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Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 449– 458

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal ho me page: www.elsev ier .com/ locate /co lsur fa

tudy of adsorption behaviors on a SiO2 surface using alkyl cationicodified starches

ong-Sung Hana,b, Yu-Mi Kima, Han-Young Kima, In-Shik Choa, Jong-Duk Kimb,∗

Central Research Laboratories, Aekyung Co., Ltd., Daejeon 305-345, Republic of KoreaDepartment of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea

i g h l i g h t s

Cationic alkyl starches were syn-thesized to observe the adsorptionbehavior by QCM-D.Adsorbed amount of cationic starchesincreased compared with cationicsurfactants.Tendency for increased rigidityafter desorption was observed in allcationic starches.Cationic alkyl starches showed moreadsorbed amount and rigidity thancationic starches.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 22 June 2013eceived in revised form 5 September 2013ccepted 1 October 2013vailable online 11 October 2013

a b s t r a c t

Quartz crystal microbalance with dissipation monitoring (QCM-D) was performed in order to study theadsorption behavior of monoelectrolytes (cationic surfactants: C12–16 trimethyl ammonium bromide) andpolyelectrolytes (cationic starches and cationic alkyl substituted starches). An adsorption step using sur-factant or polymer solutions and a desorption step of rinsing with distilled water were adopted in order toobserve the adsorption behavior. The adsorbed amount of all cationic starches increased compared with

eywords:ationic starchuartz crystal microbalanceissipation factoriscoelasticity

that of the cationic surfactants, and the adsorption gap between the adsorption and desorption processeswas remarkably small. In addition, a tendency for increased rigidity after desorption was observed. Inparticular, the cationic alkyl (C12–18) substituted starches had long alkyl chains as well as polymer back-bones. In the results, the adsorbed layer of the cationic alkyl substituted starches increased more thanthe general cationic starches and exhibited more rigid properties. These results were attributed to the

amo

urfactant hydrophobic interactions

. Introduction

Starch is one of the most abundant natural polymers in theorld. Starch, which is readily obtained from natural grains, is

nvironmentally biodegradable, renewable, and harmless to theuman-body. Because it consists of a polymer structure such as

extrose linking, starch is a useful material that has many applica-ions. Most importantly, from an industrial viewpoint, it is muchheaper than other polymer materials. Accordingly, starch is used

∗ Corresponding author. Tel.: +82 42 350 3961; fax: +82 42 350 3910.E-mail address: kjd@kaist.ac.kr (J.-D. Kim).

927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2013.10.007

ng polymers originating from long alkyl chain substitutions.© 2013 Elsevier B.V. All rights reserved.

not only for food purposes but also in the textile and adhesivesindustries, paper-making, and cosmetics [1]. A well-known starchapplication is the synthesis of biodegradable (or biodestructible)polymers blended with biodegradable synthetic polymers, such aspoly (�-caprolactone) (PCL), poly (lactic acid) (PLA), and polyvinylalcohol (PVOH) [2,3]. Furthermore, cationic modified starch iswidely used to improve water solubility by controlling the sur-face charge of particles in aqueous solutions [4–6]. Due to the highmolecular weight and cross-linking connecting structure of starch,

it is not easily dissolved in water despite having many hydrophilicOH groups. Cationic modified starch is also used as a celluloseadhesive in paper-making industries and as an adsorbent for indus-trial wastewater treatment due to its water-soluble characteristics.

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citwaahata

tccaahsi

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50 D.-S. Han et al. / Colloids and Surfaces A: P

he cationic nature of the modified starches also offers adsorptionbilities in anionic materials, which can be exploited in practicalpplications [7,8].

As starch is harmless to the human body, environmentallyriendly, and inexpensive, modified starch can be used in house-old and personal-care applications, as well as cosmetics. In termsf applications, the surface of skin, hair and cloth generally exhibitsnionic characteristics. So, conditioning products such as fabricoftner, hair rinse and hair treatment have used cationic surfac-ants and cationic polymers as main ingredients to attach them tokin, hair and cloth. Thus, cationic starches can provide efficientdsorption on their surfaces.

In this paper, the adsorption behaviors of various modifiedationic starches are investigated. The study of adsorption behav-ors is very difficult using skin, hair, and cloth directly, becausehe adsorbed amount is extremely small. Therefore, a SiO2 cellas used as the anionic surface instead of skin, hair, or cloth; the

dsorption study was undertaken using quartz crystal microbal-nce with dissipation monitoring (QCM-D). The QCM-D methodas been developed in numerous previous research papers andllows comparison of the visco-elastic properties of each adsorp-ion layer while also being used to measure the adsorbed amountnd thickness of the adsorption layers [9–13,16–18].

