Volumetric and compressibility behaviour of poly(propylene glycol) – Amino acid aqueous solutions at different temperatures

Accepted Manuscript Volumetric and compressibility behaviour of poly(propylene glycol) - Amino acid aqueous solutions at different temperatures Nosaibah Ebrahimi, Rahmat Sadeghi PII: DOI: Reference:

S0021-9614(15)00200-1 http://dx.doi.org/10.1016/j.jct.2015.06.024 YJCHT 4283

To appear in:

J. Chem. Thermodynamics

Received Date: Revised Date: Accepted Date:

7 April 2015 6 June 2015 21 June 2015

Please cite this article as: N. Ebrahimi, R. Sadeghi, Volumetric and compressibility behaviour of poly(propylene glycol) - Amino acid aqueous solutions at different temperatures, J. Chem. Thermodynamics (2015), doi: http:// dx.doi.org/10.1016/j.jct.2015.06.024

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1

Volumetric and compressibility behaviour of poly(propylene glycol) - Amino acid aqueous solutions at different temperatures

Nosaibah Ebrahimi, Rahmat Sadeghi∗ Department of Chemistry, University of Kurdistan, Sanandaj, Iran

Abstract Precise density and sound velocity measurements have been carried out for aqueous solutions of PPG725 in the absence and presence of (0.2 and 0.5) mol·kg-1 amino acids: alanine, glycine, serine and proline, and also for aqueous solutions of these amino acids in the absence and presence of 0.01 w/w PPG725 at T = (288.15, 293.15, 298.15, 303.15 and 308.15) K. From the experimental density and sound velocity values, the apparent molar volume and isentropic compressibility have been obtained and extrapolated to infinite dilution. The infinite dilution apparent molar properties for transfer of PPG from water to aqueous amino acids solutions and also those for transfer of amino acids from water to aqueous PPG solutions have been studied. Temperature dependency of the infinite dilution apparent molar volume was utilized to determine structure-breaker or structuremaker effects of the solutes. Hydration numbers of the amino acids in the investigated aqueous solutions have been evaluated from the volumetric and compressibility properties. All results are discussed based on the salting-out aptitude of the amino acids, hydrophilic-hydrophobic interactions and hydrophobic-hydrophobic interactions occurred between PPG and the investigated amino acids.

Keywords: Volumetric; Compressibility; Poly(propylene glycol); Amino Acid; Aqueous Solutions

∗

Corresponding author. Tel.: +98 871 6624133; fax: +98 871 6660075. E-mail address: [email protected] and [email protected]

2 1. Introduction The stability of native conformations of proteins and their physicochemical behaviours in aqueous solutions are strongly affected by the addition of different solutes to solutions. However, due to the structural complexities of proteins, direct studies of this phenomenon are extremely difficult [1-3]. Thermodynamics investigations of aqueous solutions containing amino acids, building stones of proteins, will be helpful in the fundamental understanding of the nature of interactions between amino acids and other solutes existing in solution, and in turn, comprehension of denaturation and other complicated biological process. On the other hand, in our previous work [4] we showed that poly(propylene glycol) (PPG) and amino acids can induce phase separation in aqueous media over part of the composition space. In other word, they form aqueous biphasic systems (ABS) consist of a PPG-rich and an amino acid-rich phase with water as solvent in both phases. Since ABS systems provide a biocompatible media for separation of biomaterials, they have a widespread use in biotechnology [5-7]. From this point of view, investigation of the various thermodynamic properties of PPG-amino acid aqueous mixtures is necessary for elucidating of phase forming behaviour of these systems and also for optimum design of industrial purification and separation process dealing with this type of ABS. Volumetric and acoustical studies of polymer-amino acid-water ternary systems can provide some unique information to extend our knowledge about the different interactions between their constituents. Despite this fact, as far as we know, only four articles [4, 810] have been published regarding to this subject. Among these articles, except one of them [4] which survey the volumetric and compressibility properties of alanine and glycine in aqueous solutions of poly(ethylene glycol)s (PEGs) and PPG400, other works [8-10] are devoted to the volumetric properties of PEG-amino acid-water systems. Sasahara et al. [8,9] determined the volume and compressibility changes on mixing of aqueous solutions of various amino acids and PEG4000, and Ayranci et al. [10] studied the volumetric and acoustic properties of glycine in aqueous PEG400 solutions.

3 In this work, in order to obtain further information about the different interactions in aqueous PPGamino acid systems, a comprehensive study of volumetric and compressibility properties at various temperatures (T = 288.15, 293.15, 298.15, 303.15 and 308.15 K) was performed for binary and ternary aqueous systems including: (i) PPG725 in water (ii) amino acids (alanine, glycine, serine and proline) in water (iii) PPG725 in aqueous solutions of 0.2 mol·kg-1 and 0.5 mol·kg-1 the mentioned amino acids and (iv) the mentioned amino acids in aqueous solutions of 0.01 w/w PPG725.

2. Experimental Section 2.1. Materials The properties of the chemicals used in this work were listed in table 1. The polymer, amino acids and alcohols were used without further purification. Double distilled and deionized water was used. 2.2. Methods All the solutions were prepared by mass on a Sartorius CP225D balance precisely within ±1×10-5 g. Density and sound velocity of the investigated solutions were measured by an Anton Paar DSA 5000 model high precision vibrating tube digital densimeter and sound velocity measuring device, with automatic viscosity corrections. It can measure the density in the range of (0 to 3) g·cm−3 and speed of sound from (1000 to 2000) m·s−1, simultaneously at temperatures from (273.15 to 343.15) K, with a pressure variation of (0 to 0.3) MPa at low frequency (approximately 3 MHz). The instrument has a built-in thermostat to maintain the samples at working temperature within ±10-3 K. The calibration of the instrument was made with degassed and double distilled water and dry air at atmospheric pressure according to the instruction manual of the instrument. The experimental uncertainties of density and sound velocity measurements were ±5×10-6 g·cm-3 and ±10-1 m·s-1 respectively. The instrument required a liquid volume of about 2.5 cm3 and measured

4 the density and sound velocity simultaneously after a thermal equilibration period of about 10 minute.

Results and discussion In order to determine how amino acids affect the volumetric and compressibility properties of PPG in aqueous solutions and also how these properties of amino acids in aqueous media influenced by the addition of PPG, in the present work, density and sound velocity measurements were carried out on aqueous solutions of PPG725 in the absence and presence of 0.2 mol·kg-1 and 0.5 mol·kg-1 the amino acids investigated, and also on aqueous solutions of the amino acids in the absence and presence of 0.01 w/w PPG725 (mass fraction of PPG725 in solvent is 0.01, means that molality of monomer of polymer in solvent is 0.17 mol·kg-1). It is necessary to note that the pH value of all the investigated solutions is about 5.5-6.5 in the whole concentration range, which is close to the isoelectric points of the amino acids under study. Therefore in these investigated solutions, the amino acids are in their zwitterionic form. The measured density and sound velocity data have been presented in the Supporting Information of this manuscript.

