Molecular and physical characterization of octenyl succinic anhydride-modified starches with potential applications in pharmaceutics

Journal Pre-proofs Molecular and physical characterization of octenyl succinic anhydride-modified starches with potential applications in pharmaceutics Carolina P. Mora, Juan Manuel Martinez-Alejo, Laura Roman, Mario M. Martinez, Teresa Carvajal, Rodolfo Pinal, Claudia E. Mora-Huertas PII: DOI: Reference:

S0378-5173(20)30147-2 https://doi.org/10.1016/j.ijpharm.2020.119163 IJP 119163

To appear in:

International Journal of Pharmaceutics

Received Date: Revised Date: Accepted Date:

6 December 2019 8 February 2020 17 February 2020

Please cite this article as: C.P. Mora, J. Manuel Martinez-Alejo, L. Roman, M.M. Martinez, T. Carvajal, R. Pinal, C.E. Mora-Huertas, Molecular and physical characterization of octenyl succinic anhydride-modified starches with potential applications in pharmaceutics, International Journal of Pharmaceutics (2020), doi: https://doi.org/ 10.1016/j.ijpharm.2020.119163

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© 2020 Published by Elsevier B.V.

Molecular and physical characterization of octenyl succinic anhydride-modified starches with potential applications in pharmaceutics

Carolina P. Moraa,e, Juan Manuel Martinez-Alejob,c, Laura Romand, Mario M. Martinezd, Teresa Carvajalc, Rodolfo Pinalb*, Claudia E. Mora-Huertasa*

a

Universidad Nacional de Colombia, Sede Bogotá, Facultad de Ciencias, Departamento de Farmacia, Grupo de

investigación en Desarrollo y Calidad de Productos Farmacéuticos y Cosméticos, Ciudad Universitaria, Carrera 30 No. 45 – 03, edificio 450, código postal 111321, Bogotá, Colombia. b

Department of Industrial and Physical Pharmacy, College of Pharmacy, Purdue University, 575 Stadium Mall

Drive, West Lafayette, IN 47907-2051, USA. c

Department of Agricultural & Biological Engineering, Purdue University, 225 South University Street, West

Lafayette, IN 47907-2093, USA. d School

of Engineering, University of Guelph, Guelph, Ontario N1G 2W1, Canada.

e Departamento

de Ciencias Farmacéuticas, Facultad de Ciencias Naturales, Universidad Icesi, Calle 18 No. 122 –

135, código postal 760031, Cali, Colombia.

∗Corresponding

authors.

Claudia E. Mora-Huertas E-mail address: [email protected]; Tel.: +57 1 3165000 Ext. 14609; Fax: 14639. Postal address: Departamento de Farmacia, Universidad Nacional de Colombia, Sede Bogotá, Ciudad Universitaria, Carrera 30 No. 45 – 03, edificio 450, código postal 111321, Bogotá, Colombia.

Rodolfo Pinal E-mail address: [email protected]; Tel.: +1 765 4966247; Fax: +1 765 4946545; Postal address: Department of Industrial and Physical Pharmacy, College of Pharmacy, Purdue University, 575 Stadium Mall Drive, West Lafayette, IN 47907-2051, USA.

Abstract Five commercially available starches modified with octenyl succinic anhydride (OSA) are characterized at a molecular, physicochemical and bulk level providing useful data for designing pharmaceutical products. The degree of substitution (DS) of the starches range from 0.017 to 0.032 and their molecular weights (Mw) and radius of gyration (Rz) are lower than those of native starch, suggesting additional modification processes besides the chemical treatment with OSA. The ability of the starches to reduce the water surface tension keeps a direct relationship with the DS and an inverse association with the Mw. Thermal properties, crystallinity assays and morphology evidence that most modified starches characterize by amorphous aggregated structures, possibly generated by gelatinization processes, which favor the flow properties of the 1

powders. Water sorption and surface energy behaviors seem to be related to the number of octenyl succinate (OS) moieties. After dispersion in water, shear-thinning and Newtonian behaviors also depend on the type of OS-starch.

Keywords Waxy corn starch, octenyl succinic anhydride, modified starch, structural properties, physicochemical characterization, pharmaceutical product.

Abbreviations ANOVA CAP CI DB DS DSC DMSO FDA FFC GU HIC IGC Ka Kb LSD Mw NCR NDS OS OSA PC PCA PG2000 PGU PSD PXRD QbD Ρ Rz RH SD SDS SEM SMSS SSA

Analysis of variance Capsul® Carr’s index Degree of branching Degree of substitution Differential Scanning Calorimetry Dimethyl sulfoxide Food and Drug Administration Flow function coefficient Glucose units Hi-Cap 100® Inverse gas chromatography Lewis acid number Lewis base number Fisher's least significant difference Weight-average molecular weight N-Creamer 46® Native or without degree of substitution Octenyl succinate Octenyl succinic anhydride Principal component Principal component analyses Purity Gum 2000® Purity Gum Ultra® Particle size distribution Powder X-ray diffraction Quality by design Dispersed molecular density z-average radius of gyration Relative humidity Standard deviation Sodium dodecyl sulfate Scanning electron microscopy Sugary maize soluble starch Specific surface area 2

WMS

Waxy maize starch

1. Introduction Although the use of starches as a pharmaceutical excipient is well-known (Singh et al., 2010), chemical, physical or enzymatic modifications are carried out to improve their performance. In fact, the United States Pharmacopoeia includes the modified starch monograph to refer to the food starch subjected to different treatments in order to change its functional properties (USP, 2018). In particular, the chemical modification of starch with OSA, i.e., esterification with DS values of approximately 0.02 as it has been established for the United States Food and Drug Administration (Miao et al., 2014), leads to materials exhibiting good properties as suspending (Kuentz et al., 2006), emulsifying and film-forming agents (Sweedman et al., 2013). In this way, OS-starches from different natural sources such as plantain (Bello-Flores et al., 2014), maize (Timgren et al., 2013), waxy maize (Miao et al., 2014), high-amylose maize (Timgren et al., 2013), sugary maize (Ye et al., 2017), potato (Hui et al., 2009), rice, waxy rice, quinoa, waxy barley (Timgren et al., 2013), and amaranth (Bhosale and Singhal, 2006), have been obtained and characterized by different techniques according to the purpose of the study. However, challenges regarding the research on OS-starch properties at the solid state, aqueous dispersion and structural level, still exist, moreover when the development of pharmaceutical products would be based on a quality by design (QbD) approach, where the selection and processing of the raw materials require extensive knowledge of their molecular, physicochemical and pharmaceutical properties. As a contribution in this regard, a comprehensive study of characterization for different OSA-modified waxy corn starches, that are commercially available to use in pharmaceutics, is reported. In this sense, our hypothesis is: “structural properties of waxy corn starches chemically modified with OSA explain the behavior of their pharmaceutical properties”. It is expected to provide the readers with useful information about modified starches as raw material to understand and predict the influence of their characteristics on their performance as excipients and make rational decisions at the different stages of the products and processes design to guarantee the quality, performance, stability, and safety of the final products.

