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lecithin solution emulsions
Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27
Electrokinetic properties of n-tetradecane/lecithin solution emulsions Agnieszka Ewa Wi˛acek Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Skłodowska University, 20031 Lublin, Poland Received 21 April 2006; received in revised form 29 June 2006; accepted 2 July 2006 Available online 7 July 2006
Abstract Properties of surface of hydrophobic liquid (e.g. n-alkane) in aqueous solutions are very important for stability of dispersed systems, i.e. emulsions and suspensions, as well as in many biochemical processes including living organisms. Application of natural stabilizers it is a key aspect of wide-ranging investigations of emulsion. One of such systems is oil/lecithin solution. The adsorbed lecithin layer at the oil droplets determines the structural and dynamic properties of oil/water emulsions. It changes the interfacial free energy and the zeta potential. The zeta potentials determined in this paper as a function of time and pH confirm that H+ /OH− ions are potential determining and the largest effect of OH− is visible at alkaline pHs. The decrease in absolute value of the zeta potential was accompanied by a growth in the effective diameter of emulsion droplets. Using both the potential and diameters electric charge at the shear plane for spherical particle was calculated. In these systems the zeta potential may be created by immobilized and oriented water dipoles and acid–base interactions. However, on the basis of the results it is concluded that the main stabilizing mechanism is the double layers repulsion, besides possible stabilizing effect of lecithin liposomes and other aggregates. © 2006 Published by Elsevier B.V. Keywords: Oil/water emulsions; Zeta potential; Multimodal size distribution; Electric charge; Dipalmitoyl lecithin
1. Introduction Important parameters determining properties of dispersed systems are: zeta potential (surface charge), droplet size, pH and ionic strength. The stability of emulsions and suspensions and electrical phenomena at nonionogenic hydrophobic surfaces in water or electrolyte solutions, still attract researches because of wide-ranging using of these systems, for example in many biochemical processes. However, the origin of electric charge and potential at the oil droplet is still not well understood [1–8]. Important aspect of emulsion investigations is application of natural emulsifiers and stabilizers. One of them is lecithin. Natural stabilizers involve a broad group of phospholipids, which are important substances for living organisms. For example, phosphatidylcholins are a key building block of cell membranes and protects cells from oxidation and largely comprises the protective sheaths surrounding the brain. Although lecithin is a fatty substance, it is also a fat emulsifier. The food industry recognizes lecithin (product of processing soybean oil) as a good stabilizer (lipophilic emulsifier), using it in many processes, like wetting, dispersion and lubrication [1]. A wide spectrum of lecithin
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applications results from its amphiphilic structure. The adsorbed lecithin layer at oil droplets surface determines the structural and dynamic properties of oil/water emulsions. Usually pH affects lecithin adsorption and hence the surface charge at the oil droplet, what should appear in the changes of zeta potential [2]. The changes in zeta potential of the oil/lecithin droplets may be due to conformation changes of the adsorbed lecithin molecules and interaction of H+ /OH− ions with the molecules. The structure of phosphatidylcholin molecule is shown in Fig. 1 [3]. An increase in zeta potential of the droplet is often desirable to stabilize the emulsion. However, despite attractive dispersion and repulsive electrostatic force originating from polar groups, if present, solvation forces should be also taken into account. In many systems they are due to hydrogen bonding interactions between hydrogen and oxygen atoms. Changes in the zeta potential may correlate with changes in the acid–base interactions, which can essentially affect stability of the systems [4–7,9]. On the basis of the earlier experimental results and literature data, it is concluded that at n-tetradecane surface water dipoles in the first layer may play an important role in the electric potential origin [10,12–15]. Using the measured zeta potentials and the droplet diameters the electric charge in the shear plane was calculated [16–21]. Taking into account water arrangement near the n-tetradecane surface and the calculated charges, it was con-
A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27
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Fig. 1. The structure of phosphatidylcholin molecule [3].
cluded that in fact the potential may be created both by OH− /H+ and oriented water dipoles. The purpose of this work was to investigate the electrokinetic properties of n-tetradecane/water emulsion in the presence of lecithin, stabilizing the droplets, and also to learn about probable stabilizing mechanism. 2. Experimental The emulsions were prepared in 100 ml-calibrated flasks. The components were added as follows. First n-tetradecane was introduced, then lecithin solution (0.5 or 1 mg/100 ml). The water was from MilliQ-Plus System. The pH of the emulsion was natural or adjusted with a help of HCl and NaOH 0.1 M solutions. Next, the flasks were placed for 15 min into a homogenizer (10,000 rpm). The lecithin (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) used was from Sigma-Aldrich Chemical Co.; n-tetradecane (p ≥99%) was from Fluka. Zeta Pals/Bi-Mass apparatus (Brookhaven Instr. Co.) was applied for measurement both the electrophoretic mobility and the effective diameter (multimodal size distribution). These parameters were determined as a function of time, after 5, 15, 30, 60, 120 min and 24 h since the moment of the emulsion preparation. The zeta potential was calculated from the electrophoretic mobility using O’BrienWhite computer program. The instrument applies dynamic light scattering for the parameters evaluation. All the experiments were carried out at 20 ◦ C. Some of them were replicated two times. For each measured sample 10 measuring runs were taken, with seven cycles in the each run, and were fixed in the Zetameter.
276 nm (2 h old emulsion) at natural pH 6.04. Larger effective diameters, above 500 nm, and their large fluctuation occurred at acidic pH (3 and 4), whereas at basic pH the changes in effective diameters on time scale were minimal and produced emulsions were quite monodisperse. The obtained results are very regular and stable, even after one day from the emulsion preparation. It is known from the literature [22,23], that any excess of lecithin forms stable unilamellar liposomes showing high pH-dependent. Neutral or slightly acidic liposome dispersions, if present in the medium, destabilize the oil/water emulsion. The presence of liposome’s aggregates may easily be misread with oil droplets in the creamed oil phase [22,23]. Hence, a higher effective diameter does not always mean a lower stability emulsion. Only CryoTEM technique allows direct observation of liposomes apart from the oil droplets. Quantity inspection of liposomal material is, however, difficult. Further experiments are necessary to better characterize lecithin emulsion. Multimodal size distribution analysis shows that in the studied emulsions mostly there is one principal population of the droplet sizes, however in some systems there are two comparable populations. Polydispersities of the emulsions were low during all the experiment time, between 0.005 and 0.2. Typical multimodal size distribution of the emulsions n-tetradecane/lecithin is shown in Figs. 3 and 4, for freshly prepared emulsion (after 5 min) and after 1 h since the emulsion preparation. In both cases there is practically only one population of the droplets, around 470 and 505 nm, respectively. Some small populations appear around 205 and 298 nm, respectively. The presence of lecithin did not affect the effective diameter much, the range
3. Droplet diameter and multimodal size distribution The changing effective diameter of the droplets as a function of time resulted from coalescence of the droplets and their flotation to the emulsion surface (phases separation). Unfortunately, with the dynamic light scattering technique it is difficult to determine the droplet concentration, however the relative changes can be well monitored [10,11]. The most stable effective diameters were obtained for the emulsion of n-tetradecane 0.2 ml/lecithin 1 mg in 100 ml. The effective diameters of these n-tetradecane/ lecithin emulsions are shown in Fig. 2. The size distributions of the droplets are usually sharp. Any decrease in the effective diameter should be interpreted as a partial separation of the dispersed phase, i.e., larger droplets float toward the surface. On the contrary, an increase in the average diameter shows that the coalesced droplets are still present in the bulk emulsion for a period of time. For example, from 258.7 nm (5 min old emulsion) to
Fig. 2. Changes of the effective diameter of n-tetradecane droplets (0.2 ml)/ lecithin (1 mg)/100 ml emulsion as a function of time and pH.
