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Interactions of divalent cations with tetrameric acid aggregates in aqueous solutions
Colloids and Surfaces A: Physicochem. Eng. Aspects 396 (2012) 238–245
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Interactions of divalent cations with tetrameric acid aggregates in aqueous solutions Lingling Ge a , Marwyn Vernon a , Sébastien Simon b , Yadollah Maham a , Johan Sjöblom b , Zhenghe Xu a,∗ a b
Department of Chemical and Materials Engineering, University of Alberta, Edmonton AB, T6G 2V4, Canada Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Sciences and Technology (NTNU), N-7491 Trondheim, Norway
a r t i c l e
i n f o
Article history: Received 19 December 2011 Accepted 29 December 2011 Available online 8 January 2012 Keywords: Tetrameric naphthenic acid Divalent cation Single molecular layer membrane Vesicle Ion transport
a b s t r a c t Interactions in aqueous solutions between a tetrameric naphthenic acid in sodium salt (BP10Na4 ) and divalent Mg2+ and Ca2+ cations were investigated by isothermal titration calorimetry (ITC) and dynamic light scattering (DLS). Colloidal aggregation of the tetrameric sodium naphthenates to vesicles and micelles enhances its binding with divalent cations. In vesicle systems, Ca2+ was found to bind first with carboxylate groups located at the external water/lipid interface of vesicles. The cations then diffuse through the membrane to interact with the inner carboxylate groups. Higher Ca2+ concentration and higher salinity gradients enhance cation diffusion. Under solution chemistry conditions where micelle dominates, Ca2+ reacts only with outer carboxylate groups of BP10Na4 without any significant cation diffusion through the aggregates. The binding of calcium with sodium naphthenate is an entropy-driven process where entropy gain comes from the dehydration of both cations and carboxylate groups of BP10Na4 . The affinity of Mg2+ to BP10Na4 is much weaker than Ca2+ due to its stronger hydration and hence larger size of hydrated magnesium ions than hydrated calcium ions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Naphthenic monoacids contain single or multiple-fused cyclopentane and cyclohexane rings where the carboxylic acid group is attached to the aliphatic side chain or to the cycloaliphatic ring. They are mostly found in immature heavy crude, and are assumed to be generated from in-reservoir biodegradation of petroleum hydrocarbons [1]. Naphthenic acids cause serious problems during the heavy oil processing [2]. They corrode production facilities [3] and enhance emulsion stability [4]. The solid deposits of naphthenic acids could cause operational problems with frequent costly shutdowns of operations for cleaning [5]. Among the naphthenic acids, a specific category with four carboxylic acidic groups was discovered by Baugh et al. [6] in 2004 from calcium naphthenate deposits collected from some offshore fields. They are reported to form network through cross-linking with calcium ions at the water/oil (W/O) interface [7]. Differential scanning calorimetry (DSC) investigations on calcium naphthenate deposits collected at W/O interface show that the naphthenic acid network is formed through strong ionic bonds between carboxylic groups and calcium ions. The naphthenate network formed as such has a high cross-linking density with very stiff and rigid bonds [8].
∗ Corresponding author. Tel.: +1 780 492 7667; fax: +1 780 492 2881. E-mail address: [email protected] (Z. Xu). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.074
A series of naphthenic acid model compounds have been synthesized by Nordgård et al. [9]. The well-defined molecular structure of the synthesized compound enables us to clarify the formation and interaction mechanisms of naphthenates at a molecular level. Early studies have shown that the tetrameric acid model compound BP10 exhibits similar physical-chemical properties as indigenous tetrameric acid [10]. The intermolecular reaction of BP10 with calcium ions results in an elastic and “solid like” film at W/O interface in the form of supramolecular network [8]. The naphthenate deposits are observed in BP10 systems containing Ca2+ , Mg2+ , Sr2+ and Ba2+ although these ions differ in size and hydration state [10]. An important question is why the naphthenate deposits found in practice contain almost exclusively calcium even though other divalent cations are also present. Potentiometric titration work showed strongest preference of BP10 to Ca2+ , and the lowest affinity for Mg2+ , with Ba2+ and Sr2+ showing intermediate and roughly equal reactivity with BP10 [11]. Additional systematic and fundamental study is needed to better understand interaction mechanisms of naphthenic acids and divalent cations present in crude oil. The aggregation of tetrameric naphthenic acid in bulk aqueous solutions into vesicles was first discovered by Ge et al. in the crude oil research [12]. Due to its special molecular structure, BP10 was found to form vesicles at BP10 concentration as low as 10−6 mol/L. Vesicles were found to co-exist with micelles at BP10 concentrations higher than its critical micelle concentration (CMC) of 1.0 × 10−3 mol/L. A well-defined lamellar liquid crystal appeared upon addition of a short chain aliphatic alcohol. To better
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system. For DLS experiments, the solutions prepared were filtrated through 0.45 m PVDF Millipore filters directly into dust free light scattering cells, which were sealed immediately to prevent dust. After filtration, the solutions were thermostated at 25 ◦ C without disturbance for more than 30 min before measurement. ITC measurements were performed at 50 ◦ C using a Nano-ITC 2G titration microcalorimeter (TA instruments, USA). The integrated injection-stirrer assembly (250 L) was filled with metal nitrate (M(NO3 )2 ) aqueous solution, which was titrated in portions of 10 L or 5 L into the calorimeter vessel that initially contained 1 mL BP10Na4 aqueous solution. The pH values of both M(NO3 )2 and BP10Na4 aqueous solutions were adjusted to 10. The reference cell was filled with Millipore Milli-Q water. The titration was carried out at the constant stirring speed of 250 rpm to ensure thorough mixing. Each injection was completed in 10 s. To allow equilibration, there was a 420-s time delay between successive injections. Raw data were obtained as a plot of heating rate (J/s) as a function of titration time (s). These raw data were then integrated to obtain a plot of enthalpy change per mole of injectant (M2+ ), Hobs (kJ/mol) referred to in this paper as molar enthalpy of reaction, as a function of M2+ to BP10Na4 molar ratio. All the titration experiments were repeated twice to ensure reproducibility within 2%. 2.2. Dynamic light scattering (DLS)
Fig. 1. Calorimetric titration isotherm of interaction between Ca2+ and BP10Na4 when 2.0 mM Ca(NO3 )2 was injected into 0.1 mM BP10Na4 at 25 ◦ C and pH 10.2: a – heat flow versus time and b – enthalpy of mixing versus Ca2+ /BP10Na4 molar ratio ([Ca2+ ]/[BP10Na4 ]).
