water systems with respect to polymer-surfactant interactions and salting-effect of surfactant on polymer in aqueous solutions

Fluid Phase Equilibria 425 (2016) 411e420

Contents lists available at ScienceDirect

Fluid Phase Equilibria 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 / fl u i d

Thermodynamic properties of anionic surfactant/polymer/water systems with respect to polymer-surfactant interactions and saltingeffect of surfactant on polymer in aqueous solutions Rahmat Sadeghi*, Saivan Solaimani, Nosaibah Ebrahimi Department of Chemistry, University of Kurdistan, Sanandaj, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 May 2016 Received in revised form 11 June 2016 Accepted 13 June 2016 Available online 16 June 2016

An extensive set of experimental measurements was carried out on several ternary polymer-surfactant aqueous solutions in order to investigation of the binding behaviors between anionic surfactants and water soluble polymers in aqueous solutions. Isopiestic measurements of different ternary polymer (polyethylene glycol 400 (PEG400), polyethylene glycol 2000 (PEG2000), polyethylene glycol 6000 (PEG6000) and polypropylene glycol 400 (PPG400)) þ anionic surfactant (sodium n-hexyl sulfonate) þ water systems indicate that, the constant water activity lines of the investigated systems show three regions with convex, concave and convex slopes respectively in low-, intermediate- and high-solutes concentration region. In the second part of this work, volumetric and compressibility properties of micellization of sodium n-hexyl sulfonate in aqueous polymer solutions were studied. Finally, the cloud point measurements of aqueous PPG725 solutions in the absence and presence of various anionic surfactants show that, anionic surfactants with short hydrocarbon chain length decrease the cloud point temperature of PPG725 aqueous solutions (salting-out effect), while anionic surfactants with long hydrocarbon chain length increase that (salting-in effect). Based on the cloud point values, the energetic parameters of the clouding process were estimated and it was found that, entropy is the driving force for biphasic formation. The obtained results were interpreted in term of different interactions existing in aqueous solutions which determine the salting-effects produced by the addition of hydrophilic solutes to aqueous solutions of water soluble polymers. © 2016 Elsevier B.V. All rights reserved.

Keywords: Salt effect Isopiestic Constant water activity line Volumetric Compressibility Phase diagram Cloud point

1. Introduction In industrial applications, polymer-surfactant systems are commonly encountered, such as in foods, cosmetics, mineral processing, paints, coating, polymer synthesis, adhesives, and pharmaceutical products. Surfactants may bind cooperatively to nonionic water-soluble polymers to form micelle-polymer complexes, and these interactions are largely confined to anionic surfactants. Goddard [1,2] gave an excellent review of the interaction between nonionic polymers and charged surfactants. The interactions between surfactants and polymers in aqueous solutions have been studied by a variety of techniques, including surface tension [3,4], ITC [5e8], conductivity [9e12], laser light scattering [4,11,13], viscosity [9,11,14,15], neutron scattering [16,17], electron

* Corresponding author. E-mail addresses: [email protected], [email protected] (R. Sadeghi). http://dx.doi.org/10.1016/j.fluid.2016.06.025 0378-3812/© 2016 Elsevier B.V. All rights reserved.

spin resonance [15], zeta potential [11], dialysis equilibrium [18,19], NMR [5,20e22], fluorescence [4,11], UV [15], size exclusion chromatography [23]. However, the structure and morphology of the polymer-surfactant complexes as well as the nature of the interaction involved in the complex have not been well established yet and although this phenomenon is well documented and has been extensively investigated in the literature, their mechanism at the molecular level is still unclear. In order to study the effect of anionic surfactants on the phase behavior of aqueous polymer solutions and in an attempt to obtain further information about the mechanism of salting-effect of surfactants on aqueous polymer solutions, experimental measurements of vapor-liquid equilibria, liquid-liquid equilibria (cloud point values), volumetric and compressibility properties were performed for ternary aqueous solutions of anionic surfactants in the presence of a large series of water soluble polymers (PEG400, PEG2000, PEG6000, PPG400 and PPG725).

412

R. Sadeghi et al. / Fluid Phase Equilibria 425 (2016) 411e420

2. Experimental 2.1. Materials The specifications of used chemicals are summarized in Table 1. Double-distilled and deionized water was used. 2.2. Experimental procedures The details of isopiestic method used in this work are similarly to the one used previously [24]. In this method, different solutions with only one common solvent, when connected through the vapor space, approach equilibrium by transferring solvent mass by distillation. At equilibrium, the chemical potential of solvent (and also solvent activity) in each of the solutions in the isopiestic system is identical. From the solvent activity of one or more standard solutions, the activity of solvent for each solution within the isopiestic system can be known. The apparatus used consisted of a multi-leg manifold attached to round-bottom flasks. Two flasks contained the standard pure NaCl solution, one flask contained the pure polymer solution, one flask contained the pure surfactant solution, two or three flasks contained the polymer þ surfactant solutions and the central flask was used as a water reservoir. The apparatus was held in a constant-temperature bath at least 5 days (depending on the solutes concentration) for equilibrium. The temperature was controlled at 298.15 K to within ±0.05 K. After equilibrium had been reached, the manifold assembly was removed from the bath, and each flask was weighed with an analytical balance with a precision of ±1$104 g. From the weight of each flask after equilibrium and the initial weight of solutes, the mass fraction of each solution was calculated. The osmotic coefficients for the standard NaCl aqueous solutions have been calculated from correlation given in the literature [25]. Density and sound velocity measurements were carried out by an Anton Paar DSA 5000 model high precision vibrating tube digital densimeter and sound velocity measuring device, with automatic viscosity corrections and proportional temperature control that kept the samples at working temperature within ±103 K. The calibration of the instrument was made with degassed and bidistilled water and dry air at atmospheric pressure according to the instruction manual of the instrument. The uncertainties of measurements were ±5  106 g cm3 for density and ±5  101 m s1 for sound velocity. All the solutions for density and sound velocity measurements were prepared by mass on a Sartorius CP225D balance precisely within ±1  105 g. The experimental apparatus employed for determination of liquid-liquid equilibria phase diagrams (cloud point curves) is a glass vessel with an external jacket. A Julabo thermostat with a precision of ±0.05 K was used to circulate water at a certain

