1170
Langmuir 1989,5, 1170-1175
Surface Ionization at Atmospheric Pressure. 2. Particles of Inorganic Alkali Salts and of Sodium Dodecyl Sulfate Leif Holmlid* and Staffan Wall Department of Physical Chemistry GU, University of Goteborg and Chalmers University of Technology, S-412 96 Goteborg, Sweden Received December 16, 1988. In Final Form: April 14, 1989 The surface ionization of alkali aerosol particles with approximate diameters of 4-200 nm, impinging on a platinum surface with a temperature between 1100 and 1500 K, is studied at atmospheric pressure. Particles of three different alkali salts are studied, NaCl, CsN03, and sodium dodecyl sulfate (SDS). It has been shown in our previous studies that just a thin layer of the particles is melted and deposited on the hot surface. This layer gives rise to the signal current. SDS forms micelles in the water solution and has its sodium end at the surface of the micelles. It is shown that the surface temperature has a great influence on the characteristicsof the ionization data, which are usually obtained as ion signals as a function of the applied voltage. The data found for different particle sizes (solution concentrations)can be transformed to be of identical shape, within solution concentration regions of at least a decade for each salt. The signal varies after transformation as c”, where c is the solution concentration and n a parameter such that 0.8 < n < 1.2. At large voltages, above the signal maximum, which appears at a variable voltage in the range 50-200 V, the signal decreases proportional to the inverse voltage, indicating a scattering process at the surface.
Introduction The surface ionization of alkali salt aerosol particles has been studied in our group1I2 in order to understand the complex behavior of a particle in contact with a surface. The initial motivation for our studies was to use the atmospheric pressure surface ionization of alkali salt particles as a detector for alkali salts in the atmosphere, much in the same way as the surface ionization method has been used in other applications as a simple detecting device. For example, surface ionization has been used as a detector of particles in exhaust gases from power station^.^ However, the nonlinearity of the signals showed that it should be possible to use the quite simple experimental device to investigate the shape and surface characteristics of particles in the size range of a few nanometers. After our initial study of the effect,‘ we have proceeded to verify more strictly the thickness of the layer which is melted off the particles in contact with the hot surface.2 In the present study, we investigate the mechanisms which give the variable shape of the ion signals as a function of the applied voltage and demonstrate the effects of the surface temperature, the gas temperature, and the applied voltage in more detail to give the information needed for a more complete theoretical description of the particle melting and scattering processes. Alkali aerosol particles are of interest in various fields. The alkali salts constitute the main part of the natural aerosol formed above the sea, which is the largest source of aerosols on earth.4 The transport of various substances from the surface of the sea, including both organic material and living organisms, and their transport and deposition are important in several r e ~ p e d s .The ~ ~ surface ~ ionization method could provide the ionization principle for the use of mass spectrometry to study such aerosols. Recent studies of the role of sea spray in photosmog reactions are (1)Andersson, L.; Olsson, J.; Holmlid, L. Langmuir 1986,2, 594. (2)Holmlid, L.; Wall, S. J . Aerosol Sci. 1988,19, 1219. (3) (a) Zarchy, A. S. 5th Symposium Instrumentation and Control of Fossil Fuel Energy Processes (1981),San Fransisco, 1981. (b) In-stack particle detector, “SIMP-l”, from Extranuclear Laboratories Inc., Pittsburgh, PA. (4) Brunner, C. R. Hazardous Air Emissions from Incineration; Chapman and Hall London, 1985. (5)Clarke, A. D.J. Aerosol Sci. 1988,19,1195. (6)Covert, D.S.; Quinn, P.; Bates, T. Book of Abstracts, European Aerosol Conf. Lund, Sweden, 1988;p 83.
