Effect of added diols (glycols) on the emulsion properties of oil, water and surfactant mixtures

Colloids and Surfaces A: Physicochem. Eng. Aspects 390 (2011) 67–73

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Effect of added diols (glycols) on the emulsion properties of oil, water and surfactant mixtures Bernard P. Binks a , Paul D.I. Fletcher a,∗ , Michael A. Thompson a , Russell P. Elliott b a b

Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, UK GlaxoSmithKline, Harmire Road, Barnard Castle, County Durham DL12 8DT, UK

a r t i c l e

i n f o

Article history: Received 16 June 2011 Accepted 5 September 2011 Available online 10 September 2011 Keywords: Emulsion Surfactant Glycols Diols Phase inversion Adsorption

a b s t r a c t We have investigated the effects of addition of various diols on the properties of water–paraffin emulsions stabilised by non-ionic surfactants. Diols do not partition to the oil phase of these emulsions to any significant extent. Diol addition causes phase inversion of the emulsions from water-in-oil (w/o) to oilin-water (o/w) and the concentrations of the different diols required for phase inversion correlates with their tendency to adsorb at the oil–water interface. Results for the type of emulsion formed, the emulsion mean drop size and the stability of the emulsions with respect to droplet coalescence are discussed in terms of the relationships between these emulsion properties and the multiphase equilibrium liquid (microemulsion) phases from which they are derived. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Emulsions are thermodynamically unstable dispersions of one liquid in a second immiscible liquid where the two liquids are most commonly a polar aqueous phase and an apolar oil phase. They may be made kinetically stable by the addition of a suitable stabiliser which adsorbs on the droplet surfaces and renders them stable with respect to the possible breakdown processes of flocculation, sedimentation or creaming, Ostwald ripening and coalescence. Stable emulsions are important in a wide range of diverse applications in product sectors including the household cleaner, personal care, cosmetic, oilfield chemical and pharmaceutical fields. Many pharmaceutical emulsions, particularly creams designed for skin application, consisting of water, oil, stabilising surfactant and pharmaceutically active species, also contain high concentrations of added polar diol (glycol) species such as propane1,2-diol (propylene glycol). Diols are added mainly to improve the solubility of the active species and, for topical emulsions, to facilitate the transport of active species across skin and to improve anti-microbial preservative efficacy (see, for example [1–3]). Diol addition can also be used to match the refractive index values of the oil and aqueous phases and produce transparent emulsions [4]. Although there are a limited number of studies of emulsions produced from immiscible mixtures of an oil with various non-aqueous

∗ Corresponding author. E-mail address: p.d.fl[email protected] (P.D.I. Fletcher). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.09.005

polar liquids [5–9], the effects of variable concentrations of polar liquid such as glycols are not fully understood. The main aim of the study described here is to understand how the addition of various glycols to emulsions affects their properties. Emulsion properties can be rationalised in terms of the properties of the equilibrium multi-phase liquid systems from which they are prepared by energy input through the emulsification process [10–13]. The equilibrium phase behaviour of systems containing equal volumes of oil and water phases can be summarised as follows. Addition of surfactant at an overall concentration less than a critical value (cacov ) yields two co-existing liquid phases containing monomeric (non-aggregated) surfactant molecules which distribute between the water and oil phases with distribution coefficient Kow [14–16]. Kow =

[surfactant monomer]oil [surfactant monomer]water

(1)

For a system with oil and water volume fractions oil and water , the overall surfactant concentration is [surfactant]ov = oil [surfactant]oil + aq [surfactant]aq

(2)

As the overall surfactant concentration is increased, the monomer concentrations in both the oil and aqueous phases increase in the ratio Kow until the critical concentration at which aggregation occurs is reached. At this concentration: cacov = oil cacoil + aq cacaq

(3)

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B.P. Binks et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 390 (2011) 67–73

Noting that Kow = cacoil /cacaq , Eq. (3) can also be written as cacov = {(1 − aq )Kow + aq }cacaq

(4)

