Fluid Interfaces: A

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Ind. Eng. Chem. Res. 2005, 44, 1129-1138

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AOT and Pluronic F68 Coadsorption at Fluid/Fluid Interfaces: A Continuous-Flow Tensiometry Study T. F. Svitova†,‡ and C. J. Radke*,† Chemical Engineering Department, University of California at Berkeley, Berkeley, California 94720-1468, and Institute of Physical Chemistry, RAS Moscow, Leninsky Pr. 31, 117915 Moscow, Russia

Efficient emulsifier surfactants reduce oil/water interfacial tensions to low values within a fraction of a second. Unfortunately, in many cases, these surfactants do not provide high coalescence resistance due to the poor structural-mechanical (viscoelastic) strength of the adsorption layer. The emulsifier interfacial layer can be fortified by including an emulsion stabilizer, usually of higher molecular weight, that imparts higher interfacial elasticity and provides long-term emulsion stability. Unfortunately, the mechanisms of emulsifier and stabilizer coadsorption at the fluid/fluid interface are not well understood. To attack this problem, we study mixed adsorption of an anionic surfactant, AOT, a well-known emulsifier, and a nonionic polymeric surfactant, Pluronic F68, at the mineral oil/water interface by applying continuousflow tensiometry. Adsorption/desorption of AOT is instantaneous. Pluronic F68, however, exhibits a strong desorption barrier and is practically irreversibly adsorbed at the air/water and mineral oil/water interfaces when washed with water. When AOT solutions are used for washout of the Pluronic F68 adsorbed layer, complete displacement of polymeric surfactant from the oil/water interface is observed at AOT concentrations above the critical micelle concentration. When adsorption occurs from mixed AOT/Pluronic F68 solutions, however, the polymeric surfactant successfully competes with AOT for the interface, but only at higher concentrations where the singly adsorbed polymer adopts a hairpin conformation at the interface. Optimized AOT/Pluronic F68 surfactant mixtures are very efficient for mineral-oil-in-water emulsification. The emulsions show long-term stability and a unique immunity against dilution. 1. Introduction Application of emulsions is broad, spanning food emulsions, pharmaceutical and cosmetic personal care products, paints and coatings, agriculture, and petrochemicals. Emulsification mechanisms and emulsion stability are reasonably well understood.1-3 Some applications, such as formulation of “surfactant-free” emulsions in pharmaceuticals and cosmetics, is mainly focused on emulsion polymerization.4,5 Other possibilities for surfactant-free emulsions remain largely unexplored. Emulsification is a dynamic process requiring the presence of surface-active substances. An ideal emulsifier should adsorb instantaneously at a newly created liquid/liquid surface, providing low interfacial tension, which is critical to achieving a desirable small droplet size.6,7 Later on, the same emulsifier should also protect the immature droplets from coalescence by creating a stable structural-mechanical barrier at the liquid/liquid interface.8 These two properties do not often come together in one particular surfactant. The most common practical solution is to use surfactant blends containing fast-adsorbing, relatively short chained traditional surfactants in combination with higher molecular weight polymeric surfactants. Although such surfactant/polymer blends are rather popular in cosmetic and personal care products, the choice of the components and their proportions is largely empirical. * To whom correspondence should be addressed. Tel.: (510) 642-5204. Fax: (510) 642-4778. E-mail: [email protected]. † University of California at Berkeley. ‡ RAS Moscow.

The mechanisms and dynamics of surfactant/polymer coadsorption at the liquid/liquid interface remain unexplored. Dynamic tensiometry now stands as a standard technique to elucidate surface-active-substance adsorption kinetics at fluid/fluid interfaces.9-17 With the advent of modern video-recording equipment and automated image-analysis software, pendant-drop dynamic tensiometry with axisymmetric drop shape analysis (ADSA) is now a robust and routine technique9-14 capable of establishing adsorption dynamics starting within a few seconds after drop formation. Unfortunately, in classical pendant-drop tensiometry, the sorption dynamics is almost always followed only in the adsorption or interface-loading direction. Emulsion stability upon dilution of the continuous phase demands irreversible adsorption of the emulsifier. Studies of surfactant and polymer desorption kinetics15,16 or washout studies are rarely, if ever, undertaken. Concomitant with the lack of desorption data is the inherent assumption of reversible sorption for most surfactants, a supposition that we show below is in error. In this paper, we describe continuous-flow-tensiometry (CFT) experiments17 performed for individual and mixed low-molecular-weight and polymeric surfactant systems at the air/water and the mineral oil/water interfaces with major emphasis on adsorption-desorption behavior at the oil/water interface. 2. Experiment 2.1. Materials. Distilled and deionized (DI) water from a MilliQ filter system (Millipore Co., Bedford, WA,

10.1021/ie049676j CCC: $30.25 © 2005 American Chemical Society Published on Web 08/26/2004

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Figure 1. Schematic of the continuous-flow tensiometer.

