Article pubs.acs.org/Langmuir
Phase Segregation in Individually Dried Particles Composed of Biopolymers Marine Nuzzo,†,‡ Jakob Sloth,§ Björn Bergenstahl,‡ and Anna Millqvist-Fureby*,† †
SP Technical Research Institute of Sweden, Chemistry, Materials and Surfaces, SE-501 15 Stockholm, Sweden Lund University, Food Technology, Engineering and Nutrition, SE-221 00 Lund, Sweden § GEA, Process Engineering A/S, Process Engineering, 2860 Soeborg, Denmark ‡
S Supporting Information *
ABSTRACT: Mixing of two biopolymers can results in phase separation due to their thermodynamically incompatibility under certain conditions. This phenomenon was first reported when the solution was allowed to equilibrate, but it has later been observed also as a consequence of drying. The challenges of this study were to observe phase segregation by confocal Raman microscopy and LV-SEM on dried film, individually dried particles, and spray dried particles. The influence of the solid content and the phase ratio (composition) of a HPMC/ maltodextrin mixture on the localization of the ingredients in the individually dried particles was investigated. We observed that phase segregation of HPMC and maltodextrin is induced by solvent evaporation in film drying, single particle drying, as well as spray drying. The phase ratio is an important parameter that influences the localization of the HPMC-enriched phase and maltodextrin-enriched phase, i.e., to the particle surface, to the core, or in a more or less bicontinuous pattern. The drying time, affected by the solids content, was found to control the level of advancement of the phase segregation.
1. INTRODUCTION Phase separation can occur when mixing two biopolymers in a common solvent.1,2 This means that the mixed system will evolve from a one-phase system to a two-phase system. It is well-known that phase separation is due to thermodynamically incompatibility of the polymers under certain conditions.3,4 The high molecular weight and the long chain length characteristic of polymers reduce polymer chain mixability and initiate phase separation.5−7 Thus, phase segregation occurs when the enthalpic contribution of one of the solute of the system becomes dominant.8 The Flory−Huggins theory describes this process.9 It is found that two mechanisms of phase separation can occur: the spinodal decomposition and nucleation and growth mechanism (presented in ref 10). Small concentration fluctuation of the polymers will provoke spinodal decomposition. Furthermore, two types of phase separation can occur.7,11,12 The phase separation is associative in the case where an attraction between the two polymers dominates. These conditions can be reached when the polymer charges are opposite. Hence, the two polymers form a concentrated phase in equilibrium with a dilute aqueous solution. Phase separation is segregative in the case of a domination of repulsive interactions between the polymers, for instance, if they have a similar charge or neutral. Hence, one of the phases tends to be continuous while the other phase is dispersed. The separation occurs above a certain concentration: for segregative separation, typically when the concentration exceeds 2−12%; for © 2015 American Chemical Society
associative separation, the critical concentration typically is much lower. Moreover, numerous parameters can influence phase separation. The polymers concentration effect on phase separation was largely studied, and it was described as follows: At low polymer concentration, the polymers can coexist in a single phase, but when the polymer concentration increases, phase separation occurs.13 As food processing is often based on solvent removal (solute concentration increase) and as biopolymers are common components in food systems, phase separation is expected to have an important role in the tuning of food properties.14 For instance, phase segregation induced by freeze-drying has been reported.4,8 Indeed, while freezing, the formation of ice crystals induces a great increase of the solute concentration that promotes molecular interactions and hence leads to phase segregation of the compounds. In his conclusion, Heller4 concluded that the stabilizer in the solution could be trapped in another phase than the one desired when phase separation occurs. Further, phase segregation as a consequence of drying has been observed in thin dried film drying experiments using AFM, for instance, in thin films formed from protein and starch.12 Additionally, SEM micrographs of the cross sections of HPMC/corn starch-dried films revealed a polymer phase segregation15 where HPMC was found at the Received: June 2, 2015 Revised: September 4, 2015 Published: September 23, 2015 10946
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The HPMC/Maltodextrin phase diagram has been obtained by experimental observations by using a titration procedure. A known HPMC/maltodextrin mix in the two phase region was stirred. As a consequence of the emulsified character, the system has a turbid appearance. A known amount of distillated water was added gradually (a drop every 10 s), while the opaque character of the stirred system was observed. When the system turned into a transparent solution (within 1 s of the addition), the added amount of water was noted, and the phase boundary data point was obtained. The liquid before drying A1, A2, A3, B1, and C1 were placed in graded test tubes overnight at room temperature to read the phase volumes presented in the Table 1. 