pubs.acs.org/Langmuir © 2010 American Chemical Society
Adsorption of Nonionic and Mixed Nonionic/Cationic Surfactants onto Hydrophilic and Hydrophobic Cellulose Thin Films I. Tucker,*,† J. Petkov,† J. Penfold,‡,§ and R. K. Thomas§ †
Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, United Kingdom, ‡ISIS Facility, STFC, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, United Kingdom, and §Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, United Kingdom Received January 4, 2010. Revised Manuscript Received January 28, 2010
The adsorption of the nonionic surfactant hexaethylene monododecyl ether, C12E6, and the mixed nonionic/cationic surfactants C12E6 and hexadecyl trimethyl ammonium bromide, C16TAB, onto the hydrophilic and hydrophobic surfaces of thin cellulose films, formed by Langmuir-Blodgett, L-B, deposition, have been studied by neutron reflectivity. For the surfactant mixtures, considerable nonideal mixing is observed at both hydrophobic and hydrophilic surfaces. The results demonstrate that the C12E6, C12E6/C16TAB mixture and solvent have a greater penetration into the cellulose film upon adsorption, compared to that observed in previous studies of C16TAB adsorbed onto cellulose, due to the presence of the nonionic surfactant. From the range of measurements made, it is concluded that both the presence of the nonionic surfactant and the nature of the cellulose films are both contributing factors to this increased penetration and swelling of the cellulose film.
Introduction Understanding the nature of surfactant and mixed surfactant adsorption at the solid-solution interface is important in the context of a wide range of key industrial, technological, and domestic applications, such as lubrication, detergency, and surface conditioning.1 In recent years, the applications of advanced surface techniques, such as atomic force microscopy, AFM,2 neutron and X-ray reflectivity,3 and ellipsometry,4 have transformed our ability to probe and our understanding of the adsorption process at the solid-solution interface. Such techniques have provided information on the nature of the adsorbed layer and the adsorption mechanism,5 and on the kinetics of adsorption.6 However, the studies to date have been primarily on highly idealized model hydrophilic or hydrophobic surfaces, on hydrophilic surfaces such as silicon, quartz, or mica, and on hydrophobic surfaces such as graphite or surfaces modified by a variety of different self-assembled monolayers, SAMs. Of increasing current interest is the study of interfaces which have specific and different functionalities, which include polymeric surfaces,7 and surfaces formed from different SAMs, for example, hydroxylated,8 amino functionalized,9 and carboxyl terminated thiols.10 Surfactants adsorbed onto such surfaces have demonstrated a variety of different surface affinities and different surface structures. Another significant trend in this general area is (1) Scamehorn, J. F. In Phenomena in mixed surfactant systems; ACS Symposium Series 311; Scamehorn, J. F., Ed.; American Chemical Society: Washington, DC, 1986. (2) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (3) Penfold, J.; Thomas, R. K. J. Phys.: Condens. Matter 1990, 2, 1369. (4) Tiberg, F. R. J. Chem. Soc., Faraday Trans. 1996, 92, 215. (5) Penfold, J.; Thomas, R.K. In Advanced chemistry of monolayers at interfaces; Imae, I., Ed.; Elsevier: London, 2006; Chapter 4. (6) Torn, L. H.; Koopal, L. K.; de Keizer, A.; Lyklema, J Langmuir 2005, 21, 7768. (7) Turner, S. F.; Clarke, S. M.; Rennie, A. R.; Thirtle, P. N.; Cooke, D. J.; Li, Z. X.; Thomas, R. K. Langmuir 1999, 15, 1017. (8) Thirtle, P. N.; Li, Z. X.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Sung, L. P. Langmuir 1997, 13, 5451. (9) Song, X.; Wang, J.; Jiana, L. J. Colloid Interface Sci. 2006, 298, 267. (10) Himmel, H. J.; Terfort, A.; Woll, C. J. Am. Chem. Soc. 1998, 120, 12069.
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the study of model surfaces that mimic specific surfaces, such as the hair cuticle,4 the corneum stratum of the skin,10 and fabrics, such as cotton or polyester.11 In the context of fabric surfaces, the formation and study of thin cellulose films is of much current interest and activity6,11-19 and is closely related to the subject of this paper. Predominantly the model thin cellulose films studied have been prepared by two different routes. Spin-coating trimethylsilylcellulose, TMSC, from a volatile solvent such as chloroform onto a substrate such as silicon produces smooth films ∼1000 A˚, depending upon concentration, solvent, and spin speed.6 Langmuir-Blodgett, L-B, deposition from a spread monolayer of TMSC produces thinner films which depend upon the surface coverage and the number of sequential depositions11 and are typically e100 A˚. In both cases, the films produced are initially hydrophobic, due to the terminal methyl groups of the TMSC. These are cleaved during a regeneration process (requiring exposure to vapor of concentrated hydrochloric acid (HCl)) to produce a predominantly hydrophilic surface, which readily wets and remains wetted when exposed to water. This latter L-B deposition approach is ideally suited to the interfacial length scale probed by neutron reflectivity, and has been used in a previous preliminary study of the adsorption of cationic surfactant, hexadecyltrimethylammonium bromide, C16TAB, onto both hydrophilic and hydrophobic cellulose surfaces.20 In that study, it was demonstrated that reproducible smooth thin (11) Swift, J. A. J. Cosmet. Sci. 1999, 50, 23. (12) dela Maza, A.; Baucells, J.; Ensenat, P. G.; Parra, J. L. J. Colloid Interface Sci. 1996, 185, 155. (13) Holmberg, M.; Berg, J.; Stemme, S.; Odberg, L.; Rasmusson, J. J. Colloid Interface Sci. 1997, 186, 369. (14) Falt, S.; Wagberg, L.; Vesterlund, E. L. Langmuir 2003, 19, 7895. (15) Alila, S.; Bouti, S.; Balgacen, M. N.; Benveneti, D. Langmuir 2005, 21, 8106. (16) Paria, S; Manohar, C; Khalir, K. C. J. Inst. Eng., Singapore 2003, 43, 34. (17) Zhang, Y.; Tim, Z.; Ritcey, A. M. Langmuir 2004, 20, 6187. (18) Zimin, D.; Craig, V. S. J.; Kunz, W. Langmuir 2004, 20, 8114. (19) Tarada, E.; Samoshina, Y.; Nylander, T.; Lindman, B. Langmuir 2004, 20, 1753. (20) Penfold, J.; Tucker, I.; Petkov, J.; Thomas, R. K. Langmuir 2007, 23, 8357.
