MIMS/CMMA seminar 2013.10.18

Effect of chemical reaction

causing contact angle variationon the spontaneous motionof an I2-containing nitrobenzene droplet

Kanagawa UniversityBen Nanzai

Aim of study
・ Quantification of droplet motion
Previous researchers have presented discussions using only phase diagrams
indicating the existence/non-existence of motion or the type of motion.

・ Determination of chemical components
Previous researchers investigated this phenomenon using a physical ormathematical approach.

・ Verification of proposed mechanism
TSA+I3- or I-HighLow

Interfacial Free Energy

Quantitative determination of droplet motion
Torus-shape flat glass plate

Nitrobenzene droplet
with 5 mM Ｉ2 (10 mL)
without KI

Petri Dish

TSAC aq.
(pH2-12)・Distance・Duration
The rotational direction was not constant.
The droplet ran on the glass substrate ata constant velocity.

・I2 concentration in oil droplet: Absorptiometry (550 nm) or RGB value analysis
・Iodic ion (I-, I3-, IO3-) concentration in aqueous solution: Ion chromatography

Running distance and duration of motion
Droplet motion was observed under alkaline conditions with the additionof NaOH, although no remarkable motion was observed in an aqueous
solution of NaCl or HCl.
4030300

20200100

100002468[NaOH] / mmol L-1

1012Duration / secDistance / mm

400

An intermittent motion consistingof the running state and resting state
was exhibited below 0.8 mM.
The running distance and duration
increased with increasing NaOH conc.

Iodine disproportionate in the alkaline conditions

I2 + 6OH- → 5I- + IO3- + 3H2O

The droplet motion was probably induced by the ionic association
between TSA+ and iodic ions such as I- and IO3-.

Quantitative determination of chemical components
The presence of I- in aqueous
solution was only identified in thesystem without TSA+.
Matsushita et al. reported in relation

to the formation of TSA-I in a similarsystem.
Colloid Surface A 395 (2012) 233.
These results support the reaction
between TSA+ and I- at the interface.
Alkaline conditions

The mass balance between I2 and I- was not break-even.
I2 + 6OH- → 5I- + IO3- + 3H2O

The formed I- would induce further reaction with I2
on the oil−water interface to produce I3- :I2 + I- → I3-

The contributions of I- and I3- to the consumptionof I2 are almost equal.

Acidic conditions

I2 consumption rate
The consumption rate of I2 increased
with increasing NaOH conc.

50 min41 minThe concentration equilibrium wasachieved earlier under the condition ofhigher NaOH conc.

323 min15 min10 min15 min0012

3

4

The rate of I2 consumption correlatedwith the velocity of droplet motion.
5[NaOH] / mmol L-1

These results agree well with the predictionthat the reaction between TSA+ and iodic ionsat the interface induces the droplet motion.

Velocity / mm sec-1[I2] / mmol L-1

6d[I2] dt-1 / 10-8 mol min-1

Droplet contact angle
For discussion in terms of the variation of the interfacial energy of oil−glass
interface, we considered the interfacial tensions and contact angles within thethree phase water–oil–glass substrate in this study.
Initial state

The oil droplet contact angle stayed high(~160 deg) in the aqueous solution without
surfactant TSAC.

End of motion

Variation of contact angle
Large contact angle

Contact angle / degree

200without TSAC
1500 min
3 min
5 min

100

The interfacial energy increased with more
TSA+ desorbing from the oil−glass interface.
In fact, the contact angle in the aqueoussolution without TSAC corresponded with theupper limit.

10 min50

1 day

Small contact angle

00

1

2

3

45

[NaOH] / mmol L-1
The contact angle would be controlled bycompetition between TSA+ adsorption on theoil−glass interface and the ion aggregation
derived from iodic ion generation.
I2 + 6OH- → 5I- + IO3- + 3H2O

The interfacial energy decreased with more
TSA+ adsorption on the oil−glass interface.
I2 was consumed over time, becoming
scarce for the ion aggregation with TSA+.

Difference of interfacial tensions
Young’s equation

TSAC aq.

gosq

gow cos qwso + gso = gsw

gow
gsw

However, the left side of equation
was smaller than the right side .

gow

oil−water interfacial tension

gsw

substrate−water

gso

substrate−oilqwso

contact angle of droplet

The difference of interfacial tensions generated
around the oil droplet at non-equilibrium.
DF = gsw − gow cos qwso− gso

gsw =gsa− gaw cos qasw

gso =gsa− gao cos qaso）

Young’s equation in air

DF = gao cos qaso − gow cos qwso − gaw cos qasw

Effect of DF on droplet motion
60

DF increased to almost 50 mN m-1
with increasing NaOH conc.
NaOH addition

DF increased slightly with increasingNaCl conc. under the condition withaddition of NaCl in which the droplet
motion was not observed.

403020DF was not 0 mN m-1,
even in the case without droplet motion.

NaCl addition

10002

4681012Concentration/ mmol L-1

The threshold for the occurrence ofdroplet motion existed.
DＦth  23.5 mN m-1

Running distance / mmDF / mN m-1

50DＦ / mN m-1

Trigger for droplet motion
We expect that a small trigger exists for the deformationof oil droplets, called “contact angle hysteresis”.
such as…
① An instability of droplet during the injection② The interfacial instability caused by the interfacial reaction③ The generation of emulsion around the droplet.

Resting StateSTART

Running State17 msec/frame

Conclusion① Iodic ions such as I−, IO3−, and I3− were produced from thedisproportionate reaction of I2 under alkaline conditions.
② These ions reacted with cationic surfactant TSA+ at the oil–waterinterface on the glass substrate.
③ The aggregate of these ions desorbed from the glass surface.
The oil–glass interfacial tension increased.
④ The oil droplet contact angle increased.
⑤ The disruption of balance of the interfacial tension around thedroplet induced spontaneous droplet motion.