Adsorption Behavior of Carboxymethylcellulose on Amino-Terminated

Brandrup, J. Immergut, E. H. Polymer Handbook, 2nd ed.; John Wiley & Sons: ..... Tamilselvan Mohan , Rupert Kargl , Karin Eva Tradt , Martin R. Kulter...
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Langmuir 2001, 17, 56-60

Adsorption Behavior of Carboxymethylcellulose on Amino-Terminated Surfaces J. Fujimoto and D. F. S. Petri* Instituto de Quı´mica, Universidade de Sa˜ o Paulo, P.O. Box 26077, CEP 05513-970, Sa˜ o Paulo-SP, Brazil Received May 30, 2000. In Final Form: August 29, 2000 The adsorption behavior of carboxymethyl cellulose onto amino-terminated surfaces was investigated by means of ellipsometry. The adsorbed amount decreases with ionic strength decrease, in agreement with the screening-reduced adsorption regime. The pH has a strong influence on the adsorption behavior. At pH values lower than 4 the surface charge is high and the segment charge low. The maximum adsorbed amount is obtained at pH 3 for fixed ionic strength. At pH values higher than 4 the adsorption is weak. The protonation of the amino groups on the substrate is unfavored, while the segment charge is high due to the strong dissociation of carboxylate groups. These findings show that the surface charge governs the adsorption behavior and that the driving force is electrostatic in nature. The dependence of the adsorbed amount on the segment charge fits scaling laws. The adsorption kinetics reveal that the adsorption equilibrium is achieved after approximately 45 min. In the first minutes the CMC chains move toward the bare substrate in a fast diffusive process. After this period, the few adsorbed chains must change their conformation in order to make room for the arriving chains. This is a slow process, because the steric and electrostatic hindrances must be overcome. This occurs more slowly in higher ionic strength and at lower pH values. After approximately 45 min the surface is completely covered.

Introduction The adsorption of polyelectrolytes at solid/liquid interfaces is one of the important steps in many technological processes, like paper coating, thickening of cosmetic and food products, and stabilization of colloidal dispersions.1,2 Much effort has been devoted in order to understand and describe the influence of ionic strength and pH on the adsorption behavior of such systems.3-8 Van de Steeg and co-workers4 developed theories for the polyelectrolyte adsorption based on numerical calculations, which take in account salt concentration, segment charge, and surface charge. This theory describes two regimes. The first is the so-called screening-reduced adsorption regime, where the adsorbed amount decreases with increasing salt concentration. This regime has been found for polyelectrolytes with low as well as with high segment charge and sufficiently high surface charge density. This effect is expected if the attraction between polyelectrolyte and surface is mainly electrostatic in nature, since salt screens not only the segment-segment repulsion but also the segment-surface attraction. The second is the screeningenhanced adsorption regime, where the adsorbed amount increases with increasing salt concentration. This case has been often found for highly charged polyelectrolyte. The idea is that at high salt concentration the strong segment repulsion is screened and the polyelectrolyte * Corresponding author. Telephone: 0055 11 3818 38 31. Fax: 0055 11 3815 55 79. E-mail: [email protected]. (1) Kroschwitz, J., Ed. In Encyclopedia of Polymer Science and Engineering; John Wiley & Sons: New York, 1985; Vol. 3. (2) Stephen, M. A., Ed, In Food Polysaccharides and their applications; Marcel Dekker Inc.: New York, 1995. (3) Cohen Stuart, M. A., Fleer, G. J.; Lyklema, J.; Norde, W.; Scheutjens, J. M. H. M. Adv. Colloid Interface Sci. 1991, 34, 477. (4) Van de Steeg, H. G. M., Cohen Stuart, M. A, de Kaizer, A., Bijsterbosch, B. H. Langmuir 1992, 8, 2538. (5) Muthukumar, M. J. Chem. Phys. 1987, 86, 7230. (6) Vermeer, A. W. P., Leermakers, F. A. M., Koopal, L. K. Langmuir 1997, 13, 4413. (7) Borukhov, I., Andelman, D., Orland, H. Macromolecules 1998, 31, 1665. (8) Lowack, K., Helm, C. A. Macromolecules 1998, 31, 823.

