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Fluid Phase Equilibria 289 (2010) 122–128

Contents lists available at ScienceDirect

Fluid Phase Equilibria

journa l homepage: www.e lsev ier .com/ locate / f lu id

uffer interactions: Solubilities and transfer free energies of TRIS, TAPS, TAPSO,nd TABS from water to aqueous ethanol solutions

ohamed Taha, Ming-Jer Lee ∗

epartment of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan

r t i c l e i n f o

rticle history:eceived 2 September 2009eceived in revised form0 November 2009ccepted 16 November 2009vailable online 24 November 2009

a b s t r a c t

Tris(hydroxymethyl)aminomethane (TRIS), N-[tris(hydroxymethyl)methyl]-3-aminopropanesulfonicacid (TAPS), N-[tris(hydroxymethyl)methyl]-3-amino-2-hydroxypropane sulfonic acid (TAPSO), and N-tris[hydroxymethyl]-4-amino-butanesulfonic acid (TABS), are useful for pH control as standard buffersin the physiological region of pH 7.0–9.5 for TRIS, 7.7–9.1 for TAPS, 7.0–8.2 for TAPSO, and 8.2–9.5 forTABS, respectively. These buffers are structurally related and contain at least TRIS groups. Densities of

eywords:iological buffersuffer interactionsolubilities

these buffers in binary ethanol + water solvent mixtures have been measured by a high precision vibrat-ing tube digital densitometer at 298.15 K under atmospheric pressure. From these data, the solubilitiesof these buffers in water + ethanol mixed solvents have been determined at 298.15 K. The solubility datawere further used to calculate the apparent transfer free energy (�G′

tr) of these buffers from water to amixed solvent of ethanol and water at 298.15 K. The contributions of the transfer free energies from var-

ere es ′

ransfer free energiesolecular interactions

ious functional groups wmolecular structures.

. Introduction

Maintaining a stable pH value in many fields, such as chem-stry, biology, pharmacology, medical science, and our daily lifes well, by adding a suitable buffer to the medium is major pre-equisite. Buffers are generally efficient only in narrow pH rangeshich depend, in part, on their dissociation constants. The buffer

hould not be an enzyme substrate or enzyme inhibitor and shouldot react with metabolites or other components. The buffer shouldherefore be inert. However, to a certain degree all of the buffersxhibit various deviations from the requirements of being inert.RIS and other primary amines can react with aldehydes [1–5].orate forms covalent complexes with mono- and olio-saccharides,ibose subunits of nucleic acids, glycerol and pyridine nucleotides.icarbonate is in equilibrium with CO2 and therefore needs a closedystem. The cost of buffer substances may be an important consid-ration, especially where large quantities of material are involved.uffers should be purifying as possible. Purity is extremely impor-ant, since contaminations (e.g., heavy metals) can easily interfereith sensitive systems.

The introduction of zwitterionic buffers by Good and his asso-iates [6–8], seemed to solve the problems specified earlier ando satisfy all the criteria for a perfect buffer. Zwitterionic buffers,y definition (from German “Zwitter”, hermaphrodite), contain

∗ Corresponding author. Tel.: +886 2 2737 6626; fax: +886 2 2737 6644.E-mail address: mjlee@mail.ntust.edu.tw (M.-J. Lee).

378-3812/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.fluid.2009.11.019

timated from (�Gtr) of the mixtures containing the buffers with different

© 2009 Elsevier B.V. All rights reserved.

both positive and negative ionizable groups. Secondary and tertiaryamines provide the positive charges, while sulfonic and carboxylicacid groups provide the negative charges. To minimize the dangerof selection an unsuitable buffer it is desirable, in the early stagesof an investigation, to carry out replicate studies using structurallydifferent types of buffers.

The biological buffer should exhibit high water solubility andminimal solubility in organic solvents so that in particular systemsthere would be very little of the buffer inside the particulate phase,and so that the buffers would pass only with difficulty through bio-logical membranes. For instance, TRIS has appreciable solubility inorganic solvents which leads to it accumulation in the biologicalphases of reaction systems. High water solubility also permits theuse of aliquots of stock buffer solutions. Most pH measurementsin biological systems are performed in the aqueous phase. How-ever, sometimes mixed aqueous-water-miscible solvents, such asmethanol or ethanol, are used for dissolving compounds of biolog-ical importance. The solubility measurements in mixed solvent ofethanol and water are of biological relevance.

