Adsorption of the Nonionic Surfactant Triton X-405 on Polystyrene

Jun 12, 1996 - An investigation of the adsorption of the nonionic surfactant Triton X-405 on a polystyrene latex by use of small-angle X-ray scatterin...
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Langmuir 1996, 12, 2906-2912

Adsorption of the Nonionic Surfactant Triton X-405 on Polystyrene Latex Particles As Monitored by Small-Angle X-ray Scattering J. Bolze, K. D. Ho¨rner, and M. Ballauff* Polymer-Institut, Universita¨ t (T.H.) Karlsruhe, Kaiserstrasse 12, 76128 Karlsruhe, Germany Received November 27, 1995. In Final Form: March 8, 1996X An investigation of the adsorption of the nonionic surfactant Triton X-405 on a polystyrene latex by use of small-angle X-ray scattering (SAXS) is given. Since the latex particles only have a low contrast toward water whereas the electron density of the added surfactant is rather high, the gradual buildup of the surfactant layer on the particles can be monitored with good sensitivity. The contrast of the particles can be matched by adding glycerol or sucrose to the dispersion medium. Thus, a detailed study of the adsorbed layer by contrast variation is possible. Small differences in the scattering patterns depending on the nature of the contrast agent (glycerol and sucrose) could be observed and are discussed. It is shown that the adsorbed Triton molecules form a thin, dense layer of approximately 1-3 nm thickness on the latex spheres. After maximum coverage of the surface is reached, the additional surfactant forms free micelles which give a distinct scattering contribution in the region of wider angles.

Introduction Adsorption of surface active agents or of macromolecular chains on the surface of colloidal particles has been a topic of major interest recently.1-3 For this purpose small-angle neutron scattering4-6 (SANS) has been proven to be a useful investigative tool which allows discrimination between the core particle and the adsorbed layer by contrast variation.7-13 The main advantage of this technique resides in the marked difference between the scattering cross sections of hydrogen and deuterium.6 Therefore the contrast of many organic and inorganic bulk materials can easily be matched by appropriate mixtures of H2O and D2O. The contrast between the core particle and the surfactant may be enhanced further by partial deuteration of the latter component.7 Thus, applying these techniques Ottewill2,14,15 and co-workers could study a broad variety of systems including surfactants adsorbed on latex particles and on inorganic colloids. Small-angle X-ray scattering5 (SAXS) has been applied much less frequently to the investigation of the fine * To whom all correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (2) Ottewill, R. H. Prog. Colloid Polym. Sci. 1992, 88, 49 and references given therein. (3) Steinby, K.; Silveston, R.; Kronberg, B. J. Colloid Interface Sci. 1993, 155, 70 and references given therein. (4) Feigin, L. A.; Svergun, D. I. Structure Analysis by Small-Angle X-Ray and Neutron Scattering; Plenum Press: New York, 1987. (5) Glatter, O.; Kratky, O. Small Angle X-Ray Scattering; Academic Press: London, 1982. (6) Higgins, J.; Benoit, H. Polymers and Neutron Scattering; Clarendon Press: Oxford, 1994. (7) Cebula, D. J.; Thomas, R. K.; Harris, N. M.; Tabony, J.; White, J. W. Faraday Discuss. Chem. Soc. 1978, 65, 76. (8) Stuhrmann, H. B.; Kirste, R. G. Z. Phys. Chem. 1965, NF 46, 247. (9) Stuhrmann, H. B.; Kirste, R. G. Z. Phys. Chem. 1967, NF 56, 334. (10) Kawaguchi, T.; Hamanaka, T. J. Appl. Crystallogr. 1992, 25, 778. Kawaguchi, T. J. Appl. Crystallogr. 1995, 28, 424. (11) Duits, M. H. G.; May, R. P.; de Kruif, C. G. J. Appl. Crystallogr. 1990, 23, 366. (12) Penders, M. H. G. M.; Vrij, A. Colloid Polym. Sci. 1990, 268, 823. (13) Philipse, A. P.; Smits, C.; Vrij, A. J. Colloid Interface Sci. 1989, 129, 335. (14) Harris, N. M.; Ottewill, R. H.; White, J. W. In Adsorption from Solution; Ottewill, R. H., Rochester, C. H., Smith, A. C., Eds.; Academic Press: London, 1983. (15) Ottewill, R. H.; Sinagra, E.; MacDonald, I. P.; Marsh, J. F.; Heenan, R. K. Colloid Polym. Sci. 1992, 270, 602 and references given therein.