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Fig. 3. Multimodal size distribution of n-tetradecane (0.2 ml)/lecithin (1 mg)/ 100 ml 5 min-old emulsions in pH 3 (effective diameter 470 nm, polydispersity 0.107).
Fig. 6. Changes of the effective diameter of n-tetradecane (0.1, 0.15 or 0.2 ml)/lecithin (1 mg)/100 ml emulsion as a function of time.
Fig. 4. Multimodal size distribution of n-tetradecane (0.2 ml)/lecithin (1 mg)/ 100 ml 1 h-old emulsions in pH 3 (effective diameter 505 nm, polydispersity 0.202).
of effective diameters is comparable like for emulsion of ntetradecane/water studied earlier [10]. Effect of oil concentration in n-tetradecane/lecithin solution (0.5 or 1 mg/100 ml) emulsions at their natural pH is visualized in Figs. 5 and 6. Very stable emulsion was obtained containing 0.1 ml of oil and 0.5 mg of lecithin in 100 ml of emulsion. Significant increase in the diameter is observed for emulsion
Fig. 5. Changes of the effective diameter of n-tetradecane (0.1, 0.15 or 0.2 ml)/lecithin (0.5 mg)/100 ml emulsion as a function of time.
of 0.15 ml n-tetradecane/1 mg lecithin. After 24 h the effective diameter has grown up twice. Since the lecithin molecule has a trimethylammonium and phosphate group separated by two methylene groups, its structure allows two ionic forms; with maximal separation of charges or a reduced separation of charges. It results from an internal salt linkage between the phosphate and trimethylammonium groups in the same molecule [22]. These different structures of lecithin molecule are of considerable interest in relation to the surface coverage and stability. Assuming that total amount of lecithin added had adsorbed to the oil droplet surface and that during the coalescence process of smaller droplets the lecithin remained on the surface of the larger droplets formed then surface coverage should be larger than 1 mg/m2 . But, this is not the case, because the calculated maximum statistical coverage of the droplets is less than 1 mg/m2 , especially in freshly prepared emulsion. 4. Zeta potential and electrokinetic charge The zeta potentials of n-tetradecane/lecithin emulsion were investigated at different pH. Fig. 7 shows that changes of zeta potentials are pH-dependent, but it settles rather fast. The values of zeta potential were calculated from O’Brien-White equation [16] using a computer program. From the effective diameters and the ionic strength of the emulsions studied κa values were calculated. κ is the Debye-H¨uckel parameter (reciprocal of the diffuse double layer thickness) and a is the particle diameter [17]. Thus calculated zeta potential values are listed in Table 1. Our experiments show that H+ /OH− ions are the potential determining for those emulsions. The largest effect of OH− ions on the zeta potential is visible at alkaline pH. As the pH of the emulsion is decreased probably the phosphate groups in the layer of lecithin molecules are neutralized, especially for dipalmitoyl lecithin (with long acyl chains) used in this paper. The increase in unsaturation of the fatty acyl chains increases the intermolecular spacing, which reduces the ionic repulsion between the polar groups, and hence strengthens the internal salt linkage. There
A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27
Fig. 7. Changes of the zeta potential of n-tetradecane (0.2 ml)/lecithin (1 mg)/100 ml emulsion as a function of time and pH.
is a vertical rather than coplanar orientation of the phosphoryl choline group with respect to the interface [22]. These conformations probably contribute to the observed zeta potential changes and resulting the electrokinetic’s charge. Jabło´nski et al. [15] conducted the acid–base potentiometric titration of octadecane suspensions in aqueous solutions of NaCl and their results showed that neither H+ nor OH− ions have adsorbed at the octadecane/electrolyte interface. The electrophoretic mobility of the octadecane suspension in 10−3 M NaCl at pH 7–8.5 was ca. −6 m cm/V s, which corresponded to −65 to 70 mV of the zeta potential. On the basis of the measured electrophoretic mobility they calculated that the surface charge (equated to the electrokinetic’s charge) should be about 1 C/cm2 (i.e. 0.01 C/m2 ). They concluded that the influence of H+ or OH− on the observed zeta potential is rather of an indirect nature and changes in water structure at the hydrophobic interface were responsible for the observed zeta potential appearance. Our measurements for paraffin [13,14] led to the conclusion that at a nonionogenic hydrophobic paraffin surface a significant role in the electric potential origin was played by the oriented water
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Fig. 8. Changes of the zeta potential of n-tetradecane (0.2 ml)/lecithin/100 ml emulsion (2 h-old) as a function of pH.
dipoles in the first layer next to a thin “vacuum” space being of about 0.1 nm thickness. It is believed that the electric double layer at such an interface, especially at a low ionic strength, possesses partially a dipolar nature. In this paper significant increase in the negative zeta potential at alkaline pH was observed. On the basis of classical double layer theory this increase in negative zeta potential can be interpreted as a result of OH− or lecithin molecules adsorption during equilibration of the system. Marinova et al. [6] concluded that negative surface charge at oil/water interface resulted from specific OH− adsorption also via hydrogen bonding with water molecules in the boundary layers, and that was inherent property of the oil/water interface. The changes in zeta potential at different pH may be considered to be also due to some restructuring of the water molecules immobilized at the n-tetradecane surface to attain an equilibrium structure and adsorption or depletion of OH− ions, which could form hydrogen bonding [13,14]. Changes in the zeta potential of n-tetradecane droplets in 2 h-old emulsions with lecithin at different pH are presented in Figs. 8 and 9. The isoelectric point (iep) of n-tetradecane/lecithin
Table 1 Solution
Lecithin
Potential from O’Brien-White methoda
Particle radius (nm)
κa
Surface chargeb (C/m2 × 103 )
No. of charges/1000 nm2
td 0.2 ml, pH 3 td 0.2 ml, pH 4 td 0.2 ml, pH 7.3 td 0.2 ml, pH 8 td 0.2 ml, pH 10 td 0.2 ml, pH 12 td 0.2 ml, pH 3 td 0.2 ml, pH 4 td 0.2 ml, pH 6.1 td 0.2 ml, pH 8 td 0.2 ml, pH 10 td 0.2 ml, pH 12 H2 O, td 0.2 ml, pH 6.5
0.5 mg
28.3 7.11 −37 −30.7 −57.2 −115.5 32.5 −4.65 −58.6 −94.9 −111.2 −95.9 −46.6
436 767 374 183 126 166 280 501 138 110 148 123 247
32 19 3.2 1.6 36 39 21 131 1.7 1.4 43 29 0.36
0.84 0.26 −1.17 −0.37 −4.01 −4.97 0.98 −0.27 −0.91 −1.73 −5.83 −4.85 −1.78
5.2 1.6 7.3 2.3 25 31 6.1 1.7 5.7 10.8 36.4 30.3 11.1
a b
1 mg
0 mg
From O’Brien-White method. From Eq. (1) for spherical double layer.
