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Phase behavior of liquid–crystalline emulsion systems
Journal of Colloid and Interface Science 349 (2010) 554–559
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Phase behavior of liquid–crystalline emulsion systems Petra Kudla a, Tobias Sokolowski a,*, Bernhard Blümich b, Klaus-Peter Wittern a a b
Beiersdorf AG, 20245 Hamburg, Germany RWTH Aachen University, Institute of Technical and Macromolecular Chemistry, 52056 Aachen, Germany
a r t i c l e
i n f o
Article history: Received 9 March 2010 Accepted 26 May 2010 Available online 1 June 2010 Keywords: Phase diagram Surfactant Lyotropic liquid crystals Lamellar phases
a b s t r a c t The phase behavior of a mixture containing a surfactant, fatty alcohols and water has been analyzed. Depending on the amount of surfactant, i.e. N-(3-dimethylaminopropyl) octadecanamide, the emulsion-like system forms different microstructures. With increasing surfactant content the formulation evolves from a system with lyotropic lamellar phases to a system with crystal layer phases. 13C-CPMAS NMR studies carried out at varying surfactant levels showed significant differences in the behavior of the system. Using 2H and 13C-CPMAS NMR, X-ray scattering, DSC and polarization microscopy a phase diagram of this system could be derived. Additionally, ultrasonic velocity measurements showed that the ripening process of the emulsions can take up to 2 weeks and longer. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Lamellar systems can be formed if fatty alcohols, cationic surfactants and water are mixed [1,2]. These phases are classified as lyotropic liquid crystals (LLC), which combine the properties of crystals and the fluidity of isotropic liquids [3]. Typically, the investigated LLC systems in this paper consist of approximately 90% (w/w) water, 7% (w/w) fatty alcohols, 1–3% (w/w) active cationic surfactants and additives, like preservative agents and pH regulators. In the past, the phase behavior of LLC systems containing CTAC and CTAB has been studied extensively [4–11]. These surfactants constitute common components of emulsion systems. Therefore, the microstructures formed by CTAC and CTAB as well as the temperature dependences of these structures are of great interest [4]. LLCs can also be formed by other surfactant systems based on amides, e.g. N-(3-dimethylaminopropyl) octadecanamide. Up to now, no detailed studies of such systems have been published. Therefore, the phase behavior and occurring structures in LLC systems with N-(3-dimethylaminopropyl) octadecanamide are described in this paper. The stabilizing effect and consistency control of emulsifiers in emulsions is known from the gel-network theory by Eccleston [11]. It describes the swelling process of liquid emulsions to LLC network phases when the excess emulsifier interacts with the con-
Abbreviations: LLC, lyotropic liquid crystal; CTAC, N,N,N-trimethyl-1-hexadecanaminium chloride; CTAB, N,N,N-trimethyl-1-hexadecanaminium bromide. * Corresponding author. Fax: +49 40 4909 3855. E-mail address: [email protected] (T. Sokolowski). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.05.085
tinuous water phase. Friberg et al. [12–14] suggested a concentration of these multilayers at the oil–water droplet interface, which is accompanied by a rapid growth in emulsion stability because of the high viscosity of LLCs. They induce a reduction of attractive forces and a delay of coalescence. In order to gain a fundamental understanding of the occurring supramolecular structures, such as LLCs, several structure analysis methods, e.g. differential scanning calorimetry (DSC), polarization microscopy, X-ray scattering and NMR, are needed. For a basic thermodynamic understanding of the properties of the structures, however, the creation of a phase diagram is very helpful as it enables to correlate macroscopic behavior and intermolecular interactions. A simplified system has been used to create a phase diagram of the system at hand (Table 1). It consists of water, a mixture of two fatty alcohols, i.e. 1-hexadecanol, 1-octadecanol, and N(3-dimethylaminopropyl) octadecanamide as cationic surfactant. The phase diagram was derived by a combination of DSC, temperature-controlled polarization microscopy and X-ray scattering analyses. Previous work showed that NMR spectroscopy as a non-invasive method is a useful tool for analyzing liquid–crystalline mesophases [15,16]. Therefore, Cross Polarization (13C-CP) Magic Angle Spinning (MAS) measurements were accomplished to analyze the different LLCs in the phase diagram. 2H NMR has been used extensively to study the temperature dependence of the LLC phases [4,17]. In the current studies 2H NMR has been used to clarify the temperature influences on the LLC structures of a system containing N-(3-dimethylaminopropyl) octadecanamide. As the total amount of surfactant is too low to detect the quadrupolar splitting, deuterated octadecanol was used as a fatty alcohol component [4,18].
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P. Kudla et al. / Journal of Colloid and Interface Science 349 (2010) 554–559 Table 1 Basic formulation. Ingredients x[surfactant]
a
Water Fatty alcoholsb Surfactantc Additivesd a b c d
Amount (w/w-%) 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
89.3 9.5 0.0 1.2
89.3 8.2 1.3 1.2
89.3 7.0 2.5 1.2
89.3 5.9 3.6 1.2
89.3 4.9 4.6 1.2
89.3 3.9 5.6 1.2
89.3 3.0 6.5 1.2
89.3 2.2 7.3 1.2
89.3 1.4 8.1 1.2
89.3 0.7 8.8 1.2
89.3 0.0 9.5 1.2
Molar ratio of surfactant and fatty alcohols. 1-hexadecanol and 1-octadecanol (ratio 1:2). N-(3-dimethylaminopropyl) octadecanamide. pH regulators, e.g. sodium chloride and lactic acid, and preservative agents, like parabenes.
2. Materials and methods 2.1. Materials N-(3-Dimethylaminopropyl) octadecanamide (100%, Tm = 340 K) was purchased from Evonik Goldschmidt (Essen, Germany). 1-Hexadecanol (95%, Tm = 322 K) and 1-octadecanol (100%, Tm = 332 K) were purchased from Cognis (Düsseldorf, Germany). n-Octadecyl1,1-d2 alcohol (99.3%, Tm = 332 K) for the 2H NMR investigations was obtained from Dr. Ehrenstorfer (Augsburg, Germany). Deionized water was used. All ingredients were used as received.
