Volumetric, viscometric, and refractive index behaviour of α-amino acids and their groups’ contribution in aqueous d-glucose solution at different temperatures

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Volumetric, viscometric, and refractive index behaviour of α-amino acids and their groups’ contribution in aqueous d-glucose solution at different temperatures
  ACTA PHYSICO-CHIMICA SINICA Volume 23, Issue 1, January 2007 Online English edition of the Chinese language journal Cite this article as:  Acta Phys. - Chim. Sin., 2007, 23(1), 79 - 83. Received: May 24, 2006; Revised: July 5, 2006. * Corresponding author. Email: anwar_jmi@yahoo.com.in; Tel: +9111-26981717-3257; Fax: +9111-26890229. Copyright   2007, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn ARTICLE Volumetric Viscometric and Refractive Index Behaviors of -Amino Acids in Aqueous Caffeine Solution at Varying Temperatures Anwar Ali * , S. Sabir, M. Tariq Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi-110025, India Abstract:   Measurements of density ( r  ), viscosity ( h ), and refractive index ( n ) were carried out on a -amino acids, DL-alanine (Ala), D-phenylalanine (Phe), and DL-threonine (Thr) (0.01  !  0.05 mol " L  !  1 ), in 0.05 mol " L  !  1  aqueous caffeine solution at 298.15, 303.15, 308.15, and 313.15 K. These measurements have been carried out to evaluate some important parameters, viz., apparent mo-lar volume ( f  v ), partial molar volume ( 0 v f  ), transfer volume ( 0 v f  (tr)), viscosity  A  and  B  coefficients of Jones-Dole equation, free energies of activation per mole of solvent ( #01 m D ) and solute ( #02 m D ), enthalpies ( #  H  * ) and entropies ( # S  * ) of activation of viscous flow, variation of  B  with temperature ( $  B / $ T  )  P  , and molar refractive index (  R  D ). These parameters have been interpreted in terms of solute-solute and solute-solvent interactions and structure making/breaking ability of solutes in the given solution.  Key Words:   Amino acids; Caffeine; Partial molar volume; Viscosity  B  coefficient; Refractive index  In recent years, attempts have been made to study the phys-icochemical behavior of amino acids in aqueous [1]  as well as aqueous-electrolyte [2] , carbohydrate [3] , and surfactant [4]  media. Amino acids (AAs) are the basic components of proteins and are considered to be one of the important model compounds of  protein molecules [5,6] . But because of complex nature of pro-teins, their direct study is somewhat difficult. Therefore, the convenient approach is to study simpler model compounds (amino acids) that are constituents of proteins [7] . AAs when dissolved in water convert into zwitterionic form because of ionization of their carboxyl (  !  COOH) and amino (  !   NH 2 ) groups. In physiological media such as blood membranes and cellular fluids, the dipolar character of amino acids (in pres-ence of certain additives) has an important bearing on their  biological functions. Therefore, a knowledge of water-amino acid interactions in the presence of electrolytes/carbohydrates/ surfactants is necessary to understand several biological proc-esses occurring in living organisms because biological fluids are not pure water but contain inorganic and organic com- pounds. As a continuation of previous studies carried out by the au-thors of this article [8  !  