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Oct 21, 2005 - The forces acting between nonpolar surfaces coated with the nonionic surfactant n-dodecyl-β-d-maltoside (β-C12G2) were investigated a...
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11836

Langmuir 2005, 21, 11836-11843

Interactions between Nonpolar Surfaces Coated with the Nonionic Surfactant n-Dodecyl-β-D-maltoside Orlando J. Rojas,*,† Cosima Stubenrauch,‡ Judith Schulze-Schlarmann,§ and Per M. Claesson|,⊥ Forest Biomaterials Laboratory, College of Natural Resources, North Carolina State University, Box 8005, Raleigh, North Carolina 27695, Department of Chemical and Biochemical Engineering, University College Dublin, Belfield, Dublin 4, Ireland, Engelhard Process Chemicals, Freundallee 23, 30173 Hannover, Germany, Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas va¨ g 51, Stockholm, SE-100 44, Sweden, and Institute for Surface Chemistry, Box 5607, Stockholm, SE-114 86, Sweden Received July 18, 2005. In Final Form: September 9, 2005 The forces acting between nonpolar surfaces coated with the nonionic surfactant n-dodecyl-β-D-maltoside (β-C12G2) were investigated at concentrations below and above the critical micelle concentration. The long-range and adhesive forces were measured with a bimorph surface force apparatus (MASIF). It was found that the effect of hydrodynamic interactions had to be taken into account for an accurate determination of the short-range static interactions. The results were compared with disjoining pressure versus thickness curves that were obtained earlier with a thin film pressure balance (TFPB). This comparison led to the conclusion that the charges observed at the air-water interface are not due to charged species present in the surfactant sample. In addition, it was observed that the stability of thin liquid films crucially depends on the surfactant’s bulk concentration (c) and thus on the packing density in the adsorbed layer. The force barrier preventing removal of the surfactant layer from between two solid-liquid interfaces increases with increasing c, while for foam films it is the stability of the Newton black film that increases with c. Finally, the results obtained for β-C12G2 were compared with those obtained for the homologue n-decyl-β-D-maltoside (β-C10G2) as well as with those obtained for nonionic surfactants with polyoxyethylene moieties as polar groups.

1. Introduction Nonionic surfactants of the alkyl polyglycoside type, also called “sugar” surfactants, are becoming an attractive alternative to other nonionic surfactants as demonstrated by their increasing use. Besides their distinctive properties, one of the reasons that makes this type of surfactant attractive is that they can be produced on a large scale from renewable raw materials. Alkyl-β-D-glucosides, alkylR-D-glucosides, alkyl-β-D-maltosides, and other derivatives have been incorporated in a broad variety of commercial formulations.1 Wide-ranging research work has been carried out on the bulk and interfacial properties of sugar surfactants.1-6 A characteristic feature of sugar surfactants is their insensitivity to changes in temperature, * Corresponding author. Telephone: (919) 513-7494. Fax: (919) 515-6302. E-mail: [email protected]. † North Carolina State University. ‡ University College Dublin. § Engelhard Technologies. | Department of Chemistry, Surface Chemistry, Royal Institute of Technology. ⊥ Institute for Surface Chemistry. (1) Nickel, D.; Forster, T.; von Rybinski, W. In Alkyl Polyglycoside, Technology, Properties and Applications; Hill, K., von Rybinski, W., Stoll, G., Eds.; VCH: Weinheim, Germany, 1996; Chapter 4. (2) von Rybinski, W. Curr. Opin. Colloid Interface Sci. 2001, 6, 146. (3) Aveyard, R.; Binks, B. P.; Chen, J.; Esquena, J.; Fletcher, P. D. I.; Buscall, R.; Davies, S. Langmuir 1998, 14, 4699. (4) Stubenrauch, C. Curr. Opin. Colloid Interface Sci. 2001, 6, 160. (5) Claesson, P. M. Interactions between Surfaces Coated with Carbohydrates, Glycolipids and Glycoproteins. In Interfacial Behaviour of Biomolecules and Biopolymers; Malmsten, M., Ed.; Marcel Dekker: New York, 1998; Vol. 75; p 281. (6) Claesson, P. M.; Kjellin, U. R. M. Sugar Surfactants. In Encyclopedia of Surface and Colloid Science; Hubbard, H., Ed.; Marcel Dekker: New York, 2002; p 4909.

which distinguishes them from the most common nonionic surfactants having polyoxyethylene headgroups. The reason for the temperature insensitivity of the properties of sugar surfactants is suggested to be due to the strength of the hydrogen bonds between the hydroxyl groups of the polar sugar headgroups and surrounding water, which significantly reduces their dehydration with increasing temperature.4 In fact, sorption calorimetry studies have shown that the hydration of the glucose unit in n-octylβ-D-glucoside is entropy driven and results in a repulsive pressure in the lamellar phase that increases with increasing temperature.7 Further, sum-frequency generation spectroscopy has identified water molecules forming exceptionally strong hydrogen bonds next to the sugar moiety of several glucoside and maltoside surfactants adsorbed at the air-water interface.8 Many applications, including particle dispersion and stabilization,9-12 utilize the adsorption of (sugar) surfactants on solid surfaces to induce the desired changes in long-range and adhesion forces. Not surprisingly, significant differences have been observed between the adsorption behavior of nonionic surfactants at hydrophilic and hydrophobic solid surfaces, respectively. The adsorp(7) Kocherbitov, V.; So¨derman, O.; Wadso¨, L. J. Phys. Chem. B 2002, 106, 2910. (8) Tyrode, E.; Johnsson, M.; Kumpulainen, A.; Rutland, M. W.; Claesson, P. M. Submitted for publication. (9) Osipow, L.; Snell, F. D.; Marra, D.; York, W. C. Ind. Eng. Chem. 1956, 48, 1462. (10) Weerawardena, A.; Boyd, B. J.; Drummond, C. J.; Furlong, D. N. Colloids Surf., A: Physicochem. Eng. Aspects 2000, 169, 317. (11) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Weerawardena, A.; Furlong, D. N.; Grieser, F. Langmuir 2001, 17, 6100. (12) Stradner, A.; Mayer, B.; Sottmann, T.; Hermetter, A.; Glatter, O. J. Phys. Chem. B 1999, 103, 6680.

