Article pubs.acs.org/jced
Volumetric Properties of Glycine in Aqueous Solutions of Some Sulfa Drugs at (288.15, 298.15, and 308.15) K Amalendu Pal* and Surbhi Soni Department of Chemistry, Kurukshetra University, Kurukshetra 136119, India S Supporting Information *
ABSTRACT: In this work, densities have been measured for glycine in aqueous solutions of (0.01, 0.02, 0.03, and 0.04) mol·kg−1 sulfanilamide, sulfanilic acid, and sulfosalicylic acid dihydrate at temperatures from (288.15 to 308.15) K, using vibrating tube digital densimeter. These solution densities were used to calculate the partial molar volume values at infinite dilution, V∞ ϕ , for the glycine. The calculated parameters are used to interpret the solute−solute and solute−solvent interactions and structure making/breaking ability of the amino acid in the given aqueous sulfa drug solution.
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different molalities, (0.01, 0.02, 0.03, and 0.04) mol·kg−1, and at different temperatures, (288.15 to 308.15) K, are determined.
INTRODUCTION Sulfonamides are a group of synthetic organic drugs extensively used for the treatment of certain infections caused by different kinds of microorganisms.1 A detailed literature survey reveals that, with the advent of antibiotics, the use of sulfa drugs has been limited2 but they are still used as antibacterial drugs.3 Although studies regarding thermodynamics and transport properties of a few sulfa drugs in different media4−8 have been reported, information is still scant on the volumetric characteristics of these drugs in aqueous solutions of amino acids. As the concentration of amino acid is increased, there is a significant decrease in drug−amino acid binding. This type of binding appears to involve unusual and complex mechanisms. This mechanisms involve the ion−hydrophilic and hydrophilic−hydrophilic interactions among −OH, −NH2, and other groups of sulfa drugs and the zwitterionic centers (COO− and NH3+) of amino acids. Despite various explanations of the phenomenon, the role of these drugs remain uncertain, we therefore planned to investigate the molecular interactions between amino acid and some sulfa drugs based on the corresponding volumetric behavior. The main goal of this study was to evaluate the effect of molality and temperature on the physicochemical behavior of three structurally related sulfonamides in the presence of glycine: sulfanilamide, sulfanilic acid, and sulfosalicylic acid dihydrate (Figure 1). With that purpose, the density of glycine in aqueous solutions of these drugs at
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MATERIALS AND METHODS Glycine was procured from Hi Media, India and used after drying over silica gel in a vacuum desiccator at room temperature for a minimum of 48 h. Sulfanilamide, sulfanilic acid, and sulfosalicyclic acid dihydrate from Loba Chemie, India were dried for 24 h in a vacuum desiccator before use. All the chemicals used are of analytical reagent grade, having a mass purity of 0.99 or more, as shown in Table 1. All solutions were Table 1. Sample Materials and Information chemical name sulfanilamide sulfanilic acid sulfosalicylic acid dihydrate glycine
purification method
mass purity
Loba Chemie Loba Chemie Loba Chemie
vacuum desiccator vacuum desiccator vacuum desiccator
