w emulsions

Advances in Colloid and Interface Science 107 (2004) 125–155

Degradation of kinetically-stable oyw emulsions夞 ´ Capek* Ignac Polymer Institute, Slovak Academy of Sciences, Dubravska cesta 9, Bratislava, Slovakia 842 36, Slovak Republic

Abstract This article summarizes the studies on the degradation of the thermodynamically unstable oyw (nano)emulsion—a dispersion of one liquid in another, where each liquid is immiscible, or poorly miscible in the other. Emulsions are unstable exhibiting flocculation, coalescence, creaming and degradation. The physical degradation of emulsions is due to the spontaneous trend toward a minimal interfacial area between the dispersed phase and the dispersion medium. Minimizing the interfacial area is mainly achieved by two mechanisms: first coagulation possibly followed by coalescence and second by Ostwald ripening. Coalescence is often considered as the most important destabilization mechanism leading to coursing of dispersions and can be prevented by a careful choice of stabilizers. The molecular diffusion of solubilizate (Ostwald ripening), however, will continuously occur as soon as curved interfaces are present. Mass transfers in emulsion may be driven not only by differences in droplet curvatures, but also by differences in their compositions. This is observed when two or more chemically different oils are emulsified separately and the resulting emulsions are mixed. Compositional ripening involves the exchange of oil molecules between emulsion droplets with different compositions. The stability of the electrostatically- and sterically-stabilized dispersions can be controlled by the charge of the electrical double layer and the thickness of the droplet surface layer formed by non-ionic emulsifier. In spite of the similarities between electrostatically- and sterically-stabilized emulsions, there are large differences in the partitioning of molecules of ionic and non-ionic emulsifiers between the oil and water phases and the thickness of the interfacial layers at the droplet surface. The thin interfacial layer (the electrical double layer) at the surface of electrostatically stabilized droplets does not create any steric barrier for mass transfer. This may not be true for the thick interfacial layer formed by non-ionic emulsifier. The interactive sterically-stabilized oil droplets, however, can favor the transfer of materials within the intermediate agglomerates. The stability of electrosterically-stabilized emulsion is controlled by the ratio of the thickness of the non-ionic emulsifier adsorption layer (d) to the thickness of the electrical double layer (ky1 ) around the oil droplets (dy(ky1))s (dk). The monomer droplet degradation can be somewhat depressed by transformation of coarse emulsions to nano-emulsion (miniemulsion) by intensive homogenization and by the addition of a surface active agent (coemulsifier) oryand a water-insoluble compound (hydrophobe). The addition of hydrophobe (hexadecane) to the dispersed phase significantly retards the rate of ripening. A long chain alcohol (coemulsifier) resulted in a marked improvement in stability, as well, which was attributed to a specific interaction between alcohol and emulsifier and to the alcohols tendency to concentrate at the oyw interface to form stronger interfacial film. The rate of ripening, according to the Lifshitz–Slyozov–Wagner (LSW) model, is directly proportional to the solubility of the dispersed phase in the dispersion medium. The increased polarity of the dispersed phase (oil) decreases the stability of the emulsion. The molar volume of solubilizate is a further parameter, which influences the stability of emulsion or the transfer of materials through the aqueous phase. The interparticle interaction is expected to favor the transfer of solubilizate located at the interfacial layer. The kinetics of solubilization of non-polar oils by ionic micelles is strongly related to the aqueous solubility of the oil phase (the diffusion approach), whilst their solubilization into non-ionic micelles can be contributed by interparticle collisions. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: OyW emulsions; Coalescence; Creaming; Solubilization; Sterically-; Electrosterically- and electrostatically stabilized emulsions; Emulsifier; Additives

夞 Originally submitted as part of the Special Issue from the Conference on Polymers and Surfactants in Colloidal Systems at Imperial College, London, July 9th –11th, 2002, Vol. 106y1–3, 2003. *Fax: q42-7-375923. E-mail address: [email protected] (I. Capek). 0001-8686/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0001-8686(03)00115-5

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1. Introduction An emulsion is a dispersion of one liquid in another where each liquid is immiscible, or poorly miscible in the other w1x. Emulsions exhibit all classical behaviors of metastable colloids: Brownian motion, reversible phase transitions as a result of droplet interactions that may be strongly modified; and irreversible transitions that generally involve their destruction. They are obtained by shearing two immiscible fluids to the fragmentation of one phase into the other. The droplet volume fraction may vary from 0 to almost 1. From diluted to highly concentrated, emulsions exhibit very different internal dynamics and mechanical properties. Emulsifiers are usually added to oilywater mixture to enhance the formation of monomer emulsions and their stability. The molecules of emulsifier adsorb to the surface of oil droplets during homogenization and provide a protective membrane that prevents the droplets from flocculating or coalescing. Under certain circumstances, emulsifiers may have a negative impact on emulsion stability, because of their ability to form micelles that enhance mass transport processes, such as solubilization and oil diffusion through the aqueous phase w2,3x. These mass transport processes can cause significant changes in droplet concentration, composition and size distribution and may therefore adversely influence the bulk physicochemical properties of an emulsion, such as appearance, rheology and stability. The mass transport among droplets is typically driven by differences in size and composition, because they impose differences in chemical potentials for the solutes in each environment. Solubilization involves the movement of oil molecules from emulsion droplets to the surrounding aqueous medium. The rapid increase of the emulsion industry is connected with the environment concerns to regulate or decrease the content of oil in the aqueous phase. Oil-in-water (oyw) emulsions often result as a consequence of cleaning industrial equipment. Because the lifetime of these emulsions may become significant, they become good candidates for various commercial applications. All these applications have already led to an important empirical control of these emulsions, from their formation to their destruction. Emulsions are kinetically stable systems that is to say their free energy of formation is greater than zero, and as such will show a tendency to break. The interfacial tension in emulsions is generally of the order of 1–10 mN my1, this in connection with the large interfacial area results in a large positive interfacial energy term. Emulsions are, however, kinetically stable due to the presence of an adsorbed layer at the oyw interface, this barrier may be electrostatic in nature, or steric. These barriers not only prevent emulsion droplets from coming into the direct contact, but also serve to stabilize the thin film of liquid between two adjacent droplets. Emul-

sions may degrade via a number of different mechanisms such as: creaming with or without aggregation and increase in the droplet size, aggregation with or without creaming, increase in the droplet diameter through the oil diffusion and droplet coalescence leading to the production of a separate oil phase. As a result of their thermodynamic instability, emulsions will tend to reduce their total free energy through an increase in droplet size, so reducing their total interfacial area. Creaming and aggregation do not involve the increase in size of the droplets, but are precursors to coalescence since this process requires the droplets to be in close proximity. Ostwald ripening, on the other hand, does not require the droplets to be close, since the process occurs by transport of dissolved matter through the dispersion medium. Monomer emulsions are supposed to contain the relatively large (1–10 mm) monomer droplets and the much smaller monomer-swollen micelles (10–20 nm) or even emulsifier micelles (3–5 nm), and hence the surface area of the micelles can be of the order of magnitudes greater than that of the monomer droplets. Consequently, the probability of interaction between large monomer droplets is very low, and most interactions appear between droplets and micelles or among micelles. This indicates one of the possible ways of mass transfer activity. Kinetically stable nano-emulsions (miniemulsions) are much more stable than the coarse emulsions, but less stable than microemulsions. Microemulsions are thermodynamically stable, since the interfacial energy term is now very small owing to the very low interfacial tensions (typically 10y1 –10y2 mN my1). Moreover, as a result of their very small size, positive entropy of formation of microemulsion droplets may be of the order of magnitudes larger than in emulsions. Besides the empirical background which is considerably widespread among the various specific applications, the basic understanding of emulsions is certainly progressing and it is aimed in this article to give an overview of the most recent advances. Review papers by El-Aasser w4x, Asua w5x and Capek and Chern w6x summarized the preparation, colloidal stability and kinetics of nano-emulsion (miniemulsion) polymerization. Taylor summarized Ostwald ripening in emulsions done up to 1997 w7x. This article reviews the main aspect concerning the colloidal stability or degradation of sterically-stabilized emulsions and nano-emulsions. 2. General modes of emulsion degradation Emulsions are thermodynamically unstable exhibiting flocculation and coalescence unless significant energetic barriers to droplet interactions are present. They degrade toward phase separation via mass transfer, and other mechanisms. Emulsions are sensitive to coarsening phenomena like coalescence and Ostwald ripening, since

I. Capek / Advances in Colloid and Interface Science 107 (2004) 125–155

Scheme 1. Schematic representation of the Oswald ripening process.

their thermodynamically most stable state is the completely demixed one. Coalescence is often considered as the most important destabilization mechanism leading to coursing of dispersions. However, coalescence can often be prevented by a careful choice of stabilizers and is mainly of interest during processing. However, Ostwald ripening will continuously occur as soon as curved interfaces are present. The curvature of particles causes higher solubilities of the dispersed phase at the particle boundary compared to in the bulk or near to large particles. The concentration gradient in the dispersed phase in the continuous phase causes large particles to grow at the expense of smaller particles. Ostwald ripening involves the movement of oil molecules from small droplets to large droplets (Scheme 1). It is, thus, the process whereby large droplets grow at the expense of small ones because the solubility of a material within a droplet increases as the interfacial curvature increases w8x. In other words, Ostwald ripening is the process by which larger particles grow at the expense of smaller ones due to the higher solubility of the smaller particles (Gibbs–Thomson or Kelvin effect) and to molecular diffusion through the continuous phase w9x. Ostwald ripening is the process whereby the higher Laplace pressure inside small drops drives the transfer of dispersed oil from small to large drops. The speed of ripening depends primary on the product of the solubility of the dispersed oil in the aqueous continuous phase C` and its diffusion coefficient D w10–12x. Oils, which are slightly water-soluble (so called ‘mobile’ oils) can transfer between droplets at significant rates whereas Ostwald ripening is negligibly slow for oils of sufficiently low aqueous phase solubility, and these oils are termed ‘immobile’. Oil transport can occur by diffusion of molecularly dissolved oil molecules through the continuous aqueous phase, but may also be enhanced by an additional mechanism of transport as solubilized oil within micellar aggregates, which are normally present in the continuous phase. Mass transfers in emulsion may be driven not only by differences in droplet curvatures, but also by differences in their compositions. This is observed when two chemically different oils are emulsified separately and the resulting emulsions are mixed. Compositional ripening involves the exchange of oil molecules between emulsion droplets with different compositions. The term compositional ripening is used when the composition of

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droplets is not uniform and concentration gradients drive mass exchange, thus inducing changes in droplet sizes w13x. The thermodynamic driving force for each of these mechanisms is the difference in chemical potential of the oil molecules between the different environments. Mass transfer from one emulsion to the other is controlled by the entropy of mixing and will proceed until the compositions of the droplets are identical. The most spectacular evidence of composition ripening comes from the so-called ‘reverse recondensation’, which occurs when the two emulsions differ significantly both in their initial size and in their rate molecular diffusion. If the larger sized emulsion is composed of the faster diffusing oil, then molecular diffusion occurs in the reverse direction, i.e. from large to small droplets w14x. The physical degradation of emulsions is due to the spontaneous trend toward a minimal interfacial area between the dispersed phase and the dispersion medium. Minimizing the interfacial area is mainly achieved by two mechanisms: first coagulation possibly followed by coalescence and second by Ostwald ripening. However, if properly stabilized against the coagulationycoalescence process, the latter can cause a substantial breakdown of the emulsion. Dissolution in wateryoily emulsifier systems takes place through mechanisms that occur at the molecular level. Kinetic studies aim at determining the limiting step(s) within such mechanisms that dictate mass-transfer rates and transient behavior. Most authors have interpreted in terms of a single ratelimiting process, either (a) the diffusion of the transferred solutes in the bulk as individual molecules w15x or incorporated in micellar aggregates w16x or (b) the interfacial resistance to mass transfer, which may refer to transport across an emulsifier monolayer or some other physical barrier when molecular dissolution prevails w17x or to the adsorption and emission of micelles that act as carriers for the transported compounds w18,19x. The kinetics of the mass transfer is supposed to be also a function of droplet size. Nano-emulsions (miniemulsions) are a class of emulsions that can be transparent (size range 50–200 nm) or ‘milky’ (up to 500 nm) w4,20,21x. Unlike microemulsions, which are transparent and thermodynamically stable w22x, nano-emulsions are only kinetically stable. However, the long-term physical stability of nano-emulsions (with no apparent flocculation or coalescence) makes them unique, and it is sometimes referred to as ‘approaching thermodynamic stability’. The inherent long-term physical stability of nano-emulsions can be well understood from a consideration of their stabilization. In most cases nano-emulsions are prepared using non-ionic emulsifiers of the ethoxylated type, which may be considered as an A–B ‘diblock’. The alkyl chain (A-chain) resides in the oil phase, leaving the poly(ethylene oxide) (PEO), B-chain, ‘dangling’ in solution. The B-chain has a thickness d

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that depends on the number of EO units; in most cases d is in the range 5–10 nm. In other words, the ratio of adsorbed layer thickness to droplet radius (d yr) is significant. This means that the system will be sterically stabilized providing that the polymeric chains are in a good solvent w23x. The attraction of nano-emulsions for application in various fields is due to the following reasons. First, the very small droplet size causes a large reduction in the gravity force and the Brownian diffusion may prevent any creaming or sedimentation. Second, the steric stabilization prevents flocculation or coalescence of the droplets. The small droplet size and the high kinetic stability make nano-emulsions suitable for the efficient delivery of active ingredients. Unlike microemulsions, which require a high concentration of emulsifiers for their preparation (usually in the range 10–30 wt.%), nano-emulsions can be prepared at moderate emulsifier concentration (in the range 4–8 wt.%). Thus, the droplet size distribution of the nano-emulsions is a complex function of breaking up and coalescence of droplets, the droplet degradation by monomer diffusion and the presence or the absence of emulsifier, coemulsifier (or surface active agent) or hydrophobe. The formation and stability of oyw (nano) emulsion is directly connected with the transport of oil through the aqueous phase. For example, the monomer required for the emulsion polymerization must be transported from the monomer droplets by diffusion of monomer through the aqueous phase to the reaction loci. On the contrary, the oil transport processes within some emulsions used in cosmetic, biomedical and pharmaceutical applications must be strongly hindered.

where dn denotes the number average particle diameter and Dm the dispersed phase molecular diffusion coefficient; a a material-dependent constant called the capillary length, defined by as2gVm yRT

