Colloids and Surfaces A: Physicochemical and Engineering Aspects

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1 Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects jo ur nal ho me page: Study of adsorption behaviors on a SiO 2 surface using alkyl cationic modified starches Dong-Sung Han a,b, Yu-Mi Kim a, Han-Young Kim a, In-Shik Cho a, Jong-Duk Kim b, a Central Research Laboratories, Aekyung Co., Ltd., Daejeon , Republic of Korea b Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon , Republic of Korea h i g h l i g h t s g r a p h i c a l a b s t r a c t Cationic alkyl starches were synthesized to observe the adsorption behavior by QCM-D. Adsorbed amount of cationic starches increased compared with cationic surfactants. Tendency for increased rigidity after desorption was observed in all cationic starches. Cationic alkyl starches showed more adsorbed amount and rigidity than cationic starches. a r t i c l e i n f o Article history: Received 22 June 2013 Received in revised form 5 September 2013 Accepted 1 October 2013 Available online 11 October 2013 Keywords: Cationic starch Quartz crystal microbalance Dissipation factor Viscoelasticity Surfactant a b s t r a c t Quartz crystal microbalance with dissipation monitoring (QCM-D) was performed in order to study the adsorption behavior of monoelectrolytes (cationic surfactants: C trimethyl ammonium bromide) and polyelectrolytes (cationic starches and cationic alkyl substituted starches). An adsorption step using surfactant or polymer solutions and a desorption step of rinsing with distilled water were adopted in order to observe the adsorption behavior. The adsorbed amount of all cationic starches increased compared with that of the cationic surfactants, and the adsorption gap between the adsorption and desorption processes was remarkably small. In addition, a tendency for increased rigidity after desorption was observed. In particular, the cationic alkyl (C ) substituted starches had long alkyl chains as well as polymer backbones. In the results, the adsorbed layer of the cationic alkyl substituted starches increased more than the general cationic starches and exhibited more rigid properties. These results were attributed to the hydrophobic interactions among polymers originating from long alkyl chain substitutions Elsevier B.V. All rights reserved. 1. Introduction Starch is one of the most abundant natural polymers in the world. Starch, which is readily obtained from natural grains, is environmentally biodegradable, renewable, and harmless to the human-body. Because it consists of a polymer structure such as dextrose linking, starch is a useful material that has many applications. Most importantly, from an industrial viewpoint, it is much cheaper than other polymer materials. Accordingly, starch is used Corresponding author. Tel.: ; fax: address: kjd@kaist.ac.kr (J.-D. Kim). not only for food purposes but also in the textile and adhesives industries, paper-making, and cosmetics [1]. A well-known starch application is the synthesis of biodegradable (or biodestructible) polymers blended with biodegradable synthetic polymers, such as poly ( -caprolactone) (PCL), poly (lactic acid) (PLA), and polyvinyl alcohol (PVOH) [2,3]. Furthermore, cationic modified starch is widely used to improve water solubility by controlling the surface charge of particles in aqueous solutions [4 6]. Due to the high molecular weight and cross-linking connecting structure of starch, it is not easily dissolved in water despite having many hydrophilic OH groups. Cationic modified starch is also used as a cellulose adhesive in paper-making industries and as an adsorbent for industrial wastewater treatment due to its water-soluble characteristics /$ see front matter 2013 Elsevier B.V. All rights reserved.

