Physico-Chemical Aspects of Asphalt Pavements Energy Relations at Interface between Asphalt and Mineral Aggregate and Their Measurements The forces acting on the surfaces and a t the interface in a system mineral aggregate-asphalt are of importance for the stability of asphalt pavements. The most prominent energy relation is the interfacial tension between solids and liquids. It is shown that typically hydrophilic solids have energy relations a t their interfaces with pure liquids which are of the same order as a t the interface between water and the same liquids. A simple method to measure the interfacial tension between solids and liquids is to allow the powdered
@A-
solids to settle freely in liquids. The volume to which a lyophilic powder settles in a pure liquid is proportional to the interfacial tension between the two phases. This method is applied to six asphalts and three aggregates-silica, limestone, and blue clay. It is shown that the interfacial tensions betw*eenasphalts and solids vary not only with the type of the aggregate but also with the type of asphalt. This is of importance with respect to the behavior of asphalt pavements under the action of water.
SPHALTS have
CHARLES MACK
found their largest field of
Imperial o i l Limited, Sarnia, Ontario, Canada
application in road construction, where they have the function of binding together mineral aggregates. In a system asphalt-aggregate, surfaces and their energies play a n important part, and the changes in surface energy which occur when asphalt comes in contact with aggregate are the most important properties in such a system. These changes in surface energy are as follows: ENERGY OF IMMERSION. If a particle, either solid or liquid, is immersed in a liquid, the surface disappears and in its place is found the same area of interface, solid-liquid or liquidliquid. The energy of immersion is equal to the surface energy of the immersed particle minus its interfacial energy against the surrounding liquid. If S denotes surface or interfacial energy and subscripts 1 and 2 represent the tnro phases then Energy of immersion = 81 -
81, z
The energy of immersion is often referred to as adhesion tension, since i t is obviously indirectly related to adhesion. ENERGY OF ADHES~ON.When a solid or a liquid coalesces with the surface of another liquid, the surface energies of the coalescing substances disappear and the energy of the interface appears. The energy involved in this reaction is equal to the sum of the energies of the meeting surfaces minus the interfacial energy:
+ Sz - SI,
Energy of adhesion = XI
2
The energy of adhesion is tne itmount of energy per sq. cm. which must be applied in order to separate the substances at the interface. Whereas the energy of cohesion, which is twice the surface tension of a substance, is a measure for the attraction of
molecules of the same type, the energy of adhesion is a measure for the attraction of molecules of different substances. COEFFICICXT OF SPREBDING. When a liquid (9)spreads over a solid or other liquid, attraction between the two phases occurs, the force of which is directed against the cohesion of the liquid which is in the position of spreading. Thus the coefficient of spreading is equal to the energy of adhesion minus the energy of cohesion of the spreading liquid: Coefficient of spreading = s 1
+ s, - SI,- ZS,= s, - -s,, 2
s 1
2
Spreading occurs only if the energy of adhesion i b greater than the energy of cohesion of the spreading liquid or, in other words, if the value of the coefficient of spreading is positive. It is readily seen from the foregoing that the interfacial tension between solids and liquids is the most important energy relation and that the problems of adhesion and wetting are directly related to it. Since the stability and durability of a3phalt pavements depend upon the adhesive forces bet%-een asphalt and aggregate, the necessity of determining this force has been felt for a long time, and several tests have been proposed already. Xicholson (7) suggests that a powdered aggregate be shaken with a benzene solution of asphalt and with water to see whether the aggregate is preferentially wetted by water or a s p h a l e i . e,, if the aggregate is hydrophilic or hydrophobic. This test. however, does not allow the estimation of the adhesive strength betreen asphalt and aggregate. Riedel and Weber (8) measure the adhesion by boiling a mixture of asphalt and aggregate with water or sodium carbonate solutions of different strengths for a definite time. The higher the concentration of sodium carbonate which is 1500
necessary to displace the asphalt from the aggregate, the higher is the adhesion between the two phases. Although this method ha5 promising features, it cannot be successfully used because variations occur in the rate of displacement with the viscosity of the asphalts, and because it does not take into con~iderationthe interfacial tensions between asphalts and the sodium carbonate solutions. The energy of immersion or adhesion tension can be determined by the Bartell and Miller method (2) of measuring the pressure developed when one liquid displaces another liquid from the pores of a po~vderedsolid. This method is so time conqumiiig that it cannot be used for routine work. On account of the importance of the interfacial tension of asphalts against mineral aggregates with respect to adhesion and vetting, it was necessary to develop a simple method. For this purpose the data available in the literature for the adhesion tension of organic liquids against solids have been related to surface energy relationships which are readily accessible.
