THE RHEOLOGICAL PROPERTIES OF FILMS ,IT CRUDE PETROLEUXWATER INTERFACES BY CHARLES G. DODD The P u r e Oil Company and University of Oklahoma, Norman, Oklahoma Received November 11, 1959
The rheologid properties of films a t liquid surfaces (gas-liquid interfaces) have been investigated by numerous workers, but little work has been done on films a t crude petroleum-water interfaces or other oil-water interfaces. Elastic properties of films a t oil-water interfaces have been measured by oscillating disk viscometers, but rotating cylinder viscometers designed for measuring general rheological properties, including time-dependent properties, of interfacial films have not been described in the literature. This paper describes the develo ment and use of an interfacial film viscometer designed esperially for rheological investigations a t oil-water interfaces. &tical design factors pertinent to interfacial film work are summarized. The viscometer has been used to study diffusion-controlled reactions a t crude oil-water interfaces and the nonequilibrium, time-dependent rheological properties acquired by films as a result of such interactions. Control of aqueous su!,strate composition and pre-treatment of oil samples permit some interpretations of film structure and of interaction mech:tnisms. The most significant rheological properties of crude petroleum-water interfacial films appear to be the pseudo-thixotropic phenomena which depend upon time elapsed after formation of the interface. Evidence is presented which indicates the film structures involve free naphthenic acids, acid anions and their salts. Geochemical implications of the data arc discussed.
The first measurements of surface viscosity apparently n-ere made by Wilson and lties,l who employed a torsional or oscillating disk type of surface viscometer. Subsequent workers using similar instruments have studied the plasticity of films sht. airwater interfaces in more Other investigators have employed slit and canal viscometers to study insoluble monolayer^.^-^ ?;-either the oscillating disk nor the slit t’ypeof surface viscometer permits t’he attainment of controlled rates of :shear, held constant during measurements, which are necessary for the characterization of thixotropic, \riscoelastic, aiid other time-dependent rheological properties. ‘The oscillating disk mebhod detects departures from Kemtonian rheological behavior only in a qualitative manner. The slit, or canal viscometers, two-dimensional analogs of capillary inst,rurnents, permit t’he rate of shear to be calculated only at the wall and suffer from a varying rate of shear across the slit unless the materic‘1 1 undergoiiig test is in the “plug flow” regime. The Couette-Hatschek- or Searle-type of rotational viscometer is t,he preferred surface viscometer for the study of time-dependent rheological propert’ies. The first such rot~atioiialsurface viscometer appears t,o have been described by Van Wazer.* An improved instrument was designed by Brown, Thuman and M ~ B a i n . The ~ ixerfacia1 film viscometer used in the work described herein was patterned aft’er the latt,er instrument. Xppropriatc modifications were made to adapt the instmineiit, for rne:~sureinentsat the oil-wutw int rfncc . ($
( I ) R. E. Wilson a n d E. D. Ries, “Surface P’ilnis a s I’lastic Solids.” Colloid Syinposiuin Monnpraph, University of Wisconsin Prrss. .\ladison. \I isc., 102,’3, 11. 143-173. (2) I. Langniuir and V. .J. Scliaefer, J . z l n ~C h e m . Soc., 6 9 , 2100
(1937). ( 3 ) E. d . Burrik rind R. C . Xewman, J . Colloid Sci., 12, 10 (19.57). (4) 1). W. Criddlc and A . I.. hleader, ,Jr., J . A p p l . f’hys., 2 6 , 838 (1953). 1.5) W. D. Harkins a n d J . G. Kirkwood, J . Chem. Phgs., 6 , R3 (1938). (I?) L. r o l l r t and W. D. Harkins, THIS.TOI-RKAL. 42, 897 ( 1 338). ( 7 ) ti. (,’. Xutting a n d W. D. Ifarkins, J . A m . Chem. Soc., 6 2 , r3155 (1940).
;!I) .A. C;. B r o w n I V . c‘. (1953).