10.1021/la051938e CCC: $30.25 © 2005 American Chemical Society Published on Web 10/21/2005

Interactions of Nonpolar β-C12G2-Coated Surfaces

tion of nonionic surfactants with different headgroups on hydrophilic surfaces (e.g., silica) is very sensitive to the nature of both the surface and the surfactant headgroup.13,14 This indicates that the adsorption is strongly influenced by specific short-range interactions such as hydrogen bonding and Lewis acid-base interactions. On the other hand, adsorption of different nonionic surfactants on hydrophobic surfaces is expected to be less dependent on the nature of the headgroup since it is the nonpolar part of the molecule that interacts with the hydrophobic surface. However, interactions between the headgroups within the layer will influence the adsorption and the interactions between nonpolar surfaces coated with nonionic surfactants. The effect of the headgroup is difficult to predict, and accurate measurements are needed in order to better understand the influence of different polar and nonpolar groups on the interfacial properties of a given system. Recently, the adsorption and wetting behavior as well as the surface forces acting between adsorbed layers of n-decyl-β-D-maltoside (β-C10G2) have been investigated.14-21 Since notable differences in interfacial properties are expected even with small changes in the surfactant structure, this study addresses the surface forces generated by another important representative of the alkyl maltoside series, namely n-dodecyl-β-D-maltoside (βC12G2). There are some studies dealing with the properties of adsorbed β-C12G2 focusing on either the single air/water interface22 or on foam films.23-26 To our knowledge there are only two studies14,27 in which the adsorption of β-C12G2 on hydrophilic solid surfaces was investigated, and likewise only a few reports address the adsorption of β-C12G2 on hydrophobic surfaces.27,28 Moreover, no studies of the interaction forces between solids coated with β-C12G2 have been reported. This lack of information is rather surprising since sugar surfactants are used in several applications involving solid surfaces. The present study deals with the adsorption of β-C12G2 on hydrophobic solid surfaces and the resulting long-range and adhesive forces between the respective surfactantcoated surfaces. Thiolated gold was used as hydrophobic solid surface. The interaction forces were measured with a bimorph surface force apparatus (the measurement and analysis of surface interaction forces (MASIF) tech(13) Kiraly, Z.; Findenegg, G. H. Langmuir 2000, 16, 8842. (14) Matsson, M. K.; Kronberg, B.; Claesson, P. M. Langmuir 2004, 20, 4051. (15) Persson, C. M.; Claesson, P. M.; Lunkenheimer, K. J. Colloid Interface Sci. 2002, 251, 182. (16) Persson, C. M.; Kjellin, U. R. M.; Eriksson, J. C. Langmuir 2003, 19, 8152. (17) Persson, C. M.; Kumpulainen, A. J.; Eriksson, J. C. Langmuir 2003, 19, 6110. (18) Persson, C. M.; Kumpulainen, A. J. Colloids Surf., A 2004, 233, 43. (19) Kumpulainen, A. J.; Persson, C. M.; Eriksson, J. C. Langmuir 2004, 20, 10935. (20) Kumpulainen, A. J.; Persson, C. M.; Eriksson, J. C. Langmuir 2004, 20, 10534. (21) Kumpulainen, A. J.; Persson, C. M.; Eriksson, J. C.; Tyrode, E. C.; Johnson, C. M. Langmuir 2005, 21, 305. (22) Liljekvist, P.; Kjellin, M.; Eriksson, J. C. Adv. Colloid Interface Sci. 2001, 293, 89. (23) Stubenrauch, C.; Schlarmann, J.; Strey, R. Phys. Chem. Chem. Phys. 2002, 4, 4504; Phys. Chem. Chem. Phys. 2003, 5, 2736 (erratum). (24) Schlarmann, J.; Stubenrauch, C. Tenside Surfactants Deterg. 2003, 40, 190. (25) Muruganathan, R. M.; Krustev, R.; Ikeda, N.; Mu¨ller, H.-J. Langmuir 2003, 19, 3062. (26) Muruganathan, R. M.; Krustev, R.; Mu¨ller, H. J.; Mo¨hwald, H.; Kolaric, B.; von Klitzing, R. Langmuir 2004, 20, 6352. (27) Zhang, L.; Somasundaran, P.; Maltesh, C. Colloid Interface Sci. 1997, 191, 202. (28) Dedinaite, A.; Bastardo, L. Langmuir 2002, 18, 9393.

