Ostwald Ripening of Oil-in-Water Emulsions Stabilized by Phenoxy-Substituted Dextrans

Journal of Colloid and Interface Science 254, 355–361 (2002) doi:10.1006/jcis.2002.8624

Ostwald Ripening of Oil-in-Water Emulsions Stabilized by Phenoxy-Substituted Dextrans V´eronique M. Sadtler,1 Pascal Imbert, and Edith Dellacherie Laboratoire de Chimie Physique Macromol´eculaire, UMR CNRS-INPL 7568 Groupe ENSIC, 1 rue Grandville, BP 451, 54001 Nancy Cedex, France Received March 21, 2002; accepted July 19, 2002

The stability of oil-in-water emulsions prepared using dextran, a natural polysaccharide, hydrophobically substituted with phenoxy groups, was studied. The evolution of the emulsion droplet size was investigated as a function of polymer concentration (Cp = 0.2 to 1% w/w in a water phase) and the degree of phenoxy substitution (τ = 4.2 to 15.7%). For the highest τ values, emulsions, which presented submicrometer droplets, were stable over more than 4 months at room temperature. The most substituted polymers clearly showed a better efficiency to lower the surface tension at the oil/water interface. DexP did not induce real viscosification of the continuous phase. The linearity of the particle volume variation with time, and the invariability of the volume distribution function, proved that Ostwald ripening was the main destabilization mechanism of the phenoxy dextran emulsions. The nature of the oil dispersed phase drastically affected the behavior of emulsions. While the emulsions prepared with n-dodecane presented a particle growth with time, only few size variations occurred when n-hexadecane was used. Furthermore, small ratios of n-hexadecane in n-dodecane phase reduced the particle growth due to the lower solubility and lower diffusion coefficient in water of n-hexadecane, which acted as a ripening inhibitor. C 2002 Elsevier Science (USA) Key Words: emulsions; polymeric surfactants; ostwald ripening; ripening inhibition; interfacial tension.

1. INTRODUCTION

In its simplest form, an emulsion is defined as a liquid–liquid dispersion system in which one of the two liquids is dispersed in another immiscible liquid in the form of microsize or nanosize droplets, the system being stabilized by a third component, the emulsifying agent. When the bulk phase is water, the emulsion formed is called an oil-in-water (O/W) emulsion (1). Emulsions are widely used in industry such as cosmetics, foods, pharmacy, detergents agrochemistry, and coating (2). The formation of emulsion droplets entails an expenditure of mechanical energy to compensate for the cost of increasing the total interface area between the two liquids. An emulsion is therefore a thermodynamically unstable dispersion because 1 To whom correspondence should be addressed at GENICO, Groupe ENSIC, 1 rue Grandville, BP 451, 54001 Nancy Cedex, France. Fax: 33 (0)3 83 17 53 19. E-mail: [email protected].

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the system is well above its global minimum energy. Any apparent stability must therefore be regarded as a purely kinetic phenomenon. The crucial interfacial phase must be taken into account, and consequently, the preparation of emulsions involves the use of emulsifiers. The emulsifying products employed in stabilizing can be greatly varied according to the particular requirements (3). Amphiphilic polymers have hydrophilic and hydrophobic subregions so they can act like low-molecular-weight surfactants. The surface properties of block copolymer are well established and make them useful as emulsifying agents (4). But relatively few detailed studies have been reported which include graft copolymers as emulsifiers (5–7). The surface-active properties of hydrophobized polysaccharides such as dextran, substituted with a phenoxy group (DexP), have been studied mainly in the coating of polystyrene particles or for the preparation of biocompatible poly(lactic acid) nanospheres by an o/w emulsion evaporation technique (8, 9). Preliminary results concerning the ability of hydrophobized dextrans to stabilize emulsions, and their interfacial activity at the oil/water interface, were recently reported (10). DexP presented good ability for oil emulsification probably due to steric stabilization with respect to their macromolecular structure. But their activity at the liquid–liquid interface seemed to indicate surfactant behavior. So to improve the stability of DexP emulsion with time, it was essential to determine the predominant stabilization mechanism that can occur. The physical destabilization of emulsions is due to the spontaneous trend toward a minimal interfacial area between the dispersed phase and the continuous one. Minimizing the interfacial area is mainly achieved by two mechanisms: first, coagulation possibly followed by coalescence and, second, Ostwald ripening. The former mechanism has been studied the most often (11–13). The process of droplet coalescence (fusion of separated droplets) is the normal way by which an emulsion coarsens with time, i.e., by which the droplet size increases upon storage. To maintain the emulsion stability and protect it against coalescence, the important factors are interfacial tension, electrostatic repulsion, steric repulsion, and viscosity of the outer phase. Upon their adsorption to the liquid–liquid interface, small surfactant molecules drastically lower the interfacial tension and 0021-9797/02 $35.00

