Emulsion inversion point

Intensive work has been carried out in order to estab lish a relationship between emulsion properties and the properties of surfactant systems. The classical HLB (hydrophile-lipophile balance) concept is widely used in emulsion science to describe the balance of the hydrophilic and lipophilic properties of a surfae tant at oil/water interfaees. The HLB value deter mines flie emulsion inversion point (EIP) at which an emulsion ehanges from W/O to 0/W type. This was of particular importance for nonionic surfactants that change their properties with ehanges in tempera ture (59). Various NMR techniques have provided significant contributions to this basic understanding of surfactant systems and some of those were reviewed in Ref. 7. The usefulness of NMR techni ques in studying surfactant solutions lies in the direct information they provide about the microstracture ofmicroheterogeneous systems (8,60— 64). It is beyond the scope of this chapter to summarize the use of NMR techniques in the study of surfactant systems, but we will present some representative examples related to emulsions. 

Dahms and Ludwig [13] discussed the two methods. Refer also to the low-energy emulsification process discussed in Sec. VI. The same method was used by Marszall [14] to determine the required HUB of the emulsifier. He used the emulsion-inversion point concept (EIP). The EIP is determined with conductivity measurements by adding increments (1 cm ) of water to a measiured amotmt (50 cm ) of oil in which the emulsifiers are dissolved (Fig. 12). He found that the required HLB corresponds to the minimum EIP. 

The effect of additives in the water and oil phases. Although the exact mechanism by which various additives affect the phase inversion is not fully understood, their presence in nSOW systems has been shown to affect the PIT as well as the emulsion inversion point, EIP. " The phase inversion temperature varies with the amount and chemical type of additives in the water phase. Shinoda and Takeda showed that inorganic salts can affect the PIT more strongly than their parent acids. Also, they showed that the effect of fatty acids and alcohols on the PIT for 1 1 volume ratio paraffin-water systems was independent of the chain length of the acid or alcohol. 

Much of the emulsion inversion point (EIP) work is due to Marzall. Brooks et a/. 2.10,11,36 gjgjj yggj jjjjg concept. The EIP is related to the inversion of W/0... 

Most often, emulsions are either 0/W or W/0. In some cases, however, emulsions have a more complex structure and the disperse phase drops contain inclusions of still smaller droplets of continuous phase. These emulsions were discovered in 1925 by Seifritz and are called multiple emulsions. These systems are typically observed as transient structures close to the emulsion inversion point and are rather unstable. ... 

Phase inversion technique the external phase is added to the internal phase. For example, if an O/W emulsion is to be prepared, the aqueous phase is added to the oil phase. First a W/O emulsion is formed. At the inversion point, the... 

The inversion point of a number of salts for such emulsions has been investigated by Bhatnagar (J.G.S. cxvii. 642,1920), but unfortunately no data on the interfacial adsorption of the mixed salts are available as yet,... 

The amount of acid in the acid-hydrocarbon reaction mixture also has an important bearing on the alkylate quality. If the reaction mixture contains less than 40% acid by volume, an acid-in-hydrocarbon emulsion results. Above this 40% inversion point, a hydrocarbon-in-acid emulsion is formed. The latter type produces the better product and consequently an acid volume of 60 to 70% of the reaction mixture is normally maintained. 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

For a constant amount of nonionic surfactant, the interfacial tension at the planar oil-water interface, for the same amounts of oil and water, passes through a minimum when plotted against the hydrophilic-lipophilic balance (HLB). The emulsion stability passes through maxima in the W/O and O/W ranges and through a minimum between the two at the phase inversion point. The minima in the two cases coincide. These observations are explained on the basis of thermodynamics. The stability of macroemulsions can be correlated with the surface excess of surfactant, which also passes through two maxima and a minimum between them [2.11]. 

The phase relationships of two-phase polymer systems also have been of considerable interest in recent years. In an important series of papers, Molau and co-workers (19-24) studied systems, which were denoted POO emulsions (polymeric oil-in-oil), prepared by dissolving a given polymer in monomer and then polymerizing the monomer. During polymerizations of this type the composition of the respective phases reverses, and a phase inversion process was proposed to explain this. A similar process has been suggested as the mechanism by which poly-butadiene forms the dispersed phase in the manufacture of high-impact polystyrenes (22,25). Recently, Kruse has pointed out that this phase-inversion point may correspond to that point on a ternary phase diagram at which the reaction line bisects a tie line (26), and we have advanced a similar point of view in our earlier reports (17,18, 27). 

