Article pubs.acs.org/Langmuir
Adsorption of Polymer−Surfactant Mixtures at the Oil−Water Interface Ian M. Tucker,† Jordan T. Petkov,† Craig Jones,† Jeffrey Penfold,*,‡,§ Robert K. Thomas,§ Sarah E. Rogers,‡ Ann E. Terry,‡ Richard K. Heenan,‡ and Isabelle Grillo∥ †
Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, United Kingdom STFC, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, United Kingdom § Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, United Kingdom ∥ Institut Laue Langevin, 6 rue Jules Horowitz, BP156, 38042 Grenoble Cedex, France ‡
ABSTRACT: Small-angle neutron scattering, zeta potential measurements, and dynamic light scattering have been used to investigate the adsorption of polymer−surfactant mixtures at the oil−water interface. The water−hexadecane interface investigated was in the form of small oil-in-water emulsion droplets stabilized by the anionic surfactant sodium dodecyl sulfate, SDS. The impact of the addition of two different cationic polymers, poly(ethyleneimine), PEI, and poly(dimethyldiallylammonium chloride), polydmdaac, on the SDS adsorption at the oil−water interface was studied. For both polymers, the addition of the polymer enhances the SDS adsorption at low SDS concentrations at the oil−water interface due to a strong surface polyelectrolyte−surfactant interaction and complexation, but the effects are not as pronounced as at the air− water interface. For PEI/SDS, the adsorption was largely independent of solution pH and increasing PEI concentration. In marked contrast to the adsorption at the air−water interface, only monolayer adsorption and no multilayer adsorption was observed. For the SDS−polydmdaac mixture, the enhanced SDS adsorption was in the form of a monolayer, and the adsorption increased with increasing polymer concentration. The strong SDS/polydmdaac surface interaction resulted in regions of emulsion instability. The zeta potential measurements showed that the combination of SDS and polydmdaac at the interface resulted in charge reversal at the interface. This correlates with the regions of emulsion stability at both high and low polymer concentrations, such that the instabilities arise in the regions of low or zero surface charge. The results presented and their interpretation represent a development in the understanding of polymer−surfactant adsorption at the oil−water interface.
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formation due to surface complex formation6 are attractive features to exploit in the context of emulsion formation and stability. Some polymer−surfactant mixtures are already used; for example, oil-in-water emulsion stabilized by anionic surfactant−gelatin mixtures are the basis of photographic coatings,9 and biopolymers, proteins, surfactants, and their mixtures are extensively used in stabilizing food-related emulsions.10,11 Given12 has recently reviewed the different technologies available for micro-encapsulation, in the context of flavors and
INTRODUCTION Micro-encapsulation is one of the emerging technologies for packaging volatile additives and providing controlled and targeted release over a wide product area, which includes cosmetics, beverages, and pharmaceuticals. Polymer−surfactant-stabilized oil-in-water emulsions are a potentially exciting route to microencapsulation of a variety of volatile components, such as perfumes, or flavors, in a wide range of applications in home and personal care products, cosmetics, foods, and pharmaceuticals. The study of emulsion formation and stability is a relatively mature and established area of research, and a number of excellent general texts exist.1−3 Polymer−surfactant mixtures have been extensively studied in solution4,5 and at interfaces.6−8 Their enhanced adsorption and surface multilayer © 2012 American Chemical Society
Received: September 4, 2012 Revised: September 28, 2012 Published: October 1, 2012 14974
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in polymer−surfactant mixtures are the abilities to measure adsorbed amounts and the surface structure directly, and neutron reflectivity measurements have made a unique contribution in this respect. Applying such techniques to the oil−water interface is not straightforward, and Zarbakhsh et al.21 have made some pioneering studies of surfactant adsorption at the oil−water interface. Hence, there are few systematic measurements on polymer−surfactant adsorption at the oil−water interface using structural methods. One of the few reported studies was the recent work of Beaman et al.,22 who used sum frequency spectroscopy, but their measurements were limited to probing the structure of poly(acrylic acid) at a CCl4−water interface. Staples et al.23 have shown how smallangle neutron scattering, SANS, can be used to study surfactant and mixed surfactant adsorption at the oil−water interface of well-defined and stable emulsion droplets to obtain adsorbed amounts, compositions, and structure. This approach was also applied to surfactant mixtures on water-in-oil emulsion droplets by Bumajdad et al.,24 and to the adsorption of triblock copolymers on hydrocarbon and fluorocarbon emulsion droplets by King et al.25 The use of scattering techniques to probe the structure of