This result predicts that the aging or the increase in the average droplet size is mainly determined by the bulk solubility C` of the dispersed phase in the continuous phase. Furthermore, the results of the LSW analyses can be summarized as follows: (1) In the stationary region, the shape of the particle distribution function is time invariant; and (2) the cube of the number-averaged particle size increases linearly with time. According to the extended Lifshitz–Slyozov–Wagner (LSW) theory, the Ostwald ripening rate for nonoemulsions (miniemulsion) containing a water-insoluble, low molecular weight coemulsifier (hydrophobe) can be predicted by the following equation w8x: vs64gDcoVmCco,` y9RT fco

3. Ostwald ripening

d3t yd3ts0s (64gDC`V2mt)y(9RT)svt,

(1)

where t is time, d mean number droplet diameter, g the interfacial tension at the oil–water interface, D the diffusion coefficient of the oil in the aqueous phase, C` the solubility of the oil in the aqueous phase, Vm the molar volume of the oil, R the gas constant, T the absolute temperature, and v the Ostwald ripening rate. Ostwald ripening rate should therefore increase as the water solubility of the oil increases or the interfacial tension increases. One of the major results of LSW theory is that in the long time limit a stationary regime is reached for which the Ostwald ripening rate is given by the expression vsd3n ydts(4y9) aDmC`

(2)

(4)

where Dco the molecular diffusivity of coemulsifier in water, Cco,` the solubility of coemulsifier in water, and fco the volume fraction of coemulsifier in the oil droplet. The Ostwald ripening rate of an emulsion can be determined from the change in mean droplet size w24x. When the dominating mass transfer mechanism is the diffusion of individual oil molecules between the droplets, the increase in droplet radius (r) with time given by Eq. (1) can be simplified to: r3t sr3ts0yvt

The Lifshitz–Slyozov–Wagner (LSW) model w24,25x predicts that once steady-state conditions have been attained, the change in mean droplet number diameter with time is given by

(3)

(5)

where v is the molecular-diffusion ripening rate. When the dominating mass transfer mechanism is the transport of oil molecules between the droplets via emulsifier micelles, the increase in droplet radius with time is given by w26x: r2t sr2ts0yvmict

(6)

where vmic means the micelle-transport ripening rate. If added in sufficiently large amounts of surfactant and after complete coverage of the oil–water interface, surfactants spontaneously form micelles in the continuous aqueous phase. The presence of micelles drastically increases the solubility of the oil phase. Therefore an effect of micelles on the Ostwald ripening may be anticipated. It might be simplistically expected that replacing the bulk oil solubility C` in Eq. (1) by the concentration of oil solubilized by the micelles and using the micellar diffusion coefficient instead of the molecular one would yield the Ostwald ripening rate in the presence of micelles. This approach predicts an

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increase of the rate by a factor of approximately 200– 1000 w7x. Several, sometimes conflicting studies report hardly any increase w10,11,27,28x. In the absence of micelles, mass transport occurs by the movement of individual oil molecules into and through the aqueous phase separating different environments. The presence of micelles, however, enhances the rates of Ostwald and compositional ripening in emulsions w11x. It is proposed that mass transport in the presence of micelles still involves the movement of individual oil molecules through the aqueous phase, but that the micelles increase the water solubility of the oil, which enhances the transport rate w27x. Other researchers have proposed that mass transport is facilitated because emulsifier micelles solubilize oil molecules in their hydrophobic interior and transport them across the aqueous phase separating the droplets w16x. Whether the oil molecules are taken up by the micelles directly from the aqueous phase surrounding the droplets or by the fusiony fission of a micelle with a droplet surface is still uncertain. Ostwald ripening takes place when the dispersed phase is soluble enough within the continuous phase and consists of a gradual coarsening of the emulsions. This process is driven by the difference in Laplace pressure between droplets having different radii. The solubility (and consequently chemical potential) of the disperse phase in the bulk phase is dependent upon the radius of curvature of that droplet, the solubility increasing with decreasing radius w29x. This results in smaller drops dissolving into the bulk phase and then diffusing to, and redeposing upon larger ones leading to an overall increase in average size of the emulsion. 4. The volume degradation

fraction

effect

on

monomer

Much work on ripening is focused on the effect of volume fraction. At finite volume fractions the diffusion field of one particle will interfere with the diffusion field around another particle. This local environment effect can be accounted for by using sink-source terms for every particle w30x. Numerical simulations showed that the size distribution becomes broader and the change in the cube of the number-averaged radius with time is faster with increasing volume fraction. For the infinitely concentrated case, the square of the number-averaged particle radius instead of the cube increases linearly with time and also a stationary state is obtained in which the distribution function can be described with an analytical function similar to the LSW distribution. These results of the effect of volume fraction on coarsening rates and the form of the distribution function were also confirmed by simple numerical simulations of the coarsening process w31x. Experimental measurements of Ostwald ripening (v) are relatively few, particularly for emulsions, but exper-

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iments conducted at low dispersed phase volume fractions generally confirm the predictions of the LSW technique w10x. However, the measured size distributions for emulsions with higher volume fractions of dispersed phase tend to be broader than predicted by the LSW theory and the absolute growth rate, i.e. the change in the cube of the mean radius with time, tends to be faster than predicted w8x. In general, the effect of the dispersed phase volume fraction has been handled by tracking the location of each drop and determining the local environment through the contribution of sourceysink terms for each drop. This approach is constrained to a limited number of drops, but various statistical techniques have been used to overcome the deficiency w8x. In all cases, the predicted drop distributions and the change in the cube of the mean radius with time are in better agreement with experimental observations than are predicted from the LSW technique. Several predictions have been made for finite dispersed phase volume fractions greater than 30%. However, concentrated emulsions with dispersed phase volume fractions as high as 70% can undergo Ostwald ripening, e.g. a creamed emulsion that does not coalescence. Hence, it is desirable to extend the solution of the ripening problem to higher dispersed phase volume fractions. Marqusee and Ross (MR) w32x have used a different analytical technique to examine in the infinitely dilute case and in an interfacially controlled diffusion case. The latter case is equivalent to Ostwald ripening in an infinitely concentrated emulsion in which the continuous phase in effect exists as a thin interface around the drops. Predictions from MR technique confirm that the cube of the mean radius increases linearly with time for infinitely dilute emulsions, but indicate that the square, not the cube, of the mean radius increases linearly with time for infinitely concentrated emulsions. Ostwald ripening at finite dispersed phase volumes was modeled successfully using linearized analytical solutions of the ripening equations and an explicit numerical routine w31x. The routine incorporated a number frequency distribution of drop radii rather than using a discrete number of drops. The effect of volume fraction was accounted for by using half the average separation distance between drops as a mass transfer boundary. Since an average separation distance is employed, the model is an approximation and the effect of different local environments is not accounted for. The numerical predictions for infinitely dilute systems match the LSW predictions almost exactly. The numerical predictions are in quantitative agreement with predictions from the MR w32x techniques for infinitely concentrated systems. However, since spherical droplet geometry is assumed in the model, the model is only valid for dispersed phase volume fractions where spherical geometry exists (fd-0.8 for polydisperse droplet sizes). The numerical model was applied to the full range of

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I. Capek / Advances in Colloid and Interface Science 107 (2004) 125–155 Table 1 Growth rates.

b9(fd)yb9(0.34)

fd

0.34 0.42 0.55 a b

Experimentala

Predictedb

1 1.34 1.83

1 1.21 1.74

w33x Ks9b9y4s9c3 y4bfd.

Ks9b9y4s9c3 y4bfd

Fig. 1. Growth rate correction factor (K) for dispersed phase volume fraction (fd) less then 80%. (a) w31x, (b) w30x, (c) Eq. (8)

dispersed phase concentrations and successfully predicted cumulative frequency distributions for the range of available experimental data at dispersed phase volume fractions between 0 and 0.3. The critical radius was found to vary between the mean radius, r0, and r1 radius, as the dispersed phase volume fraction varied between 0 and 1. Despite the variation in critical radius, the growth rate could be related to the cube of the mean radius at any dispersed phase volume fraction. A simple expression, depending solely on the dispersed phase volume fraction, was developed to predict the growth rate, dr31 ydtsc3bybfdsb9

(7)

at any dispersed phase volume fraction (where b and c are proportional constants). In addition, the values of b9 can be converted into correction factors to the LSW growth law:

(8)

Correction factors to the LSW growth law were determined for (0FfdF0.8). The correction factors agree well with previously published correction factors at fd-0.8, but have higher values, i.e. predict faster growth rates, at higher dispersed phase volume fractions. The correction factors are the ratio of the growth rate at a finite dispersed phase volume fraction to the growth rate at infinite dilution, Ksb9(fd)y b9(0). Correction factors from the full numerical solution and from the approximate method are shown in Fig. 1 and compared with the correction factors determined numerically by Enomoto et al. w30x, who showed that their correction factors are consistent with results from several other numerical approaches for the range of dispersed phase volume fractions considered, fdF0.3. The predicted correction factors agree well with experimental data at 0.34FfdF0.55 (Table 1). 5. Interfacial tension approach Emulsion droplets are normally stabilized by surfactant or amphiphilic polymer. The adsorbing surfactant causes a lowering of the interfacial tension, which makes it easier to disrupt droplets, but also provides the droplet steric or electrostatic repulsion to stabilize it against coalescence. The interfacial tension is closely related to the amount of adsorbed stabilizer and the nature of the interfacial layer (Scheme 2). The interfacial tension

Scheme 2. Schematic representation of the interfacial layer.

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decreases with the increase in surface load. The surface load is directly related to the bulk concentration of the stabilizer although, dependent on the type of stabilizer, many other effects such as history are of importance. If the interfacial area is increased and the stabilizer does not desorb, the surface load will decrease and the interfacial tension g will become higher. In this case, the interface is said to be elastic and its elastic behavior can be characterized by an interfacial dilation elasticity modulus E. This modulus is defined as the change in interfacial tension (dg) dg y(divided by the relative change in interfacial area (dAyA): Esdgy(dAyA)sdgyd ln A

(9)

If the stabilizer desorbs on compression of the interface within the time scale of deformation, the surface load will remain the same and, therefore, the interfacial tension will remain approximately the same. This idea behavior is often observed for low molecular surfactants. Whether or not a stabilizer will desorb on compression of the interface depends on the time scale of deformation and the time scale of adsorptionydesorption. The dependence of the interfacial tension on the time scale of deformation can be described by the interfacial dilatational viscosity (hd), which is defined by the difference between the interfacial tension at equilibrium (ge) and the interfacial tension during steady-state expansionycompression divided by the surface deformation rate: hds(gyge)yd ln9A with d ln9Asd ln Aydt

(10)

Elastic interfaces can be formed, for example, by proteins that adsorb in a train-loop-like manner at the interface. It is not likely that several ‘trains’ from one amphiphile molecule desorb cooperatively. However, there will always be some viscous behavior due to intraor intermolecular rearrangements. The interface will become more elastic if the amphiphiles will form extra (intra-) molecular interactions. Another way to make an elastic interface is by particles that have a certain affinity for the continuous phaseydispersed phase interface and therefore adsorb at the interface and stabilize it by Pickering stabilization w34x. According to Lucassen w34x this would be an efficient way to stabilize emulsion against Ostwald ripening. The coarsening of droplets in an emulsion (with a size distribution that initially is given by the LSW distribution) was studied by means of numerical calculations taking into account elastic interfacial behavior w35x. Droplet smaller than the critical radius will shrink while droplets larger than the critical radius will grow. For a zero interfacial elasticity the stationary LSW distribution is obtained and its coarsening rate matches theoretical values. The critical droplet radius, number-

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averaged droplet radius and volume-surface-averaged droplet radius increase with time. Low interfacial elasticity with respect to initial interfacial tension causes the initial LSW distribution to become bimodal. The size distribution of the coarsening peak can still be described by the LSW distribution. The smaller peak that accumulates in time has an average radius that depends on the ratio between the interfacial elastic modulus and the interfacial tension. For large ratios (Ey g)1), the system goes within a short time to a stable situation without changes in particle size with time. Another factor that is hardly considered in emulsion coarsening literature is the effect of the interfacial rheology. The interfacial tension (g) is incorporated in the coarsening equations in the capillary length parameter a(s(2Vm,dg)y(kT)) (Eq. (3)). A lowering of the interfacial tension will lead to a smaller capillary length and therefore a lower solubility at the droplet boundary. This will cause a lower coarsening rate, as is also clear from Eqs. (1) and (2), which shows that the coarsening rate is proportional to the capillary length. Almost all studies on Ostwald ripening assume the interfacial tension to be constant, but this may not be true, especially when considering ripening of emulsions. From theoretical ripening in a solid dispersion, it appeared that mechanical stresses around a particle could strongly influence the ripening rates w36x. The mechanical stresses around a solid particle can be compared with interfacial stresses at an emulsion droplet boundary. When oil droplets contain a sufficiently high concentration of low-polarity molecules, the interfacial tension at the oil–water boundary is high w37x. Consequently, the most emulsifier molecules will strongly adsorb to the droplet surface and stabilize the droplets against coalescence. When oil droplets contain a sufficiently high concentration of high-polarity molecules, the interfacial tension at the oil–water boundary is relatively low. In this situation, strongly surface-active molecules (e.g. Tween 20) will still adsorb to the droplet surface and provide protection against coalescence, but weakly surface-active molecules, (e.g. gum arabic) will not adsorb to the droplet surface, leading to the droplet coalescence. In the study w37x, the two different physicochemical mechanisms responsible for droplet growth were distinguished because of the differences in the evolution of the particle size distribution (PSD). For Ostwald ripening, the shape of the PSD remained relatively constant over time, but the maximum in the peak shifted upward. For coalescence, the initial monomodal PSD was converted into a bimodal distribution, and the rate and extent of droplet growth were much greater than for ripening. At the hexadecane–water interface, there was a rapid initial decrease in interfacial tension with increasing emulsifier concentration due to adsorption of emulsifier to the boundary, which was followed by a plateau region

