Article pubs.acs.org/Langmuir
Self-Assembly Behavior of Ultrahighly Charged Amphiphilic Polyelectrolyte on Solid Surfaces Hui Yang,† Huabo Duan,‡ Xu Wu,§ Min Wang,∥ Ting Chen,† Fanghui Liu,† Shizhe Huang,† Wei Zhang,† Gang Chen,† Danfeng Yu,*,†,‡ and Jinben Wang*,† †
Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ College of Civil Engineering, Shenzhen University, Shenzhen 518060, P. R. China § College of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P. R. China ∥ Biolin Scientific AB, Shanghai Representative Office, Shanghai 200120, P. R. China S Supporting Information *
ABSTRACT: The adsorption process of a geminized amphiphilic polyelectrolyte, comprising double elementary charges and double hydrophobic tails in each repeat unit (denoted as PAGC8), was investigated and characterized by means of quartz crystal microbalance with dissipation (QCMD), ellipsometry, and atomic force microscopy (AFM). By comparison, the self-assembly behaviors of a traditional polyelectrolyte without hydrophobic chains (denoted as PASC1) and an amphiphilic polyelectrolyte with a single hydrophilic headgroup and hydrophobic tail in each repeat unit (denoted as PASC8) at the solid/liquid interface were also investigated in parallel. A two-regime buildup was found in both amphiphilic systems of PASC8 and PAGC8, where the first regime was dependent on electrostatic interactions between polyelectrolytes and oppositely charged substrates, and the rearrangements of the preadsorbed chains and their aggregation behaviors on surface dominated the second regime. Furthermore, it was found that the adsorbed amount and conformation changed as a function of the charge density and bulk concentrations of the polyelectrolytes. The comparison of the adsorbed mass obtained from QCM-D and ellipsometry allowed calculating the coupling water content which reached high values and indicated a flexible aggregate conformation in the presence of PAGC8, resulting in controlling the suspension stability even at an extremely low concentration. In order to provide an insight into the mechanism of the suspension stability of colloidal dispersions, we gave a further explanation with respect to the interactions between surfaces in the presence of the geminized polyelectrolyte.
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INTRODUCTION The strong propensity of polyelectrolytes (PEs) adsorbing at oppositely charged particles or surfaces has attracted extensive attention during the past decade due to the significance to many important fields, such as stability or flocculation of colloidal systems, surface cleaning, biosensors, and, in general, in a large number of areas in colloid science.1−4 Fully dissociated polyions, generally denoted as highly charged PEs with each monomer carrying an elementary charge, are oppositely charged to surfaces or particles.5 Such adsorption is so strong that they will overcompensate or overcharge for the charge of surfaces which immersed in bulk solution, playing an important role in the electrostatic interaction between charged particles or surfaces. The nonelectrostatic interaction, such as hydrophobic interaction, may contribute to the adsorption free energy after PEs being endowed with amphiphilicity.6 In such a case, amphiphilicity PEs may self-organize or self-assemble at interfaces and in solution to minimize contact between less © XXXX American Chemical Society
compatible parts, resulting in strong stability and low critical aggregation concentrations.7 On this basis, amphiphilic PEs with high charge density and abundant behaviors can be exploited to alter the characteristics of surfaces and enhance the solubility in solution adaptively.8 Until now, knowledge about these self-assembled structures has been extremely enticing for fundamental research and technological applications. Amphiphilic PEs are able to exhibit surfactant behavior and arrange into micelles of various morphologies spontaneously in aqueous solutions, such as spheres, vesicles, cylinders, discs, and bowls.9−13 PE micelles, containing both hydrophobic and charged groups, are able to bond or interact with surface groups. And such adsorbed micelles may keep their topological structures and integrity upon adsorption so that a rough layer Received: August 24, 2016 Revised: October 11, 2016 Published: October 18, 2016 A
DOI: 10.1021/acs.langmuir.6b03144 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
cationic hydrophilic PE (PASC1, Scheme 1c) was purchased from Beijing Chemical Co. (A.R. Grade, Mw = 1.3 × 104 g/mol). Millipore Milli-Q grade water was used in all our experiments. QCM-D Measurements. A quartz crystal microbalance with dissipation (QCM-D) from Biolin Scientific AB (Q-sense E1, Sweden) and a sensor of gold-coated quartz crystal with AT-cut (QSX 301, Biolin Scientific AB, Sweden) were used. The resonator with a fundamental resonance frequency of ∼5 MHz, owning a mass sensitivity constant (C) of 17.7 ng cm−2 Hz−1, is mounted in a fluid cell with one side exposed to solution. During QCM-D experiments, frequency and dissipation shifts of all the harmonics (n = 1, 3, 5, ..., 13) are recorded, in which the former provides the information about the mass of adsorbate on the surface and the latter provides the information about the elastic property of the adsorbed film. For the sake of simplicity, only the curves of Δf and ΔD versus time at the third overtone were exhibited in this work. The uncertainty was below 0.5 ng cm−2. The introduction of a rigid and thin layer leads to a tiny decrease in resonant frequency which is proportional to the mass of the layer and appropriate for calculating the adsorbed mass on the sensor through the Sauerbrey equation:23
