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Apparent and partial molar enthalpies of some potassium p-n-alkyl-benzoate aqueous solutions
Polyhedron 21 (2002) 1497 /1502 www.elsevier.com/locate/poly
Apparent and partial molar enthalpies of some potassium p-n-alkylbenzoate aqueous solutions Emilia Fisicaro a,*, Carlotta Compari a, Yoshikiyo Moroi b a
Dipartimento Farmaceutico, Sezione di Chimica Fisica Applicata, Universita` di Parma, Viale delle Scienze, 78-43100 Parma, Italy b Department of Chemistry, Faculty of Science, Kyushu University, Higashi-ku, Fukuoka 812, Japan Received 8 November 2001; accepted 20 December 2002
Abstract Dilution heats at 298 K of aqueous solutions of potassium salts of p -n -hexylbenzoic acid (KHB), p -n -heptylbenzoic acid (KHEB), p -n -octylbenzoic acid (KOB) in KOH 0.1 m have been measured as a function of concentration by using the flow mixing cell of a Thermometric TAM microcalorimeter. From the experimental data, apparent and partial molar enthalpies versus concentration have been obtained. The group contribution of the /CH2 / group in the plateau region results /1.6 kJ mol 1 per / CH2 /. This value is comparable with that obtained in the case of cationic surfactants. The enthalpy changes upon micellization (DHm ) have been obtained by using a pseudo phase transition approach and assuming as cmc the abscissa of the first inflection point of the curves of FL versus m . Micellization enthalpies result additive with a group contribution for the methylene group of /1.5 kJ mol 1 per group, comparable with that obtained for similar anionic compounds in the same experimental conditions. The behavior of the compounds under study is compared with that of the potassium salts of 4-((alkylcarbonyl)amino)-benzoic acids in order to understand the role played by the amido group bridging the benzoic moiety and the alkyl chain. The amido group appears to behave as a part of the polar head, strongly increasing the hydrophilicity of the molecule. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Potassium p -n -hexylbenzoate; Potassium p -n -heptylbenzoate; Potassium p -n -octylbenzoate; Apparent molar enthalpies of; Partial molar enthalpies of; Micellization enthalpy; Methylene group contribution
1. Introduction Some time ago, we started [1] to study the thermodynamic properties of the aqueous solutions of potassium 4-((alkylcarbonyl)amino)-2-hydroxybenzoate (later on referred to as KPAS-Cn where n stands for the number of carbon atoms in the chain). This class of ‘functionalized’ surfactants, containing reactive functions covalently bound to the molecular structure, is very interesting because it shows, beyond the ability to form micelles in solution, a chelating polar head, able to coordinate metal ions. For this reason, they have potential applications in extraction methods, ion transport across lipophilic membranes and other analytical fields. These compounds can be solubilized in water and, in the case of sufficiently high values of n , aggregate
* Corresponding author. Tel.: /39-0521-905-028; fax: /39-0521905-006. E-mail address: [email protected] (E. Fisicaro).
only in basic environment, i.e. in conditions above the ‘critical micelle pH range’ [2]. The system is complicated by the presence of salicylic hydroxylic group: this group can be deprotonated, giving rise to a doubly charged polar head, either at high pH values or in the presence of a transition metal. The pKa of the hydroxy group for the monomer below the cmc, spectroscopically evaluated, lies around 13.09/0.2 at 25 8C [3]. It has been observed as the enthalpic properties of these solutions are strongly dependent on pH, because the dissociation of the salicylic OH affects the hydrophilic /lipophilic balance of the molecule. Some very interesting features were outlined. The thermodynamic functions of micellization vary, within the limits of the experimental error, almost linearly with pH and a micelle structural transition is evidenced in the partial molar enthalpies versus concentration curve. This behavior is very interesting because, as a rule, not much heat is involved in micellar transitions in solutions and enthalpy measurements are, in general, not sensitive enough in this respect. In order
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to better explain thermodynamic data in terms of microscopic interactions and to understand the role of intra- and/or intermolecular hydrogen bonding on the structure, we have synthesized and studied the aqueous solutions of some potassium salts of 3-((alkylcarbonyl)amino)-4-hydroxybenzoic acid (KAHBn ) and 4-((alkylcarbonyl)amino)- benzoic acid (KPABn ) [4]. The present work is devoted to the study of potassium salts of p -n-hexylbenzoic acid (HBA), p -n-heptylbenzoic acid (HEBA), p-n -octylbenzoic acid (OBA) and it is aimed to understand the role played by the amido group bridging the benzoic moiety and the alkyl chain.
0.002 8C by a Heto proportional temperature controller.
