Membrane processes for the facilitated extraction of disaccharide sugars: Parameters and mechanism
Khalifa Touaj a,b , Oussama Kamal a,b , El Houssaine EL Atmani a , Tarik Eljaddi a , Laurent Lebrun b , Miloudi Hlaïbi a,b, ⇑
a
Laboratoire d’Interface Matériaux et Chimie de l’Environnement (LIME), Université Hassan II Faculté des Sciences Aïn Chock, B.P. 5366, Maârif, Casablanca, Morocco
b
Laboratoire des Polymères, Biopolymères, Surfaces, UMR 6270 du CNRS. Université de Rouen, Faculté des Sciences, F-76821 Mont-Saint-Aignan, France
a r t i c l e i n f o
Article history:
Received 24 January 2014
Received in revised form 10 July 2014 Accepted 15 July 2014
Available online 23 July 2014
Keywords:
Facilitated extraction Supported liquid membrane Apparent diffusion coefficients Association constants Molecular recognition
a b s t r a c t
A supported liquid membrane (SLM) containing a lipophilic agent methyl cholate as carrier, previously used for the facilitated transport of monosaccharides, has been used for the selective extraction of some disaccharides: Lactulose, Lactose, Maltose, Melezitose, Sucrose and Trehalose. The membrane is made of a micro porous poly(vinylidene difluoride) film (PVDF), impregnated with a 0.01 M solution of the carrier in xylene. This prepared SLM is remarkably stable for at least 20 days. The parameters, permeabilities (P) and initial fluxes (J
0) on the facilitated extraction across the studied SLM, for these sugars, were determined.
On the basis of the flux dependence on the initial concentrations of carrier and transported substrate, the rate-determining step in the transport mechanism is shown to be the migration of the sugar–carrier com- plex (ST) in the membrane organic phase. The initial flux of transported sugar is related to the initial con- centration of this substrate in feed phase by a saturation law, which has allowed the determination of the parameters, apparent diffusion coefficient D
⁄and the association constant K
assof the formed complex (ST) in the SLM organic phase. The results clearly indicate that these all sugar-carrier complexes are unstable, which results in higher coefficients D
⁄and thus a high permeability of this membrane type to facilitated extraction of these disaccharides. While, the stability of these complexes varies widely depending on the molecular structure of each disaccharide, confirming the molecular recognition phenomenon by the carrier and can identify interaction sites (substrate–carrier) involved during the extraction these carbohydrates
Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction
Monosaccharides such as glucose are produced every day by the photosynthesis phenomenon from water and atmospheric carbon dioxide. The sugars of various natures are found in the form of sim- ple molecules mixtures after degradation reactions and which may be used as reagents for the synthesis of the novel compounds.
Indeed, the gas and oil (exhaustible resources), are almost the only organic raw materials for the chemical industry, then it is neces- sary and imperative to find new renewable raw materials for this vital industry. The sugars, abundantly produced by plants, are the most likely to be used, and the development of simple and economic methods for separation of sugar mixtures would
consequently allow the obtainment of a renewable source of molecules highly functionalized. Since most sugars are isomers which only differ in the configuration of specific CHOH groups, methods based on differences of chemical reactivities are generally unsuitable [1]. On the contrary, separative methods based on molecular recognition appear as promising tools [2–4], with the use of a selective reagent (the host) that can form (host–guest) complexes with mono or disaccharides (the guests) [5,6]. When the stability of each complex varies with the sugar structure, the components in the mixture are complexed in different proportions, and various techniques such as extraction may be used for the isolation of the resulting complexes.
In the case of mono or disaccharides compounds, a possible process is extraction into organic solvents, which is made possible by forming complexes with lipophilic host agents. However, for use on the industrial scale, extraction methods should be preferably adapted to liquid membrane processes, which currently offer, the best strategy for environmental-friendly separations [7].
Most liquid membranes consist of an organic phase that separates two aqueous (feed and receiving) phases [7], with a double
http://dx.doi.org/10.1016/j.micromeso.2014.07.025 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.
