Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects
  • G01N13/02—Investigating surface tension of liquids
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects
  • G01N13/02—Investigating surface tension of liquids
  • G01N2013/0241—Investigating surface tension of liquids bubble, pendant drop, sessile drop methods
  • G01N2013/0266—Bubble methods
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects
  • G01N13/02—Investigating surface tension of liquids
  • G01N2013/0275—Investigating surface tension of liquids involving surface-active agents

Definitions

• the invention relates to a method and a device for examining a surface-active substance.
• Surface-active substances form a molecular film at interfaces between two immiscible fluids. They allow the targeted production of molecularly defined layers and are of considerable importance, for example, in nonlinear optics.
• Surface-active, biological macromolecules for example, form a functional component in the human lung (lung surfactant) or serve as a model of the plasma membrane in the life sciences.
• the film pressure IT of the surface-active substances counteracts the surface tension of the boundary layer. For example, the following relationship applies to the boundary layer between water and air:
• ⁇ 0 is the surface tension of pure water (-72 mN / m at 25 ° C), and ⁇ is the resulting surface tension of a filmed boundary layer.
• One property here is that the film pressure increases with decreasing average area per molecule and the surface tension decreases accordingly. This relationship takes a characteristic form depending on the surface-active substance and is recorded at a constant temperature in a so-called pressure-surface isotherm, which can be measured, for example, using a "captive bubble surfactometer".
• a “Captive Bubble Surfactometer” the surface tension of an interface can be determined from the bubble geometry of a gas bubble in liquid using a mathematical method. as described, for example, by Kwok et al. in the journal “Polymer Engineering and Science”, (1998) 38: 757. Because of its buoyancy in the liquid, the gas bubble is driven onto a slightly dome-shaped agar roof and is fixed in this way. By changing the chamber pressure, then the volume and thereby the area of the gas bubble can be changed. From this the associated surface tension can be changed be determined.
• Schürch et al. “A captive bubble method reproduces the in situ behavior of lung surfactant monolayers”, J. Appl. Physiol. (1989) 67: 2389-96.
• a capillary filled with a gas is immersed in a liquid which is arranged in a closed vessel, and the volume of the liquid in the vessel is then reduced by a predetermined amount. This causes a bubble of gas with the appropriate volume to emerge from the capillary into the liquid, and the interfacial tension at the interface between the gas and the liquid can be calculated using a mathematical method.
• a less dense liquid which is immiscible with the liquid in which the gas bubble is otherwise introduced, can also be used in the two methods described.
• the pressure-surface isotherms determined with the aid of the described methods provide information about thermodynamic phenomena, such as first-order or higher-order phase transformations, miscibility in multi-component systems, etc., in connection with the surface film which comprises the surface-active substance.
• the molecular architecture of the surface film during this compression is of great interest because it allows conclusions to be drawn about the molecular basis of the characteristic behavior of a substance. It has been shown that a surface film in the area of the boundary layer between two fluids has phase boundaries between molecules in different physical states. stand may contain; in the case of mixed films, there can be a characteristic distribution of the molecules within the surface film, etc.
• the pulmonary surfactant forms a complex three-dimensional molecular architecture that is directly related to its function and has so far only been partially elucidated.
• a method for the structural investigation of surface-active substances in surface films is the fluorescence labeling of certain components and subsequent observation with a fluorescence light microscope (cf. Lösche et al., "Fluorescence microscope to observe dynamical processes in monomolecular layerst at the air / water interface", (1984) 55 : 1968-1972)
• a fluorescence light microscope cf. Lösche et al., "Fluorescence microscope to observe dynamical processes in monomolecular layerst at the air / water interface", (1984) 55 : 1968-1972
• a so-called film balance in which a substance is applied to the gas-liquid interface of a liquid-filled trough (typically with an interface size of several cm 2 ) and compressed over a movable barrier (cf. for example Ulman, A., "An introduction to ultrathin organic films” Academic Press, Boston, 1991, p. 442).
