Adsorption of Penta(ethylene glycol) Monododecyl Ether at the Solid

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Adsorption of Penta(ethylene glycol) Monododecyl Ether at the Solid Poly(methyl methacrylate)-Water Interface: A Spectroscopic Ellipsometry Study V. A. Gilchrist, J. R. Lu,* and J. L. Keddie School of Physics and Chemistry, University of Surrey, Guildford, GU2 5XH, U.K.

E. Staples and P. Garrett Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, L63 3JW, U.K. Received May 27, 1999. In Final Form: September 24, 1999 We have examined the adsorption of a nonionic surfactant, penta(ethylene glycol) monododecyl ether (C12E5), at the poly(methyl methacrylate) (PMMA)-water interface using spectroscopic ellipsometry. The solid PMMA surface was deposited by spin casting an ultrathin film onto a freshly cleaned silicon wafer. Measurements by both spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) showed that the thin PMMA film was uniform with no prominent structural features on the surface. The adsorption of C12E5 at the solid PMMA-aqueous solution interface was studied using a specially designed cell with a fixed angle of incidence of 75°, and the measurements were made over a wide concentration range around the critical micellar concentration (cmc). It was found that the adsorption is completely reversible and that there is no observable penetration of C12E5 into the PMMA. The adsorption was found to reach equilibrium well within seconds. Although spectroscopic ellipsometry cannot allow a reliable measurement of layer thickness as a result of coupling between refractive indices and layer thickness for ultrathin layers, the surface excess at a given concentration can be determined reliably. The limiting area per molecule at the cmc was calculated to be 50 ( 3 Å2, in good agreement with the value obtained from a previous neutron reflection study.

Introduction Adsorption of surfactants onto solid polymer substrates is an issue relevant to both domestic and industrial applications, ranging from cleaning aids, cosmetics, pharmaceutical preparations, paint stabilization, pulp and paper making, drilling fluids to water treatment. These processes tend to rely on the adsorption of surfactants onto surfaces either as individual molecules or as aggregates of varying size and structure. Numerous studies have aimed to understand the nature of the interaction between surfactants and solid polymer surfaces.1-6 Most of these studies have used particulate dispersions, e.g., latex particles, where surface excess can be determined by depletion measurement. Although such an approach offers an easy route for the estimation of surface excess, it provides little reliable information about the in situ structural conformation of the adsorbed layer, which is crucial to a real understanding of the mode of interaction between surfactant and polymer substrate. When a surfactant is physically adsorbed onto a polymer * All correspondence should be addressed to Dr. Jian R. Lu, Department of Chemistry, School of Physics and Chemistry, University of Surrey, Guildford, GU2 5XH, U.K. Tel: 44-(0)1483876831. E-mail: [email protected]. (1) Haq, Z.; Thompson, L. Colloid Polym. Sci. 1982, 260, 212. (2) Zisman, W. A., In Contact Angle, Wettability and Adhesion; Advances in Chemistry Series 43; American Chemical Society: Washington, DC, 1964. (3) Kronberg, B.; Stenius, P. J. J. Colloid Interface Sci. 1984, 102, 410. (4) Steinby, K.; Silveston, R.; Kronberg, B. J. Colloid Interface Sci. 1993, 155, 70. (5) Romero-Cano, M. S.; Martin-Rodriguez, A.; Chauveteaum G.; de las Nieves, F. J. J. Colloid Interface Sci. 1998, 198, 2711. (6) Gau, C.-S.; Zografi, G. J. Colloid Interface Sci. 1990, 140, 1.

surface, it might also penetrate into the polymer substrate. Few techniques are sensitive enough to detect the extent of penetration of surfactant into the polymer. The lack of reliable information about the in situ structural conformation of the surfactant layer at the polymer-water interface has seriously hindered the development of theory in this area.4,7 We have shown in a previous study that neutron reflection is an ideally suited technique for quantifying the structural conformation of the surfactant layer at the polymer-water interface.8 Its high depth resolution, combined with deuterium labeling of the surfactant and solvent, enables us to reveal the detailed structural information inside the adsorbed layer. In contrast, ellipsometry cannot resolve small variations within an interfacial layer, especially when the layer thickness is below ∼300 Å and its optical constants are unknown. However, in comparison to neutron reflection, ellipsometry has a number of attractive advantages. At the moment, a neutron source is expensive and its access is very limited. As a home laboratory technique, ellipsometry is easy and inexpensive to use. When the materials under study are properly characterized, ellipsometry can offer useful structural information to complement a neutron experiment. A prior ellipsometric study can always make neutron work more efficient. In comparison with conventional null ellipsometry, spectroscopic ellipsometry has the flexibility of performing measurements over a wide range of wavelengths, hence making the measurements more sensitive to interfacial structural profiles. (7) Zhu, B.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3813. (8) Gilchrist, V. A.; Lu, J. R.; Staples, E.; Garrett, P.; Penfold, J. Langmuir 1999, 15, 250.

