COLLOIDAL STRUCTURE O F ASPHALT
1195
SUhIJIhRY
Sols and gels of barium sulfate have been prepared in acetic acid by the interaction of barium acetate and sulfuric acid. The sols are readily coagulated with excess sulfuric acid, but excess barium acetate does not readily cause coagulation. The sols seem t o be positively charged, as determined by coagulation by electrolytes and by cataphoresis. The amount of incident light absorbed by the sols sloirly increases over a period of several weeks, except for such concentrations as form gels. With these dispersions the amount of light absorbed increases t o a maximum and then decreases. S o visible settling occurred during the year after preparation. REFERENCES (1) D ~ ~ I D sA o . IT ~ .,: J. Am. Chem. Soc. 50, 1890 (1928). (2) DATIDSOA, W., ASD M C ~ L L I S TW E. RH.: , J. A4m Chem. Soc. 52, 507 (1930) (3) ELDER, A . L., A S D BCRKARD, P. X . : J Phys. Chem. 41, 621 (1937). (A) OSTW'ALD, IVO., A S D IVTASNOW, H. &i.:Kolloid-Z 76, 159 (1936) (5) SCLSOS, H. A . : J. Phys. Colloid Chem. 51, 1103 (194i). (6) SKILES, B. F . : Master's thesis, University of S e b r a s k a , Lincoln, 1934. (7) WEISER, H. B., . ~ S DMACK,G . L.: J. Phys. Chem. 34, 86 (1930).
THE COLLOIDAL STRUCTURE OF ASPHALT H. EILERS Laboratory N . V . de Bataafsche Petroleum i v a a t s c h a p p i j , A m s t e r d a m , Netherlands Receiued Souember 1,1048 I. IKTRODUCTIOS
Petroleum asphalts are mostly obtained as residues in the distillation TYith superheated steam and in vacuum at temperatures varying from 300" to 400°C. of crude oils of the naphthenic-aromatic-asphaltic type (e.g., Mexican, Yenezuelan) (36).Sometimes asphalts are prepared in the same \Yay from the rcsiduc of thc crac1,ing distillation of heavy dihtillates. When at 230-300°C. air iz let1 through a heavy asphaltic residue, oxidized asphalts are formed. A l t atmospheric temperatures asphalts are highly viscous to apparently solit1 suhstances. The rheological properties of asphalts have been the subjcrt of many investigations, among others 11~-Lee (lo), Mack (13). and T r a d e r (32. 10), ho concentrated on the relation lietn-een stress and rate of shear, n-hereas 1,ethersich (11) and Saal (33, 34) studied also the variation of ihc rate of shear during deformation at constant stress. These investigations showed that the rheological behavior of most asphalts is that of a colloidal diqpersion in the sol or gel state. A fen of the products mentioned above show a purely viscous flon-: rate of ahear at constant stress independent of time, deformation per unit of time proportional to shearing stress, no elastic recovery (see figure 1).
3Io-t coinn~ciciul asphdts prep:Lrctl hy distillation from residues show in addition flnitic iIr,formabii?fy,11hich a t constant shearing stress is manifest from an initially decwa4ng rate of deformation and, after removal of :hc stress, in a p:wtial ehst ic. t w o w r j * ; tlie find ratc of deformation at constant shearing stress is in acmrdancc 11-ith purely vivous flow (so2 i y p e ) .
I
7
4 I
Purely viscous type
I
t
1
Viscous with elastic effect (sol type)
h Yield volue with elostic effect ond breakdown of structure (gel. type) 1;1(?. 1. 8chcrri:itic ~cpresent:itioiiof thc cleformntioli ~)licnomen:tin n s l ) h s l t . D! deformn t i o n ; t : tirnc; T . , ~ i i c : i r i n g s t r c ~ s .
VTithothci Li\piialts, *uch a 3 the typical owlized pioducts. the elastic recovery tleiormat ion5 undcr a imall i t rchc is almost conipIete; therefore, under thew c*oiiditions only clastic and no permanent viscous deformation ocrurs. When the qhearing str exceeds a certain value, the yield value, the recovery 1 ) r c ~ ~ n incomplr~tc~, t~a T i.(wus defuimntictrl occurs in addition to a n elastic one. l h r rate of defoi.mation iuidc:. conctnnt stress may even increase with time, but nftei i.cmo\-al of thc . t i ~ wthe original value of the resistance to shear is gradually rc(~~r(w (fh?n.ntr,p;y), ~1 ;)ointjnq I I ) :: rp\-pr41)le l~renkdonn of :in intcrnal struc-
:if1 PI' .light
r 7
COLLOIDAL bTHUCTUHB O F ASI'HSLT
119T
ture (gel t y p e ) , These asphalts also display a considerable age harclcning (Traxler (41, 43)). Asphalts must be considered as mixtures of an indefinitely lnrgcl nunilxr of chemical indiriduals of mainly hydrocarbon nature but varying in the structurci of the hydrocarbon chain (e.g., in aromaticity and blanching), in tlir prcmicc' of other elements than carbon and hydrogen (e.g., oxygen, nitrogen, or sulfur), and in molecular weight. All separations made in this mixture are arbitrary. The solubility in 5uch liquids as petroleum ether is often used in routine analysis and has been dio\\n to constitute an approach in the study of the colloidal structure of the product. By the addition of aromatic-free gasoline (b.p., li0-8O"C.) or diethyl ether, asphalts of the sol and gel types and most purely viscous asphalts can lie