⇑ Corresponding author at: Laboratoire d’Interface Matériaux et Chimie de l’Environnement (LIME), Université Hassan II Faculté des Sciences Aïn Chock, B.P.
5366, Maârif, Casablanca, Morocco. Tel.: + 212 2 23 06 80; fax: + 212 2 23 06 74.
E-mail address: miloudi58@hotmail.com (M. Hlaïbi).
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m i c r o m e s o
liquid–liquid extraction process, with a very small volume of solvent and limited emissions of organic vapors, these liquid mem- branes allow larger fluxes than other membrane types. Selective extraction occurs when the liquid membrane contains an extrac- tive agent (carrier) which forms the entities of different stabilities with the various sugars present in the feed phase.
The supported liquid membranes (SLMs) represent an improve- ment for these applications, and provide a convenient and elegant method for the selective extraction of some hydro soluble species by facilitated extraction process. Generally, the extracted species are metal ions [8–10], organic acids [11–14], amino acids [15], and also neutral molecules such as various drugs (antibiotics) [16,17], phenols [18,19] and especially sugars [7,20,21], for which several extractive agents have been used. This liquid membrane type used for the selective extraction of the compounds from mix- tures, requires the adoption of a lipophilic agent, soluble only in the membrane organic phase, and allows a specific interaction with each compound in the mixture according to its structure. A supported liquid membrane (SLM) is typically made of a hydropho- bic micro porous support, which is impregnated with an organic solvent not soluble in the aqueous phases, containing a quantity of extractive agent. This organic phase containing an extractive agent is immobilized in the pores of the support under the capil- lary forces action, however, the only weakness often presented by this membrane type is its lifetime, limited by a possible disso- lution of small quantities of the solvent and the extractive agent in the aqueous phases, which limits the use of this effective mem- brane type for specific industrial applications [22–24].
The facilitated separation process of mono or disaccharide com- pounds (sugars) through SLM is mainly due to the use of specific extractive agents can react selectively with the sugars to form complexes with different stabilities. Boronic acids were among the first agents used to selective extraction of some simple sugars [25–27]. Later, a derivative agent of calix[4]arene with a perfectly
symmetrical structure, was used in organic phase CCl
4for facili- tated but not specific extraction of several alditols and sugars from aqueous solutions [28–34]. For more selectivity, we have recently suggested a commercial carrier agent, having unsymmetrical structure, and derived from a non-toxic natural compound (sterol) [35,36], especially the methyl cholate ester (Fig. 1). This lipophilic agent has a rigid structure and three adjacent hydroxyl groups may be a recognition site, capable of reacting selectively with each of the extracted molecules, whose molecular structure is composed of several adjacent hydroxyl groups.
In this work we adopted the selective extraction process through a SLM containing this lipophilic extractive agent with a specific structure (Fig. 1), and reacts with each studied disaccha- ride molecule (Fig. 2) in a different way depending on its molecular structure.
Nomenclature
a slope of the plot ln(C
02C
R) = f(t)
C
0initial concentration of sugar in the feed phase (mol L
1) C
Rconcentration of transported sugar in the receiving
phase (mol L
1)
C
Fconcentration of sugar in the feed phase (mol L
1) P the membrane permeability ions (cm
2s
1)
J
0initial flux on the facilitated transport of substrate (mmol cm
2s
1)
D
⁄apparent diffusion coefficient of the complex (TS) (cm
2s
1)
K
assconstant association on the formation of the complex (TS).
l the membrane thickness (mm or l m) S the membrane active area (cm
2)
[T]
0concentration of carrier in the membrane (mol L
1) [TS] concentration of the complex in the organic phase
(mol L
1) T temperature (K)
t time (s)
V volume of the receiving compartment (cm
3)
Fig. 1. Structure of methyl cholate ester.
