Colloids and Surfaces A: Physicochemical and Engineering Aspects

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1 Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: A study of adsorption of dodecylamine on quartz surface using quartz crystal microbalance with dissipation J. Kou a,b,d.tao a,,g.xu a a Department of Mining Engineering, University of Kentucky, Lexington, KY 40506, USA b School of Civil and Environment Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing , PR China article info abstract Article history: Received 23 April 2010 Received in revised form 13 July 2010 Accepted 15 July 2010 Available online 23 July 2010 Keywords: Adsorption Quartz QCM-D Dodecylamine hydrochloride Zeta-potential FTIR In this study the adsorption characteristics of dodecylamine hydrochloride (DACL) on quartz surface have been investigated using a high sensitivity surface characterization technique referred to as quartz crystal microbalance with dissipation (QCM-D) technique in conjunction with zeta-potential and FTIR analyses. The experimental results have demonstrated the versatility and accuracy of the QCM-D for surface adsorption characterization and for the first time revealed the changes in structure and orientation of the amine adsorption film on quartz surface during the adsorption process by real-time measurements of frequency and dissipation shifts with quartz coated sensor. Five distinct adsorption behaviors were identified from the different slopes of D f plots at different concentrations of DACL at ph 6 and 9.5. The physisorption, coadsorption of dodecylammonium and dodecylamine, and surface precipitation of neutral amine molecules with variable conformation and orientation were revealed on the quartz surface by FTIR and QCM-D. Physisorption of ammonium ion and coadsorption of dodecylammonium and dodecylamine dominated at concentrations <0.11 mm at ph 6 and 9.5 forming a rigid and thin adsorption layer. A compaction stage was present at ph 9.5 at concentrations lower than 1.13 mm. Surface precipitation of neutral molecules dominated at higher concentrations at ph 6 and 9.5 to form a thick but dissipated adsorption layer. The adsorption density was calculated with Sauerbrey equation and Voigt model and the results indicated the existence of a critical concentration of 0.45 mm at ph 6 and 1.13 mm at ph 9.5 which led to a significant increase in adsorption density and a structural change in adsorption layer Elsevier B.V. All rights reserved. 1. Introduction Long-chain primary alkylammonium salts are employed in the reverse flotation of silicates as well as in the flotation of quartz, dolomite and calcite from phosphate minerals [1]. Since the amount of surfactant adsorbed, the orientation of the adsorbed molecules, and the nature of the adsorbed layer have a major influence on the hydrophobicity of mineral surface, a good understanding of adsorption mechanisms of long-chain primary alkylammonium salt on quartz surface and properties of the adsorbed layer is crucial for a variety of industrial applications [2]. The mechanism of amine-silicate interaction has been studied extensively by indirect methods and ex situ measurements such as flotation recovery response [3], adsorption experiments [4,5], AFM [6], contact angle [7], zeta-potential [8,9] and FTIR spectroscopy [10 13] in the last several decades. Many concepts have been proposed based on these studies. Corresponding author. Tel.: ; fax: address: dtao@engr.uky.edu (D. Tao). Bijsterbosch [14] investigated the mechanism of cationic surfactant adsorption on silica surfaces by electrophoresis experiments. The results supported the theory of monolayer formation at low surfactant concentrations due to interaction of opposite charges on the silica surface and the surfactant ions. The formation of bilayers at high surfactant concentration on quartz surface was demonstrated by Menezes et al. [15] using contact angle and zeta-potential measurements, which indicated that the decrease in contact angles at concentration above the CMC can be attributed to the formation of bilayers. The adsorption mechanism of cationic surfactant dodecylpyridinium chloride on quartz surface was also delineated through measurements of adsorption isotherms, zeta-potentials, suspension stability, contact angles, induction times, and flotation response by Fuerstenau and Jia [9]. The results showed that dodecylpyridinium adsorption on quartz occurred in four distinct regions as the concentration of surfactant was increased. The adsorption was controlled primarily by electrostatic interactions at low concentrations, but at higher concentrations, adsorbed surfactant ions began to associate at the interface forming hemimicelles. At the CMC, the adsorbed surfactant ions existed as a bilayer or its equivalent, with the charged head groups oriented towards /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.colsurfa

