УДК 541.18.051.3
FORMATION OF ORANGE OIL-IN-WATER NANOEMULSIONS USING NONIONIC SURFACTANT MIXTURES BY HIGH PRESSURE HOMOGENIZER
© 2010 г. Loretta R. Kourniatis, Luciana S. Spinelli, Carolinne R. Piombini, Claudia R. E. Mansur
Federal University of Rio de Janeiro, Institute of Macromolecules — Centro de Tecnologia, bloco J — Ilha do Fundгo, Rio de Janeiro, Brazil 21945970 E-mail: spinelli@ima.ufrj.br; celias@ima.ufrj.br Поступила в редакцию 19.06.2009 г.
The formation of nanoemulsions depends on the size of the droplets formed, the polydispersity and the difference in solubility and/or chemical potential between the small and large droplets. This article reports experiments to evaluate the formation of orange oil/water nanoemulsions in the presence of mixtures of nonionic surfactants, prepared in a high-pressure homogenizer. The surfactant mixtures were prepared to have different HLB values, by varying their type and concentration. The formation and stability of the nanoemulsions were evaluated as a function of the surfactant mixture used and also the processing conditions in the homogenizer. The size and distribution of the droplets formed, along with their stability, were determined in a Zetasizer Nano ZS particle size analyzer. The results showed that the optimal HLB range of the surfactant mixtures to obtain stable o/w nanoemulsions, independent of the processing conditions, is between 11 and 12. Better results were obtained with Unitol®L20/Uni-tol®L100 mixtures, in which the hydrophobic surfactant causes a reduction in the interfacial tension and the hy-drophilic surfactant promotes steric stabilization of the system.
1. INTRODUCTION
Nanoemulsions are dispersions where the size of the dispersed droplets is on a nanometric scale. In general, the most widely accepted definition of a nanoemulsion is that the great majority of the droplets have radii smaller than 100 nanometers [1], although other authors consider a greater size range, between 10 and 500 nm [2].
Nanoemulsions have some interesting physical properties that distinguish them from macroemulsions. For example, nanoemulsions have a much greater surface area ofthe dispersed phase in relation to the total volume of the dispersion than macroemulsions. Therefore, the phenomena related to droplet deformation are typically more pronounced in nanoemulsions than in other emulsions [1].
The small droplet size in a nanoemulsion gives it stability, avoiding sedimentation (creaming). Ostwald ripening is the main mechanism for destabilization of na-noemulsions, caused by polydispersion and a difference in solubility and/or chemical potential between small and large droplets [1-5].
Energy is necessary to form nanoemulsions. This energy is generally provided by mechanical devices or the chemical potential of the components [6]. Sufficient mechanical energy to promote a shear rate able to deform the droplets is usually attained by high-pressure homog-enizers or ultrasound generators. The application of high energy generates forces that can rupture the droplets of the dispersed phase, so that the difference between the internal and external pressures is overcome (Young-Laplace Law) [6, 7]. However, nanoemulsions can also be obtained by altering the physical-chemical properties
of the system. These methods are generally known as low-energy emulsification methods [8] and they are advantageous because they use the energy stored within the molecular aggregates, formed by surfactant molecules present in the emulsion, by changing the properties of the medium (temperature and conductivity) or by variation of oil and water fraction. In these systems the process can be transitional or catastrophic, respectively [9-11].
In order to produce nanoemulsions using high-energy emulsification process, with homogenizing pressures of up to 2500 bar nowadays mean droplets diameters less than 0.2 ^m are achieved. High-pressure systems can be subdivided into radial diffusers, counterjet dispergators, and axial nozzle aggregates, depending on the flow guidance. They generally consist of a high-pressure pump, mostly in the form of a one-to three-piston plunger pump which can be electrically or pneumatically actuated, and the actual high-pressure dispersion unit [12-14].
Some authors indicate that while it is not absolutely necessary, the production ofnanoemulsions generally uses mixtures of nonionic surfactants and does not require any other type of interfacial agent (co-surfactants and hydrotropes). The masses of the oil and water used in the process are very near, while the surfactant quantity is in the range of 3 to 10% of the mixture's total mass. This permits generating large emulsion volumes using low amounts of surfactants, since the nanoemulsion also maintains its particle size even when greatly diluted. The surfactant systems often used to produce nanoemulsions are combinations of surfactants that have HLB values above and below the ideal range (10-12) [15].
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Table 1. Unitol®L line surfactants [10]
Name Structure (a) HLB value (b)
Unitol L20 ethoxylated lauryl ether with 2 EO units 6.4
Unitol L100 ethoxylated lauryl ether with 10 EO units 13.9
Unitol L230 ethoxylated lauryl ether with 23 EO units 16.9
(in grams) necessary of each surfactant to obtain systems with the desired HLB values followed Eq. (1)
HLBd — HLBb
(a) Confirmed by 1H NMR.