Tliunittii
and , I . W . J I v h i n . ihid.. 8 , l!J1
The colloidal nature of crude petroleum has long been a subject of discussion and speculation. T’arious exploratory investigat’ionshave been concerned with dispersed material in pet’roleum, the films that form at crude oil-mater interfaces, the interfacially active constituent’s in petroleum, and the ability of petroleum to alter the preferential wettability of reservoir rock. Each of the workers in this field has been concerned wit’h but a few aspects of the over-all problem. Bartell and Siederhauser’O and Denekas, Carlson, Moore and Dodd” were interested in the formation of films at oil-water interfaces, the isolation aiid identification of the film material, and the significance of t’his material with respect to its role in petroleum production, especially in flooding wit,h water. In addition to devising a novel method of isolating the oil-water interfacial film substance, Bartell and Niederhauser followed the removal of the film-forming constituent,s from petroleum by measuring interfacial tensions of oil-water syst,ems before and after extraction of the film-forming constituents. They used the pendent drop interfacial t’ension instrument for t,his purpose and for relatively crude, semi-quantitative observations of interfacial film strength. Denekas, et al.,” used the same techniques. Dodd, Noore and DenekasI2 showed that the interfacial films contained trace metals in amounts considerably concentrated relative to those in the original crude oil. Ray, 7&7itherspoonand GrimI3 demonstrated, l)y use of an ultracent’rifuge, t,hat some Illinois I3asiii crude oils actually (lid contain c>olloidallydispersed materials. \Vitherspoon14 has revieired the current status of this field. The first significant) study of the inechunicd properties of petroleum-water interfacial films n-as (10) F. E. Bartell a n d I>. 0. Niederhauser, “Film Forming Constituenbs of Crude Petroleum Oils,” Fundamental Rcsearch on Occurrence and Recovery of Petroleum, 1946-1947, .in,. Pet,roleiim Inst., New York, p. 57-80, 1949. (11) R1. 0. Denekas. 12. T. Carlson, J. \V. .\\Iouie arid C’, (:, I )rdii. l n d .
[email protected]., 43, 1165 (1951). (12) C. G. Dodd, J. W. Jloore a n d 31. 0. Denekas, i o i d . . 44, 2385
(1952). (13) B. R. Ray, I’. A. Wibherspoon and R. E. tirim, T i i r s JOCRS-.SL, 61, 12Wi ( l g 5 7 ) . i l l ) 1’. .4. ~ V i ~ I i ~ ~ i s“Studics ~ ~ o ~ n o, n I ’ c t r o l ~ i i i i ~v i t l i t h e U l t r a rentrifiiec.” Re[)ril-t,of 1nvcsti~:ttivirs2 W . Illiiioiu St,;ttc Gcol. Siirvey. Urbana. Ill., 1958.
May, 1960
~ ~ H E O L O G I C A PROPERTIES L 014' FILMS AT
that of Lan-rence and Killner15 who were interested in fuel oil demulsification problems. They measured elastic properties of the films and found effective viscosities many orders of magnitude larger than the bulk viscosities of either water or crude petroleum, but they did not measure rheological properties of the films effectively. The instrument, used in their investigations was of the oscillating disk type with which they could measure only bulk elastic: properties of the interfacial phase. dears'6 also used an oscillat'ing disk viscomet'er to inake an esploratory study of the transition teniperatures of crude oil-water interfacial films and the effect of' "aging" on the elastic propert'ies of the films. Sears speculated that the interfacial films might ha1 e a, structure such as that postulat'ed for the sodium soap solution films described by Burcik, , ~ ~by Burcik sild Kewmaii.3 Sears and T i l l o t s ~ nand In this paper we wish, first, to examine the requirements i'or rotational viscometers designed for interfacial iaheology ; second, to present results of esperimentitl measurements; and, third, to interpret the data from a geochemical point of view.
