Langmuir 2002, 18, 3487-3494
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Flocculation Transitions of Weakly Charged Oil-in-Water Emulsions Stabilized by Different Surfactants R. Aveyard, B. P. Binks, J. Esquena, and P. D. I. Fletcher* Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
P. Bault and P. Villa Universite´ de Picardie, Jules Verne, Faculte´ des Sciences, Laboratoire de Chimie Organique et Cine´ tique, 33 rue Saint Leu-80039, Amiens Cedex, France Received November 28, 2001. In Final Form: February 18, 2002 This study concerns dodecane-in-water emulsions stabilized primarily by a range of nonionic surfactants. The emulsions are stable with respect to coalescence and Ostwald ripening but unstable with respect to flocculation and creaming. Sharp and reversible transitions between a flocculated state and a nonflocculated state can be induced by the addition of small mole fractions of either the anionic surfactant sodium octadecyl sulfate or the cationic surfactant octadecyl trimethylammonium bromide and were measured as a function of NaCl concentration. The flocculation transitions are modeled in terms of three types of colloidal forces: a short-range repulsion modeled as a “hard wall”, van der Waals attraction, and electrostatic repulsion. The flocculation transition is taken to correspond to the ionic surfactant concentration for which the maximum in disjoining pressure equals the capillary pressure of the drops. Comparison of calculated and experimental results shows reasonable agreement for emulsions stabilized by n-decyl-β-D-glucoside and 1-O-n-decyl-D,L-xylitol. Emulsions stabilized by n-decyl-N-methylglucamide and n-dodecyl octaoxyethylene and n-hexadecyl 20-oxyethylene glycol ethers (C12E8 and C16E20) reveal a significant negative charge on the drop surfaces in the absence of added ionic surfactant. Additionally, we find that the surface charging of the surfactants with sugar headgroups by addition of charged surfactants is correctly predicted from the concentration of added ionic surfactant with the assumption that the ionic surfactant distributes equally between the emulsion surface monolayer and the micelles present in the continuous phase. For C12E8 and C16E20, the added ionic surfactant is found to distribute preferentially to the emulsion surfaces.
Introduction and Theory Emulsions consist of thermodynamically unstable dispersions of a liquid in a second liquid with which it is partially or wholly immiscible. Emulsion separation may occur by a combination of four processes:1 creaming or sedimentation, Ostwald ripening, flocculation, and coalescence. Creaming or sedimentation rate depends on the density difference between the dispersed and continuous phases, droplet size, and continuous phase viscosity but is also affected by interdroplet interactions. The rate of Ostwald ripening depends on the solubility and diffusion rate of the dispersed phase in the continuous phase. Stability to flocculation is obtained only when the droplets are coated with a stabilizing film of adsorbed surfactant and is mainly determined by the colloidal interactions between the droplet surfaces. Stability to coalescence, again obtained only in the presence of an adsorbed surfactant film, is thought to be related to the colloidal interactions at short range together with the monolayer rigidity.2,3 Colloidal interactions between solid surfaces have been investigated extensively, both for large surface areas using the surface forces apparatus4 and between micron- and * To whom correspondence should be addressed. E mail:
[email protected]. (1) See, for example: Modern Aspects of Emulsion Science; Binks, B. P, Ed.; Royal Society of Chemistry: Cambridge, 1998. (2) Fletcher, P. D. I.; Horsup, D. I. J. Chem. Soc., Faraday Trans. 1992, 88, 855. (3) Kabalnov, A.; Wennerstrom, H. Langmuir 1996, 12, 276. (4) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992.
submicron-sized colloidal particles.5 Although there is a large body of information concerning colloidal interactions across air-liquid-air (i.e., foam) films, there is relatively little data concerning interactions between deformable liquid-liquid surfaces of direct relevance to emulsion stability. Using a novel instrument based on a flexible micropipet, Aveyard et al. measured directly the surface deformation and total force as a micron-sized half oil drop was pushed up to an oil-water interface.6-8 In addition to determining curves of disjoining pressure as a function of surface separation, both the drop and surface deformations and the total force were successfully quantitatively modeled. A different approach was taken by Leal Calderon et al. who examined oil-in-water (o/w) emulsions in which the oil drops contained magnetic particles.9-11 Using optical measurements of the drop structuring under the influence of magnetic fields, they were able to extract quantitative measurements of drop interactions as a function of separation. A third approach, developed in our own laboratories, involves comparing measured floc(5) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain; VCH: New York, 1994. (6) Aveyard, R.; Binks, B. P.; Cho, W.-G.; Fisher, L. R.; Fletcher, P. D. I.; Klinkhammer, F. Langmuir 1996, 12, 6561. (7) Cho, W.-G.; Fletcher, P. D. I. J. Chem. Soc., Faraday Trans. 1997, 93, 1389. (8) Binks, B. P.; Cho, W.-G.; Fletcher, P. D. I. Langmuir 1997, 13, 7180. (9) Leal Calderon, F.; Stora, T.; Mondain Monval, O.; Poulin, P.; Bibette, J. Phys. Rev. Lett. 1994, 72, 2959. (10) Mondain Monval, O.; Leal Calderon, F.; Philip, J.; Bibette, J. Phys. Rev. Lett. 1995, 75, 3364. (11) Mondain Monval, O.; Leal Calderon, F.; Bibette, J. J. Phys. II France 1996, 6, 1313.
