Investigation of multiple emulsion stability using rheological measurements

Colloids and Surfaces A: Physicochemical and Engineering Aspects 91(1994) 215-225

EiLo*DS A SURFACES

Investigation of multiple emulsion stability using rheological measurements C. Py a, J. Rouvike a, P. Loll b, M.C. Taelman b, Th.F. Tadros G* a Laboratoire de Physico-Chimie des Systbmes Polyphas&, URA CNRS 330, Universitc!Montpellier II, Case 016, Place EugPne Bataillon, 34095 Montpellier Cedex 5, France b ICI Surfactants, Eoerberg, Belgium ’ Zeneca Agrochemicals (Formerly part of the ICI group), Jealott’s Hill Research Station, Bracknell, Berkshire RG12 6EY, UK

Received 15 February 1994; accepted 20 April 1994

Abstract

The stability of water-in-oil-in-water (W/O/W) multiple emulsions was investigated using rheological measurements, droplet size analysis and optical microscopy. Both steady state and oscillatory measurements were carried out simultaneously. The W/O/W emulsion was prepared using a lipophilic surfactant mixture (triglycerol triricinoleate and sorbitan monooleate) and an ABA block copolymer of poly(ethylene oxide)-poly(propylene oxide) (Synperonic PEF127). The composition of the lipophilic surfactant mixture was investigated and it was shown that the triglycerol triricinoleate alone or with small additions of sorbitan monooleate gave the best stability. The effect of the Synperonic PEF127 concentration was investigated using rheological measurements as well as droplet size analysis. This showed that the PEF127 concentration should not exceed 1.2%; otherwise some oil-in water emulsion droplets are produced within the multiple emulsion. Optical microscopy investigations confirmed the results obtained using rheology and droplet size analysis. A multiple emulsion with optimum composition of surfactants remained stable for 223 days at room temperature. The results of these investigations demonstrated that very stable multiple emulsions could be produced provided the system is optimised. The rheological measurements provided a powerful tool to investigate the stability without the need of diluted systems. Keywords: Droplet size analysis; Multiple emulsion stability; Optical microscopy; Rheological measurements

1. Introduction Multiple emulsions both of the water-in-oil-inwater (W/O/W) type and the oil-in-water-in-oil (O/W/O) type find many applications in various fields including cosmetics, pharmaceuticals, agrochemicals, industrial chemicals, etc. These systems are normally prepared using a two-step pro-

*Corresponding author. 0927-7757/94/$07.000 1994Elsevier Science B.V. All rights reserved SSDI 0927-7757(94)02918-I

cess. For example, for a W/O/W multiple emulsion, a W/O emulsion is first prepared using a low HLB (hydrophilic-lipophilic balance) surfactant which is then emulsified in an aqueous solution of a high HLB surfactant. According to Florence and Whitehill [ 11, three main types of multiple emulsion may be distinguished: (i) type “A” droplets containing one large internal droplet, similar to that described by Matsumoto et al. [Z]; (ii) type ‘3” droplets containing several small internal droplets; (iii) type “c” droplets entrapping a large number

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of small internal droplets. The last type is the most common type used for cosmetic applications. As discussed before by Tadros [3], the main criteria for the preparation of stable multiple emulsions are (i) two emulsifiers (1 and 2) with low and high HLB numbers respectively (emulsifier 1 should ideally produce a viscoelastic film to reduce transport on storage); (ii) a very stable primary emulsion (coalescence should be minimised to reduce leakage; (iii) an optimum osmotic balance; (iv) the production of an effective barrier by the secondary emulsifier to prevent flocculation and coalescence. Several procedures may be applied to monitor the stability of multiple emulsions, the most common being droplet size analysis (e.g. using a Coulter Counter or a Malvern Mastersizer E) and microscopy. However, these methods suffer from the problem of having to dilute the multiple emulsion before the analysis is carried out. Recently, we have established, in our laboratory, the use of rheology for the investigation of the stability of concentrated dispersions [4] and we thought that these methods could be applied to multiple emulsions. This is the objective of the present investigations. A W/O/W multiple emulsion was prepared using a lipophilic surfactant mixture (triglycerol triricinoleate and sorbitan monooleate) and a hydrophilic surfactant poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO) ABA block copolymer. Initially, the surfactant mixture was investigated to find the optimum ratio for obtaining the most stable multiple emulsion. As we will see later, it was found that the triglycerol triricinoleate alone was sufficient to produce the most stable multiple emulsion. The effect of the concentration of the PEO-PPO block copolymer was investigated. The droplet size distribution of the multiple emulsion was measured using the Malvern Mastersizer E, and optical micrographs of one of the systems were taken at various time intervals. 2. Experimental 2.1. Materials The oil used for all the multiple emulsions was Arlamol HD, heptamethylnonane oil, supplied by ICI Surfactants (Everberg, Belgium).

