DISSOLUTION DYNAMICS OF A BINARY SWITCHABLE HYDROPHILICTY SOLVENT - POLYMER DROP INTO AN ACIDIC AQUEOUS PHASE

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DISSOLUTION DYNAMICS OF A BINARY SWITCHABLE

HYDROPHILICTY SOLVENT - POLYMER DROP INTO AN ACIDIC

AQUEOUS PHASE

Romain Billet

Department of Chemical and Materials Engineering

University of Alberta

Edmonton, T6G 1H9, Canada

Binglin Zeng

Department of Chemical and Materials Engineering

University of Alberta

Edmonton, T6G 1H9, Canada

James Lockhart

BC Research Inc.

Richmond, BC V6V 1M8, Canada

Mike Gattrell

BC Research Inc.

Richmond, BC V6V 1M8, Canada

Hongying Zhao

BC Research Inc.

Richmond, BC V6V 1M8, Canada

hzhao@bcri.ca

Xuehua Zhang

Department of Chemical and Materials Engineering

University of Alberta

Edmonton, T6G 1H9, Canada

xuehua.zhang@ualberta.ca

October 27, 2022

ABSTRACT

Switchable hydrophilicity solvents (SHSs) are solvents deﬁned by their ability to switch from their

hydrophobic form to a hydrophilic form when put in contact with an acidic trigger such as

CO2

. As

a consequence, SHSs qualify as promising alternatives to volatile organic compounds during the

industrial solvent extraction processes, as greener and inexpensive methods can be applied to separate

and recover SHSs. Furthermore, because of their less volatile nature, SHSs are less ﬂammable and

so increase the safety of a larger-scale extraction process. In this work, we study the dynamics and

in-drop phase separation during the dissolution process of a drop composed of SHS and polymer,

triggered by an acid in the surrounding aqueous environment. From 70 different experimental

conditions, we found a scaling relationship between the drop dissolution time and initial volume with

an overall scaling coefﬁcient

∼0.53

. We quantitatively assessed and found a shorter dissolution time

related with a decrease in pH of the aqueous phase or an increase in initial polymer concentration in

the drop. Examining the internal state of the drop during the dissolution revealed an in-drop phase

separation behavior, resulting in a porous morphology of the ﬁnal polymer particle. Our experimental

results provide a microscopic view of the SHS dissolution process from droplets, and ﬁndings may

help design SHS extraction processes for particle formation from emulsions.

1 Introduction

Solvent removal is one of the most common processes in the chemical industry. Being able to remove and recover the

solvent from a system is of the utmost importance and can be done by evaporation induced by heating and/or vacuum. A

common way to make such solvent removal and recovery easier is to use low boiling point Volatile Organic Compounds

(VOCs). However, these solvents are also usually hazardous pollutants known for their toxicity and smog-forming

properties [

] These VOCs are also typically highly ﬂammable, requiring special considerations in the design and

arXiv:2210.14398v1 [cond-mat.soft] 26 Oct 2022

APREPRINT - OCTOBER 27, 2022

operation of the solvent recovery processes. Therefore, to respect the principles of green-chemistry, the search for an

alternative is necessary. [3, 4]

Switchable Hydrophilicity Solvents (SHSs) constitute a family of solvents that exhibits hydrophobic (immiscibility

with water) properties in their neutral form. However, when ionized (for example by contact with an acid such as

CO2

for a cationic SHS) they become hydrophilic (miscible with water). This switching process is also reversible, and SHS

can be "switched" back to their hydrophobic form by, for example, removing

CO2

by ﬂushing with

. [

] [

] The

mechanism behind this switching process is the acid-base chemical reaction described below:

SHSorg + H+

aq −−*

)−− SHSH+

For the

CO2

trigger, it is the dissolution of

CO2

and the acidiﬁcation of water that allows the SHS to switch. This

switching behavior can more generally be observed with other kinds of acid. [7, 8, 9]

SHSs present an economically viable and green alternative to VOCs due to the simplicity to "switch on" and "off" the

hydrophobicity of the solvent with high recoverability coming from the "switching off" process. Potential applications

of SHS have already been demonstrated in soybean oil extraction[

] [

], separation of bitumen from oil sands[

biofuel extraction from microalgae [

], polystyrene foam [

] and multi-layer packaging recycling[

liquid-liquid micro-extraction (SHS-LLME) for analytical chemistry [18, 19] or latex formation [20].

