Langmuir 2007, 23, 6091-6094
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Interaction Forces in Thin Liquid Films Stabilized by Hydrophobically Modified Inulin Polymeric Surfactant. 3. Influence of Electrolyte Type on Emulsion Films G. Gotchev,† T. Kolarov,† B. Levecke,‡ Th. Tadros,§ Khr. Khristov,† and D. Exerowa*,† Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria, ORAFTI Bio Based Chemicals, Aandorenstraat 1, 3300 Tienen, Belgium, and 89 Nash GroVe Lane, Wokingham, Berkshire RG40 4HE, United Kingdom ReceiVed January 31, 2007. In Final Form: March 15, 2007 The interaction forces in emulsion films stabilized using hydrophobically modified inulin (INUTEC SP1) were investigated as a function of concentrations of electrolytes of different types (NaCl, Na2SO4, and MgSO4). At a constant disjoining pressure of 36 kPa, a constant temperature of 22 °C, and a film radius of 100 µm, the film thickness, hw, decreased with an increase in electrolyte concentration until a critical value, Cel,cr, was reached above which hw remained constant. Cel,cr decreased with an increase in electrolyte valency (Cel,cr ) 5 × 10-2 mol‚dm-3 for NaCl and 1 × 10-2 mol‚dm-3 for Na2SO4 and MgSO4). The reduction in film thickness below Cel,cr could be accounted for by the compression of the electrical double layer. The Π-hw isotherms below Cel,cr could be fitted using the DerjaguinLandau-Verwey-Overbeek (DLVO) theory (constant charge and constant potential cases were considered). At a certain pressure, the film jumped to a Newton black film. The pressure at the jump decreased with an increase in electrolyte valency as a result of the reduction of the electrostatic barrier. At electrolyte (NaCl, Na2SO4, or MgSO4) concentrations higher than Cel,cr, the jump occurred at a low pressure that was independent of the electrolyte type. The thickness of the Newton black film was independent of both the concentration and nature of the electrolytes studied. The results show clearly that the polyfructose loops and tails remain strongly hydrated both in water and in high concentrations of electrolytes of different types, and these results explain the high INUTEC SP1 emulsion stability against coalescence of emulsions prepared under such conditions.
Introduction In a previous publication from this laboratory, we have studied the interaction between two oil droplets containing adsorbed hydrophobically modified inulin (INUTEC SP1) molecules in aqueous solution and also in the presence of NaCl up to 2 mol‚dm-3 using thin liquid film studies.1 The results obtained could be correlated to the stability of oil/water emulsions stabilized by INUTEC SP1. Experimental results using atomic force microscopy (AFM) to study the interaction between a hydrophobic glass sphere and a hydrophobic glass plate both containing adsorbed INUTEC SP1 also showed very strong repulsion between the adsorbed polymer layers both in water and in the presence of high Na2SO4 concentrations.2 Investigations on emulsion stability using INUTEC SP1 also showed lack of coalescence in the presence of high electrolyte concentrations (1 mol‚dm-3 MgSO4).3To explain this high emulsion stability in the presence of different types of electrolytes, we thought that it would be essential to study emulsion films in the presence of NaCl, Na2SO4, and MgSO4 and this is the objective of the present investigation. As discussed previously, emulsion film studies could directly give information on the interaction forces between emulsion * To whom correspondence should be addressed. E-mail: exerowa@ ipc.bas.bg. † Bulgarian Academy of Sciences. ‡ ORAFTI Bio Based Chemicals. § 89 Nash Grove Lane. (1) Exerowa, D.; Gotchev, G.; Kolarov, T.; Khristov, Khr.; Levecke, B.; Tadros, Th. Langmuir 2007, 23, 1711. (2) Nestor, J.; Esquena, J.; Solans, C.; Luckham, P. F.; Musoko, M.; Levecke, B.; Tadros, Th. F. J. Colloid Interface Sci., accepted. (3) Tadros, Th. F.; Vandamme, A.; Levecke, B.; Booten, K.; Stevens, C. V. AdV. Colloid Interface Sci. 2004, 108-109, 207. (4) Exerowa, D.; Kolarov, T.; Pigov, I.; Levecke, B.; Tadros, Th. F. Langmuir 2006, 22, 5013.