Addition of further surfactant such that [surfactant]ov > cacov produces equilibrium liquid phases which contain monomeric surfactant at concentrations equal to cacoil and cacwater in the oil and water phases plus aggregated surfactant in either the oil, the water or a third phase [17–23]. A hydrophilic surfactant will form a Winsor I (WI) system consisting of an o/w microemulsion phase coexisting with an excess oil phase. A hydrophobic surfactant will form a Winsor II (WII) system of a w/o microemulsion with a coexisting aqueous phase. A surfactant of the appropriate, intermediate hydrophilicity will form a Winsor III (WIII) three-phase system consisting of excess oil and aqueous phases coexisting with a third phase containing all the aggregated surfactant in the form of either a bicontinuous microemulsion or lamellar liquid crystalline phase. The oil–water interfacial tension decreases progressively from the value for a “bare” oil–water interface (of the order of 40 mN m−1 ) down to a value of a few mN m−1 or less due to monomeric surfactant adsorption for [surfactant]ov increasing from zero to its maximum value cacov . For [surfactant]ov > cacov , the tension is independent of surfacant concentration at a constant value  c since the additional surfactant is in the form of aggregates which are not surface active. When the system conditions (e.g. surfactant hydrophilicity, temperature, electrolyte concentration) are altered such that the phase sequence WI-WIII-WII (microemulsion phase inversion) is observed, the tension  c passes though a minimum which may be ultralow, e.g. <10−3 mN m−1 . When a Winsor equilibrium multiphase system is emulsified there is a relationship between the properties of the derived emulsion and the equilibrium system from which it is prepared [10–13]. The main features of this relationship are: • Emulsification of a system with overall surfactant concentration < cacov does not produce a stable emulsion owing to less than maximum surfactant adsorption at the droplet surfaces and the increased oil–water tension. • For systems with [surfactant]ov > cacov and containing equal volumes of oil and aqueous phases, emulsification of a WI system always gives an o/w emulsion and a WII system always produces a w/o emulsion. Emulsification of a three-phase WIII system can produce a variety of multiple emulsions [24,25]. • Since emulsion formation increases the amount of oil–water interface, the energy cost is proportional to the oil–water interfacial tension. Hence, when emulsions are formed using a constant energy input, minimum emulsion drop size (maximum oil–water interface created) is observed under conditions corresponding to microemulsion phase inversion when  c is a minimum. • Emulsion stability with respect to drop coalescence is generally high under conditions far from microemulsion phase inversion and low close to microemulsion phase inversion. In this work, we describe how the type of emulsion formed, the drop sizes and emulsion stability vary with the addition of a range of pharmaceutically relevant diols. The aim is to understand whether the effects of diol addition follow the patterns of behaviour discussed above and thus to establish formulation design principles applicable to these systems. 2. Experimental 2.1. Materials Water was purified by passing through an Elgastat Prima reverse osmosis unit followed by a Millipore Milli-Q reagent

water system. Its surface tension was 71.9 mN m−1 at 25 ◦ C, in good agreement with literature values. Paraffin liquid oil (Total, Grade 783LP) was columned over neutral aluminium oxide 90 (Merck) to remove polar impurities. The glycols ethane-1,2-diol (Sigma, >99%), propane-1,3-diol (Sigma, 98%), propane-1,2-diol (Dow Corning, 98%), butane-1,2-diol (Sigma, >98%), butane-1,3-diol (Sigma, 99%), butane-1,4-diol (Sigma, 99%%) and pentane-1,5-diol (Sigma, >97%) were used as received. The non-ionic surfactants consisting of oleyl hydrophobic chains bonded to polyglycerol units are denoted here with the abbreviation On Gm . The average molecular structures were determined from the manufacturers’ information on the average number of glycerol units per molecule and the measured saponification number to derive the number of oleyl chains. O2 G2 (tradename Emulsogen OG, Clariant) and O1.4 G1 (tradename Cithrol GMO 0041, Croda) were used as supplied. It should be noted here that the On Gm notation refers to the average molecular structure; in fact these surfactants contain a mixture of species with different numbers of O and G units distributed around the averages. Pluronic F127 is a tri-block co-polymer surfactant with average structure HO–(CH2 –CH2 –O)101 –(CH2 –CH(CH3 )–O)56 –(CH2 –CH2 –O)101 –H. It was supplied by BASF and used as received.