3. Results The experimental data have been expressed in terms of apparent and partial molar quantities of the solute, as is usual in solution thermodynamics, assuming the infinite dilution as reference state. With reference to this state, we recall [1,5/10] that, the molar enthalpy of dilution, DHd, is given by: DHd FL;f FL;i
2. Experimental 2.1. Materials p-n -Hexylbenzoic acid (HBA), p-n -heptylbenzoic acid (HEBA), p -n-octylbenzoic acid (OBA), were prepared at Department of Chemistry of Kyushu University, as already described [2]. The purity of the reagents was checked by elemental analysis and gas chromatography. Inorganic chemicals of analytical reagent grade were used without further purification. Freshly boiled doubly distilled water, stored under nitrogen, was used throughout. Potassium hydroxide solutions (ca. 2 and 0.2 mol l 1) were prepared by diluting the content of Merck Titrisol ampoules with the proper quantity of water and standardized with potassium hydrogen phthalate (C. Erba; dried at 120 8C). The surfactant solutions were prepared under nitrogen adding to the acid the quantity of KOH needed both to neutralize the carboxylate group and to reach the required excess of hydroxide ion in the final solution (KOH 0.1 m excess). Solution concentrations are expressed as molality, m (mol kg 1). 2.2. Measurements The enthalpies of dilution were measured by means of the Thermometric TAM (flow mixing cell) microcalorimeter, equipped with 221 Nano Amplifier, at 298 K. The freshly prepared surfactant solutions, kept before injection at the experimental temperature by means of a Heto cryothermostatic bath, were diluted into the ‘mixing’ measuring cell of the microcalorimeter in ratio 1:1 by using CO2-free water. The injection of the solutions and of water into the calorimeter was done by means of a Gilson peristaltic pump, Minipuls 2, and their flows were determined by weight. Densities of the solutions were measured by a vibrating tube densimeter (PAAR DMA602) controlled by an IBM personal computer, calibrated with water and air. The temperature was kept constant at 9/
(1)
where FL is the apparent molar relative enthalpy and the indexes f and i stand for the final and initial concentrations, respectively. For ionic surfactant in the premicellar region: FL AL m1=2 BL m CL m3=2
(2)
where AL is the limiting Debye/Huckel slope for relative enthalpies, accounting for the long range electrostatic solute/solute interactions. Parameters BL and CL are averaged on the experimental points in the premicellar region. In the micellar region, the apparent molar enthalpies are evaluated by means of equation 1 and by graphical interpolation. The partial molar enthalpies L2 are determined by drawing the best curve for the apparent molar enthalpies versus m and then by calculating the partial molar quantities as D(mFL )/Dm from points interpolated at regular intervals. The heats of dilution and apparent and partial molar enthalpies at 298 K, obtained by means of Eq. (1) /(2), for the compounds under study are listed in Tables 1 /3. In Figs. 1 and 2, the plots of the apparent and partial molar enthalpies, respectively, as a function of molality for the different chain length are shown. The apparent molar volumes, VF of surfactants were calculated by means of the following equation: VF
M d
103 (d do ) mddo
(3)
where d is the density of the solution of molality m , M is the molecular weight of the surfactants and do is the density of the KOH 0.1 m solution.
4. Discussion In the literature, dissolution and micellization of HBA and OBA in aqueous solution were examined by the phase rule in relation to the solution pH [2]. The experimental data were interpreted under the light of
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Table 1 Enthalpies of dilution, apparent and partial molar enthalpies of KHB, in water at 298 K mi (mol Kg 1)
mf (mol Kg 1)
DHd (J mol 1)
FL ,i (J mol 1)
FL ,f (J mol 1)
L2,i (J mol 1)
L2,f (J mol 1)
0.0050 0.0080 0.0100 0.0150 0.0201 0.0240 0.0301 0.0401 0.0502 0.0602 0.0702 0.0803 0.1005 0.1511 0.2021 0.2525 0.3034 0.3548 0.4060 0.5093 0.6132
0.0024 0.0039 0.0042 0.0071 0.0098 0.0117 0.0146 0.0190 0.0244 0.0251 0.0333 0.0391 0.0488 0.0724 0.1050 0.1193 0.1439 0.1676 0.1922 0.2387 0.2823
54 96 131 195 235 269 393 487 457 427 309 204 121 5 60 123 164 214 232 280 347
170 237 282 394 512 611 766 982 1100 1150 1180 1180 1190 1180 1130 1080 1015 950 900 810 695
108 144 151 216 276 318 385 488 620 636 855 970 1090 1180 1190 1192 1189 1162 1150 1088 1040
184 414 504 794 996 1214 1565 1560 1455 1375 1310 1250 1175 1010 880 775 690 605 500 300 178
172 236 249 373 492 578 719 942 1236 1271 1570 1565 1470 1300 1152 1100 1025 960 905 800 730
the mass action model to obtain the micellization constant Kn , the micelle aggregation number n , and the number of counterion bound per micelle, m . In presence of undissolved acid as a separate phase, the pH plays a role similar to that of temperature, because the concentration of the dissociated anion showing surface active properties increases linearly with 1/aH till to a range of values (micellar pH range) at which micellization starts. This pH range, as the cmc, is typical of the