• an objective for epifluorescence light microscopy is placed at a central point above the surface
• This method has the disadvantage that the surface film on the film scale is subjected to currents which can be very strong, especially at low film pressures, but which also occur spontaneously at high film pressures. This significantly impairs the microscopic examination of the surface film. Due to the large interface exposed to the environment and the tendency of the liquid to vibrate in the trough, the system is also extremely sensitive to disturbances from the environment, such as air circulation in the laboratory or vibrations in the building. This prevents an observation of the temporal behavior of individual structures of the surface film or at least requires complex shielding of the apparatus from the surroundings.
• a change in the interface size leads to further flows and tensions in the surface film, which is described, for example, by Malcom, "The Flow and Deformation of Synthetic Polypeptide Monolayers during Compression", J. Colloid Interface Sei., (1985) 104: 520 in addition, the surface film shifts relative to the light microscope objective during compression or expansion, so that the structures of interest disappear from the field of view of the examination.
• creeping often occurs on a film scale: Instead of further compression of the molecules, they move away from the water-air interface. For example, the molecules are pushed under the movable barrier.
• the invention essentially comprises the introduction of a fluid in the form of a sample volume into another fluid which is immiscible with the one fluid, so that an interface between the one fluid and the other fluid is formed at least in a partial area of a surface of the sample volume, wherein the sample volume is formed axially symmetrically about a predefined definition axis, so that the boundary layer is formed axially symmetrically to the predefined definition axis.
• the surface-active substance is spread over the boundary layer to form a surface film in the region of the boundary layer.
• the boundary layer between the two fluids is characterized by high mechanical and temporal stability.
• the sample volume of one fluid is well shielded from acoustic disturbances and thermal fluctuations of the environment by the surrounding other fluid. If, for example, the pressure in the measuring space deviates from the ambient pressure, an almost complete acoustic decoupling from the environment is achieved. If a very small sample volume is selected, the tendency to vibrate at the interface is drastically reduced compared to the measurement with a film scale. This eliminates the need for complex and costly shielding of the equipment from the surroundings.
• High-resolution microscopes such as as the scanning probe microscopy (cf. Colton et al., "Scanning probe microscopy", Curr. Opin. Chem. Biol. (1997) 1: 370-377) can be used.
• the surface film contracts or expands symmetrically to the predetermined definition axis when the surface changes.
• the stabilization of the surface film achieved allows an investigation of dynamic structural changes of the surface-active substance in the surface film at the interface between the two fluids. Furthermore, in comparison to the known investigation using the film balance, microscopy using high-resolution methods is made possible. Furthermore, optical microscopic observation of an active interface film has so far only been possible on a gas-liquid interface.
• An axially symmetrical sample volume and thereby an axially symmetrical boundary layer can, for example, thereby. be formed that the one fluid with the lower density is introduced into the other fluid with the higher density and the buoyancy of the sample volume is limited with the aid of an element whose properties are close to those of the other fluid, so that the interface between the two fluids in the area of the surface of the sample volume is not significantly affected.
• the element With the help of the design of the element as a dome-shaped roof, self-centering of the sample volume can be achieved in a horizontal plane. Other forms of centering can also be provided, for example mechanical adjustment in the horizontal plane.
• One fluid can be introduced, for example, via a valve and / or via a syringe. Other forms of introduction are of course conceivable.
• one fluid can enter the other fluid from one capillary.
• the interface or boundary layer is not closed in an area in which the sample volume and capillary touch.
• a measurement of the surface tension is possible with the aid of an analysis of the shape of the interface.
• the pressure can be changed, as is known in connection with the “captive bubble surfactometer”
• the principle underlying the "pulsating bubble surfactometer” described above can be used to change the interface.
• the volume of the other fluid is changed, as a result of which a volume of the one fluid corresponding to the change enters or exits the capillary.
• a lateral image of the sample volume can be recorded from a direction perpendicular to the axis of symmetry of the sample volume with an optical observation device. From this, the surface tension can be calculated exactly or in good approximation using various mathematical methods.
• an acoustic excitation of the sample volume can be used to measure the surface tension.
• the resonance frequencies due to the capillary forces depend on the surface tension and can therefore be determined from a frequency spectrum.
