Interactions between aminated cellulose nanocrystals and quartz: Adsorption and wettability studies

Colloids and Surfaces A: Physicochem. Eng. Aspects 489 (2016) 207–215

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Interactions between aminated cellulose nanocrystals and quartz: Adsorption and wettability studies Robert Hartmann a , Juho Antti Sirviö a , Rafal Sliz b , Ossi Laitinen a , Henrikki Liimatainen a,∗ , Ari Ämmälä a , Tapio Fabritius b , Mirja Illikainen a a b

Fibre and Particle Engineering, University of Oulu, FI-90570, Finland Optoelectronics and Measurement Techniques Laboratory, University of Oulu, FI-90570, Finland

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Adsorption of CNCs on quartz. • Hypothesis of different orientations of CNCs on the quartz surface.

• Comparison of the specific surface free energies of rough and smooth quartz surfaces coated with CNCs.

a r t i c l e

i n f o

Article history: Received 24 August 2015 Received in revised form 12 October 2015 Accepted 13 October 2015 Available online 30 October 2015 Keywords: Cellulose nanocrystals Quartz Specific surface charge Adsorption Wettability

a b s t r a c t Cellulose nanocrystals (CNCs) are potential high-performance, biodegradable, and environmentallyfriendly alternatives to the oil-derived chemicals currently used in mineral processing. For this purpose, the understanding of phenomena associated with interactions between CNCs and minerals is crucial. In the present study, aminated CNCs with varying crystal sizes and crystal size distributions were examined in terms of their adsorption properties on quartz and the wetting properties of quartz surfaces after CNC adsorption. CNCs with varying alkyl chain lengths were obtained from consequent periodate oxidation and reductive amination, followed by mechanical high-intensity homogenisation. The adsorption of CNCs was studied based on polyelectrolyte titration. An enhanced interaction between CNCs and the quartz surface, including decreasing CNC surface charges, was obtained. Subsequently, wetting studies of CNCcoated quartz were performed using the sessile drop method. The specific surface free energies of both the rough and smooth surfaces were obtained and compared to determine the effects of roughness on wetting properties. The results showed that smooth surfaces possess a low specific surface free energy due to the absence of surface heterogeneities. For rough surfaces, the specific surface free energy was decreased as the extended alkyl chains of adsorbed CNCs modified the wetting properties of the surface. © 2015 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. E-mail address: henrikki.liimatainen@oulu.fi (H. Liimatainen). http://dx.doi.org/10.1016/j.colsurfa.2015.10.022 0927-7757/© 2015 Elsevier B.V. All rights reserved.

Compared to oil-derived chemicals, bio-based chemicals have advantages such as renewability, biodegradability and low toxicity