In this study, three types of starches were used accordingo their molecular weight (starch-oligomer, soluble starch, andorn-starch) and cationic modification was conducted using theationic reagents glycidyl trimethyl ammonium chloride (GTMAC)nd glycidyl dimethyl alkyl (C12–18) ammonium chloride [14,4]. Thedsorption characteristics of a cationic substituted starch with aydrocarbon alkyl chain were compared with those of a cationictarch without alkyl chains. This paper reports the results of thisnvestigation in detail.

. Experimental

.1. Materials

Starch-oligomer (dextrose equivalent 10; Mw 4000; poly dis-ersity 3.2 by GPC) and corn-starch were obtained as commercialroducts from DAESANG Corporation (South Korea). Soluble starchMw 7.1 × 104; poly dispersity 2.8 by GPC; extra pure grade)as purchased from Samchun Pure Chemical Co., Ltd. (Southorea). Glycidyl trimethyl ammonium chloride (GTMAC; techni-al grade; purity > 70%) was purchased from Sigma-Aldrich (Korea).-dodecyl-N-dimethylamine, N-tetradecyl-N-dimethylamine, N-exadecyl-N-dimethylamine, and N-octadecyl-N-dimethylamineere obtained from TCI (Tokyo, Japan). Dodecyl trimethyl ammo-ium bromide, tetradecyl trimethyl ammonium bromide, andexadecyl trimethyl ammonium bromide were also purchased

rom TCI (Tokyo, Japan).

.2. GTMAC (C1) substituted starch synthesis

The cationic starches (starch-g-GTMAC and starch-g-C1) wereynthesized according to the method reported by Bendoraitiene [6].hree types of starch, i.e. starch-oligomer (DE10, Mw 4000), solubletarch (Mw 7.1 × 104), and corn-starch, were used to synthesize theationic starches.

Dried starch was added to a reaction vessel containing a mix-ure of distilled water, GTMAC (C1), and aqueous sodium hydroxide1N solution). The reaction solution was then mixed until it became

omogeneous. The reaction proceeded at a temperature of 45 ◦C for4 h. After being cooled to room temperature, the reaction solu-ion was precipitated in excess isopropyl alcohol (IPA). Finally, theltered product was purified via washing with IPA three times.

chem. Eng. Aspects 441 (2014) 449– 458

The final powder product was then dried in a vacuum oven at45 ◦C for 8 h. The molar ratio of the reactants was anhydroglucose(AGU):GTMAC:NaOH:H2O = 1:(0.1–0.5):0.04:1.

2.3. Glycidyl dimethyl alkyl ammonium chloride (C12–18)substituted starch synthesis

Solutions of glycidyl dimethyl dodecyl ammonium chloride(GDMDAC, C12), glycidyl dimethyl tetradecyl ammonium chloride(GDMTAC, C14), glycidyl dimethyl hexadecyl ammonium chloride(GDMHAC, C16), and glycidyl dimethyl octadecyl ammonium chlo-ride (GDMOAC, C18) were prepared as intermediates for the cationicalkyl grafting reaction with N-dimethyl-N-alkyl (C12–18) amine andepichlorohydrin. In this paper, the data and conditions of the N-dimethyl-N-alkyl (C12–18) amine and epichlorohydrin reaction areexcluded.

Starch-oligomer, soluble starch, and starch (soluble)-g-C1 wereused to synthesize the alkyl cationic starches. The reaction con-ditions and molar ratios were the same as those employed in theGTMAC grafting experiments.

The overall synthesis scheme of the cationic starches is shownin Fig. 1.

2.4. Evaluation of the degree of substitution

An elementary analysis (C, H, N) and Cl− potentiometric titra-tion with AgNO3 were performed in order to evaluate the nitrogenand chlorine content in cationic starches [15]. In the elementaryanalysis (EA), the degree of substitution (DS) was calculated fromthe ratio of nitrogen and carbon content. For products containing asmall amount of nitrogen, potentiometric titration of Cl− was per-formed as well as the EA in order to improve the accuracy of theanalysis.

The molar ratio of nitrogen content and Cl− content was thesame, as the quaternization reagents such as GTMAC, GDMDAC,GDMTAC, GDMHAC and GDMOAC contained 1 nitrogen with 1 chlo-rine 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 wasmeasured by an equilibrium surface tensiometer (ThermoCahn,RADIAN Series 300). The measurement was conducted at 25 ◦C in0.1 wt% concentration using a pH 7.0 buffer solution.