adsorbed layers on both solid and fluid colloidal particles was reviewed by Oberdisse,26 but no examples of polymer−surfactant mixed adsorption were cited. In this Article, we report the use of SANS, light scattering, and zeta potential measurements to investigate the adsorption of polymer−surfactant mixtures at the hexadecane−water interface of submicrometer-sized emulsion droplets. Two different polyelectrolyte−surfactant mixtures, SDS−polydmdaac and SDS−PEI, are compared. These have both been extensively studied at the air−water interface27,28 by NR and show contrasting patterns of behavior at that interface. There is enhanced adsorption in the form of a monolayer at low surfactant concentrations in the SDS−polydmdaac due to the strong SDS−polydmdaac surface interaction, and partial surface depletion close to charge neutralization due to precipitation effects. For a branched PEI−SDS mixture, there is also enhanced adsorption due to surface complex formation, and close to charge neutralization the adsorption changes from a monolayer to multilayer adsorption. The multilayer adsorption is now attributed to two-phase wetting associated with the precipitate/coacervate region observed around charge neutralization.29 The motivation for this study is the potential to exploit the synergistic adsorption of polymer−surfactant mixtures at the oil−water interface to form emulsion micro-encapsulates. Characterizing the adsorption behavior and emulsion properties of two polymer−surfactant systems that have been extensively studied at the air−water interface and that achieve enhanced adsorption in two different ways will provide an important insight into some of the key features required for microcapsule formation.
beverages, and the different approaches include polymer, and protein-stabilized emulsions, emulsions stabilized by mixtures of biopolymers, proteins, and surfactants, layer-by-layer, LbL, polyelectrolyte multilayer shells, and liposomes. The advantages of mixtures of biopolymers in enhancing emulsion stability are highlighted. The LbL approach offers the advantages of droplet shell integrity and stability against applied stresses. The LbL multilayer formation has attracted much attention,13 but is associated with complex production issues. Petrovic et al.14 have recently demonstrated the potential of polymer− surfactant mixtures in micro-encapsulation. They showed how a mixture of SDS and hydroxypropylmethyl cellulose, HPMC, forms a compact and robust layer at the interface of an oil-inwater emulsion that is remarkably resistant to drying and rehydration. Their work demonstrates the possibilities of using polymer−surfactant mixtures to form adsorbed layers suitable for microcapsule formation. However, they highlight the need for further studies on polymer−surfactant interactions and their impact on the properties of the adsorbed layer at the oil−water interface. There are a number of studies in the recent literature that use a combination of interfacial tension measurements and dilational rheology to study polymer−surfactant interactions at the oil−water interface,15−17 and report increases in emulsion stability. Sharipova et al.15 observed the impact of surface complex formation in SDS/poly(allylamine hydrochloride) mixtures at the hexane−water interface from interfacial tension and dilational rheology measurements. A minimum in the interfacial tension and a corresponding maximum in the dilational elasticity were attributed to the formation of a surface network. Zhang et al.16 observed synergistic effects in the interfacial tension at the air−water and oil−water interfaces in poly(acrylamide)−surfactant mixtures due to the polymer−surfactant interactions. Alvarez et al.17 observed enhanced stability in oil-in-water (for dodecane and hexadecane) for the mixtures of the nonionic surfactant C12E4 and the anionic polyelectrolyte, hydrophobically modified poly(sodium acrylate). The enhanced stability was attributed to the formation of polymer−surfactant complexes leading to a viscoelastic interfacial film. Stamkulov et al.18 used a different novel approach to stabilize oil-in-water emulsions, but still relied on a polymer−surfactant mixture. They used an oilsoluble surfactant, hexadecyl amine, and a high MW watersoluble poly(acrylic acid). The surfactant adsorbs at the oil− water interface, making the droplets positively charged, but insufficient to stabilize the emulsion. Adsorption of the negatively charged polymer forms a steric barrier, which prevents coalescence and enhances stability. Philip et al.19,20 used the force measurements between emulsion droplets to probe the nature of the weakly interacting poly(vinyl alcohol) (PVA)−surfactant complex formation at the interface. The addition of SDS to the emulsions sterically stabilized by the PVA resulted in an increase in the repulsive force, which was attributed to a stretching of the polymer in a polyelectrolytelike bebavior due to the SDS adsorption onto the polymer. Although such studies provide an important insight into the synergistic effects of polymer−surfactant mixtures on emulsion formation and stability, it is difficult to extract adsorbed amounts or any structural information. On the other hand, polymer−surfactant interactions at the air−water interface have been extensively studied and quantified6 using techniques such as neutron reflectivity, NR. The key features of those studies and their impact upon the understanding of surface interactions