• the spectrum can be detected optically by observing standing waves at certain frequencies. Sufficient excitation energy must be applied so that the amplitudes are large enough for optical detection. When using a scanning probe microscope, amplitudes in the nm range can be detected.
• the detection of a spectrum could also be carried out via an absorption measurement, since a particularly large amount of energy for the excitation of the vibration is absorbed in the resonance and is at least partially scattered isotropically. A recording could take place, for example, with the aid of a microphone.
• the interface between the two fluids is to be changed in order, for example, to study surface-induced changes in the surface film
• this is a fluid which forms the sample volume, preferably a gas.
• the two fluids are arranged in a pressure-resistant chamber.
• a pressure change is carried out to change the volume and thus the surface of the sample volume from gas.
• an objective must be attached the sample volume can be introduced.
• the lens can be optically matched to the other fluid for optimal observation, ie, for example, be a water immersion lens when the other fluid is water, and immersed in it. Provision can also be made to observe the sample volume in the airtight chamber through a transparent disk.
• the probe of a scanning probe microscope or the observation device of another microscopic technique can also be used.
• the object plane of the microscope used to examine the surface film always lies in the lowest point on the surface of the sample volume, which is also referred to as the apex.
• the possibility of shifting the sample volume in the direction of the optical beam path is provided.
• the possibility of an adjustment perpendicular to the optical axis can be provided in order to bring the apex into the optical axis.
• FIG. 1 shows a schematic illustration to explain a method for examining a surface-active substance in a surface film
• FIG. 2 shows a schematic illustration of a “captive bubble surfactometer” for using the method for examining the surface-active substance
• FIG. 3 shows a schematic illustration to explain a change in volume and refocusing in connection with the “captive bubble surfactometer” according to FIG. 2;
• FIG. 4 shows a schematic illustration of a “pulsating bubble surfactometer” for using the method for examining the surface-active substance;
• FIG. 5 shows a schematic illustration of another “captive bubble surfactometer” with an atomic force microscope for examining the surface film
• FIG. 6 shows a schematic illustration to explain a further method for examining a surface-active substance in a surface film.
• FIG. 1 shows a schematic illustration to explain a method for examining a surface-active substance in a surface film.
• a sample volume 1 from a fluid 2 is surrounded by another fluid 3.
• a boundary layer 5 between the two fluids 2, 3 is formed on a surface 4 of the sample volume 1.
• One or more surface-active substances whose physical properties are to be investigated are applied in the area of the boundary layer 5.
• the applied surface-active substances form a surface film in the area of the boundary layer 5 on the sample volume 1.
• the sample volume 1 and thereby the boundary layer 5 are formed axially symmetrically to an axis 6. Because of the axial symmetry, the surface film cannot flow if the molecules of the surface film formed from the surface-active substance are not soluble in the two fluids 2, 3 and the two fluids 2, 3 are not miscible.
• a microscope 20 For microscopic observation of the surface film, a microscope 20 is provided, the optical axis of which suitably coincides with the axis 6. In this way, an apex 7, i.e. H. the lowest point of the sample volume 1 or the boundary layer 5 can be arranged opposite the microscope 20. A distance 40 between the microscope 20 and the apex 7 is expediently kept constant during the measurement.
• the axis 15 does not have to go through the center of the sample volume 1. It is only necessary to ensure that the entire outline or a part of the outline of the sample volume sufficient to determine the surface tension can be seen. Due to the axial symmetry of the sample volume 1 to the axis 6, the 3-dimensional shape can be determined from the outline.
• FIG. 2 shows a schematic illustration of a “captive bubble surfactometer” for using the method for examining the surface-active substance.
• the fluid 3 here is, for example, water which is filled in an airtight chamber 8.
• the fluid 2 is a gas.
• the sample volume 1 is pressed by the buoyancy against a hydrophilic dome-shaped roof 71, which is agar gel, for example, and the boundary layer 5 that is axially symmetrical to the axis 6 is formed.
• the sample volume 1 has a diameter of approximately 100 ⁇ m to 300 In principle, however, smaller or larger dimensions are also conceivable
• the sample volume 1 is fixed in the chamber 8 by the agar gel 4 and its shape.
