Langmuir 1991, 7, 653-657
653
Phase Behavior of a Nonionic Cellulose Ether in Nonaqueous Solution Arianeh A. Samii,*ytGunnar Karlstrom,l and Bjorn Lindmant Physical Chemistry 1 a n d Theoretical Chemistry, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden Received J u n e 18, 1990. I n Final Form: September 25, 1990 The phase behavior of ethyl(hydroxyethy1)cellulose (EHEC) in different polar solvents is reported. It is found that the clouding phenomenon appears only in water and formamide. It is also shown that the lower consolute temperature of a salt-free EHEC-formamide solution increases on addition of ionic surfactants. However, in the presence of a certain amount of sodium chloride, the cloud point of EHEC in formamide solutions, just as for the EHEC-water system, decreases considerably and exhibits a minimum on addition of low concentrations of ionic surfactants. It is suggested that the driving force behind the clouding phenomenon is the same in water and formamide.
Introduction
EHEC or ethyl(hydroxyethy1)cellulose is a nonionic cellulose ether synthesized from cellulose by introduction of ethyl and ethylene oxide (EO) groups. Since EHEC has extensive technical applications in several fields, it has been the subject of intensive research for many year^.'-^ EHEC as other nonionic polymers containing EO groups displays a clouding behavior in aqueous solution above a certain temperature, commonly referred to as the cloud point (CP) temperature. It is generally found that the cloud point of a nonionic polymer or surfactant containing ethylene oxide is raised by addition of ionic surfactanta435 Recently, it was shown that, in the presence of a small amount of a simple inorganic electrolyte, the cloud point of EHEC in water decreases dramatically on addition of moderate amounts of ionic surfactantse6 T h e origin of the phase separation of solutions of nonionic polymer or surfactant with ethylene oxide groups is a question that has been examined in a number of theoretical studies. In these studies, the systems containing ethylene oxide segments are modeled and different types of molecular interactions, like solvent-solvent interaction~,~>8 solute-solvent interactions,g or solute-solute interactions,1° are assumed to be the origin of this phenomenon. So far no consensus has been reached as to which of these three models is the most appropriate one and as to the origin of the interactions. In order to understand the underlying mechanism behind the phase separation in water and to be able to select between these different competing models, it is essential to examine the phase behavior of nonionic polymers and surfactants also in nonaqueous solvents. Recent results show qualitative similarities in the phase behavior of nonionic surfactants in water and in formamide."J2 It can also be noted that
* To whom correspondence should be addressed. +Physical Chemistry 1. f Theoretical Chemistry. (1) Sdnnerskog, S. Suen. Papperstidn. 1945, 48, 413. (2) Jullander, I. Ind. Eng. Chem. 1957, 49, 364. (3) Manley, R. St. J. Ark. Kemi 1956, 9, 519. (4) Karlstriim, G.; Carlsson, A.; Lindman, B. J . Phys. Chem. 1990,94, 5005. (5) Nilsson, P. G.; Lindman, B. J. Phys. Chem. 1984, 88, 5391. (6) Carlsson, A.; Karlstrom, G.; Lindman, B. Langmuir 1986,2, 536. ( 7 ) Kjellander, R.; Florin, E. J. Chem. Soc., Faraday Trans. 1981, 77, 2053. (8) Kjellander, R. J. Chem. SOC.,Faraday Trans. 2 1982, 78, 2052. (9) Goldstein, R. E. J. J . Chem. Phys. 1984, 80, 5340. (10) KarlstrBm, G. J . Phys. Chem. 1985,89, 4962.