2.2.2. Drying of a Single Particle. The drying kinetics analyzer (DKA) is based on the principle of ultrasonic levitation. The experimental setup has been described by Ullum et al.22 and is therefore only described briefly here. An ultrasonic field is generated between the transmitter and reflector. Due to the forces of this ultrasonic field, it is possible to hold a small droplet constant against gravity. While the droplet is drying, it is monitored by a CCD-camera, which records a video file of the drying process, and an infrared thermometer is used to measure the development in droplet surface temperature during drying. The levitation unit is enclosed in a small drying chamber so that the air temperature and humidity around the droplet may be set to match the conditions in a spray dryer. The drying air was kept between 70 and 75 °C. When the particle has dried, it can be collected and stored in a desiccator for further analysis. 2.2.3. Spray Drying. The liquid before drying with the HPMC/ maltodextrin ratio C1 was spray dried in a laboratory spray dryer built at SP (Technical Research Institute of Sweden). The dryer operates in cocurrent mode with a jacketed two-fluid nozzle operating with pressurized air. The dimensions of the drying chamber are 0.75 × 0.15 m. The nozzle orifice diameter is 1 mm. The feed rate used was 5 mL/ min, and the drying air flow was about 0.8m3/min. The inlet temperature of the drying air was 150 °C, and the outlet temperature was keep between 70−75 °C by adjusting the airflow. The powder was collected in a cyclone, and only the powder collected from the receiving vessel below the cyclone was used for further analyses. After drying, the powder was stored in a desiccator. The powder can be stored up to several months at room temperature and relative humidity lower than 20%. Confocal Raman images were taken within 1 week of production. 2.2.4. Dried Film. The mixture of HPMC/maltodextrin (ratio 3.2/ 10) was spread on a glass slide (75 × 25 mm) and left to dry over 2 days at ambient conditions. The mixture was poured on the glass slide until the coating was applied on the whole area of the glass slide, and the surplus was allowed to spill over. The thickness of the dried film formed was about 10−20 μm. The coated glass slide was protected by a glass cover from external pollution during drying and storage. 2.2.5. Low-Vacuum Scanning Electron Microscopy (LV-SEM). The analyses were performed in an environmental scanning electron microscope from the FEI-XL 30 Series operating in low vacuum mode. The LV-SEM images were taken under low vacuum (50−100 Pa) using 10 kV accelerating voltage. The particles were mounted on aluminum stubs using double-sided carbon tapes. The particles were uncoated and analyzed with a large field detector (LFD). 2.2.6. Confocal Raman Microscopy. The distribution of the components in the single particles was analyzed with the confocal Raman microscopy technique. The measurements were performed with a WITec alpha300 (Ulm, Germany) system. Horizontal scan XY and depth scan XZ can be obtained. The lateral resolution is 250 nm, and a vertical resolution is 500 nm. A plan achromatic objective with a numerical aperture of 0.9 and a magnification of 100 was used (Nikon Instruments Europe, Amsterdam, Netherlands). In this study, we use the depth scan mapping (surface scan mapping for the dried film) where the scan range for one image is 25 μm × 0.4 μm × 25 μm and 150 × 150 pixels using a 532 nm laser for excitation.23 Thus, a vertical slab of the particle is analyzed. The integration time per Raman spectrum is 0.1 s. One full Raman spectrum is collected in each image pixel. These spectra are compared to the reference spectra to obtain the local mapping of the compounds. The measured spectra are fitted
top layer and corn starch at the bottom layer. The order of the layers was determined by the relative density of the two polymer phases. In a study on micellar casein and guar gum mixture, Bourriot et al.16 determined the concentration of the polymers by confocal laser scanning microscopy where a bicontinuous phase of the mixture was observed. In a study of Ziemecka et al.,17 microdroplets, from aqueous polymer solution (PEG/dextran), were produced by a microfluidic device. Even at a short time scale, the formed droplets were phase separated with PEG in the core and the dextran in the shell of the droplet. Thus, the encapsulation of biomaterial is realized here by the phase separation of PEG and dextran. Few studies have been done on phase separation during spray drying, where the droplets dry in less than 1 s and the solute concentration increases by evaporation of the solvent. Recent research by Whiteside et al.18 has revealed that polymerenriched domains (PEG 6000) and drug-enriched domains (griseofulvin) can appear in spray-dried particles. By a series of XPS measurements Millqvist-Fureby et al.19 showed that the phase separation of spray-dried feed was preserved after spray drying. The aim of this study is to investigate the phase segregation in individually dried particles composed of biopolymers. The phase segregation will be observed by confocal Raman microscopy. The influence of the solid content (resulting in phase separation in the feed or possibly during the rapidly drying particle) and the phase ratio (composition) on the ingredient localization in individually dried particles made of HPMC and maltodextrin is observed. Previous studies20,21 have shown that HPMC tends to phase separate from carbohydrates, and that this concept can be used for microencapsulation.
