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There is considerable confusion on experimental and thermodynamic aspects of the formation of ionized surfactant micelles. Recent claims that the van't Hoff and calorimetric enthalpies of micellization of ionized surfactants represent different aspects of micellization are shown to be incorrect. New conductance and surface tension measurements on n-dodecyltrimethylammonium bromide (DTAB) in water and in cosolution with 0.1 M KBr over a range of temperatures are used to provide a set of critical micelle concentrations (cmc's). The molar enthalpies of micellization derived from these cmc's are compared with calorimetric enthalpies of micellization obtained from a critical survey of the literature and shown to be equivalent. This signifies that both enthalpies capture the contributions from all aspects of micellization of charged micelles. The apparent discrepancies arise from experimental problems, incorrect use of the van't Hoff equation, and failure to recognize the constraints of the Phase Rule and electroneutrality. Deficiencies in the application of the two major models of micellization, namely, the "mass-action" model and the "phase" model, are exposed. Neither model is generally valid. The van't Hoff enthalpies of micellization of DTAB in water are similar in the presence and absence of 0.1 M KBr, confirming a similar correspondence found with calorimetric enthalpies for DTAB in water and NaBr.

The lateral intermolecular forces between phospholipids are of particular relevance to the behavior of biomembranes, and have been approached via studies of monolayer isotherms at aqueous interfaces, mostly restricted to air/water (A/W) systems. For thermodynamic properties, the oil/water (O/W) interface has major advantages but is experimentally more difficult and less studied. A comprehensive reanalysis of the available thermodynamic data on spread monolayers of phosphatidyl cholines (PC) and phosphatidyl ethanolamines (PE) at O/W interfaces is conducted to identify the secure key features that will underpin further development of molecular models. Relevant recourse is made to isotherms of single-chain molecules and of mixed monolayers to identify the contributions of chain-chain interactions and interionic forces. The emphasis is on the properties of the phase transitions for a range of oil phases. Apparent published discrepancies in thermodynamic properties are resolved and substantial agreement emerges on the main features of these phospholipid monolayer systems. In compression to low areas, the forces between the zwitterions of like phospholipids are repulsive. The molecular model for phospholipid headgroup interactions developed by Stigter et al. accounts well for the virial coefficients in expanded phospholipid O/W monolayers. Inclusion of the changes in configuration and orientation of the zwitterion headgroups on compression, which are indicated by the surface potentials in the phase transition region, and inclusion of the energy of chain demixing from the oil phase will be required for molecular modeling of the phase transitions.

The combined use of small-angle X-ray scattering (SAXS) and photon correlation spectroscopy (PCS) to characterise adsorbed layers of β-casein at the solid/liquid interface is reported. The protein was adsorbed to polystyrene latex particles at room temperature, low ionic strength and neutral pH and adsorption densities assessed by a solution-depletion technique which showed a plateau in the adsorption. Results from the SAXS experiments were analysed to provide electron-density profiles. These were backed up with results from PCS which provided hydrodynamic thicknesses over the range of the adsorption isotherm. This information, together with calculated hydrophobicity and charge profiles for the protein, yielded a molecular model for the adsorbed layer. Although β-casein in solution has a largely randon coil conformation, it appears to adopt a much more compact form when it is adsorbed on polystyrene latex. Most of the protein lies close to the surface, leaving part of the chain extended into the aqueous phase. The most likely candidate for the extended chain is part of the highly charged sequence of 40 or so amino acids at the N terminus of the protein. The hydrodynamic thickness of the protein layers increases with adsorbed concentration of protein. The thicknesses reached are substantially greater than those predicted by theories of self-avoiding walks of the extended chain with volume exclusion interactions included and it is suggested that long-range electrostatic repulsive forces are involved.

Following a short historical survey of the field of insoluble lipid monolayers, the relevance of such surface studies to membrane structure and behavior is briefly discussed. Some of the main findings on phospholipid monolayers at both air—water and oil—water interfaces are then described with particular emphasis on the two-dimensional phase transitions found at high monolayer densities. Finally new surface pressure and potential data for phosphatidyl ethanolamine monolayers spread over a wide range of areas at the oil—water interface are presented and discussed in molecular terms.

Contact angles have been measured on smooth polyethylene, polytetrafluoroethylene, nylon, and polyester plates as a function of the surface pressure of insoluble monolayers at an air/salt solution interface. In all cases, surface tensions that gave zero contact angle at the solid were much lower than critical surface tensions established previously for the same solids using surfactant solutions or pure liquids. An analysis of the results shows transfer of monolayer to the more hydrophobic solid surfaces under conditions depending on the nature of the monolayer.

Insoluble monolayers of phosphatidylcholines and ethanolamines show first-order phase transitions at hydrocarbon-water interfaces, depending on the temperature and chain length. In mixed monolayers of two phosphatidylcholines of different chain lengths, or of phosphatidylcholines and ethanolamines of the same chain length, demixing occurs at the phase transition.