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cellulose films (thickness ∼ 40 A˚) could be prepared and characterized in both hydrophobic and hydrophilic states. Adsorption of C16TAB onto the hydrophilic cellulose surface was similar to that on hydrophilic silica and was in the form of surface aggregates. In contrast, adsorption onto the hydrophobic cellulose surface was in the form of a monolayer. The neutron reflectivity measurements showed that for the hydrophobic cellulose some intermixing between the cellulose and surfactant occurred, whereas there was little penetration of surfactant into the hydrophilic cellulose film. Furthermore, the measurements showed that solvent exchange between the partially hydrated cellulose and the solvent was slow. As highlighted earlier in the Introduction, there have been some relatively recent studies of surfactant adsorption onto cellulose surfaces, surfaces created from spin-coating, L-B deposition, or in the form of cellulose fibers. However, there is little information about the behavior of mixed surfactants at such surfaces. As discussed in detail in the previous paragraph, Penfold et al.20 used neutron reflectivity to study the adsorption of C16TAB onto both hydrophilic and hydrophobic cellulose surfaces, to characterize the bare cellulose surfaces, and to investigate the impact of the surface regeneration and adsorption on the cellulose surface. In a recently published paper, Notley21 used soft-contact AFM to confirm and support the nature of C16TAB adsorption onto cellulose and to provide information on the lateral surface structure. Holmberg et al.13 have used a surface force apparatus, SFA, and ellipsometry to study L-B deposited cellulose films, reporting the formation of atomically smooth surfaces and the extent of the swelling of dry films in solvent. Falt et al.14 investigated the swelling behavior of cellulose films, formed from spin-coating, with different charge densities. A similar swelling behavior, dependent upon pH and ionic strength, to that observed in cellulose fibers was reported. Alila et al.15 reported the adsorption of a range of cationic surfactants, of different alkyl chain lengths, onto cellulose fibers. Cooperative adsorption, similar to that reported by Penfold et al.20 and more generally observed on hydrophilic surfaces,5 was reported, and the role of surface charge density on the adsorption was explored. One of the few studies of mixed surfactant adsorption onto cellulose was the work of Paria et al.,16 who studied the adsorption of sodium alkyl benzene sulfonate, C16TAB, and nonionic surfactant, Triton X100, mixtures onto cellulose, where the cellulose was in the form of filter paper. These cellulose substrates were assumed to have a mixture of both hydrophilic and hydrophobic sites, and hence, it is difficult to disentangle the different contributions to the adsorption process. Torn et al.6 used ellipsometry to study the adsorption and adsorption kinetics of a range of nonionic surfactants onto cellulose surfaces. Particularly relevant to this study is the observation that the adsorbed amounts, compared to those on hydrophilic silica and hydrophobic surfaces, were large. This implies significant penetration of the nonionic surfactant into a swollen cellulose film. In related studies, the adsorption of polyelectrolytes onto and into cellulose surfaces has been recently reported.22-24 Tammelin et al.22 studied the adsorption of the polyelectrolyte poly(dimethyl dialyl ammonium chloride), poly(dmdaac), into L-B deposited cellulose films by AFM and quartz microbalance, QCM, measurements. Horvath et al.23 also studied the adsorption of poly(dmdaac) and the copolymer acrylamide(-poly(dmdaac) onto cellulose fibers. (21) Notley, S. M. J. Phys. Chem. B 2009, 113, 13895. (22) Tammelin, T.; Saarinen, T.; Osterberg, M.; Laine, J. Cellulose 2006, 13, 519. (23) Horvath, A. T.; Horvath, A. E.; Lindstrom, T.; Wagberg, L. Langmuir 2008, 24, 10797. (24) Lefebvre, J.; Gray, D. G. Cellulose 2005, 12, 127.
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Particular emphasis was placed upon the kinetics of adsorption and penetration into the fibers. Very slow penetration kinetics, which was strongly affected by electrolyte, was reported. Lefebvre and Gray24 used AFM to study the adsorption of polyelectrolytes onto cellulose surfaces, spin-coated onto silicon. From this brief review of the recent literature, it is clear that there is a relative paucity of information on the adsorption of surfactants, and especially mixed surfactants, onto cellulose surfaces, and on the impact of surfactant adsorption on the structure and integrity of the cellulose layer and surface. In particular, the evidence for the effect of surfactant adsorption on the cellulose surface is somewhat variable. Understanding such effects is especially important in the context of minimizing and controlling the damage and deterioration of cotton based fabrics. As a result, there is an overriding need to study this area in more detail. Hence, in this paper, we report an extension of our original study on the adsorption of cationic surfactants onto model cellulose surfaces,18 where we have now studied a wider range of surfactants and surfactant mixtures. In particular, we have used neutron reflectivity to characterize the adsorption of the nonionic surfactant hexaethyleneglycol-dodecyl ether, C12E6, and the nonionic/cationic surfactant mixture of C12E6/ C16TAB onto both hydrophilic and hydrophobic cellulose surfaces, and to probe the impact of the adsorption on the structure and integrity of cellulose films and surfaces.
Experimental Details The specular neutron reflectivity measurements were made on the SURF reflectometer25 at the ISIS pulsed neutron source, using the white beam time-of-flight method. Measurements were made in the Q range (where Q, the wave vector transfer normal to the surface of interface, z direction, is defined as Q = 4π sin θ/λ, λ is the neutron wavelength, and θ is the grazing angle of incidence) 0.012-0.3 A˚-1, using the neutron wavelength range 1-7 A˚ and three different grazing angles of incidence, 0.35, 0.8, and 1.8. The reflectivity measurements of the set of three angles take typically ∼60 min. The resolution in Q, ΔQ/Q, was ∼4%. The neutron beam is incident at grazing incidence at the solid-solution interface by transmission through the upper crystalline silicon phase. The surface of the silicon, Æ111æ, supplied by Crystran, was polished to a surface roughness e 5 A˚ rms, and the illuminated area was ∼30 60 mm2. The upper (unused) surface of this crystal was protected using a shaped Teflon spacer which only contacted the outer 2 mm of the crystal. The cell uses a thin solvent gap (e10 μm) and a sample volume of ∼0.2 mL. Solutions are exchanged using a Merck LaChrom chromatography HPLC mixing pump, and exchange of ∼20 mL of sample provides efficient sample changing and cleaning. The data were normalized for the incident beam spectral distribution, measurement time, and detector efficiency and established on an absolute scale by reference to the direct beam intensity.26 Specular neutron reflectivity provides information about the composition and concentration profile in the direction perpendicular to the surface or interface on a molecular scale, probing distances ∼10 to ∼3000 A˚, and is described in detail elsewhere.3,27 The specular reflectivity, R(Q), can be described using the kinematic approximation,28 or in terms of the optical description of reflectivity in thin films.29 The latter approach is used in the (25) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (26) Fragneto, G.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1996, 12, 6036. (27) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143. (28) Crowley, T. L.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Phys. B 1991, 173, 143. (29) Heavens, O. S. Optical properties of thin films; Dover: New York, 1991.
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modeling of the reflectivity data reported in this paper. The simplest model, with the least number of layers, consistent with the data and assessed by least-squares, is adopted. A key feature of the technique that is exploited here is that for neutrons the refractive index can be manipulated using H/D isotopic substitution, where H and D have vastly different scattering powers for neutrons. This gives rise to a difference in the scattering length density F(z) or refractive index, n(z), where n(z) is defined as nðzÞ ¼ 1 -
λ2 FðzÞ 2π
ð1Þ
H/D isotopic substitution can be applied to the adsorbate and solvent, and modeling the reflectivity data from the different combinations simultaneously can minimize uncertainties in the model.27 It relies on there being no significant isotopic dependence on the adsorption and the structure of the adsorbed layer, and this is well established for these types of systems.27 The thin cellulose films studied in the paper were deposited onto silicon substrates (described above) by repeated dipping using the L-B deposition method.30 The adsorption onto the cellulose surface was contrasted with that on a hydrophilic silica surface, where the silica surface was made hydrophilic by storage under water, following a “mild piranha” surface treatment, as described elsewhere.31 Prior to the cellulose deposition, the silicon blocks were first exposed to a “mild piranha” treatment 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 trimethylsilylcellulose (TMSC) from a Langmuir trough. The TMSC was spread at the air-water interface from a chloroform solution, in the concentration range 0.5-0.8 mg/mL. The trough, described in detail in our original paper,20 has dimensions 700 150 6 mm3, and the amount of material available was sufficient for a single dip/pull cycle. The trough well is 120 70 40 mm3. The well is sited 100 mm from one edge of the trough. The dipping/pulling rate was constant in all the preparations, at 5 mm/min, and the dipping/pulling of the crystal was done at a constant surface pressure of 20 mN/m. Repeating the dip/pull cycle 10 times with a high transfer ratio deposited a layer of about 60-100 A˚ with a surface roughness of less than 10 A˚. In order to render the cellulose surface hydrophilic, the films were exposed to vapor of a 10 wt % HCl solution for 60 s, followed by copious rinsing with high purity water. The characterization of the cellulose films (see Results and Discussion section) shows that the process results in some variability in the exact nature of the cellulose films produced. This is largely dependent upon the efficiency of the transfer and results in a film thickness and structure (density distribution within the film). The deuterated and hydrogeneous C16TAB, d- and h-C16TAB, were synthesized and purified as described elsewhere.32 The hydrogeneous C12E6, h-C12E6 (C12H25(OCH2CH2)OH), was obtained from Nikkol. The deuterated C12E6, d-C12E6 (C12D25(OCH2CH2)OH), was synthesized and purified as described previously.32 The neutron reflectivity measurements were made in D2O. The D2O was obtained from Sigma-Aldrich, and high purity (Elgastat Ultrapure) water was used. All the measurements were made at 30 C, to ensure that the solutions were above the Krafft point of the C16TAB. Neutron reflectivity measurements were made for the nonionic surfactant, C12E6, and the mixed cationic/nonionic surfactants, C16TAB/C12E6, onto both hydrophobic and hydrophilic cellulose surfaces. For each measurement sequence, a fresh (either hydrophilic or hydrophobic) cellulose surface was used. Measurements in D2O were used before and after the adsorption sequence to (30) deGroot, P. M. MPhil Thesis, UMIST, 2003 (31) Penfold, J.; Staples, E.; Tucker, I. Langmuir 2002, 18, 2967. (32) Lu, J. R.; Hromadova, M.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1994, 98, 1159.
