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Influence of the concentration of a polyoxyethylene glycerol ester on the physical stability of submicron emulsions
Accepted Manuscript Title: Influence of the concentration of a polyoxyethylene glycerol ester on the physical stability of submicron emulsions Author: J. Santos N. Calero J. Mu˜noz PII: DOI: Reference:
S0263-8762(15)00188-4 http://dx.doi.org/doi:10.1016/j.cherd.2015.05.027 CHERD 1893
To appear in: Received date: Revised date: Accepted date:
19-1-2015 23-4-2015 21-5-2015
Please cite this article as: Santos, J., Calero, N., Mu˜noz, J.,Influence of the concentration of a polyoxyethylene glycerol ester on the physical stability of submicron emulsions, Chemical Engineering Research and Design (2015), http://dx.doi.org/10.1016/j.cherd.2015.05.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Research Highlights: 1. The combination of techniques provides relevant information of physical stability
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2. Emulsions in the range 2-3wt% surfactant were highly stable.
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3. Emulsion with 1.5 wt% surfactant showed creaming as a predominant mechanism.
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4. Emulsions with 3.5-4wt% surfactant showed flocculation, creaming and coalescence.
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Influence of the concentration of a polyoxyethylene glycerol ester on the physical stability of submicron emulsions
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J. Santos1, N. Calero*1 and J. Muñoz1.
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Aplicada. Tecnología de Coloides. Departamento de Ingeniería Química.
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Facultad de Química. Universidad de Sevilla c/ P. García González, 1, E41012,
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Sevilla Spain.
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* Corresponding author. Tel.: +34 954 557179; fax: +34 954 556447. E-mail
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address: [email protected]
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Abstract
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The chemistry and technology of agrochemical products has undergone extensive
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changes over the last 20 years. The new formulations and ingredients should meet
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the needs of the agrochemical industry for products having greater safety to the
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user and much lower environmentally impact maintaining the same performance
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targets. A recent trend involves the use of the emulsion format for agrochemicals,
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which provides a more efficient performance than those conventionally used.
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Furthermore, the production of submicron stable emulsion is a key achievement
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especially for this product.
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This study has been focused on the development of fine emulsions containing
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ecofriendly ingredients, such as surfactants and green solvents. It has been proven
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that the optimal surfactant concentration not only may lead to emulsions with
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submicron droplet sizes but also may prevent the typical destabilization process
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occurring in these formulations. In this particular case, it has been demonstrated
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that 3wt% surfactant concentration is adequate for three reasons: a) allowing the
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lowest droplet size to be achieved, b) providing the sufficient viscosity to prevent
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creaming and c) not being an excess of surfactant that leads to depletion
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flocculation.
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Keywords: green surfactant, physical stability, emulsions, rheology, multiple light
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scattering.
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1. INTRODUCTION
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Emulsion science and technology has been used for many years to create a diverse
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range of commercial products, including pharmaceuticals, foods, agrochemicals,
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lubricants, personal care products, and cosmetics. Production of emulsion-based
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systems with specific physicochemical and functional properties often requires
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tight control over the particle size distribution (McClements, 2005). Type and
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concentration of emulsifier play an important role in the droplet size distribution
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(DSD).
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The interest in submicron emulsions has increased in recent years due to their
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very small droplet size and high stability and their applications in many industrial
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fields such as personal care and cosmetics, health care, pharmaceuticals, and
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agrochemicals. (Schultz et al, 2000; Sonneville et al, 2004; Leal-Calderon et al.,
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2007; McClements, 2005; Tadros, 2009). These emulsions whose range in the DSD
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falls typically of 100-500 nm are also sometimes referred to as ultrafine emulsions
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(Nakajima, 1997) ,mini-emulsions (El-Aasser et al, 2004) and nanoemulsions (Ying
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Tang et al, 2013). In addition, submicron emulsions can be prepared by reasonable
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surfactant concentrations (less than 10%) that may fulfil the requirements of a
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bio-based society (Brockel et al, 2007).
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There is a need to replace the traditional organic solvents by more
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environmentally favorable solvents (Anastas and Warner, 1998.) Consequently,
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the renewed interest in search of appropriate greener and alternative solvents to
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be used in emulsions has grown enormously (Sheldon, 2005). Fatty acid
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dimethylamides (FAD) are among green solvents that can find applications in
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agrochemicals (Hofer and Bigorra, 2007). N,N-dimethyldecanamide (DMA-10) is
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considered a safe biosolvent, according to the Environmental Protection Agency.
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Therefore, it is a great solvent for agrochemical use due to the lack of risk to the
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farmer. The fact of satisfying the needs of customers is the basic principle of the
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product design (Brokel, 2007).
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D-limonene, a naturally occurring hydrocarbon, is a cyclic monoterpene, which is
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commonly found in the rinds of citrus fruits such as grapefruit, lemon, lime, and in
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particular, oranges. D-limonene exhibits good biodegradability, hence it may be
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proposed as an interesting alternative to organic solvents (Walter, 2010 and
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Medvedovici et al, 2012). These solvents can meet the ever-increasing safety and
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environmental demands of the 21st Century.
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Environmentally friendly surfactants have attracted significant interest recently.
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Polyoxyethylene glycerol esters derived from cocoa oil are non-ionic surfactants
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obtained from a renewable source which fulfil the environmental and toxicological
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requirements to be used as ecofriendly foaming and/or emulsifying agents, hence
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their consideration as green surfactants (Castán and González, 2003). Their use in
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detergents and personal care products is disclosed in several patents (Lutz, 2006;
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Denolle,2011). Levenol C-201 was selected as emulsifier due to its great superficial
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and interfacial properties. Furthermore, α-pinene emulsions with Levenol C-201
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showed better stability than emulsions containing its counterpart Levenol H&B
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(Trujillo-Cayado, 2014a, 2014b)
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The main objective of this work was the study of the influence of the surfactant
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concentration (polyoxyethylene glycerol ester) on the physical stability of slightly
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concentrated O/W emulsions formulated with a mixture of green solvents (N,N-
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dimethyldecanamide and D-limonene). The optimum ratio of these solvents was
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previously studied by Santos et al, 2014. A further goal was to achieve stable fine
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emulsions, which may be used as matrices for incorporation of active agrochemical
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ingredients. According to a recent study (Santos et al, 2014), the same strategy was
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followed considering the combination of different techniques, which was proven to
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be a powerful tool to provide very interesting information at an early stage about
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the mechanisms of destabilization occurring in emulsions.
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2. MATERIALS AND METHODS
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2.1. Materials
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N,N Dimethyl Decanamide (Agnique AMD-10TM) was kindly provided by BASF. D-
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Limonene was supplied by Sigma Chemical Company. The emulsifier used was a
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nonionic surfactant derived from cocoa oil. Namely, a polyoxyethylene glycerol
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fatty acid ester, Glycereth-17 Cocoate (HLB:13), received as a gift from KAO, was
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selected. Its trade name is Levenol C-201TM. The safety data sheet provided by the
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supplier reports a value for oral toxicity (LD50) higher than 5000 mg/kg of animal
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in tests carried out with rats. It is interestingly to note that this value would be
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3000 mg/kg for salt (Hollinger, 2005).