understand these aggregation states requires investigations on the interactions between BP10 and divalent cations such as Mg2+ and Ca2+ . In this study molecular interactions was investigated mainly by isothermal titration calorimetry (ITC), which is a popular and high precision technique for chemical interactions [13]. Dynamic light scattering (DLS) is also used to study the topology change of BP10 aggregates upon binding with divalent cations. 2. Experimental 2.1. Materials and sample preparation The synthesis of BP10Na4 was performed at the Ugelstad Laboratory (NTNU, Trondheim, Norway). The detailed procedures of synthesis were reported previously in literature [14]. The name BP10Na4 originates from its molecular structure which contains a benzophenone (BP) core with each phenolic oxygen being linked to an alkyl chain of 10 methylene ( CH2 ) groups terminated with a carboxylic acid group. The molecular structure of BP10Na4 is given below:
All the chemicals including Mg(NO3 )2 ·6H2 O (>98%), Ca(NO3 )2 ·4H2 O (99.9%), acetic acid (99.8%), butyric acid (>99%), octanoic acid (>98%) and citric acid (99.5%) were obtained from Sigma–Aldrich Corp. and used without further purification. Unless otherwise stated, the pH of the systems was adjusted to 10 by the addition of NaOH (Fisher Scientific, 99.5%). The deionized water was filtered using a Millipore Milli-Q UV-Plus purification
DLS measurements were carried out at a scattering angle of 90◦ using an ALV 5022 laser light-scattering (LLS) instrument equipped with a cylindrical He–Ne laser (model 1145p-3083; output power = 22 mW at = 632.8 nm) in combination with an ALV SP-86 digital correlator of a sampling time range between 25 ns and 40 ms. The LLS cell was held in an index matching vat filled with high purity, dust-free toluene. The temperature of the experiments was controlled within ±0.02 ◦ C by a thermostat. The DLS experiments were completed in 10 min and repeated at least twice. In DLS experiments, the intensity–intensity time correlation function G(2) (t, q) was measured, where t is the decay time and q is scattering vector given by q = (4n/0 )sin (/2). G(2) (t, q) is related to the normalized first-order electric field-time correlation function |g(1) (t, q)| via Siegert relation given by [15]
2
G(2) (t, q) = A[1 + ˇg (1) (t, q) ]
(1)
where A (≡I(0)2 ) is the measured baseline. For the broadly distributed relaxation spectrum, |g(1) (t, q)| is related to characteristic relaxation time distribution G( ) by
(1) g (t, q) = E(t, q)E ∗ (0, q) =
∞
G( )e−t d
(2)
0
From Eqs. (1) and (2), the term G( ) in Eq. (2) can be calculated from Laplace transformation of the measured G(2) (t, q). The translational coefficient distribution, G(D) can be calculated from G( )
by equation = Dq2 . The hydrodynamic radius distribution is then determined by the Stokes–Einstein equation: Rh = kB T/6D, where , kB and T are the viscosity of solvent, the Boltzmann constant and the absolute temperature. In this study, the CONTIN program supplied with the correlator was used to calculated G(D) and Rh .
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Fig. 2. Scheme of interaction between calcium ions and BP10Na4 vesicle and concentration dependence of Ca2+ .
3. Results and discussion 3.1. Calcium in vesicle systems (diluted BP10Na4 systems) The heat flow of isothermally titrating interactions of calcium with BP10Na4 at pH 10 is shown in Fig. 1a. By integrating the area under each injection peak, the enthalpy change per mole of Ca(NO3 )2 injected is plotted as a function of Ca2+ to BP10Na4 mole ratio ([Ca2+ ]/[BP10Na4 ]) in Fig. 1b. The vesicle structure of BP10Na4 aqueous solutions is thought as a single BP10Na4 molecular layer (membrane) separating the inner and outer aqueous phases, with two carboxylate groups on each side of BP10 molecules being located at both inner and outer membrane-aqueous interfaces and the hydrocarbon skeleton of the BP10Na4 forming the hydrocarbon phase of the membrane. As investigated by the cryo-TEM in our previous work [12], the vesicles are spherical. The molecular illustration of this unique single molecular layer vesicle is shown in Fig. 2. The decisive step for the tetrameric acid to interact with various cations is the deprotonation of the tetrameric acids [11,14a, 16]. The deprotonation of BP10 is known to occur in a single step. The apparent pKa value of BP10 is reported to range from 6.8 to 8.0, depending on the salinity of the system [17]. This value is higher than the intrinsic pKa value of typical naphthenic acids because the tetrameric acid forms vesicles and micelles in aqueous solutions, resulting in interfaces with different pKa values. In our study, the pH value is kept constant at 10 which are about 3 units higher than the apparent pKa. At this pH, most of the carboxylic groups if not all should be in a deprotonated form. When an electrolyte such as calcium nitrate is added as the titrant into the vesicle solutions, a concentration gradient of calcium ions is created between the outer and inner aqueous phases (illustrated in Fig. 2). This concentration gradient provides a driving force for cation transport to and through the membrane. The results in Fig. 1 show three distinct processes. At the first addition of Ca2+ , a relatively high enthalpy value is determined, which decreases sharply with increasing Ca2+ concentration. This reduction in the molar entropy with increasing calcium ion concentration is attributed to the reduced amount of unbound carboxylic groups and decreased charge density of vesicle. The enthalpy curve
shows a slope that can be extrapolated to a [Ca2+ ]/[BP10Na4 ] value of 1, indicating a complete reaction of carboxylate groups on the external surface of vesicles. Within this concentration range, the added Ca2+ appears to react only with the external carboxylate groups with minimum transport through the membrane. As a result, salinity gradient is not sufficient to cause a noticeable transport of added calcium ions. After complete reaction of external carboxylate groups with calcium ions at [Ca2+ ]/[BP10Na4 ] ratio of 1, the calcium ions added are able to penetrate to the inner side of the vesicle and react with unbound carboxylate groups on the inner surface of membranes, shown by the additional enthalpy. Compared with enthalpy change of calcium ion binding with carboxylate groups on the external surface of vesicles, the enthalpy change on the inner surface is relatively higher. The higher charge density in the inner side of vesicle than in the outside of the vesicle probably results in the higher enthalpy reflected by the reduced slope of the enthalpy curve. Interestingly, the molar enthalpy also decreases linearly with increasing calcium ion addition to a [Ca2+ ]/[BP10Na4 ] value of around 2, after which no enthalpy is detectable with further increasing calcium ion addition. This finding suggests a complete binding of inner carboxylate groups of vesicle membranes by calcium ions. By considering both the structure of the vesicle and the location of the carboxylate groups, the ITC results led us to conclude that the chemical potential of reactions between Ca2+ and COO− at the inner vesicle interface could provide sufficient driving force to cause Ca2+ transport through the single molecular layer vesicle membrane even at low ionic gradient with complete complexation between carboxylate groups at both inner and outer interface at the stoichiometric [Ca2+ ]/[BP10Na4 ] ratio of 2. The results in Fig. 1 show that the interaction between Ca2+ and carboxylate group of BP10Na4 is endothermic (Hmix > 0). The binding of calcium ions with BP10Na4 vesicles is a complex process, involving dehydration of calcium ions (endothermic), dissociation of carboxylate acid groups and sodium ions (endothermic), binding of calcium ions to the carboxylic acid groups of vesicles (exothermic), hydration of sodium ions (exothermic), etc. [18]. The enthalpy measured by ITC is an overall effect of these various enthalpy contributions. The measured positive (endothermic) apparent enthalpy
