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Short-chain n-Alcohol induced changes in phase behaviors of aqueous mixed cationic/anionic surfactant system Li-Sheng Hao, Jin Wu, Ying-Rui Peng, Ying Wang, Kai Xiao, Yan Hu, and Yan-Qing Nan Langmuir, Just Accepted Manuscript • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Langmuir

Short-chain n-Alcohol induced changes in phase behaviors of aqueous mixed cationic/anionic surfactant system Li-Sheng Hao, Jin Wu, Ying-Rui Peng, Ying Wang, Kai Xiao, Yan Hu, Yan-Qing Nan* Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China

Abstract The short-chain n-alcohol induced changes in phase behaviors of aqueous mixed 1,3-propanediyl bis(dodecyl dimethylammonium bromide) (12-3-12) and sodium dodecylsulfonate (AS) system have been investigated. For the 12-3-12/AS/H2O mixed system, there are two kinds of aqueous two-phase systems with excess cationic surfactant (ATPS-C). The molar ratio of 12-3-12 to AS (MR12-3-12/AS) and the total surfactant concentration (mT) in the top phase are smaller than those in the bottom phase of ATPS-C. It is worth noting that the addition of ethanol or n-propanol leads to different influences on the ATPS-C. MD simulation results illustrate that the different influences ascribe to the difference in the cosurfactant effect of ethanol and n-propanol. When ethanol is used as additive, the difference in mT leads to the difference in interactions between surfactants and ethanol for the two coexisting phases of ATPS-C, determining the difference in their combination ability with the mixed solvent. It is the main reason for the ethanol induced phase inversion of the first kind of ATPS-C. When n-propanol is added, in addition to mT, MR12-3-12/AS is also a key factor influencing the interactions between 12-3-12 and AS, and between surfactants and n-propanol due to the stronger cosurfactant effect of n-propanol. MD simulations indicate that vesicles with smaller MR12-3-12/AS are easier and faster to form. These vesicles spontaneously accumulate at the top phase accompanied by certain amount of mixed solvent transferred from the bottom phase of ATPS-C. Meanwhile, the competition for the mixed solvent arising from the surfactant-rich bottom phase prevents the transferring. The two factors work together to cause the increase of mT in the top phase of ATPS-C with the addition of n-propanol, leading to

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n-propanol induced phase concentration inversion rather than phase inversion of ATPS-C. Based on the experimental results and MD simulations, ethanol induced phase inversion mechanism or n-propanol induced phase concentration inversion mechanism of ATPS-C has been proposed. Keywords: cationic/anionic surfactants; aqueous two-phase system; phase inversion; short chain n-alcohol; MD simulations

1. Introduction Aqueous mixtures of oppositely charged surfactants possess a rich variety of aggregate morphologies and strong synergism in mixed micelle formation and interfacial properties, and exhibit interesting and complex phase behaviors[1-4], hence are of great theoretical and practical interest. The micro-structures of aggregates and phase behaviors of the aqueous mixtures of oppositely charged surfactants are dependent on the microscopic interactions, such as the electrostatic interaction between oppositely charged headgroups[5], hydrophobic interaction between surfactant tail chains[6], and/or hydrogen binding between some groups[7]. Additives[8-12] can influence these interactions and in turn induce morphological transitions between aggregates with different micro-structures, and thus induce changes in phase behaviors. Mixtures of oppositely charged surfactants[4,13-15], or mixtures of oppositely charged polyelectrolytes[16], or mixtures of oppositely charged polyelectrolyte and ionic surfactant[17] in water often display associative phase separation. When cationic and anionic surfactants with certain concentrations are mixed at certain molar ratios and temperatures, the mixed system separates spontaneously into two aqueous phases with different densities[4] under the force of gravity. Because both phases are mainly composed of water, this kind of system at liquid-liquid phase equilibrium was referred as aqueous two-phase system (ATPS). ATPSs have attracted much attention since the pioneering work of Albertsson[18]. There are many kinds of ATPSs, such as the polymer/polymer ATPS[19,20] and polymer/salt ATPS[21], ionic liquid-based ATPS[22]


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and surfactant-based ATPS[14,23,24]. Aqueous two-phase extraction is a highly efficient and mild promising downstream separation technology, thus the investigations on the phase behaviors of ATPSs are necessary for design of extraction processes. For some ATPSs, an interesting phase inversion phenomenon was observed and investigated. It has been shown that the variations of temperature or the concentration of the constituent salt of ATPSs, and salt additives can induce phase inversion of ATPSs[4,22,25-32]. These factors can affect the concentrations or compositions of the two coexisting phases, leading to the change in the relationship between the densities of the two coexisting phases, thus resulting in phase inversion of ATPSs. Are there any other factors, such as n-alcohol additives, which can induce phase inversion of ATPSs? The short-chain n-alcohols, such as ethanol and n-propanol, are the mostly used additives in surfactant systems due to their good solubility in water and low toxicity. Investigations indicate that alcohol molecules including short-chain alcohols play two roles: cosolvent and cosurfactant effects[33]. Short-chain alcohols not only have an imperative function changing the solvent properties, such as the macroscopic interfacial tension and the dielectric constant (ε) of the solution, but also penetrating the micellar shell. Thus the addition of alcohols into aqueous mixed cationic/anionic surfactant systems affects both the electrostatic interaction between the charged headgroups and the solvophobic interaction. Based on cosolvent and cosurfactant effects, the addition of short-chain alcohols into aqueous mixed cationic/anionic surfactant systems can lead to micro-structural transformations of aggregates and changes in phase behaviors[34-37]. In addition, considering the lower density of n-alcohol than water[38-41], the surface enrichment ability[42] and self-association ability[43] of n-alcohols, n-alcohol additive may be one factor to induce the phase inversion of ATPSs. Gemini surfactants[7,44-47], which represent a new class of surfactants consisting of two amphiphilic moieties connected at the level of the headgroups or very close to the headgroups by a spacer, have attracted much attention by virtue of their appealing properties in comparison to conventional surfactants with one headgroup and one


