Viscous Oil Emulsification by Catastrophic Phase Inversion - American

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Viscous Oil Emulsification by Catastrophic Phase Inversion: Influence of Oil Viscosity and Process Conditions Johanna Galindo-Alvarez,† V
eronique Sadtler,*,† Lionel Choplin,† and Jean-Louis Salager‡ †

Laboratoire de R
eactions et G
enie des Proc
ed
es-GEMICO, UPR 3349 CNRS, Nancy Universit
e - ENSIC 1 rue Grandville, BP 20451, 54001 Nancy cedex, France ‡ Laboratorio FIRP, Universidad de los Andes, M
erida, Venezuela ABSTRACT: This study deals with the description of the influence of oil viscosity and process conditions on catastrophic phase inversion, through the analysis of the effects of formulation and process variables on the dispersed phase fraction at which the inversion is triggered. The in situ simultaneous follow-up of viscosity and conductivity measurements allowed, from a process point of view, to emphasize the effect of the aqueous phase addition rate on the catastrophic phase inversion point (PIP) and multiple w/ O/W emulsion formation. Thus if the aqueous phase is added by very small fractions, formulation dominates and the inversion phenomenon can be accelerated, as a consequence of multiple emulsion formation, that greatly increases the volume of effective dispersed phase. An increase in oil viscosity greatly increased the tendency of the oily phase to become the dispersed phase and promoted the formation of highly concentrated emulsions (about 80 to 95% in volume) after inversion.

1. INTRODUCTION In emulsification processes, conventional stirring procedures become inefficient to generate small droplets as the oily phase viscosity increases. During emulsification, drop deformation is enough whenever the shear stress is larger than Laplace’s pressure according to the critical Weber number for turbulent processes and the critical capillary number for laminar ones. Hence, an increase in viscosity ratio reduces the shear efficiency to break the droplets,1 and more mechanical energy is then dissipated in the continuous phase.2,3 Therefore, in industrial applications, emulsification of high viscosity oils, e.g., alkyd resins, epoxy resins, silicone oils, and water-borne polymers colloids is often carried out using a catastrophic phase inversion process, i.e., an alternative method involving a lower energy input but suitable emulsion characteristics.4
20 An emulsion is a thermodynamically unstable dispersion of two mutually insoluble liquids, such as water (W) and oil (O), stabilized by a surfactant. The stirring of these three components can result in different emulsion types, i.e., direct (O/W), inverse (W/O), or multiple type (w/O/W) or (o/W/O), where the lower case letter represents the most internal phase.21,22 If a pre-equilibrated surfactant-oil
water system is stirred according to a standard emulsification protocol, the attained morphology may be represented in a formulation
composition bidimensional map (Figure 1).23 The ordinate indicates the system formulation which may be quantified by the hydrophilic
lipophilic difference (HLD) concept. The HLD is a generalized expression that includes the effect of all physicochemical formulation variables,24 such as the surfactant hydrophilicity, the oil nature, the salinity, the temperature, among others. Nevertheless, in most cases only one of these variables is varied at once, i.e., the surfactant hydrophilic
lipophilic balance (HLB), characterizing only in some approximate way the surfactant affinity tendency (HLB < 10, lipophilic, HLB > 10 hydrophilic). In Figure 1, the abscissa represents the oil/water mass ratio as water mass fraction (Fw). The bold line r 2011 American Chemical Society