TRIS, TAPS, TAPSO, and TABS, are widely used in various applica-tions [9–18]. The solubilities of TRIS buffer in water and in variousorganic solvent have been studied [19,20]. Recently, the solubilitylimits of TRIS and TABS in water and in aqueous solutions of dif-

ferent ionic salts have been studied by our research group [21]. Inthis work, the solubilities of TRIS, TAPS, TAPSO, and TABS in binaryethanol + water solvent mixtures have been determined experi-mentally at 298.15 K from density measurements. These solubilitydata were used to study the apparent transfer free energy of these

se Equilibria 289 (2010) 122–128 123

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Table 1Experimental densities (�) for ternary systems of(TRIS + water + ethanol) at 298.15 K under atmo-spheric pressure.

m/mol kg−1 �/g cm−1

10% (w/w) ethanol0.3566 0.995041.0211 1.009851.7129 1.025012.3898 1.038903.0832 1.053643.5141 1.063334.0135 1.073424.4527 1.08276

20% (w/w) ethanol0.2410 0.973820.3863 0.978220.6959 0.985391.0682 0.993731.5948 1.006402.1199 1.019432.6457 1.032463.1245 1.043473.7453 1.05895

30% (w/w) ethanol0.0570 0.952520.1874 0.956480.3162 0.959050.7041 0.970111.0789 0.979231.5239 0.990912.0101 1.002982.5186 1.016633.0386 1.029653.5191 1.04201

40% (w/w) ethanol0.0570 0.952520.1874 0.956480.3162 0.959050.7041 0.970111.0789 0.979231.5239 0.990912.0101 1.002982.5186 1.016633.0386 1.02965

50% (w/w) ethanol0.1180 0.913520.3640 0.921920.7396 0.933461.0996 0.944041.4793 0.954531.8631 0.964562.1455 0.973232.4212 0.98103

60% (w/w) ethanol0.0479 0.888520.1956 0.893480.3814 0.899770.7570 0.911910.9749 0.918301.1606 0.924121.4017 0.930701.6411 0.937831.8103 0.94423

70% (w/w) ethanol0.1907 0.870070.3847 0.876750.5844 0.883370.7776 0.889730.9559 0.895891.0542 0.89881

M. Taha, M.-J. Lee / Fluid Pha

uffers and several functional group-contributions from water toqueous ethanol solutions at 298.15 K.

. Materials and methods

.1. Materials

TRIS (mass fraction purity >0.999), TAPS (mass fraction purity0.995), TAPSO (mass fraction purity >0.99), and TABS (mass frac-ion purity >0.99) were supplied by Sigma Chemical Co. (USA).thanol with purity level of 0.999 in mass fraction was purchasedrom Acros Organics (USA). All the purchased materials were usedithout further purification. Water used for preparing the aque-

us and the aqueous ethanol solutions was obtained from NANOure-Ultra pure water system that was distilled and deionized withesistance of 18.3 M�. All the aqueous solution samples were pre-ared gravimetrically.

.2. Methods

Density measurements were carried out using a vibrating tubeigital densitometer (model DMA-4500 from Anton Paar, Austria),ith an uncertainty of ±5 × 10−5 g cm−3. The densitometer hasbuilt-in thermostat for maintaining the desired temperaturesithin ±0.02 K over the temperature range of (273.15–363.15) K.

he calibration of the densitometer was performed with air andegassed distilled water.

Solubilities were determined as described by Nozaki and Tan-ord [22–24]. The detailed procedure used in this work has beenelineated in our earlier articles [21,25,26]. To a minimum of ninelass vials, each containing fixed weight of solvent (0, 10, 20, 30,0, 50, 60, 70, 80, 90, 100%, w/w ethanol + water mixtures), wasdded weighed amounts of a buffer compound to provide a seriesf mixtures with increasing composition of buffer compound. Theeighed samples were prepared such that approximately five vialsould ultimately result in unsaturated solutions and the remaining

our vials were saturated. Each vial was sealed with a Teflon coatedcrew cup. The vials were completely in a thermostatic shakerquipped with water bath (BT-350R, Yih-Der, Taiwan) at 298.15 Kor (36–48) h, and the supernatant of each solution was removedhrough syringe and filtered by 0.22 �m disposal filter (Milli-ore, Millex-GS) before performing the density measurements. Thencertainty of the solubility limit is lower than ±0.8%. The concen-ration of buffer in the samples, in units of (g of buffer/100 g ofolvent) calculated from Eq. (1).

composition gbuffer

100 gsolvent=

(weightbuffer(g)weightsolvent(g)

)× 100 (1)