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hydrocarbon tail chain. The investigation about surfactant mixtures containing gemini surfactant, such as oppositely charged mixtures of gemini and conventional surfactants[46], has become a hot topic due to their stronger synergism compared with the corresponding conventional surfactant mixtures. Alkanediyl-α,ω-bis(alkyldimethyl ammonium) dibromide, referred to as m-s-m, are the most widely investigated gemini surfactants[47]. Sodium dodecylsulfonate (AS) is one of the conventional anionic surfactant widely used in both technological applications and fundamental research[48]. Therefore in a series of our investigations[4,45,46], the oppositely charged surfactant mixture of 12-3-12/AS has been chosen as the studied system. Molecular dynamics (MD) simulation is a promising approach to model the self-assembly of surfactants and to provide insight into the process of micelle formation in aqueous solution[49,50]. It is an important tool that can provide the information of the intermolecular interaction energies in the simulated systems[51-53]. In addition, MD simulation is also an effective method to investigate the distribution of cosurfactant between the micelle and the aqueous surrounding[54,55]. In our previous work, salt-induced phase inversion in ATPS-C of 12-3-12/AS/H2O mixed system at 318.15 K have been investigated[4]. As part of the series of investigations on phase inversion in the ATPS-C of 12-3-12/AS/H2O mixed system at 318.15 K, in this work, the changes in phase behavior and micro-structures of 12-3-12/AS/H2O mixed system induced by ethanol, n-propanol and n-butanol were investigated. In addition, united-atom MD simulation has been used to explore the effect of ethanol or n-propanol on the phase behavior of 12-3-12/AS/H2O mixed system. The main aim of the present investigation is to explore whether the short-chain n-alcohol additive is a factor to induce phase inversion in the ATPS-C and explain the corresponding mechanism.

2. Experimental Section and Simulation Method 2.1 Materials. Sodium dodecylsulfonate (AS) (purity ≥ 97%) was recrystallized following the


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procedure reported earlier[4]. 1,3-propanediyl bis(dodecyl dimethylammonium bromide) (12-3-12) was synthesized and purified according to literature[44] as in our previous work[45,46]. Ethanol (purity ≥ 99.7%), n-propanol (purity ≥ 99.0%) and n-butanol (purity ≥ 99.5%), AR, were purchased from Sinopharm Chemical Reagent, Co. Ltd and were used as received. Ultra pure water (18 MΩ cm−1, Milli-Q, Millipore) was used throughout the experiments. 2.2 methods In this paper, for all the experimental 12-3-12/AS mixed systems, the total surfactant concentration mT is 0.10 mol kg-1. Stock solutions of 12-3-12 and AS with mT = 0.10 mol kg-1 were prepared using n-alcohol and water mixed solvents. Samples with different molar ratios of 12-3-12 to AS were prepared by mixing stock solutions of 12-3-12 and AS in test tubes for each kind of solvent and the temperature was kept at 318.15 ± 0.01 K. The observation for the phase behaviors of the samples is the same as in our previous work[4,45,56]. 2.3 Composition measurements The composition analysis methods for the top phase or the bottom phase of an ATPS-C are the same as in our previous work

[4,45,56]

. An elementary analyzer

(Elementar Analysensysteme GmbH VarioEL CHNS mode) made in Germany was used for elemental (nitrogen, sulfur, and carbon) analysis. The concentrations of the Br− ions in the dilute top phase of the second kind of ATPS-C were determined by the Mohr method described previously[56]. 2.4 Electron microscopy Cryo-TEM observation: Cryo-TEM samples were prepared, and the cryo-TEM micrographs were obtained as previously described[56]. Negative stained electron micrographs were obtained as previously described[46]. 2.5 concentration of ethanol in the separated phases of ATPS-C Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy


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(FTIR-7600, Lambda, Australia) was used to determine the concentration of ethanol in each phase. The ATR-FTIR spectra were recorded in the range of 750–4000 cm-1 with a nominal resolution of 2 cm-1 at room temperature. 32 scans were averaged in order to obtain an appropriate signal to noise ratio. The wavenumber of 1045 cm−1 was chosen for calibration due to the higher sensitivity and greater linearity. The calibration curve was built at 1045 cm−1 wavenumber using standard solutions of ethanol in ultra pure water over the mass fraction range of 0.02–0.14. There are no absorption bands for 12-3-12 and AS at the selected characteristic wavelength of ethanol. 2.6 Molecular Dynamic Simulations MD simulations were carried out using the GROMACS software package (version 5.1.2) with a time step of 2 fs. The initial coordinates of AS, 12-3-12, ethanol and n-propanol were generated using GaussView program. The GROMOS 53A6 united-atom force field was used with topology files generated by Automated Topology Builder (ATB). The electrostatic potential of 12-3-12 was calculated by the Merz-Kollman method using the Gaussian09 package at the B3LYP/6-31G* level of theory, then the symmetric groups were indentified, and the charges on individual atoms were averaged and reassigned[57]. A specific number (such as 24) of surfactant molecules and a specific number of additive (such as ethanol or n-propanol) molecules were inserted randomly into a cubic simulation box of 432 nm3 successively, and solvated by adding specific SPC/E water molecules into the box; all simulations began with a random distribution of the molecules in a cubic periodic box. To neutralize the charge of the ionic headgroup of surfactant, two bromide ions per 12-3-12 molecule and one sodium ion per AS molecule were added. A leapfrog integration algorithm was employed in the MD simulation. All MD simulations were performed at a constant temperature of 318.15 K using the V-rescale thermostat, with a coupling constant of 0.1 ps. Berendsen’s barostat was used to fix the pressure of the system at 1 bar with a coupling constant of 0.5 ps (assuming the isothermal compressibility to be 4.6×10-5 bar-1). A 1.2 nm cutoff was used for all non-bonded


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interactions (such as Coulombic interaction and Lennard-Jones interaction) and neighbor list was updated every 10 steps. The electrostatic (Coulombic) interactions were treated with the particle mesh Ewald (PME) method. The initial particle velocities were assigned according to the Maxwellian distribution at 318.15 K. All bond lengths were constrained using the LINCS algorithm. As a first step, energy minimizations were done using the steepest descent algorithm. Then MD simulations were performed in NPT ensemble for certain simulation time until the simulated aggregate fragment remained stable for about 20 ns. The molecular structures of 12-3-12, AS, ethanol and n-propanol haven been shown in Figure S1 in the supporting information. Table S1 in the supporting information summarizes all MD simulation systems of this study. The mT of simulation systems M6 and M9 is 0.15 mol·kg-1, the mT of the other simulation systems is 0.10 mol·kg-1. The force field parameters used in the simulations were listed in Table S2 in the supporting information.

