Food Research International 49 (2012) 580–587
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Shearing as a variable to engineer the rheology of candelilla wax organogels F.M. Alvarez-Mitre, J.A. Morales-Rueda, E. Dibildox-Alvarado, M.A. Charó-Alonso, J.F. Toro-Vazquez ⁎ Facultad de Ciencias Químicas, Centro de Investigación y Estudios de Posgrado, Universidad Autónoma de San Luis Potosí, México
a r t i c l e
i n f o
Article history: Received 30 May 2012 Accepted 26 August 2012 Keywords: Candelilla wax Organogelation LMOG Shear rate Molecular alignment Junction zones
a b s t r a c t We investigated the organogelation of candelilla wax (CW) in safﬂower oil during cooling (6 °C/min) from 90 °C to 5 °C. The gelation of 3% CW solutions was done statically, or by applying a particular shear rate (30 to 600 s−1) constantly during cooling (CS), or just during cooling from 90 °C to 52 °C then continuing the cooling under static conditions (S52). We measured the elastic (G′) and loss (G″) modulus, and yield stress (σ*) of the CW organogels as a function of time (0 to 30 min) at 5 °C. Independent of the storage time, the results showed that compared with the gels formed statically the use of CS resulted in organogels poorly structured with decreasing G′ as shear rate increased. Under quiescent conditions the gels showed microplatelets with a meshing organization, while CS produced smaller microplatelets with less extent of meshing as shear rate increased. In contrast, at all shear rates investigated S52 conditions formed organogels with larger microplatelets, a more apparent meshing organization with higher G′ and σ* than gels developed statically or with CS (P b 0.10). At 300 s−1, where S52 organogels showed the highest solid phase content (SPC), the G′ showed a maximum (P b 0.002). However, the SPC did not explain the G′ behavior fully. The behavior of the microstructure or gel development rate [i.e., d(G′)/d(time)] as a function of T°, suggested that ﬂow produced by S52 conditions induced the molecular alignment of CW components at T° above the onset for CW crystallization (39.0 °C ± 0.08 °C) and even above its melting T° (43.6 °C ± 0.32 °C). This would result in the development of mesophase precursors that upon further cooling under static conditions, crystallize and develop a three-dimensional organization with higher extent of microplatelet–microplatelet interaction and higher elasticity as shear rate increased up to 300 s−1. This shear rate seemed to provide the optimum ﬂow favoring the alignment of the CW components, probably developing mixed mesophase organizations. We concluded that shearing and the extent of its application as T° decreases, determine crystal size and the microplatelet–microplatelet interaction throughout the three-dimensional crystal network. Therefore, shearing rate, the extent of its application as cooling proceeds, and cooling rate can be used as engineering variables to tailor organogels' rheology. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction A robust approach to design low molecular-mass organic gelators (LMOG) with particular properties ought to establish correlations between the chemical structure of LMOG's, their ability to self-assemble and develop a three-dimensional network with particular topology, and the macroscopic physical properties of the material (i.e., rheology). Such correlations must be supported by the identiﬁcation and understanding of the factors that control the crystal network formation and topology of the LMOG aggregates (Tang, Liu, & Strom, 2009). These include the establishment of both the nature of the short-range, weak forces and the strong solvent dependence on the molecular selfassembling capability of the gelator (Baddeley, Yan, King, Woodward, & Badjic, 2007; Tu, Li, Wang, Liu, & Li, 2008), as well as the effect of the thermodynamic and mass transfer conditions applied through ⁎ Corresponding author at: Facultad de Ciencias Químicas-CIEP, Av. Dr. Manuel Nava 6, Zona Universitaria, San Luis Potosí, SLP 78210, México. Tel.: +52 444 8262436x101. E-mail address: [email protected]
(J.F. Toro-Vazquez). 0963-9969/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodres.2012.08.025
external forces (i.e., supercooling, cooling rate, shearing, gelator concentration). These parameters induce molecular alignment, crystal growth, and crystal network organization that might be used to engineer the desired macroscopic properties (i.e., rheological properties) of supramolecular soft materials such as organogels. Organogelation is easily achieved by heating a dispersion of a LMOG in a low polarity solvent until achieving its full solubility, and then cooling the system below the temperature at which the LMOG develops a solid phase in the solvent. This temperature (TCr), regularly determined by DSC, is associated with the LMOG's nucleation onset in the solvent. Most studies involving organogelation have been done under static conditions, where mass transfer conditions are limited and distant from those used by the industry. Under these conditions, supercooling is the main thermodynamic driving force for nucleation, crystal growth, and subsequent gelation of the LMOG. Supercooling is established as the difference between the organogel's melting temperature (TM) and the gel setting temperature (Tset), where Tset ≤ TCr. Experiments done in our laboratory with candelilla wax (CW) using safﬂower oil as the liquid phase, showed that the application of a
