- Email: [email protected]
W Cosmetic Emulsions in an Ultrasonic Field
THE FORMATION OF STABLE W/O, O/W, W/O/W COSMETIC EMULSIONS IN AN ULTRASONIC FIELD B. Tal-Figiel Institute of Chemical & Process Engineering, Cracow University of Technology, Krako´w, Poland.
This paper presents basic research results for production of W/O, O/W and W/O/W cosmetic emulsions under ultrasonic irradiation. A technology for the continuous and batch treatment of fluid mixtures with ultrasound was characterized using cosmetic emulsions as model systems. If cavitation is the dominant mechanism of droplet disruption, these results have to be taken into account for an optimal design of the emulsification apparatus geometry. The theoretical model supports experimental work. The experimental findings prove that final drop size essentially depends on specific power density. Drop size decreases with increasing residence time in the ultrasonic field until a system specific, minimum drop size is obtained. Ultrasonic irradiation leads to turbulent flow conditions on a macroscopic and a microscopic scale. The model based on the theory of turbulent drop bread-up predicts drop size and can be used to design equipment for ultrasonic emulsification. Emulsion used were stabilized using a combination of hydrophilic and hydrophobic surfactants. The ratio of this surfactants is important in achieving stable cosmetic emulsions. It was shown that emulsifier concentration plays great role in controlling the emulsification processes as well as the structured features of the droplets. The rheological behavior and morphological evolution during the ultrasonic emulsification were characterized systematically. The rheological properties were used to asses emulsion stability. Keywords: ultrasonic emulsification; cosmetic emulsions; break-up model; rheology; surfactant.
INTRODUCTION
droplets, they can penetrate through the body membranes which enables transport of active substances. Small droplet size enhances emulsion stability since the Brownian motion may be sufficient for overcoming gravity; it means that creaming or sedimentation should not occur even on prolonged storage, also the small size prevents flocculation of the droplets as well as their coalescence. The long-term physical stability of fine emulsions (with no apparent flocculation or coalescence) makes them unique and they are sometimes referred to as ‘approaching thermodynamic stability’. The recent surge in the cost of energy has led to an increased interest in emulsification processes. Intensive research on methods of carrying out this kind of processes ensures optimal utilization of resources, as well as externally applied energy. The application of ultrasound as a final step of emulsification, produces an emulsion with very small droplets, which has the effect of increasing the area of interfacial contact, and, hence, the efficiency of emulsification. The theory of emulsification by ultrasonic waves has not
Correspondence to: Professor B. Tal-Figiel, Institute of Chemical & Process Engineering, Cracow University of Technology, Ul Warszawska 24, 31-155 Krako´w, Poland. E-mail: btfi[email protected] DOI: 10.1205/cherd06199 0263–8762/07/ $30.00 þ 0.00 Chemical Engineering Research and Design Trans IChemE, Part A, May 2007 # 2007 Institution of Chemical Engineers
A wide variety of cosmetic emulsions are used as bases for skincare products for healthy and diseased skin. These products can range in consistency from a cream to a lotion or milk and even a fluid. Since today’s trends are increasingly short-lived, a manufacturer of cosmetic products needs to respond quickly to new market demands. This means that development times must be drastically reduced while maintaining the same high product quality. To be able to develop cosmetic products quickly and effectively, developers need reliable methods to obtain product stability without the need of long time testing. Here rheology has gained in importance in recent years (Kita et al., 1977; Pal, 1993; Princen, 1985). In the past few years micro- and nanoemulsions have been the subject of extensive research work, because of the vast possibilities they offer. Their properties such as improved drug solubilization and bioavailability make them very attractive for application in personal care and cosmetics as well as in health care, because, due to small size of 730
Vol 85 (A5) 730–734
THE FORMATION OF STABLE COSMETIC EMULSIONS been fully elaborated as yet, owing to the complexity of the mechanisms involved. Cavitation is likely to control droplet disruption in sonicated liquid–liquid systems. The current view is that, apart from cavitation, it is the interfacial capillary waves that are responsible for the process of dispersion in a liquid–liquidsystem in an ultrasonic field (Li and Fogler, 1978a, b; Mason, 1991; Tal-Figiel, 1990, 2005). These waves spread out across the interface without penetrating into the bulk phase. An increase of ultrasonic intensity and, hence, of the amplitude of the capillary waves, eventually results in droplets of liquid breaking away from the crests of the wave, and their ejection into the dispersing phase. The research results presented hereunder demonstrate the principles of formation of cosmetic fine emulsions by mixing and ultrasound. The objective of this work is to asses the impact of various physico-chemical properties of liquid, its flow rate, the irradiation time, amplitude or mean specific power density ,1. of ultrasound and apparatus geometry on the droplet size. This knowledge may prove helpful in composition and quality control.