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EXPERIMENTAL DETAILS
The adsorption of the polymer−surfactant mixtures was measured using SANS at a hexadecane−water interface of well-defined submicrometer emulsion droplets, as described in detail in ref 23. The measurements were made with a mixture of d- and h-hexadecane, which was neutron refractive index matched to D2O, in D2O, and for emulsion droplets ∼0.2 μm. In these circumstances and in the absence of micellar aggregates in solution, only the adsorbed layer of hydrogeneous polymer−surfactant contributes to the scattering. The size of the emulsion droplets and their stability were established using 14975
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Nano optimized for weakly scattering systems, fitted with a 50 mM Microgreen laser, 10 nm bandpass filters (532 nm), and using the new ZEN1010 high brilliance sample cell for enhanced data collection.32 The instrument uses phase analysis of the light in the crossover region of the light beam to measure the phase shift arising from the motion of the particles. A combination of up to 200 runs were required to ensure good noise free data in the phase plot data, which were subsequently Fourier transformed to give the change in frequency that is directly coupled to the change in mobility of the particles. A choice of dilution conditions showed that a 1:2000 dilution factor in UHQ water retained a conductivity that was sufficiently low to remain in the Smoluchowski limit (K−1·A < 1). From analyzing the phase shift of the light amplitude as a consequence of the electrophoretic mobility of the particles, it follows that the mobility, Ue, is related to the phase shift, Δν, by
dynamic light scattering. Phase analysis light scattering was used to determine the surface charge on the emulsion droplets. 1. Materials. Measurements were made for SDS/polydmdaac and SDS/PEI mixtures adsorbed at the hexadecane−water interface. The PEI was a 2k daltons MW branched sample obtained from Sigma Aldrich. The polydmdaac was custom synthesized, as described elsewhere27 with a MW ≈ 100k daltons, as determined by size exclusion chromatography. The hydrogeneous SDS (h-SDS) was obtained from PolySciences. The alkyl chain deuterated SDS (d-SDS) was provided by the Oxford Isotope Facility,30 and the details of the synthesis are reported elsewhere.31 Both surfactants were recrystallized before use, and their purity was verified by surface tension and neutron reflectivity. The D2O was obtained from Fluorochem, and the deuterium labeled and hydrogeneous hexadecane (d-,h-hexadecane) were obtained from Sigma Aldrich. UHQ water was used throughout, and all glassware and sample cells were cleaned prior to use in Decon90 alkaline detergent. 2. Emulsion Preparation and Characterization. Stable hexadecane in water emulsions (≤10% volume fraction) were prepared at low surfactant (SDS) concentrations (at approximately 10% coverage of the available emulsion surface) using ultrasonic homogenization, as described by Staples et al.23 The emulsions were prepared with the minimum amount of SDS required to stabilize the emulsion and to provide a sufficient electrostatic barrier to coalescence. The emulsion droplets were characterized by dynamic light scattering, DLS, using a Malvern Zetasizer Nano. The emulsion droplets had a mean diameter ∼0.2 μm and a relatively narrow polydispersity, ∼0.15, as shown in Figure 1. The particle size was
Δν = 2Ue
sin(θ /2) λ
(1)
and the Henry equation relates the mobility and zeta potential via: Ue =
2εξF(Κa) 3η
(2)
where K is the Debye screening length and a is the dimension of the particles, ε is the dielectric constant, η is the viscosity, ξ is the zeta potential, and F is Henry’s function. 4. Small-Angle Neutron Scattering. For d/h-hexadecane neutron index matched to D2O and in the absence of any bulk scattering from micelles or polymer−surfactant aggregates, the scattering arises only from the adsorbed layer of hydrogeneous polymer−surfactant at the emulsion oil−water interface. For emulsion droplets ∼0.2 μm and in dilute solution, the scattering in the Q range 0.004−0.25 Å−1 is in Porod regime, and the scattered intensity can be approximated to:
I(Q ) ≈ NV 2Q−4[(ρ1 − ρ0 )2 + (ρ2 − ρ1)2 + 2(ρ1 − ρ0 ) (ρ2 − ρ1)cos Qd]
(3)
where N and V are the emulsion droplet number density and volume, ρ0, ρ1, and ρ2 are the scattering length densities of the hexadecane, adsorbed layer (shell), and D2O, and d is the adsorbed layer thickness. In practice, the absolutely scaled SANS data are analyzed using a standard core−shell sphere model,33 taking into account polydispersity, known concentration, and instrumental resolution, to give the thickness of the adsorbed layer, d, and a scattering length density, ρf. From this, the adsorbed amount, Γ, can be obtained: Γ=
Figure 1. Emulsion size distribution from DLS measurements for (blue) 6.3% emulsion and 1.25 mM SDS, and (red) 6.3% emulsion 1.25 mM SDS following addition of 150 ppm PEI.