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Figure 1. Molecular structures of n-dodecyl-β-D-maltoside (βC12G2) and hexaoxyethylene dodecyl ether (C12E6).

nique).29,30 The results are discussed in the light of previous studies in which the interactions between the respective water/air surfaces, i.e., the disjoining pressure of foam films stabilized with β-C12G2, were investigated.23,24 To elucidate the influence that the molecular structure of the surfactant has on the interaction forces, we compared our results with those obtained for hexaoxyethylene dodecyl ether (C12E6) (see Figure 1).31 Key features of sugar surfactants (CnGm) and polyoxyethylene alkyl ethers (CiEj) will be juxtaposed and discussed. 2. Experimental Section 2.1. Solution Preparation. The nonionic surfactant n-dodecyl-β-D-maltoside (β-C12G2) was used as received from Sigma (>98% GC). The purity was checked by measuring the surface tension as a function of the concentration at 22 °C by the Du Nouy ring method using a Kru¨ss K10ST tensiometer.23 Sodium chloride was obtained from Merck (Germany) and roasted at 500 °C before use to remove organic impurities. Water used for the preparation of all solutions was purified with a Millipore Milli-Q Plus 185 water purification system. For the surface force experiments the water was deaerated using a water jet pump for 2 h immediately before use, which is essential to minimize the formation of air bubbles in the vicinity of highly hydrophobic solid surfaces. All glassware was cleaned with Deconex from Borer Chemie and rinsed thoroughly with Milli-Q water before use. Three different surfactant solutions at concentrations of 0.035, 0.14, and 0.175 mM were prepared in 0.1 mM NaCl background electrolyte. The first two concentrations are below and the last one is above the critical micelle concentration (cmc) of this surfactant (0.15 mM). In Table 1 some characteristic parameters of β-C12G2 as well as those of some other selected nonionic surfactants are listed. 2.2. MASIF Technique. Surface Preparation. The hydrophobic surfaces used for force measurements were obtained by thiolation of gold-coated glass. The glass surfaces were prepared from rods of borosilicate (Pyrex) with 2 mm diameter that were melted at one end with a butane-oxygen burner. The treatment was conducted until a surface of spherical shape with a radius of approximately 2 mm was obtained. Details about the subsequent thiolation are provided in previous reports.31,35,36 Surface Forces. The forces between surfactant-coated solid surfaces were measured with the MASIF technique,29,30,37 which basically employs an LVDT and a bimorph sensor to measure the relative distance between the surfaces (D) and the forces of interaction (F). The experimental data are presented as (F/R)-D (29) Parker, J. L. Prog. Surf. Sci. 1994, 47, 205. (30) Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W. Adv. Colloid Interface Sci. 1996, 67, 119. (31) Stubenrauch, C.; Rojas, O. J.; Schlarmann, J.; Claesson, P. M. Langmuir 2004, 20, 4977. (32) Shinoda, K.; Yamaguchi, T.; Hori, R. Bull. Chem. Soc. Jpn. 1961, 34, 237. (33) Drummond, C. J.; Warr, G. G.; Grieser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1985, 89, 2103. (34) Kjellin, U. R. M.; Claesson, P. M.; Linse, P. Langmuir 2002, 18, 6745. (35) Stubenrauch, C.; Schlarmann, J.; Rojas, O. J.; Claesson, P. M. Tenside Surfactants Deterg. 2004, 41, 174. (36) Ederth, T.; Claesson, P.; Liedberg, B. Langmuir 1998, 14, 4782. (37) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135.

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Table 1. Critical Micelle Concentrations (cmc), Area per Molecule (Acmc), and Surface Tensions (γcmc) of Different Nonionic Surfactants in Aqueous Solution at 298 Ka cmc/mM

Acmc/Å2

γcmc/mN m-1

2 0.19 2.1 0.15, 0.2 (295-297 K)22 0.86 0.73 0.05 0.064 0.073,35 0.064 mM (295-297 K)22

41 36 56 50,33 48.5,27 38 (295-297 K),22 55 (on graphite)27 50 47 42 47 52,35 50 (295-297 K)22

29.9 40.9 37.0 37.633 29.4 29.9 27.6 29.7 34.035

surfactant n-decyl-β-glucoside3

β-C10G1, β-C12G1, n-dodecyl-β-glucoside32 β-C10G2, n-decyl-β-maltoside3 β-C12G2, n-dodecyl-β-maltoside C10E424 C10E534 C12E434 C12E534 C12E6 a

Acmc and γcmc refer to the values at the cmc.

curves, where R is the harmonic radius R of the two surfaces (R ) 2R1R2/(R1 + R2)). In the MASIF experiments the distance D measured between the (bare or surfactant-coated) surfaces is not absolute but relative to the zero compliance or “hard wall” contact. Therefore, if two surfactant-coated surfaces are in contact, D ) 0 means that the two surfactant layers (one on each surface) are in direct contact. This has to be taken into account when the MASIF results are evaluated and compared with results from other techniques, e.g., the interferometric surface force apparatus (SFA). A detailed description of the experimental procedure and the data evaluation is provided in ref 31. Hydrodynamic Effects. In the MASIF the interactions are measured under dynamic conditions; i.e., the data are automatically acquired while the surfaces are moving with respect to each other (approaching or separating). Therefore, hydrodynamic effects arising from the movement of the liquid medium during the approach or the separation of the surfaces need to be taken into account, especially when high rates of displacement are used.38,39 The hydrodynamic interactions result in an extra repulsion when the surfaces approach each other and in an extra attraction when they are separated. To minimize this effect, low driving rates (during surface approach and separation) are employed. The hydrodynamic force FH between two approaching surfaces is given by38,39