LIQUIDIXT~;IWACES
545
'r
4 lq'ig. 1.-Interfacial
viscometer
Experimental Crude Oil-Water Rotational Viscometer.-A commerciitlly available Machlichael rotational viscometer, used in the paint, varnish and other related industries, was converted to an interfacial film viscometer following the general features of the design suggested by Brown, Thuman and l l c Bain.Q Their instrument, an adaptation of the CouetteHatchek-type of rotational viscometer, consisted essentially of an inverted cylinder-type of bob suspended on a torsion wire. The t:tpered edge of the bob dipped through the surface of the liquid sample held in a dish mounted on a rotating pan on t.he base of the instrument. B s the pan was rotated a t a constant rate, the shearing stress was indicated by the angular deflection of the inner, suspended bob. The following factors were found to be esscntial for the development of an instrument applicable to studies of petroleum-w:iter interfacial films. 1. The rotational speed of the outer turntable must I)(: ittijustable continuously and smoothly over a range from about 0.1 r.p.m. to about 50 r p.m., and the rate of rotat,ion must remain constant once it is set. This reqiiirca that the motor and gear train or other driving and speed control mech:tnism must operate without vibration and subst:m tidly without bitcklash . 2. The axes of rotlation of the sample vessel and of the Ijob snspenthd on the torsion wire must be co-axid. Provi sion must be macle i'or lorn-ering the bob to a predetermined and rqiroducible dcyth of submersion through the upper oil phase, interf:tcial film, and lower aqiieous phaue. This wt iing must he rcprociuced easily for eavh mcasurc~ment.. :i. The l o w r edge of the rylindriral visvomeier hob m i i d 1 ) ~t:tpereti aiiurply to minimize cnd t f f e c t s in tiic lowcr l,hase. 4 . Tllr bo))must) be consiruc*tcd of it nintpri:il wliirli will L E \vc:t hy th.2 t,wo liulk liquids undcr twL. Clean i)ra.Ps is suit:tble, for csample, unfinished or coated with a baked l a c q w r , but ;il:tstirs having est,remely low surface enwgies, siicli as T~>flt~n and ot>herfluorocarbon plastics, should not be used. TI e cout,sc.t angle with wliirh the upper phase wets the bob should, preferahly, be less than 90 degrees. 5 . The moment of inertia of t,he bob and t,he torsion aire constan[, should be optimized for application of the apparatus to crude oil-mter interfacial films. Rrady a,nd Brown18 recommend t,hat t,he moment of inertia of the bob be _ .
(15) A. S. C. Lawrence a n d W.Killner, J. Inst. Petr., 54, 821 (1948). ( l o ) J . It. S ; A M , " A Piilrdniiirlitsl Iiivrstigntioii of Surface VIYcojity a n d Stir! ice Plasticity," AI. Petr. Eugr. Thesis, Univ. of Ohlah o m i , Norman, Okla., 1952. Illotsun, J . C'uilo~dSsci., 9, 281 (17) E. J , Burcik, J. I t . Scnrb aiiu ( I %.I).
&!L&CT/ON
Fig. 3.-Cypress
(hGIWS),
crude us. distilled water.
adjust,ed t,o provide for slight under-damping, :tnd that the suspension wire be selected to provide sufficient, wnsitivity for the materials under study. Too great a sensitivity leads to difficulties in bringing the bob to a reproducible point of measurement. A 34-gauge wire was used for this work. -~
(18) A. 1'. Brndy a n d A . C:, Brown, "3leclianiual Properties of t h e Surface Films o n Aqueous Solutions of Dctergents," 3Iononiolecular Layws, 11. S u h o t l r n . Ed., . h i . .tasuc. .ldv. Sei., IYaslliiigton, 1). C.. 1,. &-(j2,
C S ~ J .IJ.
3.2, 1 9 X .
CHARLES G. DODD
546
Vol. 64
2
Q
0
/O
20
30
#
Sa Lo 70 80 Def LCC T/ON (DHPCeS),
Fig. 4.--Cypress
90
/80
NO
/20
crude us. 2% NaCl solution.
