J. Phys. Chem. B 2008, 112, 3617-3624
3617
The Solution Behavior of Poly(vinylpyrrolidone): Its Clouding in Salt Solution, Solvation by Water and Isopropanol, and Interaction with Sodium Dodecyl Sulfate Abhijit Dan, Soumen Ghosh, and Satya P. Moulik* Centre for Surface Science, Department of Chemistry, JadaVpur UniVersity, Kolkata 700032, India ReceiVed: September 26, 2007; In Final Form: December 11, 2007
This article deals with the solution properties of poly(vinylpyrrolidone) (PVP) in salt and surfactant environment. The cloud point (CP) of PVP has been found to be induced by the salts NaCl, KCl, KBr, Na2SO4, MgSO4, and Na3PO4. On the basis of CP values for a salt at different [PVP], the energetics of the clouding process have been estimated. The effect of the surfactant, sodium dodecyl sulfate (SDS), on the salt-induced CP has also been studied, and reduction in CP at low [SDS] and increase in CP at high [SDS] have been observed. The water vapor adsorption of PVP has been determined by isopiestic method. The results display a BET Type III isotherm whose analysis has helped to obtain the monolayer capacity of PVP and formation of multilayer on it. The solvation of PVP in a solution of water and a water-isopropanol mixture has been determined by conductometry from which contribution of the individual components were estimated. The interaction of PVP with SDS in solution led to formation of a complex entity, which has been studied also by conductometry adopting a binding-equilibrium scheme. SDS has been found to undergo two types of binding as monomers in the pre- critical aggregation concentration (CAC) range and as small clusters in the post CAC region. The stoichiometries of binding and binding constant were evaluated.
Introduction Nonionic surfactants like p-tert-octylphenoxy polyoxyethylene (9.5) ether (Triton X-100),1-5 polyoxyethylene (10) cetylether (Brij 56),1,2,6 polyoxyethylene (20) sorbitanmonolaurate (Tween 20),5 and polyoxyethylene (20) sorbitanmonooleate (Tween 80),5 and so forth, and water soluble polymers like poly(vinylmethyl ether),1,2 ethyl hydroxyethyl cellulose,7 triblock copolymer as Pluronic 85,2 poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (L62),8 poly(vinylalcohol)-acetate,9 and so forth, and coacervates like tetrabutyl sodium decyl,10,11 dodecyl,12,13 tetradecyl14 sulfate, and so forth exhibit clouding upon heating their solutions. Additives can influence the clouding process, and both enhancement and delay by them have been reported.1-14 The clouding has importance in the uses of surfactants and polymers in chemical and pharmaceutical processes in solution in combination with different other components in salt environments under varied thermal conditions. Essentially, elevation of temperature reduces the hydration of clouding materials thereby making the desolvated molecules assemble or aggregate and emerge in an insoluble form to manifest clouding. This is a simplified explanation of a complex process whose detailing with other involved physicochemical steps remains to be understood. The water soluble polymer poly(vinylpyrrolidone) (PVP) is a well-studied representative in aqueous solution particularly in the presence of the surfactant, sodium dodecyl sulfate (SDS), which undergoes molecular interaction with the polymer15-21 similar to other water soluble polymers such as polyethylene oxide,22-25 polyethylene glycol,26,27 block copolymer as polystyrene-b-poly(2-vinyl pyrrolidone)-b-polyethylene oxide,28 and so forth. The polymer, PVP, is biocompatible; it is a prospective * To whom correspondence should be addressed. E-mail: spmcss@ yahoo.com. Fax: +91-33-2414-6266.