droplets containing adsorbed layers of hydrophobically modified inulin.1,5 The isotherms of disjoining pressure, Π, as a function of the equivalent film thickness, hw, obtained previously in the presence of NaCl concentrations in the range 2 × 10-4-2 mol‚dm-3 allows one to distinguish between films stabilized by electrical double layer repulsion and those that are sterically stabilized.1 In this paper we will establish Π-hw isotherms at various concentrations of NaCl, Na2SO4, and MgSO4 and this allows us to compare film stability in different electrolyte types. Experimental Section Materials. Hydrophobically modified inulin, INUTEC SP1 (ORAFTI, Belgium), is the same polymeric surfactant as used before.1-4 This polymer is an ABn graft type with A being inulin (linear polyfructose with a degree of polymerization greater than 23) and B being alkyl chains (C12) that are grafted on the inulin backbone. The structure of the molecule was given before.4 The oil used in the present investigation, namely, Isopar M, is the same as described before.1,3 The electrolytes used were NaCl (Merck, with a purity higher than 99.99% that was heated to 500 °C to remove any organic impurities); Na2SO4‚10H2O (Merck, purity higher than 99.0%); and MgSO4·7H2O (Merck, 98-102% purity). Doubly distilled water (electroconductivity of ∼1 µS‚cm-1) was used for the preparation of all solutions. Methods. The interfacial tension was measured at 22 °C, using the Du Nouy ring method and a Kru¨ss K10 tensiometer (Kru¨ss, Germany). The microinterferometric method for the investigation of emulsion films has been described before.1,5 Microscopic emulsion films of 100 µm radius were studied in a measuring cell at constant capillary pressure and in a porous plate where the capillary pressure could be altered. The film thickness is monitored by measuring the reflection (5) Exerowa, D.; Kruglyakov, P. M. Foam and Foam Films; Elsevier: Amsterdam, 1998.
10.1021/la7002673 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/24/2007
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Figure 1. Effect of INUTEC SP1 concentration on interfacial tension, σ1,2, at the interfaces of Isopar M/aqueous solutions of NaCl (circles), Na2SO4 (triangles), and MgSO4 (squares) at concentrations of 2 × 10-4 mol‚dm-3 (open symbols) or 0.5 mol‚dm-3 (closed symbols).
Figure 2. Effects of NaCl (circles), Na2SO4 (triangles), and MgSO4 (squares) concentration, Cel, on the equivalent thickness, hw, of oil/water (O/W) emulsion films from 2 × 10-5 mol‚dm-3 INUTEC SP1 aqueous solutions. of monochromatic light. If the film is optically homogeneous and has a refractive index equal to that of the bulk solution, it is possible to calculate the so-called “equivalent” film thickness, hw. A specialized measuring system determined the capillary pressure (equal to the disjoining pressure in the film) versus film thickness. We named this method “film pressure balance technique”.5 This is how isotherms of disjoining pressure are depicted, that is, interaction forces in the thin liquid film (in the case of constant capillary pressure experiments, in particular).
Results and Discussion Interfacial Tension. Figure 1 shows the interfacial tension, σ1,2, versus INUTEC SP1 concentration in the presence of NaCl, Na2SO4, and MgSO4 at 2 × 10-4 and 0.5 mol‚dm-3 for each type of electrolyte. There does not seem to be a significant change in the interfacial tension between the various types of electrolytes and the various concentrations. In addition, the break point, which may be identified as the critical association concentration (CAC), is the same in all cases (5 × 10-5 mol‚dm-3 INUTEC SP1). The CAC is identical to that reported before for NaCl.1 The near constant values of interfacial tension in the presence of different electrolytes simplifies the analysis of the emulsion film studies. Influence of Electrolyte Concentration and Type on Film Thickness. Figure 2 shows the dependence of the equivalent film thickness of hw on electrolyte concentration for NaCl, Na2SO4, and MgSO4 at a constant capillary pressure Pc ) 36 Pa, at 22 °C, and at an INUTEC SP1 concentration of 2 × 10-5 mol‚dm-3. The choice of Na2SO4 and MgSO4 as a comparison to NaCl is based on two main effects. First, it is well-known that
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Figure 3. Measured disjoining pressure, Π, versus equivalent thickness, hw, for O/W emulsion films from 2 × 10-5 mol‚dm-3 INUTEC SP1 and 2 × 10-4 mol‚dm-3 NaCl (circles), Na2SO4 (triangles), and MgSO4 (squares) aqueous solutions. The corresponding pairs of lines (from top to bottom) represent the DLVO evaluation as a sum of double layer repulsion at constant potential (solid lines) and constant charge (dashed lines) modes and van der Waals attraction.
