Effect of chemical reaction causing contact angle variation on the spontaneous motion of an I 2 -containing nitrobenzene droplet

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Sylvia Reeves
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1 MIMS/CMMA seminar Effect of chemical reaction causing contact angle variation on the spontaneous motion of an I 2 containing nitrobenzene droplet Kanagawa University Ben Nanzai

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

2 Aim of study Quantification of droplet motion Previous researchers have presented discussions using only phase diagrams indicating the existence/nonexistence of motion or the type of motion. Determination of chemical components Previous researchers investigated this phenomenon using a physical or mathematical approach. Verification of proposed mechanism TSA + I 3 or I High Interfacial Free Energy Low

Quantitative determination of droplet motion Torusshape flat glass plate Petri Dish Nitrobenzene droplet with 5 mm I 2 (10 ml) without KI TSAC aq.

3 Quantitative determination of droplet motion Torusshape flat glass plate Petri Dish Nitrobenzene droplet with 5 mm I 2 (10 ml) without KI TSAC aq. (ph212) Distance Duration The rotational direction was not constant. The droplet ran on the glass substrate at a constant velocity. I 2 concentration in oil droplet : Absorptiometry (550 nm) or RGB value analysis Iodic ion (I, I 3, IO 3 ) concentration in aqueous solution : Ion chromatography

4 Distance / mm Duration / sec Running distance and duration of motion Droplet motion was observed under alkaline conditions with the addition of NaOH, although no remarkable motion was observed in an aqueous solution of NaCl or HCl An intermittent motion consisting of the running state and resting state was exhibited below 0.8 mm The running distance and duration increased with increasing NaOH conc [NaOH] / mmol L 1 Iodine disproportionate in the alkaline conditions I 2 + 6OH 5I + IO 3 + 3H 2 O The droplet motion was probably induced by the ionic association between TSA + and iodic ions such as I and IO 3.

Quantitative determination of chemical components The presence of I in aqueous solution was only

reported in relation to the formation of TSAI in a similar system. Colloid Surface A 395 (2012) 233.

The mass balance between I 2 and I was not breakeven.

5 Quantitative determination of chemical components The presence of I in aqueous solution was only identified in the system without TSA +. Matsushita et al. reported in relation to the formation of TSAI in a similar system. Colloid Surface A 395 (2012) 233. These results support the reaction between TSA + and I at the interface. The mass balance between I 2 and I was not breakeven. I 2 + 6OH 5I + IO 3 + 3H 2 O The formed I would induce further reaction with I 2 on the oil water interface to produce I 3 : I 2 + I I 3 The contributions of I and I 3 to the consumption of I 2 are almost equal. Alkaline conditions Acidic conditions

6 I 2 consumption rate min The consumption rate of I 2 increased with increasing NaOH conc. [I 2 ] / mmol L min 3 min The concentration equilibrium was achieved earlier under the condition of higher NaOH conc min 10 min 15 min [NaOH] / mmol L 1 These results agree well with the prediction that the reaction between TSA + and iodic ions at the interface induces the droplet motion. The rate of I 2 consumption correlated with the velocity of droplet motion. Velocity / mm sec 1 d[i 2 ] dt 1 / 10 8 mol min 1

the three phase water oil glass substrate in this study.

7 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 the three phase water oil glass substrate in this study. Initial state End of motion The oil droplet contact angle stayed high (~160 deg) in the aqueous solution without surfactant TSAC.

Contact angle / degree Variation of contact angle Large contact angle 200 150 100 50 0 without TSAC 0 1 2 3 4 5 [NaOH] / mmol L 1 0 min 3 min 5 min 10 min 1 day The interfacial energy increased with

8 Contact angle / degree Variation of contact angle Large contact angle without TSAC [NaOH] / mmol L 1 0 min 3 min 5 min 10 min 1 day The interfacial energy increased with more TSA + desorbing from the oil glass interface. In fact, the contact angle in the aqueous solution without TSAC corresponded with the upper limit. Small contact angle The contact angle would be controlled by competition between TSA + adsorption on the oil glass interface and the ion aggregation derived from iodic ion generation. I 2 + 6OH 5I + IO 3 + 3H 2 O The interfacial energy decreased with more TSA + adsorption on the oil glass interface. I 2 was consumed over time, becoming scarce for the ion aggregation with TSA +.

9 Difference of interfacial tensions TSAC aq. Young s equation g os q g ow g sw However, the left side of equation was smaller than the right side. g ow cos q wso + g so = g sw g ow g sw g so q wso oil water interfacial tension substrate water substrate oil contact angle of droplet The difference of interfacial tensions generated around the oil droplet at nonequilibrium. DF = g sw g ow cos q wso g so g sw = g sa g aw cos q asw g so = g sa g ao cos q aso ) Young s equation in air DF = g ao cos q aso g ow cos q wso g aw cos q asw

Running distance / mm Effect of DF on droplet motion 60 50 DF increased to almost 50 mn m 1 with increasing NaOH conc.

10 Running distance / mm Effect of DF on droplet motion DF increased to almost 50 mn m 1 with increasing NaOH conc. DF / mn m NaOH addition DF increased slightly with increasing NaCl conc. under the condition with addition of NaCl in which the droplet motion was not observed NaCl addition DF was not 0 mn m 1, even in the case without droplet motion. Concentration/ mmol L 1 The threshold for the occurrence of droplet motion existed. DF th 23.5 mn m 1 DF / mn m 1

Trigger for droplet motion We expect that a small trigger

The interfacial instability caused by the interfacial

11 Trigger for droplet motion We expect that a small trigger exists for the deformation of oil droplets, called contact angle hysteresis. such as 1 An instability of droplet during the injection 2 The interfacial instability caused by the interfacial reaction 3 The generation of emulsion around the droplet. Resting State START Running State 17 msec/frame

3 The aggregate of these ions desorbed from the glass surface. The oil glass interfacial tension increased.

12 Conclusion 1 Iodic ions such as I, IO 3, and I 3 were produced from the disproportionate reaction of I 2 under alkaline conditions. 2 These ions reacted with cationic surfactant TSA + at the oil water interface 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 the droplet induced spontaneous droplet motion.

Titration a solution of known concentration, called a standard solution

Acid-Base Titrations Titration is a form of analysis in which we measure the volume of material of known concentration sufficient to react with the substance being analyzed. Titration a solution of known