Hydration/Dehydration Behavior of Hydroxyethyl Cellulose Ether in Aqueous Solution

molecules

Article Hydration/Dehydration Behavior of Hydroxyethyl Cellulose Ether in Aqueous Solution

Kengo Arai 1 and Toshiyuki Shikata 1,2,* 1 Cellulose Research Unit, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan; [email protected] 2 Division of Natural Resources and Eco-materials, Graduate School of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan * Correspondence: [email protected]

Academic Editor: Derek J. McPhee Received: 11 September 2020; Accepted: 12 October 2020; Published: 15 October 2020

Abstract: Hydroxyethyl cellulose (HeC) maintains high water solubility over a wide temperature range even in a high temperature region where other nonionic chemically modiﬁed cellulose ethers, such as methyl cellulose (MC) and hydroxypropylmethyl cellulose (HpMC), demonstrate cloud points. In order to clarify the reason for the high solubility of HeC, the temperature dependence of the hydration number per glucopyranose unit, nH, for the HeC samples was examined by using extremely high frequency dielectric spectrum measuring techniques up to 50 GHz over a temperature range from 10 to 70 ◦C. HeC samples with a molar substitution number (MS) per glucopyranose unit by hydroxyethyl groups ranging from 1.3 to 3.6 were examined in this study. All HeC samples dissolve into water over the examined temperature range and did not show their cloud points. The value of nH for the HeC sample possessing the MS of 1.3 was 14 at 20 ◦C and decreased gently with increasing temperature and declined to 10 at 70 ◦C. The nH values of the HeC samples are substantially larger than the minimum critical nH value of ca. 5 necessary to be dissolved into water for cellulose ethers such as MC and HpMC, even in a high temperature range. Then, the HeC molecules possess water solubility over the wide temperature range. The temperature dependence of nH for the HeC samples and triethyleneglycol, which is a model compound for substitution groups of HeC, is gentle and they are similar to each other. This observation strongly suggests that the hydration/dehydration behavior of the HeC samples was essentially controlled by that of their substitution groups.

Keywords: chemically modiﬁed cellulose ether; hydroxyethyl cellulose; methyl cellulose; hydration; dehydration; dielectric spectroscopy; relaxation time

1. Introduction Cellulose is the most abundant natural organic resource on the globe. Cellulose is a high-molecular weight polysaccharide which consists of β-1,4-d-glucopyranose units [1]. Native cellulose is insoluble in most usual solvents including water, due to its highly developed inter- and intra-molecular hydrogen bonding between hydroxy groups [2,3]. To improve this insolubility and to make use of cellulose for a wide range of applications, many kinds of chemically modified cellulose derivatives have been synthesized from natural cellulose. A series of chemically modified celluloses, such as water-soluble nonionic methyl, hydroxypropyl, hydroxyethyl and hydroxypropylmethyl cellulose ethers, anionic sodium carboxymethyl cellulose ether, and organic solvent-soluble ethyl and cyanoethyl cellulose ethers, and cellulose nitrate and cellulose acetate, have been developed by several chemical companies [1]. These chemically modified cellulose samples have been widely used in various areas because they exhibit useful properties, such as viscosity thickening, surfactant activity, film formation, adhesion, and so on [1,4–7].

Molecules 2020, 25, 4726; doi:10.3390/molecules25204726 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 14

Molecules 2020, 25, 4726 2 of 14 in various areas because they exhibit useful properties, such as viscosity thickening, surfactant activity, film formation, adhesion, and so on [1,4–7]. HydroxyethylHydroxyethyl cellulosecellulose (HeC)(HeC) isis oneone ofof thethe water-solublewater-soluble cellulosecellulose ethers,ethers, whichwhich hashas beenbeen commerciallycommercially produced produced by by the the chemical chemical reaction reaction between between cellulose cellulose and ethylene and oxide.ethylene HeC oxide. molecules HeC aremolecules composed are ofcomposed glucopyranose of glucopyranose units randomly units substituted randomly substituted by hydroxyethyl by hydroxyethyl groups at the groups position at the of 2,position 3, or 6, of which2, 3, or side6, of chains which can side be chains monomers, can be dimers, monomers, or trimers dimers, [8]. or Scheme trimers1 shows[8]. Scheme the chemical 1 shows structurethe chemical of HeC structure examined of HeC in examined this study. in Due this to study. its high Due water to its solubility,high water HeC solubility, is widely HeC used is widely for a varietyused for of a applicationsvariety of applications such as cosmetics such as cosmetics [9], building [9], materialsbuilding materials [10], ﬁlm [10], forming film forming materials materials [11,12], and[11,12], pharmaceutical and pharmaceutical productions productions (drug delivery (drug delivery system) [system)13,14]. [13,14].

RO O OR n R = H or CH CH OH 2 2

Scheme 1. Chemical structure of hydroxyethyl cellulose ether. Scheme 1. Chemical structure of hydroxyethyl cellulose ether. Although the methyl cellulose (MC) and hydroxypropyl methyl cellulose (HpMC) samples, Although the methyl cellulose (MC) and hydroxypropyl methyl cellulose (HpMC) samples, which are the most prevalent two nonionic cellulose ethers, show high water solubility below room which are the most prevalent two nonionic cellulose ethers, show high water solubility below room temperature, they lose their solubility and sometimes become turbid gels at higher temperatures, i.e., temperature, they lose their solubility and sometimes become turbid gels at higher temperatures, i.e., lower critical solution temperatures (LCSTs) [15–17]. On the other hand, HeC shows high solubility in lower critical solution temperatures (LCSTs) [15–17]. On the other hand, HeC shows high solubility water over a wide temperature range even at higher temperatures than 50 C[18]. Giudice et al. [19] in water over a wide temperature range even at higher temperatures than◦ 50 °C [18]. Giudice et al. have investigated the viscoelastic properties of aqueous HeC solution by using various rheological [19] have investigated the viscoelastic properties of aqueous HeC solution by using various techniques over a wide concentration and frequency range. Koda et al. [20] have compared the water rheological techniques over a wide concentration and frequency range. Koda et al. [20] have retention capacity among MC, HeC, and hydroxypropyl cellulose (HpC) by using the compression compared the water retention capacity among MC, HeC, and hydroxypropyl cellulose (HpC) by method and differential scanning calorimetry measurements and concluded that the capacity of using the compression method and differential scanning calorimetry measurements and concluded hydroxyethyl cellulose is the strongest. Ouaer et al. [21] have determined the intrinsic viscosity and that the capacity of hydroxyethyl cellulose is the strongest. Ouaer et al. [21] have determined the the critical overlap concentration of HeC molecules in aqueous solution. However, these studies were intrinsic viscosity and the critical overlap concentration of HeC molecules in aqueous solution. carried out at room temperature ca. 25 or 0 C and very few studies have reported on the influence However, these studies were carried out at◦ room temperature ca. 25 or 0 °C and very few studies of temperature on the molecular properties of HeC in aqueous solution. Moreover, few studies have have reported on the influence of temperature on the molecular properties of HeC in aqueous focused on the reason why HeC molecules keep their high water solubility even at high temperatures, solution. Moreover, few studies have focused on the reason why HeC molecules keep their high and the reason still remains unsolved. Water solubility, such a fundamental property, is essential to water solubility even at high temperatures, and the reason still remains unsolved. Water solubility, understand the physicochemical characteristics of HeC molecules possessing a higher potential to be such a fundamental property, is essential to understand the physicochemical characteristics of HeC used in broader and more effective applications than they are currently used. molecules possessing a higher potential to be used in broader and more effective applications than To understand the water solubility of polymeric materials, the determination of hydration numbers they are currently used. for the materials is important. Although Arfin et al. [22] have investigated the hydration behavior of HeC To understand the water solubility of polymeric materials, the determination of hydration in aqueous solution by using differential scanning calorimetry and Raman spectroscopy measurements, numbers for the materials is important. Although Arfin et al. [22] have investigated the hydration they did not discuss the hydration number and its temperature dependence of HeC samples. behavior of HeC in aqueous solution by using differential scanning calorimetry and Raman Recently, our group developed a useful technique to determine the hydration numbers of spectroscopy measurements, they did not discuss the hydration number and its temperature solute molecules dissolved in water as functions of temperature using dielectric spectroscopic (DS) dependence of HeC samples. measurements performed in an extremely high frequency range up to 50 GHz (3.14 1011 s 1 in Recently, our group developed a useful technique to determine the hydration numbers× of solute− angular frequency (ω)) [23–26]. Because the relaxation strength of free water molecules can be precisely molecules dissolved in water as functions of temperature using dielectric spectroscopic (DS) evaluated in this high frequency range, the amount of water molecules hydrated to solute molecules measurements performed in an extremely high frequency range up to 50 GHz (3.14 × 1011 s−1 in can be exactly determined. In a previous study, the hydration numbers of poly(vinyl alcohol) (PVA) angular frequency (ω)) [23–26]. Because the relaxation strength of free water molecules can be and partially chemically modified PVA, which showed much higher solubility in water than PVA, were precisely evaluated in this high frequency range, the amount of water molecules hydrated to solute determined as functions of temperature by using DS techniques. The obtained results have revealed that molecules can be exactly determined. In a previous study, the hydration numbers of poly(vinyl the hydration number of chemically modified PVA is larger than that of PVA over all temperature ranges alcohol) (PVA) and partially chemically modified PVA, which showed much higher solubility in in the study [23]. Furthermore, the temperature dependencies of hydration numbers per glucopyranose water than PVA, were determined as functions of temperature by using DS techniques. The obtained unit for chemically modified cellulose, MC, HpMC, and hydroxyethylmethyl cellulose (HeMC), were results have revealed that the hydration number of chemically modified PVA is larger than that of also obtained by using DS techniques. The results have shown that additional substitution to MC