resistivity greater than 18 MΩ cm) was used for all solution preparation. Mineral oil was from Aldrich Chemical Co., Inc. (Milwaukee, WI, >98% purity). The interfacial tension of this oil against DI water is 56.4 mN/m, and its kinematic viscosity is 25 cSt. Just prior to use, the mineral oil was purified by passing it through a chromatographic column sequentially filled with baked silica and alumina (1:1 by volume) to remove surface-active impurities. The sodium salt of sulfosuccinic acid bis(2-ethyhexyl) ester (AOT), >99% purity, was from Sigma Chemical Co. (St. Louis, MO). The block copolymer of polyethylene and polypropylene glycol, (EO)76(PO)29(EO)76, Synperonic PE/Pluronic F68, >98% purity, was from Fluka Chemie AG (Buchs, Germany). Both surfactants were used as received without further purification. All experiments were performed at ambient temperature, 22 ( 0.5 °C. 2.2. Methods. A Kruss DSA 10 apparatus (Kruss, Charlotte, NC) coupled with an image-acquisition board, DT-3155 (Data Translation, Marlboro, MA), was adopted for real-time drop/bubble image acquisition. Figure 1 displays the experimental setup. A 25 mL sample of the aqueous phase was placed into a 30 mL optical glass cell with a 20 mm optical path length and 24 × 35 × 53 mm outside dimensions (Carl Zeiss, Jena, Germany). The flat end of a 4 mm diameter circular glass rod, slightly beveled (i.e., the plunger from a 1 mL Hamilton syringe) and carefully coated only on the flat end with liquid Teflon SF (DuPont Fluoroproducts, Wilmington, DE), served as the droplet/bubble holder. A typical drop size was 40 ( 5 µL. Drops of this size remain pinned to the flat end of the Teflon-hydrophobized rod during the experiment regardless of transient alterations in the interfacial tension and external-phase flow. The pinning feature is critical to the success of CFT since it eliminates contact-line motion as the surfactant adsorbs or desorbs and since it maintains an axisymmetric drop shape.17 The aqueous phase in the cell was continuously stirred, unless stated otherwise, by a Teflon-coated, 9 mm diameter, cross-shaped bar magnet (model MC-301, Scinic Co. Ltd., Tokyo, Japan) at a speed slightly over 500 rpm. Two chromatographic syringe pumps (model 260D, Isco Inc., Lincoln, NE) simultaneously supplied liquid to and withdrew liquid from the cell at matched

flow rates. The tip of the inlet flow tube was placed just adjacent to the stirring bar, but without physical interference. The outlet flow tip was located opposite the inlet line at the liquid surface to provide horizontal and vertical cross-flow to the suspended droplet. More details of the CFT device can be found elsewhere.17 To facilitate low-interfacial-tension measurement for the mineral oil/aqueous AOT solutions, a new feature was incorporated into the CFT system. Oil drops were formed at the flat, Teflon-hydrophobized tip of a thickwalled glass capillary, 6 mm outer diameter, with a 1 mm inner diameter. The tip of the capillary tube was tapered to present a 3 mm flat circular surface on which the drop was pinned. To form a drop, mineral oil was dispensed via a programmable syringe pump (J-KEM Scientific, Inc., St. Louis, MO). This tip design permitted precise regulation of the drop volume and drop formation time. It also added the capability of measuring interfacial rheology.18-24 In the present work, we used a step-strain technique to gather information about the viscoelastic properties of individual and mixed adsorbed layers. In this technique, a drop of oil, initially equilibrated with the aqueous surfactant solution under study, was instantaneously and slightly contracted with a small area perturbation, ∆A/Ao , 1. Interfacial stress relaxation was then monitored by ADSA until the tension reached a constant value. In our case, it usually took 30-60 min for complete relaxation. We found that for the solutions studied a linear response was obtained for 3% < ∆A/Ao < 8%. Accordingly, we keep ∆A/Ao ) 5 ( 1%. The dilatational relaxation modulus and its decay with time are then determined as23,24

E(t) ) Ao∆γ(t)/∆A

(1)

where Ao is the initial drop area and ∆γ(t) is the change in tension induced by drop compression. To quantify the transient behavior of the dilatational relaxation modulus, we adopt a combined Maxwell viscoelastic and diffusion-relaxation model:24

E(t) ) E∞ + AM exp(-t/τM) + AD exp(2t/τD) erfc(2t/τD)1/2 (2)

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Figure 2. Equilibrium interfacial-tension isotherms for AOT (closed circles) and Pluronic F68 (closed squares). The solid line represents a best fit to the Langmuir-Szyszkowski relation for AOT. Arrows and crossed squares, respectively, indicate the concentrations of AOT and Pluronic F68 in mixtures studied later.

where the first two terms on the right account for an irreversibly adsorbed interfacial layer and the last term reflects any reversibly adsorbed surface-active species. τM and τD represent the characteristic times for in-plane viscoelastic relaxation and diffusive exchange with the bulk solution, respectively. E∞ is the Gibbs static elasticity of the irreversibly adsorbed component of the interface, and AM and AD are constants reflecting the relative contributions to E(t) of the irreversible and reversible adsorbate material, respectively. 3. Results and Discussion 3.1. Individual Surfactants. Figure 2 gives as filled symbols the measured static interfacial tensions at the mineral oil/water interface as a function of aqueous concentration for AOT (circles) and for Pluronic F68 (squares) individually in the aqueous phase. The solid line in the figure represents a best fit to the LangmuirSzyszkowski equation25 for AOT and indicates acceptable agreement with the Langmuir adsorption model

Γ/Γmax ) Kc/(1 + Kc)

(3)

where Γmax ) 3.08 × 10-6 mol/m2 and K ) 24.7 m3/mol. Arrows in this figure point to the AOT concentrations that are later chosen for mixed adsorption and loadingwashout experiments. These concentrations are 0.22, 0.44, and 1.12 of the critical micelle concentration (cmc). The interfacial-tension isotherm for Pluronic F68 exhibits a complicated staircase shape that cannot be fit with common adsorption models. Note that the results reported in Figure 2 for Pluronic F68 correspond to a static tension taken after 24-48 h of oil-drop exposure to the polymeric surfactant solutions where the interfacial tension remained reasonably constant ((0.2 mN/m) for over 1 h. This isotherm is very similar to the Pluronic F68 surface-pressure isotherm at the air/ water interface reported by Munoz et al.,26 where several plateaus appeared at concentrations well below the cmc (i.e., about 10 mM).26 Munoz et al. attributed the observed steps to first-order surface phase transitions. In Figure 2, we find similar interfacial-tension

