Article pubs.acs.org/EF
Tailings Pond Surfactant Analogues: Effects on Toluene-Diluted Bitumen Drops in NaHCO3/K2CO3 Solution. Part 2: Dilational Interfacial Viscoelasticity Chandra W. Angle* and Yujuan Hua CanmetENERGY, CanmetÉNERGIE Natural Resources Canada, Ressources Naturelles Canada, Devon, Alberta T9G 1A8, Canada ABSTRACT: In this two-part work, we used dilational interfacial rheometry to study the interfaces associated with diluted bitumen in simulated pond water as a function of pH, for three different surfactants: two simple carboxylic acids, one with a straight chain and one having a ring in the tail, and the more complex sodium naphthenates. In part 1 (10.1021/ef400376v), dynamic interfacial tension as a function of time was used to measure adsorption. In part 2, described in the present paper, interfacial dilational rheology was studied as a function of oscillation frequency for three different concentrations of the three surfactants adsorbed at the interface between buffer and toluene-diluted bitumen. The concentrations used were selected on the basis of results from part 1 (10.1021/ef400376v). Our results show that interfacial viscoelasticity depends upon not only the tail complexity in the surfactants but also the surfactant concentration and buffer pH. The adsorbed naphthenates were synergistic with interfacially active materials indigenous to bitumen, resulting in high interfacial rigidity (reduced interfacial elasticity). The simpler carboxylic surfactants were less effective. The observed interactions provide insight into possible transport pathways for surfactants in pond water when bitumen is present.
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Clemente et al.,7 followed by Headley et al.8,9 Since publication of these works, a few studies have also been published on the actual interactions of these naphthenates and bitumen in the context of understanding interfacial film stability and rheology.10−22 The findings of these authors are summarized in the Introduction of part 1 of our study (10.1021/ef400376v). Specific studies on the bitumen films undertaken earlier1−3 at CanmetENERGY pointed to a need to understand diluted bitumen interfacial interactions with the surface-active chemicals (surfactants) dissolved in the process water. To the present date, there are no published accounts describing the effects of surfactants that occur naturally in aquifers or tailings ponds as they interact with diluted bitumen films. In part 1 (10.1021/ef400376v), we addressed the effects of adsorption of three analogue surfactants on the dynamic interfacial tensions of toluene/buffer and bitumen/buffer systems, as a function of their concentrations and pH. We determined equilibrium interfacial tensions, critical micelle concentrations (CMCs)2,23,,24 and Gibbs surface excess for each surfactant at the toluene/buffer interface.2,25 We found significant differences in responses as the surfactants became increasingly complex. In the current study, we seek to further characterize the mechanical properties of bitumen films after interactions with the same analogue pond water-soluble surfactants. Thus, current investigations focus on measurements of dilational interfacial rheological effects resulting from the interactions of toluene/ and toluene-diluted bitumen/surfactants in buffer that simulate tailings pond water. The results are expected to help determine conditions for stability−instability and future pond water treatment protocols.
INTRODUCTION Waste streams in bitumen production contain not only bitumen but also volatile organic solvent that dilutes bitumen. When spread on water, the bitumen interfacial films not only become hazardous to the environment but also represent energy losses. Bitumen interfacial films may contain asphaltenes, resins, solids, and oil-soluble organic acids, which are transported to the oil/ water interfaces over time. There, the materials adsorb and organize into structural configurations that give the film its characteristic rheological properties.1−3 The configurations of species at the interface depend upon the hydrophilic− hydrophobic property, charge, and molecular structures of surfactants and their interactions because they are masstransported and packed at the interfaces. In addition, a diluted bitumen/water interface can interact with any water-soluble surfactants on contact, potentially modifying the structural properties of the film. Such modifications of the films could make the water surfaces amenable to cleanup or could aid disintegration of the films into finely dispersed emulsions. Information on these systems would be essential for designing improved practices for tailings stream management. The surfactants in the tailings ponds may take a variety of reaction paths. Interaction with bitumen may be one such path. Many water-soluble surfactants are released during the hotwater extraction process used to separate the bitumen from the oil sand. Among these are naphthenic acids (NAs), which are typical bituminous surfactants. They generally represent mixtures of organic carboxylic acids (R-COOH). The R group can be linear, monocyclic, or polycyclic. The relevance of oil sand naphthenates and their chemistries were highlighted in 1989 by Strausz. 4 Actual extractions and chemical identifications of naphthenates and surrogate compounds of that family in tailings water were undertaken in 2001 by Holowenko et al.,5 in 2002 by Rogers et al.,6 and in 2003 by Published 2013 by the American Chemical Society