MECHANISM OF EMULSIFICATION To prepare an emulsion, oil, water, surfactant and energy are needed. The method of mixing those components is very important. The formation of large droplets, as is the case of macroemulsions, is fairly easy and hence high speed stirrers are sufficient to produce the emulsion. The formation of small drops is difficult and requires a large amount of surfactant and/or energy. To obtain emulsion with mean droplet diameter in the range 100 –500 nm, ultrasound or high pressure homogenization should be applied as the second step, since typical rotational homogenizers do not provide the required energetic effect. The high energy required for formation of fine emulsions can be understood from a consideration of the Laplace pressure P. P¼
2s r
(1)
Addition of surfactants plays a major role in the formation of emulsions by lowering the interfacial tension s, P is then reduced and smaller stress is needed to break up a drop. The amount of surfactant required to produce the smallest drop will depend on its activity a (concentration) in the bulk, which determines the reduction in s, as given by the Gibbs adsorption equation. Another important role of the emulsifier is preventing coalescence during emulsification (Rosen, 2004). An important parameter that describes droplet deformation is the Weber number We, which gives the ratio of the external stress Gh, where G is the velocity gradient and h is the viscosity over the Laplace pressure P: We ¼
Ghr 2s
(2)
The droplet deformation increases with increase in the Weber number, droplet break-up occurs, when certain value, called critical Weber number is achieved. It is clear, that for producing small droplets high stresses are required.
731
MODEL OF DROP BREAK IN ULTRASONIC FIELD There are several possible mechanisms of droplet formation and disruption under the influence of ultrasound. One is the formation of droplets as a consequence of unstable oscillation of a liquid–liquid interface. These capillary waves may occur and contribute to dispersion, only if the diameter of droplets to be disrupted is larger than the wavelength of the capillary waves. A mechanism related to that of capillary waves is the oscillation and subsequent disruption of whole droplets due to the action of sound. This mechanism has to be taken into account as one cause of droplet disruption in an acoustic field, but only for a small fraction of droplets with diameters exactly in the corresponding range. With respect to the usually broad droplet size disruption in an emulsion pre-mix, a broad range of sound frequencies would be a precondition for this mechanism to become the governing one. Under practical conditions cavitation is generally regarded as crucial element. Parameters positively influencing this phenomenon in liquids, improve emulsification in terms of smaller droplet size of the dispersed phase right after disruption. Ultrasounds generate periodic compressions and expansions in sonificated media. At sufficiently high ultrasonic intensity during the expansion phase, bubbles, filled with vapour or gas, are generated on the interface. Those bubbles grow till the critical size is obtained and then collapse. During this process, intensive shear fields are generated in the vicinity of collapsing bubbles, as well as extreme pressure fluctuations and microjets of high liquid velocity. Those phenomena are the sources of turbulent microstreams. Apart from that, dynamic pressure generates liquid streams with speed of about 1 m s (Re ¼ 8000– 30 000) below the sonoprobe; it means that the turbulent effects exist also on macroscopic scale. The model based on the Kolmogorov theory of turbulent drop break up predicts drop size and can be used to design equipment for ultrasonic emulsification (Bechtel, 1999); Rajan and Pandit, (2001). In the field of turbulent streams, there exist vortices with various diameters. Especially important for droplet disruption are the smallest eddies. Their size and speed changes in the turbulent field can be described with
0:25 vc3 k1 l
Turbulent length scale
lw ¼
Turbulent velocity scale
vw ¼ ðk1 lnc Þ0:25
(3) (4)
According to this theory, critical Weber number equals Wec ¼
rc vw2 dmax s
(5)
and maximum droplet diameter attained in a very long process can be given as dmax / de We0:6
(6)
Kolmogorov theory was derived primarily for the fluids of low viscosity and isotropic turbulence. For the case where viscosities of phases differ significantly, Davies (1985) and Bechtel et al. (1999) extended it by including the influence of viscous stresses and obtained the following expression
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A5): 730–734
732
TAL-FIGIEL or
for the generalized Weber number: Sh dmax ¼ dmax
rc vw2 Wec ¼ s=dmax þ hd uin =dmax hd uin dmax
induced viscose stress, with uin denoting inside velocity,
u ¼ ðk1 lvc Þ
0:25
for dmax ¼ lw
(8)
and obtained the following expression for the critical droplet diameter: 0:6 dmax / hd r0:5 k1l0:25 þ s r0:2 10:4 d c
for d lw
(9)
For d , lw in the viscous subrange, the flow is viscous and 0:5 1 dmax , nc
ts ¼ h c
0:5 k1 l nc
(10)
and according to Shinnar (1961): hd f 0:5 hc hc k1 l
sn0:5 c
(11)
where: f(hd/hc) is defined as f
hd 16hd =hc þ 16 ¼ 19hd =hc þ 16 hc
(12)
Bałdyga and Bourne (1995) derived the following general expression for dispersion under creeping flow conditions using multifractal concept: dmax
f
sf hd =hc ffi ts
(13)
hd h ð1 =nÞ0:5 ðlw =LÞ2(a1)=aþ3 ¼ c hc s=dmax
(14)
and dmax ¼
Apart from the theoretical derivation, many experimental correlations were obtained for the case of ultrasonic emulsification (Tal-Figiel, 1989, 1990; Rajan and Pandit, 2001).