10.1021/la011723e CCC: $22.00 © 2002 American Chemical Society Published on Web 03/27/2002
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culation transitions with calculations to test models of the colloidal interactions between emulsion drops.12 We consider o/w emulsions, stabilized primarily by a nonionic surfactant, which are stable with respect to Ostwald ripening and coalescence. The emulsion drops undergo a sharp, reversible transition between a flocculated and nonflocculated state on the addition of a small mole fraction (with respect to the total surfactant concentration) of a strongly adsorbing ionic surfactant. Comparison of the experimental flocculation transition results for different concentrations of added electrolyte allows the testing of different models of the colloidal interactions between the emulsion drop surfaces. As discussed in ref 12, models for the flocculation transition can be formulated either in terms of the total energy of interaction between two deformable drops13-15 or in terms of the disjoining pressure across the thin oilwater-oil emulsion film formed when two deformable drops adhere together. Flocculation transitions measured for dodecane-in-water emulsions stabilized primarily by n-decyl-β-D-glucoside (C10βG) doped with small amounts of ionic surfactant and containing different concentrations of NaCl were found to agree with calculated transitions based on film disjoining pressure. This simple approach, used in the present study to model flocculation transitions in emulsions stabilized by a range of different surfactants, is briefly summarized below. Three different types of colloidal interaction are considered. First, the disjoining pressure Πhw arising from a combination of steric, undulation, hydration, and any other short-range, repulsive forces (necessary for stability against coalescence) is crudely modeled as arising from a “hard-wall” potential and is characterized by a single length parameter hhw,
Πhw ) ∞ for h < hhw and Πhw ) 0 for h > hhw (1) where h is the film thickness and hhw is the range of the hard-wall interaction. Zero separation between the surfaces (i.e., h ) 0) is taken to correspond to contact between the surfaces separating the tail- and headgroups of the adsorbed surfactant monolayers. Within this definition, the lowest accessible value of film thickness h is equal to hhw, that is, h cannot equal zero, corresponding to film rupture and drop coalescence. As described in ref 12, the value of hhw is obtained from measurement of the contact angle θ formed by the emulsion film separating two adhering drops. For the measurements described here, the flocculation transitions are primarily sensitive to the longer-range interactions and are not very dependent on the nature of the short-range interactions and the value of hhw. Second, the attractive van der Waals component of the disjoining pressure, ΠvdW, is calculated using the expression
ΠvdW ) -
A 6πh3
(2)
where A is the Hamaker constant for the oil-water-oil film (where the oil drops are taken to include the surfactant monolayer tail region). For dodecane-water-dodecane (12) Aveyard, R.; Binks, B. P.; Esquena, J.; Fletcher, P. D. I.; Buscall, R.; Davies, S. Langmuir 1999, 15, 970. (13) Danov, K. D.; Petsev, D. N.; Denkov, N. D.; Borwankar, R. J. Chem. Phys. 1993, 99, 7179. (14) Denkov, N. D.; Petsev, D. N.; Danov, K. D. J. Colloid Interface Sci. 1995, 176, 189. (15) Denkov, N. D.; Petsev, D. N.; Danov, K. D. J. Colloid Interface Sci. 1995, 176, 201.