CMC of cationic surfactants and alkyl substituted cationicstarch-oligomers was measured at 25 ◦C using a Du Nuoy ring ten-siometer with a platinum ring (Kruss K100, Germany).

2.6. Particle size and zeta potential

The particle size and zeta potential of the aqueous starch (DE10)-g-C18 solution were measured at 25 ◦C as a function of DS by usinga zeta potential analyzer (Otsuka ELS-800, Japan). The particle sizemeasurement was carried out in a 1.0 wt% concentration with dis-tilled water and the zeta potential measurement was performedwith 0.5 wt% concentration without any pH adjustment. The dilutecationic starch solutions were measured at pH 6.9–7.5. Distilled

water was used instead of a 7.0 buffer solution to remove interfer-ence of ionic strength. Because the intensity of light scattering wasweak under 1500 cps at starch (DE10)-g-C12–16 series, the measure-ment was carried out only for the starch (DE10)-g-C18 series.

D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 449– 458 451

onic s

2(

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lacmsbsfqQ

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Fig. 1. Synthesis scheme of the cati

.7. Morphology study using transmission electron microscopyTEM)

TEM images were observed using an energy filtering-ransmission electron microscope (Libra-200, Carl Zeiss, Germany)t an accelerated voltage of 200 kV for comparison with the par-icle size analysis results. To prepare TEM samples, the polymerolution was dropped onto a formvarcarbon coated copper gridnd excess solution was gently removed using filter paper. Sam-les were then negatively stained with aqueous phosphotungsticcid solution (2 wt%, pH 7.4) and the excess was removed prior torying in a desiccators without vacuum at 25 ◦C overnight.

.8. Quartz crystal microbalance with dissipation monitoringQCM-D)

The adsorption behavior and visco-elasticity of the adsorbedayers were studied using QCM-D (Q-sense E4, Sweden). Eachqueous solution of cationic starch was prepared in 0.1 wt% con-entration with distilled water without any pH adjustment. Theeasurement conditions were a SiO2 cell (QSX 303) as an anionic

urface, a solution injection rate of 200 �l/min through the cham-er, and fundamental resonance frequency of 5 MHz. When cationictarch was adsorbed on the anionic surface of the SiO2 cell, therequency decreased with an increase of weight. The change of fre-

uency (�f) can be converted to the change of mass (�m) by the-tool program.

m = −C�f

n(2)

tarches and cationic alkyl starches.

where n is the overtone number and C is a constant that describesthe sensitivity of the device to changes in mass in the Sauerbreyrelation.

In addition, the dissipation factor (D (×10−6)) was measured aswell as the frequency in order to investigate the visco-elasticity(rigidity/softness) of the adsorbed layer. In the QCM-D system, thedissipation factor (D) is defined by the relation of the energy dissi-pated and the energy stored, as follows:

D = E(diss)2�E(stor)

(3)

The QCM cell oscillates at the resonance frequency. When thecurrent voltage, i.e. the driving force of the oscillation, is stopped,the oscillation is decreased. The decay rate is related to the visco-elasticity of the cell, adsorbed layer, and solution. Therefore, thesoftness of the adsorbed layer increases with an increasing �D.However, the rigidity of the adsorbed layer can be analyzed, similarto the SiO2 cell, at the smallest change of �D [10,11].

The QCM-D measurement proceeded according to the follow-ing three steps. After stabilizing the baseline of the frequency(f) and dissipation factor (D) through injecting distilled water, astable baseline was confirmed after an additional 10 min in the dis-tilled water condition. The distilled water was then exchanged inorder to dilute the cationic starch solution. The adsorption of thecationic starch was observed for 20–40 min until the adsorption

stabilized. Finally, the cationic starch solution was exchanged withthe distilled water again in order to remove the unstable adsorbedpolymers from the adsorbed layer, and stabilization of the desorp-tion was observed.

452 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 449– 458

Table 1DS (%) results of the cationic modified starch from the elementary analysis and Cl−

potentiometer: (a) starch-g-C1 synthesis, (b) starch (DE10)-g-C12–18 synthesis, and(c) starch (soluble)-g-C1 23.0 mol%, C12,18 synthesis.