10] , this study attempts to investigate the interaction of amino acids with caffeine. Caffeine is a plant alkaloid obtained from coffee, tea, and kola nuts. It is a heart stimulant, a diuretic, and is widely used in medicine [11] . It is an addictive drug and affects the central nervous system. A sur-vey of published reports indicates that no studies have been carried out on these systems from the point of view of their thermodynamic behavior. In view of the above-mentioned practical importance of the compounds selected for this study, experimentally measured densities ( r  ), viscosities ( h ), and refractive indices ( n ) of aqueous caffeine (0.05 mol " L  !  1 ) and of solutions of amino acids, DL-alanine, D-phenylalanine, and DL-threonine (0.01, 0.02, 0.03, 0.04, and 0.05 mol " L  !  1 ), in aqueous caffeine at 298.15, 303.15, 308.15, and 313.15 K are presented. From the experimental data on r  , h , and n , the apparent molar volume ( f  v ), partial molar volume ( 0 v f  ) and slope ( * v S  ), transfer vol-ume ( 0 v f  (tr)), viscosity  A  and  B  coefficients, free energies of activation per mole of solvent ( #01 m D ) and per mole of solute ( #02 m D ), enthalpies ( #  H  * ) and entropies ( # S  * ) of activation of viscous flow, and molar refraction (  R  D ) were calculated for all   Anwar Ali et al. / Acta Physico-Chimica Sinica, 2007, 23(1): 79 -  83 the amino acids in aqueous caffeine solution. The parameters were used to discuss the solute-solute and solute-solvent in-teractions in the aforementioned mixtures. 1 Experimental methods DL-alanine (Loba Chemie, India, purity >99.0%), D-phenylalanine (Thomas Baker, India, purity >99.0%), and DL-threonine (s.d. Fine Chemicals, purity>99.0%) were used after recrystallization from ethanol+water mixture and dried in vacuum over P 2 O 5  at room temperature for 72 h before use. Caffeine (s.d. Fine Chemicals, India, purity>98.5%) was used as such without further pretreatment. Aqueous caffeine solu-tion of 0.05 mol " L  !  1  concentration was prepared using deion-ized and double-distilled water and used as solvent to prepare 0.01, 0.02, 0.03, 0.04, and 0.05 mol " L  !  1  amino acid solutions. The mixtures were prepared in a dry box using Precisa XB 220A (made in Swiss) electronic balance with a precision of ± 0.0001 g. The mixtures were stored in special airtight bottles to avoid contamination and evaporation. The densities and viscosities of the solutions were measured using a pycnometer and ubbelohde-type suspended level viscometer, respectively, the methodology of which is described in previous by the au-thors of this article [8  !  10] . The accuracy of the measurement of r   and h  was found to be ± 0.1 kg " m - 3  and 3 ´ 10 - 6  N " m - 2 " s, re-spectively. Refractive indices ( n ) were measured using a thermostated Abbe refractometer (Metrex, India) after calibra-tion with distilled water and toluene at known temperatures. Measurements of h  and n  were carried out in triplicate. The error in the measurements of n  was less than ± 0.0001 units. The temperature of the solutions was maintained a constant in an electronically controlled water bath (Julabo, Germany) having a precision of ± 0.02 K. 2 Results and discussion The experimental values of densities ( r ), viscosities ( h ), and refractive indices (n) of amino acids in aqueous caffeine solutions at 298.15, 303.15, 308.15, and 313.15 K are listed in Table 1. The apparent molar volumes ( f  v ) were calculated from the measured density data using the following equation: f  v =1000(   0  !    )/ c   0 +  M  /   0  (1) where r   and   0  are the densities of the solution and solvent (0.05 mol " L  !  1  aqueous caffeine), respectively, c  is the   molar concentration of amino acid, and  M   is its molar mass. The calculated f  v   values are given in Table 2. It was observed that f  v  values varied linearly with amino acid concentration and could be least-squares fitted to the equation: f  v = 0 v f  + * v Sc 1/2  (2) where 0 v f   is the infinite dilution apparent molar volume that equals the standard partial molar volume and * v S   is the ex- perimental slope, which is sometimes considered to be a volumetric pairwise interaction coefficient [12,13] . The standard  partial molar volumes and their * v S   values are shown in Ta- Table 1 Values of experimentally measured densities ( r  ), vis-cosities ( h ), and refractive indices ( n ) of amino acids in aqueous caffeine solutions at 298.15, 303.15, 308.15, and 313.15 K m /(mol " kg - 1 ) 298.15 K 303.15 K 308.15 K 313.15 K DL-alanine +aqueous caffeine r  /(kg " m - 3 ) 0.00 999.9 998.1 996.4 994.6 0.01 999.3 997.6 996.0 994.3 0.02 999.8 998.2 996.7 995.0 0.03 1000.1 998.6 997.1 995.5 0.04 1000.3 998.8 997.4 996.0 0.05 1000.7 999.3 998.0 996.6 10 - 3 h /(N " m - 2 " s) 0.00 0.9681 0.8613 0.7801 0.6839 0.01 0.9154 0.8222 0.7485 0.6660 0.02 0.9228 0.8247 0.7498 0.6620 0.03 0.9338 0.8334 0.7579 0.6675 0.04 0.9405 0.8403 0.7634 0.6732 0.05 0.9476 0.8471 0.7725 0.6820 n  D  0.00 1.3345 1.3340 1.3336 1.3334 0.01 1.3296 1.3295 1.3294 1.3292 0.02 1.3298 1.3297 1.3296 1.3295 0.03 1.3300 1.3298 1.3297 1.3296 0.04 1.3309 1.3305 1.3301 1.3300 0.05 1.3310 1.3308 1.3305 1.3302 D-phenylalanine+aqueous caffeine r  /(kg " m - 3 ) 0.00 999.9 998.1 996.4 994.6 0.01 1000.0 998.3 996.7 995.0 0.02 1000.3 998.7 997.1 995.5 0.03 1000.7 999.1 997.6 996.0 0.04 1000.1 999.6 998.1 996.5 0.05 1001.6 1000.1 998.6 997.1 10 - 3 h /(N " m - 2 " s) 0.00 0.9681 0.8613 0.7801 0.6839 0.01 0.9175 0.8192 0.7447 0.6560 0.02 0.9231 0.8230 0.7479 0.6587 0.03 0.9327 0.8323 0.7554 0.6657 0.04 0.9412 0.8414 0.7638 0.6727 0.05 0.9483 0.8478 0.7713 0.6776 n  D  0.00 1.3345 1.3340 1.3336 1.3334 0.01 1.3358 1.3354 1.3352 1.3350 0.02 1.3360 1.3358 1.3355 1.3352 0.03 1.3362 1.3360 1.3358 1.3357 0.04 1.3371 1.3368 1.3365 1.3362 0.05 1.3379 1.3373 1.3370 1.3368 DL-threonine + aqueous caffeine r  /(kg " m - 3 ) 0.00 999.9 998.1 996.4 994.6 0.01 999.8 998.3 996.7 995.2 0.02 1000.2 998.7 997.1 995.7 0.03 1000.6 999.2 997.6 996.1 0.04 1001.0 999.6 998.1 996.5 0.05 1001.5 1000.0 998.6 996.9 10 - 3 h /(N " m - 2 " s) 0.00 0.9681 0.8613 0.7801 0.6839 0.01 0.9167 0.8187 0.7446 0.6545 0.02 0.9222 0.8240 0.7488 0.6610 0.03 0.9323 0.8312 0.7573 0.6680 0.04 0.9399 0.8415 0.7675 0.6772 0.05 0.9480 0.8507 0.7758 0.6836 n  D  0.00 1.3345 1.3340 1.3336 1.3334 0.01 1.3341 1.3339 1.3335 1.3320 0.02 1.3345 1.3342 1.3338 1.3325 0.03 1.3349 1.3346 1.3342 1.3330 0.04 1.3352 1.3348 1.3345 1.3335 0.05 1.3360 1.3350 1.3347 1.3340   Anwar Ali et al. / Acta Physico-Chimica Sinica, 2007, 23(1): 79 -  83  ble 3. The volumetric behavior of a solute at infinite dilution is satisfactorily represented by 0 v f  , which is independent of the solute-solute interaction   and provides information concerning solute-solvent interactions. It is evident that 0 v f   values are  positive for all the amino acids at all the temperatures investi-gated, suggesting the presence of solute-solvent interactions. A study of the