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On the density of sodium chloride aerosols formed by condensation
On the Density
of Sodium
Chloride
Aerosols
Formed
by Condensation
steps at right angles to the line and also evaluated in the usual manner with the same stepwidth in a light microscope. The triangles in Fig. 1 show the electron microscopic diameters, each value representing a mean over several particles. The full line represents geometric diameters obtained from aerodynamic measurement. A noticeable slight inclination of the row of points of electron microscope diameters results from difficulties in precise orientation of the foil. Deviations lie within 3%. From this excellent agreement it may therefore be justified to assume bulk density also for spherical aerosol particles of sodium chloride. The seemingly amorphous structure in electron scattering could then perhaps result from a very fine distribution of microcrystals in random orientation, giving a very diffuse diffraction pattern but no significantly different mass density.
Spherical aerosol particles of sodium chloride are generated in one-, two-, or three-stage furnaces by condensation of h’aC1 vapor either on well-defined nuclei, e.g., of NaF, or on the permanently existing background nuclei from the furnace. A recent description of these techniques was given by Kerker (1). It is known that such spherical particles do not show any sharp electron diffraction patterns and must therefore be considered as mainly amorphous (2). The question arises as to how the properties of amorphous NaCI differ from the bulk material. Espenscheid et al. (3) have extrapolated the values of refractive index and mass density of liquid NaCl to room temperature and thus obtained a value of 1.51 for GZ (bulk material 1.55 to 1.56) and a density of 93% of the bulk (1.165, “Handbook of Physics and Chemistry,” 44th Ed.). The optical properties agreed well with their theoretical calculations. On the other hand, it has been noted (4) during experiments involving aerodynamic properties of the aerosol (i.e., density and geometric shape) that large density variations seem to occur. Spheres of various sizes were reported to have shown identical aerodynamic diameters, most of them with a density far below 2.16. Armbruster et al. (5) mentioned recently that their one-stage furnace simultaneously produced spherical and irregularly shaped particles. The author has investigated densities of spherical sodium chloride particles produced by a generator consisting of one furnace with three separately controlled heating zones. A stream of filtered and dried nitrogen (1 liter/min) enters the first zone, where condensation nuclei of NaF are produced; in zone II, NaCl is evaporated (800~8SO’C) and subsequently condensed on the nuclei. Zone III acts as a reheater. The aerosol of narrow size distribution (relative standard deviation &7%) was collected on the foil of an aerosol centrifuge (ROSL-Spectrometer (6, 7)). This spectrometer has a resolution better than 1% and gives absolute Stokes diameters with an accuracy of the same range. The geometric particle sizes obtained from the aerodynamic diameters (for bulk density of 2.165) were compared to electron micrographs of the same spectrometer run, magnification being calibrated simultaneously with latex. During all manipulations of the aerosol, care was taken to avoid any humidification. The results for one run are given in Fig. 1. The deposit on the spectrometer foil, which is a narrow line of a few millimeters, was photographed in equidistant
5 055r
ILLli 13
I1
12
FIG. 1. Comparison obtained from electron aerodynamic diameters density 2.165).
I1
1
J
7 10 9 8 mm LOC.of DEPOSl T
of geometric particle microscope (triangles) (solid line, calculated
diameters and from for bulk
3.59 Copyright All rights
@ 1977 by Academic Press, Inc. of reproduction in any form reserved.
Jonrxal
of Colloid
aed Interface
S&me,
Vol. 62, No. 2, November 1977 ISSN 0021-9797
360
NOTES
Reported large density variations of NaCl aerosol in one and the same sample seem to be typical for certain models of generators. They could be explained by a ‘%uffy” structure of particles which may result from the special cooling rate of the condensing vapor. The author has noticed that furnaces with only one long heating zone and gentle slopes in temperature gradient tend to produce particles of unstable density or irregular shape. The generator used here had a marked temperature drop at the exits of the heating zones, particles were always rigorously spherical with no cavities, according to density measurements. ACKNOWLEDGMENT This work R 801 983.
was partly
supported
by EPA
Grant
Jozmal
of
M.,
Co&id
Ah.
Cnlloid
M~TIJIWIE, E., ESPXNSCHEID,TV. F., AND KERKER,
3.
ESPENSCHEID,W. I?., MATIJEVIE, E., AND KERKER,
M., J. C&id M.,
18, 91 (1963).
Clzem. 68, 2831 (1964).
MATTESON, M. J., Fox, J. J., AND PREINING, O., Nature Pltys. Sci. 238, 61 (1972). 5. ARLCBRUSTER,L., STAHLHOFEN,W., AND GEBHART, J., in “Tagungsbericht Ges. f. Aerosolforschung, Bad Soden/BRD,” (V. Bohlau and H. Straubel, Ed.), GAF Publ., 1976. 6. ROPPERT, H., AND BERNER, A., Staub Rein/a. der Luft 29, 92 (1969). i'. ABED-NAVANDI, M., BERNER, A., A~W PREINING, O., ilz “Fine Particles” (B. Y. H. Liu, Ed.) p. 447, Academic Press, New York, 1976.
GERHARD KASPER 1st Physics Institute University of Vienna 1090 Vienna, Austria
Interface
Science,
J. Phys.
Sci.
4.
Sci.
7, 107 ff Received
and Interface
Interfhe
No.
REFERENCES 1. KERKER, (1975).
2.