in the map is the standard inversion frontier that separates the regions in which oil is the continuous phase (regions: B
, Bþ, Aþ), from those in where it is the water (regions: A
, C
, Cþ). The signs (þ) and (-) indicate, respectively, a dominant lipophilic and hydrophilic affinity of the surfactant. The general phenomenology, described elsewhere,3,23 indicates that the central horizontal branch of the inversion frontier (between A
and Aþ) coincides with HLD = 0. When HLD > 0 (respectively HLD < 0) the surfactant affinity for the oil (respectively water) dominates, and the natural emulsion morphology is W/O (respectively O/W) as proposed by Bancroft’s one century ago, and corroborated by Langmuir wedge theory and other models.25 Emulsions that follow Bancroft’s rule, e. g. O/W in A
and C
on one side, and W/O in Aþ Bþ regions on the other side of the inversion frontier, are called normal emulsions. On the contrary emulsions on the B
and Cþ zones have morphologies known as abnormal, because they do not obey to the normal formulation influence (Bancroft’s rule) but rather to the preference imposed by the composition which compels the phase in higher amount to be the external phase.3,10,26Under these conditions, surfactant is present in the dispersed phase, and resulting droplets are not stable. Emulsion inversion is a process in which the curvature of the liquid
liquid interface swaps its bending from one way to the other. It occurs when one of formulation variables (i.e., surfactant affinity) or composition variables (i.e., oil/water ratio) is changed during the stirring process. If the change is rendered in the map as a vertical shift from A
to Aþ or reversely, as for instance in the continuous change in temperature, the inversion will always take place under the same conditions (at HLD = 0, so-called optimum formulation). Such a dynamic inversion, which is found to be Received: November 3, 2010 Accepted: March 22, 2011 Revised: March 15, 2011 Published: March 30, 2011 5575

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Figure 1. Formulation
composition bidimensional map at constant surfactant concentration. The bold line is the standard inversion frontier.

Figure 2. Emulsion morphology evolution during phase inversion process with intermediate multiple morphology formation.

reversible, has been called transitional because it is linked to a phase behavior transition (vertical arrow in Figure 1).27 On the other hand, when the inversion takes place by crossing through a vertical branch of the inversion line (from B
to A
or Aþ to Cþ, horizontal arrows in Figure 1), it is called catastrophic because it may be modeled by using catastrophe theory.23,28 This second dynamic inversion type is irreversible and exhibits a hysteresis zone in which the location of the inversion frontier depends on the process conditions.10,14,15,18 The fraction of dispersed phase at which the inversion phenomenon is triggered is called the phase inversion point (PIP), and it is represented here by the mass fraction Fw of added water beyond which inversion takes place in the case from B
to A
zone (from an abnormal W/O emulsion to a normal O/W emulsion). Catastrophic inversion occurs as a result of the complete coalescence of an unstable emulsion morphology (abnormal W/O emulsion) as the closest packing arrangement condition is approached. This critical packing arrangement condition results of a system composition change as a consequence of either an internal phase addition (to reach the theoretical range between 50
74% of dispersed phase for uniform drops) or by an increase of the effective dispersed phase volume via multiple emulsions formation when some amount of external phase is included as droplets in drops (to swell dispersed drops).10,14
18 In a semibatch process, catastrophic inversion from water-in-oil emulsion W/O to oil-in-water emulsion O/W (from B
to A
zones) is triggered by dispersed phase (W) addition. During this step, a dynamic balance between coalescence and breakup is established. However, as breakup is a unary process, proportional to the number of droplets per unit, and coalescence is a binary process proportional to the number of droplets per unit volume squared, the coalescence rate increases faster than the breakup rate with increasing volume fraction of dispersed phase, triggering the phase inversion.29 According to dispersed phase addition rate, filling degree of multiple emulsions, agitation, and formulation conditions, different morphologies like W/O þ o/W/O and/or W/O þ w/o/W/O can appear30,31 (Figure 2), changing the PIP, i.e. the fraction of dispersed phase at will inversion is triggered. The phase addition rate has been reported to have an

