Adsorption of Naphthenic Acids onto Mineral Surfaces Studied by

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Adsorption of Naphthenic Acids onto Mineral Surfaces Studied by Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D) Serkan Keleşoğlu,*,†,‡ Sondre Volden,† Mürşide Kes,§ and Johan Sjöblom† †

Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway ‡ The Multiphase Flow Assurance Innovation Centre (FACE), Institute for Energy Technology (IFE), NO-2007 Kjeller, Norway § Norwegian Biopolymer Laboratory (NOBIPOL), Department of Biotechnology, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway ABSTRACT: In the present research, the adsorption of naphthenic acids from n-dodecane solutions onto hydrophilic surfaces of the most common minerals in petroleum reservoirs (silica, alumina, and calcite) has been investigated using the quartz crystal microbalance with dissipation monitoring (QCM-D). The wettability and surface roughness of the mineral surfaces applied for the QCM-D measurements were determined with contact angle and atomic force microscopy (AFM) measurements. Corrections for changes in bulk density and viscosity of the naphthenic acids solutions as a function of concentration are necessary to determine the actual adsorption levels for the QCM-D measurements and have been accounted for in the adsorption calculations. The adsorption affinity of the naphthenic acids to the mineral surfaces is ranked as calcite > alumina > silica. The adsorption data for all systems can be adequately well described by the Langmuir isotherm and adsorption free energies (ΔG°) calculated from the fitted Langmuir isotherms indicate that the binding between naphthenic acids and mineral surfaces is of a physical nature. Plots of change in surface area per molecule (σ) as a function of the variation of concentration indicate that the adsorption process of the naphthenic acids changes as a function of surface coverage and suggest that the naphthenic acids chains undergo conformational alterations during the adsorption. The surface area per molecule (σ) of the adsorbed naphthenic acids onto the mineral surfaces is calcite < alumina < silica for all concentrations, which supports the adsorption affinity ranking.

1. INTRODUCTION In the petroleum science, the term “naphthenic acids” is often used for all types of carboxylic acids in crude oils. Naphthenic acids are very complex mixtures, in which alkyl-substituted acyclic and cycloaliphatic carboxylic acids are major components. Naphthenic acids are indigenous constituents of crude oils and exhibit amphiphilic character because they contain both hydrophilic carboxylic acid groups and hydrophobic hydrocarbon chains. The concentration of naphthenic acids in crude oils ranges mostly from a few ppm up to 3 wt %. The general chemical formula of naphthenic acids is given as CnH2n+ZO2, in which n denotes the carbon number and Z represents a homologous series and the saturation and cyclic character. However, experimental results indicate that naphthenic acids in crude oils are not merely comprised of cyclic or acyclic alkenic acids with this general chemical formula. For instance, UV and IR analyses confirm the presence of other structures, such as pyrroles, thiophenes, and phenols. Complete separation, quantification, and identification of individual compounds of naphthenic acids in crude oils are not easy tasks and have not been achieved yet due to very complex composition of this acid mixtures.1,2 The quartz crystal microbalance (QCM) is simple, rapid, and nanogram sensitive technique initially developed to quantify the adsorption of gases onto a quartz surface, but as researchers improved the underlying theory, it was indicated that the QCM could also be employed in liquid systems. Afterward, the QCM has been widely used in many different research areas for © 2012 American Chemical Society

various applications. Currently, adsorption of surfactants, polymers, and vesicles, coadsorption of surfactants and polymers, and viscoelastic properties of polymers have been extensively investigated using the QCM technique by many researchers.3−12 In the literature, reviews about theoretical background and working principle of the QCM technique have been given in detail by many researchers.13−15 From the existing experimental literature, one can see that aqueous solutions are typically employed in QCM studies. On the other hand, hexane and alkanethiols have been used to investigate the adsorption properties of cholesterol surfactants; toluene and heptane mixtures have been used to examine the adsorption properties of resins, and asphaltenes and dodecane has been used to investigate the adsorption properties of waxes using the QCM technique. Moreover, more viscous liquids, such as oils, have also been used in QCM studies.16−21 In all these measurements, it is of paramount importance to be aware of changes in bulk density and viscosity of liquids, as these might severely influence the calculated adsorbed mass. It has been indicated by many researchers that the soluble polar compounds, such as naphthenic acids, in crude oils adsorb onto the solid surfaces and make them more hydrophobic by changing their wettability. Therefore, retention times of crude Received: April 11, 2012 Revised: July 17, 2012 Published: July 17, 2012 5060

