Wettability of a Polymethylmethacrylate Surface by Fluorocarbon Surfactant Solutions

Abstract

To clarify the adsorption behavior of fluorocarbon surfactants on PMMA surfaces, the contact angles of two nonionic fluorocarbon surfactants (FNS-1 and FNS-2) and an anionic fluorocarbon surfactant (FAS) on polymethylmethacrylate (PMMA) surface were determined using the sessile drop method. Moreover, the effects of molecular structures on the surface tension, adhesion tension, solid–liquid interfacial tension, and adhesion work of the three fluorocarbon surfactants were investigated. The results demonstrate that the adsorption amounts for three fluorocarbon surfactants at the air–water interface are 4~5 times higher than those at the PMMA–solution interface. The three fluorocarbon surfactants adsorb on the PMMA surface by polar groups before CMC and by hydrophobic chains after CMC. Before CMC, FNS-2 with the smallest molecular size owns the highest adsorption amount, while FAS with large-branched chains and electrostatic repulsion has the smallest adsorption amount. After CMC, the three fluorocarbon surfactants form aggregates at the PMMA-liquid interface. FAS possesses the smallest adsorption amount after CMC. Besides, FNS-1 possesses a higher adsorption amount than FNS-2 due to the longer fluorocarbon chain and the lower CMC value of FNS-1. The adsorption behaviors of nonionic and anionic fluorocarbon surfactants on the PMMA surface are different. FAS forms interfacial aggregates before CMC, which may be attributed to the electrostatic interaction between the anionic head of FAS and the PMMA surface.

1. Introduction

Wettability is one of the fundamental surface properties of solid materials. Surfactant molecules can regulate the wettability of solid surfaces [1]. Therefore, surfactants play a pivotal role in coating, printing, drug-releasing, and other fields where wettability needs to be controlled. When surfactant molecules adsorb on solid surfaces in different ways, the contact angles of solid materials vary [2]. Thus, the regulation of solid surface wettability can be realized. Studying the mechanism of surfactants in the control of solid surface wettability is of great significance for the development of novel surfactants and the surface modification of solid materials.

Polymethylmethacrylate (PMMA) is widely used in biological fields such as artificial bones and dental materials. In addition, PMMA is also widely used in optics, electronics, architecture, and automotive parts. As a weakly polar polymer, PMMA can interact with surfactant molecules by multiple approaches. Owing to the -CO, -OCH₃, and -CH₃ groups, PMMA can produce hydrogen bonding, polar interactions, and hydrophobic interactions with surfactants [3]. So far, a variety of conventional surfactants have been studied on their wettability on PMMA surfaces. Harkot et al. determined the adsorption behavior of cationic dodecylethyldimethylammonium bromide (C₁₂(EDMAB)) and benzyldimethyldodecylammonium bromide (BDDAB) surfactants on the PMMA surface, and it was found that their adsorption amounts at the water–air interface were three times higher than their adsorption amounts at the PMMA–water interface [4]. Moreover, they measured the contact angles of the anionic sodium bis(2-ethylhexyl) sulfosuccinate (AOT) solution, and they found that the concentration of the AOT solution determined its wettability on the PMMA surface [5]. Zdziennicka and Szymczyk et al. explored the adsorption behaviors of binary mixtures [6,7] and ternary mixtures [8,9] of Triton X-100 (TX-100) on PMMA surfaces. Zdziennicka noted that, when cetyltrimethylammonium bromide (CTAB), 1-hexadecylpyridinium bromide (CPyB), sodium dodecyl sulfate (SDDS), and sodium hexadecyl sulfonate (SHS) mixed with short-chain alcohols, the potential of their adhesion to work on the PMMA surface depended on the concentration of the solution [10,11]. Subsequently, Zdziennicka et al. investigated the adsorption behaviors of rhamnolipid (RL) and surfactin (SF) on the PMMA surface [12] and the effect of short-chain alcohols on the adsorption, aggregation, and wettability of various surfactants on the PMMA surface [13]. Rekiel et al. analyzed the effect of ethanol on the wetting properties of SF [14] and RL [15] on the PMMA surface. Wang et al. found that the mixture of dodecyltrimethylammonium bromide (C₁₂TAB) and SDS had a synergistic effect on the wetting of the PMMA surface [16]. Zhang et al. noticed that sodium salts of bis octadecenoyl succinate (GeminiC3, GeminiC6) showed strong hydrophilic modification abilities on the PMMA surface [17].

