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nonionic surfactant system and the emulsification process on the properties of paraffin emulsions
Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 38–44
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Influence of a mixed ionic/nonionic surfactant system and the emulsification process on the properties of paraffin emulsions J. Vilasau a , C. Solans a , M.J. Gómez b , J. Dabrio b , R. Mújika-Garai b , J. Esquena a,∗ a b
Instituto de Química Avanzada de Catalu˜ na (IQAC-CSIC), CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), c/Jordi Girona 18-26, 08034 Barcelona, Spain RYLESA (Repsol-YPF Lubricantes y Especialidades SA), Madrid, Spain
a r t i c l e
i n f o
Article history: Received 28 July 2011 Received in revised form 15 September 2011 Accepted 17 September 2011 Available online 24 September 2011 Keywords: Paraffin emulsions Emulsification process Mixed surfactant system Particleboard panels Stability Particle size
a b s t r a c t Paraffin emulsions are important in technological applications such as coating in the food packaging industry or to provide waterproof properties to particleboard panels. Small particle size (about 1.0 m) and low polydispersity are required to form stable paraffin emulsions for these applications. In this context, the main objective of the present work is to study the influence of the surfactant system and the emulsification process on the properties of paraffin emulsions. A high pressure homogenizer was used to prepare the emulsions and its characterization was made by means of optical microscopy, laser diffraction and electrophoretic mobility measurements. Emulsions were prepared as a function of the ionic/nonionic surfactant ratio, the total surfactant concentration and the homogenization pressure. A simple theoretical model to predict the minimum particle size was used, assuming that surfactant is either at the oil–water interface or as monomer in the external phase. Experimental and theoretical data are on good agreement and the formation of stable emulsions is explained according to such model. This result could be of prime importance in order to formulate new paraffin emulsions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Paraffin emulsions are used in a wide range of technological applications such as coating in the food packaging industry or to provide waterproof properties to particleboard panels in the furniture industry. In order to obtain stable paraffin emulsions, for different applications, small particle size (about 1.0 m) and low polydispersity are required. Recent works [1,2] dealing with paraffin emulsions, stabilized with a mixed ionic/nonionic surfactant system, led to interesting results. At high ionic/nonionic surfactant ratios (<44/56) a significant change in emulsion properties was observed, increasing storage modulus (G ) and emulsion stability; while decreasing particle size [1]. Moreover, the phase behaviour of the surfactant system in water revealed that lamellar liquid crystalline aggregates were present at very low surfactant concentration (down to 0.2 wt% surfactant). Considering that the total surfactant concentration was about 2 wt%, the high stability could be related to the formation of a surfactant multilayer arrangement, surrounding the particles [2]. Emulsions are stabilized generally by surfactants that adsorb at interfaces, such as oil–water interface, and form normally a monolayer surrounding the droplets, which retards the destabilization processes. However, other kind of surfactant self-aggregation,
such as liquid crystalline phases, can be present at the interface. Friberg et al. described the stabilization by multilayers around the droplets, increasing the viscosity of the system and enhancing emulsion stability [3–8]. This stabilization mechanism was found in formulations with a 1/1 oil/water ratio and surfactant concentrations between 3 and 4 wt% [6]. Recent works [9–11] revealed that the surfactant concentration required to form multilayers surrounding the droplets is normally higher (between 10 and 15 wt%). It should be pointed out that the formation of such multilayers depends on many factors such as the nature of the surfactant, the interfacial area and the surfactant solubility in the continuous phase. In this context, little is known about the role of the emulsification process in the emulsion properties of such systems. Moreover, the influence of the surfactants on paraffin emulsion and the possible self-assembled structures in the continuous phase have not been subject to systematic studies. Consequently, in this work the influence of the ionic/nonionic surfactant ratio, the total surfactant concentration and the homogenization pressure on paraffin emulsion properties have been studied. 2. Experimental 2.1. Materials
∗ Corresponding author. Tel.: +34 934006178; fax: +34 932045904. E-mail address: [email protected] (J. Esquena). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.09.026
The ionic surfactant (anionic) was formed by neutralization of an acid (alkyl chain length of 21 carbon atoms) with an alkanolamine,
J. Vilasau et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 38–44
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Table 1 Compositions of the emulsions (concentrations of the nonionic surfactant and the ionic surfactant, which consists of an acid neutralized with an alkanolamine), at different ionic surfactant weight fraction, defined as I = [acid]/([acid] + [nonionic]). I
0.00
0.11
0.22
0.33
0.44
0.56
0.67
0.78
0.89
1.00
Nonionic (wt%) Acid (wt%) Alkanolamine (wt%)
1.80 0.00 0.00
1.60 0.20 0.27
1.40 0.40 0.53
1.20 0.60 0.80
1.00 0.80 1.07
0.80 1.00 1.33
0.60 1.20 1.60
0.40 1.40 1.87
0.20 1.60 2.13
0.00 1.80 2.40
which was added at 0.34 [acid]/[alkanolamine] molar ratio. This stoichiometric excess was used to ensure complete neutralization. The ionic surfactant, formed after neutralization, possesses a melting point of 61 ◦ C. The nonionic surfactant possesses an alkyl chain length of 19 carbon atoms and its HLB is 7.6. The melting point of this surfactant is 36 ◦ C. The oil phase is paraffin and consists of a mixture of saturated hydrocarbons with an average chain length of 25 carbon atoms and a melting point of 52 ◦ C. Paraffin and surfactants were used without any further purification. No additional information on these materials can be presented, due to confidentiality. 2.2. Methods 2.2.1. Emulsion preparation The aqueous phase was added to the oil phase containing paraffin, nonionic and ionic surfactant at approximately 70 ◦ C. The emulsions were firstly pre-mixed by manual agitation, introduced into a high pressure homogenizer (Microfluidizer ML100, Microfludics® ) and forced to pass one or two times (one or two emulsification steps) through a 75 m capillary, at 70 ◦ C. Otherwise stated, the homogenization pressure was approximately 270 atm. The resulting emulsions were rapidly cooled down collecting them in a beaker immersed in ice water. It should be pointed out that surfactants and paraffin have their melting points between 36 ◦ C and 61 ◦ C. Therefore, emulsification is possible at 70 ◦ C, and these components form crystalline phases at room temperature. According to the phase diagram of the same surfactant mixture [1], the Krafft point is approximately 48 ◦ C, and therefore the concentration of surfactant molecules as monomer will be rather small after cooling down. The pH was adjusted between 8 and 10. The final conductivity was between 100 and 400 S/cm, in all emulsions. Different emulsions were prepared as a function of three parameters: ionic surfactant weight fraction, defined as 1. The I = [acid]/([acid] + [nonionic]), which was increased from 0 to 1, keeping constant the total surfactant concentration ([acid] + [nonionic]) and the homogenization pressure at 1.8 wt% and 270 atm, respectively (Table 1). 2. The total surfactant concentration ([acid] + [nonionic]) was varied between 0.8 and 4.0 wt%, keeping constant the ionic weight fraction (I ), and the homogenization pressure at 0.33 and 270 atm, respectively. 3. The homogenization pressure, which was varied between 140 and 540 atm for the emulsions at 0.8, 3.0 and 4.0 wt% surfactant concentration, keeping constant the ionic weight fraction at 0.33. In all cases, the acid was fully neutralized with alkanolamine, at constant 0.34 acid/alkanolamine molar ratio and the paraffin content was kept constant at 57.4 wt%.