Molecules 2020, 25, 4726 3 of 14 by hydroxypropyl or hydroxyethyl groups is quite effective to extend solubility into water because the dehydration behavior of HpMC and HeMC with increasing temperature is much gentler than that of MC [24]. Therefore, the hydration number and its temperature dependence determined by DS measurements should be a key parameter to understand the water solubility of the materials. It is important to understand why chemically modified cellulose samples possess rather different temperature dependencies of hydration behavior and solubility, highly depending on the species of substitution groups for their many advanced practical applications, especially at high temperatures. In this study, we report the temperature dependence of hydration number per glucopyranose unit, nH, for the HeC samples in aqueous solution over a wide temperature range from 10 to 70 ◦C, determined by using extremely high-frequency DS techniques. The substitution group dependence of nH with increasing temperature is fully discussed by comparing the nH values of HeC, MC, HpMC, and HeMC. Additionally, the molar substitution number (MS) dependence of the nH values is also discussed using HeC samples with different MS quantities per glucopyranose unit by hydroxyethyl groups 3 ranging from 1.3 to 3.6. The HeC samples examined in this study were coded as HeC(he:Mw/10 ) with numerical quantities, he and Mw, which represent the molar substitution number per glucopyranose unit, MS, by hydroxyethyl groups, and the weight-average molecular weight. Finally, the reason for the high-water solubility of HeC molecules over a wide temperature range will be discussed.

2. Results and Discussion

2.1. Dielectric Behavior for Aqueous Solution of Hydroxyethyl Cellulose

1 Dielectric spectra (ε0 and ε” vs. ω) for the aqueous solution of HeC(1.3:90) at c = 0.23 mol L− (M) and T = 20 ◦C are shown in Figure1a as a typical example. The dependence of ε0 and ε” on ω was decomposed into five dielectric relaxation modes described by the Debye-type relaxation modes as follows: 5 5 X εj X εjωτj ε0 = + ε , ε00 = (1) + 2 2 ∞ + 2 2 j=1 1 ω τj j=1 1 ω τj where εj, τj, and ε represent the relaxation strength and time for a mode j and the high-frequency ∞ limiting electric permittivity, respectively. The broken lines drawn in the figure show the constituent Debye-type relaxation mode εj0 and εj”(j = 1 to 5 from the fastest relaxation mode due to the shortest relaxation time). To verify the validity of decomposition into five Debye-type relaxation modes for the spectra, the residuals of ε0–ε10–ε (red) and ε”–ε1” (blue) are also plotted in the same figure, which ∞ obviously demonstrate the presence of modes j = 2, 3, 4, and 5. The GDC values for the examined samples were lower than 400 µS. A variety of useful methods were proposed to investigate the hydration behavior of solute molecules including large biomacromolecules like proteins and DNAs, such as extended depolarized light scattering (EDLS) [27–30], nuclear magnetic resonance (NMR) [31,32], neutron scattering (NS) [33], time-resolved fluorescence decay [34] and dielectric spectroscopic (DS) [23,26,30,35–38] measurements, and molecular dynamics simulation (MD) [39–41] techniques. The DS measurement is one of the most useful methods to explore hydration behavior because it can directly detect the dynamics of water molecules quite precisely [23,26,30,35–38]. In the case of DS measurements conducted in an extremely high frequency range up to 50 GHz, it has been well-known that dielectric spectra of the collective rotational relaxation process for water molecules are well described, with a single Debye-type mode at the relaxation time of τw = 9.0 ps (at T = 20 ◦C) in pure liquid water [42]. Dielectric spectra for aqueous solutions of solute molecules are precisely described as the summation of Debye-type relaxation mode of free water molecules, additional Debye-type relaxation modes assigned to the rotational relaxation process of hydrated water molecules, and that of dipoles attached to solute molecules in aqueous systems [23,26,30,35–38]. Then, we used the summation of Debye-type modes given by Equation (1) to describe the dielectric spectra of aqueous solutions examined in this study. Molecules 2020, 25, x FOR PEER REVIEW 4 of 14 assigned to the rotational relaxation process of hydrated water molecules, and that of dipoles attached to solute molecules in aqueous systems [23,26,30,35–38]. Then, we used the summation of Molecules 2020, 25, 4726 4 of 14 Debye-type modes given by Equation (1) to describe the dielectric spectra of aqueous solutions examined in this study. 102 60 ε' (a) (b) 50 ε" τ -1 101 ε'-ε '-ε 40 1 ∞ 1 " ε ε ∞

ε"-ε " " or 1 ε 30 ' ε 100 τ -1 20 2 τ -1 5 10 τ -1 τ -1 High ω low ω -1 4 3 10 0 107 108 109 1010 1011 1012 0 1020304050607080 ω / s-1 ε'

