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J. Chem. Thermodynamics 39 (2007) 240–246

Upper critical solution temperatures for immiscible solvent systemswith halide salts, carboxylic acids, surfactants and polynuclear

aromatic compounds and benzene derivatives

Man Singh *

Chemistry Research Lab, Deshbandhu College, University of Delhi, New Delhi 110 019, India

Received 6 May 2006; received in revised form 11 July 2006; accepted 12 July 2006Available online 8 August 2006

Abstract

Upper critical solution temperatures (UCSTs) of (water + phenol) systems are reported with 0.1 mol Æ kg�1 halide salts, carboxylicacids, 1.0% PEG 200 in water, and 0.01 mol Æ kg�1 surfactants and polynuclear aromatic compounds namely benzene, naphthalene,anthracene, chrysene; and benzene derivatives namely toluene and xylene solutions in phenol. Valence electrons and shell numbers, basi-city, –CH3 and –CH2–, hydrophilic, hydrophobic and p conjugated electrons of respective additives have been noted to affect their UCSTsand mutual solubilities. The surfactants decrease the USCTs with higher mutual solubilities due to effective hydrophilic as well as hydro-phobic interactions with aqueous and organic phases, respectively. A stronger structure breaking action of 3(-OH) of glycerol outweighsthose of the 3(-COO�) and 1(-OH) of citric acid and urea does produce almost equal UCSTs as compared to glycerol. A decrease inUCSTs is noted with increasing number of conjugated p electrons of benzene, naphthalene, anthracene and chrysene. In general, dTc/dx2 values of salts for 0.20 to 0.16 mole fractions of phenol are found positive while for 0.055 to 0.052 mole fractions, the negative.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Critical solution; Surfactants; Mutual solubility; Hydrophilic; Hydrophobic interactions

1. Introduction

Currently mutual solubilities and upper critical solutiontemperatures (UCSTs) data of immiscible phases [1] areelegant tools in solution chemistry and chemical engineer-ing. A scarcity of USCT data with additives is noted inthe literature [2], thus four different series of additives havebeen studied in our work and listed as a first series contain-ing mono (NaF, NaCl, NaBr, KCl, KBr, NH4Br, andNH4SCN), di (MgCl2, CaF2, CaCl2, and SrCl2) and triva-lent (AlCl3, CrCl3, and FeCl3) salts. The second series con-tains mono (formic), di (oxalic, malonic, and succinic), andtribasic (citric) acids; glycerol and urea. The third seriesconsists of CTAB (cetyltrimethyl ammonium bromide),TEAB (tetraethyl ammonium bromide), CPC and CPB

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doi:10.1016/j.jct.2006.07.011

* Tel.: +91 11 26217579.E-mail address: mansingh50@hotmail.com.

(cetyl pyridinium chloride and bromide) cationic; LDSand SDS anionic and CA (cetyl alcohol), butanol, PEG200 (polyethylene glycol 200) and EGMDE (ethylenegly-colmonododecyl ether) nonionic surfactant. The fourthhas polynuclear aromatic compounds: benzene, naphtha-lene, anthracene, chrysene, and toluene and xylene (deriva-tives). The objective of these studies has been to make theUCST data available for a wide range of additives asmutual solubilities of immiscible solvents are of industrialuse. Thus our data would serve a wider purpose in solutionengineering of immiscible solvents for mutual solubilitieswith several additives, which do vary the dipole momentof solutions that could allow estimation of their interac-tions with industrially useful chemical compounds. TheUCST data illustrate ionic, hydrophilic and hydrophobicinteractions of respective additives to define cloud pointsof systems with different additives, which are of thermody-namic significance. Aims and objectives of these studies

M. Singh / J. Chem. Thermodynamics 39 (2007) 240–246 241

have been to procure UCST and critical composition datato formulate set of solutions of immiscible solvents havingpotential uses in soaps, detergents, textile, ink, paint andpigments, solvent extraction, disinfectant solutions. Addi-tives crucially change the dipole moment of mixed solvents,which may be very useful in developing the mixed solventfor drug delivery systems.