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[1]. Cellulose, the most abundant organic polymer on earth derived from plants, bacteria, and tunicates, has been considered as one of the most attractive raw materials for biochemicals [2]. Recently, researches have become particularly interested in nanosized cellulose particles such as cellulose nanocrystals (CNCs) and nanofibrils (CNFs), which have very high specific surface areas, high aspect ratios, and versatile chemical modification routes [3]. Biochemicals have considerable potential as additives in mineral processing, but the use of nanocelluloses as additives (e.g., flotation and flocculation aids) requires modified and selective functionalities [4]. Modifying the properties of nanocellulose properties for a given process or for particular minerals is challenging since interactions between nanocelluloses and minerals are not completely understood. A fundamental understanding of nanocellulose interactions and adsorption on mineral surfaces is required to develop nanocellulose biochemicals whose properties and performance are comparable to synthetic chemicals [5]. Several chemical methods can be used to modify the surface characteristics of nanocelluloses. One potential method is periodate oxidation to oxidise the vicinal hydroxyl groups of cellulose at positions 2 and 3, forming aldehyde groups and simultaneously breaking the corresponding carbon–carbon bond of the glucopyranose ring to form 2,3-dialdehyde cellulose (DAC) [6]. The aldehyde groups of DAC, in turn, exhibit high reactivity toward further modification and functional groups that have a tailored hydrophobicity or hydrophilicity, anionicity or cationicity, or a special functionality can be selectively introduced into the cellulose fibres. In the present work, cellulose nanocrystals were functionalized with variable alkyl chains through consequent periodate oxidation and reductive amination, followed by individualisation of the CNCs through a mechanical homogenisation process. The interaction of CNCs with quartz surfaces was analyzed by measuring the CNC adsorption on quartz in water based on the protonation phenomena of CNCs (see Fig. 1) [7]. To investigate the changes in the surface-wetting properties of CNC-coated quartz pellets, contact angle measurements were performed [8]. 2. Materials and methods 2.1. Preparation and characterisation of quartz Quartz from the deposit of Nilsiä, Finland, was delivered by Sibelco. The quartz was comminuted using a Retsch ball mill (PM 200) by adding 50 g of quartz with seven stainless steel grinding balls of 20 mm in diameter per jar and using a grinding time of 3 min at 350 rpm. The product was classified by using the e200 LS air jet sieve (Hosokawa Alpine, Germany) with mesh sizes of 20 ␮m, 45 ␮m, 63 ␮m, and 125 ␮m. The quartz remaining on top of the 125 ␮m sieve was refilled and ground again with the same settings described previously. The quartz that passed through the 20 ␮m sieve was collected and further milled for 15 min at 500 rpm to obtain submicron particles. The particle size of the quartz was determined using a Beckman Coulter LS 320 particle size analyser (shown in electronic supplements). The specific surface area of the quartz was determined using an ASAP 2020 N2 physisorption analyser. Table 1 shows the specific surface area of the quartz fractions obtained using the Brunauer–Emmett–Teller (BET) method. The high values of the specific surface area indicate that the number

of fine particles was high in each fraction. In particular, the Sauter mean diameter reveals that a significant number of fine particles were included in each fraction. The particle size fraction between 20 and 45 ␮m was used to measure the adsorption of CNCs on quartz, and the smallest fraction was used to prepare the quartz pellets for contact angle measurements. The chemical composition of the quartz was determined using an X-ray fluorescence spectrometer (XRF, PANalytical AxiosmAX ), as shown in Table 2. According to the elemental analysis, the original quartz sample mainly consisted of SiO2 without significant impurities. 2.2. Preparation of modified cellulose nanocrystals (CNCs) Bleached kraft hardwood (Betula pendula) pulp was used as the cellulose raw material for the synthesis of modified CNCs using consequent periodate oxidation and reductive amination reactions. The chemicals used in the periodate oxidation were sodium (meta) periodate, NaIO4 (> 99.0%, India), and lithium chloride, LiCl (> 98.0%, Germany), which were obtained from Sigma–Aldrich. For the amination of DAC, the following chemicals were used without any further purification: 2-picoline borane (95.0%, USA) purchased from Sigma–Aldrich, and 3 amines with variable alkyl groups including methylamine (>98.0%, Belgium), n-butylamine (>98.0%, Belgium), and n-hexylamine hydrochloride (>98.0%, Belgium) obtained from TCI. For washing the oxidized pulp, ethanol (95.0%, Finland) was purchased from VWR. The synthesis of CNCs was performed in accordance with previous studies [9]. The cellulose pulp fibres were converted to DAC with LiCl-assisted NaIO4 oxidation conducted for 3 h at 75 ◦ C. After oxidation, DAC with 3.86 mmol g−1 of aldehydes was obtained. A 10-fold excess of methyl-, n-butyl-, or n-hexylamine hydrochloride in relation to the aldehyde groups of DAC was mixed with demineralized water (300 ml) and the pH of the solution was set to 4.5 using diluted HCl solution (Merck). The DAC pulp (4 g) and a 2-fold excess of 2-picoline borane were added to the suspension based on the assessed amount of aldehyde groups, and the reaction was continued for 72 h under stirring in a closed container at ambient room temperature. The reaction was stopped by removing the active chemicals from the solution using vacuum filtration. The product was washed in three steps. First, the filter cake was washed with 200 ml of demineralized water. The product was removed and stirred in 300 ml ethanol for 5 min. The suspension was vacuum filtrated, and then washed with 600 ml of demineralized water. The washed pulp was diluted (0.4 wt%) and individualized to nanocrystals from the aminated cellulose fibres using a Microfluidics two-chamber high-shear homogeniser (M-110EH-30 Microfluidizer, USA). The diluted fibre suspensions were passed once through the 400 ␮m and 200 ␮m chambers at a pressure of 1300 bar, once through the 400 ␮m and 100 ␮m chambers at a pressure of 2000 bar, and once through the 200 ␮m and 87 ␮m chambers at a pressure of 2000 bar to obtain clear, non-viscous CNC suspensions. The CNCs were analyzed using diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) Table 2 Chemical composition of the pure quartz and the smallest fraction of quartz determined by XRF. Components (%)

