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Title:
TUNABLE SHEAR-RESPONSIVE EMULSIONS STABILIZED BY CELLULOSE NANOCRYSTALS
Document Type and Number:
WIPO Patent Application WO/2021/113975
Kind Code:
A1
Abstract:
It is provided an emulsion stabilized by cellulose nanocrystals (CNC), and methods for producing the emulsion, controlling and/or tuning the rheological properties of the emulsion. Accordingly, it is demonstrated herein that emulsions stabilized by cellulose nanocrystals have tunable flow, or rheological, properties when subjected to external load, as well as long-term storage stability. That is to say, these emulsions can be solid under no load, but flow only when modulated pressure or load is externally applied. Once the external pressure or load is relieved, the emulsion returns to solid state.

Inventors:
HAMAD WADOOD Y (CA)
MIAO CHUANWEI (CA)
MAURAN DAMIEN (CA)
Application Number:
PCT/CA2020/051700
Publication Date:
June 17, 2021
Filing Date:
December 10, 2020
Export Citation:
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Assignee:
FPINNOVATIONS (CA)
International Classes:
B01F17/52; A61K8/73; A61K47/38; A61P17/00; A61Q17/04; B01F3/08; C09K8/035
Foreign References:
CA2833673A12013-01-03
Other References:
KALASHNIKOVA, I. ET AL.: "New Pickering Emulsions Stabilized By Bacterial Cellulose Nanocrystals", LANGMUIR, vol. 27, 2011, pages 7471 - 7479, XP002692997, DOI: 10.1021/la200971f
KALASHNIKOVA, I. ET AL.: "Cellulosic Nanorods of Various Aspect Ratios For Oil in Water Pickering Emulsions", SOFT MATTER, vol. 9, 2013, pages 952 - 959
GUO, J. ET AL.: "Cellulose Nanocrystals As Water-in-oil Pickering Emulsifiers Via Intercalative Modification", COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS, vol. 529, 2017, pages 634 - 642, XP085148057, DOI: 10.1016/j.colsurfa.2017.06.056
Attorney, Agent or Firm:
NORTON ROSE FULBRIGHT CANADA S.E.N.C.R.L., S.R.L. / LLP (CA)
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Claims:
WHAT IS CLAIMED IS:

1. An emulsion comprising; an internal phase dispersed in a continuous external phase forming an interface between the internal phase and the continuous external phase; and cellulose nanocrystals (CNC) located at the interface; wherein the emulsion comprises between 10 to 95 % by volume of the internal phase.

2. The emulsion according to claim 1, wherein the cellulose nanocrystals are in a concentration of 0.001 to 5 % by weight.

3. The emulsion according to claim 1 or 2, wherein the internal phase is hydrophobic and the continuous external phase is aqueous.

4. The emulsion according to claim 3, wherein the aqueous phase comprises a dissolved salt.

5. The emulsion according to claim 4, wherein the dissolved salt is monovalent, divalent or trivalent.

6. The emulsion according to claim 1 or 2, wherein the cellulose nanocrystals have a functionalized hydrophobic surface, and wherein the internal phase is aqueous and the continuous external phase is hydrophobic.

7. The emulsion according to any one of claims 1 to 5, wherein the internal phase is water immiscible and the continuous phase is water.

8. The emulsion according to any one of claims 1 to 5, wherein the internal phase is at least 50 % by weight of the emulsion.

9. The emulsion according to any one of claims 1 to 8, wherein the emulsion has long term storage stability. 10. The emulsion according to any one of claims 1 to 9, wherein the emulsion has a controlled rheological performance.

11. The emulsion according to claim 10, wherein the emulsion is adapted to break at a predetermined threshold shear rate.

12. The emulsion according to any one of claims 1 to 11, wherein the emulsion further comprises cellulose filaments.

13. The emulsion according to claim 12, wherein the cellulose filaments are from wood pulp.

14. The emulsion according to any one of claims 1 to 13, wherein the emulsion is free of any surfactant.

15. The emulsion according to any one of claims 1 to 13, wherein the CNCs are from a biomass.

16. The emulsion according to claim 15, wherein said biomass is from cotton, grass, wheat straw, wood pulp or tunicate.

17. A method of producing an emulsion stabilized by cellulose nanocrystals, the method comprising; a) mixing an internal hydrophobic phase into an external aqueous phase comprising cellulose nanocrystals and a dissolved salt to form a precursor emulsion wherein the internal phase is less than 50 % by volume of the precursor emulsion; and b) mixing under high shear more of the internal hydrophobic phase to the precursor emulsion to obtain the emulsion stabilized by cellulose nanocrystals wherein the internal phase is at least 50 % by volume of the emulsion.