insoluble acid under study. This paper deals with the potassium salts of the above mentioned acids and of HEBA, in KOH 0.1 m excess, i.e. when the concentration of the undissociated acid is negligible and hydrolysis is avoided. In the same conditions we studied, as mentioned before, the enthalpic properties of the potassium salts of PASn , PABn , and AIBn solutions [1,4]. The comparison of the behavior of the above mentioned compounds in the same conditions could give
Table 2 Enthalpies of dilution, apparent and partial molar enthalpies of KHEB, in water at 298 K mi (mol kg 1)
mf (mol kg 1)
DHd (J mol 1)
FL ,i (J mol 1)
FL ,f (J mol 1)
L2,i (J mol 1)
L2,f (J mol 1)
0.0020 0.0040 0.0060 0.0081 0.0100 0.0131 0.0150 0.0181 0.0201 0.0301 0.0502 0.0804 0.1008 0.1511 0.2016 0.3037 0.4064 0.4579 0.5382 0.6130 0.7159
0.0010 0.0020 0.0029 0.0040 0.0048 0.0064 0.0074 0.0088 0.0097 0.0147 0.0249 0.0394 0.0487 0.0732 0.0985 0.1449 0.1925 0.2156 0.2569 0.2813 0.3387
14 36 52 132 126 27 111 159 219 224 191 189 188 188 195 227 276 294 324 372 408
64 96 132 227 235 166 60 10 16 144 278 399 453 558 645 777 906 954 1040 1122 1228
47 63 78 95 110 138 159 230 235 80 88 210 265 370 450 550 630 660 715 750 820
95 162 247 350 300 160 240 280 310 410 550 668 715 858 975 1150 1312 1390 1500 1590 1640
66 94 124 160 195 265 316 360 345 230 360 475 538 650 712 842 958 998 1070 1113 1215
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1500
Table 3 Enthalpies of dilution, apparent and partial molar enthalpies of KOB, in water at 298 K mi (mol Kg 1)
mf (mol Kg 1)
DHd (J mol 1)
FL ,i (J mol 1)
FL ,f (J mol 1)
L2,i (J mol 1)
L2,f (J mol 1)
0.0030 0.0040 0.0050 0.0060 0.0080 0.0101 0.0200 0.0401 0.0601 0.0804 0.1359 0.1816 0.2020 0.2543 0.3578 0.5088 0.6134
0.0015 0.0020 0.0024 0.0030 0.0040 0.0050 0.0098 0.0196 0.0302 0.0398 0.0657 0.0870 0.0989 0.1249 0.1720 0.2306 0.2789
189 605 911 887 779 681 415 266 212 191 180 191 194 174 257 355 375
429 18 340 460 760 1000 1415 1678 1785 1868 2010 2090 2120 2190 2320 2522 2591
618 622 571 433 18 340 990 1390 1570 1678 1810 1890 1920 1975 2060 2160 2220
503 1400 1600 1660 1710 1750 1860 1985 2070 2140 2270 2370 2430 2545 2750 2950 3000
746 485 149 486 1400 1600 1750 1855 1935 1982 2090 2160 2192 2248 2340 2495 2595
Fig. 1. Apparent molar relative enthalpies (FL , J mol 1) of potassium salts of p -n -hexylbenzoic acid (KHB, empty circles), p -n -heptylbenzoic acid (KHEB, empty rhombs), p -n -octylbenzoic acid (KOB, empty squares) in KOH 0.1 m as a function of surfactant molality, m .
Fig. 2. Partial molar relative enthalpies (L2, J mol 1) of potassium salts of p -n -hexylbenzoic acid (KHB, empty squares), p -n -heptylbenzoic acid (KHEB, empty rhombs), p -n -octylbenzoic acid (KOB, empty circles) in KOH 0.1 m as a function of surfactant molality, m .
us information about the role played by the different fragments of the molecule in the micellization process. In Fig. 1 and in Tables 1 /3, the curves of the apparent molar enthalpies versus m at 298 K are reported for different chain lengths. The general trends concord with the expectations for ionic surfactants: the curves, after increasing in the premicellar region, tend to level off at concentrations above the cmc, where they are almost parallel. The lowering of the curves in the micellar region is proportional to the number of carbon atoms in the alkyl chain. It is possible to extract the group contribution of the /CH2 / group in the postmicellar region: it results /1.6 kJ mol 1 per /CH2 /. This value is comparable with that obtained in the case of cationic surfactants [6]. This group contribution proves to apply specifically to the methylene group and to be independent of the charge in the polar head, too.
It is very interesting to evaluate the effect of the amido group, bound in para position in respect to the carboxylic moiety, by the comparison of the trends of the compounds under study with those of potassium 4((alkylcarbonyl)amino)- benzoate (KPABn ) [4]. In Fig. 3, as an example, the comparison between the apparent molar enthalpies versus m of the terms having seven carbon atoms in the alkyl chain is reported. The effect of the substitution of a methylene group by an amido group results in a strong increase in hydrophilicity for the KPABn : the effect is comparable with that brought about by a removal of a methylene group in the alkyl chain. The group contribution results /1.7 kJ mol 1. The electron withdrawing amido group behaves as a part of the polar head. Some of us have shown for a series of dodecyl cationic surfactants with the same counterion that, from the trends of the apparent and
E. Fisicaro et al. / Polyhedron 21 (2002) 1497 /1502
Fig. 3. Comparison between apparent molar enthapies (FL , J mol 1) as a function of molality, m of potassium salts of p -n -heptylbenzoic acid (KHEB, empty triangles), 4-((hexylcarbonyl)amino-benzoic acid (KPAB7, full squares) [4] and 4-((hexylcarbonyl)amino-2-hydroxybenzoic acid (KPAS-C7, empty squares) in KOH 0.1 m [1].