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structure of polymer colloids. On the other hand, the electron densities of typical polymers used for the preparation of latexes are rather low,16 and the match point can be reached by adding sucrose or glycerol to the dispersion medium.4 SAXS measurements can be extended to higher scattering angles, since there is no incoherent contribution to the background. It can be shown that parasitic scattering can be minimized as well.16 Hence, SAXS is suitable to perform a detailed structure analysis of polymer colloids by contrast variation.17 Recent investigations by this method dealt with core-shell latexes,17-19 swollen latexes,20 and micelles21 formed by block copolymers. In all cases even minute structural details down to a scale of ca. 1 nm could be resolved. In this paper these investigations are extended to the study of adsorbed nonionic surfactant molecules of Triton X-405 on the surface of polystyrene latex particles. Triton X-405 is a [p-(1,1,3,3-tetramethylbutyl)phenoxypoly(oxyethylene glycol)] having an average of 40 oxyethylene units per molecule (Scheme 1). The choice of polystyrene as core material derives from its low excess electron density compared to water (6/nm3, cf. ref 16). Thus, these particles are nearly matched by the surrounding water whereas the Triton X-405 molecules have a high excess electron density in particular due to the poly(ethylene oxide) moiety (46/nm3). From this it is evident that the scattering will originate mainly from the surface layer. These favorable conditions furthermore allow the variation of the average particle contrast from positive to strongly negative by adding glycerol or sucrose to the dispersion medium. Thus, the internal electron density variations as well as the overall shape of the particles can be investigated, and the modeling of the particle structure becomes possible with the same fit procedures described previously.17-21 The paper is organized as follows: At first a detailed study of the uncovered latex particles by contrast variation is given. The high spatial resolution allows a discussion of the radial excess electron density and of the surface (16) Grunder, R.; Urban, G.; Ballauff, M. Colloid Polym. Sci. 1993, 271, 563. (17) Dingenouts, N.; Ballauff, M. Acta Polym. 1993, 44, 178. (18) Dingenouts, N.; Kim, Y. S.; Ballauff, M. Colloid Polym. Sci. 1994, 272, 1380. (19) Ballauff, M. Macromol. Symp. 1994, 87, 93. (20) Bolze, J.; Ballauff, M. Macromolecules 1995, 28, 7429. (21) Hickl, P.; Ballauff, M.; Jada, A. Macromolecules, in press.