The density of solution obtained from each sample vial was plot-ed as a function of the composition of the vial and the densityersus composition data of the unsaturated and saturated solutionsere fitted to a straight line. The solubility limit was determined at

he point of intersection of the two fitted lines. The solubility limitsxpressed as (g of buffer/100 g of solvent) are converted to the moreppropriate units of molarity (moles of buffer/liter of solution) andolality (moles of buffers/kg of solvent). The result of TRIS solubil-

ty in aqueous solution at T = 298.15 K (S = 5.76 mol kg−1) was foundo agree very well with values 5.780 and 5.766 mol kg−1 reportedy Bates and co-workers [19] and El-Harakany and Barakat [20],espectively.

. Results and discussions

The densities of solutions of TRIS, TAPS, TAPSO, and TABS asfunction of concentration of biological buffers in water-ethanol

1.1697 0.902061.2556 0.90539

80% (w/w) ethanol0.0454 0.840270.1280 0.84332

124 M. Taha, M.-J. Lee / Fluid Phase Equilibria 289 (2010) 122–128

Table 1 (Continued)

m/mol kg−1 �/g cm−1

0.2064 0.846170.3005 0.849660.4227 0.854200.5159 0.857300.5836 0.859750.6695 0.862630.7174 0.86455

90% (w/w) ethanol0.0404 0.813050.0933 0.814590.1436 0.816170.1725 0.817190.1940 0.818070.2229 0.818970.2427 0.819650.2683 0.820400.3162 0.82147

100% (w/w) ethanol0.0206 0.785330.0289 0.785690.0404 0.786140.0504 0.78654

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Table 2Experimental densities (�) for ternary systems of(TAPS + water + ethanol) at 298.15 K under atmosphericpressure.

m/mol kg−1 �/g cm−1

10% (w/w) ethanol0.1866 1.001990.5187 1.024140.8669 1.046611.2105 1.069471.5180 1.088481.7215 1.101401.8970 1.113632.2086 1.13357

20% (w/w) ethanol0.0263 0.968700.0555 0.971480.0966 0.974980.1295 0.977950.1850 0.982790.5212 1.010360.8768 1.036691.0954 1.055431.3408 1.074741.6051 1.09625

30% (w/w) ethanol0.0197 0.952370.0526 0.955430.0900 0.958630.1344 0.962610.1796 0.966580.3580 0.979500.5294 0.993390.7469 1.009860.9183 1.022831.1456 1.03903

40% (w/w) ethanol0.0238 0.933680.0563 0.936410.0904 0.939540.1278 0.942920.1636 0.946190.2421 0.952820.3568 0.96185

0.0578 0.786800.0677 0.787210.0743 0.787520.0817 0.78779

ixtures at atmospheric pressure, up to saturated conditions, andt 298.15 K are given in Tables 1–4

and the experimental densities (�) of these buffers in water ateveral temperatures have been reported in our previous papers21,27].

The solubility limits (SB) of each buffer in the aqueous ethanololutions containing ethanol of 0, 10, 20, 30, 40, 50, 60, 70, 80,0, and 100% (w/w) (wEtOH) were obtained from density measure-ents at 298.15 K. As an example, Fig. 1 shows the density profile

f TRIS buffer in 10% (w/w) ethanol + water mixture. The solubil-ty limit and the density at solubility limit were obtained at the

ntersection of the two fitted lines for each experiment. Table 5ummarizes the solubility limits and the corresponding densities�∗

B) of the saturated solutions. Graphical representation of theiological buffers solubilities is given in Fig. 2. It is difficult to accu-ately extend all solubility measurements of zwitterionic buffers

ig. 1. Densities of TRIS in 10% (w/w) aqueous ethanol solutions vs. composition ofRIS in the solutions at 298.15 K. The solubility limit of buffer was determined at theoint of the intersection of the two fitted lines for each system.

