Study on the Formation and Properties of Liquid Crystal Emulsion in Cosmetic ()
1. Introduction
The liquid crystal emulsion is a new type of emulsion which is different from the traditional emulsion system. It is the ordered arrangement of surfactant and oil molecules formed at the oil-water interface, and this ordered arrangement makes the emulsion of liquid crystal structure showing better application performances than conventional emulsion systems in terms of stability, controlled release and moisturizing [1-3]. The formation of liquid crystal structure in emulsion depends not only on the composition of the emulsion formulations, but also the preparation processes [4,5].
The unique properties and the applications of liquid crystal emulsion have attracted many researchers in pharmaceutical and cosmetic product to do extensive studies on the special structure emulsion, including preparation and properties. Many specialty chemicals suppliers have produced emulsifiers, e.g. many non-ionic surfactants such as alkyl glycosides, polyglycerol esters, phosphates etc., for preparing emulsions with liquid crystal structure. At present, the studies of liquid crystalemulsion mainly focus on the theory of special systems, such as aligned structure of emulsifier molecules, stability and rheological behavior, as well as the examples of practical applications of emulsion systems [6,7].
In this paper, the formation and transformation of liquid crystal structure in preparation, storage and usage were investigated. Meanwhile the rheological behavior has been studied to establish the correlation between the skin sensory characteristics and the rheology properties.
2. Experimental Section
2.1. Experimental Materials and Equipments
C16-18 APG, 99% (Cognis); Steareth-21, 99% (Croda); sodium stearoyl glutamate, 99% (Cognis); Cetearyl alcohol, 99% (Cognis); caprylic/capric triglyceride, 99% (Cognis); stearic acid, 99% (Cognis); Mineral oil, 99% (Hangzhou Refinery); glycerol 99% (Sinopharm Chemical Reagent Co., Ltd.); Dimethicone, 99% (Dow Corning); 1,3-dihydroxy-5,5-DMH (Lonza).
FA25 High-speed shearing machine (Fluko); ECLIPSE E200 polarizing microscope (NIKON).
2.2. Experimental Methods
2.2.1. Preparation
The formulation of emulsions comprise of oil phase, aqueous phase and other components. The oil phase includes oil and alkyl polyglycoside emulsifiers. And the aqueous phase includes deionized water and moisturizing agents. Appropriate flavors and preservatives are examples of other components. A typical formulation looks like the following: 3% emulsifier of alkyl polyglycosides, 2% stearic alcohol, 3% caprylic/capric triglyceride, 5% mineral oil, 5% dimethicone, 5% glycerin, 0.2% preservative, 0.2% flavor, 75.6% deionized water (w/w unless otherwise indicated).
Emulsions were prepared following the typical procedures used for preparing O/W emulsions, i.e. the aqueous phase (deionized water and moisturizing agents) was heated up to 80˚C in a glass beaker. Meanwhile the oil phase was heated up to 80˚C in another glass beaker. The oil phase was added to the aqueous phase followed by homogenization with F25 Ultraturrax at 13,000 rpm for 3 min. The sample was allowed to cool down to room temperature under moderate stirring at 150 rpm, and then stored at room temperature.
2.2.2. Polarized Optical Microscope
The samples were observed with a microscope (NIKON ECLIPSE E200). For sample preparation, a pin-tip amount of the emulsion was smeared on the microscope glass slide and then quickly covered by the cover slip. The sample was finger pressed to make it as thin as possible. A 40× objective lens and 10 40× eye lens were used with cross polarizers in bright field to detect birefringence. The micrograph was taken under polarizing microscope.
2.2.3. Rheological Measurements
Steady-flow, thixotropy and dynamic viscoelastic properties were measured with a cone-and-plate geometry on a rheometer controlled by stress (TA Instruments Co., Ltd. AR-2000N). The cone diameter was 40 mm. The shear rates were from 0.0001 to 200 s−1 in steady-flow and thixotropy property measurements. The dynamic viscoelasticity was measured as a function of the frequency at a small strain in the linear regions and as a function of strain at a constant frequency. The angular frequencies were from 0.1 to 100 s−1 and the strain amplitude was 0.1%. The measuring temperature was 25˚C.
2.2.4. Moisturizing Property
The water content and the Transepidermal Water Loss (TEWL) on the skin surface were measured by Corneometer. The measurement was carried out in a room with controlled humidity (40%) and temperature (22˚C ± 1˚C).
3. Results and Discussion
3.1. Formation of Liquid Crystal Structure Emulsion
Emulsions with good stability can be obtained by using a mixture of emulsifiers with preparation processes described in this paper. The formation of liquid crystal structure, however, shows strong dependence on the composition of formulation and preparation process [6, 7]. By optimizing the composition of formulation and preparation process, the liquid crystal emulsion can be formed. In the experiment, the formation process of the liquid crystal structure during preparation of emulsion was studied. The polarized microscope was utilized to monitor the sample during emulsification and homogenization. Photos were taken under polarized light and at 10 min, 20 min after homogenization and the end of the preparation, respectively.
Figure 1 shows that a small amount of liquid crystal structure was present, though disorderly, in the emulsion droplets during the emulsification and homogenization process. Photos taken 10 and 20 minutes after homogenization show increasingly higher amount of liquid crystal structure at the oil-water interface. Furthermore, the photo taken after homogenization and cooling show well developed liquid crystal structures at the interface. Therefore, it could be concluded that the liquid crystal structure of oil-water interface in the emulsion is gradually formed with cooling after homogenization.