Langmuir 2006, 22, 5273-5281
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Wetting and Spreading of a Surfactant Film on Solid Particles: Influence of Sharp Edges and Surface Irregularities Peter J. Gerber,*,†,‡,§ Christoph Lehmann,† Peter Gehr,† and Samuel Schu¨rch| Institute of Anatomy, Faculty of Medicine, UniVersity of Berne, Berne, Switzerland, Clinic of Internal Medicine, DAIN, and Clinic of Pulmonary Medicine, DMLL, UniVersity Hospital, Berne, Switzerland, and Department of Physiology and Biophysics, Faculty of Medicine, The UniVersity of Calgary, 3330 Hospital DriVe Northwest, Calgary, Alberta T2N 4N1, Canada ReceiVed October 2, 2005. In Final Form: March 15, 2006 In addition to particle size and surface chemistry, the shape of particles plays an important role in their wetting and displacement by the surfactant film in the lung. The role of particle shape was the subject of our investigations using a model system consisting of a modified Langmuir-Wilhelmy surface balance. We measured the influence of sharp edges (lines) and other highly curved surfaces, including sharp corners or spikes, of different particles on the spreading of a dipalmitoylphosphatidyl (DPPC) film. The edges of cylindrical sapphire plates (circular curved edges, 1.65 mm radius) were wetted at a surface tension of 10.7 mJ/m2 (standard error (SE) ) 0.45, n ) 20) compared with that of 13.8 mJ/m2 (SE ) 0.20, n ) 20) for cubic sapphire plates (straight linear edges, edge length 3 mm) (p < 0.05). The top surfaces of the sapphire plates (cubic and cylindrical) were wetted at 8.4 mJ/m2 (SE ) 0.54, n ) 20) and 9.1 mJ/m2 (SE ) 0.50, n ) 20), respectively, but the difference was not significant (p > 0.05). The surfaces of the plates showed significantly higher resistance to spreading compared to that of the edges, as substantially lower surface tensions were required to initiate wetting (p < 0.05). Similar results were found for talc particles, were the edges of macro- and microcrystalline particles were wetted at 7.2 mJ/m2 (SE ) 0.52, n ) 20) and 8.2 mJ/m2 (SE ) 0.30, n ) 20) (p > 0.05), respectively, whereas the surfaces were wetted at 3.8 mJ/m2 (SE ) 0.89, n ) 20) and 5.8 mJ/m2 (SE ) 0.52, n ) 20) (p < 0.05), respectively. Further experiments with pollen of malvaceae and maize (spiky and fine knobbly surfaces) were wetted at 10.0 mJ/m2 (SE ) 0.52, n ) 10) and 22.75 mJ/m2 (SE ) 0.81, n ) 10), respectively (p < 0.05). These results show that resistance to spreading of a DPPC film on various surfaces is dependent on the extent these surfaces are curved. This is seen with cubic sapphire plates which have at their corners a radius of curvature of about 0.75 µm, spiky malvaceae pollen with an even smaller radius on top of their spikes, or talc with various highly curved surfaces. These highly curved surfaces resisted wetting by the DPPC film to a higher degree than more moderately curved surfaces such as those of cylindrical sapphire plates, maize pollens, or polystyrene spheres, which have a surface free energy similar to that of talc but a smooth surface. The macroscopic plane surfaces of the particles demonstrated the greatest resistance to spreading. This was explained by the extremely fine grooves in the nanometer range, as revealed by electron microscopy. In summary, to understand the effects of airborne particles retained on the surfaces of the respiratory tract, and ultimately their pathological potential, not only the particle size and surface chemistry but also the particle shape should be taken in consideration.
Introduction
* To whom correspondence should be addressed. E-mail: pgerber@ ana.unibe.ch. † University of Berne. ‡ DAIN, University Hospital. § DMLL, University Hospital. | The University of Calgary.
