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Equilibration in a tartaric acid emulsion system
Colloids and Surfaces A: Physicochem. Eng. Aspects 358 (2010) 135–141
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Equilibration in a tartaric acid emulsion system Jie Chen, Lingling Ge, Stig E. Friberg ∗ , Rong Guo School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 1 December 2009 Received in revised form 18 January 2010 Accepted 20 January 2010 Available online 25 January 2010 Keywords: Emulsions Evaporation Phase diagrams Skin lotions Liquid crystals Equilibration
a b s t r a c t The equilibration between phases in an emulsion of tartaric acid stabilized by a commercial non-ionic surfactant, Laureth 4, was studied by contacting layers of water, tartaric acid and surfactant and measuring the change of layer heights with time. Contrary to earlier results of similar investigations with salicylic acid, the present determinations disclosed the transfer of compounds between the solid acid phase and the liquids not to be the primary rate-determining step in the equilibration process. Instead this stage was found in the surfactant liquid and the aqueous solution of the acid generating a lamellar liquid crystal. The results revealed a surprising number of different mechanisms involved in the transfers of compounds between the phases as exemplified by the fact that initially the formed liquid crystal was dispersed in the aqueous liquid forming a birefringent layer, a significant fraction of which was the aqueous phase. At intermediate times in the process a fraction of liquid crystal from the aqueous layer was redispersed into the surfactant liquid, in which the newly generated liquid crystal was also dispersed, but now the birefringent parts were of a more complex structure. In the final stage of the investigation but little liquid crystal was produced and the birefringent generating elements in the surfactant liquid layer were spontaneously redispersed into a liquid crystal in the aqueous stratum. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Emulsions are of pronounced industrial and commercial importance and the general literature concerning them is vast [1–5]. Their processes, properties and applications involve numeral fundamental aspects, but evaporation is certainly a vital component in a number of applications; as exemplified by the preparation of nano-particles in the few years from their introduction as simple metals and oxides [6–8] to the recent advanced and complex synthesis [9,10] of such materials and the preparation of organic polymeric materials [11–14]. Of other applications attention may be called to the area of pharmaceutics and biotechnology, where micro-particles serve to alleviate the problems of extremely low solubility of active compounds [15]. The evaporation of emulsions in the area of printing inks is certainly of importance for the information technology area [16]. In spite of these many fold applications of the evaporation process, its more basic aspects have so far not been represented to a comparable degree. The contributions of varied degree of sophistication [17–19] have focused more on the physical chemistry of the product rather than the process per se. The articles on the procedure
∗ Corresponding author at: University of Virginia, Department of Chemistry, 1695 Goldentree Place, Charlottesville, VA 22911, USA. Tel.: +1 434 973 8826. E-mail address: [email protected] (S.E. Friberg). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.01.050
as such have gradually developed from the early emphasis on evaporation of emulsion drops in air [20] to the recent truly fundamental studies of the essentials of evaporation rates from two-phase emulsions, in which area the Hull group of Fletcher and Binks [21–25] have been undisputed leaders. A substantial number of emulsion applications are concerned with evaporation from solid surfaces and the fundamentals of the evaporation process of isolated drops on a solid surface have reached an advanced state [26,27] as also exemplified by the latest results [28]. The knowledge about the behavior of evaporating emulsions on a solid surface has not reached such a erudite state, but experimental evidence has related the adsorption of drops to electrostatic interaction [29] including the effect of reduced drop–drop interaction due to electrolyte screening [30,31]. In fact the proximity to a surface may lead to macroscopic phase separation; especially weighty for emulsions confined in a narrow gap between two surfaces as demonstrated by interferometric surface force and non-interferometric MASIF methods [32]. In addition the relative hydrophobicity of the surface has been demonstrated to have an effect [33] in a marked tendency for oil drops to adhere to the hydrophobic surface rendering the emulsion unstable. These initial observations of emulsion evaporation were limited to the two-phase stage of the process, even though more complex states in the latter part of the entire evaporation process had been indicated by Settone et al. [34], by Friberg et al. [35] and by Beverley et al. [23]. Realizing the potential significance of these phase
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changes for applications, a phase diagram method [36,37] was used to attain information about potential phase changes experienced by emulsions during later part of the evaporation. The ensuing results not only gave direct information as to the prospective phase changes during the process, but, in addition, also revealed vital and surprising changes in the number of phases depending on the relative O/W ratio [38,39] and, more importantly, on the relative humidity of the atmosphere. These calculations were based on the assumption of equilibrium between the compounds in all phases in the entire emulsion system; including the gas phase. In spite of this evident restriction, the predictions of the evaporation path and the concomitant phase changes have been shown to be surprisingly accurate for a selected emulsion [40]. Notwithstanding this encouraging result of the equilibrium approach, the fact remains that emulsion evaporation is a kinetic phenomenon and not only is caution necessary in the use of the phase diagram viewpoint to predict the evaporation path, but obviously more information is called for concerning the factors being affected by the non-equilibrium conditions. Such conditions were early treated by Miller and Neogi [41–46] and continued by Masliyah and Bhattacharjee [47] and Al-Bawab et al. [48] from a different perspective. Essential information, albeit of indirect nature, is also found in the reports on spontaneous emulsification [49–55] and emulsion inversion [56–60]. The interpenetration process per se of compounds was early studied [61] and was developed into an instrumental method to establish phase equilibria of water and surfactants [62]. As a contrast to these contributions the present study is concerned with a basic study of the equilibrium process as such focusing on the transfer of compounds between phases in a more complex emulsion system. This is achieved by contacting the three primary emulsion compounds and by following the development of structures with time in order to clarify the relation between the equilibration rates and the emulsion phases. The results revealed the equilibration process under the conditions investigated to be more complex than anticipated and significantly to deviate from the processes in an earlier system examined in the same manner [63,64]. The results reflect the conditions of an emulsion during the entire evaporation process and the compounds were chosen to represent one of the most common combinations in skin lotions. The authors realize that a commercial emulsion contains most significantly less fractions of surfactant and acid, and the choice of initial amounts may be open to question. However, the selection was made to be able to provide information also about the final evaporation stages, when most of the water has expired. In this manner the both the initial and final stages of the process is covered in a single experiment.
