I n d . E n g . Chem. Res. 1989, 28, 1231-1236
1231
Shimura, K. Isoleucine In The Microbial Production of Amino Acids; Yamada, K., Kinoshita, S., Tsunoda, T., Aida, K., Eds.; Wiley: New York, 1972; p 491. Skoog, D. A.; West, D. M. Analytical Chemistry, 3rd ed.; Saunders College: Philadelphia, 1980. Torii, K.; Iitaka, Y. The Crystal Structure of L-Isoleucine Acta Crystallogr. 1971, B27, 2237. Zumstein, R. C. 1. Modeling, Determination, and Measurement of Growth Rate Dispersion in Crystallization. 2. The Crystallization of L-Isoleucine in Aqueous Solutions Ph.D. Thesis, North Carolina State University, Raleigh, 1987.
Donnay, J. D. H.; Ondik, H. M. Crystal Data Determinative Tables, 3rd ed.; U.S. Dept. of Commerce, National Bureau of Standards: Washington, DC, 1972; Vol. 1. Dunn, M. S.; Ross, F. J.; Read, L. S. The Solubility of the Amino Acids in Water J. B i d . Chem. 1933, 103, 579. Greenstein, J. P.; Winitz, M. Chemistry of the Amino Acids; Wiley: New York, 1961; Vols. 1-3. Meister, A. Biochemistry of the Amino Acids; Academic Press: New York, 1965. Mullin, J. W. Crystallisation; Butterworth & Co.; London, 1971. Needham, T. E.; Paruta, A. N.; Gerraughty, R. J. Solubility of Amino Acids in Pure Solvent Systems, J. Pharm. Sci. 1971, 60, 565. Pfeifer, R.; Karol, R.; Burgoyne, R.; McCourt, D. Practical Applications of HPLC to Amino Acid Analysis Am. Lab. 1983, March, 78.
Received for review May 23, 1988 Accepted March 24, 1989
GENERAL RESEARCH Development of New Adhesion Agents for Asphalt Cement Carlo Giavarini* and Gilbert0 Rinaldi Department of Chemical Engineering, Facoltci di Ingegneria, University of Roma, La Sapienza, Roma 00184, Italy
New adhesion and antistripping additives for bitumens were prepared by reacting tetraethylenepentamine (TEPA) with formaldehyde (CH20) or with C H 2 0 and phenol. T h e additives were characterized both chemically and physically. Adhesion of two bitumens containing 0.2% by weight additive was measured by a n improved stripping test and by a U-peeling test. The results showed very good improvement of the antistripping properties of the tested bitumens, especially after water immersion. Both groups of additives (Le., TEPA-CH20 and TEPA-CH20-phenol) are suggested for use as adhesion agents. During additive formulation, basicity and distribution of polar and basic groups play a major role, together with the capability of the additive to control the bitumen viscosity and consistency. Adhesion agents are added to asphalt cement to prevent binder-to-aggregate bond separation, especially in wet conditions. If this bond is broken, water will displace the bitumen film over the aggregate surface. The adhesion agents generally used are based on amines, especially fatty polyamines, and amine derivatives such as amides, substituted imidazoline, etc. (Johnson, 1942; Blair et al., 1957; Kalinonski and Crews, 1957; Gianattasio, 1971). Tertiary nitrogen heterocyclic material is proposed to reduce moisture-induced damage in asphalt-aggregate mixtures (Plancher and Petersen, 1982). The first patents of practical interest appeared in the 1940s. Technical and scientific literature is very poor on this subject, and most additive formulations are, of course, strictly confidential. The effects of antistripping additives on the properties of asphalt cement were studied by Andersen et al. (1982), who also reviewed the literature on the subject, and by Plancher et al. (1982) and Ensley (1973), who studied asphalt-aggregate interactions. The purpose of this work was to prepare and test adhesion agents whose properties (i.e., basicity, density, viscosity, and adhesion) could be modified as needed, depending on the type of application and bitumen.
