Effect of dipotassium hydrogen phosphate on thermodynamic properties of glycine and L-alanine in aqueous solutions at different temperatures Harsh Kumar ⇑ , Kirtanjot Kaur Department of Chemistry, Dr. B.R. Ambedkar National Institute of Technology, Jalandhar 144 011, Punjab, India article info Article history: Received 24 March 2012 Received in revised form 18 April 2012 Accepted 18 April 2012 Available online 26 April 2012 Keywords: Apparent molar volume Apparent molar compressibility Amino acids Dipotassium hydrogen phosphate Hydration number abstract Densities, q, speed of sound, u for glycine, L-alanine have been measured in aqueous solutions of dipotas- sium hydrogen phosphate (DKHP) ranging from 0.2, 0.4, 0.6 and 0.8 molkg 1 at temperatures T = (288.15, 298.15, 308.15 and 318.15) K. The different parameters such as apparent molar volume, limiting apparent molar volume, transfer volume, partial molar expansibility have been derived from density data. Exper- imental speeds of sound data were used to estimate apparent molar adiabatic compressibility, limiting apparent molar adiabatic compressibility, transfer parameter and hydration number. These parameters have been discussed in the light of ion-ion and ion-solvent interactions. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Dipotassium hydrogen phosphate is a highly water-soluble salt which is often used as a fertilizer, food additive and buffering agent [1,2]. It is a common source of phosphorus and potassium. Dipotas- sium hydrogen phosphate recognized as a safe food additive is used in non-dairy creamers to prevent coagulation. It is also used as nutrient in microorganism cultures to produce antibiotics in pharmaceutical industry. Electrolytes in general influence the properties of biological molecules like proteins [3,4]. Proteins are complex molecules and their interactional behaviour is very diffi- cult to understand due to many specific interactions. Amino acids are the low molar mass model compounds or building blocks of proteins that can be used for studies which are expected to set im- pact on the solvation and conformation of proteins [5,6]. Studies on thermodynamic properties of biological molecules in aqueous solution are very important as many biochemical processes occur in aqueous solutions. Moreover, the electrolytes present in our body participate in many biological functions. Thermodynamic studies on amino acids are mainly focused on investigation of interaction present between amino acids and electrolytes as well as protein denaturation, aggregation and the hydration phenome- non [7,8]. Extensive work [9–18] has been done on thermodynamic properties of amino acids in aqueous electrolyte solutions, carbo- hydrates etc. However, thermodynamic studies are not very much directed towards the interactions of amino acids with organic salts of biological and industrial importance [19–21] like citrates and phosphates which are used in food, cosmetic, chemical and phar- maceutical industries and they are of significant importance in many biochemical processes [22–24]. Our main aim is to study the interactional behaviour of amino acids with these salts of bio- logical and industrial importance which will further help in better understanding of these classes of compounds. In earlier study [25] we studied the apparent molar volumes and transport behaviour of glycine and L-valine in aqueous solutions of tripotassium citrate at T = (288.15, 298.15, 308.15 and 308.15) K. As a part of our research program on thermodynamics studies of amino acids with salts of citrates and phosphates, here we have taken dipotassium hydrogen phosphate, the thermodynamic studies on which are very scarce. As per our knowledge, studies on liquid–liquid equilibria [26,27], osmotic and activity coefficients [28] of two phase systems con- taining PEG, 1-butyl-3-methylimidazolium bromide and dipotas- sium hydrogen phosphate have been carried out but no work has been done on thermodynamic studies of amino acids and dipotas- sium hydrogen phosphate mixtures. In the present study, we re- port the density and speeds of sound of glycine and L-alanine in (0.2, 0.4, 0.6, and 0.8) molkg 1 solutions of dipotassium hydrogen phosphate at T = (288.15, 298.15, 308.15 and 308.15) K. 2. Experimental Glycine, L-alanine, and dipotassium hydrogen phosphate with mass fraction purities > 0.99 were obtained from Merck, Germany. They were used as such without further purification. However, be- fore use, the amino acids were dried under vacuum. Thereafter, 0021-9614/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jct.2012.04.020 ⇑ Corresponding author. E-mail addresses: [email protected], [email protected](H. Kumar). J. Chem. Thermodynamics 53 (2012) 86–92 Contents lists available at SciVerse ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct
Transcript
J. Chem. Thermodynamics 53 (2012) 86–92
Contents lists available at SciVerse ScienceDirect
J. Chem. Thermodynamics
journal homepage: www.elsevier .com/locate / jc t
Effect of dipotassium hydrogen phosphate on thermodynamic propertiesof glycine and L-alanine in aqueous solutions at different temperatures
Harsh Kumar ⇑, Kirtanjot KaurDepartment of Chemistry, Dr. B.R. Ambedkar National Institute of Technology, Jalandhar 144 011, Punjab, India
a r t i c l e i n f o
Article history:Received 24 March 2012Received in revised form 18 April 2012Accepted 18 April 2012Available online 26 April 2012
Keywords:Apparent molar volumeApparent molar compressibilityAmino acidsDipotassium hydrogen phosphateHydration number
0021-9614/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.jct.2012.04.020
Densities, q, speed of sound, u for glycine, L-alanine have been measured in aqueous solutions of dipotas-sium hydrogen phosphate (DKHP) ranging from 0.2, 0.4, 0.6 and 0.8 mol�kg�1 at temperatures T = (288.15,298.15, 308.15 and 318.15) K. The different parameters such as apparent molar volume, limiting apparentmolar volume, transfer volume, partial molar expansibility have been derived from density data. Exper-imental speeds of sound data were used to estimate apparent molar adiabatic compressibility, limitingapparent molar adiabatic compressibility, transfer parameter and hydration number. These parametershave been discussed in the light of ion-ion and ion-solvent interactions.
� 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Dipotassium hydrogen phosphate is a highly water-soluble saltwhich is often used as a fertilizer, food additive and buffering agent[1,2]. It is a common source of phosphorus and potassium. Dipotas-sium hydrogen phosphate recognized as a safe food additive isused in non-dairy creamers to prevent coagulation. It is also usedas nutrient in microorganism cultures to produce antibiotics inpharmaceutical industry. Electrolytes in general influence theproperties of biological molecules like proteins [3,4]. Proteins arecomplex molecules and their interactional behaviour is very diffi-cult to understand due to many specific interactions. Amino acidsare the low molar mass model compounds or building blocks ofproteins that can be used for studies which are expected to set im-pact on the solvation and conformation of proteins [5,6]. Studies onthermodynamic properties of biological molecules in aqueoussolution are very important as many biochemical processes occurin aqueous solutions. Moreover, the electrolytes present in ourbody participate in many biological functions. Thermodynamicstudies on amino acids are mainly focused on investigation ofinteraction present between amino acids and electrolytes as wellas protein denaturation, aggregation and the hydration phenome-non [7,8]. Extensive work [9–18] has been done on thermodynamicproperties of amino acids in aqueous electrolyte solutions, carbo-hydrates etc. However, thermodynamic studies are not very muchdirected towards the interactions of amino acids with organic salts
of biological and industrial importance [19–21] like citrates andphosphates which are used in food, cosmetic, chemical and phar-maceutical industries and they are of significant importance inmany biochemical processes [22–24]. Our main aim is to studythe interactional behaviour of amino acids with these salts of bio-logical and industrial importance which will further help in betterunderstanding of these classes of compounds. In earlier study [25]we studied the apparent molar volumes and transport behaviour ofglycine and L-valine in aqueous solutions of tripotassium citrate atT = (288.15, 298.15, 308.15 and 308.15) K. As a part of our researchprogram on thermodynamics studies of amino acids with salts ofcitrates and phosphates, here we have taken dipotassium hydrogenphosphate, the thermodynamic studies on which are very scarce.As per our knowledge, studies on liquid–liquid equilibria [26,27],osmotic and activity coefficients [28] of two phase systems con-taining PEG, 1-butyl-3-methylimidazolium bromide and dipotas-sium hydrogen phosphate have been carried out but no work hasbeen done on thermodynamic studies of amino acids and dipotas-sium hydrogen phosphate mixtures. In the present study, we re-port the density and speeds of sound of glycine and L-alanine in(0.2, 0.4, 0.6, and 0.8) mol�kg�1 solutions of dipotassium hydrogenphosphate at T = (288.15, 298.15, 308.15 and 308.15) K.