0743-7463/91/2407-0653$02.50/0
Saeki et al.'3 observed clouding of poly(ethy1ene oxide) in a non-hydrogen-bonded solvent, tert-butyl acetate. In our first preliminary paper,I4we reported a clouding behavior in formamide of nonionic polymers like block copolymers poly(ethy1ene oxide)-poly(propy1ene oxide) and EHEC. We briefly reported on a major synergistic effect occurring in the presence of both a n ionic surfactant and a n electrolyte. In the present study we report more fully on these observations as well as extend the investigations by considering the phase behavior of EHEC in several other polar solvents. We also discuss possible underlying mechanisms of the change in polymer solubility in formamide, in the presence or absence of sodium chloride, as a function of added amount of ionic surfactants.
Materials and Methods The EHEC polymer (Bermocoll E 351X), with a molecular weight of 50 000, was obtained from Berol Nobel AB, Stenungsund, Sweden. On the average each anhydroglucose unit in cellulose was substituted with 1.4 ethyl groups and 0.9 ethylene oxide groups. The polymer was dialyzed against membranefiltered water (Millipore) to remove salt (NaCl) present in the polymer. All other chemicals were of analytical grade and used as supplied. Sampleswere prepared by weighing the components. EHEC and NaCl concentrations are expressed in percent by weight and surfactant concentrations in moles per kilogram of solvent (molal). The cloud point measurements were performed in a thermostated bath with, at least, three determinations for each sample. The cloud point temperature is taken as the temperature where the last visible sign of clouds disappears on cooling. In the formamide system a rather indistinctive haziness occurs instead of a distinct cloudiness as in aqueous solutions. We estimate the accuracy of our measurements to be 0.5 "C. The FT-NMR-PGSE technique for self-diffusion measurements has been described by Stilbs.I5 The measurements were performed for the 1Hnucleus on a Jeol FX-60FT NMR instrument operating at 60 MHz. The magnetic field was locked externally on the DzO signal. Temperature was adjusted to within f0.5 "C and measured by a copper-constantan thermocouple. A good signal-to-noiseratio was obtained by accumulation of 10 spectra (11) Warnheim, T.; Bokstrom, J.; Williams, Y. Colloid Polym. Sci. 1988,266, 562. (12) Warnheim, T.; Sjoberg. M. J . Colloid Interface Sci. 1989, 131, 402. (13) Saeki, S.; Kuwahara, N.; Nakata, M.; Kanenko, M. Polymer 1976,
--
17. . . , 685. -.
(14) Samii, A.; Lindman, B.; Karlstrom, G. Prog. Colloid Polym. Sci. 1990, 82, 285. (15) Stilbs, P. Prog. Nucl. Reson. Spectrosc. 1987, 19, 1.
0 1991 American Chemical Society
Samii et al.
654 Langmuir, Vol. 7, No. 4 , 1991 Table I. CP and Solubility of EHEC in Different Polar Solvents dielectric molecular dipole molecular permittivity (e) moment ( p ) , D volumes (V), A3 p/V CP 3.83 97.01 0.039 no CPb 182.4 3.73 126.81 0.029 no CPb 175.7 3.73 65.98 0.056 74 "C' 109.5 78.54 1.85 29.89 0.062 40 "C' 49.00 3.96 117.78 0.034 no CPb 37.00 3.82 127.91 0.029 no CPb
solvent N-methyll'ormamide N-methylacetamide formamide water dimethyl sulfoxide N,N-dimethylformamide
limit of the solubility of EHEC (wt ) at RTa '(
2.48 1.93 4.00 2.00 2.40
RT, room temperature, b Samples were heated up to the boiling point of each solvent. Corresponds to a minimum of CP.
0
1
2
3
4
E
5
EHEC conc./weight%
0
2
4
6
8
10
12
-2
Figure 1. (a)Partial phase diagram for the EHEC-water system. (b) Partial phase diagram for the EHEC-formamide system. (For each of the curves the one-phase region is below the curve and the two-phase region is above the curve.)
with a pulse repetition time of 2.5 s. Signals for 10-14 different values of the duration of the field gradient (8)were recorded for each sample. The range of d varied from 10 to 80 ms for each experiment. The time ( A ) between the 90" and 180" pulses was chosen to be 140 ms. The average of three intensity values (I) was taken and the intensity data were fitted to single exponentials according to I , = A exp[-G Aid2 (A - d/3)]
40
(1)
where 3 is the duration of the field gradient,D,is the self-diffusion coefficient of the ith species, G is a constant, and A is a fitting parameter. C was determined from calibration experiments where the self-diffusioncoefficient16of a trace HDO in D20 was taken as standard. Presented self-diffusion coefficients are estimated to have an accuracy of 5'(.