temperature in the external jacket around the vessel. The cloud point temperatures for solutions of PPG725 in water and in aqueous solutions with the same molality of anionic surfactants were determined by visual observation. Temperature of the stirred sample in stoppered glass vessel was slowly increased by increasing temperature of circulated water around the vessel until the sample clouded or got turbid. The sample temperature was then slowly decreased until the turbidity was vanished. The heatingecooling cycle was repeated three times for a given sample, to check the reproducibility of the measurements. The mean value of the temperature for appearance and the temperature for disappearance of clouding was considered as the cloud point temperature (TC). The samples for cloud point measurements were prepared on a Sartorius CP124S balance precisely within ±1  104 g. 3. Results and discussion In this work, three part of experimental measurements have been carried out on several ternary polymer-surfactant aqueous solutions: (i) isopiestic equilibrium measurements for the ternary {PEG400, PEG2000, PEG6000, PPG400 þ C6SO3Na þ water} systems at T ¼ 298.15 K, (ii) volumetric and compressibility properties measurements for solutions of C6SO3Na in aqueous solutions of 0.03 w/w PEG6000 at T ¼ 298.15, 303.15, 308.15, 313.15 and 318.15 K and (iii) the cloud point measurements for PPG725 in water and in aqueous solutions of anionic surfactants including C8SO4Na (0.011 mol kg1), C12SO4Na (0.002, 0.005, 0.008 and 0.011 mol kg1), C6SO3Na (0.011 mol kg1) and C12SO3Na (0.011 mol kg1) as a function of PPG725 concentration. The measured vapor-liquid equilibria, cloud point, density and sound velocity data are presented in the Supplementary Material of this manuscript. 3.1. Vapor-liquid equilibria properties For a certain polymer-surfactant aqueous solution which is in isopiestic equilibrium with a sodium chloride solution with molality mNaCl and osmotic coefficient FNaCl, the water activity, aw, and vapor pressure, p, were obtained according to:

aw ¼ exp½  2Mw mNaCl FNaCl ;
ln aw ¼ ln

p p+w

 þ

(1)

 +   + p  p+ Bw  Vw w ; RT

(2)

where Mw is the molar mass of water, B+w is the second virial co+ is the molar volume of liquid water, and efficient of water vapor, Vw p+w is the vapor pressure of pure water. The Zdanovskii-Stokes-Robinson (ZSR) rule, proposed by

Table 1 Specification of chemicals used in this work. Chemical name (abbreviation)

103$Molecular weight (kg mol1) Source

Lot number

Purification method

Final mass fraction puritya

sodium n-hexyl sulfonate (C6SO3Na) sodium n-dodecyl sulfonate (C12SO3Na) sodium n-octyl sulfate (C8SO4Na) sodium n-dodecyl sulfate (C12SO4Na) polyethylene glycol 400 (PEG400) polyethylene glycol 2000 (PEG2000) polyethylene glycol 6000 (PEG6000) polypropylene glycol 400 (PPG400) polypropylene glycol 725 (PPG725) NaCl

188.22 272.38 232.28 288.38 400 2000 6000 400 725 58.44

K35876705613 K37015808722 L55093568635 L55084433728 S6041783 019 S5415037004 S35317203 125230420206282 06228MD-197

None None none None None None None None None was dried in an electrical oven at about 110  C for 24 h prior to use

0.99 0.99

a

Declared by supplier.

Merck Merck Merck Merck Merck Merck Merck Fluka Aldrich Merck

0.99

0.995

R. Sadeghi et al. / Fluid Phase Equilibria 425 (2016) 411e420

413

Zdanovskii [26], which is a well-known empirical linear isopiestic relation (LIR) between the molalities of different ternary and binary aqueous electrolyte solutions under isopiestic equilibrium, has the following form:

X mi i

m0i

¼1

aw ¼ constant and 0 

! mi  1 ; m0i

(3)

where mi is the molality of solute i in the ternary solution and moi is the molality of solute i in the binary solution of equal aw. Stokes and Robinson [27] theoretically derived this equation for isopiestic mixed non-electrolyte aqueous solutions from the semi-ideal hydration model. According to ZSR rule, different aqueous solutions under isopiestic equilibrium exhibit no net effective interaction when mixed, that is, changes of hydration between the dissolved components on mixing are apparently absent. Since interactions between the solutes and the water occur and can be important in the binary solutions, but interactions between the solutes in mixed solutions are not evident, the behavior is termed “semi-ideal”. In fact, the semi-ideality means that the solute-solute interactions can be either mutually self-cancelled or neglected and the solute-water interactions can be simply described by a hydration number. In our previous works [24,28], we present the following form of the LIR for the ternary aqueous polymer(p)-solute(s) solutions.

ms wp þ ¼1 m0s w0p

! wp ms aw ¼ constant; 0  0  1 and 0  0  1 ; ms wp (4)

where w is mass fraction. In Fig. 1 the experimental constant water activity lines of PPG400 þ C6SO3Na þ H2O system along with those obtained from Eq (4) have been shown. The similar behavior was obtained for the other polymers investigated in this work. As can be seen, the constant water activity lines for polymer þ C6SO3Na aqueous systems show three regions with positive, negative and positive deviations from Eq (4) respectively for low-, intermediate- and highsolutes concentration region. In fact, for the aqueous ternary solutions obeying the LIR (Eqs 3 or 4), the constituent binary solutions mix ideally under isopiestic equilibrium and we can conclude that the solute-solvent interactions in the ternary solution are same as those in the binary solutions. The positive deviation from the LIR shows that the activity of water in a ternary solution in isopiestic equilibrium with certain binary solute 1 þ water and solute 2 þ water solutions are larger than those we expect in the case of semi-ideal solution (dotted lines). This behavior shows that the solute 1-water interaction becomes less favorable in the presence of solute 2 and therefore more free water molecules would be available in respect to the semi-ideal behavior in which the solutesolvent interactions in the ternary solution are same as those in the binary solutions. Therefore, the concentrations of solutes in a ternary solution which is in isopiestic equilibrium with certain binary solutions are larger than those we expect in the case of semiideal solution. On the other hand, in the case of negative deviation from the LIR, the interaction of solute 1 with water becomes more stable in the presence of solute 2 and therefore in these ternary systems less free water molecules would be available in respect to the semi-ideal behavior and then the activity of water in the ternary solutions are smaller than those we expect in the case of semi-ideal solution. Anionic surfactants exhibit strong cooperative binding interaction with uncharged water-soluble polymers in order to minimize the contact area of the hydrophobic segment and water [8,29]. At low concentrations in which C6SO3Na is in its monomeric form,