retention process, and thus, the particle-surfactant interactions are important for the following processes. Total wetting of a particle by a fluid means that the liquid spreads over the entire particle surface, resulting in particle immersion into the fluid. Partial wetting means that the surfaces are only partially wetted or an edge or corner might resist total wetting. In that case, the solid surface is covered to a certain extent and mechanical equilibrium might be established depending on the surface tension of the surfactant film in the deposition zone of the lung and the resistance to wetting of the particle. Wetting occurs at the three-phase interface [gas (air), liquid (surfactant and subphase), and solid (particle)]. Spreading is a dynamic process with rapid wetting of a surface of the solid by an advancing liquid meniscus (speed in the range of several centimeters per second).4 The physiological surface tension in the airways of the lung is different from that in the alveoli. For the trachea of horses it has been shown to be ∼32 mJ/m2,5,6 whereas for the alveoli of rabbits a minimum surface tension of less than 2 mJ/m2 was
(1) Verma, D. K.; Kurtz, L. A.; Sahai, D.; Finkelstein, M. M. Appl. Occup. EnViron. Hyg. 2003, 18 (12), 1031-1047. (2) Gauderman, W. J.; Avol, E.; Gilliland, F.; Vora, H.; Thomas, D.; Berhane, K.; McConnell, R.; Kuenzli, N.; Lurmann, F.; Rappaport, E.; Margolis, H.; Bates, D.; Peters, J. N. Engl. J. Med. 2004, 351 (11), 1057-1067. (3) Hussein, T.; Hameri, K.; Aalto, P.; Asmi, A.; Kakko, L.; Kulmala, M. Scand. J. Work EnViron. Health 2004, 30 (Suppl. 2), 54-62.
(4) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley-Interscience: New York, 1997. (5) Im, H., V.; Gehr, P.; Gerber, V.; Lee, M. M.; Schurch, S. Respir. Physiol. 1997, 109 (1), 81-93. (6) Schurch, S.; Bachofen, H.; Weibel, E. R. Respir. Physiol. 1985, 62 (1), 31-45.
A substantial number of epidemiological studies have shown that the lung is exposed to a large quantity of different particles either from the environmental air or by occupationally generated aerosols with considerable health effects to children and adults.1-3 If these particles are deposited in the airways or alveoli, the processes of retention and clearance begin. Lung cells, including epithelial cells, macrophages, dendritic cells, and tissue structures of the lung, are involved in the retention and clearing processes. Retention depends on differing properties of the particles and the inner surface of the lung. After the contact with the surfactant layer upon particle deposition, wetting of the inhaled particles is the first step of the
10.1021/la0526683 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/11/2006
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Figure 1. (a) Modified Wilhelmy balance (MWB). The two main parts are the chamber and sliding barrier (A) and the motor unit (B). For details see the Materials and Methods. (b) Overview of the entire setup as used during the experiments.
measured on deflation toward 40% total lung capacity.5,6 For hamster lungs the surface tension in the small airways was estimated to fall at least to 15 mJ/m2. Particles of different surface chemistries and shapes show distinct retention patterns7 (see also the Discussion). The detailed knowledge of the wetting processes of particles by the surfactant film in the lungs is important not only for understanding how particles due to air pollution can interact with this organ but also for the development of inhaled therapeutics. Solid and not easily soluble substances will be wetted and displaced as particles, and thus, the above-mentioned mechanisms will be crucial to their final health effect. Many investigations have been conducted on retention of different particles in the lung of animals or humans, but only a few experiments were performed in vitro to estimate the surface tension at which distinct particles (i.e., puff ball spores, Teflon spheres) are wetted. To our knowledge direct observation of the critical surface tension at which a surfactant film upon compression begins to spread over differing surface structures has not yet been performed. For the present project we constructed an apparatus to observe the wetting and spreading processes of a fluid covered by a surfactant film at a particular measured surface tension of that film on various particle structures.
Definitions. We use the term “edge” to describe the intersection of two surfaces which are differently oriented in space or the linear connection of two corners of a geometrical body. This means mathematically a line which has only one dimension. We have used
the term “corner” to describe the intersection of at least three surfaces differently oriented in space. This means mathematically a point with no dimension. In reality edges and corners are not mathematically defined entities, as they represent regions of particular dimensions and structures in the nanometer or even molecular range. Thus, we are using these terms for regions in which the curvature changes rapidly in the nanometer range. In the case of the edges this leads to a surface region which is planar or curved in one dimension. In the case of the corners this leads to a surface region which might be considered a surface of a segment of a sphere. When the curved surface has a relatively small radius as in the case of the spikes of malvaceae pollen, which have a complex shape, we use the term “highly curved surfaces”. Measurement Apparatus. Modified Wilhemy Balance (MWB; Figure 1). For visual observation of the wetting process a glass chamber was constructed. To this end a glass tube of 24 × 24 × 66 mm was produced (glassworks, Trabolt & Co. AG, Berne, Switzerland; Figure 1a, A1). To reach a stable surface tension, film leakage by wetting of the glass chamber by the surfactant film had to be minimized without the optical qualities of the glass being influenced. As glass has a relatively high surface free energy, in the hundreds of millijoules per square meter, the glass walls had to be rendered hydrophobic. To this end we chose a thin coating of paraffin, which has a surface free energy of approximately 20 mJ/m2, a value close to that of Teflon, about 18 mJ/m2. The surface tension of this paraffin-coated glass was determined according to Neumann’s equation of state approach from the contact angle and the surface tension value of a water droplet placed onto the glass surface.8 Both ends of the tubes were partially closed with Teflon-coated glass fiber plates (Figure 1a, A3) forming a chamber with two small openings for manipulation.
