J. Cent. South Univ. (2016) 23: 2182-2190

DOI: 10.1007/s11771-016-3275-x

Direct AFM measurements of morphology and interaction force at solid-liquid interfaces between DTAC/CTAC and mica

XIE Zhen(谢珍), JIANG Hao(蒋昊), SUN Zhong-cheng(孙忠诚), YANG Qin-hong(杨沁红)

School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: The adsorption of dedecyltrimethylammoium chloride (DTAC) and hexadecyltrimethylammoium chloride (CTAC) on muscovite mica substrates was examined using atomic force microscopy (AFM). Adsorption morphology images and interaction forces of cationic surfactants at solid-solution interfaces were measured in tapping mode and PicoForce mode, respectively. The images demonstrated that the adsorbed structure was varied by a variety of surfactant concentrations. The adsorbed layer on mica was monolayer at first, and then became bilayer. A striped adsorbed structure was observed in a higher concentration of CTAC, which could not be found in any other concentrations of DTAC. For force measurements, the repulsive force was exponentially decreasing with the concentration increasing till a net attractive force appeared. A largest attractive force could be observed at a certain concentration, which was close to the point of charge neutralization. The results also showed a significant impact of hydrocarbon chain length on adsorption. An adsorption simulation was established to give a clear understanding of the interaction between cationic surfactants and mica.

Key words: adsorption; cationic surfactants; interaction force; atomic force microscopy

1 Introduction

To understand the properties of adsorbed surfactant films on solid surfaces, many research works have been carried out over the last few decades. As early as 1955, in order to explain the shapes of the adsorption isotherms, surface aggregates or hemi-micelles had been proposed as partial surface coverage [1]. Direct evidence of surface micelles was gleaned principally from fluorescence probe studies [2-3] although other techniques also gave similar indications.

The atomic force microscope (AFM), invented by BINNIG et al [4], has become an important technique for imaging the topographies of surfaces and measuring interaction forces between particles in a wide range of research fields including chemistry, biochemistry and engineering [5-9]. The advantage of AFM measurements for surface properties is that AFM measurements are able to acquire information on a particular surface, even a particular point, unlike other techniques, which can only measure the integral properties of particle surfaces. The “soft-contact mode” of AFM has been employed to demonstrate a strong correlation between the bulks self-assembly structure and the morphologies of surfactant adsorbed layers [10-16]. Images of films have been interpreted as showing the adsorbed layer to consist of full spheres, flexible cylinders, and bilayers on hydrophilic substrates and rigid hemi-cylinders on graphite [13-14], and the mesh structure might be more commonly observed at the solid-solution interfaces [16].

Previous studies showed that hexadecyltrimethylammoium chloride (CTAC) and hexadecyltrimethylammoium bromine (CTAB) form micelles when the solution concentrations were greater than 1.3 mmol/L and 0.9 mmol/L, respectively [17]. The higher critical micelle concentration (CMC) for CTAC has been attributed to a smaller van der Waals interaction between Cl^- and the micelle than that between Br^- and the micelle. DUCKER et al have examined the adsorption mechanism of CTAB and CTAC onto mica in considerable details [18-21]. They found that the initially formed cylindrical structures transformed into a flat sheet over a period of approximately 18 h, and attributed this to slow exchange of CTA⁺ for the potassium ions in the mica lattice, which meant that an adsorbed layer of CTA⁺ hindered adsorption of polymer.

Colloidal forces play a crucial role in a variety of disciplines, ranging from biological systems to industrial processes [22-23]. These include measurements of electrical double layer forces and hydrophobic forces between two adsorbed surfactant films [24]. The investigation of interaction forces can be used in many fields like foods, material and mineral processing. Froth flotation, an important method in mineral processing industry, is a process for separating minerals from gangues by taking advantage of the difference in their hydrophobicity [25-26]. For this purpose, surfactant molecules (collectors) containing long hydrophobic chains are added to adsorb on the mineral surfaces either via physisorption or ionic bonding, which can render the surface hydrophobic. The necessities of particle flotation are as follows: selective agglomeration of surfactants, proper dispersion of pulp and the collision and adhesion between the particles and bubbles [27]. These processes depend largely on the interaction forces between the particles as well as particles with bubbles.

