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Equilibration in a geranyl acetate emulsion
Colloids and Surfaces A: Physicochem. Eng. Aspects 373 (2011) 110–115
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Equilibration in a geranyl acetate emulsion Ayat Bozeya a , Abeer Al-Bawab b,∗ , Stig E. Friberg c , Rong Guo c a
Hamdi Mango Center for Scientific Research (HMCSR), University of Jordan, Amman, Jordan Chemistry Department, University of Jordan, PO Box 13536, Amman 11942, Jordan c School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 12 August 2010 Received in revised form 19 October 2010 Accepted 21 October 2010 Available online 30 October 2010 Keywords: Emulsion Liquid crystal Geranyl acetate Non-ionic surfactant Equilibration
a b s t r a c t Water and a geranyl acetate solution of a non-ionic surfactant, a commercial C12 EO4 , were brought into contact in amounts to give a combination of a lamellar liquid crystal and an oil phase of equal weights at equilibrium and the equilibration transport between the layers was followed by measuring the change in layer heights with time. The initial reaction, lasting approximately two months, transferred surfactant from the oil phase to combine with water to form a birefringent layer initially containing excess water over the fraction in the liquid crystal in equilibrium with both water and oil phase. After this period the composition of the oil phase had reached a level corresponding to equilibrium with both water and a lamellar liquid crystal, while the birefringent phase, although a liquid crystal, still contained less water fraction than required for equilibrium. The final equilibration process of transferring the excess water to the formed liquid crystal was extremely slow with an estimated time to reach equilibrium of several years. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Equilibria in surfactant systems have attracted an increased interest among colloid scientists ever since Ekwall’s pioneering investigations [1], which were followed by a large number of investigations on the conditions for equilibrium in such systems [2–6]. However, a significant number of products and processes containing surfactants are not at equilibrium; emulsions being one of the most conspicuous examples [7,8]. Since emulsions are of prime importance in a wide variety of commercial products with the investigations into intermetallic emulsions as chief examples [9,10] and since the non-equilibrium conditions are vital both for their production and use, the scientific interest in the equilibration process per se has been significant from the early contributions by Neogi and Miller [11–16] to the more recent investigations [17,18]. Of more specific interest for the preparation of emulsions are the investigations into spontaneous emulsification [19–25], which process in some cases involves the complex non-equilibrium behavior during emulsion inversion [26–30]. The equilibration process has even been applied to create specific instrumentation to study phase equilibria [31,32].
∗ Corresponding author. Tel.: +962 796661601; fax: +9626 5300238. E-mail addresses: [email protected] (A. Bozeya), [email protected], [email protected] (A. Al-Bawab), [email protected] (S.E. Friberg), [email protected] (R. Guo). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.10.037
Although non-equilibrium stages are common in all facets of emulsion preparation and applications, the most general aspect of them is found in the evaporation stage and as such it has attracted a significant interest. While some of the investigations on evaporation [33–35] have been more focused on the physical chemistry of the product than the process per se, the progress in the clarification of the evaporation from two-phase emulsions has advanced rapidly from the early contribution by Blinov and Dobrynina [36], who investigated evaporation of emulsion drops in air to the efforts by the Hull group of Fletcher and Binks [37–41], who emerged as the undisputed leaders in the research on emulsion evaporation clarifying the evaporation rates from two-phase emulsions in an outstanding manner. In parallel, emulsion evaporation from a solid surface was investigated with the two-phase approach resulting in advanced knowledge of the fundamentals of the evaporation process of isolated one-phase drops on a solid surface [42–44]. In the investigations on evaporation of the bulk emulsion from a solid surface, the most primary effect determined was the electrostatic interaction [45] including the effect of reduced drop–drop interaction due to electrolyte screening [46,47]. The effect of the hydrophobicity of the surface was demonstrated to have an effect [48] in a marked tendency for oil drops to adhere to the hydrophobic surface rendering the emulsion unstable. A similar effect was detected in the more advanced experiments using interferometric surface force and noninterferometric MASIF determinations in a narrow gap between two surfaces [49] in which the proximity to a surface lead to macroscopic phase separation.