10.1021/la9906572 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/20/1999

C12E5 Adsorption at the PMMA-Water Interface

In this work we report the ellipsometric study of the adsorption of penta(ethylene glycol) monododecyl ether (C12E5) on a polymer substrate, poly(methyl methacrylate) (PMMA), deposited on the surface of a silicon wafer. We demonstrate that spectroscopic ellipsometry is sufficiently sensitive not only to the amount of surfactant adsorbed but also to a possible change of adsorption with time. Experimental Details Materials. PMMA was used as received from Aldrich (lot no. ) 04707TN, MW ) 101 000, Mw/Mn ) 2.09). Before casting, the polymer was dissolved in analytical grade toluene. The solution was sonicated for approximately 45 min (to aid dissolution) and was then left on a stirrer (to homogenize) overnight. The silicon wafers (111) used were approximately 0.40 mm thick (Compart Technology Ltd, U.K.). Prior to use, each wafer surface was cleaned in a three-stage cleaning process. First, the wafers were copiously rinsed in ethanol, then immersed in a neutral Decon solution (5%, diluted from the concentrated neutral Decon solution from Decon Laboratories, U.K.), and sonicated for 30 min. They were then rinsed and sonicated in ultrapure water (UHQ water, processed by the Elgastat water purification system) for a further 30 min. In the second stage, the wafers were heated for 6 min at 120 °C in a solution of 98% concentrated sulfuric acid (BDH) and 34% (w/v) hydrogen peroxide (both analytical grade) in a volume ratio of 6:1 in favor of the acid.9 The wafers were then thoroughly rinsed with the UHQ water. Final stage cleaning was comprised of UV/ ozone treatment for 30 min to remove any traces of organic impurity.10 The surface of each wafer was then rehydrated by soaking in UHQ water overnight and checked for complete wettability. This procedure was found to generate a highly reproducible hydrophilic surface with an average oxide layer thickness of approximately 25 ( 5 Å. Thin-layer chromatography (TLC) on analytical grade C12E5 (Fluka) indicated the possible presence of its homologue. The sample was therefore purified by passing it through a silica flash column. UHQ water was used throughout the measurements, and all the glassware and sample cells were cleaned using alkaline Decon solution diluted from Decon 90 (Decon Laboratories Ltd, UK), followed by repeated washing in UHQ water. Surface Coating. Prior to coating, each cleaned wafer was first measured on the ellipsometer to determine its average oxide thickness. Each 2.5 cm × 4.5 cm wafer was measured at seven centrally located positions. A micrometer on the ellipsometer stage made it possible to measure the same wafer reproducibly and repeatedly at the same locations. PMMA films were deposited by spincasting from solution using a photoresist spinner from Cammax Precima Ltd, U.K. (model no. PRS 14E). As will be described later, the optimum conditions for spin casting were found to be at 2000 rpm using a fast acceleration and holding for 20 s. This coating procedure has consistently been found to be highly reliable and reproducible over a wide range of film thicknesses to well within a few angstroms. The coated wafer was annealed overnight under vacuum at 160 °C, which is above the glass transition temperature (Tg) of about 120 °C11 for these PMMA thin films, to remove any residual toluene and to enable relaxation of the polymer molecules. The wafer (9) Brzoska, J. B.; Shahidzedeh, N.; Rondelez, F. Nature 1992, 360, 719. (10) Vig, J. R. J. Vac. Sci. Technol 1985, A3, 1027. (11) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Faraday Discuss. 1994, 98, 219.

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was then cooled to room temperature while still under vacuum and measured on the ellipsometer to determine the final film thickness and uniformity. Ellipsometry. Ellipsometry measures the change in the polarization state of light before and after the beam is reflected from a surface or an interface.12,13 The technique however, does not directly measure thickness or optical constants, instead it measures two ellipsometric angles: Ψ (related to the amplitude attenuation) and ∆ (related to the phase difference before and after reflection). These angles are related to the ratio of the Fresnel reflection coefficients: Rp (within the plane of reflection) and Rs (normal to the plane of reflection)

Rp/Rs ) tan Ψ exp(i∆)

(1)

