Colloids and Surfaces

A: Physicochemical and Engineering Aspects 204 (2002) 141–151

Determination of thermodynamic parameters of ethoxylatednonionic surfactants by means of reversed-phase

high-performance liquid chromatography

Marieta Balcan a,*, Dan-Florin Anghel a, Anca Voicu b, Doru-Cristian Balcan c

a Department of Colloids, ‘I.G. Murgulescu’ Institute of Physical Chemistry, Spl. Independentei 202, 77208 Bucharest, Romaniab ‘C.D. Nenitescu’ Institute of Organic Chemistry, Spl. Independentei 202 B, 71141 Bucharest, Romania

c Faculty of Mathematics, Bucharest Uni�ersity, St. Academiei 14, 70109 Bucharest, Romania

Received 13 June 2001; accepted 12 November 2001

Abstract

An approach to evaluate the thermodynamic properties of ethoxylated nonionic surfactants by using thereversed-phase high-performance liquid chromatography (RP-HPLC) is described. The enthalpies (�H°) and entropies(�S°) of transfer from the mobile to the stationary phase were determined using the change of capacity factor withtemperature. The surfactants taken into consideration were homogeneous (C10–16E8) and polydisperse (C12–18EO10)ethoxylated fatty alcohols. The stationary phase was the alkyl-silica (i.e. octyl- and octadecyl-silica). The eluents werevarious methanol–water mixtures and the detection was done at 280 nm. The obtained �H° and �S° values werenegative and decreased with the alkyl chain length of the surfactant, the water content of the eluent and the lengthof the ligand bonded to the stationary phase. The methylenic increments of enthalpy, entropy and Gibbs free energyof transfer from pure water as eluent to the nonpolar stationary phase were also determined. Their values were of−8.60 kJ mol−1, −18.71 J mol−1 K−1 and −2.74 kJ mol−1 on the octyl-silica column and, respectively of −17.28kJ mol−1, −46.40 J mol−1 K−1 and −2.76 kJ mol−1 on the octadecyl-silica column. The enthalpy–entropycompensation behavior of the surfactants was evaluated and the compensation temperatures obtained were within the455–586 K range. They agreed with those for alkylbenzenes and polycyclic aromatic hydrocarbons in the RPLCsystems. The results reveal the likeness of the interactions involved in the retention of hydrophobic compounds onalkyl-bonded stationary phases and hydro-organic eluents. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Ethoxylated nonionic surfactants; Reversed-phase HPLC; Thermodynamic parameters; Enthalpy and entropy of transfer;Enthalpy–entropy compensation

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1. Introduction

Ethoxylated nonionic surfactants are am-phiphilic compounds able to self-assemble andcreate a large variety of colloidal structures in

* Corresponding author. Fax: +40-1312-1147.E-mail address: mbalcan@chimfiz.icf.ro (M. Balcan).

0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.

PII: S0 927 -7757 (02 )00007 -9

M. Balcan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 204 (2002) 141–151142

water, oil, oil–water or solid– liquid media.Their properties and applications greatly dependon the equilibrium between the hydrophobic hy-drocarbon tail and the polar poly(ethylene ox-ide) (PEO) chain, and the structure–perfor-mance relationship is very important in bothfundamental studies and practical uses of theethoxylates. From analytical point of view, asuitable method to analyze them is the high-per-formance liquid chromatography (HPLC) [1].The normal-phase liquid chromatography(NPLC) separates the ethylene oxide oligomers[2–5], whereas the reversed-phase liquid chro-matography (RPLC) gives information aboutthe hydrophobic tail [5–7]. Some reversed-phaseapproaches also allowed the separation ofethoxylated surfactants according to the ethyleneoxide (EO) units [8–11]. In comparison to thesituation in NPLC, the retention mechanism inRPLC is still unclear. One of the major andvery old issues in the theory of RPLC iswhether eluite molecules are retained by a parti-tioning or by an adsorption-like process. Twofundamental theories exist which account for theexperimental results in reversed-phase systems:the solvophobic theory by Horvath et al. [12]and the partitioning theory by Dill and co-worker [13,14]. The main difference betweenthem originates in the fact that the stationaryphase is assumed to play a passive role accord-ing to the first theory, whereas it is consideredactive in the second. In the simplified version ofthe solvophobic theory, the contributions of themobile and stationary phase to the retention en-ergetics are separated [15]. The analysis indi-cated that the mobile phase contribution variesmuch more strongly with the mobile phase com-position than did the stationary phase contribu-tion with changes in the bonded chain length.This confirms that the mobile phase plays thedominant role in RPLC retention. Both ap-proaches, however, permit to make similarpredictions.

