Influence of Hofmeister cations and composition on carboxymethyl cellulose stabilized o/w Pickering emulsions and application in the encapsulation of vitamin E

doi:10.21203/rs.3.rs-2007199/v1

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Influence of Hofmeister cations and composition on carboxymethyl cellulose stabilized o/w Pickering emulsions and application in the encapsulation of vitamin E

https://doi.org/10.21203/rs.3.rs-2007199/v1

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Pickering emulsions have gained considerable interest due to their applications in the food, cosmetics, and pharmaceutical industries. In this study, we investigated the roles of pH, oil/water volume ratio, Hofmeister cations (Na⁺, NH₄⁺, Ca²⁺ and Mg²⁺) as well as chitosan concentrations on carboxymethyl cellulose (CMC) stabilized Pickering emulsions and their application in the encapsulation of vitamin E. Morphology, crystallinity and functional groups were investigated by Scanning Electron Microscopy (SEM), Optical Microscopic Techniques, X-ray Diffractometry (XRD) and Fourier Transform Infrared (FTIR) spectrometry. SEM images revealed a thread-like morphology for CMC, while sponge-like morphology was observed in the images of chitosan and CMC-chitosan complex (CMC-CH). The combination of CMC and chitosan resulted in a complex structure with remarkable improvement in the stability of the emulsions to both creaming and coalescence compared to systems stabilized by either CMC or chitosan alone. At fixed oil:water ratio, the transitional phase inversion of emulsions stabilized by CMC occurred upon increasing the concentration of NH₄⁺, Mg²⁺ and Ca²⁺ Hofmeister cations, whereas emulsions containing Na⁺ ions are o/w at all concentrations. An increase in contact angle upon increasing the salt concentration was observed, which might be responsible for the observed phase inversion. Emulsion stability to coalescence is more favoured at lower pH for chitosan particles, while CMC is more favoured at higher pH. A CMC-Chitosan stabilised Pickering emulsion system was successfully applied in the encapsulation of Vitamin E.

Carboxymethyl Cellulose

Chitosan

Hofmeister

Pickering emulsion

Vitamin E

The last two decades have witnessed sustained research efforts on particle-stabilized (Pickering) emulsions owing to their diverse applications in food (Zhu et al., 2019; Berton Carabin and Schroën, 2015), cosmetics, pharmaceuticals (Sodeinde et al., 2021; Marto et al., 2016), recovery of crude oil (Son et al., 2015), catalysis (Pera-Titus et al., 2015), etc. In Pickering emulsions, droplets are stabilized by solid particles without the addition of molecular emulsifiers (Aveyard, Binks & Clint, 2003). Hence, Pickering emulsions are particularly favoured in “surfactant-free” food products. The partitioning of solid particles at oil-water interface is not necessarily due to their amphiphilic nature, but the partial wetting of the particles by the water and oil phase, which allow strong anchoring of the particles at the interface (Chevalier and Bolzinger, 2013). Thus, the particles provide considerable mechanical support at the interface against coalescence. The property of the emulsions is determined by the particle wettability, and this can be quantitatively determined by measuring the contact angle $\left(\theta \right)$ which the particle makes at the oil–water interface. The contact angle is related to solid-oil tension,${\gamma }_{so, }$ solid-water tension, ${\gamma }_{sw}$, and oil-water interfacial tension, ${\gamma }_{ow}$by the Young’s equation (Binks and Olusanya, 2018; Binks and Clint, 2002):

$$\text{cos}\theta = \frac{{\gamma }_{so -{\gamma }_{sw} }}{{\gamma }_{ow}}$$

If the value of θ is greater than 90^o, water-in-oil (w/o) emulsions will be formed whereas if θ < 90^o, oil-in-water (o/w) emulsions will be produced. The variations in value of θ requires simultaneous understanding of the response of all three interfacial tensions to changes in the composition of oil and aqueous phase. From the knowledge of interfacial thermodynamics, the presence or absence of a solute at the interface will either decrease or increase the corresponding interfacial tension (de Gennes, 1985; Lyklema, 2000), which will ultimately affect the overall wettability of the solid particle and the properties of the resulting emulsion. The surface tensions are often affected by the polarizability of the ion itself and by hydration effects. Both of which are highly ion-specific. The influence of electrolytes on the wettability of solid surfaces has been studied and reported by some authors (Sedev et al., 2004; Kondiparty et al., 2011; Koopal, 2012). Recently, the effect of increasing CaCl₂ concentration and pH values on wettability of mica in decane under ambient condition was reported by Mugele et al. (2015). It was found that at low salt concentration and pH, the contact angles were very low, whereas finite contact angles were measured at concentrations exceeding a few tens of millimolar. Additionally, recent studies by Bera et a.l (2018) showed that the measured contact angle of a drop of aqueous chloride salt solutions under non-polar oil on the surface of mica increased from a value close to zero to a value greater than 10° as the Hofmeister cation concentration was increased. The ability of the cations to induce partial wetting follows this order: Na⁺, K⁺< Li⁺ <Rb⁺< Cs⁺< Ca²⁺< Mg²⁺ < Ba²⁺. The studies cited above are mainly devoted to ion effects on systems involving air-water, oil-water, and oil-solid-water interfaces. Studies describing the effect of ions along the Hofmeister series concerning stabilization of Pickering emulsion are rare in the literature.

Hofmeister series are an order of ions arranged according to their precipitating efficiency (Collins, 2004). It helps in predicting the ability of ions to strengthen hydrophobic interactions in colloidal systems (Zhang and Cremer, 2006). The specific ion effect of the Hofmeister series on the interaction between proteins, macromolecules, or colloidal particles has been qualitatively interpreted based on the ionic size, the ionic interaction and the hydration force Bostrom, Kunz and Ninham, 2005). With reference to several research papers on various systems (Kunz, 2010; Bostrom, Kunz and Ninham, 2005), the widely accepted order of the Hofmeister cation series follows this order:

Na⁺> K⁺> Li⁺> Ca²⁺> Mg²⁺> Fe²⁺> Zn²⁺> Cs⁺> Al³⁺> NH₄⁺> H⁺

Recently, the influence of three different Hofmeister cations (H⁺, Na⁺, K⁺) on the stability of oil-in-water emulsions stabilized by cellulose nanocrystals was studied by Liu et al. (2018). The Pickering emulsions were formulated using two types of oil of different polarities (high polar corn oil and non-polar hexadecane). With the addition of hydrogen ions, stable emulsions were formed by the system containing the polar oil, whereas the addition of either Na⁺or K⁺ results in the formation of stable emulsions using both the polar and non-polar oil. Lawal et al (2009) reported the effects of Hofmeister series salts on emulsions, foams and gel properties of jack bean protein. The study revealed that kosmotropic salts improved water absorption capacity and least gelation concentration of the protein than the chaotropic salts, whereas a reverse trend was observed with the foaming and emulsifying properties of the protein isolate.

