Article pubs.acs.org/Langmuir
How Electrolyte and Polyelectrolyte Affect the Adsorption of the Anionic Surfactant SDS onto the Surface of a Cellulose Thin Film and the Structure of the Cellulose Film. 1. Hydrophobic Cellulose Ian M. Tucker,† Jordan T. Petkov,† Jeffrey Penfold,*,‡,§ and Robert K. Thomas§ †
Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, U.K. STFC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K. § Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, U.K. ‡
S Supporting Information *
ABSTRACT: The nature of hydrophobic thin cellulose films, formed by Langmuir−Blodgett (LB) deposition on silica, has been studied using neutron reflectivity (NR). The impact of electrolyte and a polyelectrolyte, poly(dimethyldiallylammonium chloride) (polydmdaac), on the adsorption of the anionic surfactant sodium dodecyl sulfate (SDS) onto the surface of the hydrophobic cellulose film and upon the structure of the cellulose film has been investigated. The results show how a combination of polyelectrolytes and electrolyte can be used to manipulate surfactant adsorption onto hydrophobic cellulose surfaces and modify the structure of the cellulose film by swelling and penetration. The results illustrate how polyelectrolytes can be used to reverse adsorption and swelling of cellulose films which are not reversible simply by dilution in solvent.
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INTRODUCTION Surfactant adsorption at the solid−solution interface plays an important role in many key technological and industrial processes and commonplace applications, such as lubrication, mineral flotation, dyeing, enhanced oil recovery, detergency, and surface (hair, fabric) conditioning.1,2 Specific interactions between the surfactant and the solid surface modify the nature of the surfactant adsorption compared to the air−water or oil− water interfaces.3 At a hydrophilic solid surface the adsorption is usually cooperative, and surface self-assembly which is related to the solution aggregation occurs.4 At a hydrophobic solid surface the adsorption is generally in the form of a monolayer and is similar to that at the air−water interface.5 The use of surface techniques, such as ellipsometry,6 atomic force microscopy,7 and X-ray and neutron reflectivity,8,9 has greatly extended the ability to study adsorption at different solid surfaces. Many different model surfaces with different functional properties have been explored and include polymeric,10 hydroxylated,11 amino-functionalized,12 and carboxyl-terminated thiol13 surfaces. The types of surfaces studied have been extended to include model surfaces that mimic the properties of the hair cuticle,14 the corneum stratum of the skin,15 and fabrics such as polyester or cotton.16 About 90% of the content of cotton is cellulose,17 and hence the nature of cellulose surfaces and surfactant adsorption onto those surfaces is of much current interest.18−28 Cellulose has been studied in fibrous and gel forms and as model films. Kontturi et al.29 have comprehensively reviewed the formation of model cellulose films and the greater opportunities that they © 2012 American Chemical Society
provide for detailed fundamental studies. The model cellulose surfaces are mostly prepared by two different routes, both using trimethylsilyl cellulose (TMSC). Thin films ∼1000 Å thick are produced by spin-coating TMSC from a volatile solvent such as chloroform on to a silicon substrate.18,19 Thinner cellulose films ∼100 Å are deposited on silicon using sequential LB deposition, and this produces smoother films with a greater control of the thickness of the film.16,20 Some adsorption studies have been made using cellulose fibers.21,22 The cellulose surfaces produced by LB deposition or spin-coating from TMSC are initially hydrophobic due to the terminal methyl groups on the TMSC. To make the initially hydrophobic surface hydrophilic, exposure to the vapor of concentrated HCl23 cleaves the methyl groups and makes the surface hydrophilic. Fält et al.18 studied the effect of electrolyte and pH on the swelling of cellulose films produced by spin-coating using quartz microbalance measurements (QCM) and contrasted it with the behavior of cellulose fibers. Gunnars et al.19 reported on the development of spin-coated cellulose films and their surface characterization. Zhang et al.20 described the LB deposition of cellulose films and used X-ray and neutron reflectivity to characterize the films. Holmberg et al.16 used a surface forces apparatus (SFA) and ellipsometry to study LB deposited cellulose films in their hydrophobic and hydrophilic forms and their swelling in solvent. The swelling of cellulose Received: May 11, 2012 Revised: June 25, 2012 Published: June 26, 2012 10773
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amounts and structure of the adsorbed layer were not accessible. Notley48 and Enarsson and Wägberg49 demonstrated the impact of the highly charged polyelectrolyte polydmdaac adsorption on charged cellulose films, in inducing deswelling in water. Although surfactant and polymer−surfactant adsorption onto cellulose surfaces has been widely studied in recent years, there are a number of key issues that remain unresolved. In particular, the conditions under which surfactants can penetrate cellulose surfaces rather than just adsorb onto the surface are not established with any certainty. Furthermore, how polyelectrolytes affect the surfactant adsorption onto cellulose and the subsequent structure of the cellulose surface is not clear. We have previously demonstrated how NR can be used to study both adsorption on cellulose surfaces and changes in the structure of the surface.24,25 In this study we have used NR to understand how polyelectrolytes can be used to manipulate surfactant adsorption onto cellulose surfaces and the structure of the cellulose surface. In particular, we have investigated the adsorption of SDS onto a hydrophobic cellulose surface (before regeneration) and how adsorption of the cationic polyelectrolyte polydmdaac before and after the SDS adsorption and electrolyte NaCl modifies the adsorption and the structure of the cellulose surface. This paper is complemented by a second paper50 where the effects of polydmdaac and electrolyte on the adsorption of SDS and the structure of the cellulose thin film are investigated for hydrophilic (regenerated) cellulose films.
