Thermodynamic Properties of Sodium n-Alkanecarboxylates in Water

the apparent molar properties. From the standard partial molar properties of the substrate in pure water and in water + HP-β-CD mixtures and literatu...
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Langmuir 1998, 14, 6045-6053

6045

Thermodynamic Properties of Sodium n-Alkanecarboxylates in Water and in Water + Cyclodextrins Mixtures† R. De Lisi,‡ S. Milioto,* and A. Pellerito Dipartimento di Chimica Fisica, Universita` di Palermo, via Archirafi 26, 90123 Palermo, Italy

A. Inglese§ Dipartimento di Chimica, Universita` di Bari, via Orabona 4, 70126 Bari, Italy Received November 17, 1997. In Final Form: July 20, 1998 Densities and heat capacities of water-substrate, water-cyclodextrin, and water-substrate-cyclodextrin systems were determined at 298 K. The substrates studied are sodium n-alkanecarboxylates (CnCOONa) (from sodium acetate to sodium decanoate) and the cyclodextrins are hydroxypropyl-R-cyclodextrin (HPR-CD), hydroxypropyl-β-cyclodextrin, (HP-β-CD), hydroxypropyl-γ-cyclodextrin (HP-γ-CD) and β-cyclodextrin (β-CD). The apparent molar volumes and heat capacities of CnCOONa in water were calculated as functions of concentration. The standard partial molar properties agree with those obtained by using the additivity rule. HP-β-CD essentially does not affect the thermodynamic properties of C1COONa and C2COONa. Contrarily, the formation of the inclusion complex between cyclodextrin and substrate modifies the apparent molar properties. From the standard partial molar properties of the substrate in pure water and in water + HP-β-CD mixtures and literature values for the equilibrium constant for the inclusion complex formation, the standard partial molar properties of the complex (YoC) were calculated. The increase of VoC with the number of carbon atoms in the alkyl chain (n) is consistent with the solubilization of methylene groups in the hydrophobic cavity of the cyclodextrin and the expulsion of some water molecules from the cavity. To explain the dependence of CpoC on n, conformational effects are also invoked. Studies performed as functions of cyclodextrin concentration evidence that micellization occurs provided that all the cyclodextrin is almost complexed and that the dispersed surfactant concentration equals its critical micelle concentration in water. Data in HP-R-CD, HP-β-CD, HP-γ-CD, and β-CD indicate that the size cavity of the cyclodextrin strongly affects the thermodynamics of the inclusion complex formation while the nature of the hydrophilic shell of the cyclodextrin does not.

Introduction Cyclodextrins are water-soluble cyclic oligosaccharides; R-, β-, and γ-cyclodextrins made up of 6, 7, and 8 glucose units are well-known. Because of the hydrophilic outer surface and the inner hydrophobic cavity, cyclodextrins undergo the inclusion complex formation with hydrophobic compounds such as surfactants.2-5 There are several studies, based on different techniques such as surface tension,2 ultrasound velocity,3 conductivity,4 and emf,5 dealing with the evaluation of the equilibrium constant for the formation of the inclusion complex between surfactants and cyclodextrins (KC) and of the stoichiometry of the complex. Recently, from calorimetric studies in the premicellar region, the KC and the standard partial molar enthalpy for the complex formation between alkanecar† Since this paper was submitted for publication, Wilson and Verrall1 have reported volume data for hydrogenated and fluorinated sodium alkanecarboxylates/β-cyclodextrin inclusion complexes. The paper was revised to account for these literature results. ‡ E-mail: [email protected]. § E-mail: [email protected].

(1) Wilson, L. D.; Verrall, R. E. J. Phys. Chem. B 1997, 101, 9270. (2) Funasaki, N.; Yodo, H.; Hada, S.; Neya, S. Bull. Chem. Soc. Jpn. 1992, 65, 1323. (3) Junquera, E.; Taradajos, G.; Aicart, E. Langmuir 1993, 9, 1213 and references therein. (4) Saint Aman, E.; Serve, D. J. Colloid Interface Sci. 1990, 138, 365. (5) Wan Yunus, W. M. Z.; Taylor, J.; Bloor, D. M.; Hall, D. G.; WynJones, E. J. Phys. Chem. 1992, 96, 8979.

boxylates and R- and β-cyclodextrin (β-CD) have been derived.6 Moreover, volumes in the premicellar region for β-CD + water + sodium alkanecarboxylates (hydrogenated and fluorinated) have been recently reported.1 Investigations up to the micellar region7-9 are very scarce. In particular, calorimetric,7,9 volumetric8,9 and heat capacity8,9 studies of some cyclodextrin-surfactant-water ternary systems in a wide range of surfactant concentration were performed. It turned out that the properties of the surfactants are strongly affected by the presence of cyclodextrin in the premicellar region while they are hardly affected in the micellar one. Volume and heat capacity, obtained from direct measurements, are very accurate properties which are able to describe the interactions in solution at molecular level. From a large database for volume and heat capacity of the substrate-cyclodextrin inclusion complex formation, straightforward information on the different contributions (solubilization of the substrate in the cavity, release of water from the cavity, etc.) to the process can be drawn. Unfortunately, no volume (with the exception of data (6) Rekharsky, M. V.; Mayhew, M. P.; Goldberg, R. N.; Ross, P. D.; Yamashoji, Y.; Yoshihisa, I. J. Phys. Chem. B 1997, 101, 87. (7) Turco Liveri, V.; Cavallaro, G.; Giammona, G.; Pitarresi, G.; Puglisi, G.; Ventura, C. Thermochim. Acta 1992, 199, 125. (8) Milioto, S.; Bakshi, M. S.; Crisantino, R.; De Lisi, R. J. Solution Chem. 1995, 24, 103. (9) Delitalia, C.; Marongiu, B.; Pittau, B.; Porcedda, S. Fluid Phase Equilib. 1996, 126, 257.

S0743-7463(97)01252-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/19/1998

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reported by Wilson and Verrall1) and heat capacity changes for the complex formation between hydrophobic substrate and cyclodextrin are available. Therefore, we determined the volume and the heat capacity of water-cyclodextrinsubstrate ternary systems in a wide range of substrate concentration by choosing substrates and cyclodextrins of a different nature. In particular, the substrates are sodium n-alkanecarboxylates (from sodium acetate to sodium decanoate) while the macrocycles are hydroxypropyl-R-, -β- and -γ-cyclodextrins which are more soluble than their corresponding unmodified compounds.

VΦ,i )

3 M 10 (d - do) d mi ddo

CΦ,i ) M cp +

103(cp - cpo) mi

(2)

(3)

where mi and M are the molality and the molecular weight of the solute and d and cp are the densities and the specific heat capacities of solutions, respectively, while do and cpo are the corresponding properties for the solvent.

