Effect of aggregates in bulk and surface properties: surface tension

J . Phys. Chem. 1988,92, 3565-3568 TABLE 111: AC Resistance of Polypyrrole Membranes Prior to and after Exchange of Tosvlate with Another Anion

exchanging anion" 50:NO,-

PO,'-

resistance,* ohm 20.0 (24.0) 21.1 (28.4) 28.8 (28.6)

exchanging anion" F CIBr-

resistance,* ohm 26.2 (28.5) 29.0 (30.0) 21.6 (21.6)

"Anion concentration in all cases was 0.1 M. bThe values in parentheses refer to the membrane resistance after exchange for 45 h; see Experimental Section.

calculation^^^ and experimental data2* suggest a significant influence of counterion geometry on the electronic conductivity of the backbone polymer. Differences in conductivity can also be caused by morphological variations induced by the polymerization medium. This ambiguity could be avoided by monitoring changes in the electrical conductivity of polypyrrole after the dopant anion is exchanged with different anions. Ion exchange also affords the possibility of rapidly changing the counterion composition of the polymer while retaining the same level of partial oxidation or reduction." In this vein, we have used the ac impedance techn i q ~ e *to~ monitor variations in the electrical conductivity of (26) Druy, M. A,; Rubner, M. F.; Sichel, E. K.; Tripathy, S. K.; Emma, T.; Cukor, P. Mol. Cryst. Liq. Cryst. 1984, 105, 109. (27) Bredas, J. L.; Themans, G. B.; Fripiat, J. G.; Andre, J. M.; Chance, R. R. Phys. Rev. E: Condens. Matter 1984, 29, 6761. (28) Glatzhofer, D. T.; Ulanski, J.; Wegner, G. Polymer 1987, 28, 449. (29) We prefer the ac technique to the more standard four-probe dc method because of possible complicating effects from faradaic polarization at the membrane/contact interfaces.

3565

polypyrrole prior to and after anion exchange. Table I11 summarizes the key results. Contrasting with the findings of previous authors (variations as high as lo5 have been reported; cf. ref 12a), the conductivity varies at best in a modest fashion when the tosylate anion is exchanged with other dopant species. Thus, it seems safe to conclude that the large conductivity variations observed in earlier s t ~ d i e s ' are ~ ~ a' ~manifestation ~~~ of either the varying polymer morphology or the degree of oxidation of the membranes synthesized from different electrolytic media. A final point concerns the air stability of the exchanged polypyrrole membranes. It was observed that the exchanged material was more prone to O2 attack on prolonged air exposure as evidenced by a gradual and systematic increase in the membrane resistance with time. Subtle variations in the membrane structure undoubtedly had occurred as a result of the exchange process to render it more unstable relative to the starting material. The results presented herein coupled with the information emerging from other laboratories'-'' are illustrative of the versatility of polypyrrole to act as an anion or cation exchanger depending on its synthetic history.

Acknowledgment. This research was partially supported by grants from the Defense Advanced Research Projects Agency (monitored by the Office of Naval Research) and the Texas Advanced Technology Research Program. Several enlightening discussions with J. B. Schlenoff are also gratefully acknowledged. The authors thank R. D. Goolsby for the SEM data. Registry No. CI', 16887-00-6;OH-, 14280-30-9; CIOL, 14797-73-0; SO>-, 14808-79-8;polypyrrole perchlorate, 82200-25-7;polypyrrole tosylate, 98509-12-7.

Effect of Aggregates in Bulk and Surface Properties. Surface Tension, Foam Stability, and Heat Capacities for 2-Butoxyethanol Watert

+

Francisco Elizalde, Jesus Gracia, and Miguel Costas* Departamento de Ffsica y Qufmica Tedrica, DivisiBn de Ciencias Bcisicas, Facultad de Qufmica. Universidad Nacional Autdnoma de MZxico, MZxico D.F. 04510, MZxico (Received: August 28, 1987; In Final Form: December 2, 1987)