plateaus in exactly the same concentration regions as did Munoz et al.26 for Pluronic F68 at the air/water interface. Thus, the surface phase transitions of Pluronic F68 at the air/water interface likely occur also at the oil/water interface and are the result of analogous polymer reorientation/repacking at both interfaces. Similar molecular-reconfiguration behavior has also been observed for Pluronics other than F68 at the air/ water interface and has likewise been attributed to the rearrangement of the copolymer molecules on the surface.27 Neutron reflection studies confirm these surface molecular rearrangements for block copolymers of high-molecular-weight (i.e., 5000 and higher) Pluronics.28,29 The possibility that Pluronic F68 forms aggregates in the bulk solution at the concentrations corresponding to the different plateaus in Figure 2 should not be ruled out. Here, however, we focus on the interfacial behavior. We chose three representative Pluronic F68 concentrations, marked in Figure 2 as crossed squares, for further investigation. The lowest one was 5 × 10-8 M at just below the first plateau on the interfacial-tension isotherm. This concentration corresponds to a pronemolecule orientation where the triblock copolymer molecules lie stretched at the oil/water interface (region I).26,28 A second concentration, 1 × 10-6 M, is in the surface semidilute region where the adsorbed Pluronic molecules adopt an intermediate conformation at the interface (region II),26 and the third concentration, 2.5 × 10-5 M, lies within the region where the adsorbed molecules are closely packed in a “hairpin” conformation (region III).26 Sketches of the Pluronic F68 molecule interfacial conformations for these three different concentration regions are presented in Figure 2. The adsorption/desorption kinetics of Pluronic F68 at these concentrations, both single and in mixtures with AOT, is addressed later. To evaluate the adsorption/desorption behavior of AOT alone at the oil/water interface, we perform loading-washout experiments that provide important information on the reversibility of adsorption, as well as on the possible existence of sorption barriers.17 Figure 3 displays the interfacial-tension history for one of the load-wash sequences conducted with a 0.1 wt % AOT solution at about 0.9 of the cmc for this surfactant.30,31 In these experiments, we formed a drop of pure mineral oil in DI water and flushed the cell with 10 cell volumes of AOT solution at a flow rate of 10 mL/min. During this period, the interfacial tension fell from the initial value of 56.4 to 10 mN/m. After loading was complete, the drop was equilibrated with the AOT solution for 1 h. We then pumped DI water through the cell at the same loading flow rate, and the tension rose back to its initial value characteristic at the clean oil/water interface. Data scatter in this figure, and in those later, arises from drop-shape perturbations induced primarily by stirring.17 Note that, when stirring is stopped, the tension values are at the lower bound of the measurements with stirring, and not at the mean level. The solid line in Figure 3 represents the interfacial tension calculated using a well-stirred tank model for the surfactant concentration and local surfactant adsorption/desorption equilibrium according to the LangmuirSzyszkowski relation.17 Clearly, the experimental data are in excellent agreement with the calculated theoretical curve even though the Langmuir adsorption model is rather simple. These results indicate that at the

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Figure 3. Dynamic interfacial tension as a function of time for loading-washout of 0.1 wt % aqueous AOT at a flow rate of 10 mL/min. The solid line is calculated according to the Langmuir equilibrium model with the listed parameters.

Figure 4. Dynamic interfacial tension of 5 × 10-8 M aqueous Pluronic F68 as a function of time in a load-wash sequence. Arrows designate the beginning and end of DI washout. Data scatter is due to stirring.

contact times studied AOT is reversibly adsorbed at the oil/water interface. The same reversible adsorption/ desorption behavior and excellent agreement with theory were observed for the more dilute AOT solutions of 0.025 wt % [0.22(cmc)] and 0.05 wt % [0.44(cmc)]. Figure 4 shows the interfacial-tension history for an analogous load-wash experiment performed with a dilute 5 × 10-8 M Pluronic F68 solution (region I). In this experiment, we formed a drop of mineral oil in the Pluronic F68 solution and equilibrated it for 3 h with stirring followed by elution of the polymeric surfactant with DI water at a flow rate 50 mL/min for 20 cell volumes. Adsorption of Pluronic F68 is demonstrably slower than that of AOT in Figure 3. The desorption kinetics of Pluronic F68, however, is dramatically different. The loading interfacial tension attained after 3 h in Figure 4 is 28.2 ( 0.5 mN/m, in agreement with

Figure 5. Dynamic interfacial tension of 2.5 × 10-5 M aqueous Pluronic F68 as a function of time in a load-wash sequence. Arrows designate the beginning and end of DI washout. Data scatter is due to stirring.

the final static tension of this particular Pluronic F68 solution. After washout with DI water in Figure 4, the interfacial tension rises only slightly to 29 mN/m, a value that is substantially lower than the pure water/ mineral oil tension of 56.4 mN/m. When a fresh drop of mineral oil was formed in the aqueous phase remaining in the cell after the Pluronic F68 washout procedure, its interfacial tension was 56.4 mN/m, confirming that there was no polymeric surfactant left in the bulk aqueous phase. In Figure 4 one can clearly recognize the periods of no stirring and the corresponding tension values at the lower bound of the tensions measured during stirring. The experiment in Figure 4 shows that, in region I where the Pluronic F68 copolymer molecules adopt a prone (planar) orientation at the mineral oil/water interface, the adsorption is practically irreversible upon washing with DI water. Irreversibility is most likely caused by multiple contacts between the polymer hydrophobic poly(oxypropylene) chains and the oil phase. The probability that all contacts rupture at the same moment is low. Consequently, desorption is a rare event. We left one of the drops of mineral oil in the aqueous phase for 3 days after washout of the Pluronic F68 solution. The interfacial tension was practically unchanged. Figure 5 depicts the interfacial-tension history we observe in a similar load-wash experiment performed with the more concentrated 2.5 × 10-5 M Pluronic F68 solution (region III). Upon equilibration for 2 h with Pluronic solution, the interfacial tension (unstirred value) falls to a value of 22.0 mN/m within the first 15 min. After washing with 1 L of DI water at the flow rate 50 mL/min, and with subsequent stirring for more than 1 h, the final interfacial tension was 24 mN/m (unstirred value), an increase of only 2 mN/m. So, although a small portion of the polymeric surfactant desorbed from the interface, the major portion stayed at the interface even after intense washout. We observed this result as well for the intermediate Pluronic F68 concentration in region II. The majority of polymeric surfactant is irreversibly adsorbed at the oil/water interface for all three initially adsorbed configurations after thorough washing with DI water.

