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Adsorption of Nonionic Surfactants onto Polystyrene: Kinetics and Reversibility C. Geffroy,*,† M. A. Cohen Stuart,† K. Wong,‡ B. Cabane,‡ and V. Bergeron§ Laboratory for Physical Chemistry and Colloidal Science, DreijenPlein 6, 6700 EK Wageningen, The Netherlands, Rhodia, Centre de Recherches 52, Rue de la Haie Coq, 93308 Aubervilliers, France, and Rhodia, CRL 85, Avenue des Fre` res Perret, 69191 Saint Fons Cedex, France Received January 19, 2000. In Final Form: May 3, 2000 The reversible adsorption and desorption kinetics of nonionic surfactants at a hydrophobic surface have been studied by reflectometry. This enables the measurement of the adsorption isotherm with unprecedented accuracy. It is shown that the adsorption mechanism can be split into three processes. The surface is first covered by single monomers that experience a strong hydrophobic interaction. This is followed by a typically cooperative process leading to the formation of surface aggregates. At the saturation level, the surfactant molecules stagger in the interface such that there is a possible overlap of hydrophilic headgroups with hydrophobic tails. All this information comes directly out of the kinetics that can be modeled in terms of a local equilibrium.
Introduction The adsorption and desorption of organic matter at the solid-liquid interface have been studied for years because of industrial relevance (flotation, oil recovery, paints, papermaking). In particular, surfactant and polymer adsorption lead to various interesting interfacial properties and have been investigated in detail.1,2 However, most of the studies deal with the equilibrium state and did not address the dynamic processes of adsorption and desorption. Some papers report on the use of optical devices to characterize the dynamic properties of these interfaces, but the kinetics have not been systematically studied. The recent development of time-resolved techniques such as ellipsometry and reflectometry has made it possible to get insight into these dynamic aspects. In the case of macromolecules, such as homopolymers, proteins, or humic acids, both the adsorption and the desorption have been described.1,3,4 In some cases, it has been found that the adsorption at the solid-liquid interface is mostly a transport-controlled process. As soon as a molecule strikes the surface, it adsorbs and establishes an equilibrium with the surrounding macromolecules. This feature has been successfully modeled by Dijt5 through the local-equilibrium concept. Desorption of macromolecules is usually very slow as many segments are in contact with the surface leading to a huge energetic barrier that has to be surpassed.3,6 Slow surface processes were revealed mostly in studies of polymer-polymer exchange.5,7
Concerning surfactants, for the case where the adsorption energy per molecule is rather small, the kinetics of adsorption and desorption may show some interesting behavior. When the monomer-surface interaction is rather weak (of the order of kT per molecule, a reasonable estimate for the interaction of a nonionic surfactant with hydrophilic silica), a transport-controlled adsorption mechanism is expected and indeed has been found by ellipsometry.7-11 However, these ellipsometry experiments suffered from a lack of hydrodynamic control. The aim of this paper is to show how time-resolved reflectometry combined with controlled flow can be a useful tool to study the adsorption and desorption of small molecules such as surfactants, for which very fast kinetics are expected. Our aim is to find out how the interaction between the surface and the monomer affects the desorption kinetics. For that purpose, we have carried out experiments under well-controlled hydrodynamic conditions in the so-called stagnation point flow cell described by Dabros.12 We have performed experiments on the hydrophobic polystyrene surface. The surfactant tails are known to experience a strong binding strength with such a surface whereas the hydrophilic part of the monomer may protrude toward the aqueous solution. The effect of the polar headgroup on the adsorption and desorption kinetics has been investigated in more detail. To that aim, the kinetics of desorption upon dilution has been systematically investigated and the reversibility of the adsorption has been examined.
* Corresponding author. Email:
[email protected]. Tel: +33 (0) 149376208. † Laboratory for Physical Chemistry and Colloidal Science. ‡ Rhodia, Centre de Recherches. § Rhodia, CRL85.