In the application of this instrument it found necessary to mount it on a vibration-free base and to protect it from air currents in a suitable cabinet. Temperature control was found to be necessary for reproducibility and for obswvations of transition temperatures. The most satisfactory type of temperature control is that described by Brown, Thuman and McBains which utilized a hollowwalled cabinet through which thermostated water was al20 lowed to flow from a constant-temperature reservoir. \ Experimental Procedures.-In making a measurement with the viscometer, g!am dishes, similar to Petri dishes, were filled uniformly with 50 ml. of the aqueous phase followed by a covering of 10 ml. of the oil under study. The i: sample in the dish then was set aside for the desired pcriod of E aging before measurement or, in the case of the 0-hour measurements, was placed immediately in the outer rotatable 4 IO pan of the viscometer. The clean viscometer bob then was lowered to the predetermined depth of submersion through the oil and interfacial film and into the lower aqueous phase, Q a total of 5 mm. in most runs. After the viscometer bob had come to rest, the angular rest position was recorded and, if all materials had previously been brought to the desired temperature of the measurement, rotation was started a t a 0 slow rate, usually a little less than 1 r.p.m. A reading 0 /4 20 a0 40 50 of the angular deflection of the bob was taken as soon as it was steady, and the rate of rotation of the outer dish imh F L C C 7 / O N (&6U&bS). mediately was increased to determine the next point on the Fig. 5.--Cypress crude us. 1% XaOH solution. curve. A typical run consisted of about eight points measured a t SucceSsively increasing rates of rotation starting at 30 a little less than one and going up to 25 or 30 r.p.m., preferably in uniform increments, followed immediately by six to eight measurements at successively decreasing rates of rotation, ending a t approximately one r.p.m. This proredure permitted one to obtain reproducible curves readily interpretable with respect to characterization of timedependent rheological properties of the interfacial film. Absolute values of Newt.onian, apparent, or plastic viscosities, 20 Bingham yield values, or bulk interfacial phase elastic moduli cannot be determined directly by this procedure. It was convenient, however, to compare the curves with those obtained when 60 ml. of a Newtonian liquid, or 50 ml. of water and 10 ml. of a hydrocarbon oil, were in the cup. A subtraction of the viscometer bob deflection due to Ken-tonian liquids having the same bulk viscosities from corresponding /d data obtained with film-forming oil-water pairs permitted B approximate film flow curves to be plotted but these difference curves are not presented in this paper. !t The procedure of continuously increasing rates of rotation was found necessary because it was difficult to reproduce the rest point deflection when the instrument was stopped after each reading. Apparently, this difficulty was due to the time-dependent film properties and, possibly, the imperfect 0 elasticity of the torsion wire. h schematic drawing of the instrument is shown in Fig. 1. Materials.-Exaerimental work was done on two fresh K g . Ci.-Cjprese crude us. 2v0 NaCl O.O1"/r, NilOH solu- crude oil and waier samples obtained from Soulhrrn Illiiiois Fields. One silmple was from the Veit S o . 8 well in tion.
s.
e 3
c
*
h
+
RHEOLOGICAL PROPERTIES OF FILMS AT LIQUID INTERFACES
May, 1960
547
30
20
/4
0 /O
lb
10
40
50
&FL€CT/ON
Fig. 7.-Cypress
10 70 (fl€Gfit€S).
crude us. 2% NaCl
80
90
100
/IO
120
+ O.OOlc/b NaOH solution.
the Soble district producing from the Cypress formation. The other n-:$sobtained from J. 0. Coen No. 43 well in the Noble district producing from the 1IcClosky formation.
Results of Rheological Measurements Significant differences were found in the rheological behavior of t,he crude oil--water int'erfAcia1 filins studied with respect to the type of water that was used in the viscometer with a gi\-en crude oil arid as a fuiictioii of the time that the film was permitted to age before measurement's were started. The important t'ime-dependent rheological characteristics of these curves appear t o be the thixotropic properties as indicated by the degree of hysteresis, that is, non-reproducibilit'y of the data a t increasing a i d decreasing rates of shear. If other factors are held constantm,the degree of thixotropy in a systenl is indicated by the area of the hysteresis loop. 19.20 Newtonian Curves.-Some of the films studied were essentially Newtonian. Whenever curves representing Ken-toniaii behavior were obtained it mas assumed that the rigidity of any film formed at the oil-mater interface m s negligible and t'he rheological properties of the oil-water system were essentially those of the bulk liquid phases. h calibration curve (shown in Fig. 2 ) was obtained using a volume of water equal in volume to t8hecombined volume of mater and crude oil used normally in the -viscometerdish. This gave an indication of the t,ype of curve to be expected for a n'ewt,oiiian liquid of approximately one centipoise viscosity. When 30 nil. of water was covered by 10 ml. of kerosirie, essentially the same curve was obtained, a s sho\vii in Fig. 2 also. Similar curves, representing I-ewtoiiian behavior and essentially 110 film strength, n-ere measured on films of some of t,he crude oil--ii-ater systems under study. It is true that, the rheological diagram for a Kewtoiiiari liquid 5sho~~ld be represented by a st'raight line interwct'ing the origin of t,he diagram. The calibration curves referred to iii the last paragraph have a slight cui,vature conves t,o t,he rate-of-shear asis. The curvature is believed t o be caused by an R. N.~ V d ~ t i i a .I. n, ( 2 0 ) II. Crrei,ii a n d T I . S (19)
,
C o a r r & e i I r ,C h e m i r i r , 7 , 3U9 ( 1 O . X ) . C i i e m , 6, 228 (1946).