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During this development, emerging publications [39,50,51] revealed the fact that two-phase emulsions form additional phases during evaporation; disclosing more acute changes than the pure weight loss and prompting the use of phase diagrams combined with an algebraic system [52,53] to outline potential phase changes during evaporation. These investigations brought to light surprising consequences of initial O/W ratio [54] as well as of relative humidity on the process [55]. However, these evaporation examinations were concerned with equilibrium conditions and, in spite of excellent agreement with experiments on a selected emulsion [56], the undeniable fact that non-equilibrium factors are vital for the evaporation path prompted a series of studies by Chen et al. [57,58] to gain information as to which structures in the emulsion system caused most deviation from equilibrium. The initial contributions [57,58] were concerned with salicylic acid emulsions, for which the solid state phase was the overwhelmingly rate determining specimen. This was followed by an initial investigation [58] into the conditions in a tartaric acid emulsion with results revealing a much more complex relationship between phases and transport rates. These results were intriguing to a degree that the authors found a more general examination well justified into the equilibration interphase transport of individual compounds in emulsions. The greatest importance of emulsion evaporation is probably found within the area of fragrance compositions. The recent progress in the area has been significant taking into account the development in the enzymatic synthesis [59,60], the progress in the understanding of the evaporation and of vapor pressure of fragrance compounds from solutions [61–63] as well as of the emulsified formulations [64,65]. Considering the importance of the behavior of fragrance emulsions and the significant effect of the presence of liquid crystals on the process [66] the authors realized the benefit of an investigation of the transport from two liquid phases into a lamellar liquid crystal to be of interest. The system water, geranyl acetate and Brij 30 were chosen because of the wide application of the fragrance compound, the knowledge of its preparation per se [60] and the fact its phase diagram has been determined [67] and made available for the present investigation. 2. Experimental 2.1. Materials 98% Laureth-4 (Brij® 30) and 3,7-dimethyl-trans-2,6-octadien1-yl-acetate 98%, mixture of isomers (geranyl acetate) were obtained from Acros (Geel, Belgium) and were used without further purification. Water was deionized and distilled. 2.2. Instrumentation Weights were determined using a Mettler AJ150 Analytic Balance and YCW 04mWater bath was used to thermostate samples at 25 ◦ C, a LEICA DM EP microscope with samples between crossed polarizers was used for the liquid crystal microphotographs with a magnification of 10 × 10 (100) 2.3. Test tube calibration The 1 cm diameter test tube was weighed, closed and thermostated at 25 ◦ C overnight together with a supply of deionized water to ensure minimum deviation from correct volume values. The thermostated water was weighed into the duplicate test tubes in 1 g portions, the height was measured after each addition, the test tube was stored for 1 h and the height was measured again.
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Table 1 The weight and height of added water. Water weight (g)
Water height (cm)
1.0058 2.0113 3.0300 4.0435
2.1 4.0 6.0 7.9
Table 1 shows the weights of water added with the height after each addition. As a first approach, the numbers in Table 1 were used to obtain a first order function for volume versus column height with satisfactory R2 value of 0.9998. However, a calculation of individual points gave errors of several percent for small values of heights, errors that were in excess of the limit to measuring accuracy. Hence the numbers in Table 1 were instead fitted to a third order equation reducing the discrepancies to less than 1.5%. This is better than the measuring accuracy and the equations were used to relate volumes to heights. The deviation of water density from unity, 0.005 g/cm3 is of no concern since the results are given as fractions of total height. 2.4. Sample preparation and interlayer transport 1.4297 g of water was weighed into the calibrated test tube and a solution of 0.875 g surfactant in 1.700 g of geranyl acetate was slowly poured over the water in the tilted test tube, ensuring the oil solution to spread on the water layer forming a distinct interface. The test tube was thermostated at 25 ◦ C and the heights of the different layers were measured at regular intervals. 2.5. Samples for liquid crystal photos Five additional samples were prepared and samples from the birefringent layer were micro-photographed between crossed polarizers at 5 day intervals. 3. Fundamental background The investigations are based on the relevant parts of the phase diagram [67], Fig. 1, with equilibrium total composition chosen on the tie line between the liquid crystal and the oil phase consisting of a liquid crystal weight fraction of 0.5 referred to as 0.5LC. The relevant compositions in weight fractions (XW , XGA , XS ), in the diagram, Fig. 1, [67] are the three equilibrium phases with the liquid crystal, LC, as (0.715, 0.025, 0.260), the oil phase, Oi, as (0, 0.828, 0.172) and the water as (1, 0, 0). The amounts weighed in would give an equilibrium of equal weight fractions of the liquid crystal and oil phase with a total composition of (0.3575, 0.4265, 0.216). Experimentally, water and a composition along the oil axis (0, 0X 0 GA , XS ) were contacted in appropriate amounts and allowed to equilibrate. The initial composition of the oil phase was calculated using the equations for the concerned lines, Fig. 1. XGA =
Eq X GA (1 − XW ) (1−Eq XW )
(1)
in which the superscript Eq indicates equilibrium. XW = 0 gives 0X Eq GA XGA Eq (1− XW )
(2)
and Eq
Xs = (1 − Eq XGA )
in which superscript 0 indicates initial composition.