films, the value of A is 5.0 × 10-21 J.16 Equation 2 neglects the effects of the headgroups of the adsorbed surfactant monolayers. As discussed in ref 4, this is valid only for film thicknesses significantly in excess of twice the thickness of the headgroup region. Equation 2 also neglects effects due to retardation which cause the dispersion component of the van der Waals force to decrease to a magnitude less than that predicted by eq 2 for h greater than about 10 nm. This approximation is less serious for emulsion films than for foam films in air since the zero frequency component of A (not subject to retardation) has a greater contribution to the total value of A in the case of emulsion films. Overall, eq 2 is likely to lead to fairly accurate values of ΠvdW for thicknesses greater than about 2 nm and less than some tens of nanometers. Third, the repulsive electrostatic component of the disjoining pressure Πel is given by5
( )
Πel ) 64ckT tanh2
zeeψ exp(-κh) 4kT
(3)
where c is the concentration of inert symmetrical electrolyte of charge number z, k is the Boltzmann constant, T is the absolute temperature, e is the electronic charge, and ψ is the surface potential assumed to be located in the plane corresponding to h ) 0, that is, at the surface between the surfactant head- and tailgroups. The inverse Debye length κ is
κ)
( ) 0kT
-1/2
2z2e2c
(4)
where is the dielectric constant of the aqueous phase and 0 is the permittivity of free space. Equation 3 is valid for κh > 1, the “weak overlap” approximation, for which both the surface potential ψ and surface charge density σ remain constant with changing film thickness (see, for example, refs 4 and 5). In considering Πel, it is further assumed that the charge distribution over the drop surface is continuous and homogeneous; these latter approximations are expected to become inaccurate when the charge density is low and when the film thickness is small. However, following comparison of the predictions of several alternative expressions, the use of eq 3 was found to provide the most satisfactory fit to experimental flocculation transitions for emulsions stabilized by C10βG.12 In the presence of a concentration c of a symmetrical electrolyte of charge number z, the surface potential is related to the surface charge density by the Grahame equation,4
σ ) (x80kTc sinh(zeψ/2kT)
(5)
The surface charge density on the emulsion drop surfaces is related to the mole fraction mem of ionic surfactant within the (mainly) nonionic monolayer stabilizing the droplets. For a nonionic surfactant monolayer (assumed to be totally uncharged initially) containing a low mole fraction of ionic surfactant (assumed fully dissociated), σ is related to mem according to
σ ) memez/As
(6)
As discussed in ref 12, the latter assumption is expected to be valid for mem less than about 0.05 where the ionic surfactant molecules are widely separated in the mono(16) Hough, D. B.; White, L. R. Adv. Colloid Interface Sci. 1980, 14, 3.
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layer. In eq 6, it is implicitly assumed that the surface area occupied per ionic surfactant in a mixed monolayer is equal to that of the nonionic surfactant (As). Since the areas of the nonionic and ionic surfactants are likely to be equal within a factor of 2 and because mem is much less than 1, this approximation leads to negligible error. Equations 5 and 6 allow the surface charge properties (σ and ψ) of the emulsion drop surfaces to be obtained from the value of mem. The value of mem is related to the total (known) mole fraction with respect to the total surfactant concentration. As detailed in ref 12, the relationship between mem and mtot requires consideration of the distribution of the ionic surfactant between the emulsion drop surfaces and the surfaces of the micelles present in the continuous phase and is given by
mtot )
mem(cNem + cNmic/P) cN
(7)
where cNem and cNmic are the concentrations of nonionic surfactant in the emulsion drops and micelles, respectively, cN is the total concentration of nonionic surfactant, and P is a distribution coefficient defined as the mole fraction of ionic surfactant in the emulsion drop monolayer divided by the mole fraction in the micelles. The value of cNem is estimated from the emulsion drop surface area (calculated from the measured drop size distribution) and the area per adsorbed surfactant molecule at the oilwater interface (measured using interfacial tensiometry). The concentration of nonionic surfactant in the micelles is given by cNmic ) cN - cNem - cac, where cac is the critical aggregation concentration of the nonionic surfactant in the emulsion. For the systems considered here, the concentrations of nonionic surfactant adsorbed at the emulsion surfaces cNem were in the range 0.2-0.8 mM and the concentrations of micellized nonionic surfactant cNmic were between 3.1 and 9.8 mM for the different systems. In the derivation of eq 7, it is assumed that (i) the concentration of nonadsorbed