Table 1 The weight (g) of each compound in sample in Laureth 4/tartaric acid/water system. Compound TA
W
S
3.977
2.096
1.956
2.3. Determinations The change in the layer heights with time was recorded by direct measurement each day with an accuracy of ±0.025 cm and the test tubes photographed between crossed polarizers against a light source. The experimental arrangement did not allow the observation of the interesting initial development of myelin figures. The amounts in Table 1 were chosen to give a final location of the sample according to the mark in Fig. 1. As is evident from Fig. 1, the ratio of the weights was chosen to give final state of three phases: aqueous liquid (d, Fig. 1), liquid crystal with maximum solubilized tartaric acid (b, Fig. 1) and tartaric acid solid solution with maximum combination of surfactant and water (c, Fig. 1). 2.4. The system The sample was chosen with the weight fractions of the compounds according to Table 1 and marked in Fig. 1. The weights of the three compounds were chosen for the system at equilibrium [65] to form the three phases the aqueous phase (Aq, d, Fig. 1), the lamellar liquid crystal (LLC, b, Fig. 1) and the solid solution of the acid (Solid, c, Fig. 1), but this state was reached through transfer of compounds in a complex pattern. The weights of the three compounds are given in Table 1. They were chosen to result in weight fractions of the three phases equal to one third each. The composition of each phase of concern is given in Table 2. 3. Results The results are described in the following order. In the first section the structure of the different layers is established and in the subsequent section the quantitative changes in the layer heights are reported and comments made on these specific items. The more
2. Experimental 2.1. Materials Laureth 4 (>99%, Sigma), tartaric acid (AR, Chinese National Medicine), and analytical grade water from a Millipore Milli-Q filtration system were used. 2.2. Sample preparation The three compounds were weighed in amounts according to Table 1 and each combination was layered in a flat-bottomed test tube in the order tartaric acid, water and surfactant from the bottom. The acid was dissolved in ethyl alcohol and a flat horizontal surface of acid glass was obtained after the ethanol was evaporated by gentle heating.
Fig. 1. The phase diagram of the system water (W), tartaric acid (TA) and Laureth 4 (S). The total composition of the compounds weighed in for the experiment is marked in the diagram by a circle in the three-phase region in question. Tie lines are included in the major two-phase region for clarity (adapted from [65] with permission).
J. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 358 (2010) 135–141 Table 2 Composition in weight fractions of each equilibrium phase involved in the experiment as marked in Fig. 1. Composition
S
TA
W
b c d
0.580 0.123 0.000
0.222 0.802 0.506
0.198 0.075 0.494
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Table 3 Approximate densities of the phases. Phases
Aqueous solution
Liquid crystal
Tartaric acid solution
Surfactant
Densities (g/cm3 )
1.2
1.0
1.7
0.95
iment was done to ensure that the non-equilibrium conditions during the experiment did not cause a transient hexagonal structure to be formed. A sample was taken at the interface between an aqueous solution in contact with a lamellar liquid crystal with less than equilibrium water content giving an optical pattern according to D. In addition the fact that the pattern in Fig. 3B is from a lamellar phase was confirmed by similar optical features of a lamellar liquid crystal in a suspension of approximately equal amounts of equilibrium compositions of a liquid crystal and the aqueous solution.
3.2. Quantitative results A summary of the results are first given to present an overview of the trends; followed by a more detailed arrangement to enable an analysis of essential details.
Fig. 2. The layers after 3 days. The birefringent layers have formed between the original water layer and the liquid surfactant layer. TA = tartaric acid solid; Aq = aqueous liquid; BL, Aq = birefringent layer towards the aqueous layer; BL, I = intermediate birefringent layer; BL, S = birefringent layer towards the surfactant liquid layer, S.
complete analysis of the results is presented in the subsequent Section 4. 3.1. Layer structures After the layers were contacted, birefringent layers were formed between the water and the surfactant layers as expected from the phase diagram. In addition the solid layer of the tartaric acid and the surfactant liquid layer were reduced and the height of the aqueous layer increased. Fig. 2 gives a view of the test tube and the layers observed. Information about the structure of the layers was obtained from optical microscopy patterns with samples between crossed polarizers (Fig. 3). Photographs A and B demonstrate the gradual development of the liquid crystalline structure from a disordered birefringent medium to a better defined structure with time. The pattern B at a first glance may be taken for that of an H1 phase and a simple exper-
3.2.1. Overview The quantitative results were primarily obtained as layer heights, i.e. relative volumes. These numbers were recalculated to weight fractions with approximate densities of the layers according to Table 3. Exact density values were not available, since the densities in each layer would vary with time and location in the layer. The densities were chosen as realistic intermediates between the maximum and minimum values. The variation in the weight fractions of the layers with time is presented in Fig. 4. The sum of the height of the three birefringent layers, instead of their individual values, is shown in order to give comprehensible overview of the general development. In general the results vary as expected. The weight fraction of the tartaric acid and of the surfactant layers is reduced, that of the aqueous liquid layer is increased as is the sum of the birefringent layers. These are anticipated changes, but even after a considerable time the fractions of the layers are far from the ones calculated from the equilibrium conditions in the phase diagram, as demonstrated by the single line in Fig. 4. At first the surfactant liquid layer was not completely depleted and secondly, the aqueous layer was exceedingly larger than its equilibrium value, while the two remaining layers were too small.
Fig. 3. Microscopy photographs of the layer BL I (Fig. 2) after 3 h (A) and after 6 h (B) and of the layer BL, S (C) after 6 h. (D is the pattern in a separate experiment, in which an aqueous solution with a 0.4 weight fraction of acid in water was brought in contact with a liquid crystal with greater surfactant content than corresponds to equilibrium. The pattern is from a sample taken at the interface between the liquid crystal and the liquid).
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Fig. 4. The weight fractions of the test tube layers versus time. The added line from 40 to 50 days shows the equilibrium value fraction for the three main layers according to the phase diagram (Fig. 1) (symbol—layer: () aqueous liquid; () tartaric acid, solid solution; (×) birefringent layer; () surfactant liquid).
3.2.2. Detailed processes At a cursory glance the variation with time of the layers appears without specific features and it would be attractive to discuss the general trends against the characteristics of the phase diagram. There is in point of fact some evidence to support such an action; as exemplified by a quantitative comparison between the reduction of the acid layer and the increase in the size of the aqueous layer, Fig. 5. The numbers in the diagram correspond to the change of the volume fractions of the time range with the sign for the tartaric acid value reversed. At a first glance the values are similar, indicating the contribution from the two compounds to the birefringent layer to be at a negligible level in comparison with the transfer of acid into the aqueous layer. However, a more thorough scrutiny of the results proves this conclusion not only to be premature, but in effect excluding vital information about the mechanisms of the equilibration process. This fact is apparent from the features of the diagram, which presents the birefringent layers separately. At first the second and third layer (BL, I and BL, Aq, Fig. 2) were directed towards the aqueous layer, while the top layer (BL, S), was extended towards and into the surfactant liquid. Secondly, BL, Aq was the initial layer formed; layers BL, I and BL, S appeared first after 5 days. Furthermore, the two layers towards the aqueous liquid layer, BL, Aq and BL, I, were visibly distinguished, but their variation with time did not justify their separate analysis. In fact the curve BL, Aq became discontinuous at the site, when BL, I appeared, when separately plotted. This discontinuity is illustrated by an empirical equation covering the BL, Aq curve from two points before than the appearance of the BL, I layer to two points after that instance. The empirical function of the individual BL, Aq layer had an R2 = 0.725, while the added layers showed R2 = 0.994; amply justifying treating the two layers as one, BL, A. The changes in the weight fractions of the two layers, so selected, are characterized by three time periods (Fig. 6), the first one from
Fig. 6. The variation in the weight fractions of the two birefringent layers versus times (symbol—location of the layer: () towards the aqueous layer (BL, A); (×) towards the surfactant layer (BL, S))
zero time to 5 days, the second one from 5 to 14 days and the final period from 14 days to the end of the experiment. During the first period, 0–5 days the BL, A grew to a maximum. In the second time period 5–14 days BL, A was reduced while the BL, S grew. The final time period, 14–50 days, was characterized by a slow growth of the BL, A and a corresponding reduction of the BL, S. 4. Discussion In the discussion part the overreaching factors are first examined followed by an analysis and interpretation of essential details of the equilibrium process. 4.1. Overview Before examining the details of the transfers it is vitally essential to emphasize the profound difference in behavior between the system of this acid and that of a different hydroxy acid, salicylic acid, which also has a significant commercial application in personal care products. In the salicylic acid system [63], as a contrast to the present case, the transfer rates involving the solid acid solution were slow to such an extent that the initial activities were limited entirely to the liquid and liquid crystalline phases. The behavior in the present system is exactly opposite; the solid phase reaction is comparatively faster significantly changing the total behavior. This fact evidently has a consequence in the application field causing a difference in the interaction between the acids and the stratum corneum in skin care formulations. This problem will be examined at a different occasion. 4.2. Specific features These are concerned with the deviation from equilibrium and with the processes in the specific time periods.