2 76 J. Kou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) the solution phase. Churaev et al. [16] developed two models of Langmuir-type adsorption of cationic surfactant on quartz surfaces, including the adsorption of cations on both charged and neutral sites which formed the monolayer and the consecutive adsorption that first occurred on charged sites only, followed by adsorption on hydrophobic tails of pre-adsorbed ions, forming two layers or hemimicelles. Novich and Ring [3] investigated the flotation behavior and adsorption mechanism for the alkylamine-quartz flotation system. They demonstrated the existence of three adsorption regions in the adsorption isotherm. In the lower concentration range single ion adsorbed on the quartz surface by electrostatic effect and the adsorption density increased linearly with equilibrium concentration. At the equilibrium concentration of approximately 0.1 mm (neutral ph) the surface micelle formed with the flattened adsorption curves, which then abruptly steepened at an equilibrium concentration closed to the CMC and continued to rise linearly with the increase of concentration. Multilayer adsorption occurred at high surfactant concentrations. These results are consistent with G F model [7,17] which also postulated the formation of surface micelles and hemimicelle prior to monolayer coverage at HMC (hemimicelle concentration). The adsorption mechanisms of long-chain alkylamines and their acetate salts on quartz and mixed cationic and anionic collectors on feldspar were investigated by Vidyadhar and Rao [18,19] using Hallimond flotation, zeta-potential, FTIR, and X-ray photoelectron spectroscopy (XPS) at neutral and acidic ph. In differentiation to the electrostatic adsorption theory of G F model, their FTIR results exhibited the strong hydrogen bonds between the amine cations and surface silanol groups. The acetate counterions were found to influence the amine adsorption. The presence of neutral amine molecules together with protonated ammonium ions was also revealed by the XPS spectra above the critical hemimicelle concentration (CHC). Vidyadhar and Rao [18] also proved spectroscopically that long-chain alcohols were coadsorbed along with amine cations, which led to formation of a closely packed surface layer, as compared to the case of adsorption of pure amine alone at the same concentration. The correlation between solid surface free energy and flotation activity was verified by Chibowski and Holysz [20] with quartz conditioned with DACL dissolved in methanol. They concluded that at 0.25 statistical monolayer of DACL the sample of quartz floats in 90%, which can be interpreted by the drastic reduction in the polar component of quartz surface free energy, and the maximum flotation activity appears at one statistical monolayer (20 Å 2 was assumed for the molecule) of the amine. The hydrophobic attractive forces between mica surfaces in dodecylammonium chloride solution were reported by Yoon and Ravishankar [21]. At concentration of mm and ph 5.7 where the surfactant is in ionized form (dodecylammonium), only shortrange hydrophobic forces were observed, which can be attributed to the difficulty in forming close-packed monolayers on the mica surface. At ph 9.5 where neutral molecule of dodecylamine was formed as a result of hydrolysis long-range hydrophobic forces were observed, which may be attributed to the coadsorption of both dodecylammonium and dodecylamine, increasing the packing density of hydrocarbon chains on the surface. They also concluded that the ph at which the long-range hydrophobic force appeared corresponded to the maximum quartz flotation. McNamee et al. [22] investigated the adsorption of the cationic surfactant octadecyl trimethyl ammonium chloride (C 18 TAC) at low concentrations to negatively charged silica surfaces in water using atomic force microscopy (AFM). From the AFM images of silica surface, C 18 TAC was seen to form islands of bilayers and patches on mica at a concentration of 0.03 mm, which was identified as partial surfactant bilayers or hemimicelles. The electrostatic attraction between the bilayer islands of surfactant and bare areas of the substrate was observed at low surfactant concentrations. Most of these concepts about the adsorption of amines on silicate minerals were based on ex situ studies and indirect methods such as measurement of contact angle, zeta-potential, surface forces, and recovery response, which unfortunately cannot monitor the formation process of the adsorbed layer. The objective of the present study was to perform in situ investigation of the behavior of the adsorbed layer of dodecylamine on quartz surface by use of the quartz crystal microbalance with dissipation technique (QCM-D), which is capable of providing real-time information on the adsorption layer density, strength, and structural change under various process conditions. To our knowledge, this technique has not been employed previously for studying the adsorption of amine on quartz. Quartz crystal microbalance (QCM) measures the mass change per unit area by monitoring the change in frequency of a quartz crystal resonator. It offers an opportunity to study molecular interactions and conformation changes of adsorbed layer on many different types of surfaces with acute sensitivity [23]. QCM-D is the second generation of QCM, which cannot only determine the mass of surface bound layers, but also simultaneously give information about their structural (viscoelastic) properties based on the data of dissipation factor shift. A brief discussion of this technique is provided later in this paper. QCM-D has recently been used to study the viscoelastic properties of protein adsorbed on biosensors surface [24] and the adsorption behavior of surfactants and polymers from aqueous solutions [25 28]. In this work, in situ adsorption behavior of dodecylammonium onto the quartz coated sensor at various concentrations and ph was studied for the first time using QCM- D. The obtained data was fitted with Voigt model to get physical properties and mechanical properties of the adsorbed layer. 