(b) Data supplied by the manufacturer.
Table 2. Orange oil/aqueous solutions of surfactants interfacial tensions
Surfactant systems HLB value Interfacial tension (mN/m) (a)
8 wt % (b) 10 wt % (b)
Unitol L20 6.4 (c) (c)
Unitol L100 13.9 0.8 0.8
Unitol L230 16.9 1.8 1.8
Unitol L100 10.0 0.8 0.6
and 11.0 (c) (c)
Unitol L20 12.0 (c) (c)
Unitol L230 10.0 1.8 1.8
and 11.0 1.8 1.7
Unitol L20 12.0 1.7 1.6
(1)
(a) Orange oil/water interfacial tension = 5.4 mN/m.
(b) Concentration of surfactant in the dispersion.
(c) "Vklues of interfacial tensions below 0.8 mN/m.
In this study, we prepared orange oil-in-water na-noemulsions with the presence of mixtures of nonionic surfactants, by varying the type and HLB values of the surfactant mixtures, in a high-pressure homogenizer under different processing conditions.
2. EXPERIMENTAL 2.1. Materials
The nonionic surfactants used, obtained from Ox-iteno (Brazil), were based on ethoxylated lauryl ether (Unitol® L line), with different quantities of ethylene oxide (EO) units in their chains. The oil phases of the o/w emulsions were composed of orange essential oil, obtained from All Flavors (Brazil). Distilled and deionized water was used as the aqueous phase.
Preparation of the mixtures of commercial surfactants. The o/w emulsions were obtained with the presence of commercial nonionic surfactants in the aqueous phase. These were prepared to obtain systems with HLB values between 10 and 12. Determination of the quantity
Ma
HLBa — HLBb
where Ma — mass of surfactant a (kg), HLBd — HLB desired, HLBa— HLB of surfactant a, HLBb - HLB of surfactant b.
The mass calculated of each surfactant was then weighed directly in a transparent flask with the aid of a graduated pipette. The mixtures were homogenized for approximately 600 s, under magnetic stirring, at a temperature of 308 K.
2.2. Methods
The surfactants were characterized by hydrogen nuclear magnetic resonance (1H-NMR), to quantify the number of ethylene oxide units (EO) (Table 1) and also by measurement of the water-oil interfacial tension.
The nanoemulsions were prepared in a high pressure homogenizer (HPH). The stability of the emulsions produced was evaluated by particle size and size distribution analyses with the aid of a Zetasizer Nano ZS particle size analyzer (Malvern Instruments).
Hydrogen nuclear magnetic resonance. The 1H-NMR analyses were performed in a Varian Unity 300 1H-NMR spectrometer, operating at 300 MHz and 303 ±0.1 K. Deuterated chloroform was used as a solvent. The number of EO units was calculated based on the area ratio of the signs undergoing chemical shift of 0.83 ppm (relative to the hydrogen of the CH3 end of the molecule) and 3.5 ppm (relative to the hydrogen of the two CH2 groups ofthe EO structure and a CH2 group of the hydrocarbon chain near the oxygen). Moreover, from the identification of each region of the surfactants specters and with the relative areas of each peak it was possible to calculate the number of hydrogen presents in each part of molecules.
Measurement of water/oil interfacial tension. The water/oil interfacial tension values (in mN/m) were measured using a Kruss K10 digital tensiometer, based on the Du Nouy ring method, at a temperature of 298 K.
Aqueous solutions of the surfactants at concentrations of 8 and 10 wt % were prepared without any subsequent dilution. The oil/aqueous solution interfacial tension measures were done for several compositions to observe the behavior of these compounds at the concentrations used for formation of nanoemulsions.
All measurements were performed at least in triplicate. The interfacial tension values are shown in Table 2.
Preparation of the oil/water emulsions in the HPH. The high-pressure homogenizer used to prepare the o/w emulsions was an Emulsiflex C5. The dispersion is forced through a very narrow section and then allowed to expand virtually instantaneously, generating a partition in the dispersed droplets. This causes the diameters of the
droplets generated to decline systematically with the increase in the shear rate. Because the stream is not homogenous, it is generally necessary to pass the fluid through the homogenizer several times, or cycles, until the desired droplet size is obtained.
Before preparing the o/w emulsions, we determined the times ofeach processing cycle, in seconds, for passage of a defined sample volume at a determined pressure.
All the o/w emulsions were prepared containing a fixed oil phase concentration of 15 wt % and either 8 or
10 wt % of the nonionic surfactant mixtures.
Evaluation o
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