Sensory and instrumental characterization of fast inverting oil-in-water emulsions for cosmetic application

Enhanced stability, formulations, and rheological properties of nanoemulsions produced with microfludization for eco-friendly process

HYPOTHESIS

Eco-friendly processes that are emerging around the world require mass production of low-energy, low-cost nanoemulsions. The process involving the high-concentrated nanoemulsions and diluting them with a large amount of solvent can certainly save the cost; however, not much detailed research has been conducted on the stability mechanism and rheological characteristics of high-concentrated nanoemulsions.

EXPERIMENTS

In this study, we produced nanoemulsions via the microfluidization (MF) process, comparing their dispersion stability and rheological characteristics with macroemulsions across various oil and surfactant concentrations. Droplet mobility and dispersion stability depended on these concentrations, with Asakura-Osawa-type attractive depletion considering interparticle interaction's role in stability changes. We investigated nanoemulsions' long-term stability based on turbidity and droplet size changes over four weeks, proposing a stability diagram showing four different states depending on emulsification conditions.

FINDINGS

We explored the microstructure of emulsions under varying mixing conditions, observing their effects on droplet mobility and rheological properties. We monitored changes in rheology, turbidity, and droplet size over 4 weeks, establishing stability diagrams for macro- and nanoemulsions. The stability diagrams revealed that the stability of emulsions are sensitively dependent on the droplet size, concentrations, surfactant cocentrations and the strcture of coexistent phases in case of macroscopic segregation are significantly different depending on the droplet sizes. We identified their respective stability mechanisms and discovered the relationship between stability and rheological properties for highly concentrated nanoemulsion.

Efficient Development of Green Emulsifier/Emollient-Based Emulsion Vehicles: From RSM Optimal Experimental Design to Abridged In Vivo Assessment

Since natural-origin, sustainable ingredients are preferred by modern consumers, novel emulsifiers and emollients keep entering the market. This study hypothesizes that a combination of in silico, instrumental tools and simplified sensory studies could be used to efficiently characterize emulsions in a shorter timeframe. A total of 22 rather simple o/w emulsions were prepared by a time/energy-saving emulsification process. A natural mixed emulsifier (Lauryl Glucoside/Myristyl Glucoside/Polyglyceryl-6 Laurate) and two emollients (both with INCI name C15-19 Alkane) were used. The performed D-optimal experimental design within the response surface method (RSM) significantly narrowed down the number of samples about to enter the stage of texture, friction and sensory studies to the samples comprising 30% of a respective Emogreen emollient and 2% or 3% of the emulsifier. The sample comprising 2% emulsifier/30% Emogreen^® L15 showed significantly higher firmness (42.12 mN) when compared to the one with 2% emulsifier/30% Emogreen^® L19 (33.62 mN), which was somewhat unexpected considering the emollients' inherent viscosity values (4.5 mPa·s for L15 and 9 mPa·s for L19). The sample with 2% emulsifier/30% Emogreen^® L19 managed to maintain the lowest friction, while the one with 3% emulsifier/30% Emogreen^® L19 released its full lubricating potential in the second part of the measurement (30-60 s). The obtained results revealed the strengths and weaknesses of each formulation, narrowing down their possible applications in the early development stage.

Fast Inverted Photoprotective o/w Emulsions Loaded With Dihydroquercetin and β-Carotene: An Innovative Approach to In Vitro Assessment of Antioxidant Activity in a Bioenvironment