forms, in which way amphiphilic PEs can be used as an additive to control colloidal stability or as an encapsulation or delivery of drugs, proteins, and DNA.14−16 One classic example of the former is the use of amphiphilic PEs to prevent particle aggregation, which not only enable the phase transfer of the particles from organic solvents to aqueous solution but also act as a versatile platform for chemical modification and bioconjugation of the particles.17 Moreover, high charge density PEs are more strongly bound to the oppositely charged surface, so there are more chances for the adsorbed PE layers to interpenetrate or interplay.14 In comparison, the lower the charge density is the easier the PEs can be removed from the surface.18 Until now, there is still little understanding on the self-assembly properties of highly charged amphiphilic PEs at solid−liquid interfaces, especially the molecular level of interactions between such PEs and solid surfaces.6,19,20 We have recently developed a molecular design based on highly charged PEs which are denoted as “geminized amphiphilic PE” and in which both the ionic head groups and the hydrophobic tails are doubled.21,22 The self-assembly behavior of this material in bulk solution has been investigated for the first time, which also has been used to enhance the efficiency of solid/liquid separation, resulting in an excellent performance in regulating the phase behavior in a concentration range of 3−5 mg L−1. Here, we are particularly interested in a series of PEs, including a traditional PE without hydrophobic chains, PASC1, an amphiphilic PE with one hydrophilic headgroup and hydrophobic tail in each repeat unit, PASC8, and a geminized amphiphilic PE, PAGC8. Quartz crystal microbalance with dissipation (QCM-D), ellipsometry, and atomic force microscopy (AFM) have been employed to investigate the adsorption and kinetic properties of the adsorbed PE layers in this study. Moreover, the effect of charge density of PEs and the bulk concentration on the adsorption process and stability of colloidal suspensions have been studied, from which the interactions between PE-coating surfaces have been revealed.
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Δm = −
ρq lq Δf f0 n
= −C
Δf n
(1)
where f 0 is the fundamental frequency and ρq and lq are the specific density and thickness of the quartz crystal, respectively. The dissipation factor is defined by eq 2:24 ΔD =
Ed 2πEs
(2)
where Ed and Es are the dissipated and stored energy during one oscillation, respectively. For viscoelastic films, e.g., in the case of ΔD/Δf > 0.2 × 10−6/Hz, the Sauerbrey equation is not valid, and it will underestimate the coupled mass, since the film is not fully coupled to the motion of the sensor surface.25 So, an evaluation process based on the Voigt model is introduced, in which the adsorbed mass can be modeled by the Qtools software package (Biolin Scientific AB, Sweden). The parameters that used in here were layer density (1200 kg m−3),26 fluid density (1000 kg m−3), layer viscosity (0.001−0.1 kg ms−1), layer shear modulus (103−105 Pa), and thickness (10−9−10−7 m). All the experimental data were fitted with Voigt model, as shown in Figure S1, Figure S2, and Table S2 of the Supporting Information. Prior to each measurement, crystals were cleaned in an ultravioletozone surface treatment for 10 min, immersed into a mixture of MilliQ water, ammonia (25%), and hydrogen peroxide (30%) in a ratio of 5:1:1, and then heated to 75 °C for 5 min. At last, they were rinsed in Milli-Q water and dried by N2. All experiments were performed at 25 ± 0.02 °C. The third, fifth, seventh, and ninth overtones were used for all modeling calculations. Ellipsometry Measurements. To measure the thickness of PE layers, a spectroscopic ellipsometer (M-2000V, J.A. Woollam) has been carried out at an incidence angle of 70° and with a wavelength scan from 370.1 to 999.1 nm. The complex reflection coefficient is measured as a function of wavelength expressed by eq 3:27
EXPERIMENTAL SECTION
Materials. Two amphiphilic PEs were prepared in our laboratory. One was a cationic homopolymer of 1,3-bis(N,N-dimethyl-N-octylammonium)-2-propyl acrylate dibromide with ultrahigh charge density and double hydrophobic chains in each repeat unit (PAGC8, Scheme 1a). The other was a cationic homopolymer of acryloyloxyethyl-N,Ndimethyl-N-octylammonium bromide bearing high charge density and single hydrophobic chains in each repeat unit (PASC8, Scheme 1b). The details for synthesis of PAGC8 and PASC8 can be found in our previous work.21,22 The molecular weights and polydispersities were determined by GPC analysis system as shown in Table S1. The
Scheme 1. Chemical Structures of the Polyelectrolytes: (a) PAGC8, (b) PASC8, and (c) PASC1
tan(Ψ)eiΔ =
Rp Rs
(3)
where tan(Ψ) denotes the amplitude ratio of the reflection coefficient of p-polarized light (Rp) to that of s-polarized light (Rs) and Δ is the phase difference. The quantities of Ψ and Δ are measured directly in experiments, and physical parameters of thickness and refractive index can be obtained by numerical fitting using appropriate models. In this study, a well-established Cauchy dispersion model has been used to fit ellipsometry data as shown in Figures S3−S5. From the results of thickness, the adsorbed mass was calculated, where the density of the bulk PEs was chosen as 1.00 g cm−3.26 B