molecular structure of caffeine shows that there are several possible sites at which it can interact with the zwit-terions of AAs. The lone pair electrons on the two O atoms and one on each N atom may attract the + 3  NH ion of AAs. At the same time, the positive charge of CH 3  groups (because of  positive inductive effect of C) may interact with the COO -  terminal of the AAs. Thus, it is expected that the following types of interactions will take place in the mixtures: (i) Ion-dipolar interaction that occurs between zwitterionic centers of amino acids and dipolar parts of caffeine. (ii) Hydrophobic-dipolar interaction that occurs between nonpolar parts of the amino acid and the dipolar part of the caffeine. (iii) Hydrophobic-hydrophobic interactions that occur be-tween the nonpolar parts of amino acids and the hydropholic  part of the caffeine. The observed 0 v f   values (Table 3) are the results of the above-mentioned possible interactions in these systems. The observed 0 v f    values of the three AAs investigated in this study are found to increase in the following order: Thr<Phe<Ala, thereby indicating the trend of solute-solvent interactions. However, because the size of the side-groups of these amino acids is in the sequence Ala<Thr<Phe, it was ex- pected that 0 v f    would follow this trend [14] . Contrary to this, the observed trend in 0 v f   (Thr<Phe<Ala) may be attributed to the shrinkage as a result of the H bond between the  !  OH group of Thr/ p   electrons of benzene ring of Phe and the sur-rounding solvent molecules (the former being stronger than the latter), whereas this interaction is absent in case of Ala. Similar conclusions were arrived at by Iqbal et al  . [14] . The experimental slope ( * v S  ) values (Table 3) were found to be negative at all the investigated temperatures (except at 313.15 K for Thr), suggesting weak solute-solute interactions. Furthermore, the observed increase in * v S   with temperature may be attributed to the increased solute-solute interactions. Thus, the observed trends in * v S  , which are in contrast to those observed in 0 v f  , support the above-mentioned view regarding solute-solvent interactions. The thermodynamic transfer functions may be interpreted in terms of water structure forming or breaking ability of the solute, as has been postulated by Frank and Evans [15] . Thus, the transfer volume ( 0 v f  (tr)) of the AAs from aqueous to aqueous caffeine solution were calculated using the equation: 0 v f  (tr)= 0 v f  (aq. Caffeine)  !  0 v f  (aq.) (3) and is shown in Table 3. The 0 v f  (aq) at 298.15, 308.15, and 313.15 K were obtained from previously published re- ports [16  !  19] . It is found that the 0 v f  (aq. Caffeine) values are higher than 0 v f  (aq) values, resulting in positive transfer vol-umes, except for Thr at 313.15 K. The positive 0 v f  (tr) is at-tributed to the decrease in volume of solution in the presence of caffeine. It is well known that zwitterionic groups of AAs induce a considerable reduction in the volume of the periph-eral solvent because of electrostrictive effect [20] . This electro-strictive effect of AAs is diminished on addition of caffeine  because of amino acid-caffeine interaction. Similar conclu-sions were also drawn by others researchers regarding AAs in Table 2 Values of apparent molar volume ( f  v ) for amino acids, DL-alanine, D-phenylalanine, and DL-threonine in aqueous caffeine at 298.15, 303.15, 