Results and Discussion

Experimental Section Materials. Sodium acetate (C1COONa), sodium propanoate (C2COONa), and sodium butanoate (C3COONa) (Aldrich) were dried under vacuum at 313 K for at least 48 h before use. Sodium pentanoate (C4COONa), sodium hexanoate (C5COONa), and sodium heptanoate (C6COONa) were obtained by neutralizing their corresponding acids (Aldrich) with sodium hydroxide ethanolic solutions at 313 K. The products precipitated by cooling and were recovered by filtration. They were dried in a vacuum oven at 313 K for at least 4 days. Their aqueous solutions gave a pH ≈8.5. Sodium octanoate (C7COONa) and sodium decanoate (C9COONa), Sigma, were recrystallized from absolute ethanol and dried in a vacuum oven at 313 K for at least 4 days. β-Cyclodextrin (β-CD, Sigma), hydroxypropyl-R-cyclodextrin (HP-R-CD, Sigma), hydroxypropyl-β-cyclodextrin (HP-β-CD, Acros), and hydroxypropyl-γ-cyclodextrin (HP-γ-CD, Sigma) were used as received. The average molar substitution for each glucopyranose residue is 0.6 for HP-R-CD and HP-γ-CD and 0.43 for HP-β-CD. From several K. Fischer analyses performed by using Metrohm 655 Dosimat, it was found that for HP-R-CD, HP-β-CD, and HP-γCD the water contents were 8.49, 9.40, and 6.71% (w/w), respectively. D-(+)-sucrose (Sigma) was dried in a vacuum oven at 313 K for at least 4 days. All solutions were prepared by mass using degassed conductivity water, and their concentrations were expressed as molalities. Equipment. The solutions densities were measured at 298 K by using a vibrating tube flow densimeter (Model 03D, Sodev Inc.) sensitive to 3 ppm or better. The temperature was maintained constant within 0.001 K by using a closed loop temperature controller (Model CT-L, Sodev Inc.). The calibration of the densimeter was made with water (d ) 0.997047 g cm-3)10 and D-(+)-sucrose aqueous solutions whose densities as functions of concentration are reported in the literature.11 The relative differences in the heat capacities per unit volume ∆σ/σo ) (σ - σo)/σo were determined with a Picker flow microcalorimeter (Setaram) at 298 ( 0.001 K. Using a flow rate of about 0.01 cm3 s-1 and a basic power of 21.20 mW, the temperature increment was approximately 0.5 K. The sensitivity of the instrument is 7 × 10-5 J K-1 g-1. The specific heat capacities (cp) of solutions of density d are related to ∆σ/σo through the equation

Cyclodextrins and Sodium Alkanecarboxylates in Water. As said in the Experimental Section, cyclodextrins are hydrated compounds. Therefore, to avoid the adsorption of water, which depends on the relative humidity and exposition time,13 aqueous solutions of cyclodextrins at different concentrations were prepared under nitrogen. From their densities and specific heat capacities, the W ) and heat capacities apparent molar volumes (VΦ,CD W (CΦ,CD) of cyclodextrins in water were calculated. Both W W VΦ,CD and CΦ,CD change linearly with mW CD according to W YΦ,CD ) YoCD(w) + BYmW CD

(4)

where cpo and do correspond to the specific heat capacity and density of the solvent, respectively; they correspond to pure water for the binary system and to water + cyclodextrin mixture for the ternary system. The value of 4.1792 J K-1 g-1 for the specific heat capacity of water was taken.12 Apparent Molar Property Calculations. The apparent molar volumes (VΦ,i) and heat capacities (CΦ,i) of the solute i (cyclodextrin or sodium n-alkanecarboxylate) in the given solvent were calculated by means of the following equations

where YoCD(w) is the standard (infinite dilution) partial molar property and BY the solute-solute pair interactions parameter. The best fits of weighted experimental data to eq 4 gave the values collected in Table 1. Both the standard partial molar volumes and heat capacities increase in a nonlinear manner with the number of glucose units while it is reported that the additivity rule also holds for glucose oligomers.14 The lack of the additivity could be ascribed to the different content of the hydroxypropyl groups in HP-β-CD with respect to that in HP-R-CD and HP-γ-CD. We recall that the average molar substitution is 0.43 for the former and 0.6 for the latter. This hypothesis was verified by calculating YoCD(w) for HP-β-CD having the same hydroxypropyl groups content as HP-R-CD and HP-γ-CD. This was made on the basis of the additivity rule and YoCD(w) values for unmodified cyclodextrins.8,15,16 o The obtained values (926 cm3 mol-1 for VHP-β-CD (w) and o 2838 J K-1 mol-1 for CpHP-β-CD(w)) agree, within the experimental uncertainties, with those (927.6 cm3 mol-1 o and 2795 J K-1 mol-1) calculated from YHP-R-CD (w) and o YHP-γ-CD(w). The VoCD(w) and BV values were used to obtain a calibration curve which did permit us to easily calculate at any time the correct concentration by using a procedure which takes less time than a K. Fisher analysis. In particular, the density of a given solution was measured and the correct mW CD value was obtained by the iterative method through eqs 2 and 4. From the correct molality W was calculated through and cp experimental value, CΦ,CD W eq 3. The obtained CΦ,CD value was in good agreement with that calculated by means of eq 4 and the parameters collected in Table 1. The same procedure was used also W W and VΦ,CD values as functions of for β-CD whose CΦ,CD W 8 mCD are reported elsewhere. As far as sodium alkanecarboxylates are concerned, from W W and CΦ,S show a small C1COONa to C5COONa, both VΦ,S

(10) Kell, G. S. J. Chem. Eng. Data 1967, 12, 66. (11) Garrod, J. E.; Herrington, T. M. J. Phys. Chem. 1970, 74, 363. (12) Stimson, M. F. Am. J. Phys. 1955, 23, 614.