3

2. Materials and methods 2.1. Materials The materials used in this study were unmodified waxy corn starch (native or without degree of substitution - NDS) and five commercially available types of OSA-modified waxy corn starch: Hi-Cap® 100 (HIC), Purity Gum® 2000 (PG2000), Capsul® (CAP), N-Creamer® 46 (NCR), and Purity Gum® Ultra (PGU), that were kindly provided by Ingredion (Cali, Colombia and New Jersey, USA). Detailed structural and compositional parameters for NDS were previously reported by Martinez et al. (2018). Briefly, lengths of the amylopectin chains: 14.4 and 39.6 glucose units (GU) for the shortest (XAp1) and the longest (XAp2), respectively; molar ratio of long to short chains (hAp2/hAp1): 0.54; degree of branching (DB): 5.80; and amylose content: negligible. Deionized water was used, and all the other reagents were commercially available and analytical grade.

2.2.

Methods

2.2.1. Morphology The surface morphology of the OS-starches and NDS was observed by a scanning electron microscope (FEI Quanta 200, FEI Company, Czech Republic) operated at 15 and 30 kV and a working distance of 10 -13 mm. Previously, starch samples were dried at 40 °C for 24 h (Jouan IG 150 oven, Jouan, France), dispersed on a carbon adhesive-coated metal stubs and sputter coated with gold-palladium for 1 min at 80 mA by using a Quorum Q150R ES Sputter Coater (Quorum Technologies Ltd, Laughton, East Sussex, England). Additionally, starches were observed by an Olympus BX 51 polarized light microscope (Olympus Optical Co., Ltd., Japan) fitted with a 100x objective lens and a digital camera (Infinity1, Canada). The images were processed by the Infinity Capture software (Ver. 5.0.4.).

2.2.2. Particle size distribution The starch sample was added under stirring (2500 rpm; ultrasound: 20%) to the flow system of the particle size analyzer (Mastersizer 2000, Hydro 2000µP, Malvern Instruments, Westborough, MA, USA; 20 mL capacity) containing 95% (v/v) ethanol. The optical mode was Mie, where the refractive index of starch and ethanol were set to 1.53 and 1.36, respectively, and the obscuration 4

was between 5% and 20%. The measurements were processed by Mastersizer 2000 software Ver. 5.61. The particle size characterization was carried out in triplicate for each starch sample and the results are expressed as the diameters D[4,3] and D[3,2], and the span value.

2.2.3. Crystallinity Powder X-ray diffraction (PXRD) analyses were performed in a Panalytical X’Pert PRO MPD diffractometer (PANalytical, Holland; X’Pert Data Collector Software, Ver. 2.2a). Diffraction patterns were collected at room temperature using Ni-filtered CuKα radiation (λ = 1.5418 Å) with X-ray tube operated at 40 kV and 30 mA; diffraction angle 2-theta; range 5 – 50 °; scan rate 0.02°/70 s; and step size 0.02°. The starch samples were prepared before the analysis by drying at 40 °C for 24 h (Jouan IG 150 oven, Jouan, France).

2.2.4. Thermal properties Thermal properties of the starch samples were investigated by high-pressure differential scanning calorimetry (HP DSC 1 STARe System, Mettler-Toledo AG, Analytical, Schwerzenbach, Switzerland; STARe Software Ver. 13.00). Starch (2 mg on dry base) was placed in an aluminum pan at room temperature (20 °C), and deionized water was added to obtain a dispersion of about 30% of starch. The pans were hermetically sealed and allowed to stand for 12 h at room temperature. Sample pans were heated at a rate of 10 °C/min from 25 to 120 °C. An empty pan was used as a reference and the assays were carried out in triplicate.

2.2.5. Degree of substitution The DS of OS-starches was determined following the methodology proposed by Scheffler et al. (2010) with some modifications regarding the amount of starch sample and the volumes of the analytical reagents used. Therefore, starch samples (1.2 g) were added of 7.2 mL of 2.5 M HCl and the dispersion was stirred for 30 min. Then, 90% (v/v) isopropanol (30 mL) was added and stirring was maintained for 10 min. This dispersion was centrifuged at 3000 g for 10 min and the supernatant was discarded. The solid residue was resuspended with 90% isopropanol (30 mL) and centrifuged as previously mentioned. This procedure was repeated to remove the excess of acid until a negative test of chloride by the white-haze test with AgNO3. The resulting solid material was dried at 40 C for 30 h in a vacuum oven (VWR, Sheldon Manufacturing Inc., 5

Cornelius, OR, USA) to constant weight. Samples of 125 mg of dried solid material were dispersed in 20 mL of deionized water and kept in a boiling water bath for 30 min. Then, those mixtures were titrated with 1 mM NaOH until pH 7.00  0.1. A blank of native waxy corn starch was simultaneously titrated following the same procedure. The DS was calculated by Eq. (1): 𝐷𝑆 =

162 x 𝐴 1000 ― (210 x 𝐴)

(1)

where the values 162 and 210 are the molecular weights of the glucosyl unit and the octenyl succinate group, respectively, and A is the amount of octenyl succinate groups in one gram of modified starch, expressed in mmol/g, and calculated by Eq. (2): 𝐴=

(𝑉 ― 𝑉0) x 𝑀

(2)

𝑊

where V is the NaOH solution consumed by the octenyl succinate starch (mL), V0 is the NaOH consumed by native starch (mL, blank titration), M is the molar concentration of NaOH and W is the dry weight (g) of each sample. DS determinations were performed in triplicate.