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RD antifoam emulsion (DOW CORNING®) was used as antifoaming agent. This
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commercial product consists of an aqueous solution containing Polydimethyl
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siloxane (<10 %w/w) and Dimethyl siloxane, hydroxyl-terminated (<10 %w/w).
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Deionized water was used for the preparation of all emulsions.
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2.2. Submicron emulsion development.
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Emulsions containing 0.1 wt% antifoam emulsion, a variable surfactant
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concentration and 30 wt% mixture of solvents (75wt% AMD-10/25wt% D-
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Limonene) were prepared. The optimum ratio of solvents was previously studied
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by Santos, 2014. The surfactant concentrations studied were 1.5, 2, 2.5, 3, 3.5 and
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4wt%. These O/W emulsions were carried out using a rotor-stator homogenizer
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(Silverson L5M), equipped with a mesh screen at 6000 rpm during 60 seconds.
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2.3. Droplet size distribution measurements.
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Size distribution of oil droplets was determined by laser diffraction using
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Mastersizer X (Malvern, Worcestershire, United Kingdom). All measurements were
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repeated 3 times with each emulsion. These measurements were carried out after
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1, 3, 13, 21, 40 days aging time to analyze likely coalescence effects.
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The mean droplet diameter was expressed as Sauter diameter (D[3,2]) and volume
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mean diameter (D[4,3]).
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2.4. Rheological measurements.
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Rheological experiments were conducted with a Haake MARS controlled-stress
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rheometer (Thermo-Scientific, Germany), equipped with a sand-blasted coaxial
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cylinder Z-20 (sample volume: 8.2 mL, Re/Ri =1.085, Ri= 1 cm) to avoid slip
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effects. Flow curves were carried out from 0.05 Pa to 1 Pa at 20oC. Flow curves
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were carried out after 1, 3, 13, 21 and 40 days to check the effect of aging time. All
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measurements were repeated 3 times with each emulsion. Samples were taken at
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about 2 cm from the upper part of the container. Sampling from the top part of the
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container in contact with air was avoided.
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2.5. Multiple light scattering
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Multiple light scattering measurements with a Turbiscan Lab Expert were used in
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order to study the destabilization of the emulsions. Measurements were carried
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out until 40 days at 20oC to determine the predominant mechanism of
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destabilization in each emulsion as well as the kinetics of the destabilization
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process. Multiple light scattering is a sensitive and non-intrusive technique to
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monitor physical stability of emulsions (Allende et al, 2008 and Camino et al, 2012)
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and more complex systems such as suspoemulsions (Santos et al, 2013).
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Multiple light scattering measurements in the middle zone of the measuring cell
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also allowed the evolution of a mean droplet diameter with aging to be monitored.
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2.6 Viscosity of the continuous phase.
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The viscosity of continuous phases solutions (from 2.14 wt% to 5.71 wt%) were
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measured with an Ubbelohde glass capillary viscometer. A volume of solution was
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pipetted into the capillary viscometer, which was equilibrated at 20°C in a water
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bath for 30 minutes prior to measurements. All the measurements were performed
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at 20oC ± 0.1oC and the result is the average of five measurements. Viscosity is
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obtained from the following equation:
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(Eq 2)
where is the viscosity of the continuous phase, C is a constant that depends of the
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glass capillary, ρ is the density of the continuous phase and t is the time.
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3. RESULTS AND DISCUSSION.
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Figure 1 shows the droplet size distribution of the emulsions with different
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concentration of surfactant. All emulsions studied showed two populations of
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droplets except for emulsion with 1.5 % surfactant. The first peak is below 1
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micron and the second population is centred about three microns. This second
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peak is probably due to recoalescence phenomenon induced by an excess of
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mechanical energy-input. (Jafari et al, 2008; Santos et al, 2014)
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A decrease of the surfactant content provoked the distributions to shift towards
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greater droplet sizes. This fact is more pronounced for the emulsion containing
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1.5wt% of surfactant. In the range 1.5-2.0 wt%, the surfactant available was not
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enough to achieve the minimum droplet size that can be obtained by these
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operating conditions.
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Sauter and volumetric diameters for all emulsions are shown in table 1. All
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emulsions possess submicron mean diameters. It could be due to the low
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interfacial tension providing by the mixture of these solvents as reported by Santos
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et al, 2014. The Sauter and volumetric mean diameters levelled off for surfactant
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concentrations above 2.5 wt%.
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Figure 2 shows the flow properties for 1 day-aged emulsions studied as a function
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of surfactant concentration. All the emulsions exhibited a trend to reach a
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Newtonian region at low-shear rate regime, which is defined by the zero-shear
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viscosity, (η0). This range is followed by a slight decrease in viscosity (shear-
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thinning behaviour) above a critical shear rate. Fig. 2 also illustrates the fitting
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quality of the results obtained to the Cross model (R2 > 0.999). (Eq 3)
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is related to the critical shear rate for the onset of shear-thinning response, η0
stands for the zero-shear viscosity and (1-n) is a parameter related to the slope of
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the power-law region; n being the so-called flow index. For shear thinning
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materials, 0 < n <1. A solid material would show n = 0, while a Newtonian liquid
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would show n = 1.
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The values of these parameters are shown in Table 2 as a function of the
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concentration of surfactant. The emulsion at a low surfactant concentration of 1.5
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wt% showed the lowest Zero-shear viscosity. It is related to the fact that this
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emulsion showed the higher Sauter diameter. Zero-shear viscosity for the emulsions
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containing from 2%wt to 3.5%wt showed no significant differences, even though a slight
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tendency to increase with surfactant concentration may be observed. This is supported
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by the fact that these emulsions showed similar Sauter diameter values.
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A sudden increase in the zero-shear viscosity of the emulsion upon increasing the
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surfactant concentration from 3.5 wt% to 4 wt% was observed. Given that DSD did not
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change between 3.5 wt% and 4 wt% surfactant, this rheological change could be produced
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by either, an enhanced viscosity of the continuous phase (due to increasing interactions
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among micelles), or the occurrence of a stronger oil network due to depletion flocculation
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(Palazolo et al, 2005; Manoj et al, 1998).
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Table 3 shows the viscosity of the continuous phase prior emulsification. It is seen
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that increasing surfactant concentration from 5 wt% to 5.71 wt% provokes
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viscosity to increase from 12.89 to 17.93 mPa·s. Nonetheless, the increase in the
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viscosity of emulsions from 3.5wt% surfactant concentration to 4 wt% is about
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120%. Hence, the marked increase of the viscosity of the emulsions is more
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influenced by a depletion flocculation phenomenon than to the viscosity of the
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continuous phase.
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Therefore, it indicates that the critical surfactant concentration at which depletion
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flocculation of droplets occurs lies somewhere between 3.5 and 4wt%. Nowadays
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it is well established that in presence of high concentration of surfactant or
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polymer, the micelles can play an important role in the stability of the emulsions
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(Dickinson et al, 1997). After interface saturation by the adsorbed surfactant,
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micelles do not adsorb on the surfactant coated surface, and they can cause
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attraction between drops by a depletion mechanism. Thus, when two droplets
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approach in a solution of non-adsorbing micelles, the latter are expelled from the
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gap, generating a local region with almost pure solvent. The osmotic pressure in
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the liquid surrounding the particle pair exceeds that between the drops and
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consequently forces the droplets to aggregate. (Napper, 1983). The depletion
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flocculation could lead to a creaming and/or coalescence process.
