Induced Crystallization of Polyelectrolyte-Surfactant Complexes at the Gas-Water Interface

Synchrotron x-ray and surface-tension studies of a strong polyelectrolyte (PE) in the semidilute regime ($\sim 0.1M$ monomer charges) with varying surfactant concentrations show that minute surfactant concentrations induce the formation of a PE-surfactant complex at the gas-solution interface. X-ray reflectivity and grazing angle x-ray diffraction show the complex PE-surfactant resides at the interface and the alkyl chains of the surfactant form a two-dimensional liquidlike monolayer. With the addition of salt (NaCl), columnar crystals with distorted-hexagonal symmetry are formed.

Figure 1

Chemical structure of sodium dodecyl sulfate, SDS, and poly-diallyldimethylammonium chloride, PDAC, used in this study. Surface tension as a function of SDS concentration in water, and $0.1M$ NaCl and 2% PDAC both at 0 and $0.1M$ NaCl, practically indistinguishable.

Figure 2

(A) Reflectivity normalized to $R_F$ ($R_F$ is the reflectivity from ideally flat water interface) for (a) ${10}^{-4}M$ SDS in water, (b) ${10}^{-4}M$ SDS $0.1M$ NaCl, (c) 2% wt PDAC in pure water, (d) ${10}^{-4}M$ SDS in 2% wt PDAC solution, (e) ${10}^{-4}M$ SDS 2% wt PDAC after the addition of $0.1M$ NaCl. (Reflectivities are shifted by a decade for clarity.) (B) Electron density profiles ($Z=0$ is the molecule-solution interface) used to generate the fitted reflectivity [solid lines in (A)], the steplike functions (box model) are generated assuming no surface roughness ($\sigma =0$; ED’s are shifted for clarity).

Figure 3

(A) In-plane diffraction (GIXD) scans of pure water, and 2% wt PDAC $2\times {10}^{-5}M$ SDS and $0.1M$ NaCl as indicated and the difference between the two scans. (B) Similar differences between solutions of 2% wt PDAC (a) $0M$ SDS, (b) $2\times {10}^{-5}M$ SDS, (c) $2\times {10}^{-4}M$ SDS (scans shifted for clarity). The disruption of the structure of water at the interface is apparent.

Figure 4

(A) A scan along $Q_x$ of (10) peak at $Q_z=0.098{\AA }^{-1}$. (B) A rod scan along the same peak (logarithmic scale) also revealing the (11) peak. (C) The observed peaks in the $Q_x,Q_z$ plane. (D) A schematic illustration of the suggested model structure with the oblique unit cell. The long axes of cylindrical micelles are parallel to the liquid surface.

Figure 5

(A) Schematic pathways of complexation and subsequent crystallization. (A) Negligible surfactant concentration—the PE is repelled from the interface. (B) Below the CAC a PE surfactant is formed at the interface. (C) Critical aggregation concentration (CAC) is reached and micellization occurs. (D) The addition of salt transforms micelles to cylindrical shape and crystallizes them. The arrangement and location of the PE in the crystalline regions (shown with a dashed line) was not be determined in this study.