Aqueous interfaces, entropies

A current hypothesis, which is receiving considerable attention, is that one can indeed produce a surface which actively repels proteins and other macromolecules123 124, 133). The basic idea is presented in Fig. 25, which shows that a neutral hydrophilic polymer, which exhibits considerable mobility or dynamics in the aqueous phase, can actively repel macromolecules from the interface by steric exclusion and interface entropy methods. This method has been well-known and applied in the field of colloid stability for many years 120). The most effective polymer appears to be polyethylene oxide, probably because of its very high chain mobility and only modest hydrogen bonding tendencies 121 123>. 

Entropies of Compression of Charged Monolayers at Aqueous Interfaces... 

Here A//adS is the enthalpy of adsorption, T is the temperature, and AAads is the entropy change associated with the adsorption of the protein onto the surface. Protein adsorption will take place if AGads < 0. Considering a complex system, where proteins are dissolved in an aqueous environment, and are brought into contact with an artificial interface, there are a vast number of parameters that impact AGads due to their small size (i.e., large diffusion coefficient), water molecules are the first to reach the surface when a solid substrate is placed in an aqueous biological environment. Hence, a hydrate layer is formed. With some delay, proteins diffuse to the interface and competition for a suitable spot for adsorption starts. This competition... 

From their calculations of the surface excess entropy and volume of the electric double layer at a mercury-aqueous electrolyte interface, Hill and Payne (HP) [147] postulated an increase in the number of water molecules in the Stern inner region as the surface charge a of about 30 piC/m2, which is consistent with the results of TC on a silver surface obtained some 30 years later. HP used an indirect method to determine the excess entropy and volume by measuring the dependence on temperature and pressure of the double layer capacitance at the mercury-solution interface. 

For the above reasons, many personal care emulsions are formulated using nonionic surfactants of which the alcohol ethoxylates (the ICI Brij series), sorbitan esters (Spans), and their ethoxylates (Tween) are the most commonly used surfactants. These surfactants adsorb at the oil/water interface with the hydrophobic (alkyl) group pointing to (or dissolved in) the oil phase and the hydrophilic chain [mostly poly(ethylene) oxide (PEO)] remaining in the aqueous phase. These molecules produce a repulsive barrier as a result of the unfavorable mixing of the polar PEO chains (when these are in good solvent conditions) and the reduction in configurational entropy of the chains when these overlap. Such repulsion is usually referred to as steric stabilization (see below) (6). These nonionic surfactants, which are usually used in mixtures) have been successfully applied to prepare stable o/w and w/o emulsions. In addition, in some cases, they form liquid crystalline structures at the o/w interface and these prevent coalescence of the oil droplets (7). 

Polymer Adsorption. The driving force for adsorption typically is the en-thalpic interaction between the interface and the polymer segments. Exceptions include aqueous solutions near hydrophobic substrates. In that case, the driving force may include the hydrophobic interaction between water and the surface. The enthalpic interactions may be of various forms such as hydrogen bonding, inter-facial tension, van der Waals attraction, polar interactions, and electrostatic attractions. This enthalpic interaction is offset by the loss in conformational entropy... 

---