Influence of the oleic phase and co-surfactant addition in non-ionic microemulsified systems

Microemulsion is a thermodynamically stable dispersion consisting of an aqueous and an organic phases, both stabilized by surfactant molecules and when in need, co-active surfactant. The nature and structure of these components are essential in the formulation of microemulsified systems. For this, the construction of phase diagrams can be a fundamental tool to characterize the ideal experimental conditions for the existence and operation of microemulsions. Thus, the present work had as objective to obtain a comparison between microemulsions with different compositions through the construction of ternary diagrams, aiming to achieve the most stable system. To produce microemulsified systems, a non-ionic surfactant (Ultranex NP 60), a co-surfactant (Isopropyl Alcohol), two organic phases (pine oil and castor oil) and an aqueous phase (glycerin solution) were used. Also complementing the study, rheological tests of the oleic phases were accomplished, as well as their thermogravimetric analysis. The focus of the reached ternary diagrams was to find the system with the largest Winsor type IV region (microemulsion). It was verified this region had a significant increase by the addition of the co-surfactant in the medium and using a vegetable oil, such as pine oil, since it promotes strong surfactant-oil interactions on the interface.

The surfactant is a type of molecule that has a part with an apolar characteristic linked to another part with a polar character. Thus, this type of molecule is polar and non-polar at the same time (Formariz, 2004). In association, the addition of co-surfactants, such as short-chain alcohols (eg., methanol, isopropanol, or n-butanol), in an optimal amount, as well as ideal levels of salinity, together with the effect of the composition of the oleic phase, may lead to a microemulsion system's greater capacity of solubilization (Pietrangeli, & Quintero, 2013).
For the spontaneous formation of microemulsified systems, the interfacial tension must be close to zero. This system has some advantages such as transparency, high stability, easy preparation, and the ability to minimize the interfacial differences between water and oil (Thomas, 2004).
A classification system that defines the various equilibriums existing between the microemulsion and the aqueous and oleic phases was proposed by Winsor, 1948. Four types of systems were established: 1) Winsor I-It is represented by the balance between the ME phase and the excess oleic phase; 2) Winsor II-Represents the balance between the ME phase and the excess aqueous phase; 3) Winsor III-It is characterized by a three-phase system, where the microemulsion is in balance at the same time with an aqueous and an oil phase; 4) Winsor IV-It is a system in which only the microemulsion phase exists, that is, a macroscopically monophasic system (Winsor, 1948;Daltin, 2011).
The nature and structure of the surfactant, co-surfactant and oleic phase are paramount in the formulation of microemulsified systems. The construction of phase diagrams can be a fundamental tool to characterize in which experimental conditions the ME exist and in what proportions of the components other structures may be present, allowing to select the region of the phase diagram that represents the most appropriate condition for the objective of your research (Formariz, 2004).
No further delay, this work consists of a comparative study of the composition of microemulsions, differentiated by the organic and surfactant components of their structure, with the objective of finding a more suitable microemulsified system through the incorporation of a co-active agent in the system.

Methodology
For the construction of the ternary diagrams, the following were used: pine oil or castor oil to compose the oleic phase; as the aqueous phase, a 1:1 glycerin-water mixture; a Ultranex NP 60 surfactant; in addition to the commercial Isopropyl alcohol P.A. from the producer Neon as co-surfactant.

Obtaining Ternary Diagrams
For the construction of the diagram, 0.5 g of two components -aqueous phase, oleic phase, or surfactant/co-surfactant -were used with known mass proportions and added in a test tube. Then, the titration with the third component started, with first manual agitation in a vortex mixer 2800 rpm, ending in an analog centrifuge Centrilab 80-2B-15 mL, until the appearance of a permanent uniform region, and then the mass of the third component was determined and, subsequently, the mass fraction.
Mass fraction calculations are made by the ratio between the mass of each substance, surfactant/co-surfactant, oleic phase, and aqueous phase, by the mass of the entire system, as shown in Equations 1-3.
Where, Mtotal is the total mass of the system; mT the mass of the surfactant/co-surfactant; mFo mass of the oleic phase; and mFa mass of the aqueous phase.
With the results of mass percentage of each component, these were plotted in Origin to obtain the ternary diagrams.
This procedure was repeated for different mass proportions of both initial components in order to close the diagram curve.

Rheology
The pine and castor oils rheological tests were determined using the Brookfield DVIII Ultra rheometer. The samples were placed in the rheometer container and subjected to sufficient torque to keep the CPE-52 spindle rotation immersed in the samples. The rotation range was from zero to 90 rpm, at temperatures from 30 to 70 °C, varying every 10 °C.