Table 1 Specific surface area of quartz fractions and Sauter mean diameter according to BET analysis. Size fraction

45–63 ␮m

20–45 ␮m

Submicron fraction

BET in m2 /g Sauter mean diameter in ␮m

0.28 8.08

0.64 3.51

13.5 0.18

SiO2

MgO

Al2 O3

P2 O5

SO3

Cl

Original x < 1 ␮m

98.283 96.055

0.068 –

0.563 0.378

0.017 0.012

0.048 0.022

0.021 0.043

Original x < 1 ␮m

K2 O 0.091 0.068

CaO 0.612 0.03

TiO2 0.017 0.01

Fe2 O3 0.162 2.885

CuO 0.058 0.011

BaO 0.007 –

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209

Fig. 1. Left: neutral CNC at a medium pH level. Right: positively charged CNC due to protonation at a low pH level.

with a Bruker Vortex 80 v spectrometer (USA) to obtain the spectra in the 600–4000 cm−1 range. For each sample, 40 scans were taken at a resolution of 2 cm−1 . The number of attached functional groups was measured by determining the nitrogen content of the product using a PerkinElmer CHNS/O 2400 Series II elemental analyser. The modified cellulose nanocrystal samples were designated as methylamine-CNC (MAC), butylamine-CNC (BAC), and hexylamine-CNC (HAC).

before and after the pH value was adjusted to between 5 and 9 using diluted HCl and NaOH solutions (Merck). Afterwards, the suspension stood still for 30 min. Last, 50 ml of supernatant was collected, and 10 ml of the supernatant was titrated using 5 duplicates. All of the preparation steps were conducted at ambient temperatures. In acidic solution, the protonation reaction of amine group of CNCs is the following:

2.3. Transmission electron microscopy (TEM)

where NH2 -R represents the alkyl amine group of the CNC and NH+3 -R represents the protonated one [10]. The relationship between the total amount of alkyl amine groups and the protonated ones is given by the protonation degree P in Eq. (2):

To investigate the size and morphology of CNCs using TEM (Tecnai G2 Spirit transmission electron microscope, FEI Europe, Eindhoven, the Netherlands), the experiment used carbon-coated and glow-discharged hexagonal 200 mesh copper grids (Electron Microscopy Science, USA) and uranyl acetate (98.0%, USA) purchased by Polysciences, Inc. diluted to 2 wt% with demineralized water. Samples were prepared from CNC suspensions by diluting them with Milli-Q water to obtain 10 ppm CNC content. A small droplet of the dilution was dosed on top of the Butvar-coated copper grid. Excess samples were removed from the copper grid by touching the droplet with filter paper. The sample was negatively stained by placing a droplet of uranyl acetate (2 wt%) on top of each specimen. Grids were dried at ambient room temperature and analyzed at 100 kV under standard conditions. Images were captured with a Quemesa CCD camera, and iTEM image analysis software (Olympus Soft Image Solutions GmBH, Münster, Germany) was used to measure individual CNC widths and lengths. In total, 50 individual crystals of each CNC sample were analyzed. 2.4. Determination of the surface charge distribution by polyelectrolyte titration The surface charge was determined for quartz, 3 different amino-CNCs, and quartz-CNC mixtures with a pH ranging from 5 to 9. For the measurements, a particle charge detector (Mütek, PCD 03, Germany) was used by employing a titration with charge-compensating polyelectrolytes. The titrant for the negative surface charge was cationic polyelectrolyte poly-(diallyl-dimethylammonium chloride) (DADMAC, BTG Mütek GmBH, Germany), and for the positive surface charge, the titrant used was anionic polyelectrolyte poly-(sodium-polyethylene-sulfonate) (PES-Na, BTG Mütek GmBH, Germany). Samples were prepared from CNCs by diluting 6 ml of the 0.1 wt% CNC suspension with 54 ml of demineralized water. The conductivity of the demineralized water was 52.1 ␮S/cm, guaranteeing that the interactions would occur solely between quartz and the CNCs. For the quartz-CNC mixtures, different amounts of quartz were added, representing 25, 50, 75, and 100% of the absolute surface area of the CNCs obtained from TEM images and BET analysis, assuming that only one third of the absolute CNC surface can be in contact with a solid surface. The CNC-quartz suspension was dispersed using ultrasonic treatment for 60 s. Next, the suspension was shaken by hand for one minute