(7) Geiser, M.; Schurch, S.; Gehr, P. J. Appl. Physiol. 2003, 94 (5), 17931801.
(8) Neumann, A. W.; Good, R. J.; Hope, C. J.; Sejpal, M. J. Collid Interface Sci. 1974, 49, 291-304.
Materials and Methods
Surfactant Film Wetting and Spreading on Particles The plates were coated with Teflon to minimize their surface free energy. These plates were attached to the chamber with ethyl cyanacrylate glue (Loctite 406; KVT Koenig AG, Dietikon, Switzerland) after the contact surfaces were prepared by an aliphatic amine primer to enhance adhesion (Loctite 770; KVT Koenig AG). The joints were also coated with paraffin to prevent film leakage. The chamber could be filled and the liquid exchanged with a syringe connected to a tubing fed through inlets in the plates (Figure 1a, A5). The sliding barrier (Figure 1a, A2) consisting of hard PVC and semisoft silicon allowed us to change the surface tension by compressing and decompressing the surfactant film. The contact surface of the sliding barrier was also coated with a thin layer of paraffin to lower its surface free energy and prevent film leakage during film compression. The barrier was connected to the motor unit with an aluminum clamp. The motor unit (Figure 1a, B) consists of a base plate on which the motor (Figure 1a, B4), the high-precision spindle (Figure 1a, B3) and the slide-bearing (Figure 1a, B1) are mounted. The spindle moves the connector (Figure 1a, B2) of both the chamber and the motor unit on the slide-bearing. In the motor unit (Figure 1a, B), the motor (12 V; transmission ratio 176/1; Minimotor Agno, Switzerland; Figure 1a, B4) moves the sliding barrier through the high-precision spindle (Figure 1a, B3) and the connector (Figure 1a, B2) on a slide-bearing (Figure 1a, B1). The speed of the sliding barrier is fully adjustable between 0 and 5 mm/s. The working speed for all experiments was 3-4 mm/s. This speed was found in preliminary experiments to be the best for controlled compression of the surfactant film. The electronic switch allowed an immediate stop of the barrier. A ruler with 1/2 mm divisions (Figure 1a, B1) was attached onto the slide-bearing to facilitate reading the length of the film surface. The area of the resulting surfactant film was calculated by the product of the width and the length of the film. All the mechanical parts of the motor unit were selected and modified to prevent shocks, vibrations, or jerky movements of the sliding barrier in the chamber. The temperature in the chamber was measured with an electronic thermometer in preliminary experimental series. They showed the exact same time periods as the main experiments, and the temperature was maintained at 22 ( 0.5 °C during the entire procedure. The MWB was attached onto a manual lifting device (our own construction) which allowed us to move the setup vertically without shaking and vibrating the film, facilitating the observation of the wetting process. All of the parts were placed onto a heavy stone table. However, to avoid vibration from the environment, we conducted our experiments in the early morning or during the night, when there was less traffic outside and no people inside the laboratories. Microscope and Light Sources. We used a mineralogical/ metallurgical microscope, Nikon Optiphot (Nikon AG, Ku¨snacht, Switzerland), equipped with a Universal Epi illuminator for bright field, dark field, and differential interference contrast. A long working distance bright field objective of 5× magnification was used for all experiments. To alter continuously the angle of the observation axis, the microscope was attached onto a ball-and-socket hinge. For additional horizontal shift the microscope and the hinge were placed onto a base sliding on two poles and movable by a thread spindle. Two light sources were used to optimize illumination according to the particle geometry and for