2.2. Methods The solubilities of the glycols in the paraffin liquid oil, the oil in water and the glycols in the oil were estimated as follows. For the glycols in the oil, 5 mL of oil was accurately weighed into a thermostatted, stirred sample tube. Small amounts of the glycol were added sequentially using a 50 ␮L syringe until phase separation was observed visually. The maximum amount which could be added before phase separation was taken to equal the solubility of the glycol in the oil. The mutual solubilities of the other pairs of components were determined similarly. The emulsions were prepared by weighing the required amounts of each component into a glass sample tube which was mounted in a thermostatted vessel. The total volume of each emulsion sample was 10 mL. The samples were equilibrated for 30 min to reach the set temperature and then emulsified using an IKA UltraTurrax T25 homogeniser fitted with a 18 mm rotor–stator head and operating at 11,000 rpm for 1 min. The emulsion type (oil- or watercontinuous) was determined using two methods. First, the addition of drops of the emulsion into separate samples of the oil phase and the aqueous phase of the emulsion were observed. Emulsions with the aqueous phase as the continuous phase (o/w emulsions) tend to remain as drops in the oil and disperse in the aqueous phase, with the opposite being true for emulsions with oil as the continuous phase (w/o emulsions). Second, the electrical conductivity of the emulsion immediately after preparation was measured using a Jenway 3540 conductivity meter with platinum/platinum black electrodes. High conductivity measurements indicate an o/w emulsion, and low measurements indicate w/o emulsions. A low concentration (4 mM) of NaCl was dissolved in the water used for all emulsions to improve the identification of emulsion type using conductivity. Emulsion droplet diameter distributions were derived by analysis of transmission microscope images obtained using an Olympus BX51 M microscope equipped with a Olympus DP70 digital camera. The sample was placed in the recess of a dimpled microscope slide, diluted as required with the continuous phase of the emulsion (typically ten-fold or so) and covered with a cover slip. The number average diameters of the droplets were calculated from at least one hundred individual drop diameters measured using the digital micrographs. Overall magnification scale calibration was done using a National Physical Laboratory reference stage

B.P. Binks et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 390 (2011) 67–73

69

Table 1 Solubility of diols in paraffin oil at 20 ◦ C.

Ethane-1,2-diol Diol solubility in paraffin oil/wt%

Ethane-1,2-diol Propane-1,3-diol Butane-1,4-diol Pentane-1,5-diol Propane-1,2-diol Butane-1,2-diol Butane-1,3-diol

0.13 0.24 0.40 0.47 0.31 0.32 0.34

± ± ± ± ± ± ±

0.03 0.01 0.03 0.03 0.06 0.03 0.03

Propane-1,2-diol Propane-1,3-diol Butane-1,2-diol

100

κ /μS cm -1

Diol

125

Butane-1,3-diol Butane-1,4-diol Pentane-1,5-diol

75

50

3. Results and discussion All the diols used here are miscible in all proportions with water and only sparingly soluble in the paraffin oil (all diol solubilities in the oil are less than 0.5 wt%, see Table 1). These results indicate that the maximum extent of partitioning of the different diols from the aqueous to the oil phases of the emulsions at equilibrium is small. For this reason, all diol concentrations are expressed as mole fractions in the aqueous phase, i.e. X = moles diol/(moles diol + moles water) and it is assumed that the diol mole fractions at equilibrium are not significantly different to the “as prepared” aqueous phase values prior to mixing with the oil phases. 3.1. Effects of diol addition on emulsion type and phase inversion The effect of diol addition on the type of emulsion formed was determined by measuring the electrical conductivity of the emulsions immediately after preparation and confirmed using the drop test method. As seen in Fig. 1, systems containing equal volumes of oil and mixed diol + water aqueous phases and 4 wt% of O1.4 G1 non-ionic surfactant form w/o emulsions (with low conductivity) with no diol present; on addition of diol the emulsion type switches to o/w (with high conductivity). Diol addition causes the preferred curvature of the adsorbed surfactant monolayer to change progressively from a negative value (defined as that with the surfactant hydrophilic headgroups on the interior of the curved surface) producing w/o drops to positive values producing o/w drops. A similar effect of ethane-1,2-diol substitution for water causing increased positive preferred curvature of the surfactant film has been observed previously for microemulsion systems stabilised by an anionic surfactant [28].