Received: March 5, 2013 Revised: June 5, 2013 Published: June 5, 2013 3613
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Table 1. Measured pH and Interfacial and Physical Properties of Surfactant Solutions
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surfactant
HAA
CPPA
SN
CMC (mol kg−1) area per molecule (nm2) near CMC (mol kg−1) in bitumen−buffer study pH near CMC pH at 0.0004 mol kg−1 pH at 0.0087 mol kg−1
0.0437 0.38 ± 0.02 0.043 4.4 8.1 6.0
0.0153 0.47 ± 0.03 0.015 5.7 8.3 6.1
0.0242 0.52 ± 0.04 0.024 9.3 8.5 9.0
of the SNs was determined at 323.15 K by vapor pressure osmometry (K-7000 VPO, KNAUER, Berlin, Germany) described earlier. Solutions of surfactants required for interfacial study were made by dissolving a given mass of surfactant in the buffer. The pH of all aqueous solutions was adjusted by adding 1.0 M HCl or 1.0 M NaOH from Fisher Scientific. The pH was measured using a Fisher Research AR-50 meter and combination glass electrodes (model 300729, Denver Instrument, Bohemia, NY) after three-point calibrations using standardized pH 4, 7, and 10 solutions obtained from Fisher Scientific. Measurements of Interfacial Rheology. The interfacial rheology measurements were performed using the Tracker from Teclis-IT Concept, Marseille, France, as described in previous publications.27,28 All interfacial tensions were calculated using axisymmetric drop shape analysis (ADSA). A 250 μL syringe with an attached U-shaped needle was filled with the oil phase. The syringe was mounted in the syringe holder positioned above a quartz cuvette containing 25 mL of aqueous solution. The syringe was lowered such that the needle was immersed in the aqueous phase of the cuvette, and a 20 μL drop (interfacial area A = 34.5 mm2) was expelled to the tip of the needle. Prior adjustments in alignments were made such that the drops were in line with the optics and charge-coupled device (CCD) camera of the instrument to obtain clear visualization on the computer screen. The drops were dilated using an amplitude of 10% of the drop area A or approximately 3.45 mm2 in controlled sinusoidal oscillations for selected frequencies. Area oscillation resulted in periodic changes in the interfacial tensions for the same frequency ν, as described in the theory.29−32 The small-amplitude and controlled harmonic perturbation permitted the corresponding magnitude and phase of the interfacial tension response to be harmonic. During each oscillation, the images were processed for calculation of the interfacial tension and area variations using ADSA software. Sinusoidal oscillation was conducted in a frequency sweep mode for a range from 0.005 to 0.5 Hz. The specific frequencies employed were 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, and 0.5 Hz (periods of 200, 100, 50, 20, 10, 5, and 2 s, respectively). After an oscillation at one frequency, a 1 min rest period allowed recovery of the interface prior to implementation of a change to the next oscillation frequency. Toluene− and Bitumen−Surfactant Systems. Rheological measurements were conducted for toluene-diluted bitumen/buffer + surfactant interfaces at three different concentrations of each surfactant dissolved in the buffer. Table 1, taken from part 1 (10.1021/ ef400376v), gives the near-CMC surfactant concentrations used for the dilational studies: HAA at 0.043 mol kg−1, CPPA at 0.015 mol kg−1, and SN at 0.024 mol kg−1. Table 1 also summarizes the pH of the aqueous solutions of surfactants at (a) near CMCs, (b) trace (0.0004 mol kg−1) surfactant concentrations, and (c) full interfacial coverage or saturated (0.0087 mol kg−1) concentrations. The pH was measured before each dilational study. We encountered difficulty in rheological measurements when using SNs near the CMC. Drop instability occurred, seen as diffusion and stranding. We therefore chose to compare the rheological responses for only the two simpler surfactants at this concentration. The pH at the CMC of each surfactant varied significantly. Thus, it was important to study the systems at very low surfactant concentration, where pH is better controlled, and at the interfacial coverage concentration. The benchmark concentration of 0.0087 mol kg−1 was selected for all three surfactants because this value is common to the linear regions of σeq
EXPERIMENTAL SECTION
The dynamic responses in part 1 (10.1021/ef400376v) allowed us to establish experimental concentrations, equilibrium conditions, and the pH at which the comparative dilational rheology could be studied in part 2. The controls in part 2 are the same as in part 1 (10.1021/ ef400376v), viz., the toluene only with surfactants and toluene-diluted bitumen as a function of pH of the buffer. From our earlier observations of pH changes with surfactant additives at their CMCs, we had selected and studied effects of trace surfactant concentrations, first to control pH and second to observe how a few free molecules would interact with the interfaces. The saturated or full interface coverage concentration common to the three surfactants selected from the results of part 1 (10.1021/ef400376v) is another benchmark condition used for this study. Materials and Methods. Bitumen from Athabasca oil sand (AOSB) was obtained after Dean−Stark extraction of an oil-rich fresh bitumen froth taken from CanmetENERGY pilot extraction and froth treatment operations in Devon, Alberta. The mass fractions of saturates, aromatics, resins, and