Ethyl Cellulose Nanoparticles at the AlkaneâWater Interface and the

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Ethyl cellulose nanoparticles at the alkane-water interface and the making of Pickering emulsions Navid Bizmark, and Marios Ioannidis Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02051 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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Ethyl cellulose nanoparticles at the alkanewater interface and the making of Pickering emulsions Navid Bizmark, Marios A. Ioannidis* Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada. *To whom correspondence should be addressed, Email: [email protected]

ABSTRACT Pickering emulsions stabilized by nanoparticles have recently received great attention for their remarkable stability, in part a consequence of irreversible adsorption. In this study, we generate Pickering oil-in-water emulsions stabilized by ethyl cellulose (EC) nanoparticles without the addition of surfactants. Over a range of ionic strength and EC nanoparticle concentrations, a series of dynamic interfacial tension (IFT) measurements complemented by extended-DLVO theoretical computations are conducted to quantitatively describe the behaviour of EC nanoparticles at the interface of water with different alkanes. Regardless of ionic strength, there is no barrier against the adsorption of EC nanoparticles at the alkane-water interfaces studied and the particles tightly cover these interfaces with near maximal coverage (i.e., 91%). Remarkably, the rate of approach to maximum coverage of the alkane-water interface by EC nanoparticles 1 ACS Paragon Plus Environment

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during the later stages of adsorption is accelerated in the presence of salt at concentrations below the critical coagulation concentration (CCC), unlike the air-water interface. Above the CCC, alkane-water interfaces behave similar to air-water interfaces, showing decay in the adsorption flux which is attributed to an increase in surface blocking originating from the attachment of nanoparticles to nanoparticles already adsorbed at the interface. These findings shed light on particle-particle and particle-interface colloidal interactions at and near fluid-fluid interfaces, thereby improving our ability to use hydrophobic EC nanoparticles as emulsion stabilizers.

keywords: nanoparticle, ethyl cellulose, fluid interface, interfacial coverage, adsorption, emulsion/foam

INTRODUCTION Particle stabilized emulsions, so-called Pickering emulsions, are more stable than emulsions stabilized by surfactants. Over the years, Pickering emulsions stabilized by microparticles have found a plethora of applications,1–9 whereas more recently emulsions stabilized by nanoparticles have been considered for applications in water remediation,10,11 oil recovery12 and as templates for the synthesis of novel materials.13– 15

To a great extent, the superior stability of Pickering emulsions is related to the

irreversible nature of nanoparticle adsorption at fluid interfaces, which is due to the very large adsorption energy ( ∆E ). Recently, it was shown that ∆E can be extracted from dynamic surface tension data using a mechanistic model16 that explains the kinetics of

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irreversible adsorption of nanoparticles of uniform size (radius r ) at fluid interfaces at two asymptotic time periods (early time: t → 0 and late time: t → ∞ ), as follows

t →0:

γ = γ 0 − 2N A ∆E C0

t →∞:

γ = γ∞ +

K l ∆E (π r ) N AC0 2 2

Dt

(1)

π 1 Dt

(2)

where γ 0 and γ are surface (interfacial) tension at t = 0 (i.e., pristine interface) and at any time, respectively, D is the diffusion coefficient of nanoparticles estimated from Stokes-Einstein theory,17 C0 is the molar concentration of nanoparticles in the bulk, N A is

Avogadro’s

Kl =

number,

π r 2N AC0D Θ3∞ ka

4.64

Kl

is

a

dimensionless

adsorption

parameter

(

),18 k a is the adsorption constant, and γ ∞ is the steady state

surface (interfacial) tension. Alternatively, the adsorption energy can be determined as follows from knowledge of either the particle contact angle at the interface, θ , or the ultimate coverage of the interface, Θ∞ , with nanoparticles ( Θ ∞ = 0.91 if the coverage pattern of the surface is hexagonal and the interface is tightly packed)

∆E = γ 0π r 2 (1 − cosθ ∆E =

γ0 − γ∞ Θ∞

)

2 19

(3)

π r 2 .20

(4)

Insofar as the generation of stable emulsions is concerned, adsorption of nanoparticles at the oil-water interface is necessary, but may not be sufficient. The organization of adsorbed particles on the surface of emulsion droplets in ways that hinder coalescence


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and coarsening (Ostwald ripening) and control the creaming phenomenon is also required.21–23 Most often, liquid droplets in particle-stabilized emulsions are covered with dense particle layers that provide steric hindrance to droplet coalescence.24 Whereas the kinetics of nanoparticle adsorption are governed by the interactions between suspended particles and the more or less covered interface,16,25 the ultimate coverage of the oil-water interface at steady state is controlled by the interactions among already adsorbed particles.