(3)
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Fig. 3. The volume fractions of the layers versus time, () Oil layer, () water layer, and (X) birefringent layer.
equal weights of the oil and liquid crystal; and its volume fractions are not the average of that of the two phases. 4. Results
Fig. 1. Relevant part of the phase diagram (adapted from [67] with permission). The system consists of a geranyl acetate surfactant solution in equilibrium with water and the water-rich end of the liquid crystal. The principle path of the oil phase after addition of water is marked with an arrow; the actual concentration changes are more complex and do not involve the complex phases along the arrow. (See text.)
Solving for an equilibrium composition of (0.3575, 0.4265, 0.216) gives the initial composition of the oil phase as (0, 0.6638, 0.3362). The phase diagram [67] is based on weight fractions, while the results of layer heights give volume fractions and the key weight fractions from the phase diagram are converted to volume fractions to enable comparison. The density of the different phases in the diagram, Fig. 1, was calculated assuming the compounds occupying a volume in the composition equal to the value in the pure compound giving c = 1/˙X /c in which c stands for compound. The densities are equal to 0.928 g/cm3 for the oil phase (0, 0.828, 0.172) and 1.005 g/cm3 for the liquid crystal giving volume fractions of the liquid crystal as (0.719, 0.028, 0.253), of the oil phase (0, 0.845, 0.155), and of the equilibrium composition as (0.3582. 0.4687, 0.2097). The equilibrium composition is a combination of
The results show two stages in the transfer of compounds between the layers. In the initiation of the process the geranyl acetate solution of surfactant encounters water and a birefringent layer is immediately formed between the two initial layers. In the continuation, the birefringent layer grows by addition of surfactant from the oil layer and water from the water layer, which compounds enter the birefringent layer to form the birefringent generating structure. Micrographs of samples from the birefringent layer between crossed polarizers show the typical pattern of a lamellar liquid crystal, Fig. 2. Obviously the surfactant from the oil layer reacted with water forming a liquid crystal dispersion, separating the two liquid layers. The pattern in the micrographs became better defined with time as exemplified by the difference between the features at 5 and 10 days, indicating the liquid crystal dispersion to be less disordered, an interpretation supported by later evaluation of the results from the variation with time of the layer dimensions, Fig. 3. The volume fractions versus time of the three layers are characterized by two stages; an initial period of 69 days, during which the volume of the oil and of the water was significantly reduced, accompanied by a corresponding increase of the volume of the birefringent layer. In the subsequent period of 69–125 days, no detectable change in the size of the oil layer and only minute alter-
Fig. 2. Microphotographs of the birefringent layer between crossed polarizers after 5 (a) and 10 days (b).
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Fig. 4. Schematic description of the transport, W: water, Oi: oil phase, S: oil phase surfactant, and B: birefringent layer.
