J. Phys. Chem. 1995, 99, 3823-3831
3823
Phase Diagrams of Mixtures of a Nonionic Polymer, Hexanol, and Water. An Experimental and Theoretical Study of the Effect of Hydrophobic Modification Krister Thuresson,**tGunnar Karlstrijm: and Bjijrn Lindmant Physical Chemistry 1 and Theoretical Chemistry, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden Received: June 22, 1994; In Final Form: November 23, 1994@
The phase behavior of two ethyl hydroxyethyl celluloses, one regular (EHEC) and one hydrophobically modified (HM-EHEC), in aqueous solutions has been investigated. Phase diagrams have been obtained in the twoand three-component mixtures, with hexanol as the third component. On hydrophobically modifying the polymer, a lowering of the cloud point in the binary system, water/polymer, was recorded. A more unexpected change of the cloud point was seen in the ternary systems. In an aqueous solution of a fixed EHEC concentration, the addition of hexanol at first only gives rise to a minor cloud-point depression but when a certain hexanol concentration is reached, there is a rapid drop. With HM-EHEC the cloud point displays a smooth decrease over the whole hexanol concentration regime investigated. The observed differences in the behavior of the two polymers were analyzed and could be understood when the partial three-component phase triangles were established. The difference is explained in terms of a transfer from one two-phase region to another on increasing the hexanol concentration in the aqueous solutions of EHEC, while in the case of HM-EHEC the same two-phase region is reached on increasing the temperature independently of the hexanol concentration. The main features of the experimentally observed phase diagrams of the two polymers, and notably the differences mentioned, could be reproduced semiquantitatively using a modified FloryHuggins theory.
Introduction Hydrophobically modified polymers consist of a water-soluble backbone onto which a low number, typically less than 5 mol % based on repeating units, of hydrophobic groups have been chemically attached.' Often the hydrophobic side groups consist of long alkyl chains (C~Z-CIS).However, other groups have also been chosen, as in the studies by Winnik and others where fluorescence probes were grafted onto the polymer backbones, with the purpose of probing the interactions between different hydrophobic tails on the polymer molecules.2-9 In the present study, we have chosen a polymer that is hydrophobically modified with a branched nonylphenol. During the past decade this class of polymers has found increasing use in industrial applications, for example, as thickeners in water-based paint formulations, in shampoos, and in skin care products. In addition, there are many fields of potential use, such as stabilizers in emulsions, as dispatchers of active substances in drug formulations, and for modification of surfaces from hydrophobic to hydrophilic or vice versa. heviously, a scarce knowledge stood in sharp contrast to the growing commercial interest; however, in recent years the number of investigations has increased with a natural focus on the viscosity or the rheological behavior of the aqueous solution^.^^-^^ Typically, these polymers show an increased viscosity in the binary solution compared to their unmodified analogues. The difference in viscosity can, on addition of a third component (ranging from simple salts or alcohols to surfactants), be boosted or weakened depending on the choice.ls Perhaps surfactants are the cosolutes that display the most spectacular effect; the viscosity of an aqueous solution contain-
* To whom correspondence should be addressed. + Physical Chemistry
1.
* Theoretical Chemistry.
* Abstract published in Advance ACS Abstracts, February 1, 1995. 0022-365419512099-3823$09.00/0
ing 1% polymer can either be enhanced or reduced, depending on the concentration. Addition of a certain amount can increase the viscosity several orders of magnitude, while on further surfactant addition the viscosity drops off to a level at, or even below, the initial value. Addition of salts or alcohols induces a more moderate, but nevertheless still interesting and sometimes puzzling, change. An important complement to rheological work, and a way to quantify the interactions underlying the rheological properties, is the phase behavior. In future publications, we will present a number of phase studies, represented as cloud-point diagrams, in which we have chosen to keep the polymer concentration constant and vary the type and amount of added cosolute. In the present work, we rather focus on one cosolute, hexanol, but vary also the polymer concentration and the temperature. The results are presented in ternary triangular phase diagrams. The paper is outlined as a comparative study between a hydrophobically modified polymer and its unmodified analogue. In addition to experimental phase studies, we have used a modified Flory-Huggins theory to calculate theoretical phase diagrams.lg The calculations are successful in reproducing all the trends the experimental phase diagrams display.