Experimental Section Bitumen and Aggregate Used in the Study. The literature indicates that water stripping resistance is a 0888-5885/89/2628-1231$01.50/0
Table I. Properties of the Bitumens penetration (ASTM D5), dm softening pt (ASTM D 36), "C penetration index Fraass p t (IP 80), "C asphaltenes (IP 143), wt % acidity (ASTM D 664), mg KOH/g
Vega 56 60.0 +1.2 -16 24.6 1.35
Iranian 90 41.8 -2.6 -13 8.60 0.21
function of aggregate type and of the asphalt composition (Domaney, 1968; Fromm, 1974). However, for the tests, only one type of aggregate material was used, i.e., the Italian S. Fedelino granite (a silicate rock), because it is representative of the type of material prescribed for the surface layer of asphalt cement in this country. In order to reduce the number of variables, most experiments were carried out with a bitumen obtained from the Sicilian crude Vega; adhesion tests were repeated by using a bitumen obtained from Iranian heavy crude. The characteristics of the bitumens are shown in Table I. Additive Preparation and Characterization. Various products were prepared by reacting, in different conditions, some polyalkylenepolyamines with other substances; after preliminary adhesion tests, two groups of additives were selected with different molecular structures: (A) tetraethylenepentamine (TEPAbformaldehyde derivatives prepared by reacting various amounts of CH,O 0 1989 American Chemical Society
1232 Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 NH2-C 2 H4 -NH-C2H4-NH-C2H4-NH-C2H4-NH2
8
+ 2 CH20
+ 2 CH20
t NH-C2H4-NH-C2H4-NH-C2Hq-NH-C2H4-NH
H -MH-C2H4-NH-C2H4-NH-C2H4-NH
NH-C
z4
I
I CH20H
CH20H
I
I
CH20H
CH20H
+ CH20 NH-C I 2H4-NH-C2H4-NH-C2HQ-NH-C2Hq-NH NH-C
H -NH-C 2 4
I
CH20H
H -N-C2H4-NH-C2H4-NH 24 I CH20H
I CH20H
NH-C2H4-NH-C2H4-N-C2H4-NH-C2H4-NH
I
I
I
CH20H
CH20H
CH20H
+ . .-NH-C I
I
I
I
NH-C
I
CH20H
2 H4 -NH-CZHq-NH-
I I
*
OH
HEAT
+
(110 " C )
H -NH-C2H4-N-C2H4-NH-C2H4-NH I 2 4
1 CH20H
I
OH
/&AI
NH-C
I
CH20H H -NH-C2H4-N-C2H4-NH-CZH4-NH
NH-C
I
24
CH20H ,,*
2 H4 -NH-C2H4-NH
I CH20H
?H
CH20H
;H2
+
2H4-N-CI
I
I
,-NH-C2H4-N-C
H -NH-C 2 4
2 H4 -NH--,
I
t .
H2°
Figure 1. Group A additives: evolution of chemical structure during preparation.
N-C
I
N-C
with TEPA; (B) tetraethylenepentamine-formaldehydephenol derivatives prepared by reacting various amounts of CHzO and phenol with TEPA. The additives of group A were prepared as follows: formaldehyde (aqueous solution, 40 % by weight) was slowly added to TEPA in a stirred vessel, a t room temperature. Cooling was necessary to keep the temperature below 30 "C, the reaction being exothermic. During this stage, CHPOreacted with primary amino groups to yield methylolic groups; by increasing the CH,O/TEPA ratio, NH groups were also involved in the reaction (Figure 1A). The temperature was then gradually raised and kept at 110-120 "C for 2 h, in order to evaporate all water introduced with the CHzO solution. The temperature was raised again and kept at 160-170 "C for 3 h; during this stage, some methylene bridges were formed among molecules, as shown in Figure 1B; the final product was still water soluble. A final step, common to both groups of products, included vacuum treatment to completely free volatile matter (e.g., condensation water and CH20) that was eventually kept in the additives. The final products did not contain any free formaldehyde. The additives of group B were prepared as follows: the exact amount of CHPOnecessary to react with the primary amino groups of tetraethylenepentamine was added to TEPA as previously described (Figure 2A). Phenol was then dissolved in the solution, and the remaining amount of CHzO was slowly added to the resulting solution at room temperature. In the following step, the temperature was kept at 110-120 "C for 2 h; during this stage, water was
H -NH-C 2 q
FH2 2 H4 -N-C 2 H4 -NH-C
2 H4 -N-CI
2 H4 -NH-,
,
I
I
CH20H
2H4-NH-C2H,,-N-C2Hq-NH--. I
a
FH2 + H20
I OH
Figure 2. Group B additives: evolution of chemical structure during preparation.
evaporated and phenol was linked to the amino groups through methylene bridges, as shown in Figure 2B. Raising the temperature to 170-180 "C for 3 h produced further branching of the polyamine molecules (Figure 2C). After the vacuum treatment, the product was free from any entrained water and CH,O; the water solubility was still good. Thermal stability of the additives was tested by thermogravimetry (Stanton STA-780): the decomposition of types A and B started at temperatures higher than 230 "C. By varying the type of reagent and molar ratio, it was possible to obtain additives with different molecular weights, viscosities, densities, basicities, and chemical characteristics. The additives were characterized during and after preparation by IR spectrophotometry (PerkinElmer 1310). The molecular weight was cryoscopically measured in aqueous solution. The viscosity at 20 "C and the density were measured, respectively, by using a falling ball Hoeppler viscometer and a pycnometer for liquids, respectively. The basicity was evaluated by dissolving 2% by weight additives in water and titrating the solution potentiometrically with 1.0 N HCl.
Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 1233
I
%T
I
1900
u-0-0,
10
cm3
I
1700
Cm.l
Figure 6. IR spectrum of B, in the range 1950-1650 cm-'. Thin film, NaCl disk.
-m-
5
I
HCI
Figure 3. Titration curves of TEPA, As, and B2additives (2% by weight in water) with 1.0 N HCl. Electrodes: glass/calomel.
%T
Table 11. Composition and Characteristics of the Experimental TEPA-CH20 Additives TEPA/ MW CH20 molar density: (cryoscoadditive ratio g/cm3 pic) viscosity: CP 1.015 200 240 A, 2:l.O 1.018 400 1000 A2 1:l.O 1.019 950 '43 1:1.5 3000 1.020 1100 A4 1:2.0 5200 "At 20 "C.
Table 111. Composition and Characteristics of the Experimental TEPA-CH20-Phenol Additives TEPA/ MW CH,O/phenol density,' (cryosco- viscosity: additive molar ratio p/cm3 Dit) CP Bl 1:1:0.2 1.010 500 200 1.015 600 240 B2 1:1:0.4 1.016 540 300 B3 1:1:0.6 B4 1:1:0.8 1.025 580 400 B6 1:l:l 1.030 550 320
" A t 20 "C. ,
,
I
,
,
l
l
3c
3500 cm-'
Figure 4. IR spectra (range 3800-3000 cm-') of TEPA and of a condensation product (type A) obtained at room temperature from TEPA and CHPO (ratio l/l). Thin film, NaCl disk. %l
1
I
1400
I
I
1000
Cm-,
Figure 5. IR spectrum of product A3 in the range 1800-800 cm-'. Thin film, NaCl disk.
Figure 3 shows the titration curves of two additives of groups A and B, respectively, and of TEPA. It is interesting to note, starting from similar pHs, the slow decrease of basicity of groups A and B during HC1 addition, compared with TEPA.
Figures 4-7 show IR spectra of the additives. The intensity decrease of the band between 3600 and 3250 cm-' in Figure 4 confirms that room-temperature condensation of TEPA and CH20yields methylol-substituted products with some free amino groups. Figure 5, referring to the IR spectrum of product A2, shows that by varying the ratio of the reagents and the temperature of the reaction it is possible to retain some free methylolic groups (peaks at 1670,1150, and 1020 cm-' due to the CH20H groups). Group A additives are polyaminoalkylolic oligomers, with linear and branched chains, whose molecular weight and other characteristics depend on the reagent molar ratio and on reaction conditions. High TEPA-CH20 ratios more easily give group A compounds with long polymeric chains, while lower ratios and higher temperatures produce branching with consequent disappearance of free CH20H groups. Condensation of TEPA and CH20 in the presence of phenol yields group B oligomers with phenyl rings linked to the aminic chains. The IR spectra of the B2product (Figures 6 and 7) show the bands of the 1,2- and 1,4-disubstituted benzene rings (at 1020,1810, 1700,1670 cm-' and at 820,760,690 cm-'1; the absorption at 870 cm-' refers to the 1,2,4 substitution. A t the end of the reaction, the phenolic ring is included in the polyaminic chain of the product.
1234 Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 VlSCOSl t y
30001
800
700
600 C m - ~
Figure 7. IR spectrum of B, in the range 600-900 cm-'. Thin film, NaCi disk. 70
60
Table IV. Commercial Additive Tested for Comparison type polyalkylenepolyamine chem structure R(NHCH&H&HJ,NH, 0.893 density: g/cm3 viscosity,0 CP 170 recommended quantity, wt 70 of 0.3-0.5 bitumen
T C
Figure 8. Rotational viscosity (Paes) against temperature. Bit = Vega bitumen; A3 = Vega bitumen + 0.2% by weight additive As; B, = Vega bitumen + 0.270 by weight additive B,.
n
"At 20 "C.