split up into a fraction soluble in this solvent, the ma/tc,nrs, and onc insoluhlc in it, the asphaltenes. The asphaltenes, which arc obtainecl as u solid precipitate, form at least part of the disperse phase of the colloidal system, while at 1ca.t part of the maltenes form its continuous phase. The maltenes can be gradually refined by means of adsorption on earth (Saal (33)), or split up into several fractions (resins, oil and wax) by adsorption and extraction (Stricter (38)) or by the use of different solvents (Hoiberg (8)). It nil1 be sho\\n tliat, depending on the temperature, a greater or smaller part of thc i e4noui m:rt c1rial is inclutled in the disperse phase of the system. The amount of asphaltenes of an asphalt and tlieir composition ctepend laigely on the crude oil and the process of manufacture. The asphaltene content of most petroleum asphalt lies between 5 and 30 pcr cent of the asphalt. The composition of asphaltenes and malteney f r o m w m c rharac.teristic* a*plialts iy given in table 1. The usphaltenes in the Alelican asphalt, ulitained by distillation, w i e already present in the cmde. Part of the a+plialteneh i n the oaitlize(1 1-enczuelan asphalt were present in the crude, but during the oxidation proccs:, asphdtenes I\ ere alw iornied from maltencs by dehydrogenation and cwdeiitation, accaording to T3oihrr.g (8) ehpecially from r e m s . Thp ahphaltenei of tlic residue from the (rac.l;ing oi g:i- oil 11 et e ciitirely formeti ~liiringthc. pnogeriic tlrcomptr\itiori oi hpclroc~arboil-. To c.haiuctcmze the composition of these pi'ocluc*t~,the i :)ti0 of the numixi> of (ai tion .ml hydiopcn atom\ i> u+etl. The lo\\ PI thii 1 atio, t lit. \ i i ongei I:, tlw aliphatir (*lxiiacter of the piotluct (the C' 11 ratio oi paraffin hvtliocai*bon\ approac*hc.i 0.30); the highei the C H rdtio, the mole aicimati(. 15 the product 1 I atio of 1)enzcne. 1.0, of nnphthalrnc, 1.25) 'l'liis ratio ii, lo\\ EI \\ i t h maltenri th:m I\ itli a-pli:dtenr+. and 11 ith 17eneziwlan ant1 &Iexican :ibphalt- i t 1 i lo\\ ei than 11ith the residue obtained fiom cracking. Ahphaltenesare richer in oxygen, nit iopen, and iiiltur I ~ I thc I roi irspmciing nialtenes. 11. SOLUBILITY OF
IqPH \ L T E \ C %
Sellensteyn (13, 16, l i ) v a s the fiist to stiezs the colloidal nutuie of a ~ l ~ l ~ l i , hii fundamental study of the solubzlzty of asphultenes in organic solvents let1 t o
119s
H. EILERS
the conclusion that they are soluble in liquids nhich are mibcible in all ratios itli the maltenes and have a surface tension higher than about 2-1-26 dynes per centimeter. -kcording to our investigations, these criteria can be better formulated as folio;\ s: -1sphalteneb are soluble in a nolz-polar o~ wcaldy polar liquid hen its znterual pressure exceeds a certain limit. Xs a measure of the internal pressure, Hildebrand (G) proposed the value cJ*-l (c = surface tension, T' = molecular volume of the solvent). the cubic roots of the molecular volume of the common solvents are nearly eqiial. the surface tension alone, as applied by Seliensteyn, gives an approximation t o the inTABLE 1 Composition of maltenes and asphaltenes f r o m some asphalts CRCDE
...........................................................
MEXIC.\S
YELTZCELAS
...................................................
OXIDATIOS
DISTILLATIOS
~ _ _ _ _
,
A sphaIt;
I
Softening point of asphalt, R and B,"C.. . . . . . . . Penetration, 25°C.. . . . . . . . . . . . . . . . . . . . . . . Per cent iiisoluble in 60-80 gasoline. . . . . . . . . .
Maltenes Per cent Pel cent Pel cent Per cent Pei cent
R X S I D U E FROM CRACKING 01' G4S OIL
72 15 26
C H S
82 5 10 9 5.4
I
0 4
0 (as tliffeience)
0 8
85
51 36 26
40
2s
I
I
84 11 2 0 1
87 9 7.9 3 7
3 1 9 4 3
0 5
I
c II l a t l o
0 63
0.63
Asphallenes: Per cent C . . . . . . . . Per cent H . . . . . . . . . . Per cent S . . . . . . . . . . Per cent 1... . . . . P e r cent ash 0 (as difference). .
+
. . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . .
GI1 ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
81.5 8.1 3.5
81.4 8.0 8.3 0.6 1.7
0.85
~
0.87
0.95
'
i 1
88.9 5.9 3.0
1.26
tcrnal pressure. Table 2 shon-s that the bolubility of an a y h a l t in non-polar liquids is governed hy thi. quantity: the asphalt used x a s completely soluble in all non-polar liquid5 ~5 ith a value of UT'-' larger than about 4.5, lyhereas n ith lower values of this quantity the amount of precipitate increases n-ith decreasing internal piewire. himilar hut not identical relation nas found to exist for ethers having a permaimit polar. moment of alioiit 1 X 10-l3. Kheieas a y h a l t 3 obtained 11)- careful distillation with steam and vacuum of crude oils niodlg cwiitain no constituent- insolitble in, for example, cyclohexane, supeiheutd aqphalt.: or those made from crac~kedresidues often do. For most asphalts ethyl ether and aromatic-free petroleum spirit (b.p. GO-SOOC.)
