Long-Lived and Thermoresponsive Emulsion Foams Stabilized by Self

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Long-Lived and Thermo-Responsive Emulsion Foams Stabilized by Self-Assembled Saponin Nanofibrils and Fibrillar Network Zhili Wan, Yingen Sun, Lulu Ma, Feibai Zhou, Jian Guo, Song-Qing Hu, and Xiao-Quan Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00128 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Long-Lived and Thermo-Responsive Emulsion Foams Stabilized by

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Self-Assembled Saponin Nanofibrils and Fibrillar Network

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Zhili Wan,1 Yingen Sun,1 Lulu Ma,1 Feibai Zhou,1 Jian Guo,1 Songqing Hu1 and Xiaoquan Yang1,2,* 1

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School of Food Science and Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China

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Guangdong Province Key Laboratory for Green Processing of Natural Products and Product Safety, South China University of Technology, Guangzhou 510640, People's Republic of China

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AUTHOR INFORMATION

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Xiaoquan Yang (*corresponding author)

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E-mail: [email protected]; [email protected]

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Fax: (086) 20-87114263; Tel: (086) 20-87114262

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ACS Paragon Plus Environment

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ABSTRACT

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Nanofibrils from self-assembly of the naturally occurring saponin glycyrrhizic acid (GA) can be used

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to create an oil-in-water emulsion foam with long-term stability. Through a homogenization and

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aeration followed by a rapid cooling, stable emulsion foams can be produced from the mixtures of

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sunflower oil and saponin nanofibrils. At high temperature, the GA fibrils form a multilayer

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assembly at the interface, creating an interfacial fibrillar network to stabilize the oil droplets and air

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bubbles generated during homogenization. A subsequent rapid cooling can trigger the self-assembly

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of free GA fibrils in the continuous phase, forming a fibrillar hydrogel and thus trapping the oil

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droplets and air bubbles. The viscoelastic bulk hydrogel showed a high yield stress and storage

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modulus, which lead to a full arrest of the liquid drainage and a strong slowing down of the bubble

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coarsening in emulsion foams. The jamming of emulsion droplets in the liquid channels as well as

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around the bubbles was also found to be able to enhance the foam stability. We show that such stable

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foam systems can be destroyed rapidly and on-demand by heating due to the melting of the bulk

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hydrogel. The reversible gel-sol phase transition of the GA hydrogel leads to thermoresponsive

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emulsion foams, for which the foam stability can be switched from stable to unstable states by

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simply raising the temperature. The emulsion foams can be further developed to be photoresponsive

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by incorporating internal heat sources such as carbon black particles, which can absorb UV

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irradiation and convert the absorbed light energy into heat. This new class of smart responsive

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emulsion foams stabilized by the natural, sustainable saponin nanofibrils has potential applications in

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the food, pharmaceutical and personal care industries.

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KEYWORDS: saponin nanofibrils, self-assembly, responsive emulsion foams, interface, hydrogel

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network


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INTRODUCTION

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Many food and cosmetic products used in daily life are complex soft condensed matter systems

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containing both oil and air dispersed in an aqueous matrix, such as ice creams, whipped creams and

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toppings. Due to the presence of mixtures of oil droplets and air bubbles, these complex systems can

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be either considered as emulsion foams (or foamed emulsions), or as a specific aqueous foam whose

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interstitial fluid is doped with emulsion droplets.1-3 They also find widespread applications in many

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other fields, such as in enhanced oil recovery processes,4,5 or serving as a template for the production

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of macroporous materials.6-8 For many of these applications, foam stability is an important parameter,

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since aqueous foams are thermodynamically unstable dispersions, and can be easily destroyed by

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drainage, coarsening and coalescence.2,9-11 Therefore, the precise control of foam stability remains a

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major challenge in foam science and is required for various applications of foams. In the case of

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emulsion foams, the stabilization of foams is achieved by jamming emulsion droplets to block the

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Plateau border junctions between bubbles or the liquid films slowing down or even completely

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arresting drainage.1,12-14 However, a high fraction of oil droplets acts as effective anti-foaming agents,

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which can wet the air-water interface and subsequently break the films.15,16 The dual function of oil

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droplets makes them an attractive system for the active control of formation and stabilization of

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emulsion foams.

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For some industrial applications, such as material recovery, washing, and cleaning processes, both

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the formation of stable foams and the ability of on-demand foam destruction are required. In practice,

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foam destabilization often involves the mechanical breakage or the addition of chemical defoamers,

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which are not environmentally friendly and also change the formulation composition.17-20 In

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comparison with these invasive approaches, the application of external stimuli to noninvasively


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control the foam destabilization has received increasing interest in the last few years. Various

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strategies have been developed to design and construct stimuli-responsive smart foams, and the

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on-demand foam destabilization can be achieved either by changing the pH and ionic strength of

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foaming solution, or by using external stimuli, such as temperature, light or magnetic fields.4,17,20-32

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Compared with these responsive foams and emulsion foams stabilized using synthetic and even

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potentially toxic surfactants or polymers, the use of naturally occurring sustainable materials to

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produce such systems is highly desired considering their applications in the fields of foods,

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cosmetics, and pharmaceuticals.25,33-35 However, few studies have been carried out on the use of

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natural amphiphilic molecules to design stable and stimuli-responsive foams and emulsion foams so

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far.