material for use as serum for artificial blood preparation and has similarities with proteins because of the presence of the amide functions in the molecule. In aqueous solution, it remains well hydrated and does not manifest clouding on heating. It can form a strong purple-colored complex with I3- and can be used for deiodification purpose. The nitrogens of the pyrrolidone rings become positive centers in the chain where water molecules and other dipolar or ionic molecules, such as the dissociated anion of SDS, can favorably interact.20,29,30 The desolvation of PVP by heating at temperatures below 100 °C at atmospheric pressure is not sufficient enough to produce insolubility (or clouding) of the material in water. In continuation of our investigation on the solution behavior of PVP in water in the presence of SDS as well as in the presence of alkanol (especially isopropanol) and salts,31 we have observed its clouding at high concentrations of several salts. The salts compete with PVP for hydration, and the temperature reduces it further to make PVP behave as a clouding compound. In the present study, this property of PVP has been explored in some detail along with its solvation behavior in water and in isopropanol to get extended knowledge on the solution behavior of PVP. The interaction of PVP with SDS in water-isopropanol medium was studied by us recently31 to have information on the cosolvent effect on the process. A comparative study of the solvation of PVP by water and isopropanol in combination has been herein undertaken. In addition to this, complexation or binding of SDS with PVP has also been investigated for additional information on the solution behavior of the polymer. The results are discussed in the following sections. Experimental Section Materials. Poly(vinylpyrrolidone) (PVP, molar mass 40 k) and sodium dodecyl sulfate (SDS, 99% pure) were obtained from Sigma (U.S.A.). NaCl, KCl, KBr, LiCl, CaCl2, ZnCl2, Na2-
10.1021/jp077733r CCC: $40.75 © 2008 American Chemical Society Published on Web 02/29/2008
3618 J. Phys. Chem. B, Vol. 112, No. 12, 2008 SO4, Na3PO4, and sodium oxalate were GR grade products of Merck (India). BaCl2, MgCl2, and urea were also GR grade products of SRL (India). MgSO4, Al2(SO4)3, and SnCl4·5H2O were AR grade products of Loba Chemie (India). AR grade (NH4)4Ce(SO4)3·2H2O was obtained from Merck (Germany). The sulfuric acid (AR grade) was obtained from Merck (India). The materials were used as received. Analytical grade isopropanol (SRL, India) and doubly distilled water of specific conductance, 2-4 µS cm-1 at 303 K, were used for preparing solutions for the study. All measurements were taken at 303 ( 0.2 K unless otherwise stated. The concentration of PVP has been expressed in g % (w/v) and that of IP in vol % (v/v) throughout the text. Cloud Point (CP) Determination. The experimental procedure for CP measurements previously studied was followed.5,6,32 In the actual experiment, the measuring solution was taken in a stoppered test tube and was securely placed in a heating mantle with constant stirring and a controlled increment of heat. The point of clouding or turbidity was visually observed and was noted at the start of the phenomenon. The heating was then stopped, and the system under condition of stirring was allowed to slowly cool. The temperature for the disappearance of turbidity was also noted. The mean value of the two temperatures was considered as the CP of the system. The measured CP was accurate within (1%. Isopiestic Method. Isopiestic method was used for the study of hydration of PVP. The powdered sample of PVP was dried by keeping it in a vacuum desiccator over concentrated H2SO4. The moisture content of PVP was determined by drying a given weight of the sample until constant weight was attained. The dry weight of the sample was used in all calculations. In this study, a definite weight of the dried sample was taken in an accurately weighed sample bottle of special design. The lid of the bottle was then removed and the lidless bottle was floated on concentrated H2SO4 taken in a specially designed desiccator. The desiccator was then evacuated, and the H2SO4 solution inside was frequently stirred by a magnetic stirrer for a week when the vapor pressure equilibrium between the hydrated sample present in the sample bottle and the H2SO4 solution present in the desiccator (to be referred as reference solution) was established. The sample bottle was then taken out of the desiccator quickly, closed with the lid, and weighed. From the difference in weight, the mass (mw) of the water vapor adsorbed per gram of PVP was calculated. The concentration of H2SO4 in the reference solution was determined titrimetrically, and the corresponding value of the relative humidity (P/P0) at this acid concentration was obtained from the standard table. The water activity a1 at this state was equal to P/P0 according to the Raoult’s law. The standard deviation (SD) of measurement of mw was found not to exceed 3%. The measurement details can be found in earlier reports.33-35 Conductometry. The conductivity measurements were performed with a Jenway (U.K.) conductometer in a conductivity cell of unit cell constant. The measurements were taken after thorough mixing and temperature equilibration of the solution. The accuracy of the measurements was within (0.5%. Conductance experiments were performed to determine the solvation of PVP in water and IP-water media. The PVP solution in 5 mM NaCl was thermostated for 30 min to which a thermostated 5 mM NaCl solution was progressively added. Conductance was measured at each dilution after mixing the solution thoroughly and allowing a few minutes time to attain thermal equilibrium. Conductance of 5 mM NaCl was also
Dan et al.
Figure 1. CP vs µ profiles for PVP (2 g %) in presence of salts. b, KBr; 0, NaCl (15.5); 9, KCl (23.1); 4, MgSO4 (43.3); 2, Na3PO4 (77.3); O, Na2SO4 (85.3). Values in parentheses are slopes of the related plots.