asphaltenes (SArRA) were 0.176, 0.461, 0.173, and 0.190, respectively, as determined by American Society for Testing and Materials (ASTM) D2007 methods.26 The ash content of the bitumen that we used was 0.146 wt % and was negligible in the samples tested after toluene dilution and centrifugation. The total acid number (TAN) determined by ASTM D664 methods was 2.52 mg of KOH/g of bitumen. Fisher Scientific spectroscopic-grade toluene was used for preparing diluted bitumen. A solution of 10 wt % AOSB (86.5 g L−1) in toluene was prepared by adding bitumen to preweighed toluene in a glass jar. The jar was capped, and the contents were mixed for 2 h using a wrist-action shaker until no lumps were observed by light microscopy. In our earlier studies on the interfacial properties of bitumen-in-toluene/ NaHCO3, a critical concentration at which there is a break to constant equilibrium interfacial tension σeq (CAC) was reported to be 10.16 wt % (87.9 g L−1) bitumen in toluene.2 Above this concentration, the interface is saturated with surface-active species present in the bitumen. Throughout the present study, the dilational interfacial rheological measurements were conducted for toluene-diluted bitumen close to its CAC. The inorganic salts NaHCO3 and K2CO3 used for buffer preparation were of ACS-grade obtained from Fisher Scientific. The buffer solution was made with 0.01 M NaHCO3 plus 1.37 × 10−4 M K2CO3 and was used as the water-continuous phase in the dilational interfacial study of the diluted bitumen droplets. This buffer had a pH of 8.5. Three water-soluble surfactants were selected on the basis of their increasingly complex structures. The published chemical structures were shown earlier in part 1 (10.1021/ef400376v). The linear-chain hexanoic acid (C5H11COOH) is a simple linear carboxylic acid referred to as HAA. Similarly, the 3-cyclopentylpropionic acid (C5H9CH2CH2COOH), which has an added cyclic structure attached to the linear R chain, is abbreviated CPPA. The carboxylic acids were obtained from Sigma-Aldrich Chemical Co., Canada. The sodium naphthenates (SNs) were earlier characterized and illustrated in part 1 (10.1021/ef400376v) as a mixture of various chemical structures. SN, a white-yellow crystalline material from Eastman Kodak Company, Rochester, NY, was used without further purification. NAs generally represent a complex mixture of alkyl-substituted acyclic and cycloaliphatic carboxylic acids, denoted by the general formula CnH2n + zO2, where n is the carbon number and z is zero or a negative number that specifies a homologous series. The number-averaged molecular weight 3614
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function of the interfacial layer, and τ is the relaxation time. In the oscillating drop experiments for the given frequency, the area is defined as
versus ln cs plots for each surfactant and also because the interface was known to be at full coverage. Each surfactant was dissolved in 0.01 M NaHCO3 plus 1.37 × 10−4 M K2CO3 buffer at each of the concentrations, and these were used as water phases. The pH values of all of the surfactant solutions were also measured. Toluene plus 10 wt % AOSB in toluene was used as the oil phase throughout. Two kinds of control experiments were performed: (1) 10 wt % AOSB in buffer solution (pH 8.5) in the same buffer with the pH adjusted to the same pH as for those experiments with surfactant present in solutions and (2) toluene drops in surfactant solutions at the low concentration (0.0004 mol kg−1) and at the typical benchmark concentration of 0.0087 mol kg−1 common to the three surfactants as indicated in part 1 (10.1021/ef400376v). Dilational viscoelasticities were determined following 15 h of equilibration of droplets of both toluene control and 10 wt % AOSB in toluene in the buffer solution with and without surfactants.
A = A 0 + Ã sin(2πυt )
and the harmonic response of the interfacial tension is γ = γ0 + γ ̃ sin(2πυt + ϕ)
THEORY OF INTERFACIAL DILATIONAL RHEOLOGY Small harmonic periodic dilational expansion and contraction of an oil drop with adsorbed surface-active components at the interface33−37 result in measurable dilational stress. The periodic change in interfacial tension is defined in
E=
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A (t ) − A 0 ΔA = A A0
(1)
(2)
the interface is described as completely elastic. A smallamplitude periodic perturbation at frequency ν also results in an area perturbation expressed as ΔA = Aẽ i2πυt
(3)
where ΔA is the change in interfacial area, Ã is the amplitude of area oscillation, ν is the sinusoidal oscillation frequency, and t is time. The dilational stress is then the sum of purely elastic and purely viscous terms as in Δγ = E0α + ηα̇
(4)
where E0 is the dilational interfacial elasticity, η is the dilational interfacial viscosity, and α̇ is the rate of interfacial area deformation. A system is described as viscoelastic when adsorbed layers exhibit relaxation at the interface as well as in the bulk. The dilational viscoelasticity E is thus the sum of the real dilational elasticity E0 and the imaginary Ei = 2πνη or dilational viscous modulus. E is described as the complex viscoelastic modulus, the relationship between E and the dilational stress and area, also called the complex dilational viscoelastic modulus or dilational viscoelasticity. E=
Δγ = E0 + i 2πυη = E0 + iEi ΔA /A 0
(5)
For a low-amplitude perturbation, E(ν) or E is simply expressed in the linear Fourier formalism.35,36,38 Δγ =