© 1996 American Chemical Society

Adsorption of Triton X-405 on Polystyrene Latex

Langmuir, Vol. 12, No. 12, 1996 2907

Scheme 1

structure of the latex derived therefrom. In a second step the gradual adsorption of Triton X-405 on the particle surface is investigated. Finally the nearly fully covered particles are characterized by contrast variation. In the course of these studies possible influences of the added contrast medium, glycerol or sucrose, are discussed as well. Experimental Section Materials. Styrene (Fluka, p.a.) was first destabilized with a 10 wt % sodium hydroxide solution, washed with water, and subsequently dried over calcium chloride. Then it was distilled for further purification. Sodium dodecyl sulfate (SDS, Lancaster) was used as received. The initiator potassium peroxodisulfate (Fluka) was recrystallized twice from bidistilled water. The polystyrene latex was prepared by a conventional batch emulsion polymerization of 50 g of styrene in 525 mL of bidistilled water using 1.4965 g of SDS and 0.3120 g of K2S2O8. The polymerization temperature was 80 °C. The latex was purified by dialysis against a large volume of 0.0025 M KCl solution for three weeks. Triton X-405, D-(+)-sucrose (Fluka), and glycerol (BASF, DAB7) were used without further purification. A given amount of Triton X-405 (Fluka) was added to the latex and subsequently shaken thoroughly. SAXS measurements started at different times after the addition of the surfactant show that the adsorption equilibrium is reached within less than 1 h. Methods. The densities of the aqueous sucrose and glycerol solutions as well as of the latex were determined using a DMA60 apparatus (Paar, Graz, Austria). The electron densities (number of electrons per cubic nanometer) calculated by a leastsquare-fit from these data follow as Fm ) 332.4 + 1.31(wt % sucrose) for the sucrose solutions and Fm ) 332.8 + 0.749(wt % glycerol) for the glycerol solutions. The density of the polystyrene particles is 1.049 g/cm3. The number-average diameter and the weight-average diameter as determined by transmission electron microscopy (Hitachi 700-H) were 68.1 and 71.3 nm, respectively. The solid content of the stock latex was determined gravimetrically to be 8.2 wt %. SAXS Analysis. Small-angle X-ray scattering (SAXS) measurements were performed at room temperature using a KratkyKompakt-Kamera (Paar, Graz, Austria) with a block collimation system.16,17 The scattering intensities were registered with a position-sensitive detector (Braun, Germany) in the q-range 0.08 nm-1 < q < 5 nm-1 (scattering vector, q ) (4π/λ) sin(θ/2); θ, scattering angle; λ, wavelength of the Cu KR radiation used in the experiment). Absolute scattering intensities were obtained by the moving-slit method.22 The principal steps of data treatment are as follows: First the scattering contributions by the empty capillary and by the respective dispersion medium were subtracted. For the discussion of the curves in the region of lowest scattering angles (q < 0.5 nm-1), the data were desmeared taking into account the smearing effects of the finite height and width of the slit in the collimation system. The scattering curves discussed in the range of high q-values were not desmeared because in this region only a qualitative comparison of the respective intensities is given. Finally, all curves displayed here were normalized to the volume concentration of the polystyrene core particles. To enable comparison with given models of the radial electron density distribution, the scattering intensity originating from the density fluctuations inside the latex particles was subtracted by use of a Porod plot.5 Further details concerning the measurements as well as the subsequent data treatment including the desmearing procedures have been described elsewhere.17 As already stated previously16-18 rather high concentrations (4-7 vol %) of the latex could be investigated to obtain good counting statistics in the measurements. This is due to the fact that in the q-range under consideration here the influence of (22) Stabinger, H.; Kratky, O. Makromol. Chem. 1978, 179, 1655.

Figure 1. Comparison of the scattering intensities of the uncoated PS latex measured at different contrast by addition of glycerol or sucrose. The curves refer to the following concentrations by weight of the contrast agents: 9, 13.5% glycerol; +, 8.0% sucrose; 3, 20.5% glycerol; +, 12.0% sucrose; b, 27.4% glycerol; +, 16.0% sucrose; 4, 55.4% glycerol; +, 32.0% sucrose. interaction between the latex spheres, i.e., of the structure factor, is practically negligible.16

Results and Discussion Polystyrene Latex without Additional Surfactant. For the surface investigation of the latex particles two series of SAXS measurements were performed: In one case we used glycerol for changing the contrast, and in the other case we used sucrose. Thus possible influences of the dispersion media on the particle structure can be studied. To ensure a meaningful comparison, the amounts of glycerol and sucrose were adjusted so that exactly the same contrasts were obtained. Figure 1 shows representative data as a function of the scattering vector q. Here the scattering contribution due to the density fluctuations within the latex particles has been subtracted. From the data displayed in Figure 1 it is obvious that both agents used for raising the electron density in the dispersion medium lead to the same results. Only at lowest contrast is there some small discrepancy which must be traced back to the low scattering intensity near the match point. This result shows that the structural features monitored by SAXS were independent of the amount and the nature of the added agent, glycerol or sucrose. Since glycerol gives a much lower scattering background, it is preferred in subsequent investigations. In order to relate the scattering curves to various models of the radial electron density distribution F(r) of particles dispersed in a medium of electron density Fm, we proceed as described previously:11-13,16-21 For a dilute system of polydisperse particles the intensity per volume normalized to the scattering intensity of a single electron (e.u./nm3) follows as

I(q) )

∑i NiBi2(q)

(1)

where Ni is the number of particles of radius Ri per unit volume and Bi(q) is the scattering amplitude. For spherical symmetric particles of radius R, B(q) is given by

∫0R[F(r) - Fm]r2

B(q) ) 4π

sin(qr) dr qr

(2)

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Figure 2. Full set of scattering curves of the uncoated PS latex measured at different contrast using glycerol as contrast agent. The curves refer to the following concentrations by weight of glycerol whereas the numbers in parentheses refer to the average contrast (in nm-3): ], 0% (7.8); *, 6.3% (3.9); 9, 13.5% (-1.5); 3, 20.5% (-6.8); b, 27.4% (-11.9); O, 41.4% (-22.5); 4, 55.4% (-32.9). The lines refer to the fit curves obtained from the electron density profile shown in the inset (Fh ) 341.4 nm-3; core radius, 30.4 nm; shell thickness, 1 nm; the electron density of water was taken as reference in the profile).