measured. PVP imparted negligible conductance to the solution. The detailed measurement procedure can be found in earlier reports.36,37 To determine the binding of SDS with PVP, the conductance of SDS solution was measured both in the absence and presence of PVP. Viscometry. The viscosity measurements were performed in a calibrated Ubbelohde three-limbed viscometer with a clearance time of 319.8 s for 35 mL of water. The temperature was controlled in a water bath accurate within (0.1 °C. The accuracy of viscosity measurements was within (0.5%. Each measurement was duplicated, and the mean value was recorded and used. Results and Discussion Salt-Effect and Clouding. A large number of electrolytes of different valences were tried at different concentrations to check the stability or phase separation (i.e., clouding) of PVP by the effect of temperature. Of the tried salts (NaCl, KCl, KBr, LiCl, CaCl2, ZnCl2, BaCl2, MgCl2, Na2SO4, MgSO4, Al2(SO4)3, Na3PO4, (NH4)4Ce(SO4)4·2H2O, SnCl4·5H2O, sodium oxalate, and the nonelectrolyte, urea), NaCl, KCl, KBr, Na2SO4, MgSO4, Na3PO4, (NH4)4Ce(SO4)4·2H2O, and SnCl4·5H2O were found to initiate or induce clouding of the polymer. The results are presented in Figure 1 at 2 g % of PVP. The plots are in terms of ionic strengths (µ), and they followed the efficiency sequence of Na2SO4 (µ ) 0.9-1.8) > Na3PO4 (µ ) 1.05-1.8) > MgSO4 (µ ) 2.0-3.0) > KCl (µ ) 2.0-4.25) > NaCl (µ ) 2.5-5.0) > KBr (µ ) 6.0). The sequence revealed that the bi- and trivalent anions have shown better efficiency than the monovalents. The reduction of CP per unit ionic strength found from the slopes of the straight-line plots were 85.3, 77.3, 43.3, 23.1 and 15.5 for Na2SO4, Na3PO4, MgSO4, KCl and NaCl, respectively. The use of ceric-ammonium double salt (NH4)4Ce(SO4)4·2H2O at 0.3 M produced clouding of PVP (2 g %) that appeared at 77 °C and disappeared at 63 °C. The salt SnCl4· 5H2O at 0.125 M concentration also produced clouding of PVP (2 g %) solution that appeared at 61 °C and disappeared at 36 °C; this was confirmed by repeated experiments. Such uncommon nonreversible clouding behaviors in the presence of ceric and stannic salts warrant further investigation. The stability/instability of nonelectrolytes (the polymer in present context) are primarily guided by water structure. Hydrophilic polymers (polyvinylalcohol, polyvinylpyrrolidone, polyethyleneoxide, etc.) are insensitive to the addition of water-structure breaker species like SCN-, ClO4-, I-, Br-, Cs+, and Rb+.
The Solution Behavior of Poly(vinylpyrrolidone)
J. Phys. Chem. B, Vol. 112, No. 12, 2008 3619 TABLE 1: Energetic Parameters for Clouding of PVP in Presence of Saltsa g % PVP 0.5 1 1.5 2 2.5
Figure 2. Dependence of CP on [PVP] in presence of salts. 9, KCl (4 M); 4, MgSO4 (0.7 M); 2, Na2SO4 (0.5 M), 0, NaCl (5 M); O, Na3PO4 (0.25 M) (inset A). Inset B: SDS effect on salt-induced clouding of PVP (2 g %). b, Na2SO4 (0.5 N); 1, MgSO4 (0.7 M); 3, Na3PO4 (0.25 M).
Water-structure-forming agents like sucrose and ions like F-, OH-, H+, Li+, Na+, Ca++, Mg++, Al+++, and so forth are expected to cause insolubility of nonelectrolytes and polymers that may be termed as the salting out effect.38 Relatively, lowsoluble compounds like poly(vinylalcohol)-acetate and poly(vinylpyrrolidone)-acetate copolymers are markedly affected by salt addition (both salting in and salting out phenomena are observed). This aspect has been elaborately studied by Saito38 (see Introduction) using poly(vinylalcohol)-acetate copolymer. The presently studied water-soluble polymer, PVP, falls into the first category, and the additive effect on its stability is not expected to be drastic. Dehydration of PVP by temperature and water abstraction by salts acted concertedly toward the instability of the polymer to result clouding. Highly hydrated salts39-42 like Na2SO4, MgSO4, and Na3PO4 assisted efficient clouding of PVP at lower ionic strength than weakly hydrated salts42 like NaCl, KCl, and KBr. In agreement with a reviewer’s suggestion, a discussion on the effect of pH on the clouding of PVP in salt solution has been considered relevant as salt like Na3PO4 can form different species in solution depending on the solution pH that could affect the ionic strength of the solution against which the CP have been plotted in Figure 1. We have found that HCl up to a concentration of 1 M could not introduce clouding of PVP; similar was the effect of 1 M NaOH. Thus, the H+, Cl-, Na+, and OH- in their binary acid and base-forming combinations were not cloud-forming additives for PVP. As far as the pH of Na3PO4 was concerned, it has been found that the measured pH values of their solutions in the ranges of concentrations used in the study were 12.0-12.1 for Na3PO4 (the expected pH is 12.343). Their solutions thus contained the species Na3PO4 only, which we have considered in the ionic strength calculation. PVP Concentration Dependent CP. The [PVP] dependent CPs for NaCl (5 M), KCl (4 M), Na2SO4 (0.5 M), MgSO4 (0.7 M), and Na3PO4 (0.25 M) are illustrated in Figure 2.With the consideration of clouding as the point of phase separation (or the solubility limit), the free energy of phase separation or clouding (∆Gc) was calculated from the relation
∆Gc ) RT ln XPVP
(1)
where XPVP is the mole fraction concentration of PVP at CP. The values are presented in Table 1 with reference to clouding concentrations of PVP. The ∆Gc values at different CP (or T)
∆Gc/kJ mol-1 and (∆Sc/kJ mol-1 K-1) NaCl KCl Na2SO4 MgSO4 -35.9 (1.11) -33.8 (1.11) -32.6 (1.11) -31.8 (1.11) -31.4 (1.11)
-34.8 (0.998) -32.8 (0.997) -31.5 (0.999) -30.7 (0.998) -30.0 (0.997)
-35.9 (0.583) -33.6 (0.582) -32.3 (0.582) -31.3 (0.583) -30.6 (0.583)
-35.4 (0.533) -33.0 (0.533) -31.7 (0.533) -30.6 (0.534) -30.0 (0.532)
Na3PO4 -37.8 (0.518) -35.2 (0.532) -34.2 (0.532) -32.7 (0.533) -31.8) (0.531
a The ∆Hc evaluated in terms of eq 2 were 335, 288, 157, 140, and 144 kJ mol-1 in NaCl, KCl, Na2SO4, MgSO4, and Na3PO4 salt environment, respectively (see Figure 3).