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continuous shear (i.e., 600 s−1) during organogelation of the n-alkanes of CW (i.e., CW has 40 to 50% of n-alkanes), resulted in the formation of sol-type system with weaker gel structure than the organogels developed under quiescent conditions (Chopin-Doroteo, Morales-Rueda, Dibildox-Alvarado, Charó-Alonso, & Toro-Vazquez, 2011). Da Pieve, Calligaris, Co, Nicoli, and Marangoni (2010) observed similar results with 5% monoglyceride organogels applying shear rates between 50 and 2000 s −1, and also Lescanne et al. (2002) in organogels of 2, 3‐di‐ n‐decyloxyanthracene (5× 10−2 M) in N, N-dimethylformamide applying a constant shear stress of 3 Pa. Wang, Liu, Xiong, and Li (2006) showed that the proportion between transient (i.e., entanglement of ﬁbers or plates) and permanent (i.e., branching of ﬁbers or plates) junction zones in a ﬁbrillar gel network, determines the rheological properties of the self-assembled crystal network. It seems that the application of shear at temperatures below TCr (i.e., where the three-dimensional crystal network is getting developed), does not allow the establishment of the permanent junction zones required to provide a gel structure. On the other hand, using melts of pure n-alkanes Jabbarzadeh and Tanner (2009) reported that shearing before subjecting the melt to quiescent conditions, enhanced the degree of crystallinity as a function of time of high molecular weight alkanes (i.e., C60H122), whereas, for low molecular weight alkanes (i.e., C20H42), they detected no signiﬁcant change. Additionally, these authors observed that the critical shear rate above which crystallinity is enhanced, is inversely proportional to the size of the hydrocarbon chains (Jabbarzadeh & Tanner, 2009). Alkanes, are one of the main components of CW (Toro-Vazquez, Morales-Rueda, Dibildox-Alvarado, Charó-Alonso, Alonzo-Macias, & González-Chávez, 2007). These results are in line with those reported by Keller and Kolnaar (1997) and Balzano, Kukalyekar, Rastogi, Peters, and Chadwick (2009) for a polymer melt and a linear high density polyethylene, respectively. These authors showed that molecules with a molar mass above a critical value can be coiled or stretched due to a ﬂow induced alignment by the application of shearing at temperatures well above TCr. Upon further cooling the ﬂow induced mesophase structures, crystallize with an extended‐chain structure. In contrast, molecules with a molar mass below the critical value cannot be stretched during shearing and relax immediately, tending to dissolve back to a disordered state after the initial stretch. In the particular case of CW, Chopin-Doroteo et al. (2011) investigated the effect of shearing (i.e., 600 s −1) using 3% CW solutions in safﬂower oil applying shear during cooling just until achieving metastable conditions (i.e., up to 52 °C, 10 °C above the TCr of CW), and then allowing the organogel to form under static conditions at 15 °C. The application of shearing did not modify the orthorhombic packing of the crystals or the melting properties of the 3% CW organogels (e.g. 44.6 °C). However, pre-sheared CW solutions showed enhanced nucleation and crystal growth suggesting the development of ﬂow induced liquid structures. Additionally, the corresponding gels had higher rheological properties compared with CW organogels developed under quiescent conditions or with the application of continuous shear (Chopin-Doroteo et al., 2011). This indicated that the application of shearing just until achieving metastable conditions still enables the formation of permanent junction zones during the development of the three-dimensional crystal network (Chopin-Doroteo et al., 2011). Similar results were obtained by Ojijo, Neeman, Eger, and Shimoni (2004) in a monoglyceride-olive oil system gelled at 25 °C. In this system pre-shearing (i.e., 300 s −1) the solution for 60 s at 47 °C resulted in gels with higher elasticity that gels developed under quiescent conditions or pre-shearing at 55 °C. However, these authors associate the higher rheological properties of the gel to a shear‐induced or perikinetic aggregation of the monoglyceride crystals already present at 47 °C but not existing at 55 °C. These authors concluded that it should be a critical temperature of shear application that determines the maximum structure development of monoglyceride crystals (Ojijo et al., 2004). Within this framework, we consider that shearing and the extent of its application as supercooling changes (i.e., as temperature decreases)
have an important effect on crystal size and the proportion of transient to junction zones throughout the three-dimensional crystal network, determining the organogel's rheological properties. Thus, the primary aim of this study is to investigate the effect of different levels of shearing (i.e., 30 to 600 s −1) on the rheological properties of 3% CW organogels. The organogels were developed applying shearing continuously during cooling (6 °C/min) until achieving a given Tset (5 °C), or just until achieving metastable conditions (i.e., up to achieving 52 °C) then to allow the development of the organogel under quiescent conditions at Tset. To form the organogels we used a rheometer equipped with a true-gap system and a Peltier temperature control. This set-up allowed the application of a controlled shear rate at particular stages during the development of the organogels, and then measuring their rheological properties. The rheology, microstructure and solid content of the CW organogels developed under shearing conditions were compared with those showed by organogels developed statically. The results of this study might provide new insights about the effect of shearing to tailor particular rheological properties of CW organogels. Limited information is available describing the shearing effect on the crystallization of LMOG and its effect on the organogels' physical properties. 