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Fig. 9. Changes of the zeta potential of n-tetradecane (0.15 ml)/lecithin/100 ml emulsion (2 h-old) as a function of pH.
emulsion appears at an acidic pH and at pH 3 the zeta potential of n-tetradecane/lecithin solution emulsions is positive and amounts 30–40 mV. However, at pH 4, depending on the lecithin content it is negative or positive. Even small coverage of ntetradecane surface by lecithin probably leads to adsorption of H+ ions and hence positive values of zeta potential. In 4–8 of pH range dipalmitoyl lecithin shows the properties of uncharged layers [22]. According to Shaw and Schulman investigations [22] quick neutralization of the phosphate group as the pH decreases from 4 to 3 is related to intermolecular spacing in the layers and in turns to unsaturation of fatty acyl chains of lecithin molecules. Hence, at pH 3 adsorption of H+ ions is possible giving positive values of the zeta potential. The ability of dipalmitoyl lecithin to bind cations is high, especially at high salt concentration [22]. The effect of ions on behavior of oil/water emulsion with lecithin will be further describe in our next paper being in preparation. Moreover, depending on pH, salt concentration and rearrangement of water molecules near n-tetradecane (hydrophobic) surface the forces between droplets will be attractive or repulsive. Besseling and Lyklema [24] and Besseling [25] concluded that if a surface influences mostly local density of water in the adjoining layer, but not the local orientation of molecules, then the attractive force will appear between two surfaces. This deals with surfaces possessing both low and high affinity to water. However, if a surface causes changes in the adjoining layer, the forces between two surfaces will be repulsive [24,25]. Scatena’s experiments have confirmed this model [26]. A large portion of interfacial water molecules has remarkably weak or negligible hydrogen bonding interactions with other interfacial water molecules. This finding is opposite to hydrophobic hydration model for a hydrophobic molecule in water. Another experiments show that interaction by hydrogen bonding at a hydrophobic liquid/water interface are greatly reduced relative to vapor/water interface and that the free OH bonds interact with interfacial organic molecules [27]. This can only occur if water dipoles are oriented with their hydrogen atoms pointed into the organic phase, in our case to n-tetradecane with adsorbed lecithin. The majority of the water molecules at the interfa-
cial region are those oriented-weakly bonded and interfacial water monomers, which interact with hydrogen bond acceptors. Adsorbed on n-tetradecane dipalmitoyl lecithin monolayers have small intermolecular spacing between the phosphate groups, so negative and positive charges form two planes of high charge density, separated by two methylene groups [22]. Thus, the phosphate group is unable to bind cations at low salt concentration, but rather anions or water dipoles, which is the case in the studied systems here. For this reason probably, there is high pH-sensitivity of the emulsion with dipalmitoyl lecithin. Netz in his review article [18] demonstrated that electrostatic potential across water/alkane interface due to the orientation of the highest water layer might change by 0.5 V and water polarity in this position would prefer adsorption of negative ions. In the light of the above one can conclude that the negative zeta potential at n-tetradecane surface in water alone may originate from preferentially oriented and immobilized water dipoles with their hydrogen atoms directed toward the oil surface and interacting by London dispersion forces. This would be possible if the slipping plain is located between this first and the second oriented water layer. In that second layer the counter ions (here H+ ) possibly should be present, thus compensating the dipole charge. Such mechanism seems to be also possible in the systems studied in this paper. However, situation is more complicated to describe when lecithin is present. To get more information one can calculate the electrokinetic’s charge density at the surface and/or the number of the oriented dipoles. Generally, the calculated κa values indicate that more appropriate is to treat the double layer at the n-tetradecane droplets as a spherical than a plane one. Using the measured zeta potentials and the diameters the electric charge in the shear plane was calculated. For this purpose an analytical expression for surface potential relationship for a spherical particle derived by Zhou [19] and Ohshima [21] was applied: I=±
2Y02
− N
1 1 + 2 X0 2X0
2
2 i=1 Zi ni
N
ni −
i=1
N
1/2 ni exp(−Zi Y0 )
(1)
i=1
where I = (σe)/(εkTκ); Y = Y0 = (eψ0 )/(kT)—scaled potential; X = X0 = κa—scaled distance; ε is permittivity of the solution; k is the Boltzman constant; κ is the Debye-H¨uckel parameter. The electrolyte is composed of N ionic species of valence Zi and bulk concentration (number density) ni (i 2= 1, 2, . . ., N). The electro neutrality condition requires: N i=1 Zi ni = 0. Eq. (1) gives results with a maximal percent relative error of up to 5.0% of the exact solution. This expression is also applicable to the case of small κa [17]. Eq. (1) for 1:1 electrolyte reduces to expression: I=±
2Y02
1 1 + X0 2X02
1/2 − [2 − exp(−Y0 ) − exp Y0 ] (2)
A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27
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Fig. 10. Changes of the zeta potential of n-tetradecane (0.2 ml)/water/100 ml (or lecithin solution, 2 h-old) as a function of pH.
Fig. 11. Electrokinetic’s charge of n-tetradecane droplets (0.2 ml)/100 ml in lecithin emulsion as a function of lecithin concentration and pH.
The knowledge about the charge density can help to estimate whether OH− ions create potential and whether water dipoles may play a role. The calculated values of electrokinetic’s charge (Table 1) are small, in the range of 10−4 to 10−3 C/m2 . The highest one is calculated at pH 10 (lecithin 1 mg), which equals to −5.83 × 10−3 C/m2 , while the lowest value at pH 4 and it is identical for both concentration of lecithin (0.26 × 10−3 C/m2 or 0.27 × 10−3 C/m2 ). It is known from solubility data that both groups in the lecithin molecule (alkyl phosphate and alkyl trimethylammonium) are strongly hydrophilic and therefore tend to dissolve in a solution, but the effect of the trimethylammonium group is predominant [22]. The highest electrokinetic’s charge at pH 10 confirms this suggestion. The effect of lecithin molecules on the zeta potential of n-tetradecane/water emulsion after 2 h since the emulsion preparation is presented in Fig. 10 for two its concentrations studied. Even small content of lecithin (0.5 mg/100 ml of emulsion) is able to change the value of zeta potential n-tetradecane/water emulsion from 0.5 to 29.9 mV at pH 3, and from −29 to −8.4 mV at pH 4. The same trend of changes is noticeable for higher concentration of lecithin (1 mg/100 ml of emulsion). As can be seen in this figure the zeta potential in a broad range of pH is determined by OH− ions, which probably adsorb on the lecithin molecules. This is concluded because the negative zeta potential is higher in lecithin presence (1 mg) than in water, by about 40 mV at pH 8–9, and by ca. 30 mV at pH 10 (Fig. 10). The negative zeta potential increases with the pH increase. This “double” capability of lecithin results from its molecule structure, where positively N+ and negatively O− charged atoms exist. But, higher ‘activity’ of N+ to OH− ions can be postulated basing on the zeta potentials (Fig. 10) and the calculated surface charges (Table 1).
