Supplementary Materials

Enabling phase transition of infused lubricant in porous

structure for exceptional oil/water separation performance

Feng Wang,a Sihai Luo,b Senbo Xiao,a Wenjing Zhang,c Yizhi Zhuo,a Jianying He,*a Zhiliang

Zhang*a

a. NTNU Nanomechanical Lab, Department of Structural Engineering, Norwegian University of

Science and Technology (NTNU), Trondheim 7491, Norway.

b. Department of Chemistry, Norwegian University of Science and Technology (NTNU),

Trondheim 7491, Norway.

c. Department of Energy and Process Engineering, Norwegian University of Science and

Technology (NTNU), Trondheim 7491, Norway.

*E-mail: jianying.he@ntnu.no, zhiliang.zhang@ntnu.no

S1. An analysis of interfacial energy in lubricant infused porous systems for liquid

separation

Supplementary Figure S1. Schematic configurations of porous structure wetting states and their total

interfacial energy E. Configuration 2 represents the complete wetting state of the porous substrates by Liquid B.

Configuration 1 and 4 represent the states where the porous substrates are completely wetted by Liquid B with

Liquid A and C floating on top of it, respectively. Configuration 3 and 5 represent the states where the porous

substrates are completely wetted by Liquid A and Liquid C, respectively, and with Liquid B floating on top of the

system. E1, E2, E3, E4 and E5 are the total interfacial energies per unit area of the five Configurations as indicated. γA,

γB and γC are the interface energy per unit area of the air-Liquid A, air-Liquid B and air-Liquid C interfaces,

respectively. γSA, γSB and γSC are the interface energy per unit area of substrate-Liquid A, substrate-Liquid B and

substrate-Liquid C interfaces, respectively. γBA and γBC are the interfacial energy per unit area of the Liquid B-Liquid

A and Liquid B-Liquid C interfaces. f is the roughness factor of the porous substrate, which is defined as the ration

between the actual and projected areas of the porous substrate.

The total interfacial energy E determines the wettability of liquids on solid surfaces. The

difference in interfacial energy, ΔE, can be used to determine the stability of different wetting

states when two or more liquids in the same system, as indicated in Figure S1. The interfacial

energy difference between different configurations in Figure S1 can be denoted as:

∆ E31= f ( γSA−γ SB ) (S-1)

∆ E32=f ( γSA−γ SB )+γ A+γ BA−γ B (S-2)

∆ E52= f ( γSC−γ SB )+γ C+γ BC−γ B (S-3)

∆ E54=f ( γ SC−γ SB ) (S-4)

The above equations can be reduced to measurable quantities by using Young’s equation [1] as

below:

∆ E31=f [ ( γS−γ A cosθ A )− (γ S−γ B cosθB ) ] (S-1-1)

∆ E32= f [ ( γS−γ A cosθ A )− (γ S−γ B cosθB ) ]+γ A+γ BA−γ B (S-2-1)

∆ E52= f [ ( γS−γC cos θC )−(γ S−γ B cosθB ) ]+γC+γ BC−γ B (S-3-1)

∆ E54= f [ ( γ S−γC cosθC )−( γ S−γB cosθB ) ] (S-4-1)

where γS is the surface energy per unit area of the solid substrate, and θA, θB and θC are the

equilibrium contact angle of Liquid A, Liquid B and Liquid C on the substrate, respectively. The

equations can thus be further reduced into compact forms:

∆ E31= f ( γB cosθB−γ A cosθA ) (S-1-2)

∆ E32=f ( γB cosθB−γ A cosθA )+γ A+γBA−γB (S-2-2)

∆ E52= f ( γB cosθB−γ C cosθC )+γ C+γ BC−γB (S-3-2)

∆ E54=f ( γ B cosθB−γ C cosθC ) (S-4-2)

One can see that all the parameters are measurable in these equations. It is thus possible to

calculate the values of ΔE31, ΔE32, ΔE52 and ΔE54 using experiment measured quantities on

particular solid and liquid materials.

