The Effect of Water or Salt Solution on Thin Hydrophobic Films

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The Effect of Water or Salt Solution on Thin Hydrophobic Films L. B. R. Castro, A. T. Almeida, and D. F. S. Petri* Instituto de Quı´mica, Universidade de Sa˜ o Paulo, P.O. Box 26077, Sa˜ o Paulo, SP 05513-970, Brazil Received January 19, 2004. In Final Form: June 18, 2004 Hydrophobic films of polystyrene synthesized in bulk (PS) and by emulsion polymerization in the presence of the cationic surfactant cetyltrimethylammonium bromide (PS-CTAB) or the anionic surfactant sodium dodecyl sulfate (PS-SDS) were characterized by means of ellipsometry, contact angle measurements, and atomic force microscopy. Thin (∼65 nm) and thick (∼300 nm) films were spin-coated on hydrophilic silicon wafers. PS films presented scarcely tiny holes, while PS-CTAB and PS-SDS films presented holes and protuberances. The former were attributed to dewetting effects and the latter to surfactant clusters. The films were exposed to water or to a 0.1 mol/L NaCl solution for 24 h. Ex situ measurements evidenced strong topographic alterations after the exposure to the fluid. A model based on the diffusion of water (or electrolyte) molecules to the polymer/silcon dioxide interface through holes or defects on the film edges was proposed to explain the appearance of wrinkles and protuberances. In situ ellipsometric measurements were performed and compared with simulations, which considered either a water layer between a polymer and a silcon dioxide layer or an air layer between a polymer and water (medium). In the case of thin PS films, the ellipsometric angles evidenced a very thin (0.5-1.0 nm) air layer between water and the PS films. Upon increasing the PS film thickness, no air layer could be observed by ellipsometry. Regardless of the thickness, the ellipsometric data obtained for PS-CTAB and PS-SDS films did not indicate the presence of an air layer between them and the aqueous media. The dramatic changes in the topography of PS, PS-CTAB, and PS-SDS after immersion in salt solution were explained with proposed models. From a practical point of view, this study is particularly relevant because many hydrophobic polymers are used as substrates for biomedical purposes, where the physiological ionic strength is 0.15 mol/L NaCl.

Introduction Thin polymer films are widely used in microelectronics1 and as support layers for the development of biotechnology devices.2 The characteristics and behavior of thin films are rather different from those of bulk polymer. Confining polymer chains in spaces smaller or comparable to their radius of gyration causes large changes in their physical properties.3 Chain mobility is enhanced as the film thickness decreases, affecting the film stability.4,5 When the polymer films formed on solid substrates are annealed at temperatures higher than their glass temperatures, the films may become unstable and dewetting takes place.6,7 If the interactions between a substrate and a polymer are of the van der Waals type, the values of the Hammaker constant (A) for polymer/polymer and polymer/ substrate indicate the film stability. If Apolymer/polymer is higher than Apolymer/substrate, dewetting will be favored.6,7 While such instabilities have been extensively investigated,8-12 thin film instabilities due to the presence of * Corresponding author. Phone: 0055 11 3091 3831. Fax: 0055 11 3818 5579. E-mail: [email protected]. (1) Sheats, J. R.; Smith, B. W. Microlithography; Marcel Dekker: New York, 1998. (2) Eggs, B. R. Biosensors: an introduction; John Wiley & Sons: Chichester, U.K., 1996. (3) Binder, K. Annu. Rev. Phys. Chem. 1992, 43, 33. (4) Baschnagel, J.; Mischler, C.; Binder, K. J. Phys. IV 2000, 10, 9. (5) Rivillon, S.; Auroy, P.; Deloche, B. Phys. Rev. Lett. 2000, 84, 499. (6) Reiter, G. Langmuir 1993, 9, 1344. (7) Brochard-Wyart, F. C. R. Acad. Sci. Paris 1992, 314(II), 19. (8) Wunnicke, O.; Mu¨ller-Buschbaum, P.; Wolkenbauer, M.; LorenzHaas, C.; Cubbit, R.; Leiner, V.; Stamm, M. Langmuir 2003, 19, 8511. (9) Mu¨ller-Buschbaum, P.; Wolkenbauer, M.; Wunnicke, O.; Stamm, M.; Cubbit, R.; Petry, W. Langmuir 2001, 17, 5567. (10) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Stamm, M. Macromolecules 2000, 33, 4895. (11) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Lorenz-Haas, C.; Wunnicke, O.; Stamm, M.; Petry, W. Macromolecules 2002, 35, 2017.