(13) Tanaka, S.; Nakamura, T.; Kawasaki, N.; Kitayama, T.; Yoshiyuki, T. J. Colloid Interface Sci. 1997, 186, 180. (14) Shahidi, F.; Farrell, P. G.; Edward, J. T. J. Solution Chem. 1976, 12, 807.

cp ) cpo {1+ ∆σ/σo}do/d

(1)

Properties of Sodium n-Alkanecarboxylates

Langmuir, Vol. 14, No. 21, 1998 6047

Table 1. Standard Partial Molar Volumes and Heat Capacities and Solute-Solute Interactions Parameters for Sodium n-Alkanecarboxylates and Hydroxypropyl-r-, -β- and -γ-cyclodextrins in Water at 298 Ke Vo(w) C1COONa C2COONa C3COONa C4COONa C5COONa C6COONa C7COONa C9COONa

39.27 ( 0.02; (37.2) 53.65 ( 0.01; 53.8;b (53.1) 69.41 ( 0.01; (69.0) 84.55 ( 0.09; (84.9) 101.35 ( 0.04; 100.9;b (100.8) 116.27 ( 0.09; 116.6;b 116.8;c (116.7) 132.52 ( 0.06; 132.4;a 133.0;b 132.14;c (132.6) 164.89 ( 0.05; 164.2;c 164.0;d (164.4)

HP-R-CD HP-β-CD HP-γ-CD

791.53 ( 0.05 861.18 ( 0.09 1063.70 ( 0.05

39.2;a

BV 0.22 ( 0.03 0.5 ( 0.1 1.1 ( 0.1 -0.30 ( 0.09 -0.9 ( 0.4 2.4 ( 0.7 -1.5 ( 0.5 11.2 ( 0.1 8.8 ( 0.5

Cpo(w)

CV -0.26 ( 0.08

78 ( 1; 70a (69) 163 ( 1 (157) 250 ( 2 (245) 334 ( 1 (333) 420 ( 1 (421) 517 ( 1 (509) 597 ( 1; 593a (597) 769 ( 1 (773) 2270 ( 1 2540 ( 2 3319 ( 1

BC 6.3 ( 2 10 ( 2 10 ( 1 7(2 25 ( 7 -230 ( 11 -133 ( 17 -375 ( 10

a From ref 16. b From ref 1. c From ref 17. d From ref 18. e Units are as follows: cm3 mol-1 for Vo(w); J K-1 mol-1 for Cpo(w); cm3 kg mol-2 for BV; cm3 kg3/2 mol-5/2 for CV; J K-1 kg mol-2 for BC. Quantities in parentheses were calculated on the basis of the additivity rule.

dependence on mW S ; for the longer alkyl chain homoloW W and CΦ,S vs mW goues, the VΦ,S S profiles are typical for the micellization process. The following equation was applied to all the experimental data for shorter homologoues (from C1COONa to C5COONa) and to data in the premicellar region for the surfactants (from C6COONa to C9COONa) W W 3/2 1/2 YΦ,S ) YoS(w) + AY(mW + BYmW + ... S) S + CY(mS ) (5)

where YoS(w) and BY have the same meaning as above, CY is the solute-solute triplet interactions parameter and AY is the Debye-Hu¨ckel limiting slope. The AY values17 used for the volume and the heat capacity are 1.865 cm3 kg1/2 mol-3/2 and 28.95 J K-1 kg1/2 mol-3/2, respectively. The results of the fits are collected in Table 1 together with the literature data.1,18-20 As can be seen, the agreement between our data and the literature data is good. In Table 1 are also reported the standard partial molar properties calculated on the basis of the additivity rule by taking the following contributions to the volume and heat capacity, respectively:21 26.7 cm3 mol-1 and 178 J K-1 mol-1 for the CH3 group, 10.5 cm3 mol-1 and -109 J K-1 mol-1 for the COONa group, and 15.9 cm3 mol-1 and 88 J K-1 mol-1 for the CH2 group. As can be seen, with the exception of C1COONa, the experimental YoS(w) values agree with those calculated. The discrepancy for C1COONa is likely due to the antagonistic influence on solvation between the hydrophobic methyl group and the hydrophilic carboxylate group. Sodium n-Alkanecarboxylates in Water + Hydroxypropyl-β-cyclodextrin Mixture, 0.09 m. General aspects. The apparent molar volumes W+CD W+CD ) and heat capacities (CΦ,S ) of CnCOONa in (VΦ,S water + HP-β-CD mixtures, 0.09 m, together with the excess densities (∆d ) d - do) with respect to the solvent and the specific heat capacities (cp) are collected in Table W+CD vs mW+CD profiles, for C1COONa and C22. The VΦ,S S COONa are essentially unaffected by the presence of HPβ-CD while that for C3COONa is shifted toward higher values by about 0.8 cm3 mol-1. In the case of heat capacity, the cyclodextrin involves small effects for C1COONa and (15) Briggner, L.; Wadso¨, I. J. Chem. Thermodyn. 1990, 22, 1067. (16) Høiland, H.; Hald, L. H.; Kvammen, O. J. J. Solution Chem. 1981, 10, 775. (17) De Lisi, R.; Ostiguy, C.; Perron, G.; Desnoyers, J. E. J. Colloid Interface Sci. 1979, 71, 147. (18) Leduc, P. A.; Desnoyers, J. E.; Can. J. Chem. 1973, 51, 2993. (19) Vikingstad, E.; Skauge, A. S.; Høiland, H. J. Colloid Interface Sci 1978, 66, 240. (20) Kale, K. M.; Zana, R. J. Colloid Interface Sci. 1977, 61, 312. (21) Perron, G.; Desnoyers, J. E. Fluid Phase Equilib. 1979, 2, 239.

C2COONa (about 10 J K-1 mol-1) as well as for C3COONa (the difference being comprised between 10 and 20 J K-1 mol-1). More interesting is the behavior of the longer homologoues. In fact, the presence of HP-β-CD shifts the W+CD W+CD VΦ,S and CΦ,S vs mW+CD curves (not shown) toward S higher and lower values with respect to those in pure water, respectively. The standard partial molar properties, YoS(w+CD), were evaluated by using the same approach as that for substrates in pure water; their values are collected in Table 3. The other fitting parameters are not reported since they have no physical meaning. In fact, they reflect not only the solute-solute interactions but also the effect on the substrate-cyclodextrin complexation equilibrium. W+CD as functions of mW+CD are In Figures 1 and 2, YΦ,S S reported. To compact the ordinate scale, the properties are corrected for those at infinite dilution. As a general W+CD - YoS(w+CD) vs mW+CD trends present feature, the YΦ,S S a minimum in the case of volume and a maximum in the case of heat capacity, beyond which they tend to constant values. The peculiarities are present at lower concentration the more hydrophobic the substrate is; they could correspond to the cmc of the substrates in water + HPβ-CD mixture (cmcW+CD). However, the smooth minima and maxima for C5COONa and C6COONa do not give evidence for the cyclodextrin effect on cmc. In the case of C7COONa and C9COONa, the difference between cmcW and cmcW+CD is 0.1 mol kg-1, i.e. the cyclodextrin concentration. To support this idea, we determined from -1 for conductivity the cmcW+CD at mW CD ) 0.09087 mol kg C9COONa. The obtained value of 0.216 mol kg-1 corresponds, within the experimental uncertainties, to the sum of the cmcW19 (0.109 mol kg-1) and the stoichiometric cyclodextrin concentration. A comparison between data in water and in water + HP-β-CD mixture evidences two kinds of behavior of the substrates analyzed: for the hydrophilic solutes (C1COONa and C2COONa) the cyclodextrin essentially does not affect the thermodynamic properties of the substrate; in the case of higher homologoues, the cyclodextrin drastically affects not only the values but also the profiles of the apparent molar properties as functions of concentration. A borderline behavior is shown by C3COONa since the effect of HP-β-CD is not negligible, but at the same time, the profile is not deeply modified. Standard Partial Molar Property of the Inclusion Complex. The apparent molar volume of the substrate in the water + cyclodextrin mixture is given by W+CD VΦ,S )

VT - VB mW S

(6)

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De Lisi et al.