2. Materials and methods

2.1. Solutions preparations

Stock solutions of 0.1 mol Æ kg�1 salts, acids and 1.0%PEG 200 separately in water and of 0.01 mol Æ kg�1 eachof surfactants and aromatic compounds in phenol wereprepared, w/w, the solubility restriction permitted use of0.01 mol Æ kg�1 CaF2 and CrCl3 aqueous solutions. Initiallywater and phenol as reference liquids were taken in opticalcell in 2:8 ratio and temperature points for both the disap-pearance and reappearance of turbidity measured, about20 subsequent additions of water were made, for each addi-tion of 2 cm3 water, both the temperature points measured.After completion of (water + phenol) systems, the cell wascleaned and rinsed with solution whose cloud point tem-peratures were measured, further each of aqueous solutionswas taken with phenol in 2:8 ratio for measurements ofboth the temperature points along with subsequent addi-tions of aqueous stock solutions. Addition of each aqueousstock solution to phenol does not change the mole fractionof the additive as the stock solution itself was added thatcarries a similar addition of solvent and additive, respec-tively, and this was repeated for all additives whose solu-tions were prepared in water. Similarly it was repeatedfor phenolic solutions, thus the temperature points for dis-appearance and reappearance of turbidities were measuredfor two separate sets of either aqueous (solution + phenol)or (water + phenolic) solution in 2:8 ratio taken in a Boro-sil sample glass cell of 15 cm in length and 1.5 cm radii or3 cm inner diameter mounted on stand placed in paraffinoil filled thermostat. The cell capacity was determined withpr2h relation, r = 1.5 cm, h = 15 cm, and p = 3.142, thesegave 106 cm3 volume of cell which easily accommodatedmore than 20 subsequent additions. Some vacant volumeof the cell was left unoccupied due to safety measures forstopping evaporation of solutions during measurements,although the top end of cell was properly blocked afterinserting contact and Beckman thermometers, and animmersion rod. A gentle heating from T = 273.15 K atthe rate of 1 K Æ min�1 was made with a 25 watt-immersionrod connected to automatic electric relay through a contactthermometer followed with smooth stirring (glass stirrer) at50 rpm with wiper motor.

2.2. Turbidity measurements

At a point slightly below the temperature of the disap-pearance of the cloud or the turbidity point, the heating

rate was reduced to 0.5 K Æ min�1, as the disappearancewas very sharp. The temperature was controlled to(273.15 ± 0.01) K, read with Beckman thermometer; thephase dissolution was viewed with an eye lens of the cath-etometer kept at about 1 m away from the thermostat.Near the point of disappearance of the cloud point, theheating was stopped and the temperature at which the tur-bidity vanished was noted and taken as the transition pointtemperature. After noting this temperature, the mixturewas air-cooled till a clear turbidity was reappeared, i.e.

appearance of the cloud point, the temperature of thispoint was also noted with utmost care. For the air-cooling,slightly colder air than the temperature of the paraffin bathtemperature was passed around the cell through a coppercoil inserted in the paraffin wax bath. The air was propelledwith booster pump at a rate of 2000 cm3 Æ min�1 from anair cooler fitted separately. The temperatures of the disap-pearance of the turbidity or the phases are denoted asmutual solubility temperatures for each subsequent addi-tion, similarly the temperature of a point where the turbid-ity just reappeared during air-cooling was noted withutmost care. The 2 cm3 of each of the respective solutionswere added to the corresponding set of the solutions andwhole exercise was repeated. Similarly at least 20 separateadditions were made till the mutual solubility temperatureshowed a lower value that resulted in a parabolic relationwith mole fractions of the phenol and water in thesolutions.

2.3. Chemicals

Chemical substances (AR, Merck) of each series andphenol (AR, Merck) were used as received; benzene waspurified removing thiophene with concentrated H2SO4 fol-lowed by NaOH treatment and distillation over CaCl2.Anhydrous glycerol was obtained by treatment with con-centrated H2SO4 and demineralised water triply distilledwith KOH and KMnO4, and degassed.