Reproducibility in the preparation of emulsions, by using high pressure homogenization, was quite good. Fifteen replicas of the sample at I = 0.33 were prepared. The average particle size was 0.70 m, and the standard deviation was 0.08 m. 2.2.2. Optical microscopy Samples were observed with a Reicheirt Polyvar 2 microscope (Leica, Germany), equipped with video camera, polarizers, and interference contrast prism. Images were processed using the IM500 software supplied by Leica. This technique was used to determine the particle size and aggregation. 2.2.3. Determination of the surface area per surfactant molecule (as ) by SAXS SAXS measurements were performed in a S3-Micro instrument (Hecus X-ray Systems GMBH Graz, Austria) operating with point focalization. The scattering was detected with a linear position sensitive detector OED-50M. The temperature controller was a Peltier device. The q range is approximately from 0.02 to 6 nm−1 . Samples were introduced in glass capillaries of 1 mm diameter and 10 m wall thickness and then sealed to prevent evaporation. Blanks were carried out by measuring empty capillaries. The surface area per surfactant molecule in lamellar liquid crystalline phases was calculated by applying the following equations [12,13]: as =
VL dL
(1)
dL =
d · L 2
(2)
where d represents the repeat distance, in lamellar phases, measured by SAXS (d = 2/q, where q is the scattering vector corresponding to the first order peak), dL is the half thickness of the lipophilic part, L is the volume fraction of the lipophilic part of the surfactant respect to the system total volume, and VL is the volume of the lipophilic part in a surfactant molecule. The volume fraction L and the volume VL can be calculated from chain molecular weights and assuming that the density of alkyl chains was 0.789. Since the system is composed of a mixture of two surfactants (ionic and nonionic), it was assumed that both alkyl chains had the same density, and VL was calculated considering the weighted mean. 2.2.4. Surface tension measurements A K12 tensiometer (Krüss, Germany) was used to measure surface tension and critical micellar concentrations (CMC), by applying the Wilhelmy Plate Method [14]. 2.2.5. Laser diffraction A Mastersizer 2000 instrument (Malvern Co. Ltd., UK) was used to measure the particle size and the particle size distribution. Samples were diluted, in deionised water, directly in the dispersion unit (Hydro 2000) until the instrument detected the appropriate turbidity. The results are expressed as particle volume distribution,
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Fig. 1. (a) Average particle size and (b) polydispersity as a function of the ionic weight fraction (I ) for one and two emulsification steps. The total surfactant concentration ([acid] + [nonionic]) and the homogenization pressure were kept constant at 1.8 wt% and 270 atm, respectively.
defined as the volume percentage of particles as a function of their diameter. Average particle size (de Brouckere mean diameter, D [3,4]) and polydispersity (span) were calculated using Eqs. (3) and (4):
i=m D[4, 3] =
d4 X i=1 i i
i=m
(3)
di3 Xi
i=1
span =
d(0.9) − d(0.1) d(0.5)
(4)
where di represents the detected particle size, Xi is the percentage in volume of each particle size, m is the total number of particles, d(0.5) is the particle size at which 50% of the volume is in smaller particles and 50% is in larger particles. d(0.9) and d(0.1) indicate the particle size below which 90% and 10% of the particle volume lies, respectively.
2.2.6. Electrophoretic mobility Emulsions were measured in a Zetasizer NanoZS instrument (Malvern Co. Ltd., UK), equipped with a He–Ne red light laser of 633 nm wavelength. The instrument measures the velocity of the particles in an electric field (the electrophoretic mobility) using the laser Doppler electrophoresis method [15]. The zeta potential () could be calculated from the electrophoretic mobility (), applying the Smoluchowski equation (Eq. (5)). =
· εr · ε0
(5)
where εr represents the relative dielectric constant of water, ε0 is the vacuum permittivity and is the viscosity of the liquid. This equation is only valid for the case of low zeta potential values and when the particle size is much larger than the thickness of the electrical doubler layer, which is neglected [16–18]. Zeta potential is rather dependent on small concentrations of electrolytes, and moreover experimental error may depend on conductivity. Therefore, in order to have a constant background with enough conductivity, all emulsions were measured at 1 mM NaCl, in diluted solutions (approximately 0.06 wt% paraffin). These experimental conditions are usual in the literature [19].
3. Results and discussion 3.1. Emulsion formation and characterization The influence of the ionic/nonionic surfactant ratio; the total surfactant concentration; and the homogenization pressure on the properties of paraffin emulsions was studied systematically. The ionic/nonionic surfactant ratio was studied by varying the ionic surfactant weight fraction (I = [acid]/([acid] + [nonionic])) from 0 to 1. The total surfactant concentration ([acid] + [nonionic]) and the homogenization pressure were kept constant at 1.8 wt% and 270 atm, respectively. Emulsions were prepared by applying two emulsification steps, as described in Section 2. Emulsions with high ionic content (I > 0.56) were also prepared by applying solely one emulsification step. Fig. 1 shows the average particle size and the polydispersity, as a function of the ionic weight fraction, for both 1 and 2 emulsification steps. When emulsifying by a single step, the particle size (Fig. 1(a)) decreased slightly with the ionic content, whereas the polydispersity (Fig. 1(b)) increased. However, the behaviour was more complex when applying two emulsification steps. At high ionic content (I > 0.56), the emulsions obtained with two steps showed an increase in size and polydispersity, clearly attributed to particle aggregation, as observed by optical microscopy (Fig. 2). Micrographs of the emulsion with an ionic weight fraction of 0.67, prepared with one and two emulsification steps, are shown in Fig. 2. Therefore, a second emulsification step was detrimental at high ionic content. This behaviour was attributed to the fact that surfactant concentration was only 1.8 wt%, which may not be enough to cover larger surface area corresponding to smaller particles, and aggregation occurred. This hypothesis is described more in detail in Section 3.2. It should be pointed out that the particle size of samples that show aggregation may not be measured with precision by laser diffraction, since a high polydispersity may lead to some error in the determinations. For this reason, microscopy images were also taken, and the sharp increase in size, at I > 0.56 (Fig. 2), was confirmed. The electrophoretic mobility of the emulsions as a function of the ionic surfactant weight fraction for one and two emulsification steps is shown in Fig. 3. The results indicate that the electrophoretic mobility increases with the ionic weight fraction, reaching a plateau at I = 0.33, probably because of saturation of charges at the oil–water interface. Electrophoretic mobility does not vary with the number of emulsification steps, since it depends mostly on the charge density surrounding the particles, rather than
J. Vilasau et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 38–44
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Fig. 2. Micrographs of the emulsions with an ionic surfactant weight fraction of 0.67 prepared with (a) one and (b) two emulsification steps.
Fig. 3. Electrophoretic mobility as a function of the ionic surfactant weight fraction (I = [acid]/([acid] + [nonionic])) for the emulsions prepared applying one and two emulsification steps. The total surfactant concentration, defined as [acid] + [nonionic] and the homogenization pressure were kept constant at 1.8 wt% and 270 atm, respectively.
the particle size. The zeta potential was calculated using the Smoluchowski equation (Eq. (5)). It is defined, following the Stern model, as the difference in potential between the shear plane of ions on the particle surface and the electroneutral region of the solution [20]. It remains constant at −75 ± 3 mV, for emulsions with an ionic weight fraction higher than 0.22. This zeta potential is rather high, indicating that the particles are highly charged, as often described in other systems [19,21]. For example, in a similar system for an oil-in-water emulsion, stabilized with a mixed ionic/nonionic surfactant system with a ratio of 50/50, the zeta potential is about
−70 mV [19], which is a similar value. The emulsions can be considered as stable, thanks to electrostatic repulsions. Moreover, in samples prepared at I = 0.33, the particle size has been measured over a period of 30 months, and only a 10% increase in size has been observed. Therefore, one can conclude that the kinetic stability of these emulsions seems to be very high. The influence of the total surfactant concentration (defined as [acid] + [nonionic]) on the particle size was studied keeping constant the ionic surfactant weight fraction (I = 0.33) and the homogenization pressure (270 atm). Surfactant concentration was varied between 0.8 and 4.0 wt% and emulsions were prepared by applying two emulsification steps in the high pressure homogenizer, as described in Section 2. Fig. 4(a) shows that the average particle size decreases and polydispersity increases as a function of surfactant concentration. Clearly, higher surfactant concentration leads to larger interfacial areas and consequently large particles are obtained at low surfactant concentrations. However, both particle size and polydispersity increase abruptly at surfactant concentrations lower than 0.9 wt% (Fig. 4(a), indicated in the graph as a dotted line). At this concentration, aggregation has been observed by optical microscopy (see Fig. S1a in supporting information), and therefore the size indicated in Fig. 4(a) (dotted line) corresponds to the size of aggregates. This aggregation was attributed to an insufficient amount of surfactant molecules to fully cover the surface of the small particles produced by high pressure homogenization. In these experiments at 270 atm constant pressure, the smallest size (0.50 m) was achieved at 4 wt% surfactant. In order to decrease further the particle size, an increase in the homogenization pressure was required. To study the influence of the homogenization pressure, the ionic surfactant weight fraction and the surfactant concentration were kept constant at 0.33 and 4 wt%, respectively (Fig. 4b). Pressure
Fig. 4. Particle size and polydispersity as a function of: (a) the total surfactant concentration ([acid] + [nonionic]) at constant pressure (270 atm); and (b) the homogenization pressure at constant surfactant concentration (4 wt%). The ionic surfactant weight fraction was kept constant at 0.33 in both cases. The dotted lines remark abrupt changes.