gels and the impact of pH and electrolyte were explored by Grignon and Scallan.30 Penfold et al.24 used NR to quantify the nature of hexadecyltrimethylammonium bromide (CTAB) cationic surfactant adsorption in monolayer and cooperative adsorption on hydrophobic and hydrophilic cellulose surfaces, respectively. In a subsequent study they showed that the adsorption of a nonionic surfactant, hexaethylene glycol monododecyl ether (C12E6), and its coadsorption with CTAB resulted in swelling and penetration of surfactant and solvent into the cellulose film.25 AFM studies by Notley et al.26 agreed with the NR data for CTAB, but no penetration of the cellulose was observed for the nonionic surfactants hexaethylene glycol monotetradecyl ether (C14E6) or octaethyelene glycol monohexadecyl ether (C16E8).27 Torn et al.28 studied the adsorption and adsorption kinetics of a range of different nonionic surfactants onto cellulose using ellipsometry. The relatively high adsorption on cellulose compared to the adsorption on silica was attributed to a swelling of the cellulose and penetration of the surfactant into the cellulose surface. Alila et al.21 reported the adsorption of a range of cationic surfactants, including CTAB, onto cellulose fibers and observed cooperative adsorption on a hydrophilic surface similar to that reported by Penfold et al.24 and Notley et al.27 Paria et al.22 studied mixtures of the anionic surfactant sodium dodecylbenzenesulfonate (LAS) and CTAB with the nonionic surfactant polyoxyethylene octyl phenyl ether Triton X100. From the measurements on cellulose in the form of filter paper, it was difficult to disentangle the different contributions to the adsorption as there was a mixture of hydrophobic and hydrophilic adsorption sites. Mixed polymer−surfactant adsorption at interfaces has been extensively studied,31,32 and the strong surface interaction and surface complex formation between polyelectrolytes and ionic surfactants of opposite charge give rise to enhanced surfactant adsorption down to very low surfactant concentrations.33,34 Although polymer−surfactant adsorption has been most extensively studied at the air−solution interface, it also provides the ability to manipulate adsorption on solid surfaces. Polyelectrolytes adsorb as a thin layer onto hydrophilic surfaces of opposite charge, resulting in charge reversal.35−37 This is the mechanism responsible for the formation of polyelectrolyte multilayer structures which are exploited in encapsulation and controlled drug release.37 Penfold et al.38 showed how the charge reversal could be used to manipulate ionic surfactant adsorption on to hydrophilic silica, using the anionic polymer poly(styrenesulfonate) (PSS) and the cationic polymer polydmdaac. Numerous other studies have used a variety of surface techniques to probe polyelectrolyte−surfactant adsorption39−42 on silica. More recently, Zhang et al.43 have shown how the pH and ethoxylation of the pH-sensitive polyelectrolyte poly(ethylenimine) (PEI) results in a more complex manipulation of SDS adsorption at the hydrophilic surface. The nature of polyelectrolyte adsorption onto cellulose surfaces has also been studied.44−47 Tammelin et al.44 studied the adsorption of polydmdaac onto LB deposited cellulose films by AFM and QCM. Horvath et al.45 also studied the adsorption of polydmdaac and polyacrylamide−polydmdaac copolymers onto cellulose fibers, with particular emphasis on adsorption kinetics and penetration into the fiber surfaces. Lefebvre and Gray46 used AFM to study polyelectrolyte adsorption onto spin-coated cellulose surfaces. Salmi et al.47 also used AFM to probe polyelectrolyte adsorption onto a cellulose surface, and although strong adsorption was observed, the adsorbed
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EXPERIMENTAL METHODS AND MATERIALS
The specular reflection of neutrons from surfaces and interfaces provides information about refractive index or neutron scattering length density profile in a direction normal to the surface. The neutron refractive index, n(z), and scattering length density, ρ(z), are equated by n(z) = 1 −
λ2 ρ(z) 2π
(1)