18. The method according to claim 17, wherein the internal phase comprises less than 30 % by volume of the precursor emulsion. 19. The method according to claim 17 or 18, wherein the internal phase is at least 74 % by volume of the emulsion.

20 The method according to any one of claims 17 to 19, wherein in step b), the mixing is performed by high-speed homogenization or ultra-sonication.

21 A sunscreen composition comprising the emulsion as defined in any one of claims 1-16 and at least one ultra-violet filter dispersed in the emulsion.

22 A drilling fluid comprising the emulsion as defined in any one of claims 1-16.

23. A pharmaceutical composition comprising the emulsion as defined in any one of claims 1-16 and a pharmaceutically acceptable carrier.

24. The composition of claim 23, wherein said composition is a skin product.

25 The composition of claims 24, wherein the skin product is a cream or an ointment.

Description:
TUNABLE SHEAR-RESPONSIVE EMULSIONS STABILIZED BY CELLULOSE NANOCRYSTALS

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is claiming priority from U.S. Provisional Application No. 62/946,496 filed December 11, 2019, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates generally to tunable shear-responsive emulsions stabilized by cellulose nanocrystals (CNC), methods of making same, and products comprising same.

BACKGROUND

[0003] Emulsions are systems consisting of two immiscible phases, one of which is dispersed in the other. The dispersed phase, or internal phase, is stabilized by emulsifiers, typically surfactants. Medium internal phase emulsions, or MIPE, refer to the volume content the internal phase can accommodate, which is between 50 and 74 vol. %. When the volume content of the dispersed phase is equal to or higher than 74 vol. %, such a system is called a high internal phase emulsion or HIPE. In addition to surfactants, solid particles can also stabilize emulsions, and are then referred to as Pickering emulsions. Currently, some commercialized emulsions stabilized by surfactants have limitations regarding the control of their rheological properties.

[0004] It is thus desirable to be provided with emulsions having improved controlled rheological properties.

SUMMARY

[0005] It is provided an emulsion comprising an internal phase dispersed in a continuous external phase forming an interface between the internal phase and the continuous external phase; and cellulose nanocrystals (CNC) located at the interface; wherein the emulsion comprises between 10 to 95 % by volume of the internal phase.

[0006] In an embodiment, the cellulose nanocrystals are in a concentration of 0.001 to 5 % by weight. [0007] In a further embodiment, the internal phase is hydrophobic and the continuous external phase is aqueous.

[0008] In another embodiment, the aqueous phase comprises a dissolved salt.

[0009] In an embodiment, the dissolved salt is monovalent, divalent or trivalent.

[0010] In a further embodiment, the cellulose nanocrystals have a functionalized hydrophobic surface, and wherein the internal phase is aqueous and the continuous external phase is hydrophobic.

[0011] In an additional embodiment, the internal phase is water immiscible and the continuous phase is water.

[0012] In a further embodiment, the internal phase is at least 50 % by weight of the emulsion.

[0013] In another embodiment, the emulsion has long term storage stability.

[0014] In an embodiment, the emulsion has a controlled rheological performance.

[0015] In a further embodiment, the emulsion is adapted to break at a predetermined threshold shear rate.

[0016] In another embodiment, the emulsion further comprises cellulose filaments.

[0017] In an embodiment, the cellulose filaments are from wood pulp.

[0018] In an alternative embodiment, the emulsion is free of any surfactant.

[0019] In another embodiment, the CNCs are from a biomass.

[0020] In an embodiment, the biomass is from cotton, grass, wheat straw, wood pulp or tunicate.

[0021] It is further provided a method of producing an emulsion stabilized by cellulose nanocrystals, the method comprising mixing an internal hydrophobic phase into an external aqueous phase comprising cellulose nanocrystals and a dissolved salt to form a precursor emulsion wherein the internal phase is less than 50 % by volume of the precursor emulsion; and mixing under high shear more of the internal hydrophobic phase to the precursor emulsion to obtain the emulsion stabilized by cellulose nanocrystals wherein the internal phase is at least 50 % by volume of the emulsion.