water or salt solution are seldom investigated. The relevance of this subject lies in the fact that polymer based biosensors are frequently applied for physiological samples, which are composed of water, electrolytes, and the specific analyte. Particularly, polystyrene (PS) films are suitable substrates for cell growth,13 collagen adsorption,14 and phospholipid deposition.15 Recently, Elliott and co-workers15 observed that the topography of PS films has been strongly altered upon dipping them into aqueous solution. They point out that, if the topological properties of the film are important for a specific biological application, a rigorous control on the film characteristics before and after exposure to aqueous medium must be carried out. On the other hand, neutron reflectivity experiments on the water/ thin deuterated PS film interface evidenced the presence of a thin layer (2-5 nm) of nanobubbles at the polymer surface.16 Such nanobubbles were also observed by in situ atomic force microscopy (AFM) not only when the substrate was PS16 but also when the substrate was hydrophobic glass17 or gold.18 These findings are very important for the development of biosensors because specific molecules should be immobilized on the polymeric film without any hindrance as bubbles. However, it is not conclusive if the presence of gas nanobubbles at the water/PS interface (12) Lee, L.-T.; Silva, M. C. V.; Galembeck, F. Langmuir 2003, 19, 6717. (13) Walboomers, X. F.; Monaghan, W.; Curtis, A. S.; Jansen, J. A. J. Biomed. Mater. Res. 1999, 46, 212. (14) Dupont-Gillain, C. C.; Rouxhet, P. G. Langmuir 2001, 17, 7261. (15) Elliott, J. T.; Burden, D. L.; Woodward, J. T.; Sehgal, A.; Douglas, J. F. Langmuir 2003, 19, 2275. (16) Steitz, R.; Gutberlet, T.; Hauss, T.; Klo¨sgen, B.; Krastev, R.; Schemmel, S.; Simonsen, A. C.; Findenegg, G. H. Langmuir 2003, 19, 2409. (17) Tirrel, J. W. G.; Attard, P. Langmuir 2002, 18, 160. (18) Holmberg, M.; Ku¨hle, A.; Garnæs, J.; Mørch, K. A.; Boisen, A. Langmuir 2003, 19, 10510.

10.1021/la049828f CCC: $27.50 © 2004 American Chemical Society Published on Web 07/29/2004

Effect of Water or Salt Solution on PS Films

has any relation to the topographic changes observed after drying the PS films and how the polymer film thickness affects the gas layer. The thin and thick PS films were spin-coated PS films on silicon wafers. The present study is focused on the effect of water or a 0.1 mol/L NaCl solution on the film morphology by means of ellipsometry, scanning probe microscopy, and contact angle measurements. An ionic strength of 0.1 mol/L NaCl was chosen because it is close to the physiological one (0.15 mol/L). PS was synthesized in bulk and by emulsion polymerization in the presence of the cationic surfactant cetyltrimethylammonium bromide or the anionic surfactant sodium dodecyl sulfate to investigate the effect of residual surfactant molecules on the films’ characteristics and stability.

Langmuir, Vol. 20, No. 18, 2004 7611 Table 1. Characteristics of Spin-Coated Films Determined in the Aira polymer PS PS-CTAB PS-SDS

thickness (nm)

n632.8

θA (deg)

θR (deg)

63 ( 1 293 ( 5 65 ( 2 267 ( 3 63 ( 2 339 ( 1

1.595 ( 0.005 1.589 ( 0.005 1.610 ( 0.005 1.590b 1.592 ( 0.007 1.590b

90 ( 1 88 ( 2 76 ( 3 70 ( 2 77 ( 2 75 ( 2

88 ( 2 85 ( 2 69 ( 3 57 ( 7 69 ( 2 66 ( 2

∆θ rms (deg) (nm) 2 3 7 13 8 9

0.31 0.46 0.60 0.40 0.43 0.56

a The data correspond to mean values of measurements performed with at least four different samples just after preparation. b When the independent determination of n and d was not possible, n was kept as 1.59024,25 and d was calculated.