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Figure 6. Dynamic interfacial tension of 2.5 × 10-5 M aqueous Pluronic F68 as a function time. Open circles correspond to no stirring and no flow. Open squares reflect stirring, but no flow.

Even though the tension in Figure 5 falls within the first 15 min, it does so only to 22 mN/m, a value considerably higher than the static value in Figure 2 obtained after 24 h of equilibration. Apparently, there is an appreciable kinetic barrier for reaching the adsorbed hairpin configuration. This assertion is confirmed in Figure 6. Only after about a day does the tension level off. Thus, based on Figures 4 and 5, the final interfacial tensions reported in Figures 2 and 6 for Pluronic F68 correspond to irreversible adsorption. Hence, we refer to these values as “static” rather than equilibrium tensions. Finally, we studied Pluronic F68 adsorption/desorption kinetics at the air/water interface. For the three concentrations corresponding to regions I-III in the tension isotherm, we found exactly the same kinetic behavior for Pluronic F68 as we observed at the oil/ water interface. Upon washout with DI water, the surface tension of the Pluronic F68 rose only imperceptibly. Thus, adsorption is also irreversible at the air/ water interface for this polymeric surfactant. Our adsorption/desorption kinetic study of the individual surfactants shows that the short-chain, anionic surfactant AOT adsorbs in a completely reversible manner, whereas the 8.4 kDa EO/PO/EO triblock polymeric surfactant adsorbs irreversibly at both the air/water and oil/water interfaces, irrespective of the Pluronic molecular conformation in the adsorbed layer. 3.2. Mixed Surfactant Systems. 3.2.1. Sequential Adsorption. To establish the ability of the irreversibly adsorbed Pluronic F68 polymeric surfactant and the reversibly adsorbed AOT anionic surfactant to compete with each other at the mineral oil/water interface, we studied mixtures of the two surfactants with various loading and washout scenarios. First, sequential adsorption was investigated. Pluronic F68 desorption/ displacement by AOT was probed for an “isolated” adsorption layer (i.e., an irreversibly adsorbed layer of Pluronic F68 for which no polymeric surfactant is present in the bulk aqueous phase). A drop of mineral oil was equilibrated for 2 h with the Pluronic F68 solution of chosen concentration and then washed with DI water to remove all polymer from the bulk aqueous phase, but leaving any irreversibly adsorbed polymer

Figure 7. Dynamic interfacial tension of 5 × 10-8 M aqueous Pluronic F68 (washed drop) as a function time in a 0.025 wt % AOT (0.22(cmc)) load-wash sequence. Arrows designate the beginning and end of AOT loading, and the beginning and end of DI washout. Data scatter is due to stirring. Short-dashed horizontal lines locate the unstirred tension value.

at the oil/water interface. The cell was then loaded with a particular AOT solution and equilibrated for more than 1 h before a second washout with DI water. Interfacial-tension changes were monitored throughout all stages. Figure 7 shows the interfacial-tension history following an initial load and washout of 5 × 10-8 M Pluronic F68 solution. At time zero in Figure 7, a 0.025 wt % solution of AOT was loaded into the cell followed by washout with DI water for 70 min, as shown. Two horizontal short-dashed lines in Figure 7 give the unstirred tension values corresponding to loading and washout. Upon AOT loading and 2 h of equilibration, the interfacial tension decreased from 31 mN/m observed for the isolated Pluronic F68 layer (region I) to 18 mN/m, which is typical for singly adsorbed AOT at 0.22 of the cmc. The tension rose after washout with DI water to a final unstirred value of 49 mN/m, indicating significant desorption of both surfactants from the interface. However, since the interfacial tension did not rise to the clean oil/water tension value of 56.4 mN/m, some small amount of Pluronic F68 remains adsorbed at the oil/water interface. AOT at 22% of the cmc does not displace it completely. Figure 8 summarizes the results of the four-step load-wash experiments conducted with initially equilibrated Pluronic F68 (regions I-III) and then washout with AOT solutions of the three concentrations indicated by the arrows in Figure 2. In Figure 8, the final interfacial pressure π, the difference between the interfacial tension at the pure oil/water interface and the interfacial tension in the presence of surfactant, after the second washout with DI water is plotted as a function of the AOT concentration used for polymeric surfactant displacement from the isolated adsorbed layer. Open squares correspond to 2.5 × 10-5 M, the highest Pluronic F68 concentration studied (region III, hairpin conformation). Open circles are for 1 × 10-6 M (region II, intermediate conformation), and open triangles are for 5 × 10-8 M (region I, prone conformation). Lines connect the experimental points. Although the

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Figure 8. Final interfacial pressure at the mineral oil/aqueous phase interface following first a Pluronic F68 loading and DI washout, then AOT loading at the designated concentration in the abscissa, and finally DI washout. Three initial Pluronic F68 concentrations are reported: 2.5 × 10-5 M (open squares); 1 × 10-6 M (open circles); 5 × 10-8 M (open triangles).