In view of nonionic ethoxylated surfactant be-havior in RPLC, there are only few reports con-necting their physico-chemical properties withthe retention characteristics. As early as 1979,Melander et al. [8] observed numerous irregular-

ities in reversed-phase retention of commercialpoly(ethylene glycol)s and nonionic surfactants.They explained those irregularities by means oftwo stable conformers of the PEO chain thathave significantly different retention properties.Other studies dealing with the effect induced bythe hydrophobic and hydrophilic moieties of theethoxylates on their RPLC retention revealedthat the contribution of the hydrophobic alkylgroup is much stronger than that of the hy-drophilic PEO chain [16–18]. The correlation es-tablished between the retention data and thealkyl chain length of the surfactants allowed theestimation of the surfactant hydrophobicity byRP-HPLC. Few reports have been published un-til now on the retention mechanism of ethoxy-lates on reversed-phase systems with alkyl-silicacolumns and hydro-organic eluents, but they re-vealed that the retention process of ethoxylatesis quite complex [19,20]. It depends on the sup-port, the type of organic modifier and its con-centration in the mobile phase. Effects ofpolarity and solvation of the ethylene oxidegroups also alter the retention of these com-pounds.

Previously, we evaluated the retention data ofethoxylated nonionic surfactants by RPLC andassociated them with their structural characteris-tics (i.e. the nature of hydrophobic tail, thelength and conformation of PEO chain) [21–24].We found out good correlations between the re-tention data and the traditional hydrophobicityparameters of the surfactants such as the hy-drophilic– lipophilic balance (HLB) and the oil/water partition coefficient (Kow).

The main goal of this work is to determinethe enthalpy (�H°) and entropy (�S°) associ-ated with the transfer of ethoxylates from thehydro-organic eluent to the nonpolar stationaryphase. Another objective is to reveal the depen-dence of thermodynamic functions on the sur-factant molecular structure, the mobile phasecomposition and the alkyl chain bonded to thestationary phase. Finally, an attempt to revealthe physico-chemical basis of the enthalpy–en-tropy compensation process is done.

M. Balcan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 204 (2002) 141–151 143

2. Experimental

2.1. Materials

The nonionic surfactants used were series offatty alcohol derivatives. The homogeneousethoxylated alcohols (CnEm) had various alkylchains (n=10, 12, 14 and 16) and the same PEOmoiety (m=8). They were supplied by FlukaChemie AG, Buchs, Switzerland (C10E8 andC14E8) and Nikko Chemicals Co. Ltd., Tokyo,Japan (C12E8 and C16E8). The samples of polydis-perse ethoxylated alcohols (CnEOm, with n=12,14, 16 and 18 and, m=10), were obtained fromOltchim S.A. Ramnicu Valcea, Romania, andwere used without purification. To provide UVsignal, the ethoxylated fatty alcohols were reactedwith phenyl isocyanate as described in Ref. [2].

The methanol eluent was of HPLC-gradereagent. Double distilled water having an electri-cal conductivity lower than 1.5 �S cm−1 wasemployed. To remove particulate matter, the wa-ter was passed through 4612 Acrodisc filters (PallGelman Sciences, Vienna, Austria).

2.2. Apparatus

The chromatographic measurements were car-ried out on a Hewlett-Packard (Boeblingen, Ger-many) model 1084 B, liquid chromatograph,fitted with a variable detector model 79 875 A setat 280 nm. The analytical columns (200×4.6 mmI.D.) packed with irregular either octyl- (RP-8,10 �m) or octadecyl-silica (RP-18, 10 �m), werepurchased from Hewlett–Packard (Waldbronn,Germany). The mobile phase consisted of variousmixtures of methanol and water and the flowrate was of 1 ml min−1. The running solutionsfor HPLC had a concentration of 10 mg ml−1.They were prepared in methanol–water mixture(4:1, v/v) and stored at 4 °C. The injected vol-ume was of 10 �l.