METHODS AND MATERIALS
0:5 r u, uin ¼ c rd
Sh dmax /
(16)
(7)
where
u/
2(1a)=aþ1 lw L
s 1 aþ3Þ 2=ðaþ3Þ 2(1a)=ðaþ3Þ f h =h k1 l L rc n2=ð d c c
(15)
An experimental investigation of cosmetic type emulsions: O/W, W/O and W/O/W obtained by two step method— preliminary mechanical mixing and final ultrasonification, has been carried out. Table 1 shows all components of emulsions investigated in this work. The viscosity of the continuous phase was varied by addition of glycerol to the water phase in case of O/W systems and by using different oils for the W/O systems. Interfacial tension was lowered by adding mixture of commercially available emulsifier to both phases. For the mechanical mixing, a turbine impeller was applied. Ultrasonication was carried out using a generator with operating frequency of 22.5 kHz with piezoelectric transducer and sonoprobe (diameter ds ¼ 0.02 m). Vessel to sonoprobe diameter ratios were 1, 1.5, 2. Ultrasound specific volume power density , e . could be changed from 5.105 to 2.108 (Wm23). Effective ultrasound power was established using cavitation meter. Its measuring tip was placed at various distances below sonoprobe and from those measurements the mean value was calculated. It was further checked by measurement of acoustic velocity with laser Doppler anemometer. Simple emulsions O/W, W/O were made by stirring for 10 4 20 min at 500–3000 rpm or by ultrasonification from 15 to 300 min at various intensity. For double emulsions, two step emulsification process was used. In the first stage a W/O emulsion was prepared with a large excess of hydrophobic emulsifier, by strong homogenization (1200 4 5000 rpm), to form the smallest possible droplets. In the second stage a W/O/W double emulsion was formed by mechanical mixing at 250–500 rpm for 10–25 min or ultrasonication where irradiation times varied from 15 to 300 s in batch experiments, while for continuous flow process the residence times ranged from 15 to 50 s. The homogenizer rotation speed and time of mixing were controlled, as the premixed W/O emulsions have droplets with diameter from several to 100 mm. The emulsification was observed using a microscope with a differential interference contrast mode. A video camera was used to observe the emulsification. Pictures of droplets were taken with the microscope equipped with high resolution camera connected to a computer, and stored for further analysis.
Table 1. Properties of investigated systems. System 1 2 3 4 5 6 7 8
Disp. phase
Density (kg m23)
Viscosity (mPas)
Cont. phase
Density (kg m23)
Viscosity (mPas)
Interfacial tension
Oil 1 Oil 2 Oil 3 Water Water Water þ glycerol W/O W/O
910 920 935 998 998 1150 920 931
58.8 48.9 72.3 1 1 11.3 72.4 94.7
Water Water Water þ glycerol Oil 1 Oil 2 Oil 2 Water Water
998 998 1150 910 920 920 998 998
1 1 11.3 58.8 48.9 48.9 1 1
2.4 1.4 4.8 2.4 1.4 3.6 1.3 1.4
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A5): 730– 734
THE FORMATION OF STABLE COSMETIC EMULSIONS
733
The actual droplet diameters were measured with microscopic images analysis of the MULTISCAN program. They were used in the analysis of size distribution, which depended on the mixing process parameters, as well as emulsifier type. Some samples, after dilution by adding continuous phase, were additionally analysed by laser diffraction method. The dynamic viscosity of the oils was defined by means of a rotational viscometer. The interfacial tension between the phases was measured with droplet volume tensiometer.
EXPERIMENTS The objective of this study was to evaluate the stability of W/O, O/W and W/O/W emulsions using droplet size analysis, creaming volume and rheological properties. On the basis of droplet size distribution, their mean diameter d32 was calculated. Experiments using ultrasound at various disperse phase content f values at constant initial mass showed that the resulting droplet diameter increased and the dependence on energy consumption became weaker. A comparison of the average droplet diameter versus power consumption using different emulsifying machines showed that the smallest droplet diameters were obtained when using the ultrasound. It should be mentioned here that dispersion of liquid into drops using ultrasonic vibrations, the amplitude must be above a threshold value. This threshold amplitude is given by 2hc rc 0:333 Acrit ¼ (theoretical) (17) rc psf
Figure 2. The influence of oil/water ratio on W/O emulsion stability.
where b ¼ 0.18–0.29 for investigated systems.
Droplet break up will only occur at high , e . values, which means that the energy dissipated at low , e . levels is wasted. The dependence of mean droplet diameter on the specific volume power density is shown on Figure 1. The W/O/W had inner W/O droplets of 0.5–2 mm in size and double W/O/W droplets (the outer phase) of 10–60 mm, O/W emulsions had droplets of 0.1 –150 mm and W/O 0.1– 100 mm. The emulsion stability increased with decreasing the dispersed to continuous phase ratio (Figure 2). Figure 3 shows that the relative emulsion volume increased as surfactant concentration increased from 0.20% to 0.50%. This is because at low surfactant concentrations, the emulsion is not stable due to agglomeration of the droplets, while at high surfactant concentration, the emulsion destabilization occurs as a result of rapid coalescence. The optimal surfactant dosage for investigated O/W systems was 0.5%, for W/O 2%, for W/O/W 5%, for O/W/O 4%. As the concentration increased further, emulsion stability decreased. It was reported that increasing local dissipation energy in breaking zone, due to the rise of circulation consumption through mixer zone, is found to be the most effective method of drops diameter decrease. The purpose of stirring is to form a stable and homogeneous emulsion by breaking large liquid drops into smaller drops. The influence of emulsifier concentration in the oil phase on the emulsion viscosity for O/W emulsion is presented in Figure 4.
Figure 1. Dependence of mean droplet diameter d32 on the power volume specific density , e . for three O/W emulsions; line denotes Davies correlation (9).
Figure 3. Effect of surfactant dosage on emulsion stability (O/W emulsion).