Pluronic F68 laden AOT-washed drops show different interfacial pressures at AOT concentrations below the cmc, all three curves merge when the 1.12(cmc) aqueous AOT solution is used for Pluronic F68 displacement. The final interfacial pressure in all cases is about 5 mN/m. This result indicates that the initial Pluronic F68 adsorbed layer contains some species that cannot be displaced from the oil/water interface even by a concentrated AOT solution. We suspect that high-molecular-weight copolymers remain at the oil/water interface due to their low water solubility that arises from a substantially long hydrophobic poly(oxypropylene) block and/or a very short hydrophilic poly(oxyethylene) block. A second observation emerges from Figure 8. The lower the Pluronic F68 concentration in the initial solution, the smaller is the concentration of AOT necessary for practically total Pluronic F68 displacement from the isolated adsorption layer. Thus, the irreversible interfacial layer formed by Pluronic F68 at the mineral oil/water interface after washout with DI water can be almost completely displaced upon contact with concentrated AOT solutions (i.e., at near or above the cmc). Since AOT is reversibly adsorbed, it is then removed from the interface with DI water flushing. Consequently, in sequential adsorption, and when AOT is added in overwhelming excess, Pluronic F68 is displaced from the interface. Note, however, that, in the sequential addition case, there is no competing supply of Pluronic F68 from the bulk aqueous solution. 3.2.2. Competitive Adsorption. In the competitive adsorption experiments, six mixed AOT/Pluronic F68 solutions were studied. Two mixtures were prepared for each Pluronic F68 concentration (regions I-III), one containing 0.05 wt % [0.44(cmc)] AOT and the other containing 0.125 wt % [1.12(cmc)] AOT. After equilibration of the mineral oil/aqueous surfactant mixture interface for 2 h, washout was performed with DI water. Figure 9 displays the interfacial-tension history for one of the load-wash experiments conducted with the mixed surfactant system. The concentration of Pluronic F68 in the mixture is 5 × 10-8 M (region I, prone conformation), and the AOT concentration is 0.05 wt % [0.44(cmc)]. During the equilibration period, the inter-

Figure 9. Dynamic interfacial tension of mixed 0.05 wt % AOT/5 × 10-8 M Pluronic F68 as a function time in a load-wash sequence. Arrows designate the beginning and end of DI washout. Data scatter is due to stirring. A short-dashed horizontal line locates the unstirred tension value. Table 1. Interfacial Pressures at the Mineral Oil/Water Interface for Pluronic F68/AOT Mixtures before and after Washout mixture composition AOT C, Pluronic wt % F68 C, M 0.05 [0.44(cmc)] 0.125 [1.12(cmc)]

5 × 10-8 (I) 1 × 10-6 (II) 2.5 × 10-5 (III) 5 × 10-8 (I) 1 × 10-6 (II) 2.5 × 10-5 (III)

π, mN/m, before washout AOT alone mixture 42.5 49.7

42.2 43.0 44.5 49.7 50.1 49.3

π, mN/m, after mixture washout 4.0 17.7 27.4 3.0 3.0 26.7

facial tension of the mixture falls to 13 mN/m, that of AOT adsorbed alone. After washout, the unstirred interfacial tension, as indicated by the horizontal shortdashed line, rises to 52 mN/m. Even after the oil droplet was left in the DI water overnight, the interfacial tension remained the same, slightly lower by about 4 mN/m than that of the surfactant-free system. Thus, for this low Pluronic F68 concentration (region I) there is no significant difference between sequential and competitive adsorption. AOT essentially displaces all Pluronic F68 from the mineral oil/water interface. However, this finding is not the same for higher concentrations of Pluronic F68. Table 1 summarizes the results of load-wash runs conducted with the Pluronic F68/AOT mixtures at different Pluronic F68 and AOT concentrations. Columns 3 and 4 of Table 1 show that the interfacial tensions in the mixed systems are very close to those of AOT alone. Hence, the impression arises that Pluronic F68 has only a minor impact in competitive adsorption with AOT. This impression is correct for the dilute, 5 × 10-8 M Pluronic F68 concentration (region I), since there is practically nothing left at the interface after washout (i.e., a 3-4 mN/m surface pressure). At higher Pluronic F68 concentrations and for AOT concentrations below the cmc, however, column 5 in Table 1 indicates that Pluronic F68 in the inter-

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Figure 10. Interfacial pressure of initially equilibrated (filled bars) and washed (crosshatched bars) oil drops immersed in a mixed 0.125 wt % AOT/2.5 × 10-5 M Pluronic F68 aqueous solution as a function of equilibration time.

mediate (region II) and hairpin (region III) concentrations is able to compete with AOT at the oil/water interface. However, when the concentration of AOT in the surfactant mixtures is raised above the cmc, only at the highest Pluronic F68 concentration (corresponding to region III) is the polymer present at the interface after washout with DI water. Apparently, the higher polymeric surfactant concentrations successfully compete for interface space. Presumably, the Pluronic F68 competitively adsorbs into the mixed monolayer in hairpin-like conformations. From the surface pressures in Table 1, however, the remaining irreversibly adsorbed polymeric surfactant after DI washout relaxes to the prone configuration (region I). These results are quite different from those of the sequential-adsorption case, where we observe practically complete Pluronic F68 displacement from the isolated adsorbed interfacial layer (i.e., where there is no Pluronic F68 supply from the bulk aqueous phase during AOT loading) by AOT when applied at concentrations above the cmc. 3.2.3. Competitive Adsorption: Time Dependence. Pluronic F68 is a considerably larger molecule than AOT so that both diffusion and adsorption rates are slower (see Figures 4-6). Thus, an important question arises for practical applications: does Pluronic F68 adsorb fast enough at short times to compete with AOT at the oil/water interface? To answer this question, we kept a mineral oil droplet in a mixture of 2.5 × 10-5 M (region III) Pluronic F68 and 0.125 wt % AOT for different periods of time prior to washout (5, 20, and 120 min), with stirring in all cases. Figure 10 summarizes the results of these experiments as a bar graph with filled bars corresponding to the initial interfacial pressure of this mixture at different equilibration times, and the crosshatched bars corresponding to the interfacial pressure after washout with DI water. The initial interfacial pressure is practically independent of contact time with the aqueous surfactant/polymer mixtures; it is very close to the interfacial pressure of the 0.125 wt % AOT alone solution. This observation indicates once again that AOT is much more surface-active than Pluronic F68, and its impact on the interfacial pressure of the mixed solutions dominates that of the polymeric surfactant. Interfacial pressures after washout are also close. The difference between the 5 and 20 min equili-