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Fig. 2. Effect of emulsifier and oil type on the interfacial tension versus emulsifier concentration profiles. (h,s): hexadecane, (j,d): decanol (h,j): Tween 20 (s,d): Gum arabic

where the interfacial tension remained relatively constant due to saturation of the boundary by emulsifier. The star of the plateau region was approximately 0.001– 0.005 wt.% for Tween 20 and approximately 0.5–1 wt.% for gum arabic (Fig. 2). The steeper initial decrease in g with emulsifier concentration for Tween 20 than for gum arabic indicates that the former had a higher surface activity w38x. In addition, the interfacial tension in the plateau region (g0) was lower for Tween 20 than for gum arabic. The results suggest that both gum arabic and Tween 20 will strongly adsorb to hexadecane droplets and therefore stabilize against coalescence, but only Tween 20 will adsorb to decanol droplets and provide protection. 6. Coalescence Coalescence is one of the possible mechanisms of destruction of emulsions w39x. Coalescence occurs when the energy of adhesion between two droplets is larger than the turbulent energy causing dispersion. It consists in the rupture of the thin film that forms between adjacent droplets leading two droplets to transform into only one. The thin film that forms when two droplets are in contact is a metastable molecular assembly and its lifetime will be a key factor in determining the lifetime of the monomer emulsion. The origin of the film breaking has been viewed to two distinct mechanisms. One is a mechanical instability like that occurring in spinodal decomposition w40x. A second mechanism consists of the nucleation of a thermally activated hole

that reaches a critical size, above which it becomes unstable and grows, leading to the coalescence of two adjacent droplets w41x. Such a picture is in fairly good agreement with the behavior of non-ionic interfaces undergoing temperature changes. Temperature variation applied to crude polydisperse non-ionic dense emulsion leads to the formation of a macroscopic domain made of the dispersed phase. Kabalnov et al. w42x have shown that for a direct octaneyC12(EO)5 ywater emulsion with the emulsifier concentration set around a few percent. At the droplet volume fraction 50%, the half lifetime varies with temperature as ln t1y2 A1ykT. t1y2 is defined as the time it takes for half of the emulsion to transform into an oil layer. It has been claimed a correlation between emulsion stability and film viscoelasticity at the oil–water w43x. Emulsifier acts by forming a structural and mechanical barrier: a sort of ‘protective skin’ around droplets, which prevents them from coalescing. Some macromolecular films at the oil–water interface visibly show a skin-like appearance w44x. Occasionally, the droplets coalescence and surface rheology were closely related w45x. For an oil droplet beneath the planar oil–water interface, the buoyant force is normally taken as the primary force causing the film to thin. However, at separations below ca. 100 nm the effect of van der Waals’ attractive forces and electrostatic repulsive forces must be taken into account w46x, while steric and structural forces, usually repulsive, come in at lower separations w47x. Film rupture is a stochastic process driven by fluctuations arising from random thermal or mechanical noise. The theories of film rupture give equations for the critical thickness at which spontaneous rupture is likely to take place. Furthermore, theoretical work relevant to the coalescence problem has often been directed towards the closely related topic of the stability and rupture of membranes (film) w48x. The usual approach is to divide the coalescence process into two stages: film thinning and film rupture. In this context the word ‘film’ may refer just to an intervening layer of continuous phase in the complete absence of any adsorbed layer (film) of surface-active material. If coalescence was the driving force for instability, then the change of droplet size with time may follow the following equation w49x: 1yr2s1yr2oy8py3vrt

(11)

where r is the average droplet radius after time t, ro the value at ts0, and vr the frequency of rupture per unit of surface of the film. In the case of coalescence two extreme regimes may be identified. The first at low volume fractions where the collisions between the droplets may be the rate determining step rather than the rupture of the thin film between the coalescing droplets. At high volume frac-

I. Capek / Advances in Colloid and Interface Science 107 (2004) 125–155 Table 2 Experimental variables studies and standard recipes used 噛

Polystyreney(%)

Cetyl alcohol

1 2 3 4 5

1 1 0.5 0.05 0

none 30 mM 30 mM 30 mM 30 mM

in in in in

gel gel gel gel

phase phase phase phase

a 80 parts water (560 g), 10 mM SDS (1.64 g), 20 parts styrene (140 g).

tions, it is the rupture of the thin film rather than the collision frequency that is the rate determining step w50x. 7. Creaming Creaming has a central role in emulsion breaking. The speed(s) of the droplets increases with the square of the droplet radius, r, according to the Stokes equation w51x: us2Drgr2F(f0)y9h

(12)

where Dr and h are the density difference between the continuous and droplet phases and the continuous phase viscosity, g is the gravitational constant, respectively. F(f0) is a volume fraction (f0) dependent correction term (F(f0)s1.0 for dilute emulsions). Flocculation and coalescence increase the effective droplet size and generally accelerate creaming w52x. Creaming rates for dilute emulsions should be capable of providing an estimate of the effective droplet size in the absence of coalescence provided r and Dr are sufficiently large to negate the random effects of Brownian motion. Creaming depends on the droplet size and droplet size distribution. The smaller the droplets, the more stable is the nano-emulsion towards creaming. Usually in these studies, two different measurements of the emulsion stability can be followed: (1) the rate of creaming which is found by measuring the amount of separation of the emulsion from the aqueous phase over a period of time; and (2) the rate of coalescence which is determined by measuring the amount of oil separated from the emulsion as a function of time. While the latter study is useful in understanding the mechanism of stabilization of the emulsion, the former study is directly related to the size of the emulsion droplets produced. This is a consequence of the well-known Stokes–Einstein expression given by Eq. (12) or as follows: us{2gr2(rmyrp)} y9h

133

stable system in which the droplet creaming rate will be directly proportional to the square of the diameter of the emulsified droplets. Despite some complications, such as that the droplet sizes in many cases are polydisperse and the droplet sizes vary with time, measuring creaming rates can still provide a useful reference for determining relative droplet sizes for the different emulsion systems outlined in Table 2 w53x. The creaming rates for several of emulsions showed in Fig. 3 were measured by visually observing the phase boundary between a clear water phase and the creaming emulsion phase. During the course of the measurements (one week) no monomer phase was observed to separate from any of the emulsions indicating that they were stable to droplet coalescence. Miller et al. w53x based on the relative creaming rates of the styrene nanoemulsions prepared with different amounts of cetyl alcohol (CA) and polystyrene (PSt), concluded that the droplet sizes range as: d(CA)-d(CAq0.05% PSt)d(CAq0.5% PSt). These data show that the creaming rate is proportional to the droplet diameter. It is apparent from this study that the smallest droplets (slowest creaming rates) are produced for the miniemulsions made in the absence of polymer, and the size increases slightly with increasing polymer content. In addition, the miniemulsion prepared without CA (with 1% polymer) separated very quickly, indicating that it was probably not stable against diffusion degradation. Fig. 4 shows that the creaming is greatly retarded as the concentration of NP40 (nonylphenol polyoxyethylene with an average 40 ethylene oxides per molecule) is increased w54x. No creaming phenomenon was observed for the miniemulsions prepared by (i) 5 mM NP40q20 mM SMA (stearyl methacrylate) and (ii) 5 mM NP40q20 mM HD (hexadecane). It was concluded

(13)

where rm is the density of the continuous phase and rp the density of dispersed phase. It is apparent that, for a

Fig. 3. Creaming rates for several emulsions presented in Table 2.

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Fig. 4. Position of creaming line from bottom of the miniemulsion as a function of time: (a) 5 mM NP40q20 mM CA, (b) (1) 1.52 mM NP40 6.08 mM DMA, (c) 1.52 mM NP40q6.08 mM SMA, (d) 0.22 mM NP40q0.88 mM DMA, (e) 0.22 mM NP40q0.88 mM SMA

that at constant surfactant concentration, NP40 is more effective in preventing the monomer phase separation from occurring upon aging at 35 8C as compared to wSDSx w55x. 8. Solubilization The ability of micellar solutions to incorporate solubilized material is an important property of micelles and provides the basis for the widespread use of emulsifiers and micellar solutions in the different industrial and research fields. The location in the micelle where solubilization occurs varies with the nature of the solubilized materials and is important as it reflects the type of interaction that occurs between surfactant and solubilizate. These solubilization sites are mainly determined from studies on the solubilizates before and after solubilization, through NMR w56x, UV and fluorescence spectroscopy, X-ray diffraction, etc. w57x. For example, NMR and UV studies indicate changes in the environment of the solubilizate on solubilization and X-ray diffractograms provide information on changes in micellar dimensions on solubilization. Since NMR chemical shifts are dependent on the molecular environment of the nuclei, changes in these properties for solubilizates and surfactants as a function of concentration provide precise information on the location of a solubilizate with respect to the micellar nuclei as well as the mode of micellization. Solubilization is known to occur at a number of different sites in the micelle: (1) at the micelle–water

interface; (2) between the hydrophilic head groups; (3) in the palisade layer of the micelle, i.e. between the hydrophilic groups and the first few carbon atoms of the hydrophobic groups that comprise the outer core of the micellar interior; and (4) in the inner hydrophobic core of the micelle. Non-polar aliphatic compounds (e.g. alkanes) fall under the last category while polar componds, like long-chain alcohols, have their hydroxyl group close to the surfactant head groups and alkyl group toward the interior. The solubility of predominantly hydrophobic molecules in aqueous solutions is enhanced by the addition of emulsifiers to the solution. The added emulsifier molecules self-assemble to form micelles or vesicles, which by providing a more compatible environment for the sparingly soluble molecules increase their solubilization. Emulsifier micelles can be pictured as having a highly non-polar interior and a relatively polar interfacial region. The interior of the micelle is generally considered to be the locus of solubilization for very non-polar solubilizates such as nalkanes. Solubilizate molecules of relatively high polarity such as alcohols are believed to be solubilized in the interfacial region of the micelle so that their polar functional groups could retain their contact with water. The non-polar solubilizate molecules such as aromatic or aliphatic hydrocarbons are assumed to be predominantly located in the interior of the micelles. From the measurements of the 1H chemical shifts of the magnetically discrete protons of both the surfactants and the solubilizates as functions of substrate concentration, it was possible to elucidate the predominant substrate concentration solubilization site w58x. Various aromatic (alkyl- and methoxyphenols) and aliphatic compounds exhibited sharp and well-resolved NMR signals in SDS micelles. Aromatic compounds, especially those having the phenolic-OH group, showed an upfield shift of sodium dodecyl sulfate (SDS) methylene protons, which are closely linked to the terminal sulfate groups. Kumar and co-workers observed similar upfield chemical shifts in their studies with liner alkyl benzenesulfonate and oleate micellar systems w59x. This phenomenon of upfield shift of protons experiencing increased hydrophilic character is well documented in the literature w60x. In the case of phenolic compounds, the extent of the shift is larger than the corresponding methoxy derivatives. Phenolic compounds, being the most hydrophilic among the present set, reside at the hydrophilicyhydrophobic boundary of micelle–water interface and thus influence the resonances of SDS protons the most. Aromatic metoxy compouds and aliphatic compounds, being hydrophobic in nature, reside in the core of the micelle, thereby resulting in smaller shifts. Additionally, in the case of higher concentration of phenolic compounds, the unresolved signals of the nine straight-chain bulk methylene protons of SDS were split into a doublet with uneven intensity.

I. Capek / Advances in Colloid and Interface Science 107 (2004) 125–155

135

polarity parameter. The dependences of molar solubilization ratios against Vm (taken from Fig. 5) and gsV2y3 m y kT were described by the curve given by expressions: y2.66 Nsol yNaggrs1.43=106 Vm s22.9 wgsVm2y3ykTx

y2y3

(for CPC)

y2.51 Nsol yNaggrs8.25=105 Vm s29.5 wgsVm2y3ykTx

y2y3

(for DAC)

y2.58 Nsol yNaggrs5.39=105 Vm s13.2 wgsVm2y3ykTx

y2y3

Fig. 5. Molar solubilization ratio of hydrophobic solubilizates in 0.1 M cetyl pyridinium chloride (CPC) (a), dodecyl ammonium chloride (DAC) (b), and SDS (c). Molecular volume (Vm in A8 3)ysolubilizate: benzeney146, CCl4y161, cyclohexeney167, tolueney176, cyclohexaney179, n-hexaney217, n-decaney323.

This is discussed in terms of increased penetration of phenolic compound into the palisade layer of the micelles and shielding of the methylene protons of SDS w61x. An alternative explanation for the splitting of methylene resonances is due to the gradual change in micelle conformation structure from spherical to large rod-shaped w60x, the latter being capable of accommodating more solubilizate. One of the important characteristics of solubilization that is experimentally accessible is the ratio of the maximum number of solubilizate molecules to the number of emulsifier molecules in a micelle. This ratio, hereafter designed as the molar solubilization ratio, has been measured for a variety of solubilizates in a number of emulsifier solutions. The experimental data show that, within a chemical family of solubilizates, a rough correlation exists between their molecular volumes and the measured molar solubilization ratios w62x. For example, a reasonable correlation is obtained, with the molar solubilization ratio decreasing, as the molecular volume of the solubilizate increases (Fig. 5) w63x. Theories for micelle formation and solubilization w64x show that an important contribution to the free energy of micellization is provided by the interfacial free energy of the micellar coreywater interface. This interfacial free energy is likely to be affected by the extent of solubilization and by the interfacial tension between solubilizate and water, which depends on polarity. A quantitative measure of polarity can be provided by the interfacial tension of the solubilizate against water, gs. The dimensionless quantity gsV2y3 m ykT includes the effects of both molecular volume and the polarity of the solubilizate and is used as the combined molecular volume —

(for SDS)

(14)

where Nsol refers to the number of solubilizate molecules in a micelle and Naggr is the number of emulsifier molecules in the micelle. This behavior indicates that above a certain critical value of the molecular volume of the solubilizate (and the critical value of the interfacial tension of solubilizate) the solubilization will be strongly suppressed as well as any transfer of solubilizate within the micellar system. These data indicate that preferential solubilization is expected for the slightly polar solubilizate of a smaller molecular volume. The binary solubilization results for benzeneyhexane system show that the amount of hexane solubilized increases slightly or remains constant even as the mole fraction hexane in the bulk solubilized phase is decreased. In Fig. 6, the amount of hexane solubilized at a bulk solubilizate composition of 0.14 mole fraction benzene is greater than the amount of hexane solubilized when the solubilizate phase is pure hexane. Thus, it

Fig. 6. Molar solubilization ratio of benzene and hexane in 0.1 M CPC solution as a function of the composition of the bulk solubilizate phase: (a) hexane, (b) benzene.