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was varied between 140 and 540 atm, and two emulsification steps were applied, as described in Section 2. The particle size and the polydispersity decrease with pressure up to a critical value (410 atm) is reached. Above this pressure the emulsion particles aggregate, which was confirmed by optical microscopy (see Fig. S1b in supporting information). This particle aggregation produces an abrupt increase in size and polydispersity, also indicated as a dotted line (Fig. 4(b)). The particle size obtained by high pressure homogenization (>410 atm) would be too small in relation to the amount of surfactant available for adsorption (total surfactant concentration = 4 wt%). Consequently, probably there are insufficient surfactant molecules to stabilize an emulsion with such a small particle size (>0.31 m, which were obtained at 410 atm).
Table 2 Calculated values of surface area per surfactant molecule (as ). I represents the ionic surfactant weight fraction ([acid]/([acid] + [nonionic])); [ST ] the total surfactant concentration in weight percentage ([acid] + [nonionic]); L is the volume fraction of the hydrophobic part of the surfactant respect to the system total volume; d is the repeat distance measured by SAXS and dL is the half thickness of the lipophilic part in the lamellar phase. I
0.5
0.1
[ST ]
L
d (nm)
dL (nm)
as (nm2 )
50 45 40 35 50 45 40 35
0.37 0.33 0.30 0.26 0.37 0.33 0.29 0.26
9.0 9.5 12.1 13.4 8.2 9.1 10.1 11.5
1.7 1.6 1.8 1.7 1.5 1.5 1.5 1.5
0.35 0.36 0.32 0.33 0.38 0.38 0.38 0.38
3.2. Relationship between particle size and stability The theoretical minimum particle size of an emulsion for a given surfactant concentration was predicted considering some wellknown assumptions, namely that the surfactant concentration in the aqueous continuous phase corresponds to the Critical Aggregation Concentration (CAC), and that the surfactant concentration in the oil phase could be neglected, compared to the amount of surfactant adsorbed on the surface, which probably is much higher. Considering these assumptions the surfactant molecules would be distributed as follows: some molecules are dissolved as monomers in the aqueous continuous phase, at the CAC concentration; and almost all molecules are located at the liquid–liquid interfaces, since the molecules absorbed at the gas–liquid interface can also be neglected. Thus, the total amount of surfactant (S = SM + SI ) could be defined as the sum of surfactant dissolved, as monomers, in the aqueous phase (SM ) and surfactant adsorbed at the liquid–liquid interface (SI ). The amount of surfactant dissolved as monomers in the aqueous phase (SM ) could be calculated from the Critical Aggregation Concentration (CAC). SM = CAC · VT · (1 − 0 ) · MW
(6)
where VT represents the total volume of the emulsion, 0 is the volume fraction of the oil phase and MW is the surfactant molecular weight. The amount of surfactant adsorbed at the liquid–liquid interface (SI ) could be calculated from the surface/volume ratio of the particles, assuming particles as monodisperse perfect spheres and a complete coverage of particles by surfactant molecules. SI =
3 · VT · 0 MW · r as · NA
(7)
where r represents the radius of the particles, MW is the surfactant molecular weight, NA is the Avogadro’s number and as is the surface area per surfactant molecule. Merging these equations (Eq. (6) with Eq. (7)), the final expression to obtain the minimum particle size is: r=
3 · 0
as · NA ·
[S] M
(8)
+ CAC(0 − 1)
Determining the surface area per surfactant molecule (as ) and the Critical Aggregation Concentration (CAC), the minimum particle size could be theoretically calculated for a given surfactant concentration ([S]). In this work this equation is indicated as Classical Model. Eq. (8) is similar to a model established by Nakajima (Eq. (9)), proposed for predicting the size (r) in nano-emulsions [22], which have a higher curvature than conventional emulsions. r=
3·M as · NA · o
·R+
3·˛·M as · NA · c
+L
(9)
where o and c represent respectively the densities of the oil and the surfactant alkyl chain, ˛ is the weight fraction of the surfactant alkyl chain, R is the oil/surfactant weight ratio of and L is the thickness of the monolayer of hydrated hydrophilic moiety of surfactant. The Classical Model (Eq. (8)) does not consider the monolayer thickness, and Nakajima’s Model (Eq. (9)) does not take into account the CAC. Probably, Eq. (8) is more suitable at low surfactant concentration, and large particle sizes, where concentrations in the aqueous continuous phase could be comparable to CAC. In this case, the monolayer thickness could be neglected, compared to particle radius. In the present work, both models were used and its theoretical results were compared to experimental data. The CAC and the surface area per surfactant molecule (as ) are required for the calculations and were determined as described in Section 2. In a previous work [2], the phase behaviour of this surfactant system was studied. CAC is considered, since CMC could not be observed for both surfactants due to their low water solubility. Moreover, the Krafft point of the same surfactant mixture is approximately 48 ◦ C [1], indicating that the concentration of surfactant molecules as monomers is probably quite small at 25 ◦ C. However, as it can be seen in Fig. S2 of the supporting information, a sudden change in surface tension was observed for a specific concentration. This concentration can be considered as the Critical Aggregation Concentration (CAC), since probably aggregates, which are formed at the solubility limit, are not micelles. The CAC of the ionic/nonionic surfactant mixture (I = 0.33) was determined at 25 ◦ C and is 1.0 mM, which is considered the surfactant concentration at the aqueous continuous phase. The theoretical models were calculated for a given ionic surfactant weight fraction (I = 0.33). Table 2 shows the data of the SAXS determination for the surface area per surfactant molecule (as ) using Eqs. (1) and (2). The resulting values show some variation (±0.03 nm2 ) which could be related to experimental uncertainty in SAXS measurements. The surface area per surfactant molecule is considered to be 0.36 nm2 as the average value of all studied compositions. The surface area per surfactant molecule in a micelle could also be calculated from surface tension measurements, using the Gibbs adsorption equation (Eq. (10)): 1 1 = RT a5 =
1016 NA 1
∂ ∂ ln S
(10) T
(11)
where represents the surface excess concentration, ∂ are the decrease in surface tension, S is the surfactant concentration, T is the temperature and R is the gas constant. The decrease in surface tension (∂ ) is obtained from the surface tension vs. concentration plot, shown as supporting information
J. Vilasau et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 38–44
Fig. 5. Theoretical calculations of the minimum particle size as a function of the surfactant weight percentage and experimental data for the particle size obtained at different surfactant concentrations and homogenization pressures.