Thermogravimetric Analysis
The analyses were performed on a thermogravimetric scale for samples of glycerin, pine and castor oils. The materials were heated on a TGA Q50 balance initially at 25 ºC (room temperature) to 500 °C with a heating rate of 10 °C/min under an inert nitrogen atmosphere. Then, the TGA curves of the studied samples were obtained, which relate the loss of continuous mass with the temperature increase.    Table 1 shows the mass fractions of the points shown in Figure 5. The comparative procedure of the co-surfactant influence on the system was repeated using castor oil. However, initially, for a system with the aqueous phase 1:1 water and glycerin mixture and the use only of the NP 60 surfactant, for the different components proportions used, the result was mostly emulsion points, disqualifying the identification of a WIV region for the system.

Ternary Diagrams
The difference between emulsion and microemulsion is not only in thermodynamic stability, but also in the structural size of the micelle. While the region characterized by microemulsion is microscopically homogeneous composed of a single phase, the emulsion region has larger size and unstable micelles (Lif, A. & Holmberg, K., 2006). Figure 6 shows the ternary diagrams obtained for the tests with castor oil; next to the surfactant NP 60, with ( Fig. 6.a) and without (Fig. 6.b) isopropyl alcohol (as co-surfactant) in the C / T = 1.0 mass ratio; in addition to, for both cases, the same aqueous phase previously used with pine oil. The WIV microemulsion region is visible in the first case, in contrast to the castor oil system using only the NP 60 surfactant, but it is much smaller when compared to the system in figure 4 with pine oil.   Table 2 shows the mass fractions of the points shown in Figure 7. Castor oil is obtained from the seed of Ricinus communis plant and has atypical chemical characteristics when compared to most vegetable oils, because in addition to the presence of the ricinoleic acid triglyceride, which is an unusual hydroxylated fatty acid in vegetable oils, this acid is present in a range of 84.0% to 91.0% of its composition (Cangemi et al., 2010). The molecular structures of ricinoleic acid and ricinoleic acid triglyceride are shown in Figures 1 and 2 in Section 2.1.
Pine oil is the essential oil of pine tree and has in its composition a variety of terpenes, the main component being the alpha-Terpineol (Cangemi et. al, 2010;Huang et al., 2016), whose structure is shown in Figure 3 in Section 2.1.
The largest microemulsion WIV region found by the use of pine oil is possibly due to the fact that oil molecules with small molecular volume (short chain hydrocarbon) or high polarity (increased aromaticity), promote surfactant-oil strong solvation effects on the interface. In contrast, the increase in the length of the oil chain, as in the case of castor one, leads to a reduction in the interactions between the microdroplets, decreasing the solubilization of the microemulsion (Leung & Shah, 1987;Garnica et al., 2020).

Oleic Phase Rheology
The results of viscosity (cP) as a function of the shear rate (s -1 ) for both oils are shown in Figures 8 and 9.   According to the data presented in Figures 8 to 11, it was possible to classify pine and castor oils as Newtonian fluids at all temperatures evaluated (30, 40, 50, 60 and 70 °C).
Newtonian fluids are characterized by the proportionality between shear stress and shear rate, so their viscosity is unique and absolute for a given temperature, not depending on the speed gradient (Machado, 2002). In addition, it was possible to observe that the viscosity is strongly influenced by the temperature, because with the increase of the temperature, the viscosity decreased, since the molecules agitation in a disordered way increases the distance between the adjacent micelles. Figure 12 shows the curves obtained by thermogravimetric analyzes of glycerin, pine and castor oils. In the graphs, it is possible to highlight the phenomenon of mass loss and relate it to their respective temperatures. The difference in levels that appear between castor oil and the other organic materials for all temperatures is linked to its large carbon chain, having only a small fraction that decomposes at temperatures below 350 ºC (Howell & Ostrander, 2019).

Conclusion
The test results suggest that the addition of the Isopropyl alcohol co-surfactant to the NP 60 ethoxylated noniphenol surfactant was positive for obtaining larger microemulsion regions, regardless of the other components. Not less important, pine oil, in turn, showed better adherence than castor oil as an organic phase due to the large molecular volume of the latter, which makes it less reactive. In addition, pine oil is a low-cost and easily degradable vegetable oil. Therefore, the 1:1 glycerin and water mixture, pine oil, NP 60 and Isopropyl alcohol system presented the best classification for the consequent formulation of oil drilling fluids.