25

0 0.0

0.1

0.2

mole fraction of diol in aqueous phase Fig. 1. Conductivity of emulsions as a function of the mole fraction of diol in the aqueous phase at 20 ◦ C. All emulsions contained equal volumes of oil and aqueous phase and 4 wt% O1.4 G1 surfactant.

Fig. 2 shows the variation of diol mole fraction required to achieve emulsion phase inversion (Xpi ) with the number of carbon atoms in the diol. Higher carbon number diols (which are more hydrophobic) cause phase inversion at lower concentrations than more hydrophilic diols. In addition, the series of ␣,␻ diols and 1,2 diols fall on separate curves with the butane-1,3-diol value lying between the values for butane-1,2-diol and butane-1,4-diol. The emulsion phase inversion results described above suggest that the diols may act by co-adsorbing at the oil–water interface in a manner that swells the headgroup region of the main surfactant. This possible explanation is, in principle, testable by using tensiometry to determine the tendency of the different diols to co-adsorb in the presence of a fixed concentration of O1.4 G1 surfactant and observing whether the relative adsorptions correlate with the diol concentrations required to achieve phase inversion. However, as noted in the experimental section, the O1.4 G1 surfactant used here is a commercial product containing a distribution of species around the average structure with the consequence that tension results for these complex mixtures of surfactant species will not be amenable to a simple unambiguous interpretation. For this reason, we used tension measurements as a function of diol concentration to

mole fraction of diol at phase inversion

graticule (identifier RSG164). The microscope slides were maintained at the appropriate temperature using a stage-mounted strip heater (Linkam CO102). The fraction of continuous phase resolved (monitoring creaming/sedimentation) and dispersed phase (monitoring coalescence) was measured as a function of time to assess the stability of emulsion samples mounted in a thermostatted chamber. Oil–water interfacial tensions were measured using the static, maximum-pull du Noüy ring method using a Krüss K10 instrument thermostatted at 20 ◦ C. Glassware and the du Noüy ring were washed using ethanolic KOH, rinsed copiously with pure water and dried prior to use. The du Noüy ring (Pt/Ir, wire diameter = 0.18 mm and overall radius = 9.55 mm) was first placed in the less dense phase (oil) and the tensiometer zeroed. The ring was removed, cleaned, flame dried and then placed in the surface of the more dense phase (aqueous solution). The required volume of less dense, oil phase was then placed gently on top. The oil–water interfacial tension was then measured until a steady, equilibrium value was reached (typically 15 min). The tensiometer reading was corrected to obtain the true interfacial tension as detailed by Harkins and Jordan [26] and Zuidema and Waters [27].

0.08 α,ω-diols 1,2-diols Butane-1,3-diol

0.06

0.04

0.02

0.00

1

2

3

4

5

6

no. carbon atoms in diol Fig. 2. Variation of mole fraction of diol in the aqueous phase at phase inversion with number of carbon atoms in the diol for emulsions containing equal volumes of aqueous phase and oil with 4 wt% O1.4 G1 surfactant at 20 ◦ C.

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Ethane-1,2-diol Butane-1,2-diol Pentane-1,5-diol