3.1. Apparent molar volume Experimental density values for the systems investigated were used to calculate the apparent molar volume, Vφ , of solute (polymer or amino acid) by the following equation [11]:

Vφ =

M 1000 (d 0 − d ) + s , ms dd 0 d

(1)

where Ms is the molecular mass of solute, ms is its molality, d0 and d represent the density of the

solvent and solution, respectively. In the case of ternary systems where amino acid is the solute, {PPG725 + water (aqueous solutions of 0.01 w/w PPG725)} was taken as the solvent and for ternary systems where PPG725 is the solute, the {amino acid + water (aqueous solutions of 0.2 and 0.5 mol·kg-1}of the investigated amino acids) was considered as the solvent. In this work, in the

5 case of the state where PPG725 is the solute, we used the molecular mass of the PPG monomer (58.0791 g·mol-1) for Ms and monomer molality of PPG for ms, and therefore from the equation (1) the apparent molar volume of the monomer of PPG725 was obtained. As an example, the variations of Vφ as a function of the molality of solute at 298.15 K are presented in figure 1 (A and B). The similar behaviour was observed for the other temperatures. Figure 1A reveals that the Vφ values of PPG725 in all of the aqueous amino acid solutions investigated are smaller than those in pure water and decrease in the order: water ˃ glycine (0.2 molal) ≈ alanine (0.2 molal) ≈ serine (0.2 molal) ≈ glycine (0.5 molal) ≈ serine (0.5 molal) ˃ alanine (0.5 molal) ˃ proline (0.2 molal) ˃ proline (0.5 molal). Although in the absence of amino acids the plot of Vφ versus monomer molality of PPG has negative slope, in aqueous amino acid solutions this slope is generally positive (with a few exceptions at some temperatures, see table 1). Therefore the differences between Vφ of PPG725 in water and in aqueous amino acids solutions become smaller by increasing the concentrations of PPG725. As can be seen, the values of Vφ of PPG725 decrease by increasing molality of proline and alanine from (0.2 to 0.5) mol·kg-1 in the solvent, but there is no specific difference between Vφ of PPG725 in aqueous solutions with different molality of glycine and serine. According to the observed trend, it can be said that the effectiveness of the investigated amino acids in decreasing the Vφ of PPG725 in aqueous solutions increases with the hydrophobic nature of the amino acid. In fact

the ability of the amino acids to reduce Vφ of PPG725 follows a reverse order of the strength of these amino acids in salting-out of PPG, which previously observed as serine ≈ glycine > alanine > proline according to the binodal curves [4]. Figure 1B shows that the values of Vφ of glycine and proline in aqueous polymer solution, respectively, are larger and smaller than those in pure water. However, PPG does not have any specific effect on the Vφ values of serine and alanine in aqueous solution. Furthermore, figure 1B reveals that the Vφ values of the investigated amino acids increase

6 by increasing the number of alkyl groups present in the amino acid as obey the sequence: proline (C5H9NO2) ˃ serine (C3H7NO3) ≈ alanine (C3H7NO2) ˃ glycine (C2H5NO2). The infinite dilution apparent molar volume of solute, Vφo , which is equal to the partial one, can be obtained by extrapolating Vφ vs concentration of solute to zero concentration. For all of the solutions investigated in this work, the values of Vφ vary almost linearly with the molality of solute. Therefore, the values of Vφ were least-squares fitted to the following equation [12,13]:

Vφ = Vφ° + bV ms ,

(2)

where bV is an experimentally determined parameter. The values of Vφo and bV along with the standard deviations, sd ( Vφ ), at different temperatures are listed in tables 2 and 3. The literature data for Vφo of proline in water at T = 298.15 K are given as a footnote to table 3. As can be seen, there is a good agreement between our data and the literature values. According to our survey, the Vφo values for PPG725 in water and in aqueous amino acid solutions and also for amino acids in aqueous PPG725 solutions are being reported for the first time in this work, so a similar comparison for them cannot be made. The infinite dilution apparent molar volume of the monomer of PPG400 in water at T = (288.15, 293.15, 298.15, 303.15 and 308.15) K, are (52.9649, 53.1893, 53.4174, 53.6429 and 53.8773) cm3·mol-1, respectively [21]. A comparison between these values with those presented in table 2 for Vφo of PPG725 in water shows that the standard partial molar volume of monomer of PPG decreases by increasing the molar mass of PPG. This behaviour is similar to that observed by some authors for the infinite dilution apparent specific volume of PEGs in water, so that it vary almost linearly with the reciprocal values of the PEG molar mass [12,22]. As can be seen from tables 2 and 3, the experimental slopes bV for PPG725 in water, similar to other binary (polymer + water) systems [21,23], have negative values. However, similar to the aqueous solutions of simple electrolytes [24], bV for amino acids in water (other than proline) and in aqueous polymer solutions have positive values. The positive values of bV of the amino acids investigated in pure

7 water, which attributed to the solute-solute interactions, follow the order serine ≥ glycine ˃ alanine ˃ proline (in agreement with the literature [15, 17]). However, the bV of the amino acids in aqueous PPG725 solutions have similar values, except proline which has smaller bV than the other amino acids. Since at infinite dilution, there is no solute-solute interactions and each solute is surrounded only by the solvent molecules, the values of Vφo provide valuable information about the solutesolvent interactions existing in solutions. Figure 2A shows the infinite dilution apparent molar volume for transfer of monomer of PPG725 from water to aqueous solutions of the investigated amino acids ( ∆Vφo, trs, p = Vφo,p (in aqueous amino acid solution) - Vφo,p (in water)). Similarly, figure 2B shows the corresponding values for transfer of amino acids from water to aqueous solution of PPG725 ( ∆Vφo,trs,a = Vφo,a (in aqueous polymer solution) - Vφo,a (in water)). As can be seen from figure 2A, the ∆Vφo, trs,p values in all cases are negative and become more negative by increasing molality of amino acid in solvent from 0.2 molal to 0.5 molal. Temperature does not have any considerable effect on the values of ∆Vφo, trs,p , except in aqueous solution of 0.5 mol.kg-1 proline, in which the values of ∆Vφo,trs,p become less negative with an increase in temperature. Furthermore, proline has the greatest effect on the infinite dilution volume change of PPG, similar to that observed in figure 1A for V φ , p . Figure 2B reveals that the values of ∆Vφo,trs,a for glycine and serine are positive and decrease by increasing temperature. However, the values of ∆Vφo, trs, a for alanine and proline are negative and rather independent of temperature. It can be suggested that the behaviour observed here for ∆Vφo, trs , is a result of a delicate balance of the following factors: (a) The sharing of hydrophilic hydration co-sphere of zwitterions of amino acids (NH3+ and COO-) and hydrophobic hydration co-sphere of monomer of PPG [ ̶ OCH(CH3)CH2 ̶ ], results in overall repulsive force and therefore increases Vφo of solute (polymer or amino acid) and causes positive

8 value for ∆Vφo, trs [25,8,9]. For amino acids with more hydrophilic nature, the effect of this factor on

∆Vφo, trs is more than that for less hydrophilic amino acids. (b) Hydrophobic-hydrophobic interactions between the non-polar portion of the amino acids and the hydrocarbon chain of PPG contribute negatively to ∆Vφo, trs due to releasing some hydrophobically hydrated water molecules around non-polar groups. Hydrophobically hydrated water is known to be in an ice like structure having greater volume than the bulk water. So, when the hydrophobically hydrated water molecules are released in to the normal bulk water, their volumes decrease [10,11]. The importance of this factor increases by increasing the number of alkyl groups present in the amino acids. (c) Some of the ternary PPG-amino acid aqueous solutions can form ABS over specific threshold concentrations [4]. In aqueous solutions of solute 1-solute 2, which can form ABS, the solute1water interactions become more stable in the presence of solute 2. In other words, solute 1 and solute 2 exclude themselves from the vicinity of each other due to the their preferential hydration [4, 26-28]. Accordingly, the interaction of PPG with water in aqueous amino acid solutions is stronger than that in pure water. Similarly, the amino acid - water interaction in aqueous PPG solution is stronger than that in pure water. It is expected that the Vφo values of PPG in aqueous amino acid solutions (or the Vφo values of amino acids in aqueous PPG solution) should be smaller than those in water, therefore ∆Vφo, trs should be negative. The effect of this factor on the values of

∆Vφo, trs , increases by increasing salting-out aptitude of the amino acid. The large negative values of ∆Vφo, trs for transfer of PPG from water to aqueous proline solutions, and also for transfer of proline from water to aqueous PPG solution are attributed to the second factor (b). The positive values of ∆Vφo, trs for transfer of glycine and serine from water to aqueous PPG solution are attributed to the repulsive force due to overlap the hydrophilic hydration co-sphere of the amino acids and hydrophobic hydration co-sphere of PPG, which is mentioned as factor (a).