1199
COLLOIDAL STRUCTVRE O F .lSPH.iLT
give about the same amount of precipitate, hut some show a conqiderahle difference. Sellensteyn and Kuipers (19,20) determined the solubility of a number of asphalts in petroleum spirit (b.p. 40-60°C.) and in ethyl ether. They found that especially Saran al; petroleum asphalt, Trinidad Lake asphalt, and Gilsonite contain considerably more component5 insoluble in this gasoline than in ethyl ether. They treated the ethyl ether maltenes from Trinidad Lake asphalt ivith petroleum spirit (11.p. 4040°C.) and obtained 11 per cent "difference" asphaltenes. Comhustion analysis shon-et1 that the second precipitate had a higher o\rygen content than the first one (5.8 per cent ancl 4.4 per cent, respectively): part of the T.1BLE 2 Solltbrlltg o j a 3/~.1zcannsphalt (peneiratzon a t ?6'C. $0-60) iiz dzferent h v d i o c a r b o n s u rihout p e r m a ne n t p o l a r i i l o n l e r l t
SOLYEST
I
UOLECTLAR TVEIGHT
PER CEST ISSOL-
DESSITY BOILIZG P O I S T
d3nes on
"L
n - P e n t a n e . , . . . . . . . . . . . . . . . . . 72.15 Isoiict ane (2>2.4-trimetliylpentane). . . . . . . . . . . . . . . . . . ' 114.22 2.2.3-Trimethylbutanc . . . . 100.20 n-Heptane . . . . . . . . . . . . . . 100.20 3-blethylheptane. . . . . . . . . 114.22 n-Sonane . . . . . . . . . . . . . . . . . .128.25 Dimethylcyc1opent:tne; . . . . . . 98.18 Methylcyclohexane.. . . . . . . . . . 98.18 Ethylcyclohexane . . . , . , , 112.21 Cyclohexane. . . . . . . . . . . . . . . . . 84.16 BPnzene. . . . . . . . . . . . . . . . . . . . 78.11
' 1
36.2
0.6263
1.3578
15.9
3.27
33.5
99.3 60.8 98.4 119.1 149.4-150.8 90.5- 91.4 99.4-100.3 131.8-132.1 81.4 80.1
0.6919 0.6900 0.6837
1.3013 1.3894 1.3916 1.3987 1.4055 1.4112 1.4228 1.4330 1.4257 1.5011
18.6 1S.i 19.9 21.2 22.6 21.3 23.2 25.4 24.0 28.2
3.39 3.56 3.77 3.89 4.01 4.19 4.61 4.86 5.04 6.32
32.2 27.2 25.7 25.6 23.6 15.1 0.0 0.0 0.0
~
~
~
~
, ,
,
O.iO55 0.7182 0.7487 0.7691 0.7879 0.7778 0.8794
0.0
* Determinations irith Cenco-Du S o u 2 interf:tcinl tensiometer, corrected according t o Harkins 2nd Jordan. t T K Ograms asphalt i n 100 i d . solvent, filtered on Gooch crucibles, n-ashed with 200 ml. of solvent. Prohably a mixture of cis- and trans-1.3-diniethylcyclopentanes. second precipitate TKIS soluble in acetone and contained as much as 9.8 per cent of oxygen. The Saran-al.; asphalt gave no precipitate with ether, but it did n-ith 40-60 gasoline. Thk precipitate \vas also w r y rich in oxygen (5.4 per cent). Findly, it should he i ~ m a r k e dthat the ether nsphnltcnes of Crilsonite are vcry rich in nitrogcn-containing compounds; Srllcnsteyn antl Iiuipcrs (21) foiind 2.9 per cent of nitrogen. It. is therefore prohnhle that 40-60°C'. petroleum ether precipitates from the asphalt more components with x dcfinite polar character (due t o their oxygen antl nitrogen content ancl possihly alvo to n more pronounced aromatic structure) than does ether. On the other hand, ether is a poorer solvent for high-molecula1-n-eiglit components n-ith a pronounced aliphatic character. I n Dutch asphalt petroleum spirit (11.p. GO-SO"C.) gives no precipitate, hut ether gives a precipitate n-ith a C ' H ratio of about 0.60.
1200 Oliensis (24, 25, 27) suggested tlie occurrence of ztehomoyeneity after tlie mixing of asphalt with a fivefold quantity of a certain naphtha as a meaiure for the stability oi the internal phase relationship. This test vias proposed originally t o detect superheated or cracked asphalts. -Us0 b y mixing asphalt ivith a large quantity of parafin wax, inhomogeneity can be produced in this test, probably because the dissolving pon-er of the continuous phase of thc asphalt decreases too much. Some asphalts, e.g., from West Texas (Born (I)), appear, without superheating or mixing with aliphatic constituents, to be inhomogeneous in this test; the dissolvingpower of their maltenes seems to be insufficient to peptize the ssphaltenes after mixing with the naphtha. When using this test t o evaluate the stability of 3 z em, it 4iould he h r n e in mind that in this test the dissolving power of the solvent phase io always practically the same, m., approaching that of the naphtha ubed, \vhcreas that of the maltenes may vary, and will probably he larger with asphktlt5 irom cnicl\ring T.lI3lL 3 Successii'e hot erlractiotis Kith tiiJri~eti1 s o l w n i s o j asphalieties ( p r c c i p i l u t e d irith aroiltuticj r e e 60-80 gasoline i n t h e c o l d ) j ' r o i n (it! oxidired asphalt f r o ! , / T'enezicelan c r u d e oil ( p r i i e tration at 25"C., 21; softeniny point, R and B. 86'C.; fractiora insoluble in 60-80 pelroleirin s p i r i t , 32 p e r c e n t ) ~. ................... - .... ..... ~. -.\SI"
o,- I : . < (,F i n L ~ I : S T
ILTFSI:$
1'l:R C E S T 01' .\Sl'A%LTl
,
100
Extract obtained w i t h : a4romatic-free GO-SO gasoline Diethyl ether . . . . . . . . . Lthyl tert-:iniyl p t h c r . . . . 1Iethyl tert-uni>.l e t h e r . . . . . . . . . Benzene . . . .. ..........
