A spatial gradient in the concentration of binary protein-water systems is deliberately created by slow evaporation of a solution of solute. Such samples are scanned by optical interferometry to record the refractive index profile and by near-infrared microspectroscopy to determine the water composition along the spatial gradient. Combination of the two data sets allows the change in refractive index with concentration and its increment dn/dc to be determined beyond the usual report limited to low protein-solvent volume fractions. Further, phase behavior of protein solutions can be studied. To illustrate the method, cytochrome c is chosen; clean, bubble-free samples of extremely concentrated (ca. 80%) metastable solutions can be produced in this way, and in time, protein crystals separate from these solutions.

The aqueous-phase behavior of C12E2 was examined using the diffusive interfacial transport-near infrared method (DIT-NIR), a new isothermal swelling method fdr assembling accurate and precise phase diagrams. The system exhibits a large number of liquid-crystal phases over a remarkably small temperature and composition window including a sponge phase, two bicontinuous cubic phases, and a lamellar phase. The phase behavior of the system is similar to Class II polar lipids, such as monoglycerides, excluding the liquid-liquid miscibility gap and the sponge phase associated with ethoxylated surfactants. The data collected by the DIT-NIR method provide a marked improvement to the currently accepted phase diagram. Finally, temperature steps can create temperature-induced cubic-phase-containing emulsions in which pyramidal-shaped L1 droplets are dispersed within the V-2((1)) bicontinuous cubic-phase continuum, reflecting the epitaxy of the cubic-phase lattice.

The Diffusive Interfacial Transport (DIT) method for performing phase studies (in which all the phases that exist along an isotherm are produced by isothermal swelling and their compositions determined by using refractive index data) was introduced in 1987.(1) While this method represented a significant qualitative advance in phase studies methodology, its quantitative accuracy has since been found to be unsatisfactory. We describe herein the DIT-NIR method in which execution of the study is the same as before, but peak-area data on the bend-stretch combination band of water are used to determine composition. Peak-area data are obtained by using Fourier transform near-infrared (FT-NIR) microspectroscopy. Calibration studies with octyldimethylphosphine oxide-water mixtures demonstrated that peak areas vary linearly with composition over the entire composition range. The slopes of calibration lines follow a power-law dependence upon temperature. Phase studies of the C12E3-water system, using both DIT-NIR and isoplethal methods, validate the applicability of the DIT-NIR method to systems other than phosphine oxides. These data suggest that DIT-NIR may represent a new general method for the complete and accurate quantitative determination of binary aqueous surfactant phase diagrams. Further, the spectroscopic data can provide new information on the physical state of the material under study.

Phase transformations which accompany changes of state in binary systems can be expressed using a reaction equation format, which greatly facilitates analysis of the event. Some phase reactions occur simply, while others (those in which disproportionation occurs) display “reaction mechanisms” which are analogous to those long known in molecular chemistry. In surfactant systems the peritectic thermal decomposition of crystal hydrates invariably leads initially to metastable lamellar liquid crystal phases, from which the equilibrium products arise. Similar mechanisms likely apply to nonsurfactant systems, except that the intermediate state is a liquid phase. Phase reaction products may be determined by either thermodynamic or kinetic factors. © 1997 Elsevier Science Ltd.

Molecular aggregates of surfactant molecules consisting of one or more bilayers arranged in a hollow, closed, usually spherical geometry are termed ''vesicles'' or ''liposomes''. Often vesicles are formed by the application of intense mechanical shear to coarse dispersions of water-insoluble surfactants, in which case the thermodynamic state of the mixture evidently consists of the aqueous liquid plus a coexisting lamellar phase.In recent years it has been found that in certain systems the vesicular structure forms spontaneously and is long-lived, and it has been suggested that these structures may in fact constitute the equilibrium state in these cases (as is true of micelles). It is here argued that vesicular mixtures are invariably biphasic, and that vesicular structure is to be regarded as a dimension of colloidal structure rather than of phase structure.If phase separation occurs when a cationic and an anionic surfactant are mixed in water, two new components result and diagnosis of the state of the mixture demands use of the rules governing five-component mixtures. By imposing the restriction that the surfactant in limiting quantity reacts irreversibly to form a catanionic surfactant salt, however, such mixtures may be rigorously treated as four-component systems. If the byproduct non-surfactant salt is removed, a manageable ternary system (catanionic surfactant, ionic surfactant, and water) results. By taking this approach the thermodynamic state of these mixtures may be analyzed more simply and accurately, and a useful view of the physical science of vesicular mixtures results. (C) 1997 Published by Elsevier Science B.V.

Acid-soap crystals of stoichiometry NaH(2)A(3) (A = the RCO(2)(-) moiety of the C-12 through C-18 fatty acids) have been isolated in a pure state by crystallization from aqueous solutions and characterized by X-ray diffraction. The sodium palmitate-palmitic acid acid-soap NaH2P3 was characterized, in addition, using FT-IR, calorimetry, and solid-state C-13 NMR spectroscopy. The chains in the acid-soap crystals are tilted at 32.7 degrees relative to the normal of the lamellar sheets and display a perpendicular subcell packing. Both NMR and infrared data indicate that two distinct carbonyl carbons exist. These differ from the carbonyl carbons in pure fatty acid and soap crystals, although one is still carboxylic acid-like in structure, while the other is carboxylate ion-like in structure. These acid-soap crystals are anhydrous and do not form crystal hydrates. NaH2P3 crystals are modified above 70.0 degrees C to form a polymorph showing a distinctly different X-ray diffraction pattern. These new data require revision of the McBain sodium palmitate-palmitic acid phase diagram.

