Study of the temperature effect on the acid-base properties of cellulose acrylate by inverse gas chromatography at infinite dilution

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Study of the temperature effect on the acid-base properties of cellulose acrylate by inverse gas chromatography at infinite dilution Tayssir Hamieh a,b,∗ a Laboratory of Materials, Catalysis, Environment and Analytical Methods (MCEMA) and LEADDER Laboratory, Faculty of Sciences and EDST, Lebanese University, Hariri Campus, Hadath, Beirut, Lebanon b Technologies pour une Electro-Mobilité Avancée (TEMA), Institut franc¸ais des sciences et technologies des transports, de l’aménagement et des réseaux (IFSTTAR), 25 Allée des Marronniers, 78000, Versailles, France

a r t i c l e

i n f o

Article history: Received 19 May 2018 Received in revised form 2 July 2018 Accepted 4 July 2018 Available online xxx Keywords: Adsorption Specific enthalpy Lewis acid and base constants Amphoteric constant Surface energy Hamieh’s model

a b s t r a c t Inverse gas chromatography (IGC) at infinite dilution was used to characterize the surface and interfacial properties of polymers, oxides or polymers adsorbed on oxides. In this paper, the dispersive component of the surface energy of CA was calculated following the molecular models of the surface areas of n-alkanes proving the presence of two linear zones with two different slopes in the temperature intervals and indicating a change in the structure of CA groups. The acid-base properties in the Lewis terms of cellulose acrylate were determined. One proved that the specific enthalpy and entropy of interaction of polar probes are functions of the temperature The application of Hamieh’s model allows to the determination of the acid-base constants KA and KD and the amphoteric constant K of cellulose acrylate surface. It was proved that the constants KA , KD and K of cellulose acrylate strongly depend on the temperature. This study allowed us to determine the probability w of the specific adsorption of polar probes on the cellulose acrylate surface. This probability parameter also depends on the temperature. © 2018 Elsevier B.V. All rights reserved.

1. Introduction It is well known that cellulose is the most abundant naturally polymer on earth. Cellulose is used as natural resource, to produce polymer hydrogels due to its excellent biodegradability and biocompatibility [1,2]. Chiappone et al. [3] and Nair et al. [4] studied the physicochemical, mechanical and ionic properties of cellulose/acrylate, its derivatives and its adhesion on polymers. Bajpai et al. [5] prepared and studied an antibiotic drug Minocycline (Mic) loaded cellulose nano-whiskers (CNWs) / poly(sodiumacrylate) hydrogel films for their drug releasing capacity in physiological buffer solution (PBS) at 37 ◦ C. It was proved some fair anti-fungal and antibacterial properties [6]. Acrylates were used for polymerization on cotton and cellulose [7,8] and improved the properties of these natural products [9]. Other functionalized palm celluloses via graft-copolymerization were studied in order to obtain interesting physico-chemical properties and to improve mechanical properties compared [10–13].

∗ Corresponding author at: Laboratory of Materials, Catalysis, Environment and Analytical Methods (MCEMA) and LEADDER Laboratory, Faculty of Sciences and EDST, Lebanese University, Hariri Campus, Hadath, Beirut, Lebanon. E-mail addresses: [email protected], [email protected]

Cellulose is a nontoxic, renewable resource and very abundant. By chemical modification of cellulose, new characteristics can be obtained, e.g., improved solubility in various solvents [14]. The modified cellulose will lead to many industrial applications, more particularly in paint, food, and pharmaceutical industry [15]. Keshawy et al. [14] prepared natural modified biodegradable oil sorbents containing segments that act as targeted sites for biodegradation. By using cellulose derivatives, they prepared a crosslinked copolymer of hyddroxypropyl cellulose acrylate/octadecyl acrylate as the oil sorbent. However, because of the non-solubility of cellulose and some difficulties related to the melting processes, only several cellulose derivatives with less hydrogen bonding can be processed. Most of these commercially interesting derivatives are chemically modified from the native cellulose in heterogeneous reaction mixtures [16]. One of these cellulose derivatives is the cellulose acrylate that was synthetized and used for its more interesting properties. Many authors in literature used the inverse gas chromatography (IGC) technique to determine the surface physicochemical properties of polymers [17–29] and cellulose and its derivatives [30–34]. The surface properties, the acid-base properties in Lewis terms and the second order transition temperatures of some important polymers were studied in literature [17,18]. Zhao et al. [8] studied the solubility parameter of cellulose acrylate (CA) by using inverse

https://doi.org/10.1016/j.chroma.2018.07.025 0021-9673/© 2018 Elsevier B.V. All rights reserved.