H3 O+ +NH2 -R = NH+3 -R+H2 O

P=

CNH+ -R 3

(1)

(2)

(CNH2 -R +CNH+ -R ) 3

The combination of the equilibrium constant pKa of Eq. (1) and the degree of protonation leads to Eqs. (3) and (4):



pKa = pH-log pKa = pH-log



CNH2 -R



CNH+ -R 3

(1-P) P

(3)

 (4)

The pKa value is essential for knowing the number of charges at a given pH value, and thus is important for rationalising the interaction between CNCs. The energy required to protonate an amine group of CNCs is determined by the pKa value. The pK value determines the standard free energy of protonation derived in the following:  Gprot = RT ln10 (pH − pKa )

(5)

where R is the universal gas constant and T is the absolute temperature [11]. According to Eq. (5), the energy which is necessary to protonate or deprotonate an individual amine group at a given pH can be examined. 2.5. Contact angle measurements of CNC coated quartz pellets The contact angles of CNC-coated quartz pellets in air were measured using water, ethylene glycol (100%, France) purchased from VWR, glycerol (>99.5%, Malaysia) from Sigma–Aldrich, diiodomethane (>98.0%, Belgium) from TCI, and ␣-bromonaphthalene (97.0%, France) from Sigma–Aldrich. Quartz powder (x < 5 ␮m) was pressed for two minutes under 80 kN to obtain a pellet with a diameter of 2 cm. The roughness of the quartz pellets was determined using an optical profilometer from Bruker Contur GTK-OX (Germany). The quartz pellets were covered with CNCs by adding 3 ml of diluted CNC suspension (3 wt%) on top of the pellets, and then they were dried overnight under ambient conditions. Contact angle measurements were taken using a Krüss DSA100 sessile drop system (Germany). The instrument was equipped with

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a high-speed camera (360 fps) and analysis software. The contact angle was determined immediately after the drop formed on the solid surface. A drop volume of 1.5 ␮l was used to avoid gravitational distortion. Contact angles were analyzed using the circle-fitting method, which refers to a complete droplet shape to measure the static contact angle. For each sample, three droplets placed in different locations on the sample were investigated, and the results were averaged. For the evaluation of the surface free energy, the Owens, Wendt, Rabel and Kälble (OWRK) model was applied, which places the disp persive (␥ds ) and polar (␥s ) surface free energy of solids in relation p d to the dispersive (␴l ) and polar (␴l ) interfacial energy of different liquids according to Eq. (6): (1+cos) × ␴lg 2×



 =

␴dlg



p ␥sg



p

␴lg ␴dlg

+

␥dsg

(6)