>0.99 >0.99 >0.99
Hi Media
vacuum desiccator
>0.99
prepared by using deionized doubly glass-distilled water using (having specific conductance less than 10−6 S·cm−1 that have been freshly degassed by vacuum pump). Solutions of glycine in the concentration range of (0.05 to 0.30) mol·kg−1 were made by mass on the molality concentration scale with an accuracy of ± 1·10−5. The weighings were done on an Afcoset electronic balance (India, model ER-182A) with a precision of ± 0.01 mg. The uncertainties in the solution molalities were in the range ± 2·10−5 mol·kg−1. Densities (ρ) of glycine in aqueous sulfa drug solutions at different temperatures were Received: December 31, 2011 Accepted: November 23, 2012
Figure 1. Molecular structure of sulfa drugs. Left, sulfanilamide; middle, sulfanilic acid; right, sulfosalicylic acid dihydrate. © XXXX American Chemical Society
source
A
dx.doi.org/10.1021/je300455e | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 2. Densities, ρ, and Apparent Molar Volumes, V∞ 2,ϕ, of Glycine in Aqueous Solutions of Sulfanilamide at Different Temperaturesa ρ
m2 mol·kg
−1
V2,ϕ·106 −3
ρ
−1
m ·mol 3
kg·m
kg·m
V2,ϕ·106 −3
288.15 K 0.00000 0.05053 0.10320 0.15438 0.19429 0.25521 0.30503
999.61 1001.29 1003.00 1004.63 1005.89 1007.77 1009.29
0.00000 0.05231 0.10249 0.15392 0.20328 0.24949 0.30745
1000.06 1001.78 1003.40 1005.03 1006.58 1008.00 1009.75
0.00000 0.04933 0.09920 0.14586 0.20297 0.24783 0.30043
1000.65 1002.25 1003.85 1005.32 1007.10 1008.48 1010.06
0.00000 0.05227 0.10310 0.15046 0.20104 0.25590 0.29426
1001.14 1002.82 1004.41 1005.87 1007.40 1009.03 1010.14
41.89 42.11 42.33 42.50 42.74 42.93
42.21 42.39 42.57 42.74 42.91 43.13
42.54 42.69 42.82 43.00 43.14 43.31
42.98 43.22 43.43 43.67 43.90 44.09
0.01mB Sulfanilamide 997.52 999.17 1000.84 1002.43 1003.65 1005.47 1006.92 0.02mB Sulfanilamide 997.935 999.62 1001.21 1002.81 1004.32 1005.71 1007.42 0.03mB Sulfanilamide 998.61 1000.19 1001.74 1003.16 1004.85 1006.15 1007.63 0.04mB Sulfanilamide 998.94 1000.59 1002.16 1003.58 1005.04 1006.59 1007.63
−1
m ·mol 3
ρ kg·m
V2,ϕ·106 −3
298.15 K + Glycine
m3·mol−1 308.15 K
42.51 42.78 43.08 43.29 43.60 43.87
994.49 996.11 997.75 999.32 1000.51 1002.29 1003.71
42.98 43.32 43.63 43.87 44.23 44.51
42.75 42.94 43.16 43.36 43.56 43.80
994.81 996.48 998.07 999.68 1001.20 1002.60 1004.33
43.03 43.16 43.30 43.43 43.57 43.74
43.03 43.35 43.66 44.05 44.32 44.65
995.53 997.09 998.61 999.99 1001.62 1002.86 1004.25
43.49 43.94 44.33 44.85 45.23 45.70
43.38 43.76 44.08 44.46 44.85 45.15
995.87 997.50 999.02 1000.38 1001.77 1003.19 1004.14
43.86 44.41 44.96 45.54 46.20 46.66
+ Glycine
+ Glycine
+ Glycine
m2 is the molality of glycine in aqueous drug solution. Standard uncertainties u are u(T) = 0.01 K, u(m2) = ± 0.00002 mol·kg−1, and the combined expanded uncertainties Uc are Uc(ρ) = ± 0.02 kg·m−3, Uc (V2ϕ·106) = ± 0.03 m3·mol−1 (level of confidence = 0.98).
a
measured simultaneously and automatically, using an Anton Paar DSA 5000 instrument. The uncertainty in density measurement is ± 2·10−2 kg·m−3. The density is extremely sensitive to temperature, so it is controlled to ± 1·10−2 K by a built-in solid state thermostat. Before each series of measurements, the instrument was precalibrated with doubly distilled deionized water and dry air for the temperature range investigated.