The NaF, NaCl, NaBr, KCl, KBr, NH4Br, andNH4SCN; MgCl2, CaF2, CaCl2, SrCl2; AlCl3, CrCl3, andFeCl3 salts were of AR grade (Merck). The formic, oxalic,malonic, succinic, and citric acids (AR Merck); glyceroland urea (AR, Merck); the CTAB, TEAB, CPC, CPB,LDS, SDS, CA, PEG 200 (AR Merck), and EGMDE(Fluka); naphthalene, anthracene, chrysene, and tolueneand xylene were of AR grade (Merck). The chemical sub-stances were dried, before their corresponding melting tem-peratures, for 24 h and stored in P2O5 filled desiccator tillused as received.

2.4. Illustration of systems

For each water and phenol ratio, mutual miscibility tem-peratures were noted and plotted against correspondingphenol mole fractions in figure 1. More than 25 composi-tions with each additive have been run for mutual miscibil-ity temperatures. In general, the mutual solubility of the

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0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50xPhenol

mis

cibi

lity

tem

pera

ture

s (T

/K)

FIGURE 1. Plot of mutual miscibility temperature against mole fractionof phenol (xphenol) for each cloud point. For each composition of phenol,which becomes mutually soluble, temperatures are noted and plottedagainst the corresponding compositions. The apex of the curve depictsupper critical solution temperature; the area within curve system remainssingle phase and outside the curves there are two phases.

242 M. Singh / J. Chem. Thermodynamics 39 (2007) 240–246

binary immiscible systems varied with their compositionswhere either one of them may behave as a solute or as a sol-vent. Figure 1 depicts the mole fractions of water and phe-nol increasing in opposite directions where mole fractionsbelow the curve lead to single-phase solutions and abovethe curve to two-phase systems. Thus the component desig-nated as solvent dissolves another designated as soluteeither at normal or slightly higher temperatures but withcompositions of each the cloud point temperatures arehigher. Hence their wider compositions with higher con-centration differences are studied with several subsequentadditions. The systems with additives are ternary systemsand the mole fractions of each component are calculatedat cloud point temperatures.

2.5. Calculations

Numbers of moles for each component were determinedto calculate mole fractions of each component and theUCSTs. The critical compositions (CSs) were obtainedfrom mutual solubility temperatures plotted on the y-axisagainst the corresponding phenol mutual solubilities onthe x-axis and the curve was drawn. The maximum of thiscurve gave values of the UCSTs and critical solubilities(CSs). The data are given in table 1 where the mole frac-tions of each additive were calculated including the amountof previous subsequent additions. The dTc/dxphenol valueswere calculated from subsequent mole fractions and corre-sponding solubility temperatures (Tc) with the followingequation:

dT c=dxphenol ¼ dððT cÞ2 � ðT cÞ1Þ ¼ dððxphenolÞ2 � ðxphenolÞ1Þ:ð1Þ

3. Results and discussion

The discussion is divided into eight separate partswhere UCSTs for salts, acids, surfactants and aromaticcompounds are dealt with, and CSs are correlated withmole fractions of each component at the cloud point(table 1) with each additive. The mole fractions weredetermined from similar curves as shown in figure 1.The UCST was obtained at 0.0984 moles of phenol and0.9016 of water, respectively. The UCST and CS datafor the (water + phenol) blank systems showed a closeagreement with those of the literature [3] values withT = (273.15 ± 0.12) K and ±0.25% errors, respectively.The UCSTs and CSs with each additive are different (fig-ure1) due to their nature, thus critical solutions areobtained at different temperatures and would illustratethe influence of the additive on critical solutions, whichmay be used as critical solvents. With the rise in temper-ature, the region of liquid–liquid immiscibility decreases,until it shrinks to zero above the USCT and the liquidsare completely miscible. The UCST point at the top ofcurve in figure 1 is similar to the liquid-vapour criticalpoint of a pure substance hence it illustrates an interest-ing behaviour of immiscible solvents.