Experimental Section Film Preparation. Polystyrene (PS), 99.5% purity, was synthesized by bulk radical polymerization and kindly supplied by BASF, Ludwigshafen, Germany. PS particles were synthesized by means of a standard recipe19 of emulsion polymerization using K2S2O8 as the initiator. Either sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB) was used as the surfactant. The content of residual surfactant was estimated by elemental analysis as 2 wt %. The resulting particles were named PS-SDS and PS-CTAB, respectively. The latex dispersions were dialyzed against water with four changes daily for 1 week, or until the conductivity of the dialysis water reached 5 µS/cm. Dialyzed dispersions of PS-SDS and PS-CTAB were dried at 60 °C for 48 h. PS, PS-SDS, or PS-CTAB in the form of dried powder was dissolved in toluene at a concentration of 10 or 50 g/L. Polymeric films were obtained by spin-coating on silcon wafers with a Headway PWM32-PS-R790 spinner (Garland, TX). The wafers with dimensions of 1.0 cm × 1.0 cm were previously rinsed in a standard manner20 and dried under a stream of N2. All coatings were performed with a spinning velocity of 3000 rpm and a spinning time of 30 s. The effect of water or aqueous salt solution on the film characteristics and stability was investigated by means of ex situ and in situ experiments at 24 ( 1 °C. Distilled water with a pH of 6.3 and a conductivity of 2.0 µS/cm and a 0.1 mol/L NaCl solution were used. For the ex situ experiments, the films were dipped into the fluid for 24 h, dried under a stream of N2, and characterized in the air. Spin-coated films present homogeneous thickness in the central region. Approaching the edges of the samples, the thickness turns higher due to centrifugal effects and small defects might be found. Therefore, the characterization was always performed in the center of the film. Ellipsometry.21 The mean thickness (d) and the index of refraction (n) of the dried films were determined with a DRE-X02C ellipsometer (Ratzeburg, Germany) equipped with a He-Ne laser (632.8 nm). In situ experiments were performed in a cell filled with distilled water or a 0.1 mol/L NaCl solution, as described elsewhere.22 Contact Angle Measurements. Contact angle measurements were performed in a home-built apparatus23 equipped with a Casio QV-10 digital camera, which is connected to a computer. Sessile water drops of 10 and 5 µL were used for the advancing (θA) and receding (θR) angle, respectively. The hysteresis in the contact angle (∆θ ) θA - θR) stems from surface roughness or surface chemical heterogeneities.23 Scanning Force Microscopy. Scanning force microscopy (SFM) measurements were performed in a Nanoscope IIIA (Veeco) microscope in the tapping mode in air at room temperature. The cantilevers operated slightly below their resonance frequency of ∼315 kHz. All topographic images represent unfiltered original (19) Gilbert, R. G. Emulsion Polymerization: a Mechanistic Approach; Academic Press: London, 1995. (20) Petri, D. F. S.; Wenz, G.; Schunk, P.; Schimmel, Th. Langmuir 1999, 15, 4520. (21) Azzam, R. M.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland Publication: Amsterdam, The Netherlands, 1979. (22) Fujimoto, J.; Petri, D. F. S. Langmuir 2001, 17, 56. (23) Adamson, W. A. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: Toronto, Canada, 1990.

Figure 1. Topographic images in the tapping mode in the air of PS films that are ∼63 nm thick: (a) just after preparation; (b) after immersion in water for 24 h and drying; (c) after immersion in 0.1 mol/L NaCl for 24 h, rinsing with distilled water, and drying. Scan areas of 5 µm × 5 µm. data and are displayed in a linear gray scale. At least two samples of the same material were analyzed at different areas of the surface. The root-mean-square (rms) roughness values were calculated for scan areas of 5 µm × 5 µm.