dx.doi.org/10.1021/ef300612z | Energy Fuels 2012, 26, 5060−5068

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In the present research, most of the observed dissipation changes were less than 2 × 10−6 for each surface, and therefore, eq 1 was deemed satisfactory for all calculations. Equation 1 was initially derived for adsorption from the gas phase onto a quartz surface, and therefore, it was believed that the mechanical resonance of quartz crystal would be completely dampened by liquids. On the other hand, it was indicated by Nomura and Okuhara33 that a quartz crystal also resonates in liquids, with the frequency change influenced by bulk density and viscosity of liquids. The relation between the frequency change and both bulk density and viscosity for the adsorption from gas phase onto crystals was derived by Stockbridge.34 Kanazawa and Gordon35,36 modified Stockbridge’s equation for the adsorption from liquid phase onto crystals and proposed the following equation, which introduces the influence of bulk density and viscosity of liquids on the frequency change:

oils in the reservoirs increase. The adsorption of naphthenic acids onto the solid surfaces is mainly governed by chemical composition of naphthenic acids, type of solid surface and environment, and exhibits an important role to control the wettability of the reservoirs. Silica, alumina, and calcite are most common minerals in reservoirs, and thus, knowing the adsorption properties of naphthenic acids onto these minerals is very important in the petroleum industry.22−27 In this research, the adsorption properties of naphthenic acids from n-dodecane solutions onto hydrophilic surfaces of silica, alumina, and calcite have been reported using the quartz crystal microbalance with dissipation monitoring (QCM-D) in order to show how the adsorption is influenced by the nature of the mineral surface and has been correlated to wettability, surface roughness, and relative basicity (surface hydroxyl density) of the mineral surfaces. Corrections for changes in bulk density and viscosity of naphthenic acids solutions have been made in the adsorption calculations for the QCM-D measurements. Furthermore, the state of the adsorbed naphthenic acids layers onto the mineral surfaces has been discussed using the plots of change in apparent surface area per molecule (σ) as a function of the variation of naphthenic acid concentration.

Δf = −

2.1. Materials. Naphthenic acids were purchased from Fluka (Fluka, Sigma-Aldrich Co., Germany). The Fluka naphthenic acids are obtained by a distillation process of crude oils yielding a mixture of different acid structures. Fourier transform infrared (FT-IR), 1H NMR, and 13C NMR analyses indicate that the mixture is mainly comprised of saturated naphthenic acids with both cyclic and acyclic structures. The average molecular weight of the Fluka naphthenic acids was estimated based on the total acid number (TAN) analysis. The resulting molecular weight was 241 g/mol ±1.3 assuming only monoprotonic acids.28 The oil phase used was n-dodecane (99.9% purity) obtained from Sigma-Aldrich (Sigma-Aldrich Co., Germany). The quartz crystals coated with silicon dioxide (SiO2), aluminum oxide (Al2O3), and calcium carbonate (CaCO3) with a fundamental resonance frequency of 5 MHz were purchased from Q-Sense (Biolin Scientific AB/Q-Sense, Sweden) and used as received for the QCM-D measurements. The thickness of the crystals was 0.33 mm, and they were spin-coated with SiO2, Al2O3, and CaCO3 and optically polished by the supplier. Toluene (98% purity, VWR International), ethanol (99.8% purity, Fluka, Sigma-Aldrich Co., Germany), and sodium dodecyl sulfate (SDS, 99% purity, Sigma-Aldrich Co., Germany) were used to clean the crystals, QCM-D chamber, and connecting tubes. All chemicals were used as received. 2.2. Methods. 2.2.1. Quartz Crystal Microbalance (QCM) and Data Analysis. In 1959, Sauerbrey29 indicated that the resonance frequency (Δf) of a specifically cut quartz crystal is affected by added mass (Δm). Furthermore, the resonance frequency changes linearly if the added mass is rigid and firmly attached to the surface, for which Sauerbrey29 developed the following relation:

nf0 Δm ρq tq

=

2nf02 Δm ρq vq

=

nΔm c

πμq ρq

(2)

In eq 2, ρL denotes the density of liquid, ηL is the viscosity of liquid, and μq represents the elastic shear modulus of quartz (2.947 × 1011 g/ cm·s2). The QCM technique for simultaneous measurements of not only the frequency change but also the dissipation factor (D) of the quartz crystals was developed by Rodahl et al.37 It was indicated that the measured dissipation change is a function of the coupling between the oscillating sensor and the surroundings. Furthermore, it was proven that both viscoelasticity and slip of the adsorbed layer affect the dissipation factor. The dissipation factor is given by eq 3:

2. EXPERIMENTAL SECTION

−Δf =

nf03 ρL ηL

D=

ED 2πES

(3)

In this equation, ED is the total dissipated energy during one oscillation period and ES is the total energy stored in the oscillating system.37 The dissipation change (ΔD) is given by eq 4, which introduces the influence of bulk density and viscosity of liquids.38

ΔD = (ρq tq)−1

nρL ηL 2πf

(4)

The surface roughness of a substrate that is applied in QCM-D measurements can influence the ΔD and Δf changes due to liquid trapping by cavities and pores on the substrate surface. However, contribution of surface roughness to the ΔD and Δf changes is mostly small or can be ignored by utilizing smooth substrates in the QCM-D measurements.39,40 In the present research, the adsorption properties of the naphthenic acids from n-dodecane solutions onto mineral surfaces were investigated using a quartz crystal microbalance with dissipation facility (QCM-D 300, Q-Sense AB, Sweden) at 20 °C ± 0.05. The instrument collects first, third, fifth, and seventh harmonics of the fundamental resonance frequency, but the third harmonic of the fundamental resonance frequency was used in the data analysis. Prior to naphthenic acids adsorption measurements onto silica, alumina, and calcite coated quartz crystals, the crystals were rinsed with excess toluene and then rinsed with ethanol and milli-Q water to remove any contaminants. Afterward, the crystals were dried with air and then immersed in a 2% (w/v) of SDS solution for 1 h. Following immersion in SDS solution, the crystals were rinsed with excess milli-Q water and dried with air. The dry crystals were put in a UV chamber for ozone treatment for 15 min. After the ozone treatment, the crystals were rinsed again with excess milli-Q water and dried with air. Prior to each experiment, excess amounts of toluene, water, and ethanol were successively used to rinse the QCM-D chamber and connecting tubes. The chamber was initially flushed with n-dodecane to get a stable baseline and the signal was assumed stable when the frequency change was less than ±3 Hz during 30 min.

(1)

In this relation, n is the harmonic number, f 0 is the fundamental resonance frequency, ρq is the specific density of quartz, tq is the thickness of the quartz crystal, vq is the shear rate velocity, and c is the sensitivity constant. In the present research, n is 3, f 0 is 5 MHz, ρq is 2648 kg/m3, tq is 0.33 mm, vq is 3340 m/s, and c is 0.178 mg/m2·Hz. It should be noted that eq 1 can be satisfactorily used when the dissipation change is less than or equal to 2 × 10−6. The adsorbed species are generally recognized as viscoelastic and Voigt viscoelastic model is mostly used to identify the adsorption processes in the QCMD technique when the dissipation change is more than 2 × 10−6.30−32 5061


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Here, η denotes the viscosity of solution (mPa·s), ηd is the viscosity of n-dodecane (mPa·s), c represents the concentration of the naphthenic acids in n-dodecane (mol/L), and k2 is the slope of the line (mPa·s/ mol·L−1). Figure 1 provides the slope, intercept, and corresponding regression coefficient (R2) of the viscosity vs naphthenic acids concentration plot. 2.2.3. Determination of Minerals’ Wettability. The static contact angle measurements on silica, alumina, and calcite coated quartz crystals were performed in triplicate for each surface at the water/ solid/air interface at room temperature (22 °C ± 1) to determine the wettability of the surfaces applied for the QCM-D measurements. The sessile drop method using a contact angle meter (CAM 200, KSV Instruments, Finland) was employed. Contact angles were determined by fitting the Young−Laplace equation to the drop profile obtained at equilibrium conditions and calculated as the mean values of the left and right contact angles. Table 1 gives the measured contact angles (θ) of silica, alumina, and calcite coated quartz crystals applied in the QCM-D measurements.