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influence on PIP: a slow addition rate of dispersed phase triggers an early inversion, i.e. at a lower Fw, as a consequence of a high rate of multiple emulsion formation, in opposition to a fast addition.10,14,32
34 As far as the stirring speed influence is concerned, some tests have been carried out from laminar flow type (400 rpm) to turbulent type (10000 rpm), and no clear-cut influence on multiple emulsion formation and Fw at inversion point has been reported, probably because the shear effect and circulation path affect surfactant adsorption.18,34 The variation of the oil viscosity has been reported to affect the flow pattern,33,35 but no clear relationship with the occurrence of multiple emulsion has been proposed so far. The aim of this work is to understand the influence of oil phase viscosity in catastrophic phase inversion process from B
to A
zone, by means of the study of a surfactant-oil
water model system. The influence of oil viscosity on the position of the standard inversion frontier in the formulation
composition map will be established first. Then will be presented the influence on the inversion point PIP, of process parameters such as phase addition rate, stirring speed, and viscosity phase ratio (R), the ratio of dynamic viscosity of dispersed phase (μd) to continuous phase (μc). Finally, the results will be compared with previous works from inversion phenomena in low viscosity systems (between 0.01 and 0.50 Pa s).10,14,15,18,32,33,36

2. EXPERIMENTAL SECTION 2.1. Materials. The oil phases are linear polydimethylsiloxanes
PDMS with viscosity of 1, 5, and 12.5 Pa.s, (RHODORSIL 47 V) from BlueStar Silicones (France). The aqueous phase is a 1 wt % analytical grade NaCl in purified water (Milli-Q, Millipore). The used nonionic surfactant systems are mixtures of three commercial ethoxylated nonylphenols: Igepal CO-720, (HLB = 14), Igepal CO520 (HLB = 10), and Igepal CO-210, (HLB = 4.6), from Sigma Aldrich (France). 2.2. Formulation
Composition Map. The formulation
composition map is realized by changing the surfactant mixture HLB (as formulation variable) for different oil/water compositions. The surfactant mixture HLB value is changed from 8 to 14, and composition, as water mass fractions, Fw, from 0.04 to 0.50. According to the corresponding composition, the HLB is adjusted by mixing Igepal CO-720 (HLB = 14), Igepal CO
520 (HLB = 10), and Igepal CO-210 (HLB = 4.6). The surfactant mixture is incorporated both in the oil phase (silicon oil) and in the aqueous phase (purified water at 1 wt % NaCl), and two different surfactant concentrations are tested, i.e. five and 2 wt %. Each surfactant-oil
water system is left in contact at 25 °C to pre-equilibrate, without mixing during 48 h, and then, a standard emulsification protocol is carried out using a rotor
stator device (4000 rpm) during 3 min. The emulsion type is determined by conductivity measurements. Finally, the influence of the presence of a cosurfactant agent is also evaluated. Butan-2-ol is used as a cosurfactant in a concentration of 5 wt % of the overall emulsion and added to the aqueous phase. 2.3. Emulsification Protocol in Semibatch Process. Emulsification by catastrophic phase inversion is carried out in a semibatch process, with a controlled aqueous phase addition rate to the oil phase. Surfactant concentration is kept constant through the experiment, and surfactant mixture is present in both phases. At the beginning of the process, 15 g of oil phase (PDMS containing 5 wt % surfactant of HLB = 13, from a mixture of IGEPAL CO 720 and IGEPAL CO 520, which are 5576

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Figure 3. Rheo-reactor scheme.

blended in a 3:1 proportion) is placed in the tank, and then the aqueous phase (purified water containing 1 wt % NaCl and 5 wt % surfactant HLB = 13, from a mixture of IGEPAL CO 720 and IGEPAL CO 520, which are blended in a 3:1 proportion) is added at three different addition rates: 0.1, 0.5, and 1.0 g.min
1. Imposed stirring speeds are 100, 300, and 500 rpm. The emulsification process is carried out with three different oil viscosities: 1 Pa.s, 5 Pa.s, and 12.5 Pa.s, The combination of these variables allows to plan a 33 factorial analysis of experiment. The emulsification is carried out in a RFSII rheometer (Rheometrics Scientific, USA) set up as a rheo-reactor37 (Figure 3). The sample is placed in a thermostatted aluminum tank at 27 °C, which is placed on a rotating turntable whose motion results in an imposed shear rate. A helical ribbon is immersed in the sample to allow the measurement of the torque and to estimate the viscosity evolution as time elapses according to a Couette analogy discussed elsewhere.38 Conductivity evolution is also followed-up with a conductimeter model CMD-210 fitted with a CDC749 cell (Radiometer Analytical) dipped in the sample. These equipments and the Couette analogy allow the emulsification process to be monitored through continuous viscosity and conductivity measurements.

