Self-Retraction of Surfactant Droplets on a Superhydrophilic Surface

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Self-retraction of Surfactant Droplets on a Superhydrophilic Surface Tianyang Gao, Jinjin Li, Yuhong Liu, and Jianbin Luo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03130 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 22, 2018

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Self-Retraction of Surfactant Droplets on a Superhydrophilic Surface Tianyang Gao, Jinjin Li*, Yuhong Liu, Jianbin Luo* State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

Corresponding authors: *To whom all correspondence should be addressed. Jinjin Li E-mail: [email protected] Jianbin Luo E-mail: [email protected]

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Abstract: In the present work, an interesting droplet self-retraction phenomenon of C14TAB solution on the superhydrophilic mica surface was observed. The self-retraction could only occur within a concentration range from 0.01 to 16 cmc. The maximum variation of contact angle (from 24.99° to 76.85°) was observed in droplets with a concentration of 0.1 cmc. The self-retraction mechanism was studied based on high-speed photography, surface analyses, surface energy calculation, and model fitting. It was proved that there was a monolayer of C14TAB molecules adsorbed on the mica surface, which formed through the electrostatic interaction between the negative mica and positive charged headgroups. The formation of this monolayer took only a short time of 99% and sodium dodecyl sulfate (SDS) with a purity of >99% were purchased from Aladdin Industrial. The mica flakes were kindly provided by the State Key Laboratory of 4 / 29


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Tribology, Tsinghua University. The Si3N4 flakes were provided by Suzhou in-Situ Chips Technology, Ltd. The SiO2 flakes were provided by Beijing Zhongjingkeyi Technology Co., Ltd. The sapphire flakes were provided by Beijing Kepujia Co., Ltd. The pure water was derived from a nanopure water purification system with total organic carbon (TOC) monitoring. The settings for the experiments were as follows: 1 cmc exhibited similar trends for contact angles of 40°. (b) Contact angles before and after the self-retraction of C14TAB droplets. Droplets with different concentrations were put onto the mica surface. (c). C14TAB droplets with a concentration of 0.1 cmc put onto SiO2, Si3N4, and sapphire surfaces. From the spread-out moment to 20 s later, no obvious self-retraction phenomenon appeared on those three surfaces.

During the whole process of droplet self-retraction shown in Figure 2a, it can be seen that the self-retraction with different concentrations all exhibited a similar trend. First, all droplets spread out to a limited state with a small contact angle within a very short time (67 ms for droplets with a concentration of 0.1 cmc, as shown in Figure 1b), then they started to retract quickly for 10 s, and then the self-retraction turned gently 9 / 29


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to final stabilization. Although the initial contact angles of droplets with concentrations in the range from 0.01 to 16 cmc were 20°, the contact angles after self-retraction were different. They could be divided into two groups: Droplets with concentrations of 60º while those droplets with concentrations of >1 cmc could only reach final contact angles of 40º. The contact angle after selfretraction reached a maximum (76.85°) in the droplet with a concentration of 0.1 cmc. To better quantize the self-retraction phenomenon, here the concept of “retraction strength” (RS) is put forward as follows, RS =

𝜃SR ― 𝜃𝑖 𝜃𝑖

(1)

where θi and θSR are the initial contact angle and the contact angle after self-retraction, respectively. Obviously, the RS value reflects the strength of self-retraction; a higher RS value means stronger self-retraction. In our study, the solution’s RS values from 0.01 to 0.5 cmc were 2, while RS values decreased to 1 when the concentration varied from 1 to 16 cmc. The maximum θSR was 76.85° in the droplet with a concentration of 0.1 cmc, and its RS value was 2.08. From Figure 2b, it can be seen that the selfretraction phenomena was the most significant in droplets with a concentration from 0.01 to 0.5 cmc.

Because the mica surface was superhydrophilic, an aqueous solution was supposed to spread out on the surface with near-zero contact angle. However, the C14TAB solution had a much greater contact angle. This indicated that there should be molecular adsorption on the mica surface, which can change the surface energy and hydrophilism

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of the mica surface. Because the mica surface is negatively charged and the headgroup of C14TAB is positively charged, it is inferred that the C14TAB molecule may be adsorbed onto the mica surface by charge attraction.28-29 A hypothesis was proposed that C14TAB molecules are adsorbed onto the mica surface quickly and stably once the droplets touched the mica surface. After that, there was a hydrophobic molecular layer adsorbed onto the mica surface, which led to a large contact angle in the end.