charge density is also low up to pH 8. Then it sharply increases at pH 10–12. In Table 1 there are also presented a numbers of charges present on 1000 nm2 of the surface. It results from this table that, for instance, if 1 mg of lecithin was present the amount of the charges changes from 1.7 (at pH 4) to 36.4 at pH 10. But, even this charge at alkaline pH could be created by 54.8 immobilized water dipoles if were preferentially oriented with their negative ends at the shear plane. This amount results from water dipole moment [13,14,20,21]. Moreover, pH 4 the calculated 1.7 charges/1000 nm2 would be created by 2.5 water dipoles. Without lecithin presence at natural pH of n-tetradecane emulsion the charges amounts 11.1/1000 nm2 , it is equivalent to 16.7 water dipoles oriented at the shear plane. The above calculations show that key problem to understand the zeta potential origin at hydrophobic surface lies not only in the adsorption/desorption processes of OH− ions, but also in the vicinal water structure. Therefore, it may be concluded that immobilized and oriented water dipoles can contribute to the electrokinetic’s potential formation at n-tetradecane/lecithin interface. It also depends on the lecithin molecule conformation, like in case of an emulsion with proteins [12]. Charged phospholipid components and fatty acid salts contribute to more negative zeta potentials and thus increase the stability of the emulsions against, for example, heat-stress. Lecithin hydrolysis increases the zeta potential of the oil droplets. If pH falls the zeta potential is also lowered, which can lead to cracking of the emulsions [23]. Large fluctuation in the effective diameter of the droplets for studying emulsions can be seen at acidic pH (Figs. 12 and 13). At basic pH (from 8 to 12) the emulsions are very stable. High values of the zeta potentials are observed in this pH range, even above 100 mV at pH 12 (Fig. 10). These zeta potentials of long-time-equilibrated emulsions probably result from the mechanisms discussed above. As it was mentioned over, any excess lecithin forms stable unilamellar liposomes and aggregation of liposomes [23]. Neutral or slightly acidic liposome dispersions destabilize the oil droplets, showing that pH-dependent charge repulsion is more important for stability than the presence of additional lecithin in
5. Charge density and effective diameter relationship To better visualize the charge density versus pH in Fig. 11 there is plotted the charge at both concentration of lecithin. As can be seen the positive charge density is low and the negative
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A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27
Fig. 12. Changes of the effective diameter of n-tetradecane (0.2 ml)/lecithin/ 100 ml emulsion (2 h-old) as a function of pH.
lute zeta potential was accompanied by a growth in the effective diameter. The absolute values of zeta potential in most systems studied were higher than 20 mV (even reaching a maximum value of 120 mV), which is usually sufficient to stabilize the emulsion. The zeta potential of n-tetradecane surface in lecithin solution is determined by the lecithin molecules’ adsorption. However, a competition between the lecithin molecules and OH− /H+ ions or even water dipoles probably takes place. Anyway, it is clearly visible that the zeta potential of the emulsion with lecithin is pH-depended. The presence of OH− ions appeared in significant increase of negative zeta potentials. Thus, OH− ions play an essential role in the zeta potential formation. However, in stability of the n-tetradecane droplets the solvation forces resulting from the hydrogen bonding, and/or hydrophobic hydration, may also contribute. Additionally an effect of lecithin’ liposomes or aggregates can be regarded too. Acknowledgements The author thanks very much to Prof. E. Chibowski for his valuable remarks and discussion. Partial support from the Project No.3 T09A 04329 of Ministry of Education and Science is very appreciate. References
Fig. 13. Changes of the effective diameter of n-tetradecane (0.15 ml)/lecithin/ 100 ml emulsion (2 h-old) as a function of pH.
the form of liposomes. Excess of liposomes probably act as a ‘charge buffer’ helping to increase the tolerability of electrolyte addition to the emulsion. Comparing Figs. 11 and 12, stabilizing effect of charge repulsion is visible. Higher values of the electrokinetic’s charge at alkaline pH give small and quite stable values of the effective diameter, while at lower electrokinetic’s charge the effective diameters are large. Generally, comparing the zeta potentials and effective diameters (Figs. 8–13) it can be seen that at basic pH, where the zeta potentials are large negative, the fluctuations of effective diameter on the time scale are small. This is not the case at the acidic pH hence, an important role of OH− ions has been found both in the zeta potential and the stability. 6. Summary and conclusions Generally, the n-tetradecane/lecithin emulsions are relatively stable over the time period studied. The reduction in the abso-
[1] C.E. Stauffer, Emulsifiers, WNT, Warsaw, 2001. [2] H.M. Farrell, in: N. Wong (Ed.), Fundamentals of Dairy Chemistry, third ed., van Nostrand Reinhold, New York, 1988. [3] K. Beyermann, Chemistry for medical students, PZWL, Warsaw (in Polish). [4] M. Mirza, Y. Guo, K. Arnold, C.J. van Oss, S. Ohki, J. Dispersion Sci. Technol. 19 (1998) 951. [5] W. Wu, R.F. Giese, C.J. van Oss, Colloids Surf. B: Biointerfaces 14 (1999) 47. [6] K. Marinova, R.G. Algarova, N.D. Denkov, O.D. Velev, D.N. Petsev, I.B. Ivanov, R.P. Brovankar, Langmuir 12 (1996) 2045. [7] E. Chibowski, A.E. Wi˛acek, in: A.V. Delgado (Ed.), Interfacial Electrokinetics and Electrophoresis, Marcel Dekker, New York, 2001, p. 893. [8] N.A.M. Besseling, J. Lyklema, J. Phys. Chem. 101 (1997) 7604. [9] C.J. van Oss, Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994. [10] A.E. Wi˛acek, E. Chibowski, Colloids Surf. B: Biointerfaces 17 (2000) 175. [11] E.F. Grabowski, J.D. Morison, in: B. Dahneke (Ed.), Particle Size Distributions from Analysis of Quasi-Elastic Light Scattering Data, Wiley, New York, 1983. [12] A.E. Wi˛acek, E. Chibowski, Colloids Surf. A: Physicochem. Eng. Aspects 159 (1999) 253. [13] E. Chibowski, A.E. Wi˛acek, L. Hołysz, K. Terpiłowski, Langmuir 21 (10) (2005) 4347. [14] E. Chibowski, A.E. Wi˛acek, L. Hołysz, K. Terpiłowski, Langmuir 21 (17) (2005) 7662. [15] J. Jabło´nski, W. Janusz, J. Szczypa, J. Dispersion Sci. Technol. 20 (1999) 165. [16] R. O’Brien, L.R. White, J. Chem. Soc., Faraday Trans. 2 74 (1978) 1607. [17] R.J. Hunter, Zeta Potential in Colloid Science, Academic Press, London/San Francisco, 1981. [18] R.R. Netz, Curr. Opin. Colloid Interface Sci. 9 (2004) 192. [19] S. Zhou, J. Colloid Interface Sci. 208 (1998) 347.
A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27 [20] [21] [22] [23]
H. Ohshima, J. Colloid Interface Sci. 171 (1995) 525. H. Ohshima, J. Colloid Interface Sci. 247 (2002) 18. D.O. Shaw, J.H. Schulman, J. Lipid Res. 8 (1967) 227. www2.chemie.uni-erlangen.de/services/dissonline/data/dissertation.
[24] [25] [26] [27]
N.A.M. Besseling, L. Lyklema, J. Phys. Chem. 101 (1997) 7604. N.A.M. Besseling, Langmuir 13 (1997) 2213. L.F. Scatena, M.G. Brown, G.L. Richmond, Science 292 (2001) 908. P. Jungwirth, D.J. Tobias, J. Phys. Chem. 105 (2001) 10468.