Experimental Section Materials. Both polymers used in this study, Figure 1, are manufactured by Berol Nobel AB, Stenungsund, Sweden. Both the unmodified polymer (EHEC) and the hydrophobically modified one (HM-EHEC) are ethyl hydroxyethyl cellulose ethers with molecular weights of approximately 100 kDa. The degrees of substitution of ethyl and hydroxyethyl groups are DSethyl = 0.6-0.7 and MSEO= 1.8, respectively. The values of DS and MS correspond to the average number of ethyl and hydroxyethyl groups per anhydroglucose unit of the polymer. Molecular weights and DS and MS values were given by the manufacturer. Nonylphenol chains grafted to the cellulose 0 1995 American Chemical Society
Thuresson et al.
3824 J. Phys. Chem., Vol. 99, No. 11, 1995
EHEC: R = H HM-EHEC:R = + C H ~ - C H Z OC6H4-CgH19 ~ Figure 1. Stmcture of EHEC and HM-EHEC. Both polymers have M, = 100 OOO, DSehyl= 0.6-0.7, and MSEO= 1.8. Additionally, HM-EHEC has a nonylphenol substitution of ca. 1.7 mol %.
backbone represent the only difference between the two polymers. The hydrophobic substitution degree of HM-EHEC was determined to 1.7 mol % relative to repeating units of the polymer by measuring the absorbance of the aromatic ring in nonylphenol at 275 nm with a Shimadzu W - 1 6 0 W - v i s spectrophotometer. Phenol in aqueous solution was used as a reference. Before use, the polymer samples were purified according to the following procedure. The dry powder of HM-EHEC, as received from the manufacturer, was contacted with acetone several times to extract free nonylphenol not chemically bound to the polymer. After extraction, the powder was dried from acetone and an aqueous solution of ca. 1 g of polymer per 100 g of total weight (1 wt %) was prepared. Impurities which were insoluble in water were allowed to settle during a centrifugation at 10 OOO rpm. The clear supernatant phase was separated from the pellet and dialyzed against Millipore water in a Filtron Ultrasette device until the conductivity of the expelled water showed no further decrease. This occurred after ca. 70 h of dialysis and at a conductivity of approximately 2 pS/m. After dialysis the aqueous solution was freeze-dried, leaving a dry polymer sample which was stored in a desiccator prior to use. The same purification procedure, except for the acetone extraction, was performed with the EHEC sample. Hexanol of pro analyse quality was purchased from BDH Laboratory Supplies, Poole, Dorset, England, and used as received. For all samples, water of Millipore, Bedford, USA, quality was used. Methods. The freeze-dried polymer samples were dissolved in water to a concentration of 2 wt %. These solutions of EHEC and HM-EHEC were used as stock solutions from which all further samples were prepared. Figure 2 shows the cloud-point temperature (Tcp)as a function of polymer concentration. Tcp indicates where the solution demixes and becomes hazy at increasing the temperature. In
Figure 3 the shift in cloud-point temperature (ATcp)as a function of added hexanol at a fixed polymer concentration of 1 g of polymer per 100 g of water (1 wt %) is plotted. Tcp values were obtained by the following procedure: the desired sample was prepared by weighing the components (water, hexanol, and stock solution of polymer) and mixing with a magnetic stirrer. The ampule with the sample was immersed in a jacketed glass vessel connected to a thermostated circulating water bath with a stability within fO.l "C. The actual temperature in the sample, which at elevated temperatures was slightly lower than in the water bath, was measured with a thermocouple. On increasing the temperature, the solution turned from one phase ( l a ) to two phase (2@). The mixtures were taken to be 2@ when a ruled scale visually observed through the stirred solution appeared blurred. The temperature was then lowered and the solution turned from 2@ back into the clear 1@region. The mean value of the two temperatures, usually within 2 "C, was taken to represent Tcp. In determination of the three-component phase diagrams, Figure 4, Millipore water, hexanol, and polymer stock solution were added to small test tubes by weighing the aliquots. After the additions were completed, the test tubes were flame sealed and, when intermediate temperatures were desired, immersed in a thermostated water bath. The water bath was specially equipped with a tilting plate on which the test tubes were deposited. At higher temperatures the sealed samples were kept in an oven and mixed by turning end over end. The samples were mixed at the correct temperature for ca. 48 h to attain equilibrium. After the mixing, the samples were quickly transferred to a thermostated centrifuge preheated to the same temperature. [The centrifuge was heated by the surplus heat from the engine. Different temperatures altered the speed of the centrifuge between 2000 and 3500 rpm.] After centrifugation, which was performed until the hazy samples had reached a macroscopic phase separation, the number of phases was
Mixtures of a Nonionic Polymer, Hexanol, and Water
J. Phys. Chem., Vol. 99,No. 11, 1995 3825
1 d
1.o
0.5
I
1-
1.5
2.0
Figure 2. Cloud point as a function of polymer concentration in the binary mixtures watedpolymer. EHEC is represented by open circles and HM-EHEC by filled circles. The introduction of hydrophobic side p u p s on the EHEC molecule lowers the Tq by ca. 15 "C independently of the polymer concentration.