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This tendency is also showed in the critical shear rate. Nevertheless, the flow index
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decreases with the concentration of surfactant. This is due to the increase of the
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shear thinning character of emulsions. The emulsion with lowest surfactant
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concentration is only slightly shear thinning, whereas emulsions with higher
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surfactant concentrations are more shear thinning in nature.
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Figure 3 shows the results of the physical stability study performed at room
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temperature by a multiple light scattering technique. Figure 3a shows a plot of
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backscattering versus container height of the sample at room temperature for
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1.5wt% emulsion by way of example. This figure also includes an inset where
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backscattering is plotted versus container height in reference mode (enlargement
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of backscattering of the lower zone of the vial that contained the sample) to display
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higher resolution of backscattering changes. 2, 2.5, 3 and 3.5% emulsions showed
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the same behaviour.
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A backscattering decrease in the lower and higher zones of the vial was observed
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whereas it remained nearly constant in the intermediate zone. The drop in
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backscattering observed at the bottom of the measuring cell clearly indicated the
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occurrence of a destabilization mechanism by creaming, in that the dispersed
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phase possessed a lower density than the continuous phase (McClements, 2005).
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Thus all emulsions presented creaming process but in different degree.
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The fact that the backscattering remained nearly constant in the intermediate
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zone suggests that destabilization mechanisms such as flocculation and
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coalescence were not much significant for these emulsions. Nevertheless, with
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regards to the backscattering decrease observed at the top of the measuring cell,
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we think it may be attributed to the occurrence of some coalescence as a
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consequence of the migration of small droplets to the top (Mengual et al, 1999).
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Figure 3b shows backscattering (BS) as a function of the measuring cell height of
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the 4wt% emulsion. Their reference mode plot was also included. On the contrary
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to the other emulsions, the backscattering increases with aging time in the middle
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zone of the vial. This fact suggest that a significant flocculation and/or coalescence
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process is taking place. In addition, a decrease in BS in the low zone of the vial is
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also observed. However, this decrease is cover up later by the increase of BS in the
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whole measuring cell (see figure 3b). This fact could be due to the coalescence
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and/or flocculation is more accused than the creaming.
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Figure 4a shows the variation of BS in the low zone of the measuring cell as a
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function of aging time, which is related to the creaming process. The results for
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4wt% emulsion were only shown until day 14 because later creaming is covered
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up by flocculation and coalescence (see inset fig 3b).
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2, 2.5 and 3wt% emulsions underwent slight changes of BS in the low zone of the
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measuring cell while 1.5wt% and 4wt% emulsion showed the greatest increase of
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BS.
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Emulsion containing lowest concentration of surfactant showed a higher trend to
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creaming as a consequence of the Stokes law since its continuous phase possesses
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the lowest viscosity. In addition 1.5wt% showed the highest Sauter diameter,
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therefore it led to accelerate the creaming process. As previously mentioned,
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emulsion 4% underwent depletion flocculation as destabilisation mechanism.
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These flocs may also accelerate the creaming process, which provokes higher
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changes of BS in the lower zone of measuring cell.
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Figure 4b shows the increase of droplet diameter from the diameter at time zero in
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the middle part of the measuring cell plotted as a function of aging time for
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emulsions with different surfactant concentration. No changes of droplet size
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emulsions associated with a coalescence/flocculation phenomenon were detected
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for emulsions with containing less than 3.5wt% of surfactant. By contrast, the
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emulsion with higher surfactant content exhibited significant changes of droplet
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size as a consequence of a destabilisation process by coalescence/flocculation.
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Emulsions with higher surfactant concentration underwent a destabilisation
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process by coalescence/flocculation and by creaming. By contrast, an incipient
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creaming process was detected for the emulsions with surfactant concentration
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between 2 and 3wt%. The emulsion with 3wt% of surfactant exhibited the lowest
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increment of BS at 20 days of aging time (see table inset figure 4a).
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Figure 5a shows the volumetric mean diameter as a function of both surfactant
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concentration and aging time. Palazolo (2005) previously postulated that the
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volumetric mean diameter allows for detecting coalescence process with more
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sensitivity than the Sauter mean diameter. Thus, the emulsions containing 2, 2.5
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and 3%wt of surfactant did not show any significant changes of droplet sizes. By
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contrast, some changes of droplet size were observed for emulsions containing
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more than 3wt% or less than 2wt%. A slightly change was detected for both 1.5
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and 3.5 emulsions while a substantial change was found for the 4wt% emulsion.
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This increase may indicate the occurrence of a destabilization phenomenon by
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coalescence or Ostwald ripening.
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In the case of 1.5wt%, the increase of droplet size could be due to a destabilisation
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phenomenon by coalescence. Coalescence becomes more important when drops
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are not fully covered with surfactant, as manifested by an increase of mean droplet
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size with time. (Nazarzadeh et al, 2013). This increase could lead to a oiling off
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process, which was observed by MLS.
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In addition, the increase of the droplet size in 3.5wt% emulsion could be attributed
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to a coalescence mechanism induced by a flocculation and/or creaming process
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previously mentioned since coalescence tends to occur after the droplets have
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been in contact for extended periods such as in a cream layer or in a floc
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(McClements, 2007).
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Moreover, emulsion with 4% showed the highest change of droplet size, which
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could be due to the destabilisation process by flocculation from the moment of its
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preparation.
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Figure 5b shows the droplet size distributions for the 4wt% emulsion at different
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aging time by way of example. The emulsions that showed increase of the size of
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droplets followed the same trend: an increase of the second peak with aging time
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was detected, which resulted in a reduction of the population with smaller size.
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This usually points to the occurrence of a destabilization process by coalescence
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discarding an Ostwald ripening phenomenon, as the latter would lead to a shift of
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the DSD toward larger sizes without changing their shape (Santos et al, 2014). For
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Ostwald ripening the particle size distribution should attain a specific time-
305
independent form that moves up the size axis with time, whereas with coalescence
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a bi-modal distribution is usually observed (McClements, 2007; Weers, 1998)
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Figure 6 shows the zero shear viscosity at different aging times for all emulsions.
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As all emulsions exhibited a trend to reach a Newtonian region at low-shear rate
309
regime, followed by a slight decrease in viscosity (shear-thinning behaviour) above
310
a critical shear rate, it has fairly well fitted to the Cross model with a R-square
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greater than 0.999. The showed zero shear viscosity is a fitting parameter of this
312
model.
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Emulsion 1.5wt% did not show any significant changes, which may be due to a lack
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of sensibility of the measurements since the value of the zero shear viscosity is
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very low to be measured in the rheometer. The presence of two opposing
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mechanisms such as coalescence and creaming/flocculation may be related to this
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fact.
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Coalescence would provoke a decrease in the zero shear viscosity and
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creaming/flocculation, an increase. Hence , the zero shear viscosity may level off.
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This latter case supports the results obtained by MLS and Laser Diffraction.