2. MATERIALS AND METHODS 2.1. Materials. Hydroxypropyl methylcellulose (HPMC) (molar mass 10 000 g/mol and 1.8−2 methoxy and 0.2−0.3 propylene oxide per glucose unit) was purchased from Sigma-Aldrich, MO 63103, USA. Maltodextrin DE5 was provided by Kraft Foods, Glenview, IL. Ultra-purified water (Milli-Q, Millipore Systems, MA 01821, USA) was used as solvent. 2.2. Methods. 2.2.1. Preparation of Solutions. Stock solutions of HPMC and maltodextrin were prepared at 10% and 30% concentration w/w, respectively. The stock solutions were allowed to equilibrate for 24 h before mixing. The HPMC solution was mixed with maltodextrin in the proportions presented in Table 1. The solutions were stirred with a magnetic stirrer for at least 30 min before use.
Table 1. Composition of the Liquid before Drying or Dried Particlea HPMC wt %
maltodextrin wt %
solid content in the liquid before drying
A1 A2 A3
5.0 3.0 1.0
10.2 6.10 2.03
15.2 9.10 3.03
B1 C1
3.2 1.5
12.0 13.7
15.2 15.2
phase ratio 1.86 11.5 one phase 1.00 0.22
a
The concentration is given as the weight percentage of each component in the liquid before drying. A test tube of each liquid before drying was stored over night to reach the equilibrium; the phase volumes are registered in this table. The phase ratio is given as Vtop HPMC/ Vbottom maltodextrin. 10947
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Langmuir by a linear combination of the spectra of the pure components in each pixel to obtain the relative occurrence of each component in this pixel. From this data, an image is constructed in which the relative intensity of each component is shown. In each system, at least one major difference is present, i.e., the relative intensity of a peak or position of a peak is different (see Table 2). The measured spectrum (s) is described as
s = B·h + e , where B is the matrix of the reference spectra of the components, h is the vector providing the fractions of the components and, e is the error spectrum. The mixing values are fitted by the method of least-squares minimizing the error: (e)2 = Minimum Thus, the total information in the spectrum in each pixel is utilized to determine the composition in this pixel. As long as there are intensity differences in the peaks for different components, it is possible to generate an image describing the localization of the components. More than 10 individually dried particles were produced of each HPMC/maltodextrin ratio, and their morphology was observed by light microscopy. At least two particles representative of the set was used for confocal Raman microscopy where two areas of analysis were studied for each particle.
Figure 1. (●) Phase boundary of the uniform single phase solution as the weight percentage of the HPMC and maltodextrin. (○) Each point represents the liquid before drying composition used to dry a particle. The phase boundary based on experimental data points is shown as a solid line (b), and the estimated phase boundary is shown as dashed lines (a and c). These lines are based on an assumption of parallel tie lines.
3. RESULTS The state diagram of HPMC and maltodextrin in water was characterized using the titration methodology described in the Methods section. The boundary between the uniform solution and the two phase area is outlined in Figure 1.