because the water-rich phase was always much larger in volume compared to the other two phases. The polymer content was determined by optical rotation at 436 nm with a Jasco DIP-360 digital polarimeter. The optical rotation of the sample was averaged over ca. 10 min to reach the desired accuracy in the determination. The accuracy was estimated to be 0.05-0.1 wt % of the polymer. The total weight concentration of polymer and hexanol in the water-rich phase was determined by measuring the refractive index of the mixture compared to the refractive index of pure water. The refractive index increment is proportional to the total concentration of added solutes in the water. As the polymer concentration was known from the optical rotation, the hexanol concentration could be calculated. In the 2cP regions the partial tie lines were drawn as dashed lines between the points corresponding to the composition in the initial sample (cross marks) and the points corresponding to the composition in the water-rich phases (circles). The composition of the water-rich phase obtained after phase separation of a sample prepared in the 3@ area was determined according to the same procedure as above. The point thus determined corresponds to the water-rich comer in the 3cP triangle; see Figure 4.
ATcp I"C1
Results 0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0
-- I200
1
.so
1
.40 -60 J
C hexanol [molal]
\
Figure 3. Three-component cloud-point diagram in which AT, for EHEC and HM-EHEC are shown as a function of hexanol concentration. The polymer concentration was fixed at 1 wt %. Notice the different behavior of the EHEC solution (open circles) compared to
the HM-EHEC solution (filled circles). determined. When phase separation to three phases had been established, the polymer-rich phase was at the bottom, the waterrich phase in the middle, and the hexanol-rich phase at the top of the test tube. In the samples with two phases in equilibrium, the individual order of those was the same as in the samples in the three-phase (3'3) region. The temperatures were kept constant to within f0.5 "C in the water bath and to within f l "C in the oven and the centrifuge. An electronic thermometer with an accuracy of f0.2 OC was used to measure the temperature in the water bath, the oven and the centrifuge. Each of the experimental three component phase diagrams was made up from 60-65 different samples. The partial tie lines in the 2cP areas and the locations of the water-rich comers in the 3@ areas in the waterhexanoUEHEC phase diagrams, Figure 4, were determined according to the following procedure. A few milliliters of the relevant sample were prepared and after mixing at the desired temperature, the sample was centrifuged, at the same temperature, to promote a macroscopic phase separation. With the analytical methods used in this study it was natural to choose the water-rich phase to be investigated with respect to polymer and hexanol content. When phase separation was established, this showed to be a correct decision
Experimental Cloud-Point Diagrams. In Figure 2, the Tcp as a function of EHEC and HM-EHEC concentration is shown. At low temperatures, the polymers are water soluble while at high temperatures the sample separates into one polymer-rich phase, the bottom phase, and one water-rich phase, the upper phase. At low polymer concentration, the Tcp decreases with increasing content of the polymer, while at higher concentrations the Tq reaches a more or less constant value. Figure 3 shows that the Tcpof an aqueous solution of 1 wt % polymer is lowered as the hexanol concentration increases. The Tq decreases more smoothly for HM-EHEC than for the solution containing EHEC. In the latter case the Tcp shows only minor changes up to ca. 50 m o l a l hexanol, followed by a rapid drop on further addition. Experimental Triangular Phase Diagrams. For experimental reasons it was only possible to determine the water-rich part of the phase triangles; at high polymer content the viscosity of the samples is too high to allow phase equilibrium to be reached within reasonable time and, moreover, the preparation of the samples becomes difficult. The temperatures presented in the three-component triangular phase diagrams, Figure 4, are shifted with 65 "C for EHEC and with 50 "C for HM-EHEC. Those values corresponds to the Tcpof the polymer in the binary mixture, water/polymer, at a polymer concentration of 1 wt %, refemng to Figure 2. Considering first the phase diagrams for EHEC, given to the left in Figure 4, at the lowest temperature, Tcp -15 "C, there exists a large 1@ zone at the water/polymer side of the triangle. On addition of hexanol to the water/polymer system, a phase separation results and depending on the polymer content, different phases appear. At low EHEC contents, I1.5 wt %, we move into a 2cP region (the lower). According to the appearance of the 2@ region and the direction of the tie line, the dashed line, a hexanol-rich phase forms in equilibrium with a phase containing most of the water and polymer. At higher EHEC contents, another 2@ region (the upper) is reached on hexanol addition. As this region is quite small at this temperature, no tie lines have been determined. The two 2@ domains are separated by a small intermediate 3cP triangle. On increasing the temperature by 10 "C to Tcp - 5 "C, the I @ area decreases in size as the border between the 10 area
Thuresson et al.