Fluorocarbon surfactants are special surfactants whose hydrophobic tails are partially fluorinated or completely replaced by fluorine molecules [18]. Compared with traditional hydrocarbon surfactants, fluorocarbon surfactants have lower surface tension, better interfacial properties, both hydrophobic and oleophobic properties, and excellent thermal and chemical stability [19]. Fluorocarbon surfactants are widely used in firefighting, household products, foaming, coatings, and paints. Therefore, it is significant to study the wetting behaviors of fluorocarbon surfactants. However, there are few relative studies on the adsorption behaviors of fluorocarbon surfactants on PMMA surfaces. Szymczyk’s group conducted a series of research using nonionic fluorocarbon surfactants (FSN100 and FSO100). They initially found that, once FSN100 or FSO100 was added to the binary mixture of TX100 and TX165, the adhesion tension–surface tension curve would lose its original linear relationship [20,21]. They subsequently utilized the surface tension and contact angles of FSN100 and FSO100 to obtain quite a few adsorption parameters such as Gibbs surface excess concentration, electron-acceptor, and electron-donor of FSN100 and FSO100 on the PMMA surface [22,23]. Furthermore, they compared the differences in adsorption parameters and adsorption performance of FSN100 and FSO100 with conventional hydrocarbon surfactants on the PMMA surface, and it was found that fluorocarbon surfactants could change the acid–base components and Lifshitz-van der Waals of the PMMA surface [24]. However, few studies focus on the effect of the molecular structure on the wettability of fluorocarbon surfactants on PMMA surfaces.

Previous research has demonstrated that small differences in molecular structures have a tremendous impact on the adsorption behavior of surfactant molecules on PMMA surfaces. Jiang et al. explored the effect of branched chains on the wettability of extended anionic surfactants. They noticed that the linear L-C₁₂PO₄S molecule had a stronger hydrophilic modification ability, whereas the branched G-C₁₆PO₄S molecule had a stronger hydrophobic modification ability on the PMMA surface [25]. Hu et al. investigated the adsorption properties of alkyl carboxylbetaine (18C) and alkyl sulfobetaine (18S), and it was verified that 18C had a stronger modification ability for the wettability of PMMA than 18S [26]. Zhang et al. determined the contact angles of benzyl substituted alkyl carboxylbetaine (BCB) and alkyl sulfobetaine (BSB) on the PMMA surface. They found that the hydrophilic head and phenyl group endowed BCB molecules with a stronger hydrophobic modification ability on the PMMA surface, and the larger steric hindrance lead the phenyl group of BSB to stretch towards water [27]. Du et al. reported that the number of PO units affected the wettability of extended surfactants, and dodecyl(polyoxyisopropyl)₁₃ sulfate (S-C₁₂PO₁₃S) exhibited stronger hydrophilic and hydrophobic modification abilities on the PMMA surface than dodecyl(polyoxyisopropyl)7 sulfate (S-C₁₂PO₇S) [28]. Chen et al. also found that the hydrophobic and hydrophilic modification abilities of C₁₆EO_nC molecules were enhanced with the increase in EO numbers on the PMMA surface.

Herein, the effect of molecular structure on the adsorption behaviors of fluorocarbon surfactants (FNS-1, FNS-2, and FAS) on PMMA surfaces was investigated. This is not only beneficial for further understanding of fluorocarbon surfactants’ interfacial characteristics but also vital for the development and application of fluorocarbon surfactants in coatings, lubricants, and other fields. Understanding the effect of the branched chains on the adsorption behavior can help optimize the structure and formulation of fluorocarbon surfactants to improve their adsorption properties on PMMA surfaces. On the one hand, this is beneficial for designing coatings and lubricants with better lubricity, wear resistance, and corrosion resistance. On the other hand, as PMMA is widely used in biological fields such as artificial bone and dental materials, studying the effect of fluorocarbon surfactants on the surface properties of PMMA can help to improve biocompatibility and bioactivity, which is also useful in biomedical devices.

2. Materials and Methods

2.1. Materials

The two nonionic fluorocarbon surfactants (N-(methyl) perfluorooctane Sulfonamide polyoxyethylene ether FNS-1 and perfluorohexane ethyl polyoxyethylene ether FNS-2) and anionic fluorocarbon surfactant (sodium perfluorononyloxy benzene sulfonate FAS) with purity higher than 95% were all obtained from Shanghai Futian Chemical Technology Co. Ltd. (Shanghai, China). The structures of the three fluorocarbon surfactants are shown in Scheme 1. For nonionic fluorocarbon surfactants, FNS-2 is a linear molecule, and FNS-1 has a short branched –CH₃. The anionic fluorocarbon surfactant FAS has two branched chains (–CF₃ and –CF(CF₃)₂). The PMMA plates used for the contact angle measurements were the same as the PMMA reported by Chen et al. [29] By using a polarizing microscope (Nikon, ECLIPSE E600 POL), the measured surface roughness of the PMMA plates was about 0.015 µm. Additionally, all the fluorocarbon surfactant solutions were dissolved in double-distilled water (resistivity > 18.2 MΩ.cm⁻¹) with the addition of 1% NaCl to simulate the application environment.