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Electrokinetic properties of n-tetradecane/lecithin solution emulsions Agnieszka Ewa Wi˛acek Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Skłodowska University, 20031 Lublin, Poland Received 21 April 2006; received in revised form 29 June 2006; accepted 2 July 2006 Available online 7 July 2006
Abstract Properties of surface of hydrophobic liquid (e.g. n-alkane) in aqueous solutions are very important for stability of dispersed systems, i.e. emulsions and suspensions, as well as in many biochemical processes including living organisms. Application of natural stabilizers it is a key aspect of wide-ranging investigations of emulsion. One of such systems is oil/lecithin solution. The adsorbed lecithin layer at the oil droplets determines the structural and dynamic properties of oil/water emulsions. It changes the interfacial free energy and the zeta potential. The zeta potentials determined in this paper as a function of time and pH confirm that H+ /OH− ions are potential determining and the largest effect of OH− is visible at alkaline pHs. The decrease in absolute value of the zeta potential was accompanied by a growth in the effective diameter of emulsion droplets. Using both the potential and diameters electric charge at the shear plane for spherical particle was calculated. In these systems the zeta potential may be created by immobilized and oriented water dipoles and acid–base interactions. However, on the basis of the results it is concluded that the main stabilizing mechanism is the double layers repulsion, besides possible stabilizing effect of lecithin liposomes and other aggregates. © 2006 Published by Elsevier B.V. Keywords: Oil/water emulsions; Zeta potential; Multimodal size distribution; Electric charge; Dipalmitoyl lecithin
1. Introduction Important parameters determining properties of dispersed systems are: zeta potential (surface charge), droplet size, pH and ionic strength. The stability of emulsions and suspensions and electrical phenomena at nonionogenic hydrophobic surfaces in water or electrolyte solutions, still attract researches because of wide-ranging using of these systems, for example in many biochemical processes. However, the origin of electric charge and potential at the oil droplet is still not well understood [1–8]. Important aspect of emulsion investigations is application of natural emulsifiers and stabilizers. One of them is lecithin. Natural stabilizers involve a broad group of phospholipids, which are important substances for living organisms. For example, phosphatidylcholins are a key building block of cell membranes and protects cells from oxidation and largely comprises the protective sheaths surrounding the brain. Although lecithin is a fatty substance, it is also a fat emulsifier. The food industry recognizes lecithin (product of processing soybean oil) as a good stabilizer (lipophilic emulsifier), using it in many processes, like wetting, dispersion and lubrication [1]. A wide spectrum of lecithin
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applications results from its amphiphilic structure. The adsorbed lecithin layer at oil droplets surface determines the structural and dynamic properties of oil/water emulsions. Usually pH affects lecithin adsorption and hence the surface charge at the oil droplet, what should appear in the changes of zeta potential [2]. The changes in zeta potential of the oil/lecithin droplets may be due to conformation changes of the adsorbed lecithin molecules and interaction of H+ /OH− ions with the molecules. The structure of phosphatidylcholin molecule is shown in Fig. 1 [3]. An increase in zeta potential of the droplet is often desirable to stabilize the emulsion. However, despite attractive dispersion and repulsive electrostatic force originating from polar groups, if present, solvation forces should be also taken into account. In many systems they are due to hydrogen bonding interactions between hydrogen and oxygen atoms. Changes in the zeta potential may correlate with changes in the acid–base interactions, which can essentially affect stability of the systems [4–7,9]. On the basis of the earlier experimental results and literature data, it is concluded that at n-tetradecane surface water dipoles in the first layer may play an important role in the electric potential origin [10,12–15]. Using the measured zeta potentials and the droplet diameters the electric charge in the shear plane was calculated [16–21]. Taking into account water arrangement near the n-tetradecane surface and the calculated charges, it was con-
A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27
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Fig. 1. The structure of phosphatidylcholin molecule [3].
cluded that in fact the potential may be created both by OH− /H+ and oriented water dipoles. The purpose of this work was to investigate the electrokinetic properties of n-tetradecane/water emulsion in the presence of lecithin, stabilizing the droplets, and also to learn about probable stabilizing mechanism. 2. Experimental The emulsions were prepared in 100 ml-calibrated flasks. The components were added as follows. First n-tetradecane was introduced, then lecithin solution (0.5 or 1 mg/100 ml). The water was from MilliQ-Plus System. The pH of the emulsion was natural or adjusted with a help of HCl and NaOH 0.1 M solutions. Next, the flasks were placed for 15 min into a homogenizer (10,000 rpm). The lecithin (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) used was from Sigma-Aldrich Chemical Co.; n-tetradecane (p ≥99%) was from Fluka. Zeta Pals/Bi-Mass apparatus (Brookhaven Instr. Co.) was applied for measurement both the electrophoretic mobility and the effective diameter (multimodal size distribution). These parameters were determined as a function of time, after 5, 15, 30, 60, 120 min and 24 h since the moment of the emulsion preparation. The zeta potential was calculated from the electrophoretic mobility using O’BrienWhite computer program. The instrument applies dynamic light scattering for the parameters evaluation. All the experiments were carried out at 20 ◦ C. Some of them were replicated two times. For each measured sample 10 measuring runs were taken, with seven cycles in the each run, and were fixed in the Zetameter.
276 nm (2 h old emulsion) at natural pH 6.04. Larger effective diameters, above 500 nm, and their large fluctuation occurred at acidic pH (3 and 4), whereas at basic pH the changes in effective diameters on time scale were minimal and produced emulsions were quite monodisperse. The obtained results are very regular and stable, even after one day from the emulsion preparation. It is known from the literature [22,23], that any excess of lecithin forms stable unilamellar liposomes showing high pH-dependent. Neutral or slightly acidic liposome dispersions, if present in the medium, destabilize the oil/water emulsion. The presence of liposome’s aggregates may easily be misread with oil droplets in the creamed oil phase [22,23]. Hence, a higher effective diameter does not always mean a lower stability emulsion. Only CryoTEM technique allows direct observation of liposomes apart from the oil droplets. Quantity inspection of liposomal material is, however, difficult. Further experiments are necessary to better characterize lecithin emulsion. Multimodal size distribution analysis shows that in the studied emulsions mostly there is one principal population of the droplet sizes, however in some systems there are two comparable populations. Polydispersities of the emulsions were low during all the experiment time, between 0.005 and 0.2. Typical multimodal size distribution of the emulsions n-tetradecane/lecithin is shown in Figs. 3 and 4, for freshly prepared emulsion (after 5 min) and after 1 h since the emulsion preparation. In both cases there is practically only one population of the droplets, around 470 and 505 nm, respectively. Some small populations appear around 205 and 298 nm, respectively. The presence of lecithin did not affect the effective diameter much, the range
3. Droplet diameter and multimodal size distribution The changing effective diameter of the droplets as a function of time resulted from coalescence of the droplets and their flotation to the emulsion surface (phases separation). Unfortunately, with the dynamic light scattering technique it is difficult to determine the droplet concentration, however the relative changes can be well monitored [10,11]. The most stable effective diameters were obtained for the emulsion of n-tetradecane 0.2 ml/lecithin 1 mg in 100 ml. The effective diameters of these n-tetradecane/ lecithin emulsions are shown in Fig. 2. The size distributions of the droplets are usually sharp. Any decrease in the effective diameter should be interpreted as a partial separation of the dispersed phase, i.e., larger droplets float toward the surface. On the contrary, an increase in the average diameter shows that the coalesced droplets are still present in the bulk emulsion for a period of time. For example, from 258.7 nm (5 min old emulsion) to
Fig. 2. Changes of the effective diameter of n-tetradecane droplets (0.2 ml)/ lecithin (1 mg)/100 ml emulsion as a function of time and pH.