DS (%): EA DS (%): Cl−

(a) Cationic starch (C1) reaction ratioStarch(DE10):GTMAC 10 mol% 11.4 12.0Starch(DE10):GTMAC 20 mol% 23.2 23.2Starch(DE10):GTMAC 40 mol% 45.4 44.4Starch(soluble):GTMAC 20 mol% – 14.7Starch(soluble):GTMAC 20 mol% – 23.0Starch(corn):GTMAC 20 mol% 21.9 21.8Starch(corn):GTMAC 40 mol% 44.7 39.4

(b) Cationic starch (C12–18) reaction ratioStarch(DE10):GDMDAC 10 mol% 10.4 6.9Starch(DE10):GDMDAC 20 mol% – 11.1Starch(DE10):GDMDAC 30 mol% 15.7 15.0Starch(DE10):GDMDAC 30 mol% 21.3 17.3Starch(DE10):GDMTAC 15 mol% – 10.6Starch(DE10):GDMTAC 20 mol% 19.1 13.1Starch(DE10):GDMHAC 15 mol% – 9.7Starch(DE10):GDMHAC 40 mol% – 12.6Starch(DE10):GDMOAC 5 mol% 5.5 3.2Starch(DE10):GDMOAC 10 mol% 7.8 5.3Starch(DE10):GDMOAC 20 mol% 13.2 9.4Starch(DE10):GDMOAC 25 mol% – 10.2

(c) Cationic starch (C1, C12,18) reaction ratioStarch(soluble):GTMAC

20%:GDMDAC 5 mol%– (C1) 23.0, (C12) 3.6

Starch(soluble):GTMAC20%:GDMDAC 10 mol%

– (C1) 23.0, (C12) 4.5

Starch(soluble):GTMAC20%:GDMOAC 2 mol%

– (C1) 23.0, (C18) 1.8

(pw�

tBpdti

sstop

3

3

TmCbagsf(

Starch(soluble):GTMAC20%:GDMOAC 4 mol%

– (C1) 23.0, (C18) 2.7

The �f, �D, and �m were interpreted based on the visco-elasticVoigt) model presented by Voinova et al. to consider visco-elasticroperties [10,13,16,17,20]. The conversion of visco-elastic modelas carried out using the Q-tool program and the result data aboutf and �D were selected from the fifth overtone.In previous studies, many researchers have investigated adsorp-

ion behaviors of surfactant and polymer using the QCM-D system.ut, most of research studied them by injecting a surfactant orolymer solutions. However, in this paper, a desorption step withistilled water was included, considering that numerous applica-ions such as household, personal care, and cosmetics applicationsnvolve rinsing step [13,17].

In this paper, our objective of adsorption experiments was totudy the synergy effects caused by combination with connectedtructure of polymers and hydrophobic interaction from substi-uting long alkyl chains. And, we observed the rigidity changesf adsorbed layers after desorption about monoelectrolytes andolyelectrolytes.

. Results and discussion

.1. Synthesis of the cationic starches

The results of the cationic starch synthesis are presented inable 1. Due to the high water solubility originating from the lowolecular weight of the starch-oligomer, it was possible to graft

12–18 dimethyl ammonium chloride directly. However, the solu-le starch and corn starch could not be synthesized to directly graftlkyl dimethylammonium chloride, as starch (soluble or corn)-

-C12–18 cannot be dissolved in water. Thus, soluble starch wasubstituted by GTMAC (C1) at a high DS rate over 20 mol%. There-ore, starch (soluble)-g-C1 23.0 mol% was substituted by GDMDACC12) and GDMOAC (C18) at a low DS rate. However, corn-starch

Fig. 2. Surface tension measurement in 0.1 wt% solution with a buffer solution (pH7.0) at 25 ◦C (a) surface tension of starch-g-C1 and (b) surface tension of starch(DE10)-g-C12–18 as a function of DS.

could only graft GTMAC as a result of its lower solubility that stemsfrom having the highest molecular weight.

In the GTMAC grafting experiments, the reaction yield (%) wasvery high and sometimes over 100%. This was attributed to starchcontaining some water despite having been dried in a vacuum ovenprior to being used and the purity of the GTMAC as the reactionintermediate was over 70%. However, the reaction yield of the alkylcationic grafting was 50–70%.

3.2. Surface tension and critical micelle concentration (CMC)measurements

The results of the surface tension measurements of the cationicstarches are shown in Fig. 2. Changes in the surface tension werenot observed with increasing DS (%). However, the surface ten-sion decreased gradually with increasing molecular weight, as seenin Fig. 1(a). All GTMAC substituted starches without alkyl chainsexhibited high surface tension over 60 dyne/cm.

In contrast, the surface tension of the alkyl substituted cationicoligomer was lower than that of starch-g-C1, as seen in Fig. 2(b).The surface tension of starch (DE10)-g-C12–18 was 32–50 dyne/cm.These results were similar to those obtained with a cationic sur-factant (such as C12–16 dimethyl ammonium bromide) in thesame conditions. C12 trimethyl ammonium bromide (C12AB) wasmeasured to be 45.0 dyne/cm, C14 trimethyl ammonium bromide(C14AB) was 40.2 dyne/cm, and C16 trimethylammonium bro-

mide (C16AB) was 39.9 dyne/cm. C18 trimethylammonium bromide(C18AB) could not be measured because it precipitates at 25 ◦C.

