The Effect of High Shear Homogenization on Physical Stability of Emulsions

Emulsions are thermodynamically unstable systems where droplet size is one of the main factors that affect its physical stability and consequently their quality. In this context, this work analyses the incorporation of a high shear homogenization step in the manufacturing process of an emulsion with the objective of maintaining its physical stability. In order to demonstrate the effects of this homogenization in the manufacturing process, the emulsion characterization was carried out by microscopy, rheology, laser diffraction and analytical photo-centrifugation techniques. The effect of high shear homogenization into the emulsion physical results was dependent on the speed applied to the homogenizer pump, with an effective 3600 rpm speed. There was no evidence of change on pH attributes and emulsion density, although there was a change in volumetric relationship between the droplet family presented in each sample, reducing the population of larger droplets to form a third family of intermediate droplets and increasing the volumetric proportion of the population of smaller droplets. This profile change in the droplet size distribution contributed to increased viscosity and emulsion without the presence of separation after it was submitted to the stress condition of temperature and agitation.


Introduction
Emulsions are widely used as vehicles for cosmetic and pharmaceutical industry where various cosmetic or semi-solid pharmaceutical products are formulated as water in oil emulsion (W/O) or oil in water (O/W).Emulsions are systems with two immiscible liquids (oil and water) and may contain an emulsifying agent with the function of promoting emulsion stability due to its property of allocating in the oil and water interface decreasing the interfacial tension between these phases.Because the emulsion is considered a thermodynamically unstable system, it may present lack of long term stability.
It was reported in a publication by Raikar et al. (2011) that commonly used techniques for emulsification require the application of mechanical energy to the two immiscible phases and stabilization of the newly formed interfaces by surfactants.
Studies conducted to stabilize emulsions generally focus on the use of emulsifiers, as reported on relevant studies from Tadros (2006) and ( 2009) Baret et al. (2009).The use of emulsifiers in order to stabilize an emulsion certainly requires a change in the qualitative or quantitative formula of the product.There are other published works with guidance for stabilization of an emulsion, based on studying of the manufacturing process with the aim of improving the manufacturing step.Schultz et al. (2004) classify the different equipment used in the emulsion homogenization step as high pressure homogenizers, and high shear homogenizers; it is possible to observe that such equipment promotes reduction of droplet diameter, ensuring emulsion physical stability.
Established products already commercialized, have a registered formula composition, sometimes patented, where their physical stability was confirmed in the climate zone condition in the region in which this product is marketed.When there is a need to adapt the physical stability of the product already registered to address, for example, different climate zone conditions or manufacturing site changes, formula amendments will impose regulatory barriers.However, improvements in the manufacturing process steps will be a more viable alternative from a regulatory and financial perspective.
This work aims to study the additional step of high shear homogenization through a high shear homogenizer rotor-stator type pump at the end of an emulsion manufacturing process.Moreover, the ability of the high shear homogenizer to reduce the size and to increase uniformity of the droplets to maintain the emulsion physical stability was evaluated.Additionally, to prove the effectiveness of the homogenizing pump, the emulsion physical characterization was performed, in order to demonstrate uniformity of the droplet shape as confirmed by microscopy, and reduction of the droplet size as verified by laser diffraction as well as the reduction of phase separation by photo-analytical centrifugation under speed and temperature stress conditions.

Emulsion Preparation
Oil in water emulsion was prepared by phase inversion.Industrial lots of 1500Kg were manufactured in 2000L capacity vessel.Initially, the ingredients corresponding to the oil phase 1a (Organic phase component with oily solvent) were heated at 50°C.Subsequently, the emulsion was formed by phase inversion by adding water, phase 1b.The viscosity regulators from phase 1c were added at 50°C, under agitation and recirculation through a lobe pump (Alfa Laval ® manufacturer).The surfactant from phase 1d was solubilized in a stainless steel container with the remaining oil phase materials from phase 1d (Organic phase component with oily solvent).Subsequently, this mixture was transferred to the vessel.The emulsion was cooled down and transferred from the vessel to a stainless steel container through a lobe pump and a high shear homogenizer rotor-stator type with three stages (IKA ® manufacturer).Table 1 presents the formulation used for seven lots.T3 (3)  223 3600 (1) Process T1 -Without high shear homogenizer. (2) Process T2 -High shear homogenizer at 1800 rpm. (3) Process T3 -High shear homogenizer at 3600 rpm.