Experimental Section
(1) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. In Polymers at interfaces; London: Chapman and Hall, 1992. (2) Zhu, B. Y.; Gu, T. Adv. Colloid Interface Sci. 1991, 37, 1. (3) Avena, M.; Koopal, L. K. Environ. Sci. Technol. 1998, 32, 2572. (4) Elgersma, A. V.; Zsom, R. L. J.; Lyklema, J.; Norde, W. Colloids Surf. 1992, 65, 17. (5) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1992, 25, 5416. (6) Vermeer, A. W. P.; Van Riemsdijk, W. H.; Koopal, L. K. Langmuir 1998, 14, 2810.
Chemicals. We used four nonionic surfactants consisting of an alkyl group (C8,C12) and a short ethylene oxide oligomer (EO), see Table 1. They will be denoted as CnEm where n is the number of carbon atoms in the alkyl group and m the number of EO groups. The set of C12Em surfactants used in this study has been (7) Frantz, P.; Granick, S. Phys. Rev. Lett. 1992, 66, 899. (8) Tiberg, F.; Jo¨nsson, B.; Lindman, B. Langmuir 1994, 10, 3714. (9) Brinck, J.; Tiberg, F. Langmuir 1996, 12, 5042. (10) Tiberg, F.; Landgreen, M. Langmuir 1993, 9, 927. (11) Eskilsson, K.; Tiberg, F. Macromolecules 1997, 30, 6323. (12) Dabros, T.; Van de Ven, T. G. M. Prog. Polym. Sci. 1983, 261, 694.
10.1021/la0000769 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/12/2000
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Table 1. Chemicals Used in This Study molecule
molar mass
cmc (mmol/L)
dn/dc (L/mol)
AS (g/m2)
C8E5 C12E5 C12E6 C12E8 polyoxyethylene
350 406 450 538 300-106
11 0.057 0.087 0.092 s
0.128 0.131 0.136 0.142 0.136
0.0172 0.0181 0.0188 0.0196 0.0187
purchased by Nikko Chemicals and used without further purification. Their GC chromatogram show a high purity grade (>99%). The C8E5 has been supplied by Sigma Chemical Co. and has a stated purity of 98.5%. All the solutions have been prepared with Milli-Q water (nw ) 1.333). The refractive index increments have been measured by a differential refractometer and are listed in Table 1. The cmc’s have been measured by the Wilhelmy plate method and are given in Table 1. Substrate. We used strips of silicon wafer (nSi ) 3.85) that initially bear a very thin layer of native silica. The wafer is first washed thoroughly in toluene and then dip-coated with a 100 ppm solution in chloroform of a poly4vinylpyridine-polystyrene copolymer supplied by Polymer Source Inc. (Laval, Canada). The VP groups anchor onto the native silica layer, and the PS chain are exposed in the air. Subsequently, a polystyrene film is spincoated (2500 rpm), using a 15 g/L (in toluene) high molecular weight polystyrene solution. The wafers are then placed in an oven at 100 °C (glass temperature of PS) overnight. The so-prepared PS layer is 67 nm thick and exhibits a roughness (measured by AFM) of 0.5 nm. Its refractive index nPS is 1.585. Reflectometry. The design of the reflectometer is as follows. The polarized laser beam (λ ) 632.8 nm) is reflected off the silicon wafer at the silicon/water Brewster angle and then split into its parallel and perpendicular components. The signal S is taken as the intensity ratio of these two components. Upon adsorption of the surfactant, the measured signal changes from S0 to S0 + ∆S. The adsorbed amount Γ is given by the relative signal change ∆S/S0:
Γ)
1 ∆S AS S0
(1)
where AS is a sensitivity factor that is assessed through Abeles’ formalism.13 This factor depends on the refractive index increment of the surfactant solution and ranges for the surfactants used in this study from 0.018 to 0.02 g/m2 as listed in Table 1.
The Local Equilibrium Concept The adsorption/desorption measurements were performed in a stagnation point flow cell extensively described elsewhere.5 These well-defined hydrodynamic conditions12 allow very reproducible adsorption and desorption experiments.14 The flux J of an adsorbing molecule toward the surface depends on its diffusion coefficient D, its concentration gradient Cb - Cs (where b and s denote the bulk and the subsurface region that will be defined below, respectively) and the kinematic viscosity of the solution υ and is given by:
J ) kν1/3D2/3(Cb - Cs) ) λ(Cb - Cs(Γ))
(2)
Here, k and λ are constants that depend on the geometry and on the streaming intensity in the cell. The bulk concentration Cb is either maintained at a constant level (adsorption) or is equal to zero (desorption by pure solvent). The problem is to find a reasonable expression for Cs. For that purpose, we can use the local equilibrium concept.5,14 The central idea of the local equilibrium concept is that the adsorbed matter on the surface (represented by Γ) is in continuous equilibrium with the free matter of con(13) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380. (14) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. Adv. Colloid Interface Sci. 1994, 50, 79-101.