0.4719 0.971040.6088 0.982330.7144 0.99199

50% (w/w) ethanol0.0189 0.911270.0563 0.914740.0941 0.918270.1258 0.921000.1702 0.924960.2031 0.927810.2676 0.933530.3338 0.939700.3823 0.94413

60% (w/w) ethanol0.0234 0.888960.0382 0.890550.0518 0.892090.0732 0.893700.0962 0.895570.1192 0.897720.1385 0.899390.1706 0.902240.1965 0.90444

70% (w/w) ethanol0.0177 0.864580.0267 0.865430.0345 0.866200.0432 0.867070.0510 0.867810.0575 0.868440.0662 0.869170.0740 0.86992

M. Taha, M.-J. Lee / Fluid Phase Equilibria 289 (2010) 122–128 125

Table 2 (Continued )

m/mol kg−1 �/g cm−1

80% (w/w) ethanol0.0029 0.838730.0037 0.838820.0053 0.838980.0078 0.839250.0103 0.83951

(towTtha

Table 4Experimental densities (�) for ternary systems of(TABS + water + ethanol) at 298.15 K under atmosphericpressure.

m/mol kg−1 �/g cm−1

10% (w/w) ethanol0.1667 0.995150.3300 1.008850.4835 1.020930.8146 1.039561.2806 1.067671.6518 1.093332.4353 1.133013.1772 1.162993.8749 1.20067

20% (w/w) ethanol0.1671 0.981200.3280 0.994750.5010 1.00849

0.0132 0.839810.0164 0.840140.0197 0.84047

TAPS, TAPSO, and TABS) to 100% ethanol at 298.15 K, because ofhe extremely low solubility in ethanol. The observed solubilitiesf buffers in water follow the order of TABS > TRIS > TAPS > TAPSO,hereas those in aqueous ethanol solutions follow the order of

RIS > TABS > TAPS > TAPSO. The solubilities of TAPSO are lowerhan those of TAPS, which might be a result of intramolecularydrogen bond formation between the protonated amine groupnd the hydroxyl group on the ˇ carbon nearby the sulfonic group

Table 3Experimental densities (�) for ternary systems of(TAPSO + water + ethanol) at 298.15 K under atmo-spheric pressure.

m/mol kg−1 �/g cm−1

10% (w/w) ethanol0.0208 0.982610.0478 0.986500.0833 0.989060.1107 0.992100.1631 0.997110.3255 1.012720.4840 1.02752

20% (w/w) ethanol0.0220 0.968560.0490 0.971430.0899 0.975580.1130 0.977910.1219 0.978840.2445 0.990870.3251 0.99859

30% (w/w) ethanol0.0255 0.953240.0548 0.956150.0845 0.959140.1018 0.960810.1188 0.962620.1477 0.965330.1778 0.96856

40% (w/w) ethanol0.0228 0.934080.0447 0.936360.0606 0.937930.0771 0.939520.0883 0.940830.1080 0.942690.1253 0.94441

50% (w/w) ethanol0.0108 0.910610.0216 0.911730.0378 0.913510.0567 0.915630.0744 0.917260.0910 0.91892

60% (w/w) ethanol0.0120 0.887990.0158 0.888540.0212 0.889160.0258 0.88965

0.8041 1.025681.1500 1.053641.8259 1.092742.5468 1.125903.0816 1.15968

30% (w/w) ethanol0.1695 0.965740.3327 0.979370.5052 0.993170.6759 1.006070.9137 1.022801.3202 1.048861.6972 1.070322.0871 1.098902.4792 1.115492.7334 1.14141

40% (w/w) ethanol0.1745 0.946990.3440 0.961290.5134 0.974950.6949 0.988750.8550 1.000141.0233 1.011601.2907 1.032081.6817 1.052441.9312 1.07815

50% (w/w) ethanol0.0517 0.914390.0941 0.918260.1399 0.922390.3443 0.938810.7089 0.965090.9670 0.984081.1904 1.001481.5682 1.02846

60% (w/w) ethanol0.0237 0.888980.0909 0.895160.1862 0.903870.3677 0.921240.5429 0.934350.7186 0.947580.8076 0.954520.9091 0.96156

70% (w/w) ethanol0.0665 0.869340.1010 0.872630.1376 0.875990.2017 0.881870.2425 0.885370.2771 0.888660.3183 0.89224

80% (w/w) ethanol0.0086 0.839380.0148 0.840050.0214 0.84060

126 M. Taha, M.-J. Lee / Fluid Phase Equilibria 289 (2010) 122–128

Table 4 (Continued )

m/mol kg−1 �/g cm−1

[ptTo[zibbzsc

(s

�

w1

Fs

Table 6Apparent transfer free energies (�G′

tr ) from water to aqueous ethanol solutions at298.15 K.

wEtOH/% (w/w) �G′tr /J mol−1

TRIS TAPS TAPSO TABS

10 302.09 473.72 1066.21 314.5720 605.91 1066.63 2217.12 571.8930 914.46 1873.30 3372.73 935.6140 1280.63 2894.97 4530.42 1378.7650 1744.81 4377.02 5821.03 2051.5260 2341.08 6134.60 8103.98 3360.1670 3248.70 8573.99 5758.7380 4599.52 11825.58 9502.7490 6966.55100 9944.03

