Langmuir 1995,11,885-888
885
Relaxation Kinetics of an Expanded Oil-Water Interface in the Presence of Oil-Soluble Polyethers Patrick J. Breen Baker Performance Chemicals, Inc., 3920 Essex Lane, Houston, Texas 77027 Received June 27, 1994. I n Final Form: December 5, 1994@ Relaxation kinetics were recorded for a homologous series of polypropylene glycols and triols upon rapid expansion of a water droplet suspended in a heptane or toluene solution. The polypropylene oxide (PPO) and polypropylene oxide-polyethylene oxide polymers (PPO-PEO) used are oil-solubleand are prototypical of polymers used in industry to demulsify crude oil. Relaxation data were exponential in nature. This and the time frames of the relaxations are indicative of a non-diffusion-controlledrelaxation mechanism at the interface. The effect of changing from an aliphatic solvent (heptane)to an aromatic solvent (toluene) on relaxation rate was found to be minimal for the PPO-type polyethers,but significantwhen hydrophilic polyethylene oxide (PEO) segments were present in the structure. Increasing the percentages of PEO resulted in slower relaxation kinetics, possibly due to intra- or intermolecular associations of the PEO segments in the oil phase. This data suggeststhat for light, waxy crude oil emulsions the best performance should be from demulsifiers with minimal PEO content, consistent with empirical observation.
Introduction Polymeric surfactants derived from reacting propylene oxide and ethylene oxide with various compounds containing multiple hydroxyl functionalities, such as propylene glycol, glycerine, and sucrose have been used in the oilfield industry for years to demulsify water-in-crude oil emulsions. Much has been written concerning the specific chemical mechanisms whereby this separation is often addressing the behavior of such polymers at an oil-brine interface or in thin films. What has generally been ascertained is that the demulsification process involves the displacement of emulsifying entities such as asphaltenes, natural surfactants, or solids at the interface by a surface-active demulsifier which then also causes changes in the rheological properties of the interface. Such changes favor rapid coalescence between converging droplets of dispersed aqueous phase by increasing the rate at which the continuous oil phase can drain. More specifically, as dispersed phase water droplets approach each other and flatten to form a thin film of continuous oil phase between them, the outward drainage flow of the film can create gradients in interfacial tension which then oppose and slow such drainage (MarangoniGibbs e f f e ~ t ) . ~The , ~ presence of a demulsifier in the continuous phase, which rapidly adsorbs to prevent the formation of gradients at the interface, speeds drainage and coale~cence.~ In effect, the demulsifier serves to impart a lower elasticity, or dynamic film modulus, to the interface by minimizing changes in interfacial tension, ha, when the interface is expanded or stretched. Thus, Abstract published in Advance ACS Abstracts, February 15, 1995. (1)Krawczyk, M. A.; Wasan, D. T.; Shetty, C. S. Znd. Eng. Chem. Res. 1991,30,367. (2)Berger, P. D.;Hsu, C.; Arendell, J. P. Proceedings SPE International Symposium on Oilfield Chemistry, 1985,Paper 16285. (3)Thompson, D. G.; Taylor, A. S.; Graham, D. E. Colloids Surf. 1986,15,175. (4)Jones, T. J.; Neustadter, E. L.; Whittingham, K. P. J. Can. Pet. Technol. 1978,April-June, 100. (5)Sjoblom, J.;Soderlund, H.; Lindblad, S.;Johansen, E. J.; Skjarvo, I. M. Colloid Polym. Sci. 1990,268,389. (6)Shetty, C. S.; Nikolov, A. D.; Wasan, D. T. J . Disp. Sci. Technol. 1992,13,121. (7)Wasan, D.T.; Shah, S. M.; Aderangi, N.; Chan, M. S.;McNamara, J. J . SOC.Pet. Eng. J . 1978,18, 409. (8) Zapryanov, Z; Malhotra, A. K.; Aderangi, N.; Wasan, D. T. Znt. J. Multiphase Flow 1983,9 , 105. (9)Malhotra, A. K.; Wasan, D. T. Chem. Eng. Commun. 1987,55, 95. @
the kinetics of adsorption and the resultant dynamic elasticity of the interface are prime factors to consider in attempting to understand and predict demulsifier performance. Much work has been published concerning the relaxation kinetics of aqueous solutions of various alcohols and polyethylene glycols (PEG).1°-17 Very little work has been devoted to examining the adsorption behavior of such oilsolublepolymers as polypropyleneglycolsortriols to wateroil interfaces, although Wasan et a1.lahave recently been active in this area. Work has also been published on some oil-soluble surfactants such as cholester01~~ and some organic diols.20,21In these cases adsorption was found to be non-diffusion-controlled. In this work, the overall relaxation kinetics of an expanded oil-water interface in the presence of various polypropylene oxide (PPO) and PPO-polyethylene oxide (PEO) polymers are examined as a function of several variables, including molecular weight, ethylene oxide content, and organic solvent used. Wasan et al. have demonstrated that low dynamic interfacial elasticity is important for fast demulsification.1ap22It is the object of this study to go the next step and begin to examine the effects of specific structural variation on the behavior of demulsifying-type polymers at water-oil interfaces.