ionic surfactant is negligibly small for the very long chain species used in this work and (ii) the fraction of nonionic surfactant partitioned into the oil is also negligible. With respect to this latter assumption, the extent of distribution (to oil) of sugar-headgroup surfactants between water and toluene has been shown to be small relative to poly(oxyethylene) species with similar headgroup size.17 Using data for the partitioning of n-dodecyl octaoxyethylene glycol ether (C12E8) between water and decane,18 we estimate that the overall concentration of C12E8 located in the dodecane drops (i.e., nonadsorbed) of an emulsion containing 20 vol % oil does not exceed 0.2 mM, that is, the error in estimating the concentration of micellized nonionic surfactant present is 99%). The sample of n-hexadecyl 20-oxyethylene glycol ether (C16E20, Sigma, sold under the trade name of Brij 58) contained a distribution of chain lengths around the average head and tail chain lengths quoted. 1-O-n-Decyl-D,L-xylitol (C10X) was synthesized and purified as detailed in ref 19. Air-water surface tensions were measured using a Kru¨ss K10 instrument with a du Nou¨y ring. Dodecane-water interfacial tensions were determined using the Kru¨ss K10 (for values above 3 mN m-1) and a Kru¨ss Site 04 spinning drop tensiometer for lower values. Densities and refractive indices of the solutions required for calculation of the tensions were determined using a Paar DMA 55 densimeter and Abbe´ refractometer (Hilger & Watts), respectively. Emulsion samples were prepared by homogenization of 80 vol % of aqueous phase containing surfactant and NaCl plus 20 vol (19) Goodby, J. W.; Haley, J. A.; Watson, M. J.; Mackenzie, G.; Kelly, S. M.; Letellier, P.; Douillet, O.; Gode´, P.; Goethals, G.; Ronco, G.; Villa, P. Liq. Cryst. 1997, 22, 367.
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Aveyard et al. Table 1. Parameters Used for the Calculations of the Flocculation Transition Boundaries for the Different Nonionic Surfactantsa surfactant γ/mN m-1 C10βG C10NMG C10X C12E8 C16E20 a
1.0 ( 0.1 2.0 ( 0.1 1.5 ( 0.1 2.7 ( 0.1 7.7 ( 0.1
R/µm
As/nm2
cac/mM
A/10-21 J
3.7 ( 0.4 3.1 ( 0.3 10 ( 1 3.6 ( 0.4 6.8 ( 0.7
0.37 ( 0.05 0.45 ( 0.05 0.29 ( 0.05 0.58 ( 0.05 0.89 ( 0.05
1.5 ( 0.1 6.2 ( 0.2 0.61 ( 0.05 0.16 ( 0.01 0.0052 ( 0.0005
5.0 ( 0.5 5.0 ( 0.05 5.0 ( 0.05 5.0 ( 0.05 5.0 ( 0.05
The oil is dodecane (20 vol %), and the electrolyte is NaCl.
surfactant sodium dodecyl sulfate (SDS, Lancaster, 99%) or the cationic species Hyamine 1622 (BDH, >99%) was used as the titrant. Tension measurements were made at 25.0 °C. All other measurements were made at the ambient temperature of 22 ( 3 °C.
Results and Discussion
Figure 1. Molecular structures of the surfactants used. % mL of dodecane using an Ultra Turrax T25 homogenizer with either an 18G or 8G head. Both the homogenizer speed and homogenization time were controlled. Emulsion drop size distributions were obtained using a Malvern MasterSizer MS20 laser diffraction instrument, which allows resolution of drop diameters between 0.1 and over 200 µm. For the sizing measurements, the emulsion samples were diluted approximately 400-fold into water containing surfactant at a concentration equal to the appropriate critical aggregation concentration. Emulsion stability with respect to coalescence and Ostwald ripening was checked by sizing the emulsions over a period of several days; no significant drop growth was observed. For the microscopic observation of the emulsions, undiluted emulsion samples were placed directly onto microscope slides and covered with a cover slip. For the determination of the contact angle in adhering pairs of emulsion drops (see Figure 3) using microscopy, a small sample of the creamed emulsion was diluted approximately 100-fold with the continuous phase of the same emulsion. The diluted emulsions were held within hemocytometer cells (Weber Scientific Ltd.) giving a sample thickness of 0.2 mm. In the cells, the emulsion drops cream to the underside of the coverslip, and the microscope field of view was set to include a doublet of adhering drops of approximately equal radius. The focal plane was set to lie in the plane of the centers of the drops by initially focusing on the underside of the coverslip and moving the focus down by a distance equal to the drop radius. Digital images from the microscope (Nikon Labophot) were obtained using a DIC-U high-resolution camera (World Precision Instruments) connected to a PC. Contact angles were determined by analysis of the perimeter profile of the doublet of adhering drops extracted from the images using either Aldus PhotoStyler or Adobe Photoshop software. Zeta potentials of emulsion drops were measured using a Matec ESA8000 instrument. The two-phase Epton titration20 employing a mixed indicator was used for the determination of ionic impurities in the nonionic surfactant samples. Either the anionic (20) Reid, V. W.; Longman, G. F.; Heinerth, E. Tenside 1967, 4, 292.