Fig. 1. Plot of mass fraction of PPG400, wp, against molality of C6SO3Na, ms, for constant water activity curves of PPG400 (p) þ C6SO3Na (s) þ H2O (w) system at T ¼ 298.15 K: B, aw ¼ 0.9822; △, aw ¼ 0.9794; , aw ¼ 0.9754; , aw ¼ 0.9679; C, aw ¼ 0.963; :, aw ¼ 0.9585; -, aw ¼ 0.9286; …, calculated by Eq (4).

▫

⋄

because of the binding interaction between hydrophobic portions of surfactant and polymer, the hydration of solutes decreases and, therefore, the water activity of these ternary systems are larger than those we expect in the case of semi-ideal solution with same concentration and therefore these systems show the positive deviations from the LIR (salting-in effects). By increasing surfactant concentration and micelle formation (surfactant-surfactant interaction), the aggregation of surfactant micelle on the polymer chains occurs and the positive deviation from the LIR is observed until the polymer chains are saturated by surfactant molecules. By more increasing the surfactant concentration, the free surfactant micelles begin to form and no additional interactions between surfactants and polymer chains occur which results in the increasing the polymer-water interaction and therefore the negative deviation from the LIR was observed in the intermediate solutes concentration region (salting-out effects). In this case, the polymer and surfactant micelles exclude themselves from the vicinity of each other. By increasing the concentrations of polymer and surfactant, the polymer chains are cross-linked by surfactant micelles, leading to a strengthened association between polymer chains, and thus to an increased viscosity. In this case, we will have an associative phase separation, with one glassy phase concentrated in both solutes and one dilute solution. This involves partial dehydration of solutes and therefore the interaction of polymer with water (which is stronger in the presence of surfactant micelles) becomes weaker by phase separation and therefore, in the two-phase region (high solutes concentration region) the positive deviation from Eq (4) is observed. In Fig. 2, the SEM image of the glassy phase for the aqueous PPG400-C6SO3Na system has been shown. In the case of ternary aqueous polymer-salt solutions [24,28] capable of inducing phase separation (salting-out effect), because of the unfavorable polymer-salt interactions, the interaction of each solute with water becomes more favorable in the presence of the other solute and therefore these systems show the negative deviation from the LIR in the whole range of one-phase area. Similar to the polymersurfactant system, these systems show the positive deviation from the LIR in the two-phase area. However in the case of polymer-salt aqueous systems that don’t form aqueous two-phase

414

R. Sadeghi et al. / Fluid Phase Equilibria 425 (2016) 411e420

systems (salting-in effect), because of the favorable polyether oxygen-cation interactions, the positive deviations from the LIR have been shown in the whole concentration region [24,28]. In fact in the case of aqueous polymer-anionic surfactant mixtures we can distinguish between three concentration regions. In the first region, there is a high affinity binding of individual or micelles of ionic surfactant molecules to the polymer chains (positive deviations from the LIR). In the second region, the polymer chains become saturated by surfactant molecules and the more binding of surfactant to the polymer becomes unfavorable (negative deviations from the LIR). Finally, at high solutes concentrations, there is a cross-linking of the polymer chains by the surfactant micelles (positive deviations from the LIR). The results show that the first concentration region (surfactant concentration) expanded by increasing the polymer molecular weight and that for PPG400 is larger than that for PEG400. In Fig. 3, the effect of polymer molar mass and type of polymer on the deviation from the LIR in both low- and intermediate-solutes concentration regions are shown. As can be seen (Fig. 3a and b), for a same water activity the positive deviation from Eq (4) in the low solute concentration region increases by increasing the polymer molecular weight and this deviation for PPG400 is larger than those for PEG400. In fact the positive deviation from the LIR increases by increasing the hydrophobicity of polymer. The interactions between C6SO3Na and polymers have hydrophobic nature and therefore enhanced hydrophobicity of the polymers (PPG or high MW PEG) favors the binding process and therefore the positive deviations from the LIR increase. On the other hand, Fig. 3c and d shows that in the intermediate solute concentration region, the negative deviation from the LIR for a same water activity increases by decreasing the polymer molecular weight and this deviation for PEG400 is larger than those for PPG400. In fact in this area, the negative deviation from the LIR increases by increasing the hydrophilicity of polymer. In Fig. 4, comparison of the experimental water activity data for the binary aqueous solutions of C6SO3Na, PEG400, PPG400, PEG2000, PEG6000 and PEG10000 has been made at T ¼ 298.15 K.

As can be seen, in the high solute concentration, water activities of aqueous C6SO3Na and PEG400 solutions have similar values and their values are smaller than those of the aqueous solutions of the other investigated polymers. The values of water activity for PEG400 aqueous solutions are smaller than those for PPG400 aqueous solutions and increase by increasing PEG MW. Fig. 4 also shows that the plot for C6SO3Na þ water system exhibits a change in slope at the concentration in which micelles are formed (0.689 mol kg1). For the concentrations higher than CMC, water activities are larger than those we expect in the absence of micellization. In fact, the confinement of a fraction of the counterions to the micellar surface results in an effective loss of ionic charges and therefore the hydration number of monomeric state of surfactant is larger than the hydration number of micellar form of surfactant and therefore micellization lowers the NaCl concentrations required to achieve a certain water activity. Therefore for a certain concentration, both of water activity and vapor pressure depression of micellar solutions of surfactant are larger than those we expect in the case of supposed monomer solution. Fig. 4 also shows that at low concentrations, the values of the water activities for aqueous solutions of monomeric form of C6SO3Na are smaller than those of all the investigated polymers. 3.2. Volumetric and compressibility properties The following equations were used to calculate the apparent molar volume, V4, and apparent molar isentropic compressibility, K4, of surfactant in aqueous polymer solution from the experimental density and sound velocity data:

Vf ¼

1000 Ms ; ðd  dÞ þ ms dd0 0 d

(5)

Kf ¼

1000ðbd0  b0 dÞ Ms b ; þ ms dd0 d

(6)

where b0 and b are isentropic compressibilities of solvent and

Fig. 2. SEM image of the glassy phase for the aqueous PPG400-C6SO3Na system.