3. RESULTS AND DISCUSSION The surfactant-oil
water model system is composed of silicone oils as oil phase owing to the possibility to change oil viscosity without a drastically change in physicochemical behavior and also by their broad use on industrial process. 3.1. Formulation
Composition Map. The formulation
composition map allows to emphasize the effects of process conditions on the usual effects of composition and formulation variables on emulsion morphology. This map, which is obtained through a standard direct emulsification protocol of a pre-equilibrated system, is characteristic for each combination of emulsification protocol and nature of surfactant-oil
water system. In essentially all cases, the effect of variables other than formulation or water/oil composition (i. e., oil phase viscosity) is to shift the location of the vertical branches of the inversion line, thus widening or shrinking the extension of the central A
/Aþ region, where the formulation dominates the morphology.3,23 Therefore, in our study the position of the vertical branches of the standard inversion frontier renders the influence of oil viscosity. According to protocol described in section 2.2, a formulationcomposition map for the studied surfactant/oil/water model system is presented in Figure 4 for silicone oils of viscosities of 1, 5, and 12.5 Pa.s. This figure illustrates how the standard inversion frontier position is shifted as the silicone oil viscosity is changed.

Figure 4. Oil viscosity influence on the position of standard inversion frontier (for a standard emulsification protocol).

In many instances, the change of oil viscosity may be associated with an oil nature change that alters the global formulation of the system, thus resulting in a vertical displacement of the horizontal branch of the standard inversion frontier in the HLB scale.36,39 However, this does not happens in our system probably because the oil nature does not vary too much (owing to the silicon oil nature) and consequently because the required change in the HLB scale to keep HLD = 0 might be insignificant. In our zone of interest, i.e. the B
/A
frontier (left lower vertical branch) that separates an abnormal W/O morphology from a normal O/W one, the frontier is found to shift to the left toward a lower Fw as silicone oil viscosity increases, essentially when viscosity increases from 1 to 5 Pa.s. A slightly further shift on Fw is attained for a 12.Pa.s oil, but the change is insignificant, probably because some critical shift is reached at or below 5 Pa.s. In the Aþ/Cþ region (upper right vertical branch), the frontier is also shifted in the same direction, about the same extension from 1 Pa.s to 5 Pa.s, and then with a still notable shift for the change to the highest oil viscosity. This trend corroborates an early rule reported by Selker,35 who established a long time ago that as the viscosity of a phase increases, its tendency to become the dispersed phase also increases. Previous works have reported the displacement of the standard inversion frontier with oil viscosity increase but only in the Aþ/Cþ region, for experiments carried out up to 0.5 Pa.s.36,40 The present results show that in the Aþ/Cþ region the trend is curtailed as viscosity increases further; however, in the B
/A
region, the shift is limited by a composition restriction (100% oil concentration). The stirring speed may have an influence in the position of inversion frontier;39 however, in the case of a viscous oil phase, a slow agitation (i.e. under 1000 rpm) and concomitantly low stirring speed is imposed (to prevent energy losses), hence favoring an inversion occurrence at a lower Fw. Surfactant composition and surfactant concentration can also affect the inversion frontier position as a consequence of a surfactant partitioning between the phases41 and because of an alteration of the distribution of lipophilic and hydrophilic oligomers at the droplet interface. Nevertheless, as in our system the surfactant mixture has a low solubility in silicone oil, these effects have probably no influence on the position of inversion frontier in the B
/A
region (where the surfactant has an elevated affinity for the aqueous phase). Experiments carried-out (no data showed) with a surfactant concentration of 2 wt % and different surfactant combination (but with the same HLB) supported this 5577

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Figure 5. Influence of cosurfactant, butan-2-ol (5 wt %), on the position of standard inversion frontier for a direct emulsification protocol. Figure 7. Average values of PIP as a function of the stirring speed, N (rpm), and oil viscosity (Pa.s), for Q = 0.5 g.min
1.