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gas chromatography at infinite dilution, in order to find out some solvents or mixed solvents, which solubility parameter was close to that of the synthesized CA in order to dissolve it into homogeneous solution. Hamieh et al. [22–25] determined the surface properties of poly(␣-n-alkyl) methacrylates, and proved an important effect of the length of lateral chain on the thermal properties of such polymers in their bulk phase or when adsorbed on silica or alumina, by using IGC technique at infinite dilution. In this paper, we proposed to determine the acid-base properties of Cellulose acrylate by using inverse gas chromatography (IGC) technique at infinite dilution and Papirer’s approach [35–38] and Hamieh’s model [39–42]. We used different n-alkanes molecules and polar organic molecules. The polar molecules were used to determine the specific interactions between CA and these probes. The retention time obtained by this technique was proved to be a primordial experimental parameter to characterize the surface properties of the cellulose derivative CA. An important effect of the temperature on the acid-base constants of CA were highlighted and proved in this study. In the next section, we will summarize the theory and models of the inverse gas chromatography at infinite dilution (IGC-ID). We applied it in the experimental part to determine the specific interactions and acid-base constants of CA.

2.2. Specific interactions The free energy of adsorption G◦ of n-alkanes on the solid substrates is given by the following fundamental equation of IGC technique: G◦ = RTlnVn + C

(4)

where R is the ideal gas constant, T the absolute temperature and C a constant depending on the reference state of adsorption. The free energy of adsorption G◦ contains the two contributions relative to the dispersive and specific interactions. In the case of n-alkanes, G◦ is equal to the free energy of adsorption corresponding to the dispersive interactions Gd only. To calculate the specific interactions between the solid substrates and polar probes, several methods were used in the literature [35,36,39–42]. Two approaches are presented in the next sections. 2.3. Fowkes approach Specific interactions can be determined by applying the wellknown relationship of Fowkes which gives at the same time the dispersive component of the surface energy of solids  s d by using the geometric mean of the dispersive components (exponent d) of the surface energy of the probe  l d and the solid  s d :

2. Theory and methods

G◦ = Gd = NaWa = 2Na( l d ␥s d )1/2

Inverse gas chromatography was revealed to be an excellent surface technique, used for thirty years, to determine surface phenomena, glass transitions and acid-base properties of solid materials [17–20]. The IGC technique was advantageously applied to determine the change, as a function of temperature, of the surface properties of solid materials or nanomaterials, polymers, oxides or polymers adsorbed on oxides. Model organic molecules of known properties are injected in the column containing the solid. The retention times of these molecules, measured at infinite dilution, allow us to determine the interactions between the model polar or non-polar molecules and the solid substrates, by supposing that there is no lateral interaction between the probe molecules themselves.

Where Wa is the energy of adhesion, N is Avogadro’s number and a the surface area of one adsorbed molecule of the probe. For polar molecules, the specific interactions are added to the dispersive interactions:

The net retention volume Vn was calculated from: (1)

where tR is the retention time of the probe, t0 the zero retention reference time measured with a non-adsorbing probe such as methane, Dc the flow rate and j a correction factor taking into account the compression of the gas [43]. Dc and j are respectively given by the following expressions: Dc = Dm

Tc  (Tc ) Ta  (Ta )

(2)

and

 j=

3 2



P+ P0 P0 P+ P0 P0

2 3

G◦ = 2Na( l d  s d )1/2 + Gsp

(6) d )1/2

of n-alkanes, it is By plotting RTlnVn as a function of 2Na( l possible to deduce, from the slope of the straight line, the value of dispersive component  s d of the surface energy of the solid. If  l d ,  s d and a the cross section of an adsorbed molecule, are known, it is possible to calculate the contribution to the free energy of adsorption of the Lewis acid–base surface interactions Gs p by using Eq. (6) [44]. 2.4. Critique to the method based on Fowkes equation

2.1. Retention volume

Vn = jDc (tR − t0 )

(5)

−1 (3) −1

where Dm is the measured flow rate, Tc the column temperature, Ta the room temperature, ␩(T) the gas viscosity at temperature T, P0 the atmospheric pressure and P the pressure variation.