2. Experimental
Glycine, L-alanine, and dipotassium hydrogen phosphate withmass fraction purities > 0.99 were obtained from Merck, Germany.They were used as such without further purification. However, be-fore use, the amino acids were dried under vacuum. Thereafter,
IGURE 1. Plot of V/ for glycine in aqueous dipotassium hydrogen phosphate at= 318.5 K. (s, 0.2 mol�kg�1; D, 0.4 mol�kg�1; h, 0.6 mol�kg�1; e, 0.8 mol�kg�1).
FIGURE 2. Plot of V/ for L-alanine in aqueous dipotassium hydrogen phosphate atT = 318.5 K. (s, 0.2 mol�kg�1; D, 0.4 mol�kg�1; h, 0.6 mol�kg �1; e, 0.8 mol�kg�1).
H. Kumar, K. Kaur / J. Chem. Thermodynamics 53 (2012) 86–92 87
they were stored over P2O5 in desiccators for minimum of 48 h be-fore use. Double distilled water (specific conduc-tance < 10�6 S�cm�1) which has been freshly degassed was usedfor the preparation of the aqueous solutions. The details of thechemicals used in the present work are also given in table 1. Allthe solutions of dipotassium hydrogen phosphate with water andwith amino acids were prepared by mass on molality concentra-tion scale on a Sartorius BP 210 S balance having precision±0.0001 g. Uncertainty in the solution concentration was estimatedat ±2 � 10�5 mol�kg�1 in calculations. All the samples were madeafresh before use.
The densities, q, and speed of sound, u, of the solutions weresimultaneously, and automatically measured, using an Anton PaarDSA 5000 M densimeter. A density check or an air/water adjust-ment was performed at 20 �C with doubly distilled, degassedwater, and with dry air at atmospheric pressure. Before each seriesof measurements the densimeter was calibrated with double dis-tilled and degassed water, cyclopentane, propanol and benzenein the experimental temperature range. Both the density and speedof sound are extremely sensitive to temperature, so it was con-trolled to ±1 � 10�3 K by built-in Peltier device. The sensitivity ofthe instrument corresponds to a precision in density and speedof sound measurements of 1 � 10�3 kg�m�3 and 1 � 10�2 m�s�1.The uncertainty of the density and speed of sound are ±3 �10�3 kg�m�3 and ±1 � 10�1 m�s�1, respectively.