where λ is the neutron wavelength and z is the direction normal to the surface. The variation in reflectivity R(Q) with wave vector transfer normal to the surface, Q (where Q is defined as Q = 4π sin θ/λ and θ the grazing angle of incidence) is related to the scattering length density distribution, ρ(z), normal to the surface.51 The technique provides information about the structure and composition at the interface and has been extensively applied to surfactant and mixed surfactant adsorption at the air−water51 and solid−solution3 interfaces. The sensitivity of NR arises the different scattering powers of H and D, which means that by selective deuteration changes to ρ(z) can used to manipulate the reflectivity. The NR data can be modeled using the kinematic approximation or the optical matrix method for thin films adapted from optical reflectivity,52 and in the analysis of the data presented here the optical matrix method has been used.53 Throughout the analysis of the NR data the simplest model (least number of layers) which adequately describes the data is adopted. The scattering length density, ρ, of silicon, cellulose, D2O, and SDS is 2.1 × 10−6, 1.0 × 10−6, 6.35 × 10−6, and 0.4 × 10−6 Å−2, respectively. The specular neutron reflectivity measurements were made on the SURF reflectometer54 at the ISIS pulsed neutron source. The measurements were made in the Q range of 0.012−0.4 Ǻ −1, using the neutron wavelength range 1−7 Ǻ and three different grazing angles of incidence: 0.35°, 0.8°, and 1.8°. The resolution in Q, ΔQ/Q, was ∼4% and was determined by the collimation of the incident neutron beam. The reflection geometry is such that the neutron beam is incident at grazing incidence in the horizontal plane at the solid− solution interface by transmission through the upper crystalline silicon phase. The surface of the silicon, ⟨111⟩, supplied by Crystran, was polished to a surface roughness ≤5 Ǻ rms, and the illuminated area 10774
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was ∼30 × 60 mm2. The cell uses a thin solvent gap (≤10 μm) and a sample volume ∼0.2 mL. Solutions are exchanged using a Merck LaChrome Chromatography HPLC mixing pump, and exchange of ∼20 mL of sample provides efficient sample changing, rinsing, and cleaning. The data were normalized for the incident beam spectral distribution, run time, and detector efficiency and established on an absolute scale by reference to the direct beam intensity.55 Typical measurement times to cover the entire Q range were ∼60 min. The SDS (C12H25SO4−Na+) was obtained from PolySciences. The chemical purity of the SDS was checked using surface tension measurements, and no minimum in the surface tension was observed at the cmc. The cationic polymer polydmdaac was synthesized as described by Warren56 and had a molecular weight ∼100k Da. The NR measurements and rinsing were all made in D2O, and the D2O was obtained from Sigma-Aldrich All the measurements were made at 25 °C to ensure that the solutions were above the Krafft point of the SDS. The measurements were made in the absence and presence of electrolyte (0.1 M NaCl). The cmc of SDS in the absence of electrolyte and in 0.1 M NaCl is 9 and 1.5 mM, respectively.57 The thin cellulose films used in the study were deposited onto silicon substrates by repeated dipping using the LB deposition method.23 Prior to the cellulose deposition, the silicon blocks were first exposed to a “mild piranha” treatment55 to produce an oxide layer at the surface with a well-defined thickness and scattering length density. The surfaces were made hydrophobic by deposition of 1,1,1,3,3,3-hexamethyldisilazan (Merck) from dilute solution in chloroform, prior to the deposition of TMSC from a Langmuir trough. The TMSC was spread at the air−water interface at the air− water interface from a chloroform solution, in the concentration range 0.5−0.8 mg/mL. The transfer procedure is described in detail elsewhere,23 and the dipping/pulling sequence was carried out at a constant rate of 5 mm/min and at a constant surface pressure of 20 mN/m. Repeating the dip/pull cycle ten times with a high transfer ratio deposited a cellulose layer of about ∼100 Å. Three different cellulose surfaces on two different silicon substrates were used and are labeled as block 1/side A, block 1/side B, and block 2/side A.
