93
Reservoirs and Reservoir Fluids
as the ratio of the molecular weight in a nonpolar solvent (e.g., toluene) to the molecular weight on a
polar solvent (e.g., pyridine or nitrobenzene). Thus, aggregation factor, N is given by
N
MW
MW
toluene
pyridine
=
or
N
MW
MW
toluene
nitrobenzene
=
The method using the concept of freezing point depression determines the average molecular weight of various fractions using simple equipment. The samples must be completely soluble
and chemically inert in the solvent (benzene). Molecular weight values represent a number-average
molecular weight for the sample components.
The freezing points for benzene and for a benzene solution containing a known weight of sample
are determined from their cooling curves. From the depression of the freezing point due to the
sample, the average molecular weight is calculated by application of Raoult’s law, viz. the vapor
pressure lowering of a solvent is directly proportional to the concentration of the solute:
p p
p
n
n n
o
o
– = +
2
1
2
In this equation, n 1 and n 2 are the number of moles of solvent and solute, respectively. Thus, if w 2 g
of solute of molecular weight M 2 is dissolved in w 1 g of solvent of molecular weight M 1 , Raoult’s
law gives
p p
p
=
w /M
w M w M
o
o
2
2
1 1
2
2
-
+
For a dilute solution, n 2 may be neglected in favor of n 1 .
Thus, the calculation for molecular weight determination by cryoscopy is
M
W K
T
T S
f
=
-
◊
(
)
D
D
where
M is the number average molecular weight
W is the weight of sample in grams
K f is the cryoscopic constant for the solvent (K f for benzene = 65.6)
∆T is the freezing point depression in °C (i.e., the freezing point of benzene minus the freezing
point of the solution)
S is the moles of solvent
Many solvents have been tried, but with the best technique an accuracy of 1%–2% is to be considered good.
Boiling point elevation (ebullioscopic) methods are, in general use, more rapid and equally
accurate but tend to fall short when applied to the higher molecular weight fractions of petroleum,
heavy oil, and bitumen. Molecular weights in the low range (<500 Da) are readily determined by
vapor-density methods.
Reservoirs and Reservoir Fluids
as the ratio of the molecular weight in a nonpolar solvent (e.g., toluene) to the molecular weight on a
polar solvent (e.g., pyridine or nitrobenzene). Thus, aggregation factor, N is given by
N
MW
MW
toluene
pyridine
=
or
N
MW
MW
toluene
nitrobenzene
=
The method using the concept of freezing point depression determines the average molecular weight of various fractions using simple equipment. The samples must be completely soluble
and chemically inert in the solvent (benzene). Molecular weight values represent a number-average
molecular weight for the sample components.
The freezing points for benzene and for a benzene solution containing a known weight of sample
are determined from their cooling curves. From the depression of the freezing point due to the
sample, the average molecular weight is calculated by application of Raoult’s law, viz. the vapor
pressure lowering of a solvent is directly proportional to the concentration of the solute:
p p
p
n
n n
o
o
– = +
2
1
2
In this equation, n 1 and n 2 are the number of moles of solvent and solute, respectively. Thus, if w 2 g
of solute of molecular weight M 2 is dissolved in w 1 g of solvent of molecular weight M 1 , Raoult’s
law gives
p p
p
=
w /M
w M w M
o
o
2
2
1 1
2
2
-
+
For a dilute solution, n 2 may be neglected in favor of n 1 .
Thus, the calculation for molecular weight determination by cryoscopy is
M
W K
T
T S
f
=
-
◊
(
)
D
D
where
M is the number average molecular weight
W is the weight of sample in grams
K f is the cryoscopic constant for the solvent (K f for benzene = 65.6)
∆T is the freezing point depression in °C (i.e., the freezing point of benzene minus the freezing
point of the solution)
S is the moles of solvent
Many solvents have been tried, but with the best technique an accuracy of 1%–2% is to be considered good.
Boiling point elevation (ebullioscopic) methods are, in general use, more rapid and equally
accurate but tend to fall short when applied to the higher molecular weight fractions of petroleum,
heavy oil, and bitumen. Molecular weights in the low range (<500 Da) are readily determined by
vapor-density methods.