triricinoleate Triglycerol surfactant (Atlas El 1598), sorbitan monooleate (Span 80) and the block copolymer of poly(ethylene oxide)-poly(propylene oxide) (Synperonic PEF 127, with 212 ethylene oxide units and 69 propylene oxide units) were all supplied by ICI Surfactants. The water was doubly distilled using an all-glass apparatus. NaCl was of analytical grade, supplied by Carlo Erba, Milan, Italy. Germaben II, the preservative, was also supplied by ICI Surfactants. All the products were used without purification. 2.2. Preparation

of the multiple emulsions

The W/O primary emulsion, 60% by volume (volume fraction @i =0.6), was prepared using an Elado Ystral mixer, the emulsification being performed for 3 min at 12 000 rev min- ‘. The water contained 1 mol dme3 NaCl and 1% preservative (Germaben II) and the surfactant concentration was maintained at 10% by volume with respect to the aqueous phase (60 ml of 1 mol dmm3 NaCl solution were emulsified in an oil solution containing 34 ml of Arlamol HD and 6 ml of surfactant). The primary emulsion was then re-emulsified in an aqueous solution containing 1 mol dmm3 NaCl and various concentrations of Synperonic PEF 127. The secondary emulsion volume fraction was maintained at 0.8 (80 ml of primary emulsion were re-emulsified in 20 ml of the aqueous surfactant solution). The second emulsification process was carried out using a paddle stirrer at 500 rev min-’ for 30 min. 2.3. Droplet size analysis This was carried out using a Malvern Mastersizer E. The emulsion was diluted in the same aqueous solution (containing 1 mol dmp3 NaCl and Synperonic PEF 127). Initially 1 ml of the multiple emulsion was diluted in 5 ml of the aqueous solution and a few drops of this were placed in the tank of the Mastersizer, which was also used with the same surfactant solution. The dilution was adjusted so as to be in the ideal measuring range for the instrument. A low power laser was used to form a collimated monochromatic beam of light (analyser beam) and

C. Py et al./Colloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 215-225

this was passed through the sample. Each droplet will scatter light, and both the scattered and the remaining unscattered light were passed to the receiver lens and then to a detector. The instrument was fully automated and provided a display of the distribution. 2.4. Optical microscopy Photomicrographs of one set of multiple emulsions were obtained at various intervals of time using a Nikkon FXA optical microscope connected to a camera. The multiple emulsion was diluted with the aqueous surfactant solution and placed in a gap between a slide and a cover slide using microscope covers as spacers. This procedure was adopted to prevent any deformation of the multiple emulsion droplets. 2.5. Rheological

measurements

These were carried out at 20°C using a CarriMed CSL 100 rheometer. Two types of measurement were carried out, namely oscillatory and steady state measurements. A cone-and-plate geometry (cone angle, 2”; cone diameter, 4 cm) was used exept when the multiple emulsion was of low viscosity, whereby a double-concentriccylinder geometry (21.96 mm outer radius rotor, R3; 20.38 mm inner radius rotor, R2; 20.00 mm inner radius rotor, Rl ) was used. For the oscillatory measurements, a stress sweep was initially carried out ,to obtain the linear viscoelastic region [S]. This was then followed by an oscillatory sweep to obtain the phase angle shift 6, stress amplitudes and strain amplitude (T,, and y0 respectively). The storage modulus and the loss modulus (G’ and G” respectively) were obtained from these parameters: G’ = (zO/yO)cos 6 G” = (zO/yO)sin 6 The dynamic viscosity 11’was also calculated: q’ = G”/w where o is the frequency in radians per second. In steady state measurements, the stress 5 was

211

measured as a function of the shear rate y and the data were analysed using the Herschel-Bulkley model [6] t = 70 + Ky” where z0 is the yield stress, K is the consistency index and II is a measure of the non-Newtonian behaviour of the system.