By tuning the electrostatic interactions between biopolymers, bioparticles having combined favourable properties of both biopolymers and improved surface activity, can be prepared. Yildiz et al. (2018) prepared different complexes from the mixtures of protein (soy or pea protein) and polysaccharides (arabic gum, pectin, or modified starch). Their results showed that around the isoelectric point (i.e.p) of the protein, all the complexes exhibited improved solubility, surface hydrophobicity, and emulsifying ability, in particular, complexes derived from pea protein. Most of the studies on stabilization of Pickering emulsions involve systems containing only one biopolymer type. However, very few studies have been concerned with the application of mixtures of biopolymers in Pickering stabilization (Jafari et al., 2020).

The incorporation of vitamin E into food and pharmaceutical systems can facilitate easier design of novel functional products with improved health benefits and prolonged shelf life through its oxygen scavenging property. Despite the numerous health benefits of vitamin E to humans, its poor water solubility and degradation under ambient conditions pose a serious threat to its widespread applications. To overcome these barriers, different types of delivery systems suitable for lipophilic bioactive compounds have been developed, e.g. emulsions, liposomes, organogels, biopolymer coacervates, etc (Mao et al., 2017; Patravale and Mandawgade, 2008). Of these systems, Pickering emulsions provide the most suitable delivery vehicles for the encapsulation of functional ingredients.

In spite of the several studies conducted on the synthesis, characterization and applications of cellulose derivatives, there is paucity of relevant information on the influence of Hofmeister cations on the properties of Pickering emulsions stabilized by carboxymethyl cellulose (CMC). Recently, using the powdered particle method, we showed that stable o/w emulsions could be formed using CMC (derived from Delonix regia pods) as sole emulsifier of water and vegetable oil and the system was subsequently applied in the encapsulation of vitamin A (Sodeinde et al., 2021). The major goal of this present work is to investigate the influence of Hofmeister cations and weight fraction of chitosan on the properties of CMC-stabilized emulsions. We present the transitional phase inversion of CMC-stabilized Pickering emulsions induced by Hofmeister cations (Na⁺, NH₄⁺, Ca²⁺ and Mg²⁺) and we also provide the rationale for our experimental observation. In addition, the effects of pH and weight fraction of chitosan (w_ch) on the properties of emulsions stabilized by CMC and its practical application in the encapsulation of vitamin E are reported.

2.1 Materials

The chemical reagents used include hydrochloric acid, sodium hydroxide, sodium chloride, ammonium chloride, calcium chloride, magnesium chloride, King’s vegetable oil and chitosan (Sigma Aldrich, Germany, purity > 99%). All the chemical reagents used were analytical grade. The carboxymethyl cellulose used in this work was prepared from cellulose derived from Delonix regia pods (Sodeinde et al., 2021). The degree of substitution at 60 ^oC of the carboxymethyl cellulose is 2.84 (Sodeinde et al., 2021).

2.2 FTIR spectroscopic analysis

FTIR spectra of CMC, chitosan and CMC-Chitosan (CMC-CH) were measured using the KBr-technique on Perkin Elmer Spectrum Two TM spectrometer in the wavenumber range 4000–400 cm^− 1.

2.3 Scanning Electron Microscopy (SEM) analysis

A thin layer of sample granule was placed on aluminium specimen holder by double-sided tape. The specimen holder was loaded in a polaron SC 7610 sputter coater and coated with gold to a thickness of about 30 nm to prevent charging. The specimen holder was transferred to XL-20 series of scanning electron microscope. The particles were examined using a JEOL USA Model: JSM-7900F at an accelerating voltage of 15–20 kV.

2.4 Wide angle X-ray Diffractometry

Wide angle X-ray diffraction patterns of powdered CMC and CMC-CH nanocomposite samples were measured using Empyrean XRD diffractometer at 40 mA and 45 kV with Cu K_α(1.5418 Å) radiation at an angular incidence of 10-75^o.

2.5 Measurement of surface and interfacial tensions

The surface tensions of distilled water, oil and oil-water were measured using the ring method (BZY 102 Automatic Surface tensiometer). Before measurement was done, the tensiometer was calibrated using the standard mass. Thereafter, the platinum ring was hung on the hook in the instrument. The measuring vessel was placed in a plastic jacket on a perpendicularly moveable platform. For the measurement of air-water (or oil-air) surface tension, the ring was immersed in the vessel containing the distilled water (or oil). Then the ring was pulled out by lowering the platform slowly, creating a meniscus at the air-water (or oil-air) interface and the surface tension was then recorded using the maximum-pull method. Each measurement was reproduced at least 5 times to obtain an average value. For the oil-water interfacial tension, 2 cm³ of water was measured and then 2 cm³ of the oil was added in a vessel (height, 1.2 cm; diameter, 5.4 cm). The mixture was allowed to become completely phase separated and the initial procedure was followed to determine the interfacial tension (Destribats et al., 2014).

2.6 Contact Angle Measurements.

The water contact angles of CMC were measured by using a Dataphysics contact angle measurement system system OCA 25 (Dataphysics, Germany) with sessile drop method. Before the contact angle measurement, disc was made from the dried sample of CMC by compressing approximately 250 mg of CMC particles using a hydraulic press with a pressure of 10^− 8 N m^− 2. Various concentrations of solutions of NaCl, NH₄Cl, MgCl₂, and CaCl₂ the range from 0 mM to 40 mM were used for the measurements. The dosing volume was 5.0 µL and the dosing rate was 0.5 µL. All measurements were carried out at room temperature (30$\pm 2 ºC)$ and average of five measurements were taken for each sample.

