Predicting the solubilization preference of natural phenols to different solvents
Charis M. Galanakisa, Vlasis Goulasc, Vassilis Gekasc
a
Department of Environmental Engineering, Technical University of Crete, Chania, Greece
([email protected])
b
Department of Agricultural Sciences, Biotechnology and Food Science, Cyprus University of Technology,
Lemesos, Cyprus ([email protected])
ABSTRACT
The current investigation reports the prediction of activity coefficients of 15 natural phenols (tyrosol,
hydroxytyrosol, oleuropein, caffeic, cinnamic, p-coumaric, ferulic, gallic, p-hydroxybenzoic, phydroxyphenyl acetic, protocatechuic, rosmarinic, sinapic, syringic and vanillic acid) in 7 solvents (water,
ethanol, methanol, acetone, dichloromethane, ethyl acetate and diethyl ether) and 3 extraction temperatures
(298.15, 313.15 and 333.15 K), by using the Universal Functional-group Activity Coefficient (UNIFAC)
model. Solvents were classified for their ability to dissolve phenols and compared with experimental data of
the literature in order to observe if the solvent extraction of phenols in practice matches with our theoretical
approach. Results indicated the superiority of alcohols and acetone for the recovery of phenols in line with
experimental data of previous studies. Furthermore, activity coefficients values were found to increase with
increasing temperature. The prediction model may be particularly useful for the recovery of targeted
individual phenols in order to screen them for biological activities. Finally, recovery of phenols is proposed
to conduct initially with a polar protic solvent (hydro-ethanolic mixture) and then to progress sequentially
extraction steps with solvents of reducing polarity in order to separate the compounds of interest for each
case. This study provides a knowledge base for the selection of the most appropriate solvents for a given
phenolic compound.
Keywords: UNIFAC; extraction efficiency; phenolic acids; solubility; activity coefficient
INTRODUCTION
Broadly distributed in the plant kingdom and abundant in our diet, phenols are today among the most
discussed categories of natural antioxidants [1]. Phenols include one or more hydroxyl groups (polar part)
attached directly to an aromatic ring (non polar part) and are often found in plants as esters or glycosides,
rather than as free molecules [2]. This stereochemistry distinguishes phenols according to their polarity
variance. The above property has been utilized for the recovery of phenols from natural sources, which is
usually accomplished with solvent extraction. The yield of this process is strongly dependent on the nature of
the solvent [3]. The tendency of each phenol to be solubilized, transferred or diffused into a given solvent is
governed by thermodynamics. One of the primary thermodynamic factors describing this tendency is the
activity coefficient, which generally affects reversibly the solubility of phenols in nonpolar solvents (smaller
coefficient corresponds to better solubility) [1, 4].
The objective of the current study was to predict the activity coefficients of 15 natural phenols in 7 solvents
and 3 assayed extraction temperatures, by using the UNIFAC (UNIQUAC Functional-group Activity
Coefficient) model [5]. Thereafter, with regard to the denoted data, solvents were classified for their ability to
dissolve phenols. This classification was compared with bibliographic data obtained for agricultural wastes
extraction in order to discuss if the recovery of phenols in practice matches with theoretical approach.
MATERIALS & METHODS
UNIFAC model and thermodynamics framework
This method estimates activity coefficients in nonelectrolyte liquid mixtures remote from critical conditions
[5, 6]. The main characteristics specifying the results of the model are the geometry (shape and size), the
structure (distribution in chemical groups and subgroups) and the energy interaction between these groups
and the assayed molecule. The basic properties of the subgroups include the relative volume (Rk), relative
1
surface (Qk) and the interaction parameter αmk (in Kelvin units) between them. The properties Rk, Qk and αmk
exist in bibliography for a numerous of subgroups [6]. The activity coefficient γi of an ingredient “i” is
supplied by the following equation:
ln γ i = lnγ i + ln γ i
C
R
(1)
where “C” is the parameter of the combinatorial part (with regard to the shape and the size of the molecules)
and “R” is the parameter of the residual part (with regard to molecules interactions). The natural logarithms
of
γ i C and γ i R are calculated by the following equations:
 J
J
C
ln γ i = 1 - J i + ln J i - 5 q i 1 - i + ln i
Li
 Li



(2),

 β 
β 
R
ln γ i = q i 1 − ∑ θ k ik  − eki ln ik 
sk 
sk 
k 

(3)
The parameters Ji and Li are calculated by the following equations:
Ji =
ri
∑ rj x j
(4),
j
Li =
qi
∑qjxj
(5),
j
The parameters ri και qi are the parameters of relative volume and relative surface for the chemical
substances and arise summative from the parameters Rk and Qk of the subgroups:
ri = ∑ v k Rk
(i )
(6),
k
q i = ∑ v k Qk
(i )
(7),
k
where vk(i) is the number of subgroups “k” of a group inside a molecule “i”. The other parameters of equation
(3) for the residual part of activity coefficient are defined by the following equations:
(i )
v Qk
eki = k
qi
(8),
β ik = ∑ e mi τ mk
(9),
θk
m
∑x q e
=
∑x q
i
i
i
j
ki
(10),
j
j
s k = ∑ θ m τ mk
m
(11),
τ mk = exp
- α mk
T
(12)
The above 12 equations can be solved when the molecular fractions of the components (xi) and the absolute
temperature (T) are known, by using the values of Rk, Qk and αmk found in literature. Nevertheless, the
UNIFAC model possesses several limitations that should be accounted during calculations, i.e. the pressure
should be less than 5 bar, the temperature should be less than 150 oC, calculations are only applicable to
condensable non electrolytes, while components should not contain more than 10 functional groups [7].