-4dhesion Tension of Liquids against Solids in Relation to Changes in Free Surface Energies of Liquids against Water From the inyestigations of Bartell and eo-xorlrers, adhesion tensions of a set of liquids against carbon black and silica are ai-ailaisle ( 3 ) . These data together 711th the interfacial tensions of the same liquids against water are gii.en in Table I. T.~BLE I.
ADHESIOS
Liquids Carbon disulfide Carbon tetrachlori de a-Bromonaphthaif 'ne Toluene Benzene Chloroforni Decalin Nitrobenzene Tetralin Butyi acetate Amyl acetate Propyl acetate Ethyl acetate Aniline Amyl alcohol Isobutyl alcohol Water
1501
1KDUSTRIA.L AKD ENGINEERING CHEMISTRY
DECEMBER, 1935
TESSIOKOF LIQVIDSAGAISBT CARBOX
Surface Tension of Liquids
--31 3 26 1 44.0
28.1
28.3 26.5 31.0 43.3 32.8 24.1 24.4 23.8 23.4 42.5 23.7 22.4 72.0
SILICA A S D
Energy oi InterImmersion facial Adhesion Adhi:aion oi Tension Tension Tension Water against against against in Water Silica Carbon Liquids Dynes per centimeter-48.1 44.5 41.6 36.1 34.6 31.6 26.7 25.3 22.4 13.2 10.9 9.6 6.8 5.7 5.0 1.8 0.0
43.2 39.5 41.1 53.4 51.2 58.7
ei:e 72:l i3.7 T4.4 76.1 73.8 77.5 80. ' 81.5
9Kii 8fi. 38 88.81 82.10 81.03 7!l. 83 76.38 7!3.58 715.70 63.78 6.3.68 63.09 60.22
23.9 2i.5 30.4 35.9 37.4 40.4 45.3 46.7 49 6 58 8 61.1 62.4 65.2 66.3
55.i7
65.0
56.60 54.i 4
70.2 72.0
59.07
When the adhesion tensions of the different liquids against silica and carbon black are plotted against the energy of immersion for water in the same liquids, a straight line is obtained which indicates that the adhesion tensions of the hydrophilic silica are directly proportional to the immersion energies for water in these liquids. For the hydrophobic carbon black the reverse holds true (Figure 1). The average difference between the adhesion tensions for silica and the energies of immersion for water in the same liquids amounts to 12.8 dynes with the highest deviation of 6.5 for benzene and -5.3 for aniline. In a later statement Bartell (1) believed that the values for the adhesion tension data against silica are about 6 dynes too high, which would reduce the difference between the adhesion tension for silica and immersion energies for m t e r to 6.8. Since Bartell's method of measuring the adhesion tension is indirect, one can conclude without introducing a material error that the energy relations a t the interface between pure liquids and silica are similar to those a t the interface between the same liquids and water.
ENERGY OF
IMMERSION
FIGURE1. ADHESION TEVSIONOF LIQUIDS AG4lNST SOLIDS DS. EXCRGY OF I\I\lERSfOK OF W 4 T E R IN THE SI\IE LIQUID
Since it has been found that the adhesion tension values of pure liquids against different hydrophilic solids (barite, aluminum oxide, silica from different sources, calcium fluoride) are of approximately the same magnitude ( I ) , one might be led to conclude that all typically hydrophilic solids show energy relations at their interfaces viith pure liquids of the same order as a t the interface between water and the liquidi;.