If the amplitude is below Acrit then cavitation and emulsification would not take place and the correlations for droplet diameter cannot be used. For all experiments in this work amplitudes were significantly higher than the calculated theoretical values emulsification took place. The experiments with W/O, O/W, W/O/W emulsions prove that the final drop size essentially depends on mean specific volume power density , e .. Drop size decreases with increasing residence time in the ultrasonic field, until a system specific, minimum drop size is obtained. d32 / dmax / k1ðt Þlb
(18)
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A5): 730–734
734
TAL-FIGIEL A combination of rheological tests was employed, i.e., linear (continuous flow) and dynamic (oscillatory rheometry). All emulsions showed plastic rheological behaviour (the yield values ranging from 14 to 29 Pa), with a low level of thixotropy.
NOMENCLATURE d32 de dmax ds f lw p
Figure 4. The influence of emulsifier concentration in the oil phase on the O/W emulsion viscosity.
DISCUSSION AND CONCLUSIONS In the present study a technology for the continuous and batch treatment of fluid mixtures with ultrasound was characterized using cosmetic type O/W, W/O and W/O/W emulsions as model systems. This method is well suited for aseptic processing, an important issue in cosmetic and pharmaceutical development and production. Ultrasound emulsification is an efficient method of obtaining finely dispersed emulsions. If cavitation is the dominant mechanism of droplet disruption these results have to be taken into account for an optimal design of the emulsification apparatus geometry. The Sauter mean droplet diameter of investigated emulsions W/O, O/W, W/O/W could be decreased by 10 orders of magnitude starting from coarse pre-emulsions and yielding mean diameters of below 0.1 mm. Ultrasonic irradiation leads to turbulent flow conditions on a macroscopic and a microscopic scale. The model based on the theory of turbulent drop break-up predicts drop size and can be used to design equipment for ultrasonic emulsification. The stable O/W, W/O, W/O/W emulsions were produced using the optimal emulsifier dosage. At lower emulsifier dosages, the emulsion was unstable; the instability at higher emulsifier dosages was a result of rapid coalescence and concentration. Higher stirring intensity in the first step generated smaller droplets, and that enabled resulted in more stable final emulsion. The best effects were obtained using 1000 rpm for O/W emulsion, 1200 rpm for W/O emulsion, 2000 and 800 for W/O/W emulsion. To obtain emulsion with mean droplet diameter in the range 100–500 nm, ultrasound should be applied as the second step, since typical rotational homogenizers do not provide the required energetic effect. Using the ultrasonification with irradiation time from 180 to 300 min enabled preparation of kinetically stable emulsions, with droplets sizes as low as 100 nm. It was found that for the systems studied, the Davies formula gave the best approximation.
t Vw We nc ,e . ,e . s hc, hd rc, rd
mean Sauter diameter, m cell diameter, m critical, maximal droplet diameter, m sonoprobe diameter, m ultrasound frequency, kHz Kolmogorov length scale dynamic ultrasound pressure (or homogenizer pressure), Nm22 irradiation time, s Kolmogorov velocity scale Weber number kinematic viscosity of continuous phase, m2s21 mean specific volume power density, W21m23 mean specific mass power density, W kg21 interfacial tension, N m21 viscosities of continuous and dispersed phases, Pas densities of continuous and dispersed phases, kg m23
REFERENCES Bałdyga, J. and Bourne, J.R., 1995, Interpretation of turbulent mixing using fractals and multifractals turbulent energy dissipation rates, Chem Engng Sci, 50: 381– 400. Bechtel, S., Gilbert, N. and Wagner, H.-G., 1999, Grundlagenun¨ l/Wasser-Emulsionen im tersuchungen zur Herstellung von O Ultraschallfeld, Chem Ing Techn, 71(8): 810 –818. Davies, J.T., 1985, Drop sizes of emulsions related to turbulent energy dissipation rates, Chem Engng Sci, 40: 839–842. Kita, Y., Matsumoto, S. and Yonezawa, D., 1977, Viscometric method for estimating the stability of W/O/W type multiple phase emulsions, J Colloid Interface Sci, 62: 87– 94. Li, M.K. and Fogler, H.S., 1978a, Acoustic emulsification. Part 1: The instability of the oil-water interface to form initial droplets, J Fluid Mech, 88: 499–511. Li, M.K. and Fogler, H.S., 1978b, Acoustic emulsification. Part 2: Breakup of the large primary oil droplets in a water medium, J Fluid Mech, 88: 513– 528. Mason, T.J., 1991, Practical Sonochemistry (Ellis Horwood, New York). Pal, R., 1993, Rheological behaviour of surfactant-flocculated water-in-oil emulsions, Colloids Surf A: Physicochemical and Engineering Aspects, 71: 173 –185. Princen, H.M., 1985, Rheology of foams and highly concentrated emulsions, II: experimental study of the yield stress and wall effects for concentrated oil-in-water emulsions, J Colloid Interface Sci, 105: 150– 171. Rajan, R. and Pandit, A.B., 2001, Correlations to predict droplet size in ultrasonic atomisation, Ultrasonics, 39: 235–255. Rosen, M.J., 2004, Surfactants and Interfacial Phenomena (Wiley Interscience, New Jersey). Shinnar, R., 1981, On the behaviour of liquid dispersions in the mixing vessels, J Fluid Mech, 10: 259– 275. Tal-Figiel, B., 1990, Conditions for the instability of the liquid-liquid interface in an ultrasonic field, Int Chem Engng, 30: 526–534. Tal-Figiel, B., 1989, Intensification of liquid-liquid extraction in an ultrasonic field, Cracov University of Technology Monographs 87 (in polish). Tal-Figiel, B., 2005, Emulsions in cosmetics and medicine, Inz Chem Proc, 27: 403–419. The manuscript was received 21 October 2006 and accepted for publication after revision 23 March 2007.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A5): 730– 734
This paper presents basic research results for production of W/O, O/W and W/O/W cosmetic emulsions under ultrasonic irradiation. A technology for the continuous and batch treatment of fluid mixtures with ultrasound was characterized using cosmetic emulsions as model systems. If cavitation is the dominant mechanism of droplet disruption, these results have to be taken into account for an optimal design of the emulsification apparatus geometry. The theoretical model supports experimental work. The experimental findings prove that final drop size essentially depends on specific power density. Drop size decreases with increasing residence time in the ultrasonic field until a system specific, minimum drop size is obtained. Ultrasonic irradiation leads to turbulent flow conditions on a macroscopic and a microscopic scale. The model based on the theory of turbulent drop bread-up predicts drop size and can be used to design equipment for ultrasonic emulsification. Emulsion used were stabilized using a combination of hydrophilic and hydrophobic surfactants. The ratio of this surfactants is important in achieving stable cosmetic emulsions. It was shown that emulsifier concentration plays great role in controlling the emulsification processes as well as the structured features of the droplets. The rheological behavior and morphological evolution during the ultrasonic emulsification were characterized systematically. The rheological properties were used to asses emulsion stability. Keywords: ultrasonic emulsification; cosmetic emulsions; break-up model; rheology; surfactant.