The results of the CMC measurements of the cationic surfac-tants and typical C12–18 substituted cationic starch-oligomers are

D.-S. Han et al. / Colloids and Surfaces A: Physico

Table 2Results of the critical micelle concentration measurements (CMC): (a) cationic sur-factants and (b) alkyl substituted cationic starch-oligomers.

CMC (mg/L)

(a) Cationic surfactantsC12 trimethylammonium bromide 1741.2C14 trimethylammonium bromide 470.2C16 trimethylammonium bromide 220.4

(b) Alkyl grafted cationic starch-oligomersStarch(DE10)-g-C12 11.1 mol% 1666.7Starch(DE10)-g-C12 17.3 mol% 1156.8Starch(DE10)-g-C 10.6 mol% 1591.1

psCtFcc

3

tFo4

Fm

14

Starch(DE10)-g-C16 9.7 mol% 750.7Starch(DE10)-g-C18 10.2 mol% 822.9

resented in Table 2. The CMC range of cationic alkyl substitutedtarch-oligomers was similar to that of cationic surfactants. TheMC of the alkyl cationic starch-oligomers and cationic surfac-ants decreased gradually according to the length of the alkyl chain.rom the results of the surface tension and CMC measurement, theationic alkyl substituted starches demonstrated that they couldreate aggregates similar to surfactants.

.3. Particle size and zeta potential measurement

In order to evaluate the adsorption ability on an anionic surface,

he particle size and zeta potential measurements were performed.ig. 3 presents the results for the particle size and zeta potentialf starch (DE10)-g-C18. The particle size decreased from 10.4 nm to.5 nm with increasing DS (%). This result indicates that the particle

ig. 3. Results of the (a) particle size measurement and (b) zeta potential measure-ent of starch (DE10)-g-C18 with increasing DS (%).

chem. Eng. Aspects 441 (2014) 449– 458 453

structure became more rigid with increasing DS of the long alkylchain. The TEM images presented in Fig. 4 were compared with theresults of the particle size analyses from DLS. The spherical shape ofthe starch (DE10)-g-C18 series was confirmed from the TEM imagesand the decreasing tendency with increasing DS from the imageswas similar to that in the DLS results.

Furthermore, a tendency of increasing the positive charge of theparticles of the starch as a function of DS was observed, and the low-est DS rate product, i.e. starch (DE10)-g-C18 2.2 mol%, exhibited apositive charge of (+)35.9 mV. This demonstrates that the specimenhas sufficient adsorption ability to attach to an anionic surface at alow grafting rate.

3.4. Quartz crystal microbalance with dissipation monitoring(QCM-D)

In order to study the adsorption tendency, the �m convertedfrom the change in the value of frequency (�f) was measured ata stable point of adsorption/desorption steps. The change of thedissipation factor (�D) was also measured in order to study thevisco-elasticity of the adsorption layers. Due to the difference inthe adsorption amount of each cationic starch, the D/f factor couldbe 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× 109 (4)

In order to observe the change from the adsorption to desorptionequilibrium points, two new factors were adopted. �mAD (%) and�D/fAD (%) are defined using the following two equations:

�mAD (%) = �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)

�mAD (%) and �D/fAD (%) indicate the residual weight % and thechange of the D/f factor of the adsorbed layer after desorption com-pared with the adsorption condition respectively. Thus, an �mAD

of 50% indicates that the final adsorption amount was 50 wt% com-pared with the adsorption amount at the adsorption equilibriumpoint. When �D/fAD is below 100%, it indicates that the rigidity ofthe layer increased after desorption.

3.4.1. Cationic surfactant adsorption propertiesThe QCM-D measurements of the cationic surfactants (C12AB,

C14AB, and C16AB) as monoelectrolytes were achieved in order tocompare them with the cationic starches as polyelectrolytes. Theresults are presented in Table 3 and Fig. 5.

With the cationic surfactant, the adsorption amount and dis-sipation factor (D) tended to increase according to the length ofthe alkyl chain. According to �mAD (%) in Table 3, the residualadsorption amount of the layer was less than 30% at the desorp-tion step compared with the adsorption step. As shown in Table 3and Fig. 10(a), the D/f factor of the adsorbed layer increased morein the desorption step than the in adsorption step.