2.2.6. Weight-average molecular weight (Mw), z-average radius of gyration (Rz) and dispersed molecular density (ρ) OS-starches or NDS were solubilized as described by Jane and Chen (1992) with modifications. Thus, each starch (0.10 g) was stirred in 90% (v/v) DMSO (10 mL) for 1 h at 95 ºC and then for 24 h at room temperature. Solubilized starch (DMSO-starch) was precipitated with 40 mL of 99.5% (v/v) ethanol and centrifuged at 14000 g for 10 min, repeating it three times. Precipitated starch was dried (vacuum oven, Thermo Scientific, Thermo Fisher Scientific Inc., Columbus, OH, USA) at 30 ºC for 2 h and ground in a mortar. The weight-average molecular weight (Mw) and the z-average radius of gyration (Rz) of starches were measured by a multi-angle light scattering detector (MALS, Dawn Heleos, Wyatt Technology, Santa Barbara, CA, USA) fitted with a K5-flow cell. To this end, the dried starch (0.02 g) was re-dissolved in deionized water (10 mL) and autoclaved at 121 ºC for 20 min. Samples were transferred to a boiling water bath to keep them completely dissolved, then filtered through a 5 µm nylon membrane filter and injected (20 µL) into the HPSEC-MALS system fitted with PL aquagel-OH 60 8 µm and PL aquagel-OH 40 8 µm columns (Agilent Technologies, Waldbronn, Germany) connected in series. Samples were eluted with filtered purified water containing 0.02% (w/v) sodium azide at 6

a flow rate of 0.7 mL/min. Data were analyzed based on the light scattering theory using ASTRA software (version 5.3.4.14, Wyatt Technology Corporation, Goleta, CA, USA) and following the Zimm second order plot procedure (Harding et al., 2016). The specific refractive index increment (dn/dc) was assumed as 0.146 mL/g (Bello-Pérez et al., 2017) and the second viral coefficient (A2) was considered negligible (Yoo and Jane, 2002). The assays were carried out in duplicate. The dispersed molecular densities () were calculated by Eq. (3).

 = Mw/Rz3

(3)

where Mw is the weight-average molecular weight of the starches and Rz is the z-average radius of gyration of the biopolymers. 2.2.7. Surface tension The surface tensions of modified starches dispersions and aqueous solutions of sodium dodecyl sulfate (SDS) were measured by the Wilhelmy plate technique (Krüss K100 Mk2 tensiometer, roughened platinum plate, Krüss GmbH, Hamburg, Germany). Deionized water was used to prepare the samples of each material in concentrations ranging between 0.01 and 3.5 wt.%; the measurements were obtained in triplicate at 20 ± 2 C.

2.2.8. Rheological measurements of starch aqueous dispersions Rheological properties of modified starch aqueous dispersions (22 wt.%) were determined by using a controlled stress rheometer (Discovery HR-3 hybrid Rheometer, TA Instruments Ltd., New Castle, DE, USA) fitted with a cone/plate system (40 mm, 2º; gap value: 60 µm) and a solvent trap. Flow curves (shear stress vs. shear rate) were determined at 25 ºC increasing the shear stress in the range of 0.1 to 200 Pa depending on the investigated starch. The apparent viscosity (𝑎𝑝𝑝) of each of the starch aqueous dispersions was measured as a function of shear stress (). Experimental flow curves were fitted to the Ostwald's model (Eq. 4): 𝜏 = 𝐾𝛾𝑛

(4)

where  is the shear stress, 𝐾 is the consistency coefficient, 𝛾 is the shear rate and 𝑛 is the flow behavior index. Rheological parameters such as flow behavior index (𝑛), consistency coefficient (𝐾) and apparent viscosity (𝑎𝑝𝑝) at a shear stress of 1.0 Pa, are reported as the mean value of three measurements for each starch aqueous dispersion prepared in triplicate. 7

2.2.9. Bulk properties The true density of the starches was measured by the helium displacement method (AccuPyc II 1340, Micromeritics Instrument Corp., Norcross, GA, USA). The work conditions of the test include nominal cell 3.5 cm3; starch sample: 0.8 to 1.8 g; purge cycles: 10. The measurements were carried out at room temperature by triplicate (each one of them corresponding to the average of 5 measurements). Pycnometer calibration was performed with a steel sphere before the assay. Aerated and tapped densities were determined at room temperature, 12 runs/sample, by using a Tapped Density Tester (350, Agilent Technologies, NJ, USA) and following the USP method (USP, 2018). Carr's index (CI) and Hausner ratio were calculated from the obtained results. Powder flowability was measured by using a Freeman FT4 powder rheometer fitted with shear cell module (Freeman Technology Ltd., Worcestershire, UK; work conditions: splitting cylindrical vessel 50 mm x 85 mL; powder conditioning procedure: blade system; powder preconsolidation: 3.0 kPa, vented piston; stresses at 2.0, 1.75, 1.5, 1.25 and 1.0 kPa). Powder flow classification was made from the flow function coefficient (FFC) values according to Schulze (2008).

2.2.10. Surface properties measurements Vapor sorption isotherms were determined at 25 °C by using a symmetrical gravimetric analyzer (Model SGA-100, VTI Corporation, Hialeah, FL, USA), in the range of 10 to 90% of relative humidity (RH). Samples were dried in the instrument at 50 °C under a stream of dry nitrogen until constant weight. Sodium chloride and polyvinylpyrrolidone were used as calibration standards for the instrument. Prior to the studies, samples were stored for one week in drierite chambers to minimize the moisture content. The surface heterogeneity profiles of the starch samples were investigated according to the method reported by Martinez-Alejo et al. (2018), by using inverse gas chromatography – IGC (IGC-SEA 2000 system; Surface Measurement Systems, Ltd., NA, PA, USA, fitted with a flame ionization detector and SEA Analysis Software, Advanced version 1.4.2.0). Measurements were performed in duplicate. The Specific Surface Area (SSA) of the samples was estimated from the BET equation on nitrogen sorption isotherms carried out at -195.8 °C (Quantachrome Nova 2200e Surface Analyzer; Quantachrome Instruments, Boynton Beach, FL, USA). Prior to this test, the starch samples were degassed under vacuum at 95 °C for 18 h. 8

2.2.11. Statistical analysis Data processing was performed by the OriginPro 2016 software (OriginLab Co., Northampton, MA, USA) and the statistical analysis for the different tests was performed by Statgraphics Centurion XV software (Statpoint Technologies, Inc., The Plains, VA, USA). To this end, mean values and standard deviations (SD) were used to carry out tests of analysis of variance (ANOVA) and Fisher's least significant difference (LSD), where appropriate, at a confidence of 95%.

3.