3. Results and discussion
The densities, q, and apparent molar volumes V/ of glycine andL-alanine in binary aqueous solutions of dipotassium hydrogenphosphate (0.2, 0.4, 0.6 and 0.8) mol�kg�1 measured at tempera-tures (288.15, 298.15, 308.15 and 318.15) K are reported in tableS1. The measured speed of sounds u and values of the apparentmolar adiabatic compressibility, K/;s are reported in table S2 insupporting information. The representative equations are asfollows:
V/ ¼ M=q� 1000ðq� q0Þ=mqq0; ð1Þ
K/;s ¼ MbS=qþ 1000=mqq0ðbSq0 � bS;0qÞ; ð2Þ
where m is the molality (mol�kg�1) of the solution, M is the molarmass of the solute (kg�mol�1) and q0 and q are the densities(kg�m�3) of the solvent and solution. bS,0, and bS are the coefficientof adiabatic compressibility of pure solvent and solution, respec-tively. The coefficient of isentropic compressibility can be definedfrom Laplace–Newton’s equation
bS ¼ 1=ðu2qÞ; ð3Þ
where u is the speed of sound and q is the density of the solution.The magnitude of positive values of V/ increases with increase
in temperature and in each concentration of potassium dihydrogenphosphate. The values of V/ against the molality of glycine andL-alanine at 318.15 K are also graphically represented in figures 1and 2. Furthermore, the values of V/ increase as we move fromglycine to L-alanine at all temperatures that may be due to increasein chain length and also due to the hydrophobicity of the chain,which causes greater affinity for solvent and therefore enhancesgreater ion–solvent interactions. The positive values of V/ indicate
greater solute–solvent interactions. The values of K/;s as reportedin table S2 are mostly negative and magnitude of negative valuesof K/;s decreases with increase in concentration and increase intemperature. The K/;s values have been plotted for different valuesof molality of glycine in 0.2 mol�kg�1 concentration of DKHP at dif-ferent temperature in figure 3. The negative K/;s values show thatwater molecules around the solute are less compressible thanwater molecules in the bulk which is attributed to strong attractiveinteractions similarly as in the case of V/.
The variation of apparent molar volumes V/ and apparent molaradiabatic compressibility K/;s with the molal concentration can beadequately represented by following equations:
FIGURE 3. Plot of K/;s for glycine in 0.2 mol�kg �1 dipotassium hydrogen phosphateat different temperatures (s, T = 288.15 K; D, 0.4, 298.15 K; h, 308.15 K; e,318.15 K).
88 H. Kumar, K. Kaur / J. Chem. Thermodynamics 53 (2012) 86–92
V/ ¼ V0/ þ S�V m; ð4Þ
K/;s ¼ K0/;s þ S�K m; ð5Þ
where V0/ the limiting value of apparent molar volume is equal to
the infinite dilution partial molar volume, K0/;s is the limiting value
of isentropic compressibility and S�V ; S�K are the experimental slopes
indicative of solute–solute interactions. The values of V0/ and S�V and
K0/;s and S�K together with standard errors derived by least squares
fitting of the V/ and K/;s values to equations (4) and (5) are reportedin tables 2 and 3, respectively. The valuable information regardingion–solvent interactions may be obtained from the temperaturedependence of standard partial molar property as ion-ion interac-tions are negligible at infinite dilution [29,30]. Table 2 shows thatvalues of V0
/ of amino acids in aqueous DKHP solutions are positiveand increases with increase in the salt concentration and tempera-ture. At infinite dilution each ion is surrounded by solvent mole-cules and being infinitely distant with other ions. It followstherefore that V0
/ is free from ion-ion interactions thereby showingthe presence of strong ion–solvent interactions. The increase in V0
/
for amino acids in aqueous DKHP solutions may be attributed tothe increase in solvation of amino acids at higher temperature as
TABLE 2Limiting apparent molar volumes, V0
/ , and experimental slopes, S�V of glycine and L-alanin
well as at higher region of salt i.e. release of some solvent moleculesfrom loose salvation layers of the solute in solution. The tempera-ture and concentration dependence of V0
/ for glycine and L-alaninein different concentrations of DKHP have been graphically repre-sented in figure 4 which shows that the values of V0
/ increases aswe increase the concentration of salt, this is attributed to strongattractive interactions due to hydration of ions and by increasingthe salt concentration ion-ion interaction increases. In general V0
/
values increases with increase in molar mass of amino acids andalso due to the hydrophobicity of alkyl side chain of amino acids[31,32]. The values of V0
/ for L-alanine are higher than glycine be-cause of the longer side chain in case of L-alanine which in effectundergoes greater ion-ion interactions.
