4662
J. Phys. Chem. B 1997, 101, 4662-4666
Calorimetric Study of the Micellization of n-Butoxyethanol in Water G. Onori* and A. Santucci Istituto per la Fisica della Materia, Unita` di Perugia and Dipartimento di Fisica, UniVersita` di Perugia Via Pascoli, I-06100 Perugia, Italy ReceiVed: September 24, 1996; In Final Form: March 4, 1997X
This paper reports on critical micelle concentrations and enthalpies of micellization of n-butoxyethanol in water at three different temperatures (5, 25, and 45 °C) as determined by means of a sensitive microcalorimeter. The experimental enthalpies are compared with those calculated from the temperature dependence of the critical micelle concentration by using the phase separation model of micelle formation. Enthalpies and entropies of micelle formation are both positive and decrease as the temperature is raised indicating the hydrophobic effect in the formation of n-butoxyethanol microaggregates. These calorimetric results provide no indications of intermicellar interactions, while indicating a monomer-monomer repulsive interaction in the premicellar region.
I. Introduction Alcohol molecules contain both hydrophilic groups and hydrophobic “tails'”. This unique duality toward an aqueous environment leads to a complex self-association behavior which is not exhibited in nonaqueous solvents. The thermodynamics and spectroscopic properties of water-alcohol mixtures have been studied extensively.1-6 Quite often the measurements of these properties show maxima, minima, or inflection points at low alcohol concentrations, which are often interpreted in terms of structural changes in the solvent or molecular aggregation processes in the mixtures. However, depending on the relevant properties, these extrema or inflection points do not occur at the same cosolvent mole fraction, and several conflicting reports on the effect of added alcohol on the water structure have appeared in the literature.7 Most of the confusion arises from the absence of a well-defined structural model of the mixtures along with the absence of experimentally detailed data in the water-rich region of composition. Further, the application of different experimental techniques does not always convey the same information on the solute-solvent interactions. Quite recently, precise and detailed measurements of infrared spectra and isentropic compressibility of aqueous methanol, ethanol, 1- and 2-propanol, tert-butanol, and n-butoxyethanol as a function of temperature and alcohol concentration have been reported.7-9 A consistent picture of the molecular organization in the mixtures appears from these data. IR and compressibility data have been discussed by dividing the 0-1 mole fractions interval in three ranges defined by the mole fractions xa2 and xb2. The proposed model suggests that alcohol molecules are essentially monomolecularly dispersed and surrounded by water molecules at low alcohol concentration (x2 < xa2). With increasing cosolvent mole fraction, a point is reached (x2 ) xa2), where almost all water is involved in hydration structures. Above this concentration some kind of molecular aggregation of alcohol molecules occurs. Many properties, i.e., partial molar volumes,10 ultrasound absorption,5 dielectric relaxation,11,12 etc., show extrema or inflection points at a composition of xa2. Several of these observations indicate an increased structural integrity of water in this region. It has * Author to whom correspondence should be addressed: address, Dipartimento di Fisica, Universita` di Perugia, Via A. Pascoli, I-06100 Perugia, Italy; E-mail, SYMBIO@PERUGIA. INFN. IT. X Abstract published in AdVance ACS Abstracts, May 1, 1997.
S1089-5647(96)02935-5 CCC: $14.00
been suggested13 that an alcohol may affect the organization of neighboring water molecules, by a cooperative mechanism, producing a clathrate-like configuration having maximum of stability at the xa2 composition. In the xa2-xb2 region, IR and compressibility data indicate a progressive aggregation of alcohol molecules accompanied by strong modifications in the solvation of hydrophobic groups. Finally, in the (xb2-1) range effects due to the hydration of hydrophobic groups become negligible, and the interaction of water with the polar hydroxyl groups and associations of the alcohol molecules are predominant in determining the concentration dependence of the properties of the mixtures. In the xa2-xb2 region, where our data suggest a clustering of alcohol molecules, many properties of these mixtures show a remarkable change. In this region of composition a maximum is found in sound absorption5 and X-ray14 and light15,16 scattering. All these effects suggest clustering of the mixture components to form microscopic heterogeneities. The transition observed in the xa2-xb2 range resembles, for some aspects, the micellization process. In fact, the observed behavior is very similar to that exhibited by the concentration dependence of the C-H stretching frequencies in aqueous n-alkanoates solutions for concentration ranges encompassing the critical micelle concentration.10 As the size of the hydrophobic groups increases the width of the transition xb2-xa2 