~
4.5
3.9s
2.4 2.6 3 . -1 :i.i5
6.32
6.i
3.55 3.81
residues with n high c' 'H ratio than with the solvent usetl (ace table l j . 'Hie restilts of this tcst should t h e r c f o l ~hc used Ivith caution. InstubiIity ( i f t h o (lisperaion of asphultcnea in u paru~nic-ntiplitheiiicc r ~ ~ c l c pool. in aromatic.+. seems also t o have Ixwi drmonstrated hy I);vkstra, Beu, :111cl Tiatz (2). \ v h o ol).wi~vc~I with an dect ron micw)scope the occurrence of hcterogeneity in :in i,:as;: l'csxs cmtle oil after filtration thiorigli smcl. 111. ('OAIPOSITIOS
OF TIIK .\SPEI.\LTI~~SI:$
Aisplialteiicsran i w fr:wtion:ttcd into products ivith somr~\vhat8 tli\.cJigx>nt p i ~ i p ttrt,ies (Pfciffcr(20)) Iiy stwcsc.isive hot extractions with wrioiis sol\-c~nts (table? 3 ) . 1 he first cwrarts, highly viswus liquids, havc a some\vliat l o ~ v c r( ' 1-1 ratio than the fraction only soluble in benzene, \vhich can be Iieatcd t o decomposition without softening; however, these differences are not large. In the same ivny the asphalteiies from residues from cracking gas oil can be tlii-iclctl into a fraction (about 50 pel' cent) soluble in methyl tot-:my1 ether lvith C, I1 = 1.14 and a residue u-itli C '1% = 1.4. This last fraction Tvas solrihle in c2:trhon tlisulfidc to the esterit of 90 pc'r w n t . r ,
COLLOIDAL STRUCTURE O F ;ISP€ZALT
1201
Estimation of the molecular zcczght of nsphaltenes (Saal (33)) gives widely 1-arying results according to the method employed. K i t h the crj-oscopic methods value- ranging from 2000 t o 30,000 are iound. Honever, a value of, say, 3000 can u l s ~be obtained if ,z product is conipoyetl of 50 pcr cent components v-ith molecular 11-eight = 2300 and 30 per cent with very high molecular weight. As a rulc the average molecular weight is lon-er in asphaltenes n-ith a high C/H ratio than with those of a less aromatic character. Cryoscopic determination of the a\-erage molecular eight of the maltenea gave values varying for different asphalts from 400 to 1000. Molecular weight determinations of asphaltenes, based on measurements of the viscwsity of solutions in combination with the application of Staudinger’s relationdiips (3), leading to values of about 1000, seem not conclusive, because tliesrelations are based on the behavior of linear high polymers, a configuration nhich ia not proven for asphaltenes. Spreading tests on the Langmuir apparatus, carried out after evaporation of the d v e n t from asphaltene solutions spread on water, give much higher values for tlic particle n-eight of asphaltenes if the particles are assumed t o be cubic: 100,000 to 200,000 for asphaltenes from Mexican or Venezuelan crude and about 20,000 for asphaltenes from residues from cracking (Pfeiffer and Saal (29)). In this nicthod the largest particles will probably play theJeading part in the construction of a layer of uniform thickness. Swanson (39) considers such high values very probable on account of observations on the velocity of diff usionof asphaltenes in solutions. Investigation of a Californian crude oil by means of the electron microscope (Preckshot (31), Iiatz (9)) failed t o show any particles, but it did after treatment with a solvent like benzene or after an electric current had been passed through it. From these findings the inxestigators conclude that the original particles should be smaller than about 63 A , so that the molecular weight should be below 100,000. From the above it follows that asphaltenes cannot be seen in the microscope. I t is probable that the ultramicrons seen by Sellensteyn (22) when investigating isphalt solutions hetn-ecn moist glass plates are mainly impurities (salt, ~i ater, :tc.). ( h i 1 (WI oiliers, dc Lange and Corbet, studied tlic t n o broad bands in the .Tw y cliagram of asplialtenes-one indicating an interferential distance of about 3.3 A\,the other indicating one at 4.5 9.The hrst ha3 already been mentioned by Sellensteyn (I(;, 1 T ) and Pfeiffer 128), the second l x i > been described by 4ac.k (3T). ‘I‘he interierential dihtance 3.3 -1.m q * iiidimte thc cliataiice of six-ring planes l a in graphite, as ne11 as in liexairIethvlt,enzene, nliilc that at 4 5 -1.is found r\ ith many aliphatic hydrocarbons in the liquid state and indicates the distance set\\-een the pbr:iffin chams. Further, the diagram may contain bands due to x-ystalline impuiitie, of the asphaltcnw lilie sand, clay, etc. Tablc 4 gives yome examples in uhich the inteniity of the principal inter-
1202
11. EILERS
ferences w s evaluated with respect t o those given by the asphaltenes of Gilsonite and by Shell Carbon Black 33, respectively. It appears that the ratio of the intensities of the two bands correlates with the CtH ratio of the asphaltenes. These results do not corroborate Sellensteyn's view (18, 23) that asphaltenes must be considered as micrographitic carbon, surrounded by an absorption layer of free radicals of low molecular weight (ECH, =CH2, -CH3), but support the view that the asphaltenes from, e.g., Mexican or Venezuelan crude oil are large molecules composed of condensed, mainly naphthenic, aromatic, hydroaromatic, and thiophenic ring structures in combination with hydrocarbon chains of different lengths, while groups containing atoms of other elements may also be present (Hillman ( i ) ,Murphy (13)).I n the case of asphaltenes from cracked residues the aliphatic chains are probably shorter than with most T,IBLE 4 Relative intensities of the nzain rings in the x - r a y diagrams of some asphaltenes a n d t h e W H ratio of these products ASPElLT
PRECIPIT4TION X E D I U N _..
Dutch residual.. . . . . . Gilsonite . . . . . . . . . . . . . , Boeton n a t u r a l . . . . . . . Boeton n a t u r a l . . . . . . . . . . . . . . . . . . . . . Trinidad E p u r 6 . . . . . . . . . . . . . . . . . .. I 11esican residual. . . . . . . . . . . . . . . . Venezuelan residual. . . . . . . . . . . . . . . . Argentine residual. . . . . . . . . . . . . . . . . Venezuelan residual. . . . . . . . . . . . . . . Residue from cracking. . . . . . . . . . . . . . ~
~
'
~
Ether 60-80 gasoline 60-80 gasoline Ether 60-80 gasoline Ether 60-80 gasoline 60-80 gasoline
Etk:
Izllt \SIT\ R4TIO
__ r
r
2 1.3 1.0
1 0 0.x 0.5 0.4 0 __
c ~
H ~
0.61 0 i1 0.74 0.80 0.78 085 0 99 0 b9 0.90 1 32 -.