O OH HO HO OH
OH O
OH HO HO O
OH
HO
O O
OH
HO OH
OH
O OH OH
HO O
OH
O OH
HO OH
OH O O
HO OH HO
OH
O OH HO HO
O
O OH
OH OH O HO
HO O
OH OH
HO OH
HO
HO HO
HO HO
OH O Glucose
Lactose
Tréhalose Maltose
Lactulose Sucrose
HO
Fig. 2. Fischer representations of some disaccharide molecules.
This process is often referred to as molecular recognition phe- nomenon, wherein the extractive agent (host = methyl cholate) has a molecular recognition site may be distinguished by a specific interaction with each of studied substrates, according to its structure,
2. Theoretical models
For this facilitated transport and extraction phenomenon, several published studies [32,37–39] have shown that for this membrane type (SLM), and in the experimental conditions where the substrate is in excess relative to the carrier agent. A kinetic model based on Fick’s first law and a thermodynamic model based on the mass action law and the Fick’s second law, used to establish the relationships of Eqs. (1) and (2).
Pðt t
LÞ ¼ ðl V=2SÞ ln½C
0=ðC
02C
RÞ ð1Þ
1=J
0¼ ðl=D
Þ ½ð1=½T
0K
assÞ ð1=C
0Þ þ ð1=½T
0Þ ð2Þ P: membrane permeability, l: membrane thickness, S: membrane active area and V: receiving phase volume.
C
0, [T]
0: respectively the initial concentrations of substrate in the feed phase and extractive agent in the organic phase and C
R: substrate concentration in receiving phase at time t.
J
0: substrate initial flux through the membrane, K
ass: association constant of (substrate–carrier agent) entity (ST) and D
⁄: apparent diffusion coefficient of this entity through the SLM organic phase.
From these relations, we find that the term – ln (C
02C
R) is a linear function of time and the slope (a) of this line is used to calculate the values of macroscopic parameters (P and J
0) according to the expressions in Eq. (3), when the intercept (OO) and the slope (p) on the Lineweaver–Burk straight line 1/J
0= f(1/C
0) from Eq. (2), can calculate the microscopic parameters (K
assand D
⁄) according to the expressions in Eq. (4).
P ¼ a V l=2S and J
0¼ P C
0=l ð3Þ
K
ass¼ intercept ðOOÞ=slopeðpÞ and D
¼ ðl=OOÞ ð1=½T
0Þ ð4Þ These relationships were established from a mechanism based on the formation of complex (ST) at the interface feed phase- membrane, by rapid interaction of the substrate (S) with the carrier (T), the migration of this formed complex (ST) through the mem- brane organic phase which is the rate-determining step of this mechanism, and the rapid dissociation of this complex (ST) at the interface membrane-receiving phase. On the other hand, the expression in Eq. (2) shows that the value of the initial flux J
0is a function of carrier concentration [T]
0in organic phase of the membrane, and evolves with initial substrate concentration C
0according to a saturation law.
3. Experimental section
The extractive agent methyl cholate ester (MC) was purchased from ICN Biomedicals and all disaccharide sugars, solvents and other compounds are commercial products (Aldrich, Fluka) of the purest available grade for analysis, used as received.
Adopted polymeric support, is a microporous flat sheet of poly(vinylidene difluoride) (PVDF) film Millipore HVHP of thick- ness 96 l m and having a porosity of 60% and pore diameter of 0.22 l m. The membrane active area for the extraction of the sub- strate is 19.6 cm
2. The SLM preparation was conducted in two steps, the first is fixing the carrier agent in the pores by impregna- tion of this polymer support with a solution of methyl cholate in acetone, and then evaporated to dryness to eliminate this auxiliary solvent, and weigh the obtained support to determine the fixed
mass of the carrier agent (MC). The second step is the impregnation of this obtained support by xylene solvent, which is the adopted organic phase for this extraction phenomenon, and before use the SLM has been conditioned in pure water, by insertion in the transport cell between two aqueous solutions [32]. Known quanti- ties of sugars were dissolved in pure water to prepare solutions of studied disaccharides (0.2–0.025 M).
For this studied phenomenon, the adopted extraction cell is made of two compartments of equal volumes, separated by the used SLM [32,37,39]. This experimental cell is immersed in a thermostated bath (T = 298 K), the solutions in both compartments were stirred at a constant rate using a magnetic stirrer.