2. Materials and methods 2.1. Materials Dodecylamine hydrochloride (CH 3 (CH 2 ) 11 NH 2 HCl) and anhydrous ethanol (C 2 H 5 OH) with 99% and 99.5% purity, respectively were acquired from Acros Organics. Sodium hydroxide, which was purchased from Fisher Scientific, contained 99.8% NaOH. Deionized water was used throughout the experiments. The quartz coated sensor used in QCM-D analysis was an ATcut quartz disc (14 mm in diameter and 0.3 mm in thickness with an active sensor crystal area of 0.2 cm 2 ) with 5 nm Cr, 100 nm Au, 50 nm Ti and 50 nm SiO 2 sputter-coated onto the crystal surface successively. The sensors and Q-sense E4 system were supplied by Q-Sense Co. Zeta-potential and FTIR tests were conducted with quartz powder (50% <16.93 m, 90% <85.90 m) made of pulverized pure quartz crystals purchased from Ward s Natural Science. XRD analyses with the sample did not show any impurity Quartz crystal microbalance with dissipation All QCM-D measurements in this study were conducted at 25 C (±0.02 C). The stock solution was prepared by dissolving dodecylamine hydrochloride (DACL) in deionized water in the presence of ethanol. The solution ph was adjusted to 6.00 ± 0.05 and 9.50 ± 0.05 using 1% NaOH and 1% HCl solution. To ensure dissolution and degassing, the solutions were left in an ultrasonic bath for 5 10 min. For each experiment, the data generated with the solvent only (deionized water) was accepted as baseline when it became stable. Following this, the DACL solutions were injected into the flow module by a chemical feeding pump capable of precise control of

3 J. Kou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) flow rate. The flow rate in the experiment was kept at 0.50 ml/min. Prior to each test, the flow modules, crystals and tubings were cleaned with 2% Hellmanex II solution (an alkaline cleaning liquid from Hellma GmbH & Co. KG, Germany) for h and deionized water for 0.5 h. Then the crystal was removed and rinsed with more water or ethanol. After drying with nitrogen gas, the crystal was remounted into the module and deionized water was injected into the system. Software QTools 3.0 was used for data modeling and analysis. For a rigid, thin, uniform film the change in dissipation factor D < for 10 Hz frequency change [29] and the Sauerbrey equation (Eq. (1)) was used for the mass calculation: m = qt q f f 0 n = qv q f 2f 2 0 n = C f n (1) where the constant C has a value of 17.8 ng cm 2 Hz 1 and n is the harmonic number (when n =1,f 0 = 5 MHz). If the adsorbed film is soft (viscoelastic), it will not fully couple to the oscillation of the crystal [30], causing energy dissipation of the system. The dissipation factor D is proportional to the power dissipation in the oscillatory system (Eq. (2)) and can give valuable information about the rigidity of the adsorbed film [30]: D = E dissipated 2E stored (2) where E dissipated is the energy dissipated during one oscillation and E stored is the energy stored in the oscillating system [30]. Therefore, the changes in the property of adsorbed layer will result in the measured change in D. During the tests, QCM-D simultaneously measures the changes in resonance frequency f and dissipation D as a result of adsorption on a crystal surface. The quantitative information on the adsorbed layer on the mineral surface can be obtained by data modeling. When the adsorption caused greater shift in D value, i.e., D >1 10 6, as a result of the viscous and soft layer, Voigt modeling (Eqs. (3) and (4)) [28] was used: [ ] 1 3 ( 3 ) 2 j ω 2 f + h 2 0 h j j ω 2h j 0 ı 3 ı j=1,2 j j ω2 (3) [ ] 1 3 ( 3 ) 2 j ω 2 D + 2h 2f 0 h j 0 ı 3 ı (4) j=1,2 j j ω2 According to Voigt model for viscous adsorbed layer, f and D depend on the density (), thickness (h), elastic shear modulus () and shear viscosity () of adsorption layer (j: number of adsorbed layer). The Sauerbrey equation and Voigt model are the theoretical basis for data modeling using software QTools 3.0 (Q-Sense Co.) Zeta-potential measurements The zeta-potential measurements were made with Zeta-plus analyzer of Brook Haven Instruments Corporation. All experiments were conducted with DACL solutions with 1.0 mm KCl under ambient conditions. 1.0 g quartz powder was conditioned in 20 ml stock solution with a magnetic stirrer for 1 h. The mineral suspension was filtered using Whatman filter paper (pore size 25 m) and then poured into the rectangular cell for zeta-potential measurements FTIR analysis The infrared transmission spectra were recorded on a Thermo Nicolet Nexus 470 FTIR spectrometer. The quartz powders were conditioned with 20 ml DACL solution with ethanol at different Fig. 1. Real-time experimental data of frequency shifts (a) and dissipation shifts (b) for the third overtone (15 MHz) of QCM-D resonator for different concentrations of dodecylamine hydrochloride adsorption on quartz surface at ph 6. Arrow 1 indicates the injection of dodecylamine hydrochloride solution. ph s and different concentrations while being agitated with a magnetic stirrer for 0.5 h to make 1% suspension. The suspension was then filtered with Whatman filter paper (pore size 25 m) and the solids were air-dried overnight at the room temperature. The samples were prepared by dispersing g air-dried powder in 5 g KBr followed by pressing into a transparent tablet for scanning. The untreated (initial) quartz powder was used as reference. Each spectrum is the average of 250 scans. 3. Results 3.1. Adsorption behavior of DACL on quartz surface Fig. 1 displays the real-time experimental data of the frequency shift f (Fig. 1(a)) and dissipation shift D (Fig. 1(b)) from the third overtone (15 MHz) associated with DACL adsorption onto quartz surface at different concentrations at ph 6 (the same trend was observed at 0.11 and 6.76 mm as at 0.07 and 2.25 mm, respectively, and thus not shown in Fig. 1). Immediately after the injection of DACL at arrow 1, a gradual decrease in f (adsorption density increase) and a slight increase in D appeared with 0.07 and 0.11 mm DACL. At the steady state D was < and f was 2.5 Hz at 0.07 and 0.11 mm, which indicated the formation of thin and rigid adsorption film. After the injection of 0.45 mm DACL, f decreased gradually until it reached the steady state at about 5 Hz while D remained < , suggesting the formation of a thicker adsorption layer than at 0.11 and 0.07 mm DACL. At concentrations of 1.13, 2.25 and 6.76 mm, the injection of DACL caused an initial rapid decrease in f and a significant