DOI: 10.1021/acs.langmuir.6b03144 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Changes in frequency (Δf) and dissipation (ΔD) of (a) PASC1, (b) PASC8 (CAC = 130 ± 5 mg/L), and (c) PAGC8 (CAC = 70 ± 2 mg/ L) as a function of time at different concentrations.
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As will be discussed below, the comparison between the adsorbed mass obtained from QCM-D, mQCM, and ellipsometry, mellip, allows us to obtain the amount of water associated with layers by use of the method introduced in previous work.28,29 The water content, X, in the adsorbed film can be calculated by X≈
RESULTS AND DISCUSSION Adsorption Behavior. All the adsorption experiments were performed through QCM-D experiments under flow conditions at a constant of 20 μL min−1. As an example, Figure 1 shows the shifts both in frequency (Δf) and dissipation (ΔD) for PASC1, PASC8, and PAGC8 adsorbing onto solid surface at the third overtone. Three stages can be distinguished after the introduction of the PEs, respectively: first, the frequency of the baseline is obtained when the quartz crystal immersed in the background solution; then a sharp decrease in Δf is observed after PE solution is introduced in the measurement chamber, due to the adsorption of material on the surface of the sensor; in the third stage, the adsorption process ends (Δf = constant) because of the arrival of saturation adsorption. In the presence of PASC1, a decrease of ∼10 Hz in Δf and an increase of ∼2 × 10−6 in ΔD at different concentrations are as shown in Figure 1a, exhibiting the formation of rigid films. There is a negative charge on the gold substrate that aids the adsorption,34 and thus the electrostatic interaction between adsorption sites and polar headgroups in each polymer unit plays an important part in the adsorption process. It indicates that most of the polymer chains adsorb onto the surface and fewer chains extend to the bulk solution in the form of tight and linear conformations. By comparison, in the case of PASC8, Δf significantly decreases when the PE concentration increases above the CAC, while little changes in the dissipation signal takes place (Figure 1b), exhibiting tighter and denser conformations at higher concentrations (>CAC). It can be concluded that the main driving force for PASC8 adsorption onto surfaces is the PE−surface interaction when the concentrations are lower than the CAC. With the concentration increasing above the CAC, hydrophobic tails of PE tend to aggregate and participate in the adsorption process on the surface, and therefore both of the electrostatic and hydrophobic interactions contribute to the adsorption behavior. Compared with the adsorption processes of PASC1 and PASC8, big changes in both Δf and ΔD can be seen from Figure 1c after PAGC8 is introduced with the increase of concentration. It is noticeable that a larger increase in energy dissipation, ΔD, is observed in the case of PAGC8 than that of PASC8, although
mQCM − mellip mQCM
(4)
Characterization of Critical Aggregation Concentration. Critical aggregation concentration (CAC) values (around 100 mg L−1) of the two amphiphilic PEs in aqueous solutions were obtained through steady-state fluorescence as we reported previously.22 AFM Imaging. To examine the surface morphology of the films obtained from QCM-D measurements after equilibrium adsorption, AFM images were carried out in air by a Fastscan AFM microscope (Bruker, USA) and performed on a peakforce tapping mode. Silicon tips with a spring constant of ∼4.0 N m−1 were used throughout the measurements. The root-mean-square (RMS) roughness of the films was evaluated from the recorded AFM images. Image processing and determination of surface roughness were performed by using the Nanoscope Analysis software.30 Colloid Probe AFM Technique. In order to directly determine the interactions between solid surfaces in the presence of amphiphilic PEs, a colloidal probe was prepared and introduced in AFM method. A silica sphere of 10 μm in diameter was glued to the cantilever with an epoxy resin according to the reformative cantilever moving technique.31 Force curves were conducted through a Multimode VIII AFM with an O-ring liquid cell and in contact mode. Spring constants of the cantilevers bearing the colloidal particle were in the range of 0.3−0.4 N/m, determined through the “thermal tune” method.21,32,33 Force curves were obtained by converting the cantilever deflections and piezotube displacements in agreement with Hooke’s law. The colloidal probe and AFM fluid cell were rinsed with ethanol and dried with nitrogen prior to every experiment. The forces between a flat surface and the colloidal probe were measured as a function of separation distance. The approach speed was about 1 μm/s used in the force measurements and the ramp rate for an approach/retraction cycle was close to 1 Hz. More than 50 force curves were recorded at different locations for each experimental condition, which were analyzed by the use of the NanoScope Analysis software, and at last fitted and computed in one profile (see Figures S6 and S7). C
DOI: 10.1021/acs.langmuir.6b03144 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. ΔD vs Δf relations in the case of (a) PASC1, (b) PASC8, and (c) PAGC8 at different concentrations.