Surface tension and foam stability for 2-butoxyethanol+ water mixtures have been determined as a function of concentration at 4, 25, and 48 "C. In addition, the liquid-liquid coexistence curve has been obtained in the neighborhood of the lower critical solution temperature (LCST). Foam stability was measured with a dynamic foam meter, where nitrogen gas was bubbled at constant rates. Foam column heights display an unusual behavior in that a large peak is observed at concentrations and temperatures far from the liquid-liquid coexistence curve. As the temperature is r e d u d , this peak becomes more pronounced and occurs at higher concentrations. Surface tensions decrease rapidly at low 2-butoxyethanol concentrations and become constant at the same concentrations where foam column heights are maximized. Apparent heat capacities for 2-butoxyethanol reported in the literature show a pronounced peak occurring at approximately the same concentration at which foam heights are maximized and the surface tension becomes constant. The existence of these changes does not seen to have any close relationship with the ability of the solution to unmix at the LCST since the sharpness of the changes becomes largest as the temperature is reduced. The interrelations between bulk- and interfacial-phase behavior are discussed in terms of the presence of 2-butoxyethanol aggregates in solution.

Introduction systematic studies of the thermodynamic properties of nonelectrolytes in water have produced a great deal of information on solutesolvent and solute+solute interactions. These interactions are interesting in view of the high degree of structure in aqueous In memoriam of Gabriel Sanchez Zarza. *To whom correspondence should be addressed at Departamento Fisica y Quimica Teorica, Facultad de Quimica, UNAM, Edificio B. Planta Baja, Cd. Universitaria, Mexico D.F. 045 10, Mexico.

0022-3654/88/2092-3565$01.50/0

solutions, both in the solvent itself and in the aggregates that can exist in them. Amphiphilic molecules, ranging from simple small organic substances'-5 to conventional surfactants,6 have been (1) Kiyohara, 0.;Perron, G.; Desnoyers, J. E. Can. J . Chem. 1975, 53, 2591. ( 2 ) de Visser, C.; Perron, G.; Desnoyers, J. E. Can. J. Chem. 1977,55,856. (3) Roux, G.;Perron, G.; Desnoyers, J. E. J . Solution Chem. 1978, 7, 639. Roux, G.; Perron, G.; Desnoyers, J. E. J . Phys. Chem. 1978, 82, 966. (4) Roux, G.; Roberts, D.; Perron, G.; Desnoyers, J. E. J . Solution Chem. 1980, 9,629.

0 1988 American Chemical Society

3566

The Journal of Physical Chemistry, Vol. 92, No. 12, 1988

studied through the detailed measurement of several bulk properties that include heat capacities, volumes, expansivities and es. Heat capacity data are of special interest since they are known to be very sensitive to structural changes in sol~tion.~,'Among the systems studied, the mixture 2-butoxyethanol (2BE) + water is particularly interesting since its bulk properties undergo drastic changes as a function of both concentration and t e m p e r a t ~ r e for ; ~ example, the apparent molar heat capacity @c passes through a maximum in the water-rich region and then decreases rapidly to the value for pure 2BE. As the temperature is decreased, Le., it moves away from the lower critical solution temperature (LCST), this peak becomes more pronounced and occurs at higher concentrations. The concentration dependence of &, as well as those of the apparent molar volumes &,, expansivities & and isothermal compressibilities &, can be compared with that associated with micellization in the case of surfactank8 The analogous behavior of &, @v, &, and & for micellar systems and the mixture 2BE/water suggests that there is some form of organization or aggregation in the bulk of the solution. The formation of aggregates in the bulk of the solution must be reflected in its surface properties such as the surface tension and transient foam stability. Recently, such influence has been discussed in detail for the mixture phenol + water,9 where changes in bulk properties (molal conductivity) and surface properties (surface tension and foam stability) were attributed to the formation of phenol aggregates in the solution. This aggregation appears to take place on an almost vertical line in the T-X diagram situated around 7-8 wt % phenol, Le., it occurs at the one-liquid water-rich region of the phase diagram located below the upper critical solution temperature (UCST). Since for 2BE + water bulk properties indicate the formation of 2BE aggregate^,^ it appeared interesting to study the surface behavior of this binary mixture. Another surface study involving 2BE has been reportedla for the ternary mixture 2BE water + benzene; here, the effect of 2BE on the lowering of the interfacial tension between water and benzene has been discussed in detail. The present work reports surface tensions and transient foam stabilities as a function of concentration at 4, 25, and 48 OC for 2-butoxyethanol + water mixtures. These measurements, together with those on bulk proper tie^,^.^ provide a picture of the thermodynamics of this system that is discussed in terms of the interrelations between bulkand interfacial-phase behavior.