At known time intervals, small quantities are extracted from the receiving phase. These solutions are analyzed using an HPLC appa- ratus equipped with a 30 cm Phenomenex Resex column in cal- cium form, maintained in an oven at T = 358 K. The eluent was pure water, filtered with a cellulose ester membrane (Millipore, pore diameter 0.45 l m) and degassed. The pump was a Shimadzu LC-9A model and the flow rate was 0.6 mL/min. Detection of trans- ported substrates was achieved with a Varian RI-4 refractometer.
4. Results and discussion 4.1. Conditioning of the SLM
For this facilitated transport and extraction phenomenon of organic compounds by SLM’s, several studies with different mem- brane types [32,37,40,41], indicate that the diffusion of a sugar or an alditol, starts after a long induction period. This time period is independent of the substrate nature, its structure and its initial concentration. This phenomenon has been observed when using a PIM membrane [40] for the transport of monosaccharide sugars.
Several studies [31,32,37,40,42] have indicated that this period is a function of the solvent nature and can be spread over 15–40 h. It corresponds to the slow incorporation of water in the membrane organic phase to form an extractive agent–water complex. On the other hand, the formation of this complex is necessary because it allows the migration of the substrate across the organic phase of the SLM, by forming a binary complex carrier agent–sugar, by rapid exchange reaction, or by forming a ternary complex carrier agent–
water–sugar.
For this facilitated extraction phenomenon of studied sugars (disaccharides), we have adopted different times, from 15 to 40 h for this conditioning process of the used SLM. The results show that after the induction period, the transport rates, the calculated values of the permeabilities and the fluxes, are almost identical to 10% uncertainty for each extracted sugar with different condi- tioning times of the membrane. This important result indicates that the facilitated extraction mechanism does not depend of the conditioning time on this induction period. Finally this SLM conditioning step is important and allows removing the induction period, to facilitate the operating conditions and improve the experimental results. For all experiments, in extraction phenome- non of these sugars, we conducted the conditioning of used SLMs in pure water for a period of about 40 h.
4.2. Effect of initial substrate concentration C
0The facilitated extraction of different studied disaccharides was
performed for initial concentrations (C
0) ranging from 0.2 to
0.025 M and using the SLM’s containing 0.04 g of extractive agent
methyl cholate (MC). The C
Rconcentration of receiving phase
was calculated according to the time for all studied substrates
and straight lines for –ln (C
0– 2C
R) = f(t) functions were plotted
from Eq. (1) provided by the kinetic model (Fig. 3). These lines
clearly show that the developed kinetic model is verified, and the slopes of these straight lines were used to calculate according to the expressions in Eq. (3), macroscopic parameters P and J
0related to this facilitated extraction phenomenon of these sugars.
The experimental results for macroscopic parameters P and J
0are summarized in Table 1. The values of these parameters indicate that the methyl cholate (MC) is an effective agent for the transport of these organic compounds, and the adopted membrane type (SLM) is effective for this extraction process. These values show that the membrane permeability increases with the dilution of the extracted substrates and vice versa. However, the values of the initial fluxes for different substrates decrease with initial concentration C
0of the studied solutions.
These results express an evolution in a law saturation Michaelis–Menten type for the values of the initial fluxes (J
0). In fact, for each substrate (S), we observe a rapid increase in these values (J
0) with initial concentrations C
0, to reach a maximum value which corresponds to a complete saturation of the extracting
agent (T) in the organic phase by the substrate (S). The values of these macroscopic parameters (P and J
0) also indicate that the developed SLM is more efficient to facilitated transport and extrac- tion of monosaccharide sugars (glucose) than their disaccharide counterparts.