Experimental Section Organic oil phases were HPLC-grade toluene and heptane, purchased from Aldrich, and used without further purification. Water was ultrapurified using a Barnstead Nanopure ultrapure (10)Van Hunsel, J.; Bleys, G.; Joos, P. J.Colloid Interface Sci. 1988, 114,432. (11)Lucassen, J.; Giles, D. J. Chem. Soc., Faraday Trans. 1 1976, 71,217. (12)Nakamura, M.;Takeuchi, S. Bull. Chem. Soc. Jpn. 1978,51, 2776. (13)Lunkenheimer K.;Serrien, G.; Joos, P. J.Colloid Interface Sci. 1990,134,407. (14)Hansen, R. S. J. Colloid Sci. 1961,16,549. (15)Sauer, B. B.; Yu, H. Macromolecules 1989,22,786. (16)Van Hunsel, J.; Joos, P. Colloids S u r f . 1987,25,251. (17)Tsonopoulos, C.; Newman, J.;Prausnitz, J. M. Chem. Eng. Sci. 1971,26,817. (18)Kim, Y. H.; Wasan, D. T.; Breen, P. J. Colloids Surf. In press. (19)Van Hunsel, J.;Bleys, G.; Joos, P. J.Colloid Interface Sci. 1986, 114,432. (20)Joos, P.;Bleys, G.; Petre, G. J. Chim. Phys. 1982,79, 387. (21)Bleys, G.;Joos, P. J. Phys. Chem. 1986,89,1027. (22)Edwards, D.A,; Breener, H.; Wasan, D. T. Interfacial Transport Processes and Rheology; Butterworth-Heinemann: Stoneham, MA, 1991.
Q743-7463/95/2411-08~5~Q9.QQlQ 0 1995 American Chemical Society
886 Langmuir, Vol. 11, No. 3, 1995
Breen
Transducer Conc.=lxlO.'M
A = 400 MW Diol B = 4,000 MW Diol C = 15,000 MW Diol
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0.6
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-
0
0
20
40
60
80
100
120
140
Time, sec Figure 2. Relaxation kinetics and exponential fits for polypropylene glycols of varying molecular weight, at 1 x M in heptane at 24 "C.
Figure 1. Expanded drop apparatus.
water system. Polypropylene glycols and triols were obtained from Arc0 Chemicals and Baker Performance Chemicals. Polypropylene polyethylene glycols were obtained from BASF. I All glass surfaces were cleaned by soaking in fresh aqua regia followed by extensive rinsing with ultrapure water. Relaxation kinetics were measured using an expanded drop tensiometer, similar to others described in the l i t e r a t ~ r e , ~ ~ * ~ ~0.8 B = 3,000 MW Triol depicted in Figure 1. A low-pressure differential pressure C = 9,000 MW Triol transducer (Micro Switch) with an operatingrange of 0-0.18 psi g 0.6 was connected with a plastic tee to a 25 pL gastight syringe on one end and a glass capillary, with an inner diameter of 0.45 mm 2 and a wall thickness of 3.09 mm, on the other end. The syringe, tee, capillary, and transducer cell were filled with pure water. 0.4 The transducer was interfaced to a computer for easy data aquisition. The tip of the capillary was immersed in the organic phase and allowed to equilibrate for several minutes. Data 0.2 collection was then initiated and the plunger ofthe syringe quickly depressed to create a roughly hemispherical droplet at the tip of the capillary. This sudden expansion to the maximum radius 0 of curvature results in a sudden, sharp increase in the internal 0 20 40 60 80 100 120 140 droplet pressure in the blank sample (pure liquids), with no subsequent changes. This control experiment was conducted Time, sec prior to every measurement to ensure that no contamination Figure 3. Relaxation kinetics and exponential fits for polyprowas present; if surface active contaminants were present in the pylene triols of varying molecular weight, at 1 x M in water or organic phases, they would adsorb to the freshly formed heptane at 24 "C. interface and cause the interfacial tension, and detected droplet pressure, to decrease over time. Plots of static interfacial tension as a function of demulsifier It was observed early on that it was extremely difficult to concentration were prepared using a Fisher semiautomated du obtain a stable drop at the tip of the capillary, since the drop Nouy ring tensiometer. The surfactant was added to 50 mL of tended to slowly spread across the face of the capillary. This heptane in a 100 mL graduated cylinder to which 35 mL ofwater problem was solved by soaking the capillary tip for several hours was added. The cylinder was capped and shaken, and the in a 50/50 mixture of chlorotrimethylsilane and 1,1,1,3,3,3contents were poured into an 8 cm dish, which was covered, and hexamethyldisilazane, which made the glass surface hydrophobic allowed to equilibrate overnight, whereupon the