(Fig. S2). Applying Eq. (11) the surface area per surfactant molecule is 0.81 nm2 , for the given surfactant mixture (I = 0.33). This value is much higher than that obtained by SAXS determinations (0.36 nm2 ). Therefore, there is a discrepancy in the area per molecule depending on the method used. It could be considered that the value obtained by SAXS (0.36 nm2 ) is more reliable, because of two reasons. Firstly, our system is composed by a mixture of nonionic and ionic surfactants, which is not considered in Eq. (10). Secondly, surface tension data describes the liquid–gas interface, which is a different situation that liquid–liquid interfaces (oil–water) on emulsion droplets. However, calculations from SAXS (Eqs. (1) and (2)) do not consider any assumption, except for planar geometry. One could consider an average area per molecule in a mixed surfactant system. Therefore, the area per molecule obtained from SAXS measurements is considered more representative of the oil–water interface on the emulsion droplets. The theoretical models (Eqs. (8) and (9)) were plotted as a function of the total surfactant concentration, comparing to experimental data. Both models provided similar calculated values, as it can be seen in Fig. 5. The possible error in the Critical Aggregation Concentration (CAC) probably has little influence on the theoretical curves, since CAC ([S]/M). Probably, the highest error could arise from uncertainty in as values, which influences more the calculated values. However, the theoretical minimum particle size is on agreement with the smallest sizes obtained by experiments, obtained at different homogenization pressures. Comparing the theoretical curves with the experimental data, it can be concluded that two regions of emulsion formation can be distinguished. All experimental samples are either in the region of stable emulsions or just on the theoretical curves. Above the theoretical curves, stable emulsions can be obtained, since there is enough surfactant to stabilize the particles. However, below those curves, formation of stable emulsions is not possible, because the amount of surfactant, available for adsorption, is not enough to fully cover the particles. The fact that experimental values are close to the theoretical curves demonstrates that the emulsification was rather efficient and that particle sizes were close to the minimum size. Therefore, the concentration of surfactant in the aqueous phase was quite low, since the models assume this low concentration. On the other hand, when preparing emulsions with high-shear instruments such as high pressure homogenizers, small sizes are usually obtained and therefore a large interfacial area is generated. Thus, it seems that the particle size depends mostly on surfactant availability for covering particle surface. Consequently, surfactant adsorption probably forms monolayers, with almost no presence of multilayers, which
43
would require much higher surfactant concentrations. Most likely, there are insufficient surfactant molecules to form a multilayer structure surrounding the emulsion particles. In further experiments, the emulsification pressure was increased in order to reduce the particle size. However, as indicated earlier, there was a critical pressure above which the particles aggregated and non-stable emulsions were obtained. This aggregation, observed only below the theoretical curves, was due to either insufficient surfactant molecules to fully cover the particles (low surfactant concentration) or too small particles (high interfacial area resulting from excessive homogenization pressure). By adjusting the total surfactant concentration and/or the homogenization pressure the minimum particle size could be reached. Consequently, the above graph (Fig. 5) could be a useful tool to formulate stable emulsions, by adjusting only surfactant concentration and homogenization pressure. It should be pointed out that emulsions with small size (310 nm) were obtained, which can be considered as nano-emulsions. It seems that high pressure homogenization is an efficient method, because allows to obtain particle sizes close to the theoretical minimum size. These results are relevant in industry, allowing the prediction of particle size and optimization of surfactant concentrations. 4. Conclusions The influence of the ionic/nonionic surfactant ratio, the surfactant concentration and the homogenization pressure, were studied. Particle size decreases with ionic/nonionic surfactant ratio, up to a certain ratio above which emulsions aggregate. Regarding the surfactant concentration, particle size decreases with concentration, as expected. Moreover, at very low surfactant concentration there is insufficient amount of surfactant to fully cover particle surfaces and emulsions abruptly aggregate. Concerning the homogenization pressure, the size decreases with pressure, as also expected. However, there is a critical pressure above which emulsions also aggregate. These results have been explained in terms of surfactant availability at the particle surface. Experimental data and theoretical models for the prediction of the minimum particle size were compared, and good agreement has been observed, indicating that emulsification is rather effective and the smallest possible sizes can be reached. These theoretical models consider that surfactant concentration in the aqueous continuous phase, at the critical pressure, is equal to the CAC. Therefore, surfactant concentration in the continuous phase is probably small and stabilization by multilayer adsorption is not likely. A higher surfactant concentration, or larger particle size, would be required for such arrangement. These theoretical models could be used as a tool to predict stability of emulsions prepared by high-energy emulsification methods. Acknowledgements The authors greatly acknowledge Prof. Marisa García (Faculty of Pharmacy, University of Barcelona) for her technical support in electrophoretic mobility measurements and the Spanish Ministry of Science and Innovation (PET2006-0582 and CTQ2008-06892C03-01 projects) for the financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2011.09.026. References [1] R. Mújika-Garai, C. Aguilar-García, F. Juárez-Arroyo, I. Covián-Sánchez, J. Nolla, J. Esquena, C. Solans, M.A. Rodríguez-Valverde, R. Tejera-García,
44
[2]
[3]
[4] [5] [6]
[7]
[8] [9]
[10]
[11]
J. Vilasau et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 38–44 M.A. Cabrerizo-Vílchez, R. Hidalgo-Álvarez, Stabilization of paraffin emulsions used in the manufacture of chipboard panels by liquid crystalline phases, Journal of Dispersion Science and Technology 28 (2007) 829–836. J. Vilasau, C. Solans, M.J. Gómez, J. Dabrio, R. Mújika-Garai, J. Esquena, Phase behaviour of a mixed ionic/nonionic surfactant system used to prepare stable oil-in-water paraffin emulsions, Colloids and Surfaces A 384 (2011) 473–481. A.A. Ali, B.A. Mulley, Formation of liquid crystal and other non-fluid phases in emulsions containing non-ionic surfactants, Journal of Pharmacy and Pharmacology 30 (1978) 205–213. G.M. Eccleston, Multiple-phase oil-in-water emulsions, Journal of the Society of Cosmetic Chemists 41 (1990) 1–22. S. Friberg, Liquid crystalline phases in emulsions, Journal of Colloid and Interface Science 37 (1971) 291–295. S. Friberg, L. Mandell, M. Larsson, Mesomorphous phases, a factor of importance for the properties of emulsions, Journal of Colloid and Interface Science 29 (1969) 155–156. S. Friberg, P. Solyom, The influence of liquid crystalline phases on the rheological behaviour of emulsions, Kolloid-Zeitschrift & Zeitschrift für Polymere 236 (1970) 173–174. S.E. Friberg, C. Solans, Surfactant association structures and the stability of emulsions and foams, Langmuir 2 (1986) 121–126. O.D.H. dos Santos, J.V. Miotto, J.M. de Morais, P.A. da Rocha-Filho, W.P. de Oliveira, Attainment of emulsions with liquid crystal from marigold oil using the required HLB method, Journal of Dispersion Science and Technology 26 (2005) 243–249. G.G. Morais, O.D.H. Santos, W.P. Oliveira, P.A. Rocha Filho, Attainment of O/W emulsions containing liquid crystal from annatto oil (Bixa orellana), coffee oil, and tea tree oil (Melaleuca alternifolia) as oily phase using HLB system and ternary phase diagram, Journal of Dispersion Science and Technology 29 (2008) 297–306. R.C Pasquali, C. Bregni, Stabilized liquid-crystalline emulsions with triethanolamine stearate and stearic acid: Preparation method influence in the properties and formation of secondary droplets, Ars Pharmaceutica 47 (2006) 219–237.