3. Results and Discussion 3.1 Alcohol effect on phase behaviors of 12-3-12/AS/H2O mixed system The phase diagrams of 12-3-12/AS/ethanol/H2O and 12-3-12/AS/n-propanol/H2O mixed systems with mT = 0.10 mol·kg-1 at 318.15 K are presented in Figure 1. In which, x12-3-12 represents the mole fraction of 12-3-12 in the total surfactants, xn-alcohol represents the mole fraction of n-alcohol in the n-alcohol and water mixed solvent. Five regions can be distinguished in Figure 1. Region 1 is homogenous single-phase region where the anionic surfactant AS or the cationic surfactant 12-3-12 is in excess or xn-alcohol is higher. Region 2 is complicated heterogeneous region, usually solid suspension or floc coexists with turbid emulsion. Region 3 is liquid crystalline phase region. Region 4 represents aqueous two-phase region with cationic surfactant in excess (referred to as ATPS-C region).

Region 5 represents another aqueous

two-phase region with higher xn-alcohol and across x12-3-12 = 0.3333 (at which complete neutralization between the oppositely charged headgroups), this aqueous two-phase region is referred to as ATPS-alcohol region.


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When xn-alcohol is lower, the phase behaviors of the mixed systems are analogous to those in the absence of n-alcohol. The ethanol effect and the n-propanol effect on the phase behavior of 12-3-12/AS/H2O mixed system at the heterogeneous region 2 are similar. The addition of ethanol or n-propanol leads to the phase transition from the heterogeneous system to aqueous two-phase system (i.e. ATPS-alcohol) and finally to single-phase solution. The case is analogous to the short-chain alcohol effect on the equimolar OTAB/SDS/H2O mixed system[35]. 0.3

A

1

0.04 xethanol

4

5

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4

0.00 0.58

0.60

0.62 x12-3-12

0.64

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xethanol

0.2

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34 0.0 0.0

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0.000 0.62 0.64 0.66 0.68 0.70 0.72 x12-3-12

0.10

5

xn-propanol

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0.05

2 3 4

0.00 0.0

0.2

0.4

0.6

0.8

1.0

x12-3-12

Figure 1. Phase diagrams of 12-3-12/AS/n-alcohol/H2O mixed systems at 318.15 K A: ethanol; B: n-propanol

In order to give an insight into the short-chain n-alcohol effect, three 12-3-12/AS/n-propanol/H2O mixed systems with mT = 0.10 mol·kg-1 and x12-3-12 = 0.3333 (i.e., MR12-3-12/AS = 1:2= 8:16, 8 and 16 are the molecule numbers of 12-3-12 and AS, respectively) have been chosen for MD simulations. They are systems M1 to


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M3 with xn-propanol = 0, 0.01 and 0.20, respectively. Considering the computation time and simulation accuracy, 24 surfactant molecules were chosen to simulate the self-assembly of the aggregate fragment. To quantify the distribution of n-propanol around the surfactant molecules, the radial distribution function g(r) were computed (Figure 2). The main peak at r ≈ 0.5 nm (rmp) indicates the most probable distance between the neighboring n-propanol and surfactant molecules. At lower xn-propanol, some n-propanol molecules with r ≤ rmp are available to participate on the shell of aggregates (Figure 3), forcing surfactant molecules to pack loosely which means the increase of the distance between the headgroups and an increase in the apparent area per molecule at the interface[12]. Therefore, they are available to act as cosurfactants. When xn-propanol = 0.01, MD simulation result indicates that the n-propanol molecules with r ≤ rmp are about 40%. The aggregate fragment has a typical bilayer structure, which is in accordance with the lamellar structures for precipitates formed in aqueous mixed cationic/anionic surfactant systems[58]. At higher xn-propanol, the surfactant molecules are surrounded and dispersed by a large number of n-propanol molecules, and large aggregates no longer form, which is consistent with the phase behavior of single-phase solution observed in experiment. When xn-propanol = 0.20, the n-propanol molecules with r ≤ rmp are only about 20%, indicating that cosolvent effect rather than cosurfactant effect plays an important role. Then, how do the cosolvent effect and cosurfactant effect of n-propanol affect the interactions between surfactants, and further influence the phase behaviors of the studied systems? 7

A xn-propanol = 0.01 M2 xn-propanol = 0.20 M3

6 5

g(r)

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4 3 2 1 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

r / nm

Figure 2. The radial distribution function g(r) of n-propanol in the 12-3-12/AS/n-propanol/H2O mixed systems with mT = 0.10 mol·kg-1 and MR12-3-12/AS = 8:16 (r is the distance between the center of mass of n-propanol molecule and the surfactant molecules)


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12-3-12

xn-propanol = 0.01

AS

M2

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n-propanol

xn-propanol = 0.20

M3

Figure 3. Snapshots of aggregate fragment simulation for the 12-3-12/AS/n-propanol/H2O mixed systems with mT = 0.10 mol·kg-1 and MR12-3-12/AS = 8:16 at 40 ns of simulation time (n-propanol molecules with r ≤ rmp have been shown, the other solvent molecules have been omitted for clarity)

MD simulation results in Figure 4 indicate that the increase of xn-propanol leads to the significant increase of the solvent accessible surface area (SASA) of surfactant tail chain arising from the cosolvent effect. It means better affinity between surfactant and solvent with the rise of xn-propanol, suggesting weaker solvophobic interaction between the surfactant chains. That’s why the Lennard-Jones interaction (LJ interaction) between surfactant molecules in the aggregates becomes significantly weaker with increasing xn-propanol (Figure 5A). Meanwhile, according to Coulomb’s law, the increase of the distance between the headgroups arising from the cosurfactant effect, and the decrease of dielectric constant of the medium arising from the cosolvent effect of n-propanol lead to opposite influence on the Coulombic interaction. Therefore, the increase of xn-propanol only leads to a slight weakening of the Coulombic interaction between the surfactant molecules (Figure 5B). The total attraction (i.e. (Coulombic + LJ) interaction) between surfactants (Figure 5C) becomes significantly weaker with increasing xn-propanol.


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xn-propanol = 0

6

M1

xn-propanol = 0.01 M2

xn-propanol = 0.20 M3

SASA / nm2

5

4

3

2

0

10000

20000

30000

40000

Time / ps

Figure 4. The solvent accessible surface area (SASA) of surfactant chain (average value per surfactant molecule) for the 12-3-12/AS/n-propanol/H2O mixed system with mT = 0.10 mol·kg-1 and MR12-3-12/AS = 8:16

Literature investigations[6,15,59,60] indicate that for the oppositely charged surfactant aqueous mixed systems, weak attraction leads to the formation of stable micellar solutions, strong attraction results in the formation of the precipitate, and intermediate attraction causes the formation of ATPS. Therefore, the weakening of the total attraction between surfactants with the rise of xn-propanol leads to the above mentioned phase transition of the 12-3-12/AS/n-propanol/H2O mixed heterogeneous systems. The main driving force for the phase transition is the significant weakening of LJ interaction between surfactant molecules arising from the cosolvent effect of

A 0

.