water), respectively. The values for ρ and V2,ϕ as a function of molality and temperature are listed in Tables 2, 3, and 4. The values of the partial molar volume of the solute (V∞ 2,ϕ) at infinite dilution are computed by the least-squares fitting of the linear plots of V2,ϕ against the molality, m2, in accordance with the following equation
RESULTS AND DISCUSSION Partial Molar Volumes. The apparent molar volume, V2,ϕ, of glycine in sulfanilamide, sulfanilic acid, and sulfosalicylic acid dihydrate solutions, mB = (0.01, 0.02, 0.03, and 0.04) mol·kg−1, where mB is the molality of aqueous solution of drugs at temperatures of (288.15, 298.15, and 308.15) K, have been determined using accurate density data through the following equation.
where SV is the experimental or limiting slope indicative of solute−solute interactions. The resulting values of V∞ 2,ϕ and SV are summarized in Table 5 together with the standard errors. Plots of against m2 are linear in all cases. A sample plot for glycine in 0.03 mol·kg−1 of all drug solutions at different temperatures are shown in Figures 2, 3, and 4. At infinite dilution, the solute−solute interaction is negligible; therefore, the standard partial molar volume and its temperature dependence provide valuable information of the solute−solvent interactions.9−11 Table 3 shows that glycine studied here has positive V∞ 2,ϕ values in aqueous solutions of sulfa drugs at different temperatures, thereby suggesting the presence of strong glycine−drug/water interactions.12 Further, glycine in sulfanilic acid has a higher partial molar volume than
V2, ϕ = V 2,∞ϕ + SV m2
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V2, ϕ = M /ρ − 1000(ρ − ρ0 )/m2ρρ0
(1) −1
where M and m2 are the molar mass (kg·mol ) and molality of solute (mol·kg−1), that is, glycine in solutions, and ρ and ρo are the densities (kg·m−3) of the solution and the solution (drug + B
(2)
dx.doi.org/10.1021/je300455e | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 3. Densities, ρ, and Apparent Molar Volumes, V∞ 2,ϕ, of Glycine in Aqueous Solutions of Sulfanilic Acid at Different Temperatures ρ
m2 mol·kg
−1
kg·m
V2,ϕ·106 −3
ρ
−1
m ·mol 3
kg·m
V2,ϕ·106 −3
288.15 K 0.00000 0.05000 0.10402 0.14972 0.19937 0.25903 0.30427
999.71 1000.97 1002.44 1003.81 1005.38 1007.44 1009.14
0.00000 0.05216 0.10738 0.14976 0.20575 0.26076 0.29259
1000.41 1002.00 1003.72 1005.06 1006.87 1008.66 1009.72
0.00000 0.05121 0.09708 0.15219 0.19349 0.25337 0.30490
1000.93 1002.50 1003.93 1005.68 1007.01 1008.96 1010.67
0.00000 0.04969 0.09914 0.14469 0.20766 0.26147 0.29833
1001.64 1003.18 1004.74 1006.19 1008.23 1009.99 1011.22
49.81 48.63 47.47 46.33 44.87 43.65
44.45 44.04 43.77 43.39 43.05 42.85
44.21 43.94 43.60 43.35 42.99 42.69
43.95 43.69 43.42 43.05 42.75 42.52
0.01mB Sulfanilic Acid 0997.62 0998.77 1000.18 1001.49 1003.04 1005.12 1006.84 0.02mB Sulfanilic Acid 998.31 999.74 1001.34 1002.64 1004.44 1006.30 1007.42 0.03mB Sulfanilic Acid 998.89 1000.40 1001.60 1003.33 1004.66 1006.66 1008..42 0.04mB Sulfanilic Acid 999.53 1001.03 1002.57 1004.01 1006.05 1007.85 1009.10
m ·mol 3
ρ
−1
kg·m
V2,ϕ·106 −3
298.15 K + Glycine
m3·mol−1 308.15 K
52.02 50.44 49.08 47.68 45.81 44.40
994.60 995.67 996.98 998.26 999.81 1001.87 1003.70