distillation residues, while in the first asphalts of table 1 the aliphatic chmicter predominates. IV. VOLCMIKOSITY OF ~ i 8 P H I L T E S E SI S SOLTYIOSS
Determination of the 1-iscosity of d u t i o n s of :isphalt components in piire solvents provides a simple I\-ay t o obtain an insight into their colloidal propertic)z;, especially so \\-hen the results arc expressed in terms of tho z~o/irminosit~~ of the solute, i.e., the volume that seems to he occupied in t h e solution by the qunntity of solute of unit volume in the dry state. Table 5 gives some data on the voluminosity ( 1 ' ) of :\splialtcncs at infinite dilution in solvents with low molecular \\-eight, calculated from the Einstein equation; qp =
1
+ 2.31c
c = concentration in parts by volume. The table s h o w that in solvents n-ith high solvent pon-er for asphaltenes the voluminosity of the asphaltenes d e i w
1203
COLLOIDAL STRUCTURE OF ASPHALT
with increasing C/H ratio of the asphaltenes; this may be due t o the correlation between C/H ratio and molecular weight. I n solvents with moderate solvent power a someivhat higher value is generally found for the voluminosity than in those with high solvent power. This may point t o the formation of aggregates in poorer solvents. The voluminosity of the asphaltenes decreases with increasing temperature. The ratio of the voluminosity at 30°C. to that at 0°C. varies for asphaltenes of different base materials from 0.77 to 0.83. I n table 3 some values are listed for the voluminosity in carbon disulfide of fractions obtained from asphaltenes by extraction with various solvents. From these data it appears that the most soluble half have a lon-er voluminosity than the average; only the part soluble in benzene has a higher voluminosity. As the arithmetic mean of the products of voluminosity and quantity of the fraction is close t o the value found for the mixture, it is not probable that in very dilute TABLE 5 V o l i t m i n o s i t y o j asphaltenes in extremely dilute solutions in some solvents at 30°C. SOLVEKT
Carbon disulfide OF S O L V E S T . . .
. . . . . . . . . . .
d s p h a l t e n e s from: llesican distillation a s p h a l t . . . . . . . Venezuelan oxidized asphalt. . . . . . . T-enezuelan distillation a s p h a l t , . . East Indian oxidized asphalt. . . . . . California cracked asphalt. . . . . . . . . Residue of cracking. . . . . . . . . . . . . . . ~
~
.I
~
7.1
1
3.7
.
4.0
3.1 2.5 2.1 1.7
1
6.5
4.5
-
6.3
,
1.6
1
3.6
. ~
3.5 2.7
CfH O F ASPH\LTESES
B~~~~~~
Tetralin
2.4
0.85 0.87 0.87 1.03 1.05 1.25
solutions in carbon disulfide the molecules of the different fractions are united t o large mixed aggregates. -Ittempts to determine cryoscopically the solvation of asphaltenes in solutions in benzene containing a small quantity of cyclohexane, or in cyclohexane solutions with some benzene, showed that the asphaltenes probably do not adsorb preferentially appreciable quantities of one of these compounds in these solutions. The voluminosity of the asphaltenes in suitable solvents of low molecular weight is therefore probably due to the shape of the molecules; the latter might for instance consist of irregularly branched chains or a ring systems with several rather long side chains, so that the whole has a “spongy” structure. Also at higher concentrations the viscosity of asphaltene solutions in benzene and carbon disulfide was studied. If the relative viscosity is plotted against the rheological volume, Le., the product of concentration and voluminosity at infinite dilution, most points (figure 2) are found to lie scattered round a line situated at a lower level than that obtained by the expression qr =
(1
+ 1 -1 . 213 C. V3 3 ~ ~
1204
H. EILERS
Rheological volume = V x % vol. asphaltenes FIG 2. Relation between rheological volume and relative viscosity of asphaltene solutions in benzene or carbon disulfide a t 20°C I
~_-1SPII \LTI
-
-
\I5
hIeAicaii straight-run bitumen in benzene California cracked bitumen in carbon disulfide Indonesian blown bitumen in carbon disulfide Mesican straight-run bitumen in carbon disulfide Blown aromatic extract in carbon disulfide Venezuelan blown bitumen in carbon disulfide Bitumen from residue cracking in carbon disulfide Mexican blown bitumen in carbon disulfide
which is found for dispersions from undeformable spheres of equal size (Eilers (4)). This makes it improbable that in concentrated solutions in benzene or carbon disulfide the asphaltenes form larger aggregates than in dilute solution.