These phenomena were considered to result from the monomertype electrolyte (i.e. the cationic surfactant) was adsorbed withaggregates such as hemimicelles or micelles via a hydrophobicinteraction of the long alkyl chain in the conditions provided bythe surfactant solution nearby CMC [19]. However, the unstablyattached electrolyte was removed in the desorption conditions.

Thus, the difference in the adsorption amount between the adsorp-tion and desorption increased: according to the D/f factor results,the rigidity of the adsorption layer after desorption decreased com-pared with that after adsorption.

454 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 449– 458

F transmg and (d

3

cba

aDismsf

u

TQ

D

ig. 4. Morphology via the negative staining method using phosphotungstic acid in-C18 2.2 mol%, (b) starch (DE10)-g-C18 7.1 mol%, (c) starch (DE10)-g-C18 10.1 mol%

.4.2. Cationic polymer (starch-g-C1) adsorption propertiesThe starch-g-C1 series (starch-oligomer, soluble starch, and

orn-starch) were conducted in order to study the adsorptionehavior of polyelectrolytes without the alkyl chains and the resultsre presented in Table 4 and Fig. 6.

Tendencies were not observed in the adsorbed amount, D factor,nd D/f factor of all C1 substituted cationic starches with increasingS (%). For the C1 substituted cationic starch, the adsorbed amount

ncreased to 100–400 ng/cm2 and the D/f factor of the C1 cationictarch (starch-oligomer and soluble starch) was measured to beuch lower than that of the cationic surfactant. However, corn-

tarch, which has the highest molecular weight, exhibited a D/factor value of 80–400.

In contrast with the results of the cationic surfactants, the resid-al adsorption layer was approximately 80% at the desorption step

able 3CM-D results of the cationic surfactants (C12AB, C14AB, and C16AB) in the adsorption/de

In adsorption In desorptio

�f �m �D (×10−6) D/f �f

C12AB 2.5 44 0.16 64 0.75

C14AB 15.8 280 1.03 65 3.9

C16AB 15.8 280 1.32 84 4.2

/f = (�D/�f) × 109, �mAD (%) = �m(in a desorption)/�m(in an adsorption) × 100, �D/fAD

ission electron microscopy (TEM) of starch (DE10)-g-C18 series: (a) starch (DE10)-) starch (DE10)-g-C18 10.1 mol%.

compared with the adsorption step. In addition, the rigidity ofthe adsorption layer of all C1 cationic starches after desorptionincreased or did not change compared with the adsorption step.

The C1 cationic starch is a polyelectrolyte connected with eachcationic group via the starch backbone. However, hydrophobicinteraction was not observed among the polymers due to theabsence of long alkyl chains. Thus, it was analyzed that the C1cationic starch could not create aggregates via hydrophobic inter-action, and therefore the gap in the adsorption amount duringthe adsorption/desorption steps decreased. In addition, it is spec-ulated that the film rigidity of the polyelectrolyte after desorption

increased through swelling and vertical extension of the polymerchains during the rinsing step. The schematic structures of thesurfactant adsorption properties and starch-g-C1 adsorption prop-erties are presented in Fig. 7.

sorption steps.

n �mAD (%) �D/fAD (%)

�m �D (×10−6) D/f

14 0.06 80 32 12566 0.62 159 24 24572 1.19 283 26 337

(%) = D/f(in a desorption)/D/f(in an adsorption) × 100.

D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 449– 458 455

Table 4QCM-D results of the cationic starch in the adsorption/desorption steps: (a) starch (DE10)-g-C1 series, (b) starch (soluble)-g-C1 series, and (c) starch (corn)-g-C1 series.

DS (%) In adsorption In desorption �mAD (%) �D/fAD (%)

�f �m �D (×10−6) D/f �f �m �D (×10−6) D/f

(a)12.0 9.5 250 1.01 106 7.0 126 0.32 46 50 4323.2 9.6 172 0.46 48 8.1 144 0.19 23 84 4844.4 8.0 132 0.75 94 6.4 104 0.24 38 79 40

(b)0 1.00 18 0.33 330 0.33 6 0.03 91 33 285.7 24.2 429 1.02 42 22.6 402 0.83 37 94 8814.7 12.6 224 0.60 48 11.8 210 0.30 25 94 5223.0 14.8 266 0.88 59 13.2 238 0.50 38 89 64

(c)21.8 17.1 295 1.34 78 14.0 240 1.19 85 81 10928.5 13.4 242 2.03 151 9.8 177 2.10 214 73 14239.4 9.7 174 2.39 246 6.3 114 2.51 398 66 16254.6 8.7 153 1.48 170 8.9 159 1.61 181 104 106

Fnf

3a

w

ig. 5. QCM-D results of C12 trimethyl ammonium bromide, C14 trimethyl ammo-ium bromide, and C16 trimethyl ammonium bromide in 0.1 wt% solution as a

unction of time (f0 = 5 MHz, n = 5).