In order to utilize slippery lubricant infused porous structure (SLIPS) for separating liquid C

from A, the porous substrate must be infused with lubricants (Liquid B) that are more

preferentially to wet the solid structure than Liquid A and less preferentially to wet the solid

structure than Liquid C. In such a way, the porous substrate can repel Liquid A and at the same

time allow Liquid C to be filtered through. Thus, the Configuration 3 should always stay at a

higher energy state than the Configuration 1 and 2 in Suppl. Figure1. And Configuration 5

should always stay at a lower energy state than the Configuration 2 and 4. The validation of the

comparison in energy states is based on the following basic assumptions: 1) the thickness of the

fluid layer is much less than the capillary length of the fluid; 2)surface roughness is the same for

each configuration; 3) Liquid A, Liquid B and Liquid C are chemically no-reactive with the

solid[2].

To satisfy the principles above, firstly, we have E3−E1>0 and E3−E2>0. Since we have

defined ΔE31 and ΔE32 in (S-1-2) and (S-2-2), the equations can be expressed as

∆ E31=f ( γB cosθB−γ A cosθA )>0 (S-1-3)

∆ E32= f ( γB cos θB−γ A cosθA )+γ A+γBA−γB>0 (S-2-3)

Similarly, to filter Liquid C, we have E5−E2<0 and E5−E4<0. Combining the definitions in

(S-3-2) and (S-4-2), the equations can be expressed as

∆ E52= f ( γB cosθB−γ C cosθC )+γ C+γ BC−γ B<0 (S-3-3)

∆ E54=f ( γ B cosθB−γ C cosθC )<0 (S-4-3)

Here satisfying both (S-1-3) and (S-2-3) will ensure a stable lubricating film that can repel

Liquid A. At the same time, satisfying both (S-3-3) and (S-4-3) will ensure Liquid C to displace

lubricating film and pass through the membrane. In contrast, when none of (S-1-3), (S-2-3), (S-3-

3) and (S-4-3) are satisfied, Liquid A will pass through and Liquid C will be repelled by SLIPSs.

In the case where only one of the conditions in (S-1-3) and (S-2-3) is satisfied, Liquid A may or

may not be repelled. Similarly, in the case where only one of the conditions in (S-3-3) and (S-4-

3) is satisfied, Liquid C may or may not be filtered by the SLIPS.

Following the above energy principles, we can fabricate various sets of SLIPSs for separating

two immiscible liquids, namely filter liquid C while repel liquid A at the same time. Taking

separation of oil and water for example, there is a wide possibility of SLIPS design for such

purpose. However, there are obvious limitations in using conventional SLIPS for oil/water

separation. Firstly, the oil pass through the SLIPS is also highly preferential to replace the

lubricants in the porous structure, which leads to very short lifespan (or one-time use) of the

SLIPS. Secondly, lubricants are highly unstable in intrinsically hydrophilic porous structures that

are preferentially wetted by water (supplementary Figure S4b) [2]. These issues limited the

application of SLIPS in oil-water separation.

S2. Design principles and oil/water separation mechanism of PTSLIPS

The central design principle of PTSLIPS is to maintain high stability of lubricants in the

porous structure by phase transition (from liquid to solid). The solidified lubricant in the porous

structure can thus constantly repel water, and at the same time not easily being displaced in the

absorbing-permeating process of oil. By phase transition from liquid to solid, the lubricants can

be stably cemented in hydrophilic porous structures, which is on longer depends on surface

tension. It is thus possible to enable hydrophilic porous structures for oil-water separation, and to

provide the potential of separation of more oil types (Figure S2b-c and supplementary Figure

S4c-d)[3].

The mainly compositions of vegetable oil are generally saturated fatty acid, monounsaturated

fatty acid and polyunsaturated fatty acid (Figure S2). For instance, coconut oil is a mixture of

hexanoic acid, octanoic acid, decanoic acid, lauric acid, myristic acid, palmitic acid, oleic acid

and linoleic acid (Figure S2a)[4], which has a melting point of 25℃[5]. The components and

properties of coconut oil make it a good candidate for phase-transition lubricant in PTSLIPS.

The fatty acids in coconut oil can dissolve in alkanes and naphthenes, the mainly compositions of

petroleum[6]. The melting point of coconut oil is sufficiently high. Thus, PTSLIPS using coconut

oil as lubricant should be able to separate oil from water in broad scopes at temperature lower

than 25 . ℃

Figure S2. Schematic showing the mainly compositions of coconut oil and their water repellency and oil

permeability. (a) The fatty acid components in coconut oil, namely hexanoic acid, octanoic acid, decanoic acid,

lauric acid, myristic acid, palmitic acid, oleic acid and linoleic acid. (b) The coconut oil is in solid state at room

temperature, which intrinsically repels water. (c) Liquid oil can pass through solid coconut oil in a absorbing-

permeating process.