[0022] In an embodiment, the internal phase comprises less than 30 % by volume of the precursor emulsion.

[0023] In a further embodiment, the internal phase is at least 74 % by volume of the emulsion.

[0024] In another embodiment, the mixing is performed by high-speed homogenization or ultra-sonication.

[0025] It is further provided a sunscreen composition comprising the emulsion as defined herein and at least one ultra-violet filter dispersed in the emulsion.

[0026] It is additionally provided a drilling fluid comprising the emulsion as defined herein.

[0027] It is provided a pharmaceutical composition comprising the emulsion as defined herein and a pharmaceutically acceptable carrier.

[0028] In an embodiment, the composition is a skin product.

[0029] In another embodiment, the skin product is a cream or an ointment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Reference will now be made to the accompanying drawings, and in which:

[0031] Fig. 1A is a graph of the viscosity in function of time at a shear rate of 0.1 s 1 for samples 1 to 4 in accordance to an embodiment.

[0032] Fig. 1B is a graph of the viscosity in function of time at a shear rate of 1 s 1 for samples 1 to 4.

[0033] Fig. 1C is a graph of the viscosity in function of time at a shear rate of 10 s 1 for samples 1 to 4.

[0034] Fig. 2 is a graph of the viscosity in function of the shear rate for sample 5 in accordance to an embodiment. [0035] Fig. 3A is a graph of the viscosity in function of the shear rate for sample 6 in accordance to an embodiment.

[0036] Fig. 3B is a graph of the viscosity in function of the shear rate for sample 7 in accordance to an embodiment.

[0037] Fig. 3C is a graph of the viscosity in function of the shear rate for sample 8 in accordance to an embodiment.

[0038] Fig. 3D is a graph of the viscosity in function of the shear rate for sample 9 in accordance to an embodiment.

[0039] Fig. 4A is a graph of the viscosity in function of the shear rate for sample 10 in accordance to an embodiment.

[0040] Fig. 4B is a graph of the viscosity in function of the shear rate for sample 11 in accordance to an embodiment.

[0041] Fig. 4C is a graph of the viscosity in function of the shear rate for sample 12 in accordance to an embodiment.

[0042] Fig. 4D is a graph of the viscosity in function of the shear rate for sample 13 in accordance to an embodiment.

[0043] Fig. 5A is a graph of the viscosity in function of the shear rate for sample 7.

[0044] Fig. 5B is a graph of the viscosity in function of the shear rate for a commercial sunscreen.

[0045] Fig. 6A is a graph of amplitude sweep curves in function of the oscillation strain for sample 7.

[0046] Fig. 6B is a graph of amplitude sweep curves in function of the oscillation strain for a commercial sunscreen.

DETAILED DESCRIPTION

[0047] In accordance with the present disclosure, there is provided an emulsion stabilized by cellulose nanocrystals (CNCs). Further, there is provided methods for producing the emulsion and methods for controlling and/or tuning the rheological properties of the emulsion. [0048] Accordingly, it is demonstrated herein that emulsions stabilized by cellulose nanocrystals have tunable flow, or rheological, properties when subjected to external load, as well as long-term storage stability. That is to say, these emulsions can be solid under no load, but flow only when modulated pressure or load is externally applied. Once the external pressure or load is relieved, the emulsion returns to solid state. CNC-stabilized emulsions thus have controlled rheological behavior and mechanical performance, as well as tunable rheological properties.

[0049] The term “cellulose nanocrystals” or “CNCs” as used herein refer to a category of nanomaterials produced from biomass by controlled acid hydrolysis. They are characterized by high crystallinity (between 85 and 97%, typically greater than 90%) approaching the theoretical limit of the cellulose chains. CNCs can further be characterized by a degree of polymerization (DP) in the range 90 < DP < 110, and 3.7- 6.7 sulphate groups per 100 anhydroglucose units, and the crystallites have aspect ratios between 10 and 20. Their physical dimensions depend on the raw material used in the extraction, which ranges between 5-15 nm in cross-section and 100-150 nm in length for CNCs extracted by hydrolyzing wood pulp or cotton fibers, for instance, bleached kraft pulp. These negatively charged crystallites can be suspended in water, or other solvents if appropriately functionalized, or self-assemble to form solid materials by air, spray- or freeze-drying. Hydrogen bonding between the cellulose chains can stabilize the local structure within the CNCs, and plays a key role in the formation of crystalline domains. Crystallinity strongly influences the physical and chemical behavior of CNCs. For example, the crystallinity of CNCs directly influences their accessibility for chemical derivatization, swelling and water-binding properties. CNCs are polar and hydrophilic.

