A new approach to lamellar phases (L_α) in water – non-ionic surfactant systems

Abstract

We present new results on two unusual features concerning lamellar phases (L_α) in binary aqueous solutions of non-ionic surfactants. The first striking feature is the formation of highly dilute L_α phases down to 1 wt % of surfactant, which has been observed for a small number of non-ionic surfactants which are all of the alkyl polyglycol ether (C _i E _j ) type. So far, in binary H₂O – C _i E _j systems either the absence or the presence of a dilute L_α phase has been reported. In the latter case, the dilute and the concentrated L_α phase are always connected continuously. However, for one particular silane surfactant, namely (CH₃)₃Si(CH₂)₆(OCH₂CH₂)₅OCH₃, two disconnected L_α phases were observed. Systematic investigations of the phase behaviour of the binary H₂O – C₁₀E₄ system as well as of the pseudobinary H₂O – C₁₀E₄/C₁₀E₅ systems enabled us to conclude that: (a) the disconnected L_α phase is a general feature of non-ionic surfactants and (b) there are no structural differences between the connected and the disconnected L_α phases. We propose a mechanism for the disconnection of the L_α phase.

Notes

^†For the samples with γ=0.25, 0.40 and 0.45 the coexistence of two phases (isotropic+L_α) was observed over the whole temperature range, whereas for γ=0.30 and 0.35 a single lamellar phase surrounded by the corresponding two-phase region was detected.

^‡The two-phase regions we are dealing with are those in which the L_α phase coexists with an isotropic phase. The splitting that is observed corresponds to the coexisting L_α phase. As neither the composition nor the amount of the coexisting L_α phase are known precisely, a comparison with the splitting of the one-phase region is unreasonable. On the contrary, it is from the Δν(γ)-curve of the one-phase region that the composition of the coexisting L_α phase can be deduced

^§The hydration degree n _b was assumed to be constant to calculate the prefactor n _bΔν _b in equation (5). This is justified by the fact that we evaluated the Δν _max values, i.e. the splittings at which n _b is maximum for the given concentration γ. At this particular hydration degree the order of the lamellar phase reaches its maximum, which corresponds to a zero curvature. As the curvature in the binary systems can only be adjusted via the hydration degree of the hydrophilic head, an equal curvature is tantamount to an equal hydration degree.