The values of K0/;s as reported in table 3 are negative and val-
ues become less negative with increase in concentration as wellas temperature. These values for all amino acids are less nega-tive than their corresponding values in water. The negative val-ues of K0
/;s (loss of compressibility of medium) indicate that thewater molecules surrounding the amino acids would presentgreater resistance to compression than water molecules presentin bulk. The more negative values of K0
/;s for amino acids atlow temperature are attributed to the strong attractive interac-tions between amino acids and water [33]. With increase in tem-perature the K0
/;s values become less negative which means thatelectrostriction reduces and some water molecules are releasedto bulk. The strong attractive interactions due to hydration ofions produced from the dissociation of dipotassium hydrogenphosphate induce the dehydration of amino acid and increasethe water molecules in bulk. The electrostriction interaction be-tween amino acids and water molecules are suppressed due toformation of ion pairs between ion of DKHP and amino acids.Furthermore, the attractive interactions between ions of DKHPand water molecules induces the dehydration of amino acidsand therefore at high DKHP concentrations the water moleculesaround the amino acids are more compressible than those atlower DKHP concentrations.
The positive values of the slope S�V and S�K , respectively suggestthe presence of solute–solute interactions in the system. The posi-tive but lesser values of S�V as compared to V0
/ suggest the weakersolute–solute interactions as compared to stronger solute–solventinteractions. An irregular trend has been observed in the values ofS�V and S�K which are indicative of solute–solute interactions areinfluenced by number of effects [34].
The partial molar volume of transfer DV0/, and partial molar adi-
abatic compressibility DK0/;S of each amino acid from water to
aqueous potassium dihydrogen phosphate solutions at infinitedilution were calculated by using the equations:
DV0/ ¼ V0
/ðin aq:KDP solutionÞ � V0/ðin waterÞ; ð6Þ
e in aqueous solutions of DKHP at different temperatures.
FIGURE 4. Plot of V0/ for glycine (s, T = 288.15 K; D, 0.4, 298.15 K; h, 308.15 K; e,
318.15 K) and L-alanine (�, 288.15 K; N, 298.15 K; j, 308.15 K; �, 318.15 K) indifferent concentrations of aqueous dipotassium hydrogen phosphate.
H. Kumar, K. Kaur / J. Chem. Thermodynamics 53 (2012) 86–92 89
DK0/;S ¼ K0
/;sðin aq:KDP solutionÞ � K0/;sðin waterÞ: ð7Þ
The calculated values of DV0/ and DK0
/;S are reported in table 4.The DV0
/ values as shown in figure 5 are positive and these positiveDV0
/ values can be explained on the basis that salt interacts directlywith the amino acids through electrostatic interactions with thecharged centres of the amino acids, thereby leading to reductionin the electrostriction of the solvent. This may be explained onthe basis of the co-sphere overlap model [35,36]. This model pre-dicts that for apolar species, the hydrophobic hydration gives a po-sitive value contribution. This is because the overlap of hydrophilichydration co-spheres releases some water molecules to the bulkwhich gives rise to positive change in the volume. It is known thatat infinite dilution solute–solute interactions are absent so thepositive transfer volumes as per co-sphere overlap model are dueto solute–solvent interactions which in present study are betweenamino acids and DKHP and can be further classified as the ion-ioninteraction between K+ and carboxylate ions and also ion-ion inter-action between HPO4
2� and ammonium ions and ion–non polargroup interactions will lead to negative transfer volumes. Thismeans that hydrophilic-hydrophilic interactions are predominant
over hydrophilic-hydrophobic interactions. This positive and high-er values of transfer volume is attributed to the fact that DPHP car-ry the anion, i.e. Phosphate anion(PO4
3�) with higher valence, i.e.,HPO4� and two K+ cations which will have greater affinity forhydrogen bonding with the zwitterionic centres of amino acid.