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On the contrary, the zero shear viscosity increases slightly with the aging time for
321
the emulsions 2wt%, 2.5wt%, 3wt% and 3.5wt%. This fact is related to an incipient
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flocculation and/or incipient creaming. 2wt%, 2.5wt% and 3wt% exhibited
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creaming process in MLS measurements whereas 3.5wt% showed creaming and
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coalescence/flocculation.
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In addition, zero shear viscosity decreases slightly from day 3 to day 21 for the
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4wt% emulsion, which is an indication of an increase of droplet size. However, an
327
increase of zero shear viscosity was detected from day 21 to day 40. (McClements,
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2005). These opposed trends could be explained by the existence of two different
329
destabilization processes. Coalescence and creaming/flocculation could be
330
simultaneously coexisting. The increase of η0 is related to flocculation or/and
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creaming whereas its decrease is related to a coalescence process. The previous
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coalescence could accelerate the rate of possible flocculation and/or creaming.
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These results supports the MLS and laser diffraction measurements.
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Conclusions
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The influence in DSD, rheological properties and physical stability in the range of
336
2-3 wt% was not really significant. However, 1.5wt% of surfactant is not enough to
337
cover the surface of the interface and it led to higher Sauter and volumetric mean
338
diameters. Consequently, this emulsion has the lowest zero-shear viscosity and the
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highest flow index.
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Emulsion containing above 3.5wt% of surfactant showed an accused depletion
341
flocculation process since its preparation. The combination of measurements of
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laser diffraction, flow curves and multiple light scattering at different aging times
343
showed the destabilization phenomenon in the emulsions in short period of time.
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These techniques have complemented each other leading to the conclusions:
345
1.5 wt% emulsion showed creaming as a predominant mechanism.
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2-3 wt% emulsions exhibited low creaming rates being 3wt% emulsion
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3.5-4 wt% emulsion showed flocculation, creaming and coalescence. 4wt%
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emulsion showed the major increase in the droplet size due to the depletion
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flocculation showed since its preparation.
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The emulsions in the range 2-3wt% were highly stable and this excellent result can
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be explained by considering that the emulsion prepared at intermediate surfactant
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concentrations showed enough viscosity to prevent creaming and cover the
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interface. Also, it is not excessive surfactant concentration that may lead to a
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depletion flocculation process.
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Acknowledgments
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The financial support received (Project CTQ2011-27371) from the Spanish
359
Ministerio de Economia y Competitividad, the European Commission (FEDER
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Programme) and from V Plan Propio Universidad de Sevilla is kindly
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acknowledged. The authors are also grateful to BASF and KAO for providing
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materials for this research.
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limonene – as extractant and immiscible diluent for large volume injection in the
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Biomedical Chromatography.27: 48-57.
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art. Green Chem. 7(5), 267-278.
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Trujillo-Cayado, L. A., Ramírez, P., Pérez-Mosqueda, L. M., Alfaro, M. C., & Munoz, J.,
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2014a. Surface and foaming properties of polyoxyethylene glycerol ester
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surfactants. Colloid Surface A. 458, 195-202.
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Trujillo-Cayado, L. A., Ramírez, P., Alfaro, M. C., Ruíz, M., & Muñoz, J., 2014b.
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Adsorption at the biocompatible α-pinene–water interface and emulsifying
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Figure captions
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Figure 1. Droplet size distribution for emulsions containing 1.5, 2, 2.5, 3, 3.5 and 4
445
wt% of surfactant.
446
Fig 2. Flow curves for the studied emulsions as a function of surfactant concentration for 1
447
day aging time at 20oC. Continuous lines illustrate data fitting to the Cross model.
448
Figure 3a. Backscattering versus measuring cell height as a function of aging time
449
in normal (main figures) and reference mode (insets) for 1.5wt% emulsion at 25oC.
450
Note: the insets illustrate DBS values at the bottom of the measuring cell.
451
Figure 3b. Backscattering versus measuring cell height as a function of aging time
452
in normal (main figures) and reference mode (insets) for 4wt% emulsion at 25oC.
453
Note: the insets illustrate DBS values at the bottom of the measuring cell.
454
Figure 4a. Variation of the BS in the low zone of the measuring cell as a function of
455
aging time for studied emulsions at 25oC.
456
Figure 4b. Increase of droplet size diameter from the diameter at time zero as a
457
function of aging time for studied emulsions.
458
Figure 5a. Volumetric mean diameter as a function of aging time for the emulsions 1.5, 2,
459
2.5,3, 3.5 and 4wt%.
461 462 463
cr
us
an
M
d
te
Ac ce p
460
ip t
444
Figure 5b. Droplet size distributions for the emulsions containing 4wt% of
surfactant as a function of aging time. Emulsions kept under storage at 20oC.
Figure 6. Zero-shear viscosity as a function of aging time for the studied emulsions. Note: Standard deviation of the mean (3 replicates) for η0<8%.
464 465 466
Tables with headings.
Page 20 of 31
Table 1. Sauter and volumetric diameters for the studied emulsions as a function of
468
surfactant concentration for 1 day aging time.
D4.3 (µm)
1.5
0.51
0.75
2
0.39
0.58
2.5
0.37
0.57
3
0.35
0.57
3.5
0.36
0.56
4
0.36
0.57
an
469
cr
D3.2 (µm)
us
wt% Surfactant
ip t
467
Table 2. Flow curves fitting parameters for the Cross model for studied emulsions as a function of surfactant concentration at 1 day of aging time.
472
Standard deviation of the mean (3 replicates) for η0<8%
473
Standard deviation of the mean (3 replicates) for c<10%
474
Standard deviation of the mean (3 replicates) for 1-n<10%
Ac ce p
wt% Surfactant 1.5 2 2.5 3 3.5 4
476
.
d
te
475
M
470 471
η0 (mPa·s) 15 24 26 29 30 66
c
(s-1)
25 12.5 12.5 16.7 12.5 3.12
n 0.70 0.57 0.57 0.40 0.40 0.30
477
Table 3. Continuous phase density and viscosity values at 20oC.
478
Note: These surfactant concentrations in the continuous phase are 1.5, 2, 2.5, 3, 3.5
479
and 4wt% in emulsions.
480
Page 21 of 31
20ºC 20ºC (kg/m3) (mPa·s) 1.0013 ± 0.0001 2.03 ± <0.01 1.0017 ± 0.0001 2.84 ± 0.01 1.0022 ± 0.0001 4.04 ± 0.02 1.0027 ± 0.0001 5.58 ± 0.05 1.0031 ± 0.0001 12.89 ± 0.02 1.0036 ± 0.0001 17.93 ± 0.08
ip t
Levenol C-201 concentration in continuos phase (wt%) 2.14 2.86 3.57 4.29 5.00 5.71
cr
481 482
us
483
Ac ce p
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M
an
484
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Figure 1
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Figure 2
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Figure 3A
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Figure 3B
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Figure 4A
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Figure 4B
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Figure 5A
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Figure 5B
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Figure 6
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S0263-8762(15)00188-4 http://dx.doi.org/doi:10.1016/j.cherd.2015.05.027 CHERD 1893
To appear in: Received date: Revised date: Accepted date:
19-1-2015 23-4-2015 21-5-2015
Please cite this article as: Santos, J., Calero, N., Mu˜noz, J.,Influence of the concentration of a polyoxyethylene glycerol ester on the physical stability of submicron emulsions, Chemical Engineering Research and Design (2015), http://dx.doi.org/10.1016/j.cherd.2015.05.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Research Highlights: 1. The combination of techniques provides relevant information of physical stability
3
2. Emulsions in the range 2-3wt% surfactant were highly stable.
4
3. Emulsion with 1.5 wt% surfactant showed creaming as a predominant mechanism.
5 6
4. Emulsions with 3.5-4wt% surfactant showed flocculation, creaming and coalescence.
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Page 1 of 31
7 8
Influence of the concentration of a polyoxyethylene glycerol ester on the physical stability of submicron emulsions
9 10
J. Santos1, N. Calero*1 and J. Muñoz1.
ip t
11 1Reología
Aplicada. Tecnología de Coloides. Departamento de Ingeniería Química.
