Thermodynamics of micelle formation in water, hydrophobic processes and surfactant self-assemblies.

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".

Evolution of patients with clinical neurocardiogenic (vasovagal) syncope not subjected to specific treatment.

The purpose of this prospective study was to analyze the natural history of patients with clinical neurocardiogenic syncope. The results obtained with some therapeutic strategies in the prevention of neurocardiogenic syncope are encouraging. However, the benefit they represent when compared with the natural history of syncope has not been clearly established. Fifty-six consecutive patients with either (1) one or more episodes of syncope and a positive head-up tilt test or (2) typical history of vasovagal syncope despite a negative tilt test were monitored during a mean period of 16.1 +/- 10.2 months. Patients were discharged without any specific medication. There were no deaths during the follow-up period. Recurrences were found in 5 (8.9%) patients. The risk of recurrence was 7% after 1 year and 15% after 21 months. The prognosis of patients with clinical neurocardiogenic syncope is excellent as far as survival is concerned. Recurrences of episodes without specific treatment were uncommon after diagnosis had taken place. This finding strengthens the need to evaluate the effect of drugs with placebo control groups and to select the highest-risk group for treatment.