Coulombic interaction energies / kJ mol-1

n-propanol. The ethanol effect is analogous to that of n-propanol.

LJ interaction energies / kJ mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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.

-400

xn-propanol = 0 M1 xn-propanol = 0.01 M2

-800

xn-propanol = 0.20 M3 -1200

-1600

0

10000

20000

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xn-propanol = 0 M1 xn-propanol = 0.01 M2 xn-propanol = 0.20 M3

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30000

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(Coulombic + LJ) interaction energies / kJ mol-1

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.

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C

0

xn-propanol = 0 M1 xn-propanol = 0.01 M2 xn-propanol = 0.20 M3

-400 -800 -1200 -1600 -2000 -2400 -2800 -3200 0

10000

20000

30000

40000

Time / ps

Figure 5. The interaction energies between surfactants in the 12-3-12/AS/n-propanol/H2O mixed system with mT = 0.10 mol·kg-1 and MR12-3-12/AS = 8:16

3.2 Phase inversion or phase concentration inversion in ATPS-C induced by the addition of n-alcohol When pure water or n-alcohol and water mixed solutions with low xn-alcohol were used as solvents, two kinds of aqueous two-phase behaviors were observed. The first kind of ATPS-C has a slightly bluish birefringent top phase and a transparent isotropic bottom phase, which is adjacent to lamellar crystalline phase region 3. The second kind of ATPS-C has two transparent isotropic phases, which is adjacent to isotropic single phase region 1. Composition analysis results (Table 1) and extraction experiments (Figures S2B, C, F and G in supporting information) indicate that the isotropic bottom phases in both the first kind and the second kind of ATPS-C are surfactant-rich phases, the difference in mT for the two coexisting phases in the first kind of ATPS-C is small, while that for the second kind of ATPS-C is large. Our experimental results indicate that the influence of ethanol on the ATPS-C is quite different from that of n-propanol or n-butanol. The addition of ethanol leads to one interesting phase inversion phenomenon in the first kind of ATPS-C. When xethanol is larger, the ATPS-C in phase region 4 above the red dashed line (the inset of Figure 1A) has a transparent isotropic top phase and an opalescent birefringent bottom phase. There is an inversion of the optical properties of the two coexisting phases in ATPS-C. While, experimental results indicate that, for all the ATPS-C in the presence of ethanol, the bottom phase is surfactant-rich one.


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Table 1 Phase compositions of the two coexisting phases in some ATPS-C formed by 0.10 mol·kg-1 12-3-12/AS/n-alcohol/H2O mixed systems at 318.15 K System

Ion Top phase

ATPS1 xethanol 0.0101 x12-3-12 0.630 ATPS2 xethanol 0.0153 x12-3-12 0.630 ATPS3 xethanol 0.0202 x12-3-12 0.630 ATPS4 xethanol 0.0373 x12-3-12 0.630 ATPS5 xn-alcohol 0 x12-3-12 0.6389 ATPS6 xethanol 0.0101 x12-3-12 0.6389 ATPS7 xethanol 0.0373 x12-3-12 0.6389 ATPS8 xn-alcohol 0 x12-3-12 0.6480 ATPS9 xn-propanol 0.0018 x12-3-12 0.6480 ATPS10 xn-propanol 0.0075 x12-3-12 0.6480

Optical property

12-3-122+ AS– Na+ Br–

Optical property

12-3-122+ AS– Na+ Br–

Optical property

12-3-122+ AS– Na+ Br–

Optical property

12-3-122+ AS– Na+ Br–

Optical property

12-3-122+ AS– Na+ Br–

Optical property

12-3-122+ AS– Na+ Br–

Optical property

12-3-122+ AS– Na+ Br–

Optical property

12-3-122+ AS– Na+ Br–

Optical property

12-3-122+ AS– Na+ Br–

Optical property

12-3-122+ AS– Na+ Br–

(mol⋅kg-1) Birefrigent 0.04926 0.03065 0.03090 0.09877 Birefrigent 0.05292 0.03351 0.03379 0.1061 Birefrigent 0.05423 0.03458 0.03485 0.1087 Isotropic 0.04160 0.02212 0.02185 0.08293 Birefrigent 0.04410 0.02628 0.02637 0.08829 Birefrigent 0.04650 0.02908 0.02932 0.09324 Isotropic 0.04521 0.02433 0.02408 0.09017 Isotropic 0.00011 0.000095 0.03293 0.03306 Birefrigent 0.05489 0.03322 0.03256 0.1091 Birefrigent 0.07139 0.04181 0.04043 0.1414

Bottom phase (mol⋅kg-1) Isotropic 0.07440 0.03955 0.03927 0.1485 Isotropic 0.06945 0.03733 0.03707 0.1386 Isotropic 0.06550 0.03712 0.03697 0.1309 Birefrigent 0.1043 0.07000 0.07080 0.2094 Isotropic 0.07108 0.04080 0.03757 0.1389 Isotropic 0.07018 0.03780 0.03711 0.1397 Birefrigent 0.1128 0.07478 0.07545 0.2263 Isotropic 0.07031 0.03819 0.03535 0.1378 Isotropic 0.06842 0.03542 0.03575 0.1372 Isotropic 0.05786 0.02758 0.02967 0.1178

Global concentration Determined (mol⋅kg-1)

Global concentration prepared (mol⋅kg-1)

0.06344 0.03567 0.03562 0.1268

0.06300 0.03700 0.03700 0.1260

0.70 -3.59 -3.73 0.63

0.06381 0.03603 0.03595 0.1275

0.06300 0.03700 0.03700 0.1260

1.29 -2.62 -2.84 1.19

0.06398 0.03678 0.03668 0.1279

0.06300 0.03700 0.03700 0.1260

1.55 -0.60 -0.85 1.48

0.06267 0.03821 0.03830 0.1254

0.06300 0.03700 0.03700 0.1260

-0.52 3.27 3.51 -0.48

0.06468 0.03736 0.03491 0.1269

0.06389 0.03611 0.03611 0.1278

1.24 3.46 -3.32 -0.70

0.06479 0.03582 0.03534 0.1291

0.06389 0.03611 0.03611 0.1278

1.41 -0.80 -2.13 1.02

0.06302 0.03762 0.03762 0.1260

0.06389 0.03611 0.03611 0.1278

-1.36 4.18 4.18 -1.41

0.06534 0.03549 0.03518 0.1304

0.06480 0.03520 0.03520 0.1296

0.83 0.82 -0.06 0.62

0.06610 0.03522 0.03491 0.1319

0.06480 0.03520 0.03520 0.1296

2.01 -0.45 0.01 2.13

0.06572 0.03578 0.03606 0.1317

0.06480 0.03520 0.03520 0.1296

1.42 1.84 2.05 1.48

Error (%)