13
Facultad de Química. Universidad de Sevilla c/ P. García González, 1, E41012,
14
Sevilla Spain.
15
* Corresponding author. Tel.: +34 954 557179; fax: +34 954 556447. E-mail
16
address: [email protected]
17
Abstract
18
The chemistry and technology of agrochemical products has undergone extensive
19
changes over the last 20 years. The new formulations and ingredients should meet
20
the needs of the agrochemical industry for products having greater safety to the
21
user and much lower environmentally impact maintaining the same performance
22
targets. A recent trend involves the use of the emulsion format for agrochemicals,
23
which provides a more efficient performance than those conventionally used.
24
Furthermore, the production of submicron stable emulsion is a key achievement
25
especially for this product.
26
This study has been focused on the development of fine emulsions containing
27
ecofriendly ingredients, such as surfactants and green solvents. It has been proven
28
that the optimal surfactant concentration not only may lead to emulsions with
29
submicron droplet sizes but also may prevent the typical destabilization process
30
occurring in these formulations. In this particular case, it has been demonstrated
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Page 2 of 31
that 3wt% surfactant concentration is adequate for three reasons: a) allowing the
32
lowest droplet size to be achieved, b) providing the sufficient viscosity to prevent
33
creaming and c) not being an excess of surfactant that leads to depletion
34
flocculation.
35
Keywords: green surfactant, physical stability, emulsions, rheology, multiple light
36
scattering.
cr
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31
1. INTRODUCTION
an
38 39
us
37
Emulsion science and technology has been used for many years to create a diverse
41
range of commercial products, including pharmaceuticals, foods, agrochemicals,
42
lubricants, personal care products, and cosmetics. Production of emulsion-based
43
systems with specific physicochemical and functional properties often requires
44
tight control over the particle size distribution (McClements, 2005). Type and
45
concentration of emulsifier play an important role in the droplet size distribution
46
(DSD).
47
The interest in submicron emulsions has increased in recent years due to their
48
very small droplet size and high stability and their applications in many industrial
49
fields such as personal care and cosmetics, health care, pharmaceuticals, and
50
agrochemicals. (Schultz et al, 2000; Sonneville et al, 2004; Leal-Calderon et al.,
51
2007; McClements, 2005; Tadros, 2009). These emulsions whose range in the DSD
52
falls typically of 100-500 nm are also sometimes referred to as ultrafine emulsions
53
(Nakajima, 1997) ,mini-emulsions (El-Aasser et al, 2004) and nanoemulsions (Ying
54
Tang et al, 2013). In addition, submicron emulsions can be prepared by reasonable
Ac ce p
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Page 3 of 31
surfactant concentrations (less than 10%) that may fulfil the requirements of a
56
bio-based society (Brockel et al, 2007).
57
There is a need to replace the traditional organic solvents by more
58
environmentally favorable solvents (Anastas and Warner, 1998.) Consequently,
59
the renewed interest in search of appropriate greener and alternative solvents to
60
be used in emulsions has grown enormously (Sheldon, 2005). Fatty acid
61
dimethylamides (FAD) are among green solvents that can find applications in
62
agrochemicals (Hofer and Bigorra, 2007). N,N-dimethyldecanamide (DMA-10) is
63
considered a safe biosolvent, according to the Environmental Protection Agency.
64
Therefore, it is a great solvent for agrochemical use due to the lack of risk to the
65
farmer. The fact of satisfying the needs of customers is the basic principle of the
66
product design (Brokel, 2007).
67
D-limonene, a naturally occurring hydrocarbon, is a cyclic monoterpene, which is
68
commonly found in the rinds of citrus fruits such as grapefruit, lemon, lime, and in
69
particular, oranges. D-limonene exhibits good biodegradability, hence it may be
70
proposed as an interesting alternative to organic solvents (Walter, 2010 and
71
Medvedovici et al, 2012). These solvents can meet the ever-increasing safety and
72
environmental demands of the 21st Century.
73
Environmentally friendly surfactants have attracted significant interest recently.
74
Polyoxyethylene glycerol esters derived from cocoa oil are non-ionic surfactants
75
obtained from a renewable source which fulfil the environmental and toxicological
76
requirements to be used as ecofriendly foaming and/or emulsifying agents, hence
77
their consideration as green surfactants (Castán and González, 2003). Their use in
78
detergents and personal care products is disclosed in several patents (Lutz, 2006;
79
Denolle,2011). Levenol C-201 was selected as emulsifier due to its great superficial
Ac ce p
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Page 4 of 31
and interfacial properties. Furthermore, α-pinene emulsions with Levenol C-201
81
showed better stability than emulsions containing its counterpart Levenol H&B
82
(Trujillo-Cayado, 2014a, 2014b)
83
The main objective of this work was the study of the influence of the surfactant
84
concentration (polyoxyethylene glycerol ester) on the physical stability of slightly
85
concentrated O/W emulsions formulated with a mixture of green solvents (N,N-
86
dimethyldecanamide and D-limonene). The optimum ratio of these solvents was
87
previously studied by Santos et al, 2014. A further goal was to achieve stable fine
88
emulsions, which may be used as matrices for incorporation of active agrochemical
89
ingredients. According to a recent study (Santos et al, 2014), the same strategy was
90
followed considering the combination of different techniques, which was proven to
91
be a powerful tool to provide very interesting information at an early stage about
92
the mechanisms of destabilization occurring in emulsions.
93
te
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2. MATERIALS AND METHODS
95
2.1. Materials
96
N,N Dimethyl Decanamide (Agnique AMD-10TM) was kindly provided by BASF. D-
97
Limonene was supplied by Sigma Chemical Company. The emulsifier used was a
98
nonionic surfactant derived from cocoa oil. Namely, a polyoxyethylene glycerol
99
fatty acid ester, Glycereth-17 Cocoate (HLB:13), received as a gift from KAO, was
100
selected. Its trade name is Levenol C-201TM. The safety data sheet provided by the
101
supplier reports a value for oral toxicity (LD50) higher than 5000 mg/kg of animal
102
in tests carried out with rats. It is interestingly to note that this value would be
103
3000 mg/kg for salt (Hollinger, 2005).