the optical differentiation of the particle and the liquid phase. One light source was integrated into the microscope; the other one was a conventional halogen lamp to illuminate the object from any side as desired. The latter was used only for a short time to prevent uncontrolled heating of the chamber, the subphase, and the surfactant film. Cleaning Procedure. For the cleaning process the chamber and the plates were disconnected from the main body of the apparatus, washed three times with ethanol absolute GR (Merck KGaA, Darmstadt, Germany), and then rinsed with water from the water tap. The use of detergent at any step in the cleaning procedure was avoided as detergent tended to interfere with the paraffin coating, causing film instability. More aggressive cleaning with acids (sulfuric
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Figure 2. Calibration of the surface tension vs length of the surfactant film in the MWB. See Calibration. (a, b) Indirect measurements, FC-72 + dye, (SD ) 0.5). (c) FC72, measured values of the film length (n ) 10, SD ) 0.48) at total wetting of an FC72 droplet with a given surface tension of 12 mJ/m2. (d) FC43, measured values of the film length (n ) 10, SD ) 0.94) at total wetting of an FC72 droplet with a given surface tension of 16 mJ/m2. (e) Equilibrium surface tension of 25 mJ/m2 at a 55 mm (SD ) 0.5) surface film length. acid for instance) did not improve the results. Drying was done with CO2. Calibration. Direct measurement of the surface tension in the MWB simultaneously with the observation of particle wetting was not practical by using a Wilhelmy plate as the chamber was too narrow. A given surface tension at the corresponding area of the compressed film (surface tension below the equilibrium of 25 mJ/ m2 for a saturated DPPC film) was stable for 15-20 s as measured by using a test fluid droplet of DMP/O (dimethyl phthalate/normal octanol, Fluka) and the perfluorocarbon fluid FC-43 (3M Co., St. Paul, MN) having a surface tension of approximately 16 mJ/m2 at 22 °C, as described by Schu¨rch et al.6,9 After 15-20 s the surface tension started to increase measurably due to spreading of the film material over the paraffin-coated glass chamber wall. It is known that a certain loss of surfactant film material due to spreading along the constraining walls and barriers (leakage) is characteristic for any Langmuir-Wilhelmy balance when surfactant films are compressed beyond the equilibrium surface tension of approximately 25 mJ/m2, regardless of whether the trough is made from Teflon or the walls are coated with paraffin.10 In separate experiments without particles added to the trough, we found that in the range of approximately 4-24 mJ/m2 the change of the surface tension was proportional to the change of the compressed area of the film for DPPC (Figure 2). Thus, we measured the length of the surface area of the compressed film at particular surface tensions. The surface tension was determined by observing the spreading behavior of the perfluorocarbon test fluid droplets FC-43 (surface tensions of approximately 16 mJ/m2, 22 °C) and FC-72 (surface tension of approximately 12 mJ/m2, 22 °C).6,11 The test fluid droplets form thin lenses if placed on top of a surfactant film whose surface tension is above that of the test fluid itself. Upon lowering the film surface tension by area compression, the lens acquires progressively a more spherical shape. The droplet becomes finally totally immersed into the subphase when the film surface tension reaches a value within 0.5 mJ/m2 of that of the test fluid. (9) Schurch, S.; Goerke, J.; Clements, J. A. Proc. Natl. Acad. Sci. U.S.A. 1978, 75 (7), 3417-3421. (10) Goerke, J.; Gonzales, J. J. Appl. Physiol. 1981, 51 (5), 1108-1114. (11) Schurch, S.; Goerke, J.; Clements, J. A. Proc. Natl. Acad. Sci U.S.A. 1976, 73 (12), 4698-4702.