However, the true values of the surface area of organic molecules adsorbed on a solid substrate are not known with a good accuracy, especially because of the change of molecule positions when approaching a solid surface at certain temperature. The problem will be more complicated when the temperature increases; in such case, we cannot neglect the effect of the thermal agitation on the surface areas of adsorbed molecules. In a previous study, Hamieh et al. [45] proved the effect of the temperature on the surface area of n-alkanes and polar molecules. In this study, we proposed various theoretical models giving the molecular areas of n-alkanes: geometrical model, cylindrical molecular model, liquid density model, BET method, Kiselev results and the two-dimensional Van der Waals model by using its constant b that depends on the critical temperature and pressure of the liquid. The Redlich-Kwong equation transposed from three-dimensional space to two-dimensional space [39,41] was also used to calculate the areas of organic molecules. Table 1 summarizes the different surface area values for the used n-alkanes using the various molecular models. All above proposed theoretical models were experimentally tested by using inverse gas chromatography at infinite dilution coupled to the dynamic contact angle technique. Hamieh et al. [45] showed the areas a (T) of polar molecules adsorbed on Polyte-

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T. Hamieh / J. Chromatogr. A xxx (2018) xxx–xxx Table 1 Surface areas of various molecules (in Å2) obtained from the various models of Van der Waals (VDW), Redlich-Kwong (R-K), Kiselev, geometric, cylindrical or spherical models. Molecule

VDW

Kiselev

Cylindrical

R-K

Spherical

Geometric

C6 H14 C7 H16 C8 H18 C9 H20

52.7 59.2 64.9 69.6

51.5 57 63 69

45.5 51.8 58.1 64.4

41.3 46.4 50.8 54.5

39.6 42.7 45.7 48.7

40.7 48.5 56.2 64.0

Table 2 Equations of dispersive component of the surface energy ld (mJ/m2 ) of n-alkanes as a function of the temperature T (K). n-alkanes

ld (mJ/m2 )

C6 H14 C7 H16 C8 H18 C9 H20

ld ld ld ld

(C6) =−0.102 T + 48.34 (C7) =−0.098 T + 48.85 (C8) =−0.095 T + 49.61 (C9) =−0.093 T + 50.15

trafluoroethylene (PTFE), linearly depend of the temperature. The following relation was proved: a(T ) = a0 – ˝T

(7)

with  the slope of the straight line depending on the nature of the adsorbed molecule and solid substrate, a (T) the surface area at temperature T and a0 the molecule area extrapolated at 0 K. By applying the different theoretical models, we obtained different surface areas of the adsorbed molecules. Consequently, different values of ␥d s were obtained for the same solid at a fixed temperature depending on the model used. Therefore, it was impossible to deduce a precise value of the specific interaction for one polar molecule by using this method, because the surface areas of adsorbed molecules cannot be accurately determined. The limitations of this method are due, in part, to the fact that the molecular area a is not exactly known and varies both with the nature of the solid, and the temperature and surface coverage. Furthermore, some  l d values are not always available from the literature at any temperature used for the IGC measurements. These reasons led us to consider that the method developed by Brendlé and Papier [37,38] can be used to quantify more precisely the specific interactions, method considered up to now the most popular one. Before developing the Papirer’s method, we give below the results concerning the calculations of the dispersive component of the surface energy sd (mJ/m2 ) of CA as a function of the temperature T (K) by using the different molecular models of n-alkanes surface areas. 2.5. Determination of the dispersive component of the surface energy of CA In order to calculate the dispersive component of the surface energy of CA, we appealed to our previous results giving the various surface areas of n-alkanes using different models [39,41,45]. On the other hand, the equations of the dispersive component of the surface energy ld (T ) of n-alkanes as a function of the temperature are given in Table 2, with a linear regression coefficient equal to 1.000. Using the different models of n-alkane surface areas given in Table 1 will allow to obtain the dispersive component sd of the surface energy of CA for every chosen model. The obtained results will be more qualitative than quantitative, more particularly by obtaining some information on the surface properties of the polymer at different temperatures.