d(ρs − ρf ) NaVm(ρs − ρa )
(4)
where Na is Avogadro’s number, Vm is the surfactant molecular volume, and ρs, ρa, and ρf are the scattering lengths of the solvent, surfactant, and the value for the adsorbed layer obtained from the model fits. The SANS measurements were made on two different diffractometers, D11 and SANS2D. The SANS measurements on the D11 diffractometer at the Institut Laue Langevin, Grenoble, France,34 were made using neutrons with 6 Å wavelength and 10% wavelength resolution. Two sample-to-detector/collimator distances of 3.5/5.6 m and 11/11.2 m were used to cover a combined Q range of 0.001−0.25 Å−1. Data were normalized and converted to an absolute scale by reference to the scattering from H2O using standard procedures.35 The SANS measurements on the SANS2D diffractometer at the ISIS pulsed neutron source, UK,36 were made over a Q range of 0.004−0.8 Å−1 using the white beam time-of-flight method (using neutron wavelengths from 1.75 to 16.5 Å) at a sample-to-detector distance of 4.0 m, and the data were reduced using previously published procedures.37 The background scattering was subtracted using the approach described by Staples et al.23 All samples were measured in 1
sufficiently small to prevent creaming and sufficient to provide adequate scattering in the “Porod” region. The dynamic light scattering and zeta potential measurements, using phase analysis light scattering, were made prior to the SANS measurements. The light scattering measurements were made with emulsions prepared from hydrogeneous hexadecane and H2O in 0.01 M NaCl. The SANS measurements were made on emulsions, which were a mixture of d- and h-hexadecane (93.7 vol % d-hexadecane) in D2O, where the hexadecane is neutron refractive index matched to the D2O. The emulsion samples measured were prepared from the concentrated stock by dilution in solvent and concentrated surfactant and polymer solutions, with a final volume fraction ∼6.7%. The addition and order of addition of the surfactant and polymer to the initially SDS-stabilized emulsion had no effect on the emulsion stability outside the instability regions identified, and the initial emulsion structure was preserved. 3. Zeta Potential Measurements. The zeta potential of the emulsion droplets was measured using a hybrid Malvern Zetasizer 14976
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mm Starna quartz spectrophotometer cells, and at a temperature of 25 °C. 5. Measurements Made. SANS measurements were made for SDS/polydmdaac mixtures adsorbed at the hexadecane−water emulsion interface at fixed polydmdaac concentrations of 10, 250, and 375 ppm and at SDS concentrations from 3.2 to 26 mM, and at a fixed SDS concentration of 1.2 mM and polydmdaac concentrations of 0, 6, 10, 250, and 375 mM. The measurements were made predominantly for the isotopic combinations, h-SDS/polydmdaac/ D2O/h, d-hexadecane (93.7 vol % d-hexadecane). SANS measurements were made for SDS/PEI mixtures at the hexadecane−water emulsion interface in a range of different SDS values, and PEI concentrations at pH 3, 7, and 10. At a PEI concentration of 20 ppm, measurements were made for SDS concentrations of 1.3, 2.2, and 10 mM at pH 3, 7, and 10. At pH 7 and a SDS concentration of 1 mM, measurements were made for PEI concentrations of 10, 20, 40, 60, and 80 ppm. At pH 7 and PEI concentrations of 20 and 150 ppm, measurements were made for SDS concentrations in the range 2.6−25.9 and 1.2−20.5 mM, respectively. Dynamic light scattering, zeta potential measurements, and observation of the physical appearance of the SDS/polydmdaacstabilized emulsions were made over a wide range of polydmdaac (10− 375 ppm) and SDS (1−15 mM) concentrations.