For the various emulsion systems, interfacial tensiometry was used to determine the cac’s together with the values of the limiting area, As, occupied per adsorbed nonionic surfactant at the oil-water interface. Values of As and the post-cac tension γ were derived from measurements of the variation of oil-water tension with nonionic surfactant concentration. The data for concentrations below the cac were fitted to a second-order polynomial and differentiated to obtain As using the Gibbs adsorption equation. Values of cac, As, and γ at concentrations in excess of the cac for the different surfactant systems are summarized in Table 1 together with additional input parameters required for the model calculations. For C10βG and C10NMG, the values of As at the dodecane-water interface are similar to those at the air-water surface,17 suggesting that the limiting area per molecule is dominated by headgroup interactions and solvation. At the dodecane-water interface, there are significant differences in the monolayer surfactant packing densities between the two open chain sugar structures C10NMG (low surface density, high As) and C10X (high surface density, low As). This observation, coupled with the fact that the cac of C10X is 10 times lower than that for C10NMG, suggests that the xylitol headgroups may possess a greater degree of attractive interactions (probably due to hydrogen bonding) between adjacent molecules within the monolayers and micellar aggregates. C10βG, with a more globular, cyclic headgroup, shows cac and As values intermediate between those of C10NMG and C10X. The poly(oxyethylene) headgroup surfactants show large values of As which increase with increasing E number of the headgroup. The emulsions were found to be rather polydisperse, and median radii (defined as the radius at which the cumulative volume distribution is equal to 0.5) were used in the model calculations. As noted in ref 12, however, the calculated flocculation transitions are not very sensitive to the precise value of drop radius. For example, for C10βG emulsions with 1 mM 1:1 electrolyte, calculations using twice the actual mean drop radius (i.e., 7.4 instead of 3.7 µm) lead to a predicted mole fraction of ionic surfactant corresponding to the flocculation transition which is only 20% lower than for the actual radius. Hence, the emulsion polydispersity is not expected to have a large influence on the results described here. Figure 2 shows micrographs of emulsion samples in the nonflocculated (upper image) and flocculated state (lower image). In the nonflocculated state, the droplets are observed to move freely with a random, Brownian motion, whereas they become fixed in the flocculated state. Microscopic observations of selected
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Figure 3. Optical micrograph of a doublet of adhering emulsion droplets showing the formation of a flat thin film and a finite contact angle θ. The value of the contact angle (20°) was determined by curve fitting of the drop doublet perimeter obtained by analysis of the digitized image. The sample contained 100 mM NaCl and no added ionic surfactant, and all other conditions were as for Figure 2.
Figure 2. Optical micrographs of H2O/C10NMG/SODS/dodecane/NaCl emulsion samples in the nonflocculated (upper figure) and flocculated state (lower figure) induced by the addition of NaCl. The emulsion samples contained 0.03 mole fraction SODS with respect to the total surfactant concentration (10 mM) and 20 vol % dodecane.
samples were used to confirm the assignment of a flocculated or nonflocculated state made on the basis of creaming rate observations. A high-magnification image of a doublet of adhering drops is shown in Figure 3. The digitized images were analyzed by fitting the entire perimeters of the drops in order to obtain the film contact angle θ. The energy of interaction per unit area of the film, ∆E, is related to the oil-water tension and θ according to21
∆E ) 2γ(cos θ - 1)
(8)
Within the context of the model of colloidal interactions assumed here, uncharged drops adhere due to van der Waals attraction but achieve an equilibrium emulsion film thickness taken to be hhw. As described in ref 12, the contact angles can then be used to estimate hhw. hhw is not a directly determined separation; rather it is the apparent or effective range of a hard-wall repulsion which accounts for the net energy of interaction between (assumed) uncharged drops. Values of θ, ∆E, and hhw estimated in this way for the various surfactants are summarized in Table 2. For the three sugar surfactants (C10βG, C10NMG, and C10X), the values of ∆E and hhw are all very similar, although the glucoside headgroup is a cyclic, globular structure whereas headgroups of the other two surfactants are open chains. The value of hhw (around 0.7 nm) is reasonably similar to twice the thickness estimated for the headgroup region of the C10βG monolayer, suggesting that the short-range repulsion does indeed approximate to a hard wall. It is less clear if this is also true for the (21) Aronson, M. P.; Princen, H. M. Nature 1980, 286, 370.