[21] between 25 and 80 °C. Simulation of the 2H NMR spectra including relative concentrations and spectra processing was accomplished via Topspin software (Bruker BioSpin GmbH). 2.4. Polarization microscopy For visual investigations a temperature-controlled Leica DM4000M polarization microscope (Leica Microsystems GmbH, Wetzlar, Germany) with a 100 fold magnification was used. Temperature was controlled by a LTS350 Linkam heating stage (Linkam Scientific Instruments Ltd., Surrey, UK) with a closed constant tempered sample chamber.
2.2. Preparation of samples 2.5. Differential scanning calorimetry The concentrations of fatty alcohols and surfactant were specifically varied between 0 and 9.5% (w/w), whereby the sum was held constant at an amount of 9.5% (w/w). All analyses were carried out with a constant amount of water of approximately 90% (w/w) and a mixture of two fatty alcohols in a ratio of 1:2 (w/w). Table 1 gives an overview of the formulations used for the phase diagram studies in terms of the molar ratio of surfactant (x[surfactant]), which refers to the ratio of the variable ingredients, i.e. surfactant and fatty alcohols. All samples were prepared by melting the fatty alcohols and the surfactant in a water bath at 80–90 °C. Water was heated separately on a heating plate to the same temperature. The components were combined at about 80 °C in a Kitchen Aid agitator (Whirlpool Corporation, Benton Harbor, United States) and cooled down to room temperature afterwards. The cooling process was accompanied by sample homogenization at 55 °C using a rotor–stator homogenizer (Homozenta A, Zehnder AG, Zürich, Switzerland). 2.3. Nuclear magnetic resonance 13 C-CPMAS and 2H NMR measurements were used for the analyses, employing a Bruker AVII+ 500 spectrometer (B0 = 11.75 T) (Bruker BioSpin GmbH, Karlsruhe, Germany) with a B-VT 3000 temperature unit. Solid-state measurements were performed using a 4 mm MAS probe and ZrO2 rotors at 1.5 kHz rotation frequency. This low frequency constitutes a compromise between product stability and adequate line shapes. The chemical shifts for 13C NMR spectra were referenced according to Ishikawa’s definition, whereby the alkyl chain signal was set to 33 ppm [19]. The 13CCP pulse sequence included an increasing ramp for the 1H radio frequency field to optimize the Hartmann-Hahn condition with a contact time of 5 ms. For 1H-decoupling during acquisition a Two-Pulse Phase Modulation (TPPM) sequence was used [20]. All data was recorded with a repetition time of 5 s and a spectral width of 250 ppm. For 2H NMR studies a formulation with n-octadecyl-1,1-d2 alcohol as a fatty alcohol component was prepared [4]. The measurements were carried out with a 10 mm broadband tunable probe and a solid-echo pulse program with echo times of 10 and 20 ls
It was accomplished using a Mettler-Toledo DSC822e calorimeter (Mettler-Toledo GmbH, Giessen, Germany). Samples were analyzed in 40 ll aluminium pans relative to an empty aluminium pan. The temperature cycles were carried out between 5 and 110 °C at a heating rate of 10 K/min. Samples with x[surfactant] > 0.45 were only heated up to 80 °C to decrease evaporation because of the higher bulk water contents compared to lamellar systems. Every measurement included two heating and one cooling curve. In order to compare the different structure analyses methods the first heating curve is used for the studies. 2.6. X-ray scattering Temperature-dependent small angle (SAXS) and wide angle Xray scattering (WAXS) measurements were carried out at the Center for Applied Nanotechnology (CAN, Hamburg, Germany). A Seifert DRF-Cu0.3 measurement system (Rich. Seifert & Co. GmbH & Co. KG, Ahrensburg, Germany) with X-ray sensitive optical discs was used. The data was read by a BAS-180 II FujiFilm Scanner (FujiFilm Electronic Imaging Ltd., Hertfordshire, UK). The distance between sample and detector was determined via the known reflex position of collagen and was set to 11 cm. The studies were carried out at an X-ray wavelength of 0.1542 nm and temperatures of 25 or 55 °C. Measurements above room temperature were executed with a heatable sample holder with a Eurotherm control unit (Eurotherm GmbH, Limburg, Germany). Scattering data was averaged by the SXave-Software and fitted to theoretical scattering curves using Scatter-Software and the Levenberg–Marquardt-Algorithm [22]. A model of lamellar arrangements of homogeneous plates and a Gaussian distribution was used for the calculations. 2.7. Ultrasound A TF Instruments ResoScan™ measuring system (ResoScan system, TF Instruments GmbH, Heidelberg, Germany) with single-crystal lithium niobate ultrasonic transducers was used. Two independent cells with path lengths of 7 mm allowed a simulta-
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neous measurement of two samples. The fundamental wave has a wavelength of 14 mm which relates to a fundamental frequency of 107 kHz. Overtones of the 70th up to the 80th order were used for determination of the ultrasonic velocity, which results in a resonance frequency of 7–8.5 MHz. A temperature stability of ±0.001 °C was enabled by a metal block thermostat with a peltier element and heat exchange units. The measurement resolution of the ultrasonic velocity was 0.001 m/s. All samples were centrifuged before the measurements to minimize disturbance by air bubbles. All further structure analyses were carried out at least 2 weeks after sample preparation to ensure a fully completed ripening process. 3. Results and discussion 3.1. Phase diagram For a detailed understanding of the system at hand a phase diagram was created (Fig. 1). Therefore, the amounts of 1-hexadecanol and 1-octadecanol as well as N-(3-dimethylaminopropyl) octadecanamide were varied in steps of 0.5–1% between 0% and 9.5% in total at a constant water concentration of 90% (table 1). In order to characterize the occurring LLCs, a combination of different structure analysis methods had to be used. Measurements of the phase transitions via DSC (j) provided information about melting processes and changes in the LLCs. The exact structures were analyzed by temperature-controlled polarization microscopy. Additional information about the occurring liquid-crystalline phase structures was obtained by SAXS. Fig. 1 shows the phase diagram in terms of temperature and the molar ratio of the surfactant, x[surfactant], based on the differing mobility of the alkyl chains and their structures. Samples with a large amount of fatty alcohols (x[surfactant] = 0) show a biphasic behavior with a pure oil and water phase. The oil phase forms plate-like crystals of the fatty alcohols (SA). Slightly higher amounts of surfactant cause a phase transition to ordered lyotropic lamellar phases (La, x[surfactant] = 0.1–0.4). These ordered
