Wettability and permeation of ethanol/water mixture on porous mesh surface

Abstract

In this paper, we fabricated a series of different chemical composition and roughness copper (Cu) meshes by using a one-step self-assembly method with different mixed thiols. The wetting behavior and the permeation behavior of ethanol/water mixed solution on different meshes were studied systematically. The manipulation of the wettability and permeation of these surfaces by controlling the concentration of the ethanol/water solution and the mole fraction of HS(CH₂)₂OH in the modifier solution can be achieved. This work can not only help us to understand wettability of a mixture liquid on a surface, but also give us inspiration to design and fabricate new types of material for mixed liquid sensors.

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Introduction

Wettability is an important characteristic of solid surfaces and has a lot of potential applications in life and industry, and has been extensively studied both in theory and experiment.^1–5 In fact, the requirements for surface wettability are diverse in practical application, sometimes, someone needs the liquid to rapidly infiltrate the surface,^6–8 whereas others demand the surface to resist liquid strongly.^9–11 Our group have previously studied and developed a lot of special wettability materials both in air^1,4,12 and underwater.^3,5,13,14 Our group and other researchers' work have proved that various factors could affect the wettability of the surface including the roughness of the surface and surface chemical composition.^15–20 Besides, the liquid properties also have an important impact on wettability.^21–24 However, most explorations of surface wettability were based on a pure liquid, such as water or some organic solvent. Limited studies had been done on the wettability of a miscible liquid mixture,^25–31 which is a very practical problem worth being deeply inquired into in the industrial fields. For example, an ethanol/water mixture plays an important role in medical, wine making and chemical industries.^32–34

Recently, scientists begin to pay attention to the wettability behavior of mixture liquid on surface. For example, Lundgren et al.²⁵ investigated an ethanol/water droplet on graphite surface by molecular simulation. They found that the ethanol molecules prefer to incline to the surface and the wettability of the mixture depends strongly on the ethanol concentration. Boreyko et al.²⁶ investigated two distinct wetting transitions on a superhydrophobic surface with hierarchical two-tier roughness by using ethanol/water mixture with continuously tunable composition. They have confirmed the changes of liquid surface tension have a great relationship with the wetting transitions. Recently, Atanu K. et al.³⁰ investigated the effect of ethanol concentration on wetting behavior of smooth and rough surface using molecular dynamic simulations, and provided numerous parameters related to the wetting transition included the line tension of the mixture, hydrogen-bond distribution and surface adhesion. Though such researches have provided some insights for ethanol mixture wettability on various surfaces, how to prepare a porous surface with special wettability used for ethanol mixture selectively filtration remains a challenge. Such wettability and permeation of ethanol/water mixture on porous mesh surface are important in the fields of medical, wine making and chemical industries.^32–34

Herein, a series of different chemical composition and roughness copper (Cu) meshes were prepared by using one-step self-assembly method with different mixed thiol. We studied the wetting behavior of different mixture droplets on these surfaces, and achieved the manipulation of the wettability of the surfaces by controlling the concentration of ethanol/water solution and the mole fraction of HS(CH₂)₂OH in the modifier solution. In addition, the permeation behavior of ethanol/water mixed solution on different meshes was studied systematically for the first time. This work can not only help us to understand a mixture liquid wettability control on surface, but also give us inspiration to design and fabricate new type of material for mixed liquid sensor.

Experimental

Modification of copper mesh

The copper mesh substrates (4 cm length and 3 cm width) were firstly flushed with water to remove the dust on the surface; these meshes were sequentially cleaned ultrasonically in acetone and ethanol for about 15 min, respectively. After that, put them into the thiols ethanol solution containing the HS(CH₂)₁₁CH₃ and HS(CH₂)₂OH for 12 h and kept the temperature at 40 °C. X_OH is the mole fraction of the HS(CH₂)₂OH in the mixed thiols. It changed from 0 to 1, and the whole concentration of the mixed thiols in ethanol is fixed 10 mM. At last, the copper meshes were taken out of the solution, washed with ethanol and vacuum drying.

Determination C_p of the as-prepared copper mesh

Firstly, a series of concentration of ethanol/water solutions (the volume fraction of ethanol is from 0 to 100%) were prepared. Then the prepared copper meshes were fixed on a glass tube. Finally, the ethanol/water mixture was slowly poured in the tube. The concentration of the mixture is C_p when it just can leak from the mesh at the corresponding X_OH.