R. Sadeghi et al. / Fluid Phase Equilibria 425 (2016) 411e420

w

415

ms Fig. 3. (a) Plot of wp0 against m 0 for constant water activity curves of PEG2000 (p) þ C6SO3Na (s) þ H2O (w) (solid lines) and PEG6000 (p) þ C6SO3Na (s) þ H2O (w) (dotted lines) p s w ms systems at T ¼ 298.15 K: B, aw ¼ 0.99; △, aw ¼ 0.9867; , aw ¼ 0.9829; , aw ¼ 0.9783; …, calculated by Eq (4). (b) Plot of w0p against m 0 for constant water activity curves of PEG400 p s (p) þ C6SO3Na (s) þ H2O (w) (solid lines) and PPG400 (p) þ C6SO3Na (s) þ H2O (w) (dotted lines) systems at T ¼ 298.15 K: B, aw ¼ 0.9822; △, aw ¼ 0.9794; , aw ¼ 0.9754; …, wp ms calculated by Eq (4). (c) Plot of w0 against m0 for constant water activity curves of PEG400 (p) þ C6SO3Na (s) þ H2O (w) (solid lines) and PEG6000 (p) þ C6SO3Na (s) þ H2O (w) (dotted p s w ms lines) systems at T ¼ 298.15 K: B, aw ¼ 0.9675; △, aw ¼ 0.9630; , aw ¼ 0.9551; …, calculated by Eq (4). (d) Plot of wp0 against m 0 for constant water activity curves of PEG400 p s (p) þ C6SO3Na (s) þ H2O (w) (solid lines) and PPG400 (p) þ C6SO3Na (s) þ H2O (w) (dotted lines) systems at T ¼ 298.15 K: B, aw ¼ 0.9679; △, aw ¼ 0.9630; , aw ¼ 0.9555; …, calculated by Eq (4).

⋄

▫

▫

▫

▫

solution, respectively. The isentropic compressibility is defined as:

b¼

1 : du2

(7)

In the above equations, Ms is the molecular mass of solute, ms is its molality, d0 and d are the densities of the solvent and the solution, respectively and u0 and u are the sound velocities of the solvent and the solution, respectively. In order to obtain more information about the polymer-C6SO3Na interactions and also the effect of water soluble polymers on the micellization of C6SO3Na in aqueous solutions, precise density and sound velocity measurements were performed on aqueous solutions of C6SO3Na in the presence of PEG6000 as a function of surfactant concentration above and below the critical aggregation concentration (CAC) at different temperatures. Figs. 5 and 6 illustrate the typical plots of sound velocity and isentropic compressibility vs molality for solutions of C6SO3Na in water [30] and in aqueous solutions of 0.03 w/w PEG6000 at 298.15, 308.15 and 318.15 K. As can be seen, the plots show linear behavior with two

different slopes at the concentrations below and above the CAC at which a pseudophase transition takes place between monomers and micelles. The concentration dependence of b for monomeric form of the surfactant is more negative than those for the micellar form and polymer reduces it. In fact, by addition the polymer as well as by increasing C6SO3Na concentration, a large portion of the water molecules are electrostricted and therefore the amount of bulk water decreases causing the compressibility to decrease. Fig. 6 also shows that, the isentropic compressibility isotherms intersect approximately at a certain molality (which decreased in the presence of PEG6000) and at this molality, the values of b have no temperature dependence. This is indicating that all the water in the solution is now in the hydrated sphere of solutes. The isentropic compressibility of aqueous PEG6000 þ C6SO3Na solutions can be taken as the sum of three contributions: b (water intrinsic), b (C6SO3Na intrinsic) and b (PEG6000 intrinsic). For the temperature range investigated in this work the values of b (water intrinsic) decrease by increasing temperature but the values of b (C6SO3Na intrinsic) and b (PEG6000 intrinsic) increase as temperature increases. For dilute solutions, the b (water intrinsic) is the dominant contribution to the total value of b and therefore the temperature

416

R. Sadeghi et al. / Fluid Phase Equilibria 425 (2016) 411e420

Fig. 4. Water activity, aw, of investigated binary aqueous solutions against weight fraction of solutes, wi, at T ¼ 298.15 K: B, C6SO3Na; :, PEG400; , PEG2000; C, PEG6000; , PPG400;  , PEG10000.

▫

⋄

Fig. 6. Plot of isentropic compressibility, b, against molality of surfactant, ms, for binary C6SO3Na þ H2O solutions [30] (solid lines) and ternary solutions of C6SO3Na in aqueous solutions of 0.03 w/w PEG6000 (dotted lines) at: B, 298.15 K; :, 308.15 K; , 318.15 K.

▫

molar properties increase sharply as a result of the micelle formation. The values of CAC obtained from the intersection of the apparently straight lines of the apparent molar properties against m (log scale) are given in Table 2. The changes in volume and compressibility upon micellization processes were determined according to Gianni et al. [31] procedure. In this procedure, the following relation was used to calculate the values of the apparent molar properties for the micellar forms of the surfactant [31]:

Y f;M ¼

Y f m  Y f;m ðCACÞ  CAC ; m  CAC

(8)

where Y ¼ V or K. Yf,m(CAC) is the apparent molar property of the monomeric form of the surfactant at CAC. Values of Yf,M identify apparent molar properties of the surfactant in the micellar form. The values of the apparent molar properties of micellization, DmicY, were obtained according to: Fig. 5. Plot of sound velocity, u, against molality of surfactant, ms, for binary C6SO3Na þ H2O solutions [30] (solid lines) and ternary solutions of C6SO3Na in aqueous solutions of 0.03 w/w PEG6000 (dotted lines) at: B, 298.15 K; :, 308.15 K; , 318.15 K.