To confirm our inference, after the self-retraction of the droplets of C14TAB solution with a concentration of 0.1 cmc, XPS was first chosen to analyze the elemental composition of the modified mica surface. In the self-retraction process, the area covered by the droplet became smaller and smaller, as the borderline drew back. From the position before self-retraction to the position after it, there would be a circular region called an “R-Ring” (retraction ring; Figure 3a). The chemical composition of that “RRing” area was detected by using XPS. Figure 3a is the spectrum of C 1s. There are two peaks at 284.8 and 286.2 eV fitted with two Lorentzian–Gaussian peaks. The peak at 284.8 eV can be assigned to C-C and C-H bonds of C14TAB.30 The peak at 286.2 eV can be assigned to C-N bonds of C14TAB.30 Figure 3a inset is the spectrum of N 1s. There is only one peak at 402.855 eV, which can be assigned to N-C bond.30 It is confirmed that the C14TAB molecules could be adsorbed onto the mica surface stably. We did not observe a Br elemental peak in the XPS data, implying that the C14TAB molecules lost its Br− and attached onto the negatively charged sites during the adsorption. For comparison, SDS solutions with concentrations from 0.001 to 32 cmc

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were also put onto clean mica surfaces, and no self-retraction phenomenon was observed. The SDS is an anionic surfactant and it could not bond with mica by charge attraction. When the SDS droplets touched the mica surface, they spread out quickly to a stable state. In contrast to SDS droplets, it was demonstrated that the electrical interaction between C14TAB and mica led to the self-retraction phenomenon. The mica surfaces adsorbed with droplets of different concentrations were also analyzed by XPS. The N/O ratio increased with the growth of C14TAB concentration, as shown in Figure 3b. There was a saturation of N/O ratio at 0.1 cmc, which proved the saturation of adsorption as the concentration reached 0.1 cmc.

XPS analysis of the “R-Ring” area proved that there was an adsorbed C14TAB molecular layer between the droplets and the mica surface. Further research was focused on the properties of the C14TAB molecular layer. An AFM was used to scan the topographic pattern and the height section profile of the adsorbed molecular layer (Figure 3c). It was found that the surface was as smooth as the mica surface. The surface roughness Ra was 0.106 nm, which indicates that the adsorbed C14TAB molecules are well distributed. The height section profile showed that the maximum peak valley distance Rmax was 0.3 nm. The topographic pattern showed the C14TAB molecules distributed uniformly on the mica surface.

At the same time, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to analyze the distribution and thickness of the molecular layer (Figure 3d).

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The distribution of C17H38N+ molecules was analyzed in a 300 × 300 μm area. The result in Figure 3d showed a completely uniform C17H38N+ distribution. Both the AFM topographic pattern and the TOF-SIMS distribution proved a well-distributed molecular layer. The density of different groups was also analyzed by depth, and the results showed that, at a depth of 0.4 nm, the density of C17H38N+ and CN+ were both close to zero. This indicates that the thickness of the C14TAB molecular layer was just 0.4 nm. That result matched the section graph in the AFM analysis and was much shorter than the fully extended molecular length of C14TAB (l ≈ (0.3 + 0.15 + 0.1265n, n is the number of carbon atom in the carbon chain) nm, consisting of the length of the fully extended chain, the headgroup thickness, and the radius of the terminal methyl group).31 Therefore, it can be concluded that there was a C14TAB monolayer adsorbed onto the mica surface and the C14TAB molecules were not in a fully extended vertical position. Therefore, it was inferred that the C14TAB molecules might be in a tilted state with some curls in their carbon chain.

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Figure 3. (a) A C14TAB molecular layer on the “R-Ring” area. The C14TAB molecular layer was proven by the spectral peaks of carbon and nitrogen elements in the XPS analysis. The molecular structure of the nitrogen group is also shown. (b) N/O ratio from XPS results of mica surfaces adsorbed by droplets of different concentrations. (c) AFM topographic pattern in a 100  100 nm area and section graph with a width of 60 nm. The adsorbed molecular layer was completely smooth (Ra = 0.106 nm and Rmax = 0.3 nm). (d) Data from TOF-SIMS depth and surface distribution analysis. The thickness of the C14TAB molecular layer was 0.4 nm. The C17H38N+ group adsorbed uniformly on the mica surface. (e) Surface tension data for solutions with different concentrations. C14TAB can reduce the surface tension of the aqueous solution with the increase of concentration of 1 cmc.