Figure 1. (a) Dielectric spectra (ε0 and ε” vs. ω) for aqueous solution of HeC(1.3:90) at c = 0.23 M and Figure20 ◦C. Broken1. (a) Dielectric lines represent spectraconstituent (ε′ and ε″ vs. relaxation ω) for aqueous modes fromsolutionj = 1 of to HeC(1.3:90) 5. The residuals at c = of 0.23ε0– Mε1 0and–ε ∞ 20and °C.ε ”–Brokenε1” are lines also represent plotted in cons the sametituent figure relaxation with small modes closed from symbols. j = 1 to 5. (b The) A ColeresidualsCole of plot ε′–ε for1′–ε∞ the − anddata ε″ shown–ε1″ are in also (a). plotted A solid in semicircle the same represents figure with single small Debye-type closed symbols. presentation (b) A Cole for the−Cole major plot dielectric for the 11 1 datadispersion shown observedin (a). A solid at ω semicircle~ 10 s− .represents single Debye-type presentation for the major dielectric dispersion observed at ω ~ 1011 s−1. It has been well-known that the Cole Cole [43] and Cole Davidson [44] type relaxation functions − − are alsoIt has useful been well-known to describe that dielectric the Cole behavior−Cole [43] showing and Cole non-single−Davidson Debye-type [44] type relaxation dielectric functions spectra. Figure1b shows a so-called Cole Cole plot for the data represented in Figure1a together with the best are also useful to describe dielectric− behavior showing non-single Debye-type dielectric spectra. Figurefit single 1b Debye-typeshows a so-called behavior Cole shown−Cole byplot a for solid the semicircle data represented of major in dielectric Figure 1a dispersion together observedwith the 11 1 bestat ω fit~ 10singles− .Debye-type Non-negligible behavi deviationor shown from by thea solid solid semicircle circle is clearly of major observed dielectric in data dispersion points in observeda lower frequency at ω ~ 10 region,11 s−1. Non-negligible which represents deviation the additional from the contribution solid circle is of clearly the modes observedj = 3, 4,in anddata 5. pointsAlthough in a thelower agreement frequency between region, the which solid repres semicircleents andthe additional data points contribution in the major of dispersion the modes portion j = 3, seems reasonably well, the Cole Cole or Cole Davidson models are not the best methods to express 4, and 5. Although the agreement− between the solid− semicircle and data points in the major dispersion portionsuch DS seems spectra, reasonably showing well, a deviation the Cole from−Cole a or single Cole Debye-type−Davidson models behavior are as not observed the best inmethods this study. to expressThen, we such did DS not spectra, use these showing models a deviation to analyze from dielectric a single spectra Debye-type obtained behavior in this as study. observed However, in this it is worth noting that the Cole Davidson model is also quite helpful to describe the broad frequency study. Then, we did not use− these models to analyze dielectric spectra obtained in this study. However,dependence it is of worth the susceptibility noting that the of aqueousCole−Davidson solutions model determined is also quite by spectroscopic helpful to describe techniques the broad other frequencythan the DS dependence measurement of tothe explore susceptibility hydration of behavior, aqueous such solutions as EDLS determined [27–30] and by NS spectroscopic [33] methods. techniquesFigure other2a,b demonstratethan the DS measurement the concentration to explore dependence hydration of behavior, the dielectric such relaxation as EDLS [27 times–30] and NSstrength [33] methods. for each mode (j = 1 to 5) obtained from the aqueous HeC(1.3:90) solution. The fastest relaxationFigure time, 2a,bτ demonstrate1, was equal tothe the concentration value of the rotational dependence relaxation of the time, dielectricτw, for relaxation the pure liquid times water and strengthmolecules, for as each plotted mode in Figure(j = 1 2toa. Then,5) obtained the mode fromj =the1 isaqueous easily assigned HeC(1.3:90) to the solution. rotational The relaxation fastest relaxationmode of the time, free τ water1, was molecules equal to inthe the value aqueous of the HeC(1.3:90) rotational solution. relaxation The time, value τw of, forε1 clearly the pure decreased liquid waterwith increasingmolecules, concentration, as plotted in Figurec, as shown 2a. Then, in Figure the mode2b. The j = amount 1 is easily of free assigned water moleculesto the rotational in the relaxationaqueous solution mode of was the substantially free water molecules reduced by in the the volume aqueous eff ectHeC(1.3:90) of the solute solution. HeC(1.3:90) The value molecules, of ε1 clearlywhich possessdecreased a certain with increasing volume fraction concentration, in the aqueous c, as shown solution, in Figure and the 2b. presence The amount of hydration of free water water. moleculesIn a later section,in the theaqueous number solution of hydrated was substantially water molecules reduced to a glucopyranoseby the volume unit effect of the of HeC(1.3:90)the solute HeC(1.3:90)molecules (hydration molecules, number, which possessnH) will a be certain calculated volume from fraction the concentration in the aqueous dependence solution, of εand1. the presenceThe secondof hydration relaxation water. mode In ja= later2 possesses section, the the relaxation number timeof hydratedτ2 ~ 2.4τ 1waterand themolecules strength toε2 ais glucopyranosethe second largest unit andof the proportional HeC(1.3:90) to moleculesc, as shown (hydration in Figure number,2a,b. According nH) will be to calculated previous research,from the concentrationfor several water-soluble dependence substances of ε1. [23,26,30,35–38], MD simulation techniques [39–41], and DS and EDLS measurements [30], the second mode is assigned to the exchange process of hydrated water molecules by free ones in the aqueous solution. The obtained relaxation time, τ2, usually possesses a longer relaxation time than the value of τ1 for some factors. The ratio of τ2/τw has been called a retardation ratio (or slowdown factor) originally. The evaluated value of τ2/τ1 is essentially identical to that of τ2/τw in this study because the observed value of τ1 was very close to that of τw. Since the ratio of τ2/τ1 is ~ 2.4 as seen in Figure2a and that of other hydroxyethyl cellulose systems examined in this Molecules 2020, 25, 4726 5 of 14 study are quite similar to that reported for aqueous solutions of various saccharides, such as glucose, fructose, sucrose, and glucose oligomers obtained using EDLS and DS techniques [30,36–38,45], the hydration properties of aqueous hydroxyethyl cellulose systems are not so different from those of mono- and oligosaccharide molecules. The fact that the second relaxation strength ε2 is very close to the calculated value of nHc(εw/55.6) for the aqueous HeC solution, which is the estimated dielectric relaxation magnitude for hydrated water molecules, assuming that the same relaxation magnitude as Moleculesfree water 2020 molecules,, 25, x FOR PEER powerfully REVIEW supports the validity of this assignment. 5 of 14

FigureFigure 2. 2. ConcentrationConcentration dependence dependence of of dielectric dielectric relaxation relaxation times times ( (aa)) and and strength strength ( (bb)) for for each each mode mode observedobserved in in aqueous aqueous solution solution of of HeC(1.3:90) HeC(1.3:90) at at TT == 2020 °C◦C shown shown in in Figure Figure 1a.1a.