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appears that the solubilization of small amounts of benzene has a synergistic effect in increasing amount of hexane solubilized. The polar oils, such as middle- or long-chain alcohols, can influence the emulsifier layer curvature in the micelles when solubilized. In general, the change in emulsifier layer curvature is highly related to the placement of the solubilized oil in the emulsifier aggregates. If oil tends to penetrate in the emulsifier palisade layer and locates near the interface of the waterylipophilic emulsifier moiety w38x, the curvature would be less positive or negative, the curvature being defined as positive when the emulsifier film is convex towards water. However, if oil is solubilized deep inside the aggregates and tends to form an oil pool (swelling), the opposite change in emulsifier layer curvature is often induced w65x. It is known that the penetration tendency of amphiphilic or polar oils is large. Also, it is known that aromatic hydrocarbons or short-chain alkanes lower the cloud temperatures of aqueous non-ionic emulsifier solutions, whereas long-chain saturated hydrocarbon oils do not change or slightly increase them w66x. Such a difference is also attributed to the difference in penetration or swelling of the oil in the aggregates. When a long-chain hydrocarbon is solubilized and the emulsifier layer curvature becomes positive, the cloud point is increased w66x. Aromatic hydrocarbons were observed to penetrate in the emulsifier layer and widen the effective crosssectional area of emulsifier in the (liquid crystals) aggregates w67x. As a result, the emulsifier layer curvature becomes less positive or negative. On the contrary, long-chain saturated hydrocarbons have a swelling tendency, which causes an increase in repulsion between the hydrophilic moieties of emulsifier, and hence, the emulsifier layer curvature becomes more positive. For example, rod micelles in hexagonal liquid crystals often change to spherical micelles in discontinuous cubic liquid crystals often change to spherical micelles in discontinuous cubic liquid crystals upon addition of long-chain oils w67x. Long-chain alkanes show a weak tendency to penetrate in the emulsifier palisade layer, and they make an oil pool in emulsifier aggregates when they are solubilized. This phenomenon is called as ‘swelling of oil’, and the emulsifier layer curvature becomes more positive due to the decrease in repulsion between the hydrophilic moieties of emulsifier w65x. As a result, the cloud point is increased. The cloud point temperature of 2 wt.% octaethyleneblycol dodecyl ether {C12(EO)8} aqueous solutions decreases upon addition of sarcosinate-lauroyl isopropyl (SLIP), 1-dodecyl, and m-xylene, whereas it increases in glycerol tris(2-ethylhexanoic) ester (THE), isopropyl myristate (IPM) and saturated hydrocarbon systems w68x. The observed differences in phase behavior are attributed to the placement of solubilized oil in

micelles: In the former systems, oil tends to penetrate in the emulsifier palisade layer and induces the emulsifier layer curvature in micelles to be less positive, while the penetration tendency is small and the opposite effect on the curvature is induced upon addition of the latter oils. Solubilization and Ostwald ripening are competitive processes and the presence of the former results in a reduced rate of ripening. Kabalnov w11x has proposed three possible mechanisms to explain the observed effects of micelles on Ostwald ripening. The first being that the micelles undergo some form of collision with the emulsion droplets and transfer of disperse phase to or from the micelle may take place during this process. This was discounted on the basis of electrostatic repulsion between the droplets presenting sufficiently close approach. The two other mechanisms considered were based on the same basic process in which the micelles take up the solubilizate from the droplets via the bulk aqueous phase and redeposit it onto other droplets. The first is that the micelles are in equilibrium with the diffusion fields surrounding the droplets, that is to say the micelles can solubilize the dissolved oil rapidly. The second possibility is that the micelles are not in local equilibrium with the dissolved oil surrounding the droplets. The exchange of the oil molecules between the solution phase and the micelle is slow and the micelles cannot solubilize the oil sufficiently quickly prior to diffusing away from the droplets. The first two mechanisms clearly predict that the rate should be significantly enhanced by the presence of micelles containing solubilized oil, whilst the third predicts no significant increase in the rate with micellar concentration. It is expected that a somewhat polar solubilizate of a smaller molecular volume will be favorably solubilized by the diffusion approach. The location of non-polar solubilizate with a large molecular volume in the droplet interior disfavors both the solubilization and Ostwald ripening. The previous data indicate that the mechanism by which transfer of oil takes place through Brownian collisions between the micelles and the droplets may act in systems containing non-ionic surfactants in which the repulsion would not be present. The interparticle interaction is expected to favor the transfer of solubilizates located mainly in the interfacial layer. The residence time of micelles within the micellar (collision) aggregates is one of the parameters, which can govern the transfer of non-polar solubilizates with the micellar system. Furthermore, the rate of transfer is proportional to the number of collisions (favored for the stericallystabilized emulsions). The kinetics of solubilization on non-polar oils into ionic micelles is strongly related to the aqueous solubility of the oil phase (the diffusion approach), whilst solubilization into non-ionic micelles proceeds by the interparticle collisions. The very rapid solubilization proceeds in the sterically-stabilized emul-

I. Capek / Advances in Colloid and Interface Science 107 (2004) 125–155

sions near the cloud point is very rapid (too rapid to be explained in terms of molecular diffusion) w69x. This fast process can be attributed to the increased interaction (collision) caused by the decreased thickness of interfacial layer (the transfer of solubilizate(s) between the micelles is favored). 9. Electrostatically stabilized monomer emulsions Van der Waals–London attractive forces can induce the particle flocculation or agglomeration. In order to maintain stability of monomer or polymer dispersions, it is necessary to introduce the repulsive force to outweigh the attractive force. The electrostatic stabilization provides the repulsive forces between similar charged electrical double layers to the interactive particles w70,71x. The electrical double layer around the particles originates from the ionic emulsifier molecules (Scheme 3). Under the high density of surface charge, the degradation of emulsions is improbable by interparticle collisions. Kabalnov w11x reports that the ripening rate of undecane emulsion stabilized by SDS is enhanced by a factor 1.75. The enhanced rates are consistent with the picture of growing droplets in the presence of micelles. Furthermore, Kabalnov’s relative rate (vrel, the ratio of experimental rate to the theoretical one) enhances up to ca. 0.1 M sodium dodecyl sulfate (SDS) and then decreases (Table 3). The slight but systematic decrease of the enhancement factors with increasing SDS concentration was observed and explained by the fact that more oil molecules are solubilized per micelle as the surfactant concentration increase. However, the author reported that there is no local equilibrium between micelles and oil. Kabalnov et al. w72x mentioned that one of the possible reasons for the higher values of the experimental rates might be the Brownian motion of the emulsion droplets, not taken into account in the LSW theory, which assumes immobile particles. However, since Dparticle
137

Table 3 Variation of relative ripening rates (vrel) with emulsifier type and concentration wSDSx (mol dmy3)

vrel (1)

(2)

0.006 0.008 0.01 0.02 0.025 0.05 0.09 0.2 0.4 1.0

– – 1.5 – – – 2.5 – 2.2 1.5

2.2 2.8 3.7 4.0 – 4.7 5.9 5

(3)

7.5 – 10.3 – 13.1

wSDBSx (mol dmy3)

vrel

0.05 0.08 0.2

2.2 1.6 1.2

(4)

(1) SDS, Kabalnov; (2) Taylor; (3) Soma; (4) De Smet

purification). The relative Ostwald ripening rate for emulsions with oil volume fraction fs0.025 increases from approximately 2 at SDS concentrations just below the CMC to approximately 4–6 at higher SDS concentrations. This means that the presence of micelles enhances v by a factor of approximately 2–3, which agrees with the picture of growing droplets in the presence of a micellar solutions of oil molecules. Furthermore, the Ostwald ripening rates were observed to decrease above ca. 0.1 M SDS (Table 3). Soma and Papadopoulos w74x have reported that the relative Ostwald ripening rates increased from 3–4 below the CMC to 7–13 above the CMC. Note again that the presence of micelles increases v by a factor of approximately 2–3, in agreement with the picture of growing droplets in the presence of micelles. Ostwald ripening rate was observed to increase with SDS concentration (Table 3). The authors explained the increased rates by an increase of the effective concentration of oil in the bulk phase. They proposed model to explain the higher concentration of oil monomers in the bulk phase: micellar dissociation, monomer adsorption at the oily water interface, micelle formation and solubilization at the interface, desorption of the swollen micelle and finally diffusion of the swollen micelle into the bulk phase.

Scheme 3. Schematic representation of the electrostatic stabilization mechanism.

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138

Table 4 Variation of Ostwald ripening rate, the micellar radius, and the ratio of oil molecules per micelle Nsol to the number Naggr of surfactant molecules per micelle with the emulsifier SDBS concentrationa 噛

wSDBSx (mM)

wSDBSxfree (mM)

v (nm3ys)

rSDBS NsolyNaggr (nm)

1 2 3 4 5

33 69 110 130 150

30 66 107 127 147

13.4 11.1 10.4 10.2 9.5

2.4 3.2 3.7 3.9 4.6

a

1.42 1.88 2.18 2.3 2.7

rSDBS – SDBS micelle radius, 25 8C.

De Smet et al. w27x have reported that the ripening rates (v or vrel) increased with increasing sodium dodecyl benzenesulfonate (SDBS) concentration (Tables 3 and 4). Ca. 3 mM of SDBS was needed for the complete coverage of the oil–water interface. Thus, the initial concentration of SDBS is much above the CMC (5 mM). The observed Ostwald ripening rates are a factor 1.4–2.1 higher than the rate (6.6 nm3 ys) predicted by Eq. (1). These enhanced values follow very well the values of 1.4–2.4 estimated with Kelvin’s equation and are consistent with the model of growing droplets in the presence of a micellar solution of oil molecules. The slight but systematic decrease of the enhancement factors (from 2 to 1.5) was explained by the fact that more oil molecules are solubilized per micelle as the surfactant concentration increases. Indeed, the ratios Nsol yNaggr increased with increasing emulsifier concentration. The fact that the observed ratios Nsol yNaggr are an order of magnitude different from the ones estimated from the equilibrium solubilities (ranging from 0.09 to 0.24 in the SDS concentration range of 0.033–0.3 M, Kabalnov w11x) was taken as an indication that there is no local equilibrium between micelles and oil. The monomer droplets degradation was studied for four different alkanes (decane, undecane, dodecane and tridecane) in water emulsions (the oil volume fraction fs0.025 and 0.1 M SDBS) w27x. The decays of the scattering light intensity {I(103 Counts sy1)} of the dilute emulsion illustrate that the characteristic solubilization times of the different alkanes are in a first approximation inversely proportional to the molecular solubility of the oil in the continuous phase: Iy {(time(104 s)}: 220y0, 100y0.12, 50y0.25, 36y0.37, 0.25y0.5, 0.15y1 (tridecane) Iyalkane: 25ydecane, 105yundecane, 150ydodecane, 210ytridecane (times5=10y6 s)

(15)

This behavior was taken as the confirmation that there is the absence of a direct exchange of oil between

emulsion droplets and micelles: otherwise the solubilization rates would be proportional to the ratio of the number of oil molecules to that of surfactant molecules and independent of the chemical nature of the oil molecules. The main rate-determining factor in the exchange of oil between emulsion droplets and micelles is the molecular diffusion of single oil molecules through the continuous phase. Thus, the solubilization rate of several alkane emulsions in 0.1 M SDBS appears to be proportional to the oil solubility in water, confirming the hypothesis that there is no direct exchange of oil molecules between emulsion droplets and the oil phase solubilized by the micelles. The rate-determining factor for the transport of oil is stated to be the molecular diffusion of oil through the continuous aqueous phase. In the presence of oil sink the Ostwald ripening rate is not affected significantly, as long as the rate of withdrawal or solubilization of oil molecules is smaller than the rate of transport of oil molecules from smaller to larger droplets. The relative ripening rate scattered to approximately 1, showing that there is no significant effect on the growth rate due to the addition of micelles (micellar solution). The average ripening rate of another five samples (no addition of SDBS solution) was 10.5 nm3 sy1, that is, a factor of 1.59 larger than the theoretical rate of 6.6 nm3 sy1. The presence of ionic SDS micelles in the continuous phase had a surprisingly small effect on the rate of ripening, despite the fact that the solubility of the dispersed phase is largely enhanced w11,73x. It was argued that due to electrostatic repulsion, ionic micelles could not absorb oil directly from emulsion drops. Evolution of the average scattering light intensity {I(103 Counts sy1)} of the continuous diluted emulsion, however show a decrease after an initial increase (0.15 M SDBS): Iy {(time(104 s)}: 175y0, 210y0.12, 225y0.25, 220y0.37, 215y0.5, 155y0.75, 75y1

(16)

The initial increase is attributed to the average droplet size growth, while the subsequent decrease is due to solubilization. The maximum in the intensity is related to the moment at which solubilization takes over from Ostwald ripening, that is when the fraction of oil molecules exchanged between the droplets has decreased to the value of the fraction of oil swallowed by the sink. The reasons for the difference in the absolute rates between four studies (Kabalnov, Taylor, Soma and Papadopoulos, De Smet) can be discussed in several terms. Since electrostatic repulsive interactions are important in such systems, it may well be that the observations can be related to variation of electrostatic repulsion between droplets or micelles under different reaction conditions. For example, the different degree of

I. Capek / Advances in Colloid and Interface Science 107 (2004) 125–155

hydrolysis into dodecanol and purity of ionic emulsifiers can influence the emulsion behavior. Dodecanol is less surface-active agent than SDS and can introduce electrosteric stabilization. Furthermore, dodecanol can also act as the hydrophobe (see later). Although the surfactant SDS is probably the most widely used in scientific studies, it has disadvantage that in aqueous solution it may hydrolyze to dodecanol w75x. The rate of ripening, according to the LSW model, is directly proportional to the solubility of the dispersed phase in the dispersion medium. The increased polarity of the dispersed phase (oil) decreases the stability of the emulsion. The stability of the SDS-stabilized nanoemulsions increased with an increase in chain length of aliphatic hydrocarbons, this corresponds to a decrease in aqueous solubility of the droplet phase w76x. Authors pointed out that the SDS monolayeryhydrocarbon became more expanded with increasing alkane chain length (from 0.49 nm2 for hexane up to 0.54 nm2 for hexadecane). The rate of degradation decreased in the following order: benzene)xylene)cyclohexane4hexane)octane)dodecane)hexadecane. The stabilization of hexane in water emulsions by hexadecane was attributed to preferential adsorption of hexadecane at the interface and so preventing coalescence w77x. The rate of ripening of the emulsions markedly decreased with increasing additive chain length at concentrations 1–10 wt.%. The presence of hexadecane (HD), however, was reported to have no effect on the stability of hexane emulsions underforced coalescence or on the interfacial tension, showing that the additive had no effect on the interfacial film w78x. The stability of electrostatically-stabilized monomer emulsions was reported to increase by the addition of long chain fatty alcohols (e.g. cetyl alcohol (CA)) w79–81x. This drastically increased the solubilization capacity of anionic emulsifier (SDS) and the stability of nano-emulsions. The presence of fatty alcohols (coemulsifiers) led to the reduction of the interfacial energy and the formation of complexes at the droplet surfaces. The temporary complex emulsifieryfatty alcohol at the surface layer increases the residence time of oil in the electrostatically-stabilized droplet entities. The longer the fatty alcohol, the more stable nano-emulsion. Addition of a long chain alcohol (octadecanol) resulted in a marked improvement in stability, which was attributed to a specific interaction between the alcohol and the alkyl sulfate w77x. The presence of the alcohol markedly reduced the interfacial tension for SDS and the reduction in rate of degradation could be explained by this effect if Oswald ripening is taken to be the mechanism. Addition of decanol or hexadecanol to the wateryhexane emulsion was found to affect the rate of forced coalescence; this was attributed to the alcohols tendency to concentrate at the oyw interface to form stronger interfacial film w78x. In systems where the