Mica is one kind of layer silicate mineral, which is usually separated and concentrated by froth flotation. During this process, alkyl quaternary ammonium salts, such as dedecyltrimethylammoium chloride (DTAC) and hexadecyltrimethylammoium chloride (CTAC), have been tested and proved to be effective collectors of layer silicate mineral flotation. To clearly understand the adsorption mechanism between DTAC/CTAC and mica surface on a microcosmic level, the adsorbed layer structures of cationic surfactants on the negatively charged hydrophilic mica substrate were investigated by AFM imaging technique. It was also the objective of the present work to measure the interaction forces between mica substrate and silica probe using AFM. The results were combined to discuss and further simulate the adsorption pattern of DTAC/CTAC on mica. The atomic force microscopy measurements which can provide information in a nanoscale is of great scientific and practical importance. It gives fundamental insights into interaction mechanism between quaternary ammonium salts and mica at solid-solution interfaces, which can further provide theoretical support on the technology of layer silicate mineral flotation.

2 Experimental

2.1 Materials

Dedecyltrimethylammoium chloride (DTAC) and hexadecyltrimethylammoium chloride (CTAC) with 99% purity (China) were used as received without further purification. The solid substrates used in all AFM experiments were muscovite mica (China) since mica is a model hydrophilic surface and has been previously used to study interaction forces in solution. Prior to each experiment, the samples of mica substrate, freshly cleaved using a tape immediately, were supposed to be immersed in the solution for 20 min and then dried in air. Reagent acetic acid and absolute ethanol were utilized as cleaning solvents. The materials were thoroughly cleaned just prior each experiment to avoid the presence of any contaminants in the system. The ultra-pure water with a conductivity of 18.2 MW/cm was used throughout the work. All the experiments were performed at room temperature (approximately 20 °C) in a clean well- maintained room.

The purchased silica microspheres with 5 mm radius in the present work were used for preparing the silica probe for the interaction force measurements. Prior to each set of experiments, the silica particles were thoroughly rinsed with ultra-water, ethanol and ultra-water again, and then dried in air. The 5 mm radius silica sphere was glued onto the none-tip cantilever of probe by the two-component epoxy. Then, the glued silica particle probe was allowed to expose in the air for more than 1 h to let the epoxy dry before use.

2.2 Atomic force microscopy

Adsorbed layer structures at the solid-solution interface were examined using a multimode NanoScope V scanning probe microscope (Veeco Instruments Co., Ltd) in tapping mode in air. The probes used were tapping mode etched silicon probes with integral and sharpened tips, which were irradiated with ultraviolet light for 30 min to remove organic contaminants prior to use. All images including amplitude, height and phase images were taken at a scan rate of 10 Hz. And it is important to note that the obtained images were unmodified except for flattening along scan lines.

Direct force measurements were carried out by using an atomic force microscope in PicoForce mode. The AFM experimental set consists of a piezo translation stage, a cantilever substrate, a laser beam, a split photodiode signal detection system and a liquid cell. In our experiment, the solutions were injected into the liquid cell slowly with great care and stabilized for a given period of time before any force measurement. The mica substrate was glued onto a magnetic plate that was mounted on the piezo translation stage. And the silica probe was mounted in the liquid cell. All the force measurements were held in a fluid cell and sealed with a silicone O-ring. A more detailed description of using AFM to measure colloidal forces can be found in literature elsewhere [27]. The AFM measured the force between the silica colloid probe obtained in lab and the hydrophobic mica substrate as a function of separation distance. The measured forces (F) were normalized with respect to the radius (R) of the probe spheres. It is noteworthy that the force depends on the radius of the cantilever tip, the smaller the probe radius was, the more precise the force measurements were.

Images shown display deflection data using a soft contact method in which surfactant adsorbs on both the tip and substrate, allowing images to be recorded under the role of electrical double-layer forces. In the present study, the image measurements and force measurements are combined to analyze the relationship between adsorption structures and interaction forces.

