Relative Oxygen Absorption and Volatility Properties of Submicron Films of Asphalt Using the Quartz Crystal Microbalance W. H. King, Jr., and L. W. Corbett Esso Research and Engineering Co., Linden, N.J. 07036 The use of the quartz crystal microbalance for measuring relative oxygen uptake or absorption in submicron films of asphalt has proved to be a sensitive and reliable tool. It has shown that polar-aromatics absorb oxygen very quickly while asphaltenes absorb at a slow but prolonged rate. Saturates and naphthene-aromatics show virtually no absorption of oxygen but are relatively much more volatile. This is consistent with previous findings that relate the chemical and physical character of these components with their general resistance to oxidative type conditions. Significant differences between total asphalts can be shown, and it appears that they can be rated according to their oxygen absorption properties.
IT IS GENERALLY AGREED that oxygen plays an important role in most asphalt hardening reactions, and that this hardening is brought about by oxygen uptake through absorption or oxidative type reactions. The primary effect is an increase in asphalt viscosity or stiffness with subsequent loss of ductility and flexibility. This in turn influences the cohesive and flexibility properties ofasphal-aggregate mixtures, which then is reflected in pavement service qualities. References ( I ) through (7) provide helpful background on the specific effect of temperature, film thickness, oxygen rate, and hydrocarbon character on asphalt hardening. In addition, over 70 other general literature references have been noted that report on hardening in service, indicating considerable interest in this field. This report describes the use Of the quartz crystal microbalance for measuring the absorption and/or volatility of various asphalt paving binders and the generic fractions from one of these binders. The method is an adaption of that developed by Fischer and King (8) for the measurement of oxygen stability of elastomers. It is based on the use of a piezoelectric quartz crystal on which is deposited a 0.3-micron film of the material to be tested. When electrically excited, the crystal vibrates at a high frequency in direct proportion to the mass of the film. Changes in the mass of the film, caused by the uptake of oxygen or by its volatilization, are then detected and recorded by measuring the change in oscillation frequency. EXPERIMENTAL
Apparatus. Piezoelectric quartz crystals function as very sensitive microbalances. The use of these for microweighing was proposed by Sauerbrey (9), while King (IO) showed that (1) J. Ph. Pfeiffer, "The Properties of Asphaltic Bitumen," Elsevier Publ. Co., New York, 1950. (2) J. W. A. Labout and W. P. Van Oort, ACS Petroleum Division, Sept. 1954. (3) W. P. Van Oort, Ind. Eng. Chem., 48, 1196 (1956). (4) Blokker and Van Hoorn, 5th World Pet. Congr., June (1959). (5) J. Knotnerus, Jr., Inst. Pet., 42, 355 (1956). (6) R. G. Larsen, R. E. Thorpe, F. A. Amfield, Ind. Eng. Chem., 34, 183 (1942). (7) E. R. Booser and M. R. Fenske, ibid., 44, 1850 (1952). (8) W. F. Fischer and W. H. King, Jr., ANAL.CHEM.,39, 1265 I 1 9677 \._".,.