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The design and construction of novel functional materials based on structural building blocks from

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supramolecular self-assembly of natural small molecules have received increasing attentions in terms

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of bioavailability, biocompatibility, and biodegradability.36-38 Recently, researchers reported that

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triterpenoid saponins such as glycyrrhizic acid (GA, Figure 1a) can self-assemble in water or various

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organic solvents to form high-aspect-ratio nanostructures such as fibrils, which, upon increasing

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concentrations can further assemble and entangle to create supramolecular hydrogels or organogels,

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respectively.38,39

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thermoreversible gel-sol transition due to the noncovalent fibrillar network.38,39 Very recently, we

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have shown that the self-assembled GA nanofibrils can be used as building blocks to construct stable

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emulsion gels with a temperature-responsive switchable behavior, which is based on the multilayer

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assembly of GA fibrils at the oil-water interface as well as the formation of a thermoreversible

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fibrillar network in the continuous phase.40 We also found that the responsiveness to temperature of

These

saponin

nanofibril-based

supramolecular


architectures

exhibit

a

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GA fibril-based emulsion gels is independent of oil phase polarity.41 These studies strongly suggest

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that the GA nanofibrils would be an excellent building block for the design and fabrication of

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responsive supramolecular materials and multiphase soft matter. To stabilize an aqueous foam or

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emulsion foam, it is required that the stabilizers either form a stable interfacial layer on the bubble

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surfaces or lead to the gelation of the foam liquid channels or both.2 Therefore, based on the unique

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combination of emulsifying, foaming, and gelation properties,40-42 it is reasonable to suggest that the

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amphiphilic GA fibrillar assemblies can be used to prepare an emulsion foam with superior stability.

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Further, the switchable stability of such systems probably can be achieved by temperature stimulus

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when considering the thermoreversibility of GA fibrillar hydrogel.40,41

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In this article, we demonstrate that the self-assembled GA nanofibrils can be used to create an

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emulsion foam that is stable for weeks, through a homogenization and aeration at high temperature

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followed by a rapid cooling. The GA fibrils can self-assemble into fibrillar networks both at the

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interface and in the continuous phase. The former creates an interfacial fibril film stabilizing the oil

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droplets and air bubbles generated during homogenization, while the latter forms a bulk hydrogel,

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which can prevent bubble movement and growth. We show that not only do the GA fibrillar networks

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(at the interface and in bulk) stabilize the foams, the jamming of oil droplets around bubbles also

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contributes to the foam stability. In particular, we show that the melting of the bulk GA hydrogel

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upon heating is accompanied by a complete destruction of the foam structure. We further combined

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the carbon-based particles that are good absorbers of UV irradiation as well as have strong

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photothermal conversion effects with thermoresponsive GA fibrils to produce complex emulsion

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foams, and then explored the foam response to both temperature and light.

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EXPERIMENTAL SECTION

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Materials. Glycyrrhizic acid mono ammonium salt (GA, purity > 98%) was purchased from Acros

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Organics, USA. Carbon black particles (CBP), Monarch® 800, were purchased from Cabot, Inc.

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(Billerica, MA, USA). Sunflower oil was purchased from a local supermarket (Guangzhou, China).

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Nile Red and Thioflavin T (ThT) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All

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other chemicals used were of analytical grade.

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Preparation of Gelled Emulsion Foams. Stock solution of GA fibrils was prepared by dissolving

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the appropriate amounts of GA powder in Milli-Q water in sealed glass vials (2.5 cm internal

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diameter, 9 cm height), heating it at 80 °C under mild agitation until a clear solution was obtained. A

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droplet of GA fibril solution was deposited on freshly cleaved mica and dried on air. AFM

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characterization was performed by using a MultiMode 8 Scanning Probe Microscope (Bruker, USA)

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using silicon nitride tips (Bruker, USA) at 25 °C.

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The emulsion foams were produced by first dispersing sunflower oil in hot GA fibril solutions

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(80 °C) under mild agitation for 2 min, and then the resulting dispersions (10 g) were homogenized

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and aerated in a single step using an ultraturrax (IKA T10 basic) at 20000 rpm for 2 min. The

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resultant samples were immediately cooled in an ice bath or were slowly cooled down to room

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temperature (25 °C), to study the effect of cooling rate on the formation of gelled emulsion foams. To

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further investigate the effect of oil fraction on the formation ability of emulsion foams, samples with

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different oil fractions (5-60 wt%) were prepared at a constant GA fibril concentration of 4 wt%. The

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prepared emulsion foams were stored at 25 °C. Foam heights were measured using a ruler, and the

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overrun of the foam was calculated by using equation: overrun (%) = [(Vt - V0) / V0] × 100, where Vt

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is the total volume of the foam and V0 is the initial liquid volume.


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of

Dual

Photo/Thermo-Responsive

Emulsion

produce

dual

Preparation

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photo/thermo-responsive emulsion foams, the dispersion of carbon black particles (CBP) was first

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prepared by adding the particles to water, followed by ultrasonic treatment for 2 min using a Sonic

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Ruptor 400 sonicator (OMNI, USA) at a power output of 320 W. Then, the desired amount of CBP

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was added to the hot GA fibril solutions (4 wt%), and the mixtures were stirred at 80 °C for 2 min.

