Controllable Self-Assembly of Amphiphilic Dendrimers on a Silica

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Controllable Self-Assembly of Amphiphilic Dendrimers at Silica Surface: Effect of Molecular Topological Structure and Salinity Minghui Zhang, Jinben Wang, and Pei Zhang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b05673 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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The Journal of Physical Chemistry

Controllable Self-Assembly of Amphiphilic Dendrimers at Silica Surface: Effect of Molecular Topological Structure and Salinity Minghui Zhang,†,‡ Jinben Wang,*,† Pei Zhang*,† †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

*E-mail: [email protected], Phone: 86-10-62523395, Fax: 86-10-62523395. *E-mail: [email protected], Phone: 86-10-62652659, Fax: 86-10-62523395.

ACS Paragon Plus Environment

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ABSTRACT: The adsorption kinetics and equilibrium of amphiphilic dendrimers based on poly(amidoamine) PAMAM modified with dodecyl chain GnQPAMC12(n represents the generation number) with different generation at silica–water interface have been investigated. The effect of molecular shape with different charge characteristics on the adsorption kinetics, adsorption isotherms and the conformation of self-assembled layer has been elucidated. For the adsorption kinetics, two steps were observed including the adsorption of individual molecules at the concentrations below cmc and the predominant adsorption of aggregates above cmc. However, the adsorption isotherm, with the generation number as a function, presented exceptional characteristic, in which, a decrease of adsorption mass with different levels occurred in high generation of amphiphilicdendrimers, depending on the balance of hydrophobic interaction and electrostatic repulsion. Atomic force microscopy imaging showed that flatten films with pores (spacing) of various shapes and roughness of 3 ~ 4 nm were formed, of which the pores (spacing) decreased obviously as the generation increases. The addition of electrolyte (NaBr) has great effect on the film morphology formed by G3QPAMC12 dendrimer adsorbed at silica-water interface, showing that the film became closer with smaller pores with increased NaBr concentration.


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1. INTRODUCTION Dendrimers with highly compact and tree-like branched structure, as well as the uniform size in the nanometer size range have attracted tremendous scientific interest in many fields such as nanomaterials, biomaterials and supramolecules, etc.1-5 This simple and distinct framework provides a structure that is molecularly controllable based on the type of core, the amount and type of branching, and the functionality of the end groups. In addition, the flexible surface functionality and defined structure make them promising candidates in a range of applications from surface modification of solid materials to the control of structure formation in colloidal suspension.6-10 In order to extend potential application at the interface, it is essential to study the adsorption behaviors of dendrimer onto solid surfaces, exploring the changes in adsorption process and the factors affecting the structures of self-assembled layers. Thus pathways for engineering flexible interfacial materials that exhibit advantageous functions could be opened. Poly(amidoamine) dendrimer (PAMAM), as one of the pioneer dendritic polymers, has been extensively investigated including the adsorption at solid interface from aqueous solutions.11-15 Previous research revealed that the adsorption of PAMAM dendrimer on silica surface was mainly driven by hydrogen bonding and electrostatic interaction between the protonated terminal amine groups and deprotonated silica surface, and that the generation of dendrimer, pH, ionic strength were factors affecting the adsorption at solid-water interface.16, 17 It follows that the use of PAMAM dendrimer as building block for the construction of multilayered assemblies should lead to functional monolayer film with the property of stimulative responsiveness. Self-assembled monolayers (SAMs) are organic two-dimensional nanomaterials composed of molecular constituents forming ordered assembly spontaneously onto solid surface, being as excellent systems that have been applied in many fields including bionic design, interfacial


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reaction and catalysts modification etc.18-21 However, the self-assembled monolayers (SAMs) prepared from molecules dominated by simple alkyl chains have some significant disadvantages, such as the limitation and instability in two-dimensional surfaces.22 Therefore, there is a need to increase the dimensionality of SAMs to enhance their functional group density and to improve their substrate adhesion and stability. Potentially, a variety of supramolecular structures formed by modified dendrimers can be constructed by changing the nature and concentration of surface groups, adjusting the attachment of various functional fragments, of which the potential to develop more desirable organized films via self-assembly is also intriguing.23 It is reasonable to assume that amphiphilic dendrimer with remarkable aggregation behaviors through hydrophobic interaction can exhibit self-assembly performance interestingly different from conventional dendrimers in the formation of self-assembled monolayers. Recently, a series of amphiphilic dendrimers GnQPAMC12 (n = 1, 2, 3) were synthesized in our laboratory by grafting hydrophobic alkyl chains -C12H25 at the periphery of PAMAM dendrimer (Figure 1), which contain charged quaternary ammonium groups at their periphery as well as secondary amine groups in the interior, making them sensitive to both salt and pH. Here, we describe the construction of assembled layers through the direct adsorption of amphiphilic dendrimer onto silica surface, exploring the effect of charge density, molecular structures, and salinity on the adsorption processes and mechanisms. The explanation and theoretical models proposed in the present work are helpful to further understand the mechanism of surface modification and build functional materials.