(9) G. Z. Sauerbrey, Physik, 155, 206 (1959). (10) W. H. King, J~.,ANAL. C H E M . ,1735(1964), ~~, U.S.P. 3,164,004. 580 * ANALYTICAL CHEMISTRY
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Devices using the vibrating crystal owe their high degree of sensitivity to the high stability and small size of the crystal vibrator and also for the ability to measure frequency changes at a high level of accuracy. The crystal used here gives a 230-cycle-per-second (cps) signal per microgram of coating in which the following Equation (8) relates frequency change to mass change:
where AF is the frequency change in cps caused by mass change A W, F is the frequency of vibration in megahertz, A is the area (cmz) of the coated electrode, Wis the rnass(grams) of the sample film. Frequency is measured during the test, and the per cent change in weight of the sample calculated as foll@ws:
% Wt Change
=
Change in Frequency in ips X 100 (2) Frequency in cps due to coating
As pictured in Figure 1, the crystal plate is a 10-megacycle AT cut 0.5 X 0.5-inch square by 0.007-inch thick, with gold electrodes on each side of the crystal, each covered with nickel by electrodeposition. The crystal cover shown is a temporary one used to protect the crystal and its coating before being installed in the test cell. The detector and test reference crystals are mounted in a 2- x &inch brass cell as shown in the flow control and electrical scheme Figures 2 and 3. The cell is mounted on oscillator circuits (TRO-2, modified Pierce Oscillator), which are connected to a mixer that feeds the frequency meter and thence a recorder. The specimen cell is closely regulated in temperature to 150 i 0.15 "C with a proportional controller and is equipped to receive either nitrogen or oxygen at 150 psig. The rate of exit gas is regulated to 500 ml per minute at room conditions, using a soap film meter.
Procedure. The crystal electrode is first coated with the asphalt or asphalt fraction to be tested by spraying as 2% solution (in benzene) until a coating equivalent to 5000 rt 20 cycles is obtained. An artist's spray gun (Pasche) operating at 10 psig was found ideal for this work, spraying an equal amount on each side of the crystal. The coated crystal and an uncoated reference crystal, regulated to the same frequency, are then placed in the cell, pressurized with nitrogen to 150 psig and brought to 150 "C with a gas purge of 440 standard cc per minute. Earlier work had shown that at lower pressures and lower temperatures, the oxygen uptake is quite slow, thus making the test undesirably long. Time is important because the more volatile components are continually evaporating during the test. The use of 150 "Cpermitted some comparison with the elastomer work cited above; also, it is the approximate temperature at which most asphalt-aggregate hot mixes are made. As soon as the test conditions above stabilized, frequency measurements were taken and frequency change (loss or gain) cs. time was recorded. Figures 4 and 5 have been reproduced from the operating charts on a percentage weight change per minute basis. The average weight loss was calculated as the average of the loss lines under nitrogen before and after the introduction of oxygen. The weight gain or oxygen uptake was then measured as the average tendency to show gain plus the volatile loss that had to be overcome. This assumes that the same amount of volatile loss will take place under oxygen as under nitrogen. Although this may not be strictly true, the conditions of time, pressure, and gas rate were closely duplicated in order to maintain the relative nature of the test. RESULTS AND DISCUSSION
Drawing upon the experience with elastomers when using this method, it was believed that the relative oxygen stability
150 PSIG
FLOW CONTROL VALVE
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Figure 2. Flow control scheme for the quartz crystal microbalance of asphalts or asphalt fractions could be determined. It was soon found, however, that some modification of the procedure was necessary because of the inherent differences between elastomers and asphalt, particularly the spray application of the coating and the regulation of the gas pressure and flow rate
FOR COUNTER
2.2 K MIXER
meg
Electrical c i r c u i t for quartz crystal microbalance showing T R O - 2 oscillator circuits, mixer and the connection to the HP 5 0 0 Hewlet-Packard frequency meter. Temperature i s controlled separately by a 2 4 0 - M F & M Scientific Inc. Temperature Controller.