decreases. In the case of n-butoxyethanol/water mixtures this range is very narrow and comparable with that observed for the largest n-alkanoates.17 In this case the transition is closely approximated by a phase separation model and the concept of critical micelle concentration (cmc) appears to be significant. It is quite reasonable that beyond a critical cosolvent mole fraction x*2 = 0.02 n-butoxyethanol molecules in water are aggregated in micelles, as suggested by different experiments, such as hypersound adsorption,7,18,19 light-scattering,16 specific heat determination20 and small angle neutron scattering.21 Actually, n-butoxyethanol (C4E1 or BE) can be considered as a member of a well-known class of nonionic surfactants, i.e., the polyoxyethylene glycol monoethers, CH3-(CH2)i-1O(CH2CH2O)j hence after called CiEj.22 Many of these systems show complex phase behavior. The water-rich part of the phase diagram consists of an isotropic micellar solution which splits into two water-rich isotropic liquid phases as the temperature is increased. The phase diagram of water/BE mixtures is quite © 1997 American Chemical Society
Calorimetry of n-Butoxyethanol/Water Mixtures similar to that observed in long-chain CiEj amphiphile solution.22,23 Actually C4E1 aqueous solutions demix at a lower consolute temperature of about 49 °C for a critical mole fraction of 0.05.16,24 The important role that water plays in the self-association of BE molecules clearly results from a recent study of the effect of BE on the dielectric relaxation time τ of water.12 Experimentally, at a concentration of BE from 0 to x*2, a considerable increase in τ occurs increasing alcohol concentration with a break in the τ vs x2 behavior at x*2. The self-association of BE molecules has been observed in this case, looking at the behavior of water molecules rather than at the alcohol molecules. The data indicate that the water molecules near the aliphatic chain of BE have a lower rate of reorientation than that of bulk water molecules. These results support the generally accepted model of hydrophobic hydration that considers each solute molecule encaged by water, the hydrogen-bonded structure of which is suitably rearranged and stabilized. A further confirmation that micelles are present in the water/ BE system comes from recent measurements of the surface tension (γ) performed at different temperatures and concentrations.23,25 The surface tension behavior of water/BE system is typical of aqueous surfactant solutions forming micelles. In fact, with increasing BE concentration, the γ decreases down to a critical value x*2 after which it remains constant. As for the cmc of nonionic surfactants,22 x* 2 decreases with temperature. This is consistent with a phase separation model involving a positive standard enthalpy of microaggregates formation.23 Studies on the micellization of nonionic surfactants have shown that the negative free energy change which accompanies this process occurs with an increase in entropy overcompensating the positive change in enthalpy.22 Usually estimates of the micellization enthalpy ∆Hm, are based on measurements of the cmc and its variation with the temperature.26 The disadvantage of this indirect method is that it requires very high accuracy in the cmc determination to give satisfactory results for ∆Hm, and it is a model-dependent method. Therefore, careful calorimetric measurements are needed to provide reliable informations about enthalpy and entropy accompanying the BE microaggregate formation. Calorimetric measurements appear to be particularly suitable to study the micellization processes.27 The cmc and the micellization enthalpy can be determined from a single calorimetric experiment, and this technique has been used as an ideal method to quantitatively test the various theories and models for the micellization.28 In the present study a batchwise microcalorimeter with high sensitivity (to 1 µJ) was employed to measure the heat of dilution of BE as a function of concentration at 5, 25, and 45 °C. To test the validity of the proposed interpretation for the BE self-association, the values of the enthalpy of micellization extracted from the heat of dilution data have been compared with those calculated from the temperature dependence of x*2. A purpose of the present paper is to obtain by calorimetric measurements a confirmatory evidence for the proposed selfassociation behavior of BE in water. While the enthalpy of dilution of BE has already been measured in water,29 not enough data points were obtained in the water-rich region of composition to establish a correlation between these calorimetric data and our IR and compressibility data. II. Experimental Section n-Butoxyethanol (Fluka, analytical grade) was used without any further purification. The water used was bidistilled, and n-butoxyethanol/water mixtures studied were prepared by weighing the components. Precautions were taken to minimize
J. Phys. Chem. B, Vol. 101, No. 23, 1997 4663
Figure 1. (9) Apparent molal enthalpy ΦL of BE in water and (O) frequency shifts ∆νj, of the C-H stretching vibrations as a function of BE mole fraction x2 at 25 °C. (b) is the water-rich region.