2 450 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) The cationic nature of the modified starches also offers adsorption abilities in anionic materials, which can be exploited in practical applications [7,8]. As starch is harmless to the human body, environmentally friendly, and inexpensive, modified starch can be used in household and personal-care applications, as well as cosmetics. In terms of applications, the surface of skin, hair and cloth generally exhibits anionic characteristics. So, conditioning products such as fabric softner, hair rinse and hair treatment have used cationic surfactants and cationic polymers as main ingredients to attach them to skin, hair and cloth. Thus, cationic starches can provide efficient adsorption on their surfaces. In this paper, the adsorption behaviors of various modified cationic starches are investigated. The study of adsorption behaviors is very difficult using skin, hair, and cloth directly, because the adsorbed amount is extremely small. Therefore, a SiO 2 cell was used as the anionic surface instead of skin, hair, or cloth; the adsorption study was undertaken using quartz crystal microbalance with dissipation monitoring (QCM-D). The QCM-D method has been developed in numerous previous research papers and allows comparison of the visco-elastic properties of each adsorption layer while also being used to measure the adsorbed amount and thickness of the adsorption layers [9 13,16 18]. In this study, three types of starches were used according to their molecular weight (starch-oligomer, soluble starch, and corn-starch) and cationic modification was conducted using the cationic reagents glycidyl trimethyl ammonium chloride (GTMAC) and glycidyl dimethyl alkyl (C ) ammonium chloride [14,4]. The adsorption characteristics of a cationic substituted starch with a hydrocarbon alkyl chain were compared with those of a cationic starch without alkyl chains. This paper reports the results of this investigation in detail. 2. Experimental 2.1. Materials Starch-oligomer (dextrose equivalent 10; Mw 4000; poly dispersity 3.2 by GPC) and corn-starch were obtained as commercial products from DAESANG Corporation (South Korea). Soluble starch (Mw ; poly dispersity 2.8 by GPC; extra pure grade) was purchased from Samchun Pure Chemical Co., Ltd. (South Korea). Glycidyl trimethyl ammonium chloride (GTMAC; technical grade; purity > 70%) was purchased from Sigma-Aldrich (Korea). N-dodecyl-N-dimethylamine, N-tetradecyl-N-dimethylamine, N- hexadecyl-n-dimethylamine, and N-octadecyl-N-dimethylamine were obtained from TCI (Tokyo, Japan). Dodecyl trimethyl ammonium bromide, tetradecyl trimethyl ammonium bromide, and hexadecyl trimethyl ammonium bromide were also purchased from TCI (Tokyo, Japan) GTMAC (C 1 ) substituted starch synthesis The cationic starches (starch-g-gtmac and starch-g-c 1 ) were synthesized according to the method reported by Bendoraitiene [6]. Three types of starch, i.e. starch-oligomer (DE10, Mw 4000), soluble starch (Mw ), and corn-starch, were used to synthesize the cationic starches. Dried starch was added to a reaction vessel containing a mixture of distilled water, GTMAC (C 1 ), and aqueous sodium hydroxide (1N solution). The reaction solution was then mixed until it became homogeneous. The reaction proceeded at a temperature of 45 C for 24 h. After being cooled to room temperature, the reaction solution was precipitated in excess isopropyl alcohol (IPA). Finally, the filtered product was purified via washing with IPA three times. The final powder product was then dried in a vacuum oven at 45 C for 8 h. The molar ratio of the reactants was anhydroglucose (AGU):GTMAC:NaOH:H 2 O = 1:( ):0.04: Glycidyl dimethyl alkyl ammonium chloride (C ) substituted starch synthesis Solutions of glycidyl dimethyl dodecyl ammonium chloride (GDMDAC, C 12 ), glycidyl dimethyl tetradecyl ammonium chloride (GDMTAC, C 14 ), glycidyl dimethyl hexadecyl ammonium chloride (GDMHAC, C 16 ), and glycidyl dimethyl octadecyl ammonium chloride (GDMOAC, C 18 ) were prepared as intermediates for the cationic alkyl grafting reaction with N-dimethyl-N-alkyl (C ) amine and epichlorohydrin. In this paper, the data and conditions of the N- dimethyl-n-alkyl (C ) amine and epichlorohydrin reaction are excluded. Starch-oligomer, soluble starch, and starch (soluble)-g-c 1 were used to synthesize the