-Figure 3. Crystal oscillator drawing VOL. 41, NO. 4, A P R I L 1969
581
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ASPHALTENES
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Figure 4. Gain-loss relationship of asphalt fractions measured by quartz crystal microbalance
Figure 5. Gain-loss relationship of asphalts measured by quartz crystal microbalance
during the measurements. A higher pressure was found necessary in order to reduce volatility effects, and increased flow rates was used to speed up oxygen uptake. The test was then applied to each of the four generic fractions (11) of asphalt which are known to be distinctly different in their average chemical structure. After differences between them were found, the test was then applied to total paving binders of approximately the same consistency but from different crude sources. This provided us with background on the susceptibility to oxygen of each hydrocarbon type found in asphalt as well as a means for comparing asphalts from different crude sources. Referring to Figure 4 we find that each of the asphalt fractions respond differently under the same conditions of exposure to nitrogen and oxygen. All materials show an initial evaporative weight loss under nitrogen (downward slope) which tends to diminish as the run progresses. Upon intro-
duction of oxygen, all materials (except the saturates) showed some measure of weight or oxygen uptake (upward slope), tending to reverse the loss trend. The saturates display relatively high weight loss rates under nitrogen, whereas asphaltenes are relatively low, with the other fractions intermediate. Because there was a consistent directional loss under both nitrogen and oxygen in the case of the saturates, it was assumed that they did not absorb oxygen. Naphthene-aromatics showed only a minor oxygen uptake, and the polar-aromatics showed a great deal of uptake rather quickly followed by an early reversal toward the weight loss direction. Asphaltenes showed a relatively slow oxygen uptake with a delayed tendency for reversal. Tests on asphaltenes were discontinued after an extended exposure without much indication of a return to the loss direction, as experienced with the other fractions. A careful study of repeat asphaltenes curves showed a consistent tendency under oxygen to gain weight for a prolonged period. This is oxygen uptake but at a relatively slow rate. The data in Table I reflect average weight change per minute over the
(11) L. W. Corbett, ANAL.CHEM.,41,576 (1969).
Table I. Quartz Crystal Microbalance Measurements on Asphalt and Asphalt Generic Fractions Made at 150 OC in Submicron Films under Nitrogen and Oxygen at 500 ml/min and 150 psig
% Wt change per min Run No. 17 21 22 23 19 26 27
38 39 19 20 29 30 32 34 36 37
40 41
582
Fraction Saturates Saturates N-Aromatics N-Aromatics P-Aromatics P-Aromatics P-Aromatics Asphaltenes Asphaltenes Asphalt A Asphalt A Asphalt B Asphalt B Asphalt B Asphalt B Asphalt B Asphalt B Asphalt C Asphalt C
ANALYTICAL CHEMISTRY
Loss
Total gain
0.275 0.335
Gain 0 0
0
7
0
1
0.197 0.167
0.032 0.030
0.229 0.197
0.037 0.049 0.045
0.052 0.016 0.039
0.089
0.0035 0.0032
0.0128 0.0130
0.016 0.016
0.042 0.041
0.051 0.071
0.093 0.112
0.023 0.029 0.021 0.034 0.030 0.036
0.022 0.012 0.024 0.008 0.012 0.009
0.045 0.041 0.045 0.042 0.042 0.045
0.028 0.037
0.004 0.008
0.034 0.035
1' ] ]
1 J
Average % wt change per min Loss Gain 0.305 0 0.182
0.031
0.044
0.079
0.003
0.016
0.042
0.102
0.029
0.043
0.033
0.039
Polar-aromatics are the quickest to absorb oxygen with asphaltenes probably absorbing more but at a much slower rate. Saturates appear to be unaffected by oxygen under these conditions and naphthene-aromatics only slightly so. This is consistent with earlier observations (12) that polararomatics are the first to convert to asphaltenes under airblowing conditions, while the saturates are virtually unaffected.
Figure 5 is a comparison of total asphalts from three different Near East crude sources. Asphalt A in this case showed the greatest loss under nitrogen as well as the greatest degree of oxygen uptake. In comparison with this, Asphalt C showed the least volatility and least oxygen uptake. Asphalt B was repeat tested six times giving a coefficient of variation of 21 ”/, on loss and 5 on total gain under oxygen. This is based on all of the data as reported in Table I. The well-known Student “t” test was applied to loss and gain data on the three total bitumen from which it was found that there is no significant difference between the gain for Asphalts B and C. However, there is a significant difference at the 9 5 z probability level between the total gain data on Asphalt A cs. either Asphalt B or C . From this work we conclude that: The method shows excellent repeatability and sensitivity as a means for measuring the relative oxygen absorption at 150 “C in very thin films. Asphalt A absorbs oxygen at a significantly higher rate than either Asphalt B or C under the conditions of this test. Based on the work reported here, we recommend this method and technique for use as a means of detecting relative differences in the oxygen absorption properties of paving binders or their modified compositions.