The indices used represent the physical states of the material (s = solid, l = liquid, and g = gas), and the combination of two indices depicts the interfaces between two states. The dispersive and polar surface free energy of the solid state was determined using a linear regression by plotting the equation in a coordinate system [12]. The components of the dispersive, specific, and total surface tensions of examined liquids are given in the literature [13,14]. To evaluate the effects of roughness on the contact angle, the roughness factor r after Wenzel was used, which is given by [15]: r=

actual surface geometric surface

The geometric surface represents the ideal smooth solid surface, and the actual surface is the total solid surface with roughness taken into account. For the estimation of the actual surface, the assumption was that the particles are present as hemispheres on the solid surface and ordered in the closest possible package. The mean roughness was used as a radius of the hemispheres. A roughness factor of approximately 1.74 was obtained and the contact angle, , was related to the apparent contact angle, app , by: cosapp = r × cos The error bar was obtained by applying a linear fit using OriginPro 2015 software (OriginLab Corporation). 3. Results and discussion 3.1. Morphology and size distribution of CNCs The morphological properties of aminated CNCs were determined using TEM. One example of a TEM image for each CNC is shown in Fig. 2. The nanoscale CNCs had a lateral dimension, and they displayed a high uniformity of size and shape without visible aggregation. Despite the high dilution of the sample, the CNCs formed a network, which made it difficult to assess the length and width of the two-dimensional image. In general, the CNCs exhibited a uniform rod-like or cylindrical shape [16]. The white artefacts are uranyl acetate free spots due to air bubbles or to uneven spreading of uranyl acetate over the sample. The numerical characterisation of 50 individual particles is summarized in Table 3. All CNCs possessed similar morphological dimensions when the confidence intervals of the results were taken into account. However, the tendency to form network structures with extended alkyl chain lengths complicated the evaluation of liberated individual nanocrystals, and, on the other hand, indicated a higher affinity between the nanocrystals. For further calculations, the average surface area representing 150 individual CNCs without including the alkyl group of the amine was used.

Table 3 Length and width of CNCs with associated confidence interval t (␣;N-1) (˛ = 95%) and the specific surface area according to the assumption of the cylindrical shape of CNCs.

Length in nm Width in nm Mean specific surface area in m2 /g

MAC

BAC

HAC

142 ± 11 6.0 ± 0.3 538.7

125 ± 7 3.3 ± 0.2 937.6

144 ± 14 4.6 ± 0.3 789.1

The number of introduced aminated alkyl chains on CNCs was analyzed by determining the nitrogen content of the samples (Table 4). The number of amine groups varied depending on the alkyl chain: HAC possessed the most groups and BAC the least groups. 3.2. Surface charge distribution and adsorption of CNCs The interaction of solid surfaces and soluble compounds or particles is strongly attributed to their surface charge properties. Therefore, the surface charge distributions of CNC, quartz, and the supernatant of CNC-quartz suspensions were determined. The specific surface charge distributions of the pure substances are shown in Fig. 3. The determination of the specific surface charge of solid states is based on two primary assumptions. First, all polycations react completely with polyanions and vice versa, which means that uncomplexed cationic or anionic groups do not coexist. This assumption can be accepted due to the very high effective binding constants of the polyelectrolyte complex formation. The second assumption relies on the adsorption of the species of interest on the fluorocarbon surface of the surface charge detector. Based on this, an electrochemical double layer is formed and compensated through the addition of a titrant solution until the isoelectric point is reached [17]. The determination of the potential at the Stern layer in regard to measuring the surface charge enables the detection of absolute and reproducible values [18]. The protonation of the amine groups of CNCs occurred predominantly at low pH values, leading to specific surface charges of 43.0 C/g, 45.8 C/g, and 57.2 C/g for MAC, BAC, and HAC at a pH of 5, respectively. This sequence correlated to the degree of substitution of amino-modified celluloses (see Table 4). HAC possessed the highest specific surface charge at a low pH and the strongest decrease of the specific surface charge as the pH value increased. The isoelectric point of HAC was obtained at pH 9. The results indicated that the CNCs remained liberated over a period of 30 min and did not form precipitating aggregates. The specific surface charge of quartz was slightly negative, as previously reported in the literature, but the magnitude of the charge was markedly smaller than for the CNCs (−2.3 mC/g at pH 9) [19]. A slight increase of the specific surface charge as the pH value increased was detected. The opposite surface charges of the CNCs and quartz led to the assumption that attractive electrostatic interactions occurred, and thus an assumption of the existence of adsorption phenomena between the species. To understand the influence of electrostatic interactions on adsorption phenomena between CNCs and the quartz surface, the degree of protonation of the amine groups of CNCs with differing Table 4 Elemental composition and number of alkyl amine groups of CNCs. Sample