alkyl cationic starches. The reaction conditions and molar ratios were the same as those employed in the GTMAC grafting experiments. The overall synthesis scheme of the cationic starches is shown in Fig Evaluation of the degree of substitution An elementary analysis (C, H, N) and Cl potentiometric titration with AgNO 3 were performed in order to evaluate the nitrogen and chlorine content in cationic starches [15]. In the elementary analysis (EA), the degree of substitution (DS) was calculated from the ratio of nitrogen and carbon content. For products containing a small amount of nitrogen, potentiometric titration of Cl was performed as well as the EA in order to improve the accuracy of the analysis. The molar ratio of nitrogen content and Cl content was the same, as the quaternization reagents such as GTMAC, GDMDAC, GDMTAC, GDMHAC and GDMOAC contained 1 nitrogen with 1 chlorine as a counter ion. The degree of substitution is defined by the following equation [13]: DS (%) = mole number of N(or Cl) total mole number of AGU 100 (1) 2.5. Surface tension and critical micellar concentration (CMC) The surface tension of an aqueous cationic starch solution was measured by an equilibrium surface tensiometer (ThermoCahn, RADIAN Series 300). The measurement was conducted at 25 C in 0.1 wt% concentration using a ph 7.0 buffer solution. CMC of cationic surfactants and alkyl substituted cationic starch-oligomers was measured at 25 C using a Du Nuoy ring tensiometer with a platinum ring (Kruss K100, Germany) Particle size and zeta potential The particle size and zeta potential of the aqueous starch (DE10)- g-c 18 solution were measured at 25 C as a function of DS by using a zeta potential analyzer (Otsuka ELS-800, Japan). The particle size measurement was carried out in a 1.0 wt% concentration with distilled water and the zeta potential measurement was performed with 0.5 wt% concentration without any ph adjustment. The dilute cationic starch solutions were measured at ph Distilled water was used instead of a 7.0 buffer solution to remove interference of ionic strength. Because the intensity of light scattering was weak under 1500 cps at starch (DE10)-g-C series, the measurement was carried out only for the starch (DE10)-g-C 18 series.

3 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) Fig. 1. Synthesis scheme of the cationic starches and cationic alkyl starches Morphology study using transmission electron microscopy (TEM) TEM images were observed using an energy filteringtransmission electron microscope (Libra-200, Carl Zeiss, Germany) at an accelerated voltage of 200 kv for comparison with the particle size analysis results. To prepare TEM samples, the polymer solution was dropped onto a formvarcarbon coated copper grid and excess solution was gently removed using filter paper. Samples were then negatively stained with aqueous phosphotungstic acid solution (2 wt%, ph 7.4) and the excess was removed prior to drying in a desiccators without vacuum at 25 C overnight Quartz crystal microbalance with dissipation monitoring (QCM-D) The adsorption behavior and visco-elasticity of the adsorbed layers were studied using QCM-D (Q-sense E4, Sweden). Each aqueous solution of cationic starch was prepared in 0.1 wt% concentration with distilled water without any ph adjustment. The measurement conditions were a SiO 2 cell (QSX 303) as an anionic surface, a solution injection rate of 200 l/min through the chamber, and fundamental resonance frequency of 5 MHz. When cationic starch was adsorbed on the anionic surface of the SiO 2 cell, the frequency decreased with an increase of weight. The change of frequency ( f) can be converted to the change of mass ( m) by the Q-tool program. m = C f n (2) where n is the overtone number and C is a constant that describes the sensitivity of the device to changes in mass in the Sauerbrey relation. In addition, the dissipation factor (D ( 10 6 )) was measured as well as the frequency in order to investigate the visco-elasticity (rigidity/softness) of the adsorbed layer. In the QCM-D system, the dissipation factor (D) is defined by the relation of the energy dissipated and the energy stored, as follows: D = E(diss) 2E(stor) The QCM cell oscillates at the resonance frequency. When the current voltage, i.e. the driving force of the oscillation, is stopped, the oscillation is decreased. The decay