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Fig. 3. Multimodal size distribution of n-tetradecane (0.2 ml)/lecithin (1 mg)/ 100 ml 5 min-old emulsions in pH 3 (effective diameter 470 nm, polydispersity 0.107).
Fig. 6. Changes of the effective diameter of n-tetradecane (0.1, 0.15 or 0.2 ml)/lecithin (1 mg)/100 ml emulsion as a function of time.
Fig. 4. Multimodal size distribution of n-tetradecane (0.2 ml)/lecithin (1 mg)/ 100 ml 1 h-old emulsions in pH 3 (effective diameter 505 nm, polydispersity 0.202).
of effective diameters is comparable like for emulsion of ntetradecane/water studied earlier [10]. Effect of oil concentration in n-tetradecane/lecithin solution (0.5 or 1 mg/100 ml) emulsions at their natural pH is visualized in Figs. 5 and 6. Very stable emulsion was obtained containing 0.1 ml of oil and 0.5 mg of lecithin in 100 ml of emulsion. Significant increase in the diameter is observed for emulsion
Fig. 5. Changes of the effective diameter of n-tetradecane (0.1, 0.15 or 0.2 ml)/lecithin (0.5 mg)/100 ml emulsion as a function of time.
of 0.15 ml n-tetradecane/1 mg lecithin. After 24 h the effective diameter has grown up twice. Since the lecithin molecule has a trimethylammonium and phosphate group separated by two methylene groups, its structure allows two ionic forms; with maximal separation of charges or a reduced separation of charges. It results from an internal salt linkage between the phosphate and trimethylammonium groups in the same molecule [22]. These different structures of lecithin molecule are of considerable interest in relation to the surface coverage and stability. Assuming that total amount of lecithin added had adsorbed to the oil droplet surface and that during the coalescence process of smaller droplets the lecithin remained on the surface of the larger droplets formed then surface coverage should be larger than 1 mg/m2 . But, this is not the case, because the calculated maximum statistical coverage of the droplets is less than 1 mg/m2 , especially in freshly prepared emulsion. 4. Zeta potential and electrokinetic charge The zeta potentials of n-tetradecane/lecithin emulsion were investigated at different pH. Fig. 7 shows that changes of zeta potentials are pH-dependent, but it settles rather fast. The values of zeta potential were calculated from O’Brien-White equation [16] using a computer program. From the effective diameters and the ionic strength of the emulsions studied κa values were calculated. κ is the Debye-H¨uckel parameter (reciprocal of the diffuse double layer thickness) and a is the particle diameter [17]. Thus calculated zeta potential values are listed in Table 1. Our experiments show that H+ /OH− ions are the potential determining for those emulsions. The largest effect of OH− ions on the zeta potential is visible at alkaline pH. As the pH of the emulsion is decreased probably the phosphate groups in the layer of lecithin molecules are neutralized, especially for dipalmitoyl lecithin (with long acyl chains) used in this paper. The increase in unsaturation of the fatty acyl chains increases the intermolecular spacing, which reduces the ionic repulsion between the polar groups, and hence strengthens the internal salt linkage. There
A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27
Fig. 7. Changes of the zeta potential of n-tetradecane (0.2 ml)/lecithin (1 mg)/100 ml emulsion as a function of time and pH.
is a vertical rather than coplanar orientation of the phosphoryl choline group with respect to the interface [22]. These conformations probably contribute to the observed zeta potential changes and resulting the electrokinetic’s charge. Jabło´nski et al. [15] conducted the acid–base potentiometric titration of octadecane suspensions in aqueous solutions of NaCl and their results showed that neither H+ nor OH− ions have adsorbed at the octadecane/electrolyte interface. The electrophoretic mobility of the octadecane suspension in 10−3 M NaCl at pH 7–8.5 was ca. −6 m cm/V s, which corresponded to −65 to 70 mV of the zeta potential. On the basis of the measured electrophoretic mobility they calculated that the surface charge (equated to the electrokinetic’s charge) should be about 1 C/cm2 (i.e. 0.01 C/m2 ). They concluded that the influence of H+ or OH− on the observed zeta potential is rather of an indirect nature and changes in water structure at the hydrophobic interface were responsible for the observed zeta potential appearance. Our measurements for paraffin [13,14] led to the conclusion that at a nonionogenic hydrophobic paraffin surface a significant role in the electric potential origin was played by the oriented water
23
Fig. 8. Changes of the zeta potential of n-tetradecane (0.2 ml)/lecithin/100 ml emulsion (2 h-old) as a function of pH.
dipoles in the first layer next to a thin “vacuum” space being of about 0.1 nm thickness. It is believed that the electric double layer at such an interface, especially at a low ionic strength, possesses partially a dipolar nature. In this paper significant increase in the negative zeta potential at alkaline pH was observed. On the basis of classical double layer theory this increase in negative zeta potential can be interpreted as a result of OH− or lecithin molecules adsorption during equilibration of the system. Marinova et al. [6] concluded that negative surface charge at oil/water interface resulted from specific OH− adsorption also via hydrogen bonding with water molecules in the boundary layers, and that was inherent property of the oil/water interface. The changes in zeta potential at different pH may be considered to be also due to some restructuring of the water molecules immobilized at the n-tetradecane surface to attain an equilibrium structure and adsorption or depletion of OH− ions, which could form hydrogen bonding [13,14]. Changes in the zeta potential of n-tetradecane droplets in 2 h-old emulsions with lecithin at different pH are presented in Figs. 8 and 9. The isoelectric point (iep) of n-tetradecane/lecithin
Table 1 Solution
Lecithin
Potential from O’Brien-White methoda
Particle radius (nm)
κa
Surface chargeb (C/m2 × 103 )
No. of charges/1000 nm2
td 0.2 ml, pH 3 td 0.2 ml, pH 4 td 0.2 ml, pH 7.3 td 0.2 ml, pH 8 td 0.2 ml, pH 10 td 0.2 ml, pH 12 td 0.2 ml, pH 3 td 0.2 ml, pH 4 td 0.2 ml, pH 6.1 td 0.2 ml, pH 8 td 0.2 ml, pH 10 td 0.2 ml, pH 12 H2 O, td 0.2 ml, pH 6.5
0.5 mg
28.3 7.11 −37 −30.7 −57.2 −115.5 32.5 −4.65 −58.6 −94.9 −111.2 −95.9 −46.6
436 767 374 183 126 166 280 501 138 110 148 123 247
32 19 3.2 1.6 36 39 21 131 1.7 1.4 43 29 0.36
0.84 0.26 −1.17 −0.37 −4.01 −4.97 0.98 −0.27 −0.91 −1.73 −5.83 −4.85 −1.78
5.2 1.6 7.3 2.3 25 31 6.1 1.7 5.7 10.8 36.4 30.3 11.1
a b
1 mg
0 mg
From O’Brien-White method. From Eq. (1) for spherical double layer.