MAC BAC HAC

Elemental composition (%)

Amount of alkyl amine groups (mmol/g)

C

H

N

41.5 42.8 40.2

5.9 6.1 8.1

0.87 0.70 1.24

0.62 0.50 0.88

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Fig. 2. TEM images of cellulose nanocrystals (a) MAC, (b) BAC, and (c) HAC.

alkyl chain lengths, according to Eq. (2) and the pKa value, according to Eq. (4), were examined (shown in electronic supplements). To obtain the pKa value, it is common to determine the pH at which the degree of protonation is 50% (stated as pK50 a value). The pKa values of MAC and BAC were similar, and HAC possessed a lower pKa value than MAC and BAC over the examined pH range. Since the amine group acts as a Brønsted base, a higher pKa value corresponds to a weaker acid, and thus to a stronger base [20]. MAC was regarded as the strongest base, BAC possessed similar acid and base properties,

and HAC represented the weakest base. In contrast to the liberated amines in solution (pKa : methylamine = 10.67, butylamine = 10.56, cyclohexylamine = 10.49), the pKa value was not constant over the examined pH range, which suggests a need for further studies of the pKa value of complex organic molecules [11,21]. CNCs with longer alkyl chains possessed the inherent ability to be specifically oriented (e.g., elongated or tightly coiled) on the mineral surface, which increased the required energy for protonation of the amine group [22]. Based on the pKa value, the standard free energy of

Fig. 3. Surface charge distribution of MAC, BAC, HAC and quartz.

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Fig. 4. The standard free energy of protonation of CNCs based on pKa -, and pKa50 values.  protonation (Gprot ) represents the amount of energy which is  Gprot -distributions

required to protonate an amine group. The of the CNCs based on constant and pH-dependent pKa values, as expressed in Eq. (5), are shown in Fig. 4. The interception with the abscissa describes the situation in which the kinetics of the reaction of protonation equals the reaction of deprotonation if the thermodynamic equilibrium is reached and  the pH value is equal to the pK50 a value. The reduction of Gprot shifts

the equilibrium to more protonated amine groups. At a positive  Gprot value, the number of deprotonated amine groups exceeds  the number of protonated ones. For a constant pKa value, Gprot is a linear function of the pH value. The pH dependency of the pKa  value influences the Gprot curve over the pH value, leading to a reduction of the slope of all three curves. This indicates that the  differences of the absolute values of Gprot were smaller than for  over the constant pKa values. HAC possessed the highest Gprot

Fig. 5. Mass of adsorbed CNCs on the surface of quartz over pH value.

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213

Fig. 6. Mass of adsorbed CNCs on the surface of quartz over the degree of protonation.

the entire pH range observed in this study. The energy required to protonate HAC is higher than for MAC and BAC, and thus the energy required to protonate an amine group depends largely on the alkyl chain length, which corroborates the assumption that the specific orientation of the alkyl chain hinders the protonation of amine groups with increasing length. In addition, it must be consid ered that Gprot is not equal for all individual amine groups of one type of CNCs, but differs with their specific location. The attempt to  for individual amine groups is given in the literadetermine Gprot  ture [23,24]. In regard to the complex morphology of CNCs, Gprot can only be presented as a mean value. The presence of charges on the surface of CNCs and quartz has a significant effect on interactions between the CNCs and the quartz surface, leading to adsorption phenomena. The average mass of adsorbed CNCs on quartz as a function of the pH value is presented in Fig. 5. The adsorbed mass of CNCs on quartz was found to increase as a function of increasing pH value. Due to the negligible content of salts, the mass of adsorbed CNCs can be directly related to interactions between CNCs and the quartz surface. Between the CNCs, the alkyl chain length was found to correlate to the mass of adsorbed CNCs, and HAC had the strongest interaction with quartz in the neutral and alkaline pH ranges. At pH 9 especially, HAC and quartz formed precipitating agglomerates. At low pH values, CNCs were electrostatically stabilized in suspension due to the high specific surface charges. Adsorbed CNCs possessed a notably higher