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After that, all samples were produced according to the procedure described in the above section. The

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hot emulsion foams were dispensed directly into flat quartz cells for testing with UV light, and the

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quartz cells were immediately placed in the ice bath to obtain the final emulsion foams. The quartz

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cells containing the sample were irradiated from the top using a Philips 125 W mercury arc lamp

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operated at 365 nm. The change of emulsion foams upon UV irradiation was recorded using a digital

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camera (EOS M3, Canon).

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Microstructure Observations. The emulsion foams were visualized by using confocal laser

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scanning microscope (CLSM, Leica Microsystems Inc., Heidelberg, Germany) and polarized light

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microscope (PLM, Axioskop 40 Pol/40A Pol, ZEISS, Göttingen, Germany) equipped with a Power

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Shot G5 camera (Canon, Japan). For PLM, the samples were placed on a flat slide and covered by a

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coverslip, and each image was acquired under normal and polarized light. For CLSM visualization,

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we added Nile Red (0.1 wt%) into the oil phase prior to homogenization and aeration step. The

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samples were examined using an argon krypton laser (ArKr, 488 nm). ThT was used to label the GA

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fibrillar assemblies and fibrillar network in gelled emulsion foams, since it has been found to be able

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to bind to fibrillar structures and thus enhance its fluorescence.32,33 ThT (0.001 wt%) was first

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dissolved in GA fibril solutions prior to the sample preparation. The ThT concentration used was

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very low to avoid dye self-assembly. The 458 nm line of an argon laser was used to excite the


Foams.

To

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samples, and the emission fluorescence was observed between 470 and 560 nm.

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Field emission scanning electronic microscopy (FE-SEM) was further used to observe the

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structure of gelled emulsion foams. Sunflower oil was replaced with the volatile hexane as oil phase.

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After the formation of emulsion foams, the hexane was removed by evaporation in a Christ DELTA

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1-24 LSC freeze-dryer (Christ, Germany) for 24 h. The dried samples were mounted on a holder with

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double-sided adhesive tape and sputter-coated with gold (JEOL JFC-1200 fine coater, Japan) before

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images were recorded using a Zeiss Merlin FE-SEM.

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Macrorheology. The macrorheological measurements were carried out on a Haake RS600 rheometer

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(HAAKE Co., Germany) equipped with a Universal Peltier system and water bath (MultiTemp III,

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Amersham Biosciences) for temperature control. A parallel plate geometry of 27.83 mm diameter

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with a gap of 1.0 mm was used. All foam samples were carefully scooped onto the rheometer plate.

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Under the oscillatory mode, amplitude sweeps (stress = 0.1-300 Pa, frequency = 1 Hz) and frequency

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sweeps (0.1-100 rad/s, stress = 5 Pa, within the linear viscoelastic region) were performed to measure

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the variations of the elastic modulus (G′) and viscous modulus (G″). Temperature sweeps including

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heating from 25 °C to 80 °C and cooling back to 25 °C at a rate of 5 °C/min were also carried out at

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a constant stress of 5 Pa and a frequency of 1 Hz.

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Diffusing Wave Spectroscopy (DWS). The microstructure evolutions in emulsion foams during

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heating and cooling were monitored using a microrheology analyzer (Rheolaser Crystal,

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Formulaction Inc., France). This method is based on the principle of the multi speckle diffusing wave

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spectroscopy (MS-DWS), which enables a non-invasive measurement without any disturbance to the

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sample. The laser is multiply scattered by the particles into the sample, creating an interference

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pattern (speckle image). The variation of this image is directly related to the motion of the particles,


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and the fast motion of particles results in fast deformation of speckle image. By a mathematical

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analysis of this variation, decorrelation functions can be computed and then processed, to obtain a

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characteristic time τ, as a function of time or temperature. Values of 1/τ, or Micro-Dynamics (µD,

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Hz), are then plotted against time or temperature, resulting in characteristic peaks when the sample

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shows a microstructural evolution, such as a phase transition or any other physical events. All

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samples were heated from 25 °C to 80 °C at a rate of 3 °C/min, stayed at 80 °C for 5 min, and then

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cooled inversely. For each analysis, the same amount of sample was put in a small aluminum sample

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holder, which is covered with a coverslip on top to prevent evaporation.

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RESULTS AND DISCUSSION

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Formation and Stability of Emulsion Foams Prepared by GA Nanofibrils. We prepared the

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emulsion foam systems by simply homogenizing and aerating the mixtures of GA nanofibril solution

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(4 wt%) and liquid oil (sunflower oil) at high temperature (80 °C) followed by a fast cooling

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procedure. Amphiphilic GA fibrillar assemblies (Figure 1c) have the affinity for hydrophobic oil and

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gas phases, reducing the interfacial tensions effectively (oil-water and air-water), and are able to

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form multilayer interfacial networks to stabilize the oil droplets and air bubbles generated during

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homogenization.40,41 The subsequent cooling can strengthen the hydrogen-bond interactions between

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excess GA fibrils in the continuous phase as well as around the surfaces of oil droplets and air

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bubbles, self-assembling into a three-dimensional hydrogel network (Figures 1b and 1d),40,41 and

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then a gelled emulsion foam was obtained.

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Figure 2 illustrates the effects of oil fraction (5-60 wt%) and cooling rate on the formation of the

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emulsion foams. As can be seen from Figures 2a and S1a (see Supporting Information, SI), high


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foam overrun values (above 120%) were obtained in the oil fraction range from 5 to 20 wt%,

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suggesting the formation of wet foams. The further increase of oil fractions (40-60 wt%) led to a

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progressive decrease in foam overrun values (53-90%) due to the anti-foaming effect of oil droplets.