pH
The pH of the emulsions was determined at 25°C ± 2°C, without dilution, using a digital pH meter (Mettler Toledo ® manufacturer, Model pH / Ion meter S220).This test was performed in duplicate 24 hours after batch manufacturing.

Density
The density value of the emulsions was determined by Mettler Toledo densimeter Model 40, with temperature set at 20°C.The analysis was performed in duplicate 24 hours after batch manufacturing.

Viscosity
The viscosity of the emulsions was measured by a Brookfield viscometer model RVDV I+, with spindle number 5, temperature at 25°C and 20 rpm agitation speed.The test was performed in duplicate 24 hours after batch manufacturing.At spindle number 5 shear rate=0.

Rheology
The rheology behaviour of the emulsions was analysed using a rheometer from Malvern Kinexus manufacturer, KNX5001 model.The equipment was set to apply an upward continuous shear rate from 0.01 to 1000 Pa at 25°C.The measurement was done 30 days after batch manufacturing.

Droplet Size
The droplet size distribution from the dispersed phase of the emulsion was evaluated by laser diffraction using the equipment Mastersizer Microplus from Malvern Instruments.The required dilution was performed with purified water to obtain obscuration from 10% to 20% in the equipment.The measurement was done 30 days after batch manufacturing.The droplet size distribution (Span) was reported in the form of cumulative percentage volume d[v; 0,1], d[v; 0.5], d[v; 0.9] and mean droplet diameter (d [4,3]).The equipment also calculated the polydispersity index which is an indication of the width of the droplets, reported as range:

Microscopy
The droplet shape analysis was carried out by optical microscopy microscope Axio Imager Carl Zeiss, model M2M with DIC (interference contrast microscopy).The analysis was performed 30 days after batch manufacturing.The cover slip was gently placed in order to not break emulsion structure.

Analytical Photocentrifuge
In order to check the possibility of phase separation, the centrifuge was performed 24 hours and 30 days after emulsion manufacture stored at 25°C ± 2°C.For this study, centrifuge equipment, model Cientec CT-6000R was used.
Centrifugation was performed at 3500 rpm for 15 minutes.From the tubes where separation was observed, the separated phase was removed and weighed to compare the amount of each separated sample.
Beyond the conventional centrifuge test, predictive assessment of physical stability of the emulsion was also carried out after 30 days of manufacture stored at 25°C ± 2°C, by checking phase separation through LUMiSizer ® equipment.The equipment is an analytical photo-centrifuge system for measuring the intensity of light transmitted as a function of time and position along the entire length of the sample.The analysis was conducted at 45°C and rotation speed of 4000 rpm.

Manufacturing Process
The processes T1, T2 and T3 have in common the same manufacturing steps through the formation of the emulsion by phase inversion and differ only in the last homogenization step when the emulsion passes through a high shear pump, as summarized in the flow reported in Figure 1.
The lobe pump operating by a positive displacement (Eggert, 2011), was used in the manufacturing process with the function of recirculation in the vessel to enhance mixing of the ingredients in the formulation.Moreover, this pump was also used with the function of emulsion pumping, feeding the high shear homogenizer pump IKA ® manufacturer at a flow of 2400 liters/hours.This flowrate was confirmed weighing the product that was transferred and time transference.
The objective was to keep a steady feeding flow to the homogenizing pump.Thus, the emulsion residence time in the high shear homogenizer pump is constant for the seven tests.Keeping the residence time of emulsion in homogenizer pump, the speed of the homogenizer pump was changed for process T2 (1800 rpm) and process T3 (3600 rpm), in order to assess the effect of speed on the ability to promote different shear on the emulsion droplets.The shear rate applied in a rotor-stator homogenizer pump may increase emulsion temperature (Felton, 2013).It was also observed by Raikar et al. (2011) that substantial increases in the emulsion temperature for multiple passes occur at high pressure.Therefore, only a single pass of the emulsion through the high shear pump was performed and the initial and final temperature of the emulsion was measured using a digital thermometer.According to Table 3, it will be seen that the single pass of the emulsion by rotor-stator system does not increase the emulsion final temperature, even at the highest shear rate at 3600 rpm.