Propane-1,3-diol Butane-1,4-diol

0.06

0.04

Xpi

surface pressure / mN m-1

10

Propane-1,2-diol Butane-1,3-diol

α,ω-diols

5

1,2-diols

0.02

butane-1,3-diol

0

0 0.005

0

0.01

-20

-15

-10

-5

ΔGoads /kJ mol-1

mole fraction of diol in the aqueous phase

-5

◦ = −RT ln Gads

 d˘  dX

(5) X→0

where R is the gas constant and T is the absolute temperature. Fig. 4 shows the correlation between Xpi and G◦ ads where it can be seen that the more strongly adsorbing diols (i.e. those for which G◦ ads is large and negative) cause emulsion phase inversion at lower concentrations. However, the data do not all lie on a single curve; the ␣,␻- and 1,2-diol series remain separated. This lack of a “universal” correlation is probably a consequence of the fact that the G◦ ads values measured here reflect the tendency to adsorb to a “bare” oil water interface rather than the tendency of the diols to coadsorb competitively with the O1.4 G1 surfactant. Fig. 4 also shows a plot of G◦ ads versus number of carbon atoms in the diol where it can be seen that, for the same number of carbon atoms, the 1,2-diols are more surface active than the ␣,␻-diols. The increments in G◦ ads per additional carbon atom in the diols are −1.8 and −2.0 kJ mol−1 for the ␣,␻ and 1,2 diol series respectively. These values are lower in magnitude than the value of −3.15 kJ mol−1 per additional methylene group for a homologous series of 1-alkanols adsorbing from aqueous solution to the dodecane–water interface at 20 ◦ C [29]. This comparison suggests that, when adsorbed at the oil–water interface, the additional methylene groups of the adsorbed diols are not as fully removed from contact with water as methylene groups in adsorbed n-alkanols. Addition of diols causes emulsion phase inversion from w/o to o/w. By analogy with literature studies on the relationship between equilibrium (microemulsion) phase behaviour and emulsion type, this is driven by the preferred curvature of the stabilising surfactant film becoming progressively more positive. The preferred film curvature, and hence the conditions at which emulsion phase inversion occurs, is also affected by, inter alia, the surfactant structure, temperature, oil type and electrolyte concentration according to established and qualitatively understood

ΔGo ads /kJ mol-1

measure their tendency to adsorb to the “bare” oil–water interface in the absence of O1.4 G1 surfactant. Fig. 3 shows plots of the variation of surface pressure  (equal to the difference in oil–water tension in the absence and presence of added diol) with aqueous phase diol mole fraction for adsorption from water to the water–paraffin oil interface. For the low diol concentration range studied, the plots are linear and can be used to derive the standard molar Gibbs free energy of adsorption G◦ ads for the different diols. Choosing unit mole fraction (behaving ideally) as the solution standard state and a surface pressure of 1 mN m−1 for the surface standard state, G◦ ads is given by:

α,ω-diols 1,2-diols Butane-1,3-diol

-10

-15

-20

1

2

3

4

5

number of carbon atoms in diol Fig. 4. Upper plot: correlation between Xpi and G◦ ads for the different diols. Lower plot: G◦ ads versus number of carbon atoms for the different diols.

principles [10–13,17–23]. Consequently, changing one of these latter variables affecting preferred film curvature should induce a corresponding change in the concentration of diol required to effect emulsion phase inversion. Fig. 5 compares plots of emulsion conductivity versus propane1,2-diol addition for three different surfactants at 70 ◦ C, showing how the phase inversion point shifts with the structure of the

100

κ / μS cm-1

Fig. 3. Variation of the surface pressure of adsorbed diols at the water–paraffin oil interface with diol mole fraction in the aqueous phase at 20 ◦ C. The interfacial tension of the pure water–paraffin oil interface is 44.5 mN m−1 .

75 Pluronic F127 50

O2G2 O1.4G1

25 0 0

0.1

0.2

0.3

mole fraction of propane-1,2-diol in aq. phase Fig. 5. Conductivity of emulsions containing equal volumes of oil and aqueous phase with 10 wt% O1.4 G1 , O2 G2 or Pluronic F127 surfactant by the addition of propane1,2-diol at 70 ◦ C.

B.P. Binks et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 390 (2011) 67–73

70 C

100

15 initial average droplet diameter / μm

κ / μS cm-1

20 C 75

50

25

0 0

71

Ethane-1,2-diol Propane-1,2-diol Butane-1,2-diol Propane-1,3-diol Butane-1,3-diol Butane-1,4-diol Pentane-1,5-diol