9 Similarly, the less negative values of ∆Vφo, trs for transfer of PPG from water to aqueous solution of glycine, serine and alanine, in comparison to proline (despite their higher salting-out strength than proline and therefore more negative effect of factor c) is attributed to the factor (a). The temperature dependence of Vφo was expressed by the following equation [20]:

Vφ0 = a + bT 0.5 + cln(T ) ,

(3)

from which values of the infinite dilution apparent molar isobaric expansibility of solute, E φ0 , were obtained as [20]: E φ0 = (

∂Vφ0 ∂T

)P =

b 2T 0.5

+

c . T

(4)

The values of the fitting parameters a, b and c are given in table 4. For all of the systems investigated, the values obtained for Eφ0 are positive. Positive expansibility is a characteristic property of aqueous solutions of hydrophobic hydration. On heating, due to the increase of their motion, the solutes increase their size. This produces a slight increase of the Vφo,m and so E φo,m would be positive. Figure 3A shows that the positive values of Eφ0 for PPG725 in water and in aqueous amino acid solutions increase by increasing temperature (except for PPG725 in aqueous solutions of 0.5 mol·kg-1 proline that in which Eφ0 values display slow decrease with temperature). However, according to figure 3B, the positive values of Eφ0 for the amino acids in water and in aqueous polymer solutions decrease by increasing temperature. Furthermore, from figures 3A and 3B, it can be seen that there is no considerable difference between the values of Eφ0 for solute (PPG725 or amino acids) in water and those in ternary PPG-amino acid aqueous solutions. Since temperature dependency of Eφ0 for PPG725 shows different behaviour with the amino acids, we devote some attention to this subject. The results of previous studies show that in the binary aqueous solutions of common salts (namely (NH4)2HCit [20], NaH2Cit [29], Na2HCit [29], Na3Cit [29], NaH2PO4 [30]), ionic liquids (namely [C3mim]Br [31], [C4mim]Br [26], [C6mim]Br [31,32],

10 [C7mim]Br [31], [C8mim]Br [31]) and most amino acids [14,20], the positive Eφ0 values of solutes decrease by increasing temperature. So it is an interesting result that Eφ0 values of PPG725 in pure water increase by increasing temperature (in a reverse behaviour with the temperature dependency of E φ0 of the mentioned solutes in pure water). As far as we know the temperature dependence of the infinite dilution partial expansibility of polymers in aqueous solutions has not been studied in the literature. Since the repeating units of PPG have similar structure to alcohols, we also measured the volumetric and acoustic properties of aqueous solutions of some alcohols (the experimental data have also been presented in the Supporting Information). In figure 4, the temperature dependence of

E φ0 for aqueous solutions of some polymers, amino acids and alcohols has been compared. As can be seen, PPGs (PPG725 and PPG400) similar to the alcohols (ethanol, propanol and butanol) have positive slope for E φ0 vs T, while PEG6000, poly (ethylene glycol) dimethyl ethers (PEGDME250, PEGDME500 and PEGDME2000) as well as amino acids have negative slope for that. However, amounts of positive or negative slopes for polymers are smaller than those for alcohols or amino acids. The Eφ0 values of polyvinylpyrrolidone (PVP) are approximately independent of temperature. Furthermore, close examination of figure 4 shows that the infinite dilution apparent molar expansibility per monomer of polymers in water follows the sequence PVP10000 ˃ PPG725 ˃ PPG400 ˃ PEG6000 ≈ PEGDME2000 ˃ PEGDME500 ˃ PEGDME250. Data for the calculation of E φ0 of PPG400, PEG6000, PEGDMEs and PVP were taken from the references [21], [23], [33] and [34], respectively. Hepler [35] suggested a thermodynamic expression to classify solutes as structure-making or structure-breaking in solutions. According to that, (

value of (

∂ 2Vφ0 ∂T 2

∂C p0 ∂P

)T = − T (

∂ 2Vφ0 ∂T 2

) p and then the negative

) p is attributed to a structure-breaker solute and a positive one is attributed to a

11 structure-maker solute. As can be seen from figure 5, the values of (

∂E φ0 ∂T

)p = (

∂ 2Vφ0 ∂T 2

) p for PPG725

in water and in aqueous amino acid solutions are positive and have slightly negative slope with temperature (except for PPG725 in aqueous solutions of 0.5 mol·kg-1 proline); however the values of (

∂ 2Vφ0 ∂T 2

) p for amino acids in water and in aqueous PPG solution are negative and have positive

slope with temperature. It means that, structure-making property of PPG725 and structure-breaking property of amino acids become weaker at higher temperatures. Figure 5B shows that the values of (

∂ 2Vφ0 ∂T 2

) p for amino acids in water are more negative than those in aqueous solution of PPG725 and

at a same condition, the magnitude of (

∂ 2Vφ0 ∂T 2

) p and therefore structure-breaking property of the

investigated amino acids decrease in the order: serine ˃ glycine ˃ alanine ˃ proline; which is in agreement with their hydrophilic nature and salting-out strength [4]. A comparison for structurebreaker or maker effects of some polymers, amino acids and alcohols in water is given in Figure 6. From this figure we note that the structure-making property of the monomer of PPGs is so smaller than alcohols and also the structure-breaking property of the monomer of PEG6000, PEGDMEs and PVP is so smaller than that of amino acids. Besides, our previous study [20] showed that the structure-breaking property of amino acids is so smaller than ordinary salts such as (NH4)2HCit.

3.2. Isentropic compressibility Isentropic compressibility values of the investigated solutions, κ s , which can be taken as the sum of the values of the isentropic compressibility of solvent and solute, were calculated based on the sound velocity and density data using the Laplace-Newton’s equation: κs =

1 , du 2

(5)

where u is the sound velocity of solution. The isentropic compressibility of all the solutions investigated decreases with increasing concentration of solutes and temperature. In fact, within the

12 temperature range studied in this work (below 337.15 K), the temperature dependence of κ s for dilute aqueous solutions is the same as that for pure water and decreases by increasing temperature. Solute concentration dependence of κ s for the solutions investigated is presented in figure 7. Figure 7A reveals that the values of κ s for the aqueous solutions of PPG725 follow the order (PPG725 in water) • (PPG725 in aqueous solution of glycine) • (PPG725 in aqueous solution of alanine) • (PPG725 in aqueous solution of serine) • (PPG725 in aqueous solution of proline); and decrease by increasing molality of amino acid in solvent. From figure 7B, it can be seen that the values of κ s for aqueous solutions of amino acids (amino acids in water and in aqueous solution of PPG725) decrease in the order: glycine • alanine • serine • proline, which is exactly the same as the trend observed in figure 7A for κ s of aqueous solution of PPG725. In fact, for the dilute solutions investigated in this work, isentropic compressibility of solvent, κ s (solvent intrinsic), has the dominate contribution to the total value of κ s . Therefore the trends observed in figure 7A for κ s of different aqueous solutions of PPG725 are same as the order of corresponding κ s (solvent intrinsic). A comparison of figures 7A and 7B shows that