Results and Discussion Phase Diagrams for EHEC in Different Polar Solvents. In Table I, cloud point data are presented for this relatively hydrophobic EHEC in different polar solvents. It can be seen that the polymer displays a clouding phenomenon only in water and in formamide. The results obtained in our earlier ~ o r k ' ~ Jare ' very similar to the results presented here and also to the observations for the nonionic surfactants of the ethylene oxide type." From the similarities observed in the phase behavior of these different solutes, the reason why clouding appears only in water and formamide should be the same, as discussed in our previous paper~.'~J7In those studies, we claimed that a high dielectric permittivity or the ability of a solvent to form hydrogen bonds is not the only requirement to find clouding of nonionic polymers or surfactants in different polar solvents. In fact, as shown in Table I, no clouding phenomenon appears in N-methylformamide and N-methylacetamide, which are both hydrogen bonded solvents with higher dielectric permittivity than formamide and water. I t should be noted that the minimum in cloud point is always connected to an upper consolute temperature. From a theoretical point of view, one would expect that when this minimum is raised, the (16) Mills, R. J. Phys. Chem. 1973, 77, 685. (17) Samii, A. A., Karlstrom, G.; Lindman, B. Langmuir, in press.
Conc. 10 molal (Surfactant)
Figure 2. Effect of cetyltrimethylammoniumbromide (CTAB) and sodium dodecyl sulfate (SDS) on the cloud point of 1.5% of EHEC in formamide.
upper consolute temperature might be shifted to a lower temperature. In the situation where the polarity of the solvent is low enough, the lower and upper consolute temperatures will coincide and clouding disappears. Thus, by reasoning from the values for the dipole moment, p, and the volume of the solvent molecule, V, for different solvents studied (see Table l ) ,one would realize that it is only the solvent molecules that are small and polar like water and formamide that favor phase separation a t higher temperature. In fact as was previously f o ~ n d , only ~~J~ the solvents with the highest value of p / V induce clouding a t elevated temperatures, and the higher the value of p / V, the lower the clouding temperature. Figure 1presents the phase diagrams obtained for this particular EHEC in water and in formamide. These phase diagrams are qualitatively similar, but one should notice that, just as for nonionic block c ~ p o l y m e r s , ' ~the J ~clouding in water occurs a t a lower temperature than in formamide. (The CP minimums in water and formamide are 40 "C and 74 "C, respectively). If we assume that formamide is less polar than water (see refs 14 and 17 for more detailed explanation), the higher value of the cloud point in formamide than in water could be related to a smaller difference in polarity between the polymer and formamide than between water and polymer. Effect of Ionic Surfactants on EHEC-Formamide Solutions. It is known that the addition of an ionic surfactant increases the solubility of a nonionic surfactant5Js or a nonionic p ~ l y m e r in ~ ~water. ~ * ~ Recently, it has been shown that the phase behavior of nonionic surfactants in formamide present similar characteristics on addition of small amounts of ionic surfactant.I1 It was considered to be of interest to examine whether or not a nonionic polymer-formamide system shows the same behavior as a formamide-nonionic surfactant system or a waterpolymer system on addition of charged surfactants. In Figure 2, we present the cloud point variations for 1 . 5 % EHEC in formamide on addition of different (18) Marszall, L. Colloids Surf. 1987, 25, 279. (19) Molyneux, P. Water-Soluble Synthetic Polymers: Properties and Behauiour; CRC Press: Boca Raton, FL, 1984; Vol. 2, chapter 2. (20)Goddard, E. D. Colloids Surf. 1986, 19, 255.
