J.C.S. F ~ ~ u Y I. 1980, 76, 101-106.Partial Molar Volumes of Organic Compounds in WaterPart 7.-Sodium and Hydrochloride Salts of a,o-Aminocaboxylic AcidsBY FEREIDOQN SHAHIDITDepartment of Chemistry, University of Toronto,Toronto, Ontario M5S lA1, CanadaandDepartment of Chemistry, McGill University,Montreal, Quebec H3A 2K6, CanadaReceived 13th February, 1979The partial molar volumes of hydrochloride and sodium salts of a number of a,w-aminocarboxylicacids in water at 25°C have been measured and related to their van der Waals volumes. Volumes ofproton ionization have been determined and the electrostriction of the solvent calculated. It isconcluded that 5.85 k 0.45 water molecules hydrate an amino acid, of which 3.85 rt 0.35 molecules arebound to the dipolar ends of the molecule.In a previous paper in this series we examined the influence of the charges uponthe partial moalr volumes, P, of a variety of amino acids and showed that the ap-parent electrostriction, E, could be expressed as the sum of the true electrostriction,V", and hydrophobic hydration effects, VH.The apparent electrostriction forsufficiently long chain a,w-aminocarboxylic acids (n > 6, where n is the number ofintervening methylene groups) was essentially constant at = 16.2 cm3 mol-'. Thiswas obtained by the conventional method of comparing 5" values for a,w-amino-carboxylic acids with those for their unchanged isomers, hydroxyamides. Fromexamination of the packing density values, ref. (3), Vw/ 5", for a,w-amino acids and acomparison with those of n-alkylamine hydrochlorides and the sodium salts of n-alkane carboxylic acids, it was suggested that the solvent molecules along the hydro-carbon section of the a,w-amino acids are highly ordered and hence have largerVw/ Vo values.In this paper we report on the measurement of the partial molar volumes ofhydrochlorides and sodium salts of a,w-aminocarboxylic acids up to and includingthose of 11-aminoundecanoic acid in water at 25 "C.This serves as an alternativeroute for calculating electrostriction, as well as providing a means of evaluating thevolume changes upon proton ionization, A V;, for a,w-aminocarboxylic acids.EXPERIMENTALMATERIALSThe source of the amino acids and their purification have been described previously.*Solutions of the corresponding hydrochlorides and sodium salts were prepared immediatelyprior to use by titration with stoichiometric amounts of HCl or NaOH solution in doublydistilled, deionized water.A serial dilution method was used to obtain solutions at variousconcentrations in order to minimize experimental error.t Address correspondence to the author at the University of Toronto.10102 PARTIAL MOLAR VOLUMES OF AMINO ACIDSAPPARATUS A N D MEASUREMENTSAll measurements were carried out at 25.00+O.0l0C using a Paar ditigal density meter,(model DMA 02C), as described el~ewhere.~ Between 8 and 16 measurements were madewithin the concentration range 0.2-3.0 % for each compound. The observed apparentmolar volumes, @JS, were calculated from eqn (1)where the parameters have their usual meaning^.^TREATMENT OF DATABecause hydrochlorides of a,w-aminocarboxylic acids dissociate [process (a)] andtheir sodium salts hydrolyse [srocess (b)] in aqueous solution, corrections to theq5GbS values were applied (vide infra)+C1-H3N-(CHJn--COOH(A.MCl) s H3N-(CNJnCOO-(A) + HC1 (4H20 + H2N--(CH2),-C0O-Na(NaA) e H3N-(CH2),-C00-(A) +NaOH. (b)The procedure followed to correct 4v for process (a) is similar to the one used forhydrochlorides of a,cll-diamines. For process (b) corrected 4v values are obtainedfromwhere a is the degree of hydrolysis of the a,a-aminocarboxylic acid sodium salt, usingliterature pK v a l ~ e s . ~Total molar concentrations, Coy were deduced from weight fractions and densitydata. The molar concentration of each species in solution, C = Co(l -a), wasobtained from these data.The corrections required for hydrochloride salts were allsmall and always < 1.0 cm3 mol-l ; however, the corrections required for hydrolysisof sodium salts were significant, (up to 20 cm3 mol-l).Partial molar volumes, V O y were then obtained from a least squares fit of theresults to eqn (2), the Redlich-Meyer equationwhere Sv, the Debye-Hiickel limiting slope is 1.868 cm3 dm3i2 m ~ l - ~ ' ~ for mono-valent electrolytes in water at 25 "C? Plots of & against JC did not show anyrapid increase in @v over the concentration range studied, indicating that no m i d -lization occurred in either series.