• the fluid 3 can be supplied and optionally exchanged via dispersion pumps 50, 51.
• the dipension pumps 50, 51 are supplied via pressure and vacuum solid feedthroughs 52, 53 from “high pressure liquid chromatography” (HPLC) into the chamber 8.
• the fluid 2 for shaping the sample volume 1 can also be supplied via a valve 54.
• the surface-active substance is sprayed onto the boundary layer 5 via a syringe 55 (cf. Putz et al, "A spreading technique for forming a film in a captive bubble in a surfactometer ", Biophysical Journal 1998 75: 2229-39).
• the syringe 55 must be removed after the spreading so that an undisturbed expansion and observation of the sample volume 1 is possible.
• the sample volume 1 can be displaced in the x and y directions via a manually or motor-adjustable cross table 85 and via a motorized micrometer screw 86 or another adjusting device in the z direction.
• a mechanical contact to the motorized micrometer screw 86 is made via a plunger 81.
• a spring 82 provides mechanical contact with the motorized micrometer screw 86.
• the displacement in the z direction of the plunger 81 can also be achieved with the aid of a piezo element. In this case, the spring 82 is used to generate a preload.
• the microscopic examination of the sample volume 1 is carried out in the exemplary embodiment according to FIG. 2 with the aid of a light microscope 100.
• the light microscope 100 can be an epifluorescence light microscope, for example, which is operated either confocal or conventionally.
• an objective 21 is immersed in the fluid 3.
• the microscopic image which is, for example, a fluorescence distribution, is then imaged on a CCD chip 23 with the aid of a lens 22, digitized and sent to an evaluation in an evaluation device 70, which is, for example, a personal computer.
• lateral observation of the sample volume 1 is provided to determine the surface tension of the surface film on the sample volume 1.
• a borescope 110 is used for this.
• a bushing 111 is designed as an HPLC bushing.
• the borescope 110 takes the outline of the surface tension to determine the surface tension Sample volume 1.
• Subsequent digitization is also implemented via a CCD chip 33, onto which the image is focused using a lens 32.
• the digitized image is then evaluated using the evaluation device 70.
• the surface tension between the two fluids, which are influenced by the surface film on the sample volume 1, can be determined by means of the ADSA algorithm. With the aid of the evaluation device 70, an associated surface tension is assigned to each microscopic image and stored.
• a change in the size of the sample volume 1 and thereby the boundary layer 5 is possible via a pressure change using a disperser pump 60, which is designed in a software-controlled manner.
• a pressure sensor 61 controls the pressure in the chamber 8 and can also be read out using suitable software.
• Bushings 200, 201 of the disperser pump 60 and the pressure transducer 61 are designed as HPLC bushings.
• the disperse pump 60 changes a gas pressure in a space 202 above the fluid 3 in the chamber 8. This changes the volume of the sample volume 1. Usually, a negative pressure is generated with the aid of the disperse pump 60, but it can also be provided to apply an overpressure.
• the sample volume 1 can be shifted in the z direction in order to keep the distance 40 between the apex 7 of the sample volume 1 and the objective 21 constant, even if the volume of the sample volume 1 changes.
• FIGS. 3A to 3C show schematic representations to explain a change in volume and refocusing in connection with the “Capive Bubble Surfactometer” according to FIG. 2.
• the starting situation in the arrangement according to FIG. 2 is shown in FIG. 3A shown.
• the chamber 8 is filled, for example, with approximately 100 ⁇ l buffer.
• the sample volume 1 with a diameter of approximately 100 ⁇ m, for example, is injected.
• the diameter here relates to the largest extent of the sample volume 1 in the x direction, ie perpendicular to the optical axis 6.
• the diameter is then increased to approximately 300 ⁇ m in the selected example by applying a negative pressure.
• the observation with the light microscope 100 and the borescope 110 is started and the sample volume 1 is brought into the optical axis.
• the apex 7 of the sample volume 1 is then brought into the focal plane of the light microscope 100.
• the surfactant is now injected.
• the adsorption of the surface-active substance is examined structurally with the aid of the light microscope 100 and functionally with the aid of the borescope 110 (reduction of the surface tension).
• the size of the sample volume 1 is varied.
