Comparison between Interaction Forces at Air/Liquid and Solid/Liquid ...

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© 2005 Carl Hanser Verlag, Munich, Germany

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C. Stubenrauch1, J. Schlarmann2, O. J. Rojas3, P. M. Claesson4

Comparison between Interaction Forces at Air/Liquid and Solid/Liquid Interfaces in the Presence of Non-Ionic Surfactants Interactions in thin liquid foam films and between solid/liquid surfaces coated with the non-ionic surfactant hexaoxyethylene dodecylether (C12E6) were investigated with a TFPB (thin film pressure balance) and the MASIF (Measurement and Analysis of Surface Interaction Forces) technique, respectively. For foam films the formation of common black films (CBF) and Newton black films (NBF) was observed depending on the surfactant concentration and the applied pressure. With increasing surfactant concentration and increasing pressure the CBF is destabilized, whereas a stabilization of the NBF is observed. In addition, MASIF measurements with two different hydrophobic surfaces were made. In one case silanated glass surfaces and in the other case thiolated gold surfaces were used. Differences and similarities of the interactions between these two surfaces are shown and discussed. Furthermore, the results of the MASIF measurements are compared with the ones of the thin film pressure balance measurements and the influence of the surfactant concentration on the interactions is discussed. Key words: Thin liquid films, TFPB, MASIF, stability of colloids, disjoining pressure, surface forces, surface charges, non ionic surfactants Vergleich der Wechselwirkungskräfte an luft/flüssig- und fest/ flüssig-Grenzflächen in Gegenwart nichtionischer Tenside. Mit einer TFPB (thin film pressure balance) und der MASIF (Measurement and Analysis of Surface Interaction Forces) Technik wurden Wechselwirkungen in freistehenden Schaumfilmen und zwischen fest/flüssig-Grenzflächen für wässrige Lösungen des nichtionischen Tensids Hexaethylenglykol-Monododecylether (C12E6) untersucht. Für Schaumfilme wurde die Bildung von common black films (CBF) und Newton black films (NBF) abhängig von der Tensidkonzentration und dem angelegten Druck beobachtet. Dabei wird mit steigender Tensidkonzentration und steigendem Druck der CBF destabilisiert und der NBF stabilisiert. Zum Vergleich wurden MASIF-Messungen mit zwei unterschiedlichen hydrophoben Oberflächen durchgeführt. Zum einen wurden silanierte Glasoberflächen, zum anderen thiolierte Goldoberflächen verwendet. Unterschiede sowie Ähnlichkeiten der Wechselwirkungen zwischen diesen beiden Grenzflächen werden aufgezeigt und diskutiert. Weiterhin werden die mit der MASIF-Technik erhaltenen Ergebnisse mit denen der thin film pressure balance verglichen und der Einfluss der Tensidkonzentration auf die Wechselwirkungen diskutiert.

1 Introduction

The interaction forces between surfactant-coated surfaces are key factors in the stability of colloidal systems such as foams, emulsions and dispersions (see Fig. 1). In order to understand and thus to control the stability of colloidal systems several issues have to be considered. First, the extent of surfactant adsorption at the interface has to be known as each adsorbed molecule alters the properties of the surface and thus the respective interaction forces. Second, the nature of the surface itself has to be considered because it influences the interaction forces. This is due to the fact that the net interaction force results from contributions of the substrate (e. g. double-layer forces generated by charged surface groups) and from the structure of the adsorbed layer. For example, the adsorption of surfactants at hydrophilic and hydrophobic surfaces, respectively, results in adsorption layers of completely different structures, which in turn generate different interaction forces. Additionally, the nature of the surfactant is crucial in defining the quality and magnitude of the interaction forces. These interactions originate mainly from repulsive short-range confinement, repulsive longrange electrostatic, and short-range attractive van der Waals forces [1, 2]. In addition, the presence of an attractive longrange force, often referred to as the hydrophobic interaction, is observed between non-polar surfaces in aqueous solutions [3, 4]. For the investigation of these forces different techniques can be employed. The choice of the appropriate technique depends on the type of surface dealt with. For solid/ liquid interfaces the atomic force microscope (AFM), the surface force apparatus (SFA) and the MASIF-technique [5 – 7] are usually employed, whereas for both air/water and oil/