Colloidal interactions between two nanoparticles attached to the oil-water interface include attractive forces such as van der Waals, hydrophobic, and capillary forces and repulsive forces such as electrostatic, dipole-dipole, and induced dipole forces acting through water and through oil independently.26,27 The ultimate coverage of the interface is controlled by the net action of the aforementioned forces. pH and ionic strength have significant impacts on the electrostatic forces, and therefore, on the total interactions. If the total pair interaction is attractive, one expects adsorbed nanoparticles to aggregate at the interface resulting in the formation of irregular, non-compact clusters. These clusters would be expected to render parts of the interface inaccessible to adsorbing particles, and therefore, the ultimate coverage would be expected to be less than theoretical maximum corresponding to close hexagonal packing (i.e., 91%). If, however, the net action of interparticle forces is repulsive, the adsorbed nanoparticles would be expected to rearrange themselves in a hexagonal pattern, for which the potential exists for coverage to the maximal limit.28 One expects near maximal coverage when the


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energy barrier between two nanoparticles interacting at the interface is located at a distance that is just a few percent of the particle diameter, as shown in Figure 1.

Figure 1. The effect of spacing between monosized (diameter d) particles adsorbed at fluid interfaces on the percentage of interfacial coverage. The theoretical ultimate coverage ( Θ∞ ) is calculated from 2 Θ∞ = occupiedarea totalhexagonalarea = 3π d 2 6 3 ( d + h )  .  

While Figure 1 connects the ultimate coverage ( Θ ∞ ) to nanoparticle spacing at steady state, a simple relationship to compute Θ ∞ from experimental measurements may also be derived by eliminating ∆E between eq 3 and eq 4


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 γ −γ∞  1 . Θ∞ =  0  2  γ 0  (1 ± cos θ )

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(5)

Eq 5 shows that the fractional coverage of nanoparticle-laden fluid interfaces at steady state can be predicted from knowledge of the steady-state surface (interfacial) tension and the contact angle of a single nanoparticle at the interface.

The present study is motivated by a demonstration of the ability of hydrophobic spherical ethyl cellulose (EC) nanoparticles to stabilize O/W emulsions and focuses on their attachment and subsequent spatial organization at the alkane-water interface. EC is a non-toxic, non-biodegradable, food-grade material. At neutral pH and in the absence of salt, EC nanoparticles suspended in water carry a negative surface charge sufficiently large to ensure colloidal stability,29,30 alleviating the need to introduce ligands. EC nanoparticles are a strong stabilizer of foam30 and recently were reported to adsorb at hexadecane-water interface.31 These properties make the suspensions of EC nanoparticles an excellent model system for fundamental studies in colloid and interface science, and a promising candidate for the development of advanced pharmaceutical and sensing technologies.32–35 Here, stable Pickering emulsions are successfully generated, for the first time, using unmodified EC nanoparticles in the absence of surfactants. The stability of these emulsions is connected to the adsorption and arrangement of EC nanoparticles at the alkane-water interface, which is investigated by dynamic interfacial tension (IFT) measurements as a function of nanoparticle concentration and ionic strength using previously established methods.16,29 Regardless of ionic strength, diffusion of nanoparticles to the interface completely controls adsorption at the early stages, whereas a steric barrier to adsorption develops 6 ACS Paragon Plus Environment

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during the late stages. The adsorption energy determined from early-time dynamic IFT data agrees well with predictions based on contact angle measurements. Remarkably, the ultimate coverage of the interface is high (~0.9), irrespective of ionic strength. Extended-DLVO computations support these experimental findings.