 C 2002 Elsevier Science (USA)

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prevent a rapid coalescence of the droplets by changing the forces that exert at the new liquid–liquid interface formed during the emulsification process. With use of ionic surfactants, the emulsion is also stabilized by the electrostatic repulsion mechanism. With nonionic polymers, if the ability to decrease the interfacial tension is much lower compared with that of surfactants, however, they can stabilize the emulsion by steric repulsion, by forming upon adsorption, a polymeric layer at the particle surface. Thus, water-soluble polymers such as gums are effective compounds to protect emulsions against coalescence by raising the viscosity of the bulk phase, reducing the kinetic energy of particles, and decreasing the probability of collisions (14, 15). However, if emulsions are properly stabilized against the coagulation/coalescence process, the Ostwald ripening can cause their substantial breakdown. Ostwald ripening, or molecular diffusion, is the process whereby larger droplets grow at the expense of smaller ones because the solubility of a material within a droplet increases through the continuous phase as the interfacial curvature increases (Kelvin–Thomson effect). Hence, the dispersion changes with time, the large droplets grow, and the small ones disappear. The final outcome is a single droplet that offers the largest possible diameter and smallest possible surface area and interfacial energy. A theoretical description of Ostwald ripening in a two-phase system has been independently developed by Lifshitz, Slezov, and Wagner, referred to as the LSW theory (16, 17). One of their major results is that, in the long time limit, a stationary state is reached for which the ripening rate (ω) is given by  ω = daN3 dt = 8γ Vm2 Dm C(∞) /9RT,

[1]

where aN is the average particle radius, Dm and Vm are the molecular diffusion coefficient and the molar volume of the dispersed phase, respectively, C(∞) is the bulk solubility of the dispersed phase through the continuous one, γ is the interfacial tension, and R and T are the gas constant and absolute temperature. Hence, in the observable time, the Ostwald ripening process presents a stationary regime in which the cube of the mean particle radius linearly increases with time and the shape of the particle size distribution normalized by the mean radius is invariant (18). The process is assumed to be diffusion controlled. This result predicts that the dispersion aging or average droplet size increase is mainly determined by the bulk solubility C(∞) of the dispersed phase in the continuous one. The striking feature in Ostwald ripening is not the final outcome but the kinetics. From Eq. [1], the rate of the average radius increase is far smaller in an emulsion with radii ≥1 µm than in an emulsion of radius ≤0.5 µm. Ostwald ripening is consequently a major problem in submicrometer emulsions when the dispersed phase has a significant solubility in the bulk phase. The aim of this work was to determine the mechanism of the aging process of oil-in-water emulsions stabilized only with phenoxy-substituted dextrans. Emulsions were prepared using

various hydrophobically modified dextrans at various concentrations. The tensio-active power of the polymers was determined by interface tension measurements at the oil/water interface to evaluate the effect of the substitution ratio on the emulsifying ability. The phenoxy dextran emulsions were studied with respect to the droplet size variations as a function of time. Improvement in the emulsion stability by changing the oil dispersed phase composition was also investigated. 2. EXPERIMENTAL SECTION