Figure 3. ∆Gc/T vs CP-1 plots at fixed [salts]. Inset: ∆Hc - ∆Sc compensation plot for the clouding process. 9, NaCl (5 M); 0, KCl (4 M); O, Na2SO4 (0.5 M); 1, MgSO4 (0.7 M); 4, Na3PO4 (0.25 M).
for each electrolyte were processed according to the equation given below to get ∆Hc from the slope of the linear (leastsquares) plot between (∆Gc/T) against (1/T) shown in Figure 3 for the studied electrolytes.
d(∆Gc/T) d(1/T)
) ∆Hc
(2)
The ∆Hc values calculated are shown in the footnote of Table 1. The values were high as was observed for other clouding compounds earlier studied.1 Among the effective cloud-forming salts, the ∆Hc followed the order NaCl > KCl > Na2SO4 > Na3PO4 ≈ MgSO4. The ∆Hc values of alkali metal chlorides were 2-fold larger than the sulfate and phosphate. The GibbsHelmholtz equation was used to calculate the entropy changes. Thus
∆Sc )
∆Hc - ∆Gc T
(3)
The ∆Sc values are also presented in Table 1, which were all large and positive. The high positive entropy has suggested loose, disorderly, and desolvated PVP assemblies in solution; desolvation of the polymer made a major contribution to the positive entropy change. The large magnitudes of both ∆Hc and ∆Sc compared to ∆Gc have produced a nice correlation between ∆Hc and ∆Sc (Figure 3, inset). The compensation temperature of 326 K had excellent agreement with the average experimental temperature of 327 K. Such a good agreement between the two
3620 J. Phys. Chem. B, Vol. 112, No. 12, 2008
Dan et al.
kinds of temperature is rarely found in literature. The good compensation among the data obtained using different electrolytes has advocated the same type of physicochemical process or processes constituting the phenomenon of clouding assisted by the salts. Influence of SDS on the Salt-Induced CP of PVP. In view of the favorable interaction of SDS with PVP,8-13 its influence on the salt-induced CP of PVP was considered worthwhile to examine. The findings were interesting. Initially, CP declined with a small addition of SDS, produced a minimum, and then sharply increased (Figure 2, inset B). The effects of the surfactant on CP at 0.5 M Na2SO4 and 0.25 M Na3PO4 were nearly identical with a minimum at [SDS] ) 6.3 mM. In 0.7 M MgSO4 solution, the influence of SDS on CP was of different magnitude with a minimum at [SDS] ) 8.8 mM. According to report,20,29,30 at low [SDS] the surfactant monomers bind to positive nitrogen centers on the pyrrolidone units in PVP with the release of solvated water from the attached centers. At and above a critical concentration, called the critical aggregation concentration (CAC), small micelles of SDS form a complex with PVP and saturate it, and then the surfactant forms normal micelles in solution. The presence of salt reduces CAC inducing early binding of smaller micelles to PVP. The desolvated PVP molecules by the salting out effect and SDS binding undergo easier phase separation at lower temperature to reduce CP. The SDS-provided charge on the polymer chain also gets screened by the action of salt to make the polymer configuration compact leading them to associate and become out of phase. At higher [SDS], the compact complex either partially or fully solubilized in large free SDS micelles producing clarity to the solution and hence increasing CP. The mode of change essentially depends on the type of electrolyte and its concentration; its correlation needs a detailed investigation, which has not been done in the present study. The induction of CP by the addition of salt and its modification by the addition of relatively small amounts of SDS is an interesting physicochemically controlled process for the ternary polymer-saltsurfactant system with reference to application in chemical and pharmaceutical fields. Amphiphiles like hexadecyltrimethyl ammonium bromide (CTAB) and p-tert-octylphenoxy polyoxyethylene (9.5) ether (Triton X-100) did not produce an effect like SDS on the salt-induced clouding of PVP, as they are known to be noninteracting. Similar studies on CP with other anionic (interacting) surfactants and PVP in salt environments would be worthwhile. Hydration of PVP by Isopiestic Method. Adsorption of water vapor on a solid substance can be estimated from the isopiestic method, which for the present PVP-water vapor system produced an isotherm of the type illustrated in Figure 4. The maximum adsorption corresponded to 1.26 g/g (i.e., 1.26 g of water per g PVP). The isotherm looked like BET Type III, so that the adsorption process was multilayer forming type. The results were processed utilizing the linear BET equation shown below