2.2. Surface Tension Measurements

The surface tension values of the three fluorocarbon surfactants were determined at 25°C using a DCAT21 Interfacial Tensiometer from Dataphysics Company(Filderstadt, Germany) via the Wilhelmy plate method [30]. The platinum plate was washed and then burned under an alcohol flame to completely remove any adsorbed surfactant before each measurement. Next, the platinum plate was dipped in the solution to measure the surface tension. Measurements of the surface tension of pure water at 25°C were performed to calibrate the tensiometer and to check the cleanliness of the plate and glassware. The measurements were conducted until the measured surface tension value was constant. In all cases, more than three successive measurements were performed, and the standard deviation of the measurements did not exceed 0.2 mN/m.

2.3. Contact Angle Measurements

The contact angles of the three fluorocarbon surfactants on the PMMA surface were tested at 25 °C using the LAUDA Scientific GmbH machine from SurfaceMeter Company (Lauda-Königshofen, Germany) [29]. Each polymethyl methacrylate plate was cut from large thin plates. Before the contact angle measurement, the PMMA plates were respectively cleaned with ethanol, rinsed with acetone, and soaked in the HCl solution (1:1) for 5 h. Subsequently, the above PMMA plates were cleaned ultrasonically with ultrapure water for 20 min. Finally, the polymethyl methacrylate plates were dried by heating at 378 K for 2 h. Then, fluorocarbon surfactant droplets (2 μL) were contacted with the PMMA surface using the sessile drop method. Meanwhile, the shapes of the droplets were captured and measured to obtain the contact angles. When a fluorocarbon surfactant droplet was deposited on the PMMA surface, a rapid change existed in the contact angle because of the adsorption of the surfactants at the air–liquid and solid–liquid interfaces. An equilibrium value was achieved within about 1~2 min, and the value of 120 s was taken as the experimental data. In addition, each contact angle measurement was repeated at least three times until the standard deviation of the measured contact angle values was less than 3°.

3. Results

3.1. Surface Tension of Fluorocarbon Surfactant Solutions

The surface tension values as a function of bulk concentrations for three fluorocarbon surfactants are plotted in Figure 1. As known, the turning point in the surface tension curve is the critical micelle concentration (CMC), and the CMC values of FNS-1, FNS-2, and FAS are, respectively, 2.86 × 10⁻⁵ mol$·$L⁻¹, 5.56 × 10⁻⁵ mol$·$L⁻¹, and 5.43 × 10⁻⁵ mol$·$L⁻¹. By comparison, the CMC value of FNS-1 (2.86 × 10⁻⁵ mol$·$L⁻¹) was lower than that of FNS-2 (5.56 × 10⁻⁵ mol$·$L⁻¹). Owing to the longer fluorocarbon chain of the FNS-1 molecule, FNS-1 exhibits stronger hydrophobicity than FNS-2. Thus, FNS-1 possesses a smaller CMC value than FNS-2. Compared with FNS-1, FAS also owns a higher CMC value (5.43 × 10⁻⁵ mol$·$L⁻¹). On the one hand, FAS possesses a higher branch degree than FNS-1. On the other hand, there is a phenyl group in the FAS molecule. The existence of large-branched chains and the phenyl group can result in micelles with larger steric hindrance. In addition, the large-branched chains also improve the water solubility of FAS molecules. Therefore, the CMC value of FAS is higher than that of FNS-1.

By using the Gibbs equations below (Equations (1) and (2)), the saturated adsorption area (A_min) and the saturated adsorption amount (Γ_max) of the three fluorocarbon surfactants at the air–water interface are calculated and listed in

Table 1. The CMC, γ_CMC, Γ_max, and A_min values of three fluorocarbon surfactants.

Surfactants	$\mathbf{CMC}$ (mol·L−1)	${\mathit{\gamma }}_{\mathbf{CMC}}$ (mN·Lm−1)	${10}^{10}{\mathsf{\Gamma }}_{\mathbf{max}}$ (mol·cm−2)	Amin (nm2)
FNS-1	2.86 × 10−5	20.7	2.48	0.67
FNS-2	5.56 × 10−5	22.4	3.39	0.49
FAS	5.43 × 10−5	20.9	2.64	0.63

.

$${\Gamma }_{\max }= - ({\displaystyle \frac{1}{2.303\mathit{nRT}}}\left)\right({\displaystyle \frac{\mathrm{d}\gamma }{\mathrm{d} \log C}})$$

(1)

$${\mathrm{A}}_{\min }={\displaystyle \frac{{10}^{14}}{N_{\mathrm{A}}{\mathsf{\Gamma }}_{\max }}}$$

(2)

Due to the branched structure, the Γ_max values of FNS-1 and FAS molecules are smaller than that of FNS-2. Compared with FNS-2, FAS possesses a smaller Γ_max value and a larger A_min value, because the electrostatic repulsion between the anionic fluorocarbon surfactant molecules causes the interfacial film to be loose. On the other hand, the steric hindrance of FNS-1 is larger than that of FNS-2 due to a larger EO group and the CH₃- on the N atom, which makes the A_min value of FNS-1 larger than FNS-2.