3.1. UCST and CS for monovalent salts

The UCST varies with additives in the order of NH4-Br > NaBr > KI > RbCl > KCl > KBr > NaF > NH4SCN >NaCl and values of the CS in order of NH4SCN > NaI >NaBr > NH4Br > NaCl > NaF > KBr > KI > KCl > RbCl,which infer a sequence of strength of (ion + water) interac-tions. These influence the solubilities of water and phenolmutually to achieve a critical solvent-like behaviour withspecific UCST values. Similarly the NaBr > NaF > NaClorder of UCST and NaBr > NaCl > NaF of CS valuesinfer a greater polarisation [4–7] on Br� and effective

TABLE 1Upper critical solution temperature (UCST) for the various additives

N CS xphenol xwater xadditive UCST/K

(Water + phenol) 21 36.3 0.0984 0.9016 0 338.75Additive Monovalent saltsNaF 15 33.41 0.0587 0.611 0.00059 344.15NaCl 9 38.46 0.0702 0.5866 0.00045 341.15NaBr 17 44.35 0.0848 0.5557 0.00027 348.85NaI 17 45.48 0.0878 0.5494 0.00019 349.55KCl 9 22.92 0.0374 0.6562 0.00030 345.55KBr 17 32.95 0.0577 0.6131 0.00021 345.15KI 16 33.05 0.0579 0.6127 0.00015 346.15RbCl 17 26.6 0.0445 0.6411 0.00019 346.05NH4Br 9 39.05 0.0716 0.5837 0.00027 349.75NH4SCN 11 55.55 0.1168 0.488 0.00054 342.65

Divalent saltsMgCl2 16 44.31 0.0847 0.5559 0.00029 348.35CaF2 16 26.67 0.0623 0.8939 0.00004 339.65CaCl2 13 66.66 0.1555 0.4061 0.00035 362.65SrCl2 16 55.3 0.116 0.4897 0.00021 354.45

Trivalent saltsAlCl3 31 56.91 0.2019 0.7981 0.00025 357.75CrCl3 15 39.86 0.1069 0.8426 0.00002 340.85FeCl3 21 43.51 0.0826 0.5603 0.00017 371.95FeCl3 21 19.32 0.0307 0.6702 0.00013 370.65

Monobasic acidsHCOOH 16 33.39 0.0587 0.6111 0.00054 338.75CH3COOH 12 34.6 0.0613 0.6054 0.00042 338.25

DibasicOxalic acid 9 55.55 0.1168 0.488 0.00026 347.15Malonic acid 12 56.88 0.121 0.479 0.00032 344.75Succinic acid 14 44.39 0.0849 0.5555 0.00024 335.15

Tribasic acidsCitric acid 17 33.2 0.0582 0.612 0.00012 338.55

Nonionic surfactantsGlycerol 16 30.7 0.0529 0.6233 0.00026 343.65Cetyl alcohol 16 28.87 0.0689 0.8863 0.00001 341.95Butanol 11 36.7 0.0661 0.5953 0.00035 341.15

Ureas0.4Urea 9 38.49 0.0346 0.2889 0.00174 343.250.6Urea 9 47.26 0.0326 0.1899 0.00290 335.95Methylurea 9 47.8 0.1409 0.8037 0.00004 342.55Dimethylurea 9 48.2 0.1428 0.8015 0.00003 341.85

Aromatic compoundsBenzene 11 57.15 0.1907 0.7466 0.00004 367.65Naphthalene 11 21.74 0.0484 0.9098 0.00002 354.95Anthracene 20 36.87 0.0957 0.8555 0.00001 342.15Chrysene 20 36.86 0.0956 0.8556 0.00001 340.95

BenzeneToluene 21 34.05 0.0857 0.867 0.00003 340.05Xylene 21 31.46 0.0771 0.8769 0.00002 340.05

Cationic surfactantsCTAB 10 47.29 0.1385 0.8064 0.00001 336.15TEAB 11 40.33 0.1146 0.8854 0.00002 345.35CPC 11 57.3 0.1916 0.7456 0.00001 338.15CPB 13 47.22 0.1382 0.8068 0.00001 338.95

Anionic surfactantsSDS 8 54.54 0.1755 0.764 0.00001 333.75LDS 10 54.54 0.1755 0.764 0.00001 326.25

Nonionic surfactantsPEG 200 14 19.27 0.0306 0.6704 0.00010 348.95EGMDE 16 25.27 0.0419 0.6466 0.00011 338.05

The critical solubility (CS) is in percent, N refers to the number of data points and the reference temperature was 298.15 K. The xphenol, xwater, and xadditive

represent mole fractions for phenol, water and additives, respectively.