Results and Discussion The characteristics of spin-coated PS, PS-CTAB, and PS-SDS films are presented in Table 1. The thicknesses of the thin and thick films amounted approximately to 65 and 300 nm, respectively. The indices of refraction found are close to the literature values,24,25 indicating that the residual surfactant molecules do not affect the index of refraction, except for the thin PS-CTAB films, whose values were higher than the literature one. This effect is probably due to the presence of surfactant bromide counterions. The wettability and the hysteresis in the contact angle (∆θ) values found for PS-CTAB and PSSDS are higher than those measured for PS. The higher ∆θ values measured for PS-CTAB and PS-SDS might be a result of chemical heterogeneity on the surface and roughness. The AFM images show that PS films are smooth (Figures 1a and 2a), regardless of the thickness. There are scarcely tiny holes on the surface. However, the topographic images of the PS-CTAB (Figures 3a and 4a) films show the presence of small outgrowths and holes. Large holes are frequent on the thin PS-SDS films (Figure 5a). Frequent small holes and small protuberances are observed on the thick PS-SDS films (Figure 6a). Such features are confirmed by the root-mean-square (rms) roughness values in Table 1. The presence of holes on the films might be indicative of the initial stages of dewetting.6-12 However, this process did not evolve, since the hole dimensions remained stable over 2 months and no film disruption could be observed. Upon dissolving the dried PS-CTAB and PS-SDS particles in toluene, they lose their spherical form and (24) Siqueira, D. F.; Schubert, D. W.; Amato, J. P.; Erb, V.; Stamm, M. Colloid Polym. Sci. 1995, 273, 1041. (25) Siqueira-Petri, D. F.; Wenz, G.; Schunk, P.; Schimmel, Th.; Bruns, M.; Dichtl, M. A. Colloid Polym. Sci. 1999, 277, 673.

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Figure 2. Topographic images in the tapping mode in the air of PS films that are ∼293 nm thick: (a) just after preparation; (b) after immersion in water for 24 h and drying; (c) after immersion in 0.1 mol/L NaCl for 24 h, rinsing with distilled water, and drying. Scan areas of 5 µm × 5 µm.

Castro et al.

Figure 6. Topographic images in the tapping mode in the air of PS-SDS films that are ∼339 nm thick: (a) just after preparation; (b) after immersion in water for 24 h and drying; (c) after immersion in 0.1 mol/L NaCl for 24 h, rinsing with distilled water, and drying. Scan areas of 5 µm × 5 µm. Table 2. Characteristics of Spin-Coated Films Determined in the Aira polymer PS PS-CTAB PS-SDS

Figure 3. Topographic images in the tapping mode in the air of PS-CTAB films that are ∼65 nm thick: (a) just after preparation; (b) after immersion in water for 24 h and drying; (c) after immersion in 0.1 mol/L NaCl for 24 h, rinsing with distilled water, and drying. Scan areas of 5 µm × 5 µm.

Figure 4. Topographic images in the tapping mode in the air of PS-CTAB films that are ∼267 nm thick: (a) just after preparation; (b) after immersion in water for 24 h and drying; (c) after immersion in 0.1 mol/L NaCl for 24 h, rinsing with distilled water, and drying. Scan areas of 5 µm × 5 µm.

Figure 5. Topographic images in the tapping mode in the air of PS-SDS films that are ∼63 nm thick: (a) just after preparation; (b) after immersion in water for 24 h and drying; (c) after immersion in 0.1 mol/L NaCl for 24 h, rinsing with distilled water, and drying. Scan areas of 5 µm × 5 µm.

the residual surfactant molecules, which were just physically adsorbed onto the particles, can remain adsorbed on the hydrophobic PS chain or diffuse to the solvent. Since in toluene the thermodynamic conditions for the polar heads are not favorable, the surfactant molecules might form reverse micelles, if traces of water are present in the system. However, it is not expected because the critical micellar concentration is ∼100 times larger in nonaqueous solvents than in water,26 and the available amount of residual surfactant should be very low. These solutions (26) Langevin, D. Annu. Rev. Phys. Chem. 1992, 43, 341.