TB

0.0307 0.841510.0486 0.843220.0614 0.844400.0742 0.84523

28]. The solubilities of TABS are higher than those of TAPS, unex-ected behavior, probably because TABS has one more CH2 grouphan TAPS between the sulfonic and protonated amine groups.his methyl group decreases the intramolecular interactions, whichbserved from the crystal structure of some zwitterionic buffers28,29], between the sulfonic and protonated amine groups. Awitterion, like an electrolyte, is very soluble in water but almostnsoluble in alcohols. Decreasing in the dielectric constant, such asy the addition of alcohols, is accompanied by a decrease in solu-ility of zwitterions as of ions [30]. Therefore, the solubilities of thewitterionic buffers (TAPS, TAPSO, and TABS) in aqueous ethanololutions are substantially lower than those of TRIS buffer, espe-ially at higher concentrations of ethanol.

The apparent transfer free energy (�G′tr) of a buffer from water

w) to an aqueous ethanol solution (s) can be determined from theolubility measurements [25,26,31,32] given as Eq. (2).( ) ( )

G′

tr = RT lnnB,w

nB,ws+ RT ln

VS,ws

VS,w(2)

here nB,w and nB,ws represent the moles of buffer saturated in00 g water and in 100 g of ethanol-water mixed solvent, VS,w and

ig. 2. Solubility limits of TRIS, TAPS, TAPSO, TABS in water and in aqueous ethanololutions at 298.15 K: TRIS (�), TAPS (©), TAPSO (�), and TABS (×).

able 5uffer solubilities (SB) in water or in aqueous ethanol solutions and densities (�∗

B) at solub

wEtOH/% (w/w) SB/mol kg−1

TRIS TAPS TAPSO TABS

0 5.76 3.12 0.841 5.9110 4.85 2.41 0.533 4.5720 4.15 1.77 0.330 3.8930 3.58 1.23 0.207 3.1440 3.02 0.783 0.131 2.4950 2.46 0.420 0.079 1.8060 1.91 0.206 0.032 0.98370 1.30 0.078 0.35280 0.742 0.021 0.07790 0.286

100 0.088

Fig. 3. Apparent transfer free energy (�G′tr ) of TRIS, TAPS, TAPSO, and TABS from

water to aqueous ethanol solutions at 298.15 K: TRIS (�), TAPS (©), TAPSO (�), andTABS (×).

VS,ws are the total volumes of the aqueous solution and aqueousethanol solution containing the saturated solute on the molarityscale, respectively.

The transfer model has been a fixture in biophysical chemistrysince at least the 1930s [33], and it has contributed prominently tothe concept of hydrophobic interactions. Usually, the unfavorableinteractions of cosolvent with buffers, indicating that solubilitiesdecrease with increasing concentration of cosolvent, as well as the

′

positive contribution of transfer free energy. In contrast, if �Gtrhas a negative sign and the solubility increases with increasing theconcentration of the cosolvent, it indicates that the interactions arefavorable.

ility limits at 298.15 K.

�∗B/g cm−3

TRIS TAPS TAPSO TABS

1.11099 1.18961 1.07607 1.258251.09145 1.14655 1.03212 1.236561.06833 1.10912 0.99904 1.204631.04361 1.04654 0.97150 1.165071.01548 0.99718 0.94491 1.114380.98251 0.94730 0.91829 1.046230.94671 0.90525 0.89034 0.969390.90667 0.87032 0.895060.86542 0.84062 0.845740.821500.78804

M. Taha, M.-J. Lee / Fluid Phase Equilibria 289 (2010) 122–128 127

Scheme 1. Schematic illustration of the contribution of the TAPS side chain.

Scheme 2. Schematic illustration of the contribution of the TAPSO side chain.

S

itetaiaotgowTTtgIaofs

mgvf�Fobm

So

Scheme 5. Schematic illustration of the contribution of the additional –CH2 groupof TABS.

Table 7Some functional group-contributions of apparent transfer free energy (�g ′

tr ) ofbuffers from water to aqueous ethanol solutions at 298.15 K.

wEtOH/% (w/w) �g ′tr /J mol−1 from Scheme

1 2 3 4 5

10 171.63 764.12 12.48 592.49 −159.1520 460.72 1611.21 −34.02 1150.49 −494.7430 958.84 2458.27 21.15 1499.43 −937.6940 1614.34 3249.79 98.13 1635.45 −1516.2150 2632.21 4076.22 306.71 1444.01 −2325.5060 3793.52 5762.90 1019.08 1969.38 −2774.4470 5325.29 2510.03 −2815.2680 7226.06 4903.22 −2322.84

cheme 3. Schematic illustration of the contribution of the TABS side chain.