compression of the electrical double layer occurs at lower electrolyte concentrations for 1:2 and 2:2 electrolytes when compared with a 1:1 electrolyte. Second, it is also known that the sulfate anion is a strong “salting-out” ion and hence it is of interest to prove that the polyfructose chains may still remain hydrated in the presence of such anions. In all cases, an increase in electrolyte concentration leads to a decrease in film thickness up to a certain concentration, which we designate as the “critical electrolyte concentration” (Cel,cr).1,4,5 Above Cel,cr, the thickness remains constant and a plateau is reached in the hw-Cel curve. Cel,cr depends on the nature of the electrolyte with NaCl giving a value of 5 × 10-2 mol‚dm-3, whereas both Na2SO4 and MgSO4 give a value of 1 × 10-2 mol‚dm-3. The trend in the variation of hw with Cel can be rationalized in terms of the interaction forces that operate in the emulsion film. At a low electrolyte concentration, electrostatic interaction predominates as a result of the presence of extended double layers and this results in a high equilibrium film thickness. As the electrolyte concentration is increased, compression of the double layer occurs and the film thickness decreases, in accordance to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.6,7 At and above Cel,cr, the double layer is considerably compressed and the film thickness is now determined by the thickness of the steric layers (the loops and tails of the polyfructose chains). It is clear that significant compression of the double layer occurs at a higher electrolyte concentration (5 × 10-2 mol‚dm-3) for 1:1 electrolyte when compared to that of 1:2 and 2:2 electrolyte (1 × 10-2 mol‚dm-3). Thus, Cel,cr denotes the transition from electrostatic to steric interactions as discussed before.1,4,5 It is interesting to note that the film thickness obtained above Cel,cr (10.5 nm) is constant and does not seem to change with an increase in electrolyte concentration (for all three electrolyte types) up to 2 mol‚dm-3. This clearly shows that the film remains hydrated up to this very high electrolyte concentration, and this is an important criterion for maintaining emulsion stability against coalescence. Disjoining Pressure-Film Thickness (Π-hw) Isotherms. Figure 3 shows the Π-hw isotherms at a constant INUTEC SP1 concentration of 2 × 10-5 mol‚dm-3 and at 2 × 10-4 mol‚dm-3 (6) Derjagiun, B. V.; Landau, L. D. Acta Physicochim. URSS 1941, 14, 633. (7) Vervwey, E. J. V.; Overbeek, T. G. The Theory of the Stability of Liophobic Colloids; Elsevier: Amsterdam, 1948.
Interaction Forces in Inulin-Stabilized Films
electrolyte (well below Cel,cr). In all cases, an increase in pressure leads to a decrease in film thickness, and above a critical pressure that depends on electrolyte type, the film thickness jumps to a low and constant value that becomes independent of the applied pressure up to the highest applied pressure of 45 kPa (the maximum pressure that could be applied). This behavior can be accounted for by the change from electrostatic repulsion (that produces a common black film with higher thickness) to steric repulsion that is determined by the Newton black film. The Π-hw isotherm below the jump can be analyzed using the DLVO theory, as previously described, making use of an appropriate three-layer emulsion film model.1 Within the framework of this model, the total thickness of the emulsion film, h, can be approximated by the experimentally measured equivalent thickness, hw, and the film distance between the planes at the onset of Πel is d ) h - 3.6 nm. Thus, in the case under consideration, Π(h) ) Πel(d) + Πvw(h). Πel(d) was computed following the numerical procedures for the 1:1, 1:2, and 2:2 electrolytes based on algorithms for the solution of complete Poisson-Boltzmann equations described earlier.8,9 Both constant charge and constant potential modes were considered. The van der Waals interactions were calculated according to the equation Πvw ) -A/6πh3, where A is the effective Hamaker constant. For Isopar M, which is a mixture of C11-C15 isoalkanes, an average value of 5 × 10-21 J was taken based on the literature data for these alkanes.10 The results of these DLVO computations are shown in Figure 3 for the NaCl, Na2SO4, and MgSO4 cases (from top to bottom) by the corresponding pairs of solid and dashed lines. The latter represent the constant potential and constant charge cases, respectively. As can be seen, there is a good fit to the experimental data when the thickness is larger than ∼30 nm for NaCl and 25 nm for Na2SO4 and MgSO4. For the diffuse double layer potential at infinity, these results give values of -50, -18, and -15 mV for the cases of NaCl, Na2SO4, and MgSO4, respectively. Within the approximations made, it seems that the experimental results are between the constant potential and constant charge cases. In the transition region (hw is less than ∼25-30 nm), the experimental data cannot be fitted by the DLVO theory, since in these cases either metastable films are produced or the deficiencies of the DLVO theory at low thicknesses are manifested. Nevertheless, the tendency of decreasing diffuse double potential in the sequence of NaCl, Na2SO4, and MgSO4 is confirmed at least qualitatively. It should be mentioned that the pressure at the jump from the common black film to the Newton black film (i.e., at the transition from electrostatic to steric interaction) decreases as the valency of the electrolyte increases. This jump occurs at 4.5 kPa for NaCl, 3 kPa for Na2SO4, and 1 kPa for MgSO4. This trend is expected from the DLVO theory which shows that the electrostatic barrier decreases in the order NaCl > Na2SO4 > MgSO4, and this justifies the application of the DLVO theory for the emulsion films below Cel,cr. Figure 4 shows the Π-hw isotherms at a constant INUTEC SP1 concentration of 2 × 10-5 mol‚dm-3 and at NaCl, Na2SO4, and MgSO4 concentrations well above Cel,cr (0.05, 0.5, 1.0, and 2.0 mol‚dm-3 for each of electrolytes). Since all measured thickness values were very close to each other, an average value was taken and the error bars are indicated in the figure. In all cases, at a thickness of ∼10 nm, there is a transition to a Newton (8) Chan, D. Y.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283. (9) Kuo, Y.-C.; Hsu, J.-P. J. Colloid Interface Sci. 1993, 156, 250. (10) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: London, 1991; Vol. I, p A9.2.