FH 3πηR d(D - 2Ds) ) R 2D dt where η is the viscosity of the solution (assumed to be identical to that of water) and D is the surface separation at time t. Ds is the position of the plane of no shear and thus a measure of the thickness of an “immobile” region of liquid adjacent to each solid surface (the stick boundary conditions apply at a distance Ds out from each surface). The location of the slipping plane is not known independently, and Ds is treated as a fitting parameter. The local rate of approach of the surfaces (d(D - 2Ds)/dt) changes as the surfaces start to interact, and it is calculated directly from the experimental data. The hydrodynamic force FH is subtracted from the measured “dynamic” interaction force F, leading to the “static” interaction force.

3. Results 3.1. Interactions between Thiolated Surfaces. Nonpolar thiolated gold surfaces were prepared, and the interactions across aqueous solutions in the absence and presence of β-C12G2 were determined. The (dynamic and static) forces acting between the pure thiolated gold surfaces immersed in 0.1 mM aqueous NaCl solution are illustrated in Figure 2. This figure also includes the static force profile that is calculated by subtracting the hydrodynamic interactions from the measured (dynamic) forces. No long-range electrostatic double-layer force is present, demonstrating that the thiolated surfaces are uncharged. A long-range attractive force becomes detectable at a surface separation of about 15 nm, and the gradient of the force exceeds the spring constant when the separation has decreased to 10 nm. From this position the surfaces (38) Chan, D. Y. C.; Horn, R. G. J. Chem. Phys. 1985, 83, 5311. (39) Vinogradova, O. I. Langmuir 1996, 12, 5963.

Figure 2. Force F normalized by the harmonic radius R as a function of surface separation D. The forces were measured on approach (at a rate of 17 nm/s) between thiolated gold surfaces across aqueous solutions containing 0.1 mM NaCl in the absence of surfactant (circles). The solid line corresponds to the experimental static surface forces (calculated after subtracting hydrodynamic forces from the measured surface forces). The dashed line is the van der Waals force assuming a 2-nm-thick thiol layer on gold calculated with a distance-dependent Hamaker constant (according to Ederth40). The arrow represents the jump into contact when the force gradient d(F/R)/dD exceeds the spring constant, k/R.

jump into contact, and the magnitude of the attraction in this distance regime cannot be quantified. The calculation of the van der Waals force acting between thiolated gold surfaces is rather complex since the magnitude of the effective Hamaker constant varies with the surface separation. However, a situation identical to that in the MASIF experiment has been considered in detail by Ederth,40 and we utilized his results for calculating the van der Waals forces. The results, which are displayed in Figure 2 as a dashed line, show that the measured attraction is slightly larger than the calculated van der Waals force. Note that Ederth obtained significantly larger attractive forces for similar surfaces36 which were identified as being due to the formation of vapor cavities, a process that did not occur in the present study. In other words, the origin of the attractive force observed for the present system is not quite clear. However, the presence of attractive forces is consistent with previous reports describing the forces acting between nonpolar solid surfaces in water. For an introduction to the topic of the long-range attraction between such surfaces, the reader is referred to the review by Christenson and Claesson.41 Once in contact, the surfaces adhere strongly to each other. The adhesion force is in fact too strong to be measured with the bimorph spring used in these measurements, which means that it exceeds 80-100 mN m-1. (40) Ederth, T. Langmuir 2001, 17, 3329. (41) Christenson, H. K.; Claesson, P. M. Adv. Colloid Interface Sci. 2001, 91, 391.

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Figure 4. Force F normalized by the harmonic radius R as a function of surface separation D measured 8 h after the β-C12G2 surfactant had been injected. The forces were measured on approach (filled circles) and on separation (open circles) between thiolated gold surfaces across aqueous solutions containing 0.1 mM NaCl in the presence of 0.14 mM β-C12G2. The driving velocity was 4.3 nm/s. The black line that follows the trace of the force profile on separation corresponds to the static forces (after subtracting hydrodynamic effects).

Figure 3. (a) Force F normalized by the harmonic radius R as a function of surface separation D between thiolated gold surfaces across aqueous solutions containing 0.1 mM NaCl in the presence of 0.035 mM β-C12G2. The surface forces represented by the open circles were measured on approach (25 nm/ s; see inset) 2.5 h after injection of β-C12G2. An inward step at around 4 mN m-1 was observed in this case. The respective pulloff or adhesion forces measured on separation were too strong to be depicted in the force diagram. The solid line represents the static force computed after subtracting hydrodynamic effects. A slip plane located at 3.2 nm was assumed in the calculation to ensure a perfect match between the static forces and the experimental data in the zero-compliance zone. The dashed line represents the van der Waals forces for a threelayer model (gold/thiol layer/surfactant) calculated with a distance-dependent Hamaker constant according to ref 40. The surface force profile obtained after 19 h equilibration (for the same system) is represented by the filled circles (offset by 3.2 nm from the origin). In this case no step-in was observed. (b) Schematic diagrams of the situations depicted in (a) for the force curves between thiolated gold surfaces across 0.1 mM NaCl in the presence of 0.035 mM β-C12G2 after 2.5 and 19 h equilibration time. After 2.5 h the adsorbed surfactant molecules are driven out from between the interfaces at a compressive load of ca. 4 mN m-1. A strong adhesion due to thiol-thiol contact is observed. After 19 h the adsorbed surfactant molecules are not squeezed out (under the conditions of the experimental compressive loads applied); i.e., the surfactant polar groups on each surface are in contact.