technique for space resolving structural mapping. The analyzed spectra at single positions and reference spectra are used in the following discussion. The characteristic peaks of the HPMC and maltodextrin are presented in Table 2. The strong peaks at 1380 and 1495 cm−1 are characteristic of HPMC, while peaks at 800 and 900 cm−1 are characteristic of maltodextrin. Thus, it is clear that the two principal components are clearly distinguishable in the combined spectra. 3.2. Dried Film Analysis. A dry macroscopic film was manufactured and evaluated to further confirm that phase separation in dry film of the system HPMC and maltodextrin is mapable. Surface scans using the confocal Raman technique were performed on a film formed at a glass slide by allowing a system of 3.2 wt % of HPMC and 10 wt % of maltodextrin to dry. A typical image is shown in Figure 2. We can clearly observe phase separation between the maltodextrin (in blue) and the HPMC (in red). Isolated maltodextrin domains surrounded by a continuous HPMC matrix were observed. The size is approximately 10 μm, similar in range to the thickness of the film, in agreement with what have been observed in other comparable systems (for instance protein amylopectin systems12). The maltodextrin is encapsulated in the HPMC matrix. This identification is confirmed by the spectra at positions 1 and 2 in Figure 2. Indeed, the spectrum at position 1 has the same profile as the one of maltodextrin, and the spectrum at position 2 has the same profile as pure HPMC, which presents two sharp and strong peaks at 1380 and 1465 cm−1. This shows that confocal Raman microscopy is a suitable method for mapping and observation of phase segregation and that this system has an almost complete phase segregation, i.e., the film has come close to the equilibrium. 3.3. Individually Dried Particles Analyzed by Confocal Raman Microscopy. 3.3.1. Effect of the Solid Content on the Ingredient Localization (A1, A2, A3). Individual particles with the compositions A1, A2, and A3 were dried using the DKA method. The particles were imaged by light microscopy,
Table 2. HPMC and Maltodextrin Approximate Frequencies and Their Assignments for Raman Spectra HPMC wavenumber (cm−1) 1465 1380
maltodextrin wavenumber (cm−1) 1460 1340 1340 1250 1120
1050−1195 970 940 850
assignment
reference
CH2 scissor COH bending CH2 twist CH2OH related mode C−O, C−C, C−H symmetric C−O−C and asymmetric C−O−C stretching C−C or C−O stretching α-1,4-glycosidic linkage CH2 and CH
24−28 24,26−28 26−28 26−28 26−29 24,30 31 26−29 26−28
Based on the state diagram, an experimental plan was designed with one series crossing the phase separation line with constant ratio between the components (A1, A2, A3) and one line within the two phase area with varying polymer ratios (A1, B1, C1). The five liquid preparations before drying, A1, A2, A3, B1, and C1, were prepared to produce individually dried particles. The volume of each phase is presented in Table 1. The phase ratio of A2 is quite high as a consequence of tie lines almost parallel to line “b” in the diagram (Figure 1). On the contrary, the liquid before drying A3 is a one-phase system. 3.1. Confocal Raman Assignment. The surface as well as the internal compositional structure down to 15 μm into the particle matrix was mapped using confocal Raman microscopy. The differences between the Raman spectra of the components were first characterized to evaluate the suitability of the 10948
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Figure 2. (A) Confocal Raman image of the distribution of HPMC (red) and maltodextrin (blue) of dried film formed from a system with 3.2% HPMC and 10% maltodextrin. The concentration is given as the weight percentage of each component in the liquid system (13.2% total solids). (B) Reference spectra of (a) HPMC and (b) maltodextrin compared with the spectra of (c) position 2 and (d) position 1, realized by confocal Raman microscopy.
Figure 3. Confocal Raman images of the distribution of HPMC (red) and maltodextrin (blue) of individually dried particles A1, A2, and A3 using DKA.