.4.3. Alkyl substituted cationic polymer (starch-g-C12–18)dsorption properties

The QCM-D results of the alkyl (C12–18) cationic starch-oligomerith both a polymer structure and long alkyl chains are shown

Fig. 7. Schematic structures of the adsorbed layers from the adsorption step to

Fig. 6. QCM-D results of starch (DE10)-g-C1 23.2 mol% and starch (DE10)-g-C1

44.4 mol% in 0.1 wt% solution as a function of time (f0 = 5 MHz, n = 5).

in Table 5 and Fig. 8. In comparison with the results of the C1cationic starch-oligomer in Table 4(a), the adsorption amountof the alkyl cationic starch-oligomer was increased to approxi-mately 260–350 ng/cm2 for the starch (DE10)-g-C18 series; the

the desorption step: (a) cationic surfactants and (b) starch-g-C1 series.

456 D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 449– 458

Table 5QCM-D results of the alkyl cationic starch-oligomer in the adsorption/desorption steps: (a) starch (DE10)-g-C12 series, (b) starch (DE10)-g-C18 series, and (c) starch (DE10)-g-C14,16 series.

DS (%) In adsorption In desorption �mAD (%) �D/fAD (%)

�f �m �D (×10−6) D/f �f �m �D (×10−6) D/f

(a)6.9 18.9 425 0.66 35 13.9 267 0.24 17 63 4915.0 26.2 472 0.57 22 19.6 353 0.33 17 75 7717.3 22.4 405 0.32 14 16.0 286 0.16 10 71 7124.4 19.0 338 0.38 20 5.5 96 0.08 15 28 75

(b)2.2 20.0 356 0.61 31 15.1 272 0.10 7 76 233.2 22.5 484 1.04 46 16.4 290 0.32 20 60 435.3 25.3 450 0.81 32 16.7 296 0.30 18 66 567.1 28.0 500 0.78 28 19.3 345 0.18 9 69 329.4 26.9 479 1.26 47 16.0 284 0.35 22 59 4710.2 29.0 520 1.65 57 18.9 342 0.29 15 66 26

(c)13.1a 23.9 545 1.30 54 14.7 265 0.23 16 49 3012.6b 20.5 366 0.53 26 11.5 207 0.25 22 57 85

a Starch (DE10)-g-C14.b Starch (DE10)-g-C16.

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Because the alkyl (C12–18) cationic starch-oligomers have both astarch polymer backbone and long alkyl chains, it was consideredthat they could assume the characteristics of a cationic surfactantand the polyelectrolyte of polymers. Considering the characteristics

ig. 8. QCM-D results of starch (DE10)-g-C14 13.1 mol% and starch (DE10)-g-C18

.4 mol% in 0.1 wt% solution as a function of time (f0 = 5 MHz, n = 5).

alues of the starch (DE10)-g-C1 series were measured as to be00–150 ng/cm2. Furthermore, the D/f factor decreased at bothhe adsorption and desorption steps. A tendency of increasing

igidity after desorption was observed in all C1 and C12–18 starch-ligomers and the gap in the adsorption amount between thedsorption/desorption steps increased slightly from 15–20% (C1) to5–40% (C18) according to �mAD (%). In addition, the C12–16 grafting

ig. 9. QCM-D results of starch (soluble)-g-C1 23.0 mol%, C12 3.6 mol% and starchsoluble)-g-C1 23.0 mol%, C18 1.8 mol% in 0.1 wt% solution as a function of timef0 = 5 Hz, n = 5).

cationic starch-oligomer exhibited a similar tendency to the starch(DE10)-g-C18 series.

Fig. 10. Results of the D/f factor as a function of time: (a) C12–16 trimethyl ammoniumbromide and (b) three types of cationic starches.

D.-S. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 449– 458 457

Table 6QCM-D results of the alkyl cationic soluble starch in the adsorption/desorption steps: (a) starch (soluble)-g-C1 23.0 mol% (C12 series) and (b) starch (soluble)-g-C1 23.0 mol%(C18 series).