Fig. 5. A comparison of the changes in the height of the aqueous (×) and tartaric acid () layers during the time period of the diagram. The values for the acid are given with reversed sign and relocated to make the values at zero time identical.
4.2.1. Deviation from equilibrium In addition to these overreaching factors the present results also offer several facets that deserve a closer analysis. Foremost
J. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 358 (2010) 135–141 Table 4 Equilibrium and experimental phase fractions. Phase
Aqueous solution
Liquid crystal
Tartaric acid solution
Surfactant liquid
Equilibrium Experimental
0.333 0.519
0.333 0.210
0.333 0.206
0 0.065
of these is the discrepancy between the values of the equilibrium phase weight fractions from the phase diagram and the experimental weight fractions of the layers. The difference in values is an essential result from a scientific point of view, but the application aspects are probably even more central. The numbers are found in Fig. 4 and Table 4. The explanation’s central item is found in one of the features in the phase diagram (Fig. 1). The maximum solubility of the acid in water is a 0.580 weight fraction, while the greatest acid content of the aqueous acid solution in equilibrium with the liquid crystal is 0.507 and an interpretation of the results in view of this condition is informative. At the endpoint of the reported results the weight fractions of liquid crystal (0.210) and the acid (0.206) combined contain weights of water, 0.0571, and acid, 0.212. Subtracting these weights from the weighed in amounts gives weights in the aqueous liquid of water 0.204 and acid 0.2835; a weight fraction acid in the aqueous solution of 0.582, close to the maximum solubility of the acid 0.580. Hence, the essential part of the explanation of the discrepancy from equilibrium is the fact that the dissolution of the acid into the aqueous liquid was not brought to an end, when the fraction acid reached a level corresponding to the equilibrium with the liquid crystal composition with maximum acid. The explanation for this behavior is found in the phase diagram (Fig. 1). The maximum solubility of water into the solid acid amounts to a weight fraction of 0.17 equal to a mole fraction 0.37. If, as a first approximation, the activity of the acid follows its mole fraction [66] its chemical potential of the acid in the aqueous saturated aqueous solution is but a fraction of that of the pure acid and the increase of it in the aqueous solution is an expected event. This kinetics of mutual exchange between the aqueous liquid and the solid acid solution therefore offers a plausible explanation of the most conspicuous facet of the results, but, in addition, an examination of the details of the transport of compounds along the additional pathways offers vital insight in the process. A more comprehensive analysis follows the stages, which are indicated in Fig. 6, because the limit of the stages is characterized by sudden reversal of the trends. The weight changes of the different layers for each time range are offered in Table 5 with total weight of the samples as unity. These values were used in combination the equilibrium weight fractions of each phases according to Table 2 for the detailed analysis of the consecutive processes in the equilibration progression.
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the surfactant liquid layer is passed to the birefringent layer and, potentially, an insignificant part of that amount will experience a secondary transfer to the solid acid solution, via the aqueous layer; an extremely unlikely event considering the minute solubility of the surfactant in the water (for a system of infinite time the transfer of surfactant to the acid solution would take place through the liquid crystal). The transfer of specific compounds in the first stage may be quantitatively analyzed by postulating as a first approximation the composition of the birefringent layer to have maximum acid solubilization. This is a gross approximation, but was supported by to the ensuing result of the calculation. As evident from the phase diagram the quantity surfactant leaving its layer must with necessity enter the birefringent layer and the transfer of 0.0263 from the surfactant layer results in the addition of 0.202 × 0.0263/0.576 water and 0.222 × 0.0263/0.576 acid to the birefringent layer; a total of 0.0456 of liquid crystal per se. The birefringent layer, on the other hand, was increased by 0.1140 (Table 5) and, hence 0.0456 of the liquid crystal is dispersed in 0.0684 water plus acid. Assuming all but equal amounts of the two in the birefringent layer, the remaining water in the aqueous layer is 0.2611 − 0.0092 − 0.0337 = 0.2182. Its acid content equals 0.2059 − 0.0101 − 0.0346 = 0.1612 and the weight fraction of acid in the aqueous layer comes to 0.43 at the end of the first stage of 5 days. Using this number, Fig. 1 demonstrates the composition of the aqueous liquid to be at such an elevated level that the assumed composition of the liquid crystal is realistic. As a summary it may be concluded that these estimations establish with acceptable probability that the initial 5 days of the processes lead to the formation of a lamellar liquid crystal dispersed in 1.5 its weight of water plus acid. However, these numbers reflect the situation at the end of the initial 5-day period, but the results, in fact, also offer information as to the time dependence of the conditions during that time range. To illustrate the point the increase of the weight of the birefringent layer and the reduction of the surfactant layer with inversed sign and multiplied by an adequate factor to compensate for the additional water and acid in the layer. The two curves (Fig. 7) are virtually identical along the entire range, strongly indicating the dispersion degree of the liquid crystal to be constant during the full period, i.e. the formed liquid crystal is dispersed from the beginning of the process. Combining the information a realistic conclusion of the events during the first 5 days is a transfer of the acid into the aqueous layer to bring its fraction in the liquid layer to a level approximately at the maximum solubility in equilibrium with the liquid crystal and synchronously for the three compounds to form a lamellar liquid crystal with close to maximum solubilization of water and acid dispersed in nearly 1.5 times of its weight of acid and water. It is of interest to observe that after the initial activity during the first day of the experiment when the dimension of the birefringent
4.2.2. Initial period, 0–5 days In the first stage during the time range 0–5 days the direction of transfer of two of the three compounds is unambiguous. The loss of the acid from its layer is due to a transfer to the aqueous layer, forming the aqueous phase. To a minor extent, some of this acid will also relocate to the birefringent layer. In the same manner a part of Table 5 Weight changes in the different layers according to Fig. 6. Time range
S
BL, S
BL, A
Aq
TA
0–5 days 5–14 days 14–50 days
−0.0263 −0.093 −0.0016
0 0.12858 −0.0664
0.11398 −0.0631 0.06141
0.1182 0.09406 0.08765
−0.2059 −0.0665 −0.0809
Fig. 7. The weight of the birefringent layer () and the reduction of the surfactant layer multiplied by 2.62 (×) during the initial 5 days of the experiment (the earlier calculated factor of 1.5 was obtained from the numbers at 5 days; the value of 1.62 refers to the average conditions during the entire period.).