rate is related to the viscoelasticity of the cell, adsorbed layer, and solution. Therefore, the softness of the adsorbed layer increases with an increasing D. However, the rigidity of the adsorbed layer can be analyzed, similar to the SiO 2 cell, at the smallest change of D [10,11]. The QCM-D measurement proceeded according to the following three steps. After stabilizing the baseline of the frequency (f) and dissipation factor (D) through injecting distilled water, a stable baseline was confirmed after an additional 10 min in the distilled water condition. The distilled water was then exchanged in order to dilute the cationic starch solution. The adsorption of the cationic starch was observed for min until the adsorption stabilized. Finally, the cationic starch solution was exchanged with the distilled water again in order to remove the unstable adsorbed polymers from the adsorbed layer, and stabilization of the desorption was observed. (3)

4 452 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) Table 1 DS (%) results of the cationic modified starch from the elementary analysis and Cl potentiometer: (a) starch-g-c 1 synthesis, (b) starch (DE10)-g-C synthesis, and (c) starch (soluble)-g-c mol%, C 12,18 synthesis. DS (%): EA DS (%): Cl (a) Cationic starch (C 1) reaction ratio Starch(DE10):GTMAC 10 mol% Starch(DE10):GTMAC 20 mol% Starch(DE10):GTMAC 40 mol% Starch(soluble):GTMAC 20 mol% 14.7 Starch(soluble):GTMAC 20 mol% 23.0 Starch(corn):GTMAC 20 mol% Starch(corn):GTMAC 40 mol% (b) Cationic starch (C 12 18) reaction ratio Starch(DE10):GDMDAC 10 mol% Starch(DE10):GDMDAC 20 mol% 11.1 Starch(DE10):GDMDAC 30 mol% Starch(DE10):GDMDAC 30 mol% Starch(DE10):GDMTAC 15 mol% 10.6 Starch(DE10):GDMTAC 20 mol% Starch(DE10):GDMHAC 15 mol% 9.7 Starch(DE10):GDMHAC 40 mol% 12.6 Starch(DE10):GDMOAC 5 mol% Starch(DE10):GDMOAC 10 mol% Starch(DE10):GDMOAC 20 mol% Starch(DE10):GDMOAC 25 mol% 10.2 (c) Cationic starch (C 1, C 12,18) reaction ratio Starch(soluble):GTMAC 20%:GDMDAC 5 mol% Starch(soluble):GTMAC 20%:GDMDAC 10 mol% Starch(soluble):GTMAC 20%:GDMOAC 2 mol% Starch(soluble):GTMAC 20%:GDMOAC 4 mol% (C 1) 23.0, (C 12) 3.6 (C 1) 23.0, (C 12) 4.5 (C 1) 23.0, (C 18) 1.8 (C 1) 23.0, (C 18) 2.7 The f, D, and m were interpreted based on the visco-elastic (Voigt) model presented by Voinova et al. to consider visco-elastic properties [10,13,16,17,20]. The conversion of visco-elastic model was carried out using the Q-tool program and the result data about f and D were selected from the fifth overtone. In previous studies, many researchers have investigated adsorption behaviors of surfactant and polymer using the QCM-D system. But, most of research studied them by injecting a surfactant or polymer solutions. However, in this paper, a desorption step with distilled water was included, considering that numerous applications such as household, personal care, and cosmetics applications involve rinsing step [13,17]. In this paper, our objective of adsorption experiments was to study the synergy effects caused by combination with connected structure of polymers and hydrophobic interaction from substituting long alkyl chains. And, we observed the rigidity changes of adsorbed layers after desorption about monoelectrolytes and polyelectrolytes. 3. Results and discussion 3.1. Synthesis of the cationic starches The results of the cationic starch synthesis are presented in Table 1. Due to the high water solubility originating from the low molecular weight of the starch-oligomer, it was possible to graft C dimethyl ammonium chloride directly. However, the soluble starch and corn starch could not be synthesized to directly graft alkyl dimethylammonium chloride, as starch (soluble or corn)- g-c cannot be dissolved in water. Thus, soluble starch was substituted by GTMAC (C 1 ) at a high DS rate over 20 mol%. Therefore, starch (soluble)-g-c mol% was substituted by GDMDAC (C 12 ) and GDMOAC (C 18 ) at a low DS rate. However, corn-starch Fig. 2. Surface tension measurement in 0.1 wt% solution with a buffer solution (ph 7.0) at 25 C (a) surface tension of starch-g-c 1 and (b) surface tension of starch (DE10)-g-C as a function of DS. could only graft GTMAC as a result of its lower solubility that stems from having the highest molecular weight. In the GTMAC grafting experiments, the reaction yield (%) was very high and sometimes over 100%. This was