JEMEA Journal Vol.3 (2019.9)

学術論文
界面活性剤の濃度および分子長さを変更した時のマイクロ波照射中の界面張力減少Interfacial tension reduction during microwave irradiation with changing
concentration and molecular length of surfactant
薗部 智史 1, 柴田 洋輔 1, 南島 直哉 1, 朝熊 裕介*1, Anita Hyde2, Cuong Nguyen2, Chi Phan2
1.兵庫県立大学大学院工学研究科化学工学専攻〒671-2280 兵庫県姫路市書写 2167
2. Discipline of Chemical Engineering and Curtin Institute of Functional Molecules and Interfaces,
Curtin University
GPO Box U1987, Perth, WA 6845, Australia
Satoshi Sonobe1, Yosuke Shibata1, Naoya Minamishima1, Yusuke Asakuma*1,
Anita Hyde2, Cuong Nguyen2, Chi Phan2
1. Department of Chemical Engineering, University of Hyogo、
1. 2167 Shosha, Himeji, Hyogo 671-2280, Japancorresponding author*, e-mail address : asakuma@eng.u-hyogo.ac.jp
Keywords: Microwave, interfacial tension, Triton X-100, Triton X-405
Abstract
Previous studies revealed that microwaves have particular effects on the interfacial tension of the decane/water interface in thepresence of Triton X-100. This study investigated the reasons for the hysteresis during and after microwave irradiation byvarying the concentration and type of Triton surfactants: X-100 and X-405. As before, physiochemical changes were found to
persist after the irradiation of Triton X-100. Interestingly, no hysteresis was observed for Triton X-405. The difference oftemperature dependency was caused by different hydrophilicity of Triton X-100 and X-405. Desorption at the interface during
the irradiation is easier for the shorter surfactant (Triton X-100) than its longer surfactant (Triton X-405), due to the length of thehydrophilic group. Moreover, the shorter surfactant has a stronger hysteresis during the heating/cooling cycles due to higher
adsorption. The dynamic surfactant behaviour between two phases during the irradiation might be promisingly applicable toindustrial processes such as de-emulsification.
1. Introduction
Microwave heating has been emerging as a more effective

heating method than conventional (thermal) alternatives,

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JEMEA Journal Vol.3 (2019.9)

offering advantages such as higher heat efficiency, quick

affected by heat capacity, viscosity and thermal conductivity.

thermal response, homogenous heating, selective heating and

The competition between these two extreme cases, which

non-contact heating [1–3]. Nevertheless, further application

never occurs during conventional heating, offers promisingof microwaves in industry would be facilitated by a greater

alternatives for controlling interfacial properties. Previous

understanding of the behaviour of local heating around thestudies have measured the surface tension of the liquid-air

fluid-fluid interface [4]. As an example, microwaves caninterface during microwave irradiation through pendant drop

pass through a phase with a low dielectric constant (such as

method [6,7] and found reductions to the interfacial tension

oil or air), and selectively heat a more polarising phase (i.e.

incurred during irradiation to persist after irradiation had

water) directly. As a result, the water layer around thestopped.

interface is selectively heated by microwave irradiation,

Recently, microwave irradiation has been employed forwhile heating of low and non-polar phases occurs as this heatemulsification [8–10] and de-emulsification processes

is transferred by conduction and convection to the remainder[5,11,12]. In these studies, the interfacial behaviour wasof the solution.

mainly discussed based on thermal effects through emulsion

Microwaves have a strong effect on ions or molecules

coalescence on a microscopic scale. In contrast, this study

containing dipoles. The rapid switching direction of theaims to investigate microwave effects at the liquid-liquid

electromagnetic field exerts a force on the polarised bonds,

interface on a macroscopic scale while distinguishingcausing them to shift to align with the applied field. In

thermal and non-thermal effects.

rotating, the dipoles upset the network of intermolecular

Our previous study [4] on the oil-water interface in thebonds within the solution. The energy released by thepresence of Triton surfactants indicated that microwave

disruption of these bonds translates to heating of the solution.