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layer reached 1 cm, the continued increase during the next 4 days was linear as described by an empirical equation: h = 0.0908d + 0.7996R2 = 0.97
(1)
The formation rate of the birefringent material was evidently independent of its dimension. The rate-determining step was obviously independent of the diffusion of the aqueous solution to the interface towards the surfactant layer; i.e. the association process per se was the rate-determining step. This is a parallel to the earlier results of the salicylic acid system [63] and in fact the results also quantitatively agree to a surprising extent, considering the earlier report was concerned with a birefringent layer of the surfactant and water only. The fact that the dissolution of acid into the water was fast in comparison with other processes means the results to be relevant to commercial emulsions which are usually made with the acid predissolved in the water prior to emulsification. 4.2.3. Intermediate period, 5–14 days At 5 days (Fig. 6), a reversal of the trend takes place. A reduction in the weight of the birefringent layer towards the aqueous solution is now found and, vice versa, a birefringent layer is initiated and growing towards the surfactant liquid. This latter layer is reduced by 0.093, which amount would, if the earlier assumption of the composition of the liquid crystal were retained, amount to a layer weight of 0.162, even without an assumption of a presence of a liquid in the layer. However, the layer in question shows an increase of 0.129 and obviously the earlier assumption of the composition of the birefringent layer that extended towards the aqueous layer no longer holds for a layer towards the surfactant layer. The conditions in the triangular part of the diagram towards the surfactant corner are more complex than in the water rich part and, in addition, the potential dispersity of the liquid crystal in the surfactant liquid is virtually unpredictable due to the presence of inverse micelles. Accordingly no assumption was made as to the dispersity of the lamellar structure in the layer and instead the numbers per se were used to find the composition of the birefringent layer. With the layer weight at 0.129 and the loss of surfactant equal to 0.093 the weight fraction of surfactant comes to 0.72 in the layer. As a consequence the composition of the layer would be located along the dashed line in Fig. 8, i.e. the layer may contain acid solution crystals in addition to the lamellar phase and the surfactant liquid. This conclusion is supported by the micrograph from the layer (Fig. 2) showing solid particles. This information, albeit approximate, is highly useful for the understanding of the process. The composition of the birefringent layer is now at water fractions less than that of the liquid crystal and with the information from Fig. 8 an interpretation of the layer
Fig. 8. The dashed line shows the estimated composition limit of the BL, S layer at 14 days.
as including acid solid solution crystals in addition to the liquid crystal is realistic as conformed by micrographs of samples from that layer. 4.2.4. Final period, 14–50 days The final stage from 14 to 50 days is characterized by modest activities in comparison with the two first stages of the experiment. The action in the birefringent layers is approximately complementary. The increase in the BL, Aq almost exactly corresponds to the reduction in the BL, S layer; (a regression analysis showed the former to be 13% greater) and the reduction in the surfactant layer is insignificant, but the layer is not completely depleted. Extrapolation of the trends indicated the diminution of the BL, S layer in approximately 100 days (R2 = 0.8) and the corresponding event for the surfactant layer a multiple of that number (R2 extremely small). There was no purpose felt to extend the determinations to such times and the experiments were terminated. The only realistic conclusion for this stage is a slow transfer of material of surfactant from the Bl, S layer to the BL, Aq layer. Earlier investigation [64] clarified the rate of transport from the inverse micellar solution to the lamellar liquid crystal in the system finding the transport of the surfactant from an inverse micellar solution saturated with water into a lamellar liquid crystal with a surfactant fraction less than the maximum was fast, while the transport of water from the liquid crystal with maximum surfactant into an inverse micellar solution not saturated with water was slower. In the present case the transport of surfactant from the Bl, S layer to the BL, Aq layer is in comparison a realistic outcome, considering the complex transformations involved from the presence of crystals. This process is complicated by the dissolution of water and surfactant into the solid acid solution, but such a process is slow to an extent to be without consequence for the main purpose of the investigation; to gather information about the factors affecting the equilibrium process during evaporation of an emulsion. 5. Conclusions The results clarified the transport between different phases in an emulsion system and defined the rate-determining steps involved in the equilibration process during evaporation of the emulsions of this kind. The direct comparison with the conditions during evaporation of a model system will be made in future investigations. References [1] J. Sjoeblom (Ed.), Handbook of Emulsion Technology, Marcel Dekker Inc., New York, 2001. [2] J. Bibette, F. Leal-Calderon, P. Poulin, Emulsions: basic principles, Rep. Prog. Phys. 62 (1999) 969–1033. [3] P.B. Binks, Modern Aspects of Emulsion Science, Royal Society of Chemistry, Cambridge, 1998. [4] D.J. McClements (Ed.), Food Emulsions, Second ed., CRC Press, Boca Raton, 2004. [5] A. Aserine (Ed.), Multiple Emulsions, Wiley & Sons, Hoboken, NJ, USA, 2008. [6] H. Fan, F. Van Swol, Y. Lu, C.J. Brinker, Multiphased assembly of nanoporous silica particles, J. Non-Cryst. Solids 285 (2001) 71–78. [7] A.J. Zarur, J.Y. Ying, Microemulsion synthesis of nanostructured complex oxides for catalytic combustion, Nature 403 (2000) 65–67. [8] D.H. Chen, C.J. Chen, Formation and characterization of Au–Ag bimetallic nanoparticles in water-in-oil microemulsions, J. Mater. Chem. 12 (2002) 1557–1562. [9] P.C.A. Alberius, K.L. Frindell, R.C. Hayward, E.J. Kramer, G.S. Stucky, B.F. Chmelka, General predictive syntheses of cubic, hexagonal, and lamellar silica and titania mesostructured thin films, Chem. Mater. 14 (2002) 3284–3294. [10] N. Andersson, B. Kronberg, R. Corkery, P. Alberius, Combined emulsion and solvent evaporation (ESE) synthesis route to well-ordered mesoporous materials, Langmuir 23 (2007) 1459–1464. [11] K.E. Gonzales, J. Sjunua, M.J. Baraton, Synthesis and surface characterization of functionalized polyactide copolymer particles, Biomaterials 19 (1998) 1501–1505. [12] E. Lorenceau, A.S. Utada, D.R. Link, G. Cristobal, M. Joanicot, D.A. Weitz, Generation of polymerosomes from double-emulsions, Langmuir 21 (2005) 9183–9186.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Equilibration in a tartaric acid emulsion system Jie Chen, Lingling Ge, Stig E. Friberg ∗ , Rong Guo School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 1 December 2009 Received in revised form 18 January 2010 Accepted 20 January 2010 Available online 25 January 2010 Keywords: Emulsions Evaporation Phase diagrams Skin lotions Liquid crystals Equilibration