thickness (nm)

n632.8

θA (deg)

θR (deg)

62 ( 1 295 ( 5 64 ( 1 259 ( 2 61 ( 1 341 ( 2

1.567 ( 0.008 1.590 ( 0.005 1.610 ( 0.005 1.590b 1.590 ( 0.004 1.590b

82 ( 2 80 ( 2 71 ( 2 74 ( 2 70 ( 2 74 ( 2

64 ( 2 70 ( 1 47 ( 3 59 ( 3 44 ( 2 46 ( 4

∆θ rms (deg) (nm) 18 10 24 15 26 28

0.72 0.47 1.5 0.74 0.61 0.91

a The data correspond to mean values of measurements performed with at least four different samples after 24 h of being immersed in water. b When the independent determination of n and d was not possible, n was kept as 1.59024,25 and d was calculated.

were used to prepare spin-coated films on silicon wafers. In the first spinning moments, toluene begins to evaporate, increasing the concentration of the solution. As a consequence, the solution viscosity increases and the chain mobility decreases. The surfactant molecules, which remained adsorbed on the polymer, will tend to assume an orientation which minimizes the system free energy. The polar heads will be oriented either to the polar substrate or to the air. The orientation of the polar heads to the air could lead to the formation of ionic clusters, explaining the contact angle values and the observed clumps. Studies on latex films formed by drying the aqueous dispersion on hydrophilic substrates evidenced the migration of residual surfactant molecules to the film/ air interface surface.27-31 Table 2 shows the characteristics of polymeric films in the air after immersion into water for 24 h and drying. The thickness and index of refraction values were not altered; however, the hysteresis in the contact angles increased. With the exception of the thick PS films, all films presented higher rms values, corroborating with the increase in the ∆θ values. The advancing contact angles measured for the PS films decreased 8° in comparison with those measured before exposure to water. This effect might be a result of the increase in roughness.32 The AFM images evidenced alterations in the topography of all films. In the case of thin PS films, small protuberances and wrinkles appeared on the surface and the holes were more (27) Juhue´, D.; Wang, Y.; Lang, J.; Leung, O.; Goh, M. C.; Winnik, M. A. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1123. (28) Lam, S.; Hellgren, A. C.; Sjo¨berg, M.; Holmberg, K.; Schoonbrood, H. A. S.; Unzue´, M. J.; Asua, J. M.; Tauer, K.; Sherrington, D. C.; Montoya Goni, A. J. Appl. Polym. Sci. 1997, 66, 187. (29) Tzitzinou, A.; Jenneson, P. M.; Clough, A. S.; Keddie, J. L.; Lu, J. R.; Zhdan, P.; Treacher, K. E.; Satguru, R. Prog. Org. Coat. 1999, 35, 89. (30) Hellgren, A. C.; Weissenborn, P.; Holmberg, K. Prog. Org. Coat. 1999, 35, 79. (31) Braga, M.; Leite, C. A. P.; Galembeck, F. Langmuir 2003, 19, 7580. (32) Garbassi, F.; Morra, M.; Occhiello, E. Polymer SurfacessFrom Physics To Technology, 2nd ed.; John Wiley & Sons: Chichester, U.K., 1998.