attributed to starch containing some water despite having been dried in a vacuum oven prior to being used and the purity of the GTMAC as the reaction intermediate was over 70%. However, the reaction yield of the alkyl cationic grafting was 50 70% Surface tension and critical micelle concentration (CMC) measurements The results of the surface tension measurements of the cationic starches are shown in Fig. 2. Changes in the surface tension were not observed with increasing DS (%). However, the surface tension decreased gradually with increasing molecular weight, as seen in Fig. 1(a). All GTMAC substituted starches without alkyl chains exhibited high surface tension over 60 dyne/cm. In contrast, the surface tension of the alkyl substituted cationic oligomer was lower than that of starch-g-c 1, as seen in Fig. 2(b). The surface tension of starch (DE10)-g-C was dyne/cm. These results were similar to those obtained with a cationic surfactant (such as C dimethyl ammonium bromide) in the same conditions. C 12 trimethyl ammonium bromide (C 12 AB) was measured to be 45.0 dyne/cm, C 14 trimethyl ammonium bromide (C 14 AB) was 40.2 dyne/cm, and C 16 trimethylammonium bromide (C 16 AB) was 39.9 dyne/cm. C 18 trimethylammonium bromide (C 18 AB) could not be measured because it precipitates at 25 C. The results of the CMC measurements of the cationic surfactants and typical C substituted cationic starch-oligomers are

5 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) Table 2 Results of the critical micelle concentration measurements (CMC): (a) cationic surfactants and (b) alkyl substituted cationic starch-oligomers. CMC (mg/l) (a) Cationic surfactants C12 trimethylammonium bromide C14 trimethylammonium bromide C16 trimethylammonium bromide (b) Alkyl grafted cationic starch-oligomers Starch(DE10)-g-C mol% Starch(DE10)-g-C mol% Starch(DE10)-g-C mol% Starch(DE10)-g-C mol% Starch(DE10)-g-C mol% presented in Table 2. The CMC range of cationic alkyl substituted starch-oligomers was similar to that of cationic surfactants. The CMC of the alkyl cationic starch-oligomers and cationic surfactants decreased gradually according to the length of the alkyl chain. From the results of the surface tension and CMC measurement, the cationic alkyl substituted starches demonstrated that they could create aggregates similar to surfactants Particle size and zeta potential measurement In order to evaluate the adsorption ability on an anionic surface, the particle size and zeta potential measurements were performed. Fig. 3 presents the results for the particle size and zeta potential of starch (DE10)-g-C 18. The particle size decreased from 10.4 nm to 4.5 nm with increasing DS (%). This result indicates that the particle structure became more rigid with increasing DS of the long alkyl chain. The TEM images presented in Fig. 4 were compared with the results of the particle size analyses from DLS. The spherical shape of the starch (DE10)-g-C 18 series was confirmed from the TEM images and the decreasing tendency with increasing DS from the images was similar to that in the DLS results. Furthermore, a tendency of increasing the positive charge of the particles of the starch as a function of DS was observed, and the lowest DS rate product, i.e. starch (DE10)-g-C mol%, exhibited a positive charge of (+)35.9 mv. This demonstrates that the specimen has sufficient adsorption ability to attach to an anionic surface at a low grafting rate Quartz crystal microbalance with dissipation monitoring (QCM-D) In order to study the adsorption tendency, the m converted from the change in the value of frequency ( f) was measured at a stable point of adsorption/desorption steps. The change of the dissipation factor ( D) was also measured in order to study the visco-elasticity of the adsorption layers. Due to the difference in the adsorption amount of each cationic starch, the D/f factor could be used to evaluate the apparent rigidity of the adsorption layers [11]. The D/f factor is defined by the following equation: D/f = D f 10 9 (4) In order to observe the change from the adsorption to desorption equilibrium points, two new factors were adopted. m AD (%) and D/f AD (%) are defined using the following two equations: m AD (%) = m(in a desorption) m(in an adesorption) 100 (5) D/f AD (%) = D/f (in a desorption) D/f (in an adesorption) 100 (6) m AD (%) and D/f