Stichwörter: Thin liquid films, TFPB, MASIF, Kolloidstabilität, Spaltdruck, Grenzflächenkräfte, Grenzflächenladung, nichtionische Tenside 1 2 3 4

Institut für Physikalische Chemie, Universität zu Köln, Luxemburger Str. 116, 50939 Köln, Germany Richard Kühn GmbH & Co KG, Schulze-Delitzsch-Str. 6, 30938 Burgwedel, Germany North Carolina State University, Department of Wood and Paper Science, Box 8005, Raleigh 27695, NC, USA Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas Väg 51, 10044 Stockholm, Sweden

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Figure 1 Generally speaking, a colloidal system consists of a dispersed and a continuous phase. Anticipating that the continuous phase is an aqueous surfactant solution the dispersed phase can be air, oil or a solid so that a foam, an emulsion, or a dispersion, respectively, is to be considered. Whether or not the surfaces of the dispersed phase are charged depends on the nature of the surface. For example, pure air/water and oil/water interfaces are negatively charged due to the adsorption of OH– ions. However, in the case of solid surfaces the nature of the surface can be varied (polar, non-polar, charged, uncharged) by special preparation techniques (see text)

 Carl Hanser Publisher, Munich

Tenside Surf. Det. 41 (2004) 4


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C. Stubenrauch et al.: Comparison between interaction forces at air/liquid and solid/liquid interfaces

water interfaces the thin film pressure balance (TFPB) [7 – 11] and its modified versions [reviewed in 11] have proved to be suitable techniques. In this paper two different methods to measure the interaction forces acting in colloidal systems will be presented. One aim is to clarify the influence that the nature of the surfaces has on the interaction forces. For that purpose, two complementary techniques, i. e. the TFPB and the MASIF techniques, were used. An extensive study about the interactions between non-polar surfaces coated with the non-ionic surfactant hexaoxyethylene dodecylether, C12E6 was carried out recently [12]. The most important results of this study will be presented in order to illustrate the differences and similarities of the MASIF and the TFPB techniques. Note that investigations with solid surfaces are of particular interest since we are able to easily modify the surface. In our case, we prepared non-polar solid surfaces consisting of negatively charged silanated glass and uncharged thiolated gold surfaces, respectively. The interaction forces for these surfaces were measured in aqueous media with the MASIF and compared with those acting between air/water interfaces (measured with the TFPB). All measurements were carried out with the same surfactant solutions. The effect of the nature of the employed surfaces and the surfactant concentration on the resulting interaction forces will be discussed in the paper at hand.

Figure 3 Top: Film holder consisting of a porous glass disc and a glass tube of radius r = 1.5 mm. The disc has a diameter of 20 mm and a thickness l of 2 mm. d is the diameter of the hole drilled through the disc (usually between 1 and 2 mm) and hc the height of the liquid in the tube. Bottom: Schematic picture of the thin film pressure balance. C is the measuring gastight cell with a window W in which the film holder F is placed such that the film is exposed to the gas pressure Pg and the free end of the glass tube to the reference pressure Pr (i. e. atmospheric pressure). The syringe S regulates Pg. The cell is filled with the surfactant solution to guarantee a saturated atmosphere. The pressure difference Pg – Pr is measured by a difference pressure transducer (DPT). With the buffer volume Vb pressure fluctuations are minimized. The figure is taken from [38]