In latex formation, binary drops of SHS and dissolved polymer are made to deposit solid polymer particles by switching

off SHS hydrophobicity in an aqueous environment. A typical example is the formation of polystyrene particles by the

dissolution of N-N Dimethylcyclohexylamine (DMCHA) from binary drops of DMCHA and polystyrene. When the

DMCHA-polystyrene drop contacts an acidic phase, DMCHA reacts with protons producing the protonated counterpart

DMCHAH+ following the biphasic reaction below:

DMCHAorg + H+

aq )−

−−*DMCHAH+

The produced DMCHAH

is solubilized in the aqueous environment at the surface of the binary drop. In this way,

the DMCHA in the DMCHA-polystyrene drop begins to dissolve into the aqueous phase and the drop shrinks with

time. Subsequently, the insoluble polystyrene part of the drop is left behind and forms a solid polystyrene particle.

Understanding of the dissolution dynamics of the binary drop is required to obtain the desirable morphology, size, and

properties of ﬁnal polymer particles.

The dissolution process of drops with a low solubility in their environment is a problem that has already been studied

previously. In an early work, Duncan et al [

] showed experimentally that the diffusion-driven model for bulk

bubbles in an undersaturated liquid phase developed by Epstein [

] could be also applied to dissolution of a free oil

microdroplet. The diffusion-driven model was further developed to describe the dissolution of a drop deposited on

a substrate (i.e. sessile drop) with effects of the drop geometry taken into account. [

] The lifetime of a dissolving

sessile drop in various modes such as constant contact radius, constant contact angle, stick-slide, or stick-jump modes

of the drop have been experimentally and theoretically studied.[24, 25, 26]

To add to the diffusion-driven dissolution, the inﬂuence of gravity-induced convection in the bulk liquid on the

dissolution process has also been shown to speed up the dissolution dynamics.[

]. More recently, the dissolution

process of multicomponent drops, as opposed to pure liquid drops, has been found to exhibit complex dynamics. The

components in the drop may undergo preferential dissolution [

], phase separation [

], or self-assembling [

]. For

instance, dissolution of polymer solution drops may lead to formation of polymer capsules [

], while

snowballs of graphene oxide may develop from dissolution of colloidal drops. [35] [36]

The dynamic of the switching SHS and corresponding solvent extraction process has previously been shown to be time

consuming, in some cases reaching hours [

]. It is therefore important to understand what conditions may possibly

shorten the switching-extraction time and minimize residual solvent in a cost-effective manner. In the extraction process

assisted by

CO2

-switching SHS, two steps may play a role: (1) the slow mass transfer of

CO2

gas into the aqueous

phase, followed by the chemical reactions of dissociation and hydrolysis of

CO2

HCO−

CO2−

and

H+

, and (2)

the liquid phase reaction (switching) of neutral SHS (hydrophobic form) and dissolution of switched SHS (hydrophilic

form) into the aqueous solution. In previous work by Han et al the acceleration of the switching-extraction dynamics

was studied using a microﬂuidic device. The improved speciﬁc interfacial area between the aqueous phase and

CO2

gas

accelerated the extraction process inside the microﬂuidic device. [

] However, to the best of our knowledge, there

is no quantitative understanding on the SHS dissolution dynamics of the reaction-induced mass transfer of the SHS, and

in-particular inside a liquid drop and its impact on the ﬁnal morphology of the polymer particles post solvent extraction.

APREPRINT - OCTOBER 27, 2022

Table 1: Experimental parameters to study the inﬂuence of the initial drop composition, the pH in the aqueous phase is

ﬁxed at 2.53.

Initial polystyrene (wt%) 0 10

Drop volume (µL) 0.15 0.20 0.35 0.50 0.55 0.65 0.80 2.30 4.25 0.25 0.40 0.55 0.70

Initial polystyrene (wt%) 10 20

Drop volume (µL) 0.90 1.20 2.00 2.40 2.95 3.65 0.30 0.45 0.70 0.85 0.95 1.45 2.20