Numerical calculations of the pressure and electrical contribution to the Helmholtz interaction free energy for two uniformly charged sheets interacting across 1 : 1 electrolyte solution are presented. The model differs from that considered by most other workers in that the electrolyte is allowed to penetrate the charged surfaces. Further, in addition to the cases of constant charge and constant potential, the case where the surfaces contain reversibly ionised groups is considered. In order that the results may have some relevance to cell–membrane interactions, all the numerical calculations are done using parameters which represent approximately the conditions in physiological saline solution. The results for constant potential are identical to those obtained for impenetrable interacting charged sheets. The results for constant charge differ in an essential way. Specifically, the pressure in the limit of zero separation tends to a constant value whereas for impermeable surfaces it is, for small separations, inversely proportional to the square of the separation and thus diverges as the separation approaches zero. Consequently, the difference between constant potential and constant charge for this model is not as marked as differences obtained by other workers who studied impermeable surfaces. As one might anticipate, the results for reversibly ionised surfaces lie between the two extremes of constant charge and constant potential. Present calculations differ from those of Gingell in that the complete Poisson–Boltzmann equation is solved numerically.

Using compression techniques, surface pressure (II) against area (A) isotherms are measured as a function of temperature for insoluble monolayers of 1,2-distearoyl-sn-glycero-3-phosphorylcholine spread at the n-heptane/aqueous sodium chloride interface. Problems of spreading are resolved and accurate results at moderate and high II are presented. The results show an almost first order phase change which is very temperature sensitive. Various forms of the Clapeyron analysis are applied to the data to calculate the heats of the phase change and to assess variations arising from the choice of a close-packed area. These heats vary with temperature but not with salt concentration or pH over the range studied. Results on 1,2-dioleoyl-sn-glycero-3-phosphorylcholine show no such phase changes.

Surface pressure against area isotherms for dilute monolayers of a homologous series of 1,2-diacyl-lecithins spread at the oil/water interface are presented as a function of temperature and salt concentration. Comparative results for di-C14-cephalin monolayers are used to distinguish head-group effects. Over the whole range of areas studied the isotherms are independent of chain length and salt concentration but vary with head-group, with lecithins giving the higher pressures. From conventional Amagat-type plots the monolayers are non-ideal. Apparent co-areas for both lecithins and cephalins are large with the lecithins showing a marked positive temperature coefficient. Negative entropies of compression calculated from the lecithin data are larger than those for single chain ionised molecules spread at the oil/water interface. These phospholipids show little evidence of cluster formation at areas higher than the phase transitions reported in part 1, but undergo complex configurational changes on compression which depend on medium range dipolar interactions and steric interaction of the chains.

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Isotherms of surface pressure (π) against area per molecule (A) are reported for a homologous series of pure synthetic saturated 1,2-di-acyl glycerophosphocholines (lecithins)(C14 to C22) spread at n-heptane/aqueous electrolyte interfaces. The lecithins show second-order phase transitions which for a given temperature move to higher π and lower A as the chain length is decreased until for di-C12 lecithin no phase transition can be distinguished.The calculations of Clapeyron heats in Part 1 are extended to the new data using a single value of the reference area for the solid state for all temperatures and chain lengths. These heats vary with chain length and decrease linearly with temperature. The high heat capacity calculated in Part 1 for di-C18 lecithin at low temperatures is shown to be an artefact. Entropies of compression have been calculated from the free energies obtained by integrating the full isotherms between a reference area in the expanded region above any phase transition and another area in the solid-condensed region. These entropies vary linearly with chain length, having a slope given by the configurational entropy term R ln 3 per methylene group, where R is the gas constant. The entropy change for each phospholipid is approximately 2(n–1)R ln 3, where n is the chain length. This finding, taken together with a comparison of the heats of the monolayer phase transitions with the calorimetric heats of melting of phospholipid chains, and with related published data on the heats of fusion and the heats of solution of the alkanes, suggests that the chains of the phospholipids are fully flexible at low monolayer densities and very restricted on the condensed side of the phase transitions.

Surface pressure against area isotherms for phospholipid monolayers at the oil/water interface display second-order phase transitions which resemble the ones between “condensed” and “expanded” monolayers at air/water interfaces. Behaviour at lower surface densities indicates a strong repulsive interaction between the phospholipid molecules at the oil/water interface. A statistical mechanical model is developed in which the monolayer molecules occupy sites on a two-dimensional lattice and adopt one of two possible orientational states. By appropriate choice of the interaction energies of the molecules in these states and the use of order–disorder statistics the features characteristic of a second-order phase transition are obtained for isotherms at both interfaces. From the point of view of our present theory what is termed the “expanded” state is in fact a fluid phase whose degree of order is lower than that of the “condensed” state. The choice of a large overall attractive interaction energy between nearest-neighbours yields the liquid–vapour phase transition which has been observed in experiments at the air/water interface whereas with an overall repulsive interaction the liquid–vapour transition is absent, as in the oil/water case.

Accurate surface potentials measured by the radioactive electrode method are given for monolayers of sodium octadecyl sulphate spread on several aqueous sodium chloride solutions. In the temperature range 5–18°C, the magnitude of the surface potential increases with temperature for all salt solutions. This is interpreted as showing that increasing temperature disorients water dipoles more at the clean air/water interface than at the film-covered interface for long-chain sulphate over the temperature range studied. It is not possible from the data to determine uniquely the orientation of water molecules at the clean air/water interface, but it is almost certain that they are arranged with the oxygen atoms pointing towards the air phase.The general breakdown of the Gouy—Chapman model of the ionic double layer is confirmed at all temperatures. The Stern model as modified by van Voorst Vader and van den Tempel cannot explain the data. The classical Stern adsorption isotherm covers the results, giving Stern adsorption energies for the sodium ion which are negative (except possibly at very high charge densities) and vary significantly with salt concentration but do not vary with temperature and monolayer density in the area range 300 to 1200 A2 molecule–1.