Energy & Fuels 1995,9, 829-833
829
Temperature-Dependent Structural Changes of Asphaltenes in LMethylnaphthalenet P. Thiyagarajan" Intense Pulsed Neutron Source Division, Argonne National Laboratory, Argonne, Illinois 60439
Jerry E. Hunt and Randall E. Winans Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439
Ken B. Anderson and Jeffrey T. Miller Amoco Corporation, Naperville, Illinois 60566 Received February 7, 1995. Revised Manuscript Received May 23, 1995@
Small angle neutron scattering (SANS) was used to investigate the structural changes of a 5 wt % asphaltene solution in perdeuterated 1-methylnaphthalene (1MN-dlo) as a function of
temperature. A special stainless steel cell was constructed and used for the measurements. The SANS data measured at various temperatures from 20 to 400 "C show that the scattering intensity continuously decreases with increasing temperature. The data at 20 "C suggested rodlike morphology for the particles, and a nonlinear curve in the Guinier plot implied polydispersity in their sizes. Maximum entropy analysis using the form factor for a cylinder allowed extraction of particle size distributions in the radius and length space. At 20 "C the asphaltenes self-associate in 1MN-dloforming long rod-shaped particles whose radius was around 18 8,but vary in length over 500 8,. At 50 "C these aggregates break down, as evidenced by the decrease in signal intensity and the radius of gyration. In the temperature range of 100-320 "C the maximum length of the particles decreases and the polydispersity varies in both the radius and length dimensions. Between 340 and 400 "C, the particles become smaller having a spherical shape with a radius around 12 A. Upon returning the sample to 20 "C, the SANS signal was too weak to derive any structural information, implying irreversible thermochenlistry. We also carried out similar studies on the deasphalted oil (DAO)in 1MN-dloa t 20,100, and 200 "C, but the SANS signals were very weak and no changes in the scattering behavior were seen as a function of temperature.
Introduction Asphaltenes, the heptane-insoluble fraction of petroleum resids, are known to interfere in the upgrading and refining processes of petroleum by forming colloidal 0bjects.l This system has been extensively studied by a variety of techniques t o understand their colloidal b e h a v i ~ r . ~ -Asphaltenes l~ are a complex combination of organic compounds that contain aromatic ring struc+ This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US.Department of Energy, under contract No. W-31-109-ENG-38. Abstract published in Advance ACS Abstracts, July 15, 1995. (1)Moschopedis, S. E.; Fryer, J. F.; Speight, J. G. Fuel 1976, 55, 227-232 and references therein. (2) Speight, J. G.; Moschopedis, S. E. Fuel 1977, 56, 344-345. (3) Speight, J. G.; Wernick, D. L.; Gould, K. A.; Overfield, R. E.; Rao, B. M. L.; Savage, D. W. Rev.Inst. Fr. Pet. 1985, 40, 51. (4) Herzog, P.; Tchoubar, D.; Espinat, D. Fuel 1988, 67, 245. (5) Ravey, J. C.; Ducouret, C.; Espinat, D. Fuel 1988, 67, 1560. (6) Overfield, R. E.; Sheu, E. Y.; Sinha, S. K.; Liang, K. S. Fuel Sci. Technol. Znt. 1989, 7 , 611. (7) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497-502. (8) Sheu, E. Y.; Storm, D. A.; De Tar, M. M. J. Non-Cryst. Solids 1991,131-133,341-347. (9) Sheu, E. Y.; De Tar, M. M.; Storm, D. A. Fuel Sci. Technol. Znt. 1992, 10, 607. (10) Sheu, E. Y.; Liang, K. S.; Sinha, S. K.; Overfield, R. E. J. Colloid Interface Sci. 1992, 153, 399-410. (11) Sheu, E. Y.; De Tar, M. M.; Storm, D. A.; DeCanio, S. J. Fuel 1992, 71, 299-302. @
turee with aliphatic chains. In organic solvents, they self-i3ssemble to form reverse micelle^.^,^ The critical micellar concentration (cmc)for the formation of reverse micelles depends on the solvent type and the temperatux7t8 The reverse micelles formed are known to further aggregate into large colloids depending on the polarity of the solvent1>8and temperature.6 Several studies4~6~8-12J4 focused on the size, shape, and polydispersity of asphaltenes in solution as a function of solution polarity and temperature. The effects of temperature on the aggregation properties of asphaltenes in toluene-d clearly showed the disaggregation as the temperature is increased.6J0 Furthermore, studied1 on the kinetics of the micellization phenomenon showed the slow equilibration of the micelle formation which was attributed to the complexity of the constituents that formi reverse micelles. In the previous temperaturedependent SANS study6 the maximum temperature reached was 250 "C and limited information on the size, shape, and polydispersity of the particles was obtained. (12) Storm, D. A.; Sheu, E. Y.; DeTar, M. M. Fuel 1993, 72, 977981. . (13) Storm, D. A.; Edwards, J. C.; DeCanio, S. J.; Sheu, E. Y. Energy Fuels 1994, 8, 561-566. (14) Ravey, J. C.; Espinat, D. Prog. Colloid Polym. Sci. 1990, 81, 127-130.