surface charge than quartz at pH 5, which resulted in low surface coverage due to fewer quartz adsorption sites and repulsive electrostatic forces between the CNCs. When pH increased to 6 and 7, the portion of adsorbed CNCs on the solid surface increased slightly. The enhanced adsorption might be explained by the presence of more adsorption sites on the surface of the quartz. However, the repulsive forces between the CNCs were still strong, as evidenced by high specific surface charges. At pH 9 only, the specific surface charges of CNCs were largely reduced, and adsorption phenomena occurred between all CNCs and the quartz surface. In regard to the application of CNCs in industrial processes, the complex effects of dissolved salts on adsorption behavior must be further studied. For a more detailed illustration of the effects of specific surface charges of CNCs on adsorption interaction between CNCs and quartz, the mass of adsorbed species over the degree of protonation is shown in Fig. 6. The correlation between the mass of adsorbed CNCs and the degree of protonation was hypothesized to be independent of the alkyl chain length of CNCs. Only BAC exhibited a high adsorption tendency at pH 7 (degree of protonation 0.66). In general, the mass of adsorbed CNCs was inversely proportional to the degree of protonation. The adsorption of CNCs on the quartz surface was predominantly influenced by the degree of protonation, and thus by the ratio between protonated and deprotonated amine groups. A reduced specific surface charge was responsible for attenuated CNC–CNC interactions, and thus for enhanced CNC-quartz interactions.

Fig. 7. Orientation of CNCs on the surface of quartz. Left—high specific surface charge; middle—moderate specific surface charge; right—low specific surface charge.

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Fig. 8. Specific surface free energy of CNCs based on the contact angles of rough surfaces (left) and smooth surfaces (right).

The occurrence of adsorption phenomena of CNCs on quartz surfaces is based on the decreasing specific surface charges of CNCs and subsequently, enhanced hydrophobic interactions. The low surface charges decrease the attractive interaction with water molecules and enhance the attraction between CNCs (hydrophobic behavior). In contrast, a high specific surface charge leads to attractive interactions between the surface charges and water molecules and repulsive interactions between individual alkyl chains (hydrophilic behavior). Based on different specific surface charges, a schematic description of the varying orientations of CNCs on the surface of quartz was hypothesized according to the development of attractive and repulsive electrostatic forces between CNCs and between CNCs and quartz (Fig. 7). The orientation of CNCs on the mineral surface is expected to influence mineral processes. A high specific surface charge of CNCs (low pH values) might lead to the formation of an electrostatic barrier due to strong repulsive electrostatic interactions with additional CNCs approaching the mineral surface. In addition, a high specific surface charge hinders the agglomeration of CNCcoated particles and the efficient collision between air bubbles and particles [25,26]. Therefore, the use of aminated CNCs in mineral flocculation or flotation processes is expected to be most effective in alkaline conditions, however, the effect of salt ions has to be further studied. 3.3. Specific surface free energy of quartz powder coated with CNCs In regard to the application of CNCs in mineral flocculation or flotation processes, the effects of CNCs on surface-wetting properties is of great importance. This study measured the contact angle of different liquids on quartz pellets coated with several CNCs, after the arithmetical average roughness of the pellets was measured three times per pellet [27]. All pellets possessed similar roughness values: 12.9 ± 0.6 ␮m for pure quartz, 10.1 ± 1.2 ␮m for MAC, 10.3 ± 3.0 ␮m for BAC, and 9.7 ± 2.2 ␮m for HAC-coated quartz pellets. Consequently, the effect of the roughness is expected to be equal for all samples; Table 5 displays the contact angles of the evaluated liquids. The values of quartz and ␣-bromonaphthalene were excluded, and thus the contact angles were referred to as zero. All liquids spread immediately over the surface of the pure quartz pellets when the droplet touched the surface, indicating that the quartz pellets possessed a highly porous structure that was well-wetted