Results and discussion

3.1. Morphology and particle size As shown in Fig. 1A, in general terms, granules of NDS and CAP are polygonal, and HIC, PGU, NCR, and PG2000 exhibit spherical shapes, aggregated structures and uneven surfaces with folds where small granules are located. Actually, there is no consensus on the particle morphology of modified starches. Thus, Bello-Flores et al. (2014) and Timgren et al. (2013) state that chemical modification with OSA does not alter the surface of the starch granules, while Song et al. (2006) evidence pores and cavities on the surface of chemically modified starches related to the OSA substitution. Additionally, Yusoff and Murray (2011) report broken granules and aggregated structures for NCR. It is interesting to note that in our study, the modifications in the morphology of the hydrophobic starches might suggest that in addition to the chemical treatment carried out with OSA, other modification processes affecting the integrity of the grain structure could have been carried out, especially for the NCR, PGU, PG2000 and HIC starches. Perhaps, gelatinization and spraydrying processes could be involved throughout the industrial production of these starches, as it has been reported. For example, Fu et al. (2012) evidence shriveled granules with multiple surface folds in partially gelatinized corn starch obtained by gelatinization followed by spray drying. Likewise, Niazi and Broekhuis (2012) report irregular, shriveled, and cratered particles exhibiting high agglomeration levels, after the spray drying treatment of aqueous solutions of native starch prepared at elevated temperatures (140 °C). Besides, Basilio-Cortés et al. (2019) evidence exo-erosion and whitish points in modified starch granules obtained via acid hydrolysis and succinylation treatment with OSA. 9

On the other hand, as shown in the polarized light photomicrographs (Fig. 1B), NDS exhibit birefringence attributed to the anisotropy own of its semi-crystalline nature which is retained to some extent, in CAP granules. On their part, the lack of birefringence in NCR, PGU, PG2000 and HIC granules, evidence disruption of the molecular order among the amylopectin chains, possibly generated by a gelatinization process. Concerning the particle size distribution (PSD), in general, starch samples were unimodal (Fig. 2), and span values follow the order: CAP  HIC  PG2000 = NDS  PGU  NCR, keeping relationship with details of the SEM photomicrographs such the presence of aggregates for PG2000, HIC, PGU, NCR and, NDS starches. The D3,2 values for the modified starches follow the order: PG2000  HIC  NCR  PGU and the particles are biggest in size that NDS as it is expected considering the modification processes of the starches.

3.2. Crystallinity and thermal properties As shown in Fig. 3A, the investigated starches can be classified as completely amorphous (HIC, NCR, PG2000, and PGU) or semi-crystalline (NDS and CAP) materials, to the extent possible of the periodic arrangement of the amylopectin regions. In addition, considering that NDS and CAP starches show specific peaks at 2- angles of approximately 14.9°, 17.0°, 17.9°, and 23.0°, they could correspond to polymorph A (Song et al., 2006; Wang and Wang, 2002). Regarding thermal characterization of the modified starches, drastic differences respect to the native starch (Fig. 3B) are evidenced. Thus, NDS exhibits an endothermic signal (Tonset: 64.31 ºC; ΔH: 3.76 J/g) that could be attributed to the gelatinization process (Wang and Wang, 2002). Similarly, CAP shows a small peak (Tonset: 76.26 ºC; ΔH = 0.40 J/g) supporting the assertion that this starch held some of the semi-crystalline structure after modification with OSA. On the contrary, the lack of this thermal transition for PGU, NCR, PG2000, and HIC suggests a pregelatinized nature (BeMiller, 2019), which is in line with the amorphous behavior observed for these starches.

3.3. DS, Mw, Rz and ρ characterization Table 1 compiles the results of degree of substitution, weight-average molecular weight, zaverage radius of gyration and dispersed molecular density for the investigated starches (size exclusion chromatograms are reported in supplementary material, Fig. S1 and S2). 10

DS is the average number of hydroxyl groups substituted per anhydroglucose units (Hui et al., 2009), that in general terms, for the OS-starches investigated, follows the order: PGU  NCR  CAP  PG2000  HIC. These results might be linked to both, the ability of the starch to react with the OSA reagent and the amount of the OSA reagent used during the chemical modification process. On this respect, Miao et al. (2014) provide additional evidence in their research works on OSA-modified starches. Thus, the DS of OSA-WMS (waxy maize starch) and OSA-SMSS (sugary maize soluble starch) increases as the starting amount of OSA for the esterification process increases. Also, the DS of the modified SMSS is higher than that of the modified WMS, which was attributed to the easiness to disperse the starches into the aqueous reaction media; thus, while WMS remains suspended during the chemical modification with OSA, SMSS reaches a better dispersion because is a water-soluble polysaccharide. In our research the processes and work conditions used to obtain the modified starches under study are unknown. However, considering that HIC and PG2000 exhibit the highest DS values and the lowest Mw, it could be proposed that the Mw is a factor related to the ability of the starches to be substituted, maybe because the dispersibility into the reaction medium might be favored. On the other hand, as shown in Table 1, NDS exhibit a significantly highest Mw compared to the OS-starches that follow the order: HIC  PG2000  PGU  NCR. Regarding CAP, this is the only one exhibiting a multimodal molecular weight distribution (supplementary material, Fig. S2 and Table S1). In general, Mw of modified starches decreases as a function of the OSA substitution, which could be attributed to starch depolymerization phenomena because of the work conditions used during the esterification reaction, i.e., pH  8.5 - 9.0 and stirring, as it has been reported by Bello-Flores et al. (2014) and Miao et al. (2014). Therefore, it is possible that the increase in Mw by grafting OS groups is counteracted by the alkaline depolymerization during the OSA-conversion and possibly, by additional physical, chemical or biochemical processes leading to decreasing of the starch´s Mw. Evidence in this sense is reported by Li et al. (2014) and Ulbrich et al. (2019). According to the former, the Mw of the OS-starches decreases significantly after enzymatic hydrolysis of the -(1 4) glycosidic bonds by alpha-amylase. On their part, Ulbrich et al. (2019) found the Mw decreasing for waxy corn starch, associated with the work conditions (acid concentration and hydrolysis time) for the acid-thinning process. However, statements on this respect cannot be made. Opposite results have been also reported by Ye et al. (2017), where octenyl succinic-sugary maize soluble starch (OS-SMSS) exhibits higher Mw than unmodified starch, that was related to the amount of OS groups grafted by the starch chains.

11

Concerning the Rz value for the investigated OS-starches, it increases in the order: HIC  PG2000  PGU  NCR with characteristic values lesser than that estimated for NDS. Rz for CAP ranges between 48 and 103 nm according to the three populations of Mw evidenced. As shown in Table 1, the decrease in Rz relates to the molecular weight of the modified starches, i.e., NCR shows greater values of Mw and Rz, compared with HIC. NCR and PGU characterize by greater  values than NDS, whilst PG2000, HIC and, CAP exhibit lower ones. This might be explained from the influence of the Mw and Rz on the stretching and flexibility of the amylopectin individual clusters in the aqueous dispersion medium used for the substitution reaction, that as a consequence, determines the fine structure of the starch when it is dried (Bertoft, 2013; Miao et al., 2014). Therefore, for PG2000, HIC and CAP characterized by lower Mw and Rz values, their individual clusters could have lower stretching than that expected for NCR and PGU where the greater Mw and Rz values lead to a greater expansion of the chains.

