Solubilization of Amphiphilic Carboxylic Acids in Nonionic Micelles:  Determination of Partition Coefficients from pK_a Measurements and NMR Experiments

Abstract

The solubilization of octylamidotartaric acid (C8T) and octanoic acid (C8C) in Triton X-100 and Brij 58 nonionic micelles has been studied by pHmetric and 1H NMR self-diffusion experiments. As both C8C and C8T exhibit acid-base properties, a distinction between the partition of the neutral acidic form, in terms of the partition coefficient KPH, and the partition of the charged basic form, in terms of the partition coefficient KP-, has been made. The acidity constants, Ka, of C8T and C8C in the presence of micelles have been evaluated from pHmetric experiments. For both solutes, an increase in the pKa is observed in micellar media due to the difference in the partition of acidic and basic forms of the solutes. A model has been developed to determine KPH and KP- from the pKa shifts observed. The values obtained by this pKa shift modeling method and those from self-diffusion coefficient measurements are in good agreement. The acidic form of C8C is incorporated to a larger extent into the Brij 58 micelles than the acidic form of C8T, whereas the opposite trend is observed for the basic forms. Both the acidic and basic forms of C8T are more easily incorporated into Brij 58 micelles than into Triton X-100 micelles. The influence of the structure of the polar head on the solubilization properties is demonstrated. Moreover, evidence for the localization of the solutes in the micelles is obtained from the comparison of the partition coefficients and from 1H NMR data.

Impact of emulsifier microenvironments on acid–base equilibrium and activity of antioxidants

“…This is in accordance with results presented by other authors for a number of different compounds (Dupont-Leclerq et al, 2007;Pal & Jana, 1996;Whiddon et al, 2003). The difference between the pK a values in buffer solution and in the micellar environment (DpK a ) was smaller for FA than for iFA.…”

“…The absence of an electric potential at the surface of non-ionic micelles makes it possible to determine the influence of a micellar environment on the acidic strength of a solute (Dupont-Leclerq, Giroux, Henry, & Rubini, 2007). Thus, if the concentrations of the acid and base forms of the compound in the micellar phase are known and the pH value is assumed to be equal to the pH value of the bulk, K a,m can be calculated (Eq.…”

Deterioration of Aqueous n‐Octanoate Electrolysis with Electrolytic Conductivity Collapse Caused by the Formation of n‐Octanoic Acid/n‐Octanoate Agglomerates

“…If deposits of n-octanoic acid would block the electrode of the conductivity meter due to the exceedance of the solubility limit of n-octanoic acid in water with decreasing bulk pH during C 8 titration, than it is expected that these n-octanoic acid deposits should stick to the electrode surface for lower pH (i. e. for pH values at which the n-octanoic acid concentration is even higher), as well. [42,43] In literature, this phenomenon is explained by the local decrease of pH at micelle surfaces due to the high charge density and the effect of agglomerate formation on the partition equilibrium of C 8 in the water-micelle-system. Rather, a different pH dependent, but necessarily reversible structural effect has to explain the conductivity drop, e. g. larger agglomerates such as vesicles.…”

“…Yet, k increases again for lower pH (Figure 4), rendering n-octanoic acid deposits unlikely to cause the observed conductivity drop. [38,40,[42][43][44] Note that if an apparent shift of the pK a of the n-octanoic acid/ n-octanoate system would occur to a value of 6.5, also the solubility of n-octanoic acid in water would be exceeded at higher pH values than for a lower pK a (Table S1 vs. Table S2). Counterintuitively, the minimum of k observed during titration occurred at pH % 6.5 rather than at the pH of the pK a (4.89 [23] ) which is usually favorable for vesicle formation as described above.…”

Competitive interactions between components in surfactant–cosurfactant–additive systems

“…Moderate and long-chain alcohols or carboxylic acids are commonly thought to pack within mixed micelles oriented in the same manner as the surfactant units, the polar group being located somewhere in the outer micelle parts and the hydrophobic tail pointing towards the nonhydrated inner core [7,14]. Therefore, their main effect is to enhance the stability of micellar aggregates or to increase the flexibility and size of micellar membranes.…”