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Figure 3. Cylindrical (a; 1.65 × 1 mm, bar ) 500 µm) and cubic (b; 3 × 3 × 1 mm, bar ) 1 mm) sapphire plates as used in the experiments. Focused on the corner and the edge of the cubic plates (c; bar ) 200, 10, and 1 µm). The small particles on the surface of the plates are due to pollution from the ambivalent air. The irregular surface structure seen in part c, third picture, is due to the inhomogeneous gold coating for SEM. Note the terms “edges”, “corners”, and “surfaces” used in this work in a picture: (a) surfaces, (b) edges of cylindrical and cubic plates, (c) corners of cubic plates. To extend the estimation of lower film surface tensions, from approximately 4 to 12 mJ/m2, below those obtained from using the pure fluids, FC-43 and FC-72 were doped with 2 mg/mL blue fluorocarbon dye L-1803 (a gift from Dr. D. Danielson, 3M).6 However, for those surface tension values below 12 mJ/m2, the calibration could not be performed in the present modified LangmuirWilhelmy balance, because the droplets tended to move out of sight toward the fluid menisci to spread along the restraining walls due to the narrowing space between the barrier and the walls as the barrier was moved to compress the surfactant film. A larger more conventional surface balance, but having 4 times the surface area, equipped with a platinum Wilhelmy plate, was used for the lower surface tensions. The compression speed was chosen such that the rate of change in film area was equivalent to that in the modified balance. Film stability at a particular surface tension was approximately equal to that measured in the modified MWB. Surface tension and film area were equivalent within the estimated measuring error of (1 mN/m in the surface tension range accessible to both balances. In summary, for every film area in the chamber a corresponding surface tension could be calculated. The estimated precision and accuracy of the surface tension measurements were better than 1.0 mJ/m2. Particles. Sapphire plates (synthetic corundum, Al2O3, hexagonal crystal structure) were used in two types, cubic (3 × 3 × 1 mm)
and cylindrical (1.65 × 1 mm) (Kyburz AG, Safnern, Switzerland). They were highly inert and possessed high free energy surfaces, in the range of approximately 2-4 J/m2 12,13 (Figure 3a,b, Table 1). The precision of the cutting was (0.1 mm, and that of the polishing was 60-40 S-D (scratch max 60 µm, dig max 40 µm, and dig separation distance 20 µm; U.S. military optical norm, MIL-O13830). The edges of the cubic and the cylindrical plates had a radius of curvature of about 0.75 µm; the corners of the cubic plates had the same radius of curvature. This was determined by scanning electron microscopy (Figure 3c). Talc particles (Mg3Si4O10(OH)2) in the millimeter range were used in microcrystalline (Go¨persgru¨n, Bayern, Germany) and macrocrystalline (Hospental, Uri, Switzerland) forms and were kindly provided by Dr. Beda Hofmann (Naturhistorisches Museum der Burgergemeinde Bern and Institute of Geological Sciences, University of Berne, Switzerland). The surface free energy was ∼35 mJ/m2 depending on the fracture surface14 (Figure 4, Table 1). Thus, they are comparable to polystyrene which has a surface free energy of ∼33 mJ/m2.7 (12) Verdozzi, C.; Jennison, D. R.; Schultz, P. A.; Sears, M. P. Phys. ReV. Lett. 1999, 82 (4), 799-802. (13) Manassidis, I.; Gillan, M. J. J. Am. Ceram. Soc. 1994, 77 (2), 335-338. (14) Giese, R. F.; Costanzo, P. M.; Oss, C. J. Phys. Chem. Miner. 1991, 17, 611-616.
Surfactant Film Wetting and Spreading on Particles
Langmuir, Vol. 22, No. 12, 2006 5277 Table 1. Particle Specifications
type
surface characteristics
surface free energy (mJ/m2)
dimensions (mm)
cubic cylindrical
Sapphire, Synthetic Corundum, Al2O3, Hexagonal Crystal Structure “smooth” L ) 1, high 1 2-4 “smooth” 3×3×1 3-4
microcrystalline macrocrystalline
smooth, stacked layers smooth, stacked layers
spheres
smooth
malvaceae maize
Talc, Mg3Si4O10(OH)2 range of 1-3 range of 1-3 Polystyrene 20
spiky knobbly
Polystyrene particles with a diameter of 20 µm were kindly provided by PD Dr. Thomas Geiser (Clinic of Pulmonary Medicine, DMLL, University Hospital, Berne, Switzerland). These spherical particles had a smooth surface. Polystyrene has a surface free energy of ∼33 mJ/m2, which is comparable to that of talc (∼35 mJ/m2)7 (Table 1). Pollen of malvaceae with a spiky surface were kindly provided by Prof. Brigitta Ammann and Dr. Jacqueline van Leeuwen
Figure 4. Different detailed views of characteristic types of surfaces, edges, and corners of talc particles. Differences in the types of surfaces from the two groups of talc particles (microcrystalline (a, b) and macrocrystalline (c)). Bar ) (a) 50 µm, (b) 20 µm, and (c) 100 µm. Notice the complex shape with stacked layers, edges, and corners.