Supplementary Figure S3. Chloroform and water on different lubricant infused surfaces of (a) Silicon oil

infused hydrophobic surface, (b) Silicon oil infused hydrophilic surface, (c) Coconut oil infused hydrophobic

surface, and (d) Coconut oil infused hydrophilic surface. The chloroform was colored with Oil red O and water was

colored with methylene blue. The experiments were carried out at 20 .℃

Supplementary Figure S4. Chloroform and water on PTSLIPS at temperature above the melting point of

lubricant. (a) Coconut oil infused hydrophobic porous structure at 50℃. (b) Coconut oil infused hydrophilic porous

structure at 50℃. The chloroform was colored with Oil red O and water was colored with methylene blue.

The SLIPS is commonly fabricated by infusing silicon oil into hydrophobic porous structure [2].

Given the hydrophobicity of the porous structure, SLIPS can only separate limited number of oil

types. In this study, we fabricated porous structure with walls coated by SiO2 nanoparticles. The

coated porous structure is intrinsically hydrophilic. The coated porous structure was further

treated with silanization, and become intrinsically hydrophobic. Thus, we obtained silicon oil

infused hydrophobic and hydrophilic porous structures surfaces.

Water and chloroform, as Liquid A and C respectively, were chosen as testing mixed liquids

for separation. Silicon oil represented liquid B. In the Figure S3a-b, the silicon oil infused

hydrophobic porous structure repelled both water and chloroform. While, the silicon oil infused

hydrophilic porous structure repelled neither water nor chloroform. Either of the two samples can

not separate the oil and water mixture. Therefore, silicon oil infused SiO2 nanoparticles coated

surfaces were not suitable for oil/water separation in this test. The results above revealed the

difficulty of using SLIPS in oil/water separating applications. For lubricant infused hydrophobic

porous structure, they were omniphobic and repelled both oil and water. For lubricant infused

hydrophilic porous structures, water can pass through the pores as with oil and could not be

repelled. The performance agreed with the results of Aizenberg et al.[2].

In contrast, oil-water separation performance of porous structure with infused solid coconut

oil, PTSLIPS, is outperforming. As shown in Figure S3c-d, both coconut oil infused hydrophobic

and hydrophilic surfaces were water repellency and permeated chloroform at the same time. On

the one hand, the hydrophobicity of coconut oil after phase transition in the PTSLIPS is always

water repellent. On the other hand, the solubility of fatty acid in the organic solvents allow the

permeating of oil through PTSLIPS. There are a variety of vegetable and animal oils that can be

used for fabricating PTSLIPS [3]. It is known that the main composition of petroleum, alkanes

and naphthenes, dissolved in fatty acids [6]. The PTSLIPS can serve as a good candidate for

oil/water separation in practice.

It should be noted that PTSLIPS should be used at temperature lower than the melting point of

the lubricant. At higher temperature, the liquid lubricant infused surfaces might or might not

repel water, as example shown in Figure S4. The coconut oil infused surfaces in Figure S3 were

placed on the heating plate with temperature of 50 . The chloroform could still permeate into℃

the pores, while the water was only repelled by hydrophobic surface. For the hydrophilic surface,

water preferred to spread and permeate into the porous structure, which was invalid for oil/water

separating.

Supplementary Figure S5. The morphology and diameter distribution of SiO2 particles synthesized in this

work. The particle size has a unimodal distribution, with a mean size of 394.9 nm.

S3. PTSLIPS fabrication and durability test

Supplementary Figure S6. FT-IR spectra verifying the main compositions of fabric and PTSLIPS used in

Figure1. (a) FT-IR spectra of fabric, which verified the fabric was made of polyethylene terephthalate with the

information from the provider[7, 8]. (b) FT-IR spectra of PTSLIP in (c), which confirmed the existence of lubricant in

the PTSLIPS.

Supplementary Figure S7. The morphology of the PTSLIPS after 5 cycling tests by SEM. The membrane

samples were taken after separating chloroform-water mixture for 5 times. The tests were performed at 20 .℃

Supplementary Figure S8. Separation efficiency of chloroform by PTSLIPS after immersed in harsh solvents.