However, the influence of n-propanol on ATPS-C is quite different. No phase


Langmuir

inversion has been observed, while phase concentration inversion phenomenon occurs. The ATPS-C in phase region 4 above the red dashed line (the inset of Figure 1B) has an opalescent birefringent top phase and a transparent isotropic bottom phase. Contrary to the ATPS-C with lower xn-propanol, experimental results indicate that the birefringent top phases are surfactant-rich ones and the isotropic bottom phases are surfactant-poor ones in ATPS-C with higher xn-propanol. The case for n-butanol (Figure S2I in supporting information) is analogous to that for n-propanol. The reason why ethanol and n-propanol have different influences on the ATPS-C is worth to be explored. Our previous investigation[4] indicated that the phase volume ratio of the top phase to the bottom phase VT/VB in ATPS is an important property which is closely related to the phase inversion phenomenon. The results in Figure 6A indicate that for ATPS-C with certain xethanol, before and after phase inversion, VT/VB shows a reverse trend with the increase of x12-3-12. Before phase inversion, the property of the bottom phase is analogous to that of the isotropic single phase system locating near the boundary between phase region 4 and phase region 1, the increase of x12-3-12 is beneficial for the formation of the bottom phase, suggesting the increase of VB, thus the decrease of VT/VB. After phase inversion, because of the inversion of the optical properties, the increase of x12-3-12 leads to the increase of VT, thus the increase of VT/VB. The results in Figure 6A also illustrate that for ATPS-C with certain x12-3-12 before phase inversion, VT/VB decreases with the rise of xethanol, indicating that more solvent molecules penetrate into the bottom phase of ATPS-C. A

B

10

10

1

1

xethanol 0.1

xethanol

0 0.00769 0.0153 0.0300 0.0373

0.01 0.60

0.61

xn-propanol

V T/ V B

VT/VB

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 32

0.1

0.00616 0.0101 0.0202 0.0333

0 0.00180 0.00481 0.00601 0.00686 0.00750 0.01000

0.01 0.62

0.63

0.64

0.65

0.62

x12-3-12

0.64

0.66

0.68

0.70

x12-3-12

Figure 6. Phase volume ratio of the top phase to the bottom phase VT/VB in ATPS-C formed by 12-3-12/AS/n-alcohol/H2O mixed system at 318.15 K


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Langmuir

The case is quite different when n-propanol is used. The results in Figure 6B indicate that for all of the ATPS-C with certain xn-propanol in phase region 4, VT/VB decreases monotonically with the rise of x12-3-12. Meanwhile, for ATPS-C with certain x12-3-12, VT/VB increases with the rise of xn-propanol, indicating that more solvent molecules penetrate into the top phase of ATPS-C. The different change of VT/VB induced by ethanol and n-propanol may help us to reveal the corresponding mechanism of ethanol-induced phase inversion and n-propanol-induced phase concentration inversion of ATPS-C.

3.3 Micro-structures of the two coexisting phases in ATPS-C formed by 12-3-12/AS/n-alcohol/H2O mixed system Micro-structure observations can provide important information to reveal the phase separation mechanism. Figures 7 and 8 present the TEM micrographs of some 12-3-12/AS/n-alcohol/H2O mixed systems. The TEM images reveal that for the first kind of ATPS-C with pure water solvent or n-alcohol and water mixed solvent with low xn-alcohol (ATPS5 and ATPS9 in Table 1), lamellar stacked large ellipsoidal vesicles (Figures 7A, 8D) form in the birefringent top phase, while polydispersed spherical vesicles (Figures 7B, 8E, 8F) form in the isotropic bottom phase. Whereas, for the second kind of ATPS-C (ATPS8), spherical micelles (Figure 8A) form in the isotropic top phase with very low mT, and polydispersed spherical vesicles (Figure 8B) form in the isotropic bottom phase. For the ATPS-C with x12-3-12 = 0.6389 and xethanol = 0.0373 (ATPS7), polydispersed spherical vesicles (Figure 7C) and lamellar stacked large ellipsoidal vesicles (Figure 7D) form in the isotropic top phase and the birefringent bottom phase, respectively. The micro-structures of the coexisting phases in ATPS5 and ATPS7 (Figure 7) verify the ethanol-induced phase inversion of the first kind of ATPS-C. Lamellar stacked large ellipsoidal vesicles and polydispersed spherical vesicles form in the top phase and the bottom phase of ATPS10 (Figures 8G and 8H), respectively, which are analogous to those (Figures 8D-F) of ATPS9. It is in accordance with the fact that the two ATPS-C have similar optical properties, and no phase inversion of ATPS-C has been induced by n-propanol.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A (bar: 1000 nm)Top ATPS5

C (bar: 200 nm)Top ATPS7

Page 16 of 32

B (bar: 500 nm)Bottom xethanol = 0

D (bar: 1000 nm)Bottom xethanol = 0.0373

Figure 7. Cryo-TEM micrographs of the 12-3-12/AS/ethanol/H2O mixed ATPS-C with mT = 0.10 mol·kg-1 and x12-3-12 = 0.6389 at 318.15 K

A (bar: 100 nm)Top

D (bar: 1000 nm)Top

B (bar: 500 nm)Bottom ATPS8 xn-propanol = 0

C (bar: 1000 nm)Bottom

E (bar: 500 nm)Bottom F (bar: 1000 nm)Bottom ATPS9 xn-propanol = 0.0018


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Langmuir

G (bar: 1000 nm)Top ATPS10

H (bar: 1000 nm)Bottom xn-propanol = 0.0075

Figure 8. TEM micrographs of the 12-3-12/AS/n-propanol/H2O mixed ATPS-C with mT = 0.10 mol·kg-1 and x12-3-12 = 0.648 at 318.15 K A, C, F, H: negative stained TEM; B, D, E, G: cryo-TEM