evaporation losses during the preparation and the subsequent determination of the heat of dilution. The heat of dilution measurements were obtained by means of a flow microcalorimeter Setaram G II (sensitivity 1 µW). On using a pair of circulation mixing vessels in connection with the temperature prestabilizer and two peristaltic pumps Minipulse 2 Gilson, it was possible to inject the two liquids continuously. In all the cases a check of the flow rate was performed by weighing the liquid coming out from the calorimeter. III. Results and Discussion Calorimetric Measurements. The enthalpy of dilution was determined by mixing a given amount of BE/water solution with a measured quantity of H2O. The apparent molal enthalpy ΦL was then obtained by taking infinite dilution as the standard state. A plot of ΦL as a function of x2 at 25 °C is shown in Figure 1. Also reported in Figure 1 are the previously determined7 frequency shifts ∆νj of the C-H stretching vibra-
4664 J. Phys. Chem. B, Vol. 101, No. 23, 1997 tions vs x2 at 25 °C. The behavior of ∆νj vs x2 (Figure 1) is very similar to that exhibited by various surfactant systems for concentration ranges encompassing the critical micelle concentration. The IR data have been discussed in ref 7 by considering three mole fraction ranges defined by “signpost” mole fraction xa2 and xb2. At 25 °C xa2 ) 0.01 and xb2 ) 0.04. At low concentration, νj does not change with the concentration, similar to other parameters characterizing the C-H stretching bands, and the spectra maintain the characteristics of those of monomeric solutions in the whole 0-xa2 range. The high C-H stretching frequencies in this concentration range can be attributed to an aqueous local environment of -CH3 groups. At high concentrations, xb2 e x2 e 1, the rate of change is also minimal and the spectra are those characteristic of pure liquids. In the intermediate range xa2 e x2 e xb2, the frequency changes rapidly as a function of concentration reaching the values typical of the pure alcohol. This behavior indicates that BE remains molecularly dispersed in water for very low concentrations (lower than xa2), while above this concentration the IR data indicate an increasing amount of aggregation of alcohol molecules as the concentration is increased. The almost constant values of IR frequencies for x2 > xb2 indicate that little or no hydrocarbon chain-water contact in the aggregate is present. In this concentration range water presumably loses its hydrogenbond network completely and effects due to the hydrophobic hydration become negligible. Such a description, supported by a variety of properties of these mixtures,7 allows one to understand the trend observed in ΦL (Figure 1). The concentration dependence of this quantity parallels that of ∆νj. A linear rise in ΦL at low concentrations and a change in the slope at x2 ) xa2 are observed (Figure 1b). A transition in a narrow range of BE concentrations appears from the data (xa2 e x2 e xb2) and ΦL is almost constant for x2 > xb2 (Figure 1a). A contribution in the dissolution enthalpy at high concentration is expected as an indication of growth of local structures. In contrast, the measured ΦL in concentrated solution are nearly constant. Therefore, these calorimetric results provide no indication of intermicellar interaction and of micellar growth (for x2 > xb2) while indicating a monomer-monomer interaction in the premicellar region (x2 < xa2). In this concentration, range solvent-shared association complexes are expected to form. The positive initial slope of ΦL corresponds to monomermonomer repulsive interactions in terms of enthalpy, and it is in line with the expected trend if the main effect of interaction is a reduction of hydrophobic hydration. The observed behavior in ΦL is similar to that associated with micellization in the case of surfactants. The width of transition xb2-xa2 is narrow, and the concept of critical micelle concentration appears to be significant. Previously it has been shown that the surface tension (γ) behavior of BE/water mixtures is typical of aqueous surfactant solutions forming micelles.23 In fact, with increasing BE concentration, the γ decreases down to a critical value x* 2 after which it remains constant (Figure 2). For surfactant solutions this concentration is the cmc. The critical concentration x* 2 can be also identified from calorimetric data. Actually, the first derivative of ΦL (Figure 3a) displays a maximum at the same value of x2 where γ shows a break (Figure 2). The x*2 corresponds, therefore, to the concentration where d2ΦL/d2x2 ) 0. It was previously shown23 that the concentration dependence of the apparent molal compressibility ΦK of BE in water is similar to that observed for ∆νj and that these two sets of data, adequately normalized, superimpose within the experimental errors. Like dΦL/dx2, the first derivative
Onori and Santucci
Figure 2. Surface tension γ as a function of x2 at 25 °C.
Figure 3. The first derivative of (a) ΦL and of (b) ∆νj display an extreme value at the critical BE mole fraction.
of ΦK and of ∆νj displays an extreme value at x* 2 (Figure 3b). Thus correlation among these properties seems to be evident. The ΦL vs x2 behavior is strongly affected by the temperature (Figures 4 and 5). A marked lowering of the ΦL values at high
Calorimetry of n-Butoxyethanol/Water Mixtures
J. Phys. Chem. B, Vol. 101, No. 23, 1997 4665
Figure 4. Apparent molal enthalpy ΦL of BE in water vs x2: (O), 5 °C; (4), 45 °C.
Figure 6. Partial molal enthalpy Lh 2 of BE in water as a function of x2 at 25 °C. ∆Hm is the experimentally determined enthalpy of micellization.