3

 By plotting RTlnVn as a function of 2 Na

ld of n-alkanes at

various temperatures, we obtained the values of dispersive component sd of the surface energy of CA for the chosen molecular model. The obtained results are given in Fig. 1 for the different surface area models. All plotted curves of the Fig. 1 show for every model of surface area two linear zones with two different slopes in the temperature intervals. The first zone concerns the temperature interval [315 K, 335 K] with a strong decrease of sd of CA versus the temperature with a d negative slope d s /dT ≈ −1 mJ m−2 K−1 whereas the second zone for the interval [335 K, 350 K] is characterized with a slow decrease d of sd (T ) with a slope d s /dT comprised between −0.113 mJ −2 −1 −2 m K and −0.287 mJ m K−1 depending on the used molecular model (Table 3). The important change in the slope of sd (T ) between the two temperatures zones can be attributed to a surface modification or to a relaxation of some surface groups of CA. This type of slope change was previously observed in the cellulose fiber [46] and in cellulose acetate phthalate-polycaprolactonediol blend [32] 2.6. Papirer’s method This method allowing to obtain specific enthalpy of interaction between a probe and a solid is that developed by Papirer team [35,36] who obtained a straight line when plotting RTlnVn against the logarithm of the vapor pressure lnP0 where P0 is the vapor pressure of the probes. For a homologous series of n-alkanes, whatever the nature of the solid substrates: RTlnVn = AlnP 0 + B

(8)

Where A and B are constants depending of the nature of the solid substrate. When polar molecules are injected into the column, specific interactions are established between these probes and the solid surface and G◦ is now given by: G◦ = Gd + Gsp .

(9)

Gsp

Where refers to specific interactions of a polar molecule adsorbed on solid substrate. The choice of Papirer method was made considering that the logarithm of the vapor pressure which is closely related to the evaporation enthalpies (Hv) was representative of the capacity of interactions of two identical molecules. This method presents several advantages since P0 values are given in the literature or are computable even at relatively high measurement temperatures as long as these stay below the critical temperature of the probe. Yet, problems do appear with certain solids, possessing a high surface energy, since the representative points of some polar probes fall beneath the alkane-line. We advantageously used Papirer method to quantify the specific free energy of adsorption of polar molecules and obtain the acidbase constants of cellulose acrylate. 2.7. Determination of acid-base constants of solid substrates By plotting Gsp of the polar molecules as a function of the temperature, the specific enthalpy (Hsp ) will be obtained from: Gsp = Hsp − TSsp

(10) Hsp

Ssp

In general, this relationship (10) is linear if and don’t depend on the temperature. However, when the linear correlation coefficient is not very close to 1, then the linearity is not verified; therefore, Hsp (T) and Ssp (T) strongly depend on the temperature. The curves representing the variations of Gsp (T) versus

Please cite this article in press as: T. Hamieh, Study of the temperature effect on the acid-base properties of cellulose acrylate by inverse gas chromatography at infinite dilution, J. Chromatogr. A (2018), https://doi.org/10.1016/j.chroma.2018.07.025

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Fig. 1. Variations of the dispersive component of the surface energy sd (mJ/m2 ) of CA as a function of the temperature T (K) for two temperature intervals and using the different molecular models of n-alkanes surface areas. Table 3 Equations of dispersive component of the surface energy sd (mJ/m2 ) of CA as a function of the temperature T (K) for two temperature intervals. Temperature interval T: 315 K–335 K

 d

Model of surface area

Equation of s

VDW Kiselev Cylindrical R-K Spherical geometric

sd = −0.891 T + 318.7 sd = −0.950 T + 339.7 sd = −0.963 T + 345.6 sd = −1.006 T + 350.5 sd = −0.975 T + 336.6 sd = −1.006 T + 366.0