[0050] The emulsions according to the present disclosure comprise an internal or dispersed phase and a continuous or external phase. The content of the internal phase may be between 10 to 95 % by volume of the emulsion. The two phases have an interface in which CNCs are located to stabilize the emulsion. The internal phase may be hydrophobic and the external phase hydrophilic. The internal phase may be hydrophilic and the external phase hydrophobic. The internal phase may be water- immiscible and the external phase aqueous and vice-versa. The water-immiscible phase may be an oil, polymer, or any water-immiscible solvent.

[0051] The CNCs may be in the emulsion in a concentration of 0.001 to 5% by weight of the emulsion, preferably in the range 0.1 to 4%. [0052] Suitable CNCs can be produced from biomass, such as, but not limited to, cotton, grass, wheat straw, wood pulp and tunicate.

[0053] The emulsion may further comprise cellulose filaments (CF) or nanofibrils. The term “cellulose filaments” or “CF” as used herein refers to heterogeneous materials that can distribute evenly on oil droplet surfaces under strong mechanical mixing (e.g. , homogenization) to stabilize oil-in-water emulsions. CF may be produced from wood pulp (unbleached wood pulp and mechanical pulp).

[0054] The emulsion of the present disclosure can achieve rheological performance similar to conventional emulsions without using any rheological modifiers and impart the emulsion new characteristics.

[0055] When the dispersed phase is water-immiscible and the continuous phase is aqueous, the ionic strength of the aqueous phase needs to be controlled by adding a water-soluble salt, for example NaCI. The salt may be monovalent, divalent or trivalent. Examples of other salts include, but not limited to, LiCI, CuCI 2 , MgCI 2 , NaN0 3 , Na 2 S0 , CuS0 4 . The pH of the aqueous phase comprising the CNCs is preferably between 6 and 7. When lower or higher pH is required, e.g., 4 > pH > 10, the salt concentration would need to be accordingly adjusted to balance the ionic strength change.

[0056] When the dispersed phase is aqueous and the continuous phase is water- immiscible, the CNC surfaces require functionalization to render them dispersible in the continuous phase. For example, for non-aqueous emulsions, CNC surfaces would require functionalization to render them hydrophobic and dispersible in a less-polar oil phase. The functionalization may be performed by ion-exchange, grafting of polymers, grafting of small molecules, and adsorption of polymers or small molecules, to convert the CNC surfaces to be hydrophobic. In such a case, the hydrophobic CNCs can be dispersed in the oil phase to form water-in-oil emulsions.

[0057] By adjusting (i) the volume ratio of internal phase to external phase, for example water-immiscible (such as oil) to aqueous (such as water), (ii) the concentration of the stabilizer, CNCs or CNC/CF hybrid, and (iii) the ionic strength of the aqueous phase, the emulsion properties, specifically, the rheological properties can be tailored. When properly controlled, the resulting emulsions are responsive to external shear stress as a function of time and/or shear rate. Thus, when the shear rate is over a certain threshold value, which can be controlled by adjusting the emulsion composition, the emulsion can break up and can directly be used to stabilize emulsions such as oil-in-water emulsions.

[0058] The emulsions have a controlled rheological performance, for example, storage and loss modulus, flow point, viscosity and shear stress against shear rate, threshold shear rate where emulsion breaks. Such emulsions are stable in long-term, for instance, more than one year, in storage conditions. The emulsions may be tuned to break at a predetermined threshold shear rate. This advantage allows the release of the contents of the dispersed phase under predetermined conditions.

[0059] The emulsion may be used in a multitude of applications such as cosmetic or topical skin products, such as medical and cosmetic creams and ointment, where the functional ingredients are in the dispersed phase. During application, the emulsion can first be spread over a desired area evenly using a slow moving action, and then break up the emulsion to release the active ingredient in a controlled period of time by exerting a high shear rate, e.g., rubbing. Other applications include but are not limited to personal care products, biomedical engineering products, food, pharmaceutical compositions, microfluidics and drilling fluids.