2.1. Materials The phenoxy derivatives of dextran (DexP, Fig. 1) were prepared from dextran T40 with Mn = 28,500 g/mol and Mw = 42,800 g/mol as given by the supplier (Pharmacia, Uppsala, Sweden) by linking phenoxy groups on the dextran backbone as previously described (9). The phenoxy content of the polymers was determined by UV spectroscopy at 269 nm. The degree of substitution τ (%) was the number of phenoxy groups per 100 glucose units. The oil phases, n-dodecane and n-hexadecane (supplied from Sigma), were used as received. Water was double-distilled deionized with a Milli-Q system (Millipore). 2.2. Methods 2.2.1. Emulsion preparation and particle size measurements. Appropriate amounts of polymers were dissolved in purified water (Cp weight/weight of water %) and the solutions were equilibrated for 24 h at room temperature. Sonication (Sonifier 600 watts, Sonic Materials Inc., USA) was achieved as follows: 0.1 ml of oil was added to 4.9 ml of aqueous polymer solution. The two resulting phases were submitted to sonication with a 3-mm titanium probe for 1 min (50% pulse time, 120 W, ambient temperature). The process parameters were optimized to obtain emulsions with low particle sizes and narrow size distribution. The microfluidization process required a pre-emulsion step: 2 ml of oil and 18 ml of aqueous polymer solution were mixed for 1 min, using an Ultra-Turrax (model T25, Ika-Labotechnick), 22,000 rpm, at room temperature. The coarse emulsion was

O O OH OH O

n

CH2 – CH – CH2 – O OH FIG. 1.

Schematic structure of phenoxy dextran.

357

2.2.2. Interfacial tension measurements. Surface tension was measured at room temperature on a K9 tensiometer (Kr¨uss, Germany) based on the Lecomte de No¨uy method using a rigid platinum ring. Water and oil phases were the components used for the emulsion preparation. The oil/polymer aqueous solution system was left to equilibrate for 24 h before measurements were made (time required for obtaining constant values of the surface tension). The Harkins–Jordan correction was applied (19). Surface tension of the purified water, at 20◦ C, was 71.8 mN/m. Experimental errors were in the 0.2 mN/m range. 2.2.3. Rheological characterization. Flow properties of the polymer in purified water were investigated at 25◦ C with a Couette rheometer (RFII, Rheometric Scientific, USA). Polymer solution viscosities were determined in a range of shear rate from 0.1 to 100 s−1 . 3. RESULTS AND DISCUSSION

3.1. Effects of Polymer Concentration Cp and Polymer Phenoxy Content τ n-Dodecane-in-water emulsions were prepared using DexP with various degrees of substitution τ at three polymer concentrations Cp = 0.2, 0.5, and 1% w/w. The variations with time of the average particle diameter for emulsions prepared by sonication with DexP-9.8, and stored at room temperature, are presented in Fig. 2a. First of all, it can be seen that the higher the Cp, the lower the droplet diameter, and this order is kept with time. The same phenomenon was observed for each studied τ value (4.2, 9.8, and 15.7) and was particularly marked for the highest ones. With time, the particles grew, and creaming appeared for DexP emulsions prepared with the lowest τ (DexP-4.2), but there was no phase separation (no oil de-emulsification) and we could consider the emulsions as relatively stable with respect to the study period. Regarding the substitution ratio effect, for all the investigated polymer concentrations, the initial droplet diameter of emulsions decreased with τ increase. For example, at a concentration of 1% w/w, the particle sizes were 0.74 µm for DexP-4.2 and 0.52 µm for DexP-15.7. Intermediate behavior was observed for DexP-9.8, with a droplet diameter at 0.65 µm (Fig. 2b). After 4 months, there was an increase in the droplet diameter of all

2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

20

40

a)

Particle size (µm)

then homogenized using a Microfluidizer M110L (Microfluidics Int. Corp., USA). The number of passes through the interaction chamber and the pressure applied could be controlled and have been optimized (10). A minimum of ten cycles was used to achieve an optimal droplet size distribution. The droplet size distributions and the average droplet sizes (median diameter d0.5 expressed by volume) of emulsions were determined with a laser granulometer (Mastersizer 2000, Malvern Instr. S.A., USA) just after preparation. Then, the stability was assessed by measuring the droplet size as a function of time for the emulsions stored at ambient temperature. Error on the size measurement was in the 5% range.