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Figure 4. Measured disjoining pressure, Π, versus equivalent thickness, hw, for O/W emulsion films from aqueous solutions of 2 × 10-5 mol‚dm-3 INUTEC SP1: averaged results from the measurements at NaCl, Na2SO4, and MgSO4 concentrations of 0.05, 0.5, 1, and 2 mol‚dm-3 for each of electrolytes.
Figure 5. Measured disjoining pressure, Π, versus equivalent thickness, hw, for O/W emulsion films from aqueous solutions of 1 × 10-4 mol‚dm-3 INUTEC SP1 and 2 × 10-4 mol‚dm-3 NaCl and MgSO4 (closed circles and squares, respectively). For comparison, the results obtained at 2 × 10-5 mol‚dm-3 INUTEC SP1 and 2 × 10-4 mol‚dm-3 NaCl and MgSO4 (open circles and squares, respectively) are also shown.
black film (NBF), regardless of the electrolyte type. The pressure at which the transition occurs is ∼0.5 kPa; that is, the barrier is very low and all films transform to NBFs. The thickness of the Newton black film is the same as that in the isotherm depicted for the lower electrolyte concentration (Figure 3). As mentioned above, the Π-hw isotherms in Figure 4 were plotted at electrolyte concentrations higher than Cel,cr; that is, the electrostatic disjoining pressure was eliminated and only steric forces were operating. For the latter case, the experimental results indicate clearly that they do not affect the barrier in the Π-hw isotherms; that is, the steric disjoining pressure is not affected by the electrolyte type. This result is in agreement with the plateau of the hw-Cel curve (Figure 2), where the film thickness does not depend on electrolyte concentration. Figure 5 shows Π-hw isotherms at a higher INUTEC SP1 concentration of 1 × 10-4 mol‚dm-3 in the presence of NaCl (2 × 10-4 mol‚dm-3) and MgSO4 (2 × 10-4 mol‚dm-3), that is, well below Cel,cr. For comparison, the results obtained at 2 × 10-5 mol‚m-3 INUTEC SP1 are also plotted in the same figure. The results at higher a INUTEC SP1 concentration show that the jump from electrostatic to steric interaction occurs at a lower disjoining pressure. This may be due to the higher electrolyte content in the solution when using higher INUTEC SP1 concentrations.1 Evidence for this has been obtained before using conductivity measurements.4 Thus, it can be concluded
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that using a higher polymer surfactant concentration only affects the electrostatic component of the disjoining pressure through an increase in electrolyte concentration and does not have any effect on the steric component of the disjoining pressure. This result is of great importance when using practical emulsion systems, whereby higher INUTEC SP1 concentrations are usually used for the preparation of the emulsion.
Conclusions Emulsion film thickness was measured as a function of NaCl, Na2SO4, and MgSO4 concentration at a low capillary pressure of 36 kPa and at a constant concentration of 2 × 10-5 mol‚dm-3 hydrophobically modified inulin (INUTEC SP1). The results showed a gradual decrease in film thickness with an increase in electrolyte concentration, and above a critical electrolyte
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concentration (Cel,cr), the film thickness remained constant. These results indicate the transition from electrostatic to steric interaction. The plateau thickness reached above Cel,cr was independent of the nature of the electrolyte, indicating strong hydration of the polyfructose loops and tails under these conditions. The disjoining pressure versus film thickness isotherms also showed the transition from electrostatic to steric interaction, showing a jump from a common black film to a Newton black film. This jump occurred at a low disjoining pressure when the electrolyte concentration was well above Cel,cr. The thickness of the Newton black film was ∼7.2 nm and was independent of the nature of the electrolyte. These results explain the high stability of INUTEC SP1 emulsions prepared using such high concentrations of electrolytes of different types. LA7002673