The polarization changes are very sensitive to thin films or a layer of adsorbed molecules at an interface. Structural information is extracted by fitting the data to a model. Figure 1a shows the model used to analyze data from the polymer-solution interface. The Cauchy dispersion relation is a power series that describes the dependence of refractive index (n) on wavelength (λ):

n(λ) ) A +

B C + + ... λ2 λ4

(2)

where A, B, and C are the fitted parameters. This relation applies to any transparent dielectric substance, such as polymers or solvents. The value of n for the bulk PMMA is 1.46 at λ ) 400 nm, and it varies little with λ. It was found that it was sufficient to use only the first two terms in the Cauchy equation and B was close to 0.01. The best fit to the data is obtained by minimizing the mean square error (MSE), defined by

MSE )

1

N

∑

2N - M i)1

[( (

) )]

Ψimod - Ψiexp σΨ,iexp

∆imod - ∆iexp σ∆,i

exp

2

+

2

)

1 2N - M

χ2 (3)

The MSE is the sum of squares of the differences between experimental and modeled data (denoted by subscripts to “exp” and “mod”), where each difference is weighted by the standard deviation (σ) of that measured data point. MSE also depends on the number of parameters that the model fits (M) and the number of (Ψ, ∆) pairs (N). Ideal fits give χ2 of 1. In our work, an MSE less than 5 constitutes a good fit for the values of N, M, and σ typically used. The instrument used in this study was a variable angle spectroscopic ellipsometer (VASE) with a rotating analyzer and software written in Microsoft Windows format (J A Woollam Co. Inc., USA). In comparison with the null ellipsometer operating at a single wavelength and angle, spectroscopic ellipsometry enables measurements at or near the pseudo-Brewster angle of the silicon substrate where the technique is most sensitive to the interfacial profile. By having N much greater than M, the variables (12) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarised Light; North-Holland, Amsterdam, 1977. (13) Styrkas, D.; Doran, S. J.; Gilchrist, V. A.; Keddie, J. L.; Lu, J. R.; Murphy, E.; Sackin, R.; Su, T. J.; Tzitzinou, A. In Polymer Surfaces and Interfaces III; Richards, R. W., Peace, S. K., Eds.; Wiley: Chichester, 1999; p 1.

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Figure 1. Schematic diagrams showing (a) the multilayer model used for data fitting and (b) the solid-liquid sample cell used in the ellipsometric measurement.

are overdetermined and the confidence in determining the fitted parameters is enhanced. For measurements at the solid-water interface, a specially constructed cell with glass windows at both ends fixed at an angle of 75° was used, as shown in Figure 1b. Stress-free windows (approximately 0.14 mm thick) were used in order to reduce birefringence effects that can adversely affect the state of polarization of the probe beam. The use of the delta offset function after data fitting further minimizes these effects, as it takes into account a phase shift introduced by the windows in the sample cell. All experiments were performed at 22 ( 0.5 °C. Results and Discussion (A) PMMA Film Deposition. The thickness and the extent of the uniformity of the thin film at the silicasolution interface limit the resolution of the measurement of the adsorbed surfactant layer. If the coated film is too thick, the presence of an adsorbed layer only causes a small change in the data, and if adsorption is weak, the measurement will be insensitive. On the other hand, if the film is very thin, it is difficult to control its uniformity and ensure its continuity. An uneven surface can affect the distribution of the adsorbed surfactant layer as well as the adsorbed amount.14 The uniformity of ultrathin (14) Fragneto, G.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1996, 12, 477.

polymer films is dictated by the quality of the substrate surface and the coating conditions, both of which have been characterized in this work and will be outlined in the following. Silicon has been widely used as substrate for the coating of thin polymer films using different methods because it can be polished to give a very smooth surface. An extra advantage in using silicon substrate is that its high refractive index causes a large optical contrast with an organic layer. The silicon substrate has a layer of native oxide on the surface, and its uniformity can be determined by measuring the thickness of the layer at different positions using spectroscopic ellipsometry, while the optical constants are taken to be the same as those for bulk silicon oxide.13,14 Silicon wafers of (110) and (111) orientations were examined. In the (110) orientation, the variation in oxide thickness ranged from 25 to 50 Å on a given wafer over a typical area of 2 cm × 3 cm. The thickness variation appeared to have no particular pattern with respect to the location on the wafer. Similar measurements on wafers of (111) orientation were also made with thicknesses found to be 25 ( 5 Å, showing that the oxide layer was more uniform. Thus, all subsequent measurements were made using (111) silicon. During the course of film deposition by spin casting, we found that the following factors affect the thickness and uniformity of the polymer film: the type of solvent, the concentration of solution, the spin speed and rate of

C12E5 Adsorption at the PMMA-Water Interface

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Figure 2. Spectra of Ψ (a) and ∆ (b) obtained from a PMMA film on silicon. The incidence angles are 70° (9), 75° (b), and 80° (2). The profiles shown in (c) and (d) are Ψ and ∆ measured at the PMMA-water interface at 75° only (2) as compared with those from the oxide-water interface (b). In all cases, the continuous lines are the best fits, corresponding to a 48 Å film with bulk optical constants.

