Molecular Packing Parameter Research Articles

Estimation of the surface area of a detergent molecule at the interphase boundary of mixed lipid-detergent aggregates

A new approach was proposed for estimating the surface area of a detergent molecule, ad, at the interphase boundary of mixed lipid-detergent aggregates. The approach is based on consideration of the relationship between geometrical packing parameters of lipid and detergent molecules and the composition of mixed aggregates. The structural transitions between various aggregates are described in terms of the molecular packing parameter, δ, generalized for the case of a two-component lipid-detergent system (Goltsov, A.N. and Barsukov, L.I., J. Biol. Phys., 2000, vol. 26, pp. 27–41). The effective packing parameter, δcr, was shown to be a function of the detergent/lipid ratio required for complete solubilization of mixed aggregates, Resol, the interfacial surface area of a detergent molecule, ad, and some other molecular packing parameters dependent on the chemical structure of detergent and lipid. The ad values calculated using the proposed approach are in a fairly good agreement with the experimental data available in literature. The approach ensures effective evaluation of the dependence of ad on the medium composition, temperature, and the chemical structure of detergent in a broad range of temperatures and detergent/lipid ratios. The conditions of saturation of the lipid bilayer with detergent at the beginning of solubilization were analyzed. The relationship between Resol and the lipid/detergent ratio required for the saturation of lipid bilayer with detergent, Resat, was found. A method was developed for Resat estimation using the Resol values available in the literature. The results of the present study can be used in the theoretical analysis of mixed lipid-detergent and protein-detergent aggregates.

Correspondence between Curvature, Packing Parameter, and Hydrophilic−Lipophilic Deviation Scales around the Phase-Inversion Temperature

We show in this paper that three ways of characterizing "spontaneous" lateral packing of amphiphiles are equivalent: the spontaneous curvature, the molecular packing parameter, and the refined hydrophilic-lipophilic balance known as HLD (hydrophilic-lipophilic deviation). Recognition of this equivalence, with its underlying hypothesis of incompressible fluid with lowest surface energy, reinforces the single parameter bending energy expression implicit in the classical papers by Ninham and Israelachvili, as well as all the predictive models of solubilization developed as yet.

A crystallographic review of arsenic sulfides: effects of chemical variations and changes induced by exposure to light

Crystal data for natural and synthetic arsenic sulfides are reported and discussed. Most of them [α- and β-dimorphite, realgar, β-As4S4 phase, pararealgar, Kutoglu’s As4S4(II) phase, alacranite, uzonite, orthorhombic As4S5 phase] have a crystal structure consisting of a packing of cage-like, covalently bonded As4Sn (n = 3, 4 and 5) molecules held together by weak interactions of van der Waals character. Their structures are compared in terms of molecular packing and molecular parameters. The layered structural arrangement of orpiment, As2S3, is described and the effects of the incorporation of Se replacing for S is discussed. The structures of wakabayashilite and getchel lite, which contain mixed (As, Sb) coordination polyhedra, are also described to outline the geometric effects of the Sb → As substitution. The results of recent studies dealing with the effects of the exposure of realgar or other arsenic sulfides to visible light are reported and discussed. Their interest in the study of arsenical pigments and their preservation in artwork is outlined with some examples of application.

Molecular Packing Parameter in Bolaamphiphile Solutions: Adjustment of Aggregate Morphology by Modifying the Solution Conditions

The geometrical rule of molecular packing parameters in a bolaamphiphile solution was tested with experimental results. By modifying the solution conditions to change the molecular packing parameters, the morphology of the aggregate was successfully manipulated in a single-chain bolaamphiphile, disodium phenyl-1,4-bis (oxyhexanoate) (i.e., C6PhC6Na2 solution). Micelle-vesicle-tube transformation was observed by changing the pH and the addition of NaBr or octanol. In the mixed systems of oppositely charged bola/surfactants, the molecular packing parameter's role is related to the mixing ratio.