lamellae can be identified by the observation of maltese crosses in polarization microscopy. A further increase in the surfactant leads to a pure surfactant crystal phase (SS, x[surfactant] = 0.5), which forms stable needle-like crystals with a high water storage capacity. Samples with a molar ratio between 0.4 and 0.5 show a mixed structure of La and SS. For mixtures with more than 7% of surfactant (x[surfactant] = 0.7) the bound water of the surfactant crystals is expelled and the system becomes biphasic with a pure water and a pure surfactant phase of needle-like crystals. For temperatures above 40 °C an increase in the mobility of the system can be discovered, which can be explained by the increasing mobility of the alkyl chains. A further temperature increase melts the system at about 60 °C and leads to an isotropic phase (I). Surfactant molar ratios of 0.1–0.3, on the other hand, show a remarkably stable and uniformly ordered lamellar phase up to high temperatures, which is a measure of the high order and complete integration of all components in this LLC. In polarization microscopy oily stripes indicate the melting process of the lamellae, which results in a larger extent of isotropic liquid (I) and a decrease in the lamellae distance from 30 to 21 nm, as seen by SAXS (Fig. 2, left). Furthermore, an increase in the relative displacement from the ideal lattice points of 30% could be discovered by SAXS at higher temperatures, which refers to an unsorted random lamellar system. A combination of WAXS and SAXS showed that for these samples a high degree of order exists at room temperature, which can be seen in a small angle scattering pattern with reflexes up to the fifth order and a simultaneous WAXS pattern for ordered systems (Fig. 2, right). Highly ordered structures can be characterized by a significant wide angle reflex at 0.42 nm, i.e. a scattering vector of 15 nm 1 [3]. The temperature dependence of the lamellar structures could also be observed in 2H NMR spectra. It is known from previous studies that a quadrupolar splitting is not detectable below a certain amount of surfactant and fatty alcohols [18]. Therefore, n-octadecyl-1,1-d2 alcohol was used as a fatty alcohol component in a sample with x[surfactant] = 0.175 [4]. Temperature-controlled investigations of this system showed a superposition of an isotropic (I)
Fig. 1. Phase diagram of a mixture of water, 1-hexadecanol, 1-octadecanol and N-(3-dimethylaminopropyl) octadecanamide. The concentration of fatty alcohols and surfactant is specifically varied between 0% and 9.5% by a total amount of 9.5%. The amount of water is held constant. x[surfactant] refers to the molar ratio of surfactant and fatty alcohols. Occurring structures are displayed as polarization microscopy images. j: Phase transition data derived from DSC analysis. Dashed lines are guidelines for the eyes, derived from visual investigation and polarization microscopy, I: isotropic phase, La: lamellar phase, SA: fatty alcohol crystals, Ss: layer phase of surfactant crystals.
P. Kudla et al. / Journal of Colloid and Interface Science 349 (2010) 554–559
557
Fig. 2. (left) SAXS measurements of a lyotropic lamellar phase with x[surfactant] = 0.1 at 25 and 55 °C. d: lamellae distance, rel. disp.: relative displacement from the ideal lattice points. (right) WAXS measurements of a lyotropic lamellar phase with x[surfactant] = 0.175 at 25 °C.
and a lamellar (La) resonance (Fig. 3, right). At 25 and 55 °C an ordered lamellar phase, La,ordered, which is indicated by a quadrupolar splitting of Q 30 kHz, is detectable. For higher temperatures the mobility of the system increases, which leads to a higher amount of isotropic liquid and unsorted lamellae, La,random, with a lower quadrupolar splitting [6]. This transition is accompanied by an increase in the isotropic amount of 36%. The inherently contained amount of HDO in H2O also leads to an isotropic signal in 2H NMR spectra. Additional studies with non-deuterated samples showed that the included HDO in the isotropic signal amounts to approximately 50%. The amount of lamellar phase in the system decreases from 82% of ordered lamellae to 10% of random lamellae between 25 and 80 °C. This melting process is accompanied by a structure rearrangement, which can be seen by a non-continuous change of the order parameters (Fig. 3, left). The biggest change can be discovered between 55 and 60 °C, where the transition from ordered to random lamellae takes place (Fig. 3, left). In this narrow temper-
ature region the quadrupolar splitting changes by 40%. The phase transition and the corresponding structure changes have also been detected by DSC. 4. Ripening process From previous studies on CTAC [13] and viscometric observations (data not shown here) it is known that these LLC systems change their structure in a ripening process directly after production. Repeating 13C-CPMAS measurements at certain times after production (x[surfactant] = 0.23) at constant conditions showed that this structure formation is accompanied by an increase in the solid contents (Fig. 4, left). According to the definition of Ishikawa [19] the signal of the all-trans alkyl chain in a lamellar system is set to 33 ppm. More flexible alkyl chains with an increased amount of gauche-conformations are characterized by NMR signals at lower chemical shifts and are not detected for our systems. Consequently, a structure formation of the fatty alcohol system (A) can be seen
Fig. 3. (left) Temperature controlled 2H NMR spectra of a lamellar lyotropic emulsion of n-octadecyl-1,1-d2 alcohol, N-(3-dimethylaminopropyl) octadecanamide and water, with x[surfactant] = 0.175. The NMR spectra are a superposition of an isotropic (I) and a lamellar (La) resonance. At 25 and 55 °C the resonance of the ordered lamellar phase (La,ordered) is characterized by a large quadrupolar splitting of Q 30 kHz. For random lamellar phases (La,random) at higher temperatures it is much lower [6]. The isotropic peaks are influenced by the inherently contained amounts of HDO in H2O. The given values express the pure isotropic phase without HDO. (right) Quadrupolar splitting, Q, as a function of temperature. Dashed line shows a sigmoidal fit with an inflexion point at the phase transition of La,ordered to La,random.