3826 J. Phys. Chem., Vol. 99,No. 11, 1995
H20,Hexanol and HM-EHEC.
H20,Hexanol and EHEC.
Increasing temperature.
EHEC
HM-EHEC
t
Tcp + I5 "C
HM-EHEC
EHEC
Tcp
1
2
+5
"C
1
Hexanol
2
Hexanol
2
Hexanol
2
Hexlrnol
HM-EHEC
EHEC
Tcp - 5 "C
2
1
1
Hexanol
HM-EHEC
EHKC
Tcp - 15 "C
"20
1
2
Hcxitnol
"20
1
Figure 4. Experimental three-component triangular phase diagrams for systems of water/hexanol/polymer. The temperature increases from bottom to the top in the figure. EHEC is represented to the left and HM-EHEC to the right. Only the water-rich comers of the phase diagrams are determined.
Mixtures of a Nonionic Polymer, Hexanol, and Water
J. Phys. Chem., Vol. 99, No. 11, 1995 3827
and the other regions moves towards the waterEHEC twocomponent line. The displacement is more pronounced the higher the polymer content. At the same time, the lower 2@ area shrinks while the upper 2@ and 3@ regions grow in size. In the lower 2 0 territory, the tie lines have the same direction as in the preceding phase diagram while in the upper 2@ region the tie lines have a different direction (compared to the lower 2@ region). Here the tie lines indicate that it is the polymer, rather than the hexanol, that separates out from the solution. The water-rich, upper, phase contains a lower concentration of the polymer and a concentration of hexanol slightly lower than the original sample (and the lower phase). The bottom phase contains most of the polymer and the rest of the hexanol and water. The double broken lines in the intermediate 3@ zone are not tie lines but rather they point out the water-rich comer of the 3@ triangle. As was the case in the 2@ regions, only the water-rich phase has been analyzed. The compositions and relative volume of the phases in a sample in the 3@ triangle are given by the location of the comers and by the lever rule together with the relative distance to each comer of the triangle. On further temperature increase, to Tcp+5 OC, the upper 2@ region and the 3@ triangle continue to increase in size at the expense of the 1@ area and the bottom 2@ region. The decrease in size of the bottom 2@ region is a consequence of the waterrich comer in the 3@ triangle moving down, toward the water/ hexanol axis. The decrease in size of the l@area follows naturally from Figure 2 since it is only at low polymer content that the water/polymer mixture is l@at elevated temperatures. The tie line of the upper 2 0 area indicates a phase separation into one water-rich and one polymer-rich phase. Here we stress that the tie line does not reach the 1@ sector in the water-rich comer of the triangle as it should. The difference in polymer concentration in the water-rich phase between what is obtained in the phase border determination and what is actually determined in analyzing the water-rich phase is most likely due to the polydispersity of the polymer, both in molecular weight and in degree of substitution. The polymer fraction with a high molecular weight and a high substitution degree of hydrophobic side chains, ethyl and nonylphenol, will have a stronger tendency to phase separation (vide infra), compared to other polymer fractions. This will tend to increase the polymer content determined in the water-rich phase because some polymer fractions are still soluble even though a phase separation of the less soluble polymer fractions is seen. As could be expected, the discrepancy in the determined polymer concentration decreases on further increase of the temperature to Tcp 15 "C. Most polymer fractions phase separate at the higher temperature and only a minor part of the polymer molecules is still soluble. Besides the change in the location of the end point of the tie line the main difference seen in the phase behavior on increasing the temperature to this highest temperature investigated, T , 15 "C, is a slight decrease of the 1@area. The phase diagrams based on HM-EHEC are found to the right in Figure 4. In these triangular diagrams one can see the same trends as in the diagrams based on EHEC. At low temperatures there is a 1@ sector close to the water/HM-EHEC side of the phase triangle which decreases in size with elevating temperature. At the higher temperatures there is only a tiny remainder close to the water apex. The upper 2@ region increases in size at the expense of the 1@ sector and the 3@ triangle. The lower 2@ territory seems not to be affected appreciably by temperature changes. The main distinction between the phase diagrams based on the two different polymers is the location of the water-rich comer of the three-phase