3.4. Surface tension and flow rheological properties of starch aqueous dispersions Fig. 4A reports the results of surface tension () as a function of the starch concentration in aqueous dispersions. SDS solutions were used as a reference in this research work, because of their widely recognized ability to reduce the water when used at low concentrations; in this case, 0.5 wt.% SDS causes a drastic reduction of water, exhibiting then a relatively constant behavior as the concentration increases. Regarding the modified starches, the water is approximately reduced in half at concentrations of 2.0 wt.% of CAP, HIC and PG2000 starches and then,  value keeps relatively constant as described for SDS solutions. This finding suggests that these starches behave as classic low Mw emulsifying agents at that concentration. On the contrary, PGU and NCR do not exhibit an efficient ability to reduce the water, which might be linked to the structural properties of the modified starches such as Mw and DS. Thus, a high DS and a low Mw of the modified starches allow significant reductions of the water, as OS-starch molecules could be easily adsorbed at the air-water interface. The driving force of this process is the substantial gain in free energy of the system due to the hydrophobic effect own of the OS groups bounded to the starch chains when they move from the aqueous solution to the non-polar phase (air) (Genest et al., 2013). Therefore, the differences in the hydrophobic character of the starches due to the OS groups grafted to the chain determine how easy is the migration of the OS-starch molecules at the air-water interface. Additionally, the low Mw, which suggests smaller molecules, facilitates their adsorption and 12

packing at the air-water interface. In this way, modified starches are biopolymers exhibiting amphiphilic properties and surface activity that is worth exploring when the search of alternative emulsifying agents for pharmaceutical products is intended. On the other hand, to know the rheological behavior of aqueous dispersions of the modified starches is essential for developing pharmaceutical products considering the influence of this property, among others, on the selection of the formulation and the container closure system, the definition of the manufacturing process, and the stability and consumer acceptance of the products. Fig. 4B shows the relationship between shear stress and shear rate for aqueous dispersions at 22 wt.% of the modified starches under study. In all cases a good fit of the data to the Ostwald's model (R2  0.999) is obtained and the interpretation of the parameters of this model lead to a better approximation to the nature of these starches. Therefore, from the value of flow behavior index (𝑛), NCR, PGU and PG2000 can be classified as pseudo-plastic materials because their shear-thinning behavior (n  1) characterized by a rate of disruption of the intermolecular entanglement greater than that of reformation, result in less intermolecular resistance to flow and a lower apparent viscosity (𝑎𝑝𝑝) (Sopade, 1999; Ye et al., 2017). For their part, aqueous dispersions of HIC and CAP starches show similar behaviors to Newtonian fluids (n values nearest to 1), where 𝑎𝑝𝑝 tends to remain constant when the shear stress increases. Concerning the consistency coefficient (𝐾), a parameter related to the viscous nature of the materials, for the investigated starches, it increases in the order: HIC  CAP  PG2000  PGU  NCR, that in turn, correlates with their Mw (Table 1). Moreover, comparing the apparent viscosity (𝑎𝑝𝑝) values obtained at 1.0 Pa for the starch dispersions, those prepared from NCR exhibit a viscosity sharply higher compared to the other starches. This could be attributed to the major ability of NCR to immobilize water molecules via hydrogen bonding as this starch has the greatest Mw and consequently, the highest number of the hydroxyl groups of its glucose units. Although networks of hydrophobic interactions have also been described for hydrophobically modified graft-polymers, where the intermolecular associations between OS-groups located in the branched chains of amylopectin might allow the formation of entanglements that increase the consistency coefficient (Ortega-Ojeda et al., 2005), in this work it does not seem to be the factor governing the rheological behavior of the starches. Thus, HIC, the starch exhibiting the highest DS value, shows both low viscosity and consistency coefficient. Consequently, it could be considered that a smaller number of hydrophobic molecular interactions occurs, or that these 13

weak intermolecular links are quickly broken up under the influence of the shear stress (Dokić et al., 2012).

3.5. Bulk properties and powder flowability As a general tendency, true, aerated and tapped densities of the investigated starches keep relationship with the particle size, particle shape and amorphous or semi-crystalline nature of these materials but any correlation is possible with the DS or Mw. For this reason, PG2000, HIC, NCR, and PGU, characterized by irregular granules of amorphous nature and the highest D[3,2] values, show the lowest true densities (Fig. 5A and 5B) suggesting a high porosity, i.e., greater intraparticle spaces. On the contrary, the more defined particle shapes, the lowest particle size values and semi-crystalline nature for NDS and CAP starches could explain their highest values of true densities (1.513 g/mL and 1.493 g/mL for NDS and CAP, respectively). Regarding the aerated and tapped densities, CAP and NDS having the smallest granule sizes (D3,2) and regular shapes get a better packing of the powder, as a result, the highest aerated density values are obtained. Likewise, the powder cohesivity of these starches is high (Fig. 5A and 5B), consequently, their tapped densities are the highest as well. In addition, when the starches are constituted by fine particles (7.1 µm and 21.2 µm for CAP and NDS, respectively), the interparticle van der Waals forces could govern their packing resulting in a more cohesive material (Abdullah and Geldart, 1999). Concerning the other OS-starches, their tapped densities values follow the order: PGU  NCR  PG2000  HIC, that is consistent with the aggregates and, especially for NCR and PGU, the characteristic large particle size of these materials (39.5 µm and 61.4 µm for NCR and PGU, respectively). Consequently, the interparticle forces become weaker giving rise to a less cohesive material that rearranges itself in a less consolidated way. In the matter of the flow properties of the powders, NDS shows values of 1.44 and 31% for the Hausner ratio and the Carr's index, respectively, which are characteristic of a cohesive and poor flow material (Fig. 5C and 5D; USP, 2018). It is possible that besides to the angular shape of the NDS particles, the high surface energy of the polar interactions due to the starch structure (OHrich surface) leads to a significant water uptake that results in flow difficulties for this material. On the contrary, the results for the modified starches suggest low cohesiveness and better flow due to the weak interparticle cohesion forces because of the hydrophobic (octenyl succinate) groups on the particle surface. Additionally, the other possible modifications in which these starches could be submitted, as gelatinization or processing by spray dried, produce aggregates 14