Table 2. Apparent Molar Volumes and Heat Capacities of Sodium n-Alkanecarboxylates in Water + Hydroxypropyl-β-cyclodextrin Mixtures at 298 Ka mW+CD S

103∆d

W+CD VΦ,S

cp

W+CD CΦ,S

0.02967 0.04944 0.06941 0.08446 0.09948 0.2288

1.264 2.092 2.922 3.551 4.160 9.395

39.43 39.67 39.85 39.87 40.06 40.56

C1COONa (mCD ) 0.08863; do ) 1.033895; cpo ) 3.94345) 3.93548 54 0.3949 15.989 0.5989 23.865 3.92524 60 0.7806 30.626 3.92140 61 0.9836 38.101 3.91924 78 1.1841 45.035 3.88896 81 1.3785 51.525

40.83 41.11 41.41 41.57 41.93 42.27

3.85421 3.81620 3.78441 3.74888 3.71934 3.69168

90 101 107 110 116 120

0.04817 0.09778 0.1519 0.1990 0.2582 0.3009

1.991 3.992 6.151 7.994 10.302 11.940

54.15 54.52 54.72 54.92 55.05 55.17

C2COONa (mCD ) 0.08636; do ) 1.033068; cpo ) 3.95056) 3.93997 159 0.4511 17.664 3.92973 165 0.7007 26.779 3.91918 170 0.9293 34.766 3.90990 171 1.2030 43.941 3.89906 175 1.3870 49.869 3.89130 177 1.7708 61.716

55.35 55.73 56.04 56.36 56.56 56.93

3.86503 3.82630 3.79387 3.75546 3.73257 3.68822

182 190 196 199 201 206

0.04728 0.1006 0.1554 0.3036 0.3845 0.5120

1.808 3.805 5.815 11.145 13.981 18.336

70.59 70.83 71.08 71.37 71.50 71.71

C3COONa (mCD ) 0.08820; do ) 1.033780; cpo ) 3.94790) 3.93918 249 0.6317 22.295 3.93019 257 0.9493 32.395 3.92109 259 1.0594 35.823 3.89777 264 1.3288 43.727 3.88562 266 1.6061 51.447 3.86725 268

71.92 72.30 72.35 72.63 72.89

3.85048 3.81078 3.79825 3.77177 3.74746

270 275 277 283 288

0.04985 0.07983 0.08994 0.1193 0.2194 0.3986

1.620 2.614 2.942 3.888 7.063 12.814

89.41 89.09 89.10 89.12 89.23 88.77

C4COONa (mCD ) 0.09303; do ) 1.035641; cpo ) 3.93419) 0.5975 19.003 3.91375 230 0.7893 24.792 3.91116 230 0.9960 30.701 3.90610 249 1.2821 38.463 3.88787 272 1.5814 45.925 3.85947 292 1.9245 53.573

88.57 88.44 88.49 88.59 88.83 89.28

3.82830 3.81050 3.78804 3.76731 3.73960 3.72107

298 316 323 337 341 351

0.06448 0.08468 0.1186 0.1967 0.3045 0.4147 0.5912

1.673 2.185 3.063 5.071 7.800 10.718 15.432

91.65 91.73 91.63 91.51 91.41 90.98 90.38

C4COONa (mCD ) 0.2667; do ) 1.093780; cpo ) 3.59767) 3.58409 234 0.7905 20.379 3.57755 207 1.0276 26.239 3.57192 226 1.2785 31.718 3.55778 239 1.4365 34.815 3.53861 245 1.7644 41.320 3.52031 250 2.0444 46.225 3.49712 264

90.25 89.98 90.13 90.34 90.48 90.74

3.47750 3.45591 3.44044 3.43234 3.41267 3.39265

280 291 304 311 319 321

0.03455 0.04937 0.06961 0.08422 0.1182 0.1482 0.2076

0.812 1.177 1.697 2.100 3.090 3.870 5.589

111.44 111.08 110.52 109.96 108.73 108.68 107.75

C5COONa (mCD ) 0.09144; do ) 1.035065; cpo ) 3.93751) 3.92556 196 0.2844 7.956 3.92114 210 0.3467 9.857 3.91643 238 0.5495 15.733 3.91303 250 0.7883 22.173 3.90541 268 0.9967 27.453 3.89856 276 1.2963 34.177 3.89005 309 1.5017 37.736

106.53 105.92 105.14 104.96 104.97 105.37 106.14

3.87572 3.86858 3.84467 3.82467 3.80865 3.78080 3.75327

318 336 362 385 397 401 396

0.02187 0.02978 0.03470 0.04931 0.08073 0.1223 0.1692 0.2148

0.435 0.572 0.665 0.959 1.633 2.742 4.052 5.425

128.44 129.06 129.09 128.78 127.97 125.79 124.20 122.83

C6COONa (mCD ) 0.08935; do ) 1.034220; cpo ) 3.94280) 0.2806 7.318 0.4498 12.047 0.6361 17.368 3.92058 146 0.7958 21.025 0.9345 23.867 3.89629 212 1.2047 28.471 3.88455 247 1.4989 32.864 3.88007 298 2.0095 39.220

121.84 120.64 119.55 119.95 120.43 121.64 122.69 124.13

3.87291 3.85400 3.83357 3.81844 3.80018 3.73445 3.67804 3.58308

340 389 411 425 425 395 383 366

0.01985 0.02984 0.04969 0.07985 0.1112 0.1972 0.2990 0.4548

0.439 0.712 1.335 2.276 3.225 5.842 8.814 13.001

143.36 141.60 138.54 136.79 136.16 135.21 134.95 135.27

C7COONa (mCD ) 0.01741; do ) 1.004746; cpo ) 4.12938) 4.12354 390 0.6500 17.021 4.12229 447 0.8519 20.684 4.11856 466 0.9262 21.952 4.11537 508 1.2832 27.569 4.11267 533 1.4806 30.448 4.10306 548 2.0006 37.123 4.09534 566 2.6020 43.633 4.08488 581