Based on the above TEM observation and composition analysis for ATPS-C, there are two different cases. (1) For the second kind of ATPS-C, spherical micelles form in the top phase with very low mT, and spherical vesicles form in the bottom phase. When two vesicles approach each other, the micelles are expelled from the gap, leading to the attractive depletion interactions between vesicles. Then a large number of vesicles in the solution leads to macroscopic phase separation[14]. (2) For the ATPS-C except the second kind one, lamellar stacked large ellipsoidal vesicles form in the birefringent phase with smaller MR12-3-12 (i.e., more weakly charged), while polydispersed spherical vesicles form in the isotropic phase with larger MR12-3-12 (i.e., more strongly charged). It is analogous to the case in literature work[61,62]. When enough weakly charged large vesicles become flocculated, they will leave the solution to form a separate phase, while the more strongly charged vesicles remain dispersed in the original solution, leading to macroscopic aqueous two-phase separation[61,62]. 3.4 Short-chain n-alcohol induced phase inversion mechanism or phase concentration inversion mechanism of ATPS-C The reason why ethanol leads to phase inversion of the first kind of ATPS-C and n-propanol leads to phase concentration inversion of ATPS-C is worth to be explored, and the corresponding mechanism needs to be interpreted. The VT/VB experimental results illustrate that the increase of xethanol leads to more solvent molecules penetrate into the bottom phase of ATPS-C before phase inversion. Then the concentrations of ACS Paragon Plus Environment

Langmuir

ethanol in the two coexisting phases of ATPS-C need to be determined. Referring to the research work of Jenkins et al.[63], attenuated total reflectance Fourier transform infrared (ATR-IR) spectroscopy was used to determine the corresponding concentrations of ethanol. Figure 9 shows the ATR-IR spectra of the ethanol and water mixed solution, the separated top phase and the separated bottom phase in ATPS-C with water as the background. Similar spectra were observed, suggesting that surfactants 12-3-12 and AS with mT ≤ 0.10 mol·kg-1 almost have no influence on the ATR-IR spectra. Ethanol has a characteristic peak at 1045 cm-1 that is used for the quantitation of ethanol in each phase. The experimental results (Figure 9B, Figure S3 in supporting information) suggest that the ethanol concentration in the top phase equals that in the bottom phase, i.e., the ethanol concentration is uniform in the whole ATPS-C. The case for n-propanol is similar, i.e., the n-propanol concentration is uniform in the whole ATPS-C, too. 0.05

A

0.08

characteristic peak

0.06 absorbance

Absorbance

0.04 0.03 0.02

0.04 0.02 0.00 0.00

0.04

0.08

0.12

0.16

wethanol

0.01 0.00 -0.01 800

1200

1600

2000

Wave numbers / cm 0.04

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

B

2400

2800

2400

2800

-1

Top phase Bottom phase

0.02 0.00 -0.02 800

1200

1600

2000 -1

Wavenumbers / cm

Figure 9. (A) ATR-IR spectrum of the ethanol and water mixed solution with the mass fraction of ethanol wethanol = 0.08. (B) ATR-IR spectra of the top phase and the bottom phase in ATPS-C formed by 0.10 mol·kg-1 12-3-12/AS/ethanol/H2O mixed system with x12-3-12 = 0.6389 and xethanol = 0.0373 (ATPS7). The calibration curve (panel A inset) was created from the spectra of ethanol and water mixed solutions for peak at 1045 cm-1.


Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Now that the ethanol concentration is uniform in the whole ATPS-C, why does the value of VT/VB decrease with the rise of xethanol before phase inversion of the first kind of ATPS-C? Both MR12-3-12/AS and mT in the top phase are smaller than those in the bottom phase (Table 1) for the first kind of ATPS-C. The values of MR12-3-12/AS are usually in the range of 1.5 to 2.0. The values of mT are usually in the range of 0.07 to 0.12 mol· kg-1. In order to explore the influences of MR12-3-12/AS and mT, and further reveal the reason for the decrease of VT/VB with the rise of xethanol, three 12-3-12/AS/ethanol/H2O mixed single-phase systems with xethanol = 0.01 have been chosen for MD simulations. They are systems M4 (MR12-3-12/AS = 16:8 = 2:1, mT = 0.10 mol· kg-1), M5 (MR12-3-12/AS = 14:10, mT = 0.10 mol· kg-1) and M6 (MR12-3-12/AS = 24:12 = 2:1, mT = 0.15 mol· kg-1). The simulated aggregate fragment has a typical bilayer structure (Figure 10), can be regarded as a part of the bilayer of vesicles, which is in accordance with the fact that vesicles form in the top and bottom phases of the first kind of ATPS-C. The MD simulation results (Figure 11) indicated that the LJ interaction, Coulombic interaction and their total interaction between surfactant and ethanol in system M4 are close to the corresponding interactions in system M5. And for both systems M4 and M5, the average number of ethanol molecules with r ≤ rmp (Nethanol) is about 20. The average number of hydrogen bonds (H-bonds) between ethanol and surfactants in systems M4 to M6 (Figure 12A) is very small (~ 1), indicating weak hydrogen bonding interactions. The decrease of MR12-3-12/AS or the increase of mT leads to slight enhancement of the hydrogen bonding interaction. Meanwhile the total interactions between surfactant and the mixed solvent in systems M4 and M5 are close to each other (Figure S4 in supporting information). These results suggest that the variation of MR12-3-12/AS in 12-3-12/AS/ethanol/H2O mixed single phase systems with the same mT and xethanol has weak influence on the interactions between surfactant and ethanol (or the mixed solvent).


Langmuir

MR12-3-12/AS = 16:8

M4

ethanol

MR12-3-12/AS = 14:10

M5

Figure 10. Snapshots of aggregate fragment simulation in ethanol and water mixed solvents at 80 ns of simulation time (12-3-12/AS/ethanol/H2O mixed systems with mT = 0.10 mol·kg-1 and xethanol = 0.01) (ethanol molecules with r ≤ rmp have been shown, the other solvent molecules have been omitted for clarity)

0

A Coulombic interaction energies / kJ mol-1

B

-200

LJ interaction energies / kJ mol-1 .

0

.

-400

ethanol: n-propanol:

-600

MR12-3-12/AS 16:8 M4 16:8 M7

MR12-3-12/AS 14:10 M5 14:10 M8

-800

-1000

-1200

0

20000

40000

60000

80000 100000 120000 140000 160000

-400

-800

-1200

MR12-3-12/AS ethanol: n-propanol:

-1600 0

20000

40000

Time / ps

.