that improve their flow properties. Thus, although CAP has a smaller particle size than NDS and its granules show polygonal shapes, it exhibits adequate flow properties. This is aligned with the obtained flow function coefficient (FFC; Fig. 5E) since this parameter is mainly governed by the cohesion forces among the particles (Podczeck and Wood, 2003). Thus, NDS behaves like a cohesive material whereas CAP, PG2000, HIC, and PGU evidence easy flow. The presence of OS groups on the surface of the starch granules reduces the adhesion forces between the particles decreasing their cohesion and consequently improving their flow properties. However, particle size also seems to play a role because the FFC increases as the particle size increases (Fig. 5E). Maybe, the reduction in the contact area between granules decreases the cohesion forces in the powders. Thus, FFC and particle size show a direct correlation for PG2000, HIC, and PGU. Particular attention is required to interpret the flow behavior for NCR, whose results seem contradictory. According to the FFC value (3.45), this starch is classified as a cohesive material, but the Hausner ratio of 1.2 and Carr's index of 14.5% suggest good flow properties. This could be because of the methodology used and the associated experimental error of each method. The Hausner ratio and the Carr's index are calculated values from the aerated and tapped densities that in turn, are estimated from the mass of the material and the volume of the test cylinder (Jallo et al., 2012). Regarding the FFC value, it is obtained as a direct measurement using the Freeman FT4 powder rheometer (Hare et al., 2015). In addition, and perhaps more relevant, NCR characterizes by the largest particle sizes and the highest span values that as a whole, might facilitate the flow of this material as it is corroborated when the cohesion coefficient of the starch powders is plotted as a function of particle size (Fig. 5F). For PG2000, HIC and PGU an inverse correlation between cohesion and particle size is obtained. In summary, the investigation of the bulk properties of NDS and OS-starches evidences that unmodified starch is a cohesive and poor flow powder, whereas the modified starches exhibits good flowability properties that facilitate their handle during the industrial processing.

3.6.

Surface properties measurements

Although starch modifications aim to improve properties related to its chemical structure such as the ability to solubilize compounds, modify the surface tension of water, and act as emulsifying agents (Ong and Pinal, 2018; Simsek et al., 2015; Sweedman et al., 2013), these treatments could determine the stability and processability of these excipients as it was evidenced 15

from our research on the water vapor sorption behavior and the surface energy characterization of the starches under study.

3.6.1. Vapor sorption behavior As shown in Fig. 6, starches exhibit sigmoidal isotherms classified as type II by the IUPAC (Thommes et al., 2015), where at the beginning, water molecules strongly interact with the powder surface until a monolayer is formed, then, a plateau in the isotherm is obtained because of the formation of water multilayers and finally, a drastic increase in the water content, involving water condensation, is observed. Table 2 shows a synthesis of the investigated parameters that could affect the water sorption behavior of the modified starches, such as morphology, crystallinity, particle size, specific surface area and hydrophilic/hydrophobic nature of the material (Swaminathan and Kildsig, 2001). Any correlation is possible between water sorption and both particle size and SSA when the starches are exposed to 50 % RH. Thus, for instance, CAP exhibiting the smallest particle size and the largest SSA, shows, unlike expected, lower water sorption compared to NCR and PGU whose particle sizes are large and SSA is small. It is interesting to note that the DS of the modified starches appears to be the main factor influencing their water sorption when RH below 50% is tested. Among the investigated starches, NDS uptakes the largest amount of moisture in this zone, followed in increasing order, by PG2000  HIC  CAP  PGU  NCR. It could be related to the availability of strong water– starch interacting sites on the surface of the starch particles that enable the organization of the water molecules as a monolayer (McMinn and Magee, 1997); in other words, this correlates with the DS of the starches and the prevalence of hydrophilic sites (hydroxyl groups from the glucose monomers) on the particle surface. The substitution of hydroxyl groups with OS moieties gives the modified starches an increased hydrophobicity that decreases their affinity for the moisture. However, the lower water affinity of CAP compared to NCR and PGU could be due to both, the OS substitutions and the close-packed structure because of its semi-crystalline moieties that reduce the interaction between the particles and the water molecules. On the other hand, the morphology and the degree of crystallinity of the materials could be more relevant on their ability to absorb water when RH is higher than 50% In this way, although HIC and PG2000 exhibit the lowest water sorption values below 50% RH, attributable to the largest OS substitutions, these starches uptake moisture exponentially above 50% RH, a phenomenon that is also observed for the other starches above the 80% RH condition. This behavior could be 16

a consequence of the nature predominantly amorphous of these materials, where multilayers of water molecules may be formed on the starch particle surfaces when the relative humidity is high enough (Wan et al., 2018).

3.6.2. Surface heterogeneity profiles Data about total surface free energy is useful to characterize the chemical composition of the starches because it results of the existing non-polar forces (dispersive) and polar forces (specific) on the particle surfaces (Ho and Heng, 2013; Martinez-Alejo et al., 2018). To this end, Fig. 7A shows the energy distribution profile for the investigated starches, which was obtained from the integration of the surface energy map across the complete range of surface coverages (0 - 100%; primary data available in Fig. S3, supplementary material). For its part, Fig. 7B reports the Gibbs distribution profiles generated from the Gibbs free energy of interaction from the polar probes. Relevant quantitative parameters obtained related to these figures are shown in Table S2 (supplementary material). As a general synthesis of the obtained results, in all cases the dispersive component (non-specific interactions) is the main contributor to the total surface energy, with values greater than 30 mJ/m2; CAP and NCR exhibit similar surface energy values than NDS; PG2000, and PGU and HIC show lower values than NDS for the dispersive and specific components (polar intermolecular interactions such as hydrogen bonding and other site-specific interactions). On this basis, it is clear that the modification treatments to the NDS, such as OSA esterification, gelatinization and other possible chemical and biochemical treatments (i.e., enzymatic hydrolysis, acid-thinning process), generate changes in the energetic interaction sites (dispersive and polar) at the surface of these starches that must be considered to predict their reactivity. On the other hand, the Lewis acid (Ka) and Lewis base (Kb) numbers calculated from the polar probe's energy, by applying the Guntmann approach, lead to an approximation to the acidic/basic character and the orientation of the functional groups, at the surface of the investigated starches. As shown in Fig. 8, in general, Kb numbers are higher than Ka for all materials, i.e., the electron donor behavior predominates at the surface of the particles because of the oxygen-based functional groups from saccharides. This can also be inferred from the Ka/Kb ratios that in all cases are below 1. Among samples, HIC and PG2000 exhibit the lowest numbers for both Ka and Kb parameters followed by PGU  CAP  NCR and, this trend is similar to the one observed for the specific component in the surface energy analysis (Fig. 7A). This suggests that changes 17

