ARTICLE pubs.acs.org/EF
Influence of Alkaline pH on the Rheology of Water/Acidic Crude Oil Interface David Arla,† Lionel Flesisnki,‡ Patrick Bouriat,‡ and Christophe Dicharry*,‡ † ‡
SINTEF Petroleum Research, NO 7465, Trondheim, Norway Laboratoire des Fluides Complexes et leurs Reservoirs, UMR TOTAL CNRS 5150, Universite de Pau et des Pays de l0 Adour, BP 1155, 64013 Pau, Cedex, France ABSTRACT: Interfacial tension and rheology of water/acidic oil (w/o) interfaces at neutral and alkaline pH conditions have been investigated by means of an oscillating drop tensiometer. The acidic oil phases used for this study were a West-African crude oil and a heavy distilled fraction of this oil, both diluted in cyclohexane to decrease oil viscosity and allow interfacial rheology measurements. In contact with water of initial pH of 6.5, both oils formed a 2D-gel near its gelation point at the w/o interface. For initial pH beyond 8, ionized molecules like naphthenates formed and adsorbed massively to the w/o interface, modifying the interfacial tension and rheology. We found that naphthenates did not impede but rather delayed the formation of the 2D-gel, and their presence could significantly reduce the gel strength, decreasing the stability of the corresponding w/o emulsions. Because the gel formation was not found to be an instantaneous process, it more surely contributes to the strengthening of emulsions with aging rather to their initial stability.
1. INTRODUCTION The long-term persistence or stability of water-in-oil (w/o) emulsion, that is, water droplets finely dispersed in a continuous oil phase, gives considerable challenges to the petroleum industry because of the difficulties in separating oil from water and the problems of corrosion in both production and refinery facilities.1,2 Sedimentation and coalescence of the water droplets are the two processes to be completed as fast as possible in petroleum separators, within a few tens of minutes typically. Oil companies have now to deal with heavy and biodegraded crude oils, which compared to light oils exhibit lower American Petroleum Institute (API) degrees, higher viscosity, and substantial amounts of particles3 as well as amphiphile compounds such as asphaltenes, resins, and naphthenic acids.4 The specific properties of these crude oils induce increasing times both for the sedimentation and the coalescence, which in the latter case may be as long as several months if not treated. To avoid this detrimental situation, permanent addition of demulsifiers and installation of separator units including electrostatic coalescers are employed,5 but these solutions represent considerable capital expenditures that need to be anticipated. For these reasons, it is of paramount importance to develop reliable and convenient methods to foresee the difficulties for separating water from crude oil, especially with regard to the coalescence. The water-crude oil separation difficulties are often attributed to the presence of a rigid or viscoelastic film at the w/o interface, which acts as a physical barrier to coalescence.6 In some cases, a skin is clearly visible to the naked eye when the area of the w/o interface is rapidly contracted: the interface looks crumpled.7,8 Many works have highlighted that asphaltenes, which are defined as the fraction of the crude oil insoluble in n-pentane (or n-heptane) but soluble in toluene (or benzene), strongly contribute to the formation of the viscoelastic film and thus to the w/o emulsion stability.9-13 However, the asphaltenes are not a class of molecules chemically well-defined and may differ r 2011 American Chemical Society
substantially in terms of molecular weights, contents of heteroatoms and metals, and extracted amounts depending upon the origin of crude oil and the procedure used for the extraction.14 Furthermore, it has been shown that the interfacial properties of asphaltenes were affected by the solvent properties13 and the resins15 (the fraction of crude oil insoluble in liquid propane but soluble in n-pentane and n-heptane) as well as the type of interaction between asphaltenes and naphthenates, the conjugate base of naphthenic acids. Both cooperative1,8,10 and antagonistic16-18 interactions were observed depending upon the chemical structure of the naphthenic acids, the pH, and the presence of counterions like sodium and calcium in the aqueous phase. Initially, the term naphthenic acids referred to