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P. Kudla et al. / Journal of Colloid and Interface Science 349 (2010) 554–559
Fig. 4. (left) Ripening process of a lamellar lyotropic emulsion with x[surfactant] = 0.23: measurements at 2 h (bottom), 1.25 days (middle) and 2.5 days (top) after production in terms of 13C-CPMAS NMR spectra. A: alkyl chain signal of the fatty alcohol in a lamellar phase [19]. Spinning side bands. (right) Ultrasonic velocity as a function of time after production of a lyotropic lamellar emulsion with x[surfactant] = 0.23 at 20 °C. The half-time value is 3.3 days.
directly after production (2 h). The measurement after 2.5 days shows a sixfold growth in terms of NMR signal intensity, which indicates an increased amount of solids, i.e. an increase in lamellae. This demonstrates an ordering process of the system during the first 2–3 days after production. Isothermal ultrasonic measurements at 20 °C, which are more sensitive to structure changes in the water phase, point out that the whole structure formation takes up to at least 2 weeks after production (Fig. 4, right). This can be explained by a slow water bonding process in the lamellae, which can be verified by an exponential growth in the ultrasonic velocity. The formation of the lamellae, which is also detectable by NMR, might already be completed after 2.5 days. Thus the ripened system does not form to a reasonable extent before a certain time of rest, i.e. 3 days, after production. This was found to relate to the amount of shear energy applied to the system during the production process. 5. Conclusion With combined analysis by X-ray scattering, NMR, polarization microscopy, DSC and ultrasound the phase behavior of N-(3dimethylaminopropyl) octadecanamide LLC systems can be understood in depth. A phase diagram was derived, which provides essential information about these systems. The importance of phase diagrams for the analyses of the properties and phase behavior of emulsions is well known [23–25]. Up to now, this has only been presented for other surfactant systems [26–29]. The results of this work allow a systematic adjustment of the desired microstructures of N-(3-dimethylaminopropyl) octadecanamide. Additionally, the determined phase diagram serves as a basis for further studies. Especially the analysis of the influence of other components, such as oils and acids, on the particular LLCs is of interest. A comparison of the size of the lamellar regions in the phase diagrams of formulations with different components allows to assess their stabilities. According to the gel-network theory by Eccleston [11] the formation of lamellar LLCs by N-(3-dimethylaminopropyl) octadecanamide can be understood to stabilize the system. Our results of the ripening behavior clarify that this stabilizing effect and the corresponding built-up of lamellar multilayers with an oil–water interface is a slow process for this emulsifier. Furthermore, 13CCPMAS NMR, 2H NMR, and X-ray scattering investigations of the system indicate a high sensitivity to temperature, which has not
been observed to this extent for other surfactants [4,27]. An increase in this parameter leads to smaller lamellae distances, as could be seen by SAXS. Our results constitute essential information for further studies by giving a detailed description of systems containing N-(3dimethylaminopropyl) octadecanamide. Therefore, they close the gap to previous studies of other surfactants [26–29]. Acknowledgments The authors thank Dr. Sandra Saladin and Dr. Detlef Emeis for their help in realizing this work, as well as Frank Hetzel for performing and discussing the DSC measurements. Furthermore we thank Dr. Carsten Schellbach and Dr. Andreas Frömsdorf from the Center for Applied Nanotechnology (CAN, Hamburg, Germany) for performing and explaining the X-ray measurements, as well as Ralf Ihrig from tesa AG (tesa, Hamburg, Germany) for his help with the polarization microscopy measurements and the possibility to use the equipment. References [1] W. Umbach, Kosmetik und Hygiene, Wiley-VCH, Weinheim, 2004. [2] A.J. O’Lenick Jr, Surfactants: Strategic Personal Care Ingredients, Allured Publishing Corporation, Carol Stream, 2005. [3] H. Stegemeyer, Lyotrope Flüssigkristalle, Steinkopff, Darmstadt, 1999. [4] T.M. Alam, S.K. McIntyre, Langmuir 24 (2008) 13890. [5] G.M. Eccleston, M.K. Behan-Martin, G.R. Jones, E. Towns-Andrews, Int. J. Pharm. 203 (2000) 127. [6] A. Rapp, K. Ermolaev, B.M. Fung, J. Phys. Chem. B 103 (1999) 1705. [7] C.-Y. Cheng, L.-P. Hwang, J. Chin. Chem. Soc. 48 (2001) 953. [8] R. Krishnaswamy, S.K. Ghosh, S. Lakshmanan, V.A. Raghunathan, A.K. Sood, Langmuir 21 (2005) 10439. [9] P.M. Macdonald, V. Strashko, Langmuir 14 (1998) 4758. [10] J.S. Clawson, G.P. Holland, T.M. Alam, Phys. Chem. Chem. Phys. 8 (2006) 2635. [11] G.M. Eccleston, Colloids Surf. A 123–124 (1997) 169. [12] S. Friberg, L. Mandell, M. Larsson, J. Colloid Interface Sci. 20 (1969) 155. [13] S. Friberg, K. Larsson, in: G.M. Brown (Ed.), Liquid Crystals, vol. 2, Academic Press, New York, 1976, p. 173. [14] S. Friberg, K. Madani, Colloid Polym. Sci. 65 (1978) 164. [15] C. Alberola, B. Blümich, D. Emeis, K.-P. Wittern, Colloids Surf. A 290 (2006) 247. [16] J. Plass, D. Emeis, in: J. Fraissard, O. Lapina (Eds.), Magnetic Resonance in Colloid and Interface Science, Kluwer Academic Publishers, The Netherlands, 2002, p. 375. [17] C. Stubenrauch, C. Frank, R. Strey, D. Burgemeister, C. Schmidt, Langmuir 18 (2002) 5027. [18] C. Stubenrauch, S. Burauer, R. Strey, C. Schmidt, Liq. Cryst. 31 (2004) 39. [19] S.A. Ishikawa, J. Mol. Struct. 271 (1992) 57. [20] A.E. Bennett, C.M. Rienstra, M. Auger, K.V. Lakshmi, R.G. Griffin, J. Chem. Phys. 103 (1995) 6951.
P. Kudla et al. / Journal of Colloid and Interface Science 349 (2010) 554–559 [21] J.G. Powels, J.H. Strange, Proc. Phys. Soc. London 82 (1963) 6. [22] S. Förster, A. Timmann, M. Konrad, C. Schellbach, A. Meyer, S.S. Funari, P. Mulvaney, R. Knott, J. Phys. Chem. B 109 (2005) 1347. [23] S. Friberg, in: J. Sjöblom (Ed.), Encyclopedic Handbook of Emulsion Technology, Dekker, New York, 2001, p. 47. [24] S. Friberg, L. Mandell, J. Am. Oil Chem. Soc. 47 (1970) 149.