Table 2. Film Contact Angle θ, Excess Film Interaction Energy -∆E, and Derived Apparent Hard-Wall Thickness hhw for the Different Surfactants surfactant
θ/deg
-∆E/mJ m-2
hhw/nm
C10βG C10NMG C10X C12E8 C16E20
29 ( 5 20 ( 5 25 ( 5 23 ( 5 17 ( 5
0.25 ( 0.03 0.24 ( 0.02 0.28 ( 0.03 0.43 ( 0.03 0.67 ( 0.04
0.70 ( 0.04 0.74 ( 0.04 0.74 ( 0.04 0.57 ( 0.04 0.42 ( 0.04
open chain sugar headgroups. It may be valid if the headgroup chains form a collapsed layer, but if the chains are extended into the aqueous phase, some degree of interpenetration of the opposing monolayers may occur. For the poly(oxyethylene) surfactants, the values of hhw are anomalously low by comparison with the estimated lengths of the all-trans hydrophilic group chain lengths (approximately 2.4 and 6.0 nm for E8 and E20, respectively). For these surfactants, it is likely that the low values of hhw result from the simplifying assumptions used, that is, that the short-range interactions can be approximated by a hard-wall potential and neglect of the contribution of the surfactant headgroup region to the van der Waals attraction. Although the conclusion must be that the physical interpretation of hhw as a hard-wall thickness is invalid for large, extended chain surfactants, its use as a model-consistent apparent or effective length enables the modeling of the flocculation transitions without the introduction of additional unknown parameters. Figure 4 shows the extent of creaming as a function of time for oil-in-water emulsions in a regime in which increasing the NaCl concentration induces a transition from a nonflocculated to a flocculated state. For reasonably dilute emulsions, the creaming velocity is proportional to the square of the drop radius.1 Individual (nonflocculated) drops therefore cream slowly whereas flocculated drop clusters show rapid creaming. This effect was used as a simple and rapid means of determining the flocculation boundaries, and its validity was confirmed by direct microscopic observation of the emulsions. The effect of flocculation on creaming is an example of the coupling
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Figure 4. Cream height versus time for C10X/SODS/H2O/ dodecane emulsions containing 20 vol % oil, 0.005 mole fraction SODS, and different concentrations of NaCl spanning the flocculation transition.
that can occur between the four processes contributing to the overall breakdown of emulsions. In this connection, we note that proper control experiments were made on the emulsions used here to confirm the absence of coalescence and Ostwald ripening over the experimental time scale. Comparisons of measured and calculated flocculation transitions for the different surfactants are shown in Figure 5a-e. For each plot, the filled symbols refer to addition of the anionic surfactant SODS and the unfilled symbols refer to the cationic surfactant OTAB. The dashed lines refer to the transition boundary calculated assuming that the nonionic surfactant is uncharged in the absence of added ionic surfactant. The solid lines are calculated boundaries for the transitions induced by SODS and OTAB addition where it has been assumed that the nonionic surfactant has a nonzero charge density resulting from either adsorbed ions or the presence of a small amount of charged impurity (see later). As noted previously,12 the data for C10βG (Figure 5a) are reasonably well described by theory assuming that the nonionic monolayer contains zero charge and taking P ) 1. The flocculation boundaries for SODS and OTAB are only very slightly shifted relative to each other, indicating that the net drop charges are very similar with addition of either the negative SODS or the positive OTAB. As discussed in ref 12, the range of validity of the theoretical curve is restricted to salt concentrations less than approximately 0.1 M and mtot values of less than a few mole percent. Similar behavior is seen for the xylitol surfactant in Figure 5b where the experimental flocculation transitions are again described reasonably well by the simple model with P ) 1 and zero charge on the pure nonionic monolayer. In contrast, the SODS and OTAB boundaries for C10NMG (Figure 5c) are widely separated indicating the presence of a significant negative charge on the “pure” monolayer of this nonionic surfactant. The negative surface charge is sufficient such that, for low NaCl concentration, the C10NMG drops are nonflocculated in the absence of added ionic surfactant. Addition of OTAB at low NaCl concentration leads to an initial transition from nonflocculated to flocculated as the negative charge is neutralized followed by a second transition from flocculated to nonflocculated as the net positive charge is increased. These transitions are indicated by the italic labels on the figure. Addition of SODS leads only to a single transition at high NaCl concentration from flocculated to nonflocculated driven by the monotonic increase in negative charge (nonitalic labels). The calculated flocculation transition curve (dashed line) for an uncharged pure nonionic monolayer with P ) 1 lies in a position
Aveyard et al.