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Figure 1. Chemical structure of GnQPAMC12 (n = 1, 2, 3, representing the generation).

MATERIALS AND METHODS 2.1. Materials. A series of GnQPAMC12 (n=1, 2, 3) with different generations were synthesized and purified by our group (Figure 1). The preparation of detailed procedures and 1H NMR data were presented in the Supporting Information. Milli-Q water with conductivity of 18.2 Ω/cm-1 was used for preparation of sample solutions. The silica substrates were activated with piranha solution (H2SO4:30%H2O2 = 3:1 V/V) at 80 °C for 40 min, and rinsed with distilled water, acetone three times successively, drying under nitrogen atmosphere finally. 2.2. Surface Tension Measurement. The equilibrium surface tension was examined by Wilhelmy plate method (Data physics DCAT21, Germany) using a platinum plate of ~ 5 cm perimeter and 2 cm length. The instrument was calibrated with double-distilled water and the standard deviation for surface tension measurements was less than 0.2 mN/m. Sample solutions


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were immersed into a constant-temperature bath at the desired temperature 25.0 ± 0.05 °C. Sets of measurement were taken at certain intervals until the surface tension was constant for ~ 0.5 h. The cmc value was obtained according to the static surface tension versus log of the bulk concentration. The platinum plate was cleaned by burning under alcohol flame to obliterate the residual dendrimer before each measurement, and then was dipped into solution to begin its measurement. 2.3. Quartz Crystal Microbalance with Dissipation (QCM-D). QCM-D measurements were carried out using an E1 system (Q-Sense, Gothenburg, Sweden). The experimental equipment mainly consists of a flow cell module which can hold sample volume of 0.5 mL with one sensor. The flow of solution was controlled with a peristaltic pump (Ismatec IPC-N 4, Zürich, Switzerland) at a flow rate of 0.1 mL/min. The quartz crystals coated with one SiO2 layer were commercially available (QSX 303, Q-Sense). The QCM crystal was treated with piranha cleaning solution (H2SO4:30%H2O2 = 3:1) for 1 min before rinsed with Millipore quality water and dried with N2 gas. After measurements, the crystal was cleaned with Milli-Q water, dipped in 2% SDS solution for 30 minutes at room temperature, and then rinsed with Milli-Q water for several times. All QCM experiments were carried out at temperature of 25 ± 0.05 °C. The fundamental frequencies (f) and corresponding energy dissipation factors (D) of the odd overtones 1 to 13 of each crystal were calculated before measurement. A shift of the resonance frequency corresponds to the mass change of the substrate. If the layer is rigid, homogeneous and evenly distributed on the substrate, the adsorbed mass should be calculated according to the Sauerbrey equation (1):24,25

∆m= −

C ∆f n

(1)


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where C refers to proportionality constant (C=17.7 ng Hz-1 cm-2 for the 5MHz crystal) and n is the overtone number. △m is usually referred to the “wet mass” because a portion of solvent can be incorporated in the adsorbed layer.26 Frictional losses between the crystal and the adsorbed material lead to a damping of the oscillation with a decay rate of amplitude that depends on the viscoelastic properties of the adsorbed layer. The dissipation factor D is defined by the equation (2):27