TheThe second third relaxation relaxation modemode jj == 32 possesses demonstrates the relaxation the relaxation time timeτ2 ~ 2.4τ3τof1 and 180 the ps, strength as shown ε2 is in theFigure second2a. Althoughlargest and the proportional strength of to mode c, as shownj = 3 is in considerably Figure 2a,b. smallerAccording than to thatprevious of mode research,j = 2, for the severalvalue is water-soluble proportional tosubstancesc, as seen [23,26,30,35 in Figure2b.–38], Since MD HeC(1.3:90) simulation molecules techniques have [39 some–41], polarand DS groups and with finite dipole moments, such as hydroxy ( OH) and ether ( O ) groups, the rotational mode of EDLS measurements [30], the second mode is− assigned to the exchange− − process of hydrated water moleculesthese polar by groups free ones would in the be aqueous detected so aslution. dielectric The relaxation obtained relaxation processes. time, The value τ2, usually of relaxation possesses time a longer(τ3 ~ 180 relaxation ps) for HeC time is similarthan the to value that for of methyl τ1 for andsome hydroxypropylmethyl factors. The ratio of cellulose τ2/τw has (τ 3been~ 220 called ps) [24 a]. 1 retardationFigure3a shows ratio the(or slowdownMS dependence factor) of originally. relaxation The strength evaluated per one value mol of of τ glucopyranose2/τ1 is essentially ring, identicalε3c − , 1 1 toand that the of value τ2/τw ofinε this3c − studyis proportional because the to observedMS. The factvalue that of theτ1 was intercept very close value to of thatε3c −of ~τw 2.5. Since for HeC the 1 1 ratiois almost of τ2/ theτ1 is same ~ 2.4 valueas seen of inε3 cFigure− for 2a pullulan and that (ε 3ofc− other~ 2.6), hydroxyethyl which is a typical cellulose water-soluble systems examined natural inglucan, this study strongly are quite supports similar the validityto that reported of this assignment. for aqueous The solutions rotational of various relaxation saccharides, time of a hydroxysuch as glucose,group would fructose, be governed sucrose, and by the glucose lifetime oligomer of two kindss obtained of intramolecular using EDLS hydrogenand DS techniques bonding between[30,36– 38,45],two hydroxy the hydration groups, properties and a hydroxy of aqueous group and hydroxye an etherthyl group. cellulose HeC systems molecules are have not twoso different types of from ether thosegroups: of mono- (i) a native and etheroligosaccharide group (n-ether molecules. group) Th thate fact cellulose that the possesses second nativelyrelaxation and strength (ii) an additionalε2 is very closeether to group the calculated (ad-ether group) value whichof nHc is(εw introduced/55.6) for the by theaqueous substitution. HeC solu Thetion, rotational which relaxationis the estimated time of dielectricn-ether groups relaxation would magnitude be longer thanfor hydrated that of ad-ether water molecules, groups because assuming the rotational that the motionsame relaxation of n-ether magnitudegroups requires as free the water rotation molecules, of the glucopyranosepowerfully supports rings ofthe the validity backbone of this chains. assignment. On the other hand, the ad-etherThe third groups relaxation can rotatemode withoutj = 3 demonstrates the rotation the of relaxation the backbone time chains. τ3 of 180 Then, ps, asτ 3showncan be in assigned Figure 2a.to theAlthough rotational the relaxationstrength of time mode of hydroxyj = 3 is considerably groups and ad-ethersmaller than groups that controlled of mode j by = 2, the the lifetime value is of proportionalintramolecular to hydrogenc, as seen bonding.in Figure 2b. Since HeC(1.3:90) molecules have some polar groups with finite Thedipole relaxation moments, modes, suchj =as4 hydroxy and 5, possess (−OH) the and relaxation ether (− timesO−) groups, of τ4 ~ 3.5the and rotationalτ5 ~ 24 ns,mode respectively, of these polarfor HeC(1.3:90). groups would The be relaxation detected strengthas dielectric of these relaxation modes, processes.ε4 and ε5 ,The are proportionalvalue of relaxation to c as timeε2 and (τ3ε ~3 , 180as shownps) for inHeC Figure is similar2b. Figure to that3b representsfor methyl the and chydroxypropylmethyldependence of the strength, celluloseε4 (andτ3 ~ ε2205, for ps) all [24]. the FigureHeC samples. 3a shows This the MS figure dependence reveals that of therelaxation magnitudes, strengthε4 perand oneε5, mol are simplyof glucopyranose proportional ring, to εc3andc −1, anddepend the value on neither of ε3cM −1w isnor proportionalMS for the to HeC MS samples.. The fact Then,that the the intercept dielectric value strength of ε would3c−1 ~ 2.5 be for controlled HeC is almostonly by the the same dipole value moments of ε3c −1 offor the pullulan n-ether (ε groups3c−1 ~ 2.6), in which glucopyranose is a typical rings water-soluble and β-1,4 linkages,natural glucan, not by stronglythe dipole supports moments the of validity ad-ether of groupsthis assignment. attached toThe the rotational glucopyranose relaxation rings. time The of relaxation a hydroxy times group of wouldthe modes, be governedτ4 and τ by5, for the HeC lifetime samples of two were kinds less of dependent intramolecular on both hydrogen the MS andbondingMw values.between These two hydroxy groups, and a hydroxy group and an ether group. HeC molecules have two types of ether groups: (i) a native ether group (n-ether group) that cellulose possesses natively and (ii) an additional ether group (ad-ether group) which is introduced by the substitution. The rotational relaxation time of n-ether groups would be longer than that of ad-ether groups because the rotational motion of n- ether groups requires the rotation of the glucopyranose rings of the backbone chains. On the other hand, the ad-ether groups can rotate without the rotation of the backbone chains. Then, τ3 can be assigned to the rotational relaxation time of hydroxy groups and ad-ether groups controlled by the lifetime of intramolecular hydrogen bonding. The relaxation modes, j = 4 and 5, possess the relaxation times of τ4 ~ 3.5 and τ5 ~ 24 ns, respectively, for HeC(1.3:90). The relaxation strength of these modes, ε4 and ε5, are proportional to c

Molecules 2020, 25, x FOR PEER REVIEW 6 of 14 as ε2 and ε3, as shown in Figure 2b. Figure 3b represents the c dependence of the strength, ε4 and ε5, for all the HeC samples. This figure reveals that the magnitudes, ε4 and ε5, are simply proportional to c and depend on neither Mw nor MS for the HeC samples. Then, the dielectric strength would be controlled only by the dipole moments of the n-ether groups in glucopyranose rings and β-1,4 Molecules 2020, 25, 4726 6 of 14 linkages, not by the dipole moments of ad-ether groups attached to the glucopyranose rings. The relaxation times of the modes, τ4 and τ5, for HeC samples were less dependent on both the MS and observationsMw values. These suggest observations that the modes, suggestj = that4 and the 5, modes, are attributed j = 4 and to 5, the are local attributed motions to ofthe glucopyranose local motions ringsof glucopyranose in HeC molecules rings dissolvedin HeC molecules into water. dissolved Since the into goal water. of this studySince isthe a fundamentalgoal of this study argument is a aboutfundamental the hydration argument/dehydration about the behaviorhydration/dehydrat of hydroxyethylion behavior cellulose of hydroxyethyl samples in aqueous cellulose solution, samples a detailedin aqueous discussion solution, on a detailed the relaxation discussion modes, onj the= 4 relaxation and 5, is not modes, developed j = 4 and here. 5, is not developed here.

FigureFigure 3.3. (a) Molar substitution number, MS,, dependence of normalized dielectric relaxation strength, −11 εε33c− , ,for for mode mode jj == 3.3. (b (b) )Concentration Concentration dependence dependence of of dielec dielectrictric relaxation strengthstrength forfor modemodej j= = 44 andand 55 observedobserved inin aqueousaqueous solutionssolutions ofof HeC(1.3:90),HeC(1.3:90), HeC(2.0:220),HeC(2.0:220), andand HeC(3.6:70).HeC(3.6:70).

2.2.2.2. Temperature Dependence Dependence of Relaxationof Relaxation Times Times Figure4 represents the reciprocal temperature, T 1, dependence of dielectric relaxation times, τ Figure 4 represents the reciprocal temperature, T−−1, dependence of dielectric relaxation times, τ11 and τ , and the retardation ratio, τ /τ , for an aqueous solution of HeC(1.3:90) at c = 0.23 M as a typical and τ22, and the retardation ratio, τ22/τ11, for an aqueous solution of HeC(1.3:90) at c = 0.23 M as a typical 1 example.example. TheThe activationactivation energyenergy ofof thethemode modej j= = 11 waswas calculatedcalculatedto tobe be17.1 17.1 kJ kJ mol mol−−1 fromfrom thethe slopeslope ofof the τ data shown in Figure4. The obtained value ensures the validity of assignment for the mode j = 1 the τ11 data shown in Figure 4. The obtained value ensures the validity of assignment for the mode j because this value is close to that of the rotational relaxation time, τ , of water molecules in the pure = 1 because this value is close to that of the rotational relaxation time,w τw, of water molecules in the 1 liquidpure liquid state. Thestate. activation The activation energy energy of the modeof thej mode= 2 was j = evaluated 2 was evaluated to be 17.8 tokJ be mol 17.8− kJ, which mol−1 is, which slightly is largerslightly than larger that than of mode that jof= mode1. This j small= 1. This diﬀ erencesmall difference in the activation in the energiesactivation is energies because theis because interaction the hydrationinteraction energy hydration between energy a hydrated between water a moleculehydratedand water a hydration molecule site and of thea HeC(1.3:90)hydration site molecules of the isHeC(1.3:90) slightly larger molecules than that is betweenslightly purelarger liquid than water that molecules.between pure Similar liquid temperature water molecules. dependencies Similar of Moleculesthe τ and 2020τ, 25values, x FOR PEER were REVIEW also obtained for aqueous solutions of HeC(2.0:220) and HeC(3.6:70). 7 of 14 temperature1 2 dependencies of the τ1 and τ2 values were also obtained for aqueous solutions of HeC(2.0:220) and HeC(3.6:70). The τ2/τ1 value is approximately kept at 2.2–2.4 for HeC(1.3:90) and other HeC samples in the temperature range examined. This retardation ratio reasonably agrees with the previously reported values for hydrated water molecules in aqueous saccharide solutions determined at room temperature using EDLS and DS measurements [30,36–38], and aqueous solutions of MC and HpMC in the same temperature range as in this study using DS techniques [24]. In the case of HpMC in aqueous solution, the T−1 dependence of the τ2/τ1 value showed a break point at 30 °C with increasing T [24]. However, the T−1 dependence of the τ2/τ1 value for the aqueous HeC systems shows no break point in the measured T range. A difference in the presence of break point between HeC and HpMC would be related to the difference in the dehydration behavior of the systems.