a b s t r a c t The equilibration between phases in an emulsion of tartaric acid stabilized by a commercial non-ionic surfactant, Laureth 4, was studied by contacting layers of water, tartaric acid and surfactant and measuring the change of layer heights with time. Contrary to earlier results of similar investigations with salicylic acid, the present determinations disclosed the transfer of compounds between the solid acid phase and the liquids not to be the primary rate-determining step in the equilibration process. Instead this stage was found in the surfactant liquid and the aqueous solution of the acid generating a lamellar liquid crystal. The results revealed a surprising number of different mechanisms involved in the transfers of compounds between the phases as exemplified by the fact that initially the formed liquid crystal was dispersed in the aqueous liquid forming a birefringent layer, a significant fraction of which was the aqueous phase. At intermediate times in the process a fraction of liquid crystal from the aqueous layer was redispersed into the surfactant liquid, in which the newly generated liquid crystal was also dispersed, but now the birefringent parts were of a more complex structure. In the final stage of the investigation but little liquid crystal was produced and the birefringent generating elements in the surfactant liquid layer were spontaneously redispersed into a liquid crystal in the aqueous stratum. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Emulsions are of pronounced industrial and commercial importance and the general literature concerning them is vast [1–5]. Their processes, properties and applications involve numeral fundamental aspects, but evaporation is certainly a vital component in a number of applications; as exemplified by the preparation of nano-particles in the few years from their introduction as simple metals and oxides [6–8] to the recent advanced and complex synthesis [9,10] of such materials and the preparation of organic polymeric materials [11–14]. Of other applications attention may be called to the area of pharmaceutics and biotechnology, where micro-particles serve to alleviate the problems of extremely low solubility of active compounds [15]. The evaporation of emulsions in the area of printing inks is certainly of importance for the information technology area [16]. In spite of these many fold applications of the evaporation process, its more basic aspects have so far not been represented to a comparable degree. The contributions of varied degree of sophistication [17–19] have focused more on the physical chemistry of the product rather than the process per se. The articles on the procedure
∗ Corresponding author at: University of Virginia, Department of Chemistry, 1695 Goldentree Place, Charlottesville, VA 22911, USA. Tel.: +1 434 973 8826. E-mail address: [email protected] (S.E. Friberg). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.01.050
as such have gradually developed from the early emphasis on evaporation of emulsion drops in air [20] to the recent truly fundamental studies of the essentials of evaporation rates from two-phase emulsions, in which area the Hull group of Fletcher and Binks [21–25] have been undisputed leaders. A substantial number of emulsion applications are concerned with evaporation from solid surfaces and the fundamentals of the evaporation process of isolated drops on a solid surface have reached an advanced state [26,27] as also exemplified by the latest results [28]. The knowledge about the behavior of evaporating emulsions on a solid surface has not reached such a erudite state, but experimental evidence has related the adsorption of drops to electrostatic interaction [29] including the effect of reduced drop–drop interaction due to electrolyte screening [30,31]. In fact the proximity to a surface may lead to macroscopic phase separation; especially weighty for emulsions confined in a narrow gap between two surfaces as demonstrated by interferometric surface force and non-interferometric MASIF methods [32]. In addition the relative hydrophobicity of the surface has been demonstrated to have an effect [33] in a marked tendency for oil drops to adhere to the hydrophobic surface rendering the emulsion unstable. These initial observations of emulsion evaporation were limited to the two-phase stage of the process, even though more complex states in the latter part of the entire evaporation process had been indicated by Settone et al. [34], by Friberg et al. [35] and by Beverley et al. [23]. Realizing the potential significance of these phase
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changes for applications, a phase diagram method [36,37] was used to attain information about potential phase changes experienced by emulsions during later part of the evaporation. The ensuing results not only gave direct information as to the prospective phase changes during the process, but, in addition, also revealed vital and surprising changes in the number of phases depending on the relative O/W ratio [38,39] and, more importantly, on the relative humidity of the atmosphere. These calculations were based on the assumption of equilibrium between the compounds in all phases in the entire emulsion system; including the gas phase. In spite of this evident restriction, the predictions of the evaporation path and the concomitant phase changes have been shown to be surprisingly accurate for a selected emulsion [40]. Notwithstanding this encouraging result of the equilibrium approach, the fact remains that emulsion evaporation is a kinetic phenomenon and not only is caution necessary in the use of the phase diagram viewpoint to predict the evaporation path, but obviously more information is called for concerning the factors being affected by the non-equilibrium conditions. Such conditions were early treated by Miller and Neogi [41–46] and continued by Masliyah and Bhattacharjee [47] and Al-Bawab et al. [48] from a different perspective. Essential information, albeit of indirect nature, is also found in the reports on spontaneous emulsification [49–55] and emulsion inversion [56–60]. The interpenetration process per se of compounds was early studied [61] and was developed into an instrumental method to establish phase equilibria of water and surfactants [62]. As a contrast to these contributions the present study is concerned with a basic study of the equilibrium process as such focusing on the transfer of compounds between phases in a more complex emulsion system. This is achieved by contacting the three primary emulsion compounds and by following the development of structures with time in order to clarify the relation between the equilibration rates and the emulsion phases. The results revealed the equilibration process under the conditions investigated to be more complex than anticipated and significantly to deviate from the processes in an earlier system examined in the same manner [63,64]. The results reflect the conditions of an emulsion during the entire evaporation process and the compounds were chosen to represent one of the most common combinations in skin lotions. The authors realize that a commercial emulsion contains most significantly less fractions of surfactant and acid, and the choice of initial amounts may be open to question. However, the selection was made to be able to provide information also about the final evaporation stages, when most of the water has expired. In this manner the both the initial and final stages of the process is covered in a single experiment.