Tables I1 and I11 summarize the compositions and other characteristics of the additives. Besides the described experimental products, a commercial additive (fatty long-chain polyamine) was used for comparison, whose characteristics are shown in Table IV. Preparation of the Bitumen-Additive Mixtures. The additives were added to the bitumen by first heating the asphalt to 110 "C in a metal can. The additive, preheated to 80 "C, was slowly pipetted into the bitumen and mixed thoroughly by gentle stirring. The control bitumen was subjected to the same heating and stirring. All the bitumens were heated only once for the necessary time, in order to avoid oxidation. The additive structure was not affected by such treatment at a temperature far below its preparation and decomposition temperature. The samples necessary for further testing (penetration, viscosity, adhesion, etc.) were prepared immediately after mixing with the additive. Additive content varied from 0.2% to 0.6% by weight. The viscosities of the mixtures a t various temperatures were measured by a rotational viscometer (Haake viscotester VT 241, and the measuring cups were submerged in a thermostatic bath of silicone oil. Figure 8 shows the viscosity-temperature curves of the original Vega bitumen and of the mixtures containing 0.2% by weight A3 and B2 additives. Adhesion Tests. Two types of tests, both developed in our laboratory, were used to check the effectiveness of the antistripping additives: the first was a stripping test, the second a U-peeling test (Giavarini et al., 1972). For the stripping tests, polished granite specimens were used with the dimensions 6 X 2 X 1.5 cm; the specimens were degreased with CC1, and dried at 105 "C for 1 h. They were then weighed, covered with a uniform layer (about 1mm) of bitumen, and weighed again. The samples were suspended for 10 min in boiling water, then dried at 105 "C, and weighed again. The difference in weight before
80
'1 U
L
a b Figure 9. U-peeling test (a) and peel diagram (b): 50-mm/min pulling speed.
and after the test depended on the amount of bitumen still coating the specimen after the test: the better the antistrip additive, the higher the weight of the bitumen at the end. The advantage of this test over similar ones is that antistripping resistance is measured, not only evaluated "by sight". Ten tests were carried out for each bitumen mixture, and the results were averaged. Only the specimens covered with pure bitumens showed, at the end, some uncovered granite surface. For the U-peeling tests, a tensile test machine with constant-speed strain was used (Lloyd Instruments Ltd., London, Model T 20000). Reinforcement cotton strips (20 x 2 cm) previously impregnated with bitumen were rolled by a drum on granite plates to obtain a bitumen layer that was 0.4 mm thick. The specimens so obtained were cured in air or in water at 20 "C for periods varying from 2 to 24 h and were then tested. The granite plates were suspended from the machine, and the cotton strip was then pulled downward by the mobile clamps at a constant speed of 50 mm/min (Figure 9a).
Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 1235 Table V. Typical Results of the Stripping Tests (Weight Loss, Percent of the Original Bitumen Layer) antistripping agent, 0.2% by wt bitumen none commercial A3 B2 Vega 50 35 24 26 Iranian 35 18 18
c
Five specimens were made for each bitumen-additive mixture, and the results were averaged. A typical peeling diagram is shown in Figure 9b. The differences among the five specimens were generally lower than 10%. The results of the adhesion tests confirmed that additions of antistripping agents higher than 0.3-0.4% by weight were generally useless, the gain in adhesion strength being counterbalanced by a loss due to decreased viscosity and consistency of the bituminous layer; this is especially true for group A additives and for the peeling tests. Therefore, after the screening and preliminary tests, which led to the choice of the most suitable products, the following experiments were carried out using 0.2% of the additives; this low percentage is also advantageous from an economical point of view. Of all the additives, the most suitable for the tested bitumens and for the type of tests experimented were A, in group A and B2 in group B. Typical results of the stripping and peeling tests are shown in Tables V-VII.
tI
,
5
,
1
10
15
Figure 10. pH of water solutions containing various amounts of A3 or B2 additive.