chains behave more like uncharged polymers. Hence they can adopt conformations such as loops and tails, increasing the adsorbed amount. This situation is favored only if there is an attractive interaction between segments and surface which is not electrostatic in nature. Recently, Borukhhov and co-workers7 developed scaling laws for the adsorption of polyelectrolytes from semidilute solution to a charged surface based on the mean-field approximation. For strongly charged polyelectrolyte in a low-salt solution the adsorbed amount Γ scales as p-1/2, where p is the fractional charge per monomer. In the case of highsalt solution they found two different behaviors. For weak polyelectrolytes, Γ scales as p/cb1/2, where cb is the salt concentration. For strong polyelectrolytes Γ scales as cb1/2/p. Carboxymethylcellulose (CMC) is a polysaccharide, which can behave like a polyelectrolyte under appropriate pH. The adsorption of CMC onto inorganic9-11 and organic12 substrates has been reported in the literature. Hoogendam9 and co-workers performed a systematic study. They observed that the adsorbed amount of CMC on TiO2 and Fe2O3 increased with salt concentration. The adsorbed amount decreased with pH, indicating an electrostatic contribution to the adsorbing energy, although no dependence on the degree of substitution, i.e., on the fractional charge per monomer was found. The adsorption kinetics was divided into three stages. At the beginning it can be described by a diffusive process. After a few seconds it becomes a slower process. The arriving chains face a barrier formed by the already adsorbed chains. In the case of polyelectrolytes, beyond the steric hindrance, there is the electrostatic repulsion. A special (9) Hoogendam, C. W.; de Keizer, A.; Cohen Stuart, M. A.; Bijsterbosch, B. H.; Batelaan, J. G.; van der Horst, P. M. Langmuir 1998, 14, 3825. (10) Cohen Stuart, M. A., Fokking, R. G., van der Horst, P. M., Lichtenbelt, J. W. Th. Colloid Polym. Sci. 1998, 276, 335. (11) Areas, E. P., Galembeck, F. J. Colloid Interface Sci. 1991, 147, 370. (12) Williams, P. A., Harrop, R., Phillips, G. O., Pass, G., Robb, I. J. Chem. Soc., Faraday Trans. 1 1982, 78, 1733.

10.1021/la000731c CCC: $20.00 © 2001 American Chemical Society Published on Web 12/12/2000