Settling Volume of Powdered Solids in Liquids in Relation to Interfacial Tension of Liquids against Water In the paint and varnish industry the determination of "liquid absorption" for pigments has become a common praotice. The liquid absorpt'ion factor (4)i j expressed as the volume of liquid in cubic centimeters which is necejsary l,o wet 100 grams of powder. It appears to be generally conceded that liquid absorption values for a given pigment depend upon the degree of wetting of the solid by the liquid. Owing t o the fact that' this test cannot be carried out w k h asphalts, it was decided to allow the pori-ders to sett,le freely in the liquids. If a powder is immersed in liquid, the powder is wetted, and the air absorbed on the surface of the solid is replaced by the liquid, the extent of which depends upon the attraction intensity between liquid and powder. The less a powder is wetted by t'he liquid, the larger v d l be the contact angle betn-een the 5olid and liquid, and the more liquid will be necessary to envelop the powder grains by a film of liquid. I t is evident that the less a powder is wetted by the liquid the higher will be the volume to which it will settle. Further, if the attraction intensity between liquid and powder is low, the liquid will not be able to disperse or deflocculate particles clinging together, which act as large particles and are generally referred to as flocculated. For this investigation the following mineral aggregates (ground to pass the 200-mesh sieve) were used: Silica subsequently treated with hydrochloric and nitric acid to remove inorganic and organic impurities; after n-ashing with water the silica was heated to about 1000" C. Limestone containing 82.9 per cent of calcium and magnesium carbonate. Blue clay, a basic clay. Carbon black, a commercial lamp black from which the adhering oil was removed by extraction with benzene. The powders were tested with respect to their hydrophilic or hydrophobic character by wetting 5 grams of each with 15
INDUSTR JAL A N D ENGINEERISG CHEMISTRY
1502
VOI.. 27,
so. 12
aniline are orercome by comparing the settling volumes with the coefficients of spreading of the Coliquids on water. This relationship (Figure 3) is, Interefficient ,-Settling Volumesfacial of Lime~i~~ carbon7 in the case of silica for all the liquids used, a Surface Tension against Spreading on Silica 10 stone 5 clay 5 black 1 straight-line function, indicating that the hydroLiquid Tension Water Xater grams grams grams gram philic silica settles to a lower volume in liquids, Dynes per cm. cc. cc. cc. CC . the better they spread on water. The other hyCarbon disulfide 31.3 48.1 -7.4 8.6 7.5 14.3 11.5 drophilicpoTvdersshowthisproportionalityinthe Benzene 26.3 34.6 9.1 6.0 7 0 12.6 12.0 caSe of those liquids in which they sett'le to the Chloroform 26.5 31.6 13.9 7.9 6.8 12.5 Sitrobenzene 24 43 ,. 3 21 35 ,. 3 3 34 ., 74 78 ., 2 6 ,. 5 11.9 12,, same volumes which would correspond to their Butyl acetate Aniline 42.5 5 7 23.8 7.6 4.5 6.7 9.1 interfacial tension against water. Amyl alcohol 23 7 5.0 43.3 6.8 4.5 6.5 13.0 The lorn settling volumes of limestone and blue Water 72.0 0.0 .. 6.6 5.5 9.0 15.2 'la!. in and amy1 Tvould indicate a The carbon black did not eettle in chloroform and nitrobenzene because of insufficient differences in the specific gravities. negative values for the interfacial tensions het,ween the t,mo pliases and a higher attraction intensity between liquids containing highly polar cc. of benzene and then shaking with 15 cc. of distilled n-ater. groups and these solids than between water and these solids. The carbon black was hydrophobic; the silica, limestone, and This extremely high attraction intensity between highly blue clay were found to be hvdrophilic. polar liquids and certain solids is of great importance for the The liquids used were carbon disulfide) benzene, chloroform, durability of aiphalt paving niixtureq, as will be seen later. butyl acetate, amyl alcohol, nitrobenzene, aniline, and water. They were of c. P. grade and were dried and redistilled by the usual methods; benzene was dried by metallic sodium. For the settling experiments 10 grams of silica, 1 gram of carbon black, and 5 grams of the other porvders were shaken S T H E foregoing it was shown that the settling test reprewith a part of 40 cc. of the liquids in a graduated tube, taking sents a measure for the energy relations a t the interface care that no air remained occluded in the poTvder. The walls between solid and liquid. I n the following sections this of the tube were rinsed with the remainder of the liquid, and the tubes were kept in a vertical position until no change in method is applied to the system asphalt-mineral aggregate. the volume of the settled powder took place (about 2 m-eeks); Since the spreading and adhesion relationships of asphalts then the volume of the settled poxder vias read. In order to are connected