induced by the starch modification not only govern the hydrophobicity but also, they modify the availability of electron acceptor/donor groups at the surface of these materials. Finally, as an attempt to correlate the data of starch characterization obtained in this study, different Principal Component Analyses (PCA) were carried out. Actually, the only one providing results of interest is that on particle size distribution, surface energy and, bulk properties. For the case of surface energy, the data considered were those of the infinite dilution zone, wherein the probability of molecular interactions is low and the interaction energy represents the highest energetic sites of the surface. As shown in Fig. 9, the first two principal components (PC) account for about 80% of the total variations measured (magnitude of the eigenvalues and eigenvectors are shown in Table S3, supplementary material). PC1 exhibits a positive relationship for most of the variables, although the parameters PSD, FFC and Ka/Kb ratio are inversely correlated (vectors point towards negative values). It is worth mention that samples are arranged from hydrophilic (extreme right of PC1) to hydrophobic nature (extreme left of PC1) as result of the interplay of the variables measured which allow to identifying differences among the starch samples, particularly by IGC. Regarding PC2, the main variables are FFC, Ka/Kb and the cohesion coefficient, as noted by the direction and intensity of their vectors towards the y-axis. The approximately 180 angle between the vector of cohesion coefficient and the vectors for FFC and Ka/Kb indicates a negative correlation of these variables and suggests a relationship between the flow properties and the surface energy of the investigated materials. Thus, starches located in quadrants I and II (PGU and CAP) characterize by a better flow than those located in the quadrants III and IV (HIC, PG2000, NCR, and NDS). On the same lines, the flowability of NCR seems to be consistent with its likeness with NDS, as deduced from the surface heterogeneity profiles. Although HIC and PG2000 have marked differences concerning the surface properties of NDS, they characterize by similar values for Ka, Kb, and Ka/Kb. In addition, the improved flow observed by CAP and PGU seems to correlate with an increase of the value for Ka/Kb ratio. In synthesis, the acid/base balance at the surface of the particle starches could be an indicator of their powder flow.

18

4.

Conclusions

The present research work reports the study of the structural, morphological, pharmaceutical, surface and rheological characterization for five commercially available OS-starches, that provides useful data to understand in a comprehensive way, their molecular, physicochemical and structural nature. As a general rule, the modified starches exhibit marked differences compared to the native starch, and their advantages could be taken with the aim of using them as pharmaceutical excipients. In this sense, the modified starches have good flow, different particle sizes and most of them, unimodal PSD, that are basic requirements when the design of solid dosage forms is intended. In addition, the water sorption isotherms of the hydrophobic starches are directly related to the OS substitution, and the observed type II sorption pattern could be attractive for designing modified release dosage forms. These biopolymers exhibit surface activity and ability to increase the viscosity of aqueous media depending on their Mw; hence they could act as emulsifying and stabilizing agents for disperse pharmaceutical dosage forms. Accordingly, NCR and PGU could contribute to stabilizing liquid/liquid systems by increasing the viscosity of the aqueous phase, whilst HIC and CAP are located at the oil-water interfaces allowing the stabilization of the system via the reduction of the interfacial tension. The surface energy profiles of the OS-starches evidence differences in the starches mainly in the contribution of the dispersive and specific component to the total surface free energy, that could be derived from the action of the different physical and chemical treatments employed for their modification.

Acknowledgments

This work was supported by Vicerrectoría de Investigación, Universidad Nacional de Colombia and Dane O. Kildsig Center for Pharmaceutical Processing Research (CPPR), Purdue University. C. P. M. was supported by a Ph.D. scholarship from the Departamento Administrativo de Ciencia, Tecnología e Innovación - Colciencias, Colombia, and J. M. M. by a postdoctoral research scholarship from COMEXUS (Fulbright-Garcia Robles scholarship), Consejo Nacional de Ciencia y Tecnología (CONACYT), México, and CPPR. The authors are grateful to Ingredion, Cali (Colombia) and New Jersey (USA), who kindly donated the starches studied.

19

References

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Figure captions Fig. 1. Scanning electron microscopy images - 500x (A) and photomicrographs of polarized light microscopy - 100x (B) of the starches. Fig. 2. Particle size distribution of the starch granules. Fig. 3. X-ray diffractograms (A) and DSC thermograms (B) of the starches. Values of gelatinization properties for NDS correspond to means of three replicates ± standard deviation.

Fig. 4. Surface tension of the OS-starches in comparison to surface tension of the surfactant SDS (A) and flow sweep curves at 25 ± 0.1 C for 22 wt.% homogeneous aqueous dispersions of the modified starches (B). Fig. 5. Bulk properties of native and modified starch powders: True, aerated and tapped densities (A); Density as a function of particle size (B); Hausner ratio (C); Carr's index (D); Relationship between flow function coefficient (FFC) and particle size (E); Cohesion coefficient as a function of particle size (F). Fig. 6. Water sorption isotherms for native and modified starch powders. Fig. 7. Energy distribution map profiles for the investigated starch samples: Surface energy distribution profile (A); Gibbs energy distribution profile from the polar probes (B). Fig. 8. Acid – base heterogeneity of the investigated starch samples: NDS, PGU, HIC, PG2000, CAP and NCR. Fig. 9. Principal Component Analysis from some properties investigated for the native and modified starches. The PCA was developed with the values shown in Table S2 (supplementary material), the values of SSA and PSD (Fig. 2), and the values of FFC, Cohesion Coefficient, Carr's Index and Hausner Ratio (Fig. 5).

23

Table 1. Structural properties of OS-starches and native waxy corn starch. Starch NDS HIC PG2000 CAP NCR PGU

DS (10-2)* 0 3.17a ± 0.11*** 2.50b ± 0.11*** 2.23c ± 0.18 2.11c ± 0.11 1.69d ± 0.20

Mw (105 g/mol)** 308.05a ± 5.16 3.56b ± 0.06 35.03c ± 0.03 0.68b ± 0.02**** 157.75d ± 2.90 129.45e ± 0.35

Rz (nm)** 106.90a ± 0.71 41.30b ± 2.55 54.60c ± 0.99 47.90d ± 5.23**** 78.45e ± 0.49 67.25f ± 0.64

 (g/mol nm3)** 25.22a ± 0.08 5.10b ± 0.86 21.54c ± 1.19 0.64d ± 0.19 **** 32.68e ± 1.22 42.58f ± 1.32

Different subscript letters attached to the central tendency values in the same column indicate statistical difference (= 0.05). *Means of three replicates ± standard deviation. **Means of two replicates ± standard deviation. ***Results in line with the DS values reported by Ye et al. (2017) for HIC (0.037) and PG2000 (0.031). ****Value obtained from peak No. 2 of the chromatogram (supplementary material, Fig. S2).