SDS concentration of 1.2 mM, the transition from stable to unstable emulsions and back to a stable emulsion with increasing polydmdaac concentration is shown. For polydmdaac concentrations 250 ppm the emulsions are stable, and at concentrations intermediate between those extremes the emulsions are unstable. In Figure 2b the variation in emulsion stability with increasing SDS concentration at a fixed polydmdaac concentration of 375 ppm is shown. At low SDS concentrations, there is a narrow window of SDS concentrations over which the emulsions are unstable. From these observations, it is possible to construct a map in SDS and polydmdaac concentration of the instability region, and this is shown in Figure 3.
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RESULTS AND DISCUSSION 1. Polydmdaac/SDS. The strong interaction between polydmdaac and SDS is well established. The region of charge neutralization results in cloudy solutions and in some case precipitation,27 and it was not clear how this might impact upon the emulsion stability. Hence, a series of DLS measurements and observation of the optical properties of a range of emulsions at different polydmdaac and SDS concentrations were made to establish the regions of emulsion stability and instability. The images in Figure 2a and b show the transition from stable to unstable emulsions. In Figure 2a at an
Figure 3. Stability diagram for polydmdaac/SDS-stabilized hexadecane in water emulsion. Red denotes region of colloidal instability.
The stability diagram shows a narrow region at low SDS concentrations over which the combination of SDS and polydmdaac results in unstable emulsion droplets. In the stable regions, both zeta potential and SANS measurements were made. The variation in the emulsion zeta potential with SDS concentration at different polydmdaac concentrations shows some interesting variations, which correlate with the regions of emulsion stability/instability. This is shown in Figure 4 for 0, 10, 250, and 375 ppm polydmdaac. In the absence of polydmdaac, the emulsion droplets are anionic (negatively charged), and the surface potential increases slightly with increasing SDS concentration, from −40 to −60 mV for SDS concentrations from 1 to 13 mM. For the addition of 10 ppm polydmdaac, the surface potential is similar to that in the absence of polymer at the lower SDS concentrations, but remains roughly constant over the SDS concentration range measured. Hence, at the higher SDS concentrations the difference in surface potential can be attributed to the binding of the polydmdaac to the SDS at the oil−water interface. However, at this lowest polydmdaac concentration, there appears to be insufficient polymer to approach charge neutrality at the interface. This corresponds to a region in Figure 3 where there is no emulsion instability. At the higher polydmdaac concentrations shown in Figure 4, the variation in the surface potential is now quite different. The zeta potential measurements were made up to the point at which free micelles form in solution, and the onset of free
Figure 2. Optical appearance of hexadecane in water emulsions for (a) 1.2 mM SDS and 10, 20, 30, 63, 125, 250, and 375 ppm polydmdaac, and (b) 375 ppm polydmdaac and 1, 2, 3.2, 6.5, and 9.6 mM SDS (from left to right). 14977
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Figure 4. Variation in zeta potential with SDS concentration with addition of polydmdaac: 10 ppm (red), 250 ppm (green), 375 ppm (blue), and 0 ppm (black).
Figure 5. SANS scattering data for d/h-hexadecane in D2O emulsion stabilized by 250 ppm polydmdaac and SDS at concentrations of 3.2, 6.5, 13, 19, and 26 mM. The solid lines for 3.2, 6.5, and 13.0 mM SDS are model calculations as described in the text and for the model parameters summarized in Table 1.