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[25] Z. Zhang, T. Denler, S. Friberg, P.A. Aikens, Int. J. Cosmet. Sci. 22 (2000) 105. [26] H. Kunieda, K. Shinoda, J. Dispersion Sci. Technol. 3 (1982) 233. [27] S. Friberg, A. Al-Bawab, J.L. Barber, P.A. Aikens, J. Dispersion Sci. Technol. 19 (1998) 399. [28] S. Friberg, Langmuir 8 (1992) 1889. [29] S. Friberg, Adv. Colloid Interface Sci. 32 (1990) 167.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Phase behavior of liquid–crystalline emulsion systems Petra Kudla a, Tobias Sokolowski a,*, Bernhard Blümich b, Klaus-Peter Wittern a a b
Beiersdorf AG, 20245 Hamburg, Germany RWTH Aachen University, Institute of Technical and Macromolecular Chemistry, 52056 Aachen, Germany
a r t i c l e
i n f o
Article history: Received 9 March 2010 Accepted 26 May 2010 Available online 1 June 2010 Keywords: Phase diagram Surfactant Lyotropic liquid crystals Lamellar phases
a b s t r a c t The phase behavior of a mixture containing a surfactant, fatty alcohols and water has been analyzed. Depending on the amount of surfactant, i.e. N-(3-dimethylaminopropyl) octadecanamide, the emulsion-like system forms different microstructures. With increasing surfactant content the formulation evolves from a system with lyotropic lamellar phases to a system with crystal layer phases. 13C-CPMAS NMR studies carried out at varying surfactant levels showed significant differences in the behavior of the system. Using 2H and 13C-CPMAS NMR, X-ray scattering, DSC and polarization microscopy a phase diagram of this system could be derived. Additionally, ultrasonic velocity measurements showed that the ripening process of the emulsions can take up to 2 weeks and longer. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Lamellar systems can be formed if fatty alcohols, cationic surfactants and water are mixed [1,2]. These phases are classified as lyotropic liquid crystals (LLC), which combine the properties of crystals and the fluidity of isotropic liquids [3]. Typically, the investigated LLC systems in this paper consist of approximately 90% (w/w) water, 7% (w/w) fatty alcohols, 1–3% (w/w) active cationic surfactants and additives, like preservative agents and pH regulators. In the past, the phase behavior of LLC systems containing CTAC and CTAB has been studied extensively [4–11]. These surfactants constitute common components of emulsion systems. Therefore, the microstructures formed by CTAC and CTAB as well as the temperature dependences of these structures are of great interest [4]. LLCs can also be formed by other surfactant systems based on amides, e.g. N-(3-dimethylaminopropyl) octadecanamide. Up to now, no detailed studies of such systems have been published. Therefore, the phase behavior and occurring structures in LLC systems with N-(3-dimethylaminopropyl) octadecanamide are described in this paper. The stabilizing effect and consistency control of emulsifiers in emulsions is known from the gel-network theory by Eccleston [11]. It describes the swelling process of liquid emulsions to LLC network phases when the excess emulsifier interacts with the con-
Abbreviations: LLC, lyotropic liquid crystal; CTAC, N,N,N-trimethyl-1-hexadecanaminium chloride; CTAB, N,N,N-trimethyl-1-hexadecanaminium bromide. * Corresponding author. Fax: +49 40 4909 3855. E-mail address: [email protected] (T. Sokolowski). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.05.085
tinuous water phase. Friberg et al. [12–14] suggested a concentration of these multilayers at the oil–water droplet interface, which is accompanied by a rapid growth in emulsion stability because of the high viscosity of LLCs. They induce a reduction of attractive forces and a delay of coalescence. In order to gain a fundamental understanding of the occurring supramolecular structures, such as LLCs, several structure analysis methods, e.g. differential scanning calorimetry (DSC), polarization microscopy, X-ray scattering and NMR, are needed. For a basic thermodynamic understanding of the properties of the structures, however, the creation of a phase diagram is very helpful as it enables to correlate macroscopic behavior and intermolecular interactions. A simplified system has been used to create a phase diagram of the system at hand (Table 1). It consists of water, a mixture of two fatty alcohols, i.e. 1-hexadecanol, 1-octadecanol, and N(3-dimethylaminopropyl) octadecanamide as cationic surfactant. The phase diagram was derived by a combination of DSC, temperature-controlled polarization microscopy and X-ray scattering analyses. Previous work showed that NMR spectroscopy as a non-invasive method is a useful tool for analyzing liquid–crystalline mesophases [15,16]. Therefore, Cross Polarization (13C-CP) Magic Angle Spinning (MAS) measurements were accomplished to analyze the different LLCs in the phase diagram. 2H NMR has been used extensively to study the temperature dependence of the LLC phases [4,17]. In the current studies 2H NMR has been used to clarify the temperature influences on the LLC structures of a system containing N-(3-dimethylaminopropyl) octadecanamide. As the total amount of surfactant is too low to detect the quadrupolar splitting, deuterated octadecanol was used as a fatty alcohol component [4,18].
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P. Kudla et al. / Journal of Colloid and Interface Science 349 (2010) 554–559 Table 1 Basic formulation. Ingredients x[surfactant]
a
Water Fatty alcoholsb Surfactantc Additivesd a b c d
Amount (w/w-%) 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
89.3 9.5 0.0 1.2
89.3 8.2 1.3 1.2
89.3 7.0 2.5 1.2
89.3 5.9 3.6 1.2
89.3 4.9 4.6 1.2
89.3 3.9 5.6 1.2
89.3 3.0 6.5 1.2
89.3 2.2 7.3 1.2
89.3 1.4 8.1 1.2
89.3 0.7 8.8 1.2
89.3 0.0 9.5 1.2
Molar ratio of surfactant and fatty alcohols. 1-hexadecanol and 1-octadecanol (ratio 1:2). N-(3-dimethylaminopropyl) octadecanamide. pH regulators, e.g. sodium chloride and lactic acid, and preservative agents, like parabenes.