M. Singh / J. Chem. Thermodynamics 39 (2007) 240–246 243

244 M. Singh / J. Chem. Thermodynamics 39 (2007) 240–246

nuclear charge on F� and Cl� anions, respectively, to illus-trate the order of their anionic hydrations.

3.2. UCST and CS for di- and trivalent salts

Likewise AlCl3 > MgCl2 > NaCl of UCST, andKCl > CaCl2 and CrCl3 > FeCl3 orders of CSs elucidate adirect effect of ionic size and valence electrons with strongerionic-hydration for smaller ions than those of ions of largersize. It elucidates the salting out of phenol with strongerinteraction with water molecules that requires a higherthermal energy to dissolve strongly bound water. TheUCST and CS values in CaCl2 > CaF2 order infer strongerhydration by Cl� than that of F�, if the latter is releasedfrom a common cation and CaCl2 > MgCl2 order ofUCSTs, and CS of NaF > CaF2 and NaBr > KBr, withUCST and CS values in order of CaCl2 > SrCl2 >MgCl2 > CaF2 infer an effective hydration of cations dueto size s. The CSs for salts of Br� are lower and the UCSTsare higher than those of �SCN of NH4SCN, which may beattributed to �S � C„N M S@C@N� resonating struc-tures. The maximum UCSTs and CSs for FeCl3andCaCl2are noted with two maxima on the curve for FeCl3with two separate UCSTs, the first set of UCSTs andCSs is higher for higher mole fractions of phenol whilefor the second set the values are lower for lower mole frac-tions of phenol (table 1). The one electron transfer fromFe3+ to phenol with reduction Fe2+ develops two separateUCSTs. The Fe3+ due to stronger hydration developsstronger hydration with higher UCSTs while Fe2+ showsweaker hydration with lower UCSTs (table 1). Thus thephenol concentration increases the Fe2+ with the weaker(Fe2+ + water) interactions than those of Fe3+. Furtherfree Fe2+ forms a complex with phenol, and the half filled3d5orbital of Fe3+ and 3d6of Fe2+ may account for thecomplex formation with water as the ligand. TheFeCl3 > AlCl3 > CrCl3 of UCST and AlCl3 > FeCl3 >CrCl3 orders of CSs infer that half 3d5, fully 2s2p6 and par-tially half filled 4s�3d3 cations of these salts alter the(salt + water) interactions.

3.3. Comparative UCSTs and CSs for salts

Slightly lower values of UCST and CS for RbCl, andhigher values of UCST for NH4Br among monovalentand lower UCSTs with higher CSs for NH4SCN showstronger ðNHþ4 þ waterÞ interactions. The F� developsstronger (F� + water) interactions than those by Cl�, andslightly higher UCSTs for Rb+ than that of KCl. The low-est CSs among the monovalent series result in weaker inter-actions for larger cations of the Cl� salts. Higher UCSTsfor divalent chloride salts than those of monovalent arereported, for example CaCl. In addition, SrCl2 salts showhigher CSs, however, CaF2show lower CSs with stronger(divalent cation + water) interactions. The higher UCSTsfor trivalent salts than those of monovalents and divalentsindicate a stronger hydration, but the Cr+ produces lower

UCSTs, perhaps promoting the formation of Cr(OH)3 byfilling the d orbital. However, concentrations of CaF2

and CrCl3 are different from those of the others, thus arough comparison with respect to the others is estimated.The higher UCSTs and CSs for AlCl3 show stronger(Al3+ + water) interactions with a probable contributionof the orbitals for hydration.