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evaluate the nature of the cellulose film/surface. Between each individual neutron reflectivity measurement (at a different surfactant concentration, composition, or contrast), the surface was rinsed with solvent (D2O). For the hydrophilic cellulose surface, the C12E6 adsorption was measured for the two isotopic combinations, h-C12E6/D2O and dC12E6/D2O, at surfactant concentrations of 9 10-6, 3 10-5, 7 10-5, and 10-4 M (block 1, side 1). A similar sequence of measurements was made for C12E6 adsorption onto hydrophobic cellulose but over a slightly different concentration range: 9 10-6, 3 10-5, 7 10-5, 10-4, and 3 10-4 M (block 3, side 2). On the hydrophilic cellulose surface, the C16TAB/C12E6 adsorption was measured at a fixed surfactant concentration of 1 mM (> mixed CMC). The measurements were made at the following cationic/nonionic solution compositions: 100/0, 80/20, 60/40, 40/60, 20/80, and 0/100 mol ratio. Measurements were made sequentially for the different isotopic combinations in D2O, hC16TAB/h-C12E6, and d-C16TAB/h-C12E6, abbreviated to hh and dh. On separate hydrophilic cellulose surfaces, the measurements were made in two different ways: following the composition sequence from cationic to nonionic rich (block 2, side 2), and vice versa (block 2, side 1). A similar sequence of measurements was made for the C16TAB/C12E6 mixed adsorption onto a hydrophobic cellulose surface. The only difference, compared to the measurements on hydrophilic cellulose, is that the two different composition sequences (cationic rich to nonionic rich, and vice versa) were made on the same cellulose surface (block 4, side 2).
Results and Discussion (i). Characterization of Cellulose Surfaces. Neutron reflectivity measurements were made in D2O for both the hydrophilic and hydrophobic cellulose surfaces before surfactant adsorption and after adsorption in order to characterize the nature of the cellulose film and surface. In Figure 1, the neutron reflectivity for the cellulose surface pre- and post-surfactant adsorption is shown. The reflectivity data in Figure 1a are for the hydrophilic cellulose surface (block 1, side 1), before and after adsorption of C12E6. In Figure 1b, similar data are presented for the hydrophobic cellulose surface (block 4, side 2), before and after the adsorption of the C16TAB/C12E6 mixture. The solid lines in the two Figures are model calculations using the parameters summarized in Table 1. The parameters summarized in Table 1 include all the pre- and post-surfactant adsorption characterization of the cellulose surfaces used in this study. The simplest structural model for the cellulose films that is consistent with the data is a two layer model. The inner and outer layers are subscripted 1 and 2, respectively, and where sensible (e.g., thickness) total is their combination. d is the thickness, F is the scattering length density, and σ is the interfacial roughness of each layer. The inner layer, adjacent to the silica, is predominantly but not always thicker and has a density corresponding to a cellulose volume fraction ∼0.7-0.75. The outer layer, adjacent to the solvent, is in general thinner, is less dense, is partially solvated, and has a density which varies from film to film, with a cellulose volume fraction ∼0.2-0.5. The cellulose films used in this study have one of two different mean thicknesses, ∼80 and 115 A˚. These observations of the variations in total thickness and in the structure of the cellulose films are qualitatively similar to those previously described by Penfold et al.20 The different mean thicknesses (∼45 and 65 A˚) in ref 20 were attributed to the exposure of the different sides of the silicon block used in the L-B dipping process and the asymmetrical geometry of the Langmuir trough used for the deposition. Nominally, the same conditions were used here, and so the variations from side to side reported in Table 1 are not entirely unexpected. Although the same nominal Langmuir 2010, 26(11), 8036–8048
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Figure 1. (a) Neutron reflectivity for Si/cellulose/D2O for hydrophilic cellulose surface (block 1, side 1) before (O) and after (b) C12E6 adsorption. The solid lines are model calculations using the parameters summarized in Table 1. (b) Same as (a) but for a hydrophobic cellulose surface (block 4, side 2) and before and after exposure to C16TAB/C12E6 surfactant mixture. The post-surfactant adsorption reflectivity profiles are shifted vertically for clarity.