2.3. Methods

All the measurements were done under iso-cratic conditions. The retention times (tr) of sur-factants were obtained from at least five

individual determinations. They were determinedat 303, 308, 313, 318 and 323 K. The relativestandard deviation was lower than 1%. The deadtime (to) was determined with deuterium oxide asmarker. The capacity factor (k �) was calculatedaccording to k �= (tr− to)/to.

Assuming the eluite in the standard state atinfinite dilution, Eq. (1) relates the chromato-graphic parameter, k �, to the standard Gibbs freeenergy (�G°), enthalpy (�H°) and entropy (�S°)of transfer from the mobile to the stationaryphase:

ln k �= −�G°/RT+ ln �

= −�H°/RT+�S°/R+ ln �, (1)

where, R is the gas constant, T is the absolutetemperature and � is the phase ratio of thecolumn (i.e. the volume of stationary to that ofmobile phase). If the diagram of ln k � vs. 1/T, thevan’t Hoff plot, is linear then the enthalpy andentropy are constant and the mechanism of theretention process is invariant over the wholerange of temperature under investigation [25].Thus, �H° can be evaluated from the slope and�S° from the intercept of the regression line,provided that the column phase ratio is known.Using the approach proposed in Ref. [26], weobtained for our RP-8 and RP-18 columns thefollowing values of � : 0.328 and, respectively,0.444. The Gibbs free energy change of transferfrom the mobile to the stationary phase was cal-culated at the mean of the working temperaturerange (i.e. 313 K) by the Gibbs–Helmholtz equa-tion:

�G°=�H°−T�S°. (2)

In the reversed-phase liquid chromatographywith nonpolar stationary phase and aqueous elu-ent the retention is characterized by the linearenthalpy–entropy compensation (EEC) [25,27–31]. It denotes a linear dependence of enthalpyon the corresponding entropy when either achange in eluite structure or in the medium isproduced. The compensation effect is conve-niently expressed:

�H°=�G�° +��S°. (3)

M. Balcan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 204 (2002) 141–151144

where, � is the compensation temperature and�G�° stands for the change in the free energy ofretention at the temperature �. To minimizestatistical artifacts, the compensation phe-nomenon is described by Eq. (4) [25]:

ln k �T= − (�H°/R)[(1/T)− (1/�)]− (�G°�/R�)

+ ln �, (4)

where, k �T is the capacity factor of eluite at theevaluation temperature, T. In order to enhancethe accuracy of evaluation, the reference tempera-ture chosen for estimating � must be equal orclose to the harmonic average of the experimentaltemperatures used to calculate �H° [32]. Accord-ing to Eq. (4), a plot of ln k �T vs. −�H° forvarious eluites is linear when the compensationoccurs. That is, when the retention mechanism issimilar for all eluites. The compensation tempera-ture is evaluated from the slope of the regressionline. To estimate the thermodynamic functions ofretention and to calculate the slope of compensa-tion plots, we used the least-squares regressionanalysis.

3. Results and discussion

3.1. Thermodynamic parameters of retention

Fig. 1 illustrates the change of ln k � as a func-tion of the reciprocal absolute temperature (1/T)for homogeneous ethoxylated fatty alcohols onoctyl-silica column and methanol–water (85:15,vol.) eluent. Similar graphs were obtained for allthe other investigated systems. They show theincrease of capacity factor by decreasing the tem-perature irrespective of the alkyl chain length ofthe surfactant. Early van’t Hoff plots obtained byMelander et al. [8] for oligomers of phenylethoxylates on RP-18 column and acetonitrile–water mixtures were curved. The authors ex-plained them by the existence of two conformersof the PEO chains: the extended zigzag and thecompact meander form. The van’t Hoff plotsobtained by us are comparable with the plots ofethoxylated octylphenol (Triton X-100) onbranched polyfluoroalkylsilane column and

methanol–water eluent [11]. The linearity of theplots suggests a single conformation for the ethyl-ene oxide chain in the investigated temperaturerange. Our plots are also similar with the van’tHoff plots previously reported in the RP-HPLCliterature for nonpolar and weakly polar com-pounds [25,27–31,33,34]. For the surfactants un-der investigation, the data in Fig. 1 obey thefollowing expression:

ln k �=a+b/T, (5)

where, a and b are the intercept and, respectively,the slope of the straight line. These parameterstogether with the standard deviation of the slope(SD) and the correlation coefficients of fit (r) aregiven in Table 1. The positive slopes denote thedecrease of retention with the temperature. Thevery close to unity correlation coefficients indicatea mechanism of retention that does not changewithin the range of temperature underinvestigation.