Figure 1. Typical adsorption-desorption experiment pertaining to a reflectometric device. After 400 s of adsorption the desorption upon dilution is measured by back-switch to pure solvent.
centration Cs in the immediate vicinity of the surface (the so-called subsurface region) and that the surface region is the rate-controlling step in the adsorption or desorption process. Hence, at a given adsorbed amount Γ, the local subsurface concentration Cs(Γ) can be approximated by the concentration in equilibrium with a macroscopic adsorbed layer, i.e., the one measured through the adsorption isotherm Γ ) f(Cs). The consequences of the validity of this concept are twofold. First, at low adsorbed amounts (far from saturation), Cb . Cs and the flux of surfactant is expected to be constant in time i.e., eq 2 reduces to J ) λCb. Under these conditions, the λ parameter can be straightforwardly computed. Second, the desorption experiments become a powerful method to investigate the reversibility of the adsorption. Each desorption experiment is carried out by switching back the flowing surfactant solution to pure water at a given adsorbed amount Γ formerly in equilibrium with its corresponding surfactant concentration. Thus, Cb ) 0 and this leads the desorption rate to be written as
J)
dΓ ) -λCs(Γ) dt
(3)
where λ can be determined as mentioned above. Hence, if the adsorption is truly reversible, the desorption curve can be predicted and computed from the adsorption isotherm (or vice versa). A desorption experiment Γdes ) g(t) starting from the plateau region should provide the whole adsorption isotherm (Γ ) f(Cs)) by a simple derivative at each Γ value leading to its corresponding Cs(Γ):
Cs(Γ) ) -
[ ]
1 dg(t) λ dt
Γ
(4)
After inversion, the outcome can be compared to Γ(Cs) as obtained from static adsorption measurements at various concentrations. When the curves coincide, this proves that the adsorbed layer equilibrates rapidly. The reversibility of the adsorption-desorption experiments will then be tackled with the help of the local equilibrium concept.5,14 This method has already given satisfactory results in the case of macromolecules such as polymer15 or humic acids.3 It has been found that the desorption curve lies above the adsorption curve and resembles a high affinity isotherm.6 Actually, for a typical polymer (15) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1994, 27, 3219.
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Figure 2. Flow rate of unimers toward the polystyrene surface versus its concentration. Contribution of micelles. The flux is calculated from the slope of the linear part of the Figure 1 (Jo ≈ λCb leads to λ ) 5 × 10-6 m/s). Experiments performed with Nikko C12E5, Re ) 127.
Figure 3. Adsorption isotherm (21 °C) of Nikko C8E5, C12E5, C12E6, and C12E8 surfactants onto hydrophobic polystyrene from water at pH 6. The line is drawn as a guide for the eyes.
adsorption isotherm, the adsorbed amount decreases linearly with log(t).5 The kinetics of adsorption and desorption of a set of nonionic surfactants at the silica-water interface has been successfully described by assuming a stagnant layer close to the surface.9,10 This means that the desorption kinetics emerges directly from the shape of the adsorption isotherm. However, we need clear evidence that the local equilibrium concept is still relevant for nonionic surfactants on both hydrophobic (this study) and hydrophilic surfaces.9,16 Results A Typical Experiment. An example of a typical kinetic experiment is given in Figure 1. The adsorption kinetics of a surfactant on a hydrophobic surface show the same trends as the adsorption kinetics found for polymers.14,15 The adsorbed amount Γ first increases linearly with time. The adsorption rate then decreases strongly and the adsorbed amount reaches a plateau value Γmax. After a given adsorption time (typically 400 s), the adsorbed (16) Brinck, J.; Jo¨nsson, B.; Tiberg, F.; Langmuir 1998, 14, 1058.