With definition of the volume-average electron density Fh of the particles

∫0RF(r)r2 dr Fh ) ∫0Rr2 dr

(3)

the contrast ∆F can be defined by ∆F ) Fh - Fm. As outlined previously16,17 the polydispersity of particle sizes was modeled using a slightly asymmetric distribution. For a given size distribution and chemical composition, the radial electron density is varied until the best agreement between theory and all experimental scattering intensities at different contrasts is reached. Figure 2 displays the complete set of scattering curves of the polystyrene latex obtained by using glycerol as contrast agent. A characteristic shift of the extrema with contrast is observed, in particular near the match point. As shown previously20 the direction of this shift points to a thin shell having a higher electron density than the core. A core-shell model of the electron density exhibiting sharp boundaries (cf. inset of Figure 2) already describes the experimental data rather well (solid lines in Figure 2) if the low counting statistics for the two curves at lowest contrast is taken into account. This model gives a number average core radius of 30.4 nm and a polydispersity expressed by Rw/Rn ) 1.052, in fair agreement with the results obtained by transmission electron microscopy (R = 34 nm). In the shell of 1 nm thickness the electron density is 20/nm3 higher than that in the core. Since SAXS gives only information about the excess electron densities, the chemical nature of the shell cannot be deduced from the above results, of course. It seems to be clear, however, that the increased electron density is mainly due to the polar headgroup of the SDS molecules which were used for the latex preparation (cf. the discussion of this point in ref 23). A detailed investigation (23) Zhao, J.; Brown, W. Macromolecules 1993, 26, 2711.

Figure 3. (a) Scattering curves of the PS latex measured in water after addition of different amounts of Triton X-405. The curves refer to the following Triton concentrations expressed in milligrams of Triton per gram of PS: 0, 0 mg/g; +, 28 mg/g; 4, 61 mg/g; 9, 115 mg/g (saturation); ×, 195 mg/g. (b) Scattering curves of the PS latex measured in water after addition of different amounts of Triton X-405 beyond the saturation of the particle surface. The curves refer to the following Triton concentrations expressed in milligrams of Triton per gram of PS: ×, 195 mg/g; b, 364 mg/g; ], 694 mg/g; 3, 1020 mg/g.

of the influence of adsorbed SDS molecules on the scattering curves is under way. Adsorption of Triton X-405 on the Latex Surface. To assess the adsorption of Triton X-405, several mixtures of the latex with increasing amounts of surfactant were prepared without adding any contrast agent at this stage. Figure 3 displays the resulting SAXS intensities. Good reproducibility of the measured scattering curves was found in all cases. Even small amounts of added Triton cause a marked shift of the side maxima toward smaller scattering angles. This shift continues up to an amount of ca. 115 mg of Triton per gram of polystyrene (see Figure 3a). At higher surfactant concentrations (see Figure 3b) the side maxima remain at the same q-values but there is a strong increase of the scattering intensities in the region of higher scattering angles (q > 0.3 nm-1). If a homogeneous radial electron density is assumed, the shift of the maxima in Figure 3a could be taken as a clear indication for an increase of the particle radius. For core-shell particles, however, a gradual increase of the electron density in a shell of constant thickness results

Adsorption of Triton X-405 on Polystyrene Latex

Figure 4. Smeared scattering curves (range of high q-values) of the PS latex after addition of different amounts of Triton X-405 measured in water. The curves refer to the following Triton concentrations expressed in milligrams of Triton per gram of PS: 0, 0 mg/g; +, 28 mg/g; 9, 115 mg/g (saturation); ×, 195 mg/g; b, 364 mg/g; ], 694 mg/g; 3, 1020 mg/g.

