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Langmuir 1986,2, 284-287
Surface Activity of the System Poly(propy1ene glycol) Water
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David F. Townsend and Sydney ROSS* Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 Received November 20, 1985 The relation of the surface activity of poly(propy1ene glycol) in water to the phase diagram of the binary system is demonstrated by the foaminess of the solutions, which tend toward a maximum at an epicenter (30 OC, 0.016 M)located on the coexistence curve. Dynamic surface tensions measured through a range of concentrations and temperatures close to the epicenter show behavior approximating that of an insoluble surface monolayer, namely, extremely slow rates of adsorption out of solution and of desorption into solution. The lower consolute point of the system is reflected in a positive temperature coefficient of the rate of adsorption at temperatures below that of the epicenter.
Introduction The work of Ross and Nishioka1p2 on the foaming behavior of partially miscible liquids as related to their phase diagrams demonstrates increasing foaminess of the unsaturated solutions as they approach in composition the coexistence curve. When the foaminess is reported as isuphroic contours (lines of the same foam stability) on a phase diagram, the contour lines are seen to circulate about an “epicenter” of maximum foam stability: namely, a point on the coexistence curve that is the focus of the isaphroic contours and from which they continuously decline in value in the homogeneous phase the farther removed they are from it. The isaphroic epicenter occurs a t a concentration that, with respect to the consolute or critical point, is more dilute in the component of lower surface tension. An indicator of surface activity less complex than the foam stability of a solution is the Gibbs’ excess concentration of adsorbed solute, obtained by use of Gibbs’ adsorption equation. When this property is determined a t various concentrations and temperatures from surfacetension isotherms and is reported in the form of cosorption contours (lines of the same surface activity) on the phase diagram of two partially miscible liquids, the behavior is observed to be similar to that of the isaphroic contours: the cosorption lines also circulate about an epicenter of maximum surface activity, located a t a point on the coexistence curve that is removed from the critical point in a direction toward the component of higher surface ten~ion.~ The two epicenters do not necessarily coincide as foam stabilities are affected by the bulk viscosity of the solution, which does not affect adsorption at equilibrium. In solutions of low viscosity the two epicenters tend to coincide. These findings agree in some respects with a theoretical analysis by Cahn,4 in which regimes with different degrees of surface activity are related to positions on the phase diagram. Figure 1is a copy of phase behavior according to Cahn’s theory with respect to perfect wetting of a third phase by one of two partially miscible liquids. The important item of agreement between this theory and independent observations of the isaphroic and cosorption lines is in the predicted existence and position of a point of perfect wetting on the coexistence curve. The predicted (1) Roes, S.; Nishioka, G. M. In “Foams”,Akers, R.J., Ed.; Academic Press: London, 1976; Chapter 1. (2) Ross, S.; Nishioka, G. M. Chem. Ind. (London) 1981,47. ( 3 ) Nishioka, G. M.; Lacy, L. L.; Facemire, B. R.J. Colloid Interface Sci. 1981, 80, 197. (4) Cahn, J. W. J. Chen. Phys. 1977,66, 3667.
0743-7463/86/2402-0284$01.50/0
point of perfect wetting is readily assimilated to the observed epicenters of maximum foaminess or maximum surface activity. But neither the observed isaphroic nor the cosorption curves display any discontinuity in the one-phase region, as would be expected from the locus of first-order transitions shown in Figure 1, extending from the point of perfect wetting into the homogeneous phase. Furthermore, an experimental investigation by Ross and Kornbrekke5 of the contact angles against glass of conjugate solutions near their critical point does not confirm the theoretical prediction that perfect wetting continues all the way from ita inception to the critical point. Instead, perfect wetting, where it is observed, for it is not found in all systems that have a miscibility gap, holds only a t one point on the coexistence curve: a point that also complies with the same conditions that hold for the placement of the epicenters disclosed by plotting the isaphroic and cosorption contours. These consistent deviations of observations from theory have not yet been addressed by theorists. The present study of isaphroic contours investigates, for the first time, a two-component system with a lower consolute point, namely, the system poly(propy1ene glycol) water. An epicenter of maximum foam stability, which in these dilute solutions of low viscosity probably corresponds closely to the epicenter of maximum surface activity, is located on the coexistence curve. Surface-tension isotherms measured under conditions of temperature and concentration corresponding to the one-phase region of the phase diagram in the vicinity of the foam epicenter do not show any discontinuities, although sloping inflections reminiscent of supracritical isotherms are observed. Further evidence of the surface activity of solutions represented by this region of the phase diagram is given by measurements of dynamic surface tension. These data reflect differences in the relative rates of adsorption and desorption for the system poly(propy1ene glycol) + water, when interpreted in the light of the previous work with solutions of 1-butanol in water.6
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Materials Distilled water obtained from a general supply was redistilled from alkaline permanganate into phosphoric acid and distilled once again into a quartz container. Poly(propylene glycol) (MW = lo00 g/mol; Chemical Abstracts name, a-hydro-w-hydroxypoly[ oxy( methyl- 1,2ethanediyl)]) was obtained from Aldrich Chemical Co. and was used as is. (5)Ross,S.; Kombrekke, R. E. J. Colloid Interface Sci. 1984,99, 446. (6) Townsend, D. F.; Ross,S., submitted for publication in Langmuir.