53.76 52.17 50.58 48.81 46.75 44.82
47.73 46.75 45.99 45.03 44.11 43.58
995.27 996.60 998.13 999.36 1001.09 1002.91 1003.98
49.56 48.41 47.62 46.58 45.5 44.98
45.55 45.01 44.41 43.96 43.27 42.74
995.76 997.17 998.49 1000.18 1001.50 1003.52 1005.33
47.55 46.83 45.88 45.18 44.13 43.32
44.72 44.27 43.89 43.37 42.91 42.57
996.48 997.88 999.35 1000.76 1002.79 1004.61 1005.90
46.93 46.05 45.38 44.46 43.65 43.12
+ Glycine
+ Glycine
+ Glycine
water to aqueous solutions of sulfa drugs have been determined as
that of glycine in sulfanilamide and sulfosalicylic acid dihydrate, indicating a linear dependence of the V∞ 2,ϕ on their molar masses and a strong interaction of glycine/sulfanilic acid−water. Because of the nature of the interacting groups in the two cosolute molecules (sulfanilamide and sulfosalicylic acid dihydrate) is different, the hydrophobic effect is also different. The V∞ 2,ϕ values (Table 5) increase with the increase in mass fraction of all drugs except with sulfanilic acid and also with increasing temperature. This indicates that the cosolute drug− solvent interaction increases on increasing the mass fraction of drugs (except with sulfanilic acid) and with temperature. This interaction results from the release of some of the water molecules from loose solvation layers of the solute (glycine) in the bulk solution. That is, the maximum structure breaking effect of glycine takes place on higher concentrations of sulfanilamide and sulfosalicylic acid dihydrate drugs and lower concentrations of sulfanilic acid at higher temperature. The experimental SV values in Table 5 for glycine in different drugs are found to be positive except with sulfanilic acid suggesting that solute−solute interactions are weaker than solute−solvent interaction in all drugs. Apparent Molar Properties of Transfer. Partial molar volume of transfer, ΔtrV∞ 2,ϕ, at infinite dilution of glycine from
Δtr V 2,∞ϕ = V 2,∞ϕ(in aqueous sulfa drugs) − V 2,∞ϕ(in water) (3)
V∞ 2,ϕ
The experimental values for glycine in water at (288.15, 298.15, and 308.15) K have been taken from our previous paper.13 ∞ The ΔtrV2,ϕ values are summarized in Table S1 and illustrated in Figure F1 in the Supporting Information. The values of ΔtrV∞ 2,ϕ are by definition free from solute−solute interactions and therefore provide information regarding solute−solvent interactions. ΔtrV∞ 2,ϕ values are positive for sulfanilic acid and negative for sulfosalicylic acid dihydrate and both negative and positive for sulfanilamide at different concentrations of drug. Variations of partial molar volume of transfer of glycine in aqueous solution of sulfanilamide with molal concentration of sulfanilamide at different temperatures. It may be noted that the temperature dependence of ΔtrV∞ 2,ϕ values in all of the cases is very regular except with sulfanilic acid, where the ΔtrV∞ 2,ϕ values at 298.15 K are lower, and these increase at 308.15 K at higher concentrations of drug. In the case of sulfanilamide and sulfosalicylic acid dihydrate, the ΔtrV∞ 2,ϕ values decrease with temperature in the order of C
dx.doi.org/10.1021/je300455e | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 4. Densities, ρ, and Apparent Molar Volumes, V∞ 2,ϕ, of Glycine in Aqueous Solutions of Sulfosalicylic Acid Dihydrate at Different Temperatures ρ