long-chain molecules of the additives. Moreover, additives contain functional groups that affect the peptization or dispersion of asphaltene-like constituents, with attendant changes in bitumen consistency and cohesion forces. Commercial basic additives exert a similar effect on the strength of the interfacial bond between bitumen and rock. Many of them contain in their molecules hydrocarbon-like chains with some 10-20 carbon atoms and a polar (polyamine or polyamide) portion; the hydrocarbon or “fatty” chain improves the compatibility with bitumen, while the polar portion is responsible for the improved adhesion properties. The polarity and basicity of the experimental additives are more uniformly distributed through the molecular chain; CHzO and phenol increase the compatibility with asphalt. In our opinion, they perform a more complex chemical and bridging action between the rock, on one hand, and the polar and acid bitumen constituents, on the other hand. In fact, the adhesion of Vega bitumen, containing more polar asphaltenic constituents and acidic groups than Iranian bitumen, is better improved by A than by B, whose basicity is probably lowered by the presence of phenyl groups (as shown in Figure 10, reporting the pH of the water solutions containing various amounts of the additives). The effect of additive basicity is less important for the “less acidic” Iranian bitumen, where the improvement of viscosity and consistency induced by the additives of group B probably plays a major role.
Discussion of the Adhesion Results All the additives improved the adhesion to granite rocks and the antistripping properties of the bitumens, especially in the presence of water. Compared to the commercial product, small amounts (0.2% by weight) of the additives developed in this study showed better results; higher contents were not advantageous (Table VI). As already indicated by the literature, water stripping resistance is a function of asphalt composition (Domaney, 1968; Fromm, 19741, and the effect of the antistripping additive is asphalt-specific (Andersen et al., 1982). In fact, the two asphalts considered in this work were affected differently by the same additive. Iranian bitumen is a typical example of a good Middle-East bitumen, with relatively high adhesion properties; its antistripping resistance was further improved by the selected products A, and B2, which showed a similar effect on the bitumen (Table VI). The bitumen alone lost resistance to peeling after 1-2 h of water immersion, reaching the minimum level after 12 h; this loss of resistance was slower with the bitumen containing the additive. In spite of the very high asphaltene content, Vega is a good bitumen. Compared to the Iranian bitumen, its resistance to peeling and stripping was quite low and was strongly reduced after some hours of water immersion. The addition of 0.2% by weight B2 and A, improved the adhesion by about 50% in air and by 300-500% in water (Table VII). Peeling tests showed an appreciable difference between the two products, not demonstrated by the “coarser” antistripping tests (Table V). Considering the acidic nature of granite rock, the adhesion improvement can be ascribed to the basicity of
Conclusions Antistripping additives improve the adhesion bond between bitumen and aggregate in asphalt cements; during additive formulation, a better distribution of polar and basic groups plays a major role, together with the capability of the additive to control the bitumen viscosity and consistency. The additives developed in this work are polyamines condensed with various amounts of formaldehyde and phenol, having different chemical structures, basicities, and viscosities. Their compositions and structures can be varied within a certain range in order to reach the best
Table VI. TvDical Results of the U-Peeling Tests: Iranian Mixtures (Peel Strenath in N/m) antistripping agent, % by wt time of water curing. h none commercial, 0.2% A,, 0.2% B,, 0.2% BP, 0.4% ~
0 2 6 12 24
720 590 400 300 300
I
additive, % w t
~~
780 710 580 410 390
870 610
880 870 645 520
490
510
865 630 600 566 500
BP, 0.6% 520 365 330 300 265
I n d . E n g . C h e m . Res. 1989, 28, 1236-1241
1236
Table VII. Typical Results of t h e U-Peeling Tests: Vega Mixtures (Peel S t r e n g t h i n N/m) time of antistripping agent, 0.2% by wt water curing, h none commercial A, B2 0 200 245 335 270 2 200 245 335 270 24 65 160 300 200
antistripping properties suitable for the actual application. Registry No. (TEPA)(CH,O) (copolymer), 87868- 1G-8; (TEPA)(CH20)(phenol) (copolymer), 27233-92-7; sulfolane, 126-33-0; toluene, 108-88-3.