A sequential adsorption of the naphthenic acids onto mineral surfaces was carried out in the QCM-D measurements by injection of 0.5 mL solution for each concentration under gravitational flow. The concentration of naphthenic acids was increased stepwise from 4.15 × 10−3 mol/L to 6.22 × 10−2 mol/L, followed by desorption with 0.5 mL pure n-dodecane after equilibrium was reached for the highest concentration added. All of the QCM-D measurements were performed in triplicate to check the reproducibility, and observed differences for the measurements were between 5% and 8% (±). The averaged values for the QCM-D data were represented and used for the calculations in the present research. 2.2.2. Determination of Density and Viscosity of Naphthenic Acids Solutions. The density and viscosity measurements were performed to determine the bulk density and viscosity of the naphthenic acids solutions at different concentrations. At least three density and viscosity measurements were performed for each solution, and averaged values were reported and used for the calculations. Density and viscosity data were used to calculate the corrected adsorbed mass of the naphthenic acids onto mineral surfaces for the QCM-D measurements, using eq 1 and 2. The densities of the solutions were determined using a DMA 5000 density meter (Anton Paar, Austria) at 20 °C ± 0.01. It was found that the density is a linear function of naphthenic acids concentration in ndodecane and described by the following equation:

ρ = ρD + k1c

Table 1. Contact Angles (θ) of Mineral Surfaces used in the QCM-D Measurements

(5)

In eq 5, ρ is the density of solution (g/mL), ρD denotes the density of n-dodecane (g/mL), c represents the concentration of the naphthenic acids in n-dodecane (mol/L), and k1 is the slope of the line (kg/mol). Figure 1 provides the slope, intercept, and corresponding regression coefficient (R2) of the density vs naphthenic acids concentration plot.

surface

contact angle (θ)

silica alumina calcite

10.1 ± 1.3 57.2 ± 5.8 65.1 ± 2.3

The contact angles are below 90° and confirm that all crystals are hydrophilic. Moreover, hydrophilicity of the mineral surfaces applied for the QCM-D measurements is ranked as silica > alumina > calcite. 2.2.4. Determination of Minerals’ Surface Roughness. To estimate the surface roughnesses and height deviations of dry silica, alumina, and calcite coated quartz crystals; atomic force microscopy (AFM) measurements were performed at ambient temperature (22 °C ± 1) using a Veeco diCaliber AFM instrument (Veeco Instruments Inc.) equipped with a 10 μm Z-range scanner by applying tapping mode surface imaging using silicon cantilevers with a tip radius of curvature of 10 μm and a cantilever spring constant of 42 N/m. Before the AFM measurements, the surfaces were cleaned according to the cleaning procedure given in Section 2.2.1. During the measurements, an image of the surface was initially acquired as a two-dimensional (2D) array with 20 μm × 20 μm dimensions, and then, a 2.5 μm line was randomly selected from the image and analyzed in the Z-direction. This was done for 10 different points subsequently to get an average value of the surface roughness and height deviation for each surface; Veeco SpmLab software and the following equation were used: n

Figure 1. Density and viscosity of n-dodecane solution at different concentrations of naphthenic acids (T = 20 °C).

R a = 1/n ∑ |Zi − Z̅ | i=1

The kinematic viscosities (v) of the solutions were determined using a Cannon-Fenske viscometer (SCHOTT Instruments, Germany) at 20 ± 0.1 °C and calculated using the following relation: (6) v = Kt In eq 6, K is the capillary constant equal to 0.01516 mm2/s2 in this study, and t is the drainage time (s). The dynamic viscosities of the solutions were calculated using eq 7. η = ρv (7) In this equation, η denotes the dynamic viscosity of the solution (mPa·s), ρ represents the density of the solution (g/mL), and v is the kinematic viscosity of the solution (mm2/s). The dynamic viscosity of each solution was determined to be a linear function of the naphthenic acids concentration in n-dodecane and described by eq 8. η = ηd + k 2c

(9)

In eq 9, Ra denotes the roughness average, that is, arithmetic average of the absolute values of the measured profile height deviations (Z). It can be assumed that a quantitative measure of differences in roughness of the surfaces is obtained from Ra measurements. In the present research, all of the AFM measurements were performed on three randomly acquired AFM images with a 2D array with 20 μm × 20 μm dimensions, providing a total of 30 different surface roughness measurements and height deviation data for each surface. The reported results for the surface roughness are averages with standard deviations. On the other hand, only the average value was reported for the height deviation for each surface.