2.7 Preparation and characterization of emulsions

The powdered particle method was employed for the preparation of emulsions using water volume fraction, 𝜙_w = 0.5. The required volumes of water and oil (1:1 v/v) were measured into 25 mL vial having an internal diameter of 20 mm and a height of 100 mm. The denser liquid (water) was first measured and followed by the less dense liquid (vegetable oil). The required mass of CMC particles was subsequently measured into separate 25 cm³ screw capped vials containing the water-vegetable oil mixture. The particle-water-vegetable oil mixture was homogenized using a Hanchen lab homogenizer with 12 mm head operating at 12,000 rpm for 2 min. The drop test was carried out to determine the emulsion type. This was determined by gently placing a drop of the emulsion on the surface of pure oil or water. An oil-in-water emulsion is expected to disperse in water while it will remain as a drop in the oil and vice versa (Binks and Olusanya, 2017).

The effect of pH was studied by varying the pH of the phase from 2 to 10 using 1 wt. % of either CMC, chitosan or CMC-chitosan complexes (CMC-CH, 1:1w/w) as the emulsifiers and ϕ_w = 0.5. The CMC/chitosan complex was prepared following the procedure described by Zhu et al. (2021) with little modifications. The two biopolymers were mixed in 25 mL screw-capped vials using a Hanchen lab homogenizer with a 12 mm head operating at 15,000 rpm for 2 min and a white precipitate was formed in the process. The pH of the aqueous phase was monitored with a pH meter (Search Tech) and adjusted by adding drops of either 0.1 M sodium hydroxide solution or hydrochloric acid solution. To study the effect of salt, emulsions were prepared using equal volumes of oil and water with 1 wt.% CMC particles but the concentration of the salt in water phase was varied from 1 mM to 60 mM. To investigate the influence of chitosan on properties of CMC-stabilized emulsions, CMC-chitosan complexes (at different weight fraction; 0.2–1 wt.%) were prepared at pH 4. Emulsions containing equal volumes of water and vegetable oil were prepared at varying weight fraction (W_ch) of the chitosan particles in a mixture containing CMC-Chitosan while maintaining the total particle concentration of 1 wt.%.

The stability to coalescence of the o/w emulsions was evaluated by measuring the amount of oil ejected with time while the extent of creaming was determined by measuring the quantity of water released with time. The water or oil fraction ejected was calculated according to:

$$Fraction of water or oil= \frac{volume of water or oil released}{total volume of water or oil used}$$

Optical microscopy observation of emulsion was done using a Bresser optical microscope equipped with a CCD camera and optical microscopic images of 1280 × 720 pixels were obtained at ×4 and ×10 magnifications. The top view image software was used for processing the image. The calibration of the microscope was performed before use with a reference stage graticule. A tiny emulsion drop taken from the middle of the emulsion layer was diluted with the continuous phase before being observed under the microscope. The mean drop diameter was estimated from the digital image by measuring the diameter of 100 droplets using Image J 1.47v. The photograph of vials containing o/w emulsions was taken immediately after preparation. The zeta potential of CMC, chitosan and CMC-chitosan dispersions was determined by dynamic light scattering (DLS) techniques using a Zetasizer Nano ZS (Malvern Instruments, Malvern, U.K) with an operating wavelength of 633 nm and a measurement angle of 173º. The dispersions were prepared with HPLC-grade water and diluted to a concentration of 0.1 mg/mL. Each measurement was performed at various pH values between 2 and 13 and at room temperature (30$\pm 2 ºC)$. The zeta potential values were calculated with Dispersion Technology Software (DTS, Version 3.302002, Malvern Instruments, Malvern, U.K)

2.8 Encapsulation of vitamin E

The encapsulation ability of the CMC particles was investigated using an oil-soluble vitamin E. Vitamin E (15 mg) was dissolved in vegetable oil (5 mL). This was followed by the addition of 5 mL distilled water and 1wt%. particles of CMC. The resulting mixture was homogenized at 12,000 rpm for 2 min and the amount of oil released with time was monitored. The pictures of vials containing oil/water emulsions were taken immediately after preparation and 2 weeks later at room temperature. Encapsulation efficiency (EE) of the loaded vitamin E in the oil layer was studied using High-Performance Liquid Chromatography (HPLC, Shimadzu, Nexera MX) equipped with a UV detector at 295 nm and µbondapak C 18 column (100 mm× 4.6 mm × 7µm). The mobile phase comprised acetonitrile/water mixture (70:30%, v/v) at 2.0 mL/min flow rate. For the sample pretreatment before HPLC analysis, an aliquot of the oil phase of the encapsulated vitamin E oil/water emulsion was extracted with 10 mL petroleum ether. The extract was stabilized with 25 mL ethyl acetate in a standard volumetric flask. 5 µL of the treated sample was injected into the HPLC injection port at a column oven temperature of 40 ^oC. The EE was calculated using Eq. 4:

$$EE \left(\%\right)= \frac{{V}_{At}- {V}_{Ao}}{{V}_{At}} \times 100$$

where 𝑉_𝐴𝑡 𝑖𝑠 the total amount of vitamin E in the whole emulsion and 𝑉_𝐴𝑜 is the amount of vitamin E in the outer oil phase.

3.1 Scanning Electron Microscopy (SEM)

The SEM images of CMC, chitosan and CMC-Chitosan are shown in Fig. 1(a)-(c). In Fig. 1(a), it can be seen that CMC has a thread-like morphology. The SEM image of chitosan in Fig. 1(b) reveals coherent layer of particles with a nonporous, non-homogeneous and non-smooth sponge-like surface. For CMC-Chitosan composite (Fig. 1(c)), SEM image shows a well dispersed thread-like particles embedded in the chitosan within the composite. The composite exhibits a nonporous, non-smooth membranous phase consisting of sponge-like, microfibrillated, and dome shaped structures.

3.2 X-ray Diffraction

XRD patterns of CMC and CMC-chitosan composite are shown in Fig. 2. For CMC alone, two prominent peaks appeared at 2θ = 17^o and 23^o. In the case of CMC-Chitosan composite (1:1 w/w), there is disappearance of the peak at 2θ = 17^o and the appearance of a small, but noticeable peak at 2θ = 9.8^o and peak overlap of lower intensity around 2θ = 20^o which might be attributed to the modification of the CMC via the incorporation of the chitosan molecule.