Literature Cited Andersen, D. A.; Ducatz, E. L.; Petersen, J. C. The effect of antistrip additives on the properties of asphalt cement. Proc.-Assoc. Asphalt Paving Technol. 1982,51, 298-317. Blair, C. M., Jr.; Groves, W.; Lissant, K. J. Carbonate rock aggregate bonded with bitumen containing a polyalkylene polyaminoimidazoline. US Patent 2,812,339, Appl. Sept 2, 1957. Domaney, U‘. J. Stripping characteristics of paving grade asphalts
used in New Brunwick. Proc. Can. Techn. Asphalt Assoc. 1968, 13, 267-272. Ensly, E. K. A study of asphalt aggregate interaction and asphalt molecular interactions by microcalorimetric methods. Postulated interaction mechanism. J . Inst. Pet. 1973, 59, 279. Fromm, H. J. The mechanism of asphalt stripping from aggregate surfaces. Proc.-Assoc. Asphalt Paving Technol. 1974, 43, 191-196. Gianattasio, G. Additivi chimici utilizzati nella moderna industria delle pavimentazioni stradali. Strade Traffic0 1971, 208, 2-13. Giavarini, C.; Maura, G.; Rinaldi, G . Rivestimenti dell’acciaio con bitumi ossidati. Riv. Combust. 1972, 26, 313-317. Kalinonski, M. L.; Crews, L. T. Graf polymer-fortified bitumen additives. US Patent 2,812,339, Appl. Sept 2, 1957. Johnson, J. M. Bituminous composition having increased adhesion to mineral aggregate. US Patent 2,426,220, Appl. Sept 2, 1942. Plancher, H.; Petersen, J. C. Tertiary nitrogen heterocyclic material to reduce moisture-induced damage in asphalt-aggregate mixtures. US Patent 4,325,738, Appl. April 20, 1982. Plancher, H.; Holmes, S.; Petersen, J. C. Role of nitrogen compounds in reducing moisture-induced damage in bituminous pavements. Antek 1982, 2, 6-12.
Received for review August 10, 1988 Revised manuscript received February 23, 1989 Accepted April 12, 1989
Gas Chromatographic Study of the Evaporation from Films Composed of a Volatile Solvent plus a Nonvolatile, Nonpolymeric Liquid Reynaldo C. Castells,* M6nica
L. Casella, and Angel M. Nardillo
CIDEPINT, 52 entre 121 y 122, 1900 La Plata, Argentina, and Facultad de Ciencias Exactas, Universidad Nacional de L a Plata, 1900 La Plata, Argentina
The evaporation rates from films composed of n-octane + squalane and toluene + sulfolane were measured by a gas chromatographic method. Both the conditions at the gas/liquid interface and the transport of volatile solvent from within the liquid t o the interface determine the evaporation rate. The equations obtained by integrating Fick’s second law for a homogeneous film under the assumptions of constant diffusivity and constant film thickness fail t o interpret the experimental results. In principle, the films can be considered as heterogeneous, with a very thin surface layer where the volatile solvent has a very low diffusivity and an underlying liquid a t uniform composition.
The understanding of the mechanism of solvent-cast film formation is of great importance in several technological areas. Coating research and development laboratories have been very active in this field (see, for instance, Yoshida (1972), Newman and Nunn (1975), Kornum (19801, Ramsbotham (1980),Holten-Andersen and Hansen (1983), and the references cited therein). The vast majority of these studies have been performed by means of gravimetric techniques, making use of the Shell Thin Film Evaporometer (ANSI/ASTM, 1982),more sophisticated electrobalances, as the Evapocorder developed at the Chevron Research Company (Saary and Goff, 1973),or some other type of electrobalance installed within a wind tunnel (Eaton and Willeboordse, 1980). These instruments were designed for the measurement of total evaporation rates; they are not easily adapted for the measurement of reliable individual evaporation rates from solvent blends. Furthermore, the samples suffer an important evaporative cooling as a consequence of the relatively high flow rates employed (above 10 L/min) (Rocklin and Bonner, 1980), and the experimental results correspond to evaporation rates measured at uncertain temperatures. A method t o measure the evaporation rates of solvents 0888-5885/89/2628-1236$01.50/0
under rigorously controlled conditions has been developed in this laboratory (Castells and Casella, 1987a,b),using a gas chromatograph equipped with a flame ionization detector (FID) and an automatic gas sampling valve. A known quantity of the pure solvent or mixture under study is applied by means of a 100-pL microsyringe onto a filter paper disk (No. 31 Whatman Extra Thick, diameter 2 cm) placed within a thermostated glass cell. A t controlled temperature and flow rate, a nitrogen stream flows parallel to the paper plane, sweeping the vapors toward a sampling valve actuated at a constant frequency, thus injecting the contents of its loop into a chromatographic column where the vapors are separated before entering the FID. Both the evaporation rate of each component in the sample and the composition of the remaining liquid as functions of the time elapsed from the start of the run can be calculated by applying a material balance; the necessary information is the sample initial weight and composition, the nitrogen flow rate, the sampling frequency, and the area(s) of the peak(s) produced by each vapor injection. The FID sensitivity is several orders higher than that of the best electrobalance, thus enabling the measurement of the smaller evaporation rates occurring at low flow rates. As has been experimentally demonstrated in a former paper C 1989 American Chemical Society