2. Materials and methods 2.1. Vegetable oil and candelilla wax analysis Safﬂower oil high in triolein (HOSFO) was obtained from a local producer (Coral Internacional, San Luis Potosí, Mex.). Micronized high purity CW obtained from Euphorbia cerifera was supplied from Multiceras (Monterrey, Mex.). The HOSFO was analyzed by HPLC and CW by capillar GC–mass spectroscopy after sample silanization. The major triglyceride in HOSFO was OOO (63.3%± 0.06). The CW showed high proportion of n‐alkanes with 28–33 carbons (≈44–45%), esters of aliphatic acids and alcohols (≈6–7.4%), aliphatic acids with 18–34 carbons (≈15–18.8%), aliphatic alcohols with 24–34 carbons (≈5–7.6%), alcohols of penta‐cyclic triterpenoids (21–23%), and esters of alcohols of penta‐cyclic triterpenoids (1.9–2.2%). Of the n-alkanes present hentriacontane was the major constituent (C31H64; 75.9% ± 0.1%) with lower concentrations of nonacosane (4.2%± 0.1%; C29H60), triacontane (4.2%± 2.0%; C30H62), dotriacontane (2.8%± 0.4%; C32H66), and tritriacontane (9.9% ±0.4%; C33H68). 2.2. Treatment design and statistical analysis The shearing rates investigated were between 30 and 600 s −1. The shear rate was applied to CW solutions under two stirring conditions: constant stirring during the cooling stage from 90 °C to the Tset of 5 °C (CS), or just during cooling from 90 °C to 52 °C then continuing cooling under static conditions until achieving the Tset of 5 °C (S52). Thus, the treatment conditions investigated resulted from the factorial combinations of ﬁve levels of shear rate (30, 60, 180, 300, and 600 s −1) and two conditions for applying shear rate (CS and S52). The resulting treatment conditions, including the static conditions (0 s−1), were randomly distributed among aliquots of 3% CW solution in HOSFO in a complete randomized experiment design with three replicates (n= 3). The results were statistically analyzed as a factorial treatment design by ANOVA and contrast between the treatment means using STATISTICA V 9 (StatSoft Inc., Tulsa, OK). 2.3. Organogel formation under static and controlled shearing conditions The CW organogels were developed in a rheometer (Paar Physica MCR 301, Stuttgart, Germany) using a steel truncated cone-plate geometry (diameter 50 mm, 1°, truncation 0.047 mm; CP50-1/TG), equipped with a true-gap system. The sample temperature was controlled through a Peltier temperature control located on the base of the geometry and with a Peltier‐controlled hood (H-PTD 200). The control of the
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equipment was made through the software Start Rheoplus US200/32 version 2.65 (Anton Paar, Graz, Austria). The 3% CW solution (90 °C) was applied on the base of the geometry (90 °C), and the cone was set using the true-gap function of the software. After 30 min at 90 °C, the system was cooled at 6 °C/min applying stirring at the corresponding shear rate following the CS or S52 procedure, or no stirring (i.e., static or 0 s−1) during organogel formation (see Section 2.2). Once achieved the Tset of 5 °C, at particular times (i.e., 0, 5, 30, and 60 min) a frequency sweep between 100 Hz and 0.1 Hz was applied to the organogel. The strain used was between 0.004% and 0.05% always within the linear viscoelastical region (LVR). The elastic (G′) and loss (G″) modulus of the organogel were obtained from the LVR of the frequency sweep. For the determination of the yield stress (σ*) new organogels were developed, and after 30 min at 5 °C we applied a strain (γ) sweep between 0.001% and 100%. The σ* was calculated from the log–log plot of shear stress vs. γ (%) at the corresponding upper limit of strain. In each case at least three independent determinations were obtained and the results were statistically analyzed as described in Section 2.2. 2.4. Structural development during organogelation 3% CW solutions were processed to develop organogels statically or with the application of selected shear rates following the S52 conditions. After 30 min at 90 °C (i.e., t = 0 min), the 3% CW solution was cooled (6 °C/min) and at particular temperatures, starting at 52 °C and ending at 5 °C, a frequency sweep (100 Hz to 0.1 Hz) was applied to the system using a strain between 0.004% and 0.07%. G′ was obtained from the LVR of the frequency sweep. For each G′ measurement we used a new 3% CW solution. We evaluated the evolution of the microstructure developed during cooling of the 3% CW solutions, by plotting d(G′)/d(t) vs. temperature where d(G′)/d(t) is the absolute difference in elasticity between subsequent time–temperature conditions during cooling. At least three independent measurements were done for each shear rate and temperature condition. 