(a)

DS (C12%) In adsorption In desorption �mAD (%) �D/fAD (%)

�f �m �D (×10−6) D/f �f �m �D (×10−6) D/f

3.6 14.0 250 0.46 33 13.1 235 0.23 18 94 554.5 11.0 197 0.33 30 9.1 163 0.17 19 83 63

(b)

DS (C18%) In adsorption In desorption �mAD (%) �D/fAD (%)

�f �m �D (×10−6) D/f �f �m �D (×10−6) D/f

1.8 13.0 232 0.68 52 13.0 230 0.28 22 99 422.7 8.1 145 0.27 33 8.8 155 0.05 6 107 18

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Fig. 11. Alkyl substituted cationic starch schematic structures o

f a cationic surfactant, it was posited that the alkyl (C12–18) cationictarch-oligomers could produce aggregates through hydropho-ic interactions among the hydrocarbon long alkyl chains. Fromhe results, the adsorption amount was increased and the gap inhe adsorption amount between the adsorption/desorption stepsncreased slightly relative to the C1 substituted material as a resultsf the interaction among each polymer. However, the tendencyf the polymer structure to be a polyelectrolyte was maintained.hus, the gap in the adsorption amount between the adsorp-ion/desorption steps did not change significantly, although theotal amount of adsorption layer increased considerably. Therefore,he properties of having both a cationic surfactant and a polymertructure provided the adsorption layer with greater rigidity thanhe cationic starch-oligomer series without alkyl chains or cationicurfactants, according to the D/f factor.

The QCM-D results of the alkyl (C12,18) cationic soluble starchubstituted C1 and C12,18 are presented in Table 6 and Fig. 9. Becausehe average molecular weight of soluble starch is Mw 7.1 × 104

rom the GPC, the alkyl substituted soluble starches could not beissolved in water even at a low DS (%). Hence, the alkyl cationicodification at a low DS (<5%) was achieved after C1 grafting at

3.0 mol%/AGU of soluble starch to improve the water solubility. Inarticular, the alkyl cationic corn-starch could not be dissolved inater even with the use of C1 grafting cornstarch at 54.6 mol%/AGU

f corn-starch as an intermediate.All cationic soluble starches, including the starch (soluble)-g-C1

eries and starch (soluble)-g-C1, C12,18 series, exhibited a nearlyonstant value of 0–20% in the gap in the adsorption amountetween the adsorption/desorption steps. The adsorption amountf the starch (soluble)-g-C1 23.0 mol%, C12,18 series did not increaseompared with that of the starch (soluble)-g-C1 23.0 mol%, but theigidity of the adsorption layer increased clearly in both adsorption

nd desorption conditions according to the D/f factor. In addition, aendency of increasing rigidity after desorption was also observed.ig. 10 presents the different tendencies of the rigidity change afteresorption between the cationic surfactants and cationic starches.

dsorbed layers from the adsorption step to the desorption step.

Although the soluble starch series had characteristics of bothsurfactants and polymers, it was considered that the properties ofthe polymer structure had a greater influence than those of the sur-factant due to their high molecular weight. Thus, the discrepancybetween the two steps was very small, although the adsorptionamount did not increase compared with that of the C1 graftedcationic soluble starch. However, the adsorption layer became sig-nificantly more rigid with increasing alkyl DS (%) through thehydrophobic interaction between the polymers originating fromthe long alkyl chain substitution. The schematic structure of thealkyl substituted cationic starch adsorption properties is presentedin Fig. 11.

4. Conclusion

The surface tension, CMC, particle size, zeta potential, and QCM-D were measured in order to investigate the adsorption behavior ofpolyelectrolytes such as cationic starches and the effects of cationicalkyl (C12–18) substitutions. In addition, a desorption step to haltthe supply of monoelectrolytes and polyelectrolytes, as well as anadsorption step, were adopted to accurately observe the adsorptionbehavior.

The typical characteristics of a polyelectrolyte such as cationicstarches were observed. In particular, the amount of adsorptionincreased relative to that of a cationic surfactant, and the gap in theamount of adsorption between the adsorption and desorption stepswas remarkably small. In addition, a tendency of increasing rigidityafter desorption was clearly observed. This tendency was attributedto the connection of the polyelectrolyte with each cationic group viathe starch backbone. Consequently, it was considered that the filmrigidity after desorption increased through swelling and verticalextension of the polymer chain during the rinsing step. In addition,

hydrophobic interactions were not observed between the polymersin the grafted C1.

For the cationic alkyl (C12–18) substituted starches, thehydrophobic interaction among the polymers contributed to the

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58 D.-S. Han et al. / Colloids and Surfaces A: P

ypical characteristics of a polyelectrolyte due to the presence ofoth the starch polymer backbone and long alkyl chains. The resultsemonstrated that the adsorption layer became significantly moreigid.

cknowledgement

This work was supported by a grant from the Next-GenerationioGreen 21 program (No. PJ008080), Rural Development Admin-

stration, Republic of Korea.

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