D=

Ediss 2π Estor

(2)

where Ediss is the total dissipated energy during one oscillation cycle and Estor is the total energy stored in the oscillation system. After mounting the quartz crystal in QCM cell, it was exposed to aqueous solution with same salt concentration and pH, using for establishing a stable baseline of the frequency and energy dissipation. The adsorption experiments were then started by exchanging solution medium with sample solution. Residual sample solution was removed by addition of water after equilibrium. The Q-soft software from Q-sense was utilized to record the changes in the properties of adsorbed layer at four different overtones during adsorption process. Of these, the third overtone was used for evaluating the data basically due to its stability. The concentrations of GnQPAMC12 solution were from 3×10-6 mol/L to 1×10-4mol/L in all experiments. Each test was repeated more than three times. 2.4. AFM Imaging. The AFM experiments were carried out using a NanoscopeⅢa multimode scanning probe microscope (Digital Instruments, USA) with tapping mode. Standard silicon AFM probes (FASTSCAN-B) with cantilever spring constants of 4 N/m and resonance frequencies around 400 kHz were used. The preparation of adsorbed dendrimer films on silica surface were described briefly as follows: First, the processed silica substrates were immersed


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into GnQPAMC12 solutions for different time, and then the substrates were dried by high purity nitrogen for AFM imaging. The waiting time is no more than 30 seconds after the nitrogen flushing. The morphologies were observed and analyzed by software NanoscopeⅢ 5.12 with height and root mean square roughness (RMS). Before testing, the surface roughness was described in terms of the root mean square roughness (RMS).

3. RESULTS AND DISCUSSION 3.1. Surface Properties of GnQPAMC12(n=1,2,3). The surface tension curves of GnQPAMC12 are presented in Figure 2 and cmc values are determined from the γ‒lgC curves. The maximum surface excess concentration at air/water interface (abbreviated as Γmax-A) was calculated according to the Gibbs adsorption equation Γ max = −

1  dγ  28 The   . 2.303nRT  d ln C T

minimum average surface area/surfactant molecule (Amin) was obtained from the saturated adsorption by equation Amin = ( Ν AΓ max )−1 , where γ is the surface tension in mNm-1, C is the concentration of the surfactant in the solution, Γ is the adsorption amount in µmol/m2, NA is the Avogadro constant, Τ is the absolute temperature, R= 8.314 J mol m-1 K-1, and d γ / d ( ln C ) is the maximal slope in each case. For the ionic dendrimers, n= 4 is taken (n=4 is considered in the ideal state we assumed due to uncertain dissociation degree of ionizable groups for three dendrimers.)29, 30 The maximum surface excess concentration at solid surface (abbreviated as Γmax-S) was calculated from QCM-D experiment results. Amin was also calculated according to the equation Amin = ( Ν AΓ max)−1 . The parameters such as cmc, γcmc, Γ, and Amin values are obtained and listed in Table 1.


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80

Surface tension (mN/m)

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Ccmc(mol/L); (γcmcmN/m)

75

G1QPAMC12 (cmc=7.0E-6; γcmc=34.4)

70 65

G2QPAMC12 (cmc=1.8E-5; γcmc=33.6)

60 55

G3QPAMC12

50

(cmc=5.3E-6; γcmc=33.7)

45 40 35 30 25 1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

Concentration (mol/L)

Figure 2. Surface tension curves of GnQPAMC12plotted against concentrations. The amphiphilic dendrimers showed lower cmc values comparing to conventional cationic surfactant, suggesting an excellent micelle forming ability. However, it is not a linear decrease of cmc values with the increase of generation number but has a minimum value for G2QPAMC12, as shown in Figure 2. It’s worth noting that there is no significant difference in the value of surface tension at cmc. Especially, G1QPAMC12 with only four branched alkyl chains, possessing nearly the same capacity to decrease surface tension comparing with high generation of that with more branched chains, such as G2QPAMC12 and G3QPAMC12, indicating an unusual aggregation mechanism for such a series of highly branched macromolecular surfactants. The phenomenon signified that high generation of amphiphilicdendrimer with more hydrophobic branched chains and larger positively charged head groups arranged loosely at the air-water interface, because of the larger molecule volume and stronger repulsion interaction between molecules. As shown in Table 1, the apparent area per molecule, Amin-A, increased gradually with the increasing generation number, which relates to the increasing electrostatic repulsion and the growing molecular volume. Referring to our previous approach, theoretical radius of the headgroup and the alkyl chain length were calculated assuming a totally spread model.29 Meanwhile, saturated adsorbed radius of GnQPAMC12 was obtained, showing a much smaller