The enthalpy and entropy changes of transferfrom eluent to the nonpolar stationary phase werecalculated for each surfactant and the results aresummarized in Tables 2 and 3. Both �H° and�S° are negative. The negative enthalpies standfor an exothermic retention process. For a givenstationary and mobile phase, the �H° is more

Fig. 1. The change of capacity factor of homogeneous ethoxy-lated fatty alcohols with reciprocal temperature on the RP-8column and 15% (vol.) water in the methanol eluent: (�)C10E8; (�) C12E8; (�) C14E8, and (�) C16E8.

M. Balcan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 204 (2002) 141–151 145

Table 1Regression parameters of Eq. (5) for the CnE8 surfactants on the RP-8 column and various methanol–water eluents

Intercept Slope (K)Methanol (%, vol.) SDaSurfactant rb

C10E890 −4.15 1.09 0.10 0.9961−4.56 1.29C12E8 0.25 0.9814

C14E8 −5.50 1.69 0.02 0.9999−5.56C16E8 1.75 0.24 0.9821

−4.54 1.3985 0.10C10E8 0.9916C12E8 −4.99 1.63 0.05 0.9985

−5.68 1.95 0.06C14E8 0.9984−6.11 2.20 0.02C16E8 0.9999

80 C10E8 −6.43 2.16 0.16 0.9987C12E8 −7.37 2.60 0.25 0.9952

−8.80 3.18C14E8 0.24 0.9971−9.18C16E8 3.45 0.20 0.9983

a SD, standard deviation of the slope.b r, linear correlation coefficient.

The trends of �H°, −T�S° and �G° vs. thenumber of carbon atoms of the alkyl chain (nC) ofthe CnE8 on the RP-8 column with 90 and 80%(vol.) methanol in the eluent and on the RP-18column eluted with 10% (vol.) water in methanolare illustrated in Fig. 2. When the stationary phaseis the same, the enthalpy, entropy and free energyof surfactant transfer decrease with the watercontent of the mobile phase (see Fig. 2(A) and (B)).With the same eluent, they lower when the alkylchain length bonded to the stationary phase in-creases (see Fig. 2(A) and (C)). In all cases, thevalues become more negative as the alkyl chainlength of the surfactant increases.

Evidence for the observed retention behavior ofethoxylated nonionic surfactants in the reversed-phase systems may be provided by a thermody-namic investigation of the retention mechanism. Inthe solvophobic theory [12,15], the chromato-graphic process is described as a reversible associ-ation between the eluite and the hydrocarbon-aceous ligand of the stationary phase. It is primarilyrelated to the hydrophobic effect [35] defined asthe unfavorable interaction of nonpolar com-pounds with water. According to this model,the retention contains two conceptual processes:the hypothetical association in the gas phasethat depends on the strength of eluite– ligandinteractions, and the solvation that entails

negative as the surfactant is more hydrophobic. Forthe same stationary phase, the enthalpy decreaseswith the water content of eluent. At constant eluentcomposition, �H° lowers with the length of alkylchain bonded to silica. The �H° values obtained inthis work are within the �H° range obtained forhydrophobic compounds in RP-HPLC systems.For example, the �H° values for transfer of theC6–C16 n-alkanols from the methanol–water (4:1,vol.) eluent to the Micropack CH-10 stationaryphase, were between −11 and −29 kJ mol−1 [27].The enthalpies of transfer from methanol–water(3:2, vol.) to octyl-silica for alkylbenzenes andpolycyclic aromatic hydrocarbons (PAHs) variedfrom −10 to −28 kJ mol−1 [34].