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value suggests a predominant contribution of dehydrating calcium ions to the overall enthalpy of the reaction. Unfortunately, we can only get this qualitative conclusion. Nevertheless, this observation is consistent with the results from previous studies of calcium/lipid vesicle interactions by Sinn et al. [17]. To quantitatively determine the contributions of individual sub-processes to the overall enthalpy remains a major challenge to the world of thermodynamicists. Since Hmix > 0 as detected, it is evident that the spontaneous binding of Ca2+ to naphthenic vesicles is an entropy driven (S > 0) process to meet the thermodynamic criterion of Gmix (=Hmix − TS) < 0 for the observed spontaneous binding. Investigations of the interaction between Ca2+ and lipid membranes composed of phosphatidylcholine–phosphatidylserine mixtures showed that the entropy gain comes from the dehydration of lipid vesicles [17]. In the calcium and naphthenate vesicle system under investigation, dehydration of both calcium and carboxylate groups on bilayer membrane of single molecular layer vesicles contributes to the entropy gain. The DLS results to be reported later will shed further light on these mechanisms. 3.2. Calcium in mixture of vesicles and micelles (higher BP10Na4 concentrations) To understand the interactions of calcium ions with vesicles and micelles, the ITC experiments were conducted at BP10Na4 concentration of 10 mM which is well above its CMC (1 mM) [12]. The titration results are shown in Fig. 3a. For comparison, the ITC results obtained at BP10Na4 concentrations of 0.1 mM which is well below its CMC are also shown in Fig. 3a. The difference between the isotherms obtained at higher and lower BP10Na4 concentrations is attributed to the formation of BP10Na4 micelles [19]. In micelle dominated system (10.0 mM BP10Na4 ), the transport behaviour of calcium ions through the vesicles membrane becomes subordinate. Since divalent calcium ions unlikely penetrate into the hydrophobic core of the micelle, only reactions of calcium ions with external carboxylate groups of BP10Na4 on micelle are seen in 10 mM BP10Na4 solutions, leading to not only a lower molar enthalpy but also a gradual decrease in molar enthalpy of reaction with increasing calcium to BP10Na4 molar ratio up to 2. Naturally the complexation will end up with [Ca2+ ]/[BP10Na4 ] of 2 since all surface carboxylic groups on micelle surfaces are available for reaction. In addition, the white flocculants were observed in both systems, although it appears at much lower [Ca2+ ]/[BP10Na4 ] in 10 mM BP10Na4 system than that of 0.1 mM BP10Na4 system. This difference also contributes significantly to the lower enthalpy value as well as the roughness of the curve in 10 mM BP10Na4 system. The flocculants appeared in the system will be discussed in Section 3.5. 3.3. ITC of BP10Na4 by Mg Magnesium is another important metal ion present in both produced water and seawater [11,19]. Different types and concentrations of cations are expected to play a critical role in determining ion/vesicle interactions. As shown in Fig. 3, at a given BP10Na4 concentration, the molar enthalpy of interaction between Mg2+ and BP10Na4 is much lower than that between Ca2+ and BP10Na, indicating a weaker interaction between Mg2+ and BP10Na4 than between Ca2+ and BP10Na4 . By potentiometric titration, Sundman et al. also showed that the binding of Mg2+ was quite weak compared to Ca2+ [11]. Such a big difference can be attributed to the differences in hydration levels of the cations. Mg2+ is known to have a hydration number of 6 in comparison to 2 for Ca2+ [19]. Since the direct binding of divalent cations with carboxylate groups requires dehydration of cations, a larger dehydration enthalpy is needed for magnesium ions than calcium ions prior to their binding with
Fig. 3. Enthalpy change as a function of divalent ion to BP10Na4 molar ratio when titrating different concentrations of BP10 with Ca(NO3 )2 and Mg(NO3 )2 at 25 ◦ C and pH 10.2.
carboxylate groups, leading to an overall smaller molar enthalpy of magnesium ions than calcium ions interacting with carboxylate groups of BP10Na4 . Strongly hydrated Mg2+ seems to react with BP10Na4 at much weaker strength. This is attributed to a higher (six) hydration number of magnesium and therefore a much larger radius of hydrated ions. With a larger radius of hydration sheath, it is much more difficult for magnesium to approach and bind with carboxylic group of BP10Na4 . In fact, the binding of calcium and magnesium ions with carboxylic group of BP10Na4 does not require dehydrating all the hydrated water molecules. Dehydrating one from six hydrated water molecules around a magnesium ion would require a much less energy than dehydrating one from two hydrated water molecules around a calcium ion. That is the main reason why a lower enthalpy is measured in the system of magnesium/BP10Na4 than in calcium/BP10Na4 system. More interestingly, at both BP10Na4 concentrations, Fig. 3 shows no distinct transition between interactions of magnesium
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with outer and inner carboxylate groups. It appears that hydrated magnesium ions of larger size are unable to penetrate through BP10Na4 single layer membranes to react with inner carboxylate groups. 3.4. ITC of fatty acids by Ca and Mg The interactions between M2+ and BP10Na4 are complex, involving direct ionic complexation, conformational changes of hydrocarbon chain and ionic transport linked with different hydration states. All these interactions are collectively measured in the values of Hmix at different concentrations and M2+ /BP10Na4 molar ratios. In order to isolate some of the interactions, a set of different organic acid molecules, such as acetic acid with only one carboxylic acid group per molecule, citric acid of three carboxylic groups but without hydrocarbon chain, and butanoic and octanic fatty acids with different hydrocarbon chain length, mimicking the hydrocarbon chains of BP10Na4 were studied by ITC. The concentration of these organic acids and M2+ to acid molar ratio were chosen in such a way that no aggregation was allowed in the aqueous solution. Fig. 4a shows the enthalpy curves of these systems. It is interesting to note that compared with BP10Na4 interacting with Ca2+ , the molar enthalpy of acetic acid, butanoic acid and octanoic acid binding with Ca2+ is almost negligible. These results indicate a weak interactions of Ca2+ with monomeric acid groups. It can therefore be inferred that the higher heat value of BP10Na4 and Ca2+ interaction mainly comes from interaction of calcium ions with carboxylate groups of BP10Na4 aggregates, involving both ionic transport and conformation changes in the hydrocarbon chain of BP10. In the case of Ca2+ interacting with citric acid, the reaction is exothermic at lower Ca2+ /citric acid molar ratio. (Fig. 4a) The exothermic heat value decreases sharply upon Ca2+ injection, approaching zero at the molar ratio of about 1.5. A further increase in Ca2+ /citric acid molar ratio leads to a negligible enthalpy indicating the completion of calcium interactions with citric acids. These results are similar to those reported by Christensen et al. [20] and Keowmaneechai and McClements [21] who investigated interactions of citric acid and divalent cations under slightly different conditions. As shown in Fig. 4b, the enthalpy change in the Mg2+ /acid systems with increasing Mg2+ /acid molar ratio mirrors Ca2+ /acid systems. However, the exothermic value of citric acid interacting with Mg2+ is higher than that with Ca2+ . Nonetheless, there is no clear transition of enthalpy for both Ca2+ and Mg2+ interacting with fatty acid, in contrast to the interaction of calcium with BP104 at concentrations below its CMC, as clearly shown in Fig. 4a. 3.5. DLS measurements Our previous investigations on aggregation of tetrameric naphthenic acid, BP10Na4 in aqueous solutions revealed the formation of spherical vesicles at BP10Na4 concentration as low as 10−3 mM. The size of the vesicles was found to range from 45 nm to 200 nm, depending on BP10Na4 concentration [12]. The present investigation focuses on the effect of calcium ions on the morphology of the vesicles formed from BP10Na4 solutions. Dynamic light scattering is a proven technique to provide information on aggregate topology in solution. In this study, the concentration of BP10Na4 is kept constant at 0.1 mM, where well-defined vesicles were found to exist in the system [22], while [Ca2+ ]/[BP10Na4 ] molar ratio changes from 0 to 2.6. The results in Fig. 5a show that the correlation function decays in a single step mode, indicating one kind of aggregates, i.e., vesicles, predominating the system. The addition of Ca2+ at all [Ca2+ ]/[BP10Na4 ] molar ratio tested does not break the single molecular layer vesicles.