ation of the two other layers were observed. In the present section of the article, the comments are restricted to the final time span, the more significant and complex changes during the initial period are analyzed in Section 5. Since the transport between layers is extremely small during the last stage, the primary interest is to compare the experimental values to numbers obtained at equilibrium. The equilibrium conditions are a weight ratio of liquid crystal to oil liquid of one, which corresponds to a volume fraction ratio 0.48/0.52. The volume fraction of oil from the results in Fig. 3, equal to 0.524, is in excellent agreement with the values from the phase diagram, indicating the transport of the surfactant from the oil layer to the birefringent layer to cease because of approaching equilibrium conditions, if the birefringent layer was a pure liquid crystal. The dimensions of the birefringent and water layers, conversely, were far from equilibrium and at this stage the relevant information is obtained from calculations of the composition of the oil phase as well as the birefringent layer. The calculations, neglecting the potential transfer of geranyl acetate from the oil phase, Fig. 1, are based on the initial oil layer weight fraction composition of (0, 0.664, 0.336), corresponding to volume fractions of (0, 0.691, 0.309). With a volume fraction of the total sample equaling 0.654, the volumes of geranyl acetate and surfactant are 0.452 and 0.202. The volume loss is 0.128 from the oil phase and, assuming exclusive loss of surfactant, the composition as volume fractions of the oil phase becomes (0, 0.859, 0.141). In comparison the phase diagram weight fraction values of the oil phase, in equilibrium with the liquid crystal and water, is (0, 0.828, 0.172), corresponding to a volume composition of (0, 0.845, 0.155). The values are within 2% and the oil phase composition appears close to the final three-phase equilibrium water/oil/liquid crystal. The water and birefringent layers are far from equilibrium, but an estimate of the water fraction in the birefringent layer is useful to judge the process. With the weight fraction surfactant 0.124 leaving the oil phase, and the weight fraction water contributed from the water phase 0.202, the water weight fraction in the birefringent layc is 0.62. This value is significantly less than 0.715 for the liquid crystal at equilibrium with both water and oil phases and reflects the fact that equilibrium has not been reached in this part of the system. The conditions and mechanisms of the transports during the initial stage of the process are complex and analyzed in Section 5.
Fig. 5. Transfer of surfactant.
place within the layer, monitored by the mutual diffusion of water and surfactant from the opposite ends of the layer. As illustrated by the arrows in the middle part of Fig. 4, the diffusion coefficient of the former is magnitudes greater than that of the latter, due to its dislocations [68], and the site of the formation of the liquid crystal elements is skewed towards the oil phase. After 69 days, the transfer of surfactant from the oil phase (Oi) ceases and the activities are limited to a slow relocation of water from its layer into the birefringent layer (B) as illustrated in Fig. 5. The fact that the transfer of surfactant from the oil phase ceases at this concentration is noteworthy. Obviously the equilibrium restraints become dominant and the relocation was limited to the slow process of increasing the water content of the formed liquid crystal (slow process). The authors are aware of the fact that there is a variation of sizes of both the polar group and the hydrocarbon chain of the surfactant used and that this condition could potentially skew the results, because the diverse molecules may display different rate of interfacial transport, not to mention the possibility of temporary structural modifications of the interfacial layers. However, very early unpublished investigations in this area revealed only insignificant such effects for hydrophobic surfactants of this kind and to the best judgment could not be relevant for the present results. The composition of the layer has now reached a ratio between water and surfactant, Fig. 6, that the layer may be described as a lamellar liquid crystal and the transfer mechanism is changed from adding water to the layer to create liquid crystal elements to increasing the water content of a lamellar liquid crystal; a much slower course of action. This interpretation is supported by the weight fraction of water in the material entering the layer, top curve, Fig. 6. The numbers for the first few days are scattered, a natural consequence of the small dimensions and the less well defined interfaces of the birefringent layer, but the general trend is without doubt.
5. Discussion An overview of the transfers is provided by the schematic diagram in Fig. 4. At the initial contact between the oil (Oi) layer and the water (W), a separate birefringent layer (B) is formed, which prevents the immediate contact between the water (W) layer and surfactant (S) departing the oil phase. Instead the formation of the lamellar liquid crystal elements, causing the birefringence, takes
Fig. 6. The weight fraction of water, counted as water/(water + surfactant) only, of the material entering the birefringent layer. () Water layer, and (X) oil layer.
