Nanostructured oil in cosmetic paraffin waxes - Crystal Growth

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Nanostructured oil in cosmetic paraffin waxes Fan C. Wang, Yukihiro Miyazaki, and Alejandro G. Marangoni Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00042 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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Crystal Growth & Design

Nanostructured oil in cosmetic paraffin waxes

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Fan C. Wang1, Yukihiro Miyazaki2, and Alejandro G Marangoni1*

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1. Department of Food Science, University of Guelph, Guelph, Canada

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2. Kao Corporation, Sumida-ku, Tokyo, Japan

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*Corresponding author: [email protected]

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Abstract

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This work examined the oil-binding behaviour of cosmetic paraffin wax – mineral oil systems

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using powder X-Ray diffraction (XRD), differential scanning calorimetry (DSC), and pulsed

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nuclear magnetic resonance (pNMR). Neat paraffin wax crystals and paraffin wax oleogels

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crystallized into the same polymorphic forms; however, the oleogels had a larger lamellar

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thickness and crystal domain size. This increase in lamellar size could indicate that the oil was

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structured in between individual wax crystalline lamellae (i.e., nanostructured oil). The amount

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of crystalline material determined by pNMR of wax oleogels increased with temperature below

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their melting point, because inter-lamellar nanostructured oil lost some mobility and displayed

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solid-like behaviour. These results provide experimental evidence for the existence of

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nanostructured oil, in support of simulation studies in the literature, and shed light into the oil-

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binding mechanism of some materials.

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Solid–liquid interactions are important in determining the oil-binding capacity of a

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material. Many theoretical studies have attempted to explain the oil-binding mechanism of

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materials using computer simulations. These studies have been performed on simple and

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complex fluids, such as n-alkanes in confined model pores1 and between parallel planes2–6, and

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oil between triglycerides (TAGs) nanoplatelets7,8. N-alkane polymers were found to change their

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chain conformation in confined space9,10, arrange into layered structures parallel to the surface,

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and display periodic oscillations corresponding to the length, width, and density of the polymer

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molecules11–13. No evidence was found for an immobile liquid layer near the solid substrate in

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these studies13. Similar to n-alkanes, liquid TAG oil was shown to arrange into layers parallel to

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the surfaces of solid TAG nanoplateletes and display a density gradient as a result of nano- phase

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separation7,8. Near the solid surface, oil molecules adsorb to the surface and display slower

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molecular diffusion, while the oil density increased when closer to the solid surface7,8.

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Experimental evidence in support of these simulation studies is scarce. In the current

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study, evidence for the existence of nanostructured confined oil between crystalline lamellae of

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cosmetic paraffin wax is provided, which could support results from simulation studies. We will

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further discuss the unique crystalline structure and swelling behaviour of cosmetic paraffin wax

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oleogels as well.

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Straight chain paraffin wax (Nippon Seiro Co., Ltd., Tokyo, Japan) used in this study is a

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crystalline solid mixture of hydrocarbons (C34–C40 alkanes) with a melting point of 72–74 °C

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obtained from petroleum. The oil phase used in wax oleogels is isotridecyl isononanoate, which

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is the ester of isotridecyl alcohol and isononanoic acid, and was purchased form Nisshin Oillio

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Group, Ltd. (Yokohama, Kanagawa, Japan). Polyethylene wax was purchased from New Phase

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Technologies (Sugar Land, Texas, USA), with a melting point of 83–90 °C. To prepare paraffin


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wax oleogel samples, wax-oil mixtures containing 0 to 100% paraffin wax were melted and

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mixed at 120 °C and then stored at 0 °C for 24 h to allow for crystallization. Samples were then

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kept at room temperature for at least 6 h before analysis. Wax and wax oleogel samples were

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studied using powder XRD, DSC, and pNMR, following the method used by Miyazaki and

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Marangoni14. Detailed experimental protocols are provided as supporting information.

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Figure 1. (a) XRD patterns of paraffin wax mixed with different amounts of oil, (b) lamellar

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thickness (d001) and crystal domain size (ξ) calculated form XRD data, (c) % increase in d001 and

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ξ of paraffin wax oleogel with different oil contents relative to neat paraffin wax crystals, and (d)

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correlation between % increase in d001 and % increase in ξ.