The standard entropies in Tables 2 and 3 arewithin −25.25 and −71.44 J mol−1 K−1. Theyfall within the limits previously reported for thetransfer of nonpolar compounds from hydro-or-ganic mobile phases to alkyl-bonded stationaryphases. For example, they agree well with the rangeof −13 to −34 J mol−1 K−1 for N,N-diethylani-line on the RP-18 column and methanol–watereluents [28] and with that of −13– −46 J mol−1

K−1 for alkylbenzenes and PAHs on the octyl-sil-ica stationary phase with methanol–water eluents[34]. The negative �S° values indicate an increasedmolecular order by association of eluites with thelipophilic ligands of the stationary phase.

M. Balcan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 204 (2002) 141–151146

eluite–eluent interactions. The solvation processhas in turn two steps: the creation of an eluite-sized cavity in the eluent and the interaction ofeluite placed in the cavity with the surroundingeluent molecules. The energy of cavity formationdepends on the cohesive energy of the eluent–elu-ent interactions and on the size of the eluitemolecule. The energy of the second step of solva-tion involves the eluite–eluent interactions. Thedriving force for retention is the free energychange associated with the equilibrium betweenthe above-mentioned interactions that are, theeluent-stationary phase, the eluite-solvated sta-tionary phase and the eluite–eluent interactions.

The creation of a cavity to accommodate theeluite molecule involves eluent–eluent hydrogenbondings and van der Waals interactions. Theprocess is endothermic. The enthalpy of cavityformation in the mobile phase is larger than in thestationary phase. It gets even larger as the nonpo-lar surface area of the eluite is bigger and themobile phase is more polar. In the organicmodifier rich-eluents used in our investigations,the bonded chains are extended assuming a fur-like configuration and the retention occurs bypenetration of the eluite molecule within the inter-ligate space [36]. Consequently, the eluite prefersto reside in the stationary phase as the surfactanthydrophobic tail increases.

The free energy change of transfer from themobile to the stationary phase also comprises anentropy term corresponding to constraints ofbonded chains in the presence of eluent. Theeluite in the stationary phase will lose a portion ofits freedom compared to the eluite in the mobile

phase. The insertion of eluite into the semiorderedphase leads to a chain extension, and therefore toincreased alignment of the chains along the axisnormal to the interface, which disfavors the reten-tion. The retention of less polar organic eluentalong with the eluite on the stationary phase maycause a somewhat greater chain ordering and thisfurther disfavors eluite retention. In the case ofnonpolar eluites these effects are however negligi-ble but in the presence of polar functional groupsthe specific effects and appropriate interactionsmay appear. In the conditions of our experiments,the enthalpies and entropies of retention obtainedfor the ethoxylated nonionic surfactants indicatethat the transfer from the polar mobile phase tothe hydrophobic stationary phase is enthalpicallyfavorable and entropically unfavorable.

Comparing the enthalpic and entropic contribu-tion to the overall Gibbs free energy, it is evidentthat the magnitude of the former is always greaterthan the latter (see in Fig. 2(A–C)). This meansthat the retention process is enthalpically con-trolled. As a result, the �G° of transfer is alsonegative and it decreases slowly with the alkylchain length of the surfactant, the water contentof the eluent and the length of the hydrophobicchain bonded to stationary phase.

3.2. Effect of surfactant hydrophobicity

As illustrated in Fig. 1, in similar chromato-graphic conditions (i.e. temperature, stationaryand mobile phases composition), the retentionincreases with the alkyl chain length of theethoxylated fatty alcohols. The behavior is consis-

Table 2Standard enthalpy (−�H°) and entropy (−�S°) to transfer the CnE8 surfactants from the methanol–water eluents to the bondedphase of the RP-8 column

90% methanolSurfactant 85% methanol 80% methanol

−�H° (kJ −�S° (J mol−1 −�S° (J mol−1−�H° (kJ −�H° (kJ−�S° (J mol−1

K−1) mol−1)mol−1) mol−1)K−1) K−1)

25.259.06 44.23C10E8 18.0028.5311.5652.0921.63C12E8 32.2610.76 13.5528.69

14.03 36.54 16.23 37.99 26.48C14E8 63.9514.60 37.01 18.30 41.57 28.71C16E8 67.13

M. Balcan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 204 (2002) 141–151 147

Table 3Standard enthalpy (−�H°) and entropy (−�S°) to transfer the CnE8 surfactants from the methanol–water eluents to the bondedphase of the RP-18 column