4.3. Influence of the molecular structures on the microscopic parameters K
assand D
⁄To explain these results, to have sufficient information on the mechanism for the facilitated extraction process of these sugars by this membrane type, to identify possible interaction sites of each substrate according to its structure with the carrier agent, and to ensure the migration of this substrate from the source phase to the receiving phase, we have proceeded to the determination of microscopic parameters K
assand D
⁄. Indeed, we have plotted the straight lines 1/J
0= f(1/C
0) provided by Eq. (2), the graph in the fol- lowing figure clearly shows that the developed thermodynamic model is verified (see Fig. 4).
According to the expressions in the Eq. (4), the values of the slopes and intercepts of these straight lines, are used to determine the values of parameters K
assand D
⁄, relating to the association constant of each substrate with the carrier agent to form the entity (ST), and the apparent diffusion coefficient of this entity during its movement through the SLM organic phase. All values determined for these important microscopic parameters are summarized in Table 2. The results indicate that the entities formed in the mem- brane organic phase are low stabilities, while the diffusion of these entities across the organic phase is high. These two parameters K
assand D
⁄, are specific to each of the substrates according to its structure and their values are opposite. Indeed, we find that when the value of the constant K
assis large, then the value of the coefficient D
⁄is low and inversely.
Higher values of the apparent diffusion coefficients (D
⁄) and the evolution of the parameters K
assand D
⁄, certainly show that the movement of each substrate during its migration from the source phase to the receiving phase through the membrane organic phase is not a pure movement of diffusion. Indeed, the low values of the constants K
ass, which correspond to large values of the coefficients D
⁄, probably indicate that the substrate movement during its migration across the SLM is a result of association–
dissociation reactions by successive jumps of substrate (S) from one site to another of the extractive agent (T). Several factors can Fig. 3. Evolution of –ln (C
0– 2C
R) = f(t) functions for facilitated extraction phenom-
enon of some disaccharide molecules by SLM’s.
Table 1
Evolution of macroscopic parameters P and J
0according to initial concentrations C
0of all studied sugars: Lactulose, Lactose, Maltose, Melezitose, Sucrose And Trehalose.
Sugars c (M) P 10
7(cm2 s1) J
010
5(mmol cm2 s1)
Glucose 0.4 11.04 4.60
0.2 11.73 2.44
0.1 12.42 1.29
0.05 13.61 0.71
Galactose 0.4 7.91 3.30
0.2 9.48 2.05
0.1 10.77 1.12
0.05 11.43 0.59
Lactulose 0.2 9.27 1.93
0.1 9.45 0.98
0.05 9.60 0.50
0.025 9.70 0.25
Lactose 0.2 9.64 2.01
0.025 11.40 0.30
Maltose 0.2 9.85 2.05
0.05 11.72 0.61
Melezitose 0.2 7.31 1.52
0.025 9.07 0.24
Sucrose 0.2 10.08 2.10
0.025 12.14 0.32
Trehalose 0.2 7.99 1.66
0.025 9.89 0.26
Organic phase: xylene, pore diameter: 0.22 l m, carrier mass = 0.04 g, and T = 298 K. Fig. 4. Lineweaver–Burk representations (1/J
0= f(1/C
0)) on the facilitated extraction
process of several disaccharide molecules.
influence the values of this coefficient D
⁄, and thus, the movement nature of the substrate in the organic phase. These factors include the substrate molecular structure, and the type of substrate–carrier agent interactions (S–T), established during this facilitated extrac- tion process by the SLM. The important evolution of the experi- mental values in Table 2 on association constants K
ass, indicates that interaction sites (substrates–carrier agent) change from sugar to another, certainly depends on the position and the orientation of the OH groups of each substrate relative to the triol interaction site (HO
3,7,12) of the carrier agent (MC). The histogram in Fig. 5, con- firms the inverse evolution of parameters K
assand D
⁄, and small changes in constants K
asscause significant changes in apparent dif- fusion coefficients D
⁄.
The evolution of these microscopic parameters is probably related to the molecular structure of the substrate, the nature of its movement through the membrane organic phase, and especially the type of bonds established by interaction of each substrate (S) with the carrier (T) during its passage across the SLM phase. All these results and interpretations indicate that there is a possible molecular recognition phenomenon of each substrate by extracting agent methyl cholate (MC), which allows selective extraction of some disaccharide molecules according to their molecular structures.