∫0
t
E(̑ τ )α(t − τ )dτ
γ̃ exp(iϕ) Ã /A 0
(9)
RESULTS AND DISCUSSION Interfacial Rheology Bitumen/Buffer and pH. Control 1: pH Varied in Buffer. The interfacial viscoelasticities of toluene-diluted bitumen in buffer with pH adjusted to four specific values appear in panels a−c of Figure 1. Figure 1a shows that the total viscoelasticity E rises linearly as a function of oscillation frequency ν, for each pH isoline. The slopes appear to be the same for the isolines at pH 6.0, 6.1, 8.5, and 9.0. However, for all frequencies, the E versus ν isolines for pH 6.0−6.1 are above those for pH 8.5−9.0. As frequency increases, all of the isolines are parallel, indicating similar responses. The fact that viscoelasticity is lowered significantly at pH 8.5−9.0 indicates that, with saponification of the natural surfactants in bitumen, a reordering occurs and indigenous surfactants of bitumen pack efficiently at the bitumen/buffer interfaces to produce more interfacial rigidity. There is little difference in the E values between pH 6.0 and 6.1 or pH 8.5 and 9.0. Similar responses are reflected for isolines of elasticity E0 as a function of frequency ν, at the four pH values, as shown in Figure 1b. Increased oscillation frequencies introduced more highly elastic responses in the system. The flow properties for interfaces at increased frequencies of dilation/contraction are depicted in the changes in Ei versus ν isolines in Figure 1b. Differentiation only begins at ν of 0.05 for the pH 9.0 isoline compared to those at the other lower pH values. Viscous moduli are much higher for the bitumen/buffer interface at the basic pH isolines starting from ν = 0.1 Hz. The interface then exhibits frequency-dependent viscous moduli at larger ν. Figure 1c, which shows the phase angle versus ν isolines for these systems, confirms that the bitumen systems are dominated by the elastic modulus for higher frequencies at all pH values. The higher pH produces less elastic responses than the lower pH. Interfacial Rheology at Low Surfactant Concentrations and Basic pH. Toluene/Buffer−Surfactants. The viscoelastic behavior of toluene droplets in surfactant solutions at a low concentration (0.0004 mol kg−1) and basic pH was measured. Panels a−c of Figure 2 show the viscoelastic modulus E, elastic modulus E0, viscous modulus Ei, and phase angle ϕ plotted as functions of frequency ν for the toluene/ surfactant interface (a) HAA, (b) CPPA, and (c) SN above pH 8.0. From Figure 2a, it can be seen that the dilational moduli (E, E0, and Ei) and phase angle ϕ all show little or no lowfrequency dependence for a low HAA surfactant concentration.
where γ0 is the initial interfacial tension and γ(t) is the interfacial tension in the period or at time t. During harmonic oscillation, if the dilational interfacial stress is directly proportional to the area variation α=
(8)
where A0 is the reference interfacial area, Ã is the amplitude of the area oscillation, γ0 is the equilibrium interfacial tension, and γ̃ is the amplitude of the interfacial tension oscillation. ϕ is the phase shift between the area oscillation and the interfacial tension responses and is the same as the phase shift for the complex dilational modulus E in
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Δγ = γ(t ) − γ0
(7)
(6)
where Ȇ is the inverse Fourier transform of E(ν), the frequencydependent complex modulus also envisaged as the transfer 3615
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Figure 1. Comparisons of (a) viscoelastic modulus E, (b) elastic modulus E0 (solid symbols) and viscous modulus Ei (open symbols), and (c) phase angle ϕ versus frequency ν for 10 wt % AOSB/buffer at different pH values.
The phase angles are all very small (less than 10°), indicating that the adsorbed layer at the toluene/buffer interface is predominantly elastic in nature. Similar responses are observed for CPPA shown in Figure 2b, depicting minor differences from HAA in the low-frequency regime. Similarly, the adsorption of trace SN at the toluene/buffer interface imparts few if any frequency-dependent attributes. Figure 2c indicates that SN effects on interfacial viscoelasticity are significantly less than those for adsorbed HAA and CPPA at all frequencies. Next, we compare the interfacial rheology of toluene/buffer to that for AOSB/buffer with and without surfactants adsorbed. AOSB/Buffer and Toluene/Buffer Interfaces Compared. Low-deformation dilational viscoelastic measurements were conducted for the 10 wt % AOSB/buffer interface containing three surfactant solutions at a low concentration of 0.0004 mol kg−1. Panels a and b of Figure 3 show the elastic modulus E0 and viscous modulus Ei plotted as functions of frequency ν on a semi-logarithmic scale for all three surfactants adsorbed at AOSB/buffer and toluene/buffer interfaces compared to the control AOSB/buffer without surfactants. Panels a and b of Figure 3 indicate that both the dilational elastic and viscous
Figure 2. (a) Effect of oscillation frequency on the viscoelastic modulus E, elastic modulus E0, viscous modulus Ei, and phase angle ϕ for toluene/0.0004 mol kg−1 HAA (pH 8.1). (b) Effect of oscillation frequency on the viscoelastic modulus E, elastic modulus E0, viscous modulus Ei, and phase angle ϕ for toluene/0.0004 mol kg−1 CPPA (pH 8.3). (c) Effect of oscillation frequency on the viscoelastic modulus E, elastic modulus E0, viscous modulus Ei, and phase angle ϕ for toluene/0.0004 mol kg−1 SN (pH 8.5).