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RD antifoam emulsion (DOW CORNING®) was used as antifoaming agent. This
105
commercial product consists of an aqueous solution containing Polydimethyl
106
siloxane (<10 %w/w) and Dimethyl siloxane, hydroxyl-terminated (<10 %w/w).
107
Deionized water was used for the preparation of all emulsions.
108
2.2. Submicron emulsion development.
109
Emulsions containing 0.1 wt% antifoam emulsion, a variable surfactant
110
concentration and 30 wt% mixture of solvents (75wt% AMD-10/25wt% D-
111
Limonene) were prepared. The optimum ratio of solvents was previously studied
112
by Santos, 2014. The surfactant concentrations studied were 1.5, 2, 2.5, 3, 3.5 and
113
4wt%. These O/W emulsions were carried out using a rotor-stator homogenizer
114
(Silverson L5M), equipped with a mesh screen at 6000 rpm during 60 seconds.
115
2.3. Droplet size distribution measurements.
116
Size distribution of oil droplets was determined by laser diffraction using
117
Mastersizer X (Malvern, Worcestershire, United Kingdom). All measurements were
118
repeated 3 times with each emulsion. These measurements were carried out after
119
1, 3, 13, 21, 40 days aging time to analyze likely coalescence effects.
120
The mean droplet diameter was expressed as Sauter diameter (D[3,2]) and volume
121
mean diameter (D[4,3]).
cr
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Ac ce p
(Eq 1)
122
123
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104
2.4. Rheological measurements.
Page 6 of 31
Rheological experiments were conducted with a Haake MARS controlled-stress
125
rheometer (Thermo-Scientific, Germany), equipped with a sand-blasted coaxial
126
cylinder Z-20 (sample volume: 8.2 mL, Re/Ri =1.085, Ri= 1 cm) to avoid slip
127
effects. Flow curves were carried out from 0.05 Pa to 1 Pa at 20oC. Flow curves
128
were carried out after 1, 3, 13, 21 and 40 days to check the effect of aging time. All
129
measurements were repeated 3 times with each emulsion. Samples were taken at
130
about 2 cm from the upper part of the container. Sampling from the top part of the
131
container in contact with air was avoided.
132
2.5. Multiple light scattering
133
Multiple light scattering measurements with a Turbiscan Lab Expert were used in
134
order to study the destabilization of the emulsions. Measurements were carried
135
out until 40 days at 20oC to determine the predominant mechanism of
136
destabilization in each emulsion as well as the kinetics of the destabilization
137
process. Multiple light scattering is a sensitive and non-intrusive technique to
138
monitor physical stability of emulsions (Allende et al, 2008 and Camino et al, 2012)
139
and more complex systems such as suspoemulsions (Santos et al, 2013).
140
Multiple light scattering measurements in the middle zone of the measuring cell
141
also allowed the evolution of a mean droplet diameter with aging to be monitored.
142
2.6 Viscosity of the continuous phase.
143
The viscosity of continuous phases solutions (from 2.14 wt% to 5.71 wt%) were
144
measured with an Ubbelohde glass capillary viscometer. A volume of solution was
145
pipetted into the capillary viscometer, which was equilibrated at 20°C in a water
146
bath for 30 minutes prior to measurements. All the measurements were performed
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Page 7 of 31
147
at 20oC ± 0.1oC and the result is the average of five measurements. Viscosity is
148
obtained from the following equation:
C t
149
(Eq 2)
where is the viscosity of the continuous phase, C is a constant that depends of the
151
glass capillary, ρ is the density of the continuous phase and t is the time.
ip t
150
3. RESULTS AND DISCUSSION.
us
153
cr
152
154
Figure 1 shows the droplet size distribution of the emulsions with different
156
concentration of surfactant. All emulsions studied showed two populations of
157
droplets except for emulsion with 1.5 % surfactant. The first peak is below 1
158
micron and the second population is centred about three microns. This second
159
peak is probably due to recoalescence phenomenon induced by an excess of
160
mechanical energy-input. (Jafari et al, 2008; Santos et al, 2014)
161
A decrease of the surfactant content provoked the distributions to shift towards
162
greater droplet sizes. This fact is more pronounced for the emulsion containing
163
1.5wt% of surfactant. In the range 1.5-2.0 wt%, the surfactant available was not
164
enough to achieve the minimum droplet size that can be obtained by these
165
operating conditions.
166
Sauter and volumetric diameters for all emulsions are shown in table 1. All
167
emulsions possess submicron mean diameters. It could be due to the low
168
interfacial tension providing by the mixture of these solvents as reported by Santos
169
et al, 2014. The Sauter and volumetric mean diameters levelled off for surfactant
170
concentrations above 2.5 wt%.
171
Figure 2 shows the flow properties for 1 day-aged emulsions studied as a function
172
of surfactant concentration. All the emulsions exhibited a trend to reach a
173
Newtonian region at low-shear rate regime, which is defined by the zero-shear
174
viscosity, (η0). This range is followed by a slight decrease in viscosity (shear-
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Page 8 of 31
175
thinning behaviour) above a critical shear rate. Fig. 2 also illustrates the fitting
176
quality of the results obtained to the Cross model (R2 > 0.999). (Eq 3)
178
c
ip t
177
is related to the critical shear rate for the onset of shear-thinning response, η0
stands for the zero-shear viscosity and (1-n) is a parameter related to the slope of
180
the power-law region; n being the so-called flow index. For shear thinning
181
materials, 0 < n <1. A solid material would show n = 0, while a Newtonian liquid
182
would show n = 1.
183
The values of these parameters are shown in Table 2 as a function of the
184
concentration of surfactant. The emulsion at a low surfactant concentration of 1.5
185
wt% showed the lowest Zero-shear viscosity. It is related to the fact that this
186
emulsion showed the higher Sauter diameter. Zero-shear viscosity for the emulsions
187
containing from 2%wt to 3.5%wt showed no significant differences, even though a slight
188
tendency to increase with surfactant concentration may be observed. This is supported
189
by the fact that these emulsions showed similar Sauter diameter values.
190
A sudden increase in the zero-shear viscosity of the emulsion upon increasing the
191
surfactant concentration from 3.5 wt% to 4 wt% was observed. Given that DSD did not
192
change between 3.5 wt% and 4 wt% surfactant, this rheological change could be produced
193
by either, an enhanced viscosity of the continuous phase (due to increasing interactions
194
among micelles), or the occurrence of a stronger oil network due to depletion flocculation
195
(Palazolo et al, 2005; Manoj et al, 1998).
196
Table 3 shows the viscosity of the continuous phase prior emulsification. It is seen
197
that increasing surfactant concentration from 5 wt% to 5.71 wt% provokes
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Page 9 of 31
viscosity to increase from 12.89 to 17.93 mPa·s. Nonetheless, the increase in the
199
viscosity of emulsions from 3.5wt% surfactant concentration to 4 wt% is about
200
120%. Hence, the marked increase of the viscosity of the emulsions is more
201
influenced by a depletion flocculation phenomenon than to the viscosity of the
202
continuous phase.