0743-7463/89/2405-1170$01.50/0
of great interest in this respect.’ Potentially harmful alkali salt aerosols may also be formed in industrial processes,8 where this simple ionization device could be useful. Several processes also employ alkali salt particles for their operation, e.g., a magnetohydrodynamic power g e n e r a t ~ r . ~
Experimental Section The experimental apparatus was similar to the one described in ref 1. Some changes were made, especially concerning the voltage applied to the hot Pt wire, which could now be changed between 0 and 500 V. The aerosol is formed from salt solutions in a nebulizer driven by compressed air at 2-bar pressure and a flow rate of 5-6 dm3/min. The nebulizer was originally intended for atomic absorption studies (Perkin-Elmer). The sample solution is sucked into the nebulizer at a liquid flow rate of 1-5 cm3/min. The aerosol from the nebulizer is forced to swirl around in a small volume, where the largest part of the sample solution was collected as a liquid. A fraction of the aerosol, mainly containing small particles which can survive the “swirl chamber”,is sucked at a rate of 100 cm3/min into a small volume where the ionizing hot wire is mounted. It is a Pt wire, with a djameter of 0.25 mm, which is formed to a short spiral. A cylindric collector surrounds the wire. The particles in the aerosol which are negatively charged are driven over to the hot wire, at a positive voltage. Before they reach the hot surface, they normally dry in the warm air. A small part of each particle is melted off in contact with the hot surface, and the salt is dissociated on the surface. The alkali atoms are then surface ionized, desorbed, and collected by the collector at ground potential. Theory The theoretical description of the transport of the particles to the hot wire given in ref 1is applicable here as well. The description of the collection and transport given there says that the signal current flowing from the surface to the collector is I a niq2P/r4 (1) where ni is the number of alkali ions produced by a particle, q is the charge of a particle, U is the applied voltage between the wire and the collector, and r is the radius of (7) Zetzsch, C.; Pfahler, G.; Behnke, W. J.Aerosol Sci. 1988,19,1203. (8)Wangwongwatana, S.;Scarpino, P.; Willeke, K. J. Aerosol Sci. 1988,19,947. (9)Angrist, A. W. Direct Energy Conversion; McGraw-Hill: New York, 1968.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 5, 1989 1171
Surface Ionization of Particles the particle, assumed to be spherical. As shown already in ref 1, the signal at low voltages is independent of the solution concentration, Le., the particle size. This can agree with eq 1if q = r2 and if simultaneously ni is a constant. The first assumption is reasonable, since the charge density on the particle surface is likely to be constant, as discussed in ref 1. The second assumption is more difficult to defend, but models do exist where it is natural. Especially, the use of fractal descriptions of the particle surfaces bears great promise in containing a useful model of the particle^.^ The results so far, however, do not make it possible to draw any strict conclusions about the true nature of the condition ni = constant. When a particle approaches the hot surface, it has a certain drift velocity u = (qE/3Nm~)(2am/kT)l/~
(2)
where E is the electric field strength corresponding to the voltage U ,N the gas density, m the mass of a gas molecule, and u the (diffusion) cross section for collision between a particle and a gas molecule. When the condition q = r2 and also the first-order approximation u = ar2 are introduced into eq 2, one finds that u is independent of c, the concentration of the solution, as
(E/h')(mkT)-ll2 (3) Each particle also has a thermal velocity u with random direction added. The average component along the surface normal for the particles striking the surface is u = ( kT / (2aM))'I2 (4) u
0:
For very low voltages U , the drift velocity u will be small compared to the thermal velocity, which means that particles will impinge on the hot surface with almost equal probability from all directions. At larger voltages, the drift velocity will be larger than the thermal velocity, and thus particles will impinge on the surface more concentrated around the surface normal. At low impinging velocities, the particles will melt partially on the surface and leave the surface due to transfer of energy and momentum from the surface and also due to the pressure exerted by the vaporized salt. This type of process is difficult to describe theoretically; nevertheless, it determines to a large extent the signal shape at the maximum in the I-U plots. A t somewhat larger velocities, the duration of the interaction will be determined by the drift velocity itself, since the particle will be reflected from the surface more rapidly. In this case, we expect a signal which decreases with increasing voltage U. The signal should be proportional to the time in contact with the surface, i.e., inversely proportional to the drift velocity and thus also to the applied voltage, as shown in eq 2. Thus I a ( 3 N m u / q E ) ( k T / 2 ~ m )0l:/ l~/ U (5) is valid in this velocity range. With the same relations included as in eq 3 above, one finds I = (N/E)(mkT)'I2
(6)
Results To investigate the interaction between the particles and the hot Pt surface better, we have used a larger variety of experimental conditions in the present experiments than in ref 1. Two inorganic salts, which behaved well but not identically in the previous experiments, NaCl and CsN03, were studied in several experiments using solution concentrations of 0.01-10 mM. One organic salt, sodium dodecyl sulfate (SDS), was also studied due to the fact that
1100 K
CsNO,
1
10
100
200
300
400