with their surface energies a t their interface duplicate the result's, the powders must be well mixed and dried. with air and water, it is necessary to know their surface tensince the presence of water influences the volume of the settled powder. sions and interfacial tensions against water. The data of the settling experiments are given in Table 11. Surface Tension of Asphalts The relationship between settling volume and interfacial tension of the liquids against water is presented graphically in Surface tension measurements have already been carried Figure 2. The results indicate that, with a few exceptions, out with asphalts a t different temperatures by Nellensteyn the volume of the powder settled in a liquid is a straight-line and Roodenburg ( 6 ) . Their results indicate an abrupt transition point in the surface tension-temperature curve. Below function of the interfacial tension of the liquid against water. For the hydrophilic silica, limestone, and blue clay the settling and above the temperature of transition, the surface tension volumes of the powders decrease with the interfacial tension of asphalts is a straight-line relationFhip of the temperature. of the liquid and increase for the hydrophobic carbon black. These authors conclude that a t higher temperatures asphalts may be considered as liquids; a t low temperatures they behave The relationship between settling volume and interfacial partly as a solid and partly as a liquid, and during cooling tension of the liquid against water can be expressed by: important changes in the structure of the asphalts must take Interfacial tension = A b X settling volume, where ,4 and place. However it is not comprehensible why in asphalts a b are constant. structural change should occur so abruptly; the inflection in For hydrophylic solids, A has a negative and 6 a positive value, the surface tension-temperature curves can be rather attriband for the hydrophobic carbon black the reverse holds true. uted to an experimental error introduced by the high viscosity The constants have the following values for the hydrophilic of asphalts a t lox- temperatures. This point of view is subpowders! stantiated by the fact that these authors used the gas bubble A b method for the surface tension measurements. It was there24.05 Siiica. -158.7 fore necessary t o devise another method for the measurement 24.05 Limestone -132.3 TABLE11.
SETTLING VOLVMES O F PO\\-DERED S O L I D S I N
LIQUIDS
...
I
+
Blue ciay
- 81.0
9.0
On account of this relationship the settling test can serve as a measure for the interfacial tension between 1i q u i d s and hydrophilic solids with a relatirely fair accuracy. The exceptions from the straight-line relationship between volume of settling and interfacial tension against water for the liquids in which the settling takes place are as follows : Silica settles in nitrobenzene and in aniline to a larger volume than that which would correspond to their interfacial tensions against water. Limestone and blue clay settle to a lower volume in aniline and amyl alcohol than would be expected. The settling volumes of the powders are lower in aniline and amyl alcohol than in wat,er. Carbon black settles in aniline to a lower volume and in water to a larger volume than would be expected. The settling anomalies of silica in nitrobenzene and
I
5
1FIGURE 2.
I
6
,
I
I
B2
e I1 SETTLING VOLUME INTERFACIAL
TENSIOV CS.
I
19
I
J
/A
IN C C .
SETTLING TOLLME OF SOLIDS
INDUSTRIAL -4ND EXGINEERING CHERIISTRY
DECEThIBER, 1935
izap[i
the surface tension: of its benzene solutions are proportional to tlie concentration according to tlie following data:
of surface tencions of a y h a l t s a t low temperatures which would be free from the influence of high viscos1ty.
The method developed is based on the following facts: A thread of aaphalt suqpended vertically i3 esposed to two different forces--.iiz., gravitation and surface teniion. 10 If the thread is very short and 5 gravitation is exceeded by surface tendon, the latter tends to reduce i o the surface of the thread by reducing its length. In the reverse case the gravitatioiial f o r c e t e n d s t o -10 , I 9 elongate the thread. At a definite SETTLING VOLUME OF SILICA length of the thread the gravitaFIGURE3. COEFFItional and surface forces are in CIENT OF SI'RE4DITG us. SETTLISGVoLuirE equilibrium, and the t h r e a d will O F SILICA r e t a i n it;; original length. This length corresponds to the height t o which the asphalt would rise in a capillary tube of t'he same diameber as the thread. Hence the equation which is valid for the surface tension determination by the capillary rise method call be applied t o this method:
S
= '/?lrdg
\There S = surface tension I and T = length and radius of the thread d = specific gravity g = gravitational constant
Asphalt
2C
116 121 135 135 118 118
D E F
Penetration (100 Grams, 6 Seo.) 32" F. 77' F. 100' F. (0" C.) (25' C.) (37.8' C.)