Fig. 4. Enthalpy value change with cation to acid molar ratio when titrating various acids with M(NO3 )2 at 25 ◦ C and pH 10.2. The concentrations of acids are specified in respective figure.
Relaxation time distributions in Fig. 5b obtained by the fitting of the corresponding correlation functions show a faster relaxation of vesicles with increasing [Ca2+ ]/[BP10Na4 ] molar ratio. Since the smaller, the particle size, the faster, the relaxation [23], the results in Fig. 5b suggest a reduction in the size of vesicles with increasing [Ca2+ ]/[BP10Na4 ] molar ratio. As shown in Fig. 6, the hydrodynamic radius obtained according to Stoke–Einstein relationship [24] decreases from about 45 nm in the absence of Ca2+ to about 30 nm at [Ca2+ ]/[BP10Na4 ] = 2.0. Also shown in Fig. 6 is an initial increase in scattering intensity with the addition of Ca2+ , reaching a maximum at [Ca2+ ]/[BP10Na4 ] of 2, followed by a sharp reduction with further increasing Ca2+ additions. The transition in scattering intensity at [Ca2+ ]/[BP10Na4 ] molar ratio of 2 agrees with the result from ITC. The enhanced scattering intensity of vesicle solutions at constant scattering angle can be attributed to both the dehydration of membrane by binding of calcium ions with carboxylate groups and the consequences of vesicle-vesicle interactions by reduced charges of vesicles due to calcium ion binding. In membranes of amphiphilic molecules, two kinds of water were
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Fig. 6. Apparent hydrodynamic radii and relative scattering intensity as a function of Ca(NO3 )2 to BP10Na4 molar ratio at pH 10. The concentration of BP10Na4 is kept constant at 0.1 mM.
Fig. 5. a – DLS intensity–intensity time correlation function at various Ca(NO3 )2 to BP10Na4 molar ratio at pH 10. b – Relaxation time distributions obtained by the corresponding correlation function. The concentrations of BP10Na4 in this set of tests are kept constant at 0.1 mM.
well accepted: free water located far away from amphiphiles and free to move; and bound water attached to the hydrophilic groups [25]. Binding of divalent cations with hydrophilic carboxylic groups tends to replace bound water. Dehydration of membranes upon calcium binding was reported earlier by Sinn et al. in the vesicle
system of phosphatidylcholine–phosphatidylserine [17]. In a separate study, Berquand et al. also showed that the presence of Ca2+ in the aqueous solution reduces the thickness of water layer [26]. In the present system, the enhancement of scattering intensity of shrunk vesicles supports the dehydration of membrane, and hence the increase in the refraction index of the condensed membrane. On vesicle–vesicle interactions, the vesicles are stabilized mainly by electrostatic repulsion from the negatively charged carboxylate groups. The binding of Ca2+ with carboxylate groups on vesicles neutralizes the charges, reducing and eventually eliminating the repulsive forces between the vesicles. Such conditions lead to flocculation of vesicles and hence higher scattering intensities. In fact, small pieces of semi-transparent flocculates are visually observed to form in the samples at [Ca2+ ]/[BP10Na4 ] molar ratios higher than 2, which supports the destabilization of the vesicle system. As indicated in Section 2, all DLS samples were filtered with millipore with an average pore diameter of 0.45 m. Therefore, we can safely conclude that the sharp decrease in scattering intensity is due to the flocculation of vesicle aggregates. Fig. 7 shows optical micrographs of flocculated samples which remain stable for several months. For the mixture of carboxylic acid and alkali earth metal ions at higher concentrations, three possible situations are responsible for the solid-like phase: (1) highly ordered aggregation resulting in formation of crystals; (2) random aggregation resulting in amorphous precipitation; and (3) aggregation process intermediate between these two extremes, yielding the formation of gels [27]. The darker yellow pattern on the optical micrograph taken under normal light corresponds to the white floccules. The dark texture of the floccules under polarized light rules out the case of well-ordered liquid crystals mentioned above.
Fig. 7. Photograph of sample, and microscopy graph under normal and polarized light in the system of 0.1 wt% BP10Na4 /0.05 wt% Ca(NO3 )2 /H2 O at pH 10.
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For them to be liquid crystals, a shinning texture would be observed under the polarized light [28]. However, an interesting feature of bright boundary between cotton-like phases and clear solution phase was observed under polarized light, indicating the presence of well organized boundary layers in micrometer thickness. As also shown in our previous investigation [12], tetraacid BP10Na4 is an excellent molecular candidate to build single molecular layer membranes, liquid crystals and hydrogels with vast potential applications yet to be exploited. The gels observed in the system belong to the low-molecular-weight (LMW) hydrogels. The LMW hydrogels, formed by non-covalent self-assembly of small molecules, are of wide applications in personal care, foods, pharmaceutical and biomedical applications for their benefit of reversibility, solution chemistry responsiveness, and the relative mild gelation conditions [29–32]. One of the attractive properties of the LMW gel in the BP10Na4 /Ca(NO3 )2 /H2 O system is its low gelator content (as low as 0.1 wt%). In addition, this type of LMW gel is of high stability. These two characters are attributed to the molecular structure of four chains linked to a polyaromatic sheet, resulting in high capabilities of cross linking in the presence of divalent cations. Detailed studies of the BP10Na4 LMW gel is in progress in our lab. 4. Conclusions Formation of vesicles and micelles of tetrameric naphthenate, BP10Na4 enhances its binding with divalent cations in aqueous solutions. The interaction of divalent cations and carboxylate groups of BP10Na4 is entropy driven, with entropy gain coming from the dehydration of both the divalent cations and carboxylate groups of aggregated BP10Na4 . Two-step binding of calcium cations with naphthenate vesicles is observed, in contrast to a single step binding of calcium ions with BP10Na4 micelles. Compared to Ca2+ the binding of Mg2+ with naphthenate is weaker due to its higher degree of hydration. These findings explain the phenomenon that most of the tetrameric naphthenate deposits obtained during the heavy oil processing are exclusively calcium salts, although the total contents of Ca2+ and Mg2+ are of approximately the same order of magnitude in sea water. Acknowledgement This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Industrial Research Chair program in Oil Sands Engineering. We also like to acknowledge the financial support from members of the consortium JIP 2:3 (Akzo Nobel, Bp, Champion-Technologies, Clariant Oil Services, ConocoPhillips, Statoil ASA, Talisman Energy and TOTAL) and Research Council of Norway through the PETROMAKS programme. Canada Foundation for Innovation (CFI) was also acknowledged for the facility of DLS. References [1] B.F. Lutnaes, Ø. Brandal, J. Sjöblom, J. Krane, Archaeal C80 isoprenoid tetraacids responsible for naphthenate deposition in crude oil processing, Organic and Biomolecular Chemistry 4 (2006) 616–620. [2] S. Simon, E. Nordgård, P. Bruheim, J. Sjöblom, Determination of C80 tetra-acid content in calcium naphthenate deposits, Journal of Chromatography A 1200 (2008) 136–143. [3] E. Slavcheva, B. Shone, A. Turnbull, Review of naphthenic acid corrosion in oil refining, British Corrosion Journal 34 (1999) 125–131. [4] D. Arla, A. Sinquin, T. Palermo, C. Hurtevent, A. Graciaa, C. Dicharry, Influence of pH and water content on the type and stability of acidic crude oil emulsions, Energy and Fuels 21 (2006) 1337–1342.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Interactions of divalent cations with tetrameric acid aggregates in aqueous solutions Lingling Ge a , Marwyn Vernon a , Sébastien Simon b , Yadollah Maham a , Johan Sjöblom b , Zhenghe Xu a,∗ a b
