Temperature dependent ionic structure of phospholipid monolayers.

Radiotracer studies of calcium adsorption to dipalmitoylphosphatidyl-alkanolamine monolayers measured at various temperatures showed that the binding constant of calcium increased with temperature up to around 30 degrees C but the decreased on exceeding this critical temperature. The temperature dependent ionic structure of ampholytic phospholipid monolayers are discussed.

Effects of composition and molecular packing on ionic properties of lipid monolayer having both cationic and anionic head groups.

The effects of composition and molecular packing on the overall ionic property of mixed monolayer involving cationic, anionic and zwitterionic lipids were studied by measuring surface potential change when the concentration of sodium or calcium ions in the aqueous substrate was varied. Those lipids used were N,N-dimethyl-N-n-hexadecyl-n-octadecyl ammonium chloride (DDAC) as cationic lipid, stearic acid (StH) or sodium docosylsulfate (SDocS) as anionic lipid, and 1,2-dipalmitoyl-sn-glycero-3-phosphorylethanolamine (PE) and 1,2-distearoyl-sn-glycero-3-phosphorylcholine (PC) as ampholytic lipids. For the equimolar mixture of StH and DDAC, the reversal of apparent charge is observed when molecular packing exceeds 3.25 X 10(-10) M/cm2. The effect is rendered to the discreteness of cationic and anionic charges in the monolayer. It was also found that the addition of 30% of PC drastically changes the ionic property of PE closer to that of PC.

Time Dependent Anchoring of Adsorbed Cationic Surfactant Molecules at Mica/Solution Interface.

The nature of adsorbed cationic amphiphiles at the mica/solution interface was studied by XPS and contact angle measurements. The elemental analyses of freshly cleaved mica surfaces by XPS showed that the potassium atoms on the surface lattice of mica are not necessarily distributed equally to each surface on cleavage. The adsorbed cationic amphiphile molecules remaining on mica surfaces after rinsing with distilled water were found to be anchored to the surface by ion-exchange, replacing surface potassium and/or other cations. The ratio of adsorbed cationic amphiphile molecules with single alkyl chains to the maximum potassium ions on mica surface was estimated to be twice as large as that of amphiphiles having two alkyl chains. The contact angle of water drops placed on the adsorbed surface showed a gradual decrease with the elapse of time due to the dissolution of adsorbed surfactant into the water drop; however, the decrease was not observed for those mica surfaces when aged for more than 3 days in the adsorption bath. The anchoring of adsorbed molecules by ion-exchange was found to occur extremely slowly, however; the anchored molecules may not easily be desorbed when rinsed with deionized water. The time dependent anchoring of adsorbed molecules was studied in terms of adsorption time, alkyl chain length, and concentration of cationic surfactant. Copyright 1999 Academic Press.

Hydrophilicity of Polar and Apolar Domains of Amphiphiles.

The hydrophilicity of polar and apolar domains of various amphiphiles was systematically estimated for their homologues and analogues by measuring the molar adiabatic compressibility of an aqueous solution at infinite dilution. The homologues of protic alkyl H(CH(2))(n)-, perfluoroalkyl F(CF(2))(n)-, and alkylphenyl H(CH(2))(n)(C(6)H(5))- groups (n=0-10) were chosen to represent apolar hydrophobic domains. The polar hydrophilic domains tested were -SO(4)Na, -SO(3)Na, -COONH(4), -N(CH(3))(3)Br, N(C(m)H(2m+1))(4)Br (m=1-5), and -NH(CH(2))(n)SO(3) (n=3, 4) groups. Also tested were the tetraphenyl ionic compounds (C(6)H(5))(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 polyethylene glycols H(OCH(2)CH(2))(m)OH (m=1-4) to study the role of apolar -CH(2)- units in the hydrophilic oxyethylene group, and the zwitterionic dimethylaminoalkylsulfonate (CH(3))(2)NH(CH(2))(n)SO(3) 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 of the sound velocity and density of solutions. The introduction of laboratory automation and the numerical control of the system improved the accuracies and efficiencies of the measurements a great deal. The range of the temperature scan was 0-40 degrees C with an effective accuracy of +/-0.001 degrees C and the concentration was automatically scanned down to far below the cmc of the surfactant. The hydrophilicity of various polar and apolar substances was estimated as the decrease of molar adiabatic compressibility of the aqueous solution with increased concentration of their homologues and analogues. 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 maximum at 0 degrees C. The cationic charge of quaternary ammonium N(+)(C(n)H(2n+1))(4) was found to enhance the hydrophobic hydration of methylene groups located at a distance of 4 to 6 Å from the core nitrogen atom, while the terminal negative charge of the anionic surfactant R-SO(4)(-), R-SO(3)(-), or R-COO(-) was found to decrease the hydrophobic hydration of -CH(2)- units within the same range. The hydrophilicity of quaternary ammonium and the tetraphenyl ions should be synergistically given by both hydrophobic and ionic hydrations. The hydrophilicity of the perfluoromethylene unit -CF(2)- was found to have a value comparable to that of the protic methylene unit -CH(2)-. The hydrophobic hydration seems to offer a good measure of the hydrophilicity of apolar substances; however, it does not necessarily represent the "hydrophobicity" of the apolar segment when the "surface activity" of the amphiphile is concerned. Copyright 2000 Academic Press.