16:8 M4 16:8 M7 60000

MR12-3-12/AS 14:10 M5 14:10 M8

80000 100000 120000 140000 160000

Time / ps

0

(Coulombic + LJ) interaction energies / kJ mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

C

-500

-1000

-1500

-2000

MR12-3-12/AS

-2500

ethanol: n-propanol:

-3000 0

20000

40000

16:8 M4 16:8 M7 60000

MR12-3-12/AS 14:10 M5 14:10 M8

80000 100000 120000 140000 160000

Time / ps

Figure 11. The interaction energies between surfactants and n-alcohol in the 12-3-12/AS/n-alcohol/H2O mixed systems with mT = 0.10 mol·kg-1 and xn-alcohol = 0.01 at 318.15 K


Page 21 of 32

10

MR12-3-12/AS

A

16:8 14:10

8

ethanol

MR12-3-12/AS n-propanol

B

25

M4 M5

16:8 14:10

M7 M8

Number of H-bonds

20

Number of H-bonds

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

6

4

2

15

10

5

0

0 0

20000

40000

60000

80000

0

20000

40000

Time / ps

60000

80000

100000 120000 140000

Time / ps

Figure 12. Number of H-bonds between n-alcohol and surfactants in 12-3-12/AS/n-alcohol/H2O mixed systems with mT = 0.10 mol·kg-1 and xn-alcohol = 0.01 at 318.15 K

MD simulation results indicate that Nethanol increases from about 20 (system M4) to about 26 (system M6) with increasing mT. Meanwhile, the increase of mT enhances Coulombic interaction, LJ interaction and their total interaction between surfactant and the mixed solvent (Figure S5 in supporting information). It means that in the first kind of ATPS-C using ethanol and water mixed solvent, the bottom phase with larger mT is capable to combine with more mixed solvent molecules, leading to larger VB and smaller VT/VB with the rise of xethanol. Therefore, the rise of xethanol leads to the increase of mT in the surfactant-poor top phase and the decrease of mT in the surfactant-rich bottom phase of ATPS-C before phase inversion. It means more sharply decrease of the density in the isotropic bottom phase with the rise of xethanol, relative to the birefringent top phase, thus leading to phase inversion of the first kind of ATPS-C. In order to compare the n-propanol effect and the ethanol effect on ATPS-C, three 12-3-12/AS/n-propanol/H2O mixed systems with xn-propanol = 0.01 corresponding to systems M4 to M6, i.e., systems M7 (MR12-3-12/AS = 16:8 = 2:1, mT = 0.10 mol·kg-1 ), M8 (MR12-3-12/AS = 14:10, mT = 0.10 mol·kg-1) and M9 (MR12-3-12/AS = 24:12 = 2:1, mT = 0.15 mol·kg-1) have been chosen for MD simulations. For systems M7 and M8, the average number of n-propanol molecules with r ≤ rmp (Nn-propanol) is about 60, which is significantly larger than Nethanol (about 20) in systems M4 and M5, indicating stronger cosurfactant effect of n-propanol than ethanol. The stronger cosurfactant effect of n-propanol originates from the significantly stronger LJ interaction, Coulombic ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

interaction and hydrogen bonding interaction between surfactant and n-propanol, relative to the corresponding interactions between surfactant and ethanol (Figure 11). Analogous to the case in ethanol and water mixed solvent, the total interactions between surfactants and the mixed solvent in systems M7 and M8 are close to each other (Figure S6 in supporting information). Meanwhile, the increase of mT also enhances LJ interaction, Coulombic interaction, and their total interaction between surfactant and n-propanol (or the mixed solvent) (Figure S7 in supporting information). It is worth noting that, different from the case in ethanol and water mixed solvent, the variation of MR12-3-12/AS in 12-3-12/AS/n-propanol/H2O mixed systems has significant influence on the interactions between surfactants and n-propanol due to the stronger cosurfactant effect of n-propanol. The decrease of MR12-3-12/AS leads to stronger attraction between surfactants and n-propanol (Figure 11). The number of H-bonds between n-propanol and surfactants is much larger than that between ethanol and surfactants (Figure 12), and it increases significantly with the decrease of MR12-3-12/AS, indicating significant enhancement of the hydrogen bonding interaction between surfactants and n-propanol. Figure 13 shows a series of snapshots illustrating the spontaneous aggregation process at various simulation times. The spontaneously forming aggregate fragment has a typical bilayer structure, which is in accordance with the fact that vesicles form in the separated phases of ATPS-C in the n-propanol and water mixed solvent. Literature investigations[5,13,64] indicate that the vesicles formed in aqueous mixed cationic/anionic surfactant systems are polydisperse, having certain size and composition distribution. Our MD simulation results suggest that the mixed vesicles with smaller MR12-3-12/AS are easier and faster to form in the n-propanol and water mixed solvent (Figure 13), due to the stronger attractions between surfactants 12-3-12 and AS (Figure S8 in supporting information), and between surfactants and n-propanol.


Page 22 of 32

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

M7: 0 ns

M8: 0 ns

30 ns

20 ns

80 ns

40 ns

100 ns

150 ns

80 ns

150 ns

Figure 13. Snapshots of aggregate fragment simulation of the 12-3-12/AS/n-propanol/H2O mixed systems with mT = 0.10 mol·kg-1 and xn-propanol = 0.01) (n-propanol molecules with r ≤ rmp have been shown, and the other solvent molecules have been omitted for clarity. The arrow shows the progression of the simulation

Based on the composition analysis and TEM observation for the first kind of ATPS-C, the vesicles with smaller MR12-3-12/AS spontaneously accumulate at the top phase, while the vesicles with larger MR12-3-12/AS spontaneously accumulate at the bottom phase. Therefore, the density contribution of vesicles with smaller MR12-3-12/AS is less than that of the vesicles with larger MR12-3-12/AS. When n-propanol and water mixed solution is used as solvent, those mixed vesicles with smaller MR12-3-12/AS will enter into the top phase of ATPS-C, meanwhile, a certain amount of mixed solvent molecules will be carried to the top phase simultaneously. However, for the vesicles in the bottom phase with higher mT, the stronger attraction between surfactants and the mixed solvent prevent the transferring of the mixed solvent from the bottom phase to the top phase. The competition for the mixed solvent correspondingly reduces the amount of the mixed solvent molecules transferred to the top phase. Thus, the combination of the two factors leads to the increase of VT (or VT/VB) and mT in the top phase of the two kinds of ATPS-C with the rise of xn-propanol. In addition, for the second kind of ATPS-C with isotropic top phase whose mT is very low, the addition of n-propanol leads to the phase transition from the isotropic top phase to birefringent top phase, i.e., phase transition from the second kind of ATPS-C to the first kind of ATPS-C. Meanwhile, we can deduce that the densities of the isotropic phases consisting of vesicles with larger MR12-3-12/AS are higher than those of the


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

corresponding birefringent phases consisting of vesicles with smaller MR12-3-12/AS, thus no phase inversion occurs. However, when xn-propanol is high enough, phase concentration inversion occurs in the two kinds of ATPS-C. The case for n-butanol is analogous to that for n-propanol. Based on the above experimental results and MD simulations, the schematic diagram of short chain n-alcohol induced phase inversion or phase concentration inversion of the ATPS-C formed by 12-3-12/AS/H2O mixed system is represented by Figure 14. For the first kind of ATPS-C, the increase of xethanol leads to the inversion of the birefringent phase consisting of lamellar stacked large ellipsoidal vesicles from the top phase to the bottom one, and the inversion of the isotropic phase consisting of spherical vesicles from the bottom phase to the top one. The main driving force for the ethanol induced phase inversion is the better combination ability of the surfactant-rich isotropic phase with the mixed solvent.