ization of the cellulose surface in D2O gives rise to a welldefined interference fringe consistent with a cellulose layer with a total thickness ∼110 Å. The simplest model consistent with the data is a two-layer model, which represents an inhomogeneous distribution or a gradient of density or composition, as described in Table 1 (each layer is characterized by a thickness, d, a scattering length density, ρ, an interfacial roughness, σ, where the subscript 1 refers to the layer adjacent to the silicon solid substrate, and σs is the interfacial roughness between the substrate and the initial layer). The structure and thickness of the cellulose layer are broadly similar to those previously reported by Tucker et al.,25,50 but have some notable differences. A second characterization of the hydrophobic surface, after exposure of the surface to 0.1 M NaCl/D2O for ∼2−3 h produced a slightly different NR profile, as shown in Figure 1. The pronounced interference fringe visible in the initial data NR data is still present but is shifted to lower Q values, consistent with a thicker layer. The model fit and associated model parameters (see Table 1) show that the cellulose film is still characterized by two layers, similar to the initial characterization, but that the individual thicknesses and the total thickness have all increased. The overall thickness has increased from ∼110 to ∼125 Å. The scattering length density of the two regions of the film has increased, and this is consistent with penetration of the D2O into the cellulose film. From the initial characterization of the cellulose film the volume fraction of cellulose (calculated from ρ = φcρc + (1 − φc)ρs where ρ is the fitted value, ρc and ρs are the scattering length densities of cellulose and D2O, and φc is the volume fraction of cellulose) in the layers adjacent to the silicon and solvent are 0.5 and 0.8, respectively. After swelling the volume fractions have decreased to 0.48 and 0.68, respectively. This indicates that not only has the film swelled but that there is a change in the internal distribution of the cellulose within the film. Although the total thickness of the film is similar to that reported in refs 25 and 50, the distribution within the film is different. For the hydrophobic film studied in ref 50 the layer adjacent to the solid surface is more dense, and the opposite observation is made here for the hydrophobic film. This difference may be due to slight differences in the deposition conditions and in the initial surface preparation. Previous NR studies on hydrophobic cellulose surfaces observed no swelling of the cellulose in only solvent, but the measurements were made in the absence of electrolyte. They did, however, report swelling and solvent penetration with the addition of CTAB24 and to a greater extent by the addition of C12E6.25 Holmberg et al.16 reported the swelling of dry cellulose films by humid air and water and from AFM measurements described the film as a swollen network with some protruding chains. Fält et al.18 compared the swelling of cellulose films and fibers at different pH and added electrolyte (ionic strength). These results imply that the addition of electrolyte is an important factor in controlling the solvent-induced swelling of cellulose films. This is consistent with the observations of Grignon and Scallan,30 who observed the effects of pH and electrolyte on the swelling of cellulose gels. An important difference here is that the swelling takes place in a nominally hydrophobic cellulose film, whereas the other observations were for hydrophilic cellulose. The addition of SDS at concentrations less than 1 mM (measurements were made at SDS concentrations of 10−4, 3 × 10−4, and 7 × 10−4 M) resulted in no measurable change in the
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RESULTS AND DISCUSSION Measurements in 0.1 M NaCl. The main features of the initial sequence of NR measurements on the hydrophobic cellulose surface (block 1, side A) in the presence of 0.1 M NaCl are shown in Figure 1, and the associated key model parameters are summarized in Table 1. The initial character-
Figure 1. Neutron reflectivity for hydrophobic cellulose in 0.1 M NaCl, block 1, side A, (black) initial D2O, (red) D2O after 2−3 h, (blue) 1 mM SDS/D2O, (green) D2O (post-SDS adsorption). The solid lines are model fits using the model parameters in Table 1. The data are shifted vertically for clarity. 10775