EXPERIMENTAL PROCEDURES

Materials. An aqueous suspension of ethyl cellulose (EC) nanoparticles was prepared following an anti-solvent precipitation method30 with minor modifications.16 In previous works, using dynamic light scattering, we have previously16,29 measured a value of 44±5 nm for the radius of EC nanoparticles at neutral pH and ionic strengths below the critical coagulation concentration (i.e., 0.05 M). The original colloidal suspension was diluted to four different concentrations, 0.25 g L-1, 0.5 g L-1, 0.75 g L-1, and 1.0 g L-1, and the ionic strength was tuned from 0 M to 0.1 M by addition of NaCl. The effect of ionic strength on the stability of aqueous suspensions of EC nanoparticles has been studied elsewhere.29

Methods. A pendant drop tensiometer and associated software (VCA 2500 XE, AST Products, Billerica, MA) were used to determine the dynamic interfacial tension (IFT) between colloidal EC nanoparticle suspensions and four different alkanes: iso-octane (J. T. Baker), n-decane (Sigma-Aldrich), n-dodecane (Matheson, Coleman, and Bell), and n-hexadecane (Sigma-Aldrich).

All oils were purified prior to dynamic IFT

measurement by stirring with 2 wt.% Florisil® (Sigma-Aldrich) at a speed of 450 rpm for


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more than half a day, followed by centrifuging at 7000 rpm for 20 min.36 After purification, interfacial tensions of 51.4±0.8 mN m-1, 52.8±0.4 mN m-1, 53.4±0.4 mN m-1, and 54.4±0.5 mN m-1 were measured at 298 K for purified iso-octane (iso-C8), n-decane (n-C10), n-dodecane (n-C12), and n-hexadecane (n-C16), respectively, in excellent agreement with literature.37–39 The early stages of EC nanoparticle adsorption at oilwater interfaces (first two minutes following the formation of the pendant drop) were captured at a frame rate of 10 to 15 images/s depending on the EC concentration, whereas a rate of image capture of 6 images/min was used for the later stages of adsorption (up to 100 minutes after the formation of the pendant drop). Each set of IFT measurements was repeated at least three times and the dynamics of adsorption process was analyzed using the recently-reported16 asymptotic model, eq 1 and eq 2. The significance of two factors, i.e., ionic strength and EC concentration, on the interfacial properties (steady state interfacial tension, ultimate interfacial coverage, and adsorption energy) was statistically analyzed using a factorial design approach.

The contact angle of EC nanoparticles at the oil-water interface was determined in two different experiments using a film of EC spin-coated (at 2000 rpm for 25 s) on either glass or Teflon slides from a 4 wt.% solution of EC in ethanol. In both experiments, the EC film was in contact with water first and then it was introduced to oil. This reflects the situation during the course of emulsification when EC nanoparticles dispersed in water (solid-water interface) interact with oil blobs (oil-water interface). As shown in Figure 2(a), in the first of these experiments a drop of deionized (DI) water was placed on an EC-coated glass slide. The slide was then moved to a chamber filled with purified oil.


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The system was allowed to equilibrate for ca. 5 h before the contact angle was measured. In the second experiment, shown in Figure 2(b), oil drops were placed on an EC-coated Teflon slide previously immersed in DI water. Measurement of the contact angle in this system required an inverted dispensing apparatus (the dispensing liquid, oil, has a lower density than that of the surrounding liquid, water). The contact angles measured in the two experiments exhibit a difference of about 8 degrees which may be attributed to contact angle hysteresis. We measured an average contact angle (through water) of 89±3º and 81±3º in the first and second experiment, respectively, and pooled these measurements into a single average of 85±3º. Using the inverted dispensing apparatus, we measured a contact angle of 80±3º in the presence of 0.025 M NaCl, indicating an insignificant effect of salt on contact angle (compare to 81±3º in the absence of salt). No effect of salt on the contact angle of EC nanoparticles at the airwater interface has also been reported elsewhere.29


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(a)

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water drop EC film

CCD camera

n-C10 θ

water

glass slide

(b)

teflon slide

CCD camera

θ

n-C10 water

EC film

oil drop

Figure 2. (a) Direct and (b) inverted dispensing apparatus for measuring contact angle. An average contact angle of 89±3º and 81±3º was measured through water following direct and reverse measurements, respectively.

Pickering emulsions were generated under controlled EC nanoparticle concentration (1 g L-1) and ionic strength (0.025 M or 0.05 M) at a volume ratio of 1:2 of n-C10 to water by hand shaking for 2 min or by using a mechanical mixer (Banrant mixer series 10) equipped with a three-bladed stainless steel paddle for 3 min at a speed of 500 rpm.