3. RESULTS AND DISCUSSION AFM images, sectional analysis profiles, and estimated surface roughnesses (Ra) of silica, alumina, and calcite coated quartz crystals applied in the QCM-D measurements have been given in Figure 2. As noticed here, the morphologies of the mineral surfaces were not significantly different. The surface rough-

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Figure 2. AFM images (a) sectional profile analysis (b) and surface roughness values (c) of mineral surfaces used in the QCM-D measurements.

nesses of silica, alumina, and calcite coated crystals applied for the QCM-D measurements were low and estimated as 1.60 nm (±0.40), 1.65 nm (±0.43), and 1.50 nm (±0.45) from the AFM measurements, respectively. Due to this, we consider these surfaces as being “smooth”, and any differences in adsorption behavior can be ascribed to the surface chemistry of each surface. Figure 3 indicates the obtained frequency changes for the adsorption of the naphthenic acids from n-dodecane solutions onto mineral surfaces during the QCM-D measurements, which were done for six naphthenic acids concentrations: 4.15 × 10−3 mol/L, 1.02 × 10−2 mol/L, 2.07 × 10−2 mol/L, 3.11 × 10−2 mol/L, 4.15 × 10−2 mol/L, and 6.22 × 10−2 mol/L. First, pure n-dodecane was injected in the QCM-D chamber, and then, the naphthenic acids solutions were injected after 30, 60, 90, 120, 150, and 180 min, respectively. The sudden frequency changes in Figure 3 are caused by increasing the naphthenic acids concentration. Desorption of the naphthenic acids from the silica, alumina, and calcite surfaces was also investigated, as shown in Figure 3. The desorption measurements of the

Figure 3. Frequency change data obtained during the QCM-D measurements for the adsorption and desorption of the naphthenic acids onto mineral surfaces (third harmonic of the fundamental resonance frequency, T = 20 °C ± 0.05).

naphthenic acids from the mineral surfaces were done by injecting of pure n-dodecane after 210, 240, 270, and 300 min. 5063


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In this study, the uncorrected adsorbed mass (mol/m2) of the naphthenic acids was first calculated using the frequency change determined from the QCM-D measurements and eq 1. These results are given in Figure 4. To obtain the corrected

Figure 4. Uncorrected and corrected adsorbed mass of the naphthenic acids from n-dodecane solution onto mineral surfaces (third harmonic of the fundamental resonance frequency, T = 20 °C ± 0.05).

adsorbed mass changes in bulk density and viscosity of the naphthenic acids solutions were accounted for, and the following procedure, which is given in detail by Lundgren et al.,41 was used: first, the calculated value of Δf for the naphthenic acids solutions was obtained using eq 2, and then, the calculated value of Δf for pure n-dodecane was subtracted from the calculated value of Δf. Afterward, the calculated value of Δf was subtracted from the measured value of Δf, and a corrected value of Δf was obtained. Finally, eq 1 and the corrected value of Δf were used to calculate the corrected adsorbed mass of naphthenic acids onto mineral surfaces. Figure 4 compares the corrected and uncorrected adsorbed mass of the naphthenic acids from n-dodecane solutions onto surfaces of silica, alumina, and calcite. It is obvious that the corrections made for the change in bulk density and viscosity of the naphthenic acids solutions are important for the level of the adsorption. The difference is highest at high concentrations due to the influence of the naphthenic acids concentration on viscosity. It was also seen that the change in bulk viscosity is more significant than the change in bulk density of the naphthenic acids solutions for the measured frequency change. Similar results were also found by Lundgren et al.41 from the adsorption studies of the unsaturated fatty acids onto steel surfaces. The general picture that emerges from the comparison between the corrected and uncorrected adsorbed mass is that it is important to make corrections for the changes in bulk density and viscosity of the naphthenic acids solutions when the concentration is greater than 4.15 × 10−3 mol/L. It is clear from Figure 4 that the naphthenic acids adsorption nearly reaches a plateau at a concentration of about 3.11 × 10−2 mol/ L for both silica and alumina surfaces and 4.15 × 10−2 mol/L for the calcite surface, after the corrections have been made. At these concentrations, the adsorbed masses are calculated to be 5.54 × 10−6 mol/m2, 5.86 × 10−6 mol/m2, and 8.51 × 10−6 mol/m2 on silica, alumina, and calcite surfaces, respectively. Figure 5 gives the uncorrected desorbed mass and desorbed mass percentage of the naphthenic acids from surfaces of silica, alumina, and calcite as a function of the number of cycles where the solution volume was replaced by a pure n-dodecane solution. The data show that the naphthenic acids desorb from

Figure 5. Uncorrected desorbed mass (mol/m2) and desorbed mass percentages of the naphthenic acids from mineral surfaces as a function of number of elution with n-dodecane (third harmonic of the fundamental resonance frequency, T = 20 °C ± 0.05).