The values of DK0/;S as reported in table 4 and graphically repre-
sented in figure 6 for glycine and L-alanine in different concentra-tions of DKHP at T = 288.15, 298.15, 308.15 K, and 318.15 K arepositive and increase with a rise in concentration of DKHP. Themore positive values of DK0
/;S for glycine and L-alanine indicatethe dominance of the charged end groups NH3
+ and COO–. Theinteractions between the DKHP and zwitter ionic centre of aminoacids increases with increasing dipotassium hydrogen phosphateconcentration because of the structure making tendency of theions increase. As a result the electrostricted water is much lesscompressible than bulk water and leads to large decrease in thecompressibility with increase in DKHP concentration. Thus K0
/;s val-ues are negative and DK0
/;S values are positive for all the aminoacids which is similar to the behaviour observed for transfer vol-ume i.e. a regular increase with increase in concentration and tem-perature is observed and support our volumetric data. This can beexplained on the basis that anion-water interactions for anionswith higher valence are stronger than anion water interactionsfor anions with lower valence. In the case of ion with higher va-lence, like in our case HPO4
2�, more water molecules were associ-ated with the anion compared to the anion with lower valence andhence ion association (cation-anion interactions), a greater numberof water molecules are released into the bulk thereby making themedium more compressible and justifies that stronger cation–an-ion interactions or ion association are present in DKHP with twopotassium ions which is larger and predominant. The similar effecthas been explained where the phosphate salts of sodium has beentaken [37].
The V0/ values are related to the temperature as:
V0/ ¼ aþ bT þ cT2; ð8Þ
where T is the temperature in degrees Kelvin and a, b, and c areconstants.
The partial molar expansibilities at infinite dilution /0E were ob-
tained by differentiating equation (8) with respect to temperature
/0E ¼ ð@V0
/=@TÞP ¼ bþ 2cT: ð9Þ
The calculated values of /0E for the amino acids at different
temperature are given in table 5. The values of /0E decrease with
increase in temperature in case of glycine whereas in case ofL-alanine the values of /0
E increase with increase in temperaturein aqueous DKHP solutions. However with increase in concentra-tion of DKHP the value of /0
E for glycine and L-alanine decrease
TABLE 4Partial molar volume of transfer, DV0
/ and partial molar isentropic compressibility of transfer, DK0/;S of glycine and L-alanine in aqueous solutions DKHP solutions at different
temperatures.
m /(mol�kg�1) DV0/�106 / (m3�mol�1) DK0
/;S�106 / (m3�mol�1�GPa�1)
T = 288.15 K 298.15 K 308.15 K 318.15 K 288.15 K 298.15 K 308.15 K 318.15 K
90 H. Kumar, K. Kaur / J. Chem. Thermodynamics 53 (2012) 86–92
except at 0.8 mol�kg�1 in case of glycine and at T = 318.15 K in caseof L-alanine where it increases with increase in concentration ofDKHP. The positive values of /0
E for both glycine and L-alanine fa-vour solute–solute interactions. This indicates the release of elec-trostricted water from the loose salvation layers of the aminoacids at higher temperatures and hence favouring amino acid-ami-no acid or amino acid-DKHP interactions. The values of partial mo-lar expansibility give information regarding the size of solute andits hydrophobicity. It is observed from table 5 that L-alanine, beingthe larger molecule compared with glycine and L-alanine, has thegreatest /0
E values among both of them.The partial molar volume of the amino acids can be examined
by a simple model [38]
V0/ðamino acidÞ ¼ V0
/ðintÞ þ V0/ðelectÞ; ð10Þ
where V0/(elect) is the electrostriction partial molar volume due to
the hydration of the amino acid and can be estimated from experi-mentally measured values of V0
/(amino acid), and V0/(int) is the
intrinsic partial molar volume of the amino acid and has been cal-culated from the following expressions [38]
V0/ðintÞ ¼ ð0:7=0:6ÞV0
/ðcrystÞ ð11Þ
and
V0/ðintÞ ¼ ð0:7=0:634ÞV0
/ðcrystÞ; ð12Þ
where V0/ (cryst) (=mol.�wt./dcryst) is the crystal molar volume, 0.7 is
the packing density for molecules in organic crystals and 0.634 is
TABLE 6Hydration number nH of Glycine, L-alanine in aqueous solutions of DKHP at differenttemperatures.