Emulsion Capacity and Stability. A 0.5 g sample of the freeze-dried protein fraction was redissolved in a minimum of 0.3 M citrate-phosphate buffer at pH 7.0 and mixed thoroughly with 50 ml of 1 M NaCl for 1 min in a Sorvall Omnimixer at 1000 rpm in a one pint Mason jar set in a water bath (20°C). Crisco oil (50 ml) was added to the jar and an emulsion formed by mixing at 500 rpm with simultaneous addition of oil at the rate of 1 ml/min until the emulsion broke. The endpoint was determined by monitoring electrical resistance with an ohmeter. As the emulsion broke a sharp increase (l KS2 to 35- 0 KSi) was noted. Emulsion capacity was expressed as the total volume of oil required to reach the inversion point per mg protein. This method is similar to that used by Carpenter and Saffle (8) for sausage emulsions. To establish emulsion stability the same procedure was used except that 100 ml of oil was added and a stable emulsion formed by blending at 1000 rpm for 1 min. A 100 ml aliquot was transferred to a graduate cylinder and allowed to stand at room temperature. Observations were made of the volume of the oil, emulsion and water phases at 30, 60, 90 and 180 min. 

Spontaneous phase inversion (no shear) has been described by Keskkula [12]. This was demonstrated during the quiescent polymerization of styrene-polybutadiene mixtures containing less than 3 wt% polybutadiene. For industrially important systems (higher rubber content), a minimum amount of shear is required [13]. If no adequate agitation is applied, the system will solidify in the emulsion state before the inversion point. The final product will then consist of a continuous phase of a crosslinked polybutadiene network with dispersed SAN particles. Such a material will not have the typical properties of ABS. 

Related to particle sizing, Molau and Kesskula described the concept of type I and II occlusion [5]. The prepolymer is viscous and has a retarding effect on the phase inversion. In most cases multiple emulsions are formed after the phase inversion point. If the agitation is not extremely high these multiple emulsions survive the further copolymerization and give SAN occlusions in the rubber particles. These occlusions are called type I. Type II occlusions are formed when monomer dissolved in the rubber phase is copolymerized. Because SAN is not compatible with the rubber, separation occurs within the rubber particle, giving type II occlusions. 

The exact mechanism of inversion remains unclear, although obviously some processes of coalescence and dispersion are involved. In the region of the inversion point multiple emulsions may be encountered. The process is also not always exactly reversible. That is, hysteresis may occur if the inversion point is approached from different sides of the composition scale. Figure 18 shows the irreversible inversion of a diluted bitumen-in-water emulsion brought about by the application of shear (60). 

The effect of surfactants on the interfacial tension between water and supercritical fluids is a key property for describing emulsions and microemulsions (8), as shown in Figure 2. The v axis may be any formulation variable that influences surfactant partitioning between the phases such as the pressure or temperature. A minimum in y is observed at the phase inversion point, where the system is balanced with respect to the partitioning of the surfactant... 

At (f), ri suddenly decreases as the inverted W/O emulsion has a much lower volume fraction, k also decreases sharply at the inversion point as the continuous phase is now oil. 

When C o > Chw> 1 and a W/O emulsion forms however, if J < 1 and an O/W emulsion will form. If = Chw f = 1, a planer system will result This denotes the inversion point. 

FIG. 9 Stability behavior of dodecane-water emulsions ( = 0.5) at various salt (NaN03) concentrations (indicated above) around the inversion point (a) stability of direct emulsions decreases with increasing salt activity (b) stability of inverse emulsions increases with increasing salt concentration. The polyelectrolyte emulsifier is 60C12Na. (From Ref. 151.)... 

The crudes span geographically over large areas North Sea, European continent, Afiica, Asia, etc. This is a necessity since if the crude oils in the test matrix are interrelated one cannot universalize the results. Table 1 lists the erude oils and their origin. To start with we determined the inversion point (or alternatively, the maximum eontent of water that can be introduced into the oil without a phase separation). We have ehosen to study emulsions that are 10% below the inversion point Exeeptions in this respeet are the two European erodes with 5% water stabilized. The erode oils were eharacterized by means of density, surfaee tension, and viseosity measurements. The results are summarized in Table 2. All experiments involving emulsions were carried out at 50°C. The reason for working at elevated temperature is to melt the wax in the oils and thereby prevent the influence of the wax on emulsion stability. The elevated temperature is also more elosely related to the real working temperature used in the proeesses in the field. 

Emulsion Crude Ratio of w/o used Inversion point Crude based emulsion Model oil based emulsion... 

The inversion of emnlsions upon varying the water oil ratio often exhibits hysteresis. As shown in Figure 4.30 (Silva et al., 1998), inversion of an oil-in-water emnlsion does not take place until there is quite high oil content when the snrfactant is hydrophilic (i.e., when v lal is small). Indeed, HIPR oil-in-water emnlsions can be formed in this way, as indicated above. For the same surfactant and oil, inversion from water-in-oil to oil-in-water emulsions occurs at lower oil contents. Increasing surfactant concentration shifts both inversion points toward higher oil contents (Silva et al., 1998). A similar situation occnrs for inversion when the surfactant is lipophihc, except that it occurs at high water contents, as Figure 4.30 indicates. 

The significance of the function Q(A, X) will be discussed in the Sect. 7.3.1.2, dedicated to emulsion microrheology. Steinmann et al. suggested that, at the phase inversion point, the shape relaxation times of domains of the components meet at a maximum (Steinmann et al. 2002). 

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