2 Experimental Section 2.1 Solution Preparation

The non-ionic surfactant hexaoxyethylene dodecylether (C12E6) was purchased from Fluka (Germany) and used as received. The purity was checked by measuring the surface tensions as a function of the concentration c at 22 °C by the DuNoüy ring method using a Krüss K10ST tensiometer (see Fig. 2). 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 MASIF measurements the water was degassed using a water jet pump for 2 h immediately before use which is essential in order to minimize the formation of air bubbles in the vicinity of highly hydrophobic solid surfaces. All glassware (except the film holders of the TFPB) was cleaned with deconex from Borer Chemie (as a replacement for chromic sulphuric acid) and rinsed thoroughly with Milli-Q water before use. The film holders for the TFPB (see below) were boiled two times in acetone, six times in water and at least 0.5 l of hot water was sucked through each disc before use. Three different surfactant solutions at concentrations of 10–5 M, 5 × 10–5 M, and 10–4 M, respectively, were prepared in 10–4 M NaCl background electrolyte concentration. The concentrations were chosen such they were far below, next to, and above the cmc (see Fig. 2). 2.2 Thin Film Pressure Balance

The most prominent method for investigating the interactions between two surfactant films at the air/water interface, i. e. the interactions acting in foam films, is the thin film pressure balance (TFPB) [7 – 11] and its modified versions [reviewed in 11]. In brief, a film is formed in a film holder F consisting of a porous glass disc that is connected to a glass tube. A hole is drilled in the disc in which the film is formed. This film holder is fixed in a gas-tight cell C, a pressure is applied to the cell via a syringe S, and the film thickness h at this particular pressure is determined interferometrically. By calculating the disjoining pressure P from the applied pressure Pg one obtains the characteristic P-h curves (see Fig. 5, top). The main parts of the TFPB are shown in Fig. 3. 2.3 MASIF Figure 2 Surface tension r as a function of the C12E6 concentration c at T = 22 ± 1 °C. The experimental error is about the size of the symbols. The line represents the best Langmuir-Szyskowski fit, leading to a cmc of 8.0 ×10–5 M. The three concentrations investigated are indicated by vertical lines


The MASIF technique was used to investigate the interactions between surfactant-coated solid surfaces. MASIF is an acronym for “measurement and analysis of surface interaction forces” [6, 7, 13] which basically employs a bimorph sensor

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Figure 4 Schematic view of the MASIF apparatus. One surface is mounted at the end of a bimorph force sensor and the other is mounted at the end of a piezo electric tube actuator. The LVDT (linear variable displacement transducer) is used to directly measure the displacement of the upper surface during the measurements. The figure is taken from [7] and slightly modified

to measure the relative distance between the surfaces (D) and the respective forces of interaction (F). Note that the experimental data are presented as (F/R)-D curves, where R is the mean radius of the two surfaces. The main components of the MASIF apparatus are shown in Fig. 4. A detailed description of how to prepare the surfaces is given in [12]. Briefly, the hydrophobic surfaces on which the adsorption of C12E6 was studied were obtained by silanization and thiolization, respectively, of hydrophilic glass surfaces. Despite the fact that silanated glass is hydrophobic it carries a significant net negative charge. This charge results from the dissociation of unreacted silanol groups in the glass substrate which are not completely screened by the self-assembled silane layer [14]. The thiolated surfaces, however, are completely uncharged. 3 Disjoining pressure compared to surface forces

As already mentioned, the most prominent method of investigating the interactions in thin foam films is the TFPB. With the TFPB the disjoining pressure P and the corresponding film thickness h are measured, which leads to characteristic P-h curves. For the investigation of interactions between surfactant-coated solid surfaces the surface force apparatus (SFA) is the most widely used method. Similar to the MASIF technique described above the surface