Figure 5. The first derivative of ΦL as a function of x2: (O), 5 °C; (4), 45 °C.
BE concentrations (Figure 4) and a shift of the transition toward lower x2 (Figure 5) are observed with increasing temperature. Moreover, the positive slope of ΦL at low x2 values (premicellar region) becomes larger and the change of slope at x2 ) xa2 less evident as the critical temperature is approached (see Figure 5). Similar shifts of the transition with T were previously observed23 for the apparent molal compressibility of BE in water and surface tension of the mixtures. The partial molal enthalpy of BE in water Lh 2, was calculated from ΦL values by using the following expression:
( )
L h 2 ) ΦL + m
∂ΦL ∂m
T,P
(1)
where m is the BE solute concentration in mole Kg-1. To this aim the ΦL data were fitted to a linear relationship for x2 < xa2. For x2 > xa2 the ΦL data were found to be represented by a sigmoid function within to (2%. The method proposed by Desnoyer et al.30 was then applied to the L h 2 vs x2 curves with the maximum in dΦL/dx2 curves identified as the critical concentration x* 2, to determine the enthalpy of micellization. As an example, the results obtained from the sample at 25 °C are reported in Figure 6. The results for ∆Hm at 5, 25, and 45 °C are (11.0 ( 0.5), (8.5 ( 0.5), (6.0 ( 0.5), kJ mole-1, respectively. ∆Hm is >0 and decreases with increasing temperature. Therefore the formation of microaggregates from aqueous monomers is entropy driven, the endothermal formation enthalpy counteracts the aggregation. This result, commonly obtained for the micellization process of
Figure 7. Values of BE critical mole fraction vs temperature: (O), surface tension; (4), IR absorption; (0), dielectric relaxation; (]), compressibility; (3), calorimetric measurements. (-) is the result of the fitting.
nonionic surfactants,22 is usually connected to the release of structured water around the isolated chains on the formation of micellar aggregates (hydrophobic effect). Enthalpy of Micellization from cmc Measurements. Figure 7 shows the x* 2 data as a function of temperature determined from surface tension,23 IR absorption,7 dielectric relaxation,12 compressibility,23 and calorimetric measurements. The agreement among the different type of measurements is quite good. Thus measurements looking at surface (γ) or bulk (ΦK, ΦL) properties or looking at alcohol (∆νj) or water molecules (τ) behavior give consistent results. The temperature dependence of x* 2 can be used to calculate the enthalpy of micelle formation. According to a phase separation model, the standard free energy of micelle formation per mole of monomers31 is
∆G°m ) RT ln x*2
(2)
The standard enthalpy change per mole of monomer can then calculated by applying the Gibbs-Helmotz equation
∆H°m ) -RT2
∂ ln x* 2 ∂T
(3)
4666 J. Phys. Chem. B, Vol. 101, No. 23, 1997
Onori and Santucci change with T are consistent with the expected decrease in hydrophobic hydration during alcohol association. These calorimetric results provide no indications of intermicellar interactions, while a monomer-monomer repulsive interaction is evidentiated in the premicellar region. Acknowledgment. This work was supported in part by a contribution from the Consiglio Nazionale delle Ricerche, Rome. References and Notes
Figure 8. Symbols are (-), thermodynamic quantities calculated according to eqs 2-6 as a function of temperature; and (9), Enthalpies of micellization determined by calorimetric measurements.
and the standard entropy change from the equation
∆S°m )
(∆H°m - ∆G°m) T
(4)
The factor ∂ln x* 2/∂T in eq 2 has been obtained by calculating the polynomial 2 ln x* 2 ) A + BT + CT + ...
(5)
which best fit the data. In order to obtain satisfactory results for ∆H°m, very high accuracy in the x* 2 determination is required. So, to this aim, only the x*2 data from surface tension measurements were considered. Since a second-order equation fits the data with an accuracy close to the experimental uncertainty, higher terms in the series were not considered. By differentiating eq 5, one obtains
∆H°m ) -RT2(B + 2CT)
(6)
The thermodynamic quantities so calculated are shown in Figure 8 as a function of temperature. ∆H°m decreases with increasing temperature and coincide within the experimental errors with the heats of micellization measured calorimetrically. The sign of these thermodynamic functions and its change with T are the expected ones for processes involving hydrophobic effects. Conclusions The present results further support previously description of self-association of BE in water inferred from compressibility, IR, and surface tension measurements. The change observed h 2 vs x2 is remarkably similar to that associated with in ΦL and L micellization in the case of surfactants and it gives evidence for microheterogeneities like micelles in these binary systems. The formation of micelles from aqueous BE monomers appears entropy driven, the endothermal formation enthalpy counteracting aggregation. The sign of ∆H°m and ∆S°m and its
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