acceleration, and the annealing temperature. It is important to select a good solvent for the polymer, which completely dissolves the PMMA, as partial solubilization can lead to a nonuniform coating. The solubility of PMMA in several pure and mixed solvents was tested and the effect of these solvents on the film quality was subsequently characterized. Although PMMA has reasonably good solubility in solvents such as chloroform and a 1:1 mixture of hexane and 2-propanol (IPA), the final film produced was opaque and nonuniform. The uneven features were clearly noticeable to the naked eye, even for the films with average thicknesses under 100 Å. Both toluene and ethyl acetate were found to give uniform coating and reproducible film thickness over a wide concentration range. However, toluene was eventually chosen because it is more widely used as a carrier solvent for polymers. The spin speed and acceleration were characterized using PMMA dissolved in toluene with concentrations of 1-4 g/L. The optimum spin speed was found to be around 2000 rpm with a fast acceleration rate. The thickness of the film was found to decrease with the increase of the speed, but using higher spin speeds reduced the uniformity of the film, possibly because the polymer chains experienced greater stresses. Investigation of the effect of spin time on films at 2000 rpm with fast acceleration indicated that an optimum spin time of 20 s was sufficient to ensure uniform film formation with no wet residue remaining. Times above 20 s showed no change in film thickness and uniformity. The conditions for annealing have also been explored. It was observed that if the films were annealed overnight at 160 °C or above they would remain resistant to swelling in water. However, if annealing was made at lower temperature and for a shorter time, the films were found to swell when

immersed in aqueous solution. A similar observation has been reported by Sutandar et al.15 (B) PMMA Film Characterization. Ellipsometric measurements were first made on different PMMA films at the air-solid interface to ascertain their average thickness in the absence of water. Parts a and b of Figure 2 show the profiles of Ψ and ∆, respectively, at incidence angles of 70°, 75°, and 80° plotted against the wavelength over the range of 4000-7000 Å. Measurement at the solidwater interface at 75° only is shown in parts c and d of Figure 2. The corresponding ellipsometric profiles for the bare substrate, measured in water prior to polymer coating, are also plotted in parts c and d of Figure 2 for comparison. The differences in Ψ and ∆, shown in parts c and d of Figure 2, are quite large and are well beyond the standard deviation of Ψ and ∆. For thin films of less than 300 Å, strong coupling exists between the layer thickness (τ) and refractive index, making it difficult to determine their values independently. However, as already indicated previously, if the refractive indices and the Cauchy parameters are taken to be the same as those of the bulk polymer, the thickness of the layer can be determined. This assumption should be reasonable because in a recent neutron reflection study on the PMMA films coated in a similar manner we found that both the oxide and the PMMA film have their layer densities close to those of the corresponding bulk materials.8 As the oxide layer was already characterized before coating, the thickness of the PMMA layer could then be determined straightforwardly, and the value was found to be 48 ( 3 Å. Figure 2 shows that the calculated profiles (continuous (15) Sutandar, P.; Ahn, D. J.; Frances, E. I. Macromolecules 1994, 27, 7316.

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Figure 3. AFM images in (a) tapping mode and (b) phase mode obtained from the surface of a PMMA film with thickness around 48 Å.

lines) fit the measured profiles well, with a low MSE of 1.5. However, if both the thickness and refractive index were fitted, the thickness was found to vary between 45 and 55 Å, which is still within the acceptable range. If it is assumed that the layer is somewhat rough, or contains air-voids, the optical constants will be lower than those of the bulk PMMA. The thickness of the polymer film was subsequently fitted with various values of optical constants, and it was found that when they were decreased by 10%, a noticeable deviation occurred between the measured and fitted profiles, which was reflected in a larger MSE. This suggests that the density of the layer is close to its bulk value. The good fit of a uniform layer model to the data shows that the surface of the film is smooth. To further test this observation, atomic force microscopy (AFM) (Digital Instruments, Nanoscope III) was used to visualize the same film surface. Figure 3 shows two AFM images, one produced in tapping mode (Figure 3a) that reveals the topography of the film and the other in phase mode (Figure 3b) that is sensitive to film viscoelasticity. Both scans show no prominent features, indicating that the coated film is homogeneous and uniform. Contact angle measurement offers information about the physical nature of the film surface. If voids or pinholes exist to allow any fraction of the silicon oxide to be exposed to air, then the contact angle is expected to be lower than that of the bulk PMMA surface. We have measured the static advancing contact angles for thin PMMA films with the thicknesses of 50 ( 3, 150 ( 3, and 500 ( 5 Å and in all cases, the values were found to be 76° ( 3°, in good