interfacial and effectively prevented spreading. With such treated tips, tension was measured. perfectly steady, unchanging interfacial tensions were observed after expansion of the droplet using the pure phases (control Results and Discussion experiment). Representative plots of AulAo,, as a function of time Adsorption kinetics were measured for each demulsifier by following a sudden expansion are presented in Figures 2 adding, via microliter syringe, a set amount ofdemulsifier directly to the oil phase, which was then stirred several minutes with a and 3, for a series of polypropylene glycols and polypromagnetic stirrer (also cleaned with aqua regia). The demulsifiers pylene triols, respectively, at 1 x M, in heptane at 24 were added from stock solutions (1000 ppm) in toluene. The "C. The solid lines in these figures show t h e good fit of capillary was then lowered into the oil phase, allowed to t h e data to single exponential decay. For a diffusionequilibrate for several minutes, and expanded. In most cases controlled process, the dynamic interfacial tension should the resultant data could be easily fit to an exponential, allowing vary with time according to eq 1:25 the calculation of a rate constant to describe the approximate rate of relaxation. For display purposes the data was plotted as a function of AulAu, vs time, where A u is the difference in the A&) = Auo e x p - erfc dynamic interfacial tension at time t and at equilibrium, and Ao0 is the difference in the dynamic interfacial tension at initial time t = 0 and at equilibrium. where A&) is the difference between the interfacial tension at time t a n d at equilibrium, Ao,, is t h e difference (23) Clint, J. H.; Neustadter, E. I.; Jones, T. J. Deu. Pet. Sci. 1981, between t h e interfacial tension at time t = 0 and at 13, 135. (24) Nagarajan, R.;Wasan, D. T. J.Colloid Interface Sci. 1993,159, 164. (25) Joos, P.; Bleys, G. Colloid Polym. Sci. 1983,261,1038.
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Relaxation Kinetics in the Presence of Polyethers
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15,000MW Diol
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In C Figure 4. Static interfacial tension of a water-heptane interface as a function of In C for PPO diols, at 24 "C.The slope of the lines was used to calculate the adsorption, r. Table 1. Absorption and Calculated Diffusion Relaxation Times for Diols sample r, mol/cm2 t , SQ 400 MW diol 2.9 x 6.6 x 2000 MW diol 9.0 x 10-11 6.4 10-3 15 000 MW diol 11.3 x 1.3 x Diffusion coefficient = 1 x
equilibrium, and defined by eq 2,
t~
cm2/s.
is the diffusion relaxation time as
where D is the diffusion coeficient, r the adsorption, and C the bulk concentration. This expression not only fails to describe the relaxation data in Figures 2 and 3 but also the relaxation data obtained in toluene and for polyethers containing hydrophilic blocks of polyethylene oxide. As mentioned earlier, non-diffusion-controlled adsorption has been reported for other oil-soluble surfactants adsorbing to an expanded oil-water interface, so a nondiffusion-controlled relaxation is not altogether unexpected. To further bolster such a conclusion, the total time frame for the relaxation in a diffusion-controlled situation can be e ~ t i m a t e d lfrom ~ , ~ the ~ first term of Ward and Tordai's equation26 describing the adsorption of a surfactant to an expanded interface. The first term of this expression describes the inward diffusive flow of the surfactant and is presented as eq 3:
(3) Using the Gibbs equation (eq 4), values for r can be obtained from the slope of a plot of the static interfacial tension as a function of In C . (4)
Plots of static interfacial tension as a function of In C for three of the diols are presented in Figure 4. Values for r obtained from these plots are listed in Table 1.By estimating the diffusion coefficient to be on the order of 1x cm2/s,on the basis of reported valuesl1J5for PEO polyols, and taking the measured values of r listed in (26) Ward, A. F. H.; Tordai, L. J. Chem. Phys. 1946, 14, 453.
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,
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Concentration, M x l o 7 Figure 5. Relaxation lifetime as a function of bulk concentration for the 15 000 MW PPO diol.