(46.i) (49.4) (57.2) (57.2) (47.8) (47.8)
2 4'/* 9 6 23 21
74 70 68 55 130 120
Toosoft
272 215 175 Too soft
Too soft
Suriace Tension, 20' C. Dynes/cm. 38.9 33.8 26.3 28.3 32.0 31 5
I n order to obtain information with regard to the accuracy of this suspended thread method, the value for the surface tension of asphalt A obtained by the latter method v a s compared to the value obtained by extrapolating the surface tensions of the solutions of the same asphalt in benzene using the capillary rise method. For this asphalt it was found that T ~ B L Iv. E SETTLISG Asphalt A B
C
D E
8'
-0% 8.0 8.0 8.0 8.0 8.0 8.0
20% 8.5 8.5 8.3 8.5 8.5 8.5
SlllCZl 40% 7.8 7.8 8.2 8.3 7.8
6.8
50% 7.8
7.8
8.2 8.3 7.8 7.8
60%'
7.8 7.8 8.2 8.3 7.8 7.8
\-OLUYEM 7
0%
7.0 7.0 7.0 7.0 7.0 7.0
Surface Tension,
c.
203
Concn. of Asphalt % b~ weight
Dunes/cm. 28 3 30.5 32 3 34.6 36.5
n 20 40 60
60
Lacking definite information, it may be assumed that eutrapolation \Till probably give values uf a fair accuracy. The extrapolated surface tension of the asphalt' is 38.5 dynes per em. which compares well with the value of 38.9 dynes obtained by tile suspended thread method. The results indicat,e that the surface tensions of asphalt. are of the same order as those of liquids, ranging for the asphalts investigated between 26.3 dynes per em. for asphalt C and 38.9 dynes for asphalt A . Interfacial Tensions of -4sphalts against Water
To determine the interfacial tensions of asphalts against water, an indirect method was used. The interfacial tensions of asphalt solutions in benzene of varying concentrations were measured by the method of Bartell arid Miller (21, and the results were extrapolated to 100 per cent of asphalt. The interfacial tensions of the asphalt solutions in benzene are presented in Figure 4 and Table 111. TABLE 111.
To carry out surface tension measurements, the asphalt is rolled to a thread on a smooth glass plate a t a ternperat'ure which has to be selected according to the consistericy of the asphalt. The thread is cut int,o pieces of different length: and the eame diameter (approximately 0.1 to 0.15 cm.). The threads are suspended vertically and their changes in length are observed over a sufficient period of time. From the thread retaining its original length the surface tension is calculated. The method can be applied only to asphalts which flow under the influence of gravitational force-i. e., asphalts having no yield value. The surface tensions and characteristics of the asphalts investigated are as follows: Softening Point (Ring and Bail) 'F. (" C.)
1303
TENSIONS O F hPH.IL.I. S O L U T I O S S BENZENE AGAINST WATERAT 20' C.
INTERF4CIAL
IN
Interfacial Tension a t Asphalt Concn. of Asphalt
0%
20%
60%
40%
100%
(extrapolated)
Dunes p e r centzmetrr-A B C D E F
~~~
~
~~~~
~~
7
23 7 18.2 23.0 21.8 19.4 20.5
26.3 19.5 24.3 23.0 21 0 22.0
29 2 21.1 25.7 24.4 22.2 23.7
34.6 34.6 34.6 34.6 34.6 34.6
~
18.2 15.4 20.4 19.3
16.6 17.2
~~~~~~~~~~~~~~~
The interfacial tension-concentration curves show that above an asphalt concentration of 20 per cent by weight the interfacial tension of t,he asphalt solution is a straight-line relationship of the concentration which permits tlie extrapolation of the interfacial tension of the asphalt with a high degree of probability. Interfacial Tensions of Asphalts and Mineral Aggregates
As was shown earlier in this paper, the settling volume of powders in liquids is a measure of the interfacial tension between solid and liquid. The settling of powdered mineral aggregates cannot be carried out in asphalts at room temperature because of their high viscosities, since the rate of settling is largely influenced by this property. It was therefore decided to carry out. the settling experiments in benzene solutions of asphalts, provided that a definite relationship could be established between the volume of settling in asphalts and in their solutions.