Characterization

The morphology of the copper mesh modified with mixture thiol was observed using a scanning electron microscope (SEM, Thermo-4800 high-resolution field emission). The chemical composition of the as-prepared surface was investigated using X-ray photoelectron spectroscopy (XPS, ESCALAB250XI). The water contact angle (CA) was measured with a 5 μL droplet of distilled water at ambient temperature with a Dataphysics OCA20 contact-angle system. The average CA value was obtained by measuring the sample at three different positions.

Results and discussions

The morphology of the samples and the pure copper mesh was carefully measured. Fig. 1a showed that the original copper mesh surface is relatively smooth and has no micro/nano particles on their surface. Here the used copper (Cu) mesh is 200 meshes, in which the average diameter of the single metal wire is about 51 μm, and the average width of a square in the mesh is about 85 μm, which is about twice the wire's diameter (Fig. S1†). Fig. 1b and c are the SEM images of the mesh modified with mixed thiol with X_OH of 0, 0.3. No visible morphology changes were observed similar to that in Fig. 1a. Fig. 1d–f are the SEM images of the Cu mesh modified with mixed thiol with X_OH of 0.5, 0.7 and 1.0, respectively. These images showed micro/nano particles occurred on the Cu mesh surface after modification, the size of the particles increase with the increase of X_OH (Fig. 1d–f). At X_OH = 0.7 (Fig. 1e), the particle with the diameter of about 6 μm densely arranged on the Cu mesh surface. At X_OH = 1 (Fig. 1f), the particle density on the Cu mesh was decreased and the diameters ranged from 10 to 25 μm increased compared with that in Fig. 1e. A large number of studies show that CuO can be reduced to Cu₂O by RSH (eqn (1)),³⁵ and then the RSH further react with Cu₂O to form self-assembled monolayer which had no impact on the structure of the surface (eqn (2))³⁵.

2CuO + 2RSH → Cu₂O + RS − SR + H₂O (1)

Cu₂O + 2RSH → 2RSCu + H₂O (2)

Fig. 1 (a) SEM image of the Cu mesh, the surface is smooth; (b–f) SEM images of Cu mesh modified with mixed thiol, X_OH is 0, 0.3, 0.5, 0.7 and 1.0, respectively. From these images, we can see that the roughness of the surface increase with the increase of X_OH.

When the modifier is pure HS(CH₂)₁₁CH₃, the anchoring mechanism of HS(CH₂)₁₁CH₃ on the copper substrate can be regarded as the above reactions and the as-prepared surface was as smooth as the original substrate (Fig. 1a and b). But, the situation was changed when the HS(CH₂)₂OH was added in the modifier. Frequently, HS(CH₂)₂OH is instability and it can be oxidized easily by the oxygen in the air (eqn (3)),³⁶

4HSCH₂CH₂OH + O₂ → 4HOCH₂CH₂S − SCH₂CH₂OH + 2H₂O (3)

After the reaction, the products will eventually deposit on the surface forming special micro-nanostructure. When the content of HS(CH₂)₂OH is small (X_OH = 0–0.3), the amount of product is not enough to have a significant effect on the surface structure (Fig. 1b and c). With the increase of X_OH, there will be more copper disulfides deposited on the surface and some micro-nanostructure appeared on the surface which made the surface become rough, the surface roughness reached its maximum value when X_OH is 0.6 or 0.7 (Fig. 1e). Further increase the content of HS(CH₂)₂OH (X_OH), the disulfides particles on the surface would aggregate together form spinarak-like particles with larger size but sparse. Obviously, such changes contribute to the surface roughness decrease (Fig. 1f). As a result, it can be concluded that a controlled roughness surface can be obtained just by changing the content of HS(CH₂)₂OH in the mixed thiol.