2.5. Percentage of solids The solid phase content (SPC) of CW organogels, was determined by low resolution NMR (Minispec Bruker model mq20; Bruker Analytik; Rheinstetten, Germany) following the direct method of the AOCS ofﬁcial method Cd 16b-93 (AOCS, 2009), but using similar time–temperature conditions as for the rheological measurements. For the static conditions the organogels were developed directly in NMR tubes. The temperature was controlled with a programmable temperature control bath (Presto LH 45; Julabo, Germany). To evaluate the effect of the shear on the SPC, the CW solutions were stirred at the corresponding rate (i.e., 30 to 600 s−1) using a helical ribbon impeller ﬁtted to the mechanical spectrometer. The impeller was equipped with a sample container (23 mL) and a jacket connected to the temperature control bath. The detailed description of the impeller's geometry has been described previously (Toro-Vazquez, Pérez-Martínez, Dibildox-Alvarado, Charó-Alonso, & Reyes-Hernández, 2004). For SPC determination under CS conditions, we applied a constant stirring rate during the cooling stage (6 °C/min) from 90 °C to the Tset of 5 °C and then the sample was rapidly transferred to NMR tubes pre-cooled to 5 °C. For SPC determination under S52 conditions, after the stirring stage the CW solutions were rapidly transferred to NMR tubes (52 °C) to continue the time–temperature program under static conditions until achieving the Tset of 5 °C. The SPC of the organogel was measured at different times between 0 and 60 min. We obtained two independent determinations and the results were statistically analyzed as previously indicated. 2.6. Polarized light microscopy Polarized light microphotographs (PLM) of the organogels were obtained using a polarizing light microscope (Olympus BX51; Olympus
Optical Co., Ltd., Tokyo, Japan) equipped with a color video camera (KP-D50; Hitachi Digital, Tokyo, Japan) and a heating/cooling stage (LTS 350; Linkam Scientiﬁc Instruments, Ltd.) connected to a temperature control station (TP94; Linkam Scientiﬁc Instruments, Ltd., Surrey England) and a liquid nitrogen tank. For organogels developed under static conditions 100 μL aliquot of a melted sample was dropped over a preheated glass microscope slide (≈90 °C). To guarantee a uniform sample thickness, a drop of the melted sample was gently smeared over a preheated glass microscope slide (≈90 °C) using another glass slide at a 45° angle. The slide with the sample was placed on the platina and the same thermal treatment as for the rheological measurements was applied using a temperature control station (Linksys32 V 1.3.1; Linkam Scientiﬁc Instruments LTD. Waterﬁeld, UK). For organogels developed under S52 conditions, after the stirring period using the same stirring system as for SPC measurements (see Section 2.5), 100 μL of the solution was smeared over a preheated glass microscope slide (52 °C) as previously described. The slide with the sample placed on the platina was cooled (6 °C/min) using the temperature control station of the microscope until achieving the Tset of 5 °C. For organogels developed under CS conditions, the CW solution was constantly stirred during cooling (6 °C/min) from 90 °C until achieving the Tset of 5 °C. Then, a sample of the solution (≈100 μL) was smeared over a glass microscope slide (≈5 °C) as previously described and place on the platina previously set at 5 °C. Once at the Tset of 5 °C we obtained microphotographs of the organogels at different times between 0 and 60 min. 3. Results and analysis 3.1. Organogels developed at CS, S52, and static conditions The Log(G′) for the 3% CW organogels obtained at the different conditions investigated is shown in Fig. 1. Compared with the organogels formed statically (0 s −1), the use of CS conditions decreased the organogels' elasticity as shear rate increased. This behavior was observed in all organogels independent of the time of storage at Tset (Fig. 1). As mentioned previously, similar results were obtained with 3% CW organogels (Chopin-Doroteo et al., 2011) developed at a shear rate of 600 s −1, with 5% monoglyceride organogels (Da Pieve et al., 2010) formed at shear rates between 50 and 2000 s −1, and with 2, 3‐di‐n‐decyloxyanthracene (5 × 10 −2 M) organogels in N, N-dimethylformamide developed using a shear stress of 3 Pa (Lescanne et al., 2002). We observed that the organogels' elasticity increased as storage time at Tset increased from 0 up to 30 min (P b 0.02). After 30 min the increase in G′ was not signiﬁcant. However the increase in elasticity during storage at Tset never reached the G′ of the gels formed statically (P b 0.001; Fig. 1). Thus, although some restructuring of the crystal three-dimensional organization of the organogels occurred during storage at Tset, never attained the microstructural organization of the gels formed under quiescent conditions. The SPC in the organogels formed statically and those using CS did not change as time at Tset increased (data not shown). Therefore, the SPC values as a function of time were pooled, calculating the mean and corresponding standard deviation (Table 1). The corresponding statistical analysis showed that the SPC in the organogels was mostly independent of the shearing rate applied, showing a higher value just at 300 s −1 (P b 0.05; Table 1). Therefore, the SPC does not explain the elastic behavior of organogels formed under steady shearing. The PLM of the organogels developed statically and using CS are shown in Fig. 2. The microphotographs were obtained after 60 min at the Tset of 5 °C. The CW organogel formed under quiescent conditions showed plate-like shaped crystals (i.e., microplatelets) organized in a meshing fashion (Fig. 2A), similar to the microstructural organization previously observed in CW organogels developed using a cooling rate of 1 °C/min and a Tset of 15 °C (Chopin-Doroteo et al., 2011), and at 10 °C/min using Tset values of − 5 °C, 15 °C, and 25 °C (Toro-Vazquez, Alonzo-Macias, Dibildox-Alvarado, & Charó-Alonso,
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Fig. 1. Logarithm of the elastic modulus [Log(G′)] as a function of shearing conditions (CS: constant shearing; S52: shearing up to 52 °C), shear rate, and time under storage at Tset (°C).