0887-06249512509-0829$09.00/0 0 1995 American Chemical Society
Thiyagarajan et al.
830 Energy & Fuels, Vol. 9, No. 5, 1995
To gain a better understanding of the temperaturedependent behavior of asphaltenes and deasphalted oil (DAO), we have carried out SANS studies on these systems in perdeuterated 1-methylnaphthalene (1MNd d as a function of temperatures up to 400 "C. SANS offers the advantages of high penetration power even in thick cells at high temperatures and high contrast for hydrocarbon systems dispersed in deuterated solvents. We observed large changes in the scattering behavior of asphaltenes in 1MN-dlo as a function of temperature, while the DAO solutions did not produce any significant scattering signal at any measured temperature, implying the absence of large particles.
Experimental Section Sample Preparation. Pentane soluble, heptane precipitated asphaltenes and the deasphalted oil (DAO) from vacuum resids of Maya crude oil were obtained from Amoco. The 1-methylnaphthalene-dlo (1MN-dlo) was obtained from Aldrich. Five weight percent dry asphaltene was dissolved in lMN-dlo and the solution was stirred overnight. The sample was kept at room temperature for 2 days prior to SANS experiments. A similar procedure was used for the preparation of the DAO sample. Both samples strongly absorb in the visible region and thus were not amenable to dynamic light scattering. Small Angle Neutron Scattering. An aliquot of the 5 wt % asphaltene solution in 1MN-dlo was loaded in a stainless steel cell constructed especially for SANS measurements. The wall thickness of 1mm and a sample thickness of 3 mm make the total path length of 5 mm for neutrons. Only 90% of the volume of the cell was filled to allow expansion at temperatures above the boiling point of 1MN-dlo (241 "C). The net transmission for neutrons for the sample averaged over all wavelengths (1-14 8) was 0.61. Small angle neutron scattering was performed by placing the sample cell in a boron nitride furnace tube in the small angle diffractometer (SAD) a t the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory. The temperature of the furnace was measured using a type K thermocouple and maintained within 0.5 "C using a Micricon controller. At each temperature, the sample was equilibrated for about 30 min prior to SANS measurements. The temperature was increased at the rate of 5 Wmin. The S A D instrument uses neutrons produced in pulses by spallation due to the deposition of 450 MeV protons on a depleted uranium target, followed by a solid methane moderator (22 K) yielding a wavelength range of 1-14 8 . Detection of scattered neutrons was accomplished with a 128 x 128 array, 40 x 40 cm2area sensitive, gas-filled proportional counter, and the wavelength of the scattered neutrons was determined by their times of flight. However, only data from a circular region with a radius of 10 cm from the center of the direct beam were used due to the restrictions placed by the limited radius of the cylindrical boron nitride furnace tube. Data were corrected for unit transmission of the sample, the scattering from the stainless steel cell, and incoherent scattering. The accessible q range (q = 4n sin(@)/& where L is the wavelength of the probing neutrons and f3 is half the scattering angle) using SAD is from 0.008 to 0.2 8-l. The first set of experiments was carried out at 20,200,300, 350, and 400 "C and back to 20 "C. This took about 36 h of beam time. The SANS signal was strong at 20 "C and decreased with increasing temperatures up to 400 "C but became quite weak when returned to 20 "C. Upon removal of the sample, a precipitate was observed on the inner walls of the stainless steel cell. To assess the reproducibility of the data at various temperatures, we made a second set of SANS experiments by loading another aliquot from the same asphaltene stock solution. We recorded data at 20,50,100,150, 200,300,320,340, and 400 "C. The two data sets agreed well within the experimental error at common temperatures.