agreement with the literature value of some 80°.16 That the contact angle does not vary with layer thickness clearly shows that the surface is fully covered by PMMA, even in the thinnest film. The structure of the PMMA films was further characterized by spectroscopic ellipsometry at the solid-water interface. It is possible that the film might swell and detach as a result of poor coating. Figure 4 shows a comparison between the measured profiles after only 5 min and after 8 h of immersion in degassed water, respectively. The reason for the use of degassed solutions was that air bubbles otherwise accumulate on the film surface after several hours, making the data invalid. It can be seen from these measurements that the two sets of data are virtually identical in Ψ and ∆ (Figure 4a). The difference in Ψ and ∆ between the two runs is shown in Figure 4b. The maximum variation in Ψ is (0.09° and (2° for ∆. The difference fluctuates about zero and there is not a systematic shift in Ψ or ∆, showing that swelling has not occurred. This level of variation is comparable to that found in repeated runs, suggesting that the film is robust. Fitting of the measured data with the refractive indices taken to be the same as bulk PMMA showed that the film thickness remained unchanged at around 48 Å. Different PMMA films, with thicknesses ranging from 100 to 700 Å, were also investigated for possible swelling in water over an 8 h period under the same experimental conditions. It was found that in each case the thickness remained unchanged. Any variation was again within a few angstrom and was completely random, further con(16) Johnson, B. A.; Kreuter, J.; Zografi, G. J. Colloid Interface Sci. 1986, 17, 325.

C12E5 Adsorption at the PMMA-Water Interface

Figure 4. Spectra of Ψ and ∆ (a) obtained from the solid PMMA-water interface after the sample was immersed in water for 5 min (continuous line for Ψ and broken line for ∆) and after 8 h (9 for Ψ and 2 for ∆). (b) The difference between the two sprectra are plotted (continuous line for Ψ and broken line for ∆).

Figure 5. Dynamic scan of 50 Å PMMA film performed at λ ) 4400 Å and an angle of incidence of 75° (4 for ∆ and O for Ψ). The measurement was started at the air-solid interface. After 5 min C12E5 solution at the cmc was added. The measurement proceeded at the solid-water interface for a further 45 min. Time interval between data points is 3 s.

firming the result obtained with the thinnest film. In addition, the MSE values generated remained well under 2 for each coating, indicating a good fit between the experimental and simulated data. (C) Adsorption Kinetics and Reversibility. The adsorption of C12E5 onto the surface of PMMA might be slow, particularly if surfactant penetrates the polymer film. Time-dependent adsorption can be monitored by performing a dynamic scan at a fixed wavelength. Figure 5 shows the plot of Ψ and ∆ at λ ) 4400 Å obtained from a 48 Å PMMA film. The measurement was started at time zero when the data were obtained at the air-solid interface. Measurements of a (Ψ,∆) pair were made at 3 s intervals. C12E5 solution at its critical micelle concentration (cmc) (6 × 10-5 M) was then added, and Ψ and ∆ were observed to drop sharply as a result of the addition of the