by all liquids. In contrast, CNCs were expected to form a uniform layer covering the quartz particles, which in turn reduced the porosity of the quartz. An explanation of the strong interactions between ␣-bromonaphthalene and CNC surfaces might be given by the chemical structure of ␣-bromonaphthalene, which consists of fused benzene rings and the rough and porous structure of the pellet surface. In the case of MAC, diiodomethane also spread over the surface, and no contact angle was observed. For a more detailed evaluation of the wetting properties of CNC-coated quartz, the specific surface free energy was computed by plotting the contact angles over the dispersive and polar interfacial energies of the liquids. The results are presented in Fig. 8, which illustrates the specific surface free energy of CNCs with the effects of roughness included (left-hand side) and excluded (right-hand side). Eq. (8) was used for the calculation of the microscopic contact angle, which is independent of the roughness. In general, as the alkyl chain of CNCs lengthened, the result was a reduction of the total surface free energy. In the left-hand chart of Fig. 8, the exhibited surface free energies represent the more practical situation due to the omnipresent natural roughness of a particle. There, all samples coated with CNCs possessed a low polar component of the specific surface free energy, and a reduction of the dispersive surface free energy as the alkyl chain lengthened was observed, which is related to the enhanced water repellency of the solid surface. The right-hand chart of Fig. 8 illustrates the theoretical specific surface free energies of smooth quartz surfaces coated with CNCs. The total specific surface free energy of all samples was equal and lower compared to the rough samples, and the ratio of the polar component was higher. The comparison of both charts in Fig. 8 clarifies the importance of considering of the roughness of particles. The reduction of the specific surface free energy of rough samples obtained when alkyl chains were extended can be related to a more hydrophobic behavior, which is important for the application of CNCs in mineral processes such as flotation and

Table 5 Average and standard deviation of contact angles of different liquids using the sessile drop method.

Water Glycerol Ethylene glycol Diiodomethane

MAC

BAC

71.7 ± 1.6 38.8 ± 2.3 24.2 ± 5.2 –

84.2 68.0 30.9 40.3

HAC ± ± ± ±

3.4 5.9 1.4 4.1

93.2 70.2 48.0 47.7

± ± ± ±

1.3 3.0 2.5 2.8

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flocculation. The total specific surface free energies of smooth samples are almost independent of the alkyl chain length, which have a similar chemical structure because the dried CNCs have formed a uniform layer on the surface of the quartz. A lower total specific surface free energy is related to the absence of surface heterogeneities, for example, edges and corners, which possess higher surface free energies than planar surfaces. [28] 4. Conclusions The interaction between CNCs and the quartz surface was investigated using polyelectrolyte titration, and the wetting properties of CNC-coated quartz pellets were examined using contact angle measurements. The investigations clarified the pronounced effects on the adsorption phenomena of electrostatic interactions between CNCs and between CNCs and the mineral surface. The hypothesis was established that adsorbed CNCs with a high specific surface charge form an electrostatic barrier on the mineral surface, hindering the adhesion of additional CNCs on the mineral surface. The reduction of the specific surface charge of CNCs led to reduced electrostatic repulsive forces, and thus to an increase of the mass of adsorbed CNCs. This wettability study of CNC-coated quartz pellets demonstrated a decrease of the specific surface free energy for an extended alkyl chain length. There was a significant difference between rough surfaces and smooth surfaces in terms of the specific surface free energy. In the case of rough surfaces, the dispersive surface free energy decreased as alkyl chain length increased, and the polar surface free energy of each surface was low. Therefore, the decrease of the specific surface free energy of rough surfaces can be related to an enhanced water repellency for the extended alkyl chain length. In contrast, smooth surfaces possessed lower specific surface free energies compared to rough surfaces, and the total surface free energies of all samples were similar irrespective of alkyl chain length. Acknowledgements This work was conducted as part of the ERA-MIN project ‘CELMIN’ supported by the Finnish Agency for Technology and Innovation (TEKES), the Portuguese National Funding Agency for Science, Research and Technology (FCT), the Executive Agency for Higher Education, Research, Development and Innovation Funding, Romania (UEFISCDI) and several companies (Agnico-Eagle, Haarla, Sojitz Beralt Tin & Wolfram, and Sibelco). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2015.10. 022. References [1] A.J. Ragauskas, The path forward for biofuels and biomaterials, Science 311 (2006) 484–489.

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