Vol. 62, No. 2, November 1977
May
11, 1977;
accepted
May
13, 1977
of Sodium
Chloride
Aerosols
Formed
by Condensation
steps at right angles to the line and also evaluated in the usual manner with the same stepwidth in a light microscope. The triangles in Fig. 1 show the electron microscopic diameters, each value representing a mean over several particles. The full line represents geometric diameters obtained from aerodynamic measurement. A noticeable slight inclination of the row of points of electron microscope diameters results from difficulties in precise orientation of the foil. Deviations lie within 3%. From this excellent agreement it may therefore be justified to assume bulk density also for spherical aerosol particles of sodium chloride. The seemingly amorphous structure in electron scattering could then perhaps result from a very fine distribution of microcrystals in random orientation, giving a very diffuse diffraction pattern but no significantly different mass density.
Spherical aerosol particles of sodium chloride are generated in one-, two-, or three-stage furnaces by condensation of h’aC1 vapor either on well-defined nuclei, e.g., of NaF, or on the permanently existing background nuclei from the furnace. A recent description of these techniques was given by Kerker (1). It is known that such spherical particles do not show any sharp electron diffraction patterns and must therefore be considered as mainly amorphous (2). The question arises as to how the properties of amorphous NaCI differ from the bulk material. Espenscheid et al. (3) have extrapolated the values of refractive index and mass density of liquid NaCl to room temperature and thus obtained a value of 1.51 for GZ (bulk material 1.55 to 1.56) and a density of 93% of the bulk (1.165, “Handbook of Physics and Chemistry,” 44th Ed.). The optical properties agreed well with their theoretical calculations. On the other hand, it has been noted (4) during experiments involving aerodynamic properties of the aerosol (i.e., density and geometric shape) that large density variations seem to occur. Spheres of various sizes were reported to have shown identical aerodynamic diameters, most of them with a density far below 2.16. Armbruster et al. (5) mentioned recently that their one-stage furnace simultaneously produced spherical and irregularly shaped particles. The author has investigated densities of spherical sodium chloride particles produced by a generator consisting of one furnace with three separately controlled heating zones. A stream of filtered and dried nitrogen (1 liter/min) enters the first zone, where condensation nuclei of NaF are produced; in zone II, NaCl is evaporated (800~8SO’C) and subsequently condensed on the nuclei. Zone III acts as a reheater. The aerosol of narrow size distribution (relative standard deviation &7%) was collected on the foil of an aerosol centrifuge (ROSL-Spectrometer (6, 7)). This spectrometer has a resolution better than 1% and gives absolute Stokes diameters with an accuracy of the same range. The geometric particle sizes obtained from the aerodynamic diameters (for bulk density of 2.165) were compared to electron micrographs of the same spectrometer run, magnification being calibrated simultaneously with latex. During all manipulations of the aerosol, care was taken to avoid any humidification. The results for one run are given in Fig. 1. The deposit on the spectrometer foil, which is a narrow line of a few millimeters, was photographed in equidistant
5 055r
ILLli 13
I1
12
FIG. 1. Comparison obtained from electron aerodynamic diameters density 2.165).
I1
1
J
7 10 9 8 mm LOC.of DEPOSl T
of geometric particle microscope (triangles) (solid line, calculated
diameters and from for bulk
3.59 Copyright All rights
@ 1977 by Academic Press, Inc. of reproduction in any form reserved.
Jonrxal
of Colloid
aed Interface
S&me,
Vol. 62, No. 2, November 1977 ISSN 0021-9797
360
NOTES
Reported large density variations of NaCl aerosol in one and the same sample seem to be typical for certain models of generators. They could be explained by a ‘%uffy” structure of particles which may result from the special cooling rate of the condensing vapor. The author has noticed that furnaces with only one long heating zone and gentle slopes in temperature gradient tend to produce particles of unstable density or irregular shape. The generator used here had a marked temperature drop at the exits of the heating zones, particles were always rigorously spherical with no cavities, according to density measurements. ACKNOWLEDGMENT This work R 801 983.
was partly
supported
by EPA
Grant
Jozmal
of
M.,
Co&id
Ah.
Cnlloid
M~TIJIWIE, E., ESPXNSCHEID,TV. F., AND KERKER,
3.
ESPENSCHEID,W. I?., MATIJEVIE, E., AND KERKER,
M., J. C&id M.,
18, 91 (1963).
Clzem. 68, 2831 (1964).
MATTESON, M. J., Fox, J. J., AND PREINING, O., Nature Pltys. Sci. 238, 61 (1972). 5. ARLCBRUSTER,L., STAHLHOFEN,W., AND GEBHART, J., in “Tagungsbericht Ges. f. Aerosolforschung, Bad Soden/BRD,” (V. Bohlau and H. Straubel, Ed.), GAF Publ., 1976. 6. ROPPERT, H., AND BERNER, A., Staub Rein/a. der Luft 29, 92 (1969). i'. ABED-NAVANDI, M., BERNER, A., A~W PREINING, O., ilz “Fine Particles” (B. Y. H. Liu, Ed.) p. 447, Academic Press, New York, 1976.
GERHARD KASPER 1st Physics Institute University of Vienna 1090 Vienna, Austria
Interface
Science,
J. Phys.
Sci.
4.
Sci.
7, 107 ff Received
and Interface
Interfhe
No.
REFERENCES 1. KERKER, (1975).
2.
Vol. 62, No. 2, November 1977
May
11, 1977;
accepted
May
13, 1977