4.4. Nature and effect of the interaction sites on the selective extraction process of disaccharide sugars
This facilitated transport and extraction phenomenon by sup- ported liquid membranes (SLMs) of several hydrophilic organic compounds such as mono and disaccharides sugars, is a process of dissolving these compounds in an organic solvent (hostile phase) by interaction with a hydrophobic agent having one or more interaction sites, may react differently with these hydrophilic
compounds according to their structures. The results in Table 2 can be noted that the methyl cholate agent (MC) is able to react with studied disaccharide sugars in different ways, to perform the selective extraction of these compounds by the adopted membrane process. These results also show that the carrier agent (MC) reacts very weakly with Lactulose sugar (K
ass= 0.30) compared to other studied substrates. This low value of the association constant, on the interaction of Lactulose sugar with extractive agent (CM), is certainly related to the molecular structure of this substrate and the interaction site nature of Lactulose with the carrier agent. This low value (K
ass= 0.30), provides an important value of the apparent diffusion coefficient (D
⁄= 20.8 10
6cm
2s
1) compared to other studied compounds, and therefore, a greater recognition of Lactulose sugar by the extractive agent (MC), which results in a good extraction of this substrate by this developed membrane type. This molecular recognition phenomenon is certainly based on the molecular structure of each sugar, especially on the value of the association constant (K
ass), which depends on the position and orientation of the OH groups involved in the interaction site of the substrate with the carrier agent, during its migration through the membrane organic phase. To explain and understand these results, it is absolutely necessary to identify the OH groups involved in the different possible interaction sites established for this facilitated extraction phenomenon of these disaccharide molecules.
Previous published studies [37] have shown that in the facili- tated transport and extraction process of monosaccharide sub- strates, glucose and galactose by a SLM containing the extractive agent methyl cholate (MC), the interaction sites are triol sites formed by three OH groups in positions 2, 3 and 6 (HO
2,3,6) for Glucose sugar, and in positions 1, 2 and 6 (HO
1,2,6) for its counter- part Galactose, this difference of sites has been attributed to the annoying position of the OH group in position 4 of axial orienta- tion. Each of these sites is recognized differently by the triol recog- nition site (HO
3,7,12) of the extractive agent, therefore, facilitated extraction of each substrate (glucose and galactose) by this mem- brane type is carried out by two different interactions of this triol recognition site (HO
3,7,12) with each sites, (HO
1,2,6) for galactose and (HO
2,3,6) to its counterpart glucose. The diagram in Fig. 6 shows the probable structure of the substrate–carrier agent entity formed by association of the extractive (MC) with the substrate, via its interaction triol site (HO
2,3,6). This association (substrate–car- rier) is necessary for the migration of the substrate through the membrane organic phase from source phase to receiving phase.
The values of the parameters K
assand D
⁄corresponding to this association and this migration are specific and can quantify this facilitated extraction phenomenon of the substrate.
For facilitated extraction of the disaccharide molecules by SLM containing methyl cholate agent (MC), the results show that for the studied disaccharide sugars, we obtained two limit values for K
assparameter. A low value (K
ass= 0.30) for Lactulose substrate which corresponds to a high value of D
⁄coefficient, however, a Table 2
Evolution of the microscopic parameters K
assand D
⁄depending on the nature of the sugar and its molecular structure.
Sugar K
ass(mol
1L) D
⁄(10
6cm
2.s
1) Structure Probable site
Glucose 0.98 10.05 Py 236
*Galactose 1.39 6.37 Py 126
*Lactulose 0.30 20.8 Galp(b1–4)Fru Galp 236 steric hindrance HO in position-4
Lactose 1.21 6.77 Galp(b1–4)Glcp Glcp 236
Maltose 1.18 7.66 Glcp( a 1–4)Glcp Glcp 236
Melezitose 1.15 5.88 Glcp( a 1–2) b-Fruf (3- a 1) Glcp 236
Sucrose 1.52 5.99 Glcp( a 1–2b)Fruf Glcp 236
Trehalose 1.16 6.35 Glcp( a 1- a 1)Glcp Glcp 236
*
[37], Organic phase: Xylene, pore diameter: 0.22 l m, carrier mass = 0.04 g, and T = 298°K.
