Crossover from Normal to Inverse Temperature Dependence in the Adsorption of Nonionic Surfactants at Hydrophilic Surfaces and Pore Walls

Abstract

The adsorption of the nonionic surfactant C 8 E 4 from its aqueous solutions to the pore walls of four controlledpore silica glass (CPG) materials of different mean pore widths (10-50 nm) has been studied in a temperature range from 5 to 45 °C, that is, close to the lower critical temperature of liquid-liquid phase separation of the bulk system (T c ≈ 40 °C). Pronounced S-shaped isotherms, with a normal temperature dependence of the adsorption in the initial low-affinity region but an inverse temperature dependence in the plateau region, are found with all CPG materials. The experimental adsorption isotherms are compared with predictions of a theoretical model (Phys. ReV. Lett. 2004, 92, 135701), which takes into account H-bonding and micelle formation in the bulk and at the surface. It is found that this model reproduces all peculiarities of the adsorption in the present systems. The following conclusions emerge from this analysis: (1) At low bulk concentrations, the surfactant is adsorbed only in monomeric form (Henry's law behavior). In this regime, there is an energetic driving force for surfactant adsorption (in spite of the fact that nonselective pore walls are assumed), in agreement with the observed normal temperature dependence of the adsorption in this regime. (2) Surface aggregation observed at higher concentrations involves an energy penalty due to the loss of H bonds, which is overcompensated by the gain of (rotational) entropy of the H-bonding sites of the surfactant heads. Accordingly, surface aggregation is entropy-driven and endothermic and thus shows inverse temperature dependence. Hence, the model accounts for the observed crossover in the temperature dependence of the adsorption at the critical surface aggregation concentration.

Ordering of amphiphilic Janus particles at planar walls: A density functional study

“…Especially at low densities one would also expect the occurrence of spherical structures such as vesicles and micelles; the most "stable" structure could then be selected by comparison of the related free energies. Furthermore, in experiments of elongated amphiphilic molecules (rather than amphiphilic spheres) at surfaces, both planar and spherical structures are observed 34,47 , suggesting that different structures could also occur for the spherical (Janus) case. The strategy to include spherical self-assembled structures within our density functional approach is generally clear, as shown by Tarazona et al 15 in their study of bulk systems.…”

Surfactant adsorption and aggregate structure at silica nanoparticles: Effects of particle size and surface modification

“…[2][3][4][5][6][7][8][9] When the anchoring of the surfactant heads to the surface is weak, as in the case of nonionic surfactants at oxide surfaces, the morphology of surface aggregates may depend both on the anchoring strength 10,11 and on the curvature of the adsorbing surface.…”

“…2 is a well-known signature of aggregative adsorption of nonionic surfactants at hydrophilic surfaces. 6,9,11,20 On the other hand, the strong decrease of the maximum surface concentration of the surfactant with decreasing size of the Lys-Sil nanoparticles represents a remarkable new result. To our knowledge such a pronounced size effect on the adsorption has not been reported previously.…”

Surfactant Adsorption and Aggregate Structure at Silica Nanoparticles

“…4.2 is a well-known signature of aggregative adsorption of nonionic surfactants at hydrophilic surfaces [11,14,16,25]. On the other hand, the strong decrease of the maximum surface concentration of the surfactant with decreasing size of the Lys-Sil nanoparticles represents a remarkable new result.…”

“…Surfactant adsorption onto hydrophilic surfaces can be regarded as a surface aggregation process, reminiscent of micelle formation in solution [6][7][8][9][10][11][12][13][14]. When the anchoring of the surfactant heads to the surface is weak, as in the case of nonionic surfactants at oxide surfaces, the morphology of surface aggregates may depend both on the anchoring strength [15,16] and on the curvature of the adsorbing surface [17][18][19][20][21][22][23].…”