conditions as in ref 20 were used, the total film thicknesses reported here are systematically larger. This is attributed to a more efficient transfer process used in these latter preparations. However, taking into account the increase in total thickness, the structure of the cellulose films is broadly similar in both cases. Furthermore, as previously reported,20 there is no evidence here that the regeneration process (conversion from hydrophobic to hydrophilic cellulose by exposure to HCl vapor) has any significant systematic effect on the thickness or structure of the cellulose films. There are some differences in the structure of the films, when comparing the structures before and after surfactant adsorption, in both the thickness and density of the layers. Such variations are also observed during the surfactant deposition sequences. These differences illustrate the effect of surfactant/solvent penetration into the cellulose film structure, and will be discussed in more detail later in the General Discussion subsection. Langmuir 2010, 26(11), 8036–8048
(ii). Nonionic Adsorption onto Hydrophilic and Hydrophobic Cellulose Surfaces. (a). Adsorption onto Hydrophobic Cellulose. Neutron reflectivity measurements on the adsorption of C12E6 were made sequentially on the same surface for h, d-C12E6 in D2O at surfactant concentrations of 9 10-6, 3 10-6, 7 10-5, 10-4, and 3 10-4 M. The reflectivity data for h-C12E6/D2O at the different surfactant concentrations are shown in Figure 2a. The reflectivities for 9 10-6 and 3 10-5 M h-C12E6/D2O are similar to that of the bare cellulose surface and hence are consistent with little or no surfactant adsorption. For the higher surfactant concentrations, the reflectivity is different from that of the bare interface and is consistent with some C12E6 adsorption. From this series of measurements, there are two predominant features in the reflectivity. The pronounced interference fringe at high Q is indicative of a thin layer of adsorbed surfactant. The deviation in the reflectivity at low Q compared to that for the DOI: 10.1021/la1000057
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Tucker et al. Table 1. Key Model Parameters for Characterization of Cellulose Surfaces, Pre- and Post-Surfactant Adsorption (a) Hydrophobic Cellulose
block/side identification
surface history
d1 ((5 A˚)
F1 ((0.2 10-6 A˚-2)
σ1 ((2 A˚)
d2 ((5 A˚)
F2 ((0.2 10-6 A˚-2)
σ2 ((2 A˚)
dtotal ((5 A˚)
B3/S2 B3/S2 B4/S2 B4/S2 B4/S2
initial characterization post C12E6 isotherm initial characterization post C16TAB/C12E6 adsorption post C16TAB/C12E6 adsorption
75 52 52 52 52
2.4 2.6 2.9 2.7 2.6
5 5 5 10 10
42 43 29 42 42
3.5 3.6 5.0 3.3 3.0
5 5 5 10 10
117 95 81 94 94
(b) Hydrophilic Cellulose block/side identification
surface history
d1 ((5 A˚)
F1 ((0.2 10-6 A˚-2)
σ1 ((2 A˚)
d2 ((5 A˚)
F2 ((0.2 10-6 A˚-2)
σ2 ((2 A˚)
dtotal ((5 A˚)
B1/S1 B1/S1 B2/S1 B2/S1 B2/S2 B2/S2
initial characterization post C12E6 isotherm initial characterization post C16TAB/C12E6 adsorption initial characterization post C16TAB/C12E6 adsorption
39 40 86 86 52 70
2.3 2.4 2.4 2.4 2.8 2.7
5 5 30 30 5 5
39 39 23 23 33 38
3.4 3.6 5.1 3.3 5.4 4.8
5 5 10 20 5 5
78 79 109 109 85 108
bare interface is associated with a larger length scale and is attributed to a modification (swelling and roughening) of the underlying cellulose film. The measurements for h-C12E6 give an indication of the total adsorption at the interface. The measurements for d-C12E6/D2O, where the surfactant is now closely matched to the solvent (D2O), give a further indication of the impact of the C12E6 adsorption on the structure of the cellulose film. Figure 2b shows the reflectivity data for h-C12E6/D2O and d-C12E6/D2O at 10-4 M. The solid lines are model calculations for the parameters summarized in Table 2a. The data are modeled by assuming the same two layer model for the cellulose film, but allowing the thickness and density/ composition of the layers to vary, and by including an additional third layer to account for the adsorbed surfactant. The outer surfactant layer has a thickness of ∼15-17 A˚ and is consistent with a surfactant monolayer, as would be expected for adsorption at a hydrophobic interface. Assuming that this outer layer comprises surfactant and solvent only, then the adsorbed amount is ∼2.9 10-10 mol cm-2, which corresponds to a volume fraction of ∼0.2 and a mean area/molecule of ∼55 A˚2. This is similar to that previously reported for C12E6 adsorption at the air-water interface.33 The modeling of the data for d-C12E6 and h-C12E6 (see Table 2a) also indicates that the addition of the nonionic surfactant results in a significant change in the structure of the cellulose film. The parameters indicate that the initial layer becomes considerably thicker and rougher and that the second layer becomes thinner. That is, there is a reorganization of the cellulose in the two layers used to model the data. The changes in scattering length density also imply some solvent and surfactant penetration into the cellulose film, although it is difficult to quantify that from the limited range of measurements that were possible. It is also significant that, comparing the parameters for the cellulose layers before, during, and after the C12E6 adsorption, the swelling and roughening of the cellulose film is only temporary. After rinsing, the cellulose film relaxes back to a structure that is only slightly different to its initial characterized state (see Table 1). (b). Adsorption onto Hydrophilic Cellulose. A similar sequence of neutron reflectivity measurements were also made for the adsorption of C12E6 onto a hydrophilic cellulose surface. (33) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Tucker, I.; Penfold, J. J. Phys. Chem. 1993, 97, 8012.
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Measurements were made for h-C12E6 and d-C12E6/D2O at surfactant concentrations of 9 10-6, 3 10-5, 7 10-5, and 10-4 M, and the reflectivity data for h-C12E6/D2O are shown in Figure 3a. Figure 3b shows the data for 10-4 M h-C12E6 and d-C12E6/ D2O and their associated model calculations for the model parameters summarized in Table 2b. The neutron reflectivity for d-C12E6/D2O is identical to that for the bare surface and is indicative of no penetration into the cellulose film or disruption of the cellulose film, and this is supported by the model parameters for the cellulose layer in Table 2b. With increasing C12E6 concentration, the adsorbed layer thickness increases from 10 to 24 A˚ and the adsorbed amount increases from 1.4 to 3.3 10-10 mol cm-2. At 10-4 M C12E6, the coverage of 3.3 10-10 mol cm-2 corresponds to a fractional coverage of ∼0.45 and compares with a coverage of ∼4.5 10-10 mol cm-2 on hydrophilic silica34 at pH 2.4. As a function of pH and surface history, values in the range 10-10-3.5 10-10 mol cm-2 have been reported for C12E6 adsorption onto hydrophilic silica.35 Compared to previous studies of C12E6 adsorption onto hydrophilic silica surfaces,34,35 the thickness of the adsorbed layer, even at saturation adsorption at 10-4 M, is small given that cooperative adsorption of surface aggregates is expected. Previous studies34,35 have reported thicknesses of ∼40 A˚, where the adsorbed layer was modeled as a surface bilayer. The data analysis here has used a single layer to model the surfactant adsorption (in order to minimize the complexity of the model due to the underlying cellulose layers, and this provides an adequate description of the data). This could in part account for some of the difference. Furthermore, there could be some interpenetration of the C12E6 into the outer cellulose layer, although the parameters in Table 2b would suggest that in this case it is relatively small. There is also the possibility that the surface structure is just different. Thirtle et al.8 reported such effects for the adsorption of C12E6 onto surfaces with differing degrees of hydrophilicity. Such effects were not previously reported for C16TAB adsorption onto hydrophilic cellulose.20 However, it is well established8,34,35 that the nonionic adsorption arises from a more subtle surfacesurfactant interaction, which is highly sensitive to small changes (34) Penfold, J.; Staples, E. J.; Tucker, I.; Thompson, L. J. Langmuir 1997, 13, 6638. (35) Penfold, J.; Staples, E.; Tucker, I. Langmuir 2002, 18, 2967.
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Figure 2. (a) Neutron reflectivity for h-C12E6/D2O adsorbed onto hydrophobic cellulose surface for (O) 9 10-6 M, (b) 3 10-5 M, (4) 7
10-5 M, (2) 10-4 M, and (þ) 3 10-4 M. (b) Same as (a) but for 10-4 M and (O) d-C12E6/D2O, and (b) h-C12E6/D2O. The solid lines are model calculations using the parameters summarized in Table 2a.