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Neat paraffin wax crystals and paraffin wax oleogels at various oil contents were all in the β´ polymorphic form, indicated by reflections at d=4.1Å and 3.7 Å calculated from wide


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angle diffraction (WAXD) peaks (Figure 1a). Small angle diffraction (SAXD) data of all samples

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suggest the presence of lamellar crystalline structures, with d-spacings appearing at 1:1/2:1/3

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corresponding to the main reflection and higher order reflections from the (001) crystallographic

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plane. SAXD peaks from higher order reflections (002, 003, etc.) were not observed from

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branched chain wax or straight chain polyethylene wax with a wider molecular size distribution

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(C20–C60 alkanes).

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The crystal domain size (ξ) of each sample was calculated following the Scherrer equation:

ξ=

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λ ×

Where K is the dimensionless shape factor (0.9), λ is the wavelength of the X-ray source

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(1.54 Å for copper anode source), FWHM is the full width half maximum of a Bragg’s peak

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(usually for the (001) plane), and is the angle of where a certain diffraction peak was obtained.

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Results show that even though neat paraffin wax crystals and paraffin wax oleogels are in

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the same polymorphic form, increases in lamellar thickness (d001) and domain size (ξ) with

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increasing oil content were observed (Figure 1b). The ratio between domain size and lamellar

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thickness can be used to estimate the number of lamellae per domain. The number of lamellae

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per domain was between 6.2 and 6.8 in all the samples, therefore we could assume that all

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samples have the same number of lamellae in the domain. The % increase in d001 and ξ of

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paraffin wax oleogels with various oil contents as compared to neat paraffin wax crystals was

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then calculated (Figure 1c). d001 increased by 11.0% while ξ increased by 21.0% upon increases


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in the oil content of wax oleogels from 0 to 80%. The increment in d001 is caused by swelling of

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paraffin wax lamellae in the presence of oil; while the increase in ξ is caused by both increases in

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d001 and the amount of inter-lamellar confined oil. A linear correlation suggested that the

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increase in ξ is 1.9 time of the increase in d001, as shown in Figure 1d. If no oil were structured

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between wax lamellae, the % increase in d001 and ξ should be the same.

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Figure 2. (a) Melting profiles of paraffin wax structuring 0 to 80% (w/w) of oil, and (b)

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Hildebrand plot of paraffin wax – oil system.

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The ideal solubility behaviour between paraffin wax and oil was also predicted using the Hildebrand equation15: =

Δ / 1/ 1/


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Where X represents the mole fraction of the higher melting component (paraffin wax in

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this case), ∆Hf is the enthalpy of melting for the higher melting component (in J/mol), R is the

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universal gas constant (8.314 J/mol･K), and Tm and Tb are the melting temperatures of the higher

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melting component and the blend (in K), respectively.

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The temperature and enthalpy of melting of each sample were determined using DSC

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(Figure2a). Results show that paraffin wax and mineral oil used in the concentration range of this

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study display ideal solubility in the liquid state, indicated by a linear correlation between lnX and

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1/Tb, as shown in Figure 2b16.

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Figure 3. Amount of crystalline material determined as the solid fat content (SFC) of oil

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structured with (a) 20% wt. paraffin wax (n=4, error bars are all smaller than the symbols), and

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(b) 20% (w/w). wax that contains a blend of 90% paraffin wax and 10% straight chain

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polyethylene wax (n=2, error bars are all smaller than the symbols).


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106 Upon heating from 0 to 35˚ C, still below the melting temperature, oleogel samples

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containing 20% paraffin wax showed an unexpected increase in the solid fat content (SFC), as

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shown in Figure 3. One possible explanation is that when wax lamellae swell upon heating, more

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oil becomes trapped in between or adsorbed onto the surface of wax lamellae13. These surface-

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bound and confined oil molecules probably exchange at a lower frequency with the bulk oil

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phase and thus appear as “solid” in the pNMR measurement. A slight increment in SFC upon

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heating below the melting temperature was also observed in oleogel samples structured with 20%

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wax, where the wax phase contains a blend of 90% paraffin wax and 10% polyethylene wax,

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shown as Figure 3b. Increases in SFC upon heating seems to be specific to the paraffin wax

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oleogel, and no evidence was found in other oleogel systems, within the scope of our literature

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search.