3.2. Contact Angle Values of Fluorocarbon Surfactant Solutions at the PMMA–Water Interface

The contact angles of three fluorocarbon surfactants on the PMMA surface are shown in Figure 2. The contact angle of pure water on the PMMA surface is approximately 80°. In a wide concentration range (1 × 10⁻⁸~1× 10⁻⁵ mol/L), the contact angles of the three fluorocarbon surfactants remain approximately 80° and change little with the increase in the concentration. When approaching the CMC value, the contact angles of the three fluorocarbon surfactants decrease rapidly. Unlike conventional surfactants, contact angles of three fluorocarbon surfactants change drastically after CMC, whereas contact angles of conventional surfactants generally reach plateau values and hardly change after CMC. In Figure 2, the contact angle curve of FNS-1 decreases faster than that of FNS-2 at high concentrations. On the other hand, the contact angle curve of FAS decreases faster than those of FNS-2 and FNS-1. However, the contact angle of the anionic FAS molecule is obviously larger than that of nonionic FNS-1 and FNS-2 molecules at high concentrations. Moreover, It is worth noting that the contact angle of FAS starts to decrease before CMC.

Before CMC, three fluorocarbon surfactants’ surface tension values decrease significantly, but their contact angles change little. This indicates that the adsorption of the three fluorocarbon surfactants at the PMMA–water interface counteracts the effect of surface tension on the contact angle. Therefore, the change of contact angle cannot illustrate the adsorption behavior of fluorocarbon surfactant molecules on the PMMA surface. It is necessary to analyze the adsorption parameters of the fluorocarbon surfactants at the PMMA–water interface.

3.3. Adhesion Tension of Fluorocarbon Surfactant Solutions at PMMA

The adhesion tension is the difference between the solid–air interfacial free energy (γ_SV) and the solid–liquid interfacial free energy (γ_SL). According to Young’s equation, the relationship between air–liquid interfacial free energy (γ_LV), γ_SV, and γ_SL is shown in Equation (3).

$${\gamma }_{\mathrm{SV}}-{\gamma }_{\mathrm{SL}}={\gamma }_{\mathrm{LV}} \cos \theta$$

(3)

Therefore, γ_LVcosθ is exactly the value of the adhesive tension, and the adhesion tension–surface tension curves of three fluorocarbon surfactants on the PMMA surface are shown in Figure 3.

Based on Young’s equation and Gibbs equation, the adsorption amount at the air–solid interface (Γ_SV), the adsorption amount at the solid–liquid interface (Γ_SL) and the adsorption amount at the air–liquid interface (Γ_LV) have the following relationship (Equation (4)).

$$\left|{\displaystyle \frac{d\left({\gamma }_{\mathrm{L}\mathrm{V}}\mathrm{c}\mathrm{o}\mathrm{s}\theta \right)}{d{\gamma }_{\mathrm{L}\mathrm{V}}}}\right|=-{\displaystyle \frac{{\mathsf{\Gamma }}_{\mathrm{SV}}-{\mathsf{\Gamma }}_{\mathrm{SL}}}{{\mathsf{\Gamma }}_{\mathrm{LV}}}}$$

(4)

Since the adsorption amount of the fluorocarbon surfactants at the air–solid interface (Γ_SV) is zero, the absolute value of the slope of γ_LVcosθ − γ_LV curve is the value of Γ_SL/Γ_LV.

Szymczyk et al. [20,21,24] noticed that the adhesion tension of conventional hydrocarbon surfactants (TX100 and CTAB) decreases linearly with the increase in surface tension on the PMMA surface. However, there is no linear relationship between the adhesion tension and surface tension for fluorocarbon surfactants (FSN100 and FSO100) and even their mixtures with conventional surfactants on PMMA surfaces. By comparison, the trend of adhesion tension in this work (Figure 3) is consistent with the results reported by Szymczyk et al. The adhesion tension decreases sharply at first and then increases linearly when the surface tension increases. Meanwhile, the adhesion tension decreases before CMC when the concentration of fluorocarbon surfactant solutions increases. Furthermore, the surface tension and adhesion tension display a linear relationship. When the concentration of fluorocarbon surfactants exceeds CMC, the surface tension remains constant while the adhesion tension continues to increase. Consequently, the adhesion tension curves of three fluorocarbon surfactants show a vertical upward trend.