In earlier work a detailed examination has been made of the structure of micelles formed from the ultralong chain zwitterionic surfactant (eicosyldimethylammonio)hexanoate. In the present work the behavior of micelles has been investigated in D2O solutions of electrolytes. The electrolytes used included sodium chloride, potassium bromide, potassium iodide, and sodium sulfate. As the electrolyte concentration increased, the micellar surface potential increased, leading to strong electrostatic interactions between the micelles. The latter depended on the anion used and indicated that binding to the micellar surface occurred in the order I- > Br- > Cl-. However, sulfate ions appeared to have little affinity for the micelle surface. Electrostatic interaction also increased with decrease of pH until at a pH below ca. 3.5 precipitation of the cationic surfactant salt occurred.

A technique for studying the phase behavior of aqueous surfactant systems using near-infrared (NIR) microspectroscopy has been developed. All the phases which exist along a particular isotherm are formed by creating an interface between water and surfactant within a long (55 mm) 25-micrometers pathlength fused silica cell, and allowing the components to diffuse together. The temperature of the cell is controlled to within 0.1 degree(s)C between 25 and 85 degree(s)C. Analysis of the entire composition range is achieved using a Fourier transform infrared (FT- IR) spectrometer equipped with a CaF2 beam splitter, a tungsten-halogen source, and an infrared microscope with a InSb detector. The intensity of the water bend-stretch combination band at 5175 cm-1 is shown to be useful for quantitation. A study of the octyldimethylphosphine oxide/water system illustrates the quantitative potential of this technique.

Heating dilute aqueous mixtures of the monohydrate (X.W) crystal of dioctadecyldimethylammonium chloride (DODMAC) to just below the Krafft discontinuity produces, without intervention of intense mechanical shear, a gel-like state having elaborate colloidal structure. This "gel" is probably formed by cleavage of the X.W crystal by bulk water between the planes of ions within this crystal. It was found, using cryoelectron microscopy, that many of the particles formed display cusps or are lens-shaped. Unexpectedly, these angular structures are far more prevalent in samples quenched from above the Krafft discontinuity temperature than they are in samples quenched from below this discontinuity. Additional shear encloses many such structures within others, thereby reducing the average particle size and viscosity. It is suggested that both the structure and the composition of the membranes in these colloids are related to the structure and composition of the coexisting bulk phase. If so, DODMAC membranes below the Krafft discontinuity are relatively thin and include only 2 mol (6%) of water, while membranes above the Krafft discontinuity are about three times thicker and include 74 mol (68.5%) of water. The thermodynamic definition of an interface is reviewed, in the context of this issue. The collapse of colloidal structure requires nucleation by small crystals and produces single crystals with minimal excess surface and curvature energy. Once formed, these crystals undergo secondary aggregation into large composites. Nucleation crystals are probably formed at the air/fluid interface. Isothermal swelling of the lamellar (D) liquid crystal phase by water also produces colloidally structured mixtures of liquid and lamellar liquid crystal phases, which display myelin textures. Refractive index data suggest that the fraction of the liquid crystal phase in these mixtures is approximately 0.25.

The foundation for surfactant phase science was established largely using isoplethal phase study methods, but some use has been made of qualitative isothermal penetration experiments. In 1987 a quantitative swelling method (the Diffusive Interfacial Transport method) was reported in which the analysis of phase compositions is based on refractive index data. Experience obtained to date during use of this DIT-NDX method is reviewed.These investigations have demonstrated the considerable value of swelling studies, but revealed serious flaws in the DIT-NDX method. Swelling studies are exceptionally efficient, reliable, and provide considerable information regarding the physical science of the system in addition to the phase diagram. Unfortunately, this method is incapable of accurately defining the compositions of many birefringent phases. A DIT-IR method, presently under development, should resolve this problem. Analysis of composition using the DIT-IR method will be based on infrared data obtained using a near-infrared microscope. In addition to providing better composition data, infrared studies are expected to provide information on conformational structure, crystal hydration, and hydration thermodynamics. The future of swelling methods is discussed.

A new technique for studying the phase behavior of aqueous surfactant systems using infrared (IR) microspectroscopy has been developed. All the phases which exist along a particular isotherm are formed by creating an interface between water and surfactant within a long (55 mm) 25-mu-m pathlength fused silica cell, and allowing the components to diffuse together. Analyses of the entire composition range can be achieved using near-IR spectra. The analyses can be based on either the hydrogen-stretching fundamentals or the overtone and combination bands, depending on the percentage of water present. IR data may in some cases also serve to identify the phases present. Exploratory studies of the decyldimethylphosphine oxide/water system have been performed which demonstrate the validity of this technique.

Zwitterionic functional groups possess the greatest polarity found within the nonionic class of hydrophilic groups. Their nomenclature, common misconceptions as to their classification and properties, and their ion exchange and acid-base reactions are discussed. The hydrophilicity of zwitterionic compounds is strongly influenced by the structure and basicity of the anionic substituent group, and to a lesser degree by the length of the ion bridge that binds together these oppositely charged substituent groups. Methylene groups within the ion bridge contribute to the lipophilicity of the molecules, but to a lesser degree than do methylene groups within the long chain.