Particle size ( µm)

OSTWALD RIPENING OF DEXTRAN-STABILIZED EMULSIONS

60

80

100

80

100

120

Time (day)

2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

b)

0

20

40

60

120

Time (day)

FIG. 2. Average particle size as a function of time for emulsions prepared (a) with DexP-9.8 at Cp = 0.2% w/w (), 0.5% w/w (), 1% w/w () and (b) with DexP-4.2 (), DexP-9.8 (), and DexP-15.7 (), at Cp = 1% w/w.

the emulsions (the initial size order was maintained), and no emulsion breakdown was observed. The study of the interfacial activity of DexP showed that the interfacial tension of the oil/water system was higher when DexP-4.2 was used than that in the corresponding system with DexP-15.7. For instance, the interfacial tension between water and n-dodecane was ca. 49 mN/m. This value was lowered to 17 mN/m with DexP-15.7 for a Cp of 0.25 g/L. In the case of DexP-4.2, the interfacial tension was reduced only to ca. 32 mN/m, even for concentrations greater than 1 g/L. Thus, increasing the phenoxy substitution ratio induced a decrease in interfacial tension. Hence, in the range of our study, the higher the τ value, the higher the surfactant efficiency of DexP. In parallel, the rheological measurements of aqueous solutions of DexP at Cp = 1% w/w (which corresponded to the maximal concentration used in emulsion preparation) showed Newtonian viscosities in a wide range of shear rates. The viscosity values were 1.15 × 10−3 Pa s for DexP-4.2 and 1.3 × 10−3 Pa s for DexP-15.7. Even if the DexP-15.7 solution presented a slightly higher viscosity value than the DexP-4.2 one, the DexP samples, at studied Cp and τ , had very low viscosifying properties. Gelation of the continuous medium could not be the major parameter in the emulsion stability. The increase of DexP emulsion stability with Cp for a constant τ , and with τ at a constant Cp (as seen in Figs. 2 and 3), could be explained by reduction of interfacial tension and/or by an improvement of polymer adsorption on oil droplets, which could prevent coalescence and slow down the destabilization rate.

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Volume distribution (%)

Cube of the radius (µm^3)

1

TABLE 1 Physical Characteristics of the Two Hydrocarbons Used (27)

0 day 7 days 14 days 30 days

12 9 6

60 days 3

90 days

0

0.8

0

b)

1

2 3 Reduced diameter

4

5

Hydrocarbons

C(∞) (mol m−3 )

10−4 Vm (m3 mol−1 )

10−10 Dm (m2 s−1 )

n-Dodecane n-Hexadecane

2.3 × 10−5 9.3 × 10−8

2.27 2.92

5.4 4.6

0.6 0.4

a)

0.2

Time (day)

0 0

20

40

60

80

100

120

FIG. 3. (a) Variation of the cube of the average droplet radius for emulsions prepared with three DexP-9.8 concentrations: 0.2% w/w (), 0.5% w/w (), and 1% w/w (), as a function of time. (b) Volume fraction size distribution for the DexP-9.8 emulsion at Cp = 0.5% w/w, as a function of the reduced diameter d/d0.5 , where d0.5 is the volume median diameter.

3.2. Aging Mechanism of Phenoxy Dextran Emulsions Whether the increase in particle size could be ascribed to Ostwald ripening or to coalescence was then investigated. The emulsions were submicrometer (r < 0.5 µm), and as discussed above from Eq. [1], it could be expected that Ostwald ripening was the major destabilization mechanism. Actually, the cube of the mean droplet radius linearly increased at least over 4 months, with good correlation coefficients (Fig. 3a). The rate of increase in the cube of average radius was much smaller for the emulsion prepared with 1% w/w of DexP-9.8 than for the 0.2% w/w one (ω values from 3.4 × 10−26 to 7.9 × 10−26 m3 s−1 for 1% and 0.2% w/w, respectively; data obtained using linear regression analysis). Regarding the polymer phenoxy content, for all the investigated polymer concentrations, the emulsions presented a linear variation of the cube of the droplet radius as a function of time with a good correlation coefficient. For example, Fig. 4a 15

0 day 30 days 60 days 90 days

Volume distrtibution (%)