4 78 J. Kou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) Fig. 3. Real-time experimental data of frequency and dissipation shifts for the third overtone (15 MHz) of QCM-D resonator for DACL adsorption and rinsing with deionized water from quartz surface. The straight lines are the f and D of 2.25 mm DACL at ph 6; the dash lines are the f and D of 1.13 mm DACL at ph 6; the dash-dotted curves are the f and D of 6.76 mm DACL at ph 9.5; the dotted curves are the f and D of 0.45 mm DACL at ph 9.5. Arrows 1 and 2 indicate the injection of dodecylamine hydrochloride solution and reagent-free water, respectively. Fig. 2. Real-time experimental data of frequency shifts (a) and dissipation shifts (b) for the third overtone (15 MHz) of QCM-D resonator for different concentrations of dodecylamine hydrochloride adsorption on quartz surface at ph 9.5. Arrow 1 indicates the injection of dodecylamine hydrochloride solution. increase in D followed by a gradual decrease in f and an increase in D which became stable after 1.7 h. At the steady state, f of 9, 12 and 17 Hz of was achieved with 1.13, 2.25 and 6.76 mm DACL solution, respectively and the D was higher than at these concentrations, revealing the formation of a thick but less rigid adsorption layer with adsorbed molecules not densely compacted. Fig. 2 shows the f and D as a function of time when the quartz surface was exposed to different dosages of DACL at ph 9.5 (the same trend was observed at 0.07 mm as at 0.11 mm and therefore is not shown in Fig. 2). The first observation was the gradual decrease in f and increase in D after the injection of 0.45 mm solutions (Fig. 2, arrow 1). The steady states of f and D were reached with 0.45 mm ph 9.5 solutions 1.2 h after the solution was injected, or 0.5 h earlier than with the same concentration at ph 6. The value of D smaller than indicates that the adsorbed layer was non-dissipative. The stable D values were almost the same as those observed with the adsorption at the same concentration of 0.45 mm at ph 6 but the f was much lower at ph 9.5, which suggests an increase in adsorption density with increasing solution ph value. After the injection of 1.13, 2.25 and 6.76 mm DACL solutions (Fig. 2a, arrow 1), a sharp decrease in f was observed to a steady state value of 13, 16 and 24 Hz, respectively, which were much lower and appeared much earlier than at ph 6. It should be noted that the steady state value of for D at 1.13 mm was much lower at ph 9.5 than produced at ph 6, suggesting that a thicker and more rigid adsorption layer was formed in 1.13 mm DACL solutions at ph 9.5 than at ph 6. The D s at 2.25 and 6.76 mm were also lower at ph 9.5 than at ph 6, but still higher than Fig. 3 shows the f and D as a function of time when the quartz surface was exposed to 1.13 and 2.25 mm DACL solutions at ph 6 and 0.45 and 6.76 mm DACL solutions at ph 9.5 at arrow 1 followed by rinsing with water of the same ph value but without DACL at arrow 2. It can be observed that after the injection of water at arrow 2, f and D at all these concentrations changed significantly. With a 0.45 mm solution at ph 9.5, the D and f were immediately back to zero after rinsing with water, implying that the desorption occurred readily at lower concentrations at ph 9.5. At concentrations of 1.13 and 2.25 mm at ph 6 and of 6.76 mm at ph 9.5, D changed to about with f increased to about 2 Hz, which corresponded to mol/cm 2 adsorption density. This adsorption density was much lower than the statistical monolayer, which was mol/cm 2 on the crystal sensor (the active sensor crystal area was 0.2 cm 2 and 25 Å 2 was assumed for the molecule) [3]. It can be concluded that even at higher ph or higher concentration, almost all adsorbed DACL can be washed off from the quartz surface Adsorption density of DACL The measured data of f and D from different overtone (from 3rd to 13th) was fitted using the Sauerbrey equation for D <10 6 or Voigt model for D >10 6. The calculated adsorption density at concentrations from 0.07 to 6.76 mm is shown in Fig. 4. It can be seen that the adsorption density increased with increasing concentration at both ph values. In ph 6 solutions the adsorption density was lower than mol/cm 2 at concentrations ranging from 0.07 to 0.45 mm. The adsorption density jumped to mol/cm 2 at 1.13 mm and continued to increase to about mol/cm 2 with increasing the concentration to 6.76 mm. A thicker adsorption layer appeared at all concentrations at ph 9.5 than at ph 6. For example, the adsorption density was mol/cm 2 and mol/cm 2 at ph 6 and 9.5, respectively in 1.13 mm solutions. The dissipation shift D was only with 1.13 mm DACL at ph 9.5, which was < observed at ph 6. It can be concluded that the adsorption layer was thicker and less dissipated at higher ph.