Figure 4. Temperature dependence of dielectric relaxation times, τ1 and τ2,, and and the the retardation retardation ratio, ratio, τ2//ττ11, ,for for aqueous aqueous solution solution of of HeC(1.3:90) HeC(1.3:90) at at cc == 0.230.23 M. M.

The retardation ratio of hydrated water molecules for aqueous green fluorescent protein system was reported at two different temperatures, 7 and 30 °C, by using NS techniques. Although the retardation ratio was independent of T, the ratio was ranged from 3 to 7 depending on the scattering vector [33]. The retardation ratios of hydrated water molecules for other proteins like lysozyme and oligopeptides were also reported to be 3 to 7 using various methods such as EDLS [30], DS [30,46], and NMR [31,32] measurements, and MD simulations [40,41]. It seems that amide groups forming oligo peptides and proteins have considerably larger retardation ratios of hydrated water molecules than hydroxy and ether groups constructing polysaccharides, as described above and in the previous study of MC and HpMC [24].

2.3. Hydration Behavior

From the c dependence of ε1, the hydration number, nH, per glucopyranose unit can be determined by using the following equation (Equation (2)) [23,26,35]: −3 ε 110− Vc − 1m=−10 3Vcn ε + −3 wH (2) wm110Vc /2 where Vm and Vw are the partial molar volumes of glucopyranose units and water molecules, respectively, at each measuring temperature in the unit of cm3 mol−1. The Vm value was evaluated from the density data of the examined aqueous HeC solution, and the precise values of Vw are available in the literature. The first term of Equation (2) represents the contribution of the volume excluded by the presence of solute HeC molecules, and the second one represents the hydration effect of the solute molecules, HeC, in this study. Figure 5 shows the concentration, c, dependence of the ε1/εw values for aqueous solution of HeC(1.3:90) at 20 °C as a typical example. The broken line in the figure, which is theoretically calculated via Equation (2) assuming nH = 0, is far from the experimentally obtained ε1/εw data points. However, the agreement between the data and the solid line, calculated assuming nH = 14, is perfect over the entire c range examined. Consequently, we concluded that the nH value of HeC(1.3:90) at 20 °C is 14. According to the same procedure, the values of nH were determined successfully as a function of T from 10 to 70 °C for all the aqueous HeC solutions examined.

Molecules 2020, 25, 4726 7 of 14

The τ2/τ1 value is approximately kept at 2.2–2.4 for HeC(1.3:90) and other HeC samples in the temperature range examined. This retardation ratio reasonably agrees with the previously reported values for hydrated water molecules in aqueous saccharide solutions determined at room temperature using EDLS and DS measurements [30,36–38], and aqueous solutions of MC and HpMC in the same temperature range as in this study using DS techniques [24]. In the case of HpMC in aqueous solution, 1 the T− dependence of the τ2/τ1 value showed a break point at 30 ◦C with increasing T [24]. However, 1 the T− dependence of the τ2/τ1 value for the aqueous HeC systems shows no break point in the measured T range. A difference in the presence of break point between HeC and HpMC would be related to the difference in the dehydration behavior of the systems. The retardation ratio of hydrated water molecules for aqueous green fluorescent protein system was reported at two different temperatures, 7 and 30 ◦C, by using NS techniques. Although the retardation ratio was independent of T, the ratio was ranged from 3 to 7 depending on the scattering vector [33]. The retardation ratios of hydrated water molecules for other proteins like lysozyme and oligopeptides were also reported to be 3 to 7 using various methods such as EDLS [30], DS [30,46], and NMR [31,32] measurements, and MD simulations [40,41]. It seems that amide groups forming oligo peptides and proteins have considerably larger retardation ratios of hydrated water molecules than hydroxy and ether groups constructing polysaccharides, as described above and in the previous study of MC and HpMC [24].

2.3. Hydration Behavior

From the c dependence of ε1, the hydration number, nH, per glucopyranose unit can be determined by using the following equation (Equation (2)) [23,26,35]:

ε 1 10 3V c 1 = − m 10 3V cn (2) − 3 − w H εw 1 + 10− Vmc/2 − where Vm and Vw are the partial molar volumes of glucopyranose units and water molecules, 3 1 respectively, at each measuring temperature in the unit of cm mol− . The Vm value was evaluated from the density data of the examined aqueous HeC solution, and the precise values of Vw are available in the literature. The first term of Equation (2) represents the contribution of the volume excluded by the presence of solute HeC molecules, and the second one represents the hydration effect of the solute molecules, HeC, in this study. Figure5 shows the concentration, c, dependence of the ε1/εw values for aqueous solution of HeC(1.3:90) at 20 ◦C as a typical example. The broken line in the figure, which is theoretically calculated via Equation (2) assuming nH = 0, is far from the experimentally obtained ε1/εw data points. However, the agreement between the data and the solid line, calculated assuming nH = 14, is perfect over the entire c range examined. Consequently, we concluded that the nH value of HeC(1.3:90) at 20 ◦C is 14. According to the same procedure, the values of nH were determined successfully as a function of T from 10 to 70 ◦C for all the aqueous HeC solutions examined. Figure6 shows the dependence of nH on temperature, T, for aqueous HeC(1.3:90) solutions obtained in a concentration range from 0.14 to 0.23 M. The nH values determined from aqueous solutions of HeC at different concentrations demonstrated the same value at each examined temperature. Then, one might conclude that the nH value is successfully evaluated and has very weak c dependence in the c range examined. Furthermore, the nH value is ca. 15 at 10 ◦C, and gently reduces to ca. 10 at 70 ◦C with increasing temperature. The HeC sample possessed water solubility even in a higher temperature range and did not demonstrate a cloud point and gelation in the examined T range, as previously reported [18]. Molecules 2020, 25, x FOR PEER REVIEW 8 of 14

Molecules 2020, 25, 4726 8 of 14 Molecules 2020, 25, x FOR PEER REVIEW 8 of 14

Figure 5. Relationship between the values of ε1/εw and concentration, c, for aqueous solution of HeC(1.3:90) at T = 20 °C.

Figure 6 shows the dependence of nH on temperature, T, for aqueous HeC(1.3:90) solutions obtained in a concentration range from 0.14 to 0.23 M. The nH values determined from aqueous solutions of HeC at different concentrations demonstrated the same value at each examined temperature. Then, one might conclude that the nH value is successfully evaluated and has very weak c dependence in the c range examined. Furthermore, the nH value is ca. 15 at 10 °C, and gently reduces to ca. 10 at 70 °C with increasing temperature. The HeC sample possessed water solubility even in a higher temperature range and did not demonstrate a cloud point and gelation in the examined T Figure 5. RelationshipRelationship between between the the values of ε1//εww andand concentration, concentration, cc,, for for aqueous aqueous solution of range, as previously reported [18]. HeC(1.3:90) at T = 2020 °C.◦C.

Figure 6.6. Temperature,Temperature,T ,T dependence, dependence of theof hydrationthe hydration number, number,nH, per nH glucopyranose, per glucopyranose unit for aqueousunit for HeC(1.3:90)aqueous HeC(1.3:90) solutions solutions at several atc severalvalues rangedc values from ranged 0.14 from to 0.23 0.14 M. to 0.23 M.