Bulk Modulus of Lubricating Oils as Predominant Factor Affecting Tractional Behavior in High-Pressure Elastohydrodynamic Contacts

The phase diagrams corresponding to transition from liquid to viscoelastic solid and that from viscoelastic solid to elastic-plastic solid of Santotrac100 (SN100), mineral oil, synthetic naphthenic oil, polybutene, and tetradecane were first made up by high-pressure density measurements and others. The bulk modulus of lubricating oils under a quasi-static condition was evaluated using a phase diagram. The results indicated that the bulk modulus of lubricating oils is closely related to the oil molecular packing parameter T VE −T (where T VE is the viscoelastic solid transition temperature at pressure p, and T is the oil temperature). The constant values of the bulk modulus in the elastic-plastic range are different depending on the molecular structures of the oils. It has also been shown that SN100, mineral oil, synthetic naphthenic oil, and polybutene converted to amorphous solids at high pressures and tetradecane converted to molecular crystal. Next, the elastohydrodynamic lubrication tractions were measured by a ball-on-disk machine. The results indicated that the maximum traction coefficient is closely related to T VE −T. As a result, the importance of the bulk modulus as a predominant factor for traction characteristics of lubricating oil was pointed out.

505 高圧下で固化した潤滑油膜による摩耗防止作用(OS 摩擦・磨耗材料(2))

At high pressures or at low temperatures lubricating oils cause the solidification First, phase diagrams of paraffinic mineral oils (PN), traction oils (SN), polybutene (PB), DOS and BBPH were drawn in pressure-temperature plots of three different states (liquid, viscoelastic solid and elastic-plastic solid). Next, the four-ball tests were used to evaluate anti-wear characteristics of lubricating oils in boundary lubrication regime. The SNs showed the minimum wear scar in four-ball wear test The anti-wear characteristics of oils was closely related with EHL film thickness and the oil molecular packing parameter T_<VE>-T (T_<VE> : viscoelastic solid transition temperature at pressure p of wear test condition, T: oil temperature).

Molecular Packing Parameter and Surfactant Self-Assembly: The Neglected Role of the Surfactant Tail

The concept of molecular packing parameter is widely invoked in the literature to explain, rationalize and even predict molecular self-assembly in surfactant solutions. The molecular packing parameter is defined as vo/aelo, where vo is the surfactant tail volume, lo is the tail length, and ae is the equilibrium area per molecule at the aggregate surface. A particular value of the molecular packing parameter can be translated via simple geometrical relations into specific shape and size of the equilibrium aggregate. This is the predictive application of the concept of molecular packing parameter, as discussed in the literature. In calculating the packing parameter, the dominant notion in the literature is that the surfactant headgroup determines the surface area per molecule of equilibrium aggregates ae. It follows that, given a headgroup, the molecular packing parameter vo/aelo is fixed, because the volume-to-length ratio (vo/lo) of the tail is a constant independent of the tail length for common surfacta...

Silicone Antifoam Performance: Correlation with Spreading and Surfactant Monolayer Packing

The relationship between the spreading of antifoam oils and their performance is much discussed in the literature, but a demonstrated connection between antifoam spreading and performance has been lacking. This paper reports the performance of a poly(dimethylsiloxane) (PDMS)-based antifoam on foam produced by 12 surfactant solutions. These include single or mixed surfactant systems, including impure surfactant mixtures to model fabric washing detergents. The oil film spreading pressure, πo/w, is presented as a simple and relevant measurement of the thermodynamics of antifoam oil spreading. Antifoaming efficacy was measured as the relative reduction in the initial foam height, ΔHrel, using cylinder shake tests at a fixed antifoam dosage. ΔHrel is shown to increase with πo/w, demonstrating a strong statistical correlation between antifoam oil spreading and its performance. Antifoam effectiveness varies with surfactant concentration, surfactant type, and surfactant hydrophobe size and also with increased density of surfactant packing. Surface shear viscosity, μs, was used to quantify surfactant packing. Antifoam effectiveness decreases with increasing surface shear viscosity. This finding provides a potentially useful link between antifoam efficacy and surfactant selection based on well-established surfactant molecular packing parameters. The role of spreading of antifoam oil at the air/surfactant solution interface is investigated. Oil film spreading pressure is shown to decrease by a power law function with increasing surface shear viscosity of the surfactant film. A new fluorescence technique was used to measure the extent of PDMS spreading. Initial results suggest a correlation between the spreading distance and antifoaming performance. An antifoam mechanism is proposed that features antifoam spreading as a direct contributor to bubble film rupture and incorporates surfactant type and concentration, surfactant packing density, and antifoam oil film spreading pressure as factors contributing to antifoam efficacy.