micelle formation will be seen more clearly in the subsequent SANS data. For both 250 and 375 ppm polydmdaac, the surface potential is now positive at the lowest SDS concentration, and this is consistent with surface charge reversal and an excess of polydmdaac charge over the SDS. With increasing SDS concentration, the surface potential decreases but remains slightly positive until the highest SDS concentration measured for the highest polydmdaac concentration of 375 ppm. This implies that the polydmdaac charge is in excess over most of the SDS range measured, and shows no corresponding regions of emulsion instability. At an SDS concentration of ∼3 mM, the surface potential is sufficiently close to zero that the emulsion now shows the initial signs of unstability. At a polydmdaac concentration of 250 ppm, the surface potential switched from its initial positive state at low SDS concentrations to negative at an SDS concentration of ∼2 mM, and then remains negative with further increases in the SDS concentration. This implies that the surface changes from one where the polydmdaac charge is in excess to one where the SDS charge is in excess. The region where the surface goes through zero corresponds to the region of emulsion instability. Hence, the regions of instability are due to zero or low surface charge. From previous studies,23 it is known that there is a minimal surface charge from the adsorption of the SDS required to maintain stability. In Figure 5 the SANS data for 250 ppm polydmdaac/SDS at the hexadecane−water interface are shown. At the lower SDS concentrations of 3.2, 6.5, and 13 mM, the data are consistent with a thin layer adsorbed at the hexadecane−water interface. The slope of data at the higher Q values is determined by the thickness of the adsorbed layer, and a detailed analysis shows that in this case it is from an adsorbed layer with a thickness from ∼10 to ∼15 Å. The increased level of scattering as the SDS concentration increases reflects the increased adsorption at the hexadecane−water interface. Complementary measurements with d-SDS, where the SDS is now matched to the hexadecane and D2O, show that the adsorption is dominated by the SDS and that it is not possible to independently determine the amount of polydmdaac at the interface from the SANS data. At the higher SDS concentrations shown in Figure 5, at 19 and 26 mM SDS, the form of the scattering changes. Superimposed upon the interfacial scattering is additional scattering, which resembles that from small interacting aggregates. This was discussed and evaluated in detail by
Staples et al.23 and arises from free SDS micelles in the emulsion solution. The data in this regime have not been quantitatively analyzed, but the extrapolation of the data to the lower Q limit shows that once micellization occurs the SDS adsorption at the oil−water interface shows no further increase. The results of the detailed quantitative analysis of the scattering data using the method outlined in the Experimental Details, and before the onset of micellar scattering, are summarized in Table 1. For direct comparison, the data for SDS adsorption, in the absence of polymer, from ref 23 are reproduced in Table 2. Comparison of the adsorption data in Tables 1 and 2 shows that at 10 ppm polydmdaac there is a modest increase in the SDS adsorption at the oil−water interface due to the presence of the polydmdaac. This is further enhanced as the polydmdaac concentration is increased from 10 to 250 ppm, but further increases in polydmdaac concentrations do not further increase the SDS adsorption. Here, at 1 mM SDS, the area/molecule for the SDS decreases from ∼200 to ∼85 Å2, and at 13 mM from 47 to 35 Å2, with the addition of polydmdaac. The enhanced adsorption is due to the strong polyelectrolyte−surfactant interaction and complexation at the interface, as observed at the air−water interface.27 Although polymer adsorption at the oil− water interface is not directly visible, even with a changed “contrast”, its impact on the SDS adsorption is indirectly observed. The increase in SDS adsorption over the entire SDS concentration range evaluated and the change in zeta potential with the addition of polydmdaac imply both polymer and surfactant adsorption at the interface. At the oil−water interface, the thickness of the adsorbed layer, in the presence of polymer, is similar to that for SDS alone,23 and is ∼10−15 Å. At the air−water interface, it is typically ∼18−20 Å.27,38 The extended dodecyl alkyl chain length is ∼17 Å, and so this implies that the alkyl chains in the adsorbed layer are titled or not fully extended. At the higher SDS concentrations, the adsorbed layer is close to saturation, and hence the alkyl chains in the layer are more likely tilted. The addition of polydmdaac does not result in a substantial change in the adsorbed layer thickness, and this was also observed at the air−water interface.27,38 This implies that the 14978