308.15, and 313.15 K m /(mol " kg - 1 ) 298.15 K 303.15 K 308.15 K 313.15 K 10 5 f  v  /(m 3 " mol - 1 ) DL-alanine+aqueous caffeine 0.01 14.910 13.935 1.296 11.974 0.02 9.410 8.425 7.436 6.947 0.03 8.243 7.256 6.595 5.941 0.04 7.910 7.173 6.432 5.438 0.05 7.310 6.521 5.730 4.936 D-phenylalanine+aqueous caffeine 0.01 15.521 14.547 13.568 12.587 0.02 14.520 13.545 13.066 12.084 0.03 13.854 13.211 12.564 11.917 0.04 13.520 12.793 12.313 11.833 0.05 13.120 12.543 12.163 11.582 DL-threonine+aqueous caffeine 0.01 12.913 9.931 8.944 5.944 0.02 10.413 8.929 8.442 6.447 0.03 9.580 8.261 7.941 6.950 0.04 9.163 8.178 7.690 7.201 0.05 8.713 8.127 7.539 7.352 Table 3   Values of partial molar volumes ( 0 v f  ), experimental slopes ( * v S  ), 0 v f  (water), and transfer volumes ( 0 v f  (tr)) for amino acids, DL-alanine, D-phenylalanine, and DL-threonine in aqueous caffeine at 298.15, 303.15, 308.15, and 313.15 K 298.15 K 303.15 K 308.15 K 313.15 K DL-alanine+aqueous caffeine 10 50 v f  /(m 3 " mol - 1 ) 19.195 17.976 16.785 16.019 10 6* v S  /(m 3 " mol - 3/2 " kg - 1/2 ) - 57.494 - 55.559 - 53.414 - 53.518 10 50 v f  (water)/(m 3 " mol - 1 ) 6.049 6.101 6.114 10 50 v f  (tr)/(m 3 " mol - 1 ) 13.146 10.684 9.905 D-phenylalanine+aqueous caffeine 10 50 v f  /(m 3 " mol - 1 ) 17.329 15.982 14.710 13.261 10 6* v S  /(m 3 " mol - 3/2 " kg - 1/2 ) - 19.220 - 15.835 - 11.784 - 7.516 10 50 v f  (water)/(m 3 " mol - 1 ) 12.132 12.282 12.375 10 50 v f  (tr)/(m 3 " mol - 1 ) 5.197 2.429 0.886 DL-threonine+aqueous caffeine 10 50 v f  /(m 3 " mol - 1 ) 15.631 11.174 10.087 4.799 10 6* v S  /(m 3 " mol - 3/2 " kg - 1/2 ) - 32.656 - 14.848 - 11.784 11.806 10 50 v f  (water)/(m 3 " mol - 1 ) 7.688 7.749 7.793 10 50 v f  (tr)/(m 3 " mol - 1 ) 7.943 2.338 - 2.994   Anwar Ali et al. / Acta Physico-Chimica Sinica, 2007, 23(1): 79 -  83 aqueous alkali chloride solution [21] . However, a negative 0 v f  (tr) for Thr at 313.15 K suggests that interactions (ii) and (iii) outweigh interaction (i). The above-mentioned finding is also supported by the cosphere overlap model [22] , according to which the positive 0 v f  (tr) for the amino acids investigated in this study is due to the dominance of interaction (i) over interactions (ii) and (iii), whereas the negative 0 v f  (tr) for Thr at 313.15 K is attributed to the dominance of interactions (ii) and (iii) over interaction (i). The entire viscosity data (Table 1) were analyzed in terms of the Jones-Dole equation [23] : ! r  = ! / ! 0 =1+  Ac 1/2 +  Bc  (4) where ! r   is the relative viscosity, !  and ! 0  are the viscosities of the ternary solutions and the solvent (aqueous caffeine), re-spectively,  A  and  B  are constants and measure solute-solute and solute-solvent interactions.  A  and  B  were obtained by least-squares method as intercept and slope of the linear plots of ( !  !  1)/ c 1/2   versus   c 1/2 . Erying and coworkers [24]  proposed that the contribution per mole of solvent (aqueous caffeine) to the free energy of acti-vation ( #01 m D ) could be calculated using the following equa-tion: ! 0 =( hN  A / 01 V  )exp( #01 m D /  RT  ) (5) where h ,  N  A , and 01 V   are the Plank's constant, Avagadro number, and partial molar volume of the solvent, respectively. Rearranging the above, we obtain #01 m D =  RT  ln( ! 0 01 V  / hN  A ) (6) Feakins et al  . [25]  showed that the  B  coefficient is related to #02 m D , which is the contribution per mole of solute (amino acid) to the free energy 02 V   of activation, by the equation:  B =( 01 V   !  