xlUNIFAC tally
The solution of the aforementioned equations was processed by using the software xlUNIFAC (version 1.0,
GNU Public Licence, GRL, USA). This software is a Microsoft Excel logistic tally designed for the
calculation of activity coefficients and partial vapor pressures, while it provides the possibility to examine the
2
effect of each parameter [7]. Thereby, the data of mixture ingredients (i.e. natural phenols and solvents) were
inserted in the program and afterwards the solvent activity coefficients were calculated. Fifteen phenols
(coumaric acid, vanillic acid, caffeic acid, ferulic acid, hydroxytyrosol, sinapic acid, cinnamic acid, phydroxyphenyl acetic acid, p-hydroxybenzoic acid, tyrosol, protocatechuic acid, syringic acid, gallic acid,
rosmarinic acid and oleuropein), 7 solvents (water, ethanol, methanol, acetone, dichloromethane, ethyl
acetate and diethyl ether) and 3 temperatures (298.15, 313.15 and 333.15 K) were introduced into the
program and calculated according to the corresponding subgroups. In the case that a subgroup was not
included inside the tally, the values of Rk and Qk were found in literature. The calculation of oleuropein
activity coefficient was performed by simulating a part of the molecule with maltose and the addition of
corresponding Rk and Qk values (10.7985 and 8.752, respectively) given in the literature [8].
RESULTS & DISCUSSION
The activity coefficients of natural phenols for different solvents at 298.15 K are shown in Table 1. As a
general rule, the values obtained for all the phenols were lower in alcohols and acetone, and possessed more
or less the same decimal magnitude. Particularly, the lower values for hydroxytyrosol, tyrosol, oleuropein,
caffeic, p-hydroxybenzoic, p-hydroxyphenyl acetic, protocatechuic and rosmarinic acid were obtained in
ethanol. Other phenolic acids like vanillic, ferulic, sinapic and syringic possessed the lower values for
methanol, while gallic, cinnamic and coumaric were the only acids that did not show the lower value for
alcohols but for water, dichloromethane and acetone, respectively. Besides, all the phenols possessed the
higher value for diethyl ether, except for oleuropein, coumaric, p-hydroxyphenyl acetic and protocatechuic
acid that showed the higher activity coefficient for water.
Table 2 represents the activity coefficients for different solvents at 313.15 K. Phenols showed again similar
sympathy to alcohols and acetone, while the lower values were observed for each individual compound in the
same solvent as above. An exception was observed for gallic acid, as the lower value was obtained for
ethanol instead of water. The higher values were generally observed for diethyl ether and water (9 and 6
compounds, respectively).
The activity coefficients for different solvents at even higher temperature 333.15 K are shown in Table 3.
The lower values were observed for each individual phenol in the same solvent as in previous temperatures,
but gallic and rosmarinic acid preferred methanol and acetone, respectively. The higher values were one
more time observed for diethyl ether and water (9 and 5 compounds, respectively). Besides, many
coefficients were similar for two solvents at this case, i.e. values observed for coumaric acid in alcohols
(2.210-1), for vanillic acid in dicholoromethane and ethyl acetate (8.8-8.910-1), for hydroxytyrosol in water
and dichloromethane (3.3-3.4100), for cinnamic acid in water and ethyl acetate (1.3-1.4100) and for phydroxyphenyl acetic acid for methanol and acetone (2.110-1). With regard to the variation of activity
coefficients among the 3 assayed temperatures, values obtained for water, ethanol, methanol, acetone and
ethyl acetate were generally enhanced by increasing temperature. On the other hand, values obtained for
dichloromethane and diethyl ether were decreased by increasing temperature.