∂κ s ∂κ s • . In other words, isentropic ∂mp ∂m a

compressibility of dilute aqueous mixtures of PPG-amino acid is more sensitive to the concentration of amino acid than polymer. Since, compressibility of dilute aqueous solutions is mainly due to the effect of pressure on the bulk water, we can assume that the concentration dependence of κ s becomes greater as the number of water molecules affected by the solutes increases [36]. Therefore, as expected, this observation indicates that the hydration number per monomer of PPG725 is less than that of amino acids. Furthermore, figure 7B shows that the concentration dependence of κ s becomes greater as temperature decreases, which indicates that the water-solute interactions are weakened by increasing temperature. The isentropic compressibility increments, ∆κ s , for aqueous solutions in which PPG725 is solute, were obtained using the relation [21]:

13 ∆κ s = κ s − (wp κ s, p + (1 − wp ) κ s,0 ),

(6)

where κ s , κ s,p and κ s,0 represent the isentropic compressibility of solution, pure polymer and solvent (in the case of ternary systems, (water + amino acid) is taken as the solvent), respectively. wp is the mass fraction of polymer. The values of ∆κ s for solutions of PPG725 in water and in aqueous amino acid solutions are negative and become less negative as temperature increases. The negative values of ∆κ s implies that, difficulties to compress the mixtures are greater in respect to the ideal mixtures. Furthermore, because of the low concentration of the investigated solutions (due to the low water solubility of PPG725), there is no any specific difference between ∆κ s values in the presence and absence of the amino acids. Plots of ∆κ s versus wp are given in the supporting information of this manuscript.

3.3. Apparent molar isentropic compressibility Apparent molar isentropic compressibilities of the solutes, K φ , which defined as K φ = −(

∂Vφ

∂P

)S ,

were calculated from the density and sound velocity experimental data according to the following equation: Kφ =

1000(κ s d 0 − κ s,0 d ) ms dd 0

+

M s κs . d

(7)

Again, in the case of systems where PPG725 is the solute, ms and Ms denote monomer molality and molar mass of the monomer of PPG, respectively. Therefore we obtain the apparent molar isentropic compressibility per monomer of polymer. For all systems investigated, the apparent molar isentropic compressibility of the solute (polymer or amino acid) varies almost linearly with concentration of the solute. Hence, in order to determine the apparent molar isentropic compressibility of solute at infinite dilution, K φ0 , the values of K φ were least-squares fitted to the following equation [13]:

14 K φ = K φ° + bK ms ,

(8)

where bK is an experimentally determined parameter. The values of K φo and bK along with the standard deviations, sd ( K φ ), at different temperatures are also listed in tables 2 and 3. From these tables, it can be seen that the K φo values are negative and increase as temperature increases, so that the values of K φo of PPG725 become positive at higher temperatures. In all cases, K φo of PPG725 ˃ K φo of the amino acids. The negative values of K φo imply that the water molecules surrounding the

solute molecules are less compressible than the water molecules in the bulk solutions. The amino and carboxyl groups of zwitterions of the amino acids, -NH3+ and -COO-, are hydrated in an electrostatic manner, whereas the monomers of PPG [ ̶ OCH(CH3)CH2 ̶ ] have hydrophobic hydration. Since the compressibility of electrostricted water is less than that of the ice-like configuration or unbounded water molecules around the solutes [37], it is reasonable that K φo values of PPG725 are greater than those of the amino acids. Furthermore, table 3 shows that the negative values of K φo of the amino acids generally follows the order serine ˃ glycine ˃ alanine ˃ proline, which is similar to the order of structure breaking effect of the amino acids revealed in Figure 5B. Figure 8A shows the infinite dilution apparent molar isentropic compressibility of transfer of PPG725 monomer from water to aqueous solutions of the investigated amino acids, ∆K φo, trs, p = K φo,p (in aqueous amino acid solution) - K φo,p (in water) and Figure 8B shows the corresponding values for transfer of the amino acids from water to aqueous solution of PPG725, ∆K φo, trs,a = K φo,a (in aqueous polymer solution) - K φo,a (in water). As can be seen from figure 8A, the ∆K φo, trs, p values are generally negative and relatively independent of temperature. Figure 8B reveals that ∆K φo, trs,a values of glycine and serine are positive at low temperature and become negative by increasing temperature. However, at all temperatures investigated in this work, the values of ∆K φo, trs, a for

15 alanine and proline respectively are negative and positive and relatively independent of temperature.

3.4. Hydration number The infinite dilution apparent molar volume of a solute in aqueous solution can be described as: Vφ0 = Vφ0 (int) + Vφ0 (hyd) ,

(9)

where Vφ0 (int) is the intrinsic apparent molar volume and Vφ0 (hyd) is the hydration apparent molar volume of solute. In the case of ionic solutes, Vφ0 (hyd) is denoted by Vφ0 (elect) . The Vφ0 (elect) is a decrease in volume of solute due to electrostriction, which can be related to the number of water molecules bounded to the solute at infinite dilution, n H0 ,[36]: n H0 =

Vφ0 (elect) Vφ0,E − Vφ0,B

,

(10)

where ( Vφ0,E − Vφ0,B ) describes the maximum decrease in molar volume of water (solvent) caused by electrostriction, i.e., difference between molar volume of electrostricted water ( Vφ0,E ) and bulk water ( Vφ0,B ). The values reported for ( Vφ0,E − Vφ0,B ) for transition of bulk phase water molecules to near the amino acid solute is -3.3 cm3·mol-1 at T= 298.15 K [13]. Therefore, if we have Vφ0 (int) , Vφ0 (elect) can be obtained from equation (9) and then n H0 from equation (10). According to Millero et al. [13], Vφ0 (int) for amino acids can be calculated from the following relation: V φ0(int) =

0.7 0 Vφ (cryst) , 0.634

(11)

where 0.634 is the packing density for random packing spheres, 0.7 is the packing density for molecules in organic crystals and Vφ0 (cryst) is the crystal molar volume of amino acids where obtained from the dry-state amino acid density data. Vφ0 (cryst) =

Ms , and the values of d (cryst)

16 d(cryst) for glycine, alanine, serine and proline are (1.598, 1.371, 1.582 and 1.376) g.cm-3, respectively [38]. The values of the infinite dilution hydration number of the amino acids at 298.15 K, calculated by equation (10), are given in table 5. If Vφ0,E and n H0 are assumed to be independent of pressure, by differentiating of equation (10) with respect to pressure, we have: 0 H

n =−

K φ0 (elect) K φ0,B

=−

K φ0 (elect) κ s, 0Vφ0,B

.

(12)

By differentiating of equation (9) with respect to pressure, we obtain the following relation for K φ0 (elect) of amino acids: K φ0 (hyd) = K φ0 (elect) = K φ0 − K φ0 (int ) .