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Table 1. Key Model Parameters for Hydrophobic Cellulose in 0.1 M NaCl, Block 1, Side A, for NR Data and Model Fits Shown in Figure 1 sample
σs (±2 Å)
d1 (±5 Å)
ρ1 (±0.2 × 10−6 Å−2)
σ1 (±2 Å)
d2 (±5 Å)
ρ2 (±0.2 × 10−6 Å−2)
σ2 (±2 Å)
D2 O D2O (after 2−3 h) 10−3 M SDS/D2O D2O (post-SDS adsorption)
10 10 35 25
57 72 86 123
3.6 3.8 2.0 2.9
5 10 5 10
44 53 48
2.0 2.7 3.2
10 10 10
Table 2. Key Model Parameters for Hydrophobic Cellulose in 0.1 M NaCl, Block 1, Side A, for NR Data and Model Fits Shown in Figure 2 sample D2O (post-50 ppm polydmddac) 7 × 10−4 M SDS/D2O 4 mM SDS/D2O D2O (post-SDS adsorption)
σs (±2 Å)
d1 (±5 Å)
ρ1 (±0.2 × 10−6 Å−2)
σ1 (±2 Å)
d2 (±5 Å)
ρ2 (±0.2 × 10−6 Å−2)
σ2 (±2 Å)
20
79
3.6
10
56
3.1
5
20 35 30
73 151 122
3.4 3.2 3.0
10 10 5
15 12
2.8 0.5
5 10
d3 (±5 Å)
ρ3 (±0.2 × 10−6 Å−2)
σ3 (±2 Å)
39 46
3.6 5.3
5 10
Table 3. Key Model Parameters for Hydrophobic Cellulose, Block 1, Side B, for NR Data and Model Fits Shown in Figures 3 and 4 sample D2O 4 mM SDS/D2O 10 mM SDS/D2O D2O (post-SDS adsorption) 10 mM SDS/D2O (post-polydmdaac adsorption)
σs (±2 Å) 24 54 80 28 24
d1 ρ1 (±0.2 × 10−6 (±5 Å) Å−2) 68 182 180 64 64
3.3 2.3 2.4 2.6 2.7
σ1 (±2 Å) 5 10 5 10 10
d2 ρ2 (±0.2 × 10−6 (±5 Å) Å−2) 69 26 59 42 36
2.9 3.7 2.8 3.0 3.2
σ2 (±2 Å)
d3 (±5 Å)
ρ3 (±0.2 × 10−6 Å−2)
10 5 5 10 10
36
1.8
σ3 (±2 Å)
the initial swollen state of the cellulose film prior to the exposure to the SDS. The NR data can now be modeled by a single layer, with a thickness ∼123 Å, similar to the total thickness of the film before exposure to SDS, but with a different structure and density distribution. Hence the original state of the film structure is not entirely restored, and the effects of the swelling/penetration by SDS/solvent are only partially reversible. The scattering length density of the resultant cellulose film is consistent with some SDS being retained within the film. However, assuming no SDS retention then the volume fraction of cellulose would be ∼0.65. The parameters before and after exposure to SDS are consistent with no removal of the cellulose from the surface. The same cellulose surface (block 1, side A), post-SDS adsorption and rinsing in D2O, was exposed to a 50 ppm polydmdaac/D2O/0.1 M NaCl solution, and this is the initial profile in Figure 2. From previous studies38,43 it might be expected that the polyelectrolyte would adsorb as a thin layer on the surface or not adsorb at all. In either case, an immeasurably small change to the NR profile would be expected. The data in Figure 2 and model parameters in Table 2 show that structure of the cellulose film is restored to a form closer to the initial state of the film before the exposure to SDS. Indeed, comparing the equivalent data in Tables 1 and 2 (the comparison of the post swelling in D2O structure in Table 1 with the structure post the addition of polydmdaac in Table 2), the total film thickness and internal structures are broadly similar. That is, the values for the thickness of the two layers change from 72 and 53 Å to 79 and 56 Å and the associated volume fractions of cellulose from 0.48 and 0.68 to 0.51 and 0.61. This can be explained as some of the SDS remaining in the swollen cellulose film after rinsing in D2O/0.1 M NaCl being desorbed in the presence of the polydmdaac to form bulk
NR (data not shown), and this implies that in this concentration range the SDS did not adsorb or change the structure of the cellulose film. The addition of 10−3 M SDS upward (measurements were made at 1, 2, 4, and 10 mM SDS) resulted in a marked change in the NR profile, as shown in Figure 1 for 1 mM SDS, but the change was independent of SDS concentration in the concentration range 1−10 mM. The NR data for the addition of 1 mM SDS are modeled by twolayer structure (see Table 1) which is consistent with a further swelling of the cellulose film and a penetration of SDS and solvent into the film, rather than adsorption of SDS onto the film. However, there is insufficient information to make a quantitative estimate of the changes in terms of amounts of cellulose, D2O, and surfactant. The other feature associated with the parameters from the model calculations in the variation in σs, the interfacial roughness between the silicon substrate and the cellulose film. It has an initial value, preadsorption and swelling, ∼10−20 Å, and this accounts for the thin oxide layer and the hydrophobic hxeamethyldisilazan layer on the silicon surface. As shown in Table 2, σs increases when the cellulose