12

Cube of the radius(µm^3)

9 6

1

3

shows the variation with time of the cube of particle radius for emulsions prepared at Cp = 1% with τ values = 4.2, 9.8, and 15.7%. When τ increased, the rate of destabilization decreased (the ω values were 8.2 × 10−26 and 2.4 × 10−26 m3 s−1 for τ = 4.2 and τ = 15.7, respectively). The ripening rate obtained from the experimental data were approximately 1 order of magnitude greater than the calculated one (see Tables 1 and 2). First, the presence of free polymer in the continuous medium could be explained probably the increasing diffusion of the oil. But others studies have reported experimental ripening rates 2 or 3 times higher than theoretical values (20, 21). One way to explain this difference is that the water solubility of the long-chain hydrocarbon is not really correct. Effectively, C(∞) for oil with n CH2 > 12 is estimated by data extrapolation from short-chain alkanes (n CH2 < 12). In reality, water solubility of long-chain hydrocarbons is actually greater because the extrapolation does not take into account the break in the linear relationship between the molar volume and the logarithm of the solubility of hydrocarbons when n CH2 = 11 (21). Furthermore, for all the investigated formulations (various τ and Cp), the particle size distributions plotted as a function of the reduced diameter (diameter/average diameter, d/d0.5 ) were invariant with time. Figures 3b and 4b show two typical distributions: the peak shape slightly changed during the first week and then it remained fairly constant at least for 90 days. To confirm that the observed increase in droplet size was due to Ostwald ripening and not to particle coalescence, we wanted to study emulsions prepared with heavy oil. The Ostwald ripening rate is proportional to the solubility and diffusibility of the oil phase through a continuous medium (factors C(∞) and Dm in Eq. [1]). Since the solubility of alkanes in water significantly varies with their chain length (Table 1), a considerable variation of the ripening rate is expected on changing this length (22–24). The coalescence rate mainly depends on the initial particle size,

0

0.8

b)

0

0.6

2

4

6

Reduced diameter

TABLE 2 Calculated and Measured Ripening Rates (ω) in Alkane-in-Water Emulsions Prepared by Sonication with DexP at Cp = 1%

;

0.4

a)

0.2

Time (day)

Polymer

DexP-4.2

DexP-9.8

DexP-15.7

γi /n-Dodecane (N m−1 ) ωcalcd (m3 s−1 ) ωmeasrd (m3 s−1 )

35.7 × 10−3 9 × 10−27 8.2 × 10−26

25 × 10−3 6 × 10−27 3.4 × 10−26

19.9 × 10−3 4.9 × 10−27 2.4 × 10−26

γi /n-Hexadecane (N m−1 ) ωcalcd (m3 s−1 ) ωmeasrd (m3 s−1 )

39.3 × 10−3 5.1 × 10−29 Not measrd.

27 × 10−3 3.5 × 10−29 4.6 × 10−28

20.9 × 10−3 2.7 × 10−29 Not measrd.

0 0

20

40

60

80

100

120

FIG. 4. (a) Variation of the cube of the average droplet radius for various DexP emulsions: DexP-4.2 (), DexP-9.8 (), and DexP-15.7 (), at Cp = 1% w/w, as a function of time. (b) Volume fraction size distribution for the DexP-4.2 emulsion at Cp = 1%, as a function of the reduced diameter d/d0.5 , where d0.5 is the volume median diameter.

359

1.6

1.4

1.4

1.2

1.2

1 0.8

0.4

0.6 0.4

b)

0.2

0.3 0.2 0.1

1 r^3 (µ m^3)

Particle size ( µm)

1.6

r^3 (µm^3)

Particle size ( µm)

OSTWALD RIPENING OF DEXTRAN-STABILIZED EMULSIONS

0.8 0.6

Time (day)

0.4

120

0.2

0.4 0.3 0.2 0.1 0 0

b)

20

40

60

Time (day)

0 0

20

40

60

80

100

0

a) 0

20

40

60

80

100

0

120

Time (day)

a)