+
+
triangle, and its movement downwards with increasing temperature which is very pronounced with EHEC. Theoretical Model Outline. The purpose of this section is to present the necessary theoretical background for analyzing how an additive, such as hexanol, is distributed between two phases, and how it affects the phase behavior. The starting point for the analysis is the Flory-Huggins polymer theory and a twoconformational description of the polymer. 19g20 The basic assumption here is that each segment of the polymer may exist either in a polar or a nonpolar conformation. In a polar solvent, such as water, the polar conformations will be energetically favored while the nonpolar conformations, which are more numerous, will be entropically favored. The model was originally based on quantum chemical ab initio calculations and statistic mechanical modelling of 1,2-dimetho~yethane.~l The calculations indicated that a typical polar conformation was of type anti around the C-0 bonds and gauche around the C-C bonds. The polar conformations interact favorably with water and with other polar molecules whereas the nonpolar ones interact less favorably with water and favorably with nonpolar substances. Using these assumptions, it is possible to write the entropy and energy of the system according to Zhang et al.22
and m
Ifii
m'
Pfi?
In these equations n is the total number of moles in the system and R is the gas constant. The a's denote the mole fractions of the components and the M s are the degrees of polymerization, Pij specifies the probability that a segment of component i has conformationj, the F's measure the amount of different types of conformations for a component, the Z(i) defines the number of different conformations of compound i, and fiially the w's are interaction parameters. This type of approximation has been applied successfully to several systems of clouding polymers in water.19,23-25The interaction parameters chosen to describe the water-polymer system are the standard ones used by us in earlier work to describe such a system.19 The interested reader is referred to the original work for more details. Assuming eqs 1 and 2, the free energy of the system can be calculated according to A = U - TS. The correct way to proceed to calculate a theoretical phase diagram for a given total composition is to assume a number of phases and distribute the different components in the system among the phases in such a way that the total free energy of the system is minimized. In the present version of the program, which calculates the phase diagram, this is done by a Monte Carlo procedure which moves small amounts of the different components between the phases in such a way that the total free energy is minimized. Theoretical Triangular Phase Diagrams. First it should be stated that our intention is not to in detail theoretically reproduce the experimental phase diagrams, rather it is to rationalise the changes seen on modifying the polymer or on altering the temperature. The outline of Figure 5 resembles that of Figure 4 and is as follows. The phase diagrams for EHEC are located to the left and those for HM-EHEC to the right. The temperatures chosen in the calculations almost correspond to those in the experimental phase diagrams, if a shift in Tcpis accounted for. On going upwards the temperature increases.
3828 J. Phys. Chem., Vol. 99, No. 11, 1995
Thuresson et al.
Figure 5. Calculated phase diagrams corresponding to the experimental phase diagrams presented in Figure 4. The full three-component phase diagrams have been calculated and four triangles, two at the lowest and two at the highest temperatures, have been expanded to facilitate the comparison between experimental and theoretical results. The cloud point for both EHEC and HM-EHEC is chosen to be 388 K in the calculations. The interaction parameters used in the calculations for EHEC (in which the index 1 represents water, 2 represents hexanol, p represents the polar conformation of the polymer, and finally n represents the nonpolar conformation of the polymer) are as follows: wlp = 650.8, wln = 10654, WIZ = 8500, wzP = 2500, wzn = 7752, wpn = 6352, wm = 10172. The interaction parameters for HM-EHEC are the same except for the parameters between polymer and hexanol which are set to wzP = 1600 and wzn = 6852. All values are given in J/mol.