Table 1. Solvent activity coefficients at 298.15 K for different natural phenols.
Natural
Solvent activity coefficients
phenol
Water
Ethanol
Methanol
Acetone
(W)
(E)
(M)
(A)
Caffeic acid
1.910-1
8.410-3
2.910-2
2.610-2
Cinnamic acid
2.0103
2.9100
1.1100
8.310-1
1
-1
-1
Coumaric acid
1.710
1.410
1.510
1.310-1
Ferulic acid
1.3100
2.710-1
1.410-1
1.810-1
-4
-4
-2
Gallic acid
7.510
8.710
1.110
1.410-2
0
-1
-1
p-Hydroxybenzoic acid
3.510
1.410
2.010
2.210-1
p-Hydroxyphenyl acetic acid 8.8100
9.810-2
1.510-1
1.510-1
-1
-2
-1
Hydroxytyrosol
9.110
2.710
2.010
2.310-1
Oleuropein
1.0100
3.310-5
7.410-4
3.610-4
0
-2
-2
Protocatechuic acid
4.710
1.010
4.510
5.210-2
-2
-4
-2
Rosmarinic acid
3.910
8.410
1.610
1.510-3
Sinapic acid
4.3100
3.210-1
1.010-1
2.510-1
0
-1
-1
Syringic acid
1.910
4.910
1.510
4.110-1
Tyrosol
5.0100
2.710-1
6.710-1
7.110-1
0
-1
-1
Vanillic acid
2.810
2.810
1.910
3.210-1
Dichloromethane (D)
4.610-1
6.710-1
4.910-1
4.310-1
1.5100
1.1100
6.310-1
4.8100
6.510-3
1.2100
3.610-2
4.210-1
7.710-1
3.2100
9.710-1
Ethyl
Acetate (EA)
5.510-2
1.3100
2.4 10-1
3.510-1
3.610-2
4.110-1
2.710-1
3.710-1
1.510-3
1.110-1
3.410-3
5.110-1
8.610-1
9.910-1
6.410-1
Diethyl
Ether (DE)
9.5100
1.8100
3.6100
8.2100
7.2101
6.3100
4.2100
8.0101
5.110-1
2.0100
1.4102
1.8101
3.1101
1.9101
1.5101
3
Table 2. Solvent activity coefficients at 313.15 K for different natural phenols.
Natural
Solvent activity coefficients
phenol
Water
Ethanol
Methanol Acetone Dichloro(W)
(E)
(M)
(A)
methane (D)
Caffeic acid
4.010-1
1.410-2
3.710-2
3.510-2
4.410-1
3
0
0
-1
Cinnamic acid
1.710
2.810
1.210
8.510
6.610-1
1
-1
-1
-1
Coumaric acid
2.3 10
1.710
1.910
1.310
4.810-1
Ferulic acid
1.8101
3.110-1
1.610-1
2.110-1
4.210-1
-3
-3
-2
-2
Gallic acid
2.610
1.810
1.510
2.210
1.4100
p-Hydroxybenzoic acid
5.1100
1.710-1
2.410-1
2.510-1
1.0100
1
-1
-1
-1
p-Hydroxyphenyl acetic acid 1.210
1.310
1.810
1.710
6.210-1
0
-2
-1
-1
Hydroxytyrosol
1.810
4.010
2.110
2.610
4.0100
Oleuropein
2.3100
5.310-5
8.310-4
4.810-4
7.010-3
-1
-2
-2
-2
Protocatechuic acid
1.110
1.610
5.610
7.010
1.1100
Rosmarinic acid
1.610-1
1.810-4
1.810-2
2.310-2
3.510-2
0
-1
-1
-1
Sinapic acid
6.810
3.610
1.210
2.910
4.010-1
0
-1
-1
-1
Syringic acid
3.110
5.510
1.710
4.610
7.410-1
Tyrosol
6.3101
3.110-1
6.710-1
7.110-1
2.8100
0
-1
-1
-1
Vanillic acid
4.210
3.310
2.110
3.710
9.310-1
Table 3. Solvent activity coefficients at 333.15 K for different natural phenols.