137.07 138.43 138.83 140.21 140.70 141.72 142.53

4.03328 3.96936 3.94370 3.84043 3.79052 3.68995 3.59063

522 472 455 413 401 393 389

0.01981 0.02981 0.03963 0.05971 0.07828 0.1005 0.1494 0.1532 0.1957 0.2466 0.2958 0.3943

0.233 0.368 0.484 0.768 1.219 1.722 2.850 3.174 4.311 5.580 6.828 9.136

149.63 149.05 149.16 148.51 145.92 144.39 142.42 140.83 139.46 138.74 138.15 137.76

C7COONa (mCD ) 0.08863; do ) 1.033895; cpo ) 3.94488) 3.93510 160 0.5001 11.425 3.92970 144 0.6452 13.941 3.92529 158 0.7958 16.345 3.91661 177 0.8473 16.952 3.91252 237 0.9936 19.082 3.90665 269 1.1444 20.769 3.89706 327 1.3711 23.430 3.89749 338 1.5561 25.523 3.89287 381 1.6909 26.792 3.88684 410 1.9581 29.323 3.88172 431 2.3777 33.714 3.87208 459

137.76 138.57 139.24 139.65 140.10 140.85 141.46 141.81 142.15 142.60 142.74

3.86292 3.83415 3.80060 3.76681 3.74531 3.70113 3.65365 3.62040 3.56501 3.53130 3.44050

478 465 450 416 421 402 395 393 368 375 359

cp

W+CD CΦ,S

mW+CD S

103∆d

W+CD VΦ,S

Properties of Sodium n-Alkanecarboxylates

Langmuir, Vol. 14, No. 21, 1998 6049

Table 2 (Continued) mW+CD S

103∆d

W+CD VΦ,S

0.04973 0.06965 0.08465 0.1466 0.2516 0.3523 0.4983

0.124 0.177 0.229 0.386 1.466 3.055 5.309

149.80 149.75 149.61 149.65 146.84 144.25 142.31

C7COONa (mCD ) 0.2677; do ) 1.094079; cpo ) 3.58761) 0.6972 7.627 3.55693 150 0.9955 9.885 3.55194 169 1.2873 11.412 3.52903 187 1.5923 12.722 3.50142 239 1.7944 13.440 3.48861 299 2.0676 14.262 3.47201 345 2.3506 15.015

0.02849 0.03853 0.04990 0.07580 0.1033 0.1457 0.1799 0.1985 0.2726 0.2965

0.062 0.116 0.189 0.343 0.821 1.697 2.348 2.608 3.651 3.901

185.64 184.85 184.11 183.40 180.11 176.51 175.09 174.96 174.55 174.73

C9COONa (mCD ) 0.08920; do ) 1.034171; cpo ) 3.94271) 3.92651 194 0.3993 5.029 3.92152 212 0.4868 5.912 3.91589 223 0.5756 6.854 3.90516 263 0.7348 8.266 3.89743 318 0.8221 8.973 3.89029 396 0.9837 10.304 3.88449 431 1.2403 12.193 3.87991 437 1.4817 13.744 3.86051 448 1.9023 16.055 3.85251 444

a

cp

W+CD CΦ,S

mW+CD S

103∆d

W+CD VΦ,S

cp

W+CD CΦ,S

141.77 142.32 143.00 143.56 143.87 144.26 144.58

3.44928 3.40210 3.35632 3.32158 3.30405 3.28124 3.26214

375 379 378 385 391 397 403

175.06 175.33 175.39 175.76 175.95 176.14 176.42 176.67 177.05

3.82075 3.79145

437 426

3.69266 3.63808 3.57892 3.52538 3.44090

413 397 402 403 404

W+CD W+CD Units are as follows: mol kg-1 for mW+CD ; g cm-3 for d; J K-1 g-1 for cp; cm3 mol-1 for VΦ,S ; J K-1 mol-1 for CΦ,S . S

Table 3. Standard Partial Molar Volumes and Heat Capacities of Sodium n-Alkanecarboxylates in Water + Hydroxypropyl-r, -β, and -γ-cyclodextrin Mixtures and Equilibrium Constant and Standard Partial Molar Properties for the Inclusion Complex Formationf mW CD

VoS(w+CD)

CpoS(w+CD)

C3COONa C7COONa

0.08212 0.08728

68.6 ( 0.1 125.3 ( 0.2

HP-R-CD 203 ( 2 182 ( 26

C1COONa C2COONa C3COONa C4COONa C4COONa C5COONa C6COONa C7COONa C7COONa C9COONa

0.08863 0.08636 0.08820 0.09303 0.2667 0.09144 0.08935 0.08863 0.2677 0.08920

39.32 ( 0.05 53.94 ( 0.04 70.26 ( 0.02 89.07 ( 0.05 91.91 ( 0.09 111.30 ( 0.03 131.49 ( 0.05 152.50 ( 0.03 150.72 ( 0.02 186.28 ( 0.06

HP-β-CD 57 ( 3 157 ( 1 246 ( 2 240 ( 1 196 ( 1 179 ( 1 105 ( 2 81 ( 1 116 ( 6 161 ( 1

C3COONa C7COONa

0.08784 0.08812

69.04 ( 0.09 139.7 ( 0.1

HP-γ-CD 244 ( 1 388 ( 4

KC

∆VoC

∆CpoC

16 ( 1a 731 ( 83a

-1.4 -7.3

-81 -420

11;b 24c

740;b 2140c

8.9;d 6.5e 9.9;d 8.5e 13.1;d 11.5e 16.8;d 16.1e 20.6;d 20.3e 18.4;d 18.3e 21.7;d 21.5e

-186;d 136e -185;d -160e -318;d 279e -454;d -436e -538;d 525e -492;d -484e -617;d -611e

3.9 ( 0.4a

28.1

-980

34;b 69.7c 110;b 193c 370;b 622c

a

From ref 28. b From ref 23. c From ref 1. d Calculated by using KC values from ref 23. e Calculated by using KC values from ref 1. f Units are as follows: mol kg-1 for concentration; kg mol-1 for equilibrium constant; J K-1 mol-1 for heat capacities; cm3 mol-1 for volumes.

Figure 1. Apparent molar volume, corrected for the standard one, of substrates in water + HP-β-CD mixture at 0.09 m as a function of molality: (3) sodium pentanoate; (2) sodium hexanoate; (b) sodium heptanoate; (4) sodium octanoate; (O) sodium decanoate.