and variations in the exact nature of the surface and surface charge distribution. (iii). Nonionic/Cationic Adsorption onto Hydrophilic and Hydrophobic Cellulose Surfaces. (a). Adsorption onto Hydrophobic Cellulose. Neutron reflectivity measurements were made for the adsorption of the cationic/nonionic surfactant mixture of C16TAB/C12E6 at a surfactant concentration of 1 mM on an hydrophobic cellulose surface. The measurements were made sequentially for a range of surfactant compositions from 100/0 to 0/100 mol ratio C16TAB/C12E6. Measurements were made for the isotopic combinations of h-C16TAB/ h-C12E6/D2O (hh) and d-C16TAB/h-C12E6/D2O (dh). For the hh combination, the total adsorption is measured. Whereas, for the dh combination, the C16TAB is effectively matched to the solvent and an estimate of the C12E6 adsorption and hence the surface composition is accessible. Between each measurement, the surface is rinsed in solvent (D2O). The measurements were made following two different composition sequences: starting with nonionic rich mixtures, and then the same sequence of Langmuir 2010, 26(11), 8036–8048
measurements was repeated but starting with cationic rich compositions. In this sequence of measurements, the two sets of measurements were made on the same hydrophobic cellulose surface. The reflectivity data and associated model fits for 1 mM C16TAB/C12E6 at solution compositions of 20/80 and 60/40 mol ratio, and for the two isotopic combinations, hh/D2O and dh/D2O, are shown in Figure 4. The reflectivity data are modeled with two layers to describe the cellulose film (as described earlier) using the model parameters summarized in Table 3, and an example of the model fits for the bare cellulose is shown in Figure 1a. An additional outer layer, ∼19-20 A˚ thick, is included in the model to account for the adsorbed layer of surfactant, and this provides a good description of the adsorption data. This additional layer is synonymous with a monolayer of surfactant adsorption. Notably, here the adsorption of the C16TAB/C12E6 mixture (for both the nonionic to cationic and the reverse sequence) does not result in any significant modification of the underlying cellulose film. The only exception to this is that there is DOI: 10.1021/la1000057
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Tucker et al. Table 2. Key Model Parameters for C12E6 Adsorption onto Cellulose (a) Hydrophobic Cellulose
surfactant concentration (M) 9 10-6 3 10-5 7 10-5 7 10-5 1 10-4 1 10-4 3 10-4 3 10-4
C12E6 “contrast” h, d h, d h d h d h d
d1 ((5 A˚)
F1 ((0.2 10-6 A˚-2)
σ1 ((2 A˚)
d2 ((5 A˚)
F2 ((0.2 10-6 A˚-2)
σ2 ((2 A˚)
d3 ((5 A˚)
F3 ((0.2 10-6 A˚-2)
σ3 ((2 A˚)
75 75 95 128 119 134 119 134
2.5 2.6 2.4 2.6 2.7 2.7 2.7 2.7
5 5 70 60 30 30 30 30
42 43 30 33 26 28 26 28
3.5 3.6 3.9 3.8 4.3 3.8 4.3 3.8
5 5 10 10 10 10 10 10
15 17 15 15 15 15
1.0 3.9 1.2 5.4 1.2 5.4
10 10 10 10 10 10
(b) Hydrophilic Cellulose F1 ((0.2 F2 ((0.2 F3 ((0.2 Γ (10-10 surfactant C12E6 concentration (M) “contrast” d1 ((5 A˚) 10-6 A˚-2) σ1 ((2 A˚) d2 ((5 A˚) 10-6 A˚-2) σ2 ((2 A˚) d3 ((5 A˚) 10-6 A˚-2) σ3 ((2 A˚) mol cm-2) 9 10-6 9 10-6 3 10-5 3 10-5 7 10-5 7 10-5 1 10-4 1 10-4
h d h d h d h d
34 34 34 34 29 29 29 29
2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3
5 5 5 5 5 5 5 5
a decrease in the scattering length density of the outer cellulose layer, which implies a slight penetration of surfactant into the cellulose film. Furthermore, the post-adsorption characterization of the cellulose film results in an increase in the thickness of the outer layer from ∼30 to 40 A˚, but notably it remains constant during the adsorption sequences. Hence, this implies some swelling of the layer by the solvent post the adsorption sequence, or some residual surfactant adsorption. From the measurements and analysis of the reflectivity data for the two isotopic combinations, hh/D2O and dh/D2O, the total surfactant adsorption and composition of the surfactant layer can be obtained, and these results are summarized in Table 4. The results for the two adsorption sequences are remarkably similar and consistent (see Table 4). The total adsorption is roughly independent of surfactant composition and is constant at a mean value ∼4.3 10-10 mol cm-2. The variation in the surface composition with solution composition is shown in Figure 5 and is highly nonideal. Furthermore, there is some hysteresis in the adsorption (composition), dependent upon whether the adsorption sequence is measured starting initially with cationic or nonionic rich solutions. The extreme nonideality is in marked contrast to what is observed at the air-water interface and in micelles,36 where the surface and solution compositions show a much less marked departure from ideal mixing and were much closer to what would be expected from regular solution theory, RST.37 (b). Adsorption onto Hydrophilic Cellulose. A similar sequence of measurements for C16TAB/C12E6 surfactant mixtures at 1 mM was also made at the hydrophilic cellulose surface. The reflectivity data for the C16TAB/C12E6 compositions of 20/80 and 60/40 mol ratio and for the hh/D2O and dh/D2O combinations are shown in Figure 6. In Figure 6b, the solid lines are model calculations for the parameters summarized in Table 5. (36) Penfold, J.; Staples, E.; Cummins, P.; Tucker, I.; Thompson, L.; Thomas, R. K.; Simister, E. A.; Lu, J. R. J. Chem. Soc., Faraday Trans. 1996, 92, 1773. Penfold, J.; Staples, E.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R.; Warren, N. J. Phys. Chem. B 1999, 103, 5204. (37) Holland, P. M. Colloids Surf. 1986, 19, 171.
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38 38 38 38 39 39 39 39
3.4 3.4 3.4 3.4 3.3 3.3 3.3 3.3
5 5 5 5 5 5 5 5
10
2.7
5
1.4
13
1.9
5
2.2
24
2.7
5
3.3
Here, the parameters used to describe the underlying cellulose film show some variation during the adsorption sequence (for nonionic to cationic rich, Table 5a) and the post-adsorption characterization compared to the pre-adsorption characterization. The innermost of the two layers used to describe the cellulose film remains constant in thickness and density, but the outer layer shows some variations during the adsorption sequence. During the surfactant adsorption measurements, the outer layer appears to shrink, and post-adsorption it relaxes back to its original thickness but with a reduced scattering length density. This latter observation could be consistent with some surfactant being trapped or retained within that outer cellulose layer. An additional layer is required in the model to account for the surfactant adsorption. It is a relatively thin layer, ∼20 A˚, similar to that observed for C12E6 adsorption onto hydrophilic cellulose. Similar arguments apply here, and a thicker layer, consistent with cooperative adsorption of aggregates, would normally be expected for the adsorption onto a hydrophilic surface. However, in the models fits, the volume fraction of cellulose in the outer of the two layers used to describe the cellulose film layer is quite low compared to the inner layer. The outer layer has a cellulose volume fraction of ∼0.23 compared to ∼0.74 obtained for the inner layer in the initial characterization of the film. Hence, there is some considerable scope for the penetration of surfactant and solvent into that outer layer. In Table 6, the variation in the adsorbed amount (taking into account the outer layer of surfactant only in the model) and the surface composition with solution composition is tabulated. Similar neutron reflectivity data were also obtained for the reverse sequence of measurements, that is, from cationic rich to nonionic rich solution compositions. These results are summarized in Tables 5b and 6b. However, although the data from the two sequences of measurements are broadly similar, some notable differences exist. Apart from a slight increase in the thickness of the inner cellulose layer, the structures of the pre- and postadsorption cellulose films are rather similar. On surfactant adsorption, the outer cellulose layer is systematically thinner and contracts from ∼30 to 15 A˚. The change in the scattering length density of that layer also suggests that this is partly due to Langmuir 2010, 26(11), 8036–8048
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Figure 3. (a) Neutron reflectivity for h-C12E6/D2O adsorbed onto hydrophilic cellulose surface for (O) 9 10-6 M, (b) 3 10-5 M, (4) 7 10-5 M, and (þ) 10-4 M. (b) Same as (a) but for 10-4 M and (O) d-C12E6/D2O, and (b) h-C12E6/D2O. The solid lines are model calculations using the parameters summarized in Table 2b.