()
P C-1 P 1 + ) Cm0 P0 m(P0 - P) m0C
(4)
where P and P0 are the experimental and saturated vapor pressures of water respectively, m0 is the monolayer capacity of the adsorbent (here PVP), m is the amount of water vapor adsorbed at equilibrium pressure P, and C is a constant related to the difference of heat of adsorption between the first layer and the heat of adsorption of the subsequent layers, which are
Figure 4. Isotherm for water vapor adsorption (g/g) on PVP at 303 K determined by the isopiestic method. Inset: BET plot of the results shown in the main diagram. Unfilled points deviated from linearity. The data fitted to P/P0 up to 0.9 (usually BET plots deviate from P/P0 g 0.5).
all considered equivalent and equal to the heat of condensation of water vapor. The BET plot is presented in the inset of Figure 4, which produced deviations at higher water activities. In practice, BET plots normally deviate from linearity at P/P0 > 0.5; here the deviation occurred at higher P/P0. From the intercept and slope, the m0 and C values obtained were 0.40 g/g and 14.0 kJ mol-1, respectively, at 30 °C. On the basis of total adsorption of 1.26 g/g (as stated above), formation of three layers of water on the adsorption centers was evidenced. Solvation of PVP in Water and in IP-Water Media. The above-reported clouding of PVP in the presence of salt was the manifestation of the lowering of solvation by water abstraction from the polymer by the salt. The knowledge of solvation of PVP was thus considered as an important part of this study. The method of conductance was used for the determination of solvation. It is based on the principle of obstruction to the ion transport by dispersions or solvated nonelectrolytes, which led to the proposition of the following relation for evaluation36,37,44,45
k′ k′ ) 1 - 1.93Vh c k k
()
(5)
where k and k′ are the specific conductances of an electrolyte (say NaCl) of known but low strength in the absence and presence of the nonelectrolyte (obstractant) of concentration c (expressed in g mL-1), respectively, and Vh is the hydrated specific volume of the nonelectrolyte (here PVP). The experimental conductance data plotted in terms of the linear eq 5 yielded Vh from the slope of the plot depicted in Figure 5 . Subtracting the specific volume of anhydrous PVP (0.80 ) reciprocal of density) from Vh produced mL/g solvation of PVP (Vs) in its solution. From this, the mole of solvent bound per gram (or mole) of PVP or per mole of pyrrolidone unit in the polymer was estimated. In IP-water medium, it was composite solvation of both water and IP attached to the solvated centers. In the presence of increasing percent of IP, the conductance variation became insensitive above 40% IP. The Vh values above 40% IP were, therefore, estimated by the method of interpolation (Figure 5, inset A). In the figure, the filled points were experimental and unfilled points were by interpolation. The estimated Vs values were then split into the contributions from water and IP adopting the following procedure.
The Solution Behavior of Poly(vinylpyrrolidone)
J. Phys. Chem. B, Vol. 112, No. 12, 2008 3621 TABLE 2: Hydrated Specific Volumea (V) and Solvation Numberb (n) Obtained from Equation 8 for PVP in Water-IP Media at 303 K
Figure 5. k′/k vs k′c/k plots for 5 mM NaCl in PVP solution at 303 K. b, Aqueous medium; O, 10 vol % IP-water medium. Inset A: Solvated specific volume (Vs) vs volume fraction of IP (XIP v ) profile. Filled squares experimental; unfilled squares by interpolation. Inset B: nw vs nIP profile. Also nw or nIP dependence on % IP. 9, nw; 0, nIP.