forces F acting at a given distance D between the surfaces are measured. Details about the technique can be found in [1, 5, 7]. Thus, with both the TFPB and the SFA interactions between two surfaces are measured across the corresponding aqueous surfactant solution. Although a direct comparison of the results is possible one has to be aware of the following two differences. First, with the TFPB the film’s thickness h at a certain applied pressure is measured, whereas in the SFA the distance D is adjusted and the corresponding surface force is determined. Consequently, repulsive and attractive interactions are measurable with the SFA, while in the TFPB only repulsive pressures are accessible. Second, with the TFPB the interactions are measured as pressures, whereas with the SFA it is a force that is obtained. As the disjoining pressure is proportional to the mathematical deviation of the force, high disjoining pressures (TFPB) do not necessarily correspond to high forces (SFA), which can make it difficult to compare the results obtained by the different methods. For instance, small forces correspond to high pressures in case the slope of the force curve is steep. In this particular case, the small forces are hard to detect, while the high pressures are easily measurable, which could lead to the erroneous assumption that one method is more sensitive than the other. A comparison between the results obtained by the TFPB with those obtained by the SFA is made in [7, 15]. The MASIF technique is another experimental approach to measure surface forces acting between solid surfaces. Although the MASIF is much easier to operate (and less time-consuming) than the SFA, studies with MASIF are rare in the literature [12, 16 – 20]. In the MASIF the measured distance D between the surfaces is not an absolute value but refers to a distance relative to the “hard wall” contact between the surfaces. This “hard wall” contact is attained when the two “bare” surfaces or when two firmly attached surfactant layers coating the respective surfaces make contact. This means that any movement of one of the surfaces is directly transmitted to the other one without, presumably, any “distortion”. This is generally the case for thin surfactant layers. However, for soft adsorbed layers (e. g., thick layers of adsorbed polymers) a compression of the non-rigid adsorbed layers was observed [16]. In the case of contact between surfaces with two surfactant monolayers as intervening medium, the value of D = 0 means that the two layers are in direct contact. This has to be taken into account when the MASIF results are evaluated and compared with results of other techniques. A similar situation takes place for force profiles obtained with the

TFPB

SFA

MASIF

geometry

two flat surfaces

two crossed cylinders

two spheres

surface

air/water

solid/water

solid/water

distance

absolute, h h > 4 nm

absolute, D D < 1 nm

hard-wall contact, D model-dependent

interaction

disjoining pressure P only repulsive

surface force F attractive/repulsive

surface force F attractive/repulsive

measurement

h at a given P

F at a given D

F at a given D

resulting curves

P-h curves

F/R – D curves

F/R – D curves

duration

time consuming

time consuming

fast

Table 1 Comparison of three techniques that can be used to measure interaction forces in thin films. For the investigation of forces acting in foam films the thin film pressure balance (TFPB) is commonly used. Interactions between solid surfaces can be quantified with the surface force apparatus (SFA) and the MASIF technique, respectively

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“Atomic Force Microscope” (AFM). Finally, it is worth noting that in the MASIF (and also in the AFM) the interactions are measured under dynamic conditions, i. e. the data is automatically acquired while the surfaces are moving with respect to each other (approaching or separating) whereas in the SFA it is possible to determine “equilibrium” forces as the measurement is usually performed manually and enough time can be allowed for the surfaces to reach equilibrium at a given distance D. In any case, hydrodynamic corrections [21, 22] can be made to correct for effects related to the movement of the liquid medium as the surfaces are driven at any given rate of approach or separation. In this way it is possible to calculate the net surface force, which, in turn, can be compared with data from the SFA. Table 1 presents the most important features of the TFPB, the SFA, and the MASIF techniques. The origins of the disjoining pressure P in the foam film and of the surface force F between two solid surfaces are the same, namely short-range repulsive confinement, long-range repulsive electrostatic, and short-range attractive van der Waals interactions [1, 2]. The two latter interactions are accounted for in the classical DLVO theory [23, 24]. Structural forces [1, 11, 25] as well as attractive long-range forces, often referred to as the hydrophobic interactions [3, 4], may also be present. The measured pressure and force are usually considered to be the sum of the above-mentioned interactions. For the repulsive short-range component no simple mathematical description is known yet [2, 26]. However, in the case of adsorbed surfactants with large non-ionic headgroups the interaction force is dominated by elastic and osmotic contributions arising from the overlap of these headgroups which can be modelled by theories used for polymer brushes, for example by the theory of de Gennes [27]. To obtain the electrostatic component of the net interaction force the nonlinear Poisson-Boltzmann equation has to be solved using appropriate boundary conditions. In the present work this was done with the algorithm of Chan et al. [28], using either constant charge or constant potential boundary conditions and the theoretical Debye length j–1 (see [12] for details). The van der Waals component can be calculated according to [1] A 6ph3