203
Therefore, it indicates that the critical surfactant concentration at which depletion
204
flocculation of droplets occurs lies somewhere between 3.5 and 4wt%. Nowadays
205
it is well established that in presence of high concentration of surfactant or
206
polymer, the micelles can play an important role in the stability of the emulsions
207
(Dickinson et al, 1997). After interface saturation by the adsorbed surfactant,
208
micelles do not adsorb on the surfactant coated surface, and they can cause
209
attraction between drops by a depletion mechanism. Thus, when two droplets
210
approach in a solution of non-adsorbing micelles, the latter are expelled from the
211
gap, generating a local region with almost pure solvent. The osmotic pressure in
212
the liquid surrounding the particle pair exceeds that between the drops and
213
consequently forces the droplets to aggregate. (Napper, 1983). The depletion
214
flocculation could lead to a creaming and/or coalescence process.
215
This tendency is also showed in the critical shear rate. Nevertheless, the flow index
216
decreases with the concentration of surfactant. This is due to the increase of the
217
shear thinning character of emulsions. The emulsion with lowest surfactant
218
concentration is only slightly shear thinning, whereas emulsions with higher
219
surfactant concentrations are more shear thinning in nature.
220
Figure 3 shows the results of the physical stability study performed at room
221
temperature by a multiple light scattering technique. Figure 3a shows a plot of
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Page 10 of 31
backscattering versus container height of the sample at room temperature for
223
1.5wt% emulsion by way of example. This figure also includes an inset where
224
backscattering is plotted versus container height in reference mode (enlargement
225
of backscattering of the lower zone of the vial that contained the sample) to display
226
higher resolution of backscattering changes. 2, 2.5, 3 and 3.5% emulsions showed
227
the same behaviour.
228
A backscattering decrease in the lower and higher zones of the vial was observed
229
whereas it remained nearly constant in the intermediate zone. The drop in
230
backscattering observed at the bottom of the measuring cell clearly indicated the
231
occurrence of a destabilization mechanism by creaming, in that the dispersed
232
phase possessed a lower density than the continuous phase (McClements, 2005).
233
Thus all emulsions presented creaming process but in different degree.
234
The fact that the backscattering remained nearly constant in the intermediate
235
zone suggests that destabilization mechanisms such as flocculation and
236
coalescence were not much significant for these emulsions. Nevertheless, with
237
regards to the backscattering decrease observed at the top of the measuring cell,
238
we think it may be attributed to the occurrence of some coalescence as a
239
consequence of the migration of small droplets to the top (Mengual et al, 1999).
240
Figure 3b shows backscattering (BS) as a function of the measuring cell height of
241
the 4wt% emulsion. Their reference mode plot was also included. On the contrary
242
to the other emulsions, the backscattering increases with aging time in the middle
243
zone of the vial. This fact suggest that a significant flocculation and/or coalescence
244
process is taking place. In addition, a decrease in BS in the low zone of the vial is
245
also observed. However, this decrease is cover up later by the increase of BS in the
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Page 11 of 31
whole measuring cell (see figure 3b). This fact could be due to the coalescence
247
and/or flocculation is more accused than the creaming.
248
Figure 4a shows the variation of BS in the low zone of the measuring cell as a
249
function of aging time, which is related to the creaming process. The results for
250
4wt% emulsion were only shown until day 14 because later creaming is covered
251
up by flocculation and coalescence (see inset fig 3b).
252
2, 2.5 and 3wt% emulsions underwent slight changes of BS in the low zone of the
253
measuring cell while 1.5wt% and 4wt% emulsion showed the greatest increase of
254
BS.
255
Emulsion containing lowest concentration of surfactant showed a higher trend to
256
creaming as a consequence of the Stokes law since its continuous phase possesses
257
the lowest viscosity. In addition 1.5wt% showed the highest Sauter diameter,
258
therefore it led to accelerate the creaming process. As previously mentioned,
259
emulsion 4% underwent depletion flocculation as destabilisation mechanism.
260
These flocs may also accelerate the creaming process, which provokes higher
261
changes of BS in the lower zone of measuring cell.
262
Figure 4b shows the increase of droplet diameter from the diameter at time zero in
263
the middle part of the measuring cell plotted as a function of aging time for
264
emulsions with different surfactant concentration. No changes of droplet size
265
emulsions associated with a coalescence/flocculation phenomenon were detected
266
for emulsions with containing less than 3.5wt% of surfactant. By contrast, the
267
emulsion with higher surfactant content exhibited significant changes of droplet
268
size as a consequence of a destabilisation process by coalescence/flocculation.
269
Emulsions with higher surfactant concentration underwent a destabilisation
270
process by coalescence/flocculation and by creaming. By contrast, an incipient
271
creaming process was detected for the emulsions with surfactant concentration
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Page 12 of 31
between 2 and 3wt%. The emulsion with 3wt% of surfactant exhibited the lowest
273
increment of BS at 20 days of aging time (see table inset figure 4a).
274
Figure 5a shows the volumetric mean diameter as a function of both surfactant
275
concentration and aging time. Palazolo (2005) previously postulated that the
276
volumetric mean diameter allows for detecting coalescence process with more
277
sensitivity than the Sauter mean diameter. Thus, the emulsions containing 2, 2.5
278
and 3%wt of surfactant did not show any significant changes of droplet sizes. By
279
contrast, some changes of droplet size were observed for emulsions containing
280
more than 3wt% or less than 2wt%. A slightly change was detected for both 1.5
281
and 3.5 emulsions while a substantial change was found for the 4wt% emulsion.
282
This increase may indicate the occurrence of a destabilization phenomenon by
283
coalescence or Ostwald ripening.
284
In the case of 1.5wt%, the increase of droplet size could be due to a destabilisation
285
phenomenon by coalescence. Coalescence becomes more important when drops
286
are not fully covered with surfactant, as manifested by an increase of mean droplet
287
size with time. (Nazarzadeh et al, 2013). This increase could lead to a oiling off
288
process, which was observed by MLS.
289
In addition, the increase of the droplet size in 3.5wt% emulsion could be attributed
290
to a coalescence mechanism induced by a flocculation and/or creaming process
291
previously mentioned since coalescence tends to occur after the droplets have
292
been in contact for extended periods such as in a cream layer or in a floc
293
(McClements, 2007).
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Page 13 of 31
Moreover, emulsion with 4% showed the highest change of droplet size, which
295
could be due to the destabilisation process by flocculation from the moment of its
296
preparation.
297
Figure 5b shows the droplet size distributions for the 4wt% emulsion at different
298
aging time by way of example. The emulsions that showed increase of the size of
299
droplets followed the same trend: an increase of the second peak with aging time
300
was detected, which resulted in a reduction of the population with smaller size.
301
This usually points to the occurrence of a destabilization process by coalescence
302
discarding an Ostwald ripening phenomenon, as the latter would lead to a shift of
303
the DSD toward larger sizes without changing their shape (Santos et al, 2014). For
304
Ostwald ripening the particle size distribution should attain a specific time-
305
independent form that moves up the size axis with time, whereas with coalescence
306
a bi-modal distribution is usually observed (McClements, 2007; Weers, 1998)
307
Figure 6 shows the zero shear viscosity at different aging times for all emulsions.