Langmuir, Vol. 16, No. 2, 2000 745

solution. However, Ψ and ∆ no longer varied after this initial dip, suggesting that within the time scale of several scans there was no time-dependent adsorption. Similarly, we find very little difference between two replicate scans of C12E5 on a 50 Å PMMA film using spectroscopic ellipsometry when the sample cell is drained and then refilled with fresh degassed C12E5 at cmc. The values of Ψ and ∆ again vary only by (0.09° and (2°, respectively, which is of the same magnitude of the standard deviation as that observed in the repeated runs at the PMMA-water interface. If we compare the data before (in water) and then with surfactant for the same film as shown in Figure 6a, we can see a clear difference in Ψ and ∆. It is observed from Figure 6b that changes in Ψ are up to about 1°, whereas those for ∆ rise to about 8°. Both of these changes are well beyond the fluctuations about zero that have been recorded between replicate trials, indicating that the adsorption has indeed taken place. The reversibility of the adsorption was examined by thoroughly rinsing the sample cell and polymer film with pure water via a water pump and injection system so that neither the sample nor the cell was moved. This procedure allowed the exact same position on the sample to be measured repeatedly. The ellipsometric spectra at the pure water-solid interface were subsequently analyzed. These profiles were compared with those determined before the surface was in contact with C12E5, as shown in Figure 6c. Again, no visible difference was seen in either Ψ or ∆, showing that the adsorption was completely reversible and that no surfactant had penetrated into the PMMA film. In a recent study on the interaction of the same surfactant with the thin films formed by poly(butyl methacrylate) (PBMA), we have observed the penetration of the surfactant into PBMA films which causes change in the structure of the polymer matrix and irreversible desorption of the surfactant. The difference in Ψ and ∆ between these runs is shown in Figure 6d. There is no systematic variation in Ψ and ∆ and the fluctuations are about zero. The observation is entirely consistent with that from the consecutive water runs described previously, thus confirming that the structure of the PMMA film remains intact during surfactant adsorption. PMMA films with thickness ranging from 50 to 500 Å were also investigated for swelling effects in C12E5 solution at its cmc over a period of 8 h. Once again, the thicknesses remained unchanged; any variation was within the expected error range. Adsorption was found to reach equilibrium within a few seconds. (D) Surfactant Adsorption. The amount of surfactant adsorbed onto the PMMA surface has been determined over a range of C12E5 concentrations around its cmc. The variation of Ψ and ∆ with the adsorption of surfactant on the surface of a 50 Å PMMA film is shown in Figure 7. Clearly, the profiles measured at 2 cmc show the greatest difference from those at the solid polymer-pure water interface, reflecting the highest adsorption of all the concentrations measured. As surfactant concentration decreases, both Ψ and ∆ move toward the values obtained from pure water, showing that the extent of adsorption steadily decreases. It can be seen that over the measured wavelength range of 4000-7000 Å, Ψ shows a large variation over wavelengths between 4000 and 5000 Å, with the largest sensitivity at approximately 4400 Å, while ∆ shows a smooth variation over a wide range of wavelength. This result reflects the fact that for λ ) 4400 Å, the pseudo-Brewster angle for this sample is at 75°. From the results shown previously, we can assume that surfactant adsorption has no effect on the oxide and PMMA

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Figure 6. Spectra from a 48 Å thick PMMA film showing (a) a comparison of the scan obtained when in water (continuous line for Ψ and broken line for ∆) and in C12E5 at cmc (9 for Ψ and 2 for ∆); (b) the difference between the two scans in (a); (c) a comparison between water scans, one from the beginning and the other from the end after the surfactant has been removed; and (d) the difference between the two water scans in (c).

Figure 7. Spectra of (a) Ψ and (b) ∆ obtained from the solid PMMA-water interface measured at the 2 cmc (2), 1/5 cmc (b), 1/ 50 cmc (9). The continuous lines are the best fits.

layers. This assumption must be reliable as the results shown in Figures 4, 5, and 6 clearly indicate that the underlying layers are smooth, and surfactant adsorption does not affect the physical structure of the polymer film.

We have attempted to fit the structure of the C12E5 layer at the cmc by fitting both the layer thickness and the Cauchy parameters. This gave a value of 25 ( 5 Å for the thickness and 1.39 for the refractive index at λ ) 4400 Å. The same approach was also used for fitting the profiles measured at 2 cmc and almost identical values of τ and n were obtained, hence suggesting a constant level of adsorption when the surfactant concentration is above the cmc. In a previous measurement using neutron reflection,8 the thickness of the C12E5 at the PMMA-water interface was found to be 20 ( 3 Å. In view of the greater uncertainty involved in the thickness derived from the ellipsometric data, the agreement is reasonable. At lower concentrations, e.g., 1/20 cmc, the fitting became unstable. Under this condition, the fitting was carried out by taking the thickness of the layer to be held constant at the values obtained from neutron reflection.8 The results from neutron reflection show that as the adsorbed amount decreases, the thickness of the surfactant layer also decreases as a result of more free space available in the surface layer. It should be stressed that it is difficult to assess the reliability of the thicknesses obtained from the ellipsometric data in the absence of the results from neutron reflection. Reliable information about the structural conformation of the adsorbed layer could only be obtained by neutron reflection in combination with the use of deuterium labeled surfactants. We have shown8 that at the cmc the dodecyl chain region is only 5 ( 2 Å thick, suggesting that the alkyl chains lie virtually horizontal along the PMMA surface, while in comparison, the E5 headgroups are about 15 ( 3 Å, suggesting that the ethoxylate groups are extended into the aqueous solution. The overall dimension of the surfactant layers obtained from solutions around the cmc is consistent with the observation from ellipsometry measurement.