Effect of Water or Salt Solution on PS Films

frequent than before dipping into water (Figure 1b). Thick PS films (Figure 2b) showed very small protuberances on the surfaces, but the roughness was not enhanced, as in the case of thin PS films. In the case of PS-CTAB (Figures 3b and 4b) and PS-SDS (Figures 5b and 6b) films, frequent protuberances, holes, and some wrinkles were observed. On the thin PS-CTAB films (Figure 3b), the protuberances lie on a porous structure, which seems to be formed by many small wrinkles. The reasons for all these surface alterations might be envisaged as an effect of water molecules, which lie between the polymeric film and the silicon wafer and are removed after drying. Film defects in the central region such as tiny holes and on the edges can serve as paths for the water molecules. Although the defects on the film edges are not shown here, they are frequently present. Upon drying, the space occupied by water molecules turns empty, creating wrinkles and defects on the surface. If the film is thin, such local collapse is favored. Considering that the insertion of water molecules between the hydrophobic film and the silicon wafer is favorable because water wets silicon dioxide surfaces well, if the stream of N2 does not remove all the water molecules, the protuberances might be patches of water at the polymer/silicon dioxide interface. Therefore, the wrinkles might be due to the removal of water molecules allocated at the polymer/silicon dioxide interface, whereas the protuberances might be due to water patches, which remained after drying. Recently, a study on the interface formed by deuterated PS film (30 nm thick) and D2O by means of in situ neutron and X-ray reflectivity measurements and atomic force microscopy showed that water molecules are depleted from the polymer/ D2O interface, making room for nanobubbles.16 The thickness of such a gas layer amounted to 2-5 nm. Although the evidence of the presence of a gas layer at the water/hydrophobic surface interface is a very important finding,16-18 it is difficult to couple this information with the topographic alterations observed after drying the hydrophobic polymer films. Moreover, it seems to be a controversial subject. Mao and co-workers33 recently investigated the existence of an air film between the hydrophobic n-octadecyltrichlorosilane layer chemisorbed on silicon wafers and water by means of ellipsometry. They did not observe any air layer at the solid/water interface. In situ ellipsometric measurements were performed for PS, PS-CTAB, and PS-SDS films in water for 24 h and compared with simulations. The simulations considered two multilayer models, as schematically represented in Figure 7. In the model of Figure 7a, the thickness of the water layer between PS and silicon dioxide was varied from 0 to 10 nm, assuming the index of refraction of water to be 1.333. In the model of Figure 7b, the thickness of the air layer between PS and water was varied from 0 to 10 nm, assuming the index of refraction of air to be 1.000. The ∆ and Ψ angles were calculated assuming the thickness of the PS films to be 65 or 300 nm and the index of refraction for PS to be 1.590. Tables 3 and 4 show the theoretical changes in the ∆ and Ψ angles for a growing water or air layer, as depicted in the models of parts a and b of Figure 7, respectively. The observed changes in the ∆ and Ψ angles presented in Table 5 are rather small and some of them negligible, since the polarizers accuracy is 0.05°. In the case of thin PS films, the decrease in the ∆ values indicated the existence of a very thin (between 0.5 and 1.0 nm) air layer between PS and water. This finding corroborates with those reported by Steitz and co-workers16 and can be explained by a hydrophobic force resulting from an altered structure for the film surrounding water.34 However, no air layer was

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Figure 7. Multilayer models used in the simulations: (a) considering a water layer between PS and silicon dioxide varying from 0 to 10 nm; (b) considering an air layer between PS and water varying from 0 to 10 nm. Table 3. Theoretical Changes in the ∆ and Ψ Angles for a Growing Water Layer, as Depicted in the Model of Figure 2a, for Thin (65 nm) or Thick (300 nm) Polymer Films thin

thick

dwater (nm)

d∆theoretical (deg)

dΨtheoretical (deg)

d∆theoretical (deg)

dΨtheoretical (deg)

0 0.5 1.0 2.5 5.0 7.5 10.0

0 0.33 0.66 1.66 3.37 5.12 6.92

0 0.04 0.08 0.19 0.38 0.58 0.79

0 0.25 0.5 1.26 2.50 3.75 5.00

0 -0.01 -0.02 -0.06 -0.11 -0.17 -0.23

Table 4. Theoretical Changes in the ∆ and Ψ Angles for a Growing Air Layer, as Depicted in the Model of Figure 2b, for Thin (65 nm) or Thick (300 nm) Polymer Films thin

thick

dwater (nm)

d∆theoretical (deg)

dΨtheoretical (deg)

d∆theoretical (deg)

dΨtheoretical (deg)

0 0.5 1.0 2.5 5.0 7.5 10.0

0 -0.09 -0.21 -0.68 -2.0 -4.17 -7.46

0 -0.36 -0.73 -1.82 -3.63 -5.40 -7.08

0 0.45 0.89 2.22 4.40 6.52 8.58

0 0.90 1.76 4.16 7.76 11.08 14.22

Table 5. Experimental Changes in the ∆ and Ψ Angles Measured in Situ for PS, PS-CTAB, and PS-SDS Thin (∼65 nm) or Thick (∼300 nm) Films after 24 h in Water thin

thick

sample

d∆exp (deg)

dΨexp (deg)

d∆exp (deg)

dΨexp (deg)