saturated cyclic carboxylic acids containing one polar head COOH,19 but many different classes of acids were identified in crude oils later on.20,21 Such a complexity of the amphiphile compound chemistry shows that the prediction of the water-crude-oil separation difficulties from the fluid chemistry may not be seen as a realistic option, at least in light of the current understanding. Because interfacial tension measurements alone do not explain emulsion stability, interfacial dilatational rheology has been investigated to evaluate the mechanical properties of the interfacial film.22 Adequately probing the interface response to sinusoidal deformations offers the possibility of determining parameters such as the dilatational elasticity modulus E and the loss angle φ .23 However, the correlation of the interfacial rheological parameters with emulsion stability remains quite challenging.24 The paradigm according to which the higher is the elasticity modulus the greater is the level of emulsion stability has shown validity in some cases,4,10,13 but it cannot be generalized because there are counterexamples.25,26 Yarranton et al.27 correlated the free water resolved from emulsions with the Received: November 5, 2010 Revised: January 26, 2011 Published: February 17, 2011 1118
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Table 1. Characterization of the Oil Phases SARA fractionation results (wt %) system
content (wt %)
AH
100
viscosity (cP)
sat.
aro.
res.
asph.a
TAN (mg/g) 1.25
23.6
80
52.5
32
14.5
1
104
21.2
35.3
40.8
2.7
0.34
52.4
32.1
14.6
0.9
1.24
>520 c
recombined oil a
API (deg)
b
100
Defined as the insoluble fraction in n-heptane. b Measured at 20 °C. c Calculated from the distillate data.
crumpling ratio, defined as the projected area of the droplet when the crumpling was first observed divided by the initial projected area, and the initial compressibility of the interfacial film, which is dimensionally the reciprocal of the elasticity modulus. Bouriat et al.,28 who studied the rheology of a model water/asphaltedcyclohexane interface, interpreted the rigidity and skin appearance of the interfacial film as the result of the formation of a 2Dgel near its gelation point composed of colloidal aggregates of asphaltenes because E and φ satisfied the following equations: E µ S3fn
ð1Þ
π 2
ð2Þ
φ¼n
where S was identified as the strength of the gel, f was the oscillation frequency of the interfacial area, and n was a constant (0 < n < 1). The same behavior was also observed for different types of crude oils that favored stable emulsions.8,25,29 On the other hand, Dicharry et al.25 found that the w/o interfaces formed from deionized water and the distilled fractions of a crude oil which did not produce stable w/o emulsions, did not exhibit the rheological characteristics of a 2D-gel near its gelation point. Though significant advances have been made to relate the interfacial dilatational rheology to the stability of w/o emulsions, the crude oils are so complex and different that it is still necessary to perform further experiments to test the validity of the concepts listed previously. The objective of the present work is to confront the concept of Bouriat et al.28 to the influence of alkaline pH on water/acidic crude oil interfaces. The acidic crude oils are the class of heavy and biodegraded crude oils rich in naphthenic acids, and they are of interest because many fields have been discovered in West Africa, the North Sea, and in Venezuela. They can pose severe problems of emulsion stability1 even though in some cases the total acid number (TAN) of the crude oil is as low as 0.2 mg KOH/g.8 During the oil production process, the increase in pH due to the degassing of CO2 may lead to the gradual ionization of acidic species such as naphthenic acids (RCOOH) into naphthenates (RCOO-), which greatly affects the interfacial properties of these crude oils. A West African acidic crude oil provided by Total and a heavy distillation fraction of this crude oil were used here because the stability of the corresponding w/o emulsions had already been studied as a function of the pH and the water content.16 For the two oils, we investigated the effect of alkaline pH on the ionization of acidic species, the interfacial tension, and rheology, as well as complementary emulsion stability tests. We end with a discussion on the manner to correlate the interfacial rheology to the stability of emulsions.