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radius compared to the theoretical value. The contrast indicates that amphiphilic dendrimer molecules are not outstretched at air-water interface. The ratios of theoretical radius to saturated adsorbed radius are 1.5, 2.6 and 3.8 for generation 1 to 3, respectively, indicating a more closely packed aggregates conformation of the higher generation of dendrimers. Table 1. Гmin, Amin, Saturated Adsorbed Radius (SAR) and Theoretical Headgroup Radius (THR) of GnQPAMC12 at Air-Water Interface and Solid-Water Interface Sample

Γmin-A (µmol/m2) (air-water)

Amin-A (Å2) (air-water)

SAR (Å)

THR (Å)

G1QPAMC12 G2QPAMC12 G3QPAMC12

0.93 0.89 0.82

178 185 203

7.53 7.68 8.04

11.60 20.10 31.10

Γmin-S (µmol/m2) (solidwater) 1.73 0.72 0.44

Amin-S (Å2 ) (solidwater) 96 229 377

3.2. The Adsorption Behaviors of GnQPAMC12 on Silica Surface. The knowledge of adsorption rate and the structural arrangement at adsorbed layer are the key scientific issues in investigating the adsorption behaviors. It is reported that conventional surfactants such as CTAB adsorbed directly to silica surface in the form of micelles, meaning that the aggregation behaviors of amphiphilic molecules play an important role in the adsorption process.31 In order to understand the mechanism of amphiphilic dendrimer adsorption on the silica surface and the effect of topological structure on characteristics of adsorption, we performed the kinetic measurement using QCM-D by plotting the adsorption mass as a function of time. The equilibrium time for GnQPAMC12 at different concentrations are acquired in the plots of adsorption kinetics, as shown in Figure 3. To better understand the effect of aggregation on the adsorption kinetics, the plots of adsorption kinetics are divided into two steps, including the step at concentrations below cmc and the stage at concentrations above cmc.


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2 0

A

0

B

G1QPAMC12 C > cmc 5E-5 mol/L 1E-4 mol/L

-2 -4

∆ f/n (Hz)

-4

∆ f/n (Hz)

2

G1QPAMC12 C < cmc 3E-6 mol/L 5E-6 mol/L 1E-5 mol/L

-2

-6 -8 -10

-6 -8 -10 -12

-12

-14 -14 -16 0

1000

2000

3000

4000

5000

6000

0

500

1000 1500 2000 2500 3000 3500 4000

Time (s)

Time (s)

2 0

C

-4

0

-6 -8 -10

D

G2QPAMC12 C > cmc 1E-5 mol/L 5E-5 mol/L 1E-4 mol/L

-2

∆ f/n (Hz)

∆ f/n (Hz)

2

G2QPAMC12 C < cmc 3E-6 mol/L 5E-6 mol/L

-2

-4 -6 -8 -10

-12

-12

-14

-14 -16

-16 0

1000

2000

3000

4000

5000

6000

0

1000

Time (s)

2 0

E

2000

3000

4000

5000

Time (s)

2

G3QPAMC12 C < cmc 3E-6 mol/L 5E-6 mol/L

-2

0

F

G3QPAMC12 C > cmc 1E-5 mol/L 5E-5 mol/L 1E-4 mol/L

-2

-4

-4

-6

∆ f/n (Hz)

∆f/n (Hz)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-8 -10 -12

-6 -8 -10

-14

-12

-16

-14

-18

-16 -18

-20 0

1000

2000

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6000

0

1000

Time (s)

2000

3000

4000

5000

Time (s)

Figure 3. The time dependence of the frequency shift ∆f/n for the resonator after adsorption on a silica-coated sensor. The measured concentration of GnQPAMC12 was from 3×10-6 to 1×104

mol/L. A considerable decrease in ∆f/n was observed, which confirmed an efficient adsorption of

GnQPAMC12 molecules at silica-liquid interface. Multiple stages were noticeable depending on


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the generation and concentration of the surfactants. In the case of concentration below cmc, diverse processes existed in the adsorption, in which, the adsorption mass increased rapidly at the initial moment, following a second adsorption stage in a slow pace, reaching equilibrium gradually (Figure 3A, 3C and 3E). In the case of the concentration above cmc, the adsorption exhibited fast process and reached at equilibrium very quickly (Figure 3B, 3D and 3F). It’s worth noting that the equilibrium time at high concentration decrease enormously, especially above cmc, indicating that various aggregates forms of adsorption co-occurred near and above cmc. Considering the differences in topological structure, GnQPAMC12 possesses high positive charge density with increasing generation, which is conducive to the adsorption on negative silica surface at the initial stage. Simultaneously, the enhanced hydrophobic interaction appeared with the increase of branched alkyl chains, driving the adsorption of amphiphilic dendrimers aggregating more easily at the silica surface. To further characterize the adsorption kinetics, two models including one-step model and a two-step model were employed to fit the data obtained.32 The one-step model: qt