amount becomes constant indicating that equilibrium has been reached. Then a desorption experiment is started by switching the surfactant solution back to pure water. Then, desorption by dilution may occur. The desorption kinetics can be compared by plotting the remaining adsorbed amount Γ versus time. 1. In the first linear part of the curve, we identify the adsorption rate dΓ/dt (given by the slope of the curve) with the limiting flux J0 defined as J(Cs ) 0). The experimentally measured fluxes of the C12E5 surfactant toward the surface as a function of the total surfactant concentration have been plotted in Figure 2. The same trends have been obtained for the other surfactants. One can distinguish two regions. Below the cmc, J0 increases linearly with concentration. Above the cmc, where nonadsorbing micelles participate in the transport, the flux continues to increase with the total surfactant concentration but at a slower rate. 2. By recording the final (saturated) adsorbed amount Γmax as a function of equilibrium concentration at a fixed temperature (21 °C) one obtains the equilibrium adsorption isotherm reported in Figure 3. On one hand, one can easily observe that the equilibrium adsorbed amount
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Figure 4. Normalized desorption experiments of C12E8 at various equilibrium concentrations. Each sample adsorbs for 500 s before the desorption is initiated.
Figure 5. Residual adsorbed amount (in molecules/nm2) after desorption for all the C12 surfactants used in this study versus their equilibrium concentration.
varies with the size of the polar headgroup, ranging from 1.8 molecules/nm2 for C12E5 to 1.1 for C12E8 (plateau value). On the other hand, the length of the hydrophobic tail is of importance which can be seen from the difference between C12E5 and C8E5. The isotherm as presented here would seem to be roughly of the Langmuir type as mentioned by several authors.17,18 3. The desorption experiments plotted in Figure 4 show interesting features. First, the normalized kinetics of desorption Γ/Γ0 ) f(t) depends on the surface coverage. At low surface coverage, a fairly weak and slow desorption is measured whereas at higher adsorbed amount, the desorption rate increases. Second, all the desorption experiments show a leveling off and do not reach a zero adsorbed amount within the time of the experiment. A plot of the residual adsorbed amount as a function of the (17) Kronberg, B. J. Colloid Interface Sci. 1983, 96, 55. (18) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1990, 6, 1478.
initial equilibrium concentration (Figure 5) shows somewhat scattered values ranging between 0.35 and 0.55 molecules/nm2. Comparison of Adsorption-Desorption. The adsorption-desorption process can be repeated many times. If, after desorption, the experiment is switched back to adsorption of the initial surfactant solution, the same adsorbed amount is found and the same desorption plateau is reached after a second cycle, as shown in Figure 6. This is clear evidence that the residual fraction is not a consequence of any fractionation process of a contaminated surfactant sample but an inherent property of the pure surfactant toward the surface. The lack of desorption now brings up the question of whether the process is reversible or not. It is therefore interesting to examine the reversibility of the adsorption process in more detail. As pointed out above, in the case of a truly reversible adsorption mechanism, the adsorption isotherm should be consistent with the desorption curve. From this point
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Figure 6. Reversibility of adsorption-desorption cycle of C12E8 adsorbed on PS. C1 ) 1.7 × 10-4 M. First desorption upon dilution with C2 ) 6 × 10-7 M. Further desorption with water. Second cycle: adsorption of a 1.7 × 10-4 M surfactant solution. Desorption by water. All the solute/solvent switches are indicated by an arrow.
Figure 7. Semilogarithmic plot of the adsorption isotherm of C12E5 onto polystyrene. The arrow indicates the beginning of the association process.
of view, the lack of desorption implies that an adsorbed amount of the order of 0.4 mg/m2 should be in equilibrium with a very low surfactant concentration. This implies that we should find significant adsorbed amounts at much lower concentrations than suggested by the Langmuirlike isotherm in Figure 3. We have therefore carried out experiments at very low surfactant concentrations. It turns out that the adsorption isotherm (plotted semilogarithmically in Figure 7) is not Langmuir-like. Rather, it has three distinct parts. If we take a look at the onset of the isotherm in more detail (I), we notice that at fairly low surface coverage (
