Supplementary information

GmAOT complex characterization

The composition of GmAOT was analysed by infrared (IR) spectroscopy, MALDI-TOF/MS and elemental analysis:

- IR

For a qualitative study of the GmAOT composition the IR spectrum of GmAOT was compared with those of gentamicin sulphate and the surfactant AOT. The analyses were carried out in a Spectrum One (PERKIN ELMER, USA) spectrometer and the samples were preparedusing KBr pellets as the dispersant phase. As it is depicted in Fig. SI1, the spectrum of the complex is practically identical to that of AOT but for the shoulder at 1520 cm-1 that corresponds to the amino groups of gentamicin in the complex. This signal has shifted 15 cm-1 compared to that of the amino groups of gentamicin sulphate (1535 cm-1). The signal corresponding to the bending of the S=O group of AOT also shifts 8 cm-1 towards lower frequencies in the spectrum of the GmAOT complex. These shifts indicate that the chemical environment of both groups has changed as a consequence of the replacement of the sulphate counter ions by surfactant ions through the HIP process.

Fig. SI1. IR spectra of gentamicin sulphate, AOT and GmAOT complex. Inset: signal corresponding to the amino groups of gentamicin.

FT-IR (KBr, max in cm-1): 3473, 2961, 2932, 2877, 1738, 1623, 1520, 1465, 1393, 1213, 1043, 873, 729, 582, 528.

- MALDI-TOF/MS

Mass spectroscopy was also carried out for analysing the GmAOT complex. A Ultraflex Extreme MALDI TOF/TOF mass spectrometer (Bruker Daltonics, Germany) was used for this purpose. The peak corresponding to the molecular weight of the complex was not encountered in the spectrum, indicating that GmAOT breaks before flying. The negative mode has an intense peak (m/z = 421) assessed to AOT anion. In the positive mode the most intense peaks correspond to the complex of gentamicin with one molecule of AOT, either with or without a molecule of water (m/z = 889and 905respectively). The m/z of the complex of gentamicin with two molecules of AOT together with two molecules of water (m/z = 1349) and that of the protonated gentamicin with 4 molecules of water (m/z = 545) are also encountered.

- Elemental analysis

For a quantitative study of the complex composition elemental analysis was performed using the CARLO ERBA EA1108 analyser. The analyses of C, H, N and S were performed following the modified Pregl-Dumas method (“dynamic flash combustion”) using He as the porter gas.

GmAOT is ahygroscopic compound and contains certain amount of water. In fact, the elemental analysis data depicted in Table SI1 confirmed a 1:5 gentamicin:AOT molar ratio for a complex containing 4.25 moles of water.

Table SI1. Elemental analysis of GmAOT

C120H231N5O42S5∙4.25H2O / % C / % H / % N / % S
Theoretical % / 54.23 / 9.1 / 2.63 / 6.04
Experimental % / 54.23 / 9.22 / 2.41 / 5.92

Hot Stage Microscopy (HSM)

Thermomicroscopy was carried out by a hot-stage LTS 350 (Linkam, UK) accopled to an Olympus Bx51 microscope (Olympus, Barcelona,Spain). The different samples were observed under the microscope from 30 to 230 ºC with a heating rate of 10ºC/min. Visually observed phase transformations were noted as a function of temperature.

HSM helped us to visually confirm the results obtained by DSC. The microscope images presented in Fig.SI2show the start of melting for GmAOT and C3 that is visually detected at 208 and 196 ºC respectively. This was observed for all the prepared composites, being confirmed, also by this technique, that the pure antibiotic melts at higher temperatures than when is distributed into the polymeric matrix The image corresponding to the pure PVM/MA is depicted as a control to show that the halo observed for C3 is only due to the melting of GmAOT inside the composite.

Fig. SI2. HSM images of GmAOT, PVM/MA and GmAOT:PVM/MA (C3).

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