0 1986 American Chemical Society
Surface Actiuity of Poly(propy1ene glycol) + Water
Langmuir, Vol. 2, No. 3, 1986 285
7 Surfoce criticol pan1 bulk
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.
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composition Figum 1. Cahn's prediction of propertiea of the interface between two immiscible phases, as a function of temperature and composition. Perfect wetting occurs only when both fluid phases a and /3 are stable and close to their critical point. The first-order transition extends into the single-phase region and is supposed to terminate in a surface critical point.
Experimental Section The phase diagram of the syatem poly(propy1eneglycol) + water was determined by visual observation of cloud points. Solutions were made volumetrically and placed in a test tube with a magnetic stirring bar and a thermistor. A hot-water bath and an ice bath were used to adjust temperatures. A mixture was either allowed to cool until the liquid cleared or it was warmed up until it became turbid. The mixture was observed by means of a Tyndall beam, with occasional stirring until the cloud point was reached. By approaching the cloud point in both directions, an endpoint was found to within 1 O C . The solutions of poly(propy1eneglycol) in water that were used to determine the phase diagram were prepared by weighing the polymer into a 100-mL volumetric flask. Other solutions were prepared by weighing the polymer into a 2-L flask to form a 0.10 M stock solution, which was then diluted volumetrically. Dynamic foam stabilities were measured by the technique described by Ross and Suzin.' The foam vessel was made by fusing a 60° funnel to a graduated cylinder of 4.6-cm diameter. A water bath regulated the temperature. Foam heights were measured with a cathetometer. Humidified nitrogen gas was used to create foam. Foam Stabilities were calculated by converting steady-statefoam height (defined as the total volume, liquid plus gas, minus the original volume of liquid) to the area of the cross section of the cone at that foam height. The value of the area is raised to the three-halves powerand plotted against the flow rate of gas. The central region of the resulting sigmoidal curve is linear and ita slope, which has the unite of seconds, is called Z after the unit of foaminess suggested by Bikerman: which equals the average lifetime of a bubble in dynamic foam. Dynamic surface tensions of aqueous solutions of poly(propylene glycol) were measured by using a Wilhelmy plate in contact with an area of the solution surface trapped within the walls of a funnel of rectangular cross section. The descent and ascent of the funnel into the solution causes the area of the trapped surface to contract and extend, respectively. A detailed account of the apparatus is given elsewhere? The surface tension of a pure solvent does not change during this process; but the presence of an adsorbed layer of solute produces a surface-tensionhysteresis (7) b, S.;Suzin, Y.Langmuir 1986, 1, 145. (8)Bikerman, J. J. Trans. Faraday SOC.1938,34,634. (9) Townsend D. F.; Bock, E. J. submitted for publication in Langmurr.
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Figure 2. The variation of (area)3/2with the flow rate of gas for solutions of poly(propy1ene glycol) in water at the following conditions: ( 0 )0.0086 M and (X) 0.0037 M at 20.0 O C ; 0.00205 M at ( 0 ) 20.0, ( 0 ) 25.2, ( 0 ) 30.4, (0) 35.0, and (m) 40.0 "C.
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Figure 3. Phase diagram and interpolated isaphroic contours of the two-component system poly(propy1ene glycol) and water. loop on a contraction-extension cycle of the area. The temperature was monitored by placing a thermistor at the surface outside the contained area but as near to the Wilhelmy plate as possible. Static surface tensions were measured with the same apparatus as used to measure dynamic surface tensions; a jacketed beaker was substituted for the funnel and tank. Equilibrium was assumed to be established when no further change of surface tension occurred as the surface aged on standing. The time required for the surfaces of these solutions to age is often over an hour.