Molecular Theory of Bending Elasticity and Branching of Cylindrical Micelles

Two structural−thermodynamic characteristics of cylindrical, wormlike micelles in dilute solution are studied using a molecular-level model: (a) the bending elasticity of the micelles and (b) their tendency to form intermicellar junctions (branches). The internal (free) energy of the micelles, before and after a bending deformation and junction formation, are calculated using mean field theories for the free energies of the molecules constituting these structures. The molecular free energies, which depend on the local packing geometries, include the contributions of head group repulsion forces, the hydrocarbon−water interfacial energy, and the chain conformational free energy. We find that when only the head group and surface contributions to the packing energy are taken into account, the one-dimensional bending constant of the micelles is negligibly small. When the chain contribution is included, and when reasonable molecular packing parameters are used, we find that the persistence length, which is proportional to the bending rigidity, is typically a few tens of nanometers. The free energy change associated with the formation of a trijoint intermicellar junction upon the “fusion” of one micellar end cap with the cylindrical body of another micelle is found to be small but positive; about 10 kBT at room temperature. This conclusion does not refute the possibility that intermicellar junctions are metastable transients or that their formation may be favored entropically, due either to conformational degeneracy or excluded volume interactions between micelles. Our conclusions apply to aqueous solutions containing one, single-tail, surfactant species.

DL-Norleucine, β Form at 120K

This paper forms part VI in the series of the crystal structures of hydrophobic amino acids. A low-temperature form (120 K) of dl-norleucine (2-aminohexanoic acid, C6H13NO2), has been solved and refined in the space group C2/c. Molecular packing and cell parameters correlate very well with those proposed for the β-form of dl-norleucine as deduced by Mathieson [Acta Cryst. (1953), 6, 399–403]. Estimated standard deviations betweeen heavy atoms are 0.001–0.002 Å.

Intrinsic curvature in normal and inverted lipid structures and in membranes

The intrinsic or spontaneous radius of curvature, R(o), of lipid monolayer assemblies is expressed in terms of a lipid molecular packing parameter, V/AI, for various geometries. It is shown that the equivalent lipid length, 1, in inverted hexagonal (HII) phases, defined by a cylindrical shell of equal total lipid volume, yields an expression for R o identical to that for inverted cylindrical micelles (or, equivalently, HII phases in the presence of excess hydrocarbon). This identity is used to obtain values of the effective packing parameter for various phosphatidylethanolamines. The temperature dependence of the intrinsic radius of curvature is predicted to be negative and to be considerably greater than that for the lipid length in nearly all cases. The thermal expansion coefficient is not constant but is found to vary, depending on the value of the lipid packing parameter. A possible addition rule is constructed for the intrinsic radius of curvature of lipid mixtures, based on the linear additivity of the effective molecular volumes, V, and molecular areas, A. This relation is found to hold for mixtures of dioleoyl phosphatidylcholine (DOPC) with dioleoyl phosphatidylethanolamine, and a value of R(o) of > or = 9 A (V/AI = 1.08) is obtained for DOPC. The energetics of the intrinsic curvature and lamellar-nonlamellar transitions are also discussed within the framework of the model.