2. Materials and methods 2.1. Materials N-(3-Dimethylaminopropyl) octadecanamide (100%, Tm = 340 K) was purchased from Evonik Goldschmidt (Essen, Germany). 1-Hexadecanol (95%, Tm = 322 K) and 1-octadecanol (100%, Tm = 332 K) were purchased from Cognis (Düsseldorf, Germany). n-Octadecyl1,1-d2 alcohol (99.3%, Tm = 332 K) for the 2H NMR investigations was obtained from Dr. Ehrenstorfer (Augsburg, Germany). Deionized water was used. All ingredients were used as received.
[21] between 25 and 80 °C. Simulation of the 2H NMR spectra including relative concentrations and spectra processing was accomplished via Topspin software (Bruker BioSpin GmbH). 2.4. Polarization microscopy For visual investigations a temperature-controlled Leica DM4000M polarization microscope (Leica Microsystems GmbH, Wetzlar, Germany) with a 100 fold magnification was used. Temperature was controlled by a LTS350 Linkam heating stage (Linkam Scientific Instruments Ltd., Surrey, UK) with a closed constant tempered sample chamber.
2.2. Preparation of samples 2.5. Differential scanning calorimetry The concentrations of fatty alcohols and surfactant were specifically varied between 0 and 9.5% (w/w), whereby the sum was held constant at an amount of 9.5% (w/w). All analyses were carried out with a constant amount of water of approximately 90% (w/w) and a mixture of two fatty alcohols in a ratio of 1:2 (w/w). Table 1 gives an overview of the formulations used for the phase diagram studies in terms of the molar ratio of surfactant (x[surfactant]), which refers to the ratio of the variable ingredients, i.e. surfactant and fatty alcohols. All samples were prepared by melting the fatty alcohols and the surfactant in a water bath at 80–90 °C. Water was heated separately on a heating plate to the same temperature. The components were combined at about 80 °C in a Kitchen Aid agitator (Whirlpool Corporation, Benton Harbor, United States) and cooled down to room temperature afterwards. The cooling process was accompanied by sample homogenization at 55 °C using a rotor–stator homogenizer (Homozenta A, Zehnder AG, Zürich, Switzerland). 2.3. Nuclear magnetic resonance 13 C-CPMAS and 2H NMR measurements were used for the analyses, employing a Bruker AVII+ 500 spectrometer (B0 = 11.75 T) (Bruker BioSpin GmbH, Karlsruhe, Germany) with a B-VT 3000 temperature unit. Solid-state measurements were performed using a 4 mm MAS probe and ZrO2 rotors at 1.5 kHz rotation frequency. This low frequency constitutes a compromise between product stability and adequate line shapes. The chemical shifts for 13C NMR spectra were referenced according to Ishikawa’s definition, whereby the alkyl chain signal was set to 33 ppm [19]. The 13CCP pulse sequence included an increasing ramp for the 1H radio frequency field to optimize the Hartmann-Hahn condition with a contact time of 5 ms. For 1H-decoupling during acquisition a Two-Pulse Phase Modulation (TPPM) sequence was used [20]. All data was recorded with a repetition time of 5 s and a spectral width of 250 ppm. For 2H NMR studies a formulation with n-octadecyl-1,1-d2 alcohol as a fatty alcohol component was prepared [4]. The measurements were carried out with a 10 mm broadband tunable probe and a solid-echo pulse program with echo times of 10 and 20 ls
It was accomplished using a Mettler-Toledo DSC822e calorimeter (Mettler-Toledo GmbH, Giessen, Germany). Samples were analyzed in 40 ll aluminium pans relative to an empty aluminium pan. The temperature cycles were carried out between 5 and 110 °C at a heating rate of 10 K/min. Samples with x[surfactant] > 0.45 were only heated up to 80 °C to decrease evaporation because of the higher bulk water contents compared to lamellar systems. Every measurement included two heating and one cooling curve. In order to compare the different structure analyses methods the first heating curve is used for the studies. 2.6. X-ray scattering Temperature-dependent small angle (SAXS) and wide angle Xray scattering (WAXS) measurements were carried out at the Center for Applied Nanotechnology (CAN, Hamburg, Germany). A Seifert DRF-Cu0.3 measurement system (Rich. Seifert & Co. GmbH & Co. KG, Ahrensburg, Germany) with X-ray sensitive optical discs was used. The data was read by a BAS-180 II FujiFilm Scanner (FujiFilm Electronic Imaging Ltd., Hertfordshire, UK). The distance between sample and detector was determined via the known reflex position of collagen and was set to 11 cm. The studies were carried out at an X-ray wavelength of 0.1542 nm and temperatures of 25 or 55 °C. Measurements above room temperature were executed with a heatable sample holder with a Eurotherm control unit (Eurotherm GmbH, Limburg, Germany). Scattering data was averaged by the SXave-Software and fitted to theoretical scattering curves using Scatter-Software and the Levenberg–Marquardt-Algorithm [22]. A model of lamellar arrangements of homogeneous plates and a Gaussian distribution was used for the calculations. 2.7. Ultrasound A TF Instruments ResoScan™ measuring system (ResoScan system, TF Instruments GmbH, Heidelberg, Germany) with single-crystal lithium niobate ultrasonic transducers was used. Two independent cells with path lengths of 7 mm allowed a simulta-
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neous measurement of two samples. The fundamental wave has a wavelength of 14 mm which relates to a fundamental frequency of 107 kHz. Overtones of the 70th up to the 80th order were used for determination of the ultrasonic velocity, which results in a resonance frequency of 7–8.5 MHz. A temperature stability of ±0.001 °C was enabled by a metal block thermostat with a peltier element and heat exchange units. The measurement resolution of the ultrasonic velocity was 0.001 m/s. All samples were centrifuged before the measurements to minimize disturbance by air bubbles. All further structure analyses were carried out at least 2 weeks after sample preparation to ensure a fully completed ripening process. 3. Results and discussion 3.1. Phase diagram For a detailed understanding of the system at hand a phase diagram was created (Fig. 1). Therefore, the amounts of 1-hexadecanol and 1-octadecanol as well as N-(3-dimethylaminopropyl) octadecanamide were varied in steps of 0.5–1% between 0% and 9.5% in total at a constant water concentration of 90% (table 1). In order to characterize the occurring LLCs, a combination of different structure analysis methods had to be used. Measurements of the phase transitions via DSC (j) provided information about melting processes and changes in the LLCs. The exact structures were analyzed by temperature-controlled polarization microscopy. Additional information about the occurring liquid-crystalline phase structures was obtained by SAXS. Fig. 1 shows the phase diagram in terms of temperature and the molar ratio of the surfactant, x[surfactant], based on the differing mobility of the alkyl chains and their structures. Samples with a large amount of fatty alcohols (x[surfactant] = 0) show a biphasic behavior with a pure oil and water phase. The oil phase forms plate-like crystals of the fatty alcohols (SA). Slightly higher amounts of surfactant cause a phase transition to ordered lyotropic lamellar phases (La, x[surfactant] = 0.1–0.4). These ordered