Nonionic Cellulose Ether in Nonaqueous Solution
Langmuir, Vol. 7, No. 4, 1991 655
Table 11. The Effect of Different Cosolutes on t h e CP of 1.5% of EHEC in Different Polar Solvents.
LS(250C? SDS (M)
cloud point
a
SDS (M)
NaC1, wt 7; no CP no CP no CP no CP CP CP
solvent N-methylformamide dimethyl sulfoxide N-methylacetamide N,N-dimethylformamide formamide water
with NaCl no CP no CP no CP no CP CP CP
CTAB (M) with NaCl no CP no CP no CP no CP CP CP
LS = limit of the solubility of NaCl, SDS, and CTAB in the solutions of I
NaCl, wt r*
with NaCl
1.20 0.82 1.59 0.14
0.042
0.316 0.045 0.490
CTAB (M) with NaCl 0.052 0.076 0.047 0.071
l.sr( EHEC in different polar solvents.
1
100-
20
I 0
2
4
6
8
10
12
-2
Conc. 10 molal (CTAB)
F i g u r e 3. Partial phase diagram for a 1.5% EHEC solution in formamide as a function of CTAB concentration in the presence of different amounts of NaCl.
amounts of ionic surfactants, cationic (CTAB) or anionic (SDS). We may see that just as for the EHEC-water ~ y s t e mthe , ~ phase diagrams show a monotonic increase in cloud point with the concentration of added surfactants. From the similarities observed for aqueous and nonaqueous media (formamide), one would assume the same origin of the effect of amphiphilic cosolutes. This phenomenon, which is well-known for aqueous system^,^*^,^^ is ascribed to a balance between two forces. The attractive forces are due to the hydrocarbon tails of added surfactant, which bind to the polymer and make it less polar, but at the same time, the introduction of charged groups induces long-range electrostatic repulsive forces between the polymer molecules, thereby increasing their polarity. The balance between these two mechanisms is sensitive to the change of surfactant counterion or the presence of electrolyte in the solution. In the absence of salt, a monotonic increase of cloud point (see Figure 2) indicates that for all surfactant concentrations, the electrostatic repulsive forces dominate. In an earlier work4 where we studied a similar system but with water as the solvent, we also presented model calculations based on Flory Huggins theory, which reproduced the phase behavior in water. In this previous work we modeled the polymer with a two conformational description. Thus each segment of the polymer could either exist in a waterstabilized polar conformation or in an entropy-favored less polar conformation. The balance between these conformations leads to a clouding behavior under certain conditions in the pure polymer solvent system.1° The essential features of the parameters used in the model were that there was a fairly strong interaction between the surfactant and both the solvent as well as the polar conformation of the polymer, whereas the interaction between the less polar conformation and the surfactant was less attractive. In the model the surfactant was treated as a polymer, and the effective degree of polymerization was regarded (21) Lindman, B.; Carlsson, A.; Karlstrom, G.; Malmsten, M. Adu. Colloid Interface Sci. 1990, 32, 183.
55 7..
1 I
0
10
-2
20
30
Conc. 10 molal (SDS)
Figure 4. Partial phase diagram for a 1.5%EHEC solution in formamide as a function of SDS concentration in the presence of 0.5% of NaC1.