INTRODUCTION
droplets, they can penetrate through the body membranes which enables transport of active substances. Small droplet size enhances emulsion stability since the Brownian motion may be sufficient for overcoming gravity; it means that creaming or sedimentation should not occur even on prolonged storage, also the small size prevents flocculation of the droplets as well as their coalescence. The long-term physical stability of fine emulsions (with no apparent flocculation or coalescence) makes them unique and they are sometimes referred to as ‘approaching thermodynamic stability’. The recent surge in the cost of energy has led to an increased interest in emulsification processes. Intensive research on methods of carrying out this kind of processes ensures optimal utilization of resources, as well as externally applied energy. The application of ultrasound as a final step of emulsification, produces an emulsion with very small droplets, which has the effect of increasing the area of interfacial contact, and, hence, the efficiency of emulsification. The theory of emulsification by ultrasonic waves has not
Correspondence to: Professor B. Tal-Figiel, Institute of Chemical & Process Engineering, Cracow University of Technology, Ul Warszawska 24, 31-155 Krako´w, Poland. E-mail: btfi[email protected] DOI: 10.1205/cherd06199 0263–8762/07/ $30.00 þ 0.00 Chemical Engineering Research and Design Trans IChemE, Part A, May 2007 # 2007 Institution of Chemical Engineers
A wide variety of cosmetic emulsions are used as bases for skincare products for healthy and diseased skin. These products can range in consistency from a cream to a lotion or milk and even a fluid. Since today’s trends are increasingly short-lived, a manufacturer of cosmetic products needs to respond quickly to new market demands. This means that development times must be drastically reduced while maintaining the same high product quality. To be able to develop cosmetic products quickly and effectively, developers need reliable methods to obtain product stability without the need of long time testing. Here rheology has gained in importance in recent years (Kita et al., 1977; Pal, 1993; Princen, 1985). In the past few years micro- and nanoemulsions have been the subject of extensive research work, because of the vast possibilities they offer. Their properties such as improved drug solubilization and bioavailability make them very attractive for application in personal care and cosmetics as well as in health care, because, due to small size of 730
Vol 85 (A5) 730–734
THE FORMATION OF STABLE COSMETIC EMULSIONS been fully elaborated as yet, owing to the complexity of the mechanisms involved. Cavitation is likely to control droplet disruption in sonicated liquid–liquid systems. The current view is that, apart from cavitation, it is the interfacial capillary waves that are responsible for the process of dispersion in a liquid–liquidsystem in an ultrasonic field (Li and Fogler, 1978a, b; Mason, 1991; Tal-Figiel, 1990, 2005). These waves spread out across the interface without penetrating into the bulk phase. An increase of ultrasonic intensity and, hence, of the amplitude of the capillary waves, eventually results in droplets of liquid breaking away from the crests of the wave, and their ejection into the dispersing phase. The research results presented hereunder demonstrate the principles of formation of cosmetic fine emulsions by mixing and ultrasound. The objective of this work is to asses the impact of various physico-chemical properties of liquid, its flow rate, the irradiation time, amplitude or mean specific power density ,1. of ultrasound and apparatus geometry on the droplet size. This knowledge may prove helpful in composition and quality control.
MECHANISM OF EMULSIFICATION To prepare an emulsion, oil, water, surfactant and energy are needed. The method of mixing those components is very important. The formation of large droplets, as is the case of macroemulsions, is fairly easy and hence high speed stirrers are sufficient to produce the emulsion. The formation of small drops is difficult and requires a large amount of surfactant and/or energy. To obtain emulsion with mean droplet diameter in the range 100 –500 nm, ultrasound or high pressure homogenization should be applied as the second step, since typical rotational homogenizers do not provide the required energetic effect. The high energy required for formation of fine emulsions can be understood from a consideration of the Laplace pressure P. P¼
2s r
(1)
Addition of surfactants plays a major role in the formation of emulsions by lowering the interfacial tension s, P is then reduced and smaller stress is needed to break up a drop. The amount of surfactant required to produce the smallest drop will depend on its activity a (concentration) in the bulk, which determines the reduction in s, as given by the Gibbs adsorption equation. Another important role of the emulsifier is preventing coalescence during emulsification (Rosen, 2004). An important parameter that describes droplet deformation is the Weber number We, which gives the ratio of the external stress Gh, where G is the velocity gradient and h is the viscosity over the Laplace pressure P: We ¼
Ghr 2s
(2)
The droplet deformation increases with increase in the Weber number, droplet break-up occurs, when certain value, called critical Weber number is achieved. It is clear, that for producing small droplets high stresses are required.