ð1Þ

FvdW ðDÞ A ¼ 2; R 6D

ð2Þ

PvdW ðhÞ ¼

4 Results and Discussion

As already mentioned, the TFPB and the MASIF measurements were carried out with the same C12E6 solutions in order to compare the results obtained by these two different methods. All solutions contained 10–4 M NaCl to produce a well-defined electrostatic Debye decay length for the fits that were done on the basis of the DLVO theory. The results for the three different kinds of non-polar surfaces, namely air/ water, silanated glass, and thiolated gold surfaces, are shown in Fig. 5 and Table 2 for three surfactant concentrations

or

where A is the Hamaker constant. What is difficult about calculating the van der Waals component is the estimation of the Hamaker constant A since the systems usually consist of several layers [1, 29]. Ideally, A should be calculated using (at least) a five-layer model (e. g. air-surfactant-water-surfactant-air or solid-surfactant-water-surfactant-solid). However, if the thickness of the film core is large compared to that of the stabilizing monolayers, approximations are often sufficient. The reader is refered to [30] for a simple explanation of these issues. In the case of foam films the Hamaker constant for the air-water-air system, i. e. A = 3.7 × 10–20 J, is usually used. Finally, by adding the various interactions it is possible to compare experimental profiles such as P-h (from the TFPB) and F/R-D (from the MASIF or SFA). The parameter extracted from these calculations is the apparent surface potential wo, from which the corresponding surface charge density q0 can be calculated using the Grahame equation [1].


Figure 5 P-h curves (top) and corresponding F/R-D curves (middle, bottom) for three different concentrations of C12E6 in 10–4 M NaCl solution. For the cmc a value of 8.0 10–5 M was determined. The solid lines are calculated according to the DLVO theory assuming interactions at constant charge. All F/R – D curves were measured on approach and after the equilibrium adsorption had been established. All data are taken from [12]

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c/Ma

AC12E6,1 nm2

w0,1 mV

q0,1 mC m–2

Acharge,1 nm2

w0,2 mV

q0,2 mC m–2

Acharge,2 nm2

1 ×10–5

0.55

60

1.70

95

100

4.03

40

5 ×10

0.52

24

0.57

280

30

0.72

220

0.52

b

b

b

b

b

b


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1 ×10 a b

–5

–4

cmc (C12E6) = 8.0 ×10–5 M no measurable electrostatic repulsion under the given experimental conditions

Table 2 Surfactant concentration c, surface potentials w0, surface charges q0, and area per charge Acharge calculated from DLVO calculations for the foam films (index 1) and the silanated surfaces (index 2) as described in [12]. The calculated Debye length is j1 = 30 nm at the given electrolyte concentration of 10–4 M NaCl. In addition, the area per molecule AC12E6 at the air/water surface is given (index 1). AC12E6 was obtained from fitting the surface tension isotherm to the Langmuir-Szyskowski equation