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Table 1. Key Model Parameters for Polydmdaac/SDS at the Hexadecane/Water Interface 10 ppm polydmdaac
250 ppm polydmdaac
375 ppm polydmdaac
SDS concentration (mM)
d (±1 Å)
ρ (±0.2 × 10−6 Å−2)
Γ (±0.3 × 10−10 mol cm−2
d (±1 Å)
ρ (±0.2 × 10−6 Å−2)
Γ (±0.3 × 10−10 mol cm−2)
d (±1 Å)
ρ (±0.2 × 10−6 Å−2)
Γ (±0.3 × 10−10 mol cm−2)
1.2 3.2 6.5 9.6 13.0
10 10 10 10 10
4.8 3.8 2.0 1.8 1.3
1.1 1.8 3.0 3.8 4.2
10 12 15
3.5 4.2 3.2
2.0 1.8 3.3
10
3.5
2.0
15
3.2
3.3
15
1.8
4.8
15
1.8
4.8
The stability of the hexadecane in water emulsions on the addition of PEI/SDS was explored over a wide concentration and composition range. In marked contrast to the SDS/ polydmdaac mixture, the addition of SDS/PEI to the hexadecane/water emulsion does not result in any instability regions. As a consequence, the zeta potential measurements showed no regions of charge reversal that were associated with the instabilities observed in the addition of polydmdaac/SDS to the emulsion. The zeta potential was negative and showed little variation with SDS or PEI concentration. As changing pH had no pronounced effects on the nature of the SDS/PEI, all of the subsequent measurements were made at pH 7. The SANS data were all similar in general form to the data shown in Figure 5 for SDS/polydmdaac, and so are not reproduced here. Measurements at ∼1 mM SDS and at PEI concentrations of 20, 60, 80, and 150 ppm showed very little variation with PEI concentration. At PEI concentrations of 20 and 150 ppm, the SANS measurements were made over a wider range of SDS concentrations, and the key model parameters are summarized in Table 4. Increasing the PEI concentration from
Table 2. Key Model Parameter for SDS Adsorption at the Hexadecane−Water Interface, Reproduced from Ref 23 SDS concentration (mM)
Γ (±0.3 × 10−10 mol cm−2)
1.25 2.0 3.0 4.0 6.0 8.0 10.0 12.0
0.7 1.1 1.5 2.0 2.5 3.0 3.3 3.6
polymer adopts an extended conformation on the surface and does not protrude significantly from the surface. 2. PEI/SDS. At the air−water interface, the surface adsorption behavior of SDS/PEI, for the 2k daltons MW branched PEI, showed a strong pH dependence.28 This resulted in an evolution from monolayer adsorption to multilayer structures at the interface at pH 7 and 10, and only monolayer adsorption at pH 3. A limited range of SANS measurements were made at the oil−water interface for SDS/PEI at pH 3, 7, and 10 to establish if such pH effects occurred at the oil−water interface. The SANS measurements were made at SDS concentrations of 1.4, 2.2, and 10 mM, and the form of the scattering is similar to that shown for SDS/polydmdaac in Figure 5. The key parameters from the analysis of the data are summarized in Table 3. The results show that at the concentrations measured and at all pH value, the adsorbed layer is in the form of a thin monolayer, ∼12−15 Å thick. At an SDS concentration of 10 mM, the amount of SDS adsorbed is close to the adsorption in the absence of polymer (see Table 2), and at the lower SDS concentrations the addition of the PEI has resulted in an enhanced adsorption of the SDS. Apart from the data at an SDS concentration of 1.4 mM, changing the pH has little effect on the adsorbed amount. At a SDS concentration of 1.4 mM, the effect of increasing the solution pH from 3 to 7 was a modest increase in the adsorption. The enhanced SDS adsorption is again due to the strong surface polyelectrolyte−surfactant interaction and complexation, as observed at the air−water interface.26
Table 4. Key Model Parameters for PEI/SDS Adsorbed at the Hexadecane−Water Interface, at pH 7 20 ppm PEI
150 ppm PEI
SDS concentration (mM)
d (±1 Å)
ρ (±0.2 × 10−6 Å−2)
Γ (±0.3 × 10−10 mol cm−2)
1.2 1.4 2.2 3.2 6.5 9.6 12.9
12 12 10 10 10
4.8 4.3 2.1 1.3 0.6
1.3 1.7 3.0 3.5 4.0
d (±1 Å)
ρ (±0.2 × 10−6 Å−2)
Γ (±0.3 × 10−10 mol cm−2)
10
4.3
1.4
10
3.7
1.9
12
2.4
3.3
12
1.8
3.8
20 to 150 ppm has a minimal effect on the adsorption at the interface. However, comparing the adsorption data with that for SDS alone (Table 2), the addition of the PEI increases the adsorption for SDS concentrations