Adsorption Behavior of Carboxymethylcellulose CMC

rearrangement within the adsorbed layer turns necessary to accommodate the arriving chains and this is a timeconsuming process. In the last stage, the adsorbed amount does not change as a function of time. In this work we present a systematic study on the adsorption behavior of sodium carboxymethyl cellulose (CMC) onto amino-terminated substrates. We consider this system an interesting one because the aminoterminated surface is highly positively charged if the pH is lower than 4.0, whereas the pK0 of CMC is about 4.0.9,13 Moreover, the amino-terminated surface with the desired thickness is easily obtained.14 The aim of this work is to study the adsorption isotherms and kinetics of CMC onto amino-terminated surfaces under variable salt concentration and pH and to compare the results with theoretical predictions. Experimental Section Materials. Carboxymethylcellulose (CMC), sodium salt purchased from Aldrich (Milwaukee, WI) with a nominal degree of substitution of 0.9 and a nominal molecular weight of 250 000 g/mol was used without prior purification. The average molecular weight was determined by capillary viscometry in NaCl 0.1 M solution at 25.0° C, considering the Mark-Houwink-Sakurada constants15 as a ) 0.554 and k ) 0.519 × 10-3. It amounted to 243 179 g/mol. For the adsorption experiments solutions of CMC dissolved in NaCl, 0.1, 0.01, and 0.001 M, in a pH range 3-7 were prepared in the concentration range 0.002-5.0 g/L. To avoid CMC aggregation13 in the solution, CMC was first dissolved in distilled water under stirring during a period of 8 h. After this, NaCl was added in the corresponding amount to result in concentrations of 0.1, 0.01, and 0.001 M. Sodium chloride, ammonia hydroxide, and acetic acid reagent grade (Nuclear, Sa˜o Paulo, Brazil) were used without prior treatment. Silicon (100) wafers purchased from Crystec (Berlin, Germany) with a native oxide layer approximately 2 nm thick were used as substrates. The Si wafers were rinsed in a standard manner.14,16 After this, the surfaces were functionalized with aminopropyltrimethoxysilane, APS (Fluka, Switzerland), following a method described elsewhere.14 This method yields a flat and homogeneous amino-terminated monolayer covalent bound on silicon wafers. The pK0 for propylamino groups is reported17 to be 3.3 at the temperature of 20.0° C. Methods. Ellipsometric measurements were performed in a vertical computer-controlled DRE-EL02 ellipsometer (Ratzeburg, Germany). The angle of incidence φ was set to 70.0°, and the wavelength λ of the laser was 632.8 nm. This equipment works as a null-ellipsometer. By use of a motorized polarizer, an incident light in the state of elliptical polarization is generated. After reflection from the silicon wafer the reflected light turns linearly polarized. The intensity of reflected light is extinguished by a second polarizer. The position of this second polarizer is automatically varied until the minimum of light reaches the photomultiplier. The null setting for the polarizers yields the ellipsometric angles ∆ and Ψ, which contain information about the relative phase shift and attenuation of the component waves perpendicular and parallel to the plane of incidence, respectively. ∆ and Ψ were measured and recorded in periods of 4 s. For the data interpretation, a multilayer model composed by the substrate, the unknown layer, and the surrounding medium should be used. Then the thickness (dx) and refractive index (nx) of the unknown layer can be calculated from the ellipsometric angles, ∆ and Ψ, using the fundamental ellipsometric equation (13) Hoogendam, C. W.; de Keizer, A.; Cohen Stuart, M. A.; Bijsterbosch, B. H.; Smit, J. A. M.; van Dijk, J. A. P. P., van der Horst, P. M.; Batelaan, J. G. Macromolecules 1998, 31, 6297. (14) Siqueira-Petri, D. F., Wenz, G.; Schunk, P.; Schimmel, T. Langmuir 1999, 15, 4520. (15) Brandrup, J. Immergut, E. H. Polymer Handbook, 2nd ed.; John Wiley & Sons: New York, 1966. (16) Motschmann, H.; Stamm. M.; Toprakcioglu, C. Macromolecules 1991, 24, 3681. (17) Handbook of Chemistry and Physics, 64th ed.; CRC Press: Boca Raton, FL, 1984.

Langmuir, Vol. 17, No. 1, 2001 57

Figure 1. Schematic representation of the ellipsometric cell used for the adsorption experiments. and iterative calculations with Jones matrixes:16,18

ei∆ tan Ψ ) Rp/Rs ) f(nk,dk,λ, φ)

(1)