AD (%) indicate the residual weight % and the change of the D/f factor of the adsorbed layer after desorption compared with the adsorption condition respectively. Thus, an m AD of 50% indicates that the final adsorption amount was 50 wt% compared with the adsorption amount at the adsorption equilibrium point. When D/f AD is below 100%, it indicates that the rigidity of the layer increased after desorption. Fig. 3. Results of the (a) particle size measurement and (b) zeta potential measurement of starch (DE10)-g-C 18 with increasing DS (%) Cationic surfactant adsorption properties The QCM-D measurements of the cationic surfactants (C 12 AB, C 14 AB, and C 16 AB) as monoelectrolytes were achieved in order to compare them with the cationic starches as polyelectrolytes. The results are presented in Table 3 and Fig. 5. With the cationic surfactant, the adsorption amount and dissipation factor (D) tended to increase according to the length of the alkyl chain. According to m AD (%) in Table 3, the residual adsorption amount of the layer was less than 30% at the desorption step compared with the adsorption step. As shown in Table 3 and Fig. 10(a), the D/f factor of the adsorbed layer increased more in the desorption step than the in adsorption step. These phenomena were considered to result from the monomer type electrolyte (i.e. the cationic surfactant) was adsorbed with aggregates such as hemimicelles or micelles via a hydrophobic interaction of the long alkyl chain in the conditions provided by the surfactant solution nearby CMC [19]. However, the unstably attached electrolyte was removed in the desorption conditions. Thus, the difference in the adsorption amount between the adsorption and desorption increased: according to the D/f factor results, the rigidity of the adsorption layer after desorption decreased compared with that after adsorption.

6 454 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) Fig. 4. Morphology via the negative staining method using phosphotungstic acid in transmission electron microscopy (TEM) of starch (DE10)-g-C 18 series: (a) starch (DE10)- g-c mol%, (b) starch (DE10)-g-C mol%, (c) starch (DE10)-g-C mol% and (d) starch (DE10)-g-C mol% Cationic polymer (starch-g-c 1 ) adsorption properties The starch-g-c 1 series (starch-oligomer, soluble starch, and corn-starch) were conducted in order to study the adsorption behavior of polyelectrolytes without the alkyl chains and the results are presented in Table 4 and Fig. 6. Tendencies were not observed in the adsorbed amount, D factor, and D/f factor of all C 1 substituted cationic starches with increasing DS (%). For the C 1 substituted cationic starch, the adsorbed amount increased to ng/cm 2 and the D/f factor of the C 1 cationic starch (starch-oligomer and soluble starch) was measured to be much lower than that of the cationic surfactant. However, cornstarch, which has the highest molecular weight, exhibited a D/f factor value of In contrast with the results of the cationic surfactants, the residual adsorption layer was approximately 80% at the desorption step compared with the adsorption step. In addition, the rigidity of the adsorption layer of all C 1 cationic starches after desorption increased or did not change compared with the adsorption step. The C 1 cationic starch is a polyelectrolyte connected with each cationic group via the starch backbone. However, hydrophobic interaction was not observed among the polymers due to the absence of long alkyl chains. Thus, it was analyzed that the C 1 cationic starch could not create aggregates via hydrophobic interaction, and therefore the gap in the adsorption amount during the adsorption/desorption steps decreased. In addition, it is speculated that the film rigidity of the polyelectrolyte after desorption increased through swelling and vertical extension of the polymer chains during the rinsing step. The schematic structures of the surfactant adsorption properties and starch-g-c 1 adsorption properties are presented in Fig. 7. Table 3 QCM-D results of the cationic surfactants (C 12AB, C 14AB, and C 16AB) in the adsorption/desorption steps. In adsorption In desorption m AD (%) D/f AD (%) f m D ( 10 6 ) D/f f m D ( 10 6 ) D/f C 12AB C 14AB C 16AB D/f = ( D/ f) 10 9, m AD (%) = m(in a desorption)/ m(in an adsorption) 100, D/f AD (%) = D/f(in a desorption)/d/f(in an adsorption) 100.