3 Results and discussion

3.1 DTAC adsorption

The adsorbed layer structures of DTAB on both mica and quartz have been documented to be a laterally unstructured bilayer in previous work [13]. It was also proposed that DTAB formed cylindrical aggregates on mica. Figure 1 shows AFM images of the adsorbed layer structures at a series of DTAC surfactant concentrations on mica. It is found that the morphologies of the adsorbed structures change greatly with the increase of surfactant concentrations. The observed sequence of aggregates structures is:

Block→Mesh→Laminate & Sphere

It can be seen from Fig. 1(a) that the DTAC adsorbed layer shows a homogeneous small blockstructure of about 0.9 nm height at the concentration of 10^-5 mol/L. Then, the homogeneous small block structures become larger and connect to each other in 10^-4 mol/L DTAC solution without distinct change in the height (Fig. 1(b)). At the concentration of 5×10^-4 mol/L (Fig. 1(c)), mesh adsorbed structures of 1 nm height on the mica surface were observed. And it is important to note that these concentrations are relatively low compared with the critical micelle concentration (CMC) of DTAC, which is 9.38×10^-2 mol/L. According with the adsorption heights of Figs. 1(a)-(c) and comparing them to the DTAC molecule length of 1.7 nm, it can be inferred that the DTAC is monolayer adsorption at lower concentration. At the largest concentration of 2×10^-3 mol/L (Fig. 1(d)), which is the closest to the CMC of DTAC, the adsorbed film on the mica shows a laminated structure of 0.9 nm height. Meanwhile, a few sphere structures of 2 nm height are also observed on the laminated structure, which can be interpreted as the simultaneous formation of monolayer adsorption and bilayer adsorption at this concentration.

Fig. 1 AFM images of dedecyltrimethylammoium chloride (DTAC) surfactants adsorbed on mica with DTAC concentrations:

It is found that the largest monolayer adsorption of DTAC on mica takes place at the concentration of 5×10^-4 mol/L by comparing four images to each other. For all given concentrations, the height of DTAC adsorption layer is almost consistent at about 0.9-1 nm, indicating that DTAC molecule is relatively stable on mica surface.

Figure 2 shows the surface forces (F/R) measured between a silica sphere with 5 μm in radius and a flat mica plate in DTAC solutions as a function of the separation distance (H) between the two macroscopic surfaces. In the presence of the cationic surfactant, repulsive forces, observed at low concentrations of 10^-6, 10^-5 and 10^-4 mol/L, are decreasing exponentially with the concentration increasing. According to the adsorbed morphologies in Figs. 1(a) and (b), the adsorption capability is increasing with the DTAC content increasing. And the adsorbed layers were monolayer at the concentration lower than 5×10^-4 mol/L, indicating that a hydrophobic attractive force diminishes the repulsive force.

Fig. 2 Normalized surface force (F/R) measured between a silica sphere of radius R and a fused mica plate as a function of separation distance in solutions of different DTAC concentrations

It is our hypothesis that the repulsive forces disappear almost completely due to charge neutralization. At 10^-3 mol/L DTAC, a largest attractive force is observed. Combining the adsorbed morphologies in Figs. 1(c) and (d), it can be found that there is a mesh adsorbed structure and the largest monolayer adsorption structure on mica appears at 5×10^-4 mol/L DTAC. Thus, there must be the largest hydrophobic attractive force in the vicinity of this concentration. As the surfactant concentration is increased up to 2×10^-3 mol/L, the attractive force is slightly decreased. It can be explained that a bilayer structure of the adsorbed layer is formed. Therefore, the hydrophobic attractive force begins to decrease and the total attractive force is reduced accordingly.

Previous studies showed that the attractive forces between hydrophobic surfaces became the strongest at the surfactant concentration close to the point of charge neutralization (p.c.n.) [28-29]. The curves given in Fig. 2 show the p.c.n for mica-DTAC system is approximately 10^-3 mol/L, where the attractive force is the strongest at this concentration. At the p.c.n., most of the negative sites are occupied by the DTA⁺ ions, forming hemi-micelles in which the hydrocarbon tails of the adsorbed surfactant molecules are closely packed .

