Large molecules of dyes or other substances can play the role of a nano-adsorbent in aqueous surfactant solutions and, coated with surfactant molecules, resemble surfactant micelles with a solubilizate. Such micelle-like particles were called protomicelles. Their formation does not require a critical micelle concentration (CMC) and begins immediately when the surfactant is introduced into the solution. In this spectrophotometric study of Nile red (NR) in aqueous solutions of nonanoic acid (NOA), the case where protomicelles are more important than micelles is demonstrated for the first time. It has been shown that the solubilization of NR in both premicellar and micellar solutions proceeds through protomicelles rather than NOA micelles. The article also contains a theoretical part. In it, on the basis of the chemical potential of the nano-adsorbent, it is proved that the addition of a surfactant to a solution always increases the solubility of the nano-adsorbent. The dependence of adsorption on the curvature of the nano-adsorbent surface is considered and it is shown that the adsorption equations for flat surfaces can be used for curved surfaces with high accuracy.

Large molecules of dyes or other substances can act as nano-adsorbents in aqueous surfactant solutions and, being coated with surfactant molecules, they resemble surfactant micelles with a solubilizate. These micelle-like particles were called protomicelles (or, more precisely, adsorption protomicelles, as in this paper). Their formation does not require a critical micelle concentration (CMC) and begins immediately when a surfactant is added to a solution. In this spectrophotometric study of Nile red (NR) in aqueous solutions of nonanoic acid (NOA), the first case where protomicelles were more important than micelles was demonstrated. It was shown that NR solubilization in both premicellar and micellar solutions proceeds through protomicelles rather than through NOA micelles. In the theoretical part of the paper, on the basis of the chemical potential of the nano-adsorbent, it was proved that the addition of a surfactant to a solution always increases the solubility of the nano-adsorbent. The dependence of adsorption on the curvature of the nano-adsorbent surface was considered, and it was shown that the adsorption equations for flat surfaces are applicable to curved surfaces with high accuracy.

Electronic absorption and luminescence spectroscopies have been employed to analyze the processes of octa-crown-substituted magnesium phthalocyaninate (Mgcr(8)Pc) dissolution in water in the presence of dodecyl-, tetradecyl-, and hexadecyltrimethylammonium bromides. In the premicellar region of surfactant concentrations, surfactant-induced aggregation of Mgcr(8)Pc prevails. Above the critical micelle concentration of the surfactants, Mgcr(8)Pc is completely disaggregated to monomers. At the same time, an increase in the alkyl chain length in a surfactant molecule enhances the tendency to its monomerization, as is evident from the results of measuring the electronic absorption and fluorescence spectra, as well as the fluorescence decay kinetics data. A rise in the fluorescence lifetime of Mgcr(8)Pc with compacting micelles suggests a decline in the degree of their hydration, which leads to a decrease in the polarity of Mgcr(8)Pc microenvironment. Mgcr(8)Pc exhibits intense fluorescence in hexadecyltrimethylammonium bromide solutions both at room temperature and after rapid freezing to 77 K, thus confirming that the disaggregated state of Mgcr(8)Pc molecules remains preserved in micelles even at this low temperature. The results obtained indicate that Mgcr(8)Pc dissolved in solutions of alkyltrimethylammonium bromides can be used in photodynamic therapy of tumors in combination with cryolysis procedures.

Solubilization of nile red (NR) dye in aqueous solutions of tetradecyltrimethylammonium bromide (TB) was studied by spectrophotometric method. Experiments were carried out both with NR saturated solutions under conditions of thermodynamic equilibrium of the solution with a dye precipitate, and at an NR constant weight. The solubility of NR in water (about 2 μM) has been refined. Extinction coefficients of NR monomers in water and micellar surfactant solutions were found. The micellization critical concentration (CMC) of TB in the presence of NR was determined, and the theoretical conclusion about the decrease in the surfactant CMC under the action of solubilizate was confirmed. The solubilization thermodynamic parameters were calculated on the basis of the following experimental data: the solubilization capacity of micelles, the coefficient of the solubilizate distribution between micelles and the surrounding solution, and also the standard affinity of NR solubilization. The problem of molecular NR aggregation in water and aqueous solutions of TB was considered. It has been found that the rate of reaching aggregative equilibrium is much lower than the rate of NR dissolution, therefore published spectral data based on the presence of only NR monomers mostly refer to the insufficiently equilibrium cases. It is shown that the process of NR solubilization begins in the prеmicellar region of TB, but is most effective in the presence of micelles.

The micellization processes in the systems tetraethylene glycol monododecyl ether (C12E4)–heptane (1) and C12E4–Nile red–heptane (2) have been studied by electron absorption spectroscopy. In system 1, reverse micelles of small size appear, which makes it difficult to find the critical micelle concentration (CMC). In this regard, a method for determining CMC based on spectroscopic data was tested. The same one was done for system 2, where the appearance of Nile red as a C12E4 nano-adsorbent leads to the formation of protomicelles below the CMC. The CMC itself in system 2 turns out to be lower than the CMC in system 1, which corresponds to the predictions of the theory. The water role in the formation of reverse micelles was studied using a commercial analogue of C12E4 preparation Brij 30 (with a water content of 1%). The results obtained are consistent with theoretical predictions.