3.3 Transitional phase inversion induced by Hofmeister cations

From our previous studies, we had established the possibility of forming stable vegetable oil/water emulsions using CMC particles (Sodeinde et al., 2021). The preferred emulsion type was o/w and emulsions were stable to coalescence for more than two months of storage. The stability of the emulsions to coalescence and creaming increased with CMC concentration and only 0.5 wt.% CMC particles were required to produce emulsions stable to coalescence. Here, we studied the effect of Hofmeister cations on the properties of Pickering emulsion stabilized by CMC. For this purpose, four different Hofmeister cations viz: Na⁺, NH₄⁺, Mg²⁺ and Ca²⁺ (having the same anion; Cl⁻) were selected. Emulsions were prepared using the powdered particle method using equal volumes of oil and water but at different concentrations of Hofmeister cations in the aqueous phase. Figure 3(i) depicts the photos of vessels containing vegetable oil-water emulsions at different cation concentrations taken after two weeks. The result of the drop test indicates that emulsions in Fig. 3(i) are o/w at all concentrations of Na⁺ whereas both types of emulsions are formed in the other three systems (NH₄⁺, Mg²⁺ and Ca²⁺) as the cation concentration increases. These systems phase invert from o/w to w/o emulsions upon increasing the concentration of the cations. Depending on the Hofmeister cations, phase inversion takes place at different salt concentrations and the position of inversion is indicated by the vertical broken lines in Figs. 3(i) (b-d). In systems containing Na⁺, an aqueous layer is seen below emulsions and a little oil layer is above. For systems that phase invert, an aqueous layer is seen below before inversion takes place, whereas little or no aqueous phase is seen after inversion. Also, little or no oil is seen above emulsions before phase inversion but oil is seen above all the systems after inversion. We assessed the stability of the emulsions by measuring the amount of oil ejected due to either sedimentation of w/o emulsions or coalescence of o/w emulsions and the amount of water released due to coalescence of w/o emulsions or creaming of o/w emulsions through phase inversion (Binks and Olusanya, 2018).

The fractions of oil and water resolved after 4 weeks are displayed in Fig. 4. In emulsions containing Na⁺ (Fig. 4a), the fraction of oil ejected by the systems decreases slightly as Na⁺ concentration increases whereas the fraction of water remains constant all through. The coalescence stability of the o/w emulsions increases upon increasing the salt concentration. For systems containing NH₄⁺, Mg²⁺ and Ca²⁺_, the fraction of oil released increases slightly towards the inversion point and decreases slightly after the inversion point as the salt concentration increases. On the other hand, the fraction of water resolved decreases all through as the concentration of the salt is increased. The stability of the o/w emulsions to both coalescence and creaming decreases slightly towards inversion composition, whereas the stability to coalescence and sedimentation of the w/o emulsions increases away from the inversion point. We have probed the microstructure of the emulsions using an optical microscope, and digital images of the emulsions taken immediately after preparation are shown in Fig. 5 for both inverting and non-inverting emulsion systems at different cation concentrations. As seen in Fig. 5, the droplets are spherical, polydisperse and flocculated. Interestingly, only oil droplets are formed in systems containing Na⁺ whereas both oil and water droplets are formed by other systems depending on the salt concentration. Oil droplets are formed at low salt concentrations, while water droplets are formed at high salt concentrations.

The variation of emulsion average drop diameter with salt concentration is shown in Fig. 6. Apart from emulsions containing Na⁺ (non-inverting system), the drop size of both o/w and w/o emulsions increases through inversion composition. The change in drop size with salt concentration is consistent with changes in the extent of sedimentation and creaming of o/w and w/o emulsions, respectively. This type of phase inversion of particle-stabilized emulsions had been earlier reported (Sun, Li & Ngai, 2010, Binks and Rodrigues, 2005). Binks and Rodrigues (2005) showed that emulsion of water and hexadecane stabilized by ionizable carboxy-coated polystyrene nanoparticles at pH 10.6 phase inverted upon increasing the concentration of aqueous NaCl solution in the water phase. It is well known that the type of emulsion formed by solid particles depends on particle wettability. The wettability of a solid by a liquid is a measure of the intimate contact between a liquid and solid, which can be quantified by contact angle measurement. A low θow value means there is a high wettability and vice versa. For relatively hydrophobic particles, w/o emulsions are preferred. By contrast, for relatively hydrophilic particles, o/w emulsions are produced. As mentioned earlier, the transitional phase inversion of emulsion occurs from o/w to w/o at fixed oil:water ratio as the salt concentration increases. If the average particle wettability changes upon the addition of a salt, this can induce phase inversion in the process. To correlate the propensity of the emulsion to phase invert upon increasing the salt concentration with the particle wettability, we have measured the air-liquid contact angle (θ_la) of a drop of salt solutions at different concentrations on the surface of a disc of CMC particles and the results are summarized in Fig. 3(ii). As seen in Fig. 3(ii), a slight increase in the value of θ_la upon increasing the salt concentration was observed. The contact angle increased by about 15 to 20^o depending on the system except for the system containing Na⁺ where the value is less than 10^o. The increase in θ_la indicates that the particle wettability is enhanced by the addition of the Hofmeister cation.

