Oriented Monolayer Assemblies

SFA has been traditionally used to measure the forces between modified mica surfaces. Before the JKR theory was developed, Israelachvili and Tabor [57] measured the force versus distance (F vs. d) profile and pull-off force (Pf) between steric acid monolayers assembled on mica surfaces. The authors calculated the surface energy of these monolayers from the Hamaker constant determined from the F versus d data. In a later paper on the measurement of forces between surfaces immersed in a variety of electrolytic solutions, Israelachvili [93] reported that the interfacial energies in aqueous electrolytes varies over a wide range (0.01-10 mJ/m-). In this work Israelachvili found that the adhesion energies depended on pH, type of cation, and the crystallographic orientation of mica. 

Control of molecular orientation and packing in monolayer assemblies... 

Control of oriented antibody layer by monolayer assemblying technique. 

Various Orientation of functional groups in monolayer assemblies and its effect upon some functions. 

In order to evaluate quantitatively the orientation of vibrational modes from the dichroic ratio in molecular films, we assume a uniaxial distribution of transition dipole moments in respect to the surface normal, (z-axis in Figure 1). This assumption is reasonable for a crystalline-like, regularly ordered monolayer assembly. An alternative, although more complex model is to assume uniaxial symmetry of transition dipole moments about the molecular axis, which itself is tilted (and uniaxially symmetric) with respect to the z-axis. As monolayers become more liquid-like, this may become a progressively more valid model (8,9). We define < > as the angle between the transition dipole moment M and the surface normal (note that 0° electric field of the evenescent wave (2,10), in the ATR experiment are given by equations 3-5 (8). 

Brovelli, D., 1999. Highly oriented, self-assembled alkanephosphate monolayers on tantalum(V) oxide surfaces. Langmuir 15 (13), 4324—4327. 

By combining regiospecific synthesis with alternating hydrophilic and hydrophobic side chain moieties, Bjornholm, McCullough, and coworkers [285,286] formed regioregular amphiphilic PATs that can spontaneously self-assemble at hydrophilKc or hydrophobic interfaces to form oriented monolayers or bilayers. Finally, there are also efforts to induce chiral liquid crystalline phases. Goto et al. [287] have recently reported on a family of ferroelectric LC substituents. These materials also exhibit a rich array of... 

For the studies of LB monolayer assemblies, information about the orientation of amphiphilic molecules in the monolayers is important, as well as determination of the chemical functionalities. The Kelvin method [104] is a well-established technique for determining the contact potential differences (CPDs) between reference electrode and a metal sample. The CPD for clean metal surfaces is given by the difference in the work functions of the two materials. The work functions can be changed by adsorption of molecules with different dipole moments. Therefore, the photoin-duced dipole change of the triad molecules unidirectionally oriented in LB monolayers on electrode surface can be easily detected as a change of CPD by Kelvin method. The Kelvin method of measuring the CPD makes use of the vibrating capacitor. 

D. Brovelli, G. Hahner, L. Ruiz, R. Hofer, G. Kraus, A. Waldner, J. Schlosser, P. Oroszlan, M. Ehrat, N.D. Spencer, Highly Oriented, Self-Assembled Alkanephosphate Monolayers on Tanta-lum(V) Oxide Surfaces. Langmuir (1999) Vol. 15 pp. 4324-4327. 

Highly Oriented, Self-Assembled Alkanephosphate Monolayers on Tantalum(V) Oxide Surfaces... 

Octadecyl phosphoric acid ester appears to form an ordered, oriented, self-assembled monolayer on a flat tantalum(V) oxide surface, when deposited from an n-heptane/propan-2-ol solution. The adsorption rate is rapid, with a complete tails-up layer being formed within minutes, according to contact angle and grating-coupler results. An optimal combination of assembly rate and film quality appears to be achieved with 50 or 500 solutions... 

The intermediate location of a redox-relay between the electrode surface and the cofactor unit embedded in the en2yme is of key importance for the establishment of electrical contact between the enTyme and the electrode. For example, a PQQ monolayer assembled onto an Au-electrode was employed to reconstitute the PQQ-dependent apo-GDH [164, 165]. In this case, the PQQ plays the role of the embedded cofactor, and since no additional electron-relay was immobilized between PQQreconstituted enzyme lacks the electrical contact with the electrode. The electrochemical oxidation of glucose by the reconstituted biocatalyst was only stimulated in the presence of a diffusional electron-transfer mediator. In other cases, however, the orientation of the protein with respect to the electrode is sufficient to promote electron-transfer vrithout the need for a mediator. An Fe(111 )-protoporphyrin IX complex was assembled as a monolayer on an Au-electrode and apo-Mb was reconstituted with the heme-cofactor monolayer [166]. Although native Mb usually lacks direct electrical communication with electrode supports as a result of insulation of the heme center. 

Brovelli, D., Hahner, G., Ruiz, L., Hofer, R., Kraus, G., Waldner, A., Schlosser, J., Oroszlan, P., Ehrat, M. and Spencer, N., Highly oriented self-assembled alkanephos-phate monolayers on tantalum (V) oxide surfaces, Langmuir, 15, 4324-4327 (1999). 

Polyion complexes between acid derivatised polythio-phene (poly(thiophene-3-acetic acid)), PTAA (Figure 14.39), and a stable surface active cation have been used to construct conductive LB films. In this case, the formation of a polyion complex renders PTAA surface active and provides additional control over the molecular orientation of the polymer within the monolayer. The monolayer assembly thus obtained has a structure composed of a well ordered condensed monolayer of the surface active molecule onto which is a single monolayer of PTAA ionically bound. For example, the sodium salt of PTAA can be readily absorbed onto a monolayer of dimethyldioctadecylam-monium bromide. The monolayer assembly of the polyion complex was successfully transferred into LB film in a Z-type manner [294]. 

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