2. EXPERIMENTAL SECTION 2.1. Materials. Cyclohexane (99.9% HPLC grade) and NaOH pellets (>97%) were purchased from Sigma-Aldrich and were used without further purification. Deionized water was produced by the Millipore Milli-Q 185 E system (conductivity 520 was decreased to a few centipoises by dilution in the cyclohexane. The proportions of AH and >520 in the diluted oils were chosen to be 50 and 16 wt %, respectively, the latter in accordance with the proportion of the >520 in AH (Table1). The samples for oscillating pendant drop and final pH (pHf) measurements were prepared as follows: 10 mL of the diluted oil phase was added to 30 mL of the alkaline water of pHi in 50-mL bottle tests. The bottle tests were mounted in a stainless-steel rack, which rotated them 150° to either side of the vertical direction, completing a cycle every 5 s, during 24 h. The samples were left quiescent over 48 h to achieve a complete water-oil separation, and the pHf of the aqueous phase was measured. Then, small amounts of the oil phase (100-250 μL) and the aqueous phase of pHf (∼ 7 mL) were taken and introduced into the syringe and the thermostatted cell of the pendant drop tensiometer (see below), respectively. The emulsions were prepared by stirring the oil phase with an UltraTurrax T25 homogenizer at 8000 rpm for 2 min, while slowly pouring the aqueous phase on it at ambient temperature. The water content was fixed at 30 wt %. The emulsions were kept at 20 °C for 24 h before being centrifuged at 6000 rpm for 30 min. Emulsion stability was evaluated by measuring the amount of resolved water at the bottom of the tube after centrifugation. 2.3. Interfacial Rheology. The measurements were performed with an oscillating pendant drop tensiometer from IT Concept. An oil 1119
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Figure 1. (a) Final pH, pHf as a function of initial pH, pHi and (b) ionization rate of acidic species from eq 7 versus initial pH for a 30 mL of alkaline solution in contact with a 10 mL AH-cyclo (full symbols) and >520-cyclo (open symbols). Dashed straight line in (a) indicates the location where pHi and pHf are equal, while the curves in (b) indicate the ionization rate assuming a complete acid-base reaction. drop was formed and maintained vertically at the tip of an inverted needle in a transparent thermostatted cell containing the aqueous phase. The volume of the drop (typically from 4 to 7 μL) was adjusted to reach a bond number (ratio of the gravitational force to surface tension force) of 0.15-0.2. The apparatus is designed for programming sinusoidal variations of the drop area. Amplitude and frequency of the sinusoidal variations are applied to the syringe piston by computer-controlled servomotors. Images of the drop are recorded in real time with a CCD camera. From the drop profile and the densities of the oil and aqueous phases the interfacial tension is calculated. The relative area variation and the interfacial tension response make it possible to calculate the complex interfacial dilatational elasticity E* with eq 3: E ¼
dγ dln A
ð3Þ
where A* is the complex area of the drop and γ* is the complex interfacial tension, assuming that the interfacial area response is sinusoidal. Then one can evaluate the real and imaginary parts ε0 and ε00 of the complex interfacial dilatational elasticity: E ¼ ε0 þ jε0
0
00 ε ε0
IRð%Þ ¼ 100
ð4Þ
with j = (-1)1/2. ε0 characterizes the conservative behavior of the interface, whereas ε00 is related to dissipative or loss interfacial phenomena. The interfacial dilatational modulus, E, and the loss angle, φ, are given by the following: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð5Þ E ¼ ε02 þ ε002 φ ¼ tan-1
successive pressure drops increases the final pH leading to the gradual ionization of acids initially present in the oil phase. In our experiments, the ionization of acids took place during the mixing or contact of crude oil and alkaline water, due to acid-base reaction between the acids and sodium hydroxide NaOH initially present in the crude oil and the alkaline water, respectively. Figure 1a shows the decrease in pH of the alkaline waters after the contact with each oil phase, where pHi and pHf refer to the initial and final pH, respectively. For each crude oil, pHf was systematically lower than pHi because a certain amount of the OH- (pKa ≈ 14, strong base) had reacted with RCOOH to form RCOO- (pKa ≈ 5, weak base). To estimate the extent of the acid-base reaction, we use the ionization rate (IR) defined as the molar ratio between formed RCOO- and RCOOH initially in the oil phase. The former is deduced from the reacted OH-, which is proportional to the difference (10pHi-14 - 10pHf-14). The latter is obtained from the TAN of the corresponding oil phase. Thus, IR is given by:
ð6Þ
To evaluate the complex interfacial dilatational elasticity, E*, with eq 3, the interfacial tension response to the sinusoidal stretch of the area must be sinusoidal. The linear response regime is obtained by imposing small area amplitude variations on the drop. In our experiments, the area amplitude was smaller than 10%.