= q0 + k 0 t

The two-step model: qt = qe − Ae 1

− k1t

(3)

− A2e− k2t

(4)

where q0, qt and qe are constants, representing the adsorption mass at time t and adsorption mass at equilibrium respectively. ki is rate constant and Ai is pre-exponential terms. Previous results showed that the rate constants contain information including the diffusion of surfactants, the process of micellization from bulk solution to the interface, and the reorganization of the adsorbed surfactant molecules.32, 33 All rate constants were obtained as shown in Table 2.


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Table 2. Kinetics Rate Constants of Adsorption for GnQPAMC12 at Silica-Water Interface G1QPAMC12 Ccmc

6.15 ± 0.04

Ccmc

9.12 ± 0.05

Ccmc

12.25 ± 0.03

k1(% s-1)

(1.25 ± 0.02)×10-4

N

(3.60 ± 0.02)×10-4

N

(7.47 ± 0.03)×10-4

N

k2(% s-1)

(1.04 ± 0.03)×10-3

N

(0.96 ± 0.01)×10-3

N

(0.94 ± 0.02)×10-3

N

N stands for not applicable.C is representative of the bulk concentration. Consistent with the results discussed above, the different adsorption rates are reasonably

attributed to the different adsorption mechanisms. The positive charge number of headgroup in GnQPAMC12 increased exponentially with the increase of generation, making the aggregates possess high charge density and the zeta potential data of GnQPAMC12 (as shown in Figure 4) also demonstrate that charge density increased with the increase of generation number at a bulk concentration above cmc, which provides a better explanation of charge behaviors of aggregates. Consequently, k0 increases with the generation, which is related to the process of aggregates adsorption, dominated by the electrostatic interaction. In the case of concentration below cmc, the kinetics of adsorption include fast and slow adsorption rate, in which individual molecules can be adsorbed to the silica surface and rearrange into self-assembled structures at the same time, taking long time to achieve equilibrium, as illustrated in Figure 5. Meanwhile, little difference exists in molecular rearrangement, as manifested by k2 value.


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120

Zeta potential (mv)

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G1QPAMC12 G2QPAMC12

100

G3QPAMC12 80 60 40 20 0 0

1 2 3 5 -5 4 Concentration (10 mol/L)

6

Figure 4. The charge characteristics of GnQPAMC12 solutions with concentration below and above the cmc.

Figure 5. The schematic adsorption mechanism of GnQPAMC12 on silica surface. 3.3. Adsorption Isotherm of GnQPAMC12. The adsorption isotherm can be constructed by performing a series of adsorption experiments to determine the surface excess for different concentrations of GnQPAMC12, as shown in Figure 6A. In the case of G1QPAMC12, a classical two-step adsorption model was observed, in which, the adsorption density is strongly dependent on bulk concentration, bearing some resemblance to that of traditional cationic surfactant such as the adsorption of CTAB.34, 35 It’s important to note, however, that the tendencies of adsorption


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with bulk concentration present unusual characteristics in the cases of G2QPAMC12 and G3QPAMC12. In detail, a downward trend in the adsorption amount was observed with increasing concentration, which can be divided into two stages as discussed in the follows: a decrease of adsorption with different levels below the cmc indicates that the intermolecular interactions among individual molecules promote or hinder the adsorption, depending on the balance of hydrophobic interaction and electrostatic repulsion. At higher concentration range above cmc, conformation transitions of aggregates induced by the size ratio of headgroup to hydrophobic chain length are thought to be the decisive factor affecting the adsorption. Namely, how the molecular topological structure or molecular shape influences the adsorption process or the adsorbed layer structures is a challenging issue, especially for the G2QPAMC12 and G3QPAMC12 dendrimers systems. Previously, Brian P. Cahill et al. put forward the extended three-body RSA (random sequential adsorption) model potential involving dendrimers at charged surface, explaining the adsorption process driven by attractive forces between the dendrimers and the substrate.17 However, amphiphilic dendrimers with numerous hydrophobic chains are expected to be more complicated in the process of adsorption. To explore the rigidity of the amphiphilicdendrimers configuration, D/F is plotted versus concentration for the adsorption from aqueous solutions below and above the bulk cmc (Figure 6B). In all the cases, D/F values are less than 0.4×10-6 Hz-1, indicating a very rigid coverage in the whole concentration range, which means the strong adsorption of GnQPAMC12 dendrimer at negatively charged silica surface due to electrostatic interaction and hydrophobic interaction.


