José Miñones Trillo

Stability of dialkyldimethylammonium bromides monolayers spread at the water/air interface

Abstract The pressure/are ( π A ) isotherms of Langmuir monolayers of a series of the dialkyldimethylammonium bromides with alkyl chain lengths of 12, 14, 16 and 18 have been studied in terms of their stability at the water/air interface. Only dihexadecyldimethylammonium bromide (DHAB) and dioctadecyldimethylammonium bromide (DOAB) form stable Langmuir monolayers whereas the other compounds investigated desorb from the interface. The change of the area available for one molecule in the monolayer with time at different temperatures of the subphase (water and 0.1 M KBr aqueous solution) was investigated. The process of desorption was found to be controlled by the mechanism of diffusion. Applying the Ter Minassian-Saraga model [L. Ter Minassian-Saraga, J. Chim. Phys. 52 (1955) 181], the kinetics parameters corresponding to the desorption and diffusion, for DDAB and DTAB, were estimated.

Molecular interaction in mixed spread films at the water/air interface

Abstract The treatment of a two-dimensional system of three components of isomegetic molecules, adapted from statistical thermodynamics, is used to derive parameters which can be related to the interaction between molecules at the collapse pressure in mixed spread films at the water/air interface. The theory is illustrated by the mixed system of stearic acid and 1-octadecanol.

Interactions in Monolayers: A Study of the Behavior of Poly(methyl methacrylate)—Lysozyme Mixed Films from Surface Pressure—Area and Ellipsometric Measurements

Mixed monolayers of PMMA-lysozyme show the existence of negative deviations from the additivity of the molecular areas (A(m)) when the composition of polymer mixtures is less than X(PMMA) 0.6, regardless of the surface pressure of the monolayers. The maximum deviation occurs in the mixed monolayer with composition X(PMMA) 0.25, which is attributed to the formation of a complex consisting of one polymer molecule and three protein molecules (1:3 stoichiometry), stabilized by hydrogen bonds between the NH groups of the protein and the CO groups of the polymer as well as by van der Waals attractive forces between the hydrocarbon chains of both components. When the relative proportion of the components in the mixed films significantly differs from the value corresponding to the stoichiometry of the complex (as in mixtures with X(PMMA) > 0.6), this complex cannot be formed, causing an immiscible system where the values of the experimental molecular areas coincide with those corresponding to ideal behavior. Measurements of monolayer thickness and BAM images allow confirmation on the microscopic level of the structural characteristics deduced from the π-A isotherms.

How to obtain a well-spread monolayer of lysozyme at the air/water interfaces.

The purpose of this study is to define the conditions required to obtain a complete spreading of the lysozyme monolayer at the A/W interface. To this end, using Trurnits method, the influence of the ionic strength of the substrate, the elapsed time between the spreading of the monolayer and the beginning of its compression, and the number of lysozyme molecules spread at the interface was studied. The results obtained show that the lysozyme spreading is conditioned by the unfolding of amino acid chains which form part of its structure, so that such unfolding is hindered, either because of an excessive accumulation of lysozyme molecules on the substrate surface or because the waiting time necessary to get this unfolding is not long enough, regardless of the number of spread molecules. The complete unfolding of lysozyme, which involves the loss of tertiary and secondary structures, has been obtained in this work under certain conditions: (1) using a substrate with a high saline concentration (NaCl 3-3.5 M) and at a pH value close to the isoelectric point of lysozyme (pI=11) in order to reduce the solubility and the electrical potential of the protein, (2) increasing the elapsed time between the spreading of the protein and the beginning of its compression to 3h in order to get a complete unfolding of lysozyme, which is a very slow process, (3) using an initial spreading area of 2.1m(2)/mg or larger (equivalent to a protein surface concentration of less than 2×10(12) molec/cm(2)) to reduce the accumulation of protein molecules at the surface, thus facilitating their unfolding, and (4) using Trurnits spreading method, although for the spreading of this protein was found to be insufficiently significant.

Monolayer and Brewster Angle Microscopy Studies of Poly(methyl methacrylate)−Monopalmitin Mixed Systems at the Air−Water Interface

Mixed monolayers of poly(methyl metacrylate) (PMMA) and monopalmitin (Mp) were used for the study of their interactions. A thorough analysis of surface pressure (pi)-area (A) isotherms with the Langmuir monolayer technique, complemented with Brewster angle microscopy (BAM) images was performed. Mixed films show two phase transitions at a surface pressure of 14.5 mN/m and at 20-21 mN/m, respectively. Moreover, mixed monolayers show two well-defined collapses: one, corresponding to the lipid (at surface pressures of 50-51 mN/m) and the another one, ascribed to the PMMA, at surface pressure values of 57-58 mN/m. When the mean molecular areas of the mixed films (A(1,2)) were plotted versus film composition (X(1) or X(2)), positive deviations from the ideal behavior were observed at surface pressures below 15 mN/m, which were mainly attributed to a change in the conformation of the PMMA molecules at the surface. However, at higher surface pressures, the areas per monomer unit of the mixed monolayers obey the additivity rule, attributed to the fact that the film components form an immiscible system in these conditions.

Effect of ionization on the behavior of n-eicosanephosphonic acid monolayers at the air/water interface. Experimental determinations and molecular dynamics simulations.

Monolayers of n-eicosanephosphonic acid, EPA, were studied using a Langmuir balance and a Brewster angle microscope at different subphase pH values to change the charge of the polar headgroups (Zav) from 0 to -2. Molecular dynamics simulations (MDS) results for |Zav| = 0, 1, and 2 were compared with the experimental ones. EPA monolayers behave as mixtures of mutually miscible species (C20H41-PO3H2, C20H41-PO3H(-), and C20H41-PO3(2-), depending on the subphase pH). The order and compactness of the monolayers decrease when increasing |Zav|, while go from strongly interconnected by phosphonic-phosphonic hydrogen bonds (|Zav| = 0-0.03) through an equilibrium between the total cohesive energy and the electrostatic repulsion between the charged polar groups (0.03 < |Zav| < 1.6) to an entirely ionic monolayer (|Zav| ≈ 2). MDS reveal for |Zav| = 0 that the chains form spiralled nearly rounded structures induced by the hydrogen-bonded network. When |Zav| ≈ 1 fingering domains were identified. When Z ≈ 2, the headgroups are more disordered and distanced, not only in the xy plane but also in the z direction, forming a rough layer and responding to compression with a large plateau in the isotherm. The monolayers collapse behavior is consistent with the structures and domains founds in the different ionization states and their consequent in-plane rigidity: there is a transition from a solid-like response at low pH subphases to a fluid-like response at high pH subphases. The film area in the close-packed state increases relatively slow when the polar headgroups are able to form hydrogen bonds but increases to near twice that this value when |Zav| ≈ 2. Other nanoscopic properties of monolayers were also determined by MDS. The computational results confirm the experimental findings and offer a nanoscopic perspective on the structure and interactions in the phosphonate monolayers.