contraction but also due to some overlap between the cellulose and the surfactant/solvent in the outermost layer. Notably, for this sequence of measurements, the surfactant layer is systematically thicker, ∼30 A˚, and is hence closer to that expected for surface aggregates. The variation in surface composition with solution composition (see Figure 7) is also consistent with nonideal mixing, but compared to the data for the hydrophobic surface the departure from ideal mixing is less pronounced. However, some hysteresis in the variation in surface composition, dependent upon the order of the measurements, is observed. The other significant difference, compared to the hydrophobic surface, is that the total adsorption is less, ∼3.05 10-10 mol cm-2 compared with 4.3 10-10 mol cm2. This is somewhat unexpected, as on the hydrophobic surface monolayer adsorption is expected and observed. Whereas at the hydrophilic surface, cooperative adsorption of surface aggregates is expected, and hence an associated higher adsorbed amount would also be expected. However, comparing the data in Tables 4 and 6, the differences seem to arise Langmuir 2010, 26(11), 8036–8048
from a reduction in the amount of both C16TAB and C12E6 adsorbed at the interface. (iv). General Discussion. (a). Structure of the Cellulose Films. The structure and thickness of the cellulose films formed here by L-B deposition are broadly similar to those previously reported.20 The simplest model that describes the reflectivity from both the hydrophobic and hydrophilic cellulose surfaces in D2O is a two layer model. The inner layer (adjacent to the silica surface) has a cellulose volume fraction of ∼0.7-0.75, and the thinner outer layer (adjacent to the solvent) is less dense. The outer layer is more highly hydrated and has a cellulose volume fraction which varies from 0.2 to 0.5. The films described here are characterized by two different mean thicknesses, ∼80 and ∼120 A˚, but there is some variation from film the film in the actual thickness (see Tables 1-3, and 5). The origins of the variation in thickness and structures reported here, and their comparison with a previous related study,20 were discussed in detail earlier in the discussion of the results. The effect of surfactant adsorption on these different cellulose films depends upon the structure of the cellulose film (especially DOI: 10.1021/la1000057
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Figure 4. Neutron reflectivity for 1 mM C16TAB/C12E6/D2O adsorbed onto hydrophobic cellulose (a) for C16TAB/C12E6 mole ratio of 20/80, (O) hh/D2O, (b) dh/D2O and (b) for C16TAB/C12E6 mole ratio of 60/40. The solid lines are model calculations using the parameters summarized in Table 3.
the outer layer), whether it is hydrophilic or hydrophobic, and upon the type of surfactant. In a previous detailed study of C16TAB adsorption onto hydrophilic and hydrophobic cellulose,20 there was evidence of C16TAB/solvent penetration into the hydrophobic cellulose surface, but there was no evidence for C16TAB adsorption into the hydrophilic surface. For C12E6 adsorption onto a hydrophobic cellulose surface (see Table 2a), the structure of the cellulose changes significantly during adsorption. The cellulose film becomes thicker and rougher. Furthermore, there is a change in the internal structure, which can be attributed to a reorganization of the cellulose and some solvent/surfactant penetration. For example, the total cellulose film thickness increases from ∼120 to ∼160 A˚. This disruption to the cellulose film structure also only occurs when there is C12E6 adsorption. The structure of the cellulose film at a C12E6 concentration of 9 10-6 and 3 10-5 M (where no C12E6 adsorption occurs) is the same as the initial cellulose film (see Table 1a). Due to the limited range of “contrasts” studied, it is difficult to quantify the changes in terms of the relative cellulose, 8044 DOI: 10.1021/la1000057
solvent, and surfactant volume fractions in each layer. The characterization of the cellulose film after C12E6 adsorption (and rinsing in D2O) shows that in this case the film relaxes back to a structure similar to its original state. The adsorption of C12E6 onto the hydrophilic cellulose surface shows a different behavior. The cellulose film, as formed, has a slightly different structure, with a thinner inner layer and a thicker outer layer (see Table 1b). Compared to the structure of the initial film, C12E6 adsorption results in an increase in the thickness of the outer layer, from ∼30 to ∼40 A˚. The density of the layer does not significantly change, and hence, this is most likely to be consistent with the penetration of both some solvent and surfactant. There is also a modest decrease in the thickness of the inner layer, from ∼40 to 30-34 A˚ (see Tables 1b and 2b). Unlike the effect of C12E6 on the hydrophobic cellulose, the changes (although not substantial) in the cellulose structure here appear to be irreversible (see Tables 1b and 2b). The adsorption of the C16TAB/C12E6 surfactant mixture onto hydrophobic cellulose has the same impact independent of the Langmuir 2010, 26(11), 8036–8048
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Article Table 3. Key Model Parameters for 1 mM C16TAB/C12E6 Adsorption onto Hydrophobic Cellulose (a) For Nonionic to Cationic Rich Surfactant Composition Sequence
composition (cationic/nonionic mole ratio) 0/100 20/80 40/60 60/40 80/20 100/0
C16TAB/C12E6 “contrast” hh hh dh hh dh hh dh hh dh hh dh
d1 ((5 A˚)
F1 ((0.2 10-6 A˚-2)
σ1 ((2 A˚)
d2 ((5 A˚)
F2 ((0.2 10-6 A˚-2)
σ2 ((2 A˚)
d3 ((5 A˚)
F3 ((0.2 10-6 A˚-2)
σ3 ((2 A˚)
52 52 52 52 52 52 52 52 52 52 52
2.6 2.4 2.6 2.5 2.7 2.6 2.8 2.5 2.8 2.5 2.8
10 10 10 10 10 10 10 10 10 10 10
22 24 26 19 22 23 25 21 21 21 21
4.2 4.1 4.0 4.7 4.2 4.3 3.7 4.3 3.8 4.3 3.0
10 10 10 10 10 10 10 10 10 10 10
20 20 20 20 20 18 19 19 19 19 19
1.7 1.1 2.8 1.2 3.1 1.0 3.4 1.0 3.7 1.1 2.6
10 10 10 10 10 10 10 10 10 10 10
(b) For Cationic to Nonionic Rich Surfactant Composition Sequence composition (cationic/nonionic mole ratio) 100/0 80/20 60/40 40/60 20/80 0/100
C16TAB/C12E6 “contrast” hh dh hh dh hh dh hh dh hh dh hh
d1 ((5 A˚)
F1 ((0.2 10-6 A˚-2)
σ1 ((2 A˚)
d2 ((5 A˚)
F2 ((0.2 10-6 A˚-2)
σ2 ((2 A˚)
d3 ((5 A˚)
F3 ((0.2 10-6 A˚-2)
σ3 ((2 A˚)
52 52 52 52 52 52 52 52 52 52 52
2.4 2.8 2.5 2.8 2.5 2.9 2.5 2.9 2.6 2.8 3.0
10 10 10 10 10 10 10 10 10 10 10
21 21 22 22 20 20 21 21 21 21 20
4.2 3.2 4.3 4.0 4.5 4.2 4.4 4.4 4.7 4.7 4.9
10 10 10 10 10 10 10 10 10 10 10
20 20 18 18 20 20 19 19 19 19 20
1.3 3.0 1.1 3.7 1.2 3.9 1.0 3.7 1.0 3.2 1.7
10 10 10 10 10 10 10 10 10 10 10
Table 4. Adsorbed Amount and Composition for 1 mM C16TAB/ C12E6 Adsorption onto Hydrophobic Cellulose (a) For Nonionic to Cationic Rich Surfactant Composition Sequence solution composition (C16TAB/C12E6 mole ratio) 0/100 20/80 40/60 60/40 80/20 100/0
total adsorbed amount, Γ, ((0.2 10-10) mol cm-2
C12E6 adsorption ((0.2 10-10 mol cm-2)
surface composition ((0.04) (C12E6 mole fraction)
4.1 4.6 4.5 4.2 4.4 4.4
4.1 3.1 2.8 2.4 2.2 0.0
1.0 0.68 0.63 0.58 0.49 0.0
(b) For Cationic to Nonionic Rich Surfactant Composition Sequence solution composition (C16TAB/C12E6 mole ratio) 100/0 80/20 60/40 40/60 20/80 0/100
total adsorbed amount, Γ, ((0.2 10-10) mol cm-2
C12E6 adsorption ((0.2 10-10 mol cm-2)
surface composition ((0.04) (C12E6 mole fraction)
4.4 4.2 4.5 4.4 4.4 4.1
0.0 2.1 2.1 2.1 2.6 4.1
0.0 0.49 0.48 0.47 0.59 1.0
adsorption sequence (from cationic to nonionic rich or vice versa). Adsorption of the surfactant mixture results in little or no effect on the inner cellulose layer but results in a thinning of the outer layer (see Table 3). A similar effect was observed for C16TAB onto hydrophobic cellulose.20 Although the C16TAB/C12E6 adsorption is characterized by an additional layer, ∼20 A˚, which corresponds to a Langmuir 2010, 26(11), 8036–8048
Figure 5. Variation in surface composition with solution composition for C16TAB/C12E6 adsorbed onto hydrophobic cellulose (O) for nonionic to cationic rich sequence and (b) for cationic to nonionic rich sequence.