15


0.01

G1QPAMC12

A

G2QPAMC12 G3QPAMC12

300 280 260 240 220 200

B

G1QPAMC12 G2QPAMC12

0.00

G3QPAMC12

-1

320

-6

2

340

D/F (10 Hz )

360

Adsorbed mass (ng/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.01

-0.02

-0.03

180 160 140 0.00000 0.00002 0.00004 0.00006 0.00008 0.00010 0.00012

-0.04 0.00000

0.00002


0.00004

0.00006

0.00008

0.00010


Figure 6. The adsorption isotherms of GnQPAMC12 (A); Ratios between experimental dissipation and changes in the resonance frequency (d/f) as a function of concentration monitored during the adsorption (B). 3.4. Direct AFM Imaging. To further explore the adsorption mechanism at concentrations in the vincinity of cmc induced by the variation of topological structure, the adsorption images on silica from aqueous solutions were obtained using AFM,36 as shown in Figure 6, in which the AFM images were taken at different adsorption time for G1QPAMC12. In the case of immersing in short time (1 min), spherical aggregates were observed, with a diameter range of 50 ~ 80 nm and ca. 3.0 nm in height (Figure 7a). Furthermore, DLS results reveal that the radius of aggregates is about 70 nm, which is consistent with the AFM results considering the collapse of aggregates in dry state. The DLS result is shown in Figure S4. The results above indicate that fractional aggregates of G1QPAMC12 adsorbed on silica surface in a very short time, resulted from the electrostatic interaction between the positively charged aggregates and negatively charged silica surface. In the case of longer adsorption time, such as 2 hours, numerous spherical aggregates were formed with unchanged heights in longitudinal direction (Figure 7b). Thus, this is in good agreement with the adsorption mechanism inferred from the results by QCM.


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Figure7. The AFM height image of 100 nm×100 nm for G1QPAMC12 adsorbed onto silica surface at the concentration of 5×10-5mol/L (1cmc) as a function of time: (a) 1 min; (b) 120 min. At the stage of adsorption equilibrium, spread films with varied pores were evenly formed by GnQPAMC12 on silica surface, as shown in Figure 8, with a typical adsorption thickness of 3 ~ 4 nm. It can be seen that the morphology of G1QPAMC12 film turns into continuous and flatten, with the almost unchanged roughness, indicating that the aggregates grew horizontally rather than vertically. Furthermore, it involved into the more compact quasi-fibrous structure than that of the loose connected island-like structure of the lower generation of dendrimers. Combining the adsorption isotherms, one can speculate that the variation was resulted from the special topological structure of GnQPAMC12. Theoretically, the internal structure of quaternary ammonium PAMAM was taken as hydrophilic headgroup, and the peripheral hydrocarbon tail-groups were involved into lateral interaction between adsorbed monomers or aggregates in varying degrees, depending on the area and conformation of head group at silicawater interface. Referring to the data in table 1, there are significant differences between the minimum average surface area/molecule (Amin) at air-water and silica-water interface. In the case


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of G1QPAMC12, the value of Amin at air-water interface is double that at silica-water interface. In the cases of G2QPAMC12 and G3QPAMC12, however, a significant increase in surface area was observed at the solid surface assembled layers, indicating that the amphiphilic dendrimers of higher generations adopted more spreading conformation at the silica-water interface due to more hydrophobic interaction, forming the assembled fibrous structure with various sizes of pores.

Figure 8. The AFM height image for GnQPAMC12 molecules adsorbed onto the substrate of silica for 4 h at the concentration of 1×10-4 mol/L. (a) G1QPAMC12 (b) G2QPAMC12 (c) G3QPAMC12.