C12E5 Adsorption at the PMMA-Water Interface

Langmuir, Vol. 16, No. 2, 2000 747

Table 1. Surface Excesses ×10-6 mol/m2 Obtained from Different Methodsa SE concentration

50 Å thick

150 Å thick

cmc 5 cmc 20 cmc 1/ 50 cmc

3.3 3.1 2.5 1.3

3.4

1/ 1/

NR

DM

3.3 3.1 2.6 1.3

4.4 4.2 1.3 0.9

a SE denotes spectroscopic ellipsometry; NR denotes neutron reflection; DM denotes depletion measurement using particulate dispersions by Gau et al.6

Although ellipsometry is insensitive to the thickness of the adsorbed surfactant layer, it can determine surface excess reliably. De Feijter et al.17 have shown that for an adsorbed layer of uniform composition at a finite thickness with detectable difference between refractive indexes on either side of the interface, surface excess (Γads) of the adsorbed layer can be obtained by

Γads ) τ(n1 - n0)/a

(4)

where n1 and n0 are the refractive indices for the adsorbed layer and ambient aqueous solution. The parameter a is the refractive index increment, which is equal to dn/dc of the aqueous solution, where c is its bulk concentration. The area per molecule (A) can then be converted from the surface excess by

A ) 1/NaΓads

(5)

where Na is Avogadro’s number. It should be noted that Γads is relatively unaffected by correlation between τ and n, making it more reliable than measurement of τ or n alone. The adsorption of nonionic surfactants, such as the alkyl ethoxylates at the hydrophilic silicon oxide-water interface, has been extensively studied by Tiberg et al.18-22 In studying the adsorption of C12E5 at the solid-water interface, they use a value of the refractive index increment of 0.131 cm3/g. With this value of a, and the measured parameters, the surface excess can be calculated. The variation of Γads, with bulk concentration is given in Table 1. The adsorption of C12E5 was also studied on a PMMA film with a thickness of 150 ( 3 Å. As already explained previously, when the PMMA film is thicker, the difference in the spectra before and after surfactant adsorption becomes relatively small. Hence the number of revolutions of the rotating analyzer was increased from 40 to 120 to minimize the fluctuations in Ψ and ∆. The quality of the data can be examined by comparing the replicate scans. There is only a small fluctuation in Ψ ((0.06°) and ∆ ((0.3°), showing that the statistical error has been improved. By comparison of the data before and after surfactant addition, a clear difference between the two scans is observed once again, with changes recorded in Ψ and ∆ of up to 0.35° and 1.8°, respectively. Again, both of (17) De Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759. (18) Tiberg, F.; Landgren, M. Langmuir 1992, 9, 927. (19) Tiberg, F.; Jonsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294. (20) Tiberg, F.; Lindman, B.; Landgren, M. Thin Solid Films 1993, 234, 478. (21) Tiberg, F.; Brinck, J. In Surfactant Adsorption and Surface Solubilisation; Sharma, R., Ed.; ACS Symposium Ser. 615; American Chemical Society: Washington, DC, 1995; p 231. (22) Tiberg, F. J. Chem. Soc., Faraday Trans. 1996, 92, 531.

Figure 8. Spectra of (a) Ψ and (b) ∆ obtained from the solid PMMA-water interface for a 150 Å PMMA film showing the difference between water (b) and C12E5 at the cmc (2). The continuous and broken lines are the best fits for C12E5 and water, respectively.

Figure 9. Comparison of surface excess at the solid PMMAwater interface using spectroscopic ellipsometry (2), specular neutron reflection (9), and the depletion method using particulate dispersion by Gau et al. ([). The solid lines are a guide to the eye.

these changes are beyond the fluctuations between repeated runs. The measured spectra are shown in Figure 8. Fitting to Ψ and ∆ gives the value of τ of 22 ( 5 Å and n of 1.40 at λ ) 4400 Å, resulting in an almost identical surface excess found with thin and thick PMMA films. Figure 9 compares the surface excess from neutron reflection8 with the data from spectroscopic ellipsometry. Around the cmc the neutron and ellipsometry data suggest a trend of a slight increase with bulk concentration, but

