Thermo-optical studies of asphaltene solutions: evidence for solvent–
solute aggregate formation
S. Acevedo
a,
*
, M.A. Ranaudo
a
, J.C. Pereira
a
, J. Castillo
b
, A. Ferna
´
ndez
b
,P.Pe
´
rez
b
, M. Caetano
b
a
Laboratorio de Fı
´
sico Quı
´
mica de Hidrocarburos, Escuela de Quı
´
mica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 47102, Caracas,
1041, Venezuela
b
Laboratorio de Espectroscopı
´
aLa
´
ser, Escuela de Quı
´
mica, Facultad de Ciencias, Universidad Central de Venezuela, 47102, Caracas, 1041A, Venezuela
Received 1 November 1998; received in revised form 4 February 1999; accepted 6 February 1999
Abstract
Thermo-optical diffusivities, D, were measured for Hamaca, Cerro Negro and Furrial asphaltenes in both toluene and tetrahydrofuran. In
toluene a plot of D vs. log c where c was the asphaltene concentration, afforded an unexpected minimum near 50 mg l
Ϫ1
. Similar plots in THF
for Hamaca asphaltenes yielded the result with little or no change in D with concentration changes. The reduction in D before the minimum
(c Ͻ 50 mg l
Ϫ1
in toluene) is consistent with the trapping of a layer of solvent between solute molecules (sort of solute–solvent aggregates)
presumably resulting in a higher heat capacity for the solvent in this aggregate when it is compared with the solvent in the bulk. At higher
concentrations, and probably because of the collapse of the solute–solvent aggregates and the formation of solute–solute aggregates (dimers,
trimers, etc.), solvent is released to the bulk, leading to increases in D until a fairly constant value is reached near 2000 mg l
Ϫ1
. ᭧1999
Elsevier Science Ltd. All rights reserved.
Keywords: Asphaltenes; Thermo-optical diffusivity; Solute–solvent aggregates
1. Introduction
The molecular interaction in asphaltenes is an important
and interesting feature of these materials as it is related to
phenomena such as aggregate and flock formation, low solu-
bility and asphaltene flocculation. Naturally, interaction
studies have been focused on asphaltene–asphaltene or
solute–solute interaction rather than solute–solvent interac-
tions. However, studies on solute–solvent interactions,
particularly with aromatic solvents such as toluene, could
give important information regarding the concentration at
which the aggregation begins and the possible interaction of
asphaltene with other crude oil components such as resins
and aromatics. As is the case for any associative solute,
desolvation should occur before aggregation, and the extent
and the way of this desolvation could give important clues
regarding the solute–solvent affinity. For instance, in
micelle formation in water with ordinary surfactants,
desolvation of carbon chains occurs quite sharply at the
cmc which is due to the ‘hydrophobic effect’ [1]. That is,
the affinity of water for itself is too large compared to the
one for hydrocarbons. Hence, the main driving force for
micelle formation is water desolvation rather than solute–
solute interactions and, consequently, no water is found
within the hydrocarbon core of micelles [1].
It is a very well-known fact that in an oil medium, micelle
formation occurs by mechanism unrelated to the one in
water [2,3]. For instance, among the differences, no cmc
is found, the number of aggregation is much lower and
some oil may be trapped within the aggregates. A complete
discussion on this topic has been given by Kertes [3]. In an
oil medium [2,3] and in particular, for the asphaltene case
where dimers, trimers and higher aggregates are formed as
the concentration is increased, a stepwise mechanism, is
more likely. Probably, the reason for this is that both the
asphaltene and the media are ‘oily’ in nature and thus a
significant energy barrier for desolvation might be present.
Hence a process whereby desolvation occurs as a phase
separation (such as the micelle formation at the cmc) is
unlikely.
As suggested by the evidence discussed later, solvent
molecules are trapped during asphaltene aggregation. If
this behavior is extrapolated to the aggregates in crude oil,
then, besides asphaltenes they should contain a variety of
compounds, such as resins and aromatics. This, of course,
would have very important consequences in asphaltene
solubility.
Fuel 78 (1999) 997–1003
0016-2361/99/$ - see front matter ᭧ 1999 Elsevier Science Ltd. All rights reserved.
PII: S0016-2361(99)00027-7
* Corresponding author. Fax: ϩ 58-02-605-22-46.
E-mail address: soaceved@strix.ciens.ucv.ve (S. Acevedo)