Using the surface energy components, we also calculate the oil-water contact angle (θ_ow). The surface energy components can be expressed in various forms as:

${\gamma }_{so}$ = ${\gamma }_{oa}$ + ${\gamma }_{sa}$ $-$ 2 $\sqrt{ \gamma \begin{array}{c}d\\ oa \end{array} \gamma \begin{array}{c}d\\ sa\end{array}}$ $-$ 2$\sqrt{ \gamma \begin{array}{c}p\\ oa \end{array} \gamma \begin{array}{c}p\\ sa\end{array}}$ (5)

${\gamma }_{sw}$ = ${\gamma }_{aw}$ + ${\gamma }_{sa}$ $-$ 2 $\sqrt{ \gamma \begin{array}{c}d\\ aw \end{array} \gamma \begin{array}{c}d\\ sa\end{array}}$ $-$ 2$\sqrt{ \gamma \begin{array}{c}p\\ aw \end{array} \gamma \begin{array}{c}p\\ sa\end{array}}$ (6)

${\gamma }_{ow}$ = ${\gamma }_{oa}$ + ${\gamma }_{aw}$$-$ 2 $\sqrt{ \gamma \begin{array}{c}d\\ oa \end{array} \gamma \begin{array}{c}d\\ aw\end{array}}$$-$ 2$\sqrt{ \gamma \begin{array}{c}p\\ oa \end{array} \gamma \begin{array}{c}p\\ aw\end{array}}$ (7)

Making 𝛾_𝑠𝑜 subject of the formation in Eq. 1, we have

𝛾_𝑜𝑤cos θow + 𝛾_𝑠𝑤= 𝛾_𝑠𝑜 (8)

Equating equations 5 and 8 with little rearrangement gives

$$Cos{\theta }_{ow} =\frac{{\gamma }_{oa }- {\gamma }_{aw} + 2 \sqrt{ \gamma \begin{array}{c}d\\ aw \end{array} \gamma \begin{array}{c}d\\ sa\end{array}} + 2\sqrt{ \gamma \begin{array}{c}p\\ aw \end{array} \gamma \begin{array}{c}p\\ sa\end{array}} - 2 \sqrt{ \gamma \begin{array}{c}d\\ oa \end{array} \gamma \begin{array}{c}d\\ sa\end{array}} - 2\sqrt{ \gamma \begin{array}{c}p\\ oa \end{array} \gamma \begin{array}{c}p\\ sa\end{array}} }{ {\gamma }_{ow} }$$

where 𝛾_𝑜𝑎, 𝛾_𝑎𝑤 and 𝛾_𝑜𝑤 are the air-oil surface tension, air-water surface tension and oil-water interfacial tension respectively. The values of surface tension for pure oil and water and the corresponding oil-water interfacial tension are 34.7, 71.1 and 22.1 mN/m respectively. The values of 𝛾_𝑜𝑤 measured 5 min after addition of salt to the water phase are given in Table 1. It is noted that there is a significant lowering of the interfacial tension. Since the same type of anion is present in all the salt, the extent of lowering of the surface tension by the salt may be due to the difference in the ionic polarizability, ionic hydration, and cavity potential of the cations [33]. $\gamma \begin{array}{c}d\\ aw \end{array}$, $\gamma \begin{array}{c}d\\ oa \end{array}$ and $\gamma \begin{array}{c}d\\ sa\end{array}$ are the dispersive components of the surface tension of water, oil and solid (CMC) while $\gamma \begin{array}{c}p\\ aw \end{array}$, $\gamma \begin{array}{c}p\\ oa \end{array}$ and $\gamma \begin{array}{c}p\\ sa\end{array}$ are the polar components of the surface tension of water, oil and solid (CMC). Since the values ${\gamma }_{oa}, {\gamma }_{ow, }$ and ${\gamma }_{aw }$are known, $\gamma \begin{array}{c}d\\ aw \end{array}$, $\gamma \begin{array}{c}d\\ oa \end{array},$$\gamma \begin{array}{c}p\\ aw \end{array}$ and $\gamma \begin{array}{c}p\\ oa \end{array}$ can be calculated and details can be found in the report of Binks and Clint, 2002. The values of components of solid surface energy ($\gamma \begin{array}{c}d\\ sa\end{array}$ and $\gamma \begin{array}{c}p\\ sa\end{array}$) used for our calculation here were those reported by Shen (2009). These values are the average of four surface energy components obtained using the same method (sessile drop). The values of the oil-water contact angle calculated using Eq. 9 are given in Table 1. The oil-water contact angles were calculated using the values of 𝛾_𝑎𝑤 and 𝛾_𝑜𝑤 determined around inversion composition. For the purpose of comparison, contact angle calculation was done for systems containing no salt and a value of 66.5^o was obtained. From Table 1, for those systems that undergo transitional phase inversion (i.e. NH₄⁺, Mg²⁺ and Ca²⁺), the value 𝜃_𝑜𝑤 increased from 66.5^o to around 90^o whereas for the system containing Na⁺, 𝜃_𝑜𝑤 increased to around 80^o. These results show that the presence of Hofmeister cations in the aqueous phase has a significant effect on the overall wettability of the particles depending on the nature of the cations and concentration.

Table 1

Values of γ_ow,γ_so, γ_sw and θ_ow. *Cation concentration (around inversion composition) used in the determination of oil-water interfacial tension.
cation	[cation]/mM	𝛾_𝑜𝑤 /mNm¹	𝛾_𝑠𝑜 /mNm¹	𝛾_𝑠𝑤 /mNm¹	𝜃𝑜𝑤/^O
Na⁺	30	12.1 ± 0.2	4.1	2.1	80.4 ± 2.0
NH₄⁺	30*	7.8 ± 0.2	2.0	2.1	91.2 ± 1.0
Mg²⁺	15*	8.3 ± 0.2	2.1	2.1	90.0 ± 1.0
Ca²⁺	5*	8.0 ± 0.2	2.0	2.1	90.8 ± 1.0

3.4 Influence of pH on emulsion properties

Colloidal particles having groups such as − SiOH, −COOH, −OH or − NH2 on their surface would be sensitive to variation in aqueous phase pH and this can affect their aggregation at the interface (Amalvy et al., 2004; Amalvy et al., 2003; Binks and Lumsdon, 1999). Generally, at pH values far from the isoelectric point (i.e.p. where the particles are fully charged and more hydrophilic, the particles are endowed with high surface charge density. Contrarily, at pH values around the isoelectric point (i.e.p), colloidal particles are more hydrophobic because at this condition, they possess low surface charge density and thus stabilize emulsions. In this section, we studied the effect of pH on emulsions stabilized by particles of CMC chitosan and CMC-CH complex. The photos after 1 week of vessels containing vegetable-oil emulsions prepared at different pH values using CMC, chitosan and CMC-chitosan composite are shown in Fig. 7(i)-a-c. All the systems are o/w with an aqueous layer seen below emulsions. Apart from emulsion prepared at pH 2, no oil layer is seen above emulsions in CMC-stabilized emulsions. However, the reverse is the case in emulsions stabilized by chitosan particles (i.e. there is oiling off), where stable emulsion is formed at pH 2 alone. This may be due to the high positive charge on the surface of the particle (Fig. 7(ii)). In emulsions stabilized by CMC-chitosan, no oil layer is seen above the systems at pH values < 5 which may be attributed to the high residual negative charge on the surface of the CMC-chitosan complex.

