Hydrophobic units Interface

As early as 1969, Wlieeler and Widom [73] fomuilated a simple lattice model to describe ternary mixtures. The bonds between lattice sites are conceived as particles. A bond between two positive spins corresponds to water, a bond between two negative spins corresponds to oil and a bond coimecting opposite spins is identified with an amphiphile. The contact between hydrophilic and hydrophobic units is made infinitely repulsive hence each lattice site is occupied by eitlier hydrophilic or hydrophobic units. These two states of a site are described by a spin variable s., which can take the values +1 and -1. Obviously, oil/water interfaces are always completely covered by amphiphilic molecules. The Hamiltonian of this Widom model takes the form... 

These are molecules which contain both hydrophilic and hydrophobic units (usually one or several hydrocarbon chains), such that they love and hate water at the same time. Familiar examples are lipids and alcohols. The effect of amphiphiles on interfaces between water and nonpolar phases can be quite dramatic. For example, tiny additions of good amphiphiles reduce the interfacial tension by several orders of magnitude. Amphiphiles are thus very efficient in promoting the dispersion of organic fluids in water and vice versa. Added in larger amounts, they associate into a variety of structures, filhng the material with internal interfaces which shield the oil molecules—or in the absence of oil the hydrophobic parts of the amphiphiles—from the water [3]. Some of the possible structures are depicted in Fig. 1. A very rich phase... 

Due to their structure (Figure 17.1), all surfactants have the tendency to accumulate at interfaces because there the hydrophobic tail can be shielded from interacting with water molecules while the hydrophilic headgroup remains solvated by water molecules. As a result of this orientation, surfactant molecules displace water molecules at the interface. Consequently, the number of hydrogen bonds decreases per unit interface area. This can be... 

It was crystallized from 60% aqueous ethanol and contains, per asymmetric unit, about 85 water molecules (and 7% ethanol), of which 64 were located from the electron density map. Most of these water molecules link polar groups, but several of them are organized in the form of fused pentagons which cover a hydrophobic, intermolecular interface. 

It is common knowledge that amphiphilic molecules, such as sodium dodecyl sulfate, above a certain critical concentration in water form assembled suiictures in which the hydrophobic units are clustered together. The notice of a hydrophobic effect was brought to light by Walter Kauzmann, whilst studying forces that influenced protein denaturation [27]. An excellent critical review on interfaces and the driving forces of hydrophobic assembly was written by Chandler in 2005 [28]. 

Air entraining agents are composed of the non-polar hydrocarbon chains or the other hydrophobic units, linked with the following hydrophilic groups —COO , —COO, — SOj On the gaseous and hquid phase interface these polar groups are directed to water— they lower its surface tension, and facilitate the formation of air... 

EiJ, where x is determined by thermal fluctuations. Generally, the tail-water interaction energy is considerably higher than that of the tail-air interaction (E nE, where El is an interaction energy per hydrophobic unit, and is a number of those units in each tail). Consequently, x /x = exp 2( - EjJ 1, and transfer from the interface to bulk is a much slower process than the reverse one. If the duration of a spreading experiment is shorter than x, then, during that experiment, the surfactant can be considered as insoluble. Otherwise, if t > x, the solubihty of the surfactant in the liquid must be taken into account, hi the latter case, surfactant transfer to the bulk liquid tends to make concentration uniform both in the bulk and at the interface, and the result is a substantial decrease of the surfactant influence of that type (Marangoni flow). 

Proteins often have the same high-affinity isotherms as do synthetic polymers and are also slow to equilibrate, due to many contacts with the surface. Proteins, however, have the additional complication that they can partially or completely unfold at the solid-liquid interface to expose their hydrophobic core units to a hydrophobic surface... 

The influence of the surfactant in the modified polymers of Figure 1 on n (aqueous solutions, Table I) is not overpowering. The surfactant s influence is diminished by the amphiphilic oxyethylene units which lie interfacially flat at the aqueous-air interface. The hydrophobes are structurally similar to the surfactants providing stability to commercial latices and should be capable of competing with the classical surfactants at the latex surface, but this ability is not reflected in 7T values. The oxyethylene units have been demonstrated(18) to provide osmotic stabilization to latex particles. 

A surfactant is a molecule that accumulates at the interface between two phases and modifies the surface tension. (Surface tension is the energy per unit area needed to form a surface or interface.) One common class of surfactants for aqueous solution are molecules with long hydrophobic tails and ionic head groups, such as... 

Reduced Surface Tension. Just as surfactants self-organize in the bulk solution as a result of their hydrophilic and hydrophobic segments, they also preferentially adsorb and organize at the solution—vapor interface. In the case of aqueous surfactant solutions, the hydrophobic tails protrude into the vapor and leave only (he hydrophilic head groups in contact with the solution. The favorable energetics of the arrangement can be seen by the reduction in Ihe interracial free energy per unit area, nr surface tension, it. 

Fig. 3.36 Segment density profiles from a Monte Carlo computer simulation of adsorption of a BAB triblock at a planar interface, where the hydrophobic B block is preferentially adsorbed (Balazs and Lewandowski 1990). Profiles are plotted for different A segment-surface interaction parameters, AS, with Xas = 0 and a chain length - 30 units.

Takahashi et al.67) prepared ionene-tetrahydrofuran-ionene (ITI) triblock copolymers and investigated their surface activities. Surface tension-concentration curves for salt-free aqueous solutions of ITI showed that the critical micelle concentration (CMC) decreased with increasing mole fraction of tetrahydrofuran units in the copolymer. This behavior is due to an increase in hydrophobicity. The adsorbance and the thickness of the adsorbed layer for various ITI at the air-water interface were measured by ellipsometry. The adsorbance was also estimated from the Gibbs adsorption equation extended to aqueous polyelectrolyte solutions. The measured and calculated adsorbances were of the same order of magnitude. The thickness of the adsorbed layer was almost equal to the contour length of the ionene blocks. The intramolecular electrostatic repulsion between charged groups in the ionene blocks is probably responsible for the full extension of the... 

The hydrophilic (OH) groups in these otherwise hydrophobic polymers allow the formation of stable monolayers at an air/water interface. The high-cis polymer occupies 38 A2/monomer unit which is much larger than the 9 A2/monomer unit occupied by the... 

Note that many of these surface reactions involve the conversion of a hydrophophic polymer to one with a hydrophilic surface or vice versa. For example, the replacement of trifluoroethoxy groups at the interface by hydroxyl units changes a non-adhesive, highly hydrophobic surface to an adhesive hydrophilic one. Variations in the reaction conditions allow both the depth of transformation and the ratios of the initial to the new surface groups to be controlled. A possible complication that needs to be kept in mind for all of these surface transformations is that polymer molecular motions may bury the newly introduced functional units if the polymer comes into contact with certain media. For example, a hydrophilic surface on a hydrophobic polymer may become buried when that surface is exposed to dry air or a hydrophobic liquid. But this process can be reversed by exposure to a hydrophilic liquid. 

In Refs. [24,25], a two-dimensional thermodynamic classification of nonionic monomers was proposed, which took into account three possible preferential locations of a monomer unit (location either in the hydrophilic or the hydrophobic phase or at the interface between them). The proposed classification incorporates gradations by affinity to polar and nonpolar phases and by interfacial activity (Fig. 3). 

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