(13)

The values of K φ0 (int ) for glycine, alanine, serine and proline are (0.335×10-5, 0.270×10-5, 0.3×10-5 and 0.3×10-5) cm3·mol-1·kPa-1, respectively [13]. Based on the assumption that Vφ0,B of water in aqueous polymer solution is the same as that of bulk water, the values of Vφ0,B were determined from the density of pure water at different temperatures. The values of the infinite dilution hydration number of the amino acids calculated by equation (12) are also given in table 5. According to the volumetric hydration number ( n H0 obtained from equation (10)), in the presence of PPG725, the n H0 values of glycine and serine slightly decrease, while those of alanine and proline slightly increase. Furthermore, the volumetric n H0 values of the amino acids follow the order: serine ˃ alanine ˃ proline ˃ glycine, which is the same trend observed by other researcher [13,39] for

volumetric infinite dilution hydration number. However, this trend is not in agreement with the hydrophilic nature of the amino acids. On the other hand, the values of the compressibility hydration number ( n H0 obtained from equation (12)) for all the studied amino acids (except is proline), increased in the presence of PPG725. The compressibility n H0 values of the amino acids follow the order: serine ˃ glycine ˃ alanine ˃ proline which is same as the trend observed by other

17 authors [13,39] for compressibility infinite dilution hydration number and also is in good consistent with the degree of hydrophilicity of the amino acids. Furthermore, the sequence obtained for the compressibility n H0 values is similar to that observed in figure 5B for the structure breaking property of the amino acids. Based on this result, it should be concluded that the values of n H0 depend on the method of determination. Therefore they don’t possess any universal physical significance and also don’t reflect the real hydration number. In the case of our study, it seems that the values of n H0 obtained from compressibility method are more reasonable than those obtained from volumetric method. The increase of the compressibility n H0 values of the amino acids causing by the addition of PPG725, confirms mutual exclusion of the zwitterions of the amino acids and the polymer molecules from the vicinity of each other, and their high affinity for water (salting-out phenomenon). Further as can be seen from table 5, the values of the compressibility n H0 decrease by increasing temperature attributed to a loss of hydration by heating.

4. Conclusions

The apparent molar volume, Vφ , and isentropic compressibility, K φ , of PPG725 in water and in aqueous solutions of (0.2 and 0.5) mol·kg-1 amino acids alanine, glycine, serine and proline and also those of these amino acids in water and in aqueous solution of 0.01 w/w PPG725, have been obtained from the experimental density and sound velocity data at T = (288.15 to 308.15) K. The effectiveness of the amino acids investigated in decreasing Vφ of PPG725 increases with the hydrophobic nature of the amino acids. The values of ∆Vφo, trs and ∆K φo, trs for PPG725 are negative and generally independent of temperature. The values of ∆Vφo, trs for glycine and serine are positive while for alanine and proline they are negative. Temperature dependency of ∆Vφo, trs and ∆K φo, trs for glycine and serine shows a shallow minimum and an increasing trend at the higher temperature, while those for alanine and proline are relatively independent of temperature. Based on the positive

18 values of (

∂E φ0 ∂T

) p for PPG, and negative values of that for amino acids, PPG is a structure-maker

solute while amino acids are structure-breaker solutes. In all cases, the magnitude of (

∂E φ0 ∂T

)p

decreases by increasing temperature, i.e., structuring effect of the solutes becomes smaller by temperature. The infinite dilution hydration numbers of the amino acids have been calculated using volumetric and compressibility values. The compressibility n H0 values of all the amino acids studied, except proline, increase in the presence of PPG725. At each temperature, the (

∂E φ0 ∂T

) p and

K φo and also the compressibility n H0 values of the amino acids follow the order: serine ˃ glycine ˃ alanine ˃ proline, which reflects the hydrophilicity degree of the investigated amino acids.

References

[1] W.P. Jencks, Catalysis in Chemistry and Enzymology, Dover Publications, New York, 1969. [2] F. Franks, Biophys. Chem. 96 (2002) 117–127. [3] K.D. Collins, Methods 34 (2004) 300–311. [4] R. Sadeghi, B. Hamidi, N. Ebrahimi, J. Phys. Chem. B 118 (2014) 10285-10296. [5] P. A. Albertsson, Partitioning of Cell Particles and Macromolecules. 3 ed., Wiley-Interscience, New York, 1986. [6] H. Walter, D. E. Brooks, D. Fisher, Partitioning in Aqueous Two-Phase Systems. Academic Press, New York, 1985. [7] B. Y. Zaslavsky, Aqueous Two-Phase Partitioning, Physical Chemistry and Bioanalytical Applications. Marcel Dekker, New York, 1995. [8] K. Sasahara, H. Uedaira, Colloid Polym. Sci. 272 (1994) 385-392. [9] K. Sasahara, Colloid Polym. Sci. 273 (1995) 782-786. [10] M. Sahin, Z. Yesil, M. Gunel, S. Tahiroglu, E. Ayranci, Fluid Phase Equilib. 300 (2011) 155161. [11] F.J. Millero, in: R.A. Horne (Ed.), Structure Transport Process in Water Aqueous Solutions, John Wiley, New York, 1972, pp. 519–564 (Chapter 13). [12] L. Lepori, V. Mollica, J. Polym. Sci. Polym. Phys. Ed. 16 (1978) 1123–1133. [13] F.J. Millero, A.L. Surdo, S.C. Shin, J. Phys. Chem. 82 (1978) 784–792. [14] H. Zhao, Biophys. Chem. 122 (2006) 157 183. [15] A.K. Mishra, J.C. Ahluwalia, J. Phys. Chem. 88 (1984) 86-92. [16] M. Kikuchi, M. Sakurai, K. Nitta, J. Chem. Eng. Data 40 (1995) 935-942. [17] C. Jolicoeur, B. Riedl, D. Desrochers, L.L. Lemelin, R. Zamojska, O. Enea, J. Solut. Chem.15 (1986) 109-128.

19 [18] D.P. Kharakoz, J. Phys. Chem. 95 (1991) 5634-5642. [19] R. Sadeghi, B. Goodarzi, J. Mol. Liq. 141 (2008) 62–68. [20] R. Sadeghi, A. Gholamireza, J. Chem. Thermodyn. 43 (2011) 200–215. [21] R. Sadeghi, B. Jamehbozog, Fluid Phase Equilib. 284 (2009) 86–98. [22] S. Kirincic, C. Klofutar, Fluid Phase Equilib. 149 (1998) 233–247. [23] R. Sadeghi, R. Hosseini, B. Jamehbozorg, J. Chem. Thermodyn. 40 (2008) 1364–1377. [24] F.J. Millero, Chem. Rev. 71 (1971) 147-176. [25] K. Sasahara, H. Uedaira, Colloid Polym. Sci. 271 (1993) 1035-1041. [26] R. Sadeghi, B. Mostafa, E. Parsi and Y. Shahebrahimi, J. Phys. Chem. B 114 (2010) 16528. [27] R. Sadeghi, F. Jahani, J. Phys. Chem. B 116 (2012) 5234. [28] R. Sadeghi, T. Moradian, J. Phys. Chem. B 117 (2013) 7710. [29] R. Sadeghi, R. Golabiazar, E. Parsi, J. Chem. Eng. Data 55 (2010) 5874–5882. [30] R. Sadeghi, H. Parhizkar, J. Chem. Thermodyn. 40 (2008) 1012–1021. [31] R. Sadeghi, H. Shekaari, R. Hosseini, J. Chem. Thermodyn. 41 (2009) 273–289. [32] H. Shekaari, Y. Mansoori, R. Sadeghi, J. Chem. Thermodyn. 40 (2008) 852–859. [33] R. Sadeghi, H. Baghi Kahaki, Fluid Phase Equilib. 306 (2011) 219–228. [34] R. Sadeghi, F. Ziamajidi, J. Chem. Thermodyn. 39 (2007) 1118–1124. [35] L.G. Hepler, Can. J. Chem. 47 (1969) 4613-4617. [36] F.J. Millero, G.K. Ward, F.K. Lepple, E.V. Foff, J. Phys. Chem. 78 (1974) 1636–1643. [37] B.E. Conway, R.E. Verrall, J. Phys. Chem. 70 (1966) 3952-3961.. [38] E. Berline, M.J. Pallansch, J. Phys. Chem. 72 (1968) 1887-1889. [39] H. Kumar, M. Singla, R. J. Mol. Liq. 197 (2014) 301-314.