[12] H. Kunieda, H. Kabir, K. Aramaki, K. Shigeta, Phase behavior of mixed polyoxyethylene-type nonionic surfactants in water, Journal of Molecular Liquids 90 (2001) 157–166. [13] H. Sagitani, S.E. Friberg, Structural changes induced by addition of a hydrocarbon to water/amphiphile mixtures, Colloid & Polymer Science 261 (1983) 862–867. [14] J. Drelich, C. Fang, C.L. White, Encyclopedia of Surface and Colloid Science, Marcel Dekker, New York, 2002. [15] H.A. Lieberman, M.M. Rieger, G.S. Banker, Pharmaceutical Dosage Forms: Disperse Systems, Marcel Dekker, New York, 1988. [16] S.A. Nespolo, M.A. Bevan, D.Y.C. Chan, F. Grieser, G.W. Stevens, Hydrodynamic and electrokinetic properties of decane droplets in aqueous sodium dodecyl sulfate solutions, Langmuir 17 (2001) 7210–7218. [17] R.W. O‘Brien, L.R. White, Electrophoretic mobility of a spherical colloidal particle, Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics 74 (1978) 1607–1626. [18] H. Ohshima, T.W. Healy, L.R. White, Approximate analytic expressions for the electrophoretic mobility of spherical colloidal particles and the conductivity of their dilute suspensions, Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics 79 (1983) 1613–1628. [19] T.P. Goloub, R.J. Pugh, The role of the surfactant head group in the emulsification process: binary (nonionic–ionic) surfactant mixtures, Journal of Colloid and Interface Science 291 (2005) 256–262. [20] I. Roland, G. Piel, L. Delattre, B. Evrard, Systematic characterization of oil-inwater emulsions for formulation design, International Journal of Pharmaceutics 263 (2003) 85–94. [21] K. Backfolk, G. Olofsson, J.B. Rosenholm, D. Eklund, Study by isothermal calorimetry and electrophoresis of the interaction between latices and cellulose derivatives, Colloids and Surfaces A: Physicochemical and Engineering Aspects 276 (2006) 78–86. [22] H. Nakajima, Microemulsions in Cosmetics in Industrial Applications of Macroemulsiones, Marcel Dekker, New York, 1997.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Influence of a mixed ionic/nonionic surfactant system and the emulsification process on the properties of paraffin emulsions J. Vilasau a , C. Solans a , M.J. Gómez b , J. Dabrio b , R. Mújika-Garai b , J. Esquena a,∗ a b
Instituto de Química Avanzada de Catalu˜ na (IQAC-CSIC), CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), c/Jordi Girona 18-26, 08034 Barcelona, Spain RYLESA (Repsol-YPF Lubricantes y Especialidades SA), Madrid, Spain
a r t i c l e
i n f o
Article history: Received 28 July 2011 Received in revised form 15 September 2011 Accepted 17 September 2011 Available online 24 September 2011 Keywords: Paraffin emulsions Emulsification process Mixed surfactant system Particleboard panels Stability Particle size
a b s t r a c t Paraffin emulsions are important in technological applications such as coating in the food packaging industry or to provide waterproof properties to particleboard panels. Small particle size (about 1.0 m) and low polydispersity are required to form stable paraffin emulsions for these applications. In this context, the main objective of the present work is to study the influence of the surfactant system and the emulsification process on the properties of paraffin emulsions. A high pressure homogenizer was used to prepare the emulsions and its characterization was made by means of optical microscopy, laser diffraction and electrophoretic mobility measurements. Emulsions were prepared as a function of the ionic/nonionic surfactant ratio, the total surfactant concentration and the homogenization pressure. A simple theoretical model to predict the minimum particle size was used, assuming that surfactant is either at the oil–water interface or as monomer in the external phase. Experimental and theoretical data are on good agreement and the formation of stable emulsions is explained according to such model. This result could be of prime importance in order to formulate new paraffin emulsions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Paraffin emulsions are used in a wide range of technological applications such as coating in the food packaging industry or to provide waterproof properties to particleboard panels in the furniture industry. In order to obtain stable paraffin emulsions, for different applications, small particle size (about 1.0 m) and low polydispersity are required. Recent works [1,2] dealing with paraffin emulsions, stabilized with a mixed ionic/nonionic surfactant system, led to interesting results. At high ionic/nonionic surfactant ratios (<44/56) a significant change in emulsion properties was observed, increasing storage modulus (G ) and emulsion stability; while decreasing particle size [1]. Moreover, the phase behaviour of the surfactant system in water revealed that lamellar liquid crystalline aggregates were present at very low surfactant concentration (down to 0.2 wt% surfactant). Considering that the total surfactant concentration was about 2 wt%, the high stability could be related to the formation of a surfactant multilayer arrangement, surrounding the particles [2]. Emulsions are stabilized generally by surfactants that adsorb at interfaces, such as oil–water interface, and form normally a monolayer surrounding the droplets, which retards the destabilization processes. However, other kind of surfactant self-aggregation,
such as liquid crystalline phases, can be present at the interface. Friberg et al. described the stabilization by multilayers around the droplets, increasing the viscosity of the system and enhancing emulsion stability [3–8]. This stabilization mechanism was found in formulations with a 1/1 oil/water ratio and surfactant concentrations between 3 and 4 wt% [6]. Recent works [9–11] revealed that the surfactant concentration required to form multilayers surrounding the droplets is normally higher (between 10 and 15 wt%). It should be pointed out that the formation of such multilayers depends on many factors such as the nature of the surfactant, the interfacial area and the surfactant solubility in the continuous phase. In this context, little is known about the role of the emulsification process in the emulsion properties of such systems. Moreover, the influence of the surfactants on paraffin emulsion and the possible self-assembled structures in the continuous phase have not been subject to systematic studies. Consequently, in this work the influence of the ionic/nonionic surfactant ratio, the total surfactant concentration and the homogenization pressure on paraffin emulsion properties have been studied. 2. Experimental 2.1. Materials
∗ Corresponding author. Tel.: +34 934006178; fax: +34 932045904. E-mail address: [email protected] (J. Esquena). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.09.026
The ionic surfactant (anionic) was formed by neutralization of an acid (alkyl chain length of 21 carbon atoms) with an alkanolamine,
J. Vilasau et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 38–44
39
Table 1 Compositions of the emulsions (concentrations of the nonionic surfactant and the ionic surfactant, which consists of an acid neutralized with an alkanolamine), at different ionic surfactant weight fraction, defined as I = [acid]/([acid] + [nonionic]). I
0.00
0.11
0.22
0.33
0.44
0.56
0.67
0.78
0.89
1.00
Nonionic (wt%) Acid (wt%) Alkanolamine (wt%)
1.80 0.00 0.00
1.60 0.20 0.27
1.40 0.40 0.53
1.20 0.60 0.80
1.00 0.80 1.07
0.80 1.00 1.33
0.60 1.20 1.60
0.40 1.40 1.87
0.20 1.60 2.13
0.00 1.80 2.40
which was added at 0.34 [acid]/[alkanolamine] molar ratio. This stoichiometric excess was used to ensure complete neutralization. The ionic surfactant, formed after neutralization, possesses a melting point of 61 ◦ C. The nonionic surfactant possesses an alkyl chain length of 19 carbon atoms and its HLB is 7.6. The melting point of this surfactant is 36 ◦ C. The oil phase is paraffin and consists of a mixture of saturated hydrocarbons with an average chain length of 25 carbon atoms and a melting point of 52 ◦ C. Paraffin and surfactants were used without any further purification. No additional information on these materials can be presented, due to confidentiality. 2.2. Methods 2.2.1. Emulsion preparation The aqueous phase was added to the oil phase containing paraffin, nonionic and ionic surfactant at approximately 70 ◦ C. The emulsions were firstly pre-mixed by manual agitation, introduced into a high pressure homogenizer (Microfluidizer ML100, Microfludics® ) and forced to pass one or two times (one or two emulsification steps) through a 75 m capillary, at 70 ◦ C. Otherwise stated, the homogenization pressure was approximately 270 atm. The resulting emulsions were rapidly cooled down collecting them in a beaker immersed in ice water. It should be pointed out that surfactants and paraffin have their melting points between 36 ◦ C and 61 ◦ C. Therefore, emulsification is possible at 70 ◦ C, and these components form crystalline phases at room temperature. According to the phase diagram of the same surfactant mixture [1], the Krafft point is approximately 48 ◦ C, and therefore the concentration of surfactant molecules as monomer will be rather small after cooling down. The pH was adjusted between 8 and 10. The final conductivity was between 100 and 400 S/cm, in all emulsions. Different emulsions were prepared as a function of three parameters: ionic surfactant weight fraction, defined as 1. The I = [acid]/([acid] + [nonionic]), which was increased from 0 to 1, keeping constant the total surfactant concentration ([acid] + [nonionic]) and the homogenization pressure at 1.8 wt% and 270 atm, respectively (Table 1). 2. The total surfactant concentration ([acid] + [nonionic]) was varied between 0.8 and 4.0 wt%, keeping constant the ionic weight fraction (I ), and the homogenization pressure at 0.33 and 270 atm, respectively. 3. The homogenization pressure, which was varied between 140 and 540 atm for the emulsions at 0.8, 3.0 and 4.0 wt% surfactant concentration, keeping constant the ionic weight fraction at 0.33. In all cases, the acid was fully neutralized with alkanolamine, at constant 0.34 acid/alkanolamine molar ratio and the paraffin content was kept constant at 57.4 wt%.