Mixtures of a Nonionic Polymer, Hexanol, and Water The lower four phase diagrams refer to temperatures below Tcp while the upper four phase diagrams refer to temperatures above T,. Since the same interaction parameters between solvent and polymer are used, for both EHEC and HM-EHEC, T, is specified to be 388 K in both the binary systems. The easiest way to obtain a lower clouding temperature for the more hydrophobic polymer would have been to scale all interaction parameters with a scaling factor S. If a clouding temperature of T was required, then all interaction parameters should be scaled by S = Tl388. The interaction between polymer and hexanol is made 900 J/mol less repulsive for HM-EHEC than EHEC; it is reasonable to assume that the hydrophobic substituents on the polymer will interact favorably with the hydrophobic hexanol and decrease the repulsion between the polymer and the hexanol. All parameters used are presented in the figure caption to Figure 5 . The full three-component phase diagrams are presented at all the considered temperatures. Apart from this, we have also expanded the water-rich comer of the four phase diagrams at the lowest, 370 K, and at the highest, 410 K, temperatures. These are located in the middle of Figure 5. The expansion was made to facilitate comparison with the experimental phase diagrams which could only be determined at low polymer content (vide supra). The phase diagrams for EHEC show that on increasing the temperature, moving upwards in Figure 5 , the water-rich comer of the 3@ triangle indeed moves downwards as it does in the experimental phase diagrams. At the lowest temperature, the 2@ region reached on adding hexanol at low EHEC content is a separation into one phase rich in hexanol and one phase rich in water (the lower 2@ region). At higher temperatures the 2 0 region reached corresponds to one water-rich and one polymer-rich phase (the upper 2@ region). The phase diagrams to the right in Figure 5 show the corresponding phase diagrams for HM-EHEC. For these it can be seen that the 2@ region reached on hexanol addition, the upper, is the same independent of the temperature. The sample separates into one water-rich and one polymer-rich phase. This is a consequence of the low positioning of the water-rich comer in the 3@ triangle at all temperatures.
J. Phys. Chem., Vol. 99, No. 11, 1995 3829 TABLE 1: Data from Cloud-Point Measurements on Different EHEC Samples in 1 wt % Aaueous Solution@ HM cloud fraction degree point sample DScthyi MSm EO [mol%] (“C) source EHEC EHEC EHEC EHEC EHEC
EHEC EHEC EHEC EHEC
EHEC EHEC EHEC EHEC EHEC EHEC EHEC HEC
We start this section by discussing the two-component phase diagrams (Figure 2) and the three-component phase diagrams in which the polymer concentration is kept constant at 1 wt % (Figure 3). We also discuss the connections of the latter to the three-component triangular phase diagrams, Figures 4 and 5, with respect to the structural difference between EHEC and HMEHEC. On introduction of hydrophobic groups (here 1.7 mol % nonylphenol grafted to the polymer backbone), the EHEC molecule, as a whole, becomes more hydrophobic which promotes a phase separation from the aqueous solution. This effect parallels the change in Tcpof a hydrophobically unmodified EHEC when the balance between hydrophilic hydroxyethyl and hydrophobic ethyl groups is varied. The relative amounts of these groups determine to a high degree the Tcpof a given EHEC sample. A collection of Tcp temperatures of various EHEC samples is presented in Table l.26-30 To observe the trend more easily, we have also chosen to plot these phase separation temperatures in Figure 6 as a function of the fraction of hydroxyethyl groups, M S E ~ ( M S E O DSethyl),grafted to the polymer backbone. The plot clearly shows that an increase of the hydrophilic groups increases the phase separation temperature of the polymer in a regular way. Furthermore, Figure 6 shows that the EHEC sample used in the present investigation
+
1.5 1.4 1.01 1.07 1.1-1.2 1.0 1.12 0.9 0.84
0.75
0 EI-IEC 0.6-0.7 HM-EHEC 0.6-0.7
0.7 1.1 0.9 1.1 1.3 0.8 1.7 1.7-1.8 1.42
0.32 0.39 0.39 0.39 0.41 0.50 0.53
1.55
0.59 0.59-0.61
1.7 1.8 2.24 2.1 1.98 2.1 2.37 1.8 1.8
30 34 37 32 34 62 37
ref29 ref30
ref30
ref 30 ref 30 ref29 ref30 45 ref30 58 ref27 57 ref 27 58 ref30 55 ref26 63 ref 27 70 ref28 67 ref 27 70 ref26 >lo0 ref27 65 thispaper 50 this paper
0.55-0.56 0.58 0.64 0.67 0.70
0.70 0.74 1.0
0.72-0.75 0.72-0.75 1.7 “The substitution degrees of hydroxyethyl, MSm, and of ethyl groups, DSsthyl.are altered.