lamellae can be identified by the observation of maltese crosses in polarization microscopy. A further increase in the surfactant leads to a pure surfactant crystal phase (SS, x[surfactant] = 0.5), which forms stable needle-like crystals with a high water storage capacity. Samples with a molar ratio between 0.4 and 0.5 show a mixed structure of La and SS. For mixtures with more than 7% of surfactant (x[surfactant] = 0.7) the bound water of the surfactant crystals is expelled and the system becomes biphasic with a pure water and a pure surfactant phase of needle-like crystals. For temperatures above 40 °C an increase in the mobility of the system can be discovered, which can be explained by the increasing mobility of the alkyl chains. A further temperature increase melts the system at about 60 °C and leads to an isotropic phase (I). Surfactant molar ratios of 0.1–0.3, on the other hand, show a remarkably stable and uniformly ordered lamellar phase up to high temperatures, which is a measure of the high order and complete integration of all components in this LLC. In polarization microscopy oily stripes indicate the melting process of the lamellae, which results in a larger extent of isotropic liquid (I) and a decrease in the lamellae distance from 30 to 21 nm, as seen by SAXS (Fig. 2, left). Furthermore, an increase in the relative displacement from the ideal lattice points of 30% could be discovered by SAXS at higher temperatures, which refers to an unsorted random lamellar system. A combination of WAXS and SAXS showed that for these samples a high degree of order exists at room temperature, which can be seen in a small angle scattering pattern with reflexes up to the fifth order and a simultaneous WAXS pattern for ordered systems (Fig. 2, right). Highly ordered structures can be characterized by a significant wide angle reflex at 0.42 nm, i.e. a scattering vector of 15 nm 1 [3]. The temperature dependence of the lamellar structures could also be observed in 2H NMR spectra. It is known from previous studies that a quadrupolar splitting is not detectable below a certain amount of surfactant and fatty alcohols [18]. Therefore, n-octadecyl-1,1-d2 alcohol was used as a fatty alcohol component in a sample with x[surfactant] = 0.175 [4]. Temperature-controlled investigations of this system showed a superposition of an isotropic (I)
Fig. 1. Phase diagram of a mixture of water, 1-hexadecanol, 1-octadecanol and N-(3-dimethylaminopropyl) octadecanamide. The concentration of fatty alcohols and surfactant is specifically varied between 0% and 9.5% by a total amount of 9.5%. The amount of water is held constant. x[surfactant] refers to the molar ratio of surfactant and fatty alcohols. Occurring structures are displayed as polarization microscopy images. j: Phase transition data derived from DSC analysis. Dashed lines are guidelines for the eyes, derived from visual investigation and polarization microscopy, I: isotropic phase, La: lamellar phase, SA: fatty alcohol crystals, Ss: layer phase of surfactant crystals.
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Fig. 2. (left) SAXS measurements of a lyotropic lamellar phase with x[surfactant] = 0.1 at 25 and 55 °C. d: lamellae distance, rel. disp.: relative displacement from the ideal lattice points. (right) WAXS measurements of a lyotropic lamellar phase with x[surfactant] = 0.175 at 25 °C.
and a lamellar (La) resonance (Fig. 3, right). At 25 and 55 °C an ordered lamellar phase, La,ordered, which is indicated by a quadrupolar splitting of Q 30 kHz, is detectable. For higher temperatures the mobility of the system increases, which leads to a higher amount of isotropic liquid and unsorted lamellae, La,random, with a lower quadrupolar splitting [6]. This transition is accompanied by an increase in the isotropic amount of 36%. The inherently contained amount of HDO in H2O also leads to an isotropic signal in 2H NMR spectra. Additional studies with non-deuterated samples showed that the included HDO in the isotropic signal amounts to approximately 50%. The amount of lamellar phase in the system decreases from 82% of ordered lamellae to 10% of random lamellae between 25 and 80 °C. This melting process is accompanied by a structure rearrangement, which can be seen by a non-continuous change of the order parameters (Fig. 3, left). The biggest change can be discovered between 55 and 60 °C, where the transition from ordered to random lamellae takes place (Fig. 3, left). In this narrow temper-
ature region the quadrupolar splitting changes by 40%. The phase transition and the corresponding structure changes have also been detected by DSC. 4. Ripening process From previous studies on CTAC [13] and viscometric observations (data not shown here) it is known that these LLC systems change their structure in a ripening process directly after production. Repeating 13C-CPMAS measurements at certain times after production (x[surfactant] = 0.23) at constant conditions showed that this structure formation is accompanied by an increase in the solid contents (Fig. 4, left). According to the definition of Ishikawa [19] the signal of the all-trans alkyl chain in a lamellar system is set to 33 ppm. More flexible alkyl chains with an increased amount of gauche-conformations are characterized by NMR signals at lower chemical shifts and are not detected for our systems. Consequently, a structure formation of the fatty alcohol system (A) can be seen
Fig. 3. (left) Temperature controlled 2H NMR spectra of a lamellar lyotropic emulsion of n-octadecyl-1,1-d2 alcohol, N-(3-dimethylaminopropyl) octadecanamide and water, with x[surfactant] = 0.175. The NMR spectra are a superposition of an isotropic (I) and a lamellar (La) resonance. At 25 and 55 °C the resonance of the ordered lamellar phase (La,ordered) is characterized by a large quadrupolar splitting of Q 30 kHz. For random lamellar phases (La,random) at higher temperatures it is much lower [6]. The isotropic peaks are influenced by the inherently contained amounts of HDO in H2O. The given values express the pure isotropic phase without HDO. (right) Quadrupolar splitting, Q, as a function of temperature. Dashed line shows a sigmoidal fit with an inflexion point at the phase transition of La,ordered to La,random.