(12) L. W. Corbett and R. E. Swarbrick, AAPT, 29, 104 (1960).
RECEIVED for review October 30, 1968. Accepted January 7, 1969.
arbitrary 14-minute exposure period used for comparing the four fractions. Thus, it appears that asphaltenes will take up more oxygen in the long run although its initial rate of uptake is less. This does not invalidate findings of others that asphaltenes are highly susceptible to oxygen absorption; it only indicates that the polar aromatics in these thin films will absorb relatively more initially, but less over a longer period. In general, this seems to correlate well with the chemistry of these components summed up as follows: Saturates are the most volatile with naphthene-aromatics next and asphaltenes by far the least, Directionally this is consistent with average molecular weight measurements on these fractions and also with gel-permeation analyses which related the molecular sizes in the same order.
Mechanism of Trace Counterion Transport through Ion-Exchange Membranes W. J. Blaedel, T. J. Haupert,’ and M. A. Evenson2 Cheinistry Department, Unicersitv of Wisconsin, Madison, Wis.53706 An equation for the flux of a trace counterion between two solutions separated by an ion-exchange membrane is derived and experimentallyverified. The flux equation is based on a mechanism that considers the heterogeneous surface exchange reactions in addition to the diffusional processes occurring within the membrane and within the stationary liquid films adjacent to the membrane surfaces. The equation is tested using both synthetic anion- and cation-exchange membranes for systems containing singly and multiply charged trace and bulk ions. Experimental evidence is also presented to indicate the influence of the surface exchange reaction on the overall rate of ion transport.
INCREASED APPLICATIONS of ion-exchange membranes to analytical problems have led to an increased interest in their fundamental properties and mode of operation. Of special interest is the transport rate of a trace counterion across an ion-exchange membrane separating two solutions that also contain another electrolyte at much greater concentrations. The analytical promise of such systems in the separation and concentration of trace ions has been shown (1, 2 ) . However, ‘Present address, Chemistry Department, University of Arizona, Tucson, Ariz. 85721 ZPresent address, University Hospitals, University of Wisconsin, Madison, Wis. 53706 (1) W. J. Blaedel and T.J. Haupert, ANAL.CHEM..38, 1305 (1966). (2) W. J. Blaedel and E. L. Christensen, [bid., 39, 1262 (1967).
the efficiency of such applications depends greatly upon the rate of ionic transport between the two solutions. The present study was undertaken to obtain a quantitative understanding of the factors that influence the rate of such transport. Three general approaches have been used previously to investigate ion-exchange kinetics. These include application of the Nernst-Planck flux equation, the theory of irreversible thermodynamics, and the transition theory. These have been recently reviewed (3, 4 ) . The present approach applies the steady-state principle to a system of simultaneous rate processes to give a set of flux equations, one for each step in the transport mechanism. The set of flux equations is then solved far the overall counterion flux. Previous studies of ion-exchange kinetics consider the ion transport process to be diffusion controlled either within the membrane or within the external liquid phase (5, 4). In this paper, the influence of the heterogeneous ion-exchange reactions that occur at each solution-membrane interface is also considered. Although the possibility of rate control in ion-exchange processes by the surface exchange reaction has
__
(3) N. Lakshminarayanaiah, Chem. Rev., 65, 491 (1965). (4)G. Eisenman, J. P. Sandblom, and 3. L. Walker, Jr., Scienw, 155, 965 (1967). (5) M. A. Peterson and H. P. Gregor, J. Electrochem. SOC., 106, 1051 (1959). (6) F. Helfferich, “Ion Exchange,” McGraw-Hill. New York. N.Y.. 1962, pp 345-53. VOL. 41, NO. 4, APRIL 1969
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