Figure 14. Schematic diagram of short chain n-alcohol induced phase inversion (blue color for arrows and words) or phase concentration inversion (red color for arrows and words) of ATPS-C formed by 12-3-12/AS/H2O mixed system

The increase of xn-propanol induces the phase transition from the second kind of


Page 24 of 32

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Langmuir

ATPS-C to the first kind. That is phase transition from the very dilute isotropic top phase consisting of spherical micelles to the birefringent top phase consisting of lamellar stacked large ellipsoidal vesicles, while the isotropic bottom phase still consists of spherical vesicles. For the 12-3-12/AS/n-propanol/H2O mixed systems, both of MR12-3-12/AS and mT are important factors influencing the interactions between surfactants and n-propanol. On one hand, the decrease of MR12-3-12/AS leads to significant enhancement of the attraction between surfactants 12-3-12 and AS, and between surfactant and n-propanol. These stronger attraction leads to the easier and faster formation of the mixed vesicles with smaller MR12-3-12/AS and smaller density contribution. These mixed vesicles spontaneously accumulate at the top phase of ATPS-C accompanied simultaneously with the mixed solvent transferred to the top phase. On the other hand, the increase of mT leads to enhancement of the attraction between surfactant and n-propanol (or the mixed solvent), which prevents the transferring of the mixed solvent from the bottom phase to the top phase. They are the main driving forces to induce the phase concentration inversion rather than the phase inversion of ATPS-C by n-propanol.

Conclusions In this paper, the short-chain n-alcohol induced changes in phase behaviors of 12-3-12/AS/H2O mixed system have been investigated. We focus on whether the short-chain n-alcohol additive can induce phase inversion in the ATPS-C. The addition of ethanol or n-propanol induces the phase transition from heterogeneous systems to ATPS-alcohol and then finally to single-phase systems arising from the weakening of the total attraction between surfactants 12-3-12 and AS due to the cosolvent effect and cosurfactant effect of n-alcohol. The MD simulation results indicate that the main driving force for the phase transition is the significant weakening of LJ interaction between surfactant molecules arising from the cosolvent effect of n-alcohol. Short-chain n-Alcohol induced changes in phase behaviors of ATPS-C are mainly


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dependent on the cosurfactant effect of n-alcohol. The addition of ethanol with weaker cosurfactant effect leads to the phase inversion of the first kind of ATPS-C, while the addition of n-propanol with stronger cosurfactant effect leads to the phase concentration inversion rather than phase inversion of ATPS-C. When ethanol is used as additive, in comparison with MR12-3-12/AS, mT is the main factor influencing the interactions between surfactant and ethanol. MD simulation results indicate that the increase of mT leads to strong attraction between surfactant and ethanol (or the mixed solvent). Thus, the bottom phase with larger mT in the first kind of ATPS-C is capable to combine with more mixed solvent molecules with the rise of xethanol. This is the driving force to induce the phase inversion of the first kind of ATPS-C by ethanol. When n-propanol is used as additive, MD simulation results indicate that both of MR12-3-12/AS and mT are the important factors influencing the interactions between surfactants and n-propanol. On one hand, the decrease of MR12-3-12/AS leads to significant enhancement of the attraction between surfactants 12-3-12 and AS, and between surfactant and n-propanol due to the stronger cosurfactant effect of n-propanol. Thus the mixed vesicles with smaller MR12-3-12/AS and smaller density contribution are easier and faster to form. And these mixed vesicles will spontaneously enter into the top phase accompanied simultaneously by certain amount of mixed solvent transferred to the top phase. On the other hand, the increase of mT leads to enhancement of the attraction between surfactant and n-propanol (or the mixed solvent). It leads to the competition for the mixed solvent due to the strong attraction between the vesicles in the surfactant-rich bottom phase and the mixed solvent, correspondingly reduces the amount of the mixed solvent molecules transferred to the top phase. The two factors lead to the increase of VT and mT of the top phase with the rise of n-propanol. They are the main driving forces to induce the phase concentration inversion rather than phase inversion of ATPS-C by n-propanol. This investigation enriches the methods to induce phase inversion or phase concentration inversion of ATPS, is helpful to understand the corresponding n-alcohol induced changes in phase behaviors of ATPS more deeply and to reveal the


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corresponding phase inversion or phase concentration inversion mechanism, and is instructive to the application of aqueous two-phase systems.

ASSOCIATED CONTENT Supporting Information The molecular structures of surfactants and n-alcohols in MD simulations (Figure S1), details of simulation systems (Table S1), the force field parameters (Table S2), the photographs between crossed polarizers and extraction photographs of some ATPS-C (Figure S2), ATR-IR spectra of the top phase and the bottom phase in ATPS-C formed by 0.10 mol·kg-1 12-3-12/AS/ethanol/H2O mixed systems (Figure S3), the interaction energies between surfactants and mixed solvent (or n-alcohol) in 0.10 mol·kg-1 or 0.15 mol·kg-1 12-3-12/AS/n-alcohol/H2O mixed systems with xn-alcohol = 0.01 at 318.15 K (Figures S4 to S7), the interaction energies between surfactants in n-alcohol and water mixed solvents (Figure S8), the MD simulation results for system M10 with 72

surfactant

molecules

and

MR12-3-12/AS

=

42:30

formed

by

0.10

mol·kg-1

12-3-12/AS/ethanol/H2O mixed systems with xethanol = 0.01 (snapshots of aggregate fragment simulation, interaction energies between surfactants and ethanol, radial distribution function of ethanol and number of H-bonds between ethanol and surfactants) (Figures S9 to S12). The Supporting Information is available free of charge

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Acknowledgments We gratefully acknowledge the National Natural Science Foundation of China (21576077) for financial support of this project. References [1] Hubbard Jr., F. P.; Abbott, N. L. Effect of Light on Self-Assembly of Aqueous Mixtures of Sodium Dodecyl Sulfate and a Cationic, Bolaform Surfactant


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