02 V  )+ 01 V  [( #02 m D  !  #01 m D )/  RT  ] (7) where 02 V   (= 0 v f  ) is the partial molar volume of the solute. It can be rearranged as follows: #02 m D = #01 m D +(  RT  / 01 V  )[  B  !  ( 01 V   !  02 V  )] (8) The values of  A ,  B , #01 m D , and #02 m D  are shown in Table 4. The larger positive values of  B  coefficients as compared with  A  coefficients support the behavior of 0 v f   and * v S  , respectively, both suggest stronger solute-solvent interactions as compared with solute-solute interactions. Furthermore, it is found that  B  decreases with increase in temperature, whereas  A  increases, thereby supporting the earlier conjecture that sol-ute-solvent interactions decreases and solute-solute interaction increases with rise in temperature. Table 4 also shows that #02 m D  values are positive and larger than #01 m D , suggesting that the formation of transition state is less favored in the  presence of AAs. According to Feakins et al  . [25] , the magni-tude of #02 m D  shows the structure-forming ability of the sol-ute. The results shown in Table 4 indicate that whereas Thr contributes the most to structure formation, Ala contributes the least. This may be attributed to the increased hydrophobic character as one moves from Ala to Thr. The sign of ( $  B / $ T  )  P   is another indication of structure- forming or -breaking ability of the solute [8,26] . It is observed from Fig.1 that for all AAs, ( $  B / $ T  )  P   is negative; thus, they are classified as structure promoters in caffeine+water mix-ture. The slope of  B   versus   T   is less pronounced in the case of Ala and Phe and is maximum in the case of Thr, supporting the results inferred from #02 m D  that Thr has maximum struc-ture-promoting ability in aqueous caffeine solution. Similar  behavior was also observed in the other systems: valine+urea+ water  [27]  and alanine/valine/leucine+sodium acetate+water [28] . Table 5 shows the enthalpies ( #  H  * ) and entropies ( # S  * ), of activation of viscous flow of AAs in aqueous caffeine, calcu-lated using the relations: #  " 0# = #  H  *  !  T  # S  *  (9) and Table 4   Values of  A  and  B  coefficients of Jones-Dole equation, free energy of activation for the solvent ( #01 m D ) and solute ( #02 m D ) for the amino acids, DL-alanine, D-phenylalanine, and DL-threonine in aqueous caffeine at 298.15, 303.15, 308.15, and 313.15 K 298.15 K 303.15 K 308.15 K 313.15 K DL-alanine+aqueous caffeine 10 2    A /(dm 3/2 " mol - 1/2 ) - 87.278 - 74.886 - 69.211 - 61.236 10  B /(dm 3 " mol - 1 ) 36.347 31.124 29.409 26.954 #01 m D /(kJ " mol - 1 ) 9.3790 9.2462 9.1498 8.9603 #02 m D /(kJ " mol - 1 ) 528.9021 462.5302 443.3524 413.3324 D-phenylalanine+aqueous caffeine 10 2    A /(dm 3/2 " mol - 1/2 ) - 84.658 - 81.220 - 76.838 - 69.612 10  B /(dm 3 " mol - 1 ) 35.054 34.346 32.896 30.252 #01 m D /(kJ " mol - 1 ) 9.3789 9.2462 9.1498 8.9603 #02 m D /(kJ " mol - 1 ) 508.7144 504.3865 489.4270 456.4051 DL-threonine+aqueous caffeine 10 2  /(dm 3/2 " mol - 1/2 ) - 85.878 - 82.696 - 79.227 - 75.201 10  B /(dm 3 " mol - 1 ) 35.486 35.423 35.162 34.706 #01 m D /(kJ " mol - 1 ) 9.3790 9.2462 9.1498 8.9603 #02 m D /(kJ " mol - 1 ) 512.2886 512.6303 514.7685 507.8172 Fig.1 Variation of viscosity  B  coefficient with temperature (( $  B / $ T  )  P  ) for the amino acids, DL-alanine, D-phenylalanine, and DL-threonine   