20

Table 1 Provenance and mass fraction of the chemicals studied

Chemical

Source

Mass fraction purity

Purification method

PPG725

Aldrich

-

No

L-Proline

BDH

0.990

No

Glycine

Merck

0.997

No

S(+)-alanine

Merck

0.990

No

L-serine

Merck

0.990

No

Ethanol

Merck

> 0.999

No

1-Propanol

Duksan

> 0.999

No

1-Butanol

Merck

> 0.990

No

21

Table 2 Infinite dilution apparent molar properties ( Vφo and K φo ), empirical constants (bV and bK ) and standard deviations (sd) for PPG725 in water and in aqueous solutions of amino acids at different temperatures and 84.5 kPa

Vφo / T/K

3

-1

3

bV /

-2

sd ( Vφ ) /

(cm ⋅ mol )

(cm ·kg·mol )

(cm3·mol-1)

52.7416 52.9681 53.2427 53.5405 53.8144

-0.2453 -0.1171 -0.1730 -0.2993 -0.2801

0.0139 0.0180 0.0090 0.0228 0.0192

105 Kφo / 3

-1

105.bK /

-1

(cm ⋅ mol ⋅ kPa )

105.sd ( K φ ) /

(cm3·kg·mol2 ·kPa-1)

(cm3·mol-1·kPa-1)

-0.0189 0.1076 0.0428 -0.0473 0.0593

0.0733 0.0489 0.0561 0.0474 0.0458

0.3340 0.3463 0.2537 0.3021 0.3483

0.0103 0.0162 0.0134 0.0082 0.0096

0.8009 0.8342 0.7122 0.8908 0.7784

0.0289 0.0381 0.0392 0.0378 0.0248

1.0018 0.9827 0.8777 0.7269 0.6874

0.0680 0.0509 0.0436 0.0325 0.0275

0.3572 0.5081 0.5821 0.6116 0.6701

0.0159 0.0150 0.0082 0.0152 0.0128

0.7172 0.8016 0.7339 0.5792 0.4539

0.0186 0.0288 0.0220 0.0187 0.0140

0.9776 1.2652 1.1628 1.1973 1.3128

0.0249 0.0221 0.0261 0.0300 0.0291

1.1131 1.1135 1.0658 0.8442

0.0510 0.0639 0.0589 0.0430

PPG725 in water 288.15 293.15 298.15 303.15 308.15

-0.3068 -0.0984 0.1409 0.3733 0.5304

PPG725 in aqueous solutions of 0.2 mol·kg-1 alanine 288.15 293.15 298.15 303.15 308.15

52.6932 52.9036 53.1588 53.4773 53.7424

-0.1968 -0.0187 0.0588 -0.0593 -0.0120

0.0068 0.0066 0.0036 0.0052 0.0095

-0.3775 -0.1355 0.1081 0.3035 0.4902

PPG725 in aqueous solutions of 0.5 mol·kg-1 alanine 288.15 293.15 298.15 303.15 308.15

52.5701 52.8103 53.1042 53.3969 53.6470

0.1554 0.2598 0.0385 -0.0109 0.2344

288.15 293.15 298.15 303.15 308.15

52.6471 52.8909 53.1758 53.4175 53.6969

0.0677 0.1240 0.0261 0.1564 0.1679

0.0158 0.0188 0.0089 0.0144 0.0081

-0.3386 -0.0973 0.1359 0.3123 0.5149

PPG725 in aqueous solutions of 0.2 mol·kg-1 glycine 0.0104 0.0202 0.0078 0.0280 0.0159

-0.4917 -0.2614 -0.0228 0.2156 0.4176

PPG725 in aqueous solutions of 0.5 mol·kg-1 glycine 288.15 293.15 298.15 303.15 308.15

52.6452 52.8540 53.1298 53.3595 53.6105

0.2479 0.4377 0.3901 0.6217 0.8204

288.15 293.15 298.15 303.15 308.15

52.6969 52.9002 53.1486 53.4201 53.6908

-0.2058 0.0216 0.0530 0.0058 -0.0057

0.0116 0.0122 0.0100 0.0230 0.0167

-0.2874 -0.0976 0.0985 0.2980 0.4755

PPG725 in aqueous solutions of 0.2 mol·kg-1 serine 0.0152 0.0091 0.0070 0.0147 0.0114

-0.4271 -0.2000 0.0236 0.2519 0.4708

PPG725 in aqueous solutions of 0.5 mol·kg-1 serine 288.15 293.15 298.15 303.15 308.15

52.5595 52.8139 53.0843 53.3809 53.6120

0.7681 0.7298 0.6669 0.4829 0.7889

0.0252 0.0201 0.0151 0.0179 0.0235

-0.3280 -0.1745 0.0589 0.2624 0.4226

PPG725 in aqueous solutions of 0.2 mol·kg-1 proline 288.15 293.15 298.15 303.15

52.5226 52.7608 53.0708 53.3101

0.2467 0.3074 0.1328 0.2983

0.0088 0.0075 0.0045 0.0103

-0.5669 -0.3228 -0.0969 0.1607

22 308.15

53.6044

0.2943

0.0060

0.3560

0.8324

0.0403

4.3138 3.5376 3.1387 2.9209 2.3371

0.0727 0.0461 0.0301 0.0339 0.0260

PPG725 in aqueous solutions of 0.5 mol·kg-1 proline 288.15 293.15 298.15 303.15 308.15

52.19224 52.46536 52.82558 53.16674 53.43901

1.2882 1.3033 0.8232 0.4473 0.4887

0.0178 0.0298 0.0130 0.0157 0.0091

-0.9500 -0.6013 -0.3270 -0.0871 0.1972

Table 3 o o Infinite dilution apparent molar properties ( Vφ and K φ ), empirical constants (bV and bK ) and standard deviations (sd) for amino acid in water and in aqueous solutions of PPG725 at different temperatures and 84.5 kPa

Vφo / T/K

3

-1

(cm ⋅ mol )

288.15 293.15 298.15 303.15 308.15

59.7477 60.1074 60.3976 60.6629 60.8886

288.15 293.15 298.15 303.15 308.15

59.6820 60.0273 60.3349 60.5910 60.8540

288.15 293.15 298.15 303.15 308.15

42.2329 42.7157 43.1151 43.4301 43.7168

288.15 293.15 298.15 303.15 308.15

42.3676 42.8135 43.1866 43.4870 43.7841

288.15 293.15 298.15 303.15 308.15

59.4855 60.0468 60.5599 60.9661 61.2889

288.15 293.15 298.15 303.15 308.15

59.6437 60.1491 60.5948 60.9667 61.3243

288.15 293.15 298.15 303.15 308.15

81.8389 82.2901 82.7217c 83.1094 83.4679

3

bV /

-2

(cm ·kg·mol )

sd ( Vφ ) / (cm3·mol-1)

105.Kφo / 3

-1

105.bK /

-1

(cm ⋅ mol ⋅ kPa )

(cm3·kg·mol2 ·kPa-1)