Figure 6. Typical emulsion torque and emulsion conductivity evolutions during emulsification process.

allegation, since the inversion frontier remained in the same position. On the contrary a more significant effect might be expected in the Aþ/Cþ zone, at which attaining a stable emulsion becomes more difficult due to the lower surfactant concentration. 3.1.1. Influence of Butan-2-ol in Formulation
Composition Map. Butan-2-ol is a branched alcohol with an intermediate chain, which acts as a cosurfactant and does not significantly alter the hydrophilic
lipophilic balance of interactions at interface. Its presence in formulation is justified by its capability to pull apart adsorbed surfactant molecules to prevent liquid crystal formation.16,42 Cosurfactants, such as alcohols, are amphiphilic molecules like surfactants, but they are much less efficient as far an emulsion stabilization is concerned. A fraction of these molecules coadsorb with surfactants at interface, whereas the other fraction dissolves in aqueous and oil phases according to their solubility.24 The influence of butan-2-ol on position of standard inversion frontier in formulation-composition map (map established according to section 2.2, in the presence of 5 wt % of alcohol) is presented in Figure 5 for a silicone oil of viscosity 1 Pa.s. The slight shift of the horizontal line of inversion frontier shows that alcohol presence modifies somewhat the formulation, indicating that this type of alcohol is found to be slightly hydrophilic in presence of a PDMS oil. It also produces the widening of the A
and Cþ regions, hence favoring O/W emulsions.

3.2. Process Variables Influence on Phase Inversion Point. During the inversion process from the region B
to A
(W/O abnormal morphology to O/W normal), the viscosity increases gradually due to water inclusion as drops in the oil phase, and then to the multiple emulsion formation, until an abrupt fall takes place at the time of the morphology swap into an O/W emulsion. At this point the conductivity exhibits a significant increase due to the switch of the emulsion morphology from oil external to water external (Figure 6). In a semibatch process, the torque increases gradually as the volume increases but also because the helical ribbon is progressively more and more immersed; nevertheless the calibration of Couette analogy takes into account this fact. In Figure 6, the initial segment of constant slope represents an increment of dispersed phase fraction, and the unexpected increment of slope in the final part can be attributed principally to the multiple emulsion formation before phase inversion point. This combined method of simultaneous conductivity and viscosity measurements allows to follow up the inversion phenomena in systems whose external phase is a nonconductive phase, because the rheological response still gives information about the evolution of effective dispersed phase whereas the conductivity information is not available. The fraction of dispersed phase at which inversion is triggered (PIP) is the result of interaction between formulation and process variables. In the case of the B
f A
direction of change, this fraction represents the added aqueous phase when the inversion phenomenon is detected by an abrupt increase in conductivity and a sudden fall in viscosity (Figure 6). The stirring speed and the internal phase addition rate are interdependent variables as the mixing conditions must be energetic enough to homogenize the dispersion and to incorporate in the emulsion the added internal phase. Thus, the resulting inversion process is the consequence of a total and simultaneous coalescence of the dispersed phase.30 These experiments were carried out in a laminar flow regime driven by a helical ribbon to improve axial pumping inside the rheo-reactor (as discussed in section 2.3), in order to reduce energy loss and air inclusion. 3.2.1. Influence of Stirring Speed. The influence of stirring speed on PIP, for the three viscosities of silicon oil, is shown in Figure 7 for an internal phase addition rate of 0.5. g.min
1 (with a confidence interval of 95%). The studied range (100, 300, and 5578

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Figure 8. Average values of PIP as a function of the internal phase addition rate and oil viscosity (Pa.s), for N = 300 rpm.