Results Examples of foam curves, showing the value of area to the three-halves power vs. the flow rate of gas, are given in Figure 2. Foam stabilities, calculated from the slope of the linear part of each curve, were interpolated to form the isaphroic contours on the phase diagram in Figure 3. A portion of the phase diagram of the system poly(propylene glycol) water is shown in Figure 3 in terms of the logarithm of concentration vs. temperature. This system has a lower consolute point, so the homogeneous regime
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286 Langmuir, Vol. 2, No. 3, 1986
Townsend and Ross
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CONCENTRATION (mole/liter)
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Figure 4. Variation of the equilibrium surface tension with the logarithm of concentration for poly(propy1ene glycol) in water at 10.0, 15.0, 20.0, 25.0, and 30.0 "C.
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CONTRACTION-EXTENSION AREA (cm')
Figure 6. Variation of dynamic surface tension with area for a 0.00205 M solution of poly(propy1ene glycol) in water at 10.0 "C and a cycling frequency of 0.20 cpm. 49
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49,
CONTRACTION-EXTENSION AREA (cm'l
Figure 5. Variation of dynamic surface tension with area for a 0.00205 M solution of poly(propy1ene glycol) in water at 10.0 O C and a cycling frequency of 1.02 cpm. lies below the coexistence curve. The isaphroic contours circulate around an epicenter (30 OC, 0.016 M) and decrease in value the farther they are from it. Although only this one system with a lower consolute point has been investigated in depth, the same type of variations of surface activity with concentration and temperature (as revealed by the isaphroic curves) is probable for any system characterized by the same phase behavior. Surface-tension isotherms are shown in Figure 4 for comparison with the phase diagram. The isotherms show inflections that are relatively sharp near the phase boundary; they diminish gradually with decreasing temperature. These sloping inflections occur at concentrations and temperatures near those of the epicenter and near the isaphroic contour of Z = 40 s. Examples of the variation of surface tension on extension-contraction cycling of surface area a t 1.02,0.20, and 0.05 cycle/min for a 0.0021 M solution at 10 O C are shown in Figures 5, 6, and 7, respectively. Figure 5 shows an initial compression, which is followed by successively larger dilatations in the course of which solute is gradually pumped out of the surface, more and more slowly, until a nearly flat steady-state hysteresis loop is reached, which lies above the equilibrium surface tension. When the cycling is done more slowly, the steady-state is a hysteresis
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CONTRACTION-EXTENSION AREA (cm')
Figure 7. Variation of dynamic surface tension with area for a 0.00205 M solution of poly(propy1ene glycol) in water at 10.0 "C and a cycling frequency of 0.05 cpm. loop lying partly above and partly below the equilibrium surface tension (see Figure 6). On still slower cycling, the steady state becomes a hysteresis loop lying almost entirely below equilibrium surface tension (see Figure 7); that is, the solute is now pumped into the surface. The steadystate loop is reached within three cycles. These differences in the direction of movement of the solute on continued cycling are entirely due to varying the cycling frequency and are not due to differences of concentration; the results can be interpreted as a consequence of slow rates of adsorption and desorption of the polymer (see below for the explanation). The values of Au corresponding to surface area changes a t 0.20 cycle/min are plotted in Figure 8 as dynamic isotherms, separated into compressional and dilatational components. The difference between the equilibrium surface tension and the minimum in the steady-state hysteresis loop is taken as a compressional component to Ao, and the difference between the maximum in the steady-state hysteresis loop and the equilibrium surface tension is taken as a dilatational component; these are then plotted against concentration for 10, 15, 20, and 25 "C isotherms. Compression of the surface layer is less pronounced, while dilatation is more pronounced, a t higher
Surface Activity of Poly(propy1ene glycol)
+ Water
Langmuir, Vol. 2, No. 3, 1986 287
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Figure 8. Variation of the compressional component of the dynamic surface tension (uq - umi0 ( 0 )and the dilatational component of the dynamic surface tension (u- - uq) (O),with the concentration of poly(propy1ene glycol) in water at (a) 10.0, (b) 15.0, (c) 20.0, and (d) 25.0 "C. Cycling frequency of 0.20 cpm. values of the equilibrium surface concentration. Increasing the temperature has little effect on the compressional components in Figure 8, but the dilatational component decreases.