The solubility in dimethyl sulfoxide and disaggregation in water under the action of tetradecyltrimethylammonium bromide (С 14 TAB) have been studied for metal-free phthalocyanine modified with four alkylphosphoryl groups ( I ). Electronic absorption and luminescence spectroscopies have been employed to show that, in dimethyl sulfoxide, compound I exists not only in the associated state (as it was believed previously), but also as monomers, the amount which provides high-intensity radiation. In aqueous media at pH 7–9, aggregates of I prevail; however, they are disintegrated into individual molecules under the action of С 14 TAB. According to electronic absorption and luminescence spectroscopy data, compound I is stable with respect to aggregation in a С 14 TAB micellar solution with pH 7.4 not only at 298 K, but also upon rapid freezing to 77 K.

Solubilization of crown-substituted magnesium phthalocyaninate(I) has been studied by spectrophotometry in aqueous solutions of tetradecyltrimethylammonium(II), hexadecyltrimethylammonium(III), and hexadecyltriphenylphosphonium bromides. The experiments have been carried out with saturated solutions of I occurring at the thermodynamic equilibrium with its precipitate. The experimental data have been used to determine the following thermodynamic characteristics of the solubilization: the solubilization capacity of micelles, the coefficient of solubilisate partition between micelles and an ambient solution, and the standard solubilization affinity of I. It has been shown that, in spite of different structures of the surfactants, the standard works of a I molecule transfer to micelles of all three surfactants are almost equal. The found values of the solubilization capacity, as calculated per one molecule of I, in micelles of II and III lead to abnormally large aggregation numbers. This fact may be explained by possible development of a bimodal distribution of micelles, at which solubilisate-containing micelles coexist with “empty” micelles; as a result, the average number of solubilisate molecules in a micelle may appear to be smaller than unity.

Protomicelles represent a new concept of colloid science relevant to micelle-like aggregates of surfactants in solutions. In contrast to ordinary micelles, their formation requires no critical micelle concentration (CMC) and proceeds gradually through the adsorption of surfactant molecules or ions on a solubilization core at concentrations much lower than the CMC. A completely formed protomicelle looks like an ordinary solubilisate-containing micelle. Protomicelles can be formed on phthalocyanine monomers and dimers. In this work, the role of protomicelles in monomerization of crown-substituted magnesium phthalocyaninate (I) in aqueous sodium dodecyl sulfate (II) solutions has been studied spectrophotometrically at compound I concentrations of 0.38 and 1.2 μМ. Empirical methods have been developed for determining a new parameter, monomerization concentration (concentration of II required to initiate the monomerization of I), and it has been shown that the monomerization concentration decreases with an increase in the concentration of I. Another new parameter defined as the concentration of II corresponding to nearly complete monomerization of I has been studied in comparison with the CMC of II. For this purpose, the CMC has, for the first time, being measured (by conductometry) in the presence of protomicelles. The latter circumstance has been shown to decrease the CMC. At the same time, it has been revealed that the concentration corresponding to the completion of monomerization in the studied system nearly coincides with the CMC.

Protomicelle is a new term in colloid science that refers to micelle-like surfactant aggregates in solutions. Unlike conventional micelles, protomicelles have no critical micelle concentration (CMC) and are formed gradually via adsorption of surfactant molecules or ions on solubilization cores at concentrations significantly lower than the CMC. A completely formed protomicelle looks like an ordinary solubilisate-containing micelle. In this work, the process of monomerization of a given amount of crown-substituted magnesium phthalocyaninate (I) in a premicellar aqueous solution of tetradecyltriphenylphosphonium bromide (II) has been studied by spectrophotometry. It has been shown that compound I monomers are absent in a solution in pure water, while the content of dimers increases as surfactant II is added to the solution. This phenomenon strictly proves the existence of higher-order molecular aggregates of I in aqueous solutions. A new physicochemical parameter of phthalocyanine monomerization has been defined, namely, the monomerization concentration, i.e., compound II concentration that corresponds to the beginning of this process. It has been shown that phthalocyanine monomerization, which has begun long before the CMC of II, continues above the CMC, although at a lower rate. At this stage, monomerization of I proceeds exclusively at the expense of dimers.