The fraction of oil and water resolved after 2 weeks of preparation is shown in Fig. 8. For CMC-stabilized emulsions, all the systems are stable to coalescence for more than two weeks except emulsion prepared at pH 2, whereas in systems stabilized by chitosan, a stable emulsion is formed only at pH 2. For systems stabilized by CMC-CH complex, emulsions are only stable to coalescence at pH 2 and 4. The optical images shown in Fig. 9(a-c) reveal that the droplets are spherical and polydisperse for CMC, chitosan and CMC-CH complex stabilized emulsions. The variation in drop size with pH is shown in Fig. 9(d) for the three systems. The drop size decreases with an increase in pH for CMC-stabilized emulsion whereas there is an increase in drop size with pH in chitosan and CMC-CH complex stabilized emulsions, and a maximum value was attained at around pH 8 and 6 respectively.

3.5 Influence of weight fraction of chitosan on emulsion properties

Emulsions stabilized by both CMC and chitosan lack long-time coalescence stability at certain pH values (i.e. pH 2–4 for CMC alone and pH 4–10 for chitosan). Most natural product-based particles are inherently hydrophilic because of the presence of some polar functional groups in their molecules. Due to their hydrophilic nature, they may not spontaneously adsorb to the oil-water interface, resulting in the formation of emulsion with poor stability. Enhancing the surface activity of non-surface-active particles is an active research area in colloid science and this can be achieved through either electrostatic or covalent interaction (Jafari et al., 2020; Ghasemi, Jafari, Assadpour & Khomeiri, 2018). At a fixed pH of 4, where the two particle types are oppositely charged (Fig. 7(ii)), we study the effect of complexation of chitosan with CMC on emulsion properties by varying the weight fraction (w_ch) of chitosan in a mixture with CMC while the total mass of the particle in the mixture is fixed at 1 wt.%. The formation CMC-chitosan complex was confirmed by FTIR analysis (Fig. 10). A slight shift in the absorption band coupled with a decrease in intensity of peaks confirmed the formation of the complex (Zhu et al., 2021). Specifically, a slight reduction in the intensity of peak due to N − H deformation around 1641 cm^− 1 and − N(CH₃)₃ around 1421 cm^− 1 in the spectrum of chitosan was observed. The appearance of additional peaks between 1172 and 1031 cm^− 1 due to C − O also attests to the formation of a complex between CMC and chitosan.

Photo of vessels containing emulsions (ϕ_w = 0.5) stabilized by mixtures of CMC and chitosan at varying w_ch values taken after 10 min is shown in Fig. 11(a). All emulsions are oil-in-water. From Fig. 5, no aqueous layer is seen below in all the emulsions and no oil layer is seen above the emulsion at all w_ch. Improvement in the stability of the emulsions to both creaming and coalescence was observed compared to the system stabilized by either CMC or chitosan. This is because both biopolymers synergistically interact to form a complex with improved surface activity, adsorbing at the oil − water interface to form emulsions with excellent coalescence stability. The stability to coalescence and creaming of the emulsions stabilized by the CMC-chitosan composite at different w_ch was assessed by measuring the amount of oil and water ejected after three weeks of preparation (Fig. 11(b)). As observed in Fig. 11(b), emulsions stabilized by either CMC or chitosan at pH 4 lack long-term coalescence stability whereas emulsions stabilized by a combination of the biopolymers (composite) exhibit excellent coalescence stability. The emulsion stability might be attributed to increased steric hindrance, electrostatic interaction, etc, associated with the CMC-chitosan composite thus preventing aggregation of droplets.

The influence of the composition of the particles on the morphology of the droplets was studied as well. Figure 11(c) shows the optical micrographs of emulsions stabilized by the complex at different w_ch at pH 3. The droplets are spherical, discrete and polydisperse. The variation of average emulsion drop size with w_ch is shown in Fig. 11 From Fig. 11(d), a minimum droplet size was attained at intermediate composition (w_ch = 0.5). This shows that more particles of the complex are available at the interface to stabilize more interfacial areas created during homogenization. This further reveals that the complex is a more effective emulsion stabilizer than the individual biopolymer (i.e. CMC and chitosan). Given that both particle types interact together to form a complex with enhancing emulsification performance, a maximum number of particles of the complex is expected to be produced at intermediate compositions. Since, the particles of the complex are more effective at the oil-water interface, a minimum drop size at the intermediate composition is expected.

3.6 Encapsulation of vitamin E

Encapsulation of bioactive compounds in stable Pickering emulsion systems is essential for preservation against environmental degradation and improve their bioavailability. The result of the HPLC analysis of encapsulated vitamin E in CMC-Chitosan stabilized Pickering o/w emulsion is shown in Fig. S1. Encapsulation efficiency (EE) of the oil soluble vitamin E after 60 days is 42.3%. Amiri et al. Amiri et al. (2018) reported around 78% EE for vitamin E using γ-oryzanol, polyethylene glycol and lauric acid surfactant-based nanoliposome delivery system after 40 days. The observed variation in the EE might be attributed to differences in the duration of encapsulation, surfactant composition and emulsion templates.