3. Results and discussion 3.1. Influence of lipophilic surfactant composition

A typical example of a stress sweep curve for a multiple emulsion containing 100% EL 1598 is shown in Fig. 1. This figure demonstrates that a linear viscoelastic region is obtained. All other measurements were then carried out within this linear viscoelastic region. All the moduli were measured at a frequency of 0.1 Hz. All G’ data discussed below were obtained at this frequency. A typical example of steady state measurements for a multiple emulsion containing 100% EL 1598 is shown in Fig. 2. The curves were analysed using the Herschel-Bulkley model above. The z0 values were computed using this equation. Multiple emulsions were prepared at the same volume fraction and concentration of Synperonic PEF127 while varying the ratio of EL1598: Span 80 from 100: 0 to 0: 100. Fig. 3 shows plots of G’ (Pa) versus per cent EL1598 (Fig. 3(a)) and mean drop diameter (Fig. 3(b)). There seems to be a monotonic increase in G’ with increase in per cent EL 1598 and this is accompanied by a monotonic decrease in the mean droplet diameter. As the diameter of the multiple emulsion droplets becomes smaller, the number of contact points between them increases and this results in an increase in the storage modulus. In all cases, the loss modulus G” was quite low (an order of magnitude less than G’) and this also showed a monotonic increase with increase in per cent EL1598. The high elasticity of the system is a result of the high volume fraction of the multiple emulsion (0.8) and hence drop-drop interactions are significant, resulting in a predominantly elastic system. To test the stability of the above multiple emul-

C. Py et al.lCoNoids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 215-225

218

60-

50-.1.0

0.9 40-0.8 ‘1’ tam (Pa.s) .0.7 30..0.6 .0.5 20-0.4 .0.3 10 -0.2 .0.1

0

I 100

0

I 200

I 300

I 400

0 500

stress

I 600

I 700

(mPa)

Fig. 1. Stress sweep curves for a multiple emulsion containing 100% EL 1598.

f

0

stress (pa)

0-1

I

I

0

I

shear

,

rate

I

(I/S)

1

1200

Fig. 2. Shear stress-shear rate curves for a multiple emulsion containing 100% EL 1598.

sions, G’ was a period of reduction in time t (days)

monitored as a function of time for 14 days. All the systems showed a G’ with time. Plots of log G’ versus were linear. As an illustration, Fig. 4

presents the plot for 100% EL1598. The slope of the log G’ versus t plot was taken as an index of stability; the higher the slope, the less stable the system. Similar trends were obtained using the yield

219

C. Py et al.lColloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 215-225 160

160

140

120

TlOO k ‘0

80

60

40

20

0 0

10

20

30

40

50

60

70

80

90

100

EL1596(%)

(a)

180

160

T

L

9 Meandiameter

(b)

Fig. 3. (a) G’ as a function

of per cent EL 1598. (b) G’ as a function

values. A summary of the slopes obtained from G’ and z is given in Table 1. The results for ratios lower than 60: 40 are not given since in these cases the moduli and yield values were too low and they showed a rapid reduction with time.

of mean drop diameter.

The results of Table 1 clearly show that the multiple emulsion with 100% EL1598 is the most stable and this was chosen for a further investigation of the effect of Synperonic PEF127 concentration as shown below.

C. Py et al./Colloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 215-22.5

.

0

/

I

0

4

2

6

6 Time (days

Fig. 4. Log G’ as a function

of time for a multiple

Table 1 Summary of the slopes of the log G and log t0 versus t (days) plots for multiple emulsions at various ratios of EL 1598 and Span 80 Proportion EL 1598

of

Slope

(%)

log G vs. t

log To vs. t

100 80 60

0.078 0.160 0.170

0.078 0.136 0.150

3.2. EfSect of Synperonic PEF127 concentration Multiple emulsions were prepared using EL1598 at 6% by volume in the oil phase of the primary emulsion. The Synperonic PEF127 concentration was increased from 0.1 to 1.5%. At concentrations below 0.3%, it was not possible to prepare a multiple emulsion since the system changed totally to a W/O emulsions. Fig. 5 shows plots of G’ (after 1 day) versus per cent Synperonic PEF127 and mean droplet diameter (at t =O). The mean droplet diameter did not change significantly with time. Fig. 5(a) shows a