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The size distribution of dispersed-phase droplets in the emulsions produced were determined from analysis of microscopic images obtained by a Zeiss Axiovert 200 optical microscope.

RESULTS AND DISCUSSION

Pickering emulsion generation. The Pickering emulsion made with n-C10 and aqueous suspension of EC nanoparticles at a concentration of 1 g L-1 and ionic strength of 0.05 M is shown in Figure 3(a)-ii. The emulsion phase rests atop the excess aqueous phase suggesting that it is an oil-in-water (O/W) emulsion, as would be expected40 from contact angle measurements ( θ < 90° ). That oil is the dispersed phase is confirmed by

closing an electric circuit traversing through the emulsion. In such a circuit (shown in Figure S1 in the Supporting Information), the turning on of an LED lamp indicates current flow. This could happen only if water, which is the only conductive component in the system, is the continuous phase, in which case an O/W emulsion is verified.

That the O/W emulsion obtained is stabilized by EC nanoparticles adsorbed at the oilwater interface is verified by attempting to generate an emulsion using four different aqueous phases: (i) deionized water, (ii) aqueous NaCl solution at a concentration of 0.05 M, (iii) EC nanoparticle suspension at 1 g L-1 with ionic strength of 0.05 M, and (iv) the supernatant solution obtained after coagulation and sedimentation of EC nanoparticles at ionic strength of 0.05 M. As shown in Figure 3(a), a stable emulsion


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could be successfully generated only in the presence of EC nanoparticles in the aqueous phase.

Figure 3. (a) Attempts at generating emulsions (1:2 n-C10-to-water volume ratio) from different aqueous phases using the mixer: (i) in the absence of EC nanoparticles and (ii) in the presence of EC nanoparticles. (b) Analysis on Pickering emulsion generated by hand shaking EC nanoparticle suspension at a concentration of 1 g L-1 and at ionic strength of 0.025 M (1:2, n-C10 to water volume ratio): (i) microscopic image (scale bar is 500 µm ) and (ii) volume-surface diameter size distribution from measurement of over 250 drops.

The emulsions shown in Figure 3 are of the Bancroft-type41 since the emulsifier (i.e., EC nanoparticles) is dispersed initially in the continuous phase (water). A simple mass balance for such emulsions to determine the mean volume-surface diameter of the oil drops ( D32 ) yields.41 D32 =

8r ρ p Θ Φ ρ0 1 − Φ

(6)


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where ρ p is the density of EC (i.e., 1.14 g mL-1 at 25 °C, provided by the manufacturer),

Θ is the interfacial coverage, ρ0 is the EC nanoparticle mass concentration in the bulk (before emulsification), and Φ is the volume fraction of the disperse phase (oil) defined as Φ ≡

Voil

Vtotal

(where Vtotal = Vwater + Voil ). Assuming maximum interfacial coverage (

Θ = 0.91 ) and from knowledge of

the remaining parameters:

r = 44 ± 5 nm ,

ρ p = 1.14 g mL-1 , ρ0 = 1.0 g L-1 , and Φ = 1 3 , a value of 183 ± 21µm is calculated for the mean diameter of oil drops. Microscopic image analysis on the generated emulsions gives a size distribution showed on Figure 3(b) with a mean value of 126 ± 32 µm for the oil droplets diameter in fair agreement with theory. Such agreement implies that the coverage of the oil drops by nanoparticles is near the maximum limit. An independent and more detailed assessment of surface coverage of the oil drops by EC nanoparticles is presented below.

Irreversible adsorption of EC nanoparticles at the interface. A series of interfacial tension (IFT) measurements for the n-C10-water system at different conditions are shown in Figure 4: Figure 4(a) shows the effect of EC nanoparticle concentration and Figure 4(b) shows the effect of ionic strength on dynamic IFT. Similar data for other alkanes are delegated to the Supporting Information (see Figure S2, Figure S3, and Figure S4 for iso-C8, n-C12, and n-C16, respectively, in the Supporting Information). The steady-state interfacial tensions ( γ ∞ ) for all alkane-water systems studied in this work are obtained from the intercepts of plots of IFT against

1 t (viz. eq 2) and are

collected in Table 1. For n-C10, in particular, the IFT drops from 52.8±0.4 mN m-1, 13 ACS Paragon Plus Environment