 2

mJ/m

Temperature interval T: 335 K–350 K R2 0.9989 0.9987 0.9992 0.9996 0.9997 0.9999

the temperature will allow the thermodynamic calculation of specific enthalpy and entropy as a function of the temperature. Polar molecules used to determine the specific interactions with the solid substrates are characterized by their donor (DN) and acceptor (AN) numbers of electrons. The concept of donor-acceptor interactions is an extension of the Lewis acid-base reactions, dealing with coordinate bonds which are formed by sharing a pair of electrons between donor and acceptor species. Knowing Hsp of the various polar molecules, the acidic constant KA and basic constant KD , the two constants characterizing the solid substrate, are determined by using the following classical relationship: sp

(−H ) = (KA .DN + K D .AN)

(11)

or (−H sp /AN) = KA (DN/AN) + KD

(12)

(−Hsp /AN)

as a function of (DN/AN), according to Eq. and plotting (12). In previous papers, Hamieh et al. [39,40,47,48], corrected the relation (11) for some solid materials as polymers or metallic oxides and proposed a new relationship by adding a third parameter K reflecting the amphoteric character of the oxide according to: (−H sp ) = KA .DN + KD .AN − K.ANDN

(13)

Another relationship was also proposed [47,48]: (−H sp ) = w(KA .DN + KD .AN)

(14)

Model of surface area

Equation of sd

VDW Kiselev Cylindrical R-K Spherical geometric

sd sd sd sd sd sd



mJ/m2

= −0.265 T + 109.3 = −0.274 T + 113.5 = −0.287 T + 119.3 = −0.184 T + 75.5 = −0.113 T + 48.244 = −0.173 T + 86.8



R2 0.9989 0.9993 0.9994 0.9992 0.9996 0.9998

Where w is the weighing factor of the exchanging interactions between adsorbed molecule and solid substrate. In certain cases, results obtained by Eq. (11) and Eqs. (13) and (14) can be the same. However, in many cases, the results giving KA and KD using the two approaches are different and the conclusions are not the same.

3. Experimental part 3.1. Synthesis condition We used the inverse gas chromatography technique at infinite dilution for the physicochemical characterization of cellulose acrylate (CA). Measurements were carried out with a DELSI GC 121 FB Chromatograph equipped with a flame ionization detector of high sensitivity. The cellulose acrylate studied in this paper was synthesized following the same synthesis method of Marsano et al. [49] and Beatriz et al. [50], and used by Zhao et al. [8] CA was obtained by diluting of a convenient quantity of acryloyl chloride in a mother solution of cellulose in dimethylacetamide (∼10%) LiCl at a polymer concentration Cp = 2%(w/w). The temperature of the experiment was 50 ◦ C for 2.5 h of the reaction. The methanol was used for precipitation of the product that was washed with distilled water for several times and dried in vacuum oven at 60 ◦ C over a night. The obtained cellulose acrylate had a degree of substitution (DS) equal to 1.74.

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Table 4 Normalized donor and acceptor numbers of some polar molecules. Polar probes

DN’

AN’

DN’/AN’

Character

Chloroform Acetone Ether THF Toluene Isopropanol

0 42.5 48 50 9.75 51.25

18.7 8.7 4.9 1.9 3.3 14.8

0 4.89 9.80 26.32 2.95 3.46

higher acidity Amphoteric More basic Stronger basicity Amphoteric Amphoteric with higher basicity and acidity

3.2. Solvents Classical organic probes, characterized by their donor and acceptor numbers, were used in this study. Corrected acceptor number AN = AN-ANd , given by Riddle and Fowkes [51], who subtracted the contribution of Van der Waals interactions (or dispersion forces), was normalized by Hamieh et al. [47,48], and proposed a dimensionless donor number DN according to the following relationship: DN  = 2.5(mol/kcal)DN(kcal/mol)

(15) AN

However, if one wants to use DN in kcal/mol, can be easily transformed to the kcal/mol unit using the following relationship:





AN (kcal/mol) =

40 kcal/mol 100





AN (unitless)

(16)