139

Scheme 4. Schematic representation of the interfacial layer at the oil droplet surface.

degradation is via Ostwald ripening then the alcohol could possibly reduce the rate through either reduction of interfacial tension or by having a lower aqueous solubility than the oil (Scheme 4). The addition of CA or HD to the coarse emulsions prepared by the ordinary stirring did not improve their stability and emulsification ability w77x. Therefore, a more efficient homogenization is needed to prepare more stable nano-emulsions in the presence of coemulsifier or hydrophobe. The rate of ripening may be reduced in emulsions consisting of single component droplets by either a reduction in the interfacial tension or by the use of a less soluble oil. Polymeric emulsifiers may help in the droplet stabilization through the formation of a thick steric barrier at the oilywater interface, which may hinder the passage of the oil molecules to and from the droplets w8x. This method is attractive since the rate of ripening can be reduced by several orders of magnitude, provided a suitable additive is chosen, using only small concentrations of the additive. The mechanism by which the additive of less soluble additive to an oil droplet reduces the rate of ripening is based on the fact that the two components in the droplets show different rates of transfer between droplets as a result of their differences in solubility w8x. Initially, the concentration of additive is equal in all droplets, however, as ripening proceeds the more soluble component (1) diffuses from the smaller droplets to the larger droplets. The less soluble component (2) cannot transfer as high a rate as component 1 and is essentially trapped in the droplets at this stage. Eventually a point is reached where upon the chemical potential of component 1 is equal in all droplets as a result of the competition between the two effects. When this point is reached then there is no driving force for further transfer of component 1 and any further ripening can occur only through transfer of the less soluble component, which is at a much reduced rate. This state is referred to as the pseudo-steady of ripening. The effect of the addition

140

I. Capek / Advances in Colloid and Interface Science 107 (2004) 125–155

Scheme 5. Schematic representation of the steric stabilization mechanism.

of a less soluble additive on the rate of ripening was investigated by Kabalnov et al. w82x. Their approach differed from that of Higuchi and Misra w15x in that they considered the bulk phase solubilities of the two components within the droplets. 10. Sterically stabilized monomer emulsions The steric stabilization becomes operative when the interfacial layer consists of non-ionic amphiphiles (emulsifiers) or the amphiphiles are adsorbed or bound to the particle surface w83x. When the layers of two interacting particles overlap, the concentration of these macromolecules (or chains) increase in the overlapped region (Scheme 5). The increase in the local polymer concentration initiates the diffusion of solvent from the continuous phase to the overlapped zone. When molecules of a solvent enter the overlapped region, the local chain concentration decreases and consequently the particles separate from one another. 10.1. One oil component emulsions In the series of experiments, Hoang et al. w84x have used several emulsions stabilized by (polyethylene glycole) monolaureate (PEM) (Mws600) of different alkanes (decane and undecane). The increase in particle size was discussed in terms of Ostwald ripening and coalescence. The Ostwald ripening rate was proportional to the solubility of the alkane. Since the solubilities of alkanes in water vary significantly with the alkane chain length, a considerable variation of the ripening rate changes with the alkane chain length. On the other hand, the coalescence rate depended, for a given system, on the initial particle size and concentration. The ripening rate is reduced on replacing the decane by undecane. The ripening rates estimated from the slope of dependence of the cube of the droplet radius vs. time were estimated to be 1.45 and 0.4, respectively. The ratio 3.6 of these rates is in good agreement with the

ratio 3.55 for the bulk solubilities of the two alkanes (2=10y8 ml of undecane and 7.1=10y8 ml of decane in 2 ml of water w72x). Thus, the main aging process is Ostwald ripening and not coalescence. This implies that the main aging mechanism results from the transport of the alkane by diffusion through the continuous phase. To investigate the effect of an increasing number of PEM micelles, ripening experiments were carried out as a function of the emulsifier concentration (Table 5). The minimum emulsifier concentration of approximately 5 mM was estimated for emulsions 1–3 and approximately 10 mM for emulsions 4 and 5 (Table 5). The number of moles (ns) of emulsifier needed to cover the wateryoil oil interface of an oil volume Voil was estimated by nss3Voil y(ro As NA), where ro is the initial particle radius and As the specific surface area of emulsifier. The CMC and As were estimated to be 0.2 mM and 0.39 nm2. Assuming that the first action of the surfactant is to cover the oil–water interface, all of it is used to cover this interface and no surfactant seems to be left in the continuous phase in emulsions 1 and 2. Hence for these emulsions there are no micelles, which can effect the ripening behavior. The observed ripening rates are approximately 1.1–1.3 nm3 ys, which is significantly below the rate of 6.5 nm3 ys which can be predicted by Eq. (1). Possible explanations are: effects of impurities of the surfactant and the increase of the surface elasticity with increasing droplet size. Impurities may act as coemulsifiers, reducing the interfacial tension. The increase of surface elasticity with increasing droplets size may also slow the increase of the average droplet size. The partitioning of emulsifier between the aqueous phase and the oil phase would decrease the amount of emulsifier available for the particle stabilization or the fraction of emulsifier within the interfacial layer and so would decrease the stability of emulsion. The smaller average initial particle radius in emulsions 4 and 5 indicates that there must be enough surfactant left after complete coverage of the oilywater interface to ensure the presence of surfactant micelles. Surprisingly the presence of these micelles decreases the Ostwald ripening rate instead of increasing it. The intensity weighted average sizes of the emulsion droplets are hardly affected by the presence of the micelles. A Table 5 Variation of droplet size and Ostwald ripening rates of the undecane emulsion with emulsifier (PEM) concentration 噛 mM

wPEMx

r (nm)

v (nm3ys)

1 2 3 4 5

3.8 4.6 7.8 10.2 20

35 37 33 22 14

1.27 1.14 0.88 0.06 0.17

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Table 6 Comparison of specific solubilization capacities (Cm ) and ripening rates (v) of n-tetradecane (TD) emulsion droplets suspended in different 2 wt.% emulsifier aqueous solutionsa Emulsifier HLB

Cm (g gy1)

v (nm2 hy1)

Chain length (number of carbons)

Tween 20 16.7 Tween 40 15.6 Tween 60 14.9 Tween 80 15.0 Triton SP-190 13.0 Triton SP-175 12.0

0.023 0.04 0.055 0.058 0.12 0.25

54 26 19 73 18 14

12 16 18 18

a

(saturated) (saturated) (saturated) (monounsat.)

CMC (mM)

nEO

0.05 0.023 0.022 0.01

20 20 20 20 9 7–8

nEO – number of EO groups in emulsifier molecule.

possible explanation for the reduced ripening rate was suggested to be the micelles, which withdraw oil from the continuous phase: that is, they reduce the factor C` in Eq. (1) and hence slow the transport of oil from smaller to larger droplets. The formation of the thick interfacial layer and the accumulation of both micellar and non-micellar aggregates in both water and oil phases can somehow retard the transport of monomer. It is well known than non-ionic emulsifier can form supramicellar aggregates even at slightly elevated temperatures w85x. At relatively low non-ionic emulsifier concentrations, emulsifiers were capable of accelerating Ostwald ripening by facilitating the transport of oil molecules between emulsion droplets (Table 6) w86,87x. The most of emulsifier is supposed to be located at the interfacial zone. At high concentrations, however, some emulsifiers reduced the Ostwald ripening rate or even caused a decrease in droplet size with time. Under such a condition, the partitioning emulsifier between the aqueous and oil phases makes the process very complex. This can result from the presence of micelles in both phases and the formation of non-micellar aggregates. Comparison of Ostwald ripening and solubilization processes on similar systems indicates that there is no simple relation between Ostwald ripening and solubilization. The results suggest that those emulsifiers that are capable of rapidly solubilizing large quantities of the oil are less effective at promoting Ostwald ripening (Table 6). Both the square and the cube of the mean droplet radius increased approximately linearly with time, but there was an appreciably better correlation coefficient for the former. The droplet ripening was mainly driven by molecular diffusion at low emulsifier concentrations, but by micellar transport at higher concentrations. The main difference between the Tween-type emulsifiers was the length and degree of unsaturation of the fatty acid chains. These differences would be expected to affect the ability of the emulsifier micelles to incorporate oil into their hydrophobic interior and transport them across an aqueous phase. There was an increase in the Ostwald ripening rate with emulsifier concentration up to concentrations of approximately 7.5 wt.%

Tween 20, after which the ripening rate (v) decreased: v{(nm)2 hy1} y {Tween 20y(wt.%)}: 24y0.5, 44y1, 54y2, 92y5, 95y7.5, 41y10

(17)

Similar behavior was also observed with Tween 40. These results suggest that Tween 20 and 40 micelles are capable of accelerating ripening at relatively low concentrations, but that they retard it at high concentrations. Tween 60 and 80 show a linear increase in v with emulsifier concentration. Other workers (see above) have reported an approximately linear increase in the Ostwald ripening rate with emulsifier concentration w74x, but at the high concentrations the decrease in v was observed. The decrease in the Ostwald ripening rate at a very high non-ionic emulsifier concentration was discussed in the following items: (1) The diffusion coefficient of micelles decreases as their concentration increases, because crowding effects can increase the viscosity w16x. (2) The decreases in the average droplet size. (3) Solubilization. (4) It is possible that association colloids other than micelles are formed at relatively high emulsifier concentrations, and these transport oil differently. (5) It is possible that the emulsifiers form thick multilayers around the emulsion droplets, which retard the movement of oil molecules from the droplets to the surrounding aqueous phase. (6) Preliminary aggregates formed at low emulsifier concentration are loosely associated and likely to be slightly interacting with oil molecules. Above the CMC, thermal stability is expected to increase, while further increasing emulsifier concentration can lead to the additional (supramicellar) aggregation and decreased oil solubilization w88,89x. (7) The high oil solubility of non-ionic emulsifier can initiate the hydrophobe activity of Tween 20 oryand the formation of inverse structures in the monomer droplets. The droplet size, however, was reported to vary by a complex way with the emulsifier type and concentration w87x. The author reported that the monomer droplet stabilized by Tweens (Tw) was nearly independent of ageing time at wTw 20x4CMC. The increased stability

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Table 7 Colloidal parameters of nano-emulsiona Run 1 2 3 4

Monomer

Emulsifier

St BA St BA

SDS SDS Tw 20 Tw 20

(b)

Shelf-life (c)

1–2 min 9h 10 h 20 s )2 months – no separation

(a) 100 g water, 40 g monomer, wTw 20xs=10y3 mol dmy3; wSDSxs10=10y3 mol dmy3; rpms25 000, 25 8C; (b) 25 8C, (c) 60 8C

of monomer droplets saturated with the non-ionic emulsifier (Tw 20) was assumed to be a result of a number of possible processes. First, association of Tw 20 can form colloids other than the spherical micelles at relatively high emulsifier concentration and these transport oil differently. Secondly, the non-ionic emulsifiers can form multilayers around the monomer droplets, which retard the movement of oil molecules from the droplets to the surrounding aqueous phase. Third, the non-ionic emulsifier can act as hydrophobe especially at high temperature. Fourth, the high oil solubility, solubility of non-ionic emulsifier changes the orientation of the emulsifier molecule at the interface and increases the specific surface area. Fifth, the synergistic effect can result from the interaction of emulsifier with other ingredient(s) w90,91x. Sixth, the present emulsifier micelles can adsorb the monomer, diffusing out of the monomer droplets. The influence of the Tritons on Ostwald ripening was significantly different from that of the Tweens. As an example, the change in the square of the mean droplet size of emulsions containing different concentrations of Triton SP-190 varies as follows: (1) At relatively low emulsifier concentrations (-2 wt.%), there was an increase in droplet size with time, but at higher concentrations there was a decrease, that is, the droplets actually shrank. On the contrary, the increase in the Triton SP 175 emulsifier concentration increased the Ostwald ripening rate. As much as 18% of the total tetradecane (TD) in the emulsion could be solubilized within the aqueous phase at 7.5% Triton SP-190, which could cause a significant decrease in droplet size. Droplet shrinkage due to solubilization has been observed in experiments conducted on single oil droplets w92x. It could also occur in emulsions containing many droplets when the solubilization rate is much faster than the ripening rate. This argument, however, does not explain the data for the Triton SP-175, which is capable of solubilizing a greater amount of oil than Triton SP-190. Solubilization capacity and interfacial mass transfer coefficient both increased as the HLB number decreased from ca. 17 to 12. Furthermore, the measured rates, however, were between 1 or 2 orders in magnitude greater than the calculated rate for molecular diffusion

alone, which suggests that micelles were capable of significantly enhancing the diffusion rate. The 1 or 2 orders of magnitude increases above the expected ripening rate and is more consistent with the 100- to 1000fold increase reported in some studies w16x. In the presence of non-ionic emulsifiers, much larger increases in the ripening rate might be expected due to larger solubilization capacities and to the absence of electrostatic repulsion between droplets and micelles. In the latter case, the collision between micelles and monomer droplets leads to the formation of aggregates in which proceeds transfer of oil from droplet to the micelle. Furthermore, the large residence tome of micelleydroplet aggregate favors the transfer oil process (solubilization). Weiss et al. w93x showed that a significant increase in average diameter can be achieved for tetradecane-inwater emulsions diluted with a fresh solution of the non-ionic emulsifier Tw 20. The shelf-life and the time necessary for appearance of a visible monomer phase on the top of the sample for the (homogenized) nano-SDSybutyl acrylate (BA)y water emulsion were much shorter than those for the (homogenized) nano-Tween 20 (Tw 20)yBAywater emulsion (Table 7) w94x. A visible monomer phase on the top of Tw 20yBAywater sample did not appear even after three months (suppressed coalescence). The diluted BA nano-emulsions, however, underwent the monomer droplet degradation (Fig. 7) The dilution of original emulsion is accompanied by the fast transport of ingredients (Tw20) from the oil phase to the interface and the aqueous phase and, subsequently, the external phase would be saturated with emulsifier w95x. Furthermore, the monomer droplets generated by the homogenization of Tw 20yStyreney water emulsion w96x were less stable than the Tw 20y BAywater nano-emulsions (Table 7). The hydrophobic

Fig. 7. Variation of monomer droplet size (ddrop) of diluted BA nanoemulsion with ageing time (see the legend to Table 7).