Table 1 The weight (g) of each compound in sample in Laureth 4/tartaric acid/water system. Compound TA
W
S
3.977
2.096
1.956
2.3. Determinations The change in the layer heights with time was recorded by direct measurement each day with an accuracy of ±0.025 cm and the test tubes photographed between crossed polarizers against a light source. The experimental arrangement did not allow the observation of the interesting initial development of myelin figures. The amounts in Table 1 were chosen to give a final location of the sample according to the mark in Fig. 1. As is evident from Fig. 1, the ratio of the weights was chosen to give final state of three phases: aqueous liquid (d, Fig. 1), liquid crystal with maximum solubilized tartaric acid (b, Fig. 1) and tartaric acid solid solution with maximum combination of surfactant and water (c, Fig. 1). 2.4. The system The sample was chosen with the weight fractions of the compounds according to Table 1 and marked in Fig. 1. The weights of the three compounds were chosen for the system at equilibrium [65] to form the three phases the aqueous phase (Aq, d, Fig. 1), the lamellar liquid crystal (LLC, b, Fig. 1) and the solid solution of the acid (Solid, c, Fig. 1), but this state was reached through transfer of compounds in a complex pattern. The weights of the three compounds are given in Table 1. They were chosen to result in weight fractions of the three phases equal to one third each. The composition of each phase of concern is given in Table 2. 3. Results The results are described in the following order. In the first section the structure of the different layers is established and in the subsequent section the quantitative changes in the layer heights are reported and comments made on these specific items. The more
2. Experimental 2.1. Materials Laureth 4 (>99%, Sigma), tartaric acid (AR, Chinese National Medicine), and analytical grade water from a Millipore Milli-Q filtration system were used. 2.2. Sample preparation The three compounds were weighed in amounts according to Table 1 and each combination was layered in a flat-bottomed test tube in the order tartaric acid, water and surfactant from the bottom. The acid was dissolved in ethyl alcohol and a flat horizontal surface of acid glass was obtained after the ethanol was evaporated by gentle heating.
Fig. 1. The phase diagram of the system water (W), tartaric acid (TA) and Laureth 4 (S). The total composition of the compounds weighed in for the experiment is marked in the diagram by a circle in the three-phase region in question. Tie lines are included in the major two-phase region for clarity (adapted from [65] with permission).
J. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 358 (2010) 135–141 Table 2 Composition in weight fractions of each equilibrium phase involved in the experiment as marked in Fig. 1. Composition
S
TA
W
b c d
0.580 0.123 0.000
0.222 0.802 0.506
0.198 0.075 0.494
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Table 3 Approximate densities of the phases. Phases
Aqueous solution
Liquid crystal
Tartaric acid solution
Surfactant
Densities (g/cm3 )
1.2
1.0
1.7
0.95
iment was done to ensure that the non-equilibrium conditions during the experiment did not cause a transient hexagonal structure to be formed. A sample was taken at the interface between an aqueous solution in contact with a lamellar liquid crystal with less than equilibrium water content giving an optical pattern according to D. In addition the fact that the pattern in Fig. 3B is from a lamellar phase was confirmed by similar optical features of a lamellar liquid crystal in a suspension of approximately equal amounts of equilibrium compositions of a liquid crystal and the aqueous solution.
3.2. Quantitative results A summary of the results are first given to present an overview of the trends; followed by a more detailed arrangement to enable an analysis of essential details.
Fig. 2. The layers after 3 days. The birefringent layers have formed between the original water layer and the liquid surfactant layer. TA = tartaric acid solid; Aq = aqueous liquid; BL, Aq = birefringent layer towards the aqueous layer; BL, I = intermediate birefringent layer; BL, S = birefringent layer towards the surfactant liquid layer, S.
complete analysis of the results is presented in the subsequent Section 4. 3.1. Layer structures After the layers were contacted, birefringent layers were formed between the water and the surfactant layers as expected from the phase diagram. In addition the solid layer of the tartaric acid and the surfactant liquid layer were reduced and the height of the aqueous layer increased. Fig. 2 gives a view of the test tube and the layers observed. Information about the structure of the layers was obtained from optical microscopy patterns with samples between crossed polarizers (Fig. 3). Photographs A and B demonstrate the gradual development of the liquid crystalline structure from a disordered birefringent medium to a better defined structure with time. The pattern B at a first glance may be taken for that of an H1 phase and a simple exper-
3.2.1. Overview The quantitative results were primarily obtained as layer heights, i.e. relative volumes. These numbers were recalculated to weight fractions with approximate densities of the layers according to Table 3. Exact density values were not available, since the densities in each layer would vary with time and location in the layer. The densities were chosen as realistic intermediates between the maximum and minimum values. The variation in the weight fractions of the layers with time is presented in Fig. 4. The sum of the height of the three birefringent layers, instead of their individual values, is shown in order to give comprehensible overview of the general development. In general the results vary as expected. The weight fraction of the tartaric acid and of the surfactant layers is reduced, that of the aqueous liquid layer is increased as is the sum of the birefringent layers. These are anticipated changes, but even after a considerable time the fractions of the layers are far from the ones calculated from the equilibrium conditions in the phase diagram, as demonstrated by the single line in Fig. 4. At first the surfactant liquid layer was not completely depleted and secondly, the aqueous layer was exceedingly larger than its equilibrium value, while the two remaining layers were too small.
Fig. 3. Microscopy photographs of the layer BL I (Fig. 2) after 3 h (A) and after 6 h (B) and of the layer BL, S (C) after 6 h. (D is the pattern in a separate experiment, in which an aqueous solution with a 0.4 weight fraction of acid in water was brought in contact with a liquid crystal with greater surfactant content than corresponds to equilibrium. The pattern is from a sample taken at the interface between the liquid crystal and the liquid).
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Fig. 4. The weight fractions of the test tube layers versus time. The added line from 40 to 50 days shows the equilibrium value fraction for the three main layers according to the phase diagram (Fig. 1) (symbol—layer: () aqueous liquid; () tartaric acid, solid solution; (×) birefringent layer; () surfactant liquid).
3.2.2. Detailed processes At a cursory glance the variation with time of the layers appears without specific features and it would be attractive to discuss the general trends against the characteristics of the phase diagram. There is in point of fact some evidence to support such an action; as exemplified by a quantitative comparison between the reduction of the acid layer and the increase in the size of the aqueous layer, Fig. 5. The numbers in the diagram correspond to the change of the volume fractions of the time range with the sign for the tartaric acid value reversed. At a first glance the values are similar, indicating the contribution from the two compounds to the birefringent layer to be at a negligible level in comparison with the transfer of acid into the aqueous layer. However, a more thorough scrutiny of the results proves this conclusion not only to be premature, but in effect excluding vital information about the mechanisms of the equilibration process. This fact is apparent from the features of the diagram, which presents the birefringent layers separately. At first the second and third layer (BL, I and BL, Aq, Fig. 2) were directed towards the aqueous layer, while the top layer (BL, S), was extended towards and into the surfactant liquid. Secondly, BL, Aq was the initial layer formed; layers BL, I and BL, S appeared first after 5 days. Furthermore, the two layers towards the aqueous liquid layer, BL, Aq and BL, I, were visibly distinguished, but their variation with time did not justify their separate analysis. In fact the curve BL, Aq became discontinuous at the site, when BL, I appeared, when separately plotted. This discontinuity is illustrated by an empirical equation covering the BL, Aq curve from two points before than the appearance of the BL, I layer to two points after that instance. The empirical function of the individual BL, Aq layer had an R2 = 0.725, while the added layers showed R2 = 0.994; amply justifying treating the two layers as one, BL, A. The changes in the weight fractions of the two layers, so selected, are characterized by three time periods (Fig. 6), the first one from
Fig. 6. The variation in the weight fractions of the two birefringent layers versus times (symbol—location of the layer: () towards the aqueous layer (BL, A); (×) towards the surfactant layer (BL, S))
zero time to 5 days, the second one from 5 to 14 days and the final period from 14 days to the end of the experiment. During the first period, 0–5 days the BL, A grew to a maximum. In the second time period 5–14 days BL, A was reduced while the BL, S grew. The final time period, 14–50 days, was characterized by a slow growth of the BL, A and a corresponding reduction of the BL, S. 4. Discussion In the discussion part the overreaching factors are first examined followed by an analysis and interpretation of essential details of the equilibrium process. 4.1. Overview Before examining the details of the transfers it is vitally essential to emphasize the profound difference in behavior between the system of this acid and that of a different hydroxy acid, salicylic acid, which also has a significant commercial application in personal care products. In the salicylic acid system [63], as a contrast to the present case, the transfer rates involving the solid acid solution were slow to such an extent that the initial activities were limited entirely to the liquid and liquid crystalline phases. The behavior in the present system is exactly opposite; the solid phase reaction is comparatively faster significantly changing the total behavior. This fact evidently has a consequence in the application field causing a difference in the interaction between the acids and the stratum corneum in skin care formulations. This problem will be examined at a different occasion. 4.2. Specific features These are concerned with the deviation from equilibrium and with the processes in the specific time periods.
