Measurements of Depletion Interactions

P. Kekicheff, F. Nallet, and P. Richetti Measurement of Depletion Interaction in Semi-Dilute Solutions of Worm-Like Surfactant Aggregates. J. Phys. II France 4,... 

Winstanley CA, et al. 5-HT(2A) and 5-HT(2C) receptor antagonists have opposing effects on a measure of impulsivity interactions with global 5-HT depletion. Psychopharmacology (Berl) 2004 176(3-4) 376-385. 

In this chapter, we will limit the discussion to the details of depletion interacdon on the pair potendal level. We will outline the basics of the derivation of the commonly used equations for the modeling of depletion interaction for different types of depletants and discuss approximations and shortcomings. We will introduce a very powerful experimental technique to measure pair interaction potentials and survey a biased selection of experimental reports on depletion interaction. 

The supersolid skin contains molecules upto two layers. A comparative study using x-ray and neutron scattering, sum-frequency vibrational spectroscopy and calorimetric measurements of the interaction between water and hydrophobic surface, as well as molecular-statistical calculations of the state of water molecules in the skin prove that the boundary water layer in the vicinity of hydrophobic surface consists of a thin ( 0.5 nm) depletion layer. The density is as low as 0.4 g/cm (correspond to doo = 3.66 A) and a considerable amount (25-30 %) of water molecules with free OH groups, which is characterized by a more ordered network of H-bonds compared to liquid water [22, 23]. 

Another AFM-based technique is chemical force microscopy (CFM) (Friedsam et al. 2004 Noy et al. 2003 Ortiz and Hadziioaimou 1999), where the AFM tip is functionalized with specific chemicals of interest, such as proteins or other food biopolymers, and can be used to probe the intermolecular interactions between food components. CFM combines chemical discrimination with the high spatial resolution of AFM by exploiting the forces between chemically derivatized AFM tips and the surface. The key interactions involved in food components include fundamental interactions such as van der Waals force, hydrogen bonding, electrostatic force, and elastic force arising from conformation entropy, and so on. (Dther interactions such as chemical bonding, depletion potential, capillary force, hydration force, hydrophobic/ hydrophobic force and osmotic pressure will also participate to affect the physical properties and phase behaviors of multicomponent food systems. Direct measurements of these inter- and intramolecular forces are of great interest because such forces dominate the behavior of different food systems. 

Increasing levels of emulsification significantly depleted protein from the fat globule in the mix. The adsorbed protein content in the mix (mg m of fat surface area) correlated with major characteristic analyses describing the fat structure in ice cream (fat agglomerate size, fat agglomeration index, solvent extractable fat Fig. 6). Thus, the measurement of protein load in the mix can be used to predict ice-cream-fat stability and related structure. Structural analyses indicated enhanced interaction between fat and air as protein adsorption decreased. It was also observed that the fat content in the dripped portion collected from a meltdown test correlated well with other indices of fat destabilization. 

We have compared these theoretical predictions of the low-frequency modulus to experimental measurements on compressed emulsions and concentrated dispersions of microgels [121]. The emulsions were dispersions of silicone oil (viscosity 0.5 Pas) in water stabilized by the nonionic surfactant Triton X-100 [102, 121]. The excess surfactant was carefully eliminated by successive washing operations to avoid attractive depletion interactions. The size distribution of the droplets was moderately polydisperse with a mean droplet diameter of 2pin. The interfacial energy F between oil and water was 4mJ/m. The contact modulus for these emulsions was thus F 35 kPa. The volume fraction of the dispersed phase was easily obtained from weight measurements before and after water evaporation. Concentrated emulsions have a plateau modulus that extends to the lowest accessible frequencies, from which the low-frequency modulus Gq was obtained. Figure 11 shows the variations of Gq/E"" with 0 measured for the emulsions against the values calculated in the... 

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