around the cmc. These measurements were carried out (a) to clarify the influence of the nature of the surface and surfactant concentration on the interaction forces and (b) to gain a deeper insight into the origin of the OH– ions charging the air/water surface. In the following we will focus on the former aspect while details about the latter can be found in [12]. Air/water surface: The results from TFPB measurements are shown in Fig. 5 (top). The most important issues, which are of relevance for the following comparison, can be summarized as follows. Two different kinds of films were observed, namely electrostatically-stabilized common black films (CBF) and thin Newton black films (NBF). The latter are known to be stabilized by short-range repulsive forces. The thickness of the CBFs decreases monotonically as the disjoining pressure increases. The curves shift towards lower disjoining pressures when the surfactant concentration is increased. This shift is accompanied by an increasing tendency to form an NBF which is demonstrated by the fact that the transition from a CBF to an NBF does not appear for the lowest concentrations, whereas it is observed at intermediate concentrations via “black spot” formation in the CBF. Moreover, at the highest concentrations investigated no CBF is formed at all, which means that only an NBF, with constant thickness, is observed. Once formed, the NBFs are stable over the whole pressure range investigated. As the thickness and the stability of the NBF do not change significantly within the reported pressure range, the NBF is represented as a dashed line. In the NBF a surfactant bilayer consisting of two densely-packed monolayers creates a “force barrier” – similar to the force barriers observed in MASIF measurements (see below) – which prevents the film from rupturing. It is important to realize that for a stable NBF to form not only short-range repulsive interactions normal to the surfaces are required but also a densely packed adsorption layer (see chapter 3.4.3 and 3.4.4 in [10]). Both the P-h curves shown in Fig. 5 and the fact that the surfactants dealt with are non-ionic raise the question of the origin of the electrostatic repulsion, i. e. the charges at the air/water surface. At present, the widely accepted explanation for the origin of the charges in thin foam films stabilized by non-ionic surfactants is the presence of excess OH– ions at the air/water surface [10, 11, 31 – 33]. However, it has not been clarified yet whether the OH– ions are specifically adsorbed [32,33] or if other mechanisms such as surface reactions [12] are responsible for the excess of OH– ions at the air/water surface. Moreover, the accumulation of halide ions at the air/water interface was discussed only recently [34]. This has to be mentioned as our C12E6 solutions were prepared with an electrolyte concentration of 10–4 M NaCl (see 2.1). However, according to Garrett [34] the amount of chloride ions at the air/water interface is expected to be negligible due to their low polarizability.

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Despite the fact that the origin of the OH– ions is not clear yet, the experimental P-h curves can be fitted with the DLVO theory to obtain the surface charge density q0 (see Tab. 2). Looking at Fig. 5 and Table 2, one sees that the value of q0 decreases with increasing surfactant concentration until it is so low that a CBF can no longer be stabilized and an NBF is directly formed under the chosen experimental conditions. These results are in absolute agreement with those published by other authors [reviewed in Ref. 11]. Experimentally it has been observed repeatedly that the surface charge is constant at low surfactant concentrations and decreases significantly above a certain concentration, which is proposed to be connected with the total number of ethylene oxide units [35]. What is important to realize is that the decrease of the surface charge contrasts sharply with the adsorption of the surfactant, which changes significantly at low concentrations and stays close to constant already far below the cmc (see Tab. 2). Although this observation is not understood yet, the typical explanation of a competitive adsorption between surfactant and hydroxide can be excluded. Silanated glass surface: The silanated surface is negatively charged due to the dissociation of unreacted silanol groups in the glass substrate. Although the origin of the charge is different, the interaction forces between two silanated surfaces and two air/water surfaces are very similar (see Fig. 5 and Tab. 2). At the lowest surfactant concentration investigated the interaction forces are dominated by long-range electrostatic double-layer forces. By analogy with the air/water surface further adsorption of C12E6 reduces the net charge until long-range forces are negligible. Moreover, it is seen in Fig. 5 that the NBF formation corresponds to the appearance of a force barrier between the silanated surfaces. This barrier is a measure of the force that is needed to remove surfactant from between the two surfaces. As this barrier is located at a distance that corresponds to the thickness of a surfactant bilayer (from the contribution of one monolayer from each of the two interacting surfaces) the analogy with the NBF formation is obvious. Thus, in both cases, the adsorption of C12E6 leads to a decrease of the surface charge resulting in the formation of a densely packed bilayer which generates a force barrier. Note that a “removal” of this bilayer results in film rupturing in the case of foam films, whereas in the case of surfactant-coated water/solid surfaces it leads to a direct contact between the two solid surfaces. Thiolated gold surface: As seen in Fig. 5 the situation is different for the interaction forces between thiolated gold surfaces coated with C12E6. The substrate, i. e. the thiolated gold surface, is uncharged so that double-layer forces do not play any role either in the absence or in the presence of C12E6. However, the adsorption of C12E6 influences significantly the short-range interaction forces. In contrast to the