3.4. Systems with carboxylic acids

Comparatively higher UCSTs and lower CSs for formicacid than those of acetic acid infer a dominance of hydro-phobic over hydrophilic interactions due to the –CH3

groups. But oxalic and malonic acids result in higherUCSTs and CSs than those of monobasic acids where the–CH2– (methylene) groups of malonic acid enhancehydrophobic interactions even though the higher UCSTsindicate stronger hydrophilic interactions with higherCSs. The two –CH2– groups of succinic acid decrease theUCST by 9% as well as the CSs, whereas higher UCSTsand lower CSs for glycerol and citric acid than those of suc-cinic acid illustrate the outweighing of hydrophilic interac-tions due to the 3(-COO�) and 1(-OH) groups of citric acidover those of hydrophobic interactions arising from the3(-CH2-) groups.

Glycerol with 3(-OH) groups shows higher UCST andlower CS values than those of the tribasic acids, thus the3(-OH) groups produce a stronger structure breakingaction on water than those by the 3(-COO�) and 1(-OH)groups of the citric acid. Urea with ketonic (�C@O) andamino (–NH2) groups produces almost equal UCSTs asglycerol with higher CSs, whereas urea concentrationsdecrease the UCST and increase CSs as occurs withmethylurea.

3.5. Surfactants systems

The trends of the UCSTs and CSs for glycerol, citricacid, urea, and methyl urea have similarity with those ofthe surfactants, which decrease interfacial potential show-ing a decrease in UCSTs and an increase in CSs due tohydrophilic and hydrophobic interactions.

Thus glycerol, citric acid, and urea behave as surfac-tants. The lower UCSTs and higher CSs for SDS andLDS indicate they are more anionic than those of CPCand CPB, The cationic surfactants are observed to possessstronger interactions of SO2�

4 than those of Cl� and Br�

cationic surfactants. Likewise, actions of –OH of citricand glycerol are compared with those of butanol that con-tain 1(-CH3) and 3(-CH2-) groups. The –OH of butanol isnoted to decrease UCSTs by 275.65 K and to increase CSscompared with those of glycerol by 0.00121 mole fraction,and an increase in UCSTs by 275.72 K and CSs by 0.00062mole fraction over those of citric acid. Hence –CH3 and –CH2– enhance hydrophobic interactions. Cetyl alcoholshows a slightly higher UCST by 274.45 K and lower CSsby 0.0452 mole fractions compared with those of butanol,

M. Singh / J. Chem. Thermodynamics 39 (2007) 240–246 245

which infer a strengthening of hydrophobic interactionsdue to cetyl, a longer alkyl chain. The UCSTs for EGMDEare found to be close to the values of surfactants, valueslower in CSs by 0.0758 mole fractions, which infer strongerhydrophilic interactions of the EGMDE than those ofother non-ionic surfactants. As EGMDE has a larger num-ber of –OH groups with which to develop stronger hydro-philic interactions. Likewise, UCSTs for PEG 200 are verynear to the values of salts, which show similarity in hydro-philic interactions of PEG 200 with those of ionic interac-tions of salts.

3.6. Systems with aromatic compounds

Interestingly, the UCSTs for benzene are found to beclose to the values of tri- and divalent salts which provesthat p-conjugation of benzene develops hydrophobic inter-actions of the same strengths as those by hydrophilic inter-actions of salts. In addition, values of the UCSTs insubstances from benzene to chrysene decrease in a rationalmanner with numbers of p conjugated electrons. This sug-gests a weakening of hydrophobic interactions with p con-jugated electrons where perturbation of the latter maydevelop hydrophilic interactions in the same ratio. Agreater decrease in UCSTs for naphthalene than those ofanthracene elucidate a greater activity due to the two adja-cent rings of naphthalene. Likewise a decrease in theUCSTs by 300.76 K and CSs by 0.0458 mole fractionsfor toluene and xylene compared with aromatics prove thathyper conjugation due to –CH3 of the toluene and xyleneleads to the development of slightly stronger hydrophobicinteractions within toluene than the hydrophilic interac-tions as compared to xylene. Perhaps electron spin isresponsible for this difference. However, a negligiblechange in UCSTs with a decrease in CSs by 0.0023-molefraction from toluene to xylene infer a stronger disruptionin the resonance system of p conjugation by electron with-drawing –CH3.

