Hydrophilicity of polar and apolar domains of amphiphiles

The hydrophilicity of polar and apolar domains of various amphiphiles was s ystematically estimated for their homologues and analogues by measuring the molar adiabatic compressibility of an aqueous solution at infinite dilutio n. The homologues of protic alkyl H(CH2)(n)-, perfluoroalkyl F(CF2)(n)-, an d alkylphenyl H(CH2)(n)(C6H5)- groups (n = 0-10) were chosen to represent a polar hydrophobic domains. The polar hydrophilic domains tested were -SO4Na , -SO3Na, -COONH4, -N(CH3)(3)Br, N(CmH2m+1)(4)Br (m = 1-5), and -NH(CH2)(n) SO3 (n = 3, 4) groups. Also tested were the tetraphenyl ionic compounds (C6 H5)(4)MX (M=B/X=Na, M = P/X = Cl, M = As/X = Cl) to study the effect of the ionic sign of the core atom across the tetraphenyl apolar shell, the polye thylene glycols H(OCH2CH2)(m)OH (m = 1-4) to study the role of apolar -CH2- units in the hydrophilic oxyethylene group, and the zwitterionic dimethyla minoalkylsulfonate (CH3)(2)NH(CH2)(n)SO3 homologues to study the effect of intramolecular salt formation on the hydrophilicity of the zwitterion, The adiabatic compressibility of the solution was calculated from measurement o f the sound velocity and density of solutions. The introduction of laborato ry automation and the numerical control of the system improved the accuraci es and efficiencies of the measurements a great deal. The range of the temp erature scan was 0-40 degrees C with an effective accuracy of +/-0.001 degr ees C and the concentration was automatically scanned down to far below the cmc of the surfactant. The hydrophilicity of various polar and apolar subs tances was estimated as the decrease of molar adiabatic compressibility of the aqueous solution with increased concentration of their homologues and a nalogues. The hydrophobic hydration of nonpolar substances was found to be very small at room temperature and was barely detected above 40 degrees C; however, it became large as the temperature was lowered and attained a maxi mum at 0 degrees C. The cationic charge of quaternary ammonium N+(CnH2n+1)( 4) was found to enhance the hydrophobic hydration of methylene groups locat ed at a distance of 4 to 6 Angstrom from the core nitrogen atom, while the terminal negative charge of the anionic surfactant R-SO4-, R-SO3-, or R-COO - was found to decrease the hydrophobic hydration of -CH2- units within the same range. The hydrophilicity of quaternary ammonium and the tetraphenyl ions should be synergistically given by both hydrophobic and ionic hydratio ns. The hydrophilicity of the perfluoromethylene unit -CF2- was found to ha ve a value comparable to that of the protic methylene unit -CH2-, The hydro phobic hydration seems to offer a good measure of the hydrophilicity of apo lar substances; however, it does not necessarily represent the "hydrophobic ity" of the apolar segment when the "surface activity" of the amphiphile is concerned. (C) 2000 Academic Press.