308
As all emulsions exhibited a trend to reach a Newtonian region at low-shear rate
309
regime, followed by a slight decrease in viscosity (shear-thinning behaviour) above
310
a critical shear rate, it has fairly well fitted to the Cross model with a R-square
311
greater than 0.999. The showed zero shear viscosity is a fitting parameter of this
312
model.
313
Emulsion 1.5wt% did not show any significant changes, which may be due to a lack
314
of sensibility of the measurements since the value of the zero shear viscosity is
315
very low to be measured in the rheometer. The presence of two opposing
316
mechanisms such as coalescence and creaming/flocculation may be related to this
317
fact.
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Coalescence would provoke a decrease in the zero shear viscosity and
Page 14 of 31
creaming/flocculation, an increase. Hence , the zero shear viscosity may level off.
319
This latter case supports the results obtained by MLS and Laser Diffraction.
320
On the contrary, the zero shear viscosity increases slightly with the aging time for
321
the emulsions 2wt%, 2.5wt%, 3wt% and 3.5wt%. This fact is related to an incipient
322
flocculation and/or incipient creaming. 2wt%, 2.5wt% and 3wt% exhibited
323
creaming process in MLS measurements whereas 3.5wt% showed creaming and
324
coalescence/flocculation.
325
In addition, zero shear viscosity decreases slightly from day 3 to day 21 for the
326
4wt% emulsion, which is an indication of an increase of droplet size. However, an
327
increase of zero shear viscosity was detected from day 21 to day 40. (McClements,
328
2005). These opposed trends could be explained by the existence of two different
329
destabilization processes. Coalescence and creaming/flocculation could be
330
simultaneously coexisting. The increase of η0 is related to flocculation or/and
331
creaming whereas its decrease is related to a coalescence process. The previous
332
coalescence could accelerate the rate of possible flocculation and/or creaming.
333
These results supports the MLS and laser diffraction measurements.
334
Conclusions
335
The influence in DSD, rheological properties and physical stability in the range of
336
2-3 wt% was not really significant. However, 1.5wt% of surfactant is not enough to
337
cover the surface of the interface and it led to higher Sauter and volumetric mean
338
diameters. Consequently, this emulsion has the lowest zero-shear viscosity and the
339
highest flow index.
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Page 15 of 31
Emulsion containing above 3.5wt% of surfactant showed an accused depletion
341
flocculation process since its preparation. The combination of measurements of
342
laser diffraction, flow curves and multiple light scattering at different aging times
343
showed the destabilization phenomenon in the emulsions in short period of time.
344
These techniques have complemented each other leading to the conclusions:
345
1.5 wt% emulsion showed creaming as a predominant mechanism.
346
2-3 wt% emulsions exhibited low creaming rates being 3wt% emulsion
cr
us
which showed the greatest stability.
an
347
ip t
340
3.5-4 wt% emulsion showed flocculation, creaming and coalescence. 4wt%
349
emulsion showed the major increase in the droplet size due to the depletion
350
flocculation showed since its preparation.
M
348
The emulsions in the range 2-3wt% were highly stable and this excellent result can
352
be explained by considering that the emulsion prepared at intermediate surfactant
353
concentrations showed enough viscosity to prevent creaming and cover the
354
interface. Also, it is not excessive surfactant concentration that may lead to a
355
depletion flocculation process.
te
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d
351
357
Acknowledgments
358
The financial support received (Project CTQ2011-27371) from the Spanish
359
Ministerio de Economia y Competitividad, the European Commission (FEDER
360
Programme) and from V Plan Propio Universidad de Sevilla is kindly
361
acknowledged. The authors are also grateful to BASF and KAO for providing
362
materials for this research.
Page 16 of 31
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Figure captions
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Figure 1. Droplet size distribution for emulsions containing 1.5, 2, 2.5, 3, 3.5 and 4
445
wt% of surfactant.
446
Fig 2. Flow curves for the studied emulsions as a function of surfactant concentration for 1
447
day aging time at 20oC. Continuous lines illustrate data fitting to the Cross model.
448
Figure 3a. Backscattering versus measuring cell height as a function of aging time
449
in normal (main figures) and reference mode (insets) for 1.5wt% emulsion at 25oC.
450
Note: the insets illustrate DBS values at the bottom of the measuring cell.
451
Figure 3b. Backscattering versus measuring cell height as a function of aging time
452
in normal (main figures) and reference mode (insets) for 4wt% emulsion at 25oC.
453
Note: the insets illustrate DBS values at the bottom of the measuring cell.
454
Figure 4a. Variation of the BS in the low zone of the measuring cell as a function of
455
aging time for studied emulsions at 25oC.
456
Figure 4b. Increase of droplet size diameter from the diameter at time zero as a
457
function of aging time for studied emulsions.
458
Figure 5a. Volumetric mean diameter as a function of aging time for the emulsions 1.5, 2,
459
2.5,3, 3.5 and 4wt%.
461 462 463
cr
us
an
M
d
te
Ac ce p
460
ip t
444
Figure 5b. Droplet size distributions for the emulsions containing 4wt% of
surfactant as a function of aging time. Emulsions kept under storage at 20oC.
Figure 6. Zero-shear viscosity as a function of aging time for the studied emulsions. Note: Standard deviation of the mean (3 replicates) for η0<8%.
464 465 466
Tables with headings.
Page 20 of 31
Table 1. Sauter and volumetric diameters for the studied emulsions as a function of
468
surfactant concentration for 1 day aging time.
D4.3 (µm)
1.5
0.51
0.75
2
0.39
0.58
2.5
0.37
0.57
3
0.35
0.57
3.5
0.36
0.56
4
0.36
0.57
an
469
cr
D3.2 (µm)
us
wt% Surfactant
ip t
467
Table 2. Flow curves fitting parameters for the Cross model for studied emulsions as a function of surfactant concentration at 1 day of aging time.
472
Standard deviation of the mean (3 replicates) for η0<8%
473
Standard deviation of the mean (3 replicates) for c<10%
474
Standard deviation of the mean (3 replicates) for 1-n<10%
Ac ce p
wt% Surfactant 1.5 2 2.5 3 3.5 4
476
.
d
te
475
M
470 471
η0 (mPa·s) 15 24 26 29 30 66
c
(s-1)
25 12.5 12.5 16.7 12.5 3.12
n 0.70 0.57 0.57 0.40 0.40 0.30
477
Table 3. Continuous phase density and viscosity values at 20oC.
478
Note: These surfactant concentrations in the continuous phase are 1.5, 2, 2.5, 3, 3.5
479
and 4wt% in emulsions.
480
Page 21 of 31
20ºC 20ºC (kg/m3) (mPa·s) 1.0013 ± 0.0001 2.03 ± <0.01 1.0017 ± 0.0001 2.84 ± 0.01 1.0022 ± 0.0001 4.04 ± 0.02 1.0027 ± 0.0001 5.58 ± 0.05 1.0031 ± 0.0001 12.89 ± 0.02 1.0036 ± 0.0001 17.93 ± 0.08
ip t
Levenol C-201 concentration in continuos phase (wt%) 2.14 2.86 3.57 4.29 5.00 5.71
cr
481 482
us
483
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Figure 1
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Figure 2
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Figure 3A
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Figure 3B
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Figure 4A
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Figure 4B
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Figure 5A
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Figure 5B
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Figure 6
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