From the above analysis, it could be confirmed that the adsorbed C14TAB molecules formed monolayer onto the mica surface, which was well distributed with no defects in its structure. Further research was done by putting three kinds of liquid (water and two kinds of C14TAB solution with concentrations of 0.1 and 4 cmc) onto the adsorbed C14TAB monolayer. All three kinds of droplets spread out to a stable state with no self-retraction. The contact angle of water was 74.9°, proving that the C14TAB monolayer was much more hydrophobic than the mica surface. It was confirmed that the C14TAB could change the mica surface property after the adsorption. The droplet with a concentration of 0.1 cmc exhibited a smaller contact angle of 72.8° while the one of 4 cmc exhibited a contact angle of 46.2°. Those results of contact angles on the C14TAB molecular layer (θCML) are in accordance with θSR values (Table 1). It can be seen that θCML values of droplets of water and a solution with a concentration of 0.1 cmc were close to the θSR value of droplets with a concentration of 0.1 cmc. The values of θCML and θSR of droplets with a concentration of 4 cmc were close to each other because the surfactant molecules reduce the surface tension between vapor and liquid (Figure 3e). There is an obvious decrease of surface tension observed in Figure 3e when the concentration increases from 0.1 cmc to 1 cmc. This is similar to the decrease of θSR in Figure 2b, which means that the change of surface tension led to the change of θSR. This relation can be also proved by the constant values of both θSR and surface tension for concentrations > 1 cmc. The relation between surface tension and contact angle was based on Young’s equation as follows, 15 / 29


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(2)

𝛾𝑠 = 𝛾𝑠𝑙 + 𝛾𝑙𝑐𝑜𝑠 𝜃

where θ is the contact angle between the solid substrate surface and liquid droplets and γs, γsl, and γl are the solid surface free energy, solid–liquid interfacial energy, and liquid surface free energy (equal to surface tension), respectively. Because γs and γsl are stable, a decline of γl would lead to a decline in θ to satisfy the equation, which is in accordance with the trend of the θSR value in droplets with a concentration of >0.1 cmc in Figure 2b.

Table 1. Comparison of contact angles on the C14TAB molecular layer (θCML) and contact angles after self-retraction (θSR). Concentration (cmc)

θCML (°)

θSR (°)

0

74.9°

Close to 0

0.1

72.8°

76.85°

4

46.2°

42.49

We propose the following explanation for the difference in RS values for different concentration ranges. When the concentration is very low, there are only a few molecules adsorbed onto the mica surface, which can barely affect the properties of the mica surface, so no self-retraction phenomenon occurs and the contact angles are close to zero owing to the superhydrophilism of the mica surface. With the increase of the

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C14TAB solution’s concentration, more and more molecules are adsorbed onto the mica surface, completely covering the surface. Figure 2b shows that the θSR approached a saturation point at 0.1 cmc, meanwhile the XPS results of N/O ratio also showed a saturation at 0.1 cmc (Figure 3b). Thus it can be concluded that the monolayer reached a saturation of adsorption at 0.1 cmc. For the droplets with concentrations > 0.1 cmc, the monolayer was stable and the change of θSR was due to the change of the surface tension of liquid. For the droplets with concentrations of >1 cmc, the surface tension of liquid reached a minimum value and θSR would barely change.

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Figure 4. (a) Model of the self-retraction process (shown in Figure 1) of a C14TAB droplet with a concentration of 0.1 cmc. The droplet spread out quickly in 67 ms and retracted to final stabilization. The model of C14TAB hydrophilic and hydrophobic groups is also shown. (b) Force analysis at the contact line between the surface and the droplet in the self-retraction process and some geometrical parameters of the droplet. (c) Calculated data and experimental data of the base radius. The calculated radius was based on spherical cup model. (d) Calculated data and experimental data of the contact

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line receding velocity. The calculated velocity was based on energy conservation.

