(401i) A Multi-Scale Approach to Evaluate the Relationship between Rheological and Textural Properties of Oil-in-Water Cosmetic Emulsions

The production of higher value-added products and consumer-oriented products is a major concern in the chemical industry as a result of market globalization and competitiveness[1]. Consumer-oriented products are used in a wide variety of industrial sectors such as the pharmaceutical, food, personal care, and cosmetic [2]. A major volume of products obtained from those sectors are produced as emulsions [3]. In particular, the cosmetic industry designs emulsified products to satisfy the consumer's sensory expectation since the consumers' selection often relies on product functionality[4]. Hence, it is of significant interest to understand the relationship between the sensations experienced by the consumer and instrumental characterization of the product properties (e.g., rheological and textural characterization).

Previous studies have shown the links between the macroscopic behavior of emulsified systems (i.e., rheology) and their corresponding microscopic structure (i.e., drop size distribution and mean drop size) [5][6][7]. However, these relationships have not been established for emulsified cosmetic products. Thus, it is critical to gain insights into how the product properties at the macroscopic and microscopic scales are correlated and impact product formulation and process operating variables. To understand those relationships between macroscopic properties (i.e., rheology and texture) and microscopic properties (i.e., droplet size and droplet size distribution) for emulsified cosmetic products, the energy consumed during the emulsification process and product formulation needs to be considered. A multiscale approach is a suitable tool to study emulsified cosmetic systems since it can be used to gain insight on the interconnection between the macroscopic and microscopic properties of emulsified products[8].

Often, heuristic rules are frequently used in the design of emulsified cosmetic products. These rules aim to establish the functionality and concentration of some components in the emulsion such as the thickening agents[9]. For instance, previous studies have set a thickener concentration of 1%w/w as a rule of thumb for the design of cosmetic creams[3][10][11]. However, it is not completely understood how to select the thickeners’ concentration and type in the design of an emulsified cosmetic product. On the other hand, previous reports have highlighted the influence of synthetic (e.g., Carbopol) and natural (e.g., Xanthan gum) thickening agents on the textural perception of the product[10]. Although synthetic and natural thickeners generate different microstructures in emulsified systems[12], few studies have correlated the rheological and textural properties of emulsified cosmetic systems[4] to the product microstructure. To the authors’ knowledge, a study relating the rheology and texture of emulsified cosmetic products to their corresponding microstructure, operating variables, and product formulation (e.g., thickeners’ concentration and type) has not been published in the open literature. Those relationships can provide insights on how the product formulation, product properties, and process variables are correlated thus offering a first approach for the efficient and systematic design of emulsified cosmetic products. Those correlations may also provide a deeper understanding of the consumer sensory experience, translating the consumer perception into engineering variables that could be characterized under controlled conditions (e.g., rheological and textural measurements). Accounting for these correlations may represent a reduction in both experimental resources and time for product commercialization.

The present study aims to identify the influence of the product formulation and emulsification energy on the macroscopic (rheology and texture) and microscopic (droplet size distribution) properties of cosmetic emulsions. In particular, this study correlates cosmetic emulsions' rheological and textural properties as a first attempt to link high-controlled instrumental characterization with the sensations that the consumer may experience. Those correlations can provide insights into the role of the thickener on the consumer's perception, which may represent a suitable tool for decision making during the design process. Moreover, this work evaluates the influence of emulsification energy and product formulation on the emulsion's physical stability. To study this phenomena, oil-in-water (O/W) cosmetic emulsions were manufactured varying the thickener type (i.e., Carbopol and Xanthan Gum) and concentration (i.e., 0.1%, 0.73%, 1.37%, and 2% w/w), the dispersed phase concentration (i.e., 10%, 20%, and 30% w/w), and the agitation rate (500, 900, and 1700 rpm). These scenarios were established according to conventional cosmetic formulation proposed in literature[3][10][13].

Results from this study show that the emulsion's macroscopic and microscopic properties are dominated by the thickener type and concentration. The drop size distribution (DSD), rheology, and texture of the prepared cosmetic emulsions remain unaffected once a given thickener concentration is reached. This behavior indicates that there is a critical thickener concentration for the design of emulsified cosmetic products, which depends on the specific thickener used to formulate the cosmetic emulsion (e.g., Carbopol, Xanthan Gum). Our results show that the cosmetic emulsions tested in this work were stable. The cosmetic emulsions’ physical stability was improved as the thickener concentration is increased in the formulation. This is mostly due to the polymeric matrix that the thickener generates in the continuous phase of the emulsion. The results also show that changes in the agitation rate and dispersed phase concentration do not affect significantly the product properties when thickener is added into the product. These results indicate that the thickener type and concentration have a strong influence on cosmetic emulsions’ sensory performance. Cosmetic emulsions prepared using a natural thickener exhibit notable differences in their DSD, rheological, and textural properties compared to those prepared using a synthetic thickener. This is mostly due to the particular polymeric matrix that natural and synthetic thickeners generate, which causes different rheological and textural behavior of the cosmetic emulsions[10]. Since cosmetic emulsions prepared using thickeners of the same type present similar textural and rheological properties (i.e., natural or synthetic) [10], the correlations proposed in this study could be extended to similar emulsion systems (e.g., moisturizing creams, hand lotions.). Also, the correlations identified in this work can be potentially used to make decisions regarding formulation, product properties, and process variables during the early stages in the design of emulsified cosmetic products. As future work, we aim to expand the analysis for additional formulation scenarios to gain insights on the role of other components such as emollients, exfoliants, surfactants, and humectants.

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