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This indicates that the emulsion foam formation can be actively controlled by changing oil droplet

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concentration.1,3,15 Compared with the emulsion foams cooled at room temperature (25 °C) (Figure

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2c), the emulsion foams (especially at 10 and 20 wt% oil fractions) cooled in ice bath showed a

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higher overrun as well as a homogeneous appearance, and it is worth noting that no obvious foam

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drainage was observed in these foams (Figure 2a). Based on our previous works,40,41 it is known that

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the cooling in ice bath can rapidly trigger the formation of GA fibrillar hydrogel network in the

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continuous phase, trapping the air bubbles and oil droplets, and thus keep the bubbles well separated.

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This contributes to delaying or even suppressing the drainage and also protecting the air bubbles

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against coarsening or coalescence. Figure 2b shows the photographs of two emulsion foams

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containing oil droplets of 10 and 20 wt%, and it was found that the foam can be extruded and shaped

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with a pipette, demonstrating that it exhibits a yield stress. The gel-like behavior of these emulsion

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foams will be further studied in the following mechanical property measurements. Furthermore, the

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impact of GA fibril concentrations (0.5-4 wt%) on emulsion foam formation is presented in Figure

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S1b (SI), and the oil fraction was fixed at 20 wt%. We observed that the increase of fibril

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concentration led to an increased foamability. At a low fibril concentration (0.5 wt%), the prepared

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emulsion foam was fluid and did not show any gelation. Upon the further increase of fibril

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concentrations (1-4 wt%), as expected, we were able to achieve the self-standing emulsion foam gels,

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probably due to the formation of the viscoelastic GA hydrogel network in the continuous phase.

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To visualize the GA fibrillar network and the arrangement of air bubbles and oil droplets, the


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microstructure observations of emulsion foams were performed by using PLM, CLSM, and FE-SEM.

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PLM and optical microscopy images of the freshly prepared emulsion foams are shown in Figures 3

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and S2, respectively. As can be seen, these emulsion foams exhibited a strong birefringence under

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PLM (Figure 3), which is due to the anisotropy of the assembled GA fibrils and fibrillar network in

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bulk.39-41 The presence of a multilamellar GA fibril shell at the surface of the bubbles was confirmed,

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and the fibril shell seems to be thicker at high oil droplet concentrations (40-60 wt%) (Figures 3e and

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3f, marked by red arrows). This is in good agreement with our previous studies,40,41 which showed

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that the GA fibrils also had the multilayer assembly at the oil-water interface, yielding the multilayer

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fibril-coated emulsion droplets. This can be further observed in the PLM images of emulsion foams,

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where the halo around these smaller oil droplets appears to be more luminous (Figure 3d), due to the

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multilayer fibril shell of the oil droplets.40,41 From these images, one can also see that the emulsion

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droplets are closely packed in the continuous phase as well as around the bubbles, especially at

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higher oil fractions of 40 and 60 wt% (Figures 3d-f). This is confirmed by the CLSM images of the

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emulsion foams dyed with Nile Red (Figure S3, SI). Such a dense assembly of oil droplets jammed

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in the channels between the bubbles can increase the local viscosity and thus reduce the film

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drainage rate. The jamming of emulsion droplets, known as “active filler particles”, can allow them

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to interact strongly with each other, and thus facilitate the enhancement in the strength and stability

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of gel network.43,44

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These GA fibrous aggregates and hydrogel network in bulk can be observed more directly with

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confocal microscopy (Figures 4a-c) and FE-SEM (Figures 4d-f). For observation of the freshly

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prepared emulsion foams, the GA fibril dispersions were first labelled with ThT. As can be seen, the

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strong ThT fluorescence enhancement was clearly observed (Figures 4a and 4c), demonstrating the


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presence of GA fibrillar assemblies at the interface and in bulk, in agreement with the PLM images

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(Figure 3). For SEM observations, we used the volatile hexane as oil phase (10 and 20 wt%) to

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produce the emulsion foams. The formation of gelled emulsion foams was not influenced by the use

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of hexane. The SEM images show fibrous network in the continuous phase (Figures 4d-f), which is

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formed through interfibrillar junctions and entanglements. The porous structure with large open cells

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surrounded by some small cells demonstrates the trap of air bubbles and jammed oil droplets in the

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continuous network (Figure S4, SI). These observations are in good agreement with the results of

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PLM (Figure 3) and CLSM (Figures 4a-c).