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Table 8 Phase-behavior of wateryBAyTw 20 and wateryStyTw 20 solutionsa Temp BA (8C) Lower Phase 20 30 40 50 60 70 80 90

T (8) T (8) SBC (8) BC (8) SlM (8) M (8) M (8) M (8)

St Medium

Upper

Lower Phase

– – M (3.5) M (3) SM (2.5) SM (1.5) SM (0.5) SM (0.5)

M (4) M (4) T (0.5) T (1) T (1.5) T (2.5) T (3.5) T (3.5)

Two SM phases (13) M (6.5) M (6.5) T F(5.5) T (5.5) T (5.0) T (4)

Medium Upper

– M (6.5) – SlM (7.5) – SlM (7.5) – BC (7.5) M (0.5) BC (7.5) M (1.5) BC (7.5)

a 100 g water, 40 g monomer, 2 g Tw 20, T – transparent, M – milky, SlM – slightly milky (low turbidity), SM – strongly milky (high turbidity), SBC – slightly blue coloured (slightly turbid), BC – blue coloured. T – transparent with the foam on walls. The value in bracket means the volume (in abr. units).

styrene (St) was expected to produce more stable monomer emulsion than polar BA w54x. The results show that the reverse is true. One of the reasons could be the low degree of homogenisation. A visible monomer phase on the top of sample (capillary) did not appear in the BA nano-emulsion even at 80 8C. The St nano-emulsions were relatively stable at low temperature (20 8C) but they degradated very fast at 60 8C. The clouding temperature of the monomer (1.3 g)yTw 20 (0.8 mol.dmy3)ywater (100 g) solution was much lower for St (cloud point, CPs42 8C) than for BA (CPs62 8C). The droplet flocculation with increasing temperature was much more pronounced with St then with BA. The positive effect of BA probably results from its coemulsifier property w97x. The phase behavior shows the different data for St and BA containing solutions (Table 8). The wateryBAy Tw 20 solution equilibrated to a two-phase system (at 20–30 8C) when shaking (ca. 30 min at 400 rev.ymin) was ceased. The upper milky phase corresponded to wy o emulsion and the lower one to the aqueous micellar solution. The three-phase system appeared at 40–50 8C w91x. The upper phase was transparent (oil phase), medium milky and lower one blue coloured typical for microemulsion. The lower aqueous phase (microemulsion) transforms to the milky emulsion at high temperature (70–90 8C). The volume of lower aqueous phase was constant and independent of temperature. The threephase monomeryTw 20ywater system was transformed by a slight homogenization to milky one. Agitation of two- or three-phase system led to the reaction mixture containing together the monomer swollen micelles (microdroplets) and large monomer droplets. Variations of the square (Fig. 8) and cube (Fig. 9) of the mean droplet radius of the Tw 20-stabilized styrene emulsion with aging time indicate that neither the coalescence nor Ostwald ripening is the main driving

Fig. 8. Emulsion 1yr y2 as a function of time at 25 8C in the system wateryTw 20ySt at different amount of polystyrene. Recipe: 150 g water, 15 g St, and 7.5 g Tw 20. Mw,PSt s 252 864, MwyMns2.7. Monomer emulsions were prepared by mixing all components and then stirred for 20 min at ca. 400 rpm. PSt (g)y150 g water: 0 (a), 0.15 (b), 0.75 (c).

Fig. 9. Emulsion r y3 as a function of time at 25 8C in the system wateryTw 20ySt at different amount of polystyrene. Recipe: 150 g water, 15 g St, and 7.5 g Tw 20. PSt (g)y150 g water: 0 (a), 0.15 (b), 0.75 (c). See legend to Fig. 8.

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Table 9 Variation of the interfacial tension in the waterystyreneyTw 20 emulsions with polystyrene amounta Timeyh

0 12 24 48 72 168

gymN my1 (1)

(2)

(3)

37.8 38.5 39.8 40.4 38.6 39.0

38.9 41.0 42.8 41.9 40.7 40.3

40.0 41.5 42.4 42.5 41.1 41.2

a Recipe: 150 g water, 15 g St and 7.5 g Tw 20. Mw,PSts252 864, MwyMns2.7. PSt (g)y150 g water: 0 (1), 0.15 (2), 0.75 (3), 25 8C.

force for the droplet instability w98x. There was an initial increase in the droplet size for the systems containing the small amount of polymer and the increase was proportional to the amount of polymer. The initial increase in the droplet size can be attributed to the coalescence andyor Ostwald ripening. The fact that the peak shape appeared after the initial growth (the intensity of this peak increased with polymer amount) was more consistent with coalescence than with Ostwald ripening. The decrease in the droplet size with increasing emulsifier concentration was attributed to the solubilization. The initial decrease in the droplet size was observed in the system without polymer. The presence of polymer in the monomer phase postponed the decrease in the droplet size. The investigated emulsions were strongly diluted (VyV, monomer emulsionywater, 1y200) and therefore the observed behavior can somewhat differ from the original emulsion. Table 9 summarizes the effect of polymer (PSt) on the interfacial tension (g) of ‘original’ emulsions (without any dilution). The interfacial tension is closely related to the amount of adsorbed stabilizer; the interfacial tension decreases with an increase in surface load. The surface load is directly related to the bulk concentration of the stabilizer although, dependent on the type of stabilizer, many other effects such as history,«, are of importance. If the interfacial area is increased and the stabilizer concentration is constant, the surface load will decrease and the interfacial tension g will become higher. The dependence of g on aging is described by the curve with a maximum (Table 9). This behavior might result from the two opposing effect: (1) coalescence; and (2) solubilization. The initial increase in g can be ascribed to the dominant role of coalescence andyor Ostwald ripening. The presence of non-micellar aggregates is speculated to initiate the solubilization. The results show that the solubilization might be dominant after ca. 48 h aging. The presence of a small amount of polymer in the monomer phase was expected to increase the stability of emulsion due to (1) the decreased Ostwald ripening and (2) the release of

dissolved non-ionic emulsifier from the monomer phase (Tw 20 is not compatible with PSt). The addition of cetyl alcohol or hexadecane to the coarse emulsions prepared by the ordinary stirring did not improve their stability and emulsification ability w77x. Therefore, a more efficient homogenization may be needed to prepare more stable nano-emulsions in the presence of polymer and to follow the proper variations in g. Furthermore, the low extend of homogenization can lead to the formation of thin monomer film on the top of emulsion which could strongly influence the surface tension measurements. The oyw nano-emulsions (wateryC12(EO)4 yoil) were prepared by the phase inversion temperature emulsification method w99x. The phase inversion temperature (PIT) method was first introduced by Shinoda w100x to prepare the nano-emulsions. The conductivity of the emulsion initially increases with the increase of temperature, reaching a maximum, and then abruptly decreases (Fig. 10). The hydrophilic-lipophilic balance (HLB) temperature or phase inverse temperature was taken as the average value between the maximum and the minimum values of conductivity. The lack of continuity in the conductivity curves for the highest emulsifier concentration was attributed to the formation of liquid crystalline phases. As was described for a similar system w101x, at such high emulsifier concentration (above 5 wt.%) the transition from oyw to wyo systems passes through the formation of La (lamellar) and L3 phases. Table 10 indicates a gradual reduction of the HLB temperature with the increase in C12(EO)4 concentration. This reduction was attributed to the polydispersity of the commercial emulsifier. The chains with lower EO content will preferentially partition to the oil phase. With an increase

Fig. 10. Conductivity as a function of temperature in the system aqueous 0.01 M NaClyC12(EO)4yhexadecane at different concentrations of emulsifier, and constant oil concentration (20 wt.%). (a): C12(EO)4s 8.0 wt. %, (b): 5.0 wt.%, (c) 7.0. wt.%, 25 8C.

I. Capek / Advances in Colloid and Interface Science 107 (2004) 125–155 Table 10 Compositions and HLB temperature of samples in the system aqueous NaCl 10y2 MyC12(EO)4yhexadecane at 20 wt.% oil concentration Wt.% C12(EO)4

Oilywater ratio

THLB (8C)

3.0 3.5 4.0 5.0 6.0 7.0 8.0

20.6y79.4 20.7y79.3 20.8y79.2 21.1y78.9 21.3y78.7 21.5y78.5 21.7y78.3

57 54 49 47 46 41 41

in emulsifier concentration, there will be more accumulation of chains with low EO content in the oil phase, and this will result in a reduction of the HLB temperature. In other words, with the highest C12(EO)4 concentrations, there will be more accumulation of molecules with shorter EO chains at the interface, when compared with the case for lower C12(EO)4 concentrations. In all cases, the plots did not follow the prediction of Eq. (11), indicating that coalescence, as described by the model of Ref. w49x, may not be the mechanism of instability: 1yr2 (nm).104 ytime (h): 14y0, 6.5y1, 2y3, 1.5y6, 0.2y25 (C12(EO)4s8.0 wt.%)

(18)

However, the linear r 3 vs. time plots mean that the main driving force for instability is Ostwald ripening. Furthermore, the droplet radius decreases with the increase in emulsifier concentration because of the increase in interfacial area and the decrease in interfacial tension, g. As is well-known, g reaches a minimum value at the HLB temperature w102x. Therefore, the compositions with HLB temperatures closer to 25 8C are those having lower interfacial tensions and smaller droplet sizes. Concerning of Ostwald ripening rate, it is clear from Table 11 that v increases with the increase in emulsifier concentration. Furthermore, the values of v are 1–2 orders of magnitude higher than the theoretical (LSW) value. The LSW theory assumes that the mass transfer is due to the molecular diffusion through the continuous phase and that there is no interaction between the particles, which are spherical. Consequently, the theory applies to low dispersed phase volume fractions. The difference between theoretical and experimental rates could be due to factors not taken into account in the LSW theory, such as oil transport due to the presence of emulsifier aggregates in the continuous phase, that is, micelles w103x, and Brownian motion of the droplets w104x and other effects mentioned before. The increase of v with the increase in emulsifier concentration was discussed in terms of number of effects. First, by increasing the emulsifier concentration, the droplet size decreases, as shown in Table 11,

145

Table 11 Compositions, initial droplet radius and Ostwald ripening rates (v), at 25 8C, of nano-emulsions in the system wateryC12(EO)4 ) hexadecane at 20 wt.% oil concentration Wt.% C12(EO)4

Oilywater ratio

r (nm)

v (=1027 m3 sy1)

4.0 5.0 6.0 7.0 8.0

20.8y79.2 21.1y78.9 21.3y78.7 21.5y78.5 21.7y78.3

66 47 34 30 26

2.3 4.1 10.2 18.0 39.7

favoring the Brownian motion and increasing v. Second, the number of micelles is expected to increase with the increase in emulsifier concentration w103x and this results in a increase in the flux, J(syD(≠Cy≠x), where D is the molecular diffusion coefficient of the oil and (≠Cy≠x) the concentration gradient), of oil molecules, as given by Fick’s first law w104x. Athough diffusion of micelles is slower than diffusion of molecules, can increase by several orders of magnitude as a result of solubilization w105x. Then, the overall effect would be an increase in J and hence an increase in Ostwald ripening rate. The third reason for the increase of Ostwald ripening rate with the increase in emulsifier concentration could be due to partitioning of emulsifier molecules between the oil and the aqueous phases (see above). It is likely that accumulation of low HLB molecules results in lowering of the Gibbs elasticity, and these may result in an increase of the ripening rate. In addition, the presence of different emulsifier aggregates, as the concentration increases, would also influence Gibbs elasticity. The presence of emulsifier aggregates with a lamellar liquid crystalline structure in nano-emulsions of a similar system has been reported w20x. The HLB temperature decreases with the decrease in hydrocarbon alkyl chain length (Table 12). The initial droplet radius decreases with the decrease in HLB temperature, which can be attributed to a decrease in the interfacial tension. Table 12 also shows that the ripening rate increases with the increase of alkane solubility in water. Table 12 Compositions, HLB temperatures, molecular solubility of hydrocarbons in water (C`), and ripening rates at 25 8C of nano-emulsions in the system wateryC12(EO)4yaliphatic hydrocarbon at 20 wt.% oil and 4.0 wt.% emulsifier concentration Oil

THLB (8C)

C`=1010 (mlyml)

r (nm)

v=1027 (m3 sy1)

decane dodecane tetradecane hexadecane isohexadecane

38.5 45.5 49.5 49.8 53.0

710.0 52.0 3.7 0.3

59 62 64 66 60

20.9 9.3 4.0 2.3 8.0

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Table 13 Variation of the Ostwald ripening rates with the type of oil and emulsifier (30 8C)a Physical property

Oil type Hexadecane

3

vpredicted (m ys) vmeasured vmeasuredyvpredicted a

2.7=10 (Tw20) 1.4=10y29 (GA) -1=10y28 (Tw20) -1=10y28 (GA) -(Tw20) -(GA) y29

Decane

1-Decanol y25

1.1=10 5.0=10y26 5.8=10y24 4.7=10y24 54 (Tw20) 95 (GA)

(Tw20) (GA) (Tw20) (GA)

1.7=10y23 (Tw20) 8.3=10y24 (GA) 3.1=10y28 (Tw20) 1.4=10y20 (GA) 1.4=10y5 (Tw20) 170 (GA)

Tw20 (Tween 20), GA (gum arabic).