Anwar Ali et al. / Acta Physico-Chimica Sinica, 2007, 23(1): 79 -  83 #  " 0# = n 1 #01 m D + n 2#02 m D  (10) The values of #  H  *  and # S  *  were obtained from the inter-cepts and slopes of the plots of total free energy of activation of viscous flow ( #  " 0# ) of the solution versus   T  , where n 1  and n 2  are the number of moles of mixed solvent and solute, re-spectively. #  H  *  and # S  *  values have proved useful in yielding structural information about solute species and solute-solvent interactions. Table 5 shows that #  H  *  values are much larger than # S  *  values, both are positive and increase with the con-centration of amino acid in the solution. The values of #  H  *  increases in the order: Thr<Phe<Ala. This suggests that the formation of the activated species necessary for the viscous flow appears to be considerably easier in case of Thr, as com- pared with the other AAs. Positive #  H  *  values were also re- ported for the system urea+water+ammonium sulfate [29] . The n  data were used to calculate molar refractivity (  R  D )   of the mixtures under study using the Lorentz-Lorenz relation:  R D = ÷÷ ø öççè æ úúûùêêëé+- å = 3122 21 iii D D  M  xnn r   (11) where  x i  and  M  i  are the mole fraction and molar mass of the i th component of the mixture. The calculated values of  R  D  at 298.15, 303.15, 308.15, and 313.15 K are summarized in Ta- ble 5 and plotted in Fig.2 as a function of AAs concentration. The variation of  R  D  with mixture composition gives informa-tion on the interaction in the mixtures [30] .  R  D  increases (Fig.2) almost linearly with increase in the amount of AAs in aqueous caffeine.  R  D  being directly propor-tional to the molecular polarizability, Fig.2 suggests an in-crease in overall polarizability of all the ternary systems under study with increasing amount of AAs in the mixtures. There is not much variation in  R  D  with temperature for the systems investigated in this study. Table 5 Enthalpy ( #  H  * ), entropy ( # S  * ), and molar refractive index (  R  D ) of amino acids, DL-alanine, D-phenylalanine, and DL-threonine in aqueous caffeine at 298.15, 303.15, 308.15, and 313.15 K 10 6    R  D /(m 3 " mol - 1 ) c (mol " kg - 1 )   #  H  *  (kJ " mol - 1 ) # S  *  (J " mol - 1 " K  - 1 ) 298.15 K 303.15 K 308.15 K 313.15 K DL-alanine+aqueous caffeine 0.00 50.6306 0.0831 3.7512 3.7529 3.7552 3.7599 0.01 51.7611 0.0862 3.7061 3.7114 3.7164 3.7207 0.02 52.9890 0.0895 3.7089 3.7139 3.7184 3.7238 0.03 54.2222 0.0928 3.7125 3.7161 3.7206 3.7256 0.04 55.3897 0.0959 3.7236 3.7251 3.7263 3.7305 0.05 56.6129 0.0992 3.7258 3.7289 3.7308 3.7330 D-phenylalanine+aqueous caffeine 0.00 50.6306 0.0831 3.7512 3.7529 3.7552 3.7599 0.01 51.1793 0.0843 3.7696 3.7719 3.7759 3.7804 0.02 51.7968 0.0856 3.7760 3.7801 3.7831 3.7861 0.03 52.4475 0.0870 3.7821 3.7862 3.7898 3.7949 0.04 53.1144 0.0885 3.7954 3.7980 3.8007 3.8038 0.05 53.7846 0.0900 3.8073 3.8069 3.8096 3.8033 DL-threonine+aqueous caffeine 0.00 50.6306 0.0831 3.7512 3.7529 3.7552 3.7599 0.01 50.5360 0.0821 3.7513 3.7549 3.7569 3.7472 0.02 50.7438 0.0820 3.7577 3.7603 3.7622 3.7542 0.03 50.9976 0.0820 3.7640 3.7663 3.7682 3.7616 0.04 51.2352 0.0820 3.7694 3.7706 3.7732 3.7691 0.05 51.5209 0.0822 3.3779 3.7750 3.7772 3.7765 Fig.2 Variation of molar refraction (  R  D ) with concentration for the amino acids, DL-alanine, D-phenylalanine, and DL-threonine
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