0

10

20

30

40

50

60

70

Time (day)

the number of droplets, and the continuous phase viscosity. Hence, changing only the alkane chain length should not alter the coalescence rate. Therefore, emulsions with n-hexadecane as a dispersed phase were studied. Figure 5a shows the variation with time of particle sizes of emulsions prepared by sonication with DexP-9.8 at Cp = 1% w/w, for the two different oily phases. Emulsion stabilities were drastically different. While the n-dodecane emulsion presented a particle growing from 0.56 µm at preparation to 1.31 µm after 3 months, emulsion of n-hexadecane remained relatively invariant within the same period (from 0.82 to 0.86 µm). Consequently, for this oil, the cube of the emulsion droplet radius varied linearly with time (Fig. 5b) and the distribution function was unchanged. The experimental value of the ripening rate for the emulsion prepared with DexP-9.8 and n-hexadecane was 4.6 × 10−28 m3 s−1 for a calculated value of 3.5 × 10−29 m3 s−1 (3.4 × 10−26 m3 s−1 with n-dodecane). As expected, the rate decreased with the increasing oil molecular weight. It is to be noted that the n-hexadecane emulsion presented an initial particle size higher than that of the emulsion with n-dodecane. These two emulsions (same formulation), when prepared by the microfluidization process, presented similar initial particle diameters (0.20 and 0.18 µm for n-dodecane and n-hexadecane, respectively, Fig. 6a); then, the aging behavior was similar to that of the emulsions prepared by sonication: emulsion with n-dodecane as a dispersed phase grew with an appreciable Ostwald ripening rate (Fig. 6b), and with n-hexadecane the emulsion was very stable with little particle growth (0.28 µm after 3 months). The initial size difference observed between emulsions prepared with the two alkane phases by sonication could be attributed to the higher viscosity of the n-hexadecane relative to n-dodecane (3.34 and 1.35 cP, respectively, at 20◦ C). Higher energy should be used (higher power, longer time of emulsification) in the sonication process to reduce the initial droplet size of n-hexadecane emulsions. As microfluidization

FIG. 6. (a) Variation with time of the average particle size of emulsions prepared by microfluidization using DexP-4.2 at Cp = 0.5% w/w, with n-dodecane () and n-hexadecane (), as the dispersed phase. (b) Variation of the cube of the average droplet radius of these emulsions as a function of time; same symbols as in (a).

is a more energetic emulsification method than sonication, it could be possible to obtain fine emulsions with the different oil dispersed phases using the same process conditions (number of cycles, applied pressure). Furthermore, we confirmed by interfacial measurements at the n-hexadecane/water interface the ability of the most highly substituted DexP to lower the interfacial tension, as already observed at the n-dodecane/water interface (Fig. 7). The interfacial tension decreased from 46 mN/m (without polymer) to ca. 15 mN/m for a concentration of 2 g/L. (The minimal interfacial tension at the n-dodecane/water interface was 17 mN/m with DexP-15.7.) But for a DexP solution, at a constant Cp, there really was no difference in the surface activity at the n-dodecane/water interface than at the n-hexadecane/water one. The emulsions prepared with n-hexadecane and DexP were very stable for all studied Cp and τ , the average droplet size and the droplet-size distribution remaining essentially unchanged

50

Interfacial tension (mN/m)

FIG. 5. (a) Variation with time of the average particle size of emulsions prepared by sonication using DexP-9.8 at Cp = 1% w/w, with n-dodecane () and n-hexadecane (), as the dispersed phases. (b) Variation of the cube of the average droplet radius of these emulsions as a function of time; same symbols as in (a).

40

30

20

10

0 0.0001

Concentration (g/l) 0.001

0.01

0.1

1

10

FIG. 7. Variation of the interfacial tension as a function of polymer concentration at the n-dodecane/water interface [DexP-4.2 (); DexP-15.7 ()] and n-hexadecane/water interface [DexP-4.2 (); DexP-15.7 ()].