10

12

14

) emulsion

containing

100% EL 1598

monotonic increase of G’ with per cent Synperonic PEF127, whereas Fig. 5(b) shows a monotonic decrease. This is consistent with the increase in the number of contact points between the droplets as the Synperonic PEF127 concentration increases. However, above 1.2% Synperonic PEF127, there seems to be a small reduction in G’ while the droplet diameter is smaller. This may be due to the breakdown of some of the multiple emulsion drops as the per cent Synperonic PEF127 is increased above a certain limit. This change results in a decrease in the overall volume fraction of the system which is accompanied by a reduction in G’. Indeed, above 1.5% Synperonic PEF127, few multiple emulsion droplets were produced and the system was mainly an O/W emulsion. The stability of the multiple emulsions was investigated by following the change of G’ and r,, with time. All results showed linear plots of log G’ or log z,, versus time, and as mentioned in the previous section, the slope of the line was taken as a measure of the stability. Fig. 6 shows the variation of the slope with per cent Synperonic PEF127 and mean droplet diameter. The slope shows a monotonic increase with increase in Synperonic PEF127 con-

221

C. Py et al./CoNoids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 215-225 200

180

160

140

_I/ 120

-

$100 60

60

40

20

0

I

I

I

0

0,6

0,4

02

1

08

Synperonic PEF127

6-d

1,2

I,4

186

( %)

200 1 180

140 160 120

ZIOO

'

1

t !

‘i3 80

60 I

40

!

20 t

0

UI 0

I

I 5

10

@I

Fig. 5. (a) G’ as a function

of per cent Synperonic

(Fig. 6(a)). Fig. 6(b) shows a monotonic reduction in the slope with increase in mean diameter. This implies that the multiple emulsion becomes less stable as the concentration of Synperonic PEF127 increases. This is due to the

centration

20

15 Mean

diameter

PEF127.

25

30

35

(pm ) (b) G’ as a function

of mean drop diameter.

reduction in the droplet size and hence the possible ejection of primary emulsion droplets from the multiple emulsion drops. However, as we will see in the next section, this instability is not significant since microscopy investigations over a long period

C. Py et al./ColloidF Surfaces A: Physicochem. Eng. Aspects 91 (1994) 215-225

222

__0.8

1

1

----t-------l

I,2

1.4

196

Synperonic PEF127 (%)

0.14 t 0,12 -/

$

,o

I 0.1 t

y, 0,08 4 0,06

-c 1

0.04 _t

0.02 t

0 0)

5

10

15

20

Mean diameter (pm

25

30

35

)

Fig. 6. (a) Slope of log G vs. t curves as a function of per cent PEF127. (b) Slope of log G’ vs. t curves as a function of mean drop diameter.

of time showed that most multiple emulsion drops remained stable. Fig. 7 shows the corresponding results obtained from the zO values. The slope of the log z,, versus

t curves also shows a monotonic increase with increase in Synperonic PEF127 concentration (Fig. 7(a)) and a monotonic reduction in the slope with increase in mean diameter (Fig. 7(b)). The

223

C. Py et al.jColloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 215-22.5

0006 +

0

c--

I

0

0.2

0,4

086

0,6 Synperonic

(a)

0.09

0.06

1

PEF127

1.2

1,4

1.6

(%)

T t

0,07

c

0,06

t

0.02 t

----_-------+

0 0

~____ 5

_~__~+--_------1--__-------_ 10

(b)

Fig. 7. (a) Slope of log q, vs. t curves as a function diameter.

t-----15 Mean

20 Diameter

of per cent PEF127.

results are consistent with those obtained from the log G-time and log G/-mean diameter curves and they provide a proof that the rheology method can be used to follow the stability. Indeed,

: 25

i 30

35

(j&m)

(b) Slope of log zO vs. t curves as a function

of mean drop

the latter provides a very sensitive technique for following the stability since droplet size analysis and microscopy (see below) did not show any significant change with time.

