1664
Langmuir 2006, 22, 1664-1670
Optical Microscope Absorbance Imaging of Carbon Black Nanoparticle Films at Solid and Liquid Surfaces B. P. Binks, Z.-G. Cui, and P. D. I. Fletcher* Surfactant & Colloid Group, Department of Chemistry, UniVersity of Hull, Hull HU6 7RX, United Kingdom ReceiVed October 19, 2005. In Final Form: NoVember 30, 2005 We have used an optical transmission microscope equipped with a digital camera and fitted with a narrow-band-pass filter to obtain absorbance images consisting of an array of pixel absorbance values. Absorbance images of films of carbon nanoparticles were used to derive spatially resolved images of the carbon film thicknesses with a resolution in the thickness dimension of a few nanometers. The technique was applied to the characterization of carbon nanoparticle films at cellulose-coated glass surfaces and at the oil-water interfaces of emulsion drops. For the emulsions, it was necessary to use oil and water phases of equal refractive index to avoid artifacts due to the drops acting as lenses.
Introduction Carbon blacks are particulate forms of highly dispersed elemental carbon manufactured by controlled vapor-phase pyrolysis of hydrocarbons.1 They consist of primary particles with diameters of tens of nanometers which are irreversibly fused together within the formation process into so-called primary aggregates with diameters typically in the range of tens to hundreds of nanometers. The primary aggregates may themselves be weakly and reversibly bound together in agglomerates of larger sizes. Most types of carbon black contain over 90% elemental carbon, with most of the remainder being species containing oxygen atoms, mainly in the form of surface-bound acidic or basic functional groups. The main uses of carbon black powders are in composite materials such as tires and in inks and printing. In addition to these direct uses, soot formation and its deposition onto and cleaning from surfaces of interest are also of commercial importance, from considerations of the cleaning of sooty deposits to the understanding of the environmental effects of sooty emissions. The optical properties of carbon black films and dispersions are important in these latter considerations and are of key significance in printing inks. In this paper, we describe the application of an optical microscopic absorbance imaging method to two types of films of carbon black nanoparticles. The first type consists of carbon black particles deposited on cellulose-coated glass. The motivation here was to develop methods to determine how different types of cleaning treatment not only remove some particles but also may affect the morphology of the residual film of particles. Both the extent of removal and the film morphology affect the visual appearance of the cleaned surface. The second film type consists of particles adsorbed at the liquid-liquid interface of an emulsion drop. It is well-known that particles of the correct wettability are very strongly adsorbed at the liquid-liquid interface and can act as very effective stabilizers of the emulsions.2,3 For large particles, simple optical microscopy can be used to determine the film structure in situ in the emulsion and, in particular, whether the stabilizing particle film is a monolayer or a multilayer. Optical * To whom correspondence should be addressed. Phone: 01482 465433. Fax: 01482 466410. E-mail:
[email protected]. (1) Dannenberg, E. M.; Paquin, L.; Gwinnell, H. Encyclopedia of Chemical Technology, 4th ed.; Wiley-Interscience: New York, 1992; Vol. 4, p 1037. (2) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (3) Aveyard, R.; Binks, B. P.; Clint, J. H. AdV. Colloid Interface Sci. 2003, 100-102, 503.
microscopy resolution is insufficient for the case of nanoparticle films, and a study using electron microscopy suggests that silica nanoparticles may form multilayers around emulsion drops.4 However, the drastic sample treatment required for electron microscopy means that artifacts can never be rigorously excluded. We will show here how emulsions stabilized by carbon black nanoparticle films can be examined in situ using absorbance imaging to obtain a quantitative measure of the adsorbed film thickness. Materials and Methods Samples of carbon black powders were obtained from Cabot Corp. (Belgium). According to the manufacturers, the most hydrophilic particles (Emperor 2000) have an average primary particle diameter of 10 nm and a volatile content (i.e., species containing atoms other than carbon) of 9 wt %. The more hydrophobic particles (Mogul L) have an average primary particle diameter of 24 nm and a reduced volatile content of 5 wt %. Sodium n-dodecyl sulfate (SDS; 99%, Lancaster), perfluorotoluene (98%, Aldrich), perfluorononane (97%, Aldrich), and NaCl (>99.5%, Prolabo) were used as received. Trimethylsilyl cellulose (TMSC) was provided by Unilever Research, Port Sunlight, U.K. Water was purified by reverse osmosis followed by treatment using a Milli-Q reagent water system. Microscope slides were coated with cellulose as follows. They were cleaned with alcoholic KOH, rinsed with water, dried, and stored in a desiccator. The dry slides were spin coated using a model R6700 instrument from Speciality Coating Systems Inc. by placing 0.4 cm3 of a 10 g dm-3 solution of TMSC in chloroform onto the slide, which was then spun at 2000 rpm for 1 s followed by 25000 rpm for 24 s. The chloroform was evaporated, leaving a thin TMSC film on the surface. The silyl groups of the TMSC were removed by suspending the slide in the vapor of a 10 wt % HCl solution in a closed container for 5 min, rinsing with water, and drying for 24 h. The thickness of the dry cellulose film obtained by this method was found to be 42 ( 3 nm using a Wyko NT1100 optical profilometer. Carbon black particles were deposited on the cellulose films by a spreading-evaporation method as follows. An accurately known amount of a carbon particle dispersion in water was spread on a cellulose-coated slide and left at least 12 h at ambient temperature to achieve complete evaporation of the water. Although microscopically heterogeneous, the resultant carbon black films had a reasonably uniform visual appearance. Using appropriate concentrations and volumes of the carbon black dispersions, this deposition procedure (4) Binks, B. P.; Kirkland, M. Phys. Chem. Chem. Phys. 2002, 4, 3727.