731
MODEL OF DROP BREAK IN ULTRASONIC FIELD There are several possible mechanisms of droplet formation and disruption under the influence of ultrasound. One is the formation of droplets as a consequence of unstable oscillation of a liquid–liquid interface. These capillary waves may occur and contribute to dispersion, only if the diameter of droplets to be disrupted is larger than the wavelength of the capillary waves. A mechanism related to that of capillary waves is the oscillation and subsequent disruption of whole droplets due to the action of sound. This mechanism has to be taken into account as one cause of droplet disruption in an acoustic field, but only for a small fraction of droplets with diameters exactly in the corresponding range. With respect to the usually broad droplet size disruption in an emulsion pre-mix, a broad range of sound frequencies would be a precondition for this mechanism to become the governing one. Under practical conditions cavitation is generally regarded as crucial element. Parameters positively influencing this phenomenon in liquids, improve emulsification in terms of smaller droplet size of the dispersed phase right after disruption. Ultrasounds generate periodic compressions and expansions in sonificated media. At sufficiently high ultrasonic intensity during the expansion phase, bubbles, filled with vapour or gas, are generated on the interface. Those bubbles grow till the critical size is obtained and then collapse. During this process, intensive shear fields are generated in the vicinity of collapsing bubbles, as well as extreme pressure fluctuations and microjets of high liquid velocity. Those phenomena are the sources of turbulent microstreams. Apart from that, dynamic pressure generates liquid streams with speed of about 1 m s (Re ¼ 8000– 30 000) below the sonoprobe; it means that the turbulent effects exist also on macroscopic scale. The model based on the Kolmogorov theory of turbulent drop break up predicts drop size and can be used to design equipment for ultrasonic emulsification (Bechtel, 1999); Rajan and Pandit, (2001). In the field of turbulent streams, there exist vortices with various diameters. Especially important for droplet disruption are the smallest eddies. Their size and speed changes in the turbulent field can be described with
0:25 vc3 k1 l
Turbulent length scale
lw ¼
Turbulent velocity scale
vw ¼ ðk1 lnc Þ0:25
(3) (4)
According to this theory, critical Weber number equals Wec ¼
rc vw2 dmax s
(5)
and maximum droplet diameter attained in a very long process can be given as dmax / de We0:6
(6)
Kolmogorov theory was derived primarily for the fluids of low viscosity and isotropic turbulence. For the case where viscosities of phases differ significantly, Davies (1985) and Bechtel et al. (1999) extended it by including the influence of viscous stresses and obtained the following expression
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A5): 730–734
732
TAL-FIGIEL or
for the generalized Weber number: Sh dmax ¼ dmax
rc vw2 Wec ¼ s=dmax þ hd uin =dmax hd uin dmax
induced viscose stress, with uin denoting inside velocity,
u ¼ ðk1 lvc Þ
0:25
for dmax ¼ lw
(8)
and obtained the following expression for the critical droplet diameter: 0:6 dmax / hd r0:5 k1l0:25 þ s r0:2 10:4 d c
for d lw
(9)
For d , lw in the viscous subrange, the flow is viscous and 0:5 1 dmax , nc
ts ¼ h c
0:5 k1 l nc
(10)
and according to Shinnar (1961): hd f 0:5 hc hc k1 l
sn0:5 c
(11)
where: f(hd/hc) is defined as f
hd 16hd =hc þ 16 ¼ 19hd =hc þ 16 hc
(12)
Bałdyga and Bourne (1995) derived the following general expression for dispersion under creeping flow conditions using multifractal concept: dmax
f
sf hd =hc ffi ts
(13)
hd h ð1 =nÞ0:5 ðlw =LÞ2(a1)=aþ3 ¼ c hc s=dmax
(14)
and dmax ¼
Apart from the theoretical derivation, many experimental correlations were obtained for the case of ultrasonic emulsification (Tal-Figiel, 1989, 1990; Rajan and Pandit, 2001).