Analysis of SANS Data. The scattering intensity vs scattering vector plot in the very low q region can yield information on the morphology of the scattering particle. If the scattering intensity in the low q region exhibits an approximate power law of q-l, then it suggests that the particle morphology could be rodlike, and if the power law is q-2 then the morphology could be lamellar.15 Further analysis of the neutron scattering data to obtain size information can be made by using the Guinier appr~ximation'~ where
(1) and In eq 2, Z(0) is the intensity at q = 0, es and eP are the scattering length densities of the solvent and the particles, n is the number of asphaltene monomers in the aggregate, V is the volume of the asphaltene aggregate, N , is Avogadro's number, and M is the molecular weight of the asphaltene monomer. The use of eq 1 assumes that the interparticle interaction effects are either nonexistent or minimal. Equation 2 is useful for studying the structure and molecular weight of micelles formed by chemically homogeneous surfactant systems. The radius of gyration, R,, is the root mean squared distance of all of the atoms to the centroid of the scattering volume of the particle. R, is obtained from the slope of a line in the In Z(q) vs q2 plot15 in the q region where qR, 5 1.0. In the case of polydisperse systems, the R, and Z(0) values are respectively 2-averaged and weight-averaged quantities. For example, the 2-averaged R, value is defined as
(3) where Ni and M , and (R,)i are the number density, molecular weight, and radius of gyration of the aggregates of kind i, respectively. Since asphaltenes are chemically heterogeneous, we did not obtain information on the molecular weights of the aggregates. The size and morphology of the scattering particle can be obtained by combining information from the Guinier a n a l ~ s i s ' ~ in the low q region, the power-law behavior, if any, in the low q region of the log(Z) vs log(q) plot, and nonlinear regression analysis using the expression for the form factor of an appropriate shape. In our analysis we used either the form factor for a sphere with a radius R (eq 4)
+
I ( q ) = Z(OX(3 sin qR - qR cos qR)/(qRI3l2 B (4) or an ellipsoid (eq 5), which covers a wide range of aspect ratios to obtain information on the size and morphology of the aggregates. In eq 4, the variables for nonlinear regression are Z(O), R , and the incoherent background, B. In the presence of interparticle interactions, the measured scattering intensity is a function of the form factor, F(q), and the interparticle structure factor, S(q) , Le., Z(q) = S(q)lF(q)12.The use of eqs 4 -6 assumes that a q domain can be found where S(q) 1. In general, such domains exist in the high q region. When the fits using the ellipsoidal model yielded large aspect ratios (major semi-axis to minor semi-axis ratio) we used a modified Guinier analysis16J7 to obtain information on the crosssectional radius of gyration and eventually the radius of the rod. The form factor used for the prolate ellipsoid averaged over all the orientations in solution was
-
I ( q ) = I ( 0 ) 6 : : [ ( 3 sin X - X cos X)/x3I2cos /3 dB
+B (5)
sin2/311'2, where A, and B, In eq 5,X = qAs[cos2p + are the major and minor semi-axes of the prolate ellipsoid, (15) Guinier, A.;Fournet, G. Small Angle Scattering ofX-rays;John Wiley & Sons,Inc.: New York, 1955. (16)Kratky, 0.; Pilz, I. Q.Rev. Biophys. 1978,11, 39-70. (17)Hjelm,R. P. J.Appl. Crystallogr. 1985,18, 452-460.
Structural Changes of Asphaltenes in 1 -Methylnaphthalene
Energy & Fuels, Vol. 9,No. 5, 1995 831
1 n
f
1
c
5
Y
h
U
Y
0.1
-3
1
,
0
0.01
0.01
I
0.002
0.004
0.1
q(A-')
Figure 1. log(I) vs log(q) plots for the 5 wt % asphaltenes ( 0 ) and 5 wt % DAO (0)in 1-methylnaphthalene-dlo (1MN-dlo): the power law slope of the fit = 1.05 f 0.11, implying a rodshaped particle. and /3 is the orientational averaging angle. The variables in eq 5 for nonlinear regression are Z(O), A,, B,, and the incoherent background, B. Several methods are available to obtain the particle size distribution from the scattering data,18 of which the maximum entropy t e c h n i q ~ e has ' ~ ~been ~ ~ shown2I to be very effective for analyzing the polydispersity of particle sizes. In our modeling of SANS data using the maximum entropy analysis we have found that cylindrical form factor is sufficient t6 cover a wide range of morphologies in a variety of systems.21 Hence, we . used the form factor of a cylinder (eq 6) convoluted with the instrument resolution function of SAD22to fit the SANS data and extracted the polydispersity in the radius and length dimensions of the particles. The form factor for a cylinder is
I(q)=
nizsin 2 (qH COS e) 4J12(qR sin e) q2R2 sin 2 e sin 8 dB (6) 1(0)sB=o q 2 p c0s2 e
I
I
0.006
0.000
0.01
qZ(A-')
Figure 2. Guinier plots of the SANS data for 5 wt % asphaltenes in 1MN-dlo at 20 "C (0).50 "C (0). 150 "C (A), and 400 "C (0).