as a parameter. The main effect of this parameter on the phase behavior is to effect the steepness of the solubility curve. If larger effective polymerization numbers are used, the initial decrease at low surfactant concentrations and the subsequent increase at higher surfactant concentrations in cloud point curve become more pronounced. It can also be seen from this work that larger effective polymerization numbers lead to a displacement of the minimum in the cloud point curve to lower surfactant concentrations. Both of these observations indicate that the phase behavior observed for the EHEC-formamidesurfactant system can be understood, if the micelles are assumed to be smaller in formamide than in water. Apart from this, only very minor changes in the interaction parameters are needed to reproduce the observed phase behavior of the EHEC-formamide-surfactant systems. Combined Effect of Ionic Surfactant and Electrolyte on Phase Behavior. In Table 11, we present the cloud point as a function of added amount of sodium chloride and ionic surfactant (SDS, CTAB) for 1.5% of EHEC in different polar solvents. (The amount of ionic surfactant added varies from a small quantity (0.005 M) to the limit of the solubility and for each solvent three different concentrations of NaCl up to the solubility limit is chosen.) From this table it can be seen that no clouding behavior appears for EHEC in the presence of different amount of added cosolutes for several solvents. On the other hand for formamide we find, as seen in Figures 3 and 4, a similar synergistic salt-ionic surfactant effect as observed in water.6 These observations give further support to the idea that the mechanism of clouding is the same in formamide and water. Thus, a t low surfactant concentrations, the depression of the cloud point is due to the effect of salt, which is mainly to screen the interpolymer electrostatic repulsion induced by the head groups of the amphiphile; then the attractive hydrophobic forces, due to the hydrocarbon tails of the surfactant, become dominant leading to a decrease of the solubility. A t higher surfactant concentrations, the repulsive electrostatic effect dominates even in the presence of salt, which leads to an increase of the cloud point.
Samii et al.
Langmuir, Vol. 7, No. 4 , 1991
656
Table 111. Observed Self-Diffusion Coefficient Data for the CTA+ in Formamide in the Absence (a) and in the Presence (b) of 1.5 wt % of EHEC with and without NaCl CNaCI,
wt
( I
Dobsd,'
Ctot,"
m 2s-l
m
0
0.0300
0.68
0.0700 0.0900 0.1059 0.0700
(a) Absence of EHEC 2.87 2.88 2.83 2.78 2.71 2.78 2.57 (b) Presence of EHEC 2.25 2.20 2.13 2.11 2.08 2.05
0.1100 0
0.68
Dr,'
1o-lO m 2s-l
0.0700 0.0900 0.1100 0.0700 0.0900 0.1100
cb,
m
0.04 45%
Cf, m
O.O1 0.00
11
0.05
0.06
0.07
0.08
0.09
Free surfactant / molal
0.0153 0.0213 0.0286 0.0187 0.0250 0.0317
0.0547 0.0687 0.0813 0.0513 0.0650 0.0783
Figure 5. Binding isotherms of cetyltrimethylammonium bromide to 1.5wt $0 EHEC in formamide obtained from NMR self-diffusion measurements on the surfactant ion.
Ctot is the total CTAB concentration, Dobsd is the observed selfdiffusion Coefficient for the CTA+ ion, and Df is the self-diffusion coefficient lor free CTA+ ions obtained by extrapolating &bsd in a polymer-free solution to zero C.,,
Self-Diffusion Studies. Table I11 shows some values of the self-diffusion coefficients obtained for different CTA+ ion concentrations a t 45 "C. The systems of CTAB in formamide and CTAB in EHEC-formamide were both measured with and without salt. As a first approximation the common feature in all these systems is that the observed diffusion coefficient decreases when the concentration of surfactant increases (up to the value of the critical micelle concentration (cmc)). From Table 111, it can be seen that in the presence of EHEC, the observed diffusion coefficient (Dobsd) decreases significantly compared to that of CTA+ in formamide (about 20T lower than for CTAB in pure formamide). This reduction of Dobsd in the presence of EHEC demonstrates an interaction between CTAB and EHEC in formamide solution. Furthermore, the introduction of salt to the system leads to an overall reduction of Dobsd of CTA+ in the EHECformamide solution. From Table 111, one can notice that a t a concentration around the cmc (-0.11 m ) ,the decrease of n,,hsd is ca. 24 . The diffusion measurements on dodecyltrimethylammonium ions (DOTA+) in a salt solution of EHEC and waterzz a t a concentration around the cmc (