[0060] A two-step mixing method can be used to produce the CNC stabilized emulsions. First an internal hydrophobic phase is mixed into an external aqueous phase comprising cellulose nanocrystals and a dissolved salt to form a precursor emulsion such as the internal phase is less than 50 % by volume of the precursor emulsion, preferably less than 30 % by volume. Then, an additional quantity of the internal hydrophobic phase is mixed under high shear with the precursor emulsion to obtain the emulsion stabilized by cellulose nanocrystals where the internal phase is at least 50 % by volume of the emulsion, preferably at least 74 % by volume of the emulsion. High shear mixing can be performed, such as by high-speed homogenization or ultra-sonication.

[0061] For example, an emulsion containing low oil volume, typically less than 50 vol. %, could be formed first by applying low-shear mixing, 500-3000 rpm using a homogenizer, e.g., 1000 rpm. Then, extra oil is added into the emulsion, followed by high-shear mechanical mixing, typically in the range 8,000-30,000 rpm using a homogenizer. The formation mechanisms of this type of emulsion under different CNC concentrations is that the negatively-charged CNCs self-assemble into an ordered thin film at the oil/water interface. In CNC-stabilized MIPEs or HIPEs, hydrogen bonding interactions are formed between CNC nanoparticles on the same oil droplet and on adjacent ones owing to the CNCs’ large surface area (ca. 400 g/nri ) and abundant hydroxyl groups on the surfaces of CNCs. These interactions dominate the emulsion characteristics and can be tailored to control emulsion properties for different applications.

[0062] In the case of oil-in-water emulsions stabilized by CNCs, typically, a small portion of the dispersed oil phase is added into the continuous water phase in which the CNCs are dispersed prior to mixing. Then, mechanical mixing is applied to the mixture to convert it into a precursor emulsion. The volume content of the internal phase in this precursor emulsion must be below 50 vol. %, preferably, below 30 vol. %. Then, more internal phase liquid is added into the precursor emulsion with the application of high-shear mixing, such as high-speed homogenization or ultra- sonication, to obtain the final emulsion. The volume content of the internal phase in the final emulsion is higher than 50 vol. %, and can be higher than 74 vol. %. If the content of the oil phase is lower than 50 vol. % in the final emulsion, the oil phase can be gradually added into the aqueous phase with low-shear mechanical mixing, followed by high-shear mixing directly.

[0063] To tune the rheological properties of the emulsions of the present disclosure, it is encompassed that one or more of the following can be done: modify the volume ratio of the internal phase to the external phase, modify the concentration of CNC and/or CF, and/or modify the ionic strength of the aqueous phase.

[0064] The emulsions of the present disclosure have therefore been shown to be useful in practical applications such as application of topical skin products, such as topical medications, sunscreens, and cosmetics. Many of these products are oil-in- water emulsion systems and the effective ingredients are dispersed in the oil phase. When stabilized using surfactants, the stability over storage can be maintained using additives to specifically maintain high viscosity. However, when applied to skin, high- viscosity emulsions are not desirable because they are difficult to spread. Furthermore, surfactant-based products will remain as emulsions even after spreading, and leave white marks on the skin for some time until one of the phases in the emulsion is evaporated or absorbed by the skin. When CNC-based, shear-responsive emulsions are used in such topical skin products, they can remain stable in storage over a long period without the need for any rheology modifier, such as thickeners. When applied to the skin, for instance, they can evenly spread over the desired area using slow movement (or shear rate). Subsequently, high shear rate action, such as rubbing, can be applied, and the emulsion will break up, thereby allowing the active ingredients to be released. The release rate can be tailored, based on the requirement of the product, by controlling the emulsion compositions. Other applications that can employ the same functionality can be in drilling fluids, whereby the CNC-stabilized emulsion can modulate its rheological performance according to the level of external shear.

EXAMPLE I

Analysis of the viscosity of the emulsions at different shear rates

[0065] Three samples of emulsions were stabilized using cellulose nanocrystals, CNCs, and a fourth sample with Sodium Dodecyl Sulfate (SDS), for comparison. In the following examples, the CNCs were produced from bleached wood pulp by sulfuric acid hydrolysis and the CF was produced from bleached wood pulp by mechanical refining with energy input of about 6,500 kWh/ton. These correspond to samples 1 to 3 and 4 respectively, as shown in Table 1. For the emulsions stabilized with CNCs, the ionic strength of the water phase was controlled by adding 50 mM NaCI. The SDS-stabilized emulsion had no salt added. For these four samples, the oil phase was a light mineral oil with a density and viscosity of 0.838 g/mL and 23.7 mPa-s, respectively.