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A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27
Fig. 9. Changes of the zeta potential of n-tetradecane (0.15 ml)/lecithin/100 ml emulsion (2 h-old) as a function of pH.
emulsion appears at an acidic pH and at pH 3 the zeta potential of n-tetradecane/lecithin solution emulsions is positive and amounts 30–40 mV. However, at pH 4, depending on the lecithin content it is negative or positive. Even small coverage of ntetradecane surface by lecithin probably leads to adsorption of H+ ions and hence positive values of zeta potential. In 4–8 of pH range dipalmitoyl lecithin shows the properties of uncharged layers [22]. According to Shaw and Schulman investigations [22] quick neutralization of the phosphate group as the pH decreases from 4 to 3 is related to intermolecular spacing in the layers and in turns to unsaturation of fatty acyl chains of lecithin molecules. Hence, at pH 3 adsorption of H+ ions is possible giving positive values of the zeta potential. The ability of dipalmitoyl lecithin to bind cations is high, especially at high salt concentration [22]. The effect of ions on behavior of oil/water emulsion with lecithin will be further describe in our next paper being in preparation. Moreover, depending on pH, salt concentration and rearrangement of water molecules near n-tetradecane (hydrophobic) surface the forces between droplets will be attractive or repulsive. Besseling and Lyklema [24] and Besseling [25] concluded that if a surface influences mostly local density of water in the adjoining layer, but not the local orientation of molecules, then the attractive force will appear between two surfaces. This deals with surfaces possessing both low and high affinity to water. However, if a surface causes changes in the adjoining layer, the forces between two surfaces will be repulsive [24,25]. Scatena’s experiments have confirmed this model [26]. A large portion of interfacial water molecules has remarkably weak or negligible hydrogen bonding interactions with other interfacial water molecules. This finding is opposite to hydrophobic hydration model for a hydrophobic molecule in water. Another experiments show that interaction by hydrogen bonding at a hydrophobic liquid/water interface are greatly reduced relative to vapor/water interface and that the free OH bonds interact with interfacial organic molecules [27]. This can only occur if water dipoles are oriented with their hydrogen atoms pointed into the organic phase, in our case to n-tetradecane with adsorbed lecithin. The majority of the water molecules at the interfa-
cial region are those oriented-weakly bonded and interfacial water monomers, which interact with hydrogen bond acceptors. Adsorbed on n-tetradecane dipalmitoyl lecithin monolayers have small intermolecular spacing between the phosphate groups, so negative and positive charges form two planes of high charge density, separated by two methylene groups [22]. Thus, the phosphate group is unable to bind cations at low salt concentration, but rather anions or water dipoles, which is the case in the studied systems here. For this reason probably, there is high pH-sensitivity of the emulsion with dipalmitoyl lecithin. Netz in his review article [18] demonstrated that electrostatic potential across water/alkane interface due to the orientation of the highest water layer might change by 0.5 V and water polarity in this position would prefer adsorption of negative ions. In the light of the above one can conclude that the negative zeta potential at n-tetradecane surface in water alone may originate from preferentially oriented and immobilized water dipoles with their hydrogen atoms directed toward the oil surface and interacting by London dispersion forces. This would be possible if the slipping plain is located between this first and the second oriented water layer. In that second layer the counter ions (here H+ ) possibly should be present, thus compensating the dipole charge. Such mechanism seems to be also possible in the systems studied in this paper. However, situation is more complicated to describe when lecithin is present. To get more information one can calculate the electrokinetic’s charge density at the surface and/or the number of the oriented dipoles. Generally, the calculated κa values indicate that more appropriate is to treat the double layer at the n-tetradecane droplets as a spherical than a plane one. Using the measured zeta potentials and the diameters the electric charge in the shear plane was calculated. For this purpose an analytical expression for surface potential relationship for a spherical particle derived by Zhou [19] and Ohshima [21] was applied: I=±
2Y02
− N
1 1 + 2 X0 2X0
2
2 i=1 Zi ni
N
ni −
i=1
N
1/2 ni exp(−Zi Y0 )
(1)
i=1
where I = (σe)/(εkTκ); Y = Y0 = (eψ0 )/(kT)—scaled potential; X = X0 = κa—scaled distance; ε is permittivity of the solution; k is the Boltzman constant; κ is the Debye-H¨uckel parameter. The electrolyte is composed of N ionic species of valence Zi and bulk concentration (number density) ni (i 2= 1, 2, . . ., N). The electro neutrality condition requires: N i=1 Zi ni = 0. Eq. (1) gives results with a maximal percent relative error of up to 5.0% of the exact solution. This expression is also applicable to the case of small κa [17]. Eq. (1) for 1:1 electrolyte reduces to expression: I=±
2Y02
1 1 + X0 2X02
1/2 − [2 − exp(−Y0 ) − exp Y0 ] (2)
A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27
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Fig. 10. Changes of the zeta potential of n-tetradecane (0.2 ml)/water/100 ml (or lecithin solution, 2 h-old) as a function of pH.
Fig. 11. Electrokinetic’s charge of n-tetradecane droplets (0.2 ml)/100 ml in lecithin emulsion as a function of lecithin concentration and pH.
The knowledge about the charge density can help to estimate whether OH− ions create potential and whether water dipoles may play a role. The calculated values of electrokinetic’s charge (Table 1) are small, in the range of 10−4 to 10−3 C/m2 . The highest one is calculated at pH 10 (lecithin 1 mg), which equals to −5.83 × 10−3 C/m2 , while the lowest value at pH 4 and it is identical for both concentration of lecithin (0.26 × 10−3 C/m2 or 0.27 × 10−3 C/m2 ). It is known from solubility data that both groups in the lecithin molecule (alkyl phosphate and alkyl trimethylammonium) are strongly hydrophilic and therefore tend to dissolve in a solution, but the effect of the trimethylammonium group is predominant [22]. The highest electrokinetic’s charge at pH 10 confirms this suggestion. The effect of lecithin molecules on the zeta potential of n-tetradecane/water emulsion after 2 h since the emulsion preparation is presented in Fig. 10 for two its concentrations studied. Even small content of lecithin (0.5 mg/100 ml of emulsion) is able to change the value of zeta potential n-tetradecane/water emulsion from 0.5 to 29.9 mV at pH 3, and from −29 to −8.4 mV at pH 4. The same trend of changes is noticeable for higher concentration of lecithin (1 mg/100 ml of emulsion). As can be seen in this figure the zeta potential in a broad range of pH is determined by OH− ions, which probably adsorb on the lecithin molecules. This is concluded because the negative zeta potential is higher in lecithin presence (1 mg) than in water, by about 40 mV at pH 8–9, and by ca. 30 mV at pH 10 (Fig. 10). The negative zeta potential increases with the pH increase. This “double” capability of lecithin results from its molecule structure, where positively N+ and negatively O− charged atoms exist. But, higher ‘activity’ of N+ to OH− ions can be postulated basing on the zeta potentials (Fig. 10) and the calculated surface charges (Table 1).