Figure 77aa showsshows thethe comparison of temperature, T,T, dependenciesdependencies of of nHn Hforfor aqueous aqueous solutions solutions of ofHeC(1.3:90), HeC(1.3:90), MC(1.8:95), MC(1.8:95), which which has hasthe thedegree degree of substitution, of substitution, DS, DSby ,methyl by methyl groups, groups, Me, Me,of 1.8 of and 1.8 3 andMw/10M3w of/10 95;of HpMC(0.15:1.8:75), 95; HpMC(0.15:1.8:75), which which has the has MS the byMS hydroxypropylby hydroxypropyl groups groups of 0.15, of the 0.15, DS theby MeDS 3 byof 1.8 Me and of 1.8 Mw and/103M ofw 75;/10 andof 75;HeMC(0.20:1.5:300), and HeMC(0.20:1.5:300), which has which the hasMS theby hydroxyethylMS by hydroxyethyl groups of groups 0.20, 3 ofDS 0.20, by MeDS ofby 1.5 Me and of 1.5Mw/10 and3 ofM w300./10 Theof 300.hydration The hydration behavior behaviorof MC, HpMC, of MC, and HpMC, HeMC and has HeMC been n hasreported been already reported [24]. already The n [H24 data]. The shownH indata this shown figure inare this almost ﬁgure independent are almost of independent the concentrations of the concentrationsof solute cellulose of solute derivatives. cellulose In derivatives.a temperature In arange temperature lower than range 25 lower °C, the than nH 25value◦C, of the HeC(1.3:90),nH value of Figure 6. Temperature, T, dependence of the hydration number, nH, per glucopyranose unit for HeC(1.3:90), which possesses the lowest total substitution quantity, is the highest in the four examined whichaqueous possesses HeC(1.3:90) the lowest solutions total atsubstitution several c values quanti rangedty, is from the 0.14highest to 0.23 in M.the four examined cellulose celluloseether samples. ether samples.Figure 7b Figurerepresents7b represents temperature, temperature, T, dependenceT, dependence of the hydration of the hydration number ( numberNH) per (NH) per hydrophilic group—hydroxy ( OH) and ether ( O ) groups—for examining the cellulose hydrophilicFigure 7agroup shows— hydroxythe comparison (−OH) ofand −temperature, ether (−O− T,) groupsdependencies− − —for examining of nH for aqueous the cellulose solutions ether of ether samples in aqueous solution. Although the n value of HeC(1.3:90) is the highest in the four HeC(1.3:90),samples in aqueous MC(1.8:95), solution. which Although has the degreethe nH value of substitution, Hof HeC(1.3:90) DS, byis the methyl highest groups, in the Me, four of cellulose 1.8 and cellulose ether samples, the N values of these cellulose ethers at a low T of 10 C gather into a Metherw/10 samples,3 of 95; HpMC(0.15:1.8:75), the NH values ofH these which cellulose has the ethersMS by at hydroxypropyl a low T of 10 °Cgroups gather of into0.15,◦ a thesimilar DS by value Me similar value of ca. 2.5, irrespective of sample species. The number of ether groups for HeC increases of 1.8 and Mw/103 of 75; and HeMC(0.20:1.5:300), which has the MS by hydroxyethyl groups of 0.20, with increasing MS by hydroxyethyl, He, groups. However, the number of hydroxy groups per DS by Me of 1.5 and Mw/103 of 300. The hydration behavior of MC, HpMC, and HeMC has been glucopyranose unit for HeC is kept at the original value of three for natural cellulose with the increase reported already [24]. The nH data shown in this figure are almost independent of the concentrations in the MS value by He groups. Then, the reason why HeC(1.3:90) demonstrates the highest n value in of solute cellulose derivatives. In a temperature range lower than 25 °C, the nH value of HeC(1.3:90),H whichthe four possesses cellulose the ethers lowest is that total the substitution average number quanti ofty, hydrophilic is the highest groups in the per four glucopyranose examined cellulose unit for HeC(1.3:90) is 6.3. This value is larger than that for other cellulose ethers, such as 5.0 for MC(1.8:95), ether samples. Figure 7b represents temperature, T, dependence of the hydration number (NH) per hydrophilic5.15 for HpMC(0.15:1.8:75), group—hydroxy and (−OH) 5.2 for and HeMC(0.20:1.5:300). ether (−O−) groups Koda—for et examining al. [20] reported the cellulose that the waterether samples in aqueous solution. Although the nH value of HeC(1.3:90) is the highest in the four cellulose ether samples, the NH values of these cellulose ethers at a low T of 10 °C gather into a similar value

Molecules 2020, 25, x FOR PEER REVIEW 9 of 14

of ca. 2.5, irrespective of sample species. The number of ether groups for HeC increases with increasing MS by hydroxyethyl, He, groups. However, the number of hydroxy groups per glucopyranose unit for HeC is kept at the original value of three for natural cellulose with the increase in the MS value by He groups. Then, the reason why HeC(1.3:90) demonstrates the highest nH value Moleculesin the 2020four, 25 cellulose, 4726 ethers is that the average number of hydrophilic groups per glucopyranose unit9 of 14 for HeC(1.3:90) is 6.3. This value is larger than that for other cellulose ethers, such as 5.0 for MC(1.8:95), 5.15 for HpMC(0.15:1.8:75), and 5.2 for HeMC(0.20:1.5:300). Koda et al. [20] reported that retentionthe water capacity retention of HeCcapacity is higher of HeC than is higher that of than MC that by usingof MC the by compressionusing the compression method and method diﬀerential and scanningdifferential calorimetry scanning measurements.calorimetry measurements. The higher ThenH valueshigher ofnH HeCvalues would of HeC correspond would correspond to the higher to waterthe higher retention water capacity retention of HeCcapacity than of that HeC of than MC. that of MC.

FigureFigure 7. 7.( a()aT) Tdependence dependence ofof thethen nHH valuesvalues for HeC(1.3:90), MC(1.8:95), MC(1.8:95), HpMC(0.15:1.8:75), HpMC(0.15:1.8:75), and and HeMC(0.20:1.5:300).HeMC(0.20:1.5:300). SolidSolid arrowsarrows in this ﬁgurefigure represent the the lowest lowest cloud cloud point point observed observed in ina high a high concentrationconcentration side, side, i.e., i.e., lower lower critical critical solution solution temperature temperature (LCST). (LCST). A dotted A dotted arrow arrow represents represents a gelation a pointgelation without point turning without intoturning the into turbid the turbid statefor state aqueous for aqueous solution solution of HeMC(0.20:1.5:300)of HeMC(0.20:1.5:300) at at0.1 0.1 M.

(b)M.T dependence(b) T dependence of the ofN theH valuesNH values for for each each chemically chemically modiﬁed modified cellulose cellulose sample. sample.