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liquid oil

d

ξ

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Figure 4. Schematic diagram of cosmetic paraffin wax nanostructured oil.

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A schematic diagram of paraffin wax nanostructured oil can then be proposed (Figure 4).

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Results suggested that paraffin wax crystallized into the ß′ polymorphic form and arranged into

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lamellar structures, where oil can become trapped or bound between the lamellae. Even though

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only limited experimental evidence is available in the literature on confined/nanostructured oil, it

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may be present in other systems. One example is monoglyceride (MGs) oleogels. MGs have

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polymorphic and mesomorphic properties, and crystalize into lamellar structures that can

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immobilize both water and liquid oil17–21.Glycerol monostearate (GMS) crystallizes into a

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lamellar α phase with d001 at 50.7Å 22,23; while in a 90% oleogel structure, the d001 of the α phase

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increased to 52Å24. It is therefore plausible, based on results from the current work, that oil has

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been structured in between the GMS lamellae. However, whether nanostructured oil exists in

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MG-oil systems also remains unproven.

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To summarize, nanostructured confined oil was discovered in cosmetic paraffin wax –

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mineral oil systems in this work. Neat paraffin wax crystals and wax oleogel crystalized into the

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same polymorphic form, but oleogels had larger lamellar thickness and domain size. Increased

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lamellar thickness and domain size resulted from the swelling of wax lamellae upon

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nanostructuring of oil between them. These surface-bound/confined oil molecules had decreased

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mobility and displayed solid-like behavior at increasing temperatures. This experimental

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evidence for nanostructured oil between lamellar structures agrees with simulation studies. The

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generality of this type of oil binding at the nanoscale within lamellae and without loss in

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scattering coherence remains to be proven.

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Figure Legends

Figure 1. (a) XRD patterns of paraffin wax mixed with different amounts of oil, (b) lamellar thickness (d001) and crystal domain size (ξ) calculated form XRD data, (c) % increase in d001 and ξ of paraffin wax oleogel with different oil contents relative to neat paraffin wax crystals, and (d) correlation between % increase in d001 and % increase in ξ. Figure 2. (a) Melting profiles of paraffin wax structuring 0 to 80% (w/w) of oil, and (b) Hildebrand plot of paraffin wax – oil system. Figure 3. Amount of crystalline material determined as the solid fat content (SFC) of oil structured with (a) 20% (w/w) paraffin wax (n=4, error bars are all smaller than the symbols), and (b) 20% wt. wax that contains a blend of 90% paraffin wax and 10% straight chain polyethylene wax (n=2, error bars are all smaller than the symbols). Figure 4. Schematic diagram of cosmetic paraffin wax nanostructured oil.


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Figure 1. (a) XRD patterns of paraffin wax mixed with different oil content, (b) lamellar thickness (d001) and crystal domain size (ξ) calculated form XRD data, (c) % increase in d001 and ξ of paraffin wax oleogel with different oil content comparing to neat paraffin wax crystals, and (d) correlation between % increase in d001 and % increase in ξ. 100x62mm (600 x 600 DPI)



Figure 2. (a) Melting profiles of paraffin wax structuring 0 to 80% wt. of oil, and (b) Hildebrand plot of paraffin wax – oil system. 113x147mm (600 x 600 DPI)


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Figure 3. Amount of crystalline material determined as the solid fat content (SFC) of oil structured with (a) 20% wt. paraffin wax (n=4, error bars are all smaller than the symbols), and (b) 20% wt. wax that contains a blend of 90% paraffin wax and 10% straight chain polyethylene wax (n=2, error bars are all smaller than the symbols). 99x127mm (600 x 600 DPI)



Figure 4. Schematic diagram of cosmetic paraffin wax nanostructured oil. 36x27mm (300 x 300 DPI)


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Nanostructured oil in cosmetic paraffin waxes - Crystal Growth

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