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corresponding to the pristine n-C10-water interface, to a steady-state a value of 12±1 mN m-1 given sufficient time. Analyzing the early stages of adsorption using eq 1, we obtain the adsorption energy ( ∆E ) at the given conditions of EC nanoparticle concentration and ionic strength (see Table S1 in the Supporting Information). Factorial statistical analysis29 of these values leads to the conclusion that nanoparticle concentration and ionic strength has no significant effects on ∆E (see Table S2 in the Supporting Information). The average values of adsorption energy for each alkanewater system calculated from dynamic IFT data are listed in Table 1 and they are seen to compare very well to the values obtained from eq 4. Moreover, the value

∆E = ( 7 ± 2 ) × 104 kBT

obtained for the n-C10-water interface via eq 3 from

measurements of the contact angle is also in good agreement with other estimates of the adsorption energy. Pair-comparisons of ∆E obtained from each approach do not reveal any significant difference at the 95% confidence level. In all of these cases, the adsorption energy is several orders of magnitude larger than thermal fluctuations ( k BT ) confirming that EC nanoparticle adsorption is irreversible.


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interfacial tension / mN m-1

60

(a)

50 40 30 20 10 0 0.01

60

interfacial tension / mN m-1

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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Decane-Water 0.25 g/L 0.5 g/L 0.75 g/L 1.0 g/L 0.1

1

10 100 time / s

1000 10000

1

10 100 time / s

1000 10000

(b)

50 40 30 20 10 0 0.01

No salt 0.01 M 0.025 M 0.05 M 0.1 M 0.1

Figure 4. Interfacial tension (IFT) of the interface between n-C10 and aqueous suspension of EC nanoparticles on a log scale plot at (a) various nanoparticle 15 ACS Paragon Plus Environment

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concentrations (all salt-free) and (b) various salt concentrations (all at constant EC nanoparticle concentration of 0.5 g L-1).

Table 1. Steady state IFT and adsorption energy ( ∆E ) computed from independent approaches using eq 1 and eq 4.

iso-C8-water n-C10-water n-C12-water n-C16-water

∆E [ k BT ] (from eq 1)

∆E [ k BT ] (from eq 4)

(8±3)x104 (6±2)x104 (7±2)x104 (7±3)x104

(7±1)x104 (7±1)x104 (7±1)x104 (7±1)x104

γ ∞ [mN m-1] (from eq 2 intercept) 11±1 12±1 13±1 13±1

Turning our attention to the late stages of nanoparticle adsorption, we note that in the

(

absence of salt, d γ dt −0.5

)

t →∞

(computed from eq 2) for all alkane-water interfaces

(see Table 2) varies linearly ( R 2 = 0.99 ) with the inverse of bulk nanoparticle concentration. This shows that K l and therefore the adsorption constant ( k a ) is essentially independent of EC nanoparticle concentration. The average value of

( 4 ± 3 ) × 10−6 m s-1

obtained from the data for k a leads to an estimate16 of the barrier

against adsorption that is less than 5 k BT (i.e., of the order of thermal fluctuations) reflecting an essentially barrierless EC nanoparticle adsorption at the alkane-water interface. Therefore, similar to previous findings for the air-water interface,29 the magnitude of the adsorption flux of EC nanoparticles at the alkane-water interface is not controlled by diffusion from the bulk at the late stages of adsorption, but rather is given by j = ka N AC0B ( Θ ) where B ( Θ ) accounts for surface blocking. Nearly full coverage of the interface by nanoparticles ( Θ ≅ 0.91) would not be possible without the rearrangement

of

already

adsorbed

nanoparticles, 16

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suggesting

that

particle

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rearrangement limits the adsorption flux during the late stages of adsorption, thereby controlling B ( Θ ) . We find that when salt is added to the EC nanoparticle suspension

(dγ

dt −0.5

)

t →∞

decreases somewhat, meaning that nanoparticle adsorption reaches

steady state somewhat faster than in the absence of salt. Specifically, for salt concentrations below the critical coagulation concentration (CCC, i.e., 0.05 M),

(dγ

dt −0.5

)

t →∞

decreases by ca. 40% from its value in the absence of salt. Two

hypotheses may be made in connection to the observation of increased adsorption flux during the late stages in the presence of salt. One is that addition of salt increases the adsorption constant k a . From the data of Table 2 we estimate ka = (10 ± 5 ) × 10−6 m s−1 , which although greater, is not statistically different from the value of k a in the absence of salt. An alternative hypothesis is that salt weakens surface blocking. We find that reducing

the

exponent

m

in

the

expression

for

the

blocking

function,

m

 Θ  B ( Θ ) ≅ 2.32  1 −  , from m = 3 to m = 2.5 suffices to explain the observed Θ max  

(

reduction of d γ dt −0.5

)

t →∞

in the presence of salt. Weakening of surface blocking at

high coverage may be understood as greater ease of nanoparticle rearrangement in the presence of salt.