Pollen about 100 about 100
∼35, depending on the surface fracture ∼35, depending on the surface fracture 33 estimated 40-50 estimated 40-51
(Department of Biology, Institute of Plant Science, Section of Palaeoecology, University of Berne). In addition, to compare the behavior of spiky and smooth surfaces, we obtained pollen from maize (Gu¨mligen, Berne, Switzerland), which has fine knobbly surfaces (Figure 5, Table 1). Pollen walls are well-differentiated biological structures, and their outer layer, called exine, is highly resistant to acids and bases. The exine is composed of sporopollenin, a biopolymer of carotenoids and other polymers embedded with proteins. There is a pollen coating (pollenkitt) above this wall. It contains various proteins and lipid components depending on the species. For wind-pollinating species such as maize this coating or layer is thin and contains mainly lipids and a few proteins.15 From the chemical composition, lipids and proteins, the surface free energy is likely between 40 and 50 mJ/m2.8 Surfactant and Solutions. Monolayers of 1,2-dipalmitoyl-sn3-glycerophosphorylcholine (DPPC) (Sigma-Aldrich, Oakville, ON, Canada). DPPC was formed from a solution of 2 mg‚mL-1 in absolute ethanol on a suphase of 0.9% NaCl in distilled water (Millipore grade). FC-43 and FC-72 (3M) with surface tensions of 16 and 12 mJ/m2, respectively, at 22 °C were used as test fluid droplets for the calibration and the same fluids but with added fluorocarbon dye as described above in Calibration. This dye enhances the spreading of the perfluorocarbon droplets at a given film surface tension.6 Experimental Procedures. All experiments were done at room temperature of 22 °C ((0.5 °C). For the experiments the chamber unit of the MWB was prepared, cleaned, assembled, and filled with 0.9% NaCl solution as described above. To form the surfactant film by spreading, small droplets of ∼3 µL of DPPC solution were placed carefully onto the aqueous subphase. Three to four droplets of ∼3 µL had to be applied to the surface until no further spreading was observed and the equilibrium surface tension of approximately 25 mJ/m2 was reached. Adding an excess of DPPC did not change the results. The surfaces in contact with the surfactant film were primed three times by compressing the film by more than 75% of the initial surface area.16 After priming, a new equilibrium surface film of DPPC of 25 mJ/m2 was formed. During the experiments vibrations and shocks had to be avoided as these tended to interfere with the determination of the “critical surface tension” of wetting (see Modified Wilhelmy Balance). We could perform about 10 consecutive experiments, each time starting with a new equilibrium film for each plate, before the setup had to be disassembled for cleaning. For the sapphire experiments the sapphire plates were first cleaned with sulfuric acid and then washed for 1 min in 96% ethanol followed by another wash in ethanol absolute for 1 min. We found that cleaning the plates every time with sulfuric acid had no effect on the results; thus, we did it only once at the beginning. The sapphire plates were dried with CO2. After 30-45 s the experiments could be started. It can be assumed that in this period the sapphire plates adsorbed some pollutants from the environment,17 but this also occurs in a real, nonexperimental situation. (15) Suen, D. F.; Wu, S. S.; Chang, H. C.; Dhugga, K. S.; Huang, A. H. J. Biol. Chem. 2003, 278 (44), 43672-43681. (16) Goerke, J.; Gonzales, J. J. Appl. Physiol. 1981, 51 (5), 1108-1114. (17) Drelich, J.; Miller, J. D. J. Colloid Interface Sci. 1994, 164 (1), 252-259.
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Figure 6. (A) Cylindrical sapphire plate close to the moment of advancing of the surfactant meniscus over the plate surface (1), light-diffracting air-liquid interface (3), and subphase (2). (B) Schematic drawing to show in which direction the observation was performed: (1) plate, (2) subphase. (C) During lowering of the surface tension, the meniscus changed shapes, especially at the point where the meniscus is in transition at the air-liquid surface (arrows 4 and 5 in (A), (C), and (D)). (D) Then there was no change observable, and the next step was wetting of the edge (6).
