A majority of cleansing formulations on the market are made of surfactants that form simple micelles. This organization of the surfactants, however, often fails to provide enough yield to durably suspend insoluble ingredients such as silicone emulsions, anti-dandruff particles or moisturizing oils. Thus, they require the addition of a rheological polymer agent, e.g., carbomer or acrylates copolymer, which can jeopardize sensorial attributes of the product including foam and texture, as well as performance.
Recently, surfactant technologies forming multilamellar vesicles have been developed and introduced into cleansing formulations that durably suspend insoluble ingredients without the need for a suspending agent. Previously it has been shown that structured surfactant systems can be used to design cleansing products to deliver improved consumer benefits, such as skin moisturization and hair conditioning, over micellar systems. Structured surfactant systems also enable the design of multifunctional products.1, 2 The present article considers their effects on fragrance in cleansing products.
Fragrance plays a key role in consumer appeal and brand differentiation and is therefore a crucial component of cleansing formulations; typically, it is also the most costly. Increasing the effectiveness of perfume burst and delivery are therefore two approaches to minimizing formulation costs. This article describes how to formulate structured formulations to deliver the most fragrance to skin or hair, versus micellar systems, as well as improve its duration with no negative impact on perfume burst.
Materials and Methods
The test formulations used for the following studies are described in Table 1. The cleansing base of these products was either a combination of surfactantsa that forms multilamellar vesicles, i.e., formulas A, B and C in Table 1; or a common micellar sodium laureth sulfateb and cocamidopropyl betainec surfactant chassis; i.e., formulas D and E in Table 1. All formulations also contained 0.35% w/w of a cationic guar polymerd as a deposition aid, and 2% to 3% w/w fragrance. These fragrance loads are typical of premium cleansers marketed by fine fragrance producers and were used to ease olfactory assessments; note that mass-market formulations usually incorporate less, typically 0.2–0.5%. Two fragrances were used in the study; Fragrance 1 was an aromatic fougère fragrance with leather, violet leaves, lavender, cedarwood and amber notes. Fragrance 2 was an oriental floral fragrance with orange flower, shea butter, vanilla, cedar and tonka bean notes.
The long-term stability of the formulations in relation to viscosity and color were checked by viscosimeter and optical assessments after storage at 45°C for three months. All formulations, whether structured or micellar, displayed satisfactory stability (data not shown). The ability of the fragrances to mask the odor of the raw materials was evaluated by olfactory assessments carried out by a panel of 13 untrained individuals. The stability of the fragrance after aging the formulations also was assessed olfactorily by comparing the scent of freshly prepared formulations to formulations stored for one year at room temperature. These evaluations were carried out either by the same panel of 13 individuals or by a trained olfactory evaluation expert.
Further, the blooming effect or perfume burst during use was evaluated by diluting 2 g of the product in 100 g of hot water at 37°C. The solution was then stirred to generate foam and the trained olfactory evaluation expert rated the intensity of the fragrance in the vapor phase above the solution. Fragrance delivery and duration on skin were assessed by cleansing the forearms of volunteers with the test formulations applied by a single operator using a controlled proceduree.
The intensity of the perfume deposited on skin after rinsing was then evaluated in two different ways: olfactorily by the volunteers to mimic consumer response, the final results of which were averaged over the number of participating volunteers; and instrumentally, by collecting the components that volatize from the skin during a controlled period of time and analyzing and quantifying the collected volatiles using gas chromatography. In both cases, the evaluations were carried out at several points, i.e., 5 min, 2 hr and 6 hr after the rinsing step, to assess longer term effects.
For deposition studies related to hair, the formulations were applied on 10-g, dark brown Caucasian hair tresses by a single operator using the same controlled proceduree. Ten untrained panelists first evaluated the intensity of the perfume in wet tresses after the rinsing step, then tresses were dried for 7 min with a hair dryer and panelists evaluated them at intervals of 0 hr, 2 hr and 24 hr. For both skin and hair evaluations, the panelists recorded the fragrance intensity according to the following scale: 0 = no perfume odor; 1 = slight perfume odor; 2 = moderate perfume odor; 3 = strong perfume odor; 4 = very strong perfume odor. They also indicated if they perceived a significant difference between the two forearms or hair tresses, and if so, which smelled the most. All tests were carried out in a blind manner, whether olfactory or instrumental.