--
i
40 -
0
0
100
300
200
400
5 )O
Temperature('C)
Figure 3. Effect of temperature on the R, values of asphaltene aggregates in 1MN-dlo. 2.5
I
where H and R are respectively half-length and radius of the cylinder and J1 is the first-order Bessel function. In the case of MaxENT analysis we used the high q region where the interparticle effects are minimal.
Results and Discussion The scattering intensity as a function of scattering vector for the asphaltene and DAO solutions in 1MNd10at 20 "Care shown in Figure 1. While the intensity of the SANS signal is quite large for the asphaltene solution, it is quite weak for the DAO solution. The low q region for the asphaltenes exhibits a power law (the linear fit) with a slope of -1.05 f 0.11, implying a rodshaped particle. This contrasts with the disk morphology attributed to the aggregates formed by Safanya asphaltenes in pyridine,14 where one expects a slope close to -2, depending on the aspect ratio (ratio of their semi-axes). The difference in the morphology could be due to the difference between the chemical composition of Maya and Safanya asphaltenes. The scattering for the DAO is too weak to extract any reliable structural information. The Guinier plots and fits for the asphaltene SANS data a t 20,50,100, and 400 "C are shown in (18) Sheu, E.Y.Phys. Rev. A 1992,45,2428 and references therein. (19) Skilling, J.; Bryan, R. K. Mon. Not. R. Astrom. SOC.1984,211, 111-124. (20) Gull, S.In Developments in Maximum Entropy Data Analysis in Maximum Entropy and Baysian Methods; Skilling, J., Ed.; Kluwer Academic Press: Boston, 1988; pp 53-57. (21) Hjelm, R.P.;Thiyagarajan, P.; Sivia, D. S.; Lindner, P.; Alkan, H. A.; Schwahn, D.Prog. Colloid Polym. Sci. 1990,81, 225-321. (22) Hjelm, R. P. J.Appl. Crystallogr. 1986, 21, 618-628.
0
e
0
,
0
100
'
I
200
"
'
300
0 '
'
400
500
Temperature('C)
Figure 4. Effect of temperature on the Z(0) values of asphaltene aggregates in 1MN-dlo.
Figure 2. The scattering intensity decreased with increasing temperatures, a trend which is similar to that reported6 earlier for asphaltenes in toluene-ds. The Guinier plot of the SANS data a t 20 "C (open circle in Figure 2) shows nonlinearity in a wider q2 domain, implying the presence of polydispersity. The Guinier R, and the I(0)values obtained a t various temperatures are shown in Figures 3 and 4, respectively. While R, represents the distance correlations in the particle, I(0) is proportional to the number density, volume, contrast, and the molecular weight of the particles. The data in Figures 3 and 4 for the asphaltene solution in 1MN-dlo a t various temperatures reveal the following. At 20 "C the solution consists of large particles formed by the aggregation of the reverse micelles of a ~ p h a l t e n e s .The ~,~ forces that hold the large aggregates together seem to be very weak as the aggregates break down even with a slight increase in temperature. This is evidenced by
832 Energy & Fuels, Vol. 9, No. 5, 1995
Thiyagarajan et al. - 3 1-
-I
I
II
JI
-3.3
-4.2
0.01
0.1
i
1
-4.5
,
0
qM")
Figure 5. Ellipsoidal fits of the SANS data for asphaltene in lMN-dloat 20 "C (01, 50 "C (O), 100 "C (A), and 150 "C (0). The lines are the fits using the form factor for an ellipsoid.