The force profiles obtained after addition of β-C12G2 to a low concentration (0.035 mM) are reported in Figure 3. The surfactant molecules adsorb with the nonpolar group directed toward the surface and the polar group toward the aqueous medium. After addition of the surfactant the surfaces thus become hydrophilic and the long-range attraction present prior to addition of β-C12G2 vanishes, as expected. It takes less than 2.5 h to develop a surfactant monolayer that produces a steep repulsive force barrier at separations below 5 nm. To overcome this force barrier,

a compressive force of ca. 4 mN m-1 is needed. At short separations the surfactant monolayers (one on each surface) exert a steric repulsion as the two surfaces are brought together, producing the force barrier illustrated in Figure 3. If a sufficiently large compressive force is applied, the surfactant is squeezed out from the contact zone, leading to a contact of the two bare thiolated surfaces. Thus, the steep steric repulsion at D ∼ 3-4 nm is equivalent to twice the length of the adsorbed surfactant, and the thickness of a single sugar surfactant monolayer can be calculated to be 1.7-2 nm. The influence of hydrodynamic effects is clearly seen in Figure 3 (static force profile), where an adhesion force can be detected after subtracting hydrodynamic forces. In this case the position of the slipping plane was chosen from the separation distance at which the measured approaching velocity suddenly drops (see inset in Figure 3). This distance is estimated to be 3.2 nm, which coincides with the separation distance where a steric barrier comes into play (see step in the force profile). The observed attraction is attributed to van der Waals forces, which were calculated by assuming a distance-dependent Hamaker constant (as a multiple-layer system) according to ref 40. Indeed, the static force profile nearly matches the calculated van der Waals interactions up to the onset of the steric repulsion. Another observation worth mentioning is the fact that the magnitude of the force barrier increases with time and that a “hard wall” contact is observed after the adsorption equilibrium is reached (see force curve recorded after 19 h from injection of the surfactant solution). This steep force is due to steric repulsion forces between the maltoside headgroups of the surfactant molecules adsorbed on each surface. At a β-C12G2 concentration of 0.14 mM the cohesion of the monolayer is sufficiently strong to prevent the displacement of the surfactant from between the interfaces using forces up to F/R ) 10 mN m-1 (see Figure 4). Thus, for the 0.14 mM surfactant concentration we observe a hard wall (or MASIF constant compliance zone) due to the repulsion between the surfactant headgroups up to the maximum measurable force, which indicates that the surfactant layer is densely packed. Note that the maximum forces applied were significantly larger than the ones that can be measured accurately (limited by the saturation of the bimorph force sensor).

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However, the applied loads can be estimated from the known spring constant and piezo displacement. The outward force curve displayed in Figure 4 was obtained after applying a high load (F/R ≈ 50 mN m-1) and shows an interesting feature: when the compressive force was decreased to F/R ) 2.2 mN m-1, a sudden increase in thickness by 2-3 nm was observed. The sudden increase in the surface separation distance observed in the outward force profile has also been reported by Kjellin et al.42 and is explained by the diffusion of surfactant molecules (that were driven out from between the surfaces upon approach at a high, but not measurable, force) into the contact region due to the intralayer surface pressure at the solid-liquid interface outside the contact position. A rapid readsorption of the surfactant occurs once the repulsive force has been reduced sufficiently. Since the adsorbed surfactants were removed at high compression, the approach (inward) force curve in Figure 4 was shifted outward by 3 nm (as was the case in Figure 3). The adhesion observed after separating the surfaces (ca. 0.5 mN m-1 in the outward run shown in Figure 4) is suggested to be due to the van der Waals forces between the surfactant layers (see Figure 3). By further increasing the surfactant concentration to 0.175 mM, one observes interaction curves similar to the one obtained for 0.14 mM. An example is given in Figure 5a. Thus, for all three concentrations (below and above the cmc) the adsorbed surfactant layers are sufficiently dense to provide a stabilizing force barrier at a separation of ca. 3 nm. The magnitude of this force barrier increases with increasing surfactant concentration as the adsorption density, and thus the monolayer cohesion, is increased. An estimate of the adsorption density at the three surfactant concentrations can be obtained using the data reported by Dedinaite for β-C12G2 on silanated silica.28 The value at 0.035 mM can be estimated to be 1.4 mg/m2 (2.7 µmol/m2), whereas a value of 1.7 mg/m2 (3.3 µmol/m2) was obtained at concentrations close to the cmc. An important issue to be mentioned at this point is that Figures 3, 4a, and 5a show “featureless” inward curves up to the development of the steep steric barrier due to the contact of the adsorption layers. However, after subtracting hydrodynamic forces, it became clear that for thiolated glass surfaces the van der Waals attraction dominates at surface separations below ca. 7 nm up to the onset of the steep steric barrier (at 3-4 nm separation). In other words, the hydrodynamic correction led to a slight but important change in the force profiles revealing an additional attractive force, i.e., the van der Waals force, close to the steric repulsion. There is thus no need to include a contribution from hydrogen bonding to explain the experimental data. We note here that the adhesion between n-decyl-β-D-maltoside surfactant (β-C10G2) layers has been reported to be 0.9 mN m-1, which can also be explained as a van der Waals attraction.15 3.2. Interactions between Air/Water Surfaces. In a previous study, the disjoining pressure Π (i.e., the interaction between air/water surfaces across aqueous solutions) of foam films stabilized by β-C12G2 was measured as a function of the film thickness h, using the thin film pressure balance (TFPB) technique.23,43 Surfactant concentrations similar to those employed in the present study were used, and the background electrolyte concentration was also kept constant at 0.1 mM NaCl. Details with regard to the technique, the experimental procedure, and (42) Kjellin, U. R. M.; Claesson, P. M. Langmuir 2002, 18, 6754. (43) Stubenrauch, C.; von Klitzing, R. J. Phys.: Condens. Matter 2003, 15, R1197.

