Interaction of an organic cation with Gibbs monolayers of n-hexadecyl phosphate

How many phases and phase transitions do exist in Gibbs adsorption layers at the air–water interface?

“…For instance, Xu and Damadoran [39] proposed that there is a negative surface excess of the proteins during the induction time, whereas Sengupta et al [40] and MacRitchie and Alexander [41] suggested that the induction period represents a diffusion-controlled adsorption time that ends when the interface is saturated with protein so that the adsorbed protein monolayer creates an energy barrier for further adsorption. However, several research groups including ours have evidently showed by some microscopic techniques that this initial plateau as well as any other plateaus before reaching equilibrium is caused by a first-order phase transition in Gibbs monolayers [15,16,[23][24][25][26][27][28][29][30][31][32][33][34][35][36]38]. Thus, the π-t measurements of these monolayers clearly indicate that two-step first-order phase transitions take place at 15 • C. Since the equilibrium surface pressure is higher at lower temperatures and no other cusp point, kink or plateau except the initial plateau appears before reaching the equilibrium at 2 • C, one should conclude that only one first-order phase transition occurs at this temperature.…”

“…The surface is fully covered again by this brighter phase and the surface pressure rises slowly in the π-t curve. Such a well-ordered fractal domains in the monolayers can be classified as an LC phase [21,35,36,38]. Fig.…”

“…In the aqueous solutions of the mixtures, L-arg acts as an L-argininium cation that forms complex with the phosphate group of n-HDP by electrostatic interactions as well as hydrogen bonding [38]. Since n-HDP contains two replaceable protons, we carried out experiments by the mixtures containing n-HDP and L-arg at a molar ratio of 1:2.…”

Effect of temperature on the surface phase behavior of n-hexadecyl dihydrogen phosphate in adsorption layers at the air–water interface

“…However, their experimental findings were disputed considering the fact that the impact of surfaceactive impurities was insufficiently considered. Later several research groups, including ours, have reported the existence of the LE-LC phase transitions in Gibbs adsorption layers of highly pure and slightly water-soluble surfactants [25][26][27][28][29][30][31][32][33][34]. The phase transitions in these monolayers have usually been studied by measuring π or surface tension (γ ) with time (t ) because the measurement of π or γ as a function of A, which is generally done for the spread monolayers, is almost impossible for these surfactants.…”

“…An attempt has been made to construct a rather elaborated phase diagram of n-TDP, where a triple point of 6.5 • C is found. In some other papers, we have also investigated the surface phase behavior of n-hexadecyl dihydrogen phosphate (n-HDP) (n-C 16 H 33 OPO(OH) 2 ) and reported the existence of the LC phases in the Gibbs monolayers of this surfactant [10,34,35]. At 20 • C, the monolayers show a first-order transition at zero surface pressure from the G to a higher density I phase, which then undergoes a second-order transition to a mosaic textured LC phase [10].…”