OF POWDERED SOLIDS IN
Limestone
20% 40% 50% Cubzc cenlzmeters 4.4 4.5 4.5 4.4 4.6 4.6 4.8 5.0 5.0 4.4 4.4 4.4 4.5 4.5 4.5 4.5 4.5 4.5
ASPHALT SOLUTIOXS Blue Clav-------
7
60%
7
0%
20%
40%
50%
60%
-~
4.5 4.6 5.0 4 4 4.5 4.5
12.8 12.8 12.8 12.8 12.8 12.8
6.5 5,s 0.2 5.7 5.8 5.8
6 5
5,s 6.0 5.5 5.6 5.6
6.5 5.8 5.8 5.3 5.2 5.4
-
6.5 5.8 5.8 5.3 5.2 5.4
INDUSTRIAL AND ENGINEERING CHEMISTRY
1504
solutions the following interfacial tensions between the two phases are calculated (in dynes per em.) :
29
28 27 26
Asphalt A B
:25
f
c D w
24
I23
F
322
5
21
5
20
d
'9
I-
$ 6 *
18 17
16
'"
100 yo BY W8$OHT FIGURE 4. IKTERFACIAL TENSION OF ASPHALT SOLUTIOXS IN BENZEXEAGAINST WATER vs. COSCENTR ~TIOY ASPHAL?'
60
CONCENTRATION IN
In order to study the influence of the presence of a solvent or diluent upon the settling volume, 5 grams of silica, limestone, and blue clay were allowed to settle in solutions of amyl alcohol in benzene. The results obtained (in cc.) are as follows: Concn. of amyl alcohol, 70by weight: Silica Limest one Blue clay
I
VOL. 27, NO. 12
0 4.0 7.0 12.8
1 3.8 4.9 7.5
5 3.5 4,s 6 5
20 3.4 4.5 6.5
100 3.4 4.5 6.5
These data indicate that, in a concentration of 5 per cent amyl alcohol in benzene, the powders settled to the same volume as in pure amyl alcohol, obviously because of adsorption of the alcohol on the surface of the poivders. T'i'e can therefore expect that the settling volumes of powders in asphalt would be the same as in their solutions of high concentration. The sett'ling experiments were carried out' at 20" C. in benzene solutions of asphalts in concentrations of 20, 40, 50, and 60 grams of asphalt in 100 cc. of solution. The solutions were centrifuged to separate suspended material. For these experiments the same powdered silica, limestone, and blue clay were used as in the settling experiments in the pure liquids. It was found that 3 weeks were sufficient to complet'e the settling of the powders in the asphalt solutions. Although the c,olor of the bitumen solutions is very dark, the demarcation line between settled powder and supernatant liquid is clear enough to allow the reading of the volume of the settled ponder. The data of the settling volumes of 10 grams of silica and 5 grams of the other powdered solids in 40 cc. of bitumen solution are given in Table IV. The results indicate that in almost all cases the volumes of the powders settled in the asphalt solutions are the same for the concentrations of 40, 50 and 60 per cent and the same in all cases for the latter two concentrations. It can therefore be concluded that the powders would settle in asphalts to the same volume as in their solutions of high concentration. A comparison of the settling volumes with the concentration of the asphalt solutions leads to t,he interesting observation that the limestone powder sett'led to lower volumes in the solutions containing 20 per cent of asphalt A , B, and E than in the solutions of higher concentration. This peculiarity might be explained by changes in the adsorption with the concentration of the asphalts; i. e., the adsorption of-asphaltic constituents on the powders is apparently complete in these cases a t a concentration of 20 per cent of asphalt, whereas a t higher concentrations desorption takes place. From the settling volumes of the powders in the asphalt
--
Interfacial Tension of Asphalts against:Silica Limestone Blue clay 29 -24 -22 29 -22 -29 39 -12 - 29 19.3 41 -27 -33 16 6 29 -24 -34 17 2 29 -24 -32
Water 18.2 15.4 20.4