and all were about 600 μm in diameter and spherical with wrinkles. The ratio HPMC/maltodextrin remains constant, while the concentration is decreased. A3 is in the uniform solution region, A1 is in the two-phase region, and A2 is at the boundary. The internal mapping of the compounds in the individually dried particles was performed using confocal Raman microscopy. Representative images of the mapping of a x−z optical section are presented in Figure 3. The maltodextrin is presented in blue, and HPMC is in red. All particles show a phase segregation with an HPMC-enriched phase at the surface. Particle A1, with 15.2% solid content in the liquid before drying and separating into two phases already in the liquid before drying, presents a phase segregation between HPMC and maltodextrin in the particle as well. A representative image is shown in Figure 3a. The HPMC is present as an approximately 12 μm thick layer at the surface of the particle. Further, there are maltodextrin-rich connected domains (up to 2 μm sized) clearly observable within the HPMC layer, giving the layer a bicontinuous character. The same behavior is observed for the particle A2, which present a solid content of 9.1% (Figure 3b). The HPMC and maltodextrin phases segregate, thus HPMC is present at the
particle surface as an approximately 5−8 μm thick layer. Thin (less than 2 μm) maltodextrin-rich, somewhat connected domains are observed within the HPMC layer, giving it a slightly bicontinuous character. The phase segregation is more pronounced than for the particle A1, as fewer HPMC-depleted domains can be detected within the detection limits of the confocal Raman microscopy. For the particle A3, which has a low solid content (3%), present as a uniform solution before drying, the HPMC and maltodextrin phases segregate as well (Figure 3c). Thus, the drying induces an increase of solute concentration that promotes phase segregation. The approximately 5 μm thick HPMC is present at the particle surface, and ,from the more red color and fewer HPMC-depleted domains, the level of phase segregation appears higher than that for A1 and A2. 3.3.2. Effect of the HPMC/Maltodextrin Ratio on the Ingredient Localization (A1, B1, C1). Individual particles with the compositions A1, B1, and C1 were dried using the DKA method. The particles were imaged by light microscopy, and all were about 600 μm in diameter and spherical with wrinkles. The ratio HPMC/maltodextrin was varied, while the total concentration dry matter was kept constant at 15.2%. All 10949
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Figure 4. Confocal Raman images of the distribution of HPMC (red) and maltodextrin (blue) of individually dried particles A1, B1, and C1.
Figure 5. LV-SEM micrograph of individually dried particle A1. (a) View of the whole particle; (b) view of the surface of the particle. (c,d) LV-SEM micrograph of broken individually dried particle A1, where “O” denotes the outer layer and “I” the inner layer of the particle.
The feed of the particle B1 shows equal volume of the HPMCenriched and maltodextrin-enriched phase (Table 1). After drying, the dried particles present an HMPC/maltodextrin phase segregation, but the distribution of the phases varies even in the same particle (Figure 4). Moreover, the HPMC phasesegregated layer is found to be localized in one case at the particle surface (3 μm thick HPMC layer). In the second case, the HPMC phase-segregated layer is found at the particle inner region (more than 15 μm thick HPMC layer). Further, a
samples are obtained within the two phase region, although the phase ratio varied from 1.88 for A1 and 1 for B1 to 0.22 for C1. After drying, the internal mapping of these particles was performed using confocal Raman microscopy. Representative images of the mapping are presented in Figure 4. The feed of particle A1 presents a large volume of the HPMC-enriched phase and a small volume of the maltodextrin phase (0.65/ 0.35) (Table 1 and Figure 4). Further, after drying, a phasesegregated system is seen with HPMC at the particle surface. 10950
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Figure 6. LV-SEM micrograph of individually dried particle C1. (a) View of the inner surface in the hole; (b) view of the whole particle; (c) view of the surface of the particle. (d,f) LV-SEM micrograph of broken individually dried particle C1, where “O” denotes the outer layer and “I” the inner layer of the particle.
HPMC 2 μm adsorbed fine layer is seen at the particle surface followed by a 10 μm depleted HPMC layer below the surface and up to 7 μm HPMC-enriched patchy domains in the bulk. The feed of the particle C1 has a larger maltodextrin-enriched phase volume than the HPMC-enriched phase volume (0.18/ 0.82) (Table 1). After drying, the confocal Raman image reveals a phase segregation of HPMC and maltodextrin where the HPMC layer (about 10 μm thick layer) is at the particle inner region. Additionally, maltodextrin-enriched (higher than one micrometer) domains can be seen in the HPMC layer. Plus a thin adsorbed layer of HPMC is observed (pink color) at the particle surface. Furthermore, we found that in the case of a high HPMC/maltodextrin volume ratio, the HPMC phase segregate layer is at the particle surface. When the HPMC/ maltodextrin ratio is equal to one, the bicontinuous system shows a HPMC phase-segregated layer at the particle surface or at the inner particle region or at an intermediate stage where the particle presents an HPMC adsorbed layer and HPMC patchy domains in the bulk. When the volume ratio is below 1, the HPMC layer is at the inner region of the particle. 3.4. Characterization of the Particles A1 and C1 by LV-SEM. 3.4.1. Individually Dried Particle A1. LV-SEM micrographs were taken on particle A1 (Figure 5). The low resolution image is dominated by dents and ridges, characteristic for particles that implode at the end of the drying process (Figure 5a). The high resolution image displays a particle surface with circular domains present (about 2 μm large), Figure 5b. LV-SEM micrographs of a broken particle A1 show a smooth inner layer (I) and an outer layer (O) with circular