the surfaces when the solvent is replaced. This effect is, however, only seen for the first two flushing cycles. After this, no more desorption can be measured when replenishing the solution. It is obvious that desorption tendency of the naphthenic acids is slightly higher from a silica surface than from alumina and calcite surfaces, and this might be due to different adsorption affinity of naphthenic acids onto mineral surfaces. It should be noted that the corrected desorbed mass could not be calculated in the present study due to the illdefined conditions during the desorption protocol. Various adsorption isotherms, such as the Freundlich,42 the Langmuir,43 the Dubinin−Radushkevich,44 and the Temkin,45 are commonly employed to model the adsorption processes. Most of adsorption isotherms are empirical. However, in some cases, theoretical derivations have been adopted. The Langmuir isotherm is often employed due to its simplicity and good agreement with a lot of the experimental data, and it was applied to fit the adsorption data of the naphthenic acids onto mineral surfaces in this study. Based on the parameters obtained from the Langmuir isotherm, the physical basis of the adsorption process can be discussed. The corrected adsorbed masses of the naphthenic acids onto mineral surfaces were fitted to the Langmuir isotherm using eq 10: Γ=

ΓmaxKc 1 + Kc

(10)

In eq 10, Γ is the surface coverage of the naphthenic acids, Γmax denotes the maximum monolayer surface coverage of the naphthenic acids, K is the binding constant, and c denotes the concentration of the naphthenic acids in the bulk solution remained.46 The Langmuir isotherm is most commonly 5064


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Figure 7 gives the dissipation changes obtained during the QCM-D measurements for the adsorption of naphthenic acids

employed to model monolayer adsorption processes and assumes that adsorption takes place at specific homogeneous sites on the surface, adsorption is reversible, and there is no lateral interaction between adsorbed molecules on the surface. Furthermore, adsorption affinities to each surface can be compared by calculating K values from eq 10 when comparing the same adsorbent in the same solvent. Figure 6 indicates the fitted Langmuir isotherms and obtained isotherm parameters (Γmax, K, and R2) for the

Figure 7. Dissipation data obtained during the QCM-D measurements for adsorption and desorption of naphthenic acids onto mineral surfaces (third harmonic of the fundamental resonance frequency, T = 20 °C ± 0.05).

onto mineral surfaces. The sudden changes in dissipation are also due to increasing the naphthenic acids concentration and desorption of the naphthenic acids from the mineral surfaces. Due to the change in bulk density and viscosity of the solutions as a function of the naphthenic acids concentration, the measured dissipation change must also be corrected using the following procedure.41 First, eq 4 was employed to calculate the influence of change in bulk density and viscosity of the naphthenic acids solutions on ΔD, and this obtained value of ΔD was subtracted from the calculated value of ΔD for pure ndodecane. Finally, the calculated value of ΔD was subtracted from the measured value of ΔD and a corrected values of ΔD was obtained. Table 3 compares the corrected and uncorrected

Figure 6. Langmuir adsorption isotherms for naphthenic acids adsorption onto mineral surfaces (T = 20 °C ± 0.05).

adsorption of the naphthenic acids onto mineral surfaces from the QCM-D measurements. The amount of the adsorbed naphthenic acids increases when increasing the concentration and reaches a plateau at higher concentrations. It is obvious from the obtained Γmax and K values that the naphthenic acids have a slightly higher binding affinity for the calcite surface than for silica and alumina surfaces. Furthermore, the adsorption data for all systems fit the Langmuir isotherm well. The adsorption free energy (ΔG°) can be calculated using the K value obtained from the Langmuir isotherm and eq 11:47−49

Table 3. Uncorrected and Corrected Change in Dissipation (ΔD) for the Adsorption of the Naphthenic Acids from nDodecane Solution onto Mineral Surfacesa

(11)

ΔG° = −RT ln K

In eq 11, R denotes the universal gas constant and T is the absolute temperature. The value of ΔG° is an indication of spontaneity of adsorption and identifies the nature of the adsorption, which takes place spontaneously at a given temperature if ΔG° is negative. The adsorption is generally characterized as physisorption if ΔG° is in the range from −20 to 0 kJ/mol.47−49 Table 2 compares the calculated ΔG° values for adsorption of naphthenic acids from n-dodecane solutions onto mineral

ΔD uncorrected ( × 10−6)