▫

dependency of b is similar to b (water intrinsic). However for high surfactant concentrations, the b (C6SO3Na intrinsic) is the dominant contribution to the total value of b and similar to the b (C6SO3Na intrinsic), the values of b increase by increasing temperature. Since the temperature dependency of b (PEG6000 intrinsic) is similar to that of b (C6SO3Na intrinsic), therefore in the presence of PEG6000 the surfactant concentration range in which the b (water intrinsic) is the dominant contribution to the total value of b, decreases. In Figs. 7 and 8 the typical plots of apparent molar properties against molality (log scale) are shown for aqueous solutions of C6SO3Na in the absence [30] and presence of 0.03 w/w PEG6000 at different temperatures. As can be seen, the apparent molar properties slightly increase by increasing molality up to the CAC and at concentrations higher than the CAC, the values of the apparent

Dmic Y ¼ Yf;M ðCACÞ  Yf;m ðCACÞ;

(9)

where Yf,M(CAC) is the apparent molar property of the micellar form of the surfactant at CAC. The obtained values of DmicY are also given in Table 2. The values of DmicV and DmicK decrease with increasing temperature, as observed for sodium perfluoroheptanoate [32] and lithium perfluoroheptanoate [31], due to the larger increase with temperature of the property of the monomer in both cases. In Table 2 the infinite dilution apparent + ) and micellar (Y + ) forms molar properties of the monomeric (Yf;m f;M of the surfactant have also been given. The variations of the apparent molar volume of the monomeric and micellar form of C6SO3Na with temperature in the absence [30] and presence of 0.03 w/w PEG6000 are shown in Fig. 9. As can be seen, PEG6000 doesn’t + , but similar to have any significant effect on the values of Vf;m + DmicV, the values of Vf;M increased in the presence of the polymer. In the case of the apparent molar isentropic compressibility, + + although both of Kf;m and Kf;M increased in the presence of + + . PEG6000, the larger increase was obtained for Kf;M than Kf;m

R. Sadeghi et al. / Fluid Phase Equilibria 425 (2016) 411e420

Fig. 7. Apparent molar volume of C6SO3Na, V4, against surfactant molality (log scale), ms, for binary C6SO3Na þ H2O solutions [30] (solid lines) and ternary solutions of C6SO3Na in aqueous solutions of 0.03 w/w PEG6000 (dotted lines) at: B and C, 298.15 K; △ and :, 308.15 K; and -, 318.15 K.

▫

417

Fig. 9. Temperature dependence of the infinite dilution apparent molar volume of the + , and micellar, V + , forms of C SO Na for binary C SO Na þ H O monomeric, Vf;m 6 3 6 3 2 f;M solutions [30] and ternary solutions of C6SO3Na in aqueous solutions of 0.03 w/w + + PEG6000:  , Vf;m in pure water; B, Vf;m in aqueous 0.03 w/w PEG6000 solutions; :, + + Vf;M in pure water; △, Vf;M in aqueous 0.03 w/w PEG6000 solutions.

+ , and micelle, E + , have positive expansibilities of monomer, Ef;m f;M

values. Positive expansibility is a characteristic property of aqueous solutions of hydrophobic hydration. On heating, due to the increase of their motion, the hydrophilic tails increase their size. Although + < E + , however the variation of D Ef;m micV with temperature shows f;M that Ef,m(CAC) > Ef,M(CAC).

3.3. Liquid-liquid equilibria (cloud point properties)

Fig. 8. Apparent molar isentropic compressibility of C6SO3Na, K4, against surfactant molality (log scale), ms, for binary C6SO3Na þ H2O solutions [30] (solid lines) and ternary solutions of C6SO3Na in aqueous solutions of 0.03 w/w PEG6000 (dotted lines) at: B and C, 298.15 K; △ and :, 308.15 K; and -, 318.15 K.

▫

+ + (obtained from E+ ¼ The values of Ef;m and Ef;M f




vVf+ vT

 ) are also P

given in Table 2. Both of the infinite dilution apparent molar

In order to obtain further evidence about the salting-out effect produced by the addition of anionic surfactants to aqueous solutions of polymers and also to gain a conclusion about the main driving forces that control the phase behavior of these systems, here we have studied the effect of four anionic surfactants C8SO4Na (0.011 molal), C12SO4Na (0.002, 0.005, 0.008, 0.011 molal), C6SO3Na (0.011 molal) and C12SO3Na (0.011 molal) on the cloud point temperature values of PPG725. PPG is a thermosensitive polymer, so that, a binary PPG725 þ water system becomes turbid (two-phase system) when temperature is increased to a critical value (cloud point temperature). This phenomenon is attributed to the reduction of hydrophilicity of the polyether chains of PPG, caused by the rise in temperature. The cloud point of PPG725 þ water system can be enhanced (salting-in effect) or reduced (salting-out effect) by the addition of different solute to PPG725 aqueous solutions. Fig. 10 shows that for the same molality (0.011 mol kg1) of anionic surfactants, the magnitudes of the cloud point temperatures of

Table 2 Volume and compressibility data for the micellization of C6SO3Na in aqueous solutions of 0.03 w/w PEG6000 at different temperatures and 84.5 kPa. T/K

3 1 3 1 + /cm3 mol1 V + /cm3 mol1 D + /cm3 MPa1 mol1 K + /cm3 MPa1 mol1 D CAC/mol kg1 Vf;m CAC/mol kg1 Kf;m mol1 micV/cm mol micK/cm MPa f;M f;M

293.15 298.15 303.15 308.15 313.15 318.15

0.5941 0.5738 0.5601 0.5497 0.5406 0.5314

132.5945 133.6230 134.6023 135.5489 136.4432 137.2674

138.8379 140.2079 141.2502 142.1375 142.9619 143.7920

+ + Ef;m ¼ 0.1873 cm3 mol1 K1, Ef;M ¼ 0.1938 cm3 mol1 K1.