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Figure 5. (F/R)-D curves between thiolated gold surfaces (a) and corresponding Π-h curves (b) for three different concentrations of β-C12G2 in 0.1 mM NaCl solution. For the cmc a value of 0.15 mM was determined. All (F/R)-D curves were measured on approach and after the equilibrium adsorption had been established. The different surfactant concentrations correspond to 0.035 (diamonds), 0.14 (open circles), and 0.17 mM (filled circles). In (a) a representative static force profile (after subtraction of hydrodynamic effects) is shown for completeness (solid line). The solid lines in (b) are calculated according to the DLVO theory assuming interactions at constant charge. Data for the Π-h curves are taken from ref 23.

the interpretation of the results are given in refs 23 and 43. General conclusions can be drawn with regard to the influence of the surfactant concentration on the force profiles in the case of thiolated (see Figure 5a) surfaces as well as on the corresponding Π-h curves (Figure 5b). Since the situation displayed in Figure 5a was already discussed, we now turn our attention to the Π-h curves. As is seen in Figure 5b, film thicknesses were found to range from more than 80 nm to less than 5 nm, depending on the surfactant concentration and the applied pressure, which ranges from 200 to 9000 Pa. Two different kinds of films were observed: thick common black films (CBF) stabilized by electrostatic repulsion, and thin Newton black films (NBF) stabilized by short-range repulsion. The thicknesses of the CBFs decrease monotonically as Π increases. While the slope d(log Π)/dh is independent of the surfactant concentration, a significant shift of the curves toward lower disjoining pressures is observed by increasing the surfactant concentration from 0.034 to 0.137 mM. Moreover, at the highest concentration no CBF is formed at all, but the foam film drains directly down to the NBF. It is established that-but not yet completely understood whysthe air/water surface is negatively charged. This charge is responsible for the long-range electrostatic repulsive forces observed in foam films stabilized by

Interactions of Nonpolar β-C12G2-Coated Surfaces Table 2. Surface Excess of β-C12G2 at the Air-Water and Silanated Silica Interfaces surface

concn (mM)

surf. excess (µmol/m2)

air-water silanated silica air-water silanated silica air-water silanated silica

0.034 0.035 0.137 0.14 0.17 0.17

4.0b 2.7a 4.5b 3.3a 4.7b 3.3a

a

From ref 28. b From ref 23.

nonionic surfactants. An increase in nonionic surfactant concentration leads to a decrease of the surface charge density as more uncharged molecules (i.e., nonionic β-C12G2 surfactant) adsorb at an originally charged surface. A detailed discussion about the origin of the surface charge at the bare air/water surface can be found in refs 31 and 43-48. The electrostatic forces acting in foam films stabilized by β-C12G2 were quantified by means of the DLVO theory using constant charge boundary conditions and the theoretical Debye length of κ-1 ) 30 nm.49 These calculations led to surface charge densities of q0 ) 1.55 mC m-2 for the 0.035 mM solution and q0 ) 0.95 mC m-2 for the 0.137 mM solution, respectively. The decrease in surface charge density destabilizes the CBF until no CBF is observed for c > cmc under the given experimental conditions. At these concentrations it is the immediate formation of an NBF that is observed. The NBFs are very thin (ca. 5 nm) with an aqueous core of 1-2 nm, assuming a length of ∼2 nm for the surfactant. In other words, these films consist of two surfactant monolayers with only small amounts of water separating the headgroups (mainly hydration water). As was the case for the force barrier between nonpolar solid surfaces, densely packed monolayers are required to stabilize an NBF and thus to prevent contact between the two bare surfaces (i.e., film rupture). Hence the presence of an NBF signifies the existence of a force barrier between two air/water surfaces in analogy to the force barrier between two solid surfaces (see Figure 5a). 4. Discussion 4.1. Influence of the Surface on Interfacial Forces. The driving force for adsorption of nonionic surfactants to nonpolar surfaces is the reduction in contact area between the surface and water, as well as between the nonpolar surfactant tails and water. This leads to an adsorption with the surfactant tail directed toward the hydrophobic surface and the polar group directed toward the aqueous solution. The extent of adsorption at different nonpolar surfaces, however, varies depending on the nature of the surface. For instance, Kjellin et al. showed that the surface excesses of a range of nonionic ethoxylate surfactants were slightly larger at the air-water interface than at silanated silica.34 The value reported for the surface excess of β-C12G2 at the air-water interface23 is also slightly larger than that on silanated silica28 as shown in Table 2. Unfortunately, the surface excess of this surfactant on thiolated gold has not been reported, but as a first approximation one can assume that it is similar to that on silanated silica. (44) Karraker, K. A.; Radke, C. J. Adv. Colloid Interface Sci. 2002, 96, 231. (45) Exerowa, D.; Kruglyakov, P. M. Foam and Foams FilmssTheory, Experiment, Application; Elsevier: Amsterdam, 1998. (46) Carruthers, J. C. Trans. Faraday Soc. 1938, 34, 300. (47) Dickinson, W. Trans. Faraday Soc. 1941, 37, 140. (48) Taylor, A. J.; Wood, F. W. Trans. Faraday Soc. 1957, 53, 523. (49) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1991.