2009). On the other hand, the organogels formed using CS showed smaller microplatelets with less extent of meshing organization as shear rate increased (Fig. 2B–F). These results and the G′ behavior showed that shearing during crystallization resulted in a reduction of crystal size, probably by attrition. Additionally, the application of continuous shearing during organogelation had a deleterious effect in the establishment of the three-dimensional crystal network. It seems logical to establish that applying increasing shear rates during CW organogelation, limits the establishment of permanent junction zones among the microplatelets. Then, compared with the gels formed under static conditions the use of CS conditions resulted in poorly structured organogels (Fig. 2B–F) with lower elasticity as shear rate increased from 0 s −1 (i.e., static) to 600 s −1 (Fig. 1). S52 conditions provided organogels that had higher G′ values than gels developed statically or with CS conditions. The magnitude of G′ for the S52 organogels was independent of the storage time at Tset (P > 0.40; Fig. 1). Hence, the G′ values as a function of time were pooled, calculating the mean and corresponding standard deviation. Thus, the statistical analysis independent of storage time showed
Table 1 Percentage of solid phase content (SPC) in CW organogels developed at different shear rates after 60 min at the Tset of 5 °C using constant shearing (CS) or shearing just during cooling from 90 °C to 52 °C (S52). Shear rate s−1
0 30 60 180 300 600
2.59 (0.14) 2.56 (0.05)a,a 2.52 (0.21)a,a 2.36 (0.21)a,a 2.74 (0.29)b,a 2.55 (0.13)a,a
2.59 2.57 2.57 2.63 2.80 2.58
(0.14)a,a (0.16)a,a (0.11)a,a (0.15)a,b (0.22)b,a (0.13)a,a
* Mean and standard deviation (n ≥ 6). a,b For the same condition to apply shearing, the same ﬁrst subscript letter indicates no signiﬁcant effect of shear rate on the solid phase content. A different letter indicates a signiﬁcant effect (P b 0.05). For the same shear rate the same second subscript letter indicate no signiﬁcant effect of the shearing condition on the solid phase content. A different letter indicates a signiﬁcant effect of the shearing condition (P b 0.10).
that G′ increased as shear rate increased showing a maximum at 300 s −1 (P b 0.002; Fig. 3). A higher shearing rate (i.e., 600 s −1) produced a decrease in the organogel's elasticity attaining values similar to those showed by the gels developed at 60 s −1, but still with greater G′ than the organogels developed statically (P b 0.002) or using CS conditions (P b 0.001; Fig. 1). These results agree with the ones obtained by Chopin-Doroteo et al. (2011). These authors reported that CW organogels developed at 600 s −1 using S52 conditions, a cooling rate of 1 °C/min and a Tset of 15 °C, achieved higher rheological properties than organogels formed under quiescent conditions or with the use of steady shearing (Chopin-Doroteo et al., 2011). The PLM of the organogels developed statically and S52 conditions are shown in Fig. 4. In comparison with the microstructure developed using static (Figs. 2A and 4A) or CS (Fig. 2B–F) conditions, the microplatelets in the S52 organogels were larger showing a ﬁber-like or plate-like shape (Fig. 4B–F). Additionally, the crystals showed a meshing organization more apparent than the one developed by the organogels formed statically or with CS conditions (Fig. 2). This was more evident in the organogels formed using 300 s −1 (Fig. 4E). At this particular shear rate the organogels achieved the highest G′ (Fig. 3), a behavior that might be partially associated with the greater SPC present in these gels (Pb 0.03; Table 1). Nevertheless, the S52 organogels developed using S52 conditions had higher G′ than those formed statically or using CS at any of the shear rates investigated (Pb 0.03; Figs. 1 and 3). Except for the organogel developed at 300 s–1, all these organogels had the same SPC (P> 0.42; Table 1). Therefore, under S52 conditions CW components developed a different three-dimensional crystal organization from that obtained using quiescent or CS conditions. Recently, in a similar investigation using 3% CW solutions in vegetable oil with and without 1% of tripalmitin (TP), we reported that the application of shear (600 s−1) until reaching a temperature in the metastable zone of the gelator molecules present in CW (i.e., at 52 °C), resulted in the development of organogels with a three‐dimensional crystal network formed by larger crystals than the ones observed in gels developed under static conditions or with the application of shearing until reaching the nucleation temperature for CW (i.e., 47 °C) (Chopin-Doroteo et al., 2011). The authors proposed that the application of shearing to 3% CW and 3% CW–1% TP solutions under metastable conditions (i.e., S52)
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Fig. 2. Polarized light microphotographs of organogels developed statically (A) and using constant shearing at different shearing rates. 30 s−1 (B), 60 s−1 (C), 180 s−1 (D), 300 s−1 (E), and 600 s−1 (F).