METHODS AND MATERIALS
0:5 r u, uin ¼ c rd
Sh dmax /
(16)
(7)
where
u/
2(1a)=aþ1 lw L
s 1 aþ3Þ 2=ðaþ3Þ 2(1a)=ðaþ3Þ f h =h k1 l L rc n2=ð d c c
(15)
An experimental investigation of cosmetic type emulsions: O/W, W/O and W/O/W obtained by two step method— preliminary mechanical mixing and final ultrasonification, has been carried out. Table 1 shows all components of emulsions investigated in this work. The viscosity of the continuous phase was varied by addition of glycerol to the water phase in case of O/W systems and by using different oils for the W/O systems. Interfacial tension was lowered by adding mixture of commercially available emulsifier to both phases. For the mechanical mixing, a turbine impeller was applied. Ultrasonication was carried out using a generator with operating frequency of 22.5 kHz with piezoelectric transducer and sonoprobe (diameter ds ¼ 0.02 m). Vessel to sonoprobe diameter ratios were 1, 1.5, 2. Ultrasound specific volume power density , e . could be changed from 5.105 to 2.108 (Wm23). Effective ultrasound power was established using cavitation meter. Its measuring tip was placed at various distances below sonoprobe and from those measurements the mean value was calculated. It was further checked by measurement of acoustic velocity with laser Doppler anemometer. Simple emulsions O/W, W/O were made by stirring for 10 4 20 min at 500–3000 rpm or by ultrasonification from 15 to 300 min at various intensity. For double emulsions, two step emulsification process was used. In the first stage a W/O emulsion was prepared with a large excess of hydrophobic emulsifier, by strong homogenization (1200 4 5000 rpm), to form the smallest possible droplets. In the second stage a W/O/W double emulsion was formed by mechanical mixing at 250–500 rpm for 10–25 min or ultrasonication where irradiation times varied from 15 to 300 s in batch experiments, while for continuous flow process the residence times ranged from 15 to 50 s. The homogenizer rotation speed and time of mixing were controlled, as the premixed W/O emulsions have droplets with diameter from several to 100 mm. The emulsification was observed using a microscope with a differential interference contrast mode. A video camera was used to observe the emulsification. Pictures of droplets were taken with the microscope equipped with high resolution camera connected to a computer, and stored for further analysis.
Table 1. Properties of investigated systems. System 1 2 3 4 5 6 7 8
Disp. phase
Density (kg m23)
Viscosity (mPas)
Cont. phase
Density (kg m23)
Viscosity (mPas)
Interfacial tension
Oil 1 Oil 2 Oil 3 Water Water Water þ glycerol W/O W/O
910 920 935 998 998 1150 920 931
58.8 48.9 72.3 1 1 11.3 72.4 94.7
Water Water Water þ glycerol Oil 1 Oil 2 Oil 2 Water Water
998 998 1150 910 920 920 998 998
1 1 11.3 58.8 48.9 48.9 1 1
2.4 1.4 4.8 2.4 1.4 3.6 1.3 1.4
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THE FORMATION OF STABLE COSMETIC EMULSIONS
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The actual droplet diameters were measured with microscopic images analysis of the MULTISCAN program. They were used in the analysis of size distribution, which depended on the mixing process parameters, as well as emulsifier type. Some samples, after dilution by adding continuous phase, were additionally analysed by laser diffraction method. The dynamic viscosity of the oils was defined by means of a rotational viscometer. The interfacial tension between the phases was measured with droplet volume tensiometer.
EXPERIMENTS The objective of this study was to evaluate the stability of W/O, O/W and W/O/W emulsions using droplet size analysis, creaming volume and rheological properties. On the basis of droplet size distribution, their mean diameter d32 was calculated. Experiments using ultrasound at various disperse phase content f values at constant initial mass showed that the resulting droplet diameter increased and the dependence on energy consumption became weaker. A comparison of the average droplet diameter versus power consumption using different emulsifying machines showed that the smallest droplet diameters were obtained when using the ultrasound. It should be mentioned here that dispersion of liquid into drops using ultrasonic vibrations, the amplitude must be above a threshold value. This threshold amplitude is given by 2hc rc 0:333 Acrit ¼ (theoretical) (17) rc psf
Figure 2. The influence of oil/water ratio on W/O emulsion stability.
where b ¼ 0.18–0.29 for investigated systems.
Droplet break up will only occur at high , e . values, which means that the energy dissipated at low , e . levels is wasted. The dependence of mean droplet diameter on the specific volume power density is shown on Figure 1. The W/O/W had inner W/O droplets of 0.5–2 mm in size and double W/O/W droplets (the outer phase) of 10–60 mm, O/W emulsions had droplets of 0.1 –150 mm and W/O 0.1– 100 mm. The emulsion stability increased with decreasing the dispersed to continuous phase ratio (Figure 2). Figure 3 shows that the relative emulsion volume increased as surfactant concentration increased from 0.20% to 0.50%. This is because at low surfactant concentrations, the emulsion is not stable due to agglomeration of the droplets, while at high surfactant concentration, the emulsion destabilization occurs as a result of rapid coalescence. The optimal surfactant dosage for investigated O/W systems was 0.5%, for W/O 2%, for W/O/W 5%, for O/W/O 4%. As the concentration increased further, emulsion stability decreased. It was reported that increasing local dissipation energy in breaking zone, due to the rise of circulation consumption through mixer zone, is found to be the most effective method of drops diameter decrease. The purpose of stirring is to form a stable and homogeneous emulsion by breaking large liquid drops into smaller drops. The influence of emulsifier concentration in the oil phase on the emulsion viscosity for O/W emulsion is presented in Figure 4.
Figure 1. Dependence of mean droplet diameter d32 on the power volume specific density , e . for three O/W emulsions; line denotes Davies correlation (9).
Figure 3. Effect of surfactant dosage on emulsion stability (O/W emulsion).
If the amplitude is below Acrit then cavitation and emulsification would not take place and the correlations for droplet diameter cannot be used. For all experiments in this work amplitudes were significantly higher than the calculated theoretical values emulsification took place. The experiments with W/O, O/W, W/O/W emulsions prove that the final drop size essentially depends on mean specific volume power density , e .. Drop size decreases with increasing residence time in the ultrasonic field, until a system specific, minimum drop size is obtained. d32 / dmax / k1ðt Þlb
(18)
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TAL-FIGIEL A combination of rheological tests was employed, i.e., linear (continuous flow) and dynamic (oscillatory rheometry). All emulsions showed plastic rheological behaviour (the yield values ranging from 14 to 29 Pa), with a low level of thixotropy.