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We further studied the mechanical properties of these emulsion foams by performing oscillatory

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amplitude and frequency sweep measurements. The results of stress sweeps (at a fixed frequency of

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1 Hz) and frequency sweeps (at a fixed stress of 5 Pa) for emulsion foams with different oil fractions

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are presented in Figures 5a and 5b, respectively. As can be seen in Figure 5a, for all investigated

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cases, the storage modulus (G′) was always much larger than the loss modulus (G″), especially at

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high oil droplet fractions of 40-60 wt%. This confirms the gelation of emulsion foams, which exhibit

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mostly an elastic solid-like behavior, in agreement with the previous observation (Figure 2b). With

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increasing oil concentrations, the emulsion foams showed a higher yield stress (defined here when

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G″ = G′) and higher G′ values over the applied amplitude range, suggesting an increase in gel

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strength. As mentioned earlier, the oil droplets within the continuous network can function as active

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filler particles, and thus their close packing and connection (see Figures 3 and S3) contribute to

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increasing the strength of gel network (Figure 5a). The result of frequency sweeps is shown in Figure

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5b. As can be seen, both G′ and G″ for all the cases displayed a relatively weak frequency

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dependence, and the G′ curves had slightly positive slopes, indicating that the applied deformation


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rate did not obviously influence the rheological response of emulsion foams. The changes in G′ and

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G″ as a function of oil fractions are in good agreement with those observed in amplitude sweeps

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(Figure 5a), indicating that the strength and stability of GA fibrillar hydrogel network in emulsion

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foams can be tuned by the packing density of oil droplets.

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We found that these emulsion foams can be stable over a period of two weeks with only slight

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decrease in foam height (Figure 6). It is interesting to note that the emulsion foams do not drain

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obviously after storage, especially at higher oil fractions of 20-60 wt% (Figure 6a). Compared to the

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foam without oil droplets, the emulsion foams showed a significantly higher foam stability after

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storage, and the stability increased with the increase of oil concentration (Figure 6), indicating the

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role of emulsion droplets on improving foam stability. Previous studies have demonstrated that the

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accumulation and jamming of emulsion droplets in the Plateau borders and/or at the air-water

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interface contribute to enhancing the foam stability against drainage and bubble collapse.1,2,12-14 This

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is further supported by the results of microstructure (Figures 3 and S3) and mechanical properties

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(Figure 5) of emulsion foams, showing that at higher oil fractions (40-60 wt%) the emulsion droplets

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are packed closely around the bubbles and in the continuous phase, which can contribute to

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increasing gel strength and thus the stability of emulsion foams. However, it should be noted that

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although the emulsion foams with high oil fractions (40-60 wt%) showed high stability, their

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foamability became relatively poor (Figures 2a and S1a), which is often encountered in emulsion

283

foam systems.1,25

284

On the basis of these above analyses, we can conclude that in our emulsion foam system, it seems

285

likely that both the interfacial fibril films around the bubbles and the GA hydrogel in the continuous

286

phase are involved in providing stability for the system. We have previously demonstrated that the


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287

multilayer interfacial fibril shell with high electrostatic force can give superior stability to emulsion

288

droplets during storage and heating.40,41 In the emulsion foams, such similar multilayer GA fibrils on

289

the bubble surface (Figures 3 and 4) also can prevent the bubble coalescence. Next, we discuss in

290

more detail the contribution of the bulk hydrogel to foam stability. The observed arrest of drainage

291

seen in Figures 2a and 6a suggests that the liquid flow within the Plateau borders is fully stopped. It

292

is known that the full arrest requires that the yield stress in the network of interconnected Plateau

293

borders is higher than the local gravity-induced stress, which thus cannot provide the liquid flow.1,45

294

Herein, these typical stresses encountered inside the emulsion foams were further estimated to verify

295

whether our results can be interpreted according to the above analysis. For an initial bubble diameter

296

of the order of 100 µm, the gravitational stress (σg = ρwater·݃Ԧ) generally can reach the order of a few

297

Pa inside the Plateau borders. In Figure 5a we showed that all gelled emulsion foams had a high yield

298

stress (σy, 32.4-286.9 Pa), and the stress strongly increased with increasing oil concentration due to

299

the role of jammed oil droplets (Figures 3 and S3). So, it turns out that the yield stress of our

300

emulsion foams is easily enough to overcome the local gravitational stress (σy > σg), and as a result

301

the full arrest of liquid drainage is achieved, especially at higher oil fractions of 20-60 wt% (Figures

302

2a and 6a).

303

In addition, the viscoelastic properties of our emulsion foams also can influence the change in

304

bubble size and thereby the gas diffusion between bubbles. If the storage modulus G′ is smaller than

305

the Laplace pressure, gas from the smaller bubbles (higher pressure) can escape and diffuse through

306

the continuous phase to the larger bubbles with lower pressure, as set by Laplace’s law.46,47 However,

307

when G′ is greater than the Laplace pressure, the bubbles cannot change size, and thus the coarsening

308

can be strongly slowed down. In a bubble with diameter of around 100 µm, the Laplace pressure γ/RV


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(RV means equivalent spherical radius of a bubble with volume V) is about 103 Pa.46,47 This pressure

310

is lower than the G′ values of emulsion foams, especially at higher oil content of 60 wt%, where the

311

G′ can reach 104 Pa (Figure 5). Consequently, the emulsion foam can be stable for two weeks

312

without obvious coarsening and coalescence. Note that the presence of the layer of densely packed

313

oil droplets on the bubble surfaces (Figures 3e-f and S2c-d) also contributes greatly to decreasing the

314

gas exchange and coarsening rates.