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The initial total transfer to the birefringent layer contains a significant excess of water compared to the composition of the liquid crystal at equilibrium, indicating the layer to consist of an aqueous dispersion of lamellar liquid crystal elements. The curve shows a minimum of transferred water fraction at approximately 50 days, when the composition of the oil layer approaches that of equilibrium with both water and the lamellar liquid crystal, bottom curve, Fig. 6. Interestingly, after that period of time, the water content of the liquid crystal slowly increases and an estimate of the time for the liquid crystal to reach its equilibrium composition is in principle possible. Nevertheless, the accuracy of the numbers is not satisfactory, due to the difficulties of measuring the small dimensions of the initial layer and an equation for a linear line from day 27 to 125 gives a less than acceptable R2 value of 0.66. Added to the uncertainty due to the spread of the experimental values is the doubtful assumption of straight line change of the composition with time and the estimate of approximately 400 days to reach equilibrium must be viewed with the strictest of circumspection, but provides some illustration of the slowness of the final process.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Equilibration in a geranyl acetate emulsion Ayat Bozeya a , Abeer Al-Bawab b,∗ , Stig E. Friberg c , Rong Guo c a
Hamdi Mango Center for Scientific Research (HMCSR), University of Jordan, Amman, Jordan Chemistry Department, University of Jordan, PO Box 13536, Amman 11942, Jordan c School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 12 August 2010 Received in revised form 19 October 2010 Accepted 21 October 2010 Available online 30 October 2010 Keywords: Emulsion Liquid crystal Geranyl acetate Non-ionic surfactant Equilibration
a b s t r a c t Water and a geranyl acetate solution of a non-ionic surfactant, a commercial C12 EO4 , were brought into contact in amounts to give a combination of a lamellar liquid crystal and an oil phase of equal weights at equilibrium and the equilibration transport between the layers was followed by measuring the change in layer heights with time. The initial reaction, lasting approximately two months, transferred surfactant from the oil phase to combine with water to form a birefringent layer initially containing excess water over the fraction in the liquid crystal in equilibrium with both water and oil phase. After this period the composition of the oil phase had reached a level corresponding to equilibrium with both water and a lamellar liquid crystal, while the birefringent phase, although a liquid crystal, still contained less water fraction than required for equilibrium. The final equilibration process of transferring the excess water to the formed liquid crystal was extremely slow with an estimated time to reach equilibrium of several years. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Equilibria in surfactant systems have attracted an increased interest among colloid scientists ever since Ekwall’s pioneering investigations [1], which were followed by a large number of investigations on the conditions for equilibrium in such systems [2–6]. However, a significant number of products and processes containing surfactants are not at equilibrium; emulsions being one of the most conspicuous examples [7,8]. Since emulsions are of prime importance in a wide variety of commercial products with the investigations into intermetallic emulsions as chief examples [9,10] and since the non-equilibrium conditions are vital both for their production and use, the scientific interest in the equilibration process per se has been significant from the early contributions by Neogi and Miller [11–16] to the more recent investigations [17,18]. Of more specific interest for the preparation of emulsions are the investigations into spontaneous emulsification [19–25], which process in some cases involves the complex non-equilibrium behavior during emulsion inversion [26–30]. The equilibration process has even been applied to create specific instrumentation to study phase equilibria [31,32].
∗ Corresponding author. Tel.: +962 796661601; fax: +9626 5300238. E-mail addresses: [email protected] (A. Bozeya), [email protected], [email protected] (A. Al-Bawab), [email protected] (S.E. Friberg), [email protected] (R. Guo). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.10.037
Although non-equilibrium stages are common in all facets of emulsion preparation and applications, the most general aspect of them is found in the evaporation stage and as such it has attracted a significant interest. While some of the investigations on evaporation [33–35] have been more focused on the physical chemistry of the product than the process per se, the progress in the clarification of the evaporation from two-phase emulsions has advanced rapidly from the early contribution by Blinov and Dobrynina [36], who investigated evaporation of emulsion drops in air to the efforts by the Hull group of Fletcher and Binks [37–41], who emerged as the undisputed leaders in the research on emulsion evaporation clarifying the evaporation rates from two-phase emulsions in an outstanding manner. In parallel, emulsion evaporation from a solid surface was investigated with the two-phase approach resulting in advanced knowledge of the fundamentals of the evaporation process of isolated one-phase drops on a solid surface [42–44]. In the investigations on evaporation of the bulk emulsion from a solid surface, the most primary effect determined was the electrostatic interaction [45] including the effect of reduced drop–drop interaction due to electrolyte screening [46,47]. The effect of the hydrophobicity of the surface was demonstrated to have an effect [48] in a marked tendency for oil drops to adhere to the hydrophobic surface rendering the emulsion unstable. A similar effect was detected in the more advanced experiments using interferometric surface force and noninterferometric MASIF determinations in a narrow gap between two surfaces [49] in which the proximity to a surface lead to macroscopic phase separation.