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Table 2. Slope of late-time IFT data obtained from eq 2 for iso-C8-water and n-C10water interfaces at EC concentration of 0.5 g L-1. ionic strength [M]

0 0.01 0.025 0.05 0.1 0.2

EC nanoparticle iso-C8-water concentration [g L-1] 0.25 – 0.5 85±21 0.75 – 1.0 51±5 0.5 62±11 0.5 54±4 0.5 54±5 0.5 61±2 0.5 64±4

n-C10-water

n-C12-water

n-C16-water

226±14 98±12 63±6 57±10 83±5 57±1 107±7 84±11 –

– 97±1 – – – – – – –

– 85±8 – 47±21 – – – – –

(

Similar to the air-water interface,29 above the CCC, d γ dt −0.5

)

t →∞

increases for iso-C8-

water and n-C10-water systems, signifying a reduction of the nanoparticle flux to the nC10-water interface. This could be caused by an increase in the surface blocking, as explained elsewhere,29 because of the attachment of single nanoparticles and nanoparticle aggregates onto nanoparticles already adsorbed at the n-C10-water interface. As shown next, a detailed analysis of the interactions between adsorbed EC nanoparticles at the alkane-water interface helps explain the observed interfacial coverage at steady state, as well as the rate of approach to steady state.

Interactions between adsorbed particles at the interface. The ultimate surface coverage is the result of net interactions between colloidal nanoparticles adsorbed at the interface. According to the extended DLVO theory, the net interaction in the colloidal domain is the summation of attractive and repulsive forces. For the EC colloidal systems studied in this work, these include van der Waals ( φvdW ) and hydrophobic ( 18 ACS Paragon Plus Environment

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φhydro ) attractive interactions and electrostatic ( φelec ) and dipole-dipole ( φd −d ) repulsive interactions.26 Capillary force is another attractive force that has been considered in other studies.26,27,42 Nonetheless, due to the small size of EC nanoparticles used in this study, a very small Bond number ( O (10 −10 ) , see Supporting Information) suggests that capillary monopolar attraction may be safely neglected.42

The contact angle ( θ ) controls the fraction of adsorbed nanoparticle surface exposed to oil and water. In our approach, as in earlier studies,42 the cross-phase interactions are not considered and only interactions through the same phase are accounted for. Where possible, equations reported in the literature to compute the aforementioned interactions as a function of contact angle are used. When such equations are not available, the Derjaguin approximation is employed to derive formulae for the interaction of nanoparticles at the interface.42 Following a coordinate transformation,43 we obtained the governing equations by integrating flat meniscus approximations with respect to contact angle (see Supporting Information for the details of derivations).

Attractive van der Waals and hydrophobic interactions between two EC nanoparticles with a similar radius ( r ) separated by a distance h (surface-to-surface) are given below (superscripts O and W refer to oil and water, respectively).

attractive interactions: H O φvdW = −r θ 121 12π h

(7)


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W φvdW = −r ( π − θ )

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H131 12π h

(8)

K131 2π h

(9)

W φhydro = −r ( π − θ )

where H is the Hamaker constant for EC nanoparticle (1) interacting with another EC nanoparticle (1) at the interface through oil (2) or through water (3) and K131 is the hydrophobic interaction energy constant for interacting two EC nanoparticles at the oilwater interface (through water).