315

Foam Breakdown on Demand by Temperature. The GA supramolecular hydrogel is known to be

316

thermoresponsive with a reversible gel-sol transition at 55-60 °C, and above this temperature range,

317

its hydrogen-bonding fibrillar network initiates melting (Tm).40,41,48 So, considering the presence of

318

fibrillar hydrogel network in bulk, we speculated that the GA fibril-stabilized emulsion foams should

319

have an interesting temperature-responsive behavior. Herein, to accurately monitor the

320

microstructure evolution as a function of temperature, we first performed the non-intrusive DWS

321

microrheology measurements on the emulsion foams during heating-cooling cycle. As can be seen in

322

Figure 7, when the foams were heated from 25 to 80 °C, one major characteristic peak was observed

323

between 60 and 65 °C for all samples tested. This peak is the signature of a marked microstructure

324

evolution occurred in emulsion foams, which probably arises from the gel-to-sol transition of GA

325

fibrillar hydrogel in bulk. Compared to the foams with higher oil contents (40-60 wt%), the foams

326

with oil contents of 10 and 20 wt% (especially at 20 wt%) displayed the significantly sharper peaks

327

with higher µD values, indicating the presence of a more dramatic phase transition or any other

328

physical event in emulsion foams. This is not surprising since, at 10 and 20 wt% oil fractions, the

329

emulsion foams had higher foam volumes (Figures 2a and S1a), and the corresponding foam collapse

330

should be more dramatic during heating, thus leading to more apparent microstructure evolution


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331

(Figure 7). Accordingly, no obvious characteristic peak was observed in the DWS curves during

332

cooling.

333

We then observed the response of the emulsion foams with 10 and 20 wt% oil fractions exposed to

334

environmental temperature above and below the melting temperature (Tm, 55-60 °C), which can

335

induce the phase transition of the bulk fibrillar hydrogel. The foams (especially at 20 wt% oil) placed

336

in an oven at 50 °C (T < Tm) remained stable after 2 h of storage (Figure S5a, SI). When the

337

temperature was increased to 65 °C (T > Tm), the bulk phase between the bubbles started to

338

transform from a gel to liquid state (sol), leading to the flow of foams (Figures S5b and S6, SI). This

339

is accompanied by a marked decrease in foam volume, and finally the bubbles completely vanished

340

after 60 min (Figure S5b, SI). When the temperature was further raised up to 80 °C, as expected, a

341

rapid foam destabilization was observed and the foam completely disappeared in only 5 min (Figures

342

S5c, 8b and 8f). These observations are in good agreement with the result of DWS microrheology

343

(Figure 7). However, it should be noted that the destabilization process can be halted by rapidly

344

cooling the foam in ice bath or to room temperature (25 °C), and then the remained emulsion foam

345

can become stable again. The cooling can trigger the re-gelatinization of GA fibrils both in bulk and

346

around the bubbles (Figure 8c), thus allowing recovering the stability of the remained emulsion

347

foams. In addition, the gelled emulsion foams can also be regenerated even after the complete

348

destruction at high temperature, by similarly homogenizing and aerating the remaining dispersions of

349

the GA fibril-coated emulsion droplets at 80 °C followed by a rapid cooling in ice bath (Figures 8d

350

and 8g). These results confirm that our emulsion foams are thermoresponsive, and the foam stability

351

can be switched from stable to unstable states by simply raising the temperature.

352

We further discussed the origin of the on-demand foam destabilization stimulated by high


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temperature. As mentioned previously (see above), the bulk GA hydrogel and the multilayer

354

interfacial fibril shell are two major factors in the stability of our emulsion foams. Thus, once the

355

temperature is higher than the melting temperature (Tm), the liquid drainage and interbubble gas

356

diffusion of emulsion foams will not be effectively suppressed due to the gel-to-sol transition of the

357

bulk fibrillar hydrogel at T > Tm, leading to an onset in foam destabilization. The melting of bulk

358

hydrogel also can lead to the flow of the oil droplets covering the air bubbles, and these bulk or

359

surface disturbances can change the shape of the fragile foam lamellae,4 thus easily causing the

360

lamellae rupture. Furthermore, when the temperature is raised, we also believe that the multilayer

361

interfacial fibril networks become softer and less compact because of the weakening of interfibrillar

362

hydrogen bonding.40,41

363

Dual Photo/Thermo-Responsive Emulsion Foams. The switch from stable to unstable states of our

364

emulsion foams is shown to be controlled by raising the temperature. Instead of heating the foam

365

externally, internal heat sources can be incorporated into the foam to produce the heat. Carbon black

366

particles (CBP) are shown to be able to absorb UV light and convert the absorbed light energy into

367

heat. It has been found that for the thermoresponsive foams stabilized by fatty acid assemblies, the

368

incorporation of CBP in the foam matrix can induce a fast foam destruction upon UV irradiation,

369

developing photo/thermo-responsive aqueous or nonaqueous foams.17,23,26 Here, we attempted to

370

produce the photothermoresponsive emulsion foams by incorporating CBP into an emulsion foam

371

sample (20 wt% oil). After ultrasonic treatment, the average size of CBP (Monarch® 800)

372

dispersions was found to be around 200 nm, and the morphology observation showed that these CBP

373

were large clusters aggregated from nanoparticles with smaller size at about 20 nm (see Figure S7a,

374

SI). As can be seen from Figure S8a (SI), the prepared emulsion foams containing different CBP


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375

concentrations (0.01-1 wt%) showed similar foam height, and all these foams were shown to be

376

stable for several weeks. This suggests that the formation and stability of emulsion foams were not

377

affected by the addition of CBP. The amplitude and frequency sweep results further demonstrate that

378

the presence of CBP in the GA fibril solution (4 wt%) did not obviously affect the network formation

379

of GA fibrillar hydrogel in bulk and thus the final mechanical properties of emulsion foams (Figures

380

S8b and S8c, SI).