Figure 5. Malvaceae pollen (a; bar ) 50 µm) with a spiky surface in contrast to maize pollen (b; bar ) 20 µm) with a fine knobbly surface. (c) Malvaceae spikes (bar ) 10 µm). (d) Maize knobbles of about 272 nm (bar ) 2 µm). For the present experiments an aluminum cube of 15 × 12 × 10 mm was placed into the chamber before it was filled with 0.9% NaCl solution and the surfactant was placed onto the subphase. Then the cleaned plates where carefully placed onto this aluminum base, and the plates were immersed into the film-covered subphase up to the upper rim. This prevented the immediate and total submersion of the relatively heavy plates by gravitational force. Furthermore, it allowed us to define the distance from the upper surface level of the sapphire plates to the surface level of the liquid. This was important for
avoiding optical distortion by the liquid meniscus of the chamber wall. The cubic sapphire plates were positioned onto the aluminum base, and one corner was oriented toward the investigator. This allowed us to observe whether the two visible linear edges or the corner between was wetted first. During the preliminary experiments it had become clear that particular attention had to be given such that the plates were placed exactly parallel to the surfactant film. This had to be done to avoid wetting by mechanical disruption of the DPPC film by the corners. The plates were observed through the microscope while the film was compressed. Under microscopic observation of the plate, the surfactant film was compressed (Figure 6). At a particular compression state, corresponding to the “critical surface tension”, the surfactant film started to sweep over the edge, the movement of the barrier was stopped immediately, and the length of the remaining area was measured. The moment the surfactant film started to advance over the edge to spread over the surface was characterized by a sudden change in light reflection by the surface. As soon as the contact line advanced over the edge to the surface, the very small but brightly illuminated wetted part of the particle
Surfactant Film Wetting and Spreading on Particles
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Table 2. Wetting of Sapphire Plates at the Film Surface Tensiona cubic edgeb
surface
10.7 mJ/m2 SE ) 0.45, n ) 20 a
cylindrical b,c
edge
8.4 mJ/m2 SE ) 0.54, n ) 20
b
surfaceb,c
13.8 mJ/m2 SE ) 0.20, n ) 20
9.1 mJ/m2 SE ) 0.50, n ) 20
Mean values. SE ) standard error. b p < 0.05. c p > 0.05. Table 3. Wetting of Talc Particles (mN/m)a microcrystalline edgeb
surface
mJ/m2
edge
mJ/m2
8.2 SE ) 0.30, n ) 20 a
macrocrystalline b,c
b
surfaceb,c
mJ/m2
5.8 SE ) 0.52, n ) 20
3.8 mJ/m2 SE ) 0.89, n ) 20
7.2 SE ) 0.52, n ) 20
Mean values. SE ) standard error. b p < 0.05. c p > 0.05.
surface became visible. The measurement of the contact angle could not be done exactly, due to the optical limitations. (1) Particles including polystyrene, talc, and pollens tended to form aggregates and could not be easily separated after cleaning, and (2) the surface properties of the pollens were found to be altered by the cleaning process. The cleaning procedure altered the shape of the particle, as seen by scanning electron microscopy. Thus, only the sapphire particles were cleaned and reused. The experiments for talc particles were done as described above. Small particles including polystyrene and pollen were dusted onto the surfactant layer. A region with a few single particles (not clumped together) was chosen. The critical surface tension was taken as the compression state of the film when the small meniscus suddenly disappeared due to fluid spreading over the surface and the particles were found to immerse and sink very slowly to the bottom of the chamber. We saw small differences between the time points of wetting and submersion of different particles in the visual field, but it was difficult to distinguish between the two in our system. Statistical Analysis. Parametric data were statistically analyzed for significance between the means of two or more groups with one-way analysis of variance (ANOVA). To isolate the pairs, a pairwise multiple comparison procedure (Student-Newman-Keuls) was carried out. Nonparametric data were analyzed by a method analogous to one-way ANOVA by using an analysis of variance on ranks or a rank sum test for two single groups.
Results Wetting of the Sapphire Plates, Cubic and Cylindrical Shapes, and the Influence of Distinct Surface Features. Wetting was defined in our experiments to occur at the moment when the liquid with its surfactant film started to spread over an edge, a corner, or the total surface area. The edges and corners of the cubic and cylindrical sapphire plates were wetted at 10.7 mJ/m2 (standard error (SE) ) 0.45, n ) 20) and 13.8 mJ/m2 (SE ) 0.20, n ) 20), respectively. The difference between the two groups was statistically significant (p < 0.05). The wetting behaviors of cylindrical and cubic plates were different. Whereas the edges of cylindrical plates were mostly wetted at once under our experimental conditions, cubic plates with edges and corners seemed to be wetted first at the edges followed by the corners. In the short interval between the wetting of the straight edges and that of the corners, the surface film showed a topography of various menisci. However, we were not able to quantify or record these transient patterns because of the limitations of our imaging system. Imaging of this would require high picture rate video observation. If the cubic plates were not placed parallel to the DPPC film (see the Materials and Methods), the corner often was already wetted at the equilibrium surface tension of the DPPC film, 25 mJ/m2. The surfaces of the sapphire plates (cubic and cylindrical) were wetted at 8.4 mJ/m2 (SE ) 0.54, n ) 20) and 9.1 mJ/m2
Table 4. Wetting of Pollen (mN/m)a
a
malvaceaeb
maizeb
10.1 mJ/m2 SE ) 0.52, n ) 10
22.7 mJ/m2 SE ) 0.81, n ) 10
Mean values. SE ) standard error. b p < 0.05.