u (VI Figure 1. Temperature variation of the signal found with particles from a solution of 1m M CsN03. The applied voltage is plotted along the horizontal axis. Surface temperature is shown as the parameter. it is known to form micelles in solutions. The critical micelle-forming concentration for SDS, above which micelles exist in the water solution, is 8.5 mM. This concentration is reached rapidly during the drying of the aerosol in the hot air surrounding the hot wire. In the SDS particles, it is expected that the Na end of the long molecules is at the outer surface. The final particles formed during the drying stage of the particle formation may, however, have a rather complicated shape, as conglomerates of smaller particles, but the particles are not homogeneous concerning the sodium atoms. Thus, only a thin layer at the surface can contribute to the signal measured for SDS, while the other homogeneous salt particles may give melting and subsequent ionization of a volume, maybe in the form of a spherical segment. For each of the three salts, some typical results are shown in the figures. In all cases, the independent variable, plotted along the x axis of the figures, is the voltage applied or some quantity derived from the voltage. The measured signal current was reduced to I / c in all figures, i.e., the measured current divided by the solution concentration fed to the nebulizer. In this way, the curves are not superimposed on each other in the figures, which they would otherwise be to a large extent. The results presented here are mainly from two types of experiment: (1)those with surface temperature as the parameter, varying between 1100 and 1500 K at a constant solution concentration of 1mM, and ( 2 ) those with solution concentration as the parameter in the range 0.01-10 mM, at a constant surface temperature around 1500 K. The results in Figures 1-6 belong to the first type of experiment, while Figures 7-15 present data from the second type of experiment. Some further examples of the results are given for very small particles in Figures 16-18. Discussion Temperature Variation. The temperature variation of the signals, in Figures 1-6, shows large differences for the three salts. For &NO3, the maxima in the curves are at quite high voltages, and the signal is the highest for all three salts. This indicates that the melting process on the surface is simplified by the increased translational velocity of the particles. The maxima in the curves appear at much lower voltage for SDS than for the two other salts. This means that kinetic energy does not promote melting strongly, as it does for the two other salts. This is understandable, since only one layer will be melted away on the SDS particles: and the energy required to bring the particle in close enough contact with the surface for such
1172 Langmuir, Vol. 5, No. 5, 1989
Holmlid and Wall NaCl
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1500 K
-
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o (
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Surface temperature shown as parameter.
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0
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versus the applied voltage. Surface temperature as parameter.
Figure 3. Temperature variation of the signal found by variation of the applied voltage. A 1 mM solution of SDS was used to
-1
440
Figure 5. Square root of the signal from 1mM solution of NaC1,
j
0
200
1 4
U(V)
Figure 4. Square root of the signal for CsNOaat 1 mM, versus the applied voltage. Surface temperature as parameter.
an event is apparently rather low. The increase in signal from 1300 to 1500 K is quite small for SDS compared to NaC1. This is to be expected if the SDS particles have only one layer of Na at their surface, while the other salt particles are homogeneous. In Figures 4-6 the data are replotted for the same experiments as in Figures 1-3. The plots of ( I / c ) ~versus /~ voltage will display a quadratic voltage dependence as a linear plot. For CsN03, the dependence is linear in the figure, i.e., quadratic in voltage. This behavior is almost unchanged when the temperature is varied and agrees with eq 1. For NaCl and SDS, the curves measured at 1100 K
U(V)
Figure 6. Square root of the signal from 1mM solution of SDS, plotted against the applied voltage. The surface temperature is shown as parameter. Note that the horizontal scale is different than in Figures 4 and 5. are linear at small U up to 30 V and in fact at larger signal levels than the high-temperature curves, but then they start to increase very slowly with voltage. No real maxima are found in these two curves. This is shown very clearly in Figure 6. This unusual behavior is attributed to not completely dried particles; even a t very low voltage, the kinetic energy is large enough to coalesce the particle in contact with the surface. With increasing voltage, the scattering region at voltages above the maximum is never reached, since the fluid particles do not scatter on the surface. The low boiling point of the liquid particle, however, leads to large losses of material from the surface, and thus the signal is small and almost independent of voltage above 30 V. This explains most of the findings for the sodium salts at 1100 K. Low-Voltage Behavior. The signal varies approximately as the square of the applied voltage. This was demonstrated in ref 1. One example of an excellent quadratic behavior over a large concentration range is seen for &NO3 in Figure 7. In many cases, however, departures from this exist, as the example in Figure 8 shows for the case of NaCl at 1500 K. Similar behavior is seen in Figure 6 for SDS. The low signal levels at low voltage indicate that almost no collection of particles by the field exists there. This means that the drift velocity is too small, as derived in ref 1. The drift velocity of the particles decreases with temperature, as shown in eq 2, while their thermal velocity increases. In the case of Figure 8, the most likely reason for the low collection efficiency at low voltages is the high temperature in the gas surrounding
Langmuir, Vol. 5, No. 5, 1989 1173
Surface Ionization of Particles 3
9
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Figure 7. Square root of the signal from solutions of &NO3,
plotted against the applied voltage. Solution concentration is the parameter in mM. Surface temperature was approximately 1500
Figure 9. Signal measured with &NO3 solutions at a surface temperature of 1500 K plotted versus the inverted voltage. Solution concentration is shown in mM.