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During this development, emerging publications [39,50,51] revealed the fact that two-phase emulsions form additional phases during evaporation; disclosing more acute changes than the pure weight loss and prompting the use of phase diagrams combined with an algebraic system [52,53] to outline potential phase changes during evaporation. These investigations brought to light surprising consequences of initial O/W ratio [54] as well as of relative humidity on the process [55]. However, these evaporation examinations were concerned with equilibrium conditions and, in spite of excellent agreement with experiments on a selected emulsion [56], the undeniable fact that non-equilibrium factors are vital for the evaporation path prompted a series of studies by Chen et al. [57,58] to gain information as to which structures in the emulsion system caused most deviation from equilibrium. The initial contributions [57,58] were concerned with salicylic acid emulsions, for which the solid state phase was the overwhelmingly rate determining specimen. This was followed by an initial investigation [58] into the conditions in a tartaric acid emulsion with results revealing a much more complex relationship between phases and transport rates. These results were intriguing to a degree that the authors found a more general examination well justified into the equilibration interphase transport of individual compounds in emulsions. The greatest importance of emulsion evaporation is probably found within the area of fragrance compositions. The recent progress in the area has been significant taking into account the development in the enzymatic synthesis [59,60], the progress in the understanding of the evaporation and of vapor pressure of fragrance compounds from solutions [61–63] as well as of the emulsified formulations [64,65]. Considering the importance of the behavior of fragrance emulsions and the significant effect of the presence of liquid crystals on the process [66] the authors realized the benefit of an investigation of the transport from two liquid phases into a lamellar liquid crystal to be of interest. The system water, geranyl acetate and Brij 30 were chosen because of the wide application of the fragrance compound, the knowledge of its preparation per se [60] and the fact its phase diagram has been determined [67] and made available for the present investigation. 2. Experimental 2.1. Materials 98% Laureth-4 (Brij® 30) and 3,7-dimethyl-trans-2,6-octadien1-yl-acetate 98%, mixture of isomers (geranyl acetate) were obtained from Acros (Geel, Belgium) and were used without further purification. Water was deionized and distilled. 2.2. Instrumentation Weights were determined using a Mettler AJ150 Analytic Balance and YCW 04mWater bath was used to thermostate samples at 25 ◦ C, a LEICA DM EP microscope with samples between crossed polarizers was used for the liquid crystal microphotographs with a magnification of 10 × 10 (100) 2.3. Test tube calibration The 1 cm diameter test tube was weighed, closed and thermostated at 25 ◦ C overnight together with a supply of deionized water to ensure minimum deviation from correct volume values. The thermostated water was weighed into the duplicate test tubes in 1 g portions, the height was measured after each addition, the test tube was stored for 1 h and the height was measured again.
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Table 1 The weight and height of added water. Water weight (g)
Water height (cm)
1.0058 2.0113 3.0300 4.0435
2.1 4.0 6.0 7.9
Table 1 shows the weights of water added with the height after each addition. As a first approach, the numbers in Table 1 were used to obtain a first order function for volume versus column height with satisfactory R2 value of 0.9998. However, a calculation of individual points gave errors of several percent for small values of heights, errors that were in excess of the limit to measuring accuracy. Hence the numbers in Table 1 were instead fitted to a third order equation reducing the discrepancies to less than 1.5%. This is better than the measuring accuracy and the equations were used to relate volumes to heights. The deviation of water density from unity, 0.005 g/cm3 is of no concern since the results are given as fractions of total height. 2.4. Sample preparation and interlayer transport 1.4297 g of water was weighed into the calibrated test tube and a solution of 0.875 g surfactant in 1.700 g of geranyl acetate was slowly poured over the water in the tilted test tube, ensuring the oil solution to spread on the water layer forming a distinct interface. The test tube was thermostated at 25 ◦ C and the heights of the different layers were measured at regular intervals. 2.5. Samples for liquid crystal photos Five additional samples were prepared and samples from the birefringent layer were micro-photographed between crossed polarizers at 5 day intervals. 3. Fundamental background The investigations are based on the relevant parts of the phase diagram [67], Fig. 1, with equilibrium total composition chosen on the tie line between the liquid crystal and the oil phase consisting of a liquid crystal weight fraction of 0.5 referred to as 0.5LC. The relevant compositions in weight fractions (XW , XGA , XS ), in the diagram, Fig. 1, [67] are the three equilibrium phases with the liquid crystal, LC, as (0.715, 0.025, 0.260), the oil phase, Oi, as (0, 0.828, 0.172) and the water as (1, 0, 0). The amounts weighed in would give an equilibrium of equal weight fractions of the liquid crystal and oil phase with a total composition of (0.3575, 0.4265, 0.216). Experimentally, water and a composition along the oil axis (0, 0X 0 GA , XS ) were contacted in appropriate amounts and allowed to equilibrate. The initial composition of the oil phase was calculated using the equations for the concerned lines, Fig. 1. XGA =
Eq X GA (1 − XW ) (1−Eq XW )
(1)
in which the superscript Eq indicates equilibrium. XW = 0 gives 0X Eq GA XGA Eq (1− XW )
(2)
and Eq
Xs = (1 − Eq XGA )
in which superscript 0 indicates initial composition.