film swells due to the exposure to the higher SDS concentrations (similar observations are also reported in Tables 2 and 3). This increase is attributed to a reorganization within the cellulose film that results in lateral inhomogeneities near the silicon−cellulose interface and which are manifest as an apparent increase in the interfacial roughness/diffusiveness. This is consistent with the subsequent reduction in σs when the cellulose film relaxes back to a structure closer to its original structure. Rinsing twice in 0.1 M NaCl/D2O after the sequence of measurements at the different SDS concentrations results in a further change in the measured NR profile, as shown in Figure 1. The form of the NR profile has now changed to one closer to 10776
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et al.45 have reported the adsorption and penetration of polydmdaac and polyacrylamide−polydmdaac copolymers into cellulose fibers. Lefebvre and Gray46 and Salmi et al.47 observed strong adsorption of polydmdaac onto cellulose surfaces but were unable to determine the impact of the adsorption on the structure of the cellulose film. Tammelin et al.44 also reported the adsorption of polydmdaac onto cellulose surfaces using QCM and AFM. What is not clear from the sequence of measurements described so far is to what degree the structure of the cellulose film after the initial SDS adsorption sequence affects the impact of the polydmdaac. Hence, a further sequence of NR measurements was made for a new cellulose surface (labeled block 2, side A), and the results are presented in detail in Figure S1 and Table S1 of the Supporting Information. The structure of the cellulose film was initially characterized in 0.1 M NaCl/ D2O. The cellulose film was deposited on a fresh silicon block, and the structural parameters were broadly similar to those that were presented in Table 1, but in detail the distribution of cellulose within the film is different and closer to that reported in ref 50. The surface was then exposed to 50 ppm polydmdaac/0.1 M NaCl/D2O, and this resulted in no measurable change in the NR. The adsorption of 7 × 10−4 M and 1 mM SDS resulted in the adsorption of a monolayer of SDS and some compression of the outer surface of the cellulose, but otherwise no substantial change to the cellulose film (see Table S1) occurred. This is different to what was reported earlier and implies that at the lower SDS concentrations the history of the film and its detailed structure has some significance. However, this may also be due to a difference in the uptake of the polydmdaac. At the higher SDS concentrations (measurements were made at 2, 4, and 10 mM) the NR data are different to the data for the lower SDS concentrations and independent of SDS concentration. As shown in Figure S1 and Table S1, the data and the modeling now imply a substantial swelling of the cellulose layer and adsorption into that swollen layer. Indeed the model parameters are broadly similar to those at 4 mM as given in Table 2. Hence, it can be concluded that the cellulose film modified by the preadsorption of SDS has some impact upon the subsequent adsorption of polydmdaac and SDS and that the impact is greater at the lower SDS concentrations. However, it is also clear from both sequence of measurements that the introduction of the polydmdaac has an important impact on the modification (penetration and swelling) of the cellulose film by the SDS or SDS/polydmdaac mixtures. Measurements in the Absence of Electrolyte. A sequence of NR measurements, similar to those made in 0.1 M NaCl, were made on a fresh hydrophobic cellulose surface (labeled block 1, side B) in the absence of electrolyte. The NR data for the bare hydrophobic cellulose surface in D2O show a pronounced interference fringe, similar to that shown in Figure 3 for block 1, side A. The detailed analysis (the key model parameters are summarized in Table 3) shows that the structure of the cellulose film is similar to block 1, side A, but that the thicknesses of the two layers and the corresponding densities are slightly different. The overall thickness of the film is ∼140 Å, compared to a thickness ∼110 Å for block 1, side A, before swelling in electrolyte. A notable difference in the observed behavior was that the previous cellulose film (block 1, side A) was swelled by exposure to 0.1 M NaCl/D2O, whereas this cellulose film (block 1, side B) was not swelled by the solvent (D2O) in the absence of electrolyte.
