D. Vollhardt

Texture features of long-chain fatty acid monolayers at high pH of the aqueous subphase.

Abstract The thermodynamic properties and the condensed phase textures of Langmuir monolayers of protonated and deprotonated long-chain fatty acids (stearic acid, arachidic acid, and behenic acid) are studied. Surface pressure–area ( π – A ) isotherms and Brewster angle microscopy (BAM) images are recorded in the regions of low and high pH values. In the range of pH 11–13, the π – A isotherms of these long-chain fatty acid monolayers show a plateau region indicating a two-phase coexistence region. Under these conditions, segmented domains with highly ordered textures are formed in the stearic acid arachidic acid monolayers, whereas the domains of behenic acid monolayers are irregularly shaped without an inner texture. The best developed domains of stearic acid and arachidic acid are circularly shaped and subdivided into six almost identical segments which do not meet exactly in the centre. The long-range orientational order of the condensed phase domains is discussed. Addition of neutral salt (NaCl) to the subphase improves the stability of the long-chain fatty acid monolayers at high pH values by a salting-out effect. A time-dependent pH drift due to atmospheric CO 2 and desorption of monolayer material are characterised for realising optimal monolayer stability.

Penetration - a comparison of various models

Abstract Equilibrium penetration has been studied for the sodium dodecyl sulphate—octadecanol system. The surface composition has been calculated in terms of the monolayer density and the surfactant concentration on the basis of three different penetration models. The results have been compared and discussed. The model with least approximation does not consider the area covered by the monolayer molecules. By introducing a geometrical factor the equation obtained has a wide range of validity, thus allowing good estimation of the surface composition.

Line tension and domain shapes in insoluble monolayers of fatty alcohols

Abstract The main phase transition from the fluid state of low density to the condensed state is studied in tetradecanol monolayers using Brewster angle microscopy. The condensed phase patterns obtained on a water subphase and on sodium dodecyl sulphate (SDS) solutions are compared. The addition of SDS to the bulk phase lowers the line tension and changes the circular shape of the equilibrium condensed phase domains evolved on the water subphase. As a consequence, non-equilibrium fingering patterns are formed during compression on aqueous SDS solutions. The patterns relax slowly to equilibrium structures to minimize the total free energy.

Ordering in Langmuir monolayers of branched chain phospholipids

Abstract The ordering of branched chain glycerophosphoethanolamines (PE) with different side chain length ( n =1, 2, 5 and 14) is systematically studied and compared with that of the corresponding double chain dipalmitoyl-phosphatidylethanolamine (DPPE) monolayers. The PEs have a 2-branched (methyl, ethyl, pentyl and tetradecyl) hexadecanoyl chain at the sn -1 position of the glycerol backbone and an unbranched hexadecyl residue at the sn -2 position. Thermodynamic data obtained by π – A isotherm measurements at different temperatures are coupled with two-dimensional (2D) texture and 2D structure information obtained by Brewster angle microscopy (BAM) and X-ray diffraction at grazing incidence (GIXD), respectively. With increasing side chain length up to n =5, the ordering of the long alkyl chains is increasingly disturbed so that n -16PE monolayers are highly disordered and cannot form a condensed phase. Already the ethyl side chain reduces the ordering of 2-16PE monolayer so strongly that not only the polar tilt of the alkyl chains increases, but also in a stretched lattice a varying tilt azimuth of the alkyl chains is indicated. A side chain elongation nearly to the length of the two main chains enhances the ordering to that of a triple chain phospholipid with the alkyl chains oriented perpendicularly in a hexagonal lattice. Texture and shape of the dendritic or fractal-like condensed phase domains are similar to that known from DPPE monolayers. However, the individual differences in the inner textures found for the different side chain lengths, corroborate the GIXD results. At short side chains, domain regions of different tilt direction occur, whereas at long side chain the domains show no differences in the inner texture.

Temperature-dependent domain growth in 9-hydroxypalmitic acid monolayers

The surface pressure—molecular area isotherms and the texture features of 9-hydroxyhexa-decanoic acid monolayers are studied using the film-balance technique and Brewster-angle microscopy. In the two-phase coexistence region, homogeneously reflecting condensed-phase domains of a regular shape are formed. The 9-hydroxy-hexadecanoic acid monolayers show an unusual low, but linear, increase in the transition pressure with temperature and striking shape changes of the condensed-phase domains at different temperatures. The features of the monolayer are drastically changed by alkyl chain substitution by an OH group at the 9-position. The morphological features indicate molecular packing of nontilted alkyl chains in an orthorhombic and hexagonal lattice, respectively. OH substitution at different positions of the alkyl chain affects strongly and in a different way both the phase behaviour and the domain texture of the nonsubstituted palmitic acid monolayers.

Force measuring methods for determination of surface tension of liquids: A comparison

Abstract Three methods for the determination of the surface tension of liquids based on force measurements namely, the vertical plate method of Wilhelmy, the frame method of Lenard and the ring method of du Noüy are compared and studied in respect of a common principle of correction. It is shown that these three most important force-based methods allow the determination of the surface tension under static conditions. The force components of the corresponding liquid column below the measuring wire obtained for the straight part of the withdrawal curve up to the transition in its curved part provides exact surface tension values. The experimentally accessible value of the force component describes the physical background of the measured value correction contrary to the approximate equations obtained by mathematical way. Usually the determination of surface tension of liquids is based merely at the vertical plate method on exact equations thermodynamically derived whereas in the case of the frame and ring methods correction factors in approximate equations are used. At usual application of the force-based methods under the non-static condition of the withdrawal of a liquid column, the force maximum measured at withdrawal of the measuring object (plate, frame, or ring) is the basis for the determination of surface tension. In these cases, the measured surface tension values are compensated by correction equations for the frame and ring methods which are based on an correction factor and correction tables empirically obtained. The surface tension values obtained in this usual way agree with those obtained by using the force component of the corresponding liquid column below the measuring wire for the straight part of the withdrawal curve up to the transition in its curved part. Problems arising at the force measurements with increasing thickness of the measuring wires inside and outside the rings are discussed.

Long-range orientational order of branched chain phospholipid monolayers

Two-dimensional phase behaviour and ordering of branched chain glycerophosphoethanolamines (n-16PEs) having a 2-branched hexadecanoyl chain (the side chains are methyl, ethyl, pentyl and tetradecyl) at the sn-1 position of the glycerol backbone and an unbranched hex-adecyl residue at the sn-2 position, are systematically studied. The ordering of the alkyl chains is increasingly disturbed by short side chains of increasing length so that at n = 5 a condensed monolayer phase cannot be formed. Further side chain elongation enhanced again the ordering with the consequence that a tetradecyl side chain leads to texture and thermodynamic properties of the monolayer which correspond to those of a triple-chain phospholipid. Texture and shape of the dendritic or fractal-like condensed phase domains are similar to that known from double-chain phosphatidyl-amine monolayers. However, in agreement with the ordering, individual differences in the inner textures are observed at different side chain lengths.

Phase behavior and domain structure of 9-hydroxyhexadecanoic acid monolayers

The temperature effect on the surface pressure (π)-molecular area (A) isotherms and the texture of the condensed phase domains of 9-hydroxyhexadecanoic acid monolayers are studied. The features of the monolayer are drastically changed by alkyl chain substitution by an OH group in the 9 position. The 9-hydroxyhexadecanoic acid monolayers show unusual temperature behaviour of the n-A isotherms and striking shape changes of the condensed phase domains at different temperatures. The morphological features indicate molecular packing of non-tilted alkyl chains in an orthorhombic and hexagonal lattice, respectively.