intermediate between the SODS and OTAB boundaries. The solid lines were calculated assuming the presence of a mole fraction of a negatively charged adsorbed impurity of 5.2 × 10-3. The overall complex flocculation behavior is successfully captured. The curves for C12E8 (Figure 5d) show a small but significant separation between the SODS and OTAB curves indicating a relatively low level of surface charge. The theoretical curve shown corresponds to zero charge in the absence of added ionic surfactant with P adjusted to fit the data. The values of P estimated in this way (ranging from 15 at low [salt] to 1 at high [salt]) correspond to the added ionic surfactant distributing in favor of the emulsion surface in preference to the micelles. For low salt concentration (0.0001 M NaCl), assuming that P ) 1 yields a predicted flocculation transition at an mtot value of approximately 0.004, well above the experimental transition. In this regime, it appears that the drop interactions are either more repulsive or less attractive than calculated for P ) 1. Several possible origins of this effect were tested. First, refinements to the model to take account of the effect of the headgroup layer on the van der Waals component of the total disjoining pressure were tested but were found to have only minor effects on the calculated flocculation transitions. Second, modification of the assumed hard-wall short-range interaction (using plausible polymer overlap interaction potential functions as described in ref 5) were again found to produce only small shifts in the theoretical curves. Third, calculations of the flocculation transitions were made with the assumption that the surface plane in which the charged groups were located was shifted relative to h ) 0. The small changes in the calculated transition resulting from shifts in this plane of up to 10 nm were unable to capture the observed behavior. We therefore conclude that the most likely origin of the observed effect lies in the alteration of the distribution coefficient of the ionic surfactant between the drop surfaces and the micelles. It remains unclear why the ionic surfactant apparently distributes preferentially to the drop monolayer and not the micelles for the ethoxylate systems but seems to have no such preference in the case of sugar headgroup surfactants. The behavior of C16E20 (Figure 5e) shows the presence of a relatively high concentration of negatively charged surface impurity, and like C12E8, adjustment of P is required to fit the data. For the calculated lines, P values were taken to be identical to those for C12E8 and the impurity mole fraction was taken to be 8.2 × 10-3. We attempted to determine the concentrations of charged impurities in aqueous solutions of the different nonionic surfactants using either the Epton titration method20 or titration with NaOH for acidic impurities. However, these methods were found to be insufficiently sensitive to yield reliable results. Additionally, the negative surface charges in these systems may result not only from adsorbed impurities originating from the synthesis and/or degradation of the surfactant but could also result from unequal adsorption of anions and cations present in the solvent (and would not be revealed by the analytical methods above). Such effects have been noted for both foam and emulsions films stabilized by nonionic surfactants.8,22-25 (22) Manev, E. D.; Pugh, R. J. Langmuir 1991, 7, 2253. (23) Waltermo, A.; Manev, E.; Pugh, R.; Claesson P. J. Dispersion Sci. Technol. 1994, 15, 273. (24) Bergeron, V.; Waltermo, A.; Claesson, P. M. Langmuir 1996, 12, 1336. (25) Waltermo, A.; Claesson, P. M.; Simonsson, S.; Manev, E.; Johansson, I.; Bergeron, V. Langmuir 1996, 12, 5271.
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Figure 5. Flocculation stability diagrams for the different nonionic surfactant/ionic surfactant/H2O/dodecane/NaCl emulsion systems at 298 K. All emulsions contained 20 vol % dodecane and 10 mM total surfactant concentration with respect to the whole volume. Filled and open circles indicate the experimental points for SODS and OTAB, respectively. The dashed and solid lines show the theoretical flocculation transitions calculated as described in the text. The uncertainties in the calculated boundaries originating from the uncertainties in the parameters listed in Tables 1 and 2 are estimated to be approximately (20% in the mole fraction of ionic surfactant. The plots refer to (a) C10βG, (b) C10X, (c) C10NMG, (d) C12E8, and (e) C16E20. For the plots in (c) and (e), the italic labels indicate the flocculation transitions for OTAB and the nonitalic labels show the transitions for SODS.