Fast inverted, oil-in-water (o/w) emulsions, also known as SWitch-Oil-Phase (SWOP) emulsions, express the performances of both o/w and water-in-oil (w/o) emulsions during application to the skin, favoring their use as cosmetic carriers in sunscreen products. The objective of this study was to investigate the antioxidant potential (by 2 different methods) and the ultraviolet (UV) absorption ability of SWOP emulsion (S) with incorporated plant-based antioxidants dihydroquercetin (DHQ) and β-carotene (βC), using quercetin (Q) in a reference emulsion, in addition to the evaluation of their physicochemical properties and stability. A new biochemical extracellular model for in vitro assessment of antioxidative properties for the SWOP emulsions (S, SQ, SDHQ, and SDHQβC) was developed and compared with the results of 2, 2-diphenyl-1-picrylhydrazyl (DPPH) assay. The analyses were performed at 20 °C and 37 °C, and oxidative stress parameters were monitored and statistically analyzed. The sun protection factor (SPF) of the samples was determined in vitro. Q and DHQ incorporated into the SWOP emulsion exhibited a strong DPPH radical scavenging ability. Neither incorporated nor pure βC showed DPPH radical scavenging ability at the tested concentrations. Contrary to that, in the bioenvironment conditions, SDHQβC showed minor antioxidative effects increase and also a significant decrease in exogenous pro-oxidative effects, caused by pro-oxidant, when compared to SDHQ. The obtained SPFs of SDHQβC, SDHQ, and SQ were 5.19, 4.65, and 3.35, respectively. The physicochemical stability of the emulsions was satisfactory during 1 month storage. The presented results demonstrated that the SWOP emulsion is a suitable carrier for antioxidants with a photoprotective ability. The novel biochemical approach could be used in addition to DPPH assay with several advantages, relevant for the testing of antioxidant activity of potential active ingredients in cosmetic products.

Measurements meet perceptions: rheology-texture-sensory relations when using green, bio-derived emollients in cosmetic emulsions

OBJECTIVE

Product aesthetics and sensory performance can strongly influence a cosmetic product's acceptance by consumers. However, classic sensory analysis is time-consuming, expensive and does not provide information on the target group's preference. In the previous phase of this project, we had untrained consumers evaluate six cosmetic emulsions based on their aesthetics using a check-all-that-apply (CATA) survey. In this project, our goals were to quantitatively characterize the rheology and textural properties of the six cosmetic emulsions containing green, bio-derived emollients and identify statistical relationships between the consumers' description of products and the instrumental measurements.

METHODS

Six emulsions were prepared-three with olive oil and three with heptyl undecylenate as an emollient. Four sensory-like attributes, namely firmness, work of shear, stickiness and adhesiveness, were tested using a texture analyser. Rheological characterization included continuous flow testing and oscillatory measurements. Droplet size and stability were also evaluated. Statistical relationships were quantified between measurements in this study and sensory survey results published previously.

RESULTS

The textural and rheological results indicated that the emulsions were different-as designed. The texture and rheology measurements had analogous grouping outcomes to the consumers' discrimination. Emulsions 1 and 2 were the firmest, hardest to spread, stickiest and had the highest viscosity, while Emulsions 5 and 6 were the least firm, easiest to spread, less sticky than Emulsions 1 and 2, and had the lowest viscosity. Emulsions 3 and 4 fell in between the other two groups. Using olive oil instead of heptyl undecylenate as an emollient increased firmness, spreading, stickiness, viscosity and droplet size of the emulsions in every case-when comparing emulsions within each pair. All six emulsions had a shear-thinning behaviour. Viscosity and firmness directly correlated for the emulsions. Emulsions were visually stable at room temperature over the course of 6 months and viscosity remained relatively constant over this period also.

CONCLUSION

Certain sensory attributes can be reliably predicted with instrumental measurements. Identifying and quantifying sensory-texture-rheology relationships can contribute to achieving appropriate product characteristics tailored to suit market needs.