PS PS-CTAB PS-SDS

-0.15 -0.09 -0.28

-0.03 0.18 0.15

-0.47 -0.48 0.04

-0.05 0.14 -0.002

observed at the interface of the thick PS films and water. These findings suggest that the resolution in detecting the gas layer depends on the film thickness. Comparing mainly the direction of the angle changes, with the exception of the thin PS films, the experimental (33) Mao, M.; Zhang, J.; Yoon, R.-H.; Ducker, W. A. Langmuir 2004, 20, 1843. (34) Israelachvili, J.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98, 500.

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Table 6. Characteristics of Spin-Coated Films Determined in the Aira polymer PS PS-CTAB PS-SDS

thickness (nm)

n632.8

θA (deg)

θR (deg)

65 ( 1 295 ( 5 62 ( 3 262 ( 2 60 ( 2 345 ( 3

1.59 ( 0.01 1.590b 1.58 ( 0.01 1.590b 1.57 ( 0.02 1.590b

69 ( 3 79 ( 2 67 ( 2 68 ( 2 65 ( 3 71 ( 2

58 ( 2 62 ( 2 51 ( 3 53 ( 4 58 ( 2 59 ( 4

∆θ rms (deg) (nm) 11 17 16 15 7 12

0.65 1.2 1.2 1.0 0.91 1.2

a The data correspond to mean values of measurements performed with at least four different samples after 24 h of being immersed in 0.1 mol/L NaCl. b When the independent determination of n and d was not possible, n was kept as 1.59024,25 and d was calculated.

observations for the PS-CTAB and PS-SDS films neither match the simulations of a water layer between a polymer and silicon dioxide nor match those of an air layer between a polymer and water. The observed changes also do not match either simulation (not shown here) considering a model with both a water layer between a polymer and silicon dioxide with a thickness varying from 0.5 to 5.0 nm and an air layer between a polymer and water with a thickness between 0.5 and 10.0 nm. The lack of fit between the simulations and experimental results might be due to the film roughness. Ellipsometric measurements are very sensitive to roughness.33 The models used in the simulations consider very flat layers, but the real surfaces present defects (holes and protuberances), as shown in Table 1 and AFM images. The film roughness in the liquid might be even larger than that in the air. Moreover, a possible change in refractive index due to the loss of soluble surfactant was verified by measuring the surface tension of water, which was in contact with the PS-CTAB and PS-SDS films. The surface tension values were similar to those obtained for pure water, indicating that, if some surfactant molecules were released from the PS-CTAB or PS-SDS films to the aqueous medium, the amount was extremely low. Table 6 shows the characteristics of polymeric films in the air after immersion into a 0.1 mol/L NaCl solution for 24 h. The changes in the thickness and index of refraction values are within the standard deviations. On the other hand, the advancing contact angles (θA values) decreased and the hysteresis in the contact angle (∆θ) increased, regardless of the type of polymer and the thickness. All films presented protuberances and wrinkles on the surface (Figures 1c-6c) and high rms values, corroborating with the increase in the ∆θ values. As proposed before, the defects in the center and edges of the films may serve as paths for the insertion of water and electrolytes at the interface of silicon dioxide and the polymer film. Upon drying, the space previously occupied by water or electrolyte molecules turns empty, causing film collapse (wrinkles). The protuberances might be water, electrolyte patches, which remained even after drying, or electrolyte residues, which were not removed from the film surface with distilled water and N2. Such salt residues probably cause the decrease in the θA values and the drastic surface alteration observed in Figure 2c. The topographic images in Figures 1-6 make clear that exposing hydrophobic PS films to water or salt solution creates protuberances and wrinkles on the surface. A possible mechanism is depicted in Figure 8. Just after preparation, the PS films are smooth, but defects can appear on the edges because neither toluene nor PS wets silicon dioxide (Figure 8a). Upon exposure to water (spheres) or electrolyte (stars) molecules, these reach the hydrophilic silicon dioxide surface through the lateral

Figure 8. Schematic representation of the effect of water (spheres) or electrolyte (stars) molecules on the PS films deposited on the hydrophilic silicon wafer.