500 rpm) ensures macroscopic homogeneity according to visual observation and prevents an excessive air inclusion. The results show that stirring speed influence is not significant in this range compared to oil viscosity effect, since PIP, expressed as a Fw, decreases with oil viscosity increase instead of stirring speed. Indeed, for an agitation speed of 300 rpm, the PIP diminishes dramatically (from Fw = 0.3 to 0.1), with an increase of oil viscosity from 1 Pa.s to 5 Pa.s, but this tendency evanesces as the viscosity further increases from 5 Pa.s to 12.5 Pa.s. The studied range can be classified as a low-energy stirring regime wherein the low curvature of dispersed droplets can be easily reversed favoring formulation effect and accelerating inversion phenomenon, with respect to high-energy stirring conditions in less viscous systems.18 In our results, the PIP slightly decreases with the increase in stirring speed, in contradiction with the expected behavior (PIP decreases with stirring speed decreases). Thus, macroscopic homogeneity and shear transfer in the agitation vessel remains a critical parameter to trigger an inversion dominated by the formulation influence. 3.2.2. Influence of Internal Phase Addition Rate. From a dynamic process point of view, the internal phase addition rate controls the incorporation of the dispersed phase. The interactions between this variable and the energy input induce the actual increase in the effective dispersed phase (dispersed phase perceived by the system), via an internal phase increase or via a multiple emulsion formation. It is known that when the addition is slow enough, a multiple emulsion formation is favored, because the surfactant has enough time to migrate and support the establishment of the spontaneous curvature, as it does not happen with a fast addition rate.10,14,18 The studied range of internal addition rates (0.1, 0.5, and 1.0 g. min
1) is likely to be considered as a “slow” addition rate with respect to previous work,10,14,18 thus facilitating formulation adsorption. Consequently, the multiple emulsion formation is likely to be favored in this range. The experimental results on the influence of the internal phase addition rate for the three oil viscosities and a given stirring speed (300 rpm, and a confidence interval of 95%) are shown in Figure 8. The data indicate a statistical reduction of PIP when the internal phase addition rate increases for an oil viscosity of 1 Pa.s. Additional experiments for

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this oil viscosity (no data showed) at limit conditions (higher internal phase addition rates) showed that this reduction seems to reach the value of dispersed phase corresponding to the standard inversion frontier. For the studied system, oil viscosity influence dampens the effect of internal phase addition rate. However, the small values of PIP indicate a preference of the system to trigger the catastrophic phase inversion via a rapid formation of multiple emulsion morphology, a behavior which has been corroborated for less viscous systems (>0.2 Pa.s)33 but not described for this kind of high viscous systems. Previous works10,14,18,43 have reported a displacement of PIP values as a consequence of a change in formulation or in process conditions. If the internal phase addition rate is considered as “slow” and the agitation as “highly energetic”, the internal phase (added aqueous phase) is efficiently dispersed with no influence on the emulsion type (not multiple emulsion formation). But, if the addition is “too fast” or the mixing “too sluggish”, it is possible to create from place to place stagnant unmixed zones that promote a localized inversion phenomenon, in the direction demanded by the system formulation (as a different local water to oil ratio, WOR, is generated).40 In the present study, the formation of multiple emulsions and the creation of stagnant unmixed zones depend on the oil viscosity. Thus, as the studied range of internal addition rate may be considered as a “slow addition” (for which formulation dominates and multiple emulsion morphology formation is supported) the results may be explained by the shear transfer effectiveness and the probability of collision between dispersed drops. Multiple emulsion formation is triggered in the case of a “slow” addition rate, but this formation process occurs slowly, as a consequence of a small probability of contact between the dispersed drops. If the addition rate increases, the probability of contact between dispersed drops is enhanced, thus somehow accelerating the multiple emulsion formation and the subsequent inversion. Furthermore, an increase in oil viscosity (initial continuous phase) tends to improve the shear rate transfer to the internal (aqueous phase) phase, thus increasing the number of dispersed drops since they are smaller. Nevertheless, as a consequence of a degradation of mixing circulation paths inside the emulsification vessel, the formation of stagnant unmixed zones is favored; consequently, a rapid coalescence of the dispersed drops can take place due to formulation effects and Bancroft’s rule, thus triggering inversion phenomenon.23,35,36 3.2.3. Influence of Butan-2-ol. The inversion phenomenon depends on the interactions between the surfactant system and the aqueous and oil phases as well as the proximity of the system formulation to optimal formulation. It has been shown that an increase in surfactant concentration for low viscosity systems (up to 0.2 Pa.s) favors the formation of multiple emulsions.18,32,44 In previous sections, the exhibited effect of an oil viscosity increase has been related to the formation of multiple emulsions through an enhanced or inhibited mobility of the dispersed phase. However, in our studied system due to the poor interactions between the surfactant molecules and the silicon oil phase, there is no significant effect of surfactant concentration on the formulation
composition map and on the PIP. It is known that the presence of a cosurfactant agent can prevent the formation of liquid crystals.42 For instance the butan2-ol, acting as a cosurfactant agent, is adsorbed at the drop interface to improve the surfactant molecules mobility.17 To understand the influence of this alcohol in the system formulation and on the phase inversion phenomenon, a comparison 5579