where Rp and Rs are the overall reflection coefficients for the parallel and perpendicular waves. They are a function of the angle of incidence φ, the wavelength λ of the radiation and of the indices of refraction, and the thickness of each layer of the model, nk,dk. Ellipsometry is a technique which enables the independent determination of the index of refraction and the thickness, only if the optical contrast in the system is big enough or if the layer thickness is thick enough. However, the product nxdx is a constant value and does not depend on the adopted model or concentration profile near the wall.16 First of all, the thickness of the SiO2 layers was determined in air, considering the index of refraction for Si as n˜ ) 3.88i0.01819 and its thickness as an infinite one; for the surrounding medium (air) the index of refraction was considered as 1.00. Because the native SiO2 layer is very thin, its index of refraction was set as 1.46219 and just the thickness was calculated. The mean SiO2 thickness measured for 50 samples was 1.9 ( 0.1 nm. After the characterization, the Si wafers were functionalized by the silanization reaction with APS.14 The thickness of the aminoterminated monolayer was determined in air, considering the nominal index of refraction of the silane as 1.424. The mean thickness value calculated for the amino-terminated layer was 0.9 ( 0.1 nm. The adsorption from solution was monitored in situ with the help of a poly(tetrafluoroethylene) cell. This cell has two quartz windows, one for the incident beam and the other for the reflected beam, with the inclination angle of 70.0°, as shown in Figure 1. The measurements were done in a conditioned room, where the temperature ranged from 22 to 23 °C. The adsorbed amount Γ is determined from

Γ)

dpoly(npoly - n0) ) dpolycpoly dn/dc

(2)

where npoly and dpoly are the index of refraction and thickness of the adsorbed polymer, n0 is the index of refraction of the solution measured with an Abbe refractometer, dn/dc is the increment of refractive index determined with a differential refractometer, and cpoly is the average polymer concentration within the layer.16 For our system, n0 was measured for each concentration, and dn/dc amounted to 0.16 mL/g at the temperature of 23 °C. From the ellipsometric angles ∆ and Ψ and a multilayer model composed of silicon, silicon dioxide, amino-terminated monolayer, polyelectrolyte layer, and solution, it is possible to determine only the thickness of the adsorbed polyelectrolyte, dpoly. The small differences in the indices of refraction of the substrate, polyelectrolyte, and solution make an independent determination of npoly and dpoly impossible. Therefore, npoly was kept constant as 1.50 and dpoly was calculated. The hydrodynamic thickness could be calculated if the optical contrast in the system was higher. However, it is important to remember here that, if the index of refraction assumed for the adsorbing polymer layer lies in a reasonable range (between 1.40 and 1.60), the product npolydpoly should be a constant value.16,18 This product is the parameter necessary to calculate the adsorbed amount Γ from eq 2. When the adsorption begins, a significant decrease in ∆ and a small (18) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland Publication: Amsterdam, 1987. (19) Edward, D. P., Ed. Handbook of Optical Constants of Solids; Academic Press: London, 1985.

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Figure 2. Ellipsometric angles ∆ and Ψ as a function of time in a typical adsorption experiment. In this experiment the CMC solution was prepared in 0.01 M NaCl, pH 3 and cpoly ) 0.05 mg/mL.

Figure 3. Adsorbed amount Γ measured for CMC solution at a concentration of 1.0 g/L in 0.1 M (O), 0.01 M (4), and 0.001 M (9) NaCl onto amino-terminated surfaces as a function of pH value. increase in Ψ are observed. After a given period of time, ∆ and Ψ do not change any more. A typical measurement is shown in Figure 2.

Results and Discussion Influence of pH. The interactions between substrate and polyelectrolytes can be strongly influenced by salt concentration, segment charge, and surface charge. For the system studied here, the segment charge and surface charge are a function of the medium pH. The degree of dissociation (R) of CMC13 is affected by pH and electrolyte concentration. In 0.01 M NaCl solution R increases from 0.2 at pH 3.0 to 0.95 at pH 6.0. The amino-terminated surface is highly protonated for pH < 4, but increasing the pH value, the surface tends to be discharged. Figure 3 shows the adsorbed amount obtained for CMC from solution at the concentration of 1.0 mg/mL in NaCl 0.001, 0.01, and 0.1 M in the pH range 3-7 after 4 h of adsorption. The general behavior is that the dpoly values and the adsorbed amount Γ increases with decreasing pH values. This behavior is more pronounced for lower salt concentration. According to Hoogendam and co-workers13 the CMC is about 50% and 25% dissociated at pH 4 and 3, respectively. The protonation of the amino groups is more favored at pH 3 than at pH 4. At pH 3 there is still enough free carboxylate groups to interact with the highly charged amino surface. The negative charge might be statistically distributed along the CMC chains, yielding a conformation with many loops composed of uncharged monomers, as schematically depicted in Figure 4a. This would explain the high values of Γ at the lowest pH value. At pH 4 the segment charge is higher than at pH 3, but the surface charge is less, since the pK0 for propylamino is 3.3.17 The CMC chains might adsorb in a flatter conformation (Figure 4b) with small loops to minimize the electrostatic repulsion. In the pH range between 4