Department of Chemical and Materials Engineering, University of Alberta, Edmonton AB, T6G 2V4, Canada Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Sciences and Technology (NTNU), N-7491 Trondheim, Norway
a r t i c l e
i n f o
Article history: Received 19 December 2011 Accepted 29 December 2011 Available online 8 January 2012 Keywords: Tetrameric naphthenic acid Divalent cation Single molecular layer membrane Vesicle Ion transport
a b s t r a c t Interactions in aqueous solutions between a tetrameric naphthenic acid in sodium salt (BP10Na4 ) and divalent Mg2+ and Ca2+ cations were investigated by isothermal titration calorimetry (ITC) and dynamic light scattering (DLS). Colloidal aggregation of the tetrameric sodium naphthenates to vesicles and micelles enhances its binding with divalent cations. In vesicle systems, Ca2+ was found to bind first with carboxylate groups located at the external water/lipid interface of vesicles. The cations then diffuse through the membrane to interact with the inner carboxylate groups. Higher Ca2+ concentration and higher salinity gradients enhance cation diffusion. Under solution chemistry conditions where micelle dominates, Ca2+ reacts only with outer carboxylate groups of BP10Na4 without any significant cation diffusion through the aggregates. The binding of calcium with sodium naphthenate is an entropy-driven process where entropy gain comes from the dehydration of both cations and carboxylate groups of BP10Na4 . The affinity of Mg2+ to BP10Na4 is much weaker than Ca2+ due to its stronger hydration and hence larger size of hydrated magnesium ions than hydrated calcium ions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Naphthenic monoacids contain single or multiple-fused cyclopentane and cyclohexane rings where the carboxylic acid group is attached to the aliphatic side chain or to the cycloaliphatic ring. They are mostly found in immature heavy crude, and are assumed to be generated from in-reservoir biodegradation of petroleum hydrocarbons [1]. Naphthenic acids cause serious problems during the heavy oil processing [2]. They corrode production facilities [3] and enhance emulsion stability [4]. The solid deposits of naphthenic acids could cause operational problems with frequent costly shutdowns of operations for cleaning [5]. Among the naphthenic acids, a specific category with four carboxylic acidic groups was discovered by Baugh et al. [6] in 2004 from calcium naphthenate deposits collected from some offshore fields. They are reported to form network through cross-linking with calcium ions at the water/oil (W/O) interface [7]. Differential scanning calorimetry (DSC) investigations on calcium naphthenate deposits collected at W/O interface show that the naphthenic acid network is formed through strong ionic bonds between carboxylic groups and calcium ions. The naphthenate network formed as such has a high cross-linking density with very stiff and rigid bonds [8].
∗ Corresponding author. Tel.: +1 780 492 7667; fax: +1 780 492 2881. E-mail address: [email protected] (Z. Xu). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.074
A series of naphthenic acid model compounds have been synthesized by Nordgård et al. [9]. The well-defined molecular structure of the synthesized compound enables us to clarify the formation and interaction mechanisms of naphthenates at a molecular level. Early studies have shown that the tetrameric acid model compound BP10 exhibits similar physical-chemical properties as indigenous tetrameric acid [10]. The intermolecular reaction of BP10 with calcium ions results in an elastic and “solid like” film at W/O interface in the form of supramolecular network [8]. The naphthenate deposits are observed in BP10 systems containing Ca2+ , Mg2+ , Sr2+ and Ba2+ although these ions differ in size and hydration state [10]. An important question is why the naphthenate deposits found in practice contain almost exclusively calcium even though other divalent cations are also present. Potentiometric titration work showed strongest preference of BP10 to Ca2+ , and the lowest affinity for Mg2+ , with Ba2+ and Sr2+ showing intermediate and roughly equal reactivity with BP10 [11]. Additional systematic and fundamental study is needed to better understand interaction mechanisms of naphthenic acids and divalent cations present in crude oil. The aggregation of tetrameric naphthenic acid in bulk aqueous solutions into vesicles was first discovered by Ge et al. in the crude oil research [12]. Due to its special molecular structure, BP10 was found to form vesicles at BP10 concentration as low as 10−6 mol/L. Vesicles were found to co-exist with micelles at BP10 concentrations higher than its critical micelle concentration (CMC) of 1.0 × 10−3 mol/L. A well-defined lamellar liquid crystal appeared upon addition of a short chain aliphatic alcohol. To better
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system. For DLS experiments, the solutions prepared were filtrated through 0.45 m PVDF Millipore filters directly into dust free light scattering cells, which were sealed immediately to prevent dust. After filtration, the solutions were thermostated at 25 ◦ C without disturbance for more than 30 min before measurement. ITC measurements were performed at 50 ◦ C using a Nano-ITC 2G titration microcalorimeter (TA instruments, USA). The integrated injection-stirrer assembly (250 L) was filled with metal nitrate (M(NO3 )2 ) aqueous solution, which was titrated in portions of 10 L or 5 L into the calorimeter vessel that initially contained 1 mL BP10Na4 aqueous solution. The pH values of both M(NO3 )2 and BP10Na4 aqueous solutions were adjusted to 10. The reference cell was filled with Millipore Milli-Q water. The titration was carried out at the constant stirring speed of 250 rpm to ensure thorough mixing. Each injection was completed in 10 s. To allow equilibration, there was a 420-s time delay between successive injections. Raw data were obtained as a plot of heating rate (J/s) as a function of titration time (s). These raw data were then integrated to obtain a plot of enthalpy change per mole of injectant (M2+ ), Hobs (kJ/mol) referred to in this paper as molar enthalpy of reaction, as a function of M2+ to BP10Na4 molar ratio. All the titration experiments were repeated twice to ensure reproducibility within 2%. 2.2. Dynamic light scattering (DLS)
Fig. 1. Calorimetric titration isotherm of interaction between Ca2+ and BP10Na4 when 2.0 mM Ca(NO3 )2 was injected into 0.1 mM BP10Na4 at 25 ◦ C and pH 10.2: a – heat flow versus time and b – enthalpy of mixing versus Ca2+ /BP10Na4 molar ratio ([Ca2+ ]/[BP10Na4 ]).