Initial polystyrene (wt%) 20 30

Drop volume (µL) 2.55 3.00 3.25 0.35 0.75 1.00 1.30 1.50 2.10 2.40 2.75 2.85

In this work, we study the dissolution of a binary DMCHA/polystyrene sessile drop immersed in a controlled acidic

aqueous solution environment as it is presented in Fig. 1. We aim at understanding the reaction-induced mass transfer

of the SHS from the drop to the aqueous phase. We ﬁnd a scaling relationship between the drop dissolution time and

the initial size of the drop, and study the impact of the aqueous phase pH and initial drop composition on this scaling

law. We also follow the internal state of the drop during the dissolution process, and show the existence of a phase

separation behavior and its implication on the ﬁnal morphology of the ﬁnal polymer particle. Our ﬁndings may help to

give a quantitative understanding of the switching dynamics of SHS drops during the initial external reaction/diffusion

dominated phase, and provide useful insights for internal drop dynamics and improved design for SHS switching

processes in applications such as latex formation.

2 Experimental section

2.1 Materials, solutions and substrates

The SHS polymer solutions were prepared using a mixture of N-N Dimethylcyclohexylamine (Sigma-Aldrich, 99%,

DMCHA) and polystyrene (Sigma-Aldrich,

= 40 000 g/mol, beads >99%). The weight ratio ranged from 10 to 30

wt% of polystyrene. Both chemicals were mixed under ambient condition in sealed vials using a magnetic stirrer for 2

hours until complete dissolution of the polystyrene beads in DMCHA. Acid solutions were freshly made before each

dissolution using formic acid (Sigma-Aldrich, > 95%) and (Milli-Q) water. The solution concentration used in this

work ranged from

1.1×10−1

M to

2.7×10−4

M (0.4 vol% to 0.001 vol%). The pH of the solutions was measured

using a benchtop pH meter (Fisher Scientiﬁc, Accumet AE150) and ranged from 2.37 to 3.83. Small square (

1×1cm

)

silicon substrates were cut from wafers and thoroughly rinsed with water, ethanol and then sonicated for 20 minutes

with ethanol before ﬁnally being dried with a stream of air. The partial solubilities of DMCHA in water as well as

water inside of DMCHA were measured by progressively adding one compound in a vial of the other until saturation.

A solubility of DMCHA in water of

15.3±1.7g/L

and water in DMCHA of

155 ±7g/L

(

18.3±0.8

vol

) were

measured close to what was previously reported in the literature. [39]

2.2 Drop dissolution

To study the dynamics of this dissolution, a side view camera was used to record the drop dissolution. The drop

dissolution setup is shown in Fig. 1. The substrates were placed at the bottom of a glass cuvette (Krüss Scientiﬁc,

36 ×36 ×30 mm W ×D×H

). The polymer solution was then ﬁlled in a glass syringe that was controlled by a

connected Drop Shape Analyzer instrument (DSA-100, Krüss Scientiﬁc).

For each experiment, the glass cuvette was ﬁlled with 30 mL of the acidic solution and drops of initial volume ranging

from 0.15 to 3.65

L were introduced by the motorized syringe on the surface of the substrate inside the aqueous

acidic phase. In total, 70 dissolution conditions were studied, as summarized in Table 1 for the polystyrene content

parameter and in Table 2 for the pH levels. During the drop dissolution, the side view images were captured directly by

the DSA-100 with a 9x magniﬁcation. For the top view, an upright optical microscope (Nikon H600l) equipped with a

4x magniﬁcation lens and a camera was used to image the drops.

After the drop dissolution, the substrate was carefully removed from the acidic solution. The excess water on the sample

was gently blown with air and the remaining water left to dry under ambient conditions for 48 hours.

2.3 Image analysis and characterization

Video footage of the drops was analyzed using MATLAB. The MATLAB code used analyzed each frame of the side

view videos and detected the drop using an intensity threshold. The detected drop was then used to measure the

quantities of interest. This can be done either by assuming a spherical model and ﬁtting a spherical-cap shape to the

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摘要：

DISSOLUTIONDYNAMICSOFABINARYSWITCHABLEHYDROPHILICTYSOLVENT-POLYMERDROPINTOANACIDICAQUEOUSPHASERomainBilletDepartmentofChemicalandMaterialsEngineeringUniversityofAlbertaEdmonton,T6G1H9,CanadaBinglinZengDepartmentofChemicalandMaterialsEngineeringUniversityofAlbertaEdmonton,T6G1H9,CanadaJamesLockhartBC...

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DISSOLUTION DYNAMICS OF A BINARY SWITCHABLE HYDROPHILICTY SOLVENT - POLYMER DROP INTO AN ACIDIC AQUEOUS PHASE

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