Heat of demicellization

FIG. 2 Schematic explanation for the dependence of the heat of demicellization on chain length and temperature. The bold arrows represent the overall enthalpy of demicellization, the dotted arrows represent the excess enthalpy of hydration of the hydrophobic moieties of the amphiphiles, and the dashed arrows represent the sum of enthalpies resulting from van der Waals interactions between hydrophobic moieties in the aggregates and from hydrogen bonds between water molecules. 

When demicellization occurs, the heats of dilution of both monomers and micelles usually make only a minor contribution to the overall evolution of heat. In the following discussion we neglect these contributions. However, a more accurate evaluation of the heat of demicellization must be based on independent determination of the contribution of the heat of monomer dilution, which can be experimentally measured by dilution of a solution of a concentration below the cmc. This, of course, is possible only when the heat of dilution of monomers is... 

The infinite dilution protocol involves titration of a concentrated amphiphile solution with Dt > cmc into a cell containing no amphiphile, namely Do = 0. After titfation, the concentration in the cell is Di = DtV,/(Vc + vO, where Di < cmc. Since Vc v this protocol is denoted as infinite dilution and the heat evolution of the process reflects demicellization of aU the micelles that were present in the titrated volume. The major contribution to the heat of dilution (AQ) is the heat of demicellization of (Di cmc)Vt moles of surfactant. Hence,... 

FIG. 3 Schematic description of the expected dependence of the heat of demicellization on the total surfactant concentration in infinite dilution experiments. 

To rate the wetting tendency of surfactants for hydrophobic surfaces. A graphitic powder, for example, with its low heat of wetting in water, yields much higher heat effects if immersed in solutions of surface active agents. Heats of dilution and of demicellization can be taken into account, if desired, to arrive directly at energies of interaction. 

The observed exothermic nature of demicellization of many surfactants at low temperatures (around 25°C) (see above) must mean that the heat gain associated with hydration of the hydrophobic moieties (possibly due to stronger hydrogen bonds in the water [14]) is greater than the sum of heat losses associated with reduction of the original hydrogen bonds in the water and disruption of the van der Waals interactions between hydrophobic moieties in the aggregates. 

In another series of experiments, small volumes of aqueous solutions containing no surfactant are injected into the cell. When the initial concentration in the cell (Do) is lower than the cmc, the (relatively low) heat of dilution of monomers can be determined. When Dq cmc and Vc v the titration results in partial demicellization, the titrated volume becomes saturated with monomeric surfactant, and the total amount of surfactant that becomes dissociated equals Vt cmc so that... 

An example where these characteristic changes are easily observable, are titration experiments with micellar solutions of surfactants which are titrated into the vessel filled with pure water. Initially, dilution of the micellar solution leads to complete demicellization. When the concentration of monomers in the cell approaches the critical micellar concentration cmc the heat effect connected with the transfer of surfactants from the micelle to the aqueous solution disappears and the heat of reaction approaches zero [118-123]. 

Figure 38 shows the results of demicellization experiments with the negatively charged surfactant sodium lauroyl-alaninate (SLA) at different temperatures (A. Blume, M, Ambiihl, and H. Watzke, unpublished results). It can be seen that the heat effect due to demicellization changes sign close to a temperature of 35°C. Similar curves are obtained for a variety of other surfactants. However, the temperature where A// = 0 varies with the headgroup stnicture of the surfactant. For anionic and cationic surfactants the temperature is usually between 20 and 30°C, whereas for non-ionic surfactants, such as octylglucoside (OG), it is close to 50 °C [123] The cmc can be easily and precisely detennined from the first derivative of the curves shown in Figure 38. The cmc is slightly temperature dependent and has a minimum where the demicellization enthalpy is zero.

From the temperature dependence of the cmc as well as the enthalpy of demicellization it is possible to calculate the demicellization entropy and its temperature dependence using the Gibbs-Helmholtz relation AG AH - TAS. A plot of all three thennodynamic parameters as a function of temperature is shown in Figure 39 for the anionic surfactant sodium lauroyl-alaninate (SLA). It is clearly evident that the A(/-value for demicellization of SLA shows only a weak temperature dependence whereas the two tenns AH and TAS are strongly temperature dependent in a similar way. This type of enthalpy-entropy compensation is quite common for phenomena where changes in "hydrophobic hydration" are involved, because a considerable increase in heat capacity occurs when hydrophobic groups are exposed to water (see below) [ 124-131]. 

The enthalpy of micellization of many surfactants in aqueous solution has been determined in the past, using mostly cell type and flow microcalorimeters [6-8]. These determinations were based on measurements of the excess heat associated with dilution of a surfactant from a concentration above the cmc to a concentration below the cmc, which results in demicellization of the preexisting micelles. One diffleulty with these determinations relates to the dependence of the heat evolution (AQ) on the initial and final concentrations, probably due to secondary self-aggregations of the surfactants at high concentrations and/or pre-micellar dimer formation at low surfactant concentrations [6,9], These difficulties are at least partially responsible for the lack of consistent data on the thermodynamics of micelle formation [6]. 

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