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3.3. Microscopy

C. Py et al./CoNoids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 215-225

investigations

Fig. 8 shows photomicrographs for multiple emulsions prepared using 80 : 20 EL1598 : Span 80 and 1% Synperonic PEF127 at various time intervals. Fig. 8(a) shows a photomicrograph of the multiple emulsion at t = 0 (magnification 1250 x ). The photomicrograph clearly shows the multiple emulsion drops with only a few oil droplets in the background. Fig. 8(b) shows the corresponding photomicrograph after storage for 16 days at 20’ C. Again the multiple emulsion drops are clearly shown. One has to be careful in drawing conclusions about the change in diameter since the photomicrographs were taken at an arbitrary field of view under the microscope. Indeed, the mean drop diameter obtained using the Malvern Mastersizer showed the same average value (14.8 urn at t = 0 and t= 14 days). Fig. 8(c) shows a photomicrograph after 37 days, which again illustrates the stability of the multiple emulsion. The mean drop size obtained using the Mastersizer after 43 days was 14.2 urn. Fig. 9 shows a photomicrograph of a multiple emulsion (from a different batch) that was stored for 223 days. This micrograph provides clear evidence that the multiple emulsion prepared using the above system remains stable over a long period of time. It showed a small reduction in mean drop size over a period of 223 days (from 6.7 urn at t = 0 to 4.4 pm at t = 223 days).

4. Conclusions Multiple emulsions prepared using EL1598 (triglycerol triricinoleate) alone or with a small addition of Span 80 (sorbitan monooleate) and Synperonic PEF127 (PEO-PPO block copolymer) and Arlamol HD as the oil are quite stable over a long period of time. Optical microscopy and drop size distribution measurements (using the Malvern Mastersizer) showed insignificant changes in the values and the multiple emulsion drops were clearly visible after storage for more than 6 months at 20°C. However, the rheology experiments showed a significant reduction in the modulus or yield value over a period of 14 days. This, at first

(4 Fig. 8. Photomicrographs of multiple emulsions prepared using SO:20 EL 1598:Span 80 and 1% Synperonic PEF127 at various time intervals: (a) t =O; (b) t = 16 days; (c) t = 37 days.

sight, may indicate instability. However, one should remember that the multiple emulsions were prepared at a volume fraction of 0.8, i.e. near the

C. Py et al. JColloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 215-225

Fig. 9. Photomicrograph SO:20 EL 1598:Span 223 days.

of a multiple emulsion 80 and 1% Synperonic

prepared PEF127

using after

maximum packing fraction. In this region, the modulus and yield values show a rapid increase with increase in the volume fraction. Any small change in the volume fraction will cause a large change in the modulus or yield value. One possible mechanism of how the volume fraction may decrease slightly is through the expulsion of the primary emulsion droplets from the multiple emulsion drops. As a result of the dense packing of the primary emulsion droplets in the multiple emulsion drops (volume fraction 0.6 and diameter less than 1 urn), a high internal pressure is created in the

225

multiple emulsion drops. This results in some of the droplets at the periphery being expelled into the continuous phase, leading to a small reduction in the overall volume fraction of the multiple emulsion. This could account for the rapid decrease in the storage modulus and yield values with time. Thus rheology can be used as a sensitive tool to follow the stability of the multiple emulsion. Indeed such measurements allowed us to optimise the system and hence use EL1598 alone or with small additions of Span 80. The rheology results also showed that the Synperonic PEF127 concentration should not be increased above 1.2%; otherwise O/W emulsions will be produced together with the multiple emulsion.

References [l] [2] [3] [4] [5] [6]

A.T. Florence and D. Whitehill, J. Colloid Interface Sci., 79 (1981) 243. S. Matsumoto, Y. Kita and D. Yonezawa, J. Colloid Interface Sci., 57 (1976) 353. Th.F. Tadros, Int. J. Cosmet. Sci., 14 (1992) 93. Th.F. Tadros, Langmuir, 6 (1990) 28. J.D. Ferry, Viscoelastic Properties of Polymers, Wiley, New York, 1980. W.H. Herschel and R. Bulkley, Proc. Am. Sot. Test. Mater., 26 (1926) 621; Kolloid-Z., 39 (1926) 291.