Figure 2. Apparent molar heat capacity, corrected for the standard one, of substrates in water + HP-β-CD mixture at 0.09 m as a function of molality: (2) sodium pentanoate; (4) sodium hexanoate; (O) sodium heptanoate; (3) sodium octanoate; (b) sodium decanoate.

where VT and VB represent the bulk properties of the ternary and binary systems, respectively. Let us consider W that mW CD moles of cyclodextrin and mS moles of substrate

are added to 1 kg of water. If we assume that the stoichiometry of the inclusion complex is 1:1, at the equilibrium the solution contains mW C moles of the com-

6050 Langmuir, Vol. 14, No. 21, 1998

De Lisi et al.

plex the partial molar volume of which is VC, mW CD,f moles of uncomplexed cyclodextrin the partial molar volume of which is VCD,f and mW S,f moles of uncomplexed substrate the partial molar volume of which is VS,f. Therefore, we can write W W VT ) 55.5 V′W + mW C VC + mCD,fVCD,f + mS,fVS,f W VB ) 55.5VW + mW CD VCD

(7) (8)

where V′W and VW are the partial molar volumes of water in the ternary and binary systems, respectively, while VW CD is the partial molar volume of the cyclodextrin in pure water. If the solutions analyzed are not concentrated we can state that V′W ) VW. Moreover, since data for C1COONa and C2COONa in water and in water + HPβ-CD mixture indicate that the macrocycle practically does not play a cosolvent effect we can state that VS,f ) VW S and . Note that these reasonable approximations VCD,f ) VW CD are minimized when the standard partial molar properties of the substrate are considered. From eqs 6-8 one obtains

Table 4. Apparent Molar Volumes and Heat Capacities of Sodium Octanoate in Water + β-Cyclodextrin Mixture at 0.01729 m at 298 Ka mW+CD S

103∆d

W+CD VΦ,S

cp

W+CD CΦ,S

0.02113 0.02972 0.04609 0.07914 0.1131 0.2001 0.3064 0.4598 0.6606 0.8536 1.0013 1.2856 1.5829 1.9436 2.6701

0.453 0.702 1.197 2.230 3.298 5.974 9.066 13.179 17.317 20.782 23.282 27.741 32.004 36.670 44.390

144.14 141.95 139.55 137.22 136.11 135.07 134.91 135.27 137.11 138.46 139.19 140.20 140.93 141.61 142.68

4.12570 4.12432 4.12205 4.11746 4.11382 4.10638 4.09913 4.09043 4.03687 3.97709 3.92945

501 508 521 531 544 566 582 595 531 482 453

3.77127 3.71799 3.61228

400 406 407

a

For units see Table 2. do ) 1.004409; cpo ) 4.12962.

(12)

To evaluate ∆VoC from VoS(w+CD) and VoS(w) experimental data, KC values must be known. To our knowledge, KC literature data1,6,23 for the present substrates deal with unmodified cyclodextrins. The nature of the hydrophilic surface of cyclodextrin is scarcely relevant for the complex formation as supported by the equal, within the experimental errors, KC values22 for dodecyltrimethylammonium bromide with HP-β-CD and β-CD. On the other hand, the curves overlap of the apparent molar properties vs mW+CD S of C7COONa in β-CD and HP-β-CD 0.017 m (Table 4) in the premicellar region indicates close values for KC. On this basis, we used the KC values for CnCOONa+β-CD systems. Since, despite the similar experimental conditions, the KC values from different sources1,6,23 strongly differ from each other, we decided to use the two series of KC data1,23 more differing. The KC values for C4COONa were evaluated from graphical extrapolation of ln KC as functions of the number carbon atoms in the alkyl chain (n). As far as the heat capacity is concerned, to evaluate ∆CpoC from experimental data and eq 12, the relaxation contribution, i.e. ∆HoC (∂(ln XoS,C)/∂T), should be evaluated. For the CnCOONa + β-CD systems6 ∆HoC is smaller than 10 kJ mol-1 (in the absolute value). Also, by assuming ∆HoC to be temperature independent, from the van’t Hoff equation ∂(ln XoS,C)/∂T was calculated. It turned out that the relaxation contribution was smaller than the experimental uncertainties. The calculated ∆VoC and ∆CpoC values are reported in Table 3. As can be seen, although the two series of KC differ at least by 100%, the difference between the two series of ∆YoC ranges from 25 to 0% by increasing the alkyl chain length. Regardless of the KC series used, ∆YoC values increase and decrease with n for volume and heat capacity, respectively, tending to level off for n g 7 (not shown). The same behavior was observed for ∆VoC of CnCOONa+β-CD1 and for the standard free energy for the inclusion complex formation (∆GoC) between β-CD and CnCOONa,23 sodium alkylsulfonates24 and sodium alkylsulfates.24 The breaks in ∆GoC and ∆VoC vs n plots are sensitive to the same phenomenon; in fact, in the plot of ∆VoC vs ∆GoC the break disappears. The presence of this break excludes that the

is the standard enthalpy for the inclusion where complex formation.

(22) Junquera, E.; Pena, L.; Aicart, E. Langmuir 1995, 11, 4685. (23) Palepu, R.; Richardson, J. E.; Reinsborough, V. C. Langmuir 1989, 5, 218. (24) Park, J. W.; Song, H. J. J. Phys. Chem. 1989, 93, 121.

W+CD W W - VW VΦ,S S ) (VC - VCD - VS )XS,C ) ∆VCXS,C

(9)

where ∆VC is the volume change for the inclusion complex formation and XS,C is the fraction of the complexed W substrate (XS,C ) mW C /mS ) which is correlated to KC through

KC )

XS,C (1 - XS,C)(R - XS,C)mW S

(10)

where R is the ratio between the stoichiometric cycloW dextrin and substrate concentrations, R ) mW CD/mS . Therefore, from the apparent molar volume as a function of concentration curves both KC and VC can be obtained by means of a nonlinear least-squares fit. Indeed, in the range of the substrate and cyclodextrin concentrations analyzed in this paper, this analysis is involved since XS,C and ∆VC depend on the concentration of the present species. In the dilute concentration region, ∆VC can be assumed to be constant, and then, the use of eq 9 is simplified provided that the experimental data are sufficiently accurate. An approach similar to the latter was used by Wilson and Verrall.1 Alternatively, if the interest is devoted to the accurate determination of the change in the thermodynamic property for the complex formation, it is better to analyze the property in the infinite dilution state. Accordingly, eq 9 assumes the form

VoS(w+CD) - VoS(w) XoS,C

) ∆VoC

(11)

8 where XoS,C is the XS,C extrapolated value at mW S ) 0. Note that ∆VoC is an apparent standard partial molar property since it refers to a finite cyclodextrin concentration. By application of eq 11 to the enthalpy, from its derivative with respect to temperature, the following equation is obtained for heat capacity

CpoS(w+CD) - CpoS(w) XoS,C ∆HoC

) ∆CpoC + ∆HoC

∂(ln XoS,C) ∂T

Properties of Sodium n-Alkanecarboxylates

Figure 3. Standard partial molar volume (circles) and heat capacity (triangles) of the inclusion complexes between CnCOONa and HP-β-CD as functions of the number of carbon atoms in the alkyl chain. Key: filled symbols, calculated by using KC values from ref 1; open symbols, calculated by using KC values from ref 23.