7 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) Table 4 QCM-D results of the cationic starch in the adsorption/desorption steps: (a) starch (DE10)-g-C 1 series, (b) starch (soluble)-g-c 1 series, and (c) starch (corn)-g-c 1 series. DS (%) In adsorption In desorption m AD (%) D/f AD (%) f m D ( 10 6 ) D/f f m D ( 10 6 ) D/f (a) (b) (c) Fig. 5. QCM-D results of C 12 trimethyl ammonium bromide, C 14 trimethyl ammonium bromide, and C 16 trimethyl ammonium bromide in 0.1 wt% solution as a function of time (f 0 = 5 MHz, n = 5). Fig. 6. QCM-D results of starch (DE10)-g-C mol% and starch (DE10)-g-C mol% in 0.1 wt% solution as a function of time (f 0 = 5 MHz, n = 5) Alkyl substituted cationic polymer (starch-g-c ) adsorption properties The QCM-D results of the alkyl (C ) cationic starch-oligomer with both a polymer structure and long alkyl chains are shown in Table 5 and Fig. 8. In comparison with the results of the C 1 cationic starch-oligomer in Table 4(a), the adsorption amount of the alkyl cationic starch-oligomer was increased to approximately ng/cm 2 for the starch (DE10)-g-C 18 series; the Fig. 7. Schematic structures of the adsorbed layers from the adsorption step to the desorption step: (a) cationic surfactants and (b) starch-g-c 1 series.

8 456 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) Table 5 QCM-D results of the alkyl cationic starch-oligomer in the adsorption/desorption steps: (a) starch (DE10)-g-C 12 series, (b) starch (DE10)-g-C 18 series, and (c) starch (DE10)- g-c 14,16 series. DS (%) In adsorption In desorption m AD (%) D/f AD (%) f m D ( 10 6 ) D/f f m D ( 10 6 ) D/f (a) (b) (c) 13.1 a b a Starch (DE10)-g-C 14. b Starch (DE10)-g-C 16. cationic starch-oligomer exhibited a similar tendency to the starch (DE10)-g-C 18 series. Because the alkyl (C ) cationic starch-oligomers have both a starch polymer backbone and long alkyl chains, it was considered that they could assume the characteristics of a cationic surfactant and the polyelectrolyte of polymers. Considering the characteristics Fig. 8. QCM-D results of starch (DE10)-g-C mol% and starch (DE10)-g-C mol% in 0.1 wt% solution as a function of time (f 0 = 5 MHz, n = 5). values of the starch (DE10)-g-C 1 series were measured as to be ng/cm 2. Furthermore, the D/f factor decreased at both the adsorption and desorption steps. A tendency of increasing rigidity after desorption was observed in all C 1 and C starcholigomers and the gap in the adsorption amount between the adsorption/desorption steps increased slightly from 15 20% (C 1 ) to 25 40% (C 18 ) according to m AD (%). In addition, the C grafting Fig. 9. QCM-D results of starch (soluble)-g-c mol%, C mol% and starch (soluble)-g-c mol%, C mol% in 0.1 wt% solution as a function of time (f 0 = 5 Hz, n = 5). Fig. 10. Results of the D/f factor as a function of time: (a) C trimethyl ammonium bromide and (b) three types of cationic starches.