m2 mol ·kg
−1
kg·m
V2,ϕ·106 −3
288.15 K 0.00000 0.05292 0.10368 0.15628 0.20387 0.25965 0.30179
999.62 1001.53 1003.30 1005.07 1006.60 1008.32 1009.49
0.00000 0.04847 0.10220 0.15688 0.19997 0.25796 0.31171
1001.05 1002.79 1004.63 1006.40 1007.74 1009.42 1010.88
0.00000 0.04941 0.10294 0.14885 0.20133 0.25350 0.30670
1001.36 1003.11 1004.91 1006.36 1007.95 1009.44 1010.69
0.00000 0.05095 0.10755 0.15358 0.20465 0.25585 0.31116
1002.56 1004.35 1006.23 1007.68 1009.21 1010.65 1012.10
ρ
−1
m ·mol 3
kg·m
V2,ϕ·106 −3
m ·mol 3
−1
298.15 K 0.01mB Sulfosalicylic Acid Dihydrate + Glycine 997.48 38.83 999.37 39.42 39.36 1001.11 39.99 39.94 1002.83 40.66 40.54 1004.33 41.23 41.20 1005.98 42.02 41.94 1007.15 42.65 0.02mB Sulfosalicylic Acid Dihydrate + Glycine 998.63 39.10 1000.34 39.73 39.84 1002.14 40.58 40.70 1003.87 41.44 41.32 1005.16 42.17 42.24 1006.80 43.06 43.10 1008.22 43.90 0.03mB Sulfosalicylic Acid Dihydrate + Glycine 999.74 39.43 1001.47 40.07 40.39 1003.25 40.84 41.23 1004.69 41.59 42.02 1006.26 42.39 42.82 1007.71 43.27 43.87 1009.06 44.26 0.04mB Sulfosalicylic Acid Dihydrate + Glycine 1000.13 39.83 1001.90 40.24 40.73 1003.77 41.14 41.44 1005.20 41.84 42.25 1006.70 42.69 43.05 1008.11 43.54 43.95 1009.54 44.41
ρ
V2,ϕ·106
kg·m
−3
m3·mol−1 308.15 K
994.38 996.24 997.95 999.62 1001.05 1002.64 1003.75
39.79 40.49 41.34 42.11 42.93 43.67
994.89 996.54 998.38 1000.10 1001.39 1003.05 1004.45
39.93 40.78 41.65 42.31 43.12 44.04
996.61 998.33 1000.10 1001.55 1003.10 1004.54 1005.94
40.21 41.00 41.72 42.59 43.46 44.29
996.95 998.71 1000.57 1002.00 1003.49 1004.87 1006.23
40.55 41.31 41.98 42.85 43.79 44.87
Table 5. Limiting Partial Molar Volumes, V∞ 2,ϕ, and Experimental Slope, SV, of Glycine in Aqueous Sulfa Drug Solutions at Different Temperatures m/mol·kg−1
6 3 −1 V∞ 2,ϕ·10 /m ·mol
288.15 K
298.15 K
0.01 0.02 0.03 0.04
41.69 42.02 42.39 42.74
± ± ± ±
0.01 0.01 0.01 0.01
42.24 42.50 42.71 42.99
± ± ± ±
0.01 0.01 0.01 0.01
0.01 0.02 0.03 0.04
48.59 44.77 44.51 44.24
± ± ± ±
0.02 0.02 0.01 0.01
51.15 48.60 46.10 45.13
± ± ± ±
0.08 0.02 0.02 0.01
0.01 0.02 0.03 0.04
38.09 38.31 38.63 39.02
± ± ± ±
0.09 0.04 0.07 0.02
38.66 38.96 39.19 39.40
± ± ± ±
0.06 0.01 0.07 0.02
SV·106/m3·kg·mol 308.15 K 288.15 K Sulfanilamide 42.71 ± 0.03 4.10 ± 0.048 42.88 ± 0.01 3.57 ± 0.03 43.05 ± 0.01 3.03 ± 0.05 43.22 ± 0.02 4.55 ± 0.03 Sulfanilic Acid 53.17 ± 0.12 −16.27 ± 0.13 50.49 ± 0.05 −6.61 ± 0.10 48.43 ± 0.03 −6.02 ± 0.02 47.61 ± 0.05 −5.76 ± 0.05 Sulfosalicylic Acid Dihydrate 38.91 ± 0.04 12.29 ± 0.49 39.19 ± 0.04 15.23 ± 0.18 39.38 ± 0.03 16.90 ± 0.34 39.55 ± 0.12 15.79 ± 0.05
298.15 K
−2
308.15 K
5.35 4.23 6.46 7.26
± ± ± ±
0.05 0.08 0.06 0.05
5.80 2.78 8.80 11.60
± ± ± ±
0.16 0.04 0.06 0.10
−21.84 −17.27 −11.08 −8.55
± ± ± ±
0.39 0.14 0.08 0.07
−26.88 −19.03 −16.84 −15.14
± ± ± ±
0.62 0.24 0.13 0.24
12.95 15.88 16.22 16.08
± ± ± ±
0.31 0.08 0.34 0.10
15.61 15.46 16.00 16.65
± ± ± ±
0.21 0.18 0.16 0.62
sulfosalicylic acid dihydrate < sulfanilamide < sulfanilic acid. These results can also be explained on the basis of cosphere overlap model14,15 in terms of solute−cosolute interactions.