According to the above results, the self-retraction process was analyzed in Figure 4a. First, the C14TAB droplets with a concentration of 0.1 cmc touched the mica surface. At that time, the C14TAB molecules were in the droplet. Then the droplet spread out quickly because the mica surface was superhydrophilic and the surface needed a little time to adsorb the C14TAB molecules. After 67 ms, the C14TAB molecules were adsorbed onto the mica surface and a hydrophobic monolayer was formed. At that time, according to Young’s equation (eq 2), the stress was imbalanced at the edge of the droplet. Then the droplet started to retract. After 9.6 s, it came to a state (CA = 72°) close to final stabilization. Then the shape of the droplet slowly changed to stabilization. From this result, it is inferred that the C14TAB monolayer adsorbed stably onto the mica surface and did not detached from the mica surface with the droplet retraction. It can also be explained why the self-retraction didn’t happen or was not obvious on other surfaces. The SiO2, Si3N4 and sapphire surfaces we used were not superhydrophilic. The droplets could not spread out quickly before the molecules finished the adsorption process, which makes the self-retraction phenomenon not obvious. We also put forward a speculation that the mica surface had the strongest charge density to adsorb C14TAB molecules to form a uniform monolayer, which can make the modified surface most hydrophobic due to the maximum value in the ending contact angle.

To further confirm the above self-retraction mechanism, the dynamic process of

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retraction was simulated. Young’s equation (eq 2) can be used to analyze the stress condition at any time at the edge of the droplet. Because the solid surface was the adsorbed C14TAB molecular layer, the solid surface free energy was unknown. To calculate the free energy of the C14TAB monolayer, Neumann’s “equation of state” theory was used.32-34 In this theory, several kinds of liquid materials are used to test the contact angle and the liquid free energy. The following Neumann’s theory equation was used, cos𝜃 = ― 1 + 2

𝛾𝑠 ―𝛽(𝛾 ― 𝛾 )2 𝑠 𝑙 𝛾𝑙 𝑒

(3)

where γs and β are two unknown parameters, with γs being the solid free energy and β being a constant coefficient related to a specific solid surface, and γl and θ are two parameters that can be measured, with γl being the liquid free energy and θ being the contact angle. Eq 3 can be transformed into the following equation,35   1  cos   2  2 ln  l     2  ( s   l )  ln( s ). 2    

(4)

Five liquid materials were chosen to measure the solid–liquid interfacial free energy and the contact angle on the adsorbed C14TAB monolayer on the mica substrate under room temperature (24.5°C) conditions: water, ethanol, ethanediol, glycerol, and lactic acid. The results are listed in Table 2. Table 2. Liquid surface free energy and contact angle on the C14TAB molecular layer measured for different materials.

Material

Surface tension (mJ/m2)

Contact angle (°)

Water

72.54(0.01)

70.98

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Ethanol

22.07(0.01)

11.41

Ethanediol

41.14(0.09)

59.84

Glycerol

64.42(0.1)

67.12

Lactic acid

43.05(0.08)

43.52

To determine the parameters γs and β, a parabola curve was plotted with the left parts in eq 4 (y) against γl (x), and a second-order polynomial equation was fitted to help determine γs and β. Based on that, β was 6.7891  10-5, and the surface free energy of the adsorbed C14TAB single-molecule layer was 76.041 mJ/m2. Based on Young’s equation (eq 2), the interfacial energy between the liquid and the single-molecule layer was calculated to be 57.266 mJ/m2. The clean mica surface’s free energy had been proven to be 380 mJ/m2,36 so the C14TAB monolayer could reduce mica surface’s free energy by 80%. To analyze the self-retraction process, a model including the droplet shape, the forcebalance equation and the energy conservation was established. It was supposed that a droplet on the surface is equal to a spherical cap (parameters were shown in Figure 4b), then R(t) can be obtained in terms of the contact angle θ(t) from the following equation, 𝑅(𝑡) = 3

3𝑉𝑠𝑖𝑛3 (𝜃(𝑡)) 3𝜋(1 ― 𝑐𝑜𝑠 𝜃(𝑡))2 ― 𝜋(1 ― 𝑐𝑜𝑠 𝜃(𝑡))3

(5)

where V is the volume of the droplet (3μL). Because the contact angle at anytime in the self-retraction process could be measured, the base radius at anytime could be calculated by eq 5. As shown in Figure 4c, the calculated results are well consistent with the experimental data, which demonstrates that the droplet on the surface can be regarded as a spherical cup. Then the height of the droplet H(t) can be obtained by