(3)
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Fig. 3. The volume fractions of the layers versus time, () Oil layer, () water layer, and (X) birefringent layer.
equal weights of the oil and liquid crystal; and its volume fractions are not the average of that of the two phases. 4. Results
Fig. 1. Relevant part of the phase diagram (adapted from [67] with permission). The system consists of a geranyl acetate surfactant solution in equilibrium with water and the water-rich end of the liquid crystal. The principle path of the oil phase after addition of water is marked with an arrow; the actual concentration changes are more complex and do not involve the complex phases along the arrow. (See text.)
Solving for an equilibrium composition of (0.3575, 0.4265, 0.216) gives the initial composition of the oil phase as (0, 0.6638, 0.3362). The phase diagram [67] is based on weight fractions, while the results of layer heights give volume fractions and the key weight fractions from the phase diagram are converted to volume fractions to enable comparison. The density of the different phases in the diagram, Fig. 1, was calculated assuming the compounds occupying a volume in the composition equal to the value in the pure compound giving c = 1/˙X /c in which c stands for compound. The densities are equal to 0.928 g/cm3 for the oil phase (0, 0.828, 0.172) and 1.005 g/cm3 for the liquid crystal giving volume fractions of the liquid crystal as (0.719, 0.028, 0.253), of the oil phase (0, 0.845, 0.155), and of the equilibrium composition as (0.3582. 0.4687, 0.2097). The equilibrium composition is a combination of
The results show two stages in the transfer of compounds between the layers. In the initiation of the process the geranyl acetate solution of surfactant encounters water and a birefringent layer is immediately formed between the two initial layers. In the continuation, the birefringent layer grows by addition of surfactant from the oil layer and water from the water layer, which compounds enter the birefringent layer to form the birefringent generating structure. Micrographs of samples from the birefringent layer between crossed polarizers show the typical pattern of a lamellar liquid crystal, Fig. 2. Obviously the surfactant from the oil layer reacted with water forming a liquid crystal dispersion, separating the two liquid layers. The pattern in the micrographs became better defined with time as exemplified by the difference between the features at 5 and 10 days, indicating the liquid crystal dispersion to be less disordered, an interpretation supported by later evaluation of the results from the variation with time of the layer dimensions, Fig. 3. The volume fractions versus time of the three layers are characterized by two stages; an initial period of 69 days, during which the volume of the oil and of the water was significantly reduced, accompanied by a corresponding increase of the volume of the birefringent layer. In the subsequent period of 69–125 days, no detectable change in the size of the oil layer and only minute alter-
Fig. 2. Microphotographs of the birefringent layer between crossed polarizers after 5 (a) and 10 days (b).
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Fig. 4. Schematic description of the transport, W: water, Oi: oil phase, S: oil phase surfactant, and B: birefringent layer.