The influence of Hofmeister cations pH and composition on the properties of CMC-stabilized emulsions was studied. Using equal volumes of vegetable oil and water, we showed the transitional phase inversion of emulsions stabilized by CMC upon increasing the concentration of Hofmeister cations. It is noteworthy that emulsions containing NaCl are o/w at all concentrations whereas both types of emulsions are formed in other systems (NH₄⁺, Mg²⁺ and Ca²⁺) as the cation concentration increased. Phase inversion from o/w to w/o emulsions at different salt concentrations depending on the cation type was observed. For the effect of oil:water ratio on the stability and drop size of CMC-stabilized emulsions, no phase inversion was observed as the oil:water ratio was varied but emulsions stabilized by CMC-chitosan complex phase invert catastrophically upon increasing the oil volume fraction. A remarkable improvement in the stability of the Pickering emulsions to both creaming and coalescence was observed using CMC-Chitosan composite particles compared to systems stabilized by either CMC or chitosan particles. The synergistic interaction of both biopolymers resulted in the formation of a complex with improved surface activity. CMC-chitosan stabilised emulsion system was successfully applied in the encapsulation of vitamin E. The result of this study revealed a better understanding of interaction of Hofmeister cations and biopolymers in Pickering emulsion system thus providing suitable templates for future applications in encapsulation of food grade/pharmaceutical bioactive compounds.

Conflict of interest

All authors declare no conflict of interest

Acknowledgement

The authors acknowledge with thanks the Federal University Oye-Ekiti for providing enabling environment and the Tertiary Education Trust Fund (TETFund).

Ethical approval and consent to participate

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Consent for publication

All the authors agree to the publication of this article

Contributions

S.O. Olusanya: Investigation, Visualization, Conceptualization, Methodology, Writing - original draft,

Data curation, Validation, Writing - review & editing, Resources. D.P. Fapojuwo.: Investigation, Analysis, Formal analysis, Software, Visualization. K.O. Kehinde: Investigation, Conceptualization, Writing - original draft, Writing - review & editing, Data curation. K. Nishinari: Writing - original draft, Writing- review & editing, Project administration. H. Lindemann: Software, Formal analysis. A. Koschella: Investigation, Resources, Writing - review & editing. T. Heinze: Software, Formal analysis. O.S. Lawal: Conceptualization, Supervision, Writing - original draft, Writing- review & editing, Project administration.

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All data generated or analysed during this study are included in this published article (and the supplementary information file).