At 2×10^-3 mol/L DTAC concentration, additional DTA⁺ ions adsorb on mica, but most probably with inverse orientation formed bilayer. Such an orientation would be favored as it would cost less energy to insert additional surfactant molecules into the adsorption layer with their polar heads apart from each other. The inverse orientation exposes the head groups toward the aqueous phase and, hence, causes the surface to be less hydrophobic. Therefore, the reduction in attractive force at the higher concentration can be attributed to the inverse orientation.

3.2 CTAC adsorption

The adsorbed layer structures of CTAC on silica substrates have been documented to be micelles [21]. These micelles desorbed when the CTAC solution was replaced with water. Figure 3 shows AFM images of the adsorbed film structures of the different CTAC concentrations, which shows a significant difference compared with DTAC at high concentrations. The transformation yields overall:

Block→Connected block→Mesh→Stripe

CTAC exhibits a homogeneous small block structure of 1.1 nm height on mica at 10^-7 mol/L (Fig. 3(a)), which is the same as DTAC at 10^-6 mol/L. When the concentration increases up to 5×10^-6 mol/L (Fig. 3(b)), it can be found that the adsorbed film shows a connected block structure of 0.8 nm height and that cavities exist in this connected block structure. At 5×10^-5 mol/L CTAC (Fig. 3(c)), a mesh adsorbed structure of 0.5 nm height is observed, which is consisted by the unordered short cylinder structure.

Figure 3(d) shows an ordered stripe characteristic of adsorbed cylinders of approximate 0.4 nm at 5×10^-4 mol/L, and the anisotropic pattern is reflected in the Fourier transform. This adsorbed structure is highly different from the structures that have ever been found at other CTAC concentrations. As the CTAC concentration further increases to 10^-3 mol/L (Fig. 3(e)), the adsorbed film becomes an unordered stripe structure with 0.3-0.4 nm height and the stripes connect to each other intensively.

It can be seen from Fig. 3 that the concentration of CTAC has a significant impact on the adsorption on the mica. The height of the adsorbed layer is reduced from 1.1 nm to 0.3 nm with the concentration increasing from 10^-7 mol/L to 10^-3 mol/L. In the meantime, the CTAC adsorption capability on mica is first increased and then decreased, and the adsorbed structure is also significantly changed. It implies that the concentration of CTAC affects not only the adsorbed layer structures on mica, but also the molecule adsorption behavior. The high concentration may lead to compressing the adsorption space of CTAC molecule, causing that the polar group of CTAC adsorbs on the negative charged mica surface and the hydrocarbon chain lies flat on the mica surface at higher concentration.

Fig. 3 AFM images of hexadecyltrimethylammoium chloride (CTAC) surfactant adsorbed on mica with CTAC concentration:

Figure 4 shows the measured surface forces between a silica sphere and a mica plate in the presence of CTAC. The trend of curves is highly similar to the situation of DTAC. The repulsive forces are observed at low CTAC concentrations of 10^-6 mol/L and 5×10^-6 mol/L, and this repulsive double layer force isdominant in whole forces at these concentrations. According to images in Figs. 3(a) and (b), the adsorption capability is relatively low at 10^-7 mol/L and 5×10^-6 mol/L. Hence, there is just a small hydrophobic attractive force between the surfaces. At the concentration of 10^-5 mol/L, the repulsive force disappears completely and a net-attractive force is observed immediately. It is seen that the measured force becomes the most attractive at 5×10^-5 mol/L, indicating the p.c.n. for mica-CTAC system was in the vicinity of this concentration. As the CTAC concentration continues to increase up to 5×10^-4 mol/L, the attractive force is slightly decreased, most probably owing to the inverse orientation of the CTA⁺ ions. At 10^-3 mol/L CTAC solution, a completely repulsive force is observed. According to Figs. 3(d) and (e), the CTAC molecule should almost lie flat on the mica surface at 5×10^-4 mol/L and 10^-3 mol/L, which results in the reduction of the hydrophobic attractive force. It is interesting to note that when the largest attractive force apprears, the corresponding concentration of CTAC is much smaller than that of DTAC, with the concentration of 5×10^-5 mol/L and 10^-3 mol/L, respectively. This difference shows that CTAC has a much better hydrophobic effect on mica than DTAC.