−ΔD corrected ( × 10−6)

concn. (mol/L)

silica

alumina

calcite

silica

alumina

calcite

4.15 × 10−3

0.23

0.40

0.42

0.41

0.24

0.21

1.02 × 10−2

0.48

0.64

0.64

0.89

0.72

0.73

−2

0.99

1.13

1.14

1.69

1.55

1.55

−2

1.58

1.71

1.67

2.29

2.16

2.20

−2

2.09

2.22

2.20

3.10

2.98

2.99

−2

3.21

3.18

3.13

4.44

4.47

4.52

2.07 × 10

3.11 × 10

4.15 × 10

6.22 × 10

Third harmonic of the fundamental resonance frequency, T = 20 °C ± 0.05. a

Table 2. Adsorption Free Energy (ΔG°) for Adsorption of Naphthenic Acids onto Mineral Surfaces (T = 20 °C ± 0.05) surface

(ΔG°) (kJ/mol)

silica alumina calcite

−10.54 ± 0.41 −10.88 ± 0.35 −11.01 ± 0.32

ΔD values. It was observed that the corrections made for the change in bulk density and viscosity of the naphthenic acids solutions have great impact on ΔD. Moreover, it is obvious from the table that the corrected value of ΔD is negative for all surfaces and decreases with increasing the naphthenic acids concentration. The negative ΔD values were also observed by Lundgren et al.41 for the adsorption of unsaturated fatty acids onto steel surfaces after the bulk effects (density and viscosity) have been considered for the QCM-D measurements. This might be due to the reducing the surface roughness of the

surfaces. It is obvious from the obtained ΔG° values that the adsorption is spontaneous and feasible for all systems, while the magnitude of ΔG° indicates that the adsorption mechanism is physisorption. 5065


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Figure 8. Calculated surface area coverage, σ (Å2), per molecule of the naphthenic acids onto mineral surfaces at different concentrations (third harmonic of the fundamental resonance frequency, T = 20 °C ± 0.05).

substrate by the adsorption that somehow reduces the energy dissipation.41 To substantiate the changes of the naphthenic acids molecules during adsorption, the apparent surface area per molecule (σ) was calculated for each concentration using the following equation: σ=

SA ΓNA

on the calcite surface than silica and alumina surfaces, as is also observed directly in the QCM-D measurements (Figure 3). From studies of benzoic acid, 1-naphthonic acid, and 2naphthonic acid adsorption from nonaromatic liquids to solid surfaces, Wright and Pratt50 proposed a perpendicular orientation when the σ value is between 22 Å2 and 26 Å2, a parallel orientation when the σ value is higher than 51.5 Å2, and a mixture of parallel and perpendicular orientations when the σ value is between 26 Å2 and 51.5 Å2 for the adsorbed acid molecules. Accordingly, the naphthenic acids molecules in our study appear to be perpendicular to the mineral surfaces when the concentration is between 3.11 × 10−2 mol/L and 6.22 × 10−2 mol/L, a mixture of parallel and perpendicular when the concentration is between 1.02 × 10−2 mol/L and 2.07 × 10−2 mol/L, and almost completely parallel with the surface when the concentration is less than 1.02 × 10−2 mol/L (Figure 8). The general picture that emerges is that the naphthenic acids have slightly higher values of Γmax and lower values of σ on the calcite surface than on silica and alumina surfaces (Figures 6 and 8), indicating that the naphthenic acids molecules have different binding affinities to the mineral surfaces. It is wellknown from adsorption studies of surfactants from nonaqueous solutions to solid surfaces that adsorption is mainly governed by the polarity differences between the solvent, solute, and substrate, as well as the roughness of the substrate surface, with the former possibly being related to the acid−base characteristics of these components.51 As such, the least hydrophilic surface applied here (calcite, see Table 2) may also induce the largest adsorbed amount. It was indicated by Gutig et al.52 that surface roughness of a crystal used in the QCM-D measurements must be considered

(12)

In this equation, σ denotes the surface area per molecule, SA is the specific surface area, Γ is the adsorbed amount, and NA is Avogadro’s number. Figure 8 provides the calculated apparent surface area per molecule (σ) of the naphthenic acids onto mineral surfaces for each concentration. One can notice from the figure that the σ decreases with increasing the concentration for each surface. A qualitative explanation is that the naphthenic acids molecules go from flat orientation with the hydrocarbon chain almost parallel with the solid surface to an orthogonal orientation that allows a higher surface concentration of the molecules. The orientation of the naphthenic acids on the surface might be influenced by the surface hydrophobicity. A strongly hydrophilic surface might promote a vertical orientation of the hydrocarbon chains. Alumina and calcite surfaces are more hydrophobic than silica surface used in the QCM-D measurements (Table 1). Therefore, the hydrocarbon chains of the naphthenic acids might be more vertical on the silica surface. Furthermore, the σ values for the adsorption of the naphthenic acids onto the calcite surface at each concentration are lower than for silica and alumina surfaces. This means that naphthenic acids have a more compact layer 5066