t o t
+ t +
W t e t
+ i? 20
+
11 d
Discussion
1.5 1.7 1.4 1.7 1.9 0.8
Fraction EO groups 0.2
0.4
I
I
1-
0.6
0.8
1.0
Figure 6. Cloud point of a collection of different EHEC samples plotted against the fraction of hydroxyethyl groups, MS&(MSm DSchyl). The data are also presented in Table 1. The scattering in the experimental data might be due to the use of different methods in the determinations. The EHEC and HM-EHEC samples in the present investigation are represented by an open circle and a f i e d circle, respectively.
+
follows the suggested trend. Hydrophobic modification of the EHEC polymer by grafting nonylphenol groups to the backbone is, as indicated above, akin to a decrease in the ratio of EO groups and the Tcp should be lowered due to this, which is c o n f i i e d by Figure 2. The lowering of the phase separation temperature is also evident in Figure 6. The T, for the hydrophobically modified polymer is clearly below the trend indicated by the other polymers. To rationalize the difference between the two phase boundaries in Figure 3 on addition of hexanol is a more delicate task. The difference is, however, better understood on the basis of the complete three-component phase diagrams. The smooth decrease of T, on addition of hexanol to a solution of HMEHEC, Figure 3, corresponds to the growth of the upper 2@ area at the expense of the 1 @ sector with increasing temperature in Figure 4. In other words, the phase border of the upper 2@ domain moves closer to the water/HM-EHEC side of the triangle as the temperature increases and less hexanol has to be added
Thuresson et al.
3830 J. Phys. Chem., Vol. 99, No. 11, 1995 to reach a phase separation at a fixed polymer concentration. In the case of EHEC the situation is different. The Tcpcurve for EHEC, Figure 3, displays only minor changes up to approximately 50 mmolal hexanol, corresponding to ca. 0.5 wt %, followed by a rapid drop on further hexanol addition. Again looking for the reason in the triangular diagrams, Figure 4,it can be seen that at low temperatures, and at an EHEC concentration of 1 wt %, the lower of the two 2@ regions is reached on adding approximately 0.5 wt % hexanol. At higher temperatures, instead, the upper 2@ domain is reached, which is a result of the movement downward of the water-rich comer in the three-phase triangle on increasing the temperature. The important difference between the triangular phase diagrams based on HM-EHEC and EHEC, responsible for the difference between the Tcpcurves in Figure 3, is that the border between the lower 2@ and the l@phase areas, which is reached on adding hexanol to a 1 wt % EHEC solution at low temperatures, does not move toward the water/EHEC axis with increasing temperature. For the HM-EHEC solution, in which the upper 2@ region is reached on hexanol addition, the phase border moves considerably toward the water/polymer axis with increasing temperature, Figure 4. With the understanding gained regarding the difference in the cloud point diagrams, on the basis of the triangular phase diagrams, we will now try to get some insight into the origin of the difference between the triangular experimental phase diagrams based on EHEC and HM-EHEC, by comparing them with the calculated ones. A schematic comparison between the experimental and theoretical phase diagrams shows that the theory reproduces the experimental phase behavior; the la, the two 2@ and the 3@ regions are all found, for both EHEC and HMEHEC. Finally, it is seen that the tie lines in the 2@ regions have the correct directions, both in the lower and in the upper sectors. A closer look at the lowest temperature reveals that the major difference between the phase diagrams of the two polymers is again the position of the water-rich comer in the three-phase triangle, as it was in the experimental phase diagrams. The positioning is related to the hydrophobicity of the polymer; the more hydrophobic the easier it mixes with hexanol. This is taken into account in the choice of interaction parameter between polymer and hexanol (vide supra). On replacing EHEC with HM-EHEC the chemical potential of the polymer in hexanol is lowered for a fixed polymer concentration, and the polymer will favor the hexanol phase relative to the water phase. In the phase diagram it can be observed that the water-rich comer moves to a lower polymer concentration. At the same time the hexanol concentration in the polymer-rich phase is increased and this leads to a lowering of the water content of this phase. Now we focus on the changes seen on elevating the temperature. The shift downwards of the water-rich comer in the 3Cg triangle seen with EHEC is virtually nonexistent with HM-EHEC, which is mainly due to the fact that in the latter case the comer is already in the proximity of the water/hexanol two-component line. The displacement of the apex, which is obvious both in the experimental and in the theoretical phase diagrams of EHEC, can again be understood in terms of the hydrophobicity of the polymer, but this time as a function of the temperature. On increasing the temperature, the EHEC molecule adopts conformations that are entropically favored and hydrophobic (see Theory section above). This leads to facilitated dissolution of the polymer in the hexanol phase and the apex moves downwards; this should be compared with the different behaviour of EHEC and HM-EHEC. As the temper-
ature increases it can also be seen that the upper 2@ region increases in size at the cost of the l@sector in the experimental phase diagrams. This phenomenon is reproduced in the calculated phase triangles and is valid both for EHEC and HMEHEC. The behavior is due to the less favorable watedpolymer interaction at elevated temperatures, originating from the polymer adopting more nonpolar conformations, as mentioned above. Finally, a phase separation occurs-the clouding phenomena of the polymer, which is observed as a growth of the upper 2@ region at the expense of the l@sector.