moduli E0 and Ei resulting from AOSB interfacial interactions with all three surfactants increase monotonically with increasing frequency. The AOSB/buffer interfacial systems with HAA and CPPA were identical to that of the control AOSB/buffer interface. The SN interaction with the AOSB/buffer interface caused a reduction in both elastic and viscous moduli for all oscillation frequencies. This observation suggested interfacial modification, perhaps by molecular displacement, causing increases in rigidity and reduction in viscous flow. Because the phase angle decreased simultaneously and reflected the increasing frequency-dependent elasticity and because the 3616
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aqueous phase. The pH values of all of the surfactant solutions were measured and appear in Table 1. Recall that the concentration of 0.0087 mol kg−1 was selected because it fell in the declining linear region of the σeq versus ln cs plot shown earlier in part 1 (10.1021/ef400376v) and indicated that the interface is saturated or at full coverage for all three surfactants. The dilational rheological properties for control toluene/buffer + surfactant interfaces measured at the same surfactant concentration and pH, for each surfactant, are compared to those for AOSB/buffer under similar conditions. The summary results for the 10 wt % AOSB/buffer controls for which pH was adjusted to those of the surfactant solutions were presented above. HAA Effects. Panels a and b of Figure 4 compare the E0 versus ν and Ei versus ν isolines for three surfactants and
Figure 3. (a) Elastic modulus E0 versus dilational frequency for 0.0004 mol kg−1 surfactant interaction with AOSB/buffer and toluene/buffer interfaces at basic pH: AOSB/buffer, pH 8.5; AOSB/HAA, pH 8.1; AOSB/CPPA, pH 8.3; and AOSB/SN, pH 8.5. (b) Viscous modulus Ei versus dilational frequency for 0.0004 mol kg−1surfactant interaction with AOSB/buffer and toluene/buffer interfaces at basic pH: AOSB/ buffer, pH 8.5; AOSB/HAA, pH 8.1; AOSB/CPPA, pH 8.3; and AOSB/SN, pH 8.5.
viscoelasticity E was identical to E0, we did not include these graphs. For lower frequencies (0.01−0.02 Hz), the toluene/CPPA system is as elastic as the AOSB/CPPA and AOSB/HAA interfaces. Below 0.01 Hz, the toluene/CPPA system showed the highest elastic moduli. The order for E0 in the control toluene/surfactant systems was CPPA > HAA > SN for all frequencies. The dependence of E0 upon frequency lessened in the same order, with toluene−SN the least dependent. Figure 3b shows the isolines for AOSB/buffer plus surfactant, which indicate frequency dependence for viscous modulus Ei. Thus, interfacial flow was slower in these systems with SN. At such low concentrations, HAA and CPPA had virtually no effect on the interfacial flow of AOSB buffer. CPPA and HAA had no effect on Ei relative to the control AOSB/buffer interface. Figure 3b also shows that the viscous moduli for the toluene/buffer + surfactant systems were all the same and values were very low at all oscillation frequencies. The isolines for toluene/buffer + surfactants are significantly lower than for the bitumen/buffer + surfactant systems. Next, we examine the effects at the benchmark-saturated and common interfacial coverage concentration for these surfactants at the AOSB/buffer interfaces compared to toluene/buffer + surfactant. AOSB/Buffer Interfaces and Saturated Surfactant Concentration of 0.0087 mol kg−1. Recall that in this method, three surfactants were each dissolved in buffer to the same concentration of 0.0087 mol kg−1 and were used as the
Figure 4. (a) Elastic modulus E0 versus frequency ν and (b) viscous modulus Ei versus frequency ν for 10 wt % AOSB/buffer, 10 wt % AOSB/buffer + HAA, and toluene/buffer + HAA at pH 6.0.
control systems. In Figure 4a, the interface for toluene/buffer + HAA shows the highest interfacial dilational elasticity compared to AOSB/buffer and AOSB/buffer + HAA. The E0 versus ν isoline for AOSB/buffer + HAA is parallel to and about 10 mN m−1 lower than the isoline for the toluene/buffer + HAA system. Above 0.05 Hz, the HAA adsorption also appears to depress E0 for AOSB/buffer + HAA to values lower than that for the control AOSB/buffer. However, all systems show a linear increase in dilational elasticity as oscillation frequency increases. The comparisons of the Ei versus ν isolines for three systems in Figure 4b indicate that the viscous modulus for toluene/ buffer + HAA is unaffected by the increased dilational frequency. Above 0.02 Hz, the Ei versus ν isoline is also lower than those for AOSB/buffer and AOSB/buffer + HAA interfaces, which rise steadily as frequency increases. All systems showed the same Ei at 0.02 Hz. Above this oscillation 3617
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frequency increased. However, up to 0.02 Hz, no difference was made in Ei by CPPA, which systematically caused reductions in the Ei values from 0.05 to 0.5 Hz. All interfacial systems were dominated by the elastic property at all frequencies. SN Effects. Elastic moduli for all systems were greater than viscous moduli as dilational oscillation frequencies increased. Panels a and b of Figure 6 show E0 versus ν and Ei versus ν isolines for the three systems described above. Figure 6c shows the phase angle ϕ versus ν isolines.