+ f$v(NaA) = (GbS(NaA) - a[&(A) + VO(Na0H) - VO(H20)])/(1 - a)&-&JC= P+b& (2)RESULTS AND DISCUSSIONExperimental partial molar volumes for hydrochlorides and sodium salts of a,w-aminocarboxylic acids, together with bv values of eqn (2) are reported in table 1.Also shown in this table are calculated values, based on eqn (3) developed by Tera-sawa et aL9where Vw is the van der Waals volumelo and a and b are empirical constants depend-ing on the type of compound.As can be seen from the data presented, the bv valuesfor the hydrochlorides axe generally small, as found for other series of mine hydro-ch1orides.l. 8 s 11* l2 The corresponding values for sodium salts are all negative andV'= aVw+b (3F. SHAHIDI 103significantly larger. These values vary approximately linearly with chain length foreach series.ELECTROSTRICTIONFrom the calculated values based on eqn (3) for a,o-aminocarboxylic acids atdifferent pH conditions, using a method described elsewhere,l average values of14.5 f 1.7 and 4.5 $0.6 cm3 mol-1 have been obtained for the apparent electrostric-tion, E, for an NH3 or a Coy group, respectively.These compare with a value ofw 16.2 cm3 mol-l obtained for the total electrostriction of dipolar end groups for+TABLE ~.-PARTTAL MOLAR VOLUMES, v", AND bv VALUES FOR ~,CO-A~BCARE+OXYLIC ACIDHYDROCHLORIDES C1-H3N--(CH&--COOH AND ~,CO-AMI"CARBOXYLIC ACID SODIUMSALTS H2N--(CH2)-COO-Na+ IN WATER AT 25°C+~~~VO/~rn~rn01-~ n expt. ca1c.a bv/cm3 dm3 mo1-2a,w-a.minocarboyxlic acid hydrochlorides1234567891067.22+ 0.05b83.02$0.0599.76$ 0.06116.16+0.04133.23 $ 0.12C149.38 & 0.06165.46$ 0.23181.35k0.12197.69 & 0.5021 3.65 & 0.4767.2283.5699.89116.23132.57148.92165.80181.60197.93214.27a,o-aminocarboxylic acid sodium salts1234567891042.81 $ 0.1 5d58.78 f0.1674.29k0.1690.32+0.21106.44+_0.40e121.93 -F.0.18137.97+ 0.48154.07$ 0.38170.02$-0.28185.95k0.4042.6958.6174.5290.43106.35122.26138.16154.10169.98185.90+0.72+0.17 + 0.41 & 0.14+ 0.19+ 0.19 + 0.50+ 1.93- 0.61 k 0.47- O.89$0.70- l.18tl.01-2.24& 1.53- 4.94+ 3.09- 5.09+6.OO-0.1 1 k0.37- 1.2050.52- 2.32+ 0.42- 5.58 + 0.70- 5.63+ 1.57- 6.61 & 0.88-9.73k4.80- 16.89+ 4.90- 17.01 4 5.30- 17.04k4.21~ ~~a Calculated using eqn (3) with a = 1.596, b = -23.27 cm3 mol-l for hydrochlorides and a =1.554, b = -10.68 cm3 mol-1 for sodium salts.The chloride or sodium ion has not been includedin these calculations of a and b values. b Ref. (25) reports 67.0 cm3 mol-I and ref. (26) reports 68.3cm3 mol-'. C Ref. (25) reports 135.4 cm3 mol-I. d Ref. (25) reports 43.4 cm3 mol-l and ref. (26)reports 44.4 cm3 mol-'. e Ref. (25) reports 106.0 cm3 mo1-l.a,co-aminocarboxylic acids by comparing their vaiues with those of their cor-responding isomers, hydroxyamides.2 Using previously established values for theinfluence of the charges upon hydrophobic hydration of the aliphatic portion of thesemolecules,2 values of l l .O + 1.7 and 2.0k0.6 cm3 mol-l were obtained for the tru104 PARTIAL MOLAR VOLUMES OF AMINO ACIDS+electrostriction, V", for NE.' and COT groups, respectively. These results were usedto calculate the number of water molecules, N", hydrating an amino acid, using therelationship E Y 3.0 N e developed by Millero et aZ.l4# l5 These workers, using ap-propriate data for naturally occurring amino acids, obtained a value of 4.4 k 0.3 for N".However, naturally occurring a-amino acids suffer from incomplete hydration due tothe overlap of the hydration speheres about their charged end groups and to thepresence of alkyl side chains.2* 16-1 The data for longer chain a,o-aminocarboxylicacids and their corresponding sodium and hydrochloride salts indicate that an averageof 5.8550.45 water molecules hydrate an amino acid, of which 3.85k0.35 moleculesare bound to the dipolar end groups of these molecules.It has been suggested that water molecules align themselves along the hydro-carbon portion of a,w-a.minocarboxylic acids.20 Plots of Vw/P against Vw3 fora,@-aminocarboxylic acids, sodium n-alkane carboxylates and n-alkylamine hydro-chlorides showed that a,m-amino acids had considerably larger Vw/ v" values, whichTABLE 2.