monolayer, the reduction in the thickness of the outer cellulose layer and the associated change in its density are consistent with solvent/ surfactant overlap and penetration into that outer cellulose layer. Here, the changes observed are reversible (see Tables 1a and 3), and after rinsing in D2O the cellulose structure post the C16TAB/C12E6 adsorption is similar to the structure of the film as deposited. For the adsorption of the C16TAB/C12E6 mixture onto hydrophilic cellulose surfaces, and in spite of the structure of the cellulose films being different, the impact of the mixed surfactant adsorption is similar to that observed for hydrophobic cellulose. DOI: 10.1021/la1000057
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Figure 6. Same as Figure 4 but for a hydrophilic cellulose surface.
Solvent swelling of cellulose films has been extensively reported. Penfold et al.,20 in studies of C16TAB adsorption onto cellulose, reported the slow exchange between H2O and D2O in neutron reflectivity measurements using different solvent “contrast”. Slow solvent exchange was reported in other fibrous systems.38,39 In the absence of surfactant, there have been a number of studies reporting the swelling of dry cellulose films by aqueous solvents. Holmberg et al.13 reported the swelling of dry cellulose films by humid air or water, from ∼40 to 70 A˚. From AFM studies, they inferred that the structure of the cellulose film could be described as a swollen network with some protruding cellulose chains. Falt et al.14 compared the swelling of cellulose films and fibers in water at different pH and ionic strength (added electrolyte). In the previous study on the adsorption of C16TAB onto hydrophobic and hydrophilic cellulose surfaces,20 there was some interpenetration on the hydrophobic but not on the hydrophilic (38) Watt, I. C. J. Macromol. Sci., Rev. Macromol. Chem. 1980, C18, 169. (39) Al-Shimi, A. F.; Princen, H. M. Colloid Polym. Sci. 1987, 256, 2.
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cellulose surfaces. The difference was attributed to the packing constraints associated with the self-assembled surface C16TAB structures on the hydrophilic surface compared to the monomer adsorption on the hydrophobic surface. Although this remains a factor, it is evident from the range of measurements reported here that the introduction of the nonionic surfactant plays a greater role in promoting surfactant and solvent penetration into both hydrophobic and hydrophilic cellulose surfaces, compared to that previously reported for the cationic surfactant, C16TAB.20 However, the results indicate that the exact structure of the cellulose film (in both its hydrophilic and hydrophobic states) also plays a role in determining the degree and extent of surfactant/solvent penetration. (b). Nature of the Nonionic Surfactant Adsorption. The adsorption of C12E6 onto a hydrophobic cellulose surface is in the form of a monolayer, ∼15-17 A˚, and assuming that this layer comprises only surfactant and solvent, then the adsorbed amount is similar to that previously reported at the air-water interface,33 ∼3 10-10 mol cm-2. However, the differences in the measurements for h-C12E6 and d-C12E6 do indicate some penetration of Langmuir 2010, 26(11), 8036–8048
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Article Table 5. Key Model Parameters for 1 mM C16TAB/C12E6 Adsorption onto Hydrophilic Cellulose (a) For Nonionic to Cationic Rich Surfactant Composition Sequence
composition (cationic/nonionic mole ratio) 0/100 20/80 40/60 60/40 80/20 100/0
C16TAB/C12E6 “contrast” hh hh dh hh dh hh dh hh dh hh dh
d1 ((5 A˚)
F1 ((0.2 10-6 A˚-2)
σ1 ((2 A˚)
d2 ((5 A˚)
F2 ((0.2 10-6 A˚-2)
σ2 ((2 A˚)
d3 ((5 A˚)
F3 ((0.2 10-6 A˚-2)
σ3 ((2 A˚)
86 86 86 86 86 86 86 86 86 86 86
2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.4
50 25 25 25 25 25 39 25 25 25 25
28 12 12 12 12 12 12 12 12 12 12
4.5 5.2 5.2 4.9 5.5 4.8 3.8 4.8 5.5 4.8 3.3
2 10 2 5 5 5 5 5 5 5 12
20 20 20 20 20 20 20 20 20 20 20
3.1 2.4 3.9 2.8 4.6 2.9 4.6 2.9 5.1 2.9 3.1
5 5 5 5 5 5 5 5 5 5 5
(b) For Cationic to Nonionic Rich Surfactant Composition Sequence composition (cationic/nonionic mole ratio) 100/0 80/20 60/40 40/60 20/80 0/100
C16TAB/C12E6 “contrast” hh dh hh dh hh dh hh dh hh dh hh
d1 ((5 A˚)
F1 ((0.2 10-6 A˚-2)
σ1 ((2 A˚)
d2 ((5 A˚)
F2 ((0.2 10-6 A˚-2)
σ2 ((2 A˚)
d3 ((5 A˚)
F3 ((0.2 10-6 A˚-2)
σ3 ((2 A˚)
52 52 52 52 52 52 52 52 52 52 52
2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8
5 5 5 5 5 5 5 5 5 5 5
15 15 15 15 15 15 15 15 15 15 15
3.9 3.9 3.4 3.9 3.4 4.2 4.2 4.2 4.0 4.0 4.4
5 5 5 5 5 5 5 5 5 5 5
30 30 30 30 30 30 29 29 30 30 30
2.8 5.0 2.8 5.0 2.8 4.5 2.8 4.0 3.0 4.0 3.4
5 5 5 5 5 5 5 5 5 5 5
Table 6. Adsorbed Amount and Composition for 1 mM C16TAB/ C12E6 Adsorption onto Hydrophilic Cellulose (a) For Nonionic to Cationic Rich Surfactant Composition Sequence solution composition (C16TAB/C12E6 mole ratio) 0/100 20/80 40/60 60/40 80/20 100/0
total adsorbed amount, Γ, ((0.2 10-10) mol cm-2
C12E6 adsorption ((0.2 10-10 mol cm-2)
surface composition ((0.04) (C12E6 mole fraction)
2.8 3.4 3.1 3.0 3.0 3.0
2.8 2.1 1.5 1.5 1.1 0.0
1.0 0.62 0.5 0.51 0.37 0.0
(b) For Cationic to Nonionic Rich Surfactant Composition Sequence solution composition (C16TAB/C12E6 mole ratio) 100/0 80/20 60/40 40/60 20/80 0/100
total adsorbed amount, Γ, ((0.2 10-10) mol cm-2
C12E6 adsorption ((0.2 10-10 mol cm-2)
surface composition ((0.04) (C12E6 mole fraction)
2.9 2.9 3.1 3.1 3.1 3.1
0.0 1.2 1.6 2.0 2.4 3.1
0.0 0.38 0.53 0.66 0.80 1.0
surfactant into the very outer cellulose layer. We have not attempted to estimate the amount of C12E6 penetration into the cellulose film (for reasons discussed earlier). Torn et al.6 reported an exceptionally high adsorption of the nonionic surfactants of C12E5, C12E7 and C16E7 onto cellulose: 7 to 8 μmol/m2 compared to 4 to 6 μmol/m2 on a hydrophilic surface and 2 to 4 μmol/m2 on a hydrophobic surface. They attributed this anomalously high Langmuir 2010, 26(11), 8036–8048
Figure 7. Same as Figure 5 but for a hydrophilic cellulose surface.
