systems here. The second two types of behavior give γ-ln[S] curves with exactly the same appearance as the classical PEO/SDS and PVP/SDS curves. However, their physical significance is completely different in that the CAC is not necessarily the same as T1. To identify the CAC unambiguously in such strongly interacting systems it is necessary to make a measurement that directly examines the aggregation in the bulk solution, e.g., fluorescence or small angle neutron scattering. Surface tension cannot in general identify the CAC in strongly interacting systems, although it has often been used in this way (see, e.g., ref 11). It is hardly surprising that when the surface interactions are strong enough the surface tension behavior masks the changes occurring in the bulk solution, and this possibility was indeed pointed out by Goddard,4 although he did not discuss it in detail. The model we have used to describe the behavior of a strongly interacting polymer/surfactant system is too simple to describe most real systems. For example, it is probable that a whole sequence of surface active surfactant/polymer complexes forms. However, because of the competition between loss of configurational entropy, which increases with the number of bound sites for the individual surfactant molecules, and the gain in energy in attaching the composite hydrophobic units (surfactant + monomer fragment) to the surface, the surface activity of the complex should pass through a maximum as n varies. The difference between this and the simpler model described above will mainly be in the behavior along the plateau region. In the situation where the simple model predicts an increase in surface tension because of removal of polymer as polymer/micelle complex, a more gradual level of complexing with a decreasing surface activity of the complexes may make this rise also become more gradual. A second difference is that the polymer/micelle complex could itself be surface active and this has been suggested by Babak et al.47 One manifestation of this would be that the polymer would affect the surface tension above the cmc, although the absence of such an effect may not rule out a weaker surface activity of this complex. It is not easy to predict the consequences for the behavior below

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the cmc except that the presence of a polymer/micelle complex at the surface would probably lead to a higher surface tension than the complex between polymer and isolated surfactant molecules.

Research Council of the U.K. D.J.C. also thanks Kodak European Research, U.K., for their support. We also thank Dr. T.H. Whitesides for the gift of the Alpha gelatin.

Acknowledgment. We are grateful for financial support from the Engineering and Physical Science

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