According to the adhesion tension–surface tension curves, the slopes of FNS-1, FNS-2, and FAS are 0.23, 0.24, and 0.21, respectively. This indicates that the three fluorocarbon surfactants are likely to tile at the PMMA–liquid interface with about 1/4~1/5 of the adsorption amounts at the air–liquid interface. Therefore, the fluorocarbon surfactant molecules at the air–water interface are 4~5 times of those at the PMMA–solution interface. Szymczyk et al. [24] also found that the adsorption amount of fluorocarbon surfactants at the PMMA–solution interface is less than that at the air–water interface.

Slopes of adhesion tension curves (Γ_SL/Γ_LV) and theoretical saturated adsorption areas (A_min,theo) of three fluorocarbon surfactants and other reported surfactants in the literature are listed in

Table 2. Slopes (Γ_SL/Γ_LV) and saturated adsorption areas at PMMA calculated using Equation (4) (A_min,theo) of three fluorocarbon surfactants at PMMA and other reported surfactants before CMC.

Surfactants	ГSL/ГLV	Amin, theo (nm2/Molecule)	Surfactants	ГSL/ГLV	Amin, theo (nm2/Molecule)
FNS-1	0.23	2.91	AOT [5]	−0.1	-
FNS-2	0.24	2.04	SDS [31]	−0.17	-
FAS	0.20	3.01	CTAB [32]	−0.34	-
BDDAB [4]	−0.31	−	CPyB [32]	−0.34	-
C12(EDMAB) [4]	−0.30	−	C6 [3]	0.13	9.62
G-C16PO4S [25]	0.35	3.83	C3 [3]	0.13	8.69
G-C12PO4S [25]	0.32	4.16	L-C12PO4S [25]	0.31	3.65
18S [26]	0.28	3.01	18C [26]	0.26	2.84
BSB [27]	0.34	2.05	BCB [27]	0.30	2.00
S-C12PO13S [28]	0.32	4.63	C16E5C [29]	0.39	2.79
C16E3C [29]	0.34	2.82	C16E7C [29]	0.48	2.72

. Although the parameters in Table 2 are obtained under different experimental conditions, a qualitative comparison still can be made. Since conventional surfactants (AOT, SDS, CTAB, and C₁₂(EDMAB)) adsorb at the PMMA–solution interface via hydrophobic interactions of alkyl chains, their slopes are all negative. In contrast, the slopes of fluorocarbon surfactants before CMC are all positive, which is quite different from conventional surfactants. On the one hand, fluorocarbon surfactant molecules can rely on the polar interaction to adsorb on the PMMA surface. On the other hand, these fluorocarbon surfactant molecules can also form hydrogen bonds with functional groups of PMMA by their fluorine atoms and EO groups. Consequently, the slope values of the three fluorocarbon surfactants are positive. The more EO groups and fluorine atoms, the stronger the hydrogen bonds. In the case of little difference in the number of fluorine atoms in the three molecules, the slope of the FAS molecule is significantly lower than that of the NFS-1 and NFS-2 molecules, because the FAS molecule has no EO group. Compared to other surfactants with special structures, the three fluorocarbon surfactant molecules also have more than one hydrophilic group, and their multiple hydrophilic groups can be utilized to enhance the polar interaction with the PMMA surface. This result is in accordance with the result of previous work [25].

3.4. Interfacial Tension of Fluorocarbon Surfactant Solutions at PMMA

The effect of fluorocarbon surfactant concentration on the PMMA–water interface tension is shown in Figure 4. It is visible that the interfacial tension increases at first and then decreases as the concentration increases, and the interfacial tension curve can be divided into two stages. When the concentration is low (the first stage), the interfacial tension increases with concentration. When the concentration is in the range of 5× 10⁻⁵~1× 10⁻² (the second stage), the interfacial tension decreases with increasing concentration. What’s more, a linear relationship exists between the concentration and interfacial tension in the two stages. This trend is in agreement with the PMMA–solution interfacial tension of fluorocarbon surfactants (FSN100, FSO100) obtained by Szymczyk et al. [22] They also noted that fluorocarbon surfactants can change the Lifshitz-van der Waals and Lewis acid–base interactions of PMMA by forming a specific adsorption layer on the PMMA surface. Thus, the interfacial tension at the PMMA–solution interface is changed [23,24]. Therefore, the adsorption behaviors of fluorocarbon surfactants at these two stages are definitely different.

In the first stage, the interfacial tension increases with the increase in concentration, which indicates that fluorocarbon surfactant molecules adsorb on the PMMA surface by polar interactions. As the hydrophobic tail of the fluorocarbon surfactant orients towards water, the PMMA surface is modified to be hydrophobic at this stage. The saturated adsorption amount and adsorption area of fluorocarbon surfactants before and after CMC are calculated by Gibbs equations and shown in

Table 3. Adsorption parameters of fluorocarbon surfactants at the PMMA–solution interface.