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the difference is within the experimental error. As already indicated above, we have also performed measurement at 2 cmc (not shown in Figure 9) and the surface excess is identical to that at the cmc, showing that the adsorption has indeed reached its saturation limit and hence monolayer formation. The same system has also been studied by Gau et al.6 using PMMA latex particles. In the work by Gau et al., surface excess was calculated from the change in solution concentration before and after the solution was mixed with the PMMA particles. It can be seen in Figure 9 that while the surface excesses from both neutron reflection and spectroscopic ellipsometry are nearly overlapping over the whole concentration range, the agreement with the profile obtained by Gau et al. is very poor. At the lowest concentration of 1 × 10-5 M the values from different measurements are very close. As the concentration increases to about 2 × 10-5 M, the surface excesses from neutron reflection and spectroscopic ellipsometry are greater. As the concentration is further increased, the profiles from neutron reflection and ellipsometry gradually tend to plateau, while the surface excess by Gau et al. shows a sharp upturn. At the cmc of about 6 × 10-5 M, the surface excess from neutron reflection and ellipsometry is approximately 3.3 × 10-6 mol m-2, which corresponds to an area per molecule of 50 Å2. However, the limiting surface excess from the work of Gau et al. is about 4.3 × 10-6 mol m-2, equivalent to an area per molecule of 38 Å2. We have studied the adsorption of a number of nonionic surfactants at the air-water interface using neutron reflection23,24 and found that at the cmc the area per molecule is 37 ( 3 Å2 for C12E3, 44 ( 3 Å2 for C12E4, and 48 ( 3 Å2 for C12E5. We have also studied the adsorption of C12E4 at the hydrophobic silicawater interface and found that the area per molecule was 51 ( 3 Å2 at the cmc. (The hydrophobic surface was formed by coating a self-assembled monolayer of octadecyltrichlorosilane OTS).14) Thus, within the quoted experimental error, the area per molecule is 50 ( 3 Å2 for C12E4 and C12E5 at different interfaces. The surface excess of 38 ( 3 Å2 for C12E5 at the cmc obtained by Gau et al. is almost the same as that for C12E3 and appears to be too small. The adsorption and subsequent modification of the wettability of the solid substrate by surfactant have been extensively studied in the literature. Pyter et al.25 and others26-28 have shown that for the adsorption of surfactant onto very nonpolar surfaces such as paraffin and polyethylene from aqueous solution, the surface excess of the surfactant is within a good approximation equal to its surface excess at the air-solution interface. As already shown previously, the contact angle of PMMA surface is about 76° and is lower than that of about 111° for the OTS surface,29 suggesting that the PMMA surface is more polar. The results from both spectroscopic ellipsometry and (23) Lu, J. R.; Thomas, R. K. Mater. Res. Soc. Symp Proc. 1996, 376, 235. (24) Lu, J. R.; Su, T. J.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Tucker, I.; Penfold, J. J. Phys. Chem. B 1997, 101, 10332. (25) Pyter, R. A.; Zografi, G.; Mukerjee, P. J. Colloid Interface Sci. 1992, 89, 144. (26) Lucassen-Reynders, E. H. J. Phys. Chem. 1963, 67, 969. (27) Bargeman, D.; van Voorst. V. F. J. Colloid Interface Sci. 1973, 43, 467. (28) Wolfram, E. Kolloid-Z. 1966, 211, 84. (29) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136.

Gilchrist et al.

neutron reflection show that the surface excesses are the same and are not affected by the variation of surface hydrophobicity. This observation is however inconsistent with the results of Gau et al.6 that predict that the surface excess at the solid polymer-water interface decreases with the increased polarity of the polymeric solid substrate. The discrepancy may arise from the difference in surface properties between the coated thin PMMA film and the particles used in the work of Gau et al. But it is also likely to be caused by the assumption of zero surface excess for the surfactant at the air-solid interface, inaccurate measurements of the contact angle, the amount of the surfactant adsorbed onto the particulate dispersions as shown in Figure 9, or a combination of some of these factors. Conclusions Many studies have been made on the adsorption of surfactants onto inorganic materials such as bare silicon oxide,18-22 but there are few direct measurements on the adsorption of surfactants onto polymeric surfaces, although the latter is of much wider practical and fundamental interest. The essential step for performing the measurement at the solid polymer-water interface is to coat a thin layer of the polymeric film onto a smooth solid substrate. To obtain reliable information about the structure of the adsorbed surfactant layer, the film must be uniform and its density must be close to its bulk material. We have shown that by use of a set of defined coating conditions, the thickness and uniformity of the thin film can be manipulated. A high degree of smoothness of the film surface is suggested in our ellipsometry data and also found in the AFM analysis. Measurements of C12E5 adsorption have shown that the adsorption is completely reversible and that the film remains intact during the course of the experiment. The robustness of the film has enabled us to use a single film for a set of measurements under different solution conditions. Thus, the possible effect caused by the subtle structural differences between polymer surfaces is eliminated. Adsorption measurements on PMMA films of two different thicknesses offer identical surface excesses for the surfactant, suggesting that the thinner PMMA film has the same surface properties as the thicker one. The thickness of about 25 Å for the surfactant layer around the cmc indicates the monolayer adsorption and is in broad agreement with the previous measurement from neutron reflection. Despite its uncertainty in probing the thicknesses for the untrathin layers, spectroscopic ellipsometry offers almost identical surface excesses to those from neutron reflection over almost the entire surfactant concentration range studied, showing that spectroscopic ellipsometry can complement neutron reflection. Acknowledgment. We thank the Engineering and Physical Research Council (EPSRC) for support. V.G. thanks Unilever Research Port Sunlight for partial provision of her studentship. We would also like to thank A. J. Hook for the construction of the solid-liquid cell, T. J. Su for purifying C12E5, and A. Tzitzinou for the AFM analysis. LA9906572