partial molar enthalpies, a sort of ‘charge delocalization scale’ for the polar head could be obtained. The more delocalized is the charge, the more similar is the behavior to that of non-ionic surfactants [10]. The same observation seems to hold also for the anionic surfactants, as shown in Fig. 3. In the case of cationic surfactants, we have studied the effect of an amido group in meta position by comparing N -alkylpyridinium (Cn PC) and N -alkylnicotinamide chlorides (Cn NAC). It has been shown that when the amido group is bound in three position to the pyridinic ring, it acts in the opposite way, strongly enhancing the hydrophobicity of the Cn NAC surfactants [10]. 4.1. Changes in thermodynamic properties upon micelle formation In order to obtain the changes in thermodynamic properties upon micellization, we have applied a pseudo-phase transition model, in which the aggregation process is considered like a phase transition, taking place at equilibrium. In this model, it is assumed that, at the cmc, the partial molar properties present a discontinuity due to the formation of a pseudo-phase. The micellization parameters are obtained by extrapolating at the cmc the trends of partial molar properties before and after cmc [1,5 /10]. Rather sharp changes occur around the cmc, but they are never discontinuous, especially for surfactants with short hydrophobic chains. In order to evaluate the micellization enthalpy change, DHm of the compounds under investigation, we have to know the values of the cmc. Previous studies on solubility and micelle formation of p -n -alkylbenzoic acids have shown that the cmc values at the critical micelle pH were 4.80 /10 2 at 298 K for NaHBA and 1.30 /10 2 for NaOBA at 308 K [2]. The experimental conditions of this work are very different both as pH
1501
value at which only the basic dissociated form is present and as counterion (K instead of Na ). So it is reasonable to expect that also the cmc values are different. We have evaluated the cmc by a thermodynamic method, assuming as cmc the abscissa of the first inflection point of the curves of FL versus m . The values so obtained for the cmc and the micellization enthalpies of the compounds under investigation are reported in Table 4. The logarithms of the cmc are nicely linearly correlated (see Fig. 4) with the number of carbon atoms, the slope of the straight line so obtained being proportional to the free energy of transfer of each /CH2 / from the aqueous phase to the micellized state. It results /1.27kT per/CH2 /. This value appears to be a slightly greater quantity (in terms of absolute value) than that reported in ref [11]. The difference could be explained in terms of higher ionic strength, reducing the electrostatic repulsion between the polar head groups. The reported micellization enthalpies for the studied compounds are linearly correlated with the number of carbon atoms in the alkyl chain. It is, therefore, possible to extract a group contribution for the methylene group: thus we have /1.5 kJ mol 1 per group, which is comparable with that obtained in the case of potassium 3-((alkylTable 4 Critical micelle concentration (cmc), enthalpy changes for the micellization process (DHm ) and partial molar volume in micellar phase (Vs) for the surfactants studied
Cmc (mol kg 1) L2m (kJ mol 1) L2s (kJ mol 1) DHm (kJ mol 1) Vs (cm3 mol 1)
KOB
KHEB
KHB
0.0016 0.714 1.429 2.143 201
0.0069 0.390 0.181 0.571 184
0.020 0.980 1.847 0.857 167
Indices m and s stand for the value of the quantity for the monomer and for the micelle, respectively.
Fig. 4. Log cmc, obtained as the abscissa of the first inflection point of the curves of FL vs. m , as a function of the number of carbon atoms, n in the alkyl chain for the compounds under investigation in the experimental conditions.
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carbonyl)amino)-4-hydroxybenzoates (/1.4 kJ mol 1 per group) and 4-((alkylcarbonyl)amino)-benzoates (/1.33 kJ mol 1 per group) [4], but greater than the average group contribution obtained in the case of cationic surfactants ( /1.1 kJ mol 1 per group) [6]. In Table 4 partial molar volumes of the surfactants under study in micellar phase are also reported. It is easy to see that the values differs by 17 cm3 mol 1 per /CH2 /. The /CH2 / group contribution obtained in micellar region is the same as that obtained for pyridine and nicotinamide type cationic surfactants [6].
Acknowledgements The authors are grateful to the Italian Ministry for the University and the Scientific and Technological Research (MURST), COFIN 2000/project ‘Molecular Systems and Metallo-receptors for the Activation and Transport of Dioxygen, Nitric Oxide, and Biosubstrate’ for financial support.
References 5. Conclusions The study of thermodynamic properties of the aqueous solutions of potassium salts of KHB, KHEB, KOB in KOH 0.1 m has shown that also in the case of anionic surfactants derived from benzoic acid, the group contribution of the /CH2 / is additive both in the plateau ( /1.6 kJ mol 1 per /CH2 /) and in the micellization enthalpy changes ( /1.5 kJ mol 1 per group). The values are comparable with those obtained in the case of cationic surfactants and for similar anionic compounds in the same experimental conditions. The behavior of the compounds under study was compared with that of the potassium salts of 4-((alkylcarbonyl)amino)-benzoic acids, in order to understand the role played by the amido group bridging the benzoic moiety and the alkyl chain. Thus, we see that the amido group behaves as a part of the polar head, strongly increasing the hydrophilicity of the molecule.
[1] E. Fisicaro, E. Pelizzetti, M. Barbieri, P. Savarino, G. Viscardi, Thermochim. Acta 162 (1990) 277. [2] Y. Moroi, Y. Sakamoto, J. Phys. Chem. 92 (1988) 5189. [3] E. Pelizzetti, E. Pramauro, E. Barni, P. Savarino, M. Corti, V. Degiorgio, Ber. Busenges. Phys. Chem. 86 (1982) 529. [4] E. Fisicaro, C. Compari, G. Viscardi, P.L. Quagliotto, in preparation. [5] E. Fisicaro, M. Barbieri, E. Pelizzetti, P. Savarino, J. Chem. Soc., Faraday Trans. 87 (1991) 2983. [6] (a) E. Fisicaro, A. Ghiozzi, E. Pelizzetti, G. Viscardi, P.L. Quagliotto, J. Coll. Interf. Sci. 182 (1996) 549; (b) E. Fisicaro, A. Ghiozzi, E. Pelizzetti, G. Viscardi, P.L. Quagliotto, J. Coll. Interf. Sci. 184 (1996) 147. [7] J.E. Desnoyers, G. Perron, in: R. Zana (Ed.), Surfactant Solutions-New Methods of Investigation (and references therein), Marcel Dekker, New York, 1987, p. 1. [8] E. Fisicaro, Cell. Mol. Biology Lett. 2 (1) (1997) 45. [9] E. Fisicaro, C. Compari, B. Rozycka-Roszak, G. Viscardi, P.L. Quagliotto, Curr. Top. Coll. Interf. Sci. 2 (1997) 53. [10] E. Fisicaro, E. Pelizzetti, M. Barbieri, P. Savarino, G. Viscardi, Thermochim. Acta 168 (1990) 143. [11] K. Shinoda, T. Nakagawa, B. Tamamushi, T. Isemura, Colloidal Surfactants, Academic Press, New York, NY, 1963, pp. 37 /42.