381

For the emulsion foam containing CBP (1 wt%) in quartz cell, upon UV illumination from the top,

382

foam destabilization was observed after several minutes (Figures 9b and 9d). In contrast, the same

383

foam without CBP remained stable when exposed to the same condition of UV irradiation, and no

384

sample flow or obvious foam disappearance was observed (Figures 9a and 9c). This suggests that the

385

photoresponsive emulsion foams stabilized by GA fibrils can be successfully developed by the

386

addition of CBP. From the FE-SEM images of emulsion foam (20 wt% hexane), we observed that

387

these CBP aggregates were trapped inside the bulk hydrogel matrix between the bubbles (foam

388

lamella and Plateau borders) (see Figure S7b, SI). Under UV illumination, the CBP absorb the

389

irradiation and then increase the foam temperature through a strong photothermal conversion

390

process.23,26 When the foam temperature exceeds the gel-to-sol transition temperature (T > Tm), the

391

bulk fibrillar hydrogel in emulsion foams melts progressively (Figure S9, SI), thus leading to foam

392

destabilization (Figure 9). The rate of foam destabilization can be tuned by the CBP concentration.

393

We observed that with increasing CBP concentration, the foam destabilization rate significantly

394

increased due to the stronger photothermal effects in emulsion foams (Figures 9 and S10).

395

Additionally, the foam destruction can also be stopped by halting the UV irradiation. This result

396

demonstrates that through the synergistic combination of GA nanofibrils and CBP particles, the dual


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photo/thermo-responsive emulsion foams can be easily produced, and accordingly, the on-demand

398

foam destabilization can be better controlled.

399 400

CONCLUSIONS

401

We have shown that stable emulsion foams can be easily produced by using self-assembled GA

402

nanofibrils as natural building blocks. We showed that the foam stability is directly related to the

403

multi-interfacial fibril layers around air bubbles as well as the highly viscoelastic fibrillar hydrogel in

404

the continuous matrix, which can effectively prevent liquid drainage, bubble coarsening and

405

coalescence. Of particular interest is the presence of the GA hydrogel and the jamming of emulsion

406

droplets in bulk, endowing the gelled emulsion foams with a high yield stress (32.4-286.9 Pa), which

407

is enough to overcome the local gravitational stress, and as a result the liquid drainage in emulsion

408

foams is fully stopped. Further, the storage modulus of emulsion foams is greater than the Laplace

409

pressure (especially at higher oil fractions), which can strongly slow down the coarsening and

410

coalescence of bubbles. We then showed the on-demand foam destruction can be induced rapidly

411

upon heating, due to the melting of the bulk hydrogel and thus the flow of the emulsion droplets

412

covering air bubbles. The reversible gel-sol phase transition of the GA hydrogel network leads to the

413

thermoresponsive emulsion foams. On this basis, we demonstrate a novel simple approach to

414

produce dual photo/thermo-responsive emulsion foams, which can be destabilized by UV irradiation,

415

by synergistically combining the thermoresponsive GA fibrils with photothermal heating by CBP

416

particles. These results may open new scenarios on the design and construction of smart responsive

417

emulsion foams using natural, sustainable building blocks, e.g., saponin nanofibrils. We expect that

418

these stable and multi-responsive emulsion foams can find a broad range of applications in industrial


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419

and environmental processes such as in food, cosmetics, crude oil treatment and recovery.

420 421

ASSOCIATED CONTENT

422

Supporting Information

423

Effect of oil fractions on the foamability of emulsion foams, and photographs of emulsion foams

424

prepared at varying GA fibril concentrations; Optical microscopy images of emulsion foams with

425

different oil fractions; CLSM images of emulsion foams with different oil fractions; FE-SEM images

426

of freeze-dried emulsion foams; Photographs of emulsion foams taken at different times and

427

temperatures; Storage modulus and loss modulus of emulsion foams during heating and cooling

428

cycles; FE-SEM images of CBP dispersions after ultrasonic treatment and freeze-dried emulsion

429

foams prepared using GA fibrils and CBP; Photographs, amplitude, and frequency sweeps of

430

emulsion foams with different CBP concentrations; Photographs of GA hydrogel and emulsion foams

431

containing CBP before and after illumination with UV light.

432 433

ACKNOWLEDGMENTS

434

This work is supported by grants from the Special and General Projects of China Postdoctoral

435

Science Foundation (2017T100635 and 2016M600655), the Fundamental Research Funds for the

436

Central Universities (2017BQ101), and the National Natural Science Foundation of China

437

(31771923).

438 439


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5498-5502.

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emulsions based on the fibrillar self-assembly of natural saponin glycyrrhizic acid. Food Funct. 2017,

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properties formed by self-assembled nanofibrils of natural saponin glycyrrhizic acid for oil

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structuring. J. Agric. Food Chem. 2017, 65, 2394-2405.

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(42) Stubenrauch, C.; Gießelmann, F. Gelled complex fluids: combining unique structures with

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mechanical stability. Angew. Chem., Int. Ed. 2016, 55, 3268-3275.

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(43) Dickinson, E. Emulsion gels: the structuring of soft solids with protein-stabilized oil droplets.

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Food Hydrocolloids 2012, 28, 224-241.