Fig. 3. Logarithm of the elastic modulus [Log(G′)] as a function of shearing rate. Shearing was applied up to 52 °C.
resulted in organogels with three-dimensional crystal networks with higher extent of transient and permanent junction zones, and therefore higher elastic properties that the organogels developed statically, using steady shearing (i.e., CS) or shearing until reaching the nucleation temperature for CW. The results obtained in the present investigation are in line with these conclusions. Unfortunately, with the magniﬁcation available in the microscope, we could not establish the proportion between the permanent and the transient junction zones throughout the crystal network of the CS and S52 organogels. The three-dimensional crystal organization observed in S52 organogels might be the result of ﬂow induced molecular alignment. Results obtained with different organic compounds indicate the presence of non-crystalline structures (i.e., “liquid structures”) at temperatures above nucleation temperature (Balzano, Kukalyekar, Rastogi, Peters, & Chadwick, 2008; Bhowmik, Nedeltchev, & Han, 2008; Cebula, McClements, Povey, & Smith, 1992; Corkery, Rousseau, Smith, Pink, & Hanna, 2007; Larsson, 1972; Minato, Ueno, Smith, Amemiya, & Sato, 1997; Ueno, Minato, Yano, & Sato, 1999). Additionally, the use of shearing at temperatures above TCr, as in S52 conditions, induces molecular alignment that results in the formation of non-crystalline molecular organizations before nucleation. These mesophase structures have
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Fig. 4. Polarized light microphotographs of organogels developed statically (A) and sheared at different rates up to 52 °C. 30 s−1 (B), 60 s−1 (C), 180 s−1 (D), 300 s−1 (E), and 600 s−1 (F).
implications in the crystallization kinetics, size and shape of the crystals, and their three-dimensional organization as has been observed in polymer liquid crystals (Hongladarom & Burghardt, 1993; Hongladarom, Burghardt, Baek, Cementwala, & Magda, 1993), polyethylene (Balzano et al., 2008), polypropylene (Balzano et al., 2009), several LMOG's (Lescanne et al., 2002), and alkanes (Jabbarzadeh & Tanner, 2009). CW has different components (i.e., alkanes, esters, alcohols, fatty acids, and penta‐cyclic triterpenoids; see Section 2.1) that under static conditions might crystallize in the vegetable oil, either as independent or as mixed crystals. For instance, n‐alkanes, the main component of CW, tend to form mixed crystals with an orthorhombic subcell packing (Chevalier et al., 1999). Esters of aliphatic alcohols and acids present in CW might co-crystallize with the n-alkanes. On the other hand, penta‐ cyclic triterpenoids, also an important component of CW, can develop self‐assembled ﬁbrillar network structures in organic solvents as shown by Hu, Zhang, and Ju (2009) and Bag, Dinda, Dey, Mallia, and Weiss (2009). Therefore, although the phase behavior and gelling capability of CW have been associated mainly with the crystallization of n-alkanes, particularly with hentriacontane (Toro-Vazquez et al., 2007; Toro-Vazquez et al., 2009), given the CW composition (see Section 2.1) and the SPC of the organogels developed statically or under shearing (Table 1), 78% to 90% of the CW components evidently
crystallized during organogelation. The X-ray analysis obtained by Dassanayake, Kodali, Ueno, and Sato (2009) and Chopin-Doroteo et al. (2011), indicated that CW components crystallize with an orthorhombic subcell either in the bulk state or in 3% and 6% organogels. The SAX X-ray analysis showed that the molecular stacking of these crystals has an equivalent carbon number of 31.8, a value close enough to the actual carbon number for hentriacontane (C31), the main alkane present in CW (Chopin-Doroteo et al., 2011). Therefore, although other CW components besides hentriacontane and other n-alkanes were involved in the crystallization and the rheological properties of CW organogels, they seem to crystallize as mixed crystals with an orthorhombic subcell. We considered that despite the different shearing conditions applied, crystals in the CW organogels developed in the present investigation also have an orthorhombic subcell packing. This is the case since Chopin-Doroteo et al. (2011) found that the application of shearing does not modify the melting properties of the CW organogels nor the orthorhombic packing of the crystals. 3.2. Structure development during organogelation The behavior of Log(G′) during organogelation of 3% CW solutions using S52 conditions at different shear rates is shown in Fig. 5A. The
F.M. Alvarez-Mitre et al. / Food Research International 49 (2012) 580–587