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Amalvy, J. I., Armes, S. P., Binks, B. P., Rodrigues, J. A., Unali, G.F. (2003). Use of sterically-stabilised polystyrene latex particles as a pH responsive particulate emulsifier to prepare surfactant-free oil-in-water emulsions. Chem. Commun., 1826−1827.
Amalvy, J. I., Unali, G.F., Li, Y., Granger-Bevan, S., Armes, S. P., Binks, B. P., Rodrigues, J. A., Whitby, C. P. (2004). Synthesis of sterically stabilized polystyrene latex particles using cationic block copolymers and macromonomers and their application as stimulus-responsive particulate emulsifiers for oil-in-water emulsions. Langmuir, 20, 4345−4354.
Amiri, S., Ghanbarzadeh, B., Hamishehkar, H., Hosein, M., Babazadeh, A., Adun, P. (2018). Vitamin E loaded nanoliposomes: Effects of gammaoryzanol, polyethylene glycol and lauric acid on physicochemical properties. Colloid and Interface Science Communications, 26, 1-6.
Arriagada, F., Ugarte, C., Gunther, G., Larrain, M. A., Guarnizo-Herrero, V., Nonell, S., Morales, J. (2020). Carminic acid linked to silica nanoparticles as pigment/antioxidant bifunctional excipient for pharmaceutical emulsions. Pharmaceutics, 12(4), 376.
Aveyard, R., Binks, B. P. and Clint, J. H. (2003). Emulsions stabilised solely by colloidal particles. Advances in Colloids and Interface Science, 100, 503-546.
Bauduin, P., Nohmie, F., Touraud, D., Neueder, R., Kunz, W., Ninham, B. W. (2006). Hofmeister specific-ion effects on enzyme activity and buffer pH: Horseradish peroxidase in citrate buffer. Journal of Molecular Liquids, 123, 14-19.
Bera, B., Kumar N., Duits M. H. G., Cohen Stuart M. A., and Mugele F. (2018). Cationic Hofmeister series of wettability alteration in mica−water−alkane systems, Langmuir, 34, 13574−13583.
Berton-Carabin, C.C. and Schroën, K. (2015). Pickering emulsions for food applications background, trends, and challenges. Annu. Rev. Food Sci. Technol., 6, 263–297.
Binks B. P. and Clint J. H. (2002). Solid wettability from surface energy components: Relevance to Pickering emulsions, Langmuir, 18, 1270-1273.
Binks B. P. and Olusanya S. O.(2018). Phase inversion of colored Pickering emulsions stabilized by organic pigment particle mixtures. Langmuir, 34, 5040−5051.
Binks, B. P. and Olusanya, S. O. (2017). Pickering emulsions stabilized by coloured organic pigment particles. Chemical Science, 8(1), 708-723.
Binks, B. P., Lumsdon, S. O. (1999). Stability of oil-in-water emulsions stabilised by silica particles. Phys. Chem. Chem. Phys., 1, 3007−3016.
Bostrom, M., Kunz, W., Ninham, B. W. (2005). Hofmeister effects in surface tension of aqueous electrolyte solution. Langmuir, 21, 2619-23.
Brigelius- Flohe, R., Traber, M. G. (1999). Vitamin E: function and metabolism. FASEB J., 13, 1145–1155.
Chevalier, Y. and Bolzinger, M. A. (2013). Emulsion stabilized with solid nanoparticles: Pickering emulsions. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 439, 23 -34.
Collins, K. D. (2004). Ions from the Hofmeister series and osmolytes: effects on proteins in solution and in the crystallization process. Methods, 34: 300-311.
de Gennes, P. (1985) Wetting statics and dynamics. Rev. Mod. Phys. 57, 827.
Lyklema, J. Fundamentals of Interface and Collloid Science, Volume III: Liquid-Fluid Interfaces; Academic Press: London, 2000.
Destribats, M., Rouvet, M., Gehin-Delval, C., Schmitt, C. and Binks, B. P. (2014). Emulsion stabilized by whey protein microgel particles: towards food grade Pickering emulsion. Soft Matter, 10, 6941.
Dror, D. K. and Allen, L. H. (2011). Vitamin E deficiency in developing countries. Food and Nutrition Bulletin, 32(2), 124-143.
Ghasemi, S., Jafari, S. M., Assadpour, E., Khomeiri, M. (2018). Nanoencapsulation of dlimonene within nanocarriers produced by pectin-whey protein complexes within nanocarriers produced by pectin-whey protein complexes. Food Hydrocolloids, 77, 152–162.
Jafari, S. M., Doost, A. S., Nasrabadi, M. N., Boostani, S., Van der Meeren, P. (2020). Phytoparticles for the stabilization of Pickering emulsions in the formulation of novel food colloidal dispersions. Trends in Food Science and Technology, 98, 117–128.
Koopal, L. K. (2012) Wetting of Solid Surfaces: Fundamentals and Charge effects. Adv. Colloid Interface Sci. 179−182, 29−42.
Kondiparty, K.; Nikolov, A.; Wu, S.; Wasan, D. (2011) Wetting and Spreading of Nanofluids on Solid Surfaces Driven by the Structural Disjoining Pressure: Statics Analysis and Experiments. Langmuir 27, 3324−3335.
Kunz, W. (2010). Specific ion effects in colloidal and biological systems. Curr Opin Colloid Interface Sci., 15, 34-39.
Lawal, O. S. (2009). Specific ion effect on emulsion, foams and gels of a seed protein. Food Biophysics, 4, 347-352.
Lima, E. R. A., de Melo B. M., Baptista L. T. and Paredes M. L. L. (2013). Specific ion effects on the interfacial tension of water/hydrocarbon systems. Brazilian Journal of Chemical Engineering, 30, 55–62.
Liu, L. L., Hu, Z., Sui, X. F., Guo, J., Cranston, E. D., Mao, Z. P. (2018). Effect of counterion choice on the stability of cellulose nanocrystal Pickering emulsions. Industrial and Engineering Chemistry Research, 57, 7169–7180.
Mao, L., Wang, D. Liu, F. and Gao, Y. (2017). Emulsion design for the delivery of βcarotene in complex food systems. Critical reviews in food science and nutrition. https://doi.org/10.1080/10408398.2016.1223599.
Marto, J., Ascenso, A., Simoes, S., Almeida, A. J., Ribeiro, H. M. (2016). Pickering emulsions: challenges and opportunities in topical delivery. Expert Opin. Drug Deliv., 1–15. Patravale, V. B. and Mandawgade, S. D. (2008). Novel cosmetic delivery systems: an application update. International Journal of Cosmetic Science, 30, 19–33.
Mugele, F.; Bera, B.; Cavalli, A.; Siretanu, I.; Maestro, A.; Duits, M.; Cohen-Stuart, M.; Van Den Ende, D.; Stocker, I.; Collins, I. (2015) Ion adsorption-induced wetting transition in oil-water-mineral systems. Sci. Rep. 5, 10519.
Pera-Titus, M., Leclercq, L., Clacens, J. M., De Campo, F., Nardello-Rataj, V. (2015).
Pickering interfacial catalysis for biphasic systems: From emulsion design to green reactions. Angewandte Chemie International Ed., 54(7), 2006-2021.
Sedev, R.; Fabretto, M.; Ralston, J. (2004) Wettability and surface energetics of rough fluoropolymer surfaces. J. Adhes. 80, 497− 520.
Sedev, R.; Fabretto, M.; Ralston, J. (2004) Wettability and surface energetics of rough fluoropolymer surfaces. J. Adhes. 80, 497− 520.
Shen, Q. (2009). Surface properties of cellulose and cellulose derivatives: A review, in ACS Symposium series, Chapter 12, Vol., 1019, 259-289.
Sodeinde, K. O., Ojo, A. M., Olusanya, S. O., Ayanda, O. S., Adeoye, A. O., Dada, T. M., Lawal, O. S. (2021). Cellulose isolated from Delonix regia pods: characterisation and application in the encapsulation of vitamin A. Industrial Crops and Products, 160, 113138
Son, H. A., Yoon, K.Y., Lee, G. J., Cho, J. W., Choi, S. K., Kim, J. W., Im, K. C., Kim, H. T., Lee, K. S., Sung, W. M. (2015). The potential applications in oil recovery with silica nanoparticles and polyvinyl alcohol stabilized emulsion. Journal of Petroleum Science and Engineering, 126, 152-161.
Sun G., Li Z. and Ngai T. (2010). Inversion of particle-stabilized emulsions to form highinternal phase emulsion. Angew. Chem. Int. Ed., 49, 2163 –2166
Yildiz, G., Ding, J., Andrade, J., Engeseth, N. J., Feng, H. (2018). Effect of plant proteinpolysaccharide complexes produced by mano-thermo-sonication and pH-shifting on the structure and stability of oil-in-water emulsions. Innovative Food Science & Emerging Technologies, 47, 317–325.
Zhang, Y. Cremer, P. S. (2006) Interactions between macromolecules and ions: The Hofmeister Series. Curr. Opin. Chem. Biol., 10, 658–663.
Zhu X., Chen J., Hu Y., Zhang N., Fu Y., Chen X. (2021). Tuning complexation of carboxymethyl cellulose/cationic chitosan to stabilize Pickering emulsion for curcumin encapsulation, Food Hydrocolloids, 110, 106135
Zhu, Q., Lu, H., Zhu, J., Zhang, M., Yin, L. (2019). Development and characterization of Pickering emulsion stabilized by zein/corn fibre gum (CFG) complex colloidal particles. Food Hydrocolloids, 91, 204–213.

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Influence of Hofmeister cations and composition on carboxymethyl cellulose stabilized o/w Pickering emulsions and application in the encapsulation of vitamin E

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Abstract

Figures

1. Introduction

2. Materials And Methods

2.1 Materials

2.2 FTIR spectroscopic analysis

2.3 Scanning Electron Microscopy (SEM) analysis

2.4 Wide angle X-ray Diffractometry

2.5 Measurement of surface and interfacial tensions

2.6 Contact Angle Measurements.

2.7 Preparation and characterization of emulsions

2.8 Encapsulation of vitamin E

3. Results And Discussion

3.1 Scanning Electron Microscopy (SEM)

3.2 X-ray Diffraction

3.3 Transitional phase inversion induced by Hofmeister cations

3.4 Influence of pH on emulsion properties

3.5 Influence of weight fraction of chitosan on emulsion properties

3.6 Encapsulation of vitamin E

4. Conclusions

Declarations

References

Additional Declarations

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