Natural
Solvent activity coefficients
phenol
Water
Ethanol Methanol Acetone Dichloro(W)
(E)
(M)
(A)
methane (D)
Caffeic acid
8.710-1
2.310-2 4.810-2
4.810-2 4.310-1
Cinnamic acid
1.4103
2.7100 1.3100
8.610-1 6.610-1
Coumaric acid
3.1101
2.210-1 2.210-1
1.810-1 4.710-1
1
-1
-1
Ferulic acid
2.610
3.710
1.910
2.410-1 4.010-1
Gallic acid
9.810-3
3.910-3 2.110-3
3.610-2 1.4100
0
-1
-1
p-Hydroxybenzoic acid 7.410
2.210
2.810
3.010-1 9.910-1
1
-1
-1
p-Hydroxyphenyl acetic 1.810
1.610
2.110
2.110-1 6.110-1
acid
Hydroxytyrosol
3.4100
5.910-2 2.310-1
2.910-1 3.3100
Oleuropein
5.1100
9.210-5 9.410-4
6.810-4 7.610-3
-1
-2
-2
Protocatechuic acid
2.510
2.710
7.210
9.710-2 1.1100
-1
-3
-2
Rosmarinic acid
7.110
3.910
2.110
3.610-3 3.410-2
Sinapic acid
1.1101
4.210-1 1.310-1
3.210-1 3.910-1
0
-1
-1
Syringic acid
5.010
6.210
1.910
5.110-1 6.910-1
Tyrosol
7.8101
3.610-1 6.610-1
7.010-1 2.4100
0
-1
-1
Vanillic acid
6.510
3.910
2.410
4.210-1 8.910-1
Ethyl
Acetate (EA)
7.710-2
1.3100
2.8100
4.110-1
6.010-2
4.910-1
3.310-1
4.610-1
2.310-3
1.610-1
6.710-3
6.110-1
9.910-1
1.0100
7.510-1
Diethyl
Ether (DE)
9.5100
1.8100
3.6100
7.9100
7.0101
6.3100
4.2100
6.7101
5.310-1
2.0101
1.3102
1.7101
2.9101
1.6101
1.5101
Ethyl
Acetate (EA)
1.110-1
1.3100
3.410-1
4.910-1
1.010-1
5.810-1
4.110-1
Diethyl
Ether (DE)
9.3100
1.8100
3.6100
7.5100
6.7101
6.2100
4.2100
5.710-1
3.910-3
2.310-1
1.410-2
7.410-1
1.2100
1.1100
8.810-1
5.4101
5.410-1
1.9101
1.1102
1.5101
2.5101
1.4101
1.4101
The activity coefficient data predicted by UNIFAC model indicate that natural phenols possess a solubility
preference basically to solvents with intermediate polarity (alcohols and acetone), rather than more polar
water or less polar dichloromethane and diethyl ether. The solubility tendency can be explained by the
stereochemistry of phenols (the polar and the non polar fragment inside their molecules) and the
intermolecular forces (mainly hydrogen bonds) occurred between them and the solvents. This was more
obvious in the case of room temperature (Table 1, 298.15 K). In particular, the phenols’ hydroxyl groups can
develop hydrogen bonds with the electronegative oxygen of ethanol, methanol or acetone. The alcohols’
hydroxyl groups can also develop hydrogen bonds with the oxygen atoms occurred inside phenol molecules
(like oleuropein, ferulic, rosmarinic, vanillic, syringic or synapic acid) and this observation can explain their
preference either to polar protic (methanol or ethanol) instead of polar aprotic solvents (acetone). The
preference of phenols between methanol and ethanol can be attributed to their non polar part and the aliphatic
fragment of alcohols. Methanol contains a smaller and more flexible aliphatic fragment compared to ethanol
and thus surrounds easier phenols with substituted carbons inside their aromatic ring. This hypothesis was
rather obvious in the cases of ferulic, vanillic, syringic and synapic acids (3 or 4 substituted carbons). On the
other hand, bigger molecules (like oleuropein and rosmarinic acid), phenols with two anti-diametric
substituted carbons (like p-hydrobenzoic and p-hydroxyphenyl acetic acid) or longer aliphatic fragments (like
tyrosol and hydroxytyrosol) preferred ethanol, as it could “cover” better the gaps between the hydrogen
bonds. In the case of gallic acid where the aromatic ring is well surrounded by 3 hydroxyls and one carboxyl
group, the preferred solvent was the polar protic water molecule. Besides, the only studied phenol that did not
4
contain a hydroxyl group (cinnamic acid) preferred the polar aprotic dichloromethane, eventually due to the
dipole-dipole interactions developed between the more electronegative fragment of the solvent (chlorine
atoms) and the more electropositive fragment of the acid (hydrogen protons).