irradiation affected the liquid-liquid interface differently

Microwaves decay exponentially while penetrating thefrom conventional (thermal) heating. Evidence of finesolvent as energy is quickly absorbed by the solution. In

bubbles and emulsions around the interface raised thewater, studies have shown that microwaves penetratepossibility of metastable, localised emulsions affecting theroughly 1.3 cm from the interface [5]. In the event of rapid

interfacial tension for an extended period of time [4], andlocalised heating at the interface carried out by high-poweredsignificant thermal gradients may alter the surfactant

microwaves, sufficient energy can be absorbed in a small

solubility in the fluid layers near to the interface. However,area to cause boiling and phase change at the interface before

the study was unable to definitively determine the cause of

the energy can be dissipated by conduction or convection. In

the long-term reduction in interfacial tension, and thecontrast, consider the case of low power irradiation. Heatmechanism for the interfacial tension reduction has not beendissipates from the interface to both bulk phases throughclarified. To further our understanding of this phenomenon,

conduction and convection [6,7], preventing the interfacial

the present study investigates the non-thermal effect oflayer from heating quickly enough to boil while the bulk

microwave heating by altering surfactant concentration andfluids remain cool.

tail length during cyclic microwave irradiations.

While localised heating will always occur, the extent towhich heat will “concentrate” in the interfacial layer is

2. Experimental

directly related to the microwave power and the physical

2-1. Procedure

properties of the phases in question: the irradiation power

A small quartz cell (3×3×3 cm) was partially filled with anand dielectric properties of the solutions will influence theaqueous surfactant solution (Triton X-100 or X-405 at

rate of energy absorption in the interfacial layer, whereas thevarious concentrations) and covered with a layer of n-decane

speed of heating and heat transfer to the bulk solution are(2 mL). Decane (99%) and Triton surfactants were supplied

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JEMEA Journal Vol.3 (2019.9)

by Kanto and Kishida Chemical Companies, respectively,(a)and used without further purification. A small Teflon ballTemperature probe(6.35 mm diameter) was suspended by a thin Teflon pipe (2mm outer diameter, 1 mm inner diameter) and submerged

Cell andTeflon ballLight

until the decane-water interface showed considerable
deformation while retaining contact with the ball. A
temperature probe (FS-100, Anristu meter CO., LTD.) was

Camerainserted through the tube and through to the bottom of theball to measure the temperature of the bulk aqueous solution.

(b)

The cell and probe were placed inside of a microwave
reactor equipped with a video camera and LED light as

Decaneshown in Fig. 1a. The interface was captured on video during
and after irradiation (Fig. 1b) and the interfacial tension wasmeasured from the interfacial profile using the holm method

Water[13]. In this experiment, reflector, which is placed on theother side of the microwave reactor, was moved so that the

Teflon ball
D=6.35mm

temperature probe showed maximum heating rate.
In order to investigate the lasting effects of microwaves,heat-cool irradiation cycles were performed on theliquid-liquid interfaces. The cell was subjected to microwave
irradiation at 80 W for 60 s with a 30 minute interval

Temperature probe

between irradiation incidents, during which time the cellcools almost to room temperature. An example of the(c)temperature profiles is given in Fig. 2.
As shown in Fig. 2, the bulk fluid temperature remains
below the boiling point and the microwave power andduration is sufficiently moderate to avoid boiling at theFig. 1: Interfacial tension measurement: (a) Schematic

liquid-liquid interface. The decane layer at the top of the cell

diagram of microwave reactor; (b) image of the interface

also serves to prevent evaporation of the aqueous layer. Loss

as used for fitting; (c) Triton X (100: n=10; 405: n=45) nof decane to evaporation is minimal and does not affect the

represents the number of the periodic structure and tailshape of the interface. The experiment conditions are listed

length of Triton.

in Table 1. It should be noted that X-405 is approximately
four times longer than Triton X-100 (as shown in Fig. 1c).
Concentrations below CMC were chosen as concentrationsof each Triton X.

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JEMEA Journal Vol.3 (2019.9)

80

tension was caused by the formation of fine decane-in-water60

and water-in-decane emulsions as their solubilities changewith temperature [4,16]. As the solubility decreases while the40

irradiated sample cools [17], emulsions of water and decane

20

extract into both the aqueous and organic phases and slowly
coalescing. Surfactant adsorbs at the liquid-liquid interface

0
0

2000

4000
6000
Time (s)8000

of emulsions, stabilising the droplets. Furthermore, as theFig. 2: Temperature profile during cyclic irradiation

drops effectively increase the effective interfacial area, thenumber of surfactant molecules that can be held in the

Table 1: Experimental conditions for interfacial tension
measurement
No.

1

Surfactant

CMC (Critical micelle

Concentration

concentration) [mM][mM]

Triton X-100

0.22 [14]interfacial area increases substantially, contributing to thedepression of the interfacial tension.