3.7. Comparative UCST of acids

By comparison, the sequence oxalic > formic, cit-ric > succinic for the UCST and succinic > citric acid forthe CS, and the UCST for oxalic > formic > citric > succi-nic and the CS for oxalic > succinic > formic > citricindicates a stronger hydrophilic interaction [4–6] by–COO� and –OH than those hydrophobic interactions[8–10] by –CH2– of succinic and citric acids. This reflectsthe fact that basicity of carboxylic acids increases UCSTsand increases CSs by 0.0110 mole fractions. Similar valuesof –OH and 2(-CH2-) and 1(-CH-) of glycerol produceUCSTs in the sequence glycerol > 0.4 m urea > 0.6 m ureawith a decrease in CS values. These observations illustratestronger hydrophilic interactions [11,12] within the 3-OHof glycerol compared with those of urea by 0.00014 molefractions and citric acid by 0.078 mole fractions. The citricacid > glycerol sequence of CS values with reverse UCST

values proves higher mutual solubility by –OH and–CH2–. The sequences TEAB > CPB > CPC > CTAB forthe UCST and CPC > CTAB > CPB > TEAB of CS, theLDS > SDS for UCST values with equal CS values infera greater effect of polarisability [13–15] of tetraethyl, cetyland dodecyl alkyl chains. Four C2H5– groups of TEABdevelop stronger hydrophobic interactions [16] and theammonium ion develops stronger hydrophilic interactionswith water, respectively, which integrate both phases withhigher UCST and lower CS values. Thus cetyl and dodecylalkyl chains of CPC, CPB, CTAB, LDS and SDS surfac-tants show weaker interactions. Lower values of UCSTand higher values of CS of surfactants [17,18] than thoseof salts, acids and aromatic compounds reveal strongerhydrophilic and hydrophobic interactions.

3.8. dTc/dxphenol values

The dTc/dxphenol values for salts are found to be greaterthan one when the mole fraction phenol equals either (0.20to 0.16) or (0.055 to 0.052) with positive values for the for-mer and negative values for the latter, which infer a decreasein Tc with phenol fractions with each salt. Carboxylic acidsof similar mole fractions with oxalic acid show values of thedTc/dxphenol ratio = +0.93. However, formic, succinic, andcitric acids produce slightly lower values than that of oxalicacid. This illustrates the comparatively stronger interactionof oxalic acid with water, which allows water to mix homog-enously at slightly higher temperature. Among the aromaticcompounds, the dTc/dxphenol ratio of �1.38 for benzene forvalues of 0.055 to 0.052 mole fractions of phenol infers itsstronger interactions with phenol due to p conjugation elec-tron. Similar values of the dTc/dxphenol ratio are reportedwith cationic surfactants whereas cationic and non-ionicsurfactants have lower values that infer slightly strongerhydrophilic interactions.

4. Conclusion

The UCSTs in the sequence NaI > NaBr > NaF > NaClreflect stronger interactions of large-sized anions of thecommon cation. The K+ leads to an order of KI >KCl > KBr with stronger (I� + water) interactions, andan increase in cation size from the 3s to 4s orbital forNa+ (NaBr) and to K+ (KBr), decreases the (Br� + water)interactions by 5.6%. Further the sequence NaI > NaBr >NaCl > NaF, KI > KBr > KCl for the CSs infer stronger(Br� + water) interactions. The Cl� with its higher size ofcation (Rb+) increases the UCSTs by 280.65 K and273.85 K for Na+ and K+, respectively. The I� of NaI pro-duces an increase of 278.45 K in the UCST over those ofKI, leading to the conclusion that halide anions of thesmaller cations cause stronger hydration, but I� as a largeranion with the smaller cation shows higher UCSTs. There-fore, size of either the cation or anion determines the devel-opment of stronger ionic interactions, but Br� with NHþ4(group cation), with its stronger ðNHþ4 þ waterÞ interac-