Fig. 4 Surface force (F/R) measured between a glass sphere of radius R and a fused mica plate as a function of separation distance in solutions of different CTAC concentrations

It is likely that additional surfactant ions adsorbed with inverse orientation, rendering the surface hydrophilic. A bilayer and multilayer adsorption may be formed at high concentrations. CTAC, however, has a different adsorption behavior between low concentrations and high ones. The alkyl chain of CTAC lies flat on the mica surface to form bilayer at high concentrations, resulting in a relative low concentration for CTAC than DTAC to produce a large hydrophobic attractive force. This observation is a powerful message that longer-chain surfactants are more efficient to increase the attractive force between hydrophobic surfaces [30-31]. It is suggested that a major driving force for adsorption of a long-chain surfactant is the hydrocarbon chain association, which is commonly recognized as hydrophobic effect [32]. The longer the hydrocarbon chain is, the stronger the hydrophobic effect becomes, which in turn causes a decrease in the p.c.n. and more transformations of the adsorbed structures on mica.

3.3 Effect of hydrocarbon chain length on adsorption

The effect of hydrocarbon chain length on adsorption on solid surfaces has been studied by many researchers. For instance, LOKAR and DUCKER [8] developed experiments with cetylpyridinium chloride (CPC) and dodecylpyridinium chloride (DPC) of different alkyl chain lengths to understand the importance of chain-chain interactions between silica surfaces, finding that at surfactant concentrations over the minimum force, the surface potentials are much greater for cetylpyridinium chloride (CPC) than dodecylpyridinium chloride (DPC). It was explained that long chain surfactant was more densely packed at the same concentration. Meanwhile, in some other research, it has been noted that an increase in number of ethylene groups (—CH₂—) by two caused a 10-fold decrease in the concentration (p.c.n.) at which the strongest attractive force was observed [29].

Figure 5 shows the adsorbed images of the layers formed by DTAC and CTAC at the same concentration of 5×10^-4 mol/L. DTAC forms a mesh structure of about 1 nm height on mica surface (Fig. 5(a)), which consists of the tightly sphere structure. It means that DTAC forms spherical micelles on mica at first, and then the sphere structure begins to connect with each other closely with the concentration increasing and forms the mesh structure. This kind of transformation implies that the adsorption of surfactant on mica contains not only just one simple form, and there may be two or more complex ones in different concentration, even in a certain concentration. However, in the case of CTAC, a stripe structure of 0.4 nm height is observed at the same concentration (Fig. 5(b)), which is greatly different from DTAC. The stripes are in some kind of one direction. Condering that mica belongs to layered aluminosilicate mineral, each layer is strongly negatively charged (0.48 nm² per charge) due to isomorphous substitution of aluminum for silicon. The surface crystal of mica is some kind of stripe structure of 0.15 nm thickness. Therefore, it can be indicated that this ordered stripe adsorbed structure of CTAC is influenced by the crystal of mica which is also a stripe structure.

Based on the data obtained in the present work, it can be found that the at the same concentration of 5× 10^-4 mol/L, thicknesses of DTAC and CTAC adsorbed layer are 1 nm and 0.4 nm, respectively. And the molecule length values of DTAC and CTAC are about 1.6 nm and 2.2 nm, respectively. For DTAC, it is monolayer adsorption at this concentration. The polar group DTA⁺ adsorbs on the negative charged mica surface, with the hydrocarbon chain of DTA⁺ tilted in the solution. However, for CTAC, though the polar group CTA⁺ adsorbs on the negative charged mica surface as the same as DTA⁺, the hydrocarbon chain of CTA⁺ is almost lied flat on the mica surface. Hence, it can be concluded that the length of hydrocarbon chain of surfactant affects not only the hydrophobic force, but also the adsorption behavior of the surfactant molecule. The longer the chain is, the easier the surfactant adsorbs on the mica.