Table 1, estimates of the approximate time for the relaxation were calculated and are also listed in Table 1. As can be seen, these values are much smaller than the time frames of the relaxations observed. This can be taken as further evidence that the relaxation is not diffusioncontrolled. For the case of non-diffusion-controlled adsorption, a kinetic equation describing the adsorption from the subsurface to the surface must be used. Joos et alez0 used a simple Langmuir kinetic equation, assuming an ideal localized layer, to describe the adsorption behavior of bolaform surfactants t o an expanded oil-water interface. For such an expression, a linear change in the fitted exponential rate constant with changing bulk concentration should result, with the slope of such a plot corresponding to the ratio of the adsorption and desorption rate constants. Figure 5 is a plot of the relaxation rate constant for the 15 000 MW diol in heptane, as a function of bulk concentration. The lack of linearity in this plot indicates a relaxation process that cannot be described by a Langmuir kinetic equation, perhaps due to nonideal effects such as intermolecular interactions, a mobile surface layer, or a dependence of the adsorption rate constant on surface pressure. The data in Figure 5 are again typical of those seen for the other polyethers and indicate that the adsorbed layer cannot be treated as an ideal localized layer. Relaxation lifetimes, defined as the reciprocal of the fitted exponential rate constant, are presented in Table 2 for all of the polyethers examined in this work. Each of the values reported in Table 2 are averages of lifetimes obtained for at least three separate data traces. Standard deviations were found to vary from 5 to 10%. These data, along with the data in Figures 2 and 3, illustrate that the relaxation rate increases significantly with increasing molecular weight. This interesting effect may be related to the greater number of monomer segments in the high molecular weight homologues. Figure 6 shows the effect of equalizing the number of such segments by comparing the relaxation rate on an equal mass (ppm) basis, instead of a molecular basis, for the 400 and 15 000 molecular weight diols. In this case, at a concentration of 2 ppm, the relaxation for the 400 MW diol is actually faster than the 15 000 MW diol. Thus, on
888 Langmuir, Vol. 11,No. 3, 1995
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Breen A = 400 MW Diol
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B = 15,000MW Diol
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~~
TD, 8
sample
heptane
toluene
400 MW diol 2000 MW diol 4000 MW diol 15,000 MW diol 750 MW triol 3000 MW triol 6000 MW triol 9000 MW triol 12,000 MW triol 2000 MW diol, 10%EO 2750 MW diol, 20% EO 2650 MW diol, 30% EO
74.4 42.0 12.7 2.9 63.0 22.7 10.5 10.9 8.0 37.4 52.9 69.2
47.3 13.2 4.2 42.3 35.5 17.5 12.5 11.6 39.9 39.7 48.6
a molecular basis, higher MW polymers will be more effective a t maintaining a low interfacial elasticity, due to rapid relaxation kinetics, resulting in faster film drainage and coalescence. On an equal ppm basis, low MW homologues may be at least as effective in accomplishing this. It is interesting to consider the effect of changing the nature of the organic oil phase from aliphatic (heptane) to aromatic (toluene). The data in Table 2 include lifetimes
for three glycol block polymers of varying PEO content, M. Increased obtained in heptane and toluene at 1x PEO content leads to slower relaxation times in heptane, whereas in toluene the effect is greatly reduced. These solvent effects correspond to the relative solubilities of the polyethers in heptane and toluene. It is possible, for example, to prepare 10 wt % solutions of the PPO polyethers in heptane or toluene, with no visible turbidity. In contrast, as increasingamounts of PEO are incorporated into the polymer, visible turbidity appears at lower and lower concentrations in heptane. A 10 wt % solution of the 2650 MW diol containing 30%PEO in toluene is clear, for example, while noticable turbidity exists in a 100 ppm solution of the same polymer in heptane.27 Thus, the slower relaxation rates observed for the PEO-containing polyols are most likely a reflection of increased polymer folding and aggregation, which must then unfold during the relaxation process. The data in Table 2 illustrate that, in contrast to the PEO-containing polyols, the PPO polyols (with the sole exception of the 750 MW triol) seem to yield faster relaxation kinetics in heptane as compared to toluene. As a result, it appears that demulsifiers containing hydrophilic PEO segments are less suited for aliphatic oil emulsions than straight PPO type polymers, since the slower relaxation kinetics will allow a higher degree of elasticity at the interface. This is consistent with empirical observation and field experience.
Summary Adsorption kinetics of polypropylene oxide polymers from a hydrocarbon phase to a water-hydrocarbon interface have been shown to be non-diffusion-controlled. Measurements of relative relaxation rates for a series of homologues have demonstrated that, on a per molecule basis, high molecular weight homologues will yield the lowest dynamic interfacial elasticity, favoring rapid coalescence. The slower relaxation times observed in heptane for polyethers containing PEO segments indicate that the hydrophilic content should be minimized for optimum performance in aliphatic oil emulsions. LA940506T (27) Breen, P. J. Unpublished observations.