Monte Carlo and mean-field studies of phase evolution in concentrated surfactant solutions

A two-dimensional lattice model, originally introduced by Granek et al. [J. Chem. Phys. 101, 4331 (1994)], is used to demonstrate the intricate coupling between the intramicellar interactions that determine the optimal aggregation geometry of surfactant molecules in dilute solution, and the intermicellar interactions that govern the phase behavior at higher concentrations. Three very different scenarios of self-assembly and phase evolution are analyzed in detail, based on Monte Carlo studies and theoretical interpretations involving mean-field, Landau–Ginzburg, Bethe–Peierls, and virial expansion schemes. The basic particles in the model are ‘‘unit micelles’’ which, due to spontaneous self-assembly or because of excluded area interactions, can fuse to form larger aggregates. These aggregates are envisaged as flat micelles composed of a bilayerlike body surrounded by a curved semitoroidal rim. The system’s Hamiltonian involves one- through four-body potentials between the unit micelles, which account for their tendency to form aggregates of different shapes, e.g., elongated vs disklike micelles. Equivalently, the configurational energy of the system is a sum of micellar self-energies involving the packing free energies of the constituent molecules in the bilayer body and in rim segments of different local curvature. The rim energy is a sum of a line tension term and a 1D curvature energy which depends on the rim spontaneous curvature and bending rigidity. Different combinations of these molecular parameters imply different optimal packing geometries and hence different self-assembly and phase behaviors. The emphasis in this paper is on systems of ‘‘curvature loving’’ amphiphiles which, in our model, are characterized by negative line tension. The three systems studied are: (i) A dilute solution of stable disklike micelles which, upon increasing the concentration, undergoes a first-order phase transition to a continuous bilayer with isolated hole defects. An intermediate modulated ‘‘checkerboard’’ phase appears under certain conditions at low temperatures. (ii) A system of unit micelles which in dilute solution tend to associate into linear micelles. These micelles are rodlike at low temperatures, becoming increasingly more flexible as the temperature increases. Upon increasing the concentration the micelles grow and undergo (in 2D) a continuous transition into nematic and ‘‘stripe’’ phases of long rods. At still higher concentrations the micellar stripes fuse into continuous sheets with line defects. (iii) A system in which, already in dilute solution, the micelles favor the formation of branched aggregates, analogous to the branched cylindrical micelles recently observed in certain surfactant solutions. As the concentration increases the micelles associate into networks (‘‘gels’’) composed of a mesh of linear micelles linked by ‘‘T-like’’ intermicellar junctions. The network may span the entire system or phase separate and coexist with a dilute micellar phase, depending on the details of the molecular packing parameters.

A search for a relation between aggregate morphology and the structure of 1,4-dialkylpyridinium halide surfactants

A detailed study of the relation between the aggregate morphology and the molecular shape of 27 1-alkyl-4-(or 2-)alkyl(or n-alkoxycarbonyl)pyridinium halide surfactants is described. This shape can be expressed in a molecular packing parameter (P) as suggested by Israelachvili. For unbranched 1-methyl-4-n-alkylpyridinium iodides, the packing parameter does not depend on the length of the alkyl chain (n(c)). Spherical micelles formed from these surfactants grow into rodlike micelles. The critical rod concentration (crc) is, however, dependent on n(c). This originates from the dependence of the aggregation number of spherical micelles on n(c), in agreement with predictions based on the ladder model. Alkyl chain branching in 1-methyl-4-(C12-alkyl)pyridinium iodides affects the shape of the surfactant. Branching near the headgroup decreases P and dramatically lessens the propensity of the spherical micelle to grow. Branching near the chain end increases P and decreases the crc. Highly branched surfactant monomers associate into bilayers, which can be transformed into vesicles. The overall results indicate that the morphology of the aggregate is mainly dependent on the shape of the surfactant. The possibility for backfolding of the 1-alkyl chain of 1-alkyl-4-n-dodecylpyridinium iodide surfactants directly affects the preferred morphology of the aggregate. Bilayers are formed if backfolding occurs; otherwise spherical micelles are formed, which, on increasing surfactant concentration, grow into rodlike micelles. Interestingly, 1-methyl-4-(n-alkoxycarbonyl)pyridinium iodides associate into bilayers, independent of the length of the alkyl chain (n(c) = 10-16). This aggregation behavior probably stems from a special kind of interdigitation, which changes the geometrical constraints for packing into a bilayer.