I
,
'
I
0.002
0.004
0.008
0.006
q2M" )
Figure 6. Modified Guinier analysis of the asphaltene solution in 1MN-dlo at 20 "C. This establishes the rod-shaped morphology for the aggregates.
the substantial decrease in signal intensity, as well as
R, a t 50 "C and the progressive decrease in intensity with further increase in temperature. Furthermore, the I ( q ) data turn over in the low q region at higher temperatures (Figure 5). This is clearly due to interparticle interactions. In principle this can be modeled. However, because of the following reasons, we did not attempt to include the structure factor, S ( q ) , in our analysis. First, we do not know the exact nature of the interparticle interactions nor the chemical or physical (specific volume) nature of the aggregates. Hence, we do not know the scattering length density of the system, much less this property as a function of temperature. Furthermore, a t present we have not clearly established an expression for S(q) for the highly asymmetric particles. The question then becomes how reliable are our particle morphology and size distribution results in the presence of interparticle effects. Near ambient temperatures the aggregates are larger, but their concentration is low. In the case of data at higher temperatures we used the high q region where the interparticle effects are minimal, and hence our results should be reliable. We have used an ellipsoidal model (eq 5) t o fit the data up t o 300 "C, but beyond that the data could only be fitted with a spherical model (eq 4). The nonlinear regression analysis using the ellipsoidal form factor could not fit the SANS data for 20-50 "C as shown in Figure 5. The fits are worse especially in the high q region. The poor fits are clearly due to the polydispersity of the particles and is similar to that found for asphaltenes in toluene-ds.6J0To confirm that the morphology of the aggregates of asphaltenes is rod-like, we used an independent method, the modified Guinier analysis.16J7 The curve has a linear region (Figure 6) with a negative slope in the region of qR, < 0.8 (where R, is the cross-sectional radius of gyration) indicates that such correlations indeed exist and from the absolute slope we extracted the cross-sectional radius of 21 f 0.8 A for the particles. The consistency between the radius obtained from the modified Guinier analysis and the minor semi-axis value obtained from the ellipsoidal fit, along with the q-l.05 powerlaw behavior in the low q region of the data at 20 "C strongly support the rodlike morphology of the particles. The data for temperatures above 150 "Ccan be fitted well with either an ellipsoidal or spherical form factor, as the aggregation and the polydispersity decrease at higher temperatures. To gain a better insight into the morphology and the polydispersity of the aggregates present in the asphaltene solutions of 1MN-dlo we have used the maximum
..
0.01
0.1 q(A")
Figure 7. Maximum entropy fits for the polydispersity in the asphaltene solutions at 20 "C (0150 , "C (01, 100 "C ( A ) and 150 "C ( 0 ) .The distributions in the radius-length space are shown in Figures 8 and 9.
entropy technique.20 Since the particles are rodlike, we used the form factor for a cylinder (eq 6) to fit the scattering data. This procedure has been effectively used t o obtain the particle size distributions in the aqueous lecithin-bile-salt systems.21 We have modeled all the SANS data using this approach and excellent agreement was obtained between the data and the fit at all temperatures. The SANS data and fits from maximum entropy analysis for 20,50,100, and 150 "C are plotted in Figure 7. Comparison of the fits in Figures 5 and 7 shows the superiority of the maximum entropy fits. The particle size distribution obtained for the data at 20 "C in the region of q > 0.01 A-1 in length and radius space is shown in Figure 8. This fit also shows that the particles are rod-shaped with radii around 18 A, but lengths can vary from 100 to over 500 A. The radius from this analysis is consistent with the radius of 21 & 0.8 A obtained from the modified Guinier analysis (Figure 6). We could not obtain the upper limit of the length of the asphaltene aggregates at 20 "C because of the low q limit of the measured data. In fact we have fitted the data at 20 "C only from q > 0.01 A-1 as no Guinier region was found below that q region, but only a q-'.05 power law (Figure 1) behavior. Thus we could not obtain the upperbound limit of the length of the particles (Figure 8). The size distribution in the radial dimension seen on the left side of the plot (Figure 8) is due to symmetry in the length-radius space, as the length of these particles falls around 25 A. The size distribution that best fits the SANS data a t various temperatures indicates the presence of particles whose radii change very little, while their lengths vary sig-
Structural Changes of Asphaltenes in 1 -Methylnaphthalene
i
t i \ \\
4o
t 0
\
100
200
300
500
400
Length (A)
Figure 8. Size distribution in the radius-length space for the asphaltene solution in 1MN-dlo at 20 "C. 6 lo",
.-L5
c
I
310'-
210":
0
u'0 1 1 0 "
Energy & Fuels, Vol. 9, No. 5, 1995 833 instead of toluene-&. But the polarity of these solvents is almost the same and therefore the aggregation behavior of the asphaltenes should be similar. We have measured SANS data up to 400 "C and have repeated the measurements a t a different set of temperatures which included some common temperatures between the two sets. The agreement between the scattering behaviors measured at a temperature attained by different steps implies that the nature of aggregation is most probably determined by the thermodynamics of the asphaltene solutions. We observed a precipitate upon returning the sample to room temperature after treatment a t 400 "C. This indicates that irreversible thermochemistry should have occurred a t a certain high temperature. We believe that we have applied appropriate analysis procedures to treat the data at all temperatures. We have established the morphology of the asphaltene aggregates a t 20 "C to be rodlike and polydisperse in the length direction, but not in radius. We have used maximum entropy analysis t o fit the SANS data at various temperatures and obtained the particle morphology and the polydispersity information. Our future studies will focus on the combined effects of high pressure and temperature and also the effects of catalysts and DAO on the aggregation behavior of asphaltenes.