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and feasible onto all surfaces tested and indicative of that the adsorption interactions are of the physisorption type. The plots of the change in surface area per molecule (σ) as a function of concentration indicate that the naphthenic acids chains undergo a conformational alteration during the adsorption. The naphthenic acid molecules on the mineral surfaces seem to have a flat orientation at low concentrations and an outstretched/perpendicular orientation for the highest concentrations.

to calculate the exact amount of the adsorbed species using the roughness corrected area rather than nominal surface area, and therefore, a detailed surface analysis and computer modeling must be employed. In the present research, this point was not addressed in detail because adsorption and desorption affinities of the naphthenic acids onto mineral surfaces with the same roughness were compared. Using the QCM-D technique, it was only of interest to observe adsorption differences stemming from properties relating to the mineral surface rather than calculating the exact adsorbed and desorbed amounts. Moreover, the AFM measurements (see Figure 2) confirmed that the mineral surfaces applied for the QCM-D measurements were nearly smooth and exhibited almost similar morphological properties. Therefore, it can be concluded that there are only slight differences between the roughness corrected surface areas and nominal surface areas for the mineral surfaces used in the present research. Wu and co-workers53 investigated the effect of surface roughness on the adsorption of cetyltrimethylammonium bromide (CTAB) from aqueous solutions onto gold surfaces using the QCM-D technique. The experiments were performed on three different rough surfaces (2.3, 3.1, and 5.8 nm) of the gold. They conducted, in detail, AFM experiments on the surfaces and used a computer programming to calculate the effective areas. The effective areas were estimated as 4.52, 6.90, and 10.12 μm2 for 2.3, 3.1, and 5.8 nm rough surfaces, respectively. These results corresponded to 1.13, 1.53, and 2.56 times more than the nominal surface area of the gold crystal. Accordingly, due to the lower surface roughnesses of the mineral surfaces employed in the QCM-D measurements in the present research (see Figure 2), the effective surface areas will only be slightly higher than the nominal surface areas. The isoelectric point (IEP) of a mineral can be used to refer its relative acidity or basicity. For instance, silica is an acidic mineral because it has a low hydroxyl density and a low IEP (below 2.5). However, alumina and calcite are basic minerals due to high surface hydroxyl densities and high IEPs (8.5 for alumina and from 8 to 9.5 for calcite). It is known from adsorption studies of surfactants from nonaqueous solutions onto mineral surfaces that surfactants are adsorbed through interactions with the hydroxyl groups.51 Correlated to the QCM-D measurements, it can also be suggested that hydroxyl groups may also contribute to decide the Γmax and σ for the adsorption of naphthenic acids from hydrocarbon solutions onto silica, alumina, and calcite surfaces.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +47 735 94 149. Fax: +47 735 94 080. Email: serkan. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially financed by the Multiphase Flow Assurance Centre (FACE), a research cooperation between IFE, NTNU, and SINTEF. The center is funded by The Research Council of Norway and by the following industrial partners: Statoil ASA, GE Oil & Gas, SPT Group, FMC Technologies. The authors thank Caterina Lesaint for the contact angle and AFM measurements.

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4. CONCLUSION The quartz crystal microbalance with dissipation monitoring (QCM-D) was employed to investigate the adsorption properties of naphthenic acids from hydrocarbon solutions onto hydrophilic and 1.60 nm, 1.65 nm, and 1.50 nm rough surfaces of silica, alumina, and calcite respectively. The QCM-D measurements indicate that corrections made for the change in bulk density and viscosity of naphthenic acids solutions are necessary to properly assess the level of adsorption with viscosity being more important than density for the measured frequency change. The adsorption affinity of the naphthenic acids onto the mineral surfaces is ranked as calcite > alumina > silica. This adsorption affinity is reflected in the values of the surface coverage (Γmax), which are 7.4 × 10−6 mol/m2 on silica, 7.7 × 10−6 mol/m2 on alumina, and 10.3 × 10−6 mol/m2 on calcite. The adsorption free energy (ΔG°) based on the Langmuir isotherm indicates that the adsorption is spontaneous 5067


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Adsorption of Naphthenic Acids onto Mineral Surfaces Studied by

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