Apparent and partial molar enthalpies of some potassium p-n-alkylbenzoate aqueous solutions Emilia Fisicaro a,*, Carlotta Compari a, Yoshikiyo Moroi b a
Dipartimento Farmaceutico, Sezione di Chimica Fisica Applicata, Universita` di Parma, Viale delle Scienze, 78-43100 Parma, Italy b Department of Chemistry, Faculty of Science, Kyushu University, Higashi-ku, Fukuoka 812, Japan Received 8 November 2001; accepted 20 December 2002
Abstract Dilution heats at 298 K of aqueous solutions of potassium salts of p -n -hexylbenzoic acid (KHB), p -n -heptylbenzoic acid (KHEB), p -n -octylbenzoic acid (KOB) in KOH 0.1 m have been measured as a function of concentration by using the flow mixing cell of a Thermometric TAM microcalorimeter. From the experimental data, apparent and partial molar enthalpies versus concentration have been obtained. The group contribution of the /CH2 / group in the plateau region results /1.6 kJ mol 1 per / CH2 /. This value is comparable with that obtained in the case of cationic surfactants. The enthalpy changes upon micellization (DHm ) have been obtained by using a pseudo phase transition approach and assuming as cmc the abscissa of the first inflection point of the curves of FL versus m . Micellization enthalpies result additive with a group contribution for the methylene group of /1.5 kJ mol 1 per group, comparable with that obtained for similar anionic compounds in the same experimental conditions. The behavior of the compounds under study is compared with that of the potassium salts of 4-((alkylcarbonyl)amino)-benzoic acids in order to understand the role played by the amido group bridging the benzoic moiety and the alkyl chain. The amido group appears to behave as a part of the polar head, strongly increasing the hydrophilicity of the molecule. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Potassium p -n -hexylbenzoate; Potassium p -n -heptylbenzoate; Potassium p -n -octylbenzoate; Apparent molar enthalpies of; Partial molar enthalpies of; Micellization enthalpy; Methylene group contribution
1. Introduction Some time ago, we started [1] to study the thermodynamic properties of the aqueous solutions of potassium 4-((alkylcarbonyl)amino)-2-hydroxybenzoate (later on referred to as KPAS-Cn where n stands for the number of carbon atoms in the chain). This class of ‘functionalized’ surfactants, containing reactive functions covalently bound to the molecular structure, is very interesting because it shows, beyond the ability to form micelles in solution, a chelating polar head, able to coordinate metal ions. For this reason, they have potential applications in extraction methods, ion transport across lipophilic membranes and other analytical fields. These compounds can be solubilized in water and, in the case of sufficiently high values of n , aggregate
* Corresponding author. Tel.: /39-0521-905-028; fax: /39-0521905-006. E-mail address: [email protected] (E. Fisicaro).
only in basic environment, i.e. in conditions above the ‘critical micelle pH range’ [2]. The system is complicated by the presence of salicylic hydroxylic group: this group can be deprotonated, giving rise to a doubly charged polar head, either at high pH values or in the presence of a transition metal. The pKa of the hydroxy group for the monomer below the cmc, spectroscopically evaluated, lies around 13.09/0.2 at 25 8C [3]. It has been observed as the enthalpic properties of these solutions are strongly dependent on pH, because the dissociation of the salicylic OH affects the hydrophilic /lipophilic balance of the molecule. Some very interesting features were outlined. The thermodynamic functions of micellization vary, within the limits of the experimental error, almost linearly with pH and a micelle structural transition is evidenced in the partial molar enthalpies versus concentration curve. This behavior is very interesting because, as a rule, not much heat is involved in micellar transitions in solutions and enthalpy measurements are, in general, not sensitive enough in this respect. In order
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 0 9 5 4 - 3
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E. Fisicaro et al. / Polyhedron 21 (2002) 1497 /1502
to better explain thermodynamic data in terms of microscopic interactions and to understand the role of intra- and/or intermolecular hydrogen bonding on the structure, we have synthesized and studied the aqueous solutions of some potassium salts of 3-((alkylcarbonyl)amino)-4-hydroxybenzoic acid (KAHBn ) and 4-((alkylcarbonyl)amino)- benzoic acid (KPABn ) [4]. The present work is devoted to the study of potassium salts of p -n-hexylbenzoic acid (HBA), p -n-heptylbenzoic acid (HEBA), p-n -octylbenzoic acid (OBA) and it is aimed to understand the role played by the amido group bridging the benzoic moiety and the alkyl chain.
0.002 8C by a Heto proportional temperature controller.