Surfactant 90% methanol95% methanol 85% methanol

−�S° (J mol−1−�S° (J mol−1 −�H° (kJ−�H° (kJ −�H° (kJ −�S° (J mol−1

mol−1)K−1) mol−1)mol−1) K−1)K−1)

29.80 14.40 37.1010.80 18.19C10E8 44.6412.73C12E8 32.87 17.84 43.90 21.24 49.62

C14E8 37.2215.04 19.20 44.34 29.43 71.4441.40 23.55 54.25 –17.29 –C16E8

tent with the capability of RP-HPLC withmethanol–water eluents to separate the surfac-tants according to their hydrophobic tail [5,19,21].This is due to the fact that as the eluite becomesmore hydrophobic, it is energetically more favor-able to move it from the bulk eluent into thenonpolar stationary phase. The data in Tables 2and 3 also show that for a given chromatographicsystem, �H° and �S° decrease with the alkylchain length of surfactant. The plots in Fig. 2suggest linear relationships between the thermo-dynamic parameters of retention and the nC of thealkyl chain of ethoxylated fatty alcohols. Thedependence of −�H° on the nC obeys the follow-ing equation:

−�H°=c+dnC, (6)

where, the slope, d, is the methylenic increment of−�H°, and the intercept, c, is the enthalpychange of transfer corresponding to the hy-drophilic part of surfactant molecule.

The eluites are supposed to orient so as tominimize the energy of binding the hydrophobicmoieties to the nonpolar chromatographic sur-face. Since the surfactant molecule has two moi-eties, one may admit that the retention occurs bypenetration of the hydrophobic alkyl chainswithin the interligate space of the stationary phaseand the hydrophilic ethylene oxide chains areoriented towards the bulk of the polar mobilephase. The EO groups near to the ethoxylate alkylchains come into contact with the adsorbed mo-bile phase layer. Between the ethylene oxide unitsof the surfactant and the hydroxyl groups of theeluent hydrogen bonding interactions do occur. In

the methanol–water, in contrast to the acetoni-trile–water system, the interface layer adsorbedon the stationary phase and the bulk mobilephase are almost similar [37]. In the surfactantseries investigated by us, the ethylene oxide moi-ety is the same while the hydrophobic chainlength increases, and the changes in interactionsinvolved by the second are dominant. Conse-quently with methanol–water eluents, the reten-tion hardly depends on the ethoxylated chainlength but is strongly affected by the alkyl chainlength of surfactants.

The linear correlation established by Eq. (6)between the thermodynamic functions and thelength of the alkyl chain of surfactants, allows todetermine the methylenic contribution to the en-thalpy, entropy and free energy of transfer fromthe mobile to the stationary phase. The averageincrement of −�H°, −�S° and −�G° for anadditional methylene group, that are −�(�H°),−�(�S°) and −�(�G°), are summarized inTable 4 together with the correlation coefficientsof fit. The data fall within the range of valuesreported in the literature [29,33]. One may observethat for each stationary phase, their magnitudeincreases with the amount of water in the mobilephase as to prove that the hydrophobic interac-tions enhance when eluent–eluent interactions be-come stronger. For the same mobile phasecomposition, namely at 90% (vol.) methanol inthe eluent, the methylenic increments of −�H°,−�S° and −�G° are higher on the octadecyl-sil-ica than on the octyl-silica column. These seem tosuggest a deep penetration of the alkyl chain ofthe surfactant into the bonded alkyl layer which

M. Balcan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 204 (2002) 141–151148

involves an eluite– ligand contact closer on theoctadecyl-silica in comparison with that on theoctyl-silica column.

By extrapolation of the −�(�H°), −�(�S°)and −�(�G°) values to 100% (vol.) water in theeluent, the methylenic contribution to the en-thalpy, entropy and Gibbs free energy of transferfrom the pure water to the nonpolar stationaryphase, namely the −�(�H°)w, −�(�S°)w and−�(�G°)w were obtained. The �(�H°)w and�(�S°)w values were of −8.60 kJ mol−1 and of−18.71 J mol−1 K−1 on octyl-silica and of −17.28 kJ mol−1 and −46.40 J mol−1 K−1 onoctadecyl-silica. Both for octyl- and octadecyl-sil-ica columns, the magnitude of methylenic incre-

ments of �Hw° (8.60 and 17.28 kJ mol−1) exceedsthat of the T�Sw° term (5.86 and 14.52 kJ mol−1)proving that the RPLC retention is an enthalpi-cally driven process.