3. RESULTS AND DISCUSSION 3.1. pH and Ionization rate of Acids. In acidic oil fields, the final pH of aqueous phases is mainly controlled by the equilibrium between CO2 in the gas phase and carbonated species dissolved in the aqueous phase such as HCO3- and CO32-.31 During the production, the degassing of the CO2 due to
¼ 100
RCOOformed RCOOHinitial ð10pHi - 14 - 10pHf - 14 Þ VW TANDO dDO =MKOH VDO
ð7Þ
where MKOH, VW, TANDO, dDO, and VDO refer to the molar mass of potassium hydroxide, the volume of water, the total acid number, the density, and the volume of the diluted oil phase, respectively. Figure 1b shows the IR of both crude oils as a function of pHi. The symbols are obtained directly from eq 7 and the corresponding pHi and pHf shown in Figure 1a. Ionization of acids mainly occurred when the pHi was varied from 8 to 10.5 with >520-cyclo and from 8 to 11.5 with AH-cyclo because AH contained more acids than >520. The symbols at the highest pHi in particular with >520-cyclo could not be represented in Figure 1b because the values were much higher than the expected 100% due to a lack of accuracy in the pHf measured above 11 (ΔpHf ≈ 0.1). The curves in Figure 1b correspond to the amount of formed RCOO- assuming a complete acid-base reaction, i.e., all of the initial OH- reacted to form RCOO-. They are obtained by neglecting the term including pHf in eq 7 and allow us to determine whether the formation of RCOO- was complete over the whole range of pHi or not. With >520-cyclo, the formation of RCOO- was complete at all pHi because the symbols coincide with the theoretical curves. With AH-cyclo, the formation of 1120
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Energy & Fuels RCOO- was complete until pHi 11.5 but was incomplete from pHi 11.5 to 12. Actually, the procedure for mixing oil and water might not be optimal to allow the acid-base reaction with the last ionizable acids of AH-cyclo. On the other hand, stronger mixing conditions would have induced emulsions, which would have been more detrimental for achieving a complete oil-water separation (see Section 3.4). Lastly, the pHf of aqueous phases at a given pHi was lower with AH-cyclo than with >520-cyclo. Depending upon the range of pHi, different reasons may account for this observation. For pHi below 8, the IR of both crude oils was zero, which means that there was no RCOO- formed from the acid-base reaction. Accordingly, the pHf could not be affected by these RCOO- but potentially by low molecular weight acids from crude oils that partitioned in the aqueous phase. Indeed, it has been shown on fatty acids that even though the molecular weight had a little impact on the pKa32 it strongly affected the partitioning between oil and water.33 Because the acids of AH were essentially distributed in the light distillate fraction 520-cyclo were heavier than the ones from AH-cyclo. According to the modeling of pHf with acidic crude oils developed by Hurtevent et al.,1 the differences in pHf observed with AH-cyclo and >520-cyclo might be due to the differences in partitioning of their acids. For pHi above 10.5, there was an excess of OH- in the aqueous phase in contact with >520-cyclo compared to the aqueous phase in contact with AH-cyclo because there were fewer acids to neutralize with the former oil than with the latter. This excess of OH- explains the higher pHf observed with >520cyclo than with AH-cyclo in this range of pHi. 3.2. pH and Interfacial Tension. Figure 2 shows the interfacial tension (IFTeq) of the water/AH-cyclo and water/>520cyclo systems as a function of pHi measured 14 h after the drop formation. For both systems, the IFTeq vs pHi-curve showed a rather constant value for pHi lower than about 8 followed by a steep decrease for increasing pHi. Similar trends have already been observed with other acidic crude oils reflecting the higher surface activity of ionized species such as naphthenates.35 For pHi below 8, the IFTeq with AH-cyclo was lower than with >520-cyclo. Since