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One important difference between the two hydrophobic surfaces used in this study, the air-water interface and thiolated gold, is that the bare air/water surface is charged while the thiolated surface is uncharged (see Figure 2). The charge at the air/water surface leads to the presence of a long-range electrostatic repulsion well described by the nonlinear Poisson-Boltzmann model. Thus, the (F/ R)-D and the Π-h curves have nothing in common with regard to long-range interactions: no long-range electrostatic interactions are observed for the thiolated surfaces, while it is exactly these interactions that dominate the properties of the foam films. On the other hand, the shortrange repulsive interactions between air/water and thiolated surfaces, respectively, are similar. In both cases a direct contact between the surfaces is not observed (in the latter case a direct contact is equivalent to a film rupture). The force barrier that is built up between the thiolated surfaces is comparable to the formation of an NBF, as was discussed in detail in our previous study.31 Note that, in contrast to the bilayer formation between the thiolated surfaces, NBF formation was not observed at the lowest surfactant concentration under the given experimental conditions. However, it is not unlikely that an NBF is formed at pressures higher than 10 000 Pa. In both cases, the short-range repulsion is due to a combination of a hydration repulsion that originates from the removal of water molecules tightly bound to the polar sugar group,50 a protrusion repulsion due to the restriction in movement of surfactants in and out from the surface,51 and a conformational repulsion originating from the restricted conformation of the polar group.52 4.2. Influence of the Surfactant Polar Group on Interfacial Forces. The interaction forces and the disjoining pressures between surfaces coated with either β-C12G2 or C12E6 are compared in Figure 6. Two different surfaces are considered: thiolated gold (Figure 6a) and the air/water surface (Figure 6b). Since one can extract more information from the force profiles obtained at concentrations below the cmc, i.e., for nonsaturated adsorption layers, the respective data of the current study and of the previous C12E6 study31 are juxtaposed. Note that in both cases we are not far from the maximum air/ water surface concentration, which allows ussat least qualitativelysto compare the results. [Actual surface coverage of β-C12G2 at 0.034 mM is 4.0 × 10-6 mol m-2 compared to the maximum surface coverage of 4.7 mol m-2. Actual surface coverage of C12E6 at 0.01 mM is 3.0 × 10-6 mol m-2 compared to the maximum surface coverage of 3.2 mol m-2.31] A major difference is worth mentioning with regard to the adsorption of β-C12G2 and C12E6 at the solid surfaces. Under equilibrium conditions very high forces are needed to expel β-C12G2 from between thiolated gold surfaces, while for C12E6 compressive loads below 1 mN m-1 are sufficient (see Figure 6a). The most reasonable explanation for this observation is a higher adsorption density of β-C12G2 which results in a significant force barrier already at low concentrations. Similar observations were made in the case of silanated surfaces, where a steric force between β-C12G2-coated surfaces was evident even at low concentrations (unpublished data), whereas in the case of C12E6coated surfaces no steric force was observed at similar concentrations.31 This observation is in line with the adsorption behavior at the air/water surface: at the chosen (50) Parsegian, V. A.; Fuller, N.; Rand, R. P. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 2750. (51) Israelachvili, J. N.; Wennerstrom, H. J. Phys. Chem. 1992, 96, 520. (52) de Gennes, P. G. Adv. Colloid Interface Sci. 1987, 27, 189.

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Figure 6. (F/R)-D curves between thiolated gold surfaces (a) and corresponding Π-h curves (b) for two different nonionic surfactants (C12E6 and β-C12G2) in 0.1 mM NaCl solution. The cmc values are given in Table 1. All (F/R)-D curves were measured on approach and after the equilibrium adsorption had been established. The solid lines in (b) are calculated according to the DLVO theory assuming interactions at constant charge. Data for C12E6 are taken from 31. No correction for hydrodynamics is included in the force profiles shown in (a).

concentrations the surface concentration of C12E6 is around 25% less than that of β-C12G2. This difference is suggested to be due to the lower flexibility of the maltoside group compared to an oligooxyethylene group.22 This leads to a larger loss of conformational entropy for C12E6 layers than for β-C12G2 layers due to the confinement between two surfaces, which, in turn, provides a larger driving force for the desorption of C12E6 molecules. At higher concentrations, however, both β-C12G2 and C12E6 produce a monolayer that is very difficult to remove from the gap between the surfaces upon compression, indicating formation of densely packed surfactant layers. The layer thickness can be determined with the MASIF technique only when the surfactant is removed by compression. On the basis of these results we estimate that the β-C12G2 barrier is located at a distance of 3-4 nm, whereas a value of 4-5 nm is observed for C12E6 (see Figure 6). It is therefore concluded, as expected from the molecular structure, that under similar experimental conditions the effective length of the β-C12G2 molecule confined at the interface is shorter than that of C12E6. The attractive forces acting between the surfactant layers when they are close to each other is mainly due to van der Waals interactions. However, the depth of the attractive minimum depends on the nature of the polar part of the surfactant53-55 and is the result of an interplay (53) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468. (54) Ederth, T.; Liedberg, B. Langmuir 2000, 16, 2177.