c l t i i i ~ ~ iCi ,v l l o i d
Fig. S.-Cypress
crude us. formation water; aged 0 hours
W
/O
f
a 0
Fig. g.-Cypre,.s
crude 21s.'2( SaCl
+ 0 l',; Igepal CO-630
Ken toniitn film.
548
CHARLES G. Douu
D # a K r i o u (O#~RMS),
F,g.
10.-cC\-press crUcjP n-asiled lvith 107~ x a o solutioIl ~ and diFtilled nater us. distilled water.
1-01. 64
not to determine absolute rheological properties with the instrument. The important information sought was the relative shapes of the curves, degrees of hysteresis, and relative amounts of deflection a t varying rates of shear. As long as the iiistrumeiital constants n ere the same for different runs, their rheological diagrams could be compared. If thicknesses of the interfacial films were known, it would have been possible to calculate the elastic constants of these essentially bulk film phases, using the calibration curves to obtain iiistrumental constants. The iiiiportant point that should be recognized a t this stage is that one can differentiate clearly between the rheological behavior of a Sewt onian liquid in the sample dish and essentially the same system with a more or less rigid interfacial film at the oil-water interface. The fact that sig-
E 0
IO
to
4a
30
sa 6u 70 D ~ Ff~ L TIOU (D~GREES).
1.1~.11.--Cypress crude nnqhed n i t h (1) methanol containing 3 ' 3 ",OH Jo
3% k2 Eb
I
4 6
/o
8
d
0
D m f cm.w ( D t u f ts).
Fig. 12.-lIcClosky
crude
25.
distilled
\\
:itc.r.
artifact resulting in part from the measurement procedure referred to abOVe, hut largely t o imperfect centerillg of the hob in the sample dish. Suspended on the torsion !Tire, the 1,ob was free to osciliate aLout its axis; furthermore, the glass dishes used lvere not precisely circular. At higher rates of rotation the bob \vas more nearly centered. All of the rheological cllrves obtained with crude oilnater sptenis might have been corrected to a degree sufficient to straighten out the calibration clirvcs rneusuretl with Keirtoiiian liquids. The CUn'cCtlUllS ~ c r 1106 c illade b f ~ a l l wthe object I\
80
90
/(10
NO
/zo
and ( 2 ) distilled Rater us. 2% NaCl solution.
nificant differences can be recognized on the rurves indicates that these crude oil-water interfacial filnis have unusually high rigidities, as reported by Lawrence and Eiillner.'5 Crude Oil-Water Interfacial Films.-Many of the crude oil-water interfacial films studied were lion-Semtouian, and most were time-dependent. Most were found to have effective viscosities many times that of water. The effective viscosity of a non-Sewtonian liquid or dispersion a t a given rate of shear is proportional t o the slope of the line tlruv n from the origin of the rheological diagram t o the point on the curve measurcd a t the desir of shear. The effective viscosity of such a 11as found to be a transient quantity dependelit, among other factors, upoii the past history of the ~ r t i c ~ lsample. ar The crude oil-water films were strong enough t o make the use of more delicate surface (air-water) viicometers, such as the instrument used hy Brady, 'l'human and A l ~ B a i n ,entirely ~ uniiewssary arid useless in this work. A relatively insensitive instrument is required compared to the instrunients deqigned to measure surface viscosities on aqu ous ctettxwii d ~ t i o i ~ l'hc rheological diagramc. o?>tainecln itli t u o 1111iiois h s i t i crude oils fire shov ii in 1'igh d-15 inclusii c . Exmiiiiatiuii u l tlic c u r \ u tlisclosc. ii