the influence of ionized molecules is negligible at this range of pHi (IR ≈ 0 with both oils) and since the amount of resins and asphaltenes in AH-cyclo and >520-cyclo are the same, the main differences between the two systems are saturatearomatics ratio and content in acids (Table 1). The ratio may affect the state of solvation of asphaltenes and then their interfacial properties,36 but this effect must be tiny due to the high amounts of resins in both systems.37 The higher content in acids of AH-cyclo may explain the lower IFTeq value for AHcyclo. However, it should be emphasized that the gain in IFTeq is about 2-3 mN/m, only, for systems exhibiting large different values of TAN (∼ 0.625 and 0.05 mgKOH/g for AH-cyclo and >520-cyclo, respectively). The IFTeq of AH-cyclo and >520-cyclo started to decrease for pHi above 8, which corresponds to the beginning of ionization of naphthenic acids. At pHi around 11, the IFTeq of both systems was practically the same, but the IFTeq of AH-cyclo kept decreasing sharply at higher
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Figure 2. Interfacial tension for water/AH-cyclo (filled symbols) and water/>520-cyclo (open symbols) systems measured 14 h after the drop formation at different pHi.
pHi while the reduction with >520-cyclo was more moderate. To look at the effect of the acid content on the interfacial tension, the IFTeq has been compared with the IR. With AH-cyclo, the decrease in IFTeq correlated well to the increase of the IR from pHi 10 to 12. Above pHi 12, all the acidic molecules were ionized and the system reached interfacial tensions too low to be measured with our pendant drop tensiometer (oil drops pulled away from the needle instantaneously during their formation). We made the same observation with AH only at pHi above 12, and values as low as 0.1 mN/m were found by spinning drop tensiometry.16 The occurrence of such low IFT values suggests the presence of a complex mixture of surface active compounds at interface, probably composed of light and heavy naphthenates. Synergism in surfactant mixtures for lowering IFT has been widely reported in the literature.38,39 With >520-cyclo, the correlation IFTeq vs IR was no longer valid because the interfacial tension still decreased from pHi 10.5, the value for ionizing all the acidic molecules contained in >520-cyclo, to 12.5. By assuming that ionization of acids occurred at the latter pHi, eq 7 gives a TAN of 40 mg KOH/g for >520, which implies that the decrease in IFTeq at pHi above 10.5 cannot be due to the ionization. Instead, “salt effect” resulting from the increase in concentration of sodium by a factor 100 when the pHi of alkaline water was changed from 10.5 to 12.5 may allow more adsorption of surface agents charged negatively through screening of polar heads. In Figure 3 are shown the variations of IFT as a function of time for the water/AH-cyclo and water/>520-cyclo systems at different pHi. Even though oil and water in the pendant drop experiments were pre-equilibrated during 72 h for mixing and separation, the IFT still exhibited variations that depended upon the pHi. At pHi of 6.5 and 8, the IFT of both systems did not level off but decreased steadily with time. Although naphthenic acids, resins, and asphaltenes could all adsorb to the w/o interface, different observations support the fact that the behavior of interface was governed by the asphaltenes. The slight decrease in IFT observed over 14 h, might reflect the slow reorganization of adsorbed macromolecules already observed with model oils including asphaltenes, only.13,28,40 Besides, we measured the zeta potential of asphaltenes extracted from AH as a function of initial pH by following the procedure of Alvarez et al.41 The isoelectric point was found at pHi of 4.2 and the zeta potential was 1121
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Figure 3. Interfacial tension as a function of time for AH-cyclo (left) and >520-cyclo (right) systems at different pHi.