The �(�G°)w was of −2.74 kJ mol−1 on octyl-silica and of −2.76 kJ mol−1 on octadecyl-silicacolumn. These values are closed each other and tothe value of −2.81 kJ mol−1 reported elsewherefor the polydisperse ethoxylated fatty alcohols(CnEO10) on the octyl-silica column [21]. Compar-ing the free energy change per unit nonpolarsurface area for nonpolar and weak polar sub-stances in the reversed-phase chromatography,during the octanol–water partition and in theadsorption on activated charcoal from aqueous

Fig. 2. The thermodynamic parameters, �H° (�), −T�S° (�) and �G° (�), vs. the carbon number (nC) of the alkyl chain forhomogeneously ethoxylated fatty alcohols (CnE8) on various chromatographic systems: (A) RP-8 column and 90% (vol.) methanol;(B) RP-8 column and 80% (vol.) methanol; and (C) RP-18 column and 90% (vol.) methanol in the mobile phase.

M. Balcan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 204 (2002) 141–151 149

Table 4Methylenic contributions to the enthalpy, entropy and Gibbs free energy of transfer for the CnE8 surfactants on the RP-8 and RP-18columns with various methanol–water eluents

Stationary phase Methanol (%, vol.) −�(�H°) (kJ mol−1) −�(�S°) (J mol−1 K−1) −�(�G°) (kJ mol−1)

0.99 (r=0.9703)RP-8 2.16 (r=0.9541)90 0.32 (r=0.9935)85 1.15 (r=0.9983) 2.24 (r=0.9956) 0.44 (r=0.9995)

1.85 (r=0.9912) 4.03 (r=0.9798)80 0.59 (r=0.9999)

1.09 (r=0.9992)RP-18 1.96 (r=0.9973)95 0.48 (r�0.9999)1.44 (r=0.9825) 2.59 (r=0.9487)90 0.63 (r=0.9999)2.81 (r=0.9669) 6.70 (r=0.9400) 0.71 (r=0.9996)85

solution one found a good agreement of thesethree parameters [36]. The values were of about−3 kJ mol−1. In addition, in the micellizationprocess of CnE8, the methylenic increment of theGibbs free energy was also of −2.90 kJ mol−1

[38–40]. For all the above-mentioned processes,the methylene groups seem to interact similarlywith the aqueous phase whether the nonaqueousphase is a hydrocarbonaceous stationary phase,an organic liquid, a solid carbonaceous adsorbentor a micellar state. These results can be explainedif one admits that the same driving force, namelythe hydrophobic interaction, governs all theabove-mentioned processes.

3.3. The enthalpy–entropy compensation

The chromatographic data obtained in thisstudy for ethoxylated nonionic surfactants wereexamined for the enthalpy–entropy compensationeffects. Previously, both in RPLC and in a varietyof processes involving aqueous solutions, linearrelations between enthalpy and the correspondingentropy have been observed [25,29,31]. Their re-sult was a small variation of the free energy. Thebehavior was attributed to the mechanistic simi-larity of the processes, due to the dominant roleof the water as solvent [41]. Enthalpy–entropycompensation applied to chromatographic sys-tems allows estimating the mechanism of chro-matographic process. When the enthalpy–entropycompensation exists for various systems, it meansthat the mechanism of retention is identical forthe evaluated cases and the compensation temper-ature is a characteristic of the mechanism.

Fig. 3 presents the plot of ln k � vs. −�H° forhomogeneous ethoxylated fatty alcohols on octyl-silica column with 15% (vol.) water in themethanol eluent. The linear fit is very good havinga high degree of correlation (r=0.9978). Thisshows that in the respective chromatographic sys-tem the retention mechanism is the same for allthe investigated surfactants. The compensationtemperature, �, calculated from the slope of thisplot was equal to 507 K (with a 95% confidenceinterval of 475–541 K). The � values for homoge-neously and polydisperse ethoxylated alcoholswith octyl- and octadecyl-silica columns andmethanol–water eluents were also calculated fromthe slope of Eq. (4). They ranged from 455 to 586K. Further, we used to estimate the compensationtemperature an alternative method which is the

Fig. 3. The capacity factor vs. the enthalpy of retention forCnE8 (n=10, 12, 14 and 16) on the RP-8 column with 85%(vol.) methanol in eluent, at 313 K.

