Introduction

Vast numbers of new surfactants are introduced every year for improved features and more environment friendly products, ranging from personal care and household detergents (soaps, shampoos, sun-care and cosmetic products), industrial cleaning solutions, food industry, paints, pigments and inks, to pharmaceuticals. Formulations involving surfactants as key constituents are generally complex mixtures and, accordingly, information and control of surface tension is essential for optimizing their properties. Characterization of surfactants involves the measurement of the concentration dependence of surface tension, giving the adsorption isotherm, which further contains important properties, such as surface excess concentration and critical micelle concentration. Proper and cost-effective development of surfactant formulations requires, however, not only knowledge of the characteristics of the various individual components, but also of their interactions in solutions and micelles. The latter data is virtually impossible to predict due to the complexities related, among other things to the anisotropic nature of the air/liquid and micelle/liquid interfaces. Moreover, development of formulations routinely requires screening of a range of compositions under varying chemical environments, such as different values of pH and ionic strength.It is also of interest to note that the vast interfacial areas existing in living organisms are strongly reflected in the correlation between surface activity and in vivo (ADME/tox) properties I, II, III, IV of pharmaceutical compounds, thus opening a novel application for surface tension measurement in the prediction of these in vivo properties on the basis of in vitro physicochemical profiling. Additionally, surface tension measurements are already well established in preformulation R&D. 

Screening the properties of surfactants in a large range of conditions requires the generation of vast amounts of measured data. Conventional surface tension measurements are limited by slow throughput and high material consumption. The Delta-8 multichannel tensiometer represents a novel high throughput tool for studying how surface activity depends on the composition.

Methods

Delta-8 uses the maximum pull force method, i.e. the maximum force due to the surface tension is recorded as the probe is first immersed ca. one mm into the solution and then slowly withdrawn from the interface. The main forces acting on a probe are the buoyancy (due to the volume of liquid displaced by the probe) and the mass of the meniscus adhering to the probe (Figure 2, panel A). This is an old, reliable, and well-documented technique V, VI, VII, VIII. An important advantage of the maximum pull force technique is that the receding contact angle on the probe is effectively zero. The maximum pull force is obtained when the buoyancy force reaches its minimum, i.e. just prior to the breaking of the meniscus (Figure 2, panel B).

Principles of sample preparation

One of the most important applications of Delta-8 is the measurement of surface tension vs. concentration isotherms, to obtain adsorption isotherms. A series of concentrations can easily be prepared by dilution across a 96-well plate, where the concentration is decreased by a factor for every column using a "transfer and mix" protocol. This dilution factor describes the volumetric ratio of the transferred volume to the volume of buffer initially added into the well and ranges between 0 and 1. Using 96-well plates this procedure gives eight parallel dilution series, each containing twelve dilutions.

This approach can be used both for manual and robotic sample preparation. The plate is measured after an equilibration time, which is necessary to achieve sufficient equilibration between the surface and bulk phases. To minimize carryover the wells are measured in the order of increasing surfactant concentration. The probes can optionally be cleaned between every column of the plate.

Results and Discussion

The Delta-8 produces a wealth of data, which can be used to predict how a compound, for example a pharmaceutical ingredient, behaves in a real biological system. The following example shows how Delta-8 can be used to scan the behavior of clomipramine, a tricyclic antidepressant drug, as a function of pH. Clomipramine is a tertiary amine with a pKa ca. 9, and is therefore neutral at high pH and protonated at low pH. Adsorption isotherms, i.e. surface tension measured as a function of concentration and pH are shown in Figure 4, panel A. Importantly, the differences in the solvation of the neutral and cationic species are readily evident in the surface activity as well as the CMC/solubility (see Figure 4, panel A, B).


The neutral form is more surface active as it is less solvated than the cationic form. Accordingly, surface activity of clomipramine thus increases with pH. Additionally, at higher pH micelle formation becomes favored due to reduced Coulombic repulsion between the headgroups.

The above data clarify the behavior of clomipramine in vivo. Accordingly, the amine group will be protonated in the acidic conditions of the stomach, and hence its solubility increases while the compound's surface activity decreases. When clomipramine moves forward, into the small intestine, where the pH is considerably higher, the solubility decreases and the surface activity increases. Bile salts are excreted into the duodenum and create an anisotropic micellar environment with a hydrophilic surface and hydrophobic cores. In the presence of micelles the hydrophobic parts of clomipramine become buried in the micelle interior, while the hydrophilic amine remains on their surface, in contact with the surrounding aqueous environment. This increases the apparent solubility, which is crucial for the adsorption of many poorly soluble compounds.