Physical C
The Figure 2 s different batch

pH and De
There was no was an expect between tests Regarding the incorporate ai on Results    5. 3,367 | 3,33 3,587 | 3,57 3,006 | 3,54 3,014 | 3,21 3,809 | 3,70 4,967 | 4,80 4,788 | 4    All samples without high shear homogenization step and the sample with high shear homogenization at 1800 rpm (T2a) showed phase separation at the top of the tubes, as shown in the visual assessment in Figure 8.

Spa
For the samples with high shear homogenization at 3600 rpm no phase separation was observed at the top of the tube.Those samples which passed through the high shear process had similar light transmission profile between tests T3 (Figure 7) showing overlapping of the light transmission, confirming the benefit of the additional step of high shear homogenization for emulsion physical stabilization.The instability can be attributed to the lack of homogeneity of size and shape of the droplets present in this emulsion, as evaluated by microscopy and laser diffraction techniques.From this study, we can see that besides the presence of the emulsifier, it is also necessary to have an effective emulsification process producing a homogeneous emulsion in the shape and size distribution of their droplets.
The same qualitative and quantitative formula with the same emulsifier system gives distinct phase separation results according to the homogenization process adopted.
Using the homogenizer pump at high speed (3600 rpm) a higher emulsion shear was obtained, and consequently, there was a possible increase in the interfacial area with better distribution of the emulsifying agent around the emulsion droplets without any change in the quantities used in the formulation.

Conclusions
This work focused on studying the influence of high shear homogenization process in maintaining the physical stability of an oil-in-water emulsion, having the same qualitative and quantitative formula composition.
The effect of high shear homogenization on the physical properties of the oil in water emulsion was dependent on the speed applied to the homogenizer pump where 3600rpm speed was confirmed to be effective in improving the physical aspects of size emulsion, drop shape, and eliminating separation of phases when submitted to stress conditions of temperature and agitation.
There was higher emulsion shearing using the homogenizer pump at high speed allowing a better emulsifying agent distribution in the emulsion probably by the increase in interfacial area.
The benefit of high shear homogenization at the end of the manufacturing step was confirmed, due to modification of the volume ratio between the families of droplets sizes present in each sample, thereby reducing the population of larger drops, and forming a third family of intermediate drops and increasing the population of smaller droplets.This effect was also observed in optical microscopy where there was drop polydispersity reduction for the tests that used the homogenization at 3600 rpm, resulting in a slight increase in the emulsion viscosity.
The rheology behaviour showed similar yield stress and consistency index results for the samples that passed through the high shear pump, which differs from the sample without the high shear homogenization step.This difference indicates that the samples that passed through the pump required a lower rate to start to flow, indicating a possible benefit to the consumer, due to potentially improved spreading properties of this emulsion on the skin for samples that passed through high shear homogenization process.
The high shear homogenization step proved to be feasible in maintaining physical stability of emulsions and can be an alternative to be utilized by companies in cases where a formula composition change imposes regulatory barriers.

Figure
Figure 6 that t viscosity as the stem from pse ents with pseu served that the population of an flow curve b

Table 2
presents a summary of the conditions applied to each test.

Table 2 .
Test conditions summary

Table 8
summarizes the results of the instability index and the average droplet diameter from batches studied.It is possible to conclude that higher instability indices are obtained for samples that have a larger average droplet size, represented by d[4,3].

Table 8 .
Instability Index versus Average Droplets Sizes