Table 4 gives donor and acceptor numbers [47,48] of probe solvents used in this study. 3.3. GC conditions The CA powder was used for particles with diameters ranging from 100 to 250 ␮m. Particles of the correct size were introduced into a stainless steel column, which was 30 cm long and had an internal diameter of 2 mm. A mass of 1 g of CA was used to fill the chromatographic column. The column filled with the sample was conditioned at 120 ◦ C for 12 h to remove any impurities. The measurements of retention time were done by using the IGC technique. The column was then attached to the gas chromatograph, fitted with a flame ionization detector. Helium was used as carrier gas; its flow-rate was equal to about 20 ml min−1 . Before measurements, the CA particles were conditioned in the column under a He flow over a night at 130 ◦ C in order to eliminate physically adsorbed impurities. IGC measurements at infinite dilution were done by varying the temperature from 42 ◦ C to 77 ◦ C. The IGC system has been used to make infinite dilution (ID) pulse experiments, probes were injected manually with a 1 ␮L Hamilton syringe. The injection volume for each probe was 0.1 ␮L, in order to approach linear condition gas chromatography. In such a way that the interactions between probe molecules can be considered to be negligible and only the interactions between the surface of the solid and an isolated probe molecule are important. At least three injections were made for each probe and the average retention time, tR , was used for the calculations. The standard deviation was less than 1% in all measurements. 3.4. Results and discussion The results obtained in this part with cellulose acrylate are presented in the next paragraphs. On Fig. 2(a–c), we give the evolution of specific thermodynamic variable changes. Fig. 1(a) summarizes the variations of specific free energy change (−Gsp ) for the different polar molecules adsorbed on the cellulose acrylate as a function of the temperature T. It was observed that the values of specific interactions with the more basic probe (THF) and the more acidic molecule (Chloroform) are highly larger than the other polar

Fig. 2. Evolution of specific free energy (a), specific enthalpy (b) and specific entropy (c) of cellulose acrylate interacting with polar probes as a function of the temperature.

molecules. The values of (−Gsp (T)) of THF and chloroform are comprised between 10 and 15 kJ/mol with larger specific interaction for THF (the more basic molecule between the six polar molecules of this study). This reveals for the CA surface an important Lewis acid-base character with an accentuated acid specific interaction greater the basic one. The different curves of Fig. 2(a) also show an important amphoteric character (see for instance the curves of amphoteric molecules as acetone and ether. The specific interactions are smaller in the case of molecules with smaller amphoteric character as toluene for example. On the other hand, the calculation of the different linear regression coefficients, r2 , for specific free energy change of all polar molecules used in this paper gave values of r2 comprised between

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Table 5 Values of the interaction parameter ␹, at different temperatures [8]. T (K)

Chloroform

Acetone

Ether

THF

Toluene

Isopropanol

313 318 323 328 333 338

0.46 0.36 0.3 0.27 0.2 0.15

2.66 2.69 2.85 2.87 2.96 3.03

1.79 1.95 1.87 2.05 2.09 2.15

0.08 0.01 −0.02 −0.1 −0.15 −0.21

4.96 4.96 4.89 4.9 4.87 4.84

6.48 6.24 6.2 5.97 5.85 5.71

0.3930 and 0.8767 (Fig. 2(a)). Therefore, the linearity of (−Gsp (T)) is not perfectly verified. The values of specific entropy –Ssp and enthalpy −Hsp of adsorption of the various polar probes, obtained by supposing the linearity of −Gsp (T) cannot be considered as correct. They are not constant and extremely depend on the temperature. We then calculated these specific thermodynamic variables by using the following thermodynamic equations: −H

sp

∂ = T ∂T 2



G sp T

 S

sp

= −

∂ G sp ∂T





or H

sp

=

∂

 Gsp   T1 

∂

(17)

T

(18)