10.1021/la052816p CCC: $33.50 © 2006 American Chemical Society Published on Web 01/07/2006
Imaging of Carbon Black Nanoparticle Films
Langmuir, Vol. 22, No. 4, 2006 1665
enabled the mass per unit area of deposited carbon to be known accurately and varied over a range. Washing of the carbon films on slides was done as follows. A glass tube of 2.8 cm diameter and 9 cm height was filled to a height of 6 cm with the washing liquid. Starting from the air side, the cellulose-coated microscope slide with a deposited film of carbon black was then dipped in and out of the liquid (with complete removal into air) at a controlled speed of 8 cm/min using the dipping mechanism of a Langmuir minitrough (Joyce-Loebl). The process was repeated for a set number of wash cycles where one cycle corresponds to once in and once out of the liquid. The densities of the two types of carbon black particles were measured by dispersing a known mass of particles into the solvent to a total volume of 25 cm3. Dispersion was aided by use of either a low-power ultrasound bath (Decon model FS3006) for 30 min or a high-power ultrasound probe (Sonics & Materials Inc.) operating at 13 W for 1 min. The mass of the total dispersion was then used to calculate the density of the carbon black (assuming zero volume change on dispersion). Final density values obtained using either water or dodecane as the dispersing solvent agreed within an uncertainty of (0.04 g cm-3. Microscopic absorbance images were made using a Zeiss Axiovert S100 inverted microscope employing transmission optics and equipped with a 450 nm narrow-band-pass filter. The light source was a 100 W tungsten light emitting over the wavelength range from 400 to 800 nm. Stray ambient light was reduced to negligible levels by use of a hood over the microscope. Microscope images were collected using a digital CCD camera (Hamamatsu C4742-9512NRB) giving a digital output of a maximum of 1024 × 1024 pixels, each with 12 bit resolution of the light intensity, i.e., pixel light intensity values in the range 0-4095. The camera was connected to a PC and controlled by the digital image recording and analysis software AQM from Kinetic Imaging Ltd. Image-Pro Plus 5.1 software was additionally used to transfer image data into Microsoft Excel for further analysis. Emulsions were prepared by initially dispersing a known weight percent of the carbon black powder in either the water or oil phase using ultrasound (1 min at 13 W using a Sonic & Material instrument with a 0.3 cm tip diameter) to aid the dispersion process. Equal volumes of the oil and water were then emulsified using an Ultra Turrax homogenizer fitted with an 8 mm diameter head operating at 13500 rpm for 2 min. Microscope images of the emulsions were obtained by placing 1 drop of emulsion in the wedge-shaped gap formed by a coverslip resting on a microscope slide with one edge propped up by a second coverslip. All measurements were made at room temperature, 20 ( 2 °C.
Results and Discussion The absorbance A of a solid material varies with the light path length through it (z) according to
A ) log(I0/I) ) -(log T) ) z
(1)
where I0 and I are the incident and transmitted light intensities, T is the optical transmission, and is the absorption coefficient equal to the absorbance per unit length traveled by the light within the solid material. The microscopic absorbance imaging method can be used to obtain spatially resolved absorbance “maps” at a single fixed wavelength (450 nm was used here). This method, originally developed to characterize the microchannels in “lab-on-a-chip” devices, is fully detailed in ref 5. Briefly, an inverted transmission microscope equipped with a digital camera and a narrow-band-pass filter allowing transmission of 450 nm is used to obtain three digitized images, each consisting of an array of pixel light intensity values. The first, reference (5) Broadwell, I.; Fletcher, P. D. I.; Haswell, S. J.; McCreedy, T.; Zhang, X. Lab Chip 2001, 1, 66.