understand these aggregation states requires investigations on the interactions between BP10 and divalent cations such as Mg2+ and Ca2+ . In this study molecular interactions was investigated mainly by isothermal titration calorimetry (ITC), which is a popular and high precision technique for chemical interactions [13]. Dynamic light scattering (DLS) is also used to study the topology change of BP10 aggregates upon binding with divalent cations. 2. Experimental 2.1. Materials and sample preparation The synthesis of BP10Na4 was performed at the Ugelstad Laboratory (NTNU, Trondheim, Norway). The detailed procedures of synthesis were reported previously in literature [14]. The name BP10Na4 originates from its molecular structure which contains a benzophenone (BP) core with each phenolic oxygen being linked to an alkyl chain of 10 methylene ( CH2 ) groups terminated with a carboxylic acid group. The molecular structure of BP10Na4 is given below:
All the chemicals including Mg(NO3 )2 ·6H2 O (>98%), Ca(NO3 )2 ·4H2 O (99.9%), acetic acid (99.8%), butyric acid (>99%), octanoic acid (>98%) and citric acid (99.5%) were obtained from Sigma–Aldrich Corp. and used without further purification. Unless otherwise stated, the pH of the systems was adjusted to 10 by the addition of NaOH (Fisher Scientific, 99.5%). The deionized water was filtered using a Millipore Milli-Q UV-Plus purification
DLS measurements were carried out at a scattering angle of 90◦ using an ALV 5022 laser light-scattering (LLS) instrument equipped with a cylindrical He–Ne laser (model 1145p-3083; output power = 22 mW at = 632.8 nm) in combination with an ALV SP-86 digital correlator of a sampling time range between 25 ns and 40 ms. The LLS cell was held in an index matching vat filled with high purity, dust-free toluene. The temperature of the experiments was controlled within ±0.02 ◦ C by a thermostat. The DLS experiments were completed in 10 min and repeated at least twice. In DLS experiments, the intensity–intensity time correlation function G(2) (t, q) was measured, where t is the decay time and q is scattering vector given by q = (4n/0 )sin (/2). G(2) (t, q) is related to the normalized first-order electric field-time correlation function |g(1) (t, q)| via Siegert relation given by [15]
2
G(2) (t, q) = A[1 + ˇg (1) (t, q) ]
(1)
where A (≡I(0)2 ) is the measured baseline. For the broadly distributed relaxation spectrum, |g(1) (t, q)| is related to characteristic relaxation time distribution G( ) by
(1) g (t, q) = E(t, q)E ∗ (0, q) =
∞
G( )e−t d
(2)
0
From Eqs. (1) and (2), the term G( ) in Eq. (2) can be calculated from Laplace transformation of the measured G(2) (t, q). The translational coefficient distribution, G(D) can be calculated from G( )
by equation = Dq2 . The hydrodynamic radius distribution is then determined by the Stokes–Einstein equation: Rh = kB T/6D, where , kB and T are the viscosity of solvent, the Boltzmann constant and the absolute temperature. In this study, the CONTIN program supplied with the correlator was used to calculated G(D) and Rh .
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Fig. 2. Scheme of interaction between calcium ions and BP10Na4 vesicle and concentration dependence of Ca2+ .
3. Results and discussion 3.1. Calcium in vesicle systems (diluted BP10Na4 systems) The heat flow of isothermally titrating interactions of calcium with BP10Na4 at pH 10 is shown in Fig. 1a. By integrating the area under each injection peak, the enthalpy change per mole of Ca(NO3 )2 injected is plotted as a function of Ca2+ to BP10Na4 mole ratio ([Ca2+ ]/[BP10Na4 ]) in Fig. 1b. The vesicle structure of BP10Na4 aqueous solutions is thought as a single BP10Na4 molecular layer (membrane) separating the inner and outer aqueous phases, with two carboxylate groups on each side of BP10 molecules being located at both inner and outer membrane-aqueous interfaces and the hydrocarbon skeleton of the BP10Na4 forming the hydrocarbon phase of the membrane. As investigated by the cryo-TEM in our previous work [12], the vesicles are spherical. The molecular illustration of this unique single molecular layer vesicle is shown in Fig. 2. The decisive step for the tetrameric acid to interact with various cations is the deprotonation of the tetrameric acids [11,14a, 16]. The deprotonation of BP10 is known to occur in a single step. The apparent pKa value of BP10 is reported to range from 6.8 to 8.0, depending on the salinity of the system [17]. This value is higher than the intrinsic pKa value of typical naphthenic acids because the tetrameric acid forms vesicles and micelles in aqueous solutions, resulting in interfaces with different pKa values. In our study, the pH value is kept constant at 10 which are about 3 units higher than the apparent pKa. At this pH, most of the carboxylic groups if not all should be in a deprotonated form. When an electrolyte such as calcium nitrate is added as the titrant into the vesicle solutions, a concentration gradient of calcium ions is created between the outer and inner aqueous phases (illustrated in Fig. 2). This concentration gradient provides a driving force for cation transport to and through the membrane. The results in Fig. 1 show three distinct processes. At the first addition of Ca2+ , a relatively high enthalpy value is determined, which decreases sharply with increasing Ca2+ concentration. This reduction in the molar entropy with increasing calcium ion concentration is attributed to the reduced amount of unbound carboxylic groups and decreased charge density of vesicle. The enthalpy curve
shows a slope that can be extrapolated to a [Ca2+ ]/[BP10Na4 ] value of 1, indicating a complete reaction of carboxylate groups on the external surface of vesicles. Within this concentration range, the added Ca2+ appears to react only with the external carboxylate groups with minimum transport through the membrane. As a result, salinity gradient is not sufficient to cause a noticeable transport of added calcium ions. After complete reaction of external carboxylate groups with calcium ions at [Ca2+ ]/[BP10Na4 ] ratio of 1, the calcium ions added are able to penetrate to the inner side of the vesicle and react with unbound carboxylate groups on the inner surface of membranes, shown by the additional enthalpy. Compared with enthalpy change of calcium ion binding with carboxylate groups on the external surface of vesicles, the enthalpy change on the inner surface is relatively higher. The higher charge density in the inner side of vesicle than in the outside of the vesicle probably results in the higher enthalpy reflected by the reduced slope of the enthalpy curve. Interestingly, the molar enthalpy also decreases linearly with increasing calcium ion addition to a [Ca2+ ]/[BP10Na4 ] value of around 2, after which no enthalpy is detectable with further increasing calcium ion addition. This finding suggests a complete binding of inner carboxylate groups of vesicle membranes by calcium ions. By considering both the structure of the vesicle and the location of the carboxylate groups, the ITC results led us to conclude that the chemical potential of reactions between Ca2+ and COO− at the inner vesicle interface could provide sufficient driving force to cause Ca2+ transport through the single molecular layer vesicle membrane even at low ionic gradient with complete complexation between carboxylate groups at both inner and outer interface at the stoichiometric [Ca2+ ]/[BP10Na4 ] ratio of 2. The results in Fig. 1 show that the interaction between Ca2+ and carboxylate group of BP10Na4 is endothermic (Hmix > 0). The binding of calcium ions with BP10Na4 vesicles is a complex process, involving dehydration of calcium ions (endothermic), dissociation of carboxylate acid groups and sodium ions (endothermic), binding of calcium ions to the carboxylic acid groups of vesicles (exothermic), hydration of sodium ions (exothermic), etc. [18]. The enthalpy measured by ITC is an overall effect of these various enthalpy contributions. The measured positive (endothermic) apparent enthalpy