The existence of a special type of surfactant micelles formed on the solubilization core of phthalocyanine at concentrations significantly below the critical micelle concentration (CMC) has been established. This discovery overturns the traditional concepts of colloidal chemistry, which imply that the surfactant micelles are first formed, and then solubilization occurs in them above the CMC. In the process of solubilization, phthalocyanines (usually existing in an aqueous solution in the form of dimers) undergo monomerization, which is crucial for the manifestation of the functional properties of their molecules. Here, the spectrophotometric study of crown-substituted magnesium phthalocyaninate (I) in an aqueous solution of sodium dodecyl sulfate (II) is reported. It has been found that specific micelles of II (they can be called proto-micelles) involving dimers of I are formed significantly below the CMC. The solubilization capacity of micelles determined from experimental data leads, as calculated per molecule of I in a micelle, to an abnormally large aggregation number (309). This phenomenon can be explained by the formation of a bimodal distribution of micelles, in which micelles with solubilizate coexist with "empty" micelles, so that the average number of solubilizate molecules in a micelle can be less than unity.

Electronic absorption spectroscopy and fluorescence spectroscopy have been employed to analyze the aggregation of zinc phosphoryl-substituted phthalocyaninate by the example of zinc 2,(3),9,(10),16,(17),23,(24)-tetra(2-phosphoryl)phenoxyphthalocyaninate ([ZnPc(PO(OH)2)4]) in dimethyl sulfoxide and water at different pH values. It has been found that, in aqueous systems with pH 4–9, this compound exists in an aggregated state, while it forms a molecular solution in dimethyl sulfoxide. Moreover, ZnPc(PO(OH)2)4 is in fact insoluble in water at pH < 5.7. The addition of tetradecyltrimethylammonium bromide as a cationic surfactant to an aqueous ZnPc(PO(OH)2)4 solution leads to the complete disaggregation of the dye into individual molecules within a wide pH range (from 4 to 9.18) and a marked increase in the solubility of ZnPc(PO(OH)2)4 in strongly acidic media. Analysis of the fluorescent properties of surfactant micellar solutions containing ZnPc(PO(OH)2)4 has shown that such systems can generate singlet molecular oxygen. At the same time, the high intensity of ZnPc(PO(OH)2)4 fluorescence in micellar solutions makes it possible to visually observe the distribution of micelles that carry nonaggregated dye molecules in living tissues.

The aggregation of zinc tetra(4-carboxyphenoxy) phthalocyaninate, ZnPc(COOH)4, in organic solvents and water at different pH values has been analyzed using electronic absorption and luminescence spectroscopies. It has been found that, in aqueous systems at pH 6–9, ZnPc(COOH)4 exists in an aggregated state, while it forms molecular solutions in dimethylformamide and dimethyl sulfoxide. The use of cationic surfactants (dodecyl-, tetradecyl-, and hexadecyltrimethylammonium bromides) causes the disaggregation of ZnPc(COOH)4 into individual molecules in aqueous solutions at pH ≥ 6. This opens up possibilities of applying ZnPc(COOH)4 as a preparation for photodynamic therapy in physiological solutions, because each molecule of this compound may participate in the generation of singlet molecular oxygen.

Dynamic light scattering, conductometry, and capillary viscometry have been used to study aqueous micellar solutions of dodecyl-, tetradecyl-, and hexadecyltriphenylphosphonium bromides in a wide range of concentrations covering the first and second critical micelle concentrations (CMC1 and CMC2). It has been shown that the concentration curves for the diffusion coefficients of the ionic surfactants increase above CMC1 and, then, pass through a maximum. As the alkyl chain length increases, the slopes of the concentration curves within the range of the linear growth in the diffusion coefficient rise, the height of the maximum increases, and its position shifts toward lower concentrations. The obtained results have been explained in terms of a theory previously developed for ideal micellar systems. It has been shown that the mobility factor plays the predominant role in the range of the linear increase in the diffusion coefficient and the effect of the viscosity of a micellar surfactant solution is enhanced with a rise in its concentration.

Conductometry and viscometry have been employed to study the effect of a background electrolyte (KBr) taken in concentrations of 0.03 and 0.1 M on the critical micelle concentration of dodecyltrimethylammonium bromide (С12ТАB) and the dependence of relative viscosity η/η0 of С12ТАB micellar solutions on the overall surfactant concentration. It has been found that, as a first approximation, each of these dependences may be represented as the sum of two linear portions. Concentrations c* of С12ТАB micellar solutions, which correspond to the inflections between the two linear portions in the concentration curves of relative viscosity, have been determined. The Einstein equation η/η0 = 1 + 2.5p (p is the volume fraction of the dispersed phase and 2.5 is a theoretical parameter that takes into account the spherical shape of the particles) has been transformed into a form corresponding to the concentration dependence of the relative viscosity on the overall surfactant concentration to make it applicable to the consideration of low-concentration systems, which are uncomplicated by intermicellar interaction. In particular, the applicability of the above equation for estimating micelle radii has been studied. It has been shown that the (η/η0–1) = f(c/с01–1) concentration dependences represented in bilogarithmic coordinates (c is the overall С12ТАB concentration and c01 is the critical micelle concentration) are linear in the absence of a significant intermicellar interaction and have slopes equal to unity. This fact may be considered as a criterion for the applicability of the Einstein equation to micellar solutions.