TheThe decreasing decreasing coe coefficientfficient of of hydration hydration number,number, (∂nH//∂∂TT)c),c ,for for HeC(1.3:90) HeC(1.3:90) with with increasing increasing T isT is clearlyclearly lower lower than than that that for for other other three three cellulosecellulose ethers, as as seen seen in in Figure Figure 7a.7a. The The HeC(1.3:90) HeC(1.3:90) keeps keeps a a highhighnH nvalue,H value, even even in in a a high high temperature temperaturerange range ((nnHH ~ 10 10 at at TT == 7070 °C),◦C), and and the the nHn valueH value of HeC(1.3:90) of HeC(1.3:90) is alwaysis always much much higher higher than than that that of other of othe threer three cellulose cellulose ethers ethers in the in examined the examinedT range. T range. In the previousIn the study,previous the critical study, hydrationthe critical number hydration for number MC, HpMC, for MC, and HpMC, HeMC and samples HeMC necessary samples tonecessary be dissolved to be in dissolved in water was evaluated to be ca. 5 [24]. In Figure 7b, the NH values of MC(1.8:95) and water was evaluated to be ca. 5 [24]. In Figure7b, the NH values of MC(1.8:95) and HpMC(0.15:1.8:75) justHpMC(0.15:1.8:75) below the LCST andjust below that of the HeMC(0.20:1.5:300) LCST and that of HeMC(0.20:1.5:300) just below the gelation just below point the are gelation ca. 1. Therefore, point are ca. 1. Therefore, the critical hydration number for MC, HpMC, and HeMC samples can be the critical hydration number for MC, HpMC, and HeMC samples can be described to be ca. 1 per described to be ca. 1 per hydrophilic group. The value of NH for HeC(1.3:90) is ca. 1.7, even at T = 70 hydrophilic group. The value of N for HeC(1.3:90) is ca. 1.7, even at T = 70 C, as shown in Figure7b °C, as shown in Figure 7b and isH much higher than the critical hydration number◦ per hydrophilic and is much higher than the critical hydration number per hydrophilic group. This is the reason why group. This is the reason why HeC(1.3:90) can keep high solubility in water even in a high T range. HeC(1.3:90) can keep high solubility in water even in a high T range. Figure 8a shows temperature, T, dependence of hydration number, nH, per glucopyranose unit forFigure aqueous8a showssolutions temperature, of HeC(1.3:90),T, dependenceHeC(2.0:220), of and hydration HeC(3.6:70), number, whichnH possess, per glucopyranose different MS by unit forHe aqueous groups. solutions All the HeC of HeC(1.3:90), samples examined HeC(2.0:220), in this and study HeC(3.6:70), keep high which water possesssolubility di ffovererent a MSwideby T He groups.range Allandthe never HeC demonstrate samples examined a cloud point in this even study in a keep high high T range. water As solubility the MS by over He groups a wide increases,T range and neverthe demonstratevalues of nH afor cloud HeC point increase. even This in a highis becauseT range. the As number the MS ofby hydrophilic He groups increases,ether groups the valuesper of glucopyranosenH for HeC increase. unit increases This is becausewith increasing the number MS by of hydrophilicHe groups, and ether the groups number per of glucopyranose hydroxy groups unit increasesper glucopyranose with increasing unit MSis keptby Heat three groups, for andnatural the numbercellulose. of The hydroxy increase groups in nH pervalue glucopyranose resulting from unit is keptthe additional at three for substitution natural cellulose. by He gr Theoups increase is more inremarkablenH value resultingin a low Tfrom range; the the additional increase coefficient substitution byof He the groups nH value is moreper MS remarkable by He groups in a at low 10 T°Crange; is evaluated the increase to be (∂ coenH/ffi∂MScient)c = of 3.3. the ThenH effectvalue on per theMS nH by Hevalue groups by atan 10increase◦C is evaluatedin MS in ato high be (T∂ nrangeH/∂MS looks)c = weaker3.3. The than effect that on in the a lownH valueT range; by the an increaseincrease in MScoefficientin a high atT 70range °C is looks 2.2. Then, weaker the than T dependence that in a low of nTH range;for HeC the is substantially increase coe ffiaffectedcient at by 70 the◦C MS is 2.2. Then,value. the T dependence of nH for HeC is substantially affected by the MS value. The temperature, T, dependence of hydration number, nH, for triethylene glycol (TEG) was also investigated as a model molecule for the substitution groups of HeC. Scheme2 represents the chemical structure of TEG. Since the structure of TEG is similar to a possible side chain of HeC molecules made from three hydroxyethyl groups, TEG is a reasonable model to compare the hydration behavior of the substitution groups and that of HeC. The T dependence of nH per molecule of TEG is also shown in Figure8a. The dehydration behavior of all the HeC samples and TEG with increasing T is gentle, as seen in this figure. The reason why the T dependence of nH for HeC samples becomes greater with an increasing MS value, as mentioned above, would be the hydration characteristics of their substitution Molecules 2020, 25, x FOR PEER REVIEW 10 of 14

Molecules 2020, 25, 4726 10 of 14 groups becoming remarkable with increasing MS. Figure8b shows the T dependence of hydration number, NH, per hydrophilic group for the HeC samples and TEG. The dehydration coeﬃcients of NH for the four samples with increasing T are very similar to each other. This consideration Figure 8. (a) Temperature, T, dependence of the hydration number, nH, per glucopyranose unit for reveals that (∂N /∂MS)c ~ 0 irrespective of T, although (∂n /∂MS)c depends on T, as discussed above. HeC(1.3:90),H HeC(2.0:220), and HeC(3.6:70) and that for triethyleneH glycol, TEG, per molecule. (b) T Then, we might conclude that the hydration/dehydration behavior for the HeC samples is essentially dependence of the hydration number, NH, per hydrophilic group for the HeC samples and TEG. controlledMolecules 2020 by, the25, x substitutionFOR PEER REVIEW groups like TEG. 10 of 14

The temperature, T, dependence of hydration number, nH, for triethylene glycol (TEG) was also investigated as a model molecule for the substitution groups of HeC. Scheme 2 represents the chemical structure of TEG. Since the structure of TEG is similar to a possible side chain of HeC molecules made from three hydroxyethyl groups, TEG is a reasonable model to compare the hydration behavior of the substitution groups and that of HeC. The T dependence of nH per molecule of TEG is also shown in Figure 8a. The dehydration behavior of all the HeC samples and TEG with increasing T is gentle, as seen in this figure. The reason why the T dependence of nH for HeC samples becomes greater with an increasing MS value, as mentioned above, would be the hydration characteristics of their substitution groups becoming remarkable with increasing MS. Figure 8b shows the T dependence of hydration number, NH, per hydrophilic group for the HeC samples and TEG. The dehydration coefficients of NH for the four samples with increasing T are very similar to

each other. This consideration reveals that (∂NH/∂MS)c ~ 0 irrespective of T, although (∂nH/∂MS)c dependsFigureFigure on 8. T 8., ( as(aa) )discussed Temperature,Temperature, above. TT, ,dependence dependence Then, we ofmight of the the hydration conclu hydrationde number, that number, the nH hydration/, pernH, glucopyranose per glucopyranosedehydration unit for behavior unit HeC(1.3:90), HeC(2.0:220), and HeC(3.6:70) and that for triethylene glycol, TEG, per molecule. (b) T for thefor HeC HeC(1.3:90), samples HeC(2.0:220), is essentially and controlle HeC(3.6:70)d by the and substitution that for triethylene groups glycol, like TEG. TEG, per molecule. (b)dependenceT dependence of theof hydration the hydration number, number, NH, perNH hydrophilic, per hydrophilic group groupfor the for HeC the samples HeC samples and TEG. and TEG.