domains, Figure 5c,d. The thickness of the outer layer is around 5−10 μm. By comparing with the confocal Raman microscopy images, we believe that LV-SEM reveals the phase segregation of the HPMC and maltodextrin, where HPMC is the outer layer (observed as a grainy structure) and maltodextrin is the inner layer of the particle (observed as a smooth structure). 3.4.2. Individually Dried Particle C1. LV-SEM micrographs were taken on particle C1 (Figure 6). The particle presents a smooth surface (Figure 6c), and the inner surface has round domains (Figure 6a). The observation of the inner surface was possible due to the presence of a hole from the surface to the vacuole. The micrographs of the broken particle C1 (Figure 6d,f) show the inner layer (I) with round domains with a size of around 2−5 μm and a smooth outer layer (O). This is the opposite behavior seen for A1, where the surface presented circular domains and the inner layer was smooth. From structures observed by LV-SEM, we believe that the correlation between LV-SEM and confocal Raman microscopy (Figure 4) is in this case also valid. Indeed we observe a phase segregation of HPMC and maltodextrin where HPMC is in the inner layer and maltodextrin is in the outer layer of the particle. 3.5. Confocal Raman Microscopy on Spray Dried Particle C1. With the aim of studying phase segregation in a faster drying process, laboratory spray-dried powder was analyzed. The feed of composition C1 was spray dried. The particles obtained were in the range 5−20 μm (approximated size observed on microscopy images). The large particles (about 20 μm) where picked, and their internal composition was analyzed by confocal Raman microscopy (Figure 7). The 10951
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(particle A1 and A2), maltodextrin is found in a more or less HPMC continuous matrix. Thus, the HPMC forms a layerenriched phase located at the particle surface, and the maltodextrin-enriched phase is toward the inner layer (Figure 8d,e). This is confirmed by LV-SEM in Figure 5c,d and confocal Raman microscopy in Figure 3. In the case of a phase ratio smaller than 1 (particle C1), the system is a HPMC in maltodextrin matrix, and the HPMC-enriched phase is found in the outer layer while the maltodextrin-enriched phase is at the particle surface (Figure 8g). This is confirmed by LV-SEM in Figure 6d,f and by confocal Raman microscopy in Figure 4e. We noticed the presence of an HPMC adsorbed layer at the particle surface, Figure 4. Finally, when the phase ratio is equal to 1 (particle B1), the HPMC-enriched phase can be at the surface (case 1), or adsorbed at the surface and in the outer layer as HPMC-enriched patchy domains (case 2), or a thin HPMC adsorbed layer at the surface and a thick HPMCenriched phase in the outer layer of the particle (case 3) (Figure 8f). The three cases were possible to observe in the same particle (Figure 4b,c,d). Hence, we believe that the system is bicontinuous. The same behavior was found by Rediguieri et al.32 on a pectin/caseinate mixture (also seen in ref 33 with a gelatin/maltodextrin mixture). Actually, by confocal scanning light microscopy, they were able to observe at a 75/25 (pectin/ caseinate) ratio caseinte droplets in a pectin matrix. At a 50/50 ratio, the mixture was bicontinuous, and, at 25/75, pectin droplets were observed in a caseinate matrix. Here LV-SEM and confocal Raman microscopy are found to be valuable methods for internal mapping of phase segregation. Indeed the correlation of the size (about 2 μm) of the maltodextrin-enriched domains in the HPMC-enriched top phase in the particle A1 analyzed by Raman and the size (about 2 μm) of the spherical domains in the outer layer of the particle A1 observed by LV-SEM is confirming. The coupling LV-SEM and confocal Raman microscopy methods gave us the opportunity to proceed to a complete identification of the phase inversion between particles A1 and C1, where the outer layer of particle A1 is the HPMC-enriched phase, and the inner layer is the maltodextrin-enriched phase, and where the outer layer of particle C1 is the maltodextrin-enriched phase and the inner layer is the HPMC-enriched phase. This study allowed us to show phase segregation occurring at different time scales. Indeed, the long drying time, about 2 days, of the dried film on the glass slide reveals phase segregation of HPMC and maltodextrin (Figure 2). The phase segregation of these compounds was also found at a shorter time scale, about 3 min, in individually dried particles (Figures 3−6). Furthermore, for the first time, phase segregation in spray drying was revealed. Indeed, in Figure 7, it is possible to see that, even at short drying time, scale phase segregation occurs. The fast drying time has the effect of kinetically trapping the structure, resulting in the formation of a smaller domain of HPMC within the maltodextrin matrix.