H. Kumar, K. Kaur / J. Chem. Thermodynamics 53 (2012) 86–92 91
the packing density for random packing spheres. The values of V0/
(int) for the amino acids have been estimated from equations (11)and (12) using dcryst values for glycine and L-alanine (1.598 g�cm3
and 1.371 g�cm3) which are taken from the work of Berlin and Pal-lansch [39].
The change in volume due to electrostriction can be related tothe number of water molecules nH hydrated to the amino acid byfollowing expression [40]
nH ¼ V0/ðelectÞ=ðV0
/;e � V0/;bÞ; ð13Þ
where V0/;e is the molar volume of electrostricted water and V0
/;b isthe molar volume of bulk water (18.069 � 106 m3�mol�1 atT = 298.15 K). The reported value [38] of (V0
/;e � V0/;b) is �3.3 �
10�6 m3�mol�1 at 298.15 K. Using the value of (V0/;e � V0
/;b) and thevalues of V0
/(elect), calculated from both the two methods, the nH
values were estimated from equation (13) and are given in table 6.Further, the number of water molecules nH hydrated to the ami-
no acids were calculated by using the method given by Milleroet al. [38,41]
nH ¼ �K0/;SðelectÞ=V0
/;bK0S;b; ð14Þ
where K0S;b is the isothermal compressibility of bulk water. The va-
lue of V0/;bK0
S;b is 0.81 � 10�5 m3�mol�1�GPa�1. The electrostriction
partial molar compressibility K0/;S (elect) can be calculated from
the experimentally measured values of K0/;S (amino acid) from:
K0/;SðelectÞ ¼ K0
/;Sðamino acidÞ � K0/;SðintÞ; ð15Þ
where K0/;S (int) [28,33] = K0
/;S (isomer) for glycine (2.7 �10�6 m3�mol�1�GPa�1), L-alanine (3.35 � 106 m3�mol�1 GPa�1).Since one would expect K0
/;S (int) to be small and it is less than5 � 106 m3�mol�1�GPa�1 for ionic crystals and many organic solutesin water [38]. So, one can assume K0
/;S (int) 0. Therefore, for K0/;S
(int) 0, equation (15) becomes
K0/;SðintÞ ¼ K0
/;Sðamino acidÞ: ð16Þ
The values of nH calculated from equation (14) using the K0/;S
(elect) values determined by these two methods are also listed intable 6. The values of nH (in water) calculated by several methods,i.e., the volume and compressibility data are in good agreement.The values for all the amino acids in the presence of DKHP are lessthan in water which suggests that interactions between ions ofDKHP with the charged end groups of amino acid becomes stronger
with the concentration of co-solutes. Furthermore, the nH valuesdecreases with the increase in concentration of DKHP that againindicates the increase in the solute-co-solute interactions.
4. Conclusions
Results on density and speeds of sound measurements for gly-cine and L-alanine in aqueous solutions of dipotassium hydrogenphosphate at different temperatures are been reported in the pres-ent study. Experimental density data were used to calculate appar-ent molar volume, limiting apparent molar volume, transfervolume, and partial molar expansibility. Experimental speeds ofsound data were used to estimate apparent molar adiabatic com-pressibility, limiting apparent molar adiabatic compressibility,transfer parameter and hydration number. The apparent molar vol-ume, partial molar volume at infinite dilution and transfer volumesshow positive and increasing values with temperature as well as athigher concentration of salt which indicates greater salvation andinteraction between amino acids and DKHP. The negative valuesof K/;s and K0
/;s support our volumetric data. The partial molarexpansibilities at infinite dilution give information regarding thesize of solute and its hydrophobicity. L-Alanine being the biggermolecule out of glycine and L-alanine has higher /0
E values amongboth of them. The hydration number calculated from volumetricand compressibility data shows the presence of interactions be-tween the ions of DKHP and amino acids.
Acknowledgements
One of the authors (K.K.) is thankful to The Director and Head,Department of Chemistry, Dr B.R. Ambedkar National Institute ofTechnology, Jalandhar for providing MHRD fellowship.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jct.2012.04.020.
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