As seen from Table 6 and Fig. 3, �G′tr values are positive and

ncrease with increasing the ethanol content of the solvent mix-ures, indicating that the interactions between the buffers andthanol are unfavorable. As mentioned before, ethanol decreasesheir solubilities. Since the zwitterionic nature of the substitutedminosulfonic buffers, highly polar molecules, the unfavorablenteractions in such buffers are greater than those of TRIS buffer,s we have seen in Fig. 3. Highly positive values of �G′

tr arebtained for the interactions of TAPSO with ethanol and higherhan those of TAPS, which might be a result of the hydrophilicroup (–OH) on the ˇ carbon nearby the sulfonic group. Thebserved small, positive values of �G′

tr for the interactions of TABSith ethanol are less than those of TAPS. It may result from the

ABS molecule having one more hydrophobic group (–CH2) thanAPS. In interpreting the plots shown, it must be noted that whilehese buffers, by virtue of its –OH, protonated amine, and sulfonicroups, interact with solvent mainly through hydrogen-bonding.t is expected that the transfer of these buffers from water toqueous ethanol solutions will be accompanied by positive valuesf �G′

tr , as experimentally observed. Since �G′tr is, by definition,

ree of solute-solute interactions, it provides information regardingolute–solvent interactions.

The �G′tr values obtained from this study can be used in deter-

ining the group contributions for the constituent functionalroups of the buffers, assuming additivity of the free energy of sol-ent interactions, as shown in Schemes 1–5. The apparent transferree energy contributed from each functional group is designated as

′

Gtr . The results are listed in Table 7 and graphically illustrated inig. 4. The TRIS side chain contributions to the transfer free energiesf TAPS, TAPSO, and TABS are obtained simply from the differenceetween �G′

tr of the buffers and those of TRIS at the same experi-ental conditions, as illustrated by Schemes 1–3, respectively. The

cheme 4. Schematic illustration of the contribution of the additional –OH groupf TAPSO.

Fig. 4. Apparent transfer free energy of some functional group-contributions (�g ′tr )

from water to aqueous ethanol solutions at 298.15 K: Scheme 1 (�), Scheme 2 (�),Scheme 3 (�), Scheme 4 (×), and Scheme 5 (©).

values of �G′tr for TAPS and TAPSO residues from water to aqueous

ethanol solutions are positive and also increase with increasing theethanol concentration. In case of the TABS residue, positive �G′

trvalues were obtained, except at 20% (w/w) ethanol is negative.In order to evaluate the contributions of –OH and –CH2 groups,we used Schemes 4 and 5, respectively. From Table 7 or Fig. 4,it can be seen that the apparent transfer free energies (�G′

tr) of–OH group are positive, are attributive to their hydrophilic interac-tions with ethanol. In contrast, the values of �G′

tr are negative for–CH2 group due to their hydrophobic interactions with dioxane orethanol.

4. Conclusions

In the present study, we have measured the densities of theaqueous ethanol solutions containing TRIS, TAPS, TAPSO, and TABSat 298.15 K. The solubility limits of TRIS, TAPS, TAPSO, and TABS in

0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100% (w/w) ethanol + watermixtures at 298.15 K were determined from the results of thedensity measurements. Our findings reveal that the solubilitylimits in water follow the order of TABS > TRIS > TAPS > TAPSO,

1 se Equ

wTtttv(t

A

t2d

R

[[

[[

[[[[[

[[[[[[[[[[

28 M. Taha, M.-J. Lee / Fluid Pha

hereas those in aqueous ethanol solutions obey the order ofRIS > TABS > TAPS > TAPSO. We have determined the apparentransfer free energies (�G′

tr) of the biological buffers from watero aqueous ethanol solutions based on the solubility data. Fromhis study, we found that the biological buffers interact with sol-ent molecules mainly through hydrogen-bonding. The values of�G′

tr) were used to calculate the contributions of various func-ional groups from water to aqueous ethanol solutions.

cknowledgements

The authors gratefully acknowledge the financial support fromhe National Science Council, Taiwan, through Grant No. NSC97-221-E-011-049-MY3 and also thank to Dr. Ho-Mu Lin for valuableiscussions.

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