The electrostatic repulsive interaction through water between two EC nanoparticles adsorbed at the alkane-water interface is also obtained from integration of flat meniscus approximations with respect to contact angle (see Supporting Information for the details of derivations). Thus we find

repulsive interactions:  

W 2 = 4 (π − θ ) ε 0ε w rψ pw φelec 1−

r h + 2r

    exp ( −κ h )  ln 1 +   1+ h   r  

(10)

where ψ pw is the potential at the surface of EC nanoparticle exposed to water, κ is the inverse Debye length which varies with ionic strength, ε 0 is the vacuum permittivity, and

ε w is the relative dielectric constant of water. It is proposed that if electrostatically charged particles adsorb at a nonpolar oil-water interface, they may carry “residual charges” on their surface exposed to oil, resulting in a long-range electrostatic repulsion through oil between adsorbed particles. The dependence of this repulsion on contact angle has been reported in other studies.26,44,45


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repulsive interactions: 2  Apoσ po )  ( 1 O φelec =  8π r ε 0ε r  1 + h 2r 

(

  − 2 2 ( 3 + cosθ ) + 2 + h r  1+ h

)

2

2r

(

)

(11)

where Apo is the surface area of particle exposed to oil and σ po is the charge density on the particle surface exposed to oil. In using eq 11 we have assumed that Brownian motion has negligible effect on the EC nanoparticles adsorbed at alkane-water interfaces.

Dipole-dipole interactions are the other significant repulsive forces between adsorbed EC nanoparticles at the interface. These dipolar interactions originate from polar surface groups of hydroxyl and ethyl ether (diethyl ether) in the structure of EC molecule. The following expressions are proposed26 to compute the dipolar interactions through oil and water between particles adsorbed at an oil-water interface.

repulsive interactions:

φ

O d −d

=

φdW−d =

(πσ

d

r 2 sin2 θ

(

32πε oε 0 r + h

(A

pw

σ pw )

(

)

2

2

)

(12)

3

2

16πε 0ε w2 κ 2 r + h

2)

(13)

3

where is σ d the surface dipole moment density, ε o is relative dielectric constant of oil, is Apw the surface area of particle exposed to water, and σ pw is the charge density on the particle surface exposed to water. Detailed calculations of required parameters in eq 21 ACS Paragon Plus Environment

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7 to eq 13 are reported in the Supporting Information. Table 3 summarizes these values used for extended-DLVO calculations.

Table 3. Parameters used in eq 7 to eq 13 for extended-DLVO calculations for interactions among adsorbed EC nanoparticles at n-decane-water interface.

r : nanoparticle radius θ : particle contact angle measured through water H121 : Hamaker constant (EC-oil-EC) H131 : Hamaker constant (EC-water-EC) K131 : hydrophobic constant (EC-water-EC)

8.7x10-23 J 16

Apo : surface area of particle exposed to oil – Apo = 2π r 2 (1 − cosθ )

1.1x10-14 m2

Apw : surface area of particle exposed to water – Apw = 2π r 2 (1 + cosθ )

1.4x10-14 m2

44.5 nm 85º -21 10 J 46 10-20 J 47

ψ pw : particle surface charge (zeta potential)

– 59.07 mV 29 8.85x10-12 C2 N1 m-2 2 48

ε 0 : vacuum permittivity ε o : relative dielectric constant of oil (n-decane) ε w : relative dielectric constant of water σ po : particle surface charge density exposed to oil – [ σ po = σ pw : particle surface charge density exposed to water

σ d : surface dipole moment density

78.5 49

ψ pw ε 0ε o r

]

– 2.32x10-5 C m-2 varies with κ 2.3x10-11 C m-1

The surface dipole moment density ( σ d ) depends on the dipole moment of a polar group ( p ) and the density of that polar group at the surface of the particle exposed to oil ( σ ) as σ d = pσ . When more than one polar are present (hydroxyl and ethyl ether in EC), average values are used based on the number fraction of each polar group. The average polar group surface density is not well known for EC and a conservative estimate of 5 nm-2 for σ is made from values reported in literature for other systems.44 The net interaction energy among EC nanoparticles adsorbed at n-C10-water interface


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O W W O W + φvdW + φhydro + φelec + φelec + φdO−d + φdW−d . The results of such is computed from φtotal = φvdW 3 14442444 3 1444424444 repulsion

attraction

computations for different values of the ionic strength are plotted in Figure 5.

100

interaction energy (ϕtotal) / kBT

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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90 80 70 60

No salt 0.01 M 0.025 M 0.05 M 0.1 M

50 40 30 20 10 0 0

5 10 separation distance (h) / nm

15

Figure 5. Extended DLVO calculations for adsorbed EC nanoparticles at n-decanewater interface at different ionic strengths.