wide variety of rheological behavior encountered in the various interfacial films. Sonic of the systcnis demonstrate relatively simple Sewtonian behavior, but many of the interfacial films show definite nonKewtonian properties and most of these demonstrate irreversible time-dependent pheiiomeiia. The extent of irreuersibility, and, presumably, the degree of thixotropy or pseudothiuotropy, is ewenti:illy proportional to the size of the hysterwis loop. Another characteristic of' these data is illustrated 1)y the diflerence between the curve obtained with distilled water against Cypress crude and that against hIcClosky crude. Compare Fig. 3 and 12. Most systems, however, were Kewtonian when the pH of the water phase was high; see 1;ig. 5, 0, i 2nd 14. The power of sodium hydroxide solutions, of high pH, i o eliminate or minimize non-Sewtonian properties of the films was investigated further by extracting one sample of the Cypress crude oil with a 109; sodium hydroxide solution and another sample with a 370ammonium hydroxide solution in methanol. The rheological cnrves obtained with the extracted Cypress crude samples against distilled water are shown in Fig. 10 and should be compared with Fig. 3. These data indicate that iome but not all of the film-forming materials in the Cypress crude oil was evtracted by the alkaline solutions. More of the filmforming substancAe, however, as extracted by the sodium hydroside solution than by ammonium hydroxide in methanol. Figures 6 and 7 show the effect of various concentratioiij of sodium hydroxide plus 2% salt in the aqueous phase against Cypress crude oil. Xt a concentration of 0.01% XaOH the film denionst rated Sewtonian behavior whereas non-Sewtonian time-dependent thixotropic properties were ohserved v-ith 0.001yoSaOH. Figures 9 and 15 shorn the results obtained with aqueous substrates containing 2% salt plus various non-ionic surfactants of the type used as detergent additives for water flooding. All of the rheology curves, bcth 0-hour and aged cnrves, are U ewtonian. The ciirves for the various surfactants almost coincide. Figure i. of particular interest because it indicates 0-hoiir thivotropic behavior vith the Cypresq crude oil against its own formation water. Some crude oils studied later did not demonstrate thixotropic k)eh.ivior against their own formation water, while others resembled the Cypress system. Discussion For the first time it has been possible to measure the thixotropic, or pseudo-thixotropic, and other rheological properties of interfacial films in a semiquantitative manner. The rheological diagrams obtained appear to be the best means known at this time of characterizing time-dependent rheological properties of these colloidal systems. The results obtained tie in nicely with the data of earlier workers, cited above, which showed that crude petroleuni contains colloidally dispersed materials and that these materials are concentrated a t oil-water interfaces to form more or less rigid .nterfacial films. Dunning, X1oore and €9
40
0
80
60
/Z4
/&
D ~ F L K ~ I O fDwe&, U
Fig. 13.--.\IcClosky crude vs. 25G ZaCl solution.
25
20
/5
5
a 0
10
PO
Jo
40
50
L?.FL€CT/ON ( h 6 R 6 t S ) .
Fig. 14.--.\IrClosBy crude us 2 9 S a C l
+ 1'
SnOII solu-
tlon.
Denekas2' believed that most crude oil-water interfacially active films hax-ing low interfacial tensions contained relatively large amounts of porphyrins. Dodd, hloore and Denekas'? found that films which were less interfacially active often were stronger or more rigid films. The degree of interfacial activity was measured by the decrease in interfacial tension as determined by an instrument such as the pendent drop tensiometer. If a rigid interfacial film that constitutes essentially a third phase, separating oil and water, is formed a t the interface, the basis for the calculation of interfacial tension by the pendent drop or any other rlaqsical (21) H. N. Dunning, J. W . Moore and bI 0. Dmehas, I n d . E n g . C h e r n , 45, 1739 (1953).