K.
2
11
,
I
0
0.02
0.04
liU 0
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Figure 8. Square root of the signal obtained by using NaCl
solutions versus the applied voltage. Parameter shown is the solution concentration in mM. Surface temperature was 1500 K.
the hot wire. This conclusion is supported by the observation that in other runs at lower surface temperature NaCl shows a quadratic dependence on voltage. High-Voltage Behavior. With increasing voltage, the signal currents normally decrease more or less rapidly. As described above, it is possible that the signal can depend on 1/U, i.e., the inverted voltage. In some cases this is true, but it is by no means a general effect. In Figures 9 and 10, this is exemplified for CsN03 and SDS at 1500 K. In such cases, the linear dependence extrapolates to a finite non-zero signal when the voltage goes to infinity. This should mean that a definite partial melting takes place at the surface independent of the particle velocity at high temperature. At lower surface temperatures, the plots are not linear in 1/U, but instead they curve and approach zero at infinite voltage, indicating that the particles will scatter without any melting at high voltages. Partial Melting of Particles. The measured current for NaCl is a factor of 10 larger than that for SDS in most comparable cases, as shown in Figures 2 and 3. It is reasonable to assume that one layer melts for SDS, since the sodium atoms occupy the surface in micelles and similar particles formed by SDS in water. This leads to the conclusion that around 10 monolayers melt on the NaCl particles when they impinge on the hot surface in the experiments.2 The currents from CsN03are approximately a factor of 2 larger, which couId indicate that around 20 layers melt on such particles. A particle from a 0.01 mM solution should contain approximately lo00 molecules and
(V-1)
Figure 10. Signal measured with SDS solutions plotted versus the inverted voltage. Surface temperature was 1500 K. Solution concentration is shown in mM.
thus around 10 spherical monoatomic layers. This solution concentration is the absolute minimum for which consistent behavior has been found and then only for CsN03 particles. This conclusion is, however, complicated by the fact that the SDS particles probably are not simply spherical, since they may have the form of a conglomeration of spherical micelles and maybe also rod-like aggregates of molecules. The experimental data for SDS, however, do not indicate any complications. The results given in ref 2 and in Figures 2,6,10,14, and 15 show a very regular behavior. It is probably too early to draw any strict conclusions about the shape of the SDS particles from these results, but the conclusion concerning the thickness of the layers melted on the two other salts in comparison with SDS is quite certain. Generalized Behavior of Particles. In ref 1, we proposed a reduction of the data found at constant temperature with concentration as the parameter by plotting the signal versus U / C ~ This / ~ . was shown to work well for NaCl at a temperature of 1300 K. By this means, the signals for different solution concentrations fall into a common form. When this scheme, in modified form allowing for exponents other than 112, is applied to the present data, very interesting results are found. For all three salts, a similar behavior is found: the curves reduce to a common form, with slight shifts depending on the solution concentration. Examples are shown in Figures 11-15. For CsN03 at 1500 K, Figure 11, the signal varies as I n: c1.2, in the concentration range 0.1-1 mM. At 1300
1174 Langmuir, Vol. 5, No. 5, 1989
Holmlid and Wall
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Figure 14. Transformed signal plota for SDS at a surface temperature of 1500 K. Solution concentration is shown as the parameter in mM. Compare with Figure 15.
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Figure 13. Transformed signal plota for NaCl at a surface temperature of 1500 K. Solution concentration in mM is shown as parameter.
K, the variation with concentration is small, and the different curves reduce to parts of the common curve, as seen in Figure 12. Thus, the variation of the signal is closely proportional to c in this reduced form. For NaCl, the data are not as good as for CsN03, but in Figure 13 it is demonstrated that at 1500 K the concentration range 0.2-2 mM gives almost perfectly coinciding curves (except for 0.5 mM, which apparently was measured under somewhat different conditions). This shows a linear variation of the signal with concentration, for a somewhat different voltage
' (a.u.)