24

Table 2. Qualitative analysis of the effect of physicochemical parameters of OS-starches on their water sorption behavior. Starch

Morphology

Crystallinity

Particle size D[3,2] (µm)

SSA (m2/g)

DS

Vapor sorption behavior Net change in mass (%) RH: 0 – 50%

RH: 70%

RH: > 80%

PGU

Spherical, aggregated structures, surface folds

Amorphous

A

E

E

B

D

D

NCR

Spherical, aggregated structures, surface folds

Amorphous

B

B

D

A

C

C

HIC

Spherical, aggregated structures, surface folds

Amorphous

C

C

A

D

A

B

PG2000

Spherical, aggregated structures, surface folds

Amorphous

D

D

B

E

B

A

CAP

Polygonal

Semi-crystalline

E

A

C

C

E

E

Letters represent a descending order, with A being the highest value, while E represents the lowest value.

25

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

26

Fig. 1.

27

16

D[3,2] (µm) D[4,3] (µm) PGU NCR PG2000 HIC NDS CAP

14

Volume (%)

12

61.4 39.5 31.0 33.1 21.2 7.1

123.0 91.9 70.2 52.1 54.7 13.1

Span 2.2 2.3 2.0 1.6 2.0 0.9

SSA (m2/g) 0.24 0.37 0.26 0.27 0.41 0.68

10 8 6 4 2 0

1

10

100

1000

10000

Particle Size (m)

Fig. 2.

28

(A) PGU NCR

Intensity

PG2000 HIC CAP NDS 5

10

15

20

25

30

35

40

2-Theta (º)

(B)

0.0

PGU HIC

-0.1

PG2000 NDS

Gelatinization properties:

-0.2

Heat flux (W/g)

NCR CAP

Onset temp (ºC): Peak temp (ºC): Conclusion temp (ºC): Enthalpy (J/g starch):

-0.3

NDS 64.31 ± 0.17 71.87 ± 0.23 80.17 ± 0.24 3.76 ± 0.03

CAP 76.26 90.29 96.64 0.40

-0.4 -0.5 -0.6

Endo

-0.7 -0.8

10

20

30

40

50

60

70

80

90

100

Temperature (ºC)

Fig. 3.

29

(A)

80

HIC NCR

Surface tension (mN/m)

70

CAP PGU

PG2000 SDS

60 50 40 30 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Concentration (wt.%)

Ostwalds model n K (Pa.s) R2

10000

Shear stress (Pa)

1000

NCR PGU PG2000 CAP HIC

(B)

app(Pa.s) at 1.0 Pa

0.84 2.591 0.9991 0.92 1.050 0.9992 0.92 0.094 0.9994 1.00 0.009 1.0000 0.99 0.008 1.0000

3.221 1.023 0.073 0.009 0.008

100 10 1 0.1 0.01 0.01

0.1

1

10

100

1000

10000

-1

Shear rate (s )

Fig. 4.

30

(A) Aerated density Tapped density True density

1.6

Density (g/mL)

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 CAP

NDS

PG2000

HIC

PGU

(B)

1.6

CAP

NDS

Aerated density Tapped density True density

1.4

Density (g/mL)

NCR

1.2 PG2000

HIC

NCR PGU

1.0 0.8 0.6 0.4 0.2 0

10

20

30

40

50

60

70

Particle Size (µm)

(C)

1.8 1.7

Hausner Ratio

1.6 1.5

Poor Flow

1.4 1.3

Acceptable Flow

1.2

Good Flow

1.1 1.0 CAP

NDS

PG2000

HIC

NCR

PGU

31

(D) 40

Carr`s Index (%)

35 30

Poor Flow

25

Acceptable Flow 20

Good Flow

15 10 5 0 CAP

NDS

PG2000

HIC

NCR

PGU

(E) 12

Free Flowing 10 8

Easy Flowing

FFC

CAP

6

PGU PG2000

4

HIC

NDS

NCR

Cohesive

2

Very Cohesive 0 0

10

20

30

40

50

60

70

Particle Size (µm)

(F) Cohesion Coefficient (kPa)

1.2 1.0

Very Cohesive 0.8

Quite Cohesive

0.6 NDS

0.4 CAP

0.2

NCR

PG2000

HIC

PGU

Mild to no cohesion 0.0 0

10

20

30

40

50

60

70

Particle Size (µm)

Fig. 5.

32

40

Net change in mass (%)

35

NDS PG2000

30

PGU HIC

CAP NCR

25 20 15 10 5 0

0

10

20

30

40

50

60

70

80

90

100

Relative humidity (%)

Fig. 6.

33

(A)

Dispersive

Specific

(B)

Total

NDS

6 4 2 0 6

PGU

4

Area Increment (%)

0 6

Area Increment [%]

2

HIC

4 2 0 6

PG2000 4 2 0 6

CAP 4 2 0 6

NCR

4 2 0

0 1 2 3 4 5 6 7

30

35

40

Surface Energy (mJ/m2)

45

50

6 5 4 3 2 1 0 6 5 4 3 2 1 0 6 5 4 3 2 1 0 6 5 4 3 2 1 0 6 5 4 3 2 1 0 6 5 4 3 2 1 0

Dichloromethane Acetone

Ethyl acetate Acetonitrile

Ethanol

NDS

PGU

HIC

PG2000

CAP

NCR

4

6

8

10

12

14

16

18

20

Specific (Acid-base) Free Energy [kJ/Mol]

Fig. 7.

34

Gutmann acid and base numbers

Ka 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.00

0.02

0.04

0.06

Ka/Kb

Kb

NDS

PGU

HIC

PG2000

CAP

NCR

0.08

0.10

Coverage (n/nm)

0.12

0.14 0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Coverage [n/nm]

Fig. 8.

35

-0.2

(II)

Principal Component 2 (20.45 %)

3

0.0

0.2

0.4

(I)

CAP

FFC K /K a b

2

0.5

SSA

Ka

PGU

Ethylacetatemax

1

dmax

tmax abmax

EtOHmax Acetonemax

0

-1

NCR

HIC

0.0

CH3CNmax CH2Cl2max Kb Carr's Index Hausner Ratio

PSD

NDS

PG2000 -2

Cohesion Coefficient

(IV)

(III) -4

-2

0

2

4

6

Principal Component 1 (59.9 %)

8

-0.5 10

Fig. 9.

36

37

Highlights



Comprehensive characterization study of OSA-chemically modified starches.



Molecular, physicochemical and bulk properties of modified starches were investigated.



Molecular weight of OS-starches keeps inverse relationship with substitution degree.



OSA substitution favors the ability of starches for reducing water surface tension.



OS-starches characterize by good flowability properties.

38