(288.15 < 298.15 < 308.15) K, the reverse is the trend in the ∞ case of sulfanilic acid. Overall the ΔtrV2,ϕ values at all concentrations of drug increase in the following order: D
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group interactions contribute negatively to the ΔtrV∞ 2,ϕ values. Therefore, the observed increasing positive transfer volumes in glycine mixed solvents suggest that, in the ternary solutions sulfa drug−glycine−water, the ion−hydrophilic and hydrophilic−hydrophilic interactions among −OH and −NH2 groups of sulfanilic acid and the zwitterionic centers (COO− and NH3+) of glycine predominate over the hydrophilic−hydrophobic group interactions. The negative ΔtrV∞ 2,ϕ values observed for sulfosalicylic acid dihydrate and sulfanilamide except at higher concentrations of sulfanilamide drug and at lower temperature suggest that hydrophilic−hydrophobic and hydrophobic−hydrophobic interactions are dominating. Apparent Molar Expansibilities. The temperature variation of V∞ 2,ϕ can be expressed as V 2,∞ϕ = a + b(T − Tm) + c(T − Tm)2
(4)
where Tm represents the midpoint temperature of the range used (Tm = 298.15 K). Least-square fitting of eq 4 was done to obtain a, b, and c parameters. Differentiation of eq 4 with respect to temperature at constant pressure was done to calculate partial molar isobaric expansions:
Figure 2. Apparent molar volumes, V∞ 2,ϕ, as a function of molality, m2, of glycine in 0.03 mol·kg−1 of sulfanilamide drug at T = ■, 288.15 K; ●, 298.15 K; ▲, 308.15 K.
E2∞ = (∂V ϕ∞/∂T )P = b + 2c(T − Tm)
(5)
It follows from eq 5 that the quantity b + 2c(T − Tm) is equivalent to E∞ 2 . The calculated values of partial molar expansion are included in Table S1 in the Supporting Information. The E∞ 2 values are decreasing regularly with increase in temperature but do not follow any regular variation with the increasing concentration of sulfanilic acid and sulfosalicylic acid dihydrate. On the other hand, the E∞ 2 values decrease regularly with the increasing concentration of sulfanilamide at lower temperature. A perusal of the E∞ 2 results presented in Table S1 in the Supporting Information shows positive values, indicating strong solute−solvent interactions, which are higher in the case of sulfanilic acid. We have calculated pair and triplet interaction coefficients as discussed in our previous paper16 using the following equation
Figure 3. Apparent molar volumes, V∞ 2,ϕ, as a function of molality, m2, of glycine in 0.03 mol·kg−1 of sulfanilic acid drug at T = ■, 288.15 K; ●, 298.15 K; ▲, 308.15 K.
Δtr V 2,∞ϕ(water to aqueous cosolute solution) = 2VABmB + 3VABBmB 2 + ...