Fig. 5. Evolution of the microscopic parameters K
assand D
⁄, depending on the
nature of studied sugars.
high value (K
ass(moy) 1.25) for other substrates (Lactose, Maltose, Melizitose, Fructose and Trehalose), reflecting low extrac- tion by the adopted membrane, of these disaccharide molecules relative to Lactulose sugar. These results indicate the existence of two possible sites needed for this facilitated extraction process of these disaccharide sugars, these sites correspond to these limit val- ues of K
assparameter. Indeed, probably methyl cholate agent reacts with Lactulose substrate via the interaction triol site (HO
2,3,6), located on the galactose ring of the molecule with a high steric hindrance due to the axial orientation of HO group in position-4.
This association by the interaction triol site (HO
2,3,6) and the steric hindrance, result in the low value of the association constant (K
ass= 0.30) and thus a favorable molecular recognition phenomenon for selective extraction of this disaccharide compound by the adopted SLM. In contrast, all other studied disaccharide molecules react with methyl cholate via the triol site (HO
2,3,6), located on the glucose cycle of each disaccharide sugar, which is oriented to the recognition triol site (HO
3,7,12) of the extractant. This positive association via this well oriented triol site (HO
2,3,6), provides the high value of K
ass(1.25) and thus a negative molecular recognition phenomenon for this facilitated extraction process of these disaccharide sugars compared to their counterpart Lactulose.
These results clearly show that, on the one hand, the recognition triol site (HO
3,7,12) of the MC agent, recognizes and reacts preferentially with triol sites formed by HO groups of glucose or galactose cycle, and on the other hand, the methyl Cholate (MC) is an excellent carrier agent, which allows the extraction of Lactulose sugar with good selectivity, compared to other studied disaccharide sugars.
5. Conclusion
The facilitated extraction of disaccharide sugars by supported liquid membrane containing the methyl cholate agent was per- formed with excellent results that promise a possible selective extraction of Lactulose substrate. The developed membrane is remarkably stable for a 20 days period, these performances can consider some applications of this membrane process for the facil- itated extraction and the selective recovery of Lactulose sugar.
Indeed, several attempts and experiments based on similar pro- cesses were carried out for the separation of organic compound mixtures with satisfactory results [32,43–45].
The evolution of the flux parameter (J
0) depending on the sugar concentration is Michaelis–Menten type and indicates that the rate-determining step for the mechanism of this facilitated extrac- tion is the migration of the substrate–carrier complex through the
membrane organic phase. The determination of the association constants (K
ass) and apparent diffusion coefficients (D
⁄), allows to know the interaction sites needed for the association of each sub- strate with methyl cholate and the formation of sugars–carrier complexes, as well as the movement nature of these complexes across the SLM. The observed difference in values of association constants is very important, it has identified the interaction sites adopted by each of the substrates for this facilitated extraction process of these disaccharide sugars, and confirms the existence of a molecular recognition phenomenon, related to the recognition triol site (HO
3, 7, 12), specific to the methyl cholate agent.
Analysis of the results shows that the association constant is a capital microscopic parameter for the facilitated transport and extraction phenomenon by membrane processes. Indeed, the values of this parameter (K
ass) are specific to a substrate, a carrier and adopted membrane type, and for the facilitated extraction of organic compounds, these values are used to identify the nature of interaction sites, to explain the results and to confirm if the extractant has a molecular recognition site can distinguish and react with these compounds according to their molecular structures via specific interaction sites.
Acknowledgements
All authors wish to thank the Director and the researchers of the CNRS Laboratory PBS of Rouen University, for their Financial and logistical support.
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S S S
SLM (organic phase)
T T T T T T T
Carrier:
Methyl Cholate Resorcinarene