Figure 11. Final interfacial pressures in a three-step sequence: (1) oil drop equilibrated with 0.125 wt % AOT; (2) subsequent loading of a mixture of 0.125 wt % AOT/2.5 × 10-5 M Pluronic F68 aqueous solution; (3) washing with DI water.

brations is less than 1 mN/m; for the drops equilibrated for 2 h, the interfacial pressure is just 1.2 mN/m higher than that for the drop equilibrated for 20 min. Thus, though slower in general, Pluronic F68 adsorbs quickly enough to compete successfully with micellar AOT even at relatively short exposure times. Figure 11 addresses the important question of what happens when Pluronic F68 is added after the oil/water interface is already packed with AOT. We focus our attention on the Pluronic F68 concentration corresponding to region III. This figure diagrams the interfacial pressure for addition of AOT first, followed by AOT and Pluronic F68, and then washing with DI water. The first gray bar corresponds to the interfacial pressure for the mineral oil/water interface equilibrated with 0.125 wt % AOT solution. The second gray bar shows the interfacial pressure for the same drop now loaded and equilibrated with a mixture of 2.5 × 10-5 M Pluronic F68 and 0.125 wt % AOT. The third hatched bar corresponds to the interfacial pressure of the same drop finally washed with DI water. After the final washout, the interfacial pressure is very close to that found after a single-step washout of the same Pluronic F68/AOT mixture. This result indicates that Pluronic F68 incorporates into and partially replaces AOT from the adsorption layer previously equilibrated with AOT. Again, insertion of the polymeric surfactant into the AOT monolayer likely occurs in the intermediate or hairpin configurations; after washout, all of the surfactant and some of the polymeric surfactant desorb, leaving a partial monolayer of Pluronic F68 irreversibly attached in the prone conformation. Thus, Pluronic F68 successfully competes for the interface with AOT but only when applied at high enough concentration (i.e., corresponding to region III). 3.3. Interfacial Rheology. One reason for incorporating polymeric surfactant is to improve emulsion stability by building a strong protective adsorption layer with high structural-mechanical properties.32,33 To obtain information about the interfacial-layer elastic properties, we improved our drop-delivery system so that it permits an abrupt (1-2 s) stepwise change in drop volume and surface area of the aqueous-phaseimmersed oil drop. Thus, we perform stress-relaxation experiments to determine the dilatational relaxation

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Figure 12. Dilatational relaxation modulus of elasticity as a function of time for the mineral oil/water interface equilibrated with 2.5 × 10-5 M Pluronic F68 (filled circles) followed by washout with DI water (open circles). |∆A|/Ao ≈ 5%.

modulus, E(t) in eq 1, by suddenly decreasing the drop area and following the dynamic tension. Figure 12 presents as filled circles the dilatational relaxation modulus for Pluronic F68 adsorbed at the mineral oil/water interface at 2.5 × 10-5 M (region III). Open circles correspond to E(t) after Pluronic F68 has been removed from the bulk solution by washing with 1 L of DI water. In both cases, the adsorption layers show a very strong elastic response to the small (∼5%) area perturbation. Neither dilatational relaxation modulus approaches zero at large time, but rather both reach constant values corresponding to the static Gibbs elasticity of an irreversibly adsorbed interfacial layer (i.e., E∞ in eq 2). In Figure 12, both the equilibrated and washed Pluronic F68 laden oil/water interfaces are mechanically rather strong when the polymeric surfactant is adsorbed initially in the hairpin configuration (region III). An important feature of Figure 12 is that E(t) for Pluronic F68 grows significantly after washout and polymeric surfactant removal from the bulk aqueous phase. We argue that the slight interfacial-tension rise observed in Figure 5 after washout is due to desorption and removal of some shorter chain, mostly water-soluble species from the interfacial layer, quite possibly diblock copolymers. During interface contraction, these lowmolecular-weight components likely exchange with the bulk aqueous phase. The corresponding diffusion dissipation gives rise to the last term in eq 2. When the unwashed aqueous Pluronic F68/oil interface is perturbed, the smaller water-soluble Pluronic F68 components not only exchange with the bulk water phase, giving a faster decay time for the dilatational relaxation modulus, but also soften the viscoelastic interfacial layer. Further, the adsorbed polymeric surfactant remaining after washout reconfigures into the prone conformation making lateral interaction more likely. Thus, upon washout, the dilatational relaxation modulus both increases in strength and relaxes more slowly. The second and third rows of Table 2 show the best-fit parameters for E(t) in eq 2 corresponding to the aqueous Pluronic F68/oil interface in region III (all parameters

Figure 13. Dilatational relaxation modulus of elasticity as a function of time for the mineral oil/water interface equilibrated with mixed 0.125 wt % AOT/2.5 × 10-5 M Pluronic F68 aqueous solutions (filled circles) followed by washout with DI water (open circles). |∆A|/Ao ≈ 5%.