10

5

0.2 0.1 mole fraction of propane-1,2-diol in aq. phase

0 0.1

1

10

X/Xpi

8

125 100

average droplet diameter / μm

κ / μS cm-1

Initial

75 50 25 0

0

20

40

60

1 week 6

4

2

80

temperature /ºC Fig. 6. Upper plot: conductivity of emulsions containing equal volumes of oil and aqueous phase with 4 wt% O1.4 G1 surfactant by the addition of propane-1,2-diol at two temperatures. Lower plot: phase inversion by increasing the temperature for emulsions containing a fixed mole fraction of propane-1,2-diol equal to 0.075 in the aqueous phase.

surfactant. For the three surfactants used, we can qualitatively rank them in order of decreasing hydrophilicity (i.e. most favouring positive curvature) using the mass fraction of hydrophilic groups in the molecules to give (most hydrophilic) Pluronic F127 > O2 G2 > O1.4 G1 (least hydrophilic). From Fig. 5, we can see that the most hydrophilic surfactant (Pluronic F127) forms o/w emulsions at zero concentration of diol and further addition causes no change in emulsion type. The less hydrophilic O2 G2 and O1.4 G1 surfactants both form w/o emulsions in the absence of diol and are phase inverted to o/w by diol addition. However the less hydrophilic O2 G2 requires a lower diol concentration than O1.4 G1 for phase inversion. For non-ionic surfactants adsorbed at oil–water interfaces, increasing temperature causes dehydration of the uncharged hydrophilic headgroup and shrinks the effective headgroup size (see, for example [21]). This effect causes the preferred curvature of the adsorbed film to become increasingly negative with increasing temperature and hence emulsions stabilised by nonionic surfactants undergo phase inversion from o/w to w/o with increasing temperature. This effect can be seen in Fig. 6. Emulsions stabilised by O1.4 G1 at 20 ◦ C phase invert from w/o to o/w at a propane-1,2-diol mole fraction of approximately 0.04. At 70 ◦ C when the surfactant preferred curvature is more negative, phase inversion from w/o to o/w requires a higher diol mole fraction of approximately 0.12. As seen in the lower plot of Fig. 6, the same system with a fixed diol mole fraction of 0.075 forms o/w emulsions (i.e. positive curvature) at low temperatures and w/o emulsions (i.e. negative curvature) at high temperatures.

0 0.1

1

10

X/Xpi Fig. 7. Upper plot: initial average drop diameter as a function of diol addition through phase inversion for emulsions containing equal volumes of oil and aqueous phase and 4 wt% O1.4 G1 at 20 ◦ C. Lower plot: initial and aged (1 week) average drop diameter as a function of propane-1,2-diol addition. The emulsions are w/o for X/Xpi < 1 and o/w for X/Xpi > 1; the vertical dashed line corresponds to phase inversion at X/Xpi = 1.

3.2. Effects of diol addition on emulsion drop size A major contribution to the energy required to form an emulsion is the energy needed to create the additional oil–water interfacial area corresponding to the drop surfaces which, in turn, is proportional to the oil–water interfacial tension. When forming a series of emulsions at fixed energy input (i.e. a constant emulsification procedure), small drop size corresponding to the creation of large additional oil–water interfacial area is expected when the tension is low and large drops are expected when the tension is high. As discussed earlier, for emulsions with equal oil and aqueous phase volumes, the point of emulsion phase inversion corresponds to that at which the corresponding equilibrium liquid phases undergo microemulsion phase inversion. At phase inversion, the post-cacov tension  c is minimum [18,21,23]. It therefore follows that, for emulsions prepared with constant energy input, the drop size should be minimum around the point of emulsion phase inversion. We have tested this expectation by measuring the average drop diameters for the emulsions stabilised by O1.4 G1 and phase inverted by addition of the different diols. The results are plotted in Fig. 7 in which the drop diameter is plotted versus the diol mole fraction divided by the mole fraction required for phase inversion, X/Xpi . Using this normalised scale enables easy visual comparison of all the different diols on a common scale; w/o emulsions are