Knowing the variation of specific free enthalpy G sp (T) as a function of the temperature, we were able to calculate specific enthalpy G sp (T) and the specific entropy G sp (T) for every molecule by using Eqs. (17) and (18). Therefore, it is so clear that the slope of every point of the curve sp giving ( GT ) as a function of ( T1 ) will give H sp (T ) and the slope on every point of the curve G sp (T) as a function of T will determine −S sp (T ), and showed an important effect of the thermal agitation on the specific enthalpy and entropy of adsorption of polar molecules on the CA. On Fig. 2(b) and (c), we respectively represented the variations of specific enthalpy and entropy as a function of the temperature. The obtained results obviously showed an important dependency of these surface specific variables on the temperature. The curves of Fig. 2(b) show two types of polar molecules: the first type concerns the chloroform (the more acidic molecule), and acetone and ether (two amphoteric molecules). These three molecules highlight a slight increase of interaction with the cellulose acrylate in the temperature interval [295 K, 325 K] and then a very large increase of acid-base interaction when the temperature T increases from 325 K to 350 K. These results prove that the specific enthalpy of adsorption of CA with the first type of polar probes increases with the temperature from 8 to 18 kJ/mol to 35–55 kJ/mol. Conversely, for the second type of polar probes including THF (the more basic molecules), isopropanol and toluene (two other amphoteric molecules), the inverse is happened. The interaction highly decreases from 295 K until 325 K and then slowly decreases until 350 K. We deduced from these results that the amphoteric molecules exhibit different behaviors in a contradictory way. To understand the difference in the behavior of amphoteric molecules, we have to refer to the interaction parameter ␹ of molecules at different temperatures. Zhao et al. [8] determined the Flory–Huggins interaction parameters of the cellulose acrylate with the above molecules by inverse gas chromatography (Table 5). We can find that the interaction parameter of acetone and ether increases as a function of the temperature. Therefore, this confirms for these molecules the reason of the increase of the specific enthalpy of adsorption with the temperature. Now for the three other molecules (THF, toluene and isopropanol), the decrease of their interaction parameter (Table 5) also confirms the decrease

Fig. 3. Evolution of −Hsp /AN’ (kJ/mol) of cellulose acrylate versus of DN’/AN’ of polar molecules at different temperatures.

in their specific enthalpy with the temperature. On the other hand, the increase of specific enthalpy of adsorption of chloroform clearly shows the basic surface of the cellulose acrylate increases when the temperature increases. In fact, the Fig. 2(a) and (b) clearly prove that cellulose acrylate presents an amphoteric surface due to the presence of acrylate, methyl, methylene and OR groups in the elementary motif of CA macromolecule. The surface acid-base CA properties are very affected as shown in all Fig. 1 by the effect of the thermal energy on the surface specific variables, because of the variation of Flory–Huggins interaction parameter of the cellulose acrylate as a function of the temperature. For all curves giving the representation of the specific enthalpy of adsorption of polar molecules (Fig. 2b), we observed that the values of (−Hsp (T)), are in general positive for all temperatures, therefore, Hsp (T) is negative and then this proves that the adsorption process is an exothermic adsorption. The same behavior was found for the specific entropy change (Fig. 2(c)) that highly depends on the temperature for all adsorbed polar molecules. The negative values of Ssp (T) reflect the specific adsorption and more ordered system. In order to deduce the acid-base constants KA and KD of the cellulose acrylate, we used the experimental results to draw the evolution of −Hsp/AN’ as a function of DN’/AN’ for the various temperatures from 295 K to 350 K. The results are plotted on Fig. 3. The curves of Fig. 3 clearly show the non-linearity of the variations of −Hsp/AN’ for all temperatures chosen in this study. Consequently, the classical Eq. (11) cannot be used to deduce the values of KA and KD of CA. For all temperatures, there is no straight line and other models have to be used. 3.5. Hamieh’s model and discussion In this section, the used method for the calculation of acid-base constants of CA is that of Hamieh’s model [39,40,47,48] that proposed a new relationship by adding a third parameter K reflecting the amphoteric character of solid surfaces. Eq. (13) was used to calculate the three acid-base constants KA , KD and K of the cellulose acrylate; whereas the Eq. (14) was applied to deduce the probability of the specific adsorption w of molecules on CA surface. The obtained results are presented on Fig. 4 showing the variations of acid-base constants KD (a), KA , K (b) and of the ratio KA /KD (c) of cellulose acrylate as a function of the temperature. Fig. 4(a) shows that the cellulose acrylate has an important basic character. In fact the base constant KD is equal to 6 kJ/mol at 295 K and then decreases until 2 kJ/mol when the temperature increases. However,