below -30 mV at pHi above 6.5, which means accordingly that the interfaces between AH-cyclo, >520-cyclo, and waters of pHi of 6.5 and 8 might be composed of the asphaltenes exhibiting a negative net charge rather than non ionic molecules such as RCOOH. At pHi 10, the IFT decreased during the first 3 h, and then leveled off. In this case, interfaces with AH-cyclo and >520-cyclo also contained ionized species that adsorbed faster and required lesser time for reorganization than at the lowest pHi. At the highest pHi, a minimum in IFT as well as a brownish coloration of the aqueous phase were clearly observable. From the literature, this behavior is due to the adsorption at the w/o interface and transfer into the water phase of highly surface active species.42,43 Though RCOO- are certainly the principal contributors, ionized asphaltenes may also be involved in the observed minimum.12,44 Indeed, the slow increase in IFT, which follows the minimum, might reflect the filling with replacement acidic species from the bulk oily phase of the interfacial vacancies let by the acidic species transferred to the water phase. 3.3. pH and Interfacial Rheology. The dilatational elasticity modulus (E) of water/AH-cyclo and water/>520-cyclo interfaces as a function of time measured at a same frequency of drop oscillation (f = 0.1 Hz) and at pHi of 6.5 and 11.3 is shown in Figure 4. The interfacial rearrangement of the adsorbed macromolecules at low pHi mentioned in the latter paragraph is supported by the increase of E over a long period of time (open symbols in Figure 4). On the other hand, E more rapidly reached a constant limiting value at higher pHi (full symbols in Figure 4) suggesting faster adsorption kinetics and lesser interfacial rearrangement of the ionized acidic species. The values of E for the water/>520-cyclo interface increased slightly when pHi varied from 6.5 to 11.3, whereas it decreased drastically for the water/AH-cyclo interface. The difference in molecular weight of the acidic species present in AH and >520 could explain the opposite trends observed for E with the variation of pH. In the case of >520, which contained the heaviest acidic molecules of the crude oil, the adsorption of the ionized surface active species probably showed higher irreversibility. The electrical repulsion between them might also allow less freedom for the adsorbed molecules to reorganize at the interface. These two phenomena could be responsible for the increase of E with pHi. On the other hand, the decrease of E observed at high pHi for the water/AH-cyclo interface could result from the diffusional exchange between the interface and the bulk phases of a fraction of the adsorbed acidic species. Because diffusional exchange
Figure 4. Elasticity modulus (E) as a function of time for water/AHcyclo (triangles) and water/>520-cyclo (squares) systems at pHi = 6.5 (open symbols) and pHi = 11.3 (full symbols). The frequency of drop oscillations was fixed at 0.1 Hz.