Figure 2. Neutron reflectivity for hydrophobic cellulose in 0.1 M NaCl, block 1 side A, (black) D2O, post-SDS adsorption from Figure 1 and 50 ppm polydmdaac/D2O, (red) 7 × 10−4 M SDS/D2O, (blue) 4 mM SDS/D2O, and (green) D2O, post-SDS adsorption. The solid lines are model fits using the model parameters in Table 2. The data are shifted vertically for clarity.
surfactant/polymer complexes in solution. This implies that the complex formation is energetically more favorable than the SDS adsorption within the cellulose film. Furthermore, this would reinforce the inference from the measurements that prior to the introduction of the polydmdaac some SDS was retained within the cellulose film. However it is not possible from these measurements to conclude directly if there was any polydmdaac adsorption onto or penetration into the cellulose film. Adsorption of SDS, post exposure to the dilute polydmdaac solution, in the concentration range 10−4−10−3 M SDS (measured at 10−4, 3 × 10−4, and 7 × 10−4 M SDS) results in a slight change to the measured NR (as shown in Figure 2). The simplest model consistent with the data is a three-layer model, as summarized in Table 2, and can be explained as adsorption and swelling of the outer region of the cellulose film by the SDS and solvent. NR measurements as higher SDS concentrations (measured at 2, 4, and 10 mM) were different than those at the lower concentration, but independent of SDS concentration within that measured concentration range. The change in the measured NR profile, from 1 to 2 mM SDS, is more significant, and the interference fringe visible in the NR data is shifted to lower Q values. The model parameters (in Table 2) are consistent with a marked swelling of the film and adsorption of SDS into the film, but again it is not possible to quantify the amounts of the individual components in the layers. On rinsing in D2O/0.1 M NaCl the NR data are similar to its initial state before the initial exposure to SDS (see Table 1). Although the film can now be adequately described as a single layer of uniform density or composition, it is also similar to the data post the exposure to polydmdaac (see Table 2). Hence, the structure of the cellulose film, post-SDS adsorption, is partially restored on rinsing in D2O/0.1 M NaCl. The significant changes in the swelling behavior with SDS before and after the exposure to the polydmdaac imply that polydmdaac as well as SDS is adsorbed into the swollen cellulose film. Hence, the presence of the polydmdaac within the cellulose film is responsible for the greater degree of swelling that occurs. Some penetration into the cellulose film as well as adsorption is supported in the recent literature. Horvath 10777
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Figure 3. Neutron reflectivity for hydrophobic cellulose, no electrolyte, block 1, side B, (black) D2O, (red) 4 mM SDS/D2O, (blue) 10 mM SDS/D2O, (green) D2O, post-SDS adsorption. The solid lines are model fits using the model parameters in Table 3. The data are shifted vertically for clarity.
Figure 4. Neutron reflectivity for hydrophobic cellulose, no electrolyte, block 1, side B, (black) D2O, postexposure to 50 ppm polydmdaac/D2O, (red) 10 mM SDS/D2O. The solid lines are model fits using the model parameters in Table 3. The data are shifted vertically for clarity.
The addition of SDS in the concentration range 10−4−2 × 10−3 M (NR measurements were made at SDS concentrations of 10−4, 3 × 10−4, 7 × 10−4, 10−3, and 2 × 10−3 M) resulted in no measurable change to the NR data for D2O, and hence there was no measurable adsorption or impact on the structure of the cellulose film in this concentration range. At higher SDS concentrations, 4 and 10 mM, the NR profiles change significantly (see Figure 3) as the cellulose film swells due to the adsorption of SDS into the cellulose film. The model parameters in Table 3 indicate that the cellulose film swells significantly and that there is an outer layer that contains cellulose, solvent, and SDS which increases as the SDS concentration increases from 4 to 10 mM. The impact of SDS on the cellulose film is broadly similar to that observed in 0.1 M NaCl, except that the effects are greater. Rinsing in D2O, as shown from the parameters in Table 3, restores the structure of the cellulose film to a structure close to the initial structure, and so the impact of the adsorption and swelling is at least partially reversible. In detail, the structure has subtly changed. The layer adjacent to the silicon remains with a thickness ∼66 Å, but the volume fraction of cellulose has increased from 0.57 to 0.7, whereas the layer adjacent to the solvent has decreased in thickness from 69 to 42 Å and with a similar volume fraction of cellulose, ∼0.64. The addition of a solution of 50 ppm polydmaac in D2O has no measurable impact upon the NR profile, as was observed in 0.1 M NaCl (NR data not shown). The subsequent addition of SDS in the concentration range 10−4−10−3 M (measurements were made at 10−4, 3 × 10−4, 7 × 10−4, and 10−3 M) also resulted in no measurable adsorption or impact upon the structure of the cellulose film (NR data not shown). As illustrated in Table 3, and shown in Figure 4 for 10 mM SDS, the addition of SDS at higher surfactant concentrations (measured at 2, 4, and 10 mM) now results in a measurable change in the NR which is independent of SDS concentration between 2 and 10 mM. The data are now best described as a cellulose structure which is similar to the structure of the cellulose film before adsorption, with an additional layer of surfactant ∼36 Å on the surface. This is rather thick for the monolayer that would be expected on a hydrophobic surface.