additivity rule holds not only for ∆YoC but also for YoC since o YW CD is a constant quantity while YS(w) changes linearly o with n. For a given substrate, YC was obtained by taking W YoS(w) and YW CD calculated at the corresponding mCD value o from data in Table 1. VC increases linearly with n (with the exception of C9COONa) while the CpoC vs n trend presents a change in the sign of the slope for n between 6 and 7 (Figure 3). However, if the two properties are plotted against XoS,C (not shown), it turns out that only a break point is present at XoS,C ≈ 0.96, i.e. for n between 6 and 7. According to literature suggestions,1,24 the deviation at about n ) 7 can reflect the saturation of the cyclodextrin cavity, which cannot lodge more than seven carbon atoms. For n < 7, the CH2 group contribution to the property should reflect the solubilization in the cyclodextrin cavity and the simultaneous expulsion of water molecules from the cavity. Both contributions involve positive volume and heat capacity. Therefore, the CH2 group contribution to YoC is expected to be larger than those in apolar solvents. This is verified for the volume but not for the o is ≈20 and ≈17 cm3 mol-1 in heat capacity. In fact, VCH 2 the complex and in apolar solvents,25 respectively, while o CpCH is ≈ -50 and 30 J K-1 mol-1 in the complex and in 2 apolar solvents,27 respectively. A negative methylene group contribution to CpoC (-13 J K-1 mol-1) was also reported26 for the R-CD + alcohols complexes. According to Halle´n et al.26 the heat capacity findings can be interpreted by invoking the loss of freedom degree of the alkyl chain encapsuled in the cyclodextrin cavity and/or a conformational change of the cyclodextrin. The increase of both CpoC and VoC with n for n g 7 should reflect the solubilization in water of the additional CH2 groups. From C7COONa and C9COONa the values of 44 J K-1 mol-1 for heat capacity and 16.7 cm3 mol-1 for volume can be calculated. These values are smaller and larger, respectively, than those in pure water (reported above), indicating that the additional CH2 groups interact not only with water but also with the hydrophilic surface of cyclodextrin. It is easy to predict that for longer alkyl chain substrate the CH2 group contributions to volume (25) Inglese, A.; Mavelli, F.; De Lisi, R.; Milioto, S. J. Solution Chem. 1997, 26, 319. (26) Halle´n, D.; Scho¨n, A.; Shehatta, I.; Wadso¨, I. J. Chem. Soc., Faraday Trans. 1992, 88, 2859. (27) De Lisi, R.; Milioto, S.; Inglese, A. J. Phys. Chem. 1991, 95, 3322.

Langmuir, Vol. 14, No. 21, 1998 6051

Figure 4. Standard partial molar volume (circles) and heat capacity (triangles) of transfer of CnCOONa from water to water + HP-β-CD mixtures as functions of the fraction of the complexed substrates in the infinite dilution state: filled symbols, mCD ) 0.09 mol kg-1; open symbols, mCD ) 0.27 mol kg-1.

Figure 5. Apparent molar volume of sodium octanoate in water + HP-β-CD mixtures as a function of substrate concentration.

and heat capacity of the complex are equal to those accepted for this group in water. In fact, the correlation between YoS(w+CD) - YoS(w) and XoS,C for the complete series of the substrate analyzed (Figure 4) suggests that for high n values KC values are very large and, then, XoS,C as well as YoS(w+CD) - YoS(w) appears to be constant; the latter, which is equal to ∆YoC (see eq 12), is 23 cm3 mol-1 for volume and -760 J K-1 mol-1 for heat capacity while XoS,C ≈ 1. Consequently, for large n values the incremental CH2 group implies an increase of YoC which is equal to the increase of YoS(w). Effect of Concentration and the Nature of Cyclodextrin. To evidence the effect of cyclodextrin concentration, apparent molar volumes and heat capacities of C4COONa (which does not micellize) in the water + HPβ-CD mixture at 0.27 mol kg-1 and C7COONa (which forms micelles) in the water + HP-β-CD at 0.017 and 0.27 mol kg-1 were determined. The experimental data are collected in Table 2. As Figures 5 and 6 show, the amount W+CD vs of cyclodextrin added deeply modifies the YΦ,S W+CD mS profiles in the dilute region and hardly modifies it in the concentrated region. In particular, the curves are shifted toward larger and lower values for the volume and heat capacity, respectively, the larger mW CD is. It is to be noted that although the good correlation between the experimental points, it was impossible to evaluate with a reasonable accuracy the property at infinite dilution

6052 Langmuir, Vol. 14, No. 21, 1998

De Lisi et al. Table 6. Apparent Molar Volumes and Heat Capacities of Sodium Butanoate and Sodium Octanoate in Water + Hydroxypropyl-γ-Cyclodextrin Mixtures at 298 Ka mW+CD S

Figure 6. Apparent molar heat capacity of sodium octanoate in water + HP-β-CD mixtures as a function of substrate concentration. Table 5. Apparent Molar Volumes and Heat Capacities of Sodium Butanoate and Sodium Octanoate in Water + Hydroxypropyl-r-Cyclodextrin Mixtures at 298 Ka mW+CD S

103∆d

W+CD VΦ,S

cp

W+CD CΦ,S

C3COONa (mCD ) 0.08212; do ) 1.027257; cpo ) 3.97840) 0.03537 1.421 69.00 3.97045 212 0.06906 2.730 69.47 3.96295 213 0.1152 4.600 69.00 3.95380 222 0.1986 7.734 69.77 3.93732 227 0.2895 11.088 70.12 3.92300 241 0.3000 11.526 70.01 3.91841 231 0.4938 18.462 70.52 3.89007 249 0.7119 25.926 70.91 3.86107 260 0.9133 32.606 71.12 3.83799 269 1.1078 38.790 71.33 3.81731 275 1.3493 46.019 71.68 3.79516 282 1.5976 53.199 71.92 3.77468 288 1.7923 58.547 72.14 3.76015 292 C7COONa (mCD ) 0.08728; do ) 1.029036; cpo ) 3.96750) 0.01984 0.741 126.14 0.02981 1.083 127.06 3.95576 264 0.04952 1.749 127.93 3.94741 250 0.07984 2.751 128.62 3.93686 271 0.1116 3.670 129.98 3.93105 327 0.1966 5.995 131.94 3.92337 428 0.3048 8.841 132.97 3.91583 481 0.4432 12.149 134.03 3.90592 510 0.6542 16.021 136.25 3.86057 478 0.8415 18.844 137.83 3.80899 445 0.9993 21.033 138.79 3.76458 423 1.3082 24.990 140.06 3.69077 402 1.5863 28.212 140.84 3.63119 392 2.0374 32.750 141.81 3.55125 386 2.5851 37.407 142.65 3.47676 388 a

For units see Table 2.