Fragrancing Structured Formulations
The structured formulations disclosed in Table 1 were prepared by first solubilizing the cationic guar polymer in deionized water under moderate stirring, followed by the addition of the surfactant blend, oil phase (if any) and preservative. Then, to structure the formulation into multilamellar vesicles, two steps were necessary: first was the addition of sodium chloride, followed by the pH adjustment to about 6.0 with, for instance, a citric acid solution. Using Formula A, two processes for fragrancing the formulation were tested. The perfume either was directly added to the formulation after the first structuring step, i.e., after the sodium chloride addition (Formula A-1), or it was premixed into the oil phase and introduced before structuring, i.e., before salt addition (Formula A-2). Figure 1 illustrates the impact of these processes on the intensity of each component constituting the perfume. This shows that when the fragrance was added into the oil before structuring into multilamellar vesicles, the cumulated intensity of the perfume was five times larger than when it was introduced after structuring.
To confirm that the incorporation of the perfume into the oil phase was not the actual driver for delivery, the authors compared perfume delivery introduced before structuring (Formula A-2), as noted, with a formula containing no oil (see Formula B in Table 1). Figure 2, which shows the fragrance intensity on skin over time as assessed by a panel of volunteers, clearly shows that no difference in the intensity of the odor was perceived between formulas A-2 and B. Therefore, the oil, which is often used to provide consumer benefits such as skin moisturization or hair conditioning, was not responsible for the delivery and duration of the perfume on skin. The authors therefore concluded that to deliver the most fragrance to a surface, the fragrance must be introduced before structuring—whether including an oil or not. Structured Formulation C in Table 1 also was prepared in this way.
Masking Odors vs. Fragrance Stability
Cleansing formulations consist mainly of surfactants, which often have fatty base notes that require masking.3 For the combination of surfactants used in the structured test formulations, an acetic smell is perceived if the pH is in the low range of 4 to 5. The intensity of the smell decreases as the pH increases, and at pH 6 and above, the intensity becomes low. Thus, if formulating in a low pH range, the formulator will require fragrances with heavy notes to mask the acetic smell; i.e., fragrances with oriental, amber, vanilla or fern scents. At a higher pH, however, there will be more freedom to choose different fragrances. As noted, a panel of 13 untrained individuals assessed the smell of formulas C and E incorporating Fragrance 2 immediately after their preparation and compared them with the raw fragrance to determine which maintained the scent closest to the original. The structured formula was perceived by 60% of panelists to exhibit fragrance closer to the raw fragrance, as opposed to 40%, who perceived the micellar formulation as such. The panelists also commented that the structured formula had a more pleasant, rounded and sweeter scent than the micellar one. This suggests the structured formulation was more efficient at masking the odors while respecting the initial perfume scent.
To confirm this result, panelists were asked to smell formulations perfumed with Fragrance 1. Again, they noted the presence of an unpleasant note in the micellar formulations, i.e., formulas D and E. These formulations as well as Formula E contained 14% w/w of sodium laureth sulfate (SLES), the most widely used surfactant in cleansing products.4 This surfactant is known to often contain dodecyl aldehyde, which is responsible for the malodor.5 To determine whether this component was actually responsible for the unpleasant odor in the micellar formulations detected by the panelists, the volatiles emitted by formulations B and D were analyzed using gas chromatography–mass spectrometry (GC-MS). The only difference in the gas chromatograms of these two formulations was the presence of an additional peak for Formula D, at a retention time corresponding to that of dodecanal (data not shown). This validated the assumption that the impurity in SLES was responsible for the malodor perceived in the micellar formulations despite the high loads of fragrance, i.e., 2–3% w/w.
The long-term stability of the fragrance of formulas B and D was assessed olfactorily by comparing the scent of the freshly prepared formulations to those aged for one year and stored at room temperature. The 13 untrained panelists noticed less denaturation of the notes for the structured formula (Formula B, Table 1) than the micellar formula (Formula D, Table 1), suggesting greater fragrance stability for the structured formulation. This was confirmed by a trained expert’s evaluation, which noted that for the aged micellar formulations, the fragrance intensity was no longer sufficient to mask the odor of the raw materials; for the aged structured formulation, the scent was still acceptable, though its freshness was less pronounced.