The composite solvation of PVP by water and IP was considered to obey the relation
Xws Vws
+
XIP s VIP s
)
1
(6)
Vw-IP s
where Xws and XIP s are the volume fractions of water and isopropanol in the total solvation Vs (of PVP), and Vws , VIP s , and are the solvations of PVP in aqueous, isopropanol, and Vw-IP s IP-water mixed media, respectively. The equation is comparable with the mixed micelle forming relation of Clint46 for binary surfactant mixtures
X2 X1 1 + ) CMC1 CMC2 CMCmix
(7)
where X1 and X2 are the mole fractions of surfactants one and two and CMC1 and CMC2 are their respective critical micellar concentrations; CMCmix is the CMC of the mixed micelle. The simplified form of eq 6 can be stated as
Xws Vws
+
1 - Xws VIP s
)
1
(8)
Vw-IP s
From Vs, the total solvation split into the water and IP components at each IP-water composition according to eq 8 from which mole/mole binding of water and IP to a pyrrolidone unit in PVP was calculated. The rationale for such calculation is presented in eq 9
nt ) nw + nIP )
(
)
Vws Fw VIP s FIP MPVP + Mw MIP npy
(9)
where nw and nIP are the number of moles of water and IP, respectively, attached to one mole of pyrrolidone unit (py); nt is the total number of moles of water and isopropanol associated with one mole of pyrrolidone unit; Fw and FIP are the respective densites of water and IP; Mw, MIP, and MPVP are the molecular weights of water, IP, and PVP, respectively, and npy is the number of moles of pyrrolidone units consisting of one mole of PVP. The results are shown in Table 2.
vol % IP
Vh (Vs)
Vws (VIP s )
nw (nIP)
0 5 7 10 15 20 25 30 40 50 60 70 80 90 95 100
2.17 (1.37) 2.42 (1.62) 2.48 (1.68) 2.78 (1.98) 2.87 (2.07) 2.98 (2.18) 3.10 (2.30) 3.20 (2.40) 3.32 (2.52) 3.31 (2.51) 3.35 (2.55) 3.37 (2.57) 3.39 (2.59) 3.41 (2.61) 3.41 (2.61) 3.44 (2.64)
1.37 (0) 1.10 (0.52) 1.04 (0.64) 0.71 (1.27) 0.62 (1.45) 0.50 (1.68) 0.37 (1.93) 0.26 (2.14) 0.13 (2.39) 0.14 (2.37) 0.10 (2.45) 0.08 (2.49) 0.05 (2.54) 0.03 (2.58) 0.03 (2.58) 0 (2.64)
8.5 (0) 6.8 (0.8) 6.4 (0.9) 4.4 (1.8) 3.8 (2.1) 3.1 (2.4) 2.3 (2.8) 1.6 (3.1) 0.8 (3.5) 0.9 (3.4) 0.6 (3.6) 0.5 (3.6) 0.3 (3.7) 0.2 (3.7) 0.2 (3.7) 0 (3.8)
Specific volume of PVP ) 0.8 mL/g. Vh ) solvated specific volume (mL/g) of PVP, Vs ) solvation specific volume (mL/g) (Vh - 0.8). Vws ) solvation specific volume (mL/g) of water. VIP s ) solvation specific volume (mL/g) of isopropanol. bnw ) number of water molecules per hydrated center (pyrrolidone center) of PVP. nIP ) number of isopropanol molecules per solvated center (pyrrolidone center) of PVP. a
It is seen from the table that the solvation by pure water was 2.21 times greater than pure IP; in between the component solvation changed with composition. The lower solvation by IP than water meant hydrophobic attachment over and above charge-dipole interaction, which reduced the chances for attachment of more molecules to the sites. The small molecules of water with only charge-dipole interaction clustered around the pyrrolidone center to satisfy primary and other levels of hydration requirements. The nw and nIP values followed an inverse linear relation (Figure 5, inset B) of the form
nw ) I - SnIP
(10)
The intercept, I, and slope, S, were 8.45 (nw at 0% IP) and 2.21, respectively. The correlation coefficient was 1.0; nw and nIP had a perfect linear correlation with a standard deviation, SD ) 0.00434. The % IP dependent nw and nIP were found to be of exponential in nature in reverse direction crossing at % IP ) 21.8, where nw ) nIP (Figure 5, inset B). With regard to solvation, one molecule of IP ≡ 2.21 molecules of water. The factor 2.21 can be considered as the affinity ratio between water and IP for the pyrrolidone center in PVP. By the use of eq 5, a value of Vws ) 1.37 mL/g (≡1.36 g/g) was obtained as the hydration of PVP at 30 °C. The maximum water vapor adsorption by the isopiestic method was found to be 1.26 g/g. The agreement between two methods was fair. The formation of three layers of water on PVP was supported by both the methods. Viscosity Effect on Conductance. The measured conductance of a salt at a constant concentration in a changing viscosity of the medium should consequently change. The conductance is expected to decrease with increasing viscosity, and a product (between viscosity and conductance) called the Walden Product (WP)47 is expected to show constancy. Thus
Ληr ) Constant
(11)
where Λ is the equivalent conductance of the electrolyte, and ηr is the relative viscosity of the solution. The value of the product depends on the electrolyte-medium combination. The
3622 J. Phys. Chem. B, Vol. 112, No. 12, 2008
Dan et al. The thermodynamic binding constant (KA) should be represented by the activities of the components, which with nonionic PVP in dilute solutions of SDS were taken to be equivalent to their concentrations to represent KA as
KA )
[complex]
(13)
n
[PVP]f[SDS]f
or, log KA ) log[complex] - log[PVP]f - n log[SDS]f (14)
Figure 6. log[(ka - kp)/s] vs log[kp/s] plot for estimating the interaction parameters between PVP and SDS at 303 K and validity of Walden rule for conductance of NaCl and SDS in aqueous PVP medium at 303 K. Main plot (binding): Filled squares represent [SDS] range below CAC; open squires represent the same beyond CAC. Inset: Walden Product: 2, 5 mM NaCl; 4, 5 mM SDS. Normalized Walden Product ) equivalent (see text): 9, 5 mM NaCl; 0, 5 mM SDS. ΛPVP s conductance of salt in aqueous PVP medium; ηPVP ) relative viscosr ity of aqueous PVP solution; Λws ) equivalent conductance of salt in aqueous medium.