W+CD at mW CD ) 0.017 m due to the steep dependence of YΦ,S W+CD o W on mS . CpS(w+CD) at mCD ≈ 0.3 m is smaller for C4COONa and larger for C7COONa with respect to those o at mW CD ≈ 0.1 m while VS(w+CD) displays the opposite behavior. This strange result disappears in the plot of YoS(w+CD) - YoS(w) as functions of XoS,C (Figure 4). The YoC values, obtained according to the procedure used above (eq 12), as expected, are very close to those at mW CD ≈ 0.1 m. W+CD W+CD plots In the intermediate region the YΦ,S vs mS for C7COONa present peculiarities at higher mW+CD S values the larger the cyclodextrin concentration is; in particular, they occur at 0.45, 0.48, 0.55, and 0.75 mol -1 kg-1 for mW CD ) 0, 0.017, 0.089, and 0.27 mol kg , values the concentrarespectively. From these mW+CD S

103∆d

W+CD VΦ,S

cp

W+CD CΦ,S

C3COONa (mCD ) 0.08784; do ) 1.038272; cpo ) 3.93272) 0.02486 0.975 69.58 0.04664 1.815 69.07 0.04965 1.950 69.47 3.92384 253 0.09925 3.863 69.67 3.91481 251 0.1528 5.845 70.15 3.89682 260 0.2215 8.415 70.22 3.89541 260 0.3270 12.226 70.52 3.87980 265 0.5043 18.437 70.86 3.85565 272 0.5062 18.467 70.93 0.6096 22.170 70.96 3.84198 274 0.7196 25.680 71.17 3.82862 277 1.0830 37.142 71.65 3.78892 284 1.3385 44.664 71.98 3.76490 289 1.6354 52.862 72.36 3.73705 292 1.8293 57.921 72.61 3.72241 295 C7COONa (mCD ) 0.08812; do ) 1.038391; cpo ) 3.92574) 0.02924 0.636 139.79 0.05458 1.178 139.87 0.8163 1.761 139.80 3.90556 402 0.1116 2.385 139.90 3.89836 403 0.1977 4.241 139.58 3.87545 390 0.3072 6.680 138.98 3.85162 399 0.4589 10.011 138.48 3.82988 428 0.6425 13.561 138.66 3.79636 430 0.8500 16.853 139.39 3.74631 412 1.0651 19.655 140.28 3.69379 396 1.3056 22.492 141.01 3.63995 386 1.4974 24.556 141.49 3.60164 382 2.0779 29.895 142.60 3.51537 387 2.5865 33.751 143.29 a

For units see Table 2.

Figure 7. Apparent molar volume of sodium octanoate in water (---) and in aqueous solutions of HP-R-CD (4), HP-β-CD (3) and HP-γ-CD (2) at 0.09 m as a function of the substrate concentration.

tions of the surfactant in the complexed form (mW S,C) were W X . The obtained m values are calculated as mW S S,C S,C values as expected for the coincident with the above mW CD inclusion complex of 1:1 stoichiometry. In addition, according to the Junquera et al.3 idea, these findings show that micelles are forming provided that all the cyclodextrin is in the complexed form and the free monomer concentration of the surfactant is equal to its cmc in water. The effect of the cavity size of cyclodextrin was investigated by studying also C3COONa and C7COONa in aqueous solutions of 0.09 m HP-R-CD and HP-γ-CD. The experimental data are reported in Tables 5 and 6.

Properties of Sodium n-Alkanecarboxylates

Figure 8. Apparent molar heat capacity of sodium octanoate in water (---) and in aqueous solutions of HP-R-CD (4), HPβ-CD (2) and HP-γ-CD (O) at 0.09 m as a function of the substrate concentration.

The nature of cyclodextrin does not affect the profiles of the apparent molar properties of C3COONa as functions W+CD but slightly affects the YΦ,S values. of mW+CD S W+CD W+CD vs mS curves of C7COONa in the The VΦ,S presence of the three cyclodextrins (Figure 7) are very close to that in pure water in the micellar region while they differ in the premicellar region. In particular, in the W+CD is lower than the property presence of HP-R-CD, VΦ,S while the in pure water and increases with mW+CD S opposite is displayed by HP-β-CD; in HP-γ-CD it is still W but shows a small dependence on larger than VΦ,S W+CD vs concentration. In the premicellar region the CΦ,S W+CD mS trends are opposite to those for volumes with the exception of that in HP-R-CD. As can be seen in Figure 8, in the region around the cmc a maximum is present in the heat capacity which is more pronounced the smaller the cyclodextrin cavity is. Obviously, minima are present in the case of volumes (Figure 7) with the exception of data in HP-R-CD where only a change in the slope is detected.

Langmuir, Vol. 14, No. 21, 1998 6053

The YoS(w+CD) values, obtained by using the same procedure as above, are reported in Table 3. In the presence of HP-γ-CD, the YoS(w+CD) values do not evidence the complex formation for C3COONa while they do for C7COONa. ∆YoC dealing with HP-R-CD and HP-γ-CD were calculated by using the KC values reported elsewhere28 (Table 3). By comparing data in HP-R-CD and HP-β-CD, it turns out that ∆CpoC values are negative for both cyclodextrins while ∆VoC values have opposite sign (positive for HP-βCD and negative for HP-R-CD). However, the latter findings are not unusual since Makimoto et al.29 report negative ∆VoC for R-CD+substituted phenyl acetate systems. Similar results (∆VoC > 0 for β-CD and ∆VoC < 0 for R-CD) were observed by Høiland et al.16 even if in this case the substrates were inorganic anions. The increase of ∆VoC and the decrease of ∆CpoC for C7COONa with the increasing of the cavity size of the cyclodextrins are consistent with the conformational effects, discussed above; in fact, these effects are expected to be more important in HP-R-CD because of its smaller size cavity. This explanation is supported by the negative contribution to the volume for the gauche conformations of hydrocarbons.25 Acknowledgment. The authors are grateful to the Assessorato ai Beni Culturali della Regione Siciliana and to the Ministry of University and of Scientific and Technological Research (Cofin MURST 97 CFSIB) for financial support. Supporting Information Available: Tables of apparent molar volumes and heat capacities of cyclodextrins and alkanecarboxylates in water (4 pages). Ordering and Internet access information is given on any current masthead page. LA971252P (28) De Lisi, R.; Milioto, S.; Inglese, A. To be published. (29) Makimoto, S.; Suzuki, K.; Taniguchi, Y. J. Phys. Chem. 1982, 86, 4544.