Liquid-liquid equilibrium phase diagram of aqueous systems containing surfactant and some phosphate salts:
experimental and modeling
M. Foroutan a,, N. Heidari b, M. Mohammadlou b
a- Department of Physical Chemistry, College of Science, University of Tehran, Tehran, Iran.
[email protected], b- Department of Chemistry, University of Uremia, Uremia, Iran.
Abstract
The effect of salt type on the liquid-liquid equilibrium of the aqueous solution of surfactant polyoxyethylene cetylether (with abbreviation name Brij 58) has been investigated at four temperatures (303.15, 313.15, 323.15 and 333.15) K. The Osmotic virial equation was used to correlate the phase behavior of this system. Good agreement has been found between experimental and calculated data from both models.
Keyword: Liquid-liquid equilibrium; Surfactant; Brij-58; osmotic virial equation.
Introduction
Aqueous two-phase systems (ATPSs) have received increasing attention during the last two decades because of their ability in separating and concentrating biomaterials.
Recently liquid-liquid equilibrium data for some aqueous solutions containing surfactant have been reported in the literature [1] which shows that surfactants can be considered as an important candidate for purification of biomolecules.
Theory
There are some models describing the phase behavior of aqueous two-phase systems proposed in the literature, one based on osmotic virial expansions and the lattice theories [2,4]. In this work, following Edmond and Ogston[2] the osmotic virial
3 1 2
∑ ∑
(mm (
m m
3 m
β11
) )
equation was successfully applied for the correlation of experimental LLE data of ATPS containg surfactant Brij 58.
In the osmotic virial equation, the chemical potentials of surfactant (1) and salt (2) as a function of molality of surfactant, salt can be written as
µ = µ0 + RT(lnm + β m + β m ) (1)
1 1 1 11 1 12 2
µ = µ0 + RT(lnm
+ β m + β
m ) (2)
2 2 2 22 2 12 1
µ = µ 0 −
RTV 3
ρ (m1 + 2+ m 2 / 2 +
β m 2 / 2 + βm1m2 ) (3)
where þij is a constant characterizing the interaction between a molecule of component i and a molecule of component j, and µoi is the standard-state chemical potential of component i. The standard state of component 1 and 2 is a hypothetical state of ideal solution at unit molality.
The interaction parameters þij are evaluated from the fitting of experimental LLE data to the equation µtopi = µboti using a suitable objective function. The following objective function was used:
3 N
Fob = i , j , exptop
− top 2
i , j , cal + i , j , bot
exp
− bot 2
i , j , cal (4)
i =1 j =1
In the present work, we the report LLE data for aqueous Brij 58 with three salts (KH2P O4, (NH4)2HPO4 and NH4H2PO4) at 298.15 K.
Results and discussion
The phase diagrams and the tie lines of three surfactant-salt systems; Brij 58–salt (KH
2PO4, (NH4)2HPO4 and NH4H2PO4) has been shown in figures 1-3. The mentioned salts are common in anion type and have different cations. These figures show the caution type plays an important role in the scale of phase diagram and the extendedness of the two phase regions. Figure 4 makes the comparison between three salts. The salting-out power of salts in these systems were determined and this power can be arranged as (NH4) 2HPO
4> KH2PO4 > NH4 H2PO4, respectively. The obtained interaction parameters have been
22 12
Table 1. The parameters of osmotic virial model to correlate and predict the phase diagrams of above systems.
Aqueous solution β11 β12 β 22 Deviation= Fob/6N where N is the number of tie line.
Brij 58- NH4H2PO4 -0.2793 9.5114 -2.8281 0.0233 Brij 58-(NH4)2HPO4 -0.3201 1.5216 -3.2894 0.0059 Brij 58- KH2PO4 -2.5554 34.9499 -5.2169 0.0035
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0
0.112 0.117 0.122
(NH4)2HP O4 ( w /w %)
0.025 0.02 0.015 0.01 0.005 0
0.085 0.135 0.185
NH4H2 P O4 (w/w%)
Figure 1. Phase diagram of Brij 58-NH4
H2PO4 aqueous system at 298.15K
Figure 2- Phase diagram of Brij 58-(NH4) 2HPO4
aqueous system at 298.15K
0.06 0.01
0.05
0.008
0.04 0.03 0.02 0.01 0
0.05 0.1 0.15 0.2
KH2 P O4 (w/w%)
0.006 0.004 0.002
0
0.11 0.16 0.21
salt (w/w%)
Figure 3. Phase diagram of Brij 58- KH2PO4 aqueous at 298k
Figure 4. Phase diagram of Brij 58-salt aqueous systems at 298K; □,(NH4) 2HPO4; o, NH4 H2PO4;O,KH2PO4 .
Brij 58(w/w%) Brij 58(w/w%) Brij 58 (w/w%)Brij 58(w/w%)
Reference
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Chem. 93(1989)5612.