in Table 2 reflect an average of three separate experiments). The large increases in E∞ and τM after washout reflect a much stronger, more slowly relaxing viscoelastic interfacial layer. Moreover, AD decreases after washout, signifying a lesser amount of surface-active material that can reversibly exchange with the aqueous solution. The behavior of the dilatational relaxation modulus in the presence of AOT is given in Figure 13. Open squares correspond to AOT alone at 1.12 of the cmc (0.125 wt %). Here mechanical strength of the interface is practically absent. Also, E(t) decays to zero at long times, confirming the reversible nature of AOT adsorption. To fit these data, we use only the diffusion term in eq 2 with the fitting parameters listed in Table 2. The dilatational relaxation modulus for an aqueous mixture of 0.125 wt % AOT and 2.5 × 10-5 M Pluronic F68 (corresponding to region III) is similar in magnitude to that of AOT alone. However, the decay time is much longer. In this surfactant mixture, Pluronic F68 does indeed adsorb into the interfacial layer (see Figure 10 and Table 1). Thus, a structural-mechanical component is present in E(t), as demonstrated by the values of AM and τM in the fifth row of Table 2. However, adsorbed AOT dominates the dilatational relaxation modulus behavior of the mixed adsorbed layer, as reflected by the small values of E∞ and AD in row five of Table 2. Upon washout of the aqueous Pluronic F68/AOT mixture in Figure 13, both the dilatational relaxation modulus magnitude and decay time rise dramatically. According to Table 2, there is no longer a diffusionexchange component to E(t), and τM is a factor of 4 larger than that for washed Pluronic F68 alone. Likewise, E∞ is even larger than that for washed Pluronic F68 alone. Washout removes the most water soluble components from the adsorption layer. For Pluronic F68, these are the low-molecular-weight homologues and perhaps some diblock species that adsorb reversibly and, thus, weaken and accelerate the decay of the dilatational relaxation modulus. When adsorption occurs from a mixed Pluronic F68/AOT solution, the water-soluble polymeric surfactant components may not compete with AOT for the interface. Thus, only the irreversibly adsorbed compo-

Ind. Eng. Chem. Res., Vol. 44, No. 5, 2005 1137 Table 2. Dilatational Relaxation Modulus Parameters at the Mineral Oil/Aqueous Solution Interface from Eq 2 system

E∞, mN/m

AM, mN/m

τM, s

AD, mN/m

τ D, s

2.5 × Pluronic F68 washed 2.5 × 10-5 M Pluronic F68 0.125 wt % AOT 2.5 × 10-5 M Pluronic F68 + 0.125 wt % AOT washed 2.5 × 10-5 M F68 + 0.125 wt % AOT

4.6 18.34 0 0.76 20.1

8.12 12.2

386 1420

1.5 14.3

360 4453

8.4 1.9 8.7 4.55 0.02

200 170 167 183 193

10-5 M

nents of Pluronic F68 reside at the interface in the mixed adsorbed layer with AOT. The irreversibly adsorbed Pluronic F68 species remain at the interface even after AOT is completely washed from the adsorption layer. Apparently, in the presence of desorbing surfactant, relatively more of the lower molecular weight Pluronic F68 species are removed compared to those removed by washout from the polymeric surfactant alone. This reasoning likely explains the exceptionally high relaxation modulus in Table 2 for the mixed Pluronic F68/AOT and washed interfacial layer. The dilatational relaxation moduli in Figure 13 provide additional strong evidence of polymeric surfactant presence in the interfacial layer formed from mixed AOT/ Pluronic F68 solutions. 3.4. Application to Emulsion Stability. Oil-inwater emulsions were prepared according to the following procedure: A 1 mL sample of mineral oil was gradually added to 10 mL of a magnetically stirred aqueous phase and stirred for 10 min, followed by sonication for 15 min. For the aqueous phase, we used Pluronic F68 solutions at the concentrations studied above and their mixtures with different concentrations of AOT. We found that Pluronic F68 alone was a poor emulsifier, even at high concentrations corresponding to region III. Mixtures of Pluronic F68 with AOT show better emulsifying ability, although these emulsions tended to cream and coalesce when the AOT concentration in the mixture was below the cmc. The most stable emulsion was formed with a mixture of aqueous 2.5 × 10-5 M Pluronic F68 and 0.125 wt % AOT. We also prepared an otherwise identical oil-inwater emulsion but in a concentrated 0.5 wt % AOT [4.5(cmc)] aqueous solution alone. This emulsion was as stable as the one prepared from the mixture of 2.5 × 10-5 M Pluronic F68 and 0.125 wt % AOT. These two most stable emulsions were diluted in a 1/99 volume ratio by DI water. The original two emulsions and their diluted counterparts were then stored for over 1 year. The initial emulsions made with 0. 5 wt % AOT alone and with the 2.5 × 10-5 M Pluronic F68/0.125 wt % AOT mixture were similar in texture and color. They both showed some creaming, but otherwise were rather stable. However, the 1/99 diluted emulsions were different. The AOT-alone emulsion showed substantial creaming; small drops of mineral oil were observed floating on top of the creamed layer. Emulsions initially prepared with 2.5 × 10-5 M Pluronic F68 and 0.125 wt % AOT and then diluted showed no creaming. No oildroplet coalescence was observed. This finding is fascinating since the surfactant concentration in the diluted emulsion was extremely low: 0.00125 wt % AOT and 0.0002 wt % (2.5 × 10-7 M) Pluronic F68. We attribute the exceptional stability against dilution to the presence of irreversibly adsorbed Pluronic F68 at the interface of the emulsified oil droplets. Although AOT provides low interfacial tension during emulsification, thus promoting small droplet formation, it washes away from the interface when the emulsion is diluted. Pluronic F68 at 2 × 10-5 M successfully competes with AOT for the