The separation efficiencies of chloroform were 98.7 %, 94.6 % and 98.4 % for samples after immersed in sodium

chloride solution (10 wt%), hydrochloric acid solution (25 %) and ammonia solution (25 %) for 120 h, respectively.

All samples maintained high oil/water performance after treatment, showing no significant change in separation

efficiency comparing to results shown in Fig. 2 in the main text. The filtration system was shown in supplementary

Movie S1. All the experiments were performed at 20 . ℃

S4. Compositions of porous substrates used in this work

Supplementary Figure S9. FT-IR spectra verifying the main compositions of porous substrates used in

Figure4 for fabricating PTSLIPSs. (a) White wiper in Figure4i, which is made of non-woven cellulose/polyester

blend [9]. (b) Green paper in Figure4j, which is made of cellulose [10]. (c) Blue wiper in Figure4k is made of 55%

cellulose and 45% polyethylene terephthalatel[8, 10]. (d) Filter paper in Figure4l is made of cellulose fibers[9, 10].

The FT-IR spectra in Figure S9 show the mainly composition of the porous substrates. With

the combination of products information from provider, the main compositions of these materials

were confirmed. All porous substrates contain cellulose as one of the main composition in all

substrates, which resulted in the hydrophilicity of all samples.

S5. Oil/water separation ability of coconut oil infused bread sponge

Supplementary Figure S10. Wettability, porous structure, and oil/water separation ability of PTSLIPS

sponge from bread. (a) Wettability of PTSLIPS sponge from bread, this sponge shew water repellency and oil

absorbability. The colors and types of oil used were the same as these in Figure2b. (b) The water contact angle of the

PTSLIPS sponges, which indicated the hydrophilicity. (c) The porous structure of bread before infusing lubricant,

which were different from these in Figure5. The scale bar was 1mm. (d) ~ (f) The absorption system used and the

oil/water separation ability of sample. The oil was cyclohexane in these parts. The results indicated the PTSLIPS

originated from bread had ability of absorption oil from water, while the efficiency was not comparable with

samples in Figure5. Both porous structure and hydrophilicity of the substrates affected the properties of the

PTSLIPS sponges. All tests were performed at room temperature (20 ).℃

S6. Water wettability of porous substrate used in this work

Supplementary Figure S11. Wettability of water on various porous substrates used. (a) Porous fabric used in

Figure1. (b) ~ (e) Porous white wiper, green paper, blue wiper, and filter paper used in Figure3. (f) ~ (g) Porous

sponges used in Figure4. (h) Bread used in supplementary Figure S9. Water was colored with methylene blue. The

tests were performed at 20 .℃

S7. Changes of PTSLIPS in oil/water separating process

Supplementary Figure S12. The morphology of the as-received steel mesh by SEM. The scale bar is 200 µm,

and the original steel mesh had pore size of ~150 µm.

There are three stage of PTSLIPS utilized in oil/water separation before it loses its function, as

shown in Figure 6. At the first stage, namely oil absorbing-permeating, all pores were well

infused by lubricant, water was repelled and oil was filtered through absorbing-permeating

process as discussed in the main text. At the second stage, namely transition stage, the lubricant

in the pores were partially displaced by the oil in cycles of separation, which increased the

surfaces roughness and resulted in the increase of the water contact angle. In this stage, the main

mechanism of oil-water separation started with oil preferential wetting at the first contact with oil

and turned to absorbing-permeating finally. At the third stage, namely oil preferential wetting

stages, most lubricant in the pores ran out and only a thin lubricant film left on the wall of the

structure. In this stage, water was repelled by the surface roughness (Cassie-Baxter wetting

state), while oil was separated directly via Wenzel wetting in the rough membrane. Once all

lubricant ran out, the PTSLIPS loses its function.

Supplementary Movie S1. Separating chloroform from mixture through filtration. Chloroform was colored

with Oil red O. The separation was performed at 20 .℃

Supplementary Movie S2. Separating chloroform from mixture through absorption. Chloroform was colored

with Oil red O. The separation was performed at 20 .℃

Supplementary Movie S3. Water repellency of original steel mesh. The separation was performed at 20 .℃

Supplementary Movie S4. Water repellency of lubricant partly coated steel mesh. Water was colored with

methylene blue. The separation was performed at 20 .℃

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