Rojas et al.

of van der Waals, hydration, and confinement forces.51 The attractive minimum is marginally deeper for the surfactant with the maltoside headgroup (ca. 0.5-1 mN m-1 attractive minimum for β-C12G2 adsorbed on thiolated glass) compared to C12E6 with the same hydrophobic chain (attractive minimum of less than 0.5 mN m-1).31 One effect contributing to this is the larger refractive index of a glucoside unit compared to that of ethylene oxide, which increases the van der Waals force. Moreover, the shortrange repulsive force contributions are expected to be different for the two surfactants, which will also influence the depth of the attractive minimum. For the sake of completeness we note that in the particular case of ethylene oxide based surfactants an increase in temperature results in a larger attraction, an effect related to the temperature dependence of the hydration interaction.56 While the adsorption behaviors of β-C12G2 and C12E6 at solid surfaces are obviously different, astonishing similarities with regard to the Π-h curves are found. As is seen in Figure 6b, the curves nearly lie on top of each other, which means that the surface charge densities q0 are nearly equal. Indeed, surface charge densities of q0 ) 1.70 mC m-2 for C12E6 and q0 ) 1.55 mC m-2 for β-C12G2 were calculated from the experimental data.23,31 As the surface charge density is a property of the bare air/water surface, these values mean that a surface concentration of 3.0 × 10-6 mol m-2 C12E6 reduces the surface charge density to the same amount as a surface concentration of 4.0 × 10-6 mol m-2 β-C12G2. Moreover, the surface concentration is in both cases enough to stabilize a foam film up to 10 000 Pa. We suggest that it is the effective surface cross-sectional area covered per molecule, which is different from the average area per molecule at the interface that determines the reduction in surface charge density. Since the headgroup of C12E6 is larger than that of β-C12G2, a smaller number of the former surfactant is needed to achieve a given reduction in the repulsive doublelayer force. Similar arguments have been presented before.57,58 We conclude that for each surfactant the surface charge density is reduced with increasing adsorption. However, when comparing different surfactants, a given adsorption reduces the surface charge density to a different degree. Finally, a comparison of various oligomers of polyglycoside-type surfactants is not attempted here, but it is expected that the glucoside (β-C12G1) surfactant will form (on nonpolar surfaces) an adsorbed monolayer of higher cohesive energy and therefore will offer greater resistance against compression compared to the maltoside counterpart (β-C12G2). This is based simply on the fact that β-C12G1 is expected to adsorb more extensively due to a smaller repulsion between the headgroups. These assumptions are validated by results reported for a similar system (βC10G2 and β-C10G1 surfactants).18 5. Conclusions In conclusion, the nature of the surface at which the surfactant adsorption takes place mainly influences the interaction forces at low surface coverages. Once a densely packed surfactant layer is formed, it is the surfactant itself that determines the interaction forces. The fact that no (55) Claesson, P. M.; Kjellin, U. R. M. Modern Characterization methods of surfactant systems. In Surfactant Science Series; Binks, B. P., Ed.; Marcel Dekker: New York, 1999; Vol 83, p 255. (56) Claesson, P. M.; Kjellander, R.; Stenius, P.; Christenson, H. K. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2735. (57) Manev, E. D.; Pugh, R. J. Langmuir 1991, 7, 2253. (58) Waltermo, A.; Manev, E.; Pugh, R.; Claesson, P. J. Dispersion Sci. Technol. 1994, 15, 273.

Interactions of Nonpolar β-C12G2-Coated Surfaces

double-layer force was observed between the β-C12G2coated thiolated surfaces proves that the surfactant did not contain any charged surface-active impurities, and thus that the repulsive double-layer force observed between air-water interfaces is due to charges present at the bare air-water interface. The measurements performed in this study contribute to elucidating the influence of the molecular structure, i.e., the polar headgroup and the hydrophobic chain, on interaction forces. Comparing data obtained for β-C12G2 and C12E6, for example, demonstrates that for similar bulk concentrations the sugar surfactant forms a denser and more robust monolayer with higher cohesiveness. On the other hand, compared with the shorter tail homologue β-C10G2, β-C12G2 is anchored more strongly to thiolated gold. This is explained by a stronger interaction between the nonpolar tails within the monolayer. Our measurements indicate a thickness of the β-C12G2 monolayer under compressive loads marginally lower than

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2 nm. This value is in agreement with the surfactant molecular dimensions. It is smaller than the (compressed) monolayer thickness measured for C12E6. For both surfactants, the most distinctive feature in the force curve between surfactant-coated solid surfaces is the buildup of a steep and short-range repulsion as the surfactant adsorption approaches saturation. Acknowledgment. O.J.R. and P.M.C. are grateful to the SSF program “Colloid and Interface Technology” and the European Commission (Marie Curie RTN SOCON) for their support. C.S. is indebted to the DFG, the Fond der Chemischen Industrie, and the European Commission (Marie Curie RTN SOCON) for financial support. J.S.-S. acknowledges financial support from the DFG and the DAAD. O.J.R. acknowledges support from the National Textile Center grant C05-NS09. LA051938E