1205
COLLOIDAL STRUCTURE O F ASPHALT
The decrease in voluminosity of the asphaltenes with increasing concentration may be a consequence of deviation from the spherical shape as well as of deformability or of inequality of size of the particles. Similar determinations were carried out with solutions in carbon disulfide of the asphaltene fractions of table 3. It was found that the methyl tert-amyl ether extract gave about the same viscosity-rheological volume curve as the asphaltenes as a whole, but that the voluminosity of the fractions first obtained decreased more with rising concentration, vhile it increased with the benzene extract. I n a concentrated solution of this fraction, formation of coarse voluminous aggregates probably occurs, which is not appreciable if the other extracts are also present. From these experiments it must be concluded that in non-polar solvents with it high solvent power, such as carbon disulfide, the asphaltene molecules are largely free, but it is possible that the least soluble ones are associated with a certain amount of the more easily soluble ones. TA4BLE 6 Voliiniinosity ut injinite dilution o j asphaltenes f r o m M e x i c a n distillation asphalt (penetration at 26'C., @-SO), dissolved in the corresponding maltenes
150 125 100 25 0
2.8 3.2 3.6 12.0 15.9
Tiscosity measurements 11 ere carried out also with solutions of asphaltenes in the corresponding maltenes,-at high temperature in capillary viscometers, It low temperature in a coaxial Couette viscometer. Table 6 shows some values t'ound for the voluminosity at high dilution of asphaltenes of a distillation asohalt. If one con+ler~ that n i t h solutions in solvents of loir molecular \\eight he voluminosity also drops I\ ith rising temperature, one obtains the impression hat the asphaltenes cbssolved in mnltenes are, at high temperature, in the same ,tate as in solventh 11 ith ;L low molecular 11eight and a high solvent poi\ er. On he other hand, at lon lemperatuies the voluminosity of the asphaltenes is so nuch higher as to point strongly to the formation of larger complexes. Mack's neasurements (12) on the viscosity of solutions of asphaltenes point in the same lirection. Experiments in our Laboratory have shown that a dispersion of that part of lie a5phalteneb that ia only soluble in benzene in a liquid with maltene properties ,ives s system which has the rheological properties of an asphalt of the gel type, u t vhich passes into a system of the sol type after addition of various ether stracts of the asphaltenes. These results indicate that the least soluble ashaltene constituents have a tendency to mutual adherence when dissolved in
1206
H. EILERS
maltenes, and that the asphaltene extracts have a peptizing influence on this system. V. GEL STRUCTURE I S ASPHALT
To decide n-hether maltene components albo take part in the formation of the complexes, data derived from the study of the gel-type asphalts are enlightening. When a layer of these asphalts is covered by a fine ponder, e.g., talcum or limestone, and stored at, say 73"C., the ponder becomes greasy with oil extracted by capillary action. This oil is considered to be representative of the intermicellar phase of the asphalt ; it is a reproducible, asphaltene-free product whose composition differs from that of the maltenes, as appears from specific gravity, refractive index, viscosity, etc. By refining the maltenes of the asphalt under test with increasing quantities of decolorizing earth, a series of products can be prepared, the properties of one of which correqond t o those of the sweated oil. T.1BLE 7 T e r r a n a refining o f a n oxrdzzed asphalt (asphaltene content, 32 per cent; nialtene content, 68 per cent) m i x e d with a l a r g e q u a n t i t y of petroleiini spirit ( b . p . 60-80OC.) PROPERTIES OF R-IFFIXkTE
_
I
0.3 1.0 2.0 3.0 6.0 10.0
' 1
,
65.9 56.2 50.5 44.7 39.6 35.2
1 I
~
_
~
ceniipotses
0.937 0.920 0.909 0.898 0.888 0 . S60
354 139 86 61 46 38
1.529 1.515 1.508 1.501 1.494 1.488
The yield of this product, calculated in per cent on asphalt, is considered equal to the percentage of intermicellar phase (Pfeiffer (29); Saal (33)). Table 7 gives a survey of the refining of maltenes of an oxidized asphalt and of properties of the products obtained. Extraction of the limestone powder after a 10 days' sn-eating test of this asphalt gave 2.6 mg. of sn-eated oil per square centimeter. This oil had a specific gravity 6O0,4" = 0.920 and a viscosity at 60°C. = 179 centipoises. Comparison with the data of table 7 shows that this sweated oil constitutes 57 per cent of the asphalt. Therefore part of the 68 per cent of maltenes must be reckoned t o belong t o the immobilized phase (11 per cent on asphalt). As it consists of that part of the maltenes that is most easily adsorbed by terrana, it is assumed that it is also bound by adsorption to the asphaltenes. This result confirms Sachanen's vie\v (35) that the asphaltenes are colloidally dispersed in petroleum products, owing t o peptization by adsorbed resins and heavy polycyclic hydrocarbons. Thesystem can becharacterized by the degree 0.f sorption of the asphaltenes, i.e., the amount of maltene constituents present in the] skeleton-building phase, espressed in per cent on asphaltenes, in this case: 11/32 X 100 = 34 per cent.
120T
COLLOID.IL STRUCTURE O F .ISF”.iLT
For a series of Teaezuelan asphalts with increasing degrees oj“ qclatzon, as expressed in terms of the penetration index1 (Pfeiffer (30)), the degree of sorption was determined (table 8). The degree of sorption of the asphaltenes is lower in this series of T’enezuelan asphalts as the gelation is stronger. The degree of sorption of the asphaltenes decreases with a rise in t c m p e r a t w e , as is shon-n in table 9; this is in agreement n i t h the fact that the voluminosity of the asphaltenes in maltenes at 100°C. and up does not point to the formation of large complexes (table 6). At high temperatures the kinetic energy of the molecules overcomes their rather weak attraction forces. On cooling a bitumen down from say 130°C. to atmospheric temperature an arrangement of the molecules must take place due to adsorption affinities. The high viscosity of the medium presents a considerable obstacle, so that equilibrium is often only reached after a certain period of time; consequently these bitumens may display considerable age hardening. TABLE 8 Degree of sorption of asphaltenes at 50°C. in I’enezztelan asphalts w i t h different degrees of gelation ~
SOFTESISG POIST, R A S D
PESETRATIOX I>-DEX
PESETRATIOS A T 25.’ C.
B
1
.1.500 as aromatics, there appears t o be a positive correlation between the aromatic content of the maltenes and the degree of sorption of the asphaltenes, in asphalts without crystallizable wax. It is impossible t o convert asphalt fluxes with a high C/H ratio into a gel-type asphalt by blowing, as there will always be enough aromatics completely to peptize the asphaltenes formed, while in addition the latter are of too poor a voluminosity (lower molecular weight, shorter side chains) to build up a continuous structure. The tendency of a certain gel structure t o separate oil, i.e., its permeability, can be expressed in 7 I t , where 7 is the viscosity of the sweated oil and t is the time required for the separation of 1 mg. of oil per square centimeter of asphalt. An investigation by Saal (33) showed that the logarithm of this magnitude increases linearly with the percentage by volume of intermicellar phase in the system, but layers of oxidized asphalt containing crystalline wax a t room temTABLE 9 InJnitence os temperature on the degree of sorption of asphaltenes Venezuelan asphalt: softening point, R and l3, S5.5"C.;penetration a t 25"C., 25.5; penetration index. 3 . 4 ; asphaltene content, 30.4 per cent DEGREE OF SORPTIOS 01'THE 1SPHILTESES
Per cent I
64.5 45 12
perature, such as Iraq asphalt, are more permeable at 50°C. than can be derived from the above relation. Based on the rate of oil vqxwation, the above-mentioned investigators estimated the radius of the micclles in the oxidized Mexican asphalt from table 10 at 20 x IO+ cm., corresponding t o a micellar \\eight of 1 X lo5. This structure is milch finer than that n hich i y been 11 hen surfaces of ouidiaed bitumens are washed 11ith colvents for maltenes ('l'raxler (-1.2)); probably an artefact is formed during the rollapse of the original structure on extraction. Calculation of the viscosity of the usual asphalts from their penetration, assuming a purely viscous hehsvior, g i w s ~ n l u e sof the order of magnitude from 1Oj to 108 poises. However! the viscosit,y of the intermicellar phase separated from the gelated asphalts is about 1-10 poises at room temperature, while for the continuous phase of thc sol systems with a normal asphaltene content it may be estimated at 102-103poises. From these data it follon-s that the presence of the colloidally dispersed phase in these systems is of predominating significance for their rheological behavior.