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over a storage period of more than 3 months. This means that there was little Ostwald ripening due to the lower solubility of n-hexadecane compared to n-dodecane. Therefore, it can be concluded that in all the studied emulsions the main aging process was Ostwald ripening and not coalescence. This implies that the aging mechanism resulted from the transport of the alkane by diffusion through the continuous water phase, a process driven by the difference in Laplace pressure between droplets having different radii. 3.3. Inhibition of Ostwald Ripening Previous studies of droplet ripening in oil-in-water emulsions containing hydrocarbons with different molecular volumes also suggest that the dominant destabilization mechanism is Ostwald ripening (20–22). To reduce the rate of ripening in emulsions consisting of single-component droplets, a less soluble product can be used (25). So oil-in-water emulsions with a mixture of n-dodecane and n-hexadecane were prepared by sonication. Three oil ratios (n-dodecane : n-hexadecane, volume : volume) were used: 98 : 2, 90 : 10, and 70 : 30. Figure 8a shows the variation with time of the emulsion droplet size. First, the results confirmed that Ostwald ripening was also the mechanism of size growth; a linear variation of the cube of the average radius with time (Fig. 8b) and a constant distribution function were obtained. With only 2% w/w of n-hexadecane in the dispersed phase, a strong decrease in the rate of Ostwald ripening occurred relative to the emulsion prepared with pure n-dodecane. While the average particle diameters were 0.56 and 0.53 µm after preparation for n-dodecane and 98 : 2 dodecane : hexadecane emulsions, respectively, after 60 days, the respective diameters were 1.15 and 0.73 µm. With a higher concentration of n-hexadecane, no significant variation of the emulsion size was observed (0.60 µm at preparation and 0.62 µm after 2 months for the emulsion with a 70 : 30 dodecane : hexadecane dispersed phase).

r^3 (µm^3)

2 1.5 1 0.5

Particle size ( µm)

1.4

0

b)

1.2

0

30

Time (day)

60

1 0.8 0.6 0.4 0.2 0

a)

0

10

20

30

40

50

60

70

Time (day) FIG. 8. (a) Variation with time of the average particle size of emulsions prepared by sonication using DexP-9.8 at Cp = 1% w/w, with various n-dodecane : n-hexadecane mixtures as the dispersed phase. Weight ratios: () 100 : 0, () 98 : 2, () 90 : 10, () 70 : 30, and () 0 : 100. (b) Variation with time of the cube of the average particle radius of these emulsions.

Explanation of such a decrease of Ostwald ripening was proposed first by Higuchi and Misra (18). They theoretically demonstrated that the addition of a small amount of a less soluble substance in a disperse medium might dramatically decrease the Ostwald ripening rate due to the increasing concentration of the additives in the shrinking droplets, which equalizes the solubility of the major components between the droplets. A most complete study of this effect was given by Kalbanov et al. to take into account the additive concentration (26). The needed additive amount depends on the droplet size distribution of the emulsion and the dispersed phase volume. This is a very attractive method for reducing the Ostwald ripening rate by several orders of magnitude (25, 26). Our experimental results were in agreement with the theoretical prediction of the LSW theory. Due to the difference of solubilities of the two used oil phases, n-hexadecane acts as a ripening inhibitor when added to n-dodecane. It is hence possible to easily obtain very stable emulsions with time. 4. CONCLUSION

Phenoxy-substituted dextrans showed very good ability for preparing oil-in-water emulsions. These emulsions were relatively stable with time since no separation phases occurred for a period of at least 4 months. There was a great effect of the polymer concentration and of the phenoxy ratio τ on the emulsion characteristics. The increase in polymer concentration Cp, especially at high τ , induced a notable decrease in the droplet diameter. In spite of their moderate hydrophobicity, DexP showed surface activity at the liquid/liquid interface, particularly for the most substituted ones, which can explain, in a first approach, the emulsion size variation. All the studied DexP emulsions were resistant to coalescence, as this phenomenon did not appear during the emulsion aging in spite of no clearly viscosification of the continuous medium and the nonionic polymer character. Different experiments clearly showed the predominance of Ostwald ripening as a destabilization emulsion process. Proof was given by a constant volume distribution and a linear variation with time of the cube of the droplet radius, in good agreement with theoretical data. The LSW theory provides a basic understanding of the process. The diffusion rate can be controlled by the choice of the dispersed phase and the presence of heavy additives, according to the theory of Higushi and Misra, and the extended analysis of Kalbanov concerning the effects of low solubility additives on the dispersed phase to retard or hamper ripening. Hence, such graft copolymers are very interesting and promising emulsifying agents, as they offered, at the highest studied substitution values, the combination of sufficient surface activity, in spite of their macromolecular structure, to stabilize small droplets, steric repulsion properties to avoid particle coalescence, and an emulsion aging mechanism that could be controlled.