+

Experimental Section Materials. 2-Butoxyethanol (ethylene glycol monobutyl ether) of 99% purity supplied by Merck Co. was further purified by following the procedure described in ref 11. Through gas chromatography (Varian, column: 10% carbowax 20, chromosore 80/100) a purity of 99.92% was determined. After purification, 2BE was kept under nitrogen and in the absence of light. Water was purified from a dilute KMnO, alkaline solution through distillation in a glass apparatus with a quartz resistance. Surface Tension. The tension of the water-rich/air interface was determined with a Du Nouy (Cenco 70535) tensiometer with temperature control. The measured values were corrected as in ref 12. Foam Stability. A foam meter of the type designed by Ross and Nishioka13 was employed. It consists of a glass cylinder of (5) Lara, J.; Desnoyers, J. E. J . Solution Chem. 1981, 10, 465. (6) de Lisi, R.; Perron, G.; Desnoyers, J. E. Can. J . Chem. 1980, 58, 959. Musbally, G. M.; Perron, G.; Desnoyers, J. E. J . Colloid Interface Sci. 1974, 48, 494. Musbally, G. M.: Perron, G.; Desnoyers, J . E. J . Colloid Interface Sci. 1976, 54, 80. ( 7 ) Costas, M.; Patterson, D. J . Chem. SOC.,Faraday Trans. 1 , 1985,81, 6 3 5 . Costas, M.; Patterson, D. Thermochim. Acta 1987, 210, 161. (8) Desnoyers, J. E.; de Lisi, R.; Perron, G . Pure Appl. Chem. 1980, S 2 , 433. Desnoyers, J . E. Pure Appl. Chem. 1982, 54, 1469. (9) Gracia, J.; Guerrero, C.; Llaiies, J. G.; Robledo, A. J . Phys. Chem. 1986, 90, 1350. (10) Quirion, F.; Desnoyers, J. E. AOSTRA J . Res. 1984, 1 , 121. ( 1 1) P e r m , D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon: New York, 1980. (12) Harkins. W . D.; Jordan, H. F. J . Am. Chem. SOC.1930, 52. 1751

Elizalde et al. 505

50

c

49 5

0 I-

40

o

48 5 011

0 13

Figure 1. Liquid-liquid coexistence curve for 2-butoxyethanol

+ water.

0 01

003

005

007

000

x 2 - butoxyethonol

3-cm interior radius and 35-cm height with a temperature-control jacket. The same initial volume of solution (under constant agitation) was used for all runs; the liquid level and the foam height h were measured with a cathetometer accurate to 0.005 cm. Nitrogen was used as the foaming gas, and its flow rate was controlled with a calibrated rotameter. Measurements of foam height were carried out at a constant flow rate of 3.38 mL s-l. Liquid-Liquid Phase Diagram. The liquid-liquid coexistence curve was carefully determined in the neighborhood of the LCST by using a 20-mL glass cell with a temperature-control jacket. The experimental procedure was as follows: A solution of known concentration (prepared by weight) is introduced into the cell at room temperature and kept under constant agitation. Light from a He-Ne laser (3 mW) is passed through the solution, its intensity being recorded at 180' on a power meter (Spectra Physics). The temperature is then raised at small rates; in the one-liquid region the recorded intensity is constant, dropping abruptly to a minimum when the mixture appears opalescent, i.e. when it separates into two phases. The temperature is then decreased and increased several times at lower rates and within a 0.2 OC interval to determine more precisely the equilibrium temperature, which is recorded by using a temperature probe placed in the interior of the cell. This procedure was repeated for several concentrations, and the resulting liquid-liquid coexistence curve is shown in Figure 1. The lower critical solution temperature was located at a 2BE mole fraction of 0.058 and T = 49.05 & 0.01 "C. Previously reported values for this critical solubility point are as follows: 2BE mol fraction (temperature ( " C ) ) 0.048 (49.1),14 0.054 (48.8),15 and 0.059 (49.02).16 We estimate the accuracies to be the following: f0.3 dyn cm-' for surface tension, f0.5 cm for foam height h, and 10.01 O C for the equilibrium temperatures in the T-X diagram shown in Figure 1 . (13) Ross, S.; Nishioka, G. J . Phys. Chem. 1975, 79, 1561. Ross. S.; Kishioka, G. In Foams; Akers, R. J., Ed.; Academic: New York, 1976; p 17. (14) Cox, H. L.; Cretcher, L. H. J . Am. Chem. Soc. 1926, 48, 451. (15) Ellis, C. M . J . Chem. Educ. 1967, 44, 405. (16) Guzman, F.; Pegg, I.; McLure, I. A. Inz. Data Ser.. Ser. B. 1980, 105.