3. Results The experimental data have been expressed in terms of apparent and partial molar quantities of the solute, as is usual in solution thermodynamics, assuming the infinite dilution as reference state. With reference to this state, we recall [1,5/10] that, the molar enthalpy of dilution, DHd, is given by: DHd FL;f FL;i
2. Experimental 2.1. Materials p-n -Hexylbenzoic acid (HBA), p-n -heptylbenzoic acid (HEBA), p -n-octylbenzoic acid (OBA), were prepared at Department of Chemistry of Kyushu University, as already described [2]. The purity of the reagents was checked by elemental analysis and gas chromatography. Inorganic chemicals of analytical reagent grade were used without further purification. Freshly boiled doubly distilled water, stored under nitrogen, was used throughout. Potassium hydroxide solutions (ca. 2 and 0.2 mol l 1) were prepared by diluting the content of Merck Titrisol ampoules with the proper quantity of water and standardized with potassium hydrogen phthalate (C. Erba; dried at 120 8C). The surfactant solutions were prepared under nitrogen adding to the acid the quantity of KOH needed both to neutralize the carboxylate group and to reach the required excess of hydroxide ion in the final solution (KOH 0.1 m excess). Solution concentrations are expressed as molality, m (mol kg 1). 2.2. Measurements The enthalpies of dilution were measured by means of the Thermometric TAM (flow mixing cell) microcalorimeter, equipped with 221 Nano Amplifier, at 298 K. The freshly prepared surfactant solutions, kept before injection at the experimental temperature by means of a Heto cryothermostatic bath, were diluted into the ‘mixing’ measuring cell of the microcalorimeter in ratio 1:1 by using CO2-free water. The injection of the solutions and of water into the calorimeter was done by means of a Gilson peristaltic pump, Minipuls 2, and their flows were determined by weight. Densities of the solutions were measured by a vibrating tube densimeter (PAAR DMA602) controlled by an IBM personal computer, calibrated with water and air. The temperature was kept constant at 9/
(1)
where FL is the apparent molar relative enthalpy and the indexes f and i stand for the final and initial concentrations, respectively. For ionic surfactant in the premicellar region: FL AL m1=2 BL m CL m3=2
(2)
where AL is the limiting Debye/Huckel slope for relative enthalpies, accounting for the long range electrostatic solute/solute interactions. Parameters BL and CL are averaged on the experimental points in the premicellar region. In the micellar region, the apparent molar enthalpies are evaluated by means of equation 1 and by graphical interpolation. The partial molar enthalpies L2 are determined by drawing the best curve for the apparent molar enthalpies versus m and then by calculating the partial molar quantities as D(mFL )/Dm from points interpolated at regular intervals. The heats of dilution and apparent and partial molar enthalpies at 298 K, obtained by means of Eq. (1) /(2), for the compounds under study are listed in Tables 1 /3. In Figs. 1 and 2, the plots of the apparent and partial molar enthalpies, respectively, as a function of molality for the different chain length are shown. The apparent molar volumes, VF of surfactants were calculated by means of the following equation: VF
M d
103 (d do ) mddo
(3)
where d is the density of the solution of molality m , M is the molecular weight of the surfactants and do is the density of the KOH 0.1 m solution.
4. Discussion In the literature, dissolution and micellization of HBA and OBA in aqueous solution were examined by the phase rule in relation to the solution pH [2]. The experimental data were interpreted under the light of
E. Fisicaro et al. / Polyhedron 21 (2002) 1497 /1502
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Table 1 Enthalpies of dilution, apparent and partial molar enthalpies of KHB, in water at 298 K mi (mol Kg 1)
mf (mol Kg 1)
DHd (J mol 1)
FL ,i (J mol 1)
FL ,f (J mol 1)
L2,i (J mol 1)
L2,f (J mol 1)
0.0050 0.0080 0.0100 0.0150 0.0201 0.0240 0.0301 0.0401 0.0502 0.0602 0.0702 0.0803 0.1005 0.1511 0.2021 0.2525 0.3034 0.3548 0.4060 0.5093 0.6132
0.0024 0.0039 0.0042 0.0071 0.0098 0.0117 0.0146 0.0190 0.0244 0.0251 0.0333 0.0391 0.0488 0.0724 0.1050 0.1193 0.1439 0.1676 0.1922 0.2387 0.2823
54 96 131 195 235 269 393 487 457 427 309 204 121 5 60 123 164 214 232 280 347
170 237 282 394 512 611 766 982 1100 1150 1180 1180 1190 1180 1130 1080 1015 950 900 810 695
108 144 151 216 276 318 385 488 620 636 855 970 1090 1180 1190 1192 1189 1162 1150 1088 1040
184 414 504 794 996 1214 1565 1560 1455 1375 1310 1250 1175 1010 880 775 690 605 500 300 178
172 236 249 373 492 578 719 942 1236 1271 1570 1565 1470 1300 1152 1100 1025 960 905 800 730
the mass action model to obtain the micellization constant Kn , the micelle aggregation number n , and the number of counterion bound per micelle, m . In presence of undissolved acid as a separate phase, the pH plays a role similar to that of temperature, because the concentration of the dissociated anion showing surface active properties increases linearly with 1/aH till to a range of values (micellar pH range) at which micellization starts. This pH range, as the cmc, is typical of the