Figure 15. Transformed signal plots for SDS at a surface temperature of 1500 K. Solution concentration in mM is the parameter shown. Compare with Figure 14. normalization. Finally, SDS is the best example of the reduced representation, interestingly enough. The simple form of the curves in this case provides further support for the assumption that just one Na layer exists on the surface of the particles of SDS. In Figure 14, the curves for 1-10 mM agree very well, while in Figure 15 the same behavior is demonstrated for 0.2-2 mM. In the first case, the signal varies linearly with concentration, while in the second case (Figure 15) the variation of the signal is proportional to c0.*. The analysis thus shows that both a linear variation, I 0~ c, and nonlinear variations, I cn (where 0.8 < n < 1.2), can be found. It is now obvious that the value of the exponent in this formula varies with surface temperature, which was not known in ref 1,and that the melted fraction of each particle also varies with surface temperature. It can be shown that the volume of a spherical segment is approximatelyrh2,where r is the radius of the particle and h is the height of the segment. If a relation between the radius and the height h exists such that h 0: ?I3, the volume in the melted segment varies as c0.78 (cf. ref 1). If, instead, the height of the segment varies as #I3, the volume The nonlinear signal of the melted segment varies as variation with concentration is thus probably caused by a nonlinear variation of the melted layer thickness. Small Particles. Of special interest are the signals found for very low solution concentrations, since it is anticipated that extremely small particles will melt almost completely on the hot surface. It is estimated, on the basis of calculations and electron microscopy studies,' that the Q:
0 . 0
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Langmuir, Vol. 5, No. 5, 1989 1175
Surface Ionization of Particles w
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Fmre 16. Signals for CsN03,at very low solution concentrations,
given in m M as the parameter. Surface temperature was 1500 K.
200
4w
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Figure 18. Signal for SDS at very low solution concentrations versus the applied voltage. Surface temperature was 1500 K. Concentrations are shown in mM as the parameter.
signals tend to have their maxima at very large voltages or appear flat, independent of the concentration. This could indicate that the particles are very soft and almost liquid, maybe by retaining some water in their structure. A t large concentrations, the peaks of the SDS signals increase toward lower Concentrations, as seen in Figures 10 and 14. This is expected, since the amount of material in a two-dimensional layer (“cap”), which is the form of the melted volume in this case, varies slower than the cohcentration. However, we observe no decrease in the peak of If c with decreasing concentration, as described for CsN03 above. 1 0
0.02
0.04
liU
(V-1)
Figure 17. Same data as in Figure 16 replotted against inverted
voltage.
size of a particle from a 0.01 mM solution of NaCl or CsN03 would have a size of 4-15 nm. This is based on an average water droplet size of 0.6 pm, which was also used in the calculations of particle sizes in ref 1. The more complete melting of small particles would be seen as a transition to a behavior where the peak of the reduced signal If c is no longer independent of the concentration. A hint of this expected behavior is seen for NaC1, e.g., in Figure 13. The case is even clearer for &NO3 in Figures 16 and 17. The same data are also shown in Figure 12. There is no large change otherwise in these results but this gradual decline in the signal when the particles become smaller. The behavior of SDS particles at low concentrations is of interest, since the size of a SDS micelle in solution is 3-5 nm. However, we do not reach this small particle size in our experiments with SDS, since the molecules are large, and the number of molecules in one particle at 0.01 mM solution concentration is approximately 1000, as opposed to a number close to 70 found in one micelle. There is also a possibility that the larger particles break down into smaller fragments, maybe similar to a micelle in the solution. The results for SDS at concentrations smaller than 0.1 mM are somewhat noisy, due to the small signals found in this case, as seen in Figure 18. It is notable that the
Conclusions To summarize, the most important conclusions from this work are as follows: 1. Data taken with different solution concentrations can be transformed such that the curves have almost identical shapes. After transformation, the signal varies with concentration as cn, where c is the solution concentration and n is a parameter which varies between 0.8 and 1.2 in the temperature range 1300-1500 K. 2. At high voltages, the signal decreases proportional to the inverted voltage, since the time in contact with the surface decreases, before the particle scatters from the surface. 3. The surface temperature influences the results strongly. At sufficiently low temperatures, the particles do not dry before impact on the detector wire. 4. High surface temperature can cause a decreased collection efficiency at low applied voltages, since the thermal velocity of the particles becomes too large. 5. No special “ s m d particle” effects are observed except for a gradual decline of the signal levels with decreasing solution concentration, indicating that a large part of each particle is melted before scattering. Acknowledgment. This study was supported by grants from the Swedish Natural Science Research Council. We thank Lars Andersson, who helped to get the experiments going, and Doris Lonn, who made all the measurements with considerable skill. Registry No. SDS, 151-21-3;NaC1,7647-14-5;CsN03, 778918-6.