2-Butoxyethanol

+ Water

The Journal of Physical Chemistry, Vol. 92, No. 12, 1988

3567

Results and Discussion Surface tensions and foam column heights as a function of 2BE mole fraction a t 4,25, and 48 OC are shown in Figure 2. Also in this figure the apparent molar heat capacities bC for 2BE at 4,25, and 40 O C from ref 3 have been included. The apparent heat capacity of 2BE is calculated from the experimentally determined heat capacity of the solution CPl through & = (CPI - X2C,,20)/Xl where XIand X2 are the mole fractions of 2-butoxyethanol and water and Cp,20is the heat capacity of pure water. As such, 4c represents the contribution to the heat capacity of the solution that one would evaluate as due to the solute if the solvent component were to have the properties of the pure state. Comparison between Figures 1 and 2 shows that the dramatic changes displayed on the latter occur at concentrations and temperatures that correspond to the one-liquid phase region located well below the liquid-liquid coexistence curve. Comparison between the temperature and concentration changes observed in Figure 2 suggests immediately that there is a correlation between the behavior of the surface tension, foam stability, and the bulk heat capacities. From the results in Figure 2 three different concentration regions can be distinguished, which at 25 O C would be approximately given by 0 < X,< 0.01, 0.01 < X I < 0.017, and XI > 0.017. The first concentration region begins with pure water and corresponds to the buildup of a layer of 2BE at the waterlair interface. The decrease in surface tension and the moderate increase in foam stability are produced by the development of the dilute adsorbed layer. The surfaces of newly formed bubbles, while inside the solution and as they rise toward the liquid/air interface, are able to capture available 2BE molecules. Some of these bubbles approximate the current interfacial concentration producing the more stable liquid films, this process becoming increasingly efficient as the bulk concentration increases. The stability of the thick liquid films that constitute the foam column above the solution is also increased as 2BE becomes more tightly packed at their surfaces. In this concentration region the apparent heat capacity decreases only slightly, indicating that in the bulk solution there is a random dispersion of 2BE molecules in water. The beginning of the second concentration region is marked by a sudden increase in For many organic liquids in water C#J~ decreases in a monotonic way as the concentration is increased from zero to unity.I' In contrast, for 2BE $c goes through a maximum and then decreases sharply; qualitatively similar changes for have been found for aqueous solutions of tert-butyl alcohol? piperidine, and thiethylamine.'' The heat capacity C, is probably the most sensitive thermodynamic indicator of structure. Structure decreases the entropy S but is broken down as the temperature increases, giving a positive contribution to d S / d T and C,. The heat capacity can then be used as in indicator of structure; measurements of this quantity have proved very useful in discussing the short-range orientational order18 in long n-alkanes, e.g., n-CI6, as well as in studying the self-association of alcohol molecules in inert hydrocarbon solvents.' In the latter case, the rapid increase of alcohol & has been explained as due to the association of alcohol molecules through H bonds, i.e., as due to a structuring process taking place in the solution. Peaks for &, qualitatively similar to those in Figure 2, have been found for typical ionic surfactants such as sodium decanoate and octylamine hydrobromide.6 Here, the rapid increase in c $starts ~ at the critical micelle concentration (CMC), and the peak itself has been identified with the micellization process; micelles do not form instantaneously a t the CMC, but rather smaller aggregates probably form and grow as more surfactant is added until the optimum size is reached. The temperature variation of 4c for these surfactants is analogous to that in Figure 2 for 2BE, Le., the peak in 4c grows and displaces toward higher concentrations as the temperature is decreased. The variation of dC seen in Figure

'\

a

$00

.