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value suggests a predominant contribution of dehydrating calcium ions to the overall enthalpy of the reaction. Unfortunately, we can only get this qualitative conclusion. Nevertheless, this observation is consistent with the results from previous studies of calcium/lipid vesicle interactions by Sinn et al. [17]. To quantitatively determine the contributions of individual sub-processes to the overall enthalpy remains a major challenge to the world of thermodynamicists. Since Hmix > 0 as detected, it is evident that the spontaneous binding of Ca2+ to naphthenic vesicles is an entropy driven (S > 0) process to meet the thermodynamic criterion of Gmix (=Hmix − TS) < 0 for the observed spontaneous binding. Investigations of the interaction between Ca2+ and lipid membranes composed of phosphatidylcholine–phosphatidylserine mixtures showed that the entropy gain comes from the dehydration of lipid vesicles [17]. In the calcium and naphthenate vesicle system under investigation, dehydration of both calcium and carboxylate groups on bilayer membrane of single molecular layer vesicles contributes to the entropy gain. The DLS results to be reported later will shed further light on these mechanisms. 3.2. Calcium in mixture of vesicles and micelles (higher BP10Na4 concentrations) To understand the interactions of calcium ions with vesicles and micelles, the ITC experiments were conducted at BP10Na4 concentration of 10 mM which is well above its CMC (1 mM) [12]. The titration results are shown in Fig. 3a. For comparison, the ITC results obtained at BP10Na4 concentrations of 0.1 mM which is well below its CMC are also shown in Fig. 3a. The difference between the isotherms obtained at higher and lower BP10Na4 concentrations is attributed to the formation of BP10Na4 micelles [19]. In micelle dominated system (10.0 mM BP10Na4 ), the transport behaviour of calcium ions through the vesicles membrane becomes subordinate. Since divalent calcium ions unlikely penetrate into the hydrophobic core of the micelle, only reactions of calcium ions with external carboxylate groups of BP10Na4 on micelle are seen in 10 mM BP10Na4 solutions, leading to not only a lower molar enthalpy but also a gradual decrease in molar enthalpy of reaction with increasing calcium to BP10Na4 molar ratio up to 2. Naturally the complexation will end up with [Ca2+ ]/[BP10Na4 ] of 2 since all surface carboxylic groups on micelle surfaces are available for reaction. In addition, the white flocculants were observed in both systems, although it appears at much lower [Ca2+ ]/[BP10Na4 ] in 10 mM BP10Na4 system than that of 0.1 mM BP10Na4 system. This difference also contributes significantly to the lower enthalpy value as well as the roughness of the curve in 10 mM BP10Na4 system. The flocculants appeared in the system will be discussed in Section 3.5. 3.3. ITC of BP10Na4 by Mg Magnesium is another important metal ion present in both produced water and seawater [11,19]. Different types and concentrations of cations are expected to play a critical role in determining ion/vesicle interactions. As shown in Fig. 3, at a given BP10Na4 concentration, the molar enthalpy of interaction between Mg2+ and BP10Na4 is much lower than that between Ca2+ and BP10Na, indicating a weaker interaction between Mg2+ and BP10Na4 than between Ca2+ and BP10Na4 . By potentiometric titration, Sundman et al. also showed that the binding of Mg2+ was quite weak compared to Ca2+ [11]. Such a big difference can be attributed to the differences in hydration levels of the cations. Mg2+ is known to have a hydration number of 6 in comparison to 2 for Ca2+ [19]. Since the direct binding of divalent cations with carboxylate groups requires dehydration of cations, a larger dehydration enthalpy is needed for magnesium ions than calcium ions prior to their binding with
Fig. 3. Enthalpy change as a function of divalent ion to BP10Na4 molar ratio when titrating different concentrations of BP10 with Ca(NO3 )2 and Mg(NO3 )2 at 25 ◦ C and pH 10.2.
carboxylate groups, leading to an overall smaller molar enthalpy of magnesium ions than calcium ions interacting with carboxylate groups of BP10Na4 . Strongly hydrated Mg2+ seems to react with BP10Na4 at much weaker strength. This is attributed to a higher (six) hydration number of magnesium and therefore a much larger radius of hydrated ions. With a larger radius of hydration sheath, it is much more difficult for magnesium to approach and bind with carboxylic group of BP10Na4 . In fact, the binding of calcium and magnesium ions with carboxylic group of BP10Na4 does not require dehydrating all the hydrated water molecules. Dehydrating one from six hydrated water molecules around a magnesium ion would require a much less energy than dehydrating one from two hydrated water molecules around a calcium ion. That is the main reason why a lower enthalpy is measured in the system of magnesium/BP10Na4 than in calcium/BP10Na4 system. More interestingly, at both BP10Na4 concentrations, Fig. 3 shows no distinct transition between interactions of magnesium
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with outer and inner carboxylate groups. It appears that hydrated magnesium ions of larger size are unable to penetrate through BP10Na4 single layer membranes to react with inner carboxylate groups. 3.4. ITC of fatty acids by Ca and Mg The interactions between M2+ and BP10Na4 are complex, involving direct ionic complexation, conformational changes of hydrocarbon chain and ionic transport linked with different hydration states. All these interactions are collectively measured in the values of Hmix at different concentrations and M2+ /BP10Na4 molar ratios. In order to isolate some of the interactions, a set of different organic acid molecules, such as acetic acid with only one carboxylic acid group per molecule, citric acid of three carboxylic groups but without hydrocarbon chain, and butanoic and octanic fatty acids with different hydrocarbon chain length, mimicking the hydrocarbon chains of BP10Na4 were studied by ITC. The concentration of these organic acids and M2+ to acid molar ratio were chosen in such a way that no aggregation was allowed in the aqueous solution. Fig. 4a shows the enthalpy curves of these systems. It is interesting to note that compared with BP10Na4 interacting with Ca2+ , the molar enthalpy of acetic acid, butanoic acid and octanoic acid binding with Ca2+ is almost negligible. These results indicate a weak interactions of Ca2+ with monomeric acid groups. It can therefore be inferred that the higher heat value of BP10Na4 and Ca2+ interaction mainly comes from interaction of calcium ions with carboxylate groups of BP10Na4 aggregates, involving both ionic transport and conformation changes in the hydrocarbon chain of BP10. In the case of Ca2+ interacting with citric acid, the reaction is exothermic at lower Ca2+ /citric acid molar ratio. (Fig. 4a) The exothermic heat value decreases sharply upon Ca2+ injection, approaching zero at the molar ratio of about 1.5. A further increase in Ca2+ /citric acid molar ratio leads to a negligible enthalpy indicating the completion of calcium interactions with citric acids. These results are similar to those reported by Christensen et al. [20] and Keowmaneechai and McClements [21] who investigated interactions of citric acid and divalent cations under slightly different conditions. As shown in Fig. 4b, the enthalpy change in the Mg2+ /acid systems with increasing Mg2+ /acid molar ratio mirrors Ca2+ /acid systems. However, the exothermic value of citric acid interacting with Mg2+ is higher than that with Ca2+ . Nonetheless, there is no clear transition of enthalpy for both Ca2+ and Mg2+ interacting with fatty acid, in contrast to the interaction of calcium with BP104 at concentrations below its CMC, as clearly shown in Fig. 4a. 3.5. DLS measurements Our previous investigations on aggregation of tetrameric naphthenic acid, BP10Na4 in aqueous solutions revealed the formation of spherical vesicles at BP10Na4 concentration as low as 10−3 mM. The size of the vesicles was found to range from 45 nm to 200 nm, depending on BP10Na4 concentration [12]. The present investigation focuses on the effect of calcium ions on the morphology of the vesicles formed from BP10Na4 solutions. Dynamic light scattering is a proven technique to provide information on aggregate topology in solution. In this study, the concentration of BP10Na4 is kept constant at 0.1 mM, where well-defined vesicles were found to exist in the system [22], while [Ca2+ ]/[BP10Na4 ] molar ratio changes from 0 to 2.6. The results in Fig. 5a show that the correlation function decays in a single step mode, indicating one kind of aggregates, i.e., vesicles, predominating the system. The addition of Ca2+ at all [Ca2+ ]/[BP10Na4 ] molar ratio tested does not break the single molecular layer vesicles.