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1

0.75

0.5 Ethane-1,2-diol Butane-1,2-diol Pentane-1,5-diol

0.25

fraction of drop phase resolution after 1 week

fraction of drop phase resolution after 1 week

1

0

0.75

0.5 1% 2% 0.25

4% 10%

0 0.0

0.1

1

10

3.3. Effects of diol addition on emulsion stability with respect to droplet coalescence As noted above, Fig. 7 shows that O1.4 G1 -stabilised emulsion drop growth over 1 week is largest around phase inversion induced by propane-1,2-diol addition. For all the different diol systems, we have systematically characterised their stability by measurement of the fraction of the emulsion droplet phase (i.e. aqueous phase from w/o and oil phase from o/w) which has resolved from the emulsion after storage for 1 week at room temperature. Fig. 8 shows plots of the fractional resolution versus X/Xpi for three of the diols (the results for the other diols are broadly similar but not shown here to aid the clarity of the figure). The main features of the stability behaviour are as follows. First, the instability with respect to droplet coalescence is generally maximum around the point of emulsion phase inversion, i.e. at approximately X/Xpi = 1. As noted earlier, emulsion instability around phase inversion is a general phenomenon, irrespective of which variable is adjusted to effect inversion [10–13]. Details of why this instability to drop coalescence occurs around phase inversion and how it is related to the rigidity properties of the stabilising surfactant film are discussed in references [30–32]. Secondly, many of the diol systems (but not all) exhibit complete resolution of the drop phase at the highest diol concentrations investigated. We ascribe this second type of instability to a mechanism in which the high diol concentration increases the value of the critical aggregation concentration of the surfactant in the aqueous phase (cacaq ) and/or Kow (see equation (4)) such that cacov increases to a value which exceeds [surfactant]ov . Diol addition has been shown to increase the critical aggregation concentrations

mole fraction of diol at onset of instability

formed for X/Xpi < 1, o/w emulsions are formed for X/Xpi > 1 and phase inversion occurs at X/Xpi = 1. It can be seen that the emulsion drop size does indeed pass through a minimum at approximately the point of phase inversion for all the different diols. In the lower plot of Fig. 7, the variation of initial and aged (after 1 week) average drop diameters with X/Xpi are compared for propan1,2-diol. It can be seen that emulsion drop growth over 1 week is largest around phase inversion. For this system, drop growth, presumably due mainly to droplet coalescence, is negligibly small for both o/w or w/o emulsions far from the conditions corresponding to phase inversion.

0.4

0.6

0.8

mole fraction of propane-1,2-diol in aqueous phase

X/Xpi Fig. 8. Variation of emulsion stability with respect to droplet coalescence on the addition of diols as the phase inversion point is passed. All emulsions contained equal volumes of oil and aqueous phase and 4 wt% O1.4 G1 surfactant and the temperature was 20 ◦ C. The emulsions are w/o for X/Xpi < 1 and o/w for X/Xpi > 1; the vertical dashed line corresponds to phase inversion at X/Xpi = 1.

0.2

0.4

0.3

0.2

0.1

0

0

2

4 6 [surfactant]/wt .%

8

10

Fig. 9. Upper plot: Variation of oil drop phase resolution after 1 week with aqueous phase mole fraction of propane-1,2-diol for emulsions for o/w emulsions containing equal volumes of oil and aqueous phases and different concentrations (indicated in legend) of O1.4 G1 surfactant at 20 ◦ C. The lower plot shows X at the onset of the drop coalescence instability plotted versus the wt% of surfactant.

of different surfactants [28,33,34]. If this supposition is correct, it is predicted that, for diol concentrations greater than Xpi (i.e. when the emulsions are o/w), diol addition initially decreases the amount of oil drop coalescence/resolution; the emulsion is stabilised as the system is taken further from the phase inversion condition. However, above a certain diol concentration such that cacov exceeds [surfactant]ov , further diol concentration increases the amount of oil drop coalescence, i.e. the emulsion is destabilised. Consistent with this idea, Fig. 9 shows that progressive diol addition initially stabilises and then destabilises o/w emulsions. Furthermore, as expected, the diol concentration at which this second type of instability occurs does indeed increase with the overall surfactant concentration. Based on the arguments presented above, the surfactant concentration at the onset of the instability (dependent on the diol content) is expected to be approximately equal to the value of cacov at the corresponding diol concentration.

4. Conclusions The conclusions from this work establish principles which enable the rational formulation of emulsions containing different diols, as used in many pharmaceutical cream preparations. They can be summarised as follows:

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