9 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) Table 6 QCM-D results of the alkyl cationic soluble starch in the adsorption/desorption steps: (a) starch (soluble)-g-c mol% (C 12 series) and (b) starch (soluble)-g-c mol% (C 18 series). (a) DS (C 12%) In adsorption In desorption m AD (%) D/f AD (%) f m D ( 10 6 ) D/f f m D ( 10 6 ) D/f (b) DS (C 18%) In adsorption In desorption m AD (%) D/f AD (%) f m D ( 10 6 ) D/f f m D ( 10 6 ) D/f Fig. 11. Alkyl substituted cationic starch schematic structures of the adsorbed layers from the adsorption step to the desorption step. of a cationic surfactant, it was posited that the alkyl (C ) cationic starch-oligomers could produce aggregates through hydrophobic interactions among the hydrocarbon long alkyl chains. From the results, the adsorption amount was increased and the gap in the adsorption amount between the adsorption/desorption steps increased slightly relative to the C 1 substituted material as a results of the interaction among each polymer. However, the tendency of the polymer structure to be a polyelectrolyte was maintained. Thus, the gap in the adsorption amount between the adsorption/desorption steps did not change significantly, although the total amount of adsorption layer increased considerably. Therefore, the properties of having both a cationic surfactant and a polymer structure provided the adsorption layer with greater rigidity than the cationic starch-oligomer series without alkyl chains or cationic surfactants, according to the D/f factor. The QCM-D results of the alkyl (C 12,18 ) cationic soluble starch substituted C 1 and C 12,18 are presented in Table 6 and Fig. 9. Because the average molecular weight of soluble starch is Mw from the GPC, the alkyl substituted soluble starches could not be dissolved in water even at a low DS (%). Hence, the alkyl cationic modification at a low DS (<5%) was achieved after C 1 grafting at 23.0 mol%/agu of soluble starch to improve the water solubility. In particular, the alkyl cationic corn-starch could not be dissolved in water even with the use of C 1 grafting cornstarch at 54.6 mol%/agu of corn-starch as an intermediate. All cationic soluble starches, including the starch (soluble)-g-c 1 series and starch (soluble)-g-c 1, C 12,18 series, exhibited a nearly constant value of 0 20% in the gap in the adsorption amount between the adsorption/desorption steps. The adsorption amount of the starch (soluble)-g-c mol%, C 12,18 series did not increase compared with that of the starch (soluble)-g-c mol%, but the rigidity of the adsorption layer increased clearly in both adsorption and desorption conditions according to the D/f factor. In addition, a tendency of increasing rigidity after desorption was also observed. Fig. 10 presents the different tendencies of the rigidity change after desorption between the cationic surfactants and cationic starches. Although the soluble starch series had characteristics of both surfactants and polymers, it was considered that the properties of the polymer structure had a greater influence than those of the surfactant due to their high molecular weight. Thus, the discrepancy between the two steps was very small, although the adsorption amount did not increase compared with that of the C 1 grafted cationic soluble starch. However, the adsorption layer became significantly more rigid with increasing alkyl DS (%) through the hydrophobic interaction between the polymers originating from the long alkyl chain substitution. The schematic structure of the alkyl substituted cationic starch adsorption properties is presented in Fig Conclusion The surface tension, CMC, particle size, zeta potential, and QCM- D were measured in order to investigate the adsorption behavior of polyelectrolytes such as cationic starches and the effects of cationic alkyl (C ) substitutions. In addition, a desorption step to halt the supply of monoelectrolytes and polyelectrolytes, as well as an adsorption step, were adopted to accurately observe the adsorption behavior. The typical characteristics of a polyelectrolyte such as cationic starches were observed. In particular, the amount of adsorption increased relative to that of a cationic surfactant, and the gap in the amount of adsorption between the adsorption and desorption steps was remarkably small. In addition, a tendency of increasing rigidity after desorption was clearly observed. This tendency was attributed to the connection of the polyelectrolyte with each cationic group via the starch backbone. Consequently, it was considered that the film rigidity after desorption increased through swelling and vertical extension of the polymer chain during the rinsing step. In addition, hydrophobic interactions were not observed between the polymers in the grafted C 1. For the cationic alkyl (C ) substituted starches, the hydrophobic interaction among the polymers contributed to the

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