1209
COLLOID.4L STRUCTURE O F A S P H 1 L T
TI. C0MP;ITIBILITY O F ASPHALTS O F D I F F E R E X T T Y P E S
Mixtures of asphalts of the purely viscous type and of the sol type are producth n-hose rheological properties are intermediate betn-een those of the t ~ v obase materials, so that the viscosity of such mixtures can be approximately predicted. Khen, hoTvever, a gel-type asphalt is mixed with a product of either of the t n o other types, the outcome is often quite different, for it may be that the quantity of adsorbable aromatics added to the gelated asphalt is suffivient to peptize the asphaltenes building up the gel structure, thus eliminating the important contribution of the skeleton structure to the consistency of the product. When a bitumen of the sol type is in contact with one of the gel type it may occur that a boundary layer is formed n-ith the consistency of a lubricating oil (Oliensis (26)). Our investigation showed that this is due to the oil cn-eating from the oxidized asphalt, fluxing the sol-type asphalt. TABLE 10 S u e n f i n g test at 50°C. w i t h some oxidized asphalts f r o m dzfferent base materials but with oboict the same degree of gelation (softenzng p o i n t , R and H,ctboict 85'C :penetratzon at 25"C., 25; penetratzon i n d e x , about 3 5 ) I
..I
A l e s i c a n . .. . , . . . . . . . . . . . . . . Venezuelan.. . . . . . . . . . . . . . ..~ Texis . . . . , . . . . . . . . . . . . . . . ' Roumanian.. . . . . . . . . . . . . . . . .
- - - _ _ __ ._ - .-
AROXATICS
per
cent
32 30 2i 32
I per cent
36 50
~
~
5i
1
54
-
~
~
~
i
per cent
33 20 15 14
-
~
1
I
I , ~
103 67 55 44
,
_____
. ~~~
per cent
1
146 136 134 117
~
2.5 7.8 73 72
.
V I I . G C S E R A L SURF'EE
We are now able to correlate the variouq deformation schemes of figure 1 nith the structure of the acphalt. -1 s p h d t s with purely riscozts floui are either homogeneous, asphaltene-free substanrca. or they contain well-peptized asphaltenes of loir voluminosity and, consequently, of poor deformability. Sol-type asphalts, in which viscous flon- is accompanied hy an elastic effect, contain free micelles which, a t room temperature, consist of asphaltenes and the most polar components of the maltenea. On deformation of the system these micelles are also deformed, thus accumulating energy which causes elastic recovery after removal of the external stress. This type of deformation was mathematically treated by S a d (3-1) and by Frohlich ( 3 ) as that of a liquid containing free elastic bodies. I n sol-type asphalts n-ith a rather high percentage of asphaltenes, e.g., those of Venezuelan or Mexican crudes, the volume occupied by the micelles at room temperature is so large that the system must be considered
1210
H. EILERS
as a piling of elastic units, the voids being filled with a purely viscous liquid. Some coherence between micelles may occur. Gel-type asphalts contain an insufficient amount of the adsorbable maltene components to peptize the high percentage of asphaltenes completely, so the micelles cohere and build up a continuous gel structure in the material. This skeleton causes a high elasticity of the material and the necessity to apply stresses exceeding a yield value to obtain permanent deformation; it breaks down reversibly at larger deformations. On heating, this skeleton and even the micelles disintegrate, and its reconstruction on cooling is only S ~ O T T ,causing a definite age hardening. Intermicellar liquid is given off by these asphalts on standing in contact with a fine pon-der or another asphalt. The author thanks Nessrs. S.Y.de Bataafsche Petroleum Maatschappij for permission to publish the above survey, which is the result of the collaboration of a group at their Amsterdam Laboratories, including Dr. R . S.J. Saal and l l r . J. W. A. Labout, under the stimulating guidance of the late Dr. J. Ph. Pfeiffer. REFERESCES (1) BORK,S.: Proc. Ani. Soc. Testing 11ateri:tls 3 7 , I I , 519 (1937). (2) DYKSTRA, H., BEL-,K., . 4 S D I ~ T zD, . L.: Oil Gas J. 43, S o . 21, 79 (1944). (3) ECKERT,G. W.> AXD WEETXAS,B . : Ind. Eng. Chem. 39, 1512 (1947). (4) EILERS,H . : Kolloid-Z. 97, 313 (1941). ( 5 ) FROHLICH, H., ASD S . i c ~R , . : Proc. Roy. Soc. (London) A185, 415 (1946). (6) HILDEBRASD, J. H.: J. Ani. Cheni. Soc. 41, 1067 (1919). ( 7 ) HILLMAN, E . S., ASD BARSETT,B . : Proc. Am. Soc. Testing Materials 37, 558 (1937). (8) HOIBERG, h.J., ASD GARRIS,W .E.: Ind. Eng. Chem., Anal. E d . 