ation of the two other layers were observed. In the present section of the article, the comments are restricted to the final time span, the more significant and complex changes during the initial period are analyzed in Section 5. Since the transport between layers is extremely small during the last stage, the primary interest is to compare the experimental values to numbers obtained at equilibrium. The equilibrium conditions are a weight ratio of liquid crystal to oil liquid of one, which corresponds to a volume fraction ratio 0.48/0.52. The volume fraction of oil from the results in Fig. 3, equal to 0.524, is in excellent agreement with the values from the phase diagram, indicating the transport of the surfactant from the oil layer to the birefringent layer to cease because of approaching equilibrium conditions, if the birefringent layer was a pure liquid crystal. The dimensions of the birefringent and water layers, conversely, were far from equilibrium and at this stage the relevant information is obtained from calculations of the composition of the oil phase as well as the birefringent layer. The calculations, neglecting the potential transfer of geranyl acetate from the oil phase, Fig. 1, are based on the initial oil layer weight fraction composition of (0, 0.664, 0.336), corresponding to volume fractions of (0, 0.691, 0.309). With a volume fraction of the total sample equaling 0.654, the volumes of geranyl acetate and surfactant are 0.452 and 0.202. The volume loss is 0.128 from the oil phase and, assuming exclusive loss of surfactant, the composition as volume fractions of the oil phase becomes (0, 0.859, 0.141). In comparison the phase diagram weight fraction values of the oil phase, in equilibrium with the liquid crystal and water, is (0, 0.828, 0.172), corresponding to a volume composition of (0, 0.845, 0.155). The values are within 2% and the oil phase composition appears close to the final three-phase equilibrium water/oil/liquid crystal. The water and birefringent layers are far from equilibrium, but an estimate of the water fraction in the birefringent layer is useful to judge the process. With the weight fraction surfactant 0.124 leaving the oil phase, and the weight fraction water contributed from the water phase 0.202, the water weight fraction in the birefringent layc is 0.62. This value is significantly less than 0.715 for the liquid crystal at equilibrium with both water and oil phases and reflects the fact that equilibrium has not been reached in this part of the system. The conditions and mechanisms of the transports during the initial stage of the process are complex and analyzed in Section 5.
Fig. 5. Transfer of surfactant.
place within the layer, monitored by the mutual diffusion of water and surfactant from the opposite ends of the layer. As illustrated by the arrows in the middle part of Fig. 4, the diffusion coefficient of the former is magnitudes greater than that of the latter, due to its dislocations [68], and the site of the formation of the liquid crystal elements is skewed towards the oil phase. After 69 days, the transfer of surfactant from the oil phase (Oi) ceases and the activities are limited to a slow relocation of water from its layer into the birefringent layer (B) as illustrated in Fig. 5. The fact that the transfer of surfactant from the oil phase ceases at this concentration is noteworthy. Obviously the equilibrium restraints become dominant and the relocation was limited to the slow process of increasing the water content of the formed liquid crystal (slow process). The authors are aware of the fact that there is a variation of sizes of both the polar group and the hydrocarbon chain of the surfactant used and that this condition could potentially skew the results, because the diverse molecules may display different rate of interfacial transport, not to mention the possibility of temporary structural modifications of the interfacial layers. However, very early unpublished investigations in this area revealed only insignificant such effects for hydrophobic surfactants of this kind and to the best judgment could not be relevant for the present results. The composition of the layer has now reached a ratio between water and surfactant, Fig. 6, that the layer may be described as a lamellar liquid crystal and the transfer mechanism is changed from adding water to the layer to create liquid crystal elements to increasing the water content of a lamellar liquid crystal; a much slower course of action. This interpretation is supported by the weight fraction of water in the material entering the layer, top curve, Fig. 6. The numbers for the first few days are scattered, a natural consequence of the small dimensions and the less well defined interfaces of the birefringent layer, but the general trend is without doubt.
5. Discussion An overview of the transfers is provided by the schematic diagram in Fig. 4. At the initial contact between the oil (Oi) layer and the water (W), a separate birefringent layer (B) is formed, which prevents the immediate contact between the water (W) layer and surfactant (S) departing the oil phase. Instead the formation of the lamellar liquid crystal elements, causing the birefringence, takes
Fig. 6. The weight fraction of water, counted as water/(water + surfactant) only, of the material entering the birefringent layer. () Water layer, and (X) oil layer.
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The initial total transfer to the birefringent layer contains a significant excess of water compared to the composition of the liquid crystal at equilibrium, indicating the layer to consist of an aqueous dispersion of lamellar liquid crystal elements. The curve shows a minimum of transferred water fraction at approximately 50 days, when the composition of the oil layer approaches that of equilibrium with both water and the lamellar liquid crystal, bottom curve, Fig. 6. Interestingly, after that period of time, the water content of the liquid crystal slowly increases and an estimate of the time for the liquid crystal to reach its equilibrium composition is in principle possible. Nevertheless, the accuracy of the numbers is not satisfactory, due to the difficulties of measuring the small dimensions of the initial layer and an equation for a linear line from day 27 to 125 gives a less than acceptable R2 value of 0.66. Added to the uncertainty due to the spread of the experimental values is the doubtful assumption of straight line change of the composition with time and the estimate of approximately 400 days to reach equilibrium must be viewed with the strictest of circumspection, but provides some illustration of the slowness of the final process.
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