behavior of d(G′)/d(t) as a function of temperature is shown in Fig. 5B. The evolution of d(G′)/d(t) as a function of temperature provided an indirect measurement of microstructure or gel development rate during cooling of the 3% CW solutions. Independent of the shearing conditions applied three different zones were well deﬁned. These three zones are indicated in the Log(G′) and d(G′)/d(t) proﬁle as a function of temperature (Fig. 5A and B, respectively). Using DSC we established that at the cooling rate used the 3% CW solutions in HOSFO had a TCr of 39.0 °C ± 0.08 °C and a TM of 43.6 °C ± 0.32 °C, values that agree with the TCr of 42.2 °C ± 0.3 °C and TM of 44.6 °C ± 0.06 °C previously determined at cooling rate of 1 °C/min (Chopin-Doroteo et al., 2011). In zone I, between ≈TCr and 52 °C where a sol state prevailed (i.e., G″ >> G′), elasticity was low (Fig. 5A) and d(G′)/d(t) had values lower than 300 Pa/min (Fig. 5B). This behavior was observed independent of the shearing conditions applied. At temperatures below TCr (zone II) the Log(G′) showed a steady increase followed by a relative plateau after achieving 37 °C. In correspondence, at temperatures below TCr d(G′)/d(t) increased showing a peak with a maximum at 37 °C. The d(G′)/d(t) peak was associated with the self-assembly of CW gelator molecules and the development of a three-dimensional crystal network that provided structure to the vegetable oil. It is important to point out that shearing under S52 conditions did not modify the onset temperature nor the temperature at the peak maximum. However, the maximum gel development rate was higher as shear rate increased up to 300 s −1, and we observed no difference between these organogels and those formed at 600 s −1 (Fig. 5B). We hypothesize that shearing applied under metastable conditions induced the alignment of gelator molecules in CW, determining in zone II the development of transient
Fig. 5. Log (G′) (A) and structure development rate [d(G′)/d(t)] (B) during cooling of 3% CW solutions up to achieve the Tset of 5 °C. The legend indicates the shear rate applied up to 52 °C.
and permanent junction zones throughout the organogel's microstructure, and therefore its rheological properties. The zone III occurred from the end of the gelation stage (i.e., ≈27 °C) until achieving Tset (Fig. 5A and B). In this zone the Log(G′) behavior showed practically no effect of shear rate. In contrast, the d(G′)/d(t) behavior was dependent on the shear rate applied. Thus, under static conditions the structure rate formation shows the lowest d(G′)/d(t) values, except for a small peak around 17 °C. In comparison, the d(G′)/d(t) values for the organogels developed at 180 s−1, 300 s−1, and 600 s−1 were, in general, higher. In zone III the S52 organogels achieved the higher structure rate formation at 5 °C, with gels formed at 600 s−1 showing the lowest d(G′)/d(t) values (Fig. 5B). Incidentally, of the S52 organogels evaluated in Fig. 5 those developed at 600 s −1 showed the lowest elasticity (Fig. 3). We consider that in zone III some additional crystallization occurred, particularly in the S52 organogels, and from here the structure development rate increased (Fig. 5B). To evaluate if crystallization and changes in the organogels' microstructure occurred during storage at 5 °C, the G′ was measured after 30 min at Tset. The mean value for Log(G′) after 30 min at 5 °C, independent of the shear rate used, is indicated by the arrow in Fig. 5A. The corresponding d(G′)/d(t) in the organogels showed values equal or lower to the magnitude indicated by the arrow in Fig. 5B. Thus, during storage at 5 °the microstructural changes in the organogels were limited. From the above it is evident that organogelation (i.e., microstructure development) of CW occurred during the cooling process in zones II and III. However, the shearing applied at temperatures above the beginning of zone I, where a sol state prevailed and actually above TM of the 3% CW organogel (i.e., 43.6 °C ± 0.32 °C), resulted in the increase in the structure development rate observed in zones II and III. This, behavior was not associated with the formation of additional SPC as a result of shearing, but with a molecular alignment induced by shearing. Therefore, these results suggest that ﬂow produced by S52 conditions induced molecular alignment of CW components at temperatures > TCr and even above TM. This would result in the development of mesophase precursors that upon further cooling, now under static conditions, crystallize, grow and develop a three-dimensional organization with higher extent of microplatelet– microplatelet interaction (i.e., meshing through permanent and transient junctions), higher elasticity as shear rate increased up to 300 s −1 (Figs. 4B–E, and 3). This shear rate seemed to provide the optimum ﬂow conditions for the alignment of the CW components, probably developing mixed crystal. This is the case since