Figure 7. Confocal Raman image of the distribution of HPMC (red) and maltodextrin (blue) of spray-dried particle.
Raman image allowed us to observe a thin adsorbed layer of HPMC at the particle surface. Below the surface, a 1−2 μm thick maltodextrin-enriched layer is found and, in the bulk HPMC-enriched domains can be observed surrounded by maltodextrin. Thus, phase segregation between HPMC and maltodextrin is observed in the spray-dried particle.
4. DISCUSSION Phase segregation has been observed previously in freezedrying,4 film drying12 (also seen in Figure 2), and individually dried particles.23 The aim of this study is to reveal the influence of the solid content and the phase ratio (composition) on the ingredient localization in individually dried particles. First we assumed that a shorter drying time can promote the system being kinetically trapped at an earlier stage of phase segregation than at a longer drying time, Figure 8 A. Here we assume that low solids content in the liquid before drying promotes a longer drying time for the particle as the amount of solvent (water) is higher and the viscosity lower. Further, at high solid content (particle A1, Figure 3), the solvent evaporation is rapid so the particle drying time will be short, which may kinetically trap and freeze the structure at an early stage of the drying process (Figure 8c). Figure 8c represents the ingredient localization in the case of high solid content. In this case, the phase segregation between HPMC (top layer in red) and maltodextrin (bottom layer in blue) is observed. Large maltodextrin-enriched domains can be observed in the HPMC phase. At lower solid content (particle A2, Figure 3), the drying time will be slightly longer, which will promote a more complete phase segregation. Indeed, the maltodextrinenriched domains in the HPMC phase are smaller than for the case at higher solids content (Figure 8b). At low solid content (particle A3, Figure 3), the longer drying time allow a complete phase segregation of HPMC and maltodextrin (Figure 8a). Additionally, this study brought us to investigate the effect of the phase ratio (VHPMC‑enriched/Vmaltodextrin‑enriched) on the ingredient localization at a fixed solids content, i.e., constant drying time (Figure 8B). When the phase ratio is higher than 1
5. CONCLUSION In this study we have seen that phase segregation induced by solvent evaporation, which increases solute concentration, was present independent of the drying time. Film drying, single particle drying, and spray drying showed a similar behavior. We found that at a shorter drying time, the phase segregation of two biopolymers is less pronounced than at a longer drying time. This is explained by kinetic trapping of the state of materials at the moment the water content becomes low 10952
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Figure 8. (A) Schematic illustration of the internal mapping of individually dried particles in response to the decrease of solids content with a constant HPMC/maltodextrin ratio. (B) Schematic illustration of the internal mapping of individually dried particles in response to the decrease of phase ratio and a constant drying time. HPMC is in red, and maltodextrin is in blue.
enough to prevent or severely retard diffusion of the biopolymers. Thus, high solid content may also slow down the phase segregation phenomena. The distribution of two biopolymers can be tuned by the phase ratio used in the liquid before drying. Indeed, with the help of confocal Raman microscopy and LV-SEM, it was possible to observe a maltodextrin-enriched layer at the particle surface and an HPMC-enriched layer in the core when the phase ratio was below 1. The opposite was found when the phase ratio was higher than 1. Further, when the ratio was equal to 1, the dried particle showed a bicontinuous distribution of maltodextrin- and HPMC-enriched domains.
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Individually dried particle SEM micrograph to describe the confocal Raman microscopy scanning area (PDF)
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by the EU seventh framework programme through the “‘PowTech’” Marie Curie Initial
ASSOCIATED CONTENT
Training Network (Project No. EU FP7-PEOPLE-2010-ITN-
S Supporting Information *
264722). The SP Technical Research Institute of Sweden
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00856
gratefully acknowledges the Troëdsson Foundation for financing the Confocal Raman Microscope. 10953
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on September 23, 2015, without the Supporting Information. The corrected version was reposted on September 29, 2015.
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DOI: 10.1021/acs.langmuir.5b02023 Langmuir 2015, 31, 10946−10954