Reproducibility in the preparation of emulsions, by using high pressure homogenization, was quite good. Fifteen replicas of the sample at I = 0.33 were prepared. The average particle size was 0.70 m, and the standard deviation was 0.08 m. 2.2.2. Optical microscopy Samples were observed with a Reicheirt Polyvar 2 microscope (Leica, Germany), equipped with video camera, polarizers, and interference contrast prism. Images were processed using the IM500 software supplied by Leica. This technique was used to determine the particle size and aggregation. 2.2.3. Determination of the surface area per surfactant molecule (as ) by SAXS SAXS measurements were performed in a S3-Micro instrument (Hecus X-ray Systems GMBH Graz, Austria) operating with point focalization. The scattering was detected with a linear position sensitive detector OED-50M. The temperature controller was a Peltier device. The q range is approximately from 0.02 to 6 nm−1 . Samples were introduced in glass capillaries of 1 mm diameter and 10 m wall thickness and then sealed to prevent evaporation. Blanks were carried out by measuring empty capillaries. The surface area per surfactant molecule in lamellar liquid crystalline phases was calculated by applying the following equations [12,13]: as =
VL dL
(1)
dL =
d · L 2
(2)
where d represents the repeat distance, in lamellar phases, measured by SAXS (d = 2/q, where q is the scattering vector corresponding to the first order peak), dL is the half thickness of the lipophilic part, L is the volume fraction of the lipophilic part of the surfactant respect to the system total volume, and VL is the volume of the lipophilic part in a surfactant molecule. The volume fraction L and the volume VL can be calculated from chain molecular weights and assuming that the density of alkyl chains was 0.789. Since the system is composed of a mixture of two surfactants (ionic and nonionic), it was assumed that both alkyl chains had the same density, and VL was calculated considering the weighted mean. 2.2.4. Surface tension measurements A K12 tensiometer (Krüss, Germany) was used to measure surface tension and critical micellar concentrations (CMC), by applying the Wilhelmy Plate Method [14]. 2.2.5. Laser diffraction A Mastersizer 2000 instrument (Malvern Co. Ltd., UK) was used to measure the particle size and the particle size distribution. Samples were diluted, in deionised water, directly in the dispersion unit (Hydro 2000) until the instrument detected the appropriate turbidity. The results are expressed as particle volume distribution,
40
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Fig. 1. (a) Average particle size and (b) polydispersity as a function of the ionic weight fraction (I ) for one and two emulsification steps. The total surfactant concentration ([acid] + [nonionic]) and the homogenization pressure were kept constant at 1.8 wt% and 270 atm, respectively.
defined as the volume percentage of particles as a function of their diameter. Average particle size (de Brouckere mean diameter, D [3,4]) and polydispersity (span) were calculated using Eqs. (3) and (4):
i=m D[4, 3] =
d4 X i=1 i i
i=m
(3)
di3 Xi
i=1
span =
d(0.9) − d(0.1) d(0.5)
(4)
where di represents the detected particle size, Xi is the percentage in volume of each particle size, m is the total number of particles, d(0.5) is the particle size at which 50% of the volume is in smaller particles and 50% is in larger particles. d(0.9) and d(0.1) indicate the particle size below which 90% and 10% of the particle volume lies, respectively.
2.2.6. Electrophoretic mobility Emulsions were measured in a Zetasizer NanoZS instrument (Malvern Co. Ltd., UK), equipped with a He–Ne red light laser of 633 nm wavelength. The instrument measures the velocity of the particles in an electric field (the electrophoretic mobility) using the laser Doppler electrophoresis method [15]. The zeta potential () could be calculated from the electrophoretic mobility (), applying the Smoluchowski equation (Eq. (5)). =
· εr · ε0
(5)
where εr represents the relative dielectric constant of water, ε0 is the vacuum permittivity and is the viscosity of the liquid. This equation is only valid for the case of low zeta potential values and when the particle size is much larger than the thickness of the electrical doubler layer, which is neglected [16–18]. Zeta potential is rather dependent on small concentrations of electrolytes, and moreover experimental error may depend on conductivity. Therefore, in order to have a constant background with enough conductivity, all emulsions were measured at 1 mM NaCl, in diluted solutions (approximately 0.06 wt% paraffin). These experimental conditions are usual in the literature [19].
3. Results and discussion 3.1. Emulsion formation and characterization The influence of the ionic/nonionic surfactant ratio; the total surfactant concentration; and the homogenization pressure on the properties of paraffin emulsions was studied systematically. The ionic/nonionic surfactant ratio was studied by varying the ionic surfactant weight fraction (I = [acid]/([acid] + [nonionic])) from 0 to 1. The total surfactant concentration ([acid] + [nonionic]) and the homogenization pressure were kept constant at 1.8 wt% and 270 atm, respectively. Emulsions were prepared by applying two emulsification steps, as described in Section 2. Emulsions with high ionic content (I > 0.56) were also prepared by applying solely one emulsification step. Fig. 1 shows the average particle size and the polydispersity, as a function of the ionic weight fraction, for both 1 and 2 emulsification steps. When emulsifying by a single step, the particle size (Fig. 1(a)) decreased slightly with the ionic content, whereas the polydispersity (Fig. 1(b)) increased. However, the behaviour was more complex when applying two emulsification steps. At high ionic content (I > 0.56), the emulsions obtained with two steps showed an increase in size and polydispersity, clearly attributed to particle aggregation, as observed by optical microscopy (Fig. 2). Micrographs of the emulsion with an ionic weight fraction of 0.67, prepared with one and two emulsification steps, are shown in Fig. 2. Therefore, a second emulsification step was detrimental at high ionic content. This behaviour was attributed to the fact that surfactant concentration was only 1.8 wt%, which may not be enough to cover larger surface area corresponding to smaller particles, and aggregation occurred. This hypothesis is described more in detail in Section 3.2. It should be pointed out that the particle size of samples that show aggregation may not be measured with precision by laser diffraction, since a high polydispersity may lead to some error in the determinations. For this reason, microscopy images were also taken, and the sharp increase in size, at I > 0.56 (Fig. 2), was confirmed. The electrophoretic mobility of the emulsions as a function of the ionic surfactant weight fraction for one and two emulsification steps is shown in Fig. 3. The results indicate that the electrophoretic mobility increases with the ionic weight fraction, reaching a plateau at I = 0.33, probably because of saturation of charges at the oil–water interface. Electrophoretic mobility does not vary with the number of emulsification steps, since it depends mostly on the charge density surrounding the particles, rather than
J. Vilasau et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 38–44
41
Fig. 2. Micrographs of the emulsions with an ionic surfactant weight fraction of 0.67 prepared with (a) one and (b) two emulsification steps.
Fig. 3. Electrophoretic mobility as a function of the ionic surfactant weight fraction (I = [acid]/([acid] + [nonionic])) for the emulsions prepared applying one and two emulsification steps. The total surfactant concentration, defined as [acid] + [nonionic] and the homogenization pressure were kept constant at 1.8 wt% and 270 atm, respectively.
the particle size. The zeta potential was calculated using the Smoluchowski equation (Eq. (5)). It is defined, following the Stern model, as the difference in potential between the shear plane of ions on the particle surface and the electroneutral region of the solution [20]. It remains constant at −75 ± 3 mV, for emulsions with an ionic weight fraction higher than 0.22. This zeta potential is rather high, indicating that the particles are highly charged, as often described in other systems [19,21]. For example, in a similar system for an oil-in-water emulsion, stabilized with a mixed ionic/nonionic surfactant system with a ratio of 50/50, the zeta potential is about
−70 mV [19], which is a similar value. The emulsions can be considered as stable, thanks to electrostatic repulsions. Moreover, in samples prepared at I = 0.33, the particle size has been measured over a period of 30 months, and only a 10% increase in size has been observed. Therefore, one can conclude that the kinetic stability of these emulsions seems to be very high. The influence of the total surfactant concentration (defined as [acid] + [nonionic]) on the particle size was studied keeping constant the ionic surfactant weight fraction (I = 0.33) and the homogenization pressure (270 atm). Surfactant concentration was varied between 0.8 and 4.0 wt% and emulsions were prepared by applying two emulsification steps in the high pressure homogenizer, as described in Section 2. Fig. 4(a) shows that the average particle size decreases and polydispersity increases as a function of surfactant concentration. Clearly, higher surfactant concentration leads to larger interfacial areas and consequently large particles are obtained at low surfactant concentrations. However, both particle size and polydispersity increase abruptly at surfactant concentrations lower than 0.9 wt% (Fig. 4(a), indicated in the graph as a dotted line). At this concentration, aggregation has been observed by optical microscopy (see Fig. S1a in supporting information), and therefore the size indicated in Fig. 4(a) (dotted line) corresponds to the size of aggregates. This aggregation was attributed to an insufficient amount of surfactant molecules to fully cover the surface of the small particles produced by high pressure homogenization. In these experiments at 270 atm constant pressure, the smallest size (0.50 m) was achieved at 4 wt% surfactant. In order to decrease further the particle size, an increase in the homogenization pressure was required. To study the influence of the homogenization pressure, the ionic surfactant weight fraction and the surfactant concentration were kept constant at 0.33 and 4 wt%, respectively (Fig. 4b). Pressure
Fig. 4. Particle size and polydispersity as a function of: (a) the total surfactant concentration ([acid] + [nonionic]) at constant pressure (270 atm); and (b) the homogenization pressure at constant surfactant concentration (4 wt%). The ionic surfactant weight fraction was kept constant at 0.33 in both cases. The dotted lines remark abrupt changes.