X-ray photoelectron spectroscopy (XPS) is an effective way for analyzing the surface chemical composition. The XPS spectra of the as-prepared Cu meshes which modified by different thiol mixed solution with various X_OH were shown in Fig. 2. The Cu mesh modified with pure HS(CH₂)₁₁CH₃ was shown in Fig. 2a, the peak at 932.6 eV corresponded to the binding energy of Cu 2p_3/2, and the peak of Cu 2p_1/2 appeared at 952.2 eV. We also could find the characteristic peak of Zn 2p_3/2 and Zn 2p_1/2 appeared at 1022.2 eV and 1045.1 eV, respectively. This is because the used Cu meshes contain zinc. In addition, the characteristic peak of C, O, and S elements could be seen in the curve. The C 1s peak at 286.1 eV and S 2p peak at 162.4 eV obviously appear in the corresponding position, and the element content of C and S is about 82.5% and 4.2%, respectively. The O 1s appeared at 286.1 eV and the content is 7.1%. Fig. 2b showed the XPS of Cu mesh modified by thiol mixed solution with X_OH is 0.3, we also can find that the Cu, Zn, C, O and S elements appeared on the surface, there are no differences in the type of elements that appeared in the corresponding position, but, the element content is different. The percent content of C, O, S is 77.2%, 10.7% and 4.8%, respectively. Similarly, further increased X_OH only affect the content of surface elements (shown in Fig. 2c–e). When the X_OH is 0.5, the element percent content of C, O, S is 73.4%, 12.7% and 7.1%, respectively (Fig. 2c). The content of C, O, S is 51.4%, 27.9% and 7.8%, respectively, when X_OH is 0.7 (Fig. 2d). When the X_OH is turned to 1.0, the element percent content of C reached its minimum value (46.7%), O and S are 25.8% and 9.3%, respectively. In addition, we measured the EDX to further confirm the deposition of the thiols on the Cu surfaces (Table S1†). The C element percent content of these surfaces decreased and the percent content of O increased with an increasing of X_OH, which is consistent with that of XPS. It can observed that the C element percent content of these surfaces decreased and the percent content of O and S increased with an increasing of X_OH. This is because the micro-nanostructure deposited on the surface would hinder the reaction between HS(CH₂)₁₁CH₃ and copper, even though the sediments contain carbon element, but the length of carbon chain of HS(CH₂)₁₁CH₃ is far greater than the HS(CH₂)₂OH. In general, the element percent content of C is relatively large with a lower X_OH. Besides, more HS(CH₂)₂OH means more disulfide would deposit on the surface(eqn (3)). So there will be more O and S element loaded on the surface. When X_OH = 0.7, the surface roughness is the biggest though O content is larger than X_OH = 0, 0.3, 0.5.

Fig. 2 XPS spectra of Cu mesh modified by mixed thiol ethanol solution with X_OH = 0, 0.3, 0.5, 0.7, 1.0 respectively. (Inset image is the high-resolution XPS of S element). The appearance of the characteristic peaks of S 2p and C 1s primarily proves that the thiol molecules had been successfully assembled on the copper mesh.

The static contact angles (CAs) and dynamic CAs are used to show the wettability of the copper mesh surfaces. Obviously, the original Cu mesh has CA of about 89.5 ± 1.3° (Fig. 3a). The CA of films treated with different mixed thiol, which X_OH is 0, 0.3, 0.5, 0.7, 1.0, respectively, were 133.7 ± 0.6°, 139.0 ± 0.3°, 142.2 ± 1.2°, 152.1 ± 2.8°, 37.9 ± 0.6° (Fig. 3b–f). We have measured dynamic CAs of the samples to demonstrate the superhydrophobicity (Table S2†). The as-prepared surface modified by mixed thiol with X_OH = 0.7 has sliding angle of 6.3°, showing the superhydrophobicity. The relationship between water CA and X_OH was systematically studied (Fig. 4a). It is apparent that the CA is increasing from about 133.7° to 152.1° along with the X_OH turned from 0 to 0.7. After X_OH = 0.7, CAs began to fall and reached its minimum value (37.9°) at X_OH = 1. When X_OH is 0.6 and 0.7, the water droplet kept spherical and showed superhydrophobic. As we know, the geometrical of the surface has a great influence on the wettability of solid surface. As we discussed before, when the X_OH turned from 0 to 0.7, the surface turned to be more rough which is benefit to obtain superhydrophobic surface. So the water CAs of these surfaces increased from 133.7° to 152.1°. After X_OH is above 0.7, the CA began to diminish gradually. When the X_OH is 1.0, the size of particles formed on the surface reached the maximum value and no hydrophobic group grafted on the surface. So the prepared mesh was hydrophilic.

Fig. 3 Photograph of a water droplet on (a) the original Cu mesh with a CA of 89.5 ± 1.3°, (b) the as-prepared surface modified by mixed thiol with X_OH = 0 and the CA is 133.7 ± 0.6°, (c) X_OH = 0.3 and the CA is 139.0 ± 0.3°, (d) X_OH = 0.5 and the CA is 142.2 ± 1.2°, (e) X_OH = 0.7 and the CA is 152.1 ± 2.8°, showing the superhydrophobicity, (f) X_OH = 1.0 and the CA is 37.9 ± 0.6°.