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Fig. 4. (left) Ripening process of a lamellar lyotropic emulsion with x[surfactant] = 0.23: measurements at 2 h (bottom), 1.25 days (middle) and 2.5 days (top) after production in terms of 13C-CPMAS NMR spectra. A: alkyl chain signal of the fatty alcohol in a lamellar phase [19]. Spinning side bands. (right) Ultrasonic velocity as a function of time after production of a lyotropic lamellar emulsion with x[surfactant] = 0.23 at 20 °C. The half-time value is 3.3 days.
directly after production (2 h). The measurement after 2.5 days shows a sixfold growth in terms of NMR signal intensity, which indicates an increased amount of solids, i.e. an increase in lamellae. This demonstrates an ordering process of the system during the first 2–3 days after production. Isothermal ultrasonic measurements at 20 °C, which are more sensitive to structure changes in the water phase, point out that the whole structure formation takes up to at least 2 weeks after production (Fig. 4, right). This can be explained by a slow water bonding process in the lamellae, which can be verified by an exponential growth in the ultrasonic velocity. The formation of the lamellae, which is also detectable by NMR, might already be completed after 2.5 days. Thus the ripened system does not form to a reasonable extent before a certain time of rest, i.e. 3 days, after production. This was found to relate to the amount of shear energy applied to the system during the production process. 5. Conclusion With combined analysis by X-ray scattering, NMR, polarization microscopy, DSC and ultrasound the phase behavior of N-(3dimethylaminopropyl) octadecanamide LLC systems can be understood in depth. A phase diagram was derived, which provides essential information about these systems. The importance of phase diagrams for the analyses of the properties and phase behavior of emulsions is well known [23–25]. Up to now, this has only been presented for other surfactant systems [26–29]. The results of this work allow a systematic adjustment of the desired microstructures of N-(3-dimethylaminopropyl) octadecanamide. Additionally, the determined phase diagram serves as a basis for further studies. Especially the analysis of the influence of other components, such as oils and acids, on the particular LLCs is of interest. A comparison of the size of the lamellar regions in the phase diagrams of formulations with different components allows to assess their stabilities. According to the gel-network theory by Eccleston [11] the formation of lamellar LLCs by N-(3-dimethylaminopropyl) octadecanamide can be understood to stabilize the system. Our results of the ripening behavior clarify that this stabilizing effect and the corresponding built-up of lamellar multilayers with an oil–water interface is a slow process for this emulsifier. Furthermore, 13CCPMAS NMR, 2H NMR, and X-ray scattering investigations of the system indicate a high sensitivity to temperature, which has not
been observed to this extent for other surfactants [4,27]. An increase in this parameter leads to smaller lamellae distances, as could be seen by SAXS. Our results constitute essential information for further studies by giving a detailed description of systems containing N-(3dimethylaminopropyl) octadecanamide. Therefore, they close the gap to previous studies of other surfactants [26–29]. Acknowledgments The authors thank Dr. Sandra Saladin and Dr. Detlef Emeis for their help in realizing this work, as well as Frank Hetzel for performing and discussing the DSC measurements. Furthermore we thank Dr. Carsten Schellbach and Dr. Andreas Frömsdorf from the Center for Applied Nanotechnology (CAN, Hamburg, Germany) for performing and explaining the X-ray measurements, as well as Ralf Ihrig from tesa AG (tesa, Hamburg, Germany) for his help with the polarization microscopy measurements and the possibility to use the equipment. References [1] W. Umbach, Kosmetik und Hygiene, Wiley-VCH, Weinheim, 2004. [2] A.J. O’Lenick Jr, Surfactants: Strategic Personal Care Ingredients, Allured Publishing Corporation, Carol Stream, 2005. [3] H. Stegemeyer, Lyotrope Flüssigkristalle, Steinkopff, Darmstadt, 1999. [4] T.M. Alam, S.K. McIntyre, Langmuir 24 (2008) 13890. [5] G.M. Eccleston, M.K. Behan-Martin, G.R. Jones, E. Towns-Andrews, Int. J. Pharm. 203 (2000) 127. [6] A. Rapp, K. Ermolaev, B.M. Fung, J. Phys. Chem. B 103 (1999) 1705. [7] C.-Y. Cheng, L.-P. Hwang, J. Chin. Chem. Soc. 48 (2001) 953. [8] R. Krishnaswamy, S.K. Ghosh, S. Lakshmanan, V.A. Raghunathan, A.K. Sood, Langmuir 21 (2005) 10439. [9] P.M. Macdonald, V. Strashko, Langmuir 14 (1998) 4758. [10] J.S. Clawson, G.P. Holland, T.M. Alam, Phys. Chem. Chem. Phys. 8 (2006) 2635. [11] G.M. Eccleston, Colloids Surf. A 123–124 (1997) 169. [12] S. Friberg, L. Mandell, M. Larsson, J. Colloid Interface Sci. 20 (1969) 155. [13] S. Friberg, K. Larsson, in: G.M. Brown (Ed.), Liquid Crystals, vol. 2, Academic Press, New York, 1976, p. 173. [14] S. Friberg, K. Madani, Colloid Polym. Sci. 65 (1978) 164. [15] C. Alberola, B. Blümich, D. Emeis, K.-P. Wittern, Colloids Surf. A 290 (2006) 247. [16] J. Plass, D. Emeis, in: J. Fraissard, O. Lapina (Eds.), Magnetic Resonance in Colloid and Interface Science, Kluwer Academic Publishers, The Netherlands, 2002, p. 375. [17] C. Stubenrauch, C. Frank, R. Strey, D. Burgemeister, C. Schmidt, Langmuir 18 (2002) 5027. [18] C. Stubenrauch, S. Burauer, R. Strey, C. Schmidt, Liq. Cryst. 31 (2004) 39. [19] S.A. Ishikawa, J. Mol. Struct. 271 (1992) 57. [20] A.E. Bennett, C.M. Rienstra, M. Auger, K.V. Lakshmi, R.G. Griffin, J. Chem. Phys. 103 (1995) 6951.
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