occurs when the time scales of the applied deformation are long enough for adsorption barriers to be overcome, light naphthenic acids and naphthenates would likely be responsible for the observed behavior at the frequency oscillation employed in our experiments. This scenario is consistent with the expected lower average molecular weight, and higher abundance and interfacial activity of the acidic species adsorbed at the water/AH-cyclo interface at high pHi. An increase in solvation of asphaltenes by the formed species may also contribute to the low value of E by hindering their adsorption to the interface. Figures 5 and 6 report the dilatational elasticity modulus and the loss angle of water/AH-cyclo and water/>520-cyclo interfaces measured at different frequencies of drop oscillation and different pHi (6.5, 8, 10, and 11.3). The measurements of the rheological parameters were made on 16 h-aged drops because from Figure 4 steady-state conditions are expected to be reached after this period of time. The linear dependency of log E with log f, and the good correspondence between the measured loss angles and those calculated with eq 2 demonstrate that, for both systems and at all investigated conditions of pHi, the adsorbed amphiphile materials formed an interfacial network which presented the rheological characteristics of a 2D-gel near its gelation point.28 In a previous study,25 we found that, at a pHi of 6.5, the adsorbed layer between water and either AH, or de-asphalted-AH or >520 diluted in cyclohexane formed an interfacial gel. On the other hand, for the 1122
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Figure 5. Elasticity modulus (E) and loss angle (φ) of 16 h-aged water/AH-cyclo interfaces as a function of the frequency of drop oscillations, at 20 °C and different pHi values: (a) 6.5, (b) 8, (c) 10, and (d) 11.3. The horizontal full line corresponds to the loss angle calculated with n in eq 2.
light (520-cyclo interface. Because the number of adsorbed aggregates involved in a 2D-gel is expected to be proportional to S, their interfacial concentration should be lower for the AH system, which is consistent with the lower relative concentration of gel-building materials in AH. For the water/AH-cyclo interface, S showed a much smaller value at a pHi of 11.3 compared to pHi of 6.5, while it kept a high value for the water/>520-cyclo interface. This suggests that the interfacial concentration of gel-building materials at this pHi decreased drastically for the AH system, while it did not vary significantly for the >520 one. The observed tendencies could result from the competitive adsorption between the gel-building materials and other surface-active molecules present in the oil phase; the gel-building materials from AH having to compete with a larger amount of highly surface active species than those from >520. Arguments for this scenario can be found in the literature dealing with the interfacial adsorption of macroamphiphiles.13,44,46,47 For instance, in the review of Wilde et al.46 on the
composition, structure, and physical properties of mixed proteinemulsifier interfaces, surfactants have been reported to adsorb into packing defects of the protein adsorbed layer, forming surfactantrich domains, which weakened the protein network. The extent of weakening was found to be dependent on the surface pressure: the higher the surface pressure, the higher the weakening. Our observations are consistent with this point: the adsorbed acidic species from AH imposed, at a pHi of 11.3, a higher surface pressure than those from >520 (see Figure 2), and the resultant gel appeared weaker. The main difference is that the amphiphiles with the higher surfactancy were not added after the formation of the interfacial network as in the Wilde et al.’s review but formed in situ. In our case, the competitive adsorption between the gel-building materials and the other surface-active molecules took place as soon as the w/ o interface was formed. Under alkaline conditions, the naphthenic acids with the higher surfactancy should adsorb and ionize at the w/ o interface more readily than the acidic macromolecules, which are supposed to be the main gel-building materials. Nevertheless, the latter would manage to reach the interface, displacing a part of the already adsorbed molecules and then forming the interfacial gel. Such a process is thus likely to take place more rapidly at low pHi, where the competitive adsorption between the acidic surface active species should be weaker. To investigate this point, the rheology of the water/AH-cyclo and water/>520-cyclo interfaces was characterized at shorter time, 2 h after the formation of the w/o interface. Figure 7 shows the corresponding results obtained with the pHi of 6.5 and 11.3. Because almost 2 h was necessary to complete the rheological characterization at the different frequencies, the relative variations of E due to the aging for the water/AH-cyclo and water/ >520-cyclo interfaces were lower than 10% at pHi of 6.5 and 3% at pHi of 11.3 over the 2 h (see Figure 4). The rheological 1123
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Figure 6. Elasticity modulus (E) and loss angle (φ) of 16 h-aged water/>520-cyclo interfaces as a function of the frequency of drop oscillations, at 20 °C and different pHi values (a) 6.5, (b) 8, (c) 10, and (d) 11.3. The horizontal full line corresponds to the loss angle calculated with n in eq 2.