Fig. 5 AFM images of DTAC (a) and CTAC (b) surfactants adsorbed on mica (Both concentrations are 5×10^-4 mol/L)

3.4 Adsorption simulation

HERDER [33] estimated the thickness of the monolayer to be 0.7-0.8 nm when one DTAC surfactant molecule is adsorbed for each negative surface site and that of bilayer to be 2.7 nm. RUTLANT et al [34] obtained a monolayer thickness varying between 0.5 nm and 0.9 nm in 10^-4 mol/L DTAC solution at pH 4.7-5.8 .

It can be clearly seen from Fig. 1 that the thicknesses of adsorbed monolayer of DTAC in the given concentrations are limited in a small range from 0.9 to 1 nm, similar to each other. There is a bilayer adsorption, of which the thickness is 2 nm. Hence, it is seen that the concentration of DTAC merely affects the adsorbed layer structure, while the molecule adsorption behavior is relatively independent of concentration. The results from Fig. 3 suggest that the thickness of the absorbed layer is decreasing with the CTAC concentration increasing. Here, the model of adsorption behavior of the cationic surfactant molecule on mica is given in Fig. 6.

Fig. 6 Schematic of adsorption model of cationic surfactant molecule on mica surface:

Considering that the length of DTAC molecule is about 1.6 nm, it is indicated that when the polar group of DTA⁺ adsorbs on the negative mica surface, the alkyl chain of DTA⁺ is tilted in the solution accompanied with bending of the chain just like Fig. 6(a). The angles of alkyl chain and mica plane are about 40° and are constant in all ranging concentrations of DTAC.

As for the adsorption of CTAC, the situations are more complicated. As the length of CTAC molecule is about 2.2 nm, at low concentrations of 10^-7 mol/L and 5×10^-6 mol/L, the adsorption pattern of the CTAC is more similar to Fig. 6(a). However, the angles of alkyl chain and mica plane tend to reduce due to the effect of long alkyl chain, with the concentrations increasing from 10^-7 mol/L to 5×10^-6 mol/L. When the CTAC concentrations increase to 5×10^-5 mol/L, 5×10^-4 mol/L and 10^-3 mol/L, the adsorption pattern of CTAC turns into lying down adsorption pattern on the mica, as depicted in Fig. 6(b). In these cases, the polar group of CTAC adsorbs on the negative mica surface, while the alkyl chain of CTAC is lying low on the mica plane accompanied with the chain crossing.

The comparison of two different adsorption patterns allows us to conclude that the cationic surfactant concentration has an impact on the molecule adsorption for long chain surfactant, but hardly changes the adsorption pattern of short chain surfactant. And the difference of the adsorption pattern of CTAC and DTAC, especially in high concentration solutions, is documented to be caused partially by the alkyl chain effect.

4 Conclusions

1) There are very different adsorption morphologies between DTAC and CTAC. With the concentration increasing, the adsorbed structures of DTAC on mica are block→mesh→laminate & sphere, while the adsorbed structures of CTAC are block→connected block→mesh→stripe.

2) Force measurements shows that the repulsive force is decreasing exponentially with the concentration increasing. And there is a concentration at which the largest attractive force can be observed. The corresponding concentrations of DTAC and CTAC are 10^-3 mol/L and 5×10^-5 mol/L, respectively, which are almost related to the mesh adsorbed structures of both surfactants. Beyond this concentration, the attractive force tends to decrease and even disappear due to charge neutralization.

3) Hydrocarbon chain length affected both the hydrophobic force and adsorption behavior in the mica- surfactant system. The adsorption pattern of the surfactant molecule can be simulated as follows: the polar group adsorbs on the negatively charged mica surface with the alkyl chain tilted in the solution, and the angle between alkyl chain and mica surface in CTAC solutions is smaller than that of DTAC solutions at the same concentration.

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(Edited by YANG Hua)

Foundation item: Project(50974134) supported by the National Natural Science Foundation of China

Received date: 2015-07-09; Accepted date: 2016-03-10

Corresponding author: JIANG Hao, Associate Professor; Tel: +86-731-88836544; E-mail: jianghao-1@126.com