CHARLES G. Donn
,550
T'ol. 64
cepts of Bakerz3 relative to the migration of petroleum by solubilization in water. Baker assumed the existence of naturally occurring naphthenic acid 30 soaps in crude oils. He assumed the soaps solubilized hydrocarbons in water and could account for the transportation of relatively large quantities 25 of hydrocarbons in water in comparison with the low solubilities of hydrocarbons in water. It is likely that naphthenic acids participate in the colt w loidal structure of crude petroleum as suggested by Witherspoon, l 4 but this supposition might prove 2 9 difficult to establish experimentally. Baker did 2 present a good argument, however, for the solu8 15. bilization of hydrocarbons in water by naphthenic acid soaps. He showed experimentally that syn4 10 thetic soaps such as sodium laurate solubilize hydrocarbons in large quantities relative to their solu$ bilities in water. What is more significant, he showed that the relative amounts of different hy5 drocarbons solubilized by sodium laurate corresponded to the relative amounts of these same hydrocarbons found in crude petroleum. 0 0 /0 20 a u0 .5d The concept of a complex interfacial film made D€FL€C7/0# ( D r S a f p S ) . up from free naphthenic acids, naphthenic acid Fig. 15.-hhlcClo~kv crude us. 2% NaCl O.Olyc Igepal soaps and naphthenic acid anions, in combination CO-630 solution. with resins, waxes and asphaltenes is in harmony method is invalidated. Thus these earlier esti- with modern theories of emulsion stability and surmates of interfacial activity may be of limited va- face film viscosity. Schulmaii and Cockbain,24for lidity. Dodd, Moore and Denekas found that vana- example, have proposed that many emulsion sysdium was present in greater concentrations in the tems are stabilized by closely packed, rigid interfilms that showed the greatest mechanical rigidity facial films made up of a condensed complex of two and the least interfacial activity whereas other or more emulsifying agents. trace metals such as nickel, copper and zinc were The packing of molecules in the films observed in present in greater quantities in the films demon- the present work is thought to depena on steric, strating a higher degree of interfacial activity. fsrtors in addition to hydrogen bonding and van More recently Radchenko and Sheshina22showed der Waals bonding. Tlie process of crude oil-water that crude oils containing large amounts of vana- interfacial film formation probably involves steps dium also invariably contained large amounts of similar to those involved in the dimerization of sulfur and asphaltic materials whereas the oils fatty acids, which process depends on hydrogel) containing a predominance of nickel were more bonding. The extent of hydrolysis and film formaparaffinic in nature and contained relatively small tion, and hence the strength of the interfacial amounts of asphaltic material. films, was found to be dependent upon both acidity The work reported herein indicates that the rigid and the ionic atmosphere in the aqueous phase. interfacial films probably contain materials such as fact that the oseudo-thixotropic properties naphthenic acids. It is assumed that the acidic of The crude oil-water interfacial films appear to be time substances occur in combination with resins and various sulfur-bearing asphaltic substances, such sensitive, and that they demonstrate different deas are present in the asphaltene fractions of pe- grees of time sensitivity with difterent crude oil troleum, and, frequently, high molecular weight water systems, may indicate that the naphthenicb are of varying molecular or particle weights, hydrocarbons of a waxy nature. In combination acids resulting in ~ a r y i n grates of diffusion to the interwith naturally occurring connate waters, which contain salts, the naphthenic acids form soaps which face. The observed time sensitivity probably dehydrolyze to varying degrees depending on the pends, also, on an orientation process at the intersalt content and pH of the water. Hydrolysis of face. Both of these proposed steps are irrever\ible naphthenic acid soaps would form a complex in- rate processes requiring an activation energy nnd. cluding free acids, soaps and acid anion, and such thus, they would be temperature dependent ; hriice, occurrence of transition temperatures. .Ifter a complex probably forms the backbone of the in- the a given complexity of film structure were attained, terfacial film structure in a manner analogous to the the film would be disrupted irreversibly, to an infatty acid soap films observed by Burcik and Newman.3 In the present work no effort was made to creasing extent, as rates of shear were increaPed in explore in more detail the nature of the naphthenic the viscometer cup. (23) E 0. Baker, S c i e n c e , 129, 871 (1959). U s 0 see "Crude Oil acid groups nor the molecular structure of the films. Composition and Hydrocarbon Solubility '' paper presented as part ot a The evidence found for the presence of naph- symposium on Clremical Aspects of t h e Origin, hhgration, a n d Aethenic acids in interfacial film Pupports the con- cumulation of Oil, Arner Chem. Soc. Meeting, Chicago, S r p t 7 - 1 2 ,
s* e e
+
(22) 0 A . Radchenko a n d L. S Sheshina, Doklady Akad. Yauk S S S R , 106,1285 (1955) (Assor Tech Spr'iiccs Translation No. RT480).
1959 (Paper preprinted by Div of Petroleum Chemistry) (24) J H Schulman and E C; Cockbain Trans P ~ r a d o yS o e , 36, 6.51 (1940)