Fundamentals and applications of particle stabilized emulsions in cosmetic formulations

The cosmetic industry is one of the fastest growing industrial sectors that is constantly evolving by absorbing new technologies and incorporating innovative yet sustainable products. Cosmetic products are comprised of diverse formulations such as skin care, color cosmetics, hair care, makeup, body care products. Traditionally, cosmetic emulsions are stabilized using surfactants or polymers. Due to its adverse effects on environment, cytotoxicity effects, numerous health hazards, there is a strong drive to shift towards sustainable and surfactant free emulsions. With increasing consumer demand for a safer and more biodegradable products, formulating "surfactant- free" emulsions by replacing conventional stabilizers with particles has gained popularity. In this review, various important aspects and applications of particle stabilized emulsions in cosmetic formulations will be discussed. Importantly, novel ideas on surface modification of particles and use of Janus particles in cosmetic formulations will be discussed.

Effective tool for assessment of the quality of barrier creams - relationships between rheological, textural and sensory properties

The efficiency of barier creams (BC) in the prevention of occupational skin diseases is closely related to their mechanical, rheological but also sensory properties. The measurement procedure we elaborated, which simulates the spreadability conditions on the skin and evaluation whether the structure reconstruction occurs (hysteresis loop test, shear rate dependence of normal force), allows the assessment of the effectivness of the BC in terms of mechanical resistance and its adhesion to the skin surface. In this thesis an effort was made to define the impact of the human factor - the product application on skin - on the efficiency of medical devices for cutaneous use. Creams' performance such as the spreadability or the feeling on the skin during and after application, which mostly determine users willingness to use them systematically and rigorously, have been linked to parameters obtained during the structure analysis and to rheological properties. Moreover an attempt has been made to correlate the values of basic textural properties with rheological parameters determined by viscoelasticity and classic flow analysis. Instrumental analysis of selected BC products demonstrated a good correlation with organoleptic tests carried out on probands. The applicability of our tool for quality evaluation of BC has been confirmed.

Crystallization Velocity and UV Performance of Formulations With Oversaturated UV-Filter Content

Cosmetic oils are used to dissolve crystalline lipophilic UV filters; however, little knowledge exists about the effect of other formulation ingredients. This study investigates the influence of emulsifiers on the recrystallization speed of 4 UV filters and the impact of UV-filter crystal formation on delivered performance. The crystallization pattern of studied UV filters was assessed using X-ray diffractometry, whereas their recrystallization speed in formulations with various emulsifiers was monitored microscopically. UV-filter concentration was above the saturation level to promote recrystallization. Furthermore, to understand the kinetics of recrystallization, the conformer number of each UV-filter was calculated. For the impact on performance, the absorbance of a sunscreen was measured before and after recrystallization of the contained UV filter. This study confirmed the crystallinity of tested UV filters. The emulsifier was shown to influence the UV-filter recrystallization speed in emulsions. Continuous oil phase sunscreens were critical; all UV filters recrystallized promptly in oils and water-in-oil emulsions. Large molecule UV filters showed slowest recrystallization speed explained by a higher number of possible conformers. Finally, this work confirmed the negative impact of crystal formation on the delivered photoprotection of a sunscreen.

Relation between sensory analysis and rheology of body lotions

OBJECTIVE

Evaluation of sensory attributes of cosmetic products is traditionally based on sensory panels. However, in some cases, a suitable candidate method that can reduce time and costs is the use of instrumental analysis that can detect relatively very small changes of entry ingredients. Such approach has been already applied for emollients, salt content, stabilizers, etc. The aim of this contribution is to apply the relations between sensory analysis and rheology to a series of body lotions differing in the contents of emulsifiers and viscosity regulators.

METHODS

Sensory and rheological analyses are related. Rheological analysis can represent a good alternative to basic orientation in chosen customer's feelings. A rotational rheometer is the only instrumental device required for the measurements.

RESULTS

An empirical rheological model was proposed by means of which the selected sensory attributes were evaluated using the numerical values of adjustable model parameters. This approach exhibited a very good agreement with the results obtained by the sensory panel.

CONCLUSION

It was shown that a description of chosen sensory attributes can be responsibly carried out by rheological measurements, that is through the attained numerical values of the parameters appearing in a proposed empirical model characterizing shear viscosity of body lotions.