Alanine in watera 0.6829 0.0466 -3.0653 0.6420 0.0492 -2.7173 0.6301 0.0449 -2.4382 0.6055 0.0487 -2.2296 0.5938 0.0485 -2.0606 Alanine in aqueous solution of 1% w/w PPG725 1.1509 0.0204 -3.0533 1.1550 0.0193 -2.7351 1.1064 0.0198 -2.4607 1.1031 0.0139 -2.2550 0.9892 0.0162 -2.0836 Glycine in waterb 1.5384 0.0191 -3.4449 1.3516 0.0180 -2.9158 1.2129 0.0178 -2.6398 1.1729 0.0180 -2.4163 1.0277 0.0175 -2.2835 Glycine in aqueous solution of 1% w/w PPG725 1.2462 0.0091 -3.0827 1.1605 0.0103 -2.8239 1.0973 0.0118 -2.6197 1.0743 0.0107 -2.4509 0.9212 0.0123 -2.3181 Serine in waterb 2.0118 0.0652 -3.8398 1.7415 0.0672 -3.2525 1.2714 0.0520 -2.9304 1.1011 0.0555 -2.6588 1.0884 0.0506 -2.5154 Serine in aqueous solution of 1% w/w PPG725 1.2844 0.0084 -3.5408 1.2258 0.0172 -3.2395 1.1371 0.0211 -2.9651 1.0730 0.0115 -2.7292 0.9588 0.0124 -2.5679 Proline in water 0.0098 0.0206 -3.1659 0.0617 0.0212 -2.7700 -0.0111 0.0222 -2.3916d -0.0371 0.0231 -2.0727 -0.1195 0.0215 -1.8492 Proline in aqueous solution of 1% w/w PPG725

105.sd ( K φ ) / (cm3·mol-1·kPa-1)

0.4873 0.4044 0.3449 0.3193 0.3048

0.0151 0.0184 0.0218 0.0215 0.0177

0.6978 0.5297 0.3930 0.3516 0.3818

0.0237 0.0128 0.0104 0.0102 0.0120

1.4594 0.5614 0.3799 0.1570 0.1523

0.0287 0.0153 0.0110 0.0126 0.0103

0.4242 0.3296 0.3160 0.2738 0.2900

0.0126 0.0133 0.0074 0.0116 0.0111

1.7206 0.5611 0.2868 -0.0437 0.0280

0.0140 0.0088 0.0117 0.0087 0.0094

0.7826 0.5857 0.4165 0.1532 0.1822

0.0075 0.0090 0.0108 0.0122 0.0080

1.0673 1.1186 1.0438 0.9167 1.0815

0.0243 0.0231 0.0210 0.0215 0.0294

23 288.15 293.15 298.15 303.15 308.15

81.7123 82.1928 82.6184 82.9866 83.3954

0.4482 0.4341 0.4415 0.5123 0.2507

0.0202 0.0242 0.0231 0.0181 0.0174

a

Vφo

b

o φ

The density and sound velocity data for calculation of The density and sound velocity data for calculation of

c

Some of literature

d

Some of literature

V

-3.1104 -2.7153 -2.3280 -2.0272 -1.7689

0.9142 0.9369 0.7346 0.6840 0.6557

and

Kφo

were taken from reference [19].

and

o φ

were taken from reference [20].

K

Vφo

values are: 82.83 [13], 82.69 [14], 82.63 [15], 82.50 [16], 82.65 [17],

Kφo

values are: -2.325 [13], -2.411 [16], -2.34 [18]

0.0211 0.0220 0.0163 0.0157 0.0156

24

Table 4 Fitting parameters a, b and c of equation (3). system PPG725 in water PPG725 in aqueous solutions of 0.2 mol.kg-1 alanine PPG725 in aqueous solutions of 0.5 mol.kg-1 alanine PPG725 in aqueous solutions of 0.2 mol.kg-1 glycine PPG725 in aqueous solutions of 0.5 mol.kg-1 glycine PPG725 in aqueous solutions of 0.2 mol.kg-1 serine PPG725 in aqueous solutions of 0.5 mol.kg-1 serine PPG725 in aqueous solutions of 0.2 mol.kg-1 proline PPG725 in aqueous solutions of 0.5 mol.kg-1 proline Alanine in water Glycine in water Serine in water Proline in water Alanine in aqueous solution of 1% w/w PPG725 Glycine in aqueous solution of 1% w/w PPG725 Serine in aqueous solution of 1% w/w PPG725 Proline in aqueous solution of 1% w/w PPG725

a

parameter b

c

527.6781 734.9425 157.4828 184.3861 224.2507 673.8932 193.3032 237.2771 17.0845 -1016.6494 -1712.2152 -2082.7257 -736.0473 -724.0252 -1315.7771 -1361.9040 -635.2716

17.7588 24.2069 6.1873 6.9067 7.9553 22.1199 7.2946 8.6599 2.2804 -30.7296 -51.0840 -62.3420 -21.3165 -21.4829 -38.8509 -40.1264 -18.0674

-137.0886 -193.0205 -37.0712 -43.9629 -54.1458 -175.9847 -46.7154 -58.5790 -0.6389 282.1646 462.8960 565.1048 208.3052 202.7697 356.2549 371.2720 180.7519

25 Table 5 Infinite dilution Hydration numbers of the amino acids in water and in aqueous PPG725 solution, calculated from volumetric and compressibility method at different temperatures and 0.834 atm Amino acid Alanine in water Glycine in water Serine in water Proline in water Alanine in aqueous solution of PPG725 0.01 w/w Glycine in aqueous solution of PPG725 0.01 w/w Serine in aqueous solution of PPG725 0.01 w/w Proline in aqueous solution of PPG725 0.01 w/w

n H0

from volumerty T=298.15 K 3.439 2.652 3.874 2.927 3.458 2.631 3.863 2.958

n H0 T=293.15 K 3.711 3.873 4.319 3.733 3.768 3.797 4.344 3.700

from compressibility

T=298.15 K 3.429 3.598 3.995 3.329 3.486 3.603 4.071 3.277

T=303.15 K 3.215 3.368 3.710 2.975 3.271 3.437 3.826 2.939

T=308.15K 3.036 3.236 3.568 2.724 3.086 3.302 3.659 2.640

26 Figure Caption Figure 1. Plot of apparent molar volume ( Vφ , p , Vφ , a ), against molality of solute (mp, ma) at T =

298.15 K. (A): PPG725 (p) in water and in aqueous solutions of 0.2 mol·kg-1 and 0.5 mol·kg-1 amino acids, and (B): amino acids (a) in water and in aqueous solution of 0.01 w/w PPG725. Figure 2. (A): Plot of infinite dilution apparent molar volume of transfer of PPG725 from water to

aqueous solutions of 0.2 mol·kg-1 and 0.5 mol·kg-1 amino acids, ∆Vφo,trs , p , against temperature and (B): Infinite dilution apparent molar volume of transfer of amino acids from water to aqueous solution of 0.01 w/w PPG725, ∆Vφo,trs , a , against temperature. Figure 3. Temperature dependence of infinite dilution apparent molar expansibility ( Eφ0, p , Eφ0,a ).

(A): PPG725 (p) in water and in aqueous solutions of 0.2 mol·kg-1 and 0.5 mol·kg-1 amino acids, and (B): amino acids (a) in water and in aqueous solution of 0.01 w/w PPG725. Figure 4. Temperature dependence of infinite dilution apparent molar expansibilities of some solutes in pure water.