5 J. Kou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) Kinetics of DACL adsorption Fig. 4. Adsorption density for different concentrations of dodecylamine hydrochloride on quartz surface at ph 6 and 9.5. To characterize the adsorption behavior of DACL on quartz, the measured data of f and D from third overtone (15 MHz) in Figs. 1 and 2 were correlated in D f plots in Fig. 5. D f plots serve as the fingerprint of an adsorption process and provide the information on the energy dissipation per unit mass added to the crystal sensor [29]. The slope of the plots is defined as K (K = D/ f, absolute value) which is indicative of kinetic and structural alternation during adsorption process [29]. Small value of K indicates the formation of a compact and rigid layer, while a high value indicates a soft and dissipated layer [23,29]. Fig. 5 shows the D f plots for the adsorption of DACL on the quartz surface at (a) 0.11 mm at ph 6, (b) 2.25 mm at ph 6, (c) 0.45 mm at ph 9.5, and (d) 2.25 mm at ph 9.5. The data in Fig. 5 (a) shows only one phase (one slope) and Fig. 5(b) and (d) shows two distinct phases (two slopes) of kinetics whereas Fig. 5(c) shows three phases (three slopes). According to the previous studies conducted by Rodahl et al. [23], Hook et al. [31], and Paul et al. [29], a more rigid and compact adsorption mass is expected to yield a small K value and a soft and dissipated layer is associated with a higher K value. One slope only indicates a direct adhesion on the surface. The presence of multiple slopes suggests direct adhesion and orientational changes associated with hydrodynamically coupled water [29,23]. Data shown in Fig. 5 reveals that DACL adsorption took Fig. 5. D f plots for DACL adsorbed on the quartz surface at (a) 0.11 mm and ph 6; (b) 2.25 mm and ph 6; (c) 0.45 mm and ph 9.5; (d) 2.25 mm and ph 9.5.

6 80 J. Kou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) Table 1 Slopes in D f plots for adsorption of different concentrations of dodecylamine hydrochloride at ph 6 and 9.5 on quartz. Concentration of DACL (mm) ph 6 slope K ( 10 6 )Hz K 1 = K 1 = K 1 = K 1 = K 2 = K 2 = K 2 = K 2 = ph 9.5 slope K ( 10 6 )Hz 1 K 1 = K 1 = K 1 = K 1 = K 1 = K 1 = K 2 = K 2 = K 2 = K 3 = K 3 = K 3 = K 2 = K 2 = K 2 = place with constant kinetics at 0.11 mm in ph 6 solution. However, the adsorption in ph 9.5 solutions or in ph 6 solutions with DACL concentration higher than 0.11 mm resulted in changes in adsorption characteristics. Table 1 shows calculated K values for adsorption processes at various DACL concentrations and ph s represented in Fig. 4. Four distinct adsorption processes can be identified: (1) the adsorption layer was thin and rigid at lower DACL concentrations (0.07 and 0.11 mm) with only one K value of <0.15 at ph 6, which indicates the direct adhesion with no kinetic changes; (2) two slopes (K 1 > 0.15 > K 2 ) were observed with DACL concentrations of greater than 0.45 mm at ph 6 and 1.13 mm at ph 9.5, which indicates the formation of an initial flexible and water rich adsorption layer, followed by a structural and orientational changes as evidenced by a decrease in energy dissipation per unit adsorption mass K 1 values increased with increasing concentration of DACL at ph 6, as shown in Table 1, which indicates that the adsorption layer became more and more dissipated. (3) Three slopes (0.15 > K 1 K 3 > 0.01 > K 2 ) were observed with lower DACL concentrations (0.07, 0.11 and 0.45 mm) at ph 9.5, which shows the same slopes at the beginning and the end of the adsorption process separated by a stage with a much lower slope of <0.01. Small values of three slopes (<0.15) indicate the formation of compact and rigid layers as evidenced by D <10 6 and orientational changes during the adsorption. (4) At higher DACL concentrations of 2.25 and 6.67 mm and ph 9.5, two slopes (K 2 > 0.15 > K 1 ) were observed, indicating the rapid adhesion followed by conformational changes or water molecules trapped in the adsorption layer with low structural stability. Comparing the K values in Table 1 also shows that a smaller K 1 value was observed at ph 9.5 than at ph 6 with the same DACL concentrations, which indicates that although the initial adsorption of DACL on quartz was rapid at ph 9.5, as shown in Figs. 1 and 2, the adsorbed layer was more rigid and less dissipated than at ph 6. When the adsorption became stable, the adsorbed layer of DACL had higher adsorption density and lower D at ph 9.5 than at ph 6, which indicated better organized structure of DACL on the quartz surface at higher ph. These adsorption results can be used to explain why quartz flotation at ph 9.5 can achieve better performance than at ph 6 when amine is used as collector. According to Mielczarski et al. [32] the hydrophobicity of mineral is closely related to the structure of the adsorbed surfactant on the surface. The highly packed adsorption layer of surfactant imparts strong hydrophobicity to solid surface whereas a poorly organized structure does not. at negative sites on the quartz surface in the low-concentration region, oriented with the charged head toward the surface and the hydrocarbon tail into the solution, which accounted for the increase in zeta-potential. The QCM-D results also showed a low adsorption density of mol/cm 2 and only one slope of K 0.15 on the D f plot at 0.07 and 0.11 mm at ph 6. It can be concluded that a rigid and well-ordered monolayer was present on the quartz surface at a concentration lower than 0.36 mm at ph 6. A further increase in DACL concentration from 0.36 to 6.76 mm led to positively charged phase although the rate of increase in surface charge slowed with increasing DACL concentration. It can be postulated based on the G F model that after the monolayer coverage, a second layer forms with the charged amine heads toward the solution and the hydrocarbon tails oriented toward the surface. This is responsible for the positive zeta-potential at the beginning of the higher concentration region and agrees well with the QCM- D results that showed two slopes (K 1 > 0.15 > K 2 )onthe D f plot and significantly increased adsorption density (decreased f) at higher DACL concentrations at ph FTIR analysis Infrared transmission spectra of quartz after adsorption in DACL solution at different ph s and concentrations were investigated to confirm the adsorption mechanism of amine on the quartz surface. Fig. 7 shows the infrared transmission spectra of dodecylamine hydrochloride (DACL) as reference. The bands characteristic of alkyl chains for dodecylamine are identified at 2950, 2920, and 2850 cm 1, which were assigned to asymmetric stretching vibration in the CH 3 ( as (CH 3 )) and CH 2 radical ( as (CH 2 )) and symmetric stretching vibration in the CH 2 radical ( s (CH 2 )) [18], respectively. The broad band at 3000 cm 1 was observed, which was assigned to as (NH 3 + ) and s (NH 3 + ) [18] Zeta-potential measurement To better understand the adsorption mechanism of DACL in weakly acidic solutions, the relationship between the initial DACL concentration and the zeta-potential of quartz particles at ph 6 is shown in Fig. 6. It is evident that increasing the concentration to about 0.36 mm sharply increased the surface charge of quartz from 110 to 0 mv. Novich and Ring [3] suggested that the quartz surface will be negatively charged if the suspension ph is above the point of zero charge (ph 2.0) and cationic amine surfactant will absorb when it is placed in solution with the negatively charged quartz surface. According to G F model, alkylammonium ions specifically adsorb Fig. 6. Zeta-potential of quartz conditioned with DACL at constant ionic strength as a function of concentration.