Fujimoto and Petri

Figure 4. Representation of substrate and adsorbate charges as a function of pH: (a) pH 3, (b) pH 4, (c) pH 5.5, and (d) pH 7.

and 7 the adsorbed amount decreases with increasing pH. At pH 5 there is a weak adsorption, which is caused by the very low surface charge (Figure 4c). In this situation, CMC chains are highly charged and might stick on the few positive charged sites of the substrate, assuming a flat conformation in order to minimize the electrostatic repulsion among the carboxylate charges. At pH 7 the substrate is neutral, and although the CMC chains are highly charged, no adsorption occurs (Figure 4d). These findings show that the driving force for the adsorption in the present system is an electrostatic one and that the surface charge is the controlling parameter in this adsorption process. Adsorption Isotherms. The adsorption isotherms measured for solutions of CMC in NaCl 0.1, 0.01, and 0.001 M in pH 4 and 3 in the concentration range 0.0025.0 g/L are shown in parts a and b of Figure 5, respectively. In the case of pH 4 the adsorption plateau lies at an average value of 1.3 mg/m2, when the salt concentration is 0.1 M NaCl. On decreasing the ionic strength for NaCl to 0.01 and 0.001 M, the plateau value amounts to 1.75 mg/m2 in both (Figure 5a). At pH 3, where the surface charge on the amino-terminated surface is higher than at pH 4, the dependence of the adsorption plateau on the ionic strength is more evident. It decreases from 2.1 to 1.7 to 1.5 mg/m2 for the salt concentration of 0.001, 0.01, and 0.1 M NaCl, respectively (Figure 5b). This behavior can be explained based on the screening-reduced adsorption regime, where the adsorbed amount decreases with increasing salt concentration. This regime has been found for polyelectrolytes with low as well as with high segment charge and sufficiently high surface charge density. According to Cohen Stuart and co-workers3,4 this effect is expected if the attraction between polyelectrolyte and surface is mainly electrostatic in nature, since salt screens not only the segment-segment repulsion but also the segmentsurface attraction. The adsorption behavior of CMC onto amino-terminated surfaces follows the screening-reduced adsorption regime, as discussed above. To our knowledge, such behavior is seldom reported.20 Most systems9,11,12,21 involving the adsorption of strong polyelectrolytes follow the screeningenhanced adsorption regime, i.e., the adsorbed amount increases with the ionic strength. It is interesting to notice that the adsorption behavior of similar CMC’s onto TiO2,9 (20) Hoogeveen, N. G., Cohen Stuart, M. A., Fleer, G. J. Colloid Interface Sci. Phys. 1996, 182, 133. (21) Meadows, J., Williams, P. A., Garvey, M. J., Harrop, R., Phillips, G. O. J. Colloid Interface Sci. Phys. 1989, 132, 319.

Adsorption Behavior of Carboxymethylcellulose CMC

Langmuir, Vol. 17, No. 1, 2001 59

Figure 7. Dependence of the adsorbed amount Γ (plateau value) on p-1/2. These values correspond to the ionic strength of 0.01 M NaCl. The correlation factor is 0.94. Table 1. Values of r18, p (p is the product between r and DS), and p-1/2, as a Function of pH for 0.01 M NaCl and the Corresponding Adsorbed amount Γ (Plateau Values) pH 3, R ) 0.25 pH 4, R ) 0.5 pH 5, R ) 0.9

Figure 5. Adsorption isotherm obtained for CMC dissolved in 0.1 M (O), 0.01 M (4), and 0.001 M (9) NaCl onto aminoterminated surfaces at (a) pH 4 and (b) 3. The dotted lines represent the adsorption plateau.