A comparison of the interfacial tensions of the asphalts against the powdered aggregates with their interfacial tensions against water shows that for asphalts the relationship between settling volume and interfacial tension against water does not hold in contrast to pure liquids. Silica, for example, has interfacial tensions against the asphalts investigated which are higher than the interfacial tensions of ivater against the asphalts. Limestone and blue clay have negative interfacial tensions against the asphalts. This deviation from pure liquids has to be ascribed to a difference in t,he adsorption of certain asphaltic components on the surface of water and solids. For pure liquids there is no opportunity for selective adsorption either on mater or on mineral aggregate. The result's indicate that the interfacial tensions between asphalts and solids vary not only with the type of aggregate but also nith the type of asphalt. Coefficient of Spreading and Energy of Adhesion of Asphalts towards Mineral Aggregates With the knoxledge of the interfacial tensions of asphalts against aggregates and of the surface tensions of asphalts, it is possible to calculate the coefficients of spreading and energies of adhesion according to the equations previously given. As pointed out already, the coefficient of spreading represents the degree of wetting of a liquid towards a solid, and the energy of adhesion represents the degree of attraction intensity between the two phases. Whereas the degree of wetting of a powder for a liquid is independent of the viscosity of the liquid and the diameter of the cavities in the powder, the latter factors greatly influence the rate of wetting; i. e., the time necessary to v e t a powder increases with the viscosity of the liquid and with a decrease in pore size. These energy relations are as follows (in ergs per sq. em.): Asohalt A B C D
E
F
-Coefficient of SpreadingSilica. Limestone Blue clay 57 55 4 60 6i 9 58 75 7 71 77 3 64 74 11 64 7% 11
--Energy of Adhesion-Silica Limestone Blue Clay 82 135 133 135 77 128 127 54 110 133 59 127 138 75 128 136 75 I28
The data indicate low coefficients of spreading and low energies of adhesion for the asphalts against silica and high values of these energy relationships against h e s t o n e and blue clay. Asphalts which have a high coefficient of spreading towards an aggregate posses5 a high degree of wettability for the solid, resulting in a thin film of asphalt. The thickness of the asphalt film surrounding the aggregates has a direct bearing upon the mechanical properties of paving mixtures, especially in cases where the rupture of the pavement occurs in the asphalt layer, since the tensile strength of an adheqive layer increases as its film thickness decreases.
Behavior of Asphalt Pavements under Influence of Water Asphalt pavements come in contact with water during a large part of their life, whether i t is subsoil water or rain and snou-. The durability of asphalt pavements depends to large extent upon its resistance to the action of water, accelerated by the churning action of moving wheels.
DECEMBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
Suppose a solid body is embedded in asphalt at the interface against water. I n order to bring the solicl from the asphalt into the water, a certain amount of energy is required. This energy is directed against the interfacial energies and, according to Hofmann (5),is equal to: S solid/asphalt - S solid/water - S asphalt/water where S denoted the interfacial energy and the suffixes the interfaces. This reaction involves two possibilities : 1. The energy is zero or positive; the solid goes freely from the asphalt into the water, since energy -. is not required or is gained-in this reaction. 2. The energy is negative; the solid remains in the asphalt and energy is required to bring it from the asphalt into the water phase. This energy is positive when the mixtures of asphalt and silica come in contact with water. The water displaces the asphalts from silica in accordance with the theory. On account of the negative values for the interfacial tensions between the asphalts investigated and limestone and blue clay, the displacing energy is negative, and water does not displace the asphalts from these aggregates. Thus with the methods outlined to measure the changes
Action of Dilute Acids on Aluminum CHARLES F. POE, R. M. WARNOCK, AND A. P. W Y S s University of Colorado, Boulder, Colo.
