Orientation at interfaces

In attaining intimate molecular contact at the interface another aspect which should be considered [116-118] is the conformation of the adhesive or primer molecule when it is adsorbed onto the substrate surface. The conformation of the adsorbed molecule will be a function of many parameters, including the chemical and physical nature of the adsorbing species and the substrate surface, the concentration and molecular weight of the adsorbing molecules, the temperature, etc. 

Considering those aspects most relevant to the present discussions, then small low molecular weight molecules, such as adhesives (or primer) monomers or oligomers, will typically adsorb in the most favourable conformation from the energetics standpoint. This may lead to preferential adsorption of one particular component from a given formulation, which typically contains a mixture of resin, hardener, catalysts, etc. This aspect, and its implications, are discussed further in Section 8.3.3.5. 

Early work by Schonhorn [119] showed that orientated monolayers of amphipathic molecules, such as stearic acid, could be employed as extremely effective adhesives in the bonding of polyethylene to aluminium indeed, so effective were these adhesives that joint strengths often exceeded the cohesive strength of the polyethylene Multilayer adsorption lowered the joint strength not because of less intrinsic adhesion of the amphipathic molecule to the substrate, but because the relatively thick layer possessed low cohesive strength and thus behaved as a weak boundary layer. Chemisorption was thought to occur for the stearic acid on the aluminium oxide [120,121], and this 

There is one remaining and very significant aspect of liquid-air and liquid-liquid interfaces to be considered before proceeding to a discussion of 

The idea that unsymmetrical molecules will orient at an interface is now so well accepted that it hardly needs to be argued, but it is of interest to outline some of the history of the concept. Hardy [74] and Harkins [75] devoted a good deal of attention to the idea of force fields around molecules, more or less intense depending on the polarity and specific details of the structure. Orientation was treated in terms of a principle of least abrupt change in force fields, that is, that molecules should be oriented at an interface so as to provide the most gradual transition from one phase to the other. If we read interaction energy instead of force field, the principle could be reworded on the very reasonable basis that molecules will be oriented so that their mutual interaction energy will be a maximum. 

There is, of course, a mass of rather direct evidence on orientation at the liquid-vapor interface, much of which is at least implicit in this chapter and in Chapter IV. The methods of statistical mechanics are applicable to the calculation of surface orientation of assymmetric molecules, usually by introducing an angular dependence to the inter-molecular potential function (see Refs. 67, 68, 77 as examples). Widom has applied a mean-held approximation to a lattice model to predict the tendency of AB molecules to adsorb and orient perpendicular to the interface between phases of AA and BB [78]. In the case of water, a molecular dynamics calculation concluded that the surface dipole density corresponded to a tendency for surface-OH groups to point toward the vapor phase [79]. 

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Due to the differences in properties of the polar and These micelles are normally spherical with the nonpolar regions, surfactants tend to accumulate and nonpolar regions of the surfactant molecules gathered orient at interfaces so that each region of the surfactant in the center (core) and surrormded by a mantel of the interacts with a separate phase. The polar portion of polar regions which are in contact with the water as the surfactant will associate with the more polar phase illustrated in Fig. 13. A nonpolar drug, which is (especially if it is water) and the nonpolar portion of squeezed out of water, can locate within the micelle ... 

SHG experiments may also be used to determine molecular orientation at interfaces. By determining the polarity of the SH signal with respect to that of the incident light one may determine the independent components of y. In general, y is a third-rank tensor with 27 elements. When the composition of... 

Johnson DL, Martin LL. Controlling protein orientation at interfaces using histidine tags an alternative to Ni/NTA. J Am Chem Soc 2005 127 2018-2019. 

Insolubleoilor crystals orientate at interfaces. Form stable monolayers... 

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