Frequency Domain Fluorescence Measurements of the Aggregation Properties of C_nE_m Surfactants in Agarose Gels

Abstract

The micellar properties of C12E6, C12E8, and C12E10 surfactants in agarose gels were investigated. The aggregation number and the aggregate polarity were determined as a function of temperature, gel fiber volume fraction, and surfactant type by using steady-state fluorescence and frequency domain fluorescence quenching techniques. Application of the latter technique to the study of micellar properties in solution is described, including a method for accounting for the excitation light scattered by the gel fibers. The aggregation behavior in gels of surfactants which form spherical micelles in water is compared to those which form cylindrical micelles, and the results are interpreted by using a thermodynamic model for micelle formation in a gel matrix. For surfactants which form small spherical micelles in solution, there is little difference between the aggregation number and micelle polarity in an agarose gel and in pure solution. For surfactants which form large cylindrical micelles in solution, the aggregation number decreases with increasing gel concentration while the micelle polarity remains constant. The thermodynamic model shows that, if the agarose gel restricts the micelle size distribution, then the weight average aggregation number decreases for large cylindrical micelles and remains unchanged for small spherical micelles. The model results are consistent with the experimental results for the aggregation number of C12E6 in an agarose gel.

Influence of surfactant structure on the contribution of micelles to Ostwald ripening in oil-in-water emulsions

“…The maximum oil solubilized in the micellar solutions was quantified by extrapolating to find the oil concentration at which the steady state turbidity value goes to zero. [38] 90 [39] 90 [46,50] 90 [51] 120 [51] 40 [52] a H (Å 2 ) 107 b [43] 60 b [43] 66 [40] 61 72 [42] 79 b [43] c (mN/m) …”

Diffusion of sodium dodecyl sulfate micelles in agarose gels

“…Interestingly, previous results obtained for nonionic C 12 E 6 micelles in agarose gels showed an opposite trend, with the aggregation number decreasing in the presence of the gel [43]. C 12 E 6 micelles are non-spherical, and tend to elongate with increasing surfactant concentration.…”

“…C 12 E 6 micelles are non-spherical, and tend to elongate with increasing surfactant concentration. The growth of these rod-shaped micelles is far more substantial than that observed for SDS, and creates micellar dimensions comparable to the effective mesh size in the gel [43]. Thus, for these nonionic surfactants, the steric energy penalty created by large micelle-fiber interactions outweighs the importance of reduced micelle-micelle interactions, which are much weaker for C 12 E 6 than for SDS.…”

“…However, for 0.10 M NaCl, the reduction in D 0 at 2 wt% agarose is not clearly evident. In the next section, we examine the possibility that this is due to a change in the size of SDS micelles caused by an interaction with the gel fibers, such as that observed with nonionic C 12 E 6 micelles in agarose [43]. Another possible explanation is that a micelle-fiber interaction alters the gel's fiber network, causing changes in the effective fiber radius, a f , and the effect of the gel on hindered diffusion.…”

“…TRFQ, in particular, has been used to measure the aggregation behavior of SDS in water at various ionic strengths, for a variety of probe and quencher pairs [20,[25][26][27]29,42]. Nolan et al [43] demonstrated that TRFQ can also be applied in fibrous gels if the convolution kernel is modified slightly, and used it to measure aggregation numbers of several nonionic C 12 E m micelles in agarose gel. They showed that the presence of a network of rigid fibers can affect the size of a micelle, and consequently alter its transport behavior.…”