Conclusions Le.ngth(A)
Figure 9. Length distributions for the asphaltene solutions in 1MN-dlo: 20 "C (solid line), 50 "C(dashed line), 100 "C (dotted line), 150 "C (dashed-dotted line). See text for the radius of the particles at those temperatures.
nificantly. At 20 and 50 "C the radii are about 18 A, whereas they decrease to 15 A a t 100 "C and 13 A at 150 "C. The length distribution is plotted in Fi for particles with radii of 18 (20 and 50 "C),15 (100 "C), and 13 A (150 "C). At 20 "C the particles are quite long, with a peak around 150 A, but particles seem t o exist with lengths extending beyond 500 A in low concentration. At 50 "C the long aggregates break down such that their maximum length is around 150 A. The concentration of smaller aggregates increases by severalfold and shows a Gaussian-like distribution in the length dimension with a peak around 80 A. At 100 "C the peak concentration of the particles is 75 A, with a maximum length less than 90 A. At 150 "C two size distributions are seen corresponding to lengths around 25 and 65 A. The concentration of particles with a length around 65 A progressively decreases with an increase in temperatures up to 320 "C. At 340 "C and 400 "C the particles with lengths around 65 A disappear, and we see fairly monodisperse spherical particles with a radius around 12 A. The polydispersity in the asphaltene aggregates in toluene-& has been addressed by Sheu and coworkers.10 In that study they showed that the particles have a spherical shape and a Schultz-like (right-handed skewness) distribution in sizes and that the distribution is Gaussian-like. Overfield and co-worker@have shown the effect of higher temperatures (up t o 250 "C) on the aggregation properties of asphaltenes in the same solvent. Our study differs from these in the following ways. We used perdeuterated 1-methylnaphthalene
The present study shows that asphaltenes in 1MNdlo solutions a t ambient temperatures ag egate as rodlike particles whose radius is around 18 [but their lengths can vary beyond 500 A. On the other hand, the DAO does not aggregate at all. Maximum entropy analysis is very effective in obtaining the morphology and polydispersity information. As the temperature is increased up to 100 "C, the radius decreases only slightly, but the length of the aggregates decreases significantly with a concomitant increase in the concentration of the smaller particles. At 150 "C two types of particles, spherical particles with a radius of 12 8, and ellipsoidal particles with semi-axes of 33 and 12 A, are observed. The concentration of ellipsoidal particles decreases with increasing temperatures up to 320 "C. At 340 and 400 "C only spherical particles are seen with a radius of 12 A. However, they break down when the system is brought back to ambient temperature, implying that its chemical structure might have been irreversibly altered at high temperatures.
r9
Acknowledgment. This work was supported under a CRADA agreement between Amoco Corporation and Argonne National Laboratory through the DOE-Bartlesville Fossil Energy Project. The neutron scattering experiments were performed a t the Intense Pulsed Neutron Source a t Argonne National Laboratory supported by the Office of Basic Energy Sciences, US. Department of Energy, under contract No. W-31-109ENG-38. One of the authors (P.T.) thanks Dr. Rex P. Hjelm, Jr, LANSCE, Los Alamos National Laboratory, Los Alamos, NM, and Dr. D. S. Sivia, ISIS, England, for sharing their software to employ maximum entropy methods t o extract the polydispersity information. The authors also acknowledge the help of Denis G. Wozniak in the neutron scattering experiments. EF950031T