(6)
where the constants VAB and VABB are pairwise and triplet interaction coefficients. Here A denotes glycine and B denotes the cosolute (drugs), and mB is the molality of the cosolute. The ΔtrV∞ 2,ϕ values have been fitted with eq 6 to obtain VAB and VABB. The values of these coefficient are given in Table S2 in the Supporting Information. The pairwise interaction coefficients VAB are positive for sulfanilic acid and negative for sulfanilamide and sulfosalicylic acid dihydrate at all temperatures. Positive values for VAB strengthen our viewpoint that ionic/hydrophilic−hydrophilic interactions dominate over hydrophobic−ionic interactions between solute and cosolute molecules. The dependence of VAB is linear with increase in sulfanilic acid and decreases from sulfanilamide to sulfosalicylic acid dihydrate. The maximum positive value of the pairwise interaction coefficients VAB occurs for glycine with sulfanilic acid, suggesting that interactions occur due to the overlap of the hydration cospheres of glycine−sulfanilic acid molecules, and it increases with an increase in temperature. The values of triplet interaction coefficient VABB are positive for sulfanilamide and
Figure 4. Apparent molar volumes, V2,ϕ, as a function of molality, m2, of glycine in 0.03 mol·kg−1 of sulfosalicylic acid dihydrate drug at T = ■, 288.15 K; ●, 298.15 K; ▲, 308.15 K.
According to this model hydrophilic−ionic group interactions contribute positively, whereas hydrophobic−hydrophobic E
dx.doi.org/10.1021/je300455e | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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of Scientific and Industrial Research (CSIR), New Delhi is gratefully acknowledged.
sulfosalicylic acid dihydrate, whereas they are negative for sulfanilic acid. ∞ The values of V∞ 2,ϕ and E2 are further used to calculate the isobaric thermal expansion coefficient, α2, using following relation α2 = E2∞/V 2,∞ϕ
Notes
The authors declare no competing financial interest.
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(7)
The value of partial molar expansibility gives α2 information regarding the size of solute and its hydrophobicity. The E∞ 2 values are positive for all drugs, indicating the predominance of hydrophobic hydration over the electrostriction of water molecules around the solute glycine molecule. Thereby, it favors glycine−glycine or glycine−drug interactions. The values of E∞ 2 and α2 are higher for sulfanilic acid than for sulfanilamide and sulfosalicylic acid dihydrate. The calculated values of α2 are listed in Table S3 of the Supporting Information. Hepler17 proposed a useful thermodynamic relation, which provides qualitative information on the structure-making or structure-breaking ability of a solute in aqueous solution: (∂CP∞/∂P) = −T (∂ 2V 2,∞ϕ/∂T 2)P
(8)
where C∞ P is the heat capacity of the solute at infinite dilution. On the basis of this expression, (∂C∞ P /∂P)T values should be negative for structure-making solutes, whereas they are positive 2 for structure-breaking solutes. This implies that ∂2V∞ ϕ /∂T are negative for structure-breaking solutes and positive for the structure-making solutes.17,18 It can be seen from Table S3 of the Supporting Information that glycine has negative (∂2V∞ ϕ/ ∂T2)P values (except at higher molality of sulfanilic acid and at 0.03 mol·kg−1 of sulfanilamide), showing that it acts as a structure breaker in aqueous solutions of sulfa drugs. This features is similar to that observed for these sulfa drugs in aqueous solutions of sodium chloride.19
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CONCLUSION In summary, we have represented the volumetric properties of three sulfa drugs, namely, sulfanilamide, sulfanilic acid, and sulfosalicylic acid dihydrate. The values of limiting partial molar volumes of glycine in aqueous drug solutions are positive, indicating strong solute−solvent interactions that is, ion− hydrophilic/hydrophobic and hydrophilic−hydrophilic interactions. Solute−solvent interactions increase with the increase in the concentration of sulfa drugs except with sulfanilic acid. The negative SV values in the case of sulfanilic acid have been attributed to the self-association of these molecules. The overall V∞ 2,ϕ values at all concentrations of drug increase in the following order: sulfosalicylic acid dihydrate < sulfanilamide < sulfanilic acid.
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ASSOCIATED CONTENT
S Supporting Information *
Transfer volumes, pair and triplet interaction coefficients, parameters, and plots of tranfer molar volumes. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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dx.doi.org/10.1021/je300455e | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