characteristics of the water/AH-cyclo and water/>520-cyclo interfaces appeared to correspond to that of a 2D-gel only at low pHi. This point confirms that the competitive adsorption between the gel building materials and the other acidic surface active species of AH decreases the kinetics of formation of the 2D-gel. One can observe that the measured and calculated loss angles at low pHi matched lesser in the case of the water/>520-cyclo interface, although >520 contained a higher relative concentration of gelbuilding materials. The fact that a 2D-gel formed more readily at the water/AH-cyclo interface might result from a synergistic increase in the adsorption kinetics of the gel-building materials. Synergistic interactions between asphaltenes and naphthenic acids have already been reported in the literature.10,48 The naphthenic acids present in the 520 (Table1). As a result, the gel-building materials would adsorb more rapidly for the AH system but their interfacial concentration would be lower than for the >520 system, which led to the formation of a weaker interfacial gel. 3.4. How Emulsion Stability Correlates to the Formation of a 2D-gel at Interface. All of the emulsions made from AHcyclo, 520-cyclo, and waters at different pHi did not show any clear water phase when left under rest during 24 h at 20 °C. Consequently, the 24-h-aged emulsions were centrifuged at 6000 rpm for 30 min and then it was possible to discriminate the stability of the emulsions since different amounts of clear water were observable depending upon the investigated system. The amounts of resolved water measured with the AH-cyclo and >520-cyclo emulsions for pHi varying from 6.5 to 12.5 are reported in Table 2. In a previous work,16 we observed that the stability of the w/o emulsions with AH and >520 was reduced
gradually when increasing the pHi of water due to the appearance of substantial amounts of RCOO-. Here, the tendency was the same with AH-cyclo, whereas it was necessary to reach the highest pHi to see a decrease in stability with >520-cyclo. It is worth mentioning that the ionization rates shown in Figure 1b do not apply here because the proportions of water and oil in the mixture were different in emulsion tests from rheology experiments (30/70 vs 10/30). Let us say that at a pHi of 12.5, all of the acidic species should be ionized, both with the diluted AH and >520. Previously,25 we found that systems which formed an interfacial 2D-gel produced stable w/o emulsions against coalescence even after 24 h of rest and centrifugation. On the other hand, a spontaneous and complete separation of oil and water within few minutes was observed for the systems that did not form such an interfacial structure. In the present study, both the stability of emulsions and the occurrence of the 2D-gel at interfaces were in between. Indeed, the emulsions were stable after 24 h of rest but a separation more or less important appeared after centrifugation in almost all cases. 2D-gels of different strength S were generated depending upon the type of oil and the pHi. Nevertheless, we saw that the formation of such a structure might take hours. The last point suggests that the interfacial gel cannot be responsible for the stability of emulsions in the first instants, which must be due to steric repulsions between amphiphile molecules that adsorbed first to droplets, and/or Gibbs-Marangoni effect. The gel more surely contributes to the long-term stability of emulsions. Indeed, the emulsion lifetime seems to correlate quite well to the gel strength, S. At a pHi of 6.5, the emulsion with >520-cyclo (S = 20) was more stable than the emulsion with AH-cyclo (S = 14). Besides, we noted a gradual increase in S with time, which might 1124
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Energy & Fuels
ARTICLE
Figure 7. Elasticity modulus (E) and loss angle (φ) of 2-h-aged water/AH-cyclo (top) and water/>520-cyclo (below) interfaces as a function of the frequency of drop oscillations, at 20 °C and different pHi values: (a) and (c) pHi = 6.5, and (b) and (d) pHi = 11.3. The horizontal full line corresponds to the loss angle calculated with n in eq 2.
Table 2. Stability of Emulsions resolved watera (wt %) system water/AH-cyclo water/>520-cyclo a
pHi = 6.5 9.6 ( 0.3