The colloidal stability of the emulsion is known to depend on the oil type and the emulsifier type and concentration (see above) as well. The influence of oil type (n-hexadecane, 1-decanol, n-decane) and emulsifier type (Tween 20 – a strongly adsorbing non-ionic emulsifier, gum arabic—a slightly adsorbing emulsifier) on droplet growth in oil-in-water nano-emulsions (miniemulsions) was studied by McClements et al. w37x. Generally, the miniemulsions are prepared in two-step process: (1) emulsions are pre-treated with a high-speed blender; and (2) then sonicated using an ultrasonic generator, for example. Nano-emulsions containing oil molecules of low polarity and low water solubility (hexadecane) were stable to droplet growth, irrespective of the emulsifier, which is used to stabilize the droplets. Emulsions containing oil molecules of low polarity and relatively high water solubility (decane) were stable to coalescence, but unstable to Ostwald ripening, irrespective of emulsifier. Droplet growth in emulsions containing oil molecules of relatively high polarity and high water solubility (decanol) depended on emulsifier type. Decanol droplets stabilized by Tween 20 were stable to droplet growth in concentrated emulsions, but unstable when the emulsions were diluted. Decanol droplets stabilized by gum arabic exhibited rapid and extensive droplet growth, probably due to a combination of ripening and coalescence. It was proposed that coalescence was caused by the relatively low interfacial tension at the decanolywater boundary, which meant that the gum arabic did not absorb strongly to the droplet surfaces and therefore did not prevent the droplets from coming into close proximity. For all the emulsions studied, the measured droplet growth rate was considerably greater than the Ostwald ripening rate predicted by the LSW model w24,25x and values of the relevant physicochemical properties of the oils, with the exception of the Tween 20-stabilized decanol emulsion (Table 13). A faster droplet growth rate than expected from Ostwald ripening theory was attributed to some limitation in the Ostwald ripening theory. The interfacial tension and droplet growth studies reported by McClements et al. w37x have provided some

valuable insights into the origin of the instability mechanisms in oyw emulsions. If it is assumed that an emulsifier would provide adequate protection against coalescence provided that a sufficient quantity of it was adsorbed to the droplet surfaces, then the oil phases used in this study can be divided into three categories: – Low-polarity and low-water solubility oils (e.g. hexadecane)yw emulsions are stable to both ripening and coalescence. – Low-polarity and high-water solubility oils (e.g. decane). Oyw emulsions are stable to coalescence, but unstable to ripening. – High-polarity and high-water solubility oils (e.g. decanol). Oyw emulsions are unstable to both ripening and coalescence when stabilized by a weakly adsorbing biopolymer (gum arabic), but are stable when stabilized by a non-ionic emulsifier (Tween 20). The concentrated decanol emulsions stabilized by Tween 20 showed apparently anomalous behavior, with the observed droplet growth rate being many order of magnitude lower than that predicted by LSW model w24,25x (Table 13). The most likely explanation for this phenomenon was that the composition of the concentrated emulsion was in the region of the oil-emulsifierwater ternary phase diagram where liquid crystalline structures were thermodynamically stable w106x. Hence, Tween 20 may have formed liquid crystalline multilayers around the emulsion droplets w106x, thereby retarding the diffusion of oil molecules across the interfacial membrane. When the emulsion was diluted, it rapidly broke down because its composition was no longer in a region of the ternary phase diagram where liquid crystalline structures were thermodynamically stable. 10.2. Mixed oil-components emulsion If the dispersed phase is composed of a binary mixture, the growth may be arrested if one component is almost insoluble in the continuous phase, therefore retaining the soluble one due to the gradual loss of mixing entropy w29x. It should be noted that the rate of

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droplet growth in oyw emulsions containing mixed oils could be dramatically influenced by ‘compositional ripening’ w8x. In a polydisperse emulsion, oil moves from smaller to larger droplets because of Ostwald ripening. If emulsion droplets contain two oils that have different solubilities in water, then the more water-soluble oil will be transported more rapidly between the droplets than the less water-soluble oil. Thus, there is an accumulation of the more water-soluble molecules in the larger droplets, and an accumulation of less water-soluble oil molecules in the smaller droplets. The resultant difference in droplet composition is thermodynamically unfavorable because of entropy of mixing effects. Hence, compositional ripening tends to oppose Ostwald ripening, and may even lead to a complete suppression of droplet growth in an emulsion w8x. The issue whether the decane transfer occurs by ‘molecular’ transport or whether the emulsifier micelles present in the continuous phase act as ‘carriers’ for decane was investigated by Binks et al. w16x. For this purpose the authors used two types of emulsions. The squalane emulsions (type 1) showed no change in the size distribution over a period of weeks, indicating the absence of coalescence and Ostwald ripening. Type 2 decane emulsions showed slow growth over a period of 10–20 days, which was ascribed to Ostwald ripening. After mixing of both emulsions, the particle size (r1) of first emulsion drops increases whereas that of second emulsion drops decreases: {r1 ymm} y {timeys}: 0.55y0, 0.75y100, 0.78y200, 0.85y400, 0.88y600, 0.92y1000, 1.05y2000 (19) The degradation of emulsion was also discussed in terms of emulsifier type and concentration and solubilization as well. The extent of solubilization was observed to increase approximately linearly with emulsifier concentration: SDS (0.0006 M decaney0.1 M SDS) -tetradecyl trimethylammonium bromide (TTAB)(0.003y0.1)-C12(EO)8 (0.006y0.1) -C12(EO)6 (0.05y0.1)

(20)

These data show that the most effective solubilizer is the non-ionic emulsifier. For comparison, the solubility of decane in pure water (C`) is 4=10y7 M w107x, many orders of magnitude smaller than the solubility in the micellar solutions. C`D increases with increasing emulsifier concentration and the increase is more pronounced at the low emulsifier concentration (Fig. 11).

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Fig. 11. Derived values of C`D as a function of the emulsifier type and concentration. (h) C12(EO)6; (O) C12(EO)8 ; (j) TTAB, (d) SDS.

For two non-ionic emulsifier systems, D was assumed to vary with the swollen micelle volume fraction fmicelle according to: DsDo (1–2.1 fmicelle)

(21)

where Do is the diffusion coefficient of the swollen micelles at infinite dilution. The range of emulsifier concentration used (0.5–15 wt.%) corresponds approximately to a variation in fmicelle of 0.008–0.27. The coefficient y2.1 is appropriate for a dispersion of hard spheres, which is known to provide reasonably an accurate description of the behavior of oil swollen nonionic micelles at the solubilization limit w108x. The calculated curves are of the same order of magnitude as the experimental data. This led to conclusion that micellar-mediated transport (with zero energy barrier to exchange of oil between micelles and emulsion drops) is the dominant oil transfer mechanism. The SDS behavior is anomalous in that the experimental C`D values (derived from measuring swelling rates) are almost independent of SDS concentration (suggesting micelles are not involved in the oil transport) whereas the magnitudes of the experimental C`D values are consistent with micellar transport. In contrast to the swelling rates, ripening rates with SDS systems show almost no increase with SDS concentration and the magnitudes of the rates correspond to molecular transport. Macroscopic studies of the non-ionic emulsion systems indicate that weak flocculation occurs. Increasing the concentration of micelles in the continuous phase is expected to promote flocculation by increasing the attractive depletion interaction w109x. This was assumed by Binks et al. as one of alternative explanations for the fast rates and increase in rates with increasing micelle concentration.

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However, the initial value of C`D (Fig. 11) is a maximum or the value of C`Dy wemulsifierx decreases with increasing emulsifier concentration. This could be attributed to the lower transfer rate of oil with larger micellar aggregates. The solubilization rates of individual drops of ndecane and n-decaneysqualane mixtures in aqueous nonionic emulsifier C12(EO)8 solutions (at a concentration well above the CMC) were limited by interfacial phenomena, not by diffusion in the emulsifier solution w110x. The solubility of decane in the micellar solution was much greater than its molecular solubility in pure water, and the solubility of squalane was negligible at test conditions. The saturation of the emulsifier solution (2.5 wt.%s0.0465 M C12(EO)8) with n-decane was indicated by a persistent turbidity that reduced the transmitted light through the mixture. Saturation was reached at Cs0.0102 M which is significantly larger than the molecular solubility of decane in water (4=10y7 M at 25 8C w111x). Thus, it is plausible to assume that most of the oil dissolved is located within micellar aggregates and that the contribution of molecular solubility to the overall solute concentration in the aqueous phase is negligible. Also the CMC of C12(EO)8 in water is 7.1=10 My5s0.004 wt.% at 30 8C w112x, and therefore the amount of emulsifier forming micelles, can be taken as the whole concentration of emulsifier without introducing significant error. Two kinds of experiments were performed for the solubilization of mixtures of n-decane and squalane. Firstly, mixed drops with varying initial composition were placed in oil-free aqueous C12(EO)8 (2.5 wt.%) solution. The initial mole fraction of n-decane in the decaneysqualane mixture was xos0.8. It was found that drop radii diminished faster at early times with the rate of decrease slowly gradually, and droplets approached a limiting size in an asymptotic fashion. This limiting size coincided in all cases with the calculated radius of a hypothetical drop that would contain only the squalane initially present in the original drop. This result conforms that squalane was not solubilized in C12(EO)8 micelles to a measurable extent even in the presence of a more soluble compound such as n-decane. Therefore, it is adequate to consider this oil as immobile due to which the partitioning of squalane between the oil drops and the micelles is negligible and that the aggregates carried only n-decane. Secondly, drops of squalane were placed in partially saturated solutions of n-decane. In this case, the droplet size increased in time because n-decane is transferred from the aqueous bulk to the oil drop. This behavior does occur as estimated for interfacial and diffusion-limited mass-transfer kinetics, respectively. According to these expressions, the size of the drop at equilibrium is reached when the molar fraction of ndecane in the mixed oil equals the fractional saturation in the bulk. The data are compared with predictions

from two plausible models for mass transfer in nonionic emulsifier solutions, each assuming one of mechanisms referred to above as rate controlling. Results show that mass transport was dictated by interfacial phenomena. This is in agreement with previous investigations on the solubilization of hydrocarbons in micellar solutions of non-ionic emulsifiers w113x. The models are extended to account for mass transfer in a collection of droplets. The drops of the insoluble or immobile oil (squalane) grow while those of the slightly soluble or mobile oil (decane) diminish due to the solubilization of the latter at the interface and its further transport through the bulk phase to the squalane drops. Results for the mixed emulsion indicate that the observed ripening was also dictated by interfacial phenomena— possibly by the rates of adsorption and desorption of the micelles at the wateryoil interfaces—and not by the diffusion of micelles acting as oil carriers through the aqueous bulk phase. In addition, this work confirmed that the contribution of interfacial curvature to the difference in chemical potential between droplets is negligible compared to that of drop composition differences. Therefore, the emulsion studied by Binks et al. w15x and also considered here is a case of compositional ripening. A systematic study of the kinetics of the oil drop swelling process has been described for emulsions stabilized by non-ionic emulsifier n-dodecyl octaoxyethylene glycole ether (C12(EO)8) w114x. The mobile oil was decane, and the immobile oil was squalane. For this system, the kinetics of the drop swelling process were consistent with a mechanism in which micellarmediated transport of the decane between the oil drops dominated the oil transfer rate. The micellar-mediated transport mechanism is thought to involve exchange of oil between the aqueous micelles and the emulsion drops, possibly by a fusion–fission process of the swollen micelles with the emulsion drop surfaces w115x. The transport of oil between emulsion drops occurs by diffusion of micelles containing solubilized oil, for the system studied, zero energy barriers. This finding for a non-ionic emulsifier system is in sharp contrast to results from Ostwald ripening kinetic studies on emulsions stabilized by anionic emulsifier SDS for which a mechanism where oil transport occurs mainly by molecular diffusion w7x. Two approaches were used to distinguish between molecular and micelle mediated transport. Firstly, the measured transport rate (from either molecular diffusion or compositional ripening experiments) is compared with values calculated using the solubility and diffusion coefficient of either molecularly dissolved oil (for molecular transport) or oil solubilized within micelles. Secondly, since the extent of oil solubilization generally increases approximately proportional with emulsifier concentration, micellar-mediated transport

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rates should increase with emulsifier concentration whereas molecular transport is unaffected. In addition, the experiments of Binks et al. w114x on the dynamic behavior of droplet sizes in a mixed decaneysqualane emulsion could be interpreted by assuming mass transport to be controlled by interfacial phenomena, possibly by the relative rates of micellar adsorption and desorption at the wateryoil interfaces. Prediction from this model using a rate constant obtained from the single-drop experiments provided the best agreement with experiments with no adjustable parameters. Therefore, the mechanism that dictated masstransfer rates in the solubilization of individual drops was also that for collection of droplets found during compositional ripening of the mixed emulsion. Differences between predictions based on this mechanism and experimental data for the mixed emulsion are due to the polydispersity of the emulsion, which is not accounted for in the model. Numerical results from models built on plausible assumptions, such as ideal solution behavior of the oil mixtures and negligible effect of the interfacial curvature on the local solubility at the interface, correlated well with the data obtained in solubilization tests. In particular, data consistently showed that the partitioning of squalane between the oil drop and the micelles was negligible, a fact that eased considerably the mathematical treatment of mass transport and vindicated the proposed modeling. The addition of a second, less soluble, component to the dispersed (isohexane) phase reduced the ripening rate in oyw nano-wateryC12(EO)4 yoil emulsions w99x. The rates of Ostwald ripening for these two systems (isohexane alone and with squalane) are 8.2=10y27 and 5.5=10y27 m3 sy1. As expected the addition of squalane causes a reduction in the rate of ripening. This reduction was attributed to the partition of squalane in the isohexane droplets, which becomes different for different sized droplets. During Ostwald ripening, equilibrium is established when the difference in chemical potential between different size droplets is balanced by the difference in chemical potential resulting from partition of the two components w116x. This explains the reduction in v on addition of squalane. The mass transport rate of non-polar through an aqueous solution depends on their water solubility, and, therefore, the growth of droplets due to Ostwald ripening depends on droplet composition w117x. Oil-soluble emulsifiers dissolved within an oil droplet are capable of displacing emulsifiers from the oilywater interface, which decreases the stability of the droplets to coalescence w118x. The influence of droplet composition (hexadecane:decanol), and emulsifier type (Tween 20, gum arabic) on droplet growth in oil-in-water emulsions was studied by McClements et al. w37x. It was expected that as ratio of hexadecane (low water solubility) to decanol (high water solubility) in the droplets was increased,

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the rate of droplet growth due to Ostwald ripening would decrease. In the Tween 20-stabilized emulsions, the droplets were stable to growth at all decanol: hexadecane ratios. In the gum arabic-stabilized emulsions, the rapid growth was suppressed only when the emulsion droplets contained more than 90% hexadecane, and there was a close correlation between the increase in interfacial tension and the decrease in droplet growth. 11. Electrosterically stabilized emulsions The electrosterically-stabilized oil droplets are covered by both ionic and non-ionic emulsifiers. The mixed ionicynon-ionic emulsion system can more effectively stabilize the emulsions presumably due to the synergistic effect provided by combination of both the electrostatic and steric stabilization mechanisms w119x. The colloidal stability of the electrosterically-stabilized dispersions can be controlled by the fraction of the particle surface covered by non-ionic emulsifier (u) and the ratio of the thickness of the non-ionic emulsifier adsorption layer (d) to the thickness of the electrical double layer (ky1) around the oil droplets (d y(ky1))s(dk) w120,121x. The electric double layer around the oil droplets originates from the ionic emulsifier species. According these authors, the parameter u and the ratio dk play an important role in the interparticle interaction and bridging flocculation process. At lower values of dk (d
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Table 14 Data obtained from styreneyalkyl methacrylate miniemulsiona wNP-40xymM wSDSxymM CMCymM

0 5 4.1

1.25 3.75 0.36

2.5 2.5 0.22

5 0 0.15

d(d3m)iydty(nm3 ymin) (DMA) dm,iynm (DMA) Nm,i=1017ydm3 (DMA) dm,iynm (DMA, BD) Nm,i=1017ydm3 (DMA, BD)

8.33 169 1.06 179 0.9

4.51 140 1.88 165 1.14

4.1 138 1.96 146 1.67

3.5 1125 0.0036

d(d3m)iydty(nm3 ymin) (SMA) dm,iynm (SMA) Nm,i=1017ydm3 (SMA) dm,iynm (DMA,BD) Nm,i=1017ydm3(SMA, BD)

indep. 113 3.5 121 2.86

120 2.98 115 3.4

129 2.38 117 3.2

792 0.01 –

a CMC for SDS, SDSyNP-40 and NP-40 at 25 8C w126xx, d(d3m)iy dt (the initial Ostwald ripening rate) at 35 8C w124x and dm,i (the initial monomer droplet size) and Nm,i (the initial number of monomer droplets) at 35 8C, Blue dye (BD)s0.1 % (based on total monomer weight).