Fig. 4 (a) Relationship between water CAs and X_OH of the as-prepared mesh, the water CA is increasing from about 133.7° to 152.1° along with the X_OH increase from 0 to 0.7, after that, water CA begins to fall with the increase of the X_OH. Then the surface turns to hydrophilic while the X_OH reached 1.0. (b) Dependence of droplet CA on the ethanol volume fraction in mixture on the as-prepared surface(X_OH = 0.7), the droplet CA decreases from 152.1° to 0° with the increase of ethanol volumes in mixture, the surface become lyophilic when the volume fraction of ethanol is more than 50%. (c) Relationship between C_p (the critical volume fraction of ethanol in mixture when the solution pass through copper mesh) and X_OH, C_p tend to increase when X_OH range from 0 to 0.7 and then begin to fall. (d) Dependence of C_p on the whole concentration of thiol of the as-prepared surface modified by thiol with X_OH = 0.7, the whole concentration of 5–10 mM can maintain a larger C_p. (e) Relationship between C_p and reaction time of the as-prepared surface with X_OH = 0.7, after 12 h, the extension of the time had little effect on the C_p. (f) Recycle experiment for C_p of the as-prepared copper mesh (black line: copper substrate modified with thiol of X_OH = 0; blue line: copper substrate modified with thiol of X_OH = 0.5; red line: copper substrate modified with thiol of X_OH = 0.7).

In addition to the roughness and the chemical composition of surfaces,^19,20 the liquid properties also have an important impact on wettability. Ethanol, a liquid with low surface tension, easily spreads on these mixed thiol-modified Cu meshes. To investigate the relationship of the wettability and the liquid properties, the sample (X_OH = 0.7) was chosen as a model to measure CA of different ethanol/water mixed solution (Fig. 4b). The CAs decrease from 152.1° to 0° with the increase of the ethanol concentration in the mixture. At the beginning, CA linearly decreases, and it drops suddenly when the ethanol concentration is over 50%, then CA recover linear decrease again when the ethanol concentration is over 60%. Finally, the surface becomes lyophilic. We measured the surface tension values of different concentration ethanol/water mixed solution (Fig. S2†). The surface tension values of different concentration ethanol/water mixed solution decreased from 72.8 mN m⁻¹ to 23.1 mN m⁻¹ with the increase of the ethanol concentration, which is consistent with that reported in ref. 26. So the decrease of the mixture liquid surface tension cause the CA decrease.

We also studied the different ethanol/water mixture penetration on these as-prepared Cu meshes. Herein, we defined the critical concentration of ethanol in the mixture as C_p when this solution can pass through the Cu mesh. The change of C_p with X_OH is shown in Fig. 4c. For the surface prepared with X_OH = 0 modifier, the C_p is about 31%. When the X_OH is 0.5 and 0.7, the C_p increased to about 42% and 48%, respectively. With the further increase of X_OH, the C_p began to show a downward trend, and when the X_OH reached 1.0, ethanol solution with any concentration could permeate the Cu mesh easily. From the above result, it can be conclude that by simply changing the X_OH of the modifier solution, the C_p of the surfaces can be controlled.

To understand the ethanol/water mixture permeability behavior on the as-prepared surface and obtain surface with a higher C_p, we explore the reaction conditions influence on C_p of surface with X_OH = 0.7, such as the total concentration of mixed thiol and the reaction time. Fig. 4d shows the relationship of the whole concentration of mixed thiol and C_p, it is obviously that the C_p is relatively small when the concentration is under 5 mM. When the whole concentration of thiol is at 5–10 mM, the C_p reached it peak value of about 48%. With further increase of the concentration of thiol, the C_p began to fall. The reason is that the different number of thiol molecular grafted on Cu substrate. When the concentration of thiol is very small, the total reaction of thiol molecules with Cu is very limited, so the hydrophobic is not enough strong. When the whole concentration becomes very large, which means the content of HS(CH₂)₂OH is also become larger comparatively. This may lead to more hydroxyl groups grafted onto the surface, which can weaken the surface hydrophobicity.

Analogously, the reaction time is another factor that has an important influence on the surface wettability. Fig. 4e shows the dynamic C_p of the as-prepared Cu mesh modified with mixed thiol under different deposition time (from 2 h to 20 h). It can be seen that the C_p rise from 39% to 48% when the reaction time extend from 2 h to 12 h. After that, further extending the deposition time from 12 h to 20 h leads to few changes in C_p, and it remain about 48%. In a short period of time, the reaction between thiol molecules and Cu is not complete, the hydrophobic groups grafted onto the surface are very limited. When the reaction time reached 12 h, the coordination between thiol molecules and Cu is saturated and the C_p reaches its maximum value (48%). While the reaction time is more than 12 h, there will have no changes occurred on the surface. So the optimal reaction time is about 12 h to get a Cu mesh with a high C_p when X_OH is 0.7. In terms of the better application of these as-prepared copper surfaces, it is imperative to investigate its stability and recycle usability. Experiments show that the surface can retain such property even after one month exposure in the air without any special protection, which indicates that the meshes possess good durability (Fig. 4f). So these copper meshes exhibit good recycling performance for ethanol/water mixture permeating.