T
HE amount of aluminum dissolved when
foods have been cooked in aluminum utensils is a subject of more or less controversy. The degree to which aluminum cooking utensils would contribute to the presence of aluminum in prepared foods would depend upon the aluminum-dissolving substances, such a3 acids, which are present in the foods. The literature, however, reveals that very few quantitative results have been recorded concerning the solubility of aluminum in acids. The present paper reports the action of acids on aluminum, including some of those acids which may occur in food products. Some work has been done on the effects of inorganic acids and a few organic acids on aluminum. Most of this work has been qualitative, and the results are somewhat contradictory. S o attempt will be made to include here all of the references to the solubility of aluminum in inorganic acids, but the more important quantitative results with the organic acids will be listed. Of the organic acids, acetic has been most studied. Hodges (5) found that strong tartaric, citric, and previously boiled
1505
in energy which occur when asphalt comes in contact with mineral aggregate, it is possible to make predictions with regard to the behavior of asphalt paving mixtures on the road.
Acknowledgment The author desires to express his thanks to the directors of Imperial Oil Limited for the privilege of presenting this paper.
Literature Cited (1) Bartell, F.E.,and Jennings, H. P., J . Phys. Chem., 38,499(1934). (2) Bartell, F. E., and Miller, F. L , LND. ENG.CHEM., 20,738 (1928). (3) Bartell, F. E. and Osterhof, H. J., J . Phys. Chem., 37,549 (1933). (4) Coleman and Gardner, Paint Mfra.' Assoc. U.S., Tech. Circ. 85 (1914). (5) Hofmann, F. B., 2. physik. Chem., 83,385 (1913). (6) Nellensteyn, F. J., and Roodenburg, N. M., Kolloidchem. Beiheftc, 31, 434 (1930). (7) Nicholson, V., Proc. Assoc. Asphalt Paving Technolog., 1932, 28. (8) Riedel, W., and Weber, H., Asphalt Teer Strassenhuutech., 33,677 (1933). RECEIVEDMay 22, 1935. Presented before the Division of Petroleum Chemistry a t the 89th Meeting of the American Chemical Society, New York, N. Y . , Spril22 to 26, 1935.
acetic acids had no effect upon aluminum, but that cold saturated oxalic acid dissolved it rapidly. Seligman and Williams (18) studied the effect of several concentrated organic acids on aluminum and found their action to be slight. Maass and Wiederholt (11) found that the common inorganic acids had more effect on aluminum than did oxalic, acetic, or tartaric acids. Utz (19) stated that dilute lactic acid (0.5 to 1.0 per cent) had no effect on aluminum a t room temperature, and that a slight amount was dissolved a t higher temperatures, but not enough to produce any harm physiologically. Lunge and Schmid (10) and Lunge (9) studied the solubility of this metal in a few organic acids, and found the maximum amount dissolving in 6 days to be 6.35 mg. per 100 sq. cm. of surface exposed. Rupp (17') tested six organic acids during 8 days and found the maximum amount of aluminum dissolved to be 1 mg. The strengths of the acids ranged from 0.4 to 10 per cent. One per cent acetic acid dissolved no aluminuni. but 10 per cent acetic dissolved 0.1 mg. Hunziker, Cordes, and %sen (6) treated sheets (5.28 square inches) of aluminum for 5 days a t 70" F. with acetic, butyric, lactic, and citric acid3. The acids were 1 per cent except citric which was 0.2 per cent. The maximum amount dissolved was 2.1 mg. Klut ( 7 ) stat'es that aluminum is not attacked by dilute organic acids, sulfuric acid, or nitric acid. Ohlmciller and Heise (16), Plagge and Lebbin (If?),and Mansfeld (IS) concluded that aluminum was quite insoluble in cold dilute acetic acid, and somewhst soluble in the boiling acid. A number of authors (1-4,8,18, 14) have studied the action of certain acid foods on aluminum cooking utensils.
Procedure The purest sheet aluminum obtainable W:LS used and was composed of the following: alurniuum, 99.26 per cent; ircln, 0.53; copper, 0.03; silicon, 0.16; and manganese, 0.00. The aluminum was not heat-trea,ted. The strengths of the acids were normal, 0.1 normal, and 0.01 normal. Because of the insolubility of some of the organic acids, stronger solutions of them could not be prepared. One hundred cubic centimeters of each acid were measured into Pyrex flasks, and a piece of sheet aluminum (6 X 6 cm.) weighing about 4 grams was added. This sheet was allowed t o remain in contact with the solution a t 25" C. for 12 weeks, and was removed, washed, dried, end weighed at stated intervals. The loss in weight was recorded