non-ionic emulsifier could act as emulsifier and coemulsifier as well. When an oil-in-water emulsion is created by the application of shear force to a heterogeneous fluid containing emulsifiers, a distribution of droplet sizes results. To create an emulsion of very small droplets, the droplets must be stabilized against coalescence and diffusional instability. Stabilization against coalescence is effected by adding an appropriate (co)emulsifier. If the small droplets are not stabilized against diffusional degradation, they will disappear, increasing the average droplets size, and reducing the total interfacial area. In creating a stable monomer emulsion, diffusional stabilization is achieved by adding a quantity (1–8% wtywt based on oil) of a highly oil-soluble, water insoluble stabilizing agent. Both long chain alkanes and long chain alcohols have been used as stabilizing agents in miniemulsions w122,123x. The very thin interfacial layer formed by ionic emulsifiers alone seems to be inefficient to stabilize the monomer emulsion. These additives can penetrate into the interfacial zone due to which increases the thickness of surface layer, but decreases the degree of dissociation of ionic emulsifier (the electrostatic stabilization can transform to the electrosteric mechanism). The initial Ostwald ripening rate for into the dodecyl methacrylate (DMA) miniemulsion stabilized by SDS is somewhat slow and it is depressed when NP-40 (nonylphenol polyethoxylate with an average of 40 ethylene oxide units per molecule) is added w124,125x (Table 14): The monomer droplet degradation, however, decreases with the reaction time and the decreases is more pronounced at higher wNP-40x. The system saturated with higher wNP-40x reaches the plateau at the reaction time 4 h. This indicates that the part of NP-40 soluble in the

monomer phase increases the stability of monomer droplets, that is, it penetrates into the interfacial layer, increases the thickness of adsorption layer and makes barrier for mobile molecules. The initial droplet size decreases (the initial number of monomer droplets increases) with increasing mole fraction of NP-40 within the mixed SDSyNP-40 emulsifier system. For the stearyl methacrylate (SMA)ystyrene miniemulsion containing NP-40, dm,i is relatively insensitive to changes in wNP40x and the Ostwald ripening rate is insignificant in comparison with DMA series. The initial droplet size for the SMA system increases (the initial number of monomer droplets decreases) with increasing mole fraction of NP-40 within the mixed SDSyNP-40 emulsifier system. This indicates that SMA makes a stronger interfacial barrier than NP-40 with SDS. The d3m vs. time data suggest that DMA is not hydrophobic enough to retard diffusion of monomer from small droplets to large droplets or to form an enough thick barrier with SDS. The more hydrophobic SMA offers the miniemulsion a stronger osmotic pressure effect and it can effectively stabilize the homogenized monomer droplets. The decreased CMC of SDS by the addition of NP-40 results from the interaction between both emulsifiers, which favors the micelle formation and increases the micellar fraction of emulsifier. The synergistic effect of SDS and NP-40 interaction depends on the monomer type. In the case of DMA, the droplet size decreases with increasing wNP-40x as a result of penetration of DMA into the interfacial layer. The linear dependence of d3m vs. time obtained in the systems with CA and DMA supports the Ostwald ripening. The addition of other hydrophobe (blue dye) increased the droplet size for both DMAySDS and SMAySDS. This behavior might be attributed to the re-organization of thick interfacial layer caused by the interaction between dye and emulsifier oryand the compositional ripening. Under the circumstances, the droplet size for both DMA and SMA decreased with increasing wNP-40x. The data of dm for the nano-emulsions (miniemulsion) with DMA and SMA at various levels of wNP40x at 35 8C are summarized in Fig. 12. Both the initial and steady-state values of dm decrease with increasing wNP40x and the decrease is much more pronounced for miniemulsion with DMA. The higher water solubility of DMA favours its solubilization and consequently the more pronounced decrease in the droplet size with increasing emulsifier concentration. As shown by dm data, the lower the level of NP40, the greater is the degree of Ostwald ripening. For emulsions stabilized by the mixed SDSy C12(EO)8 system, the values of C`D increase approximately linearly with mole fraction of C12(EO)8 in the emulsifier mixture. For SDSyC12(EO)6 mixed systems, however, the variation of C`D with the mole fraction of C12(EO)8 is non-linear (high extent of solubilization).

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had the greater solubility and was able to diffuse through the dispersion medium to redeposit on the smaller droplets. This effect is called reverse condensation w132x. Initially, the system was bimodal, with peaks corresponding to the original emulsions. However, the peak at the higher radius gradually disappeared until the system became monomodal after 24 h. The effect of reverse condensation was described as being related to Ostwald ripening. The two processes rely on molecular diffusion of the dispersed phase through the medium but the driving force may be different. The dissolution of the larger drops into the smaller dropls cause a net increase in surface area. A similar situation was observed when emulsions of two different oils were mixed, but in this case the emulsions were of the same size. The resulting emulsion was of the same size as the two initial ones, but the droplets now consisted of a mixture of two oils and this was attributed to the entropy of mixing w133x. Fig. 12. Variation of average monomer droplet size (dm) upon aging at 35 8C for 4 h with NP40 concentration and coemulsifier type. (a) DMA, (b) SMA.

The addition of a small amount of SDS strongly depressed solubilization and the value of C`D as well. Microemulsion phase behavior studies of mixed nonionicyionic emulsifier systems have shown that the extent of oil solubilization varies in a highly non-linear manner with the addition of ionic emulsifier w127x. Emulsified oil droplets can experience strong attractive interactions without coalescence. Neighboring droplets are deformed and exhibit contact angles w127,128x. Adhesion between monomer droplets can lead to the formation of emulsion gels with a rigid and metastable structures span the whole system. The ionic emulsifier molecules of the mixed emulsifier system were shown to be more concentrated in the interfacial layer (contact area) w129,130x. Conversely, the non-ionic emulsifier molecules tend to be less concentrated in the contact area. This effect was indirectly deduced from the analysis of the surfactant Gibbs adsorption w131x. 12. Mixed emulsions A related effect to ripening is the effect seen when two emulsions of differing sizes and differing solubilities are mixed together w132x. Here, the larger emulsion of perfluorodecaline was turbid whereas the smaller emulsion was opalescent and almost transparent. On mixing the two emulsions, at 1:1 ratio, the resulting emulsion was initially milky white. However, as time progressed its appearance changed to being opalescent and transparent. Measurements of size of the emulsion showed that smaller emulsion was imbibing the material from the larger droplets. Of the two oils, the perfluorodecaline

13. Conclusion Emulsions are unstable exhibiting flocculation, coalescence, creaming and degradation by diffusion. They are formed by mixing of one liquid in another where each liquid is immiscible, or poorly miscible in the other. They degrade toward phase separation via mass transfer and other mechanisms. The physical degradation of emulsions is due to the spontaneous trend toward a minimal interfacial area between the dispersed phase and the dispersion medium. Minimizing the interfacial area is mainly achieved by two mechanism: first coagulation possibly followed by coalescence and second Ostwald ripening. Coalescence is often considered as the most important destabilization mechanism leading to coursing of dispersions and can be prevented by a careful choice of stabilizers. Ostwald ripening, however, will continuously occur as soon as curved interfaces are present. The rate of ripening, according to the LSW model, is directly proportional to the solubility of the dispersed phase in the dispersion medium and the polarity of the dispersed phase (oil). Mass transfers in emulsion may be driven not only by differences in droplet curvatures but also by differences in their compositions. This is observed when two or more chemically different oils are emulsified separately and the resulting emulsions are mixed. Compositional ripening involves the exchange of oil molecules between emulsion droplets with different compositions. The term compositional ripening is used when the composition of droplets is not uniform and concentration gradients drive mass exchange, thus inducing changes in droplet sizes. The degradation of emulsions can be prevented by adsorption of emulsifier molecules at the interface oily water. The ionic emulsifier can impart repulsive forces between similarly charged electrical double layers to

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emulsified oil droplets while the non-ionic emulsifier can provide the interactive particles with the steric stabilization. The colloidal stability of the electrosterically-stabilized dispersions is controlled by the fraction of the particle surface covered by non-ionic emulsifier (u) and the ratio of the thickness of the non-ionic emulsifier adsorption layer (d) to the thickness of the electrical double layer (ky1) around the oil droplets (d y (ky1))s(dk). The electric double layer around the oil droplets originates from the ionic emulsifier species. At lower values of dk (d
The monomer droplet degradation can be depressed by transformation of coarse emulsions to nano-emulsion (miniemulsion) by intensive homogenization andyor by the addition of a water-insoluble compound (hydrophobe: hexadecane) or a surface active agent (coemulsifier: cetyl alcohol). Hydrophobe decreases the fraction of oil phase in the aqueous phase, that is, it pumps the oil out of the aqueous phase. The larger the molar volume of hydrophobe (coemulsifier) the higher the anti-degradating efficiency of hydrophobic additive. A surface-active agent accumulates at the droplet surface area and so forms a barrier against the transfer of the oil phase from one droplet to another droplet through the aqueous phase. Furthermore, a more-densed interfacial layer (with higher density) can be formed by interaction between emulsifier and coemulsifier molecules. This is accompanied with the increased shelf-life of nanoemulsions. The addition of a hydrophobe to the dispersed phase significantly retards the rate of ripening as a result of decreased water solubility of disperse phase. Addition of a long chain alcohol (e.g. cetyl alcohol or octadecanol) resulted in a marked improvement in stability, which was attributed to a specific interaction (the complex formation) between the alcohol and ionic emulsifier. For example, the presence of the alcohol markedly reduced the interfacial tension for emulsifier. Addition of decanol or hexadecanol to the wateryhexane emulsion affected the rate of forced coalescence, which was attributed to the alcohols tendency to concentrate at the oyw interface to form stronger interfacial film. The micellar-mediated transport mechanism is thought to involve exchange of oil between the aqueous micelles and the emulsion drops, possibly by a fusion–fission process of the oil-swollen micelles with the emulsion drop surfaces. The transport of oil between emulsion drops occurs by diffusion of micelles containing solubilized oil, for the system studied, zero energy barrier. A non-ionic emulsifier system is in sharp contrast to results from Ostwald ripening kinetic studies on emulsions stabilized by anionic emulsifier for which a mechanism where oil transport occurs mainly by molecular diffusion. In the first system, the measured transport rate can be given by two contributions: (1) from molecular diffusion (or compositional ripening experiments); and (2) oil solubilized within micelles. In the latter system the measured rate is given predominantly by molecular diffusion. The extent of oil solubilization generally increases proportionally with non-ionic emulsifier concentration (or micellar-mediated transport rates increase with emulsifier concentration) whereas molecular transport is expected to be unaffected. The mechanism by which transfer of oil takes place through Brownian collisions between the micelles and the droplets is more favored in the sterically-stabilized emulsions. The interparticle interaction between micelles and oil droplets favors the transfer of solubilizates especially

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those located at the interfacial layer. The controlling parameter of such a transfer is the residence time of micellar (collided) aggregates. Furthermore, the rate of transfer is proportional to the number of collisions. The kinetics of solubilization on non-polar oils into ionic micelles is strongly related to the aqueous solubility of the oil phase (the diffusion approach) whilst solubilization into non-ionic micelles is also related to the interparticle interaction (too rapid to be explained in terms of molecular diffusion). This fast process can be attributed to the increased interaction (collision) and consequently to the transfer of solubilizates within the micellar aggregates. Usually, two different measurements of the emulsion stability are followed: (1) the rate of creaming which is found by measuring the amount of separation of the emulsion from the aqueous phase over a period of time; and (2) the rate of coalescence which is determined by measuring the amount of oil separated from the emulsion as a function of time. While the latter study is useful in understanding the mechanism of stabilization of the emulsion, the former study is directly related to the size of the emulsion droplets produced. This is a consequence of the well-known Stokes–Einstein expression. 14. Nomenclature A: interfacial area specific surface area of emulsifier As: BA: butyl acrylate BD: blue dye CA: cetyl alcohol CMC: critical micellar concentration solubility of the oil in the aqueous phase C`: Cco,`: solubility of coemulsifier in water, C12(EO)8: octaethyleneblycol dodecyl ether Cm: specific solubilization capacities D: mean number droplet diameter number average particle diameter dn: D: diffusion coefficient dispersed phase molecular diffusion coefficient Dm: molecular diffusivity of coemulsifier in water, Dco: diffusion coefficient of the swollen micelles at D o: infinite dilution DMA: dodecyl methacrylate D(d3m)i ydt: initial Ostwald ripening rate dm,I: initial monomer droplet size E: interfacial dilation elasticity modulus EO: ethylene oxide G: gravitational constant GA: gum arabic HLB: hydrophilic-lipophilic balance I(103 Counts sy1): scattering light intensity IPM: isopropyl myristate J: flux K: growth rate correction factor

LSW: ns:

153

Lifshitz–Slyozov–Wagner model number of moles of surfactant needed to cover the wateryoil oil interface of an oil volume

Voil: nEO: Nsol: Naggr: NP-40:

number of EO groups in emulsifier molecule. number of solubilizate molecules in a micelle number of emulsifier molecules in the micelle. nonylphenol polyethoxylate with an average of 40 ethylene oxide units per molecule initial number of monomer droplets Nm,i: OR: Ostwald ripening PEM: (polyethylene glycole) monolaureate PIT: phase inverse temperature PSt: polystyrene R: gas constant, SDBS: sodium dodecyl benzenesulfonate SDS: sodium dodecyl sulfate SLIP: sarcosinate-lauroyl isopropyl SMA: stearyl methacrylate T: absolute temperature, TD: n-tetradecane THE: tris(2-ethylhexanoic) ester Tw 20: Tween 20 u: rate of creaming molar volume Vm: a: A material-dependent constant called the capillary length, fco: volume fraction of coemulsifier in the oil droplet. fmicelle: swollen micelle volume fraction v: Ostwald ripening rate v: molecular-diffusion ripening rate vmic: micelle-transport ripening rate. hd: the interfacial dilatational viscosity g: interfacial tension ge: interfacial tension at equilibrium gs: interfacial tension of the solubilizate against water t1y2: halftime rm: density of the continuous phase rp: density of dispersed phase density difference between the continuous and Dr: droplet phases d y(ky1)sdk: ratio of the thickness of the non-ionic emulsifier adsorption layer (d) to the thickness of the electrical double layer (ky1) around the oil droplets h: continuous phase viscosity, (≠Cy≠x): concentration gradient u: fraction of the particle surface covered by nonionic emulsifier Acknowledgments This research is supported by the Slovak Grant Agency (VEGA, grant no. 1y1014y21). The author is also

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