No abstract

Inorganic Chemistrytriazole)copper(II) ( J / k = -17.9 to -19.O"K) than in ligand atoms of dichloro(l,2,4-triazole)copper(II).13 copper(I1) benzoate trihydrate ( J / k = -12.7'K), deThus, although normal magnetic moments have been spite the fact that the Cu-Cu distance of 3.40 A in the observed for these compounds a t room temperature, former is longer than 3.15 A in the latter. Therefore, the spins are not free from one another, but antiferrothe superexchange interaction rather than the intermagnetic interaction exists between neighboring spins action of direct nature is predominant. This conchin a chain. sion is supported by the existence of spin densities in the (16) M . Inoue and &I, x7torg. them., 5, 70 (1066).The repulsion between the lone electron pairs of the nitrogen atoms in the staggered hydrazine molecule influences the length of the nitrogen-nitrogen bond; diminution of the repulsion causes shortening of the bond in those derivatives of hydrazine where the lone pair is attracted to bonds. the shifts of v(h--N) are in agreement with this point of view. The band shifts from 880 toward 1000 cm-' when the repulsion between the lone pairs diminishes. I n metal complexes of hydrazine, shifts of the band are also observed, depending on the field effect of the metal cations on the lone pairs, even if the nitrogen-nitrogen bond distance remains unaltered.The infrared spectra of several hydrazine derivatives have been examined:The hydrazine molecule H2N-NH2 is composed of two tetrahedra in staggered configuration, 1, one corner in each being occupied by a lone pair. The nitrogennitrogen boiid distance is N-N = 1.46 A3 in solid hydrazine but it becomes shorter in the cation +H3N-

Abstract

The critical micelle concentration (c.m.c.) for four cationic surfactants, alkyl-trimethyl-ammonium bromides, was determined as a function of temperature by conductivity measurements. The values of the standard free energy of micellisation DeltaG degrees(mic) at different temperatures were calculated by using a pseudo-phase transition model. Then, from the diagram (-DeltaG degrees(mic)/T)=f(1/T), the thermodynamic functions DeltaH(app) and DeltaS(app) were calculated. From the plots DeltaH(app)=f(T) and DeltaS(app) = f(ln T) the slopes DeltaC(p) = n(w(H))C(p,w) and DeltaC(p)=n(w(S))C(p,w) were calculated, with the numbers n(w(H)) and n(w(S)) negative and equal and therefore defined simply as n(w). The number n(w)<0, indicating condensed water molecules, depends on the reduction of cavity that takes place as a consequence of the coalescence of the cavities previously surrounding the separate aliphatic or aromatic moieties. The analysis, based on a molecular model consisting of three forms of water, namely W(I), W(II), and W(III), respectively, was extended to several other types of surfactants for which c.m.c. data had been published by other authors. The results of this analysis form a coherent scheme consistent with the proposed molecular model. The enthalpy for all the types of surfactant is described by DeltaH(app)= -3.6 + 23.1xi(w)-xi(w)C(p,w)T and the entropy by DeltaS(app)= +10.2+428xi(w)-xi(w)C(p,w) ln T where xi(w)= |n(w)| represents the number of molecules W(III) involved in the reaction. The term Deltah(w)= +23.1 kJ mol(-1) xi(w)(-1) indicates an unfavourable endothermic contribution to enthalpy for reduction of the cavity whereas the term Deltas(w)= +428 J K(-1) mol(-1) xi(w)(-1) represents a positive entropy contribution for reduction of the cavity, what is the driving force of hydrophobic association. The processes of non polar gas dissolution in water and of micelle formation were found to be strictly related: they are, however, exactly the opposite of one another. In micelle formation no intermolecular electronic short bond is formed. We propose, therefore, to substitute the term "hydrophobic bond" with that of "hydrophobic association".

Molecular interpretations are here presented of the hydrophobic effect, which is the cause of the low solubility of apolar substances in water. The solubilization process of substances such as the noble gases consists in the formation of a cavity in the solvent with expulsion of n w water molecules. The process is associated to an entropy/enthalpy (S/H) compensation linearly dependent upon the temperature. The observed enthalpy DH app , either determined calorimetrically or by van't Hoff equation, shows DC p,app a 0 and positive. We set DC p,app ¼ n w C p,w where C p,w is the isobaric heat capacity of water. The number n w (n w 4 0) of relaxed water molecules is proportional to the size of the solute molecule and hence of the cavity. The term n w C p,w T is actually an entropy term, which compensates for part of the reaction enthalpy (DH 0 o 0). The entropy change at 298 K linearly depends on n w , thus showing that cavity formation is associated to a negative entropy change (Ds cav ¼ -23.2 J K À1 mol À1 n w À1). Beyond a temperature T min typical of each compound, the reaction becomes endothermic. The highly negative entropy change DS min (at T min we have DH app ¼ 0) is related to the loss of kinetic energy by the solute molecule when trapped in the cage. Another example of S/H compensation occurs in the formation of micelles. The resultant cage volume after formation of the micelle is smaller than the sum of the cavities previously hosting the single separated apolar moieties. Therefore, some floating water molecules need to be reintroduced into the structure of the solvent (n w o 0) to fill the void. The contraction of the cavity is associated to a positive entropy change (Ds fill ¼ 22.4 J K À1 mol À1 |n w | À1 ). Protein folding and protein-substrate association behave in a way similar to micellisation (n w o 0). The present interpretation of the complexation reactions of proteins, and also of micellisation, leads to a new formulation of the so-called 'hydrophobic bond': the positive entropy change for cavity contraction is the main driving force of hydrophobic bonding. In the denaturation process as opposite to folding, the denaturation enthalpy DH den at different temperatures T den depends on positive numbers (n w 4 0) of water molecules. The presence of polar groups and/or charges in the solute molecule, on the other hand, exerts on the water molecules the same action as that produced by micellisation (n w o 0).