Figure 1. Comparison between deposition loadings of Emperor 2000 carbon black particles on cellulose-coated glass as derived from absorbance images (Mimage) and by direct measurement (M). The “best-fit” value of the extinction coefficient used was 1.6 × 106 m-1.
image is recorded in the absence of sample using light and camera settings such that the maximum light intensity in any pixel is close to but does not exceed the limit of 4095 set by the 12 bit digital camera. This image yields an array of pixel light intensities denoted I0. The sample image is then recorded under identical conditions to give an array of pixel transmitted intensities denoted I. The third image, recorded in the absence of sample particles and with the microscope light blanked off, yields the intensity array denoted Idark. The three intensity arrays are then combined using Excel to give the final array of pixel-by-pixel absorbance values, Apixel, making up a so-called “absorbance image” according to
Apixel ) log
(
)
I0 - Idark I - Idark
(2)
Carbon Black Particles Deposited on Cellulose-Coated Glass. To convert an absorbance image into an image of z values, we require independent knowledge of the extinction coefficient, . For the carbon black particles, this was done as follows. A series of cellulose-coated glass slides were prepared, each of which contained a known loading of carbon black particles deposited using the sedimentation-evaporation method. Absorbance images were recorded of each slide and converted to z value images using a guessed value of the extinction coefficient. Summation of the z values over all pixels and conversion using the measured density value (1.55 g cm-3 for Emperor 2000 and 1.84 g cm-3 for Mogul L) yielded a derived value of the mass of carbon black per unit area (denoted as Mimage). The value of the extinction coefficient was then adjusted until Mimage agreed with the values of mass per unit area, M, known independently from the sample preparation. Figure 1 shows the good level of agreement obtained within a complete data set covering a range of M for Emperor 2000 for which was found to be 1.6 × 106 m-1. Similar results for Mogul L gave ) 2.3 × 106 m-1. The higher extinction coefficient for Mogul L may be a consequence of its lower volatile content, i.e., the higher content of graphitic carbon. Following determination of the extinction coefficient as described above, absorbance imaging was used to investigate the effects of cleaning solutions on cellulose-coated glass slides containing a film of deposited carbon black particles. Figure 2 shows absorbance images converted to z values recorded for a clean cellulose-coated slide (upper image) and a dirty slide coated with Emperor 2000 particles before (middle image) and after
1666 Langmuir, Vol. 22, No. 4, 2006
Figure 2. z value images derived from absorbance images of a clean cellulose-coated glass surface (upper plot) and a deposited film of Emperor 2000 carbon particles before (middle plot) and after (lower plot) washing with SDS solution.
(lower image) washing with 15 mM SDS. “Slices” through the data sets (Figure 3) show the following points. The amplitudes of the fluctuations in z due to random errors for the clean slide image correspond to less than 1 nm thickness of carbon black, and hence, the smallest resolvable thickness of carbon is slightly larger than this, i.e., approximately 2 nm. Before washing, the carbon black film consists of peaks with heights of 100-300 nm and widths of 1-10 µm which are on top of a fairly uniform film of 30-40 nm thickness. In the example shown here, washing removes the large particles corresponding to the big peaks but leaves a fairly uniform background film of 10-20 nm thickness. The small average thickness of this residual carbon film and the fact that it is not easily removed by further washing cycles suggests that the residual carbon nanoparticles may be embedded within the cellulose film on the glass surface.
Binks et al.