in a similar shift of the extrema. Therefore the curves may also be interpreted in terms of a densification of the adsorbed layer and need not necessarily point to an increase of the particle radius as one might expect at first. In the present case it must be kept in mind that the adsorbed surfactant has a much higher excess electron density than the polystyrene (PS) core. As a consequence, the attached layer of surfactant must lead to a pronounced core-shell structure of the coated particles. Figure 3 suggests a saturation of the particle surfaces at a Triton concentration of ca. 115 mg per gram of PS. This assumption is also consistent with the smeared scattering intensities at higher q-values displayed in Figure 4. In the wide-angle region a new broad maximum evolves beyond this point of saturation. The obvious explanation of this new feature is given by the formation of free micelles from the excess of the surfactant which are not bound to the surface of the latex particles. As a first approximation one may assume negligible interactions between the polystyrene particles and the Triton micelles. In this case the scattering intensities of both species add up throughout the entire range of scattering angles. Furthermore it is assumed that all Triton molecules adsorb on the particle surface until the surface of the particles is fully covered at the amount of 115 mg of Triton per gram of PS. Beyond this point all additional Triton will not adsorb but will form free micelles. Hence, the scattering curves of a system which consists solely of Triton micelles should be obtained when the scattering curve of the fully covered particles is subtracted from those of the ones with higher Triton concentrations. The amount of free Triton in these solutions must follow directly from the respective mass balance in which the volume fraction of the particles in the latex is taken into account. To check this prediction the scattering intensities of Triton X-405 in 3 × 10-3 M KCl solution were measured at several concentrations and the background scattering of the empty capillary and of the water was subtracted. For the above comparison the smeared data (see Figure 4) together with the smeared scattering curves from the micelles are sufficient. Figure 5 shows that for each Triton concentration the difference curves coincide with a scattering curve as measured directly from the micelles. The agreement is very good in a q-range from 0.3 nm-1 up to

Langmuir, Vol. 12, No. 12, 1996 2909

Figure 5. Scattering contribution of free Triton X-405 micelles at high q-values: Comparison of the smeared scattering curves measured from Triton X-405 solutions with those obtained from the subtraction as described in the text. The curves refer to the following concentrations by weight of Triton X-405: 0, 0.8%; 9, 5.0%; 4, 7.8%. The curves obtained from the subtraction refer to the following Triton concentrations expressed in milligrams of Triton per gram of PS: ×, 195 mg/g; ], 694 mg/g; 3, 1020 mg/g.

at least 1.5 nm-1. The discrepancy at lower angles is most probably due to difficulties when subtracting the much larger latex contribution prevalent in this region. From this comparison the amount of Triton adsorbed onto the particle surface is given by the difference of the total added amount of Triton and of the amount found in the dispersion medium. The calculated values in milligrams of Triton per gram of PS are as follows: 108, 125, 106, and 103, respectively, for increasing total amounts of surfactant added. From Figure 3a the value of 115 mg of Triton per gram of PS was deduced for full coverage of the particles. Given the various uncertainties and assumptions, the agreement between the latter figure and the above values may be considered as satisfactory. It shows that the data are consistent with the total mass balance: All additional intensity appearing in the wide-angle region beyond the point of saturation (see Figure 4) is solely due to the newly formed free Triton micelles. Obviously these micelles are not attached in a second layer on the particle surface. There is, however, a clear indication for a strong intermicellar interaction from the decrease of the scattering intensities at small angles (cf. Figure 5, uppermost curve): At high Triton concentrations the contribution of free surfactant to the scattering intensities shown in Figure 3b is appreciable already in the region of smallest angles (q < 0.2 nm-1). This is obvious from the fact that in this q-range the scattering intensities decrease slightly with increasing Triton concentration (see Figure 3b). The contribution of the free surfactant explains this finding as well as the crossing of the scattering curves at different Triton content seen in Figure 3b: The scattering intensities at highest content of Triton are lower at small scattering angles but decrease less at high scattering angles. The increased level at higher q has already been explained by the contributions of the free micelles whereas their interaction diminishes their intensity at low q. This is followed by a decrease of the entire scattering intensity in this region. Additional experiments were carried out to investigate the effect of dilution with 3 × 10-3 M KCl solution (up to a factor of three) on the adsorbed amount of Triton molecules. After background subtraction and normaliza-

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Figure 6. Check of the accuracy of contrast variation measurements: PS latex covered with Triton (103 mg of Triton per gram of PS) measured at different contrast using sucrose as contrast agent. For the sake of clarity some curves were multiplied by a factor given in parentheses. The curves refer to the following concentrations by weight of sucrose: *, 4.0% (multiplied by 0.1); 9, 8.0% (multiplied by 0.5); b, 16.0%; 4, 32.0%. The crosses refer to repeated measurement.