insoluble acid under study. This paper deals with the potassium salts of the above mentioned acids and of HEBA, in KOH 0.1 m excess, i.e. when the concentration of the undissociated acid is negligible and hydrolysis is avoided. In the same conditions we studied, as mentioned before, the enthalpic properties of the potassium salts of PASn , PABn , and AIBn solutions [1,4]. The comparison of the behavior of the above mentioned compounds in the same conditions could give
Table 2 Enthalpies of dilution, apparent and partial molar enthalpies of KHEB, in water at 298 K mi (mol kg 1)
mf (mol kg 1)
DHd (J mol 1)
FL ,i (J mol 1)
FL ,f (J mol 1)
L2,i (J mol 1)
L2,f (J mol 1)
0.0020 0.0040 0.0060 0.0081 0.0100 0.0131 0.0150 0.0181 0.0201 0.0301 0.0502 0.0804 0.1008 0.1511 0.2016 0.3037 0.4064 0.4579 0.5382 0.6130 0.7159
0.0010 0.0020 0.0029 0.0040 0.0048 0.0064 0.0074 0.0088 0.0097 0.0147 0.0249 0.0394 0.0487 0.0732 0.0985 0.1449 0.1925 0.2156 0.2569 0.2813 0.3387
14 36 52 132 126 27 111 159 219 224 191 189 188 188 195 227 276 294 324 372 408
64 96 132 227 235 166 60 10 16 144 278 399 453 558 645 777 906 954 1040 1122 1228
47 63 78 95 110 138 159 230 235 80 88 210 265 370 450 550 630 660 715 750 820
95 162 247 350 300 160 240 280 310 410 550 668 715 858 975 1150 1312 1390 1500 1590 1640
66 94 124 160 195 265 316 360 345 230 360 475 538 650 712 842 958 998 1070 1113 1215
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1500
Table 3 Enthalpies of dilution, apparent and partial molar enthalpies of KOB, in water at 298 K mi (mol Kg 1)
mf (mol Kg 1)
DHd (J mol 1)
FL ,i (J mol 1)
FL ,f (J mol 1)
L2,i (J mol 1)
L2,f (J mol 1)
0.0030 0.0040 0.0050 0.0060 0.0080 0.0101 0.0200 0.0401 0.0601 0.0804 0.1359 0.1816 0.2020 0.2543 0.3578 0.5088 0.6134
0.0015 0.0020 0.0024 0.0030 0.0040 0.0050 0.0098 0.0196 0.0302 0.0398 0.0657 0.0870 0.0989 0.1249 0.1720 0.2306 0.2789
189 605 911 887 779 681 415 266 212 191 180 191 194 174 257 355 375
429 18 340 460 760 1000 1415 1678 1785 1868 2010 2090 2120 2190 2320 2522 2591
618 622 571 433 18 340 990 1390 1570 1678 1810 1890 1920 1975 2060 2160 2220
503 1400 1600 1660 1710 1750 1860 1985 2070 2140 2270 2370 2430 2545 2750 2950 3000
746 485 149 486 1400 1600 1750 1855 1935 1982 2090 2160 2192 2248 2340 2495 2595
Fig. 1. Apparent molar relative enthalpies (FL , J mol 1) of potassium salts of p -n -hexylbenzoic acid (KHB, empty circles), p -n -heptylbenzoic acid (KHEB, empty rhombs), p -n -octylbenzoic acid (KOB, empty squares) in KOH 0.1 m as a function of surfactant molality, m .
Fig. 2. Partial molar relative enthalpies (L2, J mol 1) of potassium salts of p -n -hexylbenzoic acid (KHB, empty squares), p -n -heptylbenzoic acid (KHEB, empty rhombs), p -n -octylbenzoic acid (KOB, empty circles) in KOH 0.1 m as a function of surfactant molality, m .
us information about the role played by the different fragments of the molecule in the micellization process. In Fig. 1 and in Tables 1 /3, the curves of the apparent molar enthalpies versus m at 298 K are reported for different chain lengths. The general trends concord with the expectations for ionic surfactants: the curves, after increasing in the premicellar region, tend to level off at concentrations above the cmc, where they are almost parallel. The lowering of the curves in the micellar region is proportional to the number of carbon atoms in the alkyl chain. It is possible to extract the group contribution of the /CH2 / group in the postmicellar region: it results /1.6 kJ mol 1 per /CH2 /. This value is comparable with that obtained in the case of cationic surfactants [6]. This group contribution proves to apply specifically to the methylene group and to be independent of the charge in the polar head, too.
It is very interesting to evaluate the effect of the amido group, bound in para position in respect to the carboxylic moiety, by the comparison of the trends of the compounds under study with those of potassium 4((alkylcarbonyl)amino)- benzoate (KPABn ) [4]. In Fig. 3, as an example, the comparison between the apparent molar enthalpies versus m of the terms having seven carbon atoms in the alkyl chain is reported. The effect of the substitution of a methylene group by an amido group results in a strong increase in hydrophilicity for the KPABn : the effect is comparable with that brought about by a removal of a methylene group in the alkyl chain. The group contribution results /1.7 kJ mol 1. The electron withdrawing amido group behaves as a part of the polar head. Some of us have shown for a series of dodecyl cationic surfactants with the same counterion that, from the trends of the apparent and
E. Fisicaro et al. / Polyhedron 21 (2002) 1497 /1502
Fig. 3. Comparison between apparent molar enthapies (FL , J mol 1) as a function of molality, m of potassium salts of p -n -heptylbenzoic acid (KHEB, empty triangles), 4-((hexylcarbonyl)amino-benzoic acid (KPAB7, full squares) [4] and 4-((hexylcarbonyl)amino-2-hydroxybenzoic acid (KPAS-C7, empty squares) in KOH 0.1 m [1].