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3.5. Salinity Effect on the Adsorption of GnQPAMC12 at Silica Surface. Many factors, such as molecular architectures, pH, temperature and salinity etc., can be used to regulate the aggregation behavior and the structure of the adsorption layer of GnQPAMC12 dendrimer at silica-water surface.12, 29, 37-39 The addition of salts is one of the convenient approaches to tune the self-assembly layer of the dendrimers. There are many applications of the amphiphilic dendrimers faced with different salinity in nature such as in human organ, ground water, seawater and so on. One of the important application is the self-assembly of the amphiphilic dendrimers at the outer surface of halloysite nanotube or mesoporous silica nanoparticles (MSNs) to construct drug delivery system. In this case the structure of self-assembled layer is very sensitive to the variation of salinity, which is the basis for controllable drug release of the smart carrier in the environment with different salt concentration. Taking G3QPAMC12 as an example, the character of adsorbed layers depend obviously on the variation of electrolyte concentration, as demonstrated in the AFM images results. The in situ AFM images revealed obvious structural evolution features at concentrations from 0.05 mol/L to 1 mol/L NaBr, as indicated in Figure 9. At the NaBr concentration of 0.05 mol/L, the selfassembled layer takes the conformation of fractal and flat film with the typical roughness of 3-4 nm. Interestingly, after reaching the adsorption equilibration as the concentration of NaBr increased to 0.1 mol/L the assembled molecular layers mainly take the evenly continuous shape with varied pores and unchanged roughness.The pore size tends to decrease gradually with the increase of salt concentration. In view of the molecular interaction, the aggregates tend to interact laterally through hydrophobic interaction, enhancing the tightness of the assembled films. At the same time, the higher NaBr concentration reduces the electrostatic repulsion between headgroups more effectively and, consequently, further strengthens the molecular aggregation,


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leading to the more compact film formed at silica-water surface, which is the key factor in designing the salt-responsive controlled release system or other nanomaterials on the basis of the evolution of surface morphology.

Figure 9. The AFM height image 10µm × 10µm for GnQPAMC12 molecules adsorbed onto the substrate of silica for 4 h at the concentration of 1×10-4 mol/L with (a) 0.05 mol/L NaBr, (b) 0.1 mol/L NaBr, (c) 1 mol/LNaBr.

4. CONCLUSIONS The adsorption kinetics and equilibrium of amphiphilic dendrimer GnQPAMC12 at silica-water interface and the self-assembled layer have been studied as a function of generation and


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concentration. The adsorption kinetics was divided into two steps: at concentration below cmc, individual molecules adsorption occurs; at concentration above cmc, aggregates and individual molecules adsorption occur simultaneously. And, flatten films with pores of all sizes and thickness of 3 ~ 4 nm were formed at silica-water surface. All the results available demonstrated that the adsorption was controlled by the balance of electrostatic and hydrophobic interactions. The addition of salt has significant effect on the morphology of G3QPAMC12 at silica-water interface, which is expected to provide guidance for designing salt-responsive controlled release system or other functional materials in future.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected]. Phone: 86-10-62523395. Fax: 86-10-62523395. *E-mail: [email protected]. Phone: 86-10-62652659. Fax: 86-10-62523395. Funding Sources The Important National Science and Technology Specific Project (2016ZX05025-003-008) of China. The National Natural Science Foundation of China (Project No. 21503238). Sinopec Group Major Project (G5800-16-25-WX003). ACKNOWLEDGMENTS The authors are thankful for the Important National Science and Technology Specific Project (2016ZX05025-003-008) of China. This work also received support from the financial support from the National Natural Science Foundation of China (Project No. 21503238) and Sinopec Group Major Project (G5800-16-25-WX003).


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Supporting Information Available: GnQPAMC12 synthesis, characterization data by 1H NMR.Dynamic Light Scattering (DLS) measurements are provided.This information is available free of charge via the Internet at http://pubs.acs.org/.

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TOC IMAGE


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Table of Content Title: Controllable Self-Assembly of Amphiphilic Dendrimers at Silica Surface: Effect of Molecular Topological Structure and Salinity Authors：Minghui Zhang, Jinben Wang*, Pei Zhang*

In the case of concentration below cmc, individual molecules can be adsorbed to the silica surface and rearrange into self-assembled structures, while aggregates directly adsorbed to the silica surface at concentration above cmc.


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Controllable Self-Assembly of Amphiphilic Dendrimers on a Silica

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