7 J. Kou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) Fig. 7. Infrared transmission spectra of DACL. Curves a and b in Fig. 8A show the absorption bands ranging from 2750 to 3050 cm 1 in the FTIR spectra of the species adsorbed on quartz surface from the solutions of 4.5 mm DACL at ph 6 and 9.5 and curves c and d show the same absorption bands with 0.45 mm DACL solution at ph 6 and 9.5, respectively. Curve e shows the adsorption bands with quartz powder conditioned with ethanol Fig. 8. Infrared transmission spectra of quartz after adsorption of DACL at different concentrations and phs for (A) alkyl chain region cm 1 and (B) (OH) group and amine molecule region cm 1 (a) 4.5 mm, ph 9.5; (b) 4.5 mm, ph 6; (c) 0.45 mm, ph 9.5; (d) 0.45 mm, ph 6; (e) quartz powder with ethanol. Fig. 9. Infrared transmission spectra of quartz after adsorption of DACL at different concentrations and phs for the alkyl chain region cm 1 (a) 4.5 mm, ph 9.5; (b) 4.5 mm, ph 6; (c) 0.45 mm, ph 9.5; (d) 0.45 mm, ph 6; (e) quartz powder with ethanol; (f) DACL. solution as reference. Curves a d show the absorption peaks at 2950, 2920, and 2850 cm 1, which are undoubtedly caused by the adsorption of alkyl chain on the quartz surface since they are consistent with DACL spectra shown in Fig. 7. In comparison with curve e, the intensities of these bands in Fig. 8 were found to increase with increasing DACL concentration, which is in agreement with QCM-D measurements and zeta-potential results. According to Vidyadhar et al. [2], the amine molecule in the bulk phase in neutral and protonated forms can be identified with the appearance of the band at 3333 cm 1 and the alcohol spectrum shows a broad band around 3318 cm 1 characteristic of (OH) group. Fig. 8B (curves a e) shows the spectra in the region from 3300 to 3350 cm 1 of quartz treated with DACL solution under different conditions and with ethanol solution. The broad band on the spectra indicated the existence of H-bond of alcohol (OH) group on quartz surface at all concentrations and ph s as well as at the higher concentration of amine. The DACL spectrum in Fig. 7 also displays several bands in the region of cm 1. The band at cm 1 is often assigned to the methylene scissoring band ı(ch 2 ) on the quartz surface. But quartz also exhibits strong absorption in the same region and thus it is hard to identify these bands under mono- or submonolayer adsorption [18]. Fig. 9 shows the infrared transmission spectra for the adsorption bands in the region from 1400 to 1800 cm 1.It was observed that the absorption of DACL gave rise to characteristic absorption bands in the region of cm 1 on quartz surface and the intensity of these absorption peaks increased with increasing concentration. The FTIR results did not show significant effects of ph on the intensities of absorption peaks, which is inconsistent with flotation performance of amine at different ph s. In contrast the QCM-D results clearly showed differences in adsorption behavior at different ph s. For example, the D f plot shows K 1 > K 2 at high DACL concentrations at ph 6 and K 1 < K 2 at ph 9.5. According to Sirkeci [33] the coadsorption of ionic and molecular species is responsible for quartz flotation with dodecylamine at high phs. Sirkeci [33] further claimed that ph 9.3 represents the equilibrium between ionized and molecular amine species. Vieira and Peres [34] also reported that the most favorable ph for quartz flotation was ph 9.0

8 82 J. Kou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) for both ether monoamine and ether diamine. In good agreement with previous studies QCM-D results have shown the significant increase in adsorption density and adsorption layer stability as a result of surface precipitation of molecular amine at ph 9.5 on the quartz. 4. Discussion The previous studies on the adsorption of dodecylamine on quartz surface have revealed three adsorption mechanisms: (1) single ion adsorption at lower concentration; (2) surface micelle formation at intermediate concentration; and (3) multilayer adsorption with molecular amine precipitation at higher concentration [3]. The present QCM-D study provided for the first time the real-time adsorption data and the kinetic D f plots displayed five distinguishable adsorption stages: (1) At lower concentrations (0.07 and 0.11 mm) at ph 6, a slight decrease in f and a small D value of < with only one slope K < 0.15 was observed, which indicates the adsorption process formed a thin but rigid adsorption layer. The species distribution diagram developed by Novich and Ring [3] for 0.04 mm dodecylamine solution at different ph s suggests that cationic amine (RNH 3 + ) is the dominant species in ph 6 solution. According to Vidyadhar and Rao [18], at ph from 2 to 7, where the surface potential of silicate is negative, ammonium ions undergo physisorption and are electrostatically held at the silicate water interface much below the critical micelle concentration (CMC, 10 mm at ph 6), which is in agreement with zeta-potential measurement results. Novich and Ring [3] reported an adsorption density of mol/cm 2 at first adsorption step, in agreement with QCM-D results which showed mol/cm 2 adsorption density on quartz surface at concentrations of 0.07 and 0.11 mm. (2) Three slopes (0.15 > K 1 K 3 > 0.01 > K 2 ) were observed at lower DACL concentrations at ph 9.5, which revealed different adsorption processes from at ph 6. According to the species distribution diagram, the concentration of cationic amine starts to decrease and a small amount of RNH 2 is observed in the 0.04 mm solution at ph 9.5. Yoon and Ravishankar [21] indicated that neutral molecules of dodecylamine were coadsorbed with dodecylammonium at low concentrations at ph 9.5, which may increase