frequency, the HAA interaction with AOSB/buffer interface caused a slightly reduced viscous modulus in this case. This behavior indicated that adsorbed HAA caused a reduction in interfacial flow. However, the interfacial elasticity still dominates the viscosity. The effects of CPPA on these systems are compared next. The pH in this case is 6.1 in all three buffers. It was shown earlier (panels a−c of Figure 1) that, between pH 6.0 and 6.1, the differences between the interfaces were negligible. CPPA Effects. Elastic moduli for all systems interacting with CPPA were greater than viscous moduli. Panels a and b of Figure 5 show E0 versus ν and Ei versus ν isolines for the three
Figure 5. (a) Elastic modulus E0 versus frequency ν and (b) viscous modulus Ei versus frequency ν for 10 wt % AOSB/buffer, 10 wt % AOSB/buffer + CPPA, and toluene/buffer + CPPA at pH 6.1.
systems described above and show trends similar to those for HAA shown before. The adsorption of CPPA at the toluene/ buffer interface caused a lower dilational elastic modulus E0 than HAA (Figure 4a) for all oscillation frequencies and reached the value of 15 mN m−1 at 0.05 Hz. The E0 of the corresponding system with HAA was shown earlier to be 15 mN m−1 at 0.005 Hz. The adsorption of CPPA at the AOSB/ buffer interface caused little or no change in that interface at low dilational oscillation frequencies up to 0.1 Hz. Above this frequency, there was a slight reduction of E0. However, in both AOSB cases, there was a linear rise in E0 versus ν, while the curve for the toluene/buffer + CPPA system rose and then leveled to a plateau. The Ei versus ν isolines for the three systems are compared in Figure 5b. In the case for the toluene/buffer interface with surfactant, CPPA resulted in a slightly lower Ei versus ν isoline than the analogous HAA shown in Figure 4b. For the toluene/ buffer + CPPA system, only a slight decline in the Ei value occurred with an increased frequency. All systems were the same at 0.005 Hz. The AOSB/buffer and AOSB/buffer + CPPA interfaces showed increased viscous moduli as oscillation
Figure 6. (a) Elastic modulus E0, (b) viscous modulus Ei, and (c) phase angle ϕ versus frequency ν for 10 wt % AOSB/buffer, 10 wt % AOSB/buffer + SN, and toluene/buffer + SN at pH 9.0.
It is clearly shown in Figure 6a that there is a steady and steep rise in E0 versus ν for the AOSB/buffer system at pH 9.0. The adsorption of SN at the saturated concentration of 0.0087 mol kg−1 in solution caused the AOSB/buffer interface to show drastic reductions in E0 at all ν, and from the observation of the horizontal data isoline, we conclude that there is no dependence upon ν. E0 for toluene/buffer + SN is close to zero for all oscillation frequencies tested. 3618
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Figure 6b shows that the Ei versus ν isoline for AOSB/buffer differs drastically from that for the AOSB/buffer + SN. It appears that SN may have caused some displacement of indigenous surfactants at the interface or may have packed into available spaces by adsorption. SN caused the viscous moduli to decrease significantly at all oscillation frequencies tested. It appears that, although there is a slight rise in the Ei value as ν increased, the bitumen interfacial interaction with SN still caused the elastic modulus to dominate the viscous modulus. The Ei versus ν plot for the toluene/buffer + SN system shows that SN was adsorbed at this concentration and pH and caused little if any interfacial viscous flow, leading to interfacial rigidity. The overall change in the toluene/buffer + SN system is indicated by the ϕ versus ν plotted in Figure 6c. In this figure, the decrease in ϕ versus ν is sharp for the AOSB/buffer interface and confirms the dominance of elasticity. When SN is adsorbed, the rise in ϕ versus ν confirms the reduced elasticity and higher viscosity at the interfaces. The plots for toluene/ buffer + SN show S-shaped ϕ versus ν curves with higher ϕ values. There is no competition for space at this toluene/buffer interface; therefore, the surface-active species can organize more effectively. We next examine the three interfaces at near-breakpoint concentrations23 of the two simpler surfactants HAA and CPPA. The pH remains as it occurs for their solutions. It was not possible to make measurements for SN at this concentration. AOSB/Buffer Interface Effects at Near-CMC Surfactant Concentrations. Panels a−d of Figure 7 show the viscoelastic modulus E, elastic modulus E0, viscous modulus Ei, and phase angle ϕ plotted as functions of oscillation frequency ν on a semi-logarithmic scale for AOSB/buffer without surfactants (panels a and b of Figure 7 at pH 8.5 and 6.0) as controls and for a comparison to AOSB/buffer with HAA (Figure 7c) and CPPA (Figure 7d) at their CMCs in solution. Measurements could not be performed with SN at its CMC because the drops under these