[0066] The shear-responsive behaviour of this type of emulsion is shown in Figs. 1A-C. In Fig. 1A, at 0.1 s '1 shear rate, below the threshold value, all CNC-stabilized emulsions remained stable over long periods of time under constant shear, which is reflected by the constant viscosity over time. As shown in Fig. 1B, when the shear rate was increased to 1 s -1 , it exceeded the threshold of sample 1, yet was still below the threshold of samples 2 and 3, therefore, only sample 1 broke. With further increase of shear rate to 10 s 1 shown in Fig. 1C, both samples 1 and 2 broke up, but at different speeds (different shear sensitivity). For sample 1, the breaking occurred at a fast speed, which was reflected by the sharp decrease in viscosity. Sample 2, however, gradually broke over a longer period of time. In contrast, the surfactant-stabilized emulsion remained unaffected over the tested period regardless of the magnitude of applied shear rates. When the oil content was decreased from 80 vol. % to 65 vol. %, demonstrated by sample 3, the emulsion (sample 3) was similarly unresponsive to external shear stress as the control sample 4. Table 1

Compositions of the emulsion samples 1 to 4

EXAMPLE II

Analysis of the viscosity of sunscreen emulsions

[0067] Sample 5 was a sunscreen composition comprising 2 wt. % of surfactants in the water phase, no CNC, no CF, and no NaCI. This emulsion comprised sunscreen ingredients, including five types of UV filters and two emollients as the oil phase. The details of the composition are given in Table 2. This emulsion was stabilized using a non-ionic surfactant, Tween® 20. This sample also contained 0.25 % of xanthan gum in the water phase as rheology modifier. A rheology modifier, xanthan gum, was added in such a system to maintain emulsion stability and to control the rheology performance of the emulsion. The sample was evaluated using a rheometer by measuring the viscosity under different shear rates, which increases from 0.1 s 1 to 1,000 s 1 , followed by sweeping back to 0.1 s 1 . The results are shown in figure 2. The flow curves in the two sweeping directions are similar, suggesting sample 5 maintained the emulsion structure during shearing. Sample 5 demonstrated good stability under shearing. Table 2

Ingredients employed in the sunscreen composition

*Xanthan gum is used in the surfactant-stabilized emulsions only.

[0068] The responsiveness of CNC-based emulsions to shearing can also be tuned by controlling the ionic strength of the water phase. This kind of effect is illustrated by Figs. 3A-D and samples 6 to 9, which were emulsions comprising sunscreen ingredients (the oil phase) dispersed in water. All these examples possessed the same oil phase as sample 5, a mixture of five types of UV filters and two emollients (Table 2). The ionic strength in the water phase was controlled by addition of NaCI, and the details are given in Table 3. The performance of the emulsions was evaluated by flow sweep testing using a rheometer, where the samples were sheared under rate increasing from 0.1 s 1 to 1,000 s 1 (low to high), followed by sweeping back from 1,000 s '1 to 0.1 s 1 (high to low). The viscosity of the samples was recorded during this process. As shown in Fig. 3A, without addition of NaCI, sample 6 exhibited similar flow curves when swept from different directions. Thus, sample 6 maintained a stable emulsion structure during testing. Samples 7 and 8 contained 10 mM and 20 mM NaCI, respectively, in the water phase and they both showed stable emulsion structure within the tested flow rate range (figures 3b and 3c). However, when the NaCI concentration was increased to 30 mM in the water phase for sample 9, the viscosity could not be restored when the shear rate was swept back from high to low, suggesting the emulsion structure in sample 9 was destructed under high shear rate.

Table 3

Composition of samples 6 to 9

[0069] The response of sample 6 to shear rate was very close to sample 5 (Fig. 2), which possessed the same oil phase as sample 6, but was stabilized with a non-ionic surfactant, Tween® 20, and a rheology modifier, xanthan gum. Thus, by using CNC alone, this type of emulsion could achieve similar rheological characteristics as conventional emulsion systems stabilized by surfactants and rheology modifiers.