Fig. 4. Enthalpy value change with cation to acid molar ratio when titrating various acids with M(NO3 )2 at 25 ◦ C and pH 10.2. The concentrations of acids are specified in respective figure.
Relaxation time distributions in Fig. 5b obtained by the fitting of the corresponding correlation functions show a faster relaxation of vesicles with increasing [Ca2+ ]/[BP10Na4 ] molar ratio. Since the smaller, the particle size, the faster, the relaxation [23], the results in Fig. 5b suggest a reduction in the size of vesicles with increasing [Ca2+ ]/[BP10Na4 ] molar ratio. As shown in Fig. 6, the hydrodynamic radius obtained according to Stoke–Einstein relationship [24] decreases from about 45 nm in the absence of Ca2+ to about 30 nm at [Ca2+ ]/[BP10Na4 ] = 2.0. Also shown in Fig. 6 is an initial increase in scattering intensity with the addition of Ca2+ , reaching a maximum at [Ca2+ ]/[BP10Na4 ] of 2, followed by a sharp reduction with further increasing Ca2+ additions. The transition in scattering intensity at [Ca2+ ]/[BP10Na4 ] molar ratio of 2 agrees with the result from ITC. The enhanced scattering intensity of vesicle solutions at constant scattering angle can be attributed to both the dehydration of membrane by binding of calcium ions with carboxylate groups and the consequences of vesicle-vesicle interactions by reduced charges of vesicles due to calcium ion binding. In membranes of amphiphilic molecules, two kinds of water were
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Fig. 6. Apparent hydrodynamic radii and relative scattering intensity as a function of Ca(NO3 )2 to BP10Na4 molar ratio at pH 10. The concentration of BP10Na4 is kept constant at 0.1 mM.
Fig. 5. a – DLS intensity–intensity time correlation function at various Ca(NO3 )2 to BP10Na4 molar ratio at pH 10. b – Relaxation time distributions obtained by the corresponding correlation function. The concentrations of BP10Na4 in this set of tests are kept constant at 0.1 mM.
well accepted: free water located far away from amphiphiles and free to move; and bound water attached to the hydrophilic groups [25]. Binding of divalent cations with hydrophilic carboxylic groups tends to replace bound water. Dehydration of membranes upon calcium binding was reported earlier by Sinn et al. in the vesicle
system of phosphatidylcholine–phosphatidylserine [17]. In a separate study, Berquand et al. also showed that the presence of Ca2+ in the aqueous solution reduces the thickness of water layer [26]. In the present system, the enhancement of scattering intensity of shrunk vesicles supports the dehydration of membrane, and hence the increase in the refraction index of the condensed membrane. On vesicle–vesicle interactions, the vesicles are stabilized mainly by electrostatic repulsion from the negatively charged carboxylate groups. The binding of Ca2+ with carboxylate groups on vesicles neutralizes the charges, reducing and eventually eliminating the repulsive forces between the vesicles. Such conditions lead to flocculation of vesicles and hence higher scattering intensities. In fact, small pieces of semi-transparent flocculates are visually observed to form in the samples at [Ca2+ ]/[BP10Na4 ] molar ratios higher than 2, which supports the destabilization of the vesicle system. As indicated in Section 2, all DLS samples were filtered with millipore with an average pore diameter of 0.45 m. Therefore, we can safely conclude that the sharp decrease in scattering intensity is due to the flocculation of vesicle aggregates. Fig. 7 shows optical micrographs of flocculated samples which remain stable for several months. For the mixture of carboxylic acid and alkali earth metal ions at higher concentrations, three possible situations are responsible for the solid-like phase: (1) highly ordered aggregation resulting in formation of crystals; (2) random aggregation resulting in amorphous precipitation; and (3) aggregation process intermediate between these two extremes, yielding the formation of gels [27]. The darker yellow pattern on the optical micrograph taken under normal light corresponds to the white floccules. The dark texture of the floccules under polarized light rules out the case of well-ordered liquid crystals mentioned above.
Fig. 7. Photograph of sample, and microscopy graph under normal and polarized light in the system of 0.1 wt% BP10Na4 /0.05 wt% Ca(NO3 )2 /H2 O at pH 10.
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For them to be liquid crystals, a shinning texture would be observed under the polarized light [28]. However, an interesting feature of bright boundary between cotton-like phases and clear solution phase was observed under polarized light, indicating the presence of well organized boundary layers in micrometer thickness. As also shown in our previous investigation [12], tetraacid BP10Na4 is an excellent molecular candidate to build single molecular layer membranes, liquid crystals and hydrogels with vast potential applications yet to be exploited. The gels observed in the system belong to the low-molecular-weight (LMW) hydrogels. The LMW hydrogels, formed by non-covalent self-assembly of small molecules, are of wide applications in personal care, foods, pharmaceutical and biomedical applications for their benefit of reversibility, solution chemistry responsiveness, and the relative mild gelation conditions [29–32]. One of the attractive properties of the LMW gel in the BP10Na4 /Ca(NO3 )2 /H2 O system is its low gelator content (as low as 0.1 wt%). In addition, this type of LMW gel is of high stability. These two characters are attributed to the molecular structure of four chains linked to a polyaromatic sheet, resulting in high capabilities of cross linking in the presence of divalent cations. Detailed studies of the BP10Na4 LMW gel is in progress in our lab. 4. Conclusions Formation of vesicles and micelles of tetrameric naphthenate, BP10Na4 enhances its binding with divalent cations in aqueous solutions. The interaction of divalent cations and carboxylate groups of BP10Na4 is entropy driven, with entropy gain coming from the dehydration of both the divalent cations and carboxylate groups of aggregated BP10Na4 . Two-step binding of calcium cations with naphthenate vesicles is observed, in contrast to a single step binding of calcium ions with BP10Na4 micelles. Compared to Ca2+ the binding of Mg2+ with naphthenate is weaker due to its higher degree of hydration. These findings explain the phenomenon that most of the tetrameric naphthenate deposits obtained during the heavy oil processing are exclusively calcium salts, although the total contents of Ca2+ and Mg2+ are of approximately the same order of magnitude in sea water. Acknowledgement This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Industrial Research Chair program in Oil Sands Engineering. We also like to acknowledge the financial support from members of the consortium JIP 2:3 (Akzo Nobel, Bp, Champion-Technologies, Clariant Oil Services, ConocoPhillips, Statoil ASA, Talisman Energy and TOTAL) and Research Council of Norway through the PETROMAKS programme. Canada Foundation for Innovation (CFI) was also acknowledged for the facility of DLS. References [1] B.F. Lutnaes, Ø. Brandal, J. Sjöblom, J. Krane, Archaeal C80 isoprenoid tetraacids responsible for naphthenate deposition in crude oil processing, Organic and Biomolecular Chemistry 4 (2006) 616–620. [2] S. Simon, E. Nordgård, P. Bruheim, J. Sjöblom, Determination of C80 tetra-acid content in calcium naphthenate deposits, Journal of Chromatography A 1200 (2008) 136–143. [3] E. Slavcheva, B. Shone, A. Turnbull, Review of naphthenic acid corrosion in oil refining, British Corrosion Journal 34 (1999) 125–131. [4] D. Arla, A. Sinquin, T. Palermo, C. Hurtevent, A. Graciaa, C. Dicharry, Influence of pH and water content on the type and stability of acidic crude oil emulsions, Energy and Fuels 21 (2006) 1337–1342.
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