5.9640 6.0434 5.9359 5.7734 5.6088 5.4847

0.5171 0.5082 0.5010 0.4981 0.4973 0.4989

0.0616 0.0526 0.0442 0.0369 0.0303 0.0244

0.0150 0.0186 0.0217 0.0247 0.0276 0.0304

0.0620 0.0574 0.0537 0.0498 0.0467 0.0440

418

R. Sadeghi et al. / Fluid Phase Equilibria 425 (2016) 411e420

Fig. 10. Plot of cloud point temperature, TC, against PPG mass fraction, wp, for PPG725 in water and in aqueous solutions of anionic surfactants: C, water; , aqueous solutions of 0.011 molal C12SO4Na;  , aqueous solutions of 0.008 molal C12SO4Na; △, aqueous solutions of 0.005 molal C12SO4Na; , aqueous solutions of 0.002 molal C12SO4Na; B, aqueous solutions of 0.011 molal C12SO3Na; þ, aqueous solutions of 0.011 molal C6SO3Na; , aqueous solutions of 0.011 molal C8SO4Na.

⋄

▫

PPG725 aqueous solutions increase in the order: C12SO4Na > C12SO3Na > water > C6SO3Na > C8SO4Na. This result shows that anionic surfactants with long hydrocarbon chain length (C12SO4Na and C12SO3Na) have the salting-in effect on the PPG725 aqueous solutions and thus increase the values of TC, however those with short hydrocarbon chain length (C8SO4Na and C6SO3Na) have salting-out effect on PPG725 aqueous solutions and decrease the values of TC. As mentioned above, the polymer-surfactant interactions in aqueous solutions have hydrophobic nature and therefore enhanced hydrophobicity of the surfactants favors the binding process and therefore the cloud point temperatures of PPG725 aqueous solutions increase by increasing the hydrocarbon chain length of the surfactants. This behavior is in agreement with our isopiestic observations in which the positive deviation from the LIR increased by increasing the hydrophobicity of polymer. In fact the complex behavior of salting-effect produced by the addition of surface active ions to aqueous polymer solutions is a result of a delicate balance between hydrophobic and hydrophilic natures of those ions. The longer chain length surface active ions have stronger van der Waals interaction with polymer chains and charge the polymer chains more easily than shorter chain length ionic surfactants [33], therefore it is reasonable that anionic surfactants

possessing greater hydrocarbon proportion would be able to increase the water solubility of PPG and therefore elevate its cloud point temperature. Meanwhile, it should be noted that the values of the critical aggregation concentration of C8SO4Na, C12SO4Na, C6SO3Na and C12SO3Na in aqueous solutions at 298.15 K are 0.134 [34], 0.0083 [12], 0.571 [30] and 0.011 [12] mol.kg1, respectively. Therefore experimental concentration of anionic surfactants used in this work (aqueous solutions of 0.011 molal anionic surfactants) is much smaller than critical aggregation concentration of C8SO4Na and C6SO3Na. According to Fig. 10, the ability of C12SO4Na to increase the water solubility of PPG725 is higher than C12SO3Na, which is in agreement with our previous results [12] obtained from conductance, volumetric and compressibility measurements indicating stronger C12SO4Na-polymer interactions than C12SO3Napolymer interactions. Fig. 10 also shows that the salting-in effect of C12SO4Na on the PPG aqueous solutions, increase by increasing surfactant molality. Furthermore, it can be seen that at each molality of surfactant, the salting-in effect is dependent on the polymer concentration, so that, at lower concentration of polymer, a certain amount of C12SO4Na or C12SO3Na drastically increase the values of TC, however by increasing concentration of polymer, this effect becomes smaller or vanished. Even in the case of aqueous solution of 0.002 molal C12SO4Na, at higher concentration of polymer, the salting-out effect is observed. In fact for higher polymer concentration and in the presence of certain molality of an anionic surfactant, the totality of surface active ions which interact with polymer molecules and adsorb or aggregate on the polymer chains is not enough to induce salting-in phenomenon. With the consideration of clouding as the point of phase separation (or solubility limit), following Moulik et al. [35] the free energy of phase separation (DGm,C) can be calculated by using the relation:

DGm;C ¼ RTC ln Xp ;

(10)

where Xp is the mole fraction concentration of PPG at TC. The values of DGm,Cat different temperatures were processed according to the Gibbs-Helmholtz equation (Eq (11)) to get the enthalpy of phase separation (DHm,C) from the slope of linear plot between DGm,C/TC against 1/TC:


DG d TCm;C
 ¼ DHm;C : d T1C

(11)

Then, the following equation was used to calculate the entropy

Table 3 The free energy changes,DGm,C, entropy changes, DSm,C, and enthalpy changes,DHm,C, for clouding process of PPG725 in water and in aqueous solutions with certain molalities (mi) of anionic surfactants, as a function of cloud point temperature (TC) at 84.5 kPa. mi/mol kg1

DGm,C/kJ mol1 and (DSm,C/kJ mol1 K1) TC ¼ 303.15 K

PPG725 0 PPG725 0.002 0.005 0.008 0.011 PPG725 0.011 PPG725 0.011 PPG725 0.011

DHm,C/kJ mol1

TC ¼ 308.15 K

TC ¼ 313.15 K

TC ¼ 318.15 K

18.17 (0.3858)

20.05 (0.3856)

21.63 (0.3845)

17.71 17.45 17.08 16.93

19.60 19.22 18.78 18.50

21.38 20.61 20.22 19.79

in water 15.62 (0.3837) in aqueous solution of C12SO4Na 15.99 (0.3455) 15.44 (0.3217) 15.30 (0.3051) 15.22 (0.2816) in aqueous solution of C12SO3Na 15.63 (0.2616) in aqueous solution of C8SO4Na 17.28 (0.3508) in aqueous solution of C6SO3Na 17.03 (0.2942)

(0.3455) (0.3230) (0.3060) (0.2826)