at this shear rate SPC values achieved a maximum using both, CS and S52 conditions. However, increasing the shear rate up to 600 s−1 resulted in a decrease in SPC and G′ in the S52 organogels (Fig. 3 and Table 1). At this high shear rate the alignment and crystallization of components of CW might not be as effective as at 300 s−1. Additionally, using S52 conditions also produced CW organogels with higher σ* than gels formed under quiescent conditions (Pb 0.10). Thus, the σ* of 3% CW organogels developed statically was 106.9 Pa ± 19.1 Pa, gels developed at 180 s−1 had a σ* of 186.3 Pa ±12.6 Pa, and those formed at 300 s −1 and 600 s −1 the σ* was 127.7 Pa ± 19.5 Pa. In contrast, in the CS organogels crystallization of the mesophase precursors and the subsequent development of the three-dimensional crystal network occurred under shearing. Therefore, crystal size and their microstructural organization would depend on the extent of crystal attrition and crystal agglomeration as affected by the rate of shearing. The results obtained with the CS organogels, showed that constant shearing produced smaller microplatelets with less extent of meshing (Fig. 2B–F) and elasticity as shear rate increased (Fig. 1). It seems that CS conditions produced more crystal attrition than agglomeration, and as shearing increased the formation of permanent junction zones was limited resulting in organogels with poorly structured crystal networks (Fig. 2B–F). However, as previously stated, the microscope available did not have the resolution required to observe the shearing effect on the proportion between transient and permanent junction zones between the microplatelets.
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4. Conclusions The results obtained showed that S52 conditions developed organogels structured by larger microplatelets, with a meshing organization that provided higher G′ and σ* than gels developed statically or with CS. This, behavior was not associated with the formation of additional SPC because of shearing, but with a ﬂow induced molecular alignment of CW components occurring at temperatures > TCr and even above TM. We hypothesize that there should be a critical temperature and magnitude of shearing application that establish the maximum structure development in LMOG. These results show that shearing and the extent of its application as a function of supercooling determine the crystal size and the microplatelet–microplatelet interaction (i.e., proportion of transient to permanent junction zones) throughout the three-dimensional crystal network. Therefore, shearing rate, the extent of its application as cooling proceeds, and cooling rate can be used as engineering variables to tailor organogels' rheology. Ongoing investigation is addressing the effect of these variables in the organogelation of LMOG that develop self‐assembled ﬁbrillar networks. Acknowledgments The investigation was supported by grant # 162651 from CONACYT. The technical support from Concepcion Maza‐Moheno and Elizabeth Garcia‐Leos is greatly appreciated. F. M. Alvarez-Mitre thanks CONACYT for the scholarship provided to obtain her Ph.D. References AOCS Ofﬁcial Method Cd 16b–93 (2009). Solid fat content (SFC) by low-resolution nuclear magnetic resonance — The direct method. AOCS Methods, http://search.aocs.org/ methods/search_methods_view_method.cfm?method=cd16b_93.pdf Baddeley, C., Yan, Z., King, G., Woodward, P. M., & Badjic, J. D. (2007). Structure–function studies of modular aromatics that form molecular organogels. Journal of Organic Chemistry, 72, 7270. Bag, B. G., Dinda, S. K., Dey, P. P., Mallia, V. A., & Weiss, R. W. (2009). Self‐assembly of esters of arjunolic acid into ﬁbrous networks and the properties of their organogels. Langmuir, 25(15), 8663–8671. Balzano, L., Kukalyekar, N., Rastogi, S., Peters, G. W., & Chadwick, J. C. (2008). Crystallization and dissolution of ﬂow‐induced precursors. Physical Review Letters, 100(4), 048302. Balzano, L., Kukalyekar, N., Rastogi, S., Peters, G. W., & Chadwick, J. C. (2009). Crystallization and precursors during fast short‐term shear. Macromolecules, 42(6), 2088–2092. Bhowmik, P. K., Nedeltchev, A. K., & Han, H. (2008). Synthesis, thermal and lyotropic liquid crystalline properties of protic ionic salts. Liquid Crystals, 35, 757–764. Cebula, D. J., McClements, D. J., Povey, M. J. W., & Smith, P. R. (1992). Neutron diffraction studies of liquid and crystalline trilaurin. Journal of the American Oil Chemists' Society, 69, 130–136. Chevalier, V., Provost, E., Bourdet, J. B., Bouroukba, M., Petitjean, D., & Dirand, M. (1999). Mixtures of numerous different n-alkanes: 1. Structural studies by X-ray diffraction at room temperature - correlation between the crystallographic long c
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