Figure 5. plot of (

∂ 2Vφ0

) p against temperature for (A): PPG725 in water and in aqueous solutions ∂T 2 of 0.2 mol·kg-1 and 0.5 mol·kg-1 amino acids and (B): amino acids in water and in aqueous solution of 0.01 w/w PPG725.

Figure 6. Temperature dependence of (

∂ 2Vφ0 ∂T 2

) p of some solutes in pure water.

Figure 7. Plot of isentropic compressibility of solution ( κ s ) against molality of solute (mp or ma). (A): PPG725 (p) in water and in aqueous solutions of 0.2 mol·kg-1 and 0.5 mol·kg-1 amino acids, and (B): Amino acids (a) in water and in aqueous solution of 0.01 w/w PPG725. Figure 8. (A): Plot of infinite dilution apparent molar isentropic compressibility of transfer of

PPG725 from water to aqueous solutions of 0.2 mol·kg-1 and 0.5 mol·kg-1 amino acids, ∆K φo,trs , p , against temperature and (B): Infinite dilution apparent molar isentropic compressibility of transfer of amino acids from water to aqueous solution of 0.01 w/w PPG725, ∆K φo,trs ,a , against temperature.

27

100

53.30

82.9

95

53.25

82.7

90 53.20

82.5 0

85

0.1

0.2

0.3

Vφ,α / (cm3.mol-1)

Vφ, p /(cm3.mol-1)

53.15 53.10 53.05 53.00

80

61.0

75

60.7

70

60.4

proline in water proline in PPG725 1% serine in water serine in PPG725 1% alanine in water alanine in PPG725 1% glycine in water glycine in PPG725 1%

60.1

65

0.05

0.17

0.29

0.41

0.15

0.2

60 52.95 water glycine (0.5 m) proline (0.5 m) glycine (0.2 m) proline (0.2 m)

52.90 52.85

43.5

55

alanine (0.5 m) serine (0.5 m) alanine (0.2 m) serine (0.2 m)

43.4 43.3

50

43.2

45

0.05

0.1

0.25

40

52.80 0

0.05

0.1

0.15

0.2

0.25

mp / (mol.kg-1)

Figure 1A

0.3

0.35

0.4

0.02

0.07

0.12

ma / (mol.kg-1)

Figure 1B

0.17

0.22

28

0.0

0.2

-0.1

alanine

glycine

serine

proline

∆ V0φ , a / (cm3.mol-1)

∆ V0φ ,p / (cm3.mol-1)

0.1 -0.2

-0.3

0.0

-0.4 -0.1 alanine (0.2 m) serine (0.2 m) alanine (0.5 m) serine (0.5 m)

-0.5

glycine (0.2 m) proline (0.2 m) glycine (0.5 m) proline (0.5 m)

-0.6 285

290

295

300

T /(K)

Figure 2A

305

310

-0.2 285

290

295

300

T / (K)

Figure 2B

305

310

29

0.070

0.13

0.065

0.060

E0φ,α / (cm3.mol-1.K-1)

E0φ, p / (cm3.mol-1.K-1)

0.11

0.055

0.050

water gkycine (0.2 m) proline (0.2 m) glycine (0.5 m) proline (0.5 m)

0.045

0.09

0.07

0.05

alanine (0.2 m) serine (0.2 m) alanine (0.5 m) serine (0.5 m)

0.040

alanine in water

glycine in water

serine in water

proline in water

alanine in PPG725 1%

glycine in PPG725 1%

serine in PPG725 1%

proline in PPG725 1%

0.03 285

290

295

300

T / (K)

Figure 3A

305

310

285

290

295

300

T / (K)

Figure 3B

305

310

30

0.12

E0φ / (cm3.mol-1.K-1)

0.10 0.08 0.06 0.04 0.02 alanine serine PPG725 PEG6000 PEGDME500 PVP 1-propanol

0.00 -0.02

glycine proline PPG400 PEGDME2000 PEGDME250 ethanol 1-butanol

-0.04 285

290

295

300

T / (K)

Figure 4

305

310

31

1.40 water glycine (0.2 m) proline (0.2 m) glycine (0.5 m) proline (0.5 m)

-1.2

103.(δ δ E0φ ,a / δ T )p /(cm3.mol-1.K-2)

103.(δ E0φ, p /δ T)p / (cm3.mol-1.K-2)

1.20

alanine (0.2 m) serine (0.2 m) alanine (0.5 m) serine (0.5 m)

1.00

0.80

0.60

0.40

0.20

-1.8

-2.3

-2.9

-3.4

-4.0 0.00

alanine in water

glycine in water

serine in water

proline in water

alanine in PPG725 1%

glycine in PPG725 1%

serine in PPG725 1%

proline in PPG725 1%

-4.5

-0.20 285

290

295

300

T / (K)

Figure 5A

305

310

285

290

295

300

T / (K)

Figure 5B

305

32

alanine glycine serine proline PPG 725 PPG 400 PEG 6000 PEGDME 2000 PEGDME 500 PEGDME 250 PVP ethanol 1-propanol 1-butanol

4.0

103.(δ δ E0φ / δ T )p /(cm3.mol-1.K-2)

3.0 2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 285

290

295

300

T / (K)

Figure 6

305

310

33

4.50

4.65

4.60

4.45

4.55 4.40

4.35

107.κ s / (kPa-1)

107. κ s / (kPa-1)

4.50

4.30 water alanine (0.2 m) glycine (0.2 m) serine (0.2 m) proline (0.2 m) alanine (0.5 m) glycine (0.5 m) serine (0.5 m) proline (0.5 m)

4.25

4.20

4.45 4.40 4.35 4.30

alanine in water (288.15 K) glycine in water (288.15 K) serine in water (288.15 K) proline in water (288.15 K) alanine in water (298.15 K) glycine in water (298.15 K) serine in water (298.15 K) proline in water (298.15 K)

4.25

4.15

alanine in PPG 725 1% (288.15 K) glycine in PPG 725 1% (288.15 K) serine in PPG 725 1% (288.15 K) proline in PPG 725 1% (288.15 K) alanine in PPG 725 1% (298.15 K) glycine in PPG 725 1% (298.15 K) serine in PPG 725 1% (298.15 K) proline in PPG 725 1% (298.15 K)

4.20 0

0.05

0.1

0.15

0.2

mp / (mol.kg-1)

Figure 7A

0.25

0.3

0.35

0.4

0.00

0.05

0.10

0.15

ma / (mol.kg-1)

Figure 7B

0.20

0.25

0.30

34 0.4 0 alanine

0.3 105.∆ K0φ ,a / (cm3.mol-1.kPa-1)

105.∆ K0φ , p / (cm3.mol-1.kPa-1)

-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7

alanine (0.2 m)

glycine (0.2 m)

serine (0.2 m)

proline (0.2 m)

alanine (0.5 m)

glycine (0.5 m)

serine (0.5 m)

proline (0.5 )

glycine serine

0.2 proline

0.1

0.0

-0.1

-0.8 285

290

295

300 T / (K)

Figure 8A

305

310

285

290

295

300 T / (K)

Figure 8B

305

310

35 • Ability of amino acids in decreasing Vφ of PPG increases by their hydrophobicity • ∆Vφo,trs and ∆K φo,trs for PPG are negative and independent of temperature • (

∂Eφ0

) p shows PPG and amino acid respectively are structure-maker and -breaker solutes ∂T • Structuring effect of solutes becomes smaller by temperature ∂Eφ0 • Magnitudes of ( ) p , K φo and n H0 follow the order serine 〉 glycine 〉 alanine 〉 proline ∂T