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Figure 9. Influence of butan-2-ol on PIP (oil viscosity = 1 Pa.s, Q = 0.5 g.min
1, N = 300 rpm).

between the standard inversion frontier (obtained from a protocol like the one described in section 2.2) and the dynamic inversion frontier (obtained through a continuous addition of internal phase, protocol described in section 2.3) is presented in Figure 9, for an oil viscosity of 1 Pa.s, Q = 0.5 g.min
1, and N = 300 rpm. The dashed lines represent the inversion phenomenon in the presence of butan-2-ol, whereas the bold line shows the inversion in its absence. As a general trend, butan-2-ol presence allows to shift the PIP value toward lowers fractions (favoring O/ W emulsion). These results may be explained by a more homogeneous dispersion of the internal phase as a consequence of an improved mobility of molecules of surfactant system that increases contact between dispersed drops to trigger inversion. 3.2.4. Partial or Full Completion of Catastrophic Phase Inversion Process. Multiple emulsion formation during catastrophic phase inversion (from an abnormal emulsion, B
region to a normal emulsion, A
region) is produced by the incorporation of external phase (oily phase) inside of drops of dispersed phase (aqueous phase) (Figure 2), in agreement with surfactant affinity to stabilize O/W curvature. This emulsion morphology is composed of transitional and unstable structures that allow to obtain a W/O emulsion.3,10,14,18,30,34,45,46 However, given the oil viscosity (initial external phase), additional agitation after inversion detection may be necessary in order to enhance surfactant adsorption according to nature and affinity of surfactant molecules,7,8,13,33 because completion of the inversion process depends on process conditions. Bruggeman’s equation allows to have an estimation of the actual continuous phase fraction by means of the conductivity measurement of the emulsion.18 Hence, a comparison of the emulsion conductivity at the end of the process, with the theoretical conductivity (estimated from added aqueous phase until inversion), may give an idea of the amount of aqueous phase trapped inside the dispersed drops and the progress of inversion. In our study, if the difference between theoretical and final emulsion conductivity is more than 10%, the inversion is considered as partial and involves a multiple emulsion morphology like w/O/W, otherwise it is considered as a full completion of catastrophic phase inversion. For an oil viscosity of 1 Pa.s, Figure 10 shows a comparison between the maximum theoretical continuous phase (i.e., the added aqueous phase) and experimentally measured (or actual) continuous phase ratio (continuous phase fraction estimated from conductivity measurement). Thus, the difference, represented by the shaded area,

Figure 10. Difference between theoretical and experimental fraction of dispersed phase at phase inversion for an oil viscosity of 1 Pa.s.