(SE ) 0.50, n ) 20), respectively. The differences between them could not be discerned statistically (p > 0.05) (Table 2). The difference between the surface tension at wetting of the edges/corners compared and that of the plane surfaces of the same particle was statistically significant (p < 0.05). Thus, the plane surface of the cylindrical plate had a greater resistance to spreading of the surfactant film than the edge of the same plate. Wetting of Talc Particles. Macro- and microcrystalline talc particles were wetted at their edges and corners at 7.2 mJ/m2 (SE ) 0.52, n ) 20) and 8.2 mJ/m2 (SE ) 0.30, n ) 20), respectively. These results were statistically not discernible (p > 0.05). The entire surface was wetted at 3.8 mJ/m2 (SE ) 0.89, n ) 20) and 5.8 mJ/m2 (SE ) 0.52, n ) 20), respectively. The difference between these results was statistically significant. Thus, there is a difference in wetting between the plane surfaces of macro- and microcrystalline types of talc. For both types of talc wetting between the edges and surfaces was significantly different. The surfaces are wetted at a clearly lower surface tension than the edges (p < 0.05) (Table 3). Polystyrene spheres used for comparison with the talc particles were wetted at a surface tension of 23.6 mJ/m2 (SE ) 0.33, n ) 20), which is in agreement with literature values.7 Wetting of Maize and Malvaceae Pollen. The maize pollen with its fine knobbly surface was totally submersed at an equilibrium surface tension of about 25 mJ/m2 (mean 22.7 mJ/ m2, SE ) 0.81, n ) 10). The malvaceae pollen by contrast was “sitting” on the surface film at the equilibrium surface tension, but at 10.1 mJ/m2 (SE ) 0.52, n ) 10) it was totally wetted and thus immersed into the subphase. This difference with regard to wetting between the smooth and the spiky surfaces is significant (p < 0.05) (Table 4).
Discussion Wetting of the Sapphire Plates: Difference between Cubic and Cylindrical Shapes. We showed that cubic sapphire plates (wetting at a film surface tension of 10.7 mJ/m2) have a significantly higher resistance to spreading of a DPPC film than their cylindrical counterpart (film surface tension 13.8 mJ/m2). The corners (radius 0.75 µm) of the cubic plates are considered a segment of a sphere and the edges (radius 0.75 µm) a segment of a rod of circular cross section. Both represent curved surfaces and form together with the plane surfaces a continuum (Figure 2c). This is in contrast to the mathematical model, where corners
5280 Langmuir, Vol. 22, No. 12, 2006
are described as points, edges as lines, and surfaces as areas.18 The three-phase contact line among the DPPC film, the sapphire, and the air phase on the cylindrical plates has a constant curvature (radius 500 µm). For cubic plates the three-phase contact lines are straight on the edges but bent at the corners (radius 0.75 µm). The difference between the shapes of the corners of the cubic plates compared and the edges of cylindrical plates appears to be the most striking difference in the shapes of both types of plates. At these locations the radii of curvature differ in their magnitude by more than 500 times. However, we were not able to distinguish between the effect of the edges of the cubic and the cylindrical plates on wetting. In addition we were not able to exactly differentiate the wetting of the edges and the corners for the cubic plates. Furthermore, the wetting of the edges of cubes might be influenced by the much higher resistance to wetting of the corners near them. Therefore, we concluded that the different wetting behavior is due to the corners with their very small radii of curvature. Furthermore, it appears that for the cubic plates the resistance to spreading of liquids over edges and corners is similar to that for cylindrical plates with very small radii of curvatures. For technical reasons, we could not test smaller plates (