EXAMPLE III

Analysis of the effect of the addition of CF

[0070] The rheological performance of CNC-stabilized emulsions can also be affected by introducing other fibrillated cellulosic nanomaterials, for example, cellulose filament (CF). The composition of samples 10 to 13 is summarized in Table 4 and they had the same oil composition as shown in Table 2. Sample 10 was stabilized with both CNCs and CF. Compared to sample 7, sample 10 showed lower stability in flow sweep test (comparing Fig. 4A and Fig. 3B), even though they had the same ionic strength, 10 mM NaCI, in the water phase. In such a hybrid system, the emulsion stability under shearing decreased with increasing ionic strength as well, as shown in Figs. 4A-D. When the NaCI concentration is increased to 80 mM, sample 13 showed a completely separated flow curve when sweeping back from high to low shear rate. This means the emulsion broke completely under high shear rate.

[0071] It should be noted that all sunscreen emulsion samples (5-13) contained 0.2 % disodium EDTA, which can also contribute to the ionic strength in the water phase. The EDTA content may vary depending on the type of product, the formulation, and the applications. A complete formulation may also contain other ingredients affecting ionic strength. Therefore, the required NaCI concentration to control the emulsion rheological performance may be different for various formulations. Furthermore, multivalent salts have higher capability to affect the ionic strength than the monovalent salts under the same concentration. Lower salt concentration will be needed if such salts are used in emulsions.

Table 4

Composition of samples 10 to 13

[0072] All of the CNCs used in the examples were in near neutral suspensions (pH between about 6 to about 7). If the pH value of the aqueous phase was changed, the behavior of the emulsions could have also been affected. The pH value can influence the emulsions from two aspects. First, either an increase or a decrease of pH can lead to an increase of the ionic strength in aqueous phase, whereby the emulsion stability under shearing will be reduced as just discussed above. On the other hand, CNCs possess sulfate groups on their surfaces, which can be protonated at lower pH. With decreasing pH in water phase, the charge density on CNC surfaces will decrease, and CNC nanoparticles are more likely to form aggregates. Thus, the emulsion stability will decrease under shearing.

EXAMPLE IV

Comparative analysis with a commercial sunscreen

[0073] Fig. 5A shows the flow sweep curves of sample 7 and a commercial sunscreen product with sun protection factor (SPF) 50. The commercial sunscreen product contained several ingredients for emulsion stabilization and rheology modification, including cetyl PEG/PPG-10/1 dimethycone, neopentyl glycol diheptanoate and PEG-12 dimethicone crosspolymer. At shear rates 0.1 to 10 s 1 , both sample 7 and the commercial sunscreen showed similar viscosity. Above shear rate 10 s 1 , sample 7 exhibited lower viscosity than the commercial sunscreen, which means that sample 7 is easier to spread on skin in a real practical application. This difference is more obviously demonstrated by the shear stress vs. shear rate curves in Fig. 5B. It can be found that above shear rate of 10 s 1 , the shear stress of sample 7 remains near flat, whereas the shear stress of the commercial sunscreen increases drastically. At shear of 1,000 s 1 , the difference becomes one order of magnitude. The amplitude sweep properties of sample 7 and the commercial sunscreen sample are compared and the curves are given in Figs. 6A and B. It can be seen that both samples showed very similar storage modulus and loss modulus at low oscillation strain (linear viscoelastic region, LVE). With increasing oscillation strain, both samples started to flow at similar points, about 30% and about 25%, respectively. These results suggest that CNC-stabilized sunscreen emulsions can possess similar viscoelastic properties to those commercial products stabilized with a complex system, yet the CNC emulsions are much easier to spread on skins during application.

[0074] This shear-responsive characteristic specific to CNC-stabilized emulsions can be used for a variety of applications, where external pressure may be applied and released leading to a change in the rheology and, potentially, morphology of the emulsion system. For example, at 80 vol. % oil content, an emulsion containing 1.2 wt. % CNCs breaks at shear rate of 1 s _1 . For the same emulsion, if the CNC content is doubled, the emulsion will break at 10 s 1 . The rheology performance of emulsions can also be tuned by changing the ionic strength or pH of the water phase, by addition of CF in the system, or a combination of them.

[0075] While the disclosure has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations and including such departures from the present disclosure as come within known or customary practice within the art and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.