molecules made from Schemethree hydroxyethyl 2. Chemical structure groups, ofTEG triethylene is a reasonable glycol, TEG. model to compare the hydration behavior of theScheme substituti 2. Chemicalon groups structure and that of oftriethylene HeC. The glycol, T dependence TEG. of nH per molecule of TEGConsequently, is also shown the decreasingin Figure 8a. coe Theffi cientdehydration of nH for behavior HeC with of all increasing the HeC Tsamplesis substantially and TEG weakerwith H thanincreasingConsequently, that for T otheris gentle, the cellulose asdecreasing seen ethers in this coefficient examinedfigure. The of previouslyreason n for whyHeC the [with24 ],T dependencebecauseincreasing the T of substitutionis n substantiallyH for HeC samples groups weaker of HeCthanbecomes dothat not for greatershow other significantcellulosewith an ethersincreasing but gentle examined MS dehydration value, previo asusly behaviormentioned [24], because with above, increasing the wouldsubstitution Tbeand the groups determine hydration of HeC the hydrationdocharacteristics not show/dehydration significant of their behaviorsubstitution but gentle of the groude HeChydrationps becoming molecules. behavior remarkable That with is why withincreasing the increasing HeC T samplesand MS determine. Figure in aqueous 8b the solutionhydration/dehydrationshows possessthe T dependence high water behavior ofsolubility hydration of theover number, HeC a wide molecu NH,T perrangeles. hydrophilic That and is never why group demonstrate the for HeC the samplesHeC a cloud samples in point aqueous and and gelationsolutionTEG. The behavior,possess dehydration high even water incoefficients a highsolubilityT range of NoverH asfor a the widethe MC, four T HpMC,range samples and and with never HeMC increasing demonstrate samples T are in a aqueousvery cloud similar point solution. to and gelationeach other. behavior, This evenconsideration in a high reveals T range that as the (∂N MC,H/∂MS HpMC,)c ~ 0 andirrespective HeMC samplesof T, although in aqueous (∂nH/ solution.∂MS)c 3.depends Materials on and T, as Methods discussed above. Then, we might conclude that the hydration/dehydration behavior 3. forMaterials the HeC and samples Methods is essentially controlled by the substitution groups like TEG. 3.1. Materials 3.1. MaterialsHydroxyethyl cellulose, HeC, samples used in this study were kindly supplied by Daicel CorporationHydroxyethyl (Osaka, cellulose, Japan), and HeC, Dow samples Chemical used Japan in this Limited study (Tokyo, were Japan).kindly Tablesupplied1 summarizes by Daicel theCorporation characteristics (Osaka, of Japan), HeC samples and Dow investigated Chemical Ja inpan this Limited study. (Tokyo, The molecular Japa n). Table weight 1 distributionsummarizes M M indexesthe characteristics given by a ratioof HeC ofScheme samplesw to 2. the Chemical investigated number-average structure in of this triethylene molecular study. glycol, weightThe molecularTEG. ( n) were weight obtained distribution by using size-exclusionindexes given by chromatographic a ratio of Mw to methods the number-average and in a range molecular from 2to weight 3. All ( HeCMn) were samples obtained were by purified using bysize-exclusion dialysisConsequently, and chromatographic freeze-dried. the decreasing Triethylene methods coefficient glycol,and of in n TEGaH forrange HeC (>98%), from with was2 increasing to 3. purchased All HeC T is substantiallysamples from Tokyo were weaker Chemical purified Industrybythan dialysis that Co., forand Ltd. other freeze-dried. (Tokyo, cellulose Japan). ethers Triethylene TEG examined was glycol used previo, asTEGusly a model (>98%), [24], because compound was purchased the substitution of a trimer from of groupsTokyo ethylene ofChemical HeC oxide, i.e., do a possiblenot show side significant chain of but HeC gentle molecules dehydration made from behavior hydroxyethyl with increasing groups. T and determine the hydration/dehydration behavior of the HeC molecules. That is why the HeC samples in aqueous solution possessTable high 1. Characteristics water solubility of hydroxyethyl over a wide cellulose, T range and HeC, never samples demonstrate used in this a study.cloud point and

gelation behavior, even in a high T range as the MC, HpMC, and HeMC samples in aqueous solution.3 Sample Codes Molar Substitution Number by Hydroxyethyl Groups Mw/10 3. MaterialsHeC(1.3:90) and Methods 1.3 90 HeC(2.0:220) 2.0 220 3.1. MaterialsHeC(3.6:70) 3.6 70 Hydroxyethyl cellulose, HeC, samples used in this study were kindly supplied by Daicel Corporation (Osaka, Japan), and Dow Chemical Japan Limited (Tokyo, Japan). Table 1 summarizes the characteristics of HeC samples investigated in this study. The molecular weight distribution indexes given by a ratio of Mw to the number-average molecular weight (Mn) were obtained by using size-exclusion chromatographic methods and in a range from 2 to 3. All HeC samples were purified by dialysis and freeze-dried. Triethylene glycol, TEG (>98%), was purchased from Tokyo Chemical

Molecules 2020, 25, 4726 11 of 14

Highly deionized water with speciﬁc resistance higher than 18 MΩ cm, generated by a Direct-Q · 3UV system (Merck Millipore, Darmstadt, Germany), was used as a solvent. The concentrations of cellulose ethers ranged from 0.08 to 0.23 M in glucopyranose units.

3.2. Dielectric Spectroscopic Measurements Two systems were used to measure the dielectric relaxation behavior for aqueous solutions of the HeC samples over a frequency range from 1 M to 50 GHz. A dielectric probe kit, 8507E, equipped with a network analyzer, N5230C, an ECal module N4693A, and a performance probe 05 (Agilent Technologies, Santa Clara, CA, USA), was used for dielectric spectroscopic measurements over a frequency range from 50 M to 50 GHz (3.14 108–3.14 1011 s 1 in angular frequency, ω). A three-point calibration × × − procedure using n-hexane, 3-pentanone, and water as the standard materials was conducted prior to all the DS measurements at each measuring temperature. The details of the three-point calibration procedure used in this study have been described elsewhere [25]. In these systems, the real and imaginary parts (ε0 and ε”) of the electric permittivity were automatically calculated from the reﬂection coeﬃcients measured by the network analyzer via a program supplied by Agilent Technologies. In the lower frequency range from 1 M to 3 GHz (6.28 106–1.88 1010 s 1 in ω), an RF LCR × × − meter 4287A (Agilent) equipped with a homemade electrode cell with a vacant electric capacity of C0 = 0.23 pF was used. Open (air), short (0 Ω, a gold plate with the thickness of 0.5 mm), and load (water) calibration procedures were performed prior to sample measurements at each measuring temperature. Dielectric measurements were performed at temperatures ranging from T = 10 to 70 ◦C with an accuracy of 0.1 C using a temperature controlling unit made of a Peltier device. The ε and ± ◦ 0 ε” values were calculated using the relationship ε = CC 1 and ε” = (G G )(C ω) 1, where C, G, 0 0− − DC 0 − and GDC represent measured electric capacitance, conductance, and direct current conductance of the sample liquid, respectively. The contribution of GDC to the ε” value was adequately removed in a lower ω range.

3.3. Density Measurements Density measurements for the sample solutions were conducted using a digital density meter, DMA4500 (Anton Paar, Graz, Austria), to determine the partial molar volumes of the HeC and TEG samples at the same temperature as the performed DS measurements. The uncertainties of measured density and temperature were less than 5.0 10 5 g cm 3 and 0.03 C, respectively. × − − ◦ 4. Conclusions The temperature dependence of hydration number for hydroxyethyl cellulose, HeC, over a temperature range from 10 to 70 ◦C was determined exactly by using extremely high-frequency dielectric spectroscopic techniques. All the HeC samples examined in this study, which possess diﬀerent molar substitution numbers, MS, per glucopyranose unit by hydroxyethyl groups, 1.3, 2.0, and 3.6, well dissolve in water over the temperature range examined. The HeC molecules showed much gentler dehydration behavior with increasing temperature than that of methyl and hydroxypropylmethyl cellulose, MC and HpMC, molecules which demonstrate cloud points clearly at high temperatures. The HeC samples kept a higher hydration number, NH, per hydrophilic group, hydroxy and ether groups, than the critical NH value of ca. 1, even in a high temperature range. The high NH values of HeC molecules over a wide temperature range are the reason for their high water-solubility in the temperature range. The three HeC samples and a model compound, triethylene glycol, for their substitution groups, demonstrated a very similar decreasing coeﬃcient of NH,(∂NH/∂T)c, with increasing T. The hydration/dehydration behavior of the HeC molecules was essentially controlled by the substitution groups of HeC. Molecules 2020, 25, 4726 12 of 14

Author Contributions: Conceptualization, K.A. and T.S.; methodology, K.A.; software, K.A.; validation, T.S.; formal analysis, T.S.; investigation, K.A.; resources, T.S.; data curation, K.A.; writing—original draft preparation, K.A.; writing—review and editing, T.S.; visualization, T.S.; supervision, T.S.; project administration, T.S.; funding acquisition, T.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: All the hydroxyethyl cellulose ether samples were kindly supplied by Daicel Corporation (Osaka, Japan), and Dow Chemical Japan Limited (Tokyo, Japan). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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