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particles. J. Dispersion Sci. Technol. 1999, 20, 197-213.

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(45) Guillermic, R.-M.; Salonen, A.; Emile, J.; Saint-Jalmes, A. Surfactant foams doped with

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laponite: unusual behaviors induced by aging and confinement. Soft Matter 2009, 5, 4975-4982.

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(46) Li, T.; Nudelman, F.; Tavacoli, J. W.; Vass, H.; Adams, D. J.; Lips, A.; Clegg, P. S. Long-lived

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foams stabilized by a hydrophobic dipeptide hydrogel. Adv. Mater. Interfaces 2016, 3, 1500601.

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(47) Cantat, I.; Cohen-Addad, S.; Elias, F.; Graner, F.; Höhler, R.; Pitois, O.; Rouyer, F.; Saint-Jalmes,

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A. Foams: structure and dynamics; Oxford University Press: Oxford, 2013.

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(48) Yoshioka, H. Kinetics of the gel—sol transition of the aqueous solutions of β-glycyrrhizin

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studied by the temperature jump—spin probe method. J. Colloid Interface Sci. 1985, 105, 65-72.

549


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550

Figure 1. (a) Chemical structure of GA molecule used in this study. (b) Photographs of GA fibril

551

solutions (0.25 and 1 wt%) after storage at room temperature (25 °C) for 12 h. A transparent

552

hydrogel is formed when the fibril concentration (1 wt%) is higher than its minimum gelation

553

concentration (around 0.5 wt%). AFM height images of 0.25 wt% GA fibril solution (c) and a thin

554

layer of the 1 wt % GA fibrillar hydrogel (d).

555 556 557 558 559 560 561 562 563 564 565


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Figure 2. (a) Photographs of emulsion foams prepared at a constant GA fibril concentration (4 wt%)

567

with different oil fractions (0-60 wt%). The samples were obtained by a rapid cooling in ice bath. (b)

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Examples of emulsion foams containing 10 (above) and 20 wt% (below) oil, extruded with a pipette.

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(c) Photographs of emulsion foams stabilized by 4 wt% GA fibrils with different oil fractions (10-60

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wt%), obtained by cooling at room temperature (25 °C). Note: the obvious liquid drainage in the

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foams with 10 and 20 wt% oil fractions (c, marked by red arrows), probably due to the relatively

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slow formation of GA fibrillar hydrogel in bulk.

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Figure 3. Polarized light microscopy (PLM) images of emulsion foams stabilized by 4 wt% GA

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fibrils with oil fractions of 10 (b), 20 (c), 40 (e), and 60 wt% (f). Magnified images (a, d) of selected

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areas in the images of (b, e), respectively, showing the jamming of the oil droplets in the liquid

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channels as well as around the bubbles. The samples were obtained by a rapid cooling in ice bath.

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Note: the thick and multilamellar GA fibril shell at the bubble surface (e and f, marked by red

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arrows).

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Figure 4. CLSM images (a-c, scale bar = 200 µm) of emulsion foams stabilized by 4 wt% GA fibrils

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with 10 (a, b) and 20 wt% (c) oil fractions: (a, c) ThT fluorescence images (highlighting the GA

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fibrillar assemblies at the interface and fibrillar network in bulk); (b) bright field image. FE-SEM

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images (d-f) of emulsion foams with hexane fractions of 10 (d, e) and 20 wt% (f) prepared using 4

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wt% GA fibrils. The samples were obtained by a rapid cooling in ice bath.

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Figure 5. Amplitude (a) and frequency (b) sweeps for emulsion foams with different oil fractions

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(10-60 wt%) prepared by 4 wt% GA fibrils. Gʹ and Gʹʹ are shown as filled and open symbols,

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respectively. All measurements were performed at 25 °C.

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Figure 6. (a) Photographs of 4 wt% GA fibril-stabilized emulsion foams with different oil fractions

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(0-60 wt%) directly after formation (0 day) and after 14 days of storage at room temperature (25 °C).

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(b) The heights (OVt and OV0 = overrun at time t and initial, respectively) of these emulsion foams

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as a function of storage time at 25 °C.

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Figure 7. Micro-Dynamics (µD) values based on the diffusing wave spectroscopy (DWS) principle

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for emulsion foams stabilized by 4 wt% GA fibrils with different oil fractions (10-60 wt%),

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measured during the heating (solid line) and cooling (dashed line) cycles.

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Figure 8. (a-d) Photographs showing the temperature switchable process for emulsion foams

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containing 10 and 20 wt% oil fractions stabilized by 4 wt% GA fibrils. (a) Stable emulsion foams at

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25 °C. (b) After 5 min at 80 °C, the foams became fluid and were completely destroyed; (c) By

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cooling the emulsion to room temperature (25 °C), the systems became gelled again; (d) Stable

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emulsion foams could again be obtained after homogenizing and aerating the mixtures (c) at 80 °C

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followed by cooling in ice bath. (e-f) PLM images of the above emulsion foam with 20 wt% oil

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during the temperature-switchable process.

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Figure 9. Photographs of gelled emulsion foams (20 wt% oil) stabilized by 4 wt% GA fibrils without

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(a, c) and with (b, d) 1 wt% CBP, (a, b) before illumination with UV light and (c, d) 10 min after UV

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light illumination.

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Table of Contents

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Long-Lived and Thermoresponsive Emulsion Foams Stabilized by Self

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