Figure 11. Super multiple emulsion morphology before catastrophic phase inversion
microscopic photography 100.

may be associated with the fraction of aqueous phase encapsulated inside the dispersed phase. The results show a partial inversion with a correlation between the agitation speed and the quantity of trapped aqueous phase. Catastrophic phase inversion usually takes place through an emulsion morphology evolution of type W/Ofo/W/OfO/W. In this process, multiple emulsion formation contributes to increase the effective dispersed phase to attain a critical packing condition and to prompt the inversion phenomenon. However, during this process second level multiple emulsions like w/o/W/O (Figure 11) can appear, and at the PIP a simultaneous coalescence of external film of dispersed drops releases the internal droplets, thus triggering the inversion phase phenomenon (Figure 2). When these kinds of structures are present, the agitation conditions may not be enough to break down the remaining multiples emulsion, to achieve the inversion process. The results from the Figure 10 suggest the presence of second level multiple emulsions during the inversion process, with a likely emulsion morphology evolution of type O/Wfo/W/Ofw/o/W/Ofw/O/W. Hence, only a partial inversion is attained, and final emulsion morphology is of type w/O/W (corroborated by microscopic follow up). In contrast, oil viscosities of 5 and 12.5 Pas show a different behavior, since the difference between theoretical and experimental continuous phase ratio are lower than 15% (including experimental error), suggesting an emulsion morphology evolution of type 5580

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Figure 12. Difference between theoretical and experimental fraction of dispersed phase at phase inversion for an oil viscosity of 5 and 12.5 Pa.s.

Figure 13. Difference between theoretical and experimental fraction of dispersed phase at phase inversion for an oil viscosity of 1 Pa.s in the presence of butan-2-ol.

O/Wfo/W/Of O/W, without second level multiple emulsions formation (Figure 12). Thus, inner droplets of multiple emulsions after the PIP in the case of oil viscosity of 1 Pa.s are not stabilized by the formulation but by the viscosity ratio that slows down the coalescence process inside of multiple emulsions.17,47 It seems that as a consequence of viscosity ratio, surfactant molecules are trapped at the interfacial film. Hence, the presence of a cosurfactant, as butan-2-ol, that acts to separate the adsorbed surfactant molecules may enhance their mobility and reduce the resistance of interfacial film. Results presented in Figure 13 show the difference between theoretical and experimental continuous phase ratio in the presence of 5 wt % of butan-2-ol. Under these conditions the difference is becoming less than 5%, indicating a complete inversion process.

4. CONCLUSIONS Some relevant relationships between process and formulation variables for viscous systems on phase inversion point or PIP during emulsification by catastrophic phase inversion have been established. The oil viscosity influence has been recognized as the key parameter to trigger catastrophic inversion phenomena, since phase viscosity ratio determines the dispersed phase fraction at PIP. Depending on the viscosity of the continuous phase before inversion (oily phase, in this study), the emulsion formation may be enhanced by a very low phase viscosity ratio (μd , μc) between aqueous and oily phase. This result stresses the fact, not yet reported as far as we know, that the viscosity ratio in the inversion process may be as important as the process variables effect and even more in some cases. In the B
f A
inversion, a shift of the standard inversion frontier to a lower internal (aqueous) phase fraction (Fw) results from the oil viscosity increases. Even if this frontier is not sensitive to changes in surfactant concentration and mixture type, the presence of cosurfactant, as butan-2-ol, tends to induce an earlier inversion, i.e. at a lower Fw. The influence of stirring speed and internal phase addition rate depend on the oil viscosity, since for an oil viscosity increase, a lower shear rate allows a more efficient shear energy transfer. The internal phase addition rate has also a relevant influence on PIP, but the effect fades away as oil viscosity increases. Completion of catastrophic phase inversion is enhanced by oil viscosity increase, as the inversion mechanism seems to be more efficient. In the case of oil viscosity of 1 Pa.s, the presence of a cosurfactant, as butan-2-ol, reduces multiple emulsion persistence after the PIP, allowing a complete catastrophic phase inversion process. ’ AUTHOR INFORMATION Corresponding Author

*Phone: þ33 (0)3 83 17 50 79. Fax: þ33 (0)3 83 17 53 19. E-mail: [email protected]. 5581

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