Concluding Remarks The present investigations show that, in these systems, it is most important to know the triangular phase diagrams to fully understand the differences obtained, in the cloud-point diagrams, on hydrophobic modification of the polymer. The sharp drop in the cloud-point temperature of the EHEC solution at a certain hexanol concentration is due to a switch between different 2@ regions. Which of the two 24, regions that is reached on increasing the temperature is determined by the hexanol concentration. In the HM-EHEC solution, where the cloudpoint temperature shows a smooth decrease as a function of added hexanol, the same 2@ region is reached independently of the hexanol concentration. To get a further understanding of the experimentally observed differences, we have used a modified Flory-Huggins model to calculate theoretical triangular phase diagrams. The only change in the calculations that was needed to semiquantitatively reproduce the differences, was to make the interaction parameter between hexanol and polymer more favorable in the case of the hydrophobically modified polymer. However, even though the theoretical calculations used are satisfying in rationalizing experimentally obtained phase diagrams, it should be noted that it is not obvious how to choose the interaction parameters in a different polymer solution.
Acknowledgment. The National Board for Industrial and Technical Development (NUTEK) and Berol Nobel AB are gratefully acknowledged for financially support. Berol Nobel AB is also acknowledged for supplying the polymer samples. References and Notes (1) Polymers in Aqueous Media; Glass, J. E., Ed.; American Chemical Society: Washington, DC,1989;Vol. 223. (2) Winnik, F. M.; Winnik, M. A.; Tazuke, S.Macromolecules 1987, 20,38-44. (3) Winnik, F. M. Macromolecules 1987.20,2745-2750. (4) Winnik, F. M. J . Phys. Chem. 1989,93, 7452-7457. (5)Winnik, F. M. Langmuir 1990,6, 522-524. (6) Hu, Y.-2.;Zhao, C.-L.; Winnik, M. A. Langmuir 1990,6,880883. (7) Yekta, A.; Duhamel, J.; Adiwidjaja, H.; Brochard, P.; Winnik, M. A. Langmuir 1993,9, 881-883. (8) Richey, B.; Kirk, A. B.; Eisenhart, E. K.; Fitzwater, S.; Hook, J. J . Coat. Technol. 1991,63, 31-40. (9) Chandar, P.;Somasundaran,P.; Two, N. J. Macromolecules 1988, 21,950-953. (10)Gradzielski, M.; Rauscher, A.; Hoffman, H. J . Phys. 1993,3,6579. (11) Huldh, M.Colloids Surf. 1994,82,263-277. (12) Annable, T.; Buscall, R.; Ettelaie, R.; Whittlestone, D. J. Rheol. 1993,37,695-726. (13) Gelman, R. A. Hydrophobically modified hydroxyethylcellulose. Presented at the International Dissolving Pulps Conference, 1987. (14) Tanaka, R.; Meadows, J.; Phillips, G. 0.;Williams, P. A. Carbohydr. Polym. 1990,12, 443-459. (15) Wtner, U.;Hoffmann, H.; Danges, R.; Ehrler, R. Colloids Surf. 1994,82,279-297. (16) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B. Prog. Colloid Polym. Sci. 1992,89, 118-121. (17) Leung, P. S.;Gcddard, E. D. Langmuir 1991,7, 608-609.
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