available surface area and remains at the oil/water interface even at high aqueous-phase dilution. The irreversibly adsorbed Pluronic F68 layer possesses high mechanical strength (see Figure 13) and protects the emulsion droplets against coalescence. However, protection only happens when the initial Pluronic F68 concentration in the AOT surfactant mixture is higher than 2.5 × 10-5 M (0.021 wt %). 4. Conclusions We present a detailed study of aqueous block copolymer Pluronic F68 and AOT adsorption-desorption kinetics at the mineral oil/water interface from individual and mixed solutions using CFT. AOT adsorbs reversibly at the oil/water interface. On the time scale of our experiments, no sorption barriers are detectable for this surfactant. Conversely, Pluronic F68 (8.4 kDa) exhibits mostly irreversible adsorption at fluid/fluid interfaces for all adsorbed configurations including prone, intermediate, and hairpin. There are strong desorption barriers for this polymeric surfactant. Thus, tensions reported for Pluronic F68 are static, not equilibrium values. Further, the adsorption rate for Pluronic F68 is hindered by a noticeable adsorption kinetic barrier, but only in the higher concentration region corresponding to adsorption in the hairpin configuration. In sequential adsorption, AOT almost completely displaces Pluronic F68 from “isolated” monolayers (those for which no polymeric surfactant is present in the aqueous phase) irrespective of the polymer chain conformation. Surprisingly, Pluronic F68 in the hairpin conformation successfully competes with AOT and adsorbs irreversibly from its mixed surfactant solution even at 100-fold AOT excess (by molar ratio). Likewise, Pluronic F68 at concentrations corresponding to the individually adsorbed hairpin configuration incorporates into adsorbed layers saturated with AOT, attaching irreversibly and partially displacing some AOT from the interface. After DI washout, the polymeric surfactant remains at the interface in the prone configuration. Interfacial dilatation relaxation moduli confirm mixed Pluronic F68/AOT layer formation at the oil/water interface for systems with high polymeric surfactant concentration. In comparison with singly adsorbed Pluronic F68 or AOT layers, a significant increase of the oil/water dilatational relaxation modulus (i.e., strengthening) of the initially mixed adsorption layer was detected after AOT washout. The mechanically strong and irreversibly adsorbed Pluronic F68 interfacial film in region I (prone) explains the exceptional long-term stability and high immunity to dilution we observed for oil-in-water emulsions formed in the presence of an optimized Pluronic F68/AOT mixture. In this manner, we produce surfactant-free, yet stable, oil-inwater emulsions. Continuous-flow tensiometry is a powerful tool to study sorption kinetics at fluid/fluid interfaces. Especially useful is the examination of significant desorption

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kinetics and possible irreversible attachment to the interface. When combined with concomitant interfacial elasticity measurement, CFT opens new avenues for understanding surfactant dynamics and structures at fluid/fluid interfaces. Acknowledgment This work was partially supported by Del Amo UC/ UCM Collaborative Grant No. 10935-44 and in part by the U.S. Department of Energy under Contract No. DC03-76SF00098 to the Lawrence Berkeley National Laboratory of the University of California. We thank Dr. R. Rubio of the University of Madrid for collaboration, fruitful discussions, and supplying the Pluronic F68 samples. Literature Cited (1) Rusanov, A. I.; Prokhorov, V. A. Interfacial Tensiometry; Studies in Interface Science; Elsevier: Amsterdam, 1996; Vol. 3, Chapter 3, p 86. (2) Mo¨bius, D.; Miller, R. Drops and Bubbles in Interfacial Research; Studies in Interface Science; Elsevier: Amsterdam, 1998; Vol. 6, Chapters 2-4. (3) Loglio, G.; Pandolfini, P.; Miller, R.; Makievski, A. V.; Ravera, F.; Ferrari, M.; Liggieri, L. In Novel Methods to Study Interfacial Layers; Mo¨bius, D., Miller, R., Eds.; Studies in Interface Science; Elsevier: Amsterdam, 2001; Vol. 11, p 439. (4) Daniels, R. Surfactant-Free Emulsion Systems as Bases for Innovative Dermatics and Cosmetics. Web publication, http://www. dermotopics.de/english/issue_1_02_e/werkstattgesp_01_02_e.htm, 2002. (5) Schellenberg, C.; Tauer, K.; Antonietti, M. Film Formation of Polymeric Emulsions: Structure Set-up and the Pinhole Effect Characterized by Microscopic Techniques. J. Dispersion Sci. Technol. 1999, 20, 177. (6) Miller, R.; Joos, P.; Fainerman, V. Dynamic Surface and Interfacial Tensions of Surfactant and Polymer Solutions. Adv. Colloid Interface Sci. 1994, 49, 249. (7) Eastoe, J.; Dalton, J. S. Dynamic Surface Tension and Adsorption Mechanisms of Surfactants at the Air-Water Interface. Adv. Colloid Interface Sci. 2000, 85, 103. (8) Ferri, J. K.; Stebe, K. J. Which Surfactants Reduce Surface Tension Faster? A Scaling Argument for Diffusion-Controlled Adsorption. Adv. Colloid Interface Sci. 2000, 85, 61. (9) Lahooti, S.; del Rio, O. I.; Cheng, P.; Neumann, A. W. In Applied Surface Thermodynamics; Neumann, A. W., Spelt, J. K., Eds.; Marcel Dekker: New York, 1996; p 441. (10) Rotenberg; Y.; Boruvka, L.; Neumann, A. W. Determination of Surface-Tension and Contact-Angle from the Shapes of Axisymmetric Fluid Interfaces. J. Colloid Interface Sci. 1983, 93, 169. (11) Boyce, J. F.; Schurch, S.; Rotenberg, Y.; Neumann, A. W. The Measurement of Surface and Interfacial Tension by the Axisymmetric Drop Technique. Colloids Surf. 1984, 9, 307. (12) Cheng, P.; Li, D.; Boruvka, L.; Rotenberg, Y.; Neumann, A. W. Automation of Axisymmetric Drop Shape Analysis for Measurements of Interfacial Tensions and Contact Angles. Colloids Surf. 1990, 43, 151. (13) Cheng, P.; Neumann, A. W. Computational Evaluation of Axisymmetric Drop Shape Analysis-Profile (ADSA-P). Colloids Surf. 1992, 62, 297. (14) Kwok, D. Y.; Lin, R.; Mui, M.; Neumann, A. W. Low-Rate Dynamic and Static Contact angles and the Determination of Solid Surface Tensions. Colloids Surf., A: Physicochem. Eng. Aspects 1996, 116, 63.

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Received for review April 21, 2004 Revised manuscript received June 17, 2004 Accepted June 23, 2004 IE049676J