For higher temperatures, the obtained prediction data coincided to the above considerations, although the
induced thermal effect changed activity coefficient values. A potential explanation could be related to the
number of hydrogen bonding with water molecule that decreases upon a temperature increase [9]. The above
hypothesis was impressed in the prediction data, as activity coefficients of phenols were generally increased
in polar protic (water, methanol and ethanol) and aprotic (acetone and ethyl acetate) solvents as a function of
temperature, but decreased (or not varied) in less polar (aprotic) dichloromethane and non polar diethyl ether.
Besides, few exceptions observed for specific phenols predicted the reduction of hydrogen bonds with
increasing temperature, i.e. the coefficient of non-hydroxyl groups’ coumaric acid was decreased in polar
water and ethanol. Moreover, the increase of activity coefficients suggests that the solubility of phenols into
the aforementioned solvents is decreased as a function of temperature. However, this hypothesis is in contrast
to the experimental and theoretical data of other studies referring that the solubility of phenols (tyrosol,
protocatechuic, syringic, gallic, vanillic and coumaric acid) in water is increased by enhancing the
temperature from 298.15 to 223.15 K [2, 10, 11]. Likewise, the solubility of gallic acid in polar protic
(methanol or ethanol) and aprotic (ethyl acetate) solvents has been shown to increase smoothly with
temperature [12]. A possible explanation for this discordance could be that the temperature induced solubility
is affected more by other factors, i.e. fusion enthalpy, which should be taken account during prediction of
thermal solvent extraction. Nevertheless, the correlation between phenols solubility and thermodynamic
properties is not always clear as it has been referred for the case of several flavonoids [13].
According to bibliographic data, theoretical approach generally coincides with experimental findings, as
polar (protic or aprotic) solvents are in both cases the most popular choice. For example, olive mill waste is a
typical agricultural by-product that is known to contain all the discussed phenols of the current study (except
from rosmarinic acid) and has been utilized for their recovery by solvent extraction [14]. Particularly, ethyl
acetate has also been used for the recovery of low molecular weight phenolic acids like caffeic, ferulic and
coumaric from this waste, but other common phenols (oleuropein, protocatechuic, syringic and cinnamic
acid) were not detected in the corresponding extracts [14, 15]. The most active phenols (tyrosol and
hydroxytyrosol) were preferably recovered with polar aprotic ethyl acetate as compared to non-polar
solvents, while extraction yield enhanced with increasing polarity of the medium [16-19]. The last
observation led the researchers to investigate more the polar protic (i.e. hydro-alcoholic) solvents. For
example, water-ethanol mixtures have been reported for their advanced recovery of oleuropein,
hydroxytyrosol and tyrosol [20], while hydro-methanolic mixtures have been referred to extract phenols with
the highest yield and widest array compared to ethanol and ethyl-acetate [15]. Besides, rosmarinic acid has
preferably been recovered with ethanol from sage and lemon balm [21-22]. Moreover, Lafka et al. [23]
investigated the extraction of phenolic compounds (hydroxytyrosol, tyrosol, caffeic, syringic, vanillic and
coumaric acid) with several solvents (ethanol, hydro-ethanolic mixture, methanol and acetone) from winery
waste. According to this study, ethanolic extract possessed the highest antioxidant activity, although the
difference from methanol and acetone extracts was negligible indicating that the recovery yield was rather
similar. This result is in accordance to the theoretical estimation that the assayed phenols possessed activity
of the same decimal magnitude for the aforementioned polar solvents.
CONCLUSION
In this study a knowledge base for the extraction of 15 phenols from natural sources was developed, reliant
on the UNIFAC model and the calculation of the corresponding activity coefficients in 7 solvents. Based on
the activity coefficient data, solvents were classified for their ability to dissolve phenols in 3 different
temperatures. Predictions indicated that the studied phenols are generally solubilized easier in polar protic
mediums like ethanol and methanol, but gallic, cinnamic and coumaric acids preferred water,
dichloromethane and acetone, respectively. Increasing temperature changed the activity coefficient values of
phenols, which tend to coincide for different solvents at the temperature of 333.15 K and thus to decline the
solvent recovery preference. Despite the rather simple theoretical consideration (lower activity coefficient
correspond to solubility and solvent recovery preference), a comparison with extraction data found in
literature showed that the developed knowledge base could be utilized in order to predict the appropriate
solvent for each extraction process, as modeling predictions were generally in accordance to experimental
findings.
5
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