the packing density of hydrocarbon chains on the surface and form a higher adsorption density but lower dissipative adsorption layer. The existence of one small slope between two higher slopes may indicate that the orientational changes and layer compression step resulted in more rigid adsorption layer. (3) It should be mentioned that the QCM-D results show different adsorption behavior at concentrations of 0.45 mm at ph 6 and 1.13 mm at ph 9.5. Although the D f plot displayed two slopes and higher adsorption density at 0.45 mm and ph 6 and 1.13 mm at ph 9.5, the dissipation shift D was < These two adsorption conditions produced higher adsorption density than at concentration <0.45 mm, but lower dissipation shift than at concentration >1.13 mm. After the monolayer coverage the formation of second layer with the hydrocarbon tails oriented toward the surface and the charged amine heads toward the solution accounted for the charge reversal of zetapotential (0.36 mm at ph 6 as shown in Fig. 6). According to QCM-D results, the critical concentration for hemimicelle formation increased with increasing ph from 6 to 9.5 and the hemimicelle formed at this critical concentration was rigid and well-ordered. (4) At DACL concentrations higher than 0.45 mm at ph 6, a significant decrease in f happened simultaneously with a sharp increase in D. Two slopes (K 1 > 0.15 > K 2 ) were observed on D f plots, suggesting the formation of a thick, dissipated and less rigid structure followed by a compaction stage on the quartz surface caused by surface precipitation. According to Chernyshova et al. [8] the adsorbed film on quartz includes both neutral and protonated amine H-bonded to the surface silanols at higher concentration, which is in agreement with FTIR results shown in Fig. 8 that also showed adsorption peak intensity higher than at lower concentration and without DACL. (5) At concentrations higher than 1.13 mm and ph 9.5, a dramatic decrease in f and a significant increase in D were observed along with two slopes (K 2 > 0.15 > K 1 )on D f plots, which suggests the formation of a rigid structure followed by a surface precipitation that resulted in higher adsorption density. This result is consistent with the theory proposed by Novich and Ring [3] that there is a multilayer of adsorbates held together by Van de Waals force associated with hydrocarbon chains. McNamee et al. [35] also reported the Van der Waals attraction between the cationic surfactant and negatively charged silica surface based on AFM and zeta-potential measurements. The poorly organized multilayer structure is reflected in high dissipation shift D and low resonance frequency change f observed in the present study. The DACL adsorption density in 2.25 mm solution was estimated to be mol/cm 2 at ph 9.5, which is very close to the value of mol/cm 2 Novich and Ring [3] reported as a result of multilayer adsorption. 5. Conclusions In situ measurements of dodecylamine adsorption on quartz surface at different concentrations and ph s were conducted using the QCM-D technique to investigate the adsorption density and kinetics and the structural properties of the adsorption layer. The following major conclusions were derived from this study: (1) Three adsorption mechanisms, i.e., physisorption, formation of hemimicelle and surface precipitation of neutral molecule, were revealed by the QCM-D technique combined with Zetapotential measurements and FTIR analysis for the adsorption of dodecylamine on quartz surface. At low concentrations and ph 6, physisorption of ammonium ion formed a rigid and thin adsorption layer, resulting in a small change in both f and D. The coadsorption of both dodecylammonium and dodecylamine were observed in ph 9.5 solutions, which formed a thicker but more rigid layer than at ph 6. Surface hemimicelle with a higher density and a lower dissipation was formed at intermediate concentrations, and the critical concentration for hemimicelle formation increased from 0.45 to 1.13 mm with increasing ph from 6 to 9.5. Surface precipitation of neutral amine molecules dominated at higher concentrations and caused a rapid decrease in f and increase in D, which indicated the formation of a thick but soft and dissipated adsorption layer. (2) Different adsorption behaviors can be readily derived from the different slopes of D f plots. At lower concentrations the adsorption layer was thin and rigid with only one small slope (K < 0.15) at ph 6 and three small slopes (0.15 > K 1 K 3 > 0.01 > K 2 ) at ph 9.5. At higher concentrations two slopes were observed with K 1 > 0.15 > K 2 at ph 6 and K 2 > 0.15 > K 1 at ph 9.5, which indicated the formation of higher adsorption density and more dissipated multilayer structure. A critical concentration, i.e., 0.45 mm at ph 6, and 1.13 mm at ph

9 J. Kou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) , was observed which indicated the occurrence of surface hemimicelle with higher adsorption density and lower dissipation. (3) Adsorption densities under different solution conditions were calculated with Sauerbrey equation and Voigt model. At concentrations 0.45 mm at ph 6 and 0.11 mm at ph 9.5 the adsorption density was close to the theoretical monolayer coverage of mol/cm 2 ; the adsorption density increased significantly at concentrations higher than 0.45 mm at both ph s. (4) Zeta-potential and FTIR studies showed that the DACL adsorption on quartz involved both physisorption as a result of the electrostatic interaction at lower concentrations and surface precipitation of amine molecules on quartz surface at higher concentrations, which helped explain the different adsorption processes observed in QCM-D studies. References [1] H. Sis, S. 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