• the volume of the sample volume 1 is increased, as a result of which the distance 40 between the apex 7 and the objective 21 is reduced.
• the original shape of the sample volume 1 is shown in broken lines in FIG. 3B.
• the original distance 40 must be restored. This is achieved with the aid of the stamp 81, which is shown in FIG. 3C.
• the original position of the stamp 81 is shown in dashed lines in FIG. 3C.
• FIG. 4 shows a schematic illustration of a “pulsating bubble surfactometer” for using the method for examining the surface-active substance.
• the chamber 8 is covered with a roof 300 and completely filled with the fluid 3 in which it is
• the fluid 2 can be a gas or a liquid which is immiscible with the fluid 3.
• the fluid 2 does not form a closed boundary layer 5, but emerges from a hose 65 which, for example can be positioned with the aid of a plunger 67.
• the hose 65 is connected to a chamber volume 66, which is, for example, a gas-filled syringe with a locking ring.
• a change in the sample volume 1 can be achieved, for example, by moving the stamp 67. Another possibility would be to supply a gas via the syringe 55.
• the “creeping” that is fundamentally possible in the embodiment according to FIG. 4, ie migration of the molecules of the surface-active substance, can be minimized, for example, by the hose 65 being coated on the outside with a hydrophilic coating and on the inside with a hydrophobic coating.
• the sample volume 1 in the chamber 8 is positioned with the help of the manual cross table 85 and the motorized micrometer screw 86. This is automatically possible with the help of a software-based control.
• FIG. 5 shows a schematic illustration of another “captive bubble surfactometer” in which a scanning force microscope (SFM — “scanning force microscopy”) is provided as the microscope.
• SFM scanning force microscope
• SNOM optical near field microscope
• the cantilever 90 is attached to a piezo 91, which in turn is attached to the chamber 8 Alternatively, the piezo could also be attached to the lens 21.
• the piezo must be designed in such a way that it can move the cantilever in all three spatial directions so that a raster movement can be carried out over the surface and at the same time the distance can be maintained as it is the respective type of imaging requires
• x and y denote the displacement perpendicular to the optical axis 6.
• z denotes the axis parallel to this in the following.
• an embodiment known from scanning probe microscopy would be a piezo tube.
• the piezo could only be in the lateral direction (x and y) scan and the shift ng in the vertical direction (z direction) could be mediated by a piezo attached to the stamp. This would then move the air bubble vertically itself.
• the cantilever bending is detected by the light display principle, which is widespread in atomic force microscopy.
• the laser beam 92 from a laser 93 is directed onto the cantilever by a beam splitter 95.
• the objective 21 assumes the function of focusing the laser beam on the cantilever.
• the reflected beam 96 is then again directed via a beam splitter 97 onto a 4-segment photodiode 98.
• the signals obtained at the photodiode are evaluated in control electronics 99 and the deflection of the piezos is controlled thereby. In addition, the signals are passed on to the PC for evaluation.
• FIG. 6 shows a very simple application of the method to a liquid drop 1 which is surrounded, for example, by a gas.
• the immobility of the surface film is again guaranteed due to the axially symmetrical shape.
• the base 45 could, for example, consist of Teflon if the medium 1 is hydrophilic and of agar gel if the liquid is hydrophobic. This would ensure that the interface at the contact point is only slightly disturbed. With a lateral observation 31, the outline of the drop can be used to determine the surface tension.
• an upright microscope could be used with the objective 21 and the optimal alignment of the objective and the drop with respect to one another could be carried out using the adjusters which are commercially available.
• the surface could be changed using a syringe 48, which, however, disrupts the interface and thus enables the above-described creeping.
• a measurement method is presented for the first time with which a surface-active film can be observed microscopically directly at the interface, which stands still within the scope of the resolution of the respective microscope. Furthermore, several devices for performing the method were presented, in which the surface tension can be measured simultaneously. Furthermore, compression or expansion of the film is possible with simultaneous microscopic observation of selected surface areas without these being pushed out of the image area.

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• 2002
  • 2002-05-17 WO PCT/DE2002/001828 patent/WO2002095367A1/de active Application Filing
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