Figure 7. Comparison of the scattering intensities of the covered PS latex (103 mg of Triton per gram of PS) measured at different contrast by addition of glycerol or sucrose. For the sake of clarity one curve was multiplied by the factor given in parentheses. The curves refer to the following concentrations by weight of the contrast agents: *, 6.4% glycerol; +, 4.0% sucrose (multiplied by 0.1); 3, 20.4% glycerol; +, 12.1% sucrose; 4, 55.4% glycerol; +, 32.0% sucrose.

tion to the volume fraction of the latex the curves superimpose within the limits of error for all concentrations. Hence, this dilution has no observable effect on the adsorption equilibrium, meaning that there must be strong interaction of Triton X-405 with the polystyrene surface. The surfactant adsorbs up to a defined point of saturation beyond which free micelles are formed. The present data do not suffice to decide whether the SDS used for the synthesis of the latex particles remains on the surface or desorbs upon addition of Triton because the SDS concentration is too low. Experiments by Kronberg and co-workers24 suggest that SDS will be replaced by the nonionic surfactant. The competitive adsorption of SDS and Triton X-405 on polystyrene latex particles is the subject of an ongoing investigation by SAXS in our laboratory. Contrast Variation with the Coated Particles. To investigate the structure of the adsorbed Triton layer, a series of measurements at varying contrast has been performed. Since the discussion of the scattering intensities as a function of contrast requires excellent accuracy for measurements and sample preparation, the reproducibility was checked in detail first. For this purpose two sets of a nearly fully covered latex with the same Triton concentration of 103 mg/g of PS have been prepared. Figure 6 shows representative examples of the scattering intensities measured from these latexes at different contrasts. Here sucrose has been used as the contrast agent. In all cases the fluctuation-induced background has been subtracted. Repetitive measurements at a given contrast have been marked by crosses. Very good agreement is seen, even at low contrast. Obviously the accuracy of the present measurements is good enough to discuss even small changes of the scattering intensity up to q ) 0.4 nm-1. Beyond this point the error increases because of the small scattering contribution of the latex spheres in comparison with the one originating from the dispersion medium. As will be shown in the following section, however, the above q-range allows the discussion of small structural details of the particles.

To assess possible alterations of the surface structure by the contrast agent, we used both glycerol as well as sucrose to vary the contrast. The comparison between some representative pairs of scattering curves obtained at the same contrast using glycerol and sucrose, respectively, is shown in Figure 7. The scattering due to density fluctuations inside the particles has been subtracted in all cases. Unlike the findings on the uncovered polystyrene latex discussed in conjunction with Figure 1 there is some small difference, most notably at intermediate contrast and at higher q: The curves measured in sucrose solutions lie slightly below those measured in glycerol at the same contrast. Having shown the good accuracy and reproducibility of the measurements in the q-range under consideration (cf. the discussion of Figure 6), we conclude from Figure 7 that sucrose interacts with the adsorbed surfactant layer in a different manner than glycerol. This difference may be due to a hindered penetration of the larger sucrose molecules into the outer shell whereas glycerol is assumed to enter freely. This hypothesis is in agreement with a previous treatment of the contrast variation of Triton X-100 micelles by Kawaguchi10 where full exclusion of the sucrose molecules from the corona of the micelles was assumed. A spatial resolution of the order of 1 nm discussed herein, however, is clearly at the borderline of the resolution of the SAXS method, and quantitative results must therefore be discussed with caution. Discussing the full set of curves at varying contrast as measured in glycerol solutions (Figure 8), the structure of the surface layer can be elucidated further. From comparison with Figure 2 it becomes obvious that the variation of the internal electron density must be stronger than in the case of the uncoated latex: Near the match point the scattering curves are closer together and there is evidence for an isoscattering point10 where these curves have a common point of intersection. This crossing point has been discussed previously by a number of authors10-13,17-21 and will only occur in systems of welldefined particles with a narrow size distribution. From the characteristic shift of the extrema with contrast one may conclude that the particles must have a shell with

(24) Kronberg, B.; Lindstro¨m, M.; Stenius, P. In Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; ACS Symposium Series 311; American Chemical Society: Washington, DC, 1986.