16, 294 (1944). (9) NATZ,D. L., A S D BEL-,K . E . : Ind. Eng. Cheni. 37, 195 (1945). (10) LEE, Ai.R . , WARRES,J. 13., ASD WATER^, D. B.: J. Inst. Petroleum 26, 101 (1940). (11) LETHERSICH. W.:J. Soc. Cheni. Ind. 61, 101 (1942). (12) MACK,CH.: J . Phys. Cheni. 36, 2901 (1932). (13) MACK,CH.: J. Applied Phys. 17, 1086, 1093, 1101 (1946). (14) MURPHY, B. *I.: J . Inst. Petrolcum31, 4i5 (1945). (15) SELLESSTEYS, F. J . : Thesis, Delft, 1923. (16) SELLESSTEYN, F. J . : In J. hlesander’s Colloid Chemistry, T’ol. 111, p. 535. D. Van Sostrand Company, Inc., Xew T o r k (1931). (li) SELLESSTEYN, F. J . : In Science o j Petroleum, L-01. IV, p. 2i60 (1938). (18) XELLEXSTEYS, F. J.! ASD DORLEYN, J . : J. Inst. Petroleum 32, 582 (1946). (19) SELLESSTEYX, F. J., A S D KCIPERY,J . P . : J . Inst. Petroleum 26, 401 (1940). (20) SELLESSTEYS, F. J . , A S D I ~ U I P E J. R SP ,. : Chem. Weekblad 39,58 (1942). (21) NELLESSTEYS,F. J . , ANI) I ~ U I P EJR .P S.,: Chem. Reekblad 39, 520 (1942). (22) SELLESSTEYX, F. J . , ASD KTIPERS,J. P . : Cheni. Weeliblad 40, 141 (1943). (23) NELLESSTETS,F. J . , ASD STEFFELAAR, G. M.h.:Chem. Weeliblad 43, 105 (1947). (24) OLIEKSIS,G. L . : Proc. .hi. Soc. Testing Materials 33, 11, 715 (1933). (25) OLIETSIS,G . L . : Proc. - h i . Soc. Testiiig1I:tterials 36,11,494 (1936). (26) OLIESSIS,G . L . : Ind. Eng. Cheni., Anal. Ed. 10, 199 (1938). (27) OLIESSIS,G . L . : Proc. Ani. s o c . Testing llaterials 41, 1108 (1941). (28) PFEIFFER, J. P H . : Ingenieur (Utrecht’r 54, S o . 29, MI< 41 (1939). (29) PFEIFFER, J. PH.,ASU SAAL,R . S . J.: J. Phys. Chem. 44, 139 (1940). (30) PFEIFFER, J. PH.,.ssn VAS DOORAIAAL: P . : J. Inst. Petroleum Technol. 22, 414 (1936).
RI-IOSIC POTENTI-%LS .%CROSS POROVTS hIEhIBR.%RES. I
1211
(31) PRECKSHOT, G. W.,DELISLE,S . G . , COTTRELL, C . E., A S D KATZ,D. L . : Petroleum Techno]. 5, S o . 5, Techn. Publ. Xo. 1514 (1942). (32) ROMBERG, J . X . , A X D TRAXLER, R . S . :J. Colloid Sei. 2, 33 (1947). (33) S a . 4 ~ : R . S. J . , BAAS,P. W.,ASD HEUKELOlf, P . W.:J. Chem. Phys. 43, 235 (1916). (34) S ~ A LR ,. S . J.. ASD L - ~ B O CJ. T ,W.A , : J. Phys. Chem. 44, 149 (1940). (35) SACHASES, A . S . :Petroleum Z.21, 1441 (1925). (36) S.ICH.~SES, -4.S.: T h e Chemical C o n s t i t u t i o n of Petroleicm. Reinhold Putdishing Corporation, Sen- Tork (1945). (37) SACK,H . -4.; A S D TRILLAT, J. J.: Compt. rend. 224, 1502 (194T). (38) STRIETER, 0. G . : J. Research S a t l . Bur. Standards 26, 415 (1941). (39) S n - ~ s s o sJ. . 11.:J. Phys. Chem. 46, 141 (1942). (40) TRAXLER, R . S . :J. Colloid Sei. 2, 39 (1947). (41) TRAXLER, R . S . ,ASD Coouss, C . E.: Proc. -1m. Soc. Testing Materials 37, 549 (1937). (42) TRAXLER, R . s., A S D CoolfBs, c. E . : Ind. Eng. Chem. 30, 440 (1938). (13) TRISLER,R . S.,A S D SCHWEYER, H . E , : Proc. -1m. Soc. Testing Xaterials 36, 511 (1936).
THE OKIGIS O F BI-IOSIC POTESTL4LS ACROSS POROUS lfElIBR-4SES O F HIGH IOSIC SELECTIT‘ITY. I’ THE BI-IOSICPOTESTLIL ISD THE ~ Z E C H A X I S M OF ITS ORIGIS; THE T-ARIOIX FACTORS TYHICH DETER~IISE THE SIGA-.IYD THE >I.~GSITL-DE OF THE BI-IOSIC POTESTI.IL : THE SIMPLEST CHAISS IS WHICHBI-IOSICPOTESTLILS AIUSI.:: S T S T E M S TJ-ITH CRITIC-IL I O S S O F T H E S.1JIE S I Z E . I S D O F D I F F E R E l - T , ~ D S O R l i ABILITY
KARL SOLLSER Laboratory of P h y s i c a l Biology, E x p e r i m e n t a l Biologg a n d Medicine I n s t i t u t e , .l-cctiotinl Insfitictes of Health? Bethesda l 4 >M a r y l a n d Receiced Janitarg 6, 1949
I The present paper deals with the mechanism of the origin of the hi-ionic potentials ( 5 ) across membranes of porous character which show a high and in limiting cases an ideal degree of ionic selectivity. The hi-ionic potential (B.I.P.) has been defined as the dynamic membrane potential which arises across a membrane separating the solutions of two electrolytes at the same concentration with different “critical” ions, which are able t o exchange across the membrane, and the same “non-critical” ion species for which the membrane is impermeable (5). The critical ions in the case of electronegative membranes, such as collodion membranes, are the cations; conversely, in the case of‘ electropositive membranes the critical ions are the anions. I Presented in abstract a t the Harry B. Weiser Testimonial Symposium of the Division of Colloid Chemistry a t the 110th Meeting of the American Chemical Society at Chicago, Illinois, September 10, 1946.