246 M. Singh / J. Chem. Thermodynamics 39 (2007) 240–246

tions, shows higher UCSTs than those of Na+, K+ andRb+. However, NHþ4 with SCN� decreases the UCST by0.0226 mole fractions and increases CS by 0.0892 molefractions with stronger hydrophobic interactions due toresonating structures �S � C„N M S@C@N�. FromMg2+ to Ca2+ (table 1) and also increases the values ofUCST and CS but with lower values for Sr2+. The F� withCa+ results in lower UCST and CS values than those ofCl� due to the low solubility of CaF2 in water where disso-ciation of CaF2 and NaCl remains unaffected. The trivalentcations show a higher UCST except for CrCl3, whereasAlCl3 produces maximum CS values due to the vacant 3porbital, which should have some linkage with phenol elseits cloudy solutions could break hydrogen bonding of phe-nol and water. The –COO� of oxalic acid increases theUCST by 0.00012 mole fractions as compared with aceticacid where CH3COO� may interact with phenol and water,and partly may be in dimer form with a greater decrease inUCST and CS for oxalic acid, which then decrease with theintroduction of –CH2–, i.e. succinic acid and continues onto citric acid. The urea and –CH3 substituted N-urea causehydrophobic interactions (table 1). The –CH3 contributionfrom urea to methyl urea is 274.45 K for UCST but CS is0.0782 mole fractions, and methyl urea to dimethyl ureaUCST is 274.15 K whereas CS is 0.00015 mole fractions,having stronger hydrophobic interaction with dimethylurea than the hydrophilic interactions in urea and methylureas. Aromatic hydrocarbons with a p conjugationdecrease the UCST in the ratios 44:24.8:5.1:3.3 for3p:5p:7p:9p, with a greater decrease in CSs for naphtha-lene. This ratio may be associated with resonance energywithin the aromatic hydrocarbons. The –CH3 of toluenedoes show a change in UCSTs but the CS decreases by0.0013 mole fractions. Thus –CH3, an electron-withdraw-ing group with methyl urea and toluene, decreases the

UCSTs and CSs values, resulting in –CH3 strengtheningthe hydrophobic interactions.

Acknowledgements

Author gratefully acknowledges financial support pro-vided by University Grants Commission, New Delhi, andDr. A.P. Raste, Principal, DBC, for support.

References

[1] J. Wang, M. Nozaki, Y. Ishikawa, M. Lachab, R.S.Q. Fareed, T.Wang, Shiro Sakai, J. Colloid Interface Sci. 220 (1999) 324–328.

[2] J.W. Goodwin, R.H. Ottewill, R. Pelton, Colloid Polym. Sci. 257(1979) 61–69.

[3] Vojtech Fried, Hendrik F. Hameka, Uldis Blukis, Physical Chemistry,Macmillan/Collier Macmillan, New York/London, 1977, pp. 229–232.

[4] M.L. Mc Glashan, J. Chem. Thermodyn. 22 (1990) 653–657.[5] J.H. Wang, B.C. Baltzis, G.A. Lewandowski, Biotechnol. Bioeng. 51

(1996) 87–90.[6] J. Mandelstan, G.A. Jacob, J. Biochem. 94 (1974) 569–572.[7] R. Bhat, J.C. Ahluwalia, J. Phys. Chem. 89 (1985) 1099–1103.[8] H.M. Luis Da Silva, Watson Loh, J. Am. Chem. Soc. 104 (2000)

10069–10073.[9] W. Steven Rick, J. Phys. Chem. 104 (2000) 6884–6888.

[10] G. Nemethy, H.A. Scheraga, J. Chem. Phys. 36 (1962) 3382.[11] Daiwen Yang, J. Am. Soc. 121 (1999) 3555–3556.[12] S. Paljk, C. Klofutar, J. Chem. Soc., Faraday Trans. I 2159 (1978) 75.[13] A. Suggett, J. Solution Chem. 10800 (1976).[14] H.S. Frank, F. Frank, J. Chem. Phys. 4746 (1968) 48.[15] Y. Shinich, K. Hiroko, M. Keilko, J. Phys. Chem. 104 (2000) 10242–

10253.[16] Man Singh, J. Indian Chem. Soc. 78 (2001) 397–402.[17] R.N. Barnet, U. Landman, J. Phys. Chem. 100 (1996) 13950–13953.[18] G. Barone, B. Bove, G. Castronuovo, V. Elia, J. Solution Chem.

10803 (1981).

JCT 06-111