Carbon Black Particles Adsorbed at Liquid-Liquid Emulsion Drop Surfaces. To use absorbance imaging to determine the absorbance, and hence the thickness, of the film of stabilizing carbon nanoparticles coating liquid emulsion drops, it is necessary that the drops do not refract the light, i.e., they do not act as small lenses. As discussed in refs 6-8, light intensity distributions in microscopic images of transparent spheres are highly dependent on the refractive index difference between the sphere and its surroundings and on the position of the plane of focus used to obtain the microscope image. However, we have overcome this problem by working with emulsions in which the oil and water components are refractive index matched and hence light is not refracted or reflected by the oil-water interface. Refractive index matching between the oil and aqueous phase consisting of 10 mM NaCl was achieved within 0.001 unit by using an oil mixture consisting of 66 wt % perfluorotoluene and 34 wt % perfluorononane. Using the same aqueous and oil phases (i.e., 10 mM NaCl in water and the oil mixture of 66 wt % perfluorotoluene and 34 wt % perfluorononane), we have been able to prepare both oilin-water (o/w) and water-in-oil (w/o) emulsions of good stability using carbon black nanoparticles of the appropriate wettability. The required mass of carbon nanoparticles was initially dispersed in either the water or the oil phase using ultrasonic irradiation at 13 W power for 1 min. Equal volumes of the carbon dispersion and the second phase were then emulsified using an Ultra Turrax device operating at 13500 rpm for 2 min. Some key properties of the emulsions used here are summarized in Table 1. As expected from wettability considerations, the more hydrophilic Emperor 2000 stabilizes o/w emulsions whereas w/o emulsions are formed with the more hydrophobic Mogul L particles. For the two systems tested, no significant change in the number average drop diameter is seen following incubation for 1 week, which demonstrates that the stability with respect to both drop coalescence and Ostwald ripening is good. For the w/o emulsions, the mean initial drop diameter decreases when the particle concentration is increased from 1 to 2 wt %. This behavior is expected as the higher particle concentration is capable of stabilizing a larger oil-water surface area and hence smaller drop sizes. Without more detailed study, it is currently not clear why this trend is not found to continue for the emulsion with 0.5 wt % Mogul L particles. Because of the high density of the fluorocarbon/oil mixture relative to the aqueous phase, sedimentation of the o/w emulsion oil drops and creaming of the w/o drops occurred rapidly within 1 min or so. The drops were easily redispersed by hand shaking. Figure 4 shows a transmission micrograph recorded at a wavelength of 450 nm of an o/w emulsion which was stabilized by 2 wt % Emperor 2000 particles. For the micrograph, the o/w emulsion was diluted approximately 10-100-fold with the same aqueous phase (i.e., containing 10 mM NaCl). Figure 5 shows an absorbance image for one of the emulsion drops as derived from the intensity image of Figure 4 together with the corresponding reference and dark images which are not shown here. The absorbance image plot has a characteristic crownlike appearance where the absorbance is minimum through the poles of the drop where the light travels through a thickness of carbon equal to twice the film thickness. The absorbance is higher at the edges of the drop owing to the longer path length of the light through the carbon film. For a sphere of inner radius r coated with a uniform shell of thickness t, the light path length through (6) Ovryn, B.; Izen, S. H. J. Opt. Soc. Am. 2000, 17, 1202. (7) Elliot, M. S.; Poon, W. C. K. AdV. Colloid Interface Sci. 2001, 92, 133. (8) Kvarnstrom, M. Position estimation and tracking in colloidal particle microscopy. Ph.D. thesis, Mathematical Sciences, Chalmers University of Technology, Sweden, 2005.
Imaging of Carbon Black Nanoparticle Films
Langmuir, Vol. 22, No. 4, 2006 1667
Figure 3. Slices through the z value images of Figure 2 for a clean surface (upper plot) and a deposited film of Emperor 2000 carbon particles before (middle plot) and after (lower plot) washing with SDS solution. Table 1. Summary of the Key Properties of the Carbon Black Nanoparticle-Stabilized Emulsions Used in This Studya number av drop diam/µm carbon nanoparticles
emulsion type
initial
2 wt % Emperor 2000 2 wt % Mogul L 1 wt % Mogul L 0.5 wt % Mogul L
o/w w/o w/o w/o
41 ( 10 14 ( 6 60 ( 10 53 ( 17
after 1 week 38 ( 7 62 ( 14
a All emulsions contained equal volumes of 10 mM NaCl in water and the oil mixture consisting of 66 wt % perfluorotoluene and 34 wt % perfluorononane. The concentration of carbon nanoparticles is expressed relative to the mass of the aqueous phase. In preparing the emulsions, the hydrophilic Emperor 2000 particles were initially dispersed in the aqueous phase whereas the more hydrophobic Mogul L particles were initially dispersed in the oil phase. For each diameter, the ( value corresponds to the standard deviation of the droplet size distribution.
the film, z, is given by
z ) 2[[(r + t)2 - a2]1/2 - (r2 - a2)1/2] for 0 < a < r z ) 2[[(r + t)2 - a2]1/2] for r < a < r + t z ) 0 for
(3)
r+t