Figure 9. Schematic representation of the effect of water (spheres) or electrolyte (stars) molecules on the PS-CTAB or PS-SDS films deposited on the hydrophilic silicon wafer.

defects (Figure 8b). Water or electrolyte patches might be formed at the polymer/silicon dioxide interface (Figure 8c). After drying, part of the patches is removed, forming wrinkles, and part of them remains, leading to protuberances (Figure 8d). Residual electrolyte molecules on the dried film are also represented. In the presence of residual surfactant molecules (Figure 9), the polar heads of CTAB and SDS probably nucleate more water molecules and electrolytes than the PS films. The holes and lateral defects on the original films (Figure 9a) also help the diffusion of water (spheres) or electrolyte (stars) molecules to the polymer/silicon dioxide interface (Figure 9b and c). Upon drying, wrinkles might be formed by local film collapse and protuberances might result from remaining patches of water or electrolytes at the polymer/silicon dioxide and polymer/air interfaces (Figure 9d). If hydrophobic films are used for the immobilization of biomolecules, one must be very careful to discern defects from adsorbate. As an illustrative example, Figure 10 shows the topographic images of dried PS films (∼50 nm thick) on silicon wafers, which were exposed to 0.001 mol/L NaCl and to enolase in 0.001 mol/L NaCl at a concentration of 0.5 g/L. Comparing both images, one observes similar small clumps on both films. Should the bright clumps observed in Figure 10b be attributed to adsorbed enolase or to film defects? This example makes clear that when the hydrophobic films coated on a hydrophilic substrate are exposed to aqueous medium, one must be very careful with the results from ex situ experiments. In this

Effect of Water or Salt Solution on PS Films

Figure 10. Topographic images in the tapping mode in the air of (a) PS after exposure to 0.001 mol/L NaCl for 1 h, rinsing with distilled water, and drying; (b) PS after exposure to enolase solution in 0.001 mol/L NaCl at a concentration of 0.5 g/L for 1 h, rinsing with distilled water, and drying. Scan areas of 2 µm × 2 µm.

particular experiment, ellipsometry and contact angle measurements gave strong evidence for the adsorption of enolase,35 but in general, if only AFM is used as an experimental tool, topographic alterations and immobilized species might be confounded.

Langmuir, Vol. 20, No. 18, 2004 7615

and water in the case of thin (63 nm) PS films. Such an air layer was not observed in the case of thick (293 nm) PS films. The detection of an air layer seems to depend on the film thickness. Films of hydrophobic polymers deposited on hydrophilic substrates might present defects due to dewetting. After exposure of these samples to water or salt solution, such defects serve as paths for water or electrolyte molecules, which tend to diffuse to the substrate/polymer interface because they wet the substrate better than the polymer. Upon drying, wrinkles and protuberances appear on the surface. The former might be due to the film collapse after water removal, and the latter might be patches formed by water molecules, which were not removed. Often, such hydrophobic films are used for the immobilization of small biomolecules. Therefore, to discern the alterations on the topographic images of the films due to the water or electrolyte molecules from the adsorbates, complementary techniques such as X-ray photoelectron spectroscopy (XPS), fluorescence measurements, ellipsometry, or reflectance spectroscopy are recommended.

Ellipsometric measurements evidenced the presence of a thin (0.5-1.0 nm) air layer between the polymer film

Acknowledgment. The authors acknowledge FAPESP (grants 97/13070-2 and 00/08051-3) and CNPq for financial support. We are grateful to the Laborato´rio de Filmes Finos do IFUSP, Brazil, for the SPM facility (FAPESP grant 95/5651-0).

(35) Almeida, A. T.; Salvadori, M. C.; Petri, D. F. S. Langmuir 2002, 18, 6914.

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Conclusions