adsorption to the swollen nature of the cellulose surface and penetration of the surfactant into that surface, although no direct structural evidence was provided. The adsorption of C12E6 onto the hydrophilic cellulose surface results in an adsorbed layer which is ∼10-24 A˚ thick. The adsorbed amount varies from 1.4 10-10 to 3.3 10-10 mol cm-2. The thickness and adsorbed amount are both dependent upon the C12E6 concentration. This is low compared to the adsorption of C12E6 onto hydrophilic silica3 and the results of Torn et al.6 The thickness of the adsorbed layer is also small compared to that expected for cooperative adsorption of surface aggregates, as discussed earlier. The reflectivity measurements for the isotopic combination of d-C12E6/D2O are similar to that of the bare DOI: 10.1021/la1000057
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interface. Hence, in this case, this indicates predominantly that there is only surfactant and solvent in the outer surfactant layer, and no interpenetration of the cellulose. However, we cannot exclude the possibility of some penetration of surfactant into the outer cellulose layer, but as discussed earlier this is difficult to quantify. Taking all these factors into account, the nature of the nonionic surfactant adsorption is unexpected. It is known that the adsorption of nonionic surfactants onto hydrophilic silica depends strongly upon the surface history35 and is also highly dependent upon the exact nature of the hydrophilic surface.8 Hence, it is assumed here that the surface charge distribution is not optimal for the adsorption associated with the formation of cooperative surface structure for nonionic surfactants. (c). Adsorption of Nonionic/Cationic Surfactant Mixtures. The adsorption of the C16TAB/C12E6 mixture onto the hydrophobic cellulose surface results in monolayer adsorption and an adsorbed amount of ∼4.5 10-10 mol cm-2, independent of solution composition. The variation of surface composition with solution composition is shown in Figure 4. This shows a strong departure from ideal mixing and a trend that is outside what would be expected from RST. On the hydrophobic surface, the sequence of measurement, from cationic to nonionic surfactant rich or vice versa, makes some difference. The surface composition variation exhibits a hysteresis, with a variation in surface composition which is dependent upon the direction of the adsorption measurements. On hydrophilic cellulose, the trend in the total adsorption is broadly similar for both adsorption sequences (from cationic to nonionic surfactant rich or vice versa) and is roughly constant with changing solution composition. The variation in the surface composition with solution composition shows a similar hysteresis, which is dependent upon the order of the measurements. However, the total adsorption is significantly less than that for the cationic/nonionic surfactant mixture on the hydrophilic cellulose surface, with an adsorption of ∼3 10-10 mol cm-2 compared to ∼4.5 10-10 mol cm-2 on the hydrophobic surface. The variation in the surface composition with solution composition shows a departure from ideal mixing, but in this case it is closer to the magnitude of variation that would be predicted for the air-water interface by RST.36 The reduction in total adsorption compared to the hydrophobic surface is unexpected, but it is consistent with the trend observed for the nonionic surfactant adsorption onto hydrophilic cellulose (see earlier discussion). The observations here are different from the mixing behavior of C12E6/C16TAB mixtures at the air-water interface and in micelles, although these latter measurements were made in 0.1 M NaCl, where above the critical micelle concentration (CMC) the mixing behavior was consistent with ideal mixing.36 Below the CMC, the variation in the surface composition with solution composition at the air-water interface showed a slight departure from ideality but was significantly less than that observed here at the cellulose surface. At the hydrophilic silica-solution interface,34,40 the variation in the surface composition with solution composition for C12E6/C16TAB mixtures is close to ideal mixing over most of the composition range. The exception is at solution compositions rich in nonionic surfactant, >70 mol % nonionic, where the partitioning of the C12E6 to the interface is more prevalent. This is rather (40) Penfold, J.; Staples, E. J.; Tucker, I.; Thomas, R. K. Langmuir 2000, 16, 8879.
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different from what is observed here on cellulose, where in general the cationic surfactant is more surface active. However, a notable difference in the experimental conditions is that the measurements on hydrophilic silica were made at pH 2.4 and in 0.1 M NaBr. At this pH, the nonionic surfactant will have a greater affinity for the silica surface than the C16TAB. This was confirmed in an earlier study34 on the adsorption of C12E6/C16TAB mixtures, where at pH 7 the surface composition on hydrophilic silica was dominated by the C16TAB. Although only a limited range of measurements were made (for an equimolar C12E6/C16TAB mixture), the trend observed at pH 7 on silica was also different from that reported here for both the hydrophilic and hydrophobic cellulose surfaces. Hence, we can conclude that the relative affinity of the two surfactants in the C12E6/C16TAB mixture for a cellulose surface is different compared to the other surfaces studied.
Summary From a series of neutron reflectivity measurements, combined with D/H isotopic substitution of the surfactants studied, we have been able to characterize the adsorption of C12E6 and C12E6/ C16TAB surfactant mixtures onto both hydrophilic and hydrophobic cellulose surfaces, and to investigate the impact of the surfactant adsorption on the structure and integrity of the cellulose film. The surfactant adsorption onto the hydrophobic and hydrophilic surfaces shows some variations compared to more straightforward hydrophilic and hydrophobic surfaces, and some surfactant/solvent penetration into the cellulose surface is encountered. On the hydrophobic cellulose surface, C12E6 adsorbs a monolayer as would be expected from studies on other hydrophobic surfaces.6,20,33 Furthermore, it does penetrate into the cellulose film and promote solvent penetration to a greater extent than was observed with cationic surfactants.20 On the hydrophilic cellulose surface, the adsorption is different from that of the self-assembled structures normally adsorbed on hydrophilic surfaces.3,8,20 This is further evidence of the sensitivity of the nature of the adsorption to the exact nature of the hydrophilic surface. The hydrophilic surface also seems to be less prone to surfactant and solvent penetration, even for the nonionic surfactant. The results for the C12E6/C16TAB mixtures show that the order of addition is important and that there are some effects that are not entirely reversible. The adsorption of the CTAB/C12E6 mixture shows a pronounced departure from ideal mixing, and this is most pronounced for the hydrophobic surface. This is an indication of the relative specific interactions between the two different surfactants and the different surface states. Furthermore, the results indicate that C12E6 and the C12E6/C16TAB mixtures have a greater impact upon the cellulose film than was previously reported for C16TAB alone.20 From the range of measurements made, it is concluded that the presence of the nonionic surfactant and the nature of the cellulose film surface are both contributing factors. These results provide insight into the factors which affect surfactant and mixed surfactant adsorption onto complex surfaces, such as cellulose, and the factors which perturb the cellulose and promote greater penetration. Acknowledgment. We acknowledge the provision of beam time on the reflectometer SURF at ISIS and the invaluable assistance of the Instrument Scientists J. Webster, A. Hughes, P. P. Taylor, and S. Holt.
Langmuir 2010, 26(11), 8036–8048