(0.3460) (0.3235) (0.3065) (0.2831)

100.7

(0.3461) (0.3228) (0.3062) (0.2827)

88.7 82.1 77.2 70.2

17.32 (0.2629)

18.72 (0.2632)

19.83 (0.2625)

63.7

18.90 (0.3503)

20.79 (0.3508)

22.58 (0.3509)

89.1

18.75 (0.2950)

20.34 (0.2954)

21.69 (0.2950)

72.2

R. Sadeghi et al. / Fluid Phase Equilibria 425 (2016) 411e420

of phase separation (DSm,C):

DSm;C ¼

DHm;C  DGm;C TC

:

419

Appendix A. Supplementary data

(12)

The energetic parameters for clouding process in the PPG725anionic surfactant aqueous systems estimated by using Eqs 10e12 are presented in Table 3. As can be seen from this table, similar to polymer-salt aqueous two phase formation [35e37], entropy is the driving force for the formation of polymer-anionic surfactant ABS. Furthermore, the values of DGm,C reported in Table 3 indicate that the clouding process in the binary PPG725 þ water systems is more spontaneous than that process in the ternary PPG725 þ C12SO4Na þ water and PPG725 þ C12SO3Na þ water systems resulting from the salting-in phenomenon occurred in these systems. However, clouding of PPG725 aqueous solution in the presence of C8SO4Na and C6SO3Na is more spontaneous than that in pure water, due to salting-out effect produced in the presence of these surfactants with short hydrocarbon chain length. 4. Conclusions Isopiestic determination of vapor-liquid equilibria properties of several ternary uncharged water-soluble polymer þ anionic surfactant þ water systems at 298.15 K indicates that the slopes of the constant water activity lines show three concentration regions. In the first low concentration region, because of the binding of individual or micelles of ionic surfactant molecules to the polymer chains, the constant water activity lines have positive deviations from the LIR. The interactions between C6SO3Na and uncharged polymers have hydrophobic nature and therefore enhanced hydrophobicity of the polymers favors the binding process and increases the positive deviations from the LIR. In the second intermediate solutes concentration region, the polymer chains become saturated by surfactant molecules and the more binding of surfactant to the polymer becomes unfavorable which results in the increasing polymer-water interactions and the negative deviations from the LIR. Finally, at high solutes concentrations, because of the cross-linking of the polymer chains by the surfactant micelles, which leads to associative phase separation (one glassy phase concentrated in both solutes and one dilute solution), positive deviations from the LIR were observed. These results show that by addition of a small amount of anionic surfactant to aqueous solution of uncharged water-soluble polymers, salting-in phenomenon occurs. However, the addition of a large amount of surfactant has salting-out effect on aqueous polymer solutions. The volumetric and compressibility study of the ternary PEG6000 þ C6SO3Na þ water system indicates that, by addition of polymer to aqueous solution of surfactant, the change of infinite dilution apparent molar properties of micellar form is greater than that of monomeric form of surfactant. Besides the variations of the apparent molar properties of surfactant during micellization process (DmicV and DmicK) become smaller by increasing temperature, this is because the effect of temperature on the apparent molar properties for monomeric form is larger than that for micellar form of surfactant. The results of cloud point measurements show that, the favorable interactions between anionic surfactants and uncharged water-soluble polymers increase by increasing hydrocarbon chain length of surfactants, so that at the same surfactant molality, C8SO4Na and C6SO3Na have salting-out effect while C12SO4Na and C12SO3Na have salting-in effect on aqueous PPG725 solutions. The calculated values of Gibbs free energy, enthalpy and entropy of clouding process, indicate that entropy is the driving force for the formation of polymer-surfactant aqueous biphasic systems.