the changes in frequency, Δf, show a similar trend in the injection of PASC8 and PAGC8, attributed to a flexible aggregation among the geminized PEs and a swelling process of the adsorbed layers especially above the CAC. Figure 2a shows almost linear trends of the relationship between ΔD and Δf in all the three concentrations of PASC1, suggesting that the PE chains adsorb parallel to the surface and there are nearly no rearrangements of them, attributed to the PE−surface interaction. From Figures 2b and 2c, the curves of ΔD vs Δf relation reflect nonlinear trends and two different slopes kI and kII (as shown in Table S3), indicating that the adsorption processes of PASC8 and PAGC8 experience two regimes in the whole concentration range. In regime I, for PASC8, small slopes initially appear since the frequency decreases rapidly, suggesting a rapid transplant and adsorption of PEs from solution to surfaces. Once the frequency change exceeds about 10 Hz (regime II), the transition point in the ΔD versus Δf curve arises and corresponds to the transition point between fast and slow adsorption regime as shown in Figure 1. Different from the PASC8 system, in the presence of PAGC8, small slopes appear because frequency decreases rapidly and the dissipation increases rapidly in the meantime (regime I), indicating the adsorption of PEs from solution to surfaces with a swelling conformation. In regime II, bigger slopes can be observed in the case of PAGC8 than those values of PASC8, due to bigger dissipation values, implying an existence of rearrangements of preadsorbed chains of PE molecules and a complex balance of different forces, such as PE−surface and PE aggregate−surface interactions. QCM-D versus Ellipsometry. There is a substantial difference between the adsorbed masses calculated from QCM-D (denoted as QCM mass) and ellipsometry (denoted as ellipsometry mass) in the presence of PASC1, PASC8, and PAGC8 (Table 1, Figures 3 and 4). For PASC1, the results
Figure 3. Water content in the polyelectrolyte films calculated as the relative difference between QCM and ellipsometry mass.
Figure 4. QCM and ellipsometry mass of different polyelectrolytes on surfaces as a function of concentration.
typical conformation with tight and dense structure associating with more water, mostly because the adsorption of PASC8 is driven by both of the electrostatic and hydrophobic interactions. For PAGC8, around the CAC, the water content passes through a maximum of ∼60% and then keeps almost invariable; the QCM mass calculated through Voigt model is approximately double as compared to the ellipsometry mass with the increase of concentration, showing a good ability in associating with water. The QCM mass of PAGC8 layer is nearly twice as that of PASC8, and more interestingly, the water content is much higher. Because the doubled cations in each polymer unit of PAGC8 can bring in more water, on the other hand, the formation of a flexible aggregate conformation can provide more topological space for water molecules. From all the discussion above, it shows the following trends: in the presence of the PE without hydrophobic chains (such as PASC1), the adsorption process is mainly driven by the interaction between PE and surface; in the case of the amphiphilic PE (such as PASC8 or PAGC8), both PE−surface and PE aggregate−surface interactions contribute to the
Table 1. Adsorbed Mass and Water Content of PASC1 Adsorbed Layers adsorbed mass (ng/cm2) concn (g/L)
QCM
ellipsometry
water content (%)
0.6 3 6
115 177 267
110 170 250
4.3 4.0 6.4
show low values of water content (