Fig. 5. A comparison of the changes in the height of the aqueous (×) and tartaric acid () layers during the time period of the diagram. The values for the acid are given with reversed sign and relocated to make the values at zero time identical.
4.2.1. Deviation from equilibrium In addition to these overreaching factors the present results also offer several facets that deserve a closer analysis. Foremost
J. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 358 (2010) 135–141 Table 4 Equilibrium and experimental phase fractions. Phase
Aqueous solution
Liquid crystal
Tartaric acid solution
Surfactant liquid
Equilibrium Experimental
0.333 0.519
0.333 0.210
0.333 0.206
0 0.065
of these is the discrepancy between the values of the equilibrium phase weight fractions from the phase diagram and the experimental weight fractions of the layers. The difference in values is an essential result from a scientific point of view, but the application aspects are probably even more central. The numbers are found in Fig. 4 and Table 4. The explanation’s central item is found in one of the features in the phase diagram (Fig. 1). The maximum solubility of the acid in water is a 0.580 weight fraction, while the greatest acid content of the aqueous acid solution in equilibrium with the liquid crystal is 0.507 and an interpretation of the results in view of this condition is informative. At the endpoint of the reported results the weight fractions of liquid crystal (0.210) and the acid (0.206) combined contain weights of water, 0.0571, and acid, 0.212. Subtracting these weights from the weighed in amounts gives weights in the aqueous liquid of water 0.204 and acid 0.2835; a weight fraction acid in the aqueous solution of 0.582, close to the maximum solubility of the acid 0.580. Hence, the essential part of the explanation of the discrepancy from equilibrium is the fact that the dissolution of the acid into the aqueous liquid was not brought to an end, when the fraction acid reached a level corresponding to the equilibrium with the liquid crystal composition with maximum acid. The explanation for this behavior is found in the phase diagram (Fig. 1). The maximum solubility of water into the solid acid amounts to a weight fraction of 0.17 equal to a mole fraction 0.37. If, as a first approximation, the activity of the acid follows its mole fraction [66] its chemical potential of the acid in the aqueous saturated aqueous solution is but a fraction of that of the pure acid and the increase of it in the aqueous solution is an expected event. This kinetics of mutual exchange between the aqueous liquid and the solid acid solution therefore offers a plausible explanation of the most conspicuous facet of the results, but, in addition, an examination of the details of the transport of compounds along the additional pathways offers vital insight in the process. A more comprehensive analysis follows the stages, which are indicated in Fig. 6, because the limit of the stages is characterized by sudden reversal of the trends. The weight changes of the different layers for each time range are offered in Table 5 with total weight of the samples as unity. These values were used in combination the equilibrium weight fractions of each phases according to Table 2 for the detailed analysis of the consecutive processes in the equilibration progression.
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the surfactant liquid layer is passed to the birefringent layer and, potentially, an insignificant part of that amount will experience a secondary transfer to the solid acid solution, via the aqueous layer; an extremely unlikely event considering the minute solubility of the surfactant in the water (for a system of infinite time the transfer of surfactant to the acid solution would take place through the liquid crystal). The transfer of specific compounds in the first stage may be quantitatively analyzed by postulating as a first approximation the composition of the birefringent layer to have maximum acid solubilization. This is a gross approximation, but was supported by to the ensuing result of the calculation. As evident from the phase diagram the quantity surfactant leaving its layer must with necessity enter the birefringent layer and the transfer of 0.0263 from the surfactant layer results in the addition of 0.202 × 0.0263/0.576 water and 0.222 × 0.0263/0.576 acid to the birefringent layer; a total of 0.0456 of liquid crystal per se. The birefringent layer, on the other hand, was increased by 0.1140 (Table 5) and, hence 0.0456 of the liquid crystal is dispersed in 0.0684 water plus acid. Assuming all but equal amounts of the two in the birefringent layer, the remaining water in the aqueous layer is 0.2611 − 0.0092 − 0.0337 = 0.2182. Its acid content equals 0.2059 − 0.0101 − 0.0346 = 0.1612 and the weight fraction of acid in the aqueous layer comes to 0.43 at the end of the first stage of 5 days. Using this number, Fig. 1 demonstrates the composition of the aqueous liquid to be at such an elevated level that the assumed composition of the liquid crystal is realistic. As a summary it may be concluded that these estimations establish with acceptable probability that the initial 5 days of the processes lead to the formation of a lamellar liquid crystal dispersed in 1.5 its weight of water plus acid. However, these numbers reflect the situation at the end of the initial 5-day period, but the results, in fact, also offer information as to the time dependence of the conditions during that time range. To illustrate the point the increase of the weight of the birefringent layer and the reduction of the surfactant layer with inversed sign and multiplied by an adequate factor to compensate for the additional water and acid in the layer. The two curves (Fig. 7) are virtually identical along the entire range, strongly indicating the dispersion degree of the liquid crystal to be constant during the full period, i.e. the formed liquid crystal is dispersed from the beginning of the process. Combining the information a realistic conclusion of the events during the first 5 days is a transfer of the acid into the aqueous layer to bring its fraction in the liquid layer to a level approximately at the maximum solubility in equilibrium with the liquid crystal and synchronously for the three compounds to form a lamellar liquid crystal with close to maximum solubilization of water and acid dispersed in nearly 1.5 times of its weight of acid and water. It is of interest to observe that after the initial activity during the first day of the experiment when the dimension of the birefringent
4.2.2. Initial period, 0–5 days In the first stage during the time range 0–5 days the direction of transfer of two of the three compounds is unambiguous. The loss of the acid from its layer is due to a transfer to the aqueous layer, forming the aqueous phase. To a minor extent, some of this acid will also relocate to the birefringent layer. In the same manner a part of Table 5 Weight changes in the different layers according to Fig. 6. Time range
S
BL, S
BL, A
Aq
TA
0–5 days 5–14 days 14–50 days
−0.0263 −0.093 −0.0016
0 0.12858 −0.0664
0.11398 −0.0631 0.06141
0.1182 0.09406 0.08765
−0.2059 −0.0665 −0.0809
Fig. 7. The weight of the birefringent layer () and the reduction of the surfactant layer multiplied by 2.62 (×) during the initial 5 days of the experiment (the earlier calculated factor of 1.5 was obtained from the numbers at 5 days; the value of 1.62 refers to the average conditions during the entire period.).
