Hydrophobic solute, size

Additives, whether hydrophobic solutes, other surfactants or polymers, tend to nucleate micelles at concentrations lower than in the absence of additive. Due to this nucleating effect of polymers on micellization there is often a measurable erne, usually called a critical aggregation concentration or cac, below the regular erne observed in the absence of added polymer. This cac is usually independent of polymer concentration. The size of these aggregates is usually smaller than that of free micelles, and this size tends to be small even in the presence of added salt (conditions where free micelles tend to grow in size). 

Thus, for a hydrophobic solute is determined by quantifying the probability po of successfully inserting a hard-core solute of the same size and shape into equilibrium configurations of water, as illustrated in Figure 4. A virtue of this approach is that the thermodynamics of hydrophobic hydration characterized by is determined from the properties of pure water alone. The solute enters only through its molecular size and shape (see Fig. 4). 

Phase separation of the saturated solution from the excess solid solute is a critical process. If a filter is employed, it must be inert to the solvent, it must not release plasticizers, and its pore size must be small enough to retain the smallest particles of the solid solute. Furthermore, steps must be taken to monitor, minimize, and preferably avoid losses of the dissolved solute by adsorption onto the filter material [27-30] and/or onto the vessels, pipettes, and syringes. Typically, the first small volume of filtrate is discarded until the surfaces of the filter and/or vessels are saturated with the adsorbed solute, to ensure that the filtrate analyzed has not suffered significant adsorption losses. Adsorption can be a serious problem for hydrophobic solutes, for which filtration would not be recommended. 

The experiments which yielded the diffusion coefficients for acetaminophen in PNIPAAm gel in Fig. 16 also yielded the corresponding partition coefficients. While the diffusion coefficients fit theory, the partition coefficients as plotted in Fig. 18 do not at all. In fact, a trend opposite to theory is observed as the partition coefficients are seen to increase as the gel swelling decreases. In fact, above the transition temperature of the gel, at 35 °C, the partition coefficient is seven times the maximum possible size exclusion coefficient, 1. This implies the dominance of hydrophobic effects over steric effects, since acetaminophen is a relatively small, nonionic but hydrophobic solute, and while the gel mesh size shrinks with increasing temperature, its level of hydrophobicity increases with temperature. 

To test this, we determined the partition coefficients of two different solute extremes - vitamin B-12 and norethindrone. Vitamin B-12 is a relatively large, hydrophilic solute, while norethindrone is a relatively small, hydrophobic solute. Thus we would expect size exclusion to dominate partitioning with vitamin B-12, while hydrophobic interactions should dominate with norethindrone. This behavior is in fact observed in Figs. 19 and 20. The values of K... 

Fig. 20. Experimental partition coefficients and those predicted by size exclusion theory (Eq. (4), shown as a solid line) of norethindrone (a small, hydrophobic solute) in 10x4 PNIPAAm gel. Norethindrone is absorbed by the collapsed gel above the transition temperature, ratter than being excluded from it as predicted by Eq. (4). Error bars (standard deviation of three samples) not shown are smaller than the symbol. The dotted line is to guide the eye. Reprinted from the Journal of Controlled Release (1992) 18 1, by permission of the publishers, Elsevier Science Publishers BV [70]...

The influence of size and shape on the diffusion of hydrophobic solutes was estimated by simulations involving artificial Lennard-Jones particles those intermolecu-lar interaction parameters were based on those for ammonia or oxygen, respectively. The results on the size dependence of diffusion confirmed that the membrane interior differs strongly from a bulk hydrocarbon. In the center of the bilayer, the excess free energy for hydrophobic Lennard-Jones particles remained low irrespective of the size of the particles. This can be explained by the large fraction of accessible volume in that region. 

Ashbaugh, H. S. and Paulaitis, M. E Effect of solute size and solute-water attractive interactions on hydration water structure around hydrophobic solutes. /. Am. Chem. Soc. 123, 10721-10728 (2001). 

In conclusion, 1) Hydrophilic solutes permeate p-HEMA and p-HEMA crosslinked with lower mole % EGDMA via the "pore" mechanism. The diffusion coefficients of the solutes depend on the molecular size and may utilize the "bulk-like" water in the hydrogels. As the water content of hydrogel increases, the solute permeability increases. 2) Hydrophobic solutes permeate p-HEMA and p-HEMA crosslinked with EGDMA via either the "pore" or "partition" mechanisms. Diffusion coefficients are lower than those of hydrophilic solutes however, steroids can permeate even in p-HEMA with 5.25 mole % EGDMA due to the predominant "partition" mechanism for hydrophobic solute permeation in this membrane. Hydrophilic solutes fail to permeate the high crosslinked hydrogels. 

Sodium chloride decreases the miscibility of water and acetonitrile. A minimum of approx 20 g sodium chloride added to 1 L of 50% aqueous acetonitrile solution will cause the formation of two liquid phases, a small upper acetonitrile-rich phase and a large lower aqueous-rich phase. Continued addition of sodium chloride increases the size of the acetonitrile-rich phase as more acetonitrile is forced out of the aqueous phase. The increasing salt and decreasing solvent content of the lower phase gradually increases the partition coefficient of hydrophobic solutes for the acetonitrile-rich phase. 

Hydrophobically Associating Copolymers. Hydrophobically modified cellulose derivatives (28) and N-alkylacrylamido copolymers (24, 25, 27) were among the first nonionic associative thickeners reported in the patent literature. The concentration of hydrophobic units allowed for dissolution in aqueous solution is usually less than 1-2 mol %. Like conventional polymers, apparent viscosity is proportional to molecular weight and concentration. However, with associative copolymers, a very dramatic increase in apparent viscosity occurs at a critical concentration, C, which clearly is related to a phenomenon other than simple entanglement. Viscosity dependence on hydrophobe concentration, size, and distribution suggests mi-croheterogeneous phase formation. Surfactants enhance viscosity behavior in some instances (24), yet clearly reduce viscosity in others (i). 

Distribution statistics do not differentiate between different sizes and sh es of micelles and different locations of solute molecules with respect to the center of the micelle. The occupancy of a micelle by a solute molecule may lead to its incorporation in the micellar core, such as may be expected for hydrophobic solutes. Substrate ions, on the other hand, may replace given counterions of ionic micelles and may be firmly bound by their surface charge. For our purposes we shall only be concerned with systems in which the overall solute concentration c is large compared to [SJ, the concentration of solute molecules in the aqueous phase (c [SJ), i.e., practically all solute molecules are somehow attached to micelles. 

As indicated earlier, when the size of the monovalent ion becomes much larger than the size of a water molecule, then the ion starts to interfere with the HB network of liquid water. In addition, the solvation energy due to ion-solvent interaction decreases. According to Bom s expression, this energy decreases with size rio as 1/rion- Thus, beyond a certain size, the entropic loss to the system due to the size of a large ion becomes greater than the enthalpic stabilization due to the ion-solvent interaction. Thus, the ion can behave as a hydrophobic solute. 

Choudhury, N. and B. M. PettitL 2005a. Local density profiles are coupled to solute size and attractive potential for nanoscopic hydrophobic solutes. Molecular Simulation. 31,457. 

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