charge density is also low up to pH 8. Then it sharply increases at pH 10–12. In Table 1 there are also presented a numbers of charges present on 1000 nm2 of the surface. It results from this table that, for instance, if 1 mg of lecithin was present the amount of the charges changes from 1.7 (at pH 4) to 36.4 at pH 10. But, even this charge at alkaline pH could be created by 54.8 immobilized water dipoles if were preferentially oriented with their negative ends at the shear plane. This amount results from water dipole moment [13,14,20,21]. Moreover, pH 4 the calculated 1.7 charges/1000 nm2 would be created by 2.5 water dipoles. Without lecithin presence at natural pH of n-tetradecane emulsion the charges amounts 11.1/1000 nm2 , it is equivalent to 16.7 water dipoles oriented at the shear plane. The above calculations show that key problem to understand the zeta potential origin at hydrophobic surface lies not only in the adsorption/desorption processes of OH− ions, but also in the vicinal water structure. Therefore, it may be concluded that immobilized and oriented water dipoles can contribute to the electrokinetic’s potential formation at n-tetradecane/lecithin interface. It also depends on the lecithin molecule conformation, like in case of an emulsion with proteins [12]. Charged phospholipid components and fatty acid salts contribute to more negative zeta potentials and thus increase the stability of the emulsions against, for example, heat-stress. Lecithin hydrolysis increases the zeta potential of the oil droplets. If pH falls the zeta potential is also lowered, which can lead to cracking of the emulsions [23]. Large fluctuation in the effective diameter of the droplets for studying emulsions can be seen at acidic pH (Figs. 12 and 13). At basic pH (from 8 to 12) the emulsions are very stable. High values of the zeta potentials are observed in this pH range, even above 100 mV at pH 12 (Fig. 10). These zeta potentials of long-time-equilibrated emulsions probably result from the mechanisms discussed above. As it was mentioned over, any excess lecithin forms stable unilamellar liposomes and aggregation of liposomes [23]. Neutral or slightly acidic liposome dispersions destabilize the oil droplets, showing that pH-dependent charge repulsion is more important for stability than the presence of additional lecithin in
5. Charge density and effective diameter relationship To better visualize the charge density versus pH in Fig. 11 there is plotted the charge at both concentration of lecithin. As can be seen the positive charge density is low and the negative
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A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27
Fig. 12. Changes of the effective diameter of n-tetradecane (0.2 ml)/lecithin/ 100 ml emulsion (2 h-old) as a function of pH.
lute zeta potential was accompanied by a growth in the effective diameter. The absolute values of zeta potential in most systems studied were higher than 20 mV (even reaching a maximum value of 120 mV), which is usually sufficient to stabilize the emulsion. The zeta potential of n-tetradecane surface in lecithin solution is determined by the lecithin molecules’ adsorption. However, a competition between the lecithin molecules and OH− /H+ ions or even water dipoles probably takes place. Anyway, it is clearly visible that the zeta potential of the emulsion with lecithin is pH-depended. The presence of OH− ions appeared in significant increase of negative zeta potentials. Thus, OH− ions play an essential role in the zeta potential formation. However, in stability of the n-tetradecane droplets the solvation forces resulting from the hydrogen bonding, and/or hydrophobic hydration, may also contribute. Additionally an effect of lecithin’ liposomes or aggregates can be regarded too. Acknowledgements The author thanks very much to Prof. E. Chibowski for his valuable remarks and discussion. Partial support from the Project No.3 T09A 04329 of Ministry of Education and Science is very appreciate. References
Fig. 13. Changes of the effective diameter of n-tetradecane (0.15 ml)/lecithin/ 100 ml emulsion (2 h-old) as a function of pH.
the form of liposomes. Excess of liposomes probably act as a ‘charge buffer’ helping to increase the tolerability of electrolyte addition to the emulsion. Comparing Figs. 11 and 12, stabilizing effect of charge repulsion is visible. Higher values of the electrokinetic’s charge at alkaline pH give small and quite stable values of the effective diameter, while at lower electrokinetic’s charge the effective diameters are large. Generally, comparing the zeta potentials and effective diameters (Figs. 8–13) it can be seen that at basic pH, where the zeta potentials are large negative, the fluctuations of effective diameter on the time scale are small. This is not the case at the acidic pH hence, an important role of OH− ions has been found both in the zeta potential and the stability. 6. Summary and conclusions Generally, the n-tetradecane/lecithin emulsions are relatively stable over the time period studied. The reduction in the abso-
[1] C.E. Stauffer, Emulsifiers, WNT, Warsaw, 2001. [2] H.M. Farrell, in: N. Wong (Ed.), Fundamentals of Dairy Chemistry, third ed., van Nostrand Reinhold, New York, 1988. [3] K. Beyermann, Chemistry for medical students, PZWL, Warsaw (in Polish). [4] M. Mirza, Y. Guo, K. Arnold, C.J. van Oss, S. Ohki, J. Dispersion Sci. Technol. 19 (1998) 951. [5] W. Wu, R.F. Giese, C.J. van Oss, Colloids Surf. B: Biointerfaces 14 (1999) 47. [6] K. Marinova, R.G. Algarova, N.D. Denkov, O.D. Velev, D.N. Petsev, I.B. Ivanov, R.P. Brovankar, Langmuir 12 (1996) 2045. [7] E. Chibowski, A.E. Wi˛acek, in: A.V. Delgado (Ed.), Interfacial Electrokinetics and Electrophoresis, Marcel Dekker, New York, 2001, p. 893. [8] N.A.M. Besseling, J. Lyklema, J. Phys. Chem. 101 (1997) 7604. [9] C.J. van Oss, Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994. [10] A.E. Wi˛acek, E. Chibowski, Colloids Surf. B: Biointerfaces 17 (2000) 175. [11] E.F. Grabowski, J.D. Morison, in: B. Dahneke (Ed.), Particle Size Distributions from Analysis of Quasi-Elastic Light Scattering Data, Wiley, New York, 1983. [12] A.E. Wi˛acek, E. Chibowski, Colloids Surf. A: Physicochem. Eng. Aspects 159 (1999) 253. [13] E. Chibowski, A.E. Wi˛acek, L. Hołysz, K. Terpiłowski, Langmuir 21 (10) (2005) 4347. [14] E. Chibowski, A.E. Wi˛acek, L. Hołysz, K. Terpiłowski, Langmuir 21 (17) (2005) 7662. [15] J. Jabło´nski, W. Janusz, J. Szczypa, J. Dispersion Sci. Technol. 20 (1999) 165. [16] R. O’Brien, L.R. White, J. Chem. Soc., Faraday Trans. 2 74 (1978) 1607. [17] R.J. Hunter, Zeta Potential in Colloid Science, Academic Press, London/San Francisco, 1981. [18] R.R. Netz, Curr. Opin. Colloid Interface Sci. 9 (2004) 192. [19] S. Zhou, J. Colloid Interface Sci. 208 (1998) 347.
A.E. Wi˛acek / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 20–27 [20] [21] [22] [23]
H. Ohshima, J. Colloid Interface Sci. 171 (1995) 525. H. Ohshima, J. Colloid Interface Sci. 247 (2002) 18. D.O. Shaw, J.H. Schulman, J. Lipid Res. 8 (1967) 227. www2.chemie.uni-erlangen.de/services/dissonline/data/dissertation.
[24] [25] [26] [27]
N.A.M. Besseling, L. Lyklema, J. Phys. Chem. 101 (1997) 7604. N.A.M. Besseling, Langmuir 13 (1997) 2213. L.F. Scatena, M.G. Brown, G.L. Richmond, Science 292 (2001) 908. P. Jungwirth, D.J. Tobias, J. Phys. Chem. 105 (2001) 10468.
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