Surfactants	1010Γbelow (mol·cm−2)	${\mathbf{A}}_{\mathbf{b}\mathbf{e}\mathbf{l}\mathbf{o}\mathbf{w}}$ (nm2)	1010Γabove (mol·cm−2)	${\mathbf{A}}_{\mathbf{a}\mathbf{b}\mathbf{o}\mathbf{v}\mathbf{e}}$ (nm2)
FNS-1	0.58	2.84	2.28	0.73
FNS-2	0.83	2.01	1.42	1.17
FAS	0.56	2.95	1.23	1.35

. The adsorption parameters before the CMC value (10¹⁰Γ_below, A_below) in Table 3 are consistent with the theoretical saturated adsorption area (A_min,theo) calculated by adhesion tension in Table 2, which indicates that the results are reliable. On account of the smallest molecular size of FNS-2, FNS-2 possesses the largest adsorption amount (8.3 × 10⁻¹¹ mol·cm⁻²) and the smallest adsorption area (2.01 nm²) among the three fluorocarbon surfactants before CMC. As for the FAS molecule, on the one hand, electrostatic repulsion exists between FAS molecules. On the other hand, the large-branched chains of FAS lead to a large steric hindrance. Accordingly, FAS has the smallest adsorption amount (0.56 × 10⁻¹¹ mol·cm⁻²) and the largest adsorption area (2.95 nm²) among the three fluorocarbon surfactants before CMC.

In the second stage, the interfacial tension decreases with the increase in concentration. At this stage, fluorocarbon surfactant molecules rely on hydrophobic tails to adsorb on the PMMA surface. Since hydrophilic polar groups of fluorocarbon surfactants orient towards water, the PMMA surface is modified to be hydrophilic. According to previous reports, the trends of surfactants’ interfacial tension with increasing concentration on the PMMA surface can be divided into three cases, and the three cases are as follows:

(1) The interfacial tension increases at first and then remains constant [27], which indicates that surfactant molecules adsorb on the PMMA surface via polar interaction only.

(2) The interfacial tension increases at first and then decreases with concentration, and the two stages possess similar slope values [3,26,30]. This implies that surfactant molecules initially adsorb on the PMMA surface through polar groups, but then surfactant molecules may form a bilayer adsorption film with their hydrophilic heads towards water.

(3) The interfacial tension increases initially and then decreases, and the first stage’s slope is nearly 2~3 times lower than the second stage [17,25,28]. This phenomenon suggests that aggregates are formed at the PMMA–liquid interface, leading to a significant increase in the adsorption amount.

Obviously, the three fluorocarbon surfactants in this work conform to the third case. At the PMMA–liquid interface, FNS-1, FNS-2, and FAS molecules tend to form aggregates, resulting in much larger slopes after CMC. At high concentrations, FAS molecules’ large-branched chains and electrostatic repulsion result in the smallest adsorption amount (1.23 × 10⁻¹⁰ mol·cm⁻²) and the largest adsorption area (1.35 nm²) among the three fluorocarbon surfactants. Compared with FNS-2, FNS-1 has a longer fluorocarbon chain and a lower CMC value. Therefore, FNS-1 molecules can form aggregates more easily at the PMMA–liquid interface, and, accordingly, the adsorption amount of FNS-1 is larger than that of FNS-2.

Additionally, there is a maximum interfacial tension for the three fluorocarbon surfactants. The maximum value of interfacial tension in Figure 4 represents the maximum hydrophobic modification ability of fluorocarbon surfactants. Meanwhile, the minimum value of interfacial tension at high concentration corresponds to the maximum hydrophilic modification ability of fluorocarbon surfactants. By comparison, the hydrophobic modification ability of FNS-1 and FNS-2 is stronger than that of FAS. This can be attributed to the large-branched chain of FAS which weakens the hydrophobicity of FAS molecules. In addition, the hydrophilic modification ability of the two nonionic fluorocarbon surfactants (FNS-1 and FNS-2) is much stronger than that of the anionic fluorocarbon surfactant (FAS). Furthermore, the two nonionic fluorocarbon surfactants (FNS-1 and FNS-2) own similar interfacial tension values (19.9 mN/m) at high concentrations. This suggests that the hydrophilic modification ability of the FNS-1 and FNS-2 molecules is similar. At high concentrations, the adsorption amount of FNS-1 (2.28 × 10⁻¹⁰ mol·cm⁻²) is significantly higher than that of FNS-2 (1.42 × 10⁻¹⁰ mol·cm⁻²). However, the existence of -CH₃ affects the hydrophilic modification ability of FNS-1, which results in little difference in the hydrophilic modification ability of the two nonionic fluorocarbon surfactants (FNS-1 and FNS-2).