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silanated glass surfaces, at the lowest surfactant concentration of 1.0 × 10–5 M a force barrier is already observed. This force barrier can be overcome experimentally and the surfactant monolayers are squeezed out from between the surfaces (see the step-shape profile in the force curve). The increase of the surfactant concentration leads to an increase of the force barrier so that eventually the densely packed bilayer can no longer be removed. Comparing these results with those for the silanated surfaces, one clearly sees that the F/R–D curves obtained for surfactant concentrations of 5.0 × 10–5 M and 1.0 × 10–4 M, respectively, are hardly distinguishable. In both cases electrostatic forces are absent (or at least negligible) and the force barrier is so high that the surfactant cannot be removed from between the surfaces under the given experimental conditions. 5 Final Remarks

It was shown that for low surface coverages the nature of the surfaces onto which the surfactant adsorption takes place significantly influences the interaction force profiles. Once a densely packed surfactant layer is formed, it is the surfactant itself that determines the interaction forces. It would be of interest to quantify the surface concentration C, the area per molecule A and the conformation of C12E6 at the different surfaces. For the air/water surface C and A can be obtained from the surface tension isotherms. With a detailed neutron reflectivity study not only the surface excess but also the monolayer thickness and its roughness were obtained [36]. Moreover, by using sum-frequency spectroscopy and ellipsometry the conformation of the C12E6 molecules at the air/water surface is known [37]. What is still missing, however, is the corresponding data for the adsorption (adsorption isotherms) of C12E6 at a silanated glass and a thiolated gold surfaces, respectively. The fact that we did not observe any double-layer forces between the C12E6-coated thiolated surfaces provides evidence for the fact that the surfactant did not contain any charged surface-active impurities. This is an important result with respect to the discussion of the charge origin at the air/water surface. Work in progress deals with interaction forces between non-polar surfaces coated with the nonionic sugar surfactant n-dodecyl-b-D-maltoside (b-C12G2). These measurements are intended to clarify the influence of the headgroup on the interaction forces. References 1. Israelachvili, J.: in: Intermolecular and surface forces, 2 ed., Academic Press, San Diego, 1991. 2. Israelachvili, J. N. and Wennerström, H.: J. Phys. Chem. 96 (1992) 520. 3. Israelachvili, J. N. and Pashley, R. M.: Nature 300 (1982) 341. 4. Christenson, H. K, and Claesson, P. M.: Adv. Colloid Interface Sci. 91 (2001) 391. 5. Israelachvili, J. N. and Adams, G. E.: J. Chem. Soc. Faraday Trans. 1 (1978) 74, 975. 6. Parker, J. L.: Progr. Surf. Sci. 47 (1994) 205. 7. Claesson, P. M., Ederth, T., Bergeron, V. and Rutland, M. W.: Adv. Colloid Interface Sci. 67 (1996) 119. 8. Mysels, K. J. and Jones, M. N.: Discuss. Faraday Soc. 42 (1966) 42.


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Correspondence to Dr. Cosima Stubenrauch Institut für Physikalische Chemie, Universität zu Köln Luxemburger Str. 116, 50939 Köln Germany E-mail: [email protected]

The authors of this paper Dr. Cosima Stubenrauch studied chemistry at the universities of Münster and Freiburg and received her PhD in Physical Chemistry at the TU Berlin in 1997. After a postdoctoral year at the Université Paris Sud, she has been working as an associate researcher and lecturer at the Institute of Physical Chemistry at the University of Cologne since 1999. Dr. Judith Schlarmann studied chemistry at the University of Cologne where she received her PhD in Physical Chemistry in 2004. She is working for the Richard Kühn GmbH & Co. KG in Großburgwedel since August 2004. Dr. Orlando Rojas studied chemistry at the Universidad de Los Andes in Venezuela and received his PhD at the Auburn University in 1998. After a postdoctoral stay in Prof. Claesons’s group, he has been working as an assistant professor at the Department of Wood and Paper Science at the North Carolina State University since 2003. Prof. Per Claesson studied Chemical Engineering at the Royal Institute of Technology in Stockholm where he received his PhD in Physical Chemistry in 1986. Since 1998 he is Professor in Surface Chemistry at the Royal Institute of Technology. Prof. Per Claesson is Head of the Surface Science Group at KTH and Director of the VINNOVA competence center “Surfactants based on natural products”.

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