conditions had extremely low interfacial tensions, underwent diffusion and stranding with slight oscillation, and detached during the measurement process. Dilational rheological properties were frequency-dependent for all systems. With increasing frequency ν, the complex viscoelastic modulus E and elastic modulus E0 show parallel and similar increases. The plot of E versus ln ν is almost linear over the entire frequency range studied. All oil/water interfaces showed a viscous modulus Ei that was also frequencydependent, indicating some energy dissipation, perhaps by reorganization at the interfaces that facilitated flow. The viscous modulus Ei was lower than the elastic modulus E0 at all frequencies for all systems, which suggests that the elastic property of the oil/water interfaces was dominant in all of the AOSB buffer systems. Although the viscous modulus Ei also increased with increasing oscillation frequency, it did not show as steep a rise. The phase angle ϕ decreased as the viscoelastic modulus increased with frequency, reflecting the increase in the interfacial elasticity. Panels a−c of Figure 8 show the dilational viscoelasticity of the AOSB/buffer interface comparing the influence of pH to those of the two carboxylic acids HAA and CPPA at near CMCs. The pH of 6.0 is close to the pH for the systems with surfactants. Panels a and b of Figure 8 show that both carboxylic acids cause significant decreases in the dilational elastic modulus E0. For CPPA, the effect of reducing E0 is less than that for HAA. For HAA, the elastic modulus E0 is very
Figure 7. (a) Effect of oscillation frequency ν on the viscoelastic modulus E, elastic modulus E0, viscous modulus Ei, and phase angle ϕ for the 10 wt % AOSB/buffer interface at pH 8.5. (b) Effect of oscillation frequency ν on the viscoelastic modulus E, elastic modulus E0, viscous modulus Ei, and phase angle ϕ for the 10 wt % AOSB/ buffer interface at pH 6.0. (c) Effect of oscillation frequency ν on the viscoelastic modulus E, elastic modulus E0, viscous modulus Ei, and phase angle ϕ for 10 wt % AOSB/0.043 mol kg−1 HAA interface at pH 4.4. (d) Effect of oscillation frequency ν on the viscoelastic modulus E, elastic modulus E0, viscous modulus Ei, and phase angle ϕ for 10 wt % AOSB/buffer + 0.015 mol kg−1 CPPA interface at pH 5.7.
small, less than 1 mN m−1, even at the highest oscillation frequency. The viscous modulus Ei also decreases dramatically with the addition of the carboxylic acids. The surfactants appear to interact with or displace the surface-active materials in the oil 3619
dx.doi.org/10.1021/ef4003928 | Energy Fuels 2013, 27, 3613−3621
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can assist the future design of treatment protocols geared toward recovery of tailings bitumen and mitigation of environmental impacts. Future studies will examine the properties and behaviors of bitumen films on surface water/ air interfaces after interaction with these surfactants.
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CONCLUSION (1) The dilational moduli (E, E0, and Ei) and phase angle ϕ were frequency-dependent for all bitumen−surfactant systems, while the dilational rheological properties show little or no lowfrequency dependence for toluene/surfactant interfaces at low surfactant concentrations. (2) For all of the bitumen/surfactant interfaces, the complex viscoelastic modulus E and elastic modulus E0 show parallel and similar increases with increasing oscillation frequency ν. Viscous modulus components are all lower than elastic modulus components for all systems studied, indicating that the interface is predominantly elastic in nature. (3) The addition of a trace amount of HAA and CPPA into the aqueous buffer phase does not change the viscoelastic properties of the bitumen/buffer interface. SNs appear to displace some of the surface-active species in bitumen and form a mixed adsorption layer, while drastically reducing the dilational viscoelasticity. (4) The addition of SN at a fixed benchmark concentration for saturated conditions causes a significant decrease in the dilational moduli (E, E0, and Ei), while for HAA and CPPA, the responses are weaker. (5) The simpler surfactants adsorbed near their CMCs are the most effective at reducing the interfacial viscoelasticity of AOSB/ buffer interfaces. (6) SNs have stronger adsorption and substitution abilities than HAA and CPPA and cause the system to be significantly less elastic and more rigid.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding for this work was provided by the Government of Canada’s Panel on Energy Research and Development (PERD), the CanmetENERGY Devon Research Centre of Natural Resources Canada, and the Eco Energy Innovation Initiative of Canada (EcoEII).
Figure 8. (a) Elastic modulus E0, (b) viscous modulus Ei, and (c) phase angle ϕ versus frequency ν for AOSB/buffer with and without surfactants.