Figure 6. Adsorbed amount Γ of CMC as a function of time, after changing the polymer solution (c ) 1.05 mg/mL), by 0.01 M NaCl solution at pH 7.

Fe2O3,9 and BaSO411 follows the screening-enhanced adsorption regime. Desorption Experiments. To test the stability of the adsorbed CMC chains on the amino-terminated surfaces, the following desorption experiment was performed. First the CMC chains were allowed to adsorb in 0.01 M NaCl at pH 3 in the concentration range 0.02-5 g/L. The adsorption isotherm obtained is shown in Figure 5b. The corresponding adsorption plateau value lies at 1.7 mg/m2. After the equilibrium has been achieved, the CMC solution was changed by pure 0.01 M NaCl solution at pH 7. We observe that, on average, 50% of the CMC chains desorb after 30 min (Figure 6). After a long period of time (24 h) about 80% of the CMC chains desorb. This means that many positive charges on the amino-terminated surface are unstable if the pH is increased, destroying part of the electrostatic interactions.

p

p-1/2

Γ, mg/m2

0.225 0.45 0.81

2.11 1.49 1.11

2.2 1.7 1.2

Scaling Laws. We consider the fractional charge per monomer p as being the product between the degree of substitution and the degree of dissociation (R) of CMC as a function of pH. For a CMC with a degree of substitution (DS) of 0.99 dissolved in 0.01 M NaCl, R was determined as a function of pH by Hoogendam and co-workers.13 They found that an increase in the ionic strength facilitates the dissociation. As an approximation, we will consider these values to calculate the values of p for the system studied here. Considering the system as strongly charged polyelectrolyte in a low-salt solution, the values of p as a function of pH for 0.01 M NaCl, as well as p-1/2, were calculated. Although we just have three different pH values, the calculations are presented in Table 1 and shown in Figure 7 with the corresponding adsorbed amount Γ (plateau values). The data fit the scaling laws predicted by Borukhhov and co-workers7 for the adsorption of strong polyelectrolytes in low-salt solutions with a correlation factor of 0.94. Adsorption Kinetics. Figure 8a shows the adsorption kinetics of CMC from 0.01 M NaCl at a concentration of 0.002 mg/mL onto amino-terminated surfaces at pH 4 and 3. The kinetic curves can be divided into three stages. At the beginning they present similar behavior with the adsorbed amount Γ increasing with the same intensity until approximately 24 min (0.4 h). A linear dependence is evident in Figure 8b, where the adsorbed amount A is plotted as a function of t0.5. The kinetics adsorption of polymers is well described in the literature.16,22,23 At the initial stages the adsorption is controlled by the diffusion of free chains from the bulk solution to the bare substrate. Considering this mass transport as a Fickian diffusion, the diffusion coefficient D can be calculated by applying the relation:

Γ(t) )

2 cpoly(Dt)1/2 π1/2

(3)

(22) Siqueira, D. F., Reiter, J., Breiner, U., Stadler, R., Stamm, M. Langmuir 1996, 12, 972. (23) Siqueira, D. F., Pitsikalis, M., Hadjichristidis, N., Stamm, M. Langmuir 1996, 12, 1631.

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Figure 9. Adsorption kinetics measured for CMC solution in 0.001 M NaCl at a concentration of 0.002 g/L, pH 3 (b) and 4 (O).

Figure 8. Adsorption kinetics measured for CMC solution in 0.01 M NaCl at a concentration of 0.002 g/L, pH 3 (b) and 4 (O). Adsorbed amount Γ as a function of (a) t and (b) t0.5.