Further information on the charging of emulsion drop surfaces with either C10NMG or C16E20 with SODS and OTAB addition was obtained by comparison of measured zeta potentials with values calculated from the surface potentials estimated for varying mtot and using the values of P and the impurity concentration which were found to
fit the flocculation transitions. Zeta potentials of emulsions stabilized by C10βG, C10NMG, and C16E20 and containing 1 mM NaCl with no added ionic surfactant were all found to be in the range of -2 to -3 mV which may be consistent with the presence of trace impurities of negative charge. However, the experimental uncertainty in the magnitudes
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of SODS and OTAB are widely separated and indicate the presence of a significant negative charge present on the monolayers in the absence of added ionic surfactant. For C12E8 and C16E20, the initial theoretical model (i.e., with P ) 1) significantly underestimates the repulsion between drops at low salt concentrations. The most probable origin of this effect is related to preferential distribution of the ionic surfactant to the emulsion drop monolayers relative to the micelles present in the aqueous continuous phase, that is, P > 1. List of Symbols
Figure 6. Comparison of measured and calculated zeta potentials for emulsions containing 1 mM NaCl and added OTAB (positive values) or SODS (negative values). Open circles refer to C10NMG, and filled circles refer to C16E20. The solid line shows a slope of unity passing through the origin.
of these low values is of the order of 5 mV, and hence they cannot be used to derive meaningful quantitative estimates of the possible impurity levels. In the presence of added ionic surfactants, the zeta potentials are higher enabling valid comparison with predictions of the surface charging. For the calculations in this comparison, the plane of the surface potential ψ corresponds to h ) 0 (i.e., the plane in the monolayers at which the surfactant headgroups join the tailgroups), whereas the zeta potential ζ corresponds to the shear plane of the emulsion drops (hshear). Assuming that the potential decays exponentially with decay length equal to the Debye length κ-1, then ζ is related to ψ according to
ζ ) ψ exp(-κhshear)
(9)
The value of hshear is expected to be approximately equal to the thickness of the surfactant headgroup region of the monolayer if the Debye length (calculated for the bulk aqueous electrolyte concentration) is valid within the headgroup region. The comparison between measured and calculated ζ values for C10NMG and C16E20 is shown in Figure 6. The values of hshear were taken to be 6 and 10 nm for C10NMG and C16E20, respectively. Although these values correlate with the sizes of the surfactant headgroups, the absolute values are somewhat larger than expected, probably indicating that the assumption of the bulk water Debye length within the headgroup region is not entirely valid. Overall, the comparison of Figure 6 implies that the calculations of the surface charging for C10NMG (containing an impurity but with P ) 1) and C16E20 (containing an impurity and with P > 1) are approximately valid. Similar measurements, quoted in ref 12, show that the calculation of the surface charge for C10βG (no impurity and P ) 1) is also reliable. Conclusions We have compared flocculation transitions in oil-inwater emulsions induced by variation in the concentration of added ionic surfactant and NaCl. A simple model based on film disjoining pressure arising from a hard-wall repulsion, van der Waals attraction, and electrostatic repulsion and using no adjustable parameters shows reasonable agreement with the behavior for C10βG and C10X. For C10NMG, the flocculation curves in the presence
A As c cac cN cNem cNmic e h hhw hshear k mem mtot NAv P Pc R T z ∆E Π Πel Πhw ΠvdW 0 γ κ θ σ ψ ζ
oil-water-oil Hamaker constant area per nonionic surfactant in the monolayer for [surfactant] > cac bulk concentration of salt critical aggregation concentration of surfactant total concentration of nonionic surfactant concentration of nonionic surfactant in the emulsion drop surfaces concentration of nonionic surfactant in the micelles electronic charge emulsion film thickness range of the hard-wall interaction coordinate of the shear plane Boltzmann constant mole fraction of ionic surfactant in the emulsion drop surface overall mole fraction of ionic surfactant in the total system Avogadro’s number partition coefficient for the distribution of ionic surfactant between emulsion and micellar surfaces emulsion drop capillary pressure emulsion drop radius absolute temperature ion charge number of a symmetrical (z:z) electrolyte emulsion film interaction energy per unit area total disjoining pressure electrostatic disjoining pressure hard-wall disjoining pressure (taken to represent all short-range interactions) van der Waals disjoining pressure relative dielectric constant of continuous phase permittivity of free space oil-water interfacial tension inverse Debye length film contact angle surface charge density surface potential zeta potential
Acknowledgment. We thank Dr. Richard Buscall and Dr. Simon Davies of ICI, Wilton, for helpful discussions and gratefully acknowledge the award of an ICI Strategic Research Fund grant. LA011723E