'

l

IC1

4.C

1

Ij Y2.E W

'

001

001

0-

OD.

ocd

ow

~

( 1 7) Desnoyers, J. E.; Kiyohara, 0.;Perron, G. Adu. Chem. Ser. 1976,155, 274. de Visser, C.; Perron, G.; Desnoyers, J. E. J . Am. Chem. SOC.1977, 18, 5894. (18) Bhattacharyya, S. N.; Patterson, D. J . Phys. Chem. 1979.83, 2979.

X Z - butoxyelhmol

Figure 2. Surface tension of the water-rich/air interface (a), foam column height (b), and apparent molar heat capacities ( c ) from ref 3 for 2-butoxyethanol + water mixtures.

3568 The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 2 is then a reflection of the association between 2BE molecules, Le., a structural process in which 2BE aggregates are formed; these aggregates can probably be visualized as solvent-shared association complexes similar to clathrates. Light-scattering and Raman and IR ~pectroscopy'~ data have been found to be consistent with the existence of such solvent-shared aggregates, and more recently small-angle neutron-scattering measurements20suggested that 2BE aggregates in water resemble those of pure 2BE. Also, recent measurementsZ1of the velocity of sound in 2BE water mixtures in the same concentration region displayed in Figure 2 showed maxima in ultrasonic velocity and minima in adiabatic compressibility; in accordance with the results in Figure 2, a shift of the velocity maximum toward lower concentrations of 2BE was observed with increase of temperature, all these changes being again attributed to the formation of clathratelike structures of water and 2BE. Simultaneous with the rapid increase of dc in Figure 2, the surface tension decreases moderately as compared with the drastic drop in the first concentration region, and foam stability continues to increase. In this concentration region the adsorbed layer of 2BE at the water/air interface continues to develop, and the further increase of foam stability can be explained by the same arguments used above in regard to the first concentration region. Here, however, there must be a competition between aggregate formation and migration to the air/liquid interface. Free or monomeric 2BE molecules can either cluster together, forming aggregates, or migrate to the air/liquid interface. The third concentration region is characterized by the constancy of the surface tension, the decrease of dc, and a complete reversal in the behavior of foam stability. The constancy of the surface tension seems to indicate that the adsorbed layer has become compact and that it maintains a fixed composition independent of the bulk concentration. Surface tension data in Figure 2 were fitted by the monolayer isotherm of Szyszkowski-Langmuir,22 which indicated that at the onset of this concentration region the surface concentration is practically that of saturation, e.g., at 25 OC and X, = 0.0163 the surface is 98.4% covered with 2BE molecules. Foam films become less stable since now reductions of their surface area are not hindered by the unfavorable transfer of segregated 2BE to the bulk. Adsorbed layers can be accommodated through aggregation producing the gradual abatement of foam stability. The sharp decrease in dChas been interpreted as signaling the end of the passage to a solution that although macroscopically appearing as a normal solution is really composed of microphases coexisting in the bulk of the s ~ l u t i o n . ~This suggestion was made on the basis of the concentration behavior of the partial molar heat capacity (calculated from dc),which shows a nearly discontinuous very pronounced peak at the same concentration where dc is a maximum; the shape of cpversus X, is then approaching that expected for a phase tran~ition.~ Beyond the concentration at which the maximum occurs, dC rapidly reaches the molar value for pure 2BE (see inset of Figure 2c), indicating that 2BE is only seeing other 2BE molecules. A similar drop toward the values for pure 2BE was observed for the apparent expansivities, volumes, and compre~sibilities.~~~ It appears then that in this concentration region the bulk of the solution is characterized by the existence of some structures where 2BE molecules are held together. For surfactant systems where sizeable