At all levels of ionic strength considered, a net repulsion between two adsorbed nanoparticles is predicted.

This net repulsion leads to eventual rearrangement of the

adsorbed nanoparticles towards a hexagonal pattern. The repulsion is greatest a few nanometers away from the surface of an adsorbed nanoparticle. Assuming that the maximal proximity of adsorbed nanoparticles to one another corresponds with the 23 ACS Paragon Plus Environment

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Page 24 of 34

nanoparticle separation at maximum repulsion (see Figure 5), we obtain theoretical estimates of the maximum interfacial coverage at steady state (see Figure 1). As shown in Figure 6, the theoretical estimates compare fairly well with those obtained from experimental data via eq 5, both showing tight packing of the oil-water interface by EC nanoparticles at all levels of ionic strength considered. It must be noted that the above mentioned estimates do not account for capillary quadrupolar attraction arising from pinning and distortion of the three-phase contact line.50 The amplitude of contact line undulations is unknown, but is thought to depend on the extent of contact angle hysteresis. Contact line undulations with amplitude not exceeding 1 nm result in quadrupolar capillary attraction that is not strong enough to cause coagulation of adsorbed nanoparticles, even in the presence of salt (see Supporting Information). More significant distortions of the contact line, however, would result in net attraction between adsorbed nanoparticles at all distances in the presence of salt (see Supporting Information). The latter picture is not consistent with the experimental observation of dense coverage of the alkane-water interface with nanoparticles.


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120 110 100

Θ (%)

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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90 80 70 Theory (E-DLVO) Experiments

60 50 0

0.02

0.04 0.06 0.08 ionic strength / M

0.1

Figure 6. Interfacial coverage calculated experimentally using eq 5 and theoretically using Figure 5 and Figure 1.

Figure 5 clearly shows a significant reduction of the repulsion among EC nanoparticles adsorbed at the oil-water interface when salt is added to the aqueous phase.

As

discussed earlier, late-time dynamic IFT data are consistent with a faster adsorption rate when salt is present in amounts below the CCC. The rate of adsorption at high interfacial coverage is limited by the rate of rearrangement of already adsorbed particles which is facilitated by weakening of the repulsion among adsorbed particles. This is manifested as a reduction in B ( Θ ) in the presence of salt below the CCC, which is


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Page 26 of 34

effectively a reduction of the exponent m , since Θ∞ is observed to be largely independent of ionic strength (see Figure 6).

CONCLUSIONS Ethyl cellulose (EC) nanoparticles adsorb at the alkane-water interface and can effectively stabilize oil-in-water Pickering emulsions in the absence of molecular surfactants.

Dynamic

interfacial

tension

measurements

and

extended-DLVO

calculations of the interactions between adsorbed particles support the conclusion that oil-water interfaces are tightly covered by EC nanoparticles close to the maximal limit (91%) corresponding to a close-hexagonal pattern at all levels of ionic strength. The adsorption process is effectively barrierless irrespective of ionic strength, although salt at levels just below the critical coagulation concentration is observed to speed up the attainment of steady-state. These findings are important for understanding the emulsifying function of EC nanoparticles – a prerequisite for the design of engineering applications involving these systems.

ASSOCIATED CONTENT

Supporting Information Electric circuit for emulsion-type recognition. Dynamic interfacial tension measurements for iso-C8-water, n-C12-water and n-C16-water. Representative fits to early- and latetime interfacial tension data for n-C10-water interface. Comparison between adsorption

(

energy calculated from different approaches. Statistical analysis of d γ dt −0.5 Extended-DLVO calculations and derivations. 26 ACS Paragon Plus Environment

)

t →∞

data.

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AUTHOR INFORMATION

Corresponding Author *

E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge the Ontario Ministry of Training, Colleges, and Universities for the Ontario Graduate Scholarship (OGS) and the Natural Sciences and Engineering Research Council of Canada (NSERC, grant#: RGPIN-194309-2013) and the University of Waterloo for the financial support.


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For Table of Contents Use Only

60 40

interaction energy / kBT

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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oil water

20

water

0 0

1

2

3

4

5

6

7

-20 -40

oil water

-60 -80

-100

separation distance / nm


8

9

10

Ethyl Cellulose Nanoparticles at the AlkaneâWater Interface and the

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