3.5. Adhesion Work of Fluorocarbon Surfactant Solutions at PMMA

The adhesion work of fluorocarbon surfactants (W_A) is the work of separating a unit area of surfactant solution from the solid surface, and W_A can be calculated by Equation (5). According to Young’s equation, W_A is the sum of adhesion tension and surface tension. Therefore, W_A can also be computed using Equation (6).

$${\mathrm{W}}_{\mathrm{A}}={\gamma }_{\mathrm{S}\mathrm{V}}+{\gamma }_{\mathrm{L}\mathrm{V}}-{\gamma }_{\mathrm{S}\mathrm{L}}$$

(5)

$${\mathrm{W}}_{\mathrm{A}}={\gamma }_{\mathrm{L}\mathrm{V}}(\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\theta }+1)$$

(6)

The effect of fluorocarbon surfactant concentration on adhesion work is depicted in Figure 5. At low concentrations, the adhesion work of these fluorocarbon surfactants decreases with increasing concentration due to the decrease in surface tension. When the concentration is higher than the CMC value, the adsorption amount of fluorocarbon surfactants is saturated at the air–water interface. Accordingly, the surface tension values of fluorocarbon surfactants change little. However, these fluorocarbon surfactant molecules will continue to adsorb at the solid–liquid interface by hydrophobic interaction. As a consequence, the interfacial tension of fluorocarbon surfactant solutions decreases, resulting in the increase in adhesion work. When the adsorption amount of fluorocarbon surfactant molecules at the solid–liquid interface reaches saturation, the adhesion work reaches a plateau value. This result is in conformity with the adhesion work of fluorocarbon surfactants (FSN100, FSO100) obtained by Szymczyk et al. [20]

3.6. Adsorption Mechanism of Fluorocarbon Surfactants on PMMA Surface

Figure 6 shows the concentration dependence of adsorption parameters for fluorocarbon surfactants on the PMMA surface. Interestingly, the adsorption behaviors of nonionic and anionic fluorocarbon surfactants are obviously different. The adsorption process of nonionic fluorocarbon surfactants (FNS-1 and FNS-2) can be divided into three stages, while the adsorption process of anionic fluorocarbon surfactant (FAS) has four stages.

For the nonionic fluorocarbon surfactants, with FNS-1 as an example, its adsorption process is as follows:

(1) In the first stage (1 × 10⁻⁸~5 × 10⁻⁵ mol/L), the adsorption amount of FNS-1 molecules at the air–water interface and the PMMA–water interface increases with the increase in concentration. As a result, the surface tension decreases, and the interfacial tension increases. Because the effect of surface tension counteracts the effect of interfacial tension, the contact angle is almost constant. At this stage, FNS-1 molecules rely on their polar hydrophilic groups to adsorb at the PMMA–liquid interface. Therefore, the hydrophobic tail of FNS-1 points to water, and the PMMA surface is hydrophobically modified.

(2) In the second stage (5 × 10⁻⁵~1 × 10⁻³ mol/L), the adsorption amount of FNS-1 molecules at the air–water interface is saturated after CMC (2.86 × 10⁻⁵ mol/L). The surface tension reaches a plateau value, but the interfacial tension decreases significantly, resulting in a decreasing trend of the contact angle. At this stage, the adsorption behavior of FNS-1 changes, and FNS-1 molecules tend to adsorb on the PMMA surface via hydrophobic interactions. The adsorption amount of FNS-1 molecules increases significantly at this stage (Table 3), and this may be attributed to the aggregates that formed by FNS-1 molecules at the PMMA–liquid interface.

(3) In the third stage (1 × 10⁻³~1 × 10⁻² mol/L), the adsorption amount of FNS-1 molecules at the PMMA–water and air–water interfaces is saturated. Therefore, the contact angle keeps a stable value.

The adsorption process of the anionic FAS surfactant is as follows:

(1) In the first stage (1 × 10⁻⁸~1 × 10⁻⁵ mol/L), the adsorption amount of FAS at the air–water and PMMA–water interfaces increases as the concentration increases, which results in a decreasing surface tension and an increasing interfacial tension. As the influences of surface tension and interfacial tension cancel each other out, the contact angle changes little. FAS molecules adsorb at the PMMA–liquid interface by polar groups with the hydrophobic tail towards water. Thus, the PMMA surface is modified to be hydrophobic.

(2) In the second stage (1 × 10⁻⁵~1 × 10⁻⁴ mol/L), the adsorption amount of FAS molecules at the air–water interface is not saturated when approaching the CMC value (5.43 × 10⁻⁵ mol$·$L⁻¹). As FAS molecules continue to adsorb at the air–water interface, the surface tension decreases. However, the adsorption behavior of FAS molecules at the PMMA–liquid interface starts to transform, and the FAS molecules prefer to adsorb on the PMMA surface by hydrophobic interactions. Meanwhile, interfacial aggregates begin to form, resulting in a significant decrease in interfacial tension. Therefore, the contact angle decreases dramatically. Interestingly, FAS molecules form interfacial aggregates before CMC. This unusual phenomenon may be due to the existence of electrostatic interaction between the anionic head of FAS and the PMMA surface.