+

cp

(19) Ito, N.; Fujiyama, T.; Udagawa, Y . Bull. Chem. SOC.Jpn. 1983, 56, 379. Ito, N.; Saito, K.;Kato, T.; Fujiyama, T. Bull. Chem. SOC.Jpn. 1981, 54. 99 1 . (20) Quirion, F.; Magid, L. J. J. Phys. Chem. 1986, 90, 5193. (21) Rao, N . P.; Verrall, R. E. Can. J. Chem. 1987, 65, 810. (22) Szyszkowski, B. Z Phys. Chem. 1908, 64, 385

Elizalde et al. structures such as micelles are present in the bulk of the solution, it is e.g., aqueous solutions of sodium alkylbenzenes~lfonates,~~ found that (1) as the concentration of surfactant is increased, the surface tension drops rapidly, and foam stability increases and (2) when the surface tension reaches a constant value, Le., in the neighborhood of the cmc, the foam height reaches a maximum. With further increase of surfactant concentration, foam heights remain constant. Similarities between the behavior of conventional surfactants and 2BE suggest that 2BE acts as a small-size surfactant; while in the former, micelles are the organizations that allow the solution to accommodate more surfactant molecules once the surface has been saturated, aqueous 2BE mixtures reach the same goal through the formation of 2BE aggregates. These aggregates must result from a delicate balance between the different interactions taking place in solution:24 self-association of 2BE molecules through H bonding via their hydroxyl group, H bonding between 2BE and water via the ether oxygen and the hydroxyl group on 2BE, interactions between the hydrophobic chain of 2BE and water as well as between those chains, and finally, water-water interactions. An indication of how significant small changes can be in the balance between all these interactions is given by the fact that for lower n-alkoxyethanols, Le., methoxy-, ethoxy-, and propoxyethanol, the apparent heat capacity decreases monotonically with concentration, indicating the absence of aggregate~.~ The concentration and temperature dependences displayed in Figure 2 do not seem to have any close relationship with the ability of the solution to unmix at ca. 49 OC since the sharpness of the changes becomes largest as the temperature is reduced. A foam stability concentration dependence similar to that in Figure 2 has also been found for phenol + water mixtures9 Although not reported in ref 9, the temperature dependence of phenol/water foam stability is analogous to that for 2BE/water, i.e., the magnitude of the foam stability peak increases as the temperature is decreased. However, for phenol/water this peak occurs at approximately the same concentration (7-8 wt %) between 25 and 60 OC. Maxima on foam stability have been reportedI3 and modeled25 near the UCST in binary mixtures, e.g., diisobutylcarbinol water, and near the plait point in ternary mixtures such as ethylene glycol-butanol-water at 20 OC. By contrast, as it was previously reported,13 at temperatures and concentrations close to the LCST for 2BE water, solutions do not foam. The occurrence of sharp peaks on foam stability for phenol and 2BE aqueous solutions is unusual in the sense that they have been observed at concentrations and temperatures removed from the vicinity of the critical solution points; a similar behavior has been It appears recently found for methyl acetate + ethylene then that increasing foaminess is not necessarily linked to the proximity to the critical solution point (UCST or LCST) of the mixture. The existence of some form of aggregation in the solution can explain, as discussed above, the behavior not only of the heat capacity but also of foam stability.

+

+

Acknowledgment. We thank A. Robledo, C. Varea, and M. E. Costas for valuable discussions. This work was supported in part by Consejo Nacional de Ciencia y Tecnologia de Mexico. Registry No. 2BE, 11 1-76-2. (23) Gray, F. W.; Gerecht, F.; Krems, I. J. J. Org. Chem. 1955, 20, 5 1 1. Rosen, M. J. In Surfactants and Interfacial Phenomena; Wiley-Interscience: New York, 1978; Chapter 7. (24) Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 226. Mukerjee, P. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 931. (25) Ross, S.; Townsend, D.F. Chem. Eng. Commun. 1981, 1 1 , 347. (26) Guzman, F.; private communication.