WP should function best at the condition of infinite dilution, that is, with Λ ) Λ ∝ or in dilute solution of electrolytes. In the present NaCl-PVP solution medium at low [NaCl] ) 5 mM, constancy of WP was expected. The results with 5 mM of both NaCl and SDS are presented in Figure 6 (inset). The plots of ΛPVP ηPVP were not parallel to ηPVP or the x-axis but s r r produced positive slopes, which was greater for NaCl than SDS (which is known to be a PVP complexing surfactant). Thus, the measured conductance that was greater than expected ruled out the validity of Walden’s rule.47 The validity of the Walden rule can be seriously criticized in terms of irreversible thermodynamics. According to the Curie-Prigogine principle,48 forces of different elements of symmetry should not couple. The viscosity and conductance are thus incapable of coupling as in the rule of Walden. In the inset of Figure 6, the plots of the WP results normalized in terms of the conductance of the salts in PVP-free aqueous medium are shown where equal intercept with different slopes (lower for SDS than NaCl) were observed. SDS interacted more with PVP than NaCl. The following is an attempt to estimate the binding of SDS with PVP following a conductance measurement procedure reported earlier.49 Binding of SDS with PVP. SDS is known to efficiently bind with PVP.8-13 We have observed that reduction of ion conductance by the PVP was more for SDS than NaCl. In the case of ∼5 mM NaCl at [PVP] e 4 g %, the reduction in conductance was minor, and at [PVP] < 2 g %, it was indistinguishable. This was not so for SDS: at PVP e 1 g %, the conductance of 5 mM of SDS was perceptibly lower than without PVP. This has evidenced the binding of SDS with the polymer; the obstruction effect to reduce solution conductance was absent. The following equilibrium was considered valid:49,50 KA
n SDS 98 PVP-SDSn PVP + polymer n(surfactant) complex
(12)
where n is the number of SDS molecules in a unit complex, and KA is the binding constant.
where [PVP]f and [SDS]f are the free concentrations of PVP and SDS at equilibrium respectively. At relatively low concentration of SDS, [PVP]t ≡ [PVP]f, where [PVP]t is the total concentration of the polymer. Below the formation of free micelle in solution, the conductance-concentration linear dependence can be used for the estimation of the loss in [SDS] by complexation with PVP. Thus
[complex] )
[SDS]lost [∆SDS] ) n n
(15)
Combining eq 14 and 15 with consideration of [PVP]t ≡ [PVP]f then yields
log KA ) log
- log[PVP] - n log[SDS] ([∆SDS] n ) (16) f
On rearrangement, we get
log ([∆SDS]) ) log(nKA[PVP]) + n log[SDS]f (17) In terms of conductance of solution, eq 16 transforms into
log
{
}
{}
k a - kp kp ) log(nKA[PVP]) + n log s s
(18)
where ka and kp are the conductances of SDS solution in the absence and presence of PVP, respectively, and s is the slope of the linear conductance-concentration plot. The eq 18 can be used to calculate n and KA from the slope and intercept of the linear plots between log{(ka - kp)/s} and log{kp/s}. In the system of PVP-SDS herein studied, SDS monomers initially weakly interacted with PVP at low concentration up to a concentration called critical aggregation concentration (CAC) at which SDS monomers started to bind with the polymer in clusters (smaller than normal micelle) which upon completion led to the formation of micelles in solution at a concentration higher than normal CMC. At [PVP] ) 1 g %, the CAC and the extended CMC, that is, CMC2, were found from conductometry to be 2.25 and 17.6 mM, respectively, at 30 °C as per our previous report.31 From tensiometry and fluorimetry, Griffiths et al.18 reported CAC of SDS in 0.5 wt % PVP solution as 2.0 in aqueous and aquo-ethanol media at 25 °C. For SDS interaction, the CAC on the whole remains unchanged with [PVP], which is not so under the conditions of varied temperature, salt, and cosolvent. In Figure 6 (main plot), processing of conductance results for 1 g % PVP in terms of eq 18 is depicted. Two distinct straight lines with lower and higher slopes were obtained and the crossing point (2.11 mM) corresponded well with the CAC; the fitting of the second straight line was up to 8 mM SDS, which is the normal CMC of the surfactant, and the point from which the conductance-concentration course deviates in absence of PVP. The first straight line (by way of
The Solution Behavior of Poly(vinylpyrrolidone)
J. Phys. Chem. B, Vol. 112, No. 12, 2008 3623
TABLE 3: The Binding Parametersa of Surfactants with Nonelectrolyte Materials System
n1
PVP-SDS303 K (This study) PEG 400-SDS303 K (Ref. 49) PEG 400-CTAB303 K (Ref. 49) Triblock-copolymer-SDS298 K (Ref. 50) Cr-Al system-glycine298 K (Ref. 51)
0.11
5.8
K1A
1.0
2.0
0.86
1.41
1.0
0.52
0.85
0.0003
n2
K2A
2.64
0.036
SCHEME 1: Schematic Representation of Essential Steps for the Interaction of PVP with Solvent, Salt, and SDS
-
a The PVP-SDS interaction produced two sets of n and KA because of occurrence of two sets of binding below and above CAC. The other systems evidenced only one kind of interaction.