The interaction of cholesterol and cholest-4-en-3-one with dipalmitoylphosphatidylcholine. Comparison based on the use of three fluorophores

This study compares cholesterol-phospholipid and cholest-4-en-3-one-phospholipid interactions by their effect on thermotropic behavior of dipalmitoylphosphatidylcholine bilayers. This was approached by determining the temperature-dependent steady-state fluorescence anisotropy of three fluorophores; diphenylhexatriene (DPH), hydroxycoumarin (HC) and trans-parinaric acid (TPA). The fluorophores monitor different lateral and vertical locations of the lipid bilayers; DPH and HC average laterally the properties of the hydrophobic and headgroup regions of the bilayer, respectively, while TPA distribution is determined by the lateral organization of the bilayer. The data show that the two steroids have similar qualitative but different quantitative effects. Both diminish the pretransition and behave as ‘averagers’, broadening the main gel to liquid crystalline phase transition through ordering of the acyl chains in the liquid crystalline state and disordering of them in the gel state. However, the mechanisms by which the two molecules operate are different. Cholesterol is more effective particularly on the hydrophobic region of the bilayer, and its effect is not linear with its mole fraction. A sharp increase of the steady-state fluorescence anisotropy occurs around 20 mol% cholesterol. The effect of cholestenone is proportional to its mole fraction. The difference between the effects of the two steroids is explained by the dissimilarity in their lateral distribution. Cholesterol forms cholesterol-rich domains. The size of the boundary regions which surround the cholesterol-rich domains changes drastically at about 20 mol% cholesterol. Cholestenone, on the other hand, is randomly distributed in the bilayer plane and therefore it does not cause the formation of such defined boundary regions. This study as well as reports by others suggests that the important structural differences between the two steroids are the molecular packing parameter and the presence of small polar group at the 3β position of the steroid.

Structure of β-poly- l-alanine: Refined atomic co-ordinates for an anti-parallel beta-pleated sheet

A new model for the structure of β-poly- l-alanine is proposed. It consists of interleaved anti-parallel pleated sheets with neighbouring sheets randomly displaced ± 1 2 a ( a is the interchain distance). Optimum values of the molecular packing and conformational parameters for this statistical structure have been determined by the linked-atom least-squares method for polymer crystalline fibres (Arnott & Wonacott, 1966 a). The crystal model is supported by the infrared data and yields calculated X-ray structure amplitudes in substantially better agreement with the observed diffration than any previous model. The refined values of the standard protein conformation angles, φ, ψ and ω are 41.4 °, −45.3 ° and 1.5 °. The interchain hydrogen-bond length, which was not a fixed feature in the refinement, is 2.83 Å.

Atomic co-ordinates for an α-helix: Refinement of the crystal structure of α-poly- l-alanine

X-Ray diffraction patterns from α-poly- l-alanine have been re-measured. Intensity data, of greater accuracy and resolution than previously used, have been included in an automatic process for refining polymer crystal structures. Optimum values of the molecular packing and conformational parameters have been determined. The refinement places on a quantitative basis the crystal structure proposals of Elliott & Malcolm (1959). Our improvements in the crystal and atomic parameters are of the same kind but different in magnitude from their suggestions. Predictions of conformation and packing from simple energy considerations are strongly supported by these experimental results. The recently adopted standard protein conformation angles φ, ψ, ω were not explicit variables in this refinement but could be calculated from the refined molecular parameters to be respectively 113 ° 7′, 136 ° 19′, −1 ° 0′.