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Fig. 4. Evolution of acid-base constants KD (a), KA and K (b), the ratio KA/KD (c) and the probability of adsorption w (d) of cellulose acrylate versus the temperature T(K).

the acid character of CA is less accentuated than its base character, the acid constant KA decreases from 1.15 kJ/mol to 0.05 kJ/mol (Fig. 3(b)), whereas the amphoteric constant K of CA has relatively slow decrease as a function of the temperature. In order to appreciate the total acid-base evolution of the cellulose acrylate, we draw on Fig. 4(c) the curve of the ratio KD /KA versus the temperature. The curve clearly shows a stable variation of this ratio until 320 K and then a strong relative increase of KD /KA after this temperature even if we notice a general decrease of acid and base constants of CA as a function of the temperature. These results show a strong effect of the temperature on the acid-base surface properties of the cellulose acrylate with an accentuated basic character of this polymer and more particularly a decrease of acid-base constants when the temperature increases probably due to the variation of the interaction parameter of polar probes as a function of the temperature. This will conduct us to determine the probability w of specific adsorption of polar molecules on CA surface. The curves of the probability of adsorption of polar probes on the cellulose acrylate surface drawn on Fig. 4(d) as a function of the temperature also prove an important effect of the thermal agitation on the specific adsorption of polar probes on CA surface. For all used molecules, the probability of adsorption increases with the temperature to be 1 at 350 K, excepted for the toluene where its probability is equal to 1 until a temperature of 340 K and then decreases to 350 K; and for the isopropanol passing through a maximum of probability equal to 0.45 at 320 K and then decreases to 0.15 for higher temperature. We have to indicate the presence of a singular point at 320 K in the curve of the evolution of the ratio of KD /KA (Fig. 4c) as a function of T that shows that from this temperature, there is an increase of this ratio. Why this brutal variation of the acid base

character form this temperature? This change is also observed in Fig. 4(d) representing the variation of the probability of specific adsorption where a maximum is obtained for various polar probes. This observation isn’t due to any transition phenomena. Because if a such transition exists, it has to be observed when we draw the curves relative to the evolution of RTlnVn as a function of 1/T for n-alkane probes adsorbed on CA surface. However, when we did it, we did not observe any transition phenomena. The presence of this singular temperature can be probably attributed to small surface change at 320 K for the cellulose acrylate. In order to follow the behavior of CA as a function of acceptor and donor numbers of polar molecules at different temperatures, we draw on Fig. 4 the evolution of specific free energy, enthalpy and entropy of CA as a function of AN (a), DN (b) and DN / AN (c) The variations of −Gsp versus AN and DN represented in Fig. 5(a,b) clearly show the modification of the CA surface for all temperatures. When drawing the evolution of −Gsp as a function of DN / AN represented in Fig. 5(c), the obtained curves show an important increase of the free specific enthalpy when the basic character of polar probes relative to the acidic one increases for all temperatures. 4. Conclusion This paper constitutes a new contribution in studying the acid bas properties in Lewis terms of cellulose acrylate. The results concerning the calculations of the dispersive component of the surface energy of CA prove the presence of two linear zones with two different slopes in the temperature intervals. A strong decrease of sd of CA was observed in first zone of temperature [315 K, 335 K] with d a negative slope d s /dT ≈ −1 mJ m−2 K−1 whereas the second zone

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properties of cellulose acrylate. The specific free energy, enthalpy and entropy changes of CA strongly depend on the temperature. The study determined the acid-base constants KA , KD and K of CA surface for the different temperatures and proved the dependency of these constants on the temperature. Funding sources This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References

Fig. 5. Evolution of Specific free energy −Gsp (J/mol) versus AN (a), DN (b) and DN’/AN’ (c) of cellulose acrylate at various temperatures.

for the interval [335 K, 350 K] is characterized with a slow decrease of sd (T ). The important change in the slope of sd (T ) between the two temperatures zones can be attributed to a surface modification or to a relaxation of some surface groups of CA. Classical method cannot be used to determine the acid-base properties of CA surface. We applied the new method proposed by Hamieh’s model to determine the acidic KA , basic KD and amphoteric K constants of CA and the probability w of the specific adsorption of polar probes. The obtained results proved an effect certain of the thermal agitation on the different acid-base surface

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