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layer reached 1 cm, the continued increase during the next 4 days was linear as described by an empirical equation: h = 0.0908d + 0.7996R2 = 0.97
(1)
The formation rate of the birefringent material was evidently independent of its dimension. The rate-determining step was obviously independent of the diffusion of the aqueous solution to the interface towards the surfactant layer; i.e. the association process per se was the rate-determining step. This is a parallel to the earlier results of the salicylic acid system [63] and in fact the results also quantitatively agree to a surprising extent, considering the earlier report was concerned with a birefringent layer of the surfactant and water only. The fact that the dissolution of acid into the water was fast in comparison with other processes means the results to be relevant to commercial emulsions which are usually made with the acid predissolved in the water prior to emulsification. 4.2.3. Intermediate period, 5–14 days At 5 days (Fig. 6), a reversal of the trend takes place. A reduction in the weight of the birefringent layer towards the aqueous solution is now found and, vice versa, a birefringent layer is initiated and growing towards the surfactant liquid. This latter layer is reduced by 0.093, which amount would, if the earlier assumption of the composition of the liquid crystal were retained, amount to a layer weight of 0.162, even without an assumption of a presence of a liquid in the layer. However, the layer in question shows an increase of 0.129 and obviously the earlier assumption of the composition of the birefringent layer that extended towards the aqueous layer no longer holds for a layer towards the surfactant layer. The conditions in the triangular part of the diagram towards the surfactant corner are more complex than in the water rich part and, in addition, the potential dispersity of the liquid crystal in the surfactant liquid is virtually unpredictable due to the presence of inverse micelles. Accordingly no assumption was made as to the dispersity of the lamellar structure in the layer and instead the numbers per se were used to find the composition of the birefringent layer. With the layer weight at 0.129 and the loss of surfactant equal to 0.093 the weight fraction of surfactant comes to 0.72 in the layer. As a consequence the composition of the layer would be located along the dashed line in Fig. 8, i.e. the layer may contain acid solution crystals in addition to the lamellar phase and the surfactant liquid. This conclusion is supported by the micrograph from the layer (Fig. 2) showing solid particles. This information, albeit approximate, is highly useful for the understanding of the process. The composition of the birefringent layer is now at water fractions less than that of the liquid crystal and with the information from Fig. 8 an interpretation of the layer
Fig. 8. The dashed line shows the estimated composition limit of the BL, S layer at 14 days.
as including acid solid solution crystals in addition to the liquid crystal is realistic as conformed by micrographs of samples from that layer. 4.2.4. Final period, 14–50 days The final stage from 14 to 50 days is characterized by modest activities in comparison with the two first stages of the experiment. The action in the birefringent layers is approximately complementary. The increase in the BL, Aq almost exactly corresponds to the reduction in the BL, S layer; (a regression analysis showed the former to be 13% greater) and the reduction in the surfactant layer is insignificant, but the layer is not completely depleted. Extrapolation of the trends indicated the diminution of the BL, S layer in approximately 100 days (R2 = 0.8) and the corresponding event for the surfactant layer a multiple of that number (R2 extremely small). There was no purpose felt to extend the determinations to such times and the experiments were terminated. The only realistic conclusion for this stage is a slow transfer of material of surfactant from the Bl, S layer to the BL, Aq layer. Earlier investigation [64] clarified the rate of transport from the inverse micellar solution to the lamellar liquid crystal in the system finding the transport of the surfactant from an inverse micellar solution saturated with water into a lamellar liquid crystal with a surfactant fraction less than the maximum was fast, while the transport of water from the liquid crystal with maximum surfactant into an inverse micellar solution not saturated with water was slower. In the present case the transport of surfactant from the Bl, S layer to the BL, Aq layer is in comparison a realistic outcome, considering the complex transformations involved from the presence of crystals. This process is complicated by the dissolution of water and surfactant into the solid acid solution, but such a process is slow to an extent to be without consequence for the main purpose of the investigation; to gather information about the factors affecting the equilibrium process during evaporation of an emulsion. 5. Conclusions The results clarified the transport between different phases in an emulsion system and defined the rate-determining steps involved in the equilibration process during evaporation of the emulsions of this kind. The direct comparison with the conditions during evaporation of a model system will be made in future investigations. References [1] J. Sjoeblom (Ed.), Handbook of Emulsion Technology, Marcel Dekker Inc., New York, 2001. [2] J. Bibette, F. Leal-Calderon, P. Poulin, Emulsions: basic principles, Rep. Prog. Phys. 62 (1999) 969–1033. [3] P.B. Binks, Modern Aspects of Emulsion Science, Royal Society of Chemistry, Cambridge, 1998. [4] D.J. McClements (Ed.), Food Emulsions, Second ed., CRC Press, Boca Raton, 2004. [5] A. Aserine (Ed.), Multiple Emulsions, Wiley & Sons, Hoboken, NJ, USA, 2008. [6] H. Fan, F. Van Swol, Y. Lu, C.J. Brinker, Multiphased assembly of nanoporous silica particles, J. Non-Cryst. Solids 285 (2001) 71–78. [7] A.J. Zarur, J.Y. Ying, Microemulsion synthesis of nanostructured complex oxides for catalytic combustion, Nature 403 (2000) 65–67. [8] D.H. Chen, C.J. Chen, Formation and characterization of Au–Ag bimetallic nanoparticles in water-in-oil microemulsions, J. Mater. Chem. 12 (2002) 1557–1562. [9] P.C.A. Alberius, K.L. Frindell, R.C. Hayward, E.J. Kramer, G.S. Stucky, B.F. Chmelka, General predictive syntheses of cubic, hexagonal, and lamellar silica and titania mesostructured thin films, Chem. Mater. 14 (2002) 3284–3294. [10] N. Andersson, B. Kronberg, R. Corkery, P. Alberius, Combined emulsion and solvent evaporation (ESE) synthesis route to well-ordered mesoporous materials, Langmuir 23 (2007) 1459–1464. [11] K.E. Gonzales, J. Sjunua, M.J. Baraton, Synthesis and surface characterization of functionalized polyactide copolymer particles, Biomaterials 19 (1998) 1501–1505. [12] E. Lorenceau, A.S. Utada, D.R. Link, G. Cristobal, M. Joanicot, D.A. Weitz, Generation of polymerosomes from double-emulsions, Langmuir 21 (2005) 9183–9186.
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