OSTWALD RIPENING OF DEXTRAN-STABILIZED EMULSIONS

ACKNOWLEDGMENTS The authors thank M. Leonard, C. Rouzes, and A. De Sousa Degaldo for the synthesis of the phenoxy-dextran samples.

REFERENCES 1. Sherman, P., in “Encyclopedia of Emulsion Technology” (P. Becher, Ed.), Vol. 1, p. 405. Dekker, New York, 1983. 2. Becher, P., “Encyclopedia of Emulsion Technology,” Vol. 3. Dekker, New York, 1988. 3. Kulicke, W. M., Arendt, O., and Berger, M., Colloid Polym. Sci. 246, 276 (1998). 4. Tadros, Th. F., Dedersen, C., and Taelman, M. C., Cosmetics Toiletries 112, 75 (1997). 5. Garti, N., Acta Polym. 49, 606 (1998). 6. Pons, R., Taylor, P., and Tadros, Th. F., Colloid Polym. Sci. 275, 275 (1997). 7. Perrin, P., and Lafuma, F., J. Colloid Interface Sci. 197, 317 (1998). 8. De Sousa Delgado, A., Leonard, M., and Dellacherie, E., J. Biomater. Sci. Polym. Ed. 11, 1395 (2000). 9. Rouzes, C., Gref, R., L´eonard, M., De Sousa Delgado, A., and Dellacherie, E., J. Biomed. Mater. Res. 50, 557 (2000). 10. Imbert, P., Sadtler, V. M., and Dellacherie, E., Colloids Surf. A, in press. 11. Kabalnov, A., and Wennerstr¨om, H., Langmuir 12, 276 (1996).

361

12. Bibette, J., Morse, D. C., Witten, T. A., and Weitz, D. A., Phys. Rev. Lett. 69, 2439 (1992). 13. Leal-Calderon, F., and Poulin, P., Curr. Opin. Colloid Interface Sci. 4, 223 (1999). 14. Reichman, D., and Garti, N., in “Foods Polymers, Gels and Colloids” (E. Dickinson, Ed.), p. 549. Royal Soc. Chem., Cambridge, 1991. 15. Bobin, M. F., Michel, V., and Martini, M. C., Colloids Surf. A 152, 53 (1998). 16. Lifshitz, I. M., and Slezov, V. V., J. Phys. Chem. Solids 19, 35 (1961). 17. Wagner, C., Z. Electrochel. 65, 581 (1961). 18. Higuchi, W. I., and Misra, J., J. Pharm. Sci. 51, 508 (1962). 19. Harkins, W. D., and Jordan, H. F., J. Am. Chem. Soc. 52, 1751 (1930). 20. Kabalnov, A. S., Makarov, K. N., and Pertzov, A. V., J. Colloid Interface Sci. 138, 98 (1991). 21. Weiss, J., Herrmann, N., and Clements, D. J., Langmuir 15, 6652 (1999). 22. Nguyen Hoang, T. K., La, V. B., Deriemaeker, L., and Finsy, R., Langmuir 17, 5166 (2001). 23. Dickinson, E., Ritzoulis, C., Yamamoto, Y., and Logan, H., Colloid Surf. B 12, 139 (1999). 24. Buscall, R., Davis, S. S., and Potts, D. C., Colloid Polym. Sci. 257, 636 (1979). 25. Welin-Berger, K., and Bergenstahl, B., Int. J. Pharm. 200, 249 (2000). 26. Kabalnov, S., Pertzov, A. V., and Shchukin, E. D., Colloids Surf. A 24, 19 (1987). 27. Sakai, T., Kamogawa, K., Nishiyama, K., Sakai, H., and Abe, M., Langmuir 18, 1985 (2002).