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.fluid.2016.06.025. References [1] E.D. Goddard, Polymeresurfactant interaction. Part I: uncharged watersoluble polymers and charged surfactants, Colloid. Surf. 19 (1986) 255e300. [2] E.D. Goddard, K.P. Ananthapadmanabhan, Interactions of Surfactants with Polymer and Proteins, CRC Press, Boca Raton, FL, 1993. [3] M.J. Schwuger, Mechanism of interaction between ionic surfactants and polyglycol ethers in water, J. Colloid Interface Sci. 43 (1973) 491e498. [4] P. Yan, J.X. Xiao, Polymer-surfactant interaction: differences between alkyl sulfate and alkyl sulfonate, Colloids Surf. A Physicochem. Eng. Asp. 244 (2004) 39e44. [5] L. Bernazzani, S. Borsacchi, D. Catalano, P. Gianni, V. Mollica, M. Vitelli, F. Asaro, L. Feruglio, On the interaction of sodium dodecyl sulfate with oligomers of poly(ethylene glycol) in aqueous solution, J. Phys. Chem. B 108 (2004) 8960e8969. [6] G. Olofsson, G. Wang, Interactions between surfactants and uncharged polymers in aqueous solution studied by microcalorimetry, Pure Appl. Chem. 66 (1994) 527e532. [7] G. Wang, G. Olofsson, Ethyl(hydroxyethyl)cellulose and ionic surfactants in dilute solution. Calorimetric and viscosity study of the interaction with SDS and some cationic durfactants, J. Phys. Chem. 99 (1995) 5588e5596. [8] S. Dai, K.C. Tam, Isothermal titration calorimetry studies of binding interactions between polyethylene glycol and ionic surfactants, J. Phys. Chem. B 105 (2001) 10759e10763. [9] J. Francois, J. Dayantis, J. Sabbadin, Hydrodynamical behaviour of the poly(ethylene oxide)-sodium dodecylsulphate complex, Eur. Polym. J. 21 (1985) 165e174. [10] E. Minatti, D. Zanetti, Salt effects on the interaction of poly(ethylene oxide) and sodium dodecyl sulfate measured by conductivity, Colloids Surf. A Physicochem. Eng. Asp. 113 (1996) 237e246. [11] A.P. Romani, M.H. Gehlen, R. Itri, Surfactant-polymer aggregates formed by sodium dodecyl sulfate, poly(N-vinyl-2-pyrrolidone), and poly(ethylene glycol), Langmuir 21 (2005) 127e133. [12] R. Sadeghi, S. Shahabi, A comparison study between sodium dodecyl sulfate and sodium dodecyl sulfonate with respect to the thermodynamic properties, micellization and interaction with poly(ethylene glycol) in aqueous solutions, J. Chem. Thermodyn. 43 (2011) 1361e1370. [13] W. Brown, J. Fundin, M.G. Miguel, Poly(ethyleneoxide)-sodiumdodecylsulfate interactions studied using static and dynamic light scattering, Macromolecules 25 (1992) 7192e7198. [14] J.C. Brackmann, Sodium dodecyl sulfate induced enhancement of the viscosity and viscoelasticity of aqueous solutions of poly(ethylene oxide). A rheological study polymer-micelle interaction, Langmuir 7 (1991) 469e472. [15] M. Cao, M. Hai, Investigation on the interaction between sodium dodecyl sulfate and polyethylene glycol by electron spin resonance, ultraviolet spectrum, and viscosity, J. Chem. Eng. Data 51 (2006) 1576e1581. [16] B. Cabane, R. Duplessix, Polymer-surfactant assemblies in water. A SANS study, J. Phys. 43 (1982) 1529e1542. [17] B. Cabane, R. Duplessix, Decoration of semidilute polymer solutions with surfactant micelles, J. Phys. 48 (1987) 651e662. [18] K. Shirahama, The binding equilibrium of sodium dodecyl sulfate to polyethylene oxide in 0.1M sodium chloride solution at 30 C, Colloid Polym. Sci. 252 (1974) 978e981. [19] K. Shirahama, N. Ide, The interaction between sodium alkylsulfates and poly(ethylene oxide) in 0.1 M NaCl solutions, J. Colloid Interface Sci. 54 (1976) 450e452. [20] M.I. Gjerde, W. Nerdal, H. Hoiland, A NOESY NMR study of the interaction between sodium dodecyl sulfate and poly(ethylene oxide), J. Colloid Interface Sci. 183 (1996) 285e288. [21] Z. Gao, R.E. Wasylishen, J.C.T. Kwak, Distribution equilibrium of poly(ethylene oxide) in sodium dodecylsulfate micellar solutions: an NMR paramagnetic relaxation study, J. Phys. Chem. 95 (1991) 462e467. [22] Z. Gao, R.E. Wasylishen, J.C.T. Kwak, NMR studies in surfactant and polymersurfactant systems: micelle formation of sodium u-phenyldecanoate and interaction with poly(ethylene oxide), J. Colloid Interface Sci. 137 (1990) 137e146. [23] A. Rodenhiser, J.C.T. Kwak, Application of size exclusion chromatography with surfactant eluent to the study of polymeresurfactant interactions: oligomeric and micellar chromatographic effects, Colloids Surf. A Physicochem. Eng. Asp. 150 (1999) 191e206. [24] R. Sadeghi, F. Jahani, Salting-in and salting-out of water soluble polymers in aqueous salt solutions, J. Phys. Chem. B 116 (2012) 5234e5241. [25] E.C.W. Clarke, D.N. Glew, Evaluation of the thermodynamic functions for aqueous sodium chloride from equilibrium and calorimetric measurement below 154 ◦C, J. Phys. Chem. Ref. Data 14 (1985) 489e610. [26] A.B. Zdanovskii, Regularities in the property variations of mixed solutions, Tr. Solyanoi Lab. Akad. Nauk. SSSR 6 (1936) 5e70. [27] R.H. Stokes, R.A. Robinson, Interactions in aqueous nonelectrolyte solutions. I.

420

R. Sadeghi et al. / Fluid Phase Equilibria 425 (2016) 411e420

Solute-Solvent equilibria, J. Phys. Chem. 70 (1966) 2126e2131. [28] T. Moradian, R. Sadeghi, Isopiestic investigations on the interactions of water soluble polymers with imidazolium-based ionic liquids in aqueous solutions, J. Phys. Chem. B 117 (2013) 7710e7717. [29] S. Dai, K.C. Tam, Isothermal titration calorimetric studies on the temperature dependence of binding interactions between poly(propylene glycol)s and sodium dodecyl sulfate, Langmuir 20 (2004) 2177e2183. [30] S. Solaimani, R. Sadeghi, Thermodynamic and aggregation properties of sodium n-hexylsulfonate in aqueous solution, Fluid Phase Equilib. 363 (2014) 106e116. [31] P. Gianni, L. Bernazzani, R. Carosi, V. Mollica, Micellization of lithium perfluoroheptanoate and its aggregation on poly(ethylene glycol) oligomers in water, Langmuir 23 (2007) 8752e8759. [32] E. Blanco, A. Gonz alez-Perez, J.M. Ruso, R. Pedrido, G. Prieto, F. Sarmiento, A comparative study of the physicochemical properties of perfluorinated and

hydrogenated amphiphiles, J. Colloid Interface Sci. 288 (2005) 247e260. [33] C. Ma, T. Gu, The effect of electrolytes on the cloud point of mixed solutions of polypropylene glycol and ionic surfactants, Colloids Surf. 36 (1989) 39e47. [34] N.V. Lebedeva, A. Shahine, B.L. Bales, Aggregation number-based degrees of counterion dissociation in sodium n-alkyl sulfate micelles, J. Phys. Chem. B 109 (2005) 19806e19816. [35] A. Dan, S. Ghosh, S.P. Moulik, The solution behavior pf poly(vinylpyrrolidone): its clouding in salt solution, solvation by water and isopropanol, and interaction with sodium dodecyl sulfate, J. Phys. Chem. B 112 (2008) 3617e3624. [36] L.H.M. da Silva, W. Loh, Calorimetric investigation of the formation of aqueous two-phase systems in ternary mixtures of water, poly(ethylene oxide) and electrolytes (or dextran), J. Phys. Chem. B 104 (2000) 10069e10073. [37] R. Sadeghi, B. Jamehbozorg, The salting-out effect and phase separation in aqueous solutions of sodium phosphate salts and poly(propylene glycol), Fluid Phase Equilib. 280 (2009) 68e75.