Generally speaking, any liquids on the porous mesh membrane will exit a problem of penetration, but few researches are focus on the miscible liquid mixture like ethanol/water solution. Considering the different permeability of the ethanol/water mixture solution, we have the different meshes fixed in the glass tube and then pour different concentration of ethanol/water mixture into it to conduct preliminary penetration experiments. Fig. 5 shows the simple instrument fabricated by ourselves, three kinds of different copper meshes were embedded in the ①, ②, ③ position (the experimental condition of these meshes are X_OH = 0.7, 0.5, 0, respectively). The different concentrations of ethanol/water mixture were dyed in different colors and the volume is 5 mL. In Fig. 5a, the mixture solution can go through only the mesh of number ③ but not for ① and ②. In Fig. 5b, the red solution penetrated the number ② and ③ meshes. In Fig. 5c, the mixture solution went through all these meshes. Obviously, the permeability of mixture with various concentrations is different on these as-prepared Cu meshes. It is because the low surface tension of mixture and hydrophilic surface are benefit for the spread and penetration of the liquid. In addition, the hydraulic pressure is another important factor that can influence the penetration of solution on the Cu surface. For example, as shown in Fig. 5a ①, we believe that the solution will permeate the mesh with further increase of solution volume. The pressure of liquid on surface is determined by the liquid column height, and the solution will permeate from the Cu mesh when the pressure is above a value that the surface can support. Table 1 shows liquid penetration threshold height of different concentration of ethanol/water solution for these three kinds of surfaces. From the table, we can see that all the meshes can bear a high hydraulic pressure (about 20 cm) when using pure water for the experiment. But the threshold liquid height decreased with the increase of the ethanol concentration because of the liquid surface tension decrease. When the concentration of ethanol/water solution reached 48%, the liquid will pass through all the meshes freely with the height is 0 cm. In summary, we can control the permeation behavior of ethanol solution on the surface by changing the concentration of the liquid. The critical liquid column height (pressure) that the surface can support decrease with the decrease of the surface tension values of mixed solution. These special features may have great reference value in permeable membrane fabricating, fluid monitoring and fluid diversion.

Fig. 5 Simple instrument fabricated by ourselves was used to test the permeability of the as-prepared Cu mesh for different concentrations of ethanol/water solution, three kinds of different Cu meshes were embedded in ①, ②, ③ position (the copper meshes are prepared at X_OH = 0.7, 0.5, 0, respectively). Yellow solution is 31% volume fraction ethanol/water solution. Red solution is 42% volume fraction ethanol/water solution. Blue solution is 48% volume fraction ethanol/water solution. (a) The 31% concentration solution only can go through the ③ mesh. (b) The 42% concentration solution can penetrate the number ② and ③ meshes. (c) The 48% concentration solution can penetrate these three kinds of meshes. The volume of solution is 5 mL.

Table 1 The liquid penetration threshold height of different concentrations of ethanol/water solution for the surfaces modified with mixture thiol solution (X_OH = 0, 0.5, 0.7). The inner diameter of the tube is 1.0 cm

V/%	0	31%	42%	48%
Height/cm (XOH = 0)	16	0	0	0
Height/cm (XOH = 0.5)	21	4	0	0
Height/cm (XOH = 0.7)	23	10	3	0

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Conclusions

In conclusion, a series of hydrophobic Cu meshes were fabricated by using one step self-assembly method without any other special treatment, the roughness and surface tension of the samples can be controlled by changing the proportion of mixed modifier. The surface tension values of different concentration ethanol/water mixed solution decreased with the increase of the ethanol concentration. So the droplet CA decreases from 152.1° to 0° with the increase of ethanol volumes in mixture on the as-prepared surface(X_OH = 0.7), the surface become lyophilic when the volume fraction of ethanol is more than 50%. While the critical liquid column height (pressure) that the surface can support decrease with the decrease of the surface tension values of mixed solution. Noticeably, the obtained surfaces exhibited preferable quality of reusability, which can be used more than 30 times. Based on these characteristics, the obtained surfaces are potentially useful in many applications, such as permeable membrane fabricating, fluid monitoring and fluid diversion.

Acknowledgements

This work was supported by the National Research Fund for Fundamental Key Projects (2013CB834705, 2014CB931802), the National Natural Science Foundation of China (Grant No. 51541301).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19737j

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