least-squares analysis) produced intercept ) -1.240 and slope (n) ) 0.11. Since [PVP] was expressed in terms of [pyrrolidone unit], the result meant binding of 1 mole of SDS per 9 or approximately 10 mol of pyrrolidone. SDS population on the polymer chain was quite low. From the intercept, the value of K1A found was 5.8. Similar analysis of the second straight line yielded results of n2 ) 2.64 and K2A ) 0.036. In Table 3, the results are presented with reports on other systems.50,51 The reported values elsewhere were of different types than our results. The molecular composition and architecture of PVP was different from PEG and the other polymeric species studied. They ended up with one set of binding that for PVP produced two. The two sets of n and KA stood for monomer binding and binding of small aggregates of SDS to the polymer sites in preand post CAC stages. The n2 value represented the aggregation states of the small assemblies (approximately 3) attached to the PVP sites (pyrrolidone centers). The K2A ) 0.036 was smaller than K1A ) 5.8. The first type of binding was obviously stronger than the second type. The clusters had lower strength of association. Thus, quantification of binding of surfactant to a polymeric substance and estimation of aggregation number of the small assemblies forming at and above CAC by the method of conductance can be a modest but convenient experimental procedure.
between them with a very good correlation. The conductance and viscosity of NaCl and SDS solutions did not obey the Walden rule, which advocated nonconstancy of the products between conductance and viscosity of PVP solution in presence of an electrolyte solution. The Walden rule should be used with reservation on ground of Curie-Pregogine principle of irreversible thermodynamics that does not permit coupling between conductance and viscosity by symmetry restrictions. In dilute PVP solution of 1 g %, the conductance was affected by way of binding of SDS with the polymer, which was rationalized with an equilibrium-binding scheme to estimate the binding strength for SDS monomers and their small aggregates with PVP before and after CAC respectively. The interaction of PVP with solvent (water and IP) molecules, salt, and SDS in relation to its solvation/desolvation and clouding can be comprehensively depicted in Scheme 1. This is a simplified model of a complex system whose intricacies remain unknown and unexplored. Acknowledgment. A.D. thanks UGC, Government of India, for a Junior Research Fellowship to perform this work. Financial support by Indian National Science Academy to S.P.M. is thankfully acknowledged. S.G. is a recipient of SERC, FAST Track project (No. SR/FTP/CS-35/206) from the Department of Science and Technology, Government of India. The authors acknowledge the assistance of Mr. P. Banerjee in the isopiestic measurements in the laboratory of Professor D. K. Chattoraj at the Food Technology and Biochemical Engineering Department, Jadavpur University.
Conclusions The reported results on the physicochemical studies of PVP in solutions under different conditions led to the following conclusions. The otherwise nonclouding PVP clouded in presence of specific type of salts, namely, NaCl, KCl, KBr, Na2SO4, MgSO4 Na3PO4, (NH4)4Ce(SO4)4·2H2O, and SnCl4·5H2O. The saltinduced clouding of PVP initially decreased with small addition of SDS showing a minimum and afterward increased with further addition of SDS. The surfactant molecules caused an initial release of residual water from PVP by interaction to augment earlier clouding, and afterward the PVP-micelle complex got solubilized in larger SDS micelles to increase CP. The clouding ended up with large changes in both enthalpy and entropy, which compensated each other very well similar to the processes of micelle and microemulsion formation. Both isopiestic and electrolyte conductance methods revealed comparable values of hydration of PVP. From the BET theory, three layers of water were formed in the solvation process of the material. In solution, both water and isopropanol solvated the pyrrolidone centers of the polymer; their individual contributions were estimated to realize that the components were linearly related
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