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J. Vilasau et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 38–44
was varied between 140 and 540 atm, and two emulsification steps were applied, as described in Section 2. The particle size and the polydispersity decrease with pressure up to a critical value (410 atm) is reached. Above this pressure the emulsion particles aggregate, which was confirmed by optical microscopy (see Fig. S1b in supporting information). This particle aggregation produces an abrupt increase in size and polydispersity, also indicated as a dotted line (Fig. 4(b)). The particle size obtained by high pressure homogenization (>410 atm) would be too small in relation to the amount of surfactant available for adsorption (total surfactant concentration = 4 wt%). Consequently, probably there are insufficient surfactant molecules to stabilize an emulsion with such a small particle size (>0.31 m, which were obtained at 410 atm).
Table 2 Calculated values of surface area per surfactant molecule (as ). I represents the ionic surfactant weight fraction ([acid]/([acid] + [nonionic])); [ST ] the total surfactant concentration in weight percentage ([acid] + [nonionic]); L is the volume fraction of the hydrophobic part of the surfactant respect to the system total volume; d is the repeat distance measured by SAXS and dL is the half thickness of the lipophilic part in the lamellar phase. I
0.5
0.1
[ST ]
L
d (nm)
dL (nm)
as (nm2 )
50 45 40 35 50 45 40 35
0.37 0.33 0.30 0.26 0.37 0.33 0.29 0.26
9.0 9.5 12.1 13.4 8.2 9.1 10.1 11.5
1.7 1.6 1.8 1.7 1.5 1.5 1.5 1.5
0.35 0.36 0.32 0.33 0.38 0.38 0.38 0.38
3.2. Relationship between particle size and stability The theoretical minimum particle size of an emulsion for a given surfactant concentration was predicted considering some wellknown assumptions, namely that the surfactant concentration in the aqueous continuous phase corresponds to the Critical Aggregation Concentration (CAC), and that the surfactant concentration in the oil phase could be neglected, compared to the amount of surfactant adsorbed on the surface, which probably is much higher. Considering these assumptions the surfactant molecules would be distributed as follows: some molecules are dissolved as monomers in the aqueous continuous phase, at the CAC concentration; and almost all molecules are located at the liquid–liquid interfaces, since the molecules absorbed at the gas–liquid interface can also be neglected. Thus, the total amount of surfactant (S = SM + SI ) could be defined as the sum of surfactant dissolved, as monomers, in the aqueous phase (SM ) and surfactant adsorbed at the liquid–liquid interface (SI ). The amount of surfactant dissolved as monomers in the aqueous phase (SM ) could be calculated from the Critical Aggregation Concentration (CAC). SM = CAC · VT · (1 − 0 ) · MW
(6)
where VT represents the total volume of the emulsion, 0 is the volume fraction of the oil phase and MW is the surfactant molecular weight. The amount of surfactant adsorbed at the liquid–liquid interface (SI ) could be calculated from the surface/volume ratio of the particles, assuming particles as monodisperse perfect spheres and a complete coverage of particles by surfactant molecules. SI =
3 · VT · 0 MW · r as · NA
(7)
where r represents the radius of the particles, MW is the surfactant molecular weight, NA is the Avogadro’s number and as is the surface area per surfactant molecule. Merging these equations (Eq. (6) with Eq. (7)), the final expression to obtain the minimum particle size is: r=
3 · 0
as · NA ·
[S] M
(8)
+ CAC(0 − 1)
Determining the surface area per surfactant molecule (as ) and the Critical Aggregation Concentration (CAC), the minimum particle size could be theoretically calculated for a given surfactant concentration ([S]). In this work this equation is indicated as Classical Model. Eq. (8) is similar to a model established by Nakajima (Eq. (9)), proposed for predicting the size (r) in nano-emulsions [22], which have a higher curvature than conventional emulsions. r=
3·M as · NA · o
·R+
3·˛·M as · NA · c
+L
(9)
where o and c represent respectively the densities of the oil and the surfactant alkyl chain, ˛ is the weight fraction of the surfactant alkyl chain, R is the oil/surfactant weight ratio of and L is the thickness of the monolayer of hydrated hydrophilic moiety of surfactant. The Classical Model (Eq. (8)) does not consider the monolayer thickness, and Nakajima’s Model (Eq. (9)) does not take into account the CAC. Probably, Eq. (8) is more suitable at low surfactant concentration, and large particle sizes, where concentrations in the aqueous continuous phase could be comparable to CAC. In this case, the monolayer thickness could be neglected, compared to particle radius. In the present work, both models were used and its theoretical results were compared to experimental data. The CAC and the surface area per surfactant molecule (as ) are required for the calculations and were determined as described in Section 2. In a previous work [2], the phase behaviour of this surfactant system was studied. CAC is considered, since CMC could not be observed for both surfactants due to their low water solubility. Moreover, the Krafft point of the same surfactant mixture is approximately 48 ◦ C [1], indicating that the concentration of surfactant molecules as monomers is probably quite small at 25 ◦ C. However, as it can be seen in Fig. S2 of the supporting information, a sudden change in surface tension was observed for a specific concentration. This concentration can be considered as the Critical Aggregation Concentration (CAC), since probably aggregates, which are formed at the solubility limit, are not micelles. The CAC of the ionic/nonionic surfactant mixture (I = 0.33) was determined at 25 ◦ C and is 1.0 mM, which is considered the surfactant concentration at the aqueous continuous phase. The theoretical models were calculated for a given ionic surfactant weight fraction (I = 0.33). Table 2 shows the data of the SAXS determination for the surface area per surfactant molecule (as ) using Eqs. (1) and (2). The resulting values show some variation (±0.03 nm2 ) which could be related to experimental uncertainty in SAXS measurements. The surface area per surfactant molecule is considered to be 0.36 nm2 as the average value of all studied compositions. The surface area per surfactant molecule in a micelle could also be calculated from surface tension measurements, using the Gibbs adsorption equation (Eq. (10)): 1 1 = RT a5 =
1016 NA 1
∂ ∂ ln S
(10) T
(11)
where represents the surface excess concentration, ∂ are the decrease in surface tension, S is the surfactant concentration, T is the temperature and R is the gas constant. The decrease in surface tension (∂ ) is obtained from the surface tension vs. concentration plot, shown as supporting information
J. Vilasau et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 38–44
Fig. 5. Theoretical calculations of the minimum particle size as a function of the surfactant weight percentage and experimental data for the particle size obtained at different surfactant concentrations and homogenization pressures.