partial molar enthalpies, a sort of ‘charge delocalization scale’ for the polar head could be obtained. The more delocalized is the charge, the more similar is the behavior to that of non-ionic surfactants [10]. The same observation seems to hold also for the anionic surfactants, as shown in Fig. 3. In the case of cationic surfactants, we have studied the effect of an amido group in meta position by comparing N -alkylpyridinium (Cn PC) and N -alkylnicotinamide chlorides (Cn NAC). It has been shown that when the amido group is bound in three position to the pyridinic ring, it acts in the opposite way, strongly enhancing the hydrophobicity of the Cn NAC surfactants [10]. 4.1. Changes in thermodynamic properties upon micelle formation In order to obtain the changes in thermodynamic properties upon micellization, we have applied a pseudo-phase transition model, in which the aggregation process is considered like a phase transition, taking place at equilibrium. In this model, it is assumed that, at the cmc, the partial molar properties present a discontinuity due to the formation of a pseudo-phase. The micellization parameters are obtained by extrapolating at the cmc the trends of partial molar properties before and after cmc [1,5 /10]. Rather sharp changes occur around the cmc, but they are never discontinuous, especially for surfactants with short hydrophobic chains. In order to evaluate the micellization enthalpy change, DHm of the compounds under investigation, we have to know the values of the cmc. Previous studies on solubility and micelle formation of p -n -alkylbenzoic acids have shown that the cmc values at the critical micelle pH were 4.80 /10 2 at 298 K for NaHBA and 1.30 /10 2 for NaOBA at 308 K [2]. The experimental conditions of this work are very different both as pH
1501
value at which only the basic dissociated form is present and as counterion (K instead of Na ). So it is reasonable to expect that also the cmc values are different. We have evaluated the cmc by a thermodynamic method, assuming as cmc the abscissa of the first inflection point of the curves of FL versus m . The values so obtained for the cmc and the micellization enthalpies of the compounds under investigation are reported in Table 4. The logarithms of the cmc are nicely linearly correlated (see Fig. 4) with the number of carbon atoms, the slope of the straight line so obtained being proportional to the free energy of transfer of each /CH2 / from the aqueous phase to the micellized state. It results /1.27kT per/CH2 /. This value appears to be a slightly greater quantity (in terms of absolute value) than that reported in ref [11]. The difference could be explained in terms of higher ionic strength, reducing the electrostatic repulsion between the polar head groups. The reported micellization enthalpies for the studied compounds are linearly correlated with the number of carbon atoms in the alkyl chain. It is, therefore, possible to extract a group contribution for the methylene group: thus we have /1.5 kJ mol 1 per group, which is comparable with that obtained in the case of potassium 3-((alkylTable 4 Critical micelle concentration (cmc), enthalpy changes for the micellization process (DHm ) and partial molar volume in micellar phase (Vs) for the surfactants studied
Cmc (mol kg 1) L2m (kJ mol 1) L2s (kJ mol 1) DHm (kJ mol 1) Vs (cm3 mol 1)
KOB
KHEB
KHB
0.0016 0.714 1.429 2.143 201
0.0069 0.390 0.181 0.571 184
0.020 0.980 1.847 0.857 167
Indices m and s stand for the value of the quantity for the monomer and for the micelle, respectively.
Fig. 4. Log cmc, obtained as the abscissa of the first inflection point of the curves of FL vs. m , as a function of the number of carbon atoms, n in the alkyl chain for the compounds under investigation in the experimental conditions.
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carbonyl)amino)-4-hydroxybenzoates (/1.4 kJ mol 1 per group) and 4-((alkylcarbonyl)amino)-benzoates (/1.33 kJ mol 1 per group) [4], but greater than the average group contribution obtained in the case of cationic surfactants ( /1.1 kJ mol 1 per group) [6]. In Table 4 partial molar volumes of the surfactants under study in micellar phase are also reported. It is easy to see that the values differs by 17 cm3 mol 1 per /CH2 /. The /CH2 / group contribution obtained in micellar region is the same as that obtained for pyridine and nicotinamide type cationic surfactants [6].
Acknowledgements The authors are grateful to the Italian Ministry for the University and the Scientific and Technological Research (MURST), COFIN 2000/project ‘Molecular Systems and Metallo-receptors for the Activation and Transport of Dioxygen, Nitric Oxide, and Biosubstrate’ for financial support.
References 5. Conclusions The study of thermodynamic properties of the aqueous solutions of potassium salts of KHB, KHEB, KOB in KOH 0.1 m has shown that also in the case of anionic surfactants derived from benzoic acid, the group contribution of the /CH2 / is additive both in the plateau ( /1.6 kJ mol 1 per /CH2 /) and in the micellization enthalpy changes ( /1.5 kJ mol 1 per group). The values are comparable with those obtained in the case of cationic surfactants and for similar anionic compounds in the same experimental conditions. The behavior of the compounds under study was compared with that of the potassium salts of 4-((alkylcarbonyl)amino)-benzoic acids, in order to understand the role played by the amido group bridging the benzoic moiety and the alkyl chain. Thus, we see that the amido group behaves as a part of the polar head, strongly increasing the hydrophilicity of the molecule.
[1] E. Fisicaro, E. Pelizzetti, M. Barbieri, P. Savarino, G. Viscardi, Thermochim. Acta 162 (1990) 277. [2] Y. Moroi, Y. Sakamoto, J. Phys. Chem. 92 (1988) 5189. [3] E. Pelizzetti, E. Pramauro, E. Barni, P. Savarino, M. Corti, V. Degiorgio, Ber. Busenges. Phys. Chem. 86 (1982) 529. [4] E. Fisicaro, C. Compari, G. Viscardi, P.L. Quagliotto, in preparation. [5] E. Fisicaro, M. Barbieri, E. Pelizzetti, P. Savarino, J. Chem. Soc., Faraday Trans. 87 (1991) 2983. [6] (a) E. Fisicaro, A. Ghiozzi, E. Pelizzetti, G. Viscardi, P.L. Quagliotto, J. Coll. Interf. Sci. 182 (1996) 549; (b) E. Fisicaro, A. Ghiozzi, E. Pelizzetti, G. Viscardi, P.L. Quagliotto, J. Coll. Interf. Sci. 184 (1996) 147. [7] J.E. Desnoyers, G. Perron, in: R. Zana (Ed.), Surfactant Solutions-New Methods of Investigation (and references therein), Marcel Dekker, New York, 1987, p. 1. [8] E. Fisicaro, Cell. Mol. Biology Lett. 2 (1) (1997) 45. [9] E. Fisicaro, C. Compari, B. Rozycka-Roszak, G. Viscardi, P.L. Quagliotto, Curr. Top. Coll. Interf. Sci. 2 (1997) 53. [10] E. Fisicaro, E. Pelizzetti, M. Barbieri, P. Savarino, G. Viscardi, Thermochim. Acta 168 (1990) 143. [11] K. Shinoda, T. Nakagawa, B. Tamamushi, T. Isemura, Colloidal Surfactants, Academic Press, New York, NY, 1963, pp. 37 /42.