Discussion Three aspects of surface activity have been shown to occur in binary solutions that separate into two immiscible conjugate phases, namely, the existence of a point of perfect wetting and the existence of epicenters of the isaphroic and cosorption contours. The isaphroic contours of the system poly(propy1ene glycol) water show that a binary system with a lower consolute point exhibits the same behavior, mutatis mutandis, as systems with upper consolute points. Equilibrium surface-tension isotherms, shown in Figure 4, which were measured over the same range of concentration and temperature as the isaphroic contours, have inflections that are reminiscent of supracritical pressure vs. volume isotherms of gases. Gas p-V isotherms have a discontinuity as they cross the coexistence curve, indicating that a first-order transition is taking place. In the supracritical region, the dicontinuity vanishes; yet a "memory" of the first-order transition lingers in terms of sloping inflections, or bends in the isotherms. The bends are well defined near the critical temperature and gradually fade the farther the isotherm is removed from it. The surface-tension isotherms reported in Figure 4 likewise show inflections that are sharper near the temperature of the epicenter of the isaphroic contours and fade the farther they are from it. According to this view, the epicenter is the critical point of surface activity, and the homogeneous phase near it is a supracritical region. The dynamic surface tensions of this system have the unusual feature that the cycling frequency determines whether the adsorbed film is compressed or dilatated. Rapid cycling produces dilatations on extending the surface; but as the rate of cycling is slowed more and more, compression is observed at the contraction end of the cycle. With solutes of low molecular weight, such as 1-butanol in water, compression of the surface film on contraction
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of area is a feature of a relatively low concentration of solute in the surface; dilatation on extension of area is a feature of a relatively high concentration of solute in the surface; and the rate of cycling, within the range amenable to our control, hardly affects this aspect of behavior. That a concentrated surface layer should dilatate on extension of area is interpreted as caused by a rate of adsorption too slow to keep up with the rate a t which the area is being extended; that it should fail to compress on contraction of area indicates that the rate of desorption is rapid enough to allow surface molecules to evade being crowded together on contraction of area: they can escape by desorption. That is, in a concentrated aqueous solution of 1-butanol, the rate of desorption is more rapid than the rate of adsorption. The behavior of a concentrated solution of poly(propy1ene glycol) differs in that, even a t the slowest rate of cycling available to us, both dilatation and compression persist. Now dilatation depends on a slow rate of adsorption of solute, and compression depends on a slow rate of desorption of adsorbate, both relative to the mechanical rates of extension or contraction of surface area. The behavior of the aqueous solution of poly(propy1ene glycol) indicates rates of adsorption and desorption that are slower than the imposed rates of surface extension and contraction. An insoluble monolayer has practically zero rates of adsorption and desorption, and the aqueous solution of poly(propy1ene glycol) forms an adsorbed layer that behaves much like an insoluble monolayer. We have observed the same behavior with solutions of poly(dimethylsiloxane) in tmpheptanoate.1° The behavior may well be characteristic of a soluble adsorbed polymer. Poor compressibility of an adsorbed film is attributed in the system 1-butanol + water to a concentrated surface layer, and that seems a reasonable supposition in the system poly(propy1ene glycol) water, which is borne out by the low equilibrium surface tension. This trend in the surface concentration promotes dilatations on extensions of the surface area of the solution, as seen in the dilatational components in Figure 8. The magnitude of these dilatations decreases with increasing temperature, showing that either the rates of adsorption are increasing or the rates of desorption are decreasing or both. The compressional components, however, are relatively unaffected by the change in temperature, so we conclude that the rates of adsorption are increasing with temperature. Normally the rate of adsorption of a surface-active solute decreases with increasing temperature; but in this system the increase of temperature brings the solution closer to the phase boundary and, so, to a condition of greater surface activity. Thus the apparent anomaly is not anomalous. The increase in the rate of adsorption has an adverse effect on the attempt to correlate Au directly with foaminess. A good correlation is obtained at low temperatures, indicating that the time scale of the dynamic-surface-tension measurement is of the order of that for the foam-stability measurements, but the increase in the rate of adsorption of the polymer at temperatures above 50 "C is faster than the maximum rate of area change attainable by the apparatus, and so, the values of Aa obtained by the use of the apparatus do not correlate as well with the foam stability of aqueous solutions of poly(propy1ene glycol) as they do for the system 1-butanol + water. Registry No. Poly(propy1ene glycol), 25322-69-4.
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(10) Ross, S. 'Lubricant Foaming and Aeration Study, Part l", AFWAL-TR-84-2001, Aero Propulsion Laboratory, Air Force Systems Command, Wright-Patterson Air Force Base, OH 45433; 1984.