M. Balcan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 204 (2002) 141–151150

ratio between the methylenic increments of en-thalpy and entropy changes [29,31]. The obtainedcompensation temperatures for the above men-tioned nonionic surfactants were within the 459–556 K range. The result confirms the validity ofboth methods used to evaluate the enthalpy–en-tropy compensation parameter.

The compensation temperatures obtained in ourstudy for homogeneous and polydisperse ethoxy-lated alcohols have close values irrespective ofcomposition of the methanol–water eluent and thealkyl chain bonded to the stationary phase. Itconfirms the identity of the retention mechanismin our reversed-phase systems. The narrowness of� range indicates similar physico-chemical proper-ties of these compounds. One may suppose that atmolecular level, the retention mechanism consistsin insertion of the hydrocarbon part of surfactantbetween the chains bonded to the stationaryphase, and the hydrophilic PEO remains in thepolar mobile phase. In reversed-phase systemswith methanol–water eluents the hydrophilic moi-ety has a weak influence on the retention andtherefore the effect of the hydrophobic part ofsurfactant is dominant. When the length ofbonded ligand is higher than the length of eluitealkyl chain, the eluite chains penetrate moredeeply and are strongly associated with the sta-tionary phase. The increasing of water content ineluent leads to enhancement of hydrophobic inter-actions but the mechanism does not change. Thisexplains the likeness of compensation tempera-tures obtained by us.

Because there are no available literature data onthe compensation temperature of ethoxylates inRPLC, we compared the � values obtained by uswith those reported for hydrophobic compounds.For example, the compensation temperatures ofsubstituted benzene derivatives, substituted hy-dantoins, allantoin and phenylacetic acid werewithin 490–750 K [25]. Compensation effects havebeen also explored for monohalobenzenes andpolycyclic aromatic hydrocarbons and the com-pensation temperatures were in the 470–760 Krange [30]. In accordance with these data, one mayadmit that the intrinsic mechanism of eluite-sta-tionary phase interaction is similar for a widerange of hydrophobic organic compounds in re-

versed-phase chromatography. To support thisidea, one has to mention that in systems of polarstationary phases and nonpolar eluents the associ-ated � values are much lower. The compensationtemperatures calculated for phenols and acetophe-none on naked silica eluted with n-hexane werearound 140 K [42]. This suggests that the retentionmechanism in NPLC is quite different from thatoperating in RPLC.

4. Conclusions

We used in this work the effect of temperatureon the RPLC of ethoxylated nonionic surfactantsto determine the standard enthalpy (�H°) andentropy (�S°) of eluite transfer from the mobile tothe stationary phase. Both �H° and �S° of reten-tion decreased with the alkyl chain length ofsurfactant, the water content of hydro-organiceluent and the length of ligand bonded to thestationary phase. The negative values of enthalpywere within the range for hydrophobic compoundsand indicate the spontaneity of surfactant transferfrom the eluent to the nonpolar stationary phase.The results agreed with the solvophobic model ofthe reversed-phase liquid chromatography.

The contribution of methylene group to theenthalpy, entropy and free energy of transfer fromthe mobile to the stationary phase were deter-mined for various eluents including the pure wa-ter. The enthalpic and entropic contributions tothe Gibbs free energy showed that the retention isenthalpically controlled, and proved that all theseprocesses are governed by hydrophobicinteractions.

In the investigated systems we found that theenthalpy–entropy compensation operates, and wecalculated the compensation temperatures. Theobtained values for ethoxylated alcohols wereclosed to each other irrespective of the mobilephase composition and the length of alkyl chainbonded to the stationary phase. They were alsonot very different from those reported in literaturefor nonpolar and weak polar substances. Thissubstantiated that at molecular level, all thesecompounds obey a similar mechanism of retentionin RPLC.

M. Balcan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 204 (2002) 141–151 151

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