(3) In the third stage (1 × 10⁻⁴~1 × 10⁻³ mol/L), the concentration of the FAS solution is higher than the CMC value. The adsorption of FAS is saturated at the air–water interface, so the surface tension approaches the plateau value. However, the interfacial tension continues to decrease as more interfacial aggregates form. As a result, the contact angle also decreases at this stage.

(4) In the fourth stage (1 × 10⁻³~1 × 10⁻² mol/L), the adsorption of FAS molecules at the PMMA–water and air–water interfaces reaches saturation, and the contact angle is steady.

Owing to the differences in molecular structures, the adsorption characteristics of the three fluorocarbon surfactants on the PMMA surface are different. The possible adsorption mechanisms of FNS-1, FNS-2, and FAS molecules on PMMA surfaces are shown in Figure 7.

At low concentrations, these fluorocarbon surfactants adsorb on the PMMA surface by their hydrophilic heads. Since the FNS-2 molecule has the smallest molecular size, its adsorption area is the smallest, and its adsorption amount is the largest among the three fluorocarbon surfactants. In contrast, the FAS molecule with large-branched chains not only has electrostatic repulsion but also has large steric hindrance, so the FAS molecule possesses the smallest adsorption amount and the largest adsorption area. Moreover, the fluorocarbon chains of FNS-1 and FNS-2 are linear while the fluorocarbon chain of FAS is branched. The hydrophobicity of FAS is weakened due to the branched fluorocarbon chain. Therefore, FNS-1 and FNS-2 have a stronger hydrophobic modification ability than FAS.

At high concentrations, the large-branched chains and electrostatic repulsion lead to the smallest adsorption amount and the largest adsorption area of FAS molecules. FNS-1 has a longer fluorocarbon chain and a lower CMC value than FNS-2, so it is easier for FNS-1 to form aggregates at the PMMA–liquid interface. Thus, the adsorption amount of FNS-1 is higher than that of FNS-2. Additionally, the hydrophilic modification ability of nonionic fluorocarbon surfactants (FNS-1 and FNS-2) is much stronger than that of the anionic fluorocarbon surfactant (FAS). Since the -CH₃ group affects the hydrophilic modification ability of FNS-1 molecules, the difference in the hydrophilic modification ability of FNS-1 and FNS-2 is not significant.

4. Conclusions

The influence of branched chains on the wettability of nonionic fluorocarbon surfactants (FNS-1, FNS-2) and the anionic fluorocarbon surfactant (FAS) on the PMMA surface was investigated using the sessile drop method, and the following conclusions are obtained:

(1) FNS-1, with a longer fluorocarbon chain, has stronger hydrophobic interactions, resulting in a smaller CMC value than FNS-2. Furthermore, FAS with the phenyl group and large-branched chains owns a larger CMC value than FNS-1.

(2) As the surface tension increases, the adhesion tension of fluorocarbon surfactants decreases sharply at first and then increases linearly. Notably, the adsorption amount of the three fluorocarbon surfactants at the air–water interface is 4~5 times higher than the adsorption amount at the PMMA–solution interface.

(3) At the PMMA–solution interface, the interfacial tension of the three fluorocarbon surfactants increases initially and then decreases with increasing concentration. Fluorocarbon surfactants adsorb on the PMMA surface through polar interactions before CMC, whereas fluorocarbon surfactants rely on hydrophobic chains to adsorb on the PMMA surface after CMC. Before CMC, FNS-2 with the smallest molecular size has the largest adsorption amount and the smallest adsorption area, while FAS with large-branched chains and electrostatic repulsion exhibits the smallest adsorption amount and the largest adsorption area. After CMC, the three fluorocarbon surfactants form aggregates at the PMMA–liquid interface. FAS owns the smallest adsorption amount and the largest adsorption area after CMC. FNS-1 with a longer fluorocarbon chain and a lower CMC value is more prone to form interfacial aggregates, resulting in a higher adsorption amount than FNS-2.

(4) The hydrophobicity of FAS is weakened due to the large-branched chains, so the hydrophobic modification ability of linear FNS-1 and FNS-2 molecules is stronger than that of FAS. In addition, the hydrophilic modification ability of nonionic FNS-1 and FNS-2 molecules is stronger than that of anionic FAS molecule. Moreover, FNS-1 and FNS-2 molecules have similar hydrophilic modification abilities, because the -CH₃ group affects the hydrophilic modification ability of FNS-1.

(5) The adsorption processes of nonionic and anionic fluorocarbon surfactants on the PMMA surface are significantly different. The adsorption process of nonionic FNS-1 and FNS-2 molecules can be divided into three stages, while the adsorption process of the anionic FAS molecule can be divided into four stages. It is worth noting that FAS forms interfacial aggregates before CMC, and this phenomenon may be caused by the electrostatic interaction between the anionic head of FAS and the PMMA surface.