NOMENCLATURE d32 de dmax ds f lw p
Figure 4. The influence of emulsifier concentration in the oil phase on the O/W emulsion viscosity.
DISCUSSION AND CONCLUSIONS In the present study a technology for the continuous and batch treatment of fluid mixtures with ultrasound was characterized using cosmetic type O/W, W/O and W/O/W emulsions as model systems. This method is well suited for aseptic processing, an important issue in cosmetic and pharmaceutical development and production. Ultrasound emulsification is an efficient method of obtaining finely dispersed emulsions. If cavitation is the dominant mechanism of droplet disruption these results have to be taken into account for an optimal design of the emulsification apparatus geometry. The Sauter mean droplet diameter of investigated emulsions W/O, O/W, W/O/W could be decreased by 10 orders of magnitude starting from coarse pre-emulsions and yielding mean diameters of below 0.1 mm. Ultrasonic irradiation leads to turbulent flow conditions on a macroscopic and a microscopic scale. The model based on the theory of turbulent drop break-up predicts drop size and can be used to design equipment for ultrasonic emulsification. The stable O/W, W/O, W/O/W emulsions were produced using the optimal emulsifier dosage. At lower emulsifier dosages, the emulsion was unstable; the instability at higher emulsifier dosages was a result of rapid coalescence and concentration. Higher stirring intensity in the first step generated smaller droplets, and that enabled resulted in more stable final emulsion. The best effects were obtained using 1000 rpm for O/W emulsion, 1200 rpm for W/O emulsion, 2000 and 800 for W/O/W emulsion. To obtain emulsion with mean droplet diameter in the range 100–500 nm, ultrasound should be applied as the second step, since typical rotational homogenizers do not provide the required energetic effect. Using the ultrasonification with irradiation time from 180 to 300 min enabled preparation of kinetically stable emulsions, with droplets sizes as low as 100 nm. It was found that for the systems studied, the Davies formula gave the best approximation.
t Vw We nc ,e . ,e . s hc, hd rc, rd
mean Sauter diameter, m cell diameter, m critical, maximal droplet diameter, m sonoprobe diameter, m ultrasound frequency, kHz Kolmogorov length scale dynamic ultrasound pressure (or homogenizer pressure), Nm22 irradiation time, s Kolmogorov velocity scale Weber number kinematic viscosity of continuous phase, m2s21 mean specific volume power density, W21m23 mean specific mass power density, W kg21 interfacial tension, N m21 viscosities of continuous and dispersed phases, Pas densities of continuous and dispersed phases, kg m23
REFERENCES Bałdyga, J. and Bourne, J.R., 1995, Interpretation of turbulent mixing using fractals and multifractals turbulent energy dissipation rates, Chem Engng Sci, 50: 381– 400. Bechtel, S., Gilbert, N. and Wagner, H.-G., 1999, Grundlagenun¨ l/Wasser-Emulsionen im tersuchungen zur Herstellung von O Ultraschallfeld, Chem Ing Techn, 71(8): 810 –818. Davies, J.T., 1985, Drop sizes of emulsions related to turbulent energy dissipation rates, Chem Engng Sci, 40: 839–842. Kita, Y., Matsumoto, S. and Yonezawa, D., 1977, Viscometric method for estimating the stability of W/O/W type multiple phase emulsions, J Colloid Interface Sci, 62: 87– 94. Li, M.K. and Fogler, H.S., 1978a, Acoustic emulsification. Part 1: The instability of the oil-water interface to form initial droplets, J Fluid Mech, 88: 499–511. Li, M.K. and Fogler, H.S., 1978b, Acoustic emulsification. Part 2: Breakup of the large primary oil droplets in a water medium, J Fluid Mech, 88: 513– 528. Mason, T.J., 1991, Practical Sonochemistry (Ellis Horwood, New York). Pal, R., 1993, Rheological behaviour of surfactant-flocculated water-in-oil emulsions, Colloids Surf A: Physicochemical and Engineering Aspects, 71: 173 –185. Princen, H.M., 1985, Rheology of foams and highly concentrated emulsions, II: experimental study of the yield stress and wall effects for concentrated oil-in-water emulsions, J Colloid Interface Sci, 105: 150– 171. Rajan, R. and Pandit, A.B., 2001, Correlations to predict droplet size in ultrasonic atomisation, Ultrasonics, 39: 235–255. Rosen, M.J., 2004, Surfactants and Interfacial Phenomena (Wiley Interscience, New Jersey). Shinnar, R., 1981, On the behaviour of liquid dispersions in the mixing vessels, J Fluid Mech, 10: 259– 275. Tal-Figiel, B., 1990, Conditions for the instability of the liquid-liquid interface in an ultrasonic field, Int Chem Engng, 30: 526–534. Tal-Figiel, B., 1989, Intensification of liquid-liquid extraction in an ultrasonic field, Cracov University of Technology Monographs 87 (in polish). Tal-Figiel, B., 2005, Emulsions in cosmetics and medicine, Inz Chem Proc, 27: 403–419. The manuscript was received 21 October 2006 and accepted for publication after revision 23 March 2007.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A5): 730– 734