This implies that some polydmdaac adsorption onto the surface has made the surface more hydrophilic and hence has promoted self-assembly on the surface. Rinsing in D2O restores the surface and cellulose film to the structure obtained before adsorption. In the absence of electrolyte, SDS and polydmdaac have broadly similar effects on the cellulose thin film, but there are some key differences. In the absence of electrolyte, solvent alone does not appear to swell the cellulose film, the swelling of the cellulose film at the higher SDS concentrations is greater, and the addition of polydmdaac inhibits swelling rather than enhancing it. Furthermore, in the absence of electrolyte the surfactant adsorption and swelling of the cellulose film are generally more reversible to rinsing in solvent. Although there are a number of reports on the adsorption of polydmdaac onto cellulose,44−47 there was no detailed information about the impact of the adsorption on the structure of the cellulose film, apart from the report of penetration of the polyelectrolyte into cellulose films by Horvath et al.45 Notley48 and Enarsson and Wägberg,49 however, report directly the impact of polydmdaac on the deswelling of highly charged cellulose films. The observations reported here are different in that the impact of the polyelectrolyte and electrolyte are on the adsorption and swelling of a hydrophobic cellulose film by surfactant (SDS) and solvent, and this adds a new dimension to the phenomena previously observed.
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SUMMARY AND CONCLUSIONS In the presence of electrolyte (0.1 M NaCl) there is no SDS adsorption or change in the structure of the cellulose film for SDS concentrations less than 1 mM. For SDS concentrations greater than 1 mM the SDS penetrates and swells the cellulose film, and rinsing in D2O shows that the changes are not reversible. However, the addition of 50 ppm polydmdaac solution results in an almost complete restoration of the original cellulose film structure, and this is attributed to the more favorable formation of SDS/polydmdaac solution complexes. SDS adsorption after the exposure of the cellulose surface to a dilute polydmdaac solution (for both a fresh surface 10778
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and that previously subjected to SDS adsorption) results in a more substantial swelling of the cellulose film by SDS penetration and adsorption of SDS onto the cellulose film, and this is now partially reversible. The greater effects observed after the introduction of the polydmdaac imply that polydmdaac as well as the SDS is incorporated into the film. In the absence of electrolyte a broadly similar pattern of behavior is observed, but the onset of the effects are shifted to higher SDS concentrations, consistent with a change in the critical micellar concentration (cmc) and the critical aggregation concentration (cac) with the change in ionic strength. The adsorption onto the cellulose surface and the penetration and swelling of the cellulose film on the exposure to SDS solutions now occur for SDS concentrations greater than 4 mM. Although much greater effects are observed, they are largely reversible and the initial cellulose structure is partially restored. After exposure of the surface to a 50 ppm polydmdaac solution, the subsequent exposure to a SDS solution has a different impact. For SDS concentrations less than 2 mM there is no adsorption of an SDS layer onto the polydmdaac-coated cellulose surface and little impact upon the structure of the cellulose surface. For SDS concentrations greater than 2 mM there is now adsorption onto the cellulose surface, but no effect on the underlying cellulose structure. The results show that in some circumstances polyelectrolytes can be used to reverse or partially reverse the surfactant adsorption, penetration, and swelling of the cellulose structure, which are not reversible simply by dilution in solvent. Furthermore, the results show that electrolyte has a significant impact upon the surfactant adsorption, the swelling of the cellulose film, and the impact of the polydmdaac. Hence, the results show how electrolyte and polyelectrolyte can be used to manipulate the nature of anionic surfactant adsorption onto hydrophobic cellulose surfaces and the structure of the cellulose film.
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ASSOCIATED CONTENT
S Supporting Information *
Additional data plots (NR data) and tables of model parameters. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: jeff
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded through the provision of neutron beam time at the ISIS Facility, UK (STFC), and the Instutit Laue Langevin, Grenoble. The invaluable assistance of the ISIS scientists, Arwell Hughes, Max Skoda, and Phil Taylor, in providing a well-optimized reflectometer, SURF, for these measurements is greatly appreciated.
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