Perfume Delivery Efficacy and Duration
The efficacy of perfume delivery and duration for structured formulations B and C was evaluated both on skin and hair via blind panel studies, in comparison with micellar formulations D and E. Table 2 shows the evolution over time of the average intensity of Fragrance 1, once deposited on skin by structured (Formula B) and micellar (Formula D) cleansing systems, as perceived by the panelists. It also gives the percentage of panelists who found the fragrance intensity to be significantly higher with the structured formula.
With 75% of panelists observing a more pronounced odor on the forearm washed with Formula B, the structured body wash clearly provided the most intense scent immediately. The difference remained perceivable even 2 hr after cleansing, with 66% of panelists identifying the structured formula as the most effective. The enhanced effect of structured systems began fading between 2 hr and 6 hr, as the difference between the structured and micellar formulas was no longer perceivable for the untrained panelists at 6 hr.
Table 3 addresses the delivery of Fragrance 2 to hair from formulas C (structured) and E (micellar). It shows the average intensity of the fragrance as perceived by the panelists right after shampooing hair, then after hair drying. Again, it also gives the percentage of panelists who perceived a more intense scent from the structured formula. Similar to the results obtained on skin, the fragrance intensity on hair was more pronounced when hair was washed with the structured formula (Formula C): 90% of panelists identified the hair tress washed with the structured shampoo as having the most scent.
Expectedly, once hair was dried, fragrance intensity was reduced for all the formulas due in part to evaporation of the fragrance during drying. The extent of the fragrance reduction was similar for both products, i.e., a loss of 25–30%. However, after hair drying, 80% of the panelists still found a more pronounced perfume scent for the hair tress washed with the structured formula. This olfactory difference between the two hair samples was still perceivable 24 hr after shampooing, with 80% of panelists finding a more intense scent from the structured shampoo. Thus, for the fragrances and cleansing formulations evaluated in the study, the results of the olfactory sensory panel tests suggest that structured formulations can deposit perfumes on skin and hair to a greater extent than micellar formulations, to provide a consumer-perceivable improvement in scent for an extended period of time.
As mentioned previously, perfume burst is also key in the olfactory performance of a cleansing product. Thus, the blooming performance of structured Formula B was evaluated by a trained expert and compared with micellar Formula D. The expert rated the blooming of the structured formula as being very good and better than that achieved by the micellar product, thus indicating that structured formulations can provide improved perfume delivery with no compromise of perfume burst.
The delivery of fragrance onto skin and hair from conventional micellar cleansing products is challenging because in these products, the fragrance molecules distribute between the aqueous phase and surfactant micelles,6 and during rinsing, they tend to be washed off with the surfactants, resulting in low deposition efficiency. It is believed that the improved perfume delivery performance evidenced in this article for the structured formulations having the fragrance added before structuring comes from the encapsulation of part of the fragrance within the multi-lamellar vesicles. When fragrance is introduced after structuring, it is emulsified into the multilamellar vesicles network (see Figure 3a), whereas it can be trapped inside the multilamellar vesicles when added before structuring (see Figure 3b). The encapsulation of the fragrance inside the multilamellar vesicles is believed to provide improved resistance to rinsing, resulting in the improved fragrance deposition performance.
Formulating cleansing products with structured surfactant systems, when the perfume is added before structuring, enables the formulator to improve perfume delivery onto skin and hair, in comparison with traditional micellar systems. In addition, this approach provides improved perfume duration with no negative impact on perfume burst. The encapsulation of the fragrance inside the multilamellar vesicles is believed to be the reason for this improved performance.
It is important to note that the work presented in this article was carried out using medium to heavy fragrances. This will be expanded in a near future to light fragrances, for which delivery onto skin and hair is even more challenging.
Acknowledgments: D. Lemos, A. Sahouane, J.M. Quilis and P. Dupuis from Rhodia, and C. Mercier from Takasago, have contributed to the experimental work presented in this article. In addition, the authors wish to thank S. Catarino, A. Thomasson, L. Cavalier, V. Picquet and B. Amram from Rhodia, and O. Anthony and C. Regniez from Takasago for fruitful discussions throughout the project. They would also especially like to thank D. Bendejacq from Rhodia, as well as C. Duault for their assistance in writing this article.
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