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Figure 8. Full set of scattering curves of the covered PS latex (103 mg of Triton per gram of PS) measured at different contrast by addition of glycerol. For clarity the points were linked by lines. The curves refer to the following concentrations by weight of glycerol: ], 0%; *, 6.4%; 9, 13.4%; 3, 20.4%; b, 27.3%; O, 41.3%; 4, 55.3%.

a higher electron density in comparison to the core.20 In addition to this, the curves at lowest contrast exhibit strongly pronounced minima and maxima, as expected for hollow spheres. On the other hand, the curves far from the match point are shifted approximately parallel to the ordinate with contrast, as expected for homogeneous particles. This is due to the effect of polydispersity, which smears out the minima of the form factor of homogeneous spheres to some extent. The crossing point far away from the match point is located near this minimum. Therefore it will be affected by polydispersity much more than the crossing point near the match point where it is located around the first side maximum (cf. the discussion of this point in ref 20). The determination of the radial electron density distribution from these data was done as described above. For simplification the electron density of Triton was calculated by taking into account only the electron density of the poly(ethylene oxide) chains (379.5/nm3; cf. ref 21) and assuming that the average electron density of the unpolar moiety is similar to that of the polystyrene core. In Figure 9 a comparison of the experimental data and the curves calculated for two different coreshell models is shown. The respective electron density profiles are displayed in the inset of Figure 9a. Again the density fluctuations induced scattering has been subtracted. The first model (solid lines) assumes a compact shell of pure surfactant with a thickness of 1 nm in which the electron density is 40/nm3 higher than that in the polystyrene core. The other model describes particles with the electron density decreasing linearly in a hydrated shell of 3 nm. In this case we assumed for the modeling that glycerol can enter the shell freely. For both models the number average core radius is 30.9 nm and the overall mass balance is fulfilled. From the comparison of these two models it must be concluded that each of them describes the experimental data with equal accuracy. The diffuse model which assumes penetration of the solvent into the surfactant layer seems to be more reasonable than a dense layer of pure poly(ethylene oxide) chains. Additional model calculations showed, however, that extending the diffuse model to 5 nm thickness leads to much worse agreement of the measured and the calculated data. Thus the present

Figure 9. Scattering curves of the covered PS latex (103 mg of Triton per gram of PS; Fh ) 343.2 nm-3) measured at different contrast by addition of glycerol. The curves refer to the following concentrations by weight of glycerol whereas the numbers in parentheses refer to the average contrast (in nm-3). (a) ], 0% (9.6); *, 6.4% (5.6) (multiplied by 0.5). The lines represent the fit curves obtained from the electron density profiles shown in the inset. Solid lines: core-shell model with sharp boundaries; core radius, 30.9 nm; shell thickness, 1 nm. Dashed lines: Core radius, 30.9 nm; diffuse shell of 3 nm. The electron density of water was taken as reference in the profile. (b) 9, 13.4% (0.3) (multiplied by 0.2); 3, 20.4% (-5.0) (multiplied by 0.5); b, 27.3% (-10.1); O, 41.3% (20.7); 4, 55.3% (-31.1). The fit parameters are the same as in Figure 9a.

fits certainly lead to the conclusion that the Triton layer on the surface of the latex particles is rather dense (1-3 nm). A dense layer has been detected25 as well in a recent investigation of polystyrene latex particles coated by Triton X-100. From the mass of the adsorbed surfactant at full coverage of the particles one may estimate the average area per Triton molecule, which amounts to approximately 2.6 nm2. In view of the magnitude of this number as well as of the finding that the layer is rather thin, this figure points to monomolecular adsorption. Since the area per molecule would be much smaller if only the unpolar moiety would be taken into account, the present results point strongly to the adsorption of the entire molecules including the polar part. Hence, the poly(ethylene oxide) chains will not fully protrude into the water phase but are located near the surface of the latex particle. (25) Johnson, P. Langmuir 1993, 9, 2318.

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Conclusion The present study has shown that SAXS is highly suitable to monitor quantitatively the adsorption of the surfactant Triton X-405 on the surface of polystyrene latex particles. Due to the high contrast between polystyrene and the surfactant, the process of adsorption can be followed with very good sensitivity, even for small Triton concentrations. The analysis of the data shows that Triton X-405 is adsorbed strongly to the surface of the particles up to a well-defined point of saturation which corresponds most probably to a monomolecular layer. Beyond this point, further addition of the surfactant leads to the

Bolze et al.

formation of free micelles. The layer on the polystyrene particles as investigated by contrast variation is rather dense (1-3 nm). This fact also points to the strong interaction of both the unpolar as well as the polar part of the surfactant with the surface of the polystyrene particles. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft, by the AIF (Projekt 9749), and by the Bayer AG, Gescha¨ftsbereich Kautschuk, is gratefully acknowledged. LA951073C