(Fig. S2). Applying Eq. (11) the surface area per surfactant molecule is 0.81 nm2 , for the given surfactant mixture (I = 0.33). This value is much higher than that obtained by SAXS determinations (0.36 nm2 ). Therefore, there is a discrepancy in the area per molecule depending on the method used. It could be considered that the value obtained by SAXS (0.36 nm2 ) is more reliable, because of two reasons. Firstly, our system is composed by a mixture of nonionic and ionic surfactants, which is not considered in Eq. (10). Secondly, surface tension data describes the liquid–gas interface, which is a different situation that liquid–liquid interfaces (oil–water) on emulsion droplets. However, calculations from SAXS (Eqs. (1) and (2)) do not consider any assumption, except for planar geometry. One could consider an average area per molecule in a mixed surfactant system. Therefore, the area per molecule obtained from SAXS measurements is considered more representative of the oil–water interface on the emulsion droplets. The theoretical models (Eqs. (8) and (9)) were plotted as a function of the total surfactant concentration, comparing to experimental data. Both models provided similar calculated values, as it can be seen in Fig. 5. The possible error in the Critical Aggregation Concentration (CAC) probably has little influence on the theoretical curves, since CAC ([S]/M). Probably, the highest error could arise from uncertainty in as values, which influences more the calculated values. However, the theoretical minimum particle size is on agreement with the smallest sizes obtained by experiments, obtained at different homogenization pressures. Comparing the theoretical curves with the experimental data, it can be concluded that two regions of emulsion formation can be distinguished. All experimental samples are either in the region of stable emulsions or just on the theoretical curves. Above the theoretical curves, stable emulsions can be obtained, since there is enough surfactant to stabilize the particles. However, below those curves, formation of stable emulsions is not possible, because the amount of surfactant, available for adsorption, is not enough to fully cover the particles. The fact that experimental values are close to the theoretical curves demonstrates that the emulsification was rather efficient and that particle sizes were close to the minimum size. Therefore, the concentration of surfactant in the aqueous phase was quite low, since the models assume this low concentration. On the other hand, when preparing emulsions with high-shear instruments such as high pressure homogenizers, small sizes are usually obtained and therefore a large interfacial area is generated. Thus, it seems that the particle size depends mostly on surfactant availability for covering particle surface. Consequently, surfactant adsorption probably forms monolayers, with almost no presence of multilayers, which
43
would require much higher surfactant concentrations. Most likely, there are insufficient surfactant molecules to form a multilayer structure surrounding the emulsion particles. In further experiments, the emulsification pressure was increased in order to reduce the particle size. However, as indicated earlier, there was a critical pressure above which the particles aggregated and non-stable emulsions were obtained. This aggregation, observed only below the theoretical curves, was due to either insufficient surfactant molecules to fully cover the particles (low surfactant concentration) or too small particles (high interfacial area resulting from excessive homogenization pressure). By adjusting the total surfactant concentration and/or the homogenization pressure the minimum particle size could be reached. Consequently, the above graph (Fig. 5) could be a useful tool to formulate stable emulsions, by adjusting only surfactant concentration and homogenization pressure. It should be pointed out that emulsions with small size (310 nm) were obtained, which can be considered as nano-emulsions. It seems that high pressure homogenization is an efficient method, because allows to obtain particle sizes close to the theoretical minimum size. These results are relevant in industry, allowing the prediction of particle size and optimization of surfactant concentrations. 4. Conclusions The influence of the ionic/nonionic surfactant ratio, the surfactant concentration and the homogenization pressure, were studied. Particle size decreases with ionic/nonionic surfactant ratio, up to a certain ratio above which emulsions aggregate. Regarding the surfactant concentration, particle size decreases with concentration, as expected. Moreover, at very low surfactant concentration there is insufficient amount of surfactant to fully cover particle surfaces and emulsions abruptly aggregate. Concerning the homogenization pressure, the size decreases with pressure, as also expected. However, there is a critical pressure above which emulsions also aggregate. These results have been explained in terms of surfactant availability at the particle surface. Experimental data and theoretical models for the prediction of the minimum particle size were compared, and good agreement has been observed, indicating that emulsification is rather effective and the smallest possible sizes can be reached. These theoretical models consider that surfactant concentration in the aqueous continuous phase, at the critical pressure, is equal to the CAC. Therefore, surfactant concentration in the continuous phase is probably small and stabilization by multilayer adsorption is not likely. A higher surfactant concentration, or larger particle size, would be required for such arrangement. These theoretical models could be used as a tool to predict stability of emulsions prepared by high-energy emulsification methods. Acknowledgements The authors greatly acknowledge Prof. Marisa García (Faculty of Pharmacy, University of Barcelona) for her technical support in electrophoretic mobility measurements and the Spanish Ministry of Science and Innovation (PET2006-0582 and CTQ2008-06892C03-01 projects) for the financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2011.09.026. References [1] R. Mújika-Garai, C. Aguilar-García, F. Juárez-Arroyo, I. Covián-Sánchez, J. Nolla, J. Esquena, C. Solans, M.A. Rodríguez-Valverde, R. Tejera-García,
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[2]
[3]
[4] [5] [6]
[7]
[8] [9]
[10]
[11]
J. Vilasau et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 38–44 M.A. Cabrerizo-Vílchez, R. Hidalgo-Álvarez, Stabilization of paraffin emulsions used in the manufacture of chipboard panels by liquid crystalline phases, Journal of Dispersion Science and Technology 28 (2007) 829–836. J. Vilasau, C. Solans, M.J. Gómez, J. Dabrio, R. Mújika-Garai, J. Esquena, Phase behaviour of a mixed ionic/nonionic surfactant system used to prepare stable oil-in-water paraffin emulsions, Colloids and Surfaces A 384 (2011) 473–481. A.A. Ali, B.A. Mulley, Formation of liquid crystal and other non-fluid phases in emulsions containing non-ionic surfactants, Journal of Pharmacy and Pharmacology 30 (1978) 205–213. G.M. Eccleston, Multiple-phase oil-in-water emulsions, Journal of the Society of Cosmetic Chemists 41 (1990) 1–22. S. Friberg, Liquid crystalline phases in emulsions, Journal of Colloid and Interface Science 37 (1971) 291–295. S. Friberg, L. Mandell, M. Larsson, Mesomorphous phases, a factor of importance for the properties of emulsions, Journal of Colloid and Interface Science 29 (1969) 155–156. S. Friberg, P. Solyom, The influence of liquid crystalline phases on the rheological behaviour of emulsions, Kolloid-Zeitschrift & Zeitschrift für Polymere 236 (1970) 173–174. S.E. Friberg, C. Solans, Surfactant association structures and the stability of emulsions and foams, Langmuir 2 (1986) 121–126. O.D.H. dos Santos, J.V. Miotto, J.M. de Morais, P.A. da Rocha-Filho, W.P. de Oliveira, Attainment of emulsions with liquid crystal from marigold oil using the required HLB method, Journal of Dispersion Science and Technology 26 (2005) 243–249. G.G. Morais, O.D.H. Santos, W.P. Oliveira, P.A. Rocha Filho, Attainment of O/W emulsions containing liquid crystal from annatto oil (Bixa orellana), coffee oil, and tea tree oil (Melaleuca alternifolia) as oily phase using HLB system and ternary phase diagram, Journal of Dispersion Science and Technology 29 (2008) 297–306. R.C Pasquali, C. Bregni, Stabilized liquid-crystalline emulsions with triethanolamine stearate and stearic acid: Preparation method influence in the properties and formation of secondary droplets, Ars Pharmaceutica 47 (2006) 219–237.
[12] H. Kunieda, H. Kabir, K. Aramaki, K. Shigeta, Phase behavior of mixed polyoxyethylene-type nonionic surfactants in water, Journal of Molecular Liquids 90 (2001) 157–166. [13] H. Sagitani, S.E. Friberg, Structural changes induced by addition of a hydrocarbon to water/amphiphile mixtures, Colloid & Polymer Science 261 (1983) 862–867. [14] J. Drelich, C. Fang, C.L. White, Encyclopedia of Surface and Colloid Science, Marcel Dekker, New York, 2002. [15] H.A. Lieberman, M.M. Rieger, G.S. Banker, Pharmaceutical Dosage Forms: Disperse Systems, Marcel Dekker, New York, 1988. [16] S.A. Nespolo, M.A. Bevan, D.Y.C. Chan, F. Grieser, G.W. Stevens, Hydrodynamic and electrokinetic properties of decane droplets in aqueous sodium dodecyl sulfate solutions, Langmuir 17 (2001) 7210–7218. [17] R.W. O‘Brien, L.R. White, Electrophoretic mobility of a spherical colloidal particle, Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics 74 (1978) 1607–1626. [18] H. Ohshima, T.W. Healy, L.R. White, Approximate analytic expressions for the electrophoretic mobility of spherical colloidal particles and the conductivity of their dilute suspensions, Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics 79 (1983) 1613–1628. [19] T.P. Goloub, R.J. Pugh, The role of the surfactant head group in the emulsification process: binary (nonionic–ionic) surfactant mixtures, Journal of Colloid and Interface Science 291 (2005) 256–262. [20] I. Roland, G. Piel, L. Delattre, B. Evrard, Systematic characterization of oil-inwater emulsions for formulation design, International Journal of Pharmaceutics 263 (2003) 85–94. [21] K. Backfolk, G. Olofsson, J.B. Rosenholm, D. Eklund, Study by isothermal calorimetry and electrophoresis of the interaction between latices and cellulose derivatives, Colloids and Surfaces A: Physicochemical and Engineering Aspects 276 (2006) 78–86. [22] H. Nakajima, Microemulsions in Cosmetics in Industrial Applications of Macroemulsiones, Marcel Dekker, New York, 1997.