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drop and cause the decreases in the elastic and viscous moduli. Figure 8c shows that, although the phase angle, which reflects the increased elasticity with frequency, decreased for all systems with surfactants, the control at pH 6.0 appears to show almost constant ϕ at all oscillation frequencies. However, the interface is still dominated by the elasticity. Overall in this research, we show that the interaction of tailings pond surfactant analogues with diluted bitumen drops caused a drastic change in the interfacial dilational rheology toward more rigidity and reduced elasticity. HAA interacts more effectively than CPPA. The complex SN appears to cause the most changes in the dilational interfacial rheology of the systems through adsorption, starting from low to saturated concentrations. At the benchmark-saturated concentration and near CMC, the AOSB/buffer systems were affected less by pH reduction than by the simple carboxylic acids. The results of this work improve our understanding of the pathways taken by soluble tailings pond surfactants when bitumen is present and 3620
NOMENCLATURE σeq = equilibrium interfacial tension (mN m−1) A = interfacial area (mm2) A0 = initial interfacial area (mm2) A(t) = interfacial area at time t (mN m−1) Ã = amplitude of the area oscillation (mm2) ΔA = change in interfacial area (mm2) α = area variation ν = sinusoidal oscillation frequency (Hz) t = time (s) i = imaginary number γ0 = initial interfacial tension (mN m−1) γ(t) = interfacial tension at time t (mN m−1) Δγ = change in interfacial tension (mN m−1) γ̃ = amplitude of the interfacial tension oscillation (mN m−1) α̇ = rate of interfacial area deformation ϕ = phase angle (deg) dx.doi.org/10.1021/ef4003928 | Energy Fuels 2013, 27, 3613−3621
Energy & Fuels
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(24) Rosen, M. J. Micelle formation by surfactants. Surfactants and Interfacial Phenomena; John Wiley and Sons: Hoboken, NJ, 1989; pp 108−169. (25) Angle, C. W.; Dabros, T.; Hamza, H. A. Chem. Eng. Sci. 2006, 61, 7309−7324. (26) American Society for Testing and Materials (ASTM). ASTM D2007, Fractionation of Crudes by SARA Analysis; ASTM: West Conshohocken, PA, 2000. (27) Teclis-IT Concept. The Tracker, Automatic Drop Tensiometer for the Measurement of the Rheological Characteristics of the Interfaces; Teclis-IT Concept: Marseille, France, 2012; http://www.itconceptfr. com/Pageshtml/Tracker_Desc_VA.html. (28) Benjamins, J.; Cagna, A.; LucassenReynders, E. H. Colloids Surf., A 1996, 114, 245−254. (29) Benjamins, J.; Lyklema, J.; Lucassen-Reynders, E. H. Langmuir 2006, 22, 6181−6188. (30) Fainerman, V. B.; Aksenenko, E. V.; Lylyk, S. V.; Makievski, A. V.; Ravera, F.; Petkov, J. T.; Yorke, J.; Miller, R. Colloids Surf., A 2009, 334, 16−21. (31) Fainerman, V. B.; Mys, A. V.; Aksenenko, E. V.; Makievski, A. V.; Petkov, J. T.; Yorke, J.; Miller, R. Colloids Surf., A 2009, 334, 22− 27. (32) Lucassen-Reynders, E. H.; Cagna, A.; Lucassen, J. Colloids Surf., A 2001, 186, 63−72. (33) Leser, M. E.; Acquistapace, S.; Cagna, A.; Makievski, A. V.; Miller, R. Colloids Surf., A 2005, 261, 25−28. (34) Miller, R.; Liggieri, L. Curr. Opin. Colloid Interface Sci. 2010, 15, 215−216. (35) Ravera, F.; Ferrari, M.; Santini, E.; Liggieri, L. Adv. Colloid Interface Sci. 2005, 117, 75−100. (36) Ravera, F.; Loglio, G.; Kovalchuk, V. I. Curr. Opin. Colloid Interface Sci. 2010, 15, 217−228. (37) Ravera, F.; Loglio, G.; Pandolfini, P.; Santini, E.; Liggieri, L. Colloids Surf., A 2010, 365, 2−13. (38) Cui, X. H.; Zhang, L.; Luo, L.; Zhang, L.; Zhao, S.; Yu, J. Y. Colloids Surf., A 2010, 369, 106−112.
E = complex viscoelastic modulus or dilational viscoelasticity (mN m−1) E0 = dilational interfacial elasticity or elastic modulus (mN m−1) η = dilational interfacial viscosity (mN s m−1) Ei = dilational viscous modulus (mN m−1) Abbreviations
HAA = hexanoic acid CPPA = 3-cyclopentylpropionic acid SN = sodium naphthenate NA = naphthenic acid n-C7 = n-heptane TAN = total acid number IFT = interfacial tension VPO = vapor pressure osmometry AOSB = bitumen from Athabasca oil sands SArRA = saturates, aromatics, resins, and asphaltenes ASTM = American Society for Testing and Materials API = American Petroleum Institute CAC = critical aggregation (breakpoint) concentration for bitumen CMC = critical micelle concentration
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