-THERMODYNAMIC PARAMETERS OF PROTON IONIZATION FOR THE SERIES + +OF H3N-(CH2)n-COOH AND H3N-(CH2)n-C00- IN WATER AT 25°C an AP/cm3 mol-i AS'lcal mol-1 deg-l pK+HBN--(CH~)~-COOH1 - 5.92+ 0.34'2 - 6.72+ 0.44'3 - 8.46k0.354 - 10.86+0.335 - 10.8450.51e6 -11.5850.257 - 11.56k0.528 - 12.2550.519 - 12.95k0.7910 - 13.25k0.86H3N-(CH2),-COO-f1 0.52k0.44-f2 1.4950.543 2.00k0.454 3.23+0.535 2.95 0.796 3.1450.377 3.08 20.278 3.98k0.779 3.93k0.5710 4.563-0.79-7.1- 13.4- 17.7-20.1-23.1- 24.1 - 23.6-23.5 - 23.7- 24.6- 10.76- 9.75 - 7.86- 2.87- 3.52 - 1.93- 1.9- 0.5 - 0.43-22.343.554.044.234.374.454.524.544.584.639.8410.3610.6210.8510.9410.9710.971 1.0211.0211.070 Values of pK and ASo aretaken from ref.( 5 ) and quantities refer to processes HA+ + A+H+or A + A- + H+. b Values of for a, o-aminocarboxylic acids are taken from ref. (2) and P(Na+) =-6.61 cm3 mo1-1 [ref. (711, P(HC1) = 17.80&0.09 cm3 mol-l [ref.(l)] and p(H+) = -5.4 cm3mol-l [ref. (7), (2711. C Ref. (3) reports - 6.8 cm3 mol-l, ref. (7), (27) report - 5.9 cm3 mol-l andref. (28) reports -6.4 cm cm3 mol-l. Ref. (28) reports - 13.2 cm3 mol-l. fRef. (28) reports - 1.1 cm3 mol-'. g Ref. (28) reports - 1.6 cm3 mol-I.Ref. (28) reports -7.5 cm3 mol-lF. SHAHIDI 105was taken in support of alignment of water molecules along hydrocarbon middlesections of the a,u-amino acids.2 An examination of such plots for a,o-amino-carboxylic acids and their corresponding sodium and hydrochloride salts indicatesthat while these salts have limiting values of 0.69 & 0.01 for Vw/ P, amino acids them-selves have Vw/ Vo values levelling off at 0.73. This trend lends support to our pre-vious findings.VOLUMES OF PROTON IONIZATIONThe AV" values for the first and second steps in proton ionization of a,amt-n.ino-carboxylic acids calculated from the V o values of species involved are assembled intable 2 using a conventional method.'.21 Corresponding pK values along with ASodata for each step are also given in the table. From the data presented, values ofA Vo and AS" for the first step in the ionization of a,o-aminocarboxylic acids are allnegative and varying in the same direction, levelling off at a chain length of n > 5.The A P and AS" values for the second step, although they increase in the samedirection, have opposite signs and tend to level off at n 3 4. Furthermore, AS' andA v o limiting values for both steps of proton ionization parallel those of the corre-ponding n-alkylamine hydrochlorides and sodium n-alkane carboxylates. Thisobservation, in turn, may suggest that the electrostatic interation between a chargedamino or carboxyl group and the solvent is independent of the other polar substitu-ents present in the molecule, provided that they do not overlap with the hydrationshell around each charged group.This idea is further supported by a comparison of theexperimental partial molar volumes, v", and the calculated V* values using eqn (4)developed by Cabani et aL21V*p-(CH2),-YJ = T[X-(CH2),-H] + P[H-(CHJ,-Y] -P[H-(CH,),-H] (4)where X and Y denote the hydrophilic centres as usual.1*21 Using available literaturedata for n-alkane carboxylic acids22 n-alkylamines2 and their corresponding~ a l t ~ , ~ ~ - ~ ~ ~ ~ ~ Po values for ethane and propane24 and an increment of 15.9cm3 mol-l for the methylene group contribution for higher homologues, limitingvalues of 2 cm3 mol-l are obtained for V* - 7" in the series of apaminocarboxylicacids and their corresponding series of sodium or hydrochloride salts. This constantlimiting value implies that the slight residual interaction between hydrophilic centresmay not be electrostatic.T thank Prof.J. T. Edward and P. G. Farrell for laboratory accommodation atMcGill University, where this work was started. Also the suggestions of Dr. G. E.Langford and Prof. J. T. Edward are appreciated.Part 6, F. Shahidi and P. G. Farrell, J. SoZution Chern., 1978,7, 549.F. Shahidi and P. G.Farrell, J.C.S. Paraday I, 1978,74, 858.E. J. King, J. Phys. Chem., 1969, 73, 1220.J. T. Edward, P. G. Farrell and F. Shahidi, J.C.S. Furaday I, 1977, 73, 705.J. T. Edward, P. G. Farrell, J. L. Job and B-L Poh, Canad. J. Chem., 1978,56,1122 ; J. J. Christ-ensen, J. L. Oscarson and R. M. Izatt, J. Amer. Chem. SOC., 1968,90,5949.0. Redlich and D. M. Meyer, Chern. Rev., 1964,64,221.J. E. Desnoyers and M. Arel, Canad. J. Chem., 1967,45,359.S . Terasawa, H. Itsuki and S. Arakawa, J. Phys. Chem., 1975,79,2345.' F. J. Millero, Chem. Rev., 1971, 71, 147.l o J. T. Edward, J, Chem. 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