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(6)

𝐻(𝑡) = 𝑎𝑅(𝑡) 1

1

where 𝑎 = (sin (𝜃(𝑡)) ― tan (𝜃(𝑡))). The surficial area S(t) can be obtained by 𝑆(𝑡) = π(𝑅(𝑡)2 + 𝐻(𝑡)2)

(7)

The height of center of gravity of the droplet can be obtained by 𝐻(𝑡)

ℎ(𝑡) = (4𝑅(𝑡)2 + 2𝐻(𝑡)2)12𝑅(𝑡)2 + 4𝐻(𝑡)2

(8)

The driving force per unit length at the contact line in the self-retraction process had been analyzed as shown in Figure 4c. The driving force can be obtained by (9)

𝐹(𝑡) = 2𝜋𝑅(𝑡)(𝐹𝑙𝑐𝑜𝑠 𝜃(𝑡) + 𝐹𝑠𝑙 ― 𝐹𝑠)

where Fl, Fsl, and Fs are the surface tensions between the droplet and air, between the droplet and the surface, and between the surface and air, respectively. Based on the spherical cup model and the force analysis at the contact line, the energy transformation was analyzed. In this self-retraction process, the surface energy of the whole system was reduced, the reduction of the surface energy E(t) could be obtained by 𝐸𝑟(𝑡) = ―𝜋((𝛾𝑠 ― 𝛾𝑠𝑙)(𝑅(0)2 ― 𝑅(𝑡)2) + 𝛾𝑙(𝑆(𝑡) ― 𝑆(0)))

(10)

where 𝛾𝑠, 𝛾𝑙 and 𝛾𝑠𝑙 are the surface energy of the solid surface, liquid surface and the surface between the solid and liquid, respectively. In the self-retraction process, the reduction of surface energy transformed into other kinds of energy including the gravitational potential energy increased and the work done by the resultant of forces at the contact line. These transformation could be obtained by the following equation, 𝑅(𝑡)

𝐸𝑖(𝑡) = 𝑚𝑔(ℎ(𝑡) ―ℎ(0)) + ∫𝑅(0)2𝜋𝑅(𝑡)𝐹(𝑡)𝑑𝑅(𝑡)

(11)

where m is the mass of the droplet (m = 3 mg), g is the gravitational acceleration (g = 9.8m/s2). Combined with eq 6, eq 8, eq 10 and eq 11, the receding velocity of the contact

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line VR(t) can be obtained according to the energy conservation during the selfretraction (𝐸𝑟(𝑡) = 𝐸𝑖(𝑡)), 𝑅(𝑡)

𝑉𝑅(𝑡) =

(6 + 2𝑎2)𝑑(𝐸𝑟(𝑡) ― ∫𝑅(0)2𝜋𝑅(𝑡)𝐹(𝑡)𝑑𝑅(𝑡) + 𝑚𝑔ℎ(0)) (2𝑎 + 𝑎3)𝑚𝑔𝑑𝑡

(12)

The calculated results are shown in Figure 4d, which show that the velocity reduced with retraction progressing. They are consistent with the experimental velocity very well, as shown in Figure 4d, which confirms the proposed self-retraction mechanism. Therefore, the self-retraction mainly originates from the fast modification of the mica surface by adsorbed C14TAB monolayer, which reduces the free energy of mica surface significantly. It also indicates that the calculated model based on the energy conservation can be well used to simulate the self-retraction of droplet.

Conclusions In summary, we presented an interesting self-retraction phenomenon occurring while a droplet of C14TAB solution with a proper concentration was put onto a mica surface. It was confirmed that a stable, well-distributed, and hydrophobic C14TAB monolayer would be adsorbed onto the mica surface within 67 ms owing to electrostatic bonding, which led to the self-retraction process. The surface free energy of the adsorbed C14TAB monolayer was proved to be 76.041 mJ/m2. We explicated a selfretraction phenomenon without impact that can provide some theoretical evidence for extensive uses of surfactants in industries such as surface modification and spray coating. Acknowledgments 23 / 29


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The work is financially supported by National Natural Science Foundation of China (51775295, 51405256, and 51527901).

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Self-Retraction of Surfactant Droplets on a Superhydrophilic Surface

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