0.05, 0.1, 0.22,
0.66

2

Triton X-405

0.66 [15]0.2, 0.66, 0.86

2-2. Analytical method of interfacial tension
Movies of the oil/aqueous interface were recorded during
the experiment and analysed using a Matlab-based programwith numerical optimization to find the best-fitting
parameters for the Young-Laplace equation describing theshape of the holm meridian and contact angle () between

Fig. 3: Interfacial tension profiles during and after the first

Teflon ball and two-liquids, from which the interfacialand fifth irradiation (Triton X-100, 0.22 mM).

tension was determined [13]. Detailed theory and fitting

Fig. 4 shows interfacial tension profiles for different

procedure have been described elsewhere [4,13].

concentrations of Triton X-100 during and after microwave
irradiation. Hysteresis was observed in the interfacial tension

3. Results

profiles at each concentration. The interfacial tension

Fig. 3 shows the interfacial tension profiles of 0.2 mM

reduction occurred after each irradiation instance, althoughaqueous solutions of Triton X-100, with decane as the lightthe magnitude of the reduction decreased with successive

phase, during (filled) and after (unfilled) the first and fifth

irradiation instances, as can be seen in Fig. 5. For example,

irradiations (80 W for 60 s). As previously shown [4], thethe reduction becomes negligible after the fourth or fifthinterfacial tension increased during heating. Furthermore,

irradiation. The interfacial tension reduction, which iswhile the interfacial tension decreases as the sample cools, it

defined as difference between initial value and value at roomin fact decreases further that the original equilibrium

temperature after first or fifth cycle, is shown in Fig. 6 atmeasurement, leaving a clear hysteresis between heating anddifferent surfactant concentration.

cooling. By the fifth irradiation, the interfacial tension was
substantially depressed.

Fig. 7 shows interfacial tension profiles during and after the

Possible mechanisms for the persistent reduction in surface

first and the fifth irradiation of 0.86 mM solutions of Triton

tension were postulated in Hyde et al. (2017) [4]. A

X-405. In contrast to Triton X-100, the interfacial tension ofpromising explanation suggested that the reduced interfacialdecane/Triton X-405 decreased during microwave irradiation.

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JEMEA Journal Vol.3 (2019.9)

Furthermore, no significant hysteresis was observed between
the heating and cooling curves, nor did successive irradiation
by microwaves alter the interfacial tension (examples in Fig.
8). In the case of Triton X-405, temperature dependency ofinterfacial tension is negative. Accordingly, interfacial
tension during the irradiation becomes lower than that ofcooling because temperature at interface is higher than
measurement point due to the microwave absorption.
Fig. 6: Surface tension reduction after heating/coolingcycles: TX-100 (blue) and TX-405 (orange) after one (filled)
and five (unfilled) irradiations

Fig. 4: Interfacial tension profiles for different concentrationsof Triton X-100 during and after first irradiation.

Fig. 7: Interfacial tension profiles during and after the first
and the fifth irradiation (Triton X-405 0.86 mM)
Desorption of Triton X-405 molecules from the interface ismore difficult than Triton X-100 as its hydrophilic group isfour times longer. The longer, unwieldy molecule is lessinfluenced by microwave irradiation.
Fig. 5: Interfacial tension after each irradiation for different

Unlike conventional heating, microwaves exert a force on

concentration of Triton X-100

the on water molecules, causing them to rotate anddisrupting the hydrogen bonding network. As the highinterfacial tension of water is directly related to the unusual
strength of its hydrogen bonding network, the disruption ofthese bonds around the interface leads to a lower interfacial
tension [18,19]. Microwaves are reported to slightly decrease
the surface tension of pure water [20]. In the case of Triton
X-100,

thethermaleffectonthesurfactant

adsorption/desorption dominates the slight reduction in the50

JEMEA Journal Vol.3 (2019.9)

water-decane interfacial tension [21].

increased. However, the interfacial tension decreased as thesolutions cooled, leading to hysteresis between the heating
and cooling curves and overall depression of the interfacial
tension that increased with successive irradiation. In contrast,
the interfacial tension of decane-Triton X-405 decreased as
temperature increased without hysteresis. Desorption of
longer surfactant might be difficult and manly thermal effectby temperature difference between interface and cell bottom
was observed. To utilize microwave effectively for two
liquids system, microwave power, heating/cooling patternand surfactant adsorption capability must be considered.
Moreover, in-situ data of interfacial tension during the

Fig. 8: Interfacial tension profiles for different concentration

irradiation is needed for quantification of the hysteresis.

of Triton X-405 during and after first irradiation
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Manuscript
received:

July 5, 2019

Revised: July 25, 2019
Accepted: Aug

7, 201952