The slope (Γ/t0.5) found for pH 3 and 4 is the practically the same. By means of eq 3 we find the diffusion coefficient D ∼ 1 × 10-7 cm2/s. This value is reasonable for a polymer of molecular weight 250 kg/mol.22,23 In this case, the segment charge variation caused by the pH seems to have no influence on the diffusive process, or at least we cannot observe it within the experimental conditions. Hoogeveen and co-workers9 studied the effect of ionic strength on the adsorption kinetics of polyelectrolytes. They proposed that for a highly charged polyelectrolyte in solution with low salt concentration, the diffusion coefficient would be low due to the highly swollen chain conformation caused by the electrostatic repulsion among the charged segments. The addition of salt or the decrease of segment charge by pH change should minimize the electrostatic repulsion, increasing the transport rate. Figure 9 shows the kinetic curves of CMC from 0.001 M NaCl at a concentration of 0.002 mg/mL onto aminoterminated surfaces at pH 4 and 3. In this case, the initial stages are different. At pH 4 the adsorption is faster than at pH 3. This means that an increase of segment charge accelerates the adsorption. On comparison with Figure 8a, where the experiments were performed with 0.01 M NaCl, we conclude that at the higher ionic strength the adsorption is slower. This finding corroborates with the conclusion that the adsorption of CMC onto aminoterminated surfaces follows the screening-reduced adsorption regime. An increase of salt concentration screens not only the polyelectrolyte but also the surface charge. Therefore the probability of a charged segment to bind onto a charged site on the surface becomes smaller and the adsorption takes a longer time. After the initial stages, the already adsorbed chains must change their conformation in order to create spatial conditions for the adsorption of arriving chains. This is a

slow process because the rearrangement should be the one that minimizes the electrostatic repulsion among the segments and the steric hindrance. The CMC adsorption at this stage is slower at pH 3 than at pH 4 either in 0.01 M NaCl (Figure 8a) or 0.001 M NaCl (Figure 9). At pH 3 the segment charge is lower than at pH 4 because the degree of dissociation of CMC is lower. In this way, the few charged segments of CMC bind to the highly charged amino-terminated surface, while the neutral segments of CMC form loops avoiding contact with the surface. As proposed in Figure 3, at pH 3 the loops are bigger than at pH 4. In this way, the spatial rearrangement should consume more time in the former case. At equilibrium the surface is completely covered. The adsorption equilibrium values observed in Figures 8 and 9 are comparable within the measurement errors. They are achieved after approximately 45 min. Conclusions Carboxymethylcellulose adsorbs onto amino-terminated surfaces driven by electrostatic interactions. The adsorbed amount decreases with ionic strength, in agreement with the screening-reduced adsorption regime. The adsorption equilibrium is achieved after approximately 45 min. The charge on the amino-terminated substrate and segment charge along the CMC chains is strongly influenced by the pH. However, the adsorption behavior is mainly controlled by the surface charge. The adsorbed chains are not stable if the medium pH is increased to pH 7, leading to the desorption of about 50% of the CMC chains after 30 min. Scaling laws can fit the dependence of the adsorbed amount on the segment charge. The adsorption kinetics shows three regions. In the first stage, the CMC chains diffuse to the bare substrate in a fast process. This mass transport can be described in a purely diffusive process. The calculated value for the diffusion coefficient has an expected magnitude for the corresponding molecular weight. After this period, the few adsorbed chains must charge their conformation in order to make room for the arriving chains. This is a slow process, because the steric and electrostatic hindrances must be overcome. This occurs especially slowly in higher ionic strength and at lower pH values. In the former case a higher surface screening is responsible for the delay. In the latter case the steric hindrance among the big loops in the noncharged CMC segments is the reason for a slower rearrangement. After approximately 45 min the surface is completely covered and the equilibrium is achieved. Acknowledgment. We acknowledge FAPESP for financial support. LA000731C